xviii, 177 p. : ill., maps. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation explores several important consequences of H 2 O-rich fluids in magmatic and ore-forming systems within continental arc crust. North Sister, a stratovolcano in the Oregon High Cascades, provides a window into magma generation processes in the deep crust. Eruption of a remarkably limited basaltic andesite composition over the lifespan of this volcano may reflect last equilibration of mantle derived magma within a deep crustal hot zone. High pressure, water-undersaturated phase equilibrium experiments show that an anhydrous, augite-rich gabbro at ∼12 kbar (40 km depth) and ∼ 1175°C is the most probable lithology with which North Sister basaltic andesite with ∼3.5 wt% H 2 O last equilibrated within the deep crust before erupting. While magma often erupts at the planets surface as at North Sister, a greater volume never reaches the surface and solidifies within the upper crust. Exsolution of magmatic fluids is an inevitable consequence of crystallization of hydrous crustal magmas. The fate of these fluids is the focus of the remainder of this dissertation. Modeling of CO 2 and H 2 O variations during crystallization of granitic magma reveals that exsolution of a large mass of fluid occurs only after CO 2 is largely degassed, creating ideal conditions for hydrofracturing and formation of porphyry copper deposits. CO 2 and H 2 O solubility relations suggest that H 2 O-rich magma was required to produce the porphyry-Cu-Mo deposit at Butte, Montana, which may explain its distinctively deep generation. Electron microprobe analyses of Ti in quartz and Zr in rutile in samples from Butte yield porphyry magma temperatures (630-770°C) that overlap substantially with hydrothermal vein temperatures (<430-750°C). Veins display large temperature ranges (50-250°C) that signify variable degrees of cooling of hot magmatic fluids upon contact with cooler wall rock during vein growth. Modeling of Ti diffusion in quartz suggests that individual dikes and veins likely cooled over short timescales (10s-1000s years), indicating that porphyry systems may evolve by episodic magmatic fluid injections with discrete thermal spikes. Modeling of Ti diffusion in quartz combined with electron backscatter diffraction maps show that small hydrothermal quartz veins likely formed by epitaxial growth. This dissertation includes co-authored material both previously published and in preparation for submission. / Committee in charge: A. Dana Johnston, Chairperson, Geological Sciences; Mark Reed, Member, Geological Sciences; Paul Wallace, Member, Geological Sciences; Richard P. Taylor, Outside Member, Physics

Este estudo foca no Complexo Várzea do Capivarita (CVC), localizado no sul da Província da Mantiqueira (PM), Brasil. A fim de investigar a evolução geológica do CVC, uma abordagem multi-disciplinar foi utilizada, incluindo geologia de campo, geologia estrutural, petrografia, geoquímica de elementos maiores e traços, isótopos de Sr-Nd e geocronologia U-Pb em zircão (LA-MC-ICP-MS e SHRIMP). O complexo compreende uma variedade de orto- e paragnaisses de composição e idade diversa. Volumes subordinados de sienitos sintectônicos também perfazem o CVC. A deformação é particionada em zonas de cisalhamento do tipo thrust (D1) e transcorrentes (D2), o que sugere tectônica transpressiva. O arcabouço estrutural descrito é possivelmente relacionado a um evento colisional oblíquo. Os estudos petrológicos e geocronológicos enfatizaram os ortognaisses do CVC a fim de avaliar as fontes magmáticas e paleo-ambientes envolvidos. Idades de cristalização obtidas nos domínios de zircão com zonação tipicamente ígnea variaram entre 780 e 790 Ma. Por sua vez, idades entre 640 - 650 Ma foram obtidas em sobrecrescimentos de zircão, sendo interpretadas como o registro da idade do metamorfismo de alto grau e fusão parcial associada. Os dados geocronológicos apresentados também indicaram que ambos os regimes cinemáticos foram contemporâneos, oferecendo, dessa forma, evidencia adicional para a hipótese de colisão oblíqua. Os ortognaisses do Complexo têm composição tonalítica a granítica e são rochas calcioalcalinas meta- a peraluminosas, com razões elevadas de 87Sr/86Sr (i) variando de 0.71628 a 0.72509 e valores εNd (790) entre -7.19 a -10.06. Sua composição e padrões de elementos traços sugerem que representem um magmatismo de arco maduro continental. O magmatismo registrado no CVC é compatível com outras sequências de arco de ca. 800 Ma, incluindo parte das metavulcânicas ácidas do Complexo Metamórfico Porongos (CMP) e os ortognaisses do Cerro Bori, Uruguai. Todas essas associações têm assinatura típica de orógenos acrescionários, contendo idade TDM Meso a Paleoproterozóica, além de forte