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We present geological, bulk-rock geochemical and Sr–Nd–Hf isotopic data for mafic rocks from the West Philippine Basin (WPB). These mafic rocks comprise pillow basalts characterized by a vesicular structure. The mid-ocean ridge basalt (MORB)-normalized trace element patterns of basalts from the study area display depletions in Nb. In addition, the chondrite-normalized lanthanide patterns of basalts from the WPB are characterized by significant depletions in the light lanthanides and nearly flat Eu to Lu segments. The investigated rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703339–0.703455 and high εNd(t) values (8.0 to 8.7). Furthermore, basalts from the WPB have 176Hf/177Hf ratios that range from 0.28318 to 0.28321 and high εHf(t) from 15.2 to 16.3. Semi-quantitative modeling demonstrates that the parental melts of basalts from the study area were derived by ~20% adiabatic decompression melting of a rising spinel-bearing peridotite source. The Sr–Nd–Hf isotopic compositions of basalts from the WPB indicate that their parental magmas were derived from an upper mantle reservoir possessing the so-called Indian-type isotopic anomaly. Interpretation of the isotopic data suggests that the inferred mantle source was most likely influenced by minor inputs of a sediment melt derived from a downgoing lithospheric slab. Collectively, the petrographic and geochemical characteristics of basalts from the study area are analogous to those of mafic rocks with a back-arc basin (BAB)-like affinity. As such, the petrogenesis of basalts from the WPB can be linked to upwelling of an Indian-type mantle source due to lithospheric slab subduction that was followed by back-arc spreading.

Field observation, geochemical signatures and zircon Hf isotope data indicate that Cuomuqu ophiolite in the Bangonghu area was formed in a back-arc basin (BAB) above a supra-subduction zone (SSZ). Zircon U-Pb dating of the diabase from the Cuomuqu massif yielded an age of 164.3±1.9 Ma, thus indicating that the ophiolite complex was formed in the Middle Jurassic during back-arc extension of the mature Bangonghu-Nujiang Ocean. The zircon eHf(t) and TDMC values of the plagiogranite are similar to the eHf(t) and TDM of the diabase, respectively. The mode of occurrence of plagiogranites and their bulk-rock and Hf isotope characteristics indicate that they were derived from the mantle, associated with the surrounding gabbro and diabase, and were formed by partial melting of altered and hydrated mafic rocks under shear conditions during lateral drifting of the oceanic crust. The zircon U-Pb age of the plagiogranite is 156.4±1.4 Ma, and it is 7.9 Ma younger than the hosting diabase. In this study, zircon chronological and Hf isotopic data were tentatively analyzed to determine the genesis of plagiogranites in the Cuomuqu ophiolite complex.

The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu-Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/40Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element (PGE) data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic (161.2 ± 2.7 Ma – 165.4 ± 3.5 Ma), and experienced a second tectonic emplacement during the Early Cretaceous (137.90 ± 6.39 Ma). The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso-Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.