Magma evolution of Quaternary minor volcanic centres in southern Peru, Central Andes

Minor centres in the Central Volcanic Zone (CVZ) of the Andes occur in different places and are essential indicators of magmatic processes leading to formation of composite volcano. The Andahua-Orcopampa and Huambo monogenetic fields are located in a unique tectonic setting, in and along the margins of a deep valley. This valley, oblique to the NW-SE-trend of the CVZ, is located between two composite volcanoes (Nevado Coropuna to the east and Nevado Sabancaya to the west). Structural analysis of these volcanic fields, based on SPOT satellite images, indicates four main groups of faults. These faults may have controlled magma ascent and the distribution of most centres in this deep valley shaped by en-echelon faulting. Morphometric criteria and 14C age dating attest to four main periods of activity: Late Pleistocene, Early to Middle Holocene, Late Holocene and Historic. The two most interesting features of the cones are the wide compositional range of their lavas (52.1 to 68.1wt.% SiO2) and the unusual occurrence of mafic lavas (olivine-rich basaltic andesites and basaltic andesites). Occurrence of such minor volcanic centres and mafic magmas in the CVZ may provide clues about the magma source in southern Peru. Such information is otherwise difficult to obtain because lavas produced by composite volcanoes are affected by shallow processes that strongly mask source signatures. Major, trace, and rare earth elements, as well as Sr-, Nd-, Pb- and O-isotope data obtained on high-K calc-alkaline lavas of the Andahua-Orcopampa and Huambo volcanic province characterise their source and their evolution. These lavas display a range comparable to those of the CVZ composite volcanoes for radiogenic and stable isotopes (87Sr/86Sr: 0.70591-0.70694, 143Nd/144Nd: 0.512317-0.512509, 206Pb/204Pb: 18.30-18.63, 207Pb/204Pb: 15.57-15.60, 208Pb/204Pb: 38.49-38.64, and δ 18O: 7.1-10.0‰ SMOW), attesting to involvement of a crustal component. Sediment is absent from the Peru-Chile trench, and hence cannot be the source of such enrichment. Partial melts of the lowermost part of the thick Andean continental crust with a granulitic garnet-bearing residue added to mantle-derived arc magmas in a high-pressure MASH [melting, assimilation, storage and homogenisation] zone may play a major role in magma genesis. This may also explain the chemical characteristics of the Andahua-Orcopampa and Huambo magmas. Fractional crystallisation processes are the main governors of magma evolution for the Andahua-Orcopampa and Huambo volcanic province. An open-system evolution is, however, required to explain some O-isotopes and some major and trace elements values. Modelling of AFC processes suggests the Charcani gneisses and the local Andahua-Orcopampa and Huambo basement may be plausible contaminant

