Siderian mafic-intermediate magmatism in the SW Yangtze Block, South China: Implications for global ‘tectono-magmatic lull’ during the early Paleoproterozoic

Abstract

It is debated whether global-scale tectono-magmatic quiescence prevails on the Earth during the early Paleoproterozoic. The early Paleoproterozoic meta-mafic and intermediate rocks in the southwestern Yangtze Block of South China could provide an opportunity to constrain not only the tectonic evolution of the Yangtze Block itself but also augment our understanding of the global tectono-magmatic lull during the early Paleoproterozoic. Zircon UPb geochronological study indicates that the meta-mafic rocks were emplaced at ca. 2.32 Ga, whereas the intermediate rocks were emplaced at ca. 2.35 Ga. These two lithologies are characterized by subduction-related whole-rock geochemical (such as depleted Nb, Ta and enriched Rb, K, Pb) and unradiogenic NdHf isotopic signatures (εHf(t) < 0, εNd(t) < 0), implying a derivation from a geochemically enriched lithospheric mantle modified by ancient subduction. Thermodynamic modeling further suggests that the intermediate rocks are products of the AFC process between mantle-derived basaltic magma and TTG-like basement rocks at shallow crustal levels. We interpret this as occurring within a post-orogenic extensional setting. Integrated with the published work, we further posit that the Yangtze Block experienced a tectonic regime transition from compressive orogenesis to post-orogenic extension during the early Paleoproterozoic, and the magmatic activities in several crustal blocks are still active until ~2.3 Ga and therefore places important constraints on the beginning of the tectono-magmatic lull.