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The NE-trending Malley dyke swarm, dated herein at 2231 ± 2 Ma (U-Pb baddeleyite), extends from the central Slave craton to the vicinity of the Kilohigok basin, and may continue farther to the northeast as the geochemically similar Brichta dyke swarm, having been offset sinistrally along the prominent Bathurst fault. It carries a characteristic high unblocking temperature paleomagnetic component of single polarity directed up SE (mean direction: D = 138.3°, I = -53.8°), with corresponding paleopole at 50.8°S, 50.0°W. Lower unblocking temperature components, in some cases directed down SE, similar to ca 1.75 Ga post-Hudsonian overprints, are easily removed using combined alternating field (AF) thermal demagnetization, but difficult to remove using AF cleaning alone. The characteristic remanence has not been demonstrated primary, but is significantly older than 2.03 Ga, the age of Lac de Gras dykes, based on a baked contact test at a Lac de Gras--Malley dyke intersection. In addition, an E- to ESE-trending dyke carries a down WNW remanence, typical of 2.19 Ga Dogrib dykes near Yellowknife, suggesting that regional overprinting has not affected the study area since Dogrib emplacement, and that the Malley remanence was acquired at or shortly after Malley emplacement. Comparing Malley and Lac de Gras paleopoles with the 2.22-2.00 Ga Superior craton apparent polar wander path indicates that the two cratons were (i) not in their present relative orientation at 2.23 or 2.03 Ga, and (ii) likely not drifting in close proximity to one another as parts of a single (super)continent throughout the 2.23-2.03 Ga interval. L'essaim de dykes Malley, a tendance nord-est, est date a 2231 ± 2 Ma (U-Pb sur baddeleyite); il s'etend du craton central des Esclaves jusqu'aux environs du bassin de Kilohigok. Il pourrait s'etendre plus loin vers le nord-est en tant que l'essaim de dykes Brichta, dont la geochimie est semblable et qui a ete decale vers la gauche le long de l'importante faille Bathurst. Cet essaim Malley comporte une composante paleomagnetique de temperature de deblocage elevee caracteristique, a polarite simple dirigee vers le haut au sud-est (D = 138,3°, I = -53,8°), avec un paleopole correspondant a 50,8° Sud, 50,0° Ouest. Les composantes de temperature de deblocage inferieure, dans certains cas dirigees vers le bas au sud-est, semblables aux surimpressions post-hudsoniennes a environ 1,75 Ga, sont facilement enlevees en utilisant une demagnetisation thermique / a champ alterne, mais elles sont difficiles a enlever en utilisant uniquement une demagnetisation a champ alterne. La remanence caracteristique n'a pas ete demontree mais elle est significativement plus ancienne que 2,03 Ga, l'age des dykes Lac de Gras, selon des tests de contact a une intersection des dykes Lac de Gras-Malley. De plus, un dyke a tendance E a ESE comporte une remanence ONO vers le bas, typique des dykes Dogrib (2,19 Ga) a proximite de Yellowknife, suggerant que la surimpression regionale n' ait pas touche le secteur a l'etude depuis la mise en place des dykes Dogrib et que la remanence Malley ait ete acquise en meme temps ou peu apres la mise en place des dykes Malley. Une comparaison des paleopoles Malley et Lac de Gras avec la derive polaire apparente du craton du Superieur (2,22-2,00 Ga) indique que les deux cratons (i)n' avaient pas leur orientation relative actuelle il y a 2,23 ou 2,03 Ga et (ii) ne derivaient probablement pas pres l'un de l'autre en tant que partie d'un seul (super)continent durant tout l'intervalle 2,23-2,03 Ga. [Traduit par la Redaction], Introduction Thirty to forty Archean cratons or cratonic fragments are scattered widely on all present-day continents. Given that many have Paleoproterozoic rifted margins, it is likely that they were once [...]

