Rocks composing an early Mesozoic basaltic suite (MBS) occur throughout the Circum-Atlantic region as dikes, flows, and sheets. Numerous classification schemes have been proposed for rocks from specific geographic areas. All of these rocks, however, can be broadly classified into one of two groups—enriched or depleted—based on TiO₂-MgO relations. The enriched group, for comparable MgO contents, has TiO₂ concentrations elevated above those of the depleted group. Most of the classification schemes proposed for this group of rocks can be viewed in the context of this more general classification. Over most of the Circum-Atlantic the enriched rocks dominate. However, despite the widespread occurrence of the enriched rocks, chemical compositions of this group are remarkably uniform. This is in contrast to the geographically restricted depleted rocks, which are quite heterogeneous with respect to incompatible-element content.

Several factors have been described, any of which could be responsible for generating the chemical differences observed between the enriched and depleted MBS rocks. These include enriched subcontinental mantle, open- versus closed-system magmatism, depth and/or density-controlled partial melting, and fracture-zone or transform influence. All of these processes are consistent with our conceptual framework for MBS magmatism, the depleted rocks being derived from deeper, rift-flank source areas and the enriched rocks being derived from shallower, rift-axial source regions. It is highly possible that all of the factors discussed may have contributed to some extent to the generation of the observed chemical differences.