Petrology and evolution of transitional alkaline — sub alkaline lavas from Patmos, Dodecanesos,...

Contrib Mineral Petrol (1986) 93 : 297-311 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1986

Petrology and evolution of transitional alkaline- sub alkaline lavas from Patmos, Dodecanesos, Greece: evidence for fractional crystallization, magma mixing and assimilation G. Paul Wyers and Michael Barton Department of Geology and Mineralogy, The Ohio State University, Columbus, OH 43210, USA

Abstract. Petrographic, mineral chemical and whole-rock major oxide data are presented for the lavas of the Main Volcanic Series of Patmos, Dodecanesos, Greece. These lavas were erupted about 7 m.y. ago and range in composition from ne-trachybasalts through hy-trachybasalts and trachyandesites to Q-trachytes. To some extent, the ne-trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc-alkaline lavas of destructive plate margins. Major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz. olivine, plagioclase, clinopyroxene and Ti-magnetite in the mafic lavas, plagioclase, clinopyroxene, mica and Ti-magnetite in the evolved lavas. Apatite, which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed. However, mass balance calculations indicate that the chemistry of the hy-trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80-77% ne-trachybasalt with 20-23 % trachyandesite. It is estimated that the mixing event preceded eruption by a period of 12 h-2 weeks suggesting that mixing triggered eruption. Combined fractionation and mixing cannot explain the relatively low MgO contents of the hy-trachybasalts and it is concluded that assimilation also occurred. Assimilation, and especially addition of volatiles to the magmas, may be responsible for the evolutionary trend from ne-normative to hy-normative magmas and was probably facilitated by intensified convection resulting from mixing. A model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot primitive magma into the base of the chamber facilitates assimilation, but eventually mixing yields the hy-trachybasalts and finally the ne-trachybasalts are erupted.

Introduction

In recent years there has been increasing recognition of the complexity of the processes involved in magma evolution. It is now widely realized that the compositions of

Offprint requests to ." G.P. Wyers

many magmas erupted through the continental crust are influenced by magma mixing and assimilation in addition to fractional crystallization. As emphasized by O'Hara (1977) and O'Hara and Matthews (1981), it is essential that the operation of such processes are recognized, and the effects quantified, before attempts are made to reconstruct upper mantle source region characteristics.

The Tertiary to recent lavas from the southern Aegean Sea, Greece, have evolved via fractionation, mixing and assimilation (Barton etal. 1983; Huijsmans etal. 1983, 1986). Most of the volcanic complexes thus far described in detail occur along the Hellenic arc which is associated with the overriding of the African Plate by the Aegean microplate (Huijsmans et al. 1986). In this paper, we describe the petrology of alkaline-sub-alkaline lavas from Pat- mos, which is located some 100 km to the north of the Hellenic arc (Fig. 1). Specifically, we use mineral chemical and whole-rock major oxide data to evaluate the processes operative during evolution of the Main Lava Series (see Wyers et al. in prep.). Our major objective is to demonstrate that significant conclusions may be drawn on the basis of major element data alone. In companion papers (Wyers and Barton in prep. ; Barton and Wyers in prep.) we use trace element and isotopic data to further constrain the ideas developed in the present communication and we estimate the conditions of crystallization.

Outline of geology

Patmos is an almost entirely volcanic island with an area of approximately 38 km 2. The geology has been described in some detail by Robert (1973) who recognized the following major volcanic units: high-K, high-A1 basalts; intermediate potassic lavas (latite, quartz-latite, potassic trachyte); quartz-potassic alkaline lavas (quartz-trachytes, phonolites) and pyroclastics; and sodic alkaline lavas (phonolites, trachytes). We have adopted a classification scheme based entirely upon whole-rock chemistry. This scheme, which was previously used by Johnson et al. (1976) for alkaline volcanics on islands off the coast of Papua (New Guinea) is illustrated in Fig. 2 and is based upon the amount of normative nepheline or quartz (which includes SiO2 from normative hypersthene) and the Thornton-Tuttle 0960) differentiation index. We deliber- ately chose the scheme to avoid the necessity of using names normally associated with talc-alkaline or typically alkaline lava series and it has been used successfully for lavas on some oceanic islands (e.g., Tristan da Cunha; Baker et al. 1964).

The distribution of the major volcanic units is shown in Fig. 3, which is modified from Robert (1973). The predominant rock-types

~B U L G A R I A \ " ' ~ . I

, .#.. , '~..v. ~" ~ ( f" . . . . .J Rhodopoa

~.~ J Western ~ i SALONIKI ~ . ~ j J ~ T h r a c e - ~ "

Samotharki

Limnos ~, b Hagios

G~ Eustration

% ~ Lesbos

Corinth Samos

.-4 C 3o /r r~

b Christiane

' '~'-" )J -- Patmos~

~'Pserir Antimilos=~tl Antiparos C ~ Kos _ ~ : ~

i l l _ ~ O Y / ,.,x / ~ e,. Miles "~ Ss=ntorini ~ ' % i r A

J J 50 lOOkm ~ ~ ~ I

I ' ~ " ' " " - - - - - - ~ - " " " " " Trench

Fig. 1. Simplified geologic map for the Aegean showing the location of the Hellenic trench system, the Hellenic arc, and Patmos

are hy-trachybasaltsl, hy-trachyandesites and Q-trachytes (which form an undivided series on the map), rhyolites, trachytes and unclassified pyroclastics. Ne-trachybasalts are relatively scarce. The largest outcrop occurs on the small island of Chiliomodi, but these rocks also occur as dikes in the central and southern part of the main island and as volcanic bombs found in or near small explosion vents to the east of Grikou. Phonolites are likewise rare, occurring only at two localities in the northern part of the island. A series of steeply dipping trachytes, rhyolites, pyroclastics and mafic dykes occurs in the south, and has been called the Old Volca- nic Series by Robert (1973). The only non-volcanic rocks are marbles, which are only exposed in the south west, near Cape Ghenou- pas. Field studies indicate that the marbles have been tectonically cmplaced and that they may not necessarily represent the local basement.

According to Robert (1973), the central part of Patmos lies in a NW-SE trending graben. This implies that lavas which are found exclusively in the north or south of the island, such as the trachytes and the rhyolites, constitute the oldest eruptive products. A few K - A r age determinations are available (Fytikas et al. 1976;

1 For the sake of brevity, we have abbreviated terms such as hypersthene normative trachybasalt to hy-trachybasalt etc.

__. '_:f_g . . . . l e �9

o * �9 \ �9

I L _ _ .

f : i l ........... / .

s l t l

20 10 0 10 20 Q Ne

Fig. 2. Classification of Patmos MVS lavas in terms of CIPW normative Ne or Q and Thornton-Tuttle differentiation index. Classifi- cation scheme after Johnson et al. (1976)

Robert and Cantagrel 1977) and indicate at least two episodes of volcanic activity, one about 7 m.y. ago (Q-trachytes) and one about 4 m.y. ago (dikes and ne-trachybasalts from Chiliomodi). On the basis of major oxide, trace element and Sr-isotopic data, Wyers et al. (in prep.) recognize a main volcanic series (MVS) which consists of the ne-trachybasalts found as volcanic bombs, the hy-trachybasalts, hy-trachyandesites and Q-trachytes. Recent detailed field studies suggest that the trachytes were erupted before the basalts and that the volcanic sequence is essentially inverted (i.e. more evolved lavas were erupted before the least evolved lavas). In the present paper we consider only those lavas which be- long to the MVS as these appear to form a chemically homogenous series.

Petrography

The petrographic characteristics of the MVS lavas are summarized in Table 1. Detailed petrographic descriptions will be presented elsewhere (Wyers et al. in prep.). All rocks are strongly porphyritic with phenocryst contents up to about 30% (visual estimates).

Fresh olivine is found only in the ne-trachybasalts, in which it occurs as xenocrysts and phenocrysts and as microphenocrysts. The xenocrysts show embayed margins indicative of reaction with the host magma, and contain inclusions of brown chrome-spinel and opaque magnetite. Irregular extinction and strain lamellae are common. The phenocrysts lack strain lamellae. Serpentine often occurs along the rims or in cracks which penetrate the olivines.

Pseudomorphs after olivine occur in the hy-trachybasalts (Fig. 4a). The original olivine has been entirely replaced by deep red-brown colored material which is rimmed by opaques, or by fine-grained opaque material. These pseudomorphs are interpreted to result from reaction between olivine and melt rather than from sub-solidus hydrothermal alteration.

