Dunnage Zone Research Articles

Volcanogenic massive sulfide (VMS) deposits of the Dunnage Zone of the Newfoundland Appalachians: setting, styles, key advances, and future research

The Dunnage Zone of the Newfoundland Appalachians hosts diverse Cambrian–Ordovician volcanogenic massive sulfide (VMS) deposits. The peri-Laurentian Notre Dame Subzone contains Cu–Zn–Au mafic and bimodal mafic deposits in ∼501–485 Ma ophiolitic rocks and Zn–Pb–Cu–(Au–Ag) deposits in ∼471–465 Ma bimodal rifted continental arc sequences (e.g., Buchans). The peri-Gondwanan rocks of the Exploits Subzone host Zn–Pb–Cu–(Au–Ag) bimodal felsic, felsic siliciclastic, and Zn–Ag–Au hybrid bimodal felsic deposits in the ∼513–486 Ma Victoria Lake supergroup; Cu–Zn bimodal felsic to bimodal mafic deposits of the ∼486 Ma Wild Bight Group; and Cu–(Au) mafic siliciclastic deposits of the ∼466 Ma Great Burnt Lake/South Pond belt. Regardless of age or stratigraphic hosts, all VMS deposits are associated with specific magmatic assemblages and extensional tectonism (i.e., rifting). Gold-enriched deposits of the Rambler-Ming district are associated with felsic rocks that formed via slab melting and subsequent melt-mantle wedge interaction, which likely enhanced precious metal enrichment in these deposits. Whereas many deposits exhaled on the seafloor, some deposits formed via subseafloor replacement of host units or as re-sedimented sulfides generated in sediment-gravity flows. Metals in the deposits were derived from leaching of underlying footwall rocks; however, Au–Ag- and epithermal suite element-enriched deposits show evidence for metal contributions from magmatic hydrothermal fluids. Sulfur in deposits was derived predominantly from leaching of H2S from underlying footwall rocks and from thermochemical sulfate reduction of seawater sulfate, with lesser input from bacteria-derived H2S and magmatic-hydrothermal-derived H2S. Despite recent research advances and historic mining, numerous questions remain unresolved and provide opportunities for future study.

Latest Silurian syntectonic sedimentation and magmatism and Early Devonian orogenic gold mineralization, central Newfoundland Appalachians, Canada: Setting, structure, lithogeochemistry, and high-precision U-Pb geochronology

Abstract The eastern Dunnage Zone of the central Newfoundland Appalachians hosts Paleozoic orogenic gold mineralization along a northeast-trending, crustal-scale fault corridor that extends for more than 200 km. This orogenic gold system is characterized by polyphase, structurally controlled, quartz vein systems that cut Neoproterozoic granitoid rocks and unconformably overlying syntectonic, polymict conglomerate and associated transitional to calc-alkaline bimodal igneous rocks. High-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology confirms a widespread, latest Silurian magmatic pulse (422–420 Ma) that is attributed to a transient phase of lithospheric extension resulting from asthenospheric and crustal melting related to slab break-off. Syntectonic conglomerate was deposited as a basal unit during extension-related uplift and erosion that lasted until ca. 418 Ma in north-central Newfoundland. Orogenic gold mineralization associated with syntectonic sedimentation and magmatism is hosted within third-order shear vein systems that form offshoots within a triangle zone–like structural corridor between southeast- and northwest-dipping, second-order fault splays and shear zones. The southeast-dipping fault system formed during northwest migration of the Acadian thrust front, whereas the northwest-dipping faults and shear zones are Salinic structures that were reactivated in the Early Devonian. Primary hydrothermal rutile in the orogenic gold-mineralized quartz veins produced ages of ca. 410 Ma. These ages are consistent with quartz vein emplacement and orogenic gold mineralization as a result of hydrothermal fluid-pressure cycling related to far-field compression and thermal perturbations during the Early Devonian Acadian orogenic cycle. The setting and process evolution of the central Newfoundland gold district are remarkably similar to that of world-class orogenic gold systems of the Canadian Shield.

