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The Guerrero terrane of central and western Mexico comprises volcanic and sedimentary strata of mainly Mesozoic age, with geochemical signatures indicating formation in juvenile back-arc to slightly more evolved arc environments. At least 60 volcanogenic massive sulfide (VMS) deposits have been discovered thus far within the Guerrero terrane. They comprise two main belts: the Coastal belt mainly comprising deposits in the Zihuataneo subterrane (including the Cuale-Bramador district), and the Central belt, which includes deposits in the Teloloapan subterrane (Campo Morado, Tlanilpa-Azulaquez, Tizapa, Rey de Plata, etc.), the Guanajuato subterrane (El Gordo, Las Gavalinas), and the Zacatecas subterrane (San Nicolas and El Salvador deposits). The age of host rocks for the VMS mineralization in the various subterranes was poorly constrained prior to this study. Eleven new U-Pb zircon ages for volcanic host rocks from the Campo Morado, Tlanilpa-Azulaquez, Leon-Guanajuato, and San Nicolas-El Salvador VMS districts, together with recently determined U-Pb ages from the Cuale area, demonstrate that VMS deposits in the Guerrero terrane range from latest Middle Jurassic to early Early Cretaceous (Callovian to Valanginian) in age. The oldest ages were obtained from the Cuale district (total age range of 157.4 ±4.1 Ma for three units) and the youngest are from the Tlanilpa-Azulaquez district (total age range of 139.7 ± 2.5 Ma for two samples). Volcanogenic massive sulfide deposits in the Central belt resemble those of the bimodal-siliciclastic deposit type, whereas those in the Coastal belt are more similar to the bimodal-felsic type. Published geochemical and radiogenic isotope data from the various subterranes suggest that VMS deposits in the Coastal belt and most of those in the Central belt are hosted within juvenile to slightly evolved arc settings. The San Nicolas and El Salvador deposits of the Zacatecas subterrane are the only deposits that unequivocally formed in a back-arc setting. Our new U-Pb dating results, together with existing stratigraphic, geochemical, and radiogenic isotope data from throughout the Guerrero terrane, are consistent with the Guerrero terrane having developed as a west-facing continental margin arc that was built on mainly oceanic crust (and overlying Early Mesozoic siliciclastic fans shed from the east) along the western margin of nuclear Mexico. Slab rollback in the Middle Jurassic led to the development of a "Rocas Verdes-type" continental back-arc inboard of the arc, which is now preserved as the Arperos basin in the central and southern Guerrero terrane and the host rocks for the San Nicolas and El Salvador deposits farther to the north in southeastern Zacatecas State. We have determined Pb isotope compositions of sulfides from VMS deposits and several other styles of mineralization from throughout the Guerrero terrane. A compilation of these and previously published data shows that isotopic compositions from the various VMS deposits fall in a relatively confined field that overlaps the average upper crustal growth curve. This suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and/or from the host volcanic rocks themselves (or subvolcanic intrusive equivalents) that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption. Lead isotope compositions from the Francisco I Madera deposit in Zacatecas State indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely Tertiary in age. [ABSTRACT FROM AUTHOR]

Changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen. These disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time. If the orogen is considered at the large scale, the differences in the way in which the crust responds during accretion can be explained by large-scale strain partitioning within the same overall tectonic environment. The westernmost Canadian Cordillera records the transition from Late Cretaceous dextral strike-slip faulting to near- orthogonal compression along the orogenic axis. We postulate that the transition between strike-slip-dominated to compression-dominated tectonics represents a Late Cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny. This had a dramatic effect on subsequent Tertiary orogen-scale extension. We propose that plate readjustments in the Tertiary led to orogen-perpendicular collapse in portions of the orogen, facilitated by decoupling between the middle and lower crusts along thermally weakened layers. In contrast, localized, orogen-parallel extension occurred in other portions of the orogen, along kinematically linked, large dextral strike-slip faults where the upper crust remained coupled to the middle and lower crust. New data indicate that partitioning of strain occurs across very large regions within any orogenic system, and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes. It becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime. [ABSTRACT FROM AUTHOR]

