No description available.

The oxidation state, reflected in the oxygen fugacity (fO2), of the lithospheric mantle is both laterally and vertically heterogeneous. Depth-fO2 profiles from kimberlite-borne peridotitic mantle xenoliths from the Bultfontein kimberlite, Kimberley, South Africa and the A154-N and A154-S kimberlites of the Diavik Mine, NWT, Canada were constructed by measuring ferric iron concentrations in garnets using the flank method. These data demonstrate that mantle metasomatic re-enrichment processes had a significant effect on fO2. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO2 (FMQ) of -2 at 110 km to -4 at 210 km. The lithospheric mantle beneath Diavik is vertically layered with respect to its bulk and trace-element composition. The shallow ‘ultradepleted’ layer is oxidized, to the point that carbonate rather than graphite is the anticipated carbon host. The deeper layer is more fertile and has fO2 conditions extending down to Δlog fO2 (FMQ) -3.8. Deviations from predicted depth-fO2 trends in both xenolith localities result from metasomatic re-enrichment caused by transient fluids and melts. Diamond formation in the Kaapvaal lithospheric mantle may have occurred through the infiltration of reduced fluids into relatively more oxidized mantle. Trace-element concentrations in garnets preserve evidence of two distinct melt metasomatic enrichment events. One was a craton-wide event that is commonly observed in garnet peridotite xenoliths and xenocrysts worldwide; the other was melt infiltration event, preserved as MARID xenoliths, related to the eruption of the Group 2 kimberlites in the western portion of the Kaapvaal craton. The effect of the former melt metasomatism on fO2 is unclear ambiguous whereas the MARID event was clearly oxidizing. Diavik xenoliths preserve evidence for events similar to the fluid and ‘common’ melt metasomatism seen in the Bultfontein samples. Fluid metasomatism affected the entire depth range of xenoliths sampled from Diavik and was oxidizing. A stage of melt metasomatism affected only the deeper (>140 km) portion of the lithospheric mantle and had an overall reducing effect. The observation of sharp-edged octahedral diamonds in microxenoliths affected by the fluid metasomatic event may indicate that this was a major diamond-forming event in the mantle beneath Diavik.

No description available.

