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Anais da Academia Brasileira de Ciências

Print version ISSN 0001-3765On-line version ISSN 1678-2690

An. Acad. Bras. Ciênc. vol.74 no.3 Rio de Janeiro Sept. 2002

https://doi.org/10.1590/S0001-37652002000300021 

THE PIUMHI SUPERGROUP, SW-SÃO FRANCISCO CRATON, MG-BRAZIL

ALEXANDRE P. CHIARINI 1, 2 AND HANS D. SCHORSCHER 1

1Instituto de Geociências, USP, São Paulo, SP.
2BTX Serviços Geológicos, São Paulo, SP.

Presented by ANTONIO C. ROCHA-CAMPOS

 

Field work, petrographical and geochemical studies of the metavolcano-sedimentary sequence (VSS) of Piumhi-MG, a part of an Archaean or Palaeoproterozoic greenstone belt showed that current stratigraphical and petrographical classifications of this sequence require redefinition (Chiarini, 2001). Here, the VSS is formally defined as the Piumhi Supergroup, subdivided into: Lower and Upper groups (grupos Inferior and Superior) based on the Córrego Araras type-section. From base to top the Lower Group comprises: Andesitic Volcanic Unit (AVU), Magnesian Basaltic Volcanic Unit (MBVU) and Basaltic Volcanic Unit (BVU). The Upper Group, undivided here, comprises clastic and volcaniclastic metasediments with intercalated basic to acid volcanics. The Piumhi Supergroup exhibits middle greenschist facies metamorphism.

In the AVU predominate transitional tholeiitic to calc-alkaline basaltic andesites, dacites and rhyolites of fractional crystallization, altered by pre-metamorphic spilitization, epidotization, keratophyrization and silicification. Basaltic andesites show actinolite (substituting clinopyroxene), chlorite, albite, epidote, quartz and opaques. Porphyritic types bear relic phenocrysts of magmatic hornblende. Keratophyrized dacites and rhyolites show quartz, albite, chlorite, epidote, zircon and opaques.

Spinifex-textured MBVU-rocks have been referred to previously as komatiites or basaltic komatiites; this is not sustained mineralogically nor geochemically. These rocks present tremolite-actinolite, chlorite, albite, talc, epidote, carbonate and opaques. Pseudomorphosed acicular clinopyroxene spinifex textures predominate by far over those of bladed olivine, typical of (peridotitic) komatiites. SiO2 reaching the range of intermediate rocks, low TiO2 (0.25-0.40 wt-%), and Ti/Zr ~ 60 (50-75) relegate these rocks to siliceous magnesian basalts or low-Ti tholeiites linked to boninites, rather than komatiites with Ti/Zr > 100 (Coish 1989). These rocks may represent extrusive equivalents of the most primitive magmas, from which AVU-andesites differentiated in infracrustal magma chambers.

Differentiated magnesian basaltic flows show a spinifex-textured andesitic top and massive ultramafic cumulate base made up of pseudomorphosed euhedric medium-grained olivines and rare relic orthopyroxenes in a tremolite-chlorite intercumulus matrix with chromite as a constant magmatic intercumulus phase.

The BVU consists mainly of basic Fe(-Mg) tholeiitic flows. All three volcanic units contain subordinate volcaniclastic and chemical intertrap metasediments. — (December 14, 2001).

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