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British Geological Survey
TECHNICAL REPORT WC/9 5/6 Overseas Geology Series
THE PETROLOGY AND GEOCHEMISTRY OF NEVADOS DE CHILLAN VOLCANO, CHILE
M D MURPHY
Department of Geology University of Bristol
A Report prepared for the Overseas Development Administration under the O D N B G S Technology Development and ResearchProgramme, Project R5563
ODA ckassi/iu?nbn : Subsector: Geoscience Theme: G3 - Improve geotechnical hazard avoidance strategies in national planning Project title: Volcanic hazard mapping for development planning Reference number: R5563
Bibliographic rejerence : Murphy M D 1995. T h e petrology and geochemistry of Nevados de Chillin volcano, Chile BGS Technical Report WC/95/6
Keywords : Chile; Nevados d e ChillAn; volcano; petrology; geochcmlsrry
F m t cower illurtratwn : The volcano of Cerro Rlanco which forms the northwestcrn part of Sevados d e Chillan
0 NERC 1995
Keyworth, Nottingham, British Geological Survey, 199 5
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CONTENTS
Preface
Acknowledgements
1.
2.
3.
4.
5 .
6 .
Introduction
Whole Rock Geochemistry
Stratigraphic and geochemical grouping Geochemical grouping and general geochemistry
Petrography
Plagioclase textures High-Si dacites and mixed rocks Basaltic andesites - low-Si dacites
Mineral Chemistry and Geothermometry
Geothermometry High-Si dacites High-Si andesites Mingled and mixed samples Orthopyroxene-bearing basaltic andesites - andesites Basaltic andesites - andesites without orthopyroxene Mineral chemistry summary
Discussion: magmatic evolution
Summary
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FIGURES
Figure la: Variation diagram of Rb against SiO,
Figure lb: Variation diagram of K,O against SiO,
Figure lc: Variation diagram of Na,O against SiO,
Figure Id: Variation diagram of Zr against SiO,
Figure le: Variation diagram of FeO' against SiO,
Figure I f Variation diagram of MgO against SiO,
Figure lg: Variation diagram of TiO, against SiO,
Figure lh : Variation diagram of P205 against SiO,
Figure li: Variation diagram of A1,0, against SiO,
Figure l j : Variation diagram of Sr against SiO,
Figure lk: Variation diagram of Ba against SiO,
Figure 11: Variation diagram of Ce against SiO,
Figure lm: Variation diagram of Ni against SiO,
Figure In: Variation diagram of Cr against SiO,
Figure 2: Plot of FeO'/MgO against S O 2
Figure 3: Photomicrograph of Type 1 plagioclase within mafic inclusion in sample C14.
Photomicrograph of Type 1 plagioclase in hybrid sample C13.
Photomicrograph of Type 2 plagioclase in sample C107.
Photomicrograph of high-SiO, dacite sample C52.
Photomicrograph of sample C48 from a welded airfall deposit.
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:
Photomicrograph of microphenocryst clot in sample C3.
Photomicrograph of mingling and mixing textures in sample C14.
Figure 10: Photomicrograph of trachytic groundmass in sample C44.
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Figure 11:
Figure 12:
Figure 13:
Figure 14:
Figure 15:
Figure 16:
Figure 17:
Photomicrograph of sample C29.
Photomicrograph of sample C95.
Average temperature from Appendix 5 plotted against whole rock Si02.
Plagioclase compositions plotted on triangular diagrams as simple Or, Ab and An components.
Representative pyroxene analyses plotted as simple quadrilateral components.
Olivine compositions plotted for a representative group of samples.
Cr-spine1 compositions plotted as Cr/(Cr+ Al) against Mg/(mg +Fe2+).
REFERENCES
APPENDICES
Appendix 1 : Relationship between samples and stratigraphy.
Appendix 2: Whole rock analyses
Appendix 3: Modal analyses of a representative group of samples
Appendix 4: Electron microprobe data on mineral compositions.
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PREFACE
This report describes the petrology and geochemistry of the active volcano of Nevados de Chillin, which is situated at 36’50’s in the Southern Volcanic Zone of the Chilean Andes.
The study has been carried out at the University of Bristol as part of a broader assessment of the volcanic hazards of Nevados de Chillin, which is being undertaken in collaboration with the Servicio Nacional de Geologia y Mineria of Chile, the British Geological Survey and the University of Lancaster. The Nevados de Chillin hazard assessment in turn forms part of a wider ranging project entitled Volcanic Hazard Mapping for Development Planning (R5563), which is funded by the Emergency Aid Department of the Overseas Development Administration (ODA). It is led and co-ordinated by the British Geological Survey under the ODA/BGS Technology Development and Research Programme as part of the British Government’s provision of aid through technical assistance to the developing countries.
The objective of the Nevados de Chillin project is to undertake a volcanic hazard assessment of the volcano using both conventional and experimental mapping techniques, with a view to developing and evaluating methods for the rapid mapping of volcanic hazards and production of hazard zone maps for use by planners and related agencies in developing countries.
A first phase of field survey work was undertaken on Nevados de Chillin in February and March 1994, and a second and final phase is being carried out at present. Following the first phase of fieldwork a number of questions arose concerning the stratigraphy and evolution of the volcano, and in particular the relationships between various lava groups that have been erupted from different volcanic centres on the Nevados de Chillin complex.
The evaluation of volcanic hazards requires a knowledge and understanding of the structure and geological evolution of a volcanic centre. The petrological and geochemical study described in this report was undertaken to help elucidate the problems related to lava classification and stratigraphy at Nevados de Chillin, and also to gain an insight into the nature of the magmatic processes operating within and beneath the volcano. The investigation was carried out in close collaboration with other members of the project team who are concerned primarily with the geological mapping and hazard assessment. Through an iterative process drawing upon the petrological and geochemical data, as well as photogeological interpretations and field observations, it has been possible to refine the stratigraphy and geological map of the volcano. This has resulted in a better understanding of the evolution and eruptive history of the volcano as a whole, which will ultimately be reflected in a more realistic assessment of the nature, distribution and impact of potential hazards on the surrounding region
P N Dunkley Project Co-ordinator British Geological Survey
February 1995
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Acknowledgements
The author is grateful to Steve Sparks for reviews of earlier drafts of the report and discussions at various stages of the project. His comments and suggestions have been invaluable. Any errors or omissions, however, remain the sole responsibility of the author. Peter Dunkley is thanked for providing the initial stratigraphic framework and for innumerable discussions necessary to correlate the field observations with the laboratory data. Jennie Gilbert and Rodrigo Chaves are also thanked for advice on the stratigraphic relationships, and S teve Lane for assistance with electron microprobe analysis.
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1. Introduction
This report describes the geochemistry and petrology of rock samples
collected during the first phase of a volcanic hazard assessment of the active volcano
of Nevados de Chilliin, which is situated at 36'50' in the Southern Volcanic Zone of
the Chilean Andes.
The report should be read in conjunction with the interim report of Dunkley
and Gilbert (1995) which describes the geological history and stratigraphy of the
volcano. A major purpose of the work presented here was to attempt to constrain
field relationships between the main lithological units using whole rock
geochemistry, petrography and mineral chemistry. The methodology applied was to
make a first order geochemical grouping of the samples, by graphical analysis of
variation diagrams of the sample database using preliminary stratigraphic criteria as
a basis. Detailed petrographic studies revealed a correspondence in several cases
between mineralogy and whole rock geochemical grouping. The initial geochemical
classification has enabled refinement of the stratigraphy and permitted correlation, in
many instances, of geographically unrelated samples. Microprobe studies of selected
samples have further enhanced understanding of petrogenetic processes at the
volcano.
2. Whole Rock Geochemistry The whole rock geochemistry is discussed first as it forms the fundamental basis of
the groupings under which the petrography and mineral chemistry are considered. As
noted above, the geochemical groups are characterised in a number of cases by
specific petrographic features but these are secondary, as the geochemical groups
were defined independently. The geochemical database consists of the samples
collected by Peter Dunkley, Jennie Gilbert, Rodrigo Chaves and Steve Sparks and
analysed by XRF at Keyworth, together with a small number of samples from the
youngest eruptive centres at the volcano, collected by Rodrigo Chaves and JosC
Naranjo of the Servicio Nacional de Geologia y Mineria de Chile and analysed by
wet chemical techniques and AAS in Santiago.
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Stratigraphic and geochemical grouping
The samples are classified into fourteen geochemical groups, corresponding closely to the units defined in the interim report of Dunkley and Gilbert (1995), for the purpose of simplifying discussion and plots in this report, by grouping geochemically similar samples. It is important to appreciate that a few geochemical groups contain samples from different geographic areas or stratigraphic groups but that in many cases the stratigraphic and geochemical classifications coincide. The criteria used to define the geochemical groups are discussed below. It must be emphasised that grouping samples together here does not imply a common origin. The relationship between the stratigraphic groups and the scheme used here is given in Appendix 1. The stratigraphic terminology is used as far as possible throughout the report.
The only major difference between the stratigraphic and geochemical schemes is amongst the dacitic rocks of Volciin ChillBn. The stratigraphic unit SC6 is composed of three groups, which are geochemically and petrologically distinct from each other, termed SC6a, b and c here (Appendix 1). The SC3 and SC4 samples are chemically very similar to those of SC6a, and so have been combined as a geochemical group, termed GSC346a. The prefix "G" for geochemical is used to distinguish groups which have been defined here according to geochemical criteria. The SC6b samples form a coherent and distinctive group in themselves, termed GSC6b to avoid confusion with stratigraphic terms. The SVD stratigraphic unit, which is comprised of two mixed magma samples of uncertain origin, has been combined with the SC6c group to form the GSC6cSVD geochemical group, the members of which are all mixed or mingled magmas. This subdivision and regrouping is considered essential to maintain clarity in description and on plots.
the field workers is that sample C24, which alone forms the CB4 unit, has been grouped together with the CB3E unit to form the GCB3E4 group, and sample C92, the single member of the CB1 unit is grouped with the SC1 unit to form the GSClCBl group. The main purpose of this is to simplify the variation diagrams.
calculated as FeO, and normalised to 100% anhydrous. Some of the trace elements have abundances approaching or below detection limit and have not been utilised. The trace elements which have been applied to interpretation are Rb, Sr, Ba, Zr, Y, V, Sc, Ni and Cr. Caution has been exercised with the use of Ce, Nb and Hf which do show general internal consistency within the data set but are not determined with sufficient accuracy by
The only other differences between the terminology used here and that used by
The whole rock geochemical data are presented in Appendix 2, with total iron
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XRF to use with full confidence. The other elements have not been considered, due to excessive scatter. Direct comparison between the samples analysed by XRF and those analysed by other techniques has also been approached with caution as no common samples have been analysed and there are strong indications of systematic discrepancies between the different data sets.
Geochemical grouping and general geochemistry
The samples range in Si02 content from 53-70.5% (recalculated anhydrous). Those from the Southern Complex (Volch Chillan and associated centres) are generally dacitic although the older rocks are more mafic than most of the younger rocks and basaltic andesites do occur at higher levels in the stratigraphy. Cerro Blanco has predominantly erupted basaltic andesite to andesite. Variation diagrams of selected elements, plotted against Si02, are shown in Figs. 1 and 2, with samples grouped according to the geochemical classification. The samples are medium to high-K in the classification of Gill (198 1) and range from strongly calc-alkaline to slightly tholeiitic by the Miyashiro (1974) criteria.
initially subdivided on the basis of their Rb and Zr signatures. The mafic to intermediate rocks were classified primarily by their Ti and P trends. Subsequent examination revealed that groups defined in this way were characterised by differences in other elements (e.g. K, Na, Fe, Mg). The criteria are described in detail below.
There are four main groups of high-Si dacites (all but one sample has Si02 > 67%), and one minor group, distinguished by their Rb and Zr signatures. Two of the groups-GSC346a and GSC6b-are characterised by higher Rb and K but lower Na at similar Si02 than the SC2 and SGLD groups (Fig. la-c). The groups can be further distinguished by their Zr contents. Zr is higher in the GSC346a than the GSC6b group and is higher in the SGLD than the SC2 group (Fig. Id), thus giving a fourfold division of the high-Si dacites. The fifth group, the Du stratigraphic group, which is comprised of only two samples of uncertain origin, shows many similarities to the GSC6b group although there are some minor differences, outlined below.
particularly in the case of the SC2 samples, most of which are strongly devitrified. However the SGLD samples are fresh and have similar Rb and K signatures to the SC2 group and Na is higher in the low Rb, K groups. Although some of the Na would be in
A number of criteria serve to distinguish the groups. The high-Si dacites were
Depletions in Rb and K could be related to post-eruptive alkali element mobility,
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phenocryst plagioclase, which is not altered, a depletion in Na would be expected if significant Rb and K mobilisation had occurred. One of the Du samples is devitrified and the other fresh, yet both are almost identical in K and Rb, suggesting that devitrification has not caused remobilisation of alkali metals. It is believed therefore that post-eruptive processes have not significantly affected the whole rock geochemistry.
much higher Fe/Mg (Fig. 2) ratios than the other dacites. Although these plot in the tholeiite field of Miyashiro (1974) on Fig. 2, this is not a tholeiitic trend but is a natural consequence of extreme fractionation. At this advanced stage of magma evolution, Mg becomes highly depleted in the liquid and even very small amounts of pyroxene have a strong effect on whole rock Fe/Mg ratios. The GSC6b samples have lower Ti and P (Fig. lg-h) than the other groups. The SC2 and the SGLD groups are distinguished from each other mainly by Zr, but K and Rb also fall on higher trends in the SGLD group. The Du group differs from the GSC6b group in having higher Ti and lower Fe. Otherwise it is geochemically quite similar. The GSC6b group is petrographically unique among the high-Si dacites in that some of the samples contain small amounts of mafic material (Section 3).
The high-Si dacite groups also show differences in Al, Sr, Ba and Ce (Fig. li-1). Al is slightly higher in the SC2, SGLD and Du groups than in the GSC346a and GSC6b groups. Sr shows similar behaviour although the differences are not as clear. Ba and Ce are enriched in the GSC346a group relative to the others which overlap to some degree in both elements, although the SC2 group tends to have the lowest values.
The other groups fall broadly into two categories distinguished by trends in Ti and P. Both of these elements are relatively incompatible in the typical early fractionating phases-olivine, plagioclase and clinopyroxene-in basaltic to basaltic andesite magmas in arc settings, and tend to increase in abundance in the liquid up to the point at which titanomagnetite and apatite start to fractionate in significant amounts. Mixing between basaltic and silicic magmas with low Ti and P contents will therefore produce hybrid or mingled magmas which, at intermediate values of Si02, have lower Ti and P than magmas which have evolved predominantly by fractionation (Fig. 1g-h). Further distinctions are evident in Fe, Mg, Rb, K and Na. Several of the stratigraphically defined groups can be distinguished from each other in terms of these elements.
distinguished by their low Ti and P trends (Fig. 1g-h), indicative of a mixed origin. The samples also exhibit definite petrographic evidence for magma mixing (Section 3).
The GSC346a samples have higher Fe and lower Mg (Fig. le-f) and consequently
The GSC6cSVD samples range from 58.5-66.6% Si02 and are clearly
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The GSC 1CB 1 samples range from 53-60.1 % Si02 and are also characterised by relatively low Ti and P (Fig. lc-d). Some of these samples (C50 and C103) display evidence in their mineral chemistry for mixing or reheating (Section 4). but many other samples have no evidence for such processes. The group is also distinguished petrographically by the occurrence of abundant titanomagnetite in most samples (Section 3), in contrast to the remaining mafk-intermediate groups where Fe-Ti oxide is rare or absent in the more mafic samples. The single sample from the CB 1 stratigraphic unit, C92, differs significantly from the CB2 samples, with which it was originally classified stratigraphically. It has been included with the SC1 samples here for simplification in plotting and description, as it falls on similarly low Ti, P trends. There is no implication that it is petrogenetically related to the other rocks, but has probably evolved in a similar way.
Both the GSC6cSVD and GSClCB 1 groups are characterised by lower Fe/Mg ratios (Fig. 2) than the other mafic-intermediate groups and plot well within the calc- alkaline field of Miyashiro.
The SC7 group is predominantly low-Si dacite but ranges from 63.7-67.4% Si02. These are the samples analysed at Santiago. They are anomalous in a number of respects. They tend to plot on a relatively low trend in Ti but are quite high in P. All the samples analysed by XRF show strong coherence between these two elements. The SC7 samples plot well within the tholeiite field on Fig. 2 but this is due to their low Mg (Fig. le). The lack of coherence between Ti and P and the very low Mg are suspicious. The group is also anomalous in several other elements, in particular Al and Ni. The samples have very high Ni (Fig. lm) yet very low Cr (Fig. In), often close to or below detection limit. If the high Ni contents were due to mixing, similar trends should be apparent in many other elements (e.g. P, Cr, Mg) and the samples should lie along trends similar to the SC6SVD group, as the Ni contents would indicate a very mafic mixing end-member. This further suggests that the data should be treated with suspicion. It is recommended that samples from this stratigraphic unit be obtained and analysed by XRF in order to perform reliable comparison. The samples are not discussed in any detail in this report and no inferences are made about petrogenesis by comparison with the samples analysed by XRF.
between the groups can be made using the relative steepness of trends in these elements. Some of these groups are predominantly mafic, others span part or all of the range from basaltic andesite to dacite.
