Structures and melt fractions as indicators of rheology in cordierite-bearing migmatites of the Bayerische Wald (Variscan belt, Germany)

The relation between melt volume fraction, melt segregation and deformation mechanisms of minerals in migmatites, and the controlling effects of all t hese factors on the bulk mechanical behaviour; were investigated using rock s from the Bayerische Wald (Bohemian Massif, Variscan belt, Germany). Bioti te dehydration melting at 800-850 degrees C and 0.5-0.7 GPa was the migmati te-forming process in this al ea. Four migmatite types were distinguished t hat alternate on the scale of several decimetres to several tens of metres. Type MIG1 is massive and undeformed. types MIG2 and MIG3 al-e both stromat ic (leucosome-mesosome interlayering), but differ in the degree of deformat ion, Type MIG4 has an interlayering of melanosome and leucosome and is stro ngly deformed. Melt volume fractions found by volume estimation of pure-mel t leucosomes on outcrop, hand-specimen and thin-section scales are 20-40 vo l. % and coincide fairly well with melt volume fractions produced in dehydr ation melting experiments with similar bulk compositions at the relevant pr essure and temperature conditions (20-30 vol. %). The degree of melt remova l from mesosomes and melanosomes (melt segregation) increases in the order MIG1-MIG3-MIG2-MIG4 and is controlled by melt volume fraction and by strain partitioning between the different types as a result of strength contrasts . The degree of melt segregation controls the formation of microstructures in the four migmatite types. In migmatites with no or only little melt segr egation (MIG1 and MIG3), cordierite microstructures are indicative of growt h within interconnected layers of melt. In migmatites with considerable mel t segregation (MIG2 and MIG4), cordierite shows evidence of intracrystallin e plasticity, indicating deformation within a load-bearing framework of min erals. Biotite microstructures in all migmatite types indicate passive rota tion within a melt, but the degree of their shape-preferred orientation inc reases with increasing melt segregation and differential stress. The micros tructures suggest that deformation mechanisms and hence the bulk mechanical behaviour of migmatites change in space and dime during partial melting. T he observed complex interplay of melt volume fraction, melt segregation, bu lk strength contrasts, mechanical properties of different minerals and lime cannot be described by, a single flow lam Detailed mapping of migmatite ar eas, along with microstructural observations. deformation experiments consi dering heterogeneous melt distribution and numerical models that integrate different flow laws for various stages of partial melting, may serve to der ive quantitative models for rite bulk mechanical behaviour of crustal secti ons In the future.