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In the Wrangell Volcanic Belt (WVB) a northwesterly increase in volume of calc-alkaline versus transitional (sodic alkaline/calc-alkaline) magmatism is accompanied by a migration in the locus of magmatic activity. The space-time-composition relationships reflect oblique convergence between the North American and Pacific plates over the last 17.3 million years. Compositional- temporal trends are particularly well preserved in the four stages of volcanic stratigraphy in the St. Clare Creek field (17.3-6.5 Ma). Initially, alkaline olivine basalts, hawaiites and mugearites were erupted from small, isolated shield volcanoes in the axis of a continental molasse basin. The alkaline lavas were followed by an early stratovolcano stage of transitional trachybasalts and high-Fe basaltic trachyandesites, succeeded by basaltic trachyandesites, trachyandesites, trachytes, rhyolites and rare basaltic andesites. Widespread basaltic fissure eruptions dominated the third volcanic stage. The late stratovolcano stage consisted of renewed eruption of intermediate and felsic transitional lavas. A systematic temporal-chemical relationship between early alkaline and younger transitional and calc-alkaline lavas in the St. Clare Group is illustrated by a decrease in FeO/MgO, Na+K/Si, NB/Zr/Y, and an increase in Rb/Zr with increasing stratigraphic levels. Primitive basalts are non-primary and show variable degrees of fractionation between large ion lithophile (LILE) and high field strength element abundances. A model is proposed in which the alkaline shield volcano and early stratovolcano stage magmas formed by progressive melting of a rising mantle diapir in response to local extension along the Duke River fault. Early Fe-rich magmas may have undergone clinopyroxene fractionation at high pressures, but most magmas appear to have differentiated in the near surface environment via fractional crystallization and local magma mixing. With the onset of Yakutat subduction, progressively larger amounts of slab-derived, LILE-enriched fluids metasomatised overlying peridotite, which in turn melted to form primitive, late-stratovolcano stage magmas.

Lower Paleozoic continental margin rocks of the North American Cordillera, from Yukon to Nevada, include coeval platformal carbonate and basinal clastic strata that are offset along rift‐transfer faults, including the Liard, St. Mary‐Moyie, and Snake River structures. The Dawson fault is a prominent east‐weststriking structure in central Yukon that is interpreted herein to have been active as a rift‐transfer fault by late Cambrian time. This hypothesis is supported by new zircon U‐Pb dates that range from 501.98 ± 0.17 Ma to 497.57 ± 0.70 Ma from alkaline mafic volcanic rocks concentrated along the Dawson fault. The development of a sub‐Jiangshanian unconformity immediately post‐dates this alkaline magmatism and indicates that final continental breakup and establishment of the northern Cordilleran margin occurred by the late Miaolinginan. Alkaline magmatism caused by local decompression partial melting of the mantle may have been triggered by the release of in‐plane tensile stresses during lithospheric rupture and edge‐driven mantle convection. Upper Ordovician alkaline mafic volcanic and plutonic rocks that occur along a northwest‐southeast striking segment of the Dawson fault erupted ∼50 Myr after breakup and represent an example of post‐rift magmatism along a rift‐transfer fault. New bedrock mapping, and geochronological, paleontological, and petrological results from Upper Ordovician rocks indicate that there was localized basin development and punctuated volcanism along the Dawson fault from 453 to 447 Ma. Late Ordovician extension and post‐breakup magmatism in central Yukon is compatible with dextral strike‐slip reactivation of the Dawson rift‐transfer fault associated with counterclockwise rotation of Laurentia.

Late Triassic to Early Jurassic age (~220-185 Ma) intrusions comprise one of the most widespread and volumetrically significant plutonic suites in central and western Yukon, and eastern Alaska, but have received very limited study thus far. A new research project has been initiated that will examine the temporal, geochemical and petrotectonic evolution of this magmatic event, and the nature and origin of associated Cu, Au and PGE mineralization. The intrusions are mainly hornblende- and biotite-bearing granodiorites and quartz monzonites, although granitic phases and rare ultramafic phases (as at Pyroxene Mountain) are also present. Several bodies of coarse-grained muscovite granite that are included within the suite have been recognized in southwestern Dawson, and central and western Stewart River map areas. Most intrusions give preliminary U-Pb zircon and titanite ages of ~195 Ma to ~185 Ma, although scattered bodies give ages up to 218 Ma. Geochemical studies completed thus far indicate that most intrusions are metaluminous and formed in a volcanic arc environment, although some of the muscovite-granite phases in western Yukon are peraluminous and trend into the anorogenic (within-plate) granite field on various tectonic discriminant plots. Dating studies at Minto and Williams Creek indicate that copper-gold mineralization in both areas is hosted in part by deformed intrusions dated at ~194 Ma and is crosscut by massive, post-mineralization Granite Mountain batholith dated at ~190 Ma. The mineralization is therefore intimately associated with the Triassic-Jurassic magmatism, and we tentatively interpret the deposits as deformed copper-gold porphyries.

The early Tertiary magmatic episode in the northern Canadian Cordillera is linked to the restructuring of the Kula-North American plate system from orthogonal to oblique convergence. Resultant volcanism was widespread, and remnant successions outcrop along the eastern margin of the Coast Plutonic Complex (CPC). The Sifton Range volcanic complex of southwestern Yukon is a member of the Paleogene Sloko-Skukum Group, and comprises a 900-m thick, shallow-dipping, volcanic succession dominated by intermediate to evolved lava and pyroclastic rocks deposited in a northwesterly trending half-graben. Locally, the volcanic sequence is intruded by alkali-feldspar granites of the CPCs Nisling Plutonic Suite dated at 57.5 Ma. Felsite sills radiate from the main intrusive body, and together with numerous basaltic to dacitic dykes traverse the volcanic package. Both the felsic volcanic rocks and epizonal granitoids exhibit anomalous enrichments in large-ion lithophile elements indicating crustal contributions during the late-stage petrogenesis of the complex. In addition, the Sifton Range intrusive rocks exhibit modal mineralogy reflective of lower ambient pressures relative to the compositionally similar Annie Ned granites along the Alaska Highway between Stony Creek and Mendenhall, 20 km south of the complex. The amount of post-Eocene uplift (ca. 30 m/Ma) that exposed the contact between the intrusive and corresponding volcanic rocks is constrained by the presence of a calc-silicate bed at an elevation of 1830 m within the upper volcanic stratigraphy.

