In the Pilgangoora Belt of the Pilbara Craton, Australia, the 3517 Ma Coonterunah Group and 3484-3468 Ma Carlindi granitoids underlie the 3458 Ma Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE, and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. Indeed, such a model may be applicable to most other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4.0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3.5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4.0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A Suíte Intrusiva Santa Clara está inserida na Província Estanífera de Rondônia, na porção SW do Cráton Amazônico. Essa suíte intrusiva é composta pelos maciços Santa Clara, Oriente Velho, Oriente Novo, Manteiga-Sul, Manteiga-Norte, Jararaca, Carmelo, Primavera e das Antas. Os litotipos que perfazem a Suíte Santa Clara ocorrem hospedados nas rochas do Complexo Jamari, uma associação polideformada composta por gnaisses ortoderivados e paraderivados. Características observadas em campo e em análises petrográficas permitiram subdividir o Maciço Santa Clara em cinco fácies distintas: fácies porfirítica, fácies isotrópica, fácies fina, fácies piterlítica e fácies viborgítica. Os litotipos observados correspondem a hornblenda-biotita granitos e biotita granitos intermediários a ácidos, com composições médias semelhantes àquelas verificadas para sienogranitos e monzogranitos. Geoquimicamente, três magmas podem ser identificados. O magma menos evoluído corresponde às rochas das fácies porfirítica e equigranular, e o mais evoluído compreende as fácies de granulometria fina e piterlítica. A fácies viborgítica representa o terceiro líquido magmático, e aparentemente é diferente de todas as outras fácies em termos de aspectos de campo e geoquímica. A análise litogeoquímica indica que estes granitoides são subalcalinos, bastante empobrecidos em MgO e exibem caráter metaluminoso a fracamente peraluminoso. Os padrões de elementos-traços evidenciam que tais granitóides possuem alto conteúdo em elementos incompatíveis (Rb, Zr, Y, Ta, Ce) e ETR, com exceção do Eu. Além disso, também exibem leve enriquecimento em LILE, forte depleção em elementos como Sr e Ti, e leve empobrecimento de Ba, indicando que o fracionamento de minerais como plagioclásio e titanita foi importante na evolução do líquido magmático analisado. A anomalia negativa de Nb indica envolvimento de material crustal nos processos magmáticos que geraram estes granitoides. Os litotipos analisados possuem características típicas de granitos tipo-A ferroan, e as razões FeOt/MgO entre 4,27 e 26,22 sugerem tratar-se de uma série de granitos félsicos fracionados. Os padrões de ETR observados para os litotipos analisados exibem um considerável enriquecimento em ETRL, e anomalia negativa de Eu, sugerindo fracionamento de feldspato durante o processo de diferenciação do líquido magmático. Diagramas discriminantes de ambientes tectônicos sugerem que os litotipos do Maciço Intrusivo Santa Clara são típicos de ambiente intraplaca, do tipo-A2, isto é, associados a ambientes pós-colisionais/pós-orogênicos. As características isotópicas observadas para os granitoides do Maciço Santa Clara sugerem que os mesmos foram gerados a partir da fusão parcial de uma crosta inferior pré-existente. As idades U-Pb entre 1,07 e 1,06 Ga são compatíveis com um magmatismo ocorrido nos estágios finais da colagem do supercontinente Rodínia (1,2-1,0 Ga) e estágios finais do Ciclo Orogênico Sunsás-Aguapeí (1320-1100 Ma). Sugere-se ainda que na verdade o Maciço Santa Clara seja formado por uma coalescência das três intrusões graníticas que são representadas pelos três magmas anteriormente descritos. / The Santa Clara Intrusive Suite in the Rondônia Tin Province (SW Amazonian Craton) comprises the Santa Clara, Oriente Velho, Oriente Novo, Manteiga-Sul, Manteiga-Norte, Jararaca, Carmelo, Primavera and das Antas massifs. The rocks of the Santa Clara Intrusive Suite are emplaced in the Jamari Complex, an association of ortho and paragneisses which underwent several and complex metamorphic processes. Characteristics observed during both geological mapping and petrographic analyses allowed, for the first time, to subdivide the Santa Clara Massif (SCM) granitoids into five different facies: porphyry, equigranular, fine-grained facies, pyterlitic and wiborgitic facies. These lithotypes comprise hornblende-biotite granites and biotite granites that range from intermediate to acids, and show compositions similar to syenogranites and monzogranites. Geochemical analyses suggest that these granitoids may be divided into three different magmas, considering field aspects and geochemical characteristics. Therefore, the less evolved magma is represented by porphyritic and equigranular facies, and the most evolved magma comprises both fine-grained and pyterlitic facies. The wiborgitic facies representd the other magma, and is geochemically different from all the others facies. Geochemical analyses also show that the granitoids of the Santa Clara Massif are subalkaline, have very low MgO contents and have metaluminous to slightly peraluminous character. Trace elements patterns show that these granitoids have high contents of incompatible elements (Rb, Zr, Y, Ta, Ce) and REE, with exception of Eu. Moreover, they are also slightly enriched in LILE, strongly depleted in elements such as Sr and Ti, and slightly depleted in Ba, pointing out the importance of plagioclase and titanite fractioning during the evolution of these magmatic liquids. Negative anomalies of Nb, along with other geochemical features, suggest the participation of crustal material in the magmatic processes responsible for these granitoids generation. The lithotypes have typical characteristics of ferroan A-type granites, and FeOt/MgO ratios ranging from 4.27 to 26.22 indicate that these are fractionated felsic granites. REE patterns show a remarkable enrichment in LREE along with negative Eu anomaly. Tectonic discriminant diagrams for the Santa Clara Massif granitoids suggest that these are intraplate granitoids, A2-type, that is, related to post-collisional/post-orogenic settings.The isotopic characteristics observed for the Santa Clara Massif granitoids suggest that these were generated through partial melting of a preexistent lower crust. The U-Pb ages between 1,07 e 1,06 Ga are compatible with a magmatism taken place during the final stages of the supercontinent Rodinia agglutinations and the final stages of the Sunsás-Aguapeí Orogenic Cycle. It is also suggested that the Santa Clara Massif represents the coalescence of three different granitic intrusions, which comprise the magmas described above.