36. Bickerton, L; Colpron, M; Gibson, D; Thorkelson, DJ; Crowley, JJL.The northern termination of the Cache Creek terrane in Yukon: Middle Triassic arc activity and Jurassic-Cretaceous structural imbrication.Can. J. Earth Sci., 2020, 57: 227-248 The northern termination of the Cache Creek terrane in Yukon: Middle Triassic arc activity and Jurassic-Cretaceous structural imbrication
Cache Creek; Kutcho; Cordillera; terrane accretion; intra-oceanic arc
The northernmost part of the Cache Creek terrane lies in south-central Yukon and comprises metavolcanic rocks, hemipelagic chert and shale, newly identified volcaniclastic and clastic rocks (Michie formation, informal), pyroxenite and gabbro intrusive rocks with an arc to back-arc geochemical signature, as well as tectonized and serpentinized ultramafic rocks. The proximally sourced Michie formation yielded zircon from two samples with unimodal peaks at 245.85 +/- 0.07 and 244.64 +/- 0.08 Ma. These dates are likely close to the depositional ages and compare favourably with those from the Kutcho assemblage of northern British Columbia. The Michie formation is exposed along the northwestern flank of Mount Michie and represents singular detrital input from a nearby eroding island-arc. The Cache Creek terrane rocks are imbricated with epiclastic and carbonate rocks of the Stikinia and Lower Jurassic siliciclastic rocks of the synorogenic Whitehorse trough. This imbrication records two compressional deformation phases in the region: (1) an initial phase of west-verging thrusting along the Judas Mountain fault that placed the Cache Creek terrane rocks over the arc and basinal rocks of Stikinia and Whitehorse trough; and (2) a second phase of east-verging thrusting along the Mount Michie fault that repositioned rocks of Stikinia and the Whitehorse trough structurally above those of the Cache Creek terrane. Deformation in the centre of the study area was followed by emplacement of a coarse-grained syenite that yielded Ar-40/Ar-39 biotite and muscovite cooling ages of 165-160 Ma. DOI
35. Rabayrol, F; Hart, CJR; Thorkelson, DJ.Temporal, spatial and geochemical evolution of late Cenozoic post-subduction magmatism in central and eastern Anatolia, Turkey.Lithos, 2019, 336: 67-96 Temporal, spatial and geochemical evolution of late Cenozoic post-subduction magmatism in central and eastern Anatolia, Turkey
Indentation of the Arabian platform into the eastern domain of the Anatolide-Tauride Block (ATB) led to subhorizontal rupture and break-off of the Arabian segment of the subducting southern Neotethyan oceanic slab beneath ATB in the late Cenozoic. Although this rupture has been well-defined by seismic tomography and numerical modeling, its relationship with concomitant magmatism in central and eastern Anatolia has not been firmly demonstrated. To address this issue, we compiled, integrated and interpreted an extensive database of previously-published age and geochemical data. Our analysis shows that late Cenozoic magmatism in both eastern and central Anatolia was nearly continuous over a distance of similar to 1000 km, and is herein named the Eastern Anatolian Magmatic Belt. Magmatism initiated at 23 Ma in response to rupturing of the north-dipping subducting oceanic slab below eastern Anatolia and upwelling of hot asthenospheric mantle. The onset of initial rupturerelated magmatism is similar to 12 Ma older than previously proposed. The magmatic belt migrated westward from eastern Anatolia (23-15 Ma) to central Anatolia (12 Ma-present) and is a proxy for the westward progression of the slab rupture and related asthenospheric infiltration. Magmatism generated from the shallow melting of the previously-metasomatized Anatolian subcontinental lithospheric mantle and asthenosphere by decompression due to the impingement of the Arabian sub-slab asthenospheric mantle. In eastern Anatolia, slab-rupture magmatism was followed by a second phase of magmatism that formed in response to destabilization and convective removal of the thick Anatolian lithospheric root (13 Ma-present). This new magmatic model emphasizes that the geochemical signature of post-subduction magmatism, as recorded by primitive basalts that constantly evolved through time and space. Magmatic sources recorded the shifting of mantle melt source domains that were controlled by slab segmentation and induced mantle flow. (C) 2019 Elsevier B.V. All rights reserved. DOI
34. Polat, A; Frei, R; Longstaffe, FJ; Thorkelson, DJ; Friedman, E.Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting.Tectonophysics, 2018, 737: 1-26 Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting
Canadian Cordillera; Mantle xenoliths; Alkaline basalt; Melt pocket; Mantle metasomatism; Lithospheric delamination; Slab window
Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel Iherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar. The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in epsilon Nd values (-8.2 to +9.6) and Nd depleted mantle model ages (T-DM = 66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term ( > 3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents. The oxygen isotope compositions of the xenoliths (average delta O-18 = +5.1 +/- 0.5 parts per thousand) are similar to those of depleted mantle. The average delta O-18 values of olivine (+5.0 +/- 0.2 parts per thousand), opx (+5.9 +/- 0.6 parts per thousand), cpx (+6.0 +/- 0.6 parts per thousand) and spinel (+4.5 +/- 0.2 parts per thousand) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting. Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow litho spheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source. DOI
33. Rogers, C; Kamo, SL; Soderlund, U; Hamilton, MA; Ernst, RE; Cousens, B; Harlan, SS; Wade, CE; Thorkelson, D.Geochemistry and U-Pb geochronology of 1590 and 1550 Ma mafic dyke swarms of western Laurentia: Mantle plume magmatism shared with Australia.Lithos, 2018, 314: 216-235 Geochemistry and U-Pb geochronology of 1590 and 1550 Ma mafic dyke swarms of western Laurentia: Mantle plume magmatism shared with Australia
Gawler Craton; Olympic Dam breccia; Wemecke breccia; Mafic magmatism; Large igneous provinces (LIPs); Geochronology; Geochemistry; Isotope geochemistry; Paleomagnetism; Laurentia; South Australia Craton; Belt-Purcell Basin; Tobacco Root Mountains
Three new U-Pb ID-TIMS isotopic ages confirm previous evidence for two geochemically distinct, early Mesoproterozoic pulses of mafic magmatism along the western margin of Laurentia. The first is a U-Pb baddeleyite age of 1590 +/- 5 Ma from a dyke swarm in the Tobacco Root Mountains of western Laurentia. It is the first evidence for magmatism of this age in west-central Laurentia. The second is a U-Pb baddeleyite age of 1592.4 +/- 2.5 from the Western Channel Diabase, 2000 km to the north in NW Laurentia, which supports two previous U-Pb baddeleyite ages of ca. 1590 Ma for these intrusions. The third is a U-Pb baddeleyite age of 1551 +/- 5 Ma, also from the Tobacco Root Mountains, and provides evidence for a distinct younger pulse of mafic magmatism. We propose that the ca. 1590 Ma mafic intrusions in northwestern and west-central Laurentia represent components of a large igneous province (LIP), which we name the "ca. 1590 Ma Mammoth-Western Channel LIP." This 1590 Ma LIP is geochemically similar to the contemporaneous volcanic rocks in the Gawler Craton and Curnamona Province of Australia. Furthermore, the 1599 Ma Wernecke Breccias near the Western Channel Diabase are geologically similar to the 1590 Ma Olympic Dam Breccias on the South Australian craton. We propose that a mantle plume at ca. 1590 Ma centered between the Laurentian and Southern Australian cratons, located by converging dyke swarms, fed the intrusions on Laurentia plus the Gawler Range Volcanics, the Hiltaba Suite granites and the Ninnerie Supersuite on the South Australian craton. Additionally, magmatic underplating from the plume set up the hydrothermal system responsible for the formation of the Wernecke and Olympic Dam Breccias. The younger 1551 Ma magmatism in the Tobacco Root Mountains, which shows less contamination of a metasomatized subcontinental lithospheric mantle than the older 1590 Ma pulse, may represent an early stage of rifting that pre-dates the ca. 1470 Ma Belt-Purcell Basin extension of western Laurentia. Felsic magmatism, hypothesized to have accompanied the 1590-1550 Ma LIP magmatism, could be a potential source for detrital zircon, thereby reducing the requirement for a non-Laurentian source for detrital zircon ages within the 1610-1490 Ma "North American Magmatic Gap." Additionally, the 1590 Ma and 1550 Ma ages on western Laurentia dykes provide tighter constraints for previous paleomagnetic studies. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved. DOI
