Massif-wide metamorphism and fluid evolution at Nanga Parbat, Northern Pakistan

Abstract

The Nanga Parbat-Haramosh Massif (NPHM) is located in northwestern Pakistan at one of the two great syntaxial bends of the Himalayas. The NPHM has experienced a complex tectonic and metamorphic history involving at least an earlier Himalayan metamorphic event associated with subduction and crustal thickening and a later metamorphic overprint associated with rapid uplift and exhumation. Here we present the results of a massif-wide petrologic and fluid study, compiling and characterizing variations in metamorphic mineral assemblages, thermobarometric data, P-T path interpretations, and subsurface fluid compositions. Metamorphic mineral assemblages reveal steep metamorphic gradients away from the granulite grade cordierite-sillimanite-bearing massif core into amphibolite facies rocks toward the flanks. New and compiled thermobarometric data show distinct variations within the massif with rocks in the rapidly denuding core recording low-pressure metamorphic conditions and rocks toward the eastern margin recording higher-pressure conditions similar to adjoining Ladakh terrane rocks. Compiled P-T paths are variable within the massif but generally show near isothermal decompression in the massif core and fragments of clockwise P-T histories toward the massif margins. We suggest that the systematic variation of mineral assemblages, thermobarometric data, and P-T paths is consistent with a recently proposed "pop-up structure" model for the NPHM, whereby recent asymmetrical structural uplift of the massif core is accommodated along high angle faults and shear zones within the massif. Consistent with the pop-up structure model, fluid inclusion data from quartz veins on a transect across the massif suggest a dual hydrothermal system composed of both meteoric and metamorphic fluids. Subsurface fluids are dominantly water-rich in the massif core, becoming more CO2-rich toward the eastern massif margin, suggesting that infiltration of meteoric water into the subsurface is greatest in the massif core and is driven by steep topography generated by rapid uplift Oxygen isotope ratios of quartz veins indicate that fluids readily equilibrate with host rocks suggesting a rock-dominated hydrothermal system.