白垩纪羌塘地体西南缘的陆-陆碰撞:来自班怒带西段的花岗岩约束

Continent-continent collision at the southwestern margin of the Cretaceous Qiangtang terrane:Constraints from granite in the western Bangong-Nujiang Suture Zone

羌塘地体西南缘布木错—雄巴日—羌多—赛登地区分布着大量呈岩株或岩枝状的中酸性基性侵入岩,其岩石地球化学特点是低SiO_(2)(67.99%~73.32%)、高Al_(2)O_(3)(12.82%~15.02%),属于过铝质花岗岩,里特曼指数(2.09~2.63)σ<3.3,属钙碱性花岗岩类;稀土元素显示为轻稀土较强富集和重稀土强亏损的特征,岩浆结晶分异程度较高。岩石地球化学特征和稀有元素特征图解指示花岗岩构造环境有一定的差异性,以I型为主,其形成及侵位应与古特提斯洋的闭合、新特提斯洋的扩张有关,属同碰撞型造山花岗岩。通过对该区多个花岗岩体锆石LA-ICPMS测定,测得其形成年代为116~107 Ma,与早白垩世晚期羌塘地体南缘的陆陆碰撞作用时间相吻合。

A large number of moderately felsic to basic intrusive rocks in the forms of rock strains or rock branches are distributed in the southwestern margin of the Qiangtang terrane.These intrusive rocks are characterized by petrographic analysis as peraluminous granite with low SiO_(2)(67.99%73.32%)and high Al_(2)O_(3)(12.82%15.02%)contents,and defined as calc alkaline based on the Ritmann serial indices of 2.092.63(σ<3.3).They show strong enrichment of light rare earth elements(REE)and strong depletion of heavy REE,with a high degree of magma crystallographic differentiation.The geochemical characteristics of the rocks and REE in granite indicate its tectonic environment is mainly type I,with some variability,and its formation and intrusion should be related to the closure of the ancient Tethys Ocean and the expansion of the new Tethys Ocean,which suggests an intracontinental collisional orogenic granite origin.The zircon laser ablation inductively coupled plasma-mass spectrometry(LA-ICPMS)dating of several granite masses in this area yielded a formation age of 107116 Ma,which is consistent with the time of continent-continent collision at the southern margin of the Qiangtang terrane in the late Early Cretaceous.