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Nd Model Age Mapping of the Central Gneiss Belt In the western Grenville Province of Ontario, Canada.North, Robert 04 1900 (has links)
<p> Nd isotope analysis is well suited for mapping major tectonic boundaries in highly
metamorphosed orogenic belts. In this study, approximately 80 samples have been
analyzed to map 2 such boundaries in the Central Gneiss Belt of the Grenville Province
of Ontario. In Central Ontario, lithotectonic terranes with mapped outcrops of gneisses
intruded by eclogites and/or coronitic metagabbro have Nd model ages less than 1.8 Ga
are interpreted as components of the allochthonous polycylic belt. More northerly
terranes are comprised of similar gneissic materials, but have different types of mafic
intrusives and have model ages greater than 1. 8 Ga. These terranes are interpreted as
fragments of the parautochthonous belt. These two belts are divided by a major thrust,
termed the Allochthon Boundary Thrust (ABT) (Rivers, et. al., 1989). Continuing to the
north, another step in the Nd model ages has been used to identify and map a cryptic
suture between Archean and early Proterozoic crustal materials (Dickin & McNutt,
1989). </p> <p> Along the Georgian Bay coastline, between Pointe Au Baril and Parry Sound, the
Shawanaga Shear Zone has been interpreted as the location of the ABT (Culshaw, et. al.,
1994). Analysis of over 50 samples are used to map the crustal formation ages in this
region and have confirmed this interpretation. Orthogneisses of the Britt Domain have
Nd model ages in the range 1.8- 1.9 Ga. Reworking of the original crust has given these
rocks U-Pb crystallization ages of~ 1.45 Ga, which means that these rocks have been
metamorphosed prior to the Grenvillian event. Crossing the ABT, the orthogneisses of
the Shawanaga Domain have a younger range of crustal formation ages, 1.4 - 1.7 Ga.
The U-Pb crystallization ages of these rocks are ~ 1.36 Ga, and they lack signs pre-Grenvillian
metamorphism. To the south of Franklin Island, the location of the ABT is
difficult to map, as outcrop lies beneath the waters of Georgian Bay. Results of the Nd
isotope analyses suggest that the ABT passes through the western edge of the Snake
Islands, rather than to their east, as previously interpreted (Culshaw, et. al., 1994). </p> <p> Approximately 15 Nd isotope analyses were used to investigate a recently
proposed location of the ABT (Ketchum & Davidson, 2000) in the vicinity of the
Powassan Batholith. Results from near Arnstein, Restoule and Magnetewan agreed with
the existing location of the ABT. To the east of the Powassan Batholith, 3 Nd model
ages coupled with a lack of mappable eclogites and/or coronitic metagabbros suggest that
earlier interpretations of the position of the ABT may be correct and that further studies
in this region are necessary. </p> <p> A cryptic suture identified by crustal formation ages has been the focus of several
previous studies (Dickin & McNutt, 1989, 1990; Holmden & Dickin, 1995; Dickin, 1998;
Guo & Dickin, 1996). This suture has been mapped from the Georgian Bay coast
through Lake Nipissing to the Ontario-Quebec border. New Nd isotope analyses and
studies of the regional magnetics have identified a thrust slice between the Grenville
Front tectonic zone (GFTZ) and the parautochthonous belt. The cryptic suture appears to
coincide with a previously undescribed tectonic boundary west of the Key River. To the
west of this boundary, straight orthogneisses within the thrust slice have Nd model ages
greater than 2.2 Ga. These differ from the orthogneisses and metaplutonic tonalites to the
east of this boundary, which exhibit kilometer-scale isoclinal folds and crustal formation
ages between 1.8- 2.0 Ga, the previously identified range for the Britt Domain. </p> <p> Major steps in the depleted mantle model ages are observed in all three regions,
allowing mapping of the ABT and the Penokean Suture. It is concluded that, in
metamorphic orogenic belts, such as the Grenville Province, detailed mapping of major
tectonic boundaries is greatly enhanced by the use of Nd isotope analysis. </p> / Thesis / Master of Science (MSc)
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The Evolution of the Grenville Province in the Mattawa Region of Ontario: Evidence from Neodymium and Constraints from Aeromagnetic DataHerrell, Michael 04 1900 (has links)
<p> Approximately eighty new neodymium model ages were determined on grey
orthogneisses in the Mattawa region of Ontario and were used to develop a Grenvillian
tectonic model in the region. A residual-magnetic field map developed from
aeromagnetic data provides an additional constraint on the terranes mapped based on Nd-model
ages. The field area was divided into three sections: the northern section, the
Mattawa klippe, and the southern section. </p> <p> The objectives of mapping in the northern section were two-fold: 1) to map the
location of erosional remnants of the allochthon boundary thrust (ABT) and 2) to test the
hypothesis that northwestward thrusting of the ABT over the parautochthonous belt
caused decollement of a magmatically reworked Archean terrane, creating a duplex thrust
sheet and consequent northwestward thrusting of the magmatically reworked Archean
parautochthon. </p> <p> Two allochthonous units previously mapped in this region are the Lac Watson
nappe and the Lac Booth klippe. These allochthonous units, characterized by Nd-model
ages <1.8 Ga are bound by a magmatically reworked Archean terrane (referred to as the
reworked Archean parautochthon) with a Nd-model age range of 1.9-2.6 Ga. Likewise,
