Origin and age of Mn deposits of the Nantuo Formation in the Sinian System

摘要

THE tillites in the Nantuo Formation of the Sinian System is considered contemporary with the Varanger ice age. The manganese deposits in the Nantuo Formation are most suitable to isotopic dating and may record the precious information on paleo-oceanchemistry. However, there have been different views and disputations about their origin and age for a long time. The Sinian System in the Yangtze region is divided into 3 formations from base to top: the Nantuo Formation, the Doushantuo Formation and theDengying Formation. The Nantuo Formation comprises 3 members: the Lower Tillite Member, the Datangpo Member and the Upper Tillite Member, The Lower Tillite Member and the Upper Tillite Member consist of glacial tills and glacio-fluvial deposits. The Datangpo Member includes mostly grey shale and black carbonaceous shale rich in organic matter. The manganese carbonate beds or lens widely occur at the bottom of the Datangpo Member. The samples under study were collected from 5 manganese deposits of the Datangpo Member or equivalent beds in Changyang of Hubei, Xiushan of Sichuan, Songtao of Guizhou, Huayuan and Ningxiang of Hunan, respectively, all of which were located within the range of the same ocean basin at that time. The samples are black and laminated and consist mainly of rhodochrosite, manganocalcite and magnesio-rhodochrosite.