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We review currently available Canadian and international lithostratigraphic and lithodemic schemes and find most of them inadequate for classifying intrusive and strongly metamorphosed rocks of Canada and beyond. A new system is proposed, one that unifies, complements, and extends components of the revised North American Stratigraphic Code, the British Geological Survey Rock Unit Classification System of 2021, and the International Stratigraphic Guide of 1999. This new Cooperative Lithodemic and Stratigraphic System (CLASS) is intended to serve as a practical guide to geoscientists who need to classify and report on lithodemic units in North America and has broad applicability to other jurisdictions. It is built with database management practices in mind and employs the concept of inheritance of root characteristics between seven formal rock unit ranks, which will allow incorporation of the scheme into compact object-oriented and relational databases designed for purpose. Broad application of CLASS, especially to historically problematic lithodemic units, should help to foster jurisdictional interoperability, data sharing, global compilations, and thematic studies. At the heart of this proposed system is a subdivision and accompanying nomenclature that allows classification of rocks into seven formal ranks. Following the well-established lithostratigraphic supergroup, group, subgroup, formation, member, submember, and bed approach, CLASS proposes three classes of lithodemic subdivisions for formally naming lithodemic rock units. [ABSTRACT FROM AUTHOR]

Onset and termination of Eocene felsic volcanism in the Chilcotin Plateau of central British Columbia is constrained between 54.6 and 46.6 Ma by 33 new U-Pb and [sup.40]Ar/[sup.39]Ar isotopic age determinations. Dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the Ootsa Lake Group. The resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate, with no spatial migration of the volcanic activity. Rhyolitic compositions are oldest; and are overlain by dacitic rocks with varied phenocrysts assemblages. In many parts of the Chilcotin Plateau, the Eocene stratigraphy is capped by distinctive vitreous black dacite lavas, which are contemporaneous with andesitic lavas of the Endako Group in the Nechako Plateau to the north. Crystallization ages from Ootsa Lake Group rocks of the Chilcotin Plateau overlap age determinations from correlative rocks of the Nechako Plateau and southern BC. Collectively, this geochronological dataset supports previous suggestions of a voluminous Early Eocene-aged (~55-46 Ma) period of volcanism in the Intermontane Belt. The abrupt initiation of volcanism, as well as the wide extent, thickness, and compositions that characterize Eocene volcanic rocks may be explained by cessation of subduction and formation of a slab gap beneath British Columbia in the Early Eocene. Le volcanisme felsique a l'Eocene dans le Plateau de Chilcotin dans le centre de la Colombie Britannique est limite entre 54.6 et 46.6 Ma par trente-trois nouveaux ages isotopiques U-Pb et [sup.40]Ar/[sup.39]Ar. Ces ages sont issus d'echantillons de laves et breches volcaniques felsiques qui forment le Groupe Ootsa Lake. En consequence, la chronostratigraphie des roches volcaniques montre que les compositions magmatiques ont evolue dans le temps de felsique a intermediaire, mais aucune migration de l'activite volcanique en surface n'est observee au cours de cette periode. Les roches rhyolitiques sont les plus anciennes, et sont recouvertes par des roches dacitiques avec divers assemblages de phenocristaux. Dans de nombreuses localites du Plateau de Chilcotin, la stratigraphie de l'Eocene est dominee par des coulees distinctives de lave dacitique noire et vitreuse, contemporaines de laves andesitiques du Groupe Endako dans le Plateau de Nechako au nord. Les ages de cristallisation du Groupe Ootsa Lake dans le Plateau de Chilcotin sont en accord avec des ages existants provenant du Plateau de Nechako et du sud de la Colombie Britannique. Dans son ensemble, cette base de donnees geochronologiques confirme l'hypothese d'un episode volcanique volumineux a l'Eocene precoce (~55-46 Ma) dans la chame Intermontane. L'initiation brusque de cet episode volcanique, ainsi que la superficie, volume et composition des roches volcaniques peuvent etre expliques par l'arret de la subduction et une une discontinuite dans les plaques subductees en-dessous de la Colombie Britannique a l'Eocene precoce., Introduction Evidence of widespread Eocene magmatism in the North American Cordillera is preserved in a 3000 km long, 150-1000 km wide belt, extending from eastern Alaska to Idaho. In south-central [...]

The western quarter of North America consists of accreted terranes--crustal blocks added over the past 200 million years--but the reason for this is unclear. The widely accepted explanation posits that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today's western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building., Continents grow through subduction magmatism and collision of arcs and other buoyant crustal fragments at their margins. Poorly understood, such collisions are of broad scientific interest because they cause rapid [...]

