皖南和赣东北蛇绿岩成因及其构造意义：元素和Sm－Nd同位素制约

PETROGENESIS OF OPHIOLITES FROM SOUTH ANHUI AND NORTHEAST JIANGXI, AND THEIR TECTONIC IMPLICATIONS: CHEMICAL AND Sm-Nd ISOTOPIC CONSTRAINTS

本文对扬子块体南缘构造侵位于板溪群（及相应地层）的皖南和赣东北晚元古代蛇绿岩进行了系统的元素和Ｓｍ－Ｎｄ同位素地球化学研究，结果表明两套蛇绿岩具有类似的元素地球化学特征，即二者的火山岩元素地球化学特征均类似于形成于弧后盆地构造环境的拉斑玄武岩。但是，两套蛇绿岩的Ｎｄ同位素组成明显不同。赣东北蛇绿岩具有相当均一的高εＮｄ（Ｔ）值（＋５．５±１．２），表明来源于一较亏损的上地幔源区，并且没有受到明显的较成熟地壳物质的混染。相反，皖南蛇绿岩的εＮｄ（Ｔ）值低，并有较大的变化范围（＋４．５──１．０），并和Ｓｍ／Ｎｄ、Ｎｄ、ＭｇＯ、ＳｉＯ２呈明显的二元混合相关关系，表明蛇绿岩是由亏损幔源岩浆（初始εＮｄ（Ｔ）≈＋５．５）在结晶分异过程中与地壳组分（初始εＮｄ（Ｔ）≈－１）混合形成的。皖南和赣东北蛇绿岩的Ｎｄ同位素和元素地球化学特征以及该地区的同时代花岗岩和火山岩的时空分布特征，可以用华南和扬子元古宙陆－弧－陆碰撞模式来解释。大约在１．０Ｇａ前，扬子和华南块体之间存在一多岛弧洋盆。大陆边缘岛弧和大洋岛弧在扬子块体南缘发育演化。皖南和赣东北蛇绿岩就是在大陆边缘盆地和弧间盆地演化过程中形成的洋壳构造侵位的碎片。该地区Ｉ型?

Geochemical and Sm-Nd isotopic data are presented for two Late Proterozoic ophiolite suites (～970Ma), which tectonically emplaced into the Banxi Group and the equivalent sequences of the Yangtze Block in southern Anhui and northeastern Jiangxi Provinces, southeast China, during the Late Proterozoic (0.8￣0.9Ga). These two ophiolite suites are similar to each other in terms of geochemical features, with volcanics from both showing geochemical affinities to tholeiites forming in back-arc basin tectonic environment. However, these two suites differ significantly from each other in their initial Nd isotopic compositions. The Jiangxi suite displays uniform initial εNd values of around+5.5, implying that it was derived from a depleted upper mantle source without significant contamination by evolving continental materials.In contrast, the Anhui suite shows a large range of initial εNd values (+4.5 to -l.0) which are strongly correlated with Sm/Nd, Nd, MgO and SiO2, consistent with binary mixing between a depleted mantle component (initial εNd ≈ +5.5) and an evolving crustal component (initial εNd ≈ -l), accompanied by later crystal fractionation.The geochemical and Nd isotopic signatures of these two distinctive ophiolite suites, as well as the spatial and temporal distribution of different types of syntectonic granitoids and volcanic series in the region, can be interpreted in terms of continent-arc-continent collision between the Yangtze and the South China Blocks during the Late Proterozoic. According to this model, the Yangtze and the South China Blocks were separated by a multi-arc oceanic basin during the early Late Proterozoic (～l.OGa?), with both oceanic and continental margin island arcs developing to the south of the Yangtze Block. The Jiangxi and Anhui ophiolite suites are tectonically emplaced fragments of oceanic crust forming in the inter-arc and continental-margin tinental-margin basins, respectively. The S-and I-type granitoids in the region are generated by crustal anatexis of the accretionary complex during successive continental collision processes, with the S-type granites derived from evolving continentalmargin sediments, whilst the I-type granitoids resulted from partial melting of juvenile island-arc crust.