藏北那朗花岗岩体地球化学特征及其构造意义

GEOCHEMICAL FEATURES OF NALANG GRANITIC BODY IN NORTHERN TIBET AND ITS TECTONIC SIGNIFICANCE

藏北冈底斯地体北缘中段的那朗花岗岩体以前被认为是一个大岩基,实际调查表明,它不是一个完整的岩体,可分为两期多个岩石单元,Rb-Sr同位素年龄表明其成岩时代为燕山晚期。岩石学、微量元素、稀土元素和同位素研究表明,两期岩石的成岩环境不同,早期壳幔混源型形成于火山弧环境,并受同碰撞环境的影响;晚期壳源型形成于同碰撞环境。对比那朗、班戈等岩体,认为班公湖—怒江板块结合带代表了中特提斯洋壳的消亡地带,冈底斯地体是从冈瓦纳古大陆裂解出的微地块。

The Nalang granitic body located in the middle of the north edge of the Gandise terrane in Northern Tibet has always been regarded as a large batholith bedrock body,but now the practical research work indicates that it is not a whole rock body,but parted by many rock units in two periods.The isotopic age of RbSr shows that the mineralization could have happened in the late Yanshan period.Studies on petrology,trace elements,rare earth elements and isotope verify that the diagenetic environment in the two periods is different from each other.The early crustmantle type is formed in volcanic arc environment while the late crustsource type is formed in syncollision environment.The comparison of the Nalang and Bange granitic bodies,suggests that the juncture area of the Banggong lakeNujiang river plate is the representative of the consuming boundary zone of the midTethys oceanic crust and the Gandise terrane belongs to micromassif separated from the Gondwanaland.