32. Verbaas, J; Thorkelson, DJ; Crowley, J; Davis, WJ; Foster, DA; Gibson, HD; Marshall, DD; Milidragovic, D.A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga.Precambrian Res., 2018, 305: 19-39 A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga
Columbia; Nuna; Proterozoic; Paleogeography; Laurentia; Australia supercontinent; Detrital zircon; Wernecke Breccia; Olympic Dam
The Columbia (Nuna) supercontinent existed from approximately 1.9 Ga to 1.3 Ga. Laurentia was part of Columbia, and the western edge of Laurentia (current coordinates) was likely proximal to a large landmass during parts of this interval. Here, we present detrital zircon ages of a Paleoproterozoic sedimentary succession in northern Yukon, Canada, that bear on the evolution of Columbia. The sedimentary succession is preserved as lasts within 1.60 Ga hydrothermal megabreccias. Analyses of detrital zircon reveal abundant 1.78-1.68 Ga zircon with evolved Hf isotope values (-16.1 < epsilon Hf(t) < +1.4). Sm-Nd isotope analysis on clasts yields epsilon Ndi from -5.3 to -5.5 and model ages from 2.4 to 2.2 Ga. The detrital zircon age distribution is strikingly similar to those from sedimentary megaclasts in the ca. 1.59 Ga Olympic Dam Breccia Complex on the Gawler Craton of Australia. The whole rock Sm-Nd ratios are consistent with derivation from the Gawler Craton. We propose that the sedimentary material contained in both breccia complexes was derived from an overlap assemblage deposited on Australia and Laurentia at ca. 1.6 Ga. This model supports a previous hypothesis that the Gawler Craton was connected to northwestern Laurentia at ca. 1.6 Ga, and that these regions shared a single hydro thermal province that is recognized in northwestern Laurentia as the Wemecke Breccia and in the Gawler Craton as the Olympic Dam Breccia Complex and associated IOCG deposits. The sedimentary overlap succession was deposited after collision between Australia and Laurentia. Australia was subsequently translated southward along the Laurentian margin, placing the Gawler Craton next to southwestern Laurentia and the Mt. Isa Inlier adjacent to northwestern Laurentia by 1.5 Ga. DOI
31. Verbaas, J; Thorkelson, DJ; Milidragovic, D; Crowley, JL; Foster, D; Gibson, HD; Marshall, DD.Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada.Lithos, 2018, 316: 243-260 Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada
The Hart River sills are a set of mafic to intermediate intrusions that occur in northern Yukon, Canada. The largest sills are over 500 m thick and over 200 km long. New U-Pb dates of 1382.15 +/- 0.39 Ma and 1382.14 +/- 0.36 Ma were obtained via chemical abrasion thermal ionization mass spectrometry on zircon. Whole rock initial neodymium isotopic compositions of the Hart River sills are juvenile and have epsilon Ndi from +1.5 to +4.0. The primary mineralogy of the Hart River sills is predominated by clinopyroxene and plagioclase. Geochemical modeling indicates that the Hart River sills lie on a common liquid line of descent defined by a fractionating assemblage of plagioclase, clinopyroxene and minor olivine. The Hart River sills have rare earth element and high field strength abundances similar to normal mid-ocean ridge basalts (N-MORB) but are enriched in large ion lithophile elements. The Sm/Yb and Dy/Zr ratios indicate >8% partial melting of spinel-bearing mantle. During the emplacement of the Hart River sills, western Laurentia was juxtaposed with Australia and eastern Antarctica within the supercontinent Columbia. The degree of partial melting, similarity to N-MORB, and juvenile isotopic signature are consistent with an episode of rifting at 1.38 Ga. Coeval magmatism and intracontinental rift basins farther south on Laurentia provide additional evidence for rifting of supercontinent Columbia at 1.38 Ga. (C) 2018 Elsevier B.V. All rights reserved. DOI
30. Eyuboglu, Y; Dudas, FO; Thorkelson, D; Zhu, DC; Liu, Z; Chatterjee, N; Yi, K; Santosh, M.Eocene granitoids of northern Turkey: Polybaric magmatism in an evolving arc-slab window system.Gondwana Res., 2017, 50: 311-345 Eocene granitoids of northern Turkey: Polybaric magmatism in an evolving arc-slab window system
Slab window; Continental arc; Convergent margin; Arc magmatism; Granitoid
The Eastern Pontides Orogenic Belt offers critical clues on the origin of Early Cenozoic continental arc magmatism in the Alpine-Himalayan system. Systematic geological, geochemical and chronological studies indicate that there are three subgroups among the Early Cenozoic intrusions in the Eastern Pontides Oro genic Belt - Late Paleocene-Early Eocene adakitic intermediate-felsic intrusions, Eocene mafic intrusions, and Eocene non-adakitic granitoid intrusions. Here we focus on the petrology and geodynamic setting of the Eocene non-adakitic granitoid intrusions that are well exposed in a belt between the Thanetian-Ypresian adakitic intrusions in the south and the Lutetian gabbroic intrusions in the north. We also present data on enclaves and surrounding Eocene volcanics. The studied intrusions can be grouped into two main categories, based on their field and petrographical characteristics: granodiorite and monzodiorite-dominated and syenite-dominated bodies. They can be further subdivided into four groups of differing K2O content: low-K2O (cevrepinar, Kaletas, Saricicek and Uzengili), mixed (Sorkunlu, Kozluk and Tamdere), and high-K2O (Dolek, Mesebasi, cakirbag, and Arslandede) rocks are granodioritic and monzodioritic, whereas shoshonitic (Kosedak, Meydanli and Bademli) bodies are syenitic. Zircon U-Pb age determinations reveal that these granitoids were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt between 47 and 42 Ma, in Lutetian time, simultaneously with the gabbroic intrusions in the north. Mineral compositions and P-T calculations are consistent with the interpretation that crustal melting or magma storage started at mid-crustal depth (similar to 25 km), with a magma system that subsequently extended to shallow levels (<4 km). The studied granitoids, enclaves and volcanics exhibit geochemical signatures typical of subduction-related arc magmas, however, the shoshonitic intrusions are younger than most of the other Lutetian intrusions, and indicate a temporal change in arc magmatism. The Sr-Nd-Pb isotopic data indicate that the Lutetian rocks are mixtures of three or four end-member compositions. Considering all geological, geochemical and chronological data, we conclude that the Early Cenozoic magmatism was generated by slab window processes related to ridge subduction in a south-dipping subduction zone below the Eastern Pontides Oro genic Belt. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. DOI
29. Friedman, E; Polat, A; Thorkelson, DJ; Frei, R.Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: The Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada.Gondwana Res., 2016, 33: 209-230 Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: The Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada
The Canadian Cordillera; Mantle xenolith; Alkaline basalt; Mantle metasomatism; Continental lithospheric mantle
