A geochemical and Sr-Nd isotopic study of the Vendian greenstones from Gorny Altai, southern Siberia: Implications for the tectonic setting of the formation of greenstones and the role of oceanic plateaus in accretionary orogen

The Central Asian Orogenic Belt is considered to be the most important site of juvenile crustal formation since the Neoproterozoic. Gorny Altai of southern Siberia represents the first stage of tectonic evolution in the Central Asian Orogenic Belt. Greenstones of Gorny Altai occur either as blocks within an accretionary complex surrounded by a sedimentary/serpentinite matrix, or as a coherent ophiolite complex. The greenstones are overlain or intercalated by micritic limestones indicating that they formed in shallow water in an intra-oceanic environment. The phenocryst assemblage of the greenstones (clinopyroxene + plagioclase, plagioclase and clinopyroxene) is, however, different from that of mid-oceanic ridge basalt (MORB). Based on the geochemical characteristics, these greenstones are divided into four groups. Group 1 greenstones show light rare earth element (LREE) depletion but no negative Nb-Ta anomaly in primitive mantle normalized multi-element plots; they resemble normal (N-) MORB or back-arc-basin basalt (BABB). Group 2 rocks have slight LREE depletion or show flat REE patterns; they have a negative Nb-Ta anomaly and are similar to island arc basalt (IAB) or BABB. Group 3 rocks have LREE-enriched patterns, no negative Nb-Ta anomaly and are similar to enriched (E-) MORB or oceanic island basalt (OIB). Group 4 greenstones are represented by LREE-enriched patterns, with negative Nb-Ta anomaly; they resemble IAB. Although the co-existence of such heterogeneous geochemical signatures may suggest that the rocks were formed in different tectonic settings followed by tectonic mixing and aggregation in a subduction zone, the field relationships, petrological and mineralogical composition, and geochemical characteristics suggest that the greenstones were formed as part of an oceanic plateau generated by a mantle plume source which contained a recycled crustal component. An important implication is that oceanic plateaus may represent an important constituent in the development of accretionary orogens.