西天山吐拉苏岩体中花岗岩的年代学和地球化学特征及其地质意义

Geochronology and Geochemistry of Granite in Tulasu Pluton in West Tianshan, and Its Geological Significance

吐拉苏岩体位于西天山吐拉苏盆地的东北缘,由石英闪长玢岩、辉石闪长玢岩和花岗岩组成,其中花岗岩主要为花岗闪长玢岩和花岗斑岩.它们表现出高硅、高铝、富碱的特征,里特曼指数(σ)=1.61~3.18,A/CNK=1.00~1.09,A/NK=1.07~1.68,为准铝质-弱过铝质高钾钙碱性花岗岩;相对富集Rb、Th、U等大离子亲石元素,明显亏损Nb、Ta、Sr、P、Ti等元素;轻重稀土分馏明显,(La/Yb)N=7.48~12.12、中等Eu负异常(δEu=0.47~0.62),无Ce异常.但花岗斑岩较花岗闪长玢岩碱含量更高,更加接近准铝质钾玄岩系列,Sr、P、Ti等元素的亏损程度更强,且轻稀土更富集,可能说明花岗斑岩中壳源物质含量增加.锆石LA-ICP-MS U-Pb定年研究获得花岗斑岩的侵位年龄为305.9±5.9 Ma.锆石Hf同位素组成显示,εHf(t)值介于0.4~8.7,表明花岗岩的岩浆源区有幔源物质的加入;TDM2值介于766~1294 Ma,表明岩浆源区为中元古代-新元古代新生地壳.综合吐拉苏岩体中两类花岗岩的地质、地球化学特征、年代学和区域地质背景,认为从花岗闪长玢岩到花岗斑岩结晶分异增强、壳源物质的贡献增大,花岗岩形成于后碰撞环境,岩浆源区为中元古代-新元古代新生地壳,但有不同比例幔源物质的加入.

Tulasu pluton in Chinese West Tianshan is located in the northeastern of Tulasu basin,composed of three lithofacies,i.e.quartzdiorite,pyroxene diorite and granite.The granites are composed of granodiorite porphyrite and granite porphyry,characterized by high silicon,high aluminum,enrichment of alkali,Rittman index(σ)=1.61-3.18,A/CNK=1.00-1.09,A/NK=1.07-1.68,belonging to metaluminous-weakly peraluminous high-K calc-alkaline granite.They are relatively enriched in LILEs(Rb,Th,U),extremely poor in Nb,Ta,Sr,P,Ti,etc.,obviously fractionated in LREE and HREE,(La/Yb)N=7.48-12.12,showing a medium negative Eu anomaly and no Ce anomaly,δEu=0.47-0.62.But granite porphyry has higher alkali content,more close to shoshonite series and Sr,P,Ti strongly negative,indicating increased involvement of crustal materials.Zircon LA-ICP-MS U-Pb dating for the granite yields an age of 305.9±5.9 Ma.Analyses on zircon Hf isotopic compositions yield positiveεHf(t)values varying from+0.4 to+8.7,implying that the involvement of mantle-derived magmatic materials during the formation of granite;TDM2 varies from 766 to 1294 Ma,showing that magma was derived from Proterozoic-Neoproterozoic crust.By comparison,it is concluded that fractional crystallization enhanced,involvement of crustal materials increased from granodiorite to granite porphyry.The granite was formed in post-collisional setting.