Reavaliação da evolução geológica em terrenos Pré- cambrianos brasileiros com base em novos dados U-Pb shrimp, parte iii: Províncias Borborema, Mantiqueira Meridional e Rio Negro-Juruena(*)

Abstract

This paper discusses new SHRIMP U-Pb data for 17 key-exposures (mostly granites and orthogneisses) from the Borborema, Southern Mantiqueira (Pelotas Orogen) and Rio Negro-Juruena provinces. In the Borborema Province (Ceara state) two samples from the Cruzeta Complex TTG orthogneisses, ascribed to the Paleoproterozoic basement, were studied. One revealed Paleoarchean crystallization minimum age of ca. 3270 Ma. Accordingly, the gneiss is interpreted as the oldest continental crustal remnant already recognised in Ceara. The other sample, from the Saboeiro-Aiuaba Granite gave a crystallization age of ca. 625 Ma, suggesting the correlation of this syn-orogenic pluton with the Brasiliano II orogenic system (climax at 630 Ma). In the Paraiba state the granodioritic gneiss pluton ascribed to the Mesoproterozoic Sume Complex showed a crystallization age of ca . 640 Ma, also indicating that its evolution is associated with the Brasiliano II orogenic system. In the Pernambuco state one widespread orthogneissic unit within the Pernambuco-Alagoas Massif (Belem do Sao Francisco Complex), mapped as a component of the Mesoproterozoic Cariris Velho Orogen, yielded a crystallization age of ca. 2079 Ma and metamorphic overprinting at ca. 655 Ma (1 s ), without evidence of a Mesoproterozoic (Cariris Velhos) reworking. In the southern part of the province, near the northern margin of the Sao Francisco Craton, the Santa Maria da Boa Vista (S-type) orthogneiss yielded a crystallisation age of ca. 3070 Ma. In the southern Mantiqueira Province/Pelotas Orogen a foliated granitic pluton (mylonitic) from the Florianopolis Batholith showed Paleoproterozoic protolithic age of ca . 2175 Ma and imprecise Brasiliano age on reprecipitated overgroths. Both results match previous ages obtained on the orthogneisses protoliths from the Aguas Mornas complex, the main exposure of reworked basement within the batholith. The large, zoned calc-alkaline pluton of the Maruim Suite, confirmed its complex post-collisional history relatively to the ca. 650-630 Ma Pelotas Orogen. Two tonalitic rocks yielded crystallization ages of ca . 611 Ma and ca . 608 Ma, whereas a granitic end member a crystallisation age of ca . 580 Ma. An attempt to determine the age of rifting that originated the post-colisional foreland Itajai Basin, a resedimented volcaniclastic felsic layer was investigated. The CL imagery revealed a very heterogeneous zircon population with a dominant detrital group ranging in age between ca. 1730-1800 Ma. One euhedral volcanic crystal, yielded an apparent age of ca . 608 Ma, interpreted as the best estimate age for the onset of the Itajai volcanic-sedimentary basin, and a minimum age for the volcanogenic episode. In the SW domain of the Rio Negro- Juruena Province in Mato Grosso state, a pluton related to the Aripuana Granitic Suite, revealed a crystallization age of ca . 1540 Ma. As the granite is related to Au and Cu-Zn hydrothermal mineralization, this precise geochronologic constraint on its emplacement age is also an important clue for exploratory purposes. In the central domain of the province, in Rondonia state, two orthogneissic units exposed in the vicinities of Ariquemes and Mutum-Parana were dated at ca . 1660 Ma and ca . 1728 Ma respectively, and attributed to the Jamari Complex. In the same domain, two mylonitic leucogranites exposed close to Cacoal and Espigao do Oeste, belonging to the Serra da Providencia Intrusive Suite yielded crystallisation ages of ca . 1522 Ma and ca . 1545 Ma, respectively. The former, showed also solid-state external overgrowths dated at ca . 1400 Ma, owing to recrystallizing processes at the roots of deep seated mylonitic shear zones. Finally, two orthogneissic units with crystallization ages of ca . 1555 Ma and ca . 1545 Ma - coeval with the crytallization age of the Serra da Providencia Intrusive Suite - showed metamorphic overgrowths aged at ca . 1325 Ma, suggestive of overprinting by a regional metamorphic event, not reported in previous works on the suite, but already recognised in other associations in the region.