Petrological evidence for Neoproterozoic subduction in the Kabul Block, Afghanistan

The Kabul Block in central Afghanistan exposes Neoarchean to Neoproterozoic basement units that record two distinct metamorphic events: a Paleoproterozoic granulite-facies metamorphism and a Neoproterozoic pressure-dominated amphibolite-facies metamorphism. The granulite-facies basement units that experienced an amphibolite-facies overprint are tectonically overlain by younger amphibolite-facies units. Here we present the first evidence for Neoproterozoic blueschist-facies metamorphism in the Kabul Block, documented by mineral textures in amphibolite-facies micaschists from the upper units. These textures record a complex metamorphic evolution involving an earlier high-pressure/ low-temperature event followed by a Neoproterozoic Barrovian-type metamorphic overprint. Thermodynamic calculations and diffusion modelling on two garnet generations were applied to these rocks to constrain metamorphic P-T evolution of these two events and duration of the Barrovian metamorphism. The results, in combination with available age data, indicate that this metamorphism was related to the Neoproterozoic crustal thickening that occurred subsequent to subduction and was coeval with the Grenvillian orogeny. In the Kabul Block, this event took place around 900 Ma and, according to diffusion modelling, lasted for 20–50 Ma, depending on the peak temperature reached (612–677 °C).