Browsing by Author "Buick, Ian S."
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Item 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 deThe 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.Item An assessment of monazite from the Itambé pegmatite district for use as U–Pb isotope reference material for microanalysis and implications for the origin of the “Moacyr” monazite.(2016) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Gerdes, Axel; Kamo, Sandra L.; Corfu, Fernando; Marinho, Moacyr Moura; Chaves, Alexandre de Oliveira; Valeriano, Claudio de Morisson; Nalini Júnior, Hermínio AriasLarge quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U–Pb and Sm–Nd isotope reference materials for LA-ICP-MS and to track the origin of a piece of theMoacyrmonazite (termed here Itambé), awidely used reference material for LA-ICP-MS U–Pb geochronology. Monazite fromthe largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraíso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U–Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7 ± 1.3 (207Pb⁎/235U) and 513.6 ± 1.2 Ma (206Pb⁎/238U)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508 Ma. The Bananeiro monazite was assessed as a LA-ICPMS U–Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials.MREE/HREE fractionation (ie, (La/Gd)N and (Gd/Lu)N values), Eu/Eu⁎ and the chondrite-normalized REE patterns suggest that the “Itambé” monazite aliquot is very similar to that fromthe Coqueiro pegmatite. This similarity is likewise apparent in their Sm–Nd isotope compositions. Moreover, the εNdi values of the “Itambé” monazite fragment (εNdi=−4.2) and those fromall the major pegmatites in the district, are distinct fromother reference materials (eg, Managountry; εNdi = −22.3) as well as gem-quality monazite from c. 490–520 Ma pegmatites from the Araçuaí Orogen, further to the south. The εNdi can provide a further distinction for tracing Brazillian gemquality monazite reference materials.Item Assessing the U-Pb, Sm-Nd and Sr-Sr isotopic compositions of the sume apatite as a reference material for LA-ICP-MS analysis.(2022) Lana, Cristiano de Carvalho; Gonçalves, Guilherme de Oliveira; Mazoz, Ariela Oliveira; Buick, Ian S.; Kamo, Sandra L.; Cipriano, Ricardo Augusto Scholz; Wang, Hao; Moreira, Hugo Souza; Babinski, Marly; Queiroga, Gláucia NascimentoThis study has determined the trace element and the U-Pb, Sm-Nd and Sr-Sr isotope composition of the Sume apatite (from NE Brazil) to assess its suitability as a primary/secondary reference material for LA-ICP-MS. Reproducibility tests demonstrate that one batch (~ 100 g) of the Sume apatite (Sum e-570) is uniform in terms of Nd-Nd and U-Pb isotope compositions. Bulk isotope dilution TIMS/ICP-MS and LA-MC-ICP-MS analyses confirm that the apatite is well suited for use as quality control material for Nd isotopes and U-Pb geochronology. U-Pb ID-TIMS analyses yield weighted mean ratios of 0.09211 0.00053 (2s; 206Pb*/238U) and 0.06120 0.00063 (2s; 207Pb*/206Pb*) and a weighted mean 206Pb*/238U date of 568 3 Ma (95% c.l.). U-Pb LA-(SF/MC)-ICP-MS runs using Sume-570 as a primary RM and reproduces the dates of other established RMs within 1% deviation (except for Durango 2–4%). Major and trace element abundances show that Sume-570 is a fluorapatite derived from a syenitic source. It also strongly shows LREE-enriched chondrite-normalised REE patterns with significant negative Eu anomalies, due to crystallisation of plagioclase in the residue.Item Characterization of xenotime from datas (Brazil) as a potential reference material for in situ u-pb geochronology.(2018) Vasconcelos, A. D.; Gonçalves, Guilherme de Oliveira; Lana, Cláudio Eduardo; Buick, Ian S.; Corfu, F.; Cipriano, Ricardo Augusto Scholz; Alkmim, Ana Ramalho; Queiroga, Gláucia Nascimento; Nalini Júnior, Hermínio AriasThis study investigatesfive megacrysts of xenotime (XN01, XN02, XN03, XN04, and XN05) aspotential reference materials (RMs) for U-Pb geochronology. These crystals belong to a 300 g xenotimeassortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser Ablation-InductivelyCoupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses show that the selected crystals are internallyhomogeneous for most rare earth element, (REE, except some light REE) but are relatively