Processos de fusão parcial e metassomatismo foram identificados em xenólitos mantélicos da Província Ígnea Alto Paranaíba (PIAP), na região de Coromandel (Minas Gerais), pela combinação de técnicas de análise textural (petrografia, microscopia eletrônica de varredura), análises químicas minerais pontuais de elementos maiores (por Microssonda Eletrônica), traços (por LA-Q-ICPMS) e isótopos (Sr em clinopiroxênio por LA-MC-ICPMS). Os xenólitos, oriundos dos kimberlitos Indaiá 1, Limeira 1 e Forca, foram divididos em três grupos exibindo texturas, mineralogia e química distintas: peridotitos com espinélio ou Cr- espinélio, interpretados como fragmentos de fácies espinélio no manto litosférico raso; xenólitos com flogopita e/ou clinopiroxênio secundário representando porções do manto litosférico que sofreram metassomatismo por interação com fluidos ou magmas; e dunitos e harzburgitos com textura mosaico porfiroclástica a fluidal compatíveis com porções que sofreram efeitos de um fluxo térmico maior, possivelmente associadas a astenosfera ou a zonas de maior permeabilidade na litosfera. De modo geral os xenólitos mantélicos da Província Ígnea do Alto Paranaíba foram afetados por intensa fusão parcial, indicada especialmente pelo alto valor de mg# em olivina e ortopiroxênio, e pelo empobrecimento de Al em clinopiroxênio e minerais do grupo do espinélio. Boa parte desses xenólitos foi modificada por metassomatismo modal e/ou críptico, que atingiu inclusive níveis rasos do manto litosférico (fácies espinélio). Os harzburgitos e dunitos com textura porfiroclástica com mosaico têm baixos valores de mg# em olivina e ortopiroxênio, e altas concentrações de de Ti, Ca e Na em relação aos demais grupos de amostras sugerindo um processo de metassomatismo críptico de alta temperatura (astenosfera, ou zonas permeáveis da litosfera). Estimativas com base em razões \'Y IND.N\' e \'Yb IND.N\' (normalizadas por Manto PrimitivoPM) em clinopiroxênio, indicam que os peridotitos com Cr-espinélio sofreram uma porcentagem maior de fusão parcial (15-20%), em relação aos peridotitos com espinélio (5-8%), e preferencialmente fracionada. Em algumas amostras nos peridotitos com Crespinélio ainda são encontrados padrões enriquecidos de \'ETRL IND.N\', indicando processos de metassomatismo críptico. Os cristais de clinopiroxênio das amostras de xenólitos metassomatisados têm padrão de distribuição de \'ETR.IND.N\' semelhantes entre si, com padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' (\'La IND.N\' e \'Lu IND.N\', respectivamente 5-14xPM e 0,3-1xPM). Uma amostra de harzburgito com bolsões de clinopiroxênio (In1-12), tem o clinopiroxênio com as maiores razões de \'ETR IND.N\' do conjunto, mantendo o padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' \'(La IND.N\' e \'Lu IND.N\', respectivamente 2543xPM e 1-2,5xPM) dos peridotitos metassomatisados. Razões \'ANTPOT.87 Sr\'/ANTPOT. 86 Sr\' de clinopiroxênio desta última amostra (0,706-0,714) e de flogopita ilmenita wehrlito (0,706-7,10) sugerem que o metassomatismo seria derivado de porção da astenosfera ou litosfera com material proveniente de subducção de crosta. Um valor mais baixo de razão \'ANTPOT. 87 Sr\'/\'ANTPOT.86 Sr\' para clinopiroxênio de peridotito com espinélio (0,703),indica, juntamente com aspectos texturais e de química de maiores e traços, pouca ou nenhuma, influencia do metassomatismo nesta amostra. / Partial melting processes and metasomatism were identified in mantle xenoliths of the Alto Paranaíba Igneous Province (APIP), next Coromandel (Minas Gerais), by combining textural analysis techniques (petrography, scanning electron microscopy) and specific mineral chemical analyzes of major elements (for Electronic Microprobe), trace elements (LA-Q-ICPMS) and isotopes (Sr in clinopyroxene by LA-MC-ICPMS). The xenoliths which are from Indaiá 1, Limeira 1 and Forca kimberlite were divided into three groups displaying different textures, mineralogy and chemistry: peridotites with spinel or Cr-spinel, were interpreted as fragments in the shallow lithospheric mantle; xenoliths with phlogopite and/or secondary clinopyroxene representing the lithospheric mantle portions that suffered metasomatism by interaction with fluids or magmas; and dunites and harzburgites with porphyroclastic texture with mosaic are compatible with portions that have suffered the effects of a higher heat flow, possibly associated with asthenosphere or zones of greater permeability in the lithosphere. Mantle xenoliths from APIP were affected by intense partial melting, suitable for high-value mg# of olivine and orthopyroxene, and the depletion of Al in clinopyroxene and spinel minerals from the group. Many of these xenoliths were modified by modal and/or cryptic metasomatism, which reached even shallow levels of the lithospheric mantle (facies spinel). Harzburgites and dunites with prophyroclastic texture with mosaic have low values of mg# and high concentrations of Ti, Ca and Na in olivine and orthopyroxene compared to other groups of samples suggesting a cryptic metasomatism process of high temperature. \'Y IND.N\' and \'Yb IND.N\' (normalized by Primitive Mantle-PM) ratios in clinopyroxene indicate that the peridotites with Cr-spinel suffered a higher percentage of partial melting (15-20%) compared to the peridotites with spinel (5-8%), and preferably fractional. Some samples in peridotites with Cr-spinel are still found \'LREE IND.N\' enriched ratios patterns, indicating cryptic metasomatism processes. The clinopyroxene crystals of metasomatized xenoliths samples have similar \'REE IND.N\' ratios pattern, with convex \'LREE IND.N\' pattern with higher ratios compared to \'HREE IND.N\' (LaN:5-14xPM and LuN 0,3-1xPM). A harzburgite with insterticial pockted clinopyroxene (In1-12), has the clinopyroxene with the highest \'REE IND.N\' of the set, the convex and enriched \'LREE IND.N\' pattern as well (\'La IND.N\': 25-43xPM and \'Lu IND.N\' 1-2.5XPM) of metasomatized peridotites. Clinopyroxene \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' ratios of the latter sample (0,706 to 0,714) and from phlogopite wehrlite ilmenite sample (0,706 to 0,710) suggest that metassomatism fluids from astenosphere or lithosphere were derivative from crust subduction processes. A lower value ratio \'ANTPOT.87 Sr\'/ANTPOT.86 Sr\' for clinopyroxene in peridotite with spinel (0,703), is in agreement with textural aspects, major and trace elements information, that this group of xenolith little, or no influence of metasomatism.

