Abstract
Fresh samples of basalts were collected by dredging from the Nanyue intraplate seamount in the Southwest sub-basin of the South China Sea (SCS). These are alkali basalts displaying right-sloping, chondrite-normalized rare earth element (REE) profiles. The investigated basalts are characterized by low Os content (60.37–85.13 ppt) and radiogenic 187Os/188Os ratios (~0.19 to 0.21). Furthermore, 40Ar/39Ar dating of the Nanyue basalts showed they formed during the Tortonian (~8.3 Ma) and, thus, are products of (Late Cenozoic) post-spreading volcanism. The Sr–Nd–Pb–Hf isotopic compositions of the Nanyue basalts indicate that their parental melts were derived from an upper mantle reservoir possessing the so-called Dupal isotopic anomaly. Semiquantitative isotopic modeling demonstrates that the isotopic compositions of the Nanyue basalts can be reproduced by mixing three components: the average Pacific midocean ridge basalt (MORB), the lower continental crust (LCC), and the average Hainan ocean island basalt (OIB). Our preferred hypothesis for the genesis of the Nanyue basalts is that their parental magmas were produced from an originally depleted mantle (DM) source that was much affected by the activity of the Hainan plume. Initially, the Hainan diapir caused a thermal perturbation in the upper mantle under the present-day Southwest sub-basin of the SCS that led to erosion of the overlying LCC. Eventually, the resultant suboceanic lithospheric mantle (SOLM) interacted with OIB-type components derived from the nearby Hainan plume. Collectively, these processes contributed crustal- and plume-type components to the upper mantle underlying the Southwest sub-basin of the SCS. This implies that the Dupal isotopic signature in the upper mantle beneath the SCS was an artifact of in situ geological processes rather than a feature inherited from a Southern Hemispheric, upper mantle source.
Highlights
A number of Cenozoic extensional basins occur along the eastern continental margin of Eurasia, extending over 5.5 × 103 km from northeastern Siberia to Indochina
Four out of six unaltered basalt samples were selected from the Nanyue seamount for bulk-rock analyses of major element oxides and trace elements
In order to probe the nature of the mantle source domain(s) that contributed to post-spreading magmatism in the Southwest sub-basin of the South China Sea (SCS), we will first explore the impacts of melt fractionation, crustal contamination, and postmagmatic alteration on the compositions of the Nanyue basalts
Summary
A number of Cenozoic extensional basins occur along the eastern continental margin of Eurasia, extending over 5.5 × 103 km from northeastern Siberia to Indochina. The Late Cenozoic postspreading basalts from the intraplate seamounts of the SCS are generally characterized by: (i) ocean island basalt (OIB)-type incompatible element distributions, (ii) depleted to moderate Sr–Nd isotopic compositions, and (iii) high 208 Pb/204 Pb and 207 Pb/204 Pb ratios for a given Pb/204 Pb [2,3,4,5]. These isotopic signatures are commonly reported from basalts produced by melting of upper mantle domains with a Dupal-like isotopic anomaly
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