| Summary: | 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 orthogneiss) and associated low pressure andalusite, staurolite and garnet grade metamorphism (Ml). 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 SiO -quartz-plagioclase geobarometers indicates peak M2 P-T conditions of 696 f 20 °C at 8.6 ± 0.7 kbar (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 baseo f the thickened crust caused widespread crustal melting and intrusiono f 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.
|
|---|