Neoproterozoic oceanic crust remnants in northeast Brazil

作者：Caxito Fabricio^*; Uhlein Alexandre; Stevenson Ross; Uhlein Gabriel J

来源：Geology, 2014, 42(5): 387-390.

DOI：10.1130/G35479.1

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 epsilon(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.

• 出版日期2014-5