Tectonic and prospect implications of cryptic magmatic structures in the subtrappean crust of the South American Paraná Magmatic Province (PMP), inferred from natural Electromagnetic Induction ground systems

Abstract:

The PMP in SE South America and its extension in Etendeka, SW Africa, present a variety of magmatic events, magma-types and intrusion/extrusion structures, foremost by voluminous tholeiitic flood basalts that lie directly over and within sedimentary layers and conspicuous alkaline-carbonatitic complexes, exposed mostly abreast the Paraná Basin borders. Because of the signal blanketing and distorting effect of the former, geophysical surveys within the limits of the basalt cover are impeded to properly illuminate unperceived magmatic structures in the crust and possible mineral prospect effects on the overlying sedimentary units. Thus, a regional blunt image of the electrical conductivity distribution of the crust beneath the basin and adjacent terrains was estimated from transfer functions data obtained by a large but sparse array of fluxgate magnetometers. Inverting data from close-spaced magnetotelluric (MT) soundings along transects crossing several conductivity anomalies resulted in independent 2D and 3D resistivity models that delineated long tubulated and ellipsoidal conductors that have been interpreted, respectively, as related to traces and residues of mafic magma pathways or of alkaline-carbonatite chambers in the crust. As an example, the inference of a highly conductive oval body at about 20 km depth is interpreted as the cause of the ring-shaped disturbance of the sedimentary layers above. The emplacement either uplifted the terrain during ascent or allowed a collapse after cooling, by which hydrothermal fluids upwelling along fractures probably resulted in widespread upper crust veining and mineralization. In the lithosphere, some of the mapped crustal anomalies appear to prolong down into high conductivity zones in the upper mantle in a probable connection to the magma sources. Noting that such sources are consistent with the presence of heterogeneous mantle enrichment zones by metasomatic processes, given by geochemical and isotopic data from such deep rocks, the conductivity anomalies constrained by long period MT data is better explained by the re-fertilization on the lithosphere during the Neoproterozoic compression event and decompression melting during plate tectonic reorganization during the Cretaceous, that contradicts the presence of a typical craton under the basin.