Petrology of the 2006-2007 tephras from Ubinas volcano, southern Peru

[ESP] El volcán Ubinas (16º 22 'S, 70º 54' O) se encuentra en el rango volcánico cuaternario en el sur de Perú, ~ 60 km al este de la ciudad de Arequipa (Fig. 1). Ubinas es históricamente el volcán más activo en el sur del Perú con 24 eventos volcánicos (VEI 1-3) registrados desde 1550 AD (Hantke y Parodi, 1966; Simkin y Siebert 1994; Rivera et al. 1998). Estos eventos son episodios de desgasificación en gran parte intensos, con algunas caídas de cenizas y bloqueos balísticos (<10.106m3) producidos por actividad explosiva vulcaniana y freatomagmática (Thouret et al. 2005; Rivera et al. 1998). Los eventos causaron daños a los cultivos y al ganado y afectaron a aproximadamente 3,500 personas que viven en seis aldeas a 12 km del volcán (Fig. 1). La actividad explosiva más reciente comenzó el 27 de marzo de 2006 y duró dos años con eventos eruptivos intermitentes, mientras que la desgasificación aún continúa. Según las características de la actividad y los productos en erupción, el episodio eruptivo ha progresado en cuatro etapas: 1) actividad freática y freatomagmática inicial (del 27 de marzo al 19 de abril de 2006), incluidas las columnas de alta erupción que dispersaron la caída de cenizas a una distancia de hasta 7 km del cumbre; 2) las explosiones vulcanianas (~ 20 de abril al 11 de junio de 2006) formaron columnas de 3 a 4 km de altura que expulsaron bloques de hasta 40 cm de diámetro a distancias de 2 km del respiradero (Fig. 2). La lava fresca llegó al fondo del respiradero el 20 de abril; 3) fuerte desgasificación intercalada con al menos 12 eventos que produjeron columnas de 2 a 3 km de altura entre mediados de junio de 2006 y abril de 2007, dispersando cenizas hasta 40 km del respiradero; 4) la desgasificación suave produce un penacho permanente de 200 a 800 m de altura y ocasionalmente cenizas ligeras alrededor de la cumbre (mayo de 2007 hasta el presente). Las columnas de babosas duraderas y de corta duración, explosiones tipo cañón, pequeñas cantidades de material juvenil y la composición andesítica de las bombas de corteza de pan indican un estilo de comportamiento vulcaniano en Ubinas. El comportamiento es similar a la primera fase de la erupción del Nevado Sabancaya en 1990-1998 (Gerbe y Thouret, 2004) o al comportamiento de Sakurajima, Japón desde 1955 (Morrisey y Mastin, 2000), y a Ngauruhoe, Nueva Zelanda en 1974 -1975 (Hobden et al. 2002). Las características petrográficas y geoquímicas de los bloques juveniles y las escoria erupcionadas durante la actividad explosiva 2006-2007 permiten la descripción del magma recién erupcionado y, por lo tanto, conducen a una mejor comprensión del origen de la erupción

Rol de la contaminación crustal en el magmatismo de los Andes del sur peruano: ejemplo del volcán Misti

El volcán Misti (16º17’ S; 71º24’ O) es uno de los siete volcanes activos situados en la cadena volcánica Plio-Cuaternaria del sur peruano, perteneciente a la ZVC (Zona Volcánica Central) de los Andes. Este volcán se encuentra localizado a 17 km del centro de la ciudad de Arequipa (Fig. 1), la segunda ciudad en términos de población del Perú, con aproximadamente 1 millón de habitantes. Numerosos autores han estudiado la estratigrafía del volcán Misti, cuya actividad se inició hace ~833 ka (e.g. Thouret et al., 2001). Thouret et al. (2001) han dividido la evolución de este volcán en cuatro etapas: "Misti 1" (833 - 112 ka), "Misti 2" (112 - 40 ka), "Misti 3" (38 - 14 ka) y "Misti 4" (<11 ka). En este trabajo, nos hemos focalizado en los mecanismos de génesis y evolución de magmas ocurridos durante los últimos 112 ka, ya que durante este tiempo, el Misti ha presentado variados tipos de dinamismos eruptivos: erupciones explosivas (plinianas, freatomagmáticas, vulcanianas), erupciones efusivas y episodios de construcción y destrucción de domos, así como fenómenos de inestabilidad de flanco que han generado al menos dos depósitos de avalancha de escombros

Neogene ignimbrites and volcanic edifices in southern Peru: Stratigraphy and time-volume-composition relationships

In the Central Andes of Peru, four volcanic arcs, termed Tacaza, Lower and Upper Barroso, and Frontal arc, have been active over the past 30 Ma (Fig. 1). They form five units between Moquegua and Nazca (14°30– 17°15’°S and 70–74°W). The ‘Neogene ignimbrites’ (<25 Ma) comprise six generations of widespread sheets (>500 km2 and >20 km3 each), representing a major crustal melting event, triggered by thickening and advective heat input from the mantle wedge. Also, four generations of edifices (i.e shields, composite cones, and dome clusters) and monogenetic fields mostly overly the ignimbrites based on ages, stratigraphy and mapping

