Mantle-Derived, UHP Garnet Pyroxenite and Eclogite in the Moldanubian Gfohl Nappe, Bohemian Massif: A Geochemical Review, New P-T Determinations, and Tectonic Interpretation

摘要

The Gfohl nappe, the uppermost structural unit in the Moldanubian Zone of the Variscan Bohemian Massif, contains a distinctive association of HP crustal granulite (900-1000 deg C, 15-18 kbar) and UHP mantle garnet peridotite (875-1150 deg C, 33-60 kbar). Ultrahigh-pressure (UHP) garnet peridolite is host to layers and lenses of garnet pyroxenite and edogile, which formed by high-pressure crystal accumulation of garnet and pyroxene (± trapped melt) from transient melts in subcontinental lithosphere. The source of such melts was subducted, hydrothermally altered oceanic crust. New analyses of garnet websterite, orthopyroxene eclogite, and kyanite eclogite yield temperatures of 840-950 deg C and pressures of 34-43 kbar, comparable to those of enclosing peridotite, although kyanite eclogite at one locality (Uhrov) yields significantly different values of 1030-1200 deg C and 17-22 kbar. Most petrological and geochemical features of the Gfohl crustal and mantle association can be explained in terms of Devonian (Emsian to Famennian) convergence and subduction of Moldanubia beneath Tepla-Barrandia, culminating in Early Carboniferous (Tournaisian) continental collision. However, this tectonic scenario fails to account for a pressure gap of approx 20 kbar between HP granulite and UHP peridotite-pyroxenite-edogite, which remains problematic.