New field evidence for silicic ignimbrites and proximal lavas and their distribution in the Parana Basin, Brazil
Abstract:Silicic ignimbrites and lavas are sometimes very difficult to distinguish from one another in the field. Their accurate identification provides the basis for a better understanding of the origin of these deposits, including processes related to magma extraction, fragmentation and eruption style. We are studying the silicic volcanics in the Paraná Rift Basin in southern Brazil (the ~130 to 135 Ma Palmas member of the Serra Geral Formation), the counterpart to the Etendeka silicic volcanics in Africa. We present new field evidence for the identification of tabular-shaped ignimbrite-like packages, which contrast with dome-shaped proximal viscous lavas and domes.
The tabular-shaped packages are highly welded and devitrified, but primary feldspar and pyroxene crystals can still be identified in outcrops. Flow/cooling unit boundaries can be seen at outcrop scale as well as at the kilometer scale where successive flat-topped terraces have been cut into the volcanic landscape. These packages bear conspicuous swarms of vesicle-poor black lens-shaped features set in a light colored matrix. The lenses range in their aspect ratio (2D height vs. length) and can be > 2 m long. Horizontal jointing is superimposed on the black lens-bearing outcrops and their spacing appears to coincide with the aspect ratio of the lenses, i.e. thinner lenses have a narrower joint spacing. Locally wider lenses grade down into completely stretched and/or flattened lenses that resemble conventional flow banding. We interpret these tabular packages as ignimbrites and the vesicle-poor black lenses as juvenile magma blobs (distinct from typical pumice or fiamme).
A notably different flow-banded, irregularly deformed, and sometimes obsidian-bearing lithology appears in a number of locations. It supports topography characterized by rolling hills that contrasts markedly with the tilted plateaus supported by ignimbrites. The difference in morphology between ignimbrites and lavas is most readily observed in the north-south trending southern Brazil escarpment, where up to ~700 m of tabular-shaped packages are juxtaposed against domical features.
Future work will include detailing the genetic relationships between these two different lithologies, as well as the study of the magmatic conditions that led to their eruption.