北祁连九个泉玄武岩的形成环境及地幔源区特征:微量元素和Nd同位素地球化学制约

Tectonic setting and mantle source characteristics of Jiugequan basalts, North Qilian: Contraints from trace elements and Nd-isotopes.

北祁连九个泉蛇绿岩中的玄武岩为 MORB,根据其地质产状和地球化学特征又可以分为两部分 ,剖面下部的玄武岩为 N - MORB,上部的玄武岩主要为 E- MORB。玄武岩多数具有 Nb负异常。从下向上 ,九个泉玄武岩的 Th,Nb,L REE,Zr等含量及 (L a/ Yb) N,(L a/ Sm ) N,Ce/ Zr,Zr/ Y,Th/ L a,Th/ Yb比值逐渐增加 ,并伴随着 Y,Yb,L u,Sc含量、Zr/ Nb和L a/ Nb比值以及εNd(t)的逐渐减小。不相容元素比值及εNd(t)之间具有很好的相关性。上述特征反映不均一地幔部分熔融过程中 N- MORB源区和富集地幔之间的混合作用。微量元素和 Nd同位素地球化学特征表明九个泉蛇绿岩形成于弧后盆地中的海山环境 ,玄武岩的化学成分在垂向上的变化记录了海山生长并逐渐远离扩张脊的动态的地质过程。

Basalts from Jiugequan ophiolite, North Qilian mountains, are of MORB-type, and are composed of N-MORBs in the lower section and E-MORBs in the upper section. Most of the basalts show negative Nb anomaly. From the bottom to the top, Th, Nb, LREE and Zr contents and (La/Yb)(N), (La/Sm)(N), Ce/Zr, Zr/Y, Th/La, Th/Yb ratios of the basalts are gradually increased, companioned with continuous descreasing of Y, Yb, Lu and Sc contents, Zr/Nb and La/Nb ratios, as well as epsilon (Nd)(t) In addition, the samples show significant correlations between incompatible element ratios and epsilon (Nd)(t), that is, incompatible elements and isotopes are coupled. We interpret these correlatios as mixing trends of N-MORB source and OIB source resulting from melting of a heterogeneous mantle domain. Trace elemental and Nd-isotopic characteristics indicate that the Jiugequan ophiolite was from a seamount in a back-are basin, and the geochemical variations records the motive geological processes that the seamount being gradually developed and moved away from the spreading ridge. It is proposed that many ophiolites were actually from seamounts.