Cache Creek Terrane Research Articles

Detrital geochronology of the Cunningham Lake formation: an overlap succession linking Cache Creek terrane to Stikinia at ∼205 Ma

Detrital zircon from three coarse-grained marine siliciclastic rocks was analyzed for U–Pb, Lu–Hf, and trace element compositions to constrain the timing of deposition and sediment provenance of the Cunningham Lake formation (formerly siliciclastic unit of the Sitlika assemblage) in north-central British Columbia. This strategy tests previously proposed sedimentary linkages between the Cache Creek terrane and the westerly rocks of the Stikine terrane. All three samples indicate maximum depositional ages at ca. 205–202 Ma (Rhaetian). The samples contain a predominant ca. 225–215 Ma detrital population, sourced from proximal contemporaneous volcanic arcs, and minor Permian to Middle Triassic and Carboniferous arc-derived detrital populations. The absence of Precambrian grains is consistent with the strongly suprachondritic zircon compositions (εHf(t) = +7 to +20), and indicates exclusively juvenile sources for the Cunningham Lake formation. Late Triassic sources of zircon are not known in the Cache Creek terrane and, except within western Stikine terrane, are uncommon among the Intermontane terranes that amalgamated with the Cache Creek terrane during Late Triassic–Early Jurassic. The Stikine suite (ca. 230–214 Ma) and coeval volcanic rocks in western Stikinia are the most probable sources of Late Triassic detritus for the Cunningham Lake formation. Stikinia’s Paleozoic basement is the probable source of Carboniferous detrital zircon. Volcanic arc–backarc complexes in the Cache Creek terrane are the most likely sources of Permian to Middle Triassic detritus in the Intermontane terranes. Accordingly, the siliciclastic rocks of the Cunningham Lake formation represent an overlap sedimentary succession that links Stikinia to the Cache Creek terrane by the latest Triassic.

The Jurassic Laberge Group in the Whitehorse Trough of the Canadian Cordillera: Using Detrital Mineral Geochronology and Thermochronology to Investigate Tectonic Evolution

The Laberge Group is an Early to Middle Jurassic sequence of mostly siliciclastic sedimentary rocks that were deposited in a marginal marine environment in the northern Canadian Cordillera. It forms a long narrow belt with a total thickness of 3–4 km extending for more than 600 km across southern Yukon and northwestern British Columbia. These sedimentary rocks overlap the Yukon-Tanana, Stikinia and Cache Creek terranes that form the main components of the Intermontane superterrane. The Laberge Group contains a record of the erosion of some of these terranes, and also offers some constraints on the timing of their amalgamation and accretion to the Laurentian margin. The Laberge Group was deposited with local unconformity on the Late Triassic Stuhini Group (in British Columbia) and correlative Lewes River Group (in Yukon), both of which are volcanic-rich, and assigned to the Stikinia terrane. The Laberge Group is in turn overlain by Middle Jurassic to Cretaceous clastic rocks, including the Bowser Lake Group in BC and the Tantalus Formation in Yukon. Clast compositions and detrital zircon populations within the Laberge Group and between it and these bounding units indicate major shifts in depositional environment, basin extent and detrital sources from Late Triassic to Late Jurassic. During the Early Jurassic clast compositions in the Laberge Group shifted from sediment- and volcanic-dominated to plutonic-dominated, and detrital zircon populations are dominated by grains that yield ages that approach or overlap their inferred depositional ages. This pattern is consistent with progressive dissection and unroofing of (an) active arc(s) to eventually expose Triassic to Jurassic plutonic suites. Detrital rutile and muscovite data from the Laberge Group indicate rapid cooling and then exhumation of adjoining metamorphic rocks during the Early Jurassic, allowing these to contribute detritus on a more local scale. The most likely source for such metamorphic detritus is within the Yukon-Tanana terrane, and its presence in the Laberge Group may constrain the timing of amalgamation and accretion of the Yukon-Tanana and Stikinia terranes. Thermochronological data also provide new insights into the evolution of the Laberge Group basin. Results from the U–Th/(He) method on detrital apatite suggest that most areas experienced post-depositional heating to 60°C or more, whereas U–Th/(He) results from detrital zircon show that heating to more than 200°C occurred on a more local scale. In detail, Laberge Group cooling and exhumation was at least in part structurally controlled, with more strongly heated areas situated in the footwall of an important regional fault system. The thermochronological data are preliminary, but they suggest potential to eventually constrain the kinematics and timing of inversion across the Laberge Group basin and may also have implications for its energy prospectivity. In summary, the Laberge Group is a complex package of sedimentary rocks developed in an active, evolving tectonic realm, and many questions remain about the details of its sources and evolution. Nevertheless, the available information demonstrates the potential of combined geochronological and thermochronological methods applied to detrital minerals to unravel links between regional tectonics, basin development and clastic sedimentation.

