Hybridization of granitic magmas in the source: The origin of the Karakoram Batholith, Ladakh, NW India

Abstract

Many magmatic bodies have a hybrid isotopic signature suggesting that somewhere during genesis, transport and emplacement, magmas assimilated other rocks or mixed with other magmas. Where and how hybridization takes place is seldom documented. Here, we investigate a magmatic system in the Eastern Karakoram, Ladakh, NW India, comprising an anatectic zone, and a network of sheets, stocks and plutons exposed in the Pangong Metamorphic Complex within the Karakoram Shear Zone, as well as the Karakoram Batholith. These granitic rocks have an isotopic signature indicative of a mixture between mantle and crustal sources. In the anatectic region, calc-alkaline granitoids and their meta-sedimentary country rocks underwent water-fluxed partial melting at upper amphibolite facies between 20 and 14Ma ago. Anatexis gave rise to leucosomes and intrusive rocks that have a range in composition from leucotonalite to leucogranite. Those related to the partial melting of calc-alkaline rocks contain hornblende, whereas those related to Bt-psammites contain two micas±garnet. Leucosomes rooting in different source rocks merge with each other and homogenize as they link up to form a hierarchy of magma channels, feeding into stocks, plutons and ultimately into the Karakoram Batholith. This interpretation is supported by Sr and Nd isotopes. Initial 87Sr/86Sr and εNd values are distinct for each of the magma protoliths in the anatectic zone and for the magmatic products. Calc-alkaline granitoids have initial 87Sr/86Sr=0.7042 to 0.7077 and εNd=+0.6 to +2.4, indicative of a slightly depleted mantle source region. This is in contrast to the meta-sedimentary rocks that yield initial 87Sr/86Sr=0.7115 to 0.7161 and εNd=-10.0 to -9.6, suggesting a stronger crustal component. Leucogranitic rocks, including a variety of leucosomes in the anatectic zone and samples from the Karakoram Batholith, yield intermediate values of initial 87Sr/86Sr=0.7076 to 0.7121 and εNd=-3.6 to -7.1 that can be modelled by mixing of the two source rocks. The hybrid signature of leucosomes and their similarity to intrusive leucogranites indicate that magma hybridization must have taken place within the source region as a result of the confluence of magmas to form the escape channels. We conclude that the voluminous leucogranites of the Miocene Karakoram Batholith result from water-fluxed intracrustal melting of sources with crustal and mantle signatures, and that mixing occurred within the source.