heterogeneous forU and Th. The xenotime REE patterns are consistent with an origin from hydrothermal quartz veins in theDatas area that cut greenschist-facies metasediments and that locally contain other accessory phases such asrutile and monazite. High-precision U-Pb Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS)analyses showed slight age heterogeneity for the XN01 crystal not observed in the XN02 sample. The twocrystals have slightly different average206Pb/238U ages of 513.4 ± 0.5 Ma (2 s) and 515.4 ± 0.2 Ma (2 s),respectively. In situ U-Pb isotope data acquired via LA-(Q,SF,MC)-ICP-MS are within the uncertainty of theID-TIMS data, showing homogeneity at the 1% precision of the laser ablation (and probably ion microprobe)technique. U-Pb LA-(MC, SF)-ICP-MS analyses, using XN01 as a primary RM, reproduced the ages of otherestablished RMs within less than 1% deviation. Other Datas crystals (XN03-05) also display a reproducibility inPb/U dates better than 1% on LA-ICP-MS, making them good candidates for further testing by ID-TIMS.Item Geodynamic and metamorphic evolution of the Araçuaí orogen (SE Brazil).(2018) Schannor, Mathias; Lana, Cristiano de Carvalho; Lana, Cristiano de Carvalho; Storey, Craig Darryl; Cipriano, Ricardo Augusto Scholz; Zincone, Stéfano Albino; Buick, Ian S.Ciclos repetidos de amalgamação e quebra de supercontinentes influenciaram profundamente a evolução da crosta, atmosfera, hidrosfera e biosfera da Terra a partir do final do Arqueano. A formação de supercontinentes é caracterizada pela formação de margens convergentes, que por fim resultam em colisão continental. Espessamento crustal e produção de calor devido a acumulação de elementos produtores de calor e/ou aumento da contribuição de calor mantélico em margens convergentes resultam em fusão parcial de litologias pré-existentes e produção de grandes volumes de magma granítico, contribuindo para a diferenciação da crosta terrestre. Este trabalho tem como foco o orógeno Araçuaí no sudeste brasileiro, o qual é um dos diversos cinturões colisionais do ciclo Brasiliano/Pan-Africano, edificados durante a convergência dos proto-continentes Sul-Americano e Africano ao final do Neoproterozóico, resultando na formação do supercontinente Gondwana Ocidental. A colisão entre os crátons São Francisco e Congo resultou em um prolongado magmatismo granítico, agora exposto na porção interna do orógeno Araçuaí. Enquanto este magmatismo é bem caracterizado, a evolução metamórfica do orógeno e a evolução geodinâmica relacionada aos blocos continentais que antecederam a amalgamação do Gondwana Ocidental são menos estudados, apesar da grande exposição de rochas metassedimentares. Um estudo combinado de trabalhos de campo, petrografia, geoquímica de rocha total e análises isotópicas de U–Pb–Hf em zircões detríticos foi realizado em rochas metassedimentares de forma a melhor entender a evolução geodinâmica dos blocos continentais pré-Gondwana Ocidental. Processos de retrabalhamento crustal durante a formação de supercontinentes foram estudados utilizando análises isotópicas U–Pb–Hf em zircões de intrusões graníticas e rochas do embasamento. Além disso, geocronologia U–Pb in situ em granadas e modelamento metamórfico foram aplicados na reconstrução da evolução metamórfica do orógeno Araçuaí de forma a compreender os processos orogenéticos e fontes de calor que produziram fusões parciais durante a colisão continental. Análises isotópicas U–Pb–Hf e dados de elementos traço em zircões detríticos de rochas metassedimentares do orógeno Araçuaí forneceram evidências da quebra do cráton Congo-São Francisco. O espectro de idades U–Pb dos zircões detríticos variou de 900–650 Ma e definiram uma idade máxima de deposição de ca. 650 Ma. Elementos traço dos zircões e química de rocha total indicaram um arco de ilhas como fonte dos protólitos dos metassedimentos. Valores positivos de εHf(t) dos zircões indicam uma fase extensional Neoproterozóica e formação de crosta oceânica em uma bacia precursora ao orógeno Araçuaí. A natureza juvenil, espectro de idades e composição de elementos traço dos zircões detríticos correspondem com zircões de rochas metassedimentares e restos crosta oceânica dos cinturões orogênicos Brasilianos adjacentes. Isto sugere o rifteamento e geração de crosta oceânica em