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.
Highlights
During the Cenozoic Era, the Southeast (SE) Asian region underwent substantial geological changes related mainly to the Indo–Asia collision
We describe the first rock association of adakitic granodiorites with Nb-enriched basalts (NEBs) from samples recovered from the Xiaozhenzhu Intraplate Seamount near the Zhongnan samples recovered from the Xiaozhenzhu Intraplate Seamount near the Zhongnan Fault of the South China Sea (SCS)
The spatial, temporal and geochemical relationships between the granodiorite and basalt samples collected from the Xiaozhenzhu Seamount in the SCS represent an association of rocks that share a common petrogenetic history
Summary
During the Cenozoic Era, the Southeast (SE) Asian region underwent substantial geological changes related mainly to the Indo–Asia collision. There is a range of views on the Mesozoic tectonic evolution of the northern SCS and the southeastern coastal region of China, meaning that the tectonic regimes and associated geodynamic processes, as well as the timings and spatial extents of influence of these regimes, have yet to be clearly understood. The fresh rupture faces, angular shapes and the wide range of grain sizes of the dredged samples preclude long-distance fluvial or coastal transport, and the source rock must be situated in the slopes of the SCS, near continent–ocean transition (COT) [22,23]. We describe the first rock association of adakitic granodiorites with NEBs from samples recovered from the Xiaozhenzhu Intraplate Seamount near the Zhongnan samples recovered from the Xiaozhenzhu Intraplate Seamount near the Zhongnan Fault of the SCS (Figure 1).
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