Keywords: 
• authigenic monazite /
• Phengite mudstone /
• Precambrian Sinian /
• U /
• Th- Pb isotopic /
• timing

Abstract: A paper entitled discovery of authigenic monazite in Precambrian sedimentary roc k of Dalian area and its significance has been issued in the《Acta Sedimentologi ca Sinica》 (1999). After that we have got financial support for continuing stud y by the Chinese National Natural Scientific Foundation (No.40172044).This pape r introduces the new achievements of the mudstone study as follows: (1) characte ristic chemical composition: K₂O 11.94%, Al₂O₃ 16.47%, SiO ₂ 61.91%; REE 330.69,μg/g, LREE 266.15μg/g, Ce 130.80μg/g; La(N)/Yb(N) 12.44,δEu 0.75,δCe 1.17;(2)the main rock-frming mineral composition is phengite,which is determin ed by electron probe analysis of enery spectrum and wave spectrum methods and co mpared with X-ray diffraction curve. The phengite was altered from illite by in fluence of deep diagenesis and Mesozoic thrust tectonic movement; (3) the authig enic monazite was also suffered by the diagenetic and tectonic influences showin g in the crystal form types such as scattered mini-grains, single needle-like crystal, aggregate of crystals and connected crystals, etc. The size of authigen ic monazite crystals is from 1 to 60 μm, which are quiet different from the sh ort monoclinic prism taken from metamorphic, magmatic rocks or clastic placers. According to above, this paper concludes that the mudstone is a K and REE rich one, which created favorable inner-outside environment for authigenic monazite formation. Meanwhile this paper emphasized on that the wide spread Proterozoic strata in North China might contain many REE-rich beds with authigenic monazite to be used for U, Th-Pb isotopic timing study. It is proposed that the REE-ri ch beds might be the mineral source of the huge rare earth mineral deposits of t he Baiyun Obo along the margin of North China platform.