Petrogenetic implications of magmatic garnet in granitic pegmatites from southern Norway

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The Canadian Mineralogist Vol. 50, pp. 1095-1115 (2012) DOI : 10.3749/canmin.50.4.1095

PETROGENETIC IMPLICATIONS OF MAGMATIC GARNET IN GRANITIC PEGMATITES FROM SOUTHERN NORWAY

Axel MÜlleR§

Geological Survey of Norway, Postboks 6315 Sluppen, NO–7491 Trondheim, Norway, and Natural History Museum, Department of Earth Sciences, Cromwell Road, London SW7 5BD, UK

Anton KeARSleY, John SPRAtt And ReiMAR SeltMAnn

Natural History Museum, Department of Earth Sciences, Cromwell Road, London SW7 5BD, UK

AbStRAct

The magmatic garnet of seven granitic NYF (niobium–yttrium–fluorine-enriched) granitic pegmatites from the Froland and Evje–Iveland areas in southern Norway were studied with respect to their major- and trace-element composition and intracrystalline distribution of major and minor elements. The increasing average MnO/(FeO + MnO) values of the garnet grains investigated reflect the increasing fractionation of pegmatites from abyssal heavy REE to muscovite rare-element REE pegmatites. At a crystal scale, the MnO/(MnO + FeO) values show various trends controlled by coexisting Mn–Fe-consuming minerals. Back-scattered electron imaging revealed a great variety of structures, including large-scale (>100 mm) concentric growth-induced zoning, fine-scale oscillatory growth and replacement overgrowths. The structures predominantly correlate with the distribution of Y and HREE in the crystals. It is presumably the first time that such diversity has been reported from magmatic garnet originating from one area. The average Y and HREE concentrations are related to the bulk composition of the pegmatite-forming melt, whereas the intracrystalline zoning reflects the absence or presence of Y-bearing minerals or, in the case of the Slobrekka pegmatite, diffusion-controlled crystal growth. Sharp drops of the Y and HREE content record the abrupt change of “normal” peraluminous melt composition to a Na-rich aqueous silica-bearing fluid enriched in F, Rb, Cs, Ta, Mn in the case of the Solås and Hovåsen pegmatites. These Na-rich aqueous silica-bearing fluids are responsible for the formation of “amazonite”–“cleavelandite” replacement units. The regional implications are, first, that the Froland pegmatites are characterized by a shorter range of pegmatite fractionation. The parent melts seem to be more primitive with respect to the differentiation of a granitic magma compared to the Evje–Iveland pegmatites, as reflected by the elevated Ca content and the smaller negative Eu anomaly of garnets at Froland. Rough estimates of the bulk composition of the pegmatites could not reveal such a difference. Second, garnet of the Evje–Iveland pegmatites shows a wider range of chemical variability and patterns of Y–HREE zoning compared to the Froland samples. Thus, the gradients of pegmatite fractionation are much stronger within the Evje–Iveland field. The Evje–Iveland melts were probably more enriched in volatiles, being partially responsible for the crystallization of common rare-metal and REE minerals.

Keywords: garnet, granitic pegmatite, NYF type, LA–ICP–MS, Evje, Iveland, Froland, Norway.

§ E-mail address: [email protected]

intRoduction

Garnet is a common accessory and in some cases a minor mineral in peraluminous granitic pegmatites (e.g., London 2008). It is well known that its growth history, expressed by chemical zoning and inclusions of other minerals, is a useful indicator of fractionation trends in pegmatites and leads to a better insight into the evolution of pegmatite-forming melts (e.g., Černý et al. 1985). Reactions between garnet and coexisting minerals provide information about the chemical changes in the abundance of Fe, Mn, Y, the REE and Sc (e.g., Baldwin & von Knorring 1983, Nakano &

Nishiyama 1992, Whitworth 1992). On the other hand, garnet composition is also sensitive to temperature and pressure changes (Spear 1993, Menard & Spear 1993).

In general, changes in composition from the core to rim of garnet crystals are characterized by a continuous decrease or increase of components, and in some cases abrupt changes of components, with possible reversal trends and secondary overprints. Oscillatory zoning, which has been reported from open-system environ-ments (Jamtveit 1991, Jamtveit et al. 1993, Jamtveit & Andersen 1992) has been rarely documented for magmatic garnet (Galuskina et al. 2005, Hönig et al. 2010). But this might be because of the lack of recent

1096 the cAnAdiAn MineRAlogiSt

studies of magmatic garnet using advanced imaging methods.

In this study, magmatic garnet from peraluminous, F-poor NYF-type pegmatites from the Froland and Evje–Iveland pegmatite fields was investigated using imaging from a scanning electron microscope (SEM), electron-probe microanalysis (EPMA), and laser abla-tion – inductively coupled plasma – mass spectrometry (LA–ICP–MS). The two pegmatite fields are part of the south Norwegian pegmatite belt formed about 1 Ga ago during the Sveconorwegian orogenesis. Garnet crystals from primitive to most fractionated pegmatite subtypes were sampled from both fields to cover the local span of fractionation of the pegmatite-forming melt.

The aims of the study are (1) the examination of the overall evolution of the garnet composition in both fields and a comparison to reveal possible regional differences in pegmatite genesis, (2) the documenta-tion of the intracrystalline chemical zoning of major and trace elements in garnet to better understand the evolution of the melt during pegmatite crystalliza-tion. This approach has been applied because the bulk composition of pegmatites and the major- and minor-element composition of feldspars and micas are hardly distinctive in the pegmatite fields investigated (Larsen 2002, Müller et al. 2008). However, Larsen (2002) detected minor regional variations in the REE patterns of K-feldspar in both pegmatite fields.

geologicAl bAcKgRound

The garnet grains investigated are from peralumi-nous, F-poor NYF-type pegmatites of the Froland and Evje–Iveland pegmatite fields. Both fields are part of the south Norwegian pegmatite belt, which formed during the Sveconorwegian orogeny (1.14–0.90 Ga) at the western margin of the Fennoscandian shield, when the Bamble Complex was thrust over the Telemark Block along the Porsgrunn–Kristiansand Fault Zone (PKFZ) at 1.09–1.08 Ga (Bingen et al. 2008; Fig. 1). However, pegmatites of the two fields differ slightly in age, tectonometamorphic setting and accessory mineralogy.

The Froland pegmatites, including the Kleivmyr and Bjortjørn localities sampled, are situated above the PKFZ thrust zone in the Bamble Complex. The field is 20 km long and 5 km wide and strikes NE–SW parallel to the PKFZ. The precise age of the pegmatites in Froland is uncertain, but U/Pb dating of euxenite from the Gloserheia pegmatite near Arendal yields an age of 1060+8/–6 Ma (Baadsgaard et al. 1984). The Gloserheia pegmatite, which occurs ca. 15 km east of the Froland field, is considered to be of the same chemical and genetic type as the Froland pegmatites. The Froland pegmatites form large tabular bodies and dykes emplaced in an isoclinally folded sequence of steeply dipping and NNE–SSW-striking banded biotite–hornblende gneisses. The gneisses were affected

by upper-amphibolite-facies metamorphism, possibly transitional to the granulite facies, as indicated by orthopyroxene-bearing felsic gneisses (Cosca et al. 1998) during the Sveconorwegian deformation (e.g., Bingen et al. 2008). The emplacement of the pegmatites seems unrelated to adjacent granite plutons, namely the Herefoss pluton southwest of the field. The Froland pegmatites comprise simple abyssal (barren) HREE pegmatites in terms of the classification by Černý & Ercit (2005). The pegmatites have variable contents of quartz, alkali feldspar, plagioclase, biotite, and minor white mica; there are about 105 major granitic pegmatite bodies (Ihlen et al. 2001, 2002). Garnet, REE minerals and other striking accessories are rare. The mineralogy, the composition of minerals, the structures and the genesis of the pegmatites in these fields have been studied by Andersen (1926, 1931), Bjørlykke (1937), Ihlen et al. (2001, 2002), Larsen (2002), Larsen et al. (2004), Henderson & Ihlen (2004) and Müller et al. (2005, 2008). The Bjortjørn pegmatite represents the most fractionated, muscovite-rich pegmatite subtype in the Froland area, whereas the Kleivmyr locality is a typical primitive abyssal HREE Froland pegmatite. The Bjortjørn pegmatite is the only muscovite – rare-element REE pegmatite described from the area so far (Müller et al. 2005).

