U-Pb and LA-ICP-MS precision geochronology on zircon: a contribution to the knowledge of the tectonic evolution of South America

Abstract

U-Pb SHRIMP and LA-ICP-MS precision age determinations on zircons: A contribution to the knowledge of the tectonic evolution of the South American continental crust. Most geochronological data available for South America were obtained by means of conventional radiometric methods, such as K-Ar, Rb-Sr and Sm-Nd. These methods, although important, in many cases lack the necessary precision to characterize the geological history of many of the existing tectonic units. Therefore, this research project aims to provide age data with better precision and interpretative value, especially for these areas in which the understanding of the regional tectonic evolution is harmed by the shortage in geochronological information. This is the case of the Amazonian region, where the situation must be improved, although still at the reconnaissance scale. On the other hand, for the eastern Brazilian region, there are key areas of fundamental importance in terms of tectonic evolution which would greatly benefit from new and robust geochronological data. In this research project, we opted to acquire U-Pb zircon ages, considered the most precise and adequate for the refinement of the geological history of the terrains in question. The fundamental method to be employed is the dating of isolated zircon crystals by means of SHRIMP or LA-ICP-MS analyses. Zircon, ZrSiO4, seems to be capable in most cases of maintaining permanently its chemical constitution after the formation of the crystalline structure. The rock samples to be analysed will be carefully chosen in order to cover the areas of major deficiency of geochronological data for the South American Platform. The ages to be obtained in this research will surely represent a relevant contribution to the knowledge of the regional tectonic evolution and also for an important update of the Tectonic Map of South America. The selection of the areas for the geochronological work was done taking into account the geological conditions and the already existing coverage of each area considered of importance to be targeted. The following regions will be initially considered: (1), the Maroni-Itacaiunas and Ventuari-Tapajós tectonic provinces of the Guyana Shield of the Amazonian Craton; (2), The Rio Negro-Juruena e Rondoniana tectonic province of the Guaporé Shield of the Amazonian Craton: (3) - The anorogenic magmatism of the Amazonian Craton; (4), The Araguaia and Paraguay mobile belts in the Tocantins Province; and (5), Some selected specific areas of the Brazilian eastern region. The following steps will be used in each of the selected regions:1 - Compilation and interpretation of the previous work made for the tectonic units under study. 2 - Acquisition of adequate samples from already existing collections or through donations from interested researchers. Field trips for sample collection could be planned, if desirable. 3 - Complete petrographic study and acquisition of important geological data for each sample to be analysed. 4 - Sample preparation for the geochronological analyses, including zircon separation. 5 - Acquisition of cathodo-luminescent images for the characterization of the internal structure of the zircon grains to be analysed. 6 - U-Pb SHRIMP and;or LA-ICP-MS analytical work on the selected crystals after the CL study. 7 - Making of the regional interpretation, taking into account the previous work, together with the new petrographic, geochemical and geochronological data. 8 - Preparation of reports, regional scientific articles and a general synthesis reporting the obtained achievements for the knowledge of the tectonic evolution of South America. (AU)