Magnetic anisotropy of an ancient volcanic system: Flow dynamics of post-collisional Ediacaran volcanism in southernmost Brazil

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.