The Wrangellia Terrane along the northwest margin of North America is an extensive accreted oceanic plateau. These volcanic sequences erupted onto an extinct island arc in less than 5 million years at ca. 230 Ma. Triassic Wrangellia basalts and intrusions form a 1 to 10 km-wide linear belt of mafic and ultramafic rocks extending 300 km across southwest Yukon. A total of 85 samples were collected for geochemical and isotopic analysis from 10 widespread areas along the entire length of the linear belt. Field observations during the summer of 2004, and a synthesis of previous research for the Yukon portion of Wrangellia, are part of a larger research project involving Wrangellia basalts extending from Vancouver Island to central Alaska. The Wrangellia volcanic sequences represent one of the finest examples of an accreted oceanic plateau worldwide. They provide an excellent opportunity to gain a better understanding of the mantle source of oceanic plateaus and to assess the role of accretion of oceanic plateaus in continental growth.

In the southeastern Yukon Territory, Quaternary continental alkaline basalts have erupted across an important crustal suture, the Tintina Trench, which separates the accreted terranes of the Canadian Cordillera from the ancestral North American craton. The lavas from the Rancheria region from the west side of the Tintina Trench are basanites (BASAN), alkaline olivine basalts (AOB), and hypersthene-normative basalts (HYN). They display fractionated rare earth element (REE) profiles and are enriched in light rare earth elements (LREE) and high field strength elements (HFSE). The compositional spectra of the Rancheria alkaline magmas appears to represent the progressive melting of an amphibole-bearing garnet lherzolite. The involvement of amphibole in the petrogenesis of the Rancheria alkaline magmas indicates that these magmas were generated within the lithosphere. At the eastern end of the Rancheria suite, on the east side of the Tintina Trench, the AOB from Watson Lake have higher Zr contents than Rancheria AOB to the west of the Trench. The high Zr contents of the Watson Lake AOB are similar to those observed in the Hoole Eocene tholeiitic basalts, on the east side of the Tintina Trench, further to the north. The Eocene basalts from the Hoole River region are olivine tholeiites which have experienced closed-system crystal fractionation of olivine at low pressure. The estimated primary magma for these Eocene basalts appears to have been derived by partial melting of an incompatible-element enriched lithospheric mantle source, during which garnet was not a residual phase. The Nb-Zr systematics of the Watson Lake basalts indicate that they may be derived by mixing between melts produced by melting of an amphibole-bearing residue and a lithospheric mantle similar in composition to that of the Hoole basalts. Therefore, these compositional differences in the alkaline basalts across the Tintina Trench appear to reflect the juxtaposition of chemically distinct continental lithospheric mantles, indicating that the Tintina Fault is a steep lithospheric suture.

Pentlandite-bearing serpentinized ultramafic flows with a komatiitic composition have been identified within volcano-sedimentary stratigraphy in the Nadaleen Range. Associated listwaenites or silica-carbonate-fuchsite-altered serpentinites carry locally significant gold, copper, nickel and cobalt values. The occurrence of laterally extensive ultramafi c units at the northern edge of the Selwyn Basin remains difficult to explain within the current scope of geological knowledge in the area. However, it represents a new style of exploration target for copper-nickel-bearing massive sulphide deposits, as well as listwaenite-associated gold.

Volcanic rocks in the Dawson map-area occur as isolated lenses within the early Paleozoic Selwyn Basin assemblage and as sets of flows and complexes within Proterozoic carbonate rocks near the edge of the Mackenzie Platform. Mount Harper complex, an example of the latter group, has been studied in greater detail than the others because it is more completely exposed, and contains two mafic to felsic volcanic cycles. Unlike the sedimentary rocks, whose correlation with established units in the Wernecke Mountains and Alaska is now well advanced, understanding of the stratigraphic position and internal relations of the volcanic piles is less definite. This report, based on two field mapping seasons, has two parts. Characteristics of the two volcanic groups and correlative occurrences are discussed in the first part; the second presents interpretations from the stratigraphy of the Mount Harper complex as an illustration of the style of volcanism in the region.

The Paleoproterozoic Slab volcanics occur in three localities in the Wernecke Mountains. The largest exposure is at Slab Mountain and consists of a 0.6 x 0.25 km block of thin, steeply dipping mafic to intermediate lava flows. A zone of Wernecke Breccia (1.60 Ga), which crops out along the exposed margin of this volcanic block suggests that the megaclast foundered into the breccia zone from a higher crustal level. The volcanic rocks are typically aphyric. The groundmass consists mainly of laths of plagioclase (commonly altered to scapolite), anhedral biotite and magnetite. The scapolite, and possibly the biotite and magnetite, likely grew during hydrothermal alteration associated with Wernecke Breccia emplacement. Primary igneous mineralogy is uncertain. The Slab volcanics appear geochemically similar and are probably comagmatic with some of the 1.71 Ga Bonnet Plume River Intrusions. No correlative volcanic strata have been found in the Wernecke Mountains or in the neighbouring Ogilvie Mountains.