The Quaternary Tasse basalts are exposed near the north shore of Quesnel Lake in southeastern British Columbia. They host a variety of mantle xenoliths consisting predominantly of spinel Iherzolite with minor dunite and pyroxenite. Mineralogically, the xenoliths are composed of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo(82-93)), enstatite (En(90-92)), diopside (En(45-50)-Wo(40-45)-Fs(5)), and Cr-spinel (6-11 wt% Cr), respectively. All of the mantle xenoliths are coarse-grained and show granoblastic textures. Clinopyroxene and spinel display textural evidence for chemical reactions with percolating melts. The mantle xenoliths are characterized by restricted Mg-numbers (89 - 92) and low abundances of incompatible elements (Ba = 2-11 ppm; Sr = 3-31 ppm) and Yttrium (1-3 ppm). On the basis of REE patterns, the xenoliths are divided into three groups reflecting the various degrees of mantle metasomatism: (1) Group 1 consists of concave-up LREE patterns (La/Sm-cn = 0.48-1.16; Gd/Yb-cn = 0.71-0.92); (2) Group 2 possesses flat to moderately LREE-enriched patterns (La/Sm-cn = 1.14 - 1.92; Gd/Yb-cn = 0.87-1.09); and (3) Group 3 is characterized by strongly LREE-enriched patterns (La/Sm-cn = 1.53 - 2.45; Gd/Yb-cn = 1.00-1.32). On MORB-normalized trace element diagrams, the majority of the xenolith samples share the enrichment of LILE (Rb, Ba, K), U, Th, Pb, Sr and the depletion of HFSE (Nb, Ta, Ti, Y) relative to REE. These geochemical characteristics are consistent with a compositionally heterogeneous subcontinental lithospheric mantle source that originated as subarc mantle wedge peridotite at a convergent plate margin. The Tasse basalts have alkaline compositions characterized by low SiO2 (44 - 46 wt.%) and high alkali (Na2O + K2O = 5.1 - 6.6 wt.%) contents. They are strongly enriched in incompatible elements (TiO2 = 2.4-3.1 wt.%; Ba = 580 - 797 ppm; Sr = 872 - 993 ppm) and, display OIB-like trace element patterns (La/Sm-n = 3.15 - 3.85; Gd/Yb-n = 3.42 - 4.61). They have positive epsilon Nd (+3.8 to +5.5) values, with 338 - 426 Ma depleted mantle model ages, and display uniform OIB-like Sr (Sr-87/Sr-86 = 0.703346-0.703591) and Pb ((206)pb/(204)pb = 19.40-19.58; (207)pb/(204)pb = 15.57 - 15.60; Pb-208/Pb-204 = 38.99-39.14) isotopic compositions. The basalts erupted discontinuously along a >1000 km long SE-NW-trending linear belt with minimal compositional variation indicative of a homogenous mantle source. The Sr-Nd-Pb isotope and trace element systematics of the alkaline basalts suggests that they originated from partial melting of an upwelling asthenospheric mantle source. Melting of the asthenospheric mantle might have stemmed from extension of the overlying lithosphere in response to the early stages of back-arc basin opening in the Omineca and Intermontane belts. Ridge subduction beneath the Canadian Cordillera might have played an important role in the weakening of the lithospheric mantle prior to its extension. Alternatively, melting of the upwelling asthenosphere in response to the delamination of the lithospheric mantle beneath the Rocky Mountain Trench might have generated the alkaline lavas. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. DOI
28. Furlanetto, F; Thorkelson, DJ; Rainbird, RH; Davis, WJ; Gibson, HD; Marshall, DD.The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China.Gondwana Res., 2016, 39: 14-40 The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Wernecke Supergroup; Laurentia; Columbia; Paleoproterozoic; Detrital zircon geochronology; Geochemistry
The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U-Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 +/- 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850-1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan-Dahongshan-Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan-Dongchuan-Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia. (C) 2016 International Association for Gondwana Research. Published by Elsevier B. V. All rights reserved. DOI
27. Medig, KPR; Turner, EC; Thorkelson, DJ; Rainbird, RH.Rifting of Columbia to form a deep-water siliciclastic to carbonate succession: The Mesoproterozoic Pinguicula Group of northern Yukon, Canada.Precambrian Res., 2016, 278: 179-206 Rifting of Columbia to form a deep-water siliciclastic to carbonate succession: The Mesoproterozoic Pinguicula Group of northern Yukon, Canada
Mesoproterozoic; Deep-water carbonate; Low-energy slope; Precambrian succession; Stromatolite; Basin
The Mesoproterozoic Pinguicula Group (<1.38 Ga) is exposed in the Wernecke and Hart River inliers in northern Yukon, Canada. The Pinguicula Group records deposition of non-cyclic siliciclastic and carbonate strata on low-energy slopes affected by rare high-energy deposits in a tectonically active epicratonic setting. The succession is similar to 1.4 km thick at its measured type sections and comprises three newly formalised formations: the Mount Landreville, Pass Mountain, and Rubble Creek formations (formerly units A, B, and C, respectively). The Mount Landreville Formation is a predominantly siltstone succession with minor conglomerate and sandstone deposited below storm wave-base on a relatively gentle slope. The Pass Mountain Formation is a wispy- to planar-laminated carbonate succession deposited on a low-energy slope mostly below storm wave-base and is punctuated by rare high-energy gravity-flow deposits including debrites, grain-flows, turbidites, and micro-turbidites. The Rubble Creek Formation is dominated by repetitive centimetre- to decimetre-scale lime mudstone beds; it is distinguished from the Pass Mountain Formation by abundant zebra texture (an alternating dark grey and white banding caused by late diagenetic or hydrothermal fluid influx) and a lack of sediment gravity-flow deposits. The Pinguicula Group is the middle of five, unconformity-bounded, Proterozoic stratigraphic successions deposited on the northwestern margin of Laurentia (ancestral North America). The Pinguicula basin was epicratonic and deepened to the south (present coordinates). The basin formed during an amagmatic extensional event that contributed to the break-up of Columbia and the separation of Laurentia from Australia. Whereas most preserved Mesoproterozoic basins are dominated by shallow-water lithofacies deposited in rift and epicratonic settings, with few deep-water lithofacies preserved, the carbonate strata of the Pinguicula Group provide a rare insight into deeper-water carbonate environments. (C) 2016 Elsevier B.V. All rights reserved. DOI
26.Thorkelson, DJ; Laughton, JR.Paleoproterozoic closure of an Australia-Laurentia seaway revealed by megaclasts of an obducted volcanic arc in Yukon, Canada.Gondwana Res., 2016, 33: 115-133 Paleoproterozoic closure of an Australia-Laurentia seaway revealed by megaclasts of an obducted volcanic arc in Yukon, Canada
Precambrian; Laurentia; Volcanic; Obduction; Columbia
The Slab volcanics are a late Paleoproterozoic volcanic succession in the Wernecke Mountains of Yulcon Territory in northwestern Canada. Fragments of the succession are preserved as megaclasts in km-scale zones of hydrothermal breccia. The largest clast is 160 x 380 m and consists of 31 mafic lava flows and minor intercalated sandstone and tephra. Hydrothermal activity during the brecciation led to extensive metasomatic alteration, both sodic and potassic. Despite the alteration, an igneous geochemical signal is discernible. The rocks are mafic to intermediate and moderately alkalic. Their trace element profiles indicate derivation from a hydrated, enriched mantle source, consistent with an origin as a volcanic arc affected by a plume, rift or slab window. Major and trace element patterns demonstrate a cyclical magmatic evolution involving intervals of fractional crystallization punctuated by recharge with mafic magma. The Slab volcanics were previously thought to have been deposited on Laurentia but are herein regarded as part of the exotic terrane Bonnetia that was obducted onto the Laurentian continental margin in late Paleoproterozoic time. Bonnetia may have developed on or near the eastem margin of Australia as a continental or fringing arc. In that scenario, Bonnetia grew through arc magmatism as oceanic crust between Australia and Laurentia was consumed prior to terrane obduction and continental collision in the late Paleoproterozoic to early Mesoproterozoic. Sediment or tephra derived from the arc was transported westward into the interior of Australia, raising eNd values of late Paleoproterozoic sedimentary strata. Vigorous hydrothermal brecciation followed and led to fragmentation of the crust, including the obducted terrane Bonnetia. Giant blocks of the Slab volcanics and other units of Bonnetia foundered into breccia zones and moved downward for thousands of meters. Erosion removed the obducted terrane and the uppermost Laurentian crust, exposing the megaclasts. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. DOI
