242-14 Exsolution Textures in Garnet Xenocrysts from the Colorado Plateau: Applications for Reconstructing Tectonic Histories Across Lithologies

Abstract:

Inclusions in garnets from diatremes give insight into the petrologic conditions of garnet formation and subsequent exhumation. We present analysis of a suite of garnet xenocrysts from the Garnet Ridge ultramafic diatreme in the Navajo Volcanic Field of the Colorado Plateau. The garnets contain combinations of rutile, ilmenite, clinopyroxene, Na-Ca clinoamphibole, and quartz as lamellar to acicular microinclusions (LAMs) oriented parallel to <111>_garnet. We use large electron backscatter diffraction (EBSD) datasets and lattice matching calculations to show that the oxide and silicate LAMs represent exsolved precipitates from high-Ti and majoritic garnets. The exsolution textures indicate that the garnets experienced slow cooling and/or decompression that reduced solubility of high-temperature and/or high-pressure chemical substitutions at sufficient temperatures for long-range diffusive transport of LAM-forming components. Electron dispersive spectroscopy (EDS) analyses of major and trace element chemistry show that the garnets can be assigned to both mantle and crustal sources in the classification scheme of Schulze (2003) for garnet xenocrysts from kimberlites. The garnets with exsolved silicates have major element compositions consistent with majoritic garnets from natural samples and laboratory experiments on metasediment, eclogite, and ultramafic bulk compositions formed at 5-10 GPa (Thomson et al. 2021).

We contextualize our findings within the geologic history of the lower crust and mantle of the Colorado Plateau, particularly Navajo Volcanic Field xenoliths attributed to the geologically recent materials of the subducted Farallon Plate that passed beneath the Colorado Plateau lithosphere in the last 100 Myr. The decomposed majoritic garnets in our sample suite that are comparable to those known from eclogites and metasediments point to a role for deep subduction of surficial materials in assembling the lithosphere of the (proto-)Colorado Plateau.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-20928

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