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Neogene volcanism in the Fort Selkirk area began with eruptions in the Wolverine Creek basin ca. 4.3 Ma and persisted to ca. 3.0 Ma filling the ancestral Yukon River valley with at least 40 m of lava flows. Activity at the Ne Ch’e Ddhawa eruptive center overlapped with the last stages of the Wolverine Creek eruptive centers. Hyaloclastic tuff was erupted between ca. 3.21 and 3.05 Ma. This eruption caused or was coincident with damming of Yukon River. The first demonstrable incursion of a Cordilleran ice sheet into the Fort Selkirk area was coincident with a second eruption of the Ne Ch’e Ddhawa eruptive center ca. 2.1 Ma. The Ne Ch’e Ddhawa subglacial mound was erupted beneath at least 300 m of glacial ice (Ne Ch’e Ddhawa Glaciation). The Eruption of the Fort Selkirk center occurred between the last eruption of Ne Ch’e Ddhawa and Fort Selkirk Glaciation (ca. 2.1–1.5 Ma). Till and outwash from Fort Selkirk Glaciation are conformably overlain by nonglacial sediments that contain the Fort Selkirk tephra (fission track dated at ca. 1.5 Ma). These nonglacial sediments also preserve a short magnetic reversal (reversed to normal) identified as the Gilsa polarity excursion. Temporal control and sedimentology constrain Fort Selkirk Glaciation and the Fort Selkirk Local Fauna to marine isotope stage 54. Rapid and extensive eruption of the Pelly eruptive center filled the Yukon River valley with 70 m of lava which buried these glacial and nonglacial sediments and dammed Yukon River. Local striations and erratic pebbles occur on the last of these lava flows. They document a subsequent incursion of glacial ice during the last 500 ka of the Matuyama Chron (Forks Glaciation). The last major eruption of mafic lava occurred in the middle Pleistocene west of (early Holocene) Volcano Mountain in basin of Black Creek: lava flowed down the valley presently occupied by Black Creek and dammed Yukon River in the area of the Black Creek confluence. This eruption predated the middle Pleistocene Reid Glaciation. Minor volcanism has continued in this area since the middle Pleistocene at Volcano Mountain.

Two lava flows with interbedded palaeosols outcrop c. 40 km SW of Mount Kenya, near the Amboni River north of Mweiga, Kenya along the Nyeri/Thompson Falls Road, at 0°18′S; 37°48′E. These flows, overlain by loess, are principally trachyandesite and form the base of the Mount Kenya Volcanic Series which, in the early literature, is described as being of probable Miocene/Pliocene age. Here we report 39 Ar/ 40 Ar dates ( c. 5.2–5.5 Ma) and reversed magnetizations which establish a Latest Miocene to Earliest Pliocene age for these flows. Weathering characteristics of palaeosols interbedded with the lavas indicate generally dry climatic conditions during the Late Miocene, punctuated with humid events during the Pliocene and Quaternary. These Late Miocene–Quaternary palaeosols depict a relatively long and complex weathering history, followed by loess deposition. The palaeosols appear to have been episodically deflated, initially in phase with the deposition of lavas when surfaces were devoid of vegetation and later during periods of climatic deterioration when wind systems intensified. Such weathering histories within palaeosol profiles are also documented on nearby Mount Kenya, where well-weathered lower palaeosol horizons developed on Matuyama-age tills are overlain by much younger less-weathered horizons developed on Brunhes-age loess. The geochronology of Late Miocene lavas reported here provides maximum ages for weathering histories of palaeosols formed in a xeric tropical highland climate.

