Architecture of Late Orogenic Plutons in the Araçuaí-Ribeira Fold Belt, Southeast Brazil

Abstract

Post-collisional to late orogenic magmatism (580 to 480 Ma) in the Araçuaí-Ribeira Fold Belt, SE Brazil, is characterized by the predominance of high-K metaluminous, allanite-titanite-bearing granitoids. Small lenses of coronitic gabbro, anorthosite, pyroxenite and phlogopite-peridotite are also common in deeper exposed areas of this fold belt. In the region of southern Espírito Santo State, the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions: a tendency to funnel-shaped bodies, with sub-vertical hemi-ellipsoidal/conical roots changing upwards to shallow-dipping tops. Associated stocks, sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes, local ductile shear zones and parallel to fold hinges. The contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e.g., Santa Angélica, Venda Nova, Mimoso do Sul and Várzea Alegre, but at shallow levels e.g., Castelo, Pedra Azul and Conceição de Muqui agmatic stoping zones occur along the borders. A magmatic foliation within the granitoids is usually well marked, but the schistosity in the surrounding gneisses wraps around the plutons. The intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores (gabbro, diorite to tonalite) surrounded by a mingled (marble cake) zone where basic and acid rocks are interfingered, and an external zone of syenomonzonite and granite. Widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and/or syenomonzonitic compositions is characteristic for all intrusive complexes. We suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust. The mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis. The bimodal plutons result from interaction among the contrasting magmas. Ascent and emplacement followed older regional structures, such as regional fold hinges and ductile shear zones.