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

Zhifei Liu,Xun Yu

doi:10.1038/s41598-020-65174-y

Abstract

The mantle plume process is thought to be the prevailing dynamic mechanism for the South China Sea opening, but controversy persists due to the lack of critical evidence of magma in the initial seafloor spreading. International Ocean Discovery Program (IODP) Expedition 367 successfully recovered at Site U1500 the mid-ocean ridge basalt (MORB) representing the magma activity of the initial spreading of the South China Sea during the earliest Oligocene. Here we present the whole-rock and olivine phenocryst geochemistry of the basalts to constrain the potential influence of the Hainan mantle plume on the evolution of the South China Sea. Major and trace elemental compositions indicate that the basalts were mainly influenced by fractional crystallization of olivine and formed by melting of a spinel peridotite source without any pyroxenite in mantle source. The calculated mantle potential temperature of those most primitive basalts is much lower than plume-related MORB of Iceland, but similar to normal MORB elsewhere. Both lithological composition and mantle potential temperature clearly contradict with the mantle plume model, signifying that the mantle plume didn’t exist at the earliest Oligocene. Therefore, the initial spreading of the South China Sea should be caused by non-plume processes, most likely by the westward subduction of the Pacific Plate.

Highlights

Dynamic processes of continental breakup and subsequent initial generation of igneous oceanic crust have been identified as two end-members: magma-rich type and magma-poor type[1,2]
The evidence include: (1) seismic observations indicate a low-velocity zone beneath the South China Sea (SCS) that extends to the lower mantle, supporting the existence of a deep plume[15,16]; (2) the mantle potential temperatures of Hainan ocean island basalts (OIB), as well as the olivine crystallization temperatures of mid-ocean ridge basalts (MORB) from the east sub-basin of the SCS, are similar to those of hotspot-related OIB and higher than those of normal MORB (N-MORB)[12,17]; (3) The EM2-like Sr, Nd, and Pb isotopic geochemistry of SCS OIB and MORB are consistent with the isotopic features of Hainan OIB13,14, which are considered to origin from a deep mantle plume[18,19]
The MORB samples reported from Site U1431 (IODP Expedition 349) represent oceanic crust that formed toward the end of extension (16–15 Ma) of the SCS basin[10,13,14,20]

Summary

Introduction

Dynamic processes of continental breakup and subsequent initial generation of igneous oceanic crust have been identified as two end-members: magma-rich type and magma-poor type[1,2]. To determine whether the Hainan plume played an important role in opening of the SCS basin, obtaining the mantle potential temperature (Tp) of the upper mantle and understanding the mantle source lithology of basalts formed at the initial spreading stage is the key. Major- and trace-element whole-rock and olivine phenocryst data are reported for the basalts of IODP Expedition 367 Site U1500 These basalts represent oceanic crust produced during the initial spreading of the SCS at ~33 Ma5,21. Four sites were drilled by IODP Expeditions 367 and 368 in the northern margin of the east sub-basin, one on the outer margin high and three seaward on the outer margin high basement ridges[21] These ridges are within the continent-ocean transition zone going from outer margin high to the steady-state oceanic crust of the SCS. Site U1500 basalts are classified as normal MORB (N-MORB) and transitional MORB (T-MORB) on the MORB classification diagram, whereas Site U1431 basalts are identified as

Methods

Conclusion