Tanana Terrane Research Articles

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide deposit (VMS) that is hosted by back-arc affinity rocks of the Yukon–Tanana terrane in the Finlayson Lake VMS district, Yukon, Canada. Massive sulfide zones occur as stacked and stratabound lenses subparallel to the volcanic stratigraphy, surrounded by pervasive white mica and/or chlorite alteration. Remnant clasts of volcanic rocks and preserved bedding occur locally within the massive sulfide lenses and indicate that mineralization formed through subseafloor replacement of pre-existing strata. Three mineral assemblages occur at the ABM deposit: (1) a pyrite–chalcopyrite–magnetite–pyrrhotite assemblage that is associated with Cu–Bi–Se–Co-enrichment and occurs at the center of the massive sulfide lenses; (2) a pyrite–sphalerite assemblage, which occurs more commonly towards lens margins and is enriched in Zn–Pb–Ag–Au–Hg–As–Sb–Ba; and (3) a minor assemblage comprising chalcopyrite–pyrrhotite–pyrite stringers associated with pervasive chlorite alteration, which occurs mostly at the sulfide lens margins. Petrographic observations of preserved primary, zone refining, and metamorphic textures in combination with in situ geochemistry show that the pyrite–sphalerite assemblage formed at lower temperatures (< 270 °C) than the other two mineral assemblages (~ 270–350 °C), and that mineral chemistry in all mineral assemblages was affected by greenschist facies metamorphism, although the effects are limited to recrystallization, small-scale remobilization (< 1 m) and trace element redistribution.

Lateral Variations of Attenuation in the Crust of Alaska Using Lg Q Tomography

ABSTRACT We have conducted a crustal seismic (QLg) attenuation tomography study across Alaska using recordings from the EarthScope USArray from 2014 to 2019. The resolving power of the inversion is 150 × 150 km for most of Alaska, and it is 75 × 75 km in central and southern Alaska. Numerous fault systems and high mountain ranges are present across Alaska and accommodate compression in the north–south direction and shearing of southern Alaska toward the west. These mountain ranges include the Brooks range in the north, the Alaska range in central Alaska, and the Aleutian range in the southwest. The average LgQ for all of Alaska is significantly higher than in the western United States and Canada. This lower average attenuation impacts seismic hazard estimates for the region. According to the tomographic results, we see a significant variation of the QLg values from low to high across the southern part of the Brooks range. Also, we found higher attenuation in the southeast region of Alaska, where the Wrangell volcanoes are located. Moreover, we see an area of lower attenuation associated with weak frequency dependence in the south-central region of Alaska next to Anchorage. Another anomaly with lower attenuation can be seen extending from central Alaska to southeast Alaska, possibly associated with the Yukon–Tanana terrane. There are a few areas like southwest Alaska associated with the Togiak terrane and an area next to Fairbanks in Alaska’s interior that shows lower attenuation with lower frequency dependence and higher attenuation with higher frequency dependence, respectively, for low frequencies up to 3 Hz. Our model’s highest η zones (η≳95) are mostly confined to major tectonic terranes and other major tectonic elements such as faults and fractures. Regional variations in crustal attenuation can impact local seismic hazard estimates if incorporated into the hazard analysis.

Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia

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.

Syn-accretionary multistage assembly of an Early Jurassic Alaskan-type intrusion in the Canadian Cordillera: U–Pb and 40Ar/39Ar geochronology of the Turnagain ultramafic–mafic intrusive complex, Yukon–Tanana terrane

