A suíte de xenólitos ultramáficos do Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represeta o Manto Litosférico Continental nesta região. Estas rochas guardam registros dos processos a que o manto esteve sujeito durante todo o período de evolução e diferenciação do próprio manto litosférico e de formação de crosta continental. Utilizando-se dados de petrologia, de geoquímica mineral e rocha total de elementos maiores e menores e dados isotópicos, é possível demonstrar que os xenólitos do Cerro de los Chenques possuem paragênese mineralógica primária formada por olivina, enstatita, diopsído e espinélio e que esta paragênese está em equilíbrio, como pode ser observado nos dados de química mineral. Desta forma estas rochas são classificadas como espinélio lherzolitos, espinélio harzburgitos e espinélio olivinawebsteritos, com temperatura e pressão de equilíbrio variando de 782°C a 1029ºC e 14kbar a 19 kbar. Mesmo que a paragênese primária esteja em equilíbrio, ocorrem instabilidades locais com formação de fusão na forma de bolsões de vidro silicáticos e de uma paragênese secundária composta por olivina, diopsídio e espinélio. Os dados geoquímicos de rocha total mostram depleção nos elementos alcalinos e nos elementos traço em relação ao manto primitivo, ilustrando que houve processo de fusão parcial atuando nestas rochas. As amostras estudadas apresentam também feições características de processos de metassomatismo em momentos distintos e guardando características particulares em cada um deles. O primeiro evento metassomático foi determinado como sendo originado por uma pluma de ascenção astenosférica, sendo denotado principalmente pelo enriquecimento nos HFSE (Nb e Ta) e alguns elementos incompatíveis LILE. Este evento é responsável pela metassomatização dos lherzolitos e harzburgitos e pela formação dos olivina-websteritos a partir de um manto granadalherzolítico, onde a granada se torna instável para a formação de clinopiroxênio e espinélio. Eventos metassomáticos posteriores a este também foram identificados, sendo desta vez relacionados a líquidos provenientes da desidratação e fusão de placas oceânicas em zonas de subducção. Um desses eventos está relacionado a colagem dos micro-continentes Maciço Norte Patagônico e Maciço del Deseado, por volta de 350 Ma caracterizado principalmente pelo enriquecimento nos ETR leves em relação aos pesados. O outro, mais recente, relacionado à subducção da Placa Oceânica de Nazca, onde o principal evento é o enriquecimento nos elementos calcófilos (Pb, Sn, W e Sb), que também está registrado em todas as suítes de xenólitos da Patagônia. Foi determinado ainda, que os basaltos de platô de back-arc, resposnsáveis por trazer os xenólitos à superfície não infiltram nos xenólitos, a ponto de alterar a química de suas rochas. / The ultramafic xenolith set from Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represents the Sub Continetal Lithospheric Mantle (SCLM) of this region. All rocks record processes in the mantle that happened during all period of lithospheric mantle differentiation and crust formation. Using the petrologica data, mineralochemistry, and major, trace and isotope element geochemistry, it is possible to demonstrate that the xenoltihs equilibria mineral assemblage is olivine, enstatite, diopside and spinel. These rocks are classified as spinel lherzolites, spinel harzburgite and spinel olivine websterite, with temperature and pressure equilibrium of 782° to 1029°C and 14 19 kbar, respectively. It is also possible to point out that metassomatic process occurred in the xenolith rocks, which resulted in the crystallization of a secondary assemblage formed by olivine, diopside and spinel, and the formation of silicate melt pockets around spinel and clinopyroxene. Geochemistry data show alkalis and trace elements depletion in relation to primitive mantle as a result of the melting events. The studied samples also show metasomatic events in different periods, with different characteristics. The first metasomatic event was related to an upwelling of an asthenospheric plume, responsible for the HFSE (Nb and Ta) and some incompatible elements (LILE) enrichment in all lithologies, and by the formation of the olivinewebsterite from a garnet-lherzolite, in which garnet reacts out to form clinopyroxene and spinel. Two others metasomatic events must have happened, following this one, both related to fluids and melts originated from the dehydration and melting of the subducting slab. One of these events is related to the collage of the Norte-Patagônia massif to del Deseado massif micro-continets, around 350 Ma, which resulted in the light REE enrichment. The other one, more recent, is related to the Nazca subducting slab, responsible for the enrichment in chalcophile elements (Pb, Sn, W and Sb), which is also observed in all mantle xenoliths from Patagonia. We also discard any infiltration of the host-basalt as the responsible for the ultramafic xenoliths chemical modification.

