DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Resultados da Pesquisa
Item Quantifying rates of dome-and-keel formation in the Barberton granitoid-greenstone belt, South Africa.(2010) Lana, Cristiano de Carvalho; Tohver, Eric; Cawood, Peter AnthonyThe Barberton granitoid-greenstone belt is a classic dome-and-keel province, characterized by kilometerscale gneiss domes and elongate keels of largely folded supracrustal rocks. Combined U–Pb SHRIMP data and structural mapping demonstrate that the geometry of the Barberton belt reflects events that occurred over ∼30 million year interval, from ca. 3230 and 3203 Ma. Early deformation with NW–SE shortening in the upper crust was accompanied by emplacement of tonalite-trondhjemite-granodiorite TTG magmas at 3234±12 and 3226±9 Ma. Much of the structural grain of the greenstone belt relates to a long episode of post-orogenic extension, with NE-directed extension in the lower crust leading to exhumation of high-grade gneisses in the southern Barberton terrane. Advective heat transfer during emplacement of kilometer-scale (TTG) plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence. The end of this tectonic cycle is punctuated by the emplacement of the undeformed, 3203±7Ma Dalmein pluton, which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high-grade gneisses.Item 3230-3200 Ma post-orogenic extension and mid-crustal magmatism along the southeastern margin of the Barberton Greenstone Belt, South Africa.(2011) Lana, Cristiano de Carvalho; Buick, Ian S.; Stevens, Gary; Rossouw, Riana; Wet, Willem deThe Barberton Granitoid-Greenstone Terrain (SouthAfrica) preserves a complex and protracted evolution involving several events of magmatism and terrain accretion along convergent tectonic boundaries. Recent studies propose that the main period of tectonic accretion and arc-related magmatism is linked to a system of divergent subduction zones above which voluminous TTG magmas were emplaced between ca. 3236 and 3227 Ma. Our structural and LA-ICP-MS U–Pb geochronology study along the southeasternmargin of the BarbertonGreenstoneBelt (BGB) ties the waning stages of this TTG magmatism to a short (ca. <30 Ma) period of mid-crustalextension, between 3228 and 3205 Ma. We document a major NE-trending detachment that juxtaposed upper sequences (Moodies Group clastic sediments) against mid-crustal 3418 ± 10 Ma amphibolite-facies rocks of the base of the greenstonebelt (Onverwacht Group rocks). Several granodiorite bodies - intruded along this detachment - contain well-preserved (syn- and post-magmatic) fabrics that are demonstrably related to extensional shearing and exhumation. Field observations and U–Pb zircon data from these granitoids are consistent with the deformation taking place at 3228 ± 10 Ma – contemporaneous with the voluminous (3236–3227 Ma) TTG magmatism in the northwestern margin of the BGB. The timing of the granodiorite emplacement also constrains a minimum age for the deposition of the Moodies Group clastic sediments, which for much of the southern and southeastern parts of the BGB must have happened before ca. 3228 Ma. 3205 ± 9 Ma subvolcanic dykes intruded into the granodiorite complex indicate that the period of exhumation and cooling of the crystalline rocks along the extensional detachment was relatively short (<30 Ma), between 3228 and 3205 Ma. Our observations combined with previously published structural data from the northwestern and southern margin of the belt suggest that the main mechanism of large-scale infolding of the supracrustal strata was shortly followed by the extension-related magmatism and subsequent, solid-state diapiric movement of the arc-related plutons.