Magnetic fabrics, rock magnetism, cathodoluminescence and petrography of apparently undeformed Bambui carbonates from São Francisco Basin (Minas Gerais State, SE Brazil): An integrated study

Abstract

Magnetic measurements were performed on apparently undeformed limestones and carbonate shales from 44 sites in nearly horizontal stratigraphic layers mainly from the basal units of the Neoproterozoic Bambui Group in the southern part of the São Francisco Basin. Rock magnetism, cathodoluminescence, transmitted and reflected light microscopy analyses reveal that there is a mix of ferromagnetic minerals, mainly magnetite and pyrrhotite, in most sites. In some sites, however, the ferromagnetic minerals are magnetite and hematite. Fine-grained pyrrhotite and pyrite accompany rare fine-grained graphite and probably amorphous carbon in some of stylolites, while pyrrhotite is also present as larger interstitial masses in coarse-grained domains outside, but close to the stylolites. Magnetic fabrics were determined applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AAR). The AAR tensor was less well defined than the AMS fabric due to the low ferromagnetic mineral content. The analysis at the individual-site scale defines three AMS fabric types. The first type (two sites) shows K min perpendicular to the bedding plane, while K max and K int are scattered within bedding plane itself. This fabric is usually interpreted as primary (sedimentary-compactional), typical of totally undeformed sediments. The second type shows the three well-clustered AMS axes with K min still perpendicular to the bedding plane. This fabric is the most important since it was found in the majority of the sites. The third type (two sites) is characterized by well-clustered K max in the bedding plane, while K min and K int are distributed along a girdle. The second and third fabric types are interpreted as combinations of sedimentary-compactional and tectonic contributions at the earliest, and at a slightly later stage of deformation, respectively. AMS represents the contribution of all the rock-forming minerals, while AAR isolates the contribution of remanence-bearing minerals from the matrix minerals. However, rock magnetism shown that anhysteretic remanence only reaches grains with coercivity < 100 mT because the maximum AF in the majority of the available instruments is 100 mT. Therefore, hematite and pyrrhotite probably do not contribute to AAR, which is due to the shape-preferred orientation of magnetite grains. For some sites, the AMS and AAR fabric orientations are different, mainly with respect to the lineation orientations ( K max and A max, respectively). In general, K max is well developed and follows the trend of the main regional thrusts, fold axes and faults generated in the first deformational phase, while A max follows both this trend and that of structural lineaments formed during the second deformational phase. These deformation phases arose from the compression, which occurred during the evolution of the Brasília fold belt during the last stages of the Brasiliano event. The magnetic fabrics of the apparently undeformed Bambui limestones are typical of very weakly deformed sediments, in which the depositional-compaction fabric has been partly overprinted by a tectonic one, with minimum susceptibility direction remaining perpendicular to bedding. This result is in agreement with the textures given by the petrographic observations.