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 clinopyroxene 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 206Pb/238U 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 eNd(t) values (~4.0). The zircons show positive eHf(t) values (+5 to +8), and have elevated δ18O values (average δ18O = 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 ~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.