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 40Ar/39Ar 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 40Ar/39Ar plateau dates of 187.4 ± 1.5 (2σ) Ma on hornblende and 188.6 ± 1.2 Ma on phlogopite, and a concordant 206Pb/238U date of 190.3 ± 4.6 Ma on titanite. Stage 3 diorite has a 206Pb/238U zircon crystallization age of 188.11 ± 0.13 Ma. Stage 4 wehrlite and leucodiorite yield 206Pb/238U zircon crystallization ages of 185.68 ± 0.19 and 185.30 ± 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 (&gt;189 to &lt;185 Ma) and initially generated northeast-vergent folds in the country rocks (&gt;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 (&lt;185 Ma). Contrary to previous interpretations, new geophysical data support an allochthonous origin for the host rocks of the Turnagain intrusion. The Turnagain host succession is correlated with late Paleozoic arc-related and basinal lithostratigraphic units developed at the inboard margin of the accreted Yukon–Tanana terrane and peripheral to the Slide Mountain marginal ocean basin. The geochronological results rigorously constrain the timing and style of Early Jurassic deformation attending the initial accretion of allochthonous arc terranes in the northern Cordillera.

A petrochronological approach for the detrital record: Tracking mm-sized eclogite clasts in the northern Canadian Cordillera

Sutures recording the accretion history of the Canadian Cordillera terranes are poorly preserved. The Whitehorse trough syn-orogenic basin formed during early Mesozoic terrane accretion at the western margin of Laurentia and contains a ∼300 m thick horizon that includes eclogite clasts possibly sourced from a suture zone. By applying petrochronological micro-analytical techniques to the mm-diameter eclogite clasts, including thermobarometry and in situ rutile thermochronology, as well as detrital zircon geochronology and thermal diffusion modeling, we constrain a source-to-sink path for the clasts. The eclogite clasts likely reached peak metamorphic conditions of 2.2–2.9 GPa and ≥800 °C, cooled through Pb closure in rutile during Early Jurassic at ≥610 °C and were deposited into the basin by latest Pliensbachian/earliest Toarcian. This history implies minimum mean cooling and exhumation rates on the order of ∼38 °C/myr and ∼4.1 km/myr, respectively, consistent with rates reported for subduction-related eclogite worldwide. We suggest the most likely source for the clasts is the suture between the Yukon–Tanana and Stikinia terranes, involving a latest Triassic collision, followed by rapid Early Jurassic exhumation of the lower plate Yukon–Tanana terrane, either by buoyant extrusion or in a plate boundary zone metamorphic core complex. Our study demonstrates that micro-analytical techniques used for petrochronology can be applied to very small lithic clasts in the sedimentary record towards the tectonic reconstruction of accretionary orogens.

Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon

In 2004, two seismic reflection lines were shot across the Mesozoic Whitehorse trough and adjacent terranes. Three-dimensional first-arrival tomographic inversion is used to constrain lithology to 800–1200 m depth, and surface structures are extrapolated into the middle crust using the coincident reflection data. In the Yukon–Tanana terrane, the metasedimentary Snowcap assemblage is characterized by velocities of 4.5–5.5 km/s, while in Quesnellia, velocities of 5.0–6.0 km/s occur at 500 m depth, and probably represent igneous rocks of the Tatchun batholith. Across the Whitehorse trough, velocities &gt;4.0 km/s correspond to clastic rocks of the Jurassic Laberge and Triassic Lewes River groups; velocities &lt;4.0 km/s probably present the clastic Jurassic to Cretaceous Tantalus Formation. Several near-surface units with velocities of 2.0–3.0 km/s are identified; some correlate well with volcanic rocks of the Upper Cretaceous Carmacks Group, but others could be attributable to alluvial deposits or faulting. The Big Salmon fault is interpreted to dip southwest, implying that rocks of the Yukon–Tanana terrane extend beneath Quesnellia. Stikinia and Quesnellia underlie up to 5–8 km of Triassic to Early Cretaceous sedimentary strata, and appear to be a single allochthon within an 18–20 km deep synform above the Yukon–Tanana terrane, which we name the Northern Intermontane synform. In general, reflection geometries in the upper crust are complex, but are consistent with large-scale imbricate structures that have been dissected into numerous blocks by displacement along moderately to steeply dipping strike-slip faults, which may be part of a crustal-scale flower structure extending to the base of the crust.

