Planet Earth experienced tremendous geodynamic and environmental changes during the Archaean, from 3.85 to 2.50 Ga. The continents grew from more primitive crust, the atmosphere and oceans evolved to oxic conditions, and the first evidences of biological activities emerged in sedimentary rocks. Metals such as gold, iron, and nickel were deposited in quantities greater than ever observed during Earth’s evolution. To establish when and how these global geodynamic and environmental changes have taken place, whether simultaneous, or first in some locals and later in others are some of the major challenges for the geoscientists and the conceptual basis for the environmental changes that mankind faces today. The geological records of these complex systems of continents, oceans, atmosphere and biosphere are found in the cratons, and are represented by granite-gneiss terranes, granite-greenstone associations, sedimentary basins, mafic-ultramafic complexes, and mafic dyke swarms. Brazil has several cratonic domains (Fig. 1) but interdisciplinary studies on these areas are meagre. This thematic project aims at contributing to the understanding of Archaean terrains by collecting high quality geological data on the above referred to rock assemblages from the Brazilian territory and comparing the results with similar terrains worldwide. The São Francisco Craton and the Borborema Province (Fig. 1) are the selected areas for the proposed studies because these areas contain rocks with ages between 3.5 and 2.5 Ga. The project participants include Brazilian and international researchers of different disciplines, as well as undergraduate and graduate students. The methods and techniques to be employed include among others fieldwork, cutting edge U-Pb (SHRIMP, LA-ICP-MS) and Sm-Nd geochronology, major and trace element geochemistry, radiogenic and stable isotope geochemistry, Raman spectroscopy, and palaeomagnetism. All these research facilities are available in the project’s team Brazilian universities and in others abroad on a collaborative basis.
In the end the project will provide enough data to help understand important issues, such as (i) the nature and composition of the first continental nuclei, (ii) the relative contribution of oceanic basalts (ocean floor, plateau, ocean island), arc basalts and komatiites to the Archaean crust, and (iii) how these data fit into models of bulk Earth cooling, cyclicity, and when life and plate tectonics began on planet Earth.
Map of selected areas of this project