The Evje–Iveland pegmatites, including the sampled Steli, Li gruve, Slobrekka, Solås and Hovåsen, are located in the interior of the Telemark Block, in the footwall of the PKFZ (Fig. 1). The N–S-striking Evje–Iveland field is 30 km long and up to 10 km wide. A recent gadolinite U/Pb date from an unspecified pegmatite from the Evje field yielded an age of 910 ± 14 Ma (Scherer et al. 2001). The pegmatite field is spatially related to the Høvringsvatnet granite intrusion, which is at the northeastern margin of the field (Fig. 1); it has an emplacement age of 971+63/–34 Ma (Andersen et al. 2002). The genetic relationship between the Høvringsvatnet intrusion and pegmatite formation is still a matter of discussion. The Evje–Iveland field comprises ca. 350 major pegmatite bodies emplaced within banded amphibolite gneisses (1.31–1.27 Ga), the Iveland–Gautestad metanorites (1.26–1.29 Ga), and the Flåt metadiorite formed during the crustal formation of the Telemark block (Pedersen et al. 2009). According to Černý & Ercit (2005), the pegmatites are classified as rare-element REE and muscovite rare-element REE pegmatites consisting of K-feldspar, plagioclase, quartz, biotite and muscovite, minor magnetite and garnet, with variable contents of a range of accessory rare-metal and REE minerals, among which are beryl, allanite, monazite, euxenite, aeschynite, gadolinite, columbite. The pegmatites are zoned, consisting of a granitic wall zone, a megacrystic intermediate zone, and a core zone with crystal sizes of up to several meters. Some of the pegmatites exhibit late-pegmatitic (metasomatic), REE-depleted replacement units containing “cleavelandite”,

gARnet in gRAnitic PegMAtiteS fRoM SoutheRn noRwAY 1097

“amazonite”, topaz, garnet, beryl and tourmaline. The mineralogy of the pegmatites has been described in detail by Andersen (1931), Barth (1931, 1947) and Bjør-lykke (1934, 1937). More recent studies were carried out by Frigstad (1999), Larsen (2002) and Larsen et al. (2000, 2004). The investigated Steli and Li gruve pegmatites represent relatively primitive rare-element –

REE pegmatites (monazite and euxenite subtypes; Table 1) of the Evje–Iveland field. The Slobrekka pegmatite is a relatively highly fractionated pegmatite, very rich in accessory Y–REE minerals such as gadolinite-(Y), alla-nite-(Ce), aeschynite-(Y), and euxenite-(Y). The Solås and Hovåsen pegmatites represent the most fractionated muscovite rare-element – REE pegmatites in the area.

fig. 1. Geological map of Evje and Froland areas in southern Norway, with locations of the Froland and Evje–Iveland pegmatites sampled: 1. Kleivmyr, 2. Bjortjørn, 3. Steli, 4. Li gruve, 5. Slobrekka, 6. Solås, 7. Hovåsen.


The Solås pegmatite is famous for its “cleavelandite”–“amazonite” replacement units. The Hovåsen pegmatite seems to be more evolved than the Solås pegmatite, but replacement units are not exposed.

SAMPleS And AnAlYticAl MethodS

The investigated pegmatites are well exposed in vertical sections owing to historical and ongoing feld-spar mining and blasting related to mineral collector tourism. Between four and six representative samples of garnet were collected from each pegmatite. The origin of the garnet in relation to the pegmatite zoning is described in the following chapter. The size of the crystals collected, commonly embedded in feldspar and quartz, ranges from 0.5 to 20 mm. We prepared surface-polished, 300 mm thick sections fixed with epoxy on 4.8 3 2.4 mm glass slides.

Back-scattered electron imaging (BSE) and phase identification were carried out on a JEOL 5900LV SEM equipped with an Oxford Instruments INCA energy-dispersive X-ray spectrometer (EDX) system operated at 20 kV and 2 nA. The concentration of major, minor and some trace elements was determined in garnet from Froland and Evje–Iveland pegmatites using a Cameca SX100 electron microprobe at the Natural History Museum of London. Instrument beam conditions used were 20 kV and 20 nA with a 1 mm probe size. Standards used for all of the elements sought and detections limits are presented in Appendix 1, available from the Depository of Unpublished Data on the Mineralogical Association of Canada website [document Magmatic garnet CM50_1095]. Interelement interferences, particularly for the rare-earth elements, were corrected with the procedure given in Williams (1996). End-member components of the garnet crsytals were calculated according to Locock (2008). Analytical profiles on two to three crystals from each pegmatite were acquired, resulting in a total of approximately 500 EPMA analyses. The most representative results are provided and discussed.

Laser-ablation ICP–MS was used to determine the in situ concentrations of Y, REE, Th, U, Sc, V, and Cr in garnet. The analyses were performed on the double-focusing sector field mass spectrometer, model ELEMENT–1 from Finnigan MAT, which is combined with the excimer-based NewWave UP193FX laser probe. The 193 nm ArF laser had a repetition rate of 10 Hz, a spot size of 50 mm, and an energy fluence of about 11 mJ/cm2 on the sample surface. A raster of an area of approximately 150 3 100 mm was ablated in the standards and garnet grains. The approximate depth of ablation determined using an optical microscope was about 50 mm. The carrier gas for transport of the ablated material to the ICP–MS was He mixed with Ar. External calibration was performed using the four silicate glass reference materials, NIST SRM 610, 612, 614 and 616, the soda–lime float glass NIST SRM 1830, the

high-purity silica BCS 313/1 reference sample from the Bureau of Analysed Samples, UK, and the BHVO–2G basalt glass from the U.S. Geological Survey. Certified, recommended and proposed values for these reference materials were taken from the certificates of analysis where available, or otherwise from the web site Geolog-ical and Environmental Reference Materials (GeoReM 2011). The isotope 29Si was chosen as internal standard. Each isotope mass was scanned 15 times sequentially. From each pegmatite, one representative crystal of garnet was chosen for the LA–ICP–MS analysis based on the EPMA results. On each crystal, 12 LA–ICP–MS analyses were performed. All reference materials were measured at the beginning of the analytical sequence. In addition, NIST SRM 612 was monitored repeat-edly throughout the sequence after every 12th sample analyses to document instrumental drift. An Ar blank was run before each measurement of reference mate-rial and sample. The background signal was subtracted from the instrumental response of the reference material before normalization against the internal standard, to avoid memory effects between samples. The calculation of absolute element concentrations was performed by applying Excel worksheets macros. A weighted linear regression model, including several measurements of the different reference materials, was used to define the calibration curve for each element. Signal outlines caused by micro-inclusions of monazite, zircon, and gadolinite, among others, superimposed on the signal of the garnet, were separated manually. Ten sequential measurements on a “SiO2 blank” crystal were used to estimate the limits of detection (LOD), which were based on 33 standard deviation (3s) of the 10 measure-ments. Respective values of LOD are 0.01 ppm for Y, REE, Th and U, 0.03 ppm for Sc, and 0.04 ppm for V. The analytical error ranges within 10% of the absolute concentration of the element based on 20 measurements of the NIST SRM 612.

We analyzed the plagioclase in the pegmatite by X-ray fluorescence spectrometry (XRF) in order to determine the CaO content. The equipment used was a Phillips PW1480 spectrometer with a (Sc)W X-ray tube at the Geological Survey of Norway in Trondheim, Norway. About 0.5 kg of macroscopic homogeneous megacrysts were sampled and first crushed down to a grain size of 0.25 mm. Grains containing inclusions of foreign minerals were removed by careful hand-picking. However, microscopic mineral inclusions cannot be removed entirely by this procedure. The purified and washed sample material was milled down to ~40 mm. About 3 g of the powder was fused to prepare glass discs at 1030°C for 10 min. Loss on ignition (LOI) was determined gravimetrically, and it was used as an approximate measure of volatiles such as H2O and CO2. The limit of detection was 0.01 wt.% for CaO.

