Angela Beatriz de Menezes Leal

Evolução crustal do setor ocidental do Bloco Arqueano Gavião, Cráton do São Francisco, com base em evidências U-Pb, Sm-Nd e Rb-Sr

The western part of the Gaviao Block (Sao Francisco Craton) is one of the oldest fragments of the South American Platform. It comprises the Riacho de Santana Gneissic Migmatitic complex, the Santa Isabel complex and the Riacho de Santana greenstone belt. T DM ages of country rocks of these units reveal that the main accretion periods were: 3.9 Ga, 3.2 - 3.0 Ga and 2.6 Ga. The Gneissic Migmatitic complex yields U-Pb age of 3648 ± 69 Ma, while the negative eNdt value is compatible with crustal reworking. The Rb-Sr isochron provided age of 3247 ± 120 Ma, tentatively interpreted as migmatization event. U-Pb data indicate that crystallization of a rock from the Santa Isabel Complex occurred at 2954 ± 100 Ma ago, with migmatization at 2748 ± 100 Ma ago (Rb-Sr isochron). This complex displays T DM ages of 3.3 - 3.1 Ga and variable eNdt parameters (-4.7 to +0.3), which suggest juvenile and reworking processes from Mesoarchean protholiths with short crustal residence time. Relative the metamafic (tholeiitic) rock from the Riacho de Santana greenstone belt, at least one of the samples indicates anomalous fractionation, yields a 2.6 Ga T DM age whereas the U-Pb crystallization age is 2218 ± 18 Ma. This suggests crustal contamination of the particular mantle source. Shortly after, the country rocks were intruded by the (U/Pb = 2054 ± 8 Ma) Guanambi batholith which was mainly derived from Archean protholiths (T DM ages of 2.8 - 2.7 Ga). Eventually, tectonic and thermal overprints took place in the study area during Meso- and Neoproterozoic periods, as indicated by K-Ar apparent ages and lower intercept age of a U-Pb discordia line from rocks of the Riacho de Santana Gneissic Migmatitic Complex. We believe that these younger episodes reflect the onset of the Northern Espinhaco intracratonic system and the Neoproterozoic dynamics, respectively, as similar as recorded over the Paramirim Corridor.