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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 [sup.40]Ar/[sup.39]Ar 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. Le volcanisme felsique a l'Eocene dans le Plateau de Chilcotin dans le centre de la Colombie Britannique est limite entre 54.6 et 46.6 Ma par trente-trois nouveaux ages isotopiques U-Pb et [sup.40]Ar/[sup.39]Ar. Ces ages sont issus d'echantillons de laves et breches volcaniques felsiques qui forment le Groupe Ootsa Lake. En consequence, la chronostratigraphie des roches volcaniques montre que les compositions magmatiques ont evolue dans le temps de felsique a intermediaire, mais aucune migration de l'activite volcanique en surface n'est observee au cours de cette periode. Les roches rhyolitiques sont les plus anciennes, et sont recouvertes par des roches dacitiques avec divers assemblages de phenocristaux. Dans de nombreuses localites du Plateau de Chilcotin, la stratigraphie de l'Eocene est dominee par des coulees distinctives de lave dacitique noire et vitreuse, contemporaines de laves andesitiques du Groupe Endako dans le Plateau de Nechako au nord. Les ages de cristallisation du Groupe Ootsa Lake dans le Plateau de Chilcotin sont en accord avec des ages existants provenant du Plateau de Nechako et du sud de la Colombie Britannique. Dans son ensemble, cette base de donnees geochronologiques confirme l'hypothese d'un episode volcanique volumineux a l'Eocene precoce (~55-46 Ma) dans la chame Intermontane. L'initiation brusque de cet episode volcanique, ainsi que la superficie, volume et composition des roches volcaniques peuvent etre expliques par l'arret de la subduction et une une discontinuite dans les plaques subductees en-dessous de la Colombie Britannique a l'Eocene precoce., Introduction Evidence of widespread Eocene magmatism in the North American Cordillera is preserved in a 3000 km long, 150-1000 km wide belt, extending from eastern Alaska to Idaho. In south-central [...]

Neoproterozoic to Paleozoic slope-to-basin facies continental margin strata underlie area ~700 x 200 km across central Yukon Territory, Canada, and collectively define the Selwyn Basin. In a Cordilleran framework, Selwyn Basin strata form a strongly deformed and thrust-faulted package located between the Mackenzie foreland fold-and-thrust belt, and accreted terranes and displaced elements of the ancient North American continental margin. Orogeny commenced in the Jurassic as exotic elements of the composite Ynkon-Tanana terrane overrode the ancient continental margin. Collision-related deformation had ceased by ca. 100 Ma, and was followed by a Late Cretaceous (post-85 Ma) dextral transcurrent regime, which laterally displaced elements of the newly assembled continental margin along the orogen-parallel Tintina fault. In western Selwyn Basin, more than 100 km of structural overlap was accommodated on two main detachments, the Robert Service and underlying Tombstone thrust faults. Internal deformation within the thrust sheets is intense, characterized by shear-related folds and fabrics. Metamorphic grade reaches lower to middle greenschist facies at the deepest structural levels exposed, and is characterized by chlorite-muscovite schists. The onset of deformation is constrained by the Late Jurassic age of the youngest units deformed during orogeny. The end of ductile deformation is constrained by new [sup.40]Ar/[sup.39]Ar ages fur metamorphic muscovite that range from 104 to 100 Ma. Due to the low metamorphic grade, these ages are interpreted to closely follow the waning of deformation. At ca. 93 [+ or -] 3 Ma, isolated granitic intrusions of the Tombstone-Tungsten magmatic belt were emplaced across the western Selwyn Basin in a tensional, postcollisional regime. Restoration of displacement on the Tintina fault places the western Selwyn Basin adjacent to the Yukon-Tanana terrane uplands of east-central Alaska in the Early to mid-Cretaceous. Despite their adjacent positioning in cross-orogen section during orogenesis, the two elements feature some significant differences in Jurassic-Cretaceous deformation. Most notably, the Yukon-Tanana terrane uplands record a significant extensional event at 120-105 Ma, which resulted in NW-SE-oriented extension, exhumation of deep structural levels, and voluminous felsic plutonism. In contrast, western Selwyn Basin did not undergo equivalent uplift and extension, and features temporally and spatially restricted plutonism. Within an orogenic framework, the Yukon-Tanana terrane uplands can therefore be considered to represent an exhumed core characterized by high heat flow, whereas the western Selwyn Basin represents an immediate northeastern salient to the exhumed core. These differences have important implications for the geodynamic setting of mid-Cretaceous plutonism across these two major lithologic-tectonic entities of the northern Cordillera. Keywords: Selwyn Basin, orogenesis, thrust sheets, At-At, Yukon-Tanana terrane.

