Quartz chemistry in polygeneration Sveconorwegian pegmatites ...

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Eur. J. Mineral. 2008, 20, 447–463 Published online June 2008

Paper presented at the symposium “Granitic Pagmatites: the State of the Art”, Porto, May 2007

Quartz chemistry in polygeneration Sveconorwegian pegmatites, Froland, Norway Axel MÜLLER1∗ , Peter M. IHLEN1 and Andreas KRONZ2 1

Geological Survey of Norway, 7491 Trondheim, Norway *Corresponding author, e-mail: [email protected] 2 Geowissenschaftliches Zentrum Göttingen, Goldschmidtstr. 1, 37073 Göttingen, Germany

Abstract: Element concentrations in quartz, feldspar and biotite of Sveconorwegian (1.13–0.9 Ga) granitic pegmatites in Froland, Norway, were analysed by LA-ICP-MS, EPMA and XRF, respectively, in order to determine chemical variations between different pegmatite types and within individual pegmatitic bodies. A refined classification of the syn-, late- and post-orogenic granitic pegmatites of Froland is presented basing on the pegmatite structure, bulk composition and mineral chemistry. Syn-orogenic pegmatites (1.13–1.06 Ga) are relative primitive with respect to granite differentiation. Late-orogenic pegmatites linked to the Herefoss pluton (0.93 Ga) have the most primitive composition and contain Fe phlogopite. Post-orogenic zinnwaldite pegmatites (< 0.93 Ga) are the most evolved. Pegmatitic quartz has an astonishingly consistent trace element signature between and within syn-orogenic pegmatites. Average concentrations are in the range of 6–10 μgg−1 for Li, 34–44 μgg−1 for Al, 4–8 μgg−1 for Ti, and 0.9–1.8 μgg−1 for Ge. Al, Li, Fe, Ge, and Ti in quartz of late- and post-orogenic and contact-metamorphosed syn-orogenic pegmatites are more variable. Micro-mylonitisation and contact metamorphism caused the lowering of Li and Al and the increase of Ti and Ge in pegmatitic quartz of some syn-orogenic granites. Several generations of secondary quartz replaced pegmatitic quartz at the micro scale (< 1 mm) during retrograde fluid-driven overprint. Secondary quartz is depleted in Al, Ti and Li compared to the host quartz. In contrast to quartz, the feldspar and biotite chemistry depends largely on the differentiation degree of the pegmatites and varies significantly within structurally-zoned pegmatite bodies. Feldspar and biotite chemistry reflects changes in melt composition within pegmatites, which includes a decrease of Mg and Sr and increase of Li, Rb, and Ba. The syn-orogenic pegmatites were formed during the crustal accretion on the western margin of Fennoscandia under constant PTX-conditions causing the homogeneous trace element signature of quartz. Key-words: pegmatite, quartz, LA-ICP-MS, cathodoluminescence, Froland, trace elements.

1. Introduction The pegmatites in southern Norway have attracted the attention of mineralogists for more than a century due to their contents of rare-metal minerals (e.g., Brøgger, 1906), many of them being discovered and described for the first time in the world (e.g., Schetelig, 1922). However, few attempts have been made to give details of the most common pegmatite minerals quartz, feldspar and mica. The chemistry of quartz in the Froland pegmatite field, that developed during the Sveconorwegian orogeny (1.13–0.9 Ga) will be the object of this study. The 20 km NE-SW striking and 5-km wide Froland field is in the centre of the south Norwegian pegmatite cluster is framed by the fields of GlamslandLillesand, Evje-Iveland, Arendal, and Kragerø (Fig. 1a, b). The mineralogy, mineral chemistry, geochronology, structures and genesis of the pegmatites in these fields have been studied by Andersen (1926, 1931), Bjørlykke (1937), Åmli (1975, 1977), Baadsgaard et al. (1984), Ihlen et al. (2001, 2002), Larsen (2002), Larsen et al. (2004), Henderson & DOI: 10.1127/0935-1221/2008/0020-1822

