Olivine thermometry and source constraints for mantle fragments in the Navajo Volcanic Field, Colorado Plateau, southwest United States: Implications for the mantle wedge

Abstract

The evolution of the mantle wedge below the Colorado Plateau during low‐angle subduction has been investigated by analysis of fragments from the Navajo Volcanic Field (NVF), most from serpentinized ultramafic microbreccia (SUM) diatremes. In most SUM‐hosted olivine, concentrations of Al and V are < 1 ppm, and Cr, Ca, and Na concentrations also are unusually low: most temperatures from three olivine thermometers (Al, Cr, and V‐based) are in the range 530°C to 650°C. The temperatures are consistent with the presence of chlorite in some inclusions, and they support the utility of olivine thermometry for diverse mineral assemblages in cool peridotite. Most pressures calculated for discrete diopside grains in SUM correspond to depths in the range 80 to 120 km. Diopside is relatively high in Li (~3.5 ppm), and two of five diopside grains have high Sr/Nd. SUM diatreme sources are inferred to be serpentine‐rich mélange tectonically eroded from the forearc, transported above the Farallon slab, and incorporated into the lithosphere of the Plateau. Garnet peridotite xenoliths from minettes of the NVF record deeper depths in the range 120 to 150 km. These garnet peridotites also may be from forearc mantle emplaced during Farallon subduction. Calculated temperatures preclude the possibility that asthenosphere was in contact with that lithosphere at or near 150 km depth for tens of m.y. before NVF formation. Structures observed in seismic images of mantle to depths of 150 km below the central Colorado Plateau may be related to Farallon subduction, not inherited from Proterozoic lithosphere.

Heat-flow data in the Four Corners area suggest Neogene crustal warming resulting from partial lithosphere replacement in the Colorado Plateau interior, southwest USA

“…These processes may have both enhanced density instability and produced a lowered viscous friction layer by magmatic heating ; both processes are favorable to initiate mantle lithosphere fl oundering and displacement above ~150 km. However, delaminating a mantle lithosphere with the LAB at 150 km would be contradictory to studies indicating that pre-Neogene depths of ~140-150 km are not in contact with the asthenosphere (Smith, 2013). In the present model, pre-Neogene mantle below ~140-150 km, in place or placed by subduction (which may suggest different material than above), remains cooler than the LAB at ~208 km depth.…”

“…This occurs because bringing the asthenosphere to the Moho results in an excess of heat. The result would be the same if an asthenosphere at 150 km was brought up to the Moho, concurring with mineralogical studies that indicate that asthenosphere temperatures are not in contact with depths of 140-150 km (Smith, 2013).…”

“…Relatively cool temperatures below 140-150 km are suggested from the above-mentioned studies, indicating that depths of 140-150 km are not in contact with asthenosphere and friction from a shallow subducting plate is not observed (Smith, 2013;Riter and Smith, 1996). The Colorado Plateau lithosphere appears to have had a complex history.…”

“…The Colorado Plateau lithosphere appears to have had a complex history. A relatively cool lithosphere may have been in place below the potential Proterozoic upper mantle suggested by Roden et al (1990), or slices of eroded cool forearc mantle may have been placed under the Colorado Plateau by Farallon plate subduction before 30 Ma (Smith, 2013). Some Colorado Plateau asthenosphere may have remained (Jones et al, 2011), but would have had to be deeper than 140-150 km because the lithosphere at 150 km was not in contact with asthenosphere.…”

“…First, the pre-Neogene upper mantle is relatively cool to at least 140-150 km; it is not warmed by friction from a subducting slab, and it is not in contact with the asthenosphere at these depths (Riter and Smith, 1996;Smith, 2013). Second, extensive studies indicate that for many early Proterozoic areas the LAB is at 200-220 km depth (1300 °C; Artemieva and Mooney, 2001); note that the Four Corners area is Paleoproterozoic in age (Karlstrom et al, 2004).…”