Detrital zircon provenance and palaeogeographic implications of the Ediacaran Shigu Group in the Zhongza Terrane, SW China

The palaeogeographic positions of South China in the Rodinia and Gondwana supercontinents have long been controversial. We report petrological, detrital zircon U-Pb geochronological, trace element geochemical and Lu-Hf isotopic compositions for metasedimentary rocks from the Shigu Group in the Zhongza Terrane, SW China. The Shigu Group has been regarded as the Precambrian basement of the Zhongza Terrane, and comprises mainly a suite of medium-/low-grade metamorphosed terrigenous clastic rocks. The youngest zircon age population of two metasedimentary rock samples from the bottom Shigu Group yields a weighted average age of 633 ± 9 Ma, indicative of a late Neoproterozoic (ca. 630 Ma) maximum deposition age. The youngest detrital zircon age of a metasedimentary rock from the upper Shigu Group is 597 ± 7 Ma, it may suggesting a considerably later deposition. Detrital zircon age distribution patterns of the Shigu Group metasedimentary rocks are similar to those of coeval sedimentary rocks in the western Yangtze Block. According to Hf isotope compositions and zircon morphology, the Neoproterozoic Shigu Group detritus may have sourced from Neoproterozoic magmatic rocks in the western Yangtze Block, whilst the pre-Neoproterozoic zircons might originate from recycling of the underlying metasedimentary rocks. Our results indicate that the basement of the Zhongza Terrane was a continuation of the Yangtze Block, which implies that Precambrian Yangtze basement rocks were present beyond the current boundary of the Block. In the late Neoproterozoic (ca. 630–570 Ma), the western Yangtze Block may have been facing the Proto-Tethys and received detritus mainly from the local Neoproterozoic magmatic rocks. During the late Neoproterozoic-earliest Palaeozoic (ca. 570–520 Ma), the seaway closure between the Yangtze (incl. Zhongza) Block and northern India likely created new pathways for delivery of the earliest Neoproterozoic and Pan-African detrital zircons into the lowermost Palaeozoic sequences in South China.

华南在罗迪尼亚（Rodinia）与冈瓦纳（Gondwana）超大陆中的古地理位置长期存在争议。本文报道了中国西南仲扎地体（Zhongza Terrane）石鼓群（Shigu Group）变质沉积岩的岩石学、碎屑锆石U-Pb年代学、微量元素地球化学及Lu-Hf同位素组成数据。石鼓群被认为是仲扎地体的前寒武纪基底，主要由一套中低级变质陆源碎屑岩构成。对石鼓群底部两件变质沉积岩样品的研究显示，其最年轻锆石年龄群体的加权平均年龄为633±9 Ma，指示新元古代晚期（约630 Ma）的最大沉积年龄；而石鼓群上部一件变质沉积岩样品的最年轻碎屑锆石年龄为597±7 Ma，暗示其沉积时代可能更晚。石鼓群变质沉积岩的碎屑锆石年龄分布模式与扬子地块（Yangtze Block）西部同期沉积岩较为相似。结合Hf同位素组成与锆石形态特征，石鼓群新元古代碎屑物质可能来源于扬子地块西部的新元古代岩浆岩，而新元古代之前的锆石则可能源自下伏变质沉积岩的再循环。本研究结果表明，仲扎地体的基底属于扬子地块的延伸，这意味着扬子地块前寒武纪基底的分布范围超出了当前地块边界。在新元古代晚期（约630~570 Ma），扬子地块西部可能面向原特提斯洋（Proto-Tethys），碎屑物质主要来源于当地的新元古代岩浆岩。在新元古代晚期至早古生代（约570~520 Ma），扬子（含仲扎）地块与印度北部之间的海道闭合，这一过程可能为最早新元古代及泛非期（Pan-African）碎屑锆石输入华南下古生界地层提供了新的运移通道。