Allanite U-Th-Pb Micro-Chronology Method And Research Of Epidote-group Minerals In Dabie-Sulu Orogen

The first work in this dissertation is to analyze U-Th-Pb isotope and trace elements of allanite in REE granite from Guangdong Xinfeng by laser ablation ICP-MS. Our aim is to build in-situ geochronological method for allanite and search for allanite standard for in-situ dating.Allanite is a common accessory mineral, which is an important carrier of LREE, Th, U, Sr and other trace elements in rocks. Th, U can be enriched in allanite lattice in the form of isomorphism, so allanite is a good geochronological tool. However, the variable incorporation of common lead in allanite makes it difficult to subtract the common Pb, which limits the application of allanite U-Th-Pb dating. In this work, procedures have been developed to analyze the allanite from Guangdong Xinfeng rare earth element granite by laser ablation ICP-MS.232Th/206Pbc—208Pb/206Pbc isochron assumes age, and then common Pb correction is used according to the lead isotopic evolution of the two-stage mode. Using this method, the allanite Th-Pb age of Guangdong Xinfeng rare earth granite has been dated to be of-160Ma, which is very similar to159Ma-165Ma as dated by magmatic zircon U-Pb dating method. Trace elements,8Eu, Th/U ratio and common lead content of allanite indicate the magmatic origin. Allanite dating is the first time used in China. The dating method is simple, convenient and accurate for determination the age of the rocks containing allanite. The method may play an important role in geochronology, and has a very good and broad application prospect.The second work in this dissertation is based on the lack of systematic research and controversy on the epi dote-group minerals, to analyze major and trace element of whole rock and epidote-group minerals, together with U-Th-Pb isotope in gneiss of Shima, Shuanghe in Dabieshan, Fangshan in Sulu orogeny and granite of Jingshan. We do this with microphysiography to explore the reasons for epidote-group minerals formation and their geological meaning.There is allanite surrounded by epidote in gneiss of Dabie-Sulu and the allanite-epidote grain is usually more than1mm with zonings. Allanite-epidote grains have a variety of elements zoning:Fe and Ca contents increase from core to rim, and LREE decrease from core to rim with a sudden rise. Sr represents complex zoning with relatively low contents in the core and a sudden rise similar to LREE and Th. Composition of dramatic changes in allanite-epidote grains reflect different degree water-rock reaction over the whole metamorphic period. Increasing pressure can make the feldspar decomposition so that increase the content of Sr in alianite, and fluid effects can be recorded by LREE and Th in allanite-epidote grains. Trace elements and common lead character demonstrate the metamorphic origin of allanite-epidote in gneiss of Dabie-Sulu, U-Th-Pb isotope results show that the formation time all over the metamorphic period and mainly focus on the retrograde metamorphic process. The large grain alianite in Jingshan granite has corrosion core and magmatic zoning rim, with50～70%common208Pb in the core and less than30%in the rim. The alianite core is peritectic and rim is magmatic, whose age is-160Ma, consistent with the granite crystallization age.