Plagioclase is the dominant phenocryst phase in all of the lavas. It shows radial and oscillatory zoning. The phenocrysts show complex morphologies which are reminiscent of those shown by feldspars in oceanic basalts (Dungan and Rhodes 1978; Rhodes et al. 1979; Kuo and Kirkpatrick 1982) and in calc-alkaline lavas (Luhr and Carmichael 1980) inasmuch as sieve textures and embayed margins are common. The cores of some phenocrysts in the ne- trachybasalts, hy-trachybasalts and hy-trachyandesites are riddled with inclusions of groundmass material and are surrounded by rims of clear feldspar (Fig. 4b). On the other hand, some of the phenocrysts in the ne-trachybasalts and hy-trachyandesites show embayed, inclusion-free cores surrounded by a sieve-textured mantle around which there is a rim of clear feldspar (Fig. 4c). Textures such as these suggest periods of rapid growth or partial reaction with the host melt and are absent in the phenocrysts in the most evolved lavas (i.e. the Q-trachytes). The plagioclase phenocrysts in the hy-trachybasalts, trachyandesites and Q-trachytes commonly have a very thin outer rim of sanidine - a feature often

LEFKES

ISLE OF PATMOS

(SIMPLIFIED FROM ROBERT 1973) 0 1 2Km

_ A M B I I l I

C.GHERANOU

~C.KOUMANAS C)

C,GHENOUPAS

~CHILIOMODI

- - '':'-~:=~i~ii!~,,. iKOU

~ R ~ ~ Ne Trachy basalt Hy - Trachy basalt Hy - Trachy andesite

~ l ~ ~ / ~ . ~ . Q- Zrac h yte Trachyte

�9 . ~ Rhyolite ...... :~ ,::(: !(i~:~!i ~ ~ Phonolite �9 ,i;il ii;:ii ~ i i i i i ~ ! ~ Pyroclastics

o .:.......,.i:-~i.::~!-##i~}5~)~ ~ ~ Volcanic Breccias F ~ "] Silicified Rocks

Old Volcanic series Marble

~ C.PRASSONISSI I I Quaternary

Table 1. Summary of petrographic characteristics of Main Volcanic Series lavas

Fig. 3. Geologic map of Patmos, simplified from Robert (1973)

Lava type Ol Plag Cpx Fe - Ti Mica K-spar Ap Oxide

Ne-trachybasalt P,M,X P,M P,M P,M - - l Hy-trachybasalt S P,M,X P,M,X P,M M,X X M Hy-trachyandesite - P,M P,M P,M P,M P,M M Q-trachyte - P,M P,M P,M P,M P,M M

P = phenocryst; M = microphenocryst; X = xenocryst; S = pseudomorph; I = inclusion in other phenocryst phases

considered to be characteristic of shoshonites. The calcic feldspars in some lavas show varying degrees of alteration to clay minerals and sericite.

Clinopyroxene, which occurs as euhedral-anhedral phenocrysts and as microphenocrysts is generally the second most abundant

phenocryst phase. The pyroxene phenocrysts in the mafic lavas show extremely complex zoning patterns. The phenocrysts in the ne-trachybasalts are medium-green to colorless and ar characterized by radial, oscillatory and sector zoning (Fig. 4d). Some phenocrysts have abundant inclusions of groundmass material in the

Fig. 4. a Pseudomorph of olivine in hy-trachybasalt. The olivine is replaced by deep red-brown colored material, b Plagioclase phenocryst with sieve textured core in hy-trachybasalt, e Plagioclase phenocryst with clear, embayed core, sieve textured mantle and clear rim in hy-trachybasalt, d Pyroxene phenocryst in ne-trachybasalt showing sector and oscillatory zoning

cores and others have embayed margins suggesting resorbtion. This is especially true in the case of the hy-trachybasalts which contain two distinct types of phenocryst, one with a colorless core, a light- green mantle and a darker-green rim, the other with an embayed, dark-green core, a light-green mantle and a darker-green rim. The mantles and rims of these two types of phenocryst are identical in appearance and the mantles are identical to the microphenocrysts, the rims identical to the groundmass pyroxenes. These pyroxenes are almost identical to those described by Barton et al. (1982) from the K-rich lavas of Vulsini (Italy). The green cores are characterized by radial zoning and occasionally contain inclusions of opaques and groundmass material. The colorless cores show radial, oscillatory and sector zoning. In contrast, only radial zoning is shown by the pyroxenes in the hy-trachyandesites and Q-trachytes. The phenocrysts in these lavas vary in color from dark-green (hy-trachyandesites) to light yellowish-green (Q-trachytes). It should be apparent that the colorless and green cores of the pyroxene phenocrysts in the hy-trachybasalts are similar in appearance and zoning characteristics to the pyroxenes in the ne-trachybasalts and hy-trachyandesites respectively.

Magnetite occurs as anhedral phenocrysts and microphenocrysts in all lavas though it is relatively rare in the ne-trachybasalts.

Biotite is a phenocryst and microphenocryst phase in the hy- trachybasalts, trachyandesites and Q-trachytes. The phenocrysts always show a thin rim of opaque material (? magnetite) which presumably reflects oxidation and resorbtion during eruption. However, the biotite "phenocrysts" in the hy-trachybasalts show extensive resorbtion features (complete or nearly complete replace- ment by turbid, opaque material) and most probably represent

xenocrysts. Needle-like inclusions of rutile are common in the bio- tites of the traehyandesites.

Sanidine is found as resorbed xenocrysts in the hy-trachybasalts and as phenocrysts in the trachyandesites and Q-trachytes. The sanidines in the trachyandesites show some resorbtion effects (sieve texture, embayed margins), but it is not clear from petrographic observations whether these represent phenocrysts or xenocrysts (see below).

Apatite is a ubiquitous acessory phase in the MVS lavas. It occurs mostly as small inclusions, orientated parallel to the crystal faces, in plagioclase, elinopyroxene and biotite phenocrysts, but it also forms microphenocrysts in the hy-trachybasalts, trachyandesites and Q-trachytes. The microphenocrysts in the hy-trachyandesites are anhedral and are dusted with fine, brown specks and are interpreted to be xenocrysts.

All of the minerals described above may occur in crystal clots or glomeroaggregates and, in general, the minerals found in the glomeroaggregates are those found as phenocrysts in the host lavas. However, sanidine does not occur in glomeroaggregates in the hy- trachybasalts which supports the contention that the anhedral sanidine crystals in these lavas represent xenocrysts.

The groundmasses of the lavas are mostly fine-grained, holo- crystalline, and commonly display pilotaxitic texture. The groundmass of the Q-trachytes occasionally includes minor brown glass. The main crystalline phases are plagioclase, sanidine, pyroxene, magnetite, apatite and biotite. Carbonate occurs in the groundmass of some lavas and is interpreted to be secondary. In some cases, the carbonate clearly replaces clinopyroxene.

Some of the MVS lavas contain xenoliths and inclusions. We

have not yet completed detailed studies of these, but the occurrence of a distinctive type of inclusion in some of the trachyandesites is worthy of note. The inclusions are sub-spherical with a diameter of 2 cm. Contacts with the host lava are not sharp, but are finely contorted. The inclusions are holo-crystalline and, in contrast with the lava, contain numerous irregular vesicles. The dominant mineral constituents are plagioclase and skeletal opaques which often form radial aggregates. Clinopyroxene and biotite (rimmed by opaques) occur in subordinate amounts. Inclusions showing tex- tural features similar to those described here have been reported from intermediate and acid lavas (e.g. Eichelberger and Gooley 1977; Van Bergen et al. 1983).

Analytical techniques

Reconnaissance mineral analyses were performed at the Free Uni- versity, Amsterdam, using a Cambridge Mark 9 automated microprobe and wavelength dispersive techniques. Operating conditions were: accelerating voltage - 20 kV, sample current - 20 nA, counting time - 20 s for each pair of elements. Detailed mineral chemical studies were carried out at the State University of Utrecht using a TPD microprobe fitted with a Tracor Northern Energy Disper- sive System. Operating conditions were: 15 kV accelerating voltage, 3-4 nA sample current and 60-100 s counting time. All analyses are fully corrected for deadtime, background, atomic number, absorption and fluorescence. Agreement between analyses obtained on the two instruments is excellent.

Major oxide whole-rock analyses were done at Utrecht by automated XRF on fused glass discs. The instrument was regularly calibrated using a variety of international and internal standards. Precision of the XRF analyses is the same as that given by Barton and Huijsmans (1986) (see Huijsmans et al. 1986, for a detailed discussion), viz. SiO2-0.7%, A1203-0.5%, TiO2-0.04%, FeO- 0.3%, MgO-0.3%, MnO-0.02%, CaO~0.3%, Na20-0.4%, K20- 0.08%, P205-0.04%. These figures are based upon replicate analyses of well-characterized standards and are reported as 2 a devia- tions in wt.% from the true value.