The other groups are characterised by higher trends in Ti and P. Distinctions
The stratigraphic CBG unit is comprised of three samples, which range from 61.2-66.8% Si02. They fall on the descending leg of a fractionation trend on the Ti and
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P plots. The two evolved samples are very similar to the SC2 dacites in their Rb, K and Zr signatures and also show strong petrographic similarities (Section 3).
The SGLA stratigraphic unit ranges from 55.9-59% Si02 and is distinguished by very high Ti and P (Fig. 1g-h). These samples mostly plot close to or within the tholeiite field on the Fe/Mg plot (Fig. 2).
The CB2 stratigraphic group ranges from 58.1-62.3% Si02 and also has very high P, but is not quite as high in Ti and FeMg as the SGLA samples. Both of these groups are petrographically distinct from all the other mafic-intermediate groups in that the samples are fine grained and poorly porphyritic, whereas the others tend to be mostly coarse grained and have high contents of phenocrysts (Section 3). However, the CB2 and SGLA groups can be distinguished from each other by their mineral assemblages (Section 3).
The remaining three groups are very similar to each other in their Ti and P characteristics. The SC5 group is a stratigraphic unit with a narrow range in Si02 from 56.5-57.9%. Because of the narrow range in Si02, it is difficult to discern trends in the group but they appear to form a steep trend in Ti and P. This group is petrographically distinct from the mafic members of GCB3E4 as orthopyroxene is absent, whereas it is relatively abundant in the latter group at similar Si02 content.
the former ranging 55.5-66% Si02 and the latter 56.5-64.4% Si02. A single sample C10, from the CB3W group lies on a lower Ti, P trend. This sample is the product of magma mingling as evidenced by the geochemical trend and the presence of a small mafic inclusion, evident in thin section. The sample plots at higher values of Ti and P than the GSC6cSVD mixed group, implying that one or both of the mixing end-members were geochemically different than those in the GSC6cSVD group.
Distinctions between some of the mafic-intermediate groups can be made on some of the incompatible element plots. The GSClCBland CB3W groups fall on slightly higher trends on the Rb and K plots than the SC5 and GCB3E4 groups, although there are exceptions and overlaps. The differences are most evident between the GSC 1CB 1 and GCB3E4 groups. Zr, Ba and Ce show similar behaviour. Na shows the opposite amongst the more mafic samples. The differences are not likely to be caused by different proportions of phenocrysts as all of these groups are similarly highly porphyritic (Section 3), although the abundance of phenoctysts complicates comparison and interpretation. Further distinguishing features of the GSClCB 1 group, in terms of compatible trace elements, are higher Ni and Cr in many samples (Fig. 1m-n) at Si02 around 55-57% compared to the other groups and in a few cases lower Sc and V, although there is
The GCB3E4 and CB3W groups span the range from basaltic andesite to dacite,
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considerable scatter in V. None of the elements discussed in this paragraph have not been used to distinguish the groups but the contrasts are noted here as they may be significant in relation to petrogenesis, discussed in Section 5.
and C50 from the GSClCB 1 group. This is not simply due to excess plagioclase accumulation. Several other porphyritic samples have similar or higher proportions of plagioclase, although many appear to have experienced enrichment in feldspar relative to the mafic minerals. C50 plots above the general trend on the AI diagram, but neither Ca nor A1 is anomalous in C22, which is also characterised by higher Ba, Ce and to a lesser extent by higher K and Zr when the general trend is extrapolated to 53% Si02.
elements show similar behaviour to Zr throughout the range of the sample set , although they are more variable. Sc and V also show much scatter.
Another feature worth noting is the exceptionally high Sr contents of samples C22
Elements which have not been plotted are Nb, Hf, Y, Sc and V. The first three
3. Petrography
The petrography is described by rock type rather than by stratigraphic or geochemical group, as there are many similarities between samples of similar Si02 content. Geochemical distinctions discussed in the previous section are in many cases reflected in the petrography. Distinguishing features are highlighted below. The descriptions are generalised because of the number of samples and there are some exceptions to the broad characteristics reported. Individual samples are discussed in more detail in Section 4. Modal analyses of selected samples, mostly those which have been analysed by microprobe, are given in Appendix 3. The following terminology is used in the thin section descriptions.
Grainsize Coarse > 2 mm Medium > 1 mm Fine > .5 mm Very Fine > .25 mm Microphenocryst < 250 pm down to about 100 pm
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Groundmass Texture Holohaline: Completely glassy Holocrystalline: Completely crystalline Intersertal: Microlites with interstitial glass. Trachytic: Used here to describe groundmass which is predominantly composed of plagioclase microlites, aligned to varying degrees, with very little or no interstitial glass. Coarse, medium and fine refer to the relative size of plagioclase microlites. These terms are used in a qualitative descriptive sense only.
Plagioclase Textures
Two different plagioclase textures can be distinguished, based mainly on zoning patterns and the type of melt inclusions within the crystals. Although these textures ultimately require microprobe analysis for absolute distinction, they can often be recognised under the microscope. Similar textures are described by Kawamoto (1992) from Japanese volcanic rocks and are very common in lavas at Sollipulli volcano at 39"s in the Chilean Andes (Murphy, in prep.).
framework of glass and plagioclase, usually on a scale of less than 10 pm, determined by microprobe. The dusty sieve-textured regions may be confined to the rims or can permeate the entire grain. They consist of an intergrowth of glass and plagioclase which is almost invariably more calcic than the normally clear core. In some cases rim overgrowths of more sodic clear plagioclase are present.
Type 2 (Fig. 5) is characterised by large melt inclusions, often elongate and oriented parallel to the long axis of the grain. The inclusion size is sometimes positively correlated with the grainsize. This type may or may not exhibit patchy zoning often with a large compositional range. The inclusions are almost always similar in composition to or more evolved than the groundmass, to which they are often connected by cracks. They usually consist of glass or microcrystalline material.
The Type 1 texture is typically related to resorption of relatively sodic plagioclase phenocrysts consequent on interaction with more mafic magma, often producing reverse zoned crystals. Rapid growth of more calcic plagioclase, in equilibrium with the hotter liquid, results in trapping of melt inclusions and forms the intricate network.
Kirkpatrick, 1982), the high growth rate resulting in the formation of skeletal grains.
Type 1 (Fig. 3,4) has a very fine sieve-texture, composed of an intricate
The Type 2 texture is generally considered to form by rapid growth (e.g. Kuo and
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Subsequent growth of more sodic plagioclase at a later stage of magmatic evolution gives rise to the characteristic patchy zoning. Remaining cavities become infilled by liquid in the late stages of crystallisation of the magma. No examples have been found here or in a detailed study of this texture in lavas from Sollipulli volcano (Murphy, in prep.) where the inclusions represent more primitive trapped liquid.
always more sodic than the Type 2, although if present, sodic patches in the latter may overlap in composition with the Type 1 cores. The composition of plagioclase in the Type 1 dusty areas is similar to or more calcic than the most calcic parts of the Type 2 grains. Because of the potential complexity of the processes involved in their formation, there are no hard and fast rules for interpreting these textures. The Type 1 texture is diagnostic of magma mixing and occurs in many samples which show other evidence for mixing or mingling. The Type 2 texture is seen in a range of rock types formed under different conditions. Many circumstances can be envisaged in which rapid growth may occur. It may in some instances be associated with a mixing process, for example if a mafic magma ponds initially below a cooler silicic magma, resulting in rapid undercooling of the mafic magma at the interface between the two bodies or in a hybrid layer. However, Type 2 plagioclase textures at both Sollipulli and Nevados de ChillSn are common in rocks which display no evidence for interaction between mafic and silicic magmas.
In some cases it is impossible to distinguish these textures under the microscope, as some grains with very fine inclusions are probably not formed by resorption but by skeletal growth and are therefore Type 2 grains. The terms Type 1 and 2 are used here to save repetition in describing the petrography and mineral chemistry. Clear crystals (without inclusions) and crystals with small, sparse melt inclusions (usually in dacitic samples) are also common.
When the two types coexist in a single sample, the Type 1 cores are almost
High-Si Dacites and Mixed Rocks
Samples described in this section are from the five dacitic geochemical groups as well as the evolved samples from the CBG group. All have Si02 > 65.5% and the majority have over 68% Si02. The mixed and mingled samples of the GSC6cSVD group are also described under this heading, as they are essentially high-Si dacites which have mixed with mafic magma, probably basaltic andesite.
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The samples are predominantly fine-grained, mostly with less than 10% phenocrysts although some have up to 15% (Appendix 3). Assemblages of plagioclase, ortho- and clinopyroxene, titanomagnetite and ilmenite, with trace amounts of apatite, are typical. Amphibole phenocrysts are not observed in any of the Nevados de Chillfin rocks, although hydrous minerals are sometimes found as alteration products of mafic phenocrysts in the less evolved rocks. The GSC6b group differs from the other high-Si dacites in that minor amounts of mafic inclusions are present in some samples. Rare olivine, derived from the mafic material, is present in the dacitic parts of some of these samples.
or interstitial glass, similar in colour but usually less crystalline than the bulk of the groundmass. Plagioclase tends to be more coarse grained than pyroxene in any given sample. It is normally euhedral, equant or lath-shaped, and is generally clear or contains small sparse brown melt inclusions. Larger grains sometimes have inclusions of other minerals. The GSC6b group samples often contain Type 2 plagioclase, with large inclusions (20-100 pm approx.). In some of the GSC6b samples, Type 1 plagioclase is also present and has a very fine sieve-texture towards the rims. Orthopyroxene is usually very fine grained and forms equant euhedral crystals. Clinopyroxene is fine to medium grained and sometimes has reaction rims. Microphenocrysts of ilmenite and titanomagnetite occur as single grains and in clots. Euhedral microphenocrysts of apatite occur in clots with the other minerals. Apatite is also found as acicular inclusions in plagioclase and pyroxene. Olivine has been observed only in the GSC6b group.
Groundmass textures are intersertal, predominantly plagioclase microlites in brown glass. The SC2 samples have partially to completely devitrified groundmass. One of the Du and one of the CBG samples are also devitrified. Many samples show flow alignment of phenocrysts and groundmass microlites, sometimes with very fine scale flow banding and folding (Fig. 6).
Phenocrysts occur as isolated grains and in clots, some of which have glassy rinds
C48 is an exceptional sample which alone forms the SC3 stratigraphic unit and is categorised here in the GSC346a group. It was collected from a welded air-fall deposit. It has a similar phenocryst assemblage to the other samples but has a completely glassy groundmass. Its original fragmental origin is evident from the presence of very fine strings of flattened pumice, the internal texture of which is not optically resolvable, in a welded glassy matrix (Fig. 7).
The GSC6b group is exceptional among the high-Si dacites in that small sparse diktytaxitic mafic inclusions are present in many samples. Olivine, plagioclase and pyroxene occur as phenocrysts in some of these inclusions, whereas other inclusions are
10
D
B
m e
e
e 8 e
e e e m 0
e 0
e 0
0 0
0 rn 0
0 0
0
aphyric. The presence of the mafic inclusions has had only a minor effect on the whole rock geochemistry. As well as diktytaxitic inclusions, microphenocryst clots, sometimes with glassy rinds, and often with high proportions of pyroxene and Fe-Ti oxides, are common (Fig. 8). Similar microphenocryst clots and small clusters of fine acicular plagioclase occur in some of the other dacites but these appear to be compositionally similar to the larger phenocrysts (Section 4).
Much larger mafic inclusions are present in many of the GSC6cSVD samples (Fig. 4,9). The inclusions form a framework of fine grained plagioclase with interstitial voids, sometimes partially filled or lined with glass, similar to that of the dacitic groundmass. Mafic inclusions of this type have been described from volcanoes in Western North America by Bacon (1986) and are abundant in dacites at Sollipulli volcano (Murphy, in prep.). The inclusions in the GSC6cSVD tend to have coarser groundmass than the GSC6b samples. Interstitial voids are also much larger, some having longest dimensions > 1 mm. The mineral chemistry of some typical samples is presented in Section 4.
of mafic origin (determined by microprobe analyses) also occur. Dacitic phenocrysts are also present in the inclusions. Sodic plagioclase in the inclusions often has Type 1 texture, due to resorption upon incorporation into hotter magma than that in which it has grown. Olivine derived from the mafic magma also occurs in the dacitic parts of some samples.
The phenocryst assemblages in the dacitic parts of the mingled samples are identical to those in the dacites, described above, with the addition of abundant xenocrystic olivine. Plagioclase textures are very distinctive and Type 1 plagioclase is very common, showing various degree of resorption. Clear plagioclase also occurs. The resorption is undoubtedly related to magma mixing in these samples. Grainsize tends to be medium to very fine although coarse grains are present in some samples. The clear plagioclase is generally euhedral, tabular or prismatic. Pyroxene and Fe-Ti oxide textures are similar to those in the dacites. Pyroxene is generally fine to very fine grained, forming equant euhedral grains, often in clots with titanomagnetite and ilmenite. Oxides occur as microphenocrysts. Olivine is medium to coarse grained, euhedral to subhedral, and often contains Cr-spine1 inclusions.
mingled, as they contain few or no inclusions, but have disequilibrium phenocryst assemblages with coexisting sodic and calcic plagioclase, forsteritic olivine and multiple generations of pyroxenes. The phenocrysts are texturally similar to those in the mingled
Olivine is the main phenocryst in the inclusions but plagioclase and clinopyroxene
Samples C13, C45 and C46 from the GSC6cSVD group are mixed rather than
11
D B B B B
B
0
e
0
0
0
I)
0
0
@ 0 0
0 0 a 0 e
e 0
0 0
rocks. Sample C14 shows partial hybridisation and has three distinct areas of groundmass: original host, inclusion and mixed parts (Fig. 9).
Basaltic Andesites-Low-Si Dacites
The mafic to intermediate rocks can be broadly classified into three petrographic categories. Categories A and B are comprised of the more mafic rocks (most are < 60% Si02), and C is mainly high-Si andesite and low-Si dacite.
A) This category encompasses all samples from the GSClCB 1 and SC5 groups and the more mafic members of the GCB3E4 and CB3W groups. All samples have Si02 < 60%. Although there is much variation, the main petrographic features which distinguish samples within this category are phenocryst content and grainsize. Samples are highly porphyritic, often containing > 30% phenocrysts, generally coarse grained compared to the other petrographic groups and tend to have coarser groundmass than the dacites. All samples contain plagioclase, and most contain olivine and clinopyroxene. This category can be subdivided on the basis of titanomagnetite abundance which is present in GSClCB 1 samples but very sparse or absent in the mafic samples of the other groups at similar Si02 content (< 58% approx.), a feature which is reflected in the whole rock geochemistry, as samples which crystallise titanomagnetite will tend to evolve on lower Fe-Ti trends (Figs. 1g-h). The presence of orthopyroxene also corresponds to some extent with the geochemical and stratigraphic grouping. It is absent in the SC5 samples, rare in the CB3W mafic samples but relatively abundant in the otherwise petrographically similar GSC 1CB 1 and GCB3E4 samples.
Plagioclase has a large range in grainsize but is normally much coarser grained than in the dacites and other andesites. It is usually euhedral, equant or lath-shaped, occumng in clots and as individual grains. Type 2 and clear plagioclase are by far the most common, but some GSClCB 1 samples contain Type 1 grains in addition (Section 4). C24, the single member of the CB4 stratigraphic unit, placed here in the GCB3E4 group, also contains some Type 1 plagioclase.
Olivine occurs as rounded microphenocrysts in many samples, with or without larger phenocrysts, which are common in the more mafic samples. When two size populations of olivine are present, they usually have different mineral chemistry, the larger ones being more magnesian (Section 4). Olivine almost invariably has pigeonite
12
D B D D D B D B
@
D
B
0 0 D
rims except in the GSC 1CB 1 samples. Cr-spine1 inclusions are present in some of the larger phenocrysts.
groundmass phase in most samples. No consistent differences in clinopyroxene phenocryst textures have been observed between different groups. It tends to become more abundant in the more evolved rocks but there are exceptions, some of the more mafic rocks containing more clinopyroxene than olivine. In some samples it occurs intergrown with plagioclase as coarse grained clots without interstitial groundmass and is probably of cumulate origin.
although there are exceptions as with clinopyroxene. It commonly replaces olivine, which occurs as remnant patches within the pyroxene. It varies in grainsize from medium to very fine.
samples. It is sparse or absent in the mafic samples from the other groups.
coarse grained, glass being a minor constituent in most samples. Trachytic textures are common. Plagioclase and augite are almost ubiquitous, and pigeonite, olivine, orthopyroxene and titanomagnetite are the other common groundmass phases. Pigeonite occurs in the groundmass of samples in which it overgrows olivine. Groundmass olivine occurs in some of the more mafic rocks, sometimes rimmed by pigeonite. Orthopyroxene is the low-Ca groundmass pyroxene in the GSClCB 1 group. Titanomagnetite becomes more abundant in the more evolved samples.
Clinopyroxene varies in grainsize from coarse to very fine. It is also common as a
Orthopyroxene varies in abundance, tending to increase with whole rock Si02,
Titanomagnetite forms microphenocrysts and is abundant only in the GSClCB 1
Groundmass textures are variable but are typically holocrystalline, often relatively
B) The distinctive characteristics of this category are fine grainsize of phenocrysts (many are microphenocrysts), low phenocryst contents and trachytic groundmass in many cases (Fig. 10). The CB2 group, most of the SGLA samples (with the exception of ClOl which is highly porphyritic) as well as a single sample, C29, from the GCB3E4 group are discussed in this section. All of these samples fall on high Ti and P trends.