Great Mining Camps of Canada 7. The Bathurst Mining Camp, New Brunswick, Part 1: Geology and Exploration History

The Bathurst Mining Camp of northern New Brunswick is approximately 3800 km2 in area, encompassed by a circle of radius 35 km. It is known worldwide for its volcanogenic massive sulphide deposits, especially for the Brunswick No. 12 Mine, which was in production from 1964 to 2013. The camp was born in October of 1952, with the discovery of the Brunswick No. 6 deposit, and this sparked a staking rush with more hectares claimed in the province than at any time since. In 1952, little was known about the geology of the Bathurst Mining Camp or the depositional settings of its mineral deposits, because access was poor and the area was largely forest covered. We have learned a lot since that time. The camp was glaciated during the last ice age and various ice-flow directions are reflected on the physiographic map of the area. Despite abundant glacial deposits, we now know that the camp comprises several groups of Ordovician predominantly volcanic rocks, belonging to the Dunnage Zone, which overlie older sedimentary rocks belonging to the Gander Zone. The volcanic rocks formed during rifting of a submarine volcanic arc on the continental margin of Ganderia, ultimately leading to the formation of a Sea of Japan-style basin that is referred to as the Tetagouche-Exploits back-arc basin. The massive sulphide deposits are mostly associated with early-stage, felsic volcanic rocks and formed during the Middle Ordovician upon or near the sea floor by precipitation from metalliferous fluids escaping from submarine hot springs. The history of mineral exploration in the Bathurst Mining Camp can be divided into six periods: a) pre-1952, b) 1952-1958, c) 1959-1973, d) 1974-1988, and e) 1989-2000, over which time 45 massive sulphide deposits were discovered. Prior to 1952, only one deposit was known, but the efforts of three men, Patrick (Paddy) W. Meahan, Dr. William J. Wright, and Dr. Graham S. MacKenzie, focused attention on the mineral potential of northern New Brunswick, which led to the discovery of the Brunswick No. 6 deposit in October 1952. In the 1950s, 29 deposits were discovered, largely resulting from the application of airborne surveys, followed by ground geophysical methods. From 1959 to 1973, six deposits were discovered, mostly satellite bodies to known deposits. From 1974 to 1988, five deposits were found, largely because of the application of new low-cost analytical and geophysical techniques. From 1989 to 2000, four more deposits were discovered; three were deep drilling targets but one was at surface.

Detrital zircon U-Pb geochronology of the Magog Group, southern Quebec – stratigraphic and tectonic implications for the Quebec Appalachians

In the Quebec Appalachians, the Laurentian continental margin (Humber Zone) and adjacent oceanic domain (Dunnage Zone) were amalgamated during the Ordovician Taconian orogeny. The Dunnage Zone includes ophiolites and overlying synorogenic deposits of both the Saint-Daniel Melange and Magog Group. The latter consists of a ∼3 km-thick pile of sandstone, felsic volcaniclastic rocks and graphitic slate at the base (Frontiere, Etchemin and Beauceville formations) overlain by a ∼7 km-thick turbiditic flysch sequence, constituting the Saint-Victor Formation. The maximum upper age limit of the Magog Group was considered to be Darriwilian based on graptolite fauna. This was proven consistent with a 462+5/−4 Ma (U-Pb ID-TIMS) from a felsic tuff of the Beauceville Formation, but contradicts a detrital zircon U-Pb age of 424 ± 6 Ma recently measured in the Saint-Victor Formation. A new detrital zircon U-Pb geochronology study (HR-LA-ICP-MS and ID-TIMS), focused on the Saint-Victor Formation, yields young detrital populations that suggest that the Saint-Victor Formation is not exclusively Ordovician and extends into the Silurian, as indicated by a maximum age of sedimentation around 430 Ma. Detrital zircon U-Pb geochronology associated with fossil age constraints and stratigraphic correlations in adjacent areas attest that the Saint-Victor Formation should be considered as an upper Magog Group sequence that is separated from lower units (the Frontiere-Etchemin-Beauceville formations) by an unconformity corresponding to a sedimentary hiatus of ≤ 10 m.y. Regional tectonic considerations imply that the Magog Group evolved from a syn-Taconian forearc basin in Middle-Late Ordovician time to a syn-Salinic peri-continental basin in early Silurian time. Several NW and SE erosional sources are invoked for the sedimentation of the Magog Group, evolving from the erosion of both the southern Quebec ophiolites and adjacent sedimentary rocks of the Laurentia margin to the NW, and volcanic arc rocks of the Ascot Complex, Shelbourne Falls and Bronson Hill massifs to the SE. Potential sources for ca. 430 Ma zircons found towards the top of the Saint-Victor sequence are the Silurian Frontenac Formation and the East Inlet granitic pluton, both located in the vicinity of the Quebec-Maine border.

