Origin of the pegmatite veins within the skarn body at Vev\xc4\x8dice near Znojmo (Gföhl Unit, Moldanubian Zone)

[摘要] The lower crustal Gföhl Unit (Moldanubian Zone of the Bohemian Massif) consists of various felsic orthogneisses to migmatites (the so-called Gföhl gneiss) that are commonly associated with the high-pressure felsic granulites, serpentinized garnet/spinel peridotites, garnet pyroxenites, eclogites and skarns. The relatively small skarn bodies at Vev\xc4\x8dice in southern Moravia have the prevailing assemblage of medium- to coarse-grained granoblastic clinopyroxene + garnet ± amphibole ± epidote. The skarn is cut by narrow veins of diorite pegmatite (SiO2 = 46-56 wt. %) which is geochemically distinct from granitic melt generated by migmatization of surrounding orthogneisses. Major pegmatite minerals are plagioclase (An18-44), amphibole (K-rich hastingsite and potassic-hastingsite) and quartz, accompanied by minor K-feldspar, garnet, allanite-epidote, clinopyroxene, and accessory titanite. The pegmatites are characterized by high contents of Fe2O3tot (11.2-17.5 wt. %), CaO (7.5-10.1 wt. %), Na2O (2.9-3.3 wt. %) and low contents of MgO (1.4-1.6 wt. %), K2O (1.2-1.6 wt. %), Rb (43-86 ppm). Whole-rock and mineral chemistry of diorite pegmatites are consistent with their origin as a product of melt infiltration from surrounding migmatites and subsequent contamination by the country-rock skarn, with or without, fractional crystallization/crystal accumulation. Similarity in composition of amphiboles in the skarn and in the pegmatite can indicate formation under similar conditions, at c. 750 °C and 0.7-0.8 GPa.

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