中天山南缘那拉提碱性花岗岩岩石成因——来自锆石微量元素和Hf同位素的证据

Petrogenesis of Nalati alkali granites in South Central Tianshan Mountains: Evidence from zircon trace elements and Hf isotope

位于中天山南缘的早二叠世那拉提碱性花岗岩以近东西向狭长条带状侵入于志留系巴音布鲁克组之中,花岗岩锆石的CL图像均显示有清晰的振荡环带和较高的Th/U比值。其中一类显示出典型岩浆锆石的稀土元素球粒陨石标准化配分样式;另一类锆石则具有相对较低的Ce/Ce*比值和较高的(La/Sm)N比值,通过锆石微区微量元素特征和U-Pb同位素测年,可以推断此类锆石形成于较低的fO2环境中,且可能与流体作用的改造无关。那拉提碱性花岗岩具有较高的锆饱和温度和低Sr高Yb的全岩地球化学特征,所有被测锆石的重稀土元素呈强烈富集型,表明它们应形成于正常或减薄的地壳压力环境之中。此外,被测定的锆石微区都具有较高的Hf同位素比值和正εHf(t)值,且二阶段模式年龄均大于花岗质岩浆的结晶时代,说明那拉提花岗岩应是新生的玄武质地壳在早二叠世部分熔融的产物。结合本地区的花岗岩同位素年龄和火山岩岩石成因研究,认为那拉提花岗岩为石炭纪—二叠纪天山全区岩石圈强烈拉伸作用下,中天山南缘花岗质岩浆活动的地质记录。

Lying in South Central Tianshan Mountains, the Early-Permian Nalati granites intruded into Silurian strata. The zircons of the Nalati granites all have well-developed oscillatory growth zoning in cathodoluminescence images and high Th/U ratios. Some of the zircons show typical REE patterns, which are similar to characteristics of the magmatic zircons, and the other zircons with low Ce/ Ce^＊ ratios might have been formed in a low-fO2 environment, and their high （La/Sm）N ratios should be related to the influence of in- clusions rather than to that of the meta-fluid. The chemical features of low Sr and high Yb, and high zircon saturation temperatures indicate that they were formed in a normal or decompression environment. The analyzed domains of zircons have high 176^Hf/177^Hf ratios and positive εHf（t） values, and the zircon second-stage Hf model ages are older than the crystallization ages of granites. The above evidence suggests that Nalati granites were the melted product of juvenile crust in Permian. Combined with previous researches on granites in this region and petrogenesis of Neopaleozoic volcanic rocks, the authors have reached the conclusion that the Nalati gran- ites are geological records of Carboniferous and Permian magmatic activities in an intense lithosphere extension process in China＇s Tianshan region.