Repeated partial melting events in the polymetamorphic Carlos Chagas batholith : implications for the tectono-metamorphic evolution of the Araçuaí orogen (southeastern Brazil)

De Melo, Marilane Gonzaga (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The Araçuaí 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 São Francisco paleocontinents. From the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the Araçuaí 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 Espírito 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 cuspate-lobate 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 Araçuaí orogen. Data from assemblage 1 suggests P-T conditions for the first granulite-facies metamorphic event (M1) at 790-820 ºC and 9.5- 10.5 kbar, while the assemblage 2 records P-T conditions for a second granulite-facies metamorphism (M2) of around 770 ºC 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 anateksis 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 solidstate 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 anateksis in the CCB. The M2 event can be associated to asthenosphere upwelling during extensional thinning and gravitational collapse of the orogen, this produced anateksis in parts from the CCB that had been re-fertilized by retrogression along shear zones following the first granulite facies event.

AFRIKAANSE OPSOMMING: Die Araçuaí orogen (AO), geleë in die suidooste van Brasilië, en sy Afrikaanse eweknie, die Weste Kongogordel, is deel van die Pan-Afrika-Brasiliano orogenetiese stelsel ontwikkelde tussen die Kongo en São Francisco kratons. Van die vroegste (ongeveer 630 Ma) pre-botsing plutone om die nuutste (ongeveer 480 Ma) post-botsing rotse, die Araçuaí orogen rekords 'n uitstekende opvolging van graniet produksie gebeure in ruimte en tyd. Hierdie studie ondersoek die petrogenese van die Carlos Chagas batoliet (CCB), 'n baie groot liggaam (~ 14.000 km2) bestaan uit S-tipe graniet toegeskryf word aan die botsing plutonism in die back-boog streek van die AO, om die oos van die Rio Doce magmatiese boog. CCB strek in 'n N-S rigting tussen breedtegrade 17°S en 19°30'S in die state van Espírito Santo, Minas Gerais en Bahia. Die BSB sluit 'n dominante graniet ryker in granaat as in biotiet, waarin drie mineraal samestellings kan geïdentifiseer word: 1) Qz + Pl + Kfs + Bt + Grt + Ilm ± Rt; 2) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil; en 3) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil + Spl. Tekstuur bewyse vir die teenwoordigheid van die voormalige smelt, erken in al bestudeer CCB monsters, sluit in: silikaat smelt insluitings in granaat, pseudomorphed dun films van smelt omliggende beide generasies van granaat, pseudomorphed smelt poele aanliggend aan Garnet en biotite, en plagioklaas en kwarts met cuspate-lobate vorms voorkom tussen matriks korrels. Beide geslagte van granaat kristalle (Grt1 en Grt2) is unzoned in terme van belangrike element konsentrasie, bevat klein ronde insluitings van Ti-ryk biotiet en, benewens, die Grt2 kristalle bevat insluitings van oorblyfsel sillimanite naalde. Mikrostrukturele bewyse, in kombinasie met minerale chemie, dui daarop dat die granaat kristalle gegroei gedurende twee afsonderlike metamorfiese-anatectic gebeure, soos die