stratigraphy, structure and regional correlations of crustal sequences...
TRANSCRIPT
Stratigraphy, structure and regional correlations of crustal sequences of the Appalachian ophiolites of
Southern Québec
Alain Tremblay Département des Sciences de la Terre et de l’Atmosphère Université du Québec à Montréal Montréal, Qc, H3C 3P8
Jean H. Bédard
Commission géologique du Canada Centre géoscientifique de Québec
Québec, Qc, G1V 4C7
Field Trip B2 Guidebook
18-20 May 2006
Montréal 2006 Committee
CONTENTS page INTRODUCTION ……………………………………………………………………….. 1 THE SOUTHERN QUÉBEC APPALACHIANS …………………………………….… 2 THE SOUTHERN QUÉBEC DUNNAGE ZONE ……………………………………… 4 Structure and metamorphism …………………………………………………….. .. 5 TECTONIC EVOLUTION ………………………………………………………………. 6 THE SOUTHERN QUÉBEC OPHIOLITES BELT …………………………………. … 7 The Thetford-Mines ophiolite …………………………………………………….… 10 The Asbestos ophiolite ……………………………………………………………… 10 The Lac-Brompton ophiolite …………………………………………………….….. 10 The Mont-Orford ophiolite ………………………………………………………….. 10 FIELD TRIP ROAD LOG ………………………………………………………………… 13 Day 1 – The mantle and lower of the Thetford-Mines ophiolite …………………….….. 13 Stop 1.1 : The metamophic sole and the gabbroic sequence of the
Thetford-Mines ophiolite at Breeches lake ………………………………………….. 13 Stop 1.2 : The upper mantle at Vimy Ridge …………………………………………. 14 Stop 1.3 : Scenic view of mine Lac d’Amiante at Black Lake ……………………… 15 Stop 1.4 : The lower crust at the American Chrome mine ………………………….. 15 Stop 1.5 : Syn-oceanic deformation features of the lower crust at Mamelon Nadeau ……………………………………………………………………. 16
Day 2 – The upper crust and sedimentary cover of the Thetford-Mines ophiolite ……. 18 Stop 2.1 : Upper gabbros and the brecciated hypabyssal sequence …………………. 19 Stop 2.2.: Boninitic pillows and feeder dykes ………………………………………. 20 Stop 2.3 : Sedimentary rocks of the Rivière-de-l’Or section ………………………… 20
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Stop 2.4 : Metamorphic clast-bearing debris flows ………………………………… 21 Stop 2.5 : Interbedded siltstones and mudstones overlying ophiolitic gabbros …….. 21 Stop 2.6 : The Coleraine Breccia at the type-locality ………………………………. 22 Stop 2.7 : Turbidites of the Saint-Daniel Mélange at Ham-Sud ……………………. 23 Stop 2.8 : Pebbly mudstone of the Saint-Daniel Mélange ………………………….. 23 Day 3 – Regional variations and correlations of the sedimentary cover sequence ……. 24 Stop 3.1 : Scenic view of mine Jeffrey at Asbestos ……………………………… …. 24 Stop 3.2 : Pillowed lavas and overlying debris flow of the Asbestos ophiolite ……… 25
Stop 3.3 : Well-bedded sedimentary sequence of the Saint-Daniel Mélange ……….. 26 Stop 3.4 : The metamorphic sole of the Lac-Brompton ophiolite …………………… 26 Stop 3.5 : Siltstones overlying the dunitic lower crust in the Lac-Brompton Ophiolite ……………………………………………………………………………… 28 Stop 3.6: Conglomerate in depositional contact with the metamorphic sole in the Lac-Brompton ophiolite ………………………………………………………. 29 Stop 3.7 : The contact between the Saint-Daniel Mélange and the Magog Group ….. 29
REFERENCES ……………………………………………………………………………… 30
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FIGURES Figure 1. Geological map of the southern Québec Appalachians. Figure 2. Diagram summarizing age constraints for the deformation and metamorphic events in the southern Quebec Appalachians. Figure 3. Schematic model for structural evolution of Laurentian margin in southern Quebec. Figure 4. Geological map showing the location of major ophiolites and the inferred distribution of the various facies of the Saint-Daniel Mélange in the southern Québec Appalachians. Figure 5. A) Inferred regional tectonic setting of the Southern Québec Appalachians during the Taconian orogeny, and schematic sedimentary and tectonic evolution of the Saint-Daniel Mélange and the Magog Group. B) Schematic composite structural profile across the Laurentian margin and the adjacent oceanic domain. Figure 6. Geological map of the Thetford-Mines ophiolitic Complex and location of stops. Figure 7. Stratigraphic columns for different locations of the Thetford-Mines ophiolitic Complex. Figure 8. Structural profile A-A’ of the Thetford-Mines ophiolite. Figure 9. Geologic map of the Belmina Ridge area and location of stop 1.1. Figure 10. Field sketch of the contact ophiolite-margin at the Belmina Ridge amphibolitic sole. Figure 11. Geological map of part of the Adstock-Ham Massif showing the location of normal(?) faults interpreted as syn-magmatic structures. Figure 12. Detailed map of the American Chrome mine (stop 1.4). Figure 13. Field sketch for stop 1.5 illustrating the inferred syn-magmatic faults preserved at the base of the ophiolitic crust. Figure 14. Schematic illustration of a possible evolutionary scenario for the main crust-forming event of the Thetford-Mines ophiolite. Figure 15. Stratigraphic section of the Saint-Daniel Mélange as exposed in the Rivière de l’Or (stop 2.3). Figure 16. Schematic illustration of facies variations of the ophiolitic sedimentary cover sequence between stops 2.4 and 2.5. Figure 17. 40Ar/39Ar age spectra for two muscovite samples from a metamorphic clast of supra-ophiolitic debris flows.
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Figure 18. Stratigraphic section of the Saint-Daniel Mélange as exposed in the Mont-Ham section (stop 2.7). Figure 19. Geological map and structural profile of the Asbestos ophiolitic Complex. Figure 20. Stratigraphic column of a part of the supra-ophiolitic sedimentary sequence near Asbestos. Figure 21. Geological map of the Lac-Brompton ophiolite (stops 3.4, 3.5 and 3.6) and the northern part of the Mont-Orford ophiolite. Figure 22. Synthetic diagram for the stratigraphy of the Saint-Daniel Mélange in the Thetford-Mines area and correlation with Saint-Daniel facies mapped in the vicinity of the Lac-Brompton and Mont-Orford ophiolites. Figure 23. A) Tectonic setting of the southern Québec Appalachians during the Taconian orogeny. B) Schematic sedimentary and tectonic evolution of the Saint-Daniel Mélange along the western edge of a syncollisional forearc basin.
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Stratigraphy, structure and regional correlations of crustal sequences of the Appalachian ophiolites of Southern Québec
Alain Tremblay
Département des Sciences de la Terre et de l’Atmosphère Université du Québec à Montréal Montréal, Qc, H3C 3P8
Jean H. Bédard
Commission géologique du Canada Centre géoscientifique de Québec
Québec, Qc, G1V 4C7
ABSTRACT This 3-days fieldtrip focuses on the stratigraphy and the structural characteristics of the
Thetford-Mines ophiolitic Complex (TMOC) with a particular emphasis on pre- to syn-obduction structures and associated lihological variations in the crustal section of the ophiolite, and its overlying sedimentary cover which belongs to the Saint-Daniel Mélange. On the basis of litho-tectonic features established for the TMOC and in order to discuss various aspects of the regional correlation of ophiolitic units of the Southern Québec Appalachians, we will visit various key outcrops of the Asbestos, Lac-Brompton and Mont-Orford ophiolites.
RÉSUMÉ
Cette excursion de 3 jours est axée sur la stratigraphie et la caractérisation structurale du Complexe ophiolitique de Thetford-Mines (COTM) avec un accent particulier sur les structures pré- et syn-obduction ainsi que les variations lithologiques de la section crustale de l’ophiolite et de sa couverture sédimentaire appartenant au Mélange de Saint-Daniel. Sur la base des caractéristiques litho-tectoniques définies dans le COTM et afin de discuter certains aspects de corrélation régionale des unités ophiolitiques des Appalaches du Sud du Québec, nous visiterons certains affleurements-clés des ophiolites d’Asbestos et du Mont-Orford.
INTRODUCTION This 3-day field trip focuses on the lithological characteristics and structural features of the Ordovician ophiolites from the oceanic domain (Dunnage Zone) of the southern Québec Appalachians. A particular emphasis will be put on the characterization of pre- to syn-obduction structures and associated lihological variations in the crustal section of the Thetford-Mines ophiolite and its overlying sedimentary cover; which will serve as a template from which to understand ophiolitic and supra-ophiolitic rocks of the
Southern Québec Appalachians on a regional scale. To this end we will visit key outcrops of the Asbestos, Lac-Brompton and Mont-Orford ophiolites. Previous fieldtrips to the Thetford-Mines and Asbestos ophiolites include Hébert & Laurent (1977, 1979), St-Julien & Hubert (1979), Laliberté et al. (1979), Laurent & Baldwin (1987), and Hébert & Bédard (1998). The stratigraphical and structural analysis of the Thetford-Mines ophiolite presented in this fieldguide results from detailed geological mapping and petrological work in 2000-2002 by graduate students under the supervision of
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both leaders (Schroetter, 2004; Bécu, 2005, and Pagé, in progress); providing an integrated view of ophiolite genesis and evolution. Comparison and correlation of structures from the Thetford-Mines ophiolite with regionally extensive fabrics developed in rocks of the adjacent Laurentian continental margin (Tremblay & Castonguay, 2002), allow ophiolitic structures to be subdivided into pre-, syn- and post-obduction phases. The framework established for the Thetford-Mines ophiolite has then been applied to the rest of the Southern Québec ophiolitic belt, allowing along-strike, regional-scale lithological and structural correlations (Schroetter et al., 2003, 2005, 2006). This fieldtrip incorporates numerous discussions with scientists involved in the geology of southern Québec. The authors wish particularly to express their gratitude to R. Hébert, R. Laurent and the late P. St-Julien for introducing us to the area and sharing their knowledge during earlier field seasons, to B. Brassard (former director of the exploration department at Ressources Allican) for his inspired contribution to the initiation of Thetford-Mines ophiolite project in 1999-2000, to P. Cousineau and the late lamented G. Kessler for volcanological and sedimentological insights, and to the Thetford-Mines (S. Schroetter, P. Pagé, V. Bécu) and Lac-Brompton (C. Daoust and S. DeSouza) teams of graduate students for their passion for field geology, and their interest in ophiolites. It must also be remembered that this document represents our own vision of the Southern Québec ophiolitic Belt, and that other interpretations exist. We hope that this fieldtrip will be a forum for discussion of ophiolite genesis, and about how they can serve as analogues to modern oceanic crust.
