The retrograde metamorphism in the Niedzwiedz Amphibolite Massif (SW Poland): from incipient melting to pumpellyite crystallization

摘要

The Niedzwiedz Amphibolite Massif (eastern part of the Fore Sudetic Block, NE Bohemian Massif) consists of foliated amphibolite, which contains subordinate irregular nests and lenses of garnet-hornblende granofels and zones of intensely foliated zoisite amphibolites. The granofels is accompanied by epidote-bearing leucotonalite schlieren and clinopyroxene-plagio-clase aggregates. The granofels records the peak metamorphic conditions at ca 12-13 kbar and 790 deg C, under which incipient partial melting produced leucotonalitic melts and clinopyroxene-plagioclase restite. The peak P/T conditions show that Niedzwiedz Massif is the exhumed part of a thickened Variscan lower crust. Retrograde metamorphism connected with foliation development converted the mineral assemblage garnet-hornblende I into plagioclase-hornblende II. It was followed by further retrogression in zones of high strain, which produced the mineral assemblage zoisite-pumpellyite-albite-actinolite. The retrogressive mineral assemblages consist of minerals of highly variable composition, showing that they did not equilibrate. The Niedzwiedz Massif is an example of mafic rocks where the mineral assemblage during the retrograde metamorphism can not be ascribed to certain P-T, but occurred during continuously evolving conditions from high- to low-grade corresponding to those of the pumpellyite-actinolite facies.