Structural and thermal evolution of the Karakoram crust

Abstract

Prior to the Eocene (c. 50 Ma) collision of the Indian and Asian plates, the southern margin of Asia along the Karakoram plate was an Andean-type margin dominated by tonalitic-granodioritic magmatism of Jurassic––Lower Cretaceous age (Hushe gneiss, Muztagh Tower gneiss and K2 ortho-gneiss) and associated low pressure andalusite, staurolite and garnet grade metamorphism (M1). Following India-Asia collision, crustal shortening, thickening and regional Barrovian metamorphism (M2) occurred between 50–37 Ma. Thermobarometry of kyanite-grade metapelites indicate burial to depths of around 30–35 km. Simultaneous solution of the garnet-biotite geothermometer with the garnet-muscovite-biotite-plagioclase and garnet-AI 2 , SiO 5 ,-quartz-plagioclase geobarometers indicates peak M2 P - T conditions of 696±20°C at 8.6±0.7kbar (860 MPa). Temperatures may have exceeded 700 °C in sillimanite-grade metapelites to produce in situ partial melting and leucogranitic melt pods. Peak M2 metamorphism occurred prior to 37 ±0.8 Ma, the crystallization age of the Mango Gusar two-mica granite pluton which cross-cuts syn-metamorphic deformation fabrics. Post-M2 thermal relaxation followed from 37–25 Ma, after which localized high heat concentrations at the base of the thickened crust caused widespread crustal melting and intrusion of the Baltoro granite batholith at 25–21 Ma. A high temperature-low pressure thermal aureole (M3) along the northern contact is synchronous with the 21 ±0.5 Ma zircon age of the Baltoro granite. Andalusite hornfels along the northern contact of the batholith (Mitre thermal aureole) indicates maximum pressures of 3.75 kbar (375 MPa). A 75 °C increase of temperature in kyanite-sillimanite grade gneisses approaching the southern granite contact of the Baltoro granite is interpreted as the thermal upwarping of pre-37 Ma Barrovian metamorphic M2 isograds around the 21 Ma contact aureole M3 isotherms.