Tonian and Cryogenian ⁴⁰Ar/³⁹Ar hornblende and muscovite ages for the São Gabriel Terrane, Dom Feliciano Belt, southern Brazil

Abstract

Belt, Brazil, associated with previous structural geology and geochronology studies, allow for a better understanding of Neoproterozoic events in this region. The Sanga da Cachoeira Complex and metasedimentary rocks of Arroio Marmeleiro Complex (U-Pb provenance age > 1.5 Ga) show 40 Ar/ 39 Ar spectra with no successful plateau age, attributed to the alteration or excess 40 Ar. Tonian (ca. 729 ± 9 Ma) 40 Ar/ 39 Ar hornblende age of metadiorites of the Cambaí Complex (U-Pb protolith age ca. 750 Ma) is interpreted to register the age of an early structure in this unit, where hornblende is a mineral of the composite banding fabric formed by remnants of original bedding, locally preserved, and added by metamorphic recrystallization. A cooling rate of at least 5 C/Ma is suggested for the Cambaí Complex, from its crystallization to its metamorphic event. The Cryogenian (<690 ± 8 Ma) 40 Ar/ 39 Ar hornblende and muscovite ages belonging to the Imbicuí Complex (protolith age ca. 869 Ma) whose ages are related to the late structure formed by a short and intense activity of the Cerro Branco Shear Zone, which experienced a rapid cooling of 12 C/Ma. The ages from the rocks belonging to such a diverse tectonic setting, including continental and intraoceanic island arc, oceanic crust low-K amphibolites and meta-harzburgites, and metasediments, presenting similar nature and fabric orientation, indicate that major tectonic-magmatic-metamorphic activity was responsible for the accretion of these lithotectonic units that gave rise to the São Gabriel Terrane in southern Brazil.