Strongly foliated amphibolite clasts are found embedded within the ophiolitic melange underlying the Xigaze Ophiolite near Bainang and Angren, Yarlung Zangbo Suture Zone, Southern Tibet. These high-grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction. They include garnet-clinopyroxene amphibolites, clinopyroxene amphibolites and common amphibolites. Petrographic descriptions, mineral chemistry and thermobarometry of these rocks can be found in a companion paper [Guilmette, C., Hebert, R., Dupuis, C., Wang, C.S., Li, Z.J., 2008. Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic melange beneath the Yarlung Zangbo Ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32, 423–437.]. The geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust. Rare Earth Elements contents are suggestive of an N-MORB origin. However, enriched LILEs and depleted Nb–Ta–Ti when compared to N-MORBs rather suggest a suprasubduction zone influence. A large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics, suggesting the protoliths of the amphibolites might have crystallized in the same environment as the Xigaze ophiolitic crust, likely in a back-arc basin. 40 Ar/ 39 Ar step-heating dating of hornblende concentrates from three samples yielded ages of 123.6 ± 2.9 Ma, 127.7 ± 2.2 Ma and 127.4 ± 2.3 Ma. These cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite. All these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone. The demise of the early subduction circa 130 Ma forced the inception of a new subduction zone at the SSZ spreading axis, burying SSZ mafic rocks underneath a SSZ ophiolitic mantle.

The Aberdeen gneiss complex is composed of complexly deformed migmatitic orthogneiss and paragneiss situated within the core of the Vernon antiform, a structure defined by a series of subparallel reflectors visible at upper to middle crustal depths (6–18 km) in seismic reflection data from the Vernon area of the Shuswap metamorphic complex. The Vernon antiform and the Aberdeen gneiss complex lie within the footwall of the gently west dipping (top to the west) Kalamalka Lake shear zone. Migmatitic gneiss exposed within the antiform records evidence (recorded as age domains in complexly zoned zircon grains) of three metamorphic events, occurring at 155–150, 90, and 66–51 Ma. The timing of magmatic events within the antiform includes emplacement of diorite at ~232 Ma, tonalite at ~151 Ma, granodiorite at 102 Ma, and monzonite at 52 Ma. Middle to Late Jurassic metamorphism resulted in widespread migmatization. Early Tertiary metamorphism (66–51 Ma) was coeval with the emplacement of granitic rocks and exhumation typical of other areas of the Shuswap metamorphic complex. Highly deformed orthogneiss situated within the hanging wall of the Kalamalka Lake shear zone, comprising the superstructure, was emplaced at ~171 Ma. Ductile deformation had ceased by 162 Ma. The complex metamorphic and magmatic evolution of the Vernon antiform, which is similar to other areas of the southern Canadian Cordillera including the Nicola horst, Mount Lytton – Eagle plutonic complex, Cariboo Mountains, and Mica Creek area, may reflect episodic tectonic activity at the plate margin.

The Saint-Daniel Melange is an orogen-scale sedimentary basin of the Quebec Appalachians that is commonly interpreted as the remnant of a subduction complex formed during the Taconian orogeny. However, geochronological, structural, and stratigraphical data from Laurentian continental margin rocks and adjacent ophiolitic rocks in southern Quebec indicate that the Saint-Daniel Melange is an olistostromal, syncollisional piggyback basin that represents the base of the Magog Group forearc basin. The regional stratigraphic framework of the Saint-Daniel Melange and its relationships with underlying and overlying rock units have been established on the basis of six stratigraphic sections from the Thetford-Mines, Asbestos, and Mont-Orford ophiolitic complexes. Our results imply that (1) the Saint-Daniel Melange is a sedimentary sequence that unconformably overlies different structural and pseudostratigraphic levels of the southern Quebec ophiolites; (2) it is made up of four distinct and laterally discontinuous units that record the obduction of ophiolites onto Laurentia and exhumation of basement rocks, followed by subsidence and deposition of the overlying Saint-Victor Formation of the Magog Group; (3) the sedimentary rocks of the Saint-Daniel record a transition from ophiolite-dominated to continental sources, indicating progressive exhumation of both the ophiolite and the continental margin upon which it was obducted; 4 0 Ar/ 3 9 Ar analysis of muscovite from metamorphic rock fragments in debris flows of the melange yield an age of 467 ′ 2 Ma, which is within the range of Ar ages measured in metamorphic rocks that underlie the ophiolites, and indicates the uplift of metamorphosed continental deposits during or shortly after obduction; (4) the Saint-Daniel Melange is stratigraphically overlain by the Magog Group and represents the base of a syncollisional basin developed in a forearc setting during the Taconian orogeny.

