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

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 nonplume processes, most likely by the westward subduction of the Pacific Plate. 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. For the magma-rich type, the rifted margin is characterized by massive igneous activity in a relatively short period of time (~1-3 Myr) during breakup and initial seafloor spreading, which is caused by mantle plume. The pair of conjugate margins of Greenland and northwest Europe is a typical example 3. For the magma-poor type, weakening of mantle lithosphere and facilitating of plate rupture is caused by serpentinization over a period of time which is a result of tectonic extension and mantle exhumation. The Newfoundland and Iberia conjugate margin is a typical example 4. However, the South China Sea (SCS) margin doesn't meet the expected characteristics of neither magma-rich nor magma-poor types, instead showing a rapid transformation from continental breakup to initial spreading without massive magmatism 5. Revealing the formation process of such a unique type margin can help us understand the dynamic mechanism of the marginal sea evolution. Meanwhile, it also provides an important supplementary knowledge for the continental breakup and oceanic formation on the Earth. Various models have been put forward to explain the formation of the SCS, such as tectonic extrusion by India-Eurasia collision 6,7 , extension related to mantle plume upwelling 8-10 , and regional extension by subduction and retreat of the Pacific plate 11. Among them, the mantle plume is considered as the most popular model 10,12-14. The evidence include: (1) seismic observations indicate a low-velocity zone beneath the SCS...

Mantle Heterogeneity and Melting Processes in the South China Sea: Thermal and Melting Models Constrained by Oceanic Crustal Thickness and Basalt Geochemistry

“…The variation trends of Nb/Th and Ta/U values are relatively close to the lower continental crust (Figure 13), suggesting that the magma source is probably contaminated by lower continental crust (Sun et al, 2020). The major element compositions and REE patterns of basalt samples from site U1500 (Yu & Liu, 2020) at the northern margin of the east subbasin are comparable to that of sites U1433 and U1434 in the SW subbasin ( Figures S1 and S2). The variation trends of Nb/Th and Ta/U ratios of U1500, which are not sensitive to fractional crystallization and could be used to reflect the origin of the mantle (Niu & Batiza, 1997), are also consistent with that of U1433 and U1434, revealing the influence of lower continental crust (Figure 13).…”

“…The basalt compositions of IODP sites are selected from Zhang et al. (2018b) and Yu and Liu (2020). The gray dots represent global mid‐ocean ridge basalt compositions for segments with half‐spreading rate less than 4 cm/yr, which are modified from Gale et al.…”

“…The bulk-rock major and trace element contents of IODP sites U1431, U1433, and U1434 are available in Zhang et al (2018b). The bulk-rock major and trace element contents of site U1500 are selected from Yu and Liu (2020). The seismically determined crustal thicknesses of the east and southwest subbasins are available in Zhao et al (2018) and Qiu et al (2011), respectively.…”

“…Site U1502 was flooded by remnants of continental lithosphere, which recorded the magmatism during the final stage of continental rifting and breakup (Larsen et al., 2018). Sites U1500 and U1503 sampled the oceanic crust formed during the incipient seafloor spreading (Childress et al., 2019; Yu & Liu, 2020). The bulk‐rock major and trace element contents of site U1500 are selected from Yu and Liu (2020).…”

“…Sites U1500 and U1503 sampled the oceanic crust formed during the incipient seafloor spreading (Childress et al., 2019; Yu & Liu, 2020). The bulk‐rock major and trace element contents of site U1500 are selected from Yu and Liu (2020). Major element contents of site U1500 are consistent with that of sites U1433 and U1434 in the SW subbasin (Figure S1).…”