U-PB GEOCHRONOLOGY OF THE MAGNET COVE IGNEOUS COMPLEX, AR: NEW AGE CONSTRAINTS FROM PEROVSKITE, KIMZEYITE AND SCHORLOMITE GARNET

The Magnet Cove Igneous Complex (MCIC) in Central Arkansas is one of the many expressions of Cretaceous magmatism in the North American Midcontinent. It forms a concentric structure comprised of a suite of intrusive and extrusive silica-undersaturated rocks. The core of the complex contains masses of carbonatite within ijolite, surrounded by intermediate rings of trachyte and phonolite, and an outer ring of nepheline syenites. The nepheline syenites were interpreted to have been emplaced first, with the carbonatites in the center of the complex being the youngest (1). The MCIC is the host of zirconium- and titanium-rich minerals, such as perovskite, titanite, and schorlomite garnet, and is the type locality of kimzeyite garnet. This study examines the U-Pb geochronology of perovskite and kimzeyite from the Kimzey Calcite Pit, a quarry located in a central carbonatite of the MCIC. We use laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to refine the age of emplacement for the MCIC and examine the suitability of these Ti- and Zr-rich minerals as U-Pb reference materials. Prior U-Pb results from the MCIC indicate at least a two-phase magmatic emplacement history between ca. 102 Ma and 98 Ma (2). However, the precise age of the carbonatite and therefore a test for the outside-in emplacement hypothesis had yet to be determined.

The age range determined for MCIC emplacement is contemporaneous with other Si-undersaturated magmatism (e.g. Sulphur Springs, AR, 3), including lamproite and kimberlite magmatism in Kansas (e.g. 4). It is somewhat younger than the Oka carbonatite in Canada (5). The new geochronological data provide improved constraints on possible causes for this enigmatic period of magmatic activity, with edge-driven mantle convection (6) as the preferred model to explain the wide temporal and regional spread of Cretaceous, mantle-derived magmatism.

(1) Erickson, R.L., Blade, L.V., 1963. USGS Prof. Paper, 425, 95 pp.; (2) Möller, A., Pell, D., 2023. GSA Abstracts, 55, p. 394821; (3) Zartman, R.E., Howard, J.M., 1987. Geol. Soc. Am. Spec. Paper 215, p. 235-240; (4) Lupini, I. et al., 2024. International Kimberlite Conference: Extended Abstracts, v. 12; (5) Chen, W., Simonetti, A., 2014. Minerals, 4(2), p. 437-476; (6) Kjarsgaard, B.A. et al., 2017. Gcubed, 18(7), p. 2727-2747.