Search

Recent eruptions from volcanoes with no previously known historical activity in Chile and Indonesia have raised the importance of the early identification of potentially active centers for the purpose of hazard assessment. Here we bring radiometric evidence (C-14,Ar-39-Ar-40) of previously unrecognized but significant magmatic activity at partly eroded Imbabura volcano (Ecuador) in late Pleistocene to early Holocene times, on whose perimeter live more than 300,000 persons. Following an effusive stage from 50 to 30 ka with the emplacement of andesitic lava flows on different flanks of its edifice, the activity became explosive with the generation of andesitic block-and-ash flows on its eastern side, beginning at similar to 35 ka cal BP. Subsequently a flank collapse associated with a volcanic blast occurred on the volcano's SW flank at similar to 30 ka cal BP. The resulting debris avalanche and blast breccias cover an area now heavily populated around San Pablo Lake and its source was later concealed by successive dome building episodes at Huarmi which produced similar to 2.8 km(3) of silicic andesite. Renewed dome activity at the edifice's Taita summit occurred at similar to 17 ka cal BP and continued intermittently into early Holocene times, as indicated by pyroclastic flow deposits overlying a palaeosoil dated at similar to 9 ka cal BP. In summary, this study reveals an eruptive behavior characterized by a low recurrence rate but with quite large eruptions, a pattern which is also observed at other silicic volcanoes of Ecuador's Western Cordillera. It is now imperative to reconsider the origin and source of the many tephra layers catalogued in Holocene lacustrine sediments in the Imbabura area. Tephra and lava volume estimates for Imbabura volcano converted to Dense Rock Equivalent values yield a minimum magmatic output rate of 0.13 km(3)/ka in the past 35,000 years, which argues for sustained magma production for this volcano in recent geological times. The lmbabura example thus raises the question of how to improve population preparedness for volcanoes with infrequent eruptions, and how to guide authorities' decisions concerning the development of urban areas and infrastructures near presently inactive but potentially highly dangerous volcanoes.

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (

suggesting that slab melts can be responsible for the metasomatism Extensive sampling of the Antisana volcano in Ecuador (Northern of the mantle wedge beneath the NVZ in Ecuador. If mantle Volcanic Zone of the Andes) has revealed the presence of adakite-like convection can drag down this modified mantle beneath Antisana rocks throughout the edifice, i.e. rocks with geochemical characteristics volcano, destabilization of metasomatic amphibole at appropriate close, but not identical, to those of slab melts. Two main volcanic pressures in this modified garnetiferous mantle can adequately explain groups have been distinguished, characterized by two distinct evoluthe formation and the geochemical features of Antisana lavas. tionary trends. The AND group, mostly composed of andesites, shows the clearest adakitic characteristics such as high La/Yb and Sr/Y ratios and low heavy rare earth element (HREE) contents. The CAK group, composed of high-K andesites and dacites, displays

New geochemical and geochronological data on the Miocene–Pliocene Quimsacocha volcanic center (QVC) have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern Ecuador and northern Peru. The QVC, located in the presently inactive southern part of the Ecuadorian arc, was built up during three distinctive volcanic phases. The first phase generated a basal edifice with mainly andesitic lava flows, while the second phase is characterized by the emplacement of cryptodomes, domes and related outflow breccias comprised of andesites and some dacites. The last phase released rhyolitic ignimbrites associated with the formation of a large caldera, which was later partly filled by dacitic–rhyolitic domes. Geochemical data for the QVC indicate higher Al2O3, TiO2, Na2O, Zr and Sr contents and lower Fe2O3*, MgO, Y, MREE and HREE abundances, compared to other eruptive rocks of the Plio–Quaternary volcanic front of Ecuador. Such geochemical features, as well as the frequent presence of an associated epithermal gold deposit, are characteristic of the involvement of slab melts, also known as adakites [1] , [2] , in the generation of these magmas. After a calc-alkaline arc magmatism phase, slab horizontalization – in response to the subduction of a buoyant oceanic plateau – results in increased involvement of a slab melting component in the magmas produced. However, pristine adakites were generated and emplaced during a relatively short period, as indicated by zircon fission-track ages. Then volcanic activity stopped and a volcanic gap formed. The identification of these adakites, their location and age support a model of slab melting associated with flat slab subduction [M.A. Gutscher et al., Geology 28 (2000) 535–538].

