Further Reading

Kious, Jacquelyne, and Robert I. Tilling. "U.S. Geological Survey. This Dynamic Earth: The Story of Plate Tectonics." Available online. URL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Modified March 27, 2007.

Kusky, T. M., ed. Precambrian Ophiolites and Related Rocks, Developments in Precambrian Geology 13. Amsterdam: Elsevier, 2003. Kusky, Timothy M., Jianghai Li, and Robert T. Tucker. "The Archean Dongwanzi Ophiolite Complex, North China Craton: 2.505 Billion-Year-Old Oceanic Crust and Mantle." Science 292 (2001): 1142-1145. Kusky, Timothy M., and Julian Vearncombe. "Structure of Archean Greenstone Belts." In Tectonic Evolution of Greenstone Belts, edited by Maarten J. de Wit and Lewis D. Ashwal, 95-128. Oxford Monograph on Geology and Geophysics. Oxford: Oxford Science Publications, 1997.

Grenville province and Rodinia At several times in the history of the planet, most of the continental landmasses have aggregated or joined together to form large supercontinents. The most recent of these was the fairly familiar supercontinent of Pan-gaea, which contained most of the planet's continents between 300 and 200 million years ago. Before that, the supercontinent of Gondwana formed at about 570 million years ago and lasted only a short geological time (the exact amount is still under debate and investigation). As we explore further back in geological time, the evidence for older supercontinents becomes harder and harder to interpret. Despite this, in the past decade geologists have been able to reconstruct an older supercontinent, known as Rodinia, that formed about 1 billion years ago and broke up around 700 million years ago.

The Grenville province is the youngest region of the Canadian shield; it is outboard of the Labrador, New Quebec, Superior, Penokean, and Yavapai-Mazatzal provinces. It is the last part of the Canadian shield to experience a major deformational event, this being the Grenville Orogeny, which was responsible for forming many folds and faults throughout the entire region during the amalgamation of numerous continents to form the supercontinent of Rodinia. The other ancient, highly eroded mountain belts around the world that formed during the collision of the other continents to form Rodinia have also become known as Grenvillian belts, named after the excellent type exposures of deeply eroded mountain belts of this age in the Grenville province. The Grenville province has an aerial extent of approximately 600,000 square miles (1,000,000 km2). The subterranean extent of Grenville rocks, however, is much greater in area. Phanerozoic rocks cover their exposure from New York State down the length of the Appalachian Mountains and into Texas.

The Grenville province formed on the margin of the continent of Laurentia (an early or immature stage in the development of North America) in the middle to late Proterozoic. The rocks throughout the province represent a basement and platform sedimentary sequence intruded by igneous rocks. subsequent to this intrusive event in the late Proterozoic, the entire region underwent high-grade metamorphism and was complexly deformed. But before this highgrade metamorphic event, the rocks of the Grenville province experienced multiple pulses of metamor-phism and deformation, including the Elsonian (1,600-1,250-million-year-old) and the Elzevirian (1,250-1,200-million-year-old). Orogenies. The ottawan orogeny was the last and most intense in the Grenville province, culminating 1.1 billion years ago, and overprinting much of the earlier tectonic history. This has made it difficult for geologists to describe the earlier orogenies and also to determine the tectonic evolution of the Grenville province. For these reasons, the term Ottawan Orogeny is usually used synonymously with the term Grenville Orogeny.

The Grenville province is subdivided into numerous subprovinces including the central gneiss belt (CGB), central metasedimentary belt (CMB), and central granulite terrane (CGT), and one major structural feature: the Grenville front (GF).

The central gneiss belt (CGB) is located in the western part of the Grenville province and contains some of the oldest rocks found in the province. The majority of the rocks are 1.8-1.6 billion-year-old gneisses intruded by 1.5-1.4 billion-year-old granitic and monzonitic plutons. Both the metasedimentary and the igneous rocks of the CGB are metamorphosed from upper amphibolite and locally granu-lite facies. The CGB is bounded by the Grenville front to the northwest and lies in tectonic contact with the central metasedimentary belt to the southeast. The dominant structural trend is northeast, but changes to the northwest near Georgian Bay. The CGB has been divided into smaller terranes including the Nipissing, Algonquin, Tomiko, and Parry sound, based on lithology, metamorphic grade, and structures, namely, shear zones. These terranes are considered to be mainly parautochthonous (mean ing that they have not traveled far from their place of formation) terranes. The shear zones that separate the various terranes contain kinematic indicators that suggest northwest directed tectonic transport, and tectonic transport is thought to have occurred between 1.18 and 1.03 billion years ago.

