Zircon U-Pb SHRIMP dating of gneissic basement of the Dom Feliciano Belt, southernmost Brazil

The age of a basement gneiss of the Dom Feliciano Belt along the coast of R io Grande do Sul has been determined by zircon U-Pb SHRIMP to be about 2.08 Ga for the K-granitic magmatism and 800-590 Ma for the associated low-angl e and sub-vertical shear zone deformations. The gneiss is the G3 granitic p hase of the Arroio dos Rates Complex of previous authors, and it now define s a geotectonic environment of juvenile accretion of island arcs in the Pal eoproterozoic. The superposition of deformation events during the Neoproter ozoic precludes the precise determination of the age of each event in this investigation, but we suggest that the collisional low-angle shear zones oc curred at ca. 800 Ma and the sub-vertical shear zones at ca. 600 Ma. Th/U r atios are typically magmatic (about 0.4) in the homogeneous cores of zircon s (about 2000 Ma), but are metamorphic (similar to0.01) in the zoned euhedr al rims (about 590 Ma). All the Paleoproterozoic gneisses in the region are part of the Encantadas Complex. Archean units, such as the Santa Maria Chico granulites, were all deformed in this major event of the Transamazonian Cycle. The dated gneiss may be correlative with the Epupa Complex north and south of the Kaoko Belt of SW Africa. Ages of the Neoproterozoic deformation are younger in the Ka oko Belt of Namibia than in its Brazilian counterpart. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.