塔里木西部奥依塔克斜长花岗岩锆石U-Pb年龄和Hf同位素研究

Studies on the U-Pb ages and Hfisotopes of zircons in the Aoyitake plagloclase granite.west Tarim.

作者对塔里木西部奥依塔克斜长花岗岩中的锆石进行了配套的原位微区U-Pb年龄和Hf同位素分析。两个样品的锆石U-Pb平均年龄分别为:337.5±6.5Ma和339.2±6.5Ma(Th/U比值在0.3～1.0之间),与前人所获得的SHRIMP年龄基本一致。所有锆石的_(176)Hf/_(177)Hf比值均分布在0.2829～0.2831之间,并具有极高的正ε_(Hf)值(12～17),其平均Lu-Hf亏损地幔模式年龄为359.1Ma,与其U-Pb年龄十分接近。上述Hf同位素特点反映出其源区可能为新生的玄武质下地壳。此外,所有锆石的_(176)Hf/_(177)Hf比值和ε_(Hf)值基本位于亏损地幔演化线上,指示该新生的玄武质下地壳来源于亏损地幔。结合前人对该岩体以及区域地质的研究,推测奥依塔克岩体应与巴楚—喀什一带石炭纪似层状杂岩体相关,其形成可能受石炭纪时导致天山裂谷发育的地幔柱活动的影响。

In situ U-Pb ages and I-If isotopes of Aoyitake plagioclase granite in west Kunlun were analyzed on the basis of mircro-district studies upon zircons selected from two samples. Zircon U-Pb ages of the two samples are 337.5 +/- 6.5Ma and 339.2 +/- 6.5Ma, respectively with their Th/U ratio of 0.3 similar to 1.0, which is consistent with the SHRIMP ages tested by Zhang et al. ( 2006). The Aoyitake plagioclase granite zircons, with their (176)Hf/(177)Hf ratios varying in 0.2829 similar to 0.2831, have high positive epsilon(Hf) (12 similar to 17) and depleted mantle model ages (averaged at 359.1 Ma), which is almost equal to their U-Pb ages. In the (176)Hf/(177)Hf-epsilon(Hf) discrimation diagrams, these zircons were plotted on the depleted mantle evolution line, indicating that this granite was derived from the new basaltic lower crust which is originated from depleted mantle, through a series of crystallization differentiations. Compared with current data from studies on the layered complex mass in Bachu- Kashgar zone, Aoyitake granite bears similar characteristics of its primary magma with the former, which were supposed to be originated from partial melting of the young basaltic lower crust related to the mantle plume action during the carboniferous Tianshan rift period.