Letteratura scientifica selezionata sul tema "Sm-Nd isotope composition"

Research subject. AGV-2 and BHVO-2 geochemical reference materials for studying the Sm, Nd and Rb, Sr isotope systems along with various rocks and mineral samples. Materials and Methods. An analysis of the isotopic composition of Sm, Nd and Rb, Sr was carried out using Neptune Plus and Triton Plus mass spectrometers. Aim. To implement analytical techniques for the isotopic composition of Sm and Nd, Rb, and Sr in various rock and mineral samples using two types of multicollector mass spectrometers – inductively coupled plasma NeptunePlus and thermal ionization TritonPlus (Thermo-Fisher), as well as a description of the procedure of processing experimental data and the experience in using techniques at the “Geoanalitik” shared research facilities of the IGG UB RAS for the period 2015-2023. Results. The analytical techniques implemented included (1) column chromatography using various ion-exchange resins, optimised for the ratio of labour costs/quality of analytical results; (2) the measurement of isotope ratios using two types of mass spectrometers; (3) the correction of mass bias of isotope ratios and the determination of Sm and Nd, Rb, and Sr concentrations by the isotope dilution method using 149Sm+150Nd and 85Rb+84Sr spikes. Testing of the techniques was carried out using the AGV-2 and BHVO-2 geochemical reference materials; their metrological characteristics were presented. When using TritonPlus, the reproducibility (BHVO-2, n=60) of measurements of 143Nd/144Nd, 147Sm/144Nd isotope ratios and Sm and Nd concentrations are ±0.000020, ±0.0004, ±1.3 and ±0.4, respectively; indicators of correctness of determining the ratios 143Nd/144Nd and 147Sm/144Nd – 0.001 and 0.25% and concentrations of Sm and Nd – 2%; the reproducibility (BHVO-2, n=63) of measurements of 87Sr/86Sr, 87Rb/86Sr isotope ratios and concentrations (Rb and Sr) are ±0.0025%, ±1.5%, ±2%, respectively. The uncertainty of a single measurement of the 143Nd/144Nd and 87Sr/86Sr isotope ratio, represented by the standard error of the average single measurement in the sample, does not exceed 0.0025%. Conclusions: the results obtained for the geochemical reference materials are in satisfactory agreement with those provided in the GeoReM database, as well as with the certified values provided by the United States Geological Survey (USGS). The described analytical techniques are used at the “Geoanalitik” shared research facilities of the IGG UB RAS to analyse various rock and mineral samples. The work presents a number of experimental results obtained, which are subsequently used in geochronological applications.

Research subject. The Sm/Nd isotope system was investigated using inter-laboratory natural samples of apatite, titanite, allanite, monazite, as well as intra-laboratory samples of apatite (from carbonatites, Ilmenogorsk massif, Ural), monazite (from pegmatites of the Aduy granite massif and its framing, Middle Urals) and titanite (from calcite veins, Saranov skoye chromite deposit, Middle Urals and from alkaline pegmatite, Shpat mine, Vishnevy mountains, South Urals). The Sr isotope system was investigated using inter-laboratory natural apatite samples and intra-laboratory apatite samples (from the apatite-carbonate vein, Slyudyanogorskoe deposit, Irkutsk region and from carbonatites, Ilmenogorsk massif, Ural).Methods. The research was carried using a Neptune Plus multicollector mass spectrometer with inductively coupled plasma (ThermoFisher) equipped with an NWR 213 (ESI) laser ablation attachment, located in a room of ISO class 7 at the “Geoanalyst” Center for Collective Use (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). Results. The article describes methodological approaches for studying Sm/Nd and Sr isotope systems in natural phosphate and silicate minerals by inductively coupled plasma mass spectrometry with laser ablation, implemented on the equipment of the Center for Collective Use “Geoanalyst” (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). A comparative analysis of the obtained results with those reported in literature showed their satisfactory agreement. The developed analytical approaches were used to study apatite samples (analysis of the Sr isotope system) and those of apatite, monazite, titanite (analysis of the Sr isotope system). Conclusions. The developed approaches to the analysis of Sm/Nd and Sr isotopic systems can be recommended for investigating such minerals, as apatite, titanite, allanite, monazite (analysis of the Sm/Nd isotope system); apatite (analysis of the Sr isotope system). The achieved analysis errors allow the results to be used for interpreting various geochemical processes.

