Grenville Province Tectonic Framework: Collisional Models and Rodinia Supercontinent

The tectonic framework of the Grenville province remains a topic of considerable debate. Numerous theories and models have been proposed, yet no single model is universally accepted. Nevertheless, researchers agree on certain aspects, particularly that the Grenville province represents a collisional boundary. Support for this interpretation includes seismic data and granulite facies metamorphism, both indicating that the crust was doubly thickened during peak deformation and metamorphism.

Crustal thickening can occur through several mechanisms: thrusting, volcanism, plutonism, and homogeneous shortening. One or a combination of these processes must have operated during the late Proterozoic to produce granulite facies metamorphism in the Grenville province. Two modern models account for similar large-scale tectonic crustal thickening observed on Earth’s surface today.

Andean-Type Margin Model. The first model is based on the Andean-type margin. This scenario proposes that relatively warm, buoyant oceanic crust is subducted beneath continental crust. Several implications arise from this model. The oceanic crust subducted under the South American plate is relatively young, having insufficient time to cool and become dense. The low density of this young oceanic crust resists subduction, causing it to descend at a relatively shallow angle.

A shallow subduction angle generates a compressional stress regime throughout the margin. This results in crustal shortening accommodated by fore-arc frontal thrusts. Additionally, the subducting oceanic plate induces plutonism and volcanism, further contributing to crustal thickening. This model thus explains both mechanical shortening and magmatic addition to the crust.

Himalayan Orogen Model. The second model is derived from the Himalayan Orogen. Here, crustal thickening occurs via a continent-continent collision. This process is somewhat similar to the Andean model, except that continental crust replaces the warm, buoyant oceanic crust. The subducting continental crust resists subduction due to its buoyancy, causing it to become tucked under the overriding continental plate.

The underriding crust never descends into the asthenosphere; instead, it underplates the overriding continental crust, resulting in crustal thickening. The Andean model may precede the Himalayan model; therefore, a combination of both processes likely worked together to produce the Grenville orogen during the Proterozoic. This composite model accounts for both early arc-related thickening and later collisional stacking.

A Simplified Tectonic Model for the Grenville Province. A simplistic tectonic model for the Grenville province attempts to explain the broad-scale processes responsible for large-scale features. An arc-continent collision was followed by a continent-continent collision in the late Proterozoic. This likely involved southeastward-directed subduction for the continent-continent collision, consistent with kinematics in domain boundary shear zones (within the Central Grenville Belt, CGB) that preserve an overall northwesterly direction of tectonic transport. Such kinematics align with northwestward stacking of crustal slices.

The calc-alkaline trends of the Elzevirian batholith suggest this body represents an island arc-type batholith. Thus, the Elzevir terrane was probably an island arc before it collided with North America. Elzevirian age metamorphism resulted from the collision of the Central Metasedimentary Belt (CMB) and the Central Granulite Terrane (CGT). Ultimately, southeastward subduction along the western CMB margin led to a continent-continent collision with the CGB.

Late Proterozoic Plate Reconstructions. Plate reconstructions for the Late Proterozoic are currently an area of active investigation. Recent research in geochronology, comparative geology, stratigraphy, and paleomagnetism has provided a wealth of new information useful for correlating rocks on a global scale. Geologists use these correlations to determine temporal and spatial plate configurations for the Late Proterozoic. Such reconstructions have offered new insights into the study of the Grenville province.

Advances in geochronology have been the greatest contributor to global rock correlations. Field mapping in previously unmapped areas and improved paleomagnetic techniques further help narrow possible plate configurations. With this knowledge, geologists take present-day continents, strip away their margins (specifically all post-Grenvillian age rocks), and attempt to piece together cratons that may once have been conjugate margins.

The Rodinia Supercontinent. In 1991, researchers including Canadian Paul Hoffman (now at Harvard University), Eldridge Moores (University of California), and Ian Dalziel (University of Texas) proposed that a supercontinent existed in the Late Proterozoic. This supercontinent, named Rodinia, 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. Geologists believe these orogenic events in the Late Proterozoic produced the Grenvillian belts found throughout the world.

Most Late Proterozoic plate reconstructions place the Canadian Grenville province and the 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 similar isotopic ages of 1.4 billion years for Grenvillian belts found on the Amazonian and Congo cratons, matching the Laurentian Grenville province.

Limitations of Proterozoic Reconstructions. Plate reconstructions for the Late Proterozoic are not absolute. Unlike the Mesozoic and Cenozoic, the Proterozoic lacks hard evidence such as hot spot tracks and oceanic magnetic reversal data to determine plate motions. Furthermore, definitive sutures that would strongly demonstrate a collisional margin—such as ophiolite sequences and blueschist facies terrains—are deformed and few in number. This scarcity makes it difficult to determine the exact location of the Grenvillian suture. The expansive time interval that ensued, later orogenic events, rifting events, and erosion have all contributed to altering and destroying the geologic record.

Early Stages of Grenvillian Evolution. Most tectonic models for the Grenville province are broadly similar for the late stages of evolution 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 ca. 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 1,450–1,300 million years for the entire Central Metasedimentary Belt. These ages suggest 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, as evidenced by widespread northwestward-directed deformation and tectonic repetition in the Central Metasedimentary Belt at that time.

Following amalgamation, some researchers interpret that subduction stepped southeastward to lie outboard of the composite arc and dipped westward beneath a newly developed active margin. This generated a suite of ca. 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. Many geologists regard the Adirondack Highlands-Green Mountains block as a single arc complex, based on abundant ca. 1,350–1,250-million-year-old calc-alkaline tonalitic to granodioritic plutons in both areas. This 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 ca. 1,207-million-year-old Antwerp-Rossie arc magmas and the 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. Additionally, 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. This collision emplaced the Lowlands over the Highlands. Early localized delamination beneath the collision zone may have elevated crustal temperatures and generated crustal melts of the ca. 1,172-million-year-old Rockport and Hyde School granites; the Wells leucocratic gneiss also belongs 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.

