Hornblende Chemistry in Meta- and Diatexites and its Retention in the Source of Leucogranites: an Example from the Karakoram Shear Zone, NW India

A suite of calc-alkaline arc rocks in the Pangong Metamorphic Complex in the Karakoram Shear Zone, Ladakh, NW India, was migmatized at upper amphibolite facies under water-fluxed conditions. Meta- and diatexites contain abundant hornblende in both leucosome and melanosome. These form idiomorphic, poikilitic megacrysts in the leucosome and in melanosome rims around leucosomes. Hornblende chemistry, particularly its rare earth element (REE) pattern, is highly variable in protoliths and neosomes, and in most cases the REE contents do not reflect hornblende partition coefficients reported in the literature. These variations are the result of the complex crystallization histories of the migmatites. REE variations in whole-rock diatexites and leucogranites demonstrate the role of hornblende in controlling the REE budget. Formation of heavy REE (HREE)-enriched hornblende and its accumulation in the source results in leucogranitic magmas with high La-n/Yb-n and Sr/Y values typical of adakites. We show that leucogranitic magmas, depleted in HREE, can be generated by water-fluxed partial melting of arc-related rocks in the mid-crust without requiring garnet retention in the source.