Volcanic Arc Complex Research Articles

The Carboniferous Arc of the North Pamir

Abstract In this study, we investigate the age and geochemical variability of volcanic arc rocks found in the Chinese, Kyrgyz, and Tajik North Pamir in Central Asia. New geochemical and geochronological data together with compiled data from the literature give a holistic view of an early to mid-Carboniferous intraoceanic arc preserved in the northeastern Pamir. This North Pamir volcanic arc complex involves continental slivers in its western reaches and transforms into a Cordilleran-style collision zone with arc-magmatic rocks. These are hosted in part by Devonian to Carboniferous oceanic crust and the metamorphic Kurguvad basement block of Ediacaran age (maximum deposition age) in Tajikistan. We discuss whether a sliver of Carboniferous subduction-related basalts and intruded tonalites close to the Chinese town of Mazar was part of the same arc. LA-ICP-MS U-Pb dating of zircons, together with whole rock geochemistry derived from tonalitic to granodioritic intrusions, reveals a major Visean to Bashkirian intrusive phase between 340 and 320 Ma ago. This clearly postdates Paleozoic arc-magmatic activity in the West Kunlun by ~100 Ma. This observation, along with geochemical evidence for a more pronounced mantle component in the Carboniferous arc-magmatic rocks of the North Pamir, disagrees with the common model of a continuous Kunlun belt from the West Kunlun into the North Pamir. Moreover, Paleozoic oceanic units younger than and west of the Tarim cratonic crust challenge the idea of a continuous cratonic Tarim-Tajik continent beneath the Pamir.

The closure of the Vardar Ocean (the western domain of the northern Neotethys) from the early Middle Jurassic to the Paleocene time, based on the surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central Hellenides.

Seismic tomographic images of the mantle below the Hellenides indicate that the Vardar Ocean probably had a composite width of over 3000 km. From surface geology we know that this ocean was initially located between two passive margins: Pelagonian Adria in the west and Serbo-Macedonian-Eurasia in the east. Pelagonia was covered by a carbonate platform that accumulated, during Late Triassic to Early Cretaceous time, where highly diversified carbonate sedimentary environments evolved and reacted to the adjacent, converging Vardar Ocean plate. We conceive that on the east side of the Vardar Ocean, a Cretaceous carbonate platform evolved from the Aptian to the Maastrichtian time in the forearc basin of the Vardar supra-subduction volcanic arc complex. The closure of the Vardar Ocean occurred in one episode of ophiolite obduction and in two episodes of intra-oceanic subduction. 1. During the Middle Jurassic time a 1200-km slab of west Vardar lithosphere subducted beneath the supra-subduction, 'Eohellenic', arc, while a 200-km-wide slab obducted onto Pelagonia between the Callovian and Valanginian times. 2. During the Late Jurassic through to the Cretaceous time a 1700-km-wide slab subducted beneath the evolving east Vardar-zone arc-complex. Pelagonia, the trailing edge of the subducting east-Vardar Ocean slab, crashed and underthrust the Vardar arc complex during the Paleocene time and ultimately crashed with Serbo-Macedonia. Since the late Early Jurassic time, the Hellenides have moved about 3000 km toward the northeast while the Atlantic Ocean spread.

Ordovician–early Silurian intrusive rocks in the northwest part of the Upper Allochthon, mid-Norway: Plutons of an Iapetan volcanic arc complex

Ordovician–early Silurian intrusive rocks in the northwest part of the Upper Allochthon, mid-Norway: Plutons of an Iapetan volcanic arc complex

Detrital zircon U–Pb ages of the Palaeozoic Natal Group and Msikaba Formation, Kwazulu-Natal, South Africa: provenance areas in context of Gondwana