Bulk analyses of 20–40 kg samples of coarse-grained (0.5–3 cm) granitic rock from the wall zone of the zoned pegmatites were performed at ACME Analytical Labo-


TABLE 1. PETROGRAPHY, COMPONENT COMPOSITION AND INVENTORY OF INCLUSIONS IN THE GARNET INVESTIGATED_______________________________________________________________________________________________________________

pegmatite pegmatite garnet occurrence crystal habit average micro-inclusion common macroscopicname type within the and size composition inventory accessories(field) pegmatite of garnet Sps-Alm-Prp-Grs (alphabetical order)

(%) a b c

_______________________________________________________________________________________________________________

Kleivmyr abyssal HREE in batches of euhedral, core (n = 20): zircon, xenotime, magnetite(Froland) leucocratic aplite 0.5-20 mm 29-63-2-3 thorite

in the upper wall rim (n = 5):zone 29-62-2-4

Bjortjørn muscovite – accumulations euhedral to core (n = 21): zircon, xenotime, allanite-(Ce), monazite,(Froland) rare-element of crystals around subhedral, 48-37-4-4 uraninite, pyrite, samarskite-(Y),

REE the quartz core 0.5 to 12 mm rim (n = 3): thorite yttrotantalite-(Y)50-38-4-2

Steli rare-element – upper intermediate euhedral, core (n = 22): (Sc-bearing) allanite-(Ce), beryl,(Evje- REE monazite zone in western 0.5-50 mm 35-59-2-0 columbite, Nb-rich columbite, euxenite-(Y),Iveland) part of the rim (n = 6): rutile, rutile, Nb-rich rutile,

pegmatite 35-61-2-0 euxenite-(Y), magnetite, monazite,muscovite samarskite, xenotime,

zircon

Li gruva rare-element – layered granitic euhedral to core (n = 7): euxenite-(Y), euxenite-(Y), (Evje- REE euxenite pegmatite of the subhedral, 46-45-4-0 zircon, monaziteIveland) lower intermediate 1-8 mm rim (n = 21): gadolinite-(Y)

zone 46-47-3-0 in late cracksassociated withMs and Bt

Slobrekka rare-element – intermediate zone euhedral to (n = 30): xenotime, aeschynite-(Y),(Evje- REE gadolinite immediately above anhedral 56-36-1-0 gadolinite-(Y), allanite-(Ce), apatite,Iveland) the core zone occasionally Y-F-Ca alumino- bastnäsite-(Ce),

with dendritic silicates on columbite-(Fe),overgrowth secondary cracks euxenite-(Y),5-200 mm associated with Ms fergusonite-(Y),

gadolinite-(Y),ilmenite, magnetite,monazite, polycrase-(Y),xenotime-(Y), zircon

Solås muscovite (1) intermediate zone (1) euhedral to core (n = 6): core: gadolinite-(Y) wall, intermediate and(Evje- rare-element immediately above subhedral, 54-38-2-0 in micro veins inter- core zone: bastnäsite,Iveland) - REE allanite, the core zone 2 to 20 mm rim (n = 19): secting the core: beryl, columbite-(Fe),

“amazonite”- (2) “amazonite”- (2) euhedral to 58-38-2-0 yttrofluorite, microlite- euxenite-(Y), ilmenite,“cleavelandite” “cleavelandite” anhedral, group minerals, magnetite, monazite,replacement replacement unit 0.01 to 60 mm hellandite-(Y) samarskite, xenotime-units margin: yttrofluorite, (Y), zircon

microlite-group replacement unit:minerals, hellandite fluorite, microlite-group-(Y) minerals, topaz,

tourmaline, yttrotantalite-(Y)

Hovåsen muscovite intermediate zone euhedral to core (n = 19): monazite, allanite-(Ce), beryl,(Evje- rare-element - immediately above subhedral, 62-29-2-0 xenotime, columbite-(Fe),Iveland) REE Li-poor the core zone 0.5 to 20 mm rim (n = 6): zircon euclase, monazite

beryl-columbite 61-34-2-0_______________________________________________________________________________________________________________

The classification of pegmatites is according to Èerný & Ercit (2005). The discrimination of core and rim compositions was made by a b

means of significant differences in the Y content. n: number of analyses. c


ratories Ltd. in Vancouver, Canada, applying solution ICP–MS (AcmeLabs 2011; Group 4A4B analysis). The REE concentrations in the bulk samples were used for comparison with the garnet compositions under the assumption that the samples represent approximately the bulk composition of the pegmatites.

ReSultS

Garnet petrography

Garnet is the most common accessory mineral besides magnetite in all investigated pegmatites. Examples of the macroscopic appearance of garnet in the outcrops are shown in Figure 2. In the following, the

origin of samples in respect to pegmatite zoning, crystal size and habit, intracrystalline zoning visualized by BSE imaging and micro-inclusion inventory are described.

The Froland pegmatites

Garnet from Kleivmyr occurs in irregular batches of leucocratic aplite in the upper wall zone of the western part of the 150-m-long, ENE–WSW striking pegmatite lens. With regard to the large size of the pegmatite, garnet crystals are very rare. The euhedral, brown grains are 0.5 to 20 mm in size and have wide (>100 mm), weakly and smoothly contrasted concentric zoning in the BSE images (Fig. 3A). The crystals contain inclusions of euhedral, metamict zircon (3–60 mm)

fig. 2. Macroscopic appearance of garnet in the outcrops. A. Layered granitic pegmatite at the lower intermediate zone of the Li gruve pegmatite, Evje–Iveland. Layers enriched in euhedral crystals of garnet (small black isometric dots, e.g., black arrow) alternate with quartz- and feldspar-rich layers. The irregular dark grey patches are quartz, and the bright grey megacryst is K-feldspar (Kfs). The lower edge of the photograph corresponds to 1.2 m (see car key for scale at the left edge). B. Euhedral crystals of spessartine (Sps) intergrown with altered needles of allanite (Aln) in the intermediate zone immediately above the core zone of the Slobrekka pegmatite, Evje–Iveland. C. Subhedral spessartine with dendritic garnet–quartz overgrowth embedded in plagioclase (Pl), quartz (Qtz) and muscovite (Ms). Slobrekka pegmatite, Evje–Iveland. D. Irregular mass of orange spessartine (Sps) surrounded by beryl (Brl), “cleavelandite” (Clv) and quartz (Qtz) in the “amazonite”–“cleavelandite” replacement unit of the Solås pegmatite, Evje–Iveland.


commonly overgrown by xenotime (8–40 mm). The latter is partially altered and contains secondary thorite (0.2–3 mm). Zircon and xenotime are less altered within garnet crystals than in the host rock.

The garnet crystals from Bjortjørn, 0.5 to 12 mm in size, form accumulations of euhedral to subhedral, dark brown crystals around the massive quartz core of the 100 m long and 30 m wide zoned body of pegmatite. In the BSE images, the garnet is homogeneous; only the outermost margin appears slightly darker. Inclusions of euhedral to subhedral zircon 5 to 300 mm in size are very common, with needle-like crystal shapes. The zircon grains are strongly metamict, with inclusions of secondary thorite (0.2–2 mm). Less common are xeno-time (10–40 mm), uraninite (20–100 mm) and pyrite inclusions (20–80 mm; Fig. 3B).

The Evje–Iveland pegmatites

Garnet from Steli occurs as dark brown, perfectly shaped octahedral crystals up to 5 cm in size in the upper intermediate zone in the western part of the 90-m-long pegmatite lens. The locality is a well-known collecting site for high-quality specimens. The crystals seem homogeneous in BSE images. Micro-inclusions of Ta-poor, Sc-bearing columbite (5–40 mm), associated niobian rutile (10–60 mm), rutile (5–20 mm) and musco-vite (20–100 mm) are rare and occur only in the crystal cores. Some of the columbite inclusions are porous and marginally overgrown by euxenite-(Y) (Fig. 3C).

Layered granitic pegmatite in the lower intermediate zone hosts the garnet at Li gruve (Fig. 2A). The zoned pegmatite is more than 1 km long and up to 25 m thick; it is one of the largest pegmatites in the area. Euhedral to subhedral dark reddish brown crystals between 1 and 8 mm in size are enriched in layers (3–12 cm) alternating with quartz-rich and feldspar-rich layers. The garnet crystals do not show zoning in the BSE images. Needle-like inclusions of euxenite-(Y) (2–40 mm) are common besides zircon (20–100 mm). Gadolinite crystals (5–60 mm) occasionally intergrown with muscovite (20–150 mm) and biotite (30–120 mm) occur in open cracks and formed presumably after garnet crystallization (Fig. 3D). Macroscopic gadolinite had not been described from Li gruve.

