Mémoire (M.Sc.T.)--Université du Québec à Chicoutimi, 1996. / Document électronique également accessible en format PDF. CaQCU

Thèse (M.Sc.T.) -- Université du Québec à Chicoutimi, 2004. / Bibliogr.: f. 151-161. Document électronique également accessible en format PDF. CaQCU

Thesis (Ph. D.)--University of Washington, 1992. / Vita. Includes bibliographical references (leaves 150-155).

M.Sc. / The Central Zone of the Limpopo Complex displays two major structural features: the roughly east-west oriented Tshipise Straightening Zone Paleoproterozoic in age and a “Cross Folded Zone” to the north of the Straightening Zone comprising large-scale sheath and cross folds suggested to have developed during a Late- Archaean high grade tectono-metamorphic event. This study presents and discusses structural-metamorphic data showing that two closely associated folds (Ga-Tshanzi and Campbell) in the eastern part of the Cross Folded Zone near Musina, record different structural and metamorphic histories that may be applied to the evolution of the entire Central Zone of the Limpopo Complex. The Ga-Tshanzi structure has an ovate-shaped closed outcrop pattern approximately 4km long, and 3km wide with the long axis of the fold pattern oriented in a westerly direction. The fold geometry, characterized by a central fold axis that plunges steeply to the SSW, is very similar to other closed folds in the Central Zone previously interpreted as sheath folds. The Ga-Tshanzi fold deforms rocks of the Beit Bridge Complex (calc-silicate, metaquartzite, metapelite and magnetite quartzite and quartzofeldspathic Singelele Gneiss), and members of the Messina Layered Suite. The ovate structure is characterised by a gneissic fabric comprising peak metamorphic mineral assemblages. This regional gneissic fabric that occurs throughout the Central Zone also defines the shape of the neighbouring Campbell fold. Mineral lineations and fold hinges in the Ga-Tshanzi fold mainly present within metaquartzites and calc-silicates, plunge steeply to the southwest, parallel to its central fold axis indicating a NNE-SSW transport direction during fold formation. A decompression-cooling P-T path calculated for metapelitic gneisses from the Ga-Tshanzi fold shows that the closed fold developed under high-grade, deep crustal conditions. Peak P-T conditions of 7.5kbar/799ºC were followed by decompression and cooling down to 5.23kbar/605ºC. Water activity during this event was low, ranging from 0.122 at peak conditions, and decreasing to 0.037 at the minimum calculated conditions. The Ga-Tshanzi closed fold and the closely associated Campbell cross fold were thus formed at deep crustal levels and partially exhumed along a similar decompression-cooling P-T path to mid-crustal levels during the early orogenic event. The Campbell fold, described as a cross fold in the literature, is approximately 15km long and has a V shaped outcrop pattern that tapers from 12km in the southeast to 2 km in the northwest. This fold is developed in lithologies similar to those of the Ga-Tshanzi fold as well as in Sand River Gneisses. It has a near isoclinal fold geometry with both limbs dipping towards the southwest and a fold axis that plunges moderately to the west-southwest. This fold, that is interpreted to have developed during the same deformational event as the Ga-Tshansi structure has, however, subsequently been affected at mid- to upper crustal levels by shear movement along the Tshipise Straightening Zone displaying widespread development of younger planar and linear structural features. Planar features include north-south-trending high temperature shear zones that crosscut the regional fabric and flexural slip planes particularly evident in quartzites. Linear features from the Campbell fold that are mainly developed in younger shear and flexural slip planes, indicate, in contrast to the Ga-Tshanzi fold, an ENE-WSW directed crustal movement that is in accordance with the sense of movement suggested for the Tshipise Straightening Zone. The calculated decompression-cooling P-T path for sheared metapelitic gneisses from discrete high temperature shear zones deforming rocks of the Campbell cross fold shows that this superimposed shear deformational event occurred under peak P-T conditions of 4.98kbar/681ºC, followed by decompression and cooling down to 3.61kbar/585ºC. Water activity during this shear event was high, ranging from 0.217 at peak conditions and decreases to 0.117 at minimum calculated conditions. Structural and metamorphic data for the two folded areas thus indicate two distinct tectono-metamorphic events: (i) a late Archaean peak metamorphic and deformational event responsible for the formation of the Ga-Tshanzi fold, and similar folds throughout the Central Zone including the Campbell cross fold that was accompanied by steep NNE-SSW transport of crustal material, and (ii) a shear deformational event linked to the Paleoproterozoic Tshipise Straightening Zone that partially obliterated the early structural and metamorphic history of the Campbell fold during mid to upper crustal conditions during relatively shallow ENE-WSW directed movement of crustal material. The fact that this superimposed event had no apparent metamorphic effect on the studied metapelitic rocks of the closely associated Ga-Tshanzi closed fold, suggests that shearing was constrained to discrete north-south orientated zones.