Intracontinental Miocene: Reconstruction of hydrology and paleoclimate change in the Forez Basin, France (Part II)

International audienceSedimentary successions of palustrine deposits from the Forez Graben recorded changes in depositional and hydrological conditions throughout the Middle to Late Miocene of France. Paleontological, mineralogical and geochemical data combined with mass-balance calculation allowed the identification of modern, diagenetic and eogenetic stages of authigenic precipitation within the sedimentary pile. Stratigraphic log contains six clay- and sand-rich sequences, some associated with freshwater ostracods. delta C-13, delta O-18 values and Sr isotopic ratios of whole-rock samples and ostracods indicate that the Montrond-les-Bains upper sequence (8 to 175 mbs) is a continental lake system that recorded continental weather condition (15 to 25 degrees C) during the upper Miocene time. Geochemical changes permit the estimation of variations of evaporation-inflow changes (E/I: 8 to 75% and delta O-18(rainfall) -6 to -8 parts per thousand) associated with two sequences. delta O-18 and E/I progressive changes were related to runoff to colder climate conditions. large E/I values in below 100 mbs are associated with the appearance of authigenic calcite and dolomite precipitation. Euhedral dolomite seems to be related to desiccation period that induced capillary pumping of water undersaturated with respect to calcite but saturated for dolomite. The lower part of the Middle Miocene sedimentary series (175 to 300 mbs) hosts continental deposits with scarce fossil, a larger proportion of dolomite and massive pyrite levels related to mixed eogenetic and diagenetic precipitation. Scarce preserved "eogenetic levels" with unrealistic E/I ratios suggest either delta O-18 values similar to the upper section but with temperature lower than 15 C, or a similar temperature but very low delta O-18(rainfall). Calculated delta O-18(H2O) values have to be as low as -15 parts per thousand. delta O-18(H2O) values are similar to present-day melted snow. This stable isotope suggests episodes of regional warming melting surrounding snow, and able to mobilize as well as deposit thick sand levels. Eogenetic to diagenetic calcite, dolomite and pyrite precipitation involves Sand Mg-fluid influxes associated with methanogenic process (eogenesis) mixed with recrystallized pyrite and dolomite with diagenetic fluid circulation

Multi-stage growth of the trachytic lava dome of the Puy de Dôme (Chaîne des Puys, France). Field, geomorphological and petro-geochemical evidence

International audienceUnderstanding lava dome eruptions is a major concern in volcanology regarding the assessment of associated hazards. This question is usually addressed through the study of active calc-alkaline domes, however, alkaline domes remaining poorly known. The Puy de Dôme is the most prominent volcano of the Chaîne des Puys Quaternary intraplate volcanic field in the French Massif Central. It is a complex trachytic lava dome ca. 11 ka old. Field investigations, geomorphological and petro-geochemical studies were performed to understand the magmatic system of this dome and reconstruct its volcanic evolution. The different lava facies were fully characterised and their relationships with the volcano building stages have been established. A new four-step evolution is proposed for the growth of this edifice. Mineralogical and geochemical variations document magmatic differentiation with self-mixing and minor crustal contamination in a zoned reservoir within the upper crust. Magma mixing and crustal contamination through fluids interactions during magma ascent and the involvement of fumarolic activity throughout the edifice building are also documented. Further, this work highlights the specific features of the Puy de Dôme compared to calc-alkaline domes: lack of lava flows and almost complete magma degassing during ascent, resulting in the lack of significant explosive activity until the last eruption step

Intracontinental Miocene: Climate and paleolake volumes in the Forez Basin, France (Part I),

