T33F-2995
Geochemical and Sr–Nd–Pb Isotopic Insights of the Low-Ti basalts from Paraná–Etendeka Igneous Province, Southern Brazil: Constraints on Petrogenesis and the Role of Crustal Contamination

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Carla Joana Barreto, UFRGS Federal University of Rio Grande do Sul, Rio Grande, Brazil
Abstract:
The south hinge of the Torres Syncline in southernmost Brazil hosts a volcanic succession of pahoehoe and rubbly Gramado-type lavas belonging to the ~132 Ma Paraná–Etendeka Igneous Province. We employ local-scale stratigraphy in three distinct profiles (Santa Cruz do Sul-Herveiras, Morro da Cruz and Lajeado geologic sections) as guidelines for geochemical and Sr-Nd-Pb isotope studies in order to discuss the petrogenesis of lava flows in a single magma type and to quantitatively evaluate the role of crustal contamination and the potential contaminants involved. In all profiles, the lava flows exhibit compositions ranging from basalt to andesite with tholeiitic affinity. The compositional and isotopic variations are not systematic according to stratigraphy, implying that the magma chamber could have undergone periodic replenishments or distinct magma pulses through time or multiple plumbling systems may have existed. The andesites (SiO2 55–58 wt.%) with ponded pahoehoe morphology represent evolved melts at early stages of volcanism with strong susceptibility to crustal contamination as they established pathways to the surface. The olivine basalts (SiO2 47–50 wt.%) and basaltic andesites (SiO2 51–56 wt.%) showing compound morphology and simple pahoehoe morphology, respectively, could be explained by longer time residence of liquids in the crust with higher degrees of crustal assimilation than the ponded pahoehoe lavas. The basaltic andesites (SiO2 52–56 wt. %) with rubbly morphology are related to late differentiation process in shallow magma chambers. Crustal assimilation process accounts for the high initial 87Sr/86Sr ratios at 0.707798–0.715751, very low εNd between −8.36 and −5.41, high 206Pb/204Pb ratios at 18.424–18.865, with intermediate 207Pb/204Pb and 208Pb/204Pb ratios at 15.649–15.710 and 38.618–39.369, respectively. The isotopic variations require assimilation of both Paleoproterozoic and Neoproterozoic contaminants at variable degrees.