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 strati graphically 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 w as 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. Le terrane Cache Creek nord dans la Cordillere canadienne comprend un complexe de subduction qui demontre l'existence d'un bassin oceanique Paleozoi'que tardif--Mesozoique. Ce terrane fournit une occasion d'evaluer les processus d'accretion impliquant le transferi de materiel d'une plaque en subduction vers une plaque superieure. Les donnees lithogeochimiques provenant de roches basalliques indiquent que le lerrane Cache Creek nord est domine par deux composanles petrogenetiques differentes: une suite dominante de roches intrusives el extrusives sub-alcalines ayant principalemeni une affinite d'arc et une suite moins volumineuse de roches volcaniques alcalines d'affinite intra-plaque. Les roches intrusives et extrusives sub-alcalines constituent une section de lithosphere oceanique qui. selon les interpretations, aurait occupe une position d'avant-arc durant le Trias tardif et le Jurassique precoce avant d'etre accretee au cours d'une orogenese de collision an Jurassique moyen. Les roches volcaniques alcalines dans le terrane Cache Creek nord sont stratigraphiquement associees a des strates carbonatees contenant des faunes tethysiennes qui sont exotiques par rapport au reste de l'Amerique du Nord; ensemble, ces roches sont interpretees comme etant des lambeaux de monts sous-marins et (ou) de plateaux oceaniques. Les roches volcaniques constituent une composante mineure de la stratigraphie carbonatee et il semble que la plus grande partie du socle volcanique etait soit completement subductee a la bordure de convergence, soit sous-plaquee a une plus grande profondeur dans la zone de subduction. En resume, l'accretion dans le nord de la Cordillere canadienne s'est surtout effectuee par l'accretion d'arcs insulaires et la mise en place d'ophiolites d'avant-arc au cours de l'orogenese de collision. Le transfer! des sediments oceaniques et des parties superieures des monts sous-marins de la plaque en subduction vers une bordure d'accretion ne compte que pour de petits volumes de croissance de la plaque superieure. [Traduit par la Redaction], Introduction Accretionary processes involve the transfer of material such as oceanic sediments, seamounts, island arcs, oceanic plateaux, and microcontinental fragments from a subducting plate to an upper plate. Evidence of [...]

Constraints on the thickness of mantle lithosphere involved in collisional orogenesis are fundamental for understanding the geodynamics of mountain building and the overall growth of continents by accretionary tectonics. Garnet peridotite and ultrahigh-pressure (UHP) crustal rocks provide such a constraint in many collisional orogens but have hitherto been unrecognized in western North America's Cordillera. Here we show the first evidence for exhumation of UHP (>2.8 GPa) garnet peridotite and eclogite and for deposition of these rocks as detritus in an Early Jurassic forearc basin (Laberge Group, Yukon Territory and British Columbia). Our results suggest that collision in this part of the North American Cordillera must have been thick skinned, involving a Proterozoic continental mass with a lithosphere >100 km (and possibly to 150 km) thick. Our discovery also provides insight into the vigor of uplift and erosion of deep-seated rocks in a nascent continental arc. Keywords: mantle, garnet, peridotite, exhumation, uplift, Cordillera.

VL Hansen and C Dusel-Bacon's views on the structural and kinematic evolution of the Yukon-Tanana upland tectonites invite comment. Their proposed tectonic model for the Yukon-Tanana terrane is flawed because of their biased interpretation of structural, geochemical and geochronological data sets. They are, however, commended for synthesizing tectonite-derived kinematic data with previous lithotectonic, geochronologic and thermobarometric information that has resulted in a more refined interpretation of the nature and assembly of the Yukon-Tanana terrane.

Efforts to describe the stratigraphic relationships of rocks in British Columbia, particularly in northern Cordillera, reflects the unending pursuit of earth scientists to reinterpret terrane and assemblage relationships. Although these researches provide new insights on the causes of surface deformations, they have undermined the value of terrane analysis. By adopting inconsistent methods for performing terrane analysis, researchers have over emphasized rock differences at the expense of their similarities.

Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390,660 and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910,1770, and 2450 Ma and highly radiogenic [.sup.87 Sr]/[.sup.86 Sr] ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

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 20.5 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 poq)hyry 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 pierite 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 fore-arc 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 back-arc may track the leading edge of the slab gap, with the formation of Cu-Au and younger Au-rich poqihyries 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. [ABSTRACT FROM AUTHOR]

Onset and termination of Eocene felsic volcanism in the Chilcotin Plateau of central British Columbia is constrained between 54.6 and 46.6 Ma by 33 new U-Pb and 40Ar/39Ar isotopic age determinations. Dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the Ootsa Lake Group. The resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate, with no spatial migration of the volcanic activity. Rhyolitic compositions are oldest; and are overlain by dacitic rocks with varied phenocrysts assemblages. In many parts of the Chilcotin Plateau, the Eocene stratigraphy is capped by distinctive vitreous black dacite lavas, which are contemporaneous with andesitic lavas of the Endako Group in the Nechako Plateau to the north. Crystallization ages from Ootsa Lake Group rocks of the Chilcotin Plateau overlap age determinations from correlative rocks of the Nechako Plateau and southern BC. Collectively, this geochronological dataset supports previous suggestions of a voluminous Early Eocene-aged (∼55-46 Ma) period of volcanism in the Intermontane Belt. The abrupt initiation of volcanism, as well as the wide extent, thickness, and compositions that characterize Eocene volcanic rocks may be explained by cessation of subduction and formation of a slab gap beneath British Columbia in the Early Eocene. [ABSTRACT FROM AUTHOR]

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