The Nipissing terrane is located in the western portion of the central gneiss belt. Part of the Nipiss-ing terrane occupies a region known as the Grenville front tectonic zone (GFTZ), an area that lies within 30 miles (50 km) of the Grenville front. The litholo-gies here are strongly deformed with northeast-striking foliations and zones of cataclasis and moderately plunging southeast lineations. The heterogeneous gneisses of the Nipissing terrane fall into two categories: Archean and Lower Proterozoic migmatitic gneisses that are likely reworked units of the southern and superior provinces and Middle Proterozoic metasedimentary gneiss. These rocks were intruded by 1.7 and 1.45 billion-year-old granitic plutonic rocks, both of which are less deformed than the host rocks. Postdating this intrusive event, the region underwent high-grade metamorphism, experiencing temperatures of 1,200°F-1,280°F (650°C-750°C) and pressures of 8.0-8.5 kilobars.

The Tomiko terrane is located in the extreme northwestern portion of the central gneiss belt. The most striking aspect of the Tomiko terrane is the relative abundance of metasedimentary rocks, but it also contains metamorphosed granitic rocks that are Middle Proterozoic in age. The Tomiko terrane is allochthonous (far-traveled) with respect to the Nipissing terrane. Evidence to support this is the distinct detrital zircon population in the Tomiko metaquartzites, dated at 1,687 million years old. This is in sharp contrast to the metaquartzites of the Nipissing terrane, where the detrital zircons are Archean to Lower Proterozoic in age. This suggests that the Nipissing terrane was already adjacent to the superior province at the time of the Nipissing quartzite formation. Further evidence for the alloch-thonous nature of the Tomiko terrane is the presence of iron formations in the Tomiko terrane, which are not present elsewhere in the CGB. The metamorphic conditions experienced by the Tomiko terrane are temperatures of fewer than 1,290°F (700°C) and pressures of 6.0-8.0 kilobars.

The Algonquin terrane, the largest terrane in the CGB, consists of numerous domains. The rocks in this terrane are meta-igneous quartzo-feldspathic gneisses and supracrustal gneisses. Generally, the foliations strike northeast and dip to the southeast; down-dip stretching lineations are common. The southern domains have been interpreted as thrust sheets with a clear polarity of southeasterly dips, and the entire Algonquin terrane may be parautochtho-nous. The metamorphic temperatures and pressures range from 1,240°F-1,520°F (670oC-825oC) and 7.9-9.9 kilobars, respectively.

The Parry Sound terrane, the most studied ter-rane in the CGB, is located in the south-central por tion of the CGB and contains large volumes of mafic rock, marble, and anorthosite. The age of the Parry Sound terrane ranges from 1,425 to 1,350 million years. Both the lithologies and the age of the Parry

Southern

Superior province

Eastern i J Rae

Southern

Superior province

Eastern i J Rae

Domain boundaries

Parautochthonous belt

Allochthonous polycyclic belt

Allochthonous monocyclic belt

Long Range

Domain boundaries

Parautochthonous belt

Allochthonous polycyclic belt

Allochthonous monocyclic belt

Long Range

CENTRAL GNEISS BELT

Superior province JJ Tomiko gftz

CENTRAL GNEISS BELT

Algonquin terrane

_terrane^

_terrane^

Algonquin terrane

Superior Terrane

Age of crust of major period of Major rock units of the magmatism for CGB and CMB Adirondack Highlands

1450-1350 Ma >1350 Ma 1300-1250 Ma 1170-1100 Ma

Charnockite and granitic gneiss Meta-anorthosite Mangerite, syenitic gneiss Hornblende-granitic gneiss

Terrane boundary known, inferred

Lithological contact, geographical boundary

50 miles

50 km

Maps compiled from McLelland and Isachsen, 1985; Rivers, et al., 1989; McLelland, et al., 1996; Carr, et al., 2000.