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A Suíte Intrusiva Santa Clara está inserida na Província Estanífera de Rondônia, na porção SW do Cráton Amazônico. Essa suíte intrusiva é composta pelos maciços Santa Clara, Oriente Velho, Oriente Novo, Manteiga-Sul, Manteiga-Norte, Jararaca, Carmelo, Primavera e das Antas. Os litotipos que perfazem a Suíte Santa Clara ocorrem hospedados nas rochas do Complexo Jamari, uma associação polideformada composta por gnaisses ortoderivados e paraderivados. Características observadas em campo e em análises petrográficas permitiram subdividir o Maciço Santa Clara em cinco fácies distintas: fácies porfirítica, fácies isotrópica, fácies fina, fácies piterlítica e fácies viborgítica. Os litotipos observados correspondem a hornblenda-biotita granitos e biotita granitos intermediários a ácidos, com composições médias semelhantes àquelas verificadas para sienogranitos e monzogranitos. Geoquimicamente, três magmas podem ser identificados. O magma menos evoluído corresponde às rochas das fácies porfirítica e equigranular, e o mais evoluído compreende as fácies de granulometria fina e piterlítica. A fácies viborgítica representa o terceiro líquido magmático, e aparentemente é diferente de todas as outras fácies em termos de aspectos de campo e geoquímica. A análise litogeoquímica indica que estes granitoides são subalcalinos, bastante empobrecidos em MgO e exibem caráter metaluminoso a fracamente peraluminoso. Os padrões de elementos-traços evidenciam que tais granitóides possuem alto conteúdo em elementos incompatíveis (Rb, Zr, Y, Ta, Ce) e ETR, com exceção do Eu. Além disso, também exibem leve enriquecimento em LILE, forte depleção em elementos como Sr e Ti, e leve empobrecimento de Ba, indicando que o fracionamento de minerais como plagioclásio e titanita foi importante na evolução do líquido magmático analisado. A anomalia negativa de Nb indica envolvimento de material crustal nos processos magmáticos que geraram estes granitoides. Os litotipos analisados possuem características típicas de granitos tipo-A ferroan, e as razões FeOt/MgO entre 4,27 e 26,22 sugerem tratar-se de uma série de granitos félsicos fracionados. Os padrões de ETR observados para os litotipos analisados exibem um considerável enriquecimento em ETRL, e anomalia negativa de Eu, sugerindo fracionamento de feldspato durante o processo de diferenciação do líquido magmático. Diagramas discriminantes de ambientes tectônicos sugerem que os litotipos do Maciço Intrusivo Santa Clara são típicos de ambiente intraplaca, do tipo-A2, isto é, associados a ambientes pós-colisionais/pós-orogênicos. As características isotópicas observadas para os granitoides do Maciço Santa Clara sugerem que os mesmos foram gerados a partir da fusão parcial de uma crosta inferior pré-existente. As idades U-Pb entre 1,07 e 1,06 Ga são compatíveis com um magmatismo ocorrido nos estágios finais da colagem do supercontinente Rodínia (1,2-1,0 Ga) e estágios finais do Ciclo Orogênico Sunsás-Aguapeí (1320-1100 Ma). Sugere-se ainda que na verdade o Maciço Santa Clara seja formado por uma coalescência das três intrusões graníticas que são representadas pelos três magmas anteriormente descritos. / The Santa Clara Intrusive Suite in the Rondônia Tin Province (SW Amazonian Craton) comprises the Santa Clara, Oriente Velho, Oriente Novo, Manteiga-Sul, Manteiga-Norte, Jararaca, Carmelo, Primavera and das Antas massifs. The rocks of the Santa Clara Intrusive Suite are emplaced in the Jamari Complex, an association of ortho and paragneisses which underwent several and complex metamorphic processes. Characteristics observed during both geological mapping and petrographic analyses allowed, for the first time, to subdivide the Santa Clara Massif (SCM) granitoids into five different facies: porphyry, equigranular, fine-grained facies, pyterlitic and wiborgitic facies. These lithotypes comprise hornblende-biotite granites and biotite granites that range from intermediate to acids, and show compositions similar to syenogranites and monzogranites. Geochemical analyses suggest that these granitoids may be divided into three different magmas, considering field aspects and geochemical characteristics. Therefore, the less evolved magma is represented by porphyritic and equigranular facies, and the most evolved magma comprises both fine-grained and pyterlitic facies. The wiborgitic facies representd the other magma, and is geochemically different from all the others facies. Geochemical analyses also show that the granitoids of the Santa Clara Massif are subalkaline, have very low MgO contents and have metaluminous to slightly peraluminous character. Trace elements patterns show that these granitoids have high contents of incompatible elements (Rb, Zr, Y, Ta, Ce) and REE, with exception of Eu. Moreover, they are also slightly enriched in LILE, strongly depleted in elements such as Sr and Ti, and slightly depleted in Ba, pointing out the importance of plagioclase and titanite fractioning during the evolution of these magmatic liquids. Negative anomalies of Nb, along with other geochemical features, suggest the participation of crustal material in the magmatic processes responsible for these granitoids generation. The lithotypes have typical characteristics of ferroan A-type granites, and FeOt/MgO ratios ranging from 4.27 to 26.22 indicate that these are fractionated felsic granites. REE patterns show a remarkable enrichment in LREE along with negative Eu anomaly. Tectonic discriminant diagrams for the Santa Clara Massif granitoids suggest that these are intraplate granitoids, A2-type, that is, related to post-collisional/post-orogenic settings.The isotopic characteristics observed for the Santa Clara Massif granitoids suggest that these were generated through partial melting of a preexistent lower crust. The U-Pb ages between 1,07 e 1,06 Ga are compatible with a magmatism taken place during the final stages of the supercontinent Rodinia agglutinations and the final stages of the Sunsás-Aguapeí Orogenic Cycle. It is also suggested that the Santa Clara Massif represents the coalescence of three different granitic intrusions, which comprise the magmas described above.

Idades radiométricas absolutas recentemente publicadas levantaram questões sobre a evolução deposicional da Formação Sete Lagoas (FSL). Esta unidade é composta predominantemente por carbonatos com rochas siliciclásticas subordinadas e constitui a unidade basal do Grupo Bambuí, sobreposta aos depósitos glaciogênicos da Formação Jequitaí no Cráton São Francisco (CSF). Este estudo combina quimioestratigrafia isotópica (C, O e Sr) e geocronologia (U-Pb e Hf em zircão detrítico e Sm-Nd em rocha total) em amostras de cinco seções da FSL na região de Lagoa Santa, MG, sul do CSF, com a finalidade de responder a tais questionamentos. As seções Vespasiano (VS) e Ana Paula (AP) são constituídas por calcários cinzas e dolomitos beges, com altos teores de sedimentos terrígenos e pobres em matéria orgânica. Os valores de \'delta\'\'POT.13C\' mais representativos do ambiente deposicional oscilam em uma estreita faixa ao redor de 0%o. Estes dados permitem posicionar estas seções na primeira sequência deposicional da FSL, acima dos carbonatos de capa Sturtianos (~740 Ma) da base desta unidade. As seções Bairro da Lapinha (BL), Pedra do Baú (BAU) e Parque da Gruta da Lapinha (PGL) são compostas por