藏南特提斯喜马拉雅晚二叠世酸性岩浆作用及其构造意义

Late Permian felsic magmatism along the Tethyan Himalaya,South Tibet and tectonic implications

A型花岗岩的岩石成因对于评估伸展构造背景下的岩浆作用极为关键。我们对藏南特提斯喜马拉雅带内的哲古错花岗闪长岩进行了锆石U-Pb定年、全岩地球化学和同位素(Sr和Nd)组成分析测试。结果表明,该花岗闪长岩形成于260.3±2.8Ma,并且被一约146Ma的闪长岩侵入。该花岗闪长岩属于高K钙碱性、偏铝质系列,其A/CNK和A/NK分别为0.74~0.76和1.87~2.72,富集轻稀土元素(LREE),亏损重稀土元素(HREE),有明显的Nb、Ta和Ti的负异常,Zr和Hf正异常,ε_(Nd)(t)值为+1.1~+2.3;岩相学也显示出碱性花岗岩特征。以上所有特征揭示了该花岗闪长岩和A2型花岗岩相似,但其与典型的弧岩浆岩不同,而是与伸展构造背景下形成的岩浆岩类似。哲古错花岗闪长岩的源区是为经亏损地幔物质改造过的沉积岩或变沉积岩,基于本研究数据及已有数据表明,本研究中的晚二叠世酸性岩浆作用是一次与冈瓦纳大陆北缘裂解和新特提斯洋开启有关的岩浆活动。

Documenting the occurrence of A-type granites is crucial to evaluating the nature and the mechanism for magmatism in extensional tectonic background.We present zircon U-Pb age,whole-rock geochemical and isotopic(Sr and Nd)data for Zhegu Co granodiorites in Tethyan Himalaya,South Tibet.The granodiorites were emplaced at 260.3±2.8Ma and intruded by a diorite with an age of 146Ma.The granodiorites are high-K,calc-alkaline,and metaluminous,with A/CNK and A/NK values in range 0.74~0.76 and 1.87~2.72,respectively.They are enriched in light rare-earth-elements(LREE)and depleted in heavy rare-earth-elements(HREE),display negative Nb,Ta and Ti anomalies,and positive Zr and Hf anomalies,andε_(Nd)(t)values of+1.1 to+2.3,characteristics of alkaline granite are also shown in petrography,all of which are similar to those in A2-type granites.However,the Zhegu Co granodiorites are distinct from typical arc magmatic rocks,and they are similar to magmatic rocks formed in extensional tectonic setting.The granodiorites are derived from partial melting of sediments or metamorphosed sediments modified by depleted mantle material.Based on these observations and combined with literature data,our new results suggest that the Late Permian felsic magmatism is related to the rifting of northern Gondwana and the opening of Neo-Tethyan Ocean.