THE SOUTHERN QUÉBEC APPALACHIANS
The southern Québec Appalachians
comprise three lithotectonic assemblages (Fig. 1): the Cambrian-Ordovician Humber and Dunnage Zones (Williams, 1979), and the Silurian-Devonian successor sequence of the Gaspé Belt (Bourque et al., 2000). The Humber and Dunnage Zones are remnants of the Laurentian continental margin and of the adjacent oceanic domain, respectively. The boundary between the Humber and Dunnage Zones corresponds (on the surface) to a zone of dismembered ophiolites and serpentinite slices defined as the Baie Verte-Brompton line (BBL; Williams & St-Julien, 1982). The Dunnage zone is locally unconformably overlain by Upper Silurian and Devonian rocks of the Gaspé Belt.
The Humber Zone is subdivided into External and Internal Zones (Tremblay & Castonguay, 2002). The External Humber Zone consists of very low-grade sedimentary and volcanic rocks deformed into a series of northwest-directed thrust nappes (Fig. 1). The Internal Humber Zone is made of greenschist to amphibolite facies metamorphic rocks (the Sutton-Bennett Schists on Fig. 1) that represent distal facies of the External Humber Zone units. The highest-grade metamorphic rocks occur in the cores of doubly-plunging dome structures (i.e. the Sutton Mountains and Notre-Dame Mountains anticlinoria; Fig. 1). Regional deformation phases include a S1-2 schistosity and syn-metamorphic folds and faults, that were overprinted by a penetrative crenulation cleavage (S3 of Tremblay & Pinet, 1994), which is axial-planar to hinterland-verging (southeast) folds and ductile shear zones rooted along the northwestern limb of the Internal Humber Zone (Pinet et al., 1996; Tremblay & Castonguay, 2002; Fig. 1).
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Figure 1. Geological map of the southern Québec Appalachians. TMOC, Thetford-Mines ophiolitic Complex; AOC, Asbestos ophiolitic Complex; LBOC, Lac-Brompton ophiolitic Complex; MOOC, Mont-Orford ophiolitic Complex. From Schroetter et al. (2006). Figure 1. Carte géologique des Appalaches du sud du Québec. TMOC, Complexe ophiolitique de Thetford-Mines; AOC,Complexe ophiolitique d’Asbestos; LBOC, Complexe ophiolitique du Lac-Brompton.Tiré de Schroetter et al. (2006).
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Tremblay & Castonguay, 2002; Fig. 1). Amphibole and mica 40Ar/39Ar ages from
the internal Humber Zone vary between 431 and 410 Ma (Fig. 2). Ordovician high-temperature step ages (462-460 Ma; Fig. 2) suggest that the geochronologic imprint of typical Taconian metamorphism is only locally preserved (Castonguay et al., 2001; Tremblay & Castonguay, 2002). To the southeast, the Internal Humber Zone is bounded by the Saint-Joseph Fault (Pinet et al., 1996) and the BBL, which form a composite east-dipping normal fault system marking a boundary with less metamorphosed rocks in the hangingwall (Fig. 1). East of the Saint-Joseph-BBL fault system, continental metamorphic rocks, which yielded Middle Ordovician 40Ar/39Ar muscovite ages (469-461 Ma; Whitehead et al., 1995; Castonguay et al., 2001) are locally exposed in the core of antiformal inliers.
The Dunnage Zone occurs in the hangingwall of the Saint-Joseph-BBL fault system and comprises ophiolites, mélanges, volcanic arc sequences, and marine flysch deposits. In southern Québec it is made up of four lithotectonic assemblages (Fig. 1): (1) the Southern Quebec ophiolites, mainly represented by four massifs, the Thetford-Mines, Asbestos, Lac-Brompton and Mont-Orford ophiolites; (2) the Saint-Daniel Mélange; (3) the Magog Group forearc basin; and (4) the Ascot Complex volcanic arc (see Tremblay et al., 1995 for a review).
THE SOUTHERN QUÉBEC
DUNNAGE ZONE
The ophiolites of the Thetford-Mines and Asbestos areas are characterized by well-preserved mantle and crustal sections, whereas only the mantle and a dissected part
of the oceanic crust is exposed in the Lac-Brompton ophiolite. U/Pb zircon dating from felsic rocks
Figure 2. Diagram summarizing age constraints for the deformation and metamorphic events in the southern Quebec Appalachians. Figure 2. Sommaire des contraintes d’âges pour la déformation et le métamorphisme dans les Appalaches du sud du Québec. of the Thetford-Mines and the Asbestos ophiolites yielded ages of 479 ± 3 Ma and 478-480 +3/-2 Ma, respectively (Fig. 2; Dunning et al., 1986; Whitehead et al., 2000). These three ophiolitic massifs are dominated by magmatic rocks with boninitic affinities (with subordinate tholeiites), a feature which has been attributed to their genesis either in a forearc environment (Laurent & Hébert, 1989; Hébert & Bédard, 2000; DeSouza et al., 2006), and/or in a backarc setting (Oshin & Crocket, 1986; Olive et al., 1997). In contrast, only the crustal section is present in the Mont-Orford ophiolite, which contains a greater diversity of magma types, interpreted in terms of arc-backarc (Harnois & Morency, 1989;
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Laurent & Hébert, 1989; Hébert & Laurent, 1989) or arc-forearc to backarc environments (Huot et al., 2002). The Mont-Orford ophiolite has a maximum age of 504 +/- 3 Ma (David & Marquis, 1994).
Amphibolites from the dynamothermal sole of the Thetford-Mines ophiolite and adjacent continental micaschists yielded 40Ar/39Ar ages of 477±5 Ma (Whitehead et al., 1995) and 469-461 Ma (Fig. 2; Castonguay et al., 2001), respectively, suggesting that intra-oceanic detachment of the ophiolite (ca. 477 Ma) occurred immediately after oceanic crust formation (ca. 480 Ma); with emplacement against continental margin rocks and associated metamorphism occurring afterwards (ca. 470-460 Ma).
The Saint-Daniel Mélange (Fig. 1) is a Llanvirn lithostratigraphic unit that represents the lowermost series of the western (present coordinates) part of a forearc basin that lies on a partly-eroded ophiolite basement and which is mainly represented by the Magog Group (Schroetter et al., 2006). The lower contact of the mélange represents an erosional unconformity marking the base of the forearc basin. The processes that formed the chaotic and breccia units of the mélange were the successive uplift, erosion, and burial by heterogeneous and localized debris flows of different parts of the ophiolite and of the underlying metamorphic rocks during the emplacement of the ophiolite. A 467 Ma 40Ar/39Ar muscovite age yielded by metamorphic fragments of basal debris flows of the Saint-Daniel mélange (Schroetter et al., 2006) is within the age range of regional metamorphism in rock units structurally below the ophiolites and implies that the exhumation of these rocks occurred during or shortly after the emplacement of the ophiolite onto the continental margin.
The Magog Group (Fig. 1; Cousineau &
St-Julien, 1994) overlies the Saint-Daniel Mélange. It is made up of four units: (i) lithic sandstones and black shales of the Frontière Formation; overlain by (ii) purple-to-red shales, green siliceous siltstones and fine-grained volcaniclastic rocks of the Etchemin Formation; overlain by (iii) pyritous black shales and volcaniclastic rocks of the Beauceville Formation; overlain by (iv) sandstones, siltstones and shales with occurrences of tuff and conglomerate constituting the Saint-Victor Formation, which makes up over 70% of the thickness of the Magog Group. Graptolites, Nemagraptus gracilis, found in the Beauceville and Saint-Victor formations are Late Llandeilian to Early Caradocian (Middle Ordovician).
The Ascot Complex (Fig. 1) has been interpreted as the remnant of a 460 ± 3 Ma volcanic arc sequence (Tremblay et al., 1989; Tremblay et al., 2000). It is made up of various metavolcanic rock series, in fault contact with laminated and pebbly phyllites that have been correlated with the Saint-Daniel Mélange (Tremblay & St-Julien, 1990).
Structure and metamorphism
In the Southern Québec Dunnage Zone, regional deformation and metamorphism are related to the Middle Devonian Acadian orogeny (Tremblay 1992; Cousineau & Tremblay, 1993). Peak metamorphism varies from greenschist grade in the south (i.e., in the vicinity of the Québec-Vermont border), to prehnite-pumpellyite grade in the Chaudière river area (Fig. 1). 40Ar/39Ar dating of greenschist-grade metamorphic rocks of the Ascot Complex yielded 380-375 Ma (Fig. 2; Tremblay et al., 2000). The Magog Group is characterized by tight regional folds, generally overturned to the NW. Folds plunge gently or
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moderately to the SW or the NE. Evidence for intense Ordovician (Taconian) metamorphism and deformation is absent.
Schroetter et al. (2005) have shown that the Humber zone, the southern Quebec ophiolites and the overlying Saint-Daniel mélange share a similar structural evolution. Detailed mapping in the Thetford-Mines ophiolite has been used to discriminate pre-, syn- and post-obduction structures. Syn-obduction (Taconian) structures include shear zones and ductile fabrics developed in the ophiolitic metamorphic sole, and in the immediately overlying mantle and underlying continental margin rocks. Two generations of post-obduction structures are recognized: (i) SE-verging backthrusts and backfolds correlated with the Late Silurian-Early Devonian deformational episode recorded in the Humber Zone (Fig. 2); and (ii) NW-verging folds and reverse faults attributed to the Acadian Orogeny (Tremblay & Castonguay, 2002). However, as a result of normal faulting along the Saint-Joseph-BBL fault system (see Fig. 1b), the backthrust deformation found in the ophiolites has a lower metamorphic grade than backthrust deformation occurring in the margin.
TECTONIC EVOLUTION
In the northern Appalachians, the Taconian orogeny was historically interpreted as the result of a collision between Laurentia and an island arc terrane that was formed over an east-facing subduction zone (e.g. Osberg, 1978; Stanley & Ratcliffe 1985). The Acadian orogeny is considered to be the consequence of the accretion of terrane(s) from the east by either a renewed tectonic convergence (Osberg et al., 1989) or by polarity flip of a Taconian subduction zone (van Staal et al. 1998).
