Morfologia e padrões petrográficos dos derrames pahoehoe e'a'ā da Formação Serra Geral na Sinclinal de Torres (Rio Grande do Sul)

The Serra Geral Formation volcanic sequence in Torres Syncline (Rio Grande do Sul state, Brazil) may be divided into main valley, intermediate zone, and southern hinge. The first volcanic episode along the syncline is characterized by low eruption rates ( ; 5 - 10 m³/s). The formation of 'a'ā flows occurred on a plain relief. Pahoehoe and 'a'ā flows have similar chemistry, and the difference in the flow type is related to an increase in eruption rates. The 'a'ā lavas are plagioclase and clinopyroxene phyric, or glomeroporphyritic. All present an intergranular or intersertal microcrystalline groundmass of plagioclase, clinopyroxene, and opaque minerals (<< 0.1 mm in diameter). The pahoehoe lavas differ from the 'a'ā ones in that they have a coarser-grained microcrystalline groundmass. The pahoehoe flows are microcrystalline with glomeroporphyritic and diktytaxitic textures and a plagioclase-poor matrix when compared to 'a'ā lava flows. The higher content of microlites in the 'a'ā flows is attributed to undercooling, higher rate of eruption, and degassing before and during emplacement. When it comes to regional stratigraphic correlation between the Serra Geral Formation flows, the morphological type of the lavas should be taken into account since 'a'ā flows, unlike pahoehoe, do not reach longer distances from the source.