The emplacement of continental flood basalt provinces is often ascribed to fissure eruption. However, in many provinces the locations of actual vent complexes remains illusive. In southern Africa, the Karoo continental flood basalt province was erupted during the Jurassic between 183 and 179 Ma. The southernmost outcrop of the Karoo continental flood basalt lavas is the Lesotho remnant; and is comprised of the Drakensberg Group which forms the uppermost portion of the Karoo Supergroup. The geochemical stratigraphy for the Drakensberg Group is well established. At the base, there are a number of small volume compositionally diverse units, which form the Barkly East Formation, overlying which, there are larger volume, compositionally less variable units of the Lesotho Formation, which form the bulk of the volcanic sequence. The Lesotho remnant is associated with an abundance of dykes in the adjacent vicinity. This suggests that the lavas were fed from local rather than distal eruption sites. This study presents whole rock major and trace element data for 94 dykes and three sills from the northern Lesotho - northeastern Free State region and demonstrates that on the basis of geochemistry, all 97 intrusions can be correlated with various units of the northern Barkly East and Lesotho Formations. In addition, the petrographical; characteristics, orientation and distribution of the dykes do not correlate with geochemistry. Geochemical discrimination diagrams have been used to identify five compositionally diverse dykes, which are similar to the northern Barkly East Formation units. Three dykes are characteristic of the Letele unit and two are compositionally similar to the Wonderkop unit. Although the geochemical characteristics of the Lesotho Formation units are rather well constrained, the composition of these units is typified by considerable overlap in composition. Therefore, since unambiguous classification of dykes with geochemical similarities to the various units of the Lesotho Formation is unachievable using an empirical approach, the multivariate forward-stepwise discriminant function analysis (DFA) technique was used to facilitate the classification of the remaining 89 dykes and three sills. Forward-stepwise DFA classified 23 dykes as having compositional similarities to the Mafika Lisiu unit, 29 as having compositions of the Maloti or Senqu types; and 32 as having the composition of the Mothae type. In addition, eight dykes are compositionally similar to the Oxbow dykes, which intrude the Senqu unit in northern Lesotho. These results suggest that that the Lesotho remnant was fed from local eruption sites and that long distance magma transport for the bulk of the Lesotho remnant basalt lavas is unlikely.

Orientador: Antonio José Ranalli Nardy / Banca: Marcos Aurélio Farias de Oliveira / Banca: José Paulo Peccinini Pinese / Resumo: Este trabalho trata da investigação petrográfica e geoquímica dos derrames da Formação Serra Geral da Bacia do Paraná, distribuídas em quatro regiões distintas: Jaú, Ribeirão Preto, Franca e Fernandópolis, cobrindo praticamente toda a área de exposição dessas rochas no estado de São Paulo. As amostras coletadas nessas regiões foram analisadas através de microscopia de luz polarizada, evidenciando que os basaltos estudados são constituídos fundamentalmente por plagioclásio (30-40%), piroxênios (20-30%), magnetita (5-15%) e apatita (menos de 1%), apresentando textura predominantemente intergranular e suas variedades (intersertal, hialofítica e pilotaxítica). Os dados geoquímicos revelaram que as amostras estudadas correspondem a basaltos de natureza toleítica, com TiO2 ≥ 1,8%, característicos da região norte da Bacia do Paraná. Dados geoquímicos de elementos traços e terras raras mostraram que as rochas estudadas podem ser agrupadas em três diferentes magmas-tipo: Paranapanema (basaltos de Fernandópolis), Urubici (basaltos de Franca) e Pitanga (basaltos de Ribeirão Preto e de Jaú). Os padrões de distribuição desses elementos mostram que eles não podem ser gerados a partir de fontes mantélicas astenosféricas, mas sim, de fontes litosféricas distintas. / Abstract: A petrographical and geochemistry characterization of flood basalts of Serra Geral Formation of Paraná Basin is here presented. The investigated area is sited in four different regions of São Paulo state: Jaú, Ribeirão Preto, Franca e Fernandópolis. It represents almost the total exposure area of lava flows in São Paulo State. The petrographical data of these rocks reveals that the basalts are constituted mainly by plagioclase (30-40%), pyroxene (20-30%), magnetite (5-15%) and apatite, that characterized a intergranular, intersertal, hialophitic and pilotaxitic textures. The geochemical data show a basic and tholeiitic composition of the studied basalts with TiO2 ≥ 1,8%, characterized of the northern of Paraná Basin. Three different magmatypes were recognized: Paranapanema (basalts of Fernandópolis region), Urubici (basalts of Franca region) and Pitanga (basalts of Ribeirão Preto and Jaú regions). The distribution patterns of these elements in a spider diagram have showed that they are generated by lithospheric mantle. / Mestre

