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Dating coeval mafic magmatism and ultrahigh temperature metamorphism in the Anápolis–Itauçu Complex, Central Brazil

2010; Elsevier BV; Volume: 124; Issue: 1-2 Linguagem: Inglês

10.1016/j.lithos.2010.11.004

1872-6143

María Emilia Schutesky, Márcio Martins Pimentel, César Fonseca Ferreira Filho, Maria Helena Bezerra Maia de Hollanda,

earthquake and tectonic studies

Dating granulites has always been of great interest because they represent one of the most extreme settings of an orogen. Owing to the resilience of zircon, even in such severe environments, the link between P–T conditions and geological time is possible. However, a challenge to geochronologists is to define whether the growth of new zircon is related to pre- or post-P–T peak conditions and which processes might affect the (re)crystallization. In this context, the Anápolis–Itauçu Complex, a high-grade complex in central Brazil with ultrahigh temperature (UHT) granulites, may provide valuable information within this topic. The Anápolis–Itauçu Complex (AIC) includes ortho- and paragranulites, locally presenting UHT mineral assemblages, with igneous zircon ages varying between 760 and 650 Ma and metamorphic overgrowths dated at around 650–640 Ma. Also common in the Anápolis–Itauçu Complex are layered mafic–ultramafic complexes metamorphosed under high-grade conditions. This article presents the first geological and geochronological constraints of three of these layered complexes within the AIC, the Damolândia, Taquaral and Goianira–Trindade complexes. U–Pb (LA-MC-ICPMS, SHRIMP and ID-TIMS) zircon analyses reveal a spread of concordant ages spanning within an age interval of ~ 80 Ma with an "upper" intercept age of ~ 670 Ma. Under cathodoluminescence imaging, these crystals show partially preserved primary sector zoning, as well as internal textures typical of alteration during high-grade metamorphism, such as inward-moving boundaries. Zircon grains reveal homogeneous initial 176Hf/177Hf values in distinct crystal-scale domains in all samples. Moreover, Hf isotopic ratios show correlation neither with U–Pb ages nor with Th/U ratios, suggesting that zircon grains crystallized during a single growth event. It is suggested, therefore, that the observed spread of concordant U–Pb ages may be related to a memory effect due to coupled dissolution–reprecipitation process during high grade metamorphism. Therefore, understanding the emplacement and metamorphism of this voluminous mafic magmatism is crucial as it may represent an additional heat source for the development of the ultrahigh temperature paragenesis recorded in the paragranulites.