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The magmatic evolution of the Early Jurassic Turnagain Alaskan-type intrusion is inextricably linked to terrane accretionary events at the North American continental margin. We have calibrated the multistage assembly of the Turnagain ultramafic-mafic complex using high-precision air and chemical abrasion ID-TIMS U-Pb and [.sup.40]Ar/[.sup.39]Ar geochronology. Our results are the first published geochronological calibration of zoning in an Alaskan-type intrusion. Three of four intrusive phases (Stages 1-4, oldest to youngest) were successfully dated. Cooling ages obtained on Stage 2 dunite-wehrlite include statistically identical [.sup.40]Ar/[.sup.39]Ar plateau dates of 187.4 [+ or -] 1.5 (2[sigma]) Ma on hornblende and 188.6 [+ or -] 1.2 Ma on phlogopite, and a concordant [.sup.206]Pb/[.sup.238]U date of 190.3 [+ or -] 4.6 Ma on titanite. Stage 3 diorite has a [.sup.206]Pb/[.sup.238]U zircon crystallization age of 188.11 [+ or -] 0.13 Ma. Stage 4 wehrlite and leucodiorite yield [.sup.206]Pb/[.sup.238]U zircon crystallization ages of 185.68 [+ or -] 0.19 and 185.30 [+ or -] 0.12 Ma, respectively. The ca. 189 Ma phlogopite date represents a minimum crystallization age for Stage 2, indicating that assembly of the Turnagain intrusion spanned at least ~4 million years from ca. 189 Ma to 185 Ma. Regional progressive contractional deformation in the Early Jurassic is constrained by the geochronological results (>189 to 189 Ma), deformed Stage 1 wehrlite-clinopyroxenite prior to emplacement of Stages 2-4, and subsequently thrust the intrusion and its host rocks onto the ancient North American continental margin ( Key words: Alaskan-type intrusion, geochronology, Early Jurassic, terrane accretion, Yukon-Tanana terrane. L'evolution magmatique de l'intrusion de type alaskien de Turnagain, d'age jurassique precoce, est inextricablement reliee a des episodes d'accretion de terranes a la bordure du continent nord-americain. Nous avons etalonne l'assemblage en plusieurs etapes du complexe ultramafique-mafique de Turnagain en utilisant la geochronologie U-Pb de haute precision par ID-TIMS avec abrasion chimique et a l'air et la geochronologie [.sup.40]Ar/[.sup.39]Ar. Nos resultats constituent le premier etalonnage publie de la zonation geochronologique dans une intrusion de type alaskien. Des ages ont ete obtenus pour trois des quatre phases intrusives (phases 1-4, de la plus vieille a la plus jeune). Les ages de refroidissement de la dunite-wehrlite de la phase 2 comprennent des ages plateaux [.sup.40]Ar/[.sup.39]Ar statistiquement identiques de 187,4 [+ or -] 1,5 (2[sigma]) Ma sur hornblende et 188,6 [+ or -] 1,2 Ma sur phlogopite, et un age [.sup.206]Pb/[.sup.238]U concordant de 190,3 [+ or -] 4,6 Ma sur titanite. La diorite de phase 3 a un age de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 188,11 [+ or -] 0,13 Ma. La wehrlite et la leucodiorite de la phase 4 donnent des ages de cristallisation [.sup.206]Pb/[.sup.238]U sur zircon de 185,68 [+ or -] 0,19 et 185,30 [+ or -] 0,12 Ma, respectivement. L'age de ~189 Ma sur phlogopite represente un age minimum de cristallisation pour la phase 2, qui indique que l'assemblage de l'intrusion de Turnagain s'est etale sur au moins ~4 millions d'annees, de ~189 Ma a 185 Ma. Une deformation regionale progressive par contraction au Jurassique precoce delimitee par les resultats geochronologiques (>189 Ma a 189 Ma), la deformation de la wehrlite-clinopyroxenite de phase 1 avant la mise en place des roches des phases 2 a 4 et le chevauchement subsequent de l'intrusion et des roches encaissantes sur l'ancienne bordure continentale nord-americaine ( Mots-cles : intrusion de type alaskien, geochronologie, Jurassique precoce, accretion de terranes, terrane de Yukon-Tanana., Introduction The Early Jurassic Turnagain Alaskan-type intrusion in north-central British Columbia, situated 70 km east of Dease Lake (Fig. 1 inset), belongs to a global class of ultramafic-mafic intrusions emplaced [...]