Late Triassic to Jurassic Magmatic and Tectonic Evolution of the Intermontane Terranes in Yukon, Northern Canadian Cordillera: Transition From Arc to Syn‐Collisional Magmatism and Post‐Collisional Lithospheric Delamination

AbstractEnd‐on arc collision and onset of the northern Cordilleran orogen is recorded in Late Triassic to Jurassic plutons in the Intermontane terranes of Yukon, and in development of the synorogenic Whitehorse trough (WT). A synthesis of the extensive data set for these plutons supports interpretation of the magmatic and tectonic evolution of the northern Intermontane terranes. Late Triassic juvenile plutons that locally intrude the Yukon‐Tanana terrane represent the northern extension of arc magmatism within Stikinia. Early Jurassic plutons that intrude Stikinia and Yukon‐Tanana terranes were emplaced during crustal thickening (200–195 Ma) and subsequent exhumation (190–178 Ma). The syn‐collisional magmatism migrated to the south and shows increasing crustal contributions with time. This style of magmatism in Yukon contrasts with coeval, juvenile arc magmatism in British Columbia (Hazelton Group), that records southward arc migration in the Early Jurassic. Exhumation and subsidence of the WT in the north were probably linked to the retreating Hazelton arc by a sinistral transform. East of WT, Early Jurassic plutons intruded into Yukon‐Tanana record continued arc magmatism in Quesnellia. Middle Jurassic plutons were intruded after final enclosure of the Cache Creek terrane and imbrication of the Intermontane terranes. The post‐collisional plutons have juvenile isotopic compositions that, together with stratigraphic evidence of surface uplift, are interpreted to record asthenospheric upwelling and lithospheric delamination. A revised tectonic model proposes that entrapment of the Cache Creek terrane was the result of Hazelton slab rollback and development of a sinistral transform fault system linked to the collision zone to the north.

Timing of Cache Creek Ocean closure: insights from new Jurassic radiolarian ages in British Columbia and Yukon and their significance for Canadian Cordillera tectonics

The closure of the Cache Creek Ocean during the Jurassic led to the structuration of the Canadian Cordillera. To reassess the timing of this major tectonic event, this study proposes a biochronological review of all the youngest radiolarian-bearing localities of the Cache Creek Terrane in British Columbia and Yukon using updated radiolarian biozonations and taxonomy, complemented by new data from the type locality. The results show that all ages are Early Jurassic and fairly homogenous along the entire terrane. All the youngest siliceous rocks are restricted to the late Sinemurian – early Toarcian interval (∼195–180 Ma). The youngest well-dated pelagic cherts are early Pliensbachian, with slightly younger siliceous argillites of Pliensbachian and possibly early Toarcian age. These results suggest that Cache Creek deep pelagic sedimentation ceased fairly synchronously along the entire belt, probably during the Pliensbachian (∼191–183 Ma), predating the final closure of the basin when Cache Creek units were thrust over Stikinia and Quesnellia in the Middle and Late Jurassic. This apparent synchronicity should result in re-evaluating the tectonic mechanism of the Cache Creek “entrapment model” in its present form. The study also shows that pelagic sedimentation ceased 22–31 million years earlier in the Cache Creek Terrane than in the Bridge River Terrane, confirming the stratigraphic discrepancy between the two terranes during the Jurassic.

The northern termination of the Cache Creek terrane in Yukon: Middle Triassic arc activity and Jurassic–Cretaceous structural imbrication

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 ± 0.07 and 244.64 ± 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 40Ar/39Ar biotite and muscovite cooling ages of 165–160 Ma.

Element and isotopic signature of re-fertilized mantle peridotite as determined by nanopowder and olivine LA-ICPMS analyses