Ophiolite obduction in the Quebec Appalachians, Canada — 40Ar/39Ar age constraints and evidence for syn-tectonic erosion and sedimentation1Geological Survey of Canada Contribution 20100430.2GEOTOP (Centre de recherche en géochimie et géodynamique) Contribution 2011-0002.3This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

Detailed field work conducted in the Dunnage zone of the Quebec Appalachians, is herein combined with 40Ar/39Ar dating on a series of ophiolitic massifs, crosscutting granites, and associated metamorphic rocks occurring along the Baie Verte–Brompton line, the Taconian suture between Laurentia and Lower Paleozoic peri-Laurentian oceanic terranes. Studied massifs are the Lac-Brompton ophiolite and the Rivière-des-Plante Ultramafic Complex in southern Quebec, and the Nadeau Ophiolitic Mélange in the Gaspé Peninsula. Our work suggests that these massifs form remnants of eroded ophiolitic nappes, which are unconformably overlain by the Saint-Daniel and Rivière-Port-Daniel mélanges, and correlate with the Thetford-Mines and Mont-Albert ophiolitic complexes. Our 40Ar/39Ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen. The timing of obduction and mélange formation varies according to the irregular geometry of the Early Paleozoic Laurentian margin, with earlier collision occurring along, or at the margins of promontories. Obduction was initiated with the formation of infraophiolitic metamorphic soles between ca. 479 and 472 Ma in southern Quebec and the Nadeau Ophiolitic Mélange, and possibly as late as ca. 470–466 Ma for the Mont-Albert Complex. These sole rocks were later exhumed and translated onto the Laurentian margin with the overlying ophiolites between 475 and 460–457 Ma. The uplifting and erosion of the orogenic wedge during the waning stages of obduction, has resulted in the sedimentation of olistostromal mélanges and onlapping flysch units above the ophiolitic nappes, as well as foredeep flysch successions during the latest Arenig(?) to earliest Caradoc.

Laurentia-derived detritus in the Badger Group of central Newfoundland: deposition during closing of the Iapetus Ocean1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2Geological Survey of Canada Contribution 20110273.

The Dunnage zone of central Newfoundland records the closing of an oceanic tract — the Iapetus Ocean. Within this tract, the Red Indian line separates arc terranes developed close to Laurentia (Notre Dame subzone) from those associated with the peri-Gondwanan terrane Ganderia (Exploits subzone). Sandstones and conglomerates of the Badger Group were deposited on the Exploits subzone, southeast of the Red Indian line, from Late Ordovician (Katian) to early Silurian (Llandovery) time. Two samples were collected from the base and top of the Badger Group for detrital zircon U/Pb dating. The lower sample yielded detrital zircon populations with a large statistical peak at ca. 1.0 Ga, and other features characteristic of derivation from Laurentian sources. Paleozoic zircons with ages from Late Cambrian to Late Ordovician were probably derived from the Notre Dame arc on the margin of Laurentia. The upper sample yielded a comparable age distribution, but with a much smaller proportion of Mesoproterozoic relative to Paleozoic detrital zircon. These results date the earliest arrival of Laurentia-derived detritus on the peri-Gondwanan Exploits subzone crust in the early Katian (ca. 455 Ma). However, the absence, in both samples, of Neoproterozoic grains suggests that there was still a seaway separating the Gander margin from arc terranes accreted to Laurentia until at least the middle Llandovery (ca. 436 Ma).

An Ordovician “Lost City” — venting serpentinite and life oases on Iapetus seafloor

Highly brecciated carbonate-rich serpentinites (or ophicalcites) of Early Ordovician age in the Dunnage Zone of Quebec are host to fracture-fill, high-temperature (80–230 °C) carbonate cements. Away from, or crosscut by the fractures, centimetre- to decimetre-thick crusts made up of massive to laminated micrite, peloidal layers and threads are associated with low-temperature botryoidal calcite cements. The peloidal masses are characterized by a clotted texture that is reminiscent of interpreted fossilized Ordovician microbial communities. The ophicalcite contains carbonate botryoids, morphology commonly found at shallow reefal margins but also at modern cold CH4 seeps and recently documented at hot CH4 vents from serpentinite, such as the modern Lost City hydrothermal field in the Atlantic Ocean. The δ18OVPDB ratios of the calcite botryoids and peloidal layers indicate formation and (or) precipitation out of cold Ordovician deep-marine waters. To the contrary of botryoids at cold methane seeps, the botryoids associated with the modern and Ordovician hot vents do not show the negative δ13CVPDB ratios indicative of microbial isotopic fractionation. Therefore, any microbial-derived HCO3– has been overwhelmed by the isotopically heavier marine-derived carbon during the open-system diagenesis. Carbonate-rich serpentinites should be carefully revisited in the search for evidence of microbial life in the Precambrian, as the negative δ13CVPDB ratios used as fingerprints of biological activity are not always reliable.

