Metamorphic Evolution of Cordierite-Bearing Migmatites from the Bayerische Wald (Variscan Belt, Germany)

Abstract

A detailed petrological study was carried out on cordierite-bearing migmatites of the Bayerische Wald (Germany, Bohemian Massif, Variscan Belt) to constrain 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 four migmatite types and their intercalations preserve a range of mineral assemblages and reaction textures that vary with bulk composition. All rocks followed the same clockwise P–T path, which can be divided into four stages on the basis of reaction textures and garnet zoning patterns. Prograde evolution is characterized by biotite dehydration melting in the absence of an aqueous fluid phase. The operating dehydration melting reactions changed with bulk composition on small scales and produced locally varying modal cordierite, garnet, spinel and orthopyroxene along with melt. Minimum estimates of peak temperatures (800–850°C) and pressure constraints (0.5–0.7 GPa) emerge from experimental results. Phase compositions, however, were last equilibrated on the retrograde part of the P–T path but still record very high temperatures close to peak conditions (770–846°C and 0.44–0.51 GPa) as indicated by the results of geothermobarometry (e.g. Na-in-cordierite, garnet–cordierite, garnet–orthopyroxene equilibria). The calculated temperatures are significantly higher than those previously obtained for similar 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 crustal levels of 15–20 km depth subsequent to Variscan collision and crustal thickening.