GEOCHRONOLOGY AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM A LARGE RIFT SYSTEM IN SOUTHERN BRAZIL: PRELIMINARY DATA FROM THE CAMAQUã AND ITAJAí BASINS

The Camaquã Basin of Southern Brazil (southern Rio Grande do Sul state) comprises an exceptionally well-preserved suite of sedimentary, volcanic, and intrusive igneous rocks that formed during the late Proterozoic and early Phanerozoic. It and the more northern Itajaí Basin are two of more than a dozen Ediacaran- to Cambrian-age basins that extend ~1500 km throughout Uruguay and southeastern Brazil (Almeida et al., 2010). Previous geologic studies on this region have interpreted the basins and their tectonic histories individually; however, recent structural, volcanic, and sedimentary correlations suggest that the basins together may represent one extensive rift system (Almeida et al., 2010). Prior geochronologic studies of volcanics from these basins have relied on a variety of dating methods (e.g. Ar/Ar, U/Pb zircon TIMS and SHRIMP; see Janikian et al., 2012), which encumbers age-correlation within and across basins.

This study brings a consistent approach to dating volcanic rocks from the Paleoproterozoic basins and correlates zircon ages with zircon geochemistry, aimed to better constrain the evolution of this system of basins in Southern Brazil. We used LA-ICPMS U-Pb zircon geochronology, bulk rock and zircon geochemistry, and SEM-CL zircon imaging to investigate the timing and duration of silicic volcanism and its relationship to tectonism within the Camaquã and Itajaí basins using 7 (6 Camaquã and 1 Itajaí) bulk rock geochemical samples; zircon dating was performed using 4 of the Camaquã samples and 1 Itajaí sample. Camaquã rhyolites (lavas and tuffs) are highly evolved, with 72-76 wt% SiO₂, 5-60 ppm Sr, and distinctive negative Eu anomalies. The Itajaí sample has 69 wt% SiO₂, ~120 ppm Sr, and a less pronounced Eu anomaly. U-Pb ages for samples from the Camaquã Basin are concentrated in an interval ~550-560 Ma, though many zircons as young as 500 Ma are present. The Itajaí rhyolite is younger, with a weighted average U-Pb age of 508±11 Ma (2σ; n=52, MSWD = 2.1).