Geochemical Characteristics And Tectonic Significance Of Mashan Basalt In Southeastern Guangxi

The Qinhang metallogenic belt is not only an important tungsten-tin polymetallic metallogenic belt in the south of China,but also a unique place for scholars and experts at home and abroad to study the evolution of geological structure in South China in view of its unique geographic location-?the junction of Yangtze block and Cathaysian block,the joint influence of Tethys tectonic domain and Pacific tectonic domain?.The Mashan Complex in southeastern Guangxi is located in the southwest section of Qinhang tectonic belt.Because it has a complete series of potassium basaltic rocks,the study of its age and petrogenesis is of great significance to the geological tectonic evolution of South China.Zircon chronology,major trace and isotope geochemistry of Mashan basalt have been systematically studied in this paper.The purpose of this study is to provide a basis for the early Mesozoic dynamic background in South China.?1?Mashan basalt belongs to potassium basalt series.The LA-ICP-MS zircon U-Pb age of the rock body is 246.1+5.5 Ma?MSWD=0.36?,indicating that it was formed in the Early Triassic.Rock samples are alkali-rich?ALK=6.72-8.86?,high K₂O/Na₂O ratio?0.66-2.04?,high Ti content?TiO₂=1.86-2.13%?and rich in large ion lithophile elements Rb,Ba,Th,U and light rare earth elements.High field strength elements such as Nb,Ta and Ti are not significantly depleted,showing typical characteristics of intraplate potassium basaltic rocks.The light and heavy rare earth elements?La/Yb?_N=11.64-19.72,?Gd/Yb?_N=2.08-2.63)are obviously different.The distribution curves of rare earth elements in chondrite and primitive mantle are similar to those of oceanic island basalt?OIB?.?2?The average values of Nb/Ta and Zr/Hf of Mashan basalt are 19.41 and 42.29,respectively,which are close to the original mantle values?Nb/Ta=17.5 and Zr/Hf=36.27?.The ratio of Nb/La is higher?0.83-1.1?.Its isotope characteristics show relatively low Sr isotope composition and low positive?_Nd?t?,I_Sr value is 0.703991-0.704732 and?_Nd?t?value is 0.90-1.94.It is suggested that the source area may come from partial melting of phlogopite-bearing garnet peridotite,which is less contaminated by the crust.?3?On the discriminant map of tectonic setting,the samples of Mashan potassium basalt all fall in the intraplate environment.We infer that Mashan basalt was formed in the intracontinental rift environment,possibly related to the extension of the fault thrust-nappe structure in the southeastern part of Guangxi,which resulted in the intrusion and eruption of basaltic magma.