On the basis of age data for arc volcanism
and ophiolite genesis in southern Québec, as well as similar lithological and structural settings of ophiolites from southern Québec and western Maine, Pinet and Tremblay (1995) proposed an alternative hypothesis for the Taconian orogeny. In their model, the Taconian deformation and metamorphism of the Laurentian margin is attributed to the obduction of a large-scale ophiolitic nappe that predates any collisional interaction with the volcanic arc.
The structural evolution of the Laurentian continental margin and adjacent Dunnage Zone of southern Québec have been summarized by Tremblay & Castonguay (2002). The Taconian stage (ca. 480 to 445 Ma) involves stacking of northwest-directed thrust nappes (Fig. 3a). D1-2 deformation progressed from east to west, from ophiolite emplacement and related metamorphism in the underlying margin in the early stages of crustal thickening, to the piggyback translation of accreted material toward the front (west side) of the accretionary wedge. Obducted oceanic crust remained relatively undeformed except for minor tectonic slicing. Underplating of the overridden margin and foreland (westward) translation of metamorphic rocks because of frontal accretion have led to progressive exhumation of deeper crustal levels of the orogen (hence preserving Ordovician isotopic ages), parts of which are now preserved below the ophiolite in the downthrown side of the St-Joseph-BBL fault system.
D3 deformation began in latest Early Silurian time (ca. 430 Ma), and lasted until the Early Devonian (ca. 410 Ma; Fig. 3b). 40Ar/39Ar age data suggest that D3 first consisted of ductile shear zones defining a major upper plate-lower plate (UP-LP) boundary, i.e. the Bennett-Brome fault, and culminated with normal faulting along the St-
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Joseph fault and the Baie Verte-Brompton line
Figure 3. Schematic model for structural evolution of Laurentian margin in southern Quebec. 1- Grenvillian rocks, 2- St. Lawrence Lowlands platform, 3- External Humber zone, 4- Internal Humber zone, 5-6- ophiolites and sedimentary rocks of Dunnage zone, respectively. Figure 3. Modèle schématique de l’évolution structurale de la marge Laurentienne dans le sud du Québec. 1- Grenville, 2- plate-forme des Basses-Terres du St-Laurent, 3- zone de Humber externe, 4- zone de Humber interne, 5-6- ophiolites et roches sédimentaires de la zone de Dunnage, respectivement. (Fig. 3b). The upper plate is made up of a folded stack of D1-2 nappes of deformed and
metamorphosed rocks of the Taconian accretionary wedge and includes metamorphic rocks that retain Ordovician ages. Low- and high-angle normal faulting was probably activated in Late Silurian-Early Devonian time (Figs 2 and 3b) and crosscut the UP-LP boundary, which led to the juxtaposition of metamorphic rocks from different crustal levels on both sides of the St-Joseph-BBL fault system. East of the St-Joseph-BBL fault sytem, the D3 event thus accounts for the presence of external-zone rocks, their juxtaposition with ophiolites or underlying metasedimentary rocks, and the presence of SE-verging recumbent folds (originally interpreted as gravity nappes by St-Julien & Hubert, 1975).
Acadian compression resulted in the folding of D1-2 and D3 structures and in the passive rotation and steepening of high-angle normal faults (Fig. 3c). Tectonic inversion of normal faults has probably occurred.
THE SOUTHERN QUÉBEC OPHIOLITE BELT
The ophiolites of southern Québec have
been historically considered as km-scale, fault-bounded blocks within the Saint-Daniel Mélange, which was interpreted as a subduction-accretionary complex (Cousineau & St-Julien, 1992; Tremblay et al., 1995) in fault contact both with the ophiolites and the Magog Group. Mapping in key areas of the ophiolitic belt (Schroetter et al., 2003, 2005, 2006) indicates, however, that the Saint-Daniel Mélange is a sedimentary sequence that stratigraphically overlies the ophiolites and is, in turn, depositionally overlain by the Magog Group (Fig. 4). As such, the Saint-Daniel Mélange forms the lowermost part of a
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Figure 4. Geological map showing the location of major ophiolites and the inferred distribution of the various facies of the Saint-Daniel Mélange in the southern Québec Appalachians. Compiled from mapping by Cooke (1938, 1950), Brassard and Tremblay (1999), St-Julien and Slivitsky (1985) and Schroetter (2004). Figure 4. Carte géologique montrant la localisation des principales ophiolites et la distribution présumée des différents faciès sédimentaires du Mélange de Saint-Daniel dans les Appalaches du sud du Québec. Compilé des travaux de cartographie de Cooke (1938, 1950), Brassard et Tremblay (1999), St-Julien et Slivitsky (1985), et Schroetter (2004).
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Figure 5. A) Inferred regional tectonic setting of the Southern Québec Appalachians during the Taconian orogeny, and schematic sedimentary and tectonic evolution of the Saint-Daniel Mélange and the Magog Group. B) Schematic composite structural profile across the Laurentian margin and the adjacent oceanic domain. From Schroetter et al. (2005). Figure 5. A) Contexte tectonique inféré pour les Appalaches du sud du Québec pendant l’orogénie Taconienne, illustrant aussi l’évolution sédimentaire et tectonique du Mélange de Saint-Daniel et du Groupe de Magog. B) Profil structural composite au sein de la marge Laurentienne et le domaine océanique. Tiré de Schroetter et al. (2005). piggyback basin that records the infilling of gan inherited topography of the forearc oceanic crust (Fig. 5a). Schroetter et al. (2005) have suggested that the southern Québec ophiolites were probably accreted to the margin as a single, large slab of supra-subduction oceanic lithosphere. These ophiolites should not be, therefore, considered as genetically unrelated tectonic slices
incorporated into a subduction complex (i.e. the Saint-Daniel Mélange), but as a coherent segment of oceanic crust (although structurally complex and partially dismembered) which extends laterally for over a hundred kilometres of strike, and that has experienced at least two episodes of regional deformation after obduction (Fig. 5b).
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The Thetford-Mines Ophiolite The Thetford-Mines ophiolite crops out as a NE-trending belt, 40 km in length and 10-15 km in width (Fig. 4). It is divided into the Thetford-Mines (TM) massif to the northwest and the Adstock-Ham Mountains (AHM) massif to the southeast (Figs. 6, 7 and 8). The TM massif has a ca. 5 km thick mantle section (Laurent et al., 1979; Pagé et al., 2003) and a 0.5 to 1.5 km-thick crustal section (Schroetter et al., 2005). The oceanic mantle is not preserved in the AHM massif. The crustal section in both massifs consists of dunitic, pyroxenitic and gabbroic cumulates, crosscut by mafic to ultramafic dikes (all of boninitic affinity), which locally grade up into a sheeted dike complex (Bédard et al., 2001; Schroetter et al., 2003). The extrusive sequence is extremely variable (Fig. 7); both in thickness and lithology, but boninitic lava flows and felsic pyroclastic rocks dominate. The ophiolitic extrusive sequence is overlain by discontinuous debris flows (Coleraine Group of Riordon, (1954) and Coleraine breccia of Hébert, (1981), which are characterized by cm- to m-scale angular fragments, which are typically ophiolite- and continentally-derived (ultramafic, volcanic, sedimentary, and metamorphic clasts). The debris flows wedge out laterally into fine-grained siliciclastic rocks, and grade up into turbidites, argillites, siltstones and pebbly mudstones of the Saint-Daniel Mélange (Schroetter et al., 2006).
The Asbestos Ophiolite The Asbestos ophiolite is located
approximately 20 km to the southwest of the southernmost extremity of the Thetford-Mines ophiolite (Fig. 4). It preserves a thinner (2000-2500m) but very similar ophiolitic sequence, consisting of harzburgitic mantle, overlain by ultramafic-to-mafic cumulates (dunite, pyroxenite and gabbro), and capped by
diabasic and volcanic rocks (Hébert, 1980). The ophiolitic lavas are overlain by fine-grained volcaniclastic rocks and flow breccias, and then by the Saint-Daniel Mélange (Lavoie, 1989).
The Lac-Brompton Ophiolite
Ophiolitic plutonic rocks occurring in the vicinity (south) of the Saint-François River (Fig. 4), consist of dunitic peridotite, gabbro, lava and volcaniclastic rocks, and can be correlated withrocks of the Asbestos ophiolite (Schroetter et al., 2005). This sequence of ophiolitic rocks extends almost continuously southwards until it merges into the Lac Montjoie ophiolitic mélange (Lamothe, 1978), previously described as a serpentinite diapir. Recent fieldwork suggests that Lac Montjoie is part of an ophiolitic sequence (mantle and lower crustal peridotites) forming the Lac-Brompton ophiolite (Daoust et al., 2005). These ultramafic rocks are overlain by Asbestos-type mafic lavas and tuffs, and by the Saint-Daniel Mélange, with the whole sequence defining a northward-plunging anticline. The continuity of lithologies and exposures suggest that ophiolitic rocks crop out discontinuously between Asbestos and Lac-Brompton. Given the resemblance between the Thetford—Mines, Asbestos and Lac-Brompton ophiolites, this implies that the ophiolitic rocks of this large area (over 100 km of strike length) may originally have formed a single panel of obducted oceanic lithosphere (Schroetter et al., 2005; DeSouza et al., 2006).
The Mont-Orford Ophiolite The Mont-Orford ophiolite (Fig. 4) occurs
as two main masses, the Mont Chauve and the Mont Orford-Chagnon massifs, which are dominated by gabbro, and overlain by basalt and various types of volcaniclastic rocks
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Figure 6. Geological map of the Thetford-Mines ophiolitic Complex and location of stops. From Schroetter et al. (2005). Figure 6. Carte géologique du Complexe ophiolitique de Thetford-Mines et localisation des arrêts. Tiré de Schroetter et al. (2005).
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Figure 7. Stratigraphic columns for different locations of the Thetford-Mines ophiolitic Complex. From Schroetter et al. (2003). Figure 7. Colonnes stratigraphiques de différentes localités du Complexe ophiolitique de Thetford-Mines. Tiré de Schroetter et al. (2003). (Rodrigue, 1979; Laurent & Hébert, 1989; Huot et al., 2002). The tectonostratigraphic link between the Asbestos and Mont-Orford ophiolites can be inferred from structural relationships shown by the ophiolitic rocks in the Lac-Brompton area (Fig. 4) where the southern extremity of the Lac-Brompton ophiolite is separated from the Mont-Orford
ophiolite by an extensive metamorphic unit of micaschists and albite-chlorite laminated greenschist (Schroetter et al. 2005; Daoust et al. 2006).
According to Schroetter et al. (2005), the Mont-Orford ophiolite and overlying Saint-Daniel Mélange are structurally overlain by the metamorphic rock unit mentioned above,
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and by the Lac-Brompton ophiolite; the structural juxtaposition occurring along a northwest-dipping fault zone (the ϕ3 fault of Fig. 5b) which is folded by a north-plunging Acadian antiform (Fig. 5b).