peritektiese produkte van vloeistof-afwesig smelting reaksies wat verbruik biotiet, kwarts en plagioklaas, in die geval van Grt1, en wat verbruik biotiet, kwarts, plagioklaas en sillimanite in die geval van Grt2. P-T bereken deur Theriak-Domino, in kombinasie met die U-Pb monasiet en sirkoon dating, verskaf nuwe beperkings op die termiese evolusie van die back-boog streek van die Araçuaí orogen. Data van mineraal samestellings 1 dui PT toestande vir die eerste granulietterreine-fasies metamorfiese gebeurtenis (M1) by 790â820 ºC en 9.5â10.5 kbar, terwyl die mineraal samestellings 2 rekords PT toestande vir 'n tweede granulietterreine-fasies metamorfose (M2) van sowat 770 ºC en 6.6 kbar. Ti-in-sirkoon thermometry is in ooreenstemming met die faseewewig modellering, wat daarop dui dat die twee metamorfe gebeurtenisse in die batoliet bereik granulietterreine fasies voorwaardes. 'N Wye verskeidenheid van monasiet en sirkoon ouderdomme (> 825 Ma tot ongeveer 490 Ma) is gevind in CCB rotse, opname van 'n komplekse geskiedenis van kors herwinning, magmatiese kristallisasie en anateksis gedurende die evolusie van die AO. 582 Ma magmatiese sirkone is gekenmerk deur soortgelyke Hf isotoop komposisies en REE patrone aan dié van geërf kern (ongeveer 825-600 Ma), wat aandui dat die chemiese ondertekening van hierdie kristalle het waarskynlik geërf uit die bron. Die U-Pb ouderdomme en aanvanklike 176Hf / 177Hf verhoudings van anatectic en / of metamorfiese sirkoon domeine is in ooreenstemming met 'n twee-stadium metamorfiese evolusie gekenmerk deur kontrasterende meganismes van groei sirkoon en dekristallisatie gedurende die orogenie. Die oudste metamorfiese episode (ongeveer 570-550 Ma) is aangeteken deur die ontwikkeling van dun anatectic oorgroei op ouer gebiede en deur groei van nuwe anatectic sirkoon kristalle. Beide gebiede hoër aanvanklike 176Hf / 177Hf waardes in vergelyking met oorblyfsel cores en vertoon REE patrone tipies van sirkoon wat gelyktydig gegroei met peritektiese Grt1 deur vloeistof-vrye biotiet smelting. U-Pb ouderdomme verkry in monasiet en sirkoon (ongeveer 569-552 Ma) ingesluit in Grt1, verder te ondersteun die interpretasie dat die Grt1 kristalle gegroei gedurende die eerste anatectic episode. HF isotope en chemiese bewyse dui daarop dat 'n tweede anatectic episode (ongeveer 535-500 Ma) is slegs aangeteken in sommige CCB rotse (mineraal samestellings 2 en 3). In hierdie rotse, is die groei van nuwe anatectic sirkoon en / of oorgroei word gekenmerk deur 'n hoë aanvanklike 176Hf / 177Hf waardes en ook deur die opwekking van Grt2, soos aangedui deur petrografiese waarnemings en REE patrone. Aan die ander kant, sommige rotse het sirkoon korrels gevorm deur vastetoestand dekristallisatie van voorafbestaande sirkoon, met 'n soortgelyke Hf isotoop komposisie diegene te geërf / magmatiese domeine. Die M1 gebeurtenis waarskynlik ooreenstem met die treffende kors verdikking en wydverspreide anateksis in die CCB. Die M2 gebeurtenis geassosieer kan word om opwelling astenosfeer gedurende extensieve dunner en gravitasie ineenstorting van die orogen, hierdie geproduseer anateksis in dele van die CCB wat was weer bevrug deur agteruitgang saam skuifskeursones na aanleiding van die eerste granulietterreine-fasies metamorfiese.

