Melting Dynamics of Late Cretaceous Lamprophyres in Central Asia Suggest a Mechanism to Explain Many Continental Intraplate Basaltic Suite Magmatic Provinces

Abstract

AbstractIntraplate small‐volume mafic magmatism is spatially and temporally widespread in central and eastern Asia, but the relevant melting dynamics have remained enigmatic. Here, we report Ar‐Ar ages, mineral and whole‐rock compositions on newly found ∼81 Ma lamprophyre dykes from central Asia, aiming to constrain the source characteristics and the melting dynamics in this intraplate setting. Mineral chemistry of the lamprophyres shows that pre‐emplacement magmas equilibrated at 970–1060°C (probably at the base of the crust) and contained 1.4–2.1 wt% water. The source region of the lamprophyres is shown to have H contents equivalent to H2O = ∼150 ppm, and to be lithologically heterogeneous with silica‐deficient pyroxenite embedded in peridotite. Because of the thermodynamic complexities involved in calculating melting scenarios with major elements for such mantle domains, we have conducted a grid search with a forward‐modeling methodology that can simulate the adiabatic decompression melting of a lithologically heterogeneous mantle using incompatible trace elements. The modeling results indicate that original melting occurred at a potential temperature of ∼1400°C under a thinned lithosphere (∼70 km, corresponding to a final melting pressure of 2.3 GPa). Combining these conditions with constraints from regional geology and the tectonic history, the lamprophyres are inferred to have formed by decompression melting induced by small‐scale asthenospheric upwellings due to the edge effects under a corrugated lithospheric lower boundary. We suggest that this scenario has global relevance and represents a likely mechanism for the initiation of relatively small‐scale intraplate magmatism within continents.