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The internal metamorphic-plutonic zone of the middle Paleoproterozoic Wopmay orogen, northwestern Canadian Shield, exposes an oblique section through a prograde Buchan-type metamorphic sequence. New petrographic, petrologic, and geochronologic analyses of pelite and quartz arenite samples from the eastern margin of the internal zone quantify the evolving conditions and timing of metamorphism and enable the discriminant evaluation of competing tectonic models. Study of fabric development and mineral growth relationships indicates that prograde metamorphism occurred late within a craton-verging, crustal-shortening ("Calderian") deformation event. Phase equilibria modeling was used to calculate pressure-temperature (P-T) path segments and determine peak conditions from andalusite-muscovite grade at ~3.3 kbar and 575 °C to sillimanite-K-feldspar ± granitic leucosome grade at ~4.8-6.2 kbar and 670-745 °C. In situ monazite geochronology indicates that thermal peak metamorphic conditions were reached between 1878.6 ± 6.5 Ma and 1872 ± 9 Ma. Ages of detrital zircon within quartz arenite across the width of the metamorphic sequence document a diagnostic easterly Slave craton-Thelon tectonic zone derivation that links the metasedimentary rocks, including those previously considered to be "exotic" in a proposed tectonic klippe model for the internal zone, to the adjacent craton. Integration of these results with observations on the relatively low state of strain and the bracketing of a common (single) metamorphic field gradient for all garnet-bearing mineral zones confirm that the latter have preserved their prograde isogradic continuity from time of formation, and that the leading edge of a tectonic klippe consisting of rocks exotic to the Coronation margin does not traverse the study area. [ABSTRACT FROM AUTHOR]

Paleoproterozoic metasedimentary successions of the northwestern Canadian Shield provide records of tectonic events, but the definition of depositional ages has proved elusive. Although previously poorly understood, the Montresor belt of western Nunavut yields new insight into the 2.2-1.8 Ga time window. On the basis of U-Pb analyses of detrital zircon in sedimentary rocks and igneous zircon in sills, we conclude that arenite of the lower Montresor group was deposited between 2.194 and 2.045 Ga, and arkose of the upper Montresor group after 1.924 Ga, adding constraints on the Rae cover sequence. The lower Montresor arenite yielded an older group (3.05-2.58 Ga) and a younger, more tightly constrained group (2.194 ± 0.014 Ga). Four of six zircon grains analyzed from a gabbro sill within the lower Montresor have discordant 207Pb/206Pb ages (2.71, 2.66, 2.53, and 2.39 Ga) and are considered to be inherited, whereas two grains provide an age of 2045 ± 13 Ma, interpreted to date crystallization and providing a minimum age for the lower Montresor package. Upper Montresor arkose contains detrital zircon with probability density peaks at 2.55-2.25 and 2.1-1.92 Ga, together with scattered older grains (3.8-2.65 Ga). The youngest grain yields an age of 1924 ± 6 Ma, establishing a maximum age for sandstone deposition. Provenance is inferred to have been from the west, where igneous sources of 2.5-2.3 Ga (Queen Maud block) and 2.03-1.89 Ga (Thelon orogen) are known. Collectively, the new ages suggest a minimum 120 million year gap between deposition of the pre-2045 ± 13 Ma lower and post-1924 ± 6 Ma upper parts of the Montresor group. Similar age constraints may apply to other parts of the Rae cover sequence. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Wopmay orogen is modeled as a simple arc-continent collision involving the Archean Slave craton and an enigmatic continental arc terrane termed the Hottah terrane. Hottah is a composite terrane that includes a juvenile 2.6-2.1 Ga basement and a younger 1.93-1.90 Ga continental arc. The extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin, and eventual role in the development of the orogen. Multi-isotopic (U-Pb, Hf, O) analyses of detrital zircon from rocks of the Hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics. The data confirm that ca 2.6-2.1 Ga crust is an important component of the terrane but Hf isotopic data indicate mostly non-juvenile compositions consistent with recycled Neoarchean crust sources. Ages of detrital zircon are distinct from those expected of the Slave craton but are similar to adjacent 2.0-1.90 Ga Paleoproterozoic terranes of northwestern Laurentia. The similarity to terranes such as the Ksituan/Buffalo Head and Taltson-Thelon magmatic zones, south and east of the Wopmay Orogen indicate that Hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the Nuna continent between 2.0 and 1.9 Ga, and was rifted prior to the collision with the Slave margin that produced the Calderian orogeny. [ABSTRACT FROM AUTHOR]