Geometries of sills intruded into the lava pile of the Faroe Island Basalt Group (FIBG), which were targeted in this study, were mostly recorded by conventional mapping methods where measured distances and positions were plotted onto accurate topographic maps aided by the use of high-quality photos of relevant outcrops. These data were subsequently used to manually plot 2D profiles along selected tracks and to produce electronic 3D maps using ArcGIS software. The general geometries of the investigated sills, measured at lateral scales ranging from a few metres to a few kilometres and at vertical scales ranging from a few metres to a few hundred metres, differ somewhat from typical sill geometries reported previously for sills intruded into sedimentary successions. The ubiquitous saucer-shapes of the sills from this study, which generally curve upwards in a gradual manner from inner sub-horizontal sections to steeper outer margins, contrast with the common angular transitions from inner sub-horizontal to outer steeper sections of sills reported from sedimentary host-rocks. In this thesis we explore possible alternatives to already existing theories on sill emplacement in sedimentary successions. Major and trace element compositions for samples representing most of the sills exposed in the Faroe Islands have been determined by means of XRF and ICP-MS analyses. Geochemically most of these sills can be grouped into two main categories characterised either by high or by low TiO2 contents. Different sorts/types of metasomatism of source rocks to high-TiO2 versus low-TiO2 sills are indicated by different Nb and Ta anomalies. Modelling by means of REE and other trace elements suggest that much of the compositional differences between these two main categories can be explained by various degrees of partial melting of broadly similar mantle sources. Additional fractionation and accumulation of plagioclase modified some of the melts that gave rise to the actual sills. The initial partial melting event probably occurred at depths slightly shallower than the lower limit of the garnet stability field at ~85 km while plagioclase crystallisation/accumulation most likely occurred at depths shallower than ~18 km. Isotopic compositions may point to very slight contamination of some sills with crustal material.

Made available in DSpace on 2014-06-11T19:26:13Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-05-05Bitstream added on 2014-06-13T19:54:18Z : No. of bitstreams: 1 squisato_e_me_rcla.pdf: 1767564 bytes, checksum: 7402c6efa5c703b95454872a8a0b9bb4 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Este trabalho trata da investigação petrográfica e geoquímica dos derrames da Formação Serra Geral da Bacia do Paraná, distribuídas em quatro regiões distintas: Jaú, Ribeirão Preto, Franca e Fernandópolis, cobrindo praticamente toda a área de exposição dessas rochas no estado de São Paulo. As amostras coletadas nessas regiões foram analisadas através de microscopia de luz polarizada, evidenciando que os basaltos estudados são constituídos fundamentalmente por plagioclásio (30-40%), piroxênios (20-30%), magnetita (5-15%) e apatita (menos de 1%), apresentando textura predominantemente intergranular e suas variedades (intersertal, hialofítica e pilotaxítica). Os dados geoquímicos revelaram que as amostras estudadas correspondem a basaltos de natureza toleítica, com TiO2 ≥ 1,8%, característicos da região norte da Bacia do Paraná. Dados geoquímicos de elementos traços e terras raras mostraram que as rochas estudadas podem ser agrupadas em três diferentes magmas-tipo: Paranapanema (basaltos de Fernandópolis), Urubici (basaltos de Franca) e Pitanga (basaltos de Ribeirão Preto e de Jaú). Os padrões de distribuição desses elementos mostram que eles não podem ser gerados a partir de fontes mantélicas astenosféricas, mas sim, de fontes litosféricas distintas. / A petrographical and geochemistry characterization of flood basalts of Serra Geral Formation of Paraná Basin is here presented. The investigated area is sited in four different regions of São Paulo state: Jaú, Ribeirão Preto, Franca e Fernandópolis. It represents almost the total exposure area of lava flows in São Paulo State. The petrographical data of these rocks reveals that the basalts are constituted mainly by plagioclase (30-40%), pyroxene (20-30%), magnetite (5-15%) and apatite, that characterized a intergranular, intersertal, hialophitic and pilotaxitic textures. The geochemical data show a basic and tholeiitic composition of the studied basalts with TiO2 ≥ 1,8%, characterized of the northern of Paraná Basin. Three different magmatypes were recognized: Paranapanema (basalts of Fernandópolis region), Urubici (basalts of Franca region) and Pitanga (basalts of Ribeirão Preto and Jaú regions). The distribution patterns of these elements in a spider diagram have showed that they are generated by lithospheric mantle.