25. Medig, KPR; Thorkelson, DJ; Davis, WJ; Rainbird, RH; Gibson, HD; Turner, EC; Marshall, DD.Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic.Precambrian Res., 2014, 249: 88-99 Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic
Yukon; Mesoproterozoic; Columbia; Nuna; Supercontinents; Geochronology
Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9 to 1.3 Ga, and Rodinia from ca. 1.1 to 0.75 Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499 +/- 3 Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493 +/- 8 Ma to 1508 +/- 4 Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420 Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved. DOI
24. Furlanetto, F; Thorkelson, DJ; Gibson, HD; Marshall, DD; Rainbird, RH; Davis, WJ; Crowley, JL; Vervoort, JD.Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis.Precambrian Res., 2013, 224: 512-528 Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Wernecke Supergroup; RackIan; Paleoproterozoic; Bonnetia; Columbia
The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wemecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wemecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wemecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present. (c) 2012 Elsevier B.V. All rights reserved. DOI
23. Nielsen, AB; Thorkelson, DJ; Gibson, HD; Marshall, DD.The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia.Precambrian Res., 2013, 238: 78-92 The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia
Geochemistry; Tectonics; Wernecke; Proterozoic; Yukon; Metasomatism
The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U-Pb titanite at 1599 Ma. Four U-Pb zircon ages on the Wernecke igneous clasts (1714-1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (< 1.64 Ga) and indicate that the clasts were not derived from dykes within the Wernecke Supergroup. Instead, the clasts were derived from an obducted terrane named Bonnetia. Geochemical characteristics of the Wernecke igneous clasts infer that Bonnetia formed as a volcanic arc with a component of within-plate magmatism. Neodymium mantle depletion ages of 2080-2760 Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly Wilt on a rifted fragment of Laurentia, on another continental fragment, or possibly on the leading edge of another continent. The subsequent event of breccia-formation may represent a hydrothermal response to abduction-caused tectonic loading of the crust. The characterization of Bonnetia as a volcanic arc complex that underwent obduction requires that northwestern Laurentia was flanked by an ocean basin in the late Paleoproterozoic. (C) 2013 Elsevier B.V. All rights reserved. DOI
22. Brown, SR; Gibson, HD; Andrews, GDM; Thorkelson, DJ; Marshall, DD; Vervoort, JD; Rayner, N.New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone.Lithosphere, 2012, 4: 354-377 New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone
The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an similar to 1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64-89 km of WNW-directed horizontal extension based on an original shear zone angle of similar to 15 degrees. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic-Mesozoic layered sequence of sedimentary rocks. DOI
21. Galicki, M; Marshall, D; Staples, R; Thorkelson, D; Downie, C; Gallagher, C; Enkin, R; Davis, W.Iron Oxide +/- Cu +/- Au Deposits in the Iron Range, Purcell Basin, Southeastern British Columbia.Econ. Geol., 2012, 107: 1293-1301 Iron Oxide +/- Cu +/- Au Deposits in the Iron Range, Purcell Basin, Southeastern British Columbia
The Iron Range iron oxide +/- Cu +/- Au deposits in southeastern British Columbia comprise massive lenses and veins of hematite and martite with lesser magnetite in the Iron Range fault zone, which crosscuts the Proterozoic Aldridge Formation and Moyie sills. The mineralized zones are flanked by albite-quartz-iron oxide breccia within sedimentary rocks and by chlorite-altered iron oxide breccia where they are in contact with Moyie sills. Oxygen isotope analyses indicate 340 degrees to 400 degrees C precipitation temperatures for the albite-quartz-magnetite assemblages in the mineralized zones. Magnetite trace element compositions closely compare with those in iron oxide-(copper-gold) (IOCG) to porphyry-type mineralization worldwide. Paleomagnetic studies show consistent paleopole orientations concordant with Cretaceous poles and support links to a porphyry-type genesis associated with phases of the nearby SO to 105 Ma magnetite-bearing Bayonne Suite plutonic rocks. The Iron Range iron oxide mineralized zones share many characteristics of major IOCG deposits, with the exception of economic Cu (+/- Au) concentrations in the exposed rocks; however, recent drilling intersected minor chalcopyrite, pyrite, and gold. DOI
20.Thorkelson, DJ; Madsen, JK; Sluggett, CL.Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry.Geology, 2011, 39: 267-270 Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry
The Northern Cordilleran slab window formed beneath western Canada concurrently with the opening of the Californian slab window beneath the southwestern United States, beginning in Late Oligocene-Miocene time. A database of 3530 analyses from Miocene-Holocene volcanoes along a 3500-km-long transect, from the northern Cascade Arc to the Aleutian Arc, was used to investigate mantle conditions in the Northern Cordilleran slab window. Using geochemical ratios sensitive to tectonic affinity, such as Nb/Zr, we show that typical volcanic arc compositions in the Cascade and Aleutian systems (derived from subduction-hydrated mantle) are separated by an extensive volcanic field with intraplate compositions (derived from relatively anhydrous mantle). This chemically defined region of intraplate volcanism is spatially coincident with a geophysical model of the Northern Cordilleran slab window. We suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge, which had developed during extensive early Cenozoic arc and backarc volcanism in western Canada. High heat flow throughout the western Canadian Cordillera is broadly coincident with the field of intraplate volcanism and is linked to slab window-induced mantle upwelling. DOI
19. Xue, G; Marshall, D; Zhang, S; Ullrich, TD; Bishop, T; Groat, LA; Thorkelson, DJ; Giuliani, G; Fallick, AE.Conditions for Early Cretaceous Emerald Formation at Dyakou, China: Fluid Inclusion, Ar-Ar, and Stable Isotope Studies.Econ. Geol., 2010, 105: 339-349 Conditions for Early Cretaceous Emerald Formation at Dyakou, China: Fluid Inclusion, Ar-Ar, and Stable Isotope Studies
The Dyakou emerald occurrence is located in Malipo County in the province of Yunnan, southern China. The occurrence lies in the northern part of the Laojunshan-Song Chay metamorphic core complex, which is exposed in an area of approximately 2,000 km(2) and extends across the border between China and Vietnam. Emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed Proterozoic biotite-muscovite granofels and schist. Hydrogen and oxygen isotope results from the emerald channel waters and emerald, respectively, are consistent with an igneous fluid source. The delta(18)O fractionation between emerald and quartz yields vein temperatures of 365 to 420 degrees C. Fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10.5 mass percent NaCl equiv. Fluid inclusion isochores intersected with delta(18)O data yield pressures changing along the geothermal gradient from 1,500 to 3,300 bars. Ar-Ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 +/- 1 Ma. These constraints combined with field observations indicate that the Dyakou emerald deposit is consistent with the igneous-related model for emerald formation. DOI