evidência da participação de proscessos de assimilação crustal/ contaminação. Desta forma, o conjunto de dados apresentados permite interpretar essas associações como parte do mesmo magmatismo, ou pelo menos como fragmentos de arcos magmáticos similares. As assinaturas Sr-Nd e geoquímica sugere que ao menos parte das metavulcânicas do CMP represente os protólitos dos ortognaisses de alto grau inclusos no CVC. Adicionalmente, as evidencias isotópicas também apontam similaridade entre as rochas sedimentares de ambas as unidades, sugerindo que o CVC e o PMC são, ao menos em parte, expressões do mesmo contexto, onde a atividade magmática e sedimentar ocorreu em um mesmo ambiente de arco continental. A corroboração desta premissa é o objetivo principal de estudos de proveniência em andamento, cujos resultados prévios apontam para o caráter vulcano-sedimentar dos metapelitos do CVC e sua relação co-genética com os ortognaisses do CVC. Os dados isotópico Sr-Nd sugerem que os protólitos dos ortognaisses foram gerados por processos de assimilação crustal associados à cristalização fracionada. O modelamento binário (binary mixing model) realizado indica que o magmatismo estudado teria se originado de fontes mantélicas do tipo EM II. Uma seqüência paleoproterozóica de rochas TTG pertencente ao Complexo Arroio dos Ratos (CAR) é possivelmente o principal contaminante crustal assimilado. Em conjunto com as idades de herança descritas no CVC em ca. 2.0 Ga é sugerido que a fusão crustal que gerou o magmatismo do CVC em ca. 790-780 Ma foi predominantemente similar ao CAR. / This study focuses in the Várzea do Capivarita Complex (VCC), exposed in the southern part of the Neoproterozoic Mantiqueira Province (PM), Brazil. To investigate the evolutionary processes that lead the VCC construction, a multidisciplinary approach is taken, which includes field and structural geology, petrography, major and trace-element geochemistry, Sr-Nd isotope and U-Pb zircon geochronology by LA-MC-ICP-MS and SHRIMP. The complex comprises a compositional and age variety of ortho- and paragneisses tectonically interleaved during a high grade event. Subordinate volumes of syntectonic syenites are also part of CVC. The VCC deformation is partitioned into thrusting (D1) and transcurrent (D2) shear zones, suggestive of transpressive tectonics. This structural framework is possibly related to an oblique collision event. Petrological and goechronological studies emphasize the VCC orthogneisses in order to evaluate magmatic sources and related paleo-environments. Igneous crystallization ages obtained in the typical magmatic domains presenting oscillatory zoning in zircons vary between 780 and 790 Ma. Zircon overgrowths have ages mostly in the 650 – 640 Ma range and are interpreted to record the timing of high-grade metamorphism and associated partial melting. Geochronological data presented also indicates that boths kinematic regimes are contemporaneous, offering, therefore, further evidence for the oblique collisional event hypothesis. The VCC ortogneisses comprise tonalitic to granitic compositions and are metaluminous to peraluminous, calc-alkaline rocks, with high 87Sr/86Sr (i) ratios from 0.71628 to 0.72509 and εNd (790) values from -7.19 to -10.06. Their geochemical composition and trace-element patterns are compatible with a continental mature arc. VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, including part of the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori, Uruguay. All these associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and present strong evidences of crustal assimilation/contamination. Thus, these sequences may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. Geochemical and Sr-Nd signatures suggest that at least part of the PMC metavolcanic rocks may represent the protoliths of the VCC high grade orthogneisses. This, together with the isotope evidence of similarity between the sedimentary fractions of both unities, suggest that the VCC and PMC are, at least in part, expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. The corroboration of this premise is the main goal of provenience studies in prep, which previous results points to the volcano-sedimentary character of part of the VCC metapelites and its co-genetic relation with the VCC orthogneisses. Sr-Nd isotope data suggest that the orthogneiss protoliths were generated by crustal assimilation processes associated with fractional crystallization. Binary mixing models indicate that the VCC magmatism originates from evolved EM II mantle sources. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant assimilated. Together with the small inheritance contribution at ca. 2.0, this suggests that the melted crust at ca. 790-800 Ma was predominantly like ARC.