um grande sistema orogênico conectado e de forma contemporânea, provavelmente relacionado à abertura de um grande oceano. Isto também implica que os blocos cratônicos envolvidos nesta evolução orogenética estavam conectados há 900 Ma. Gnaisses migmatíticos Paleoproterozóicos do embasamento do orógeno Araçuaí representam adição de crosta juvenil em um cenário de acreção orogenética entre 2.2–2.0 Ga, evidenciado pelos valores positivos de εHf de seus zircões. A composição isotópica de Hf dos zircões dos granitóides pré-colisionais definem um grande intervalo de valores negativos de εHf, consistente com a mistura ineficiente de fusões derivadas de fontes máficas e félsicas Paleoproterozóicas. Grãos de zircão de intrusões máficas contemporâneas exibem composição isotópica de Hf enriquecida, indicando fusão de anfibolitos Paleoproterozóicos ao invés de adições mantélicas. O magmatismo do orógeno Araçuaí relacionado à amalgamação do Gondwana Ocidental é, desta forma, dominantemente um evento de retrabalhamento crustal. Observações petrográficas, química mineral e modelamento metamórfico restringe o pico metamórfico das rochas metassedimentares não-anatéticas do domínio central do orógeno em 640 ± 20 °C e 8.75 ± 0.75 kbar. Sem precedentes, geocronologia U-Pb in-situ em granadas de baixo-U (<1 μg/g) produziram idades consitentes para o pico metamórfico entre 585–570 Ma. Zircões metamórficos com idades de ca. 630 Ma. são relacionados à um evento de acreção de um terreno não exposto atualmente antes da orogênese, enquanto as idades U–Pb em granada registram o pico metamórfico devido à orogênese, revelado pelo desequilíbrio entre o particionamento de elementos terras raras (ETR) entre os dois minerais. A combinação das condições metamórficas define uma evolução P–T em sentido horário para o orógeno Araçuaí, caracterizado pelo lento soterramento a profundidades de 26–30 km, seguido por descompressão praticamente isotérmica de ~10 a 6 kbar. Diferenças substanciais entre as temperaturas de pico metamórfico nos diversos domínios do orógeno Araçuaí são consistentes com diferentes litologias em diferentes seções crustais, causando uma maior concentração de elementos produtores de calor no domínio anatético, causando fusão parcial das rochas metassedimentares. Isto sugere que os elementos produtores de calor foram uma fonte significativa de calor para o retrabalhamento crustal durante a tectônica convergente do Gondwana Ocidental.Item A LA-ICP-MS and SHRIMP U/PB age constraint on the timing of ree mineralisation associated with bushveld granites.(2011) Buick, Ian S.; Lana, Cristiano de Carvalho; Gregory, C.Monazite occurs in association with fluorite and polymetallic sulphides in brecciated pegmatites emplaced within discordant pipelike structures, and spatially associated mineralised quartz veins that cut hydrothermally altered Bushveld-age Nebo Granite (Lebowa Granite Suite) on the farm Houtenbek 194 JR. Mineralisation of this style has been viewed as having a high-temperature origin related to a late-stage of Bushveld-age granite emplacement. However, early attempts to date Houtenbek monazite in the 1950 to 1960s, using thermal ionisation mass spectrometry, gave a range of very imprecise 207Pb/206Pb ages from isotopically moderately to strongly discordant grains that cannot be used to confidently make this link. Here we present new high spatial resolution SHRIMP and LA-ICP-MS weighted mean 207Pb/206Pb ages of 2059.2 ± 3.5 Ma and 2046 ± 10 Ma for a museum specimen of Houtenbek monazite. These estimates overlap with the 2054 Ma emplacement age of the main phase of the Lebowa Granite Suite. Together with the monazite REE patterns, which indicate a magmatic origin, the new age determinations reinforce the connection between REE mineralisation and Bushveld-age granites.Item LA-ICP-MS U–Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt, SE Brazil.