Highlights: [•] Zircon from lower crustal xenoliths record complex crust formation histories. [•] Cores record crust formation at ca. 3.5–2.6Ga and ca. 2.0–1.9Ga thermal reworking. [•] Metamorphic rims record regional lower crustal reworking at ca. 1.75Ga. [•] Lower crustal reworking likely resulted from 1.75Ga magma underplating. [•] Intracrystalline Pb-disturbance related to 1.75Ga granulite-facies metamorphism. [Copyright &y& Elsevier]

New U-Pb zircon dates from gneissic xenoliths in an Early Cambrian lamprophyre point to late Mesoproterozoic metamorphism and magmatism in the crust beneath northern Yukon. The data indicate a previously unrecognized thermal event in Yukon and extend the recognition of widely spaced 1.3-1.0 Ga igneous, metamorphic and tectonic events along the western margin of Laurentia. Together, these events demonstrate that the north-western margin of Laurentia was thermally active during a period of traditionally inferred tectonic quiescence. The tectonic setting, intensity and general nature of this late Mesoproterozoic tectonothermal activity are poorly constrained, but their recognition is key to expanding our understanding of the Proterozoic geology of North America. The recognition of this event places new constraints on the palaeotectonic reconstructions of the supercontinent Rodinia. [ABSTRACT FROM AUTHOR]

The Boothia mainland region of the north-central Rae domain is underlain by remnants of a Neoarchean volcano sedimentary sequence dismembered by two regionally extensive Neoarchean high-potassium granitoid suites with rare occurrences of a structurally interleaved, Paleoproterozoic sedimentary cover sequence. The granitoids and their gneissic equivalents are dominated by variably deformed and metamorphosed I-type, metaluminous, polyphase, commonly porphyritic to augen, biotite ±hornblende monzogranite, and subordinate granodiorite, with rare tonalite. New geochronological results, the first for this area, demonstrate that the widespread Neoarchean granitoid plutonism is dominantly 2.61-2.59 Ga, with a less prominent 2.66 Ga plutonic event. The age of zircon recrystallization suggests that ca. 2.60 Ga Archean metamorphism and fabric development (S1) affected the 2.66 Ga plutons prior to or contemporaneously with intrusion of the voluminous ca. 2.6 Ga suite. εNd(t) for the ca. 2.61-2.59 Ga suite range from 1.4 to -1.9, overlapping with the ca. 2.66 Ga suite that range from 1.4 to 1.5. The Nd isotopic data, coupled with the presence of inherited ca. 2.65, 2.70, and 2.85-2.90 Ga zircon, suggests recycling of older, Neoarchean to Mesoarchean crust in the formation of these suites. Metaplutonic rocks preserve Paleoproterozoic deformation (F4 and F5) and amphibolite-facies metamorphism, sporadically recorded in zircon rims that formed at 1.81 Ga. This event strongly reoriented the Neoarchean fabrics in metaplutonic rocks, generally without the development of a new coaxial Paleoproterozoic fabric, and we attribute this strain and metamorphism to the Hudsonian orogeny. La région terrestre Boothia du domaine Rae centre-nord repose sur des vestiges d'une séquence volcano-sédimentaire datant du Néoarchéen; cette séquence est démembrée par deux suites granitoïdes d'échelle régionale, à teneur élevée en potassium et possédant de rares occurrences d'une séquence structurellement intercalée d'une couverture sédimentaire datant du Paléoprotérozoïque. Les granitoïdes et leurs équivalents gneissiques sont dominés par un monzogranite à déformation variable, métamorphisé, de type I, métalumineux, polyphasé, souvent porphyrique à oeillé et à biotite hornblende avec une granodiorite subordonnée et une rare tonalite. De nouveaux résultats géochronologiques, les premiers pour ce secteur, démontrent que le plutonisme granitoïde néoarchéen étendu date surtout de 2,61 à 2,59 Ga, avec un événement plutonique moins important vers 2,66 Ga. L'âge de la recristallisation du zircon suggère que, vers 2,60 Ga, le métamorphisme archéen et le développement de la fabrique (S1) aient affecté les plutons âgés de 2,66 Ga avant ou en même temps que l'intrusion de la volumineuse suite vers 2,6 Ga. Les valeurs εNd(t) pour la suite datant de ~ 2,61 à 2,59 Ga varient de 1,4 à -1,9, chevauchant la suite datant ~2,66 Ga, dont les valeurs εNd(t) varient de 1,4 à 1,5. Les données d'isotopes Nd, jumelées à la présence de zircons hérités dont les âges sont ~2,65, 2,70 et 2,85-2,90 Ga, suggèrent un recyclage de l'ancienne croûte néoarchéenne à mésoarchéenne pour la formation de ces suites. Des roches métaplutoniques ont préservé la déformation paléoprotérozoïque (F4 et F5) ainsi que le métamorphisme au faciès des amphibolites, retrouvé sporadiquement dans les bordures des zircons qui se sont formées à 1,81 Ga. Cet événement a grandement réorienté les fabriques néoarchéennes dans les roches métaplutoniques, généralement sans le développement d'une nouvelle fabrique coaxiale paléoprotérozoïque; nous attribuons cette déformation et ce métamorphisme à l'orogenèse hudsonienne. [ABSTRACT FROM AUTHOR]

