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Miocene strata of the Claremont, Orinda, and Moraga formations of the Berkeley Hills (California Coast Ranges, USA) record sedimentation and volcanism during the passage of the Mendocino triple junction and early evolution of the San Andreas fault system. Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) age spectra indicate a change in sedimentary prove nance between the marine Claremont formation (Monterey Group) and the terrestrial Orinda and Moraga Formations associated with uplift of Franciscan Complex lithologies. A sandstone from the Claremont formation produced a detrital zircon chemical abrasion–isotope dilution–thermal ionization mass spec trometry (CA-ID-TIMS) maximum depositional age of 13.298 ± 0.046 Ma, indicating younger Claremont deposition than previously interpreted. A trachydacite tuff clast within the uppermost Orinda Formation yielded a CA-ID-TIMS U-Pb zircon date of 10.094 ± 0.018 Ma, and a dacitic tuff within the Moraga Formation produced a CA-ID-TIMS U-Pb zircon date of 9.974 ± 0.014 Ma. These results indicate rapid progression from subsidence in which deep-water siliceous sediments of the Claremont formation were deposited to uplift that was followed by subsidence during deposition of terrestrial sediments of the Orinda Forma tion and subsequent eruption of the Moraga Formation volcanics. We associate the Orinda tuff clast and Moraga volcanics with slab-gap volcanism that followed the passage of the Mendocino triple junction. Given the necessary time lag between triple junction passage and the removal of the slab that led to this volcanism, subsidence associated with ca. 13 Ma Claremont sedimentation and subsequent Orinda to Moraga deposition can be attributed to basin formation along the newly arrived transform boundary.

The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620-600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma. Key words: U-Pb geochronology, Appalachian metamorphism, Bras d'Or terrane, Cape Breton Island, McMillan Flowage Formation, detrital zircon, plutonic ages, Introduction The Bras d'Or terrane of central Cape Breton Island, Nova Scotia (Fig. 1), consists of mainly Ediacaran and Cambrian metamorphic and plutonic rocks overlain unconformably by Carboniferous sedimentary rocks [...]

The Coldbrook Group and related plutons in the Caledonian Highlands of southern New Brunswick contain voluminous late Ediacaran silicic rocks formed in a magmatic event not recognized in other parts of Avalonia in the northern Appalachian orogen. To better constrain the age and origin of these rocks, we used U-Pb zircon dating by laser ablation inductively coupled plasma mass spectrometry to check for older inherited zircon and obtain trace element data, followed by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-TIMS) to obtain precise dates. Four silicic samples were dated from the Coldbrook Group, one from the Bonnell Brook pluton, and, for comparison, a felsic lithic-crystal tuff sample from the older arc-related Broad River Group. Overlapping CA-TIMS dates of 551.57 [+ or -] 0.23, 551.38 [+ or -] 0.24, and 551.70 [+ or -] 0.20 Ma for samples from the lower, middle, and upper Coldbrook Group, respectively, and 551.71 [+ or -] 0.19 Ma for granite from the Bonnell Brook pluton show that these units crystallized in 760 000 years or less, consistent with a super-eruption event. Rhyolite from the uppermost unit of the Coldbrook Group yielded a younger date of 549.18 [+ or -] 0.09 Ma, but the large extent of that unit is consistent with the possibility of a second younger super-eruption. The felsic lithic-crystal tuff sample from the Broad River Group yielded a date of 615.48 [+ or -] 0.16 Ma, consistent with previously published dates from that group and associated plutons. Differences in zircon chemistry between the Broad River Group sample and the late Ediacaran samples are consistent with the contrasting subduction-related vs within-plate extensional tectonic settings as suggested by previous studies of whole-rock petrological characteristics of the two age groups. Key words: U-Pb zircon dating, super-eruptions, late Ediacaran, Avalonia, zircon chemistry, Introduction The Caledonia terrane of southern New Brunswick, Canada, is a component of Avalonia in the northern Appalachian orogen (Fig. 1, inset). The original relations among the now-geographically separated parts [...]