A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of 5 km2 for the K-bentonite, covering part of southern Quebec, New York State and eastern Ontario. The deposit coincides with the first influx of distal turbdites into this shale basin, associated with Taconic flysch, and simultaneous abrupt ventilation of the once anoxic deep-water basin, which had formed initially after foundering of the Upper Ordovician carbonate platform. Concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation, palaeoceanography and volcanism were linked to a regional external process. Compositionally, the bentonite is distinct from the older Ordovician platform deposits in eastern North America. The deposit contains abundant titaniferous phlogopite with 1.6% BaO, fluoroapatite with 2.5% F, and dynamically shaped glass spherules now altered to clay. The spherules and clay matrix constitute 45% of the bed and, compositionally, define an illite (>90%)–smectite (I/S) structure with about 7.5% K2O%. Age-dating by Ar–Ar analysis of the phlogopite crystals yielded a younger than expected (440–445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalachians.

The age of the Fish Canyon Tuff (FCT) has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the K/Ar and U/Pb isotopic systems. Minerals from the FCT (notably sanidine and biotite) are in widespread use as 40Ar/39Ar fluence monitors (“standards”) and have been employed as both primary and intercalibrated standards. In attempting to constrain the true age of FCT, the distinction between primary vs. intercalibrated ages is important, as only the primary age determinations are valid. An investigation of both Fish Canyon sanidine (FCs) and Fish Canyon biotite (FCT-3) was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain-to-grain scale. Electron microprobe results show that FCs is chemically homogeneous, verifying previous studies, although there are some very small zones of minor K depletion; FCT-3 biotite commonly contains numerous and small mineral inclusions. 40Ar/39Ar experiments further show that FCs is remarkably homogeneous, confirming previous observations, whereas FCT-3 shows much greater isotopic variability due to contamination from the mineral inclusions. By setting the age of FCs equal to 27.98±0.15 Ma (2σ), as intercalibrated to the U/Pb system by Villeneuve et al. (2000), the resultant intercalibration factor between FCs and FCT-3 is 1.005±0.017 yielding an age of 28.13±0.47 Ma (2σ) for FCT-3, although the significant intragrain chemical and isotopic variability precludes its use as an effective 40Ar/39Ar fluence monitor. The true age of the FCT is problematic; a survey of all primary ages for FCT shows that mean K/Ar ages for plagioclase, hornblende, biotite, and sanidine are consistently younger than mean U/Pb ages from titanite and zircon. In conjunction with recently published petrological results which indicate that the FCT contains material from multiple crystallization events, this significant discrepancy between the K/Ar and U/Pb data sets demonstrates that the Fish Canyon Tuff cannot serve as a golden standard between the K/Ar and U/Pb systems.

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.

Knowledge of the time-scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development. Detailed petrological and chemical study of the mid-Cretaceous, composite Emerald Lake pluton, from the northern Canadian Cordillera, Yukon Territory, coupled with U^Pb and 40 Ar/ 39 Ar geochronology, indicates that this pluton was intruded as a series of magmatic pulses. Intrusion of these pulses produced a strong petrological zonation from augite syenite, hornblende quartz syenite and monzonite, to biotite granite. Our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time-scale on the order of the resolution of the 40 Ar/ 39 Ar technique (V1 Myr), and that emplacement occurred at 94.3 Ma. Simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion. These calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70 < 5 kyr. These results demonstrate that production, transport and emplacement of the different phases of the Emerald Lake pluton occurred essentially simultaneously, and that these processes must also have been closely related in time and space. By analogy, these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development. ? 2002 Elsevier Science B.V. All rights reserved.

