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Berger, 1999 estructuras y reologia en migmatitas con cordierita Alemania

Berger, 1999 estructuras y reologia en migmatitas con cordierita Alemania

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JOURNAL OF PETROLOGY VOLUME 40 NUMBER 11 PAGES 1699–1719 1999 Structures and Melt Fractions as Indicators of Rheology in Cordierite-Bearing Migmatites of the Bayerische Wald (Variscan Belt, Germany) ALFONS BERGER * AND ANGELIKA KALT MINERALOGISCHES INSTITUT, UNIVERSITY OF HEIDELBERG, IM NEUENHEIMER FELD 236, D-69120 HEIDELBERG, GERMANY RECEIVED OCTOBER 1, 1998; REVISED TYPESCRIPT ACCEPTED MAY 21, 1999 a melt, but the degree of their shape-preferred orientation increases The relation between melt volume fraction, melt segregation and with increasing melt segregation and di ff erential stress. The mi- deformation mechanisms of minerals in migmatites, and the con- crostructures suggest that deformation mechanisms and hence the trolling e ff ects of all these factors on the bulk mechanical behaviour, bulk mechanical behaviour of migmatites change in space and time were investigated using rocks from the Bayerische Wald (Bohemian during partial melting. The observed complex interplay of melt Massif, Variscan belt, Germany). Biotite dehydration melting at volume fraction, melt segregation, bulk strength contrasts, mechanical 800–850 ° C and 0·5–0·7 GPa was the migmatite-forming process properties of di ff erent minerals and time cannot be described by a in this area. Four migmatite types were distinguished that alternate single flow law. Detailed mapping of migmatite areas, along with on the scale of several decimetres to several tens of metres. Type microstructural observations, deformation experiments considering MIG1 is massive and undeformed. Types MIG2 and MIG3 are heterogeneous melt distribution and numerical models that integrate both stromatic (leucosome–mesosome interlayering), but di ff er in the di ff erent flow laws for various stages of partial melting, may serve degree of deformation. Type MIG4 has an interlayering of me- to derive quantitative models for the bulk mechanical behaviour of lanosome and leucosome and is strongly deformed. Melt volume crustal sections in the future. 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 KEY WORDS: deformation mechanisms; melt fractions; microstructures; segregation) increases in the order MIG1–MIG3–MIG2–MIG4 migmatites and is controlled by melt volume fraction and by strain partitioning between the di ff erent 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 INTRODUCTION melt segregation (MIG1 and MIG3), cordierite microstructures are indicative of growth within interconnected layers of melt. In mig- The mechanical behaviour of migmatites plays a major matites with considerable melt segregation (MIG2 and MIG4),
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