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

2004 Journal of Petrology  
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 (M 1 ) grew during fabric formation in the rocks (D 1 ) and is characterized by the mineral assemblage Pl þ Bt þ Sil þ Kfs þ Qtz. Peak metamorphic conditions (M 2 ) are characterized by the assemblage Bt þ Crd þ Kfs þ Pl þ Grt þ liq þ Qtz and are inferred to have developed during D 2
more » ... eveloped during D 2 folding of the rocks at T ¼ 750-800 C and P ¼ 7 kbar. M 3 reaction textures overprint the M 2 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 M 2 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.
doi:10.1093/petrology/egh058 fatcat:ljhdlcir3ncqponr3odlfnosn4