The Slobrekka pegmatite, 250 m long and up to 12 m thick, contains dark reddish brown to almost black megacrystic garnet in the intermediate zone immedi-ately above the central core zone. The crystals form either euhedral to subhedral crystals up to 8 cm across or irregular massive crystals up to 20 cm in size. Garnet is commonly intergrown or close to allanite, mostly developed as decimeter-long needles (Fig. 2B). Some of the euhedral crystals grade into a dendritic, marginal quartz–garnet overgrowth (Fig. 2C). Our BSE imaging revealed fine-scale oscillatory growth-zoning (Fig. 3E). Inclusions are relatively rare and comprise xenotime

(10–80 mm), gadolinite (10–60 mm) and Y–F–Ca aluminosilicates (3–40 mm), possibly tritomite-(Y) or kuliokite-(Y). The xenotime inclusions are partially concentrically distributed. Large crsytals of xenotime (80–600 mm) occur at garnet margins. The Y–F–Ca aluminosilicates are associated with muscovite (20–250 mm) occur in open cracks and were presumably formed after garnet crystallization.

The sheet-like Solås pegmatite, up to 5 m thick, is the only locality investigated where garnet occurs in two adjacent zones: (1) in the intermediate zone immediately above the core zone and (2) in the “amazonite”–“cleavelandite” replacement unit. The euhedral to subhedral grains of garnet of the interme-diate zone (type-1 garnet) are dark red, 2 to 20 mm in size and occasionally intergrown with allanite similar to the garnet in the Slobrekka pegmatite. Garnet grains of the replacement units (type-2 garnet) are commonly orange; their size varies between 0.01 to 60 mm. They generally occur as irregular masses in the quartz and between “cleavelandite” blades (Fig. 2D). One of the investigated samples (Solås 1 + 2) originating from the contact of the intermediate zone and the replacement unit contains dark reddish brown, anhedral (dendritic-like) garnet with an orange brown overgrowth. The red dark core represents type-1 garnet, and the orange over-growth, the type-2 garnet of the replacement unit. In the BSE images, the core appears bright and porous and is marginally dissolved and overgrown by dark, unzoned garnet (Fig. 3F). The porous core contains numerous inclusions of gadolinite (3–60 mm), yttrofluorite (3–20 mm) and microlite-group minerals (5–200 mm), which occur in micro-veins of Y-poor garnet intersecting the Y-rich garnet cores (see below for further explanations; Figs. 3G, H). Macroscopic gadolinite has not been described from Solås. The second type of investigated samples (Solås 2) comprises brownish orange, anhedral garnet of type 2, which seems homogeneous in BSE images.

In the Hovåsen pegmatite, garnet has accumulated in the southern edge in the intermediate zone immedi-ately above of the core zone of the 100-m-long lens. The reddish brown, euhedral to subhedral crystals are 0.5 to 20 mm in size, intergrown with allanite and occasionally show a dendritic overgrowth. In the BSE images, the crystal core seems homogeneous with a marginal dark grey overgrowth. The micro-inclusions comprise relatively large euhedral grains of monazite and small euhedral grains of metamict zircon (5–120 mm). Anhedral inclusions of xenotime (10–60 mm) occur along open cracks. Columbite, which occurs as decimetric crystals in the pegmatite, was not found included in garnet.



the coMPoSition of gARnet

Major-element zoning

The garnet grains from Bjortjørn (Froland), Slobrekka (Evje–Iveland), Solås (Evje–Iveland), Hovåsen (Evje–Iveland) have a predominant spessartine component with a subordinate almandine component (Tables 1, 2; Appendix 2), which is typical of garnet in pegmatitic assemblages (e.g., London 2008). Note that Appendix 2 is available from the Depository of Unpub-lished Data on the Mineralogical Association of Canada website [document Magmatic garnet CM50_1095]. In garnet from Kleivmyr (Froland), Steli (Evje–Iveland) and Li gruve (Evje–Iveland), the almandine compo-nent slightly prevails over the spessartine component. The pyrope, grossular and andradite components are less than 5 mol.%. Other components containing Ti, Cr and V are mostly negligible, and concentrations of TiO2, Cr2O3 and V2O5 are <0.4, <0.02 and <0.02 wt.%, respectively. In a spessartine versus almandine plot, the garnet compositions follow a linear trend from Fe-rich garnet typical of less fractionated pegmatites (abyssal HREE and rare-element REE types) to Mn-rich garnet typical of more fractionated muscovite rare-metal REE pegmatites (Fig. 4). The trend of the Froland suite of garnet is slightly shifted to lower spessartine–almandine concentrations owing to the additional grossular compo-nent, 2 to 5 mol.%. The Evje–Iveland suite of garnet compositions have a negligible grossular component. Across individual crystals, the Mn/(Mn + Fe) value is either constant (Kleivmyr, Bjortjørn, Slobrekka), increases continuously from core to rim (Solås), or decreases continuously from core to rim (Li gruve, Hovåsen; Fig. 5). In the case of Steli, the value increases from the center of the garnet outward to about halfway across the radius, at which point it diminishes, reaching a minimum at the periphery.

In the relative Ca-rich (1.4–2.8 wt.% CaO) garnet from Froland, the Ca content either decreases continu-

ously from core (2.9 wt.%) to rim (1.5 wt.%; Bjortjørn) or increases from core (1.4 wt.%) to rim (2.2 wt.%), with a steep increase at the outermost margin (Kleivmyr; Figs. 5A, B). In the Ca-poor (≤0.7 wt.% CaO) garnet from Evje–Iveland, the Ca content tends to decrease slightly at the margins, from 0.6–0.7 to 0.3–0.5 wt.%. Concentrations of MgO vary between 0.1 and 1.0 wt.% and do not show any particular zoning or trends with other elements (Fig. 6A).

Trace-element zoning

The abundance and zoning patterns of Y deter-mined by EPMA and LA–ICP–MS are comparable for garnet crystals from one pegmatite, but highly variable among the deposits. Overall, the average Y content in garnet increases with increasing differentiation of the pegmatites and, thus, with the MnO/(MnO + FeO) value of garnet (Fig. 6B). The intracrystalline Y distribu-tion shows partially the opposite trend; the Y content decreases with progressing growth (Kleivmyr, Li gruve, Solås, Hovåsen; Fig. 5). In marginal growth-zones with MnO/(MnO + FeO) values greater than 0.55, the Y2O3 content might drop from >1.0 wt.% to <0.5 wt.% (Solås and Hovåsen; Figs. 5F, G). The intracrystalline pattern of Y zoning in the individual grains is described in the following paragraph. Other significant correlations between major and trace elements could not be detected except for a negative correlation between Y and Ca in the garnet from Kleivmyr.

The Kleivmyr garnet shows a more or less contin-uous decrease of the Y content from the core (0.062 apfu; 1.35 wt.% Y2O3) to the rim (0.006 apfu; 0.15 wt.% Y2O3; Fig. 5A). The Y content in Bjortjørn sample is relatively constant across the central part of the crystals. At the outermost margin, the Y content drops from about 0.078 apfu (1.7 wt.% Y2O3) down to 0.047 apfu (1.1 wt.% Y2O3; Fig. 5B). The Steli garnet has the lowest Y content of the samples investigated (mean 0.002 apfu; 0.05 wt.% Y2O3). Concentrations

fig. 3. Back-scattered electron images of garnet crystals and their mineral inclusions. A. Weakly zoned almandine from Kleivmyr, Froland. The black dots correspond to the analytical profile shown in Figure 5A. B. Composite inclusion of uraninite (Urn), thorite (Thr), zircon (Zrn), pyrite (Py) in spessartine of the Bjortjørn pegmatite, Froland. C. Assemblage of porous columbite (Col) with euxenite-(Y) (Eux) overgrowth and muscovite (Ms) in the almandine of the Steli pegmatite, Ivje–Iveland. The radiation damage caused by euxenite-(Y) resulted in the high porosity (dissolution) of the surrounding almandine. D. Euhedral zoned crystals of gadolinite (Gad) embedded in muscovite (Ms) and biotite (Bt) forming an inclusion in the spessartine of the Li gruve pegmatite, Evje–Iveland. E. Spessartine of the Slobrekka pegmatite with oscillatory growth-induced zoning. The black dots correspond to the analytical profile shown in Figure 5E. F. Spessartine from the Solås pegmatite showing a bright grey, Y-rich core (type 1) with a dark grey, Y-poor overgrowth (type 2). The black dots correspond to the analytical profile shown in Figure 5F, G. Edge of a spessartine crystal from Solås with a late homogeneous Y-poor overgrowth (type 2; dark grey) truncating the inner Y-rich parts of the crystal (type 1; bright grey). The highly porous Y-rich spessartine core is penetrated by a dense network of microveins and domains of Y-poor spessartine, which originate from the overgrowth. Inclusions of gadolinite, yttrofluorite, and microlite-group minerals occur within the microveins. The structures suggest that the older, Y-rich spessartine was replaced by younger Y-poor spessartine. H. Detail of the contact between the Y-rich core and the Y-poor margin in spessartine from Solås. The yttrofluorite inclusion (Y–Fl) occurs in a microvein of Y-poor spessartine intersecting the Y-rich spessartine.


