西藏纳木错地区晚白垩世过铝质花岗岩成因及对构造演化的制约

Genesis and constraints on the tectonic evolution of the Late Cretaceous peraluminous granite in the Namtso area,Xizang

对纳木错地区过铝质花岗岩开展锆石U-Pb定年及岩石地球化学研究,精确限定其形成时代,探讨其成因机制及构造演化。纳木错地区过铝质花岗岩为二云母钾长花岗岩及(含红柱石)二云母二长花岗岩,与NW、NE向逆冲走滑断裂构造较为密切。岩石A/CNK值为1.05~1.28,具有高CaO/Na_(2)O值(0.30~0.71)、低Al_(2)O_(3)/TiO_(2)值(25~80)的特征,属钾玄岩–高钾钙碱性过铝质S型花岗岩系列,为“高温”型后碰撞花岗岩。岩石的锆石^(206)Pb/^(238)U年龄为76~75 Ma。结合区域地质资料,认为纳木错地区早白垩世中期(120 Ma左右),班公湖–怒江洋盆最终闭合,羌塘地块与拉萨地块开始陆陆碰撞,在晚白垩世早期进入后碰撞阶段(100?~75 Ma),地壳增厚不明显(≤50 km)。岩浆源岩可能为拉萨地块基底的念青唐古拉群变质砂屑岩发生部分熔融的产物,可能代表了北拉萨地块碰撞岩浆事件的谢幕,火山–岩浆(弧)的总体构造格局形成。

The petrogenesis and tectonic properties of the Late Cretaceous intrusive rocks in the Lhasa Block have long been debated by scholars.This study studied zircon U-Pb dating and peraluminous granite geochemistry in the Namtso area to limit the formation age and discuss their genetic mechanisms and tectonic evolution.The peraluminous granites in the Namtso area were two-mica potassium feldspar granite and(andalusite)two-mica monzonite.The rock mass was closely related to the thrust and strike-slip faults in the NW and NE directions.The A/CNK of the rock ranged between 1.05 and 1.28,characterized by high CaO/Na_(2)O(0.30–0.71)and low Al_(2)O_(3)/TiO_(2)(25–80).The rocks belong to the shoshonite high-potassium calc-alkaline strong peraluminous S-type granite series and the“high temperature”post-collisional granite.The LA-ICP-MS zircon ^(206)Pb/^(238)U dating of the rocks ranged between 76 Ma and 75 Ma.Combined with the regional geological data,it is concluded that the Bangong Lake-Nujiang ocean basin finally closed in the middle of the Early Cretaceous(approximately 120 Ma),and the Qiangtang terrane and the Lhasa Block began to collide.In the early Late Cretaceous,the post-collision stage(100-75 Ma),the crustal thickening in this area is not notable(50 km).The magmatic source rocks were derived from the partial metamorphic arenaceous rock melting,which,in the Nyainqentanglha Group at the base of Lhasa Block,may represent the end of the Lhasa Block collision magmatic event and the formation of the overall volcano-magmatic(arc)tectonic framework.