东昆仑东段都兰热水花岗岩锆石U-Pb年龄、地球化学及构造意义

Zircon U-Pb dating and geochemistry of granite in the Reshui area of Dulan County,eastern section of east Kunlun orogen and its tectonic implications

都兰热水地区位于东昆仑造山带东段,发育着大量花岗岩岩石组合,主要岩石类型为二长花岗岩和花岗闪长岩,本文报道了对都兰热水地区二长花岗岩和花岗闪长岩的地球化学、LA-ICP-MS锆石U-Pb定年的研究结果,为建立完善的年代学格架和构造演化提供了新资料。锆石U-Pb同位素定年研究表明东昆仑东段都兰热水地区的二长花岗岩和花岗闪长岩的结晶侵位时代分别是232.4±1.3 Ma、230.8±1.1 Ma,属中三叠世花岗岩浆作用的产物。岩矿特征和岩石地球化学特征显示二长花岗岩和花岗闪长岩属高钾钙碱性I型花岗岩,具较高的K2O含量(2.2%~4.74%);铝饱和指数A/CNK值都小于1.1,显示准铝质特征;P2O5与SiO2之间存在明显的负相关性,还表现出富集轻稀土元素、大离子亲石元素(如K、Rb、La),亏损重稀土元素和高场强元素(如Nb、Ta、Ti、P)及Eu负异常特征。结合前人区域地质研究,我们认为东昆仑东段都兰热水地区花岗岩岩石组合是受幔源岩浆的底侵作用导致下地壳部分熔融而形成,幔源岩浆与壳源岩浆发生不同比例混合,并在岩浆演化过程中发生了一定的分离结晶作用。晚二叠世阿尼玛卿洋向东昆仑板块俯冲,直至中三叠世都兰热水地区仍处于洋壳俯冲而产生的火山弧环境,二长花岗岩和花岗闪长岩就是这一阶段的典型产物。

The Reshui area is located in Doulan county,eastern section of the East Kunlun orogenic belt.There are a large number of granitic rock combinations.The main rock types are monzogranite and granodiorite.This paper reports on the geochemical and LA-ICP-MS zircon U-Pb dating results of monzogranite and granitic in Reshui area,the results provide new informations for establish a complete chronological framework and tectonic evolution.The zircon U-Pb isotope dating study shows that the crystallization ages of the monzogranite and granodiorite are 232.4±1.3 Ma and 230.8±1.1 Ma,which is the product of granitic magmatism during Middle Triassic.The characteristics of rock and geochemistry indicate that the monzogranite and granodiorite are high-potassium-calcium-alkaline I-type granites,with high K2 O content(2.2%~4.74%);aluminum saturation index A/CNK values are both less than 1.1,showing quasialuminum characteristics;there is a significant negative correlation between P2 O5 and SiO2,and also exhibits enrichment of light rare earth elements and large ion lithophile elements(such as K,Rb,La),loss of heavy rare earth elements and high field strength elements(such as Nb,Ta,Ti,P),and Eu negative anomalies.Combined with the previous regional geological researchs,we believe that the granitic rock combinations in Reshui area is formed the partial melting of the lower crust that effected by the underplating of mantle-derived magma,The mantle-derived magma and the crustal-derived magma are mixed in different proportions,and during the magma evolution occurred fractional crystallization.Animaqing ocean subducted to the east Kunlun plate in Late Permian,Reshui area was still in the volcanic arc environment produced by the subduction of the oceanic crust until to Middle Triassic,The monzogranite and granodiorite are typical products in this stage.