Petrogenesis And Tectonic Significance Of The Late Mesozoic Mafic Dike Swarms In Shexian County,Southern Anhui Province

The tectonic position of Shexian area in southern Anhui belongs to the southeast margin of Yangtze plate,the eastern part of Jiangnan orogenic belt and the north of Jiangshao fault zone.The area has experienced multiple magmatic-tectonic activities in geological history,forming a more complex geological structure and various types of intrusive rocks.In the study area,the Late Mesozoic mafic dike swarms developed and produced as dyke swarms,and penetrated into the silty-mudaceous sericite phyllite of the Neoproterozoic Xikou Group.Previous studies on the late Mesozoic in southern Anhui mainly focused on granite rock mass,but the studies on petrology,geochemistry and tectonic environment of basic rocks in the region were still insufficient,which restricted the understanding of the late Mesozoic tectonic evolution in southern Anhui.Therefore,this paper is based on the project of Nanjing Geological Survey Center regional geological survey in Wuyishan Songxi-Qingyuan area.In this paper,the Late Mesozoic mafic dyke swarms in Shexian area of southern Anhui Province are selected as the research objects.The petrological,geochemical and zircon U-Pb chronology characteristics of the dyke swarms are studied to determine the formation age,explore the petrogenesis and magmatic source characteristics,and then analyze the tectonic environment for its formation,so as to provide petrological data for the tectonic evolution of southern Anhui.The mafic dike swarms is mainly composed of diabase.The rock is gray-green,rust-colored,with a diabase structure and massive structure.The main mineral composition is pyroxene and plagioclase.The mafic dike swarms is low in aluminum and calcium,but high in silicon,alkali and titanium.The content of SiO₂ varied from 49.54%to 55.64%,with an average of 52.36%;TiO₂ ranges from 2.43%to 4.91%with an average of 3.24%;The content of Fe₂O₃^T is low,ranging from 0.77%to 3.65%,with an average of 2.38%;CaO content ranged from 4.59%to 5.52%,average 5.07%;Al₂O₃ content ranged from13.77%to 15.76%,with an average of 14.61%;MgO content ranged from 3.79%to 5.82%,with an average of 4.95%.Mafic dike is diabase of calc-alkaline series.Rare earth elements are characterized by enrichment of light rare earth elements and relative loss of heavy rare earth elements.The fractionation degree of light and heavy rare earth elements is medium.The standardized distribution pattern of chondrite is?slowly right-leaning?.Rock enrichment of large ion lithophile elements K,Rb,Th,U,high field strength elements Nb,Ta,Zr relative loss;it shows slight negative Eu anomaly(average value of?Eu is 0.89),negative Nb,Ta anomaly and positive Pb anomaly,which are similar to typical arc magma.The LA-ICP-MS zircon U-Pb dating results of the Mafic dike swarm are 137.1±3.4Ma(MSWD=1.4),indicating that the emplacement age is Early Cretaceous.The results of zircon Lu-Hf isotope show that the variation range of?_Hf(t)is large,ranging from 7.53?1.39,indicating that the nature of magma source area is complex.The values of T_DM(average 919Ma)and T_DMc(average 1383Ma)are much larger than their U-Pb ages,indicating that the lithospheric mantle was formed with the addition of reconstructed ancient crustal materials.The rocks have the geochemical characteristics of volcanic arc basalt(CAB).La/Ta>22,La/Nb>1.5 and(Th/Nb)_N,(Th/Ta)_Nvalues are relatively high,this indicates that the primary magma of this basic dyke swarms originated from the lithospheric mantle metasomatism by subduction fluid,and the product of partial melting of diopside in spinel stable region.Subduction fluids metasomatic source area lithospheric mantle is the main reason for rocks showing"arc magma"properties.The magma ascending process is contaminated by crustal materials to a low degree.Geochemical characteristics show that the mafic dike swarms was formed in the tectonic environment of the continental plate extending in the late Mesozoic.This extensional tectonic system may be related to the subduction and retreat of the ancient Pacific plate.The determination of the mafic dike swarms indicates that the early Cretaceous southern Anhui was in the dynamic background of extensional thinning.