The lithospheric mantle should be depleted in base- and precious-metals as these elements are transferred to the crust during partial melting. However, some melt-depleted mantle peridotites are enriched in these ore-forming elements. This may reflect re-fertilization of the mantle lithosphere and/or sequestering of these elements by residual mantle phase(s). Both processes remain poorly understood because of the low abundances of incompatible elements in peridotite and the nugget-like distribution of digestion-resistant mantle phases that pose analytical challenges for conventional geochemical methods. Herein we report new major and trace element concentrations for a suite of mantle peridotite and pyroxenite samples from the Late Permian to Middle Triassic Nahlin ophiolite (Cache Creek terrane, British Columbia, Canada) using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis of nanoparticulate powders and olivine. Compatible to moderately incompatible element concentrations suggest that Nahlin ophiolite peridotites represent residues after ≥20% melt extraction. Pyroxenite dykes and replacive dunite bands are folded and closely intercalated with residual harzburgite. These field relationships, coupled with the presence of intergranular base metal sulphide, clinopyroxene and Cr-spinel at the microscale, point to percolating melts that variably re-fertilized melt-depleted mantle peridotite. Radiogenic Pb (206Pb/204Pb = 15.402–19.050; 207Pb/204Pb = 15.127–15.633; 208Pb/204Pb = 34.980–38.434; n = 45) and Os (187Os/188Os 0.1143–0.5745; n = 58) isotope compositions for a subset of melt-depleted peridotite samples further support metasomatic re-fertilization of these elements. Other ore-forming elements are also implicated in these metasomatic reactions because some melt-depleted peridotite samples are enriched relative to the primitive mantle, opposite to their expected behaviour during partial melting. New LA-ICPMS analysis of fresh olivine further demonstrates that a significant proportion of the highly incompatible element budget for the most melt-depleted rocks is either hosted by, and/or occurs as trapped inclusions within, the olivine-rich residues. Trapped phases from past melting and/or re-fertilization events are the preferred explanation for unradiogenic Pb isotope compositions and Paleozoic to Paleoproterozoic Re-depletion model ages, which predate the Nahlin ophiolite by over one billion years.

Precious metal mobility during serpentinization and breakdown of base metal sulphide

Serpentinization of ultramafic rocks drives geochemical exchange between the hydrosphere, biosphere and lithosphere in surficial environments and at depth. Precious metals are implicated in these hydration reactions because of the well-known association between peridotite-hosted Au and placer platinum-group mineral (PGM) deposits sourced from serpentinized ophiolite complexes at surface. However, the distribution of precious metals in mantle rocks and their mode of occurrence at the onset of serpentinization are not well understood because early-stage features are typically obliterated with progressive hydration. Herein we report electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICPMS) spot and mapping results for a suite of base metal sulphide (pentlandite, pyrrhotite, chalcopyrite), native metal (Cu and Fe) and Ni-Fe alloy (awaruite) from variably serpentinized peridotite and pyroxenite (Late Permian to Early Triassic Nahlin ophiolite, Cache Creek terrane; Atlin, British Columbia, Canada). Pentlandite and pyrrhotite occur with magmatic clinopyroxene and Cr-spinel as ultrafine inclusions (few μm) and as coarser interstitial base metal sulphides (≤200 μm) that are enveloped by native metal (Cu and Fe) and Ni-Fe alloy. Because native Fe- and awaruite-bearing mineral assemblages require reduced and/or low-fS2 conditions, we suggest that these replacement textures document destabilization of base metal sulphide phases during the conversion of olivine to serpentine and magnetite. Desulphurization reactions decoupled precious metals from relict pentlandite at the microscale, with Ag, Pd, Pt and Au partitioning from sulphides into both awaruite and native metal (Cu and Fe) at concentrations up to 100s of ppm. New high-resolution LA-ICPMS maps also point to clusters of ultrafine PGM, tellurides, bismuthides and metal alloys that were either remobilized within the serpentinized mesh and/or represent the completely desulphidized product of ultrafine, intergranular base metal sulphide. Other PGE (Os and Ir) are mostly hosted within relict pentlandite and/or pyrrhotite that were preserved during incomplete desulphurization, along with lesser microscale remobilization of these elements into ultrafine veins. New whole-rock PGE (nickel-sulphide fire-assay; NiS-FA) results demonstrate that precious metal remobilization was limited to the microscale during the earliest stages of serpentinization, which, in the case of Au, is consistent with its low solubility within such reduced, low fS2 fluids. Precious metal mobility during the early stages of serpentinization may therefore depend on the stability of Ni-Fe alloy, native metal (Cu and Fe) and PGM. Progressive serpentinization, complete replacement of olivine and destabilization of the reduced, low-fS2 mineral assemblage likely represents an important process to liberate, transport and concentrate precious metals within more oxidized and/or sulphur-bearing fluids in surficial and deep subduction environments.

