Lithostructural control and geophysical of Planalto da Serra Alkaline Complex, Mato Grosso, Brazil

Abstract

The Planalto da Serra Alkaline Complex is inserted in the metasediments of the Grupo Cuiabá and is composed by alkaline ultramafic rocks, such as peridotites, pyroxenites, carbonatites, apatitites and bebedourites. The first studies on the Planalto da Serra Alkaline Complex indicated that they were cretaceous rocks correlated to the alkaline magmatism occurring in Azimuth 125º lineament. However, recent geochronological studies, by the Rb/Sr and Sm/Nd methods, concluded that the age of the complex is about 600 My, reassembling, thus, an intrusion model of dikes and stocks, different from that already described. In order to understand the relation between the alkaline rocks and the rocks of Cuiabá Group, both in temporal relationship and in the question of intrusion model, two working methods were used: the geophysics, using gamma spectrometry and magnetometry, and the geological mapping, with lithostructural on the occurrences of alkaline intrusions (Chibata and Lau-Massao). The high contrast between magnetic characteristics of Cuiabá Group metasediments and the alkaline rocks allowed us to identify and delimit seven magnetic anomalies related to alkaline intrusions. Low potassium concentrations in gamma spectrometry data allowed mapping the alkaline bodies. Features such as penetrative deformation and mineral reorientation present in the edges of the alkaline dykes, corroborates with an intrusion model syn to tardi-tectonic. The relationship between age, structural and metamorphic characteristics indicates that the tectonic records verified in the alkaline rocks are correlated with those observed in the Cuiabá Group’s metamorphic rocks, Paraguay Belt.