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PROVENIÊNCIA E AMBIENTAÇÃO TECTÔNICA DE METASSEDIMENTOS DE ALTO GRAU: NAPPE TRÊS PONTAS-VARGINHA, SUL DO CRÁTON SÃO FRANCISCO

2004; Sociedade Brasileira de Geologia; Volume: 34; Issue: 1 Linguagem: Inglês

10.25249/0375-7536.20043414958

2177-4382

Maria da Glória Motta Garcia, Mário da Costa Campos Neto, Valdecir de Assis Janasi,

Geological formations and processes

Samples of granulite- to amphibolite-facies metasedimentary rocks of the Três Pontas-Varginha Nappe, south of São Francisco Craton, were analised. The results indicated that the kyanite-bearing gneisses from the base of the nappe, Três 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 /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 kyanitc/sillimanite-bearing gnesseis from the Varginha area are more homogeneous and show high SiO 2 /AlO 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 (5 1-65) 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.