The Middle Mountain Metamorphic Domain of the Montana Metasedimentary Terrane, northwestern Wyoming Craton, within the northwestern Tobacco Root Mountains, mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic (hornblende) gneiss, both of which are cut by metamorphosed mafic rocks. Together, these gneisses are defined as Middle Mountain Gneiss. Archean tonalitic gneiss from west of, and amphibolitic gneiss from east of, the Bismark Fault give, from chemically and air-abraded zircon grains, U-Pb ID-TIMS ages of 3325.5 ± 1.7 and 3340 Ma, respectively. These results reflect primary magmatic ages and show that the Middle Mountain Gneiss extends into the northern area of the Central Fault Block, between the Bismark and Mammoth faults. Older crustal processes in the tonalitic gneiss are evidenced by inherited grains, the oldest of which is >3460 Ma. A metabasite hosted in tonalitic gneiss in the Bismark Fault selvage zone yields a zircon age of 2468 Ma, which is interpreted as the time of metamorphism. This date and other ca. 2470 Ma dates known in the region reflect a series of thermotectonic events designated here as the Beaverhead--Tobacco Root Orogeny. Geochemical evidence in the Central Fault Block metabasites suggests that their >2470 Ma precursors evolved in a back-arc--arc-rift setting, whereas their equivalents west of the Bismark Fault were largely mid-ocean ridge basalt-related tholeiites and east of the Central Fault Block were back-arc tholeiites showing some continental affinity. The metabasite was metamorphosed, deformed, and intruded by pegmatite at 1756 Ma during the Big Sky Orogeny. This orogenic event also produced new zircon growth in Archean tonalitic gneiss. Monazite with an age of 75 Ma, found at one location, reflects nearby intrusion of the Cretaceous Tobacco Root Batholith. Le domaine meetamorphique de Middle Mountain dans le terrane metaseedimentaire du Montana, nord-ouest du craton Wyoming, nord-ouest des monts Tobacco Root, comprend surtout des gneiss tonalitiques migmatises interstratifies avec des gneiss amphibolitiques (hornblende); les deux sont recoupes par des roches mafiques metamorphisees. Ensemble, ces gneiss forment le gneiss de Middle Mountain. Un gneiss tonalitique archeeen provenant d'un site a l'ouest de la faille Bismark et un gneiss amphibolitique provenant d'un site a l'est de cette faille ont donnee des ages U-Pb ID-TIMS respectifs de 3325,5 [+ o -] 1,7 et 3340 Ma, sur des zircons pretraites par abrasion chimique ou par abrasion a l'air. Ces resultats refletent des ages magmatiques primaires et montrent que le gneiss de Middle Mountain s'etend dans le secteur nord du Central Fault Block, entre les failles Bismark et Mammoth. Des processus plus anciens de la croflte dans le gneiss tonalitique sont mis en evidence par des grains herites, dont le plus ancien date de >3460 Ma. Une metabasite encaisseee dans un gneiss tonalitique dans la zone de salbande de la faille Bismark a donne un age sur zircon de 2468 Ma, ce qui est interprete comme le moment du meetamorphisme. Cette datation et d'autres autour de 2470 Ma connues dans la region sont le reflet d'une serie d'evenements thermo-tectoniques designes ici sous l'appellation orogene Beaverhead--Tobacco Root. Les evidences geeochimiques dans les metabasites du Central Fault Block suggerent que leurs precurseurs >2470 Ma aient evolue dans un environnement d'arriere-arc--arc de distension, alors que leurs equivalents a l'ouest de la faille Bismark etait largement des tholeeiites reliesa des basaltes de dorsale meedio-oceeanique et, a l'est du Central Fault Block, etaient des tholeiites d'arriere-arc montrant une certaine affinite continentale. La metabasite etait metamorphisee, deformee et une pegmatite l'a penetreea 1756 Ma durant l'orogene Big Sky. Cet evenement orogeenique a aussi engendre une nouvelle croissance de zircons dans le gneiss tonalitique archeeen. Une monazite, agee de 75 Ma, trouveeea un site, reflete l'intrusion avoisinante du batholite Tobacco Root, au Cretace., Introduction The Tobacco Root Mountains comprise a Laramide (Late Cretaceous) block uplift in the northwestern Wyoming Province (southwestern Montana) that contains extensive exposures of Archean basement gneiss. The adjacent Great [...]