calcários de coloração cinza escura a negra, pobres em sedimentos detríticos e ricos em matéria orgânica. São caracterizados por valores de \'delta\'\'POT.13\' C bastante positivos (> 6%o) e razões \'87 ANTPOT.Sr\'/\'86 ANTPOT.Sr\' próximas de 0,7075. Estas seções estão posicionadas na segunda sequência deposicional da FSL. Zircões detríticos foram separados de três amostras de marga das seções VS, AP e PGL e datados pelo método U-Pb. Os resultados indicam que os sedimentos siliciclásticos da FSL na área de estudo provêm de fontes de longa residência crustal do Orógeno Araçuaí-Oeste Congo. A população expressiva mais jovem tem 557 Ma e determina a idade máxima de deposição para a segunda sequência da unidade e para a maior parte do Grupo Bambuí. Além disso, grãos de zircão concordantes mais jovens com idades de 537 ± 4 Ma e 506 ± 7 Ma para a primeira e segunda sequência, respectivamente, refutam a existência de um hiato deposicional expressivo entre as duas sequências, como recentemente proposto, e endossam uma idade deposicional do limite Ediacarano/Cambriano para a FSL. Se existe uma discordância, esta está posicionada entre os carbonatos de capa Sturtianos e as seções com valores de \'delta\'\'POT.13\'C\' ao redor de 0%o. Estas idades indicam que grande parte do Grupo Bambuí foi depositada em uma bacia de foreland, após o fechamento do Oceano Adamastor que culminou com a edificação da Faixa Araçuaí a leste do CSF. As razões \'87 ANTPOT.Sr\'/\'86 ANTPOT.Sr\' obtidas nos carbonatos da FSL contrastam com as recentes curvas de evolução de Sr para os oceanos, especialmente no Cambriano, quando razões maiores que 0,7085 são esperadas. É provável que esta unidade tenha sido depositada em um mar epicontinental restrito e a correlação global por meio de isótopos de Sr não é confiável nestes casos. / Recently published geochronological data has arisen questions on the Sete Lagoas Formation (SLF) depositional evolution. This unit is mainly composed by carbonate rocks with subordinated pelitic intercalations and represents the basal unit of the Bambuí Group, which overlies the glacial deposits of the Jequitaí Formation in the São Francisco Craton (SFC). This study combines isotope chemostratigraphy (C, O, Sr) and geochronology (U -Pb and Hf on detrital zircons and Sm-Nd on whole rock samples) in five sections of the SLF in the Lagoa Santa (MG) region, southern part of SFC, in order to answer such questions. Vespasiano (VS) and Ana Paula (AP) sections are composed by gray limestones and beige dolostones, with high contents of detrital sediments and poor in organic matter. The most representative \'delta\'\'POT.13C\' values obtained oscillate within a narrow range around 0%o. These data allow positioning these sections in the first depositional sequence of the SLF, above the basal Sturtian cap carbonates (~740 Ma) of this unit. Bairro da Lapinha (BL), Pedra do Baú (BAU) and Parque da Gruta da Lapinha (PGL) sections comprises dark gray to black limestones, with low detrital sediments contents and rich in organic matter. They are characterized by very positive \'delta\'\'POT.13\'C values (> 6%o) and \'87 ANTPOT.Sr\'/ \'86 ANTPOT.Sr\' ratios close to 0.7075. These sections belong to the second sequence of the SLF. Detrital zircons were retrieved from three marl samples from sections VS, AP and PGL and dated by the U-Pb method. The results indicate that the siliciclastic sediments of the SLF come from sources of long crustal residence time located in the Araçuaí-West Congo Orogen. The youngest population is 557 Ma aged and sets the maximum depositional age for the second sequence of SLF and most of the Bambuí Group. Furthermore, younger concordant zircon grains with ages of 537 ± 4 Ma and 506 ± 7 Ma for the first and second sequence, respectively, refute the hypothesis of a major sedimentation gap between the sequences, as recently proposed, and endorse an Ediacaran/Cambrian age for the SLF. If such gap does exist, it lies between the Sturtian cap carbonates and the sections with \'delta\'\'POT.13\' C around 0%o. These ages also indicate that the deposition of most of the Bambuí Group took place in a foreland basin, after the closure of the Adamastor Ocean which led to the edification of the Araçuaí Belt to the east of the SFC. The \'87 ANTPOT.Sr\'/ \'86 ANTPOT.Sr\' ratios obtained on the SLF carbonates contrast with the recently proposed Sr evolution curves, especially for the Cambrian, from where ratios higher than 0.7085 would be expected. It is possible that the SLF was deposited on a restricted eipiric sea and global correlations based on Sr isotopes are not reliable in such cases.

M.Sc. / Please refer to full text to view abstract

There exist various glacial units in the Neoproterozoic strata of southern Namibia (Kalahari Craton). They were recognised and discussed in the scientific literature for at nearly 100 years (e.g. Coleman, 1926; Gevers, 1931; Schwellnuss, 1941; Martin, 1965). The Snowball Earth theory (Hoffman et al., 1998) had an huge impact on Neoproterozoic geosciences and especially outcrops of the Otavi Group in northern Namibia helped to strengthen and support this idea. Nevertheless, the Neoproterozoic glacial horizons in southern Namibia were difficult to interpret and even more difficult to correlate, due to their tectonic overprint and their scarce outcrops. In order to correlate and differentiate the various Neoproterozoic glacial units of southern Namibia (western rim of Kalahari Craton) a multi‐method approach based on isotopic analyses on zircon grains, whole rock geochemistry, grain size measurements combined with extensive field work, mapping and sampling was applied. In total, ten sections were mapped and measured from which 33 samples were chosen for further analyses. Two of these samples represent local basement rocks, 19 the siliciclastic Neoproterozoic sedimentary cover including glacial diamictites, and twelve carbonate samples. 3474 single zircon grains were picked and measured for their dimensions (width and length). Of those, 2404 zircons were analysed with LA‐ICP‐MS techniques for their U‐Pb and Th‐U ratios in order to calculate detrital zircon ages and to obtain information about the source magma. 