Time calibration of a P-T path from a Variscan high-temperature low-pressure metamorphic complex (Bayerische Wald, Germany), and the detection of inherited monazite

A temperature-time path was constructed for high-temperature low-pressure ( HT-LP) migmatites of the Bayerische Wald, internal zone of the Variscan bel t, Germany. The migmatites are characterised by prograde biotite dehydratio n melting, peak metamorphic conditions of approximately 850 degrees C and 0 .5-0.7 GPa and retrograde melt crystallisation at 800 degrees C. The time-c alibration of the pressure-temperature path is based on U-Pb dating of sing le zircon and monazite grains and titanite separates, on Ar-40/Ar-39 ages o btained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and on Ar-40/Ar-39 spot fusion ages of bi otite determined in situ from sample sections. Additionally, crude estimate s of the duration of peak metamorphism were derived from garnet zoning patt erns, suggesting that peak temperatures of 850 degrees C cannot have prevai led much longer than 2.5 Ma. The temperature-time paths obtained for two ar eas approximately 30 km apart do not differ from each other considerably. U -Pb zircon ages reflect crystallisation from melt at 850-800 degrees C at 3 23 Ma (southeastern area) and 326 Ma (northwestern area). The U-Pb ages of monazite mainly coincide with those from zircon but are complicated by vari able degrees of inheritance. The preservation of inherited monazite and the presence of excess Pb-206 resulting from the incorporation of excess Th-23 0 in monazite formed during HT-LP metamorphism suggest that monazite ages i n the migmatites of the Bayerische Wald reflect crystallisation from melt a t 850-800 degrees C and persistence of older grains at these temperatures d uring a comparatively short thermal peak. The U-Pb ages of titanite (321 Ma ) and Ar-40/Ar-39 ages of hornblende (322-316 Ma) and biotite (313-309 Ma) reflect cooling through the respective closure temperatures of approximatel y 700, 570-500 and 345-310 degrees C published in the literature. Most of t he feldspars' ages (305-296 Ma) probably record cooling below 150-300 degre es C, while two grains most likely have higher closure temperatures. The te mperature-time paths are characterised by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 degrees C/my at temperatures between 850-800 and 700 degrees C to 11-16 degrees C/m y at temperatures down to 345-310 degrees C. Further cooling to feldspar cl osure for Ar was probably even slower. The lack of decompressional features , the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating, rapi d uplift and extension due to lithospheric delamination as proposed elsewhe re. Instead, the high peak temperatures at comparatively shallow crustal le vels along with the short thermal peak require external advective heating b y hot mafic or ultramafic material.