The MacCodrum Formation is a classical 'lower' Cambrian unit in southeastern Cape Breton Island stratigraphy, described since the 1800s. The age of this formation and its correlation with other Avalonian Cambrian units in eastern Newfoundland and southern New Brunswick have remained uncertain through numerous revisions. Here we present U-Pb CA-TIMS ages from an ash bed in the basal part of the MacCodrum Formation in its type-section on MacCodrum Brook that fix the maximum time of deposition at 531.86 [+ or -] 0.34 Ma. Organic-walled microfossils sampled throughout the MacCodrum Formation type-section yield acritarch taxa identifying the Asteridium-Comasphaerdium Zone, whereas the first acritarchs of the Skiagia-Fimbriaglomerella Zone appear in the overlying Canoe Brook Formation in other sections. The radiometric age and acritarch zonation place the MacCodrum Formation in the upper Fortunian, Cambrian Stage 2. Among trace fossils in the MacCodrum Formation, the meandering trace fossil Didymaulichnus dailyi comb. nov. is of particular note and morphologically identical to the type material from the lower part of the Ratcliffe Brook Formation in New Brunswick. The new radiometric and biostratigraphic data presented here provide the first firm constraints on the age of the MacCodrum Formation and enable more precise correlation with sections in southern New Brunswick and eastern Newfoundland. Key words: acritarch, trace fossils, Cambrian, U-Pb zircon, Avalonia, correlation, Introduction Cambrian rocks in southeastern Cape Breton Island, Nova Scotia (Fig. 1), have been visited and described by geologists since the 1800s (e.g., Fletcher 1878; Matthew 1903; Hutchinson 1952; Landing [...]

Key words: acritarchs, trace fossils, Cambrian, U-Pb zircon, Avalonia, correlation, We thank Landing and Geyer for their Discussion of our paper, thus drawing further attention to its significance. We do not wish to engage in a detailed 'they say--we say' [...]

New zircon U-Pb, trace element, and Lu-Hf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data are presented for five Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA. These samples were previously analyzed by U-Pb LA-ICP-MS and whole-rock geochemical methods and the new data generally corroborate the earlier results. Coarse-grained alkali-feldspar granite from northwest of the Fundy magnetic anomaly, the interpreted fault boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 414 [+ or -] 2 Ma. Southeast of the inferred fault, crystallization ages are 385 [+ or -] 3 Ma and 386 [+ or -] 3 Ma for two crystal tuff samples near the fault, 403 [+ or -] 3 Ma for an alkali-feldspar granite -50 km southeast of the fault, and 399 [+ or -] 5 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 [+ or -] 15 Ma and 558 [+ or -] 9 Ma. Lu-Hf LA-ICP-MS data for zircon retaining igneous crystallization ages have [[epsilon].sub.Hf(t)] between 2.9 and 13.1 and model ages based on felsic sources between 0.52 and 1.04 Ga, refLecting a mix of late Mesoproterozoic (Avalonian?) basement and primitive melt, possibly in an extensional setting. Zircon Nb/Hf ratios generally greater than 0.001 indicate a predominately within-plate/anorogenic/rifr setting, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams show ocean island to continental arc signatures, with a stronger continental arc signature for the syenogranite. Most zircon grains have Eu/Eu* values less than 0.1, indicating a crustal thickness of ~30 km or less at the time of their crystallization. De nouvelles donn es de datation U-Pb sur zircon, d'analyse d' l ments traces et de spectrom trie de masse plasma inductif et ablation par laser (LA-ICP-MS) des fins de datation par le Lu-Hf sont pr sent es par rapport cinq roches granitiques et volcanofelsiques du D vonien pr coce moyen provenant de la suite ign e Cashes Ledge, dans le centre du golfe du Maine, aux tats-Unis. Les chantillons en question avaient pr c demment t analys s par les m thodes de l' tude g ochimique sur roche totale et LA-ICP-MS de datation U-Pb, et les nouvelles donn es corroborent de fa on g n rale les r sultats ant rieurs. Le granite alcalo-feldspathique