Tectonics of the northern Canadian Cordillera imaged using modern magnetotelluric analysis

Magnetotelluric data from LITHOPROBE SNORCLE Corridor 2 in western Canada crossing the Rocky Mountain Trench-Tintina Fault between 56° and 60°N have been analyzed using modern MT methods to provide new insights into the tectonics of the North American Cordillera and Ancestral North America. Resistivity images of the crust were obtained using two independent sets of unconstrained 2-D inversions and constrained 2-D inversions incorporating seismic constraints. Structure to the west of the Tintina Fault includes uniform, moderate, mid to lower crustal resistivity in a seismically-defined westward-tapering wedge of Ancestral North American rocks. To the east, between the Tintina Fault and the Liard line, a transfer zone in asymmetric Neoproterozoic rifting, the wedge includes a major synformal conductor that parallels seismic reflections. The large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting. The synformal conductor is attributed to graphitic rocks from the Aida Formation of the Mesoproterozoic Muskwa Assemblage that were re-distributed in a sag basin on the lower plate margin in the Blackwater Block. Spatially-complex upper crustal resistivity in the Cordillera can be interpreted in terms of lithological variations and alteration processes. An east-dipping conductor in Stikinia is attributed to east-dipping Jurassic and relatively high conductivity beneath the Eskay rift may be explained by crustal fertilization. A conductor in the Cache Creek terrane is attributed to listwanite alteration of ultramafic bodies on the King Salmon Fault, and high conductivity in the Slide Mountain terrane within the Sylvester Allochthon to localized carbonaceous alteration. A series of conductors above North American Basement is attributed to deformation and metamorphic processes associated with Mesozoic thrust faulting. To the west of the Tintina Fault the source material for these conductors is likely from the Stikinia terrane, and to the east it is likely from the Muskwa Assemblage.

Heterogeneity of conodont faunas in the Cache Creek Terrane, Canada; significance for tectonic reconstructions of the North American Cordillera

The oceanic Cache Creek Terrane has long been interpreted to be exotic relative to the neighbouring Stikine and Quesnel terranes, on the basis of its unusual fossil faunas. Current tectonic models for the amalgamation of these three terranes have to account for the position of the exotic Cache Creek Terrane between two volcanic island arc terranes, and competing models invoke either terrane entrapment or strike-slip motion to explain this configuration. Although previous paleobiogeographic studies have attempted to assess the degree of faunal variability among terranes of the North American Cordillera, none have yet measured the amount of internal faunal heterogeneity and considered the effect that this may have on tectonic reconstructions. The use of multivariate statistics demonstrates that the Late Paleozoic to Triassic conodont faunas from the Cache Creek Terrane show distinct heterogeneity. The degree of heterogeneity among faunas from different regions of the terrane varies with time, and does not correlate with temporal changes in depositional environment, conodont ecology, or species endemism. Instead, it is proposed that the Cache Creek Terrane may consist of a number of sub-terranes which moved independently of each other prior to the final accretion of the Intermontane terranes. Periods of low faunal heterogeneity may correspond to times when the sub-terranes were close together, whereas periods of high heterogeneity may indicate that the sub-terranes had moved further apart. This interpretation requires that tectonic models for the formation of this part of the Cordillera emphasise lateral movement both within and between terranes prior to accretion.

Geochemistry of volcanic and plutonic rocks from the Nahlin ophiolite with implications for a Permo–Triassic arc in the Cache Creek terrane, northwestern British Columbia

In northwestern British Columbia, the Permian Nahlin ophiolite in the northern Cache Creek terrane comprises spinel harzburgite tectonite with minor lherzolite, lower crustal mafic and ultramafic cumulates, gabbroic rocks including dikes intruding mantle harzburgite, and basaltic volcanic and volcaniclastic rocks. New lithogeochemical data from the Menatatuline Range area confirm that plutonic and volcanic rocks of the ophiolite are tholeiitic and arc related, while only a minor component of volcanic rocks are alkaline intraplate basalts. Tholeiitic basalts of the Nahlin ophiolite represent the products of 2%–20% fractional melting, and their complementary residue may be peridotite from the ophiolite mantle section. Correlative tholeiitic volcanic sections can be found elsewhere in the northern Cache Creek terrane, and they may be linked to a regionally extensive (∼200 km) intraoceanic arc. The arc tholeiite geochemistry of the plutonic and volcanic rocks, and the highly depleted nature of the mantle residues, imply that the Nahlin ophiolite formed in a supra-subduction zone environment. The Nahlin ophiolite therefore occupied the upper plate during intraoceanic collision prior to emplacement of the Cache Creek terrane. The volumetrically minor ocean island basalt type volcanic rocks in the northern Cache Creek terrane are associated with carbonate successions bearing Tethyan fauna. These sequences are likely fragments of oceanic plateaux and their carbonate atolls sliced off of the subducting plate and are unrelated to the Nahlin ophiolite-arc system.

