西昆仑早古生代岩浆弧大同岩体中埃达克质岩石的成因及地质意义

大同岩体位于西昆仑柯冈-库地-其曼于特和麻扎-康西瓦两条蛇绿混杂岩带之间。为进一步了解西昆仑早古生代岩浆弧中某些具体岩石类型、岩浆产生机理、地质意义及成因联系,本文从岩相学、主微量元素地球化学、锆石U-Pb年龄和Hf同位素等方面,对大同岩体中和外围新发现的埃达克质岩石进行了研究。埃达克质岩石由石英二长花岗岩和黑云母二长花岗岩两种岩性组成,呈独立的岩体和大同主岩体中的脉体两种产状,高硅(w(SiO₂)≥60.34%)、高铝(w(Al₂O₃)≥14.73%)、富碱(w(K₂O+Na₂O)≥6.40%)、低镁(w(MgO)≤2.35%),以及高锶(w(Sr)≥504×10^-6)、低钇(w(Y)≤17.20×10^-6)和高Sr/Y(平均为70.88),富集轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素,不同程度负Eu异常及Ta、Nb、P和Ti亏损。锆石U-Pb测年显示,埃达克质岩石年龄为(443.6±1.4)~(462.0±1.0)Ma,与大同主岩体的形成年龄相当。锆石ε_Hf(t)为-7.28~4.56(平均-0.84),我们认为埃达克质岩石是由原特提斯洋洋壳及部分洋壳之上的陆源沉积物向南俯冲过程中,发生部分熔融形成的熔体上升过程中与地幔楔橄榄岩反应,最后定位于地壳浅层的结果。通过与大同岩体主岩体对比,对早古生代岩浆弧的形成和演化有了更深入的了解。

Formation and Significance of Adakitic Rocks in Datong Pluton of Early Paleozoic Magmatic Arc of Western Kunlun Orogen

Datong pluton is located between Kegang Kudi Qimanyute ophiolite belt and Mazha Kangxiwa ophiolite belt in the western Kunlun orogen .This paper presents the study of petrography , major and trace elements ,geochemistry ,zircon U Pb chronology ,and Hf isotopic data of the adakitic rocks ,which are in and surround Datong pluton .The adakitic rocks consist of biotite monzonitic granite and quartz monzonitic granite ,occurring in small isolated plutons and dykes in the shoshonitic Datong pluton .The adakitic rocks are characterized by high SiO2(≥60 .34% ) ,Al2O3(≥14 .73% ) ,total alkali (w(K2O+Na2O)≥6 .40% ),low MgO(≤2 .35% )and high Sr(≥504×10-6),low Y(≤17 .20×10-6), and high Sr/Y(average 70 .88) .They are prominently enriched in LREE and LILE ,depleted of HREE and HFSE ,with variably negative Eu anomalies and Ta ,Nb ,P ,and Ti depletion .The zircon U Pb dating for the adakitic rocks from both isolated and within Datong pluton yields the weighted mean ages of (443 .6 ± 1 .4) Ma (462 .0 ± 1 .0) Ma ,which are consistent with the formation of Datong pluton .The zircon εHf (t) values for the adakitic rocks are in the range of -7 .28 4 .56 (average -0 .84) .Combined with the regional tectonic evolutionary history ,the adakitic rocks eventually emplaced in the shallow level of crust after partial melting of the oceanic crust and the overlain terrigenous sediments during the southward subduction and interaction with the overlain mantle wedge subsequently . The comparison between the shoshonitic Datong pluton with the adakitic rocks is of important significance to understand the formation and geological evolution of the early Paleozoic magmatic arc of the western Kunlun orogen .