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SHRIMP U/Pb Zircon Geochronology of Neoproterozoic Juvenile and Crustal-Reworked Terranes in Southernmost Brazil

1998; Taylor & Francis; Volume: 40; Issue: 8 Linguagem: Inglês

10.1080/00206819809465232

1938-2839

Jayme Alfredo Dexheimer Leite, LéO A. Hartman, Neal J. McNaughton, Farid Chemale,

earthquake and tectonic studies

Abstract U/Pb zircon ages of six rock units from southernmost Brazil were obtained by means of SHRIMP II techniques. The studied rocks occur on either side of the Neoproterozoic contact between a juvenile accreted terrane (the Vila Nova Terrane) and a crustal-reworked belt (the Dom Feliciano Belt). This contact is located in the western portion of the exposed Precambrian rocks in the state of Rio Grande do Sul, Brazil. Contrasting patterns of U/Pb ages are shown by zircons hosted in metamorphosed basic and granitic rocks of the Cerro Mantiqueiras region and in the post-tectonic Lavras Granite, both located in the Vila Nova Terrane and in the syntectonic crust-derived Cacapava Granite in the Dom Feliciano 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 Ga, εNd(740 Ma) values are between +2 and +6 and 87Sr/86Sr ratios are low, ∼0.704 (Babinski et al., 1995; Leite, 1997). In the Dom Feliciano Belt, in contrast, Nd model ages are older, close to 2.2 Ga; εNd(620 Ma) values are strongly negative, at ∼−15; and 87Sr/86Sr ratios are higher than 0.710 (Babinski et al., 1996). SHRIMP U/Pb zircon data from the Cerro Mantiqueiras region indicate the occurrence of two main tectonometamorphic events during the development of the Brazilian/Panafrican orogenic cycle-the Sao Gabriel event at ∼750 Ma and the Dom Feliciano event at ∼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/Pb zircon data for the Cacapava Granite, in the Dom Feliciano Belt, range from nearly 2.4 Ga to 580 Ma; the older ages are interpreted as inheritance from an older crust, whereas the younger ones represent the magmatic age of the Cacapava Granite, corresponding to the late stages of the Dom Feliciano event. This event is represented in the Cerro Mantiqueiras region by the intrusion of the post-tectonic Lavras Granite, with a magmatic age of 580 Ma.