Xenolith constraints on the thermal history of the mantle below the Colorado Plateau

A rootless rockies—Support and lithospheric structure of the Colorado Rocky Mountains inferred from CREST and TA seismic data

“…However, the analysis of Neogene volcanism indicates that CRM basalts were primarily derived from a lithospheric source [Beard and Johnson, 1997] while the southern Plateau has been more strongly modified by asthenospheric melts [Crow et al, 2011], which implies a thicker CRM lithosphere. Additionally, we note that both the Precambrian diamondiferous diatremes in the Front Range [Lester and Farmer, 1998;McCallum et al, 1979] and xenoliths from 140 km depth erupted in the Navajo Volcanic Field (28-19 Ma) [Riter and Smith, 1996] indicate the presence of thick lithosphere in the past and it remains unclear, given the lack of major extension in the CRM, what mechanisms would be responsible for thinning the lithosphere.…”

Insights into the evolution of the uppermost continental mantle from xenolith localities on and near the Colorado Plateau and regional comparisons

“…Olivine is the dominant mineral in these xenoliths, and its composition is insensitive to subsolidus processes in spinel peridotite; percent forsterite decreases during melt extraction and may increase in subsequent melt interactions. Most data for spinel peridotite xenoliths hosted by young basaltic rocks on the Colorado Plateau province were for the Grand Canyon field [Best, 1974;Alibert, 1994;Riter and Smith, 1996;Smith and Riter, 1997;Riter, 1999]. Mineral analyses were also available for xenoliths hosted by ultramarie breccia and minette of the 25 Ma Navajo field [Smith and Levy, 1976;Smith, 1979 • Nine of the 10 analyzed xenoliths are Cr-diopside lherzolite and harzburgite, one of which is composite lherzolite plus green-pyroxene pyroxenite.…”

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“…Many follow Bird (1988) in invoking a basal shear stress, yet with the rheology used, this results in removal of continental mantle lithosphere from the western U.S.; however, such pre-Cenozoic continental mantle survived the Laramide orogeny over much of the region (Livaccari and Perry, 1993;Perry and Livaccari, 1994;Griffi n et al, 2004), and the continued presence of mantle that remained at lithospheric temperatures in the Sierra Nevada Saleeby, 1996, 1998) and Colorado Plateau Riter and Smith, 1996) indicates that this mantle remained as lithosphere and could not have been removed and returned, as suggested by Bird (1994). The presence of this lithosphere limits the degree to which crust could have been transported from west to east in the Laramide orogeny.…”

“…Similarly, although the modern seismological estimate of a 120-150-km-thick lithosphere from surface waves (West et al, 2004) for the Colorado Plateau agrees with xenolith-based lithosphere thicknesses (Griffi n et al, 2004), these could differ greatly from lithospheric thicknesses at 75 Ma: plateau xenoliths are all associated with post-Laramide magmatic activity. These xenoliths often record cooling through the Laramide (e.g., Helmstaedt and Schulze, 1991;Smith et al, 2004;Riter and Smith, 1996), so it is possible that modern lithosphere thicknesses are greater than those prior to the Laramide. Given these uncertainties, we posit that at 75 Ma Wyoming had an ~250-km-thick lithosphere that was cold and neutrally buoyant and that the lithosphere under the Colorado Plateau was thinner and probably not suffi ciently chemically distinct to be neutrally buoyant, and so less likely than Wyoming lithosphere to interfere with asthenospheric counterfl ow.…”