Mantle xenoliths and host basalts record the Paleo-Asian oceanic materials in the mantle wedge beneath northwest North China Craton

Abstract

Oceanic subduction is an important trigger for mantle heterogeneity, which further increases melt production and controls the compositions of intraplate basalts. Such a role played by the (Paleo-) Pacific subduction have been extensively studied and well constrained based on the widespread mantle xenoliths and intraplate magmatism in the eastern North China Craton. By contrast, the recycled materials from other Phanerozoic subducted slabs beneath the craton are relatively poorly recognized. Here, a reappraisal is made to the recently reported peridotite xenoliths and ~89 Ma host basalts from the Langshan area in the northwest North China Craton and regional data on xenoliths and basalts, with the emphasis on the mass transfer in the mantle wedge from the subducted Paleo-Asian oceanic slab. The Langshan peridotites are fertile in composition and record complex melt extraction and metasomatism. One episode of metasomatism is likely induced by silicate melts with concomitant enrichments in large ion lithophile elements and high field strength elements and positive Eu anomaly, suggestive of the contribution from recycled materials. This metasomatism should take place in Paleozoic according to the diffusion modelling. The host basalts are comparable with partial melts of pyroxenite under 2–3 GPa and have oceanic island basalts-like trace-element compositions. Positive Sr and Eu anomalies, low Rb/Sr and high Ba/Rb ratios, and the moderated depleted but slightly decoupled Sr–Nd isotopes suggest the involvement of subducted oceanic crustal materials in the mantle source. The Pb isotopic compositions are best modeled by the mixing between depleted mantle, altered oceanic crust with minor young (<500 Ma) sediments. These basalts are interpreted to be partial melts of pyroxenite-bearing asthenosphere containing slab-derived materials. Collectively, the mantle wedge beneath the northwest part of the craton is pervasively modified by slab-derived melts. The infiltrating melts were gradually consumed by interaction with country peridotites of the melt conduit during the migration from the slab-asthenosphere contact. This mass transfer process triggered the formation of pyroxenite in the asthenosphere at high melt/rock ratio and metasomatism in the lithospheric mantle at low melt-rock ratio. Considering this Paleozoic melt infiltration, the regional geological records and the tectonic locality, the recycled materials in the mantle wedge beneath northwest part of the craton are likely derived from the subducted Paleo-Asian oceanic slab. Keywords: Mantle heterogeneity, Recycled materials, Northwest North China Craton, Paleo-asian oceanic slab