引用本文：旷红伟,耿元生,柳永清,王玉冲,夏晓旭,彭楠,范正秀,陈骁帅,白华青.2019.扬子克拉通北缘“神农架群底界”的再厘定及其地层学意义[J].地质学报,93(1):72-93. KUANG Hongwei,GENG Yuansheng,LIU Yongqing,WANG Yuchong,XIA Xiaoxu,PENG Nan,FAN Zhengxiu,CHEN Xiaoshuai,BAI Huaqing.2019.Revision of “the bottom of Shennongjia Group” and stratigraphic implication in the northern margin of Yangtze Craton[J].Acta Geologica Sinica,93(1):72-93.
Abstract:Since long time, the definition of the base of Shennongjia Group is controversial. Solving this problem is the key point to decide whether Shennongjia group could be a potential stratotype section. The profile in the northeast part of Huangling Dome, tens of kilometers southeast to Shennongjia area, is composed of the Meso- Neoproterozoic deposits containing Xichahe, Wujiatai, Jiaoyuanshan, Nantuo, and Doushantuo Formations. According to Hu et al. (2012), Wujiatai Formation in this area belongs to the Middle- Lower part of Shennongjia Group. This infers that the base of the Shennongjia Group is probably located in Wujiatai Formation. Three detrital zircon samples were collected in the Jiaoyuanshan profile in this area, north to Zhangcunping located in the north of Kongling. They are from the quartz- schist (0529- 1) of Xichahe Formation, the conglomerate (0228- 4) and sandstone (0228- 5) of the base of Wujiatai Formation, respectively. The age information provided by the LA- ICP- MS analysis of these zircons, combined with the contact relationship of adjacent strata, as well as the petrological and sedimentary features of the deposits, helped to confine the formation time of Wujiatai Formation. In the Jiaoyuanshan profile, Wujiatai Formation overlies Xichahe Formation unconformably. The U- Pb age spectrum of zircon from Xichahe Formation shows two obvious peaks corresponding to Archean Eon and 20 Ga respectively, while that of the zircon from Wujiatai Formation displays not only the two peaks mentioned above, but also an extra weak peak corresponding to 08 Ga. Therefore, Xichahe Formation is assumed to be younger than 20 Ga. As Xichahe Formation is dominated by amphibolite, which is lithologically different from the metamorphic crystal basement formed before the final consolidation of Yangtze craton (18 Ga), it is estimated to be formed during Meso- proterozoic. Wujiatai Formation should be younger than 08 Ga, and the moraine of Nantuo Formation overlying it confined its formation time between Late Qingbaikou and Early Nanhua Period. Furthermore, the composition of conglomerates of Wujiatai Formation infers that it should be equivalent to Liantuo Formation in age. The provenances of Xichahe and Wujiatai Formations are the Meso- Archean TTG gneiss and the Neo- Archean granitic gneiss from the core of Huangling Dome, the Paleo- Proterozoicvolcanic and magmatic rocks from the central of Kongling complex,and the Neo- Proterozoic magmatic rocks from the south flank of Huangling Dome.