IN SITU U-PB GEOCHRONOLOGY OF PEROVSKITE, TITANITE AND SCHORLOMITE, KIMZEYITE GARNETS FROM MAGNET COVE, AR: NEW AGE CONSTRAINTS ON ALKALINE MAGMATISM AND SUITABILITY AS REFERENCE MATERIALS

The Magnet Cove Igneous Complex (MCIC) in Arkansas consists of multiple ring dikes of silica undersaturated rocks including nepheline syenite, ijolite, phonolite, and carbonatite [1]. The emplacement of the rocks is hypothesized to be from outside-in, with the outermost syenite ring emplaced first, and the carbonatite last [1]. There is an abundance of titanium- and zirconium-rich minerals including perovskite, titanite, and schorlomite and kimzeyite garnet. This study uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) after petrography and scanning electron microprobe (SEM) analysis to date emplacement of the MCIC and test if its minerals are suitable as U-Pb reference material. Results will also contribute to the discussion on the cause of Cretaceous magmatism in the midcontinent US.

Previous studies [2,3] reported ages of 94-100 Ma using Ar/Ar, K-Ar, and Rb-Sr and fission track dates for the MCIC, with uncertainties of 5 m.y. and higher. A recent study obtained a 96.4 ± 1.8 Ma U-Pb age by laser ablation on schorlomite [4]. U-Pb dating should yield more robust dates than other radiometric methods due its high closure temperatures and resilience to alteration. This should provide accurate data for the datable Ti and Zr minerals and their crystallization. Because these minerals exist in certain rock types of the MCIC, accurate ages may provide a more detailed order of emplacement.

Perovskite occurs abundantly in large, euhedral grains throughout the MCIC, but most frequently in the carbonatite. Schorlomite and kimzeyite both occur as black to dark brown phenocrysts in the carbonatite, while schorlomite is also found in the surrounding ijolite ring intrusion. Our first laser ablation U-Pb data on these perovskites is younger than previous data, with a poorly constrained and unusually low 207Pb/206Pb. Anchoring 207Pb/206Pb at 0.6 yields an age consistent with the U-Pb schorlomite age of [4]. U-Pb dating schorlomite, kimzeyite and titanite by LA-ICP-MS is ongoing and will test their use as potential reference materials in future studies.

[1] Erickson & Blade 1963, USGS Prof. Paper v. 425, pp. 94; [2] Zartman et al. 1967, Am. J. Sci. v. 265; p. 848-870 [3] Eby & Vasconcelos 2009, J. Geol. v. 117, p. 615-626; [4] Yang et al., 2019, JAAS, v. 33, p. 231-239.