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 Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered a key area for understanding the evolution of the northern margin of the West African craton during the Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the West African craton under blueschist metamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanian or Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted by greenschist conditions.

Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M 2 metamorphic episode. Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.