The Gawler Craton in South Australia consists of an Archaean to Palaeoproterozoic core surrounded and intruded by a series of Palaeo- to Mesoproterozoic metasediments and igneous suites. The region has experienced a protracted c. 1700 Myr tectonic history from the Archaean through to the Mesoproterozoic, experiencing numerous cycles of deformation, magmatism and basin development. Despite hosting a number of mineral deposits, including the immense Olympic Dam iron oxide-copper-gold deposit, the tectonothermal evolution of the Gawler Craton remains poorly constrained. A significant ambiguity in our current understanding of the geological framework of the Gawler Craton revolves around the timing and spatial distribution of the tectonic events within the craton and their metamorphic evolution. This study addresses some of this ambiguity by unravelling the timing and tectonothermal evolution of the reworked southern Gawler Craton, using a combination of structural and metamorphic analysis, coupled with targeted geochronology. These methods have been applied to three locations representing different lithologies across the southern Gawler Craton. Putting absolute time into structural and metamorphic analysis is a vital tool for unravelling the development of ancient and modern orogenic systems. Electron Probe Micro-Analysis (EPMA) chemical dating of monazite provides a useful method of obtaining good precision age data from monazite bearing assemblages. This technique was developed at the University of Adelaide in order to constrain the timing of reworked assemblages from the southern Gawler Craton. EPMA measurements carried out on samples of known age, from Palaeoproterozoic to Ordovician, produce ages which are within error of the isotopically determined ages, indicating the validity of the developed setup. This technique, together with SHRIMP monazite and titanite and garnet Sm-Nd geochronology, was used on selected samples from the southern Gawler Craton to determine the timing of high-grade metamorphism and deformation. The results show that the Sleaford Complex records evidence of an early D₁event during the c. 2450 Ma Sleaford Orogeny recorded within structural boudins. The majority of the data indicates that the region underwent subsequent reworking and thorough overprinting during the 1725–1690 Ma Kimban Orogeny. In the Coffin Bay region, Palaeoproterozoic peraluminous granites of the Dutton Suite are reworked by a series of migmatitic and mylonitic shear zones during the Kimban Orogeny. Peak metamorphic conditions recorded in mafic assemblages indicate conditions of 10 kbar at 730°C. The post-peak evolution is constrained by partial to complete replacement of garnet – clinopyroxene bearing mafic assemblages by hornblende – plagioclase symplectites, which record conditions of c. 6 kbar at 700°C, implying a steeply decompressional exhumation path. The Shoal Point region consists of a series of reworked granulite-facies metapelitic and metaigneous units which belong to the late Archaean Sleaford Complex. Structural evidence indicates three phases of fabric development with D₁retained within boudins, D₂consisting of a series upright open to isoclinal folds producing an axial planar fabric and D₃, a highly planar vertical high-strain fabric which overprints the D₂ fabric. Geochronology constrains the D₁ event to the c. 2450 Ma Sleafordian Orogeny while the D₂the D₃events are constrained to the 1730–1690 Ma Kimban Orogeny. P-T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5–6 kbar and 750–780 °C. Subsequent Kimban-aged reworking reached peak metamorphic conditions of 8–9 kbar at 820–850 °C during the D₂ event. Followed by near isothermal decompression to metamorphic conditions <6 kbar and 790–850 °C associated with the development of the D₃high-strain fabric. The Pt Neill and Mine Creek regions are located in the core and on the flank of the crustal scale Kalinjala Shear Zone, which forms the main structural element of the poorly exposed Kimban Orogen. Samples record a similar structural development with a dextrally transpressive system resulting in a layer parallel migmatitic gneissic to mylonitic KS₁ fabric which was subsequently deformed and reworked by upright folds and discrete KD₂ east-side-down sub-solidus mylonitic shear zones during east-west compression. Geochronology constrains the timing of deformation and metamorphism to the Kimban Orogeny between 1720 and 1700 Ma. Metamorphic P-T analysis and pseudosections constrain the peak M₁ conditions in the core of the shear zone to 10–11 kbar at c. 800 °C reflecting lower crustal conditions at depths of up to 30 km. On the flank of the shear zone the M₁ conditions reached 6–7 kbar at 750 °C followed by sub-solidus reworking during KD₂ at conditions of 3–4 kbar at 600–660 °C, suggesting a maximum burial of <24 km. Cooling rates suggest that the core of the shear zone cooled at rates in excess of 40–80 °CMa⁻¹ while the flank underwent much slower cooling at < 10°CMa⁻¹. The rapid cooling and inferred decompression in the core of the shear zone reflects rapid burial and exhumation of lower-crustal material into the mid-crust along the Kalinjala Shear Zone. The absence of evidence for extension indicates that differential exhumation and the extrusion of lower-crustal material into the mid-crust was driven by transpression along the shear zone and highlights the role of transpression in creating large variations in vertical exhumation over relatively short lateral extents. Garnet is a vital mineral for determining constrained P-T-t paths as it can give both the P-T and t information directly. However, estimates of the closure temperature of the Sm-Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. Five igneous garnets of varying size from an undeformed 2414 ± 6 Ma garnet – cordierite bearing s-type granite from the Coffin Bay region, that were subjected to high-T reworking during the Kimban Orogeny, have been dated to examine their diffusional behaviour in the Sm-Nd system. Garnets were compositionally profiled and then dated. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element and REE traverses reveal homogonous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggests that diffusion rates of REE’s are considerably slower than that of the major cations, in disagreement with recent experimental determinations of the diffusion rates of REE in garnet. The retention of REE zoning and the lack of resetting in the largest grains suggests that Sm-Nd closure temperature in garnet is a function of grain-size, thermal history and REE zoning in garnet. The findings of this study provide the first temporally constrained tectonothermal model of the evolution of the southern Gawler Craton. The P-T conditions obtained from the earliest D₁ fabric provide the first quantitative constraints on the P-T conditions of the southern Sleafordian Orogeny. The P-T-t evolution determined for the 1725–1690 Ma Kimban Orogeny indicate it developed along a clockwise P-T path, and dominates the structural and metamorphic character of the southern Gawler Craton. The large variations in exhumation over short lateral extents reflect the exhumation of lower crustal rocks during the Kimban Orogeny driven by transpression during the development of a regional transpressional ‘flower structure’. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1372052 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

