THE MYSTERY OF KIMBERLITES IN KANSAS

The occurrence of kimberlites is typically associated with Archean cratons, but Cretaceous-age kimberlites and lamproites are present in eastern Kansas. They erupted through basement rocks consisting predominantly of juvenile and continental margin volcanic arcs that were accreted to the SE margin of North America during the Proterozoic (1.8-1.6 Ga), i.e. far from the thick (i.e. >200 km) Archean craton typically thought to provide the necessary mantle conditions that give rise to volatile and trace element-enriched melt formation. Petrological models [1] for the origin of Kansas kimberlites are consistent with small degree partial melts of carbonate-bearing garnet lherzolite or harzburgite but with no explanation for how the conditions for melt generation are met. Geodynamic models have linked them to mantle plumes or hotspots [2] or large-scale extension inboard of the continental margin [3]. Currie and Beaumont [4] hypothesized the existence of a lithosphere-scale structure called the Cretaceous “kimberlite corridor”, which extends from northern Canada to the Gulf of Mexico and includes the Kansas kimberlites; they relate this feature to flat-slab subduction of the Farallon Plate. In contrast, [5] recognized that most Cretaceous kimberlites occur adjacent to strongly attenuated lithosphere at the edge of the North American craton; they attribute kimberlite genesis to edge-driven decompression melting of an OIB-type deep-mantle source. The Kansas kimberlites occur within this transition zone, but they are also ~65 km west of, and sub-parallel to, the axis of the late Precambrian Midcontinent Rift System (MCR). A deep lithospheric connection with the MCR has never been explored. This presentation will review the current petrological, geochemical and geophysical data for kimberlites of northeast Kansas in light of the various models for kimberlite petrogenesis. Our analysis suggests the edge-driven convection model of [5] is most likely, but with an important role for the adjacent MCR. [1] Cullers et al., 2012, http://www.kgs.ku.edu/Publications/OFR/2012/OFR12_17/index.html; [2] Chu et al., 2013, Nature Geosci, v. 6, p. 963; [3] Duke et al., 2014, EPSL, v. 403, p. 1014; [4] Currie and Beaumont, 2011, EPSL, v. 303, p. 59; [5] Kjarsgaard et al., 2017, G3, v. 18.