18. Breitsprecher, K; Thorkelson, DJ.Neogene kinematic history of Nazca-Antarctic-Phoenix slab windows beneath Patagonia and the Antarctic Peninsula.Tectonophysics, 2009, 464: 10-20 Neogene kinematic history of Nazca-Antarctic-Phoenix slab windows beneath Patagonia and the Antarctic Peninsula
Ridge subduction; Quadruple junction; Trench-ridge-ridge-trench junction; Scotia Basin; Heat flow; Mantle upwelling
The Patagonian slab window is a subsurface tectonic feature resulting from subduction of the Nazca-Antarctic spreading-ridge system (Chile Rise) beneath southern South America. The geometry of the slab window had not been rigorously defined, in part because of the complex nature of the history of ridge subduction in the southeast Pacific region, which includes four interrelated spreading-ridge systems since 20 Ma: first, the Nazca-Phoenix ridge beneath South America, then simultaneous subduction of the Nazca-Antarctic and the northern Phoenix-Antarctic spreading-ridge systems beneath South America, and the southern Phoenix-Antarctic spreading-ridge system beneath Antarctica. Spreading-ridge paleo-geographies and rotation poles for all relevant plate pairs (Nazca, Phoenix, Antarctic, South America) are available from 20 Ma onward, and form the mathematical basis of our kinematic reconstruction of the geometry of the Patagonia and Antarctic slab windows through Neogene time. At approximately 18 Ma, the Nazca-Phoenix-Antarctic oceanic (ridge-ridge-ridge) triple junction enters the South American trench; we recognize this condition as an unstable quadruple junction. Heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction. From 16 Ma onward, the geometry of the Patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction. The majority of the slab window's areal extent and geometry is controlled by the highly oblique (near-parallel) subduction angle of the Nazca-Antarctic ridge system, and by the high contrast in relative convergence rates between these two plates relative to South America. The very slow convergence rate of the Antarctic slab is manifested by the shallow levels achieved by the slab edge (<45 km); thus no point on the Antarctic slab is sufficiently deep to generate "normal" mantle-derived arc-type magmas. Upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the Scotia Sea. (C) 2008 Elsevier B.V. All rights reserved. DOI
17. Groome, WG; Thorkelson, DJ.The three-dimensional thermo-mechanical signature of ridge subduction and slab window migration.Tectonophysics, 2009, 464: 70-83 The three-dimensional thermo-mechanical signature of ridge subduction and slab window migration
Slab windows; Ridge subduction; Three-dimensional numerical models; Deformation kinematics; Metamorphism; Topographic uplift
We present a series of three-dimensional numerical models investigating possible thermal and mechanical effects of ridge subduction and slab window migration. Ridge subduction is a fundamental process of plate tectonics, and the geologic manifestations in the overriding lithosphere have been investigated in many convergent margin settings. Many of the geologic effects of slab window migration (e.g., anomalous high-T metamorphism in the forearc, non-arc-like magmatism in the volcanic arc) are explained primarily by the introduction of a region of hot, upwelling asthenospheric mantle in the subduction zone environment. Using end-member boundary conditions and idealized geometries, our models address the thermal and corresponding mechanical manifestations of slab window migration, and our results provide a general agreement with observations from interpreted regions of slab window migration. Our models show that protracted heating in the forearc region should result in protracted high-temperature metamorphism, associated rheologic weakening and strain partitioning and changes in the topographic uplift pattern in regions affected by slab window migration. Although these models are idealized, they do provide valuable insight into the geodynamics of slab window environments and provide a valuable starting point for further exploration. (C) 2008 Elsevier B.V. All rights reserved. DOI
16. Ickert, RB; Thorkelson, DJ; Marshall, DD; Ullrich, TD.Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window.Tectonophysics, 2009, 464: 164-185 Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window
Adakite; Slab window; Volcanism; Eocene; Geochemistry; British Columbia
The Princeton Group is an assemblage of terrestrial volcanic and clastic sedimentary rocks in south-central British Columbia, and is part of the Challis-Kamloops belt that stretches from central British Columbia to the northwestern United States. The volcanic rocks were largely deposited as cinder cones and composite volcanoes, and are composed of basaltic andesite (olivine+clinopyroxene), andesite and dacite (hornblende + plagioclase+clinopyroxene), and rhyolite (biotite +quartz+ K-feldspar), with calc-alkaline affinity. New Ar-40/Ar-39 dates on homblende and groundmass separates, and whole rock indicate that magmatism took place during the Early to Middle Eocene, from 53-47 Ma. New neodymium isotopic measurements, in conjunction with previously published results, indicate that the Princeton Group has an epsilon Nd-50 = 1.2-6.4 and therefore represents primarily juvenile additions to the continental crust. The major and trace element abundances of Princeton Group rocks resemble those of many modem continental arcs. The compositions are notable, however, because they have an "adakitic" signature that extends throughout their entire compositional range, including high-Mg# basaltic andesite. Trace element modelling indicates that this signature was not derived from anatexis of normal oceanic crust, but from an "arc-like" source enriched in large-ion lithophile elements. This source may have been basaltic dykes that were emplaced into the lithospheric mantle during Mesozoic arc magmatism and subsequently partially melted during an event of lithospheric heating in the Eocene. The heating may have been caused by upwelling asthenosphere related to a slab window or slab tear. (C) 2007 Elsevier BY All rights reserved. DOI
14. Madsen, JK; Thorkelson, DJ; Friedman, RM; Marshall, DD.Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America.Geosphere, 2006, 2: 11-34 Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America
tectonics; magmatism; geochronology; forearc; slab window; ridge subduction; western North America; Cordillera
Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and Ar-40-Ar-39 dating indicates that the magmatism was concurrent in widely separated areas. Eight new conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from forearc intrusions on Vancouver Island (51.2 +/- 0.4, 48.8 +/- 0.5 Ma, 38.6 +/- 0.1, 38.6 +/- 0.2, 37.4 +/- 0.2, 36.9 +/- 0.2, 35.4 +/- 0.2, and 35.3 +/- 0.3 Ma), together with previous dates, indicate that southwestern British Columbia was a particularly active part of the forearc. The forearc magmatic belt has been largely attributed to ridge-trench intersection and slab window formation involving subduction of the Kula-Farallon ridge. Integration of the new and previous ages reveals shortcomings of the Kula-Farallon ridge explanation, and supports the hypothesis of two additional plates, the Resurrection plate (recently proposed) and the Eshamy plate (introduced herein) in the Pacific basin during Paleocene and Eocene time. We present a quantitative geometric plate-tectonic model that was constructed from 53 Ma to present to best account for the forearc magmatic record using ridge-trench intersection and slab window formation as the main causes of magmatism. The model is also in accord with Tertiary to present inboard magmatic and structural features. DOI