Foram determinadas as concentrações de terras raras (La, Ce, Nd, Sm, Eu, Tb, Yb e Lu) e outros elementos-traço (Cs, Rb, Ba, U, Th, Ta, Hf, Co e Sc) em 51 amostras representativas de derrames e soleiras das regiões norte e nordeste da Província Magmática do Paraná, que ocorrem no norte do estado de São Paulo e no sul de Minas Gerais. O método utilizado para estas determinações foi a técnica de ativação com nêutrons térmicos e epitérmicos, seguida de espectrometria gama de alta resolução, que forneceu resultados com elevados níveis de precisão e de exatidão (geralmente com valores inferiores a 10%), conforme atestado pelas concentrações determinadas no material geológico de referência JB-1. Os dados obtidos foram analisados conjuntamente com determinações de elementos maiores, menores e dos traços Sr, Y, Zr, Nb, Cr e Ni, realizadas por fluorescência de raios ? X. As rochas investigadas são quimicamente representadas por basaltos toleíticos, andesi-basaltos toleíticos e lati-basaltos, enquanto as mais evoluídas (MgO < 3%), que ocorrem de modo muito subordinado, são representadas por latitos e lati-andesitos, além de um riodacito. As rochas básicas (SiO2 < 55% e/ou MgO > 3%) são caracterizadas por apresentarem conteúdos de TiO2 maiores que 3%, sendo que grande maioria possui fortes semelhanças com os magmas-tipo Pitanga, embora tenham sido encontrados alguns derrames, localizados no norte do estado de São Paulo, geoquimicamente semelhantes aos magmas-tipo Urubici (típicos do sul da província). Rochas básicas com concentrações de TiO2 entre 2% e 3% foram também raramente encontradas, as quais representam equivalentes intrusivos de derrames Paranapanema. As rochas mais evoluídas só foram encontradas em soleiras e possuem características geoquímicas que indicam diferenciação a partir de magmas do tipo Pitanga. O comportamento de elementos maiores, menores e traços nas rochas intrusivas e extrusivas do tipo Pitanga, incluindo as mais diferenciadas, é compatível com um processo de evolução por cristalização fracionada envolvendo plagioclásios, clinopiroxênios e titano-magnetitas. A comparação entre derrames e soleiras do tipo Pitanga indica a atuação significativa de processos de diferenciação in situ nas rochas intrusivas, que causaram uma maior variabilidade composicional (soleiras: MgO entre 2,3% e 6,4%; derrames: MgO entre 3,2% e 5,5%). As rochas mais diferenciadas são também caracterizadas por anomalias positivas de európio, que sugerem o acúmulo de plagioclásio devido a esse processo de fracionamento. Os dados obtidos também reforçam que não houve participação significativa de componentes astenosféricos do tipo N-MORB, E-MORB e OIB na gênese das rochas básicas investigadas, as quais possuem grande similaridade geoquímica com os basaltos da Cadeia Rio Grande (sítio 516F) e Cadeia Walvis (sítio 525A), especialmente com os desta última, indicando o envolvimento do componente mantélico EMI. / It is presented the determination of rare earth (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) and other trace (Cs, Rb, Ba, U, Th, Ta, Hf, Co and Sc) element concentrations in 51 representative samples of flows and sills from northern and northeastern Paraná Magmatic Province, particularly those located at north of São Paulo State and south of Minas Gerais. The employed analytical method to those determinations was thermal and epithermal neutron activation analysis, followed by high resolution gamma ray spectrometry, which provided high precision and accuracy results (in general better than 10%), as verified by determinations in the international geological reference material JB-1. The obtained results were integrated with major, minor and trace (Sr, Y, Zr, Nb, Cr and Ni) elements determined by X-ray fluorescence. The investigated rocks are mainly represented by tholeiitic basalts, tholeiitic andesi-basalts and lati-basalts, with subordinate relatively evolved (MgO < 3wt%) lithotypes, which are represented by latites and lati-andesites, besides one rhyodacite. The basic rocks (SiO2 < 55% and/or MgO > 3%) are characterized by presenting TiO2 contents higher than 3wt%, very similar to Pitanga magma-type, although some flows, located