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ABSTRACT

This field guide covers a half-day tour of three localities representative of the Cretaceous kimberlites of Riley and Marshall Counties in northeastern Kansas: Stockdale, Bala, and Winkler Crater. The kimberlites are porphyritic with phenocrysts of olivine and smaller amounts of pyroxene, magnetite, ilmenite, pyrope garnet, and/or phlogopite. Most of the kimberlites include abundant shallow crustal xenoliths of altered shale or sedimentary carbonate; igneous and metamorphic xenoliths from the lower crust and upper mantle occur but are less common. The kimberlites can be divided into two types, micaceous and non-micaceous. Examples of both kimberlite types will be examined during the field trip—Stockdale and Winkler Crater are micaceous kimberlites, whereas Bala is non-micaceous. The existence of kimberlites in the midcontinent of North America has long been controversial. In Kansas, they erupted through Proterozoic (1.8–1.6 Ga) basement that was accreted to the southeastern margin of North America, raising the question of how the conditions for kimberlitic melt genesis can be met so far from thick Archean craton (e.g., thick lithosphere >200 km where volatile and trace element–enriched melts can form and accumulate). However, recent geophysical data indicate that the kimberlites were generated within the boundary between thick Proterozoic lithosphere to the east, where depth to the lithosphere-asthenosphere boundary (LAB) is at least 200 km, and regions to the west where the LAB is <190 km as thick cratonic lithosphere thins into Mesozoic extensional basins. New ground gravity data show that the kimberlite pipes are oriented in a NE–SW direction, parallel to the Precambrian Midcontinent rift, which occurs in the subsurface west of the kimberlite field. The data indicate that the kimberlites took advantage of preexisting structural weaknesses that were reactivated in the Cretaceous.

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