Gabriel J. Uhlein

Neoproterozoic oceanic crust remnants in northeast Brazil

The Borborema Province of northeast Brazil occupies a strategic position in the central portion of West Gondwana, linking three of its major cratonic constituents: the Sao Francisco–Congo, Amazon, and West Africa cratons. The southern portion of the province, the Riacho do Pontal fold belt, comprises an association of exhalative rocks with metabasalts having transitional mid-oceanic-ridge basalt geochemistry (Monte Orebe Complex) that suggests the preservation of remnants of Neoproterozoic oceanic crust. This view is supported by geophysical data: the Riacho do Pontal fold belt corresponds to the inflexion of a paired positive-negative Bouguer anomaly similar to other Precambrian suture zones. Sm-Nd isotope data for the metabasalts yield a whole-rock isochron age of 819 ± 120 Ma with an initial e Nd (t) = +4.4, indicating derivation from a depleted mantle source. The onset of subduction within the Borborema Province (ca. 630 Ma) caused the inversion of basins and obduction of slices of oceanic crust. In this scenario, the Riacho do Pontal fold belt represents a complete late Neoproterozoic plate tectonics cycle involving the collision of the Sao Francisco craton (lower plate) with the Pernambuco-Alagoas block (upper plate). This interpretation challenges current views that the Borborema Province has acted as a coherent block since Paleoproterozoic time (part of the Atlantica supercontinent), suggesting instead a dynamic setting, where multiple plates interacted during the Proterozoic. The Monte Orebe ophiolite provides a link with other Cryogenian oceanic crust occurrences in central Brazil and West Africa, indicating the preservation of a transcontinental Neoproterozoic suture zone in the heart of West Gondwana.

Análise de fácies, estratigrafia de sequências e quimioestratigrafia da Formação Sete Lagoas (Grupo Bambuí), norte do Estado de Minas Gerais, Brasil: Evidência de um carbonato de capa depositado sobre o Alto de Januária.

Manuscript ID: 20160112. Received in: 09/14/2016. Approved in: 01/19/2017. ABSTRACT: Sedimentary rocks of the Sete Lagoas Formation, exposed in the left margin of the São Francisco river (Minas Gerais State, Brazil), were deposited on the Januária‐Itacarambi basement high. They show both lateral and vertical rock stack‐ ing along continuous outcrops, allowing us to carry out detailed facies analysis and sequence stratigraphy studies. Our studies also integrate data from geological mapping, macro and micro‐ scopic petrography and high‐resolution C and O isotope analy‐ sis. Eight facies and four facies associations make up a sequence composed by a transgressive tract in the base, and a high stand tract in the upper portion, separated by a maximum flooding surface. The high stand tract shows a progradation stacking from the basement high apex towards progressively deeper base‐ ment areas. This stratigraphic framework, associated with oth‐ ers stratigraphic and isotopic features, indicates that the now exposed Januária basement high also represents a paleo‐high during the sedimentation event. Aragonite pseudomorphs and dolomites coupled with δ13C values of ‐5‰ characterize the basal carbonate of the transgressive tract as a cap carbonate. Records of the Cloudina fossil, recognized in the high stand tract, indicate a Late Ediacaran age for the upper portion of the studied stratigraphic sequence.Rochas sedimentares da Formacao Sete Lagoas afloram na margem esquerda do Rio Sao Francisco, na regiao de Januaria-Itacarambi, Minas Gerais, em secoes relativamente continuas, tanto lateralmente como no empilhamento vertical. Boas exposicoes associadas a preservacao de estruturas/texturas primarias favorecem o estudo de facies, e de estratigrafia sequencial. Este estudo estratigrafico foi realizado com a integracao de dados de mapeamento geologico, petrografia macro e microscopica e de tres perfis estratigraficos chaves, onde foram realizados levantamentos quimioestratigraficos (C e O) de alta resolucao. Foram reconhecidas oito facies e quatro associacoes de facies distribuidas em uma sequencia composta por um trato de sistema transgressivo na base, e um trato de sistema de mar alto na porcao superior, separados por uma superficie de inundacao maxima. O padrao de empilhamento do trato de mar alto apresenta uma progradacao das facies, do alto do embasamento, em direcao a areas onde o topo do embasamento e progressivamente mais profundo. Este arcabouco estratigrafico, associado com outras feicoes estratigraficas e isotopicas, indica que o alto de Januaria atuou como um alto do embasamento durante o evento sedimentar da Formacao Sete Lagoas. Estudos quimioestratigraficos e petrograficos permitiram caracterizar o carbonato basal da Formacao Sete Lagoas como um carbonato de capa na base do trato transgressivo, onde ocorrem dolomitos e pseudomorfos de aragonita associados a valores de δ13C de ate -5‰. Registros do fossil Cloudina, recuperados no trato de mar alto, indicam uma idade associada ao Ediacarano Superior para a porcao superior da sequencia estratigrafica estudada.