U–Pb and Hf isotopic analyses of detrital zircons from the Taku terrane, southeast Alaska

The Taku terrane consists of metamorphosed Carboniferous through Triassic marine clastic strata, volcanic rocks, and limestone that occur along the western margin of the Coast Mountains in southeastern Alaska. These rocks are juxtaposed along mid-Cretaceous thrust faults over Jura-Cretaceous basinal strata of the Gravina belt to the west and beneath Proterozoic through Carboniferous metamorphic rocks of the Yukon–Tanana terrane to the east. This paper presents U–Pb ages and Hf isotope analyses of detrital zircons from the Taku terrane, and compares these values with information from the adjacent Wrangellia, Alexander, and northern and southern portions of the Yukon–Tanana terrane (YTTn and YTTs). These comparisons suggest that (i) Carboniferous strata of the Taku terrane were shed mainly from mid-Paleozoic igneous rocks of YTTs, (ii) Permian strata of the Taku terrane were shed from mid-Paleozoic igneous rocks and intraformational Lower Permian volcanic rocks of YTTs as well as Upper Permian volcanic rocks exposed in YTTn, and (iii) Triassic sandstones were shed from mid-Paleozoic igneous rocks of YTTs, whereas conglomerates were shed mainly from mid-Paleozoic arc rocks in YTTn. Hf isotope analyses of Paleozoic zircons record increasing continental input during Silurian–Devonian and Permian phases of magmatism. Similarities in isotopic characteristics, combined with stratigraphic and geochemical information presented by previous workers, suggest that strata of the Taku terrane accumulated on (and partly as lateral equivalents of) rocks of YTTs, and that the combined assemblages formed outboard or along strike of YTTn.

The P–T path of eclogites in the St. Cyr klippe, Yukon, Canada: Permian metamorphism of a coherent high-pressure unit in an accreted terrane of the North American Cordillera

The St. Cyr klippe, part of the Yukon–Tanana terrane in Yukon, Canada, preserves fresh and retrogressed eclogite hosted by quartzofeldspathic schists. Petrology, mineral chemistry and compositional zoning combined with isochemical phase equilibrium (pseudosection) modeling shows that eclogites followed a clockwise pressure–temperature path. An amphibolite facies, pre-eclogite stage (pre-Stage I) preceded early eclogite-facies conditions of about 16 kbar and 510 °C (Stage I) and the peak pressure (Stage II), which reached up to 20 kbar and 670 °C. The peak observed and modeled eclogite-facies assemblage is garnet, omphacite, Na-rich amphibole, phengite, quartz and rutile. Pseudosections were modeled for varying Fe 3+ contents, and Fe 3+ = 20 % of total Fe is found to best match the pyrope and grossular content observed in garnet. Retrogression back to amphibolite-facies conditions (Stage III) followed a clockwise path as temperature decreased during decompression without passing through the chlorite stability field. U-Pb SIMS (secondary ion mass spectrometry) dating and trace-element analysis of zircon shows that the protolith of the eclogites formed within the Yukon–Tanana terrane during early arc activity, between 364 and 380 Ma. The rocks were then subducted to mantle depths and metamorphosed to eclogite-facies conditions during the Late Permian, between 267 and 271 Ma. These medium-temperature eclogites are part of a coherent high-pressure (HP) terrane that was tectonically eroded from the Yukon-Tanana composite arc in a subduction zone environment.

Determining the provenance of Triassic sedimentary rocks in northeastern British Columbia and western Alberta using detrital zircon geochronology, with implications for regional tectonics

