EM - Escola de Minas

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6

Notícias

A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.

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Deciphering the source of multiple U–Pb ages and complex Hf isotope composition in zircon from post-collisional charnockite-granite associations from the Araçuaí orogen (southeastern Brazil).
(2020) Melo, Marilane Gonzaga de; Lana, Cristiano de Carvalho; Stevens, Gary; Hartwig, Marcos Eduardo; Pimenta, Marcel Sarcinelli; Nalini Júnior, Hermínio Arias
Giant post-collisional magmatic systems are marked by an intricate diversity of zircon ages and Hf isotopic signatures. Granites and charnockites from such systems are in high contrast with simple isotopic composition of modern granites, which often record no more than one crystallization age. Despite the complexity, the assortment in ages and isotopic compositions can be explored to extract vital information to understand magma histories, isotope fractionation and magma sources. Here we focused on a charnockite-granite association (Barra do Sao ˜ Francisco pluton - BSFP) formed during the post-collisional stage of the Araçuaí orogen (southeastern Brazil). The chemical composition of these rocks points towards a metaluminous to slightly peraluminous character, ferroan and calc-alkalic to alkalic signature. Zircons extracted from the BSFP show a large scatter in the U–Pb age distributions (from 614 to 498 Ma), indicating a complex history of magmatic processes during the wanning stages of the orogeny. Detailed discrimination analysis based on texture, chemistry and U–Pb data established well-defined age groups (groups I to III); each of which reveales a significant part of this post-collisional magmatic evolution (ca. 614-552 Ma, ca. 528-513 Ma and ca. 510-498 Ma). Zircon xenocryst cores of Group I have a broad age distribution (ca. 614-552 Ma) that mirror the age distribution within the magmatic and inherited zircon populations from the country rock (Carlos Chagas batholith). The other two age groups are present in all samples and are interpreted here as antecrysts (Group II: ca. 528-513 Ma) and autocrysts (Group III: ca. 510-498 Ma). All charnockite samples show negative εHf(t) (from − 7.4 to − 0.5) and Calymmian to Ectasian model ages (TDM = 1.58-1.21 Ga), indicating contribution from heterogenous sources. The granites exhibit more negative εHf(t) values (from − 9.3 to − 5.9) and largely older model ages (TDM = 1.68-1.49 Ga), indicating that the charnockite and granite magmas were derived from the different crustal sources. These Hf isotopes data are a robust evidence of different source rocks for these magmas with different water activities. The age of the zircon autocrysts and antecrysts can be interpreted as an evidence for a prolonged pluton assemblage probably combined with slow magma cooling during the post-collisional stage of the Araçuaí orogen.
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 de
The 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.
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 Sá
From 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.
Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt, Brazil : evidence for crystallization in the lower crust.
(2017) Narduzzi, Francesco; Farina, Federico; Stevens, Gary; Lana, Cristiano de Carvalho; Nalini Júnior, Hermínio Arias
Magmatic garnet, together with epidote, is a rare mineral association in cordilleran-I-type granitoids and of special petrogenetic significance. The metaluminous to slightly peraluminous (ASI = 0.97–1.07) Galiléia batholith (Brazil) is a large (ca. 30,000 km2), Neoproterozoic (ca. 632–570 Ma) weakly foliated calc-alkaline granitoid body, characterized by the widespread occurrence of garnet (grossular 25–43 mol%) and epidote (pistacite 9.3–22.7 mol%). Field, petrographic and mineral chemical evidence indicates that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia granitoids and other garnet-free cordilleran-type granitoids worldwide. This evidence strongly suggests that the origin of the uncommon garnet+epidote parageneses is related to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison between the mineral assemblages andmineral compositions fromthis study and those recorded in crystallization experiments on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous volcanic rocks and granitoids, indicates that the supersolidus coexistence of grossular-rich garnet, epidote and white mica is consistentwith magma crystallization at pressures greater than 0.8 GPa (above 25 km depth) and at temperatures below 700 °C, i.e. near the water saturated solidus. Furthermore, resorption textures around garnet (plagioclase ± quartz coronas) and epidote suggest that these minerals have been partially consumed prior to complete crystallization. These findings demonstrate that at 630 Ma the crust underneath the Araçuaí Orogen was already at least 25–30 km thick and relatively cool. However, this contrasts with the marked high heat flow registered from the neighbour Carlos Chagas Batholith located 50 km to the east. In fact such granitoids record granulite-facies metamorphism at the same pressure and time (ca. 570 Ma) of Galiléia granitoids crystallization. Thus, a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen. Eventually, field, petrographic and mineral chemical analogies with similar garnet-bearing granitoids located in the fore-arc settings of the British Columbia subduction zone, possibly imply that the Galiléia granitoids represent “rare” garnet- and epidote-bearing metaluminous Cordilleran-Itype granites which can only form in a fore-arc setting.
