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Published December 2004 | public
Journal Article

Limited migration of leucosomes in a migmatite and effects of progressive partial melting on strain partitioning


The migration of melts in the crust and mantle is one of important problems in geology. As a product of crustal anatexis, leucosomes in a migmatite provide a unique opportunity to investigate the factors that affects the migration and transportation of crustal melts in the lower and middle continental crust. We carried out a detailed major element geochemistry and structural analyzes on a suite of leucosomes from the Goat Ranch migmatite complex, Southern Sierra Nevada, California, and theoretical calculations to evaluate their migration distances. Field observations show that: (1) the leucosomes have a thickness ranging from millimeter to centimeter and (2) metamorphic temperatures, metamorphic grades, and degrees of partial melting in the Cretaceous metapelite increase with the decreasing proximity of the Goat Ranch pluton. In the localities of partial melting of high degrees (>10%), the loading framework is dominated by IWL (Interconnected Weak Layers) in which the strain is accommodated by weak interconnected leucosomes. In contrast, LBF (Load bearing framework) dominated those areas having a low degree of partial melting (<5%) where leucosomes occurred as isolated blobs or pockets. The strain was accommodated by strong matrix. It seems that melts represented by the leucosomes acts as weak phases during deformation. During migmatization, the presence of melts greatly affects the bulk strength and the strain partitioning for a rock undergoing syn-metamorphism deformation. Using the Shaw's viscosity model and modified Stoke's equation, we calculated the viscosities of the leucosomes based on their major element compositions and estimated their migration distance. These results show that these leucosomes had experienced limited migration due to high viscosities. They have viscosities ranging from 10^9 to 10^(12)Pas, which are 3 to 8 orders of magnitude higher than 10^4 ~ 10^6 Pas for typical granites. Results from this study provide strong support for using migmatites to characterize the geochemical and isotopic geochemistry of crustal anatexis and examine the mechanic properties of molten rocks.

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© 2004 Science Press.

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