Abstract The sulfide minerals from old mafic intrusion rocks from the Kola Peninsula were analyzed on stable sulfur isotopes. These samples were already dated by the Sm-Nd method. These sulfide samples were evaluated upon a geochemical composition by ICP-MS. The sulfide mineral samples were selected from the main ore-bearing rocks of the Monchetundra layered intrusion. The analyzed sulfides formed several generations of mineralization associated with primary and hydrothermal stage of formation of the deposits. Isotopic studies confirm a few consecutive stages of mineralization. These data were compared with the results of Sm-Nd dating of sulfide mineralization. The results of geochemical and geochronological studies indicate a complementarity in the context of determining the mineralization stages.

The Vorontsovskoye gold-ore deposit (Northern Urals) generally corresponds to the Carlin-type. One of the deposit features is the presence of numerous dykes of mafic and middle chemical composition, of normal and increased alkalinity. Sm-Nd isotope studies for the first time confirmed the dykes formation age 340 ± 35 Ma. These data can be indicative of the post-collisional stage of magmatism in this area. The positive Nd values suggest a probable abyssal magmatic source of Nd.

AbstractThe Doba gabbro was collected from an exploration well through the Cretaceous Doba basin of southern Chad. The gabbro is composed mostly of plagioclase, clinopyroxene and Fe–Ti oxide minerals and displays cumulus mineral textures. Whole-rock40Ar–39Ar step-heating geochronology yielded a Late Permian plateau age of 257 ± 1 Ma. The major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios (i.e. La/YbN> 7; Sm/YbPM> 3.4; Nb/Y > 1; Zr/Y > 5) indicative of a basaltic melt derived from a garnet-bearing mantle source. The moderately enriched Sr–Nd isotopes (i.e. ISr= 0.70495 to 0.70839; ɛNd(T)= −1.0 to −1.3) fall within the mantle array (i.e. OIB-like) and are similar to other Late Permian plutonic rocks of North-Central Africa (i.e. ISr= 0.7040 to 0.7070). The enriched isotopic composition of the Doba gabbro contrasts with the more depleted compositions of the spatially associated Neoproterozoic post-Pan-African within-plate granites. The contrasting Nd isotope composition between the older within-plate granites and the younger Doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the Saharan Metacraton.

Abstract —We consider the isotope-geochemical features of epithermal fluorite deposits in Transbaikalia, including the REE compositions, Sr isotope ratios, Sm–Nd systems, and isotope compositions of oxygen, carbon, hydrogen, and sulfur. The 87Sr/86Sr ratios in fluorites are within 0.706–0.708, and the εNd values are negative. Oxygen in quartz, the main mineral of the deposits, has a light isotope composition (δ18O = –3.4 to +2.6‰), and the calculated isotope composition of oxygen in the fluid in equilibrium with quartz (δ18O = –9 to –16‰) indicates the presence of meteoric water. The latter is confirmed by analysis of the isotope compositions of oxygen and hydrogen in gas–liquid inclusions in fluorites from three deposits. These isotope compositions are due to recycling caused by the impact of shallow basic plutons. The isotope composition of sulfur indicates its deep source. During ascent, sulfur became enriched in its light isotope (δ34S = –1.8 to –7.7‰). We assess the association of fluorite ores with basaltoids widespread in the study area. The isotope and geochemical parameters suggest their spatial proximity. Probably, the basaltoids were responsible for the recycling of meteoric water. It is shown that the epithermal fluorite deposits formed by the same mechanism as fissure–vein thermal waters in western Transbaikalia.

