Abstract
The Great Oxidation Event (GOE) and Lomagundi-Jatuli Event (LJE) during Paleoprterozoic Era represent significant global environmental changes in Earth's history and their geological records have been identified from many ancient cratons. However, reliable records of GOE and LJE are rare in the North China Craton (NCC) due to high-grade metamorphism on Paleoproterozoic strata during Orosirian Period and poor age constraints. Weakly metamorphosed to unmetamorphosed Proterozoic strata termed as Fanhe Group of around 8 km thick are well preserved in Fanhe Basin in the Archean Longgang Block in northeastern NCC and the Guanmenshan Formation from the lower part of the Fanhe Group is characterized by positive (delta C-13(earb) excursion of around +5 parts per thousand. Since the Fanhe Group was usually considered as Meso-Neoproterozoic in ages, the reasons for positive (delta C-13(earb) excursions in the lower Fanhe Group are unclear. Our new LA-(MC-)ICP-MS U-Pb dating on a dolerite sill intruding into the Guanmenshan Formation yields two upper intercept Pb-207/Pb-206 ages of 2065 + 20 Ma (95% confidence, MSWD = 1.4, N = 26) and 2062 +/- 22 Ma (95% confidence, MSWD = 1.05, N = 22), respectively. Combined with the weighted mean Pb-207/Pb-206 age of the youngest detrital zircon population of the Daposhan Formation (2199 +/- 19 Ma, 95% confidence, N = 8, MSWD = 0.47) and their contact relations, the deposition ages of the Daposhan, Kangzhaungzi, Guanmenshan and Tongjiajie formations from the lower part of Fanhe Group were constrained to be Rhyacian Period from ca. 2200 Ma to ca. 2060 Ma, not Mesoproterozoic Era as previously regarded. These new age constraints provide solid evidence for the existence of weakly metamorphosed stratigraphic records of GOE and LJE in lower part of Fanhe Group in the NCC. Stratigraphic correlations with other ancient cratons show that the Guanmenshan Formation in Fanhe Basin in the northeastern NCC can be correlated with the Dashiling Formation of the Hutuo Group in the central NCC, the Lucknow Formation in West Australia, the Upper Pretoria Formation in South Africa, the Gordon Lake Formation in North America and the Tulomozero and Kuetsjarvi formations in Fennoscandia, and therefore the Fanhe Group provides excellent stratigraphic records to study GOE and LJE in the NCC.
NSTL
Elsevier