The Gawler Craton in South Australia consists of an Archaean to Palaeoproterozoic core surrounded and intruded by a series of Palaeo- to Mesoproterozoic metasediments and igneous suites. The region has experienced a protracted c. 1700 Myr tectonic history from the Archaean through to the Mesoproterozoic, experiencing numerous cycles of deformation, magmatism and basin development. Despite hosting a number of mineral deposits, including the immense Olympic Dam iron oxide-copper-gold deposit, the tectonothermal evolution of the Gawler Craton remains poorly constrained. A significant ambiguity in our current understanding of the geological framework of the Gawler Craton revolves around the timing and spatial distribution of the tectonic events within the craton and their metamorphic evolution. This study addresses some of this ambiguity by unravelling the timing and tectonothermal evolution of the reworked southern Gawler Craton, using a combination of structural and metamorphic analysis, coupled with targeted geochronology. These methods have been applied to three locations representing different lithologies across the southern Gawler Craton. Putting absolute time into structural and metamorphic analysis is a vital tool for unravelling the development of ancient and modern orogenic systems. Electron Probe Micro-Analysis (EPMA) chemical dating of monazite provides a useful method of obtaining good precision age data from monazite bearing assemblages. This technique was developed at the University of Adelaide in order to constrain the timing of reworked assemblages from the southern Gawler Craton. EPMA measurements carried out on samples of known age, from Palaeoproterozoic to Ordovician, produce ages which are within error of the isotopically determined ages, indicating the validity of the developed setup. This technique, together with SHRIMP monazite and titanite and garnet Sm-Nd geochronology, was used on selected samples from the southern Gawler Craton to determine the timing of high-grade metamorphism and deformation. The results show that the Sleaford Complex records evidence of an early D₁event during the c. 2450 Ma Sleaford Orogeny recorded within structural boudins. The majority of the data indicates that the region underwent subsequent reworking and thorough overprinting during the 1725–1690 Ma Kimban Orogeny. In the Coffin Bay region, Palaeoproterozoic peraluminous granites of the Dutton Suite are reworked by a series of migmatitic and mylonitic shear zones during the Kimban Orogeny. Peak metamorphic conditions recorded in mafic assemblages indicate conditions of 10 kbar at 730°C. The post-peak evolution is constrained by partial to complete replacement of garnet – clinopyroxene bearing mafic assemblages by hornblende – plagioclase symplectites, which record conditions of c. 6 kbar at 700°C, implying a steeply decompressional exhumation path. The Shoal Point region consists of a series of reworked granulite-facies metapelitic and metaigneous units which belong to the late Archaean Sleaford Complex. Structural evidence indicates three phases of fabric development with D₁retained within boudins, D₂consisting of a series upright open to isoclinal folds producing an axial planar fabric and D₃, a highly planar vertical high-strain fabric which overprints the D₂ fabric. Geochronology constrains the D₁ event to the c. 2450 Ma Sleafordian Orogeny while the D₂the D₃events are constrained to the 1730–1690 Ma Kimban Orogeny. P-T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5–6 kbar and 750–780 °C. Subsequent Kimban-aged reworking reached peak metamorphic conditions of 8–9 kbar at 820–850 °C during the D₂ event. Followed by near isothermal decompression to metamorphic conditions <6 kbar and 790–850 °C associated with the development of the D₃high-strain fabric. The Pt Neill and Mine Creek regions are located in the core and on the flank of the crustal scale Kalinjala Shear Zone, which forms the main structural element of the poorly exposed Kimban Orogen. Samples record a similar structural development with a dextrally transpressive system resulting in a layer parallel migmatitic gneissic to mylonitic KS₁ fabric which was subsequently deformed and reworked by upright folds and discrete KD₂ east-side-down sub-solidus mylonitic shear zones during east-west compression. Geochronology constrains the timing of deformation and metamorphism to the Kimban Orogeny between 1720 and 1700 Ma. Metamorphic P-T analysis and pseudosections constrain the peak M₁ conditions in the core of the shear zone to 10–11 kbar at c. 800 °C reflecting lower crustal conditions at depths of up to 30 km. On the flank of the shear zone the M₁ conditions reached 6–7 kbar at 750 °C followed by sub-solidus reworking during KD₂ at conditions of 3–4 kbar at 600–660 °C, suggesting a maximum burial of <24 km. Cooling rates suggest that the core of the shear zone cooled at rates in excess of 40–80 °CMa⁻¹ while the flank underwent much slower cooling at < 10°CMa⁻¹. The rapid cooling and inferred decompression in the core of the shear zone reflects rapid burial and exhumation of lower-crustal material into the mid-crust along the Kalinjala Shear Zone. The absence of evidence for extension indicates that differential exhumation and the extrusion of lower-crustal material into the mid-crust was driven by transpression along the shear zone and highlights the role of transpression in creating large variations in vertical exhumation over relatively short lateral extents. Garnet is a vital mineral for determining constrained P-T-t paths as it can give both the P-T and t information directly. However, estimates of the closure temperature of the Sm-Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. Five igneous garnets of varying size from an undeformed 2414 ± 6 Ma garnet – cordierite bearing s-type granite from the Coffin Bay region, that were subjected to high-T reworking during the Kimban Orogeny, have been dated to examine their diffusional behaviour in the Sm-Nd system. Garnets were compositionally profiled and then dated. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element and REE traverses reveal homogonous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggests that diffusion rates of REE’s are considerably slower than that of the major cations, in disagreement with recent experimental determinations of the diffusion rates of REE in garnet. The retention of REE zoning and the lack of resetting in the largest grains suggests that Sm-Nd closure temperature in garnet is a function of grain-size, thermal history and REE zoning in garnet. The findings of this study provide the first temporally constrained tectonothermal model of the evolution of the southern Gawler Craton. The P-T conditions obtained from the earliest D₁ fabric provide the first quantitative constraints on the P-T conditions of the southern Sleafordian Orogeny. The P-T-t evolution determined for the 1725–1690 Ma Kimban Orogeny indicate it developed along a clockwise P-T path, and dominates the structural and metamorphic character of the southern Gawler Craton. The large variations in exhumation over short lateral extents reflect the exhumation of lower crustal rocks during the Kimban Orogeny driven by transpression during the development of a regional transpressional ‘flower structure’. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1372052 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

Results from two-station surface-wave inversions across the Archean Kaapvaal craton of southern Africa are compared with seismic velocities estimated from approximately 100 mantle xenoliths brought to the surface in kimberlite pipes. As the xenoliths represent a snapshot of the mantle at the time of their eruption, comparison with recently recorded seismic data provides an opportunity to compare and contrast the independently gained results. These cratonic xenoliths from the southern Kaapvaal, all less than 100Ma in age, have been analyzed geothermobarometrically to obtain the equilibrium P-T conditions of the cratonic mantle to about 180km depth [James et al 2004]. Seismic velocity-depth and density-depth profiles calculated on the basis of these P-T data and the mineral modes of the xenoliths are used to produce theoretical surface-wave dispersion curves and to generate roughly the upper 200km of a starting/reference model. A regionally-developed crustal structure [Niu and James 2002] was used for the crust and 300km of mantle values taken from PREM filled in down to 500km depth. This composite model was used as the starting/reference model for a Neighbourhood Algorithm surface-wave inversion using fundamental-mode Rayleigh-wave phase velocities for 16 paths within the Kaapvaal Craton from five events. The velocity structures found by that inversion are consistent with those derived from the xenolith data. Hence the velocity structure (i.e. thermal structure) of the mantle to a depth of 180km beneath the Kaapvaal craton is basically the same today as it was 80-90Ma. Further, synthetics runs show that for this surface-wave dataset, there is no strong low-velocity zone at depths shallower than at least 200km. / Master of Science

. 21 ont des teneurs en Os > 1 ppb et des rapports 187Re/188Os < 3. Leurs TRD ERUPT varient de 0.7 à 2.3 Ga (moyenne 1.7 Ga). Les analyses isotopiques du Re/Os de 9 péridotites résiduelles (0.1-1.2% Al2O3) donnent des âges TRD ERUPT variant de 1.4 à 2.3 Ga (maximum de probabilité 2.1 Ga) Les cratons sont de vastes portions de lithosphère continentale formés d’une croûte continentale type TTG et d’un manteau lithosphérique épaissi (>200km), froid (40 mW/m²) et composé majoritairement par des péridotites fortement réfractaires (Mg#WR>0.92, Al2O3-CaO <1%), échantillonnées sous forme de xénolites lors des éruptions kimberlitiques. Il est admis que les péridotites cratoniques ont été formées par de hauts degrés de fusion partielle, cependant, les fortes teneurs en SiO2 de certaines xénolites impliquent des mécanismes de fusion très particuliers. De plus, les fortes teneurs en FeO, TiO2, REE, en cpx et en grenats d’un grand nombre d’échantillons ne sont pas cohérentes avec une origine résiduelle. En effet la majorité des péridotites cratoniques a subi des transformations qui masquent actuellement leur composition initiale. C’est pourquoi les conditions de la fusion et les processus post-formation qui affectent le manteau cratonique demeurent très mal contraints. Udachnaya est la seule kimberlite du craton Sibérien qui fournit des xénolites de péridotite de tailles suffisantes pour les études géochimiques et isotopiques (>>100g). Cette thèse se base sur une grande collection de xénolites de péridotites (>50) particulièrement fraîches i.e. les olivines et les autres minéraux sont préservés de l’altération et les pertes au feu des roches totales (LOI) sont <2%. Les péridotites sont des harzburgites et des lherzolites, généralement pauvres en cpx (<6%) du faciès à spinelle et à grenat, avec des microstructures aussi bien grenues que déformées. Cette collection est représentative de l’ensemble du profil lithosphérique (du Moho jusqu’à 220km). La majorité des péridotites à spinelle est pauvre en opx (<20%). Les péridotites riches en silice sont moins courantes dans le manteau cratonique sibérien que ce que l’étude de Boyd et al. (1997) a montré. Seul ¼ des péridotites à spinelle de notre collection a des compositions en opx >30%. Les péridotites à spinelle, pauvres en opx ont des compositions en éléments majeurs proches d’un résidu de fusion. Contrairement aux autres études, leurs compositions en Al2O3, FeO et Mg#WR définissent des trends qui, comparés aux études expérimentales, indiquent une origine par 38% d’une fusion fractionnée par décompression entre 7-4 GPa et ≤1–2 GPa ; ce qui est en accord avec les modélisations des éléments en trace. L’origine des péridotites riches en silice est incertaine, et l’hypothèse d’une interaction avec des liquides de subduction est peu probable : leurs compositions en SiO2 et FeO diffèrent des péridotites d’arc, et leur δ18OWR ~5.3 ± 0.2‰ est proche de la valeur référence pour le manteau (5.5‰). 1/3 des péridotites à grenat grenues a des compositions similaires aux péridotites à spinelle et indiquent donc des conditions de formation similaires. Le grenat, dans la majorité de ces péridotites, est d’origine résiduelle, contrairement au cpx qui lui est clairement d’origine métasomatique. La majorité des péridotites à grenat forme des séquences d’enrichissement en FeO, TiO2 et REE par rapport aux péridotites résiduelles. Cela traduit un métasomatisme modal (précipitation de cpx et de grenat), et ce par interaction entre le protolite des péridotites avec un magma riche en Si, Al, Fe (Ti, REE). Les cpx sont en équilibre avec un liquide de composition kimberlitique. Les kimberlites sont des magmas pauvres en Si, Al, Fe et ne peuvent pas être responsables du métasomatisme modal, mais un magma parent peut avoir interagi avec la roche peu de temps avant l’éruption. 28 péridotites ont été analysées pour obtenir leur composition isotopique Re/Os et leur composition en PGE / English abstract not supplied by the author. Résumé en anglais non fourni par l'auteur.