New bedrock mapping completed as part of the Nechako NATMAP Project indicates that the area between Babine and Takla lakes in central British Columbia is underlain by rocks of the Early Permian Asitka, Late Triassic Takla, and Early to Middle Jurassic Hazelton volcanic-arc assemblages of the Stikine Terrane. These are cut by large composite stocks of quartz diorite, granodiorite, and quartz monzonite previously mapped as the Late Triassic to Early Jurassic Topley intrusions. New U/Pb (n = 6) and laser 40Ar/39Ar (n = 10) isotopic age dates reported in this paper suggest there are two distinct ages of plutons: the Topley intrusive suite with isotopic ages between 218 and 193 Ma; and, east of Babine Lake, the new Spike Peak intrusive suite with isotopic ages ranging from 179 to 166 Ma. West of the main plutonic belt is a thick volcanic succession of subaerial, porphyritic andesite flows, volcanic breccias, and rhyolitic ash-flow tuffs that have isotopic ages between 185 and 174 Ma. These rocks are assigned to the Saddle Hill Formation of the Hazelton Group. The plutonic roots of this proximal arc assemblage are most likely the coeval and compositionally similar plutons of the Spike Peak intrusive suite that have been unroofed in the area east of the Takla Fault. Major oxide and trace element data support the interpretation that the Topley and Spike Peak granitic rocks formed in a juvenile volcanic-arc environment and that magmatism is related to melts generated above a long-lived subduction zone of unknown orientation.

New U/Pb and 40Ar/39Ar isotopic dating in the Babine porphyry copper district of central British Columbia documents three distinct magmatic events at 107–104, 85–78, and 54–50 Ma. The earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the Rocky Ridge Formation. The rhyolite domes and related dacitic to basaltic volcanic rocks gave a U–Pb age of 107.9 ± 0.2 Ma and an 40Ar/39Ar age of 104.8 ± 1.2 Ma. The rhyolites, which were previously mapped as Eocene, are reinterpreted to be part of a previously unrecognized mid-Cretaceous cauldron subsidence complex. The regionally extensive Late Cretaceous magmatic event is also recognized in the Babine district and is represented by 40Ar/39Ar ages of 85.2 ± 2.8 and 78.3 ± 0.8 Ma on two Bulkley intrusions, one of which has associated porphyry copper mineralization. The final magmatic event is the most widespread and involved emplacement of the Babine intrusions and formation of numerous porphyry copper deposits including the Bell and Granisle past producers. Twenty-one new 40Ar/39Ar isotopic ages for these intrusions and coeval andesites of the Newman Formation have a narrow range from 53.6 ± 0.9 to 49.9 ± 0.6 Ma, whereas previous K–Ar isotopic dating had a possible range of 15 Ma. The mid-Cretaceous, Late Cretaceous, and Eocene magmatic suites in the Babine district are interpreted to be part of a long-lived volcano-plutonic complex that was the site of periodic magmatism and porphyry copper mineralization over a 60 Ma time period. This complex may have evolved within a zone of extension (pull-apart basins) situated between dextral strike-slip faults that were active during periods of rapid oblique plate convergence.

The Endako batholith is a composite batholith that consists of early foliated hornblende ± biotite diorites, intermediate-age unfoliated hornblende ± biotite diorites, and late granodiorites to monzogranites. These latter plutons host the Endako molybdenite deposit, a major porphyry deposit located in the central Canadian Cordillera. Extensive K-Ar dating on the batholith had given relatively reproducible ages centered on 140 Ma, indicating a short life span for magmatic activity. However, a combined U-Pb and 40Ar/39Ar study has resulted in a substantial reappraisal of the age and time span of intrusive activity and its association with molybdenite mineralization. The new data show that the batholith had a protracted emplacement history, covering ~75 m.y., with clear evidence for periods of magmatic quiescence. The oldest magmatic suite of the Endako batholith, the Stern Creek suite, is dated at ~220 Ma and comprises foliated gabbros and diorites. Mafic to intermediate plutons of the Stag Lake suite range in age from 180 Ma to 161 Ma. The Francois Lake suite is divided into two subsuites: the Glenannan subsuite dated at 157 Ma to ~155 Ma and the 149 to 145 Ma Endako subsuite that hosts the Endako molybdenite deposit. 40Ar/39Ar dating of hydrothermal biotite indicates molybdenite mineralization is genetically associated with terminal stages of magmatic activity. Biotite within the mine had its 40Ar/39Ar systematics reset at ~145 Ma by hydrothermal fluids with temperatures in excess of 300°C. This resetting is associated with intrusion of the 145.1 ± 0.2 Ma Casey phase monzogranite, the youngest phase within the Endako subsuite. A further partial resetting of biotite is interpreted to be coeval with Eocene intrusion of the high-level, miarolitic Sam Ross Creek phase and numerous mafic dikes that cut through the region. Hydrothermal fluids associated with this latter event were below biotite closure temperature and do not appear to have any relationship to molybdenite mineralization within the Endako deposit. The new geochronologic data, coupled with detailed mapping, show a general age progression from mafic plutonism at the margins to a central core of high-level granodiorite to monzogranite. The data document the existence of a long-lived, relatively stationary magmatic center. Pulsed magmatism, evident even within individual magmatic suites, may play a major role in concentrating and ultimately depositing molybdenite in some porphyry systems. At Endako, molybdenite would have formed within the higher level portions of the batholith, a position that would normally have rendered the deposit prone to erosional removal. However, subsequent Eocene extensional block faulting may explain the preservation of this Jurassic deposit, which is the oldest economic molybdenite deposit in the Cordillera of North America.

Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma , corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after ∼0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma . The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levees, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended ∼2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km2. Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km3. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended ∼400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of ∼90 m s−1 can be calculated at this point; lower velocities of ∼45 m s−1 are calculated where distal toes ascend ∼200 m slopes. It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism.

The increasing analytical precision of 40Ar-39Ar geochronological data and its use in studies that combine U-Pb and 40Ar-39Ar data has necessitated derivation of a means to intercalibrate ages between the two isotopic systems. The primary determinants inhibiting accurate cross calibration are uncertainty in decay constant values and uncertainty in 40Ar∗/40K (usually presented as apparent age) for 40Ar-39Ar standards, notably those derived from the Fish Canyon Tuff. By utilizing a monazite- and biotite bearing Oligocene ash flow tuff (MAC-83) as the primary standard material, precise U-Th-Pb and 40Ar-39Ar ages can be derived. Because both isotopic systems should reflect closure during the instantaneous eruption event, the 24.21 ± 0.10 Ma 207Pb/235U age should match the age resulting from 40Ar-39Ar biotite analysis. This forced equivalence will partly offset the effects of decay constant uncertainty. Furthermore, by co-irradiating Fish Canyon Tuff sanidine with MAC-83 biotite, the apparent age for Fish Canyon Tuff sanidine is back-calculated to 27.98 ± 0.15 Ma, relative to the MAC-83 207Pb/235U age. Hence, intercalibration between the two isotopic systems in a relative sense can be accomplished, allowing for a direct and valid comparison of ages. It is also shown that forcing equivalence of the two isotopic systems minimizes the effect of decay constant uncertainty for samples of Phanerozoic age.

A combined U–Pb geochronology and Nd isotopic analysis of clastic metasedimentary rocks from the Archean Slave Province, Northwest Territories, Canada is presented. A series of clastic sedimentary rocks with deposition age of ∼3.13 Ga to ∼2.58 Ga was collected from the west-central Slave Province. These samples include conglomerates as well as finer sedimentary rocks such as greywacke, arkose and quartzite. Although it is generally agreed that the Nd model age ( T DM ) of clastic sedimentary rocks represents the average sedimentary provenance age, T DM of samples studied here is generally older than the average U–Pb age of detrital zircons extracted from the same sample. Rather, there seems to be a better ∼1:1 relationship between the T DM and U–Pb age of the oldest zircon from each sample. A simple model relationship between U–Pb geochronology and T DM suggests that such correlation is a result of Archean crustal evolution of the Slave Province being dominated by crustal reworking (i.e. intracrustal processes) rather than addition of juvenile materials from the mantle. One exception is the period between ∼3.1 and ∼3.2 Ga, during which period a significant portion of detrital zircons have U–Pb ages exceeding the T DM , indicating a large input of juvenile material and/or decreasing level of crustal reworking. An alternative explanation for the U–Pb vs. T DM trend may be a mixing of older mafic juvenile crust (containing no zircon) and younger felsic juvenile crust. However, the general lack of extensive pre-2.8 Ga mafic crust in the Slave Province makes this scenario unlikely. The results of this study cannot constrain the change in the volume of continental crust at any given period of time, since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle. However, a dominance of crustal reworking during the early- to mid-Archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events. Since the majority of zircon may be lost during crustal reworking processes (unless they are preserved as an inherited core), absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early Earth.