One of the current directions of the Sm-Nd isotope systematics development is a dating of the ore process using sulfide minerals. Yet, the issue of the existence of rare earth elements (REE) in sulfides is still a matter for discussion. Sulfides from ore-bearing rocks of Proterozoic (2.53–1.98 Ga) Cu-Ni and platinum group elements (PGE) deposits of the Fennoscandian Shield were studied. It is found that the most probable source of REE in sulfide minerals from Cu-Ni-PGE complexes could be submicronic fluid inclusions, which are trapped at the mineral crystallization stage. In such a case, fluid or melt inclusions are specimens of the syngenetic parental melt, from which the base mineral formed, and these reflect a composition of the parental fluid. The mineral–rock partition coefficients for Nd and Sm can be used as “fingerprints” for individual deposits, and these are isotope-geochemical indicators of the ore-caused fluid that is syngenetic to sulfide. Moreover, the DNd/DSm ratio for various sulfide minerals can be used as a prospective geochemical tool for reconstructing a mineral formation sequence in ore complexes. On the other hand, differences in isotope compositions of sulfide neodymium could be markers of some ore-caused fluids and related to certain generations of sulfide minerals.

The neodymium isotope composition of micritic limestones from the Devonian–Carboniferous carbonate platform of the Greater Karatau (southern Kazakhstan) was investigated to test the ability of calcite micrite to archive Nd isotope signatures of seawater. The carbonate fraction that displays seawater-like rare earth element (REE + Y) signatures is often more radiogenic than the dispersed terrigenous material in the samples. Hence, its Nd isotope composition is interpreted to correspond to the seawater from which the micrite was precipitated. The seawater on the Karatau platform exhibited an extremely wide range of εNd(t) values from –9.3 to +4.3 (the most radiogenic value measured for past seawater to date) and very uniform Sm/Nd ratios, from 0.19 to 0.22, lying within the range characteristic for modern oceanic water. The temporal trend in εNd(t) values is interpreted to document the final closure of the Uralian–Turkestan Ocean. It shows that the subduction along Kazakhstan's active margin had already started at the beginning of the Tournaisian (c. 355 Ma), at least 23 Myr earlier than previously thought. The application of Nd isotope time series on biostratigraphically dated carbonates opens a new direction for geotectonic studies. This approach has the potential to provide useful constraints for the precise dating of the duration of geotectonic and volcanic events.Supplementary material: Nd isotope and REE concentration data, summary of stratigraphic and lithological data, field photographs and additional geochemical plots are available at: https://doi.org/10.6084/m9.figshare.c.5110163

Abstract We report results of geological, mineralogical-petrographic, geochemical, isotope-geochemical (Sm–Nd, Rb–Sr), and geochronological (U–Pb, 40Ar/39Ar) studies of acid and intermediate intrusive rocks (granodiorites, leucocratic granites, subalkaline granites, and subalkaline leucocratic granites, diorites, and quartz diorites) of the Bukeschen and Samyr small plutons in the western part of the Yana–Kolyma gold belt (northeast Asia). These rocks are combined with Late Jurassic (151–145 Ma) dikes of basic, intermediate, and acid compositions into a single complex of small intrusions. They intrude the Upper Triassic–Middle Jurassic terrigenous deposits of continental margin blocks in the eastern part of the Verkhoyansk–Kolyma folded area. Our new U–Pb data for zircon (SHRIMP-II) indicate that the Bukeschen and Samyr pluton granitoids formed in the Berriasian, at 144.5 and 143 Ma, respectively. The small-intrusion granitoids have geochemical and isotope (Sm–Nd and Rb–Sr) characteristics similar to those of Late Jurassic dikes of varying composition. Therefore, they can be united into a single complex of small intrusions generated from a mixed source with the participation of mantle (OIB- and E-MORB type), lower crust, and subduction components and with Paleoproterozoic–Mesoproterozoic Sm–Nd model age estimates for the magma sources. Late Jurassic–Early Cretaceous magmatic and postmagmatic events and cooling of the intrusions played an important role in the processes of gold localization in the western part of the Yana–Kolyma gold belt. This is reflected in two tectonothermal stages (accounting for closing temperatures of the U–Pb, 40Ar/39Ar, and Re–Os isotope systems for different minerals) estimated at 151–141 and 138–137 Ma. These results for the small-intrusion complex agree with the tectonic model of the evolution of an active continental margin (northeastern Siberia) in the Mesozoic era, whose final development stage in the Berriasian age saw the formation of mostly small granitoid plutons.