O significado da idade do magmatismo, reconhecimento das séries magmáticas e ambientes tectônicos que controlaram a origem das rochas metavulcânicas do Greenstone Belt da Vila Nova (GBVN), é uma importante ferramenta para entender a evolução do Escudo das Guianas durante o Paleoproterozóico. Este trabalho apresenta novos dados geoquímicos de rochas metavulcânicas e de U/Pb em zircão (LA-MC-ICPMS), que foram suplementados por petrografia, dados estratigráficos e estruturais obtidos através de mapeamento geológico e descrição de furos de sondagem. Na região de Vila Nova, as unidades do GBVN repousam sobre o embasamento Arqueano composto por ortognaisses, metagranitos e anfibolitos do Complexo Tumucumaque. A base do GBVN é composta por metabasaltos e metabasaltos andesíticos, sotoposto por um domínio superior metassedimentar com rochas químico exalativas e metavulcânicas subordinadas. As rochas metavulcânicas incluem anfibolitos e anfibólio xistos, cujos corpos estão alongados segundo a xistosidade regional de direção NW-SE. A geocronologia pelo método U-Pb em zircão mostra uma idade de 2.154 ± 6 Ma para um meta-andesito da porção inferior do GBVN. O evento de metamorfismo orogênico esteve associado a três eventos deformacionais. Os eventos D1 e D2 formaram a xistosidade (S1), preservada como dobras intrafoliais (F2) e a clivagem de crenulação (S2), originadas a partir de movimentos de cavalgamento. O pico metamórfico (M1) está marcado pela assembleia plagioclásio+hornblenda+granada e, plagioclásio+hornblenda+diopsídio, indicando temperaturas entre 450 e 650 °C e pressão entre 4 e 6 kbares. Os metabasitos compreendem Fe-toleítos e Mg-toleítos com afinidade komatitica, composições geoquímicas enriquecidas em LILE e ETR e empobrecidas em HFSE (com anomalias negativas de Nb, Ti e P) e padrões de ETR semelhantes ao MORB. As características observadas indicam um magmatismo toleítico relacionado à bacias de retro-arco e arco de ilhas de 2.15 Ga no Escudo das Guianas. / The significance of the age of magmatism, magma series and tectonic settings that controls the origin of metavolcanic rocks of Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of Guiana Shield in Paleoproterozoic times. This work presents new U-Pb LA-ICP-MS geochronological and geochemical analyzes carried out on zircon grains and metavolcanic rocks of the Vila Nova greenstone belt (VNGB) which were supplemented by petrography, and stratigraphic and structural data acquired through description of boreholes and field work. In the Vila Nova region, the VNGB units rest on the Archaean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB is composed of metabasalts and andesitic metabasalts, supported by an upper metasedimentary domain with exhalative chemical rocks and subordinate metavolcanic rocks. The metavolcanic rocks include amphibolites and amphibole schists, whose bodies are elongated according to the regional NW-SE schistosity. U-Pb zircon geochronology data showed an age of 2154 ±6 Ma for a meta-andesite of the lower portion of VNGB. Orogenic metamorphism event followed by three deformation events were recognized. The D1 and D2 events formed the schistosity (S1), preserved as intrafolial folds (F2) and the crenulation cleavage (S2), derived from thrust movements. The assemblages plagioclase + hornblende + garnet and plagioclase + hornblende + diopside define the metamorphic peak (M1-M2) with temperatures from 450-650 °C and lithostatic pressure between 4 and 6 kbars. The metabasites comprise Fe-tholeiites and Mg-tholeiites with komatiitic affinity, geochemical compositions enriched in LILE and REE and depleted in HFSE (with negative Nb, Ti and P anomalies) and MORB-like REE patterns. The observed features indicate an expressive magmatism related to back-arc basins and island arcs at 2.15 Ga in Guiana Shield.