The U-Pb geochronology of three granitoid plutons and three granitic pegmatite dykes, largely from the Thompson Nickel Belt located along the northwestern Superior craton margin, was investigated to place constraints on the timing of felsic magmatism associated with closure of the Manikewan Ocean and final continent-continent collision to form the Trans-Hudson Orogen. These data indicate that 1840-1820 Ma granite magmatism along the Superior margin was more active than previously thought and that some magmatism extended beyond the Thompson Nickel Belt sensu stricto, including the 1836 ± 3 Ma Mystery Lake granodiorite, 1822 ± 5 Ma Wintering Lake granodiorite, and the 1825 ± 8 Ma Fox Lake granite located in the Split Lake Block. Granitic pegmatites within the Thompson Nickel Belt were emplaced late in the collisional history in the period 1.79-1.75 Ga and include a 1770 ± 2 Ma dyke exposed at the Thompson pit, a 1767 ± 6 Ma dyke at the Pipe Pit, and a 1786 ± 2 Ma dyke located at Paint Lake. The final stage of crustal amalgamation in the eastern Trans-Hudson Orogen involved Superior Province crustal thickening and partial melting forming 1.84-1.82 Ga granite magmas and then final collision at ~1.8 Ga between the Superior Province and a continental block to the west consisting of the previously amalgamated Sask and Hearne cratons. Heating of the Superior craton margin and granitic magmatism continued past peak metamorphism (1790-1750 Ma); this thermal event is represented by the emplacement of numerous late pegmatite dykes and evidenced by cooling dates recorded by metamorphic minerals (e.g., titanite) in reworked Archean gneisses and Proterozoic intrusions. La geeochronologie U-Pb de trois plutons granitoldes et de trois dykes de pegmatite granitique, provenant en grande partie de la Ceinture de nickel Thompson situee le long de la bordure nord-ouest du craton du Superieur, a ete etudieee afin de delimiter le moment du magmatisme felsique associe a la fermeture de l'ocean Manikewan et la collision finale continent a continent pour former l'orogene trans-hudsonien. Ces donnees indiquent que le magmatisme granitique (18401820 Ma) le long de la bordure du Superieur est plus commun que cru auparavant et qu'il s'etend au-dela de la Ceinture de nickel Thompson proprement dite, incluant le granodiorite de Mystery Lake (1836 ± 3 Ma), le granodiorite de Wintering Lake (1822 [+ o -] 5 Ma) et le granite de Fox Lake (1825 [+ o -] 8 Ma) situee dans le bloc Split Lake. Les pegmatites granitiques a l'interieur de la Ceinture de nickel Thompson ont ete mises en place tard dans l'historique de collision dans la periode 1,791,75 Ga et elles comprennent un dyke (1770 [+ o -] 2 Ma) affleurant dans la fosse Thompson, un dyke (1767 [+ o -] 6 Ma) dans la fosse Pipe et un dyke (1786 [+ o -] 2 Ma) situee au lac Paint. L'eetape finale de l'amalgamation de la croflte dans l'orogene transhudsonien oriental impliquait un eepaississement de la coflte de la province du Superieur et une fusion partielle formant des magmas granitiques (1,84-1,82 Ga) et la collision finale a ~1,8 Ga entre la Province du Superieur et un bloc continental a l'ouest qui comprenait les cratons amalgamees de Sask et de Hearne. L'eechauffement de la bordure du craton du Superieur et le magmatisme granitique ont continue aprees la crete du metamorphisme (1790-1750 Ma) et ils sont representees par la mise en place de nombreux dykes de pegmatites tardifs et des dates de refroidissement telles qu'enregistreees par des mineeraux meetamorphiques (p. ex. la titanite) dans des gneiss archeens et des intrusions proterozoiques retravaille;s. [Traduit par la Redaction], Introduction The Trans-Hudson Orogen in northern Manitoba and Saskatchewan is one of several Paleoproterozoic accretionary and collisional orogens in North America that formed during the breakup of one or more [...]