Abstract: Monazite can be used for U–Pb and Th–Pb dating of a wide variety of rocks and geological processes, particularly metamorphism. In many instances ion microprobe analysis is the preferred method for dating monazite, combining good analytical precision with μm-scale spatial resolution. However, ion microprobe dates can be compromised by matrix effects on the ionisation efficiencies of different secondary ionic species, which can bias monazite Pb+/ThOn + and Pb+/UOn + data by as much as ∼40% and ∼30%, respectively, for known monazite compositions. These effects appear to be entirely attributable to the sputtering process, in which case similar effects exist in data from all ion probes. The effects on data ratios are modified, sometimes exacerbated or reversed, when data are subjected to the correlation-based Pb/Th and Pb/U calibrations that are now commonly used. One-dimensional calibrations of 206Pb+/270[UO2]+ and 208Pb+/264[ThO2]+ are recommended. Corrections derived from data for a suite of reference materials with diverse compositions can reduce biases in 208Pb/232Th ages to <∼2% and in 206Pb/238U ages to <∼1%. The matrix effects come predominantly from variations in the concentrations of Th, U, Pb, light rare earth elements (LREE) and Y. La/Ce is an effective proxy for REE composition in many monazites, but for some monazites it would also be necessary to monitor Nd. Matrix corrections can be applied without measuring all LREE, using ion probe data alone. Other interacting data complications, such as Pb/Pb fractionation and an isobar at 204Pb+, can be addressed without iterative data reduction. [Copyright &y& Elsevier]

Highlights: [•] A back-arc basin developed along the southeastern Yangtze Block at 842–825Ma. [•] Neoproterozoic juvenile crustal growth in the southeastern Yangtze Block. [•] Orogen-parallel motion played a significant role in amalgamation of South China. [ABSTRACT FROM AUTHOR]

Abstract: The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic–Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600Ma. The six youngest U–Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640Ma. Lu–Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600Ma and 1370Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present. [Copyright &y& Elsevier]

Abstract: U–Pb zircon SHRIMP geochronology and Lu–Hf zircon LA–MC–ICPMS isotope geochemistry are used to constrain the ages of Paleoproterozoic felsic magmatism and their crustal sources in the Aillik Group, Makkovik Province of Labrador. This information is important for understanding the extent of crustal reworking vs. juvenile crustal growth in subduction-related magmatism during the Paleoproterozoic and improves framework knowledge for uranium exploration in the Central Mineral Belt of Labrador. U–Pb SHRIMP zircon geochronology from four felsic tuff samples collected from two different areas of the Aillik Group yield magmatic 207Pb/206Pb ages of 1852±7Ma (2σ), 1854±7Ma (2σ), 1861±7Ma (2σ), and 1862±7Ma (2σ). The ages fall within the range of previous U–Pb zircon dates reported for felsic volcanic rocks of the Aillik Group. Altogether these dates indicate that felsic volcanism spanned at least 30m.y. and was perhaps most intense during the last 10m.y. of that interval. A foliated syn-deformational monzogranite in the Aillik Group (Cross Lake granite) yields a 207Pb/206Pb zircon date of 1805±6Ma (2σ), which indicates that development of a pervasive planar fabric associated with northwestward thrusting of the Aillik Group continued after this time. Initial values for magmatic zircons from the four felsic tuffs (−4.8 to −1.9, −1.7 to +2.2, −11.8 to +3.8, −9.0 to −4.6) and monzogranite (−4.8 to −1.5) exhibit significant variability, which indicates heterogeneous sources. Most of the zircon grains have negative initial values and calculated crust formation Hf-isotope T DM model ages point to substantial contributions from melts of ancient crust, ranging in age from ca. 2200 to 2900m.y. and clustering at ca. 2500m.y. Inherited zircon grains in the felsic tuffs and monzogranite, mainly have U–Pb ages of ca. 1880–1920Ma, though others are older, up to ca. 2743Ma. Hf-isotope T DM model ages for the inherited zircons are similar to those of the magmatic zircons. The Hf-isotope results indicate that subduction-related felsic magmatism in the Makkovik Orogen involved significant crustal reworking (∼80%) during formation of the Aillik Group. [Copyright &y& Elsevier]