the perimeter of the reworked Archean parautochthon is truncated by a pristine Archean terrane which exclusively hosts Nd-model ages> 2.6 Ga, indicating that there are three
crustal stacking levels in the northern section. </p> <p> Nd-model age mapping was employed in the Mattawa klippe region with the
same objectives as in the northern section and additionally to provide constraints on the
methodology that is to be used when mapping first-order tectonic boundaries such as the
ABT. Ketchum and Davidson (2000) suggested that the ABT trended northward in this
region based on the presence of 1.16 Ga coronitic metagabbros which Ketchum (1994)
concluded were exclusively confined to the allochthonous polycyclic belt (APB). It was
detennined here that although the metagabbros are confined to allochthonous crust, based
on Nd-model ages, they are contained within an allochthonous klippe (Mattawa klippe)
overlying the reworked Archean parautochthon, that transported the coronitic
metagabbros northwestward. Therefore, this klippe represents an erosional remnant ofthe
APB but the main ABT is located further south. </p> Nd-model age mapping in the southern region identified three distinct crustal
terranes. The Mattawa fault was detennined to be a brittle fault following approximately
along a pre-Grenvillian suture. This separates the reworked Archean parautochthon from
the Paleoproterozoic parautochthon, which had a Nd-model age range from 1.8-1.9 Ga
The Paleoproterozoic parautochthon was found to be truncated by the main body of the
APB, containing Nd-model ages < 1.8 Ga. The boundary between the Paleoproterozoic
parautochthon and the APB was interpreted as the location of the main ABT. </p> <p> Based on the distnbution of Nd-model ages in the region, four distinct crustal
terranes were identified representing two thrusting events. Initially the ABT was
overthrust on the parautochthonous belt causing decollement of the reworked Archean
parautochthon. The entrainment of the reworked Archean parautochthon under the APB
generated a duplex thrust sheet, which resulted in the consequent northwestward thrusting
of the reworked Archean parautochthon over the pristine Archean terrane. Finally, the
ABT was offset by post-Grenvillian normal faulting associated with Ottawa-Bonnechere
graben. </p> / Thesis / Master of Science (MSc)
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Nd Model Age Mapping of the Central Grenville Province in the Lac St. Jean Region, QuebecMartin, Christopher L. 09 1900 (has links)
<p> Detailed Neodymium ( Nd ) and Lead ( Pb ) isotopic mapping was performed on gray gneisses from the Lac St. Jean region of Quebec, in the Central Grenville Province. The Nd model ages determined were based upon the Depleted Mantle Model of DePaolo ( 1981c ). The use of this model is justified by a Sm-Nd whole-rock isochron as well as common Pb-Pb whole-rock dating. </p> <p> The field area was divided up into three sections: Western, Central, and Eastern. The boundaries of these divisions corresponded to abrupt changes in Nd model ages. The Western section was located in the Parautochthonous Belt of Rivers et al., ( 1989 ) and almost exclusively consisted of tonalitic gray gneisses with Archean Nd model ages. Whole-rock Pb isotopic analyses produced a Transposed Palaeoisochron, implying that the rocks of the Western section were significantly affected by the Grenville event approximately 1.1 Ga ago. Major and trace element analyses indicate that the Western section represents an arc and may be the exhumed basement of the Abitibi Volcanic Belt to the north, consistent with previous work by Ciesielski ( 1992 ).</p> <p> In contrast to the Western section, the Central section which is separated from the Western section by the Allochthon Boundary Thrust of Rivers et al., ( 1989 ) has a very wide range of Nd model ages from 1.65 Ga to 2.35 Ga. The temporal and spatial range of Nd model ages, coupled with major and trace element analyses, indicates that the Central section represents an Andean style ensialic continental margin arc. The diapirs that reworked these rocks contain varying amounts of Archean material implied by the Early-Proterozoic Nd model ages of some samples, particularly in regions close to the Allochthon Boundary Thrust. Whole-rock Pb isotopic analyses produce a Transposed Palaeoisochron indicating significant U depletion caused by the Grenville orogeny.</p> <p> The Eastern section is composed almost exclusively of gneisses with Nd model ages of ca. 1.5 Ga. The boundary between the Central and Eastern sections is based solely on lithological differences and an isotopic age break form the variable Nd model ages of the Central section, to the consistent model ages of the Eastern section. A Sm-Nd isochron for all samples with ca. 1.5 Ga Nd model ages produces an age of 1.50 +/- 0.07 Ga, in close agreement with previous work ( Dickin and Higgins, 1992 ). The consistency of model ages ( > 100,000 Km^2 when combined with data from Dickin and Higgins, 1992 ) coupled with major and trace element analyses, indicates the presence of an island arc which formed at ca. 1.5 Ga and was sutured on to the Laurentian craton within 100 Ma of its formation.</p> <p> Whole-rock Pb isotope analyses indicate that the Eastern section was not affected by the Grenville event as much as the Central or Western sections were.</p> <p> Pb-Pb zircon dating for samples from the Central and Eastern sections failed to place a tight constraint between crystallization ages and Nd model ages because of excessive Pb loss.</p> / Thesis / Master of Science (MSc)
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