kilometre scale folding in the teslin zone northern canadian cordillera and its tectonic implications for the accretion of the yukon tanana terrane to north america reply the teslin zone in south central yukon has previously been described as a discrete zone with a steep foliation unique to the zone it includes the anvil assemblage and the narrowest portion of the

development of late paleozoic volcanic arcs in the canadian cordillera an example from the klinkit group northern british columbia and southern yukon the late paleozoic volcanic rocks of the northern canadian cordillera lying between ancestral north america to the east and the accreted terranes of the omineca belt to the west record early arc and rift magmatism along the paleo pacific margin of the north american craton the mississippian to permian volcano sedimentary klinkit group extends discontinuously over 250 km in northern british columbia and southern yukon the two stratotype areas are as follows 1 in the englishman range southern yukon the english creek limestone is conformably overlain by the volcano sedimentary mount mccleary formation lower clastic member alkali basalt member and volcaniclastic member and 2 in the stikine ranges northern british columbia the screw creek limestone is conformably overlain by the volcano sedimentary butsih formation volcaniclastic member and upper clastic member the calc alkali nature of the basaltic volcaniclastic members of the klinkit group indicates a volcanic arc setting la yb n 2 77

geochemistry of late proterozoic amphibolites and ultramafic rocks southeastern canadian cordillera late proterozoic amphibolites and ultramafic rocks from the southeastern canadian cordillera have been analysed for major and trace elements in order to determine the nature and origin of the protoliths geologic relations indicate that these rocks were produced during an episode of continental rifting in the precambrian based on rare earth element ree patterns immobile incompatible element ratios and characteristic elemental abundances amphibolites are subdivided into alkaline and tholeiitic metabasalts alkaline basalts are recognized by their steep ree patterns high zr y high tio2 and p2o5 abundances and low y nb and ti zr tholeiitic basalts are subdivided into three groups i high mg high field strength element hfse depleted light ree lree enriched tholeiites with flat heavy ree hree patterns ii lree enriched tholeiites depleted in hree and iii low mg tholeiites with flat ree patterns ultramafic rocks occur as boudins of partially recrystallized cr spinel bearing harzburgi

the sylvester allochthon upper paleozoic marginal basin and island arc terranes in northern british columbia the sylvester allochthon is a composite klippe of upper paleozoic ophiolitic island arc and pericratonic assemblages which rests directly on the cassiar terrane a displaced sliver of ancestral north america each tectonic assemblage occurs at a distinct and consistent structural level within the allochthon they are assigned respectively to the slide mountain harper ranch and yukon tanana terranes the sylvester allochthon provides a view of the structural relationships between these terranes prior to early cretaceous early tertiary strike slip dismemberment as well as possible sedimentological links to late paleozoic north america slide mountain terrane assemblages designated divisions i and ii form the lowest structural panels chert quartz sandstones are interbedded with lower mississippian deep water sediments in division i and ocean floor basalts and deep water sediments in division ii they are similar in age and character to sandstones in the autochthonous earn group division ii as

nd sr isotopic constraints on the interactions of the intermontane superterrane with the western edge of north america in the southern canadian cordillera sr and nd isotopic compositions of the late paleozoic metavolcanics and late triassic to early tertiary granitoids from four magmatic episodes in the southern canadian cordillera from the kootenay arc to the fraser fault have been used to i identify the sources of these rocks ii constrain the compressive tectonic history from middle jurassic to paleocene and iii constrain the western boundary of the basement in this region the 215 190 ma old primitive granitoids end 3 1 to 8 7 87sr 86sr 0 7028 0 7043 of the late triassic and early jurassic magmatic episode were emplaced in the paleozoic oceanic crust of quesnellia end 2 9 to 9 3 prior to its obduction over the basement in contrast during the younger magmatic episodes middle late jurassic cretaceous and early tertiary the granitoids from western quesnellia show primitive isotopic compositions and those from eastern quesnellia show eastward increasing crust contaminated compositions the contaminated characters of the middle