Ihlen (2004), and Müller et al. (2005). The Froland pegmatites comprise simple abyssal pegmatites with variable contents of quartz, alkali feldspar, plagioclase, biotite, and minor white mica forming about 105 major granitic pegmatite bodies (Ihlen et al., 2001, 2002). REE minerals and other striking accessories are rare and thus the Froland field has been of minor interest for mineralogists, but of large interest for the procelain and glass industry. 76 of the Froland pegmatites were mined for feldspar and/or quartz since the 19th century. Some pegmatites were mined for flaky mica and REE minerals. ˇ Pegmatites can display both regional (e.g., Cerný, 1992; Malló et al., 1995; London, 1996) and internal compositional zoning (e.g., Cameron et al., 1949; Norton, 1983; London et al., 1989; London, 1996; Roda-Robles et al., 2004) on the basis of characteristic minerals and paragenesis as well as the chemistry of minerals, such as feldspar, mica, tourmaline, garnet and accessory rare-metal minerals. However, the chemistry of quartz has not been considered until the beginning of the 21st century mainly 0935-1221/08/0020-1822 $ 7.65 c 2008 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 

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Fig. 1. a – Location of the pegmatite fields in southern Norway. b – Pegmatite occurrences and fields in southern Norway. c – Simplified geologic map of the Froland area showing the distribution of major and sampled pegmatite occurrences. Sample localities: 1 – Våtåskammen, 2 – Haukemyrliene, 3 – Lille Kleivmyr, 4 – Hellheia Middle, 5 – Hellheia North, 6 – Bjortjørn, 7 – Skåremyr, 8 – Sønnristjern, 9 – Løvland, 10 – Vaselona, 11 – Fossheia West, 12 – Husefjell, 13 – Heimdal, 14 – Fossheia East, 15 – Metveit.

due to analytical limitations. Due to the increasing economic interest for quartz as raw material for high-T glass moulds in the production of solar-grade silicon and other high-tech end uses, the study of pegmatite quartz deposits has been intensified over the last years (e.g., Ihlen et al., 2001, 2002; Larsen et al., 2004; Müller et al., 2005). This is promoted by recent developments of micro-beam techniques enabling the precise determination of trace element in quartz (Flem et al., 2002; Müller et al., 2003b). The aim of this study is to reveal possible variations in the quartz chemistry between different pegmatite types and within individual pegmatites in Froland. For that purpose 14 pegmatite localities were investigated representing different structural and compositional pegmatite types (Fig. 1c; Ihlen et al., 2001, 2002). Larger pegmatites (> 50 m) were sampled along several traverses across different compositional zones. Quartz was investigated by scanning electron microscope cathodoluminescence (SEM-CL) prior to trace element analyses in order to reveal different quartz generations (primary and secondary quartz) at micro-scale (0.001 to 10 mm). Trace elements in quartz (Li, Be, B, Ge, Na, Al, P, K, Ca, Ti, Fe) were analysed with laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Al, Ti, K and Fe of secondary quartz were analysed by electron probe micro analysis (EPMA) due to small volumes of secondary quartz. The chemical signature of quartz is compared with the composition of feldspar and biotite to detect chemical relationships between these co-genetic minerals. This study is a continuation of the work done by Larsen et al. (2004) who provided a general overview of the quartz and feldspar chemistry of the Evje and Froland pegmatite fields.