Processos de fusão parcial e metassomatismo foram identificados em xenólitos mantélicos da Província Ígnea Alto Paranaíba (PIAP), na região de Coromandel (Minas Gerais), pela combinação de técnicas de análise textural (petrografia, microscopia eletrônica de varredura), análises químicas minerais pontuais de elementos maiores (por Microssonda Eletrônica), traços (por LA-Q-ICPMS) e isótopos (Sr em clinopiroxênio por LA-MC-ICPMS). Os xenólitos, oriundos dos kimberlitos Indaiá 1, Limeira 1 e Forca, foram divididos em três grupos exibindo texturas, mineralogia e química distintas: peridotitos com espinélio ou Cr- espinélio, interpretados como fragmentos de fácies espinélio no manto litosférico raso; xenólitos com flogopita e/ou clinopiroxênio secundário representando porções do manto litosférico que sofreram metassomatismo por interação com fluidos ou magmas; e dunitos e harzburgitos com textura mosaico porfiroclástica a fluidal compatíveis com porções que sofreram efeitos de um fluxo térmico maior, possivelmente associadas a astenosfera ou a zonas de maior permeabilidade na litosfera. De modo geral os xenólitos mantélicos da Província Ígnea do Alto Paranaíba foram afetados por intensa fusão parcial, indicada especialmente pelo alto valor de mg# em olivina e ortopiroxênio, e pelo empobrecimento de Al em clinopiroxênio e minerais do grupo do espinélio. Boa parte desses xenólitos foi modificada por metassomatismo modal e/ou críptico, que atingiu inclusive níveis rasos do manto litosférico (fácies espinélio). Os harzburgitos e dunitos com textura porfiroclástica com mosaico têm baixos valores de mg# em olivina e ortopiroxênio, e altas concentrações de de Ti, Ca e Na em relação aos demais grupos de amostras sugerindo um processo de metassomatismo críptico de alta temperatura (astenosfera, ou zonas permeáveis da litosfera). Estimativas com base em razões \'Y IND.N\' e \'Yb IND.N\' (normalizadas por Manto PrimitivoPM) em clinopiroxênio, indicam que os peridotitos com Cr-espinélio sofreram uma porcentagem maior de fusão parcial (15-20%), em relação aos peridotitos com espinélio (5-8%), e preferencialmente fracionada. Em algumas amostras nos peridotitos com Crespinélio ainda são encontrados padrões enriquecidos de \'ETRL IND.N\', indicando processos de metassomatismo críptico. Os cristais de clinopiroxênio das amostras de xenólitos metassomatisados têm padrão de distribuição de \'ETR.IND.N\' semelhantes entre si, com padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' (\'La IND.N\' e \'Lu IND.N\', respectivamente 5-14xPM e 0,3-1xPM). Uma amostra de harzburgito com bolsões de clinopiroxênio (In1-12), tem o clinopiroxênio com as maiores razões de \'ETR IND.N\' do conjunto, mantendo o padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' \'(La IND.N\' e \'Lu IND.N\', respectivamente 2543xPM e 1-2,5xPM) dos peridotitos metassomatisados. Razões \'ANTPOT.87 Sr\'/ANTPOT. 86 Sr\' de clinopiroxênio desta última amostra (0,706-0,714) e de flogopita ilmenita wehrlito (0,706-7,10) sugerem que o metassomatismo seria derivado de porção da astenosfera ou litosfera com material proveniente de subducção de crosta. Um valor mais baixo de razão \'ANTPOT. 87 Sr\'/\'ANTPOT.86 Sr\' para clinopiroxênio de peridotito com espinélio (0,703),indica, juntamente com aspectos texturais e de química de maiores e traços, pouca ou nenhuma, influencia do metassomatismo nesta amostra. / Partial melting processes and metasomatism were identified in mantle xenoliths of the Alto Paranaíba Igneous Province (APIP), next Coromandel (Minas Gerais), by combining textural analysis techniques (petrography, scanning electron microscopy) and specific mineral chemical analyzes of major elements (for Electronic Microprobe), trace elements (LA-Q-ICPMS) and isotopes (Sr in clinopyroxene by LA-MC-ICPMS). The xenoliths which are from Indaiá 1, Limeira 1 and Forca kimberlite were divided into three groups displaying different textures, mineralogy and chemistry: peridotites with spinel or Cr-spinel, were interpreted as fragments in the shallow lithospheric mantle; xenoliths with phlogopite and/or secondary clinopyroxene representing the lithospheric mantle portions that suffered metasomatism by interaction with fluids or magmas; and dunites and harzburgites with porphyroclastic texture with mosaic are compatible with portions that have suffered the effects of a higher heat flow, possibly associated with asthenosphere or zones of greater permeability in the lithosphere. Mantle xenoliths from APIP were affected by intense partial melting, suitable for high-value mg# of olivine and orthopyroxene, and the depletion of Al in clinopyroxene and spinel minerals from the group. Many of these xenoliths were modified by modal and/or cryptic metasomatism, which reached even shallow levels of the lithospheric mantle (facies spinel). Harzburgites and dunites with prophyroclastic texture with mosaic have low values of mg# and high concentrations of Ti, Ca and Na in olivine and orthopyroxene compared to other groups of samples suggesting a cryptic metasomatism process of high temperature. \'Y IND.N\' and \'Yb IND.N\' (normalized by Primitive Mantle-PM) ratios in clinopyroxene indicate that the peridotites with Cr-spinel suffered a higher percentage of partial melting (15-20%) compared to the peridotites with spinel (5-8%), and preferably fractional. Some samples in peridotites with Cr-spinel are still found \'LREE IND.N\' enriched ratios patterns, indicating cryptic metasomatism processes. The clinopyroxene crystals of metasomatized xenoliths samples have similar \'REE IND.N\' ratios pattern, with convex \'LREE IND.N\' pattern with higher ratios compared to \'HREE IND.N\' (LaN:5-14xPM and LuN 0,3-1xPM). A harzburgite with insterticial pockted clinopyroxene (In1-12), has the clinopyroxene with the highest \'REE IND.N\' of the set, the convex and enriched \'LREE IND.N\' pattern as well (\'La IND.N\': 25-43xPM and \'Lu IND.N\' 1-2.5XPM) of metasomatized peridotites. Clinopyroxene \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' ratios of the latter sample (0,706 to 0,714) and from phlogopite wehrlite ilmenite sample (0,706 to 0,710) suggest that metassomatism fluids from astenosphere or lithosphere were derivative from crust subduction processes. A lower value ratio \'ANTPOT.87 Sr\'/ANTPOT.86 Sr\' for clinopyroxene in peridotite with spinel (0,703), is in agreement with textural aspects, major and trace elements information, that this group of xenolith little, or no influence of metasomatism.

