ABSTRACT The McDowell Mountains of central Arizona contain one of the best preserved and thickest sections of pre–Apache Group Mesoproterozoic rocks in the state. The oldest formation, ash-flow tuff, has an age of ca. 1650 Ma and is overlain by a quartzite-shale-quartzite triplet. These two units are interpreted to correlate with the Red Rock Group and Mazatzal Group, respectively. Significantly, these formations are overlain by ~6 km of ash-flow tuffs and minor psammite and metabasalt. Preliminary U-Pb analysis of igneous zircons in the youngest ash-flow tuff (the Taliesin tuff) provides an age of 1546 ± 11 Ma. The oldest granite pluton, the Antiguo granite, intrudes the Taliesin tuff and has a U-Pb age of 1525.6 ± 9.5 Ma. If these preliminary results are accurate, they record volcanism, burial and metamorphism, and plutonism that occurred within the Proterozoic ‘magmatic gap’ (between ca. 1.6 and 1.45 Ga) and may be the only voluminous igneous rocks within this age range yet recognized in North America. Additionally, the stratigraphic section was deformed into km-scale folds during an episode of contraction that formed two northwest-vergent thrust faults in the northern part of the mountain range. Both thrusts were subsequently intruded by mafic hypabyssal sills and then buried to greenschist-facies depths and intruded by the coarse-grained Carefree granite at 1425 Ma.

ABSTRACT 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.