Mesozoic Northward Subduction Along the SE Asian Continental Margin Inferred from Magmatic Records in the South China Sea

Abstract

During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled by Tethyan or paleo-Pacific Ocean subduction. Herein, we report ~124 Ma adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount of the SCS. Granodiorites have relatively high Sr/Y (34.7–37.0) and (La/Yb)N (13.8–15.7) ratios, as well as low Y (9.67–9.90 μg/g) and Yb (0.93–0.94 μg/g) concentrations, typical of adakites. Their Sr/Y and (La/Yb)N values coupled with their relatively low initial 87Sr/86Sr ratios (0.70541–0.70551), relatively high K2O contents (3.31–3.38 wt%), high Th/La ratios (0.33–0.40), negative εNd(t) values (−3.62 to −3.52), and their variable zircon εHf(t) values (−3.8 to +5.2) indicate that these rocks were formed by melting of subducted oceanic crust and sediments. The Nb-enriched basalts show enrichment in high field strength elements (HFSE) and have εNd(t) values of +2.90 to +2.93, as well as relatively low initial 87Sr/86Sr ratios of 0.70341–0.70343, demonstrating that they were derived from a depleted-mantle (DM) source metasomatized by silicate magmas originating from melting of a subducted oceanic lithospheric slab. By combining our findings with data from other Late Mesozoic arc-related magmatic rocks and adakites from the broader study area, we propose a geotectonic model involving subduction of young oceanic lithosphere during the Late Jurassic and northward subduction of the proto-South China Sea (PSCS) along the SE Asian continental margin during the Early Cretaceous. This conceptual model better explains the two-period Mesozoic magmatism, commonly reported for the SE Asian continental margin.

NW Pacific‐Panthalassa Intra‐Oceanic Subduction During Mesozoic Times From Mantle Convection and Geoid Models

“…Reconstruction 3 follows Reconstruction 2 but adds a Panthalassa‐Eurasia subduction zone along the eastern Eurasian margin after 180 Ma based on arc continental magmatism (Figures 2k–2o) (Akinin & Miller, 2011; Cai et al., 2019; Dong et al., 2018; F.‐Y. Wu et al., 2019; Hourigan & Akinin, 2004; Xinmin et al., 2006; Zhou & Li, 2000).…”

The Earth’s Crust and Upper Mantle of the East China Sea (Seimotomographic and Gravity Models)

“…На поверхности здесь широко распространены вулканические образования, отсутствующие на южном и северном отрезках дуги [14]. Присутствие в этих вулканитах большой группы редких элементов (LREE) [10], характерно для мантийных адакитоподобных магм [9].…”

Editorial for the Special Issue “Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes”

“…By integrating geochronological (U-Pb and 40 Ar/ 39 Ar) and isotope (Sr-Nd-Pb-Hf) data, a geotectonic model is proposed for Mesozoic magmatic rocks from the South China Sea by involving subduction of an early Neo-Tethyan lithospheric domain [2]. The geochronological and geochemical signatures of adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount outline contemporaneous formation of these products during northward subduction of the proto-South China Sea along the SE Asian continental margin in the Early Cretaceous [2]. A singularity analysis (frequency anomalies) is also applied to U-Pb ages of zircons from the Great Xing'an Range to characterize the causative relationship between age abnormality and Pacific Plate subduction [3].…”