AbstractThe Natal Group and Msikaba Formation remain relatively poorly understood with regards to their provenance and relative age of deposition; a much-needed geochronological study of the detrital zircons from these two units was therefore undertaken. Five samples of the Durban and Mariannhill Formations (Natal Group) and the Msikaba Formation (Cape Supergroup) were obtained. A total of 882 concordant U–Pb ages of detrital zircon populations from these units were determined by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Major Neoproterozoic and secondary Mesoproterozoic detrital zircon age populations are present in the detrital zircon content of all the samples. Smaller contributions from Archean-, Palaeoproterozoic-, Cambrian- and Ordovician-aged grains are also present. Due to the presence of a prominent major population of 800–1000 Ma zircons in all the samples, late Stenian – Tonian ancient volcanic arc complexes overprinted by Pan-African metamorphism of Mozambique, Malawi and Zambia, along with areas of similar age within Antarctica, India and Sri Lanka, are suggested as major sources of detritus. The Namaqua–Natal Metamorphic Complex is suggested as a possible source of minor late Mesoproterozoic-aged detritus. Minor populations of Archean and Palaeoproterozoic zircons were likely sourced from the Kaapvaal and Grunehogna Cratons. Post-orogenic Cambrian – Lower Ordovician granitoids of the Mozambique Belt (Mozambique) and the Maud Belt (Antarctica) made lesser contributions. In view of the apparent broad similarity of source areas for the Natal Group and Msikaba Formation, their sedimentation occurred in parts of the same large and evolving basin rather than localized in small continental basins, and the current exposures merely represent small erosional relicts.

Uncoupled O and Hf isotopic systems in zircon from the contrasting granite suites of the New England Orogen, eastern Australia: Implications for studies of Phanerozoic magma genesis

The Permo-Triassic granites of the New England Orogen, eastern Australia, were emplaced into a volcanic arc complex accreted to the eastern Gondwana margin in the Late Devonian or Early Carboniferous. Zircon U–Pb dating shows that the S-type Hillgrove (∼297Ma) and Bundarra (∼287Ma) Supersuites predated intrusion of the I-type Moonbi Supersuite (∼250Ma) by up to 50Ma. The high δ18Ozrn of the S-type granites (10.0–11.5‰), and range of U–Pb ages (∼370–300Ma) and δ18Ozrn (∼5–10‰) of their inherited zircon cores, show that their source rocks were predominantly weathered Carboniferous volcaniclastics, the youngest deposited<25Ma before the granites were emplaced. In contrast, the lower δ18Ozrn (6.9–7.8‰) and lack of inheritance in the I-type granites is consistent with a zircon poor, more juvenile source, probably a mafic igneous underplate mixed with a small amount of volcanogenic and/or oceanic sediment. Despite the differences in source materials, the εHf(t) values of all granites, both S- and I-type, are similar (+5.0±0.5 cf. +5.9±0.5), consistent with both the mafic and sedimentary components in the granite sources being relatively young and similar in Hf isotopic composition at the time of granite genesis. In young, isotopically juvenile orogens, the O isotopic composition of well-dated igneous and inherited zircon can be a much more sensitive indicator of petrogenetic processes than the zircon Hf isotopic compositions alone.

The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia

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 1599Ma. Four U–Pb zircon ages on the Wernecke igneous clasts (1714–1706Ma) demonstrate that the clasts are older than the Wernecke Supergroup (<1.64Ga) 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 2080–2760Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly built 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 obduction-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.

Source rock potential of Lower to Middle Jurassic black clastic sequences of the Intermontane belt1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

Lower to Middle Jurassic clastic sequences are widespread within the interior of the Canadian Cordillera. These successions cap waning Jurassic volcanism in many Cordilleran arc terranes and are succeeded by clastic sequences of the Intermontane basins. Fine-grained, carbonaceous lithologies, which locally contain elevated levels of organic carbon, characterize these clastic successions. These include sections of the Spatsizi Formation (Abou Member) and Smithers Formation in northern and western Bowser basin, respectively, Ashcroft Formation and equivalent strata in central Quesnellia, and Last Creek Formation and Junction Creek rocks below the Tyaughton–Methow basin. These rocks locally contain metre-thick sections with total organic carbon (TOC) levels &gt;5 wt.% and others have thicknesses approaching 100 m with TOC between 3 and 5 wt.%. Thermal maturation levels are high in many sections, suggesting original organic contents were greater and that these rocks may have been excellent source beds. Associated bitumen in these successions, together with Mesozoic oil in some overlying Intermontane clastic rocks, also suggests these sequences may have been effective oil source rocks. TOC levels, thermal maturity, and thickness of some sections are comparable with shale gas sequences being exploited elsewhere in North America. TOC concentrations within these rocks, together with other elemental abundances, indicate anoxic conditions during deposition. The age of these clastic rocks brackets the global Toarcian anoxic event and that of other organic-rich sequences in North America. Elemental abundances suggest predominantly volcanic-arc complexes as source terranes, although continental signatures are suggested by rocks in western Quesnellia.

