Journal of Petrology | Volume 40 | Number 11 | Pages 1699-1719 | 1999
© Oxford University Press 1999
Structures and Melt Fractions as Indicators of Rheology in Cordierite-Bearing Migmatites of the Bayerische Wald(Variscan Belt, Germany)
Mineralogisches Institut, University of Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany
Received October 1, 1998; Revised typescript accepted May 21, 1999
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Abstract |
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The relation between melt volume fraction, melt segregation and deformation mechanisms of minerals in migmatites, and the controlling effects of all these factors on the bulk mechanical behaviour, were investigated using rocks from the Bayerische Wald (Bohemian Massif, Variscan belt, Germany). Biotite dehydration melting at 800–850°C and 0.5–0.7 GPa was the migmatite-forming process in this area. Four migmatite types were distinguished that alternate on the scale of several decimetres to several tens of metres. Type MIG1 is massive and undeformed. Types MIG2 and MIG3 are both stromatic (leucosome–mesosome interlayering), but differ in the degree of deformation. Type MIG4 has an interlayering of melanosome and leucosome and is strongly deformed. Melt volume fractions found by volume estimation of pure-melt leucosomes on outcrop, hand-specimen and thin-section scales are 20–40 vol. % and coincide fairly well with melt volume fractions produced in dehydration melting experiments with similar bulk compositions at the relevant pressure and temperature conditions (20–30 vol. %). The degree of melt removal 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 segregation (MIG1 and MIG3), cordierite microstructures are indicative of growth within interconnected layers of melt. In migmatites with considerable melt segregation (MIG2 and MIG4), cordierite shows evidence of intracrystalline plasticity, indicating deformation within a load-bearing framework of minerals. Biotite microstructures in all migmatite types indicate passive rotation within a melt, but the degree of their shape-preferred orientation increases with increasing melt segregation and differential stress. The microstructures suggest that deformation mechanisms and hence the bulk mechanical behaviour of migmatites change in space and time during partial melting. The observed complex interplay of melt volume fraction, melt segregation, bulk strength contrasts, mechanical properties of different minerals and time cannot be described by a single flow law. Detailed mapping of migmatite areas, along with microstructural observations, deformation experiments considering heterogeneous melt distribution and numerical models that integrate different flow laws for various stages of partial melting, may serve to derive quantitative models for the bulk mechanical behaviour of crustal sections in the future.
KEY WORDS: deformation mechanisms; melt fractions; microstructures; migmatites
* Corresponding author. Present address: Mineralogisch-petro graphisches Institut, University of Bern, Baltzerstr. 1, CH-3012 Bern, Switzerland. Fax: ++41 31 6314843. e-mail: berger{at}mpi.unibe.ch