天山托木尔峰花岗质岩石的同位素地球化学特征

Isotopic geochemistry of granitic rocks from Tuomuer Peak region, Tianshan, China

分析了西天山托木尔峰地区花岗质岩石和基底岩石的稀土元素 ,微量元素以及 O、 Pb、 Sr、 Nd同位素组成。本区北部伊犁地块基底岩石的 Nd同位素模式年龄约为 1.9Ga,南部南天山褶皱带的约为 2 .2 Ga。高的 Sr同位素初始比值(87Sr/ 86 Sr(T)约为 0 .70 6到 0 .733)表明本区花岗质岩石主要为地壳熔融产物。 O、Pb、Sr、Nd同位素组成指示位于伊犁地块和南天山褶皱带的花岗岩源区岩石的 Nd模式年龄都相当于伊犁地壳 ,但源区性质不同 ,前者源区δ1 8O低 ,μ值低 ,Th/ U比低 ,后者相反。于 2 90 Ma前后形成的岩体 ,无论产出于伊犁地块或南天山褶皱带 ,其 Nd模式年龄均很低 (1.4～ 1.5 Ga) ,指示一次幔源物质加入事件 。

Rare earth and other trace elements as well as O, Pb, Sr and Nd isotopic compositions were analyzed for granitic and basement rock samplers from Tuomuer Peak region, west Tianshan, China. Nd isotopic model ages of basement rocks suggested that the crustal age of Ili plate in the north (1.9Ga) is younger than that of the South Tianshan (STS) fold belt in the south (2.2Ga). Because of high 87Sr/ 86Sr(T) (0.706 to 0.733), granitic rocks occurring in the entire region are mostly products of melting of the crust. However, O, Pb, Sr and Nd isotopic compositions suggested that granitic rocks in the Ili plate and those in the STS fold belt were derived from the crust with Nd model age similar to that of the Ili crust. The geochemical characteristics of the two sources are slightly different. Granitic rocks in Ili plate were generated by crustal rocks with low δ 18O, lower μ and Th/U ratio, while those in the STS fold belt by rocks having higher δ 18O, highter μ and Th/U ratio. Granitic rocks in both Ili plate and STS fold belt formed 290Ma ago show significantly lower Nd model ages (1.4~ 1.5Ga), suggesting a mantle input event which was contemporaneous with emplacement of mafic-ultramafic igneous rocks in the region.