Metamorphism and exhumation of the NW Himalaya constrained by U–Th–Pb analyses of detrital monazite grains from early foreland basin sediments

Abstract

Single detrital monazite grains from the Dharamsala and Lower Siwalik Formations (early to mid-Miocene continental foreland basin sediments in NW India) have been dated by two techniques; isotope dilution thermal ionization multicollector mass spectrometry (ID-TIMS) and laser ablation plasma ionization multicollector mass spectrometry (LA-PIMMS). The results give U–Th–Pb isotopic ages of c. 400–1300 Ma and 28–37 Ma and suggest that the source of detritus shed from the uplifting Himalayan mountains and captured in the foreland basin included (1) the protolith to the High Himalayan Crystalline Series (HHCS), i.e. rocks unaffected by the Himalayan metamorphism, (2) Cambro-Ordovician granites and (3) HHCS affected by the M 1 phase of Barrovian metamorphism (Eo-Himalayan) related to the Indo-Asian collision. Deposition of the Dharamsala Formation was coeval with M 2 sillimanite grade Himalayan metamorphism and crustal melting. The youngest monazite ( c. 28–37 Ma) ages imply that Indian plate rocks, having experienced the earliest Himalayan metamorphic event which occurred within 10–20 Ma of collision were exhumed, eroded and deposited within c. 10–20 Ma of metamorphism. This indicates a minimum cooling rate of between 60 and 40°C Ma −1 for the period 30–20 Ma. After 20 Ma our study suggests no change in source area and that this same sequence, comprising both metamorphosed and unmetamorphosed rocks, was supplying detritus and being progressively incised by erosion for at least a further 8 million years.