Triassic rocks of the Western Canada Sedimentary Basin (WCSB) have previously been interpreted as being deposited on the passive margin of North America. Recent detrital zircon provenance studies on equivalent Triassic rocks in the Yukon have suggested that these rocks were in part derived from the pericratonic Yukon–Tanana terrane and were deposited in a foreland basin related to the Late Permian Klondike orogeny. Detrital zircons within a number of samples collected from Triassic sediments of the WCSB throughout northeastern British Columbia and western Alberta suggest that the bulk of the sediment was derived from recycled sediments of the miogeocline along western North America, with a smaller but significant proportion coming from the Innuitian orogenic wedge in the Arctic and from local plutonic and volcanic rocks. There is also evidence of sediment being derived from the Yukon–Tanana terrane, supporting the model of terrane accretion occurring prior to the Triassic. The age distribution of detrital zircons from the WCSB in British Columbia is similar to those of the Selwyn and Earn sub-basins in the Yukon and is in agreement with previous observations that sediment deposited along the margin of North America during the Triassic was derived from similar source areas. Together these findings support the model of deposition within a foreland basin, similar to the one inferred in the Yukon. Only a small proportion of zircon derived from the Yukon–Tanana terrane is present within Triassic strata in northeastern British Columbia, which may be due to post-Triassic erosion of the rocks containing these zircons.

Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

Eocene rotation of the Coast Plutonic Complex and Intermontane Belt: paleomagnetism of Eocene plutons along the Klondike Highway

Paleomagnetic results are reported for the ∼59 Ma Skagway, ∼54 Ma Fraser, ∼53 Ma Summit Lake, and ∼48 Ma Clifton felsic plutons of the eastern Coast Plutonic Complex (CPC) that outcrop along the south Klondike Highway in Alaska and British Columbia. Thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization (ChRM) directions for all 29 sites of normal, reversed, and mixed polarity. The ChRM resides in single or pseudosingle domain magnetite and (or) pyrrhotite that is shown to be primary by contact tests with the ∼47 Ma vertical White Pass mafic dikes. Paleopoles from six 56 to 50 Ma (mean 52 ± 2 Ma) Intermontane Belt – Yukon–Tanana terrane (IMB–YTT) units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 Ma (mean 52 ± 4 Ma) eastern CPC paleopoles. Both paleopole populations show nonsignificant poleward (northward) translation relative to North America (IMB–YTT, 3.7° ± 5.3°N; CPC, 4.3° ± 6.4°S; overall, 1.2° ± 4.9°S), indicating that northward translation of the accreted terranes ended by ∼58 Ma. Conversely, both populations show clockwise (CW) rotation that is either highly significant or substantial (IMB–YTT, 19.3° ± 10.5 °CW; CPC, 7.1° ± 16.1 °CW; overall 12.8° ± 10.9 °CW). The results are best explained by tectonic rotation from ∼50 to ∼45 Ma of the IMB–YTT as a thin-skin on top of North America during emplacement and co-incident rotation of the massive Eocene plutons of the eastern CPC along the North American margin.

Chilliwack composite terrane in northwest Washington: Neoproterozoic–Silurian passive margin basement, Ordovician–Silurian arc inception

The Chilliwack composite terrane in northwest Washington is part of an assemblage of mid-Paleozoic arc terranes extending from California to Alaska. Some terranes bear evidence of exotic origin, whereas others apparently formed proximal to western Laurentia, posing a complex problem in unraveling the Paleozoic accretionary history of the Cordillera. In our proposed broader definition, the Chilliwack composite terrane includes the volcanic and sedimentary East Sound and Chilliwack groups, and the plutonic and metamorphic Turtleback and Yellow Aster complexes. New zircon ages indicate that the plutonic and volcanic rocks are mutually related as parts of the same arc complex and that its inception was as old as Late Ordovician to Silurian, older than most other parts of the mid-Paleozoic terrane assemblage. Basement to the arc complex is a passive margin assemblage of metamorphosed quartzose sandstone and calc-silicate rock of the Yellow Aster Complex, bracketed in age by ca. 1000 Ma detrital zircons and 418 Ma intrusive rocks. This association of paragneiss basement and overlying and (or) intruding arc resembles that of older parts of the extensive Yukon–Tanana terrane in the northern Cordillera. Detrital zircon ages support a western Laurentian pericratonic origin for the paragneiss basement and the overlying arc. However, an early to mid-Paleozoic connection of this assemblage to the exotic outboard Alexander terrane is also indicated, based on (1) Mesoproterozoic and early Paleozoic detrital zircons in Devonian sedimentary rocks of the arc, and also in certain other pericratonic Devonian terranes and strata of the miogeocline; (2) Late Ordovician – Silurian igneous ages; and (3) an earliest Devonian or older metamorphic age of the basement paragneiss.

