东昆仑东段早泥盆世花岗岩成因:来自龙漠卡岩体的证据

Genesis of Early Devonian Granitoid in East Segment of East Kunlun:Evidences from Longmoka Pluton

龙漠卡岩体位于东昆仑造山带东段昆北构造带都兰—沟里一带。通过龙漠卡岩体锆石UPb定年、全岩地球化学及锆石Hf同位素研究,对岩体成因及其动力学背景进行探讨。结果表明:龙漠卡岩体主要为花岗闪长岩,其锆石206Pb/238 U加权平均年龄为(409±2)Ma,属于早泥盆世早期侵入体;全岩SiO_2含量(质量分数,下同)为66.25%~69.61%,具有较高的Al_2O_3、Na_2O含量和较低的K_2O、MgO、CaO含量,相对富Na,铝饱和指数(A/CNK)为1.04~1.19,属于过铝质,具有Ⅰ型花岗岩特征;花岗闪长岩稀土元素总含量较低,轻、重稀土元素分异不明显,相对富集Rb、Ba、Pb、Sr等大离子亲石元素,亏损Th、Ta、Nb、Ti等高场强元素,具有较高的w(Sr)/w(Y)值(56.9~80.8)和Eu正异常,显示出与埃达克岩相似的地球化学特征;εHf(t)为5.6~11.7,地壳二阶段Hf模式年龄为659~867 Ma,表明其可能为幔源物质上隆诱发新生玄武质下地壳部分熔融的产物;东昆仑造山带在早泥盆世可能处于后碰撞伸展构造环境。

Longmoka pluton is located in Dulan-Gouli area of the east segment of North Kunlun tectonic belt,East Kunlun orogen.The zircon U-Pb dating,geochemistry of whole rocks and zircon Hf isotope of Longmoka pluton were studied,and the genesis and dynamic background of pluton were discussed.The results show that Longmoka pluton is mainly composed of granodiorites,the zircon dating yields an weighted average 206 Pb/238 U age of(409±2)Ma,belonging to the early Early Devonian;the SiO 2 contents(mass fractions)of whole rocks vary from 66.25%to 69.61%,and the granodiorites are characterized by relatively high Al 2O 3 and Na 2O contents,and low K 2O,MgO and CaO contents,and they are relatively rich in Na and have high A/CNK of 1.04-1.19,showing anⅠ-type granite affinity with peraluminous;the granodiorites have low REE and poor fractionation between LREE and HREE,and they are enriched in LILE(Rb,Ba,Pb,Sr)and depleted in HFSE(Th,Ta,Nb,Ti),the granodiorites show similar geochemical characteristics to adakites with high Sr/Y(56.9-80.8)and positive Eu anomalies;theεHf(t)of granodiorites ranges from 5.6 to 11.7 with T DMC of 659-867 Ma,indicating the magma are probably produced by partial melting of juvenile basaltic lower crust caused by the upper mantle uplift;East Kunlun orogen may be under a tectonic setting of post-collision extensional stage in Early Devonian.