From explosive breccia to unidirectional solidification textures: magmatic evolution of a phosphorus- and fluorine-rich granite system (Podlesí, Krušné hory Mts., Czech Republic)


Authors: Breiter K

Published in: Bulletin of Geosciences, volume 77, issue 2; pages: 67 - 92; Received 20 March 2002; Accepted in revised form 19 May 2002;

Keywords: granite, brecciation, layered intrusion, undirectional solidification textures, chemical composition, fluorine, phosphorus, Krušné hory Mts., Bohemian Massif,

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The Podlesí granite stock in western Krušné hory Mts. represents the most highly fractionated part of the late Variscan Nejdek-Eibenstock pluton. Internal fabric of the stock has been studied in several boreholes up to 350 m deep. The stock is composed of two tongue-like bodies of albite-protolithionite-topaz granite (stock granite) coalesced at depth, which were emplaced into Ordovician phyllite and biotite granite of younger intrusive complex (YIC) of the Nejdek pluton. The uppermost part of the intrusion is bordered by a layer of marginal pegmatite (stockscheider) up to 50 cm thick. Explosive breccia was found as an isolated block at the southwest contact of the stock. It is comprised of fragments of phyllite several millimetres to 5 cm in size cemented with fine-grained granitic matrix similar to the stock granite, but very fine-grained.
Within the uppermost 100 m, the stock granite is intercalated with several mostly flat-lying dykes of albite-zinnwaldite-topaz granite (dyke granite). Upper and lower contacts of the dykes are sharp, flat, but in detail slightly uneven. The thickest dyke (about 7 m) outcrops in an old quarry. A prominent example of layering with unidirectional solidification textures (UST) was found in the upper part of this major dyke. Individual Q-Afs laminae are separated by comb quartz layers and/or by layers of oriented fan-like zinnwaldite aggregates. A pegmatite-like layer with oriented megacrysts of Kfs up to 6 cm long was encountered in the uppermost part of the dyke. One thin layer of fine-grained quartz with oriented Kfs-megacrysts was found within the stock granite.
Post-magmatic processes, particularly greisenisation, developed only to a limited degree. The uppermost flat dyke of the dyke granite was partly greisenised into white quartz-rich (+topaz, Li-mica, wolframite) greisen. Scarce thin, steep stringers of biotite greisen were encountered over the entire outcrop and in drilled parts of the stock granite and surrounding biotite granite.