Philippe Erdmer

Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence

We present geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblage greenstones from the Teslin tectonic zone of the northern Canadian Cordillera. This study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed. For the Nisutlin assemblage metasedimentary rocks, the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments. One source region was felsic, upper crustal material with Nd isotopic compositions compatible with ultimate derivation from the North American continent (depleted mantle model age (TDM) 2.5–2.8 Ga). The second source region is deduced to be chemically primitive crust (basaltic-andesitic) with a short crustal residence history (<0.9 Ga). For the Anvil assemblage greenstones, immobile trace element abundances are dissimilar to within-plate, oceanic arc and normal mid-ocean ridge basalts and similar to calc-alkaline basalts in active continental margin settings. We interpret the paleosetting for the Nisutlin assemblage to be at the outermost margin of the ancestral North American continent, in an area which received detrital input from the distal North American craton in the early Paleozoic. However, this area also received detritus from a chemically and isotopically juvenile magmatic arc source, a source type not known from Paleozoic metasedimentary rocks from the miogeoclinal sequence. On the basis of similar Nd isotopic relationships recorded elsewhere in deformed Paleozoic rocks of the orogen, we infer that these geochemical signals reflect tectonic processes of regional extent. The trace element geochemistry of the Anvil assemblage greenstones does not support a correclation with known Paleozoic greenstones of the Slide Mountain terrane, which some tectonic models have advocated.

North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: Inferences from geochemical and Nd isotopic characteristics of Triassic metasedimentary rocks

New geochemical and Nd isotopic data from Triassic (ca. 220 Ma) clastic metasedimentary rocks assigned to the Quesnel terrane (Quesnellia) in southeastern British Columbia show that detritus was derived from both primitive and evolved sources. Twenty-four samples yield eNd 2 2 0 values between-9.1 and +5.6, independent of sample location. These samples have Eu anomalies between 0.6 and 1.0, La N /Yb N ratios between 3.8 and 11.4, La/Sc ratios ranging from 0.38 to 4.76, and Th/Sc ratios between 0.09 and 1.62. This range of isotopic and geochemical values overlaps that of samples from upper Paleozoic to Mesozoic rocks of the North American miogeocline and Upper Triassic Nicola Group volcanic rocks of the Quesnel terrane. The range in Nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental North America (evolved cratonic source) and a volcanic arc (primitive source) developing on the continental margin. These data do not support previous interpretations of the Quesnel terrane as an intraoceanic arc. The inferred depositional link between North America and the Quesnel arc in Triassic time is ca. 40 Ma older than the hypothesized obduction of the Quesnel terrane onto the North American continental margin and, when combined with existing geological information, strongly supports a pericratonic, nonexotic origin for the Quesnel terrane in southeastern British Columbia.

Paleozoic and Mesozoic high-pressure metamorphism at the margin of ancestral North America in central Yukon

Eclogite and blueschist in the Yukon-Tanana and Slide Mountain terranes in the Yukon and Alaska preserve a partial record of middle Paleozoic, late Paleozoic, and early Mesozoic subduction and subsequent unroofing. Near Faro, eclogites are commonly mylonitized and partially retrogressed. The analysis of Fe-Mg exchange between garnet and clinopyroxene suggests temperatures ( T ) in the 400–500 °C range for eclogite, and jadeite (∼40 mol%) + quartz suggest minimum pressures ( P ) of 11–13 kilobar at that temperature range; clinopyroxene-garnet-epidote-quartz suggest pressure near 15 kilobar ( P H 2 O = P S ). Eclogite near Last Peak, which contains brown hornblende, late-stage oligoclase, and biotite, yielded P-T estimates up to 625 °C at a minimum pressure of 14 kilobar. Eclogite near Ross River yielded temperatures near 525 °C and a minimum pressure near 13 kilobar. The previously proposed Permian-Triassic age of high-pressure tectonism for the entire Yukon-Tanana terrane in the Yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities. In this study, isotopic 40 Ar/ 39 Ar analysis of white mica from eclogite in the Simpson range yielded a date of 344 ± 1 Ma. Eclogite and blueschist from Ross River yielded white mica 40 Ar/ 39 Ar dates of 267 ± 3 Ma and 273 ± 3 Ma, respectively. White mica in eclogite from Faro was dated at 260 ± 3 Ma. White mica in two eclogites from Stewart Lake, about 2 km apart, yielded 40 Ar/ 39 Ar dates of 228 ± 1 Ma and 346 ± 3 Ma. White mica in Last Peak eclogite, for which a concordant U-Pb zircon date is 269 Ma, yielded an integrated date of 236 ± 1 Ma. Overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others. The cooling ages indicate significant plate convergence as early as Carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone. When this is considered together with the range of ages of other high-pressure occurrences, the North American Cordillera is seen to be characterized by multiepisodic and diachronous high-pressure metamorphism.

