黔中地区新元古代清水江组沉积时代厘定及古地理格局

Sedimentary age determination and palaeogeographic pattern of Neoproterozoic Qingshuijiang Formation in Central Guizhou Province

【研究目的】黔中开阳地区清水江组是新元古代在扬子地块边缘形成的一套浅变质陆源碎屑岩夹火山碎屑岩组合,本文通过对陆源碎屑岩沉积特征和火山碎屑岩年代学的研究,旨在准确厘定其沉积时限、物源及其古地理格局。【研究方法】本次在详实的野外地质调查基础上,采用LA-ICP-MS分析方法对其顶部沉凝灰岩碎屑锆石进行U-Pb年龄测定,并进行区域地层对比。【研究结果】测得锆石U-Pb年龄主要分为两组,即古元古代2488~1821 Ma和新元古代838~779 Ma,最小年龄组的加权平均值为(802±24)Ma,火山碎屑含量的统计结果显示该时期的岩浆活动存在明显的幕式分布特征和周期性;区域地层对比结果显示黔中开阳地区出露的清水江组与铜仁江口、印江地区所出露的相似,可能相当于黔东南地区清水江组的底部。【结论】黔中地区清水江组最晚的沉积时限为800 Ma左右,物源主要来自扬子陆块西缘的川滇黔地区,其丰富的火山碎屑可能来自新元古代中期超大陆裂解过程中构造-岩浆活动的第二幕至第三幕,并在随后的雪峰运动中发生海陆转换,形成了南高北低、呈北东向延伸的古陆,奠定了新元古代中期南华纪的古地理格局。

This paper is the result of geological survey engineering.[Objective]The Qingshuijiang Formation in Kaiyang area of Central Guizhou is a set of epimetamorphic terrigenous clastic rocks intercalated with pyroclastic rocks,which formed at the margin of the Yangtze block in Neoproterozoic.In order to accurately determining the depositional age,the provenance and the paleogeographic pattern of the Qingshuijiang Formation,the sedimentary characteristics of terrigenous clastic rocks and the chronology of pyroclastic rocks are studied in this paper.[Methods]Based on the detailed geological survey,the detrital zircon U-Pb age of the tuffites at the top of this Formation was determined by LA-ICP-MS analysis,and the regional stratigraphic correlation was carried out.[Results]Zircon U-Pb ages are divided into two groups,2488-1821 Ma in Paleoproterozoic and 838-779 Ma in Neoproterozoic,respectively,with a weighted average of(802±24)Ma for the youngest age group.The statistical results of pyroclastic content shows that the magma activity in this period has obvious episodic distribution and periodicity.Regional stratigraphic correlation results show that the exposure nature of the Qingshuijiang Formation in Kaiyang area is similar to Tongren and Yinjiang area,and may be equivalent to the bottom of the Qingshuijiang Formation in southeast Guizhou area.[Conclusions]The latest depositional age limit of the Qingshuijiang Formation in Central Guizhou is about 800 Ma.The provenance mainly comes from Sichuan-Yunnan-Guizhou region on the western margin of the Yangtze Landmass.and the abundant volcanics may come from the second to third episodes of tectonic magmatic activity during the breakup of the supercontinent in the middle Neoproterozoic.The sea-land conversion occurred in the subsequent Xuefeng movement,and formed an ancient land extending in the northeast with a higher distribution of altitude in the south than the north,which established the paleogeographic framework of the Nanhua period in the middle Neoproterozoic.