Field relations, geochemical, isotopic and age studies of the Teru Volcanic Formation, northern Kohistan Terrane, Pakistani Himalayas

This research presents new field, structural, geochemical, isotopic 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 comprises a 3-km thick sequence of subaerially-erupted mafic to felsic lavas and associated volcaniclastic rocks. The Shunji Pluton bounds this sequence to the south and the sequence is overthrust to the north by the mid-Cretaceous Chalt Volcanic Group (intra-oceanic arc phase) along the south-verging Ghizer Thrust. These volcanics are lithologically comparable to similar-age suites elsewhere in the Himalayas. These lavas are dominated by a typical convergent margin assemblage of clinopyroxene, amphibole, plagioclase and magnetite. The Teru Volcanic Formation ranges from basalt through rhyolite, and have tholeiitic, calc-alkaline, and shoshonitic affinities. Trace element features also suggest subduction-related patterns which is also supported by isotopic studies. Isotopic and trace element studies show that these lavas originated from a mantle source which experienced a combination of variable subduction components added to a variable mantle wedge composition. Mixing calculations performed on 87Sr/86Sr versus 143Nd/144Nd specify that addition of 0.2–0.6% of Indus margin sediments and/or 2–4% of Indus margin sediment fluids can generate Teru volcanics composition. 40Ar/39Ar studies of Teru Volcanics show well-defined plateau age of 43.8 ± 0.5, and 32.5 ± 0.4 Ma. These ages makes the Teru Volcanics the youngest reported volcanics in Kohistan terrane. The country rock for these volcanics, the Shunji Pluton, shows a 65 Ma Rb-Sr whole rock age.