Genesis And Geodynamic Mechanism Of Volcanic Rocks In The Early Stage Of Late Jurassic In Danxia Basin,Northern Guangdong Province

The Nanling Tectonic Belt is an ideal place to study the plate conversion between the ancient Pacific Tectonic Domain and the ancient Tethys Tectonic Domain.The Danxia Basin in northern Guangdong Province is located in the middle of the Nanling Tectonic Belt.The Jurassic Sandong Formation at the bottom developed a volcanic sequence of basaltic andesite+andesite+andesite tuff.At present,the focuses of previous studies are mainly on the diagenesis and landscape evolution mechanism of the Danxia red layer in the basin,ignoring the geodynamic background during the evolution of the basin.The formation time of the volcanic rocks at the bottom of the Sandong Formation in the Danxia Basin represents the initial time of basin expansion,which is of great significance for inverting the plate properties of the Nanling Tectonic Belt in the late Mesozoic and determining the volcanic eruption sequence in the Danxia Basin.Therefore,this article is selected from 14 volcanic rock samples in the stratum of the Sandong Formation within3-5km north of Danxia Town.The research methods of petrography,chronology,petrogeochemistry,and isotope geochemistry were used to compare with the Nanling The Sr-Nd isotopic characteristics of the volcanic rocks around the site provide reliable evidence for exploring the tectonic setting and magmatic evolution in the basin.In this paper,the formation ages of basaltic andesite and andesite in the lower section of the Sandong Formation in the Danxia Basin are measured by LA-ICP-MS dating to160.3±1.3Ma and 155.2±0.53 Ma,indicating that the volcanic lava at the bottom of the Sandong Formation was formed late.Early Jurassic.The main elements of the volcanic rock sample at the bottom show geochemical characteristics such as alkali-rich,high-potassium,peraluminous,and low-titanium.At the same time,trace elements mainly deplete high-field strength elements,and are enriched with large ionophilic elements.Right-inclined light rare earth element-enriched type,and Ce and Eu were normal.On the tectonic environment discriminative map,the andesite at the bottom of the Sandong Formation is projected on the basalt region within the plate,and at the same time falls within the basalt range of the continental tension zone,indicating that the basin is in a structural environment that extends within the plate.On the ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb and²⁰⁶Pb/²⁰⁴Pb-²⁰⁸Pb/²⁰⁴Pb discriminant map,the andesite at the bottom of the Sandong Formation is projected to the right of the Earth's isochrone and above the northern hemisphere reference line,and between the upper crust and lower crust area,near the EM?-enriched mantle area,reflecting Enrichment characteristics of the source region;combined with the comparative analysis of the Sr-Nd isotopic geochemical characteristics of volcanic rocks around the Nanling area of South China,the andesite at the bottom of the Sandong Formation had similar Sr-Nd during the Late Jurassic to Early Cretaceous Isotope geochemical characteristics.Based on previous studies on the Mesozoic in South China,this paper believes that the compressional tectonic environment of the early Yanshanian had a significant impact on South China and even the entire mainland of China.After this strong compression event,an important tectonic attribute conversion occurred in East Asia The performance of Nanling area is obvious:Middle Jurassic,dominated by compression environment and serious strata lack;Late Jurassic,with intraplate tension environment,developed A-type granite,potash alkaline rock,potash basalt As a feature,the extension is a continuation of the intracontinental environment after the Indosinian movement in the Guttis tectonic domain,and has nothing to do with the subduction of the Pacific plate.