Figure 8. Structural profile A-A’ of the Thetford-Mines ophiolite. See Fig. 6 for location. Figure 8. Profile structural A-A’ de l’ophiolite de Thetford-Mines. Voir la Fig. 6 pour la localisation. FIELD TRIP ROAD LOG
DAY 1 - THE MANTLE AND LOWER CRUST OF THE THETFORD-MINES
OPHIOLITE The first day of the fieldtrip focuses on the mantle and lower crust of the Thetford-Mines ophiolite, with a particular emphasis on magmatic and structural features interpreted to be the result of pre-obduction (syn-magmatic) faulting. Stop locations are shown on Figs. 6, 9 and 11. Figures 7 and 8 present stratigraphic columns established from different sites of the ophiolite, and a NW-SE trending structural profile, respectively. Detailed mapping in the Thetford-Mines ophiolite has shown the presence of sub-vertically dipping, N-S-striking faults, spaced
~1 km apart on average (Fig. 6; Schroetter et al., 2003). In the lower crust, the faults are manifested as sheared dunites and syn-magmatic breccias, and correspond to breaks in lithology. The kinematic analysis suggests that these structures are normal faults separating a series of tilted blocks. In the upper part of the crust, the N-S-striking fault blocks contain N-S-striking dykes that locally constitute a sheeted complex. The faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks (Fig. 7), are locally marked by prominent subvolcanic breccias, and have upwardly decreasing throws suggesting that they are growth faults. STOP 1.1: The metamophic sole and the gabbroic sequence of the Thetford-Mines ophiolite at Breeches lake
Location: From Montréal, take Hw 20 East until the Thetford-Mines – Plessisville exit (approx. 2 hours driving). At the exit, follow road 263 South until Plessisville, and then 265 South until the town of Black-Lake (approx. 70 Km). At the intersection with road 112, turn right, drive to the town of Disraeli and turn right at the flashing light. Just after the bridge, turn right on road 263 North, and drive until Lake Breeches (approx. 8 km). The dynamothermal sole outcrop is a roadcut located 1.3 km west of the rest area on the lake, whereas the ophiolitic gabbro crops out just in front of the lake (Fig. 9).
Field description: The dynamothermal sole. From east to west, the outcrop exposes (Fig. 10), 1) serpentinized ultramafic rocks, 2) approximately 20 m of amphibolite with a strong foliation (S1) defined by the preferred orientation of sodic hornblende and epidote crystals (also rutile and albite), garnet is absent in this outcrop,
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Figure 9. Geologic map of the Belmina Ridge area (from Whitehead et al., 1995) and location of stop 1.1. Figure 9. Carte géologique de la region de Belmina Ridge (tirée de Whitehead et al., 1995) et localisation de l’arrêt 1.1. and 3) garnet-bearing micaschists belonging to the Laurentian margin sequence. The contact between (2) and (3) is sharp, strikes N340o and dips 65oE. Microprobe mineral compositions determined on clinopyroxene-garnet-amphibole and garnet-amphibole assemblages suggest that near its upper contact, the amphibolite reached equilibrium temperatures of 780-850oC, and that temperatures decreased to 380-500oC away from the contact. Inferred pressures varied between 5 and 7 kbars. The data indicate an
Figure 10. Field sketch of the contact ophiolite-margin at the Belmina Ridge amphibolitic sole (stop 1.1). Figure 10. Schéma de terrain du contact ophiolite-marge sur le site de l’amphibolite de Belmina Ridge (arrêt 1.1). inverse thermal gradient of 40ºC/Km (Feininger, 1981). The gabbroic sequence. These outcrops expose complexly-deformed and hydrothermally metamorphosed layered gabbro, pyroxenite and boninitic dykes. Layered gabbro is locally transformed to high-temperature amphibolite + trondhjemite in a 1st syn-ridge event. Igneous rocks are dissected into 10 x 2 m phacoidal slivers by tightly-spaced brittle-ductile greenschist facies shear zones. Epidozite veins abound in these shear zones, with older epidote veins being folded and then cut by younger epidote veins. Prominent chloritic haloes surround this generation of veins. The volume of epidozite suggests a focussed hydrothermal discharge, with fluid volumes most consistent with a ridge-related system. A younger generation of breccia-veins are filled with quartz-prehnite-calcite crosscuts all older structures. The age and origin of this youngest event is not known. STOP 1.2: The upper mantle at Vimy Ridge
Location: Drive back to Disraeli and follow road 112 West until the town of St-Joseph-de-Coleraine. In town, turn left on Vimy road and drive for 7 Km until the Vimy
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settlement. Park in front of the house before the last one on your left. The outcrop is located just behind that house and is accessible via the drive-way.
Field description: This outcrop exposes harzburgitic mantle peridotite with subordinate dunitic pods. There are numerous orthopyroxenite dykes, and abundant Asbestos veins (stockwerk) showing various orientations. This outcrop is part of the Caribou Mountain Block of Pagé et al. (in preparation). It shows a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N-S, parallel to the regional orientation of seafloor-spreading related paleo-normal faults in the crust. The nature of fabrics and textures suggests a lithospheric deformation, possibly related to tectonic denudation (oceanic core complex; Tremblay et al., 2006; Pagé et al., in preparation). This would explain problematic lava/mantle contacts in the area (Fig. 6). Mineral chemical and whole-rock geochemical data imply that the harzburgite is residual from extensive partial fusion (Hébert, 1985; Hébert & Laurent, 1989). STOP 1.3: Scenic view of mine Lac d’Amiante at Black Lake
Location: From the last stop, continue straight ahead until the intersection with road 265. Turn right and drive to the traffic light at the intersection between roads 265 and 112. Turn right on road 112, drive for ca. 4 Km and park at the Belvedere on your right.
Field description: From the Belvedere, we can see the open pit mine in the upper mantle sequence of the Thetford-Mines ophiolite. A closer examination of the open pit reveals the presence of light-coloured intrusions that are foliated to unfoliated, and partly rodingitized granodiorites and granites (Laurent & Baldwin, 1987). These two-mica
granitoids are peraluminous, have high 87Sr/86Sr initial ratios and igneous zircons with low 208Pb/206Pb ratios, and have yielded 469+/-4 to 470+/-5 Ma crystallization ages (Fig. 2; U/Pb on zircon, Whitehead et al., 2000), suggesting that they derived by the partial fusion of continental margin sediments during emplacement of the ophiolite (Clague et al., 1985; Whitehead et al., 2000). STOP 1.4: Dunites, chromitites and structure of the lower crust at the American Chrome mine
Location: From the Belvedere, continue East onto road 112. After ca. 3.5 Km, turn left onto Cariboo Lake road. Stay on this road for ca. 4.5 Km. At the intersection with the Petit-Lac-St-François road, keep your left and take the next dirt road on your right (ca. 1 Km from the intersection). Drive that dirt road for ca. 1.5 Km and park (just after the swamp). Follow the path marked by the red flagging tape until you reach exploration trenches and cleared areas of the American Chrome mine (ca. 800 m from parking).
Field description: This outcrop exposes various facies of dunitic crust of the AHM massif (Fig. 6). Two different generations of dunite are present, (1) a pale green dunite with centimetric layers and schlieren of chromitite defining a magmatic bedding, and (2) pale brown, breccia-like dunitic bodies that crosscut and brecciated the older dunite. Centimetric and metric rodingitized felsic dykes are also exposed (Fig. 12). All lithologies are cut by NE-trending brittle-ductile shear zones. Rodingite dykes can be used as markers and clearly indicate that these shear zones represent a well-developed network of NW-verging reverse faults. These minor structures are interpreted as Acadian faults genetically associated with a major
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Figure 11. Geological map of part of the Adstock-Ham Massif showing the location of normal(?) faults interpreted as syn-magmatic structures. From Schroetter et al. (2003). Stops 1.5 and 2.1 are located on the map. Figure 11. Carte géologique d’une partie du Massif d’Adstock-Ham montrant la localisation de failles normales(?) interprétées comme des structures syn-magmatiques. Tirée de Schroetter et al. (2003). Les arrêts 1.5 et 2.1 sont localises sur cette carte. reverse fault at the contact between the AHM and the Saint-Daniel Mélange, less than 100 metres west of the American Chrome mine (see structural profile of Fig. 8).
In terms of regional structures, these reverse faults and related folds are responsible for : the formation of (a) a dome-and-basin interference pattern with antiformal culminations corresponding to the Carinault and Bécancour antiforms (Fig. 6), and (b) a NW-directed major fault that juxtaposed the lower crust of the AHM against Saint-Daniel
sediments, and (c) folding of the Saint-Daniel Mélange and the overlying Magog Group. STOP 1.5: Syn-oceanic deformation features of the lower crust at Mamelon Nadeau
Location: Drive back to road 112 and drive East towards St-Joseph-de-Coleraine. At the entrance of town, turn left (just after the local arena) onto Bisby Lake road. Stay on the main gravel road for 4 Km and park just in front of a cleared area on your left. The next
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Figure 12. Detailed map of the American Chrome mine (stop 1.4) showing late (Acadian) NE-trending, SE-dipping reverse faults crosscutting the dunitic zone of the AHM massif. From Schroetter et al. (2005). Figure 12. Carte détaillée du site de la mine American Chrome (arrêt 1.4) montrant une série de failles inverses (acadiennes) orientées NE et à pendage SE recoupant la zone dunitique du massif d’Adstock-Ham. Tirée de Schroetter et al. (2005). outcrop is a 500 metres walk. It is located on top of the smaller and nearest hill to the north, the so-called Mamelon Nadeau.