are slightly enriched at the outermost margin (0.25 wt.% Y2O3). Similar to the Steli sample, the garnet from Li gruve has a low level of Y (mean 0.21 apfu;

wt.% Y2O3). The core is slightly enriched in Y (0.010 apfu; 0.68 wt.% Y2O3). The garnet from Slobrekka has the highest concentrations of Y (mean 0.102 apfu; 2.31

TABLE 2. AVERAGE COMPOSITIONS OF CORE AND RIM OF THE GARNET CRYSTALS_______________________________________________________________________________________________________________

Froland Evje-Iveland_________________________ ___________________________________________________________

Kleivmyr Bjortjørn Steli Li gruve Slobrekka Solås Hovåsentype 1 type 2

core rim core rim core rim core rim core rim core rimn 20 5 21 3 22 6 7 21 30 6 19 19 6_______________________________________________________________________________________________________________

2SiO wt.% 36.00 36.44 35.94 35.94 35.65 35.86 36.00 36.34 35.45 35.28 36.48 35.45 35.20

2TiO <0.02 <0.02 0.11 0.06 0.07 0.05 0.11 0.20 0.09 0.21 0.02 0.19 0.09

2 3Al O 20.52 20.49 19.80 19.83 20.27 20.35 20.26 20.28 20.32 20.17 20.56 19.97 20.22

2Na O 0.06 <0.03 0.09 0.07 <0.03 <0.03 0.03 <0.03 0.10 0.07 0.03 0.04 <0.03CaO 1.57 2.12 2.74 1.86 0.55 0.41 0.53 0.54 0.47 0.59 0.46 0.34 0.33FeO 27.95 28.08 17.78 17.98 26.47 26.98 21.77 22.65 17.62 17.70 17.66 15.05 16.16MnO 12.39 12.52 20.75 21.36 15.11 14.85 19.92 19.88 23.84 23.05 24.95 26.61 25.69MgO 0.51 0.47 0.91 0.93 0.58 0.56 0.89 0.68 0.29 0.46 0.37 0.59 0.56

2 3Sc O 0.04 0.01 <0.01 <0.01 0.01 <0.01 0.07 0.04 <0.01 0.10 0.01 0.05 0.01

2 3Y O 0.98 0.23 1.73 1.49 <0.03 0.20 0.53 0.12 2.31 1.95 0.59 1.15 0.06

2 3Dy O <0.07 <0.07 0.11 0.08 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07

2 3Er O 0.24 0.13 0.17 0.15 0.10 0.10 0.09 0.10 0.17 0.19 <0.05 0.08 0.07

2 3Yb O 0.26 0.08 0.15 0.12 <0.05 <0.05 0.05 <0.05 0.20 0.88 0.10 0.11 <0.05

Total 100.56 100.62 100.31 99.89 98.25 98.95 100.29 100.88 100.93 100.67 101.19 99.67 98.47

Si apfu 2.963 2.980 2.958 2.970 2.964 2.975 2.959 2.970 2.931 2.936 2.976 2.947 2.947Al 0.037 0.020 0.042 0.030 0.036 0.025 0.041 0.030 0.069 0.064 0.024 0.053 0.053IV

total Z 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

Al 1.954 1.955 1.878 1.902 1.944 1.965 1.921 1.922 1.911 1.915 1.953 1.904 1.940VI

Ti 0.001 0.001 0.007 0.004 0.005 0.003 0.007 0.012 0.005 0.013 0.001 0.012 0.005Sc 0.003 0.001 0.000 0.000 0.001 0.000 0.005 0.003 0.000 0.007 0.001 0.004 0.001Zr 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.000 0.000 0.000V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe 0.045 0.056 0.089 0.065 0.081 0.047 0.083 0.078 0.061 0.038 0.046 0.076 0.1003+

Fe 0.002 0.000 0.026 0.029 0.000 0.002 0.000 0.000 0.023 0.025 0.002 0.009 0.0002+

total Y 2.004 2.012 2.000 2.000 2.031 2.017 2.016 2.016 2.000 2.000 2.004 2.005 2.046

Fe 1.878 1.865 1.109 1.149 1.786 1.823 1.413 1.470 1.136 1.168 1.156 0.962 1.0312+

Mn 0.864 0.867 1.447 1.495 1.060 1.044 1.387 1.376 1.670 1.625 1.724 1.874 1.8192+

Mg 0.063 0.058 0.111 0.115 0.074 0.069 0.109 0.082 0.035 0.057 0.045 0.073 0.070Ca 0.138 0.185 0.242 0.165 0.049 0.036 0.047 0.047 0.041 0.052 0.040 0.030 0.030Na 0.010 0.003 0.015 0.011 0.000 0.002 0.005 0.003 0.016 0.012 0.005 0.006 0.001Y 0.043 0.010 0.076 0.065 0.000 0.009 0.023 0.005 0.102 0.086 0.026 0.051 0.003total X 2.996 2.988 3.000 3.000 2.969 2.983 2.984 2.984 3.000 3.000 2.996 2.995 2.954

“yttrogarnet” mol.% 1.2 0.3 1.4 1.0 0.0 0.3 0.8 0.2 2.3 2.1 0.7 1.5 0.1“schorlomite-Al” 0.0 0.0 0.0 0.0 0.2 0.1 0.3 0.6 0.0 0.0 0.0 0.2 0.3morimotoite 0.0 0.0 0.7 0.4 0.0 0.1 0.0 0.0 0.5 1.2 0.1 0.5 0.0Sc garnet 0.1 0.0 0.0 0.0 0.0 0.0 0.2 0.1 0.0 0.3 0.0 0.1 0.0spessartine 28.8 28.9 48.2 49.8 35.3 34.8 46.2 45.9 55.7 54.1 57.5 62.5 60.6pyrope 2.1 1.9 3.7 3.8 2.5 2.3 3.6 2.7 1.2 1.9 1.5 2.4 2.3almandine 62.6 62.2 37.0 38.3 59.3 60.7 45.4 47.3 36.4 37.6 37.9 28.8 33.9grossular 3.0 4.4 3.6 2.2 0.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0andradite 1.4 1.7 3.8 3.0 1.3 0.8 0.9 0.8 0.8 0.2 1.2 0.2 0.6“skiagite” 0.0 0.0 0.0 0.0 0.2 0.1 1.7 1.7 1.4 1.3 0.6 3.1 0.5_______________________________________________________________________________________________________________

These data were acquired with an electron microprobe. Elements below the detection limit (V, Cr, K, Zr, La, Ce, Pr, Nd, Sm, Gd) areexcluded from the list. The end-member components of the garnet were calculated according to Locock (2008). End-member componentsamounting to less than 0.1 mol.% are not shown. Cation proportions are calculated on the basis of 12 atoms per formula unit (apfu).


wt.% Y2O3) compared to the other samples (Fig. 6B). High concentrations of Y correlate with bright zones of the oscillatory growth-zoning evident in the BSE images (Fig. 3E). Concentrations vary among mm-wide growth-zones between 0.055 and 0.132 apfu (1.27–2.98 wt.% Y2O3). The highest detected Y2O3 values are 2.98 wt.% (EPMA) and 2.50 wt.% (LA–ICP–MS). The Solås sample is characterized by a Y-rich core (mean 0.086 apfu; 1.95 wt.% Y2O3) and a margin with a relatively low content of Y (0.026 apfu; 0.59 wt.% Y2O3; Fig. 5F). Within the margin, the Y content decreases smoothly toward the edge. At the core–margin boundary, Y drops from 0.088 apfu (1.99 wt.% Y2O3) to 0.034 apfu (0.77 wt.% Y2O3). Similar to the Solås garnet, the Hovåsen sample shows a strong drop in concentration, where the high Y content of the core (mean 0.051 apfu; 1.15 wt.% Y2O3) decreases to 0.003 apfu (0.06 wt.% Y2O3) at the margin (Fig. 5G). It is important to note that garnet from the most evolved pegmatites, Solås and Hovåsen, has a Y-rich core and a margin depleted in Y. In the case of Solås, the Y-poor margin forms not only an overgrowth;

it partially replaces the Y-rich core. Figures 3G and H show late overgrowths of Y-poor spessartine truncating inner Y-rich parts of the crystal, suggesting that the Y-rich core was replaced by the Y-poor overgrowth. In addition, a dense network of microveins and domains of Y-poor spessartine intersect and replace the Y-rich core.

