ABSTRACT: Age and Petrology of the Jackson Dome Igneous-Volcanic Complex, Mississippi: Implications for the Tectonic History of the Mississippi Salt Dome Basin

SAUNDERS, J. A., Department of Geology, Auburn University, Auburn, AL, and D. W. HARRELSON, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS

The Jackson dome is cored by an exotic suite of alkalic igneous intrusions of Cretaceous age, some of which vented to produce a volcanic island. The igneous rocks fall into two general types: (1) phonolites, and (2) mafic alkalic rocks. Phonolite of the Jackson Dome typically consists of phenocrysts of sanidine and nepheline in a fine-grained groundmass of similar composition. Mafic alkalic rocks contain variable amounts of nepheline, clinopyroxene (aegerine, aegerine-augite, or titanaugite), biotite, magnetite, garnet, and sphene. The latter rocks are best classified as ijolite, melteigite, nephelinite, and jacupirangite based on their relative mineral abundances. Whole-rock chemical analyses indicate that the rocks are nepheline-normative, are silica deficient, with SiO[2] as low as 26.7 wt. %, are enriched in titanium (TiO[2] as high as 8%) and alkalis, with Na + K typically exceeding 14%. Igneous rocks of the Jackson dome are mineralogically and geochemically very similar to the Cretaceous Magnet Cove carbonatite complex of central Arkansas, indicating that the Jackson Dome may be a carbonatite complex as well. K-Ar age dates were determined on either whole-rock samples or biotite separates from four core samples from two different wells. Age dates range from 79 +/- 2.9 to 69 +/- 2.9 Ma indicating that the alkalic igneous rocks of the Jackson dome are younger than other documented intrusive complexes in the northern Gulf Coast subsurface. The intrusion of alkalic magmas during this time interval indicates that basement rifting continued to at least the beginning o the Tertiary in the Mississippi Salt Dome basin.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)