Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

High-pressure (HP) granulites are commonly regarded as indicators of plate convergence and collision following the subduction of oceanic or continental crust. In this study we report the discovery of Silurian HP basic granulites from Central Qiangtang on the Tibetan Plateau. Detailed petrology and geochronology reveal a three-stage metamorphic history based on inclusions, reaction textures, and garnet zoning patterns. Peak metamorphism at 830-860 degrees C and 1.15-1.45 GPa (M-1) is defined by high-Ca garnet cores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas of orthopyroxene + plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions at ca. 810-830 degrees C and 0.65-0.85 GPa (M-2). Kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 590-650 degrees C and 0.62-0.82 GPa (M-3). These results help define a sequential P-T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. Identification of mineral inclusion assemblages in zircons dated by U-Pb SHRIMP and LA-ICP-MS reveals peak HP metamorphism at ca. 427-422 Ma, subsequent near-isothermal decompression with associated retrograde reactions at ca. 392-389 Ma, and continued cooling at ca. 360 Ma. The P-T-t path of HP basic granulites reflects collision followed by extensional exhumation during early Paleozoic orogenesis. The present results indicate the occurrence of a collisional event along the northern margin of Indo-Australian Gondwana during the Silurian. Renewed Gondwana-directed subduction and subsequent collision probably led to the opening of the Paleo-Tethys Ocean. (C) 2014 Elsevier B.V. All rights reserved.