Abstract The Choquequirao Formation is a >3 km-thick amphibolite-grade succession that outcrops in the Central Andes of southern Peru. To constrain its age and tectonostratigraphic setting, detrital zircon and metamorphic zircon, titanite, and rutile U–Pb isotopic analyses were conducted. Mantle-derived c. 640 Ma detrital zircons constrain the maximum age of the lower part of the succession and 550–490 Ma metamorphic zircon domains constrain its minimum age. The absence of early Paleozoic detrital zircons suggests that deposition predated early Paleozoic orogenesis in southwestern Gondwana. The close similarity of detrital zircon age spectra to those from sediments deposited on the Arequipa basement suggests that the Choquequirao Formation was deposited on the Arequipa Terrane. Metamorphic titanite dates are highly overdispersed, yet they overlap with c. 460 Ma peak metamorphism recorded by metamorphic zircon. Pb-loss pathways displayed by metamorphic titanite have a lower intercept that overlaps with c. 325 Ma metamorphic rutile, which corresponds to Hercynian orogenesis. A poorly constrained upper intercept of c. 510 Ma may correspond to Pampean and/or early Famatinian orogenesis. We interpret the Cryogenian–Ediacaran Choquequirao Formation as having been deposited during the opening of the Palaeo-Iapetus (Puncoviscana–Clymene) Ocean between eastern Arequipa and southern Kalahari prior to the subsequent collision with southwestern Amazonia during the Pampean Orogeny.

ABSTRACT The Montana metasedimentary terrane (MMT) forms the NW margin of the Wyoming Province in present coordinates. The MMT preserves a multistage Paleoproterozoic tectonic history that clarifies the position of the Wyoming craton during assembly and breakup of the Precambrian Kenorland supercontinent and the subsequent assembly of Laurentia’s Precambrian basement. In SW Montana, burial, metamorphism, deformation, and partial melting attributed to orogeny were superimposed on Archean quartzofeldspathic orthogneisses and paragneisses at ca. 2.55 and ca. 2.45 Ga during the Tendoy and Beaverhead orogenies, respectively. Subsequent stability was disrupted at 2.06 Ga, when probable rift-related mafic dikes and sills intruded the older gneisses. The MMT was profoundly reworked by tectonism again as a consequence of the ca. 1.8–1.7 Ga Big Sky orogeny, during which juvenile metasupracrustal suites characteristic of an arc (the Little Belt arc) and back-arc basin collapsed against the Wyoming craton continental margin. The northern margin of the Wyoming craton occupied an upper-plate position south of a south-dipping subduction zone at that time. Lithostratigraphic correlations link the southeastern Wyoming and southern Superior cratons at ca. 2.45 Ga with the Wyoming craton joined to the Kenorland supercontinent in an inverted position relative to present coordinates. This places the MMT along an open supercontinental margin, in a position permissive of collision or accretion and orogeny during a time when other parts of Kenorland were experiencing mafic volcanism and incipient rifting. The ca. 2.45 Ga Beaverhead orogeny in the MMT was most likely the consequence of collision with one of the Rae family of cratons, which share a history of tectonism at this time. The Beaverhead collision enveloped the Wyoming craton in a larger continental landmass and led to the 2.45–2.06 Ga period of tectonic quiescence in the MMT. Breakup of Kenorland occurred ca. 2.2–2.0 Ga. In the MMT, this is expressed by the 2.06 Ga mafic dikes and sills that crosscut older gneisses. The Wyoming craton would have been an island continent within the Manikewan Ocean after rifting from Kenorland on one side and from the Rae family craton on the MMT side. Subduction beneath the MMT in the Wyoming craton started no later than 1.87 Ga and was active until 1.79 Ga. This opened a back-arc basin and created the Little Belt arc to the north of the craton, contributed to the demise of the Manikewan Ocean, and culminated in collision along the Big Sky orogen starting ca. 1.78 Ga. Collision across the Trans-Hudson orogen in Canada occurred during a slightly earlier period. Thus, docking of the Wyoming craton reflects the final stage in the closure of the Manikewan Ocean and the amalgamation of the Archean cratons of Laurentia.

ABSTRACT The Grenville Province is the largest and most structurally complex orogenic belt that formed in the Mesoproterozoic, representing the amalgamation of the supercontinent Rodinia. The Mattawa domain, part of the Algonquin domain in Ontario, Canada, hosts some of the most deeply buried metamorphic rocks of this orogen. This high-grade metamorphic terrane consists of large areas of felsic orthogneiss and kilometer-sized mafic boudins. Dark-colored metabasite cropping out near Mattawa, Ontario, Canada, contains relict mineral assemblages and decompression textures indicative of high-pressure eclogite. Garnet porphyroblasts surrounded by plagioclase coronae are hosted in fine-grained symplectic intergrowths of diopside + plagioclase ± amphibole, which compositionally reintegrated into an omphacite composition (Na/[Na + Ca] ~0.5). Phase equilibria analysis revealed an eclogite-facies peak pressure of ~2 GPa at 850 °C. This temperature is consistent with the zirconium contents of rutile inclusions in garnet (up to 1725 ppm Zr). Despite high-temperature metamorphism, garnet growth zonation is partially preserved. Diffusion modeling of representative garnet profiles yielded a time scale of <0.1 m.y. for decompression from the peak pressure to ~1.2 GPa, suggesting an average exhumation rate of several decimeters per year. Decompression was followed by fast cooling within hundreds of thousands of years. Such fast decompression and cooling rates contrast with the protracted metamorphic evolution recorded in most of the Grenville orogen and likely resulted from local extrusion of lower-crustal material in response to localized extension during the early Ottawan stage. Since very few examples of Precambrian short-duration regional metamorphism have been documented, the fast decompression documented in this study provides valuable constraints for the geodynamic transition to a modern plate-tectonic regime.

ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

ABSTRACT Nearly all models of Earth’s oxygenation converge on the premise that the first notable rise of atmospheric oxygen occurred slightly above the Archean-Proterozoic boundary, with the second notable rise occurring just below the Proterozoic-Phanerozoic boundary. Plate tectonic–driven secular changes found above the Archean-Proterozoic boundary are thought to have been partly or wholly responsible for the initial rise in atmospheric O 2 in the Great Oxidation Event; however, the role of plate tectonics in oxygen levels thereafter is not well defined. Modern plate tectonics undoubtedly play a role in regulating atmospheric O 2 levels. Mountain building, for example, promotes high erosion rates, nutrient delivery to oceans, and efficient biogeochemical cycling of carbon, resulting in the net burial of organic carbon—thought to be the primary regulator of atmospheric O 2 levels on geological time scales. The trajectory of atmospheric O 2 and oceanic redox conditions in the Proterozoic Eon, representing almost 2 b.y. of geological history, shows a dynamic history with global trends that indicate overall high-low-high O 2 levels throughout the Proterozoic Eon, with low-oxygen conditions established by ca. 2.0–1.8 Ga. This contravenes the tenet that major orogenic events (e.g., the Himalaya-scale Trans-Hudson orogen and other coeval orogens that formed the supercontinent Nuna) should yield higher O 2 levels, not lower. The contrast of higher O 2 early in the Paleoproterozoic with lower O 2 later in the Paleoproterozoic is particularly striking, and mechanisms that might have caused this secular change remain unclear. This contribution explores feedbacks related to the tectonic evolution associated with the building of proto-Laurentia and Earth’s first supercontinent, Nuna, and how this impacted the trajectory of atmospheric O 2 in the latest Paleoproterozoic Era.