Petrology of the Lower Mesozoic diabase dikes in Georgia and Alabama

Lower Mesozoic tholeiites in Georgia and Alabama represent some of the most chemically diverse rocks in the Eastern North American (ENA) Mesozoic igneous province. The Eastern North American province is part of the much lager central Atlantic magmatic province (CAMP), a once-vast suite of extrusive and intrusive mafic rocks associated with Pangean rifting.;Samples from Georgia mid Alabama (G/AL) can be distinguished into quartz-normative and olivine-normative groups. Of the quartz-normative samples, the HFQ and LTQ ENA magma types are present. The olivine-normative samples can be separated into three geochemical groups. Unlike other classifications of olivine-normative samples in the ENA province, which rely on gradational differences in trace-element characteristics, the G/AL olivine-normative samples can be separated into distinct groups defined by major-element compositions. Two of the groups have higher Ti contents than any other olivine-normative samples observed in the ENA province.;Samples of both quartz-normative groups display trace-element evidence of continental crustal components. Thermal considerations and numerical modeling of trace elements produces better results for mantle source contamination than for magmatic contamination within the continental crust. Some of the olivine-normative samples also display evidence of an isotopic- and LILE-enriched component; however this component does not resemble upper crustal lithologies and may be associated with subduction-related processes.;Trace-element characteristics of many of the G/AL magmas, as well as most of the Carolinas-Virginia magmas, bear striking similarities to certain island-arc basalts. The sub-continental parisphere model of Anderson and co-workers can explain the chemical diversity of the G/AL magmas. The perisphere is envisioned as a viscous layer in the lower part of the sub-continental lithosphere that acts as a reservoir for enriched and volatile products of subduction processes. The perisphere may also retain continentally-derived sedimentary deposits emplaced during subduction. I propose that the geochemical diversity of the G/AL magmas resulted from the interaction of perisphare and depleted convecting mantale material as asthenospheric mantle welled up and displaced the perisphere during continental rifting. I suggest that the greater diversity of magma types in the G/AL region is due to the apparently more advanced stage of rifting, and thus of perisphere-asthenosphere, mixing in the southern portion of the ENA province.