Lucy Gomes Sant'Anna

Cenozoic tectonics of the Fonseca Basin Region, eastern Quadrilátero Ferrífero, MG, Brazil

Abstract Cenozoic tectonics of the region of the Fonseca Basin, eastern Quadrilatero Ferrifero, Minas Gerais State, were investigated through integrated lithostructural and morphotectonic analysis of the area of the Cenozoic Fonseca and Chapada de Canga Formations and their Precambrian basement rocks. The Eocene Fonseca Formation underwent brittle deformations, as evidenced by post-sedimentary, generally NE and NW normal faults. The directions of stress axes are concordant with the post-sedimentary structural style of the basin in response to a regional regime of extensional tectonics. Evidence of synsedimentary tectonics in the Fonseca sediments could not be found. The probably Quaternary Chapada de Canga Formation, a succession of continental itabiritic ironstone pebble conglomerates, directly overlies both the Fonseca Formation and the Precambrian basement in angular unconformity. The Chapada de Canga deposits are cut by NNE and WNW clastic dykes and fractures (brittle faults and joints) indicating, respectively, syn- and post-sedimentary tectonics. The tectonic events also deformed old planation surfaces (SulAmericana and Velhas), whose remnants are recognized in the region of the Fonseca Basin. Integrated analysis of Cenozoic structural features and major Precambrian tectonic discontinuities of regional extent reveal that all the Cenozoic tectonic events consisted mainly of the reactivation of pre-existing structures in the Precambrian basement, thus characterizing resurgent tectonics, and that the youngest, probably neotectonic processes strongly influenced the development of the present day landscape, including the morphology and the drainage network.

Ocorrência de fosfato de terras raras na Formação Resende, Paleógeno do Rift Continental do Sudeste do Brasil

Petrographic studies and SEM/EDS and XRD analyses revealed the presence of a phosphate with Ca and REE (La, Nd) in a mudstone deposit of the Resende Formation, located on the northern border of the Resende Basin. The morphological characteristics (prismatic hexagonal habit) and chemical composition of the phosphate allow including this mineral in the rhabdophane group. This phosphate is authigenic and its origin is related to the dissolution of detritic apatite grains, probably by alkaline hydrothermal solutions. These solutions also provided some of the elements involved in the rhabdophane crystalization.