Regional Metallogeny and Genesis of a New Deposit Type—Disseminated Awaruite (Ni3Fe) Mineralization Hosted in the Cache Creek Terrane

Eight occurrences and one large resource of broadly disseminated awaruite mineralization discovered since 2008 are all hosted in serpentinized ultramafic units of the Cache Creek complex in northern British Columbia and southern Yukon. The most significant discovery that has been delineated to date is the Baptiste deposit at the Decar property, with an indicated resource of 1.160 billion tons (Bt) at 0.124% magnetically recovered nickel and an inferred resource of 0.87 Bt at 0.125% magnetically recovered nickel. It is the only known deposit consisting solely of awaruite to generate a positive preliminary economic analysis. The disseminated awaruite (Ni3Fe) within mineralized zones in the Decar property has compositions that average 76.9% Ni, 21.6% Fe, 0.6% Co, and 0.8% Cu. This awaruite has very high magnetic susceptibility (higher than magnetite) and a high specific gravity of 8.2. These properties make awaruite amenable to magnetic and gravity separation that yields a high-quality product containing minimal sulfur. Alloy grains range in size from 400 μ m, whereas awaruite <10 μ m cannot be recovered easily using mechanical processes. Peridotites containing 10 to 30% medium to coarse orthopyroxene and minor clinopyroxene are the main host of significant awaruite mineralization at the Decar and Mich properties. The homogeneous and coarser ( 200 μ m) awaruite mineralization is associated with antigorite-lizardite-magnetite assemblages that formed at temperatures >~300°C in the Decar and Mich properties, contrasting with the fine-grained sized (2–20 μ m) awaruite-bearing assemblages associated with lower temperature ( 2 mm) to precipitate coarser grains as temperatures increased from 300° to 450°C. The Cache Creek terrane accreted to ancestral North America sometime after the lower Mesozoic, generating later north-northwest shear and regional metamorphic events that included serpentinization, the process that generated the awaruite mineralization in this region. Depleted upper mantle material, predominantly harzburgite but also lherzolite and lesser amounts of dunite, within the Cache Creek ophiolite sequence typically contains 0.22 to 0.30% Ni that initially resided primarily in olivine. In alpine or suture settings, microfracturing and foliations are generally subparallel to northwest shears or faults, and other fracture systems that provided high permeability to focus continental or meteoric waters. Regional metamorphism, shearing, and exothermic serpentinization generated a source of heat to produce the pervasive awaruite-bearing antigorite-lizardite-magnetite assemblages. The ferrous component in magmatic olivines maintained low ![Formula][1] values during mineralizing and serpentinization events and was buffered by relict olivines in mineralized zones. This condition caused oxidation of iron and the precipitation of magnetite in addition to generating a high H2-rich metamorphic fluid, with Ni and Fe ions reduced, mobilized, and stabilized as awaruite during serpentinization. Serpentinization of orthopyroxene increased ![Formula][2] , reduced the production of brucite, and possibly aided the growth of coarse-grained awaruite. Peridotites in abyssal, subduction, and mantle settings are not likely to have generated the coarse-grained awaruite mineralization caused by serpentinization in these environments; however, peridotites from any of these settings could be a potential host following accretion and shearing in an alpine or suture setting assuming attendant higher temperature serpentinization and magnetite-awaruite mineralization. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif

Seismic interpretation of crustal-scale extension in the Intermontane Belt of the northern Canadian Cordillera

The crustal thickness of the Canadian Cordillera, which may have reached 50–65 km during the Cretaceous, is now only 32–38 km along its length. In the south, postorogenic extension during the Eocene resulted in the formation of core complexes and crustal-scale extensional shear zones, whereas in the north evidence for postorogenic extension is either limited to relatively minor basin formation in a transtensional environment or has not been recognized. Using new images of Lithoprobe seismic reflection line 2a in northern British Columbia, which has been previously interpreted in terms of terrane accretion, I make an alternative interpretation of crustal-scale postaccretion extension in the middle and upper crust. As early as the late Permian to Early Triassic, the Quesnellia and Stikinia arc terranes were accreted to ancestral North America along a crustal ramp, which is imaged in the lower crust. An extensional shear zone, which extends from close to the surface to 20 km depth, is identified from the geometry of reflections in the overlying structural basin and a lateral change in seismic velocity. The oceanic Cache Creek terrane, which is <4 km thick in the vicinity of line 2a, was preserved, because it was imbricated with its overlap assemblage and dropped into the 10-km-deep structural half-graben, which may correspond approximately to the combined extent of the Whitehorse trough and the Cache Creek terrane in southern Yukon and northern British Columbia.

Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon

The Whitehorse trough is an Early to Middle Jurassic marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. Detrital zircon dates from eight Laberge Group sandstones from various parts of the trough all display a major Late Triassic–Early Jurassic peak (220–180 Ma) and a minor peak in the mid-Paleozoic (340–330 Ma), corresponding exactly with known igneous ages from areas surrounding the trough. Source regions generally have Early Jurassic (ca. 200–180 Ma) mica cooling dates, and the petrology of metamorphic rocks and Early Jurassic granitoid plutons flanking the trough suggests rapid exhumation during emplacement. These data suggest that subsidence and coarse clastic sedimentation in the trough occurred concurrently with rapid exhumation of the shoulders. Isolated occurrences of sandstone and conglomerate units with similar detrital zircon signatures occur west and east of the trough, as well as overlapping the Cache Creek terrane, indicating that either the trough was once more extensive, or isolated basins tapped similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera were coeval with the onset of orogenic activity in the hinterland of the southern Canadian Cordillera, and subsidence in the western Canada foreland sedimentary basin. The Whitehorse trough is interpreted as a forearc basin that progressively evolved into a collisional, synorogenic piggyback basin developed atop the nascent Cordilleran orogen. Upper Jurassic–Lower Cretaceous fluvial deposits overlapping the Whitehorse trough have detrital zircons that were mainly derived from recycling of the Laberge Group, but they also contain zircons exotic to the northern Intermontane terranes that are interpreted to reflect windblown detritus from the Late Jurassic–Early Cretaceous magmatic arc that developed either atop the approaching Insular terranes to the west or southern Stikinia.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako–Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11–12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45–6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east–west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.

Tectonic Controls on Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au Ag-Pt-Pd-Mo) Within the Canadian Cordillera

Paired belts of alkalic and calc-alkalic porphyry deposits are genetically linked to early Mesozoic volcanic arcs preserved as the Quesnel and Stikine terranes in western Canada. These parallel arc terranes extend for 2,000 km along the axis of the Canadian Cordillera. They are joined at their northern ends, but are otherwise separated by relics of Tethyan ocean basin and oceanic arc rocks collectively referred to as the Cache Creek terrane. Porphyry Cu ± Au-Mo-Ag deposits are concentrated within the Stikine-Quesnel arc terranes, with most of their economic metal endowment a consequence of a 15-m.y. mineralizing epoch. A particularly prolific period within the mineralizing epoch is a 6-m.y. pulse centered on 205 Ma when more than 90% of the known copper endowment was acquired. Distinct trends of Cu-Au ± Ag-Mo mineralization within both arc terranes coincide in time and space with events that are attributed to effects of slab subduction. Stratigraphy within both Stikine and Quesnel arc terranes is equivalent, with greater variation along the arcs than between them. Magmatism within both arcs began in the Late Devonian, following initiation of subduction near eastern Panthalassa and formation of the back-arc Slide Mountain ocean basin margin adjacent to Laurentia. Both terranes record further arc development in the lower Carboniferous and cessation in the Permian. Between the Late Permian and Middle Triassic, both arcs foundered, and then recorded a cryptic Early Triassic deformational event, coincident with closing of the Slide Mountain ocean. Permo-Triassic subduction and arc magmatism are recorded in the Sitlika-Kutcho-Venables arc, but this occurred far to the west, within the Tethyan realm. Early in the Late Triassic (early Carnian), both the Stikine and Quesnel arcs reignited. Later, near the end of the Triassic (late Norian), the arcs were uplifted, picrites were erupted locally, and magmatism was dominantly strongly alkaline. Monzonitic plutons hosting alkalic Cu porphyry deposits were emplaced along the arcs, followed by a widespread erosional hiatus that extends throughout much of the latest Triassic (Rhaetian). Late Triassic events culminating in generation of Cu porphyry deposits are attributed to a model wherein delivery of the Permo-Triassic Sitlika-Kutcho-Venables arc to the Stikine-Quesnel arc trench led to stalled subduction and arc-parallel tearing of the slab. Ingress of hot subslab mantle into the tear resulted in high-degree partial melts, as recorded by picrite within the arc circa 210 Ma. Widening of the tear and decay of the initial thermal spike raised temperatures across broad regions of the metasomatized mantle wedge, with low-degree partial melts generated from the most hydrated, metal-enriched portions circa 203 Ma. These melts produced the well-defined belt of alkalic Cu-Au-Ag porphyries. Parts of the slab tear may have nucleated near the forearc to generate Mo-rich Cu porphyries, such as the Gibraltar deposit, which partly cuts the accreted Cache Creek terrane. Metal-rich hydrous melts may have traveled along preexisting intrusive pathways to invade batholithic bodies along the arc axis, such as at Highland Valley, where 204 Ma mineralization is hosted within a larger ~210 Ma plutonic complex. Migration of the locus of mineralized porphyry intrusions toward the backarc may track the leading edge of the slab gap, with the formation of Cu-Au and younger Au-rich porphyries until ~195 Ma. Late alkalic porphyry formation, following emplacement of the Quesnel arc onto the ancestral North American margin at ~186 Ma, may have been produced through cannibalism of mineralized intrusions emplaced at deeper crustal levels during the ~203 Ma Cu porphyry event. Two calc-alkalic porphyry deposits that predate Sitlika-Kutcho-Venables arc collision—Fin and Schaft Creek (~220 Ma)—are temporally related to arc uplift and erosion across the northern Stikine arc, but little is known about the extent and cause of this event and how it relates to porphyry Cu genesis. Slab tears may play a dominant role in the formation of porphyry deposits globally. Previous workers have linked episodes of porphyry formation with subduction of ocean plateaus, changes in subduction polarity, or termination of subduction. In each case, slab tears are the likely underlying cause of porphyry formation. We believe them to be fundamental to mineralization in the early Mesozoic arc terranes of Stikine and Quesnel.