Sedimentation, metamorphism and granite generation in a back-arcregion : records from the Ediacaran Nova Venécia Complex, Araçuaí Orogen, Southeastern Brazil.
(2016) Richter, Fabiana; Lana, Cristiano de Carvalho; Stevens, Gary; Buick, Ian S.; Soares, Antônio Carlos Pedrosa; Alkmim, Fernando Flecha de; Cutts, Kathryn Ann
The Nova Venécia migmatite-granulite-granite Complex (NVC) in the core of the Arac¸ uaí Orogen (AO,630–480 Ma), southeast Brazil, exposes a mid-crustal section with abundant evidence for high-grademetamorphism linked to production, extraction and emplacement of peraluminous melts. Although theAO represents the textbook example of a confined orogen, there is surprisingly a lack of detailed studieson its metamorphic evolution related to widespread granitogenesis occurring from the Neoproterozoic toearly Paleozoic. In this study, we combine U–Pb geochronology and metamorphic petrology to constrainthe evolution of the NVC migmatitic metasedimentary granulites, from deposition to high-grade meta-morphism, and to correlate the metamorphic history of the terrain with the several episodes of granitemagmatism (G1–G5) in the AO. The sedimentation of the NVC can be bracketed within a maximum 13 Myperiod, between its maximum depositional age at ca. 606 Ma and the intrusion of early syn-collisionalgranitoids at 593 Ma. Compilation of available U–Pb data shows that the bulk of the magmatic rocks in theAO (G1 + G2 rocks) crystallized contemporaneously over a period of 15 My (ca. 595–570 Ma) with a peakat ca. 575 Ma. Although it is inferred a protracted period of crustal heating in the AO (from ca. 640–480),U–Pb ages of metamorphic and magmatic zircons and monazites suggest at least two major heat pulsesat ca. 593–560 and 523–495 Ma. The timing of peak regional metamorphism is constrained from 575 to560 Ma, which temporally overlaps with the crystallization of the youngest granitoids. Phase equilibriummodeling of metasedimentary granulites from three different localities within the NVC, indicates thatall areas record similar peak P–T conditions of 750–850◦C and 5300–7500 bar. This is followed by hightemperature retrograde evolution to 640–800◦C and 4500–6000 bar. A post-collisional thermal eventlinked to the intrusion of large norite bodies (520–480 Ma) is recorded in our metagreywackes (monaziteU–Pb) and in granites (monazite and zircon U–Pb) from 523 to 495 Ma.
Zircon geochronology and Hf isotopes of the Dwalile Supracrustal Suite, Ancient Gneiss Complex, Swaziland : insights into the diversity of Palaeoarchaean source rocks, depositional and metamorphic ages.
(2017) Schijndel, Valby van; Stevens, Gary; Zeh, Armin; Lana, Cristiano de Carvalho
We report the results of combined U-Pb and Lu-Hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano-sedimentary rocks of the Dwalile Supracrustal Suite (DSS) from the ca. 3.66 to 3.20 Ga Ancient Gneiss Complex (AGC) in Swaziland. The results indicate that the DSS is made up by Palaeo- and Mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the Neoarchaean. The oldest Dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca. 3.46 Ga, and are derived from the same magma source as the Tsawela Gneisses. Based on the combined U-Pb-Hf isotope data, two groups of metasedimentary rocks can be distinguished: an older Dwalile Group I that is similar in age to the Hooggenoeg and Kromberg Formations (3.46–3.40 Ga), but with a different source area. A younger Dwalile Group II is coeval with the Fig Tree and possibly Moodies Groups (<3.23 Ga) and the data suggest there was a similar provenance. The U-Pb-Hf isotope data and petrological observations demonstrate that the DSS was affected by three stages of metamorphic overprint: an event at 3.23 Ga, pervasive amphibolite-facies metamorphism at 3.15 Ga, and low-P granulite-facies metamorphism at 2.99 Ga. Metamorphic zircon in the amphibolitefacies rocks resulted from new growth in aqueous fluids at 3.15 Ga, whereas in the granulite-facies rocks new zircon was formed in the presence of melt. The 3.23 Ga event coincides with the last significant metamorphic overprint documented in the Barberton Greenstone Belt (BGB), whereas as two younger events are restricted to the AGC. The data support the model suggesting amalgamation between the AGC and BGB terranes at ca. 3.23 Ga. In addition, they require crustal thickening in the AGC terrane at ca. 3.15 Ga, causing Barrovian-type metamorphism in the DSS and granulite-facies metamorphism in the Luboya-Kubuta Terrane. Low-P granulite-facies metamorphism at ca. 2.99 Ga is most likely related to crustal extension during the onset of Pongola Basin rifting, causing mantle upwelling, mafic magmatism, and heat transfer from the mantle into the thinned crust.
Age and field relationships of Mahamba orthogneisses and Mkhondo Valley metamorphic suite paragneisses from the Mkhondo river, ancient gneiss complex, Swaziland.
(2013) Taylor, J.; Stevens, Gary; Lana, Cristiano de Carvalho
