Further evidence of Rhyacian arc magmatism in the basement of the Brasilia Belt, western São Francisco pericraton

摘要

A recently mapped area in central Brazil revealed a slice of Paleoproterozoic basement within Neoproterozoic metasedimentary rocks of the Brasilia Belt. The area, named here the Artulandia area for its proximity to the homonymous village, contains metatonalites and metagranodiorites in tectonic contact with a meta-volcanosedimentary sequence that encompasses felsic, intermediate and mafic metavolcanic rocks interlayered with clastic and chemical metasedimentary rocks. A sample of felsic orthogneiss from the metavolcano-sedimentary sequence, yielded a U-Pb zircon age of 2142 +/- 4.4 Ma, which is interpreted as the crystallization age of the volcanic protolith. Samples from the metagranite unit returned U-Pb zircon ages of 2130 +/- 7.4 Ma for a biotite metatonalite, 2157 +/- 9 for a biotite metagranite, and 2147 +/- 5 Ma for a sulfide-bearing meta-granodiorite, indicating roughly contemporaneous plutonic activity. Whole-rock geochemistry shows calcalkaline signatures for both units, and trace element results are compatible with an extensional setting, such as back-arc or intra-arc, for the metavolcano-sedimentary sequence, and a magmatic arc system for the metagranite unit. These rocks were generated in an episode of juvenile crust formation, as suggested by epsilon(Nd)(2.14Ga) values between 0 and +3.18. Based on lithological association, age and chemical signature, we suggest that the studied rocks represent a lateral continuation of the Campinorte Arc beyond the Rio Maranhao Thrust. Thus, the occurrence of Campinorte Arc rocks on both sides of the thrust argues against the thrust being a Neoproterozoic collisional suture. The geology of the Artulandia area in the context of the Campinorte Arc attests to the complexity of the Rhyacian amalgamation that generated the Sao Francisco Paleocontinent during a Rhyacian Orogeny.