(2020) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Kamo, Sandra L.; Corfu, Fernando; Queiroga, Gláucia NascimentoThe presence of hydrothermal rutile enables the dating of mineralization related to hydrothermal processes. Refinement of the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique is key to enabling the quick analysis of individual rutile crystals in high spatial resolution and dating of crystals with low U contents, as a prerequisite for the reliable characterization of hydrothermal deposits. For this study rutile crystals were selected from quartz and topaz veins that extend along the southeastern portion of the Araçuaí Belt, part of the long-lived (630 - 490 Ma) polyphase Brasiliano Orogen that formed during West Gondwana amalgamation. The purpose was to allow a systematic evaluation of in situ dating of hydrothermal rutile by LA-ICPMS. U–Pb ages from different rutiles range between 527 ± 6 Ma and 487 ± 5 Ma, indicating that different parts of the orogen were affected by episodic fluid flow under oxidizing conditions over a similar time interval. Our U–Pb dating results of rutiles associated with the Imperial Topaz mineralization in the Quadrilátero Ferrífero (southern portion of the São Francisco Craton, SE Brazil) constrains the age of those deposits to 500- 498 Ma, which is within the range of U–Pb ages of hydrothermal systems and mineralized zones previously reported for the region. The chemical composition of the studied hydrothermal rutiles, as determined by electron microprobe and LA-ICP-MS analysis, together with their U–Pb ages, suggests that the extensive fluid production required to form the hydrothermal deposits was generated from multiple sources. Fluid production was related to extensional collapse of the Araçuaí Orogen. The findings of this research clearly shows the utility of U–Pb rutile dating via LA-ICP-MS for geochronological study of a range of hydrothermal deposits.Item LA-ICP-MS U–Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt, SE Brazil.(2020) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Kamo, Sandra L.; Corfu, Fernando; Queiroga, Gláucia NascimentoThe presence of hydrothermal rutile enables the dating of mineralization related to hydrothermal processes. Refinement of the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique is key to enabling the quick analysis of individual rutile crystals in high spatial resolution and dating of crystals with low U contents, as a prerequisite for the reliable characterization of hydrothermal deposits. For this study rutile crystals were selected from quartz and topaz veins that extend along the southeastern portion of the Araçuaí Belt, part of the long-lived (630 - 490 Ma) polyphase Brasiliano Orogen that formed during West Gondwana amalgamation. The purpose was to allow a systematic evaluation of in situ dating of hydrothermal rutile by LA-ICPMS. U–Pb ages from different rutiles range between 527 ± 6 Ma and 487 ± 5 Ma, indicating that different parts of the orogen were affected by episodic fluid flow under oxidizing conditions over a similar time interval. Our U–Pb dating results of rutiles associated with the Imperial Topaz mineralization in the Quadrilátero Ferrífero (southern portion of the São Francisco Craton, SE Brazil) constrains the age of those deposits to 500- 498 Ma, which is within the range of U–Pb ages of hydrothermal systems and mineralized zones previously reported for the region. The chemical composition of the studied hydrothermal rutiles, as determined by electron microprobe and LA-ICP-MS analysis, together with their U–Pb ages, suggests that the extensive fluid production required to form the hydrothermal deposits was generated from multiple sources. Fluid production was related to extensional collapse of the Araçuaí Orogen. The findings of this research clearly shows the utility of U–Pb rutile dating via LA-ICP-MS for geochronological study of a range of hydrothermal deposits.Item A new appraisal of sri lankan bb zircon as a reference material for LA-ICP-MS U-Pb geochronology and Lu-Hf isotope tracing.