ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT Two models have been proposed to explain continental crust generation in accretionary orogens. One model suggests that accretionary orogens are formed by the successive collision of juvenile arcs. The second model invokes tectonic switching, which is the repeated cycles of slab rollback and extensional backarc basin formation followed by basin collapse caused by collision, shallow subduction, and/or increased convergence rate. The northern Colorado Front Range, specifically in and around the Big Thompson, Rist, and Poudre Canyons, offers excellent exposures of Paleoproterozoic rocks to test which accretionary model best explains crust generation for a portion of the Yavapai Province. In this contribution we have two goals: The first is to provide a field-trip guide that augments Mahan et al.’s (2013) field guide, which uses many stops that have become inaccessible or have changed because of catastrophic flooding that occurred in September 2013. This more current guide focuses on a variety of mostly Paleoproterozoic rocks within what some call the Poudre Basin. These rocks include clastic metasedimentary rocks, amphibolite, the Big Thompson Canyon tonalite suite, the northern Front Range granodiorite, granitic pegmatites, and Mesoproterozoic Silver Plume granite. The second goal is to present and synthesize new and existing geochemistry, geochronology, and isotopic data, and then discuss the origins, age, deformation, and metamorphism of these rocks in the context of the proposed tectonic models. These data were synthesized into the following tectonic model for the Poudre Basin. At ca. 1780 Ma, the juvenile Green Mountain arc, located today along the Colorado-Wyoming border, formed and extended shortly thereafter during slab rollback, resulting in the extensional backarc Poudre basin between the diverging arc fragments. Sedimentation within the basin began at inception and continued to ca. 1735 Ma when basin rocks were intruded by the Big Thompson Canyon tonalite suite and the northern Front Range granodiorite, all of which were subsequently metamorphosed and deformed at ca. 1725 Ma. Felsic magmatism and deformation within the basin were perhaps driven by the northward shallow subduction of an oceanic plateau or seamount. This suggests that following accretion of the Green Mountain Arc, tectonic switching explains formation and collapse of the Poudre Basin and creation of some of northern Colorado’s crust.

ABSTRACT Analysis of detrital zircon U-Pb ages from the Phanerozoic sedimentary record of central Colorado reveals variability in sediment transport pathways across the middle of the North American continent during the last 500 m.y. that reflects the tectonic and paleogeographic evolution of the region. In total, we present 2222 detrital zircon U-Pb ages from 18 samples collected from a vertical transect in the vicinity of Colorado’s southern Front Range. Of these, 1792 analyses from 13 samples are published herein for the first time. Detrital zircon U-Pb age distributions display a considerable degree of variability that we interpret to reflect derivation from (1) local sediment sources along the southern Front Range or other areas within the Yavapai-Mazatzal Provinces, or (2) distant sediment sources (hundreds to thousands of kilometers), including northern, eastern, or southwestern Laurentia. Local sediment sources dominated during the Cambrian marine transgression onto the North American craton and during local mountain building associated with the formation of the Ancestral and modern Rocky Mountains. Distant sediment sources characterize the remaining ~75% of geologic time and reflect transcontinental sediment transport from the Appalachian or western Cordilleran orogenies. Sediment transport mechanisms to central Colorado are variable and include alluvial, fluvial, marine, and eolian processes, the latter including windblown volcanic ash from the distant mid-Cretaceous Cordilleran arc. Our results highlight the importance of active mountain building and developing topography in controlling sediment dispersal patterns. For example, locally derived sediment is predominantly associated with generation of topography during uplift of the Ancestral and modern Rocky Mountains, whereas sediment derived from distant sources reflects the migrating locus of orogenesis from the Appalachian orogen in the east to western Cordilleran orogenic belts in the west. Alternating episodes of local and distant sediment sources are suggestive of local-to-distant provenance cyclicity, with cycle boundaries occurring at fundamental transitions in sediment transport patterns. Thus, identifying provenance cycles in sedimentary successions can provide insight into variability in drainage networks, which in turn reflects tectonic or other exogenic forcing mechanisms in sediment routing systems.

