A study of geochronology, geochemistry and genesis of Maqigang beschtauite pluton, south-eastern Guangxi

Maqigang beschtauite pluton is located in the southeast of the Bobai-Wuzhou fault, southeastern Guangxi.The zircon UPb geochronology, elemental geochemistry and Sr-Nd-Hf isotopic components as well as petrogenesis of Maqigang beschtauite were investigated in this paper.The LA-ICPMS zircon U-Pb age is 90.2±1.5Ma (MSWD=1.7).The beschtauite is characterized by rich alkalis (ALK=7.38%~8.14%) and K (K₂O=4.41%~4.78%).The rare earth elements exhibit the light REE enrichment type, with trace elements characterized by enrichment of LILE (e.g., Rb, Th, U, K, Pb, LREE) and depletion of HFSE elements (e.g., Nb, Ta, P, Ti, HREE).The geochemical characteristics of Maqigang beschtauite show shoshonitic features.The beschtauite has relatively high Mg^# values (42.82~50.35) and zircon saturation temperatures (860~883℃) and low Sr values (268.00×10^-6~304.00×10^-6).The Nb/Ta ratios (11.24 on average), Zr/Hf ratios (38.20 on average) and Th/La ratios (0.17 on average) of the beschtauite are remarkably different from those of upper crustal rocks, indicating that the beschtauite originated from lower crustal or mantle source.In addition, the beschtauitic magma was less contaminated by the upper crust substance in the process of emplacement.The Sr-Nd isotopic com-positions show that they have the characteristics of the EMⅡ source.Nd isotopic two-phase model age (t_DM2=1.33~1.36Ga) is similar to the Hf isotopic two-phase model principal age (t_DM2=1.20~1.50Ga), inmplying that the beschtauite was derived dominantly from mafic rocks in the middle Proterozoic.The Maqigang beschtauite was formed in an intra-plate extensional environment.There was a huge stretching event throughout South China at 90Ma±.The dynamic mechanism of the event was connected with low angle sub-duction of the ancient Pacific plate.