(A) Tectonic subdivisions of the Grenville province according to the classification of Toby Rivers (1989) and Carr, et al. (2000), showing also the older domain boundaries of Wynne-Edwards (1972) and others; (B) Terranes and shear zones of the central gneiss belt (CGB), the central metasedimentary belt (CMB), and major geological features of the Adirondack Highlands. Abbreviations as follows: BCS-Baie Comeau segment; CCMZ-Carthage-Colton mylonite zone; CLM-Chain Lakes massif; CMBBZ-central metasedimentary belt boundary zone; CGB-central gneiss belt; CGT-central granulite terrane; EGP-eastern Grenville province; GFTZ-Grenville Front tectonic zone; GM-Green Mountains.

Sound terrane are different from the rest of the CGB. Not surprisingly, therefore, this terrane is considered as allochthonous and overlying the parautochtho-nous Algonquin domains. Because the Parry Sound terrane is completely surrounded by the Algonquin terrane, structurally it is considered a klippe. The metamorphic conditions reached by the Parry Sound terrane are in the range of 1,200°F-1,470°F (650°C-800°C) and 8.0-11.0 kilobars.

The central metasedimentary belt (CMB) has a long history of geologic investigation. One of the reasons is the abundance of metasedimentary rocks, which makes it a prime target for locating ore deposits. The CMB was originally named the Grenville series by Sir William Logan in 1863 for an assemblage of rocks near the village of Grenville, Quebec, and is the source of the name for the entire Grenville province. Later, the Grenville series achieved supergroup status, but presently Grenville Supergroup is a term limited to a continuous sequence of rocks within the CMB.

The CMB contains Middle Proterozoic metasediments that were subsequently intruded by syn-, late-, and post-tectonic granites. The time of deposition is estimated to have been from 1.3 to 1.1 billion years ago, with the bulk of the material having been deposited before 1.25 billion years ago. After their deposition, the rocks of the CMB underwent deformation and metamorphism from in the Elzevirian Orogeny (1.19-1.06 billion years ago). The effects of the Elzevirian orogeny were all but wiped out by the later Ottawan Orogeny, which deformed and metamorphosed the rocks to middle-upper amphibo-lite facies. The CMB contains five distinct terranes: Bancroft, Elzevir, Sharbot Lake, Mazinaw, and Frontenac. The Frontenac is correlative with the Adirondack Lowlands.

The Bancroft terrane is located in the northwestern portion of the CMB. The Bancroft is dominated by marbles but also contains nepheline-bearing gneiss and granodioritic orthogneiss metamorphosed to middle through upper amphibolite facies. The Bancroft terrane contains complex structures, such as marble breccias and high-strain zones. The orthogneiss occurs in thin structural sheets, suggesting that it may occur in thrust-nappe complexes. The thrust sheets generally dip to the southeast with dips increasing toward the dip direction. Rocks of the Bancroft terrane possess a well-developed stretching lineation that also plunges in the southeast direction. Both of these structural orientations suggest northwest directed tectonic transport.

The Elzevir terrane, located in the central portion of the CMB, is known for containing the classic Grenville Supergroup. The Elzevir is composed of 1.30-1.25-billion-year old metavolcanics and metasediments, intruded by 1.27-billion-year-old tonalitic plutons ranging in composition from gab-bro to syenite. The largest of these calc-alkaline bodies is the Elzevirian batholith. The calc-alkaline signature of the batholith suggests that it may have been generated in an arc-type setting. The Elzevir terrane also contains metamorphic depressions, areas of lower metamorphic grade, such as greenschist to lower amphibolite facies. These depressions may be related to the region's polyphase deformation history, and in contrast to surrounding high-grade ter-ranes, they contain sedimentary structures enabling the application of stratigraphic principles to determine superposition.

The Mazinaw terrane was once mapped as part of the Elzevir terrane; it also contains some of the classic Grenville Supergroup marbles and the Flinton Group. The rocks encountered here are marbles, calc-alkalic metavolcanic and clastic metasedimen-tary rocks. The Flinton Group is derived from the weathering of plutonic and metamorphic rocks found in the Frontenac terrane. Furthermore, the complex structural style of the Mazinaw terrane is similar to the Frontenac and the Adirondack Lowlands.

The Sharbot Lake terrane was once mapped as part of the Frontenac terrane but is now considered a separate terrane. The Sharbot Lake principally contains marbles and metavolcanic rocks intruded by intermediate and mafic plutonic rocks and may represent a strongly deformed and metamorphosed carbonate basin. Metamorphic grade ranges from greenschist to lower amphibolite. The lithologies, metamorphic grade, and lack of exposed basement rocks to the Sharbot Lake terrane imply that these rocks may be correlative with the Elzevir terrane.

