eletrica

Revista Brasileira de Geociências

Márcio M. Pimentel, Carlos A. Spier & Cesar F. Ferreira Filho

32(3):xxx-xxx, setembro de 2002

ESTUDO Sm-Nd DO COMPLEXO MÁFICO-ULTRAMÁFICO
BACURI, AMAPÁ: IDADE DA INTRUSÃO, METAMORFISMO E NATUREZA DO
MAGMA ORIGINAL
MÁRCIO MARTINS PIMENTEL1, CARLOS ALBERTO SPIER2 & CESAR FONSECA FERREIRA FILHO1

ABSTRACT
Sm-Nd STUDY OF THE BACURI MAFIC-ULTRAMAFIC COMPLEX, AMAPÁ: AGE OF INTRUSION,
METAMORPHISM AND NATURE OF THE ORIGINAL MAGMA The Bacuri mafic-ultramafic complex is a metamorphosed and deformed intrusion made of amphibolite and metaultramafic rocks (serpentinite and chromitite), exposed in centralsouthern Amapá, within the Paleoproterozoic Maroni-Itacauinas mobile belt, northern Brazil. Its age, nature of original magma and tectonic significance are still poorly constrained. A whole-rock Sm-Nd “errorchron” (MSWD = 5,1) for the mafic and ultramafic rocks indicates the age of 2.22 ± 0.12 Ga and eNd(T) of –3.8. Despite the large uncertainty, this represents the only estimate available so far for the age of crystallization of the complex. The initial Nd isotopic composition indicates that the original magma is strongly contaminated with older sialic crust. Gneiss country-rocks have TDM model ages between ca.
3.1 and 3.3 Ga and might represent the crustal contaminants. Garnet amphibolites of the Vila Nova Group yielded whole rockgarnet isochron indicating the age of 2.08 ± 0.02 Ga for the regional metamorphic event. This is roughly coeval with the voluminous late-orogenic granitic magmatism in the Maroni-Itacaiunas mobile belt. The isotopic data agrees with previous field and petrological studies which concluded that the intrusion is a layered body intruded into and contaminated with older continental crust. This kind of intrusion in northern Brazil may represent a significant source of metals such as Cr, PGE, FeTi-V, Cu-Ni, enlarging the possibilities for mineral exploration in the Guiana Shield.
Keywords: Bacuri,