DIRECT AND INDIRECT EVIDENCES OF PALEOPROTEROZOIC MAGMATIC ARC IN THE JANUÁRIA HIGH - NORTH OF MINAS GERAIS

Abstract

The studied rocks belong to the São Francisco Craton basement, nearly of Bonito de Minas (MG). They are exposed along dry creeks of the Januária High and are part of the homonymous complex. Supported on U-Th-Pb ages of monazite crystals from a migmatitic gneiss as indirect guides of the local granitogenesis, integrated to the direct preliminary petrography and geochemistry data of these rocks, which constitute the Januária Complex, it is here proposed that they are the representatives of a Paleoproterozoic orogenic cycle. Five crystals (A, B, C, D e E) of monazite-Ce have been dated. The monazite A presents average age of 2329 ± 55 Ma. This age data could be correlated to formation and intrusion of pre-collisional juvenile type – I biotite granodiorite and biotite granites, calcic to calcic-alkaline, most of the high potassium, formed in a continental magmatic arc environment, contemporaneous to the beginning of the metamorphism and anatexis process of the pre-existing TTG rocks of Januária high. The monazite B has an average age of 2120 ± 56 Ma, interpreted as being formed during the metamorphic peak and correlated to the sin-collisional phase of the Rhyacian period. The monazites C and D had mean ages of 1923 ± 28 and 1903 ± 29, respectively, indicating an orogenic collapse. With the collapse, it would have occurred the biotite pink-granite intrusion. Despite the similarities with type-A granitoids, it was interpreted as post-collisional granite, formed in a high oxygen fugacity environment. The monazite crystal E outlines an average age of 1817 ± 37. Possibly, this age reports the anorogenic extensional phase related to the formation of the Espinhaço rift, recorded by mafic dykes of the Januária swarm. According to these results, it is suggest that the Januária Complex would be a portion of the Rhyacian-Orosirian orogen in the central region of the São Francisco Craton at Minas Gerais State.

Keywords: Januária Complex; petrography; geochemistry; monazite U-Th-Pb dating; magmatic arc