M.Sc. / The Limpopo Complex (LC) of southern Africa is one of the best-studied Precambrian granulite facies terrains in the world, yet workers still disagree on fundamental aspects of the geological evolution of this complexly deformed high-grade terrain. Most workers agree that the two marginal zones were exhumed in the late-Archaean, but disagree on the timing of major tectono-metamorphic events that affected the Central Zone (CZ) of Limpopo Belt, and the mechanism/s of its formation. There are currently two main schools of thought: The first school regards the LC as a late-Archaean orogenic zone that resulted from a north-south collision of the Zimbabwe and Kaapvaal cratons. Granitic plutons throughout the entire LC are considered to be accurate time-markers for this orogeny. The second school suggests that the CZ evolved as a result of a major Paleoproterozoic tectono-metamorphic event based mainly on the interpretation of metamorphic mineral ages. The present study focuses on two aims, namely (i) to provide a synthesis of published data as a basis to understand the ongoing age controversy concerning the evolution of the CZ, and (ii) to show that specific fold types in the CZ can be related to either the late-Archaean or the Paleoproterozoic event. New age, structural, metamorphic, and petrographic data are presented to show that (i) major sheath folds reflect the peak tectono-metamorphic event that affected the CZ in the late-Archaean, while (ii) major cross folds developed as a result of a transpressive event in the Paleoproterozoic. The age of formation of the Avoca sheath fold located about 40 km west of Alldays is accurately constrained by the age of emplacement of different structural varieties of precursors to the Singelele Gneiss: penetratively deformed syn- to late-tectonic Singelele gneisses with a zircon SHRIMP age of 2651 ± 8 Ma, date the time of formation of the sheath fold that is characterized by a single population of linear elements that define the central fold axis. The Avoca sheath fold documents top-to-the-NNE movement of material during the exhumation of the high-grade CZ rocks. Weakly foliated late-tectonic L-tectonites with a zircon SHRIMP age of 2626.8 ± 5.4 Ma, outcrop near the centre of the sheath fold, and provide a minimum age for the shear deformation event. An almost undeformed (post-tectonic) variety of the Singelele Gneiss was emplaced after the shear event. A detailed metamorphic study of metapelitic gneisses from the large Baklykraal cross fold, located about 20 km east of the Avoca sheath fold, documents a single decompression-cooling (DC) P-T path for the evolution of this structure. Three studied metapelitic samples characterized by a single generation of garnet provide a Pb-Pb age of 2023 ± 11 Ma, that accurately constrain the time of formation of this major fold to the Paleoproterozoic. A metapelitic sample characterized by two generations of garnet provide a slightly older Pb-Pb age of 2173 ± 79 Ma, that is interpreted to also reflect the late-Archaean event. The Baklykraal cross fold is characterized by two populations of linear elements: the one population defines the shallow N-S oriented fold axes, while the second population is associated with top-to-the-NNE movement of material during exhumation, resulting in folds with a nappe-like geometry. A DC P-T path for the Campbell cross fold (Van Kal, 2004) located just west of Musina, suggests that cross folds developed under significantly lower P-T conditions than is the case with sheath folds, providing an explanation for the lack of significant anatexis associated with the Paleoproterozoic event. The late-Archaean orogeny in contrast, was accompanied by widespread anatexis during a major magmatic event that is characterized by an abnormal high radiogenic signature. This study, for the first time, provides evidence that link specific fold types, and thus deformational events, to different tectono-metamorphic events. The main conclusion is that the CZ was exhumed as the result of two distinct orogenies, one in the late-Archaean, and the other in the Paleoproterozoic.

