前寒武纪碎屑岩沉积作用时间的限定:沉积自生磷钇矿定年

沉积岩地层是研究古环境、古地理和构造演化历史等的重要素材。生物地层、火山岩夹层定年是最常用的且最精确的两种限定地层沉积时代的方法，然而对于前寒武纪地层，生物标志地层方法基本不能用，在缺乏夹层火山岩时，其沉积时代的确定成为地质年代学难题之一。近年来，碎屑沉积岩中自生磷钇矿的原位微区U-Th-Pb定年逐渐成为厘定沉积时代的有力测试手段。自生磷钇矿是在沉积作用发生之后，在沉积物与水体交界面以下，元素Y、稀土元素（REE）和磷酸盐供给充分的地方形成，因此，自生磷钇矿的形成时代近似于沉积时代。由于磷钇矿和锆石晶体结构相同，自生磷钇矿大多在碎屑锆石颗粒的表面形成三角锥状至不规则次生加大边。由于可能有类似产状的后期热液磷钇矿干扰，沉积岩中磷钇矿年龄解释存在一定的多解性。以长城系碎屑沉积岩磷钇矿定年为实例,总结了自生磷钇矿的特点、寻找和定位方法、年代学分析流程、数据解析和成因判别等。结果表明:长城系最下部的常州沟组粗砂岩中磷钇矿为热液成因，而上覆串岭沟组细砂岩中发育有沉积自生磷钇矿，其最老一组年龄为(1 716±3)Ma，限定了长城系的最小沉积时代。

Determining the Depositional Age of Precambrian Clastic Sedimentary Rocks: Diagenetic Authigenic Xenotime Dating

Sedimentary rocks are important material to investigate paleoenvironment, paleogeography and tectonic evolution history. Biostratigraphy and volcanic rocks dating are two most commonly used and accurate methods to limit the depositional age. However, it is rather difficult to constrain the depositional age of Precambrian sedimentary rocks without volcanic interlayers, as the stratigraphical paleontology methods are hardly applicable for these rocks. Consequently, in-situ U-Th-Pb dating on authigenic xenotime is a powerful tool to constrain the depositional age of such kinds of rocks. Shortly after burial in siliciclastic sediments, the authigenic xenotime grows within tens of meters of the sediment-water interface, where Y, REE and phosphate are supplied sufficiently. Therefore, the forming age of authigenic xenotime is close to that of the sedimentary rock. In most cases, the authigenic xenotime occurs as pyramidal to irregular overgrowths on detrital zircon grains, because the crystal structure of xenotime is the same as that of zircon. Due to the indistinguishable petrography of hydrothermal xenotime and diagenetic xenotime, the age of xenotime shows multiple interpretations in sedimentary rocks. Taking the xenotime from the Changcheng System as an example, the characteristics and discrimination of authigenic xenotime, and the methods to search, locate, analyze authigenic xenotime, as well as to process the data were summarized. The results show that the xenotime in coarse-grained sandstone of the lowermost Changzhougou Formation is hydrothermal origin, while the xenotime in medium-fine sandstone of the Chuanlinggou Formation is diagenetic origin; the oldest age of (1 716±3)Ma is interpreted as the minimum depositional age of the Changcheng System.