Ancient and young recycled materials in the Hainan Plume

08/28/2020

Subduction processes introduce crustal materials into the mantle, while mantle plumes return them back to the surface. However, when and how the subducted materials were recorded in the plume-related basalts remains unclear. Here we investigated geochronology, bulk-rock composition, and Sr-Nd-Pb isotopes of Cenozoic basalts from Southeast China, occurring near the west Pacific subduction zone and the seismically detected Hainan plume. Volcanism beginning in the late Oligocene in the continental margin of SE China consistently becomes younger landward.

(a) Simplified geological map of eastern Asia with distributions of Cenozoic basalts and the major regional faults. The thin light-gray lines with the numbers beside denote the depth contours of the present upper boundary of the subducting Pacific slab estimated from seismicity. The thick red line shows the boundary of the gravity anomaly in East China. The thick blue line represents the western edge of the flat slab in the mantle transition zone (MTZ) beneath East Asia, which was estimated from a tomographic model. The solid sawtooth lines denote the westward subduction zones of the Philippine Sea Plate and the Pacific Plate. The four pink lines with capitals at the beginning and the end denote locations of the vertical cross-sections shown in panel c. The shadowed areas represent the Cenozoic basalts in East China with EM1-type and EM2-type isotopic affinities, the latter also representative of the basalts from the Korean Peninsula. (b) The major subduction zones surrounding the margin of South China and the Indochina peninsula (c) Vertical cross-section of P-wave tomography along the profiles in panel a. The red and blue colors denote low and high velocity perturbations, respectively, as shown by the color bar with numbers below. The red triangles atop each cross-section denote the locations of Cenozoic volcanoes. The background seismicity of each profile is shown in white circles. The two dashed black lines denote the 410-660 km discontinuities of the MTZ. The flat high-velocity anomaly in the MTZ was interpreted as representing the stagnant Pacific Plate beneath East Asia, while the sub-vertical low-velocity anomaly crossing the MTZ into the upper mantle was indicated as the thermal upwelling beneath the Hainan Island.

Together with a compilation of published results of the synchronous basalts from the South China Sea seamounts and the Indochina peninsula, the volcanoes close to the Pacific subduction zone exhibit more radiogenic Pb and Sr isotopes associated with less radiogenic Nd isotopes compared to those of the inland volcanoes. Such spatiotemporal variations in radiogenic isotopes imply oceanic crusts of different ages in the source, each corresponding to a different geographical volcanic belt. Major-element features such as low CaO, high TiO2 and high Fe/Mn ratios imply that pyroxenite/eclogite could serve as a source lithology of the SE China basalts. Specific trace element signatures reveal the important roles of recycled oceanic crust along with surface sediment, which was inconsistently dehydrated during subduction.

(a) A map of SE China, the Indochina peninsula and the South China Sea (SCS) showing sample locations and volcano ages. The ranges of ages for each location, labeled with red triangles, are shown in a box near each of them. The volcanoes within orange and blue rectangles represent two geochemical groups, and the colors of the data symbols in the compositional and isotopic plots indicate the geochemical group from which a sample was taken. The three brick-red lines denote locations of the vertical cross-sections shown in panel b. (b) Vertical cross-sections of the S-wave velocity anomaly along the three profiles shown in panel a. Contours are at ±40, ±60, ±80, and ±100 m/s. The abscissa values represent the distance from the starting point of each profile (e.g., A−A’) shown in panel a. The tick space on the section lines is 500 km. The red triangles on the top denote locations of Cenozoic volcanoes from SE Asia. Inn., Inner belt in SE China; Out., Outer belt in SE China. (c) A conceptual model of the geochemical geometry of the Hainan plume. The compositions of the Hainan plume in different evolution stages and the associated volcanism in SE Asia are shown. The syn-spreading stage represents the period when the SCS seafloor was spreading (ca. 32-16 Ma); the post-spreading stage represents the period from 16 Ma to the present. Colors of the volcanoes labelled by triangles atop the cylinder model correspond to the colors of their mantle components in the plume. (d) A brief summary of the enriched domains in the source of outer and inner belt lavas in SE China.

A geologically, geochemically, and geophysically plausible scenario is proposed to illustrate the time-space-source correlation of the late Cenozoic basaltic lavas in SE Asia. The Hainan plume delivered the ancient subducted crust (1.5 Ga) from the core-mantle boundary and, subsequently, the subducted Pacific plate crustal materials from the mantle transition zone to the shallow mantle as a result of mantle convection induced by continuous subduction of the Pacific plate. Such recycled materials of different ages contributed to the geographic compositional heterogeneities of the late Cenozoic basaltic lavas in SE Asia.

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