东昆仑沟里地区晚奥陶世花岗闪长岩地球化学特征及其对原特提斯洋演化的制约

Geochemical Characteristics of Late Ordovician Granodiorite in Gouli Area,Eastern Kunlun Orogenic Belt,Qinghai Province:Implications on the Evolution of Proto-Tethys Ocean

原特提斯洋的俯冲时限以及俯冲极性还存在较大争议,对位于东昆仑造山带东端的昆南亚带中敖洼得花岗闪长岩体开展详细的LA-ICP-MS锆石U-Pb年代学、元素地球化学和同位素地球化学研究.结果表明,其单颗粒锆石LA-ICP-MS U-Pb定年结果为454±2Ma,指示侵位时代为晚奥陶世.岩石Na2O/K2O比值为0.92~2.68,Mg#为39~43,A/CNK为1.00~1.03,属准铝质钙碱性-高钾钙碱性系列.岩石稀土元素标准化图显示右倾,(La/Yb)N介于17~46,具有明显的轻重稀土分异及中等-弱的Eu负异常(0.48~0.96).花岗闪长岩富集大离子亲石元素(Rb、Ba、K、La、Sr),亏损高场强元素(Nb、Ta、P、Ti),Cr、Ni含量较低,Sr/Y均值为55.全岩(87Sr/86Sr)i较为均一(0.705 9~0.706 3),εNd(t)变化范围为-4.1^-1.9,对应的二阶段模式年龄为1.3~1.5Ga,锆石εHf(t)变化范围为+5.4^+9.2.敖洼得花岗闪长岩具有低镁埃达克质岩的特征,为俯冲洋壳部分熔融的产物,熔融后的洋壳残留应为含石榴子石角闪岩,岩浆演化过程中经历了角闪石和斜长石的分离结晶.微量元素组成特征指示岩体形成于火山弧环境,结合昆中断裂带内蛇绿岩及其两侧相关变质岩、弧花岗岩、弧后盆地火山岩等的综合对比,笔者认为昆中缝合带原特提斯洋(昆中洋)在晚奥陶世存在双向俯冲,敖洼得岩体形成于原特提斯洋的南向俯冲消减过程.

The Aowade granodiorited in the east-end of Kunnan terrain,a part of Kunlun Orogenic belt,belongs to Proto-Tethyan tectonic domain.Detailed study of the granodiorite can offer important information to constrain the evolution of the Proto-Tethys Ocean.Detailer study of the granodiorite can offer important information to constrain the evolution of the Proto-Tethys Ocean.In this study,LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 454±2 Ma for the intrusion.It is characterized by low K2O/Na2O （0.92-2.68）and Mg#（39-43）,and the A/CNK is between 1.00 and 1.03.The REE exhib-its right-dipping patterns with negative anomalies of Eu/Eu＊（0.48-0.96）and high （La/Yb）N ratios.LILE（Rb,Ba,K,La, Sr）is enriched in this intrusion,while HFSE（Nb,Ta,P,Ti）is depleted relative to the primitive mantle,and the concentra-tions of transition elements such as Cr and Ni are low.The ratios of Sr/Y （average of 55）are relatively high.All the samples have high initial 87 Sr/86 Sr values （0.7059-0.7063）andεHf （t）（十5.4 to 十9.2）,but with relatively lowεNd （t）（-4.1 to-1.9）and old two stage model age （T2DMNd=1.3-1.5 Ga）.In combination with the lithology,geochemical signature and iso-topic compositions of the intrusion,we propose that the Aowade granodiorite was derived from melting of the subducted ocean-ic crust with minor involvement of continent crust.The possible residual of the source region is garnet amphibolite.Fractional crystallization of pyroxene,amphibole and plagioclase may have also occurred during the evolution of magma.Integrating evi-dences from the geodynamic setting of the Aowade granodiorite,the ophiolites in the Kunzhong structure zone,and the meta-morphic rocks,arc-related igneous rocks on either side of the Kunzhong structure zone,it is concluded that the Aowade grano-diorite was formed during the subduction of Proto-Tethys Ocean,which further reveals the nature of bidirectional subduction of the Proto-Tethys in the Late Ordovician.