大连地区前寒武纪沉积岩中发现自生独居石及其意义

Discovery of Authigenic Rare Earth Mineral-Monazite in Precambrian Sedimentary Rock of Dalian Area and its Significance

自生的稀土元素矿物一独居石在前寒武纪沉积岩中发现尚属首次。产地是大连地区金县金石滩，地层是震旦系的十三里台组。自生独居石发现于灰绿色绢云母化伊利石泥岩中，泥岩作为铁质浸染的红色叠层石灰岩的夹层也包含有针铁矿的细脉，因为许多独居石聚晶是被包裹在针铁矿细脉中。可以确定蓝细菌的生物矿化作用对于自生独居石和铁质浸染的形成都起了作用。电镜扫描和电子探针的能谱、波谱分析证实：自生独居石为10μm左右的半自形柱状单晶。单晶的柱面和轴面清渐可辨，集合体最大直径50μm左右。经波谱分析自生独居石的化学成份为（％）：Ce2O333.28，La2O39．52，Nd2O317．26，P2O330．04，ThO22．73，Pr2O33．12，Sm2O32．01，Gd2O30．59，UO20．19，Fe2O50.34，SiO20.56，CaO0．14，Al2O30．08，TiO20．11，Na2O0．01，总计99．98。因此，自生独居石应属于Ca，La，Nd的独居石变种。自生独居石的发现为利用U，Th-Pb法测定无化石的前寒武纪地层同位素年龄显示了可能性。

The rare earth mineral monazite((Ce, La, Th)PO4)is usually found in magmatic rocks or related metamorphosed rocks as one of seldom associated minerals. Sometimes the monazite can be found in clastic rocks or placer. Although the authigenic monazite has been found in bauxite deposits or karstification sediments, but lt is the first time that discovery of authigenic monazite in Precambrian sedimentary rock of Dalian area China.The authigenic monazite is found in the Shisanlitai Formation of Precambrian Sinian System of the Jinshitan Beach, Dalian area. This formation is composed of abundant reddish stromatolite limestone. The authigenic monazite has been found in the intercalated green mudstone between Iimestone beds. It was supposed that Cynobacteria biomineralization promoted formation of the authigenic monazite. The authigenic monazitebearing mudstone is consisted entirely of illite (sericitized) by X-ray diffraction determination. Exactly the authigenic monazite is involved in the hematite veinlets of the illite mudstone. The electron scanning image analysis of monazite shows subhedral crystal form in 10μm diameter. The chemical composition of the authigenic nlonazite is determined by electron-probe analysis (Wave Spectrum Analysis) as follows (%): Ce2O3 33. 28,La2O3, 9. 52,Nd2O3, 17. 26,P2O5, 30.04, ThO2, 2. 73,Pr2O3 3. 12,Sm2O3, 2. 01, Gd2O3, 0. 59, UO2 0.19,Fe2O3 0.34,SiO2 0. 56, CaO 0. 14,Al2O3, 0. 08,TiO2, 0. 11,Na2O3 0. 01,total 99. 98. Therefore,the authigenic monazite belollgs to Ce, La, Nd, monazite varities.The significance of the discovery of authigenic monazite in Precambrian sedimentary rock are discussed in this paper, e. g., the favourable sedimentary environment of authigenic monazite, the possible origin of rare earth elements of the famous Baiyun obo rare earth mineral deposits and so on. Besides,the discovery of auhigenic monazite shows the possibility of isotopic age determination by U,Th= Pb method for the non-fossil Precambrian strata.