13. Laughton, JR; Thorkelson, DJ; Brideau, MA; Hunt, JA; Marshall, DD.Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada.Can. J. Earth Sci., 2005, 42: 1033-1044 Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada
In the Yukon, the oldest known supracrustal succession, the Wernecke Supergroup, was deposited in a marine basin before 1.71 Ga. The earliest orogenic event to disturb these strata was the Racklan orogeny, which produced folds and fabrics at peak temperatures of 450-550 degrees C. These features and those of the correlative Forward orogeny are recognized at the surface and in the subsurface throughout much of northwestern Canada. Zones of Wernecke Breccia (hydrothermal breccias, 1.60 Ga) were emplaced into the Wernecke Supergroup after Racklan deformation and metamorphism. Two main types of breccia are recognized: grey sodic breccias and colourful potassic breccias. In the Slab Mountain area, a belt of grey breccias contains abundant megaclasts of country rock including blocks of a subaerial lava succession, the Slab volcanics. These grey breccias are interpreted as a vent facies of Wernecke Breccia, and their emplacement into the stratigraphically lowest unit of the Wernecke Supergroup infers that at least 9 km of exhumation occurred in the core of a major Racklan anticline prior to brecciation. The Slab volcanics are preserved only as clasts in Wernecke Breccia and are interpreted as fragments of a former valley-filling basalt succession which overlay deformed and deeply incised strata of the Wernecke Supergroup. DOI
12.Thorkelson, DJ; Abbott, JA; Mortensen, JK; Creaser, RA; Villeneuve, ME; McNicoll, VJ; Layer, PW.Early and Middle Proterozoic evolution of Yukon, Canada.Can. J. Earth Sci., 2005, 42: 1045-1071 Early and Middle Proterozoic evolution of Yukon, Canada
This paper provides a comprehensive synthesis of virtually all units and events of Early and Middle Proterozoic age in the Yukon, spanning similar to 1 Ga. Early and Middle Proterozoic time was dominated by a series of extensional-basin-forming events punctuated by orogenesis, magmatism, and hydrothermal activity. Basinal deposits include the Wernecke Supergroup (> 1.71 Ga), Pinguicula Group (similar to 1.38 Ga), and Mackenzie Mountains Supergroup (1.00-0.78 Ga). Igneous rocks include the Bonnet Plume River Intrusions (1.71 Ga), Slab volcanics (>= 1.6 Ga), Hart River sills and volcanics (1.38 Ga), and Bear River (Mackenzie) dykes (1.27 Ga). A voluminous hydrothermal event generated the widespread Wernecke breccias at 1.60 Ga. The Racklan orogeny deformed the Wernecke Supergroup prior to emplacement of the Wernecke Breccia. The Corn Creek orogeny deformed Mackenzie Mountains Supergroup and older rocks prior to deposition of the Windermere Supergroup (< 0.78 Ga). Long intervals with scanty rock records extended for as much as 300 Ma and appear to represent periods of crustal stability and subaerial conditions. By the time of Windermere rifting (< 0.78 Ga), the supracrust of northwestern Laurentia was a mature, largely denuded orogenic belt with a composite sedimentary-metamorphic-igneous character. New isotopic data include Nd depleted mantle model ages for the Wernecke Supergroup (2.28-2.69 Ga) and Wernecke Breccia (2.36-2.96 Ga), a U-Pb zircon age for a Hart River sill 1381.9(-3.7)(+5.3) (Ma), detrital U-Pb zircon ages from the basal Pinguicula Group (1841-3078 Ma), detrital muscovite ages from the Mackenzie Mountains Supergroup (1037-2473 Ma), and muscovite Ar-40/Ar-39 cooling ages from the Wernecke Supergroup (788 +/- 8 and 980 +/- 4 Ma). DOI
11.Thorkelson, DJ; Breitsprecher, K.Partial melting of slab window margins: genesis of adakitic and non-adakitic magmas.Lithos, 2005, 79: 25-41 Partial melting of slab window margins: genesis of adakitic and non-adakitic magmas
slab window; ridge subduction; adakite; anatexis; mantle; restite
When a mid-ocean spreading ridge subducts, it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window. We examine the fate of the slab edges using simplified slab window geometries, specific thermal parameters, and assumptions regarding shear stress and slab hydration. Six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions. Non-adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5-65 km, whereas adakitic melts form proximal to the edges at depths of 25-90 km. As anatexis proceeds, the subducted crust is converted to a migmatite of slab melt and eclogitic restite. Much of the migmatite may shear away from the slab and become incorporated into the mantle. The melts will rise and leave behind fragments of restite within mantle peridotite. If the peridotite is part of the overriding plate, then the restite fragments may become long-term residents of the continental lithospheric mantle. However, if the restite becomes entrained in the asthenosphere, it may undergo upwelling and melting, or flow away as ductile streaks to become long-term mantle heterogeneities. Slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle. (C) 2004 Elsevier B.V. All rights reserved. DOI
10. Schwab, DL; Thorkelson, DJ; Mortensen, JK; Creaser, RA; Abbott, JG.The Bear River dykes (1265-1269 Ma): Westward continuation of the Mackenzie dyke swarm into Yukon, Canada.Precambrian Res., 2004, 133: 175-186 The Bear River dykes (1265-1269 Ma): Westward continuation of the Mackenzie dyke swarm into Yukon, Canada
dyke; Cordillera; Yukon; mantle plume; Proterozoic; Mackenzie
The 1.27 Ga Mackenzie dyke swarm, the largest on Earth, radiates from a point in the western Canadian Arctic and extends across much of the Canadian Shield. Possible western extensions of the swarm are largely obscured by younger sedimentary cover. Paleoproterozoic inliers in northern Yukon host the Bear River dykes (BRD), herein dated by U-Pb baddeleyite and zircon methods at 1268.5 +/- 1.5 Ma and 1264.6 +/- 1.2 Ma. The BRD share similar geochemical and Nd-isotopic characteristics with the Mackenzie dykes and the coeval Coppermine River basalts, and are regarded as products of the same plume. Hydrothermal activity at similar to1270Ma in breccia of the Nor mineral occurrence, northwest of the BRD, was probably generated by BRD at depth. The BRD generally strike northwest, approximately 90degrees to the orientation predicted by the radiating dyke model. This difference may be explained by an anomalous local stress field at the time of BRD emplacement, or reorientation of BRD during subsequent events of Cordilleran deformation. Using the Nor mineral Occurrence as the westernmost locale of Mackenzie dyking and assuming a model of uniform dyke radiation, the arc of the Mackenzie swarm is at least 50degrees greater than previously recognized, bringing the total are of dyke radiation to >150degrees. (C) 2004 Elsevier B.V. All rights reserved. DOI
9. Breitsprecher, K; Thorkelson, DJ; Groome, WG; Dostal, J.Geochemical confirmation of the Kula-Farallon slab window beneath the Pacific Northwest in Eocene time.Geology, 2003, 31: 351-354 Geochemical confirmation of the Kula-Farallon slab window beneath the Pacific Northwest in Eocene time
Challis-Kamloops; Tertiary; geochemistry; ridge subduction; British Columbia; adakite
Plate tectonic models indicate subduction of the Kula-Farallon spreading ridge, and thus imply formation of the Kula-Farallon slab window, beneath western North America from Late Cretaceous to middle Eocene time. Seafloor magnetic anomalies, however, provide few constraints on the configuration of the subducted ridge and the location of the slab window. We use geochemical data from Eocene igneous rocks to provide an estimate of the slab window's position at 50 Ma. Contouring of the trace element ratios K2O/SiO2, Rb/Zr, and Ta/Ce for lavas of the Eocene Challis-Kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character. This trend is consistent with a Pacific Northwest position for the slab window; this is supported by the presence of adakites (slab melts) near the Canada-United States border, and by ca. 51 Ma forearc intrusions and volcanic rocks on Vancouver Island that are regarded as products of ridge subduction. A new reconstruction of the slab window's geometry over time indicates that its chemically defined position is kinematically viable. DOI
