Metamorphic pressure‐temperature conditions of Indian Plate rocks south of the main mantle thrust, lower swat, Pakistan

Abstract

The Lower Swat rock sequence, in northern Pakistan, is composed of greenschist and amphibolite facies Indian plate rocks that crop out in a dome directly south of the Main Mantle thrust zone and Kohistan arc terrane. The metamorphism is Eocene to Oligocene in age and records the collision of the Indian plate with the Kohistan arc. This paper presents mineral assemblage, mineral composition and garnet zoning data on the Lower Swat rock sequence in order to estimate the pressure‐temperature conditions of metamorphism and to infer a possible pressure‐temperature‐time path. The stratigraphic sequence consists of Precambrian to Cambrian(?) Manglaur formation unconformably overlain by late Paleozoic to early Mesozoic Alpurai group. The Alpurai group is further subdivided into the Marghazar, Kashala, Saidu and Nikanai Ghar formations. The Manglaur formation, in the core of the dome, is at kyanite grade. Metamorphic grade decreases in the overlying Alpurai group where a garnet “isograd” is mapped in calcareous schist of the Kashala formation. Mineral composition geothermometers and geobarometers indicate final metamorphic equilibrium conditions of about 600°–700°C and 9–11 kbar in the Manglaur and Marghazar formations. These conditions are associated with a phase of static recrystallization which postdates the main phases of deformation. Correlation of the structural‐tectonic history with the pressure‐temperature estimates, and with the garnet zoning analysis, suggests that in the Eocene the Lower Swat rock sequence was subducted to a depth of 35 to 45 km beneath the MMT suture melange. Subduction was followed immediately by exhumation. The initiation of exhumation is believed to have been triggered by a change from subduction to strike slip motion in the MMT, thereby allowing the relatively buoyant Lower Swat crust to rise. The entire metamorphic‐deformational cycle lasted from 7 to 16 m.y., ending in the late Eocene about 38 Ma when the rocks cooled through the argon blocking temperature in hornblende (≈ 550°C).