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GSA Bulletin; July 2008; v. 120; no. 7-8; p. 994-1009; DOI: 10.1130/B26227.1
© 2008 Geological Society of America
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Magma migration, folding, and disaggregation of migmatites in the Karakoram Shear Zone, Ladakh, NW India

Roberto F. Weinberg1,{dagger} and Geordie Mark1,*

1 School of Geosciences, Monash University, Clayton, Vic 3800, Australia

Correspondence: {dagger}E-mail: roberto.weinberg{at}sci.monash.edu.au

Efficient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. We investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, NW India, in which leucosome distribution is inferred to record the permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to the fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding were destroyed. A number of structures developed during this process such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and flow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.

Key Words: diatexite • metatexite • migmatite • folding • transposition • granites




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