Where was Brazil in the Precambrian?

Geophysical characterization of the Azimuth 125° lineament with aeromagnetic data: contributions to the geology of central Brazil

Authors:

Rocha et al

Abstract:

This study analyzes a portion of the Azimuth 125° lineament (Az 125°), located in the Brazilian states of Goiás and Minas Gerais, based on its magnetic signature and previous studies. In particular, the determination of a chronology of events that shaped the Az 125° lineament is the focus of this contribution. Geological provinces with highly economically valuable mineralizations occur along Az 125, including the most important carbonatite and kimberlite complexes in Brazil. Az 125° is the main structural feature associated with these complexes, consisting of an extensive set of NW-SE oriented faults (approximately 850 km long and 70 km wide), however it is not mapped on geological maps at a regional scale as a continuous structural feature, because it is not consistently visible on the surface or in satellite images. Magnetic signature characterizes the lineament as a set of linear features with regional continuity in the subsurface. It is characterized by a higher magnetic susceptibility response that contrasts with the response exhibited by surrounding host rocks. Field data support geophysical data: occasional outcrops resulting from active erosion reveal dikes formed by gabbroic rocks and diabase and define the Az 125° lineament as a set of dikes of different ages. Other magnetic anomalies that occur along Az 125° are associated with intrusive rocks from the Goiás and Alto Paranaíba alkaline provinces. Based on magnetic signatures and mapped igneous rocks chronology, the Az 125° is partitioned, within the study area, into three main systems (called L1, L2, and L3). This subdivision, in conjunction with the dates assigned to various basic dikes within the azimuth, reveals that the evolution of the azimuth can be associated with three events. The first event occurred during the Brasiliano (950 Ma to 520 Ma). The second event occurred during the Gondwana fragmentation (starting circa 180 Ma). The third event is related to the tectonomagmatic activity of The Trindade plume (90 Ma to 80 Ma). We posit that the L1 system includes the older dikes, and the L3 system includes the youngest dikes; the L2 system which is intersected by L3, is therefore, of intermediate age. The determination of a chronology of events that characterizes the segmented structure of the Az 125° lineament, here defined as L-systems (L1, L2, and L3), based on both geological and geophysical data, and not yet reported in earlier scientific articles, is the main contribution of this study.