8. Dostal, J; Breitsprecher, K; Church, BN; Thorkelson, D; Hamilton, TS.Eocene melting of Precambrian lithospheric mantle: Analcime-bearing volcanic rocks from the Challis-Kamloops belt of south central British Columbia.J. Volcanol. Geotherm. Res., 2003, 126: 303-326 Eocene melting of Precambrian lithospheric mantle: Analcime-bearing volcanic rocks from the Challis-Kamloops belt of south central British Columbia
ultrapotassic; volcanism; Eocene; isotopes; trace elements; slab window
Potassic silica-undersaturated mafic volcanic rocks form a minor portion of the predominantly calc-alkaline Eocene Challis-Kamloops volcanic belt, which extends from the northwestern United States into central British Columbia (Canada). Their major occurrence is in the Penticton Group in south central British Columbia, where they reach a thickness of up to 500 in and form the northwestern edge of the Montana alkaline province. These analcime-bearing rocks (similar to 53-52 Ma old) are typically rhomb porphyries of ternary feldspar (An(28)Ab(52)Or(20)). Additional phenocryst phases include clinopyroxene, analcime, phlogopite and rare olivine. The rocks are characterized by high total alkalis, particularly K2O ( > 4.5 wt%) as well as by a distinct enrichment of large-ion lithophile elements versus heavy rare-earth elements and high-field-strength elements. They have unusual isotopic compositions compared to most other rocks of the Challis-Kamloops belt, particularly high negative epsilon(Nd) values and elevated but relatively uniform initial Sr-87/Sr-86 ratios (similar to 0.7065). The potassic silica-undersaturated rocks overlie Precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere, which was modified in a Precambrian subduction setting. The alkaline rocks of the Challis-Kamloops belt are related to a slab-window environment. In particular, they were formed above the southern edge of the Kula plate adjacent to the Kula-Farallon slab window, whereas the Montana alkaline province situated well to the southeast was formed directly above the Kula-Farallon slab window. Upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle. (C) 2003 Elsevier Science B.V. All rights reserved. DOI
7. Smith, AD; Thorkelson, D.Geochemical and Nd-Sr-Pb isotopic evidence on the origin and geodynamic evolution of mid-Cretaceous continental arc volcanic rocks of the Spences Bridge Group, south-central British Columbia.Geol. J., 2002, 37: 167-186 Geochemical and Nd-Sr-Pb isotopic evidence on the origin and geodynamic evolution of mid-Cretaceous continental arc volcanic rocks of the Spences Bridge Group, south-central British Columbia
arcs; North American Cordillera; Cretaceous; adakites; intraplate signatures
The mid-Cretaceous Spences Bridge Group (SBG) comprises a series of basaltic to rhyolitic lavas and related volcaniclastic rocks (Pimainus Formation) overlain by a succession of mainly amygdaloidal andesites (Spins Formation) related to the closure of the Methow-Tyaughton basin and accretion of the Insular terrane in the North American Cordillera. Geochemical variation in the SBG is related primarily to metasomatic processes in the mantle wedge. Pimainus lavas include low- to high-K, tholeiitic and calc-alkaline types, and have isotopic compositions (epsilon(Nd)(100Ma) = + 5.2 to + 7.0, epsilon(Sr)(100Ma) = - 10 to - 20, Pb-206/Ph-204 = 18.82 to 18.91, Pb-207/Ph-204 = 15.55 to 15.60, Pb-208/Ph-204 38.24 to 38.43) between the ranges for primitive arcs and accreted terrane crust. Crustal sources are identified only for some low-medium K dacite and rhyolite compositions. The occurrence of intermediate compositions with high MgO contents (up to 6 wt%) and the presence of adakitic trace element features in medium-high K felsic lavas attests to metasomatism of the mantle wedge by slab melts during Pimainus volcanism. Spins lavas have comparable K2O and Ph isotopic compositions to the Pimainus, even higher MgO (up to 9.2 wt%), and display a mild intraplate character in having up to 0.6 wt% P2O5, 15 ppm Nb, and 240 ppm Zr. Spius Nd - Sr isotopic compositions (epsilon(Nd)(100Ma) = + 5.3 to + 6.9, epsilon(Sr)(100Ma) = - 14 to - 25) define an array extending from Pimainus to alkaline seamount compositions. The low epsilon(Sr) values, elevated high field strength element contents, and moderate silica contents suggest Spins volcanism was related to the introduction of small melt fractions from the asthenosphere into the mantle wedge which had previously generated Pimainus melts. The range of compositional types in the Pimainus Formation constrains tectonic scenarios to include an elevated slab thermal regime, likely from approach of an ocean ridge system toward the continental margin. Spins volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab-hinge rollback on closure of the Methow-Tyaughton basin. Copyright (C) 2002 John Wiley Sons, Ltd. DOI
6.Thorkelson, DJ; Mortensen, JK; Creaser, RA; Davidson, GJ; Abbott, JG.Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia.Can. J. Earth Sci., 2001, 38: 1479-1494 Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia
Northwestern Laurentia, after cratonization at about 1.85 Ga, underwent a series of tectonic and magmatic events during the Proterozoic that were followed by separation of Laurentia from another landmass, probably Australia. The oldest magmatic event produced the Bonnet Plume River Intrusions (BPRI), which intruded the Wernecke Supergroup as short dikes and small stocks. The BPRI are hydrothermally altered tholeiitic diorites, gabbros, and subordinate anorthositic and syenitic rocks, with trace element signatures consistent with a rift origin. Depleted mantle model ages range from 2.29 to 2.57 Ga and epsilon (Nd) values range from +0.7 to -1.7. An increasing crustal component is apparent in rocks with more evolved compositions. Four U-Pb zircon ages (1705.9 +/- 0.7, 1709.4 +/- 1.4, 1711.1 +/- 5.1, and 1713.6 +/- 12.7 Ma) indicate a Paleoproterozoic age for the BPRI. These dates constrain the age of the Wernecke Supergroup to greater than or equal to ca. 1710 Ma, making it the oldest supracrustal succession in western Laurentia, e.g., >240 Ma older than the Belt Supergroup of southeastern British Columbia and the northwestern United States. The Wernecke Supergroup was deposited in the first rift basin to open along the western margin of Laurentia, but was later inverted by the pre-1.6 Ga Racklan Orogeny, an event possibly influenced by transmission of compression from the Yavapai and Mazatzal orogenies in southern Laurentia. The Neoproterozoic southwestern United States - east Antarctica (SWEAT) reconstruction, which places Australia next to northwestern Laurentia, is supported by linkages between Paleoproterozoic and Mesoproterozoic geological features in northwestern Canada and Australia. DOI
5. Johnston, ST; Thorkelson, DJ.Continental flood basalts: episodic magmatism above long-lived hotspots.Earth Planet. Sci. Lett., 2000, 175: 247-256 Continental flood basalts: episodic magmatism above long-lived hotspots
flood basalts; mantle plumes; magmatism; hot spots
The eruption of continental flood basalt (CFB) may reflect episodic magmatism above long-lived mantle plumes, The Iceland and Yellowstone hotspots have generated successive CFB provinces, large intrusive complexes, anomalous uplift, basin formation, and rifting events, and linear volcanic chains dating back > 120 and > 70 Ma, respectively. Amagmatic intervals occurred: (1) when ascent of plumes to shallow levels-was impeded by impenetrable lithosphere, resulting in sub-lithospheric pending of plume mantle; and (2) in response to dispersion by subducting oceanic lithosphere in convergent margin settings. By comparison with the plume eruptive potential of typical oceanic hotspots, it is apparent that preservation of only a small portion of plume mantle ponded during an amagmatic interval is necessary to account for large volume of CFBs. Thermal erosion, lithospheric attenuation, translation of ponded hotspot mantle to the base of thinner penetrable lithosphere, and passage of plume mantle through slab windows in subducting oceanic lithosphere led to subsequent breakthrough and eruption of CFB. Since both mantle plume and plate tectonic processes have been operating since the Archean, it seems likely that the migration of continents over hotspots, with attendant magmatic and tectonic consequences,,is a common occurrence in the geological record. (C) 2000 Elsevier Science B.V. All rights reserved. DOI
