东昆仑东段沟里地区战红山过铝质流纹斑岩年代学、岩石成因及构造意义

Geochronology,Petrogenesis and Tectonic Significance of Zhanhongshan Peraluminous Rhyolite Porphyry in Gouli Area,Eastern Section of East Kunlun

东昆仑三叠纪花岗质岩石的研究主要集中于具I型花岗岩特征的大型花岗岩基,而对少量出露的过铝质花岗岩研究较少。东昆仑造山带东段沟里地区战红山花岗斑岩LA−ICP−MS锆石U−Pb同位素定年结果显示,战红山流纹斑岩的结晶年龄为(245±1)Ma。战红山流纹斑岩具有高Si(SiO2=74.50%~75.59%)、富Na(Na2O=4.04%~4.06%),高Na2O/K2O值(1.26~1.76)和铝饱和指数(A/CNK=1.07~1.14),呈弱过铝质−过铝质中钾−高钾钙碱性系列。岩石稀土含量较低,轻、重稀土元素分馏明显,Eu具轻微的负异常和正异常(δEu=0.80~1.06);富集Ba、Rb、Th、K、U等大离子亲石元素,亏损Nb、Ta、Nd、P、Ti等高场强元素,εHf(t)同位素主体呈富集特征(εHf(t)=-4.7~+0.9)。战红山过铝质流纹斑岩具I型花岗岩特征,为早期俯冲洋壳经过幔源岩浆的底侵和外来流体的加入部分熔融的结果。战红山流纹斑岩具弧岩浆岩地球化学特征,结合东昆仑造山带东段岩浆岩分布以及沉积地层特征显示,早三叠世东昆仑地区处于古特提斯洋的俯冲阶段。

The study of Triassic granitic rocks in East Kunlun mainly focuses on the large granite batholith with the characteristics of I−type granite,while the study of a small amount of peraluminous granite is less.The LA−ICP−MS zircon U−Pb dating of The Zhanhongshan rhyolite porphyry shows that the crystallization age of the Zhanhongshan rhyolite porphyry is 245±1 Ma.The Zhanhongshan rhyolite porphyry is characterized by high silica(SiO2=74.50%~75.59%),rich Na(Na2O=4.04%~4.06%),high Na2O/K2O ratio(1.26~1.76),and aluminum saturation index(A/CNK=1.07~1.14),which indicate weak peraluminous medium potassium and high potassium calc−alkaline series.The rocks is characterized by low REE content with obvious fractionation of LREE and HREE,slight negative and positive Eu anomalies(δEu=0.80~1.06),also enrichment of LILE and depletion of HFSE.They have enriched Hf isotopic compositions withεHf(t)isotope values of−4.7~+0.9.It is concluded that the peraluminous Zhanhongshan rhyolite porphyry has the characteristics of I−type granite,which is the result of the underplating of the subducted oceanic crust through mantle−derived magma and partial melting with the addition of foreign fluid,and has arc magmatic geochemical characteristics.Combined with the previous data,the study shows that the East Kunlun area was in the subduction stage of the Paleo−Tethys ocean in the Early Triassic.