Fate of the lower lithosphere during shallow-angle subduction: The Laramide example

Abstract

Continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert were dislodged, transported eastward during Laramide shallow-angle subduction, and attached to the base of the Colorado Plateau transition zone (central Arizona, USA) and further inboard. We identify here two late Oligocene xenolith localities from the transition zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. Geochemical, isotopic, and thermobarometric data from garnet clinopyroxenite, the dominant xenolith type at both studied localities, strongly suggest a continental arc residue (“arclogite”) rather than a lower plate subduction (“eclogite”) origin. Zircon grains extracted from these nodules yield a bimodal age distribution with peaks at ca. 75 and 150 Ma, overlapping ages of continental arc magmas emplaced into the Mojave Desert (the southern California batholith) and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks from SW Arizona, with age peaks at ca. 60 and 170 Ma, do not provide as close a match. In light of these results, we suggest that a mafic keel to the southern California batholith: (1) formed in two discrete (Late Jurassic and Late Cretaceous) pulses; (2) was transported along the Moho ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (3) was affixed to the base of the crust in central Arizona. Titanite U-Pb and garnet Sm-Nd ages spanning ca. 60–30 Ma suggest that displaced arclogite remained at >600 °C for tens of millions of years following its dispersal and until entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in ca. 15 Ma and younger volcanic host rocks and the presence of a vertical high-seismicvelocity anomaly beneath the western Colorado Plateau suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since the early Miocene.