Getrogenesis And Tectonic Significance Of The Ordovician Paleomarine Volcanic Rocks In The Jiurui Area Of The Middle And Lower Reaches Of The Yangtze River

Marine volcanic rocks are the rock formation formed by the submarine volcanism,common rock types are the quenched lava of the overflow phase and the pyroclastic rocks of the explosive facies.The vitreous clastic structure is a special structural structure of marine volcanic rocks compared to continental volcanic rocks.Jiurui area is located in the northwestern part of Jiangxi Province,which is one of the important metallogenic districts of the iron-copper polymetallic metallogenic belt in the middle and lower reaches of the Yangtze River.For a long time,there is a great controversy over whether this submarine volcanic activity had occurred in this area.In this paper,a detailed study of petrology and geochemistry is carried out on the silicified breccia which is considered to be a structural fracture zone in Jiurui area,and it is determined to be a set of Paleomarine volcanic rocks,confirming the formation age of the set of Paleomarine volcanic rocks.The origin of the rock and the tectonic setting formed.The marine volcanic rocks have certain significance for the exploration of the Early Paleozoic tectonic evolution of the Yangtze plate.At the same time,through the study of marine volcanic rocks in this area,it has theoretical and practical significance for exploring the tectonic-magmatic evolution,geological prospecting and resource exploration of the regional crust.Through detailed petrographic studies,it is determined that the paleomarine volcanic rocks in the Jiurui area are an intrusive-burst facies,generally?clustered?breccia-structured quench pyroclastic lava.The breccia and cement share the same composition and a similar lava structure.The self-crushing breccia is significantly different from the continental volcanic breccia in brecciated mechanism.The set of vitreous breccia lava is integrated on the tidal flat facies wary limestone and argillaceous limestone,under the shallow sea shelf facies shale and argillaceous siltstone,so it is meant to be a deposition in the continental margin of the shallow sea basin environment,which is a product of submarine volcanic activity.The set of paleomarine glacial breccia lava is gray-dark gray,weathered as light yellow-brown,with a flow-like and thick-massive structure in its macroscopic characteristics.It has a common contemporaneous breccia structure.The size of the breccia is relatively uniform,the most of which is less than 5cm,generally below 3cm,and few reaches the agglomerate;the shape is irregular,mostly isometric,and sharp-angled-angular-sub-angular shape,which shows it had not been transported and rounded,mixed size,no orientation and partial cavities represent obvious gravity accumulation characteristics.Its petrological characteristics are characterized by breccia lava structure.The breccia under the microscope is angular-sub-angular,the internal structure of the breccia is uniform,and the fine structure of the rhyolite lava is almost all of the microcrystalline-cryptocrystalline quartz.The breccia boundary is relatively clear,the internal cracks develop,and the breccia has good collage.The breccia and gravel interstitials have similar structural features,almost all composed of microcrystalline quartz;partially visible residual micro The plaque structure,there are star-shaped quartz phenocrysts in the breccia,and silicified feldspar pseudo phenocrysts;there are more quartz and silicified feldspar phenocrysts in the matrix.Due to the widespread and uniform hydrothermal alteration quartz lithology in the same sedimentary diagenesis,Al₂O₃,Fe₂O₃,MgO,CaO,K₂O,Na₂O and other components in the original rock are taken out,and SiO₂ is relatively enriched and trace elements,light rare earth elements?Sr,Ba,LREE,etc.?undergo different degrees of migration and activation.The SiO₂ content is above 90%,mostly from 91.18%to97.26%,which is much higher than the siliceous component of any continental volcanic rock.The total amount of rare earths??REE?is mainly concentrated in33.95×10^-671.56×10^-6,obviously enriched in LREE,lean HREE,LREE/HREE ratio is 7.38²4.28,average is 13.13,?La/Yb?_N The ratio is 11.01⁸2.33 and the average is38.63.The?Eu value is 0.57⁰.79,the average value is 0.63,showing a significant negative anomaly;the?Ce value is 0.52¹.09,and the average value is 0.83,with a weak negative anomaly.The zircons of the paleomarine volcanic rocks have typical magmatic zircons with rhythmically oscillating rings.The zircon has a depleted light rare earth element and gradually enriches the heavy rare earth element,that is,gradually increases from Sm to Lu,and shows the positive anomaly of Ce and Eu.The negative anomaly characteristics,the LA-ICP-MS U-Pb age is 447.2±4.4Ma,and the formation period is Late Ordovician.This is in good agreement with the age of the porphyry?441-453Ma?,which is widely distributed around the Yangtze block.It is the response record of the Yangtze block to the Ordovician Sandb-Hirnant period global tectonic movement-volcanic activity.Through detailed petrology,petrography and geochemical characteristics,and using the inactive components HFSE?Nb,Ta,Zr,Hf,Ti?and HREE?Yb,Y?to judge the characteristics of the original rock.This paleomarine volcanic rocks have porphyry minerals such as bay-shaped ablation quartz,high-temperature feldspar and magmatic zircon,which is characterized by typical moderate-acid volcanic rocks.The zircon trace element characteristics and zircon U/Yb ratio are 0.56¹.68.The zircon characteristics of the continental rock are similar to those of the continental rock,indicating that the zircon in this area is mainly derived from the continental magma crystallization;the comprehensive Zr/TiO₂-Nb/Y,?Y+Nb?-Rb,Hf-Rb/30-3Ta diagram and its zircon trace elements U/Yb-Nb/Yb and lg?U/Yb?-lg?Nb/Yb?discriminant diagrams indicate that the magma source area should be medium-acid volcanic rocks with rhyolite and alkaline composition lithology.The magma source area may have the characteristics of arc volcanic rocks,and its formation environment is the continental marginal extension basin environment.The Late Ordovician vitrified breccia lava found in the Jiurui area of the middle and lower reaches of the Yangtze River is a volcanic crater and near crater facies.It identifies a Late Ordovician volcanic activity center in the middle and lower reaches of the Yangtze River.The Ordovician bentonite is discovered by the predecessors in the periphery of the Yangtze block has a relatively reliable and close-range source attribution,which accumulated new data for the Late Ordovician tectonic-magmatic activity in the study area.New evidence is provided for studying the tectonic evolution of the Early Paleozoic Yangtze block and the time limit of magmatism.