No abstract

No 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

Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño-Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U-Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry was used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from -4.7 to -7.9, whole rock εNd(i) and 87Sr/86Sr(i) values range from -9.4 to -11.9 and 0.7064 to 0.7094, and quartz δ18OVSMOW values range from 6.8‰ to 9.4‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels and is supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison to other Laramide-age anatectic granites in southeast Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences which reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early-Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies.

A growing body of evidence suggests that continental arc lower crust and underlying mantle wedge assemblages native to the Mojave Desert (i.e., the southern California batholith) were displaced eastward during Laramide shallow-angle subduction, and reattached to the base of the Colorado Plateau Transition Zone (central Arizona) and farther inboard. On this field trip, we highlight two xenolith localities from the Transition Zone (Camp Creek and Chino Valley) that likely contain remnants of the missing Mojave lithosphere. At these localities, nodules of garnet clinopyroxenite, the dominant xenolith type at both studied localities, yield low jadeite components in clinopyroxene, chemically homogeneous “type-B” garnet, and peak conditions of equilibration at 600–900 °C and 9–28 kbar. These relations 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 southern California batholith pluton ages, and suggesting a consanguineous relationship. In contrast, Mesozoic and early Cenozoic igneous rocks native to 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 Transition Zone xenoliths: (1) began forming in Late Jurassic time as a mafic keel to continental arc magmas emplaced into the Mojave Desert and associated with eastward subduction of the Farallon plate; (2) experienced a second ca. 80–70 Ma pulse of growth associated with increased magmatism in the southern California batholith; (3) were transported ~500 km eastward along the leading edge of the shallowly subducting Farallon plate; and (4) were reaffixed to the base of the crust at the new location, in central Arizona. Cenozoic zircon U-Pb, garnet-whole rock Sm-Nd, and titanite U-Pb ages suggest that displaced arclogite remained at elevated temperature (>700 °C) for 10s of m.y., following its dispersal, and until late Oligocene entrainment in host latite. The lack of arclogite and abundance of spinel peridotite xenoliths in Miocene and younger mafic volcanic host rocks (such as those at the San Carlos xenolith locality), and the presence of seismically fast and vertically dipping features beneath the western Colorado Plateau, suggest that arclogite has been foundering into the mantle and being replaced by upwelling asthenosphere since Miocene time.