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 [...]

ABSTRACT Machias Seal Island in the northern Gulf of Maine consists of fine- to medium-grained weakly foliated quartz monzodiorite. Although previously inferred to be as young as Devonian, the quartz [...]

The eastern Coast Plutonic Complex in west-central British Columbia (51-54[degrees]N) consists of a broad belt of Jurassic to Eocene granitic to dioritic intrusive rocks that collectively represent more than 140 m.y. of nearly continuous, subduction-related magmatism. Seven distinct intrusive suites have been identified in the Bella Coola region, including, from oldest to youngest, the Howe Lake, Stick Pass, Firvale, Desire, Fougner, Big Snow, and Four Mile suites. PreLate Cretaceous rocks were produced by episodic subduction-related magmatism characterized by partial melting of preexisting lower-arc crust, with minimal incorporation of evolved continental material. The eastern Coast Plutonic Complex experienced a magmatic lull between ca. 110 and 95 Ma, coincident with sinistral transpression potentially linked to translational displacement of the Wrangellia-Alexander composite terranes. Crustal thickening in Late Cretaceous time (ca. 100-90 Ma), possibly due to underplating of the Wrangellia-Alexander terrane, produced a deep crustal root (>40 km) that likely extended into the eclogite transition zone. Adakitic magmas (ca. 95-67 Ma) were produced by partial melting of an amphibolitic protolith in this thickened lower-arc crust. Adakitic plutons are pre-, syn-, and postkinematic with respect to Late Cretaceous high-angle shear zones, and they are coeval and spatially associated with post-kinematic two-mica granites (ca. 70-75 Ma) having a hybrid A-type geochemistry. We suggest that delamination of this dense crustal root led to voluminous magmatism, extension, and crustal exhumation in Paleocene-Eocene time. The successive intrusive events in the eastern Coast Plutonic Complex may reflect a predictable evolutionary progression common to all continental arc systems.

Abstract: This study presents isotope dilution--thermal ionization mass spectrometry (ID-TIMS) U-Pb data for megacrystic zircon from the Cabonga Nepheline Syenite Complex of the Reservoir Cabonga terrane (Grenville Province). This terrane [...]

New results from integrated biochronologic and geochronometric studies on the basal Jurassic section at Puale Bay (Alaska Peninsula) improve the calibration of the Early Jurassic time scale. Previously, the interval around the Triassic-Jurassic system boundary was poorly dated, which hampered our understanding of geologic and biotic events, e.g., the end-Triassic mass extinction and subsequent recovery. Published suggestions for the presence of the earliest Hettangian (Planorbis Zone) and a continuous boundary section at Puale Bay are not substantiated. Although the Kamishak Formation is likely to contain an uninterrupted sedimentary record, pre-middle Hettangian strata are locally faulted, resulting in an apparent Rhaetian to early Hettangian gap in the fossil record. The Hittangian ammonite zonal schemes developed locally for Nevada and the Queen Charlotte Islands permit reliable correlation with Alaska, but have limited applicability. The faunal succession recorded at Puale Bay is useful in the development of a regional zonation for North America. We obtained three U-Pb zircon dates that are tied into an ammonite biochronology at the zonal level. A middle Hettangian tuff layer from near the top of the Kamishak Formation is dated at 200.8(super +2.7)/(sub 2.8) Ma. Tuffs from the overlying Talkeetna Formation are bracketed by middle and late Hettangian ammonites and yield crystallization ages of 197.8(super +1.2)/(sub 0.4) and 197.8 +/- 1.0 Ma. These new calibration points require that the Hettangian-Sinemurian boundary be younger than 199 Ma. The Triassic-Jurassic boundary is likely to fall between 200 and 205 Ma. Similar studies are needed for the uppermost Triassic to obtain tighter constraints. Zircon U-Pb systematics of two samples revealed strong Proterozoic and late Archean inheritance patterns that require revised tectonic models to account for the proximity of the Talkeetna arc to evolved crustal blocks.