Este estudo foca no Complexo Várzea do Capivarita (CVC), localizado no sul da Província da Mantiqueira (PM), Brasil. A fim de investigar a evolução geológica do CVC, uma abordagem multi-disciplinar foi utilizada, incluindo geologia de campo, geologia estrutural, petrografia, geoquímica de elementos maiores e traços, isótopos de Sr-Nd e geocronologia U-Pb em zircão (LA-MC-ICP-MS e SHRIMP). O complexo compreende uma variedade de orto- e paragnaisses de composição e idade diversa. Volumes subordinados de sienitos sintectônicos também perfazem o CVC. A deformação é particionada em zonas de cisalhamento do tipo thrust (D1) e transcorrentes (D2), o que sugere tectônica transpressiva. O arcabouço estrutural descrito é possivelmente relacionado a um evento colisional oblíquo. Os estudos petrológicos e geocronológicos enfatizaram os ortognaisses do CVC a fim de avaliar as fontes magmáticas e paleo-ambientes envolvidos. Idades de cristalização obtidas nos domínios de zircão com zonação tipicamente ígnea variaram entre 780 e 790 Ma. Por sua vez, idades entre 640 - 650 Ma foram obtidas em sobrecrescimentos de zircão, sendo interpretadas como o registro da idade do metamorfismo de alto grau e fusão parcial associada. Os dados geocronológicos apresentados também indicaram que ambos os regimes cinemáticos foram contemporâneos, oferecendo, dessa forma, evidencia adicional para a hipótese de colisão oblíqua. Os ortognaisses do Complexo têm composição tonalítica a granítica e são rochas calcioalcalinas meta- a peraluminosas, com razões elevadas de 87Sr/86Sr (i) variando de 0.71628 a 0.72509 e valores εNd (790) entre -7.19 a -10.06. Sua composição e padrões de elementos traços sugerem que representem um magmatismo de arco maduro continental. O magmatismo registrado no CVC é compatível com outras sequências de arco de ca. 800 Ma, incluindo parte das metavulcânicas ácidas do Complexo Metamórfico Porongos (CMP) e os ortognaisses do Cerro Bori, Uruguai. Todas essas associações têm assinatura típica de orógenos acrescionários, contendo idade TDM Meso a Paleoproterozóica, além de forte evidência da participação de proscessos de assimilação crustal/ contaminação. Desta forma, o conjunto de dados apresentados permite interpretar essas associações como parte do mesmo magmatismo, ou pelo menos como fragmentos de arcos magmáticos similares. As assinaturas Sr-Nd e geoquímica sugere que ao menos parte das metavulcânicas do CMP represente os protólitos dos ortognaisses de alto grau inclusos no CVC. Adicionalmente, as evidencias isotópicas também apontam similaridade entre as rochas sedimentares de ambas as unidades, sugerindo que o CVC e o PMC são, ao menos em parte, expressões do mesmo contexto, onde a atividade magmática e sedimentar ocorreu em um mesmo ambiente de arco continental. A corroboração desta premissa é o objetivo principal de estudos de proveniência em andamento, cujos resultados prévios apontam para o caráter vulcano-sedimentar dos metapelitos do CVC e sua relação co-genética com os ortognaisses do CVC. Os dados isotópico Sr-Nd sugerem que os protólitos dos ortognaisses foram gerados por processos de assimilação crustal associados à cristalização fracionada. O modelamento binário (binary mixing model) realizado indica que o magmatismo estudado teria se originado de fontes mantélicas do tipo EM II. Uma seqüência paleoproterozóica de rochas TTG pertencente ao Complexo Arroio dos Ratos (CAR) é possivelmente