Thông tin tài liệu
GRENVILLIAN OROGENY 155 GRENVILLIAN OROGENY R P Tollo, George Washington University, Washington, DC, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Orogenies are defined by extended periods of mountain building, usually resulting from convergence of tectonic plates Such episodes in Earth’s history typically 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 southeastern Canada, which is the most thoroughly studied portion of this composite orogen, this period of orogenesis 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 orogen was the last major tectonic event to affect the Precambrian core of Laurentia, and marked the final stage in assembly of the Mesoproterozoic supercontinent 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 Appalachian 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 fragments 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 important evidence bearing on the mechanisms of plate tectonics and the physical evolution of the planet The Grenvillian Orogeny represents an episode of unusually widespread tectonism that profoundly affected Earth’s palaeogeography; like other major orogenies, such as the Permo-Carboniferous Appalachian 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 mapping, 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 Grenvillian Orogeny During this time, researchers recognized 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 constrained to the interval 1080–1050 Ma, and that this deposition occurred during a tectonic hiatus separating two major episodes of orogenesis: (1) a pre1080-Ma period of arc-related magmatism, uplift, and erosion and (2) a post-1050-Ma period of widespread regional metamorphism (see column A in Figure 3) The earlier episode of orogenesis was referred to as the Elzevirian Orogeny; the later episode is known as the Ottawan Orogeny Both periods of orogenesis were considered part of the Grenville
Ngày đăng: 26/10/2022, 10:59
Xem thêm:
