Such episodes in Earth’s history typic-ally involve a series of geological environments that reflect changes in the tectonic setting as convergence proceeds.. The Grenvillian Orogeny is
Trang 1GRENVILLIAN OROGENY
R P Tollo, George Washington University, Washington,
DC, USA
ß 2005, ElsevierLtd All Rights Reserved.
Introduction
Orogenies are defined by extended periods of
moun-tain building, usually resulting from convergence of
tectonic plates Such episodes in Earth’s history
typic-ally involve a series of geological environments that
reflect changes in the tectonic setting as convergence
proceeds The Grenvillian Orogeny is named after the
village of Grenville in Que´bec, and the term is widely
used to refer to a range of Mesoproterozoic tectonic
events that occurred between 1.3 and 1.0 Ga,
resulting in development of a series of orogens that
may have stretched across the globe for nearly
10 000 km Within the Grenville Province of
south-eastern Canada, which is the most thoroughly studied
portion of this composite orogen, this period of
oro-genesis included (1) an early accretionary stage at
1.3–1.2 Ga, (2) an interval of widespread magmatism
at 1.18–1.08 Ga, (3) and a period of continent–
continent collision at 1.08–0.98 Ga that was rapidly
followed by uplift and exhumation of the orogenic
core Development of the widespread Grenville
oro-gen was the last major tectonic event to affect the
Precambrian core of Laurentia, and marked the final
stage in assembly of the Mesoproterozoic
supercon-tinent of Rodinia In North America, this widespread
tectonism is recorded by a broad swath of igneous
and metamorphic rocks extending 2000 km from the
Atlantic coast of southern Labrador to Lake Huron
in Canada and the Adirondacks in the United States
(Figure 1) The belt of affected rocks continues
south-westward for another 1500 km, mostly in
the subsurface, to the Mississippi embayment in the
United States, reappearing to the west in Texas and
Mexico Igneous and metamorphic rocks of similar
age and tectonic affinity also occur in a series of
internal and external massifs associated with the
Ap-palachian orogen in the United States (Figure 2)
Grenvillian rocks also constitute the Sveconorwegian
Province of southern Norway and Sweden and are
recognized as inliers within the Caledonides of
Northern Ireland, Scotland, and Norway Recent
palaeogeographic reconstructions suggest that
frag-ments of the dismembered orogen are also present in
Antarctica, South America, and Australia
Major geological events such as the Grenvillian
Orogeny that result in assembly of supercontinents
are relatively rare events in Earth’s history, and the causal mechanisms are not yet well understood Such events may be driven by global-scale geodynamic mechanisms, such as mantle downwelling, or may involve periodic random amalgamation of cratons resulting from subduction of the oceanic lithosphere Geological studies of orogens and of the orogenies that produce them are typically spurred by economic factors, because orogenic belts contain much of the world’s metallic mineral resources Because orogens and orogenies are manifestations of large-scale Earth processes, enhanced understanding of the geological factors involved in their genesis also provides import-ant evidence bearing on the mechanisms of plate tec-tonics and the physical evolution of the planet The Grenvillian Orogeny represents an episode of un-usually widespread tectonism that profoundly affected Earth’s palaeogeography; like other major orogenies, such as the Permo-Carboniferous Appa-lachian Orogeny, the Grenvillian Orogeny marked the end of a major era in the geological time-scale of Earth’s history
Definition of the Grenvillian Orogeny
Geologists working in south-eastern Ontario and western Que´bec recognized in the early part of the twentieth century that the Grenville Province was structurally distinct from the rest of the Canadian Shield Advances resulting from detailed field map-ping, structural studies, and application of isotopic dating techniques in the 1960s and 1970s led to a more comprehensive understanding of the internal geology of the province and the timing of the Gren-villian Orogeny During this time, researchers recog-nized that many areas of the orogen preserved tracts
of older recycled crust that had not been completely overprinted by the effects of Grenvillian orogenesis
By 1980, geologists had determined that deposition
of the Flinton Group, a Precambrian succession of metamorphosed clastic and carbonate rocks, was con-strained to the interval 1080–1050 Ma, and that this deposition occurred during a tectonic hiatus sep-arating two major episodes of orogenesis: (1) a pre-1080-Ma period of arc-related magmatism, uplift, and erosion and (2) a post-1050-Ma period of wide-spread regional metamorphism (see column A in
Figure 3) The earlier episode of orogenesis was referred to as the Elzevirian Orogeny; the later epi-sode is known as the Ottawan Orogeny Both periods
of orogenesis were considered part of the Grenville
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