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

geochemistry and paleotectonic setting of felsic volcanic rocks in the finlayson lake volcanic hosted massive sulfide district yukon canada the finlayson lake volcanic hosted massive sulfide vhms district represents one of canada s most recent vhms discovery regions with 34 million metric tons mt of massive sulfide mineralization found since the mid 1990s felsic volcanic rocks are associated with three units the fire lake unit the kudz ze kayah unit and the wolverine succession significant accumulations of polymetallic felsic vhms deposits kudz ze kayah gp4f and wolverine have only been discovered in the kudz ze kayah unit and wolverine succession to date in the hanging wall of the money creek thrust felsic volcanic and high level intrusive rocks in the fire lake unit have calc alkalic and tholeiitic affinities with low high field strength element hfse contents and intermediate zr sc 9 4 43 4 and zr tio2 254 864 ratios these rocks are interlayered with mafic rocks with arc geochemical signatures to date devoid of significant vhms mineralization and represent bimodal magmatism within an evolving devonian mississippian continental arc system the kudz ze kayah unit in the footwall of the money creek thrust stratigraphically overlies the fire lake unit and consists of felsic volcanic rocks with high hfse contents within plate a type signatures and high zr sc 15 3 190 3 and zr tio2 630 2 185 ratios the kudz ze kayah unit felsic rocks are crosscut and overlain by alkalic mafic rocks are associated with abundant carbonaceous sedimentary rocks and represent magmatism within a devonian mississippian ensialic back arc rift basin environment the wolverine succession unconformably overlies the kudz ze kayah unit felsic rocks below the wolverine deposit have geochemical attributes similar to the kudz ze kayah unit with high hfse contents within plate a type signatures and high zr sc 29 9 84 2 and zr tio2 391 1 220 ratios in contrast aphyric rhyolite flows in the hanging wall of the deposit have much lower hfse contents and the lowest zr sc 3 5 27 7 and zr tio2 181 591 ratios in the district all the felsic rocks of the wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by midocean ridge basalt morb like basaltic rocks the wolverine succession is interpreted to have formed within an early mississippian ensialic back arc basin environment that eventually evolved to sea floor spreading the variation in the hfse budgets of the felsic rocks of the finlayson lake district likely reflects variations in the source and or temperature of crustal melting in particular felsic rocks of the fire lake unit have higher nb ta and lower ti sc ratios than other volcanic rocks in the district suggesting possible derivation from mafic crustal sources and or lower crustal fusion temperatures the kudz ze kayah unit and footwall rocks to the wolverine deposit are inferred to have formed from high temperature partial melting of continental crust the hanging wall aphyric rhyolites from the wolverine deposit may have formed from either lower temperature continental crustal melting or may have been derived from the mixing of hfse depleted n morb mafic magmas and evolved continental crust polymetallic felsic volcanic associated vhms deposits within the finlayson lake district are preferentially associated with hfse enriched felsic rocks with high zr sc 15 3 190 3 and zr tio2 391 2 105 ratios the hfse and rare earth element ree systematics of vhms associated felsic rocks of the finlayson lake district are different from prospective felsic rocks from archean vhms environments in the superior province and are displaced toward higher zr y and la ybn ratios their hfse and ree systematics are similar to many phanerozoic vhms environments in particular those at least partially to fully underlain by evolved continental crust the geochemical differences between the felsic rocks of the finlayson lake district and those from archean vhms environments most likely reflect differences in the substrates from which the felsic rocks were derived e g evolved versus juvenile

triassic jurassic silica undersaturated and silica saturated alkalic intrusions in the cordillera of british columbia implications for arc magmatism alkalic igneous rocks of early mesozoic age are found in both the quesnel and stikine terranes in the canadian cordillera and include both silica undersaturated and silica saturated types the saturated complexes are most abundant in quesnellia and are multiphase complexes dominated by monzonite to diorite intrusions undersaturated complexes are distributed through both terranes are dominated by syenite with lesser monzonite and pyroxenite and when present as a single intrusion are characterized by concentric zoning igneous layering and planar mineral fabrics both types of complex are associated with cu au mineralization accompanied by potassic and distinctive sodic and calc potassic alteration assemblages although undersaturated and saturated alkalic intrusions are petrographically distinct a petrogenetic association is suggested by their spatial coincidence in some districts and similarities in their tectonic environment and associated alteration the undersaturated complexes represent a distinctive suite of alkalic intrusion with magmatic arc affinities and their emplacement into both stikinia and quesnellia between 210 and 200 ma suggests that these terranes were either linked at that time or have shared unusual but similar magma generating tectonic events at identical times

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

introduction to special section on alkaline arc magmatism alkaline magmas are an uncommon but intriguing member of many of the earth s volcanic arc systems this peculiar magmatism is generally associated with perturbations to the normal tectonic setting of arc magmatism in december 1986 we convened a symposium at the agu fall meeting to explore connections between the tectonic environments of alkaline arc magmatism and specific petrologic processes associated with their genesis twenty five contributions reported occurrences of alkaline arc magmas including modern and ancient examples from both intraoceanic and continental margin volcanic arcs