Zircon age constraints on the provenance of Llandovery to Wenlock sandstones from the Midland Valley terrane of the Scottish Caledonides

Synopsis Detrital zircon populations within the Llandovery to Wenlock sandstones of the southern Midland Valley of Scotland indicate that the recycled orogenic provenance for these sedimentary rocks was essentially bimodal, comprising a younger Lower Palaeozoic component and an older predominantly Mesoproterozoic component. The Lower Palaeozoic contribution is dominated by Arenig/Llanvirn ( c . 475 Ma) zircons interpreted as having been derived from a volcanic-plutonic source located within the Midland Valley terrane. The dominant Mesoproterozoic component within the sandstones is c . 1000 Ma and is thought to represent detritus shed from a Grenvillian ( c . 1000–1800Ma) basement to the Midland Valley terrane. The scarcity of Archaean zircons precludes the Grampian metamorphic terrane Dalradian Supergroup as a supplier of sediment to the Ordovician–Silurian basins located along the southern margin of the Midland Valley. The age profiles of detrital zircon populations do not fit with a simple model of unroofing of a volcanic-arc complex. Rather they point to the periodic uplift of fault-bound, dismembered blocks of volcanic and plutonic rocks during a prolonged (Llandovery through to at least early Devonian) period of sinistral strike-slip deformation, and it was this which controlled basin development, sedimentary facies distribution and deformation along the southern side of the Midland Valley terrane. Appendices 1 &amp; 2 can be found at http://www.geolsoc.org.uk/SUP18370

Deposition and taphonomy of the Hound Island Late Triassic vertebrate fauna: Fossil preservation within subaqueous gravity flows

Late Triassic marine vertebrates occur in the Hound Island Volcanics exposed on the eastern shore of Hound Island, southeastern Alaska. This fossil assemblage derived from slope deposits within an intra-arc basin of a volcanic island arc complex. It is associated with the allochthonous Alexander Terrane prior to its northern translation from tropical latitudes to its current latitude. Fossils occur within a 13-m-thick succession of interbedded calcareous shale and volcaniclastic-rich bioclastic limestone. The limestone layers are skeletal (vertebrate and invertebrate) packstones to wackestones and are interpreted as episodic thin-bedded turbidite and debris flow deposits that resulted from redeposition of slope sediments. Forty-seven vertebrate fossils were collected that consist of complete and fragmented cranial and postcranial elements belonging to Eosauropterygia, Thalattosauria, and Ichthyosauria, including the genus Shonisaurus. Bones are frequently disassociated and show predepositional abrasion and breakage. Another less common taphonomic mode is observed, characterized by several large associated elements, representing a single individual, found parallel to bedding and draped by shale. Rare Osteichthyes and Chondrichthyes teeth are also present. Bivalve and conodont fossils indicate a middle Norian age for the assemblage.

Diversity of the Palaeoproterozoic granitoids of the Kédougou inlier (eastern Sénégal): Petrographical and geochemical constraints

Field, petrographical and geochemical studies of the Palaeoproterozoic granitoids of the Kédougou inlier (Eastern Sénégal) allow to define four compositional types: (i) the Layered Plutonic Complex consisting of sodic calc-alkaline granitoids defining a trend from Opx-, Cpx- and amphibole-bearing layered gabbros to amphibole-bearing tonalite; the latter shows similarities with adakite-like rocks; (ii) amphibole-bearing granitoids with TTG features occurring as composite batholiths and as isolated oval-shaped plutons cross cutting the entire Birimian metavolcanic and metasedimentary formations; (iii) potassic granitoids defining a trend from Opx-, Cpx-bearing gabbros to amphibole-bearing monzodiorite and preferentially emplaced within the calc-alkaline metavolcanites; (iv) peraluminous biotite-bearing granitoids emplaced within the metasediments of the upper part of the Birimian lithostratigraphical pile. The granitoid suites were emplaced within a time interval of about 80 Ma from about 2160 Ma (emplacement of the LPC) to 2080 Ma (emplacement of the youngest plutons such as the amphibole-bearing TTG granitoid or the pyroxene- and K-feldspar-bearing granitoids). The granitoid suites were emplaced in an immature arc environment [Dia, A., Van Schmus, W.R., Kröner, A., 1997. Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou inlier, eastern Senegal, West Africa. J. Afr. Earth Sci. 24 (3), 197–213]. Parental magmas were probably derived from the partial melting of a mantle wedge above a subduction zone. Tonalite magmas of distinctive composition were probably produced by partial melting of the subducting slab itself. They could characterize the early stage of the slab-melting, precursor of the TTG magmatism. The close association of calc-alkaline, trondhjemitic and peraluminous granitoids and the successive emplacement of granitoid groups more and more enriched in K 2O recognized in the Kédougou inlier is typical of the Paleoproterozoic granitoid series from West Africa.

