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CHAPTER 1: INTRODUCTION
1.1
NATURE OF PROBLEM
The Iapetus Ocean, the predecessor to the modern day Atlantic, began its life near
the end of the Proterozoic with a rifting event that would separate Laurentia from the
supercontinent of Rhodinia. No definite age has been placed on the opening of the
Iapetus. However, magmatism related to its initial rifting is believed to have generated
various dyke swarms and volcanic flows along Laurentia’s margins. The Long Range
dyke swarm of Newfoundland’s northern peninsula and coastal Labrador is an example
that has yielded a U-Pb date of 615 Ma (Kamo et al. 1989). There is also an undated
dyke swarm on the north shore of the St. Lawrence River, across from the Lac Matapédia
area of the Gaspé Peninsula, which may be related to a triple-junction suggested by
Kamo et al. (1995). The Grenville dykes of Ontario and Quebec occur in a failed arm of
this triple junction and yield U-Pb ages of 590 Ma (Kamo et al., 1995). The Tibbit Hill
volcanics of Quebec (Figure 1) are also thought to be related to this triple junction and
yield accurate U-Pb dates of 555 Ma (McCausland and Hodych, 1998). It has been
suggested that the Lac Matapédia basaltic flows to be studied in this thesis are
correlatives of the Tibbit Hill volcanics since they too are alkalic basalts (Camiré et al.,
1995).
The 577 Ma Callandar Bay intrusive complex of Southern Ontario (Figure 1)
gives an important paleopole for Laurentia, placing it near the south pole at 577 Ma
(Figure 2) (Symons & Chiasson, 1990). Primary remanence was suggested by a single
baked contact test; however, recent testing at three other baked contact sites were
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inconclusive thereby placing the reliability of this paleopole in question (Frampton,1999).
Investigation of Western Newfoundland’s Skinner Cove volcanics (Figure 1) by
McCausland and Hodych (1998) provides another valuable paleomagnetic result for
Laurentia, placing it near the equator at 550 Ma. The remanence from the Skinner Cove
volcanics was shown to be primary through a conglomerate test performed on a
conglomeratic tuff that lay within the series of flows. These two paleomagnetic results
suggest that Laurentia drifted rapidly northward at approximately 34cm per year between
577 and 550 Ma, twice as fast as any Cenozoic plate (McCausland and Hodych, 1998)
(Figure 3).
However, since the reliability of the Callandar Complex remanence is
somewhat questionable, and since the Skinner Cove Volcanics are allochthonous and
hence may not represent Laurentia, paleomagnetic studies of other Laurentian rocks of
this age range should be undertaken to test this postulated very rapid northward
movement of Laurentia.
Paleomagnetic studies so far undertaken at other sites (i.e. Sept Iles, Catoctin,
Johnnie Rainstorm) do not have proof of primary remanence.
The Lac Matapédia
basaltic flows to be studied in this thesis were chosen because they should be alkalic
enough to yield zircons for precise U-Pb dating and because they provide opportunity for
testing whether the basalts were magnetized before their early Paleozoic folding.
1.2
PREVIOUS WORK
As the study area for this paper is within the Appalachian Orogen, the Geological
Society of America’s publication on Appalachian geology (Williams, 1995) provides an
introduction to the geological setting. For information specifically pertaining to the
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geology of the Lac Matapédia area, the reader is referred to a publication by St-Julien et
al. (1990).
Paleomagnetic technique is discussed by Strangway (1970), Tarling (1971), and
most thoroughly by Butler (1992). If a better explanation of the paleomagnetic apparatus
used to gather the data for this paper is desired, the text by Collinson (1983) can be
consulted.
Tuzo Wilson was the first to propose the idea of oceans opening and closing using
the Iapetus as his example (Wilson, 1966). Evidence for Iapetus opening and closing has
since been gathered through paleomagnetic measurements. Recent paleomagnetic results
regarding the Late Proterozoic movement of Laurentia (McCausland and Hodych, 1998;
Symmons and Chiasson, 1990) have prompted interest in the very rapid rate at which
Laurentia migrated and have led to the suggestion that it was the pole rather than
Laurentia that moved. Discussion of this possibility of true polar wander can be found in
publications by Evans (1998) and Meert (1998).
