中国北方元古宙沉积岩中自生稀土矿物特征及其意义——以北京、大连地区为例

Characteristics of REE Minerals from Proterozoic Sedimentary Rocks of Northern China and Their Significance—Case Studies of Beijing and Dalian Areas

大连震旦系十三里台组首次发现自生稀土元素矿物独居石后 ,在北京十三陵中元古代长城系常州沟组、串岭沟组和大红峪组又发现了自生的独居石以及其他磷酸盐和硅酸盐稀土矿物。自生稀土矿物的形成和岩石中稀土元素含量较高有关 ;电子探针背散射图像和 P、Th、L a、Ce、Nd、Y等元素面分布图像研究表明 ,沉积岩中的自生稀土矿物与岩浆岩、变质岩和碎屑成因的截然不同。本文首次报道了元古宙长城系常州沟组沉积岩中碎屑锆石边部成岩过程中形成的自生磷钇矿。中国北方元古宙泥砂质碎屑沉积岩普遍具有高稀土组合 ,许多地区都可能存在独居石等自生稀土矿物 ,如天津蓟县、辽西、辽南和宣化等地 ,为利用离子探针 (SHRIMP)确定其同位素地质年代提供了可能。此外 ,本文对比了大连震旦系自生独居石和内蒙白云鄂博矿区东矿的独居石晶形和化学成分的相似性 ,再一次提出中国北方元古宙富稀土地层可能是内蒙古白云鄂博巨型稀土元素矿床的矿源层问题。

The Precambrian Strata in Beijing and Dalian areas can represent Proterozoic sedimentary sequences of northern China. Since the discovery of authigenic monazite for the first time in the Shisanlitai Formation of Sinian System of Dalian area, we have discovered authigenic REE minerals of phosphate and silicate from the Changzhougou, Chuanlinggou and Dahongyu formations of Mesoproterozoic sedimentary rocks in Beijing area. There is a relationship between contents of REE in sedimentary rocks and presentation of authigenic REE minerals. According to the study of back scattered electron images and X-ray mapping of elements P, Th, La, Ce, Nd and Y by means of electron microscopic probe analysis, there are quite different properties between authigenic REE minerals and those from magmatic, metamorphic or associated heavy minerals of clastic sediments. This paper introduces for the first time the xenotime overgrowing on the clastic zircon grain margins in sedimentary rocks of the Changzhougou Formation, Changcheng System during the diagenesis process. Based on previous experiences there will be widely distributed authigenic REE minerals in Proterozoic sedimentary rocks of northern China, such as Jixian of Tianjin, western and southern Liaoning Province, and Xuanhua of Hebei Province. Those authigenic REE minerals can be used to determine their isotopic geological ages with the SHRIMP. Besides, in this paper we compared the similarity of crystalline forms and chemical compositions between the authigeneic monazite of the Sinian System, Dalian area and the monazite in host rock slate of eastern iron mining area of Bayan Obo, Inner Mongolia. So we once more emphasize the suggestion that the Proterozoic K- and REE-enriched sedimentary rocks in northern China may be the source rocks for the huge REE ore deposits in Bayan Obo, Inner Mongolia.