NEW (U-TH)/HE AGE CONSTRAINTS ON THE EMPLACEMENT OF KIMBERLITE PIPES IN NORTH-EASTERN KANSAS

Located above the mid-continent rift system in Riley and Marshall Counties, Kansas, twelve kimberlite pipes have been identified by geological and geophysical exploration. Over the past decades, multiple studies have speculated about the timing and origin of ultra-potassic magmatism in the mid-continent. Limited geochronological work yielded inconclusive estimates for the timing of these intrusives and thus have made the critical evaluation of the different geodynamic models difficult. K/Ar work on kimberlitic phlogopite yielded irreproducible ages ranging from ~112 to 380 Ma and are likely affected by excess Ar. The seemingly most reliable ages for the kimberlite pipes are from early fission track work suggesting an age of 115 Ma for the Bala pipe and 123 Ma for Stockdale. This study presents new age constraints for the age of kimberlite emplacement using (U-Th)/He dating on apatite and several silicate and oxide phases from the Stockdale, Bala, Tuttle and Leonardville kimberlites. Our results demonstrate that (U-Th)/He thermochronometry can be successfully employed to provide reliable age constraints on kimberlite emplacement into the upper crust. Furthermore, (U-Th)/He dating of different xeno- and phenocryst phases from kimberlites can provide a reliable alternative to other dating techniques that depend on the presence of rare or unaltered mineral phases. For our case study, we determined single- and multi-grain (n=4) laser apatite (U-Th)/He ages on surface and core samples from several kimberlites. Reproducible (U-Th)/He ages range in age from 67.3+/-4.5 Ma for the Stockdale pipe to 61.2+/-8 Ma for the Leonardville pipe, suggesting a latest Cretaceous to earliest Tertiary time of emplacement. Titantite (U-Th)/He ages from the Tuttle kimberlite give ages as young as ~65 Ma, but commonly contain zircon inclusions yielding irreproducible old ages. Apatite (U-Th)/He analyses from sandstone core samples from a borehole in the vicinity of the kimberlites yield Jurassic ages indicating that there was no regional thermal event that could have reset kimberlitic apatite after emplacement. Ongoing efforts to evaluate the validity of the apatite (U-Th)/He ages focus on Sm/Nd dating of kimberlitic mantle xenoliths and on the continued development of (U-Th)/He dating applied to other U- and Th-bearing phases.