北大别灰色片麻岩原岩性质的探讨:Sm-Nd同位素年龄及同位素成分特点

Approach to the source of the gray gneisses in North Dabie Terrain:Sm-Nd isochron age and isotope composition

北大别变质杂岩主要是一套英云闪长岩质片麻岩。以全岩样品测定得到 10 47± 44 .5 Ma的 Sm - Nd等时线年龄 ,εNd值 - 8.99,r=0 .998。 Nd同位素和 Pb同位素给出的模式年龄也显示这套灰色片麻岩的古老来源。同位素成分研究显示 ,岩石普遍具有较高的 Sr同位素 (>0 .70 7)和较低的 Nd同位素 (<0 .5 12 4)现代比值 ,所有岩石的现代 εNd值均 <0 ,Pb同位素明显偏低 ,这些特征可以与中 -下地壳的同位素成分对比。北大别灰色片麻岩源自一个活化的古老地壳 ,中 -下地壳物质重熔所生成的岩浆经分异形成了北大别灰色片麻岩 ,而它们的形成和演化过程与华北板块向杨子板块下俯冲作用密不可分 。

The North Dabie Metamorphic Complex (NDC) is a suite of gray gneiss, mainly of tonalitic gneiss. Sm-Nd isochron age of 1047±44.5Ma, ε Nd =-8.99 and r=0.998 are obtained. Nd model ages (T DM) and Pb model ages also indicate an ancient crust-derived source for the gray gneisses. Either the 87Sr/ 86Sr present ratios (>0.707) and the present ε Nd value (-8.84^-23.56), or the ε Nd value (-8.99) determined by isochron clearly constrained a crust source for the gray gneisses in NDC. Hence it can be inferred that the gray gneisses of NDC were derived from an reworking ancient middle to lower continental crust. The generation and evolution process is closely related to the subduction of the Yangtze plate beneath the North China plate. The intrusion of the Yanshan granitic magma caused the last and the strongest tectonothermal event for the gray gneisses.