Miles Canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern Yukon. The volcanic rocks are part of the Northern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows at the Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4 Ma). The largest exposure of the Miles Canyon basalt occurs at the Alligator Lake volcanic complex where two nearly concordant Pliocene Ar-Ar dates indicate eruption at ca. 3.2 Ma. K-Ar analyses from other sites yield dates of 2.4 and 7.1 Ma and indicate an episodic Neogene volcanic history for the region. There is no evidence of Quaternary or postglacial volcanism. The dates are older than assumed by previous workers, and in some cases the K-Ar dates are strongly discordant from Ar-Ar determinations. More accurate Ar-Ar determinations may result from the method's ability to select smaller amounts of better material for analysis. Excess 40Ar was not encountered. As a result, the accuracy of any single or several discordant K-Ar determinations for Neogene subaerial volcanic rocks, particularly low-K rocks such as basalts, should be questioned and resulting interpretations made with caution. Models accounting for the eruption of the Northern Cordilleran volcanic province lavas have typically relied upon extension along north-trending faults that were generated by stresses along the continental margin. However, we consider a slab window model which better accounts for the initiation and distribution of northern Cordilleran Neogene volcanic activity.

The composition, chronology, and structural relations of the Saskatchewan Craton and enveloping mylonitic rocks exposed in basement windows of the Glennie Domain, Trans‐Hudson Orogen, have been determined by geochemical, geochronologic, and structural studies accompanying detailed field mapping. Basement windows lie along the hinge zone of a regional crustal culmination and consist mostly of 2.4–2.5 Ga felsic plutonic rocks enveloped by the Nistowiak Thrust. The Nistowiak Thrust is a folded, 1–2 km thick, upper amphibolite facies mylonite zone formed during emplacement of the Flin Flon–Glennie Complex across the Saskatchewan Craton. It is likely correlative to the Pelican Thrust, which envelops basement windows in the Hanson Lake Block ∼100 km to the east. An internal high strain zone within the overlying nappe pile, the Guncoat Thrust, is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites. U‐Pb geochronological results suggest calc‐alkaline plutonism from 1889–1837 Ma, thr...

New U–Pb, Re–Os, and 40Ar/39Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu–Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U–Pb and Re–Os ages for porphyritic stocks (U– Pb, zircon), associated prograde skarn (U–Pb, hydrothermal titanite), and retrograde stage skarn (Re–Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K–Ar ages obtained on various hydrothermal minerals. The new Jurassic age for the Nambija district is slightly younger than the 40Ar/39Ar and Re–Os ages for magmatic–hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic arc magmatism (∼190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization style (Au-skarn) to the metal endowment of this belt.

Data reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses).

On a warm, dark evening in early October, I had the privilege of finding myself in Pretoria, South Africa with a group of nurse colleagues from many southern African nations, listening as various leaders brought messages of greetings before we were to share dinner. The minister of health had fallen ill that day and was replaced at the last minute by her director general. The minister’s illness turned out to be an interesting turn of fate for us, because the comments of the director general amounted to a master class in policy – and he was able to say things that she surely never could have said.

There are reports of current nurse shortages in all but a few OECD countries. With further increases in demand for nurses expected and nurse workforce ageing predicted to reduce the supply of nurses, shortages are likely to persist or even increase in the future, unless action is taken to increase flows into and reduce flows out of the workforce or to raise the productivity of nurses.This paper analyses shortages of nurses in OECD countries. It defines and describes evidence on current nurse shortages, and analyses international variability in nurse employment. Additionally, a number of demand and supply factors that are likely to influence the existence and extent of any future nurse shortages are examined. In order to resolve nurse shortages, the paper compares and evaluates policy levers that decision makers can use to increase flows of nurses into the workforce, reduce flows out of the workforce, and improve nurse retention rates.Although delayed market response may have been ...