gros grains de l'anomalie magn tique du nord-ouest de Fundy, limite interpr t e de la faille au large entre les micro continents gondwaniens de Ganderia au nord-ouest et d'Avalonia au sud-est, a accus un ge de cristallisation de 414 [+ or -] 2 Ma. Au sud-est de la faille inf r e, les ges de la cristallisation sont de 385 [+ or -] 3 Ma et 386 [+ or -] 3 Ma dans le cas de deux chantillons de tuf cristallin pr s de la faille, 403 [+ or -] 3 Ma dans le cas d'un granite alcalo-feldspathique une cinquantaine de kilom tres au sud-est de la faille, et 399 [+ or -] 5 Ma dans le cas du sy nogranite environ 25 km au sudest de la faille, qui a aussi livr des grains h rit s d'environ 1,3 Ga et d'entre 613 [+ or -] 15 Ma et 558 [+ or -] 9 Ma. Les donn es LA-ICP-MS de datation par le Lu-Hf du zircon r v lant les ges de la cristallisation ign e situent les ges entre 2,9 et 13,1 s'inspirant de sources felsiques entre 0,52 et 1,04 Ga, ce qui t moigne d'un m lange d'une fusion primitive et d'une fusion du socle tardive m soprot rozoique (avalonienne?), possiblement dans un milieu d'extension. Les rapports Nb/Hf sur zircon g n ralement sup rieurs 0,001 signalent un milieu intraplaque/anorog nique/de rift correspondant la composition chimique de sa roche totale. Des sch mas de discrimination des milieux tectoniques U/Yb-Nb/Yb et U/Yb-Hf affichent des signatures d' les oc aniques et des arcs continentaux dans le cas du sy nogranite. La majorit des grains de zircon pr sentent des concentrations d'Eu/Eu* de moins de 0,1 t moignant d'une paisseur de la cro te d'une trentaine de kilom tres ou moins au moment de leur cristallisation., INTRODUCTION We obtained zircon U-Pb, trace element, and Lu-Hf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data to investigate the nature, tectonic setting, and basement of five Early to Middle [...]

The Fire Bay Formation of Trettin (1998), Clements Markham belt, Ellesmere Island, Canada, includes volcanic rocks described as Silurian in age based on Llandovery graptolites in adjacent clastic rocks. New field observations suggest the Llandovery fossils are from packages of the Silurian Danish River and/or Lands Lokk formations that are fault-bounded rather than stratigraphically tied to Ordovician sections that contain a 470.0 [+ or -] 0.2 Ma lithic tuf, volcaniclastic units with maximum depositional ages (MDAs) of 466 [+ or -] 2 and 462 [+ or -] 2 Ma based on detrital zircon, volcanic clasts with ages of 498 [+ or -] 6, 478 [+ or -] 4, and 477 [+ or -] 8 Ma, and Ordovician conodonts and graptolites of Darriwilian and Sandbian age, respectively. Since the Fire Bay Formation of Trettin (1998) lacks a type section and is fault-bounded with ambiguous age relationships, Ordovician volcanic units and fault-bounded clastic rocks correlated with the Hazen Formation are both included in the Fire Bay assemblage following the original interpretations of Trettin and Nowlan (1990). The Fire Bay assemblage records juvenile Ordovician arc magmatism proximal to the Pearya terrane. The adjacent Lands Lokk Formation yields bimodal age peaks at 440-430 and 465 Ma, MDA of 424 [+ or -] 3 Ma, and [epsilon][Hf.sub.(t)] values of -5 to +10. The signature matches Ordovician Pearya units and Silurian circum-Arctic arc sources but there is no evidence for Silurian arc magmatism between the Pearya terrane and Laurentian margin, compatible with Pearya accretion during oblique Ordovician arc collision and Silurian sinistral translation along the northern Laurentian margin. Key words: U/Pb dating, Pearya terrane accretion, Ordovician arcs, biostratigraphy, detrital zircon, Ellesmere Island La Formation de Fire Bay deTrettin (1998), de la ceinture de Clements Markham de l'île d'Ellesmere (Canada), comprend des roches volcaniques décrites comme étant d'âge silurien à la lumière de graptolites llandovériens dans des roches clastiques attenantes. De nouvelles observations de terrain portent à croire que les fossiles llandovériens proviennent de paquets des formations siluriennes de Danish River ou de Lands Lokk qui sont bordés par des failles plutôt que d'être reliés stratigraphiquement à des séquences ordoviciennes qui