Dissertations / Theses on the topic 'U-Pb monazite geochronology'

La compréhension des processus orogéniques nécessite un couplage toujours plus étroit entre données pétrologiques visant à contraindre le trajet pression-température des roches et les données géochronologiques donnant accès aux âges et durées absolues. Cette thèse vise en premier lieu à étudier le comportement des monazites et des zircons, utilisés en géochronométrie U-Th-Pb dans les granulites de ultra-haute température (UHT) du Rogaland (sud de la Norvège). Nous montrons que la datation ponctuelle in-situ U-Th-Pb, combinée à l'analyse des éléments majeurs et traces contenus dans la monazite permet d'identifier et de quantifier deux incursions à ultra-haute température du Rogaland à 1030-1005 Ma et à 940-930 Ma. En effet, l'examen des relations de phases à l'équilibre entre monazite, xénotime et huttonite a permis de démontrer que les monazites étaient capables de cristalliser et d'enregistrer des températures supérieures à 880 °C. D'autre part, la caractérisation chimique et isotopique U-Th-Pb-O des néocristallisations et surcroissances des zircons permet de définir un intervalle de temps de 60 Ma entre les deux pics de métamorphisme, au cours duquel la croûte moyenne était partiellement fondue à des températures supérieures à 800°C et à basse pression (0.7-0.4 GPa). Ces travaux de thèse soulignent par ailleurs la variété des facteurs susceptibles de conduire à la remise à zéro, partielle ou non, des âges U-Th-Pb dans les monazites et les zircons. Nous montrons que compte tenu de l'histoire T-t du domaine étudié, le système chronométrique U-Th-Pb est largement contrôlé dans le zircon par le degré d'amorphisation de ce dernier lié à son auto-irradiation, alors que dans la monazite, ce sont les processus de dissolution-recristallisation en présence de fluides silicatés ou aqueux qui sont prépondérants. Nous montrons également qualitativement l'influence des conditions d'oxydo-réduction dans l'incorporation du S comme sulfate dans le réseau cristallin de la monazite et par conséquent le potentiel que représente la monazite pour sonder l'état d'oxydo-réduction lié aux différents évènements géologiques, dans les roches métamorphiques. Finalement, nous mettons en évidence une corrélation spatiale et temporelle entre magmatisme mantellique et métamorphisme de ultra-haute température qui ne peut être expliquée avec les modèles actuellement acceptés pour la genèse du métamorphisme de UHT. Ces observations peuvent néanmoins être expliquées en prenant en compte la différence de composition et de température du manteau Protérozoïque comparé à l'actuel, favorisant le développement d'orogènes ultra-chauds et de phénomènes gravitaires
Understanding mountain building processes requires a better integration of petrological and peochronological data in order to link pressure-temperature paths to absolute ages. This work focuses on the behaviour of monazite and zircon, which are used as geochronometers, in ultra-high temperature granulites of Rogaland (South Norway). We show that linking in-situ U-Th-Pb dating of monazite with its major- and trace-element composition lead to the recognition of two ultra-high temperature (UHT) metamorphic events in Rogaland at c.1030-1005 Ma and c. 940-930 Ma. Indeed, the examination of monazite-xenotime-huttonite phase relationships suggests that monazite may record crystallization age at or near ultra-high temperature. Besides, the chemical and U-Th-Pb-O isotopic characterization of zircon neo-crystallization or overgrowths indicates that the Rogaland crust remains molten (> 800 °C at 0.7-0.4 GPa) at least during 60 My between the two identified UHT excursions. This manuscript also highlights the various factors responsible for U-Th-Pb (partial) resetting in the course of granulite facies metamorphism. Zircon behaviour is mostly controlled its level of amorphization, enhancing Pb loss during annealing, whereas monazite resetting is dominated by dissolution-precipitation processes in the presence of a melt or fluid phase. More specifically, we point out that monazite may be used to monitor the redox conditions of its crystallizing medium since monazite may incorporate the redox-sensitive element S in its lattice as sulphate. Finally, we demonstrate a spatial and temporal correlation between magmatism and UHT metamorphism in Rogaland. The timescale, P-T path and tectono-magmatic history however cannot be explained by currently accepted models for UHT. We suggest that physical and thermal specificities of Proterozoic mantle may explain the observed ultra-hot orogen style and the occurrence of gravity driven processes during orogeny

In situ SHRIMP U-Pb geochronology of xenotime and monazite is a powerful method for dating hydrothermal gold mineralization. This method has been applied to the northern Capricorn Orogen of Western Australia, providing a new geological framework that reveals a more complex mineralization history than previously recognised. The study has identified three gold mineralization events (2.4, 1.77 and 1.68 billion years ago) and demonstrates a direct link between fault reactivation and hydrothermal fluid flow during orogeny.

