Tectonic development of the Colorado Plateau Transition Zone, central Arizona: Insights from lower lithosphere xenoliths and volcanic host rocks

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.

The role of crustal anatexis in porphyry copper ore formation during flat-slab subduction: Insights from the Laramide Belt, SW USA

“…1.0-2.8 GPa and 600-850°C 26 , contain copper-bearing sulphides 25 and have been dated by U-Pb zircon at ca. 150 Ma and 75 Ma 26,27,28 , yet their origins remain unclear. It is possible they originate from cumulate 'arclogites' derived from juvenile mantle-derived magmas 25,26,27,28 .…”

Evolution of Microstructural Heterogeneity in the Deep Arc Lithosphere During Delamination

“…These vents also host granulite and websterite xenoliths (Esperança et al, 1988;Smith et al, 1994), indicating that the lithosphere in this region is heterogenous, though this study focuses on the arclogites. The eruption age for the Sullivan Buttes host latite has been estimated to be 25 Ma through K-Ar geochronology (Krieger et al, 1971) and, while no ages are available for Camp Creek, it has been estimated to have erupted around 26.5 Ma (A. D. Chapman et al, 2019). These vents are separated by 115 km.…”

“…These vents are separated by 115 km. There are no systematic differences in the arclogites between vents, other than a lower degree of host magma infiltration in the Sullivan Buttes xenoliths (A. D. Chapman et al., 2019), thus we discuss samples from each vent as a whole group.…”

Plate convergence, consumption, collision, coupling, capture, and formation of mantle waves—Linkages to global orogenesis and epeirogeny

“…The exhumation episodes correlate with major tectonic unconformities and with key deformation events at adjacent plate boundaries, suggesting a causative link despite the large distances (up to 1500 km) separating the closest plate boundaries from the zones of exhumation at the time of each episode (Holford et al, 2009). 2) emplace prisms of wet sediment to great depth (Grove et al, 2008), where migmatization is a common process; (3) induce subduction of oceanic flat slabs (Chapman, 2016;Chapman et al, 2019); and, in locations where terranes have been isolated from the main plate mass, (4) force deep underthrusting of crust to a depth of 100 km or more, commonly recorded by formation of eclogite (Brueckner, 2006, and references therein).…”

“…Tectonic map of southwestern North America showing plate setting during the development of the Gulf of California(Oskin and Stock, 2003). Baja California, which is separated from western North America by a regional structural basin floored by oceanic crust, is postulated to represent the type of tectonic assemblage susceptible to deep emplacement under a continental plate margin, which in this case would be western North America, upon arrival of an east-moving mantle "wave" (cf Chapman et al, 2019)…”