Provenance of North American Triassic strata from west-central and southeastern Yukon: correlations with coeval strata in the Western Canada Sedimentary Basin and Canadian Arctic Islands

New detrital mineral age and whole-rock geochemical data provide the first constraints on the composition and source of North American Triassic strata in the northern Canadian Cordillera. Conodont-bearing Triassic strata collected from five locations across west-central to southeastern Yukon contain trace-element ratios ((La/Yb)N = ∼8; Eu/Eu* = ∼0.66), εNd(248 Ma) values (–9 to –10), and detrital zircon ages (400–680, 980–1200, 1500–1650, 1800–2000 Ma) that correspond with those of coeval rocks in the Canadian Arctic Islands and the Western Canada Sedimentary Basin of British Columbia and Alberta. The majority of detrital zircons were cannibalized from Ellesmerian clastic wedge and western Laurentian margin strata and recycled into Triassic rocks. Conspicuous early Paleozoic and Neoproterozoic detrital zircons may have been ultimately derived from allochthonous rocks of Caledonian–Baltican affinity in northern North America, such as the Pearya and Arctic Alaska – Chukotka terranes. One Early Triassic unit in eastern Yukon contains ca. 360 Ma detrital muscovite, and samples from several localities include single-grain occurrences of Mississippian detrital zircon. Mississippian detrital mineral ages likely record a partial source from mid-Paleozoic rocks of the allochthonous Slide Mountain and Yukon–Tanana terranes following their Late Permian – Early Triassic emplacement onto the Cordilleran margin. More substantial evidence of terrane-derived sediment deposited along the North American margin may be further identified within Triassic strata that are exposed to the west (outboard) of our sample sites, immediately adjacent to the Slide Mountain and Yukon–Tanana terranes.

Arc imbrication during thick-skinned collision within the northern Cordilleran accretionary orogen, Yukon, Canada

Abstract We present the results of geological mapping and geochronological studies of the Tally Ho shear zone (THSZ) and adjacent rocks. The shear zone crops out near the west margin of Stikinia, an oceanic arc and the largest of the accreted terranes within the Cordilleran orogen of western North America. The hanging wall of the largely flat-lying shear zone consists of coarsely crystalline leucogabbro and cumulate pyroxenite interpreted as the lower crustal and possibly lithospheric mantle roots of a magmatic arc. Rocks in the footwall consist of volcanic and volcano-sedimentary sequences of the Lewes River Arc, a Late Triassic magmatic arc characteristic of Stikinia. Because the shear zone places lower crustal plutonic rocks over a supracrustal sequence, we interpret it as a crustal-scale thrust fault. Kinematic indicators imply top-to-the-east displacement across the shear zone. The geometry of folds of the shear zone is consistent with deformation in response to displacement over ramps in deeper-seated thrust faults kinematically linked to the THSZ. Crystallization of the hanging-wall leucogabbro at 208±4.3 Ma provides a maximum age constraint for deformation, whereas a post-kinematic granitoid pluton that plugs the shear zone and that crystallized at about 173 Ma provides a lower age limit. The THSZ is, therefore, coeval with: (1) a series of latest Triassic–Early Jurassic shear and fault zones that characterize the length of the west margin of Stikinia; (2) the termination of isotopically juvenile arc magmatism of the Lewes River Arc; (3) crustal loading of Stikinia giving rise to a foreland basin that rapidly filled with westerly derived orogenic molasse that includes clasts of ultrahigh-pressure metamorphic rocks; and (4) juxtaposition of Stikinia against continental crust of the Nisling Assemblage of the Yukon–Tanana terrane to the west. These constraints are consistent with a model of deformation in response to the entry of the continental Nisling Assemblage into the trench of the west-facing Lewes River Arc, terminating subduction and imbricating the arc along a series of east-verging thrust faults, including the THSZ.