F%O3 and FeO have not been determined independently because there is evidence (petrographic) of post-eruptive oxidation and alteration in many of the lavas. For similar reasons, all analyses have been recalculated to 100% on a volatile-free basis. Mea- sured LOI's range from 0.68-3.43 wt.%.

Mineral chemistry

Olivine

Representative analyses are available from G.P. Wyers upon request. The cores of the xenocrysts are highly forster- itic (Fo9o-88) and contain 0.3-0.4 wt.% NiO. They are thus comparable in composition to the olivines which occur in xenoliths of upper mantle material (cf. Boyd and Nixon 1975; Reid etal. 1975). The Fo content decreases to Fo8o-81 at the rim. The phenocrysts show a wide range in composition, mostly from FOBs (cores) to Fo77 (rims). The data indicate, therefore, that the olivine xenocrysts partially re-equilibrated with the host melt. The microphenocrysts are relatively iron-rich (Fo81-~1) and are also normally zoned.

Spinel

The small size of the crystals made analysis difficult. The spinels are rich in A1203 ( ~ 16 wt.%). SiOz, TiOz, MnO, NiO and ZnO are present in low amounts (<0 .5 wt.%). The spinels are similar in composition to those which occur in lherzolite xenoliths entrained in alkali basalts and basan- ites.

Plagioclase

The plagioclases show a wide range in composition but the average composition changes progressively from bytow- nite in the ne-trachybasalts to andesine in the Q-trachytes. The phenocrysts in individual lavas show extensive zoning (up to 35% An), as illustrated in Fig. 5a, which is in most cases normal inasmuch as Ca decreases and Na and K increase towards the rim. BaO contents are low (<0.05 wt.%).

The cores of the majority of the phenocrysts in the ne- trachybasalts are An83-77 whereas the rims vary from An78_66. The cores of a few phenocrysts are richer in anorthite (An89-83) but the rims of these crystals are compositionally identical to those of the other phenocrysts. The cores of the microphenocrysts have compositions which fall within the range shown by the phenocryst rims.

The plagioclases in the hy-trachybasalts exhibit complex compositional variations. The cores of the phenocrysts define two distinct populations (Fig. 5b), one with An84-8o and the other with An64 4o. The microphenocrysts are intermediate in composition (~An66). In general, crystals with anorthite-rich cores show normal zoning to ~Ans8, although many crystals have a very thin outer rim which is very poor in An (Ans_6). Crystals with cores of An64-4o commonly show abrupt reversals in zoning, the core giving way to a mantle of An31-34 which is in turn surrounded by a zone of intermediate composition (An65) and by a thin outer rim of extremely An-poor material.

The phenocrysts in the trachyandesites and Q-trachytes show normal zoning. The cores of the phenocrysts in the trachyandesites are mostly in the range An68-44, but a few are more anorthitic (An79) and these most probably represent xenocrysts. The rims of the phenocrysts and xenocrysts show a restricted range of composition (An43_41) whereas the microphenocrysts (An48-gs) are of intermediate composition. The cores of the phenocrysts in the Q-trachytes range from An59 to An44, and the rims (An45-4o) have similar compositions to the cores of the microphenocrysts (An45 -4o).

Clinopyroxene

The clinopyroxene phenocrysts show a relatively restricted range of composition in terms of Mg/(Mg + ~ Fe z +), from 0.84 to 0.68. The cores of the phenocrysts in the ne-trachybasalts tend, on average, to be characterized by higher Mg/ ( M g + ~ Fe 2+) than the cores of the phenocrysts in the more evolved lavas (0.84 and 0 .74-0 .78 respectively), but there is a poorly defined correlation between M g / ( M g +

Fe z+ ) and whole-rock chemistry due partly, no doubt, to the extensive zoning of the phenocrysts. It is, however, noteworthy that the clinopyroxenes in the trachyandesites are indistinguishable in composition from those in the Q- trachytes. In terms of the conventional pyroxene quadrilateral (Fig. 6a), the clinopyroxenes range in composition from diopside to smite.

A1, Cr, Ti and Fe 3+ (calculated assuming that ~ cations = 4,000 on a 6 oxygen basis) contents are also variable but are, on average, higher in the clinopyroxenes in the ne-trachybasalts than in the clinopyroxenes in the trachyandesites and Q-trachytes. The appropriate ranges in composition shown by the pyroxenes in the ne-trachybasalts and trachyandesites/Q-trachytes are: A1 0.116-0.393 and

: '-" or

\or An

0.016-0.075 afu, Ti 0.012-0.066 and 0.001-0.030 afu, Fe 3+ (expressed as Fe203/~ FeOT) 0.53-0.96 and 0.21-0.32 respectively. Na contents (0.012-0.074 afu) are approximately the same in pyroxenes in all lava types.

As would be expected from the petrographic descriptions, zoning patterns in the pyroxene phenocrysts in the ne-trachybasalts and hy-trachybasalts are complex but they are, in general, similar to those described by Barton et al. (1982) for pyroxene phenocrysts in Vulsini leucite tephrites. The cores of the clinopyroxene phenocrysts in the hy-trachybasalts define two compositionally distinct populations (Fig. 6b), one with M g / ( M g + ~ Fe z+) 0.820-0.851, the other with M g / ( M g + ~ Fe 2+) 0.702-0.748. The mantles surrounding both types of core have intermediate composi-

Fig. 5. a Feldspar compositions plotted in terms of the compo- nents A n - A b - O r . NeTb nepheline trachybasalt; HyTb hypersthene trachybasalt; HyTa hypersthene trachyandesite; QT quartz-trachyte, b The cores of phenocrysts in the hy-trachybasalts define two distinct populations whereas rims and microphenocrysts are of intermediate composition

tions which are identical to those of the microphenocrysts in these lavas.

It proved impossible to obtain satisfactory analyses of the strongly resorbed mica phenocrysts in the hy-trachybasalts so only analyses of the microphenocrysts are considered. The micas in these lavas are phlogopites and are richer in Mg than those in the trachyandesites and Q-trachytes. In the latter micas, Al is just about sufficient to balance Si-deficiencies in the tetrahedral sites whereas in the former, (A1+Si)<8,000. Ti contents are uniformly high

N. Tb / .y Tb

/ 6 0 . PAT 54 �9 / w:/

5 % # "

/ �9 , N ~ Q �9 �9 o o O

% oo oo =

40 . . . . .

0 10 20 Fs

En / b

Fig. 6. a Compositions of clinopyroxenes plotted in the conventional C a - M g - F e quadrilateral. The relatively wide range in Ca-content undoubtedly reflects complex oscillatory and sector zoning. Abbreviations as for Fig. 5. b The cores of pyroxene phenocrysts in the hy-trachybasalts define two distinct populations. The mantles around the phenocrysts and the microphenocrysts occupy an intermediate field

(0.61-0.72afu) and do not correlate with Mg/(Mg+ FEZ+). Ba contents are relatively low, <0.09 afu.

Magnetite

The magnetites contain up to 16.2 wt.% TiO2, 5.3 wt.% AlzO3 and 2.6 wt.% MgO. TiO2 contents are highest in the magnetites in the ne-trachybasalts and are lowest in the magnetites in the Q-trachytes. Calculations based upon stoichiometry (i.e. assuming ~ cations = 3,000 on the basis of 4 oxygens) indicate 30-40 wt.% F%O~. Some magnetites contain exsolution lamellae of ilmenite, but unfortunately the coarseness of the exsolution prohibits successful reinte- gration of the component phases to yield a reliable estimate of the composition of the original phase.

Sanidine

The phenocrysts range in composition from Or56An7 to Or75Anl and individual crystals are normally zoned - that is, Or contents decrease and An contents increase towards the rims. However, the range of composition found in each type of lava is relatively small, viz. Or69Anl. 9 - Or56An6. 9 in the hy-trachybasalts, Or69An2-Or66An 3 in the trachyandesites and OrTsAn2-Or63Ana in the Q-trachytes. The rims are identical in composition to the micropheno- erysts and to the rims of the plagioclase phenocrysts in the same lava. Note that the maximum Or content ofpheno-

crysts in the hy-trachybasalts is the same as that of phenocrysts in the trachyandesites. The analyses are plotted in the A n - A b - O r diagram in Fig. 5a.

Major oxide whole-rock analyses

Representative whole-rock major oxide analyses together with CIPW norms are given in Table 2. Since only total iron was analysed, the norms were calculated assuming F%O3/FeO = 0.15 (Brooks 1976).

