Grenville orogeny
Encyclopedia
The Grenville Orogeny was a long-lived Mesoproterozoic
mountain-building event associated with the assembly of the supercontinent
Rodinia
. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador
to Mexico
, as well as to Scotland
. Orogenic
crust of mid-late Mesoproterozoic age (circa 1250-980 Ma) is found worldwide, but generally, only events which occurred on the southern and eastern margins of Laurentia
are recognized under the “Grenville” name. These events are known as the Kibaran orogeny in Africa, the Dalslandian orogeny in western Europe.
Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende, biotite, and potassium feldspar suggest that extension was occurring when compression had momentarily ceased.
River’s 2008 paper has now examined the timing of the different periods of the orogeny and reconstructed the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian Orogeny occurs from 1240-1220 Ma, the Shawinigan occurs from 1190-1140 Ma and is no longer part of the Grenville Cycle, the Ottawan (now 1090-1020 Ma) and Rigolet (still 1010-980 Ma) become phases which are grouped into the Grenvillian Orogeny.
margins until the beginning of continental collision. This type of subduction
(B-type) tends to emplace magmatic arcs
on or near the edge of the overriding plate in modern subduction zones, and evidence of contemporary (ca. 1300-1200 Ma) island arcs can be found throughout the Grenville orogen. The Andes
of South America are considered a modern analogue. From about ca. 1190-980 Ma (the actual timing varies by locality) two separate continental blocks collided with Laurentia. Both of these collision events are thought to be analogous to the collision driving modern-day growth of the Himalaya range. For some time one of the blocks was believed to be the continent of Amazonia, but paleomagnetic evidence has now proven that this is not the case.
These periods of thrusting and metamorphism
were not continuous, but rather interrupted by comparatively quiet periods, during which AMCG (anorthosite
/ mangerite
/ charnockite
/ granite
) plutons were intruded into the country rock. Polarities of subduction (which plate overrode which) vary by region and time. Some island arc remnants were emplaced on the Laurentian margin, and some were accreted during orogeny. Timing of these events is constrained by cross-cutting relations observed in the field as well as SHRIMP (sensitive high-resolution ion microprobe) and TIMS (thermal ionization mass spectrometry) uranium- lead dating
.
The first period of tectonic activity was the accretion of an island arc at some point during the Elzevirian Orogeny. Before the accretion of the island arc took place, subduction between a continental plate and presumably an oceanic plate was taking place. Slab pull and far-field drivers such as ridge push were aiding in closing the distance between the island arc and the continent. Depending on the angle of subduction deformation of the continental crust was already taking place and thickening the lithosphere. By 1.19 Ga the Elzevir back arc basin was closing.
From 1.18 to 1.14 Ga extension was occurring in the area. Whether due to lithospheric cooling also known as thermal subsidence or the compressional activity in the area reactivated some extensional faults the extension is marked by the isotopic ages of the previously mentioned rocks. Additionally there is the formation of sedimentary basins which means the margin was quiescent enough that sediments could accumulate. However in some areas from 1.16 to 1.13 Ga, coeval with extension, there is evidence there was still thrusting and emplacement of terranes occurring.
According to one model westward thrusting occurred from 1.12 to 1.09 Ga and then extension was the primary tectonic activity until 1.05 Ga. It was at this point that the Central Granulite Terrane was exhumed and minor magmatism occurred.
The precise reason for change from compression to extension is unknown but may be the result of gravitational collapse, mantle delamination, the formation of a plume underneath a supercontinent, changes in far-field drivers on the distribution of stress, or any combination of reasons originating from the fact that our planet is dynamic. The cyclic compression and extension history of this area is similar to the Wilson Cycle. In this area of the world the Wilson Cycle would be creating the basin for the proto-Atlantic Ocean (Iapetus Ocean
)
and granulite
facies, that is, medium to high temperature and pressure alteration. Eclogitized
metagabbros (very high pressure ultramafic metamorphic rocks) are found in some localities, and likely represent areas of deepest burial and/or most intense collision. Throughout the orogen, these sequences of high pressure metamorphic rocks are cut by intrusive AMCG suite plutons, generally interpreted as syn- or post-tectonic. AMCG plutonism is generally associated with asthenospheric
upwelling under thinned lithosphere
. This is derived from the theory that AMCG plutonism is driven by ponding of olivine
tholeiite
basalt at the base of the continental crust
during tectonic extension. The lithosphere may be thinned either convectively or by delamination
, in which the bottom portion of the lithosphere is stripped off. Both models have been proposed for the Grenville orogeny.