Paleogeography of the Matapédia basin in the Gaspé Appalachians: initiation of the Gaspé Belt successor basin

The Matapédia basin consists of the uppermost Ordovician – lowermost Silurian deep-water, fine-grained carbonate–siliciclastic rocks of the Honorat (Garin Formation) and Matapédia groups (Pabos and White Head formations), the lower rock assemblage of the Gaspé Belt in the Gaspé Appalachians. Paleogeographic maps of eight time slices from the Caradocian to the Llandoverian are presented to better understand the tectonosedimentary evolution of the Matapédia basin. Deposition evolved from siliciclastic (Garin Fm.) to argillaceous limestones (Pabos Fm.), to limestones (White Head Fm.). The overall change from terrigenous (Garin Fm.) to limestone facies (White Head Fm.) reflects a change in the source area. Paleocurrent directions and composition of sandstones indicate an orogenic source area to the south for the Garin Formation, which is believed to be the inliers of the Humber and Dunnage zones in the southern Gaspé and New Brunswick Appalachians. Lime muds deposited by turbidity currents coming from the north suggest the Anticosti active carbonate platform as the source area for the White Head Formation. The Matapédia basin was filled from south to north. First deposits, the Garin Formation, occurred south of the Taconian thrust sheets (Humber Zone) and also south of the Grenville basement. This region was the domain of the Ordovician Iapetus Ocean (Dunnage Zone). The northern limit of the basin migrated northward during deposition of the Matapédia Group in Ashgillian–Llandoverian times and reached its actual northern limit at the very end of the Llandoverian (C6), when siliciclastic facies of the lower Chaleurs Group were deposited.

Integration and spatial analysis of high-resolution geophysical and geological data, eastern Gaspé Peninsula

In the vicinity of the Town of Gaspé, the relationships between the Silurian-Devonian sedimentary succession of the Gaspé Belt and the Humber and the Dunnage zones are complex. To unravel these relationships, we used high-resolution aeromagnetic data and regional gravimetric data coupled with field tectonostratigraphic information. The magnetic vertical derivative located several magnetic anomalies associated with near-surface features in the Silurian–Devonian cover sequence. In particular, a conglomerate with magnetic fragments that overlies the Late Silurian Salinic Unconformity is clearly recognizable. Large ovoid anomalies of significant intensity located in the Silurian–Devonian sedimentary cover area cannot be associated with any known geological feature. The interpretation of the high-pass and low-pass filtered aeromagnetic survey indicates that the ovoid anomalies originate below the Silurian–Devonian cover sequence. The most significant of the ovoid anomalies is associated with a gravimetric anomaly. It is proposed that these geophysical anomalies are probably associated with ultramafic and (or) volcanic rocks correlative in the subsurface with outcrops of the Cambrian–Ordovician lithologies of the Lady Step Complex and (or) the Shickshock Group.

A new stratigraphic framework for the Gaspé Belt in southern Quebec: implications for the pre-Acadian Appalachians of eastern Canada

The post-Taconian units in the Quebec and northern New Brunswick Appalachians constitute the Gaspé Belt and geological studies have mostly focussed on its eastern Quebec segment. Biostratigraphic data indicate that the succession in southern Quebec is no older than Late Silurian and extends into the Early Devonian. Two distinct stratigraphic assemblages are present. The first assemblage (Saint-Luc, Cranbourne, and Lac Aylmer formations, and Glenbrooke Group) unconformably overlies the Humber and Dunnage zones. The units show a basal alluvial conglomerate that passes progressively to deeper marine facies upsection, which have recorded a post-Late Silurian transgressive event. The second assemblage (Saint-Francis Group and Frontenac Formation) is faulted against either Dunnage units or autochthonous post-Taconian units. It locally unconformably overlies units of the Dunnage Zone; the succession shows progressively deeper marine conditions upsection and also has recorded a post-Late Silurian transgressive event. The biostratigraphic framework suggests that some of the units that were assumed to be vertically stacked are rather laterally equivalent. Independant evidence supports the hypothesis that the Gaspé Belt in southern Quebec formed after the collapse of the Taconian orogen in Late Silurian time. This event is ascribed to the Salinian Orogeny. The framework from southern Quebec is incorporated in a regional scenario. The Gaspé Belt experienced a Pridolian–Lochkovian sea-level rise. In Pragian time, shallower marine conditions were established in southern Quebec, whereas in the Gaspé Peninsula, the shallower conditions only occurred in early Emsian time.