Upper Ordovician Garin conglomerates (base of the Gaspe belt, Quebec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Quebec, Newfoundland) subzone lavas. Clasts (monzonite-rhyodacite) give U-Pb zircon ages between 465 and 466 Ma, precluding correlation with Exploits rocks. We suggest that the suite represents peri-Laurentian continental arc magmas coeval with the Red Indian Lake Group of Newfoundland. Correlation of lavas from a Gaspe well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspe. (2) Mafic tholeiitic-to-alkaline clasts resemble New Brunswick and Maine (Exploits subzone) alkaline lavas. We interpret them as magmas associated with initiation of a back-arc basin, possibly on the Laurentian margin. (3) Mafic-intermediate tholeiitic to calc-alkaline clasts have an oceanic subduction component and correlate with Notre Dame subzone lavas (Quebec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea melange lavas to the calc-alkaline intermediat-felsic and mafic-intermediate suites and interpret them as samples of a Notre Dame subzone terrane coeval with the Red Indian Lake Group of Newfoundland during initiation of a successor arc–back-arc, a terrane which is no longer preserved in New Brunswick and Maine but that provides some evidence for possible extensions of Newfoundland9s geology into the Gaspe Appalachians of Quebec. A mid-ocean-ridge basalt–like intrusion in the McCrea melange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.

The Mesoproterozoic (1.47-1.40 Ga) Belt basin in western North America contains a thick succession of siliciclastic strata dominated by sediment derived from source areas that lay to the west and southwest of the present basin. The age and identity of this source area is important for reconstruction of the Mesoproterozoic history of western North America. Over 400 new SHRIMP U-Pb ages for detrital zircons, supplemented by Ar-Ar ages of detrital muscovite from sandstones within the Belt basin, document distinctive provenances of the basin fill; the predominant sediment influx came from a western craton, whereas subordinate input came from Laurentian sources to the east and south of the basin. During deposition of the lower two-thirds of the Belt basin, the western source was composed predominantly of Proterozoic rocks with 1920-1460 Ma ages; abundant grains have 1610-1500 Ma ages. These latter ages are unrepresentative of Laurentian sources, falling within the North American magmatic gap (1610-1490 Ma), but are widespread in central and eastern Australia and Baltica, the former being the most likely candidate for a former fluvial connection to western North America. The presence of detrital-zircon grains (1480-1440 Ma) with ages contemporary with sedimentation in the Belt basin suggests that the western source area hosted active magmatism during Belt sedimentation. The Missoula Group in the upper Belt records the sedimentologic effects of basin reorganization, which introduced sediment from a new southwestern source that, in contrast to earlier Belt history, resembles crust of the southwestern United States and lacks grains within the North American magmatic gap. Evidence for extension is widespread in the Belt basin in the form of mafic volcanic rocks, rapid rates of sedimentation, and intrabasinal syndepositional faults. The predominant contribution of detrital material from the western source area throughout Belt deposition, in concert with the large-scale facies patterns and evidence for rapid rates of subsidence, is taken to suggest that the Belt basin represents an extensional basin with a tectonically active western side. Although a wholly extensional environment has been suggested by previous workers, we suggest that an alternative model, wherein the Belt basin developed within an extensional domain along a collisional/convergent plate margin, analogous to Tethyan sedimentary basins such as the Black and Caspian Seas, is equally viable and paints a different picture of western North America at ca. 1450 Ma.

