TECTONIC SETTING AND U PB ZIRCON DATING OF THE PLUTONIC SOCORRO COMPLEX IN THE TRANSPRESSIVE RIO-PARAIBA-DO-SUL SHEAR BELT, SE BRAZIL/

The Precambrian Rio Paraiba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending o ver 1000 km of the southeastern coast of Brazil. Granulitic, gneissic- migmatitic, and granitoid terrains as well as low- to medium-grade met avolcano-sedimentary sequences are included within it. These rocks wer e affected by strong contractional, tangential tectonics, due to west- northwestward oblique convergence of continental blocks. Subsequent tr anspressional tectonics accomodated large dextral, orogen-parallel mov ements and shortening. The plutonic Socorro Complex is one of many def ormed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ducti le shear zones. The tectonic and magmatic structural features of the S erra do Lope Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening or country rocks. Geochronological data indicate that tile main igneous activity of Socorro Complex spanned at least 55 million years, from the late st age of the northwestward ductile thrusting (650 Ma), through right-lat eral strike slip (595 Ma) deformation. The country rocks yield discord ant age data, which reflect a strong imprint of the Transamazonian tec tono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the ori gin of calc-alkaline granites of the Ribeira Belt by partial melting o f the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have fo cused granite migration from deeper into higher crustal levels along t hese shear zones.