The Grenville Orogeny can be categorized into three sections based on structure, lithology, and thermochronology. The three sections, respectively called the Gneiss Belt, Metasedimentary Belt, and the Granulite terrane are all separated by shear zones.
The Gneiss Belt is made up of felsic gneisses and amphibolites that were metamorphosed in the upper amphibolite to granulite facies. Thrusting in this section was low angle, but would have the potential to increase and rotate as it continued and evolved. Shear in this area is referred to as ductile shear meaning the material was cooling and becoming solid, but still behaving viscously or plasticly.
The age of this belt is approximately 1.8 to 1.18 Ga. Regional metamorphism is believed to have deformed this area at approximately 1.4 Ga and metamorphic thrusting at approximately 1.16 to 1.12 Ga. The area called the Appalachians falls into this category.
The Metasedimentary Belt is predominantly sedimentary and volcanic rocks which have undergone greenschist to granulite facies metamorphism. Subdivisions of this belt include the Bancroft, Elzevir, Sharbot Lake, and Frontenac Domains and the Adirondack Lowlands. In this belt magmatism is known to have occurred between 1.42 to 1.04 Ga depending on location. As with the Gneiss Belt, metamorphism is believed to have occurred at approximately 1.16 Ga.
The Granulite Terrane is composed of meta-igneous gneisses including anorthosite massifs. Anorthosites form in plutons and are composed mostly of plagioclase. The rocks of the Grenville Province in Canada are included in this category. The oldest magmatism known in this area dates to 1.32 Ga approximately. Granulite facies metamorphism began around 1.15 Ga and continued for about 150 Ma after the onset, however the continuity of the metamorphism cannot be determined.
Texas and Mexico represent the southern margin of Laurentia, and likely collided with a different continent than that involved in the eastern collision. The Zapotecan Orogeny of Mexico is coeval with the later stages of the Grenville orogeny, and they are generally considered to be one and the same. Mesoproterozoic igneous protoliths (metamorphosed to granulite
facies during the orogeny) fall into two age groups in Mexico; ca. 1235-1115 Ma and ca. 1035-1010 Ma. Rocks of the former group bear geochemical signatures implying island arc
and back-arc basin
provenance. The latter group represents AMCG magmatism. These AMCG rocks are somewhat anomalous throughout the Grenville orogen, there is no known orogenic event which immediately predates their emplacement. It is suggested that the regime of subduction under the Laurentian margin (currently in Texas, north of the accreted Mexican terrane
) ended around 1230 Ma, and that subduction
polarity reversed to bring the colliding continent north, since the Llano uplift, which records the history of the Grenville in Texas, bears no evidence of arc magmatism after this time.
contain small, isolated exposures of the Grenville orogen. The largest of these, the Shenandoah and French Broad massif
s, comprise the Blue Ridge
province of Virginia. Blue Ridge rocks consist of various gneisses of upper amphibolite
and granulite facies, intruded by charnockites and granitoid rocks. These igneous rocks were intruded in three intervals: ca. 1160-1140 Ma, ca. 1112 Ma, and ca. 1080-1050 Ma, and are massive to weakly foliated
in texture.
separates the dome into the Highlands to the southeast and the Lowlands to the northwest. It is believed that the shear zone (the Carthage-Colton) was a transpressional boundary during the Ottawan, when the Highlands were thrust over the Lowlands.
in Quebec
, and constitutes the youngest portion of the Canadian Shield
. Since the area has not undergone any regional metamorphic overprinting since the orogeny, it is considered an ideal study area for Grenville and pre-Grenville age tectonics. Hence, most of what is known about the orogeny and its processes is derived from the Grenville Province.
o Models of supercontinents, such as Rodinia, rely on information about what parts of which continents were connected and orogenies in many cases provide rocks like ophiolites or exotic terranes which can be examined and compared to other continents. Even similar ages of rocks can provide a starting point for such investigations. For example, when a connection can be made between two continents based on a type of rock present on opposing coast with similar ages and isotopic ratios aids in understanding continental movements and locations.
• Allows for the investigation of the behavior of the deep crust during orogenesis.
o Since geologists cannot directly observe the movement of the deep crust and mantle during the mountain building process, the deep mountain exposures present in the Grenville Orogeny area (formed due to long term erosion) allow for observation of the mountain belt assembly.