A suíte de xenólitos ultramáficos do Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represeta o Manto Litosférico Continental nesta região. Estas rochas guardam registros dos processos a que o manto esteve sujeito durante todo o período de evolução e diferenciação do próprio manto litosférico e de formação de crosta continental. Utilizando-se dados de petrologia, de geoquímica mineral e rocha total de elementos maiores e menores e dados isotópicos, é possível demonstrar que os xenólitos do Cerro de los Chenques possuem paragênese mineralógica primária formada por olivina, enstatita, diopsído e espinélio e que esta paragênese está em equilíbrio, como pode ser observado nos dados de química mineral. Desta forma estas rochas são classificadas como espinélio lherzolitos, espinélio harzburgitos e espinélio olivinawebsteritos, com temperatura e pressão de equilíbrio variando de 782°C a 1029ºC e 14kbar a 19 kbar. Mesmo que a paragênese primária esteja em equilíbrio, ocorrem instabilidades locais com formação de fusão na forma de bolsões de vidro silicáticos e de uma paragênese secundária composta por olivina, diopsídio e espinélio. Os dados geoquímicos de rocha total mostram depleção nos elementos alcalinos e nos elementos traço em relação ao manto primitivo, ilustrando que houve processo de fusão parcial atuando nestas rochas. As amostras estudadas apresentam também feições características de processos de metassomatismo em momentos distintos e guardando características particulares em cada um deles. O primeiro evento metassomático foi determinado como sendo originado por uma pluma de ascenção astenosférica, sendo denotado principalmente pelo enriquecimento nos HFSE (Nb e Ta) e alguns elementos incompatíveis LILE. Este evento é responsável pela metassomatização dos lherzolitos e harzburgitos e pela formação dos olivina-websteritos a partir de um manto granadalherzolítico, onde a granada se torna instável para a formação de clinopiroxênio e espinélio. Eventos metassomáticos posteriores a este também foram identificados, sendo desta vez relacionados a líquidos provenientes da desidratação e fusão de placas oceânicas em zonas de subducção. Um desses eventos está relacionado a colagem dos micro-continentes Maciço Norte Patagônico e Maciço del Deseado, por volta de 350 Ma caracterizado principalmente pelo enriquecimento nos ETR leves em relação aos pesados. O outro, mais recente, relacionado à subducção da Placa Oceânica de Nazca, onde o principal evento é o enriquecimento nos elementos calcófilos (Pb, Sn, W e Sb), que também está registrado em todas as suítes de xenólitos da Patagônia. Foi determinado ainda, que os basaltos de platô de back-arc, resposnsáveis por trazer os xenólitos à superfície não infiltram nos xenólitos, a ponto de alterar a química de suas rochas. / The ultramafic xenolith set from Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represents the Sub Continetal Lithospheric Mantle (SCLM) of this region. All rocks record processes in the mantle that happened during all period of lithospheric mantle differentiation and crust formation. Using the petrologica data, mineralochemistry, and major, trace and isotope element geochemistry, it is possible to demonstrate that the xenoltihs equilibria mineral assemblage is olivine, enstatite, diopside and spinel. These rocks are classified as spinel lherzolites, spinel harzburgite and spinel olivine websterite, with temperature and pressure equilibrium of 782° to 1029°C and 14 19 kbar, respectively. It is also possible to point out that metassomatic process occurred in the xenolith rocks, which resulted in the crystallization of a secondary assemblage formed by olivine, diopside and spinel, and the formation of silicate melt pockets around spinel and clinopyroxene. Geochemistry data show alkalis and trace elements depletion in relation to primitive mantle as a result of the melting events. The studied samples also show metasomatic events in different periods, with different characteristics. The first metasomatic event was related to an upwelling of an asthenospheric plume, responsible for the HFSE (Nb and Ta) and some incompatible elements (LILE) enrichment in all lithologies, and by the formation of the olivinewebsterite from a garnet-lherzolite, in which garnet reacts out to form clinopyroxene and spinel. Two others metasomatic events must have happened, following this one, both related to fluids and melts originated from the dehydration and melting of the subducting slab. One of these events is related to the collage of the Norte-Patagônia massif to del Deseado massif micro-continets, around 350 Ma, which resulted in the light REE enrichment. The other one, more recent, is related to the Nazca subducting slab, responsible for the enrichment in chalcophile elements (Pb, Sn, W and Sb), which is also observed in all mantle xenoliths from Patagonia. We also discard any infiltration of the host-basalt as the responsible for the ultramafic xenoliths chemical modification.

