The Origin of Late Cenozoic Magmatism in the South China Sea and Southeast Asia

Abstract

Basaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐3He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.

Bilateral Heterogeneity in an Upwelling Mantle via Double Subduction of Oceanic Lithosphere

“…The earlier studies for the basalts in Central Vietnam (An et al, 2017;Hoang et al, 2018) and the other volcanic fields adjacent to Central Vietnam (SE China and EVS/SCS; Y.-Q. Li et al [2020]; Qian et al [2021]) also estimated a similar composition for this component.…”

“…Another recent study (Qian et al, 2021) also found geochemical characteristics which support the insignificant role of a pyroxenite source for the basalts from EVS/SCS seamounts. The study reported that olivine phenocrysts in basalts from EVS/SCS seamounts have a Ni abundance lower than those in Vietnamese basalts, but comparable to those found in MORB, yet the basalts from these two regions share common trace-element and isotopic characteristics.…”

“…Some EVS/SCS basalts from the sites U1433 and U1434 show Sr-Nd-Pb isotope compositions comparable to D-DMM of Workman and Hart (2005). Major-and trace-element compositions of these EVS/SCS basalts are also well within the range of global normal (N)-MORBs (Qian et al, 2021;Zhang, Luo, et al, 2018;Figure S4 in Supporting Information S1). EVS/SCS basalts from the site U1431 plot on a radiogenic extension of Pb-isotopic array formed with those from the sites U1433 and U1434 (Figure 5).…”

“…A greater contribution of the EM2‐like component is found in some basalts of the Central Group (Con Co Island and Ly Son‐Quang Ngai) as well as basalts in adjacent volcanic fields. It is noted that the EM2‐type basalts dominantly occur in the northeastern region of the Indochina Peninsula (Northern Vietnam and Hainan) and the EVS/SCS (Figure 5; Hoang et al., 1996; Qian et al., 2021; Tu et al., 1992; Wang et al., 2013; Yan et al., 2008; Zhang, Luo, et al., 2018; Zhang, Sun, et al., 2018). This component is characterized by high Pb/Ce and Th/La ratios (Figure 4) and high time‐integrated Rb/Sr and Th/U ratios (Figure 5).…”

“…The observed isotopic variations in EVS/SCS are interpreted as the mixing of two end-member components. Zhang, Luo, et al (2018) attributed compositional variations of EVS/SCS basalts to the mixing of melts derived from the mantle similar to the source of N-MORB and that from the lower continental crust (LCC), whereas Qian et al (2021) ascribed it to mixing of melts derived from mantle similar to the source of N-MORB and that from recycled young oceanic crust. In either scenario, the isotopically-depleted source is considered to represent the uppermost asthenospheric mantle dominated by refractory peridotite.…”