Igneous Rock Associations 21. The Early Permian Panjal Traps of the Western Himalaya

The Early Permian (290 Ma) Panjal Traps are the largest contiguous outcropping of volcanic rocks associated with the Himalayan Magmatic Province (HMP). The eruptions of HMP-related lava were contemporaneous with the initial break-up of Pangea. The Panjal Traps are primarily basalt but volumetrically minor intermediate and felsic volcanic rocks also occur. The basaltic rocks range in composition from continental tholeiite to ocean-floor basalt and nearly all have experienced, to varying extent, crustal contamination. Uncontaminated basaltic rocks have Sr–Nd isotopes similar to a chondritic source (ISr = 0.7043 to 0.7073; eNd(t) = 0 ± 1), whereas the remaining basaltic rocks have a wide range of Nd (eNd(t) = –6.1 to +4.3) and Sr (ISr = 0.7051 to 0.7185) isotopic values. The calculated primary melt compositions of basalt are picritic and their mantle potential temperatures (TP ≤ 1450°C) are similar to ambient mantle rather than anomalously hot mantle. The silicic volcanic rocks were likely derived by partial melting of the crust whereas the andesitic rocks were derived by mixing between crustal and mantle melts. The Traps erupted within a continental rift setting that developed into a shallow sea. Sustained rifting created a nascent ocean basin that led to sea-floor spreading and the rifting of microcontinents from Gondwana to form the ribbon-like continent Cimmeria and the Neotethys Ocean.RÉSUMÉLes Panjal Traps du début Permien (290 Ma) constituent le plus grand affleurement contigu de roches volcaniques associées à la province magmatique de himalayienne (HMP). Les éruptions de lave de type HMP étaient contemporaines de la rupture initiale de la Pangée. Les Panjal Traps sont essentiellement des basaltes, mais on y trouve aussi des roches volcaniques intermédiaires et felsiques en quantités mineures. La composition de ces roches basaltiques varie de tholéiite continentale à basalte de plancher océanique, et presque toutes ont subi, à des degrés divers, une contamination de matériaux crustaux. Les roches basaltiques non contaminées ont des contenus isotopiques Sr–Nd similaires à une source chondritique (Isr = 0,7043 à 0,7073; eNd (t) = 0 ± 1), alors que les roches basaltiques autres montrent une large gamme de valeurs isotopiques en Nd (eNd (t) = –6,1 à +4,3) et Sr (Isr = de 0,7051 à 0,7185). Les compositions de fusion primaire calculées des basaltes sont picritiques et leurs températures potentielles mantelliques (TP de ≤ 1450°C) sont similaires à la température ambiante du manteau plutôt que celle d’un manteau anormalement chaud. Les roches volcaniques siliciques dérivent probablement de la fusion partielle de la croûte alors que les roches andésitiques proviennent du mélange entre des matériaux de fusion crustaux et mantelliques. Les Traps ont fait irruption dans un contexte de rift continental qui s’est développé dans une mer peu profonde. Un rifting soutenu a créé un début de bassin océanique lequel conduit à une expansion du fond océanique et au rifting de microcontinents tirés du Gondwana pour former le continent rubané de Cimméria et l'océan Néotéthys.

Download Full-text

Geochemistry and uranium-lead isotopic ages of volcanic rocks associated with Ladakh batholith, western Himalaya: Implications for petrogenesis and tectonic evolution

10.5194/egusphere-egu2020-113 ◽

2020 ◽

Author(s):

Nongmaithem Lakhan Singh ◽

Athokpam Krishnakanta Singh

Keyword(s):

