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Journal of Petrology Advance Access originally published online on August 27, 2004
Journal of Petrology 2004 45(11):2303-2323; doi:10.1093/petrology/egh058
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Journal of Petrology 45(11) © Oxford University Press 2004; all rights reserved

Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil

A. KÜHN1,*, K. STÜWE1 and R. A. J. TROUW2

1 DEPARTMENT OF GEOLOGY AND PALAEONTOLOGY, UNIVERSITY OF GRAZ, HEINRICHSTRASSE 26, 8010 GRAZ, AUSTRIA
2 UNIVERSIDADE FEDERAL DO RIO DE JANEIRO, DEPARTAMENTO DE GEOLOGIA, CEP 21910-900 RIO DE JANEIRO, BRAZIL

Migmatitic granulites and arc-related felsic intrusives of Pan-African age form the bedrock in the Rio de Janeiro area, SE Brazil. These rocks preserve a partial record of three parageneses. The earliest assemblage (M1) grew during fabric formation in the rocks (D1) and is characterized by the mineral assemblage Pl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M2) are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt + liq + Qtz and are inferred to have developed during D2 folding of the rocks at T = 750–800°C and P = 7 kbar. M3 reaction textures overprint the M2 assemblage and comprise symplectitic intergrowth of cordierite(II) and quartz that formed after garnet, whereas secondary biotite formed as a result of reactions between garnet and K-feldspar. By comparing the observed modal abundances with modal contours of garnet, cordierite and quartz on the relevant pseudosection a post M2 P–T vector indicating contemporaneous cooling and decompression can be deduced. The inferred equilibrium assemblage and reaction textures are interpreted to reflect a clockwise P–T path involving heating followed by post-peak decompression and associated cooling. We infer that metamorphism occurred in response to advective heating by the abundant syn-collisional (arc-related) I-type granitoids in the region, consistent with the unusually high peak T/P ratio.

KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; P–T pseudosection

* Corresponding author. Present address: Institute for Geosciences, University of Mainz, Becherweg 21, D-55099 Mainz, Germany. Telephone: 0049 (0)6131 3924368. Fax: 0049 (0)6131 3924769. E-mail: kuehna{at}uni-mainz.de


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