The aim of this study is to define mineralogical and geochemical characteristics of phlogopite-pargasite enriched ultramafic-mafic cumulate rocks from Kuran&ccedil / ali (Kirsehir) and their implications for petrology and regional geological setting. The Kuran&ccedil / ali rocks, found within an allochthonous sliver, are representative for the isolated members of the Central Anatolian Ophiolites, derived from closure of Izmir-Ankara-Erzincan branch of Alpine Neotethys. The rocks overthrust the Metamorphic Ophiolitic M&eacute / lange (the uppermost part of the Central Anatolian Metamorphics) and cut by felsic dykes of the Late Cretaceous Central Anatolian Granitoids. The Kuran&ccedil / ali rocks are unusually enriched in phlogopite and pargasite with varying crystal sizes. They are also composed of diopsidic augite, plagioclase, rutile, ilmenite, sphene, apatite and pyrite. The rocks are divided into six types / clinopyroxenite, clinopyroxenite-with-hydrous minerals-plagioclase, phlogopitite, hornblendite, layered gabbro and diorite. Evaluation of detailed EMP data from constituent minerals of different rock types showed that phlogopite with high Fe2+-Fe3+-Al[6]-Ti, diopsidic-augite with high Ca-Al(t)-Ti, Si-undersaturated pargasite with high Al[4]-K-Na-Ti-contents and intercumulus plagioclase with a wide range of composition (an%=40.61-98.58) display unusual compositions. Substitution mechanisms and elemental variations of the minerals suggest crystallization from hydrous metasomatized mantle, high water pressure and oxygen fugacities during formation of the Kuran&ccedil / ali rocks. Major oxide, trace and rare earth element abundances of the rock units were used to evaluate petrological characteristics. Chemical and tectonic discrimination diagrams, and parallel multi-element and REE patterns with highly enriched in LILE and LREE relative to HREE and HFSE show strong calc-alkaline affinity with slight alkaline features. Troughs at Nb-Ta and Ti characterize the rocks but these elements are slightly enriched than N-MORB. The rocks show high LREE/HREE ratios. Both unusual mineralogical and geochemical features of the rocks show that the rocks were generated in an arc environment. Moreover, they require a mantle wedge source strongly influenced by metasomatic components (fluid/melt) derived from subducting slab and/or OIB-like alkaline melt. Comparison of the rocks with tectonically well-defined rocks displays that they are generated in an intra-oceanic arc environment, but owe a comparison with fore-arc back-arc Central Anatolian Ophiolites within supra-subduction zone environment revealed that Kuran&ccedil / ali rocks are different and generated in an arc basement.

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.

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.

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.