Oceanic Crust ◽

Volcanic Rocks ◽

Crustal Contamination ◽

Primary Source ◽

Western Himalaya ◽

Mature Stage ◽

Northern Margin ◽

Felsic Volcanic ◽

Compositional Diversity ◽

Felsic Volcanic Rocks

We present zircon U-Pb ages and whole-rock geochemistry along with mineral chemistry of the Khardung volcanic rocks outcropped in the northern margin of the Ladakh batholith in order to constrain their origin and tectono-magmatic history. These volcanic rocks are sandwiched between the Ladakh batholith in the south and the Shyok suture zone in the north and span a continuous compositional range from basalt to rhyolite, although mafic rocks are minor and intermediate to felsic rocks are volumetrically predominant. New zircon U-Pb dating for andesite coupled with two rhyolitic rocks yield 69.71 Ma, 62.49 Ma, and 66.55 Ma, defining the probable span of their magmatism from Late Cretaceous to Palaeogene. Based on their mineralogical and geochemical compositional diversity, the Khardung volcanic rocks are categorized as intermediate volcanic rocks (basaltic andesite-andesite) and felsic volcanic rocks (dacite-rhyolite). The intermediate volcanic rocks are marked by low SiO2 (52.80-61.31 wt.%), enriched LREEs, and negative HFSEs (Nb, Ti, Zr) anomalies whereas,&#160; felsic volcanic rocks are characterized by high SiO2 (64.52-79.19 wt.%), pronounced negative Eu anomalies, enriched LREE and concave-downward HREE&#8217;s and negative HFSE&#8217;s (Nb, Ti) anomalies. Both the intermediate and felsic volcanic rocks exhibit quartz, sanidine, albite, bytownite, and diopside as their dominant mineral phases. Geochemical signatures indicate that the fractional crystallization and crustal contamination played a significant role in the evolution of the Khardung volcanic rocks and their geochemical diversity probably resulted from the partial melting of the common primary source, which had been metasomatized by variable contributions of fluids released from down going Neo-Tethyan oceanic crust. Thus, the Khardung volcanic rocks could be considered as a product of mature stage of arc magmatism during the subduction of the Neo-Tethyan oceanic crust, which occurred during Early Cretaceous to Palaeogene, prior to the main collision between the Indian and Asian plates.

Download Full-text

Geochemistry and U–Pb zircon age of comenditic metafelsites of the Tibbit Hill Formation, Quebec Appalachians

Canadian Journal of Earth Sciences ◽

10.1139/e89-117 ◽

1989 ◽

Vol 26 (7) ◽

pp. 1374-1383 ◽

Cited By ~ 46

Author(s):

P. Stephen Kumarapeli ◽

Greg R. Dunning ◽

Hillar Pintson ◽

Jim Shaver

Keyword(s):

Triple Junction ◽

Volcanic Rocks ◽

Rift System ◽

Low Grade ◽

Volcanic Event ◽

Zircon Age ◽

Iapetus Ocean ◽

Silicic Volcanic ◽

Felsic Volcanic ◽

Sea Floor Spreading

Metafelsites in Waterloo area, Quebec, represent the only known silicic volcanic rocks in the predominantly basaltic Tibbit Hill Formation. Low-grade metamorphism accompanied by hydration and albitization has converted the felsic volcanic rocks mainly to muscovite–quartz–albite schists. The volcanic parent of these metafelsites was formed partly as lava flows and partly as tuffs. The principal compositional type was a comendite. A component of intermediate rocks is also present but its extent is undetermined and probably minor. U–Pb zircon studies of the metafelsites have yielded a reliable age of [Formula: see text]. This Early Cambrian age is probably representative of the age of the Tibbit Hill Formation as a whole.The Tibbit Hill Formation accumulated at one of the clearest examples of a RRR (rift–rift–rift) triple junction–the Sutton Mountains triple junction–of the continental rift system formed as a prelude to the opening of the Iapetus Ocean. Its volcanic rocks are products of the youngest major episode of rift-related volcanism known from the continental margin of Laurentia. The volcanic event may have occurred as a harbinger of the onset of sea-floor spreading at the Sutton Mountains triple junction.

Download Full-text

Geochemistry and Isotope Composition of Paleoproterozoic Granites and Felsic Volcanics of the Elash Graben: Evidence of the Heterogeneity of the Early Precambrian Crust

Russian Geology and Geophysics ◽

10.2113/rgg20204206 ◽

2021 ◽

Vol 62 (10) ◽

pp. 1175-1187

Author(s):

A.D. Nozhkin ◽

O.M. Turkina ◽

K.A. Savko

Keyword(s):

