DADOS Rb-Sr E Sm-Nd DA REGIÃO DE JUSSARA-GOIÁS-MOSSÂMEDES (GO), E O LIMITE ENTRE TERRENOS ANTIGOS DO MACIÇO DE GOIÁS E O ARCO MAGMÁTICO DE GOIÁS

Abstract

Rb-Sr AND Sm-Nd DATA FROM THE JUSSARA-GOIÁS-MOSSÂMEDES REGION (GO, BRAZIL), AND THE LIMIT BETWEEN ANCIENT TERRANES OF THE GOIÁS MASSIF AND THE GOIÁS MAGMÁTIC ARC The nature of the limits between the Archean/Paleoproterozoic terrains of the Goiás Massif and the Neoproterozoic (ca. 900 - 650 Ma) juvenile terrains of the Goiás Magmatic Are is still poorly understood. In this study we have investigated southwestern and southern áreas of the massif, in the region between the towns of Goiás, Jussara and Mossâmedcs. Several granitic, gneissic and mylonitic rock were recognized and dated (Rb-Sr and Sm-Nd methods) units in the transect between Goiás, situated in the Goiás Massif and Jussara, in the magmatic are. From east to west they are: (i) Uvá tonalitic gneiss (GT) (2564 ± 140 Ma; i.r. = 0.7017 ± 0.0003; TDM=3.0-3.5 Ga); (ii) Uvá granite (GU) (2851 ± 180 Ma; εNd(T) = +0.3; TDM = 3.0-3. l Ga); (iii) Itapirapuã mylonitic gneiss; (GMI) (iv) syn-tectonic coarse-grained porphyritic granite (GPG) (759 ± 62 Ma; i.r. = 0.7053 ± 0.0002; TDM = 2.0-2.3 Ga); (v) Granitic gneiss (GG) (2601 ± 209 Ma; i.r. = 0.7016 ± 0.0007; TDM = 2.4 - 2.5 Ga), (vi) Mylonitic gneiss (GM). GT and GU display predominantly E-W tectonic foliation, with a local and weak superimposed N-S deformation. N-S structures become progressively more important towards the west, with the development of wide dextral shear zones, which are typical in the rocks of the magmatic are. The structural and geochronologic data indicate that the E-W deformational fabric is Archaean or Paleoproterozoic in age, and the N-S fabric is Neoproterozoic. Felsic metavolcanic rocks from the Mossâmedes área, soutlvof the Goiás Archaean terrains, yielded Rb-Sr isochron ages of 1978 ± 55 Ma and 1582 ± 101 Ma, with initial 87Sr/ 86Sr ratios of ca. 0.7023 and 0.7053, respectively. Therefore, these rocks are not part of the Neoproterozoic magmatic are, and are better interpreted as part of the southern edge of the Goiás Massif. The data presented suggest that the rocks investigated here are part of the southern and southwestern edges of the São Francisco-Congo continental plate. According to this reconstruction the Goiás massif represented an indentation of that plate during the continental collision that originated the Brasília Belt.