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

Pressure–temperature–time (P–T–t) pathsof orogenic granulites provide important information on thethermal and chemical structure of the lower continental crustthrough time, and constraints on tectonic processes. We presentthe first detailed petrological investigation of granulitesfrom the Variscan Schwarzwald. Pelitic granulites from the CentralSchwarzwald Gneiss Complex (CSGC) are characterized by the peakassemblage garnet + rutile + kyanite + antiperthite ±quartz. Felsic to intermediate granulites from the SouthernSchwarzwald Gneiss Complex (SSGC) exhibit different peak assemblageswith 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°Cand 1·4–1·8 GPa for the granulites of theCSGC, which followed a clockwiseP–T path. The retrogradepath is characterized by initial isothermal decompression, associatedwith partial melting, followed by isobaric cooling. Peak conditionsfor the SSGC are 1015°C and 1·5 GPa (minimum temperature,maximum pressure). No prograde relics are preserved, and isothermaldecompression was less pronounced than in the CSGC. Other VariscanHP–HT granulites from Central Europe show similar lithologies,equilibration temperatures and ages (340–335 Ma). Theheat for widespread high-temperature metamorphism in the Variscanlower crust could have been supplied by repeated intrusion ofsubduction-related basic magmas. Rapid, near-isothermal decompressionof the granulites may have been facilitated by considerablevolumes of partial melt and by orogenic extension. KEY WORDS: granulites; near-isothermal decompression; two-feldspar thermometry; HT metamorphism; Variscan Schwarzwald