The Yukon–Tanana terrane: Part of North America at ∼ 215 Ma from paleomagnetism of the Taylor Mountain batholith, Alaska

The massive unmetamorphosed 212 Ma Taylor Mountain batholith underlies ∼ 648 km 2 of the Yukon–Tanana terrane (YTT) in east-central Alaska, and intrudes klippen thrust sheets of the Fortymile River assemblage and the Chicken Metamorphic Complex. Structural evidence indicates that the batholith is neither tilted nor rotated about a vertical axis. Paleomagnetic analysis of 295 specimens from 24 sites in the batholith and cross-cutting mafic dikes isolates a stable primary characteristic remanent magnetization in magnetite with unblocking temperatures of 500 to 585 °C that places the batholith's paleopole at 61.4°N, 102.2°E ( N = 17, α 95 = 5.2°) on the North American apparent polar wander path (APWP) at 215 ± 5 Ma (1 σ). The Taylor Mountain paleopole alone and all YTT paleopoles together show that the YTT has been a para-autochthonous part of the North American margin since the Late Triassic. Further, the Intermontane Belt (IMB) paleopoles are distinct from the North American and YTT paleopoles prior to the mid Eocene and provide two APWP segments. One IMB segment includes the Quesnel, Cache Creek, Eastern Coast Belt, and northern Stikine terranes, and the second more-speculative IMB segment includes the central and southwestern Stikine and Methow terranes. This analysis supports tectonic models for the northern Cordillera in which the IMB terranes were obducted onto North America during the Late Cretaceous to mid Eocene, co-eval with the Laramide orogeny.

Correlation of mid-Cretaceous granites with source terranes in the northern Canadian CordilleraLithoprobe Publication 1475.

This study presents a broad geochemical and isotopic synthesis of mid-Cretaceous granites in the southern Yukon as well as a comparative data set for granites sourced from, and hosted by, accreted terranes in the west through to ancient cratonic rocks in the east. We present data from a traverse perpendicular to the strike of the northern Canadian Cordillera allowing comparison with the growing body of such data derived from the host terranes. Trace elements, specifically the “subduction signature,” allow the discrimination of oceanic verses continental crustal sources. Comparison of isotopic ratios of Sr and, particularly, Nd with published data further refine the correlation of granites with their source terranes. Granites are initially divided based upon their host morphogeological belts, however, our study indicates that the source terranes transcend these traditional boundaries. For Intermontane Belt hosted granites three distinct sources can be identified: an isotopically primitive (Sri, 0.7050; ϵNdT, 2.3 to –1.2), subduction-related source probably associated with the mid-Cretaceous continental margin; an isotopically primitive (Sri, 0.7032 to 0.7035; ϵNdT, 4.2 to 1.4), non-subduction-related source identified as the host Cache Creek terrane; and an isotopically slightly more evolved (Sri, 0.7094 to 0.7101; ϵNdT, 4.5 to –7.3), subduction-related source identified as the host Stikine terrane. Immediately east of the Teslin Tectonic Zone (TTZ), pericratonic Omineca granites (Sri, 0.7032 to 0.7076; ϵNdT, 2.0 to –5.4) do not correlate with their host terranes, but instead show marked similarities with granites immediately to the west of the TTZ suggesting that the same, or similar crustal sources extend further east in the subsurface than previously thought. In the eastern pericratonic Omineca Belt, there is a substantial jump to more evolved isotopic values (Sri, 0.7172 to 0.7354; ϵNdT, –16.6 to –21.7) for granites that extend to the most easterly exposed plutons of the cratonic Omineca Belt. These more isotopically evolved granites correlate with isotopic values for the pericratonic Yukon–Tanana and Cassiar terranes, as well as cratonic North America.

Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario)

Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are < 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n = 53) and horizontal K max for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow.

