Archean and Proterozoic Basins

We are currently using isotopes to investigate a number of Archaean and Proterozoic basins in South America. Amongst the most important isotope methods used are: U-Pb detrital zircons, Boron in detrital tourmalines, and high-precision U-Pb geochronology of volcanic rocks

The main basis studied are the Maquiné Basin in the Rio das Velhas Greenstone Belt, the Minas Basin of the Quadrilatero Ferrifero, and the Espinhaço Basin all in southeast Brazil.

The Minas Basin, Southeast Brazil 

The Minas Basin of the Quadrilatero Ferrifero Province is a world class Precambrian  Basin that was developed around the margins of the Southern São Francisco Craton (SSFC) at around 2500 Ma.  The Basin hosts a world-class iron deposit (Banded Iron Formations) that was precipitated at ca 2400 Ma.

The BIF is the main ore exploited in the region, but there are several unexplored mineral resources such as U-Th and Gold that await further studies. Also, several questions surrounding 1 – the tectonics of opening and basin closure, and 2 the various sources of  the sediments remain to be investigated.

The Minas sequence is represented by alluvial sandstones, conglomerates and shallow-water pelites (Caraça Group), banded iron formations and carbonates (Itabira Group), deltaic, shallow water strata (Piracicaba Group) and a syn-orogenic (flysch) succession of turbidites, tuffs, volcaniclastics and conglomerates (Sabará Group). The Caraça and Piracicaba Groups contain zircons derived predominantly from the Archaean granitoids, with the grains ranging in age from 3250 to 2580 Ma (Caraça Group), and from 3353 to 2775 Ma (Piracicaba Group). Lead isotopic studies indicate that the subsequent chemical sedimentation of the carbonates and iron formations of the Itabira Group occurred at ca. 2420±20 Ma, whereas the accumulation of the shallow water deltaic sequence of the Piracicaba Group must have taken place during a period of slow subsidence of the entire basin after 2420 Ma. In contrast, the overlying Sabará Group is considerably younger, containing detrital zircons of ca. 2120-2130.  The deposition of this unit is linked to a collisional event along the eastern margin of the craton during the 2100-2000 Ma Trans-Amazonian Orogeny.

Fig. 2.Stratigraphic column of the QuadrilateroFerrifero showing a compilation of U-Pb age data for individual supracrustal units.

The goal of this study is to understand the source of the sediments that make up most of the Minas Basin from the Caraça/Tamandua to Itacolomi Groups. Several students from UFOP and the University of Portsmouth are working on a sampling program to develop a high profile Age and Trace Element bar code of the various stratigraphic units of the Minas Basin. The idea is to correlate the various segments of the basin and their respective sources.