1535 of those gave concordant ages (90‐110 % of concordance). Further, selected zircon grains (in total 346) with concordant U‐Pb ages were analyses for their εHf(t) values. To gather more information and to be able to provide absolute ages for the Neoproterozoic glacial units the new technique of LAICP‐MS U‐Pb dating on carbonate samples was tested and gave reliable results for ten out of twelve samples (representing seven different sample locations). Field work revealed two sections containing the Sturtian as well as the Marinoan glacial diamictites in relatively undisturbed succession that qualified as reference profiles for Neoproterozoic strata in southern Namibia: the Dreigratberg and the Namuskluft section in the Gariep Belt close to the Orange River. All analysed samples contain a very similar detrital zircon isotopic record and the whole rock geochemical analyses confirm this interpretation. All siliciclastic samples show a general felsic provenance, with zircon ages mainly divided into two age groups (Mesoproterozoic 1.0 – 105 Ga and Palaeoproterozoic 1.7 – 2.1 Ga), reflecting four different growth and recycling events of Mesoproterozoic to Archaean crustal units. The samples have a geochemical signature of continental island arc and the zircon grain dimensions (width vs. length) are also very similar for all samples. Direct age dating of the samples based on detrital zircons was not possible caused by the lack of ages reflecting deposition times. Nevertheless, the most important differences between the various glacial horizons were found in petrographic features (diamictite pebble contents) and the age peak shift of detrital zircon U‐Pb ages (P/M ratio). Based on these and the two reference profiles correlations to other sections were achievable and the differentiation of four distinct Neoproterozoic glacial horizons for southern Namibia was possible. Furthermore, these new results provide new insights into the Neoproterozoic Gariep Belt formation comprising Tonian rifting events, Cryogenian formation of the Arachania Terrane and final Ediacaran collision of the Rio de la Plata and Kalahari cratons. The combination of all results reflects a continuous sedimentary recycling on the western Kalahari Craton. Comparison and statistical similarity tests based on zircon age data bases for possible source areas defined the Namaqua Natal and Gariep belts as the most likely sedimentary source areas, providing the rock material that got recycled for at least 200 Ma from Kaigas glaciation at ca. 750 Ma to Vingerbreek glaciation at ca. 550 Ma. In addition, the lack of exotic detrital zircon ages within the two Snowball Earth events of this study suggests the interpretation of none or only very minor glacial movement confirming the idea of a completely ice‐covered Earth. The assumed Sturtian and Marinoan ages of Numees Fm and Namaskluft Mbr diamictites were confirmed by the results of U‐Pb cap carbonate dating. Based on these, a minimum duration of ca. 8 Ma for the Sturtian and of ca. 14 Ma for the Marinoan glaciation can be assumed.:Abstract Kurzfassung Contents List of Figures List of Tables List of abbreviations Scientific question and thesis structure 1 Introduction 1.1 The Neoproterozoic era: Supercontinent dispersal and global glaciations 1.1.1 Rodinia supercontinent: Formation, dispersal, and location of Kalahari Craton 1.1.2 Glacial events during the Neoproterozoic era 1.1.2.1 A brief history on the discovery of Snowball Earth events 1.1.2.2 Formation and termination of a Snowball Earth event: The Snowball Earth flow chart 1.1.2.3 Hypotheses for cap carbonate formation 1.1.2.4 Survival of life during a Snowball Earth event 1.2 The Kalahari Craton 1.2.1 Evolution of the Kalahari Craton 1.3 Overview over the Geology of Namibia under special consideration of southern Namibia (Kalahari Craton) 2 Characteristics of southern Namibian Neoproterozoic glacial samples and sides 3 The problematic correlations of Neoproterozoic glacial deposits of the Kalahari Craton (southern Namibia) 4 Methods 4.1 Field work 4.2 Whole Rock geochemical analyses 4.3 Heavy mineral separation and SEM analyses on zircon grains of siliciclastic samples 4.4 Zircon grain size analyses 4.5 LA‐ICP‐MS analyses on zircon grains 4.5.1 U‐Pb analyses with LA‐SF‐ICP‐MS 4.5.2 Th‐U ratio determination on zircon grains 4.5.3 Hf‐isotope measurements with LA‐MS‐ICP‐MS 4.6 LA‐ICP‐MS U‐Pb dating on carbonates 4.7 Provenance interpretations and likeness tests based on zircon U‐Pb age data bases 5 Study I: “The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana” 5.1 Introduction and geological setting 5.2 Samples and methods 5.3 Results 5.4 Discussion and interpretation 5.5 Summary 6 Study II: “The four Neoproterozoic glaciations of southern Namibia and their detrital zircon record: The fingerprints of four crustal growth events during two supercontinent cycles” 6.1 Introduction 6.2 The samples 6.3 Methods 6.4 Results 6.5 Interpretation and discussion 6.6 Conclusion/Summary 7 Study III: “Correlation of Neoproterozoic diamictites in southern Namibia” 7.1 Introduction 7.2 Sample sites 7.2.1. The Kaigas and Sturtian Numees diamictites at the Orange River section 2.1.1. Outcrops of the Kaigas Fm diamictites 7.2.1.2 Outcrop of the Numees Fm diamictites (Sturtian) 7.2.2 The Sturtian diamcitite of the Blaubeker Fm (Witvlei Grp) at the farmgrounds Blaubeker and Tahiti 7.2.2.1 The Blaubeker diamictite at Blaubeker Farm (type locality) 7.2.2.2 The Blaubeker diamictite at Tahiti Farm (Gobabis‐syncline) 7.2.2.3 Correlation of Blaubeker diamictite at Blaubeker and Tahiti farms 7.2.3 The Sturtian diamictite at the Trekpoort Farm section 7.2.4 The Sturtian and Marinoan diamictites at Namuskluft section (reference profile) 7.2.5 The Sturtian and Marinoan diamictites at Dreigratberg section 7.2.6 Sturtian diamictite and Marinoan‐type cap carbonate at Dreigratberg North section 7.2.7 The Marinoan diamictite at the Witputs Farm section 7.2.8 The post‐Gaskiers Vingerbreek diamictite 7.2.8.1 The Vingerbreek diamictite along the Orange River 7.2.8.2 The Vingerbreek diamictite at Tierkloof Farm (Klein Karas Mountains) 7.3 Methods 7.4 Data and Results 7.4.1 Results of the U‐Pb detrital zircon data 7.4.2 Results of the U‐Pb carbonate dating 7.4.3 Results of zircon grain width and length measurements 7.4.4 Results of the Th‐U zircon ratios 7.4.5 Results of Lu‐Hf isotopic measurements 7.4.6 Geochemical results of the siliciclastic and basement samples 7.4.7 Geochemical results of the carbonate samples 7.5 Discussion and Conclusion 8 Sediment provenance and Snowball Earth ice dynamics 9 Implications on the evolution of the Gariep Belt 10 Conclusions and outlook 11 References Supplementary Material / Die neoproterozoischen Einheiten des südlichen Namibias (Kalahari Kraton) umfassen verschiedene glaziale Einheiten, die schon seit fast 100 Jahren bekannt sind und wissenschaftlich beschrieben wurden (z.B. Coleman, 1926; Gevers, 1931; Schwellnuss, 1941; Martin, 1965). Die Schneeball Erde Theorie (Hoffman et al., 1998) hatte einen enormen Einfluss auf die geologischen Studien des Neoproterozoikums, wobei besonders Aufschlüsse der Otavi Gruppe Nordnamibias die Theorie stärken und bestätigen. Im Gegensatz dazu sind neoproterozoische glaziale Horizonte Südnamibias aufgrund ihrer tektonischen Überprägung und der wenigen Aufschlüsse schwer zu interpretieren und zu korrelieren. Mit dem Ziel, die neoproterozoischen glazialen Einheiten Südnamibias zu unterscheiden und zu korrelieren, wurde ein Multimethodenansatz basierend auf Isotopenanalysen an Zirkonmineralen, Gesamtgesteinsgeochemie, Mineralkorngrößenmessungen und intensiver Feldarbeit angewandt. Insgesamt wurden zehn Profile kartiert und vermessen, von denen 33 Proben zur weiteren Analyse ausgewählt wurden. Zwei dieser Proben stammen vom lokalen Grundgebirge, 19 aus den sedimentären Einheiten darüber (inklusive der glazialen Ablagerungen) und zwölf repräsentieren Karbonatgesteinsproben. 