Field description: The location of this outcrop is shown on Fig. 11. This map area has been extensively described by Schroetter et al. (2003), these outcrops corresponding to their sector 1. The Mamelon Nadeau is entirely contained within the Dunitic Cumulate Zone (Fig. 11). From West to East, massive dunite with disseminated chromite gives way (over 0.5 m) to a thick (100s of m) breccia composed of angular, 1-10 cm, clasts of dunite, locally with chromitite beds within them, in an orthopyroxenitic stockwork as exposed at the base of the hill (stop 5a on Fig. 13). As the main fault plane is approached,
sheared serpentinites appear in the dunite, culminating in 2-3-m-wide serpentinite mylonites that mark the core of faults. These breccias and mylonites are cross-cut by undeformed, tabular websterite and lherzolite intrusions (30-50-m-wide), that are oriented N-S, parallel to the main series of faults. At the top of the Mamelon Nadeau (stop 5b on Fig. 13), the rhythmically-layered chromitites are cut by shallowly-dipping, E-W striking websterite dykes that lack chilled margins against their dunitic hosts, suggesting that these rocks were still quite hot at the time of dyke emplacement. A series of faults associated with serpentinite veins are parallel to dyke contacts and offset chromitite beds to the East. On the same outcrop, these same
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Figure 13. Field sketch for stop 1.5 illustrating the inferred syn-magmatic faults preserved at the base of the ophiolitic crust. Figure 13. Schéma de terrain de l’arrêt 1.5 illustrant les failles syn-magmatiques présumées préservées dans la section crustale de l’ophiolite. websterite dykes are chopped up into cm-scale horst-and-graben structures by a series of conjugate, steeply-dipping, N-S-striking normal faults. 50 metres further north, another exposure shows these same dykes being sheared and boudinaged due to the occurrence of another fault with a peridotite intrusion in its hanginwall (stop 5c on Fig. 13).
From this series of exposures, the history of syn-magmatic deformation can be divided into three increments (E1 to E3; Schroetter et al., 2003). The early event (E1) corresponds to the localized development of a high-temperature layering-parallel fabric (chromitite schlieren and isoclinal folds). Restoration to the horizontal of the chromitite beds gives the websterite dykes and parallel faults of increment E2 a sub-vertical orientation, E2 faults are thus interpreted as having originally been steeply-dipping, normal faults. The last increment of deformation (E3) defines a horst-and-graben system marking a continuation of extension.
End of day 1.
Drive back to road 112 and drive toward Thetford-Mines (via road 112 East) – Beer Time.
DAY 2 - THE UPPER CRUST AND SEDIMENTARY COVER OF THE THETFORD-MINES OPHIOLITE
The second day of the fieldtrip focuses on the upper crust and overlying sedimentary sequence of the Thetford-Mines ophiolite. We will show examples of lateral variations of facies of the upper crust and the overlying sedimentary rocks. We will also show evidence that the upper contact of the ophiolite represents a major erosionnal surface (Fig. 7). The upper crust of the Thetford-Mines ophiolite consists of gabbros, hypabyssal facies rocks and basaltic volcanic rocks overlain by volcaniclastic and sedimentary rocks. The Gabbroic Zone is up to 1200 m thick and consists of interlayered norites and gabbronorites at the base, gabbros in the middle, and an upper complex composed of hornblende gabbro, hornblendite, dykes, trondhjemitic intrusions and breccia veins. Two types of hypabyssal facies rocks occur: dyke swarms and breccias. The dykes (30 cm to ~ 1 m thick) are mafic to ultramafic, show microgabbroic or aphanitic textures, and are commonly oriented N-S. In some sectors, the dykes constitute 40-100% of the outcrop over 100s of meters, and are mapped as a sheeted dyke complex (Fig. 11). The breccia facies reaches a maximum of 150 m in thickness and separate plutonic and volcanic sequences. Where it has been studied in detail (Schroetter et al., 2003), the breccia facies caps the gabbroic sequence and is overlain by boninitic lavas and volcaniclastic deposits. The volcanic and volcaniclastic rocks exhibit marked lateral changes in thickness (Figs. 6 &
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11). The volcaniclastic rocks are commonly blocky tuffs containing rounded pillow-lava fragments in a sandy volcaniclastic matrix. Vesicular pillow lavas are intercalated with massive flows, hyaloclastite breccias, and submarine talus breccias. Abundant pyroclastic flow breccias containing rounded clasts of dacite, gabbro and pyroxenite, with intercalated fine-grained dacitic tufs and argillites are locally present. At Lac de l’Est (Fig. 6), a 1-2-m red argillite separates a lower volcanic unit composed of tholeiites and boninites from an upper unit dominated by boninites (Hébert, 1983; Hébert & Bédard, 2000). STOP 2.1: Upper gabbros and the brecciated hypabyssal sequence at Aux Boulettes section Location: Drive back to stop 1.5 of day 1 and continue onto Bisby Lake road until the next road intersection. Turn left and park after 1.8 Km (i.e. at the right-angle bend of the road). Follow the trail going NW from that point and keep going for ca. 100-150 metres in the same direction. Field description: This series of outcrops (Fig. 11) exposes hypabyssal breccias. The breccia matrix is generally igneous, and the jigsaw-puzzle morphology of the rocks seems compatible with some type of magmatic hydro-fracture mechanism, perhaps complemented by phreato-magmatic explosions caused by ascent of magma into rocks impregnated with seawater (Fig. 14; Schroetter et al., 2003). Amygdaloidal intrusions are injected into the microgabbroic breccia, and then are stretched and offset by faults (Fig. 11), suggesting that faulting and magmatism were coeval. Field observations suggest that syn-magmatic faulting must have played a role in brecciation, because some
Figure 14. Schematic illustration of a possible evolutionary scenario for the main crust-forming event of the Thetford-Mines ophiolite. From Schroetter et al. (2003). Figure 14. Illustration schématique d’un scénario d’évolution possible pour la formation de la croûte de l’ophiolite de Thetford-Mines. Tiré de Schroetter et al. (2003).
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breccias have a cataclastic matrix, and field mapping shows that the brecciated hypabyssal facies is preferentially developed along the extension of the major N-S normal faults described in the area (Fig. 11). STOP 2.2: Boninitic pillows and feeder dykes of the Mont Adstock section Location: Continue towards East on the gravel road where you were parked. At the intersection with St-François Lake road, turn left. Follow this road for ca. 9 Km. The outcrop is located along the right side of the road, in the vicinity of a right-angle bend of the road towards the left. Field description: This outcrop exposes a well-developed sequence of pillowed boninitic lavas and breccia crosscut by mafic dykes. Vesicular pillow lavas of 1-1.5 m in size alternate with smaller pillows (0.5 m average), with intercalated massive flows and with hyaloclastite and pillow breccias. Pillows are almost undeformed and their alignment and shape suggest East-West trending volcanic flows and a south- to southwest-directed way-up. The mafic dykes are also of boninitic composition. The dykes show asymmetrical chilled margins and dykes-in-dykes structures indicating that they represent feeder conduits to the volcanic sequence. STOP 2.3: Sedimentary rocks of the Rivière-de-l’Or section Location: Drive back south on the same road and turn left at the next intersection. Cross the bridge and park. The outcrop is located in the river bed, a few tens of metres north of the bridge. Field description: The Rivière-de-l’Or section (see Fig. 6 for location) is 600 meters thick (Fig. 15). The bedding is subvertical and parallel to the regional schistosity. At the base of the section, the sedimentary rocks overly an
Figure 15. Stratigraphic section of the Saint-Daniel Mélange as exposed in the Rivière de l’Or (stop 2.3). From Schroetter et al. (2006). Figure 15. Section stratigraphique du Mélange de Saint-Daniel tel qu’exposée dans la Rivière de l’Or (arrêt 2.3). Tiré de Schroetter et al. (2006). ophiolitic substrate made up of pillow basalts, volcanic breccias and intermediate to felsic pyroclastic rocks. Sedimentary facies show an evolution (base to top) from massive clast-supported breccias, to interbedded matrix-supported microconglomerates and sandstones, to well-bedded and fine- to coarse-grained sandstones, and finally to
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interbedded fine-grained sandstones, siltstones and mudstones. This outcrop exposes the uppermost part of the section. It shows conglomerates containing both ophiolitic and metamorphic rock fragments, and which are interbedded with, and grade into black mudslates typical of the Saint-Daniel Mélange (Fig. 15). STOP 2.4: Metamorphic clast-bearing debris flows overlying ophiolitic gabbros Location: Drive back to St-François Lake road. Turn left and drive back to St-Joseph-de-Coleraine via Bisby Lake road. Turn left at the intersection with road 112, and take the second road to your right (St-Julien road). Turn left at the next intersection after approximatively 1.5 Km. Park in the entrance of the first farm on your left. Walk the farm road towards East. The outcrop is in a cleared area located 700 metres away on the right side of the farm road. Field description: Stops 2.4 and 2.5 (Fig. 6 for locations) illustrate lateral variations of sedimentary facies that characterizes the basal part (Unit 1 of the Saint-Daniel Mélange of Schroetter et al., 2006) of the cover sequence of the Thetford-Mines ophiolite (Fig. 16). This outcrop exposes the contact between brecciated upper gabbros and a metamorphic clast-bearing debrite horizon (Fig. 16). The gabbroic breccia is composed of centimetric to decimetric angular clasts of aphanitic ‘dolerite’, layered gabbro, medium-grained to pegmatitic hornblende gabbro, and microgabbro. The angularity of clasts indicates only limited transport, while jigsaw-puzzle textures imply in-situ brecciation. The matrix is typically igneous (microgabbro), but hydrothermal assemblages are also present. The debrite horizon is a few metres thick and is made up of angular, centimetric clasts
Figure 16. Schematic illustration of facies variations of the ophiolitic sedimentary cover sequence between stops 2.4 (debris flows on top of gabbros) and 2.5 (well-bedded, fine-grained sedimentary sequence overlying gabbros). Figure 16. Illustration schématique de la variation latérale de faciès dans la sequence sédimentaire supra-ophiolitique entre les arrêts 2.4 (coulées de débris par-dessus les gabbros) et 2.5 (série sédimentaire à grains fins, bien stratifiée, surmontant les gabbros). of quartzite, micaschist and quartz veins. The debrite is devoid of sedimentary textures but grades up (towards the north) into sandstones with graded-bedding (Fig. 16). 40Ar/39Ar analysis of micaschist clasts from a similar horizon in the Mont Adstock area yielded a 40Ar/39Ar muscovite plateau age of 467±2.6 Ma (Fig. 17; Schroetter et al., 2006), which is within the range of 40Ar/39Ar muscovite ages (469-461 Ma, see Fig. 2) measured in metasedimentary rocks structurally underlying the ophiolite. This indicates that metamorphic rocks underlying the ophiolite are the most probable source for metamorphic fragments, and more significantly, that these metamorphic source rocks were rapidly uplifted to the surface during or shortly after the emplacement of the oceanic crust onto the continental margin.