Concentrations of Y, REE, Th, U, Sc, V, and Cr were determined by LA–ICP–MS (Table 3, Figs. 7, 8). Owing to the relative large laser-ablation raster (150 3 100 mm), element concentrations of garnet with growth-zones thinner than 200 mm are commonly integrated over several growth-zones. This is in particular the case of the fine-scale oscillatory-zoned Slobrekka garnet. However, large-scale zoning, >500 mm, can be spatially resolved during analysis as has been done, for example, for the Solås material. Laser spot-analyses were not applicable because of the relative long acquisition-time, about 30 s; also, a smaller spot (<50 mm) would results in higher detection-limits.

Garnet crystals and individual growth-zones with high Y are enriched in HREE as well, and SREE and

fig. 4. Spessartine (Sps) versus almandine (Alm) component plot illustrating the general trends (arrows) of increasing MnO/(MnO + FeO) values with increasing fractionation from primitive abyssal HREE (Froland) and rare-metal REE (RE–REE; Evje–Iveland) to muscovite rare-metal REE pegmatites (MSRE–REE). The inset show the triangular almandine (Alm) – spessartine (Sps) – grossular (Grs) diagram.


fig. 5. Profiles of the Ca and Y content in atoms per formula unit (apfu) and the Mn/(Mn + Fe2+

Y) value of garnet grains. The analyses were done with an electron microprobe. For explanation see text.


Y show positive, approximately linear correlations (Fig. 7A). The slopes of the linear trends, however, differ slightly between garnet samples. In garnet from Solås, Steli and Kleivmyr, the SREE/Y value is close to 1. Overall, the chondrite-normalized patterns are strongly HREE- enriched, with increasing normalized abundance from La to Lu. The LaN/YbN ratios are generally lower in the Froland garnet, indicating a stronger enrichment of HREE contents compared to the Evje–Iveland garnets (Fig. 7B). The (sub)parallelism of the HREE patterns of the bulk rock and garnet indicates that garnet mainly

controls the HREE content of the pegmatite except in the Kleivmyr pegmatite, where garnet is very rare (Fig. 8). Normalized LREE values are strongly depleted. All REE patterns show a distinct negative Eu anomaly due to preferential partitioning of Eu into plagioclase. The Eu anomaly is less distinct in the flat bulk-rock REE patterns, indicating that fractionation of some plagioclase occurred prior to garnet crystallization and that plagioclase continued to crystallize during garnet formation. The Eu anomaly is generally less pronounced in garnet in the Froland suite, as indicated by higher Eu/

fig. 6. Major- and minor-element plots of garnet compositionss. Concentrations were determined with an electron microprobe. The MnO/(MnO + FeO) values were calculated by using the absolute concentrations of MnO and FeO.


fig. 7. Trace-element plots of garnet compositions. Concentrations were determined by laser-ablation–ICP–MS. The LaN/YbN and Eu/Eu* (Eu/Eu* = EuN/[{SmN + GdN}/2]) values were calculated using chondrite-normalized values.

Eu* values (Eu/Eu* = EuN/[{SmN + GdN}/2]) compared to the Evje–Iveland suite (Fig. 7B). Garnet in the Steli suite has the lowest REE. The REE pattern is relatively consistent within the Kleivmyr, Bjortjørn and Slobrekka pegmatites (Figs. 8A, B, E). In the Li gruve, Steli and Solås suites, garnet exhibits a slight variation, particu-larly in the HREE, which is related to zoning (Figs. 8C, F). In the case of Solås, the marginal low-Y zones are depleted in both LREE and HREE compared to the Y-rich core. The Y–HREE-depleted margins of garnet in the Hovåsen suite could not be analyzed because of the narrow width of the zone, smaller than the applied ablation raster of the ICP–MS equipment.

Scandium concentrations vary between 13 and 1820 ppm. Garnet crystals from Li gruva and Hovåsen are relatively rich in Sc (mean >700 ppm), whereas at Bjortjørn and Slobrekka, the garnet is depleted in Sc (mean <40 ppm). A distinct enrichment of Sc is observed in the core of the Solås garnet (about 1000 ppm). All the garnet crystals studied contain a low level of U, in the range of 0 to 12 ppm. Material from Li gruva and Hovåsen has the highest U, about 2 ppm.

diScuSSion

The established trends and zoning patterns of the major elements Ca, Fe, Mn and the trace elements Y and


fig. 8. Chondrite-normalized REE patterns of garnet grains. The grey-shaded REE patterns result from bulk analyses of the coarse-grained wall zone of the pegmatite, representing roughly the bulk composition of the pegmatite at the time of emplacement.


HREE are discussed in the following in order to better understand the chemical changes in the pegmatite-forming melt during garnet crystallization and reactions between garnet and other coexisting minerals.

Zoning of the garnet in major elements Ca, Fe and Mn

The most distinct compositional difference between garnet in the Froland suite and that in the Evje–Iveland suite is the higher Ca content of the Froland samples. Examination of the Ca content and zoning in alman-dine reported by other investigators shows that the Ca content is not sensitive to variations of other major cations such as Fe, Mn and Mg in garnet (e.g., Chernoff & Carlson 1997). Spear (1993) and Menard & Spear (1993) showed that the composition of metamorphic garnet, namely the Ca, Fe, Mn and Mg content, is to some extent controlled by temperature and pressure changes. The pressure during pegmatite emplacement was ~6 kbar in Froland (Cosca et al. 1998) and 3–4 kbar in Evje–Iveland (Hansen et al. 1996). The crystal-lization temperatures were 535 ± 31°C for the Froland pegmatites and 582 ± 48°C for the Evje–Iveland pegma-tites (Müller & Ihlen 2012). According to Menard & Spear (1993), the higher pressure and lower temperature of crystallization could theoretically cause the crystal-lization of garnet with higher Ca in the Froland pegma-tites than in the Evje–Iveland pegmatites. However,

the major controlling factors of the composition of magmatic garnet are the melt composition (e.g., Černy et al. 1985) and coexisting minerals (e.g., Chernoff & Carlson 1997). The main Ca-bearing coexisting mineral in both pegmatite fields is plagioclase. The average CaO content of plagioclase from the investigated Evje–Iveland pegmatites is 2.03 wt.% (n = 4; XRF) and in Froland, 3.33 wt.% (n = 11; XRF) suggesting that the Froland pegmatite-forming melts were simply more enriched in Ca than the Evje–Iveland melts and, thus, more Ca was available to be incorporated in the garnet in the Froland system.

The observed overall fractionation-induced trend characterized by the increase in MnO/(MnO + FeO) of garnet from relative primitive abyssal HREE (Froland) and rare-element REE pegmatites (Evje–Iveland) to more fractionated muscovite rare-element REE pegma-tites, is similar in both pegmatite fields. The trend is typical of garnet in pegmatitic systems (e.g., Baldwin & von Knorring 1983, Černý et al. 1985, Solokov & Khlestov 1990) and for this to be the case, other mafic minerals in which Fe is far more compatible than Mn must control the MnO/(MnO + FeO) value of the melt (London et al. 2001).

In pegmatites of the lithium–cesium–tantalum type (i.e., LCT), the MnO/(MnO + FeO) value of garnet varies according to its position within the pegmatite body (Baldwin & von Knorring 1983). These authors noted that there is a distinct tendency for the late-formed

TABLE 3. REPRESENTATIVE RESULTS OF ANALYSES OF GARNETFOR THE REE AND RARE METALS

___________________________________________________________________________________

LOD Kleivmyr Bjortjørn Steli Li gruve Slobrekka Solås Hovåsencore core core rim core rim core rim core

type 1 type 2___________________________________________________________________________________

