Repeated partial melting events in polymetamorphic Carlos Chagas Batholith : implications for tectono-metamorphic evolution of the Ara?ua? orogen, southeastern Brazil.

Programa de P?s-Gradua??o em Evolu??o Crustal e Recursos Naturais. Departamento de Geologia. Escola de Minas, Universidade Federal de Ouro Preto.O or?geno Ara?ua? (OA) localizado no sudeste do Brasil e sua contraparte africana (cintur?o Congo Ocidental) constituem o sistema orog?nico Brasiliano Pan Africano desenvolvido entre as paleocontinentes Congo e S?o Francisco. O or?geno Ara?ua? registra uma sucess?o not?vel de eventos de produ??o de granitos no espa?o e no tempo, que engloba desde os primeiros pl?tons pr?-colisionais (~ 630 Ma) at? as mais recentes intrus?es p?s-colisionais (~ 480 Ma). Este estudo investiga a petrog?nese do bat?lito Carlos Chagas (BCC), um enorme corpo (~ 14.000 km2) composto por granitos do tipo S atribu?dos ao plutonismo colisional (supersu?te G2) na regi?o de retro arco do OA, a leste do arco magm?tico Rio Doce. BBC estende-se em uma dire??o N-S entre as latitudes 17 ? S e 19?30'S, nos estados de Esp?rito Santo, Minas Gerais e Bahia. BBC inclui predominantemente granitos ricos em granada que em biotita, em que tr?s assembleias minerais podem ser identificadas: 1) Qz + Pl + KFS + Bt + Grt + Ilm ? Rt; 2) Qz + Pl + KFS + Bt + Grt + Ilm + Sil; e 3) Qz + Pl + KFS + Bt + Grt + Ilm + Sil + Spl. As rochas que cont?m as assembleias 2 e 3 cont?m duas gera??es de granadas. Evid?ncias microestruturais para a presen?a de melt s?o identificadas em todas as amostras estudadas, tais como: inclus?es de melt silic?tico em granadas poiquilobl?sticas, finos filmes de melt pseudomorfizados ao redor de ambas gera??es de granada, ?piscinas? de melt adjacentes a granada e biotita, al?m de plagiocl?sio e quartzo com formas cuspade a lobulados que ocorrem entre os gr?os da matriz. Ambas gera??es de granada (Grt1 e Grt2) n?o apresentam zoneamento em termos de elementos maiores e cont?m pequenas inclus?es arredondadas de biotita rica em Ti. Al?m disso, os cristais Grt2 tamb?m cont?m inclus?es de remanescentes agulhas de sillimanita. Evid?ncias microestruturais, em combina??o com a qu?mica mineral, indicam que os cristais de granada cresceram durante dois eventos metam?rficos/anat?ticos distintos, como produtos perit?ticos de rea??es de fus?o parcial em regime de fluido ausente que consumiram biotita, quartzo e plagiocl?sio, no caso de Grt1, e que consumiram biotita, quartzo, plagiocl?sio e silimanita, no caso de Grt2. Pseudose??es calculadas via Theriak-Domino forneceram novas condi??es metam?rficas para a regi?o do retro arco do OA. Dados da assembleia mineral 1 sugerem que o primeiro evento metam?rfico de f?cies granulito (M1) ocorreu a temperaturas de 790-820 ?C e press?es de 9.5-10.5 kbar, enquanto a assembleia mineral 2 registra condi??es PT para um segundo metamorfismo de f?cies granulito (M2) a aproximadamente 770 ?C e 6.6 kbar. Dados do term?metro Ti em zirc?o s?o consistentes com modelamento metam?rfico, indicando que os dois eventos metam?rficos no bat?lito atingiram condi??es de f?cies granulito. Variadas idades foram encontradas em gr?os de monazita e zirc?o (> 825 Ma a 490 Ma) das rochas do BCC, registrando uma complexa hist?ria de reciclagem crustal e heran?a, cristaliza??o magm?tica e anatexia durante a evolu??o do OA. Os zirc?es magm?ticos (~ 582 Ma) mostram composi??o isot?pica de Hf e padr?es de ETR similar aos dos n?cleos herdados (825-600 Ma), indicando que a assinatura qu?mica destes cristais provavelmente foi herdada da fonte. Idades U-Pb e raz?es iniciais 176Hf/177Hf dos dom?nios de zirc?es anat?ticos e/ou metam?rficos s?o consistentes com dois est?gios metam?rficos caracterizados por processos de crescimento/sobrecrescimento e recristaliza??o de gr?os de zirc?o durante a orogenia. O epis?dio metam?rfico mais antigo (570-550 Ma) ? caracterizado pelo sobrecrescimento anat?tico em n?cleos mais velhos e pelo crescimento de novos cristais de zirc?o anat?tico. Ambos os dom?nios t?m elevados valores iniciais de 176Hf/177Hf em compara??o com n?cleos mais antigos e exibem padr?es de ETR t?picos de zirc?o que cresceu simultaneamente com granada perit?tica (Grt1) atrav?s de rea??es de fus?o parcial em regime de fluido ausente. Idades U-Pb obtidas em gr?os de monazita e zirc?o (569-552 Ma) inclusos na Grt1, corroboram a interpreta??o de que os cristais Grt1 cresceram durante o primeiro epis?dio anat?tico. Composi??o isot?pica de Hf e evid?ncias qu?micas indicam que um segundo epis?dio anat?tico (535-500 Ma) ? registrado em parte das rochas do bat?lito (assembleias 2 e 3). Nestas rochas, o crescimento e/ou sobrecrescimento de novos zirc?es anat?ticos ? marcado por elevada raz?o inicial de 176Hf/177Hf e tamb?m pela gera??o da Grt2, como indicado pelas observa??es petrogr?ficas e padr?es de ETR. Por outro lado, algumas rochas t?m gr?os de zirc?o formados por processos de recristaliza??o no estado