Geocronologia de rochas gnáissico-migmatíticas e sienograníticas do Núcleo Setuva (Pr): implicações tectônicas

Abstract

Geochronology of gneiss migmatitic and syenogranite rocks of the Setuva Nucleus (Pr): tectonic implications The Setuva Nucleus is located in the eastern part of Paraná, south of the Lancinha Shear Zone. It occurs as an elongated sygmoidal shape within Metassedimentary sequences of the Capirú Formation. It is represented by promomylonitic to mylonitic rocks, predominating gneissic-migmatitic lithotypes in the southern portion and syenogranitic to monzogranodioritic rocks in the central – northern portions. The U-Pb isotopic pattern in zircons characterizes the formation of gneissic-migmatitic rocks during the Archean (3.200-3.000Ma), and the proto – to mylonitic syenogranitic lithotypes (northern portions) during the Archean (~2.650Ma)) and Paleoproterozoic (~2.100Ma). In general, the analyzed zircons show overgrowths of doubtful Neoproterozoic ages, obtained in the lower intercepts of Concordia Diagram. On the other hand, the zircons of protomylonitic syenogranite rocks (Setuva’s central sector) don’t show any overgrowth features. U-Pb analytical data, when plotted in corcodia diagram, cluster close to the lower intercept, yielding 2.140±8Ma. Such value, related to the paleoproterozoic, is interpreted as the time of zircon crystallization and consequently formation of the syenogranitic rocks. Aditionally, K-Ar and 40Ar-39Ar analyses carried out in biotites of theses rocks indicated Neoproterozoic ages (590Ma). Such values indicate emplacement according to isotherms of temperatures lower than 250°C-300°C during the Neoproterozoic. The geochronological pattern joined with the structural pattern suggests that the emplacement, largely controlled by shearing, at upper crustal levels occurred during the Neoproterozoic. The initial, low-angle tectonics, associated with transcurrent shear systems and later folding, should be responsible for the allochthony of these terrains and consequent emplacement at upper crustal levels. Such isotopic pattern is very similar to the observed for the Atuba Complex gneissic migmatitic terrains located south of Setuva