Petrogenesis And Geological Significance Of The Mesozoic Igneous Rocks In Southern Dayaoshan,Guangxi

The Dayaoshan uplift is located in the southwest of the junction zone between the Yangtze plate and the Cathayia plate(Qinzhou-Hangzhou junction zone),and there is strong magmatic activity in the area.In this paper,U-Pb chronology,petrology,major microelements and Hf isotope geochemistry of Mesozoic igneous rocks(mafic,neutral and acidic rocks)were studied in Dafoding in the south and Nuodong and Anping in the southeast.The formation age,petrogenesis and tectonic setting are discussed,which provides important geochemical basis for revealing the deep dynamic mechanism of Mesozoic magmatic generation and volcanic activity in Dayaoshan area.The main insights gained are as follows:(1)The formation age and petrogenesis of the Dafoding pluton in southern Dayaoshan,Guangxi,and to discuss its source property and tectonic significance,LA-ICP-MS zircon U-Pb dating,zircon in-situ Hf isotope analysis and whole-rock geochemistry were carried out.The results show that the emplacement age of the Dafoding pluton is 158-157 Ma.The εHf(t)values range from-10.77 to 3.44,with mostly negative values,and the two-stage model ages(TDM2)range from 1.86 to 1.16 Ga,indicating that the magma mainly originated from the Mesoproterozoic ancient crust with a small amount of mantle-derived material added.The pluton is rich in aluminum(Al2O3 is 17.75%-17.82%),high in potassium(K2O is 2.49%-2.66%),low in calcium(CaO is 4.07%-4.30%),and Mg#is 38-39,and has a low total rare earth elements(REEs)(∑REE is 88.62×10-6-121.62 ×10-6)with moderate fractionation between light and heavy REEs(LaN/YbN=12.41-12.69)and δEu values of 0.92-0.98.The large ion lithophile elements(LILEs)such as Ba,K,Sr and high field strength elements(HFSEs)such as U,Pb,Nd are enriched.Ti,P,Ta,Th,Nb and other elements show negative anomalies,with low Zr+Nb+Y+Ce contents,low 10000×Ga/Al values and A/CNK ratio,belonging to typical quasialuminous I-type granite.The Dafoding I-type granites with island-arc type characteristics are basaltic rocks preserved at the bottom of the island arc(lower crust)in the paleo-subduction zone.They were formed by partial melting under the background of lithospheric extension in the early Yanshanian period and rising and emplacement with the addition of a small amount of mantle derived materials.Therefore,the geochemical characteristics of previous island arc-like rocks are inherited.It is related to the disturbance of the subduction of the paleo-Pacific plate under the South China continent.(2)Zircon U-Pb dating and geochemical data are reported for actinolitization amphibolites(metamorphic basic rocks)and intermediate-basic volcanic breccias of Nuodong-Anping plutons,southeastern Guangxi.The data show that the formation age of the Nuodong pluton is later than 123 Ma,and the Anping pluton record two important magmatic activities.One was in Yanshanian period(or later)with the youngest age of～138 Ma,and the other was in Early Paleozoic(452-450 Ma).A large number of captured zircons ranging from Middle ArcheanPaleoproterozoic(3076-1806 Ma),Mesoproterozoic-Neoproterozoic to Early Paleozoic in both plutons indicate the existence of the Middle Archean-Paleoproterozoic basement materials and the Mesoproterozoic,Neoproterozoic and Early Paleozoic magmatism.Geochemical data exhibit that the Nuodong pluton is characterized by a flat or slightly loss rare earth elements(REEs)distribution model,with relatively enriched in large-ion lithophile elements(LILEs)(Th,U and Pb)and depleted in high-field-strength elements(HFSEs)(Nb,Ta,Zr,Hf and Ti).The Anping pluton is also enriched in LILEs(Rb,Ba,Th,U and Pb)and depleted in HFSEs(Nb,Ta,P and Ti),but they show enrichment in light REEs and depletion in heavy REEs contents.The geochemical characteristics are similar to the Caledonian(metamorphic)basic rocks in the study area,indicating that the geochemical characteristics of rocks have limitations and multiple solutions in determining the tectonic setting.