Distribution and Diversity of Carboniferous and Permian Colonial Rugose Coral Faunas in Western North America: Clues for Placement of Allochthonous Terranes

Colonial rugose corals are common in western cratonal North America and in some of the allochthonous terranes, now amalgamated against its western margin. Throughout the Late Paleozoic, the coral faunas in these two different settings were significantly different. Comparisons of these faunas suggest that during the Mississippian the Alexander terrane probably was southwest of Arctic Alaska and the Stikine terrane probably lay west of the southern part of the North American craton. The Cache Creek terrane lay far out in the Paleopacific Ocean. The Pennsylvanian faunas suggest that the Quesnellia and Eastern Klamath terranes were situated southwest of Arctic Alaska and the Alexander terrane was somewhat farther southwest and farther from cratonal North America. The Stikine terrane continued to be positioned west of the southern part of the North American craton. During the Early Permian, terranes with a cratonal faunal aspect may have lain 2000–3000 km west of cratonal North America and latitudinally generally southwest of their present positions. In the Middle Permian these terranes were carried southward relative to the North American craton. Simultaneously the Tethyan Realm expanded eastward.

Early Triassic peritidal carbonate sedimentation on a Panthalassan seamount: the Jesmond succession, Cache Creek Terrane, British Columbia, Canada

The Jesmond succession of the Cache Creek Terrane in southern British Columbia records late Early Triassic peritidal carbonate sedimentation on a mudflat of a buildup resting upon a Panthalassan seamount. Conodont and foraminiferal biostratigraphy dates the succession as the uppermost Smithian to mid-Spathian. The study section (ca. 91 m thick) is dominated by fine-grained carbonates and organized into at least 12 shallowing-upwards cycles, each consisting of shallow subtidal facies and overlying intertidal facies. The former includes peloidal and skeletal limestones, flat-pebble conglomerates, stromatolitic bindstones, and oolitic grainstone, whereas the latter consists mainly of dolomicrite. The scarcity of skeletal debris, prevalence of microbialite, and intermittent intercalation of flat-pebble conglomerate facies imply environmentally harsh conditions in the mudflat. The study section also records a rapid sea-level fall near the Smithian-Spathian boundary followed by a gradual sea-level rise in the early to mid-Spathian.

Nature and significance of the diachronous contact between the Hazelton and Bowser Lake groups (Jurassic), north-central British Columbia

