Unusual intraplate volcanism at the Conrad Rise, Indian Ocean: the role of inherited continental lithosphere

Intraplate volcanism occurring far from active plate boundaries is commonly attributed to mantle plumes or lithospheric stress reorganization. However, several oceanic rises exhibit magmatic histories that challenge these conventional models. The Conrad Rise in the southern Indian Ocean represents a particularly enigmatic case of oceanic plateau formation. The Conrad Rise was previously interpreted as a Late Cretaceous oceanic plateau, but its origin and magmatic evolution remained poorly constrained.

Recent geochronological and isotopic analyses of volcanic rocks from the Conrad Rise (Sato et al., 2024) have significantly revised this perspective. ⁴⁰Ar/³⁹Ar dating demonstrates that the primary volcanic edifices formed during distinct intraplate episodes in the middle–late Eocene (~40 Ma) and late Miocene (~8.5 Ma), significantly younger than the surrounding oceanic lithosphere (ca. 84 Ma). Furthermore, the Sr–Nd–Pb–Hf isotopic signatures cannot be explained by a single depleted mantle or plume-derived source and instead indicate contributions from enriched reservoirs, including components consistent with lower continental crust compositions.

In addition to these volcanic constraints, dredging at the Conrad Rise has recovered granitoid and high-grade metamorphic rocks with clear continental affinities. These rocks record Proterozoic to early Paleozoic crustal histories comparable to those of the Gondwana terranes in East Antarctica and eastern India. The occurrence of continental-derived rocks in such a remote offshore setting recalls similar observations from the Rio Grande Rise in the South Atlantic. While alternative explanations, such as iceberg-rafted debris, must be considered, the size, abundance, and lithological diversity of the recovered rocks, together with the geochemical signatures of the associated volcanism, collectively suggest the involvement of continental material within or beneath the rise.

We propose that the unusual episodic intraplate magmatism of the Conrad Rise may result from interactions between mantle upwelling and inherited lithospheric heterogeneity associated with continental components. This “hotspot-less” model, distinct from classical plume-head- or ridge-related mechanisms, drives episodic melt generation and compositional diversity, underscoring the critical influence of inherited lithospheric structures on offshore intraplate volcanism.