Neodymium isotopic composition of Cambrian–Ordovician biogenic apatite in the Baltoscandian Basin: implications for palaeogeographical evolution and patterns of biodiversity

Biogenic apatite preserved in 148 samples of conodonts and organophosphatic-shelled brachiopods from Cambrian through Ordovician successions of the Baltoscandian Basin (Baltica Plate) preserves a sensitive record of early Palaeozoic sea-water chemistry interpreted via neodymium isotope ratios. Consistent ${\uvarepsilon}$Nd(t) values of −9.6 to −8.3 for Lower to Middle Cambrian samples suggest no significant lateral or temporal variation across the region. Average Upper Cambrian values are −7.2 to −7.7. Sedimentary analysis suggests that the influence of continental weathering from Baltica as a major source of radiogenic Nd was negligible. Ordovician samples show a rise to −5 to −6 in the early Arenig, early–mid Llanvirn and late Caradoc. Sea-water mixing from the southeast Iapetus Ocean was a constant factor throughout Cambrian–Ordovician times. The rise reflects erosion of obducted volcanic arc complexes along the Caledonian margin, and probably also relates to pollution of the Baltica sector of Iapetus from the approaching Avalonia Plate. Patterns of evolutionary biodiversity and palaeobiogeographical linkages support the geochemical signatures in interpreting the tectonic history of the region. Extinction of lingulate brachiopod faunas in the Tremadoc, followed by subsequent recovery and emergence of benthic assemblages typical of the Ordovician Evolutionary Fauna in the Billingen–early Volkhov regional stages coincide with significant changes in geochemical characteristics of water masses across the Baltoscandian basin. The early and mid Ordovician (Arenig to Llandeilo) brachiopod faunas of the North Estonian Confacies Belt are characterized by high endemism and low turnover rates, whereas increased immigration resulted in the extinction of a number of local lineages in the late Llanvirn. From the mid Caradoc to mid Ashgill, when Baltica was drifting on a course to collide eventually with Avalonia and gradually approach Laurentia, brachiopod assemblages were characterized by higher turnover rates. At the same time they gradually became more cosmopolitan and less influenced by the invasion of new faunas.

Introduction to special issue of Canadian Journal of Earth Sciences: The Trans-Hudson Orogen Transect of Lithoprobe

Dedication: Dr. John F. Lewry (1939–1999; see Saskatchewan Geological Survey 1999) dedicated his career to investigations of the Saskatchewan–Manitoba segment of the Trans-Hudson Orogen (THO), one of the principal Paleoproterozoic orogens associated with the assembly of Laurentia. Indeed, one can make a strong case that Lithoprobe's Trans-Hudson Orogen Transect (THOT) was designed to test the tectonic models proposed by John Lewry. He delineated the distinct tectonic provinces in the western part of the THO, predicted the presence of an Archean craton trapped within the THO, and recognized and interpreted the significance of the Pelican Thrust between the juvenile Paleoproterozoic volcanic arc complex of the western Flin Flon Domain and the Archean craton, now called the Sask craton. The research published in Lewry and Stauffer (1990), and many of his ideas, provided the framework for the design of the THOT geophysical and geological studies. John Lewry was co-leader of the THOT until he passed away in 1999 after a battle with cancer. This Special Issue of the Canadian Journal of Earth Sciences is dedicated to him.

Stratigraphic and geochemical evolution of an oceanic arc upper crustal section: The Jurassic Talkeetna Volcanic Formation, south-central Alaska

The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tuffs, and volcaniclastic debris-fl ow and turbidite deposits, showing signifi cant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals ~7 km in thickness, similar to modern western Pacifi c arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calc-alkaline and shows no clear trend to increasing SiO 2 up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in meltincompatible versus compatible high fi eld strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy REE-depleted mafi c lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a northdipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America.

Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.

Willsher Group and geology of the Triassic Kaka Point coastal section, south‐east Otago, New Zealand

Willsher Group (new) and component formations in the Kaka Point Structural Belt are defined and described: Karoro Formation (Nelsonian, late Early Triassic), Potiki Siltstone (?Malakovian, late Early to early Middle Triassic), Bates Siltstone, Kaka Point Volcanic Sandstone, Tilson Siltstone, and Steney Voe Formation (Etalian, Middle Triassic), Pilot Point Sandstone (Etalian or Kaihikuan), Short Bay Formation (late Etalian or Kaihikuan), Port Molyneux Siltstone and Waituti Siltstone (Kaihikuan, latest Middle to early Late Triassic). Sparse body fossils include ammonoids, bivalves, gastropods, brachiopods, and crustacean, crinoid, and rare bone fragments. Bentonitic and zeolitised ash beds are copiously developed in thick sequences of laminated siltstones with subordinate mostly highly felsic volcaniclastic sandstones; a laharic origin is suggested for some. Sedimentary structures indicate current directions and slope directions from an easterly quarter. Analcime‐quartz and albite are progressively advanced dehydration products of heulandite and clinoptilolite replacement of glass with accompanying movement of mobile elements. Some analcime has formed without heulandite precursor. Differences from coeval rocks in the Southland Syncline of the Murihiku Terrane in inferred source directions, sedimentary facies, very low‐grade metamorphism, and geochemistry imply distinct provenances and tectonic histories, although sources were probably parts of the same volcanic arc complex. The Kaka Point Structural Belt is fault‐bounded and was displaced relative to the Southland Syncline sequence during accretion or later. Its relationship to the Dun Mountain‐Maitai Terrane is at present unclear. The belt may be regarded as a displaced splinter of either the Murihiku Terrane or the Dun Mountain‐Maitai Terrane, or it may be a fragment of a separate but related suspect terrane.

Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa

The Palaeoproterozoic Kedougou basement in the Mako area in eastern Senegal consists of four major rock complexes: 1) high-grade amphibolitic and gneissic rocks occuring as large xenoliths (the Sandikounda amphibolite-gneiss complex); 2) voluminous bimodal volcanic rocks (the Mako volcanic-plutonic complex); 3) the Sandikounda layered plutonic complex; and 4) trondhjemitic to calc-alkaline intrusive granitoids (the Laminia-Kaourou plutonic complex). Zircons from xenoliths of the Sandikounda amphibolite-gneiss complex yielded ages of ∼2200 Ma, which are slightly older than those of the Mako Supergroup volcanic rocks. These xenoliths also have ɛNd(t) values ranging from +2.4 to +4.1. The volcanic sequence represents an important bimodal volcanic event with Mg-tholeiites and subordinate felsic calc-alkaline lavas having ɛNd(t) values ranging from +2.6 to 3.8. The plutonic intrusions yield ages between 2158 and 2079 Ma with positive ɛNd(t) values of +2.6 to +4.7. Isotopic evidence indicates that the Mako Palaeoproterozoic crust is a juvenile, arc-related terrain derived from a depleted mantle source, emplaced between 2300 and 2079 Ma and subsequently accreted to the growing West African Craton. An early magmatic event, represented by the protolith of the amphibolite and the gneiss, is inferred to represent the initial stage of this Palaeoproterozoic crustal growth and may represent the root of the Mako volcanic-plutonic complex. Thus, the Birimian-Eburnian development of the western part of the Kedougou Inlier is constrained to the interval 2300-2079 Ma; this is slightly older than other parts of the Birimian-Eburnian domains elsewhere in western Africa.

Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada

The Lac des Iles area is located in the Wabigoon Subprovince, an Archean granite-greenstone terrain in the Superior Province in Ontario, Canada. The area consists of several unmetamorphosed plutons of biotite- and hornblende-tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss. The mafic/ultramafic bodies are concentric in shape and consist of periodotite, pyroxenite, gabbro and hornblende gabbro cumulates and diorite. The largest intrusion, the Lac des Iles complex, consists of several mafic and ultramafic centers. An internal SmNd isochron from the southern ultramafic complex gives an intrusion age of 2736 ± 28 Ma (MSWD = 1.5; ϵ Nd(t) = 1.2). This age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons. The tonalitic gneiss represents older continental crust and had an ϵ Nd value of −1 at this time. The ϵ Nd(t) values of the felsic and mafic/ultramafic suites are positive, ranging from 0 to +2. This range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust, such as the tonalite gneiss, during their ascent. This implies that the tonalite magmas in the Lac des Iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere. The morphology and the rock assemblages of the mafic and ultramafic intrusions in the Lac des Iles area resemble Alaskan-type intrusions which are common components of Phanerozoic magmatic arcs associated with subduction zones. In addition, the crystallization sequence in the mafic/ultramafic intrusions suggests that they are derived from a parental magma with high-Al basalt affinities. The mafic/ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements, and the tonalites belong to calc-alkaline rock series displaying fractionation trends towards trondhjemitic compositions. Thus, the Archean Lac des Iles area and the root zones of Phanerozoic active continental margins share many geological, geochemical and petrological features which would suggest that the Lac des Iles area formed in an Andean-type continental margin.

