The Kohistan Island Arc, an oceanic arc obducted on top of the Indian Plate, formed in the equatorial region of the Tethys Ocean during the Mesozoic to Cenozoic eras. The Chilas Complex, a unit of this arc, contains gabbronorite, ultramafic, anorthosite and diorite rocks, and represents exhumed lower arc crustal sections. Previous studies suggest the Chilas Complex as a potential back-arc basin. This study focuses on the source and chemical modification of the Chilas gabbronorite. Our data reveals that the evolved lower crust of the oceanic arc is geochemically similar to the continental lower crust. The primitive magma was derived from a mixture of depleted mid-ocean ridge basalt mantle (DMM) and material from the recycled sediments of the subducted slab. The zircon U-Pb dating reveals that the Chilas gabbronorite crystallized at 87.5-83.5 Ma. The zircon trace elements and Hf isotope compositions also support that the mafic rocks are contaminated by the continental crust materials. The amalgamation of the continental component in the Chilas magma took place through metasomatism of its mantle source rocks by subduction-derived fluids. We suggest that the pelagic, detrital and some terrigenous sediments of the Greater India were deposited in the Tethys Ocean, which in turn descended with subduction and mixed up with the Chilas magma. The oceanic arc depleted magma plays a significant role in the growth of the lower continental crust.