Derivation of Jurassic HIMU-like intraplate basalts from mantle transition zone in South China: New geochemical constraints from olivine-hosted melt inclusion

Abstract

The petrogenesis of high-μ (HIMU, μ = 238U/204Pb) basalt in a continental setting remains highly controversial. Here we present a comprehensive geochemical (including major and trace element and isotopic compositions of whole-rock, olivine and melt inclusion) data on Jurassic HIMU-like basalts from the interior (Antang) of South China. This study identified high-H2O (up to 5.0 wt%) olivine-hosted melt inclusions in these basalts. After stripping off the effects of degassing, post-entrapment crystallization and kinetic diffusion, the primary magma of the Antang basalts was estimated to contain ≥2.65 wt% H2O and the corresponding mantle source contains >1000 ppm H2O, which is much higher than the water content of convecting asthenosphere and lower mantle. In addition, the relative depletion of fluid-mobile elements and positive Nb-Ta anomalies in bulk rocks and melt inclusions, and the distance far away from the contemporaneous paleo-Pacific subduction zone, preclude an origin from a slab fluid-metasomatized mantle. The strong depletion of CaO in both olivine phenocrysts and whole-rock compositions further indicates that the basaltic magmas originated from an eclogitic or pyroxenitic mantle source. The combined geochemical data suggest that the Antang basalts were most likely derived from a hydrous source, which was composed mainly of ancient recycled oceanic crust that had stagnated in the mantle transition zone (MTZ) for more than one billion years. Our results therefore provide a new perspective on the MTZ origin of intracontinental basalts and imply that the hydrous MTZ can preserve recycled crustal components for long periods of time and form distinct mantle reservoirs for intraplate basalts.