| dc.description.abstract | The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥ 30% in volume, the OGB is unique among other greenstone belts in the world. In this study, Ipresent a comprehensive geochemical and isotopic data for the OGB rocks, in order to better constrain their origin and the geodynamic process involved in their formation in the Archean time. Rhenium-osmium isotopic data of the ultramafic rocks yield TMA model age of 29603020 Ma, comparable to the formation age of the OGB at 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, which are highly refractory residual mantle rocks evidently indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs. Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of metasomatic overprint. Unlike having a cumulate origin for most Archean ultramafic rocks, the OGB dunites were mantle residues after high degree of partial melting (>30%), subsequently metasomatized by the subduction-related melt/fluid. On the other hand, the OGB volcanic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the residual dunites, reinforcing subduction-related processes involved in some of their formation, despite extra mid-ocean ridge and plume activities associated with other mafic rocks. Tholeiitic basalts yield variable REE patterns from depleted, flat, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to those found in typical supra-subduction zone (SSZ) ophiolites. Such elemental characteristics with combined lower εNd(t) and negative Nb-Ta anomalies are most likely a result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requires a mantle plume to transport and melt at deeper depth. Additionally, the OGB has documented the occurrence of magnesian andesites, andesites, rhyolites, and Nb-enriched basalts. Magnesian andesites show high Mg# (52) with elevated Cr and Ni content. Andesite-rhyolite display LREE enriched patterns and negative Nb and Ti anomalies, similar to Cenozoic adakites. They could be generated by melting of subducted slab. Nb-enriched basalts (NEBs) exhibit elevated concentrations of Na2O, P2O5, TiO2, and high Nb contents (>6 ppm). They are characterized by LREE enrichment with negative Nb anomalies. They are most likely the result of mantle wedge metasomatized by Olondo adakitic magmas during magma ascent.
The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time. The subduction initiation was triggered by a mantle plume, which also provided higher thermal conditions for slab melting. The NEB-Mg andesite-adakites assemblage is evidence of a young, hot subduction process.
The OGB, as some other greenstone belts in Mesoarchean, was formed in a combined plume-arc setting. This suggests that the Mesoarchean time might mark the transition stage from dominantly plume to plate tectonic regime on the Earth. | en_US |