Abstract
Triassic volcanic rocks, including basalts and dacites, were drilled from Meiji Atoll in the South China Sea (SCS), which represents a rifted slice from the active continental margin along the Cathaysia Block. In this study, we present apatite and whole rock geochemistry of Meiji dacites to decipher their petrogenesis. Apatite geochronology yielded U-Pb ages of 204–221 Ma, which are identical to zircon U-Pb ages within uncertainty and thus corroborate the formation of the Meiji volcanic rocks during the Late Triassic. Whole rock major elements suggest that Meiji dacites mainly belong to the high-K calc-alkaline series. They display enriched patterns in light rare earth elements (LREE) and flat patterns in heavy rare earth elements (HREE). They show enrichment in large-ion lithophile elements (LILE) and negative anomalies in Eu, Sr, P, Nb, Ta, and Ti. The dacites have initial 87Sr/86Sr ratios of 0.7094–0.7113, εNd(t) values of -5.9–-5.4 and εHf(t) values of -2.9–-1.7, whereas the apatite has relatively higher initial 87Sr/86Sr ratios (0.71289–0.71968) and similar εNd(t) (-8.13–-4.56) values. The dacites have homogeneous Pb isotopes, with initial 206Pb/204Pb of 18.73–18.87, 207Pb/204Pb of 15.75–15.80, and 208Pb/204Pb of 38.97–39.17. Modeling results suggest that Meiji dacites can be generated by <40% partial melting of amphibolites containing ∼10% garnet. Therefore, we propose that the Meiji dacites were produced by partial melting of the lower continental crust beneath the South China block, triggered by the underplating of mafic magmas as a response to Paleo-Pacific (Panthalassa) subduction during the Triassic. Meiji Atoll, together with other microblocks in the SCS, were rifted from the South China block and drifted southward due to continental extension and the opening of the SCS.
Highlights
As one of the largest marginal basins, the South China Sea (SCS) in the western Pacific (∼3,500,000 km2) has been extensively studied in recent years (e.g., Taylor and Hayes, 1983; Savva et al, 2014; Zhang G.-L. et al, 2018; Ye et al, 2018; Ding et al, 2020; Cui et al, 2021; Miao et al, 2021)
Outside of the SCS area, Permian–Triassic volcanic rocks have rarely been reported along the eastern coastal region of the South China block (SCB), which is interpreted to be the result of high rates of tectonic uplift and erosion of the active continental margin volcanic arc (Yamamoto et al, 2009; Safonova et al, 2015; Gao W. et al, 2017)
The northern and southern continental margins of the SCS that opened during the Cenozoic preserve the record of late Mesozoic arc magmatism in the SCB related to Paleo-Pacific subduction according to large positive magnetic anomalies and Mesozoic silicic to intermediate arcrelated intrusives and volcanic rocks (e.g., Zhou et al, 2006; Yan et al, 2010; Wu et al, 2011; Yan et al, 2014; Xu et al, 2017; Li et al, 2018)
Summary
As one of the largest marginal basins, the South China Sea (SCS) in the western Pacific (∼3,500,000 km2) has been extensively studied in recent years (e.g., Taylor and Hayes, 1983; Savva et al, 2014; Zhang G.-L. et al, 2018; Ye et al, 2018; Ding et al, 2020; Cui et al, 2021; Miao et al, 2021). Before the initial rifting and seafloor spreading, the SCS region is considered to be a convergent continental margin with an Andean-type volcanic arc during much of the Jurassic and Cretaceous, and represented the southernmost extent of the South China mainland (Holloway, 1982). The northern and southern continental margins of the SCS that opened during the Cenozoic preserve the record of late Mesozoic arc magmatism in the SCB related to Paleo-Pacific subduction according to large positive magnetic anomalies and Mesozoic silicic to intermediate arcrelated intrusives and volcanic rocks (e.g., Zhou et al, 2006; Yan et al, 2010; Wu et al, 2011; Yan et al, 2014; Xu et al, 2017; Li et al, 2018)
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