Ba-Mg isotopic evidence from an OIB-type diabase for a big mantle wedge beneath East Asia in the Early Cretaceous

Abstract

Beneath Eastern Asia, the world's spatially most extensive, and possibly extremely long-lived big mantle wedge (BMW) occurs, formed by the Izanagi-Pacific slab residing in the mantle transition zone (MTZ). It provides a fascinating model accounting for the intraplate magmatism and earthquake >1800 km distal from the trench which are incompletely considered by classic plate tectonics. Since when the BMW exists is crucial for global and regional geodynamic reconstructions and modeling of material transfer. However, temporal constraints from the geophysical, geodynamic model, tectonic, and magmatic studies yield conflicting results. To contribute to this endeavor, we have characterized BaMg isotopic features of the ∼123 Ma-old, OIB-type Daixi diabase in Eastern China, which is located ∼300 km west of the inferred Izanagi subduction zone. Results show that the diabase has δ138/134Ba (−0.08‰ to 0.04‰) slightly lower than the terrestrial mantle (∼0.03 to 0.05‰). The δ138/134Ba values are negatively correlated with (87Sr/86Sr)i, K/Ti, K/La, and K/Na of the diabase, pointing to the incorporation of subducted sediments into the mantle source. In addition, the diabase also has lower δ26Mg (−0.67‰ to −0.35‰) than the terrestrial mantle (−0.25 ± 0.04‰) and low (87Sr/86Sr)i (0.704254–0.706300), reflecting source contributions of marine Mg‑carbonates. Thus, BaMg isotopes record the source contribution of carbonate-bearing sediments which must have melted in the MTZ (20–22 GPa) according to experimental investigations. High K/U (∼30,500) and Hf/Hf* (∼1.68) ratios of the melts derived from the enriched source moreover require partial melting in the presence of peritectic liebermannite and majorite. These two peritectic phases are formed in the MTZ (410–660 km) after early incipient melting of carbonate-bearing sediments during which the loss of Na-rich, Ca‑carbonate melts has resulted in the removal of Th, U, and some LREE and retention of K, Ba, Zr, and Hf. These findings provide robust evidence that the Daixi diabase originated from an enriched mantle source metasomatized by melts/fluids derived from the stagnant slab at the MTZ. They also place important constraints on the existence of the Izanagi BMW beneath eastern Asia in the Early Cretaceous (∼123 Ma). It reconciles magmatic with previous tectonic and metallogenetic models that showed lithospheric thinning (peak at ∼125 Ma) and gold mineralization flare-up (peak at ∼120 Ma) at the Eastern China continental margin. Thus, our study is important to understand not only the materials cycling and geodynamic processes within a BMW system but also the tectonic-magmatic effects and interactions between the subducted slab and the overlying continental plate.