Figure 1.
Location of the study region in the Araçuaí orogen shown within a paleotectonic fit including the African counterpart and a simplified geological map of the main occurrence region of the Salinas Formation (modified from Pedrosa-Soares & Oliveira 1997Pedrosa-Soares A.C., Oliveira M.J.R. 1997. Geologia da Folha Salinas. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S. (eds.), Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 419-542.; Pedrosa-Soares 1997aPedrosa-Soares A.C. 1997a. Geologia da Folha Araçuaí. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S. (eds.), Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 715-852., bPedrosa-Soares A.C. 1997b. Geologia da Folha Jenipapo. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S. (eds.). Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 1053-1198.; Oliveira et al. 1997Oliveira M.J.R., Mourão M.A., Noce C.M, Grossi-Sad J.H. 1997. Geologia da Folha Grão Mogol. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S. (eds.). Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 620-677.; Pedrosa-Soares & Grossi-Sad 1997Pedrosa-Soares A.C., Grossi-Sad J.H. 1997. Geologia da Folha Minas Novas. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S. (eds.). Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 952-1052.; Baars et al. 1997Baars F.J., Grossi-Sad J.H., Fonseca E. 1997. Geologia da Folha Capelinha. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S (eds.), Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG, p. 1373-1503.; Guimarães & Grossi-Sad 1997Guimarães M.L.V., Grossi-Sad J.H. 1997. Geologia da Folha de Malacacheta. In: Grossi-Sad J.H., Lobato L.M., Pedrosa-Soares A.C., Soares-Filho B.S (eds.). Projeto Espinhaço em CD-ROM. Belo Horizonte, COMIG , p. 1505-1574.).
Figure 2.
The Salinas Formation in outcrops of the type area (UTM: 801065 - 8222433; BR-251 highway roadcuts, 24 km to the east from the junction with MG-404 road), showing rocks without orogenic deformation but metamorphosed in the biotite zone of the greenschist facies: A, succession of banded to laminated wacke (BLW), convolute wacke (CW) and brecciated wacke (BrW), between massive wacke (MW) layers; B, water-escape structure marked by biotite-rich (i.e., mud-rich) darker upright flames in massive wacke; C, detail of brecciated wacke; D, detail of convolute wacke; E, cataclastic laminated wacke filling space in a syn-sedimentary extensional (growth) fault cutting massive wacke; F, syn-sedimentary (growth) micro-faults and graben-horst micro-pattern in laminated to banded wacke with coarsening-up graded layering done by the gradual decreasing of biotite content from the base (darker) to top (lighter); G, syn-sedimentary fault with drag microfolds in laminated wacke; H, water-escape structures along syn-sedimentary faults, cutting across coarsening-up graded, banded to laminated wackes; I, flame structures outlined by biotite-rich (i.e., mud-rich) bands, covered by massive wacke.
Figure 3.
Clast-supported metaconglomerate (meta-orthoconglomerate) lenses of the Salinas Formation: A, outcrop in a quarry for dimension stone, showing the contact of a conglomerate lens and massive wacke (UTM: 789473 - 8220347; road MG-404, 12 km to the north of the road junction with the BR-251 highway); B, detail of the same metaconglomerate, showing stretched and rotate clasts (v, volcanic rocks); C, porphyritic volcanic rocks (v) of intermediate to felsic composition; D, pebble of porphyritic dacite; E, lens of clast-supported metaconglomerate intercalated with quartz-mica schist in the Minas Novas corridor (left bank of the Araçuaí River in front of the mouth of the Fanado River).
Figure 4.