Reconstruction of Taconian and Acadian paleostress regimes in the Quebec and northern New Brunswick Appalachians

A numerical analysis of fault populations was applied in the Quebec and northern New Brunswick Appalachians to characterize paleostress fields attributed to the Taconian and Acadian orogenies. The first brittle deformation documented in the external domain of the Humber Zone is associated with the thin-skinned tectonics of the Taconian orogen. The Taconian brittle deformation, characterized by north–south to NE–SW reverse conjugated brittle faults, evolved under a pure shear compressional regime (vertical σ3 axes) with σ1 axes oriented ESE–WNW. The ENE–WSW dextral and NW-SE sinistral faults that are observed in the St.Lawrence Lowlands are also attributed to Taconian compression. The first brittle deformation recognized in the Dunnage Zone and the Gaspé Belt is attributed to an ESE–WNW compression during the Acadian Orogeny. In the Dunnage Zone of southern Quebec, the Acadian paleostress axes are similar to the directions of σ1 axes and to the pure shear compressional regime computed for the Silurian and Devonian rocks of the Gaspé Belt. In the Gaspé Peninsula, the Acadian stresses are oriented ESE–WNW and are mechanically compatible with transpressional regimes. Reverse brittle faults and pure shear stress are identified along regional and secondary sets of NE–SW brittle faults, whereas a strike-slip stress regime is reconstructed along major east–west faults. In the Humber Zone of the Quebec Appalachians, a second compression is identified. It is characterized by conjugate ENE–WSW dextral and NW–SE sinistral strike-slip faults that crosscut Taconian thrust faults. This late deformation event is attributed to a distal expression of the Acadian Orogeny.

COMPOSITION OF BIOTITE FROM GRANITIC ROCKS OF THE CANADIAN APPALACHIAN OROGEN: A POTENTIAL TECTONOMAGMATIC INDICATOR?

Biotite, the dominant ferromagnesian mineral in Paleozoic granitic rocks of the Canadian Appalachians, has been analyzed with an electron microprobe (wavelength dispersion) for major elements and by 57Fe Mossbauer spectroscopy. We sampled a wide variety of rock types, ranging from gabbro, diorite, syenite to granite, but by far mostly granitic ( sensu lato ). The most pronounced variations are in total Al contents and Fe/(Fe + Mg) values. In the biotite quadrilateral (annite –siderophyllite –phlogopite – eastonite), biotite from A-type granites of the Humber and Avalon zones in Gaspe (Quebec) and New Brunswick is characterized by low mean Al contents, ~1.15 atoms per formula unit ( apfu ), and variable Fe/(Fe + Mg) values in the range 0.4 to 0.9. In the granites of the Notre Dame arc of the Dunnage zone in Newfoundland, biotite has moderate mean values of Al (~1.40 apfu ) and Fe/(Fe + Mg) (~0.58). In granites of the Gander zone of New Brunswick and Newfoundland, biotite has a mean Fe/(Fe + Mg) value of 0.6 and shows a pronounced trend of increasing total Al (1.05 to 1.75 apfu ), confirming significant contributions of aluminous supracrustal material to the magmas, either by assimilation or anatexis. Finally, in granites of the Meguma zone, derived entirely from metasedimentary material, biotite exhibits a remarkable increase in total Al (1.30 to 2.00 apfu ) and considerable iron-enrichment [Fe/(Fe + Mg) in the range 0.4 to 1], with compositions nearing the siderophyllite end-member. The biotite from most zones plots on or above the NNO buffer, indicating moderately oxidizing conditions, whereas that from the Meguma zone plots mainly between the QFM and NNO buffers, implying fairly reducing conditions during crystallization. Assuming a reasonable range of crystallization temperatures of 750 to 900°C, oxygen fugacities ranged from 10−10 to 10−16.9 bars during crystallization. The composition of biotite reflects primarily the nature of the host magmas. It cannot readily be used for tectonomagmatic characterization of these rocks without the aid of other types of data.

