Proveniência e ambientação tectônica de metassedimentos de alto grau: nappe três pontas-varginha, sul do Cráton São Francisco

Abstract

Samples of granulite- to amphibolite-facies metasedimentary rocks of the Tres Pontas-Varginha Nappe, south of Sao Francisco Craton, were analised. The results indicated that the kyanite-bearing gneisses from the base of the nappe, Tres Pontas area, can be divided in two groups with different chemical characteristics. The first one presents high SiO 2 /Al 2 O 3 ratios (3.28-4.69), low K 2 O/Na 2 O (0.45-1.06) ratios, and low IAQ values (48-60). The second group exhibits high and varied SiO 2 /Al 2 O 3 ratios (2.55-4.71) and high K 2 O/Na 2 O ratios (2.30-11.71), probably constituting products of relatively pure pelitic sediments. The ASI values are high (67-79) and suggestive of a long weathering period marked by intense chemical weathering. Passive margin settings or, alternatively, old crustal sources are the most probable origin for these sediments. The upper unit of kyanite/sillimanite-bearing gneisses from the Varginha area are more homogeneous and show high SiO 2 /Al 2 O 3 ratios (3.03-4.59) and relatively low K 2 O/Na 2 O ratios (0.23-2.48), which suggest an origin from feldspar-rich psamitic sediments, probably wackes. The low ASI values (51-65) indicate that rocks from the source area were lesser affected by chemical weathering, suggesting a general prevalence of physical weathering or other short-termed weathering processes. These characteristics are compatible with post- Archean active margin settings.