托莱牧场中-新元古代火山岩Sm-Nd等时线定年及地质意义

Sm-Nd isochron dating and geological significance of volcanic rocks with Middle Proterozoic-Neoproterozoic in Tuole ranch area,Qinghai

为了确定在祁连托莱牧场一带新发现的中-新元古代火山岩的形成时代及岩石类型,并对其构造环境进行分析探讨。采取野外实地调查,并结合Sm-Nd等时线年龄测定、岩石学、岩石化学、岩石地球化学等方面的成果,进行综合分析研究。结果表明,新发现的中-新元古代火山岩由单一的玄武岩组成,呈夹层透镜体或构造岩块近东西向赋存于新划分的中-新元古代托勒片岩岩组中,Sm-Nd等时线年龄(950±53)Ma;岩石具斑状结构,基质具变余间粒结构,为铝过饱和型(A l2O3>K2O+Na2O+CaO)岩石,属拉斑玄武岩系列;稀土、微量元素特征证明,其具有岛弧拉斑玄武岩和N-MORB的双重地球化学特征;同位素143Nd/144Nd=0.512 61～0.511 845,εNd(t)为-2.39。反映出火山岩浆源于偏基性的壳幔混合区,是陆缘裂谷的产物。

Field investigation was integrated with methods of Sm-Nd isochron age,lithology,lithochemistry and lithogeochemistry to determine the formation age and rock types of newly discovered volcanic rocks from a single basalt with Middle Proterozoic-Neoproterozoic in Tuole ranch area of Qilian,Qinghai,and to analyze its tectonic setting.The volcanic rocks with laminated lenses Or structure blocks occurred in the EW schist formation with Middle Proterozoic-Neoproterozoic,and with porphyritic texture and matrix with variable odd grain structure.The Sm-Nd isochron age is about 950±53Ma,and belongs to tholeiitic series with excess saturated aluminum of rock（Al2O3K2O＋Na2O＋CaO）.Its isotope 143Nd/144Nd=0.51261-0.511845,εNd（t） is-2.39.From the above,we have conclusion that magma derived from lower crust of partial mafic,and was the product of continental margin rift.