Rates of thermal equilibration at the onset of subduction deduced from diffusion modeling of eclogitic garnets, Yukon-Tanana terrane, Canada

Well-preserved eclogitic rocks near Faro (Yukon-Tanana terrane, Canada) underwent a prograde evolution from ~510 °C and ♢1.1 GPa to 690 °C and ♢1.5 Gpa followed by nearisobaric cooling to ~540 °C. A remarkable feature of the garnet porphyroblast cores is the presence of minute garnet “inclusions” of distinctly different composition. Preservation of a sharp compositional gradient at the interface between the host and the inclusion garnets and results of diffusion modeling indicate that the counterclockwise pressure-time evolution took place on a very short time scale of about 0.2 m.y. Minimum rates of heating (950 °C/m.y.) and burial (7 cm/yr) calculated along the prograde part of the path are in good agreement with values extracted from thermal models of newly initiated subduction zones.

Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?

Coherent oceanic strata in the French Range belong to the exotic Cache Creek terrane of the Canadian Cordillera. They were metamorphosed to blueschist grade, tectonically extruded, eroded, and intruded by plutons—perhaps in <2.5 m.y. Sodic amphibole overprint chert as young as late Pliensbachian to Toarcian age (ca. 191 to ca. 177 Ma). Blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 Ar/ 39 Ar as 173.7

Petrogenesis of the Cretaceous Cassiar batholith, Yukon–British Columbia, Canada: Implications for magmatism in the North American Cordilleran Interior

The ca. 100 Ma Cassiar batholith is the single largest plutonic body in the hinterland of the Canadian Cordillera and is part of widespread middle Cretaceous to Eocene magmatism that occurred in the Omineca crystalline belt. The Cassiar batholith is dominated by muscovite-biotite granite and biotite ± muscovite granodiorite along with subordinate biotite ± hornblende granodiorite, quartz monzodiorite, and quartz monzonite. The batholith has a wide range of strontium, neodymium, and oxygen isotope compositions [( 87 Sr/ 86 Sr) i = 0.706 to 0.734; ϵ Nd = −2.7 to −17.1, T DM = 2000 to 980 Ma; δ 18 O = +7.6‰ to +11.6‰], but the batholith9s lead isotope compositions are limited ( 206 Pb/ 204 Pb = 19.14 to 19.24; 207 Pb/ 204 Pb = 15.70 to 15.72; 208 Pb/ 204 Pb = 39.14 to 39.26). These data require that the Cassiar magmas have been generated from at least two distinct sources—an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition. We propose that the felsic source comprised Mesoproterozoic to Neoproterozoic metasedimentary rocks and Paleoproterozoic basement rocks of the North American cratonic margin. Determining the mafic source is more problematic, but most likely it was Proterozoic basement rocks of mafic to intermediate composition, such as have been reported from various parts of the Omineca crystalline belt. The Cassiar batholith is lithologically, geochemically, and isotopically similar to other major batholiths in the Cordilleran Interior, including the batholiths of southeastern British Columbia and the Idaho batholith. We interpret these similarities to reflect melting of common crustal source materials, namely Proterozoic basement and cover rocks of western Laurentia. The Cassiar batholith and other Cordilleran Interior granitoids are distinct from granitoids generated in known subduction-zone environments in terms of their mineralogical, geochemical, and isotopic characteristics. These differences reflect a predominantly or exclusively crustal source region for the Cordilleran Interior granitoids. In contrast, subduction-zone magmatism contains a significant mantle input, even in cases where subduction takes place beneath thick continental crust. These results strongly suggest that Cordilleran Interior granitoids did not form directly from a subduction setting. Rather, an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension.

An examination of the cataclastic fabrics and structures of parts of Nisutlin, Anvil and Simpson allochthons, central Yukon: test of the arc-continent collision model

Abstract Collision of an arc complex against the Yukon part of the North American craton during the Mesozoic resulted in northeastward transport of arc rocks onto the craton. The arc rocks comprise three distinguishable tectonostratigraphic assemblages called: Nisutlin Allochthon, quartz muscovite and chlorite schist derived from sedimentary and intermediate volcanic protoliths; Anvil Allochthon, amphibolite, gabbro, ultramafic rocks and sepentinite; and Simpson Allochthon, granitic and granodioritic rocks and schist derived from them. Deformation, recovery and recrystallization structures show that parts of the allochthons are blastomylonite, formed at temperatures of 350–700°C and at depths between 15 and 40 km. The stacking order of the allochthons is inconsistent and complex, and locally, the cataclastic rocks are overthrust by autochthonous strata. The allochthons are truncated by late steep faults that have mainly strike-slip displacement, and may be contemporaneous with Tintina Fault. The proposed tectonic history of the region is supported by detailed results of the study.

