The multistage exhumation history of the Kaghan Valley UHP series, NW Himalaya, Pakistan from U-Pb and 40Ar/39Ar ages

Abstract

Amphibole and mica 40Ar/39Ar ages as well as zircon, rutile and titanite U-Pb geochronology of eclogites and associated host rocks from the Higher Himalayan Crystalline Nappes (Indian Plate) in the Upper Kaghan Valley, Pakistan allow distinction of a multistage exhumation history. An Eocene age for peak-pressure metamorphism has been obtained by phengite 40Ar/39Ar (47.3 ± 0.3 Ma) and zircon U-Pb (47.3 ± 0.4 and 47.4 ± 0.3 Ma) ages from cover and basement gneisses. A very short-lived metamorphic peak and rapid cooling is documented by an amphibole 40Ar/39Ar age of 46.6 ± 0.5 Ma and a rutile U-Pb age of 44.1 ± 1.3 Ma from eclogites. Phengite and biotite ages from cover and basement sequences metamorphosed during the Himalayan orogeny are 34.5 ± 0.2 to 28.1 ± 0.2 Ma whereas youngest biotites, yielding 23.6 ± 0.1 and 21.7 ± 0.2 Ma, probably reflect argon partial resetting. The amphibole age, together with those derived from phengite and zircon demonstrate a rate of initial exhumation of 86–143 mm/a i.e . an extremely rapid transport of the Indian Plate continental crust from ultra-high pressure (UH P ) conditions back to crustal levels (47–46 Ma for transport from 140 to 40 km depth). Subsequent exhumation (46–41 Ma, 40–35 km) slowed to about 1 mm/a at the base of the continental crust but increased again later towards slightly higher exhumation rates of ca . 2 mm/a (41–34 Ma, 35–20 km). This indicates a change from buoyancy-driven exhumation at mantle depths to compression forces related to continent-continent collision and accompanied crustal folding, thrusting and stacking that finally exposed the former deeply-buried rocks.