桂东南马其岗石英二长斑岩年代学、地球化学特征及成因

A study of geochronology,geochemistry and genesis of Maqigang beschtauite pluton,southeastern Guangxi

马其岗石英二长斑岩体出露于桂东南博白-梧州断裂带南东侧,对其开展了锆石U-Pb年代学、地球化学、Sr-Nd-Hf同位素及成因研究。结果表明,岩体LA-ICP-MS锆石U-Pb年龄为90.2±1.5Ma(MSWD=1.7)。岩体富碱(ALK=7.38%~8.14%)、富钾(K_2O=4.41%~4.78%),稀土元素特征为轻稀土元素富集型,微量元素特征为富集大离子亲石元素(Rb、Th、U、K、Pb、LREE),亏损高场强元素(Nb、Ta、P、Ti、HREE),符合钾玄岩系列的岩石特点。岩体有较高的Mg~#值(42.82~50.35),较低的Sr含量(268.00×10^(-6))~304.00×10^(-6)),以及较高的锆石饱和温度(860~883℃),同时Nb/Ta(平均值为11.24)、Zr/Hf(平均值为38.20),Th/La值(平均值为0.17)明显不同于大陆地壳特征,表明岩浆主要来自下部地壳或地幔,在上升侵位过程中受到地壳大规模混染程度较小,具有EMⅡ富集地幔端元的Sr-Nd同位素特征。二阶段Nd模式年龄(t_(DM2))变化于1.33~1.36Ga之间,二阶段Hf模式年龄(t_(DM2))主体为1.20~1.50Ga,两者模式年龄较一致,显示马其岗岩体可能是中元古代中期镁铁质岩石部分熔融的产物。石英二长斑岩形成于板内伸展环境,整个华南在90Ma左右存在一次大规模的伸展事件,其动力学机制与古太平洋板块低角度俯冲有关。

Maqigang beschtauite pluton is located in the southeast of the Bobai-Wuzhou fault,southeastern Guangxi.The zircon UPbgeochronology,elemental geochemistry and Sr-Nd-Hf isotopic components as well as petrogenesis of Maqigang beschtauitewere investigated in this paper.The LA-ICPMS zircon U-Pb age is90.2±1.5Ma(MSWD=1.7).The beschtauite is characterized byrich alkalis(ALK=7.38%~8.14%)and K(K2O=4.41%~4.78%).The rare earth elements exhibit the light REE enrichment type,withtrace 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 relativelyhigh Mg#values(42.82~50.35)and zircon saturation temperatures(860~883℃)and low Sr values(268.00×10-6~304.00×10-6).TheNb/Ta ratios(11.24on average),Zr/Hf ratios(38.20on average)and Th/La ratios(0.17on average)of the beschtauite are remarkablydifferent 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 EMⅡsource.Nd isotopic two-phase model age(tDM2=1.33~1.36Ga)is similar to the Hf isotopic two-phase model principal age(tDM2=1.20~1.50Ga),inmplying that the beschtauite was derived dominantly frommafic rocks in the middle Proterozoic.The Maqigang beschtauite was formed in an intra-plate extensional environment.There was ahuge stretching event throughout South China at90Ma±.The dynamic mechanism of the event was connected with low angle sub?duction of the ancient Pacific plate.