o principal contaminante crustal assimilado. Em conjunto com as idades de herança descritas no CVC em ca. 2.0 Ga é sugerido que a fusão crustal que gerou o magmatismo do CVC em ca. 790-780 Ma foi predominantemente similar ao CAR. / This study focuses in the Várzea do Capivarita Complex (VCC), exposed in the southern part of the Neoproterozoic Mantiqueira Province (PM), Brazil. To investigate the evolutionary processes that lead the VCC construction, a multidisciplinary approach is taken, which includes field and structural geology, petrography, major and trace-element geochemistry, Sr-Nd isotope and U-Pb zircon geochronology by LA-MC-ICP-MS and SHRIMP. The complex comprises a compositional and age variety of ortho- and paragneisses tectonically interleaved during a high grade event. Subordinate volumes of syntectonic syenites are also part of CVC. The VCC deformation is partitioned into thrusting (D1) and transcurrent (D2) shear zones, suggestive of transpressive tectonics. This structural framework is possibly related to an oblique collision event. Petrological and goechronological studies emphasize the VCC orthogneisses in order to evaluate magmatic sources and related paleo-environments. Igneous crystallization ages obtained in the typical magmatic domains presenting oscillatory zoning in zircons vary between 780 and 790 Ma. Zircon overgrowths have ages mostly in the 650 – 640 Ma range and are interpreted to record the timing of high-grade metamorphism and associated partial melting. Geochronological data presented also indicates that boths kinematic regimes are contemporaneous, offering, therefore, further evidence for the oblique collisional event hypothesis. The VCC ortogneisses comprise tonalitic to granitic compositions and are metaluminous to peraluminous, calc-alkaline rocks, with high 87Sr/86Sr (i) ratios from 0.71628 to 0.72509 and εNd (790) values from -7.19 to -10.06. Their geochemical composition and trace-element patterns are compatible with a continental mature arc. VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, including part of the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori, Uruguay. All these associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and present strong evidences of crustal assimilation/contamination. Thus, these sequences may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. Geochemical and Sr-Nd signatures suggest that at least part of the PMC metavolcanic rocks may represent the protoliths of the VCC high grade orthogneisses. This, together with the isotope evidence of similarity between the sedimentary fractions of both unities, suggest that the VCC and PMC are, at least in part, expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. The corroboration of this premise is the main goal of provenience studies in prep, which previous results points to the volcano-sedimentary character of part of the VCC metapelites and its co-genetic relation with the VCC orthogneisses. Sr-Nd isotope data suggest that the orthogneiss protoliths were generated by crustal assimilation processes associated with fractional crystallization. Binary mixing models indicate that the VCC magmatism originates from evolved EM II mantle sources. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant assimilated. Together with the small inheritance contribution at ca. 2.0, this suggests that the melted crust at ca. 790-800 Ma was predominantly like ARC.