s?lido de zirc?o pr?existente, que exibem composi??o isot?pica de Hf similar ?queles dom?nios magm?ticos/herdados. O evento M1 ? associado ao espessamento crustal e anatexia no bat?lito. O evento M2 pode ser associado a ascens?o da astenosfera durante o colapso gravitacional do or?geno. Isto produziu anatexia em partes do BCC que tinham sido refertilizado ao longo de zonas de cisalhamento ap?s o primeiro evento de f?cies granulito.The Ara?ua? orogen (AO), located in the southeastern Brazil, and its African counterpart, the West Congo belt, is part of the Pan-African-Brasiliano orogenic system developed between the Congo and S?o Francisco paleocontinents. From the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the Ara?ua? orogen records an outstanding succession of granite production events in space and time. This study investigates the petrogenesis of the Carlos Chagas batholith (CCB), a very large body (~ 14,000 km2) composed of S-type granites ascribed to the collisional plutonism (G2 supersuite) in the back-arc region of the AO, to the east of the Rio Doce magmatic arc. CCB extends in a N-S direction between latitudes 17?S and 19?30?S in the states of Esp?rito Santo, Minas Gerais and Bahia. The CCB includes a dominant granite richer in garnet than in biotite, in which three mineral assemblages can be identified: 1) Qz + Pl + Kfs + Bt + Grt + Ilm ? Rt; 2) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil; and 3) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil + Spl. Rocks which contain mineral assemblage 2 and 3 all contain two generations of garnet. Textural evidence for the presence of former melt, recognized in all studied CCB samples, includes: silicate melt inclusions in poikiloblastic garnet, pseudomorphed thin films of melt surrounding both generations of garnet, pseudomorphed melt pools adjacent to garnet and biotite, and plagioclase and quartz with cuspatelobate shapes occurring among matrix grains. Both generations of garnet crystals (Grt1 and Grt2) are unzoned in terms of major element concentration, contain small rounded inclusions of Ti-rich biotite and, in addition, the Grt2 crystals also contain inclusions of remnant sillimanite needles. Microstructural evidence, in combination with mineral chemistry, indicates that the garnet crystals grew during two distinct metamorphic-anatectic events, as the peritectic products of fluid-absent melting reactions which consumed biotite, quartz and plagioclase, in the case of Grt1, and which consumed biotite, quartz, plagioclase and sillimanite in the case of Grt2. P-T pseudosections calculated via Theriak-Domino, in combination with U-Pb monazite and zircon dating, provide new constraints on the thermal evolution of the back-arc region of the Ara?ua? orogen. Data from assemblage 1 suggests P-T conditions for the first granulite-facies metamorphic event (M1) at 790-820 ?C and 9.5- 10.5 kbar, while the assemblage 2 records P-T conditions for a second granulite-facies metamorphism (M2) of around 770 ?C and 6.6 kbar. Ti-in-zircon thermometry is consistent with the phase equilibrium modelling, indicating that the two metamorphic events in the batholith reached granulite facies conditions. A wide range of monazite and zircon ages (> 825 Ma to ca. 490 Ma) have been found in CCB rocks, recording a complex history of crustal recycling and inheritance, magmatic crystallization and anatexis during the evolution of the AO. 582 Ma magmatic zircons are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825-600 Ma), indicating that the chemical signature of these crystals has likely been inherited from the source. The U-Pb ages and initial 176Hf/177Hf ratios from anatectic and/or metamorphic zircon domains are consistent with a two stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. The oldest metamorphic episode (ca. 570-550 Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic Grt1 through biotite-absent fluid partial melting. U-Pb ages obtained in monazite and zircon (ca. 569-552 Ma) included in Grt1, further support the interpretation that the Grt1 crystals grew during the first anatectic episode. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535-500 Ma) is only recorded in part from the CCB rocks (assemblage 2 and 3). In these rocks, the growth of new anatectic zircon and/or overgrowth is marked by high initial 176Hf/177Hf values and also by generation of Grt2, as indicated by petrographic observations and REE patterns. On the other hand, some rocks have zircon grains formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition those to inherited/magmatic core domains. The M1 event likely corresponds to the striking crustal thickening and widespread anatexis in the CCB. The M2 event can be associated to asthenosphere upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts from the CCB that had been re-fertilized by retrogression along shear zones following the first granulite facies event