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 Newfoundland to Mexico, as well as 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.^[1] These events are known as the Kibaran orogeny in Africa, the Dalslandian orogeny in western Europe, and the Uralian orogeny in Siberia.

Extent (orange regions) of the Grenville orogeny, after Tollo et al (2004) and Darabi (2004)

[edit] Timescale

The problem of timing of the Grenville Orogeny is an area of some contention today. The timescale outlined in Toby Rivers’ recent work^[2] 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.^[1] Regional Variations discusses local deviations from Rivers’ timeline, presented here.

Timeline of the Grenville orogeny, after Rivers (2002)

[edit] 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 convergent 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.^[1] 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.^[3][1] 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.^[1] 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.^[4][5] 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.^[6]

[edit] 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 amphibolite 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.^[7] 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.^[8][1] 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.^[9][1] 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.^[9]

[edit] 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 [N. Atlantic Ocean], the two share largely the same history.^[1][10]

[edit] 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.^[3] 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.^[11] 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.^[11] 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.^[5]

[edit] Appalachians

The Appalachian mountains contain small, isolated exposures of the Grenville orogen. The largest of these, the Shenandoah and French Broad massifs, 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 3 intervals- ca. 1160-1140 Ma, ca. 1112 Ma, and ca. 1080-1050 Ma, and are massive to weakly foliated in texture.^[1]

[edit] 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 zone separates the dome into the Highlands to the southeast and the Lowlands to the northwest. It is believed^[12][13] that the shear zone (the Carthage- Colton) was a transpressional boundary during the Ottawan, when the Highlands were thrust over the Lowlands.^[1]

[edit] Grenville Province

The Grenville province is named for the village of Grenville 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.^[1]

[edit] See also

• Laurentian Mountains in Quebec
• Adirondack Mountains in New York State
• Blue Ridge Mountains in Appalachia
• Llano Uplift in Central Texas

[edit] External links

• The Grenville

[edit] References