SHRIMP U PB ZIRCON GEOCHRONOLOGY OF NEOPROTEROZOIC JUVENILE AND CRUSTAL-REWORKED TERRANES IN SOUTHERNMOST BRAZIL/

U/Ph zircon ages of sire rock units fi om southernmost Brazil were obt ained by means of SHRIMP II techniques. The studied rocks occur on eit her side of the Neoproterozoic contact between a juvenile accreted ter race (the Vila Nova Terrane) and a crustal-reworked belt (the Dom Feli ciano Belt). This contact is located in the western portion of the exp osed Precambrian rocks in the state of Rio Grande tio Sul. Brazil. Con trasting patterns of U/Ph ages are shown by zircons hosted in metamorp hosed basic and granitic rocks of the Cerro Mantiqueiras region and in the post-tectonic Lavras Granite, both located in the Vila Nova Terra ne and in the syntectonic crust-derived Cacapava Granite in the Dom Fe liciano Belt. Contrasting isotopic characteristics also are supported by Nd and Sr isotopes. For the Vila Nova Terrane, Nd model ages are no older than 1.3 Ca, epsilon Nd(740 Ma) values are between +2 and +6 an d Sr-87/Sr-86 ratios are low, similar to 0.704 (Babinski et al., 1995; Leite. 1997). In the Dom Feliciano Belt, in contrast. Nd model ages a re older, close to 2.2 Ga: epsilon Nd(620 Ma) values are strongly nega tive, at similar to-15, and Sr-87/Sr-86 ratios are higher than 0.710 ( Babinski et al., 1996). SHRIMP U/Pb zircon data from the Cerro Mantiqu eiras region indicate the occurrence of two main tectonometamorphic ev ents during the development of the Brazilian/Panafrican orogenic cycle -the Sao Gabriel event at similar to 750 Ma and the Dom Feliciano even t at similar to 580 Ma. The possible existence of a third. older event is suggested by an 890 Ma age for the Passinho Metadiorite. In marked contrast. U/Ph zircon data for the Cacapava Granite, in the Dom Felic iano Belt, range from nearly 2.4 Ga to 550 Ma: the older ages are inte rpreted as inheritance from an older crust, whereas the younger ones r epresent the magmatic age of the Cacapava Granite, corresponding to th e late stages of the Dom Feliciano event. This event is represented in the Cerro Mantiqueiras region by the intrusion of the post-tectonic L avras Granite, with a magmatic age of 580 Ma.