Abstract:After the late Cryogenian glaciation the central region of Brazil was the site of extensive deposition of platformal carbonates of the Araras Group. This group includes a basal cap carbonate sequence succeeded by transgressive, deep platform deposits of bituminous lime mudstone and shale. Facies and stratigraphic data combined with carbon and oxygen isotopic analyses of the most complete section of the transgressive deposits, exposed in the Guia syncline, were used to evaluate the depositional paleoenvironment… Show more
“…The first sector was developed during the Brasiliano Cycle [8,9], and the stratigraphy mainly consists of metaepiclastites and low-grade metasedimentary units of the Cuiabá Group (850-630 Ma). The lower unit contains metalimestones of the Araras Group (approximately 580 Ma); the upper unit includes low-grade metasedimentary formations of the Alto Paraguay Group (590-545 Ma) [10][11][12].…”
Abstract:The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125 • lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90-80 Ma we question the subsistence of a stationary mantle plume for so long time.
Keywords
“…The first sector was developed during the Brasiliano Cycle [8,9], and the stratigraphy mainly consists of metaepiclastites and low-grade metasedimentary units of the Cuiabá Group (850-630 Ma). The lower unit contains metalimestones of the Araras Group (approximately 580 Ma); the upper unit includes low-grade metasedimentary formations of the Alto Paraguay Group (590-545 Ma) [10][11][12].…”
Abstract:The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125 • lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90-80 Ma we question the subsistence of a stationary mantle plume for so long time.
Keywords
“…The cap dolostone of the Mirassol d'Oeste Formation is succeeded by transgressive, deep-platformal deposits of dark gray laminated lime-mudstone and shale of the Guia Formation, reaching up to 250 m thick in the middle shelf domain Riccomini et al 2007;Alvarenga et al 2004Alvarenga et al , 2008Alvarenga et al , 2011. These trangressive deposits are exposed along more than 200 km from West to East (Alvarenga et al 2008.…”
This report reviews and incorporates new elemental and isotope chemostratigraphic data for correlation of Neoproterozoic carbonate-dominated successions in South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay). These thick mixed carbonate/siliciclastic successions were largely deposited in epicontinental basins or accumulated on passive margins on the edges of cratons (e.g. São Francisco, Amazonia, Rio Apa Block, Pampia and Río de la Plata paleocontinents) during extensional events related to the rifting of the Rodinia Supercontinent. From the stratigraphic point of view, these successions occur as three mega-sequences: glaciogenic, marine carbonate platform (above glaciomarine diamictites or rift successions), and dominantly continental to transitional siliciclastics. In the orogenic belts surrounding cratons, carbonate-dominated successions with important volcanoclastic/siliciclastic contribution have been, in most cases, strongly deformed. The precise ages of these successions remain a matter of debate, but recently new paleontological and geochronological data have considerably constrained depositional intervals. Here, we report high-resolution C, O, Sr, and S isotope trends measured in well-preserved sample sets and mainly use Sr and C isotopes
RESUMO: Esta revisão incorpora novos dados isotópicos e quími-cos para correlação quimioestratigráfica de sucessões
“…O hábito gregário deste organismo , junto com sua freqüente associação em grandes acumulações com estromatólitos e trombólitos, ou mesmo preservado em posição vertical ou sub-horizontal dentro de trombólitos, apoiam esta concepção, que também é reforçada pela presença de outros metazoários de morfologia compatível a um estilo de vida séssil, como Namacalathus e Namapoikia, no mesmo contexto deposicional Horfmann & Mountjoy 2001;Wood et al 2002;Wood 2011). Estas observações sugerem uma comunidade composta por metazoários filtradores ou microcarnívoros, oportunistas, cujo hábito gregário favoreceria estes organismos na competição por espaço, e também na estabilização do substrato (Jackson 1983;Wood 2011 (Babinski et al 2006Nogueira et al 2007;Riccomini et al 2007), sendo que a ocorrência de metazoários é limitada ao último grupo Boggiani et al 2010). (Almeida 1965;Del'Arco et al 1981;Boggiani 1998;Gaucher et al 2003;Oliveira 2010 Esta formação é a mais fossilífera do Grupo Corumbá, e reveste-se de grande importância por conter os mais antigos metazoários no registro paleontológico brasileiro: Cloudina lucianoi e Corumbella werneri Walde et al 1982;Zaine e Fairchild 1985;Gaucher et al 2003).…”
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