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第21186号
人博第858号
新制||人||204(附属図書館)
29||人博||858(吉田南総合図書館)
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 小木曽 哲, 教授 石川 尚人, 教授 酒井 敏, 准教授 飯塚 毅
学位規則第4条第1項該当

Le Crétacé est caractérisé par d'importants changements paléogéographiques, géodynamiques et climatiques, incluant notamment la formation de nouveaux océans dont l'Océan Atlantique équatorial, la fragmentation continue du Gondwana et des périodes à effet de serre. Tous ces changements sont potentiellement contenus dans les enregistrements sédimentaires des bassins. Le système Andes-Amazonie-Marge équatoriale est un vaste système sédimentaire dont l'évolution géodynamique et paléoenvironnementale est enregistrée dans les bassins d'avant-pays retroarc amazonien, les bassins intracratoniques brésiliens et ceux de la marge équatoriale. Dans la partie occidentale (région andine), des études récentes suggèrent que le début de l'orogenèse andine s'est produit au cours du Crétacé supérieur, mais le développement des systèmes de drainage et la source des sédiments déposés au cours du Crétacé restent encore peu connus. Le Crétacé inférieur est enregistré dans des bassins brésiliens intracratoniques et équatoriaux et témoigne des phases de rifting crustal suivant la fragmentation du Gondwana. Le bassin d'Araripe est un bon exemple de ces bassins puis qu'il contient un registre sédimentaire complet et particulièrement bien étudié. Cependant, l'origine des sédiments issus des différentes étapes tectoniques de la formation de bassin ainsi que la provenance crétacée des incursions marines l'ayant affecté restent à préciser. Cette thèse vise à proposer une reconstruction paléogéographique du système Andes-Amazonie-Marge équatoriale pendant le Crétacé et à analyser les changements géodynamiques et tectoniques qu'ont affecté. Pour ce faire, nous avons utilisé les méthodes de la provenance et en particulier la géochimie des éléments majeurs et traces, la composition isotopique en Sm et Nd, les âges U-Pb sur zircons. Les résultats obtenus ont permis d'identifier les zones sources du bassin d'Araripe pendent les phases pré, syn et post-rift et ainsi de mieux comprendre la configuration paléogéographique de la région nord-est du Brésil au début du Crétacé. Les sédiments du bassin d'Araripe proviennent principalement des terrains Paléoprotérozoïque (2,3-1,6 Ga) de la province de Borborema et des plutons granitiques du cycle Brasiliano (720-541Ma). Le bassin possède une histoire sédimentaire liée aux phases de rupture de l'Atlantique Sud et équatorial. Pendant la phase pré-rift (~152-135 Ma), le bassin faisait partie de la dépression Afro-Brésilienne et recevait des sédiments en provenance du N-NO. Lors la phase rift (~135-125Ma), il y a eu un changement de source due à rupture continentale et à la formation de l'Atlantique Sud, avec un apport de sédiments en provenance de la partie orientale de la province de Borborema. Pendant la phase post-rift (121-113 Ma), le bassin a cessé d'être alimenté par la partie orientale et les incursions marines ont dominé l'environnement de sédimentation. Enfin, au cours de la phase post-rift II (113-100 Ma) un dernier changement de sources a été observée, avec des sources au NE qui reflètent le moment final de la fragmentation du Gondwana. L'étude de la provenance des sédiments des bassins de l'Amazonie occidentale (Acre et Madre de Dios) indique que les sources sont cratoniques, provenant principalement des terrains Ventuari-Tapajos (2,0 - 1,82 Ga) et Rio Negro - Juruena (1,82 - 1,54 Ga). Ces résultats indiquent qu'un vaste réseau de drainage intracontinental ("Sanozama") qu'a émergé en réponse au soulèvement de l'Arche de Purus et des régions cratoniques du Brésil et des Guyanes pendant l'ouverture de l'océan Atlantique équatorial. L'intégration des résultats obtenue compilée avec les données de la littérature a permis de proposer quatre cartes paléogéographiques de la partie septentrionale de la plate-forme sud-américaine. Ils mettent en évidence comment les mécanismes géodynamiques liées à l'ouverture des océans atlantiques sud et équatoriaux ont contrôlé et façonné la paléogéographie de cette région
