Print version ISSN 0001-3765
An. Acad. Bras. Ciênc. vol.73 no.3 Rio de Janeiro Sept. 2001
Sm-Nd ISOTOPIC DATA FOR METABASIC ROCKS IN CENTRAL BRAZIL: CONSTRAINTS FOR THE TIMING OF MAFIC MAGMATISM AND REGIONAL MAGMATISM
MÁRCIO M. PIMENTEL
Instituto de Geociências, Universidade de Brasília, 70910-900 Brasília, DF
Mafic rock associations of varied nature and ages represent an important component of the Brasília Belt in central Brazil. The emplacement and metamorphic ages of mafic associations allied with their initial Nd isotopic compositions are essential to understand the tectonic evolution of the belt. Despite the poor precision normally achieved by the Sm-Nd isochron data, the ages allied with initial ratio values provide important information on the geological history of Precambrian terrains.
The oldest mafic-ultramafic rocks in central Brazil are komatiitic rocks of the Crixás greenstone belt: ca. 3.0 Ga and Nd(T) of ca. +1.5, possibly representing Archean oceanic floor preserved within the oldest continental fragment preserved in central Brazil. In the sialic basement of the supracrustal rocks of the belt, the mafic sequence of Silvânia yielded an isochron age of ca. 2.1 with Nd(T) of ca. +2.3. This is representative of the juvenile Paleoproterozoic of the basement of the Neoproterozoic orogenic belt. Approximately 1.9 Ga mafic intrusions represent a Paleoproterozoic continental rift ( Nd(T) of ca. -2.5): the lower unit of the Niquelândia mafic ultramafic body. The western part of the Barro Alto and Niquelândia intrusions are formed by 1.3 Ga rocks with oceanic affinity, indicating oceanic crust formation towards the end of the Mesoproterozoic. Garnet ages of ca. 0.74-0.76 Ga reveal an ''early Brasiliano'' metamorphic event. Oceanic 1.0-0.9 Ga rocks form the basal parts of island arc sequences in Mara Rosa. These are the oldest representatives of the Neoproterozoic intraoceanic arcs in the western part of the Brasília Belt. Late- to post-orogenic gabbro-dioritic intrusions dated at ca. 0.6 Ga (e.g. the Americano do Brasil and Lajeado intrusions) represent the last precambrian mafic event in the orogenic belt, associated with uplift following continental collision. ( May 18, 2001 )