藏北双湖山字形山玄武岩锆石U-Pb年龄、地球化学特征及其地质意义
Zircon UPb Age, Geochemical Characteristics and Geological Significance of Shanzixingshan Basalt in Shuanghu, Northern Tibet

摘要： 次对藏北双湖山字形山玄武岩进行了同位素年代学和岩石地球化学研究。锆石LAICPMS UPb测年结果表明，玄武岩形成时代为中三叠世Ladinian期((2358±27) Ma)。主量元素SiO2质量分数为4284%~5222%，TiO2为161%~269%，FeOT/MgO为152~194，属亚碱性系列拉斑玄武岩。稀土元素∑REE含量为11409×10-6~20847×10-6，(La/Yb)N为490~650，相对富集LREE。微量元素配分模式曲线与OIB型玄武岩相似。岩石成因研究表明岩浆在演化过程中主要受分离结晶作用控制，La/Nb、La/Ta、Zr/Ba等不相容元素比值与Ti含量表明，山字形山玄武岩可能是软流圈地幔与岩石圈地幔相互作用的产物。玄武岩较高的Zr含量与Zr/Y比值显示其处于板内伸展构造背景。结合区域地质资料推测，中三叠世玄武质岩浆是南、北羌塘地块碰撞造山过程中板片断离、软流圈物质上涌熔融的产物，而晚三叠世偏铝质-过铝质岩浆岩及高压变质岩折返剥露为造山带垮塌引起的伸展作用的结果。
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