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...
mountain-building event associated with the assembly of the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
Rodinia
Rodinia
In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador
Labrador
Labrador is the distinct, northerly region of the Canadian province of Newfoundland and Labrador. It comprises the mainland portion of the province, separated from the island of Newfoundland by the Strait of Belle Isle...
to Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
, as well as to Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. Orogenic
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
crust of mid-late Mesoproterozoic age (circa 1250-980 Ma) is found worldwide, but generally, only events which occurred on the southern and eastern margins of Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
are recognized under the “Grenville” name. These events are known as the Kibaran orogeny in Africa, the Dalslandian orogeny in western Europe.
Timescale
The problem of timing of the Grenville Orogeny is an area of some contention today. The timescale outlined in Toby Rivers’ recent work is derived from the well-preserved Grenville Province, and represents one of the most detailed records of the orogeny. This classification considers the classical Grenville designation to cover two separate orogenic cycles; the Rigolet, Ottawan and Shawingian orogenies compose the Grenville Cycle, and the Elzevirian Orogeny stands on its own. Due to the great size of the area affected by Grenville events, there is some variance in timing across the orogenic belt. Regional Variations discusses local deviations from Rivers’ timeline, presented here.Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende, biotite, and potassium feldspar suggest that extension was occurring when compression had momentarily ceased.
River’s 2008 paper has now examined the timing of the different periods of the orogeny and reconstructed the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian Orogeny occurs from 1240-1220 Ma, the Shawinigan occurs from 1190-1140 Ma and is no longer part of the Grenville Cycle, the Ottawan (now 1090-1020 Ma) and Rigolet (still 1010-980 Ma) become phases which are grouped into the Grenvillian Orogeny.
General Tectonics
Reconstruction of the events of the orogeny is ongoing, but the generally-accepted view is that the eastern and southern margins of Laurentia were active convergentConvergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
margins until the beginning of continental collision. This type of subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
(B-type) tends to emplace magmatic arcs
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
on or near the edge of the overriding plate in modern subduction zones, and evidence of contemporary (ca. 1300-1200 Ma) island arcs can be found throughout the Grenville orogen. The Andes
Andes
The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...
of South America are considered a modern analogue. From about ca. 1190-980 Ma (the actual timing varies by locality) two separate continental blocks collided with Laurentia. Both of these collision events are thought to be analogous to the collision driving modern-day growth of the Himalaya range. For some time one of the blocks was believed to be the continent of Amazonia, but paleomagnetic evidence has now proven that this is not the case.
These periods of thrusting and metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
were not continuous, but rather interrupted by comparatively quiet periods, during which AMCG (anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
/ mangerite
Mangerite
Mangerite is a plutonic intrusive igneous rock, that is essentially a hypersthene-bearing monzonite. It often occurs in association with norite, anorthosite, charnockite and rapakivi granite in Proterozoic metamorphic belts....
/ charnockite
Charnockite
Charnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene , as an end-member of the charnockite series.-Charnockite series:...
/ granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
) plutons were intruded into the country rock. Polarities of subduction (which plate overrode which) vary by region and time. Some island arc remnants were emplaced on the Laurentian margin, and some were accreted during orogeny. Timing of these events is constrained by cross-cutting relations observed in the field as well as SHRIMP (sensitive high-resolution ion microprobe) and TIMS (thermal ionization mass spectrometry) uranium- lead dating
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
.
The first period of tectonic activity was the accretion of an island arc at some point during the Elzevirian Orogeny. Before the accretion of the island arc took place, subduction between a continental plate and presumably an oceanic plate was taking place. Slab pull and far-field drivers such as ridge push were aiding in closing the distance between the island arc and the continent. Depending on the angle of subduction deformation of the continental crust was already taking place and thickening the lithosphere. By 1.19 Ga the Elzevir back arc basin was closing.
From 1.18 to 1.14 Ga extension was occurring in the area. Whether due to lithospheric cooling also known as thermal subsidence or the compressional activity in the area reactivated some extensional faults the extension is marked by the isotopic ages of the previously mentioned rocks. Additionally there is the formation of sedimentary basins which means the margin was quiescent enough that sediments could accumulate. However in some areas from 1.16 to 1.13 Ga, coeval with extension, there is evidence there was still thrusting and emplacement of terranes occurring.
