The Thermal, Metamorphic and Magmatic Evolution of a Rapidly Exhuming Terrane: the Nanga Parbat Massif, Northern Pakistan.

Whittington, Alan Geoffrey (1997). The Thermal, Metamorphic and Magmatic Evolution of a Rapidly Exhuming Terrane: the Nanga Parbat Massif, Northern Pakistan. PhD thesis The Open University.

Abstract

The Nanga Parbat-Haramosh Massif (NFHM) is the most northerly outcrop of Indian Plate rocks in the Himalayan orogen, exposed by rapid recent exhumation through the structurally overlying Kohistan Island Arc in northern Pakistan. Exhumation has been achieved by erosion in the apparent absence of extensional tectonics. Metabasaltic dykes intruded into polymetamorphic basement rocks provide time markers that distinguish early from late fabrics that probably correlated with Precambrian and Himalayan deformation respectively. Nanga Parbat is anomalous within the Himalayan orogen for both the timing and nature of metamorphism and anatexis. A multidisciplinary approach allows the thermal history to be tightly constrained.

Metamorphism is characterised by cordierite growth during near-isothermal decompression. Garnet inclusion assemblages record P-T conditions of 720 ± 50°C and 7.5 ±1.5 kbar, while spinel-cordierite intergrowths unique to the massif interior formed at 710 ± 60°C and 5.0 ± 1.1 kbar. Lower grade assemblages are exposed towards the massif margins. Tourmaline leucogranite plutons and sheets are characterised by similar trace element geochemistry to Miocene High Himalayan granites, indicative of vapour-absent crustal melting as would result from rapid decompression, but were emplaced more than 10 million years later. More recently, fluid infiltration into conjugate shear zones accommodating vertical stretching in the core of the massif resulted in anatexis and the formation of restitic cordierite-bearing pods. 40Ar-39Ar studies indicate regional cooling below 400°C as recently as 1 Ma, and the distribution of excess 40Ar in basement samples is indicative of magmatic/metamorphic fluids at depths > 10 km.

One-dimensional thermal modelling indicates that increasing cooling rates through time do not require increasing exhumation rates because advection of heat results in a steepened near-surface geotherm. These models predict both the style and timing of metamorphism and anatexis in this rapidly exhuming terrane, and are consistent with petrological and geochemical constraints determined from the NPHM basement.

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