Geologic history
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The geologic history ofSouth America can be summarized in three different developmental stages, each corresponding to a major division ofgeologic time. The first stageencompassedPrecambrian time (about 4.6 billion to 541 million years ago) and was characterized by a complex series of amalgamations and dispersals of stable blocks of protocontinental crust calledcratons. The second stage coincides with thePaleozoic Era (about 541 to 252 million years ago), during which time the cratons and material accreted to them contributed to the formation first of thesupercontinentGondwana (or Gondwanaland) and then of the even largerPangea (Pangaea). The third stage, in which the present continental structure emerged, occurred in theMesozoic andCenozoic eras (about the past 252 million years) and includes the breakup of Pangea and Gondwana, the opening of the SouthAtlantic Ocean, and the generation of theAndean cordillera.
The present tectonic framework of South America consists of three fundamental units: the ancient cratons, the relatively recent Andean ranges, and a number ofbasins. Five cratons—Amazonia, São Francisco, Luis Alves, AltoParaguay, andRío de la Plata—represent the Precambrian core of South America, and (with the exception of theAlto Paraguay craton) those now appear as upwarped massifs arrayed from north to south in the immense eastern portion of the continent; a number of other Precambrian crustal blocks also were accreted along the margins of South America over geologic time. The lofty ranges and intermontane plateaus of the Andes rise along the entire western margin of thecontinent and represent the collision in the Cenozoic Era (about the past 66 million years) of the Pacific and South American plates brought about by the opening of the South Atlantic. Finally, vast, downwarped, sediment-filled basins are found between the cratons and along the entire eastern margin of the Andes.
ThePrecambrian
Precambrian rocksconstitute the oldest rocks of the continent and are preserved in the five core cratons. Those rocks are represented by high- to low-grade metamorphosed assemblages along heavily deformed belts of plutonic (intrusive), metavolcanic (metamorphosed extrusive igneous rocks), and metasedimentary rocks. Rocks ofArchean age (2.5 to 4 billion years old) are known in theAmazonia,Luis Alves, andSão Francisco cratons, although precisely dated rock samples are scarce. Ages older than 3 billion years have been reported in the Imataca Complex ofVenezuela and in the Xingu area ofBrazil, both in the Amazonia craton. The oldest rocks found so far—with ages of some 3.4 billion years—are in the São Francisco craton in the Brazilian state ofBahia. In the other cratons (e.g., theRío de la Plata craton inUruguay) the dating of Archean rocks has been inconclusive.Greenstone belts, which are remnants of Archeanoceanic crust emplaced in the suture zones (convergent plate boundaries), contain most of South America’s known largegold deposits, such as those located nearBelo Horizonte, Brazil. Two major cycles of crustal deformation occurred in the Precambrian, widely separated in time from each other. The first, called the Trans-Amazonian cycle, took place approximately 2.2 to 1.8 billion years ago; and the second, the Brazilian cycle, between about 900 and 570 million years ago.
TheTrans-Amazonian cycle
Trans-Amazonian rocks can be subdivided into three distinct groups:orogenic belts, such as the Maroni-Itacaiúnas belt of the Amazonia craton or the Salvador-Juazeiro belt of the São Francisco; stable cover rocks, such as the Chapada Diamantina formation in Bahia or the Carajás and Roraima platform deposits; and large extensionaldike swarms (groups of tabular intrusions ofigneous rock into sedimentary strata). The orogenic belts represent old mountain chains that had been formed either along the margins of the continent as geosynclines (downwarps ofEarth’s crust) and thenuplifted, such as the Maroni-Itacaiúnas belt, or were the result of collisions between continental blocks, such as the Tandil belt inBuenos Aires,Argentina.
Such collisions are believed to have formed a supercontinent (sometimes called the firstPangea) some 1.8 billion years ago. The sedimentary cover of that supercontinent (preserved on the Amazonia craton), consisting of postcollision rhyolites and clastic shelf deposits, was deep and widespread and obliterated earlier suture boundaries. Extensive stratified iron and manganese deposits are found in those sequences, such as near Carajás, Brazil. Early phases of continental-plate dispersal produced extensive dike swarms ofmafic rock, including a zone some 60 miles (100 km) wide in west-centralUruguay where hundreds of gabbro dikes are now emplaced along a 150-mile (240-km) stretch.
TheBrazilian cycle
Rocks of the Brazilian cycle today aremanifested in a series of orogenic belts—developed mainly on previously deformed continental crust—that were formed during the amalgamation of the Precambrian cratons into the first supercontinent in late Proterozoic time (about 1 billion to 541 million years ago). Most of present-day South America,encompassing the platforms of Brazil,Guyana, and southern Venezuela, was accreted at that time—together with Africa—to form the western part of the huge southern supercontinent ofGondwana; Precambrian blocks that were not part of Gondwana—notably theSanta Marta Massif inColombia, theArequipa block inPeru, andPatagonia in Argentina—were accreted later during Paleozoic times.
TheBrasilides in the southern Brazilian state ofMato Grosso represent the type locality of the Brazilian orogenic cycle. There, important sequences of green schists, platform limestones, and quartzites, as well as red bed molasse formations (associated with granitoids), permit a reconstruction of thecollision between the Amazonia craton’s passive (i.e., without active volcanoes) margin and the Alto Paraguay craton’s active margin (now partially covered by theParaná River basin). The interpreted suture zone between the two cratons corresponds to the Paraguay-Araguaia line, along which mafic and ultramafic rocks are found today.
Several other Brazilian belts are known, such as the structurally complex Borborema belt and theDom Feliciano belt in southern Brazil and Uruguay, which resulted from the collision between the Río de la Plata craton and the Kalahari craton of present-dayAfrica. The Dom Feliciano belt represents a complex suture zone where rocks typical of a late Proterozoic arc system were trapped between the two cratons; those rocks were then covered byplateaus of rhyolites during the earlyCambrian Period (about 541 million years ago). A striking coincidence exists between that suture, which is known as theBrazilian–Pan-African suture, and the inception of the future rift system that opened the Atlantic Ocean. The Pampean Sierras in Argentina are a good example of a Brazilian belt formed by accretion of an island-arc system and several small continental plates.