The Prospect Rock thrust: western limit of the Taconian accretionary prism in the northern Green Mountain anticlinorium, Vermont

This paper presents evidence for an early Taconian, west-directed fault in northern Vermont, the Prospect Rock thrust, which represents the trace of the contact between the Taconian oceanic accretionary prism (Dunnage Zone) and less allochthonous, continental margin rocks (Humber Zone). Mapping at 1 : 24 000 over the last decade has led to reassessment of units within the Camels Hump Group, resulting in newly defined lithotectonic packages: the Green Mountain slice and the Prospect Rock slice. Rocks in these slices may be of similar age, but those in the Green Mountain slice were originally deposited as more proximal sediments and remained on the Humber side of the sub duction zone, whereas those in the Prospect Rock slice were transferred by underplating to the accretionary prism. Both slices contain ultramafics. Motion on the Prospect Rock thrust (D1) preceded Taconian garnet-grade metamorphism and subsequent east-directed back-folding. However, D1 structures were diachronous across the orogen. Late Taconian (D2) structures record a change from east-verging back-folds in northern Vermont and southern Quebec to west-verging folds farther south. The Prospect Rock thrust does not correspond exactly to Cameron's line nor to the Baie Verte – Brompton line as originally defined. However, our understanding of the geometry of the Prospect Rock thrust demonstrates that the Dunnage Zone extends farther west than previously recognized and may also explain features farther to the east.

Late Middle Ordovician olistostrome formation and magmatism along the Red Indian Line, the Laurentian arc – Gondwanan arc boundary, at Sops Head, Newfoundland

The Sops Head Complex of Badger Bay, central Newfoundland, includes olistostromal and tectonized mélanges and marks the Red Indian Line, the boundary between the peri-Gondwanan Exploits and peri-Laurentian Notre Dame subzones of the oceanic Dunnage Zone. Basalts in the olistostromal mélange preserve peperitic contacts with mudstone and limestone within slumped sedimentary units, demonstrating that magmatism was coeval with olistostrome formation. Conodonts from the limestones date these events as late Darriwilian ("Llandeilian," Middle Ordovician) in age. The conodont fauna consists entirely of North Atlantic Faunal Realm elements, suggesting a peri-Gondwanan or mid-Iapetan provenance for limestone of the Sops Head Complex. Basalt geochemistry suggests magmatism in a mature volcanic-arc to back-arc setting. The Sops Head Complex shows similarities in age, provenance, and stratigraphic setting to the Dunnage Mélange, suggesting that the two units are correlative.

Geochemistry and tectonic setting of metabasic rocks of the Gneiss Dome Belt, SW New England Appalachians