As shown in Fig. 2, the ne-trachybasalts are alkaline according to the definition of Shand (1922), whereas the intermediate and evolved lavas are sub-alkaline. The switch from alkaline to sub-alkaline occurs at an early stage in the evolution of the lava series and in this respect the Pat- mos Main Volcanic Series differs from truly alkaline volcanic series as found, for example, on Tristan da Cunha (Baker et al. 1964), St. Helena (Baker 1969) and Skye (Thompson et al. 1972). There are other differences between the Patmos ne-trachybasalts and the alkali-olivine basalts of oceanic islands. The former have higher SiO2 and lower TiO2 and total iron, characteristics often associated with calc-alkaline basalts. To this extent, the Patmos MVS may be considered to be transitional between the alkaline series of intra-oceanic islands and the talc-alkaline series of destructive plate margins.

Major oxide variations shown by the MVS are illustrated in variation diagrams in Fig. 7, in which SiO2 has

Table 2. Whole-rock chemical analyses and CIPW norms of selected Patmos lavas

Ne -- Tb Hy -- Tb Hy -- Ta Q - T

Pat 127 Pat 126 Pat 54 Pat 181 Pat 74 Pat 75 Pat 58 Pat 56

SiO2 50.3 50.3 54.5 54.8 61.7 62.60 65.9 66.4 TiOz 1.17 1.17 1.13 1.31 0.87 0.88 0.59 0.59 A1203 18.3 17.7 17.2 18.2 17.0 17.1 16.3 16.0 FeOT" 7.09 7.29 6.72 6.66 4.88 45.59 3.41 3.40 MnO 0.17 0.13 0.14 0.38 0.10 0.16 0.07 0.07 MgO 6.51 7.14 3.65 2.70 1.84 1.82 0.87 0.89 CaO 9.22 10.1 7.83 6.39 4.33 4.39 2.98 2.99 Na~O 3.44 3.03 2.92 3.15 3.13 3.06 3.46 3.31 K20 2.95 2.71 5.13 5.55 5.62 5.59 6.19 6.05 PzO5 0.72 0.70 0.78 0.88 0.49 0.48 0.26 0.29

Mg 0.621 0.636 0.492 0.419 0.402 0.414

( M g + ~ Fe 2+) 0.313 0.318

CIPW norms

Q . . . . 9.3 10.1 13.8 15.1 or 17.7 16.0 30.3 32.8 33.2 33.0 36.6 35.7 ab 19.5 19.4 24.7 26.6 26.4 25.9 29.3 28.6 an 25.l 26.7 18.7 19.1 15.9 16.4 10.7 11.1 ne 5.3 3.4 . . . . . . di 13.3 15.0 12.4 5.7 2.0 1.9 2.0 1.5 hy - - 1.5 3.4 9.3 8,9 5.2 5.4 ol 13.6 13.9 6.9 6.3 . . . . mt 1.7 1.8 1.6 1.6 1.2 1.1 0.8 0.8 il 2.2 2.2 2.1 2.5 1.7 1.7 1.1 1.1 ap 1.7 1.7 1.9 2.1 1.2 1.1 0.6 0.7

" Total Fe as FeO. Analyses normalized to 100.0 on a volatile-free basis. Norms calculated with Fe203 =0.15 FeO

Si02Wt % 22

il 7 ~0 0 21 20

eo % ~8

Si02Wt.%

3 I �9 I

8 63 68

SiO2Wt.%

' "8 ~'3 "8 5 3

Si02Wt%

iL 11 F ~ 7 ~~ . �9

o 8 o i~, � 9 , , 4 ~ ,

5 8 6 8

Si02Wt.% SiO2Wt.%

il 8 ~ o o o o �9

Si02Wt. % SiO 2 Wt.%

Fig. 7. Major oxides plotted against SiO2. The trends shown by most oxides are broadly consistent with fractional crystallization

8 Ne-Trachybasalts

~; Hy-Trachvandesites & Q-Trachytes

77, 0 i , J i i i , , i

48 53 58 63 68 Wt.% SiO 2

Fig. 8. A plot of MgO versus SiO2 suggests the existence of two magma series

Table 3. Broad-beam electron microprobe analyses of groundmasses

Pat 26 Pat 75

giO 2 53.32 66.30 TiO2 1.37 0.26 A1203 19.60 17.13 FeO 6.52 1.44 MnO - - MgO 2.96 0.84 CaO 7.93 1.21 Na20 1.88 3.85 K20 5.99 8.80 P205 0.40 -

Analyses normalized to 100%. Total Fe reported as FeO

been used as a differentiation index. MgO, FeOT, CaO and A1203 decrease with increasing SiO2, whereas Na20 remains approximately constant and K20 increases. TiO2 and PzO5 show slight initial enrichment and then decrease. These variations are qualitatively explicable in terms of crystal-liquid differentiation processes involving crystallization of the observed phenocryst phases viz., olivine, clinopyroxene and plagioclase in the mafic lavas and biotite, clinopyroxene and plagioclase in the more evolved lavas; the change in the rate of enrichment of K20 at SiO2 ~ 54 wt.% marks the onset of biotite crystallization. The behaviour of Na20 indicates that plagioclase was the dominant crystalline phase, in accordance with petrographic observations. The decrease in TiO2 and P205 in lavas with > 54% SiO2 requires involvement of Ti-magnetite, biotite, and apatite in the differentiation of the intermediate and evolved lavas. However, extrapolated P2Os and TiO2 contents at 40 wt.% SiO2 (the minimum SiO2 content of any realistic crystalline assemblage) are >0, indicating that crystallization of Ti- magnetite, biotite, and apatite must have occurred in the ne-trachybasalts, for which there is petrographic evidence.

Nevertheless, detailed examination of the variation diagrams reveals that certain oxides exhibit unusual behavior. Na20 and A12Oa contents remain approximately constant during differentiation, which suggests that the processes involved in the evolution of the Patmos MVS were not identical to those involved in the evolution of other alkaline lavas. Also, the variations of MgO and to a lesser extent, FeO depart from a linear or a smooth curvilinear trend which is normally developed as a result of differentiation. Indeed, MgO contents of the hy-trachybasalts are so low in comparison to those of the ne-trachybasalts and trachyandesites as to suggest the existence of two distinct lava series - one from ne-trachybasalt to hy-trachybasalt, the other from ne- trachybasalt through trachyandesite to Q-trachyte (Fig. 8). The existence of two series is precluded upon the basis of field relationships, but it is noteworthy that the hy-trachybasalts also appear to be relatively enriched in P205, TiO2 and K20 in addition to FeO (see Fig. 7). These observations strongly suggest that processes other than fractional crystallization have been involved in the evolution of the magmas.

Analyses of groundmasses/mesostasis

The groundmasses of two lavas were analysed with the electron microprobe using a wide (20-50 gm) spot diameter.

Care was taken to avoid microphenocrysts and the larger groundmass crystals so that the analyses most probably do not represent the composition of the liquid quenched upon eruption. Rather, they represent the final liquid after crystallization of part of the groundmass. Analyses are reported in Table 3. Relative to the whole-rock compositions, the groundmasses are enriched in SiO2 and K20 and are depleted in MgO, FeOT, CaO and A1203. They thus con- form to the trends shown by the whole-rock analyses, as shown if the groundmass analyses are compared to whole- rock analyses with about the same SiO2 content. Although it is impossible to find a perfect match between the groundmass analyses and the whole-rock analyses - presumably because the former represent the final liquid after partial crystallization of the groundmass - it is noteworthy that high K20 contents and high K20/Na20 ratios can be gen- erated during crystallization. The low NaaO content, relative to the whole-rock analysis, of the mesostasis of Pat 26 may reflect the crystallization of albitic feldspar as a groundmass phase.

Discussion

Fractional crystallization - quantitative modeling

Although there is evidence that more than one process may have operated during evolution of the Patmos MVS, we first consider fractional crystallization as this process is readily amenable to quantitative modeling. Major oxide variations were modeled using unweighted least-squares mixing calculations (Bryan et al. 1969) involving subtrac- tion of the observed phenocryst phases from successive parental magmas to generate more evolved compositions. For each model, a large number of plausible solutions were obtained depending upon a) the type and number of minerals used and b) the compositions of the minerals, especially the compositions of the F e - Ti oxides and plagioclase. Un- fortunately, there is no sure way to discriminate between the various solutions (Le Maitre 1982). Some previous workers have arbitrarily accepted models as being satisfactory if the sum of the squares of residuals (SSR) is less than 1.5 (Luhr and Carmichael 1980), or less than 0.1 (Le Roex and Erlank 1982). Others have applied a number of criteria, including the absolute and/or relative difference for each oxide between the calculated daughter and the proposed daughter (Fisk et al. 1982). We adopted a two-

Table 4. Results of least-squares mass balance calculations which test the feasibility that the magmas of the MVS are related by fractional crystallization

Pat 26-Pat 54 Pat 54--Pat 74 Pat 74-Pat 58

Pat 26 Pat 54 Pat 74 Obs. Calc. Diff. Obs. Calc. Diff. Obs. Calc. Diff.

