Geochemistry of the Wyoming Craton from eclogite and pyroxenite xenoliths from the Aultman, Maxwell, and Schaffer kimberlites at the Colorado-Wyoming State Line

The Wyoming Craton is one of the best-preserved Archaean fragments of Laurentia. For billions of years it's consisted of a thick Archaean root surrounded by Proterozoic suture zones. In this study we characterise the geochemistry of 20 eclogitic xenoliths and, for five samples determine the ferric-ferrous ratios and oxygen fugacities, from the Aultman, Maxwell, and Schaffer kimberlite pipes from the Colorado–Wyoming State Line kimberlite district with EPMA, LA-ICP-MS, and Synchrotron Mössbauer spectroscopy. We measured major and trace element concentrations in garnet and clinopyroxene, which were used to reconstruct bulk-rock compositions and determine the pressure-temperature conditions along the craton's 38 mW/m 2 geotherm. We subdivide xenoliths into gabbroic eclogites (with an ancient oceanic gabbro protolith), pyroxenitic (of various origins), high-Mg eclogites (derived from oceanic basalts), and one high-Ca eclogite (likely with an oceanic crust protolith). Major element compositions show a range between predominantly pyrope- and almandine-rich garnet. Clinopyroxenes range from Al–Na-rich diopside (in the more Mg-rich xenoliths) to omphacite in gabbroic eclogites, with a wide range of jadeite (18–54 mol%). All xenoliths formed at depths between ∼90 and 170 km. No strong metasomatic overprint is observed in any samples except for one, which exhibits strong LREE enrichment and high Sr N /Y N and Ce N /Yb N ratios (elements normalised to MORB). Approximately half of the xenoliths show evidence for partial melting manifested as LREE depletion. Garnets are depleted in ferric iron (0.02–0.04 Fe 3+ /Fe (tot) ) except for one sample (0.08 Fe 3+ /Fe (tot) ). Clinopyroxenes contain 0.17–0.41 Fe 3+ /Fe (tot) . Calculated oxygen fugacities show a range between −2.3 and − 5.6 relative to the fayalite–magnetite–quartz redox buffer, indicating a wide spread of redox conditions in the protolith, and possibly a small degree of metasomatic re-enrichment instrumental in increasing f O 2 during residence in the cratonic mantle. In agreement with previous work on xenoliths from nearby kimberlites, our study suggests that the Colorado–Wyoming State Line xenoliths sample ancient, depleted, and partially melted oceanic crust. This interpretation accords with a model in which proto-continental lithosphere was underthrust and wedged during Proterozoic collisional tectonics related to Yavapai terrane accretion along the Cheyenne Belt, thereby stacking Proterozoic (eclogitic) slabs against Archaean (peridotitic) lithospheric fragments.