Heterogeneous Mantle Metasomatism Controlled by Continental Subduction: Evidence From Post-Collisional Mafic Rocks in the North Qaidam Orogen

Abstract

Post-collisional mafic rocks not only record geodynamic processes at the end of the orogenic cycle but also retain various clues regarding preceding interactions between subducted slabs and mantle wedges. However, the latter and related indicators have often been overlooked; in particular, how the continental crust interacts with the mantle in subduction zones and modifies its nature remains ambiguous. This study of post-collisional mafic rocks in the North Qaidam orogen provides new insights into sophisticated crust–mantle interactions via variable continental subduction. These post-collisional mafic rocks are consistent with the geochemical nature of arc lava and originated from partial melting of the antecedent metasomatic mantle. Although they possess relatively uniform whole-rock Sr–Nd isotopes, the mafic rocks from the ultrahigh-pressure metamorphic terranes present more enriched zircon Hf isotopes and remarkable signals of melt-driven mantle metasomatism than the other rocks within the non-ultrahigh-pressure metamorphic zone. This is attributed to spatially variable continental subduction and the consequent differentiated crust–mantle interactions. Trace element modeling also reveals that a greater proportion of continental component-derived metasomatic melts are needed to form mafic rocks in ultrahigh-pressure metamorphic terranes. The whole-rock magnesium isotopes are between −0.15‰ and −0.38‰, which are negatively correlated with (⁸⁷Sr/⁸⁶Sr)_i and positively correlated with ɛ_Nd(t) and ɛ_Hf(t). These findings indicate that these mafic rocks exhibit lighter magnesium isotopes when their mantle source is metasomatized by more continental component-derived melts enriched with radiogenic isotopes. The geochemical distinctions of the post-collisional mafic rocks verify the contributions from continental subduction to heterogeneous mantle metasomatism and magmatic diversity.