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Figure 17. 40Ar/39Ar age spectra for two muscovite samples from a metamorphic clast of supra-ophiolitic debris flows. From Schroetter et al. (2006). Figure 17. Spectre d’äge 40Ar/39Ar de deux échantillons de muscovite provenant d’un fragment de micaschiste des coulées de débris supra-ophiolitiques. Tiré de Schroetter et al. (2006). STOP 2.5: Interbedded siltstones and mudstones overlying ophiolitic gabbros Location: Drive back to the St-Julien road and turn right towards St-Joseph-de-Coleraine. Turn right on the street just before the CN railway. The street continues straight ahead into a dirt road, drive it for 600 metres (keep your right) and park. There is a footpath going West, walk this trail for ca. 100 metres. The hill to your left consists of brecciated gabbros, the overlying sedimentary rocks are exposed at the toe of that hill. Field description: This outcrop is located ca. 800 metres East of stop 2.4, at the same stratigraphic level (Fig. 16). Gabbros forming the relief to the south are identical to the brecciated gabbro sequence of stop 2.4. The debris flow horizon is, however, absent here. The sedimentary sequence directly
overlying the gabbro consists of a metre-thick microconglomeratic horizon that grades-up into green, tuffaceous, parallel-laminated siltstone overlain by red mudstone. Lateral facies and thickness variations in the Saint-Daniel Mélange are most prominent in the basal unit (U1). This unit is 250 to 600 meters thick in the Thetford-Mines area but is absent from the Mont-Ham section. Lateral variations towards increasing proportions of continentally-derived debris (Fig. 16) suggest that parts of the continental margin upon which the ophiolite was emplaced was being uplifted, presumably through compression of the margin during the Taconian orogeny, while other parts of the ophiolite were being progressively buried beneath fine-grained deposits. STOP 2.6: The Coleraine Breccia at the type-locality Location: Drive back to the intersection between roads St-Julien and 112. Turn right on road 112, then left on the second street to your left. The outcrop is located in the cemetery. Field description: This outcrop and other exposures in the surrounding area (Fig. 6) constitute the type-locality for the Coleraine breccia (Riordon, 1954; Hébert, 1981). This is a polygenic debris flow breccia characterized by cm-to-m-sized fragments in a fine- to medium-grained matrix of greywacke. Fragments include basaltic lava, fine-grained sedimentary rocks (argillite and siltstone), gabbro, peridotite, hornblende diorite, and rare metasedimentary rocks. Large fragments (up to 1 meter) of red mudstone showing soft-sediment deformation structures are present within the breccia and are interpreted as large rafts or rip-up clasts. The occurrence of such argillitic fragments also indicates that part of the marine sedimentary sequence of the
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oceanic crust was reworked by debris flows formed during its emplacement onto the continental margin. Such breccias are overlain by, and interbedded with, a sequence of metre-thick beds of wackes and greywackes of the same composition as the breccia matrix. STOP 2.7: Turbidites of the Saint-Daniel Mélange at Ham-Sud Location: Drive road 112 East for ca. 30 Km, until Weedon-Centre. At the entrance of town, turn right on road 257 North. Drive this road to St-Joseph-de-Ham-Sud. In town, turn right and drive for 3.1 Km. The outcrop is a roadcut located at the intersection between the main gravel road and a dirt road going North (on the left). Field description: This outcrop is part of the Mont-Ham section (Figs. 6 and 18), which has been used by Schroetter et al. (2006) to illustrate the relations between their units U-2, U-3 and U-4 of the Saint-Daniel Mélange. The ophiolitic basement crops out 1 Km along the road to the north and is made up of the upper volcanic series of the Thetford-Mines ophiolite (Hébert, 1980). At Mont-Ham, the ophiolite is directly overlain by interbedded black sandstone and laminated, black and green argillite of unit U-2 as shown here. These rocks correspond to the S3 subfacies of the Saint-Daniel Mélange as described by Cousineau and St-Julien (1992). The black sandstone beds (fine- to coarse-grained, 0.1-2 meters-thick) show basal channelling, cross-bedding and parallel laminations. The argillite beds (5-80 cm thick) show evidence of syn-sedimentary deformation. A similar sedimentary succession, at the same stratigraphic level, has been described in the Asbestos area by Lavoie (1989), who interpreted it as a marine turbidite sequence.
Figure 18. Stratigraphic section of the Saint-Daniel Mélange as exposed in the Mont-Ham section (stop 2.7). From Schroetter et al. (2006). Figure 18. Section stratigraphique du Mélange de Sanit-Daniel telle qu’exposée dans le secteur du Mont Ham (arrêt 2.7). Tirée de Schroetter et al. (2006). STOP 2.8: Pebbly mudstone of the Saint-Daniel Mélange in Nicolet-Centre river Location: Drive back to Ham-Sud and turn right onto road 257. Follow road 257 until Saint-Adrien (ca. 12 Km), then turn left on road 216 West and drive for ca. 8 Km. Park just after crossing the bridge over
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Nicolet-Centre river. The outcrop is located in the river bed on the eastern side of the bridge.
Field description: This outcrop exposes the archetypical pebbly mudstone of the Saint-Daniel Mélange (see Fig. 4 for location), recognizable for more than 200 km of strike-length in the southern Québec Appalachians (see Tremblay et al. 1995 for a review).
This unit has conformable contacts against underlying sedimentary rocks (Fig. 18). Typically, as here, the lithology consists mainly (60-70%) of a black shale matrix containing pebble- to cobble-size clasts of sedimentary rocks, including black sandstone clasts similar to those of unit U-2, and siltstone, dolomitic siltstone and mudstone clasts identical to those of unit U-3 (see Fig. 18). Mudstone fragments are commonly flattened whereas sandstone and siltstone fragments are subangular to subrounded. Cousineau & St-Julien (1992) have suggested that such chaotic lithologies represent mud volcanoes formed within an accretionary prism. However, they could also be interpreted as mud flows that reworked the underlying sedimentary units (Schroetter et al., 2006).
End of day 2. Continue on road 216 West, and turn right on road 255 for an overnight stay in Asbestos – Beer Time again (life is tough).
DAY 3 – REGIONAL VARIATIONS AND CORRELATIONS OF THE
SEDIMENTARY COVER SEQUENCE The third and last day of the fieldtrip focuses on lithological variations and regional correlations of the uppermost ophiolitic crust and overlying sedimentary sequence of the Saint-Daniel Mélange between the Thetford-
Mines and Mont-Orford areas (see Figs. 19 and 21 for location). We will argue that the sub-ophiolitic sedimentary sequence is almost the same everywhere between these two areas (Fig. 22), but depending on the erosion level of the obducted oceanic crust, the basement over which were deposited these sediments varies greatly between the Thetford-Mines ophiolite to the north and the Lac-Brompton ophiolite to the south (Fig. 23). STOP 3.1: Scenic view of mine Jeffrey at Asbestos
Location: From the Hotel parking lot, turn right on road 255 and take the first street on your left. Drive this street for approximately 1 Km and park close to the Belvedere on your right. Field description: From the Belvedere, you are looking westward at the Jeffrey mine. The contact between the Asbestos ophiolite and the Humber zone to the West, defining the BBL, can be observed in the open pit (Fig. 19). It is marked by distinctive rock colours on both sides, dark for the Humber Zone metasedimentary rocks, and pale green for ophiolitic rocks. The continental margin rocks consist here of black and rusty schists. At the contact between the margin rocks and the ophiolite, there is a ca. 50 metre-wide fault zone that shows well-developed C/S fabrics, indicating down-to-the-east normal faulting (Schroetter et al., 2005). This contact (and hence the BBL) is therefore interpreted to be a major normal fault, which can be correlated with the St-Joseph fault of the Thetford-Mines area. Together, these structures constitute the St-Joseph-BBL normal fault system (Fig. 1), which is attributed to the relaxation phase of the Late Silurian-Lower Devonian SE-verging deformation (Fig. 3; Tremblay & Castonguay, 2002).
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Figure 19. Geological map and structural profile of the Asbestos ophiolitic Complex. The limit of the open pit of the Jeffrey mine is indicated by the broken line From Schroetter et al. (2005). Figure 19. Carte géologique et profile structural du Complexe ophiolitique d’Asbestos. Les limite du puits de la mine Jeffrey sont indiquées par la ligne tiretée. Tirée de Schroetter et al. (2005). STOP 3.2: Pillowed lavas and overlying debris flow of the Asbestos ophiolite Location: Drive back to road 255 and turn right. Turn right again at the intersection with road 249 and drive it for 3.6 Km. Turn right and drive for approximately 5.5 Km. Turn right at the intersection and, at approximately 1.3 Km, left onto the entrance road for the Sintra quarry.
Field description: The first part of the outcrop is an old quarry on the right side of the road towards the main mining site. It exposes the top of the volcanic sequence of the Asbestos ophiolite which consists here of massive and pillowed mafic lavas. Spectacular columnar joints are also exposed. The second part of the outcrop is located a few tens of metres further south along the
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road, and exposes a typical Coleraine-type, polygenic debris flow breccia overlying the volcanic sequence. Centimetric fragments of basalt, gabbro, red chert and other types of sedimentary rocks are present, whereas the matrix seems to be a mixture of volcanic and siliciclastic material. This unit is very similar to the breccia unit that overlies the upper crust of the Thetford-Mines ophiolite. It is part of unit OP-13 of Lamarche (1973) and unit 2a (Fig. 20) of the Saint-Daniel Formation as mapped by Lavoie (1989). STOP 3.3: Well-bedded sedimentary sequence of the Saint-Daniel Mélange – the pebbly mudstone protolith? Location: Drive back to the main road and turn right. Take the next road on your left, drive for 2.8 Km and park. The outcrop is a roadcut and an adjacent cleared area on the right side of the road. Field description: The outcrop exposes a well-bedded sequence of alternating mudslate, dolomitic siltstone and fine-grained sandstone of the Saint-Daniel Mélange. Sandstones with gradded bedding (up to 1.5 metres thick) are interlayered with cm-thick, brown-cloured dolomitic siltstones. Siltstone interbeds are locally slumped but remain cohesive throughout the outcrop. This sedimentary sequence directly overlies the volcanic rocks and debris flows of the previous stop, and correlates with the Saint-Daniel turbidites of the Mont Ham area (i.e. stop 2.7). Note that the overall composition of interlayered lithologies shown here is the same as those of fragmental components of the overlying pebbly mudstone unit (compare with stop 2.8), which is consistent with the Schroetter’s et al. (2006) hypothesis that the Saint-Daniel pebbly mudstone occupies the uppermost part of its stratigraphy (Fig. 22) and originates from the reworking (either by
mudflow deposition or mud volcanism) of underlying units.