SiO2 50.04 50 .01 -0.0334 54.49 54.58 0.0899 61.69 61.69 -0.0026 TiO2 1.t7 1 . 0 9 -0.0804 1.13 1.56 0.4343 0.87 0.94 0.0716 A1203 17.73 17.80 0.0731 17.21 17.19 -0.0209 17.04 17.02 -0.0197 FeO a 7.29 7.31 0.0156 6.72 6.65 -0.0740 4.88 4.88 -0.0027 MnO 0.13 0.16 0.0291 0.14 0.12 -0.0154 0.10 0.07 -0.0295 MgO 7.14 7.16 0.0236 3.65 3.72 0.0745 1.84 1.84 0.0004 CaO 10.06 10.It 0.0509 7.83 7.60 - 0.2295 4.33 4.34 0.0053 Na~O 3.03 2.30 - 0.7277 2.92 2.74 -0.1834 3.t3 3.27 0.1389 K20 2.71 3.15 0.4354 5.13 4.43 - 0.6980 5.62 5.61 - 0.0054 P205 0.70 0.63 - 0.0670 0.78 1.08 0.3022 0.49 0.48 - 0.0070

Mix Propor- S.D. Mix Propor- S.D. Mix Propor- S.D. Variable tions Variable tions Variable tions

Pat 54 0.6051 0.0356 Pat 74 0.6444 0.0352 Pat 58 0.7855 0.0067 O1 0.0917 0.0150 Cpx 0.0780 0.0272 Cpx 0.0141 0.0039 Cpx 0.0740 0.0352 Plag 0.1544 0.0278 Plag 0.1149 0.0054 Plag 0.2072 0.0254 Phlog 0.0728 0.0265 Bi 0.0705 0.0053 Ti-mag 0.0155 0.0059 Ti-mag 0.0293 0.0060 Ti-mag 0.0090 0.0012 Ap 0.0037 0.0092 Ap 0.0178 0.0096 Ap 0.0076 0.0016

SSR = 0.7408 SSR = 0.8732 SSR = 0.0258

" All Fe calculated as FeO

Pat 26 = ne-trachybasalt; Pat 54 = hy-trachybasalt; Pat 74 = hy-trachyandesite, Pat 58 = Q-trachyte. Analysed phenocryst phases in each lava used as input data

stage approach to the problem. Initially, we accepted all solutions for which the SSR was less than 1.2 as solutions in this range are within TOTAL analyt ical uncer ta inty for the daughter magma. We then at tempted, via adjustment of the mineral composi t ions, to reduce the absolute differences for each oxide between the calculated daughter and proposed daughter to the values of analyt ical uncertainty for each oxide. Before presenting the results, three points should be emphasized; 1) The best SSR's (generally < 0.1) for each model were obta ined using plagioclase compositions which are far too albit ic to represent the average composi t ion o f the removed feldspar and solutions involving unrealistic plagioclase composi t ions were thus rejected; 2) Variat ions in plagioclase and F e - T i oxide composi t ion have a d ispropor t ional ly large influence upon F, the frac- t ion of l iquid remaining; 3) The role of sanidine in magma evolut ion is ambiguous - in most cases, inclusion o f sanidine (an observed phenocryst phase in the evolved lavas) significantly improves the SSR (e.g. the hy- t rachybasal ts to the trachyandesites) but often sanidine is a redundant phase inasmuch as it must be added to parent magmas from which other phases must be removed. Since sanidine is a redundant phase in models involving the hy- t rachyan- desites and Q-trachytes - in which sanidine occurs as a phenocrysta l phase - we have ignored all solutions for the hy- t rachybasal ts which involve sanidine.

In Table 4, the most sat isfactory solutions for three mixing calculations involving a ne-trachybasal t , a hy- t rachyba- salt, a t rachyandesi te and a Q-trachyte are listed. Al l meet the cri terion that SSR < 1.2, but two fail the cri terion that the m i s f t for individual oxides should be similar to the

analytical uncertainty. These models involve the ne-trachybasalts, hy- t rachybasal ts and trachyandesites and the poor solutions presumably reflect the anomalous composi t ions of the hy-trachybasal ts which were documented in the pre- ceeding section. In part icular , the fits for KzO, N a 2 0 , P205 and TiO2 are poor and cannot be reconciled with the hypothesis that major oxide variat ions reflect fract ional crystal l ization alone. Nevertheless, the results o f the calculations suggest that fractional crystall ization was an impor- tant process in the evolution o f the MVS and confirm that olivine, cl inopyroxene and plagioclase were the major phases removed from the mafic magmas whereas plagioclase, cl inopyroxene and phlogopi te /biot i te were the major phases removed from the intermediate and evolved magmas. Fur thermore , the solutions to the mixing models re- quire that minor F e - Ti oxide and apat i te played an impor- tant role in magma evolution in accordance with quali tat ive predict ions made on the basis o f major oxide variat ions and with pet rographic observations. I t is noteworthy that the low P205 contents o f the mafic MVS lavas preclude apat i te sa tura t ion (Green and Watson 1982) in these liquids and that the apat i te appears to have crystallized in response to the local increase in P205 in liquids adjacent to growing phenocryst phases.

I t is not possible to identify other processes which have affected the Patmos MVS from the results of the mass- balance calculations alone. Two addi t ional processes, magma mixing and assimilation, are discussed in the following sections. To avoid confusion, it must be emphasized that these conclusions do not necessarily conflict with the da ta presented for the groundmasses of two of the lavas.

The latter demonstrate that compositions showing extreme enrichment in K20 and high KzO/Na20 ratios can be gen- erated by crystallization alone, but it must be borne in mind that these liquids do not correspond in detail with any of the erupted lavas and that the analyses represent the products of extreme fractionation on the scale of a thin section. As noted previously, certain characteristics of the analyses of the groundmasses (i.e., the low Na20 content of the groundmass of Pat 26) are suggestive of modification of the liquid composition by quench crystallization during eruption. It is thus unlikely that such compositions are gener- ated during equilibrium or fractional crystallization involving the observed phenocryst and microphenocryst phases.

Evidence for magma mixing

Petrographic and mineral chemical data provide the most compelling evidence that magma mixing has occurred. In particular, the complex zoning patterns shown by the clinopyroxene and plagioclase phenocrysts in the hy-trachybasalts are difficult to reconcile with a simple, isobaric, fractional crystallization model. Similar features have been described for phenocrysts in MORB's, calc-alkaline lavas and alkaline lavas and have been interpreted to indicate polybaric fractionation (Fisk et al. 1982) or magma mixing (Dungan and Rhodes 1978; Rhodes et al. 1979; Barton et al. 1982; Kuo and Kirkpatrick 1982). As noted by Barton and Van Bergen (1981) and Barton et al. (1982), complex zoning patterns could also result from disaggregation of cognate or non-cognate xenoliths with reaction between li- berated xenocrysts and the host magma. This possibility is the most difficult to evaluate in the case of the Patmos lavas, as many of the minerals which are obviously xeno- crystal in the hy-trachybasalts occur as glomeroaggregates in other lavas and hence could be derived by disaggregation and partial ingestion of cumulates. The arguments presented by Barton et al. (1982) against this possibility in the case of pyroxene xenocrysts in the lavas of Vulsini are also applicable to the hy-trachybasalts. There is no evidence, in the form of partially disaggregated xenoliths, to support the assimilation hypothesis and, moreover, it is difficult to envisage how two compositionally distinct types of pyroxene and plagioclase phenocryst cores originate by disaggregation of xenoliths and why such cores are found only in the hy-trachybasalts. The occurrence of diopsidic and salitic cores in a wide variety of alkaline lavas (Brooks and Printzlau 1978; Barton and Van Bergen 1981) also mitigates against this hypothesis.

The occurrence of diopsidic and salitic pyroxene cores is also unlikely to reflect variation infH2o and / o f f 02 during crystallization (Frisch and Schmincke 1969), as discussed in some detail by Barton et al. (1982). In particular, there is no evidence that crystallization of magnetite brought about a change in the Fe 2 + and Fe 3 + content of the melt and thereby influenced the composition of the pyroxene, as appears to have occurred in the case of K-rich lavas from Muriah, Java (I.A. Nicholls, pers. comm. 1981).

