Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes. From 230 to 214 Ma (during Late Triassic time), extensive mafic volcanism occurred in the Wrangell, Quesnel, and Stikine terranes. Volcanic-related ultramafic complexes are found scattered through the two latter terranes. Large calc-alkaline granitic plutons are known only in a belt crossing Stikinia in northern British Columbia. At the same time, blueschists formed in the Cache Creek accretion wedge. From 214 to 200 Ma (end of Triassic and part of Early Jurassic time), Early to Middle Jurassic arc magmatism began in Wrangellia and in the northern Quesnel, Stikine, and Yukon terranes. A distinct magmatic event is recognizable only in southern Quesnellia. Magmatism was absent on the North American craton. The Cache Creek and Quesnel terranes were definitely linked, Stikine and Cache Creek terranes were probably linked, and a regional metamorphic episode was completed in the Yukon Terrane by this time. From 200 to 155 Ma (late Early to early Late Jurassic time), magmatism was extensive in the Wrangell, Quesnel, Stikine, and Yukon terranes. Magmatism over-lapped into North America only east of southern Quesnellia after about 180 Ma. By the middle of this time interval, the southern Quesnel-Slide Mountain-North America linkage was complete, and major deformation and metamorphism had affected the Omineca Belt in British Columbia. Early to Middle Jurassic magmatism in southern Wrangellia (Vancouver Island) is distinctly older than the Middle to Late Jurassic magmatism that occurred in central Wrangellia (Queen Charlotte Islands). From 155 to 140 Ma (during Late Jurassic time), a few last-gasp plutons of the late Early to early Late Jurassic episode and other rocks with partially reset 155- to 145-Ma dates occur in the Wrangell, Quesnel, and Stikine terranes. Late Jurassic magmatism (160 to 140 Ma) occurred in the Alexander Terrane (Saint Elias region). From 145 to 138 Ma (latest Jurassic and beginning of Early Cretaceous time), plutonism occurred in the Endako area of central British Columbia (Francois Lake suite) but is virtually unknown elsewhere. From 135 to 125 Ma (during Early Cretaceous time), there was a magmatic lull of major significance present throughout western North America. From 110 to 90 Ma (middle Cretaceous time), widespread plutonism occurred across all terranes. Dual culminations are evident: the Coast Plutonic and Ominica belts. Before this time all sutures except those outboard of Wrangellia had been closed. From 80 to 70 Ma (during Late Cretaceous time), a narrow, sinuous belt of magmatism persisted, mostly in the southeastern Coast Plutonic Belt, southwestern Yukon Territory, and scattered across the Skeena and Stikine arches. From 70 to 60 Ma (latest Cretaceous to Paleocene time), a distinct lull in magmatism occurred. Rare plutons of this time interval are known in the Coast Plutonic Belt, on the Skeena Arch, and in the southern Intermontane Belt. From 55 to 45 Ma (latest Paleocene to Middle Eocene time), widespread and voluminous magmatism occurred in all terranes. The early Cenozoic volcanic front crossed the Coast Plutonic Complex from its east side in the south to its west side in the north. Associated thermal and tectonic effects were strong even into the Omineca Belt, producing large reset metamorphic areas in the Coast and Omineca belts. This was a short-lived event, synchronous from southern British Columbia through the Yukon Territory. West of the volcanic front, offshore of Wrangellia, Metchosin volcano growth was underway at this time. Late in this time interval, the 50?–45–36-Ma Catface–Leech River event(s) of southern Wrangellia occurred. There is also time overlap with a diffuse Massett magmatic event in the Queen Charlotte Islands, and with Baranoff Island and Yakutat–Saint Elias region magmatism. Initial 87 Sr/ 86 Sr ratios and petrographic characteristics of Canadian Cordilleran igneous rocks are reviewed in the time frame just described. These reflect the nature of underlying crust, contemporaneous lithosphere thickness, and distance from the subduction zone. Comparisons with other parts of the Circum-Pacific Magmatic Belt shows both out-of-phase magmatism (Japan and southwestern Alaska) and perfect matching of some episodes (Sierra Nevada). Major magmatic episodes correspond to times of increased westward motion of North America with respect to hot spots or to times of increased convergence between western North America and the Farallon Plate.