A more than 2000 m thick sequence of Hadrynian to Paleozoic Snowshoe Group metasedimentary rocks of the Omineca Belt (OMB) is exposed near Niagara Peak in the western Cariboo Mountains, central British Columbia. This package contains the northern extremity of the Shuswap Metamorphic Complex and lies 30 km northeast of the accretionary boundary with Intermontane Belt (IMB) Mesozoic sedimentary and volcanic rocks (Quesnellia Terrane) and Upper Paleozoic (?) ophiolitic and sedimentary rocks (Slide Mountain Terrane). Four phases of folding (D₁-D₄) are recognized. D₁ consists of isoclinal folds and transposed compositional layering. D₂ commonly forms southwest verging, open to close folds with subhorizontal axes and moderately northeast dipping axial surfaces. In the eastern part of the area, divergent fanning of D₂ axial surfaces and a reversal of vergence direction occur about a map-scale synform characterized by greater strain, bimodal fold style and a locally penetrative axial planar cleavage. D₃ and D₄ form orthogonal upright open buckles with respective northwest and northeast trending axes. Steeply dipping normal and minor reverse faults crosscut all fold structures, displaying minor offsets. Prograde regional metamorphism reached greenschist grade late in D₁. Staurolite and kyanite growth accompanied D₂, followed by postkinematic sillimanite generation under conditions of approximately 635° C and 5 kb. D₃ associated sericite-chlorite retrogression of porphyroblasts occurs in sub-sillimanite grade rocks in the western part of the area. Synmetamorphic veins represent polyepisodic hydraulic fracture development during progressive dewatering of a sedimentary pile by prograde metamorphism. Eastward obduction of Quesnellia and Slide Mountain Terranes onto theOmineca Belt took place in the Middle Jurassic. Shortly following this event, the IMB-OMB tectonic suture was deformed, forming map-scale folds of cuspate/lobate geometry. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Detailed structural and petrologic mapping near Azure Lake, British Columbia provides an overview of geologic relations along the northeast margin of the Shuswap Metamorphic Complex. Four phases of deformation have been recognized in the Shuswap Complex and the adjacent lower grade metasediments of the cover sequence. The first deformation consists of west-verging isoclinal folds plunging north and northwest. The second phase resulted in large upright folds with a shallow northwest or southeast plunge. The third and fourth phases are only locally developed as faults, fractures, and brittle folds trending north and northeast, respectively. Mineral assemblages range from garnet-biotite through first sillimanite zones of the Barrovian facies series. Metamorphic grade increases toward the southwest. Regional metamorphism is associated with the first phase of deformation. The Complex is separated from the adjacent cover sequence by a first phase tectonic slide. Structural and metamorphic discontinuities across this slide probably resulted from reactivation of the slide surface during the second phase of deformation. Microprobe analyses have been combined with linear regression techniques to outline probable sillimanite-forming reactions in pelites of the Complex. The regressions show that reaction textures are partly preserved because of the exhaustion of rutile as a reactant phase. Metamorphic conditions in the Complex are estimated from the mutual intersection of experimentally studied mineral equilibria. These conditions are: P=7600 ± 400 bars, T=705 ± 40°C, aH₂O =0.5 ⁺⁰•⁵­₀․₂ . Carbonate mineral assemblages initially buffered fluid phase compositions to high X CO₂ values near 0.75 during metamorphism. Therefore the fluid phase was not homogeneous in composition throughout all rock