The presence, in any lava, of two compositionally distinct generations of pyroxene and plagioclase - one of which is in equilibrium with the host magma - might be ascribed to polybaric fractionation (Fisk et al. 1982; Ven- huis and Barton 1985), but it is extremely improbable that this process can account for the presence of three generations of pyroxene and plagioclase, two of which are out

of equilibrium with the host magma, which is the case for the Patmos hy-trachybasalts. Indeed, the complex zoning patterns of the plagioclase and pyroxene phenocrysts provide strong evidence for the mixing of two magmas, one of which was relatively primitive and carried phenocrysts of diopsidic pyroxene and anorthitic plagioclase whereas the other was more evolved and carried phenocrysts of salitic pyroxene and more albitic plagioclase. During mixing, these phenocrysts may be resorbed, depending upon the proportions in which the magmas mix and upon the thermal curvature of the liquidus surface (Barton et al. 1982), since they are out of thermal and chemical equilibrium with the host magma. After mixing, pyroxene and plagioclase of intermediate composition precipitate from the hybrid magma as microphenocrysts or nucleate and grow as mantles around the xenocrysts (Fig. 9). This indicates that the melt was homogenized prior to eruption and it is noteworthy that the mantles do not occur on the fractured surfaces of crystals which were broken during eruption. The thin, extreme rims of the crystals and the groundmass crystals nucleated and grew during eruption.

The xenocrysts of biotite and sanidine in the hy-trachybasalts were presumably derived from the relatively evolved magma involved in the mixing event whereas the resorbed olivines were derived from the relatively primitive magma. The latter mineral was undoubtedly resorbed during or after the mixing event because the hybrid magma did not lie on the olivine saturation surface. The small, dusty-brown crystals of apatite probably also represent xenocrysts derived from the evolved magma since the P205 contents of the hy-trachybasalts are too low for these magmas to be saturated with apatite (cf. Green and Watson 1982). Some resorbtion of this phase would be expected to occur, therefore, during mixing.

Petrographic and mineral chemical data allow the end- members involved in the mixing process to be identified and an estimate of the proportions in which the end- members mixed to be made, Both the hy-trachyandesites and the Q-trachytes carry phenocrysts of salitic pyroxene, plagioclase, sanidine, biotite and apatite, but the compositions of the most sodic plagioclase cores and the sanidine xenocrysts indicate that hy-trachyandesite is the most likely evolved end-member. The occurrence of olivine and diopsidic pyroxene suggests that the mafic end-member was similar in composition to the most primitive analysed ne-trachybasalt, but it may have been slightly less evolved (i.e. higher M g/(Mg + ~ Fe z +)) in terms of major oxide chemistry since the diopsidic cores of pyroxene phenocrysts in the hy-trachybasalts are slightly more magnesian and contain more Cr than the phenocrysts in the ne-trachybasalts. Mixing proportions may be estimated from the compositions of the pyroxene and plagioclase phenocryst cores and mantles in the hy-trachybasalts and are 80-77% mafic end- member and 20-23% evolved end-member. In calculating these proportions we have assumed that only one cycle of mixing occurred, which is reasonable on the basis of the available data, and that no other process (e.g. assimilation) operated concurrently with mixing (see, however, below).

As emphasized above, and elsewhere (Barton et al. 1982), sufficient time elapsed between mixing and eruption to allow the hybrid magma to become completely homogenized. An approximate estimate of the length of time can be made because olivine phenocrysts became completely resorbed during residence in the hybrid magma. Thornber

~/~henocryst cores derived f roq ne-trschybasalt /

/ , I "qd.,iv ,rom hy / ] M a n t l e s . microoh no I I trachysndesite

4 0 / I " . " ' " ' m ~ " ~ " I I " /f Icrysts. precipitated by ] / /

Phenocryst cores

derived from ne-

trachybasalt

Mantles, microphenoc rysts

precipitated by hybrid magma

I Phenocryst cores derived from |

I hy-trachyandesite

A b Or

Fig. 9. Interpretation of compositional characteristics of plagioclase and pyroxene phenocrysts in the hy-trachybasalts in terms of magma mixing. Note especially the similarity to the diagrams presented by Barton et al. (1982) which illustrate effects of mixing on pyroxenes in the lavas of Vulsini

and Huebner (1982) and Donaldson (1984) have published data for the rate of dissolution of olivine in basaltic melts. Using these data it may be shown that the olivine phenocrysts in the hy-trachyandesites would be resorbed over a period of 8-375 h, depending upon the degree of superheat- ing of the hybrid magma and upon the extent of chemical disequilibrium between olivine and melt. Using data presented by Donaldson (op. cit.) for the rate of dissolution of plagioclase in basalt, we estimate that the resorbtion features observed in the hy-trachybasalt could have developed in 12-14 h. It is thus apparent that eruption occurred shortly after mixing and such data support models in which mixing triggers eruption (Sparks et al. 1977; Huppert and Sparks 1980).

Although the most convincing evidence for mixing is found in the hy-trachybasalts, it seems possible that other evolved rock-types represent hybrid magmas. Definitive mineral-chemical evidence is lacking, but the occasional An-rich plagioclase phenocryst cores, di-rich pyroxene phenocryst cores and the vesicle-rich inclusions in the trachyandesites are certainly suggestive of mixing.

Evidence for assimilation

Magma mixing can account for some of the unusual chemical characteristics of the hy-trachyandesites- the high P205 contents for example - but it cannot account for all of them. In terms of magma chemistry, mixing is a linear process (Brooks and Printzlau 1978) and a hybrid magma should thus be a linear combination of the end-members. This is clearly not so in the case of the hy-trachybasalts (Fig. 10) which are relatively enriched in K20 and FeO and relatively depleted in MgO. Such characteristics could conceivably reflect post-mixing fractional crystallization but we are reluctant to accept this possibility because analyses of microphenocrysts and mantles around the pyroxene

048 ' ' ' ' 5/3 ' ~ '

Hy-Trachyandesites & Q-Trachytes

Hy-Trachybasalts

Ne-Trachybasalts

J 518 ~ i , 613 , i , , 618

SiO 2 Wt.%

Fig. 10. A plot of K20 versus SiO= for the MVS lavas illustrating the effects of mixing (hybridism) between ne-trachybasalt and trachyandesite. Note that the elevated K20 contents of the hy-traehy- basalts are inconsistent with mixing alone (see also Fig. 8)

and plagioclase phenocrysts provide no evidence for post- mixing fractionation. Grove et al. (1982) have recently shown that magmas undergoing reaction with olivine may become enriched in FeO at approximately constant SiO2, but this does not explain the enrichment in K20 or the depletion in MgO shown by the hy-trachybasalts. It is thus necessary to invoke the operation of another process during evolution of the intermediate lavas and the most likely process is assimilation of crustal material. Since we have not found, to date, evidence for assimilation in the form of xenocrysts or xenoliths and hence cannot deduce the nature of the crustal end-member involved, it is impossible to predict the effects of assimilation with any certainty. However, it is known that, in general, assimilation will lead to an increase in K20 content and because of this the inability of the least-squares mixing calculations to fully describe K20 variation is believed to be highly significant. Further- more, K20 is likely to be enriched relative to Na20 during assimilation (Watson 1982) which may explain the unusual behavior of these oxides in the Patmos MVS and the poor

fits for both of these oxides in the least-squares mass balance calculations. Assimilation will normally be accompa- nied by fractional crystallization (Bowen 1928) and if the assimilated material is poor in MgO (as are most crustal rocks) this could lead to relative depletion in MgO as is observed in the hy-trachybasalts.

Simultaneous operation of assimilation and fractionation probably explains another unusual feature of the Pat- mos MVS. The transition from ne-normative to hy-norma- rive trachybasalts occurs at any early stage of magma evolution, corresponding to a DI of about 55. Many alkali-basalt - trachyte suites on oceanic islands are entirely ne-normative, for example those on Tristan da Cunha (Baker et al. 1964) and St. Helena (Baker 1969), whereas in the continental alkali-basalt - trachyte suite of Skye (Thompson et al. 1972) the transition occurs at a DI of about 72. The lavas of Ascension are similar to those of Patmos inasmuch as ne-normative mafic lavas are associated with hy-normative intermediate lavas (Harris 1983). In terms of the conventional basalt tetrahedron (Yoder and Tilley 1962), the transition from ne-normative to hy-normative mafic magmas is equivalent to crossing the plane ol-di-plag which approxi- mates a thermal divide at low pressures and therefore places severe constraints upon the path followed by crystallizing magmas. Presnall et al. (1978) have shown that the plane is, in fact, a complex curved surface which at 1 atm. is located mostly in the tholeiite volume of the basalt tetrahedron but partly (due to solid solution toward monticellite in olivine) in the alkali basalt volume. These workers have also shown that the tendency for clinopyroxene to become more aluminous as pressure increases, shifts the whole surface into the alkali basalt volume, and that the divide ceases to operate at a pressure of 3-5 kb. We show elsewhere (Wyers and Barton 1984; Barton and Wyers in prep.) that the Patmos MVS evolved at P = 2-3 kb, so that the crystallization trend of these lavas requires explanation.