Este estudo foca no Complexo Várzea do Capivarita (CVC), localizado no sul da Província da Mantiqueira (PM), Brasil. A fim de investigar a evolução geológica do CVC, uma abordagem multi-disciplinar foi utilizada, incluindo geologia de campo, geologia estrutural, petrografia, geoquímica de elementos maiores e traços, isótopos de Sr-Nd e geocronologia U-Pb em zircão (LA-MC-ICP-MS e SHRIMP). O complexo compreende uma variedade de orto- e paragnaisses de composição e idade diversa. Volumes subordinados de sienitos sintectônicos também perfazem o CVC. A deformação é particionada em zonas de cisalhamento do tipo thrust (D1) e transcorrentes (D2), o que sugere tectônica transpressiva. O arcabouço estrutural descrito é possivelmente relacionado a um evento colisional oblíquo. Os estudos petrológicos e geocronológicos enfatizaram os ortognaisses do CVC a fim de avaliar as fontes magmáticas e paleo-ambientes envolvidos. Idades de cristalização obtidas nos domínios de zircão com zonação tipicamente ígnea variaram entre 780 e 790 Ma. Por sua vez, idades entre 640 - 650 Ma foram obtidas em sobrecrescimentos de zircão, sendo interpretadas como o registro da idade do metamorfismo de alto grau e fusão parcial associada. Os dados geocronológicos apresentados também indicaram que ambos os regimes cinemáticos foram contemporâneos, oferecendo, dessa forma, evidencia adicional para a hipótese de colisão oblíqua. Os ortognaisses do Complexo têm composição tonalítica a granítica e são rochas calcioalcalinas meta- a peraluminosas, com razões elevadas de 87Sr/86Sr (i) variando de 0.71628 a 0.72509 e valores εNd (790) entre -7.19 a -10.06. Sua composição e padrões de elementos traços sugerem que representem um magmatismo de arco maduro continental. O magmatismo registrado no CVC é compatível com outras sequências de arco de ca. 800 Ma, incluindo parte das metavulcânicas ácidas do Complexo Metamórfico Porongos (CMP) e os ortognaisses do Cerro Bori, Uruguai. Todas essas associações têm assinatura típica de orógenos acrescionários, contendo idade TDM Meso a Paleoproterozóica, além de forte evidência da participação de proscessos de assimilação crustal/ contaminação. Desta forma, o conjunto de dados apresentados permite interpretar essas associações como parte do mesmo magmatismo, ou pelo menos como fragmentos de arcos magmáticos similares. As assinaturas Sr-Nd e geoquímica sugere que ao menos parte das metavulcânicas do CMP represente os protólitos dos ortognaisses de alto grau inclusos no CVC. Adicionalmente, as evidencias isotópicas também apontam similaridade entre as rochas sedimentares de ambas as unidades, sugerindo que o CVC e o PMC são, ao menos em parte, expressões do mesmo contexto, onde a atividade magmática e sedimentar ocorreu em um mesmo ambiente de arco continental. A corroboração desta premissa é o objetivo principal de estudos de proveniência em andamento, cujos resultados prévios apontam para o caráter vulcano-sedimentar dos metapelitos do CVC e sua relação co-genética com os ortognaisses do CVC. Os dados isotópico Sr-Nd sugerem que os protólitos dos ortognaisses foram gerados por processos de assimilação crustal associados à cristalização fracionada. O modelamento binário (binary mixing model) realizado indica que o magmatismo estudado teria se originado de fontes mantélicas do tipo EM II. Uma seqüência paleoproterozóica de rochas TTG pertencente ao Complexo Arroio dos Ratos (CAR) é possivelmente o principal contaminante crustal assimilado. Em conjunto com as idades de herança descritas no CVC em ca. 2.0 Ga é sugerido que a fusão crustal que gerou o magmatismo do CVC em ca. 790-780 Ma foi predominantemente similar ao CAR. / This study focuses in the Várzea do Capivarita Complex (VCC), exposed in the southern part of the Neoproterozoic Mantiqueira Province (PM), Brazil. To investigate the evolutionary processes that lead the VCC construction, a multidisciplinary approach is taken, which includes field and structural geology, petrography, major and trace-element geochemistry, Sr-Nd isotope and U-Pb zircon geochronology by LA-MC-ICP-MS and SHRIMP. The complex comprises a compositional and age variety of ortho- and paragneisses tectonically interleaved during a high grade event. Subordinate volumes of syntectonic syenites are also part of CVC. The VCC deformation is partitioned into thrusting (D1) and transcurrent (D2) shear zones, suggestive of transpressive tectonics. This structural framework is possibly related to an oblique collision event. Petrological and goechronological studies emphasize the VCC orthogneisses in order to evaluate