at northern São Paulo State, geochemically similar to Urubici magma-type (typical of southern Paraná) were also found. Basic rocks having TiO2 contents between 2 and 3 wt% were rarely encountered and represent intrusive equivalents of Paranapanema flows. The more evolved rocks were just found in sills and have geochemical characteristics that indicate differentiation from Pitanga magma-types. Major, minor and trace element behaviour of intrusive and extrusive rocks of Pitanga type, including those differentiated ones, is compatible with evolution by fractional crystallization of plagioclases, clinopyroxenes and titanium magnetites. The comparison between flows and sills of Pitanga type indicates the significant role of in situ differentiation in the genesis intrusive rocks, causing its larger compositional variability (sills: MgO between 2.3 and 6.4wt%; flows: MgO between 3.2 and 5.5wt%). The most evolved rocks are also characterized by positive europium anomalies, which suggest plagioclase accumulation caused by such fractionation process. The obtained data also corroborate the lack of significant participation of N-MORB, E-MORB and OIB asthenospheric mantle components in the genesis of basic rocks, which present very close geochemical similarity with Rio Grande Rise (site 516F) and Walvis Ridge (site 525A) basalts, particularly with the last ones, indicating the involvement of EMI mantle component.

O objetivo primário deste estudo é aprimorar o conhecimento acerca das fontes do manto e dos mecanismos envolvidos na gênese dos basaltos da Província Magmática do Paraná, que constitui uma das maiores manifestações de basaltos continentais do mundo. Para tanto, foram determinadas as concentrações de terras raras (La, Ce, Nd, Sm, Eu, Tb, Yb e Lu), outros elementos traço (Cs, Rb, Ba, U, Th, Ta, Hf, Co e Sc) e elementos altamente siderófilos (Os, Ir, Ru, Pt, Pd e Re), juntamente com razões isotópicas dos sistemas Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os em basaltos com alto-Ti (Paranapanema e Pitanga) que ocorrem no norte da PMP. Além disso, foram determinadas as concentrações de elementos altamente siderófilos e as razões isotópicas de 187Os/188Os amostras representativas de basaltos com baixo-Ti (Esmeralda) do sul da PMP. Os dados geoquímicos e as razões isotópicas de Sr, Nd e Pb obtidos são consistentes com dados da literatura, porém, refinam as variações (extremos) isotópicas dos magmas-tipo Paranapanema e Pitanga. Esses dados, juntamente com as concentrações de elementos altamente siderófilos e das razões isotópicas de Os, inéditas na literatura, sugerem que as fontes dos basaltos (astenosfera ou manto litosférico subcontinental) sofreram metassomatismo significativo, com a intrusão de veios piroxeníticos, relacionado a antigas subducções e/ou processos de delaminação. / The primary goal of this study is to improve the understanding about the mantle sources and the mechanisms involved in the basalt genesis from Paraná Magmatic Province (PMP), which is one of the largest known continental flood basalts of the world. Therefore, the concentrations of rare earths (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu), other trace elements (Cs, Rb, Ba, U, Th, Ta, Hf, Co and Sc) and highly siderophile elements (Os, Ir, Ru, Pt, Pd and Re) were determined, along with isotope ratios regarding Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os systematics in high-Ti basalts (Paranapanema and Pitanga) from northern PMP. In addition, the highly siderophile element concentrations, as well as 187Os/188Os isotope ratios, were measured in selected samples of low-Ti basalts (Esmeralda) from southern PMP. The geochemical and Sr-Nd-Pb isotope results of the present study are consistent with literature data, but refine the isotope variations (extreme) for the Paranapanema and Pitanga magma-types. These data, along with the concentrations of highly siderophile elements and Os isotope ratios suggest that the basalt mantle sources (asthenosphere or subcontinental lithospheric mantle) were affected by significant metasomatism (piroxenitic vein hybridization), related with old subduction and/or delamination processes.

