Growth and chemical evolution of the Kohistan-Ladakh Arc crust, Northern Pakistan, Western Himalayas

The 55 km-thick Kohistan-Ladakh Arc (Northern Pakistan) was an intra-oceanic arc for 120 Ma prior to becoming an Andean-type continental arc from 50 to 40 Ma. We present new geochemical data from the Kohistan-Ladakh Arc and use existing geochemical and geochronological literature data from this arc to constrain the growth and chemical evolution of the arc crust. The ultramafic to mafic lower crust of the Kohistan-Ladakh Arc is older than their intermediate to felsic middle to upper crust. Olivine, clinopyroxene, anorthite and amphibole fractionated from basaltic magmas to form the intermediate to felsic plutons of the Kohistan Batholith. The middle to upper crust of the Kohistan-Ladakh Arc has higher Th/La and lower ¹⁴³Nd/¹⁴⁴Nd ratios than the older lower crust. The later intermediate to felsic plutons of this arc formed by fractional crystallisation of basaltic magmas and were more influenced by slab sediment melts than crustal contamination. The 150 Ma Matum Das Pluton and Hunza Formation were cogenetic. The 67-65 Ma plutonic rocks of the eastern Kohistan Batholith are genetically related to the 65-60 Ma Teru Formation and Dir Group. The high (Dy/Yb)_N ratios of some of the mafic plutonic rocks imply deeper melting of the Kohistan-Ladakh sub-arc mantle once the arc had attained a thickness of >42 km Late Cretaceous. The older volcanic and plutonic rocks in the Kohistan-Ladakh Arc resemble those of the active Izu-Bonin-Mariana and Tonga intra-oceanic arcs, whereas the younger igneous rocks are more similar to those in the active Cascade continental arc.