U-PB EMPLACEMENT AGES OF KANSAS LAMPROITES AND KIMBERLITES

Three tectonic models have been proposed to explain the formation of kimberlites and lamproites in Kansas including association with mantle plumes [1,2], subduction of the Farallon plate [3], and edge-driven mantle-convection cells [4,5]. Each of these models has implications for the timing and duration of this alkaline volcanism. Emplacement ages for kimberlites from NE Kansas were previously reported by [6] as 110-85 Ma, with a younger reheating event recorded at ~65 Ma. K-Ar geochronology of 3 phlogopite separates from the lamproites of SE Kansas suggest they are younger than the kimberlites, with an age of ~90 Ma [7]. However, the limited amount of data and their uncertainty make it difficult to assess the relationship between the two magmatic events. To better constrain the timing of emplacement, we conducted in situ U-Pb perovskite geochronology on the Hills Pond (KS) lamproite, along with the Tuttle and Bala kimberlites. Perovskite was chosen for analysis because it is one of the few primary phases that occurs in both rock types AND has survived the pervasive alteration observed [5]. Our data for Bala are consistent with previous analyses [6}; results for Tuttle are more precise and suggest that kimberlite emplacement started at least 10 m.y. earlier than previously thought. The different ²⁰⁷Pb/²⁰⁶ Pb compositions observed for Bala and Tuttle, as well as the two age arrays obtained for Bala spanning ~ 30 Ma suggest that the KS kimberlites represent multiple batches of kimberlitic melt generation from different sources. Our new geochronology data for the Hills Pond lamproite suggest that it was emplaced near the end of kimberlite volcanism but before the reheating event. The significant age span of the kimberlites and lamproites is most consistent with an edge-driven convection model for their formation [4, 5].

[1] Heaman et al., 2004, Lithos, v. 76, p. 377; [2] Chu et al., 2013, Nat. Geosci., v. 6, p. 963; [3] Currie and Beaumont, 2011, EPSL, v. 303, p. 59; [4] Kjarsgaard et al., 2017, G3, v. 18; [5] Kempton et al., 2019, GSA Field Guide 52, p. 37; [6] Blackburn, et al., 2008, EPSL, v. 275, p. 111; [7] Zartman et al., 1967, AJS, v. 265, p. 848.