O objetivo primário deste estudo é aprimorar o conhecimento acerca das fontes do manto e dos mecanismos envolvidos na gênese dos basaltos da Província Magmática do Paraná, que constitui uma das maiores manifestações de basaltos continentais do mundo. Para tanto, foram determinadas as concentrações de terras raras (La, Ce, Nd, Sm, Eu, Tb, Yb e Lu), outros elementos traço (Cs, Rb, Ba, U, Th, Ta, Hf, Co e Sc) e elementos altamente siderófilos (Os, Ir, Ru, Pt, Pd e Re), juntamente com razões isotópicas dos sistemas Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os em basaltos com alto-Ti (Paranapanema e Pitanga) que ocorrem no norte da PMP. Além disso, foram determinadas as concentrações de elementos altamente siderófilos e as razões isotópicas de 187Os/188Os amostras representativas de basaltos com baixo-Ti (Esmeralda) do sul da PMP. Os dados geoquímicos e as razões isotópicas de Sr, Nd e Pb obtidos são consistentes com dados da literatura, porém, refinam as variações (extremos) isotópicas dos magmas-tipo Paranapanema e Pitanga. Esses dados, juntamente com as concentrações de elementos altamente siderófilos e das razões isotópicas de Os, inéditas na literatura, sugerem que as fontes dos basaltos (astenosfera ou manto litosférico subcontinental) sofreram metassomatismo significativo, com a intrusão de veios piroxeníticos, relacionado a antigas subducções e/ou processos de delaminação. / The primary goal of this study is to improve the understanding about the mantle sources and the mechanisms involved in the basalt genesis from Paraná Magmatic Province (PMP), which is one of the largest known continental flood basalts of the world. Therefore, the concentrations of rare earths (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu), other trace elements (Cs, Rb, Ba, U, Th, Ta, Hf, Co and Sc) and highly siderophile elements (Os, Ir, Ru, Pt, Pd and Re) were determined, along with isotope ratios regarding Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os systematics in high-Ti basalts (Paranapanema and Pitanga) from northern PMP. In addition, the highly siderophile element concentrations, as well as 187Os/188Os isotope ratios, were measured in selected samples of low-Ti basalts (Esmeralda) from southern PMP. The geochemical and Sr-Nd-Pb isotope results of the present study are consistent with literature data, but refine the isotope variations (extreme) for the Paranapanema and Pitanga magma-types. These data, along with the concentrations of highly siderophile elements and Os isotope ratios suggest that the basalt mantle sources (asthenosphere or subcontinental lithospheric mantle) were affected by significant metasomatism (piroxenitic vein hybridization), related with old subduction and/or delamination processes.

Foram determinadas as concentrações de terras raras (La, Ce, Nd, Sm, Eu, Tb, Yb e Lu) e outros elementos-traço (Cs, Rb, Ba, U, Th, Ta, Hf, Co e Sc) em 51 amostras representativas de derrames e soleiras das regiões norte e nordeste da Província Magmática do Paraná, que ocorrem no norte do estado de São Paulo e no sul de Minas Gerais. O método utilizado para estas determinações foi a técnica de ativação com nêutrons térmicos e epitérmicos, seguida de espectrometria gama de alta resolução, que forneceu resultados com elevados níveis de precisão e de exatidão (geralmente com valores inferiores a 10%), conforme atestado pelas concentrações determinadas no material geológico de referência JB-1. Os dados obtidos foram analisados conjuntamente com determinações de elementos maiores, menores e dos traços Sr, Y, Zr, Nb, Cr e Ni, realizadas por fluorescência de raios ? X. As rochas investigadas são quimicamente representadas por basaltos toleíticos, andesi-basaltos toleíticos e lati-basaltos, enquanto as mais evoluídas (MgO < 3%), que ocorrem de modo muito subordinado, são representadas por latitos e lati-andesitos, além de um riodacito. As rochas básicas (SiO2 < 55% e/ou MgO > 3%) são caracterizadas por apresentarem conteúdos de TiO2 maiores que 3%, sendo que grande maioria possui fortes semelhanças com os magmas-tipo Pitanga, embora tenham sido encontrados alguns derrames, localizados no norte do estado de São