Y 0.01 8249 13010 156 2329 3032 1382 18142 21945 6068 11255La 0.01 0.01 0.01 0.08 0.05 0.31 0.04 0.06 0.23 0.09 0.06Ce 0.01 0.03 0.07 0.20 0.06 3.49 0.27 0.10 0.25 0.09 0.22Pr 0.01 0.01 0.12 0.12 0.02 3.98 0.49 0.08 0.06 0.05 0.31Nd 0.01 0.24 2.94 3.73 0.57 70.4 13 2.58 2.28 0.71 4.36Sm 0.01 1.8 37.7 26.3 10 155 62.9 21.2 17.3 7.9 46Eu 0.01 0.08 1.06 0.03 0.06 0.15 0.19 0.16 0.11 0.08 0.13Gd 0.01 37.1 376 49.4 87.9 260 145 239 186 80.1 225Tb 0.01 27.4 162 7.6 29.5 44.3 23.5 108 86.8 36.1 84.1Dy 0.01 519 1578 27.8 293.1 306 141 1299 1135 419 797Ho 0.01 290 283 1.9 63.1 57.3 23.5 329 445 125 152Er 0.01 1301 929 4.6 271.7 217 80 1305 3043 652 624Tm 0.01 341 155 0.8 52.5 44.3 16.4 231 1052 158 149Yb 0.01 2882 1065 5.4 533 408 138 1673 12835 1440 1556Lu 0.01 517 144 0.6 82.6 78.5 23.3 255 3364 267 236Th 0.01 0.00 0.02 0.08 0.00 0.48 0.00 0.00 0.17 0.08 0.09U 0.01 0.02 0.28 0.18 0.03 5.20 0.36 0.05 0.25 0.04 1.45Sc 0.03 308 43.1 246 116 539 443 63.6 1067 169 675V 0.04 1.80 4.28 27.7 5.07 10.2 6.3 3.75 5.60 4.95 1.33Sum LREE 2.1 40.8 30.5 10.7 233 76.6 24.0 20.1 8.8 51Sum HREE 5915 4694 98 1413 1416 591 5439 22146 3178 3824Eu/Eu* 0.015 0.017 0.003 0.004 0.002 0.006 0.004 0.004 0.006 0.003

N NLa /Yb (×10 ) 3 6 114 67 522 213 26 13 45 26–6

___________________________________________________________________________________

These data were acquired by laser-ablation ICP–MS. The concentrations of chromium are below thelimit of detection (<0.1 ppm). LOD: limit of detection.


garnet, in replacement units and zones surrounding the core, to have a higher MnO/(MnO + FeO) value. In our study, the garnet grains occur only in one particular zone of the pegmatites, representing a specific stage of pegmatite crystallization. The Solås pegmatite is the only locality where garnet occurs in two adjacent zones. The garnet of the intermediate zone (type-1 garnet; Y-rich core) has a lower MnO/(MnO + FeO) value than garnet of the replacement unit (type-2 garnet), confirming the findings of Baldwin & von Knorring (1983).

On the scale of a crystal, the MnO/(MnO + FeO) values show various trends (Fig. 5). Where garnet crystallizes from a melt containing both Mn2+ and Fe2+, the Mn is preferentially incorporated in the garnet (e.g., Feenstra & Engi 1998, London et al. 2001). Where garnet alone is responsible for the Fe and Mn contents of the melt, the melt becomes depleted in Mn relative to Fe, and the composition of garnet changes progres-sively from Mn-rich (core) to Fe-rich (rim), as is the case for the garnet from Steli, Li gruve and Hovåsen. Where Fe-compatible mafic minerals such as biotite, magnetite, allanite, columbite and black tourmaline coexist with garnet, the MnO/(MnO + FeO) value increases from core to rim, as observed in the case of Solås. Manganese-rich type-2 garnet from Solås is the only common mafic accessory in the replacement units other than some very rare black tourmaline. The residual melt of the replacement unit (see below) was strongly depleted in Fe and, thus, the decreasing MnO/(MnO + FeO) ratio in the garnet reflects the increasing absence of Fe in the pegmatite-forming melt. Frigstad (1999) detected up to 89% of the spessartine component in garnet of the replacement units in Evje–Iveland pegma-tites. In the case of the garnet from Kleivmyr, Bjortjørn and Slobrekka, the MnO/(MnO + FeO) value of the pegmatite-forming melt was constant during garnet growth in spite of the coexistence of Fe-compatible minerals.

Yttrium and HREE zoning

Yttrium and the HREE are highly compatible in garnet, particularly in Mn-rich garnet, owing to the substitution of Y3+(REE3+)Al3+ for Mn2+Si4+ (Jaffe 1951, Geller & Miller 1959). However, few studies have been made on the distribution of Y in magmatic garnet. Harrison (1988) reported an absence of zoning in the Y-rich garnet of the Cairngorm granite, but Whitworth & Feely (1994) reported appreciable amounts of Y in a garnet core from a pegmatite in Galway, Ireland. Smeds (1994) reported high concentrations of Y in garnet of moderately fractionated pegmatites and low Y in garnet of primitive and high fractionated pegmatites of the Falun area, Sweden. The pattern was interpreted as an initial increase of Y in the pegmatite-forming melts due to fractionation and, owing to the appearance of other

Y-bearing species, the depletion in the melt producing more fractionated pegmatites. Wang et al. (2003) detected the presence of a Y-rich core with a Y-poor margin in garnet of the Xihuashan granite intrusion, suggesting that the Y-poor margin crystallized during fluid-phase accumulation, which was unfavorable for the entrance of REE in the garnet structure.

The established overall increase in the average Y and HREE content of the garnet crystals investigated with increasing fractionation of the pegmatites is consistent with enrichment of Y and HREE during melt fraction-ation. Patterns of intracrystalline Y zoning are, however, manifold and show partially reversed trends because coexisting Y–HREE accessory phases may control the shape of the profile. Four types of intracrystalline Y zoning were generally detected:

1) The more or less continuous decrease of the Y content from core to rim (Kleivmyr and Bjortjørn; type A).

2) Garnet with low Y and barely significant varia-tions in Y (type B). This type includes the Steli sample, with slight Y enrichment at the crystal margin, and the Li gruve sample, with a slightly Y-enriched core. Both samples represent relatively less fractionated Evje–Iveland pegmatites.

3) Garnet with high average Y and fine-scale oscilla-tory zoning with a high amplitude of variation in content (Slobrekka; type C).

4) Garnet with a negative concentration shoulder separating a Y-rich core and a Y-poor margin (Solås, Hovåsen; type D).

Previous studies have shown that the presence, absence, growth and consumption of accessory phases enriched in Y (and HREE as well) play an important role in controlling the distribution of Y throughout the rock (e.g., Wark & Miller 1993, Förster 2010, Trumbull & Förster 2010). Xenotime is known to control largely the distribution of Y in granitic rocks where present; it can be considered a Y-saturation phase, which buffers the Y activity in the melt (Pyle & Spear 1999), despite the fact that Y (and HREE) are highly compatible in Mn-rich garnet. The common occurrence of xenotime in the Froland pegmatites and as micro-inclusions in garnet suggest that, first, the pegmatite-forming melts were relatively rich in Y and HREE and, second, the type-A zoning of yttrium on garnet at Kleivmyr and Bjortjørn was suppressed by the simultaneous crystal-lization of xenotime. With progressive crystallization of xenotime and garnet, the Y content of the remaining melt decreased and, thus, the Y content of the outer growth-zones of the garnet. An alternative explanation is that the growth rate of garnet exceeded the diffusion rate of Y.

In the case of the low-Y Evje–Iveland garnet (Steli and Li gruve; type B), presumably the low Y of these less fractionated pegmatite-forming melts was pref-erentially consumed by euxenite-(Y), which is the


most common primary inclusion in the garnet. Other Y-compatible accessories of these localities are mona-zite, zircon and columbite (Table 1). The gadolinite inclusions examined, together with muscovite and biotite along cracks in the Li gruve garnet, seem to have crystallized after garnet solidification.

The oscillatory zoning of the Slobrekka garnet (type C) seen in the BSE images is caused by the high and oscillating Y and HREE concentrations. The Slobrekka pegmatite is famous for its Y-rich mineral assemblage, from where up to 500-kg gadolinite-(Y) crystals and arm-thick allanite crystals more than 50 cm in length were reported (Revheim 2009). Other Y-rich minerals, such as xenotime-(Y), polycrase-(Y), euxenite-(Y), monazite, and zircon, are common as well (Table 1). Despite the large number and variety of Y-consuming and Y-buffering minerals, the average Y content of the Slobrekka garnet is one of the highest reported in this study (Fig. 6B) and among the highest reported from Evje–Iveland pegmatites (Frikstad 1999), indi-cating that the Slobrekka pegmatite-forming melt was significantly enriched in Y and HREE. Some of the LA–ICP–MS datasets acquired on garnet from the Solås and Bjortjørn bodies show similar or somewhat higher Y values compared to the Slobrekka garnet (Fig. 7A).

