PETROGRAPHY, GEOCHEMISTRY, AND GEOTHERMOMETRY OF XENOLITHS AND XENOCRYSTS FROM THE FANCY CREEK KIMBERLITE, RILEY COUNTY, KANSAS

Kimberlites and their entrained xenoliths give us a rare glimpse into otherwise inaccessible regions of the lithosphere. This study provides the first geochemical, petrographic and geothermometric analysis of lower crustal xenoliths from the Fancy Creek kimberlite (Riley County, KS), as well as new major element mineral compositions of garnet xenocrysts from the same locality.

The granulite xenoliths studied are 3-4 cm in size and consist of 35-40% plagioclase, 15-25% clinopyroxene and 10-15% orthopyroxene. The samples are virtually free of secondary alteration or decompression melting textures, but plagioclase shows strongly developed deformation twinning and both pyroxenes show evidence of exsolution. An estimate of the bulk rock compositions for the samples, based on major element mineral chemistry and modal mineral proportions determined by point counting, indicates that the rocks are mafic, with high Mg#s of 0.78 - 0.85. A plot of SiO₂/Al₂O₃ vs. Mg#s suggests that the granulites originated as cumulates that crystallized from basaltic parental magmas. Such rocks reported elsewhere globally have been attributed to magmatic underplating at the base of the crust. The two-pyroxene geothermometer of Brey and Kohler (1990) yields values of 728°C to 770°C. Assuming a 25°C/km geothermal gradient, these temperatures suggest the Fancy Creek granulites are derived from a depth of ~30 km, i.e. near the base of the crust for this region of Kansas, where depth to the MOHO is 36 km. Petrographically similar granulites from the southern Basin and Range have been associated with Cenozoic magmatism and extension.

The garnet xenocrysts collected from the kimberlite are all pyrope in composition. Plots of Cr₂O₃ vs. CaO show that the garnet xenocrysts entrained by the Fancy Creek kimberlite are derived from peridotitic and eclogitic mantle rocks, although these had largely been disaggregated. This suggested that the kimberlite originated within the mantle deep enough for garnet to be a stable phase. Given that the Riley County kimberlites are believed to be Cretaceous in age (85-110 Ma), the magmatic event that gave rise to these granulites must at least be older than the age of entrainment. An area for future research is to explore whether these rocks are associated with extension as part of the mid-continent rift system at ~1.1 Ga.