New 40 Ar/ 39 Ar geochronology and global cyclostratigraphic calibration provide high-resolution insights into the timing of geochemical fluctuations across oceanic anoxic event II (OAE II). Five new 40 Ar/ 39 Ar data from a single section in western Canada suggest an age of 96.4 ± 1 Ma for the Cenomanian-Turonian (C-T) boundary. This is ∼3 m.y. older than previously dated. Time-series analysis (e.g., wavelet transform) reveals that OAE II was synchronously deposited worldwide over a period of ∼320 k.y. The preservation and stationarity of Milankovitch cyclicity in sections across the Atlantic indicates that no major fluctuations in sedimentation rate occur across the OAE II, except for a minor short- term decrease in the rate just before the C-T boundary. A δ 13 C org increase of >2‰ took place at the beginning of OAE II over a period of ∼110 k.y., and likely represents a period of worldwide excessive organic carbon burial during transgression, leading to a 12 C depletion of seawater and sedimentation. This interval was followed by a three-step decrease to pre-OAE II δ 13 C values.

The Anton terrane of the western Slave Province has been interpreted as a remnant of a meso-Archean (3500–2800 Ma) microcontinent. The largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex, herein termed the Snare domain. The granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso-Archean basement in the Slave Province. The domain consists of interlayered 3100–2624 Ma orthogneisses and paragneisses and 2600–2585 Ma granitoid rocks; its structure was largely established during 2610–2590 Ma orogeny throughout the province that culminated in ca. 2590 Ma granulite facies metamorphism. Pressure-temperature ( P-T ) determinations yield peak conditions of 775–875 °C at 6–7 kbar. Shortly thereafter, 2–4 kbar of decompression occurred before the domain cooled below 450 °C. Decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults. U-Pb zircon age data suggest that the Snare domain was partially exhumed from mid-crustal levels ca. 2585–2580 Ma. We attribute exhumation to tectonic denudation of an overthickened crustal welt in the western Slave Province. Localization of the crustal welt in this part of the Slave Province may be related to the presence there of meso-Archean crust.

Lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle-derived magmas and continental crust and the three independent decay paths in the U-Th-Pb system. Preserved within Wopmay Orogen (1.94 to 1.84 Ga) are the remnants of three magmatic arcs. The isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with $^{206}Pb/^{204}Pb = 15.59-15.88$, $^{207}Pb/^{204}Pb = 15.27-15.36$, and $^{208}Pb/^{204}Pb = 35.15-35.54$, except for samples from the only plutonic suite within Wopmay Orogen observed to intrude Archean crust. Plutons intruding Archean crust have $^{206}Pb/^{204}Pb = 15.29-15.49$, $^{206}Pb/^{204}Pb= 15.23-15.31$, and $^{208}Pb/^{204}Pb = 35.21-35.30$. Lead isotopic compositions do not correlate with rock composition: both diorites and peraluminous granites have the same lead isotopic composition. This isotopic composition...

The accuracy of published Jurassic time scales is compromised by a paucity of sufficiently accurate and stratigraphically well-constrained isotopic dates. U–Pb dating of volcaniclastic rocks within fossiliferous marine successions is a method suitable of providing additional useful calibration points. Stikinia and Wrangellia, two of the major accreted terranes of the Canadian Cordillera, comprise volcanosedimentary assemblages of magmatic arc origin. Within them we selected sections from which we report 14 new U–Pb zircon ages that are integrated with ammonite biochronology at the zonal level. Poor zircon recovery and complex U–Pb systematics made it difficult to obtain precise ages for many of the samples. The following ages were determined (with 2σ errors): 194.0+9.1–1.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian), 191.5 ± 0.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian) to the Whiteavesi Zone (lower Pliensbachian), 185.6+6.1–0.6 Ma from the Freboldi Zone (lower Pliensbachian), 184.7 ± 0.9 Ma from the Kunae Zone (upper Pliensbachian), 180.4+8.0–0.4 Ma from the Kanense to Planulata zones (lower to middle Toarcian), 179.8 ± 6.3 Ma from the Yakounensis Zone (upper Toarcian) to the lower Bajocian, 167.2+10.5–0.4 Ma from the Rotundum Zone (upper Bajocian), 158.2+1.9–0.4 Ma from the upper Bathonian, and 162.6+2.9–7.0 from the upper Bathonian to lower Callovian. Five other dates are inferior in precision, but still provide some constraints for time-scale calibration. The U–Pb dates reported here together with a wealth of other recently obtained Cordilleran isotopic dates will be used for a significant revision of the Jurassic time scale.