Metamorphism and Melting of the Lithosphere Due to Rapid Denudation, Nanga Parbat Massif Himalaya

Abstract

The Nanga Parbat-Haramosh Massif of the western Himalaya has undergone a complex metamorphic and denudational history over the past 1.8 Ga. Metamorphism in the Tato region is characterized by a Barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures >550°C. Mineralogical and textural evidence for this path includes garnet zoning patterns, late-stage migmatization, abundant cordierite both in schists and leucogranite dikes, replacement of high-pressure assemblages by low-pressure assemblages, and abundant low-density inclusions. Thermobarometry on the nonmigmatized gneisses reveals conditions of 540-740°C at 7.1-13.1 kbar. Thermobarometry in the migmat-ized rocks reveals final equilibration at 608-675°C at 3.9-6.8 kbar. Early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression. The last fluid inclusions to be trapped occur in microfractures that locally crosscut several grain boundaries and, hence, record conditions after quartz grain boundary migration ceased. Examination of all the petrologic and fluid inclusions data conclusively show a very rapid denudation event late in the collisional process. Rapid denudation resulted in the advection of isotherms to shallow crustal levels causing an elevated geotherm (~60°C/km) in the upper crust, metamorphism along shear zones, and partial melting of the crust at depths of ca. 22 km.