In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900 °C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700 °C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab. In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes

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

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

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900 °C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700 °C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab. In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes