Early Jurassic accretion of retrograde eclogites and granulites in the Amdo complex, Bangong–Nujiang suture zone, central Tibet

Abstract

The timing of ocean closure and thermal evolution of the crust in the Bangong–Nujiang suture zone (BNSZ) in central Tibet, which separates the Qiangtang terrane to the north and the Lhasa terrane to the south, are controversial. In this study, we combine petrology, zircon U–Pb geochronology and phase equilibrium modeling to determine the P–T–t paths of newly discovered retrograde eclogites and retrograde granulites from the Amdo complex in the central part of BNSZ. Retrograde eclogites, which occur in the north as lenses in the orthogneisses with a prominent migmatitic texture, record decompression with heating P–T paths from medium-temperature eclogite-facies P–T fields at ~23–24.5 kbar and ~610–655 °C to high-pressure granulite-facies P–T fields at ~10.5–16.5 kbar at ~680–775 °C, followed by nearly isothermal decompression to the P of ~9 kbar at c. 182–181 Ma and further decompression. By contrast, the south orthogneisses without any migmatitic structure host lenses of retrograde granulites. They record “hairpin” clockwise P–T path characterized by compression and heating from ~5 kbar at ~450 °C to peak granulite-facies P–T conditions of 11.5–13 kbar at 640–725 °C, followed by an isothermal decompression to the P of ~8–10 kbar at c. 182 Ma and near isobaric cooling. The spatial relations (retrograde eclogites in the north) and higher peak P–T conditions of the retrograde eclogites, suggest the Amdo complex was a product of northward subduction beneath the Qiangtang terrane. Based on the protolith age for retrograde granulites (c. 184 Ma) and one young gabbro from the Amdo ophiolite (c. 184 Ma) that represents the intra-oceanic arc–backarc basin complex between the Qiangtang terrane to the north and the Amdo complex, we propose that both retrograde eclogites and granulites experienced fast burial to ~13–24 kbar before exhumation to ~9 kbar within 2–3 Ma.