According to one model westward thrusting occurred from 1.12 to 1.09 Ga and then extension was the primary tectonic activity until 1.05 Ga. It was at this point that the Central Granulite Terrane was exhumed and minor magmatism occurred.
The precise reason for change from compression to extension is unknown but may be the result of gravitational collapse, mantle delamination, the formation of a plume underneath a supercontinent, changes in far-field drivers on the distribution of stress, or any combination of reasons originating from the fact that our planet is dynamic. The cyclic compression and extension history of this area is similar to the Wilson Cycle. In this area of the world the Wilson Cycle would be creating the basin for the proto-Atlantic Ocean (Iapetus Ocean
Iapetus Ocean
The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale . The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia...
)
General Lithology
Today, the Grenville orogen is marked by northwest verging fold-and-thrust belts and high pressure metamorphic regimes, as well as distinctive AMCG suite magmatism. Metamorphism is commonly of amphiboliteAmphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
and granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
facies, that is, medium to high temperature and pressure alteration. Eclogitized
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
metagabbros (very high pressure ultramafic metamorphic rocks) are found in some localities, and likely represent areas of deepest burial and/or most intense collision. Throughout the orogen, these sequences of high pressure metamorphic rocks are cut by intrusive AMCG suite plutons, generally interpreted as syn- or post-tectonic. AMCG plutonism is generally associated with asthenospheric
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
upwelling under thinned lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
. This is derived from the theory that AMCG plutonism is driven by ponding of olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
tholeiite
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
basalt at the base of the continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
during tectonic extension. The lithosphere may be thinned either convectively or by delamination
Delamination (geology)
In geophysics, delamination refers to the loss and sinking of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.This can occur when the lower portion of the lithosphere becomes more dense than the surrounding mantle...
, in which the bottom portion of the lithosphere is stripped off. Both models have been proposed for the Grenville orogeny.
The Grenville Orogeny can be categorized into three sections based on structure, lithology, and thermochronology. The three sections, respectively called the Gneiss Belt, Metasedimentary Belt, and the Granulite terrane are all separated by shear zones.
The Gneiss Belt is made up of felsic gneisses and amphibolites that were metamorphosed in the upper amphibolite to granulite facies. Thrusting in this section was low angle, but would have the potential to increase and rotate as it continued and evolved. Shear in this area is referred to as ductile shear meaning the material was cooling and becoming solid, but still behaving viscously or plasticly.
The age of this belt is approximately 1.8 to 1.18 Ga. Regional metamorphism is believed to have deformed this area at approximately 1.4 Ga and metamorphic thrusting at approximately 1.16 to 1.12 Ga. The area called the Appalachians falls into this category.
The Metasedimentary Belt is predominantly sedimentary and volcanic rocks which have undergone greenschist to granulite facies metamorphism. Subdivisions of this belt include the Bancroft, Elzevir, Sharbot Lake, and Frontenac Domains and the Adirondack Lowlands. In this belt magmatism is known to have occurred between 1.42 to 1.04 Ga depending on location. As with the Gneiss Belt, metamorphism is believed to have occurred at approximately 1.16 Ga.
The Granulite Terrane is composed of meta-igneous gneisses including anorthosite massifs. Anorthosites form in plutons and are composed mostly of plagioclase. The rocks of the Grenville Province in Canada are included in this category. The oldest magmatism known in this area dates to 1.32 Ga approximately. Granulite facies metamorphism began around 1.15 Ga and continued for about 150 Ma after the onset, however the continuity of the metamorphism cannot be determined.
Regional Variations
It is important to separate local from large-scale tectonic history of the orogenic belt in order to understand the orogeny. For this purpose, the Grenville orogen is generally broken into four localities: the southern extent in Texas and Mexico, the Appalachians, the Adirondacks and the well-studied Grenville Province of Canada. A portion of the orogen can be found in Scotland, but due to Scotland’s proximity to the Grenville province prior to opening of the Iapetus Ocean (modern day Atlantic Ocean), the two share largely the same history.Texas and Mexico
Texas and Mexico represent the southern margin of Laurentia, and likely collided with a different continent than that involved in the eastern collision. The Zapotecan Orogeny of Mexico is coeval with the later stages of the Grenville orogeny, and they are generally considered to be one and the same. Mesoproterozoic igneous protoliths (metamorphosed to granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
facies during the orogeny) fall into two age groups in Mexico; ca. 1235-1115 Ma and ca. 1035-1010 Ma. Rocks of the former group bear geochemical signatures implying island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
and back-arc basin
Back-arc basin
Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...
provenance. The latter group represents AMCG magmatism. These AMCG rocks are somewhat anomalous throughout the Grenville orogen, there is no known orogenic event which immediately predates their emplacement. It is suggested that the regime of subduction under the Laurentian margin (currently in Texas, north of the accreted Mexican terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
) ended around 1230 Ma, and that subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
polarity reversed to bring the colliding continent north, since the Llano uplift, which records the history of the Grenville in Texas, bears no evidence of arc magmatism after this time.