Si02) are readily distinguished from each other by their phenocryst assemblages. Clinopyroxene is extremely rare or absent in the CB2 samples, the only phenocrysts being olivine and plagioclase. In contrast, at similar Si02 content, clinopyroxene predominates over olivine in the SGLA group. In some SGLA samples, olivine has been pseudomorphed by aggregates of a brown fibrous fine-grained mineral, probably iddingsite. Minor amounts of titanomagnetite are present in some SGLA samples. The
The more mafic samples of the SGLA and CB2 groups (those between 57.9-5996
13
B
B D D
D D
B B
B D I)
I)
D
I,
0
t rn
0 rn rn 0
SGLA sample, C118, is more porphyritic and has a finer grained groundmass than the other two samples from this group (C44 and C125). The mineral assemblage is similar although titanomagnetite is more abundant than in the less porphyritic samples. The more evolved samples in this petrographic category contain plagioclase, clinopyroxene, and lesser amounts of orthopyroxene, olivine and titanomagnetite.
2 grains are sometimes present. Olivine is usually very fine grained and rounded, and almost invariably has pigeonite rims (determined by microprobe). Clinopyroxene generally occurs as small euhedral grains. When present, orthopyroxene is very fine grained and magnetite often forms euhedral microphenocrysts.
interstitial glass. Sample C29 has a more glassy groundmass (Fig. 11).
Plagioclase in most samples occurs as clear euhedral laths or equant grains. Type
Groundmasses are mostly trachytic, either holocrystalline or with minor
C) This category consists of the more evolved samples from CB3W group, sample C36 from the CBG group and all the SC7 samples.
Texturally the samples are somewhat similar to the high-Si dacites. They generally have low to medium abundances (5-20%) of medium to fine grained phenocrysts, and have medium to fine intersertal groundmass textures. The main distinction between this category and the more evolved dacites is the presence of minor amounts of olivine microphenocrysts with pigeonite rims or remnant olivine inclusions in pyroxene in several samples. This olivine is distinct from the xenocrystic olivine in some of the high-Si dacites as it is not associated with the presence of mafic inclusions and has reacted with the melt to form pyroxene.
olivine in some cases. The phenocrysts are similar to those in the dacites described above. Plagioclase is mostly clear and euhedral although some Type 2 grains occur. Pyroxene is typically equant and euhedral. The SC7 samples are similar to the others in terms of phenocryst assemblages but some are slightly coarser grained and many are vesicular. Phenocryst clots are common in most samples.
microlites with brown glass. In some samples the groundmasses are heterogeneous. Samples C91, C95 and C96 as well as the more mafic samples C97, C98 and C116, considered under category A, all have dark glassy blebs, which have been variably deformed during flow and sometimes intricately folded, flattened and stretched (Fig. 12). In some cases the blebs appear to have chilled margins, as the outer parts are sometimes more glassy and occasionally vesicular. Microprobe analyses reveal no compositional
The typical assemblage is plagioclase, two pyroxenes, Fe-Ti oxides and minor
The groundmass textures are usually intersertal, predominantly plagioclase
14
e 0
e 0
0 0 0
0
0 0
a 0 * e e
0
0 0
e e
e
0 0 0
0
U
0 e 0
contrast between the different groundmass areas (Section 4, sample C95), nor between phenocrysts in the blebs and in the normal matrix. The blebs may have formed from the same magma as their hosts but have had a different history and subsequently been incorporated back into the main body of magma. Whether this occurred in the magma chamber or during flow is open to question. The presence of what appear to be chilled margins is difficult to account for in the terms of the latter scenario. The origin of this texture is subject to speculation. Sample C36 also has heterogeneous groundmass texture, but this is similar to the normal flow banding in many of the dacites (Fig. 6).
4. Mineral Chemistry and Geothermometry
Electron microprobe analyses of the samples selected for detailed study are presented in Appendices 4a-g. The analyses were performed at Bristol University on a Jeol JXA 8600 four spectrometer instrument with LINK analytical x-ray analysis system and LEMAS automation. Online data reduction used the ZAF matrix correction methods. Run conditions were 15 kV accelerating voltage, with a 15 nA beam current at minimum probe diameter of 1-2 pm. Groundmass analyses were made in a few cases using a 20 km beam diameter at 7 nA beam current.
The samples were chosen on preliminary stratigraphic criteria in the early stages of the project, before the geochemistry had been examined in any detail, in order to have thin sections prepared in time. Most of the stratigraphic units are represented in the probe data set, but no samples from the CB2 unit were chosen as C92 (CB 1) was at that time considered to belong to this group. Nor have either of the Du samples been analysed, although these are very similar to other dacites which have been studied. The SC7 unit samples, collected by the Chilean workers, have not been analysed by microprobe, as suitable thin sections were not available. None of the GSC6b group samples were chosen, as the geochemical divisions had not been made at the time, although three samples from the SC6 stratigraphic unit have been examined in detail. However it is unlikely that there are any features of the mineral chemistry unique to these samples, as all have equivalents among the analysed samples, which have been studied in detail. For completeness, further microprobe work could be performed on the unstudied groups. The time scale of this project did not permit a completely exhaustive study but a total of about four thousand five hundred analyses have been made on twenty three samples. The data are presented in synoptical form in Appendix 4a-g.
15
0
0 0
e 0
0
0 0 0
0
e 0 0 0 * I)
0
0 0
0 0 0 0 0
0
0 0 0 0
0
a 0
0
In order to obtain a realistic insight into the mineral chemistry of the samples, it is necessary to perform a large number of analyses on several grains in each sample, as heterogeneity is the rule, both within grains and over the scale of a microprobe section. A number of grains of the principal minerals have been analysed in the majority of samples. In cases where there are textural differences between grains of the same mineral (mainly plagioclase), the different types have been analysed. For the silicate minerals, between three and five analyses were made of the rims in most cases and usually between seven and fifteen of the cores, normally as a traverse from the outer margin to the core. Random core analyses were made of Fe-Ti oxides as these rarely exhibit compositional zoning.
In general the groundmasses have not been analysed in detail, although broad beam traverses have been made in some samples where the microlite grainsize is fine enough to produce reasonable results. The technique involves performing traverses across areas of microcrystalline groundmass and calculating an average composition. Between fifteen and one hundred analyses were performed on each sample studied. Acceptable results are obtained when the matrix minerals are similar in size to or smaller than the beam diameter. In samples with coarse grained groundmass such as many of the mafic to intermediate rocks, poor quality results were obtained. Holes and pits between the coarse grained microlites and surface roughness all provide obstacles to the effective use of this technique, due to difficulty in randomly traversing a sufficiently large area to obtain an estimate of the groundmass composition. Analyses have been made andor ED spectra taken of groundmass minerals in many samples. These data are presented only where considered particularly relevant to the petrogenesis, as the quality of the analyses is often poor due to the small grainsize of the minerals.
being used in the case of silicate minerals. Oxides always have low totals due to the presence of ferric iron and it is not generally possible to determine the quality of the analysis from the total as the stoichiometry must be assumed in order to calculate Fe2+/Fe3+. Mineral components referred to in the text are the simple components in all cases (see Appendix 4).
The raw data were filtered, only analyses with totals between 98.5 and 101.5%
16
0
0 0 0
a 0 0 0 0
0
m 0 0 0
0 0 0 0
0 m e 0 0
rn 0 0 0 e
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0 0
0
Geothermometry
Pyroxene temperatures have been calculated using the QUILF program of Andersen et al. (1993), which incorporates a numerical version of the graphical pyroxene thermometer of Lindsley and Andersen (1983). All temperatures are calculated for a pressure of 1 bar. The pressure dependence of the thermometer is negligible and well within the limits of analytical error. The method used was to calculate individual pyroxene temperatures, as the ortho- and clinopyroxene thermometers are independent of each other and can be used separately, as long as the other mineral is assumed present and in equilibrium (Andersen et al., 1993, p. 1345). Using the single pyroxene method alleviates the problem of initially deciding which pairs are actually in equilibrium. It enables assessment of equilibrium, as the temperatures should be in reasonable agreement if equilibrium growth of ortho- and clinopyroxene has occurred. Two-pyroxene calculations were also performed initially but the program often yields erratic and unrealistic values in this calculation mode, in some cases outside the range of the single pyroxene values, particularly when the single pyroxene temperatures for the pair are not close to each other. However the two-pyroxene temperatures for pairs which yield coherent single pyroxene temperatures are always close to the individual values.
using a projection scheme from the QUILF program, which takes account of non- quadrilateral components and yields different values from the simple components. Both sets of components are listed below the analyses. Orthopyroxene components can also be calculated using the QUILF program, but as the non-quadrilateral components are present in very low abundance in all grains analysed here, the difference is usually zero or negligible for geothermometry and always has the effect of raising the calculated orthopyroxene temperatures. In a number of samples, orthopyroxene yields higher temperatures than clinopyroxene (see below). In the few cases where the QUILF and simple components differ, the effect of using the QUILF projection for orthopyroxene is to increase the discrepancy between clino- and orthopyroxene temperatures. Only the simple components have therefore been used for orthopyroxene.
for each analysis. Temperatures were calculated for the average composition of cores and rims (in many cases) of each individual grain. These are tabulated in Appendix 4c. Mean temperatures for each sample, calculated from the averages of the individual grains, are reported in Appendix 5 and plotted in Fig. 13. Some samples, particularly those which have experienced magma mixing, contain different generations of pyroxene. In such
For the purpose of thermometry, the clinopyroxene components are calculated
The design of the QUILF program makes it impractical to calculate temperatures
17
I)
0 0
0
I,
0
0 0
0
0 0
0
0 0 0
e 0
0
0
0
0 0
0 m I)
0
0
0 m 0
0
0 0
0
cases, the averages of the different generations have been taken. The absolute error on the estimated temperature is about f30-50"C. Statistical standard errors are not quoted as they are much smaller than the true error and are meaningless due to the method used. The relative differences between and within samples must reflect actual temperature differences, as there are real and significant compositional differences between grains. Although there is no precise negative correlation between whole rock Si02 content and temperature, there is a systematic decrease in temperature from basaltic andesite to dacite (Fig. 13). The very good correspondence between pyroxene and olivine-liquid temperatures (see below) provides a measure of confidence in the results.
In many cases, there is good agreement between temperatures calculated for the majority of grains in a given sample. In some cases however, there are systematic discrepancies between the calculated ortho- and clinopyroxene temperatures, the former often being somewhat higher. Because the thermometers are based on exchange of Ca between the pyroxenes, the low abundance of Ca in orthopyroxene and the steepness of the orthopyroxene side of the solvus curve may make the calculated temperatures more susceptible to error. It seems petrologically unintuitive to suggest that orthopyroxene has crystallised at higher temperature than clinopyroxene. It is possible that a small positive error in the measurement of Ca could cause this effect as this would result in higher ortho- and lower clinopyroxene temperatures. However there are a number of cases where there is very good agreement between the two thermometers, particularly where pairs are intergrown. The effect is not related to fluctuations on the microprobe, as samples which give both types of result were analysed during the same run and re- analysis did not alter the previous results. Nor is it related to the temperature range, as the effect is seen across the entire spectrum of temperatures. A further possibility is a secondary fluorescence effect for Ca. However the contrast in Ca content between the matrix and orthopyroxene is not generally large and it would certainly be unexpected in the dacitic compositions. This effect may be expected to be greatest where the two pyroxenes are intergrown but this is not observed.
pigeonite field. In some samples the orthopyroxene has high Ca content (approaching Wog). Although the QUILF program can deal with pigeonite, it requires separate projection using a different algorithm and a warning is given by the authors of the program about using this mineral for thermometry. Projection using the pigeonite algorithm on the high-Ca analyses here yielded highly erratic results. Similarly, calculations using true pigeonite, which is common in many of the andesites as reaction
Another problem with orthopyroxene is determining the exact boundary with the
18
I)
0
0 0 0
0 0 0
0
0
0
0 0 rn 0 0
0 0
0
0 0 0
0 0
0 0
0 0 0
0 0
0
rims around olivine and as a groundmass phase, produced implausible values. These results have been discarded.
The pigeonite problem does not account for the systematic discrepancies between ortho- and clinopyroxene, the origin of which is not resolved. However there is a good correlation between orthopyroxene and olivine-liquid temperatures, discussed below. It is possible that the clinopyroxene may have continued to re-equilibrate with the liquid during cooling in some cases. Some clinopyroxene grains, mainly in the more evolved rocks, have very narrow reaction rims. However there is no direct correspondence between this texture and the discrepant temperatures. Kinetic factors during crystallisation may have had an influence, as growth rates can have significant effects on major and minor element compositions of clinopyroxene (Lofgren, 1981, Skulski et al., 1994).
There are many samples in which the two pyroxenes do produce concordant results. Greater confidence is placed in the orthopyroxene temperatures, where these differ from clinopyroxene values, due to their closer correspondence in most cases (with one exception) to the olivine-liquid temperatures.
Two-oxide temperatures, calculated by QUILF and two other methods (Stormer, 1983 and Ghiorso and Sack, 1991), are lower in all cases by 100-200 "C than the pyroxene temperatures in the dacitic rocks, which contain titanomagnetite-ilmenite assemblages. Two-oxide thermometry often yields lower values than pyroxene thermometry (e.g. Honjo et al., 1992) and the very large differences here are almost certainly due to low temperature re-equilibration of the oxides. Exsolution lamellae are evident in some titanomagnetite crystals. Average temperatures and oxygen fugacities calculated by the Stormer (1983) method are tabulated in Appendix 4e for completeness but the results have not been used in interpretation as the pyroxene thermometers are undoubtedly the more accurate. The QUILF two-oxide values are even lower than those from the other thermometers.
Olivine-liquid thermometry has also been used, mainly to constrain values obtained from pyroxene equilibria. Several problems arise when attempting to apply liquid thermometers to the Nevados de Chillin porphyritic rocks, the main one being in the estimation of liquid compositions. None of the olivine-bearing rocks are glassy except for minute interstitial patches, too small for accurate microprobe analysis. These almost certainly represent final liquids and not the melt from which the olivine has grown. The groundmass minerals are in most cases too coarse for accurate broad beam analysis.
19
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0 0 0
0 0 0
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0 0
e 0 m 0 0
0
0 0 0
0 0
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0 0
e 0
0
0 e e
An alternative method of estimating liquid composition is to approximate the composition of the mineral assemblage using microprobe data and modal analyses, corrected from volume to mass proportions, and to subtract this from the whole rock composition. However other problems arise with this method when applied to the porphyritic andesites of Nevados de Chillin. In a number of cases, plagioclase greatly predominates over the mafic minerals (Appendix 3), strongly suggesting accumulation of plagioclase, due probably to its lower density. Calculations will therefore yield erroneous liquid mole fractions of Fe and Mg, the most important parameters in olivine-liquid thermometry, although the Fe/Mg ratio will be only slightly affected. Even if the phenocryst assemblage was present in cotectic proportions, this technique is only a crude approximation of liquid composition, due to compositional variations within and between crystals of the same mineral and inevitable errors in point counting.
samples, with two or more distinct generations present (Section 5). The presence of pigeonite rims on olivine in many samples is firm evidence that it is no longer in equilibrium with the liquid. The occurrence of different generations of olivine is not ascribed to magma mixing, for which there is no evidence in most of the more mafic samples, but is considered to be due to retention of early formed more forsteritic phenocrysts.