O campo vulcânico Patagônico é composto pelo vulcanismo datado do Quaternário ao Cretáceo e está amplamente distribuído no ambiente geotectônico de extra back-arc continental. Onze vulcões associados à ocorrência de xenólitos mantélicos estão situados dos 36°13’S aos 44°52’S. Estes vulcões são dominantemente compostos por basanitos e a basaltos alcalinos, que são divididos em dois grupos com base em aspectos petrográficos, geoquímicos e isotópicos. (Grupos I e II). Estes Grupos estão relacionados a fontes mantélicas similares, mas foram submetidos a diferentes processos metassomáticos. Os Grupos I e I foram gerados a partir de baixas taxas de fusão a partir de uma fonte mantélica do tipo OIB na zona de estabilidade da granada, mas o Grupo II tem características de manto enriquecido (EMII) possivelmente herdadas de um agente metassomático relacionado à zona de subducção, enquanto que o Grupo I demonstra assinatura geoquímica de magmas tipo OIB relacionados a fontes mantélicas ricas em flogopita. Os basaltos alcalinos do vulcão Agua Poca (37º01’S - 68º07’W) pertencem ao Grupo II e são traquibasaltos. O vulcão Agua Poca é definido é piroclástico monogenético, é composto por intercalações de camadas de spatter e cinder, hospeda xenólitos mantélicos e está localizado a oeste da Província de La Pampa, no extremo norte da Argentina. As amostras de xenólitos mostram textura protogranular, protogranular a porfiroclástica, porfiroclástica e porfiroclástica a equigranular e são compostos por olivina (fosterita), ortopiroxênio (enstatita), clinopiroxênio (diopsídio) e espinélio (sp). Os xenólitos estudados são peridotitos da fácies espinélio e piroxenitos anidros em basaltos alcalinos do Pleistoceno com #Mg em rocha total de 89 a 91. As assinaturas geoquímicas desses xenólitos mostram correlação negativa entre os principais óxidos quando dispostos contra o #Mg e estão empobrecidos em elementos incompatíveis em relação ao manto primitivo (MP). Os xenólitos do vulcão Agua Poca são caracterizados pelo empobrecimento de ETRP e ETRM normalizados para o MP e pelo fracionamento de ETRL em relação aos ETRP (CeN/YbN = 0,15-0,5), com exceção da amostra HAP10 (1,46). Esse comportamento indica que os xenólitos do terreno Cuyania são o resultado de 1 a 10% de fusão do DMM (Manto Depletado) ou de 8 a 17% do MP (Manto Primitivo). Em geral, os peridotitos mostram anomalias positivas de Ba, U, Ta, Pb, Zr e Ti; e anomalias negativas de Rb, Th, Nb, La e Y, enquanto que os piroxenitos mostram anomalias positivas de Ba, U, Ta e Pb; e anomalias negativas de Th, Nb, La, Zr, Hf, Ti e Y. Curvas de mistura calculadas para o resíduo de fusão do MP/DMM com a composição de fluídos/sedimentos derivados de ambientes de subducção indicam interação do manto com até 3% de fluídos/sedimentos. As razões 87Sr/86Sr (0,702874 - 0,704999, com média de 0,704035) são muito similares àquelas definidas para peridotitos com fonte tipo OIB. Agua Poca tem razões 87Sr/86Sr, que estão abaixo daquelas definidas para peridotitos metassomatizados (usualmente >0,705). As razões de Nb/Ta sugerem a presença de um reservatório eclogítico refratário subductado fusão parcial gerando líquidos alcalinos com razões Nb/Ta supercondríticas. / The Patagonian Volcanic Field composed of late Cretaceous to Quaternary volcanism is widely distributed in a continental extra back-arc geotectonic environment. Eleven monogenetic volcanoes accompanied with ultramafic xenoliths are situated from 36°13′S to 44°52′S. These volcanoes are dominantly composed of basanite to alkaline basalt, which are divided into two groups, based on mineralogy, geochemical and isotope compositions (Groups I and II). These Groups are originated from the similar subcontinental mantle sources, but were undergone to different metasomatism processes. Groups I and II were generated from low melting degrees of an OIB-like garnet peridotite, but the Group II has enriched mantle (EMII) characteristics possibly inherited from on-going subduction related metasomatism, while Group I demonstrates the OIB-like signature, which might result from phlogopite-bearing in the subcontinental lithosphere. The alkaline basalts from Agua Poca volcano (37º01’S - 68º07’W) belong to the Group II and are trachybasalts. The Agua Poca volcano is a monogenetic pyroclastic volcano composed by intercalation of spatter and cinder layers, host ultramafic mantle xenoliths and is located in the West of the La Pampa Province, Northernmost of Argentine Patagonia. The xenoliths show protogranular, protogranular to porphyroclastic, porphyroclastic and porphyroclastic to equigranular textures, and are composed of olivine (fosterite), orthopyroxene (enstatite), clinopyroxene (diopside) and spinel (sp). The studied xenoliths are anhydrous spinel-bearing peridotite and pyroxenite xenoliths in Pleistocene alkali basalts with whole rock Mg# from 88 to 91. Geochemical signatures of the mantle xenoliths show negative correlation between main oxides against Mg# and depletion in incompatible elements compared to primitive mantle (PM). Agua Poca mantle xenoliths are characterized by flat Sun & McDonough (1989) primitive mantle (PM) normalized HREE and MREE patterns, and depletion of LREE compared to HREE (CeN/YbN = 0.15-0.5), with exception of the HAP10 (1.46) sample. These characteristics suggest that partial melting event is the main process responsible for the generation of these xenoliths. Model calculations suggest that the xenoliths are the result of 1 to 10% of DMM (Depleted Mantle MORB) or 8 to 17% of PM partial melting. Peridotite samples show positive anomalies of Ba, U, Ta, Pb, Zr and Ti; and negative anomalies of Rb, Th, Nb, La and Y, while the pyroxenite samples show positive anomalies of Ba, U, Ta and Pb; and negative anomalies of Th, Nb, La, Zr, Hf, Ti and Y. Mixing curves calculated to mixtures of melting residue of PM/DMM and fluid or sediment compositions related to subduction tectonic setting end members suggest up to 3% of interaction of the fluid sediment on the depleted mantle residue. 87Sr/86Sr ratios (0.702874 - 0.704999, with average of 0.704035) are similar to those defined to peridotites with OIB source (87Sr/86Sr = 0.70244 to 0.70502), being close to Depleted Mantle (DM; 87Sr/86Sr = 0.7023 to 0.7032) values. Nb/Ta ratios suggest that Agua Poca xenoliths were undergone to partial melting processes that generated alkaline magmas with superchondritic Nb/Ta ratios.

