The Geochronology And Tectonic Implications Of Granite Gneiss And Xiaohe Granite In Taihua Group Of Xiaoqinling Area

The aim of this paper is to analyze the formation age and cause of pegmatitic gneiss, granite gneiss and Xiaohe granite of Taihua Group in Xiaoqingling area by way of petrological geochemistry, LA-ICP-MS zircon U-Pb chronology, zircon trace element and Hf isotopic test on the basis of summarizing previous accomplishment and progress.Geochemical data reveals that pegmatitic gneiss of Taihua Group in Xiaoqingling area belongs to Sodium-rich and Aluminum-oversaturated granite and the REE partition pattern shows strongly positive Eu anomalies, which may be due to the entrance of partial melting of plagioclase to primary magma. Granite gneiss belongs to Aluminum-oversaturated granite. The trace elements exhibits enrichment of Rb, Th, Sm, etc. and depletion of Sr, Ta, P, Ti, etc. and REE distribution pattern displays a right deviation type and obviously negative Eu anomalies. Xiaohe granite belongs to quasi-Aluminous calc-alkaline series and the trace element shows the enrichment of Rb, Th, Sm, etc. and the depletion of Sr, P, Ti, Ta, Nb, etc. and strong fractionations between LREE and HREE ((La/Yb)N=7.5), with negligible Eu anomalies, which mirrors that differentiation of various degrees may has been through in the process of magmatic crystallization.With the application of LA-ICP-MS zircon U-Pb dating, an analysis of zircons from pegmatitic gneiss, Xiaohe granite, granite gneiss and it`s veins of Taihua Group in Xiaoqingling area. The analysis result shows high Th/U of zircons from granite gneiss(0.20～0.70) , a left deviation type of REE distribution patterns of zircons, clearly positive Ce anomalies, and negative Eu anomalies. The dating age is 2346±28 Ma. Th/U of zircons from pegmatitic gneiss is high with ratios of 0.46～1.05 and the dating age is 1866±19 Ma, the dating age of the veins of pegmatitic gneiss is 1881±24 Ma which indicates pegmatitic gneiss, granite gneiss and it`s veins of Taihua Group in Xiaoqingling Area belongs to Palaeoproterozoic Era. From cathodeluminescence (CL) images of zircons, it can be found that zircons are dark and endow with distinctly magmatic oscillatory zoning. Th/U of zircons is high and ratios are more than 0.25. Properties of magmatic origin can be conjectured from REE distribution pattern and CL images of zircons. The age of rock is confirmed to be 2328±14 Ma, belonging to Palaeoproterozoic Era instead of Mesoproterozoic Era, by dating result.Hf isotope test indicatesεHf(t)of zircons from xiaohe granite range between-2.49～2.58, which shows magma derives from remelting of newborn materials and ancient crustal materials of late Archean; And theεHf(t) of zircons from granite gneiss of Taihua Group in Xiaoqinling area ranges between 0.44～3.77, which indicates the primary magma of granite gneiss in Xiaoqinling Area derives from the Late Archean differentiated from depleted mantle source.