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It has been established that almost all known mantle plumes since the Mesozoic formed above the two lower mantle large low shear velocity provinces (LLSVPs). The Hainan plume is one of the rare exceptions in that instead of rising above the LLSVPs, it is located within the broad global mantle downwelling zone, therefore classified as a “lone plume”. Here, we use the Hainan plume example to investigate the feasibility of such lone plumes being generated by subducting slabs in the mantle downwelling zone using 3D geodynamic modelling. Our geodynamic model has a high-resolution regional domain embedded in a relatively low resolution global domain, which is set up in an adaptive-mesh-refined, 3D mantle convection code ASPECT (Advanced Solver for Problems in Earth's ConvecTion). We use a recently published plate motion model to define the top mechanical boundary condition. Our modelling results suggest that cold slabs under the present-day Eurasia, formed from the Mesozoic subduction and closure of the Tethys oceans, have prevented deep mantle hot materials from moving to the South China Sea from regions north or west of the South China Sea. From the east side, the Western Pacific subduction systems started to promote the formation of a lower-mantle thermal-chemical pile in the vicinity of the future South China Sea region since 70 Ma ago. As the top of this lower-mantle thermal-chemical pile rises, it first moved to the west, and finally rested beneath the South China Sea. The presence of a thermochemical layer (possible the D″ layer) in the model helps stabilizing the plume root. Our modelling is the first implementation of multi-scale mesh in the regional model. It has been proved to be an effective way of modelling regional dynamics within a global plate motion and mantle dynamics background. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the processes responsible for the intra-plate early Paleozoic Wuyi–Yunkai orogeny (>460Ma to 420–415Ma) in the South China Block (SCB) is important for deducing the interactions of this block with other continents at that time, as well as the tectonic evolution of East Asia. One salient feature of the orogen is that despite the wide occurrence of syn- to late-orogenic (440Ma to 420–415Ma) granites in the orogen, neither syn- to late-orogenic volcanic rocks nor mafic rocks of any type have been reported. Such mafic rocks could shed clues about any mantle–crust interaction during such a major orogeny, thus help to understand the dynamics of the orogenic event. We present here, for the first time, geochronological, isotopic and geochemical data for a mafic-intermediate volcanic succession in northern Guangdong, near the edge of the metamorphic core of the orogen. The volcanic rocks unconformably overlie strongly deformed Cambro-Ordovician strata, but are in low-angle unconformable contact with overlying post-orogenic mid-Devonian strata. LA-ICP-MS and SHRIMP U–Pb dating of zircons from two andesitic and dacitic samples gives a consistent crystallization age of ca. 435Ma, younger than the 460–445Ma peak metamorphism of the orogeny but synchronous with the widespread late-orogenic (ca. 440–415Ma) granitic intrusions. Nine least crustally-contaminated basaltic samples are characterized by high MgO (12.3–19.2wt.%), Ni (214–715ppm) and Cr (724–1107ppm), but low TiO2 (0.6–0.8wt.%), Al2O3 (10.2–12.8wt.%) and Fe2O3 T (total Fe as Fe2O3) (8.7–11.4wt.%) contents. The basalts also exhibit low Nb/La ratios (0.4–0.8) and constant εNd(t) values (−8.0 to −8.4) with variable SiO2 (44.8–51.5wt.%) contents, suggesting a likely sub-continental lithospheric mantle origin. These high-magnesian basalts have chemical compositions similar to their primary magma, which was estimated using geochemical modeling at SiO2 ≈50wt.%, MgO≈14wt.% and FeOT ≈9wt.%. The estimated potential temperature for the melts is >1300°C, much higher than that of a normal sub-continental lithosphere. This implies that the magma was likely generated from partial melting of lithospheric peridotite heated by hot upwelling asthenosphere. The high-magnesian andesites are interpreted as the products of differentiation and AFC processes from the same basaltic magma source, as supported by their negative zircon εHf(t) values (−21.7 to −6.3) and high zircon δ18O values (7.3–9.0‰). Overall, we interpret that this post-kinematic basalts, plus andesites and dacites as differentiates, resulted from a late-orogenic lithospheric delamination which led to an orogenic collapse, melting of the SCLM, and widespread late-orogenic granitic intrusions in the orogen. [Copyright &y& Elsevier]

Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90·7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2–25 kbar (dominantly at 10–15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th (≥ 4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90·7) composition and the most primitive bulk-rock samples (MgO > 9·0 wt % and CaO >8·0 wt %), assuming a constant Fe–Mg exchange partition coefficient of KD = 0·31 and Fe3+/FeT = 0·1. The effective melting pressure (Pf) and melting temperature (T) of the primary melts are Pf = 18–32 kbar (weighted average = 23·8 ± 1·8 kbar) and T = 1420–1520°C for the tholeiites, and Pf = 25–32 (weighted average = 28·3 ± 1·4 kbar) and T = 1480–1530°C for the alkali basalts. The Pf –T data form an array that plots systematically above the dry lherzolite solidus but below the base of the lithosphere (∼55 km) and intersects the dry peridotite solidus at a pressure of about 50 kbar. The mantle potential temperature beneath Hainan Island, based on the estimate primary melt compositions, varies from about 1500 to 1580°C with a weighted average of 1541 ± 10°C. The high-magnesian olivine phenocrysts, high mantle potential temperature, and the presence of recycled oceanic crust in the source region provide independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. The Hainan plume thus provides a rare example of a young mantle plume associated with deep slab subduction. [ABSTRACT FROM AUTHOR]

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss (TL) data in the shadow zone of deep water. We propose a multiple-step TL inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid half-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very well. [ABSTRACT FROM AUTHOR]

The horizontal-longitudinal correlations of the acoustic field in deep water are investigated based on the experimental data obtained in the South China Sea. It is shown that the horizontal-longitudinal correlation coefficients in the convergence zone are high, and the correlation length is consistent with the convergence zone width, which depends on the receiver depth and range. The horizontal-longitudinal correlation coefficients in the convergence zone also have a division structure for the deeper receiver. The signals from the second part of the convergence zone are still correlated with the reference signal in the first part. The horizontal-longitudinal correlation coefficients in the shadow zone are lower than that in the convergence zone, and the correlation length in the shadow zone is also much shorter than that in the convergence zone. The numerical simulation results by using the normal modes theory are qualitatively consistent with the experimental results. [ABSTRACT FROM AUTHOR]