THE PANGONG INJECTION COMPLEX, INDIAN KARAKORAM - A CASE OF PERVASIVEGRANITE FLOW-THROUGH HOT VISCOUS CRUST

In the Karakoram Range of NW India, the Tangtse gorge cuts across deep crustal rocks exhumed by transpression along two strands of the dextr al Karakoram fault. Crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesc ed and expanded to form kilometre-scale plutons. Sheets resulted from slow seeping of granite into foliation planes (magma wedging) rather t han From dyking, driven by magma buoyancy and possibly syn-intrusive t ectonic deformation. High temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the int rusive leucogranite and in situ partial melt in the migmatites, and re flected in the ductile structures developed. Whereas low country rock viscosity inhibited dyking, high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion. The injecti on complex exposed along the Tangtse gorge, rather than representing t he final emplacement structures, represent transient structures of the granite pathways or, their upward journey to build the Karakoram bath olith, which crops out a few kilometres north, at structurally shallow er levels. We suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold c rust and it may be one of several factors controlling whether dyking o r diapirism becomes dominant during late ascent.