东昆仑洪水川地区科科鄂阿龙岩体锆石U-Pb年代学、地球化学及其地质意义

Zircon U-Pb Geochronology, Geochemical Characteristics and Geological Significance of Cocoe A'Long Quartz Diorites Body from the Hongshuichuan Area in East Kunlun

东昆仑地区出露大量呈带状分布的中生代花岗岩,是我国一条巨型花岗质岩浆岩带,它的发育与该地区岩石圈动力学演化有着密切联系。对东昆仑造山带东段南缘洪水川地区的科科鄂阿龙石英闪长岩体的岩相学、岩石地球化学和LA-ICP-MS锆石U-Pb同位素年代学研究结果显示,岩石具有3组谐和锆石年龄:第一组年龄大于402Ma,为捕获围岩的锆石年龄;第二组年龄为243.9±3.0Ma(MSWD=0.94),代表该区玄武质岩浆底侵事件;第三组年龄为218.3±1.4Ma(MSWD=0.52),代表岩体的结晶年龄。地球化学分析表明该岩体具有高Sr,低Y、Yb,高Sr/Y比值,轻重稀土分异明显,Eu异常不明显,其微量元素显示出埃达克质岩石特征。其较低的Nb、Ta和较高的Mg#、Cr、Ni含量,高Nb/Ta比值,说明了在金红石稳定域内部分熔融形成的熔体交代了岩石圈地幔。岩体的年代学及地球化学特征表明,东昆仑地区在218Ma的晚三叠世时期发生了岩石圈地壳拆沉作用,它是早期俯冲形成的玄武质岩浆底侵而形成的加厚下地壳,在高压缺水的条件下相变为榴辉岩相并拆沉进入软流圈,部分熔融的熔体在上升过程中先与岩石圈地幔反应,然后与地壳围岩发生同化混染作用,最终形成了科科鄂阿龙石英闪长岩。

The stripe-like distributed Mesozoic granites developed in the East Kunlun Orogenic Belt （EKOB） is a gigantic granitic magma belt, which has inseparable relationship with the evolution of lithospheric dynamics in the area. This study carried out comprehensive analyses on petrography, petro-geochemistry, and LA-ICP-MS zircon U-Pb chronology for the Cocoe A%ong quartz diorite pluton in the Hongshuichuan area in the south margin of the eastern EKOB. Our results show that three groups of zircon concord ages： ① the first group is older than 402Ma, representing the age of xenocryst; ② the second one is about （243.9±4-3.0） Ma （MSWD=O. 94）, marked perhaps the age of underplating of the basaltic magma in the area; and ③（218.3±1.4） Ma （MSWD=O. 52） for the last one, giving the crystallized age of the plutons. Geochemical analysis shows that the pluton is rich in Sr, low in Y and Yb, a high ratio of Sr/Y, and strong differentiation between HREE and LREE, and no distinct Eu anomaly, and this presents the lithological feature of adakitic rock. Additionally, low Nb and Ta, and high Cr and Ni, and high Mg~ and Nb/Ta ratio show that the mantle peridotite underwent metasomatism in rutile existing phase through partial melting. Some conclusions can be drawn on the data overall that the Cocoe A＇Long pluton was firstly derived from a thickened lower crust that formed by the early basaltic magma of subduction and fell into asthenosphere as eclogite facies triggered by lithosphere delamination under the condition of HP（high pressure） and dehydration later than, after that the melt undergone chemical reaction with lithospheric mantle and contaminated by crust in order while ascending. All of these phenomenon indicates an event of lithospheric crust delamination that happened at 218 Ma in Late Triassic in EKOB.