Eocene-Oligocene strike-slip faulting along the Denali fault system in Yukon Territory produced several local strike-slip basins that were filled by coarse-grained alluvial, fluvial, and lacustrine sediment referred to as the Amphitheatre Formation. The Amphitheatre Formation provides an excellent opportunity to study in detail the composition of siliciclastic sediment in a major, accretionary strike-slip orogen. Petrographic analyses of sandstones and clast counts of conglomerates reveal a lithologically diverse provenance. On standard QFL and QmFLt diagrams, sandstones from the Burwash and Bates Lake basins are arkosic, overlapping the uplifted basement and dissected magmatic arc provenance fields of Dickinson and Suczek (1979). Conglomerates were derived from volcanic, plutonic, and medium- to low-grade metasedimentary rocks that currently are found in several accreted terranes associated with the Denali fault system. Wrangellia and the Alexander terrane provided voluminous volcanic, metavolcanic (greenstone), sedimentary, and low-grade metasedimentary material. The Yukon Crystalline terrane provided medium-grade (gneissic and schistose) material and plutonic material, and the Gravina-Nutzotin belt provided pelitic and metasedimentary material. These data support the contention that the Amphitheatre Formation was derived mainly from local, high-relief sources associated with major uplift in the eastern St. Elias Mountains, as well as the southern Yukon Crystalline terrane. The Eocene-Oligocene climatic event apparently had no significant effect on Amphitheatre sandstone composition. This is attributed to the proximal character of the sandstones studied; steep local relief kept soil residence times to a minimum in spite of climatic change. In addition, the persistence of coal- and stream-dominated alluvial fan facies throughout the entire stratigraphic section indicates that climate remained humid in southwestern Yukon Territory, possibly owing to local orographic effects. It is proposed that, in general, climate should not have a major impact on detrital sand composition in large strike-slip orogens, where sediment accumulates within the orogen itself and is subject neither to long distances of transport nor to long-term residence in lowland soil profiles where most compositional modification is expected.

ABSTRACT The Coast Mountains batholith (CMB) is one of the largest continental margin batholiths in the world. It is nearly continuously exposed for >1700 km along the west coast of North America in British Columbia through southeastern Alaska into southwestern Yukon Territory. This guide, prepared for the GSA Thompson Field Forum held in August 2018, describes the geology along the readily accessible Skeena River transect of the CMB in British Columbia. At this latitude, the CMB is bounded on the east by generally low-grade stratified rocks and subordinate Jurassic to Eocene plutons. These rocks are bounded on the west by a Paleogene, low-angle, top-to-the northeast detachment (the Eastside detachment). West of the detachment, the Central Gneiss Complex (CGC), which forms the lower plate of the detachment, consists of amphibolite to granulite-facies schist, gneiss, and orthogneiss, intruded by Late Cretaceous to Paleogene plutons. The CGC is characterized by regionally consistent Eocene 40 Ar/ 39 Ar and K-Ar cooling dates. This core belt is bounded on the west by the Paleogene Coast shear zone, a steep crustal-scale structure. Paleogene plutons do not occur west of this belt. West of the Coast shear zone, schists of the Western metamorphic belt show evidence for southwest-verging thrusting, and form an inverted metamorphic sequence with grade dramatically decreasing to the west. These rocks are intruded by Jurassic to Late Cretaceous plutons. We use this transect as a basis to examine the growth of the CMB as a whole, emphasizing commonalities and variations with the batholith and how these traits may reflect magmatic processes that create this and other convergent-margin batholiths. We conclude by highlighting a few of the many open questions regarding the evolution of this complex batholith.

Abstract The Carboniferous System in Western Canada Basin (Fig. 4E.1-4E.3) is a thick succesion of strata deposited on the downwarped and downfaulted western margin of the ancestral North American plate, the central to western cratonic platform, and southern Yukon Fold Belt. This succession, representing the upper Kaskaskia sequenceand lower Absaroka sequence of Sloss (1963), comprises two main lithofacies assemblages. The lower assemblage is basinal shale and generally thickens southwestward or basinward (see Fig. 4 E . 9 - 4 E . 1 3) . Upward and northeastward, it passe into an upper assemblage of platform and ramp carbonates (Fig. 4E.4, 4E.5, 4E.9-4E.13) and sandstone-dominated siliciclastic facies, deposited in deep-water slope to continental settings. Both assemblages consist of numerous formations, some separated by regional disconformities. Subaerial erosion during the Late Carboniferous, Permian, and subsequent periods removed large parts of the succession, particularly in the Interior Plains, the region west of the Rocky Mountain Front Ranges, and the Cordillera between southwestern District of Mackenzie and northern Yukon Territory. Where the Carboniferous remains, it is generally unconformably overlain byeither Permian or Mesozoic strata. Carboniferous formations are preserved in twomain regions. The southern one, which includes much of the eastern Cordillera andsouthern to western Interior Plains, extends from southwestern Manitoba to southwestern District of Mackenzie. The northern area includes the eastern Cordillera ofnorthern Yukon Territory and northwestern District of Mackenzie (Fig. 4E.1). Betweenthese regions, erosional remnants are present in the Mackenzie and Selwyn Mountains of east-central Yukon and west-central District of Mackenzie.

