川西北塔公石英闪长岩地球化学特征和岩石成因

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134～136 Ma，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（¹⁴³Nd）/N（¹⁴⁴Nd）值为0.512 135～0.512 196，εNd(t)值均为负（-8.6～-7.5），Nd模式年龄为1.33～1.41 Ga，Hf模式年龄为1.13～1.37 Ga。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。

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

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(⁸⁷Sr)/N(⁸⁶Sr) is 0.712 589-0.713 009 with the initial N(⁸⁷Sr)/N(⁸⁶Sr) of 0.709 503-0.709 878, N(¹⁴³Nd)/N(¹⁴⁴Nd) is 0.512 135-0.512 196 with negative εNd(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 undifferentiation Ⅰ-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.