Canoas geophysical anomaly: A possible alkaline body or unusual anomaly caused by mafic dykes in the Ponta Grossa Arch, Brazil?

Abstract

A geophysical anomaly in the Canoas Seaside, in State of Paraná, Brazil, was observed in airborne magnetic data. This anomaly does not have outcropping evidences, nor have drill holes within its area. We used magnetic and gravity data to investigate and propose a geophysical model to explain this anomaly, constrained by geological framework of the Ponta Grossa Arch (PGA) and paleomagnetic data. The source of the anomaly indicated an approximately 5.5 km body subparallel to the preferential northwest-southeast direction of the PGA, with its shallower portion varying from a depth of 20 m, in its central portion, increasing to a depth of 460 m towards the borders. We modelled the whole set of data, achieving a body analogue with mapped alkaline complexes, and cut by highly magnetized subvertical dykes. The remanent magnetization of this body was indirectly estimated with inclination of 50°, declination 192°, and intensity 4.8 A/m, which allowed to calculate an apparent Virtual Geomagnetic Pole (aVGP) at 78°S, 251°E (α95% = 1°, N = 39). This aVGP is coherent with reference poles for South America at 135 Ma, indicating that this body was intruded slightly before to contemporaneous with the PGA mafic dyke swarm.