2. Geological setting The Froland pegmatite field is situated in the BambleLillesand block of southern Norway at the southwestern margin of the Fennoscandian shield (Andersen, 2005; Fig. 1a, b). The pegmatites form large tabular bodies and dykes emplaced in an isoclinally folded sequence of steeply dipping and NNE-SSW striking banded biotite-hornblende gneisses of volcano-sedimentary origin (Alirezaei, 2000). The gneisses are affected by amphibolite facies metamorphism, possibly transitional to granulite facies as indicated by orthopyroxene-bearing felsic gneisses (Elders, 1963) during the Sveconorwegian deformation in the period 1.14 to ca. 0.9 Ga (e.g., Bingen et al., 1998). The precise age of the pegmatites in Froland is uncertain, but U/Pb dating elsewhere in the Bamble-Lillesand block yields ages in the range 1128–1060 Ma (Baadsgaard et al., 1984; Cosca et al., 1998). This crustal block representing a segment of the Sveconorwegian orogeny (1.13–0.9 Ga) comprises an exhumed mid-crustal portion of a volcanic arc complex (Knudsen & Andersen, 1999), thrusted over the Telemark block along the Porsgrunn-Kristiansand Fault Zone (PKFZ) during the early Sveconorwegian (1.15–1.10 Ga; e.g., Bingen et al., 2001, 2002). The 5-km wide Froland pegmatite field is situated in the hanging wall of the PKFZ where it can be followed over a distance of ca. 20 km in NE-SW direction. The PKFZ is interpreted as a northwestdirected long-lived and polyphase fault zone initiated under amphibole facies conditions (Starmer, 1993; Henderson & Ihlen, 2004) when the injection of the majority of pegmatites occurred in the Froland and Glamsland-Lillesand fields (Henderson & Ihlen, 2004). These two pegmatite

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fields, that together comprise a belt of pegmatites with similar bulk compositions, are separated by the Herefoss granite pluton (Ihlen et al., 2002). The circular Herefoss pluton (diameter 18 km) intruded at 0.93 Ga (Andersen, 1997; Andersen et al., 2002) into the central part of the older Glamsland-Lillesand-Froland pegmatite belt (Fig. 1b). The pluton carries mega-enclaves of gneisses (several km in length) in its northern and central part where these gneisses host similar compositional types of pegmatites as in the Froland pegmatite field to the north outside the pluton.

3. Classification of Froland pegmatites The pegmatite field of Froland comprises different types of granitic pegmatites of the abyssal class, some transitional ˇ to AB-HREE pegmatites (Cerný & Ercit, 2005). Representative localities of the different pegmatite types which were studied are characterised in Table 1, which is deposited and freely available on the EJM website at GeoScienceWorld (http://eurjmin.geoscienceworld.org/). For more detailed information see Müller et al. (2005). The Froland pegmatites formed during the Sveconorwegian orogeny, i.e. syn-orogenic (1.13–1.06 Ga), lateorogenic (0.93 Ga; syn-genetic in respect to the emplacement of the Herefoss pluton) and post-orogenic (< 0.93 Ga; Ihlen et al. 2002). The largest volume of pegmatites formed during the syn-orogenic stage (Henderson & Ihlen, 2004). Ihlen et al. (2002) subdivided the syn-orogenic pegmatites into a number of sub-groups on the basis of their major mineral composition including pegmatitic granites (PGr), granite pegmatites (GP), plagioclase-dominant pegmatites (NaP, Fig. 2a), zoned granitic pegmatites (ZoP, Fig. 2b, c) and K-feldspar-dominant pegmatites (KP), and white mica pegmatites (MP1). Their injection is roughly coeval, although several magma pulses can be distinguished by cross-cutting relationships in the individual areas. The distinction between these pegmatite types is not always obvious, because transitional pegmatites also occur. Some of the ZoP (Vaselona and Fossheia West) which are in the contact aureole of the Herefoss pluton were affected by contact metamorphism and micro-shearing (sheared zoned pegmatites – sZoP). PGr, GP, NaP, and ZoP are crosscut by fine- to medium grained biotite granite dykes (BtGr). The late-orogenic pegmatites form one structural type, i.e. zoned pegmatites linked to the Herefoss pluton emplacement. The Herefoss pluton itself consists of four major granite facies (HGr1-4). HGr1 and HGr2 show development of pegmatite segregations in their interior and frequently along their endocontacts. The pegmatite at Heimdal (HP1) is related to the megacrystic leucogranite HGr1 and the pegmatites at Fossheia East and Metveit (HP2) to the coarse-grained biotite quartz monzonite (HGr2; Fig. 1c). The post-orogenic pegmatites are represented by zinnwaldite pegmatites (MP2) which comprise < 1 m thick dykes. These straight MP2 dykes crosscut all older pegmatite generations. They contain comb quartz and Kfeldspar and show internal banding similar to layered

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pegmatites related to highly fractionated granites rich in Li, F and Mn (e.g., Morgan & London, 1999).

