东昆仑大格勒农场南辉长岩锆石U-Pb年代学、地球化学特征及地质意义

Zircon U-Pb Chronology,Geochemistry,and Geological Significance of Gabbros in southern Dagele farm,East Kunlun

东昆仑志留纪岩浆活动强烈,记录了东昆仑造山带原特提斯洋演化晚期大洋俯冲阶段与碰撞阶段转换地质过程。本次研究对东昆仑大格勒农场南志留纪基性侵入岩开展了详细的岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究,以确定其形成时代、岩石成因和构造环境。研究结果表明大格勒农场南辉长岩的结晶年龄为430.2±2.4 Ma,形成时代为中志留世。辉长岩SiO_(2)(51.40%~53.66%)含量较低,MgO(18.56%~22.16%)和FeO^(T)(7.54%~10.64%)含量高,Mg^(#)为79~82。微量元素富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有高Ni(276~973μg/g)、Cr(2152~3574μg/g)和Sc(35.5~53.1μg/g)含量,低Nb/Ta值(3.23~8.68)和εHf(t)值(-7.3~-3.6)。大格勒农场南辉长岩的地球化学组成具有岛弧基性岩特征,其成因可能是流体和俯冲沉积物加入的富尖晶石橄榄岩部分熔融的结果。综合区域资料表明,大格勒农场南辉长岩形成于东昆仑原特提斯洋俯冲晚期阶段,进一步限定原特提斯洋晚期俯冲与碰撞转换时限为中志留世,随后转入碰撞造山阶段。

The Silurian,which is characterized by the intensive magmatic activity,is a key period for the Proto-and Paleo-Tethyan tectonic transformation in the East Kunlun orogenic belt.There are still controversies concerning the petrogenesis of the igneous rocks,deep dynamic mechanism,and Proto-Tethyan tectonic evolution during this period.LA-ICP-MS zircon U-Pb dating shows that the gabbros in the southern Dagele farm were formed at 430.2±2.4 Ma.The gabbros have low SiO_(2)(51.40%-53.66%),high MgO(18.56%-22.16%),FeO^(T)(7.54%-10.64%),and Mg^(#)value(79-82).Gabbros are enriched in large ion lithophile elements(LILE)such as Rb,Th,Ba,and Cs,and depleted in high field strength elements(HFSE),such as Nb,Ta,and Ti,with high Ni(276-973μg/g),Cr(2152-3574μg/g),and Sc(35.5-53.1μg/g)contents,low Nb/Ta(3.23-8.68),and lowεHf(t)value(−7.3 to−3.6).It can be inferred that gabbros in the southern Dagele farm perhaps resulted from partial melting of enriched spinel lherzolite which was metasomatized by low Nb/Ta ratio fluid with input of subducted sediments.The island arc geochemical features as well as the formation age that is obviously earlier than the peak age of the Early Paleozoic ultra-high pressure metamorphism in East Kunlun indicate that the formation of gabbros in the southern Dagele farm is related to the subduction of the Proto-Tethys Ocean.Together with the regional data,we suggest that the gabbros represent the latest magmatic records of the subduction of the Proto-Tethys Ocean in the Central Kunlun area,and the transition from subduction to continent occurred in Middle Silurian.