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

Abstract

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% SiO2, 12.18%-18.30% Al2O3, 0.55%-0.79% TiO2 and 2.53%-3.54% K2O. 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, 1600 Ma, and 2500 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.

Keywords

• zircon geochronology
• the indus-tsangpo suture zone
• zhongba block
• gongzhu co
• gneiss