Jurassic metasomatised lithospheric mantle beneath South China and its implications: Geochemical and Sr-Nd isotope evidence from the Late Jurassic shoshonitic rocks

Abstract

It is well established that the Late Jurassic magmatism in the southeastern area of the South China Block (SCB) was associated with an extensional event. However, the triggering mechanism for this event remains unclear, with models commonly invoking either back-arc extension or intra-continental extension in response to slab roll-back of the Pacific plate or asthenospheric upwelling. The key issue about these models revolves around whether the subducted Pacific slab contributed to the mantle source beneath the SCB during the Jurassic, or not. Basalts erupted during the Late Jurassic can provide significant clues for probing the nature of the lithospheric mantle and underlying convective mantle. In this study, we present detailed zircon U-Pb geochronological, whole-rock element and Sr-Nd isotope data for Late Jurassic trachybasalt and trachyandesite from the Mashan Complex, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA-ICP-MS zircon U-Pb dating suggests that these volcanic rocks erupted in the Late Jurassic (~158 Ma). All volcanic samples have shoshonitic geochemical affinities with high K2O (2.03–4.89 wt%) and K2O/Na2O (0.55–2.50). They are strongly enriched in LILE and LREE with positive K anomalies, positive εNd(t) values (from +0.8 to +3.0), moderate Dy/Yb (1.98–2.36) and low Ba/La (11.1–30.7), Th/Yb (2.29–5.77), U/Th (0.18–0.35), Th/Nb (0.16–0.27) and Th/Ce (0.08–0.15) ratios. Such geochemical signatures suggest that the Mashan volcanic rocks were derived from low-degree (1–5%) partial melting of a metasomatised lithospheric mantle with little evidence supporting the involvement of the Pacific slab in the genesis of the magmas. The synthesis of the available data shows that the Jurassic shoshonitic and mafic rocks have higher εNd(t) values than the Triassic shoshonitic and mafic rocks (εNd(t) < 0), reflecting source transformation from a Triassic enriched lithospheric mantle to a Jurassic depleted source. The metasomatism of the lithospheric mantle is likely to be associated with asthenospheric upwelling prior to the Late Jurassic. The asthenosphere-lithosphere interaction might have produced a newly metasomatised zone within the lowermost lithospheric mantle. It is inferred that the Jurassic magmatism in the southeast of the SCB might be associated with asthenospheric upwelling in an intra-continental extension setting.