Paulo, geoquimicamente semelhantes aos magmas-tipo Urubici (típicos do sul da província). Rochas básicas com concentrações de TiO2 entre 2% e 3% foram também raramente encontradas, as quais representam equivalentes intrusivos de derrames Paranapanema. As rochas mais evoluídas só foram encontradas em soleiras e possuem características geoquímicas que indicam diferenciação a partir de magmas do tipo Pitanga. O comportamento de elementos maiores, menores e traços nas rochas intrusivas e extrusivas do tipo Pitanga, incluindo as mais diferenciadas, é compatível com um processo de evolução por cristalização fracionada envolvendo plagioclásios, clinopiroxênios e titano-magnetitas. A comparação entre derrames e soleiras do tipo Pitanga indica a atuação significativa de processos de diferenciação in situ nas rochas intrusivas, que causaram uma maior variabilidade composicional (soleiras: MgO entre 2,3% e 6,4%; derrames: MgO entre 3,2% e 5,5%). As rochas mais diferenciadas são também caracterizadas por anomalias positivas de európio, que sugerem o acúmulo de plagioclásio devido a esse processo de fracionamento. Os dados obtidos também reforçam que não houve participação significativa de componentes astenosféricos do tipo N-MORB, E-MORB e OIB na gênese das rochas básicas investigadas, as quais possuem grande similaridade geoquímica com os basaltos da Cadeia Rio Grande (sítio 516F) e Cadeia Walvis (sítio 525A), especialmente com os desta última, indicando o envolvimento do componente mantélico EMI. / It is presented the determination of rare earth (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) and other trace (Cs, Rb, Ba, U, Th, Ta, Hf, Co and Sc) element concentrations in 51 representative samples of flows and sills from northern and northeastern Paraná Magmatic Province, particularly those located at north of São Paulo State and south of Minas Gerais. The employed analytical method to those determinations was thermal and epithermal neutron activation analysis, followed by high resolution gamma ray spectrometry, which provided high precision and accuracy results (in general better than 10%), as verified by determinations in the international geological reference material JB-1. The obtained results were integrated with major, minor and trace (Sr, Y, Zr, Nb, Cr and Ni) elements determined by X-ray fluorescence. The investigated rocks are mainly represented by tholeiitic basalts, tholeiitic andesi-basalts and lati-basalts, with subordinate relatively evolved (MgO < 3wt%) lithotypes, which are represented by latites and lati-andesites, besides one rhyodacite. The basic rocks (SiO2 < 55% and/or MgO > 3%) are characterized by presenting TiO2 contents higher than 3wt%, very similar to Pitanga magma-type, although some flows, located at northern São Paulo State, geochemically similar to Urubici magma-type (typical of southern Paraná) were also found. Basic rocks having TiO2 contents between 2 and 3 wt% were rarely encountered and represent intrusive equivalents of Paranapanema flows. The more evolved rocks were just found in sills and have geochemical characteristics that indicate differentiation from Pitanga magma-types. Major, minor and trace element behaviour of intrusive and extrusive rocks of Pitanga type, including those differentiated ones, is compatible with evolution by fractional crystallization of plagioclases, clinopyroxenes and titanium magnetites. The comparison between flows and sills of Pitanga type indicates the significant role of in situ differentiation in the genesis intrusive rocks, causing its larger compositional variability (sills: MgO between 2.3 and 6.4wt%; flows: MgO between 3.2 and 5.5wt%). The most evolved rocks are also characterized by positive europium anomalies, which suggest plagioclase accumulation caused by such fractionation process. The obtained data also corroborate the lack of significant participation of N-MORB, E-MORB and OIB asthenospheric mantle components in the genesis of basic rocks, which present very close geochemical similarity with Rio Grande Rise (site 516F) and Walvis Ridge (site 525A) basalts, particularly with the last ones, indicating the involvement of EMI mantle component.