Un volumineux magmatisme anorogénique a recouvert une large partie (1.500.000 km2) du craton Amazonien à 1880 Ma et définit une province magmatique felsique qu'on appelle l'événement Uatumã. L'objectif de ce travail est d'étudier le paléomagnétisme ainsi que la pétrologie de ces roches afin de préciser le cadre spatio-temporel de cet événement et de définir la place du craton Amazonien au sein du Supercontinent Columbia. Deux régions d'études localisées dans le sud-ouest du craton Amazonien (Pará) ont permis de collecter les échantillons nécessaires : (1) la région de Tucumã où 16 filons felsiques, 7 filons mafiques, un filon gabbroïque et 3 sites du socle archéen ont été collectés. (2) la région de São Felix do Xingu où on a échantillonné 7 sites de laves rhyolitiques, 2 sites d'ignimbrites, un filon felsique et un site de brèches volcaniques qui appartiennent à la formation Santa Rosa. 6 sites de la formation Sobreiro (roches volcanoclastiques andésitiques) ainsi qu'un filon felsique de la suite Velho Guilherme ont aussi été collectés. Un des résultats majeurs de la pétrologie des filons felsiques de Tucumã (1880.9 ± 6.7 Ma U-Pb sur zircon) a été de montrer qu'ils représentent le système filonien associé à la formation volcanique Santa Rosa. L'aimantation rémanente des filons felsiques est portée par la magnétite et l'hématite. Cette hématite est syn- à post-magmatique et sa formation, à partir des fluides hydrothermaux, peut être quantifiée grâce certaines propriétés magnétiques. Les désaimantations (en champ alternatif, thermiques) montrent une composante A caractéristique de direction nord-ouest avec une inclinaison positive dont la moyenne par site donne un pôle paléomagnétique localisé à 52.9°S, 76.4°E (A95= 10.4°, K= 13.52). Une réaimantation régionale mésozoïque en relation avec les filons de la CAMP (Central Atlantic Magmatic Province) est observée dans cette région. Les meilleurs résultats paléomagnétiques ont été obtenus dans la région de São Felix de Xingu. Deux nouveaux pôles paléomagnétiques primaires, ont été déterminés: (i) Le pôle SF1 (319.7°E, 24.7°S, N= 10; A95= 16.9°) est obtenu pour des andésites et des rhyolites datés à 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICPMS), son origine primaire est confirmée par un test de contact inverse (> 1853 Ma). La pétrographie montre que la minéralogie magnétique de cette composante est l'hématite formée par des fluides hydrothermaux syn- à post-magmatiques. (ii) Le pole SF2 (220.1°E, 31.1°N, N= 15; A95= 9.7°) est déterminé par l'aimantation rémanente du filon felsique de la Suite Velho Guilherme, mais aussi par l'aimantation secondaire dans les échantillons de la formation Santa Rosa et Sobreiro. Un âge de 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICPMS) est calculé pour le filon felsique portant SF2, dont l'origine primaire est confirmée par un test de contact positif. Les pôles SF1 et SF2 sont très différents, malgré une différence d'âge de seulement ~25 Ma. Des résultats paléomagnétiques similaires ont été obtenus pour les pôles de même âge dans d'autres cratons (Inde, Supérieur (Laurentia), Slave (Laurentia), Kalahari, Baltica et Sibérie), et peuvent être expliqués par un événement de Vrai Dérive Polaire (VDP). Cette époque (~1880 Ma) est marquée par une forte activité du manteau, qui aboutit à la formation du Supercontinent Columbia, autour de 1850 - 1800 Ma. La formation de superpanaches est une conséquence possible de l'assemblage du supercontinent et de l'effet d'isolation thermique du manteau qui en résulte, ou bien lui est concomitante. Les superpanaches peuvent provoquer des perturbations de densité modifiant le tenseur inertiel de gravité de la Terre. Un rapide événement de Vrai Dérive Polaire (VDP) peut ainsi avoir eu lieu, ce qui va déplacer rapidement les continents et les superpanaches vers l'équateur. Ces événements peuvent être liés à une réorganisation du manteau dans son ensemble à la suite d'une période de faible activité magmatique entre 2400 et 2200 Ma. / An anorogenic magmatism covered a large part (1.500.000 km2) of the Amazonian craton at ca. 1880 Ma and defined a Silicic Large Igneous Province (SLIP) called the Uatumã event. The aim of this work is to study the paleomagnetism and petrology of these rocks to define the space-time framework of the Uatumã event and to try to elucidate the Amazonian craton evolution during the Columbia supercontinent amalgamation. Two regions were selected in the southwestern Amazonian craton (Pará) for sampling: (1) The Tucumã area where 16 felsic dikes, 7 mafic dikes, a gabbroic dike and 3 sites of the Archean basement were collected, and (2) the São Felix do Xingu area where, 7 sites of rhyolitic lava flows, 2 sites of ignimbrites, a felsic dike and a volcanic breccia belonging to the Santa Rosa Formation were sampled, and also 6 sites of the Sobreiro Formation (volcanoclastic rocks, andesitic) and one felsic dike of the Velho Guilherme Suite were collected. Petrology of the felsic dikes of Tucumã (1880.9 ± 6.7 Ma U-Pb zrn) showed that they represent the dike swarm associated with the Santa Rosa volcanic Formation. The remanent magnetization of the felsic dikes is carried by PSD magnetite and hematite. This hematite is syn- to post magmatic derived from hydrothermal fluids. Magnetic mineralogy can be used as a proxy to quantify the hydrothermal alteration. AF, thermal, LTD + AF and LTD + thermal demagnetizations show a northwest direction with a positive inclination (component A), whose site mean directions gives a paleomagnetic pole located at 52.9°S, 76.4°E, A95= 10.4 °, K= 13.52). However, this component seems to represent a remagnetization, probabily occurred at Neoproterozoic times. Another magnetic component (named component B) was also isolated for these rocks, and it was associated to a Mesozoic regional remagnetization related to the Central Atlantic Magmatic Province (CAMP). Yet, a third southwestern direction with low positive inclination (component C) was also isolated for some sites. This component was interpreted to be related with the ca. 1760 Ma Velho Guilherme magmatic intrusion. The best paleomagnetic results were obtained in the São Felix do Xingu area. Two new primary paleomagnetic poles have been determined: (i) SF1 pole (319.7°E, 24.7°S, N= 10; A95= 16.9 °) was obtained for andesites and rhyolites dated to 1877.4 ± 4.3 Ma (U-Pb zrn, LA-ICPMS), and its primary origin is confirmed by an inverse baked contact test (> 1853 Ma). Petrography shows that the magnetic mineralogy of this component is hematite formed by hydrothermal fluids syn- to post magmatic. (Ii) SF2 pole (220.1°E, 31.1°N, N= 15 ° A95= 9.7 °) was determined by the remanent magnetization of the felsic dike of the Velho Guilherme Suite but also as secondary magnetizations in samples of the Santa Rosa and Sobreiro Formations. An age of 1853.7 ± 6.2 Ma (U-Pb zrn, LA-ICPMS) is calculated for the felsic dike carrying SF2, whose primary origin is confirmed by a positive baked contact test. The SF1 and SF2 poles have a significant difference in angular distance, for a time interval of only ~25 Ma. Similar coeval paleomagnetic discrepancies were observed for other cratons (India, Superior (Laurentia), Slave (Laurentia), Kalahari, Baltica and Siberia), which can be explained by a True Polar Wander (TPW) event at ca. 1880 - 1860 Ma. This period is marked by a high mantle activity, which results in the amalgamation of the Columbia supercontinent, formed at ca. 1850 - 1800 Ma. Amalgamation of supercontinent may cause the formation of superplume and thermal insulation which can disturb mass distribution in mantle and alter the inertial gravity tensor of the Earth. A True Polar Wander (TPW) event may thus have taken place, which will move the cratons and the superplumes towards the equator. These conditions may be related to a reorganization of the whole mantle following a global magmatic quiescence between 2400 and 2200 Ma.

No description available.

O significado da idade do magmatismo, reconhecimento das séries magmáticas e ambientes tectônicos que controlaram a origem das rochas metavulcânicas do Greenstone Belt da Vila Nova (GBVN), é uma importante ferramenta para entender a evolução do Escudo das Guianas durante o Paleoproterozóico. Este trabalho apresenta novos dados geoquímicos de rochas metavulcânicas e de U/Pb em zircão (LA-MC-ICPMS), que foram suplementados por petrografia, dados estratigráficos e estruturais obtidos através de mapeamento geológico e descrição de furos de sondagem. Na região de Vila Nova, as unidades do GBVN repousam sobre o embasamento Arqueano composto por ortognaisses, metagranitos e anfibolitos do Complexo Tumucumaque. A base do GBVN é composta por metabasaltos e metabasaltos andesíticos, sotoposto por um domínio superior metassedimentar com rochas químico exalativas e metavulcânicas subordinadas. As rochas metavulcânicas incluem anfibolitos e anfibólio xistos, cujos corpos estão alongados segundo a xistosidade regional de direção NW-SE. A geocronologia pelo método U-Pb em zircão mostra uma idade de 2.154 ± 6 Ma para um meta-andesito da porção inferior do GBVN. O evento de metamorfismo orogênico esteve associado a três eventos deformacionais. Os eventos D1 e D2 formaram a xistosidade (S1), preservada como dobras intrafoliais (F2) e a clivagem de crenulação (S2), originadas a partir de movimentos de cavalgamento. O pico metamórfico (M1) está marcado pela assembleia plagioclásio+hornblenda+granada e, plagioclásio+hornblenda+diopsídio, indicando temperaturas entre 450 e 650 °C e pressão entre 4 e 6 kbares. Os metabasitos compreendem Fe-toleítos e Mg-toleítos com afinidade komatitica, composições geoquímicas enriquecidas em LILE e ETR e empobrecidas em HFSE (com anomalias negativas de Nb, Ti e P) e padrões de ETR semelhantes ao MORB. As características observadas indicam um magmatismo toleítico relacionado à bacias de retro-arco e arco de ilhas de 2.15 Ga no Escudo das Guianas. / The significance of the age of magmatism, magma series and tectonic settings that controls the origin of metavolcanic rocks of Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of Guiana Shield in Paleoproterozoic times. This work presents new U-Pb LA-ICP-MS geochronological and geochemical analyzes carried out on zircon grains and metavolcanic rocks of the Vila Nova greenstone belt (VNGB) which were supplemented by petrography, and stratigraphic and structural data acquired through description of boreholes and field work. In the Vila Nova region, the VNGB units rest on the Archaean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB is composed of metabasalts and andesitic metabasalts, supported by an upper metasedimentary domain with exhalative chemical rocks and subordinate metavolcanic rocks. The metavolcanic rocks include amphibolites and amphibole schists, whose bodies are elongated according to the regional NW-SE schistosity. U-Pb zircon geochronology data showed an age of 2154 ±6 Ma for a meta-andesite of the lower portion of VNGB. Orogenic metamorphism event followed by three deformation events were recognized. The D1 and D2 events formed the schistosity (S1), preserved as intrafolial folds (F2) and the crenulation cleavage (S2), derived from thrust movements. The assemblages plagioclase + hornblende + garnet and plagioclase + hornblende + diopside define the metamorphic peak (M1-M2) with temperatures from 450-650 °C and lithostatic pressure between 4 and 6 kbars. The metabasites comprise Fe-tholeiites and Mg-tholeiites with komatiitic affinity, geochemical compositions enriched in LILE and REE and depleted in HFSE (with negative Nb, Ti and P anomalies) and MORB-like REE patterns. The observed features indicate an expressive magmatism related to back-arc basins and island arcs at 2.15 Ga in Guiana Shield.