The Thompson Nickel Belt marks the boundary between the Archean Superior Province and the Trans-Hudson Orogen in Canada. It comprises Archean gneisses, and Paleoproterozoic rocks with metasediments and metavolcanites (Ospwagan Group) and intrusions. The gneisses are frequently migmatitic and host numerous pegmatites. The western belt boundary is a fault contact with the Kisseynew Domain of the Reindeer Zone. In the south, the transition zone between the belt and the Kisseynew Domain comprises granitoids and a detrital sequence (Grass River Group), part of which grades into turbidites in the Kisseynew Domain. The eastern belt boundary is a diffuse zone where the Archean east-west (E-W) structural trend changes into the north-northeast (NNE) trend of the belt. This paper presents U-Pb ages for granitoids and [sup.207]Pb/[sup.206]Pb detrital zircon ages from the Ospwagan and Grass River groups. Ages and a comparison of events in the belt and in the eastern Reindeer Zone have major implications. The change from stable platform deposits to syn-tec-tonic filling and emplacement of mafic intrusions in the Ospwagan Group are attributed to the convergence between the Reindeer Zone and the Superior Province at 1891-1885 Ma. At ca. 1850 Ma, continuing convergence led to drowning of marginal basins of the Superior craton and to the development of a transpressive regime in the belt, the onset of which could be as old as ca. 1885 Ma. Metamorphic ages of 1818-1785 record closure of the Kisseynew basin and crustal thickening. Collision of the new continental block with the Superior Province was accommodated by transpression until 1750-1720 Ma. La ceinture de Thompson est situee entre le Craton archeen du Superieur et l'orogene paleoproterozoique du Trans-Hudson, au Canada. Elle comprend des gneiss archeeens souvent migmatitiques, et des roches paleeoproteeroiques, metasediments et metavolcanites (Groupe Ospwagan), et intrusions. A l'ouest, une zone de faille marque le contact entre la ceinture et le domaine Kisseynew de la Zone de Reindeer. Vers le sud, cette zone de transition entre la ceinture et le domaine Kisseynew comprends des granitoldes et une sequence deetritique (Groupe Grass River), qui passe graduellement aux turbidites du domaine Kisseynew. Sur la bordure orientale de la ceinture, le grain structural E-W du Craton archeen se reoriente dans le grain NNE de la ceinture. Cet article presente des ages U-Pb pour differents granitoides et des ages [sup.207]Pb/[sup.206]Pb pour les zircons deetritiques des groupes Ospwagan et Grass River. Les ages et une comparaison des eveenements affectant la ceinture et l'est de la zone Reindeer ont des implications importantes. Dans le groupe Ospwagan, le passage de depots de type plate-forme a des depots syn-tectoniques, et l'intrusion de roches basiques, sont attribueees a la convergence entre la zone Reindeer et le Craton du Superieur vers 1891-1885 Ma. Vers 1850 Ma, la convergence conduit a l'enfouissement des bassins de la marge du Craton et a un regime transpressif pouvant avoir debute des ca. 1885 Ma. La fermeture du bassin de Kisseynew et l'epaississement crustal sont enregistres par des ages de meetamorphisme entre 1818 et 1785 Ma. Le regime transpressif se poursuit jusqu'a ca. 1750-1720 Ma. [Traduit par la Redaction], Introduction The structure and evolution of Paleoproterozoic orogens is of considerable interest for understanding early continental tectonic processes because they involve convergence of Archean cratons and accretion of juvenile crust. [...]

