Back-arc basins, characterized by intense extension behind island arcs or along continental margins adjacent to trenches, are commonly attributed to subduction processes involving multiple thermal and mechanical factors (Molnar & Atwater, 1978;Sdrolias & Müller, 2006;Stern, 2002). However, it remains a matter of debate if subduction alone is sufficient for initiating the back-arc extension (Mantovani et al., 2001; Mc-Cabe, 1984;Uyeda & Kanamori, 1979;Wallace et al., 2005), because back-arc basins are missing in some subduction zones, or extension has ceased while subduction is still active.

The Permian Emeishan large igneous province (LIP) in Southwestern China is recognized as a remnant of mantle plume activity. Its crust composition has a significant bearing on understanding the magmatic plumbing system in a LIP and the volume of magmas generated, which is critical to the viability of the mantle plume model and its potential environmental impact but is difficult to constrain. Here, a SiO2‐VP/VS‐VS relationship, compiled from existing laboratory measurements, is used to estimate the crustal SiO2 content of the Emeishan LIP based on the joint constraint of the crustal bulk VP/VS ratio and average VS. The crust underneath the northeastern inner zone of the Emeishan LIP has a SiO2 content of ∼55.7 wt%, in contrast with the bulk continental crust's SiO2 content of 62.1 ± 2.1 wt% but largely overlapping with the exposed mafic‐ultramafic intrusions. Such an unusually “mafic” crust is attributable to the intensive magmatic underplating and intraplating associated with the Emeishan mantle plume. It further allows a minimum estimate of the volume of ∼1.45 × 106 km3 for erupted magmas and ∼2.39 × 106 km3 for total emplaced magmas, which is comparable to the volume of typical plume‐generated LIPs.