新疆中天山雅西辉长岩体的锆石U-Pb年龄和Hf同位素特征

Zircon U-Pb Dating and Hf Isotopes of the Yaxi Gabbro Body and Their Tectonic Significances in Cent ral Tianshan Mountains,Xinjiang Autonomous Regions,China

新疆中天山雅西辉长岩体出露于阿其克库都克—沙泉子断裂南侧,主要岩相为辉长岩,含少量辉石岩和闪长岩,岩体有明显的钒钛磁铁矿化。利用LA-MC-ICP-MS技术开展对辉长岩中锆石U-Pb年代学和Hf同位素研究,结果表明,锆石加权平均年龄为(311.7±3.5)Ma(MSWD=0.104,n=14),代表岩体侵位于晚石炭世。锆石(176Hf/177Hf)i值为0.282637~0.282945,对应的εHf(t)值为+2.1^+13,平均值为+4.7,计算得到的锆石单阶段Hf模式年龄TDM1为430~865 Ma,表明岩浆主要来源于亏损地幔并有壳源物质的加入。结合已有报道的东天山地区辉长岩形成时代和Hf同位素组成特征,本文认为雅西辉长岩的年代学和Hf同位素组成特征为准噶尔古洋壳在晚古生代期间向南俯冲提供了重要证据。

The Yaxi gabbro intrusion in Central Tianshan Mountains,Xinjiang,China is outcropped in the south side of the Aqikekuduke-Shaquanzi fault. It contains main lithofacies of gabbro,which is obviously V-Ti magnetite mineralized,and minor lithofacies of pyroxenite and diorite. The LA-MC-ICP-MS technique is applied to date U-Pb ages and to analyze Lu-Hf isotopes of zircons of the Yaxi gabbro intrusion in this paper. The results show that the gabbro was emplaced in Late Carboniferous as the 206 Pb/238 U weighted average age of its zircons is 311. 7 ± 3. 5 Ma（ MSWD= 0. 104,n = 14）. The(176 Hf/177 Hf)ivalues of zircons are varied from 0. 282637 to 0. 282945,with corresponding εHf（ t） values varying from+2. 1 to +13（ average +4. 7）. The calculated Lu-Hf single-stage mantle mode ages vary from 430 Ma to 865 Ma,with mean age of 772 Ma which is older than U-Pb age. These characteristics indicate that the parental magma was mainly derived from the depleted mantle with minor contamination of crustal material. Combining with previous researches on the characteristics of Hf isotopes and ages of gabbros in the eastern Tianshan mountains,it is believed that the chronological and Hf isotopic data of the Yaxi gabbro body have providedimportant evidences for the view that the Junggar paleoceanic crust was southward subducted in Late Palaeozoic.