The Cretaceous is characterized by major paleogeographic, geodynamic, and climatic changes, including the formation of new oceans such as the equatorial Atlantic Ocean, the continued fragmentation of Gondwana, and greenhouse effect periods. All these changes are potentially contained in the sedimentary records of the basins. The Andes-Amazonia-Equatorial Margin system is a vast sedimentary system whose geodynamic and palaeoenvironmental evolution is recorded in the Amazonian retroarc foreland basins, the Brazilian intracratonic basins, and those of the equatorial margin. In the western part (Andean region), recent studies suggest that the onset of the Andean orogeny occurred during the Late Cretaceous, but the development of drainage systems and the source of sediments deposited during the Cretaceous are still poorly understood. The Lower Cretaceous is recorded in Brazilian intracratonic and equatorial basins and bears witness to crustal rifting phases following Gondwana's break-up. The Araripe Basin is a good example of these basins since it contains a complete and particularly well-studied sedimentary record. However, the origin of the sediments from the various tectonic stages of basin formation and the Cretaceous origin of the marine incursions that affected it remain to be determined. The aim of this thesis is to propose a palaeogeographic reconstruction of the Andes-Amazonia-Equatorial Margin system during the Cretaceous and to analyze the geodynamic and tectonic changes that affected it. To do this, we used provenance methods, in particular major and trace element geochemistry, Sm and Nd isotopic composition, and U-Pb ages on zircons. The results obtained have enabled us to identify the source zones of the Araripe Basin during the pre-, syn- and post-rift phases and thus to gain a better understanding of the palaeogeographic configuration of the north-eastern region of Brazil at the beginning of the Cretaceous. The sediments in the Araripe basin come mainly from the Palaeoproterozoic (2.3-1.6 Ga) terrains of the Borborema province and the granitic plutons of the Brasiliano cycle (720-541Ma). The basin has a sedimentary history linked to the South Atlantic and equatorial break-up phases. During the pre-rift phase (~152-135 Ma), the basin was part of the Afro-Brazilian depression and received sediments from the N-NW. During the rift phase (~135-125Ma), there was a change in source due to continental break-up and the formation of the South Atlantic, with an influx of sediments from the eastern part of the Borborema province. During the post-rift phase (121-113 Ma), the basin ceased to be fed by the eastern part and marine incursions dominated the sedimentation environment. Finally, during the post-rift II phase (113-100 Ma) a last change in sources was observed, with sources to the NE reflecting the final moment of the fragmentation of Gondwana. A study of the provenance of sediments from the basins of western Amazonia (Acre and Madre de Dios) indicates that the sources are cratonic, originating mainly from the Ventuari-Tapajos (2.0 - 1.82 Ga) and Rio Negro - Juruena (1.82 - 1.54 Ga) terrains. These results indicate that a vast intracontinental drainage network ('Sanozama') emerged in response to the uplift of the Purus Arch and the cratonic regions of Brazil and the Guianas during the opening of the equatorial Atlantic Ocean. The integration of the results obtained compiled with data from the literature has made it possible to propose four palaeogeographic maps of the northern part of the South American platform. They highlight how geodynamic mechanisms linked to the opening of the South Atlantic and equatorial oceans controlled and shaped the paleogeography of this region