Isotope Composition ◽

Volcanic Rocks ◽

Rare Metal ◽

Temporal Association ◽

Metal Deposit ◽

Wide Range ◽

Rare Metal Deposit ◽

Geochronological Study ◽

Felsic Volcanic ◽

Felsic Volcanic Rocks

Abstract —The paper presents results of a petrogeochemical and isotope–geochronological study of the granite–leucogranite association of the Pavlov massif and felsic volcanics from the Elash graben (Biryusa block, southwest of the Siberian craton). A characteristic feature of the granite–leucogranites is their spatial and temporal association with vein aplites and pegmatites of the East Sayan rare-metal province. The U–Pb age of zircon from granites of the Pavlov massif (1852 ± 5 Ma) is close to the age of the pegmatites of the Vishnyakovskoe rare-metal deposit (1838 ± 3 Ma). The predominant biotite porphyritic granites and leucogranites of the Pavlov massif show variable alkali ratios (K2O/Na2O = 1.1–2.3) and ferroan (Fe*) index and a peraluminous composition; they are comparable with S-granites. The studied rhyolites of the Tagul River (SiO2 = 71–76%) show a low ferroan index, a high K2O/Na2O ratio (1.6–4.0), low (La/Yb)n values (4.3–10.5), and a clear Eu minimum (Eu/Eu* = 0.3–0.5); they are similar to highly fractionated I-granites. All coeval late Paleoproterozoic (1.88–1.85 Ga) granites and felsic volcanics of the Elash graben have distinct differences in composition, especially in the ferroan index and HREE contents, owing to variations in the source composition and melting conditions during their formation at postcollisions extension. The wide range of the isotope parameters of granites and felsic volcanic rocks (εNd from +2.0 to –3.7) and zircons (εHf from +3.0 to +0.8, granites of the Toporok massif) indicates the heterogeneity of the crustal basement of the Elash graben, which formed both in the Archean and in the Paleoproterozoic.

Download Full-text

Geochemistry and U/Pb geochronology of the Williams Brook area, Tobique-Chaleur zone, New Brunswick: stratigraphic and geotectonic setting of gold mineralization

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2020-0094 ◽

2021 ◽

Author(s):

Dennis Sánchez-Mora ◽

Christopher R.M. McFarlane ◽

James A Walker ◽

David R. Lentz

Keyword(s):

New Brunswick ◽

Gold Mineralization ◽

Tectonic Setting ◽

Volcanic Rocks ◽

Crustal Contamination ◽

Volcanic Belt ◽

Temporal Variations ◽

Lower Percentage ◽

Oblique Convergence ◽

Felsic Volcanic

Gold mineralization at Williams Brook in northern New Brunswick is hosted within the Siluro-Devonian, bimodal, volcano-sedimentary rocks of the Tobique-Chaleur Zone (Wapske Formation). Gold mineralization occurs in two styles: 1) as disseminations (refractory gold) in rhyolite, and 2) in cross-cutting quartz veins (free gold). Dating of the felsic volcanic host rocks by in situ LA-ICP-MS zircon U-Pb geochronology returned ages of 422 ± 3, 409 ± 2, 408 ± 3, 405 ± 2, 401 ± 9 Ma. Zr/Y of subvolcanic felsic intrusion (<8 for syn-mineralization and >8 for post-mineralization) suggests evolution from transitional to more alkalic affinities. Two mineralizing events are recognized; the first is a disseminated mineralization style formed at ~422–416 Ma and the second consists of quartz vein-hosted gold emplaced at 410–408 Ma. Felsic rocks from Williams Brook and elsewhere in the Tobique Group (i.e. Wapske, Costigan Mountain, and Benjamin formations), and the Coastal Volcanic Belt have similar Th/Nb ratios of ~0.1 to 1, reflecting similar levels of crustal contamination, and similar Nb and Y content, suggesting A-type affinities. These data indicate a similar environment of formation. Regionally, mafic rocks show similar within-plate continental signatures and an E-MORB mantle source that formed from partial melts of 10–30%. Mafic volcanic rocks from Williams Brook have a more alkaline affinity (based on Ti/V), and derivation from lower percentage partial melting (~5%). The chemical and temporal variations in the Williams Brook rocks suggest that they were erupted in an evolving transpressional tectonic setting during the oblique convergence of Gondwana and Laurentia.

Download Full-text

Early Proterozoic (1.88–1.87 Ga) tholeiitic magmatism in the New Québec orogen

Canadian Journal of Earth Sciences ◽

10.1139/e93-129 ◽

1993 ◽

Vol 30 (7) ◽

pp. 1505-1520 ◽

Cited By ~ 23

Author(s):

Thomas Skulski ◽

Robert P. Wares ◽

Alan D. Smith

Keyword(s):