types during metamorphism. Whole rock Rb-Sr dates of 138 ± 12 Ma (all five samples) and 163 ± 7 Ma were obtained for granodiorite stocks in the Azure Lake area. Two biotite-whole rock ± hornblende dates of 119 ± 11 Ma and 77 ± 20 Ma indicate isotopic resetting. Initial Sr⁸⁷ -Sr⁸⁶ ratios vary from 0.7061 ± 0.0001 to 0.7103 ± 0.0002 for rock and mineral dates. These dated stocks cross-cut structural trends for the first two deformations and impose a hornfelsic contact aureole on regional metamorphic assemblages. Therefore regional metamorphism and deformation were completed by Late Jurassic time. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Orientador: Ticiano José Saraiva dos Santos / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociências / Made available in DSpace on 2018-08-18T11:36:53Z (GMT). No. of bitstreams: 1 Ancelmi_MatheusFernando_M.pdf: 13152442 bytes, checksum: 9332eaa1da5b9097c8fe1da5caf15557 (MD5) Previous issue date: 2011 / Resumo: Na porção NW da Província Borborema, Domínio Ceará Central, a existência de retroeclogitos posicionados a oeste do Arco Magmático de Santa Quitéria são importantes registros geológicos para caracterização geotectônica, principalmente pelo fato destes tipos de rochas comumente marcarem zonas de subducção oceânica e colisão de blocos continentais. Com mapeamento geológico, em escala 1:10.000, e estudo petrográfico foram caracterizados dezenas de lentes de retroeclogitos numa faixa N-S ao longo de mais de 16 km. Tais rochas estão encaixadas em gnaisses orto- e paraderivados, e raramente associadas a rochas cálcio-silicáticas. O retrometamorfismo ocorreu de forma heterogênea nestes corpos metamáficos, sendo mais intenso nas bordas devido sua interação com fluídos contidos nos gnaisses encaixantes. Assim, são sugeridas as seguintes reações de retrometamorfismo: 1º) Grt + Cpx + Qtz + H2O ? Pl + Amp ou Grt + Cpx + Rt + Qtz + H2O ? Amp + Ilm + Pl; 2°) Omp ? Di + Pl; 3º) Grt + Di + Pl + H2O ? Amp; 4°) Grt + Pl + Qtz + H2O ? Grt2 + Pl (An) + Amp. Cabendo ressaltar a incipiente formação de Fe(Mg)- clorita e actinolita que são minerais de fácies xisto verde. Já os gnaisses encaixantes estão migmatizados e apresentam a paragênese de sillimanita (depois de cianita) + granada + biotita + álcali-feldspato + rutilo + quartzo ± plagioclásio, típica de fácies granulito de alta pressão. Estas associações compõem a Faixa Eclogítica de Forquilha, que estruturalmente está em contato tectônico de empurrão com faixas dominantemente constituídas por metapelitos e pequenas porções de metacalcários e metamáficas com diferentes graus metamórficos pertencentes ao Grupo Ceará. Estudos anteriores mostraram que a idade do protólito destes retroeclogitos é de ca. 1,57 Ga, com dois eventos de metamorfismo entre 650 a 630 Ma e 620 a 600 Ma. Para avaliar a relação destes retroeclogitos e suas encaixantes, foi realizado um estudo de proveniência de sedimentos, baseado em U-Pb (LA-ICP-MS) em zircões detríticos e isótopos de Nd, que identificou que esta bacia recebeu sedimentos continentais exclusivamente de fontes paleoproterozóicas com picos entre 2,0 e 2,2 Ga. A idade máxima de sedimentação é estimada em ca. 1850 Ma e os embasamentos paleoproterozóicos do Domínio Ceará Central e do Domínio Rio Grande do Norte são as prováveis fontes de sedimentos em uma margem passiva ou uma bacia intracratônica. Adicionalmente, estudo de proveniência foi realizado nas rochas do Grupo Ceará, e neste caso os zircões detríticos mostraram um espectro de idades desde o Paleoproterozóico até o Neoproterozóico. Zircões mais jovens que ca. 650 Ma possuem característica de grãos metamórficos por conta da baixa razão Th/U e textura do tipo ovóide e soccer ball. A idade máxima de deposição foi estimada pelo zircão que forneceu a idade de 659 ± 4 Ma (206Pb/238U, Th/U = 0,56), indicando que esta bacia teve um curto período de existência (ca. 15 Ma), sendo possivelmente parte de um ambiente de margem ativa. Estes dados e as relações de campo mostram que a Faixa Eclogítica de Forquilha foi exumada e colocada tectonicamente com unidades de cobertura de uma margem ativa / Abstract: In the NW Borborema Province, Ceará Central Domain, retrograded eclogites occur in the west side of the Santa Quitéria Magmatic Arc. These high-pressure metamafic rocks are important geological features, due to the fact that this type of rock is typical of subduction and collisional zones. Based on 1:10.000 geological map and petrography of the lithological units established in this work, it is possible to character that the Forquilha retrograded eclogites occur along more than 16 km in a N-S trend belt. These metamafic rocks are enclosed in migmatized ortho- and paragnaisses, and rarely in calc-silicate rocks. The retrogressive metamorphism occurred heterogeneously on these metamafic bodies, being more intensive at the outer parts and obliterating higher mineral paragenesis, possibly due to a greater interaction with fluids of the host rocks. Thus, it is suggested the following retrometamophic reactions: 1º) Grt + Cpx + Qtz + H2O ? Pl + Amp or Grt + Cpx + Rt + Qtz + H2O ? Amp + Ilm + Pl; 2°) Omp ? Di + Pl; 3º) Grt + Di + Pl + H2O ? Amp; 4°) Grt + Pl + Qtz + H2O ? Grt2 + Pl (An) + Amp, besides the incipient formation of Fe(Mg)-chlorite and actinolite that are typical of greenschist facies conditions. The paragneisses that enclose these rocks are migmatized and present the mineral paragenesis of sillimanite (after kyanite) + garnet + biotite + alkali-feldspar + rutile + quartz ± plagioclase, which is typical of high-pressure granulite facies conditions. Together, these rocks establish the Forquilha Eclogite Zone, which is in tectonic contact with rocks of Ceará Group that are dominantly constitute by metapelites and minor portions of metacalcareous and metamafic rocks metamorphosed in different levels. Previous geochronological studies on these retrograded eclogites show a ca. 1.57 Ga protolith age, and two metamorphic ages ranging from 650 to 630 Ma and 620 to 600 Ma. A sediment provenance study, based on LA-ICPMS detrital zircon U-Pb ages and whole rock Nd isotopic data, were performed on the metasedimentary rocks that host these retrograded eclogites, in order to determine the maximum depositional age and the possible sources of sediment material. The zircon age distribution, Nd isotopic rations and TDM model ages demonstrated that these rocks exclusively received sediments from Paleoproterozoic sources older than ca. 1.85 Ga, with peaks at 2.0 to 2.2 Ga. The Paleoproterozoic basement of Rio Grande do Norte Domain and Ceará Central Domain are estimated as the main possible sources of sediments on a passive margin or intracratonic basin. It also was analyzed detrital zircons of non-migmatized metasedimentary rocks of Ceará Group, which are in tectonic contact with FEZ. In this case, the samples showed an age distribution from Paleoproterozoic to Neoproterozoic. Zircons grains younger than 650 Ma present low Th/U ratio, and ovoid and soccer ball textures typical of metamorphic grains. Because of this, the maximum deposition age of this sequence is constrained by the youngest zircon grain dated at 659 ± 4 Ma (206Pb/238U age, Th/U = 0.56), what indicates that this sedimentary basin had a short life (ca. 15 Ma) and possible have been part of an active margin setting. These data and structural field observations suggest that the FEZ was exhumed and tectonically juxtaposed with cover units of a active margin setting / Mestrado / Geologia e Recursos Naturais / Mestre em Geociências

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