Timing of subduction initiation in the Proto-Tethys Ocean: Evidence from the Cambrian gabbros from the NE Pamir Plateau

In order to determine the timing of subduction initiation for the Proto-Tethys Ocean and to better understand the assembly of the Precambrian terranes in east Asia to Eastern Gondwana, we report ages and geochemical data for newly identified gabbro sheets/stocks emplaced in the lower part of the early Paleozoic accretionary wedge at the NE Pamir, the western section of the West Kunlun Orogenic Belt (WKOB). The gabbros from the NE Pamir consist of dinopyroxene and plagioclase with minor olivine, orthopyroxene, Ti-Fe oxides, and accessory minerals apatite and zircon. SHRIMP U-Pb dating of co-magmatic zircons yield weighted mean Pb-206/U-238 ages of ca. 530 Ma. Whole-rock and mineral chemistry indicates that these gabbros crystallized from an evolved tholeiitic mafic magma. Isotopically, whole-rock gabbros exhibit positive epsilon Nd(t) values (similar to 4.0). The zircons show positive epsilon Hf(t) values (+5 to +8), and have elevated delta O-18 values (average delta O-18 = 6.52 +/- 0.22) relative to mantle zircons. Our isotopic and chemical data suggest that the Cambrian gabbros in the NE Pamir were derived from a depleted asthenospheric mantle source in a fore-arc setting. The Cambrian gabbros in the NE Pamir and the coeval gabbros in the Tianshuihai terrane at the eastern section of WKOB reveal that the southward subduction of the Proto-Tethys Ocean initiated at similar to 530 Ma. This long-term southward subduction led to the formation of a massive early Paleozoic accretionary wedge in WKOB and the final closure of the Proto-Tethys Ocean at ca. 440 Ma. As a result, the terranes along the northern margin of the Tibetan Plateau were docked along the northern fringe of the Eastern Gondwana at ca. 440 Ma. (C) 2018 Elsevier B.V. All rights reserved.