Abstract

In the Karakoram Range of NW India, the Tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral Karakoram fault. Crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre-scale plutons. Sheets resulted from slow seeping of granite into foliation planes (magma wedging) rather than from dyking, driven by magma buoyancy and possibly syn-intrusive tectonic deformation. High temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites, and reflected 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 injection complex exposed along the Tangtse gorge, rather than representing the final emplacement structures, represent transient structures of the granite pathways on their upward journey to build the Karakoram batholith, which crops out a few kilometres north, at structurally shallower levels. We suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent.

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