Abstract Deposition of the Sauk megasequence of northwestern Canada, from Peace River in British Columbia (north of lat. 56°N) to the Beaufort Sea in the Northwest Territories and Yukon Territory (lat. ∼70°N), occurred along a complex segment of the rifted western margin of Laurentia. This synthesis describes the stratigraphy, depositional history, tectonic setting, and resource potential associated with the development of the Cambrian-Ordovician great American carbonate bank. There have been few detailed studies of the Sauk megasequence in these remote areas of Canada, and much information on platform and equivalent facies remains at a reconnaissance scale. Platform rocks occur as relatively flat-lying strata of the Interior Platform that thin eastward onto the Canadian shield and correlate westward to strata within the deformed belt of the Canadian Cordillera. It is generally accepted that the Sauk megasequence was deposited along the subsiding rifted margin of Laurentia because facies pass from shallow-marine settings to deeper water slope and basinal settings from east to west. Abrupt facies changes, thickness changes, and intercalated volcanic rocks in most successions suggest a setting more complex than a simple passive margin during the development of the Cambrian-Ordovician great American carbonate bank. During Sauk megasequence deposition, positive and negative tectonic features controlled deposition along the continental margin and are represented by distinct present-day physiographic regions. The northern Rocky Mountains region lies between 56 and 60°N and is included in this summary because its geologic history is more similar to regions to the north, instead of to the southern Cordillera. North of 60°N latitude, five main regions span the vast northern Canadian mainland sedimentary basin: Beaufort-Mackenzie, Interior Plains, Mackenzie Arc, Northern Yukon, and Selwyn-Cordillera. The lower Paleozoic succession is poorly known in the “Beaufort-Mackenzie Basin,” a term that typically refers only to uppermost Cretaceous to Holocene Mackenzie Delta sediments. The northern Rocky Mountains region lies north of Peace-Athabasca arch. The arch was a positive area and forms the southern limit to the Macdonald platform and the Kechika trough, a graben system that flanked the platform to the west. The Kechika trough accumulated mainly shallow-water siliciclastic sediments until the Late Cambrian, and Sauk I and II super-sequences are characterized by complex facies changes. Carbonate deposition was prominent in the Middle Cambrian when bioherms developed on tilted fault blocks in the western part of the Kechika trough. The uppermost Cambrian–Ordovician Sauk III stratigraphy records deposition during postrift passive-margin thermal subsidence; however, during the middle Sauk III supersequence, a distinct east-to-west platform-to-basin setting evolved in response to renewed extension along the margin. The Interior Plains region lies mainly in the Northwest Territories and contains a subsurface record of the Mackenzie platform. In the Cambrian, the area was an epicontinental marine basin consisting of several depocenters that opened to the south and were segmented by positive high elements such as the Mackenzie arch. Lower and Middle Cambrian units are siliciclastic dominated and discontinuous because of the extent of paleogeographic highs. Extensive Sauk III platform carbonates of the Mackenzie platform were deposited beginning in the latest Cambrian. The Mackenzie Arc region is the eastern part of the foreland belt in Yukon and the Northwest Territories. It offers a more continuous stratigraphic record compared with the Interior Plains, but abrupt facies changes from north to south suggest phases of extension. The region contains deformed strata of the Mackenzie platform, the Mackenzie arch, and the eastern part of Selwyn Basin, which extended into the platform as Misty Creek embayment. The Sauk I supersequence is siliciclastic dominated, and carbonates are present only in distal slope settings. Sauk II and III supersequences are carbonate dominated, representing expansion of the Macdonald platform across the Mackenzie arch. Prominent slope facies in the Sauk II and III supersequences suggest an abrupt platform-to-basin setting that also accumulated submarine volcanics during phases of extension. The Selwyn-Cordilleran region extends from southwestern Yukon and northeastern British Columbia to the international border with Alaska. It is an area of deformed incompetent strata that were deposited in the Selwyn Basin, west of the Mackenzie Arc. The region contains autochthonous North American strata east of the Tintina Fault, and parautochthonous and al-lochthonous terranes west of the fault. One parautochthonous terrane thought to be a displaced slice of the North American margin is the Cassiar terrane or Cassiar platform. The Sauk mega-sequence of Selwyn Basin is generally similar to that of the Mackenzie Arc region, although the Sauk I and II supersequences contain additional units of distal basinal siliciclastic facies. The northern Yukon region contains several mountain belts and