西藏冈底斯中段晚侏罗世许如错花岗岩成因:锆石U-Pb年龄、地球化学及Sr-Nd-Pb-Hf同位素证据

ZIRCON U-Pb DATING,GEOCHEMISTRY AND Sr-Nd-Pb-Hf ISOTOPIC EVIDENCE FOR THE LATE JURASSIC XURUCUO GRANITES IN THE MIDDLE GANGDESE AREA,TIBET

西藏冈底斯中段晚侏罗世许如错花岗岩的岩石成因对理解特提斯洋演化具有重要的意义。本次系统研究了许如错地区似斑状黑云母二长花岗岩的岩石学、锆石U-Pb年代学、锆石Lu-Hf同位素、全岩元素地球化学和Sr-Nd-Pb同位素。许如错花岗岩的锆石U-Pb年龄为152.3±0.7 Ma,形成于晚侏罗世。SiO_(2)和Al_(2)O_(3)质量分数较高,平均值分别为71.3%和14.2%,铝饱和指数(A/CNK)较高(1.10~1.23),属于强过铝质高钾钙碱性系列岩石;富集大离子亲石元素(Rb,Th和K)、轻稀土元素和Pb等;亏损高场强元素(Nb,Ta,Ti和P),重稀土元素,Eu,Sr和Ba等。锆石εHf(t)值(-12.5~-16.9)、全岩^(87)Sr/^(86)Sr_((t))初始值(0.715~0.717)、εNd(t)值(-14.2~-14.5)和Pb同位素初始值(^(208)Pb/^(204)Pb_((t)),^(207)Pb/^(204)Pb_((t))和^(206)Pb/^(204)Pb_((t))平均值分别为39.1,15.7和18.6)均指示其起源于中拉萨地块的中—下地壳古老物质的重熔。结合前人研究,本次研究认为许如错晚侏罗世花岗岩大地构造背景为新特提斯洋洋壳板片向北俯冲于拉萨板块之下的构造汇聚。

The petrology,zircon U-Pb chronology,zircon Lu-Hf isotope,whole rock element geochemistry and Sr-Nd-Pb isotope are used to study porphyritic biotite monzogranite in Xurucuo area,Tibet,to understand the petrogenesis and its significance in the evolution of the Tethys Ocean.Chronological analysis reveals that Xurucuo granite was formed in the late Jurassic(zircon U-Pb age=152.3±0.7 Ma).Geochemical studies show that the rocks have high w(SiO_(2))and w(Al_(2)O_(3))contents,with an average value of 71.3%and 14.2%,respectively,and high Al saturation index(A/CNK)(1.10-1.23),indicating that the rocks belong to the strong peraluminous high-k calc-alkaline series.The rocks are enriched with large ion lithophile elements(Rb,Th and K),light rare earth elements and Pb,etc.High field strength elements(Nb,Ta,Ti and P),heavy rare earth elements,Eu,Sr and Ba are depleted.ZirconεHf(t)values are(-12.5 to-16.9),whole rock ^(87)Sr/^(86)Sr_((t))initial values are(0.715~0.717),εNd(t)values are(-14.2 to-14.5),and the mean values of Pb isotope initial values(^(208)Pb/^(204)Pb_((t)),^(207)Pb/^(204)Pb_((t))and ^(206)Pb/^(204)Pb_((t)))are 39.1,15.7 and 18.6,respectively,indicating that the rock originated from the remelting of the middle-lower crust material in Lhasa block.Based on this study and previous work,the tectonic setting of the late Jurassic Xurucuo granite is considered to be the tectonic convergence of the Neo-Tethyan Ocean subducted northward under the Lhasa block.