DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Item Water-present eclogite melting – the effects of the phengite and water in the partial melting of basaltic rocks in earth’s early crust development.(2014) Alkmin, Leonardo Azevedo Sá; Lana, Cristiano de Carvalho; Stevens, Gary; Lana, Cristiano de Carvalho; Novo, Tiago Amâncio; Nalini Júnior, Hermínio Arias; Stevens, GaryAs suítes tonalíticas-trondjemíticas-granodioríticas (TTGs) de granidóides eram dominantes durante a formação da crosta oceânica félsica no Éon Arqueano. TTGs são granitóides caracterizados por alto teor de sílica (tipicamente superior a 68 wt.% SiO2) e sódio, com baixos valores das razões potássio-sódio (Na2O ~4.65 wt.%; K2O/ Na2O ~0.4), baixas razões La/Yb (~48) e baixos valores de concentração de Yb (~0.6 ppm). Laurie e Stevens (2012) demonstraram que magmas TTG com composição similar à média dos trondhjemitos arqueanos podem ser produzidos através da fusão parcial de rochas basálticas saturadas em água, sob condições de pressão e temperatura (PT) consistentes com a subducção arquena em fácies eclogíticas. O Barberton Granite Greesntone Terrane (BGGT), localizado no oeste da África do Sul e leste de Swazilândia é uma das províncias crustais mais antigas preservadas na Terra e provê um laboratório ideal para os estudos da evolução planetária nos seus estágios mais precoces da formação de crosta continental. Esse terreno é formado por rochas supracrustais associadas a três eventos de magmatismo TTG datados em ca 3.55, 3.42 e 3.23 Ga. Existe ainda um estágio de magmatismo potássico tardio de afinidade granítica-monazítica-sienítica (GMS) , originado a partir da fusão parcial dos plutons TTG. Esse estágio potássico não está relacionado, entretanto, com alguns seixos e rochas vulcânicas documentadas em Sanchez-Garrido et al. (2011) e Diegaardt (2013). Tratam-se de clastos peraluminosos de granitóides que ocorrem na forma de conglomerados e vulcanismo félsico no greenstone belt e possuem idades 3.550±45 Ma, 3.460±34 Ma e 3.270±36 Ma. Eles se diferem de típicos granitóides tipo-S da suíte GMS por seu caráter pouco sódico (0.30 wt.% em média) e concentrações razoáveis de Sr (12 – 330 ppm). Para esses granitos foi proposta uma gênese através da anatexia de rochas sedimentares subductadas junto com a crosta oceânica e isso explicaria a produção contemporânea desses granitóides peraluminosos e dos magmas TTG. Esse estudo investigou experimentalmente a possibilidade de magmas TTG e granitóides potássicos serem produzidos a partir da mesma fonte como consequência da fusão parcial em elevadas pressões. Os experimentos utilizaram-se de composições propostas por Smithies et al. (2009) para a composição média da crosta oceânica arqueana, em conjunto com condições de PT consistentes com as estruturas termais das zonas de subducção arqueanas propostas por Laurie et al. (2012). Os experimentos conduzidos demonstraram que em temperaturas relativamente baixas (800°C a 850°C a 2.5 GPa) a fonte máfica produz magmas graníticos peraluminosos com baixo teor de CaO (0.88 wt.% em média).Item Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil : clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon.(2016) Melo, Marilane Gonzaga de; Lana, Cristiano de Carvalho; Stevens, Gary; Soares, Antônio Carlos Pedrosa; Gerdes, Axel; Alkmin, Leonardo Azevedo Sá; Nalini Júnior, Hermínio Arias; Alkmim, Fernando Flecha deThe Carlos Chagas batholith (CCB) is a very large (~14,000 km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U–Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny.Magmatic zircons (ca. 578–588Ma) aremarked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825–600Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U–Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a twostage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the twometamorphic events in the batholith reached granulite facies conditions (N800 °C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570– 550Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of newanatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grewcontemporaneouslywith peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535–500Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after the intrusion of the batholith. This introduced the batholith to a depth in excess of 30 km and produced widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts fromthe CCB that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies event.Item Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen, southeastern Brazil : evidence from the polymetamorphic Carlos Chagas batholith.(2017) Melo, Marilane Gonzaga de; Stevens, Gary; Lana, Cristiano de Carvalho; Soares, Antônio Carlos Pedrosa; Frei, D.; Alkmim, Fernando Flecha de; Alkmin, Leonardo Azevedo SáFrom the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the Araçuaí orogen (SE Brazil) records an outstanding succession of granite production events in space and time. The Carlos Chagas batholith (CCB) is the largest (~14,000 km2) granitic body ascribed to the collisional plutonism (G2 supersuite) in the back-arc region of the Araçuaí orogen, to the east of the Rio Doce magmatic arc. A wide range of monazite and zircon ages (N725Ma to ca. 490Ma) have been found in CCB granites, recording a rich history of crustal recycling and inheritance, magmatic crystallization and anatexis. 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, pseudomorphedmelt pools adjacent to garnet and biotite, and plagioclase and quartzwith 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 in situ 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 °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. Monazite and zircon within garnets from the different assemblages give age peaks at 570–550 Ma (M1) and 535–515 Ma (M2), recording two anatectic events in the CCB during a single orogenic cycle. The PT conditions for these metamorphic events can be related to: i) M1, striking crustal thickening, probably involving thrusting of the magmatic arc onto the back-arc region; and ii) M2, decompression related to the gravitational collapse of the Araçuaí orogen.