(2017) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Buick, Ian S.; Schmitz, Mark D.; Kamo, Sandra L.; Gerdes, Axel; Corfu, Fernando; Tapster, Simon; Lancaster, Penelope; Storey, Craig Darryl; Basei, Miguel Ângelo Stipp; Tohver, Eric; Alkmim, Ana Ramalho; Nalini Júnior, Hermínio Arias; Krambrock, Klaus Wilhelm Heinrich; Leite, Cristiano Fantini; Wiedenbeck, MichaelA potential zircon reference material (BB zircon) for laser abla tion-in ductively coupled plasma-mass spectrometry (LA-ICP-M S)U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from SriLanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with somevariation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains,the variation in U content for all analysed crystals ranged from 227 to 368 lgg-1and the average Th/U ratios werebetween 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between thegrains – mean176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 3 1018g-1for a numberof BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the samecrystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurementreproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that arewithin uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of theoxygen isotope systematic of four BB crystals (13.16‰ VSMOW).Item Obtaining and maintaining the distribution of natural reference materials for UPb geochronology by LA-ICP-MS : protocols and future perspectives.(2020) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Santos, Maristella MoreiraItem 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 AnnThe 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.Item The Diamantina monazite : a new Low‐Th reference material for microanalysis.(2018) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Buick, Ian S.; Gerdes, Axel; Kamo, Sandra L.; Corfu, Fernando; Rubatto, Daniela; Wiedenbeck, Michael; Nalini Júnior, Hermínio Arias; Oliveira, Luiz Carlos Alves deMost monazite reference materials (RMs) for in situ U-Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix-matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low-U and low-Th Diamantina monazite from the Espinhac o Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA-(Q, SF, MC)-ICP-MS and SIMS) are within uncertainty of the ID-TIMS data. U-Pb LA-(Q, MC)-ICP-MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and eNd(495 Ma) of -18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina’s relative homogeneity at test portion masses below 1 ng.Item Twenty million years of post-orogenic fluid production and hydrothermal mineralization across the external Araçuaí orogen and adjacent São Francisco craton, SE Brazil.(2019) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Buick, Ian S.; Alkmim, Fernando Flecha de; Cipriano, Ricardo Augusto Scholz; Queiroga, Gláucia NascimentoThe Araçuaí orogen (AO) is a hot orogenic system that is known for its widespread and long-lived magmatism (630–480 Ma).Within the last c. 20 million years of its evolution, the AO saw the widespread development of hydrothermal systems in its lower-grade external fold-and-thrust belt and its Foreland Domain (eastern margin of the adjoining São Francisco craton). U\\Pb crystallization ages for hydrothermal monazite, rutile, and xenotime indicate that fluid circulation along the entire AO occurred between 515 to ca. 495 Ma, largely overlapping with U\\Pb ages of hydrothermal systems and mineralized zones in the adjacent São Francisco craton. Titanium-inquartz thermometry suggests that most hydrothermal systems closed at temperatures of ca. 360 °C. The Sm\\Nd isotope composition of the vein monazite samples ranges from εNd500Ma −16.8 to −17.8, and one less-evolved sample with εNd500Ma of −5.9. The Sm\\Nd isotope compositions overlap with the bulk rock Sm\\Nd data for the Espinhaço Supergroup, suggesting that the hydrothermal veins in this study are mainly related to metamorphic fluids derived from the prograde dehydration of low-grade metasedimentary rocks. The U\\Pb ages, Sm\\Nd isotope composition, and vein mineralogy are all consistent with a 20 My period of channelized fluid flow, characterized by an oxidizing, low pH, REE-, P-, and Ti-bearing aqueous metamorphic fluid. The expression of supercritical fluid flow in the core of the AO is given by the emplacement of large volumes of bare/mineralized pegmatite bodies and regional resetting of monazite ages in host rocks. On a regional context, this widespread fluid flow is associated with a high-T, low-P metamorphism that marks the beginning of the post-collisional decompression of the orogeny, which is associated with lithospheric mantle delamination. The reorganization of mass and heat, due to extensional collapse, resulted in multi-sourced fluid production and mineralization from deep to shallow crustal levels that affected an area of over 400.000 km2 along the eastern border of the São Francisco Craton.