Assembly of the Sao Francisco-Congo paleocontinent: New SHRIMP U-Pb data from the eastern Brazilian margin

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Da Silva, Luiz۱; Pedrosa Soares, Antonio Carlos۲; Noce, Carlos Maurیcio۲; Delgado, Inacio Medeiros۱; Armstrong, Richard۳; McNaughton, Neal Jesse۴
Holding Date 17 September 2008

This SHRIMP U-Pb zircon study aims to discriminate crystallization and metamorphic ages of Paleoproterozoic high grade terranes of the Eastern Bahia Belt (EBB), located in eastern Brazil, in order to cast new lights on the reconstruction of the Sao Francisco-Congo (SFC) paleocontinent. Results from the EBB led to the discrimination of two distinct orogenic zones. The coastal (eastern) zone, with crystallization ages between 2170 to 2090 Ma for the magmatic protoliths, corresponds to an orthogneissic-granulitic belt with chemical and isotopic signatures similar to modern continental margin (Andean-type) arcs. The western zone (Itabuna block), in turn, represents the exhumed roots of a large syn-collisional orthogranulitic belt that overprinted Archean TTG protoliths, with scanty new Paleoproterozoic magmatic addition. The collisional high-grade metamorphism took place from 2090 Ma to 2050 Ma in both zones. Therefore, the EBB evolution can be correlated with the second collisional phase recorded from NW Gabon to South Cameroon. Accordingly, the coastal zone can be considered as the SW projection of the juvenile (2100-2030 Ma) coastal orthogneisses of the Eburnean Nyong Series, exposed in NW Gabon. Farther south, the Paleoproterozoic Mineiro Belt (MB), despite of its more complex and protracted accretionary magmatic evolution, correlates with the EBB. The African counterpart of the MB is the Paleoproterozoic Kimezian Complex, an unit of the West Congo Belt basement. The EBB-MB orogenic system and its Eburnean-Kimezian counterpart amalgamated the SFC paleocontinent at ca. 2050-2000 Ma. This paleocontinent only was partially broken by the Neoproterozoic precursor basin of the Araçuaí-West Congo orogen, from an Early Tonian continental rift to an oceanic spreading center around 750-650 Ma, but a continental bridge remained linking the EBB and its counterpart in Gabon, the Bahia-Gabon cratonic bridge. This cratonic bridge records the Tonian-Ediacaran extensional event only in discrete zones where mafic dykes and alkaline intrusions were emplaced. The Brasiliano orogenic event (ca. 630-500 Ma) inverted the precursor basin of the Araçuaی-West Congo orogen, but affected only the edges of the Bahia-Gabon bridge that survived until the opening of the Atlantic Ocean in Cretaceous times.