Notwithstanding these difficulties, trial calculations were attempted and surprisingly acceptable results, concordant with pyroxene thermometry, were obtained using the graphical method of Roeder and Emslie (1970). The temperature is calculated from the molar proportions of Fe and Mg in the liquid, the olivine composition simply being used to check for equilibrium. The evidence for disequibrium from the pigeonite rims and the fact that the more relevant temperatures are the highest ones, suggested that whole rock compositions be used in the calculations, although liquid compositions, calculated as described above, were also used. This essentially assumes that the whole rock composition represents the liquid from which the most forsteritic olivine has grown, an invalid assumption in the light of the evidence for plagioclase accumulation. Subtraction of varying amounts of plagioclase from the whole rock composition was found to have only a minor effect on the temperatures, increasing values by a maximum of 15-2OoC, which is within the limit of analytical error. Because there is no firm control over the actual amount of plagioclase likely to have accumulated in any given sample, the quoted results are for unmodified whole rock compositions.
formula of Kilinc et al. (1983). Experimentation using both whole rock and estimated
Another significant problem concerns the variation in olivine composition in most
The Fe2+ content of the liquid at NNO and QFM was approximated using the
20
0 0
0
0 0
0 0 0 0
0
0
a 0
0
0 0 0 0 0 0
0
0 0 a a 0 e 0
0
0
0 0
0
liquids revealed that the best results were obtained using whole rock, uncorrected for Fe3+, as calculated liquid Fe2+ was too low in almost all cases to be in equilibrium with the most magnesian olivine even at QFM. Calculated Fe2+/Fe3+ increases with decreasing oxygen fugacity. Conditions significantly more reduced than QFM are unlikely in arc magmas. The difference in calculated temperature using different Fe2+ values from Fe* (all Fe as Fe2+) to Fe2+ at NNO is minimal (10-15OC) and well within analytical error but the Fe* values yield calculated olivine compositions very close to actual measured values. The whole rock values using Fe* give results consistent with both pyroxene thermometry and olivine-liquid equilibria. In most samples, the most Mg- rich olivine composition is consistent with equilibrium conditions when whole rock Fe* compositions are used. In the case of two samples (C24 and C25, both of which contain two pyroxenes), the only olivine analysed is too Fe-rich to be in equilibrium with the whole rock composition. It is suggested that this is due either to loss of early magnesian olivine by fractionation or that such olivine is present but was not analysed, as there is excellent agreement between olivine-liquid and pyroxene temperatures (Fig. 13). The calculated temperatures using whole rock compositions and Fe* are tabulated in Appendix 5. Because the estimation method is graphical, values are approximate and are quoted to the nearest 10°C. Temperatures calculated from liquid estimates are not reported as they are unrealistic in most cases, due to the fact that olivine is no longer in equilibrium with the liquid. At best they represent cooling temperatures as they are generally 30-10O0C lower than the calculated whole rock temperatures.
of Sisson and Grove (19934, which is applicable only to samples without orthopyroxene and with Si02 < 55%, was used on the whole rock analysis of the single suitable sample, C22, and gives results which are reasonably close to the Roeder and Emslie values (Appendix 5). The liquid thermometer of Grove and Juster (1989), which requires the four-phase assemblage of olivine, plagioclase and two pyroxenes was also tested on whole rock and calculated liquid compositions for appropriate samples. Results based on
calculated liquids tend to be somewhat higher than the Roeder and Emslie whole rock and two pyroxene QUILF values but in some cases there is good agreement. However whole rock values from the Grove and Juster thermometer are much higher (more than 5OOC) than the whole rock Roeder and Emslie values in all cases. The validity of comparing temperatures, calculated from whole rock values for the Roeder and Emslie thermometer and estimated liquids for the Grove and Juster formulation, is doubtful and the results are not reported. Furthermore the thermometer generates values that are much too high for all of the more evolved (andesitic) samples.
Two other liquid thermometers have been used. The olivine-liquid thermometer
21
0
0
0 0
0
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0 0
0
0 0
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0 0
e e 0
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a e e 0
0 a 0
0 e 0
Trial calculations to estimate f 0 2 of the some of the mafic samples were attempted using the oxygen barometer of Ballhaus et al. (1991). There are no assemblages truly suitable for spine1 oxygen barometry in the mafic magmas as the barometer requires an olivine-orthopyroxene-spine1 assemblage and is recommended only for olivine of Fog5 or greater. Results gave values about 3 log units above QFM. Such oxidising conditions are unlikely and contradict qualitative evidence, from the presence of pigeonite rims on many olivines, for evolution close to the QFM buffer in many samples (Section 5).
High-Si Dacites
Samples C4, C52, C59 and C37 were analysed in detail. Modal analyses are given in Appendix 3. All samples contain plagioclase, two pyroxenes and two oxides, with minor apatite. The assemblage is dominated by plagioclase. Orthopyroxene is more abundant than clinopyroxene in some samples but in others they are present in subequal amounts. Titanomagnetite is generally much more abundant than ilmenite. Apatite occurs as small euhedral phenocrysts in clots with the other minerals in some samples and is often present as acicular inclusions in plagioclase and pyroxene.
C4: (SGLD, 67.9% Si02) This sample is fine to very fine grained with less than 10% phenocrysts. There are two slightly different plagioclase compositions but no textural differences. One type has cores which vary from An3847 and rims An3-0, the other has cores Au7-52 and rims An50-52. Pyroxenes generally show no consistent zoning, except for Opxl which is normally zoned, with core averaging E n a , distinctly more magnesian than other orthopyroxene in the sample, and rims ranging En55-59, Wo2.3-3.3, similar to the other orthopyroxene. With the exception of the Opxl core, the orthopyroxene appears to be in equilibrium with the clinopyroxene. Opx3 and Cpx3 are intergrown and yield very similar temperatures (Appendix 4c). The average clinopyroxene and orthopyroxene temperatures, 950°C and 966°C respectively (Appendix 5 ) are in very good agreement, when the Opxl temperature is omitted. Fe-Ti oxide temperatures are much lower, averaging 843°C with logf02 of -13.6 (Appendix 4e).
C52: (SC4, SC346a, 69.3% Si02) This sample is petrographically similar to C4, containing about 10% phenocrysts and is slightly vesicular (Appendix 3). There are two
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different plagioclase compositions with no obvious textural differences. One type is unzoned and varies from An3542, the other is more calcic and shows normal zoning, with cores An51-54 and rims An41-42. The more calcic plagioclase occurs in clots with clino- and sometimes orthopyroxene. The other occurs only with orthopyroxene. However the orthopyroxene is generally quite magnesian, En6248, Wo2.g-3.7 and gives significantly higher temperatures than coexisting clinopyroxene. Opx2 1 is sector zoned. One sector is less magnesian but more calcic and yields similar temperatures to the other orthopyroxene. Fe-sulphide inclusions are present in some orthopyroxene. Clinopyroxene shows little variation, ranging En3840, Wo3941. Orthopyroxene temperatures average 1007"C, whereas clinopyroxene averages 945OC. Two-oxide temperatures are again much lower, averaging 863°C with logfO2 of -13.3.
C59: ( SC2,68.9% Si02) This sample is slightly more coarse grained than C4 and C52 with about 8% phenocrysts. The groundmass is partially devitrified, a common feature of some of the older dacites. Most plagioclase is unzoned or normally zoned in the range An45-33. Orthopyroxene varies from E n 6 2 4 , Wo2.7-3.2 and clinopyroxene ranges En40_ 43, Wo40-43. There is a small very fine grained clot of acicular plagioclase with pyroxene and oxide microphenocrysts, labelled MP in Appendix 4. The clot plagioclase ranges from An63-34. However compositions of pyroxene microphenocrysts in the inclusion are almost indistinguishable from the other phenocrysts. Average orthopyroxene temperatures, 988OC, are in good agreement with clinopyroxene temperatures, 979°C. The microphenocrysts have not been included in the calculation of mean temperatures. Two-oxide temperatures are very low in comparison, averaging 805OC and logf02 of - 14.7.
C37: (CBG, 66.4% Si02) This sample is similar to C59 in some respects being coarser grained than many of the other dacites and having a devitrified groundmass. A microphenocryst clot (grains labelled 1, Appendix 4) with plagioclase and clinopyroxene was analysed as well as several larger phenocrysts. Plagioclase is normally zoned and uniform in composition throughout the section with no difference between clot microphenocrysts and phenocrysts. Some grains show oscillatory zoning. Rims range from An4452 and cores from A w l . Clinopyroxene microphenocrysts in the clot are very slightly more Fe-poor than the phenocrysts. One microphenocryst has distinctly higher A1 than the other. Clinopyroxene varies between En41-44, W041-44 except for the high-Al microphenocryst. Orthopyroxene varies between En64-69, W02.62.9. There appear to be two distinct clusters of clinopyroxene temperatures, one averaging 925°C
23
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and the other 979OC, the latter in good agreement with the orthopyroxene temperatures which average 965°C. The high-A1 grain was not used for thermometry as it gives erroneous results. Its distinctive composition is most likely due to rapid crystallisation, as indicated by its fine grainsize and in accordance with the tendency towards high A1 contents in rapidly grown clinopyroxene (Lofgren, 1980). Large clinopyroxene phenocrysts yield the lower temperatures. Oxides temperatures average 778OC with l0gfO2 of -14.8.
High-Si andesites
Samples C11 and C95 are from the upper stratigraphic units of Cerro Blanco. All have assemblages of plagioclase, two pyroxenes, and titanomagnetite. Plagioclase is predominant and clinopyroxene is more abundant than orthopyroxene in all samples (Appendix 3). Olivine is present in very low abundance in all three. It occurs as individual fine grained phenocrysts in C29 but is present only as remnant patches in orthopyroxene in the other two. In the case of C11 it is too small to analyse by microprobe. All three samples have less than 10% total phenocrysts, C29 having less than 3%. The whole rock analysis of this sample is probably the closest to a true liquid composition among the andesites.
C11: (CB3W, 62.9% Si02) This sample is mostly fine to very fine grained, with numerous clots of fine to very fine grained plagioclase, pyroxene and titanomagnetite. Plagioclase also occurs as medium to coarse grained phenocrysts whereas the other minerals are invariably small. The groundmass is quite vesicular (19% vesicles approx.). Two fine grained clots (grains labelled 1 and 2, Appendix 4) were analysed. Most clot plagioclase is quite uniform in composition, ranging from Aw3-58, with little difference between rims and cores, although one grain exhibits slight reverse zoning. This is not likely to be related to magma mixing and is probably simple oscillatory zoning. The large phenocryst plagioclase P13 is patchy zoned, ranging from cores An35-80 to rims An49-54, and is therefore much more mafic than the clot plagioclase. Groundmass plagioclase is about An35 Pyroxenes show little compositional variation except for the extreme rims of some grains which are more Fe-rich or in the case of some orthopyroxene have pigeonite compositions. Pigeonite also occurs in the groundmass. Clinopyroxene varies from E w w , WO3942 and orthopyroxene ranges from En67-70, Wo3.54.1. Olivine occurs as tiny patches, too small to analyse, in orthopyroxene, by
24
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which it has presumably been replaced. Average orthopyroxene temperature is 1O7O0C, higher than those calculated for clinopyroxene, which average 1034°C.
C95: (CB3W, 62.1% Si02) This sample has an inhomogeneous groundmass with blebs of dark glassy material which have been mechanically mixed into the surrounding matrix, discussed in Section 3 (Fig 9). Analysis of phenocrysts and groundmass in both areas reveals no compositional differences. Averages of the groundmass analyses, performed by diffuse beam traverse, are given in Appendix 4g. Grains labelled 1 are from a microphenocryst clot, those labelled 2 are larger phenocrysts and those labelled 3 are from a glassy bleb. Plagioclase is normally zoned or unzoned, with rims An38-59 and cores A-. Orthopyroxene ranges between E n 6 7 2 , Wo3.3-3.8. Clinopyroxene ranges En42-44, W o 3 9 4 . Olivine ranges Fo69-70. The core of Cpx2, a large phenocryst, and the small phenocrysts in the glassy bleb are slightly less calcic than the other grains and yield an average temperature of 1042OC, close to the average orthopyroxene temperature of 1066°C. The other clinopyroxene averages 1006°C. Olivine-liquid temperatures were outside the range of the Roeder and Emslie calibration (< lO00"C) and are not reported.
C29: (Group 9, CB3E, 60.1% Si02) This sample has less than 3% of fine to very fine grained phenocrysts. There appear to be two slightly different generations of plagioclase which exhibit no texturally distinctive features. P11 is more calcic than the others and is slightly reverse zoned with cores An5343 and rims An64-67. The other plagioclase crystals show oscillatory zoning varying from An45-59. It is unlikely that the reverse zoning is due to magma mixing or reheating. Orthopyroxene ranges En67-70, W03.w.2 and clinopyroxene varies from En4146, w03942. Olivine varies from Fo67-72. Average orthopyroxene and clinopyroxene temperatures are similar, 1055°C and 1066°C respectively. A temperature of 102OOC was obtained from olivine liquid thermometry. This is the only case where the Roeder and Emslie thermometer yields lower values than QUILF. This sample falls at the low end of the temperature range calibrated by Roeder and Emslie and it is suggested that the pyroxene temperatures are more accurate.
25
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Mingled and mixed samples
The mineral chemistry of the mixed and mingled rocks is complex, with dacitic phenocrysts occurring in the mafic inclusions and mafic phenocrysts in the dacitic hosts, as well as phenocrysts which may have grown after mixing occurred. Samples C5 and C14 are mingled dacites with mafic inclusions. Sample C13 is a mixed hybrid and does not have any mafic inclusions but has a disequilibrium phenocryst assemblage, reflecting its mixed origin.
C5: (GSC~CSVD, SVD, 66.7% Si02) The probe section contains a few diktytaxitic mafic inclusions, in a dacitic host, which resembles the other dacites described above in many respects, but differs in that it contains Mg-rich olivine. The mineral chemistry of the dacitic phenocrysts clearly records interaction with mafic magma although there are no distinctive Type 1 plagioclase textures in this sample. However reverse zoned plagioclase does occur. P13 has a core ranging A ~ 8 - 5 9 and rims A n 6 7 0 . Other plagioclase ranges An43-56 and averages An48-50. P14 from a mafk inclusion is more calcic, with cores An62-73 and rims An6347. There are two generations of olivine. Most are magnesian, with cores F079-82 and rims F074-79, and contain Cr-spine1 inclusions. 013, which is intergrown in a clot with the reverse zoned P13, is more Fe-rich, Fo69-74. Cpx3, in the same clot, is more Mg-rich and Ca-poor than clinopyroxene elsewhere in the slide and is reverse zoned, with core E n q 3 4 , wO38-39 and rim E w 7 , Wo42-43. A clinopyroxene grain with quite similar composition occurs as an inclusion in P15, which is a large relatively sodic grain. Other clinopyroxene is mostly En38.42, W o 4 ~ 3 . Orthopyroxene is generally quite uniform, ranging mostly En-3, Wo3-3.4, although Opx5 is much more calcic ranging from W o u . 9 , bordering on pigeonite composition. Parts of Opx22 are also highly calcic. There is little difference in the Mg or Fe content of these grains and patches and the more normal dacitic compositions. Calculated temperatures are difficult to interpret, because of the variation in pyroxene composition, representing growth under different conditions. Average temperatures have been calculated by separating grains of obviously different composition. There are three distinct temperature clusters among the clinopyroxenes, one averaging 932OC, a second at 979°C and a third from Cpx3, the reverse zoned grain in a mafic inclusion which averages 1064°C. Orthopyroxene temperatures average 989OC but Opx5 gives a temperature of 1106OC. It is likely that these high values have significance and reflect further growth of previously existing phenocrysts at higher temperature. The significance of the difference between the medium and low values is open to interpretation. Olivine-
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liquid temperatures cannot be calculated for any of the mixed or mingled samples as there is no way of estimating liquid compositions for the mafic magmas. Broad beam microprobe analysis cannot be used on the mafic inclusions, due to the high proportion of voids in the groundmass. A broad beam traverse was made on the host groundmass (Appendix 4g), indicating Si02 content of about 7 1%.
C13: (GSC~CSVD, SC6, 60.9% Si02) Although this slide contains no discrete mafic inclusions, there are several features indicative of a hybrid origin. The occurrence of numerous reverse zoned dusty-rimmed Type 1 plagioclase, together with pyroxene of dacitic origin and numerous high-Mg olivines with spine1 inclusions is firm evidence of magma mixing. The Type 1 plagioclase grains have clear cores with dusty overgrowths composed of more calcic plagioclase and glass, too fine to analyse by diffuse beam. P11 is a typical Type 1 grain with core An32-39 and rim An4945 P14 is similar with core An3543 and rim An54-56. In general, the dusty areas are confined to the rims and do not have further clear overgrowths. Other plagioclase in the section tends to be clear and shows normal zoning. In detail there are probably two other distinct types of plagioclase besides the Type 1 grains. P12 is similar in composition to the cores of the dusty grains, ranging An3340, and presumably grew prior to the mixing event but was unaffected. P13 is slightly more calcic, An39-50, but is too sodic to be derived from the mafic magma. Groundmass plagioclase is An5657 similar to the rims of the dusty grains. Olivine is high-Mg with cores Fog244 and rims F070-71. Clinopyroxene is dacitic in composition, ranging En3439, W040-42. Orthopyroxene ranges from cores En5942, W03.4-4.1 to rims E n 5 Wo3.63.9. None of the pyroxenes are reverse zoned. Clinopyroxene temperatures average 946°C whereas orthopyroxene temperatures are much higher, averaging 10 19OC. Groundmass orthopyroxene is much more Mg and Ca-rich than the phenocrysts, ranging En72-74, W03.74.8. Groundmass clinopyroxene is also more magnesian and slightly less calcic than the phenocrysts ranging En4344 Wo40. The more mafic character of the groundmass minerals must reflect the mixing and heating event. Temperatures calculated from the groundmass minerals are 108 1°C and 1133°C for clinopyroxene and orthopyroxene respectively.