Crustal structure of the Middle Urals based on reprocessing of Russian seismic reflection data

SUMMARY Reprocessed Russian seismic reflection data, combined with existing Russian geological maps and seismic interpretations, provide the basis for a revised tectonic interpretation of the upper crust in the Middle Urals. The Main Uralian Fault (MUF)—the principal suture of the Urals separating the East European Craton to the west from accreted terranes to the east—is found to penetrate to depths of at least 18 km, and perhaps much deeper, with a uniform eastward dip of 35° (after migration). As the MUF appears to truncate reflections in both the foreland and hinterland of the Urals, it is arguably one of the youngest structural features of the orogen. Basement rocks of the East European Platform underthrust the suture and comprise the deep crust a minimum of 30 km east of the surface expression of the MUF. The Tagil volcanic arc complex, greater than 1000 km in length along strike, comprises at least the upper 10 km of the crust east of the MUF. This terrane shallows eastwards, where it appears to be truncated by the west-dipping Serov-Mauk fault based on correlation with surface geology. The hinterland of the Urals, east of the MUF, is characterized by a previously unrecognized zone of discontinuous mid-crustal reflectivity at depths of 14–16 km that is discordant with upper crustal reflections. This mid-crustal zone of reflectivity is interpreted to represent either a major subhorizontal structural detachment or a zone of magmatic intrusion.

Conglomeratic fan deltas and submarine fans of the Jurassic Laberge Group, Whitehorse Trough, Yukon Territory, Canada: fore-arc sedimentation and unroofing of a volcanic island arc complex

The Lower and Middle Jurassic Laberge Group consists of conglomerates, sandstone turbidites and shales that were deposited within a deep-water fore-arc trough. The conglomerate of the Laberge Group accounts for most of the preserved basin-fill within the Whitehorse Trough, and was derived from exhumed plutons, comprising multiple intrusives and extrusives of varying composition, that formed an island arc terrane in the western margin of the Whitehorse Trough. The island arc complex docked against North America in the Late Triassic and the Laberge conglomerates record the sedimentary and unroofing processes that predominated in the intervening trough during arc-continent collision and accretion. Coarse-grained sediment shed from the arc constructed fan deltas along the western margin of the basin. These deposits were resedimented into deep water (below wave base) through shelf-break chutes and submarine channels. Gravel was emplaced within submarine channel-and-fan complexes; sandy turbidites formed lobes along the margins of the gravelly submarine fans and muds were deposited in inter-fan settings. Stratigraphic change within the Laberge Group reflect: (1) collision of the arc, with plutonism and resultant uplift, and subsequent unroofing of the arc highlands; and (2) overlap of coarse-grained deep-water fans across the shelf as a response of crustal flexure to collisional tectonism and/or a eustatic sea-level change. Later deposition of the overlying Tantalus Formation records a diachronous overlap of successor paralic-alluvial deposits over the Laberge sediments associated with final-stage accretion events in the Whitehorse Trough.

The concealed Caledonide basement of Eastern England and the southern North Sea — A review

Field mapping, analysis of borehole core and studies of geophysical potential field and seismic data can be used to demonstrate the existence of a number of distinct crustal blocks within Eastern Avalonia beneath eastern England and the southern North Sea. At the core of these blocks is the Midlands Microcraton which is flanked by Ordovician volcanic arc complexes exposed in Wales and the Lake District. A possible volcanic arc complex of comparable age in eastern England is concealed by late Palaeozoic and Mesozoic cover. These volcanic arc complexes resulted from subduction of oceanic lithosphere beneath Avalonia prior to collision with Baltica and Laurentia in late Ordovician and Silurian time, respectively. The nature of the crust north and east of the concealed Caledonides of Eastern England and south of the lapetus Suture/Tornquist Sea Suture, which forms the basement to the southern North Sea, is unclear. Late Ordovician metamorphic ages from cores penetrating deformed metasedimentary rocks on the Mid-North Sea High suggest these rocks were involved in Avalonia-Baltica collision before final closure of the lapetus Ocean between Laurentia and Avalonia.