Thesis (M.S.)--Kent State University, 2008.
Title from PDF t.p. (viewed Oct. 5, 2009). Advisor: Peter Dahl. Keywords: Geochronology; Radiometric Dating; Plate Tectonics. Includes bibliographical references (p. 106-109).

Thesis (M.S.)--Kent State University, 2009.
Title from PDF t.p. (viewed Feb. 12, 2010). Advisor: Peter Dahl. Keywords: Black Hills; Crook Mountain granite; Homestake gold mine; gold mineralization; magmatism; metamorphism; metapelite; g monazite; zircon; titanite; geochronology; thermotectonism Includes bibliographical references (p. 97-106).

La transition Archéen – Protérozoïque (~2,45 Ga) est marquée par des bouleversements environnementaux majeurs dans l’histoire de la Terre (oxygénation de l’atmosphère et glaciations globales). Les séquences sédimentaires qui enregistrent ces événements ont été très étudiées en Amérique du Nord (séquence Huronienne) et en Afrique du Sud (Transvaal). Une séquence analogue sur le craton de Pilbara en Australie Occidentale (groupe de Turee Creek, TCG) contient 3 niveaux de diamictites glaciaires. 3 forages continentaux (Turee Creek Drilling Project, TCDP) ont été réalisés pour étudier la séquence. La géochronologie U-Pb de monazite et zircon dans des échantillons de forage et de surface a daté la première glaciation à 2,45 Ga et la seconde à ~2,34 Ga. Des analyses Re-Os de pyrites de la seconde diamictite ont donné une isochrone à 2,31 Ga. Ces résultats ont permis de proposer un scenario de corrélation des glaciations paléoprotérozoïques enregistrées sur différents continents. Une ‘Snowball Earth’ aurait eu lieu à ~2,45 Ga sur au moins 5 cratons (Pilbara, Kaapvaal, Supérieur, Wyoming, Karélia). Une seconde glaciation aurait pu avoir lieu à ~2,31 Ga sur 4 cratons. L’ensemble des zircons détritiques du TCG ont une distribution d’âge avec des pics à 2,45, 2,54, 2,68, 2,82, 2,95 et 3,2 Ga. Un cristal de zircon Hadéen a été découvert et indique la présence d’une probable croûte différenciée dans le craton de Pilbara à 4,0 Ga. 70% des zircons ont des compositions isotopiques en Hf juvéniles, suggérant que le TCG a incorporé du matériel volcanique provenant de larges provinces ignées continentales à travers l’érosion des groupes sous jacents de Hamersley et Fortescue
The Archean – Proterozoic transition at 2.45 Ga is marked by major environmental changes in Earth’s history (atmosphere oxygenation and global glaciations). The origin and the relationships between these events are debated. Sedimentary sequences that record these events have been widely studied in North America (Huronian sequence) and South Africa (Transvaal). An analog sequence in the Pilbara craton in Western Australia (Turee Creek Group, TCG) contains 3 glacial horizons. 3 continental drill cores (Turee Creek Drilling Project, TCDP) were performed in order to investigate the sequence. U-Pb geochronology of monazite and zircon from drill core and surface samples allows to date the first paleoproterozoic glaciation at 2.45 Ga and a second glacial event at ~2.34 Ga. Re-Os analyses of pyrites from the second diamictite yielded an isochron at 2.31 Ga. These results enable to propose a correlation scenario of the paleoproterozoic glacial events recorded on different continents. A ‘Snowball Earth’ would have occured at 2.45 Ga at least on 5 cratons. A second glaciation could have occured at ~2.31 Ga on 4 cratons. Detrital zircons age spectrum highlights age peaks at 2.45, 2.54, 2.68, 2.82, 2.95 and 3.2 Ga. A crystal of Hadean zircon was discovered and indicate the existence of a probable differentiated crust within the Pilbara craton at 4.0 Ga. 70% of the analyzed zircons have juvenile Hf isotope composition, suggesting that the TCG incorporated volcanic material from continental large igneous provinces through the erosion of the underlying Hamersley and Fortescue groups

