Metamorphic evolution of cordierite-bearing migmatites from the BayerischeWald (Variscan Belt, Germany)

A detailed petrological study was carried out on cordierite-bearing migmati tes of the Bayerische Wald (Germany Bohemian Massif; Variscan Belt) to cons train the physical-chemical conditions of metamorphism and partial melting. Four types of migmatites (MIG1-MIG4) can be classified mainly on the basis of field appearance. microstructures and degree of melt extraction, The fo ur migmatite types and their intercalations preserve a range of mineral ass emblages and reaction textures that vary with bulk composition. All rocks f ollowed the same clockwise P-T path, which can be divided into four stages on the basis of reaction textures and garnet zoning patterns. Prograde evol ution is characterized by biotite dehydration melting in the absence of an aqueous fluid phase. The operating dehydration melting reactions changed wi th bulk composition on small scales and produced locally varying modal cord ierite garnet, spinel and orthopyroxene along with melt, Minimum estimates of peak temperatures (800-850 degrees C) and pressure constraints (0.5-0.7 GPa) emerge from experimental results. Phase compositions, however were las t equilibrated on the retrograde part of the P-T path but still record very high temperatures close to peak conditions (770-846 degrees C and 0.44-0.5 1 GPa) as indicated by the results of geothermobarometry (e.g. Na-in-cordie rite, garnet-cordierite, garnet-orthopyroxene equilibria). The calculated t emperatures are significantly higher than those previously obtained for sim ilar rocks of the Bayerische Wald. The P-T path and conditions derived here suggest that high-temperature metamorphism in the Moldanubian Zone of the Bohemian Massif was induced by anomalously high heat influx to shallow crus tal levels of 15-20 km depth subsequent to Variscan collision and crustal t hickening.