C14: (GSC~CSVD, SC6,63.8% Si02) This sample is a dacite with a large diktytaxitic mafic inclusion. Modal analyses of host and inclusion are given in Appendix 3. Phenocrysts from the mafic magma have been incorporated into the dacite and vice versa. Variably resorbed Type 1 plagioclase is quite common, occurring in both the host and inclusion. Clear plagioclase is present also in both areas. Dacitic clear plagioclase
27
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(P14, P17 and P172) is either unzoned or has slight normal zoning, ranging An3347 in the cores and An33-39 at the rims. PI4 is part of a medium grained dacitic phenocryst clot with two pyroxenes and ilmenite, in the maiic inclusion. Basaltic plagioclase (P12), intergrown with olivine in the inclusion, has cores ranging An73-85 and r ims An54-63. A dusty Type 1 plagioclase, P18, in the dacitic part of the section, with an almost completely resorbed core and clear rim, is almost as calcic in parts as the basaltic grain, ranging An24-72 in the core, an inner rim ranging h(j7-71 and outer rim An52-55. Groundmass plagioclase in the inclusion ranges An7332 and in the host is An54-56. Olivine has cores ranging Fog345 and rims Fo73-77 in both inclusion and host. Cr-spine1 of variable composition is present as inclusions in most olivine and also in the basaltic plagioclase where it is more Fe and Ti rich but contains significant amounts of Cr (Appendix 4e). Pyroxenes appear to be mostly of dacitic origin although one clinopyroxene microphenocryst from the inclusion is quite magnesian ranging En4547, Wo42-43, suggesting growth in the mafic magma. A few pyroxenes have strongly reverse zoned rims. Cpx3 from the mafic inclusion has a core ranging En3842, W04.1 and rim E-50,
W03.84.1, indicating initial growth in the dacite and further growth at higher temperature in the mafic magma. The remaining clinopyroxene in the dacite and the inclusion is quite uniform in composition and unzoned, ranging En39-42, W 0 3 ~ . Opx4, also from the inclusion, is strongly reverse zoned in Mg and slightly in Ca, with core averaging En55, Wo3.5 and rim En69, Wo3.7. Two other orthopyroxenes in the dacite (Opx7 and 0 ~ x 7 2 ) are unaffected by the magma mixing and show normal zoning. They also differ from each other, the former ranging En4g-52, wO3.3-3.6 in the core and En4g-50, Wo3-3.2 at the rim whereas the latter ranges h54-56, Wo3.43.7 in the core and En54-55, Wo3.3-3.6 at the rim. Clinopyroxene is present in the inclusion groundmass and orthopyroxene is present in the host. As in sample C5, there are several different temperature clusters. Most clinopyroxene averages 966°C. There are two orthopyroxene clusters, one averaging 982OC, the other 921°C. The reheating is reflected in the very strong reverse zoning of Cpx3 and Opx3, the rims of which, together with the clinopyroxene microphenocryst in the mafic inclusion, average 1089°C. Fe-Ti oxide thermometry using ilmenite from a dacitic clot within the inclusion and magnetite from the host gives a temperature of 921°C (Appendix 4e). If this value represents an equilibrium temperature, it is consistent with the lower pyroxene results, and would be the only plausible two-oxide temperature from all the samples analysed. However it is likely that the value is artificial as the two grains used are in separate parts of the rock. The average Si02 content of the host groundmass is 7096, determined by broad beam traversal (Appendix 4g).
28
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CS7: This sample was not analysed by XRF and its stratigraphic affinities have not been defined. It is a diktytaxitic inclusion, somewhat more evolved than those in the dacites, judging from its mineral assemblage and mineral chemistry. It contains plagioclase, olivine, two pyroxenes and magnetite. It has not been examined in detail as it was chosen accidentally for microprobe analysis, before realising that no whole rock analysis was available. Plagioclase ranges An4948 in the core and An5744 at the rim. Two generations of olivine are present, one with core Fo78 and rim Fo76, the other has core ranging Fo-9 and rim F065. Orthopyroxene ranges En67-70, Wo3.u .3 and clinopyroxene ranges En42-45, W o w . It is likely that it the magma was andesitic in composition. The sample provides evidence for the involvement of more evolved mafk end-member magmas in mixing processes at the volcano.
Orthopyroxene-bearing basaltic andesites-andesites
Most samples described under this heading are highly porphyritic and medium to coarse grained. In some samples (C19 and C25), orthopyroxene is relatively coarse grained, similar in size to coexisting clinopyroxene, although it tends to be finer grained than clinopyroxene in most samples.
C24: (GCB3E4, CB4,56.7% Si02) Three plagioclase types can be distinguished. Type 1 grains, similar to those in the mixed samples, have resorbed dusty areas towards the rims, which are more calcic than the cores. There are further clear outer rims in some cases. The compositional contrast between core and dusty areas is generally not as great as similar grains in the mingled dacites. Plag62 is an example of a Type 1 grain, with core An50-55, dusty area An39-72 and outer calcic rim An&-74. The second type consists of patchy reverse zoned grains, without dusty textures. P13 is an example of this type and shows definite reverse zoning with core An4942 and rim An69-74. The third type is clear and shows no consistent zoning. P163, which ranges from An51-63, is an example. Olivine occurs as fine grained phenocrysts or microphenocrysts and ranges between Fo69-71 with pigeonite rims. Clinopyroxene shows no consistent zoning or variation between different grains and ranges between En4345 Wo38-39. There is some reverse zoning of orthopyroxene. Opxl shows an increase in Ca at the rim with cores En6743, Wo3.63.9 with rims En6748. W03.8-4.2. Opx2 shows an increase in Mg towards the rim
with cores En-, Wo3.74.1 and rims En6g-70, W03.94. Part of the rim has a pigeonite composition En69-70, Wog. Clinopyroxene temperatures average 1053°C and orthopyroxene averages 1093°C. In the case of Opx2, the rim temperatures (1 113°C) are
29
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higher than the cores (1075°C) by about 40°C. The textural evidence from the Type1 plagioclase strongly suggests that reheating or magma mixing occurred so that the apparent orthopyroxene temperature increase towards the rim is significant. The olivine- liquid temperature is 1080°C in very good agreement with the pyroxene temperature. Calculated equilibrium olivine would be about Fogo, a composition which, if present in the rock, was not analysed, most olivine being closer to Fo70 (Appendix 5). Considering the good agreement with the pyroxene temperatures, it is probable that such olivine was present during early crystallisation but has either fractionated out or has not been analysed.
C25: (GCB3E4, CB3,55.5% Si02) There are two plagioclase types. Type 2 grains are patchy zoned with cores An54435 and normal rims An5543 (Plagl and Plag22) The other type is clear and shows no consistent zoning, varying from An5342. Olivine most commonly occurs as microphenocrysts, F o ~ g , with pigeonite rims. More magnesian olivine, Fo72, is present in the cores of some clinopyroxene. This sample contains coarse grained orthopyroxene. It shows no consistent zoning pattern or compositional contrast between grains of different sizes and is highly magnesian, En71-74, Wo35-40. Neither does clinopyroxene show any consistent zoning or differences between grains, ranging En44-47, Wo3943. Average temperatures of clino- and orthopyroxene are in very good agreement, 1105°C and 1109°C respectively. The olivine-liquid temperature is 11 10°C, remarkably close to the pyroxene values. As in the case of C24, the analysed olivine is too Fe-rich to be in equilibrium with the whole rock. The same explanation as above is offered.
C103: (GSClCBl, SC1,58.5% Si02) This sample and C50 are similar in a number of respects. Both have coarse grained plagioclase, which can be classified into two distinct textural types, similar to those in the Group 5 mixed rocks. Type 1 grains, of which P112 is an example, have very fine dusty areas. In this sample, the outer cores are dusty and composed of glass and plagioclase An75-76, similar to the clear rim which ranges An72- go. The inner core is clear and significantly more sodic, with a very narrow range around An55 This texture is very likely to be related to resorption due to reheating. The other type is clear and similar in composition to the cores of the Type 1 grains but shows some reverse zoning with cores An42-55 and rims An55-61. These grains would appear to be unresorbed equivalents of the first type, and have simply experienced overgrowth of more calcic plagioclase without resorption. The latter type is much more abundant than the former. Olivine is quite Mg-rich with cores F O g N 2 and rims around Fo79. Pyroxenes
30
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a a a a a a a a a a a a a a a a a a a a a a a 0 a 0
a a a a1
a
do not show any evidence of reheating and are unzoned or normally zoned at the outer rims, some clinopyroxenes having more Fe-rich but Mg and Ca-poor rims. Most grains analysed range E n 4 5 4 , W 0 4 2 4 . One grain has a composition similar to the extreme rims of the others ranging En42-44, w03941. Orthopyroxene ranges En65-46, W044.3. Temperatures average 1045°C for clinopyroxene and 1 100°C for orthopyroxene. Olivine-liquid temperature is 1 100°C and calculated olivine corresponds very closely to measured values of the most magnesian grain analysed (Appendix 5 ) . It is suggested on this basis that the orthopyroxene and olivine temperatures are the more realistic.
CSO: (GSClCBl, SC1; 56.9% Si02) There are two distinct textural types of plagioclase, similar to those in sample C103. Many of the Type 1 grains have cores which are almost completely resorbed and clear rims. They are more abundant than in sample C103. One large Type 1 grain, P13, was analysed. This ranges from An40-69 in the core with rim Ar4j7. The second type is clear and can be unzoned or patchy zoned. P11 ranges A n 5 1 4
in the core and A n 5 2 4 at the rim. PI32 is patchy zoned ranging An4747 in the core to An47-54 at the rim. Olivine is magnesian, Fo7g430 in the core and ranges Fo75-76 at the rim. Smaller olivine phenocrysts have cores similar to the rims of larger grains. clinopyroxene ranges En42-44, W03840 in the core and En4347. Wo3741 at the rim. Orthopyroxene ranges En68-69, wO3.94.1 in cores and En73-74, Wo3.3-3.5 at the rim. It is clearly reverse zoned in Mg but normally zoned in Ca. The clinopyroxene also shows zoning with an increase in Mg and decrease in Ca (both elements implying temperature increase from core to rim in the case of clinopyroxene). It is possible that in the case of samples C50 and 103 that there has not been actual magma mixing as there is no real evidence for disequilibrium phenocryst assemblages. Reheating without mixing could produce the Type 1 plagioclase textures and reverse zoning. Temperatures average 1063°C for clinopyroxene and 1082°C for orthopyroxene. Olivine-liquid temperature is 1080°C, in excellent agreement with the pyroxene results. Calculated and measured olivine are also concordant.
C19: (GSClCBl, SC1,56.1% Si02) This is a highly porphyritic sample, containing coarse grained orthopyroxene. Plagioclase is relatively uniform, coarse to medium grained with clear and Type 2 grains present. There is no compositional difference between these types and all grains are normally zoned or unzoned. Cores range h a - 7 6
and rims An52-73. Most olivine ranges FO72-76 with little difference between rim and core. One large olivine is more Mg-rich and zoned F o g m l in the core, Fo74-75 at the rim. Orthopyroxene is highly magnesian and shows no discernible zoning, ranging
31
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e 0
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a a a 0
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0 0
e e a 0
0 0 0 8 0
0 0
0
En73-77, W03.24.2. Clinopyroxene temperatures average 1 1 17°C and orthopyroxene averages 1092°C. Olivine-liquid temperature is 1 100°C, with very good correspondence between calculated and measured olivine compositions.
C92: (GSC 1CB 1, CB 1,60.1% Si02) This sample contains coarse grained plagioclase and clinopyroxene, with more fine grained orthopyroxene. Plagioclase occurs as clear and Type 2 grains, usually patchy zoned. A coarse grained clot of clear plagioclase and clinopyroxene was analysed (P11 and Cpx 1). There are no significant differences between the plagioclase in the clot and clear plagioclase elsewhere. The clear plagioclase (P11 and P122) ranges A n 5 1 4 with no observable difference between rims and cores. The patchy grains are texturally very similar to those in other samples and range An5g43 in the cores and An60-70 at the rims. Clinopyroxene compositions vary slightly. Cpx3 1 is a coarse individual grain and is almost identical to Cpxl in the coarse clot, ranging En42-44, W042-43 in the cores. The rim compositions are slightly different, Cpxl having a less calcic and more Fe-rich rim, similar to the core of Cpx32 (a small phenocryst) in composition. The rim of Cpx32 is similar to the cores of the coarser grains. The lower Ca results in higher calculated temperatures. Orthopyroxene is quite uniform in composition ranging mostly between E n w , Wo3.2-3.7, although the rim of Opx1, a small phenocryst is more calcic and in places has a pigeonite composition. Clinopyroxene temperatures average 994°C whereas orthopyroxene averages 1034°C. Olivine is present in trace quantity in this sample and was not analysed by microprobe. However when the liquid composition is projected onto the Roeder and Emslie diagram, a temperature of 1040°C is obtained, concordant once again with the orthopyroxene temperature. The higher temperature is also geologically more plausible. Compared to the other samples, a temperature of around lO00"C is low for a rock of this composition.
C98: (CB3W, 58% Si02) This sample is coarse to medium grained, somewhat less porphyritic than many of the other andesites. Plagioclase occurs as clear and Type 2 grains. P11 is a large patchy zoned Type 2 grain with core ranging An52-79 and rim An57-58. P12 is a clear crystal with core ranging A-54 and rim An53-54. P13 is slightly reverse zoned, with core An5658 and rim An57-71. Olivine occurs as fine grained crystals, Fo69, with pigeonite rims. Clinopyroxene varies in grainsize from fine to coarse but exhibits no consistent compositional differences. Most grains range En42- 46, W04044. The rims of the larger grains (Cpx4 and Cpx42) are more variable but no consistent trend is apparent from core to rim in general. Orthopyroxene is fine grained and compositionally uniform, varying En6g-70, w03.84.6. Clinopyroxene temperatures
32
0
0
0
0 0 0
0
0
0
0 0
0
0 0 0
0
0 0
0 0 0
0
0 0 0 0
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0 0 0
average 1049°C whereas orthopyroxene temperatures are much higher averaging 1 12OoC, due to their high Ca contents. The olivine-liquid temperature 1O6O0C, this being the sole case of better agreement with clinopyroxene rather than orthopyroxene. The clinopyroxene temperatures are probably the more accurate as they concord with average temperatures from other samples of similar Si02.
Basaltic andesites-andesites without orthopyroxene
Samples which do not contain orthopyroxene are described under a single heading. Three of the samples belong to the SC5 group, one from SGLA and one from GSClCBl. Sample C22 from the latter group, is the most mafic sample in the database and does not contain orthopyroxene, unlike all the other members of its group.
C17: (SC5, 56.5% Si02) This is a coarse grained sample with clear and Type 2 plagioclase, the latter being much more abundant. The clear plagioclase is unzoned or normally zoned and the Type 2 is patchy zoned. P11 is a typical example of a Type 2 grain. It has a sodic outer rim An-, a highly calcic inner rim Ang1-u and a patchy core ranging from An64-82. Plagl2 is clear and normally zoned, An5042 in the core to An30-31 at the rim. Plagl3 is clear, ranges An5141, with a slightly more calcic rim, A n 5 8 4 . Large olivines are magnesian, with cores Fo7240 and rims Fo68. Smaller olivines have cores Fo70-75 and rims F o m 9 . Microphenocrysts range from Fo62-68. Pigeonite rims are common on smaller olivines. Spine1 inclusions are present in some olivine. They vary in composition according to the composition of the olivine host. The more Mg-rich olivine has spine1 inclusions which are relatively Cr-rich (Cr203 = 22%) whereas the smaller olivine has much more Fe and Ti-rich inclusions (Appendix 4d). Chopyroxene is unzoned and uniform in composition ranging from En4545 Wo4143. The olivine-liquid temperature is 1080°C. Although olivine is strongly zoned, equilibrium is permitted by calculated and measured values using the most Mg-rich composition.
C39: (SC5,56.5% SiO2) This sample is similar to C17 in terms of whole rock geochemistry and mineral chemistry although there are some differences. Plagioclase is texturally and compositionally similar to that in C17, with clear and Type 2 crystals. P11 has an outer rim ranging Ang-58, inner rim An73-76 and patchy core ranging An47-73. P13 is similar but has a more extreme range, h 3 5 - 8 0 , in the core, with rims An53-59.
33
0
0
0
0
0
0
0
0 0
0 0
0
0 0
0 0
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0 0
0 0
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0
P112 is clear and shows no consistent zoning with rim An4849 and core An5242. Plagl3 is clear and is highly calcic, Ang0-83. There are two ohvine populations very Similar to C17. Larger grains have cores between Fo74-80 and rims Fo70, whereas smaller grains range from FOfj9-74. The more Mg-rich olivine contains Cr-spine1 inclusions which are quite similar to those in C17 but are more Cr-rich, Al and Mg-poor (Appendix 4d). Most olivine has pigeonite rims. There are also differences between the clinopyroxenes analysed. Cpx 1, which is part of a clot with the more Fe-rich 011, has higher Ca and lower Fe than Cpx 2. The former has a composition similar to clinopyroxene in C17. The overall range in pyroxene composition is very similar to that in C17, En45-46, W o w 4. Olivine-liquid temperature is 11 10°C and there is good correspondence between calculated olivine and the most magnesian composition analysed.
C107: (SC5,58% Si02) This sample is similar to C17 and C39 but is slightly more silicic. Plagioclase is generally coarse grained, most crystals having Type 2 texture, with large melt inclusions, but without patchy zoning. Clinopyroxene and olivine are generally medium to fine grained although a few coarse pyroxenes occur. Plagioclase is quite calcic, ranging from An72-81, with no definite zoning pattern. It is similar in composition to the most calcic grains in C17 and C39. Inclusions in Type 2 plagioclase and groundmass were analysed by broad beam traverse, as the groundmass grainsize is fine enough and the inclusions sufficiently large to obtain meaningful results. The data are tabulated in Appendix 4g. The inclusions studied consist of partially recrystallised glass, compositionally more evolved than the main groundmass. The interpretation offered here is that they represent the final liquid before complete crystallisation. The plagioclase is far too calcic to have grown in a liquid of this composition and can not possibly be dacitic in origin. There is no evidence of magma mixing in this sample. An attempt was made to analyse similar inclusions in C 17 but the groundmass grainsize is too coarse and the sample too pitted to use the broad beam technique over a sufficiently large area to obtain a meaningful average. Olivine phenocrysts range from cores of Fo75- 76 to rims F070-74, similar to the more Fe-rich populations of C17 and c39. Olivine microphenocrysts range from F065-68. Spine1 inclusions are present in some of the phenocryst. These differ from those in both C17 and C39 being more Fe-rich and Al, Cr and Mg-poor (Appendix 4d). All olivine has pigeonite rims. Clinopyroxene is unzoned and ranges from En44-45, W042-44, very similar to that in C17 and C39, particularly Cpxl from C39. Olivine-liquid temperature is 104OOC and there is reasonable agreement between calculated and measured olivine composition.