Detailed field mapping (Map, Appendix B) has been conducted in and around the boundaries of a 14x18km, volcanic complex 35km northeast of Molteno in the Eastern Cape Province, South Africa. The structure is interpreted as a subsidence structure, and is filled with two volcaniclastic breccias, numerous lava flows, a number of sedimentary facies, and lies on a base of Clarens Formation overlying Elliot Formation rocks. This is an important study because 'widespread, voluminous fields of basaltic breccias are very rare (see Hanson and Elliot, 1996) and this is the first time that this type of volcanic complex and its deposits have been described. Detailed analyses of the two volcaniclastic breccias revealed changes in colour, clast types, clast sizes, and degree of alteration over relatively short distances both vertically and laterally within a single breccia unit. The variation in clast sizes implies a lack of sorting of the breccias. The lower of the two volcaniclastic breccias fills the subsidence structure, and outcrops between the Stormberg sedimentary sequence and the overlying Drakensberg basalts and was produced from phreatomagmatic eruptions signalling the start of the break-up of Gondwanaland in the mid-Jurassic. The upper volcaniclastic breccia is interbedded with the flood basalts and is separated from the lower breccia by up to 100m of lava flows in places, it is finer-grained than the lower volcaniclastic breccia, and it extends over 10km south, and over 100km north from the volcanic complex. The upper breccia is inferred to have been transported from outside the study area, from a source presumably similar to the subsidence structure in the volcanic complex. The pyroclastic material forming the upper breccia was transported to the subsidence structure as a laharic debris flow, based on its poorly sorted, unwelded and matrix-supported appearance. However, both breccias are unlikely to have been derived from epiclastic reworking of lava flows as they contain glass shards which are atypical of those derived from the autoclastic component of lava flows. The breccias are therefore not "secondary" lahars. There is also no evidence of any palaeotopographic highs from which the breccias could have been derived as gravity-driven flows. Based on the occurrence of three, 1m thick lacustrine deposits, localised peperite, fluvial reworking of sandstone and breccia in an outcrop to the south of the subsidence structure, and channel-lags encountered only in the upper units of the Clarens Formation and only within the subsidence structure, the palaeoenvironment inferred for the subsidence structure is one of wet sediment, possibly a shallow lake, in a topographic depression fed by small streams. Magmatic intrusions below the subsidence structure heated the water-laden, partly consolidated Clarens Formation sandstones, causing the circulation of pore fluid which resulted in the precipitation of minerals forming pisoliths in the sandstones. Intruding magma mixed, nonexplosively, with the wet, unconsolidated sediments near the base of the Clarens Formation (at approximately 100m below the surface), forming fluidal peperite by a process of sediment fluidisation where magma replaces wet sediment and cools slowly enough to prevent the magma fracturing brittly. Formation of fluidal peperite may have been a precursor to the development of FCIs (Fuel Coolant Interactions) (Busby-Spera and White, 1987). The breccias may represent the products of FCIs and may be the erupted equivalents of the peperites, suggesting a possible genetic link between the two. The peperites may have given way to FCI eruptions due to a number of factors including the drying out of the sediments and/or an increase in the volume of intruded magma below the subsidence structure which may have resulted in a more explosive interaction between sediment and magma. Phreatic activity fragmented and erupted the Clarens Formation sandstone, and stream flows reworked the angular sandstone fragments, pisoliths and sand grains into channelised deposits. With an increase in magmatic activity below the subsidence structure, phreatic activity became phreatomagmatic. The wet, partly consolidated Clarens Formation, and underlying, fully consolidated Elliot Formation sediments were erupted and fragmented. Clasts and individual grains of these sediments were redeposited with juvenile and non-juvenile basaltic material probably by a combination of back fall, where clasts erupted into the air fell directly back into the structure, and backflow where material was erupted out of the structure, but immediately flowed back in as lahars. This material formed the lower volcaniclastic breccia. A fault plane is identified along the southwestern margin of the subsidence structure, and is believed to continue up the western margin to the northwestern corner. A large dolerite body has intruded along the inferred fault plane on the western margin of the structure, and may be related to the formation of the lower volcaniclastic breccia, either directly through fluidisation of wet sediment during its intrusion, or as a dyke extending upwards from a network of sill-like intrusions below the subsidence structure. Geochemical analysis of the Drakensberg basalt lava flows by Mitchell (1980) and Masokwane (1997) revealed four distinct basalt types; the Moshesh's Ford, the Tafelkop, the Roodehoek, and the Vaalkop basalts. Basalt clasts sampled from the lower volcaniclastic breccia were shown to belong to the Moshesh's Ford basalt type which does not outcrop in situ within the subsidence structure. This implies that the Moshesh's Ford basalts were emplaced prior to the formation of the lower volcaniclastic breccia, and may have acted as a "cap-rock" over the system, allowing pressure from the vaporised fluids, heated by intruding basalt, to build up. The Moshesh's Ford basalt type was erupted prior to the resultant phreatomagmatic events forming the lower volcaniclastic breccia.

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