International audienceEuropean Tertiary sedimentary basins as the Forez Graben, France, are potential records of continental paleoclimates. The Forez Basin hosts deposited and precipitated sediments of Oligocene to Miocene age. Geochemical data of carbonates indicate strictly continental origin starting at Eocene-Oligocene with tropical to temperate climate conditions, then during the Middle Miocene a temperate continental climate prevails. Combining volume of calcite deposits and their geochemical data, volumes of large lakes and evaporation/inflow ratios were reconstructed. The Late Miocene in the Forez Graben has been affected by dissolution and secondary precipitation of calcite, barite, which is the result of wetter and colder climate conditions. These lake volume calculations represent the first estimation of large lakes volumes in Western Europe during the Miocene

Mineralogical and numerical approaches to establish the pre-eruptive conditions of the mafic Licán Ignimbrite, Villarrica Volcano (Chilean Southern Andes)

International audienceThe Licán Ignimbrite represents a large explosive basaltic andesite event in the eruptive history of the Villarrica Volcano, an active stratovolcano of the Southern Andean Volcanic Zone. A mineralogical study of this ignimbrite indicates important disequilibrium features. Resoption textures were observed around Ca-rich (An74-92; Group 1) and more sodic (An35-44; Group 2) plagioclase phenocryst cores. Rims on these phenocrysts as well as microlites grew with an intermediate composition (An47-63; Group 3). Low- and high-Mg clinopyroxene and orthopyroxene phenocrysts were also identified; most of them showing reverse Mg zoning. Based on these mineralogical data and thermodynamic modeling, an increase of about 200 °C in temperature (from ~ 900-1000° to ~ 1100-1150 °C), a decompression of ~ 400 bar (equivalent to ~ 1.5 km) and a decrease in H2O content in the melt of ~ 1.6 wt.% are identified during crystallization. Addition of hot magma from below would have caused heating of the reservoir with the consecutive ascent of magma pulses, H2O exsolution and convective mixing (compositional and/or thermal mixing). A numerical model using the Finite Element Method was developed to assess the response of the Licán Ignimbrite reservoir upon conductive heating by 200 °C by mafic underplating; this model indicates a rapid (tens of years) temperature homogenization of this system, with ascending magma plumes with velocities as high as 6 m/h. Because the Licán Ignimbrite erupted at the end of the Pleistocene glaciation, glacial unloading could have also contributed to depressurization of the reservoir

Mineralogical and numerical approaches to establish the pre-eruptive conditions of the mafic Licán Ignimbrite, Villarrica Volcano (Chilean Southern Andes)

International audienceThe Licán Ignimbrite represents a large explosive basaltic andesite event in the eruptive history of the Villarrica Volcano, an active stratovolcano of the Southern Andean Volcanic Zone. A mineralogical study of this ignimbrite indicates important disequilibrium features. Resoption textures were observed around Ca-rich (An74-92; Group 1) and more sodic (An35-44; Group 2) plagioclase phenocryst cores. Rims on these phenocrysts as well as microlites grew with an intermediate composition (An47-63; Group 3). Low- and high-Mg clinopyroxene and orthopyroxene phenocrysts were also identified; most of them showing reverse Mg zoning. Based on these mineralogical data and thermodynamic modeling, an increase of about 200 °C in temperature (from ~ 900-1000° to ~ 1100-1150 °C), a decompression of ~ 400 bar (equivalent to ~ 1.5 km) and a decrease in H2O content in the melt of ~ 1.6 wt.% are identified during crystallization. Addition of hot magma from below would have caused heating of the reservoir with the consecutive ascent of magma pulses, H2O exsolution and convective mixing (compositional and/or thermal mixing). A numerical model using the Finite Element Method was developed to assess the response of the Licán Ignimbrite reservoir upon conductive heating by 200 °C by mafic underplating; this model indicates a rapid (tens of years) temperature homogenization of this system, with ascending magma plumes with velocities as high as 6 m/h. Because the Licán Ignimbrite erupted at the end of the Pleistocene glaciation, glacial unloading could have also contributed to depressurization of the reservoir

Oxic and post-oxic chemical changes related to eogenesis and mesogenesis in a Miocene paleolake

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