The Karakoram Shear Zone dike swarm: syn-kinematic magma transfer linking source to batholith

The migration of granitic magmas is debated, at least in part, because the physical link between migmatites and plutons and batholiths is rarely documented. Within the Karakoram Shear Zone, Ladakh, NW India, the syn-kinematic transfer of magma through an interconnected magma flow network can be traced from the anatectic source region to the Karakoram Batholith through a complex dike swarm. In situ and intrusive Miocene hornblende leucogranites, biotite leucogranites and garnet-two mica leucogranites resulted from water-fluxed melting at upper amphibolite facies. Meta- and diatexites in calc-alkaline granitoids contain abundant idiomorphic, poikilitic hornblende megacrysts in leucosome and melanosome rims, while migmatitic biotite psammites and pelites contain no peritectic minerals. The root of the magma network in the anatectic zone is characterized by syn-magmatic folding and shearing of migmatites, and by pervasive, irregular magma migration paths forming injection complexes. The magma transfer zone, mostly through non-anatectic rocks, is characterized by dikes forming anastomozing systems with rare cross-cutting relationships suggesting interconnectivity of magma in the dike system, and by dike intersections oriented parallel to the dominant mineral stretching lineation. The network comprises a range of styles: (a) dike swarms parallel to the shear plane, (b) dikes following the two major foliation planes in the shear fabric, (c) magma sheets in conjugate pairs of ductile fractures, or (d) chaotic dike complexes. Network geometries are controlled by the regional stress field, strain distribution, pre-existing anisotropies and rheological contrasts. The dike swarm feeds a number of sills, stocks, plutons, and, ultimately, the leucogranitic Karakoram Batholith. We propose that magma developed an interlinked, continuous dike swarm from source to sink by interacting with deformation and pre-existing anisotropies. Unlike theoretical and laboratory predictions, dikes are typically oriented at high angles to the maximum shortening axis.