We contend that assimilation combined with fractionation may alter the course of crystallization sufficiently that the thermal divide is breached at low pressures. Interaction with crustal material may not directly induce a trend towards silica enrichment if a thermal barrier is operative between the assimilant and the magma (O'Hara 1980), but assimilation could lead to enrichment of the magma in oxygen (cf. Osborn 1959) and other volatiles (especially HzO) which would promote crystallization of minerals which significantly alter the course of crystallization. A number of workers (e.g. Yoder and Tilley 1962; Presnall et al. 1978) have suggested that crystallization of spinel allows magmas to breach the olivine-gabbro divide and this mechanism is of particular interest as the mafic Patmos lavas contain phenocrysts and microphenocrysts of Ti-magnetite. Osborn (1959) showed that early crystallization of magnetite is fa- vored by high f O 2 (cf. Wyers and Barton 1984) and will lead to residual liquids enriched in silica. It is noteworthy that Thompson et al. (1972) concluded that the transition from he-normative to hy-normative lavas on Skye reflects crystallization of abundant magnetite in the benmoreites.

However, comparison with alkali-basalts from oceanic islands suggests that other factors must be involved. For example, early crystallization of magnetite in the basalts of Tristan da Cuhna (Baker et al. 1964) and St. Helena (Baker 1969) has not led to the development of by-normative derivative magmas. The results of the major oxide mass balance calculations for the Patmos MVS indicate that crys-

tallization of mineral assemblages involving two silica-flee phases (Ti-magnetite and apatite) in the ne-trachybasalts can yield hy-normative residual liquids and it seems significant that these two phases have been identified in the lavas of both Skye and Ascension. HzO and/or F, which are required to stabilize apatite could be added to the magma during assimilation, and it may be noted that Thompson et al. (1983) have recently argued that the Skye basalts have been contaminated by continental crust.

It seems likely, therefore, that apatite plays a crucial role in the derivation of hy-normative liquids from ne-normative parental magmas at low pressures and that the volatiles necessary to stabilize magnetite and apatite, are, in at least some cases, enriched in the magmas by interaction with the continental crust.

Conclusions

1. Fractional crystallization can account for much of the compositional variation displayed by the MVS but cannot account for the alkali oxide and MgO contents of the hy-trachybasalts.

2. Petrographic and mineral chemical data provide conclu- sive evidence that the hy-trachybasalts are hybrid magmas formed by mixing between ne-trachybasalts and trachyandesites. The mixing event preceded eruption by a very short time period - less than 375 h - during which olivine xenocrysts were resorbed and mantles grew around pyroxene and plagioclase xenocryst cores. The hybrid magmas were completely homogenized during or after the mixing event.

3. Mixing combined with fractionation cannot account for the low MgO contents of the hy-trachybasalts which indicates that assimilation also occurred. Assimilation also accounts in part for the large increase in K20/Na20 which accompanies evolution of the MVS.

4. Assimilation, combined with fractionation, caused the transition from ne-normative to hy-normative compositions at any early stage of evolution. In particular, volatiles (Hi0 , F, O2, etc.) may be added to the magmas and induce crystallization of F e - Ti oxides and apatite (in some cases, perhaps, amphibole) which can significantly affect the liquid line of descent. We envisage the following scenario for emplacement

and eruption of the MVS magmas (see Fig. 11). Emplace- ment of primitive trachybasalt into shallow magma chambers (Wyers and Barton 1984) where fractional crystallization occurs to produce more evolved magmas. During fractionation, repeated influx of hot primitive magma leads to vigorous convection which facilitates assimilation (Van Bergen and Barton 1984) and triggers eruption. Eventually, mixing of primitive and evolved magma yields the hy-trachybasalts. Following eruption of the latter, the ne-trachybasalts are erupted either because al evolved magmas have been expelled from the chambers or because they followed a different plumbing system and were not intercepted by the evolved products in the chambers. Throughout much of the volcanic history, the shallow chambers thus acted as a density filter, preventing eruption of ne-trachybasalt and the magma in these chambers was compositionally stratified.

The Patmos MVS thus experienced a complex evolutionary history involving fractionation, assimilation and mixing and may serve as an example for the evolution of

~ - - Some mixing of Q-

, i TM chyte and Trachyandesite

convee l ion Underplating by hot ne- l !7;fZ,

hy Trachyandesite

4 (' 2-:.',)

t - Disaggregat~on of wall-rock; assimilmion

lit Fig. 11, Schematic diagram showing the probable sequence of emplacement and eruption of the MVS magmas. 1 Ne trachybasalt is emplaced into a relatively shallow magma chamber and differentiates to produce trachyandesite and trachyte, probably in a zoned chamber. 2 Underplating by hot, primitive ne- trachybasalt instigates vigorous convection in the chamber and induces partial digestion of wall-rock material. Some mixing of trachyandesite and Q-trachyte may occur. 3 Mixing occurs between trachyandesite and ne-trachybasalt. 4 Eruption of hybrid hy- trachybasalt. 5 Eruption of ne-trachybasalt, probably along graben faults but possibly also from original vents

alkaline and arc magmas in other regions i.e. the alkaline series of Central Italy (Barton et al. 1982).

Acknowledgements. The analytical work upon which this paper is based was done largely at the State University of Utrecht, The Netherlands, and we wish to thank R.D. Schuiling in particular for making the facilities available. J. van der Wal and P. Anten helped with the whole-rock analyses and it is appropriate to acknowledge the efforts made by J.P.P. Huijsmans to improve the quality of whole-rock analyses at Utrecht. The microprobes in Utrecht and Amsterdam receive financial and personnel support from ZWO-WACOM. We also wish to thank H.S. Pietersen for assistance in the field and in the laboratory, W.J. Lustenhouwer and C. Kieft for reconnaissance microprobe analyses, and M.J. van Bergen for maintaining the microprobe in Utrecht. We are especially grateful to IGME, Athens, for permission to do field- work on Patmos and to the State University of Utrecht and the Stichting Molengraaff Fonds for financial support for field work. Finally, M.B. wishes to acknowledge Hatton S. Yoder, Jr. and Tim L. Grove whose perceptive comments at or after the 1984 Fall AGU meeting prompted him to critically re-examine the evidence for fractionation, mixing and assimilation. However, we accept full responsibility for the views expressed in this paper.

References

Baker I (1969) Petrology of the volcanic rocks of Saint Helena Island, South Atlantic. Geol Soc Am Bull 80:1283-1310

Baker PE, Gass IG, Harris PG, Le Maitre RW (1964) The volcano- logical report of the Royal Society expedition to Tristan da Cunha. Phil Trans Roy Soc Lond Ser A 256:439-578

Barton M, Huijsmans JPP (1986) Post-caldera lavas from the San- torini volcanic complex, Aegean Sea, Greece: an example of the eruption of lavas of near constant composition over a 2,200 year period. Contrib Mineral Petrol (in press)

Barton M, Salters VJM, Huijsmans JPP (1983) Sr-isotope and trace

element evidence for the role of continental crust in calc-alkaline volcanism, Santorini and Milos, Aegean Sea, Greece. Earth Planet Sci Lett 63:273-291

Barton M, Van Bergen MJ (1981) Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming. Contrib Mineral Petrol 77:101-114

Barton M, Varekamp JC, Van Bergen MJ (1982) Complex zoning of elinopyroxenes in the lavas of Vulsini, Latium, Italy: Evi- dence for magma mixing. J Volc Geotherm Res 14:361-388

Bowen NL (1928) Evolution of the igenous rocks. University Press, Princeton

Boyd FR, Nixon PH (1975) Origins of the ultramafic nodules from some kimberlites of northern Lesotho and the Monastery mine, South Africa. Phys Chem Earth 9:431-454

Brooks CK (1976) The Fe203/FeO ratio of basalt analyses: an appeal for a standardized procedure. Bull Geol Soc Denmark 25:117-120

Brooks CK, Printzlau I (1978) Magma mixing in marie alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions. J Volc Geotherm Res 4:315-331

Bryan WB, Finger LW, Chayes F (1969) Estimating proportions in petrographic mixing calculations by least-squares approxi- mation. Science 163:926-927

Donaldson CH (1984) Crystal dissolution rates in a basaltic melt. In: Progress in experimental petrology. NERC, London, pp 175-178

Dungan MA, Rhodes JM (1978) Residual glasses and melt inclusions in basalts from DSDP Legs 45 and 46: Evidence for magma mixing. Contrib Mineral Petrol 67 : 417-431

Eichelberger JC, Gooley R (1977) Evolution of silicic magma chambers and their relationship to basaltic volcanism. Geophys Monogr Am Geophys Union 20: 57-77

