西藏冈底斯带北缘波果日则地区A型花岗岩年代学、地球化学特征及其地质意义

波果日则地区位于西藏冈底斯带北缘的班公湖—怒江缝合带中段,目前有关班公湖—怒江洋盆的闭合时间存在争议。在波果日则地区出露的A型花岗岩以岩株侵位于卧荣沟组一段,岩性为中—细粒黑云花岗斑岩。岩石具有富SiO₂(含量(质量分数,下同)为70.61%～74.94%)、K₂O(4.96%～7.48%),贫Al₂O₃(11.90%～13.79%),较高FeO^T/MgO值(7.47～25.88),较低TiO₂(含量为0.16%～0.25%)的特征,属弱过铝质—准铝质碱性系列(A/CNK值为0.90～1.12,A/NK值为1.10～1.34)。稀土元素总含量为(179.80～370.29)×10^-6,(La/Yb)_N值为5.36～8.70,Eu异常为0.28～0.61,球粒陨石标准化稀土元素配分模式表现为右倾的“海鸥”型,属于典型的铝质A型花岗岩。在原始地幔标准化微量元素蛛网图上,大离子亲石元素和高场强元素Rb、Th、U、K、Zr、Hf明显富集,而Ba、Sr、Nb、Ta、Ti明显亏损。较高的Zr、Nb、Ce和Y含量及较高的Y/Nb、Yb/Ta和Ce/Nb值显示波果日则地区A型花岗岩为A₂型花岗岩。岩相学、地球化学和区域地质资料综合显示,波果日则地区A型花岗岩形成于碰撞后岩石圈伸展背景,是下地壳拆沉导致软流圈物质上涌并加热上覆地壳,致使上覆地壳变质泥岩发生部分熔融并经历了一定程度分离结晶作用的产物。A型花岗岩LA-ICP-MS锆石U-Pb年龄为(110.8±1.3)Ma,表明其形成于早白垩世晚期,推测班公湖—怒江洋盆至少在早白垩世晚期就已闭合。

Geochronology and Geochemical Characteristics of A-type Granites in Boguorize Area,the Northern Margin of Gangdese Belt,Tibet, China and Their Geological Significance

Boguorize area is located in the central part of Bangonghu-Nujiang suture zone, the northern margin of Gangdese belt, Tibet. At present, the closure time of Bangonghu-Nujiang ocean basin is controversial. The A-type granite outcropped in Boguorize area intrudes into the first member of Woronggou Formation in the form of rock stocks, and its lithology is mid-fine-grained biotite granite porphyry. The rock is characterized by high SiO₂(the contents are 70.61%-74.94%), K₂O(4.96%-7.48%), lean Al₂O₃(11.90%-13.79%), high ratio of FeO^T/MgO(7.47-25.88), and low TiO₂(0.16%-0.25%), which belongs to weak aluminum-quasi-aluminum alkaline series(A/CNK is 0.90-1.12, A/NK is 1.10-1.34). The total contents of REE are(179.80-370.29)×10^-6,(La/Yb)_N is 5.36-8.70, and Eu anomaly is 0.28-0.61. The chondrite-normalized REE pattern is right-leaning seagull type, which belongs to typical aluminous A-type granite. On the primitive mantle-normalized trace element spider diagram, the large ion lithophile elements and high field strength elements(Rb, Th, U, K, Zr and Hf)are obviously enriched, while Ba, Sr, Nb, Ta and Ti are obviously depleted. The higher contents of Zr, Nb, Ce and Y, and the higher ratios of Y/Nb, Yb/Ta and Ce/Nb indicate that the A-type granite is A₂-type. The combination research of petrography, geochemistry and regional geological data shows that the A-type granite in Boguorize area forms in the extension of the lithosphere after the collision. It is the product of the lower crust displacing, which causes the asthenosphere material upwelling and heats the metamorphic mudstone in the upper crust, and make the metamorphic mudstone occur partially melt and undergo a certain degree of separation crystallization. The LA-ICP-MS zircon U-Pb age of A-type granite is(110.8±1.3)Ma, indicating that it is formed in late Early Cretaceous. It is estimated that Bangonghu-Nujiang ocean basin is closed at least in late Early Cretaceous.