Volcanic Rocks ◽

Crustal Contamination ◽

Passive Margin ◽

Magma Chambers ◽

Sedimentary Sequences ◽

The North ◽

Marine Sedimentation ◽

Lithospheric Extension ◽

Felsic Volcanic ◽

Ocean Ridge

The New Québec orogen contains two volcano-sedimentary sequences bounded by unconformities. Each sequence records a change from continental sedimentation and alkaline volcanism to marine sedimentation and tholeiitic volcanism. The first sequence records 2.17 Ga rifting and the development, by 2.14 Ga, of a passive margin along the eastern part of the Superior craton. The second sequence developed between 1.88 and 1.87 Ga in pull-apart basins that reflect precollisional dextral transtension along the continental margin. Second-sequence magmatism comprises (i) carbonatitic and lamprophyric intrusions and mildly alkaline mafic to felsic volcanic rocks; (ii) widespread intrusion of tholeiitic gabbro sills, and submarine extrusion of plagioclase glomeroporphyritic basalts and younger aphyric basalts and picrites; and (iii) late-stage, mafic to felsic volcanism and intrusion of carbonatites. Crustal thinning allowed primitive tholeiitic magmas to equilibrate at progressively lower pressures before more buoyant derivative liquids could erupt. Early primitive melts were trapped at the base of the crust and crystallized olivine and orthopyroxene with minor crustal contamination. Derivative melts, similar to transitional mid-ocean-ridge basalts, migrated upward into mid-crustal magma chambers where they became saturated in calcic plagioclase. Subsequent tapping of these magma chambers allowed plagioclase ultraphyric magmas to intrude the sedimentary pile and erupt on the sea floor. Prolonged lithospheric extension resulted in more voluminous mantle melting and eruption of picrites and basalts in the south. Primitive magmas in the north were trapped beneath thicker crust and crystallized wehrlite cumulates. Resulting basaltic melts intruded the volcano-sedimentary pile, or erupted as aphyric basalts.

Download Full-text

The genesis of agates and amethyst geodes

The Canadian Mineralogist ◽

10.3749/canmin.1900028 ◽

2019 ◽

Vol 57 (6) ◽

pp. 867-883 ◽

Cited By ~ 2

Author(s):

Ingrid N. Kigai

Keyword(s):

Lava Flow ◽

Volcanic Rocks ◽

Silica Sol ◽

Low Grade ◽

Wide Range ◽

Diluted Solution ◽

Mafic Magmas ◽

Felsic Volcanic ◽

Low Pressures ◽

Felsic Volcanic Rocks

Abstract Practically all aspects of agate genesis generate debate. The time is ripe to clarify the most important enigmas concerning the environments of formation of agates and the related famous amethyst geodes of Brazil and Uruguay. Agates form over a wide range of temperatures, from those of basaltic and andesitic melts (about 1100 °C) down to about 50 °C, and at rather low pressures. Their formation in liquid mafic magmas is indicated by a correlation between (1) the orientation of amygdules and the inclination of onyx banding in them and (2) the attitude of amygdules in the lava flow layers. The correlation arises because lava moves at a different rate close to and far from the upper and lower rims of a flow. The alkaline supercritical fluid fills gas vesicles in lavas and dissolves silica, mainly, from ambient lava or rock to produce a silica sol. If the pressure on the fluid causes percolation of water from amygdules, the sol coagulates on the walls of the vesicle to form a concentric lining. If the pressure in amygdules falls below the maximum osmotic pressure of a sol (about 0.1 MPa for a silica sol), percolation of fluid stops, and coagulation leads to the formation of horizontal onyx banding. Multiple repetitions of precipitation of various gel layers can be caused by overlapping fresh flows upon the cooling older agate-bearing lava flow. In a submarine setting, phase separation of the fluid and the formation of a film of gel between vapor (or diluted solution) and brine stimulate the osmotic processes, which result in growth of hollow membrane tubes and branching moss-like arrays at the bottom of amygdules. Some agates exhibit numerous channels as a result of repeated extrusion of fluid or gel from inner zones to the periphery of amygdules that were compressed under the burden of new flows. Previously, such channels were interpreted to be feeding channels for silica supply in amygdules. Periodic compression of amygdules after percolation of fluid from them requires no additional supply of silica because the volume of the amygdules is reduced in proportion to the loss of fluid. The concentric and horizontal banding and mossy textures of agates from the lithophysae of felsic volcanic rocks were created during active volcanism as well. The agates from dissolution-induced cavities in carbonate rocks and the famous amethyst druses of Brazil and Uruguay formed at the moderate temperatures associated with low-grade burial metamorphism, as indicated by the lack of moss textures and onyx banding.

