Download Stratigraphy, structure and regional correlations of crustal

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 …………………………………………………….…
The Asbestos ophiolite ………………………………………………………………
The Lac-Brompton ophiolite …………………………………………………….…..
The Mont-Orford ophiolite …………………………………………………………..
10
10
10
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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page
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
iii
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 supraophiolitic debris flows.
iv
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 ThetfordMines 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.
v
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 synobduction 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
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 19992000, 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 hinterlandverging (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).
2
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 LacBrompton.Tiré de Schroetter et al. (2006).
3
of the oceanic crust is exposed in the LacBrompton ophiolite. U/Pb zircon dating from
felsic rocks
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 hightemperature 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 (469461 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 ThetfordMines, Asbestos, Lac-Brompton and MontOrford 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).
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 MontOrford ophiolite, which contains a greater
diversity of magma types, interpreted in terms
of arc-backarc (Harnois & Morency, 1989;
THE SOUTHERN QUÉBEC
DUNNAGE ZONE
The ophiolites of the Thetford-Mines and
Asbestos areas are characterized by wellpreserved mantle and crustal sections,
whereas only the mantle and a dissected part
4
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 finegrained 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 SaintVictor 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 SaintDaniel Mélange (Tremblay & St-Julien,
1990).
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
40
Ar/39Ar ages of 477±5 Ma (Whitehead et al.,
1995) and 469-461 Ma (Fig. 2; Castonguay et
al., 2001), respectively, suggesting that intraoceanic 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
40
Ar/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 &
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
5
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).
40
Ar/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-
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. Synobduction (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) NWverging 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
6
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.
Joseph fault and the Baie Verte-Brompton line
THE SOUTHERN QUÉBEC
OPHIOLITE BELT
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.
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 SaintDaniel 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 SaintDaniel Mélange forms the lowermost part of a
(Fig. 3b). The upper plate is made up of a
folded stack of D1-2 nappes of deformed and
7
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).
8
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 suprasubduction 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).
9
diabasic and volcanic rocks (Hébert, 1980).
The ophiolitic lavas are overlain by finegrained volcaniclastic rocks and flow
breccias, and then by the Saint-Daniel
Mélange (Lavoie, 1989).
The Thetford-Mines Ophiolite
The Thetford-Mines ophiolite crops out
as a NE-trending belt, 40 km in length and 1015 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 SaintDaniel Mélange (Schroetter et al., 2006).
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 LacBrompton 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 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 (20002500m) but very similar ophiolitic sequence,
consisting of harzburgitic mantle, overlain by
ultramafic-to-mafic
cumulates
(dunite,
pyroxenite and gabbro), and capped by
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
10
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).
11
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 ThetfordMines. 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 SaintDaniel Mélange are structurally overlain by
the metamorphic rock unit mentioned above,
12
~1 km apart on average (Fig. 6; Schroetter et
al., 2003). In the lower crust, the faults are
manifested as sheared dunites and synmagmatic 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.
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).
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,
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 (synmagmatic) 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 subvertically dipping, N-S-striking faults, spaced
13
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 hightemperature 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 quartzprehnite-calcite crosscuts all older structures.
The age and origin of this youngest event is
not known.
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 clinopyroxenegarnet-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
STOP 1.2: The upper mantle at Vimy
Ridge
Location: Drive back to Disraeli and
follow road 112 West until the town of StJoseph-de-Coleraine. In town, turn left on
Vimy road and drive for 7 Km until the Vimy
14
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).
granitoids are peraluminous, have high
87
Sr/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 PetitLac-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 brittleductile 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
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
15
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
16
Figure 12. Detailed map of the American Chrome mine (stop 1.4) showing late (Acadian) NEtrending, 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).
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
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,
17
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 &
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 hightemperature
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.
18
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 crustforming 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).
19
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 southwestdirected way-up. The mafic dykes are also of
boninitic composition. The dykes show
asymmetrical chilled margins and dykes-indykes structures indicating that they represent
feeder conduits to the volcanic sequence.
Figure 15. Stratigraphic section of the SaintDaniel 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).
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
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 clastsupported breccias, to interbedded matrixsupported
microconglomerates
and
sandstones, to well-bedded and fine- to
coarse-grained sandstones, and finally to
20
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 StJoseph-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 ThetfordMines 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, mediumgrained to pegmatitic hornblende gabbro, and
microgabbro. The angularity of clasts
indicates only limited transport, while jigsawpuzzle 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, finegrained 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
40
Ar/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.
21
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.
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 2.6: 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 softsediment 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
STOP 2.5: Interbedded siltstones and
mudstones overlying ophiolitic gabbros
Location: Drive back to the St-Julien
road and turn right towards St-Joseph-deColeraine. 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
22
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 ThetfordMines ophiolite (Hébert, 1980).
At Mont-Ham, the ophiolite is directly
overlain by interbedded black sandstone and
laminated, black and green argillite of unit U2 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 SaintDaniel 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 SaintDaniel 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
23
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 SaintDaniel Mélange (see Fig. 4 for location),
recognizable for more than 200 km of strikelength 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).
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).
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 Thetford24
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
25
mudflow deposition or mud volcanism) of
underlying units.
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 SaintDaniel 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
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 supraophiolitique 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
26
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 greenschistgrade 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
27
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 MontOrford 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-LacBrompton. 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
28
(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 LacBrompton 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 SaintDaniel Mélange and the Magog Group: the
Ruisseau-Castle section
Location: From the last stop, drive back
29
2000. Paleogeography and tectonosedimentary 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 SaintDaniel 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 UPb 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.
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 SaintDaniel 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.,
30
Geological Association of Canada Meeting,
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DeSouza, S., Daoust, C. Tremblay, A. & Gauthier,
M. 2006. Stratigraphical setting and
geochemical characteristics of the LacBrompton 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 A13 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 B3, 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.
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