In the mid-fifteenth century, one of the largest eruptions of the last 10 000 years occurred in the Central New Hebrides arc, forming the Kuwae caldera (12x6 km). This eruption followed a late maar phase in the pre-caldera edifice, responsible for a series of alternating hydromagmatic deposits and airfall lapilli layers. Tuffs related to caldera formation (≈ 120 m of deposits on a composite section from the caldera wall) were emitted during two main ignimbritic phases associated with two additional hydromagmatic episodes. The lower hydromagmatic tuffs from the precaldera maar phase are mainly basaltic andesite in composition, but clasts show compositions ranging from 48 to 60% SiO2. The unwelded and welded ashflow deposits from the ignimbritic phases and the associated intermediate and upper hydromagmatic deposits also show a wide compositional range (60–73% SiO2), but are dominantly dacitic. This broad compositional range is thought to be due to crystal fractionation. The striking evolution from one eruptive style (hydromagmatic) to the other (magmatic with emission of a large volume of ignimbrites) which occurred either over the tuff series as a whole, or at the beginning of each ignimbritic phase, is the most impressive characteristic of the caldera-forming event. This strongly suggests triggering of the main eruptive phases by magma-water interaction. A three-step model of caldera formation is presented: (1) moderate hydromagmatic (sequences HD 1–4) and magmatic (fallout deposits) activity from a central vent, probably over a period of months or years, affected an area slightly wider than the present caldera. At the end of this stage, intense seismic activity and extrusion of differentiated magma outside the caldera area occurred; (2) unhomogenized dacite was released during a hydromagmatic episode (HD 5). This was immediately followed by two major pyroclastic flows (PFD 1 and 2). The vents spread and intense magma-water interaction at the beginning of this stage decreased rapidly as magma discharge increased. Subsequent collapse of the caldera probably commenced in the southeastern sector of the caldera; (3) dacitic welded tuffs were emplaced during a second main phase (WFD 1–5). At the beginning of this phase, magma-water interaction continued, producing typical hydromagmatic deposits (HD 6). Caldera collapse extended to the northern part of the caldera. Previous C14 dates and records of explosive volcanism in ice from the south Pole show that the climactic phase of this event occurred in 1452 A.D.

In Vanuatu, Tongoa and Epi islands once formed part of a larger landmass, Kuwae, which was partly destroyed during a cataclysmic seismo-volcanic event that is recorded in local folklore. It led to the formation of a 12-kmlong and 6-km-wide oval-shaped submarine caldera with two distinct basins and a total area of - 60 km2 at the level of the rim. The age ofthis eniption, 1420-1430 A.D., and the structure of the related collapse are discussed and a composite log ( 143 m) of the pyroclastics surrounding the caldera is presented. They comprise thick hydromagmatic deposits belonging to a terminal hydromagmatic phase of the pre-caldera edifice, which grade upwards into two major sequences of pyroclastic flow deposits, clearly related to the caldera event. Collapse near the caldera edge was at least in the range 650 to 950 m, and may have been as much as 800 to 1100 m. The volume of rocks engulfed during the caldera formation is - 32-39 km3, suggesting the same volume of magma was erupted. Even if two coalescent collapse structures were formed, it is worth noting that the Kuwae caldera is not a reactivated structure, but the result of a single event of short duration which occurred in the first half of the Fifteenth century. This event is one of the seven biggest caldera-forming events during the last 10,000 years, and is comparable with the Santorini Minoan eruption and the Crater Lake eruption.

