EXPLORING POSSIBLE CRUSTAL XENOLITH LOCALITIES IN THE SOUTHERN ROCKY MOUNTAIN REGION: HENRY MOUNTAINS, UTAH AND ELK RANGE, COLORADO

Assessing the Earth’s deep continental crust and mantle is difficult outside of geophysical means because they are rarely exposed at the surface. Xenoliths, although also somewhat rare and localized geographically, can provide direct samples from the deep lithosphere, making them important for understanding Earth’s composition, properties, and history. In the broader southern Rocky Mountain region, there are only a few well-documented deep crustal xenolith localities, including the Devonian State Line Kimberlite District along the Wyoming-Colorado border and the 30-20 Ma Navajo Volcanic Field centered on the Four Corners of the Colorado Plateau. This project explores two more possible crustal xenolith localities in the region, the ca. 30 Ma Henry Mountains plagioclase-hornblende-porphyry laccolith complex in southern Utah (Mt. Hillers) and a granodiorite dike of probable ca. 30 Ma age in the Elk Range near Crested Butte, Colorado. Preliminary observations suggest some of the inclusions are co-magmatic enclaves while others are true xenoliths. The Mt. Hillers' xenoliths appear to be dominantly mafic with hornblende-plagioclase assemblages and varying amounts of garnet, orthopyroxene, clinopyroxene, rutile, quartz and accessory apatite and zircon. They are also most commonly foliated, with fabric intensities ranging from weak to strong, gneissic to penetrative, and some mylonitic. Our initial textural observations indicate that some samples have an early mineral assemblage that includes orthopyroxene, clinopyroxene, and/or garnet with amphibole reaction rims suggesting a history involving hydration. The Elk Range inclusions may be more commonly co-magmatic enclaves, but at least one contains abundant sillimanite suggesting a xenolithic origin that is likely metasedimentary. Preliminary micro-X-ray fluorescence data supports the mineralogical and textural observations above and ongoing work is aimed at gathering a more detailed understanding of the samples using optical, chemical, and textural analyses, as well as thermobarometry.