Syn-kinematic, water-fluxed melting and ascent of granitic magmas in the Karakoram Shear Zone, Ladakh, NW India

Abstract

The Karakoram Shear Zone in Ladakh, NW India, forms part of a crustal-scale shear zone system that developed in response to the northward push of the Indian into the Eurasian continent. In its central part, the ~750 km long, NW-SE trending shear zone is split into two parallel mylonitic strands that bound deeply exhumed rocks of the Pangong Metamorphic Complex. This region provides a rare opportunity to study the conditions of magma formation, its segregation from the source and subsequent ascent through the crust to feed a batholith. Metamorphism at upper amphibolite facies gave rise to anatexis of a suite of calc-alkaline granitoids of the Muglib Batholith and a meta-sedimentary sequence, and to formation of leucogranite dykes, stocks and plutons. Timing of anatexis is constrained by U-Pb SHRIMP analyses on zircons and titanites of a leucogranite and two calc-alkaline granitic rocks. These yielded Miocene (~18 Ma) crystallization ages for the leucogranite and a contemporaneous Miocene metamorphic event in the Cretaceous (~71 Ma) calc-alkaline rocks. Meta- and diatexites of calc-alkaline protolith have abundant euhedral, poikilitic hornblende megacrysts in the leucosome, interpreted to be peritectic and indicative for water-fluxed conditions. Hornblende locally forms accumulations in the migmatitic source, and is generally absent in the leucogranites derived from these source rocks. REE contents in hornblende are unusual and vary between samples, and have significant influence on the REE concentrations of the magmatic products. Geochemical modelling by using own determined partition coefficients for these hornblendes, shows that retainment of hornblende in the source explains adakite-like high Lan/Ybn and high Sr/Y values of leucogranites, without involvement of garnet in the residue. Rb-Sr and Sm-Nd isotope systematics show that magma stemming from two distinct sources, calc-alkaline granitoids with a signature indicative of a mantle source, and meta-sedimentary rocks with a stronger crustal signature, hybridized to form magmatic rocks with an intermediate hybrid isotope signature. Magma hybridization is a common feature of magmas world-wide, but how and where hybridization takes place has remained debatable. Here it is demonstrated that granite magmas hybridize already within the source by confluence into shared flow channels. The link between anatectic rocks and the Karakoram Batholith is characterized by a transported zone dominated by dyke swarms linked to local stocks and plutons. Dykes are generally interconnected, with rare cross-cutting or truncation relationships, and have a common intersection direction parallel to the dominant mineral lineation. All dyke networks are oriented at high angles to the orientation of the axis of maximum compression during shearing, and not, as expected from the literature, at high angles to the minimum compression axis. Isotope systematics was also used to investigate relationships between the Muglib Batholith, and the contemporaneous Ladakh Batholith to the south. The…