藏南仲巴地体早奥陶世构造—热事件及其地质意义

Early Ordovician Tectono-Thermal Event in Zhongba Terrane and Its Geological Significance

仲巴地体是雅鲁藏布江缝合带西段南北蛇绿岩带之间的重要构造单元,目前对其是否存在早古生代构造-热事件记录及构造属性判别尚不明确.通过野外观测、岩石地球化学研究和年代学分析,在仲巴地体中段的公珠错一带识别出一套侵入到黑云斜长片麻岩中的片麻状二长花岗岩,其锆石U-Pb年龄为~478Ma,表明岩石形成时代为早奥陶世;该套花岗岩具有高Si、富Al和总碱含量较高的特点,铝饱和指数A/CNK=1.13~1.20,富集大离子亲石元素Rb、K,相对亏损Ba、Sr、Nb、Ti等,属于钙碱性强过铝质花岗岩.这是首次在雅鲁藏布江缝合带西段仲巴地体内部识别出代表早奥陶世构造-热事件的地质记录,该套花岗岩形成过程与原特提斯洋向冈瓦纳大陆北缘俯冲结束后的上地壳熔融相关,证明仲巴地体在早古生代应为东冈瓦纳大陆北缘的组成部分.

The Zhongba terrane is an important tectonic unit sandwiched between the northern and the southern ophiolitic sub-belts in the western segment of the Yarlung Zangbo Suture Zone（YZSZ）,Southwest Tibet.Currently it is still unclear as to whether there exists the Early Paleozoic tectono-thermal events in this terrane as well as its tectonic affinity.BasedBaesed on field and petrological investigations,a suite of gneissic monzogranite that intruded into a biotite plagioclase gneiss was recognized near the Gongzhu Co area in the middle part of the Zhongba terrane in this study.U-Pb dating of zircon from the monzogranite yieldsed ages of^478Ma,indicating the deformation age for such rock at the Early Ordovician.These monzogranites are characterized by high Si and Al,relatively high contents of total alkalis,A/CNK=1.13-1.20,enrichment of large lithophile elements of Rb and K and depletion of Ba,Sr,Nb and Ti,which should be calc-alkalic alkaline,strong peraluminous granite.To our knowledge,this represents the first report of the Early Ordovician tectono-thermal events taken place within the Zhongba terrane from the western YZSZ.These gneissic monzogranites were formed by the upper crustal melting after subduction of the Proto-Tethys Ocean to the northern margin of the Gondwana supercontinent,and suggesting that the Zhongba terrane should belong to the northern margin of the eastern Gondwana supercontinent at the Early Paleozoic.