The age of deep, steep continental subduction in the NW Himalaya: Relating zircon growth to metamorphic history. Comment on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005)

Abstract

Leech et al. [Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83–97], present 3 clusters of ages for growth stages in zircon from quartzo-feldspathic gneisses hosting coesite-bearing eclogite from the Tso Morari Complex, NW India. These age clusters, from oldest to youngest, are interpreted to represent the age of ultrahigh-pressure metamorphism, a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of Indian crust to have started earlier than previously accepted. However, no petrographic evidence, such as inclusions in the zircons relating to particular metamorphic events, is presented to substantiate the proposed sequence of metamorphic stages. Previously published data from eclogites of the same area indicate that coesite–eclogite is not the first but at least the second eclogite facies stage. In addition, the newly proposed time interval between coesite–eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet–garnet couples in eclogite. Neither the age of ultrahigh-pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data.