Acadian structural domes in the Connecticut Valley Zone in western Connecticut expose polydeformed, amphibolite and upper amphibolite facies pre-Silurian rocks correlative with the Notre Dame subzone (western Dunnage zone) of Williams et al. [Geologic Survey of Canada Paper 88-1B, 1988, pp. 91–98] of the Canadian Appalachians. We have analyzed the geochemistry of 99 metabasic rocks from the Cambrian-Lower Ordovician Rowe Schist (RS), the Arenig or older Taine Mountain Formation (TMF), and the overlying Arenig Collinsville Formation (CF) to identify the tectonic setting of mafic volcanism in the Gneiss Dome Belt (GDB) in western Connecticut and southwestern Massachusetts. The metabasites record epidote-amphibolite to lower amphibolite facies metamorphism and exhibit equilibrium microstructures and only limited retrogression. We used our data to test whether the GDB represents part of a Taconic volcanic arc in southwestern New England. RS and TMF metabasites that can be interpreted as dikes range in composition from subalkaline basalt to basaltic andesite and exhibit overall arc-like signatures. Their REE patterns at 10–25× chondrite are mildly LREE enriched with (La/Yb) N=1.50–1.94. Enriched HFSE from Th to Ce and lack of negative Ta–Nb anomalies indicate that the RS and TMF metabasites experienced some degree of crustal contamination. The majority of CF metabasites range from basaltic andesite to subalkaline basalt. The CF metabasites are geochemically diverse and have a wide range of Ti/V ratios characteristic of boninites and low-Ti arc tholeiites, arc tholeiites, ocean floor basalts, and ocean island and alkali basalts. Based on their geochemical characteristics, CF metabasites have been subdivided into three major types: (1) boninitic and low-Ti IAT metabasites (20% of the analyzed samples), (2) arc-like metabasites (27%), and (3) MORB-like and MORB/WPB transitional metabasites (53%). Boninitic CF metabasites are characterized by extremely depleted HFSE, enriched LILE, and distinctive U-shaped REE patterns below the 10× chondrite level. Arc-like CF metabasites have moderately depleted HFSE from P to Yb and relatively enriched LILE; REE patterns at 10–25× chondrite are mostly mildly LREE enriched with (La/Yb) N=1.68–2.12. HFSE from Th to Ce are relatively enriched indicating a small degree of crustal contamination in these metabasites. MORB-like CF metabasites display signatures characteristic of BABB; they have flat, weakly enriched HFSE and moderately enriched LILE typical of evolved MORBs. REE patterns at 15–35× chondrite are flat to slightly LREE enriched with (La/Yb) N=1.08–1.49. On tectonic discriminant diagrams, the majority of MORB-like CF metabasites define MORB-WPB trend. Four samples with the most elevated REE patterns at 20–100× chondrite and moderately LREE enriched have a transitional MORB/WPB character. The tectonic position and geochemical signatures of the RS and TMF dikes suggest that they represent extension-related magmatism, either in an arc or forearc setting, prior to the establishment of Arenig- to Caradoc-aged magmatism in this part of western New England. The coexistence of boninitic, arc-like, and MORB-like compositions is consistent with a backarc setting as the eruptive environment of the CF metabasites in the GDB in western Connecticut and southwestern Massachusetts as predicted by Crawford et al.'s [Earth, and Planetary Science Letters 54, 1981, 346–356] model of boninite generation. Whether the initial rifting and opening of this backarc basin occurred in the forearc or as intra-arc splitting remains unclear. MORB-like compositions among the CF metabasites are well represented and denote a more evolved stage of this backarc basin development. Boninitic and arc-like CF metabasites conclusively prove a supra-subduction zone setting, but the polarity of this subduction remains equivocal. Our results indicate that Taconic accretion in western New England Appalachians was more complex than previously proposed and likely involved forearc, arc, and backarc complexes.

Devonian Extension in Northwestern Newfoundland: 40Ar/39Ar and U‐Pb Data from the Ming’s Bight Area, Baie Verte Peninsula

Abstract The Ming’s Bight Group of northwestern Newfoundland, an outlier of Humber Zone continental margin rocks, is entirely surrounded by ophiolitic rocks of the Dunnage Zone. Structures in the Ming’s Bight Group and adjacent units record three main phases of deformation. The earliest structures relate to Silurian sinistral transpression previously documented in the region. Two later phases of extensional deformation produced a series of dextral oblique‐normal shear zones and faults that now separate the Ming’s Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. 40Ar/39Ar and U‐Pb data constrain the times of oblique‐normal shear and cooling. Metagabbro in the Point Rousse Ophiolite Complex, which lies in the hangingwall, preserves disturbed Ordovician hornblende 40Ar/39Ar ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record 40Ar/39Ar ages of 390–380 Ma. Synkinematic titanite...

Age and composition of the Ruisseau Isabelle Mélange along the Shickshock Sud fault zone: constraints on the timing of mélanges formation in the Gaspé Appalachians

Mélanges are common in the Humber and Dunnage zones of the Quebec Appalachians. Humber Zone mélanges are spatially associated with Taconian nappes and believed to be of tectonic origin. Dunnage Zone mélanges are interpreted as relicts of an Ordovician accretionary prism. The Ruisseau Isabelle Mélange (RIM) in the Gaspé Appalachians was first interpreted as a Dunnage Zone mélange, because of its tectonic position along the Baie Verte – Brompton Line and lithological components, such as dark mudstone, blocks of ultramafic rocks, and greenish chromite-bearing sandstone. The RIM comprises sedimentary rock assemblages (Composite Shale, Black Shale, and Chromite-Bearing Sandstone assemblages), an Exotic Block, a Pebbly Mudstone Assemblage and slivers of Serpentinized Peridotite, and Metamorphic Tectonite. Most mélanges of the Dunnage Zone in the Quebec Appalachians contain partly similar rocks, but their age is not well constrained. Recent work on the RIM and Cap-Chat Mélange of the Humber Zone and new fossil finds in the RIM rock assemblages allow us to conclude that previous distinction between mélanges of the Dunnage and Humber zones on the basis of rock-type associations is not valid. According to the age of the RIM rock assemblages, mélanges in the northern Gaspé Appalachians continued forming after the mid-Caradocian, later than the Dunnage Zone mélanges of southern Gaspé Appalachians. The juxtaposition and structural features of the RIM rock assemblages result from repeated faulting along the Shickshock Sud fault from Late Ordovician to Middle Devonian.