intermontane basins or lowlands that contain a succession with overall similarities to the North American Sauk mega-sequence, although the record is poorly understood in this remote area. The main tectonic element is a broad area of platform carbonates, the Yukon stable block, which is bordered to the east by the Richardson trough and to the south by the Selwyn Basin. The lower Sauk I supersequence is absent because of uplift of the Ogilvie arch. Block faulting influenced a mostly siliciclastic-dominated Sauk II supersequence, although deposition of carbonate resumed during deposition of the upper Sauk II supersequence and persisted throughout the Sauk III supersequence deposition. Sauk megasequence carbonates that may have been part of the great American carbonate bank underwent abrupt lateral facies changes indicative of syndepositional extension during the Early to Middle Cambrian, and again in the Middle Ordovician. These tectonic complexities, coupled with the lack of detailed studies, preclude a comprehensive sequence-stratigraphic analysis and determination of relative or eustatic sea level changes. The base of the Sauk megasequence is a prominent sub-Cambrian unconformity in most regions; however, eastward (toward the craton), this surface represents a merger of several unconformities, and westward (basinward), various Cambrian units overlie Proterozoic rocks. The base of the Tippecanoe megasequence, although muted and less clear in thicker shelf and offshelf successions, lies in the Middle or Upper Ordovician. The Sauk I supersequence is dominated by shallow-water siliciclastic deposits, many of which are localized in extent, with rare carbonate in distal settings such as the Selwyn Basin. Carbonate units of the middle to upper Sauk I supersequence were most likely deposited basinward of a carbonate platform. The best record of Sauk II carbonate deposition occurs along the margins of the Selwyn Basin or is represented bynarrow tracts associated with block faulting. The basal parts of the Sauk III supersequence are poorly known, but significant Upper Cambrian terrigenous siliciclastic rocks in some regions suggest a phase of uplift. The Sauk III supersequence is otherwise characterized by widespread deposition of carbonate platform and equivalent slope and basin deposits at the edge of platforms during widespread transgression and continued margin subsidence. The vast area of northwestern Canada has both hydrocarbon and mineral resource potential in rocks of the Sauk megasequence, although exploration is limited by remoteness and lack of infrastructure. Early Paleozoic intracratonic and platform-to-basin settings of the margin accumulated both potential source and reservoir rocks for fluid hydrocarbons. The Colville Hills area of the Interior Plains region has the most significant discovery of gas and some condensate within Cambrian strata. This basal Cambrian siliciclastic play potentially exists across the Interior Plains. Porous reservoir facies of Cambrian–Ordovician platform carbonates constitute another potential play, and abundant source rocks are present within equivalent basinal facies. Although mountainous regions have little hydrocarbon potential because of their geologic history, some Cambrian–Ordovician rocks host mineralized zones, particularly in the Selwyn-Cordillera region such as the metallogenic belt of Zn-Pb-Ag sedimentary exhalative deposits of the Anvil district along the western margin of the Selwyn Basin.

Abstract Cambrian-Lower Ordovician carbonate rocks that likely formed as part of the Laurentian continental margin, and may thus have been part of the Cambrian-Ordovician great American carbonate bank, occur in east-central Alaska in the Nation Arch area. These strata accumulated on the southwestern margin (present-day coordinates) of the Yukon stable block, a broad area of early Paleozoic carbonate platform deposition in the northern Yukon Territory, and constitute two successions. The first consists of approximately 900 m (∼2950 ft) of shallow-water limestone and dolostone that are in part silicified, laminated, oolitic, and pisolitic, and make up the lower member of the Jones Ridge Limestone. Conodonts, trilobites, archaeo-cyathids, and brachiopods indicate an age of Early Cambrian to early Early Ordovician (Tremadoc; Ibexian) and have Laurentian biogeographic affinities. Upper Ordovician bio-clastic limestone (the upper member of the Jones Ridge Limestone) unconformably overlies these strata. A roughly coeval, but somewhat deeper water, succession crops out near the Jones Ridge Limestone and consists of, in ascending order, the Funnel Creek Limestone, Adams Argillite, and Hillard Limestone. The Funnel Creek (15-400 m [50-1310 ft] thick) is mainly nonfossilif-erous, extensively silicified, commonly oolitic limestone and dolostone and is assumed to be Lower Cambrian in age. It is overlain by argillite, siltstone, cross-laminated quartzite, and oolitic to sandy limestone of the Adams Argillite (90-180 m [295-550 ft] thick). This unit contains the trace fossil Oldhamia and Lower Cambrian archaeocyathids and trilobites that have Siberian affinities. The