contiennent un tuf lithique de 470,0 [+ ou -] 0,2 Ma, des unités volcanoclastiques d'âge de dépôt maximum (ADM) de 466 [+ ou -] 2 Ma et 462 [+ ou -] 2 Ma à la lumière de zircons détritiques, des clastes volcaniques ayant produit des âges de 498 [+ ou -] 6 Ma, 478 [+ ou -] 4 Ma et 477 [+ ou -] 8 Ma et des conodontes et graptolites ordoviciens d'âge darriwillien et sandbien, respectivement. Comme la Formation de Fire Bay de Trettin (1998) n'a pas de coupe type et est bordée par des failles dont les relations d'âge sont ambiguës, des unités volcaniques ordoviciennes et des roches clastiques bordées par des failles corrélées à la Formation de Hazen sont incluses dans l'assemblage de Fire Bay, suivant les interprétations initiales de Trettin et Nowlan (1990). L'assemblage de Fire Bay témoigne d'un magmatisme d'arc ordovicien proximal par rapport au terrane de Pearya. La Formation de Lands Lokk attenante donne des pics d'âges bimodaux à 440-430 Ma et 465 Ma, un ADM de 424 [+ ou -] 3 Ma et des valeurs de [epsilon][Hf.sub.(t)] de -5 à +10. Cette signature concorde avec des unités ordoviciennes de Pearya et des sources d'arc circumarctique siluriennes, mais il n'y a aucun indice de magmatisme d'arc silurien entre le terrane de Pearya et la marge laurentienne, ce qui est compatible avec l'accrétion de Pearya durant la collision oblique d'un arc ordovicien et une translation senestre silurienne le long de la marge septentrionale de la Laurentie. [Traduit par la Rédaction] Mots-clés : datation U/Pb, accrétion du terrane de Pearya, arcs ordoviciens, biostratigarphie, zircon détritique, île d'Ellesmere, Introduction The Pearya terrane on the northern margin of Laurentia plays a critical role in tectonic models for the Paleozoic evolution of the circum-Arctic region that advocates for large-scale transfer [...]

Current investigations into the Albian–Cenomanian sedimentary record within the Western Interior have identified multiple complex tectono‐sedimentary process–response systems during the ongoing evolution of North America. One key sedimentary succession, the upper Cedar Mountain Formation (Short Canyon Member and Mussentuchit Member), has historically been linked to various regionally and continentally significant tectonic events, including Sevier fold‐and‐thrust deformation. However, the linkage between the Short Canyon Member and active Sevier tectonism has been unclear due to a lack of high‐precision age constraints. To establish temporal context, this study compares maximum depositional ages from detrital zircons recovered from the Short Canyon Member with that of a modified Bayesian age stratigraphic model (top‐down) to infer that the Short Canyon Member was deposited at ca 100 Ma, penecontemporaneous with rejuvenated thrusting across Utah [Pavant (Pahvant), Iron Springs and Nebo thrusts]. These also indicate a short depositional hiatus with the lowermost portion of the overlying Mussentuchit Member. The Short Canyon Member and Mussentuchit Member preserve markedly different sedimentary successions, with the Short Canyon Member interpreted to be composed of para‐autochthonous orogen–transverse (across the Sevier highlands) clastics deposited within a series of stacked distributive fluvial fans. Meanwhile, the muddy paralic Mussentuchit Member was a mix of orogen–transverse (Sevier highlands and Cordilleran Arc) and orogen–parallel basinal sediments and suspension settling fines within the developing collisional foredeep. However, the informally named last chance sandstone (middle sandstone of the Mussentuchit Member) is identified as an orogen–transverse sandy debris flow originating from the Sevier highlands, similar to the underlying Short Canyon Member. During this phase of landscape evolution, the Short Canyon Member – Mussentuchit Member depocentre was a sedimentary conduit system that would fertilize the Western Interior Seaway with ash‐rich sediments. These volcaniclastic contributions, along with penecontemporaneous deposits across the western coastal margin of the Western Interior Seaway, eventually would have lowered oxygen content and resulted in a contributing antecedent trigger for the Cenomanian–Turonian transition Oceanic Anoxic Event 2. [ABSTRACT FROM AUTHOR]

