PRESSURE-TEMPERATURE CONDITIONS OF INDIAN PLATE ROCK WITHIN THE INDUS SYNTAXIS, NW HIMALAYA, PAKISTAN

The Indus syntaxis is a large northerly-trending anticline (the Indus River anticline) cored by Early Proterozoic schist, amphibolite, and gneiss, and flanked by Late Proterozoic to Triassic schist, marble, amphibolite, and gneiss. Rock units within the syntaxis trend north-south at a high angle to the general east-west trend of the Himalayan orogen and at a high-angle to the nearly east-west trending Indus Suture zone which abruptly truncates the rock units along their north side. Metamorphism increases in all rock units from greenschist facies in the southern part of the syntaxis surrounding Tarbela Lake where garnet is absent, to amphibolite facies in the north where garnet is widespread and where kyanite and/or fibrolitic sillimanite occur sporadically in rocks of appropriate bulk composition. Temperature-pressure conditions were calculated for 12 rocks from an area which extends 75 km southward from the suture zone and 35 km east to west on both limbs of the Indus River anticline. All 12 rocks contain the assemblage garnet-biotite-muscovite-plagioclase-quartz, ±kyanite, ±fibrolitic sillimanite. They include three garnet schists from the Gandaf and four from the Tanawal formation, two granitic gniesses each from the Kotla and Mansehra complexes, and a rare garnet schist from the lower part of the Indus suture zone. The Gandaf is intruded by the Kotla which has a U-Pb zircon age of 1836 ±1 Ma. The Tanawal unconformably overlies both the Gandaf and Kotla and is intruded by Upper Cambrian Mansehra complex. Eight rocks from the northern part of the syntaxis record 650 ±50 °C and 9.5 ±2.0 kbar. The schist from the suture zone appears to be an imbricated lens of Late Paleozoic Duma formation and records 580 °C and 9.7 kbar. Three rocks from the Tanawal formation in the southern part of the syntaxis record 540 ±10 °C and 6.8 ±1.3 kbar. Metamorphic conditions are consistent with those in the Loe Sar dome to the west and in the Hazara area to the east. Ar/Ar hornblende ages from 38 to 67 Ma within the syntaxis suggest Late Cretaceous or early Paleogene peak metamorphism. The metamorphism is attributed to Late Cretaceous underthrusting and partial subduction of the Indian plate beneath ophiolitic melange of the Indus suture zone beginning prior to initial collision of India with Kohistan at ca. 54 Ma.