冈底斯南缘变形花岗岩锆石U-Pb年龄和Hf同位素组成:新特提斯洋早侏罗世俯冲作用的证据  

Zircon U-Pb and Hf isotopic composition of deformed granite in the southern margin of the Gangdise belt, Tibet:Evidence for early Jurassic subduction of Neo-Tethyan oceanic slab

作　　者：张宏飞[1] 徐旺春[1] 郭建秋[2] 宗克清[1] 蔡宏明[1] 袁洪林[3] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室和地球科学学院,武汉430074 [2]中国地质大学地质调查研究院 [3]西北大学大陆动力学国家重点实验室,西安710069

出　　处：《岩石学报》2007年第6期1347-1353,共7页Acta Petrologica Sinica

基　　金：国家自然科学基金(40521001);教育部长江学者和创新团队发展计划(IRT0441);中国地质调查局西藏冲江矿调项目资助.

摘　　要：本文对产于冈底斯南缘一个变形花岗岩进行了主量元素、微量元素、原位锆石 LA-ICP-MS U-Pb 定年和锆石 Hf 同位素组成研究。结果表明,变形花岗岩为高硅(SiO_2=73.38%～76.06%)钙碱性岩系,K_2O/Na_2O=0.69～1.17,铝指数(A/CNK)=1.03～1.07,为弱过铝质岩石。变形花岗岩微量元素组成显示贫大离子亲石元素(e.g.Rb=47×10^(-6)～71×10^(-6))和高场强元素(e.g.Nb=1.3l×10^(-6)～3.09×10^(-6),Ta=0.23×10^(-6)～0.54×10^(-6)),具有岛弧型花岗岩的地球化学属性。变形花岗岩锆石 U-Pb 年龄为178±1Ma,该年龄代表岩浆结晶年龄。锆石 Hf 同位素组成显示ε_(Hf)(178Ma)值变化于+14.1～+17.7,表明变形花岗岩岩浆来自初生地壳物质的部分熔融。变形花岗岩的岩石成因与新特提斯洋向欧亚板块南缘的俯冲消减作用存在联系,其岩浆结晶年龄反映了新特提斯洋发生俯冲消减的开始时代不晚于早侏罗纪,说明了新特提斯洋经历了较长时间的演化。This paper reports major and trace element compositions, LA-ICP-MS zircon U-Pb age and Hf isotopic compositions from a deformed granite occurring in the southern margin of the Gangdese belt, Tibet. The results show that the deformed granite are high silicon calc-alkaline series, with SiO2 = 73.38% - 76.06%, K2O/Na2O = 0.69 - 1.17 and Al index （A/CNK） = 1.03 - 1.07. The deformed granite is characterized by low LILE contents （e. g. Rb =47 × 10^-6 -71 × 10^-6） and low HFSE contents （e. g. Nb = 1.31× 10^ -6 - 3.09 × 10^ -6, Ta = 0.23 × 10^-6 - 0.54× 10^ -6 ）, indicating that the granite has an island-arc-type geochemical affinity. Obtained U-Pb zircon age of 178 ± 1Ma for the granite represents its magma crystallization age. Zircon Hf isotopic compositions from the granite display δHf（178Ma）values ranging from ＋ 14. 1 to ＋ 17.7, suggesting that the magma was derived from partial melting of juvenile crust. The petrogenesis of the granite is linked to Neo-Tethyan oceanic slab subduction towards the southern margin of the Europe-Asia Plate. Therefore, the formation time of the granite indicates that the beginning time of the Neo-Tethyan oceanic slab subduction is not later than Early Jurassic. The Neo-Tethyan ocean has long lasting tectonic evolution.

关 键 词：变形花岗岩 U-Pb定年 HF同位素组成 冈底斯块体 新特提斯洋 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]