Thermal Model for the Baltoro-Muztagh Karakoram
Abstract
Following the Eocene (50 Ma.) collision of the Indian and Karakoram plates, crustal thickening and shortening in the Karakoram north of the Neo-Tethyan suture zones resulted in a widespread regional Barrovian metamorphism (M2) south of the Karakoram batholith. Pelitic lithologies are characterised by the assemblage: kyanite-staurolite-biotite-garnet-muscovite-plagioclase-quartz with sillimanite-muscovite and sillimanite-K-feldspar assemblages locally developed. Hornblende and diopside-bearing marbles and garnet-bearing amphibolites are also widespread. The age of this high pressure-high temperature metamorphism is constrained as post-50 Ma. (age of India-Asia collision) and pre-37 Ma. (age of cross-cutting, post-collisional granite plutons).
Intrusion of the 21 Ma. Baltoro batholith comprising compositions ranging from biotite monzogranite to two-mica leucogranite caused a high temperature low pressure thermal aureole of contact metamorphism in the country rock. Andalusite- bearing hornfels along the northern contact of the batholith indicates maximum pressures of 3.5 kbars (350 MPa). A 75˚ increase in temperature in kyanite-sillimanite grade gneisses approaching the granite contact along the Baltoro glacier is modelled as the thermal upwarping of pre-36 Ma. Barrovian metamorphic isograds around the 21 Ma. contact aureole isotherms along the margins of the Baltoro batholith. Post-metamorphic folding of the early Barrovian metamorphic isograds is related to thrust culminations along the hanging-wall of the Main Karakorarm Thrust˗ a reactivated breakback thrust along the older Shyok suture zone.
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