A systematic compilation of the geochemical data collected during the starmer programme is presented, in addition to all published data gathered along the North Fiji Basin (NFB) spreading system. From the 194 samples selected, the geochemical variations observed along the different spreading segments can be related to uni- or multi-modal origin of distinctive magma sources. These variations are interpreted as directly linked to the geodynamical features which surround the NFB. In the southern NFB, on the N174°E segment, N-MORB are present, but the low rate (or dead) subduction along the Hunter ridge still marks its influence as several basalts show a significant subduction-related contamination with negative Nb anomaly and high LOI. Several others are marked by a weak E-MORB source contribution, that may be related to subducted-OIB seamounts from the South Fiji Basin. In the central NFB, along the N-S segment which represents the most active and morphologically regular spreading ridge of the NFB, the magma source produces only N-MORB, with depleted LILE, HFSE, and LREE patterns, except in its northernmost part where magmatic signatures similar to that of the N15° segment appear. Along the N15° segment, three distinctive sources coexist and produce a wide range of geochemical signatures on the basalts collected; (1) a N-MORB source signature; (2) a transitional towards E-MORB source signature with a negative Nb anomaly indicating that some subduction related contamination still exists beneath the NFB (the basalts derived from this source might also have been called BABB previously); and (3) a source signature transitional towards E-MORB or OIB marking the start of an influence that increases towards the northern NFB as the Rotuma-Samoan hot spot lineament is approched. Around 17°S, on the Kaiyo station 4 site explored and sampled by the Nautile deep-sea submersible, as well as in the triple junction area, the same variability derived from three distinctive mantle sources is observed. Along the N160° segment, the three sources still coexist, but the influence of the E-MORB or OIB source (hot spot-related) increases, whereas the influence of the subduction-related source decreases. Along the Pandora-Rotuma ridge, the OIB-derived lava type is the only one present, the eventual contribution from other sources being completely diluted. Thus, the geochemistry of the NFB basalts is directly influenced by (1) the regional geodynamic environment, such as subduction zones and/or hot spot trails, (2) the geodynamical regime and stability of this type of back-arc system. The influence of the New Hebrides subduction, located some 500 km west of the active spreading ridge, is still perceptible, although weak, in the whole northern half of the NFB. However, this infuence is not directly linked to the presently active subduction, but originates rather from the partial melting of an upper mantle source that suffered subduction contamination during the clockwise rotation of the New Hebrides arc leading to the opening of the NFB during the past 12 Myr. In the northern NFB, many basalts result from the mixing of an N-MORB and a OIB source, similar to transitional alkalic lavas from oceanic intra-plate magmatism. This sources mixing increases northwards from 18°20′S to 12°S.

Lagabrielle, Y., Auzende, J.-M., Eissen, J.-P., Janin, M.-C. and Cotten, J., 1994. Geology and geochemistry of a 800 m section through young upper oceanic crust in the North Fiji Basin (Southwest Pacific). In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STARMER French-Japanese Program. Mar. Geol., 116: 113-132. We report the results of geological and structural observations made during one dive of the French submersible Nautile during the STARMER Cruise in 1989 in the vicinity of the 16”40’S triple junction in the North Fiji Basin. This dive provided a spectacular, 800 m continuous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction. A number of volcanic facies have been encountered including massive and pillowed lavas, sheeted dikes and spectacular columnar-jointed massive lavas. Pillowed lavas have been observed only at the base and at the top of the section. The ratio between massive and pillowed basalts is high suggesting that the crust in this area grew mostly during a stage of high eruptive rate. Dikes have been observed in the middle part of the section. They probably do not represent the top of a classical “dike complex” but an isolated set of sheeted dikes, possibly the feeders of overlying flows. The abundance of vertical tectonic breccias observed during the first part of the dive confirms that the basal part of the wall can be regarded as a major tectonic boundary along which occurred significant vertical and strike-slip motions. Micropaleontological data from sedimentary rocks collected during the dive bring new constraints to the evolution of the triple junction. The oceanic crust in the surveyed area is at least as old as 1.9-1.3 Ma, based on the age of sedimentary rocks collected in talus at the base of the wall and observed at the summit of the section. The collected basalts include (1) N-type MORBs, (2) E-type MORBs and (3) BABBs. This emphasizes the heterogeneous nature of the mantle beneath the central NFB. Some portions of the depleted mantle underlying the central NFB triple junction area have recorded contamination from melts or fluids from an ancient subduction zone (probably .the New Hebrides during the opening of the NFB), whereas adjacent mantle areas show the influence from an alkali-enriched source. Such alkali-enriched characters are reported from recent basalts of the northern NFB (South-Pandora ridge-Rotuma island).

Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust.

International audience; The Mojanda-Fuya Fuya Volcanic Complex consists of two nearby volcanoes, Mojanda and Fuya Fuya. The older one, Mojanda volcano (0.6 to 0.2 Ma), was first constructed by andesites and high-silica andesites forming a large stratovolcano (Lower Mojanda). This edifice was capped by a basaltic andesite and andesitic cone (Upper Mojanda), which collapsed later to form a 3-km-wide summit caldera, after large phreatomagmatic eruptions. The Lower Fuya Fuya edifice was constructed by the extrusion of viscous Si-rich andesitic lavas and dacitic domes, and the emission of a thick sequence of pyroclastic-flow and fallout deposits which include two voluminous rhyolitic layers. An intermediate construction phase at Fuya Fuya is represented by a mainly effusive cone, andesitic in composition (San Bartolo edifice), the construction of which was interrupted by a major sector collapse in the Late Pleistocene. Finally, a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre, forming the Upper Fuya Fuya volcanic centre. This paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source. Although all the rocks of the complex are included in the medium-K field of continental arcs, the Fuya Fuya suite (61-75 wt.% SiO2) shows depletion in Y and HREE and high Sr/Y and La/Yb values, compared to the less silicic Mojanda suite (55-66.5 wt.% SiO2). The Mojanda calc-alkaline suite was generated by partial melting of an adakite-metasomatised mantle source that left a residue with 2% garnet, followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow, intra-crustal depths. For Fuya Fuya, geochemical and mineralogical data suggest either (1) partial melting of a similar metasomatised mantle with more garnet in the residue (4%), followed by fractional crystallization involving plagioclase, amphibole and pyroxene, or (2) mixing of mafic mantle-derived magma from the Mojanda suite and slab melts, followed by the same fractional crystallization process.

The North Fiji Basin (NFB) is a 12 m.y. old back-arc basin that has a complex multi-stage history. The presently active spreading system can be divided into four segments between 16°S and 22°S, which from north to south trend N160, N15 and N-S (the fourth segment is the N-S trending segment located near 174E). The main N-S segment is morphologically similar to other medium-rate oceanic ridges, whereas the other segments have rougher morphologies which have been severely disturbed by a triple junction at 16°45′S and several instability features such as overlapping spreading centres (OSCs) and propagating rifts. The spreading rate seems to diminish from 7.8 cm/yr near 20°S to 4.6 cm/yr near 18°S. Mineralogical, pertological and geochemical data were obtained on 24 new stations located along all the four segments. The petrogenesis of the basalts collected is essentially controlled by low-pressure crystal fractionation of plagioclase±olivine±clinopyroxene (plagioclase>olivine>clinopyroxene) with 52% of the NFB basalts reaching the four-phase cotectic. Locally, some magma mixing occurs, but this is limited to magma batches of closely related composition, as might be expected to occur inside a magma reservoir. The N-S segment, which, since 3 m.y., is the only steady-state segment, is also petrologically and geochemically very comparable to other medium-rate oceanic spreading centres, producing moderately evolved LILE and LREE-depleted N-MORB. In contrast, the three other segments produce basalts of much more variable petrology and geochemistry characterized by LILE and slightly LREE-enriched magmas cf back-arc basin basalt (BABB) affinity (but not as enriched as, for example, the Mariana BABB); MORB is, however, also found on the N160, N15 and 174E segments. Diagrams using Ba, Rb, K/P and (K/Ti)N (Normalized to the chondrites) plotted against latitude clearly show along-strike variations. Beneath the recently formed segments, the mantle source is heterogeneous, and locally has some BABB affinities, whereas beneath the more steady-state N-S segment the magma source is more homogeneous, being generally depleted in LILE and REE as is the case for classical N-MORB mantle sources. Simple evolution from an early stage of BABB production to MORB described in the Lau basin and proposed for the NFB does not seem to occur. Present-day activity still produces large amounts of BABB along the less-stable and more recently created segments, and MORB was produced in the earlier stages of the development of the NFB.

