北京十三陵地区前寒武纪岩石中稀土矿物的发现及对同位素测年的意义

Discovery of REE Minerals from Precambrian Rocks of the Ming Tomb District,Beijing and Its Implications for SHRIMP Isotopic Dating

华北中、新元古代地层的年龄数据很混乱(表1),本文建议将北京十三陵地区新发现的稀土矿物用SHRIMP方法测年,有助于问题的解决。北京十三陵地区保存了新太古代五台群和新—中元古代较完整的地层,电子耦合等离子体分析(即原子收光谱分析)(ICP)的定量分析数据表明,在这些地层中,岩石中所含钾和稀土元素含量都比北美页岩(NASC)、欧洲页岩(ES)和澳大利亚后太古宙页岩(PAAS)高出很多,经电子扫描+能谱仪+波谱仪(SEM+EDS+WDS)分析证明,在岩石中包含独居石(碎屑的和自生-成岩的)和磷钇矿(自生-成岩的),并首次发现钍石-独居石环带状混合矿物(变质的)以及显微脉状稀土硅酸盐矿物(地下流体形成的)等稀土矿物。利用激光拉曼光谱鉴定发现稀土矿物的分布状态包括:1在太古宙五台群的片麻岩中,云母、石英和长石之间有非自形晶独居石,而且在石英单晶里还有独居石的自形晶包裹体;并发现独居石和钍石-独居石环带状混合矿物,这些稀土矿物都是变质成因的;2在新太古代五台群片麻岩的准平原化风化面上,沉积的元古宙常沟组的底砾岩中发现了碎屑的独居石,这些独居石的同位素年龄对于元古宇的底界定年意义重大;3在常州沟组下部压扁-透镜状层理的粉砂岩中,普遍发现碎屑锆石的外缘生长出自生-成岩磷钇矿,磷钇矿的同位素定年对于常州沟组的地层年代有代表性意义;4串岭沟组的粉砂岩中发现了无形晶状自生-成岩独居石和磷钇矿,并且较多出现在显微缝合线内外,可作为SHRIMP测年的对象;5大红峪组粉砂岩中除了发现碎屑独居石外还发现脉状硅-铝稀土矿物,可能与后元古宙热液活动有关。事实上这些自生-成岩的稀土矿物的形成,都是源自太古宙富含稀土元素的变质岩石,其形成机理也与地下流体活动有关。笔者认为北京十三陵以及至华北地区,前寒武系富稀土元素形成的自生-成岩的稀土矿物,有助于用SHRIMP方法对前寒武纪地层的同位素测年研究。

There are very confused isotopic geochronological data of Neo-Meso-Proterozoic strata in the North China area（Table 1）. It is a suggestion of this paper to solve this problem by SHRIMP method to test the new finding rare earth minerals from the Ming Tomb District, Beijing. There is a well preserved outcrop of strata beds of the Late Archean Wutai Group and the Neo-Meso-Proterozoic Strata in the Ming Tombs District Beijing. According to geo-chemical analysis data tested by Inductively Coupled Plasma （-Atomic Spectrum Analyzer） （ICP-AS） all of those rocks shows that their REE element contents are much higher than the North American Shale （NASC）, Europe Shale（ES） and Post-Archean Shale of Australia（PAAS） and tested by electron micro-probe analysis （SEM-EDS-WDS） demonstrates that there are monazite （metamorphosed, clastic and authigenic-diagenic）, xenotime （authigenic-diagenic）, and the first finding zonal mixed monazite-thorite mineral （metamorphosed） as well as micro-veinlet-silieate REE mineral （under grand aqueous effected） etc. Tested by Laser-Raman- Spectrum Analysis the distribution types of REE minerals were preserved in Precambrian rocks are. （a） metamorphosed monazite as unhedral crystals however some single euhedral monazite crystal in quartz and feldspar as inclusions and being the first finding of zonal mixed monazite-thorite mineral in gneiss of the Late Archean Wutai Group. All of those REE minerals are metamorphosed. （b） clastic monazite preserved in base conglomerate of the Proterozoic Changzhougou Formation which laid on unconformity surface of the Archean gneiss, determination of those clastic monazite being important significance for the geo-age between Archean and Proterozoic unconformity surfacer （c） in lenticular-flaser beddings of siltstone of the Chuanglinggou Formation it is always found the xenotime to be secondary growth around clastic zircon grains as rim or re-growth euhedral crystals . It is supported that test the geo-age of the xenotime by SHRIMP method would be representing geo-age of the Changzhougou Formation;（d） there are dispersed plenty grains of authigenic- diagenic unhedral monazite and xenotime in siltstone especially around suture structure of the Chuanglinggou Formation. It is supported that those REE minerals would be a good targets for SHRIMP test geo-age of the Chuanglinggou Formation; （e） there have been found REE-silicate mineral in siltstone as micro-veinlets of the Dahongyu Formation. In fact the original source of elements of above REE minerals might be taken from Archean REE-rich rocks and related with underground aqueous-flow activity. This paper issues that wide distributed authigenic-diagenic REE minerals in Precambrian rocks of the Ming Tombs District, Beijing would be favorable for the SHRIMP geo-age determination of Precambrian strata of the North China.