Abstract In north-central British Columbia, the contact between the Lower to Middle Jurassic Hazelton Group and the Middle Jurassic to Cretaceous Bowser Lake Group is an upward gradational change from fine-grained, commonly tuffaceous sedimentary lithofacies to commonly coarser and more heterogeneous lithofacies assemblages that include abundant chert clasts and generally lack distinct tuff beds. Paleontologic dating reveals that the lowest Bowser Lake Group is generally older (Bajocian) in north-most exposures and younger (Early Oxfordian) to the southwest toward the centre of the Bowser Basin. Interpretation of stratigraphic changes relies upon age data from ammonites, and to a lesser extent marine bivalves; a new bivalve species with biostratigraphic significance, Myophorella richardsi n. sp., is named herein. In many places the contact between the Hazelton and Bowser Lake groups is younger than previously understood. Earlier treatments of this boundary over-emphasized paleontologic content and time correlations rather than lithologic criteria. The diachroneity of the base of the Bowser Lake Group resulted from: southwestward migration of the initial front of coarse, chert-bearing, clastic detritus derived from the Cache Creek terrane, which started in the Bajocian; and subsequent northward migration of coarse facies containing volcanic clasts shed from a volcanic highland near the south margin of Bowser Basin. Although volcanic activity in the immediate vicinity of the Bowser Basin diminished significantly in the Bajocian, volcanism continued to the south of the basin into the latest Jurassic. This southern volcanism was likely the source for tuff beds in the uppermost Hazelton Group in the immediate vicinity of Bowser Basin. Continued southern volcanism and input of ash to the Bowser Lake Group is recorded by U-Pb ages of detrital zircons that are coeval with depositional ages inferred from fossils. The large volume of clastic detritus in the Bowser depositional systems, however, diluted the influx of ash and prevented the accumulation of distinct tuff beds within the Bowser Basin proper.

Sedimentation styles and depositional processes in a Middle to Late Jurassic slope environment, Bowser Basin, northwestern British Columbia, Canada

The Middle to Upper Jurassic Todagin assemblage in northwestern British Columbia, Canada, was deposited in the Bowser Basin above arc-related rocks of the Stikine terrane. Sedimentary structures indicate that a variety of gravity flow processes were involved in transport and deposition in deep-water slope environments. At Mount Dilworth, laterally continuous and channelized turbidites are interbedded with and overlain by mass-transport deposits in which sedimentary clasts are supported in a mudstone matrix. More than 50% of the succession consists of mass-transport deposits, indicating significant slope instability. A 300 m thick mass-transport complex exposed near the top of the succession is interpreted to result from tectonic activity, which triggered a major change in sediment supply from a local source area. At Todagin Mountain, a channel complex displays three successive channel-fills with associated overbank sedimentation units. Mass-transport deposits are rare, and confined to channel axes. Channels 1 and 2 are characterized by 40–50 m thick, ungraded pebble clast-supported conglomerate while the uppermost Channel 3 contains graded beds and occasional traction structures. The gradual change from erosive and amalgamated channel deposits at the base, to more aggradational channels at the top, is related to elevation of the equilibrium profile. Creation of accommodation favored aggradation on the mud-dominated slope succession and construction of well-developed channel-levee systems. The vertical succession exposed at Todagin Mountain is consistent with normal progradation of the slope under high sedimentation rates. In the Mount Dilworth area, extensional faulting associated with development of the restricted Eskay rift in the early Middle Jurassic produced a dissected basement above which the Todagin assemblage was deposited. These structures were inverted during collision of the Stikine and Cache Creek terranes, and likely played a major role in the stratigraphic evolution of the deep-water architectures.

Geochemistry of the northern Cache Creek terrane and implications for accretionary processes in the Canadian Cordillera

The northern Cache Creek terrane in the Canadian Cordillera includes a subduction complex that records the existence of a late Paleozoic – Mesozoic ocean basin and provides an opportunity to assess accretionary processes that involve the transfer of material from a subducting plate to an upper plate. Lithogeochemical data from basaltic rocks indicate that the northern Cache Creek terrane is dominated by two different petrogenetic components: (1) a dominant suite of subalkaline intrusive and extrusive rocks mostly of arc affinity and (2) a volumetrically less significant suite of alkaline volcanic rocks of within-plate affinity. The subalkaline intrusive and extrusive rocks constitute a section of oceanic lithosphere that is interpreted to have occupied a fore-arc position during the Late Triassic and Early Jurassic before it was accreted during collisional orogenesis in the Middle Jurassic. Alkaline volcanic rocks in the northern Cache Creek terrane are stratigraphically associated with carbonate strata that contain Tethyan fauna that are exotic with respect to the rest of North America; together, they are interpreted as remnants of oceanic seamounts and (or) plateaux. The volcanic rocks are a minor component of the carbonate stratigraphy, and it appears that the majority of the volcanic basement was either subducted completely at the convergent margin or underplated at greater depth in the subduction zone. In summary, accretion in the northern Canadian Cordillera occurred principally by the accretion of island arcs and emplacement of fore-arc ophiolites during collisional orogenesis. The transfer of oceanic sediments and the upper portions of oceanic seamounts from the subducting plate to an accretionary margin accounts for only small volumes of growth of the upper plate.