PORTUGUESE RESUMO: O orógeno Araçuaí (OA) localizado no sudeste do Brasil e sua contraparte africana (cinturão Congo Ocidental) constituem o sistema orogênico Brasiliano Pan Africano desenvolvido entre as paleocontinentes Congo e São Francisco. O orógeno Araçuaí registra uma notável sucessão de eventos de produção de granitos no espaço e no tempo, que engloba desde os primeiros plútons pré-colisionais (~ 630 Ma) até as mais recentes intrusões póscolisionais (~ 480 Ma). Este estudo investiga a petrogênese do batólito Carlos Chagas (BCC), um enorme corpo (~ 14.000 km2) composto por granitos do tipo S atribuídos ao plutonismo colisional (supersuíte G2) na região de retro arco do OA, a leste do arco magmático Rio Doce. BBC estende-se em uma direção N-S entre as latitudes 17 ° S e 19°30'S, nos estados de Espírito Santo, Minas Gerais e Bahia. BBC inclui predominantemente granitos ricos em granada, em que três 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 contêm as assembleias 2 e 3 contêm duas gerações de granadas. Evidências microestruturais para a presença de melt são identificadas em todas as amostras estudadas, tais como: inclusões de melt silicático em granadas poiquiloblásticas, finos filmes de melt pseudomorfizados ao redor de ambas gerações de granada, “piscinas” de melt adjacentes a granada e biotita, além de plagioclásio e quartzo com formas cuspade a lobulados que ocorrem entre os grãos da matriz. Ambas gerações de granada (Grt1 e Grt2) não apresentam zoneamento em termos de elementos maiores e contêm pequenas inclusões arredondadas de biotita rica em Ti. Além disso, os cristais Grt2 também contêm inclusões de remanescentes agulhas de sillimanita. Evidências microestruturais, em combinação com a química mineral, indicam que os cristais de granada cresceram durante dois eventos metamórficos/anatéticos distintos, como produtos peritéticos de reações de fusão parcial em regime de fluido ausente que consumiram biotita, quartzo e plagioclásio, no caso de Grt1, e que consumiram biotita, quartzo, plagioclásio e silimanita, no caso de Grt2. Pseudoseções calculadas via Theriak-Domino forneceram novas condições metamórficas para a região do retro arco do OA. Dados da assembleia mineral 1 sugerem que o primeiro evento metamórfico de fácies granulito (M1) ocorreu a temperaturas de 790-820 ºC e pressões de 9.5-10.5 kbar, enquanto a assembleia mineral 2 registra condições PT para um segundo metamorfismo de fácies granulito (M2) a aproximadamente 770 ºC e 6.6 kbar. Dados do geotermômetro Ti em zircão são consistentes com modelamento metamórfico, indicando que os dois eventos metamórficos no batólito atingiram condições de fácies granulito. Variadas idades foram encontradas em grãos de monazita e zircão (> 825 Ma a 490 Ma) das rochas do BCC, registrando uma complexa história de reciclagem crustal e herança, cristalização magmática e anatexia durante a evolução do OA. Os zircões magmáticos (~ 582 Ma) mostram composição isotópica de Hf e padrões de ETR similar aos dos núcleos herdados (825-600 Ma), indicando que a assinatura química destes cristais provavelmente foi herdada da fonte. Idades U-Pb e razões iniciais 176Hf/177Hf dos domínios de zircões anatéticos e/ou metamórficos são consistentes com dois estágios metamórficos caracterizados por processos de crescimento/sobrecrescimento e recristalização de grãos de zircão durante a orogenia. O episódio metamórfico mais antigo (570-550 Ma) é caracterizado pelo sobrecrescimento anatético em núcleos mais velhos e pelo crescimento de novos cristais de zircão anatético. Ambos os domínios têm elevados valores iniciais de 176Hf/177Hf em comparação com núcleos mais antigos e exibem padrões de ETR típicos de zircão que cresceu simultaneamente com granada peritética (Grt1) através de reações de fusão parcial em regime de fluido ausente. Idades U-Pb obtidas em grãos de monazita e zircão (569-552 Ma) inclusos na Grt1, corroboram a interpretação de que os cristais Grt1 cresceram durante o primeiro episódio anatético. Composição isotópica de Hf e evidências químicas indicam que um segundo episódio anatético (535-500 Ma) é registrado em parte das rochas do batólito (assembleias 2 e 3). Nestas rochas, o crescimento e/ou sobrecrescimento de novos zircões anatéticos é marcado por elevada razão inicial de 176Hf/177Hf e também pela geração da Grt2, como indicado pelas observações petrográficas e padrões de ETR. Por outro lado, algumas rochas têm grãos de zircão formados por processos de recristalização no estado sólido de zircão pré-existente, que exibem composição isotópica de Hf similar àqueles domínios magmáticos/herdados. O evento M1 é associado ao espessamento crustal e anatexia no batólito. O evento M2 pode ser associado a ascensão da astenosfera durante o colapso gravitacional do orógeno. Isto produziu anatexia em partes do BCC que tinham sido refertilizado ao longo de zonas de cisalhamento após o primeiro evento de fácies granulito.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101122
This item appears in the following collections: