Laurentian Margin Research Articles

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Tsakkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in Jamtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and Jamtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.

Late Ordovician to Silurian ensialic magmatism in Liverpool Land, East Greenland: new evidence extending the northeastern branch of the continental Laurentian magmatic arc

AbstractNew U–Pb ID-TIMS geochronological and whole-rock geochemical data from the Hurry Inlet Plutonic Terrane in Liverpool Land provide evidence of a Late Ordovician to Silurian magmatic arc in the East Greenland Caledonides. These voluminous granitoid rocks range from meladiorite to monzonite and granite, they are alkali-calcic to calc-alkaline and magnesian, and have characteristic arc granitoid trace element signatures. Zircon data give ages of 446 ± 2 and 438 ± 4 Ma for two phases of the Hurry Inlet Composite Pluton, 426 ± 1 Ma for a meladioritic xenolith in the anatectic Triaselv granite, and 424 ± 1 Ma for the Hodal-Storefjord Pluton. The Late Ordovician plutons can be correlated with similar plutons in the uppermost nappes of the Scandinavian Caledonides, likely representing the northern branch of magmatic arcs formed on the Laurentian margin. Magmatism appears to have continued sporadically until about 425 Ma when a major, short-lived, magmatic event formed the bulk of the batholith on Liverpool Land. This activity was likely mantle-driven and can be correlated with the Newer Granites in Scotland, for which a slab break-off mechanism has been proposed. The increased heat flow from this process can also explain the generation of the crustally derived, syntectonic, two-mica granites, which are the areally most important Caledonian suite in East Greenland.

Sutton hotspot: Resolving Ediacaran-Cambrian Tectonics and true polar wander for Laurentia

Hotspot tracks represent plate motions relative to mantle sources, and paleomagnetic data from magmatic units along those tracks can quantify motions of those mantle anomalies relative to the Earth's magnetic field and rotational axis. The Ediacaran Period is notable for rapid and large paleomagnetic apparent polar wander (APW) for many continents. Whereas magmatic units attributed to the mantle plume suggest a practically stationary hotspot track, paleolatitudes of Laurentia for that interval vary dramatically; geologic and paleomagnetic data are at odds unless true polar wander (TPW) is invoked to explain a majority of APW. Here we test the plume-TPW hypothesis by generating the predicted Sutton hotspot track for a station- ary plume under a moving plate along the Laurentian margin during the interval from 615 to 530 Ma. Our model is the first to provide a kinematic framework for the extensive large igneous province associated with opening the Iapetus Ocean.

Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands

More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation. Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia?

Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land–Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.

Age constraints and geochemistry of the Ordovician Tyrone Igneous Complex, Northern Ireland: implications for the Grampian orogeny

Abstract: The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c . 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c . 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c . 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus. Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467 .

Provenance and paleoweathering reconstruction of the Neoproterozoic Johnnie Formation, southeastern California

Petrologic and geochemical data confirm that mudstones and sandstones of the Johnnie Formation were the initial siliciclastic deposits laid along the Cordilleran Laurentian margin following the Neoproterozoic break-up of Rodinia. Sedimentary rocks of the Johnnie Formation have corrected CIA values between 63 and 83 (or higher), which suggest moderate to intense weathering of crystalline source rocks or recycling. Based on modeling the fresh source rocks likely consisted of 90% granodiorite and 10% high-K granite. This conclusion is based on petrographic observations, major element geochemistry, and investigation of the REE: (La CN/Sm CN = 4.19 ± 1.26, Gd CN/Yb CN = 1.34 ± 0.38, Eu/Eu* = 0.63 ± 0.09 and La CN/Yb CN = 9.55 ± 2.27). Feldspars are unevenly distributed in the finer grained sedimentary rocks. Observed fluctuations in feldspar content throughout the Johnnie Formation are interpreted as a result of abrasion and hydrodynamic sorting, which concentrated feldspars in the finer grained sediment. None of the mudstone samples, including those collected just below and above the flat-pebble conglomerate in the upper Johnnie Formation, show evidence of true cold weather depositional conditions. Consequently, Johnnie Formation mudstone geochemistry does not record evidence of an extreme paleoclimatic environmental shift in the succession. Textural characteristics of Johnnie Formation sandstones are consistent with quiescent tectonic conditions characterized by low relief, and deposition of Johnnie Formation strata in a passive-margin setting.

Tectonic escape of a crustal fragment during the closure of the Rheic Ocean: U–Pb detrital zircon data from the Late Palaeozoic Pulo do Lobo and South Portuguese zones, southern Iberia

Abstract: The Pulo do Lobo Zone, which crops out immediately north of the allochthonous South Portuguese Zone in southern Iberia, is classically interpreted as a polydeformed accretionary complex developed along the southern margin of the Gondwanan parautochthon (Ossa–Morena Zone), during the late Palaeozoic closure of the Rheic Ocean. This closure was a major event during the amalgamation of Pangaea. U–Pb laser ablation inductively coupled mass spectrometry dating of detrital zircons from late Palaeozoic Devono-Carboniferous clastic units in the South Portuguese Zone and Pulo do Lobo Zone yield contrasting age populations and attest to the exotic nature of both zones. Detrital zircons from the South Portuguese Zone display populations typical of detritus derived from either Gondwana (Ossa–Morena Zone), or peri-Gondwanan terranes. In contrast, rocks from the Pulo do Lobo Zone contain populations consistent with derivation from Baltica, Laurentia or recycled early Silurian deposits along the Laurentian margin. An example of one such deposit is the Southern Uplands terrane of the British Caledonides. Taken together, these data can be reconciled by a model involving tectonic transport of a crustal fragment that was laterally equivalent to the Southern Uplands terrane between the allochthonous South Portuguese Zone and Gondwana as a result of an early Devonian collision between an Iberian indenter with Laurussia. Supplementary material: U–Pb data tables, concordia diagrams, methods and representative back-scattered electron images are available at http://www.geolsoc.org.uk/SUP18441 .

Palaeoproterozoic accretion processes of Australia and comparisons with Laurentia

The Palaeoproterozoic rocks of Australia and Laurentia preserve an excellent record of the accretionary tectonics associated with transitions between the Columbia (Nuna) and Rodinia supercontinents. The geologic records of Australia and Laurentia suggest that the dominant tectonic driver was one or more subduction zones in which several episodes of crustal accretion occurred between ca. 1790 Ma and 1620 Ma. Correlated orogenic events include the ca. 1800–1780 Ma Yapungku–Yambah (Australia)–Medicine Bow (Laurentia) orogenies, ca. 1740–1690 Ma Strangways–Kimban (Australia)–Nimrod (Antarctica)–Yavapai (Laurentia) orogenies, and the ca. 1650–1620 Ma Leibig–Ooldea (Australia)–Mazatzal (Laurentia) orogenies. There are major differences in the style of accretion: Laurentia is characterized by accretion of dominantly juvenile arc terranes, whereas accreted Australian terranes are more evolved and are isotopically similar to the continental nucleus. Adjacent to its plate margin, the Australian continent contained regions of elevated heat production compared with the Laurentian margin. Brace-Goetze lithospheric strength models for ca. 1700 Ma indicate that the Australian plate margin was significantly weaker than that of Laurentia. This variation in lithospheric strength is interpreted to impact the behaviour of the overriding plate during subduction roll back. Attenuation of the Australian lithosphere during ductile extension caused rifting of large continental fragments from the plate margin. Their subsequent accretion resulted in the re-amalgamation of pre-existing continental lithosphere similar to the lithosphere in the overriding plate. Subduction roll back adjacent to the cold and rigid Laurentian margin (e.g. the Wyoming Craton) had relatively little impact on the overriding plate, with oceanic back-arc basins and juvenile arc terranes developing outboard of the plate margin. Inversion of these arc and oceanic back-arc terranes resulted in episodic continental growth.

Obduction of Tethyan-type ophiolites—A case-study from the Thetford-Mines ophiolitic Complex, Quebec Appalachians, Canada

Oceanic plagiogranites and peridotite-hosted granites (PHG) of the Thetford-Mines ophiolite (TMO) yield mean U–Pb zircon ages of 479.2±1.6Ma and 469.5±2.8Ma, and are interpreted to be the products of fractionation of basaltic magma and melting of the Laurentian margin, respectively. The existing 40Ar/39Ar age for the ophiolitic metamorphic sole of the TMO is a hornblende isochron age of 477±5Ma for amphibolite facies rocks. Sites previously sampled for U–Pb and 39Ar–40Ar analyses were resampled for more accurate 39Ar–40Ar dating, in order to better constrain the thermo-tectonic evolution of the ophiolite and underlying Laurentian margin rocks on which they were overthrust, and quantify the time required for obduction of Tethyan-type ophiolites in the Canadian Appalachians. Amphiboles from TMO gabbro and plagiogranite yield 39Ar–40Ar plateau ages of 477–475Ma. In contrast, muscovites from PHG yield cooling ages of 466–465Ma, and show evidence for thermal resetting at ca. 460Ma for both a granitic mylonite and a crosscutting pegmatite facies. The amphibolitic metamorphic sole yields 39Ar–40Ar ages of ca. 471Ma in amphibole, and high-temperature ages of ca. 466Ma in muscovite, with evidence for recrystallization to ages as young as 457Ma. Muscovites from the sub-ophiolitic Laurentian metasedimentary basement, and micaschist clasts in supraophiolitic debris flows of the TMO, yield similar pseudo-plateau and high-temperature ages of ca. 463–464Ma. The U–Pb and 39Ar–40Ar data suggest that the obduction of peri-Laurentian oceanic crust lasted approximately 5 to 10 m.y. Since both facies formed approximatelym.y. after the ophiolitic crust, this supports models whereby the still-hot ophiolitic mantle provided the heat needed to generate the PHG and the metamorphic sole between ca. 475 and 470Ma. Muscovite ages of PHG record cooling below ca. 450–500°C by ca. 465Ma, but 39Ar–40Ar ages from both a mylonitized PHG facies and micaschist clasts suggest that the emplacement of the oceanic lithosphere on the Laurentia margin had nearly ended by ca. 460Ma, likely due to foreland-directed downward thrust propagation and related exhumation of the collisional wedge. The new age data imply that the southern Quebec ophiolites were thrust 100s of km over the Laurentian margin, which has major implications for the location of the subduction suture and the early stages of the Taconic orogeny in the Northern Appalachians.

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200–1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (TDM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613–1634) and smallest epsilon Nd (eNd) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss–Rockport Granite suites (1155–1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

Tectonic evolution of western Amazonia from the assembly of Rodinia to its break-up

Plate tectonic reconstructions of the late Mesoproterozoic–Neoproterozoic supercontinent of Rodinia juxtapose the western margin of Amazonia against eastern Laurentia based on palaeomagnetic, isotopic, and geological evidence. Mesoproterozoic (‘Grenvillian’) orogenic belts are of crucial importance to these reconstructions as they act as key tectonic tracers for Amazonia–Laurentia interactions. They include orogenic belts sited on Amazonia (such as the Sunsás Orogen), para- autochthonous elements such as the Mesoproterozoic metamorphic basement inliers within the Andean Belt (e.g. in the Colombian Andes), exotic terranes accreted to Amazonia during Rodinia assembly (such as the Arequipa Massif), and orphaned fragments of Amazonian basement in Central and North America. A review of the timing of Sunsás orogenesis demonstrates that it occurred from about 1.2–1 Ga in eastern Bolivia and the western Amazon region of Brazil. This is significantly older than the timing of metamorphism in Mesoproterozoic basement inliers of the Colombian Andes, which record a late metamorphic event between 0.9 and 1.0 Ga. Orphaned fragments of Amazonian basement in Laurentia (such as the Blue Ridge/Mars Hill terrane) suggest collision between southeastern Laurentia and Amazonia at ca. 1.15 Ga. The Arequipa Massif (and Antofalla Basement) most likely represents an exotic basement terrane that was caught up in the collision of southeastern Laurentia with western Amazonia. Recent palaeomagnetic data suggest that Amazonia moved northeastwards along the eastern Laurentian margin during Grenvillian collision. Amazonia evidently collided with southern Laurentia at ca. 1200 Ma and, as a result of progressive dextral transcurrent movement, it encountered the Labrador–Greenland sector of eastern Laurentia (and possibly Baltica) by 980 Ma. The timing of the Rodinia break-up in western Amazonia is poorly constrained. Evidence exists supporting a Neoproterozoic western Amazonian active margin, which would imply its at least partial separation from the conjugate rift margin of eastern Laurentia (i.e. formation of a proto-Iapetus Ocean) prior to ca. 650 Ma. This rifting event may be linked to A-type magmatism at ca. 770–690 Ma which is documented in both southeast Laurentia and western Amazonia. Final separation must have been completed by Early Cambrian times based on the unequivocal evidence for drift-related sedimentation on the Laurentian margin of the Iapetus Ocean.

Provenance of detrital zircons from the late Neoproterozoic to Ordovician sandstones of South China: implications for its continental affinity

AbstractThe U–Pb geochronology of 687 detrital zircons from the voluminous Upper Neoproterozoic–Ordovician succession in the Wuyishan Fold Belt of South China reveals a common dominant c. 1200–950 Ma group, indicative of an outboard provenance terrane with a Grenville-age province to the southeast during the late Neoproterozoic–Early Palaeozoic. Compared with coeval samples from the Gondwanan and eastern Laurentian margins, our data show a scarcity of distinctive Gondwanan provenances (c. 650–500 Ma) and reveal some Laurentian signatures. These results argue against the peri-Gondwanan setting for South China during the late Neoproterozoic–Ordovician, instead implying a Laurentian affinity.

Prolonged Paleozoic Magmatism in the East Greenland Caledonides: Some Constraints from U-Pb Ages and Hf Isotopes

New U, Pb, and Hf isotopic data from thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry from intermediate calc-alkaline rocks in the Caledonides of central East Greenland provide new evidence of a prolonged period of Paleozoic subduction-related magmatism. The new data point toward two groups of plutons: one older generation crystallizing at ca. 450 Ma and a younger generation at ca. 423 Ma. There is additionally a titanite-forming event at ca. 440 Ma. The Hf isotopes of the younger group have a larger variation and less juvenile character than those of the older group, but data indicate a mixed source with both crustal and mantle components for both groups. These new data combined with already published geochronological data testify to a prolonged and complex magmatic history starting markedly earlier in the southernmost segment of the East Greenland Caledonides than farther north. The varied geochemical compositions of the plutons and the prolonged intrusive activity are in contrast to the short-lived homogeneous S-type leucogranitic magmatism found elsewhere in the East Greenland Caledonides and bear more similarities to arc-related terranes now found in the Irish, Scottish, and Scandinavian Caledonides. It is speculated that the earlier plutons in East Greenland represent the northernmost expression of continental arcs formed in a west-directed subduction system along the full length of the Laurentian margin in the Paleozoic. The later plutons formed concomitant with leucogranite intrusions within a collisional setting.

Detrital zircon U–Pb ages of Silurian–Devonian sediments from NW Svalbard: a fragment of Avalonia and Laurentia?

Abstract: Detrital zircon populations from Silurian–Devonian clastic rocks of NW Svalbard were analysed by U–Pb laser ablation inductively coupled plasma mass spectrometry to investigate the pre-Caledonian provenance of Svalbard's Northwestern Terrane. Changes in the resulting age spectra suggest a major shift in sources from the Laurentian–Avalonian suture in the latest Silurian to the local metasedimentary basement of the Northwestern Terrane in the Late Silurian–Early Devonian, and in the Lochkovian to Grenvillian–Sveconorwegian sources. These data, together with structural, additional geochronological and metamorphic data from Svalbard, East Greenland and Avalonia, support the amalgamation of Svalbard as the result of long-distance transport along sinistral strike-slip faults. A unifying model for the final amalgamation of Svalbard, consistent with the stratigraphical and tectonothermal history of Svalbard, involves fragments from the Grampian orogen and Avalonian crust originally accreted to the Laurentian margin being subsequently transported northward along sinistral strike-slip faults during Scandian deformation. Supplementary material: The sample preparation procedures, laser ablation ICP-MS analytical methods, and data table are available at http://www.geolsoc.org.uk/SUP18421 .

A revision of the Ordovician stratigraphy for the western Ireland Caledonides and implications for tectonic models

AbstractThe western Ireland Ordovician stratigraphy has been previously used to constrain the timing of docking of an island arc and its fore‐arc basin with the margin of Laurentia for the British and Irish Caledonides. New field relationships and age data indicate that one of the key formations, the Rosroe Formation (459.2 ± 0.8 and 465.1 ± 2.1 Ma), and its supposed lateral equivalent, the Maumtrasna Formation are younger than previously interpreted. New age data for a tuff band in the Maumtrasna Formation (468.9 ± 1.3 Ma) also support previous studies showing it can be correlated to the adjacent Mweelrea Formation. The new field evidence, age data and geochemistry contradict some previous studies and show that the Maumtrasna, Rosroe and Derrylea formations can no longer be considered lateral equivalents. Based on the new stratigraphy a revised tectonic model is required with sedimentation in this part of the Caledonides taking place in a fore‐arc basin outboard of a continental arc and the oceanic arc was an along‐strike equivalent of this arc situated in an embayment of the Laurentian margin. Copyright © 2010 John Wiley & Sons, Ltd.

Late Ordovician (Sandbian) brachiopods from the Mweelrea Formation, South Mayo, western Ireland: stratigraphic and tectonic implications

AbstractA Late Ordovician (Sandbian), Scoto‐Appalachian brachiopod fauna from the Mweelrea Formation in western Ireland confirms a location for the South Mayo Trough adjacent to the Laurentian margin, characterized at this time by a succession of marine excursions over fluviatile environments. The new, younger biostratigraphical data help to constrain the timing of late Grampian folding of this part of the South Mayo Trough. Copyright © 2010 John Wiley & Sons, Ltd.

The geochemistry of mafic gneisses from the Renzy terrane, western Grenville Province, Quebec: Implications for the geodynamic setting of the early Mesoproterozoic Laurentian margin

This study presents new geochemical data on mafic gneisses from the Renzy terrane (RT), located in the Grenvillian Parautochthonous Belt of western Quebec. This region is of particular interest since it contains one of the rare Ni–Cu–Co massive sulfide deposits ever mined in the Grenville Province. RT mafic gneisses have been divided into 3 groups based mainly on major element compositions, trace element profiles normalized to the primitive mantle, and incompatible trace element ratios. The behaviour of high field strength elements (HFSE) (Ta, Zr, Hf, Ti, Th) and large ion lithophile elements (LILE) (Ba, Rb) are particularly useful for discriminating RT mafic gneisses. Major and trace element compositions of group 1 mafic/ultramafic gneisses are very similar to those of the RT ultramafic sheets; the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets. The trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source. These rocks have high Th/Ta ratios and show strong to moderate negative anomalies in Ti, Zr, and Hf on multi-element plots normalized to the primitive mantle. Mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings. These rocks generally have low concentrations of HFSE and high concentrations of LILE, typical features of mafic rocks formed in a compressional environment; however, some of them are Fe–Ti rich, with minor or no negative anomalies in HFSE and low to moderate concentrations of LILE on multi-element diagrams normalized to primitive mantle, features more typical of an extensional environment. Mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt (OIB). These gneisses have no negative anomalies in HFSE and have low concentrations of LILE. RT mafic gneisses are similar in composition to mafic granulites in the Bondy gneiss complex in the Central Metasedimentary Belt and to Shawanaga Domain amphibolites from the Central Gneiss Belt, two paleoarc/back-arc environments in the Grenville Province. Based on the contrasting geochemical signatures found in the RT mafic gneisses and the similarities with known arc/back-arc settings in the Grenville Province, we propose that the RT was formed by arc/back-arc magmatic activity most likely associated with a period of arc magmatism along the Proterozoic Laurentian margin. OIB-like mafic gneisses found in the RT are similar to those from the Kipawa region, formed by intraplate continental magmatism in the Parautochthonous Belt of the Grenville Province.