magmatic sources and related paleo-environments. Igneous crystallization ages obtained in the typical magmatic domains presenting oscillatory zoning in zircons vary between 780 and 790 Ma. Zircon overgrowths have ages mostly in the 650 – 640 Ma range and are interpreted to record the timing of high-grade metamorphism and associated partial melting. Geochronological data presented also indicates that boths kinematic regimes are contemporaneous, offering, therefore, further evidence for the oblique collisional event hypothesis. The VCC ortogneisses comprise tonalitic to granitic compositions and are metaluminous to peraluminous, calc-alkaline rocks, with high 87Sr/86Sr (i) ratios from 0.71628 to 0.72509 and εNd (790) values from -7.19 to -10.06. Their geochemical composition and trace-element patterns are compatible with a continental mature arc. VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, including part of the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori, Uruguay. All these associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and present strong evidences of crustal assimilation/contamination. Thus, these sequences may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. Geochemical and Sr-Nd signatures suggest that at least part of the PMC metavolcanic rocks may represent the protoliths of the VCC high grade orthogneisses. This, together with the isotope evidence of similarity between the sedimentary fractions of both unities, suggest that the VCC and PMC are, at least in part, expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. The corroboration of this premise is the main goal of provenience studies in prep, which previous results points to the volcano-sedimentary character of part of the VCC metapelites and its co-genetic relation with the VCC orthogneisses. Sr-Nd isotope data suggest that the orthogneiss protoliths were generated by crustal assimilation processes associated with fractional crystallization. Binary mixing models indicate that the VCC magmatism originates from evolved EM II mantle sources. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant assimilated. Together with the small inheritance contribution at ca. 2.0, this suggests that the melted crust at ca. 790-800 Ma was predominantly like ARC.

The Andean Southern Volcanic Zone (SVZ) is a vast and complex continental arc that has been studied extensively to provide an understanding of arc-magma genesis, the origin and chemical evolution of the continental crust, and geochemical compositions of volcanic products. The present study focuses on distinguishing the magma/sub-arc crustal interaction of eruptive products from the Azufre-Planchon-Peteroa (APP 35°15’S) volcanic center and other major centers in the Central SVZ (CSVZ 37°S - 42°S), Transitional SVZ (TSVZ 34.3-37.0°S), and Northern SVZ (NSVZ 33°S - 34°30’S). New Hf and Nd isotopic and trace element data for SVZ centers are consistent with former studies that these magmas experienced variable depths of crystal fractionation, and that crustal assimilation is restricted to the lower crustal depths with an apparent role of garnet. Thermobarometric calculations applied to magma compositions constrain the depth of magma separation from mantle sources in all segments of the SVZ to(70-90 km). Magmatic separation at the APP complex occurs at an average depth of ~50 km which is confined to the mantle lithosphere and the base of the crust suggesting localized thermal abrasion both reservoirs. Thermobarometric calculations indicate that CSVZ primary magmas arise from a similar average depth of (~54 km) which confines magma separation to the asthenospheric mantle. The northwards along-arc Sr-Nd-Hf isotopic data and LREE enrichment accompanied with HREE depletion of SVZ mafic magmas correlates well with northward increasing crustal thickness and decreasing primary melt separation from mantle source regions indicating an increased involvement of lower crustal components in SVZ magma petrogenesis. The study concludes that the development of mature subduction zones over millions of years of continuous magmatism requires that mafic arc derived melts stagnate at lower crustal levels due to density similarities and emplace at lower crustal depths. Basaltic underplating creates localized hot zone environments below major magmatic centers. These regions of high temperature/partial melting, and equilibration with underplated mafic rocks provides the mechanism that controls trace element and isotopic variability of primary magmas of the TSVZ and NSVZ from their baseline CSVZ-like precursors.

