Coexistence of Hainan Plume and Stagnant Slab in the Mantle Transition Zone beneath the South China Sea Spreading Ridge: Constraints from Volcanic Glasses and Seismic Tomography

Zhikun Ren,Jie Liao,Shuang-Shuang Chen,Tong Hou,Rui Gao,Cheng Yan,Zewei Wang

doi:10.2113/2021/6619463

Abstract

Abstract The influence of Hainan mantle plume and subducting recycled oceanic crust beneath the spreading ridge of the South China Sea (SCS) have been widely proposed recently, but still controversial and ambiguous. Here, we present seismic tomographic evidence, new major and trace element, and Pb isotopic compositions of volcanic glasses from one International Ocean Drilling Program drill core (Site U1434) in the SCS spreading ridge. The volcanic glasses are relatively enriched in alkalis and light rare earth elements (LREEs) and depleted in heavy REEs (HREEs), exhibit slightly positive anomalies in Nb, Ta, Zr, and Hf as well as a positive Nb relative to La and Th, and show relatively high 207Pb/206Pb and 208Pb/206Pb isotopic ratios, suggesting ocean island basalt- (OIB-) type and enriched mantle 2- (EM2-) type geochemical features likely related to a mantle plume. These geochemical features are consistent with those of late Cenozoic volcanic rocks in Hainan and surrounding areas associated with a mantle plume, likely providing the influence of Hainan mantle plume beneath the spreading ridge of the SCS. The SCS primary-melt and volcanic glasses indicate that the source mantle involved 18.5% eclogite (dense, recycled oceanic crust from the stagnant subducted slab) and 46.1% garnet pyroxenite (produced by the reaction between the peridotite melt and recycled oceanic crust). The existence of Hainan mantle plume and stagnant subducted slab is further supported by geophysical evidence from a recent three-dimensional P-wave seismic tomographic model.

Highlights

The South China Sea (SCS) is a western Pacific Cenozoic marginal sea surrounded and affected by motions of the Eurasian, Indian, and Philippine/Pacific plates, which has been commonly divided into eastern and southwestern subbasins taking the Zhongnan Fault as the boundary (Figure 1)
We present a comprehensive set of chemical data of volcanic glasses to elucidate the existence of the Hainan mantle plume beneath the SCS spreading center, calculate the primary-melt compositions to evaluate the influence of recycled oceanic crust from the stagnant subducted slab, and present a three-dimensional P-wave seismic tomographic model to verify our conclusions
This study presents seismic tomographic evidence, new major and trace elements, and Pb isotopic compositions of volcanic glasses from one International Ocean Drilling Program (IODP) drill cores (Site U1434) in the SCS spreading ridge

Summary

Introduction

The South China Sea (SCS) is a western Pacific Cenozoic marginal sea surrounded and affected by motions of the Eurasian, Indian, and Philippine/Pacific plates, which has been commonly divided into eastern and southwestern subbasins taking the Zhongnan Fault as the boundary (Figure 1). The results of microfossils in the interflow claystone [1] and deep-tow magnetic anomaly lineations [2,3,4] indicate that SCS continental-margin breakup and seafloor spreading occurred at 32–33 Ma in the northeastern SCS [1, 4], this spreading ridge jumps 20 km southward in the East Subbasin which occurred at ~25 Ma [2], and the seafloor spreading in the Southwest Subbasin propagated 400 km southwestward during the period of 23.6-21.5 Ma [4]. The collision between the northern Luzon Arc and the South China continent occurred at approximately

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