A reappraisal of the global tectono-magmatic lull at ∼ 2.3 Ga

Abstract

There is a 87–86% drop in frequency in both detrital and igneous zircon U/Pb ages during the Tectono-Magmatic Lull (TML, 2365–2235 Ma) and a significant decrease in frequency of granitoids, but no recognized age gap. The TML is transferred in detrital zircon ages through all younger depositional time windows, indicating it is a globally robust feature. This seems to require a decrease in production rate of felsic magma during the TML, and possibly also recycling of crust of this age into the mantle. However, εHf zircon and εNd whole-rock felsic igneous data show that juvenile continental crust continued to be produced and that the ratio of juvenile/reworked crust remained about the same or increased relative to the ratio before and after the lull. During the TML there is a large decrease in the frequency of LIPs, with only four that fall in the 2365–2235 Ma time frame, and there is a global LIP age gap at 2340–2260 Ma. Komatiite frequency in greenstones decreases rapidly at 2600–2000 Ma and enrichment in incompatible elements in greenstone basalts at 2700–2400 Ma reflects increasing enriched components in mantle sources. Although 13 orogens, all with convergent margin characteristics, are recognized between 2400 and 2200 Ma, only four are known with major deformation in the TML. Paleomagnetic data indicate that the relative positions of the Superior, Kaapvaal and India cratons changed significantly between 2435 and 2175 Ma, implying the existence of plate tectonics during this time interval. Also, paleomagnetic positions from several cratons confirm that average minimal plate speeds are relatively low during the TML (<8 cm/yr) and that there are no fast plates (>10 cm/yr) between 2.35 and 2.25 Ga.We propose a new testable model for the TML related to a mantle overturn event at 2.7 Ga that initiated widespread subduction. Sinking slabs covered the core-mantle boundary decreasing the rate of mantle convection so that oceanic lithosphere was consumed faster than it was produced. This led to a slow-down in plate speeds some 400 Myr later during the TML, with consequent decreases in magma production, mantle plume generation and orogenic activity.