In hydrothermal garnet, oscillatory zoning is consid-ered to be related to open-system environments (Jamt-veit 1991, Jamtveit et al. 1993, Jamtveit & Andersen 1992). There are however, no structural or mineralogical indications that the Slobrekka pegmatite crystallized under open-system conditions. In addition, the concen-trations oscillate consistently around the mean value across the crystals, and the major-element composition remains constant. Therefore, we suggest that the fine-scale oscillatory zoning is related to self-organized diffusion-controlled crystal growth (e.g., Allègre et al. 1981). In this case, the Y and HREE content in garnet is controlled by the intercrystalline diffusion of these elements within the pegmatite-forming melt. Self-organization of growth is controlled by cyclic competition between the growth rate and the diffusion rate of garnet-forming components, including trace elements within the crystal–melt reaction zone and boundary layer. Saturation of Mn, Fe, Al and Si in the reaction zone increases the growth rate of garnet during the cooling of the melt. The increasing growth-rate results in the decrease of these elements if the growth rate exceeds the diffusion rate of Mn, Fe, Al and Si. Simultaneously, trace elements such as Y and HREE are accumulated in the reaction zone and boundary layer. The high growth-rate favors the incorporation of trace elements owing to the planar change of the cellular interfaces with rather high specific free energy (Allègre et al. 1981). The growth rate will slow down when garnet growth is so fast that Mn, Fe, Al and Si become depleted in the reaction zone and boundary layer. Consequently, the diffusion rate becomes the

dominant growth-controlling process. The growth rate starts to rise again as soon as the Mn, Fe, Al and Si in the reaction zone have been recovered. Physical or chemical changes in the bulk pegmatite-forming melt are not required to develop such fine-scale oscillatory zoning of trace elements superimposed on a consistent major-element composition.

The marginal drop in the concentration of Y and the HREE in the Solås and Hovåsen garnet (type D) is related to Mn-rich growth zones with MnO/(MnO + FeO) values greater than 0.55. This pattern is developed only in garnet of the most evolved MSRE–REE pegma-tites in Evje–Iveland. The abrupt change in the Y and HREE content seems to reflect a correspondingly abrupt change in the composition of the melt from which the garnet crystallized. The Solås garnet was collected from the intermediate zone immediately above the core zone, close to the “amazonite”–“cleavelandite” replacement unit. The replacement units of Evje–Iveland are inter-preted as highly fractionated, volatile-enriched residual melts representative of a specific chemical environment characterized by high Na, Rb, Cs, Ta, Mn and F, and lower K, Fe, Nb, Ti, Y and REE compared to the parent peraluminous pegmatite-forming melt (Frigstad 1999). These units comprise less than 1 vol.% of the pegmatites in which they occur. Owing to the enrichment of Na, H2O, other fluxes and lithophile elements, these residual melts interacted with and replaced crystallized parts of the pegmatite, a process that could be called “auto-metasomatism” of the pegmatite. These melts probably do not resemble a normal silicate melt, but rather a transitional aqueous silicate fluid such as that found within fluid inclusions in pegmatites (e.g., London 1986, Thomas et al. 2006). As mentioned above, “amazonite”–“cleavelandite” replacement units are not exposed at Hovåsen, but there are mineralogical indi-cations that replacement units are hidden in unexposed parts of the pegmatite. Similar processes as discussed for the Solås pegmatite may have occurred at Hovåsen.

The drop in concentration of the Y and the HREE in the garnet is interpreted to reflect the chemical switch from peraluminous composition to an Na-rich aqueous silica-bearing fluid. The chemical change is also reflected in the micro-inclusion inventory of the Solås garnet; the core contains gadolinite-(Y) micro-inclusions, and the margin contains yttrofluorite and microlite-group minerals. The Y-rich core of the Solås garnet crystals was partially dissolved, indicating a temporary instability of garnet before the Y–HREE-poor, Mn-rich garnet overgrowth. The growth and stability of garnet at lower temperatures are mainly controlled by the Mn and Fe content of the melt and their diffusion (e.g., Baldwin & von Knorring 1983). The Na-rich aqueous silica-bearing fluid is strongly depleted in Fe but relatively rich in Mn. The decreasing availability of Fe in the replacement units might be responsible for the temporary instability of garnet.


In addition, masses of coexisting, finely crystalline Fe-rich tourmaline have been observed within the Solås replacement unit; it constitutes a Fe–(Mn) buffer (e.g., London 2008).

SuMMARY

An investigation of major- and trace-element abundances and their distribution in garnet crystals from a suite of various fractionated granitic pegma-tites from Froland and Evje–Iveland reveals a great variety in average garnet composition and patterns of intra crystalline zoning. This is probably the first study in which such a great variability is reported from a relatively limited area, in particular in the case of the Evje–Iveland pegmatite field. The major results of the study can be summarized as follows:

1) The average MnO/(FeO + MnO) value of garnet crystals is a sensitive indicator of the degree of fraction-ation of pegmatite-forming melts. The ratio increases with increasing fractionation of the melt. At a scale of a crystal, the MnO/(MnO + FeO) values may show various trends of minor changes controlled by coex-isting Mn–Fe-consuming minerals.

2) A great diversity of BSE-documented structures, including large-scale (>100 mm) concentric growth-induced zoning, fine-scale oscillatory growth-zoning and overgrown resorption-induced surfaces, has been detected. The structures are predominantly related to the distribution of Y and HREE in the crystals. It is presumably the first time that such diversity has been reported from magmatic garnet originating from one area. This however, may be due to a lack of studies of garnet crystallized in NYF-type pegmatites.

3) The Y and HREE concentrations are very sensi-tive to large scale (bulk pegmatite) and as well as small, local changes of melt composition and the assemblage of coexisting minerals. The average Y and HREE content of garnet is related to the bulk composition of the pegmatite-forming melt, whereas the intracrystalline zoning reflects the absence or presence of Y-bearing minerals or, in the case of Slobrekka, diffusion-controlled crystal growth. The drop in the concentration of the Y and the HREE records the abrupt change of a “normal” peraluminous melt composition to a Na-rich aqueous silica-bearing fluid enriched in F, Rb, Cs, Ta, Mn in the case of the Solås and Hovåsen samples. The chemical switch possibly reflects the separation of a residual immiscible melt at the final stage of pegmatite crystallization.

4) Overall, the micro-inclusion inventory of garnet reflects the macroscopic assemblage of accessory phases. Changes in the macroscopic assemblage during garnet crystallization are also recorded in the zonal arrangement of inclusions, as in the case of garnet in the Solås pegmatite. Gadolinite-(Y) inclusions in the

garnet of Solås and Li gruve, however, have not been reported at a macroscopic scale.

5) The Froland field is characterized by a shorter range of pegmatite fractionation, and the parent melts seem to be more primitive with respect to granitic magma differentiation, as reflected by the elevated Ca content and the smaller Eu anomaly of Froland garnet compared to that at Evje–Iveland. Rough estimates of the pegmatite bulk compositions could not reveal such differences.

6) Garnet crystals of the Evje–Iveland pegmatites show a wider range in chemical variability and patterns of Y–HREE zoning compared to the Froland samples. The gradients of pegmatite fractionation thus are much stronger within the Evje–Iveland field. The pegmatite-forming melts were probably more enriched in volatiles, as indicated by the broader reaction-induced halos in the host rocks (e.g., conversion of amphibolite-bearing host rocks to biotite-bearing assemblages) and the occur-rence of “amazonite”–“cleavelandite” replacement units. The volatile enrichment might be one reason why the Evje–Iveland pegmatites are much richer in rare-metal accessories than the Froland pegmatites.

Our study shows that garnet composition shows great potential for an improved understanding of the crystallization and fractionation of pegmatite-forming melts. More studies of garnet in pegmatites are neces-sary, however, to document the entire range of major- and trace-element profiles and their intracrystalline patterns of distribution.

AcKnowledgeMentS

The study was granted by the EU Synthesys GB–TAF–5516 project of the Natural History Museum of London. We are grateful to Chris Stanley for discussions and improvements of the manuscript. Critical comments and suggestions of the guest editor Milan Novák and the reviewers Adriana Heimann and Giulio Morteani improved the quality of the manuscript and are highly appreciated.

The paper is dedicated to Prof. Petr Černý. Over the years, we met Petr at numerous conferences and excur-sions, and he always impressed us with his unique expe-rience, approach, insight and passion about the subject of granitic pegmatites. His classical landmark papers about the Tanco pegmatite, the classification and genesis of granitic pegmatites improved our understanding tremendously, and inspired our work on pegmatites. For all those reasons, we are happy to thank Petr and to participate in this tribute.

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Received August 28, 2011, revised manuscript accepted August 2, 2012.

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Petrogenetic implications of magmatic garnet in granitic pegmatites from southern Norway