The Magmatic, Thermal and Exhumation History of the Nanga Parbat-Haramosh Massif, Western Himalaya

Abstract

This study presents important new constraints on the geological evolution of a structurally-anomalous basement massif in the western Himalaya. In northern Pakistan, Indian continental crust of the Nanga Parbat-Haramosh massif (NPHM) has been recently exhumed from beneath the Kohistan-Ladakh island arc. Integrated structural, metamorphic, geochronological and geochemical techniques have provided valuable evidence for the thermal, magmatic and denudation history of the NPHM and adjacent parts of the Kohistan arc. The northern part of the Kohistan arc is composed of a range of intrusive igneous rocks and associated metasediments, whilst the adjacent NPHM is characterised by orthogneiss with subsidiary intercalations of pelitic gneiss and calcsilicate. The contact zone between the western margin of the NPHM and the Kohistan arc is characterised by a steeply-inclined, high-grade shear zone, along which there has been a significant variation in movement sense. In the south of the area, the rocks of the NPHM have been thrust north-westwards over Kohistan, resulting in the active exhumation of Indian continental crust. In contrast, in the north of the area, the uplift of the NPHM has been accommodated within the shear zone by dextral transtension. The metamorphic rocks of the NPHM have mostly equilibrated under 600-750 °C and 7-10 kbar, whilst those from adjacent lithologies in the Kohistan arc have equilibrated under 550-700°C and 6-8 kbar (as determined by rim compositions). The change in estimated peak metamorphic grade across the region is gradual, and major metamorphic breaks are absent. The majority of samples from both the NPHM and Kohistan record retrogressive P-T pathways, which is thought to reflect the extended exhumation history of the region. The variable results of Gibbs modelling are interpreted as indicating lithological control on garnet growth, resulting in the preservation of different segments of the same PT path. The most recent equilibration of metamorphic assemblages occurred at > 30 Ma in Kohistan and < 25 Ma in the NPHM, suggesting that contrasting structural levels can be exhumed without preserving substantial metamorphic discontinuities. 40Ar/39Ar and Rb-Sr mica ages mostly lie in the ranges 1-8 Ma and 17-26 Ma for the NPHM and the Kohistan arc, respectively, and are considered to be exhumation-related cooling ages. The interpretation of such cooling ages in terms of cooling rates and exhumation rates is dependant on the reliable estimation of mineral closure temperatures and palaeogeotherms. However, in general the ages indicate that cooling through the relevant closure temperatures has occurred more recently and more rapidly for the NPHM compared with the Kohistan arc, confirming the recent differential exhumation of the NPHM relative to Kohistan. Cooling ages for deformed samples collected from the shear zone arc highly variable, which is probably a reflection of variable closure temperatures or variable degrees of isotopic resetting of rocks characterised by contrasting strain-states, lithologies and fluid-contents. The observed variation in mineral ages may also partly be a result of the late-stage juxtaposition of different structural levels by shearing and/or open system behaviour. The variable geochemistry of Himalayan-age granite magmatism in the region is a reflection of contrasting source compositions. In Kohistan, undeformed 26-50 Ma granitoid sheets are characterised by relatively primitive initial Sr and Nd isotopic compositions (87Sr/86Sri=0.7045-0.7054; ͼNd(T)=0.1-2.7), indicating that locally, underthrusting of relatively evolved Indian continental crust beneath Kohistan had not occurred before 30 Ma. The majority of the granites were generated by the fluid-absent breakdown of biotite in an isotopically juvenile, LILE and LREE-enriched source. Proximal to the NPHM, the isotopic and trace element compositions of deformed granites have been significantly affected by both the assimilation of Indian continental crust and sub-solidus fluid mobility. The tourmaline leucogranites of the NPHM display a wide range of geochemically and isotopically evolved compositions (87Sr/86Sri=0.72-0.97; ͼNd(T)=-24 to -26), and have been generated by the fluid-absent melting of mature pelitic rocks within the NPHM. The leucogranites are low melt fractions that probably result from the decompression melting of Indian crustal material with unusually high heat productivity during rapid tectonic exhumation.