Absolute palaeointensity determinations of recent lavas from Mt. Etna have been carried out using the Koenigsberger‐Thellier‐Thellier (KTT) method but with two different data presentations. The first, which is the standard method, compares the natural remanent magnetization (NRM) demagnetized and the thermoremanent magnetization (TRM) acquired in successive temperature intervals cumulatively. The second compares the NRM and TRM in the successive intervals in an incremental manner. The latter method appears to distinguish aberrant behaviour better than the classical method. Such aberrant behaviour can be related to changes in hysteresis ratio parameters, saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force, which are reflected by displacements in the Day et al. (1977) plot. These changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling. The changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations.
SUMMARY
Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolution. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti-rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.
The Paraná-Etendeka Magmatic Province is associated with the distensive tectonics that caused the rupture of the Gondwana continent during the Lower Cretaceous and generated an intense volcanism that covers South America and the NW portion of Namibia in Africa. In Brazil, this volcanic sequence is named Serra Geral Group and predominantly consists of basalts and subordinated silicic rocks. The goal of this study is to characterize the geomorphological features observed in the Aparados da Serra region, southern Brazil, and to evaluate the relationship between these structures and the primary silicic volcanic structures. The geomorphological features were fi rst identifi ed using remote sensing and then correlated with fl ow structures observed in the fi eld, as well as petrographic and geochemical data. AMS data were used to determine magnetic patterns and the direction of magmatic fl ow of the rocks. Despite the low degree of anisotropy, clear patterns of lineation and foliation were identifi ed in the studied rocks. Our data shows that Units I and II correspond to silicic lava fl ows linked to effusive fi ssure eruptions, presenting a dome morphology caused by differential erosion. Unit III rocks may correspond to true volcanic domes, whereas the Unit IV corresponds to the effusive feeder structures.
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