The accretionary Mesoproterozoic Grenville Orogeny (ca. 1300 – 980 Ma) involving the Central Metasedimentary Belt is a key building block of the eastern Laurentian margin. A petrographic, mineralogical, geochemical and geochronological study of the migmatite complex in Otter Lake (QC) within the Marble Domain is used to resolve regional metamorphic and magmatic events primarily recorded in the leucosome accessory minerals (i.e. monazite). The relationship between the different stages of monazite and garnet growth and dissolution during the tectonic evolution of the orogenic history for the interpreted metasomatic (injected) and anatectic (in situ) monazite-bearing neosomes from this study supports published thermochronological work in the area and challenges the claim that the Central Metasedimentary Belt was not heated above 500 °C during the Ottawan phase. Instead, the region shows Grenville magmatic and anatectic events were overprinted by high-temperature, fluid-rich Ottawan-phase metamorphism recorded within both injected (monazite-bearing) and in situ (monazite- and garnet-bearing) neosomes.

New and existing U–Pb and 40Ar/39Ar geochronological data, and oxygen and hydrogen stable isotope data, are combined with structural and metamorphic data from Thor–Odin, the southern culmination of the Monashee Complex. This leads to a new interpretation of the timing of deformation and metamorphism. Amphibolites in Thor–Odin with hornblende 40Ar/39Ar dates between ~75–70 and ~51 Ma experienced more 18O- and D-depletion than amphibolites with older dates. The younger dates that were previously interpreted as cooling ages, may have resulted from complete or partial Ar loss in the presence of meteoric fluids that were introduced into the rock during extension.

Monazite crystals in pelitic schist, quartzite and orthogneiss, which have U–Pb ages younger than 40Ar/39Ar hornblende ages in amphibolite in northwest Thor–Odin, may have grown during tension in the presence of fluids. Titanite, xenotime and zircon dates may be interpreted in the same way. Thus, the U–Pb dates that were previously interpreted as representing peak of metamorphism and the hornblende 40Ar/39Ar dates that were previously interpreted as representing cooling ages, may be interpreted as reflecting meteoric fluid penetration of the crust during regional extension. This implies that the age of the thermal peak of metamorphism is older than ~75–70 Ma. Migmatisation in a basement orthogneiss in Thor–Odin occurred at ~1.8 Ga. Dissolution rims are preserved in zircon between ~1.8 Ga domains and 52 Ma overgrowths. Because growth of new zircon (and possibly other U–Pb accessory phases) did not take place, any geological event that occurred during the ~1.8 Ga to 52 Ma time interval is not recorded. Cordilleran deformation and metamorphism may have taken place within that time interval, e.g. in the Middle Jurassic and/or mid- to Late Cretaceous, the time of Cordilleran deformation and metamorphism in the rocks overlying the Monashee Complex.

The Joss Mountain orthogneiss, west of the Monashee Complex in the Selkirk Allochthon, is dated at 362 +/– 13 Ma. F3 folding in pelitic schist at Joss Mountain is constrained between ~73 and ~70 Ma. Existing structural, metamorphic and geochronological data in, and close to, the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model.

The Tibetan plateau is a vast, elevated region located in central Asia, which is underlain by the thickest crust known on Earth (up to 90 km). An outstanding question of importance to many fields within geology is how and why did the Tibetan plateau form? Models attribute the growth of the plateau to a consequence of the ongoing India-Asia continental collision, but differ in the details of how the crustal thickening was accommodated: was it by underplating of Indian lower crust or by homogeneous shortening? High-grade metamorphic rocks sampled from the region potentially hold the key to answering this question, as they contain a record of past tectonic events that can discriminate between the various proposed models. This record can be decoded by integrating field, thermobarometric and geochronological techniques, to elucidate a detailed thermotectonic understanding of a region. This methodology was applied to three case studies, each of which targeted rare tectonic windows into the mid-crust of the plateau. These regions comprise Danba in eastern Tibet, Basong Tso in south-eastern Tibet and the Western Nyainqentanglha in southern Tibet. Each case study documents previously unreported metamorphic events that have allowed original interpretations to be made regarding tectonic evolution: in Danba, all metamorphism is shown to be early Jurassic; in Basong Tso, two metamorphic belts are documented that reveal a late Triassic--early Jurassic orogenic event; and in the Western Nyainqengtanglha, Cretaceous--Neogene magmatism is shown to overprint late Triassic metamorphism. Integration of the results has enabled commentary on the large scale evolution of the Tibetan plateau from the Permian until the present day, and even hinted at its future. The results indicate that the closure of the Paleotethys played an important role in the construction of the Tibetan plateau, and suggest that homogeneous crustal thickening is not a viable model for the documented exposure levels.