Download Full-text

Petrological and geodynamic evolution of the Late Cretaceous subduction-related volcanism in the eastern Sakarya Zone, NE Turkey

10.5194/egusphere-egu2020-21241 ◽

2020 ◽

Author(s):

Simge Oğuz Saka ◽

Faruk Aydin ◽

Cüneyt Şen ◽

Abdurrahman Dokuz ◽

Thomas Aiglsperger ◽

...

Keyword(s):

Late Cretaceous ◽

Volcanic Rocks ◽

Geochemical Data ◽

Spinel Lherzolite ◽

Volcanic Event ◽

Geochemical Composition ◽

Basaltic Rocks ◽

Rock Types ◽

Sakarya Zone ◽

Felsic Volcanic

Based on the volcanostratigraphic studies, zircon U-Pb dating and geochemical data, the Late Cretaceous volcanic rocks (LCVs) from the Artvin region in the eastern Sakarya zone (NE Turkey) consist of mafic/basaltic (S1-&#199;atak and S2-&#199;a&#287;layan) and felsic/acidic (S1-K&#305;z&#305;lkaya and S2-Tirebolu) rock types that occurred in two successive stages: (i) first stage (S1: Turonian to Early Santonian) and (ii) second stage (S2: Late Santonian to Campanian). In both stages, the basaltic rocks contain generally calcic plagioclase and lesser augite crystals, whereas the acidic samples commonly contain quartz, sodic plagioclase and K-sanidine phenocrysts. Data from clinopyroexene thermobarometry point to the S2-&#199;a&#287;layan basaltic rocks having crystallised at higher temperatures and under deeper crustal conditions (T = 1128 &#177; 15 oC, P = 6.5 &#177; 0.7 kbar and D = 19.5 &#177; 2.1 km) than those of the S1-&#199;atak rocks (T = 1073 &#177; 11 oC, P = 2.2 &#177; 1.0 kbar, D = 6.6 &#177; 3.0 km).The LCVs show a wide compositional spectrum, ranging from tholeiite to calc-alkaline/shoshonite and are typically represented by a geochemical composition resembling subduction-related arc rocks although the 87Sr/86Sr(i) (0.7044&#8211;0.7071) and &#603;Nd(i) values (-0.63 to +3.47) as well as 206Pb/204Pb(i) (18.07 to 18.56), 207Pb/204Pb(i) (15.57 to 15.62) and 208Pb/204Pb(i) (37.12 to 38.55) ratios show very limited variation. The average &#948;18O isotope values of the S1-K&#305;z&#305;lkaya (5.3 &#177; 0.5&#8240;) and S2-Tirebolu (4.9 &#177; 0.8&#8240;) zircons are quite consistent with average mantle values (5.3 &#177; 0.3&#8240;). The similar isotopic compositions of the studied mafic and felsic volcanic rocks, and the relatively high Mg# values (up to 0.4&#8211;0.51) of the felsic samples indicate a cogenetic origin. The parent magmas of the S1-&#199;atak and S2-&#199;a&#287;layan mafic volcanic rocks were derived from underplated basaltic melts that originated by partial melting of metasomatised spinel lherzolite and spinel-garnet lherzolite, respectively. It is proposed that the compositions of the S1-K&#305;z&#305;lkaya (mainly dacitic) and S2-Tirebolu (rhyolitic to trachytic) felsic rocks were particularly controlled by metasomatised mantle&#8211;crust interaction and MASH zone plus shallow crustal fractionation processes.Our data, together with data from previous studies, suggest that the S1- and S2-mafic and felsic rock types of the LCVs (~95&#8211;75 Ma) are the products of two-stage volcanic event that took place during the northward subduction of the northern Neotethys Ocean.&#160;AcknowledgementThis study was financially supported by Scientific and Technological Research Council of Turkey (TUBITAK) with grant# 112Y365.

Download Full-text

Geochemistry and isotope studies of the metavolcanic rocks of Shimoga greenstone belt, Western Dharwar craton - an effort to deduce the Petrogenesis.