This study documents for the first time the ages and structural field relationships of banded orthogneisses, and associated highgrade paragneisses, from the Mkhondo River in the Ancient Gneiss Complex of Swaziland, whose age and regional context has been disputed. LA-ICP-MS U-Pb zircon dating shows that the orthogneiss unit mapped as Mahamba gneiss is composed of a 3215 ± 12 Ma granodiorite phase that was intruded by a 3162 ± 11 Ma felsic granitic phase. A hornblende-bearing tonalite dated at 2889 ± 13 Ma, which occurs structurally beneath the Mkhondo Valley Metamorphic Suite (MVMS) metasediments, experienced a metamorphic overprint at 2735 ± 11 Ma that coincided with granulite facies metamorphism and partial melting in the MVMS (Taylor et al., 2010). Both the MVMS and the Mahamba gneisses are characterised by north-northwest aligned, syn-peakmetamorphic deformation structures, indicating a period of west-southwest to east-northeast directed crustal shortening. Neither the Mahamba gneisses, nor the hornblende tonalite are intrusive into the structurally overlying MVMS sediments. This suggests that: (1) the Mahamba gneisses form part of the exposed basement in the area; and (2) the MVMS is a relatively young, late Archaean sedimentary succession that is unrelated to ≥ 3.23 Ga high-grade paragneiss units documented to the east and northeast of the MVMS.
3230-3200 Ma post-orogenic extension and mid-crustal magmatism along the southeastern margin of the Barberton Greenstone Belt, South Africa.
(2011) Lana, Cristiano de Carvalho; Buick, Ian S.; Stevens, Gary; Rossouw, Riana; Wet, Willem de
The Barberton Granitoid-Greenstone Terrain (SouthAfrica) preserves a complex and protracted evolution involving several events of magmatism and terrain accretion along convergent tectonic boundaries. Recent studies propose that the main period of tectonic accretion and arc-related magmatism is linked to a system of divergent subduction zones above which voluminous TTG magmas were emplaced between ca. 3236 and 3227 Ma. Our structural and LA-ICP-MS U–Pb geochronology study along the southeasternmargin of the BarbertonGreenstoneBelt (BGB) ties the waning stages of this TTG magmatism to a short (ca. <30 Ma) period of mid-crustalextension, between 3228 and 3205 Ma. We document a major NE-trending detachment that juxtaposed upper sequences (Moodies Group clastic sediments) against mid-crustal 3418 ± 10 Ma amphibolite-facies rocks of the base of the greenstonebelt (Onverwacht Group rocks). Several granodiorite bodies - intruded along this detachment - contain well-preserved (syn- and post-magmatic) fabrics that are demonstrably related to extensional shearing and exhumation. Field observations and U–Pb zircon data from these granitoids are consistent with the deformation taking place at 3228 ± 10 Ma – contemporaneous with the voluminous (3236–3227 Ma) TTG magmatism in the northwestern margin of the BGB. The timing of the granodiorite emplacement also constrains a minimum age for the deposition of the Moodies Group clastic sediments, which for much of the southern and southeastern parts of the BGB must have happened before ca. 3228 Ma. 3205 ± 9 Ma subvolcanic dykes intruded into the granodiorite complex indicate that the period of exhumation and cooling of the crystalline rocks along the extensional detachment was relatively short (<30 Ma), between 3228 and 3205 Ma. Our observations combined with previously published structural data from the northwestern and southern margin of the belt suggest that the main mechanism of large-scale infolding of the supracrustal strata was shortly followed by the extension-related magmatism and subsequent, solid-state diapiric movement of the arc-related plutons.
Stabilization of the southern portion of the São Francisco craton, SE Brazil, through a long-lived period of potassic magmatism.
(2012) Romano, Rafael Cotta; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Stevens, Gary; Armstrong, Richard
Although the southernportion of the SãoFranciscocraton in southeast Brazil encompasses one of the largest segments of Meso- to Neoarchaean crust in the South American plate, there is little understanding of the processes leading to cratonization of this region. Our combined field and U-Pb age data show that the crust experienced massive intrusions of potassic (K-rich) granitoids between 2750 and 2600 Ma, and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain, with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions. Thus, deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the SãoFrancisco crust attained buoyancy to survive recycling. Regional mapping indicates that the potassic granitoids are distributed over an area of ∼25000 km2, and are related to one of the most prolific periods of potassicmagmatism in the craton. LA-ICP-MS and SHRIMP U-Pb data from 16 samples of potassic granitoids together with published TIMS ages suggest alonglivedperiod of magmatism from 2750 to 2700 Ma, and a small pulse of granite crystallisation at ca. 2612 Ma. The U-Pb SHRIMP and LA-ICP-MS data also indicate that the Archaean granitoids do not record evidence of U-Pb zircon resetting. Our interpretation is that partial melting and transport of granitoid melts (rich in heat producing elements) to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment (in the craton area) became sufficiently refractory and resistant to further partial melting.

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