The Saglek-Hebron Gneiss Complex (SHC) is located in Northern Labrador within the Nain Province. It has recorded multiple magmatic events over more than 1 billion years, making it ideal to study the evolution of mantle-derived rocks through time. Here we present a 147Sm-143Nd isotopic study focussing on the different generations of mantle-derived rocks in the SHC. A total of 83 samples have been analysed, including: 1) mafic metavolcanic rocks; 2) ultramafic rocks divided into two distinct groups (a Fe-rich group enriched in incompatible elements and more depleted ultramafic rocks with lower Fe contents); 3) mafic metamorphosed dikes called the Saglek dikes; and 4) undeformed mafic dikes. Some samples exhibit evidence of post-magmatic geochemical and isotopic disturbance but only the least disturbed samples have been considered to constrain the timing of formation of the different lithologies and the isotopic composition of their mantle source. The mafic metavolcanic rocks combined with the co-genetic low-Fe ultramafic rocks yield an isochron age of 3819 ± 190 Ma (MSWD=34, n=25) with an initial εNd value of +2.3 ± 0.6. The high-Fe enriched ultramafic rocks yield a younger age of 3433 ± 220 Ma (MSWD=10.4, n=10) with an initial εNd= +1.8 ± 0.5. The two generations of mafic dikes appear to have been emplaced in the Mesoarchean and the Neoarchean. The Saglek dikes yield an isochron age of 3565 ±120 Ma (MSWD=1.17, n=10) with an initial εNd value of +1.7 ± 0.1, while the Sm-Nd isochron age for the undeformed mafic dikes is 2694 ±79 Ma (MSWD=3.2, n=21) with an initial εNd value of +1.7 ± 0.1. All generations of mantle-derived rocks yield positive initial εNd values, where only the Eoarchean rocks display an initial Nd isotopic composition similar to the depleted mantle. The Mesoarchean ultramafic rocks, Saglek dikes and Neoarchean mafic dikes display almost identical initial εNd values, despite an age difference of ~800 Ma. This could suggest the contribution of distinct mantle sources or, if all generations of mantle-derived rocks in the SHC were produced from the same mantle source, it implies that this source evolved with a nearly chondritic Sm/Nd ratio for almost the whole Archean Eon. The fact that the initial isotopic compositions of the mantle-derived rocks appear to deviate from the depleted mantle with time, could also suggest an increasing interaction with older evolved crust.