O campo vulcânico Patagônico é composto pelo vulcanismo datado do Quaternário ao Cretáceo e está amplamente distribuído no ambiente geotectônico de extra back-arc continental. Onze vulcões associados à ocorrência de xenólitos mantélicos estão situados dos 36°13’S aos 44°52’S. Estes vulcões são dominantemente compostos por basanitos e a basaltos alcalinos, que são divididos em dois grupos com base em aspectos petrográficos, geoquímicos e isotópicos. (Grupos I e II). Estes Grupos estão relacionados a fontes mantélicas similares, mas foram submetidos a diferentes processos metassomáticos. Os Grupos I e I foram gerados a partir de baixas taxas de fusão a partir de uma fonte mantélica do tipo OIB na zona de estabilidade da granada, mas o Grupo II tem características de manto enriquecido (EMII) possivelmente herdadas de um agente metassomático relacionado à zona de subducção, enquanto que o Grupo I demonstra assinatura geoquímica de magmas tipo OIB relacionados a fontes mantélicas ricas em flogopita. Os basaltos alcalinos do vulcão Agua Poca (37º01’S - 68º07’W) pertencem ao Grupo II e são traquibasaltos. O vulcão Agua Poca é definido é piroclástico monogenético, é composto por intercalações de camadas de spatter e cinder, hospeda xenólitos mantélicos e está localizado a oeste da Província de La Pampa, no extremo norte da Argentina. As amostras de xenólitos mostram textura protogranular, protogranular a porfiroclástica, porfiroclástica e porfiroclástica a equigranular e são compostos por olivina (fosterita), ortopiroxênio (enstatita), clinopiroxênio (diopsídio) e espinélio (sp). Os xenólitos estudados são peridotitos da fácies espinélio e piroxenitos anidros em basaltos alcalinos do Pleistoceno com #Mg em rocha total de 89 a 91. As assinaturas geoquímicas desses xenólitos mostram correlação negativa entre os principais óxidos quando dispostos contra o #Mg e estão empobrecidos em elementos incompatíveis em relação ao manto primitivo (MP). Os xenólitos do vulcão Agua Poca são caracterizados pelo empobrecimento de ETRP e ETRM normalizados para o MP e pelo fracionamento de ETRL em relação aos ETRP (CeN/YbN = 0,15-0,5), com exceção da amostra HAP10 (1,46). Esse comportamento indica que os xenólitos do terreno Cuyania são o resultado de 1 a 10% de fusão do DMM (Manto Depletado) ou de 8 a 17% do MP (Manto Primitivo). Em geral, os peridotitos mostram anomalias positivas de Ba, U, Ta, Pb, Zr e Ti; e anomalias negativas de Rb, Th, Nb, La e Y, enquanto que os piroxenitos mostram anomalias positivas de Ba, U, Ta e Pb; e anomalias negativas de Th, Nb, La, Zr, Hf, Ti e Y. Curvas de mistura calculadas para o resíduo de fusão do MP/DMM com a composição de fluídos/sedimentos derivados de ambientes de subducção indicam interação do manto com até 3% de fluídos/sedimentos. As razões 87Sr/86Sr (0,702874 - 0,704999, com média de 0,704035) são muito similares àquelas definidas para peridotitos com fonte tipo OIB. Agua Poca tem razões 87Sr/86Sr, que estão abaixo daquelas definidas para peridotitos metassomatizados (usualmente >0,705). As razões de Nb/Ta sugerem a presença de um reservatório eclogítico refratário subductado fusão parcial gerando líquidos alcalinos com razões Nb/Ta supercondríticas. / The Patagonian Volcanic Field composed of late Cretaceous to Quaternary volcanism is widely distributed in a continental extra back-arc geotectonic environment. Eleven monogenetic volcanoes accompanied with ultramafic xenoliths are situated from 36°13′S to 44°52′S. These volcanoes are dominantly composed of basanite to alkaline basalt, which are divided into two groups, based on mineralogy, geochemical and isotope compositions (Groups I and II). These Groups are originated from the similar subcontinental mantle sources, but were undergone to different metasomatism processes. Groups I and II were generated from low melting degrees of an OIB-like garnet peridotite, but the Group II has enriched mantle (EMII) characteristics possibly inherited from on-going subduction related metasomatism, while Group I demonstrates the OIB-like signature, which might result from phlogopite-bearing in the subcontinental lithosphere. The alkaline basalts from Agua Poca volcano (37º01’S - 68º07’W) belong to the Group II and are trachybasalts. The Agua Poca volcano is a monogenetic pyroclastic volcano composed by intercalation of spatter and cinder layers, host ultramafic mantle xenoliths and is located in the West of the La Pampa Province, Northernmost of Argentine Patagonia. The xenoliths show protogranular, protogranular to porphyroclastic, porphyroclastic and porphyroclastic to equigranular textures, and are composed of olivine (fosterite), orthopyroxene (enstatite), clinopyroxene (diopside) and spinel (sp). The studied xenoliths are anhydrous spinel-bearing peridotite and pyroxenite xenoliths in Pleistocene alkali basalts with whole rock Mg# from 88 to 91. Geochemical signatures of the mantle xenoliths show negative correlation between main oxides against Mg# and depletion in incompatible elements compared to primitive mantle (PM). Agua Poca mantle xenoliths are characterized by flat Sun & McDonough (1989) primitive mantle (PM) normalized HREE and MREE patterns, and depletion of LREE compared to HREE (CeN/YbN = 0.15-0.5), with exception of the HAP10 (1.46) sample. These characteristics suggest that partial melting event is the main process responsible for the generation of these xenoliths. Model calculations suggest that the xenoliths are the result of 1 to 10% of DMM (Depleted Mantle MORB) or 8 to 17% of PM partial melting. Peridotite samples show positive anomalies of Ba, U, Ta, Pb, Zr and Ti; and negative anomalies of Rb, Th, Nb, La and Y, while the pyroxenite samples show positive anomalies of Ba, U, Ta and Pb; and negative anomalies of Th, Nb, La, Zr, Hf, Ti and Y. Mixing curves calculated to mixtures of melting residue of PM/DMM and fluid or sediment compositions related to subduction tectonic setting end members suggest up to 3% of interaction of the fluid sediment on the depleted mantle residue. 87Sr/86Sr ratios (0.702874 - 0.704999, with average of 0.704035) are similar to those defined to peridotites with OIB source (87Sr/86Sr = 0.70244 to 0.70502), being close to Depleted Mantle (DM; 87Sr/86Sr = 0.7023 to 0.7032) values. Nb/Ta ratios suggest that Agua Poca xenoliths were undergone to partial melting processes that generated alkaline magmas with superchondritic Nb/Ta ratios.