Comment on “Detrital U–Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides” by Peter D. Clift, Andrew Carter, Amy E. Draut, Hoang Van Long, David M. Chew, Hans A. Schouten, Tectonophysics 479 (2009), 165–174 (doi:10.1016/j.tecto.2008.07.018)

Clift et al. [Clift, P.D., Carter, A., Draut, A.E., Van Long, H., Chew, D.M. & Schouten, H.A., 2009.Detrital U–Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides. Tectonophysics, 479, 165–174] present U–Pb detrital zircon age data for one sample from each of three formations of the Ordovician South Mayo Trough sedimentary sequence and interpret these in the context of a ‘Taiwan model’ of arc-continent collision on the Laurentian margin of Iapetus. Their interpretation relies on an overly simplistic treatment of their data and does not take cognisance of other data relevant to the relative time of deposition of the sampled horizons within the orogenic history and the evolving availability of sediment source rocks.

New insights on the 1.7–1.0 Ga crustal evolution of the central Grenville Province from the Manicouagan – Baie Comeau transect

Integrated field relationships and U–Pb geochronology of rocks exposed along the southern part of the Manicouagan reservoir (central Grenville Province, Quebec) provide insights into protolith formation and thermal events that range in age from ca. 1.7 to 0.9 Ga and share common features with the rest of the Grenville Province and beyond. From presently lower to higher structural levels, main findings include the recognition of: (a) ca. 1.70–1.75 Ga granitic magmatism and metamorphism in the southern part of the parautochthonous Gagnon terrane, correlative in age with late stages of magmatism in the Makkovik Orogen to the northeast; (b) a ca. 1.69 gabbro-anorthosite suite (Island domain) which is inferred to represent the southwesternmost exposure of Labradorian crust in the central Grenville Province; (c) a 1.4 Ga volcaniclastic sequence and associated plutonic rocks (Canyon domain), which based on lithological association and age, is inferred to represent the NNE extension of the Montauban island arc; and (d) a ca. 1.2 Ga bimodal felsic-mafic volcanic to intermediate volcaniclastic sequence (Banded complex), inferred to have originated in an intra-continental rift system, contemporaneously with the Composite Arc Belt (southwestern Grenville Province) and with widespread extensional magmatism elsewhere in the Canadian Shield. In addition, 1.3 Ga granitoid plutons are documented to intrude Labradorian units at the NW boundary of Canyon domain. The significance of these plutons is ambiguous, as they may represent either stitching plutons, marking the accretion of the 1.4 Ga Canyon domain to the Laurentian margin, or the first stage in the development of the ca. 1.2 Ga continental rift system. The hinterland units (Island domain, Canyon domain and Banded complex) were metamorphosed under medium-pressure granulite facies conditions during the culmination of the Grenvillian orogeny between 1.08 and 1.04 Ga. This metamorphism was accompanied by ca. 1.07 Ga A-type granitic magmatism in Canyon domain. In addition, locally abundant ca. 0.98 Ga late-tectonic felsic pegmatite and ultrapotassic dykes occur in Canyon domain. This event is coeval with titanite ages documented in a wide range of units. The data on the Grenvillian evolution suggest mantle-derived magmatism and associated metamorphism in the hinterland during both the culmination and the waning stages of the Grenvillian orogeny.