Zircon and baddeleyite U-Pb geochronology of the Shennongjia Group in the Yangtze Craton and its tectonic significance

Abstract

The Shennongjia Group, spreading on the northern margin of the Yangtze Craton, is a well-preserved succession of the Mesoproterozoic strata in South China, unconformably overlain by the Macaoyuan Group. Zircon and baddeleyite U-Pb dating results of dolarenite, tuff of the Shennongjia Group and mafic dike intruding the Shennongjia Group are reported in this paper. The dating results demonstrate that there are four age peaks 1.4Ga, 1.8Ga, 2.0Ga and 2.7Ga in the detrital zircon age spectra of the Dayanping Formation of the Lower Shennongjia Group. The zircon U-Pb age of the tuff of the Yemahe Formation in the upper part of the Shennongjia Group is ca. 1220Ma. The baddeleyite and zircon U-Pb ages of the mafic dike intruding the Shicaohe Formation are 1115Ma and 1083Ma respectively. Based on our new age data, combining with the regional geological setting analysis, we arrive at the following preliminary conclusions. (1) The age span of the Shennongjia Group can be precisely put in 1.4 ∼ 1.1 Ga; the sedimentary materials of the Shennongjia Group came mainly from the ancient basement of the Yangtze Craton with minor probably from the Cathaysian or Laurentian. (2) The unconformity between the Shennongjia Group and the overlying Macaoyuan Group was formed at about 1.1 ∼ 1.0Ga probably in the process of the initial assembly between the Yangtze Craton and the Cathaysia Massif, which might be the tectonic response of the Rodinia supercontinental assembly event. (3) The emplacing time of the mafic dike intruding the Shicaohe Formation is 1115 ∼ 1083Ma; coeval mafic intrusion events are widespread in Laurentia, Africa, Australia and Antarctica. It is still an unresolved problem whether this phase mafic intrusion event in the Shennongjia area resulted from the Rodinia supercontinental assembly or was genetically related with the coeval global-scale mantle plume. (4) The determination of the sedimentation age of the Shennongjia Group provides a potential candidate for the establishment of the standard section of the 1.4 ∼ 1.1Ga strata of China. (5) The ∼ 1.45 Ga detrital zircon U-Pb age peak of the Dayanping Formation provides new evidence to support the opinion that the Cathaysia was located in between Laurentia and Antartica in the Nuna (Columbia) supercontinent configuration.