The Earn Group of central Yukon records the transition from a passive to an active margin along western Laurentia in the Late Devonian. Fine-grained clastic rocks and chert of the lower Earn Group contain late Early to Middle Devonian fossils and were deposited in an offshelf environment. The upper Earn Group comprises a mixture of sandstone and conglomerate, fine-grained siliciclastic rocks, and widespread crystal lithic tuff. Zircons from this succession are precisely dated using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) methods on igneous (ca. 363 Ma) and detrital (ca. 378-363 Ma) grains and the ages were confirmed by Frasnian to Famennian fossils. Abrupt, along-strike facies changes within the upper Earn Group of the Glenlyon-Tay River area occur across mapped faults that are inferred to have originated as syndepositional extension faults in the Late Devonian. Occurrences of ca. 363 Ma tuff horizons within all facies of the upper Earn Group provide a temporal correlation across the area. Diorite plutons intrude lower Paleozoic rocks in the area and have U-Pb zircon crystallization dates of ca. 364 Ma. The diorite has calc-alkaline composition consistent with arc magmatism or crustal contamination. The Late Devonian magmatism in the Earn Group is coincident with onset of arc magmatism in the allochthonous Yukon-Tanana terrane, and extension related to rifting and opening of the Slide Mountain ocean in a back-arc setting. Magmatic rocks in the Earn Group of central Yukon thus represent part of a remnant continental arc and back-arc stranded behind the Slide Mountain ocean in the Mississippian. Key words: Late Devonian, magmatism, Earn Group, extension, western Laurentia, biostratigraphy. Le Groupe d'Earn dans le centre du Yukon preserve des preuves d'une transition d'une marge passive a une marge active le long de la Laurentie occidentale au Devonien tardif. Des roches clastiques a grains fins et deschertsdu Groupe d'Earn inferieur renferment des fossiles d'age devonien precoce tardif a devonien moyen et ont ete deposes dans un milieu oceanique. Le Groupe d'Earn superieur comprend un melange de gres et de conglomerats, de roches silicoclastiques a grains fins et de tuf cristallolithique ubiquiste. Des zircons provenant de cette succession sont dates de maniere precise par des methodes de spectrometrie de masse d'ionisation thermique d'ionisation d'isotope d'abrasion chimique (CA-ID-TIMS) appliquees a des grains ignes (~363 Ma) et detritiques (~378-363 Ma), ces ages etant confirmes par des fossiles frasniens et famenniens. Des changements de facies abrupts paralleles a la direction des strates dans le Groupe d'Earn superieur de la region de Glenlyon-riviere Tay sont observes de part et d'autre de failles cartographiees interpretees comme etant, a leur origine au Devonien precoce, des failles d'extension synsedimentaires. La presence d'horizons de tuf de ~363 Ma dans tous lesfaciesduGrouped'Earn superieur fournit une correlation temporelle sur l'ensemble de la region. Des plutons de diorite recoupent des roches du Paleozo'ique inferieur dans la region et donnent des ages U-Pb de cristallisation des zircons de ~364 Ma. La diorite presente une composition calco-alcaline que pourraient expliquer un magmatisme d'arc ou une contamination crustale. Le magmatisme d'age devonien tardif dans le Groupe d'Earn coincide avec le debut du magmatisme d'arc dans le terrane allochtone de Yukon-Tanana et avec l'extension associee au rifting et a l'ouverture de l'ocean de Slide Mountain dans un milieu d'arriere-arc. Les roches magmatiques dans le Groupe d'Earn du centre du Yukon representent donc une partie des vestiges d'un arc continental et d'un arriere-arc echoues derriere l'ocean de Slide Mountain au Mississipien. [Traduit par la Redaction] Mots-cles: Devonien tardif, magmatisme, Groupe d'Earn, extension, Laurentie occidentale, biostratigraphie., Introduction The western edge of North America (or Laurentia in the Paleozoic) was a passive continental margin from early Cambrian to Middle Devonian (Bond et al. 1983; Bond and Kominz [...]

New whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the "Leka sector" of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere. Plain Language Summary: The Leka Ophiolite Complex (LOC) represents a preserved fragment of oceanic crust that formed during subduction in the Iapetus Ocean. Geochemical information recorded in the LOC rocks shows that it formed during the initial phase of subduction. The age of subduction initiation in the Iapetus Ocean is estimated at 491.36 million years ago based on isotopic dating of minerals within the LOC rocks. Other fragments of preserved oceanic crust with similar ages are found in the Appalachian–Caledonian mountains; however, their geochemical information suggests that they may have formed during different stages of the subduction zone development. We consider the variations in the oceanic crustal record to reflect selective preservation of different parts of the variable oceanic crust formed during the development of a subduction zone in the Iapetus Ocean. Key Points: The Leka Ophiolite Complex (LOC) preserves a record of geochemical variation from forearc basaltic to boninitic magmatism, reflecting formation during initiation and early evolution of a subduction zoneA 491.36 ± 0.17 Ma U–Pb zircon date for an LOC forearc basalt is considered to date subduction initiation in the "Leka sector" of the Iapetus OceanDifferences between the LOC pseudostratigraphy and the model Izu‐Bonin‐Mariana forearc may result from selective preservation of the spatially variable forearc lithosphere in addition to the specific history of formation, obduction, deformation, and uplift/erosion records [ABSTRACT FROM AUTHOR]

The northernmost part of the Cache Creek terrane lies in south-central Yukon and comprises metavolcanic rocks, hemipelagic chert and shale, newly identified volcaniclastic and clastic rocks (Michie formation, informal), pyroxenite and gabbro intrusive rocks with an arc to back-arc geochemical signature, as well as tectonized and serpentinized ultramafic rocks. The proximally sourced Michie formation yielded zircon from two samples with unimodal peaks at 245.85 [+ or -] 0.07 and 244.64 [+ or -] 0.08 Ma. These dates are likely close to the depositional ages and compare favourably with those from the Kutcho assemblage of northern British Columbia. The Michie formation is exposed along the northwestern flank of Mount Michie and represents singular detrital input from a nearby eroding island-arc. The Cache Creek terrane rocks are imbricated with epiclastic and carbonate rocks of the Stikinia and Lower Jurassic siliciclastic rocks of the synorogenic Whitehorse trough. This imbrication records two compressional deformation phases in the region: (1) an initial phase of west-verging thrusting along the Judas Mountain fault that placed the Cache Creek terrane rocks over the arc and basinal rocks of Stikinia and Whitehorse trough; and (2) a second phase of east-verging thrusting along the Mount Michie fault that repositioned rocks of Stikinia and the Whitehorse trough structurally above those of the Cache Creek terrane. Deformation in the centre of the study area was followed by emplacement of a coarse-grained syenite that yielded [.sup.40]Ar/[.sup.39]Ar biotite and muscovite cooling ages of 165-160 Ma. Key words: Cache Creek, Kutcho, Cordillera, terrane accretion, intra-oceanic arc. La partie la plus septentrionale du terrane de Cache Creek est situee dans le centre sud du Yukon et renferme des roches metavolcaniques, des cherts et shales hemipelagiques, de roches volcanoclastiques et clastiques nouvellement decouvertes (formation de Michie, nom informel), des roches intrusives pyroxenitiques et gabbroiques presentant une signature geochimique d'arc a