前寒武纪沉积岩中自生独居石的发现及其意义

由于受到葛家屯组中发现自生方铅矿的启发 ,首次于大连前寒武纪震旦系十三里台组泥岩中发现了自生独居石 ,这一发现为在中国北方前寒武纪沉积岩中寻找自生独居石提供了重要的线索。它为进一步探索 U、Th- Pb同位素测年拓展了新的研究领域和提供了可能性。本文列举了中国北方前寒武纪沉积岩中 Ce元素异常以及 REE较高的例证 ,认为上述地区都有可能发现自生独居石。同时介绍了最近又在北京十三陵中元古代的常州沟组和串岭沟组所发现自生独居石的新资料。研究表明 ,自生独居石的电子探针扫描形态 ,与岩浆岩、变质岩及砂矿中截然不同。在地质年代分布上 ,元古宙 REE相对丰度较高 ,特别是 L REE较高的泥质岩多数来自古陆壳上。资料对比结果显示 :在 L a+Ce+Nd、Yb+Y、Sm+Gd+Dy三角图中 ,北京十三陵元古宙泥质岩、大连震旦系十三里台组泥岩和辽南 -辽西中元古代泥质岩都属于近古陆的沉积类型。首次提出 ,中国北方元古宙沉积与南方震旦系磷块岩沉积环境存在明显的不同并反映在三角图中 ,前者离 L a+Ce+Nd端点近 ,而后者由于成因上属于洋流上升沉积物而远离该端点。按照大连震旦系十三里台组沉积环境特点 ,自生独居石应为生物成矿作用的产物 ,含矿物泥岩形成于总体氧化环境中的局部还原亚环境中。基于此 ,建立了

Discovery of Authigenic Monazite in Precambrian Sedimentary Rocks and Its Signi ficance

The authigenic monazite have been discovered for the first time in the Shisanlitai Formation, Sinia System of Precambrian sedimentary rock from Dalian area. That was inspired by the discovery of auhtigenic galena in the Gejiatong Formation. There are listed in this paper same examples of North China, where are abnormal high contents of Ce and REE in the Precambrian sedimentary rocks being expected to find authigenic monazite either in above areas or to offer the possibility using U,Th-Pb isotopic method for geo-age determination. This paper introduces new data of authigenic monazite, which recently have been found in Mid-Proterozoic strata of the Ming Tombs District, Beijing. The electron scattered forms of authigenic monazite are quiet different with crystalline forms of magmatic, metamophzeic, and placer monazite grains. This paper recognizes that high contents of REE, especially, higher LREE of Proterozoic mudstones are deposits near old continental crust. The statistical data of La+Ce+Nd, Yb+Y, and Sm+Gd+Dy ratio and plotting in a triangle diagram with Preterozoic mudstones of the Ming Tombs District, Beijing, Golden Pebble Beach, Dalian, and Liaoning Province shows that those samples with more La+Ce+Nd belong to near old continental crust deposits. This paper compares for the first time with the Proterozoic sedimentary rocks of North China and phosphate rocks of Sinian System of South China showing the later plotted points far away from La+Ce+Nd end point due to the phosphorus element from ocean upwelling. According to analysis of sedimentary environment of the Shisanlitai Formation, Sinian System of Dalian the authigenic monazite should be formed by biomineralization processes. It is demonstrated that local sub-reducing environment under entire oxidation environment created authigenic monazite formation, based on which a reef-back lagoon biomineralization model has been established in this paper. Meanwhile, it is supposed that the Precambrian sedimentary rocks with rich rare earth elements of North China might be the ore-source beds of the huge rare earth mineral deposits of the Baiyun Obo, Inner Mongolia.