Acadian metamorphism in the Dunnage zone of southern Quebec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism

Research Article| January 01, 2000 Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism Alain Tremblay; Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Search for other works by this author on: GSW Google Scholar Sébastien Castonguay Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Publisher: Geological Society of America Received: 17 Aug 1998 Revision Received: 02 Mar 1999 Accepted: 12 Mar 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (1): 136–146. https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Article history Received: 17 Aug 1998 Revision Received: 02 Mar 1999 Accepted: 12 Mar 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alain Tremblay, Gilles Ruffet, Sébastien Castonguay; Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism. GSA Bulletin 2000;; 112 (1): 136–146. doi: https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40Ar/39Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Québec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician 40Ar/39Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism

Research Article| January 01, 2000 Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism Alain Tremblay; Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Search for other works by this author on: GSW Google Scholar Sébastien Castonguay Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (1): 136–146. https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Article history received: 17 Aug 1998 rev-recd: 02 Mar 1999 accepted: 12 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alain Tremblay, Gilles Ruffet, Sébastien Castonguay; Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism. GSA Bulletin 2000;; 112 (1): 136–146. doi: https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40Ar/39Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Québec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician 40Ar/39Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Palaeozoic Palaeogeography in Hercynian and Appalachian Belts by Sm-Nd Isotopic Studies of Shales

The Sm-Nd isotopic studies of shales give important informations about the crustal growth and the provenance of the sediments. Because shales integrate the diversity of eroded rocks and because the Sm-Nd isotopic system is left undisturbed by surface processes, the Nd isotopic initial composition reflects an average of the different mechanically eroded sources. This composition reflects a balance between juvenile inputs from the mantle by volcanism and reworked inputs from the continental crust. However, the Sm-Nd isotopic signal is a biased average in which pre-existing sediments and young mountains appear to be over represented. The Nd isotopic initial composition can be expressed by depleted mantle model ages (TDM). Assuming that similar TDM values can characterize a common tectonic domain, comparisons between data from several areas can give new palaeogeographical constraints. First, we test this assumption for the Hercynian belt and we expand it to the Appalachian belt in eastern Canada. Our data from the Hercynian belt concern Sm-Nd isotopic data on 46 shales from the Montagne Noire (southern French Massif Central). The sedimentary record, which comprises one of the most complete Palaeozoic sequences in Europe, was sampled from Early Cambrian to Early Jurassic (560 to 180 Ma). This sequence is interrupted by two hiatuses. The first is situated in Middle Ordovician (490 to 450 Ma) and the second during Carboniferous (330 to 300 Ma). We analysed volcanogenic materials and lavas in order to identify possible source materials. During the studied time span, most of the TDM's are around 1.6 Ga. We observe some rapid TDM changes toward older ages at 520 and 270 Ma and toward younger ages at 300 Ma. These variations are related to different sedimentary settings and to the volcanic event (300 Ma) detected in a continental basin. Our results indicate that in any case, younger TDM do not reflect large Palaeozoic juvenile inputs related to orogenic processes in the south of the French Massif Central. We compare our data with others published data from Brittany (Medio North Armorican Domain) and from Spain (Central Iberian Domain and Cantabrian Domain). The similar TI~M calculated for these areas from Early Ordovician to Early Carboniferous (490 to 330 Ma) suggest that these three areas belonged to the same palaeogeographic entity at least for this time span. This contradicts models that propose there is an oceanic suture between Brittany and Spain and between the two Spanish zones. Conversely, our observations are in agreement with palaeontological data indicating that Brittany and Spain were adjoining during this time span. On the other hand, because the Montagne Noire has younger TI~M from 490 to 400 Ma compare to Brittany and Spain, we propose that this area probably undergone a different tectonic evolution during this time span. The Appalachian belt was created by several accretion events of exotic terranes to the North American shield (Laurentia) during the Palaeozoic. The origin, the timing and the link between the accreted terranes are a matter to debate. In order to give new constraints, we analysed shales from the Dunnage zone, which represent eroded products from the Laurentia, and shales from accreted terranes: the Avalon and Meguma zones. For the Dunnage zone, we observe a rapid TDM shift toward younger ages near 440 Ma. This result is interpreted as an evidence for the accretion of Avalon to the Laurentia around that time.