Structural divergence and transpression in the Teslin tectonic zone, southern Yukon Territory

The structural and tectonic evolution of the Teslin tectonic zone, a complex belt of ductilely deformed rocks forming the southern extension of the Yukon-Tanana terrane in the northern Canadian Cordillera, is elucidated with new field data and by the reinterpretation of existing data. The zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the Nisutlin and Anvil assemblages characterized by S and L-S tectonite fabrics. Primary contact relationships and ages show that most rocks in the zone were contiguous by Mississippian time. Mapping identified a number of strain domains which preserve S fabrics dipping northeasterly or southwesterly, variable mineral lineation orientations, and a variety of shear directions including easterly and westerly vergent thrust shear, down-to-the-east and down-to-the-west normal shear, and dextral strike-slip shear. Regional constraints and microstructures suggest that latest ductile deformation was Late Triassic to Early Jurassic in age. Structural characteristics are explained most effectively by convergent-dominated transpression in combination with tectonic wedging and associated back thrusting above a low-angle west dipping detachment. Oblique eastward and upward movement of the rocks over the detachment, which may have coincided with the top of thinned North American continental crust, produced easterly vergent shear, and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear. The transpressive deformation postdates formation of the Nisutlin assemblage by Mississippian time and subduction of the Slide Mountain Ocean beneath the Nisutlin assemblage in Permian to Early Triassic time. The western margin of the Teslin tectonic zone was truncated by Cretaceous strike-slip faults and translated northward. Rocks of the peraluminous orthogneiss assemblage of the Yukon-Tanana terrane in central Yukon acted as the tectonic wedge and were overthrust by rocks of the Nisutlin and Anvil assemblages in Early Jurassic time before unroofing by local extension in Cretaceous time.

A 1200-km-long Eocene metamorphic-plutonic belt in the northwestern Cordillera: Evidence from southwest Yukon

Distinctive regional Eocene metamorphic-plutonic units of the Coast Belt in southwestern Yukon match similar units in southeastern Alaska and British Columbia, and possibly in the eastern Alaska Range, and together form a belt originally >1200 km long. The final high-grade metamorphic event in the Aishihik and Kluane assemblages between the Ruby Range batholith and the Denali fault zone occurred in latest Paleocene to early Eocene time. Structural, petrologic, and geochronologic evidence indicates that metamorphism occurred during a major period of west-southwest-vergent deformation and metamorphism. The final phase of consumption of the Kula plate at ∼43 Ma may be linked to the culmination of metamorphism, intrusion, and deformation along the early Cenozoic Cordilleran margin.

Hot-side-up aureole in southwest Yukon and limits on terrane assembly of the northern Canadian Cordillera

Near Aishihik Lake in southwest Yukon, Canada, Paleozoic and older pericratonic metamorphic rocks of the Nisling terrane are overlain by foliated granodiorite of the Aishihik batholith (186.0 ± 2.8 Ma), part of the Lower Jurassic Aishihik plutonic suite. Isograd geometry in the Nisling rocks indicates that metamorphism resulted from intrusion of the batholith. Metamorphic minerals define a series of zones, including staurolite, staurolite-kyanite, kyanite, and sillimanite, which are parallel to schistosity and dip beneath the batholith. They form a hot-side-up aureole in which metamorphic grade increases upward. Petrogenetic relations and geothermobarometry indicate peak metamorphism at pressures of 8 to 10 kbar and temperatures in excess of 720° C in the sillimanite zone. A U-Pb age determination on titanite from the aureole indicates cooling through 600° C at 184 ± 2 Ma, soon after crystallization of the batholith, consistent with emplacement of the Aishihik batholith into the Nisling terrane. Early Jurassic arc magmatism is therefore a characteristic of Nisling and Stikinia. Isotopic and stratigraphic data point to the construction of a single arc across these previously juxtaposed terranes, although it is possible that coeval Lower Jurassic arcs may have developed on both terranes in response to the closure of intervening basins. This magmatism distinguishes Nisling from conterminous North America, which lacks significant Lower Jurassic igneous rocks.