4. Johnston, ST; Thorkelson, DJ.Cocos-Nazca slab window beneath Central America.Earth Planet. Sci. Lett., 1997, 146: 465-474 Cocos-Nazca slab window beneath Central America
subduction zones; spreading centers; transform faults; island arcs; slabs; mantle plumes; lithosphere; asthenosphere; geochemistry
Integration of petrologic and tectonic data favours a model of slab window formation beneath Central America in the Pliocene-Pleistocene. Central America has been the site of voluminous Cenozoic are volcanism. The Cocos and Nazca plates, which are subducting beneath Central America, are diverging along the east-trending Cocos-Nazca spreading ridge. Since 25 Ma the Americas have advanced about 1800 km west over the ridge-transform system. Since at least 8 Ma, plate integrity and the ridge-transform configuration have been preserved during convergence, resulting in subduction of the spreading ridge and development of a slab window. The Panama fracture zone, an active transform fault, is the part of the ridge-transform system currently being subducted. The ridge-transform system formerly adjoining the northern end of the Panama fracture zone is likely to have been left-stepping. We use present-day plate motions to design a slab window to fit known variations in igneous composition, hypocentre distribution, and mantle anisotropy. The modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 Ma. Cessation of ridge subduction occurred between 1 and 3 Ma, when subduction of the Panama fracture zone is considered to have begun. The slab window is continuing to expand and migrate northeastward below the Central American volcanic are. The absence of a Wadati-Benioff zone from southeastern Costa Pica through Panama corresponds to the position of the slab window. Within this region, dacitic and rhyolitic volcanic rocks have ''adakitic'' compositions, and are thought to result from anatexis of the young, buoyant crust which forms the trailing edges of the slabs bounding the window. Basalts in this area were derived from an enriched ocean-island type mantle source, whereas basalts from the rest of the arc, in Nicaragua, El Salvador and Guatemala, are mainly derived from slab-modified depleted mantle, characteristic of volcanic arcs. The presence of ocean-island type mantle beneath southern Costa Pica and Panama is explained by eastward flow of enriched asthenosphere from the Galapagos plume-head through the slab window and into the volcano source region. Eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the Nazca and Caribbean plates. The flow of peridotite is a consequence of progressive shrinkage of the Pacific mantle reservoir and concurrent expansion of the Atlantic mantle reservoir. DOI
3. Irving, E; Wynne, PJ; Thorkelson, DJ; Schiarizza, P.Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma.J. Geophys. Res.-Solid Earth, 1996, 101: 17901-17916 Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma
Using paleomagnetically derived estimates of latitudinal displacement, within the northern Cordillera we identify two major tectonic domains that were active during Late Cretaceous through Eocene time (roughly 83 to 45 Ma). At 100 to 90 Ma, the Interior Domain (comprising much of interior British Columbia, central Yukon, and eastern and central Alaska) was situated in the latitudes of Oregon and northern California, and the Coast Domain (southeastern Alaska, much of the Coast Ranges and islands of British Columbia, and the Cascade Mountains of Washington) was in latitudes similar to those of northern Mexico. Subsequently, both domains moved northward, reaching their present positions before 45 Ma. Within each domain, displacement estimates from northern locations are greater than those from southern locations, as if the domains had been elongated in an orogen-parallel sense. Evidently, during the latest Cretaceous through Eocene, the Cordillera was a huge dextral shear system, which (83 to 65 Ma) carried the Coast Domain about 2000 km northward until it combined with the Interior Domain and then (65 to 45 Ma) drove the combined domains an additional 1000 to 2000 km north northwestward, spreading them out along the continental margin. Following previous workers, we ascribe these motions to the rapid oblique subduction of the oceanic Kula Plate beneath the continental western margin of the North American Plate, and derive a model based, in part, on comparisons with the oblique subduction of the Nazca Plate beneath the central Andes. Between the Interior Domain and the craton, geologic evidence in the north has been interpreted as indicating displacements comparable in magnitude to paleomagnetic estimates; in the south, the record has been obscured by Eocene extension. Between the Interior and Coast Domains, geological relationships are complex, and most current geologically based interpretations propose that post-mid-Cretaceous relative motions between them did not exceed a few hundred kilometers, whereas paleomagnetic evidence indicates 2000 km. Our reading of the geologic evidence is that it neither requires nor precludes the very large displacements inferred from paleomagnetic observations. Hence we argue that although the paleomagnetically derived displacements far exceed those proposed by geologic interpretations, they are not inconsistent with the geologic evidence itself. DOI
2.Thorkelson, DJ.Subduction of diverging plates and the principles of slab window formation.Tectonophysics, 1996, 255: 47-63 Subduction of diverging plates and the principles of slab window formation
Consumption of an ocean basin by subduction commonly brings a sea-floor-spreading ridge toward a deep-sea trench. If plate divergence and convergence continue after the ridge intersects the subduction zone, a slab window forms between the subducted parts of the diverging oceanic plates, producing anomalous thermal, physical and chemical effects in the surrounding asthenospheric mantle. In turn, these conditions alter the tectonic and magmatic evolution of the overriding plate, usually disturbing ordinary fore-are and are regimes. Differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate. Anomalous magmatism from fore are to back are may be accompanied by fore-are metamorphism, strike-slip faulting, uplift, extension and, in extreme cases, rifting. The shape and size of the window are controlled mainly by the pre-subduction ridge-transform-trench configuration, slab dip angles and vectors of plate convergence. Subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates. Subducted transform faults continue to be active, usually as oblique-slip faults, until the plates separate. As transform faults subduct, they become longer on the plate which occupies the acute angle between ridge and trench, and shorter on the other plate. Trains of isolated windows produced by subduction of a segmented ridge-transform system progressively expand during descent, commonly merging together to form a composite slab window. Oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows, one at each site of ridge-trench intersection. Above a slab window, are volcanism diminishes and may be replaced by volcanism of mid-ocean ridge or rift affinity. The change in chemical character reflects various processes including elevated heat flow, decreasing hydration of the upper mantle, juxtaposition of supra- and sub-slab mantle reservoirs, asthenospheric upwelling and melting of the trailing plate edges. If the slab window migrates, the anomalous magmatic regime may be replaced by renewed are volcanism. Identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications. DOI
