新疆四顶黑山花岗岩体锆石LA-ICP-MS U-Pb年龄及地质意义

LA-ICP-MS zircon U-Pb age of the Sidingheishan granitic pluton from east Tianshan,Xinjiang,China,and its geological implications

新疆东天山位于中亚造山带南缘,天山—兴安造山系北天山造山带东段,发育大量泥盆纪—石炭纪花岗岩,其形成过程多与觉罗塔格洋的俯冲作用有关。四顶黑山花岗岩体位于东天山觉罗塔格构造岩浆带的东端,岩体在地表呈不规则状产出,侵位于元古界片岩-变火山岩、奥陶系变玄武岩和泥盆系雀儿山群火山岩;主要岩石类型为花岗岩和花岗闪长岩。花岗岩的LA-ICP-MS锆石U-Pb年龄为380.4 Ma±3.7 Ma,表明岩体形成于晚泥盆世。岩石表现出高硅(w(SiO_2)=64.87%~74.71%)、高碱(w(K_2O)=3.35%~4.68%,w(Na_2O)=2.26%~3.85%)、富铝(w(Al_2O_3)=11.82%~14.13%)和低MgO(w(MgO)=0.56%~2.12%)、CaO(w(CaO)=1.68%~3.74%)、TiO_2(w(TiO_2)=0.26%~0.57%)、P_2O_5(w(P_2O5)=0.01%~0.19%)特征,A/CNK=0.83~1.00;富集Ba、K、La、Ce、Nd,亏损Th、Nb、Ta、Sr、Ti,铕负异常较明显(δEu=0.70~0.73);具有岛弧型花岗岩特征。四顶黑山花岗岩体形成于岛弧环境,属于觉罗塔格洋向南俯冲过程中的产物。四顶黑山花岗岩体两侧的镁铁-超镁铁质岩体的形成时代应晚于晚泥盆世。

The east Tianshan orogenic belt,Xinjiang,NW China,is located at east part of the north orogenic belt of Tianshan-Xinganling orogenic system in the south margin of the Central Asian Orogenic Belt(CAOB).The Jueluotage palaeoocean crust subduction-related Devonian-Carboniferous granites are well developed here.The Sidingheishan granitic pluton in the eastern part of the Jueluotage tectonic belt,east Tiuanshan emplaced in Proterozoic shist and metavolcanic rocks,Ordovician metabasalt and Devonian volcanic-sedimentary rocks of the Que’rshan group and outcrops irregularly at surface.It is composed mainly of granite and granodiorite.The LA-ICP-MS zircon U-Pb dating of the granite is 380.4±3.73 Ma reflecting that the it was formed at late Devonian period.Petrological and geochemical data show that the pluton is characterized by high silicon(w(SiO 2)=64.87%~74.71%),high alkali(w(K 2O)=3.35%~4.68%,w(Na 2O)=2.26%~3.85%),abundant aluminum(w(Al 2O 3)=11.82%~14.13%)and low MgO(w(MgO)=0.56%~2.12%),CaO(w(CaO)=1.68%~3.74%),TiO 2(w(TiO 2)=0.26%~0.57%),P 2O 5(w(P 2O 5)=0.01%~0.19%),ratio of A/CNK=0.83~1.00.enrichment of Ba,K,La,Ce,Nd,depletion in Th,Nb,Ta,Sr and Ti,and lowδEu value(δEu=0.70~0.73).All of these show that the Sidingheishan granitic pluton fromed in the island arc environment and related with the S-dipping subduction of the Jueluotage palaeoocean.The mafic-ultramafic intrusion on either side of the granitic pluton should be formed after the late Devonian.