GEOCHEMICAL AND SM-ND ISOTOPIC STUDY OF NEOPROTEROZOIC OPHIOLITES FROM SOUTHEASTERN CHINA - PETROGENESIS AND TECTONIC IMPLICATIONS

Geochemical and Sm-Nd isotopic data are presented for two Neoproterozo ic ophiolite suites (similar to 970 Ma), which were tectonically empla ced into the Banxi Group and equivalent sequences of the Yangtze Block in the southern Anhui and northeastern Jiangxi Provinces, southeast C hina during the Neoproterozoic (0.9-0.8 Ga). The two ophiolite suites are similar to each other in terms of geochemical features, with volca nics showing affinity to basalts forming in subduction-related environ ments, typical of the 'supra-subduction zone (SSZ)' ophiolites. Howeve r, the two suites differ significantly in their initial Nd isotopic co mpositions, The Jiangxi suite displays uniform initial epsilon(Nd) val ues of +5.5 +/- 1.2, implying derivation from a depleted upper mantle source without significant contamination by evolved continental materi als. In contrast, the Anhui suite shows a large range of initial epsil on(Nd) values (+4.5 to -1.0) which are strongly correlated with Sm/Nd, Nd, MgO and SiO2, consistent with binary mixing between a depleted ma ntle component (initial epsilon(Nd) similar to +5.5) and an evolved cr ustal component (initial epsilon(Nd) similar to -1), accompanied by cr ystal fractionation at different stages. The geochemical and Nd isotop ic signatures of the two distinct ophiolite suites, as well as the spa tial and temporal distribution of different types of syntectonic grani toids and volcanic series in the region, can be explained in terms of continent-arc-continent collision between the Yangtze and Huanan Block s during the Neoproterozoic. In this model, it is proposed that the Ya ngtze and Huanan Blocks were separated by a multi-are oceanic basin du ring the early Neoproterozoic (-1.0 Ga), with both oceanic and contine ntal margin island arcs being developed to the south of the Yangtze Bl ock, The Jiangxi and Anhui ophiolite suites are considered as tectonic ally emplaced fragments of oceanic crust forming in the inter-are and continental-margin basins, respectively.