High-pressure, low-temperature metamorphism preserved in the Indus-Yarlung suture zone of the eastern Himalaya: Overprinting at amphibolite facies and comparison with occurrences in the western Himalaya

Abstract

Eclogite and blueschist are important monitors of subduction zone processes and can record complex polymetamorphic histories during the protracted evolution of subduction systems. However, the rarity of such high-pressure, low-temperature metamorphic rocks in the Indus-Yarlung suture zone of the Himalaya hinders our understanding of the convergence of India and Asia. Here, we examine high-pressure, low-temperature metamorphic rocks from the Milin area of the eastern Indus-Yarlung suture zone. A comprehensive microtextural, mineral chemistry, and phase-equilibrium modeling investigation shows that low-temperature eclogite-facies metapelites with different parageneses (garnet-mica schists) experienced a cold subduction history along a low-thermal gradient, with peak pressures of ∼1.6 GPa at ∼550 °C. This represents an approximate thermal gradient of ∼10 °C km−1. After peak pressure, these rocks underwent a two-step exhumation history with initial exhumation accompanying heating at ∼590−600 °C and ∼0.8−1.0 Gpa; this was followed by the second stage of exhumation to lower amphibolite-facies conditions. We interpret these low-temperature eclogite-facies metapelites to represent sedimentary rocks that overlaid Neo-Tethyan oceanic crust during subduction. Exhumation paths of high-pressure, low-temperature rocks in the eastern Indus-Yarlung suture zone include an amphibolite-facies overprint, which is absent from high-pressure, low-temperature rocks in the western Indus-Yarlung suture zone. This result is caused by the different timing of exhumation relative to the terminal collision of India and Asia.