Figure 20. Stratigraphic column of a part of the supra-ophiolitic sedimentary sequence near Asbestos. From Lavoie (1989). Figure 20. Colonne stratigraphique d’une partie de la séquence sédimentaire supra-ophiolitique près d’Asbestos. Tirée de Lavoie (1989). STOP 3.4: The metamorphic sole of the Lac-Brompton ophiolite Location: Drive straight ahead until the intersection with road 249. Turn right onto road 249 South until Windsor (ca. 30 Km) and
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Figure 21. Geological map of the Lac-Brompton ophiolite (showing the location of stops 3.4, 3.5 and 3.6) and the northern part of the Mont-Orford ophiolite. From Schroetter et al. (2005). Figure 21. Carte géologique de l’ophiolite du Lac-Brompton (montrant la localisation des arrêts 3.4, 3.5 et 3.6) et de la partie nord de l’ophiolite du Mont-Orford. Tiré de Schroetter et al. (2005). St-François-Xavier-de-Brompton (6 Km to the SW of Windsor). From St-François, continue for 9 Km on road 249 South. Turn right and immediately to the left towards Brompton Lake. Follow this road for ca. 3 Km and turn right, and right again on Bouffard road. Turn right at the next road intersection towards La Rocaille. Turn left at the next intersection, and left again on the next one. Follow that road for 800 metres and turn left on the private entrance. The outcrop is located just behind the garage on your left.
Field description: This series of outcrops expose amphibolites and greenschist-grade metasedimentary and metavolcanic
rocks interpreted to represent the metamorphic sole of the Lac-Brompton ophiolite (Fig. 21), similar to the Belmina Ridge amphibolite in the Thetford-Mines area (see stop 1.1). In the Lac-Brompton area, these metamorphic rocks are locally overlain by, and in depositional contact with ophiolitic clast-bearing, polymictic conglomerate belonging to the St-Daniel Mélange (i.e. stop 3.6). The LBO amphibolite has been divided into three compositional groups (Daoust et al., 2006); (1) a lower group of albite-epidote amphibolites with edenitic amphiboles, (2) an intermediate group with pargasitic
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Figure 22. Synthetic diagram for the stratigraphy of the Saint-Daniel Mélange in the Thetford-Mines area and correlation with Saint-Daniel facies mapped in the vicinity of the Lac-Brompton and Mont-Orford ophiolites. From Schroetter et al. (2006). Figure 22. Diagramme synthétique de la stratigraphie du Mélange de Saint-Daniel dans la region de Thetford-Mines et des corrélations proposes avec les faciès du Saint-Daniel dans les environs des ophiolites du Lac-Brompton et du Mont-Orford. Tiré de Schroetter et al. (2006). hornblendes, and (3) an upper group of amphibolites with tchermakitic hornblendes. The compositional zonation of amphiboles is indicative of decreasing P-T conditions during the progressive obduction of the oceanic crust. Epidote and chlorite occur as retrograded metamorphic mineral assemblages.
Compared to Belmina ridge, the LBO metamorphic sole shows a lower metamorphic grade and appears to originate from alkaline basalts rather than the tholeiites of Belmina (Daoust et al., 2006). Although the LBO belongs to the same oceanic tract as the Thetford-Mines ophiolite, it seems that it was obducted onto on a different type of basement.
STOP 3.5: Siltstones overlying the dunitic lower crust in the Lac-Brompton ophiolite Location: From the last stop, drive back to the road 249 South. From the gas station, drive 2.2 Km and turn right towards Petit-Lac-Brompton. Take the second private entrance to your right, drive up the hill and park. The outcrop is the rock garden of this private property, please be careful not to cause damage when you walk around. Field desciption: This outcrops exposes a depositional contact between the mantle peridotite of the Lac-Brompton ophiolite and well-bedded, fine-grained sedimentary rocks
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(Fig. 21). The mantle peridotite here is a massive to brecciated, serpentinized dunite. This peridotite is strongly altered and characterized by a mineralogical assemblage of serpentine-calcite-tremolite+/-chromite and magnetite. The overlying sedimentary rock is a pale-green, laminated mudstone/siltstone sequence. In this area, these rocks are interlayered with red mudstone and slate (Fig. 22). STOP 3.6: Conglomerate in depositional contact with the metamorphic sole in the Lac-Brompton ophiolite Location: From the last stop, drive back to road 249 South, and follow it until the intersection with road 220 (approximately 9 Km south of St-Denis-de-Brompton). Turn right and follow road 220 for 6.7 Km. Turn right just after the bridge over road 220 and left onto the Lac-Brompton road. Follow that road for ca. 4 Km and turn left into a private entrance going up the hill. Park on top, in the parking lot near a house. Once again, the outcrop is a rock garden, so be cautious. Field description: The outcrop exposes a sequence of metasedimentary rocks (quartz -muscovite – plagioclase - chlorite phyllites) belonging to the tectonic sole of the Lac-Brompton ophiolite (i.e. stop 3.4) in contact with conglomerate and sandstone included in the Saint-Daniel Mélange (Fig. 22). Approximately 1 metre of overburden covers the contact between both lithologies. There is, however, no structural evidence for major faulting at the contact and it has been interpreted as an erosional unconformity (Daoust et al., 2005). The polygenic conglomerate overlying the metamorphic rocks is mainly characterized by centimetric angular clasts of pyroxenite, gabbro, volcanic and fine-grained sedimentary rocks in a lithic sandstone matrix. The outcrops along the
Figure 23. A) Tectonic setting of the southern Québec Appalachians during the Taconian orogeny. B) Schematic sedimentary and tectonic evolution of the Saint-Daniel Mélange along the western edge of a syncollisional forearc basin. From Schroetter et al. (2006). Figure 23. A) Contexte tectonique des Appalaches du sud du Québec au cours de l’orogénie Taconienne. B) Évolution sédimentaire et tectonique schématique du Mélange de Saint-Daniel sur la marge ouest d’un bassin avant-arc synorogénique. Tiré de Schroetter et al. (2006). entrance road show that conglomerate is interbedded with beds of feldspathic sandstone. Dark-coloured, amphibole-rich fragments (igneous amphibolite?) are abundant towards the lower end of the roadcut. STOP 3.7: The contact between the Saint-Daniel Mélange and the Magog Group: the Ruisseau-Castle section Location: From the last stop, drive back
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to road 220 East. At the intersection with road 249, turn right and drive until the intersection with Hw 10. Follow Hw 10 West for approximately 9 Km and take the Mont Orford/Magog exit. Follow the Magog direction and turn right at the next road intersection. Follow that road for 1.2 Km, and park just after the bridge. The outcrop is located on the eastern side of that bridge, in the Castle Brook river bed. Field description: The Ruisseau-Castle section (Schroetter et al., 2006) is one of the few localities of the southern Québec Appalachians where the contact between the Saint-Daniel Mélange and the overlying Magog Group is exposed (Figs. 21 and 22). Here, the Saint-Daniel – Magog contact is unequivocally depositional. Black mudstone with cm- to dm-sized clasts of black and grey sandstone, typical of unit U-4 of the Saint-Daniel Mélange (Schroetter et al., 2006), are overlain by a sequence of fossiliferous (i.e. graptolites) graphitic slate and grey siltstone belonging to the Magog Group (Cousineau and St-Julien, 1994), interpreted to be an onlapping forearc sequence (Fig. 23).
This is the end of the fieldtrip. We hope that you enjoyed it. Drive back to Montreal via Hw 10 West (approximately 1½ hour driving). REFERENCES
Bécu, V., 2005, Pétrologie et géochimie de l’indice de chromitites platinifàres Star Chrome, Complexe ophiolitique de Thetford-Mines, Québec. M.Sc. thesis, INRS-Eau, Terre & Environnement, Québec, 177 p.
Bédard, J.H., A., Tremblay, J.-M., Schroetter, V., Bécu, and P., Pagé (2001) Structural and magmatic evolution of the Thetford Mines Ophiolite: Preliminary results from new mapping and analysis, GAC-MAC Abstracts with programs, 26, p. 11. Bourque, P. -A., Malo, M., and Kirkwood, D.,
2000. Paleogeography and tectono- sedimentary history at the margin of Laurentia during Silurian-earliest Devonian time: the Gaspé Belt, Québec. Geological Society of America Bulletin 112, 4-20. Castonguay, S., Ruffet, G., Tremblay, A., and Féraud, G., 2001. Tectonometamorphic evolution of the southern Québec Appalachians: 40Ar/39Ar evidence for Ordovician crustal thickening and Silurian exhumation of the internal Humber zone. Geological Society of America Bulletin 113, 144-160. Clague, D., Rubin, J. and Brackett, R. 1985, The age
and origin of the garnet amphibolite underlying the Thetford Mines ophiolite, Québec. Canadian Journal of Earth Sciences 18, 1257-1261.
Cousineau, P.A., 1990, Le Groupe de Caldwell et le domaine océanique entre St-Joseph-de-Beauce et Ste-Sabine: Ministère de l'Energie et des Ressources, Québec, MM 87-02, 165 p.
Cousineau, P.A. and St-Julien, P., 1992, The Saint- Daniel mélange: evolution of an accretionary complex in the Dunnage terrane of the Québec Appalachians: Tectonics, v. 11, p. 898-909.
Cousineau, P.A. and St-Julien, P. 1994, Stratigraphie et paléogéographie d'un bassin avant-arc
ordovicien, Estrie-Beauce, Appalaches du Québec. Canadian Journal of Earth Sciences 31, 435-446.
Cousineau, P.A. and Tremblay, A. 1993, Acadian deformation in the southwestern Québec Appalachians. Geological Society of America Special Paper 275, 85-99. Daoust et al., 2006,
David, J. and Marquis, R. 1994, Géochronologie U- Pb dans les Appalaches du Québec: application aux roches de la zone de Dunnage. La Revue géologique du Québec 1, 10-15.
Daoust, C., DeSouza, S., Tremblay, A. & Gauthier, M. 2005. The Lac-Brompton ophiolitic Complex : stratigraphy and structure of a newly-recognized ophiolite in the Southern Québec Appalachians, Canada. Geological Society of America NE meeting, Saratoga Springs, NY.
Daoust, C., DeSouza, S. & Tremblay, A., 2006, Structural, metamorphic and geochemical characteristics of the Lac-Brompton ophiolite tectonic sole, Southern Québec Appalachians.
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Geological Association of Canada Meeting, Montréal.
DeSouza, S., Daoust, C. Tremblay, A. & Gauthier, M. 2006. Stratigraphical setting and geochemical characteristics of the Lac-Brompton Ophiolite, Québec, Canada. Geological Association of Canada Meeting, Montréal.
Dunning, G.R. and Pedersen, R.B. 1988, U-Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides, implications for the development of Iapetus. Contributions to Mineralogy and Petrology 98, 13-23.