3474 Einzelzirkone wurden hinsichtlich ihrer Breite und Länge vermessen, wovon 2404 Minerale mittels LA‐ICP‐MS nach ihren U‐Pb und Th‐U‐Gehalten analysiert wurden. 1535 dieser Minerale ergaben konkordante Alter (90 – 110% Konkordanz). Darüber hinaus wurden von 346 ausgewählten konkordanten Zirkonen die εHf(t) Werte bestimmt. Um das Datenset zu vervollständigen wurden LA‐ICP‐MS U‐Pb Analysen an Karbonatgesteinen an zehn von zwölf Proben erfolgreich getestet. Im Zuge der Feldarbeiten kristallisierten sich zwei Profile nahe des Oranje heraus, welche die Sturtian und die Marinoan Vereisung in nahezu ungestörter Lagerung enthalten und sich deshalb als Referenzprofile qualifizieren. Alle analysierten Proben zeichnen sich durch sehr ähnliche Zirkonisotopenwerte aus, was durch die Gesamtgesteinsgeochemieanalysen weiterhin bestätigt wird. Alle siliziklastischen Proben zeigen eine generelle felsische Provenienz mit Zirkonaltern welche sich hauptsächlich in zwei Altersgruppen unterteilen lassen (mesoproterozoisch 1.0 – 1.5 Mrd Jahre, paläoproterozoisch 1.7 – 2.1 Mrd Jahre). Diese reflektieren vier verschiedene krustale Entwicklungsstadien vom Mesoproterozoikum bis Archaikum. Die geochemische Signatur aller Proben deutet auf einen kontinentalen Inselbogen hin und auch die Zirkonmineralgrößen sind für alle Proben ähnlich. Eine direkte Altersdatierung auf Grundlage der detritischen Zirkone war aufgrund fehlender junger Alter nicht möglich. Dennoch ist eine Unterscheidung der glazialen Schichten Südnamibias basierend auf den petrographischen Eigenschaften und dem sich verschiebenden Alterstrend der detritischen Zirkone möglich (P/M Verhältnis). In Kombination mit den zwei Referenzprofilen ist eine umfassende Korrelation aller untersuchten Profile möglich und die Unterscheidung von vier Neoproterozoischen glazialen Schichten in Namibia gelungen. Die Ergebnisse geben weitere Einblicke in die neoproterozoische Entwicklung des Gariep Gürtels, welcher durch Riftvorgänge im Tonium, die Bildung des Arachania Terranes während des Cryogeniums und die ediakarische finale Kollision zwischen den Rio de la Plata und Kalahari Kratonen geprägt ist. Die Kombination aller Ergebnisse zeigt ein kontinuierliches Sedimentrecycling auf dem westlichen Kalahari Kraton. Vergleiche und statistische Ähnlichkeitsanalysen basierend auf U‐Pb Zirkonalterdatenbanken ergaben, dass der Namaqua Natal und der Gariep Gürtel die wahrscheinlichsten Liefergebiete sind. Das Recycling fand für mindestens 200 Millionen Jahre zwischen der Kaigas Vereisung (etwa vor 750 Millionen Jahren) und der Vingerbreek Vereisung (etwa vor 550 Millionen Jahren) statt. Darüber hinaus zeigt das Fehlen fremder Zirkonalter für die Schneeball Erde Proben, dass sich die Eispanzer kaum oder nur sehr wenig bewegt haben können, was die Theorie einer komplett zugefrorenen Erde unterstützt. Die Ergebnisse der U‐Pb Karbonatgesteinsdatierungen bestätigen des angenommene Sturtian und Marinoan Alter der Numees Fm und des Namaskluft Mbr. Basierend auf diesen Analysen kann eine Mindestlänge von etwa 8 Millionen Jahren für das Sturtian und etwa 14 Millionen Jahren für das Marinoan Schneeball Erde Ereignis angenommen werden.:Abstract Kurzfassung Contents List of Figures List of Tables List of abbreviations Scientific question and thesis structure 1 Introduction 1.1 The Neoproterozoic era: Supercontinent dispersal and global glaciations 1.1.1 Rodinia supercontinent: Formation, dispersal, and location of Kalahari Craton 1.1.2 Glacial events during the Neoproterozoic era 1.1.2.1 A brief history on the discovery of Snowball Earth events 1.1.2.2 Formation and termination of a Snowball Earth event: The Snowball Earth flow chart 1.1.2.3 Hypotheses for cap carbonate formation 1.1.2.4 Survival of life during a Snowball Earth event 1.2 The Kalahari Craton 1.2.1 Evolution of the Kalahari Craton 1.3 Overview over the Geology of Namibia under special consideration of southern Namibia (Kalahari Craton) 2 Characteristics of southern Namibian Neoproterozoic glacial samples and sides 3 The problematic correlations of Neoproterozoic glacial deposits of the Kalahari Craton (southern Namibia) 4 Methods 4.1 Field work 4.2 Whole Rock geochemical analyses 4.3 Heavy mineral separation and SEM analyses on zircon grains of siliciclastic samples 4.4 Zircon grain size analyses 4.5 LA‐ICP‐MS analyses on zircon grains 4.5.1 U‐Pb analyses with LA‐SF‐ICP‐MS 4.5.2 Th‐U ratio determination on zircon grains 4.5.3 Hf‐isotope measurements with LA‐MS‐ICP‐MS 4.6 LA‐ICP‐MS U‐Pb dating on carbonates 4.7 Provenance interpretations and likeness tests based on zircon U‐Pb age data bases 5 Study I: “The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana” 5.1 Introduction and geological setting 5.2 Samples and methods 5.3 Results 5.4 Discussion and interpretation 5.5 Summary 6 Study II: “The four Neoproterozoic glaciations of southern Namibia and their detrital zircon record: The fingerprints of four crustal growth events during two supercontinent cycles” 6.1 Introduction 6.2 The samples 6.3 Methods 6.4 Results 6.5 Interpretation and discussion 6.6 Conclusion/Summary 7 Study III: “Correlation of Neoproterozoic diamictites in southern Namibia” 7.1 Introduction 7.2 Sample sites 7.2.1. The Kaigas and Sturtian Numees diamictites at the Orange River section 2.1.1. Outcrops of the Kaigas Fm diamictites 7.2.1.2 Outcrop of the Numees Fm diamictites (Sturtian) 7.2.2 The Sturtian diamcitite of the Blaubeker Fm (Witvlei Grp) at the farmgrounds Blaubeker and Tahiti 7.2.2.1 The Blaubeker diamictite at Blaubeker Farm (type locality) 7.2.2.2 The Blaubeker diamictite at Tahiti Farm (Gobabis‐syncline) 7.2.2.3 Correlation of Blaubeker diamictite at Blaubeker and Tahiti farms 7.2.3 The Sturtian diamictite at the Trekpoort Farm section 7.2.4 The Sturtian and Marinoan diamictites at Namuskluft section (reference profile) 7.2.5 The Sturtian and Marinoan diamictites at Dreigratberg section 7.2.6 Sturtian diamictite and Marinoan‐type cap carbonate at Dreigratberg North section 7.2.7 The Marinoan diamictite at the Witputs Farm section 7.2.8 The post‐Gaskiers Vingerbreek diamictite 7.2.8.1 The Vingerbreek diamictite along the Orange River 7.2.8.2 The Vingerbreek diamictite at Tierkloof Farm (Klein Karas Mountains) 7.3 Methods 7.4 Data and Results 7.4.1 Results of the U‐Pb detrital zircon data 7.4.2 Results of the U‐Pb carbonate dating 7.4.3 Results of zircon grain width and length measurements 7.4.4 Results of the Th‐U zircon ratios 7.4.5 Results of Lu‐Hf isotopic measurements 7.4.6 Geochemical results of the siliciclastic and basement samples 7.4.7 Geochemical results of the carbonate samples 7.5 Discussion and Conclusion 8 Sediment provenance and Snowball Earth ice dynamics 9 Implications on the evolution of the Gariep Belt 10 Conclusions and outlook 11 References Supplementary Material

