PRECISE LA-ICP-MS U-PB SCHORLOMITE GARNET AND PEROVSKITE AGES: NEW CONSTRAINTS ON EMPLACEMENT OF THE MAGNET COVE IGNEOUS COMPLEX, AR

The Magnet Cove Intrusive Complex (MCIC), AR, is one of the enigmatic mid-Cretaceous magmatic complexes in North America for which very different mantle melting models have been proposed. The ring complex includes jacupirangite, ijolite, phonolite, nepheline syenite and carbonatite units. It is a renowned host of unique or rare minerals. Potentially suitable for U-Pb geochronology are perovskite, Ti-rich (schorlomite) and Zr-rich (kimzeyite) garnet, as well as titanite. Earlier geochronological studies using K/Ar, Ar/Ar, Rb-Sr and fission track dating established an age of 94-100 Ma for intrusion and fast cooling, with individual uncertainties of 5 Ma or more on individual dates.

We investigate the potential for U-Pb dating of the mentioned Ca-Ti-Zr minerals and their suitability as reference materials (RM) motivated by the expansion of laser ablation U-Pb dating of such non-traditional minerals.

First results on perovskite from the carbonatite give clustered, but discordant U-Pb results. The data do not form a long linear array, not suitable as a primary RM, but they have potential as a secondary, validation RM. A suitable comPb composition can be established from analysis of cogenetic calcite or from published whole rock data.

Schorlomite garnets from four samples give precise (1-2%) U-Pb dates in two groups, 97-100 Ma and 100.5-103.5 Ma. The perovskite result lies in between these groups. The results do not bear out the bulls-eye outside-in younging pattern proposed earlier. Future work will include more rock types and minerals, and will establish feasibility to obtain larger, clean material quantities for use as RMs.