Le but de ce mémoire n'est pas de faire une présentation exhaustive de toutes mes activités de recherche mais d'essayer de cibler celles qui à la fois me correspondent le mieux, celles qui me tiennent le plus à cœur, parmi les activités de recherche que j'ai menées depuis que je suis partie de Potsdam (thèse). Je présenterai donc dans une première partie une synthèse de mes travaux sur les effets de l'irradiation, liée à la radioactivité, intra-ou inter-minéraux, naturelle ou artificielle. Je donnerai ensuite des exemples d'études ciblées sur des processus (diffusion, altération) dont la cinétique peut être considérablement accrues par les effets de la radioactivité. Dans une troisième partie je détaillerai un exemple particulier d'effet d'irradiation, celle induite par un faisceau laser femtoseconde et je terminerai sur des perspectives à plus ou moins long terme, ciblées sur les interactions entre les minéraux radioactifs (ou minéraux irradiés) et les fluides et les conséquences sur le signal géochronologique.

L'évolution thermomécanique de l'unité d'Andriamena est marquée par la superposition de quatre événements thermiques : 2. 7 Ga, 790 Ma et ~500 Ma. La signification de l'événement Archéen (2. 7 Ga) reste encore problématique : l'événement fini-Archéen (2. 5 Ga) correspond au métamorphisme de UHT (1050°C, 11. 5 kbar). Le Néoprotérozoïque moyen (790Ma) est marqué par un magnatisme basique-ultrabasique contemporain d'un métamorphisme granulithique (850°C, 7 kbar) interprété comme le témoin d'un contexte du type arc continental. L'événement Cambrien (~500 Ma) est à l'origine du champ de déformation finie : il résulte de la superposition de deux phases D1 et D2 synchrones d'un métamorphisme amphibolitique (700°C, 5-6 kbar) compatible avec un raccourcissement horizontal Est-Ouest résultant de la convergence lors de la consolidation du Gondwana. Les Mg-granulites de UHT préservent de nombreuses textures minéralogiques permettant de retracer un trajet PT pétrographique apparemment continu. Les données géochronologiques obtenues par datation in-situ sur monazite à la microsonde électronique montrent que ce trajet pétrographique doit être considéré comme un trajet discontinu résultant de la superposition de deux événements : 2. 5 Ga et 790 Ma. De plus, une partie du trajet pétrographique correspond à un trajet apparent sans signification tectonique, qui résulte de l'équilibration des paragenèses réfractaires de UHT à basses pressions lors de l'événement 790 Ma. La distinction qui existe entre trajet pétrographique et réel montre l'importance de déterminer l'âge absolu des différentes paragenèses. La datation in-situ permet de dater des minéraux dans leur contexte textural et donc de corréler âge et assemblages métamorphiques. Par une nouvelle approche dans leur contexte textural et donc de corréler âge et assemblages métamorphiques. Par une nouvelle approche de datation in-situ qui utilise les méthodes chimiques (microsondes) et isotopiques (ID-TIMS), on combine haute résolution spatiale et haute précision analytique sur les mêmes les grains extraits par micro-forage en lame mince

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The southern margin of the North Australian Craton (NAC) has been suggested to represent a long-lived (c. 1860 to 1600 Ma) active margin that preserves a cryptic record of the growth and assembly of the Australian continent. The Warumpi Province is juxtaposed against the southern Aileron Province, and has been interpreted as exotic to the NAC, though the timing of collision between the Warumpi Province and the southern Aileron Province is contentious. U-Pb zircon and monazite LA-ICP-MS geochronology from granulite facies metapelites and granitic gneisses along the southern margin of the Aileron Province and northern margin of the Warumpi Province, has shown it is characterised by c. 1780-1740 Ma magmatic rocks and c. 1640-1615 Ma magmatic and metamorphic rocks. The evidence for these events is preserved in kilometre-scale migmatitic boudins and low-strain zones enveloped by pervasive E-W trending higher strain belts. The overprinting high strain fabrics are Grenvillian age and constrained to c. 1175-1070 Ma. Phase equilibria modelling on a garnet-sillimanite-cordierite metapelite dated at c.1616 Ma, from a low-strain domain within the southern Aileron Province, indicates that peak metamorphic conditions were ~7-8 kbar and between 740-900 °C, and were associated with a down-pressure or decompressional P-T history. A metamorphic monazite age of c.1620 Ma was also preserved in a granitic gneiss located in an older, low-strain domain. The presence of the c. 1760 Ma and c. 1640 Ma timelines in both the Warumpi and Aileron Provinces calls into question the proposed exotic nature of the Warumpi Province. A speculative interpretation is that the Liebig-aged metamorphism and magmatism, seemingly associated with relatively shallow orientated, low strain fabrics, represents a period of extension rather than collision.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 2012

