川西北塔公石英闪长岩地球化学特征和岩石成因

Geochemistry and Petrogenesis of Quartz Diorite in Tagong Area of Northwest Sichuan

川西北塔公地区位于青藏高原东部,属于松潘—甘孜造山带的东南部边缘。塔公石英闪长岩侵位于晚三叠世地层中,岩体的K-Ar同位素年龄为134~136Ma,形成于早白垩世。岩石SiO2质量分数为61.37%~62.25%,铝饱和指数为0.93~0.95,全碱质量分数为5.46%~5.77%,里特曼指数为1.62~1.74,样品属于亚碱性准铝质高钾钙碱系列石英闪长岩。岩石Mg#值较高,总体具有高Sr含量、低Nd含量的同位素地球化学特征。N(87Sr)/N(86Sr)值为0.712 589~0.713 009,初始N(87Sr)/N(86Sr)值为0.709 503~0.709 878,N(143 Nd)/N(144 Nd)值为0.512 135~0.512 196,εNd(t)值均为负(-8.6^-7.5),Nd模式年龄为1.33~1.41Ga,Hf模式年龄为1.13~1.37Ga。该岩体是早白垩世期间,川西北地区陆缘-陆内造山环境下由松潘—甘孜造山带古老的下地壳镁铁质物质局部熔融形成的晚碰撞-碰撞后非分异Ⅰ型花岗岩类。

Tagong area of Northwest Sichuan is located in the east of Qinghai-Tibet Plateau,and belongs to the southeast margin of Songpan-Ganzi orogenic belt.The quartz diorites in Tagong area form in Early Cretaceous with K-Ar isotopic ages of 134-136 Ma,and emplace into Late Triassic strata.Mass fractions of SiO2 are 61.37%-62.25%,A/CNK values are 0.93-0.95,mass fractions of total alkali are 5.46%-5.77%,Rittmann indexes are 1.62-1.74,so that the samples belong to subalkaline metaperaluminous high-K calc-alkaline series quartz diorites.The rocks have relative high Mg# values with the isotopic geochemical characteristics of high contents of Sr and low contents of Nd.N （87 Sr）/N （86 Sr）is 0.712 589-0.713 009 with the initial N （87 Sr）/N （86 Sr）of 0.709 503-0.709 878,N （1 43 Nd）/N （1 44 Nd）is 0.5 12 135-0.5 12 1 96 with negativeεNd （t）of -8.6--7.5;Nd and Hf model ages are 1.33-1.41 Ga and 1.13-1.37 Ga,respectively. The quartz diorites belong to undifferentiationⅠ-type granitoids with late- to post-collision caused by partial melting of ancient lower crust mafic materials at the continental margin-intercontinental orogenic setting in Northwest Sichuan.