O Quadrilátero Ferrífero (MG) é uma importante unidade geotectônica do Cráton do São Francisco que alberga jazidas de ferro e ouro hospedadas na seqüência greenstone belt Rio das Velhas e cuja evolução geodinâmica esteve regida por vários eventos tectonometamórficos. Depósitos de ouro gerados por fluidos epitermais e remobilizações orogênicas proterozóicas se associam intimamente com camadas bandas de ferro e raramente disseminados em metapelitos no topo do greenstone. Nas mineralizações é comum uma paragênese que cristalizou mediante diversos processos e em distintos estágios com predomínio de sulfetos de ferro. Em geral os minérios possuem forte controle estrutural por dobras, falhas e cisalhamentos transcorrentes, desenvolvidos durante vários eventos deformacionais de diversa índole e estreita relação com processos de alteração hidrotermal como sericitização, carbonatação, cloritização, entre outros de menor incidência. O depósito de ouro Córrego do Sítio hospeda-se na seqüência silisiclástica de origem turbidítica metamorfisada em fácies xisto verde denominada Associação de Litofácies Ressedimentada que faz parte do Grupo Nova Lima, se apresenta disseminado em metapelitos e em veios de quartzo com carbonato. A mineralização que acompanha o ouro cristalizou em vários estágios principais nos que predominaram (i) pirita + pirrotita, (ii) arsenopirita + pirrotita + pirita com texturas finas e orientadas, (iii) arsenopirita com pirrotita e sulfosais em veios de quartzo, (iv) pirita grossa pervasiva em todas as rochas hospedeiras. O estudo da composição isotópica Pb, Sr e Nd, permitiu identificar que as fontes dos fluidos mineralizantes foram as rochas hospedeiras, que na evolução policíclica da mineralização participaram eventos metamórficos e hidrotermais a 2.2, 2.0 Ga. e sobreimpostos a ~600 Ma. os quais induziram os processos de circulação hidrotermal na área de estudo. Mediante comparativa com outros depósitos da região, sugere-se que os metais formadores do minério depositaram-se em Córrego do Sítio à idade de 2.2 Ga. originados a partir da BIF tipo São Bento e remobilizados por fluidos através de falhas inversas. Análises geocronológicas U-Pb (SHRIMP) e K-Ar embora não decisivos em quanto à determinação da idade da mineralização, revelaram a idade de ~2.7 Ga. como o limite temporal para a deposição das rochas hospedeiras. No contexto geológico aplicado se estabelece a potencialidade para a prospecção aurífera que apresentam as rochas metapelíticas do topo do Greenstone Belt Rio das Velas associadas a camadas bandadas de ferro com ouro, e que os alvos mais relevantes seriam zonas com falhas inversas e cisalhamentos transcorrentes.
The Quadrilátero Ferrífero is an important geotectonic unit of the São Francisco Craton that host iron and gold deposits within the vulcanosedimentary rocks of the greenstone belt Rio das Velhas. The geodynamic evolution of this greenstone includes several tectonometamorphic events. Gold deposits were formed by epithermal fluids, remobilized during the Proterozoic orogenic processes, are associated whit the banded iron formations and are sporadically disseminate in metapelitic rocks within the top of the greenstone sequence. In the ores is common the crystallization of iron sulfide minerals during diverse stages. Gold deposits have an important structural control by folds, reverse faults and strike-slip shearing zones, and are commons several hydrothermal alteration processes like sericitization, chloritization and carbonate usually related to the deformation. The Córrego do Sítio gold deposit is hosted in turbiditic siliciclastic rocks that are metamorphosed in the greenschist facies. The gold is associated with the iron sulfide arsenopyrite and pyrite, which are disseminated in metapelitic rocks and quartz-carbonate veins. The mineralization includes vary stages of crystallization, (1) pyrite and pyrrotite (2) arsenopyrite, pyrrotite and fine pyrite, (3) arsenopyrite whit pyrrotite and sulphosalts in quartz vein, and (4) pervasive pyrite. An integrated Pb, Sr e Nd, isotopic study of the Córrego do Sítio deposit allowed to identify that sources of the mineralizing fluids was the host rocks, that in the policiclic evolution the gold mineralizaton had participation events of metamorphism and hidrothermalism to age 2.2, 2.0 and 0.6 Ga. respectively the wich induced the hydrothermal circulation processes in the study area. The isotopic character of this deposit and its comparison with similar ones, suggest that the gold and its metals associated it is deposited in Córrego do Sítio to age 2.2 Ga. and that metals were derived from the banded iron formations, and remobilized by hidrothermal fluids across reverse faults. Although U-Pb (SHRIMP) and K-Ar geochronologic data were not conclusive to constrain the age of the mineralization, they provide to age ~2.7 Ga. that a good limit for the sedimentation of the host rock. It is suggest that there is a potential for gold prospection within the metapellitic rocks associated with banded iron formation of the Greenstone Rio das Velhas, and that the more relevant targets include areas with inverse and strike-slip fault.

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Eclogites from the Usagaran Orogenic Belt of Tanzania have been reliably dated at 2.0 Ga and are the oldest reported subduction-related eclogites within a well-preserved orogenic belt. Based on limited geochemistry from two samples of eclogite from the Usagaran Belt, Möller et al. (1995) concluded that the protoliths were similar to MORB. This study analyzed a larger number of eclogitic samples and a suite of structurally intercalated mafic and pelitic rocks to establish the tectonic setting of the Usagaran Orogenic Belt rocks. Eclogitic rocks from the Usagaran Orogenic Belt display LILE and LREE enrichment relative to present-day MORB. Variations in εNd values from depleted mantel at 2.0 Ga supported this interpretation. The mantle-derived mafic rocks show strong Nb depletions, indicating that they are subduction-related. Enrichment of mafic rocks in LILE and LREE are likely caused by dehydration of the subducting slab with some contamination from crustally derived materials perhaps via subducted sediment. The intercalated pelites are mainly derived from the Tanzanian Craton, with a significant mafic input evidenced by high Cr & Ni values. Based on the geochemical isotopic compositions and field relationships, the eclogites, mafic rocks and pelites all formed in a subduction setting that operated around 2.0 Ga. Despite the fact that the Earth was hotter in its early history, modern plate tectonics, (i. e., subduction of cold oceanic crust into a warm mantle resulting in high-pressure low-temperature metamorphism), occurred and was recorded in the Usagaran Belt during the Palaeoproterozoic. Thus modern-style plate tectonics have operated since at least 2.0 Ga.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2009

