The thrust contact between the Canastra and Vazante groups in the Southern Brasília Belt: structural evolution, white mica crystallinity and implications for the Brasiliano orogeny

O contato do empurrão dos grupos Canastra e Vazante na Faixa Brasília Meridional: evolução estrutural, cristalinidade de mica branca e implicações para a Orogenia Brasiliana

ABSTRACT:

Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D₁ and D₂) are ductile, while the third and fourth ones (D₃ and D₄) are brittle-ductile. D₁ developed a slaty S₁ cleavage subparallel to the primary layering, with shallow to steep dips to NW. D₂ developed a crenulation cleavage (S₂) that dips moderately to NW and is associated with tight to isoclinal folds. D₃ and D₄ phases developed crenulations and open folds and kink bands. S₃ dips steeply to NW, while S₄ has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.

KEYWORDS:
Neoproterozoic; Zinc-Lead; X-Ray Diffraction; Thrust-fold Belt