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Phosphorus and Rubidium in Alkali Feldspars: Case Studies and Possible Genetic Interpretation

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Phosphorus and Rubidium in Alkali Feldspars: Case Studies and Possible Genetic Interpretation Bulletin of the Czech Geological Survey, Vol. 77, No. 2, 93–104, 2002 © Czech Geological Survey, ISSN 1210-3527 Phosphorus and rubidium in alkali feldspars: case studies and possible genetic interpretation KAREL BREITER1 – JIŘÍ FRÝDA1 – JAROMÍR LEICHMANN2 1Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic; e-mail: [email protected], [email protected] 2Department of Geology and Paleontology, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic; e-mail: [email protected] .cz Abstract. Potassium feldspar (Kfs) and albite (Ab) are major hosts of phosphorus in peraluminous highly fractionated granitic systems. Kfs is also an important carrier of Rb. The absolute contents of P and Rb, their homogeneous, zoned, or disturbed distribution within individual Kfs and Ab crystals, the ratio of P contents in the whole rock (WR) vs. Kfs and the ratio of P contents in Kfs vs. albite can serve as useful indicators of the grade of parental magma fractionation and late- and/or post-magmatic crystal–fluid reaction. The P and Rb distributions in feldspars were studied in detail in four peraluminous granitic systems widely differing in intensity of influence of late- to post-magmatic fluids: dominantly magmatic Podlesí gran- ite system (Bohemia), Beauvoir granite (France) influenced by late magmatic fluids, hydrothermaly altered Křížový Kámen granite (Bohemia – Bavaria) and a quartz-Kfs-amblygonite pegmatite-like dyke at Vernéřov (Bohemia). Rubidium is firmly bound in the Kfs lattice an d preserves well its magmatic signature during post-magmatic processes. In contrast, phosphorus can be easily released from the feldspar lattice and P content serves as a sensitive indicator of the late- and/or post-magmatic fluid-related reactions. Key words: alkali feldspar granite, pegmatite, chemical composition, alkali feldspar group, magmatism, fractional crystallization, phosphorus, rubidium Introduction The Podlesí granite stock (Czech Republic) and the Beauvoir granite stock (France) represent two well-de- Alkali feldspars are major hosts of phosphorus and ru- scribed P- and Rb-rich, strongly fractionated granitic sys- bidium in peraluminous highly fractionated granitic sys- tems with similar chemistry of parental melts but different tems. The absolute contents of P and Rb, their late-magmatic and fluid evolution. This, taken together, al- homogeneous, zoned, or disturbed distribution within in- lows to estimate the influence of magmatic and post-mag- dividual Afs crystals, the ratio of P contents in the whole matic processes on the concentration and distribution of rock (WR) vs. Kfs and the ratio of P contents in Kfs vs. al- both elements. In addition, some complementary data bite can serve as useful indicators of the grade of parental from P-rich granites and pegmatites from Bavaria and magma fractionation and late- and/or post-magmatic crys- western Bohemia are also presented. tal-fluid reaction. High P content in crystal lattice of Kfs was first de- scribed from the pegmatite at Hagendorf, Bavaria (Mücke Brief characteristic of the studied granitic systems 1966). Later, high P content in both Kfs and albite was identified as an important feature of peraluminous granites Podlesí by London (1992). P2O5 content in granitic Kfs may reach 1–2 wt%, while P content in coexisting albite is usually The Podlesí granite system is located in the western lower (London 1992, Frýda and Breiter 1995). London et part of the Krušné hory Mts. within the Saxothuringian al. (1993) calibrated the distribution coefficient for geo- Zone of the Bohemian Massif, Czech Republic. It is the Afs/melt logically relevant conditions DP = 2.05*ASI-1.75 youngest intrusion within the multistage late Variscan per- Kfs/Ab and DP = 1.2. During late magmatic and/or post-mag- aluminous tin-specialized Eibenstock-Nejdek granite plu- matic reactions with fluid, phosphorus is much more easi- ton, intruding the Lower Paleozoic phyllites and Variscan ly released from the lattice of albite than from Kfs, and the biotite granite. The Podlesí granite stock is generally disequilibrium in P content between Kfs and Ab may indi- formed by a tongue-like intrusion of albite-protolithionite- cate some kind of fluid-involved post-magmatic alteration topaz granite (stock granite), at the upper contact rimmed (Breiter 1999). by marginal pegmatite (stockscheider) (Fig. 1). The stock High Rb content