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Guimaraes, L. F.; De Campos, C. P.; Janasi, V. A.; Lima, E. F.; Dingwell, D. B. (2018): Flow and fragmentation patterns in the silicic feeder system and related deposits in the Parana-Etendeka Magmatic Province, Sao Marcos, South Brazil. In: Journal of Volcanology and Geothermal Research, Vol. 358: pp. 149-164
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Abstract

Voluminous silicic volcanic products from the Paran-Etendeka Magmatic Province are exposed in the southern Brazil. A regional synformal structure (Torres Syncline) controls a tick sequence of these units in the region of Sao Marcos (Rio Grande do Sul). Parallel to the axis of Torres Syncline, a NW-SE-trending zone constituted by the alignment of complex structures was interpreted as the feeding system of silicic volcanism, characterized as fissural. Through a combination of field mapping and determination of fractal dimension (FD), we have identified and described distinct intermingled morphological domains, one related to flow processes and one related to fragmentation processes. Vertical flow directions in this zone exhibit two preferential directions: NW, ranging from N272 degrees to N 355 degrees and NE, varying from N20 degrees to N85 degrees. The systematic repetition and crosscutting relations of domains point towards a cydicity of flow and fragmentation processes in this zone. This feeder system intrudes previous basaltic and silicic volcanic rocks, the later being composed by banded deposits, interpreted as rheoignimbrites, and proximal volcanic breccias. This set is overlapped by hybrid deposits, lava flows and, finally, massive deposits. The silicic volcanism can be summarized at an early pyrodastic stage, characterized by rheoignimbrites and proximal volcanic breccias. A transition between explosive and effusive events is suggested by the occurrence of hybrid deposits, which gradually give place to lobated lavas. Local alternation with new (possibly) rheoignimbritic deposits suggests oscillatory fragmentation processes at the conduit, related to decompression and volatiles ex-solution in an effective permeable system.