P–T Evolution of a Variscan Lower-Crustal Segment: a Study of Granulites from the Schwarzwald, Germany

Pressure–temperature–time (P–T–t) paths of orogenic granulites provide important information on the thermal and chemical structure of the lower continental crust through time, and constraints on tectonic processes. We present the first detailed petrological investigation of granulites from the Variscan Schwarzwald. Pelitic granulites from the Central Schwarzwald Gneiss Complex (CSGC) are characterized by the peak assemblage garnet + rutile + kyanite + antiperthite ± quartz. Felsic to intermediate granulites from the Southern Schwarzwald Gneiss Complex (SSGC) exhibit different peak assemblages with clinopyroxene, orthopyroxene, ternary feldspar, garnet, quartz and sillimanite, and manifold retrograde reaction textures. Peak P–T conditions were calculated by two-feldspar thermometry, garnet–orthopyroxene thermometry and various geobarometers. Minimum estimates for peak conditions are 950–1010°C and 1·4–1·8 GPa for the granulites of the CSGC, which followed a clockwiseP–T path. The retrograde path is characterized by initial isothermal decompression, associated with partial melting, followed by isobaric cooling. Peak conditions for the SSGC are 1015°C and 1·5 GPa (minimum temperature, maximum pressure). No prograde relics are preserved, and isothermal decompression was less pronounced than in the CSGC. Other Variscan HP–HT granulites from Central Europe show similar lithologies, equilibration temperatures and ages (340–335 Ma). The heat for widespread high-temperature metamorphism in the Variscan lower crust could have been supplied by repeated intrusion of subduction-related basic magmas. Rapid, near-isothermal decompression of the granulites may have been facilitated by considerable volumes of partial melt and by orogenic extension.