ABSTRACT The Permian marks a time of substantial climatic and tectonic changes in the late Paleozoic. Gondwanan glaciation collapsed after its earliest Permian acme, aridification affected the equatorial region, and monsoonal conditions commenced and intensified. In western equatorial Pangea, deformation associated with the Ancestral Rocky Mountains continued, while the asynchronous collision between Laurentia and Gondwana produced the Central Pangean Mountains, including the Appalachian-Ouachita-Marathon orogens bordering eastern and southern Laurentia, completing the final stages of Pangean assembly. Permian red beds of the southern midcontinent archive an especially rich record of the Permian of western equatorial Pangea. Depositional patterns and detrital-zircon provenance from Permian strata in Kansas and Oklahoma preserve tectonic and climatic histories in this archive. Although these strata have long been assumed to record marginal-marine (e.g., deltaic, tidal) and fluvial deposition, recent and ongoing detailed facies analyses indicate a predominance of eolian-transported siliciclastic material ultimately trapped in systems that ranged from eolian (loess and eolian sand) to ephemerally wet (e.g., mud flat, wadi) in a vast sink for mud to fine-grained sand. Analyses of U-Pb isotopes of zircons for 22 samples from Lower to Upper Permian strata indicate a significant shift in provenance reflected in a reduction of Yavapai-Mazatzal and Neoproterozoic sources and increases in Grenvillian and Paleozoic sources. Lower Permian (Cisuralian) strata exhibit nearly subequal proportions of Grenvillian, Neoproterozoic, and Yavapai-Mazatzal grains, whereas primarily Grenvillian and secondarily early Paleozoic grains predominate in Guadalupian and Lopingian strata. This shift records diminishment of Ancestral Rocky Mountains (western) sources and growing predominance of sources to the south and southeast. These tectonic changes operated in concert with the growing influence of monsoonal circulation, which strengthened through Permian time. This resulted in a growing predominance of material sourced from uplifts to the south and southeast, but carried to the midcontinent by easterlies, southeasterlies, and westerlies toward the ultimate sink of the southern midcontinent.

We used laser ablation–inductively coupled plasma–mass spectrometry to determine the U-Pb ages for 1206 detrital zircons from 15 samples of the Lemhi subbasin, upper Belt Supergroup, in southwest Montana and east-central Idaho. We recognize two main detrital-zircon provenance groups. The first is found in the Swauger and overlying formations. It contains a unimodal 1740–1710 Ma zircon population that we infer was derived from the “Big White” arc, an accretionary magmatic arc to the south of the Belt Basin, with an estimated volume of 1.26 million km 3 —a huge feature on a global scale. The ɛ Hf(i) values for magmatic 1740–1710 Ma zircons from the Lawson Creek Formation are +8–0, suggesting that they were derived from more juvenile melts than most other Lemhi subbasin strata, which have values as evolved as −7 and may have been derived from an arc built on Proterozoic or Archean crust in the Mojave Province. Since paleocurrents in cross-bedded sandstones indicate northward flow, the proximate source terrane for this sand was to the south. The second provenance group is that of the Missoula Group (and Cambrian strata recycled from the Missoula Group), with significant numbers of 1780–1750 Ma grains and more than 15% Archean grains. This provenance group is thought to represent mixing of Yavapai Province, Mojave Province, and Archean Wyoming Province sources. Both of these provenance groups differ from the basal Belt Prichard Formation, and strata of the Trampas and Yankee Joe Basins of Arizona and New Mexico, which contain a major population of 1.61–1.50 Ga non–North American grains. The 12 youngest grains from the several Swauger Formation samples suggest the formation is younger than 1429 Ma. The three youngest grains from Apple Creek Formation diamictite suggest the rock is younger than 1390 Ma. This makes the Apple Creek diamictite the youngest part of Belt Supergroup strata south of the Canadian border. Though the Big White magmatic arc was produced before 1.7 Ga, the sediment may have been recycled several times before being deposited as locally feldspathic sandstone in the Lemhi subbasin depositional site 300 m.y. later. Because the detrital-zircon provenance does not change from Idaho east to Montana, our data do not support the existence of a major Great Divide megashear separating the Lemhi subbasin from the Belt Basin. In southwest Montana, unfossiliferous sandstones of Cambrian age contain the same detrital-zircon assemblages as the Swauger Formation and Missoula Group, suggesting reworking of a local Belt Supergroup source.