34
0
0
0 0
0
0 0
0 0
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e 0
0 0
e 0
0 0 0 e 0
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0 0 0
C44: (SGLA, 58.2% Si02) This rock is very tine grained and has a very low phenocryst content (5-6%), with clinopyroxene predominating over olivine. All grains are normally zoned or unzoned. Two of the plagioclase grains analysed are very similar. Cores range from An40-62, and rims range from Ang1-59 but the averages are similar. A third grain is more calcic ranging from An76431 in the core to An-1 at the rim. Olivine is quite Fe- rich, F o w g , and present in only minor quantities. All olivine has pigeonite rims. Clinopyroxene is more or less uniform in composition, with no consistent differences between cores and rims. Compositions range from En3944 W041-44 and are generally quite aluminous (3-5% Al2O3) probably due to rapid crystallisation, as the grainsize is typically very fine. Olivine-liquid temperature is 1040°C with good correspondence between measured and calculated olivine composition.
C22: (GSC 1CB 1, SC 1,5396 Si02) This is the most mafic sample in the database and has a relatively straightforward mineral chemistry. It is mostly medium to fine grained but larger crystals of olivine and plagioclase are present. Plagioclase is calcic, with cores ranging An6742 and nms mostly ,41169-78 although one grain, P14, ranges An3849 at the rim and An7682 in the core. Olivine is highly magnesian although there are slightly different populations. Some range Fog244 in the core and Fo76-79 at the rim, whereas others have cores Fo78-80 with rims FO74-76. There is no correlation between grainsize and Mg content of the phenocrysts. Microphenocrysts are Fo75-76. Olivine contains Cr- spine1 inclusions, which show a great variation in composition and no systematic variation with the Mg content of the host crystal (Appendix 4d). Clinopyroxene ranges E w 7 , W o w , but the average values for cores and rims are quite uniform ranging En4344 W O ~ . The Roeder and Emslie temperature is 1150OC with good agreement between measured and calculated olivine. The olivine-liquid thermometer of Sisson and Grove (1993a) was also used on the whole rock composition and yields a value of 1127OC, with an estimated H20 content of 1.78%.
Mineral chemistry summary
A diagrammatic synopsis of the major features of the mineral chemistry is given in Figs. 14-17. The high-Si dacites are quite similar to each other in their mineral chemistry. Plagioclase compositions do not vary greatly between samples and are mostly at the more calcic end of the range expected in rocks of this composition (Fig. 14). This concords with the relatively high temperatures recorded by pyroxene thermometry. There
35
0
0
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0
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8 m 0 0
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0 0 0 0
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0 a 0 a 0
0 a 0
are slightly different generations of plagioclase in some samples but this does not signify magma mixing. Small amplitude compositional oscillations are very common in plagioclase of volcanic origin, as composition is dependent on a number of different factors including pressure, temperature and P ~ 2 0 , all of which could fluctuate in a chemically homogeneous magma chamber. Pyroxene compositions show some variation between and within samples (Fig 15). With the exception of C52, ortho- and clinopyroxene temperatures are generally concordant. The overall average temperature is around 970°C which is quite high for high-Si dacites.
chemistry. Plagioclase is generally more calcic than in the dacites (Fig 14). Some grains show slight reverse zoning but this is not likely to be connected to magma mixing, of which there is no indication in these samples. Orthopyroxene is very similar in composition in all three samples and average temperatures calculated from orthopyroxene fall in a remarkably small range from 1065-1070°C. Clinopyroxene is somewhat more variable and calculated temperatures are lower than those from orthopyroxene. Olivine has similar composition in C29 and C95.
The mineral chemistry of the mixed and mingled samples is very complex. All the samples contain phenocrysts which reflect early growth in dacitic magma at temperatures similar to the other dacites, followed by reheating leading to resorption, re- equilibration and new growth. The effects can be very local and variable. Some plagioclase grains are almost totally resorbed, others are only partially resorbed and in other cases there are no textural features indicative of interaction with mafic magma, but the presence of strong reverse zoning is evidence of reheating. Nearby grains may show no such effects. In some cases the groundmass minerals have compositions reflecting growth at higher temperatures than many of the phenocrysts. Pyroxenes also show reverse zoning in some cases, with rims giving much higher temperatures than cores. The general scenario seems to have been initial crystallisation at temperatures between 920- 980°C and subsequent heating to temperatures close to 1100°C in the case of C5 and C14. Such high temperatures are not recorded by any of the phenocrysts analysed in the hybrid sample C 13, but groundmass pyroxenes yield high temperatures. Olivine is generally normally zoned with highly magnesian cores, similar or slightly more Mg-rich than that in the most mafic lava sample C22 (Fig. 16). Further growth of olivine may have occurred after mixing, as evidenced by the presence of rare more Fe-rich grains in the dacitic part of sample C5.
suggests a complex history. The presence of Type 1 plagioclase in samples C24, C50 and
The high-Si andesites are also quite straightforward in terms of their mineral
The mineral chemistry of the basaltic andesite-andesite samples in some cases
36
I)
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C103, in some cases together with reverse zoned pyroxene, suggests interaction with hotter more mafic magma. The cores of reverse zoned grains are generally more calcic than those in the mingled dacites although there is overlap (Fig. 14). The composition of plagioclase in the dusty areas shows a similar range to the resorbed parts of dacitic Type 1 grains. Pyroxenes are all more magnesian than those in the dacites (Fig. 15). It is likely that, if mixing did occur, the silicic end-member was andesitic rather than dacitic. However, actual mixing may not have occurred as there is no evidence of true disequilibrium assemblages. Reheating by influx of hotter but denser more mafic magma, ponding at the base of a magma chamber containing basaltic andesite magma, could cause reverse zonation of previously existing phenocrysts.
The remaining mafic to intermediate rocks, which do not contain Type 1 plagioclase, have quite straightforward mineral chemistry. Most of the samples contain Type 2 plagioclase, which may or may not be patchy zoned. The melt inclusions which characterise the texture are invariably similar to or more evolved than the groundmass to which they are usually connected.
often two distinct generations present (Fig 16). The more magnesian grains range up to about Fogo, except in the most mafic sample C22, where they have compositions up to Fog4 Small olivine is very common in many samples and is generally around Fo70. Pyroxene compositions are variable and are difficult to summarise briefly due to the number of components. In general Ca contents decrease in orthopyroxene and increase in clinopyroxene with decreasing temperature, although the Ca content of clinopyroxene depends on whether or not orthopyroxene is present during growth. Mg and Fe contents increase and decrease respectively with temperature. In most cases olivine-liquid and orthopyroxene temperatures are in very good agreement. Clinopyroxene yields concordant values in some samples but temperatures are consistently lower in others.
Cr-spine1 compositions are not considered in detail but are plotted in Fig. 17. There is a general trend of decreasing Cr, A1 and Mg and increasing Fe with evolution, as expected, although this does not directly correlate with whole rock Si02, due to probable olivine retention, as discussed in relation to the olivine-liquid thermometry. Because of oxidation and/or low temperature re-equilibration, the oxide compositions are of limited value in quantifying conditions of magmatic evolution. Titanomagnetite and ilmenite are not plotted, as low temperature processes have overprinted their primary compositions.
Olivine compositions in the mafic-intermediate samples are variable and there are
37
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5. Discussion: magmatic evolution
Magmatic evolution at Nevados de Chillin can be understood in terms of three main processes-fractional crystallisation, magma mixing and assimilation of continental crust. The role of crustal assimilation in magma genesis at the volcano is speculative as there is no absolutely definitive evidence for crustal melting, as discussed below. The scarcity of analyses of mafic rocks in the database precludes investigation of the early stages of magma genesis. Only one sample has Si02 e 55%. Speculations about early magmatic history in the mantle or lower crust are therefore ruled out. Neither quantitative modelling of magma evolution nor geochemical comparison with other volcanoes in the region are attempted here, due to time constraints. This section provides a qualitative discussion of magmatic evolution and attempts to coordinate the observations reported in Sections 2 4 .
why do they fall into distinct geochemical groups and what are the characteristics of the parental magmas? The elevated K and Rb contents of the GSC346a, GSC6b and Du groups compared to the SGLD and SC2 groups (Figs. la-b) could be explained in terms of different parental andesitic magmas or alternatively could be due to different evolutionary histories. The enrichments in K and Rb over other incompatible elements (e.g. Zr) often signify crustal input (Davidson et al., 1987; McMillan et al., 1989). Sample C34, a granitic nodule, has very elevated Rb (180 ppm) and high K20 (5.05%).
Assimilation of granitic material with these characteristics by andesitic magmas, coupled with slightly different subsequent fractionation histories andor small amounts of magma mixing, for which there is evidence in several GSC6b samples, could qualitatively account for many of the geochemical differences between the high-Si dacites. However, the fact that there are small but significant differences in Rb and K amongst the intermediate groups suggests the possibility that crustal assimilation may have occurred at an even earlier stage of magmatic evolution (basalt or basaltic andesite). As noted in Section 2, the GSC 1CB 1 and CB3W groups fall on slightly higher trends in Rb and K than the SC5 and GCB3E4 groups. Early crustal assimilation would not rule out subsequent assimilation in the andesite to dacite stage. The steep trend in K from andesite to dacite, evident on the K20 plot (Fig. lb) where the high Rb, K dacites fall within the high-K field, is suggestive of continued crustal assimilation, assuming that their parental magmas were similar to the andesites. The steepness of the trend is unlikely to be due to phenocryst accumulation in the porphyritic andesites (which would
A fundamental question in relation to the petrogenesis of the high-Si dacites is
38
D D B B B D B B B B B B D D
I)
8 0
0
0 0
0
0 0
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0 0
give lower whole rock abundances of incompatible elements) as the intermediate rocks with low phenocryst contents are indistinguishable from the porphyritic rocks on the plot.
Sr isotope data may prove useful in evaluation of the various alternatives, as differences between the andesites, due to early assimilation of crust, may be reflected in the isotopes. A further possibility, over which there is presently no constraint whatsoever, is that the primary mantle derived magmas may have had fundamentally different geochemical characteristics, determined by source composition or melting processes and there has been no crustal input.
by assimilation of C34 type material, as the granite has low contents of all of these elements relative to the dacites, particularly if assimilation was accompanied by plagioclase fractionation. The GSC6b group may have been slightly more evolved than the GSC346a group prior to magma mixing, although the very small amounts of mafic material, evident in thin section, appear to have had only a minor effect on the whole rock composition.
groups have evolved from different andesitic parents. The parental magmas to the high Rb groups have probably experienced crustal assimilation together with fractional crystallisation (AFC) but this is likely to have occurred also at the more advanced evolutionary stages. The GSC6b group has also mixed with minor amounts of mafic magma lowering Fe/Mg ratios. Differences in other elements between the high Rb groups (Zr, Ba, Ce, Ti) may be simply due to magma mixing, although slightly different parents, fractionation paths, assimilants (or proportions thereof) cannot be ruled out. Differences between the low Rb dacite groups are subject to similar speculations, although different parents andor fractionation histories are probably the most plausible explanations. They are generally similar in most compatible elements, the main differences being in incompatible elements, particularly Zr and to a lesser extent Rb, K, Ba and Ce all of which are higher in the SGLD than the SC2 group, when the general trends are extrapolated to similar Si02.
The GSC6cSVD group displays incontrovertible whole rock and petrographic evidence for magma mixing. Extrapolation of the mixing line to the high Si end of the spectrum on the Rb plot (Fig. la) suggests that the silicic magma was similar or identical to the GSC346a group. The broad beam microprobe analyses give average Si02 contents in the dacitic groundmasses of 70-7 1% (Appendix 4g). Sample C3 is slightly lower in Zr and may have affinities with the GSC6b group. The high-Mg olivine, up to Fog4 which is present in the mafic inclusions and hybrid lavas, indicates a basaltic to basaltic andesite
The lower Al, Na and Sr of the high Rb, K dacite groups may also be explained
From the foregoing arguments, it is speculated that the high and low Rb dacite
39
B D D D I)
D
D B B B
0
0
0
0 0
0
e e e 0 0 e a
a a a
0
mafic end-member. By comparison with Sollipulli, similar basaltic inclusions with Si02 of around 5 1 % and MgO of around 8-996 contain Fog546 olivine and yield temperatures around 120OOC by the Sisson and Grove method (Murphy, in prep.). Since whole rock analyses of mafic inclusions at Nevados de Chillin are unavailable at present, temperature calculations are not possible. Temperatures around 1 150°C are recorded using the Roeder and Emslie method and 1125°C by the Sisson and Grove thermometer for the lava sample C22, which has olivine up to Foa, similar to the most magnesian olivine in the inclusions (max. Fog5). It is likely that the mafic inclusion magmas had similar temperatures. The contrast in temperature between the interacting mafic and silicic magmas would therefore have been around 150-200°C. The diktytaxitic textures of the inclusions are formed by rapid quenching of the hotter mafk magma upon incorporation into the cooler silicic magma. The reason for the formation of hybrid rather than mingled magmas, in the case of C13 for example, is not resolved. There is no evidence from the mineral chemistry for a lower temperature contrast between the magmas. A simple explanation is that the proportion of mafic to silicic magma was higher in the case of hybrid formation, in accordance with the general model of Sparks and Marshal1 (1986). This would heat up the silicic magma, lower the viscosity contrast between the two magmas, and permit intimate mixing.
The petrogenesis of the remaining groups can be viewed mainly in terms of varying fractionation trends, with the likelihood that the basaltic andesite groups were in some cases derived from different parents. A major consideration in comparing the geochemistry of the mafic to intermediate rocks is the highly porphyritic nature of many samples. It is not coincidental that the SGLA and CB2 groups, which have the highest P trends, have the lowest phenocryst contents. This is most likely due to the fact that the whole rock analyses are closer to liquid compositions in these groups than in the case of the more porphyritic samples.
difficult to account for, partly because the parental magma compositions are unknown. The slightly higher incompatible element contents of the CB3W group samples, in common with the GSClCB1, may be inherited features of their parental magmas. An alternative possibility is that these groups have experienced slightly higher degrees of fractionation, at similar Si02 content, than the GCB3E4 and SC5 groups, all being derived from similar parents. Higher degrees of fractionation would raise incompatible element abundances in the liquid. However, this is an unlikely explanation in the case of the GSClCB 1 group, as fractionation of even minor amounts of titanomagnetite will dramatically increase the Si02 content of the liquid, compared to an equivalent mass of
The origins of the differences in crystallising assemblages between the groups are
40
B
B D D D D D D B B B B B D B
0 0
0
0 0 0 0 0 0 0 0
0
0 0 0 0
fractionating silicate minerals, so that less fractionation is required to reach a given Si02 content. Furthermore, Ni and Cr contents of the GSClCB 1 group are higher at similar Si02 (55-57%) than the other groups, suggesting less fractionation than the other groups. The most plausible explanation is that the parental basaltic andesites were different, the GSClCBl group having been derived from magmas with higher Rb, K and other incompatible elements. The origin of these differences is unconstrainable without isotopic data, as discussed above.
The GSC 1CB 1 group is distinct from all the other mafic-intermediate groups in falling on lower Ti and P trends and having lower FeLMg. This is reflected in the petrography by the earlier crystallisation of titanomagnetite, which is abundant in these samples (Section 3). As noted in Section 4, the mineral chemistry in some cases suggests reheating in a few samples (C50 and C103) but there is no evidence for the presence of phenocrysts of dacitic origin nor of any true disequilibrium assemblages. If magma mixing has occurred, it is likely that the silicic end-member was andesitic rather than dacitic, facilitating mixing due to the lower contrast in temperature and viscosity (Sparks and Marshall, 1986). In most samples there is no evidence of magma mixing and the more calc-alkaline trend is most simply explained by titanomagnetite control. In other words, at Si02 of 55-57%, the GSC 1CB 1 group is less fractionated than the other groups, due to earlier crystallisation of titanomagnetite, which has also removed Fe from the liquid, elevating Mg/Fe ratios. This is most probably a function of oxygen fugacity, as titanomagnetite is stabilised by higher f 0 2 and may reflect higher magmatic H20 content in this group. Cr-spine1 cannot be responsible for the trend as the samples are higher in Cr than the other groups. It is probable that titanomagnetite has only begun to influence the fractionation trend at about 53% Si02, as it is present in only minor amounts in sample C22. It could be speculated that the higher fO2 together with the evidence for a more incompatible element (Rb, K) enriched parent is related to partial melting of hydrous minerals (mica) in the continental crust. Current thought on the role of spine1 and titanomagnetite in generating calc-alkaline differentiation trends is comprehensively reviewed by Sisson and Grove (1993b). A further indication of evolution at higher f 0 2
in the GSClCBl magmas is the absence of pigeonite rims on olivine or of pigeonite in the groundmass (Section 3), in contrast to its almost ubiquitous presence in most of the other mafic to intermediate groups. Grove and Juster (1989) showed experimentally that orthopyroxene versus pigeonite stability is a function of both temperature and oxygen fugacity, pigeonite being the stable phase at higher temperature and lower f 0 2 . The change in the stable low-Ca pyroxene occurs close to the NNO buffer.
