Abstract: This paper reports the isotopic geochronology and geochemistry of Shangzixingshan basalt in Shuanghu, Northern Tibet for the first time. Zircon LAICPMS UPb dating yields a mean age of (2358±27) Ma, indicating that these lava should form in the Ladinian period. The geochemical characteristics of the basalt suggest that the SiO2 contents range from 4284% to 5222%, TiO2 contents range from 161% to 269%, FeOT/MgO ratios range from 152 to 194, which can be attributed to tholeiitic basalt of subalkaline series. For Shanzixingshan basalt, the ∑REEs range from 11409×10-6 to 20847×10-6, (La/Yb)N ratios range from 490 to 650, and relatively enriched in LREE. The trace element patterns for Shanzixingshan basalt are very similar to those of the OIBtype basalts. Petrogenesis research shows that crystallization differentiation played important roles in the magma evolution. The incompatible element ratios (La/Nb, La/Ta, Zr/Ba) and Ti content indicate that the basaltic magma may be generated by the interaction of the asthenosphere and lithosphere mantle. The high Zr content and Zr/Y ratio show the characteristics of intra plate basalts in the extensional setting. Combined with regional geological available data, we suggest that the Middle Triassic basaltic magma was originated from the partial melting of the upwelling asthenosphere due to the slab window opening, which probably caused by slab breakoff in the process of the SouthQiangtang Block and the NorthQiangtang Block collision, whereas the exhumation of Late Triassic metaluminousperaluminous magmatic rocks and highpressure metamorphic rocks is resulted from extensional tectonics in response to orogenic collapse.

Key words: Longmu CoShuanghu suture zone, basalt, zircon UPb age, geochemistry, slab breakoff