Geoquímica de rocha total, geocronologia de u-pb e geologia isotópica de sm-nd das rochas ortognáissicas e unidades litológicas associadas da região ipameri - Catalão (Goiás)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The part of the southern Brasília Belt, Tocantins Province, exposed in central Brazil, comprises metasedimentary rocks, metavolcano-sedimentary sequences, amphibolites and intrusive granites, and a large orthogneiss belt. These units are tectonically imbricated, and vary in metamorphic grade from lower greenschist to amphibolite facies, characterized by a 720 and 800 Ma extensional, and 660 Ma collisional events. The amphibolites indicate gabbroic and basaltic protoliths, typically of tholeiitic affinity, showing Fe enrichment, low Ti content, low La/Yb ratios, small negative Eu anomalies and εNd(T) close to zero, associated to primitive magma sources varying from transitional to enriched mid-ocean ridge (T-MORB to E-MORB) basalts. One amphibolite is alkaline, with high TiO2 and La/Yb ratio, small negative Eu anomaly and general rift-related characteristics. Geochemical data of the intermediate to acid metavolcanic rocks of the Araxá Group and Maratá Sequence suggest formation in 720 and 800 Ma extensional events. TDM model ages varying from 1.91 to 2.08 Ga and εNd(T) values between -7.95 and -9.30 indicate that the parental magmas derived from crustal melting of varied protoliths. Model ages between 1.77 and 1.97 and negative εNd (-7.26 to 9.48) of metapelitic rocks show a continental crust source indicating provenance of paleoproterozoic rocks. The REE from these rocks indicates heterogeneous patterns, suggesting a different provenance for sedimentary protoliths of the western domain of the Araxá Group in the area. εNd(T) and geochemical data allow the distinction of two groups of granitoids. One comprises the less deformed, slightly peraluminous to metaluminous, A-type, Pires Belo and Davinópolis granites, dated at 798 2 Ma and 775 5 Ma. Their model ages are 1.29 and 1.52 Ga, εNd(T) 1.31 and 0.77, respectively, suggesting important contribution of more primitive and juvenile sources. The second group is formed by more deformed, gneissified peraluminous S-type granites with model ages between 1.79 and 2.41 Ga with one crystallization age of c. 663 Ma, with εNd(T) of 9.68, reflecting a reworked crustal source. The meso-neoproterozoic orthogneisses from Ipameri-Nova Aurora occur within the central part of the studied area. From west to east, geological, geochemical and Sm- Nd isotopic data and U-Pb zircon age determinations allow to discriminate three domains: (1) Nova Aurora 1.2 Ga orthogneiss; (2) Goiandira 640 Ma orthogneiss; and (3) Ipameri 790 Ma orthogneiss. Petrography and whole rock geochemistry of selected samples characterize their protoliths as tonalite to granite in composition, with calc-alkaline and peraluminous affinity. They vary from calcic to alkaline-calcic, with variated εNd(T) range (+2.52 to - 11.69), probably formed in a continental margin arc environment. The westernmost domain, around Nova Aurora, comprises mesoproterozoic orthogneisses, dated at 121913 Ma, εNd(T) = -4.89 and 1259 7 Ma, εNd(T) = -4.57. Model ages of this group vary between 1413 and 2624 Ma and εNd(T) between -6.52 and +2.52, suggesting mixed juvenile and older heterogeneous sources. This leads to the suggestion that this domain could have been a potential source for part of the mesoproterozoic detrital zircon population reported from the southern Brasília Belt. The orthogneiss domain around Goiandira yielded neoproterozoic ages between 6349 and 71739 Ma. Model ages range between 1371 and 2541 Ma and calculated εNd(T) values between -4.15 and -11.69. The third orthogneiss domain is exposed in the Ipameri area. U-Pb zircon ages of two samples yielded 796 64 Ma and 771 13 Ma, εNd(T) between -3.65 and -9.83 and TDM model ages between 1913 and 2057 Ma, with one lower value of 1284 Ma. Some dioritic, tonalitic and granitic orthogneiss in the Ipameri-Nova Aurora region have a different behavior. They have very high- to high-K character and peraluminous affinity with probably alkaline nature. The Goiandira 640 Ma and Ipameri 790 Ma domains are related to a continental magmatic arc, built-up at the margin of the plate that includes the São Francisco Craton. Their northward continuity constitutes the Anápolis-Itauçu granulite terrain. As for the 1230-1260 Ma orthogneiss domain in Nova Aurora, we suggest that it is an exotic fragment of a mesoproterozoic arc built elsewhere.The overall evidence demonstrates that the bimodal magmatism presented in the rocks outcropping in the Ipameri-Catalão region, are related to an extensional continental rift event occurred between 720 and 800 Ma, evolving to an oceanic crust with T-MORB to E-MORB affinity. At 660 Ma there is evidence of continental collision magmatism related to compressional movement between the southwestern portion of São Francisco Craton and Paranapanema Block.

ASSUNTO(S)

arco continental tocantins province orthogneiss brasília belt magmatismo bimodal província tocantins continental arc eventos extensional e colisional extensional and collisional events faixa brasília ortognaisses bimodal magmatism geologia regional

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