A suíte de xenólitos ultramáficos do Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represeta o Manto Litosférico Continental nesta região. Estas rochas guardam registros dos processos a que o manto esteve sujeito durante todo o período de evolução e diferenciação do próprio manto litosférico e de formação de crosta continental. Utilizando-se dados de petrologia, de geoquímica mineral e rocha total de elementos maiores e menores e dados isotópicos, é possível demonstrar que os xenólitos do Cerro de los Chenques possuem paragênese mineralógica primária formada por olivina, enstatita, diopsído e espinélio e que esta paragênese está em equilíbrio, como pode ser observado nos dados de química mineral. Desta forma estas rochas são classificadas como espinélio lherzolitos, espinélio harzburgitos e espinélio olivinawebsteritos, com temperatura e pressão de equilíbrio variando de 782°C a 1029ºC e 14kbar a 19 kbar. Mesmo que a paragênese primária esteja em equilíbrio, ocorrem instabilidades locais com formação de fusão na forma de bolsões de vidro silicáticos e de uma paragênese secundária composta por olivina, diopsídio e espinélio. Os dados geoquímicos de rocha total mostram depleção nos elementos alcalinos e nos elementos traço em relação ao manto primitivo, ilustrando que houve processo de fusão parcial atuando nestas rochas. As amostras estudadas apresentam também feições características de processos de metassomatismo em momentos distintos e guardando características particulares em cada um deles. O primeiro evento metassomático foi determinado como sendo originado por uma pluma de ascenção astenosférica, sendo denotado principalmente pelo enriquecimento nos HFSE (Nb e Ta) e alguns elementos incompatíveis LILE. Este evento é responsável pela metassomatização dos lherzolitos e harzburgitos e pela formação dos olivina-websteritos a partir de um manto granadalherzolítico, onde a granada se torna instável para a formação de clinopiroxênio e espinélio. Eventos metassomáticos posteriores a este também foram identificados, sendo desta vez relacionados a líquidos provenientes da desidratação e fusão de placas oceânicas em zonas de subducção. Um desses eventos está relacionado a colagem dos micro-continentes Maciço Norte Patagônico e Maciço del Deseado, por volta de 350 Ma caracterizado principalmente pelo enriquecimento nos ETR leves em relação aos pesados. O outro, mais recente, relacionado à subducção da Placa Oceânica de Nazca, onde o principal evento é o enriquecimento nos elementos calcófilos (Pb, Sn, W e Sb), que também está registrado em todas as suítes de xenólitos da Patagônia. Foi determinado ainda, que os basaltos de platô de back-arc, resposnsáveis por trazer os xenólitos à superfície não infiltram nos xenólitos, a ponto de alterar a química de suas rochas. / The ultramafic xenolith set from Cerro de los Chenques, Patagônia (44°52’19”S/70°03’57”W), represents the Sub Continetal Lithospheric Mantle (SCLM) of this region. All rocks record processes in the mantle that happened during all period of lithospheric mantle differentiation and crust formation. Using the petrologica data, mineralochemistry, and major, trace and isotope element geochemistry, it is possible to demonstrate that the xenoltihs equilibria mineral assemblage is olivine, enstatite, diopside and spinel. These rocks are classified as spinel lherzolites, spinel harzburgite and spinel olivine websterite, with temperature and pressure equilibrium of 782° to 1029°C and 14 19 kbar, respectively. It is also possible to point out that metassomatic process occurred in the xenolith rocks, which resulted in the crystallization of a secondary assemblage formed by olivine, diopside and spinel, and the formation of silicate melt pockets around spinel and clinopyroxene. Geochemistry data show alkalis and trace elements depletion in relation to primitive mantle as a result of the melting events. The studied samples also show metasomatic events in different periods, with different characteristics. The first metasomatic event was related to an upwelling of an asthenospheric plume, responsible for the HFSE (Nb and Ta) and some incompatible elements (LILE) enrichment in all lithologies, and by the formation of the olivinewebsterite from a garnet-lherzolite, in which garnet reacts out to form clinopyroxene and spinel. Two others metasomatic events must have happened, following this one, both related to fluids and melts originated from the dehydration and melting of the subducting slab. One of these events is related to the collage of the Norte-Patagônia massif to del Deseado massif micro-continets, around 350 Ma, which resulted in the light REE enrichment. The other one, more recent, is related to the Nazca subducting slab, responsible for the enrichment in chalcophile elements (Pb, Sn, W and Sb), which is also observed in all mantle xenoliths from Patagonia. We also discard any infiltration of the host-basalt as the responsible for the ultramafic xenoliths chemical modification.