The Frontenac terrane is located in the southeastern portion of the CMB. This terrane extends into the Northwest Lowlands of the Adirondack Mountains. The Frontenac terrane is composed of marble with pelitic gneisses and quartzites. The relative abundances of the gneisses and quartzites increase toward the southeast, while the relative abundances of metavolcanic rocks and tonalitic plutons decrease in the same direction. A trend also exists in the meta-morphic grade from northwest to southeast. In the northwest the metamorphic grade ranges from lower amphibolite to upper amphibolite-granulite facies but then decreases in the southeast to amphibolite facies. Rock attitudes also change, dipping southeast in the northwest, to vertical in the central part, to the northwest in the Northwest Lowlands.

Throughout the CMB, large-scale folds are present. These folds indicate crustal shortening. More important, however, is the recognition of main structural breaks that lie both parallel to and within the CMB. The structural breaks are marked by narrow zones of highly attenuated rocks, such as mylonites. The Robertson Lake mylonite zone (RLMZ), one such structural break, lies between the Sharbot Lake terrane and the Mazinaw terrane. The RLMZ has been interpreted as a low-angle thrust fault and also as a normal fault caused by unroofing.

To the east of the central metasedimentary belt lies the central granulite terrane. These two subprovinces are separated by the Chibougamau-Gatineau Lineament (CGL), a wide mylonite zone. The fact that the CGL is well-defined on aeromagnetic maps suggests that it is a crustal-scale feature. The CGL roughly trends northeast-southwest, where it ranges from about 10 feet (a few meters) to more than four miles (7 km) wide. The CGL may be correlative with the Carthage Colton mylonite zone in the Adirondack Mountains of New York State.

The central granulite terrane (CGT), originally named by Canadian geologist H. R. Wynne-Edwards in 1972, is located in the central and southeastern portion of the Grenville province and is correlative with the Adirondack Highlands. The CGT is often referred to as the core zone of the Grenville oro-gen, and is the site where the majority of the Gren-villian plutonic activity occurred. This subprovince underwent high-grade metamorphism with paleo-temperatures ranging up to 1,470°F (800°C) and paleopressures up to 9.0 kilobars. To explain these high pressures and temperatures, a double thickening of the crust is required. For this reason, geologists have suggested that the Grenville province represents a continent/continent collision zone.

The most abundant rock constituent of the central granulite belt is anorthosite. The larger anor-thosite bodies are termed massifs, such as the Morin massif. The anorthosites, along with a whole suite of rocks, known as AMCG (anorthosite, man-gerite, charnokite, granite) suite, are thought to have intruded at approximately 1,159-1,126 million years ago according to uranium-lead zircon analysis. These dates are in agreement with uranium-lead zircon ages of the AMCG rocks in the Adirondack Highlands (1,160-1,125 million years old). This places their intrusion as postdepositional with the sediments of the CMB and before the Ottawan Orogeny. The anorthosites were emplaced at shallow levels somewhere between the Grenville supergroup and the underlying basement. A major tectonic event such as continental collision must have occurred to produce the high paleotemperatures and paleopressures recorded in the anorthosites.

For the most part the Grenville front (GF) marks the northwestern limit of Grenville deformation and truncates older provinces and structures. The zone is approximately 1,200 miles (2,000 km) long and is dominated by northwest-directed reverse faulting that has been recognized since the 1950s. The GF is recognized by faults, shear zones, and metamorphic discontinuities. Faults, foliations, and lineations dip steeply to the southeast. Interpretation of the Grenville front has changed with time. In the 1960s, with the advent of the theory of plate tectonics, the Gren-ville front was immediately interpreted as a suture. This suggestion was refuted because Archean age rocks of the Superior craton continue south across the Grenville front, implying that the suture should lie to the southeast of the GF. It is possible that the suture is reworked somewhere in the Appalachian orogen. There are still several unresolved questions about the tectonic nature of the Grenville front, considering that the GF marks the limit of Grenvillian deformation: (1) the adjacent foreland to the northwest contains no evidence of supracrustal assemblages associated with the Grenville orogen, (2) the zone lacks Grenville age intrusives that are prevalent to the southeast, and (3) the front divides older rocks from a belt of gneisses that appear to be their reworked equivalents.

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