DEFINITION OF GRENVILLE PROVINCE

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 complexly deforming the

entire region. 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 Laurentia

in the middle to late Proterozoic. The rocks throughout

the province represent a basement and platform sedimentary

sequence that was intruded by igneous rocks. Subsequent to

this intrusive event in the late Proterozoic, the entire region

underwent high-grade metamorphism and was complexly

deformed. However, prior to this high-grade metamorphic

event, the rocks of the Grenville province experienced multiple

pulses of metamorphism 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), central granulite terrane

(CGT), and one major structural feature: the Grenville

front.

The 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-yearold

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 granulite 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 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 billion and 1.03 billion years ago.

The Nipissing terrane is located in the western portion of

the central gneiss belt. Part of the Nipissing terrane occupies

a region known as the Grenville front tectonic zone (GFTZ),

an area that lies within 30 kilometers of the Grenville front.

The lithologies here are strongly deformed with northeaststriking

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 billion- 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 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 allochthonous 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 less than 1,290°F (700°C) and pressures of

6.0–8.0 kilobars.

The Algonquin terrane is the largest terrane in the CGB

and 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 parautochthonous. The metamorphic

temperatures and pressures range from 1,240°F–1,520°F

(670°C–825°C) and 7.9–9.9 kilobars, respectively.

The Parry Sound terrane is the most studied terrane in the

CGB. It is located in the south-central portion of the CGB and

contains large volumes of mafic rock, marble, and anorthosite.

The age of the Parry Sound terrane ranges from 1,425 million

to 1,350 million years. Both the lithologies and the age of the

Parry Sound terrane are different from the rest of the CGB.

Therefore, it is not surprising that this terrane is considered as

allochthonous and overlying the parautochthonous Algonquin

domains. In fact, since 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 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

was raised to supergroup status, but presently the 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 billion 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 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 amphibolite

facies. The CMB contains five distinct terranes: Bancroft, Elzevir,

Mazinaw, Sharbot Lake, 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 welldeveloped

stretching lineation that also plunges in the southeast

direction. Both of these structural orientations suggest

northwest-directed tectonic transport.

The Elzevir terrane is located in the central portion of the

CMB. It 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-billionyear-

old tonalitic plutons ranging in composition from gabbro

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.

These are 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 terranes, they contain

sedimentary structures enabling the application of stratigraphic

principles in order to determine superposition.

The Mazinaw terrane was once mapped as part of the

Elzevir terrane and 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 metasedimentary 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

metamorphic 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) is

one such structural break and 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 separat-

ed by the Chibougamau-Gatineau Lineament (CGL), which is

a wide mylonite zone. The CGL is well defined on aeromagnetic

maps suggesting that it is a crustal-scale feature. The

CGL roughly trends northeast-southwest, where it ranges

from a few meters to seven kilometers 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) was originally

named by Wynne-Edwards in 1972. It 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 orogen and is

where the majority of the Grenvillian plutonic activity

occurred. This subprovince underwent high-grade metamorphism

with paleotemperatures ranging up to 1,470°F (800°C)

and paleopressures up to 9.0 kilobars. In order to explain

these high pressures and temperatures, a double thickening of

the crust is required. For this reason, it has been 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 anorthosite bodies are

termed massifs, such as the Morin massif. The anorthosites

along with a whole suite of rocks, known as AMCG

(anorthosite, mangerite, charnokite, granite) suite, are

thought to have intruded at approximately 1,159–1,126 million

years ago based on U-Pb zircon analysis. These dates are

in agreement with U-Pb 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 in order 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 northwestdirected

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 GF has changed

with time. In the 1960s, with the advent of the theory of

plate tectonics, the GF was immediately interpreted as a

suture. This suggestion was refuted because Archean-age

rocks of the Superior craton continue south across the GF,

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 GF, 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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