Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya

Abstract

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 ± 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720–770 °C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ∼30–80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya.