DEFINITION OF TECTONIC EVOLUTION OF THE GRENVILLE PROVINCE

The tectonic framework of the Grenville province is a topic

of considerable debate. Many theories and models have

been proposed, although there is no one universally accepted

model. Nevertheless, there are some aspects of the tectonic

framework researchers do agree upon, in particular

that the Grenville province represents a collisional boundary.

This model is supported by seismic data and the granulite

facies metamorphism, both of which suggest that the

crust was doubly thickened during peak deformation and

metamorphism.

There are a few ways to thicken the crust: thrusting,

volcanism, plutonism, and homogeneous shortening. One or

a combination of these mechanisms must have occurred in

the late Proterozoic to produce granulite facies metamorphism

in the Grenville province. Two models account for

similar large tectonic crustal thickening presently occurring

on the Earth’s surface.

The first model is fashioned after the Andean-type margin.

This model suggests that relatively warm, buoyant

oceanic crust is subducted under continental crust. This

model has several implications. The oceanic crust subducted

underneath the South American plate is relatively young.

Therefore, it has not had a sufficient time interval to cool and

become dense. The relative low density of the young oceanic

crust resists subduction. Consequently, the oceanic crust

subducts at a relatively shallow angle. A shallow subduction

angle creates a compressional stress regime throughout the

margin. This has the effect of crustal shortening accommodated

by fore-arc frontal thrusts. The subducting oceanic

plate also induces plutonism and volcanism that adds to the

crustal thickening process.

The second model is based on the Himalayan orogen.

This model thickens the crust by a continent-continent collision.

This model is somewhat similar to the Andean model,

except that the warm, buoyant oceanic crust is replaced by

continental crust. The subducting continental crust resists

subduction due to its buoyancy, causing the subducting continental

crust to get tucked under the overriding continental

crust. The underriding crust never subducts down into the

asthenosphere but rather underplates the overriding continental

crust; hence, crustal thickening. It can be quickly seen that

the Andean model may be the predecessor to the Himalayan

model. Therefore, it is fair to conclude that a combination of

the two models may have worked together to produce the

Grenville orogen in the Proterozoic.

A simplistic tectonic model for the Grenville attempts to

explain the broad-scale tectonic processes that may account

for the large-scale features. An arc-continent collision was followed

by a continent-continent collision in the late Proterozoic,

probably involving southeastward-directed subduction

for the continent-continent collision. Consistent kinematics in

domain boundary shear zones (in the CGB) preserve an overall

northwesterly direction of tectonic transport, consistent

with northwestward stacking of crustal slices.

The calc-alkaline trends of the Elzevirian batholith suggest

that this is an island arc-type batholith. Thus, the Elzevir

terrane was probably an island arc before it collided with

North America. The Elzevirian age metamorphism resulted

from the collision of the CMB and the CGT. Ultimately, the

southeastward subduction along the western CMB margin

resulted in a continent-continent collision with the CGB.

Plate reconstructions for the Late Proterozoic are currently

an area of active investigation. Recent research, in the form

of geochronology, comparative geology, stratigraphy, and

paleomagnetism, have provided a wealth of new information

that has proven useful in correlating rocks on a global scale. It

is these correlations that geologists use to determine the temporal

and spatial plate configurations for the Late Proterozoic.

Such plate reconstructions have been a new source of

insight in the study of the Grenville province.

Advances in geochronology have been the greatest contributor

in helping to correlate rocks globally. Field mapping

in previously unmapped areas and improved techniques

in paleomagnetic determination further help to narrow possible

plate configurations. With this knowledge, geologists

take present-day continents and strip away their margins—

more precisely, all post-Grenvillian age rocks—and try to

piece together the cratons that may once have been conjugate

margins.

In 1991 researchers including Paul Hoffman, Eldridge

Moores, and Ian Dalziel proposed that a supercontinent

existed in the Late Proterozoic. This supercontinent has been

named Rodinia and was formed by the amalgamation of Laurentia

(North America and Greenland), Gondwana (Africa,

Antarctica, Arabia, Australia, India, and South America),

Baltica, and Siberia. The joining of these plates resulted in

collisional events along the Laurentian margins. It is these

orogenic events in the Late Proterozoic that are thought to

produce the Grenvillian belts found throughout the world.

Most Late Proterozoic plate reconstructions place the

Canadian Grenville province and Amazonian and Congo cratons

in close proximity. Therefore, Amazonia and Congo

were the probable Late Proterozoic continental colliders with

the eastern margin of Laurentia, resulting in the Ottawan

orogeny. Evidence supporting this correlation includes the

similar Neodynium (Nd) model ages of 1.4 billion years of

the Grenvillian belts found on the Amazonian and Congo

cratons, the same as the Laurentian Grenville province.

Plate reconstructions for the Late Proterozoic are not

absolute. There is no hard evidence, such as hot spot tracks

and oceanic magnetic reversal data, to determine plate motions

for the Proterozoic as there is for the Mesozoic and Cenozoic.

Furthermore, definitive sutures, such as ophiolite sequences

and blueschist facies terrains, are deformed and few, making it

difficult to determine the exact location of the Grenvillian

suture, that would strongly demonstrate a collisional margin.

This may be due to the expansive time interval that has

ensued, later orogenic events, rifting events, and erosion, all of

which help to alter and destroy the geologic record.

Most tectonic models for the Grenville province are

broadly similar for the late stages of the evolution of this orogenic

belt but differ widely in the early stages. The earliest

record of arc magmatism in the central metasedimentary belt

comes from the Elzevir terrane or composite arc belt, where

circa 1,350–1,225-million-year-old magmatism is interpreted

to represent one or more arc/back arc basin complexes. The

Adirondack Lowlands terrane may have been continuous with

the Frontenac terrane, which together formed the trailing

margin of the Elzevirian arc. Isotopic ages for the Frontenac

terrane fall in the range of 1,480–1,380 million years, and

between 1,450 million and 1,300 million years for the entire

central metasedimentary belt, suggesting that the Elzevirian

arc is largely a juvenile terrane. The Elzevirian arc is thought

to have collided offshore with other components of the composite

arc belt by 1,220 million years ago, because of

widespread northwestward-directed deformation and tectonic

repetition in the central metasedimentary belt at that time.

Following amalgamation, subduction is interpreted by some

to have stepped southeastward to lie outboard of the composite

arc and dipped westward beneath a newly developed active

margin. This generated a suite of circa 1,207-million-year-old

calc-alkaline plutons (Antwerp-Rossie suite) and 1,214 ±

21–million-year-old dacitic volcaniclastics, metapelites, and

diorite-tonalitic plutons. Other models suggest that the

Adirondack Highlands and Frontenac/Adirondack Lowlands

terranes remained separated until 1,170–1,150 million years

ago, when the Frontenac and Sharbot Lakes domains were

metamorphosed and intruded by plutons.

The Adirondack Highlands–Green Mountains block is

regarded by many workers as a single arc complex, based on

abundant circa 1,350–1,250-million-year-old calc-alkaline

tonalitic to granodioritic plutons in both areas. The Adirondack

Highlands–Green Mountains block may have been continuous

with the Elzevirian arc as well, forming one large

composite arc complex. Neodymium model ages for the

Adirondack Mountains–Green Mountain block fall in the

range of 1,450–1,350 Ma, suggesting that this arc complex

was juvenile, without significant reworking of older material.

Collision of the Adirondack Highlands–Green Mountain

block with Laurentia occurred between the intrusion of the

circa 1,207-million-year-old Antwerp-Rossie arc magmas,

and formation of the 1,172-million-year-old Rockport-Hyde-

School-Wellesley-Wells intrusive suite. This inference is based

on the observation that peak metamorphic conditions preceded

intrusion of the 1,180–1,150-million-year-old intrusive

suite in the Frontenac terrane. Also, metamorphic zircon and

monazite (presumably dating the collision) from the central

metasedimentary belt fall in the range of 1,190–1,180 million

years. The Carthage-Colton mylonite zone may represent a

cryptic suture marking the broad boundary along which the

Adirondack Highlands–Green Mountain block is juxtaposed

with Laurentia from a collision that emplaced the Lowlands

over the Highlands. Possible early localized delamination

beneath the collision zone may have elevated crustal temperatures

and generated crustal melts of the circa 1,172-millionyear-

old Rockport and Hyde School granites, and we add the

Wells leucocratic gneiss to this group. However, the present

geometry with relatively low-grade rocks of the Lowlands,

juxtaposed with high-grade rocks of the Highlands, suggests

that the present structure is an extensional fault that may

have reactivated an older structure.

