The geochemical and tectonic evolution of the central Karakoram, North Pakistan

Abstract

The Main Karakoram Thrust (MKT) separates the Karakoram Plate from the accreted Kohistan—Ladakh Terranes and Indian Plate to the south. Within the central Karakoram three geologically distinct zones are recognized: from south to north (i) the Karakoram metamorphic complex, (ii) the Karakoram batholith and (iii) the northern Karakoraih sedimentary terrane. Magmatic episodes of Jurassic and mid-Upper Cretaceous age are recognized before India-Asia collision at ca. 50-45 Ma. Both reveal subduction-related petrographic and geochemical signatures typical of Andean-type settings. Associated with the Jurassic event was a low-pressure metamorphism (Ml). Synchronous with the mid-Upper Cretaceous episode was the passive accretion of the Kohistan-Ladakh terrane to the Karakoram and closure of the Shyok Suture Zone (SSZ). The main collision between the Indian and Asian Plates resulted in crustal thickening beneath the Karakoram and development of Barrovian metamorphism (M2). Early postcollisional plutons dated at 36-34 Ma cross-cut regional syn-metamorphic foliations and constrain a maximum age on peak M2 conditions. Uplift of the Karakoram metamorphic complex in response to continued crustal thickening was synchronous with culmination collapse along the inferred Karakoram Batholith Lineament (KBL). A combination of thermal re-equilibration of thickened continental crust and the proposed addition of an enriched mantle component promoted dehydration, partial melting and generation of the Baltoro Plutonic Unit (BPU). It was subsequently emplaced as a hot, dry magma into an extensional mid-crustal environment. A contact aureole (M3) was imposed on the low-grade sediments along the northern margin, whereas isograds in uplifted metamorphic rocks to the south were thermally domed with in situ migmatization.