4. Analytical methods 4.1. Laser ablation inductively coupled plasma mass spectrometry Laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS, was applied for the in situ determination of Li, Be, B, Ge, Na, Al, K, Ti and Fe in quartz. The ICP-MS used in this study is a double focusing sector field instrument (model-ELEMENT-1, Finnigan MAT, Bremen, Germany) combined with a Finnigan MAT UV laser probe. Operating conditions of the LA-ICP-MS are listed in Table 2. The 266-nm laser had a repetition rate of 20 Hz, and pulse energy of 1.5–1.6 mJ with continuous ablation on an area of approximately 180 × 200 μm. The laser beam was adjusted to give a spot size of approximately 20 μm. External calibration was done using four silicate glass reference materials produced by the National Institute of Standards and Technology (NIST SRM 610, NIST SRM 612, NIST SRM 614, NIST SRM 616). In addition, the standard reference material NIST 1830, soda-lime float glass (0.1 wt.% Al2 O3 ) from NIST, the high purity silica BCS 313/1 reference sample from the Bureau of Analysed Samples, UK, the certified reference material “pure substance No. 1” silicon dioxide SiO2 from the Federal Institute for Material Research and Testing, Berlin, Germany and the Qz-Tu synthetic pure quartz monocrystal provided by Andreas Kronz from the Geowissenschaftliches Zentrum Göttingen (GZG), Germany, were used. Each measurement consists of 15 scans of each isotope, with a measurement time varying from 0.15 s per scan of K in high resolution to 0.024 s per scan of, e.g. Li in low resolution. An Ar-blank was run before each standard and sample measurement. The background signal was subtracted from the instrumental response of the standard before normalisation against the internal standard. This was done to avoid memory effects between samples. A weighted linear regression model including several measurements of the different standard was applied for calculation of the calibration curve for each element. 10 successive measurements on the Qz-Tu were used to estimate the limits of detections (LOD). LOD are based on 3 times standard deviation (3σ) of the 10 measurements divided by the sensitivity S. LOD are 1.6 μgg−1 for Li, 0.3 for Be, 0.3 for B, 50 for Na, 4 for Al, 10 for P, 0.2 for Ge, 0.5 for Ti, 1 for K, and 0.2 for Fe. Flem et al. (2002) gave a more detailed description of the measurement procedure.

4.2. Electron-microprobe analysis Electron-microprobe analysis (EPMA) was applied to determine the Al, Ti, K and Fe distribution across domains of secondary quartz, since this method provides in situ trace element data with a very good spatial resolution down to 5 μm. The analysis spot of the LA-ICP-MS is too large

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Fig. 2. Three cross sections of representative pegmatites exposed by historical mining activity. The insets right below the cross sections simplify the pegmatite zoning. a – View of the SE-NW striking wall of the Hellheia Middle quarry. b – SW-NE striking wall of the Skåremyr quarry. c – SW-NE striking wall of the Sønnristjern quarry.

(180 × 200 × 80 μm) to be placed accurately inside secondary quartz which normally forms domains < 100 μm. Moreover, the high number of fluid inclusions within the secondary quartz would probably cause the adulteration of many of the LA-ICP-MS analyses by elements originating from the trapped fluids (e.g., Na, K, B). The microprobe analyses were performed with a JEOL 8900 RL electron microprobe at the Geowissenschaftliches Zentrum

Göttingen, Germany. For high precision and sensitivity, a beam current of 80 nA, a beam diameter of 5 μm, and counting times of 15 s for Si, and of 300 s for Al, Ti, K, and Fe were used. Detection limits (3σ of single point background) were 60 μgg−1 for Al, 18 μgg−1 for K, 33 μgg−1 for Ti, and 27 μgg−1 for Fe. Müller et al. (2003a, 2003b) gave a more detailed description of the measurement procedure.