The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the 'Betsimisaraka suture' is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times. Le bouclier precambrien de Madagascar est reevalue en se basant sur de recentes compilations de donnees geologiques et de nouvelles determinations geochronologiques U-Pb parmicrosonde ionique haute resolution (<< SHRIMP >>). Deux domaines archeens sont reconnus : le domaine oriental Antongil-Masora et le domaine central Antananarivo, ce dernier domaine incluant des ceintures particulie resmetamafiques de gneiss et de schistes (complexe de Tsaratanana). Dans le domaine oriental, la periode de debut de formation de la croute est prolonge jusqu'au Paleo-Mesoarcheen (3,32-3,15 Ga) et une sequence supracrustale (Groupe de Fenerivo), deposee il y a 3,18Ga et metamorphosee a 2,55 Ga, est identifiee. Dans le domaine central, une periode neoarcheenne de metamorphisme de haut grade et d'anatexie qui a touche a la fois les gneiss felsiques (suite de Betsiboka) et les gneiss mafiques (complexe de Tsaratanana) est documentee. Nous proposons donc que le domaine d'Antananarivo ait ete amalgame dans le << Greater Dharwar Craton >> (Inde et Madagascar) au cours d'un evenement d'accretion crustale au Neoarcheen (2,55 a 2,48 Ga); cet evenement se characterisant par lamise en place de roches ignees juveniles, un metamorphisme de haut grade et la juxtaposition de ceintures disparates de gneissmafiques et de schistes (ceintures de roches vertes). Le concept de << suture de Betsimisaraka >> est refute et la zone est re-definie comme un domaine de roches metasedimentaires (Groupe de Manampotsy) etmeta-ignees (suite d'Itsindro-Imorona) d'age Neoproterozoique forme durant une periode d'extension continentale et d'activite magmatique entre 840 et 760 Ma. La convergence orogenique plus recente (560-520 Ma) a provoque des charriages a vergence Est dans la patrie Centre-Sud de Madagascar; le pendage, parfois accentue, est responsable d'une inversion locale des domaines dans le centre de Madagascar. Sur une partie de son trace, le Groupe de Manampotsy recouvre la limite entre les domaines archeens oriental et central. Se superpose au Groupe Manampotsy; la zone d'intense deformation d'Angavo-Ifanadiana qui est interpretee comme une zone de faiblesse crustale durant tout le Cretace jusqu'a la periode actuelle. [Traduit par la Redaction], Introduction The island of Madagascar occupies a keystone position within the reconstructed East African Orogen (Fig. 1) between the Archean cratons of East Gondwana (India-Antarctica-Australia) and West Gondwana (South America--Africa). [...]

Zircon U-Pb dating, whole-rock major oxide, trace element and Sr-Nd isotopic data are presented for the Late Mesozoic mafic intrusive rocks from Yiwulushan of western Liaoning along the eastern segment of the Yanshan belt, North China craton, with two episodes of magmatism documented. Middle Jurassic hornblende-rich gabbros show enrichment of large ion lithophile elements and light REE, and prominent depletion in high field strength elements, and possess moderately enriched isotopic compositions with [([sup.87]Sr/[sup.86]Sr).sub.i] ranging from 0.7056 to 0.7065 and [[epsilon].sub.Nd](t) from -5.0 to -7.1. These features suggest that the gabbros were derived from an amphibole-bearing harzburgitic lithospheric mantle source metasomatized recently by slab-derived fluids. By contrast, Early Cretaceous mafic dykes are gabbroic dioritic to dioritic in composition, with comparable trace element characteristics to continental crust and depleted isotopic signatures ([([sup.87]Sr/[sup.86]Sr).sub.i] = 0.7048-0.7055, [[epsilon].sub.Nd](t) = 0 to -3.0). They probably originated from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with subsequent lower crustal assimilation and fractional crystallization. These two contrasting mafic intrusive suites, together with multiple Mesozoic mafic volcanic rocks from western Liaoning, documented a localized lithospheric thinning process, mainly through prolonged hydro-weakening or melt rock interaction and triggered by gravitational collapse, possibly within an evolved post-collisional to within-plate extensional regime. Keywords: mafic intrusions, Late Mesozoic, petrogenesis, lithospheric thinning, North China craton. doi: 10.1017/S0016756810000373

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ~1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to [less than or equal to] 35[degrees]C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ~850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events. doi: 10.1130/G30980.1