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

the nicola group late triassic and early jurassic subduction related volcanism in british columbia mafic lavas of the nicola group are divided into three distinct petrographic and geochemical types type 1 lavas are strongly augite porphyritic picrites basalts and andesites that belong to a high potassium to shoshonitic rock series type 2 lavas are augite and plagioclase porphyritic basalts and andesites that belong to a low potassium calc alkaline series and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites low concentrations of ti zr y and nb and moderate to high concentrations of k rb ba and sr in type 1 and 2 lavas clearly indicate a subduction related tectonic setting of eruption type 3 lavas show chemical affinities intermediate between modern day island arc and intraplate volcanics type 1 shoshonitic lavas generally lie east of and are younger than type 2 calc alkaline lavas a relationship that implies an east dipping early mesozoic subduction zone beneath the nicola arc these interpretations resolve previous uncertainties regarding th

ancient continental margin assemblage in the northern coast mountains southeast alaska and northwest canada geologic relations indicate that quartz rich metasedimentary rocks in the northern coast mountains separate strata to the east that belong to the stikine terrane from strata to the west of the alexander wrangellia and taku terranes the quartz rich rocks structurally overlie western terranes along a mid cretaceous thrust fault and are overlain structurally originally stratigraphically by strata of the stikine terrace these rocks are interpreted to be a continental margin assemblage that belongs to the yukon crystalline terrane u pb and nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of proterozoic rocks

stratigraphic and isotopic link between the northern stikine terrane and an ancient continental margin assemblage canadian cordillera 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 model 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 87 sr 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

postglacial vegetation and fire history of the southern cascade range oregon abstract the cascade range of southwestern oregon contains some of north america s most diverse forests but the ecological history of this area is poorly understood a 7900 yr long pollen and charcoal record was examined to better understand past changes in vegetation and fire activity in relation to large scale climate variability from 7900 to 3500 cal yr bp the dominance of xerophytic species and the frequent fires are consistent with a climate that was warmer and drier than at present the period from 3500 cal yr bp to present experienced an abundance of mesophytic taxa and reduced fire frequency suggesting cooler and wetter conditions the regional history of abies indicates that it was most widespread during the late glacial period its range contracted during the early holocene thermal maximum and it steadily expanded during the middle and late holocene in contrast pseudotsuga was restricted in range during the glacial period became abundant at low elevation sites in the coast and northern cascade ranges during the early holocene and was more prevalent in southern mid elevation sites as the climate became cooler and wetter in the late holocene the sensitivity of these species to past climate change suggests that biogeographic responses to future conditions will be highly variable in this region

the slide mountain terrane and the structural evolution of the finlayson lake fault zone southeastern yukon the finlayson lake fault zone forms a fundamental but little studied tectonic boundary between strata of autochthonous north america and the accreted slide mountain and yukon tanana terranes in southeastern yukon a structural and petrologic study was undertaken to examine the depositional environment of the slide mountain terrane its tectono thermal evolution in the fault zone and its relationship with the yukon tanana terrane the slide mountain and yukon tanana terranes are divisible into units dominated by metavolcanic and metasedimentary rocks field observations and whole rock geochemistry indicate that slide mountain greenstone is ocean floor basalt deposited in a deep submarine basin with a proximal terrigenous sediment influx either a marginal or ocean basin setting is supported by the data slide mountain greenstone is thrust northeastward over metasedimentary rocks of slide mountain terrane and southwestward over rocks of the yukon tanana terrane regional metamorphic grade ranges from su

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

nina creek group and lay range assemblage north central british columbia remnants of late paleozoic oceanic and arc terranes in north central british columbia a belt of upper paleozoic volcanic and sedimentary rocks lies between mesozoic arc rocks of quesnellia and ancestral north america these rocks belong to two distinct terranes the nina creek group of the slide mountain terrane and the lay range assemblage of the quesnel terrane the nina creek group is composed of mississippian to late permian argillite chert and mid ocean ridge tholeiitic basalt formed in an ocean floor setting the sedimentary and volcanic rocks the mount howell and pillow ridge successions respectively form discrete generally coeval sequences interpreted as facies equivalents that have been interleaved by thrusting the entire assemblage has been faulted against the cassiar terrane of the north american miogeocline west of the nina creek group is the lay range assemblage correlated with the harper ranch subterrane of quesnellia it includes a lower division of mississippian to early pennsylvanian sedimentary and volcanic rocks some with con

new u pb age constraints on latest cretaceous magmatism and associated mineralization in the fawnie range nechako plateau central british columbia new u pb ages and k ar dates primarily for rocks proximal to mineral occurrences in the fawnie range of central british columbia document latest cretaceous ca 74 66 ma continental arc igneous activity and date associated base and precious metal mineralization u pb ages of ca 73 69 ma for the capoose pluton and hypabyssal to extrusive garnet rhyolites at the capoose prospect demonstrate a latest cretaceous age for mineralization and a likely plutonic source for mineralizing fluids a u pb age of ca 67 ma for a late mineralized felsic dyke and two k ar dates ca 70 and 68 ma for hornfelsed jurassic volcanic rocks at the blackwater davidson prospect constrain a latest cretaceous age for mineralization a u pb age of ca 74 ma for a fine grained diorite sill that cuts a significant epithermal gold vein at the tsacha prospect places a minimum age on mineralization at this probable jura cretaceous deposit and documents latest cretaceous magmatism latest cretaceous k ar dates are reported for an andes