Quartz chemistry in pegmatites

Table 2. Operating parameters of the LA-ICP-MS and key method parameters. Plasma conditions plasma power auxiliary gas flow sample gas flow cone CD-1 guard electrode Data collection scan type no. of scans

1075 W 0.89 l/min 1.1–1.2 l/min high performance Ni yes E-scan 15

4.3. Scanning electron microscope cathodoluminescence Scanning electron microscope cathodoluminescence (SEM-CL) images were obtained from polished thin sections coated with carbon using the LEO 1450VP analytical SEM with an attached CENTAURUS BS BIALKALI type cathodoluminescence (CL) detector. The applied acceleration voltage and current at the sample surface were 20 kV and ∼ 3 nA, respectively. The BIALKALI tube has a CL response range from 300 (violet) to 650 nm (red). It peaks in the violet spectrum range around 400 nm. The CL images were collected from one scan of 43 s photo speed and a processing resolution of 1024 × 768 pixels and 256 grey levels. The brightness and contrast of the collected CL images were improved with the PhotoShop software. SEM-CL has been applied to quartz in order to reveal on micro-scale (< 1 mm) growth zonation, alteration structures and different quartz generations. Grey-scale contrasts visualised by SEM-CL are caused by the heterogeneous distribution of lattice defects (e.g., oxygen and silicon vacancies, broken bonds) and trace elements in the crystal lattice (e.g., Sprunt, 1981; Ramseyer et al., 1988; Perny et al., 1992; Stevens Kalceff et al., 2000; Götze et al., 2001; 2004, 2005). Although the physical background of the quartz CL is not fully understood, the structures revealed by CL give information about crystallisation, deformation and fluiddriven overprint.

5. Chemical characterisation of major pegmatite minerals The scope of the study is to characterise the the trace element composition of quartz in different pegmatite generations to indentify processes that generate high purity quartz. In this context, the chemistry of feldspar and mica is important in order to evaluate the degree of fractionation of the pegmatitite melts and their precise crystallisation conditions. One sample of K-feldspar, plagioclase, biotite and/or muscovite was taken per sample point, if the mineral occurred less than 0.5 m away from the sampled quartz. Five of the larger pegmatites (Løvland, Hellheia Middle, Skåremyr, Sønnristjern, Lille Kleivmyr) were multiple sampled along longitudinal traverses crossing the pegmatite bodies in order to reveal possible distribution patterns among the elements. The distances between sample

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points of the traverses were 4 to 65 m depending on pegmatite heterogeneties, structures, size of exposures, and dimensions of the pegmatites.The longest sampling longitudinal traverse of 208 m was taken from the Sønnristjern pegmatite crossing the zoned core (ZoP) and the host granite pegmatite (GP).

