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We used a newly developed Pn tomography method to obtain high-resolution uppermost mantle velocity and anisotropy structures beneath the Northwest Pacific region. The observed Pn velocities are consistent with the local tectonic background, where high Pn velocities are observed beneath the Japan Trench area and Songliao Basin, and low Pn velocities beneath the Kuril Islands, Japan Archipelago-Izu Islands, Kyushu Island, Changbaishan-Jingpohu volcanoes, Korea Peninsula, and Japan Basin. The new Pn velocity image outlines the subducting slabs along the trenches and the young seafloor within the Japan Basin. Our results also support the existence of hot upwelling feeding the Changbaishan, Jingpohu, and Chuga-Ryong volcanoes, where small-scale mantle convection may exist below the Northeast China region. Further east, both trench-parallel anisotropy below arcs and trench-perpendicular anisotropy within the back-arc region suggest subduction-dominant mantle flow, where anisotropy may be attributable to the lattice-preferred orientation of olivine induced by flow-related strain. The highly accurate uppermost mantle velocity and anisotropy structures provide crucial information outlining the complex dynamic processes near convergent plate boundaries.

The CL images, LA-ICP-MS in situ trace elements analysis, and U-Pb dating for zircons indicate that the metamorphic ages of the sillimanite-garnet-biotite gneiss and the garnet-amphibole gneiss from eastern Taxkorgan of the Western Kunlun Mountains are 220±2 and 220±3 Ma respectively, and their protolith ages are younger than 253±2 and 480±8 Ma respectively. Two samples were collected at the same outcrops with HP mafic granulite and HP pelitic granulite. Mineral assemblage of the sillimanite-garnet-biotite gneiss (Grt+Sill+Per+Q) is consistent with that of HP pelitic granulite at early high amphibolite-granulite facies stage. Mineral assemblage of the garnet-amphibole gneiss (Grt+Amp+Pl+Q) is consistent with retro-metamorphic assemblage of HP mafic granulite at amphibolite facies stage. The dating results suggest that these HP granulites underwent peak metamorphism at 220±2 to 253±2 Ma. Thus, the Kangxiwar tectonic zone was probably formed by subduction and collision of the Paleo-Tethys Ocean during Indosinian. Protolith ages of the two samples, together with previously published U-Pb zircon dating age, suggest that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit, not a part of the Paleoproterozoic Bulunkuole Group. Western Kunlun Mountains, LA-ICP-MS zircon dating, Bulunkuole Group, Indosinian, Kangxiwar tectonic zone Citation:Yang W Q, Liu L, Cao Y T, et al. Geochronological evidence of Indosinian (high-pressure) metamorphic event and its tectonic significance in