A ambiência geotectônica das rochas máfico-ultramáficas do sudoeste do Cráton Amazônico é, de uma maneira geral, pouco conhecida. A maioria dos trabalhos desta porção cratônica está enfocada nos estudos geocronológicos em granitóides, de modo que pouco se sabe sobre a origem e significado tectônico destas rochas. Neste contexto, esta pesquisa buscou contribuir para o conhecimento da evolução geotectônica do sudoeste do Cráton Amazônico, através da caracterização de um complexo ofiolítico Mesoproterozóico, correspondente ao Complexo Trincheira, de idade Calimiana. Desta forma, uma nova proposta de modelo tectônico é aqui apresentada, a qual explica muitas das características anteriormente enigmáticas da história Pré-cambriana desta área-chave e possibilita outras alternativas para a reconstrução do supercontinente Columbia. O ofiolito Trincheira é composto de rochas extrusivas (anfibolitos derivados de basaltos maciços e almofadados), intrusivas máfico-ultramáficas, chert, formação ferrífera bandada, pelitos, psamitos e pequena proporção de rochas cálcio-silicáticas. A composição geoquímica das rochas extrusivas e intrusivas máfico-ultramáficas mostra semelhanças com os basaltos toleiíticos modernos, as quais possuem moderado a forte fracionamento de elementos terras-raras leves, padrão quase horizontal dos elementos terras-raras pesados e moderada a forte anomalia negativa dos elementos de alto campo de força (especialmente Nb), uma assinatura geoquímica típica de zona de subducção. As unidades basais do ofiolito Trincheira são quimicamente similares aos modernos basaltos de cadeia meso-oceânica (MORB). Esse comportamento químico muda para as unidades de topo as quais apresentam uma assinatura similar aos toleiítos de arco-de- ilha (IAT). Portanto, o ofiolito Trincheira deve ter sido originado em um ambiente intra-oceânico de supra-subducção composto de um sistema de arco/retro-arco. Os dados isotópicos de Sm, Nd e Sr para essas rochas indicam valores iniciais de Nd de moderados a altamente positivos (+2.6 a +8.8) e muito baixa razão inicial de 87Sr/86Sr (0,7013 – 0,7033), sugerindo que esses magmas foram originados a partir de uma fonte mantélica empobrecida e nada ou fracamente contaminados por componentes de subducção. O complexo ofiolítico foi deformado, metassomatizado e metamorfisado durante o desenvolvimento da Faixa Móvel Guaporé, um orógeno acrescionário-colisional Mesoproterozóico (1,47-1,35 Ba), constituído pela zona de sutura Guaporé, a qual une o Cráton Amazônico com o Bloco Paraguá. A fase colisional que marca o encaixe final dessas duas massas continentais ocorreu por volta de 1,35 Ba, onde o metamorfismo atingiu temperaturas entre 780 a 853°C nos granulitos máficos e 680 a 720°C nos anfibolitos, com pressão média de 6,8 kbar. A sutura Guaporé foi reativada no final do Mesoproterozóico e evoluiu para a abertura de um rift intracontinental, com a sedimentação das rochas dos Grupos Nova Brasilândia e Aguapeí, o qual marca a fragmentação final do supercontinente Columbia, por volta de 1,3-1,2 Ba. Granulitos máficos, anfibolitos e trondhjemitos da porção meridional do Cinturão Nova Brasilândia, representativos da última fase compressional que afetou o sudoeste do Cráton Amazônico, forneceram idades U-Pb de 1110 Ma, as quais datam o metamorfismo de alto grau e o fechamento do rift, processo resultante da acresção do microcontinente Arequipa-Antofalla ao Cráton Amazônico. Portanto, a fragmentação do supercontinente Columbia foi seguida rapidamente pela aglomeração de outras massas continentais, formando o supercontinente Rodínia, por volta de 1100 Ma. / The tectonic framework of the ultramafic-mafic rocks of the southwestern Amazon Craton is generally little known. Most of work this cratonic portion is focused on the geochronological studies of granitoids, so that little is known about the origin and tectonic significance of these rocks. In this context, this study contributes to the knowledge of the tectonic evolution of the southwestern Amazon Craton, through the characterization of a Mesoproterozoic ophiolitic complex, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation, pelites, psammitic and a smaller proportion of calc-silicate rocks. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. These rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB). This behavior changes to an island arc tholeiites (IAT) signature in the upper units of the Trincheira ophiolite. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Mafic-ultramafic rocks of the Trincheira ophiolites display moderate to highly positive initial Nd values of +2.6 to +8.8 and very low values for the initial 87Sr/86Sr ratio (0.7013 - 0.7033). It is suggested that these magmas originated from a depleted mantle source, which experienced low degree of contamination by variable subduction components. The ophiolitic sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary-collisional orogen that represents the Guaporé suture zone. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The Guaporé suture zone is defined by the ESE–WNW trending mafic-ultramafic belt formed during a Mesoproterozoic (ca. 1.47-1.43 Ga) accretionary phase, and overprinted by upper amphibolite-granulite facies metamorphism during collisional phase in the Ectasian (~1.35 Ga), which mark the docking final of the Amazon craton and Paraguá Block. This suture was reactivated and evolved from the development of an intracontinental rift environment, represented by Nova Brasilândia and Aguapeí Groups, which mark the final breakup of the supercontinent Columbia in the late Mesoproterozoic (ca. 1.3-1.2 Ga). Mafic granulites, amphibolites and trondhjemites in the northernmost portion of the Nova Brasilândia belt yield U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift, due to the accretion of the Arequipa-Antofalla basement to the Amazon craton. Therefore, the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1100 Ma.

