Mantle Source and Magmatic Evolution of the Dying Spreading Ridge in the South China Sea

Abstract

The genesis of seamount chains along fossil spreading ridges reflects the mantle melting dynamics of dying spreading center. The South China Sea developed a seamount chain along its fossil ridge shortly after cessation of spreading. International Ocean Discovery Program Expedition 349 has recovered three volcanic layers, a shallow volcanic breccia layer and two deeper (upper and lower sections) basalt layers, at Site U1431 near the fossil ridge of the South China Sea. Despite the mid‐ocean ridge basalt (MORB)‐like geochemistry of the lower and upper section basalts, they show distinct geochemical differences that indicate a change of mantle source and a transition from spreading to intraplate volcanism. While the lower section basalts could reflect the last stage volcanism before the cessation of ridge spreading, the upper section basalts with whole‐rock MgO up to ~20 wt.% were formed by intraplate volcanism with significant olivine accumulation once the spreading center had become inactive. The MORB type basalts at Site U1431 have Hawaii‐like high olivine Ni and Fe/Mn and low olivine Ca and Mn and bulk‐rock low CaO that are distinctly different from normal global MORBs, suggesting an anomalous mantle source in lithology. We suggest that an eclogite‐/pyroxenite‐rich component, possibly sourced from the Hainan hot spot, played a fundamental role in the transition from spreading to intraplate volcanism of the dying spreading ridge of the South China Sea.

Non-mantle-plume process caused the initial spreading of the South China Sea

“…The Tp values show that the upper mantle that produced Site U1500 basalts had a similar thermal state to that of MORB-producing upper mantle. These conditions are significantly different from those inferred for basalts from Site U1431, which record a pyroxenite source and the presence of a thermal anomaly at the end of spreading 10 . Therefore, geochemical features attributable to plumes were absent during the initial stages of SCS spreading and increased towards the end of spreading 10,13,14 , meaning that the effects of mantle plumes can be neglected during initial spreading.…”

“…S5a). These olivines are similar to those of Hawaiian and Hainan OIB, indicating that Site U1431 basalts were produced by melting of a mantle source that contained significant amounts of recycled oceanic crust (e.g., garnet pyroxenite) 10,48 . In contrast, olivine within the basalts of Site U1500 has low Ni contents, similar to average MORB and the calculated composition of olivine fractionated from melts of fertile peridotite ( Supplementary Fig.…”

“…Average N-MORB is used as the starting magma during the modeling. Reference data for basalt samples from Sites U1431 and U1433 are referred to references 10,14 . Average elemental data for N-MORB and E-MORB are from Gale et al 60 .…”

“…Lavas with CaO contents lower than those defined by the dotted line are potential pyroxenite-bearing partial melts. Reference data for basalt samples from Sites U1431 and U1433 are referred to references 10,14 . In (b), the calculated primitive melt compositions for Site U1500 basalts are shown in red circles while calculated primitive melt compositions for Site U1431 are shown in grey diamonds.…”

“…The olivine-liquid thermometer (T°l -liquid ) does not require a specific lithology in the mantle source. In this study, we used the SiO 2 -based thermobarometer 50 , olivine-liquid equilibria based 10,14 , and data of Hainan OIB 17,61,62 are collected for comparison which are represented by white circles. Average elemental data for N-MORB and E-MORB are from Gale et al 60 .…”