Study Of Formation Of The Neoproterzoic Mamianshan Group And Its Evolution Of Deformation In Central Fujian Province, SE China

The central Fujian province is located on the junction zone of the Wuyishanpaleo-uplift, the Yongmei late Paleozoic depression and the eastern Fujian volcanicrift faulting zone. The Mamianshan Group is a main part of the Precambrianmetamorphic basement developed in the area. Based on SHRIMP zircon dating, it issuggested that the Mamianshan Group, including the Longbeixi Formation, theDongyan Formation and the Daling Formation from bottom to top, was formed duringNeoproterozoic.According to geochemical characteristics, the bimodal volcanics in theNeoproterozoic Mamianshan Group might be formed in arc rift, or in the settingsimilar to the marginal sea of southeastern China.Based on geological investigation and analysis, which are microscopicidentification, the petrofabric analysis of X-ray diffractogram of oriented specimenquartz, this research suggests that the basement metamorphic series in central FujianProvince underwent four stages of tectonic deformation, corresponding to four stagesof tectonic events, namely Paleo-meso proterozoic (early Jinning epoch),Neoproterzoic (late Jinning epoch), early Paleozoic (Caledonian) and Mesozoic(Indosinian to Yanshanian epoch) respectively. So the Paleoproterozoic MayuanGroup underwent four deformation stages, while the Neoproterozoic MamianshanGroup underwent three stages, among which the tectonic stress field of theMamianshan Group is the same as that of the latter three deformation stages of theMayuan Group. The tectonic stress field in Caledonian epoch is nearly EW-trending,NNE-trending in Indosinian epoch, and NW-trending in Yanshanian epoch, but thedirection of the tectonic stress field in Jinning epoch and the former can not berecovered. The protolith formation, metamorphism, tectonic character of the Mayuan Groupis obviously different from that of the Mamianshan Group, suggesting a long hiatusbetween their formation eras, which is presumely angular unconformity. Thisunconformity was reformed to be fault zone or mylonitic zone by later tectonism. Inthe light of the definition of the Zhuzhou conglomerate in the Dikou area, combinedwith new SHRIMP U-Pb zircon age, it is showed that the Zhuzhou conglomerate isnot supposed to be the upper basal conglomerate in the angular unconformity,deducing that the Mesoproterozoic Dikou movement does not exist.This research also reveals that the metamorphism and deformation in theCaledonian epoch was very intense—it established the base of tectonics in southChina to some extent. The brittle-ductile deformation of this stage occurred in thedeep crust (not shallow crust), which led to the previous deformation to homogenizeand not easy to identify, or it became a rootless hooked fold whose axial plane wasconsistent with foliation.After slight Hercynian tectonic event, the area was activated in the Indosinianepoch. Various tectonic activities, such as compression, fold, uplift, thrust,decollement, etc., were obvious and crust-derived S-type granite was intruded in thearea as well.Succeedingly a strong Yanshanian tectonic event occurred in the area, asevidenced by the great change of the tectonic framework in central Fujian Provinceeven southeast China. Together with the transformation from the nearly EW-trendingTethyan tectonic domain to NE-trending Pacific tectonic domain, the dynamic systemchanged from compression to extension. Coarse-grained composite terrigenous clasticcoal-bearing formations were developed in the Mesozoic basins, and large-scalemagmatism occurred in central Fujian Province. Large-scale slip shear zones wereformed, which obliquely thrust from SE to NW. Meanwhile, red compositeterrigenous clastic formation, intermediate-acid and/or alkaline volcanic rock seriesand A-type granite were developed in the later tensile fault depression zones.