Este trabalho caracteriza a evolucao tectónica, identificando a cronologia dos principais eventos tectono-magmáticos do Pré-Cambriano Boliviano. A complexa evolucao geológica do Oriente da Bolívia se estende desde o Paleo a Mesoproterozoico compreendendo as provincias Rio Negro Juruena, Rondoniana San Ignacio e Sunsás na regiao conhecida como Bloco Paragua. Diversos métodos de estudo foram adotados na pesquisa tendo em vista tratar-se de um terreno com evolução policíclica e incluiram, alem do mapeamento geológico e petrografía dos principais tipos de rocha, a metodologia U-Pb para determinação da idade de corpos graníticos e a metodologia Sm-Nd na estimativa de idade das fontes destes corpos plutônicos e inferências de ordem petrogenética, bem como dados geoquímicos obtidos para detalhamento das interpretações petrogenéticas. Nas interpretações houve ainda a avaliação critica da literatura recente, a integração de dados de campo, aeromagnéticos e aero-radiométricos, inclusive embasadas na experiência profissional do autor. Os dados obtidos na última década modificaram substancialmente a concepcao do Pré-Cambriano Boliviano, tendo sido caracterizados tres conjuntos litológicos temporalmente distintos antecedendo a orogenia San Ignacio. O granito Correreca na parte meridional da area possui idade 207Pb-206Pb de 1,92 1,89 Ga, com modelo de idades TDM de 2,8 a 2,9 Ga e valores de Nd(t) de -8,5 e -9,4. A Suite Yarituses composta pelos granitos La Cruz, Refugio e San Pablo possui quimismo calcio-alcalino. Os dados U-Pb SHRIMP, TIMS e abrasão por laser-ICPMS indicam a formação desta suíte no lapso temporal entre 1673 a 1621 Ma. A idade de cristalização U-Pb SHRIMP do granito La Cruz é de 1673 ± 21 Ma, idade modelo TDM de 1,83 Ga e valor de Nd(t) de + 2.1 indicativo de derivação mantélica. O granito Refugio tem idade U-Pb TIMS de 1673 ± 25 Ma e o pluton San Pablo idade ICPMS por laser ablasion de 1621 ± 80 Ma (idade TDM de 1,7 Ga e valor de Nd(t) de + 3,5). Este conjunto de dados sugere uma derivação mantelica principal para a suite Yarituses. O granodiorito San Ramón possui uma idade de cristalização de 1429 ± 4 Ma (SHRIMP), TDM de 1,7 Ga, e Hf(t) entre + 3,49 e +5,47 e representa um evento de geração da crosta, a partir de material juvenil. O magmatismo, deformação e metamorfismo da orogênese San Ignácio constitui o principal evento representado na área de estudo, cujo maior representante é o Complexo Granitoide Pensamiento com seus plutons sin a tardi-cinemáticos e tardi a pos-cinemáticos. Os granitos San Martín, La Junta e Diamantina possuem idades de cristalizacao de 1373- 1340 Ma, idades modelo TDM de 1,6 a 2,0 Ga, com valores de Nd(t) de + 2.0 ate -4,0. Os granitos Las Maras, Talcoso, Limonal e San Andrés produziram idades de cristalização de 1347 a 1275 Ma. As idades TDM dos granitos Limonal e San Andrés correspondem a 1,9 e 1,8 e Nd(t) de -1,4 e 1,6 respectivamente. A geoquímica em rocha total indica uma composição compatível com arco magmático, corroborando a assinatura acima dos parâmetros petrogeneticos. Em suma, a orogênese San Ignácio representa um arco acrescionário de natureza continental que construiu a arquitetura final da província Rondoniana-San Ignacio pela colisão entre o Bloco Paraguá e a província Rio Negro-Juruena. A evolução mesoproterozoica finaliza com a formação da faixa colisional Sunsás. Esta orogênese produziu plutonismo sin a tardi cinematico e tardi a cinemático marcando o limite com o bloco Paragua. A natureza alóctone e colisional do orogeno Sunsás como o evento mais jovem do Cráton Amazônico é marcada por frentes tectônicos, bem definidos de sentido sinistral, convergentes para o Bloco Paragua. / This work characterizes the tectonic and magmatic evolution of the Precambrian shield of Bolivia. The complex geological evolution of the eastern Bolivia extends from the Paleo- to Mesoproterozoic, and can be related with the magmatic and metamorphic events that are ascribed to the Rio Negro - Juruena (1.78-1.60 Ga), Rondonian - San Ignacio (1.56-1.30 Ga) and Sunsás Aguapei (1.25-1.00 Ga) provinces, known in Bolivia as the Paragua block. Several methods of study were adopted in the research with the scope that this is a land with polycyclic evolution. As such our study included, besides the geological mapping and petrography of major rock types, the U-Pb age determinations of granitoid rocks, Sm-Nd and Rb-Sr isotopic analyses, as well as geochemical data. At the interpretation there was the critical evaluation of recent papers, the integration of field data, aeromagnetic and aero-radiometric, including the field experience of the author. The data obtained in the last decade have substantially changed the geology of the Bolivian Precambrian shield. It has been characterized three temporally distinct granite suites preceding the San Ignacio orogeny (1.37-1.30 Ga): the Correreca granite in the southern part of the area has 207Pb/206Pb age from 1.92 to 1.89 Ga, with TDM model ages of 2.8 to 2.9 Ga and values of Nd(t) of -8.5 and -9.4; the Yarituses suite (La Cruz, Refugio and San Pablo granites) shows calc-alkaline signature. Data U-Pb SHRIMP, TIMS and ICPMS laser ablation indicate the formation of this suite between 1673 to 1621 Ma. The U-Pb SHRIMP crystallization age of La Cruz granite is 1673 ± 21 Ma, TDM model age of 1.83 Ga and Nd(t) of +2.1 indicative of a predominantly mantle source. The Refugio granite has U-Pb TIMS age of 1673 ± 25 Ma and the San Pablo pluton yields a ICPMS Laser ablation age of 1621 ± 80 Ma (TDM age of 1.7 Ga and Nd(t) +3.5). These data suggest again a mantle source for the Yarituses suite. The San Ramon granodiorite event has a crystallization age of 1429 ± 4 Ma (SHRIMP), TDM of 1.7 Ga, and Hf(t) between +3.49 and +5.47 and represents a juvenile accreted episode. The magmatism, deformation and metamorphism of San Ignacio orogeny is the main event of the study area, represented by the Pensamiento Granitoid Complex with sin to late-kinematic and late to post-kinematic plutons. The San Martín, La Junta and Diamantina granites have crystallization ages of 1373 - 1340 Ma, TDM model ages from 1.6 to 2.0 Ga, with values of Nd(t) from 2.0 up to -4.0. The Las Maras, Talcoso, Limonal and San Andrés granites yielded crystallization ages of 1347-1275 Ma. The TDM ages of Limonal and San Andrés granites are between 1.9 and 1.8 Ga and the Nd(t) values of -1.4 and +1.6 respectively. The whole rock geochemistry of these granites indicates a composition consistent with the magmatic arc. Thus the San Ignacio orogeny represents a continental accretionary arc that built the final architecture of the Rondonian-San Ignacio province (1.56-1.30 Ga) by the collision between the Paragua block and the Rio Negro -Juruena province (1.78-1.60 Ga). The Mesoproterozoic evolution of the SW margin of the Amazonian craton ends with the formation of the Sunsás collisional belt that produced sin to-late and late topost- kinematic plutonism. The allochthonous and collisional nature of the Sunsás orogeny is marked by tectonic fronts, with well-defined sinistral sense, converging towards the Paragua block.

