AGE CONSTRAINTS ON THE TIMING OF KIMBERLITE AND LAMPROITE MAGMATISM IN KANSAS BASED ON U-PB GEOCHRONOLOGY OF PEROVSKITE

At least four geodynamic models have been proposed to explain the occurrence of kimberlites and lamproites in Kansas, e.g., mantle plumes, Farallon slab subduction, edge-driven convection (EDC), and association with Large Low Shear Velocity Provinces (LLSVPs). Each model has implications for the timing and duration of magmatism, but assessing the relationship between the kimberlites and lamproites, as well as the models for their formation, hase been hampered by limited data. Kimberlite emplacement ages were previously reported as 110-85 Ma, with a younger ‘reheating’ event at 65 Ma [1]. Lamproite ages of ~ 90 Ma [2] were limited to three K-Ar analyses of phlogopite. This study presents in-situ U-Pb perovskite geochronology for the Hills Pond and Rose Dome lamproites (SE KS), along with new data for the Tuttle and Bala kimberlites (NE KS), to better constrain their emplacement ages. Perovskite was chosen for analysis as it is a primary mineral common to both rock types that has survived the pervasive alteration.

Tuttle kimberlite perovskites yield a U-Pb age of 105.6 ± 1.9 Ma, consistent with the 106.6 ± 1.0 Ma Rb-Sr phl-cpx age of [1]. In contrast, Bala kimberlite perovskites yield two age populations at 59.6 ± 7.4 Ma and 81.7 ± 8.6 Ma. The younger age is within uncertainty of the 64.3 ± 7.5 Ma (U-Th)/He apatite age from [1], whereas the older perovskite population is > 20 m.y. younger than their 103.0 ± 7.5 Ma age based on (U-Th)/He in magnetite. However, our in-situ analytical approach shows that the younger age is not due to hydrothermal reheating. Instead, it indicates recurrence of kimberlitic magmatism within the same pipe for at least 22 m.y.

Hills Pond and Rose Dome lamproite perovskites yield ages of 86.9 ± 3.5 Ma and 97.1 ± 12.6 Ma, respectively, both within error of the recalculated phlogopite K-Ar age range of 92-93 ± 5 Ma for Hills Pond and 90 ± 4 Ma for Rose Dome [2]. The results indicate kimberlite and lamproite magmatism overlap in time, but the kimberlite magmatism started earlier and lasted longer. The significant age span of the kimberlites and lamproites is most consistent with an EDC model for their origin and may also explain the presence of other Mid-Cretaceous alkaline magmatism across the region.

[1] Blackburn et al., 2008, EPSL 275: 111-120; [2] Zartman et al., 1967, Am J Sci 265:848-870