Syntectonic Granites and Deformation. The ca. 1,172-million-year-old collisional granites (Rockport, Hyde School gneiss, Wellesley, Wells) are largely syntectonic. Emplacement of these magmas may have slightly preceded the formation of large-scale recumbent nappes, including F1 folds. 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 ca. 1,162 million years ago, demonstrating 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, as shown by the 1,161–1,157-million-year-old Kingston dikes and Frontenac suite plutons. These bodies cross-cut Elzevirian fabrics and cut the Robertson Lake shear zone. This timing constrains the duration of major tectonic activity in this portion of the orogen.

The AMCG Suite and Delamination. The widespread monzonitic, syenitic, and granitic plutons (the AMCG suite) that intruded the Frontenac terrane from 1,180 to 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. Jim McLelland, Tim Kusky, and others have suggested that separation of the subcontinental lithospheric mantle beginning around 1,180–1,160 million years may have proceeded to large-scale delamination beneath the orogen. This process would have exposed the base of the crust to hot asthenosphere, causing melting and triggering the formation of the AMCG suite. The 1,165-million-year-old metagabbro units are related to this widespread melting and intrusive event in the Adirondacks.

The Ottawan Orogeny. The culminating Ottawan orogeny from ca. 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 Rodinia. The event is characterized by 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 Highlands-Green Mountain block and Laurentia was reactivated as an extensional shear zone during this event. This reactivation partly accommodated orogenic collapse and exhumation of deep-seated rocks in the Adirondack Highlands. The relative timing of igneous events and folding in the Adirondacks shows that the F2 and F3 folding events in the southern part of the Highlands postdated 1,165 and predated 1,052 million years ago, demonstrating that these folds and later generations of structures are related to the Ottawan orogeny. Thus, the Ottawan orogeny in this area is marked by the formation of early recumbent fold nappes overprinted by upright folds.

Regional Chronology of Folding. The regional chronology and overprinting history of folding related to the Ottawan Orogeny are generally poorly known. In 1939, Buddington noted isoclinal folds dated ca. 1,149 million years old in the Hermon granite gneiss in the Adirondack Highlands. Very large granulite facies fold nappes have been emplaced throughout the Adirondack region. These folds refold an older isoclinal fold generation, thus are F2 folds and are related to this regional event.

The youngest rocks showing widespread development of fabrics attributed to the Ottawan orogeny are the ca. 1,100–1,090-million-year-old Hawkeye suite. These record “peak” conditions of about 1,470°F (800°C) at 12–15 miles depth (20–25 km). Such conditions existed from about 1,050 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.

Grenville Belts and the Rodinia Supercontinent. The Proterozoic Eon witnessed the development of many continental-scale orogenic belts. Many of these have recently been recognized as parts of global-scale systems that reflect the formation, breakup, and reassembly of several supercontinents. Paleoproterozoic orogens include the Wopmay orogen in northern Canada, interpreted as a continental margin arc that rifted from North America and then collided soon afterward, closing a young back-arc basin. Numerous 1.9–1.6 Ga orogens exist worldwide, including the Cheyenne belt in the western United States, interpreted as a suture marking the accretion of Proterozoic arc terrains of the southwestern U.S. with the Archean Wyoming Province.

The supercontinent Rodinia formed in Mesoproterozoic times by the amalgamation of Laurentia, Siberia, Baltica, Australia, India, Antarctica, and the Congo, Kalahari, West Africa, and Amazonia cratons between 1.1 and 1.0 Ga ago. The joining of these cratons resulted in terminal collisional events at convergent margins on many of them, including the ca. 1.1–1.0 Ga Ottawan and Rigolet orogenies in the Grenville Province of Laurentia’s southern margin. Globally, these events have become known as the Grenville orogenic period, named after the Grenville orogen of eastern North America.

Grenville-age orogens are preserved along eastern North America, as the Rodinia-Sunsas belt in Amazonia, the Irumide and Kibaran belts of the Congo craton, the Namaqua-Natal and Lurian belts of the Kalahari craton, the Eastern Ghats of India, and the Albany-Fraser belt of Australia. Many of these belts now preserve deep-crustal metamorphic rocks (granulites) tectonically buried to 20–25 miles (30–40 km) depth. Subsequently, the overlying crust was removed by erosion, forcing the deeply buried rocks to the surface. Since 20–25 miles (28–30 km) of crust still underlies these regions, they likely had double crustal thickness during peak metamorphism. Such thick crust is produced today in regions of continent-continent collision and locally in Andean arc settings. Because Grenville-aged orogens are so linear and widely distributed, they are generally interpreted to mark the sites of continent-continent collisions where the various cratonic components of Rodinia collided between 1.1 and 1.0 Ga.

FURTHER READING: Dalziel, Ian W. D. “Neoproterozoic-Paleozoic Geography and Tectonics: Review, Hypothesis, Environmental Speculation.” Geological Society of America 109 (1997): 16-42.
“Pacific Margins of Laurentia and East Antarctica-Australia as a Conjugate Rift Pair: Evidence and Implications for an Eocambrian Supercontinent.” Geology 19 (1991): 598-601.
Davidson, Anthony. “An Overview of Grenville Province Geology, Canadian Shield.” In *Geology of North America, vol. C-1, Geology of the Precambrian Superior and Grenville provinces and Precambrian Fossils in North America*, edited by S. B. Lucas and M. R. St-Onge, 205-270. Denver, Colo.: Geological Society of America, 1998.