Appalachians
The Appalachian mountainsAppalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
contain small, isolated exposures of the Grenville orogen. The largest of these, the Shenandoah and French Broad massif
Massif
In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...
s, comprise the Blue Ridge
Blue Ridge Mountains
The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. The mountain range is located in the eastern United States, starting at its southern-most...
province of Virginia. Blue Ridge rocks consist of various gneisses of upper amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
and granulite facies, intruded by charnockites and granitoid rocks. These igneous rocks were intruded in three intervals: ca. 1160-1140 Ma, ca. 1112 Ma, and ca. 1080-1050 Ma, and are massive to weakly foliated
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
in texture.
Adirondacks
This region consists of a massive dome of Proterozoic rock on the New York-Canada border. Both the Elzevirian (ca. 1250-1190 Ma) and Ottawan (ca. 1080-1020 Ma) orogenic pulses are recorded in the Adirondacks, producing high-grade metamorphic rock. A northwest-trending high-strain shear zoneShear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
separates the dome into the Highlands to the southeast and the Lowlands to the northwest. It is believed that the shear zone (the Carthage-Colton) was a transpressional boundary during the Ottawan, when the Highlands were thrust over the Lowlands.
Grenville Province
The Grenville province is named for the village of GrenvilleGrenville, Quebec
Grenville is a village and municipality in the Argenteuil Regional County Municipality, Quebec, Canada. It is located opposite Hawkesbury, Ontario, on the Ottawa River.-History:...
in Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, and constitutes the youngest portion of the Canadian Shield
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
. Since the area has not undergone any regional metamorphic overprinting since the orogeny, it is considered an ideal study area for Grenville and pre-Grenville age tectonics. Hence, most of what is known about the orogeny and its processes is derived from the Grenville Province.
Importance
• Aids in determining the formation of the continents.o Models of supercontinents, such as Rodinia, rely on information about what parts of which continents were connected and orogenies in many cases provide rocks like ophiolites or exotic terranes which can be examined and compared to other continents. Even similar ages of rocks can provide a starting point for such investigations. For example, when a connection can be made between two continents based on a type of rock present on opposing coast with similar ages and isotopic ratios aids in understanding continental movements and locations.
• Allows for the investigation of the behavior of the deep crust during orogenesis.
o Since geologists cannot directly observe the movement of the deep crust and mantle during the mountain building process, the deep mountain exposures present in the Grenville Orogeny area (formed due to long term erosion) allow for observation of the mountain belt assembly.
See also
- Laurentian MountainsLaurentian mountainsThe Laurentian Mountains are a mountain range in southern Quebec, Canada, north of the St. Lawrence River and Ottawa River, rising to a highest point of 1166 metres at Mont Raoul Blanchard, north east of Quebec City in the Reserve Faunique des Laurentides. The Gatineau, L'Assomption, Lièvre,...
in QuebecQuebecQuebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level.... - Adirondack MountainsAdirondack MountainsThe Adirondack Mountains are a mountain range located in the northeastern part of New York, that runs through Clinton, Essex, Franklin, Fulton, Hamilton, Herkimer, Lewis, Saint Lawrence, Saratoga, Warren, and Washington counties....
in New York State - Blue Ridge MountainsBlue Ridge MountainsThe Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. The mountain range is located in the eastern United States, starting at its southern-most...
in AppalachiaAppalachiaAppalachia is a term used to describe a cultural region in the eastern United States that stretches from the Southern Tier of New York state to northern Alabama, Mississippi, and Georgia. While the Appalachian Mountains stretch from Belle Isle in Canada to Cheaha Mountain in the U.S... - Llano UpliftLlano UpliftThe Llano Uplift is a roughly circular geologic dome of Precambrian rock, primarily granite, in Central Texas in the United States. It is located in the eastern region of the Edwards Plateau, west of the Texas Hill Country...
in Central Texas - Wilson Cycle