The Lavras do Sul Shoshonitic Association: implications for the origin and evolution of Neoproterozoic shoshonitic magmatism in southernmost Brazil

The Neoproterozoic Lavras do Sul Shoshonitic Association in southernmost Brazil is related to the last stages of Brasiliano Orogeny and comprises a wide compositional range of intrusive and extrusive basic to acid rocks. It includes potassic trachybasalts, shoshonites, dacites, rhyolites, quartz diorites and monzodiorites, quartz monzonites, granodiorites, monzogranites and syenogranites. Leucodioritic cumulates and spessartitic lamprophyres have also been identified. The whole range of plutonic and extrusive rocks are probably derived of parental shoshonitic basaltic magmas dominantly through crystal fractionation processes, as suggested by mass balance calculations for major elements and by coefficient partition studies for trace elements. Mineral fractionation was dominated by olivine, clinopyroxene and plagioclase from basic to intermediate liquids. Amphibole, biotite and Ti-magnetite, along with plagioclase, were important during the evolution of intermediate to acid liquids. Petrographic features and field relations support the same conclusion. Low initial 87SrSr86 ratios (about 0.704), slight negative ϵNd values (about −4) and a single U-Pb age population determined in zircons of shoshonitic granodiorites, suggest a negligible crustal involvement in the differentiation of this magmatism. Normalized spiderdiagrams and trace element ratios are similar to those observed in post-collisional shoshonitic associations. Geochemical data, including isotopes, are consistent with the evolution of a parental magma generated through melting of an CO2-enriched mantle source, previously metasomatised by a Brasiliano subduction.