Hillard (30-150 m [100-490 ft] thick) is chiefly limestone, with local ooids, edgewise and boulder conglomerate, and phosphatic horizons, and likely formed in a platform-margin setting. Trilobites and brachiopods from this unit are Early Cambrian to earliest Ordovician in age and have mainly Laurentian affinities. Slope and/or basinal rocks of the Road River Formation that are as old as Early Ordovician (early middle Arenig; Ibexian) unconformably overlie the Hillard Limestone. Abrupt facies transitions between the two Nation Arch area carbonate successions may reflect relatively steep paleoslopes and/or telescoping of facies by imbricate thrust faults. Carbonate strata of Cambrian–Ordovician age are also found north of the Nation Arch area in the Porcupine terrane. These rocks have been little studied, and their precise Stratigraphic succession and paleogeographic setting are uncertain. The few fossil collections indicate mainly Laurentian affinities and include Cambrian(?) trilobites and Lower and Middle Ordovician conodonts. Lower Paleozoic strata of the Porcupine terrane probably formed at or near the northwestern edge (present-day coordinates) of the Yukon stable block. Cambrian–Ordovician carbonate strata occur widely in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes) and interior Alaska (Farewell terrane). These rocks share distinctive lithologic and faunal features and were deposited in a range of shallow-shelf to basinal environments. Carbonate platform successions in northern and interior Alaska include fossils of both Laurentian and Siberian biotic provinces and may have formed on a single crustal fragment that rifted away from the Siberian craton during the late Proterozoic. These Alaskan strata were most likely in faunal exchange with, but not physically attached to, the great American carbonate bank. Lower–Middle Ordovician carbonate and siliciclastic rocks are also found in the White Mountains, Livengood, and Ruby terranes of interior Alaska, the Alexander terrane in southeastern Alaska, and the Goodnews terrane in southwestern Alaska. These successions were likely not attached to Laurentia during their deposition, although some authors have proposed Laurentian origins for the White Mountains and Livengood terranes. Little detailed information is available on the resource potential of Cambrian–Ordovician successions in Alaska. Most have low porosity and are too thermally mature to be prospective for oil and gas, although a few units in east-central and northern Alaska may have some potential as petroleum source and reservoir rocks. Strata of this age have potential for metallic mineral resources; strata-bound Zn-Pb ± Ag occurrences are known in the Funnel Creek Limestone in east-central Alaska, as well as several units of possible Cambrian and/or Ordovician age in northern and interior Alaska.

Abstract Carlin-type ores have been reported in various locations around the world, but to date, the major economic deposits have been restricted to the Great Basin of the southwestern United States. Recent discoveries in east-central Yukon have many characteristics of Carlin-type deposits, and hold promise of great potential for new discoveries of economic importance. Both regions share commonalities of geologic history, including initial deposition of Proterozoic-Paleozoic calcareous host rocks on the passive margin of the fragmented Rodinian supercontinent. This was followed by compressional tectonism and continental accretion that included thrust faulting and plutonism through the late Paleozoic and Mesozoic. Many deposits in the Great Basin are associated with post-accretionary magmatism as the tectonic environment shifted to an extensional regime. However, at this early stage of investigation, the timing of mineralization in Yukon is not clear; the deposits may be geologically related to Late Cretaceous post-accretionary plutons. Associated gold skarn-style mineralization is present in both regions. Following mineralization, both regions experienced significant right-lateral transcurrent tectonism on their western margins; no known Carlin-type mineralization is associated with this latest tectonism. Mineralization in both areas comprises finely disseminated gold associated with arsenian pyrite hosted in calcareous siltstones-sandstones to silty carbonates, although other rock types locally host significant mineralization. Other hydrothermal minerals present in these systems include realgar/orpiment, stibnite, fluorite, barite, and quartz. Temperature of deposition appears to be near 225°C. Hydrothermal alteration consists of decarbonatization, silicification, and argillization. Gold/silver is typically high at 1:1 or higher. Trace elements that show good correlation with gold include thallium, arsenic, antimony, mercury, and to a lesser extent antimony and silver. In general, the deposits from the two areas are quite similar in terms of their tectonic history, and the processes and geochemistry appear to be very similar. The presence of extension and ore-related magmatism in Nevada appears to be a component that is much less clear in the Yukon Territory.