Outcrops of deformed and metamorphosed rocks of the Salinas Formation in the Minas Novas corridor: A, despite the rock composition, outcrops generally are dark gray to black owing to coats rich in manganese oxide (the so-called desert varnish); photo A also shows quartz veins, locally rich in gold, host by quartz-mica schist and metawacke (see Pedrosa-Soares & Leonardos, 1996Pedrosa-Soares A.C., Leonardos O.H. 1996. O Distrito Aurífero das Minas Novas do Araçuaí: Uma abordagem fundamentada em dados históricos e geológicos. Revista Escola de Minas (REM), 49(4):33-38.); B, quartz-mica schist hosting pegmatite veins (close to Coronel Murta); C, folded banded quartz-mica schist with concordant quartz veins (close to Virgem da Lapa); D, muscovite schist with biotite and garnet porphyroblasts (close to Minas Novas); E, banded to laminated metawacke; F, carbonate schist (dark grey upper part), quartzose metawacke (light gray middle part) and laminated metawacke (lower part). The outcrops A, E and F are located in the Funil area, Araçuaí River banks, south of Virgem da Lapa.
Figure 5.
Al2O3-(CaO*+Na2O)-K2O (in molar proportion) diagram for the Salinas samples, together with the chemical index of alteration (CIA) scale. The black arrow shows the compositional trend of the studied samples. The light grey dotted arrows indicate weathering trends of some common igneous rocks.
Figure 6.
Chemical compositions of rocks of the Salinas Formation plotted in the SiO2/Al2O3 - K2O/Na2O - MgO/CaO diagram of Pedrosa-Soares (1995Pedrosa-Soares A.C. 1995. Potencial Aurífero do Vale do Araçuaí, MG: História da exploração, geologia e controle-tectono metamórfico. PhD Thesis, Instituto de Geociências, Universidade de Brasília, Brasília, 177 p.). The dashed black arrow follows the curved trend shown by the sample set.
Figure 7.
Chemical classification diagrams for rocks from the Salinas Formation: (a) Log (Na
2O/K
2O) versus log (SiO
2/Al
2O
3) of
Pettijohn et al. (1972Pettijohn F.J., Potter P.E., Siever R. 1972. Sand and Sandstone. New York, Springer-Verlag, 618 p.); (b) Log (FeOt/K2O) versus log (SiO
2/Al
2O
3) of
Herron (1988Herron M.M. 1988. Geochemical classification of terrigenous sands and shales from core or log data. Journal of Sedimentary Petrology, 58(5):820-829. https://doi.org/10.1306/212F8E77-2B24-11D7-8648000102C1865D
https://doi.org/10.1306/212F8E77-2B24-11...
).
Figure 8.
Bivariant diagrams of SiO2
versus (a) TiO2, (b) Al2O3, (c) MgO, (d) CaO, (e)Na2O, (f) K2O, (g) P2O5, (h) FeOt; and (i) MnO. Upper continental crust (UCC) values are from Taylor & McLennan (1985Taylor S.T., McLennan S.M. 1985. The Continental Crust: Its Composition and Evolution. Oxford, Blackwell, 312 p.).
Figure 9.
Chondrite-normalized REE patterns for the Salinas metawackes. Chondrite values from Boynton (1984Boynton W.V. 1984. Cosmochemistry of the rare earth elements; meteorite studies. In: Henderson P. (ed.). Rare earth element geochemistry. Amsterdam, Elsevier Science Publishing Company, p. 63-114.), UCC values from Taylor & McLennan (1985Taylor S.T., McLennan S.M. 1985. The Continental Crust: Its Composition and Evolution. Oxford, Blackwell, 312 p.).
Figure 10.
Major-elements diagrams for provenance discrimination of protoliths of Salinas rocks: (a) Discriminant-function multi-dimensional diagram for high-silica sediments, DF1 versus DF2 of
Verma & Armstrong-Altrin (2013Verma S.P., Armstrong-Altrin J.S. 2013. New multi-dimensional diagrams for tectonic discrimination of siliciclastic sediments and their application to Precabrian basins. Chemical Geology, 355:117-133. https://doi.org/10.1016/j.chemgeo.2013.07.014
https://doi.org/10.1016/j.chemgeo.2013.0...
); (b) Discriminant-function multi-dimensional diagram for low-silica sediments, DF1 versus DF2 of
Verma & Armstrong-Altrin (2013Verma S.P., Armstrong-Altrin J.S. 2013. New multi-dimensional diagrams for tectonic discrimination of siliciclastic sediments and their application to Precabrian basins. Chemical Geology, 355:117-133. https://doi.org/10.1016/j.chemgeo.2013.07.014
https://doi.org/10.1016/j.chemgeo.2013.0...
