DETERMINING THE U-PB EMPLACEMENT AGE OF LAMPROITES IN SOUTHEASTERN KANSAS

In southeastern Kansas, twelve kimberlites and two lamproites constitute the southern portion of a ~5000 km stretch of Mid Cretaceous age alkaline magmatism in central N. America. Three main geodynamic models have been proposed to explain this alkaline igneous corridor, including association with a mantle plume [1,2], subduction of the Farallon plate [3], and plate edge driven convection cells [4,5]. Blackburn et al. [6] reported emplacement ages for the KS kimberlites of 110 to 103 Ma, with a reheating event recorded at 65 Ma. Zartman et al. [7] reported K-Ar emplacement ages for the KS lamproites of 91-88 ± 4-5 Ma. However, they were unable to verify these ages using Rb-Sr geochronology due to alteration. Given the uncertainties on the existing age data, it is unclear whether the lamproites were emplaced near the start of kimberlite volcanism or toward its end, or if lamproite emplacement was a harbinger of the reheating event observed in the kimberlites. To answer these questions, we are conducting U-Pb perovskite geochronology on the SE KS lamproites, as well as on two kimberlite localities, Tuttle and Bala, which also contain perovskite. Perovskite is one of the few primary phases that occurs in both the KS lamproites and kimberlites AND has survived the extensive alteration observed [5]. By focusing on perovskite, we can make direct comparisons between the ages of the two rock types, avoiding complications that could arise by using mineral and isotope systems with different closure temperatures. The new temporal constraints will contribute to more robust evaluation of the different models proposed for formation of the Mid Cretaceous kimberlites and lamproites in KS and possibly further afield across central N. America. Current analysis suggests plate edge driven convection [4] is the most plausible formation model.

[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.