Age, geochemical and Sr–Nd–Pb isotopic constraints for mantle source characteristics and petrogenesis of Teru Volcanics, Northern Kohistan Terrane, Pakistan

This paper presents new geochemical and geochronology data for the Teru Volcanic Formation (previously known as the Shamran Volcanics) exposed west of Gilgit in the Kohistan terrane of the Pakistani Himalayas. The Teru Volcanic Formation ranges from basalt through andesite to rhyolite and has subalkaline and midalkaline affinities. Trace-element compositions and isotopic characteristics suggest these magmas were formed in a subduction zone setting; isotopic studies also support this conclusion. It is suggested that these lavas originated from a depleted mantle source, which experienced contamination by variable subduction components. Model mixing calculations using 87Sr/ 86Sr and 143Nd/ 144Nd data suggest that addition of 0.2–0.6% of Indus margin sediments and/or 2–4% of fluids derived from Indus margin sediment can generate the compositional variation of the Teru Volcanic Formation. Two samples from the Teru Volcanic Formation yielded 40Ar/ 39Ar ages of 43.8+0.5 and 32.5+0.4 Ma. These ages make the volcanic rocks of the Teru Volcanic Formation the youngest reported in the Kohistan terrane. These volcanic rocks unconformably overly the Shunji Pluton, which has a 65 Ma Rb–Sr whole-rock isochron age. The results of this research suggest that subduction-related volcanism was active until 33 Ma in the India–Asia collision zone.