arriere-arc, ainsi que des roches ultramafiques tectonisees et serpentinisees. Deux echantillons de la formation de Michie, de source proximale, ont produit des zircons qui ont donne des pics monomodaux a 245,85 [+ or -] 0,07 et 244,64 [+ or -] 0,08 Ma. Ces ages s'approchent vraisemblablement des ages de depot et ressemblent a ceux de l'assemblage de Kutcho du nord de la Colombie-Britannique. La formation de Michie est exposee le long du flanc nord-ouest du mont Michie et represente un unique apport de detritus issus de l'erosion d'un arc insulaire situe a proximite. Les roches du terrane de Cache Creek sont imbriquees avec des roches epiclastiques et carbonatees de la Stikinie et des roches silicoclastiques du Jurassique inferieur de la fosse synorogenique de Whitehorse. Cette imbrication temoigne de deux phases de deformation en compression dans la region, soit (1) une phase initiale de chevauchement vers l'ouest le long de la faille du mont Judas, qui a place les roches du terrane de Cache Creek sur les roches d'arc et de bassin de la Stikinie et de la fosse de Whitehorse, et (2) une seconde phase de chevauchement vers l'est le long de la faille du mont Michie, qui a repositionne des roches de la Stikinie et de la fosse de Whitehorse structuralement au-dessus de celles du terrane de Cache Creek. La deformation dans le centre de la region a l'etude a ete suivie de la mise en place d'une syenite grenue qui a produit des ages de refroidissement [.sup.40]Ar/[.sup.39]Ar sur biotite et muscovite de 165-160 Ma. [Traduit par la Redaction] Mots-cles : Cache Creek, Kutcho, Cordillere, accretion de terranes, arc intraoceanique., Introduction The Mesozoic evolution of the western margin of North America involved a complex history of crustal growth by successive terrane accretions that resulted in development of the Cordilleran orogenic [...]

Alkalic magmatism occurred in the Selwyn Basin, Yukon, Canada throughout the Paleozoic, concurrent with extension during passive margin sedimentation. To examine magmatism associated with this extension, geochemical data were obtained from several locations in the Selwyn Basin (MacMillan Pass, Anvil District, Keno Hill, and the Misty Creek Embayment). Volcanic rocks from the Anvil District and the Misty Creek Embayment are dominated by alkalic basalts with light rare earth element (LREE)-enriched geochemical signatures, whereas metavolcanic and dike samples from Keno Hill comprise subalkaline basalts with enriched mid-ocean ridge basalt (E-MORB) signatures. The Early Ordovician Menzie Creek volcanic rocks of the Anvil District display trace element geochemical signatures intermediate between ocean island basalt (OIB) and E-MORB, whereas the Middle–Late Ordovician volcanic rocks from the Misty Creek Embayment have OIB signatures. Differences in the trace element geochemistry of the sample suites are attributed to the degree of partial melting. The Menzie Creek volcanic rocks formed from large volume melts of enriched mantle that diluted incompatible element signatures in the Early Ordovician. Late Ordovician magmatism produced the Misty Creek Embayment samples, where restricted melt volumes of an enriched mantle source resulted in the most enriched samples geochemically. The Keno Hill samples represent the shallowest melting of the analysed samples and may have resulted from melting of heterogenous subcontinental lithospheric mantle. Magmatic zircons from a Keno Hill metavolcanic sample analysed by chemical abrasion thermal ionization mass spectrometry resulted in a primary deposition age of c. 296 ± 0.36 Ma (Early Permian), during a period with little magmatism in the Selwyn Basin and representing a previously unknown unit in the area. [ABSTRACT FROM AUTHOR]