Plate tectonics in action in the Mesoarchean: Implication from the Olondo greenstone belt on the Aldan Shield of Siberian Craton

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, we present the most up-to-date geochemical and isotopic data for the ultramafic-mafic rocks of the OGB, in order to better constrain their mantle sources and the plate tectonic process involved in the formation of OGB at ca. 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, and are highly refractory as residual mantle phase as indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs for highly siderophile elements (HSE). Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of late metasomatism in their mantle source. Rhenium-Osmium isotopic compositions of these dunites yield mantle model age (TMA) of 2960–3020 Ma, comparable to the formation age of the OGB at ca. 3 Ga. Together, the data suggest that, unlike mantle cumulate origin for most of the Archean ultramafic rocks, the OGB dunites were mantle residuals after a high degree of partial melting (>30%), which subsequently interacted with the subduction-related melt/fluid. On the other hand, the OGB mafic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the ultramafic residuals that reinforce a subduction-related regime as their formation setting, despite extra mid-ocean ridge and plume settings. Tholeiitic basalts yield variable REE patterns from depleted, chondritic, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to mafic rocks in typical supra-subduction zone (SSZ) ophiolites. Such mafic rocks with combined lower εNd(t) and negative Nb-Ta anomalies were most likely the 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 requiring a mantle plume to transport and melt at such a depth. 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.