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LA-ICP-MS U-Pb monazite and zircon geochronology from granulite facies metapelites and granites indicate Inkamulla-aged metamorphism has occurred in the southern Aileron province, immediately east of the Mt Hay and Mt Chapple massifs. Gneissic metasediments and a granitic gneiss from an EW-striking structural belt in the southern Aileron Province yield ages reflective of the Inkamulla Igneous Event (1754-1741 Ma) and the Chewings Event (1593-1545 Ma), along with magmatic ages of 1627 and 1641 Ma. The Chewings age is interpreted to represent structural reworking associated with discrete shear zones along the northern margin of the EW belt. Magmatic ages of ca. 1640 Ma typically associated with Warumpi Province magmatism and deformation are found within the study area, which weakens the argument that the Warumpi terrane is exotic from the Aileron Province. The metamorphic conditions of 780-920°C and 5-10 kbars indicate an elevated geothermal gradient.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 2012

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The lowermost sediments of the Adelaide Supergroup near Arkaroola display pronounced structural complexity and anomalously high metamorphic grade. The timing of the structures, fabrics and metamorphism in the region remain unconstrained, although all are currently interpreted to have occurred during the Delamerian Orogeny. Monazite U/Pb geochronology provides a useful technique for dating amphibolites facies rocks and, combined with map scale and micro structural observations, suggests the peak metamorphism and at least one deformation event occurred significantly earlier than the Delamerian Orogeny. Monazite derived 206Pb/238U age estimates place the timing of peak metamorphism at 705.2±7.4 Ma with observations from thin section revealing a bedding parallel fabric pre-dating peak metamorphism. U/Pb analysis also reveals a second monazite growth event at 644.2±8 Ma the cause of which is hypothesised to be fluid related. Regional folding and axial planar fabric generation post-dates this metamorphism and can be seen to overprint the earlier fabric and wrap the cordierite porphyroblasts. This final deformation event remains largely unconstrained as in situ dating of monazite was not possible in this study. The inferred parasitic relationship between the folds within the studied region and the Arkaroola Syncline which folds the youngest sediments in the Adelaidian Sequence allows for the deposition to be constrained subsequent to the conclusion of the Adelaide Rift Complex.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2013

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The Kanmantoo Cu-Au deposit has been in episodic operation since 1846, one decade after the capital city of Adelaide was established some 40 kilometres to the NW. Regionally and within the host stratigraphy there exists archetypal evidence of the Cambrian Delamerian Orogeny through a complex structural, metamorphic and intrusive history. Consequently, numerous theories exist within the literature regarding a syngenetic or epigenetic style of mineralisation and the debated contribution, if any, of magmatic hydrothermal fluids. This study has documented numerous felsic intrusive vein sets within the Kanmantoo Cu-Au deposit which have been utilised to constrain the role of igneous activity on mineralisation within a wider Delamerian context. Monazite U–Pb ages of felsic veins show that intrusion first occurred at syn-peak metamorphic, syn-orogenic conditions (495.11 ± 2.79 Ma), continuing periodically until post-peak metamorphic, extensional conditions (483.43 ± 2.52 Ma). Intrusions are coeval with mineralisation and are temporally and geochemically analogous to magmatic activity in the adjacent Monarto and Murray Bridge provinces. Analysis of trace elements in monazites identifies the Kanmantoo Cu-Au deposit as a syn- to post-peak metamorphic hydrothermal anomaly which, combined with the presence of felsic veins, indicates that mineralisation resulted partly from fluids generated by a pluton at depth. These findings broadly confirm the prospectivity of Delamerian-affected terranes throughout large parts of South Eastern Australia where pervasive intrusive geology exists.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2018

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The Warumpi Province has been interpreted to be exotic and accreted to the Northern Australian Craton (NAC) during the Liebig Orogeny at c.1640Ma. However, phase equilibria modelling of melt-deficient, Mg-Al rich granulite facies rocks at Hill 830, in the Mount Liebig area show contradictory evidence of a decompressional pressure-temperature path and a high metamorphic gradient of 90°Ckbar. This interpretation in conjunction with abundant c.1780- 1740Ma and c.1640Ma magmatism in the southern Aileron and Warumpi Provinces, indicate that the tectonic regime at c.1640Ma may instead be a south-migrating, extensional scenario, compared to the previously accepted collisonal regime; speculating that the Warumpi Province is not ‘exotic’ to the NAC.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2015