5.1. Composition of feldspars Rb, Sr, and Ba in K-feldspar and plagioclase are sensitive to igneous differentiation and to the differentiation of pegmatite-forming melts (Mehnert & Büsch, 1981; Long & Luth, 1986; Cox et al., 1996). Generally, Ba and Sr decrease and Rb increases in feldspar during magmatic differentiation. However, granite magmas crystallise under equlibrium conditions whereas pegmatite melts crystallise under super-cooled conditions far from the equilibrium or granite liquidus (e.g., Chakoumakos & Lumpkin, 1990; Morgan & London, 1999; Webber et al., 1999). Therefore, the crystal and, thus, the element fractionation is different (London, 2005), i.e. of Rb, Sr and Ba. The composition of 79 pegmatitic feldspar crystals between 0.1 and 2.5 m in size were determined. Concentrations of major and trace elements are shown in Fig. 3 to 5. The analytical results for feldspar are listed in Table 3, which is deposited and freely available on the EJM website at GeoScienceWorld (http:// eurjmin.geoscienceworld.org/). The average bulk composition of K-feldspar and plagioclase from syn- and lateorogenic pegmatites vary from Or79 Ab21 to Or84 Ab16 and from Ab82 An12 Or6 to Ab75 An21 Or4 , respectively. Feldspars from Hellheia North (locality 5) and Vaselona (locality 10) were presumably affected by albitisation resulting in a higher Ab content in K-feldspars (Ab23−25 ) and plagioclases (Ab87−88 ). ZoP, GP and PGr contain the most potassium rich K-feldspars (> 13 wt.% K2 O) and, thus, the K2 O content in K-feldspar seems to be related to the pegmatite type. High K2 O (> 13 wt.%) is the requirement for glass- and ceramic-grade K-feldspar and, thus, the Kfeldspar of ZoP, GP, and PGr has high feldspar quality. The bulk composition of K-feldspars from post-orogenic pegmatites and granites (HP) is more variable (Or75 Ab24 An1 to Or83 Ab16 An1 ). Ba, Rb, Sr, Ga, and Pb in K-feldspar and Sr and Rb in plagioclase show only slight variations between the different pegmatite types and distinct variations within the individual pegmatites (e.g., Hellheia Middle, Sønnristjern, Lille Kleivmyr; Fig. 3). The element variation is higher for large ZoP and GP pegmatites than for small pegmatites. K-feldspar of the Lille Kleivmyr locality cover the broadest range of Rb/(Sr+Ba) ratios. Lille Kleivmyr is the largest of the investigated pegmatites. The variation of Rb/Sr across the pegmatites at Løvland (KP) and Våtåskammen (PGr) is minor which is in agreement with the compositional homogeneity of these pegmatites. The Rb/Sr ratios of NaP plagioclase, e.g. Hellheia Middle, are almost constant. Plagioclase from Skåremyr (ZoP) shows a strong increase in differentiation from the SE towards the NW edge of the pegmatite (Fig. 2b). A similar scenario is also obtained

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Fig. 3. Concentration variation diagrams of major and trace elements in K-feldspar and plagioclase from Froland pegmatites. Grey arrows indicate the general magmatic differentiation trend.

from Haukemyrliene PGr. A strong zoning in Rb/(Sr+Ba) of K-feldspar and of Rb/Sr in plagioclase is developed across the Sønnristjern pegmatite (Fig. 4 and 5). The granite pegmatite hosting the zoned core has feldspar with consistently low ratios. The ratios strongly increase within the zoned pegmatite core. Increasing ratios reflect higher fractionation of the residual melt from which the feldspar grew, as long as ratios are not disturbed by secondary feldspar alteration. Generally, the fractionation trends can be better obtained from the Rb/Sr ratios of plagioclase than by Rb/(Sr+Ba) of K-feldspar. Megacrystic feldspars (> 1 m) in the core of ZoP and GP can show internal chemical zoning with higher Rb/Sr and Rb/(Sr+Ba) in the core than at the margin. For example, the core of plagioclase (sample 2009411) from Sønnristjern has a more primitive composition (Rb/Sr = 0.04) than the crystal margin (Rb/Sr = 0.22). The core composition corresponds to the primitive composition of plagioclase from the pegmatite contact (e.g., sample 2009402 in Fig. 4). Generally, feldspar megacrysts were sampled at their margins to produce comparable data. By summarizing, the following differentiation trends for the Froland pegmatites can be revealed by feldspar chemistry. Feldspars from MP dykes at Skåremyr and Hellheia Middle, Løvland (KP), Hellheia North (NaP), Vaselona (sheared ZoP) have the most evolved chemistry. However, the high degree of differentiation exhibited by the NaP Hellheia North is in conflict with its plagioclase-

dominance and biotite chemistry (see following chapters) and the primitive composition of the related and neighbouring Hellheia Middle pegmatite. The Hellheia North plagioclases were presumably affected by albitisation which resulted in re-distribution of Sr, Ba and/or Rb. Sønnristjern (ZoP), Skåremyr (ZoP), Våtåskammen (PGr), Lille Kleivmyr (GP), Vaselona (sheared ZoP), Bortjørn (NaP) contain feldspars of chemical composition reflecting moderate differentiation of the pegmatitic melt. Pegmatites related to the Herefoss pluton (HP) exhibit a relative primitive differentiation reflected by high Ba and low Rb/Sr in the feldspars. The chemically most primitive plagioclase occurs at Hellheia Middle (NaP). However, the trace element signature of feldspar in the Froland pegmatite field is relatively primitive compared to feldspars from other granitic pegmatite fields elsewhere in the world (Shearer et al., 1992; Abad-Ortega et al., 1993; Larsen, 2002).