The circa 1,172-million-year-old collisional granites

(Rockport, Hyde School gneiss, Wellesley, Wells) are largely

syntectonic and we suggest that emplacement of these magmas

may have slightly preceded formation of large-scale recumbent

nappes including the F1 fold documented here. These large

nappes may be responsible for complex map patterns and repetition

of units in the CMB and CGT. High-temperature deformation

of monzonites in the Robertson Lake shear zone took

place at circa 1,162 million years ago and demonstrated that

deformation continued for at least 10 million years after intrusion

of the 1,172-million-year-old magmatic suite. Deformation

had apparently terminated by 1,160 million years ago,

however, as shown by the 1,161–1,157-million-year-old

Kingston dikes and Frontenac suite plutons, which cross-cut

Elzevirian fabrics and cut the Robertson Lake shear zone.

The widespread monzonitic, syenitic, and granitic plutons

(AMCG suite) that intruded the Frontenac terrane in the

period of 1,180–1,150 million years ago swept eastward

across the orogen forming the AMCG suite in the Highlands

at 1,155–1,125 million years ago. It has been suggested that

separation of the subcontinental lithospheric mantle that

started around 1,180–1,160 million years ago may have proceeded

to large-scale delamination beneath the orogen. This

would have exposed the base of the crust to hot asthenosphere,

causing melting and triggering the formation of the

AMCG suite. We suggest that the 1,165-million-year-old

metagabbro units are related to this widespread melting and

intrusive event in the Adirondacks.

The culminating Ottawan orogeny from circa 1,100–1,020

million years ago in the Adirondacks and Grenville orogen is

widely thought to result from the collision of Laurentia with

another major craton, probably Amazonia. This collision is

one of many associated with the global amalgamation of continents

to form the supercontinent of Rodinia. The event is

associated with large-scale thrusting, high-grade metamorphism,

recumbent folding, and intrusion of a second generation

of crustal melts associated with orogenic collapse. The

putative suture (Carthage-Colton mylonite zone) between the

accreted Adirondack Highlands–Green Mountain block and

Laurentia was reactivated as an extensional shear zone in this

event, partly accommodating the orogenic collapse and

exhumation of deep-seated rocks in the Adirondack Highlands.

The relative timing of igneous events and folding in the

Adirondacks has shown that the F2 and F3 folding events in

the southern part of the Highlands postdated 1,165 million

and predated 1,052 million years ago, demonstrating that

these folds, and later generations of structures, are related to

the Ottawan orogeny. The Ottawan orogeny in this area is

therefore marked by the formation of early recumbent fold

nappes, overprinted by upright folds.

The regional chronology and overprinting history of

folding related to the Ottawan orogeny are generally poorly

known. In 1939 Buddington noted isoclinal folds dated circa

1,149 million years old in the Hermon granite gneiss in the

Adirondack Highlands, and very large granulite facies fold

nappes have been emplaced throughout the Adirondack

region. These folds refold an older isoclinal fold generation,

so are F2 folds, and are related to this regional event. The

youngest rocks that show widespread development of fabrics

attributed to the Ottawan orogeny are the circa 1,100–1,090-

million-year-old Hawkeye suite, that show “peak” conditions

of ~ 800°C at 20–25 km depth. These conditions existed

from about 1,050 million through approximately 1,013 million

years ago. Older thrust faults along the CMB boundary

zone were reactivated at about 1,080–1,050 million years

ago. The latter parts of the Ottawan orogeny (1,045–1,020

million years ago) are marked by extensional collapse of the

orogen, with low-angle normal faults accommodating much

of this deformation. Crustal melts associated with orogenic

collapse are widespread.

See also ADIRONDACK MOUNTAINS; CONVERGENT PLATE

MARGIN PROCESSES; STRUCTURAL GEOLOGY.

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