The Formation Conditions And Chronological Study Of Wuhe Group In East Bengbu Uplift Area

Bengbu Uplift Area is a significant section of the North China Craton which located in its southeastern margin. However, it has been restricted for people to have a comprehensive understanding on how Bengbu Uplift Area formed and evolved due to the weakness of previous research on analyzing the metamorphic conditions, timing and tectonic settings of the Bengbu uplift area’s metamorphic crystal basement. It still remains controversial on the tectonic setting background of Wuhe Group’s formation. Therefore, in this research, detailed geological survey, metamorphic petrology, petrologic and mineralogical geochemistry analysis and LA-ICP-MS zircon U-Pb chronology have been conducted on granitic gneisses, biotite plagioclase gneiss and recently found garnet-bearing granitic gneisses of Zhuding-Shimen Mountain. On the basis of above-mentioned studies, this paper also discussed Wuhe Group’s metamorphic conditions, tectonic settings and evolution process of in Bengbu Uplift Area, which has remarkable meanings on revealing the tectonic evolution of southeastern margin of the North China Craton.By means of electron probe microanalysis(EPMA) and P-T value estimation on garnet-bearing granitic gneisses, the P-T conditions have obtained which are T=736-788℃ and P=0.76-0.89GPa. It indicates that they formed under the low-pressure granulite facies condition. Zircon LA-ICP-MS U-Pb dating analysis has recorded the magmatic crystallization events and metamorphic events, for the ages are(2562±34)Ma～(2584±25)Ma, (2433±31)Ma～(2466±31)Ma and (2204± 68.0)Ma. The results of those above-mentioned experiments manifest that the evolution of east Bengbu Uplift Area is in accordance of that in North China Craton, i.e. generally through the metamorphic conditions from amphibolite facies to granulite facies and multi-phases evolution processes.The result of geochemical analyses on granitic gneisses shows that the granitic gneisses are magmatic rocks belonging to the ortho-metamorphic rocks. These granitic gneisses also possess the characteristic of high contents of Na2O、K2O,which belonging to metaluminous to weakly peraluminous rocks and calcium-alkali series. The granitic rocks are enriched LILE with Rb、Ba and K, depleted HFSE with Nb、 Ta and Ti, which have features of the crustal-melted magma. Combined Zircon U-Pb isotope dating results and the rock tectonic discrimination diagrams, the granitic gneisses are classified as volcanic arc granite, which mainly formed in the island-arc-related tectonic environment. It is considered that the granitic gneiss of Bengbu uplift area in the southeastern margin of North China Craton is formed under the Neoarchean oceanic island arc tectonic environment, and it represents the period of plate subduction and accretion event.