5.2. Composition of micas The composition of 40 pegmatitic micas were determined and plotted in Fig. 4, 6 and 7. The analytical results for mica are listed in Table 4, which is deposited and freely available on the EJM website at GeoScienceWorld (http://eurjmin.geoscienceworld.org/). Their compositions are used to estimate the degree of fractionation of the

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Fig. 4. Sønnristjern pegmatite with outline of the 10-m deep quarry and access tunnel. The exploited zoned pegmatite core is hosted by granite pegmatite which intruded hornblende gneisses. The black columns illustrate the relative values of Rb/(Ba+Sr), Rb/Sr, and (MgLi)/Fe# in K-feldspar, plagioclase and biotite, respectively. Columns are placed at the sampling point.

pegmatites since micas are useful monitors of PTX during ˇ magmatic processes (e.g., Cerný & Burt, 1984). Generally, the biotites in the Froland pegmatites have a relatively homogeneous composition which is characteristically primitive in respect to granitic differentiation. All biotites plot in the Mg-siderophyllite and Fe-phlogopite field in the discrimination diagram of Tischendorf et al. (2001; Fig. 6). Biotites in different pegmatite types exhibit slight compositional variations. The most primitive compositions (Fe-phlogopite) are comprised by the micas from pegmatites related to the Herefoss pluton. NaP has micas transitional between Fe-phlogopite and Mg-siderophyllite. Biotites of GP and ZoP plot exclusively in the Fe-phlogopite field reflecting a slightly evolved differentiation. A number of pegmatites are crosscut by zinnwaldite pegmatites (MP1 and MP2). The mica of these dykes have zinnwaldite (Fe polylithionite) composition except for the mica occurring in the Vaselona pegmatite that represents a Li-Fe muscovite with relative high Ti. Sampling profiles across the Hellheia Middle, Skåremyr, Sønnristjern and Lille Kleivmyr pegmatite reveal no obvious zonation across pegmatites due to the limited chemical variation of biotite (Fig. 4). In Fig. 7 Rb/Sr in plagioclase and Rb/(Ba+Sr) in K-feldspar are plotted against Mg-Li of associated biotite. The plots reveal poorly defined tends

although the small Mg-Li variation. Mg-Li of biotite decreases with increasing Rb/Sr and Rb/(Ba+Sr) of feldspar during progressive fractionation. Thus, biotites of the four localities show weak chemical zonation in pegmatites that follows the compositional zonation of feldspar.

5.3. Micro-textures in cathodoluminescence images of quartz Late- to post-magmatic fluid-driven overprint causes smallscale quartz dissolution and precipitation (healing) along grain boundaries and micro-cracks resulting in the formation of newly crystallised (secondary) quartz which appears dark grey to black in SEM-CL images. If the CL intensity of the primary quartz is low or if the CL intensity decreases during electron bombardment these structures are hard to detect. For this study the knowledge about different quartz generations and abundance of secondary quartz is a necessity in order to interpret the trace element analyses of quartz properly (e.g., Müller et al., 2002a). Different CL intensities may indicate variable trace element contents (e.g., Götze et al., 2001). Henderson (2002) gave an overview of secondary micro structures observed in pegmatitic quartz from Froland.

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Fig. 5. Stacked column diagram of Rb/(Ba+Sr) and Rb/Sr in feldspars (upper part) and of Al, Ti, Li and Ge in pegmatite quartz along a 210 m long profile crossing the Sønnristjern pegmatite. Concentrations of trace elements in quartz are the average of two LAICP-MS measurements. The numbers between the columns in the lower line corresponds to distance between two sample points in meter. n.d. – not determined due to lack of feldspar at the sample point.