Modelos de reconstruções paleogeográficas envolvendo o Cráton Amazônico para 1100 Ma são motivos de controvérsia devido à carência de dados paleomagnéticos de qualidade para esta unidade geotectônica. Com intuito de contribuir para o esclarecimento da participação do Cráton Amazônico na evolução do ciclo continental, este trabalho apresenta o estudo paleomagnético realizado para o Complexo Máfico- ultramáfico Rincón del Tigre localizado na região sudeste da Bolívia, sudoeste do Cráton Amazônico. Uma recente datação U-Pb em badeleítas forneceu idade de 1110,4 ± 1,8 Ma para esta unidade. Para o estudo paleomagnético, 101 amostras cilíndricas orientadas foram coletadas de 15 sítios de composição litológica variada, compreendendo ultramáficas, ortopiroxenitos adcumuláticos, gabro noritos e serpentinitos. Um total de 359 espécimes cilíndricos de rocha de 2.2 cm de altura por 2.5 cm de diâmetro foi preparado para os tratamentos por campos magnéticos alternados (AF) e térmicos, assim como, para medidas de anisotropia de susceptibilidade magnética (ASM). As mesmas amostras foram preparadas para os experimentos de mineralogia magnética: curvas termomagnéticas, curvas de histerese e curvas de magnetização remanente isotérmica (MRI). Os resultados obtidos a partir da análise de anisotropias de suscetibilidade magnética (ASM) indicam, para boa parte das amostras analisadas, trama magnética aproximadamente horizontal coerente com a colocação de sills e lineação magnética para NW/SE, a qual indica que estas rochas sofreram influência da tectônica de deformação de direção NE-SW que as afetou durante a orogênese Sunsás. O estudo da mineralogia magnética indica magnetita como principal portador magnético presente nas rochas analisadas. Os tratamentos por campos alternados e térmico foram eficientes para separar as componentes de magnetização através da análise vetorial, sendo que direções coerentes foram obtidas para boa parte dos espécimes analisados para cada sítio. Todavia, a mesma coerência não é observada para as direções médias por sítio. Assim, correções tectônicas foram efetuadas e para um grupo de sítios obteve-se um teste de dobra positivo, com direção média Dm=327,9°, Im=53,5° (95=13,1°, K=22,6, N=7), a qual forneceu o polo paleomagnético situado em 271,7°E, 28,6°N (A95=17,6°). Supondo uma trama magnética horizontal (k3=90°) para os sills acamadados que constituem o Complexo Rincón del Tigre, a direção média (declinação e inclinação) do eixo k3 para cada sítio foi utilizada para corrigir as direções de magnetização para a situação de trama horizontal. Após a correção de ASM, outro grupo de sítios apresentou direções consistentes, cuja direção média Dm=118,6°, Im=20,7° (95=16,5°, K=12,2, N=8) forneceu o polo paleomagnético situado em 28,5°E, 30,0°S (A95=12,8). Os parâmetros estatísticos foram também significativamente melhorados após a correção de ASM. Com base nos dois polos determinados para o Complexo Rincón del Tigre e polos selecionados para o Cráton Amazônico e Laurentia são propostas paleogeografias para 1265 Ma, 1200 Ma, 1150 Ma, 1100 Ma e 1000 Ma que apoiam o modelo que propõe a ruptura do supercontinente Columbia, por volta de 1270 Ma atrás, e o posterior movimento de rotação horária do Cráton Amazônico/Oeste-África e da Báltica até estes blocos cratônicos colidirem novamente há 1000 Ma atrás com a Laurentia, ao longo do cinturão Grenville, para formar o supercontinente Rodínia. / Paleogeographic reconstructions at 1100 Ma involving the Amazonian Craton are controversial due to the absence of key paleomagnetic poles for this geotectonic unit. Trying to elucidate the participation of the Amazonian Craton in the continental cycle, this work present a paleomagnetic study of the Rincón del Tigre mafic-ultramafic complex from southeast Bolivia, southwestern Amazonian Craton. A recent U-Pb dating on baddeleyites of a rock from this complex yielded an age of 1110.4 ± 1.8 Ma for this unit. For the paleomagnetic study, 101 cylindrical cores were sampled from 15 sites with variable lithologies, comprising ultramafics, adacumulatic orthopyroxenites, gabbro norites and serpentinites. A total of 359 cylindrical specimens (2.5 cm diameter x 2.2 cm height) were prepared for the AF and thermal treatments, and for the anisotropy of magnetic susceptibility (AMS) measurements. The same samples were prepared for magnetic experiments: thermomagnetic curves, hysteresis curves and isothermal remanent magnetization (IRM) curves. The ASM results indicate nearly horizontal magnetic fabric for many of the analyzed sites, which agrees with that originated by sills emplacement, and a NW/SE magnetic lineation, which suggests that these rocks were tectonically affected by the Sunsás orogen. The magnetic mineralogy studies indicate magnetite as the main magnetic carrier in the rocks. The AF and thermal treatments were effective in isolating magnetic components through vector analysis, and coherent magnetic directions were disclosed for much of the specimens from each site. However, the same consistency was not observed for the between-site directions. Tectonic corrections were applied for the site mean directions, which yielded a positive fold test for a group of sites: mean direction Dm=327.9°, Im=53.5° (95=13.1°, K=22.6, N=7), which yielded the paleomagnetic pole at 271.7°E, 28.6°N (A95=17.6°). Supposing an horizontal magnetic fabric (k3=90°) for the layered sills that originally formed the Rincón del Tigre Complex, the mean direction (declination and inclination) of the k3 axis calculated for each site, was used to correct site mean magnetization directions for the situation of horizontal magnetic fabric. After ASM correction, another group of sites yielded consistent directions, whose mean direction Dm=118.6°, Im=20.7° (95=16.5°, K=12.2, N=8) yielded the paleomagnetic pole at 28.5°E, 30.0°S (A95=12.8). Statistical parameters were also greatly improved after ASM corrections. Paleogeographies at 1265 Ma, 1200 Ma, 1150 Ma, 1100 Ma and 1000 Ma were constructed based on the Rincón del Tigre poles and other selected poles from the Amazonian Craton and Laurentia, which support the model where soon after Columbia rupture at around 1270 Ma, the Amazonian Craton/West Africa and Baltica executed clockwise rotations until they collide again at 1000 Ma with Laurentia, along the Grenvillian belt, forming Rodinia supercontinent.