Spät-miozäne bis quartäre Vulkanite der vulkanischen Front und der Back-arc Region der Zentralen Vulkanischen Zone in den Anden weisen eine weite Spannbreite von delta 11B Werten (+4 bis &ndash;7 &permil;) and Borkonzentrationen (6 bis 60 ppm) auf. Die positiven delta 11B Werte der Vulkanite der vulkanischen Front zeigen eine Beteiligung einer 11B-reichen Komponente am Aufbau der andinen Vulkanite, die am wahrscheinlichsten aus Fluiden der alterierten ozeanischen Kruste der abtauchenden Nazca-Platte stammt. Diese Beobachtung macht einen alleinigen Ursprung der untersuchten Laven aus der kontinentalen Kruste und/oder dem Mantelkeil unwahrscheinlich. Der Trend zu systematisch negativeren delta 11B Werten und kleineren B/Nb Verhältnissen von der vulkanischen Front zum Back-arc wird als Resultat einer Borisotopenfraktionierung einhergehend mit einer stetigen Abnahme der Fluidkomponente und einer relativ konstanten krustalen Kontamination, die sich durch relativ gleichbleibende Sr, Nd und Pb Isotopenverhältnisse ausdrückt, interpretiert. Weil die delta 11B Variation über den andinen vulkanischen Bogen sehr gut mit einer modellierten, sich als Funktion der Temperatur dynamisch verändernden, Zusammensetzung des Subduktionszonenfluides übereinstimmt, folgern wir, dass die Borisotopenzusammensetzung von Arc-Vulkaniten durch die sich dynamisch ändernde delta 11B Signatur eines Bor-reichen Subduktionsfluides bestimmt ird. Durch die Abnahme dieses Subduktionsfluides während der Subduktion nimmt der Einfluss der krustalen Kontamination auf die Borisotopie der Arc-Vulkanite im Back-arc zu. In Anbetracht der Borisotopenfraktionierung müssen hohe delta 11B Werte von Arc-Vulkaniten nicht notwendigerweise Unterschiede in der initialen Zusammensetzung der subduzierten Platte reflektieren.<br /> Eine Dreikomponenten Mischungskalkulation zwischen Subduktionsfluid, dem Mantelkeil und der kontinentalen Kruste, die auf Bor-, Strontium- und Neodymiumisotopendaten beruht, zeigt, dass das Subduktionsfluid die Borisotopie des fertilen Mantels dominiert und, dass die primären Arc-Magmen durchschnittlich einen Anteil von 15 bis 30 % krustalem Materiales aufweisen. / Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of delta 11B values (+4 to -7 &permil;) and boron concentrations (6 to 60 ppm). Positive delta 11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The delta 11B values and B concentrations in the lavas decrease systematically with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta 11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of B-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux towards the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Modelling of fluid-mineral B-isotope fractionation as a function of temperature fits the across-arc variation in delta 11B and we conclude that the B-isotope composition of arc volcanics is dominated by changing delta 11B composition of B-rich slab-fluids during progressive dehydration. Crustal contamination becomes more important towards the back-arc due to the decrease in slab-derived fluid flux. Because of this isotope fractionation effect, high delta 11B signatures in volcanic arcs need not necessarily reflect differences in the initial composition of the subducting slab. <br /> Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the B composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30 % crustal material.