右江盆地构造属性及地幔特征:来自凭祥基性岩地球化学及Sr-Nd同位素的证据

Tectonic attributes and mantle features of Youjiang basin:evidence from Pingxiang basic rock geochemistry and Sr-Nd isotope

对凭祥辉绿岩岩石进行了全岩主量、微量元素地球化学及Sr-Nd同位素研究。结果表明,该辉绿岩属于碱性玄武岩系列,具有低SiO2(46.27%~47.39%)和高MgO(5.46%~7.18%)的特征;相对富集LREE亏损HREE,(La/Yb)N为4.37~9.45,δEu为0.98~1.08,无明显Eu异常;具有Sr的正异常,呈现出从E-MORB向OIB过渡的地球化学特征。辉绿岩的(87Sr/86Sr)i为0.705568~0.706617,εNd(t)为-0.65^+1.69,具有向富集地幔Ⅱ(EMⅡ)演化的趋势,揭示了其源区来自于轻微亏损的MORB型软流圈地幔至OIB型富集地幔的过渡源区。La/Sm-Sm/Yb判别图指示了基性岩来自石榴石二辉橄榄岩的部分熔融,结合Th/Yb-Ta/Yb判别图及区域地质背景,认为凭祥基性岩可能形成于古特提斯洋俯冲体系与峨眉山地幔柱相互作用引起的弧后拉张环境。

Mafic dykes(diabase)are exposed in Pingxiang area of southwestern Guangxi.The geochemistry study of diabase is helpful to understand the petrogenesis and tectonic nature of the Youjiang basin.In this paper,the major trace elements and Sr-Nd isotope studies of Pingxiang mafic rocks are carried out.The results show that the diabase belongs to the alkaline basalt series.It has the characteristics of low SiO2(46.27%-47.39%)and high MgO(5.46%-7.18%),relatively enriched in LREE and low HREE.(La/Yb)Nis 4.37-9.45,δEu=0.98-1.08,no obvious Eu anomaly,positive anomaly with Sr.It shows geochemical characteristics of transition from E-MORB to OIB.The(87 Sr/86 Sr)iof the diabase is 0.705568-0.706617,and theεNd(t)is-0.65-+1.69,a tendency to evolve to the enriched mantle Ⅱ(EMⅡ),a transition from asthenospheric mantle to OIB enriched mantle.It is revealed that the source transition is from the slightly depleted MORB-type asthenosphere mantle to the OIB-type enriched mantle.The La/Sm-Sm/Yb discriminant diagram indicates that the basic rock is partially melted from the garnet-lherzolite.Based on the Th/Yb-Ta/Yb discriminative map and the regional geological background,it is believed that the Pingxiang basic rock may be formed in the post-arc tension environment by the interaction of the paleo-Tethys ocean subduction system and the Emeishan mantle plume.