The nature of Early Palaeozoic Kwangsian orogenic event in the South China Block: constraints from detrital zircons in Cambrian strata

The tectonic nature of Early Palaeozoic Kwangsian orogeny in the South China Block (SCB) is still debated, due to controversy whether there was a Cambrian ocean between the Yangtze Block and the Cathaysia Block. Similarly, continental affinities of the SCB with India and Australia in East Gondwana during the Cambrian are also controversial. Herein, we present detrital zircon U-Pb geochronology of Cambrian sandstone samples from the southern SCB. Cambrian samples (including our new samples and published samples) from the Cathaysia Block and the Yangtze Block show similar detrital zircon age spectra with one prominent age cluster of ~1000-900 Ma and several subordinate age groups, as well as similar cumulative proportion curves (CPC). Such signatures, together with other geological records, suggest that a Cambrian ocean did not exist between the Cathaysia Block and the Yangtze Block, and that Early Palaeozoic Kwangsian orogeny resulted from an intra-continental collision. Cambrian samples from the SCB have much more similar detrital zircon age spectra and CPC to those from Himalaya and Qiangtang than those from Australia, but detrital zircons in Cambrian samples from the SCB, Himalaya, Qiangtang and Australia have a similar prominent age group of ~1000-900 Ma. Thus, the SCB were likely located between Australia and India during the Cambrian, receiving sediments both from India and Australia.

华南地块（South China Block, SCB）早古生代广西运动（Kwangsian Orogeny）的构造属性仍存在争议，其核心争议在于扬子地块与华夏地块之间是否存在寒武纪洋盆。同样，寒武纪时期华南地块与东冈瓦纳的印度、澳大利亚地块的大陆亲缘关系也颇具争议。本文针对华南地块南部的寒武纪砂岩样品开展了碎屑锆石U-Pb年代学研究。华夏地块与扬子地块的寒武纪砂岩样品（包括本次新获取的样品与已发表数据）均呈现相似的碎屑锆石年龄谱，以~1000-900 Ma的显著年龄簇为核心，伴随多个次要年龄组，且累积比例曲线（cumulative proportion curves, CPC）形态一致。上述特征结合其他地质记录表明，华夏地块与扬子地块之间并不存在寒武纪洋盆，早古生代广西运动实为陆内碰撞作用的产物。相较于澳大利亚地块的寒武纪砂岩样品，华南地块、喜马拉雅与羌塘地块的寒武纪砂岩样品在碎屑锆石年龄谱与累积比例曲线方面的相似性显著更高；但上述四个地块（华南、喜马拉雅、羌塘、澳大利亚）的寒武纪砂岩样品中的碎屑锆石均以~1000-900 Ma为显著年龄组。据此推测，寒武纪时期华南地块可能位于澳大利亚与印度之间，同时接收来自印度与澳大利亚的沉积物。