in Kfs was reported especially from granite is intercalated with flat dykes of the albite-zinn- the pollucite-type pegmatite. Magmatic microcline con- waldite-topaz granite (dyke granite) at a depth of tains up to 2–4 wt% Rb2O, triclinic feldspars formed in 40–100 m with locally well-developed magmatic layering. subsolidus conditions may exceed the limit of 50 mol% in The stock granite is strongly fractionated, rich in P Rb-feldspar rubicline (Teertstra et al. 1997, 1998). The re- (c. 0.5% P2O5), F (0.5–1.2%), Rb (c. 1000 ppm), Li ported Rb contents in granitic Kfs are much lower (Par- (500–1000 ppm), and Cs (100–150 ppm) (Tab. 1). The sons 1994, Deer et al. 2001). dyke granite represents extremely fractionated residual 93 Karel Breiter – Jiří Frýda – Jaromír Leichmann Table 1. Average of chemical compositions of granite types from Podlesí (borehole PTP-1, wt%). REE etc. The Beauvoir granite is the Unit upper stock granite UST dyke granite homogen dyke granite deeper stock granite latest intrusion in a peraluminous n 7 1 8 20 granitic complex composed of three SiO2 73.5–74.4 67.3 68.8–72.5 72.5–74.0 successively emplaced units: the hid- TiO2 0.03–0.08 0.02 0.02–0.05 0.03–0.05 den more-or-less hypothetical La Al2O3 13.8–14.1 17.5 14.8–15.9 13.8–14.5 Fe2O3 0.1–0.5 0.15 0.1–0.3 0.1–0.3 Bosse granite, the Colettes two-mica FeO 0.5–0.9 0.6 0.8–1.1 0.7–1.0 granite, and the Beauvoir topaz-lepi- MnO 0.020–0.025 0.052 0.04–0.18 0.02–0.03 dolite-albite granite (Cuney and Au- MgO 0.04–0.07 0.02 0.02–0.04 0.03–0.06 tran 1987, Cuney et al. 1992). The CaO 0.40–0.55 0.29 0.16–0.64 0.35–0.50 Li2O 0.17–0.21 0.32 0.20–0.40 0.13–0.20 900 m deep drillhole GPF1 allowed Na2O 3.9–4.1 3.2 3.8–4.7 3.5–3.8 to study the vertical evolution of the K2O 4.1–4.6 7.6 3.5–4.6 4.2–4.5 Beauvoir granite in an interval more P2O5 0.41–0.46 0.82 0.62–0.87* 0.44–0.50 than 700 m thick. Three granitic F 0.90–1.15 1.57 1.5–2.4 0.9–1.2 Rb 0.11–0.13 0.30 0.17–0.19 0.11–0.14 units, designated B1 to B3, were dis- tinguished by Rossi et al. (1987) and * in outcropping part of the dyke up to 1.5% of P2O5 Raimbault and Azencot (1987). Later, a detailed geochemical study subdivided the Beauvoir granite into melt even more enriched in P (c. 1% P 2O5), F (1–1.5%), two substantial units: B and B’ (Raimbault et al. 1995). and Rb (up to 3000 ppm). Prominent manifestation of K- Unit B builds the upper part of the stock and represents feldspar-dominated unidirectional solidification textures geochemically more evolved, more fluid-enriched part of (UST sensu Shanon et al. 1982) were found within both the Beauvoir initial magma. Unit B’, smaller in volume, the stock and dyke granite types. The system was studied represents relatively less evolved and later emplaced por- along a vertical section by up to 350 m deep drillholes tion of the Beauvoir magma. Separation of melts of units PTP-1 and PTP-2 (Lhotský et al. 1988) and PTP-3 and B and B’ took place in the early stage of the granite sys- PTP-4a (Breiter 2002). tem evolution. Later, both melts fractionated in fact inde- pendently. Upper unit B can be divided into three Beauvoir sub-units from the uppermost, ultimately fractionated unit B1 (depth 98–423 m) through unit B2 (depth c. The Beauvoir granite stock is a small body (<0.2 km 2) 423–571 m) to unit B3 (depth 765–790 m). Within the at the southern edge of the late Variscan Echassieres gran- lower unit B’, a relatively more fractionated unit B’2 ite pluton in the northern part of the French Massif Cen- (depth c. 571–746 m) and less fractionated unit B’3 tral, about 50 km NNW of Clermont-Ferrand, France. (depth c. 850–870 m) can be distinguished. An idealized Geochemically it is a highly specialized, strongly peralu- cross-section of the Beauvoir granite stock is shown in minous, rare metal-bearing granite enriched in P, F, Li, Fig. 2, characteristic chemical analyses of individual ge- Rb, Nb, Ta, Sn, and W, and depleted in Si, Fe, Ti, Mg, Sr, netic granite units are presented in Tab. 2. NS 0 200 m Table 2.Chemical compositions of granite types from Beauvoir (borehole GPF-1, wt%). No. 3059 3061 3063 3067 3069 Unit B1f B1 B2 B’2 B’3 Depth (m) 109 232 522 660 858 SiO2 67.8 69.2 69.7 71.1 72.4 TiO2 udl 0.11 0.10 0.01 0.03 Al2O3 17.60 17.7 17.40 15.90 15.7 Fe2O3tot 0.19 0.20 0.24 0.46 0.74 MnO 0.035 0.04 0.11 0.09 0.095 MgO udl 0.14 0.03 0.16 udl CaO 0.85 0.27 0.25 0.41 0.29 Li2O 1.22 1.54 0.99 0.58 0.49 Na2O 4.33 4.83 4.73 4.11 4.41 K2O 3.7 3.38 3.62 4.02 3.74 phyllite biotite granite stock granite P2O5 1.12 1.35 1.52 0.97 0.82 F 1.6 2.6 1.99 1.52 1.2 stockscheider dyke granite UST-layers Rb 0.41 0.42 0.25 0.20 0.18 Fig.
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