Le gisement de talc-chlorite de Trimouns est situé dans le massif nord-pyrénéen du Saint Barthélémy (Ariège, France). Il est l'objet d'une attention particulière pour ses ressources minérales exceptionnelles (tant par la qualité que l'abondance), et pour sa position stratégique dans l'histoire géologique des Pyrénées. Le but de cette étude est de caractériser les conditions de formation du gisement et de les intégrer dans le contexte géologique régional. Pour aborder cette problématique nous avons choisi trois grands axes d'étude : 1) au moyen de l'analyse cartographique et structurale, nous procédons à une description de la géométrie du gisement, et nous proposons une histoire des relations entre minéralisation et déformation ; 2) à l'aide d'analyses thermométriques via plusieurs méthodes, nous cherchons à définir quelle est l'histoire thermique enregistrée dans les différentes unités du gisement ; 3) avec des datations in situ sur un large panel de minéraux, nous positionnons des repères temporels sur plusieurs objets géologiques afin de dater le ou les épisodes minéralisateurs sur Trimouns. Les résultats obtenus expriment le caractère polyphasé du gisement, et ce pour les trois axes d'études suivis, déformation, thermicité et âge de la minéralisation. Les travaux sur les structures montrent que la minéralisation principale scelle une déformation probablement varisque, minéralisation qui se forme elle-même en contexte dynamique et qui est à son tour déformée. L'étude thermique met en évidence que la ou les minéralisations sont associées à un ou plusieurs événements froids, et qu'une empreinte thermique chaude est préservée dans les roches du toit et du mur du gisement. Les résultats géochronologiques montrent que la formation du talc et des chlorites à Trimouns est polyphasée et associée à une succession d'événements métasomatiques. L'Albien est l'évènement hydrothermal majeur mais des épisodes plus anciens sont enregistrés au Jurassique à Trimouns, voire au Permien dans d'autres gisements du massif et de l'ouest de la chaîne des Pyrénées. La synthèse de nos travaux nous permet de proposer un modèle de la formation du gisement de talc-chlorite de Trimouns. Ce modèle s'inscrit dans un contexte géodynamique extensif post-varisque que l'on peut mettre en relation avec les phénomènes extensifs pré-orogéniques pyrénéens du Crétacé (120-85 Ma). Ce contexte pré-orogénique est associé à l'exhumation du manteau, source probable du magnésium nécessaire à la formation du gisement. Nos travaux montrent également que le massif du Saint Barthélémy, et possiblement d'autres massifs nord pyrénéens semblables, ne sont pas des massifs "simplement" varisques mais qu'ils ont pu être profondément affectés par les évènements du cycle alpin. / The Trimouns talc-chlorite deposit is located in the north Pyrenean Saint Barthelemy massif (Ariège, France). This deposit is remarkable in its minerals' quality and quantity as well as for its strategic position in the Pyrenees geological history. This study aims at understanding the formation conditions of the talc-chlorite deposit and at integrating them in the alpine pre-orogenic context. To this, we focus on three main themes : 1) Using geological and structural mapping studies, we describe the rocks and their organization as to estimate how much variscan and alpine orogenies affected them. 2) With thermometric analyses using different methods, we seek to define what is the thermal history recorded in the different units of the deposit. 3) Using in-situ dating on a wide range of minerals, we locate temporal references on various geological objects to document hydrothermal events. Achieved results demonstrate the polyphase caracteristics of the deposit, in the three focal areas used : deformation, thermal approach, and geochronology. Structural analysis shows that the mineralisation seals an other deformation, probably of the variscan period. This mineralisation is also formed in a dynamic context and then deformed as well. The thermal study highlights that mineralizations are associated with one or more cold events, and a hot thermal foot-print is preserved in the footwall and the hanging wall of the deposit. Dating results show that the formation talc and chlorite in Trimouns is multiphase, associated with succession of metasomatic events. The Albian event is the major hydrothermal event but older episodes are recorded in Jurassic at Trimouns and in Permian in other fields on the west-ern Pyrenees. The synthesis of our works allows us to propose a model of the formation of the Trimouns talc-chlorite deposit. This model is part of a post-Variscan extensive geodynamic context that can be related to the Pyrenean extensive pre-orogenic phenomena during Cretaceous period (120-85 Ma). This pre-orogenic context is associated with mantle exhumation, likely source of magnesium necessary for the formation of talc. Our studies also show that the Saint Barthelemy massif and possibly other similar north Pyrenean massif are not "simply" Variscan but have been deeply affected by the events of the Alpine cycle.

Les laves alcalines sous-saturées riches en éléments volatils sont les marqueurs du rôle important des fluides dans le manteau et des interactions fluide-roche et magma-roche, processus clés pour comprendre la dynamique du manteau convectif et les interactions asthénosphère-lithosphère en domaine intracontinental. L’objectif de cette thèse est d’apporter de nouvelles contraintes sur la genèse des magmas alcalins en caractérisant les conditions de cristallisation, la source et les processus de fusion partielle à l’origine des néphélinites à olivine, des néphélinites à pyroxène et des basanites du champ volcanique du Jbel Saghro dans l’Anti-Atlas marocain. L’étude pétrologique et géochimique des roches et des minéraux, couplée à l’analyse des inclusions fluides a permis de contraindre les conditions pré-éruptives des néphélinites de Saghro à 1.7–2.2 GPa et ~1350 °C. Les minéraux montrent que les magmas néphélinitiques sont riches en éléments volatils (Cl, F, S), et les inclusions fluides indiquent que les magmas étaient saturés en fluide riche en CO2 à des pressions > 590 MPa. Les différents assemblages minéralogiques des néphélinites et la présence de xénolites péridotitiques suggèrent une ascension rapide des néphélinites à olivine et des processus plus complexes en profondeur pour les néphélinites à pyroxène. La modélisation des processus de cristallisation fractionnée et de fusion partielle des laves mafiques de Saghro a permis de déterminer qu’elles sont issues de faibles taux de fusion partielle (0.6–2.5 %) d’une péridotite carbonatée enrichie en éléments incompatibles, au niveau de la transition grenat–spinelle (~80–85 km) et en présence d’amphibole. Les néphélinites de Saghro montrent une évolution temporelle avec une légère augmentation du taux de fusion et une diminution de la