Retrograde metamorphism in a regional shear zone and related chemical changes: The Kaplice Unit of muscovite-biotite gneisses in the Moldanubian Zone of southern Bohemia, Czech Republic

Abstract

The geochemistry and petrology of the Kaplice Unit, a regional zone of muscovite-biotite gneisses 55×8 km in size, was studied. Geochemistry of the gneisses is characterized using 27 samples for major element analyses and 15 samples for minor and trace element analyses. The data are compared with element abundances in gneisses of the adjacent Monotonous and Variegated Units, with the aim to define respective chemical differences. K, Rb, Cs, Ba, H2O+ and Fe2O3/FeO are significantly enriched and Na, Ca, Sr, P and U are distinctly depleted in comparison to biotite gneisses of both the Monotonous and the Variegated Units; the ratio Rb/Sr increased by a factor of 3.5. The contents of Ti, Al, Fe1, Mg, Cr, Ce, Y, Hf, Ta and Th are closely comparable, which corresponds to behaviour of relatively immobile elements. Extensive hydration was accompanied by significant oxidation and other chemical changes comparable to synmetamorphic acid leaching. Microstructural relations indicate sillimanite- and K-feldspar-consuming reactions producing muscovite intergrown with biotite in recrystallized quartz-feldspar fabric. Residual sillimanite occurs as minute aggregates armoured by muscovite. Abundant quartz segregation lenses aligned in the dominant S3 foliation indicate high silica activity and carry occassional large crystals of andalusite, rare kyanite, cordierite or rutile. Concurrent reactions in calc-silicate gneiss interbands resulted in replacement of diopside by actinolite or low-Al magnesiohornblende and partial replacement of anorthite by clinozoisite ± minor garnet. Structural and petrological data indicate that the unit represents a regional shear zone (D3 planar foliation sets), functioning as a major conduit with a high, synmetamorphic fluid flux in the NW roof domain of the Moldanubian Pluton in southern Bohemia. Since the intrusion of the Eisgarn pluton post-dates the S3 planar fabric and largely pre-dates extension D4 crenulation, it is possible that the cooling two-mica granite body, structurally underlying the Kaplice Unit, served as a source of fluids, important in the retrograde alteration of paragneisses and the accompanying chemical changes.