Pan-African magmatism and regional tectonics of South Brazil

Abstract

The Dam Feliciano Belt is a Pan-African mobile belt from the Ribeira orogen of southern Brazil. A detailed field and geochemical traverse along the BR392 road section between Pelotas and Caҫapava do Sul identifies two major tectonic domains; the Pelotas Batholith and the Santana Metamorphic Belt, striking NNE-SSW parallel to the major foliation of the belts. The two belts are separated by a Triassic basin with flat lying red beds and interbedded andesites and rhyolites. The Santana Metamorphic Belt is a NW-verging fold belt with a metamorphosed shelf sequence of quartzites, marbles and graphitic schists and a polydeformed Lower Proterozic gneissic basement deformed during the Pan-African orogeny. Detailed mapping recognized four phases of deformation in the basement gneisses, three of which are recorded in the basement schists and cover sequence. There is also evidence of late NE-verging thrusting post-dating the formation unmetamorphosed Paleozoic sediments. Late extension caused NW-SE and NE-SW normal faulting. Metamorphism occurred contemporaneously with D2 and 03, and PT conditions for peak: metamorphism have been calculated as 8.6Kb and 600°C within the basement schists corresponding to garnet growth during D3 deformation. Three phases of granite intrusion are recognized in the Santana Metamorphic Belt The Santana Granite (8oom.y.) represents the first phase intruding the basement and it is folded by 02 and 03. The Campinas Granite (500m.y.) has a D3 foliation and the Caҫapava Granite (474m.y.) is post-tectonic and intrudes the NW portion of the Santana Metamorphic Belt The Pelotas Batholith is almost entirely composed of granitoids of Pan-African age (600- 4S0m.y.). Both D2 and D3 are recognized in the batholith. A two fold subdivision based on geochemical and field criteria distinguishes the following categories of granitoid; foliated granitoids and unfoliated granites. Geochemically all granitoids of the two belts have a strong crustal signature. The foliated granitoids of the Pelotas Batholith (600-550m.y.) are metaluminous, calkalkaline granodiorites, and have volcanic arc characteristics with Sr initial ratios between 0.708 and 0.710 and model ages in the range of 1.6-1.4b.y. The unfoliated granites (500-450m.y.) of the Pelotas Batholith are slightly peraluminous and their field relations, trace element and isotopic data suggest a post-collision setting for their formation. Their higher Sr initial ratios (0.710-0.770), Nd model ages of 1.2-1.1b.y. and high source Rb/Sr, support the proposal that they are melts of the migmatized granodiorites. The Canguҫu Red Granite and the Capao do Leao Granite are also unfoliated but appear to be geochemically distinct from the other unfoliated granites. They have high Fe/Mg, Rb/Sr and Rb/Ba ratios, flat rare earth element patterns and negative europium anomalies, all suggestive of A-type granites. From isotopic studies, the Canguҫu Granite has very high Sr initial ratios of 0.750 to 0.81 and old model ages of 2.0-3.0b.y. In the Santana Metamorphic Belt, the foliated Santana Granite has a more depleted trace element chemistry than foliated granites of the Pelotas Batholith and an earlier emplacement age of 8oom.y. Trace element and isotope modelling suggest it to be a melt of the basement gneisses. The Campinas Granite is strongly peraluminous with high Rb/Sr, Rb/Ba and Sm/Nd and initial ratios between 0.780 and 0.820 and model ages of 2.0 b.y. suggesting that it is a crustal melt from a pelitic source. The unfoliated Caҫapava Granite of the Santana Metamorphic Belt has low Rb/Ba, a low Sr initial ratio (0.705) and high model age (2.5b.y.) suggesting it is a lower crustal melt. The unfoliated Encruzilhada Granite is fault bounded and outcrops between the two belts. It is distinctive from granitoids of both belts in terms of its trace element and isotopic characteristics which suggest that it formed from a basaltic source in a within plate tectonic setting. There appears to be no systematic change in geochemistry with distance along the cross-section and any changes appear to be temporal. Combined Sr and Nd isotope studies suggest that the Pan-African Ribeira orogen represents a period of crustal reworking rather than crustal growth and in this respect resembles the Damara orogen of Namibia which is dominated by crustal reworking, in contrast to the rapid crustal growth observed in the Arabian shield.