quantité d’amphibole au résidu des plus anciennes (néphélinites à olivine, 9.6 Ma) aux plus récentes (néphélinites à pyroxène, 2.9 Ma). Les basanites forment un système indépendant des néphélinites et sont issues de taux de fusion plus élevés. Les fortes variations dans leur composition chimique suggèrent qu’elles ont subi de la cristallisation fractionnée lors de leur ascension. Les caractéristiques particulières des néphélinites et basanites de Saghro (enrichissement en éléments incompatibles, anomalies négatives en K, Zr, Hf et Ti, rapports Ca/Al et Zr/Hf élevés) indiquent que leur source a subi un métasomatisme principalement carbonatitique. L’influence de ce métasomatisme est plus forte pour les néphélinites à pyroxène que pour les néphélinites à olivine, impliquant une évolution temporelle de l’intensité du métasomatisme. Ces résultats suggèrent des interactions fluide-roche sous le craton Nord-Ouest Africain, entraînant la formation d'un manteau métasomatisé par des composants carbonatitiques riches en CO2 au niveau de la transition lithosphère-asthénosphère. L’origine du métasomatisme provoquant l’enrichissement de la source et la formation de veines d’amphibole pourrait être liée à la fusion de reliquats de croûte océanique subductée. Les températures de fusion relativement faibles (< 1350 °C) suggèrent l’absence d’anomalie thermique sous le Jbel Saghro, et favorisent donc un modèle de délamination de la lithosphère comme initiateur du volcanisme. Cependant, l’augmentation du taux de fusion partielle au cours du temps, également observée dans le Moyen Atlas, et les similitudes isotopiques et géochimiques avec les laves alcalines des îles Canaries ne permettent pas d’exclure une influence du panache des Canaries sur la source du volcanisme alcalin du Jbel Saghro. / Volatile-rich, silica-undersaturated alkaline lavas record the important role of fluids during fluid-rock and magma-rock interactions in the mantle, which are key processes to understand the dynamics of the convective mantle and lithosphere-asthenosphere interactions in intracontinental settings. The aim of this thesis is to bring new constraints on the genesis of alkaline magmas by characterizing the crystallization conditions, the source and the partial melting processes taking part in the genesis of olivine nephelinites, pyroxene nephelinites and basanites from the Jbel Saghro volcanic field in the Moroccan Anti Atlas. The petrological and geochemical study of rocks and minerals coupled with the analysis of fluid inclusions constrains the pre-eruptive conditions of Saghro nephelinites to 1.7–2.2 GPa and ~1350 °C. Minerals show that nephelinitic magmas are rich in volatile elements (Cl, F, S), and fluid inclusions indicate that magmas were saturated with a CO2-rich fluid at pressures > 590 MPa. The various mineralogical assemblages and the presence of peridotite xenoliths suggest a rapid ascent for olivine nephelinites and more complex processes at depth for pyroxene nephelinites. Fractional crystallization and partial melting modelling of Saghro mafic lavas indicate that they are low-degree melts (0.6–2.5 %) of an amphibole-bearing carbonated peridotite enriched in incompatible elements, at the garnet-spinel transition (~80–85 km). Saghro nephelinites display a temporal evolution with a slight increase of the degree of melting and a decrease of the amount of residual amphibole from the oldest (olivine nephelinites, 9.6 Ma) to the most recent (pyroxene nephelinites, 2.9 Ma). Basanites form a system that is independent from nephelinites and are slightly higher-degree melts. Important variations in their chemical composition suggest variable amounts of fractional crystallization during ascent. The peculiar characteristics of Saghro nephelinites and basanites (enrichment in incompatible elements, negative anomalies in K, Zr, Hf and Ti, high Ca/Al and Zr/Hf ratios) indicate that their source was affected by carbonatitic metasomatism. The influence of this metasomatism is stronger for pyroxene nephelinites than for olivine nephelinites. These results suggest fluid-rock interactions beneath the Northwest African Craton, leading to the formation of a metasomatized mantle by CO2-rich carbonatitic components at the lithosphere-asthenosphere transition. The origin of the metasomatism inducing source enrichment and the formation of amphibole veins could be attributed to the melting of relict subducted oce anic lithosphere. The relatively low melting temperatures (< 1350 °C) suggest the absence of a thermal anomaly beneath the Jbel Saghro, and thus support a lithosphere delamination model as precursor of Saghro volca0,3nism. However, the increasing degree of partial melting over time, also observed in the Middle Atlas, together with the isotopic and geochemical similarities with Canary Islands alkaline lavas does not allow us to discard the influence of a deviation of the Canary mantle plume beneath northwest Africa.

Cette étude vise caractériser le manteau lithosphérique du massif du Hoggar (Algérie) et son évolution, grâce à une étude multidisciplinaire (pétrologique, géochimique et pétrophysique) d'enclaves mantelliques échantillonnées par le volcanisme cénozoïque. L'échantillonnage provient de deux districts volcaniques (Tahalagha et Manzaz) situés respectivement en périphérie et au coeur du bombement du Hoggar. Le district de Tahalgha est par ailleurs situé à cheval sur un grand cisaillement pan-africain (le 4°35), séparant deux domaines structuraux majeurs du socle du Hoggar : le Hoggar Central Polycyclique à l'Est (domaine LATEA) et le Hoggar occidental à l'Ouest (bloc d'Iskel). Les xénolites étudiés apportent des informations sur l'évolution du manteau lithosphérique depuis l'orogenèse pan-africaine, au cours de laquelle s'est structuré le socle de cette région (le Bouclier Touareg), jusqu'aux événements cénozoïques responsables du bombement topographique et du volcanisme.L'héritage pan-africain est essentiellement préservé dans les échantillons du district périphérique de Tahalgha, sous la forme de lherzolites équilibrées à basse température (750 - 900°C), à clinopyroxènes appauvris en terres rares légères. Ces échantillons sont considérés comme représentant la lithosphère sous-continentale à l'issue des processus de réjuvénation qui ont marqué les derniers stades de l'orogenèse pan-africaine. Ils montrent des textures de déformation (porphyroclastiques à equigranulaires) bien préservées, attribuées à ces événements et caractérisées par des orientations cristallographiques préférentielles (OPRs) de l'olivine (axiales-[010]) compatibles avec un régime transpressif. Les événements cénozoïques sont marqués par un recuit partiel de ces textures, particulièrement prononcé à Manzaz et dans les échantillons de Tahalgha équilibrés à des températures moyennes à élevées (900-1150°C), et affectés par différents degrés de métasomatisme. Les