The magnitude of late Mesozoic terrane translation along the western North American continental margin is controversial. Paleomagnetic data suggest more than 3000 km of dextral displacement of the Insular superterrane and associated assemblages between ca. 74 and 50 Ma. Conversely, geologic evidence is more compatible with less than 500 km of offset during this time. Precambrian cratonic rocks older than 2.5 Ga are restricted to northern latitudes (above 40 [degrees] N) in North America, and the presence or absence of Archean zircons within easterly derived Cretaceous strata has been proposed as a 'crucial test' of the major translation hypothesis. There is no plausible source of Archean and Early Proterozoic zircons found in the Queen Charlotte, Nanaimo, and Methow basins other than the Canadian Shield or closely associated Proterozoic strata. The coexistence of Archean-Early Proterozoic, Middle Proterozoic (1.5-1.6 Ga) and Mississippian detrital zircon in these basins indicates deposition in northern latitudes, incompatible with the Baja B.C. hypothesis.

We report two early Tertiary U-Pb zircon ages for pegmatite from the Black Mountains of Death Valley, California. These ages, 54.7 [+ or -] 0.6 Ma and 56 [+ or -] 3 Ma, are unique for much of southeastern California. The samples belong to a pegmatite suite that occupies part of the footwall of the Badwater turtleback, a late Tertiary extensional feature; similar but undated pegmatite intrudes the footwalls of the Copper Canyon and Mormon Point turtlebacks farther south. The pegmatite suite demonstrates that fabric development on the turtlebacks was at least a two-stage process. Fabrics cut by these pegmatites likely formed during the Mesozoic, whereas those that involve them formed during late Tertiary extension.

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics. [ABSTRACT FROM AUTHOR]

The Lower Main zone of the Lorraine alkalic porphyry Cu-Au district, BC, Canada, is hosted in an intrusive complex that comprises pre- and late mineral biotite pyroxenites, monzonites, and syenites. Some of the biotite pyroxenites have interstitial sulfides, which appear similar to net-textured sulfides in magmatie ore deposits; however, the pyroxenites have undergone several stages of hydrothermal alteration. Main-stage mineralization produced sulfide zonation patterns consisting of a bornite-chalcocite or bornite-chalcopyrite core grading outward to domains of chalcopyrite, chalcopyrite > pyrite, and a peripheral domain of pyrite with minor chalcopyrite. Syenite in the inner bornite-chalcopyrite zone typically contains abundant turbid K-feldspar (>70%), whereas syenite marginal to the bornite-chalcopyrite core contains less K-feldspar (50-70%), indicating an increase in K metasomatism of syenites toward the core of the deposit. Main-stage mineralization predominantly occurs as fine-grained disseminated sulfides in syenite, biotite pyroxenite, and fine-grained K-feldspar biotite rock. Textural analyses have shown that (1) primary magmatie diopside in contact with sulfides has corroded and actinolite-altered margins, suggesting alteration of primary minerals, (2) scalloped relicts of biotite and diopside occur as inclusions in sulfides, implying that sulfides have replaced primary minerals, and (3) the deposit-scale sulfide zonation patterns overprinted numerous rock types, including the biotite pyroxenites that contain interstitial sulfides. These results provide evidence for replacement-style mineralization at Lorraine, where primary magmatie biotite and diopside were totally or partially replaced by sulfide minerals during main-stage alteration. U-Pb zircon dates from pre- and late mineral syenite dikes show that the timing of main-stage mineralization occurred between 178.8 and 176.0 Ma. Lorraine is hosted within the Quesnel island arc terrane, and mineralization and magmatism at Lorraine postdate accretion of the Quesnel terrane to ancestral North America by approximately 7 to 10 m.y. The Lorraine deposit represents the youngest known alkalic Cu-Au porphyry deposit within the Quesnel terrane, and appears to coincide with the last gasp of alkalic magmatism within the Quesnel terrane during the Mesozoic. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)