41
B
D
B B D D D B B B
B D D D D B D
0 0 0 0 0 0
0
0 a
0 0 0 a a
The SC5 group is distinct from the mafic samples of the other porphyritic groups, GCB3E4 and CB3W, in that orthopyroxene is absent. Although the SC5 samples have a narrow compositional range between 56557 .9% Si02, mafic samples from the other two groups, at similar Si02, do contain orthopyroxene. The SGLA and CB2 groups are also characterised by the absence of orthopyroxene except for the single high-Si andesite sample from the CB2 group. These groups are further distinguished from each other by the scarcity (almost complete absence except for a few microphenocrysts) of clinopyroxene in the mafic CB2 samples and its predominance over olivine in the SGLA samples at similar Si02. There appears to be no evidence of magma mixing in any of the samples analysed by microprobe from these groups, with the possible exception of sample C24 from the GCB3E4 group. This sample is in fact the sole member of the CB4 stratigraphic group. The evidence for actual mixing in contrast to reheating, as discussed previously, is inconclusive.
Magmatic oxygen fugacity may therefore be the dominant control on the different fractionation trends, with an increase in f 0 2 from orthopyroxene absent (SC5, SGLA), through orthopyroxene present but titanomagnetite scarce or absent (GCB3E4 and CB3W), to both phases present (GSClCB 1 ) at similar Si02. Although there is no way of quantifying f 0 2 , evolution of the more reduced magmas is likely to have been close to and below the NNO buffer as it is in this region of T-fO2 space that the changeover from orthopyroxene to pigeonite stability occurs in the compositions studied by Grove and Juster (1989). There is no evidence for systematic temperature differences at similar Si02 between the groups.
account for as the samples are quite evolved (58-59% Si02). Although no samples from this group were analysed by microprobe, temperatures between 1040'-1050°C for C88 and C89 were obtained using the Roeder and Emslie method. By comparison with other samples, these are reasonable values and indistinguishable from those for C44 from the SGLA group (104OOC). Both thermal and compositional considerations suggest that clinopyroxene should be present at this stage of magma evolution, even if the magma were very hydrous, suppressing crystallisation of all phases. However there is no titanomagnetite which would be expected in hydrous andesite, compared to the SGLA group which contains trace amounts of Fe-Ti oxide. The small sizes and low proportions of phenocrysts in both groups suggest crystallisation shortly before or during eruption at pressures close to atmospheric. Pigeonite rims on olivine have not been confirmed in this group as the samples have not been probed. Olivine-plagioclase assemblages are common in basalt to basaltic andesite compositions just below the liquidus but are
The almost complete absence of clinopyroxene in the CB2 group is difficult to
42
0 0
0 0 a 0 0 0 a
e
a
0
a
a a a a a a 0
a 0
a 0 0 0
0
0 e e 0
anomalous in relatively low temperature andesites. No reference to such assemblages has been found in the literature surveyed and no realistic explanation can be offered at present.
6. Summary
The samples which comprise the present database from Nevados de Chillan span the range from basaltic andesite (53% Si02) to high-Si dacite (70% Si02). The different stratigraphic units, defined in the field and retrospectively from geochemical considerations, have evolved in different ways and are readily distinguishable by petrographic criteria.
Magma mixing between basaltic andesite and high-Si dacite has produced the most calc-alkaline rocks. Differences between the high-Si dacite groups are probably related to their evolution from different andesitic parents, with some crustal input likely in the case of the high Rb and K groups. Fractionation at different oxygen fugacities may have been the dominant process determining differences in fractionating mineral assemblages amongst the intermediate groups. The parents to the andesitic magmas are unknown but may have been geochemically distinct from each other, due to any or all of the following processes: different mantle sources, melting processes, variations in slab component, early fractionation histories and crustal assimilation. At present there are no constraints over early magmatic evolution at the volcano due to the scarcity of mafic samples.
to 92OOC in the dacites although the silicic rocks average around 970OC. These temperatures are towards the higher end of the spectrum reported for arc magmas and suggest that quite high temperature basalts are involved in the early stages of magmatic evolution at the volcano.
Magma temperatures range from about 1150°C in the most mafic basaltic andesite
43
0
0 0 e 0
0 0
0
0 0
0 0
0 0 0
0
0 0 0
0
0 0
0 e e 0 0
0 0 0
0 0 0
0
Fig. 1: Variation diagrams of selected elements against Si02 distinguished according to the geochemical grouping discussed in the text. Major elements are in weight 9% oxide, normalised to 100% anhydrous after conversion of Fe203 to FeO. Trace elements are in parts per million. The SC7 samples are anomalous on many plots and are not discussed but have been plotted to demonstrate the discrepancies noted in the text..
a) The Rb plot shows clear distinctions between the high-Si dacites. There are also differences between some of the andesite groups, although there is considerable overlap. However the GS 1CB 1 group is generally higher at a given Si02 than the SC5 or CB3E4 groups. A mixing line is drawn through the GSC6SVD mixed magma group to illustrate the probable end-member compositions.
b) K20 plot with field boundaries after Gill (198 1). K and Rb show very similar behaviour in terms of distinctions between different groups. The steepness of the trend from andesite to high-Si dacite in the case of the groups enriched in K and Rb is very evident on this plot.
c) The Na20 plot shows a general anti-correlation between Na and K 2 0 (and Rb). Dacite groups which are enriched in the latter elements are depleted in Na. The differences amongst the andesites are less clear but the GS 1CB 1 tend to plot at lower Na values than the CB3E4.
d) The Zr plot illustrates distinctions between the dacite groups which plot together on the previous diagrams.
e) The FeO* plot illustrates the distinctly lower trend in the GS 1CB 1 group, the only samples which contain significant amounts of titanomagnetite below about 58% Si02.
f) The MgO plot clearly shows the magma mixing trend of the GSC6SVD group, all of which plot at higher values than groups which have evolved predominantly by fractionation.
g) The Ti02 plot again illustrates the different trends produced by mixing and fractionation. The GS 1CB 1 group plots on a lower trend than the other mafic- intermediate groups. This is probably due to titanomagnetite fractionation although there is evidence from the mineral chemistry in some samples for interaction with more mafic magma. Also notable is the Ti enrichment in the SGLA group, which is characterised by very low phenocryst contents.
h) The P2O5 plot is similar to the TiO2 plot. However the CB2 group also plots at very high P. This group also has low phenocryst contents but has fractionated some titanomagnetite, reducing Ti and Fe in the two more mafic samples. The enrichments in P relative to the porphyritic groups is partly due to the low phenocryst contents. Analyses of samples from these two groups, particularly the SGLA, are probably closest to true liquid compositions.
i) A1203 shows considerable scatter due most likely to plagioclase accumulation in the porphyritic andesites. Differences are apparent among the dacites however as these have generally low and similar phenocryst contents. A1 shows similar behaviour to Na and anti-correlates with K and Rb amongst the high-Si dacite groups.
e 0
e a 0
0
0 e a e 0 a a e e 0 0
0
0
e e 0
e 0
e e 0
6 a 0
0
e e e
j) Sr is similar to A1 due to plagioclase accumulation in the andesites and shows similar behaviour to A1 and Na in the dacites. The very high values in some of the mafic samples are not simply due to high proportions of plagioclase as neither sample is excessively enriched in plagioclase compared to the other basaltic andesites. It may be a source characteristic or be a signature of assimilation of mafic continental crust.
k and 1 ) Ba and Ce show similar behaviour to Zr in terms of relative abundances in the different groups but distinctions between the groups are not as clear as there is more scatter, particularly for Ce..
m and n) Both Ni and Cr show considerable scatter. They are close to or below detection limit in many of the dacitic rocks and are not plotted below 10 ppm Ni and 20 ppm Cr. All of the SC6SVD samples have anomalously high Ni and Cr contents, compared to other rocks at similar Si02. This is a mixing signature and further demonstrates that the mafic end-member was basalt or low-Si basaltic andesite. Many of the GSClCB 1 samples have higher contents of both elements than the other groups. This is particularly evident on the Ni plot between 55-57% Si02, although there is some overlap.
120
100
80
40
20
Fig. l a
4 +
A f l *E
Mixing line 0
+f' e
0 0 0
0
0 50 55 60 65 70
3.5
Fig. l b
3
2.5
0 c 2
1.5
1
0.5
Si02
c
I-
/
High-K / 0
50 55 60
Si02
65 70
0 GSC346a Du + GSClCB1 V CB2
' 0 GSC6b A SC6cSVD X SGLA CBG
I V SGLD rn s c 7 4 GCB3E4 I 1 0 sc2 s c 5 A CB3W I
I
0 0
0
0
0
0
0
e 0
0
0
0
0
0 * 0 0 0
0
e 0 0 e e
0 0 e 0
0
0 0
e 0
6
5.5
5
q 4.5 a Z
4
3.5
3
Fig. l c
V 0 0
w
4 f;+ + + A
3< ++ + +
+
A =
50 55 60 65 70 Si02
375 ,
325 L Fig* I d
1 6 225
1 1 75
125 I
X
0 + +
0
A m.
4 / , , I , / , , , 1 1 1 , # , , , 75
50 55 60 65 70
Si02
I 0 GSC346a Du + GSClCBl V CB2 '
0 GSC6b A SC6cSVD X SGLA 0 CBG
V SGLD s c 7 4 GCB3E4
0 s c 2 s c 5 A CB3W 1
0
0
0 0 0
0 0 0 0
0
0 0
0 0 0 0
0
0
0
0 0
0
0
0 0
0 0 0
0
0 0 0
8
7.5
7
6.5
6
b 5.5 a, LL
5
4.5
4
3.5
3
7
6
5
4
3
2
1
0
A
+ X X +
4 4d A A Fig l e
4 m 7% % +
+ 4 A + +
AA . : A w
*.#4
50
Fig If
50
55 60
5102 65 70
+ \
55 60
5102 65 70
+ GSClCBl V CB2
0 GSC6b
V SGLD 4 GCB3E4
0 s c 2 A CB3W
0 0
0 0 0
0
0 0 0
0 0
0 0
0 0
0
0 0
0
0 0 0
0
0 0
0 0 0 0
0 0
0
0
0
1.5
1.4
1.3
1.2
1.1
8 1 i=
0.9
0.8
0.7
0.6
0.5
0.5
0.45
0.4
0.35
0.3
0" a* 0.25
0.2
0.15
0.1
0.05
X X
Fig. l g
4 \
50 55 60 65 70
5102
Fig. l h
' X
X
'I 'X
X
t 4 A A A
+
0 A A
4
. .=. .. 43 . A
A A
tl
50 55 60 65 70
5102 , Cl GSC346a Du + GSClCBl V CB2
0 GSC6b A SC6cSVD X SGLA 0 CBG ~
1 1 1 V SGLD s c 7 4 GCB3E4 1 1 1
I 0 s c 2 s c 5 A CB3W 1
0 0
e e e 0
0
0 0
e 0
0 e e e 0
0
0
0
e 0 0
m
e e 0 0 0 0
0 0
0
0
+ + + 4
900
800
6 17 c 3 L c\1 I
Fig l j + -
+ - i
i-
600 L v)
500
400
300
200
100
+
+ - I + ~
AAw ym I I
-
-
A+ V.. ~
44 A & -
- 9. 1 I 0
/ / I / I ~ I 1 / , 1 / , I , , , I , ,
4?!l
0 GSC346a Du + GSClCB1 V CB2
0 GSC6b A SC6cSVD X SGLA
V SGLD . s c 7 4 GCB3E4
. . .=
I 700 I
0 0
e e 0 0
e 0 e e 0
e e e a 0 a e a 0 e e 0
e e 0 0 e a 0
e 0
e 0
V
550
500
450
400
350 (d m
300
250
200
150
100
70
60
50
a,
40
30
20
10
Fig 1 k
V A 3
+ + X
4
50 55 60 65 70
Si02
Fig 11
+
+ 4&
4
A A
V
X
+ 4
O %
# 0
50 55 60 65 70
Si02
~ 0 GSC346a Du + GSClCBl V CB2
0 GSC6b A SCGcSVD X SGLA
V SGLD s c 7 4 GCB3E4
0 s c 2 s c 5 A CB3W CBG ~
0
0
0 0 0 0 0 0
0
0 0
0 0
0 0 0
0 0
0 0 0 0
0 0 0
0 0 0 0
0 0
0
0 0
15
10
60 ,
A #A+ + m.4
- X A
I I / , I , , , / I t , , , l , , , , l ,
55
50
45
40
.- z 35
Fig. lm +
+
+ +
A
A
A
M +
++ H
X& A + +
4 @ A
8 . w A
90
80
70
60
6 50
40
30
20
Fig. I n + - A
A + + -
+ A
A -
+ +
4 H
+ A
50 55 60 65 A 70
Si02
+ GSClCBl 'I CB2 1 0 GSC6b GSC346a A SC6cSVD ;i: X SGLA 0 CBG 1 V SGLD 4 GCB3E4
0 s c 2 A CB3W
0
@ e e 0 0 0
0
0 0
e 0 0 0 e 0
e 0
0 0 0
0
0
0
0 e 0
0 e 0
0
0 0
e
0 r" b \
Q, LL
7.5
6.5
5.5
4.5
3.5
2.5
1.5
0.5
Fig 2
Thole i i te
E
o rite
Ca Ic-alkal ine
50 55 60 65 70
Si02
0 GSC346a Du + GSClCBl V CB2
0 GSC6b A SC6cSVD X SGLA 0 CBG
V SGLD s c 7 4 GCB3E4
0 s c 2 En s c 5 A CB3W
Figure 2: Plot of FeO*/MgO against SO2. The boundaries between the calc-alkaline and tholeiite fields are after Miyashiro (1974). 'The SC6SVD group is the most calc-alkaline. This is due to magma mixing, producing a hyperbolic mixing trend. The rapid increase in Fe/Mg in the GSC346a group is not a tholeiitic trend but a result of Mg depletion in the late stages of fractionation. The minor amount of magma mixing in the GSC6b group, evident in thin section, is apparent on this plot, as even very small amounts of olivine or other mafic minerals dramatically lower the whole rock Fe/Mg ratio. The GSClCBl group is more calc-alkaline than the other mafic-intermediate groups due to fractionation of titanomagnetite. One CB3W sample (C10) plots well within the calc-alkaline field. This is also a mixed rock, containing a mafic inclusion.
0 0
0 0
0 0 0 0 0 0
0
0
0
0
0
0 0
0
0 0 0 0
0 0 0
0 0 0
0
0
0 0
0 1 '
0
1150
1100
1050 0 I-
1000
950
900
0
I T olivine 1 1 V Tcpx 1 1
0
V
V 0 0 0 V
0 V o v
0 0
V V
V
0
0 V 9
0 V V 0 0
v v fl
0
50 55 60 65 70
5102
Fig 13: Average temperatures from Appendix 5 plotted against whole rock SiO2. The diagram illustrates the general decrease in temperature from basaltic andesite to dacite and highlights the similarities and contrasts between values calculated by the different methods, discussed in the text.
B B
B D B B B B B
B
B B B
D B
D B
B B B B b B B
B B B B B B B B
) I
D
Dacites (C4, C52,)
1 I y- so+ I I 1
Ab An
I I 1 + 1- +I n j I 1 \\\ Andesites (C11, C29)
Ab An
I I I +
Basaltic Andesite (C19, C25)
1 O w i o Q-HI
Ab An
1 I I 1 I = + I
Basaltic Andesite (C22)
0
Ab An
Mixed and mingled lavas (C5, C13, C14)
I I I + x 10 01 y jy I +
An i !
Ab
I I I 09 0 1
Ab An
Basaltic Andesite (C24, C103)
1 + * I
+ cores o rims x gms
Fig 14: Plagioclase compositions plotted on triangular diagrams, as simple Or, Ab and An components, calculated as described in Appendix 4. The diagrams illustrate key features of the mineral chemistry and are not intended to be comprehensive. The top four plots show the increase in An content from dacite to basaltic andesite. Most grains are either normally zoned or unzoned. Small amplitude reverse zoning is present in some of the andesite samples. The two bottom diagrams illustrate the strong reverse zoning in the mingled lavas and Type 1 grains in the basaltic andesites. The original dacitic plagioclase cores show a range similar to grains in the unmixed dacites but the rims (or dusty areas) range to much more calcic values. Type 1 cores in the basaltic andesites are similar or slightly more An-rich than the most calcic cores from the mingled rocks. Rims or dusty areas in both types have similar compositions but range to slightly more calcic compositions in the basaltic andesites. Groundmass plagioclase from inclusion and host dacite in C14 are also plotted.
