A database of 492 whole-rock analyses, about two-thirds of which include both major and trace elements, was compiled for early Mesozoic diabase dikes from central Virginia through South Carolina. The database was examined for compositional patterns that could be related to magma modification caused by crystal-melt equilibria (“mineral control”), source effects, or both.

The principal conclusion drawn from consideration of major and compatible trace elements, as well as phenocryst assemblages, is that compositions of the rocks were affected by low-pressure crystal fractionation/accumulation involving some combination of olivine, chromian spinel, plagioclase, and clinopyroxene. The sequence of crystallizing minerals affecting melt compositions, from first to last, apparently was (1) olivine + spinel, (2) olivine + plagioclase, (3) olivine + plagioclase + clinopyroxene, and (4) plagioclase + clinopyroxene. This sequence is quite similar to that commonly found in mid-ocean ridge basalts.

A bimodal distribution of compositions exists that can be related to dike attitude and not to sampling bias. The more evolved samples, about half of which are compositionally basaltic andesites, are generally found in dikes of the north-south swarm and are of the chemical high-Fe quartz-normative (HFQ) magma type. Most dikes with more primitive compositions are in the apparently older northwest swarm and are olivine normative. Phyric primitive samples contain either olivine + spinel or olivine + plagioclase as the dominant phenocryst assemblages, whereas most evolved samples contain granophyre in the mesostasis. Most phyric evolved samples have clinopyroxene, commonly pigeonite rimmed by augite, and plagioclase as phenocrystic phases.

The main conclusions drawn from an examination of incompatible-element compositions are (1) tectonic discrimination diagrams utilizing high field-strength elements at the very best yield ambiguous results for these diabases; (2) the evolved as well as primitive diabases can be divided into a relatively incompatible-element enriched and a relatively incompatible-element depleted group, which could be related to a number of factors; and (3) several parallelisms exist (both literally and figuratively) between dikes of the northwest swarm and transform-related MORBs (mid-ocean ridge basalts). Diabases in the northwest swarm are likely the precursors for modern transform-related basalts of the mid-Atlantic Ocean.