extending the ancient margin outboard in the canadian cordillera record of proterozoic crust and paleocene regional metamorphism in the nicola horst southern british columbia the nicola horst exposes plutonic and amphibolite grade metamorphic rocks and is surrounded by low grade arc rocks of the late triassic nicola group we present new geological mapping and u pb nd and metamorphic data for the nicola horst near bob lake 40 km south of kamloops british columbia the bob lake assemblage includes felsic to intermediate metavolcaniclastic rocks metaconglomerate schist phyllite and other rock types from u pb zircon analysis a felsic metaporphyry clast in metaconglomerate is 1 04 ga old the oldest detrital zircons in metaconglomerate and schist are also near 1 ga the bob lake assemblage was intruded by 230 ma tonalite 219 ma diorite and 64 ma leucogranite and includes 161 ma felsic porphyry and 157 ma rhyodacite amphibolite grade metamorphism and ductile flattening and stretching affected all rocks except crosscutting paleocene granite and granodiorite the high grade rocks may be exposed as a result of latest cretaceous eocene extensional ductile flow beneath a thin brittle upper crust a thickness of 20 km of juvenile crust beneath the proposed quesnel terrane is inconsistent with the evidence of proterozoic source rocks at surface we infer that most of the crustal lithosphere in this part of the intermontane belt is continental which does not preclude possibly thick arc rocks in other parts the local thinness of the nicola group however is inconsistent with emplacement as a regional allochthon and thus with their inclusion in a quesnel terrane the nicola arc succession appears to have been built on the ancient continental margin

superimposed quesnel late paleozoic jurassic and yukon tanana devonian mississippian arc assemblages cassiar mountains northern british columbia field u pb and igneous petrochemical evidence allochthons in the cassiar mountains of northern british columbia contain assemblages belonging to two distinct canadian cordilleran terranes yukon tanana ytt and quesnellia these assemblages of pre late devonian devonian mississippian pennsylvanian permian and early jurassic age occur in intrusive and depositional as well as structural contact with each other the allochthons are gently dipping thrust panels interrupted by the mid cretaceous cassiar batholith a key element for correlation across the batholith is the mississippian and older pericratonic dorsey complex new devonian mississippian u pb ages for deformed plutons within it document an igneous suite like those in type yukon tanana exposures farther north other characteristics of the dorsey complex that ally it with ytt are orthoquartzites and grits and amphibolite bodies with transitional mid ocean ridge basalt morb to ocean island basalt oib petrochemical signatures unconformities deformed clasts in the late paleozoic sequences and a shared mid permian intrusive suite show that later arcs onlapped the mid paleozoic and older ytt assemblage the early jurassic intrusive suite cuts all major contacts and fabrics except the terrane bounding fault between the slide mountain and combined ytt quesnel terranes it represents a northern continuation of a plutonic belt that extends the length of the mesozoic quesnel magmatic arc these relationships carry important implications for cordilleran terrane history and the tectonic evolution of the north american margin at least some of the major terranes were not unrelated entities prior to their accretion to the continent but a system of superimposed and interconnected arcs that developed over a protracted time interval with complex and evolving paleogeographic configurations much like the modern western pacific province

multiple burial exhumation episodes revealed by accessory phases in high pressure granulite facies rocks rae craton nunavut canada the big lake shear zone blsz is a 60 km long deep crustal structure adjacent to the northern segment of the snowbird tectonic zone stz nunavut canada in a region characterized by intensely sheared high pressure granulite facies rocks the units exposed here provide an exceptional record of the lower crust that preserves crucial evidence bearing on interpretation of superimposed neoarchean and paleoproterozoic tectono metamorphic events deformation along the blsz postdates 2650 ma the age of a mylonitized monzogranite and predates 2190 ma the age of cross cutting macquoid mafic dykes metamorphic assemblages and p t modeling suggest crustal thickening in the neoarchean with peak conditions of 800 c and 14 15 kbar at ca 2530 ma mylonitic fabrics developed at ca 2505 ma t 800 c p 10 c ma and partial exhumation post ca 2505 ma followed by re burial at 1900 ma rather than slower isobaric cooling at depth these results indicate that the long term residence models proposed by several authors for domains in the central stz are not applicable to the blsz region and are more compatible with a ca 1 9 ga collisional setting

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem