OVERVIEW OF CRETACEOUS ALKALIC MAGMATISM: SOUTH-CENTRAL USA

Between ~110 and 64 Ma, alkalic magmatism occurred in a broad area of the south-central U.S. in Kansas, Arkansas, Louisiana, Mississippi, and Texas. There were multiple pulses of activity at each center, with repeated pulses in Kansas occurring over ~50 million years, and magmatism in Arkansas occurring over ~20 million years.

In Kansas, four pulses of kimberlite magmatism occurred at ~110-106 Ma, ~103-99 Ma, and ~88-85 Ma, with reheating at ~67-64 Ma. Initial ⁸⁷Sr/⁸⁶Sr from two phlogopite samples are 0.7045 (~106.6 Ma) and 0.750 (~88.4 Ma), the latter likely being xenocrystic (Blackburn et al., 2008). In Arkansas, two distinct magma sources are revealed by isotopic compositions of olivine lamproites vs. other alkalic rocks, including carbonatite, ijolite, lamprophyres, tephrite, jacupirangite, phonolite, and syenite. The older lamproites (~106-108 Ma) have initial εNd values of -10 to -19, and younger samples (~102.7-88.6 Ma) have εNd of +1.85 to +4.27. In Arkansas, lithospheric components dominated early melts, producing lamproitic magmas, and later melts were asthenospheric (Duke et al., 2014).

In LA and MS, drill core samples from the Monroe Uplift include phonolite (~81, 75, 67.2 Ma, and 64 Ma), alkali basalt (~80.3, 75, and 67 Ma), and lamprophyre (~80.4, 76.7 Ma). A phonolite from Jackson Dome, MS, is ~64 Ma (Baksi, 1997). In the Balcones Igneous Province of Texas, two age groups were identified—an earlier mafic alkalic pulse (84.1-81.5 Ma) and a later felsic alkalic pulse (78.8 -76.2 Ma). Rock types are nephelinites, alkali basalts, phonolites, and phonotephrites. The older mafic alkalic rocks have high εNd values of +5.1-8.2, and initial ⁸⁷Sr/⁸⁶Sr values of 0.7032-0.705 (Griffin et al., 2010).

Cretaceous magmatism is limited to the region west of a N40ºW trend extending from MS-LA to KS. A hot-spot track has been proposed for this igneous activity, but the widespread, diffuse magmatism, as well as prolonged activity, points to another process. One model that may account for the N40ºW edge is that of focusing upwelling mantle along the edge of an oceanic slab in the transition zone or lower mantle. A window or slab gap to the west could have allowed for mantle upwelling to directly access the base of the lithosphere to the west of the slab edge.