Multiple Use of Magma Pathways: Mechanism for Hybridization

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking the Pangong Range anatectic terrane with leucogranites of the Karakoram Batholith. Water-fluxed Miocene anatexis occurs at upper amphibolite conditions, and was contemporaneous with shearing. The network is characterized by continuous and interconnected leucosomes and dykes, with only rare cross-cutting relationships, forming dyke swarms and more chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system was always controlled by solid framework suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. This meso- to macroscale complex network is also reflected at microscale. Microstructural observations indicate that these magmatic rocks consist dominantly of Qtz, Plg and Kfs in two very distinct appearances, as large irregularly-shaped grains with cuspate boundaries, or/and as fine-grained minerals with lobate boundaries. These two show intimate spatial relationship with fine-grained material forming semi- to continuous corridors to wide channels that links together and form an extensive network branching around large grains. We suggest, that the large minerals represent early formed solid granitic framework that was later invaded by a new melt batch that exploits microfractures in between and through the framework forming crystals giving rise to this interconnected network. The presence of later crystallized melt and its interaction with the solid rock was inferred from the following microstructures: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) compositional zoning of early-crystallized plagioclase and K-feldspar; (iii) quartz overgrows documented by CL imaging; (iv) corrosion of early-formed grains; and (v) different CPO of early-formed quartz and its overgrowths. In summary, the early formed dykes provided a pathway exploited by new magma batches. Once formed, the magma channels remained open either intermittently or continuously and the new melt batches migrated through following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. Accordingly, hybrid signature results from the microscopic interaction between previously crystallized magmatic rock and new magma batch, through local equilibration, not from magma mixing. We conclude that leucosomes and magmatic bodies formed by the magma that flushed through them have a complex origin and composition that is reflected in the geochemistry and isotope chemistry. Final composition is a result of the accumulation of magma residue. This in turn depends on compositional changes of magma influx, P-T conditions, and the interaction of new magma with early crystallized magmatic products.