Fisk MR, Bence AE, Schilling J-G (1982) Major element chemistry of Galapagos Rift Zone magmas and their phenocrysts, Earth Planet Sci Lett 61:171-189

Frisch T, Schmineke H-U (1969) Petrology of clinopyroxene-amphibole inclusions from the Roque Nublo volcanics, Gran Can- aria, Canary Islands. Bull Voleanol 33:1073-1088

Fytikas MD, Giuliani O, Innocenti F, Marinelli G, Mazzuoli R (1976) Geochronological data on recent magmatism of the Ae- gean Sea. Tectonophysics 31:29-34

Green TH, Watson EB (1982) Crystallization of apatite in natural magmas under high pressure, hydrous conditions, with particular reference to the "orogenic" rock series. Contrib Mineral Petrol 79 : 96105

Grove TL, Gerlach DC, Sando TW (1982) Origin of calc-alkaline series at Medicine Lake volcano by fractionation, assimilation and mixing. Contrib Mineral Petrol 80:160-182

Harris C (1983) The petrology of lavas and associated plutonic inclusions of Ascension Island. J Petrol 24:424-470

Huijsmans JPP, Barton M, Salters VJM (1983) Evidence for multi- ple processes in the genesis of calc-alkaline lavas from Santor- ini, Greece. EOS Trans Am Geophys Union 64:45

Huijsmans JPP, Barton M, Salters VJM (1986) Tectonic setting, volcanic stratigraphy and geochemistry of calc-alkaline volcanism on Santorini, Aegean Sea, Greece. J Volc Geotherm Res (in press)

Huppert HE, Sparks RSJ (1980) The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma. Contrib Mineral Petrol 75:279-289

Johnson RW, Wallace DA, Ellis DJ (1976) Feldspathoid-bearing potassic rocks and associated types from volcanic islands off the coast of New Ireland, Papua, New Guinea: a preliminary account of geology and petrology. In: Johnson RW (ed) Volca- nism in Australia, Elsevier, New York, pp 29%316

Kuo L-C, Kirkpatrick RJ (1982) Pre-eruption history of phyric basalts from DSDP legs 45 and 46: evidence from morphology and zoning patterns in plagioclase. Contrib Mineral Petrol 79 : 13-27

LeMaitre RW (1982) Numerical petrology: Statistical interpretation of geochemical data. Elsevier, Amsterdam

LeRoex AP, Erlank AJ (1982) Quantitative evaluation of fractional crystallization in Bouvet Island lavas. J Vole Geotherm Res 13 : 309-338

Luhr JE, Carmichael ISE (1980) The Colima volcanic complex, Mexico. I. Post-caldera andesites from Volcan Colima. Contrib Mineral Petrol 71 : 343-372

O'Hara MJ (1977) Geochemical evolution during fractional crystallization of a periodically refilled magma chamber. Nature 266 : 503-507

O'Hara MJ (1980) Non-linear nature of the unavoidable long-lived isotopic, trace and major element contamination of a develop- ing magma chamber. Phil Trans Roy Soc Lond Ser A 297 : 215-227

O'Hara MJ, Matthews RE (1981) Geochemical evolution in an advancing, periodically replenished, periodically tapped, con- tinuously fractionated magma chamber. J Geol Soc London 138 : 237-277

Osborn EF (1959) Role of oxygen pressure in the crystallization and differentiation of basaltic magma. Am J Sci 257 : 609-647

Presnall DC, Dixon SA, Dixon JR, O'Donnell TH, Brenner NL, Schrock RL, Dycus DW (1978) Liquidus phase relations on

the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma. Contrib Mineral Petrol 66:203-220

Reid AM, Donaldson CH, Brown RW, Ridley WL, Dawson JB (1975) Mineral chemistry of periodotite xenoliths from the La- traine Volcano, Tanzania. Phys Chena Earth 9: 525-543

Rhodes JM, Dungan MA, Blanchard DP, Long PE (1979) Magma mixing at mid-ocean ridges: evidence from basalts drilled near 22 ~ N on the Mid-Atlantic ridge. Teetonophysics 55:35-61

Robert U (1973) Les roches volcanique de l'ile de Patmos (Dode- canese, Grece). These 3 ~ Cycle, Laboratoire de Petrographic, Universit6 Paris-6

Robert U, Cantagrel JM (1977) Le volcanisme basaltique dans le Sud-Est de la Mer Egee. Donnees geochronologiques et relations avec la tectonique. Sixth Coll on the Geology of the Ae- gean Region, Athens, pp 961-967

Shand SJ (1922) The problem of the alkaline rocks. Proc Geol Soc S Africa 25 : 19

Sparks RSJ, Pinkerton H, MacDonald R (1977) Transport of xenoliths in magmas. Earth Planet Sci Lett 35 : 234-238

Thompson RN, Esson J, Dunham AC (1972) Major element chemical variation in the Eocene lavas of the Isle of Skye, Scotland. J Petrol 13:21%253

Thompson RN, Morrison MA, Dickin AP, Hendry GL (1983) Continental flood basalts ...arachnids rule OK? In: Hawkes- worth C J, Norry MJ (eds) Continental basalts and mantle xenoliths. Shiva, Nantwich (UK), pp 158-186

Thornber CR, Huebner JS (1982) Dissolution rates of olivine in basaltic liquids. EOS Trans Am Geophys Union 63:452-453

Thornton CP, Turtle OF (1960) Chemistry of igneous rocks. I. Differentiation Index. Am J Sci 258 : 664-684

Van Bergen MJ, Barton M (1984) Complex interaction of aluminous metasedimentary xenoliths and siliceous magma: an example from Mt. Amiata (Central Italy). Contrib Mineral Petrol 86:374-385

Van Bergen MJ, Ghezzo C, Ricci CA (1983) Minette inclusions in the rhyodacitic lavas of Mt. Amiata (Central Italy) : mineral- ogical and chemical evidence of mixing between Tuscan and Roman type magmas. J Volc Geotherm Res 19:1-35

Venhuis G-J, Barton M (1985) Major oxide chemistry of Precam- brian dolerite dikes of tholeiitic affinity from Rogaland, south- western Norway. Norsk Geol Tidssk (in press)

Watson EB (1982) Basalt contamination by continental crust: some experiments and models. Contrib Mineral Petrol 80:73-87

Wyers GP, Barton M (1984) Transitional alkaline - subalkaline lavas from Patmos, Greece: an example of a magma series formed by fractional crystallization, assimilation and magma mixing. Trans Am Geophys Union 65:1123

Yoder HS Jr, Tilley CE (1962) Origin of basalt magmas: an experimental study of natural and synthetic rock systems. J Petrol 3 : 34~532

Received October 15, 1985 / Accepted March 14, 1986

Petrology and evolution of transitional alkaline — sub alkaline lavas from Patmos, Dodecanesos,...

Documents

PETROLOGIC EVOLUTION OF BONINITE LAVAS FROM THE …

Patmos PatmosTimes Guide 2015

Patmos Herbst 2010

Ilkka O. Lavas yrittäjätarina ja urapolku

Petrology, Geochemistry and Geochronology of Kaua‘i Lavas ...€¦ · Shield lavas and associated fragmental deposits com-prise 495vol. % of Kaua‘i. The subaerial shield lavas

PATMOS: JOHN'S VISION OF THE EXALTED CHRIST

SEVEN CHURCHES OF REVELATION & GREEK ISLAND PATMOS

Empreendimento PATMOS LIFESTYLE RESIDENCE

John of Patmos author of Revelations - Imune of Patmos...John of Patmos (also called John the Revelator, John the Divine or John the Theologian ; Greek: Ἰωάννης ὁ Θεολόγος)

Cruising the waters around patmos

Sub Aqueous Lavas Class

PATMOS: JOHN’S VISION OF THE EXALTED CHRIST · PATMOS: JOHN’S VISION OF THE EXALTED CHRIST Situated in the sapphire waters of the Aegean Sea, the desolate island of Patmos was

A HISTÓRIA DO PROFETA DE PATMOS - WordPress.com

SEER OF PATMOS

Patmos Verlagsprogramm Frühjahr 2010

de pentecostes a patmos

Alkaline and Alkaline Earth Metals

Untangling differentiation in arc lavas: Constraints from ...searg.rhul.ac.uk/pubs/handley_etal_2010 Salak volcano Java.pdf · Untangling differentiation in arc lavas: Constraints

Las lavas de El Pedregal...químžcos de las lavas del Pedregal En términos genet-ales, fas lavas Clef Pedregal se hao argalizado desde panto de vista guímicõ. 91 sj02 TABLA ANAL

The PATMOS Workshop Series Reiner Hartenstein TU Kaiserslautern IEEE fellow FPL fellow SDPS fellow PATMOS 2015, the 25 th International