Morphological and petrographic patterns of the pahoehoe and ´a´ā flows of the Serra Geral Formation in the Torres Syncline (Rio Grande do Sul state, Brazil)

Abstract

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 m 3 /s) forming compound pahoehoe flows near the dunes and ponded ones in interdune areas. The second episode is constituted by simple flows that rarely exceed 3 m in thickness, erupted with similar eruption rates. The third is defined by ´a´ā flows that are thought to be developed when effusion rates are higher (> 5-10 m 3 /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.

Stratigraphical framework of basaltic lavas in Torres Syncline main valley, southern Parana-Etendeka Volcanic Province

“…Previous studies in the Paran a CBP have described the simple flows from Unit II as 0 a 0 a lava flows (Hartmann et al, 2010;Lima et al, 2012;Waichel et al, 2012). However, we identified a fourpart structure characterized by an autobreccia top, a coherent vesicular upper crust, a dense core and a lower vesicular crust similar to the rubbly pahoehoe flows defined by Keszthelyi (2000.…”

“…Locally high-TiO 2 lavas (Ubirici type) occur in the south and are contemporaneous with the Gramado type (Peate et al, 1999). Waichel et al (2006) described pahoehoe flows in the central portion of Parana CBP and recent works Lima et al, 2012) have also identified pahoehoe and 0 a 0 a lava flows in the southern portions of the province in the Torres Syncline area.…”

Characterization of volcanic structures associated to the silicic magmatism of the Paraná-Etendeka Province, in the Aparados da Serra region, southern Brazil

“…The region known as "Aparados da Serra" in southern Brazil consists of the silicic volcanic rocks of the Serra Geral Group, which is linked to the PEMP. These silicic rocks are related to tabular deposits of great extension, whose origin is still a subject of discussion, and the debate regarding its genesis by lava flows (Bellieni et al 1988, Comin-Chiaramonti 1988, Henry & Wolff 1992, Umann et al 2001, Lima et al 2012b, Polo et al 2017a, b, Guimarães et al 2018, Lima 2018, Simões 2018 or pyroclastic flows (Petrini et al 1989, Roisenberg 1989, Whittingham 1989, Milner et al 1992, Luchetti et al 2018 remains. In the investigation of the Serra Geral volcanism, the emphasis is usually given to the lithochemical and isotopic aspects, usually organizing the data on a regional scale, where aspects related to local structures, flow regime and effusion are suppressed or overlooked (Waichel 2006).…”

“…Pumice, fiamme, crystalloclasts, and lithoclasts were not observed, and a vitrophyre under scanning electron microscopy indicated a compact holohyaline condition, with crystallites and microliths of feldspars randomly distributed, typical of silicic flows. Lima et al (2012bLima et al ( , 2018 identified feeder systems related to the silicic lavas and, according to these authors, the presence of a feeder root suggests that, in certain portions, the structures may assume dome shapes in response to the resistance to the flow. Polo (2014) concludes that the silicic volcanic sequences of the Soledade region are widely dominated by effusive deposits and that the extrusion occurred through fissures.…”

“…Recent studies, focusing on a more detailed work scale, have suggested new stratigraphic models for the SGG, based on the facies architecture. These models have been proposed for the Torres Syncline region (Lima et al 2012b, Waichel et al 2012 and are based on the morphology of basic and silicic flows, paleotopography and effusion rates, complementing similar studies based on the volcanism stratigraphy of the Etendeka Province (e.g. Mountney et al 1998, Jerram et al 1999, 2000 in the Huab Basin, Namibia.…”