Contrasting corona structures in mafic granulite from the Blansky Les Complex, Bohemian Massif, Czech Republic

Saint Mary's University, Department of Geology, Halifax, NS, Canada

Quartz-bearing mafic granulite from Holubov, in the Blansky Les complex, Bohemian Massif, Czech Republic contains two types of corona structures: pyroxene+plagioclase rim on garnet (Type-1 corona), and garnet rim on anorthite+spinel (An+Spl) mosaics (Type-2 corona). Significantly, Type-2 structures are enclosed by an orthopyroxene-depleted halo which, in places, seems to embay the Type-1 coronal pyroxene, indicating that the Type-2 coronas are a late feature. Type-1 coronas formed during uplift by the resorption of garnet and quartz: 2 Grs+Prp+3 Qtz = 3 Di+3 An and 2 Alm+Grs+3 Qtz = 3 Fs+3 An. Decompression is also supported by patterns of zoning in garnet (rimward decrease in X _Ca , compensated by reverse zoning in andesine) and the formation of the An+Spl nucleus of Type-2 coronas at the expense of early kyanite and grossular-rich garnet: 5 Grs+(Prp,Alm)+12 Ky = 15 An+3 (Spl,Hc) Coronal grains of garnet are interpreted to have formed during cooling at substantially lower pressures as the An+Spl mosaics reacted with matrix (and, locally, coronal) orthopyroxene. These contrasting corona structures record near-isothermal ( approximately 900-750 degrees C) decompression of the granulite from approximately 14 kbar to approximately 2-3 kbar, and subsequent near-isobaric cooling. They are indicative of a clockwise P-T-t path.

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