); (c) Ternary diagram K
2O+Na
2O - SiO
2/20 - TiO
2+FeO
t+MgO of
Kroonenberg (1994Kroonenberg S.B. 1994. Effects of provenance, sorting and weathering on the geochemistry of fluvial sands from different tectonic and climatic environments. In: International Geological Congress, 29., Kyoto. Proceedings..., p. 69-81.); (d) Al
2O
3/(CaO+Na
2O) versus FeO
t+MgO of
Bhatia (1985Bhatia M.R. 1985. Rare element geochemistry of Australian Paleozoic graywackess and mudrocks: Provenance and tectonic control. Sedimentary Geology, 45(1-2):97-113. https://doi.org/10.1016/0037-0738(85)90025-9
https://doi.org/10.1016/0037-0738(85)900...
); (e) Al
2O
3/SiO
2 versus FeOt+MgO of
Bhatia (1985Bhatia M.R. 1985. Rare element geochemistry of Australian Paleozoic graywackess and mudrocks: Provenance and tectonic control. Sedimentary Geology, 45(1-2):97-113. https://doi.org/10.1016/0037-0738(85)90025-9
https://doi.org/10.1016/0037-0738(85)900...
); (f) TiO
2 versus FeOt+MgO of
Bhatia (1985Bhatia M.R. 1985. Rare element geochemistry of Australian Paleozoic graywackess and mudrocks: Provenance and tectonic control. Sedimentary Geology, 45(1-2):97-113. https://doi.org/10.1016/0037-0738(85)90025-9
https://doi.org/10.1016/0037-0738(85)900...
); (g) Discriminant function diagram from
Bhatia (1985Bhatia M.R. 1985. Rare element geochemistry of Australian Paleozoic graywackess and mudrocks: Provenance and tectonic control. Sedimentary Geology, 45(1-2):97-113. https://doi.org/10.1016/0037-0738(85)90025-9
https://doi.org/10.1016/0037-0738(85)900...
), Discriminant Function 2 versus Discriminant Function 1; (h) K
2O/Na
2O of
Roser & Korsch (1986Roser B.P., Korsch R.J. 1986. Determination of tectonic setting of sandstone-mudstone suites using SiO2 and K2O/Na2O ratio. Journal of Geology , 94(5):635-650.). Fields: Col- collisional; A, oceanic island arc; B, continental magmatic arc; C, active continental margin; D, passive continental margin.
Figure 11.
Trace element diagrams for provenance discrimination of protoliths of Salinas rocks:
Bhatia & Crook (1986Bhatia M.R., Crook K.A.W. 1986. Trace elements characteristics of graywackess and tectonic setting discriminations of sedimentary basins. Contributions to Mineralogical and Petrology, 92(2):181-193. https://doi.org/10.1007/BF00375292
https://doi.org/10.1007/BF00375292...
): (a) Th-La-Sc, Zr/10-Th-Co and Sc-Th-Zr/10 ternary diagrams; (b) Ti/Zr versus La/Sc. Fields: A, oceanic island arc; B, continental magmatic arc; C, active continental margin; D, passive continental margin.
Figure 12.
Cathodoluminescence (CL) images showing U-Pb spots in representative zircon grains of sample OPU-1995.
Figure 13.
Corcordia diagram showing ages for the pebble-cobble and matrix of sample OPU-1995. Insets showing the Concordia age of 620 ± 10 Ma, yielded by eleven spots.
Figure 14.
Concordia diagram for the Salinas type area metawacke (sample SP-54; data from Lima et al. 2002Lima S.A.A., Martins-Neto M., Pedrosa-Soares A.C., Cordani U.G., Nutman A. 2002. A Formação Salinas na área tipo, NE de Minas Gerais: uma proposta de revisão da estratigrafia da Faixa Araçuaí com base em evidências sedimentares, metamórficas, e idades U-Pb SHRIMP. Revista Brasileira de Geociências, 32(4):491-500.).