Geochemistry and petrogenesis of volcanic rocks from Daimao Seamount (South China Sea) and their tectonic implications

Abstract

The South China Sea (SCS) experienced three episodes of seafloor spreading and left three fossil spreading centers presently located at 18°N, 17°N and 15.5°N. Spreading ceased at these three locations during magnetic anomaly 10, 8, and 5c, respectively. Daimao Seamount (16.6 Ma) was formed 10 my after the cessation of the 17°N spreading center. Volcaniclastic rocks and shallow-water carbonate facies near the summit of Daimao Seamount provide key information on the seamount's geologic history. New major and trace element and Sr-Nd-Pb isotopic compositions of basaltic breccia clasts in the volcaniclastics suggest that Daimao and other SCS seamounts have typical ocean island basalt-like composition and possess a 'Dupal' isotopic signature. Our new analyses, combined with available data, indicate that the basaltic foundation of Daimao Seamount was formed through subaqueous explosive volcanic eruptions at 16.6 Ma. The seamount subsided rapidly (N 0.12 mm/y) at first, allowing the deposition of shallow-water, coral-bearing carbonates around its summit and, then, at a slower rate (b0.12 mm/y). We propose that the parental magmas of SCS seamount lavas originated from the Hainan mantle plume. In contrast, lavas from contemporaneous seamounts in other marginal basins in the western Pacific are subduction-related.

Geochemistry and Geochronology of the Accreted Mafic Rocks From the Hengchun Peninsula, Southern Taiwan: Origin and Tectonic Implications

“…The alkaline OIBs within the Hengchun accretionary prism exhibit strong covariation in the primitive mantle‐normalized multielement diagram (Figure f) and are uniformly depleted in Rb, Ba, and Sr but enriched in Pb. The OIB‐type samples show obvious positive Nb and Ta anomalies, similar to the OIBs from the seamounts in the SCS (Yan et al, , ) and the late‐stage volcanic clasts recovered at IODP Site U1431 (G. L. Zhang et al, ), which also show OIB affinity. The alkaline OIBs within the prism have high contents of HFSEs relative to MORB and exhibit a progressive decrease from Th to Lu (except for Pb and Sr) in the multielement variation diagram (Figure f), which is indicative of intraplate basaltic rocks derived from a geochemically enriched mantle source.…”

“…The Nd isotopic compositions ( 143 Nd/ 144 Nd = 0.512929–0.512967, εNd = 5.66–6.42) of the alkaline OIBs from the Hengchun Peninsula are similar to those of the Daimao seamount basalts ( 143 Nd/ 144 Nd = 0.512957–0.512967 and εNd = 6.22–6.42) from the SCS (Yan et al, ). Based on the geochemical similarity between the OIB‐type samples and the Daimao seamount (16.5 Ma), as well as the age of the Shihmen Conglomerate (middle Miocene), their isotopic compositions are corrected to an assumed age of 15 Ma (Table S4).…”

“…Huatung Basin data are from Hickey‐Vargas et al (). Data for OIB‐like seamount basalts in the SCS are from Yan et al (, ). Data for samples recovered from the SCS at Integrated Ocean Drilling Program sites are from G. L. Zhang et al (, ).…”

“…Their major element contents are described as follows: 46.36–53.56 wt.% SiO 2 , 3.88–8.88 wt.% MgO, 15.00–18.48 wt.% Al 2 O 3 , 1.39–3.50 wt.% TiO 2 , and 0.12–0.97 wt.% P 2 O 5 . The ΣREE contents of the OIB‐type samples (98.18–269.33 ppm, average 169.61 ppm) are generally lower than those of typical OIB (198.9 ppm; Sun & McDonough, ) and mostly fall in the range of alkaline OIB‐type seamount basalts from the SCS (112.17–429.19 ppm, average 215.98 ppm; Yan et al, , ).…”

“…The SCS underwent three episodes of seafloor spreading: Seafloor spreading commenced in the East and NW subbasins at 33 and ~30 Ma, respectively. And then, right after the ridge jump in the East subbasin (~23.5 Ma), spreading started in the SW subbasin (Yan et al, ; Yu et al, ). A series of seamounts, with ages of 3.49–18.61 Ma, generally formed along the fossil ridges ~6–12 Ma after the cessation of each spreading stage (Figure a; Wang et al, ; Yan et al, , , ).…”