Rifting of a Mississippian continental arc system: Little Salmon formation, Yukon–Tanana terrane, northern Canadian Cordillera

The Yukon–Tanana terrane in the northern Canadian Cordillera records the development of a series of mid- to late Paleozoic arc systems, punctuated by intra-arc deformation, uplift, and episodic rifting coeval with back-arc extension, built upon a metasedimentary basement of northwestern Laurentian affinity. In central Yukon, the Little Kalzas formation records the development of one of these Mississippian continental arcs, whereas the Little Salmon formation records the development of an intra-arc rift basin within a continental arc. The Little Salmon formation lower succession comprises mainly volcaniclastic rocks derived from erosion of Early Mississippian and older units, including rocks of the Little Kalzas continental arc. Above a medial limestone member, the upper succession of the Little Salmon formation includes alkali basalt, breccia, and crystal and ash tuffs in the north and predominantly epiclastic rocks interbedded with crystal and ash tuffs in the south. The alkali basalts have the geochemical characteristics of ocean-island basalts and their positive ε Nd 340 (+7.3) and low 87Sr/86Sr values (0.705) suggest a primitive magma source with little or no involvement of continental crust. The transition between the northern and southern facies of the upper succession of the Little Salmon formation coincides with a northeast-trending synvolcanic fault inferred to have controlled alkali basalt eruptions and deposition of Mn-bearing exhalite in the north and basin plain sedimentation in the south. The environment of deposition of the Little Salmon formation resembles that of the modern Sumisu rift in the Izu–Bonin–Mariana arc system or the early stages of development of the Japan island-arc system.

Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon‐Tanana terrane, Yukon, Canada: P–T evolution linked with in situ SHRIMP monazite geochronology

AbstractTextural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure–temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT).A TIMS U/Pb titanite age of 365–350 Ma is interpreted to date low‐P metamorphism (M1) and D1 deformation associated with arc plutonism above an east‐dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (∼9 kbar and 600 °C) at c. 239 Ma. These data help to establish a c. 260–240 Ma tectonometamorphic event (M2–D2) reflecting intra‐arc thickening during west‐dipping subduction of Slide Mountain Ocean. Another transition from low‐ to high‐P (M3–D3; 7.8 kbar and 595 °C), dated by c. 195–187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton.The Mt Burnham (north‐eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates ∼9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114–107 Ma reflects low‐P (&lt;4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.

Detrital zircon geochronology and provenance of the southeastern Yukon–Tanana terrane

Two samples of late Paleozoic grit and Late Mississippian quartzite–chert conglomerate collected from southeastern Yukon–Tanana terrane (YTT) — a composite thrust sheet resting structurally above North American parautochthonous strata and intervening imbricate sheets of the late Paleozoic oceanic Slide Mountain terrane — yielded, respectively, 89 and 74 concordant or nearly concordant (&lt;20% discordant) U–Pb ages on single detrital zircons. They provide constraints on the provenance of this allochthonous pericratonic terrane. Zircons in the grit range from 1770 to 2854 Ma, with a well-defined Early Proterozoic peak between 1800 and 2100 Ma. Precambrian zircons in the conglomerate also show a dominant peak between 1800 and 2100 Ma and smaller peaks between 2200 and 3200 Ma, with a few older grains, and younger grains with ages of 998, 1219, 1255, 1256, and 1417 Ma. The conglomerate also yielded three Devonian grains, with ages of 366 ± 23, 373 ± 12, and 379 ± 23 Ma. Their ages are approximately coeval with the oldest felsic to intermediate arc- and rift-related magmatism in the YTT. The age spectra from southeastern YTT units compare closely with those from Mississippian and older pericratonic units in the Coast Mountains, confirming correlations previously made on lithologic grounds. They also strongly resemble detrital zircon populations from craton-derived Paleozoic units of the northern North American autochthon. This robust U–Pb data set lends support to the idea that YTT once formed part of the outer, active margin of the North American continent, prior to Mississippian rifting and marginal ocean basin development.