SUMMARY This paper reviews the geology, mineralogy, and origin of the gem varieties of beryl, including emerald (green) and aquamarine (blue); it focuses on western Canada, especially the Yukon Territory, [...]

New Re-Os molybdenite dates from two lode gold deposits of the Tintina Gold Belt; Alaska, provide direct timing constraints for sulfide and gold mineralization. At Fort Knox, the Re-Os molybdenite date is identical to the U-Pb zircon age for the host intrusion, supporting an intrusive-related origin for the deposit. However, [sup.40]Ar/[sup.39]Ar dates from hydrothermal and igneous mica are considerably younger. At the Pogo deposit, Re-Os molybdenite dates are also much older than [sup.40]Ar/[sup.39]Ar dates from hydrothermal mica, but dissimilar to the age of local granites. These age relationships indicate that the Re-Os molybdenite method records the timing of sulfide and gold mineralization, whereas much younger [sup.40]Ar/[sup.39]Ar dates are affected by post-ore thermal events, slow cooling, and/or systemic analytical effects. The results of this study complement a growing body of evidence to indicate that the Re-Os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems. Keywords: Re-Os, molybdenite, geochronology, gold, Alaska.

Efforts to describe the stratigraphic relationships of rocks in British Columbia, particularly in northern Cordillera, reflects the unending pursuit of earth scientists to reinterpret terrane and assemblage relationships. Although these researches provide new insights on the causes of surface deformations, they have undermined the value of terrane analysis. By adopting inconsistent methods for performing terrane analysis, researchers have over emphasized rock differences at the expense of their similarities.

The Late Cretaceous Carmacks Group, a thick subaerial volcanic succession that once covered much of southwest Yukon, was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit. Coeval hydrothermal activity resulted in widespread alteration and gold mineralization. The lavas are shoshonites, enriched in large ion lithophile and light rare earth elements, but depleted in high field strength elements. Ankaramitic absarokite flows in the upper Carmacks Group range up to 15 wt% MgO, requiring a high liquidus temperature (1400 [degrees] C at 1 bar, dry). High [K.sub.2]O contents (> 3%) of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle. Although previously interpreted as subduction related, the Carmacks Group was erupted during a Cordilleran-wide magmatic lull and lacks coeval calc-alkalic batholiths. The lavas are petrologically similar to plume-related Eocene to Pliocene potassic lavas of the western United States. New paleomagnetic collections, combined with previous work, place the Carmacks Group 17.2 [degrees] [+ or -] 6.5 [degrees] (1900 [+ or -] 700 km) south of its present position relative to the craton during deposition, near the paleolocation of the Yellowstone hotspot. The spatial coincidence, similarity of tectonic setting, and lithologic similarity of the Carmacks Group and Yellowstone volcanic successions suggest that the Carmacks Group is the 70 Ma effusion of the Yellowstone hot-spot. Subsequent northward displacement of the Carmacks Group is attributed to coupling with the Kula plate. Correlation of the Carmacks Group and the Yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern Intermontane belt at 70 Ma.

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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]

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. 40Ar/39Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of ∼165 Ma, which dates cooling through ∼280 °C.Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by ∼9°.Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N (dp=5°, dm=5°) and a tilt-corrected paleopole at 81°W, 55°N (dp=5°, dm=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated. [Copyright &y& Elsevier]