Feininger, T. 1981, Amphibolite associated with the Thetford-Mines Ophiolite Complex at Belmina Ridge, Québec. Canadian Journal of Earth Sciences 18, 1878-1892.
Harnois, L., and Morency, M., 1989, Geochemistry of Mount Orford Ophiolite Complex, Northern Appalachians, Canada, Chemical Geology., 77, 133-147. Hébert, R., 1981, Conglomérats polygéniques
ophiolitiques : anciens éboulies de talus de fond océanique ? Canadian Journal of Earth Sciences 18: 619-623.
Hébert, R., 1985, Pétrologie des roches ignées océaniques et comparaison avec les complexes ophiolitiques du Québec, de Chypre et de l’Apennin. Ph.D. thesis, Université de Bretagne Occidentale, Brest.
Hébert, R. and Bédard, J.H., 1998, Architecture and petrogenesis of arc ophiolites: example from the ophiolitic complexes of southern Québec. GAC-MAC, Joint Annual Meeting, Québec 1998, Fieldtrip A4 Guidebook, 51 p.
Hébert, R., and Bédard, J.H., 2000. Les ophiolites d’avant-arc et leur potentiel minéral: exemple des complexes ophiolitiques du sud du Québec : Chronique de la Recherche Minière, v. 539, p. 101-117.
Hébert, R., and Laurent, R., 1989, Mineral chemistry of ultramafic and mafic plutonic rocks of the Appalachian ophiolites, Quebec, Canada. Chemical Geology 77: 265-285. Hébert, Y. and Laurent, R., 1979, Thetford-Mines
ophiolite Complex. GAC-MAC, Joint Annual Meeting, Québec city, Fieldtrip Guidebooks A-13 and B-5, 17 p.
Huot, F., Hébert, R., and Turcotte, B., 2002. A multistage magmatic history for the genesis of the Orford ophiolite (Quebec, Canada): a study of the Mt Chagnon massif: Canadian Journal of
Earth Sciences, v. 39, p. 1201-1217. Laliberté, R., Spertini, F. and Hébert, R., 1979, The
Jeffrey asbestos mine and the ophiolitic complex at Asbestos. GAC-MAC, Joint Annual Meeting, Québec city, Fieldtrip Guidebook B-3, 18 p.
Lamarche, R.Y., 1973. Complexe ophiolitique d’Asbestos. Ministère de l’Energie et des Ressources du Québec, DP-144, GM 28558, 9 pages.
Lamothe, D. (1978) Analyse structurale du Mélange ophiolitique du Lac Montjoie, M.Sc. thesis, 68 p., Université Laval, Sainte-Foy, Quebec, Canada.
Laurent, R. 1975, Occurrence and origin of the ophiolites of southern Québec, Northern Appalachians. Canadian Journal of Earth Sciences 12, 443-454.
Laurent, R. & Baldwin, B., 1987, Thetford-Mines ophiolite, Quebec. Geological Society of America Centennial Field Guide – Northeastern Section, pp 369-372.
Laurent, R. and Hébert, R. 1989, The volcanic and Intrusive rocks of the Québec Appalachians ophiolites and their island-arc setting. Chemical Geology 77, 265-286.
Laurent, R., Hébert, R. and Hébert, Y., 1979, Tectonic features and petrological features of the Québec Appalachians ophiolites. In: Ophiolites of the Canadian Appalachians and Soviet Urals. Malaps, J. & Talkington, R.W. editors, Memorial University of Newfoundland, Dept. Of Geology, Report no. 8, Contribution IGCP Project 39, pp. 53-77.
Lavoie, D., 1989. Géologie de la Formation de Saint- Daniel et du Groupe de Magog, région de Richmond, Rapport final, Estrie. Ministère de l’Energie et des Ressources du Québec, MB 89 – 06, 44.
Olive V., Hébert R., and Loubet M., 1997. Isotopic and Trace element constraints on the genesis of boninitic sequence in the Thetford-Mines ophiolitic complex, Québec, Canada: Canadian Journal of Earth Sciences, v. 34, p. 1258-1271.
Osberg, P.H. (1978) Synthesis of the geology of north-eastern Appalachians, USA, in Caledonian-Appalachian Orogen of the North Atlantic region, E.T. Tozer and P.E. Schenk, Editors, Geological Survey of Canada Paper, 78-13, 137-147. Osberg, P.H., Tull, J.F., Robinson, P., Hon, R., and
Butler, J.R., 1989, The Acadian orogen. In The Appalachian-Ouachita orogen in the United
32
States (eds R.D.Jr. Hatcher, W.A. Thomas, and G.W. Viele), pp. 179-232, Geological Society of America., Geology of North America, v. F-2.
Oshin, I.O., and Crocket, j.h., 1986, The geochemistry and petrogenesis of ophiolitic volcanic rocks from Lac de l’Est, Thetford Mines Complex, Quebec, Canada. Canadian Journal of Earth Science 23: 202-213. Pagé, P., Bédard, J.H., Schroetter, J.-M. & Tremblay,
A., 2006, Mantle petrology and mineralogy of the Thetford-Mines ophiolitic Complex. (in preparation).
Pagé, P.; Bédard, J.H.; Tremblay, A.; and J.M. Schroetter (2003) The Thetford Mines Ophiolite Complex: focus on the petrology, mineralogy and geochemistry (REE, PGE) of a supra-subduction mantle section, EOS Trans. A.G.U., 84, F1538. Pinet N., and Tremblay, A., 1995, Tectonic
evolution of the Québec-Maine Appalachians: from oceanic spreading to obduction and collision in the northern Appalachians: American Journal of Science 295, 173-200.
Pinet, N., Tremblay, A., and Sosson, M., 1996. Extension versus shortening models for hinterland-directed motions in the southern Quebec Appalachians. Tectonophysics 267, 239-256. Riordon, P.H., 1954. Région de Thetford Mines-Black
Lake, Province de Québec, Minisitère des Mines, Rapport Préliminaire, 295, 23.
Rodrigue, G., 1979. Etude pétrologique des roches ultramafiques du Mont Orford, Québec. M.Sc. thesis, Université Laval, Québec, Canada, 148.
Schroetter, J.-M., 2004. Caractérisation structurale et stratigraphique du Complexe ophiolitique de Thetford-Mines: implications géodynamiques pour la zone de Dunnage du sud du Québec, Canada. Unpubl. Ph.D. thesis, INRS-ETE, 255 pages.
Schroetter, J.-M., Pagé, P., Bédard, J.H., Tremblay, A., and Bécu, V., 2003. Forearc extension and seafloor spreading in the Thetford Mines Ophiolite complex, In Y. Dilek and P.T. Robinson (eds) Ophiolite in Earth History, Special publication of the Geological Society of London, 218, pp 231-251.
Schroetter, J.-M., Tremblay, A. and Bédard, J.H., 2005. Structural evolution of the Thetford-Mines ophiolitic Complex, Canada: implications for the Southern Quebec
Ophiolitic Belt: Tectonics, v. 24, TC2099, Loi:10.1029/2005TC001799.
Schroetter, J.-M., Tremblay, A., Bédard, J.H. and Villeneuve, M., 2006, Syncollisional basin development in the Appalachian orogen – the Saint-Daniel Mélange, southern Québec, Canada. Geological Society of America Bulletin, 118: 109-125.
Stanley, R.S., and Ratcliffe, N.M., 1985, Tectonic synthesis of the Taconian orogeny in western New England: Geological Society of America Bulletin, 96, p. 1227-1250.
St-Julien, P., and Hubert, C., 1975, Evolution of the Taconian orogen in the Québec Appalachians: American Journal of Science, v. 275-A, p. 337- 362.
St-Julien, P. and Hubert, C., 1979, Contexte structural du complexe ophiolitique de Thetford-Mines. GAC-MAC, Joint Annual Meeting, Québec city, Fieldtrip Guidebook B-10, 30 p.
Tremblay, A. 1992, Tectonic and accretionary history of Taconian oceanic rocks of the Québec Appalachians. American Journal of Science 292, 229-252.
Tremblay, A., and Castonguay, S., 2002. Structural evolution of the Laurentian margin revisited (southern Quebec): implications for the Salinian orogeny and successor basins. Geology 30, 79-82. Tremblay, A. and St-Julien, P. 1990, Structural style
and evolution of a segment of the Dunnage zone from the Québec Appalachians and its tectonic implications. Geological Society of America Bulletin 102, 1218-1229.
Tremblay, A., and Pinet, N., 1994, Distribution and characteristics of Taconian and Acadian deformation, southern Québec Appalachians: Geological Society of America Bulletin 106, 1172-1181.
Tremblay, A., Hébert, R. et Bergeron, M. 1989. Le Complexe d'Ascot des Appalaches du sud du
Québec: pétrologie et géochimie. Canadian Journal of Earth Sciences 26, 2407-2420.
Tremblay, A., Malo, M., et St-Julien, P. 1995. Dunnage Zone - Québec; Canadian Appalachians Region. In Geology of the Appalachian/Caledonian Orogen in Canada and Greenlan, (ed H. Williams), pp. 179-197, Geological Survey of Canada, Geology of Canada, no. 6. Tremblay, A., Ruffet, G., and Castonguay, S., 2000. Acadian metamorphism in the Dunnage zone of
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southern Québec, Northern Appalachians: 40Ar/39Ar evidence for collision diachronism. Geological Society of America Bulletin 112, 136-146. Tremblay, A., Pagé, P., Bédard, J.H. & Mashi, A.
2006. Oceanic core complexes and ancient oceanic lithosphere : insights from Albanian and Southern Québec ophiolites. Geological Association of Canada Meeting, Montréal.
van Staal, C.R., Dewey, J.F., Mac Niocail, C., and McKerrow, W.S., 1998. The Cambrian-Silurian tectonic evolution of the Northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus. Geological Society of London Special Publication 143, 199-242. Whitehead, J., Reynolds P.H., et Spray, J.G., 1995.
The sub-ophiolitic metamorphic rocks of the Québec Appalachians: Journal of Geodynamics, v. 19, p. 325-350.
Whitehead, J., Dunning, G.R., and Spray, J.G., 2000. U-Pb geochronology and origin of granitoid rocks in the Thetford Mines ophiolite, Canadian Appalachians: Geological Society of America Bulletin, v. 112, p. 915-928.
Williams, H. 1979, The Appalachian orogen in Canada. Canadian Journal of Earth Science 16, 792-807.
Williams, H., and St-Julien, P., 1982, The Baie Verte-Brompton Line - Early Paleozoic continent-ocean interface in the Canadian Appalachians: Geological Association of Canada Special Paper 24, p. 177-208.