xénolites de Tahalgha représentent un cas d'étude exemplaire du métasomatisme mantellique, couplant variations texturales, minéralogiques et chimiques le long de gradient locaux de température. Une modification des OPRs d'olivine est observée, qui résulterait à la fois de l'infiltration de liquides métasomatiques et d'une réactivation des accidents pan-africains en cisaillement pur.Des implications importantes de cette étude résident dans l'échelle des variations de premier ordre attribuées aux interactions lithosphère-asthénosphère au Cénozoïque. Celles-ci sont essentiellement à l'échelle du bombement du Hoggar (différences entre Manzaz et Tahalga, c'est-à-dire entre Hoggar central et périphérique) ou à celle de conduits magmatiques et de leurs épontes (variabilité locale des xénolites de Tahalgha). Par contre, les résultats obtenus montrent peu de variations significatives pour les échelles intermédiaires, notamment pour des localités de Tahlagha situées de part et d'autre ou à différentes distances du 4°35. Ceci favorise plutôt, pour l'origine du bombement volcanique du Hoggar, les modèles faisant appel à des structures d'assez grande échelle telle qu'un panache mantellique ou une cellule de convection asthénosphérique de type « Edge Driven Convection », plutôt qu'un processus essentiellement lié à la réactivation des failles lithosphériques pan-africaines. / This study aims to characterize the lithospheric mantle of the Hoggar swell (Algeria) and its evolution through time via a multidisciplinary (petrological, geochemical and petrophysical) study of mantle xenoliths sampled by Cenozoic volcanism. The samples were collected in two volcanic districts (Tahalagha and Manzaz) located in the periphery and in the central part of the Hoggar massif, respectively. The Tahalgha sampling also straddles a mega pan-African shear zone (the 4°35 fault) between two major structural domains of the Tuareg Shield basement: the Central Polycyclic Hoggar to the East (LATEA terranes) and the Western Hoggar domain to the West (Iskel block). The studied xenoliths provide information on the evolution of the lithospheric mantle from the Pan-African orogeny – i.e. the period when the Tuareg Shield was structured – to the Cenozoic events responsible for topographic upwelling and volcanism in the Hoggar swell.The Pan-African heritage is found in xenoliths from the peripheral Tahalgha district. These samples are distinguished by low equilibrium temperatures (750-900°C) and LREE-depleted clinopyroxene compositions. They are considered to represent the sub-continental lithosphere after the rejuvenation process that marked the later stages of the Pan-African orogeny. They show well preserved deformation textures (porphyroclastic to equigranular) assigned to these events and characterized by preferential crystallographic orientations (CPOs) of olivine (axial-[010]) consistent with a transpressional regime. The Cenozoic events are marked by partial annealing of these textures, particularly pronounced in the Manzaz samples, as well as in the Tahalgha xenoliths equilibrated at medium to high temperatures (900-1150°C). These samples were affected by different degrees of metasomatism. The Tahalgha xenoliths represent a rather unique case study of mantle metasomatism, where coupled textural, mineralogical and chemical variations occur along local temperature gradients. The Cenozoic events were also responsible for a change in olivine CPOs, resulting from both infiltration of metasomatic fluids and reactivation of Pan-African accidents in a pure-shear regime.Important implications of this study lie in the scale at which the first-order lithosphere modifications ascribed to the Cenozoic event are observed, i.e. either at the scale of the whole Hoggar swell, as shown by the increasing degree of textural annealing and metasomatism from Tahalgha to Manzaz (i.e. from outer to central Hoggar), or at the small scale of magma conduits and their wall rocks, as shown by the local variability registered by the Tahalgha xenoliths. Conversely, our data show little changes at intermediate scales, as might be expected, for instance, among the Tahalgha localities situated on either sides - or at different distances - from the 4°35. As regards the origin of the Hoggar volcanic swell, this result favours the models involving relatively large-scale structures such as a mantle plume or "Edge Driven Convection", rather than a process involving merely the reactivation of pan-African lithospheric faults.

In the Middelplaats mine area of the Kalahari manganese field, two drill holes (MP53 and MP54) intersected anomalously high-grade manganese ore sitting stratigraphically just above an igneous body (likely a dike or sill). Manganese ore located within approximate 5 meters of the contact with the underlying igneous rocks has been substantially metasomatically upgraded from 25 percent manganese, to over 40 percent whilst the dominant manganese species within the ore has been altered to hausmannite. This report demonstrates the metasomatic alteration is related to devolatilization (removal and/or remobilization of H₂O, CO₂ and CaO) due to contact metamorphism caused by the underlying igneous rocks. The Middelplaats mine is situated in the southwest corner of the Kalahari manganese field where the paleo basin shallows out and ends. Within the mine area, several stratigraphic units pinch out or are truncated by the side of the basin. This pinching out of lithological formations has led to the underlying Ongeluk Formation being in contact with the much younger units of the Hotazel Formation. Therefore, geochemical investigation into the nature and source of the igneous rocks was also undertaken to see if the rocks from the two drill holes were related to one another and/or the underlying Ongeluk Formation. Results of these geochemical studies have demonstrated that the Middelplaats igneous rocks (dolerites) from the two drill holes (MP53 and MP54) share a co-genetic source region. There is also reasonable geochemical evidence that the source region of the Middelplaats igneous rocks was substantially similar to the source region of the Ongeluk Formation. This may indicate that the source region of the Ongeluk Formation was reactivated at some later stage resulting in the emplacement of doleritic dikes or sills in the Middelplaats mine area. The Middelplaats igneous rocks were also found to have undergone a slight but pervasive potassic alteration; with most of the original plagioclase feldspar showing some level of replacement by a potassium enriched feldspar. Although no source for this potassic fluid was found, the devolatilization reaction within the manganese ore appears to have released some potassium into the surrounding rocks. This additional potassium may be responsible for some localized potassic alteration.

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