Modelos de reconstruções paleogeográficas envolvendo o Cráton Amazônico para 1100 Ma são motivos de controvérsia devido à carência de dados paleomagnéticos de qualidade para esta unidade geotectônica. Com intuito de contribuir para o esclarecimento da participação do Cráton Amazônico na evolução do ciclo continental, este trabalho apresenta o estudo paleomagnético realizado para o Complexo Máfico- ultramáfico Rincón del Tigre localizado na região sudeste da Bolívia, sudoeste do Cráton Amazônico. Uma recente datação U-Pb em badeleítas forneceu idade de 1110,4 ± 1,8 Ma para esta unidade. Para o estudo paleomagnético, 101 amostras cilíndricas orientadas foram coletadas de 15 sítios de composição litológica variada, compreendendo ultramáficas, ortopiroxenitos adcumuláticos, gabro noritos e serpentinitos. Um total de 359 espécimes cilíndricos de rocha de 2.2 cm de altura por 2.5 cm de diâmetro foi preparado para os tratamentos por campos magnéticos alternados (AF) e térmicos, assim como, para medidas de anisotropia de susceptibilidade magnética (ASM). As mesmas amostras foram preparadas para os experimentos de mineralogia magnética: curvas termomagnéticas, curvas de histerese e curvas de magnetização remanente isotérmica (MRI). Os resultados obtidos a partir da análise de anisotropias de suscetibilidade magnética (ASM) indicam, para boa parte das amostras analisadas, trama magnética aproximadamente horizontal coerente com a colocação de sills e lineação magnética para NW/SE, a qual indica que estas rochas sofreram influência da tectônica de deformação de direção NE-SW que as afetou durante a orogênese Sunsás. O estudo da mineralogia magnética indica magnetita como principal portador magnético presente nas rochas analisadas. Os tratamentos por campos alternados e térmico foram eficientes para separar as componentes de magnetização através da análise vetorial, sendo que direções coerentes foram obtidas para boa parte dos espécimes analisados para cada sítio. Todavia, a mesma coerência não é observada para as direções médias por sítio. Assim, correções tectônicas foram efetuadas e para um grupo de sítios obteve-se um teste de dobra positivo, com direção média Dm=327,9°, Im=53,5° (95=13,1°, K=22,6, N=7), a qual forneceu o polo paleomagnético situado em 271,7°E, 28,6°N (A95=17,6°). Supondo uma trama magnética horizontal (k3=90°) para os sills acamadados que constituem o Complexo Rincón del Tigre, a direção média (declinação e inclinação) do eixo k3 para cada sítio foi utilizada para corrigir as direções de magnetização para a situação de trama horizontal. Após a correção de ASM, outro grupo de sítios apresentou direções consistentes, cuja direção média Dm=118,6°, Im=20,7° (95=16,5°, K=12,2, N=8) forneceu o polo paleomagnético situado em 28,5°E, 30,0°S (A95=12,8). Os parâmetros estatísticos foram também significativamente melhorados após a correção de ASM. Com base nos dois polos determinados para o Complexo Rincón del Tigre e polos selecionados para o Cráton Amazônico e Laurentia são propostas paleogeografias para 1265 Ma, 1200 Ma, 1150 Ma, 1100 Ma e 1000 Ma que apoiam o modelo que propõe a ruptura do supercontinente Columbia, por volta de 1270 Ma atrás, e o posterior movimento de rotação horária do Cráton Amazônico/Oeste-África e da Báltica até estes blocos cratônicos colidirem novamente há 1000 Ma atrás com a Laurentia, ao longo do cinturão Grenville, para formar o supercontinente Rodínia. / Paleogeographic reconstructions at 1100 Ma involving the Amazonian Craton are controversial due to the absence of key paleomagnetic poles for this geotectonic unit. Trying to elucidate the participation of the Amazonian Craton in the continental cycle, this work present a paleomagnetic study of the Rincón del Tigre mafic-ultramafic complex from southeast Bolivia, southwestern Amazonian Craton. A recent U-Pb dating on baddeleyites of a rock from this complex yielded an age of 1110.4 ± 1.8 Ma for this unit. For the paleomagnetic study, 101 cylindrical cores were sampled from 15 sites with variable lithologies, comprising ultramafics, adacumulatic orthopyroxenites, gabbro norites and serpentinites. A total of 359 cylindrical specimens (2.5 cm diameter x 2.2 cm height) were prepared for the AF and thermal treatments, and for the anisotropy of magnetic susceptibility (AMS) measurements. The same samples were prepared for magnetic experiments: thermomagnetic curves, hysteresis curves and isothermal remanent magnetization (IRM) curves. The ASM results indicate nearly horizontal magnetic fabric for many of the analyzed sites, which agrees with that originated by sills emplacement, and a NW/SE magnetic lineation, which suggests that these rocks were tectonically affected by the Sunsás orogen. The magnetic mineralogy studies indicate magnetite as the main magnetic carrier in the rocks. The AF and thermal treatments were effective in isolating magnetic components through vector analysis, and coherent magnetic directions were disclosed for much of the specimens from each site. However, the same consistency was not observed for the between-site directions. Tectonic corrections were applied for the site mean directions, which yielded a positive fold test for a group of sites: mean direction Dm=327.9°, Im=53.5° (95=13.1°, K=22.6, N=7), which yielded the paleomagnetic pole at 271.7°E, 28.6°N (A95=17.6°). Supposing an horizontal magnetic fabric (k3=90°) for the layered sills that originally formed the Rincón del Tigre Complex, the mean direction (declination and inclination) of the k3 axis calculated for each site, was used to correct site mean magnetization directions for the situation of horizontal magnetic fabric. After ASM correction, another group of sites yielded consistent directions, whose mean direction Dm=118.6°, Im=20.7° (95=16.5°, K=12.2, N=8) yielded the paleomagnetic pole at 28.5°E, 30.0°S (A95=12.8). Statistical parameters were also greatly improved after ASM corrections. Paleogeographies at 1265 Ma, 1200 Ma, 1150 Ma, 1100 Ma and 1000 Ma were constructed based on the Rincón del Tigre poles and other selected poles from the Amazonian Craton and Laurentia, which support the model where soon after Columbia rupture at around 1270 Ma, the Amazonian Craton/West Africa and Baltica executed clockwise rotations until they collide again at 1000 Ma with Laurentia, along the Grenvillian belt, forming Rodinia supercontinent.

Le Bloc du Gaviao situé à l'ouest du craton de Sao Francisco (BahiaBrésil), enregistre les âges les plus anciens de l'Amérique du Sud. Grâce aux datations 207Pb/2o6Pb sur monozircon et monazite, aux données isotopiques du Sr et 1 du Nd et aux modélisations géochimiques des sources, trois domaines ont pu être 1 distingués: 1) Un domaine archéen juvénile constitué de gneiss gris (massif de Bernada) qui a été extrait du manteau vers 3,3 Ga. 2) Un domaine archéen recyclé, ou juvénile avec contamination crustale, constitué de gneiss gris trondhjémitiques (massif d'Aracatu) et d'orthogneiss monzogranitiques calco-alcalins riches en potassium (massifs de Mariana et d'Eixo). Les âges sont, respectivement, de 3240 ± 10,.3259 ± 7 et 3158 ± 5 Ma et les âgés-modèles Nd varient de 3,3 à 3,7 Ga. Le massif d'Eixo est recoupé par des granites alcalins vers 2,5 Ga. 3) Un domaine protérozoïque inférieur recyclé, représenté principalement par les granites peralumineux d'Umburanas dont les âges des zircons hérités sont compris entre 2780 ± 6 et 3130 ± 7 Ma tandis que l'âge monazite est de 2049 ± 6 Ma. Des granites cal co-alcalins riches en potassium recoupent le massif d'Aracatu à 2135 ± 4 Ma et le massif de Mariana à 1944 ± 7 Ma. Le massif de Serra da Franga, un granite de même nature, a été daté par monozircon à 2039 ± 11 Ma. La présence d'événements plus récents a été détectée à travers la datation des monazites du massif d'Aracatu (1735 ± 5 Ma) et l'influence thermique de l'orogenèse brésilienne à partir de datations Rb-Sr sur biotites et roches totales des* massifs de Serra do Eixo (533 ± 11 Ma) et d'Aracatu (508± 10 Ma) Le Bloc du Gaviao est l'exemple d'une croûte continentale archéenne qui a été recyclée par des mécanismes de migmatisation et d'anatexie liés à l'épaississement crustal résultant de l'orogenèse transamazonienne (2,0-2,1 Ga).