Geochemical and mechanical interactions within sheet-like granite intrusion during stress-field changeover (Moldanubian Batholith; Central European Variscides)

Abstract

Late-orogenic granitoids carry unique geochemical and structural evidence of post-collisional exhumation, commonly associated with decompression melting, mixing and mingling of individual magma pulses and differentiation. The Weinsberg Type Granitoids (WTG) forming the 313.6 Ma Kaplice Sheet and 322 Ma Hojná Voda Sheet (northwestern Weinsberg Composite Pluton) have geochemical signature that demonstrates the substantial role played by the partial melting of metasedimentary sequences and the magmatic fractionation of K-feldspar, plagioclase and biotite during magma ascent and emplacement. The composition of highly peraluminous WTG were partly modified by the assimilation of host metasedimentary xenoliths. The textures of the WTG are characterised by straight crystal size distribution (CSD) consistent with in-situ crystallization and crystal/melt separation, forming abundant K-feldspar phenocryst-rich domains (KPD). The KPD were formed by redistribution of K-feldspar phenocrysts (crystal accumulation) in a suspension during magma movement followed by interstitial melt extraction and escape from the highly crystallised mush. The multiple fabric pattern within the Kaplice Sheet is the result of changeover of local stress field by varying increment of regional stress conditions and buoyancy forces at the time of magma ascent and emplacement. The steeply dipping foliation (M1) reflects the magma flow with the increment of regional stress-field (ENE–WSW oriented extension). The superimposition of flat-lying foliation (M2) with abundant particle accumulations (i.e. K-feldspar phenocrysts, enclaves and host-rock xenoliths) were associated with (a) increasing buoyancy force in ascending magma above the rate of the regional stress field or (b) collapse of ascending magma after the decreasing the buoyancy force resulting in the mush moving down.