Polyphase Archean evolution in the Campo Belo metamorphic complex, Southern Sمo Francisco Craton, Brazil: SHRIMP U-Pb zircon evidence

Reconnaissance SHRIMP U-Pb geochronology on three zircon populations from a migmatite of the Campo Belo Complex reveals a polyphase Archean history. Group 1 zircons (three grains) yielded a weighted mean 207Pb/206Pb age of 3,205±17 Ma, interpreted to be a relict age component within the migmatite. This age compares well with most of the TDM ages available for the Archean metamorphic complexes in the Southern São Francisco Craton and indicates the first emergence of continental crust. Group 2 zircons (three grains) yielded a weighted mean 207Pb/206Pb age of 3,047±25 Ma, interpreted to be a second tectonic event in the area. This event is compatible with the formation of medium to high-grade gneisses with εNd (t) parameters close to zero and mantle-like single stage Pb signature (238U/204Pb=8.18). Group 3 zircons (three grains) yielded a weighted mean 207Pb/206Pb age of 2,839±17 Ma, which is considered to be the crystallization age of the neosome—the third tectonic event in the CBC. The fact that the migmatite sample contains three generations of melts is coherent with the observed variation of the εNd(2.84 Ga) values of the CBC gneissic rocks between +0.9 and −3.0. This indicates that reworking processes have had an important role during the crustal evolution. oarchean final evolution of the area is associated tectonically with the Rio das Velhas orogeny (2,780–2,700 Ma), during which migmatitic rocks were formed 2,690±56 Ma ago, in the southern edge of the Campo Belo Complex. In this region, mafic-ultramafic intrusions emplaced at ca. 2.75 and 2.66 Ga ago are associated with major distensional phases of this orogeny. Finally, a low-temperature episode took place at ca. 2,650 Ma ago within the Complex, as evidenced by resetting of the Rb–Sr and Pb–Pb whole rock isotopic systematics of granitoid rocks long after their primary crystallization.