<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)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19348 |
| Date | 04 1900 |
| Creators | North, Robert |
| Contributors | Dickin, A. P., Geology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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