The Journal of Petrology, Volume 38, Issue 12: December 1997.

Mineralogical evidence for two magmatic stages in the evolution of an extremely fractionated P-rich rare-metal granite: the Podlesi stock, Krusne Hory, Czech Republic

K Breiter^1,*, J Fryda¹, R Seltmann² and R Thomas²

¹Czech Geological Survey Praha, Geologicka 6, CZ-15200 Praha, Czech Republic, ²Geoforschungszentrum Potsdam, Telegrafenberg A50, D-14473 Potsdam, Germany, ^*Corresponding author e-mail: breiter@cgu.cz

The Podlesi granite stock in the western Krusne Hory (Erzebirge) Mountains, Czech Republic, represents an extremely fractionated, strongly peraluminous, F- and P-rich, rare-metal granite system of Late Variscan age. The stock, studied in drill core of 300 m length, is formed by albite-protolithionite-topaz granite ('stock granite', depth 30-300 m) and shows geochemical and textural zoning. The stock granite is rich in P ( 0.5 wt %), F (0.5-1.2 wt %), Rb ( 1000 ppm), Li (500-1000 ppm) and Cs (100-150 ppm, and poor in Ti, Mg, Fe, Ca, Sr, Ba, Zr and rare earth elements (REE). Within the uppermost part of the stock (depth 57-115 m), the stock granite is intercalated with albite-zinnwaldite-topaz 'dyke granite' layers of 0.5-7 m thickness and a few thin flat dykes of pegmatite. The dyke granite and pegmatite are even more enriched in P ( 1 wt % P₂O₅), F (1.0-1.5 wt %) and Rb (up to 3000 ppm), and are also rich in Nb (up to 100 ppm), and Ta (up to 50 ppm). The chemical data for the rock-forming minerals show distinct differences between the stock granite and the dyke granite and pegmatite. No chemical zoning of the rock-forming minerals, either from the upper, rapidly cooled, or from the lower, slowly cooled parts of the stock granite was observed. These rock-forming minerals formed during only one stage of crystallization from a parental melt with moderate contents of P, F, and Li (0.5 wt % P₂O₅ in K-feldspar, 5-7 wt % F and 3 wt % Li₂O in Li-mica). In contrast, alkali feldspars and zinnwaldite from the dyke granite and pegmatite show well-developed chemical zoning. The rims of K-feldspars are strongly enriched in phosphorus (up to 2 wt % P₂O₅), and rims of zinnwaldite grains are enriched in fluorine (8-9 wt % F). Both zinnwaldite and topaz from the dyke granite have the OH-F sites fully occupied by fluorine. Also, accessory phosphates were formed during late magmatic crystallization of the strongly P, F-enriched residual melt. Thus, the mineralogical data reflect the presence of two crystallization events within the dyke granite and pegmatite. The presence of two melts, parental and residual, the latter strongly enriched in phosphorus, has been also confirmed by study of melt inclusions in quartz from samples of the stock granite in its dyke-bearing uppermost part. Mineral, melt inclusion and whole-rock chemical data from all rock types provide evidence for two stages of granite evolution, namely crystallization from parental and residual melts. This means that, in addition to the granites in Beauvoir (France) and Argemela (Portugal), the Podlesi stock is another well-documented example of a two-stage evolution of a P-rich granite system.

Key words: alkali feldspars; Li-rich micas; granites; Czech Republic

Pages 1723-1739