ABSTRACT Southeastern New England is largely composed of Ediacaran granitoid and related volcanic rocks formed during the main phase of arc-related magmatism recorded in West Avalonian lithotectonic assemblages extending through Atlantic Canada to eastern Newfoundland. In situ Lu-Hf analyses presented here for zircons from the Dedham, Milford, and Esmond Granites and from the Lynn-Mattapan volcanic complex show a restricted range of εHf values (+2 to +5) and associated Hf-TDM model ages of 1.3–0.9 Ga, assuming felsic crustal sources. The most evolved granites within this suite lie in a belt north and west of the Boston Basin, whereas upfaulted granites on the south, as well as the slightly younger volcanic units, show more juvenile Hf isotopic compositions. Similar inferences have been drawn from previously published Sm-Nd isotopic signatures for several of the same plutons. Collectively, the isotopic compositions and high-precision U-Pb geochronological constraints now available for southeastern New England differ in important respects from patterns in the Mira terrane of Cape Breton Island or the Newfoundland Avalon zone, but they closely resemble those documented in the Cobequid and Antigonish Highlands of mainland Nova Scotia and New Brunswick’s Caledonia terrane. Particularly significant features are similarities between the younger than 912 Ma Westboro Formation in New England and the younger than 945 Ma Gamble Brook Formation in the Cobequid Highlands, both of which yield detrital zircon age spectra consistent with sources on the Timanide margin of Baltica. This relationship provides the starting point for a recent model in which episodic West Avalonian arc magmatism began along the Tonian margin of Baltica and terminated during diachronous late Ediacaran arc-arc collision with the Ganderian margin of Gondwana.

New isotopic analyses (Sm-Nd, O, and U-Pb), targeted geological mapping, and previously published whole-rock geochemical data and high-resolution aeromagnetic surveys define ten crustal domains across the central Thelon tectonic zone. In the eastern Slave Craton, granitoid rocks in the Overby Lake domain are more isotopically evolved than in the Tinney Hills domain and include tonalite dated at 2.71 Ga. The 400 km long main leucogranite belt separates most early (ca. 2.07-1.95 Ga) Thelon tectonic zone plutonic belts from the Queen Maud Block. Oxygen isotopes support its formation via melting of a sedimentary source during peak metamorphism, which coincides with three, new 1.925-1.91 Ga leucogranite ages. Modelling of Nd-Sm isotopes indicates Neoarchean crust as basement to early Thelon tectonic zone plutonic belts. Detrital zircon geochronology suggests a 2.5 Ga basement component that is not recognized in exposed crustal domains, but is compatible with the Dharwar Craton, which can be paleomagnetically reconstructed adjacent to the Slave Craton at 2.2 Ga. Two tectonic models are discussed for the evolution of the Thelon tectonic zone in the convergent margin tectonic setting indicated by the whole-rock geochemistry and mantle-like oxygen isotopic compositions of plutonic rocks. In one model, ca. 2.1 Ga extension precedes east-dipping subduction, which leads to 1.97 Ga collision of the Slave Craton with a composite Thelon tectonic zone basement-Rae Craton, upper plate. The second model proposes a ca. 2.05 Ga Slave-microcontinent (Thelon tectonic zone basement) collision, followed by a polarity flip with west-dipping subduction, leading to ca. 1.95 Ga collision of the Rae Craton.