10.5194/egusphere-egu2020-21529 ◽

2020 ◽

Author(s):

Anshuman Giri ◽

Rajagopal Anand

Keyword(s):

Fractional Crystallization ◽

Greenstone Belt ◽

Active Continental Margin ◽

Volcanic Rocks ◽

Crustal Contamination ◽

Dharwar Craton ◽

Metavolcanic Rocks ◽

Depleted Mantle ◽

Felsic Volcanic ◽

Felsic Volcanic Rocks

The archaean greenstone belts, dominated by mafic to felsic volcanic rocks followed by younger granitic intrusions occurs associated with volcano-sedimentary sequences. The Dharwar Super group (2600 to 2900 Ma) of rocks in western Dharwar craton, underlie the older TTG gneisses. The Shimoga greenstone belt (SGB) of WDC constitute the basal polymictic conglomerate along with quartzite, pyroclastic rocks, carbonaceous rocks, greywacke-argillite sequences with a thick pile of mafic and felsic metavolcanic rocks (BADR). These rocks are suffered from greenschist to lower amphibolite grade of metamorphism. The Medur metavolcanic volcanic rocks give an age of 2638 &#177; 66 Ma (1), whereas the Daginakatte felsic volcanic rocks give an age of 2601 &#177; 6 Ma (2). The present studied age of 2638 &#177; 66 Ma, tells about the cessation of mafic magmatism in WDC. The metavolcanic rocks of the Medur formation are tholeiitic to calc-alkaline in nature. These rocks show flat to LREE enriched REE pattern with negative europium anomaly. And also show enrichment in LILE and depletion in HFSE elements with significant Nb-Ta anomaly. The geochemical and the isotope data suggest the involvement of partial melting of the depleted mantle by the slab components and assimilation fractional crystallization (AFC) processes for the magma generation. The SGB metavolcanic rocks have 143Nd/144Nd ratios (0.511150 to .513076) and &#949;Nd values of -3.1 to -5.5 and the negative &#949;Nd values &#160;for the rocks is due to the crustal contamination of the magma in a shallow marine subduction setting. The parental magmas were derived from melting in the mantle wedge fluxed by slab derived fluids and slab components followed by assimilation fractional crystallization (AFC) processes involving continental crust in an active continental margin.<ul><li>(1) Giri et al., 2019. Lithos, 330-331, 177-193</li> <li>(2) Trendall et al., 1997a. J. Geol. Soc. India, 50, 25-50.</li> </ul>

Download Full-text

Geochemistry of the northern Cache Creek terrane and implications for accretionary processes in the Canadian Cordillera

Canadian Journal of Earth Sciences ◽

10.1139/e09-066 ◽

2010 ◽

Vol 47 (1) ◽

pp. 13-34 ◽

Cited By ~ 8

Author(s):

Joseph M. English ◽

Mitchell G. Mihalynuk ◽

Stephen T. Johnston

Keyword(s):

Volcanic Rocks ◽

Minor Component ◽

Oceanic Lithosphere ◽

Ocean Basin ◽

Late Triassic ◽

Canadian Cordillera ◽

Basaltic Rocks ◽

Subducting Plate ◽

A Minor ◽

Collisional Orogenesis

The northern Cache Creek terrane in the Canadian Cordillera includes a subduction complex that records the existence of a late Paleozoic – Mesozoic ocean basin and provides an opportunity to assess accretionary processes that involve the transfer of material from a subducting plate to an upper plate. Lithogeochemical data from basaltic rocks indicate that the northern Cache Creek terrane is dominated by two different petrogenetic components: (1) a dominant suite of subalkaline intrusive and extrusive rocks mostly of arc affinity and (2) a volumetrically less significant suite of alkaline volcanic rocks of within-plate affinity. The subalkaline intrusive and extrusive rocks constitute a section of oceanic lithosphere that is interpreted to have occupied a fore-arc position during the Late Triassic and Early Jurassic before it was accreted during collisional orogenesis in the Middle Jurassic. Alkaline volcanic rocks in the northern Cache Creek terrane are stratigraphically associated with carbonate strata that contain Tethyan fauna that are exotic with respect to the rest of North America; together, they are interpreted as remnants of oceanic seamounts and (or) plateaux. The volcanic rocks are a minor component of the carbonate stratigraphy, and it appears that the majority of the volcanic basement was either subducted completely at the convergent margin or underplated at greater depth in the subduction zone. In summary, accretion in the northern Canadian Cordillera occurred principally by the accretion of island arcs and emplacement of fore-arc ophiolites during collisional orogenesis. The transfer of oceanic sediments and the upper portions of oceanic seamounts from the subducting plate to an accretionary margin accounts for only small volumes of growth of the upper plate.

Download Full-text