1.3
PURPOSE AND SCOPE
This thesis will concentrate on a paleomagnetic study of the Lac Matapédia
basalts of Quebec’s Gaspé Peninsula. It is hoped that the results from a fold test will
prove positive, suggesting that the magnetic remanence carried by these basalts is
primary. This would then provide a paleolatitude for Laurentia at ~555 Ma, assuming
that the Lac Matapédia basalts are correlatives of the Tibbit Hill volcanics. With Richard
Cox’s help, we shall also attempt to extract zircons from the basalts and date them using
the laser ablation ICP-MS (inductively coupled plasma mass spectrometer). As it is
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believed that Laurentia migrated at an unusually high rate of speed during this time, the
results from this experiment should help constrain Laurentia’s location with more
accuracy. The Lac Matapédia basalts were chosen for this study because of their Late
Hadrynian to Early Cambrian age (Brisbois et al., 1991), and their relatively unaltered
state (Camiré et al., 1993), both difficult characteristics to find in rocks of the Laurentian
margin.
1.4
GEOLOGICAL SETTING
The Gaspé Peninsula of Quebec is located within the Northern Appalachians, a
fold and fault belt running the coastal length of Eastern North America (Figure 4). The
Appalachians have been divided into five distinct zones based upon lithology and
tectonic history. From west to east, these zones are the Humber, Dunnage, Gander,
Avalon, and Meguma.
The most westerly of these, the Humber Zone, records the
development and destruction of Laurentia’s passive margin.
The Dunnage Zone,
comprised of volcanic sequences and melanges, contains the remnants of the Iapetus
Ocean. The Gander, Avalon, and Meguma zones represent elements on the opposing side
of the Iapetus (Williams, 1995).
The Humber Zone has been further subdivided into internal and external tectonic
domains based upon the deformational and metamorphic styles observed. The external
domain, lying on the western edge of the Appalachian fold belt, is comprised of a
crystalline basement overlain unconformably by an arkosic clastic unit and localized
mafic flows. The Lac Matapédia basalts are thought to represent such flows (Camiré et
al., 1995). This basal unit is then succeeded by Cambrian shales and quartzites and a
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thick Cambrian to Middle Ordovician carbonate sequence. This is then overlain by a
Middle Ordovician shale and sandstone unit that is in turn capped by mélanges and
transported igneous and sedimentary rocks of Taconic Allochthons (Williams, 1995). As
deformation of the Humber Zone proceeded from west to east, the easterly, internal
domain has undergone higher degrees of deformation and metamorphism.
It is
comprised primarily of psammitic and pelitic schists with some chlorite schists and
marble units (Williams, 1995). Basement rocks are visible in some areas but are often
highly deformed and difficult to distinguish from the overlying schists (Williams, 1995).
1.5
LOCAL GEOLOGY
The Schickshock Group of the Gaspé Peninsula is Hadrynian to Early Cambrian
in age (Brisbois et al., 1991) and was originally divided into four distinct units, the Lac
Cascapédia, Lac Guelph, Bras au Saumon, and Lac Matapédia assemblages (Crickmay,
1932). The Lac Cascapédia assemblage is considered the metamorphic equivalent of the
Orignal Formation, a younger, overlying mudrock unit. The Lac Guelph assemblage
consists of meta-arkoses, conglomeratic meta-arkoses, chloritic schists, alternating
siltstones and chloritic schists, and laminated metabasalts. The bras au Salmon unit is
composed of metabasalts exhibiting diverse volcanic textures and minor tuff. Finally, the
Lac Matapédia assemblage is comprised of alternating basic volcanics, arkoses, and
minor amounts of red slate that have undergone low grade (pumpellyite facies)
metamorphism (St-Julien et al., 1990).
It has been suggested that the Lac Matapédia assemblage no longer be considered
part of the Schickshock Group but rather a part of the Trois Pistoles Group. The
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Schickshock Group contains numerous lavas of tholeiitic affinity whereas the Lac
Matapédia lavas were shown to be transitional to alkaline (Camiré et al., 1993). Further,
there is a concordant contact between the Lac Matapédia assemblage and the Trois
Pistoles Group and both are within the St. Anne River Nappe, the Schickshock Group
being confined to the Mount Logan Nappe. These factors seem to indicate that the Lac
Matapédia assemblage would indeed be better situated within the Trois Pistoles Group.
As well, because the Lac Matapédia assemblage is concordantly overlain by the Upper
Cambrian – Lower Ordovician Trois Pistoles Group (Brisebois et al., 1991), it must be at
minimum Upper Cambrian in age.