西藏阿里仲巴地块公珠错片麻岩锆石U-Pb年代学及其地质意义

Zircon U-Pb geochronology of the Gongzhu Co gneiss in the Zhongba block from Ali, Tibet and its geological significance

仲巴地块是藏南雅鲁藏布江缝合带西段的重要构造单元,对于研究冈瓦纳大陆北缘微地块的构造属性与古地理重建具有重要意义。目前,对于该地块内部是否存在陆块基底尚未有统一认识。通过野外观测、岩石地球化学和锆石U-Pb年代学分析,本研究在仲巴地块中段公珠错南地区识别出一套黑云斜长片麻岩,其w(SiO_(2))为64.09%~69.87%,w(Al_(2)O_(3))为12.18%~18.30%,w(TiO_(2))为0.55%~0.79%,w(K_(2)O)为2.53%~3.54%,LREE相对富集,轻重稀土分馏明显,推测其原岩以长石砂岩为主,形成于活动大陆边缘环境。锆石U-Pb年代学分析结果显示,其继承核年龄峰值为2 500,1 600,950 Ma,最年轻的谐和年龄为630 Ma。此外,还获得变质锆石的加权平均年龄为550 Ma。根据上述结果,研究认为公珠错片麻岩形成于新元古代晚期(630~550 Ma),其变形-变质程度较高,能够代表仲巴地块的陆块基底,并进一步证明仲巴地块是印度大陆北缘的组成部分。由此,本次研究为明确雅鲁藏布江缝合带西段仲巴地块的陆块基底特征及构造属性提供了直接的地质证据。

The Zhongba block, an important tectonic unit of the western segment of the Indus-Tsangpo suture zone(ITSZ) in southern Tibet, is significant for determining tectonic affinity and paleogeographic reconstruction along the northern margin of the Gondwana. However, whether the continental basement of the Zhongba block exists is still a controversial issue. In this contribution, integrated with field observations, whole-rock geochemistry and zircon geochronology allow us to identify a biotite-plagioclase gneiss suite in the southern region of Gongzhu Co from the central Zhongba block. The gneiss has 64.09%-69.87% SiO_(2), 12.18%-18.30% Al_(2)O_(3), 0.55%-0.79% TiO_(2)and 2.53%-3.54% K_(2)O. It is enriched in light rare earth elements(LREEs) and has a high LREE/HREE ratio.It was proposed that the protoliths of the gneiss may be sedimentary rocks, dominated by feldspar sandstones, which had formed in an active continental marginal environment. Zircon dating result shows that the peak ages of inherited zircon cores are 950 Ma, 1 600 Ma, and 2 500 Ma, respectively, with the youngest age of ~630 Ma. Moreover, the metamorphic zircon rim yielded a weighted mean age of ~550 Ma. Based on these results, the Gongzhu Co gneiss was supposed to form in the Late Neoproterozoic(630-550 Ma) and subsequently underwent a high-grade metamorphism. Combined with previous results, the Gongzhu Co gneiss should represent the continental basement of the Zhongba block. This further implies that the Zhongba block belonged to northern margin of the Indian continent. Therefore, this study provides vigorous evidence for clarifying the characteristics of continental basement as well as tectonic affinity of the Zhongba block.