A ambiência geotectônica das rochas máfico-ultramáficas do sudoeste do Cráton Amazônico é, de uma maneira geral, pouco conhecida. A maioria dos trabalhos desta porção cratônica está enfocada nos estudos geocronológicos em granitóides, de modo que pouco se sabe sobre a origem e significado tectônico destas rochas. Neste contexto, esta pesquisa buscou contribuir para o conhecimento da evolução geotectônica do sudoeste do Cráton Amazônico, através da caracterização de um complexo ofiolítico Mesoproterozóico, correspondente ao Complexo Trincheira, de idade Calimiana. Desta forma, uma nova proposta de modelo tectônico é aqui apresentada, a qual explica muitas das características anteriormente enigmáticas da história Pré-cambriana desta área-chave e possibilita outras alternativas para a reconstrução do supercontinente Columbia. O ofiolito Trincheira é composto de rochas extrusivas (anfibolitos derivados de basaltos maciços e almofadados), intrusivas máfico-ultramáficas, chert, formação ferrífera bandada, pelitos, psamitos e pequena proporção de rochas cálcio-silicáticas. A composição geoquímica das rochas extrusivas e intrusivas máfico-ultramáficas mostra semelhanças com os basaltos toleiíticos modernos, as quais possuem moderado a forte fracionamento de elementos terras-raras leves, padrão quase horizontal dos elementos terras-raras pesados e moderada a forte anomalia negativa dos elementos de alto campo de força (especialmente Nb), uma assinatura geoquímica típica de zona de subducção. As unidades basais do ofiolito Trincheira são quimicamente similares aos modernos basaltos de cadeia meso-oceânica (MORB). Esse comportamento químico muda para as unidades de topo as quais apresentam uma assinatura similar aos toleiítos de arco-de- ilha (IAT). Portanto, o ofiolito Trincheira deve ter sido originado em um ambiente intra-oceânico de supra-subducção composto de um sistema de arco/retro-arco. Os dados isotópicos de Sm, Nd e Sr para essas rochas indicam valores iniciais de Nd de moderados a altamente positivos (+2.6 a +8.8) e muito baixa razão inicial de 87Sr/86Sr (0,7013 – 0,7033), sugerindo que esses magmas foram originados a partir de uma fonte mantélica empobrecida e nada ou fracamente contaminados por componentes de subducção. O complexo ofiolítico foi deformado, metassomatizado e metamorfisado durante o desenvolvimento da Faixa Móvel Guaporé, um orógeno acrescionário-colisional Mesoproterozóico (1,47-1,35 Ba), constituído pela zona de sutura Guaporé, a qual une o Cráton Amazônico com o Bloco Paraguá. A fase colisional que marca o encaixe final dessas duas massas continentais ocorreu por volta de 1,35 Ba, onde o metamorfismo atingiu temperaturas entre 780 a 853°C nos granulitos máficos e 680 a 720°C nos anfibolitos, com pressão média de 6,8 kbar. A sutura Guaporé foi reativada no final do Mesoproterozóico e evoluiu para a abertura de um rift intracontinental, com a sedimentação das rochas dos Grupos Nova Brasilândia e Aguapeí, o qual marca a fragmentação final do supercontinente Columbia, por volta de 1,3-1,2 Ba. Granulitos máficos, anfibolitos e trondhjemitos da porção meridional do Cinturão Nova Brasilândia, representativos da última fase compressional que afetou o sudoeste do Cráton Amazônico, forneceram idades U-Pb de 1110 Ma, as quais datam o metamorfismo de alto grau e o fechamento do rift, processo resultante da acresção do microcontinente Arequipa-Antofalla ao Cráton Amazônico. Portanto, a fragmentação do supercontinente Columbia foi seguida rapidamente pela aglomeração de outras massas continentais, formando o supercontinente Rodínia, por volta de 1100 Ma. / The tectonic framework of the ultramafic-mafic rocks of the southwestern Amazon Craton is generally little known. Most of work this cratonic portion is focused on the geochronological studies of granitoids, so that little is known about the origin and tectonic significance of these rocks. In this context, this study contributes to the knowledge of the tectonic evolution of the southwestern Amazon Craton, through the characterization of a Mesoproterozoic ophiolitic complex, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation, pelites, psammitic and a smaller proportion of calc-silicate rocks. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. These rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB). This behavior changes to an island arc tholeiites (IAT) signature in the upper units of the Trincheira ophiolite. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Mafic-ultramafic rocks of the Trincheira ophiolites display moderate to highly positive initial Nd values of +2.6 to +8.8 and very low values for the initial 87Sr/86Sr ratio (0.7013 - 0.7033). It is suggested that these magmas originated from a depleted mantle source, which experienced low degree of contamination by variable subduction components. The ophiolitic sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary-collisional orogen that represents the Guaporé suture zone. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The Guaporé suture zone is defined by the ESE–WNW trending mafic-ultramafic belt formed during a Mesoproterozoic (ca. 1.47-1.43 Ga) accretionary phase, and overprinted by upper amphibolite-granulite facies metamorphism during collisional phase in the Ectasian (~1.35 Ga), which mark the docking final of the Amazon craton and Paraguá Block. This suture was reactivated and evolved from the development of an intracontinental rift environment, represented by Nova Brasilândia and Aguapeí Groups, which mark the final breakup of the supercontinent Columbia in the late Mesoproterozoic (ca. 1.3-1.2 Ga). Mafic granulites, amphibolites and trondhjemites in the northernmost portion of the Nova Brasilândia belt yield U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift, due to the accretion of the Arequipa-Antofalla basement to the Amazon craton. Therefore, the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1100 Ma.