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

Reconnaissance SHRIMP U-Pb geochronology on three zircon populations from a mignatite of the Campo Belo Complex reveals a polyphase Archean history. G roup 1 zircons (three grains) yielded a weighted mean Pb-207/Pb-206 age of 3,205 +/- 17 Ma, interpreted to be a relict age component within the migmat ite. This age compares well with most of the TDM ages available for the Arc hean metamorphic complexes in the Southern Sao Francisco Craton and indicat es the first emergence of continental crust. Group 2 zircons (three grains) yielded a weighted mean Pb-207/Pb-206 age 3,047 +/- 25 Ma, interpreted to be a second tectonic event in the area. This event is compatible with the f ormation of medium to high-grade gneisses with epsilon(Nd) (t) parameters c lose to zero and mantle-like single stage Pb signature (U-238/Pb-204 = 8.18 ). Group 3 zircons (three grains) yielded a weighted mean Pb-207/Pb-206 age of 2,839 +/- 17 Ma, which is considered to be the crystallization ape of t he neosome-the third tectonic event in the CBC. The fact that the migmatite sample contains three generations of melts is coherent with the observed v ariation of the epsilon(Nd(2.84 Ga)) values of the CBC gneissic rocks betwe en +0.9 and -3.0. This indicates that reworking processes have had an impor tant role during the crustal evolution. The Neoarchean 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 Com plex. 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 th e Complex, as evidenced by resetting of the Rb-Sr and Pb-Pb whole rock isot opic systematics of granitoid rocks long after their primary crystallizatio n. (C) 1998 Elsevier Science Ltd. All rights reserved.