Magnetic fabric and rock-magnetic character of the Mesozoic flood basalts of the Paraná Basin, Brazil

Abstract

We have studied the magnetic fabric of Mesozoic basaltic flows from five sequences of the Paraná Magmatic Province (PMP), southern Brazil, to infer paleoflow direction and to locate possible magma feeders. A well defined orientation pattern, indicative of the flow direction has been evidenced by the low field anisotropy of magnetic susceptibility (AMS). One sequence from the southern part of the basin, JS (27 flows, 513 specimens), shows maximum AMS ellipsoid trending approximately NW–SE. Two sequences from the west–central part of the basin, IC (13 flows, 173 specimens), and PA (17 flows, 324 specimens), trend E–W. Two sequences from the south-eastern part of the basin, CV (24 flows, 436 specimens) and BV (20 flows, 103 specimens) show maximum AMS ellipsoid trends approximately NE–SW. In all cases the minimum axes of the AMS ellipsoids are tightly grouped vertically or sub-vertically to the bedding with a relatively weak degree of anisotropy, indicative of the primary origin of the magnetic fabric. Rock-magnetic parameters of some representative samples, such as isothermal remanent magnetization (IRM), high field hysteresis loops and thermomagnetic curves suggests that the dominant magnetic mineral is a pseudo-single to small multi-domain grain size of magnetite. These and other observations are consistent with the conclusions that flows emanating from different sources may align their maximum susceptibility directions parallel to drainage that channel the flow or they will reflect regional preflow topographic structure and magma-source distributions.