Geogical characteristics of tuffs and its constraints on the tectonic regimes transformation in the southern Ordos Basin

Tuff is the product of volcanic activity.Tuff layers in sedimentary basins are often isochronous, which is an important link for exploring the tectonic evolution of sedimentary basins and adjacent orogenic belts.The Phanerozoic magmatism in the Ordos Basin is very limited.However, in the late Early Paleozoic and Late Triassic Yanchang periods, there are many layers of tuff interlayers widely distributed in the southern basin.It indicates a close relationship between magmatic activities in adjacent orogenic belts.Predecessors have conducted a certain discussion on each of the two periods of tuff, but there is a lack of overall comprehensive research and analysis on why tuff is distributed in the southern part of the basin, and what genetic connections and tectonic significance it has.Based on previous research results, this paper systematically analyzes the distribution characteristics, source rock discrimination, development time and regional tectonic background of the above-mentioned two stage tuffs.The results show that the tuffs in the Ordos Basin are developed in deep water environments, gradually thinning from south to north.At the same time, geochemical discrimination reveals that they are all intermediate and acidic magmas, which is closely related to the origin of volcanic island arc eruption.The development time are respectively concentrated in the Late Ordovician Sandbian-Katian period(449~453 Ma)and the Late Triassic Ladinian-Carnian period(227~242 Ma), which are closely related to the key period of the Ordos Basin evolution in time and space.Combined with regional tectonics analysis, it is believed that the above-mentioned two stages of tuff development is the responses and records of volcanic events in adjacent basins during the subduction and closure of Caledonian Shangdan Ocean and Indosinian Mianlue Ocean in the Qinling Orogenic Belt.It shows that the tectonic evolution of the Qinling Orogenic Belt has an obvious control on the early Paleozoic marine basin disappearance and the development of the Mesozoic large intracontinental lake basin in the southern Ordos Basin.The above understanding has certain guiding significance for in-depth exploration of the long-term basin-mountain evolution and petroleum exploration during the Phanerozoic in the southern Ordos Basin and the Qinling Orogenic Belt.