U-Pb GEOCHRONOLOGY OF GARNET, PEROVSKITE, APATITE AND CALCITE FROM THE MAGNET COVE IGNEOUS COMPLEX, AR

The Magnet Cove Igneous Complex (MCIC), AR, has close temporal relationships with other Cretaceous alkaline complexes, kimberlites and lamproites in North America, proposed to be mantle-derived melts by craton edge-driven convection. It is known for its rare mineral species, and has previously been mined for different commodities and recently become the focus of critical minerals evaluations. U-Pb analyses has been conducted by LA-ICP-MS to establish a more refined crystallization history than previously established, and to provide an initial test of some minerals suitability as U-Pb reference materials.

Perovskite from the carbonatites in the MCIC occurs as mm to cm-sized euhedral, inclusion-rich grains. Laser spot traverses across a ca. 3 mm wide crystal yields discordant results that cluster tightly, evidence for limited zoning of U and Pb. Cogenetic calcite from the carbonatite was conducted to confirm the upper, non-radiogenic intercept. The anchored perovskite U-Pb age is ca. 97 Ma, consistent with previous results on the MCIC obtained by other methods. Apatite from carbonatite and ijolite yield moderately radiogenic results and ages of about 98 Ma. However, all these results carry uncertainties of 4 to 7 %. Schorlomite garnet in occurs as euhedral and subhedral crystals in fine-grained ijolites, and as very strongly zoned overgrowths on oxides/perovskite biotite-ijolite and in pseudoleucite syenite. Regardless of rock type, texture and major element zoning, all garnet samples yield precise ages in a narrow range of 98-103 Ma, defined by limited dispersion at the lower, radiogenic end of Tera-Wasserburg discordia arrays. Uncertainties are 1-2 %, at the limit of the LA-ICP-MS technique and calibration material uncertainty. The zoned overgrowths are the older age group at 101-103, while the euhedral schorlomites from the fine-grained ijolites are younger, ca. 98 Ma.

All samples appear suitable as secondary references by in-situ dating, whereas only the euhedral material would lend itself to easy mineral separation for grain mounts. In summary, schorlomite garnet yield precise U-Pb ages within a narrow timeframe at the earliest Late Cretaceous. Together with the (less precise) perovskite and apatite ages the dataset does not support an outside-in younging, nested, assembly of the MCIC.