An AMS study of different silicic units from the southern Paraná-Etendeka Magmatic Province in Brazil: Implications for the identification of flow directions and local sources

Abstract

In the Southern portion of the Paraná-Etendeka Magmatic Province in Brazil, extensive silicic (dacite-rhyolite) deposits occur at the top of a sequence of low-Ti pahoehoe to rubbly basalts. The internal architecture of the silicic deposits and their eruptive style, as well as the location of their sources are still unsatisfactorily known. In an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive, and test the hypothesis of local sources, we carried out anisotropy of magnetic susceptibility (AMS) studies on the two main silicic units (Caxias do Sul dacites and Santa Maria Rhyolites) with the best exposures in an area previously mapped in detail.Magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites. Rock magnetism properties indicate that “pseudo-single-domain” magnetite carries the fabrics and the remanence. Magnetic fabrics were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Both AMS and AARM tensors are coaxial, indicating that the AMS fabric is not affected by the effect of magnetite single-domain grains.Magnetic data from several dacitic coulées (Caxias do Sul unit) indicate flows from SE to NW. The location and spatial distribution of these lavas support the hypothesis of local sources, aligned along a NE-SW trend. These data are in agreement with the alignments of structures (dome-shaped hills) observed in field work and DEM images. On the other hand, magnetic data obtained in Santa Maria rhyolites indicate that flow directions in two different areas are distinct (towards NW/NE and W), suggesting that they derived from different emission centers.So, regarding the silicic volcanism in the studied region, our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit. In contrast, we propose the occurrence of local volcanic events, implying in the existence of different sources, possibly characterized by local emission centers.