New U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104-80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75-55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75-65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65-55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58-56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59-51 Ma occurred before a notable magmatic lull from ca. 51-44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44-37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35-26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic. [ABSTRACT FROM AUTHOR]

Onset and termination of Eocene felsic volcanism in the Chilcotin Plateau of central British Columbia is constrained between 54.6 and 46.6 Ma by 33 new U-Pb and 40Ar/39Ar isotopic age determinations. Dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the Ootsa Lake Group. The resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate, with no spatial migration of the volcanic activity. Rhyolitic compositions are oldest; and are overlain by dacitic rocks with varied phenocrysts assemblages. In many parts of the Chilcotin Plateau, the Eocene stratigraphy is capped by distinctive vitreous black dacite lavas, which are contemporaneous with andesitic lavas of the Endako Group in the Nechako Plateau to the north. Crystallization ages from Ootsa Lake Group rocks of the Chilcotin Plateau overlap age determinations from correlative rocks of the Nechako Plateau and southern BC. Collectively, this geochronological dataset supports previous suggestions of a voluminous Early Eocene-aged (∼55-46 Ma) period of volcanism in the Intermontane Belt. The abrupt initiation of volcanism, as well as the wide extent, thickness, and compositions that characterize Eocene volcanic rocks may be explained by cessation of subduction and formation of a slab gap beneath British Columbia in the Early Eocene. [ABSTRACT FROM AUTHOR]

Abstract: Intrusive suites containing anorthosite–mangerite–charnockite–granite (AMCG) are a characteristic feature of Mesoproterozoic magmatic activity in the Grenville Province of eastern North America. In this study, the U–Pb and 40Ar/39Ar geochronology of two AMCG suites (Saint-Urbain and Havre-Saint-Pierre), and associated rutile-bearing hemo-ilmenite deposits, within the allochthonous polycyclic belt of the Grenville Province in Québec are used to constrain their crystallization ages and cooling histories, and to assess their tectonic setting. Both AMCG suites, separated by 700km along the St. Lawrence River, have similar crystallization ages (ca. 1050–1060Ma) and durations of magmatism (10–15 million years). The 450km2 Saint-Urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite (1046.2±3.1Ma) compared with anorthosite to the north and south (1055.0±2.4Ma and 1053.6±2.6Ma, respectively). The age of a deformed oxide–apatite gabbronorite (1057.4±1.5Ma) along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites, whereas the oldest intrusion in the Saint-Urbain area is the regionally extensive Saint-Anne du Nord orthopyroxene granodiorite (1060.8±2.8Ma). The large 11,000km2 Havre-Saint-Pierre anorthosite suite comprises a number of intrusive phases, including the 2700km2 Lac Allard anorthosite, which contains the giant Lac Tio ilmenite deposit and Big Island ilmenitite dyke. U–Pb zircon ages of three anorthosites from the Lac Allard intrusion, including the host to the Lac Tio deposit, overlap within error (1061.6±3.0Ma, 1060.5±1.9Ma, 1057.4±8.4Ma) and the crosscutting Big Island dyke yields an age of 1052.9±6.5Ma. These results indicate that the Havre-Saint-Pierre anorthosite should be divided into an older geon 11 part (ca. 1129Ma) and a younger geon 10 part (ca. 1060Ma). Average cooling rates of 3–4°C/million years were estimated for the two AMCG suites by combining crystallization ages (U–Pb zircon) and cooling ages (U–Pb rutile, 40Ar/39Ar biotite/plagioclase) and reflect emplacement of the intrusions into a hot, long-duration orogen. Emplacement of these two AMCG suites at ca. 1050–1060Ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional Ottawan orogenic phase of the Grenville orogeny. The results of this study provide strong support for proposals that link AMCG magmatism associated with the Grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle. [Copyright &y& Elsevier]