Fig. 6. Compositions of micas from the Froland pegmatite field plotted in classification diagram of Tischendorf et al. (2001). MP1 micas of the Hellheia Middle, Hellheia North, Skåremyr, and Vaselona pegmatite plot in the Fe polylithionite and Li-Fe muscovite fields. tFe – Fe total.

Four major types of secondary quartz (sqz1 to sqz4) replacing primary pegmatitic quartz (pqz) can be distinguished. The features and abundance of the secondary quartz generations are summarised for the different pegmatites in Table 5. The different types of secondary quartz are described in the order from young to old:

Fig. 7. Minor element plots of biotite versus plagioclase (a) and K-feldspar (b). The dashed lines of exponential regression defines poorly differentiation trends.

sqz1: Thin (< 5 μm), intra- and transgranular healed cracks connecting non-luminescent domains around secondary fluid inclusions. These structures appear black in the SEM-CL image (Fig. 8a, b, c, e). sqz2: Irregular domains of low-luminescent quartz extending from and commonly enveloping sqz1. Occasionally, the envelops extend outwards as preferentially oriented zones in certain directions which may correspond to the crystallographic plans or sets of micro-fractures. Similar to sqz1, sqz2 originates in grain and sub-grain boundaries (Fig. 8a, d), spz1-healed micro-fractures (Fig. 8b) or fluid inclusions (Fig. 8c). These structures appear grey in the SEM-CL image. sqz3: Diffuse alteration rims of relative constant width parallel to grain and sub-grain boundaries and contacts to feldspar and mica. In contrast to sqz1, sqz3 is not transgranular and it exhibits sporadically diffusional, wavy zoning. However, in some cases sqz1 and sqz3 are hard to distinguish. Sqz3 structures appear dark grey in the SEM-CL image (Fig. 8a and d). sqz4: Non-luminescent (black), thin crystal coatings and interstitial fillings at triple-junction boundaries of recrystallised quartz (Fig. 8g and h). Sqz4 dominates the quartz samples from the Fossheia West and Vaselona pegmatites.

Quartz chemistry in pegmatites

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Fig. 8. SEM-CL images of pegmatitic quartz. a – Quartz from Bjortjørn showing the distribution and spatial relations of sqz1, sqz2 and sqz3. The network of sqz1 is more dense and irregular than in other pegmatitic quartz. b – Quartz from Sønnristjern showing network of sqz1 and sqz2, the latter with preferred orientation. c – Fluid inclusion (FI) in quartz bordered by thin film of sqz1 (black). The fluid inclusion and sqz1 are hosted by sqz2. d – Strongly altered quartz (pqz) from Lille Kleivmyr which is nearly totally replaced by sqz2. Mica appears black. The structure of sqz3 corresponds the former fluid pathway. The inset shows the enlargement of the bright CL halo around a zircon inclusion. e – Quartz from Metveit. Sqz1 forms straight thin healed cracks connecting non-luminescence domains around secondary fluid inclusions. f – Detailed SEM-CL image of granite quartz from Husefjell. The quartz exhibits non-luminescent spots (black; some spots are marked with white arrows) which are interpreted as hydrogen-rich defect clusters. g – Mylonitised and recrystallised quartz from Vaselona. Boundaries of recrystallised quartz are covered and healed with non-luminescent (black) sqz4. h – Mylonitised and recrystallised quartz from Fossheia West. The grain size of recrystallised grains, which are healed with sqz4, is much smaller than in quartz of Fossheia West.

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A. Müller, P.M. Ihlen, A. Kronz

Table 5. Observed micro-textures of secondary quartz in pegmatitic quartz and their roughly estimated abundance in vol.%. The volume of sqz2 and sqz3 is summed up, because these generations are often hard to distinguish. Loc. Nr 1 2 3 4 5 6 7 8 9

Sub-type PGr GP GP NaP NaP NaP ZoP ZoP KP

Sqz1 (vol.%) ∼1