D
B
D
D D D B
D D
B
D
D
B D
B B B
a
t W
c W
c W
L .e a, -c Y
Y
a 0
a a 0
0
0
a a a a a 0
a a 0
0
a 0
a a 0
a a 0
a 0 0
a
a a a
Basaltic andesite (C22)
n n 1 - 7 ' 1 I I " " '
0.9 0.85 0.8 0.75 0.7 0.65 0.6
Mingled lavas (C5, C 14)
n 1 - 1 I 1 I ' I ' 1
0.9 0.85 0.8 0.75 0.7 0.65 0.6
Basaltic andesite (C19)
I " " I " In r ' ' ' Q V Q - Q O l ' I " " I ' " ' j
0.9 0.85 0.8 0.75 0.7 0.65 0.6
Basaltic andesite (C17)
I t " ' : 0.9 0.85 0.8 0.75 0.7 0.65 0.6
Basaltic andesite (C25) n n n
1 ' ? ' -14-1 ' ' I 0.9 0.85 0.8 0.75 0.7 0.65 0.6
Andesite (C44) n m w n
I ' ' ' ' I ' ' ' ' I ' ' ' ' 1 ' ' - I ' I I " " / 0.9 0.85 0.8 0.75 0.7 0.65 0.6
Andesite (C29)
I " " I " " I " " I " " I n ' l n l n l l M , " 1 0.9 0.85 0.8 0.75 0.7 0.65 0.6
Fo
cores o rims
Fig. 16: Olivine compositions, plotted for a representative group of samples. There is a general decrease in Fo content from the mafic to intermediate rocks as expected. Olivine compositions in the mafic inclusions are similar to those in the most mafic basaltic andesite, C22. Zonation, when it exists, is always normal. Many samples have two olivine populations, related in most cases to cooling and retention of more Mg-rich olivine which has crystallised earlier than the Fe-rich population. No Mg-rich olivine was found in C25, yet the sample records high temperatures suggesting that more magnesian olivine has crystallised but has fractionated out. More Fe-rich olivine in the mingled samples has in some cases crystallised in the dacite after reheating has occurred, the more magnesian olivine having already grown in the dacite prior to mingling.
0
0 0
e 0 0 0
0
0 0
0
0 0 e 0
0 0
0
0
0
0
0
0
a a 0 0 0 0 0 0 0 0
0
t 0.80
1 Boninites 0.70
0.60 1 3
6 0.40 + 1
O m 3 0 1 ii
0.20 t
0.10 t I
Mt. Shasta
1 Basaltic And3ite - \ /
I \ - /o /
I
I / n
/ ’ MORB
A /
I / a n 0
0
U
0 I OO
A
A
0.00 I I 1 I I I I
1 .oo 0.80 0.60 0.40 0.20 0.00
Mg / Mg + Fe2+
0 A Basaltic andesite (C22)
0
Mafic inclusions and hybrid ((25, C 13, C 14)
Basaltic andesite to andesite (C50, C17, C39, C107)
Fig 17: Cr-spine1 compositions plotted as Cr / (Cr + Al) against Mg / (Mg + Fez+). Fe2+ was calculated using the method of Stormer (1983), which assumes perfect stoichiometry. There is a general tendency for spinel to become more Fe-rich as the Fe content of the host olivine increases, which should correlate positively with whole rock Si02. Because of the effects of olivine retention, discussed in the text, this correlation is diffuse in the Nevados de Chillan basaltic andesites. Spine1 does not generally occur in the more Fe-rich olivines. The effects of re-equilibration and oxidation during cooling prevent the use of spinel compositions in quantifying f02 , poor results being obtained in trial calculations. The fields for Mt. Shasta basaltic andesites in the Western USA and MORB after Baker et al. (1994) have been sketched in for comparison.
0
0
a a a 0 0
0 0
0 a 0 0 0 a a a
a
e e
0
0
e
0
a 0 0 0 0 0
0 rn 0
References Andersen, D.J., Lindsley, D.H. and Davidson, P.M. (1993): QUILF: a PASCAL program to assess equilibria among Fe-Mg-Ti oxides, Pyroxenes, Olivine and Quartz. Computers and Geosciences 19, 1333-1 350.
Ballhaus, C., Berry, R.F. and Green, D.H. (1991): High pressure experimental calibration of the olivine-orthopyroxene-spine1 oxygen geobarometer: implications for the oxidation state of the upper mantle. Contrib. Mineral. Petrol. 107, 2 7 4 0 .
Bacon, C.R. (1986): Magmatic inclusions in silicic and intermediate volcanic rocks. J. Geophys. Res. 91B, 609 1-6 1 12.
Baker, M.B., Grove, T.L. and Price, R. (1994): Primitive basalts and andesites from the Mt. Shasta region, N. California: products of varying melt fraction and water content. Contrib. Mineral. Petrol. 118, 11 1-129.
Davidson, J.P., Dungan, M.A, Ferguson, K.M. and Colucci, M.T. (1987): Crust-magma interactions and the evolution of arc magmas: the San Pedro-Pellado volcanic complex, southern Chilean Andes. Geology 1 5 , 4 4 3 4 6 .
Dunkley, P and Gilbert, J.S. (1995): Interim Report on the Geology and Volcanic Hazards of Nevados de ChillBn. Unpublished internal report, British Geological Survey.
Ghiorso, M.S. and Sack, R.O. (1991): Fe-Ti oxide thermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas. Contrib. Mineral. Petrol. 108, 485-5 10.
Gill, J.B. (198 1): Orogenic andesites and plate tectonics. Springer-Verlag, Berlin Heidelberg New York.
Grove, T.L. and Juster, T.C. (1989): Experimental investigations of low-Ca pyroxene stability and olivine-pyroxene-liquid equilibria at 1 -atm in natural basaltic and andesitic liquids. Contrib. Mineral. Petrol. 103, 287-305.
Honjo, N., Bonnichsen, B., Leeman, W.P. and Stormer, J.C. Jr. (1992): Mineralogy and geothermometry of high-temperature rhyolites from the central and western Snake River Plain. Bull. Volcanol. 54, 220-237.
Kawamoto, T., (1992): Dusty and honeycomb plagioclase: indicators of processes in the Uchino stratified magma chamber, Izu Peninsula, Japan. J. Volcanol. Geothemz. Res. 49, 191-208.
Kilinc, A., Cannichael, I.S.E. and Sack, R.O. (1983): The ferrous-ferric ratio of natural silicate liquids equilibrated in air. Contrib. Mineral. Petrol. 83, 136-140.
Kuo, L-C, and Kirkpatrick, R.J. (1982): Pre-emption history of phyric basalts from DSDP Legs 45 and 46: evidence from morphology and zoning patterns in plagioclase. Contrib. Mineral. Petrol. 79, 13-27.
Lindsley, D. and Andersen, D.J. (1983): A two-pyroxene thermometer. Proc. 13th Lunar Planet Sci. Conf. Part 2. J. Geophys. Res. 88, A887-906.
Lofgren, G. ( 1980): Experimental studies on the dynamic crystallisation of silicate melts. in Hargreaves, R.B. (ed.), Physics of Magmatic Processes., Princeton University Press, Princeton, New Jersey, 487-55 1.
McMillan, N.J., Harmon, R.S., Moorbath, S., and Lopez Escobar, L. (1989): Crustal sources involved in continental arc magmatism: A case study of VolcAn Mocho- Choshuenco, southern Chile. Geology 17, 1152-1 156.
Miyashiro, A. (1974): Volcanic rock series in island arcs and active continental margins. Am. J. Sci 274, 321-355.
Murphy, M.D. (in prep.): Petrology and geochemistry of evolved magmas at Sollipullj volcano, 39"S, southern Andes. To be submitted to J. Petrology.
Roeder, P.L. and Emslie, R.F. ( 1970): Olivine-liquid equilibrium. Contrib. Mineral. Petrol. 29, 275-289.
Sisson, T.W. and Grove, T.L. (1993a): Temperatures and water contents of low-MgO high-alumina basalts. Contrib. Mineral. Petrol. 113, 167-1 84.
Sisson, T.W. and Grove, T.L. (1993b): Experimental investigations of the role of H20 in calc-alkaline differentiation and subduction zone magmatism. Contrib. Mineral. Petrol. 113, 143-166.
Skulski, T., Minarik, W. and Watson, E.B. (1994): High-pressure trace-element partitioning between clinopyroxene and basaltic melts. Chem. Geol. 117, 127-147.
Sparks, R.S.J. and Marshall, L.A. (1986): Thermal and mechanical constraints on mixing between mafic and silicic magmas. J. Volcanol. Geothenn. Res.
Stormer, J.C. Jr. (1983): The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of multi-component iron-titanium oxides. Am. Mineral. 68, 586-594.
29, 99-124.
S tratigraphic Unit
CBG
CB4
CB3E
CB3W
C3 1, C36, C37
C24
Appendix 1
Sample Number
C25, C26, C27, C28, C29. C30, C35
C6, C7, C8, C9, C10, C11, C91, C95, c96, c97, c98
s c 7
SC6
s c 5
Cerro Blanco
CB2 C88, C89, C90
CB 1 C92
Southern Complex
S151A, S152A, Sl53, Sl54, Sl55, S156, SI57
a: C23, C99, C100, C106. Cl 15, C122, C130, C132 b: Cl, C2, C85,C102, Cl 10, C120, C121, c: C13, C14, C45, C46, C3?
C16, C17, C39, C107, C112, C116, C117
s c 4 C52, C57, C58
s c 3
s c 2
sc 1
C48
C53, C59, C61, C71, C75
C18, C19,C20, C21, C22, C50, C73, C83, C103, C131
Geochemical Group
CBG
GCB3E4
GCB3E4
CB3W
CB2
GSC 1 CB 1
5c7
GSC346a g5c6b
GSC6cSVD
5c5
GSC346a
GSC346a
5c2
GSC 1 CB 1
Lithological units of uncertain affinity
GSC6cSVD SVD C5, C42, C3?
DU
SGLD
C104, C105
C4,C43,C113,C114
DU
SGLD
SGLA C44, C101, C118, C125 SGLA
Appendix 1: The table shows the relationship between the stratigraphic units of Dunkley and Gilbert (1995) and the groupings used in this report. The SC6 unit is subdivided into three groups, two of which are combined with other units to form the geochemical groups used here. No other stratigraphic groups have been subdivided, but some have been merged with other groups here for convenience in discussion and to reduce the amount of noise on the variation diagrams. The prefix G is used to identify these groups and the nomenclature simply merges the stratigraphic names.
a a
a a
a a a
a a
a
a a a
a a a a
0
0
0
0 0 0
0
0 0
0
0 0
UI
0 8
w m
u m
W m 0 m
-3 w m 0 0
-3 w m u 0
s m m U a
0 0
0 0
0
0 0
0
0
0
0 0 0
0 0
0 0 0
0
0 0 0
0
0
0
0 0
cv X 'CJ S aa Q
.I
2
m W -8 m w cl
m
m s 8 0
a 0 0 0 0 0
0 0 0 0 0 0 0
0 0
0 0 0
0 0 0 0 0 0
0 0
0 e 0 0 0 0 0
X D E Q) Q Q
.-
a x m U
z 0 U U J Q
2 m U
0
0
0 0 0
0 0
0
0 0
0
0
0 0
0 0
0
0 0 0
0
0 0 0 0
0 0
0
0 0
0
0
@ 0
0 0
0 0
0
0
0
0
0 0
e 0
0 0
0
0
0 0
0 0 0
0
0
0 0
0
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c 4
C52
c59
c37
S156
c11
c95
C29
c 5
C 13
C14S
C14M
C24
c25
C50
C103
C19
C92
C98
C17
c39
C44
C 107
c22
CSS
PI
5.2
8.9
6.3
10.6
13.0
6.5
6.5
2.0
8.7
5.3
10.3
1.5
34.5
30.7
28.5
23.5
31.5
27.2
11.7
23.8
27.0
4.1
16.8
15.7
9.3
01
Tr
0.1
0.1
2.7
1.1
2.7
1.7
1.6
4.4
3.4
2.9
Tr
0.2
3.4
4.5
0.3
1.7
5.7
2.2
Appendix 3
CPX
0.6
0.8
0.3
1.8
1.3
1.3
1.6
0.4
1.4
0.4
1.8
1 .o 1.9
2.7
2.9
4.0
2.6
5.5
1.9
1.3
2.8
0.9
0.6
1.5
OPX
1 .o 0.7
1 .o 1.2
1.8
0.6
0.6
0.2
1.8
0.3
0.9
0.7
1.5
1.3
1.1
1.6
2.1
3.5
0.3
ox 0.5
0.6
0.6
1.1
0.7
0.8
0.4
0.1
1 .o 0.2
0.6
0.2
0.3
1 .o 0.2
0.9
0.1
Tr
0.1
0.2
0.1
AP
0.2
0.1
Tr
0.1
0.1
Tr
Gms
92.5
87.0
91.8
85.3
60.6
71.7
89.6
97.2
83.4
91.1
85.4
66.3
60.1
63.7
62.1
67.4
55.5
62.9
81.2
71.5
61.6
94.7
76.4
74.5
88.6
Ves
1.9
22.6
19.1
1.3
3.6
27.6
5.3
4.5
4.1
4.3
2.5
Appendix 3: Modal analyses of a representative group of samples, the majority of which have been analysed by microprobe. Between 1500 and 2000 point counts were made on the thin sections (not the probe sections). The suffixes S and M on C14 represent the silicic and mafic parts of the thin section. Distinction between phenocryst and groundmass is often subjective. The typical phenocrysts in some very fine grained samples are similar in size to the groundmass of the coarse grained samples. Counts of vesicle contents may be underestimates in some cases, as small vesicles are difficult to distinguish.
0 a 0 0 0
0 0 0
0
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0 0
0 0 0 0
0 0
0
Appendix 4: The microprobe data are presented as averages of several analyses for each grain. In many cases, core and rim analyses are averaged separately. The number of analyses from which the average is calculated is given in row 3. The maximum and minimum values of the principal components in each set are also presented. For example, in the case of any particular plagioclase crystal, the average of the rims and cores are presented, as well as the maximum and minimum anorthite (An) content. In the case of the pyroxenes, the maximum and minimum enstatite (En) and wollastonite (WO) components are listed and for olivine the maximum and minimum forsterite content (Fo). The components are calculated from the simple cation proportions in all cases. In the case of plagioclase, An is calculated as Ca++ / (Ca++ + Na+ + K+) and similarly for the other components. For olivine, Fo is calculated as Mg++ / (Mg++ + Fe++) . For pyroxenes, En is calculated as Mg++ / (Mg++ + Fe++ + Ca++) and so on. In the case of clinopyroxene, the components used in thermometry are also reported. These are calculated using the projection scheme from the QUILF program.
Cation proportions are calculated from the raw probe data, with all Fe as FeO, except for ilmenite and magnetite used for thermometry. These are calculated using the scheme of Stormer (1983) which assumes perfect stoichiometry.
Temperatures calculated from single pyroxene thermometry are presented in the bottom row (see text for details). The two-oxide temperatures and oxygen fugacities are calculated as averages for each sample. The temperatures are considered to be implausibly low compared to the pyroxene temperatures but are presented for completeness.
traversal, are reported in Appendix 4g. Average groundmass compositions of some samples, measured by broad beam
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Sample No
c 4 C5 2 c59 c37
c11 c95
C29 c5
C13 C14
C24 C25 C50 C103 c19 C92 C98 C87 C107 C17 c39 c44
c22
Si02
67.94 69.37 68.90 66.43
62.94 62.09
60.05 66.70
60.91 63.84
opx-cpx 56.69 55.54 56.90 58.48 56.06 60.12 57.95
57.98 56.51 56.50 58.23
53.04
T (Cpx) "C
950 (6, 41) 945 (4, 26) 979 (2, 10) 985 (4, 16) 930 (5, 24) 1034 (7, 38) 1006 (5, 39) 1042 (2, 15) 1055 (4, 24) 932 (4, 27) 979 (3, 17) 1064 (2, 11) 946 (3, 29) 966 (5, 42)
1089 (3, 14)
11 oa (5,34) 1053 (2, 20)
1063 (6, 28) 1045 (8,51) 1117(7,51) 994 (6, 42) 1049 (6, 47) 1025 (2,17)
01-liq(S & G )
1127
Appendix 5
T (Opx) "C T 01-Liq "C 01 (calc) 01 (meas)
965 (4, 29) 1007 (6, 35) 988 (6,441 965 (6, 27)
1070 (3, 19) 1065 (8, 50)
1066 (4, 28) 989 (5, 33) 1106 (1, 5)
1019 (2, 10) 982 (2, 14) 921 (3, 22)
1093 (5, 27) 1 1 09 (3, 23) 1082 (5, 19) 1100 (2, 10) 1092 (a, 64) 1034 (3, 23) 1120 (4, 18) 1101 (2, 15)
1020 68 69-72
1 oao
1 oao 1110
1100 1100 1045 1060
1040 73 75-76 1080 80 72-79 1110 79 74-80 1040 70 66-69
77 70-7 1 80 72 78 78-80 80 80-8 1 80 ao-a 1 74 69-70
1150 82 83-84
Mean pyroxene temperatures for each sample. calculated from the mean value for each grain, are tabulated in columns 3 and 4. The numbers i n brackets refer to the number of grains analysed and the total number of analyses from which the mean compositions used for thermometry were calculated. The error is about L30-50"C on the pyroxene temperatures. Statistical errors on the results are much less (+lO"C) than the true errors. Olivine-liquid temperatures were calculated using the graphical thermometer of Roeder and Emslie (1970) are given in column 4. The calculated equilibrium olivine and the actual measured composition of the most magnesian olivine analysed are tabulated in columns 5 and 6. Because of the approximations used i n calculating the olivine-liquid values, no realistic error can be quoted but the agreement wi th the orthopyroxene values is notable. The temperature of 1089°C for C13 is :in average of ortho- and clinopyroxene rims, reflecting reheating.
