Zircon U-Pb geochronology, geochemistry of the Alasimu gabbro in eastern section of East Kunlun Mountains and the closing time of Paleo-ocean basin

Late Paleozoic intermediate-acid granitoids are widely distributed in the East Section of East Kunlun, but the records of basic magmatism are very insufficient. Alasimu mafic rocks on the southern margin of the arc magmatic rock belt in northern East Kunlun Mountains was selected as the study objective so as to provide a new basis for the research on the Late Paleozoic magmatic evolution of the east section of East Kunlun Mountains. LA-ICP-MS zircon U-Pb analysis yielded the crystallization age of 241±1Ma, suggesting that Alasimu Mafic rocks were formed in the Middle Triassic. All the rock samples are characterized by relatively low TiO₂ (0.17%-0.37%, averagely 0.25%) and high Mg^# (72.94%-77.32%, averagely 74.95%) and high Al₂O₃ (13.08%-23.20%, averagely 18.9%), enrichment of large ion lithophile elements (LILE:Sr, Rb, Ba), apparent depletion of high field strength elements (HFSE, Nb, Ta, Zr and Hf). MgO shows some correlation with Fe₂O₃^T, TiO₂, Ni and Co in the diagram of Harker, which reveals some degree of fractional crystallization of olivine and Ti-Fe oxide. Alasimu mafic rocks have similar chemical composition to island arc basalts, and they formed earlier than the collision and post-collision time of tectonic-magmatic activity. Combined with regional studies, the authors hold that Alasimu gabbro rocks ought to be the latest magmatism records in the East Kunlun region related to Middle Triassic Paleo Tethys oceanic crust subduction, which indicated that the tectonic transition from ocean subduction to collisional orogeny might have commenced in Middle Triassic.