The Atacazo-Ninahuilca Volcanic Complex (ANVC) is located in the Western Cordillera of Ecuador, 10 km southwest of Quito. At least six periods of Pleistocene to Holocene activity (N1 to N6) have been preserved in the geologic record as tephra fallouts and pyroclastic flow deposits. New field data, including petrographic and whole-rock geochemical analyses of over forty soil and tephra sections, 100 pumice and lithic samples, and 10 new C-14 ages allow us to constrain: (1) the tephra fall isopachs and detailed characteristics of the last two events (N5-N6) including volume estimates of the tephra and pyroclastic flow deposits and the corresponding volcanic explosivity index (VEI); (2) the petrographical and geochemical correlations between domes, tephras, and pyroclastic flow deposits; and, (3) the timing of the last 4 eruptive events and a period of quiescence that endured a few thousand years (1000-4000).The last two eruptive events (N5 and N6) took place at around 4400 +/- 35 yr BP and 2270 +/- 15 yr BP, producing huge plinian and pyroclastic flow deposits. Taking into account the widely spread deposits of these VEI 5 eruptions, the present population of about 70000 people, and the current infrastructure; the development of mitigation plans and deployment of monitoring systems at ANVC is highly recommended. (C) 2008 Elsevier B.V. All rights reserved.

International audience; The Puyo scoria cones and the Mera lava flows, two newly recognized volcanic formations dated between Late Pliocene to Middle Pleistocene, extend the limits of the Ecuadorian rear-arc volcanic province some 100 km to the south. The Puyo scoria cones have erupted K-rich absarokites containing olivine, diopside and phlogopite, whereas the Mera lava flows display a basic andesite composition, with olivine and minor augite phenocrysts. In addition to high contents in LILE, LREE and HFSE, the Puyo absarokites exhibit many characteristics of primitive melts, namely high Cr (590–310 ppm) and Ni (330–154 ppm) contents, high Mg# (64–70) and they contain forsteritic olivine (Fo82–89). The composition of the most primary Puyo absarokite was used in petrogenetic models, in order to constrain the genesis of these high-K magmas. Major and trace elements models, as well as isotopic data, indicate that the source of Puyo magmas is a hydrated phlogopite- and garnet-bearing lherzolite. Phlogopite crystallization in the mantle wedge is triggered by the metasomatism by 3–5% of a SiO2-, H2O-rich liquid generated by slab melting. Partial melting of the subducted oceanic crust beneath Ecuador is allowed by the subduction of the young and warm Carnegie Ridge, which modifies the thermal regime of the Benioff zone. A low degree (1–4%) of partial melting of the metasomatized mantle wedge, leaving a variable garnet (4–7%) ± phlogopite (0–4%) lherzolitic residual assemblage, leads to the compositions of the entire Puyo absarokite series and is consistent with previous petrogenetic models developed for the Ecuadorian volcanic arc. Indeed, the homogeneity of isotopic data across the arc suggests a similar source for the whole Ecuadorian magmas.

A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004-05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November-December 2002. Silica contents range from 62 to 58 wt.% SiO2 for the November 3 pyroclastic deposits to 58-56 and 54-53 wt.% SiO2 for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56-54 wt.% SiO2) between November 2004 and August 2005. Volatile contents in matrix glasses and glass inclusions from the November 3 pyroclastic deposits allow us to estimate the total amount of SO2 and HCl released into the atmosphere during the paroxysmal phase (i.e. 80 kT of SO2 and 280 kT of HCl). Pre-eruptive pressure-temperature conditions of the magmas range from 300 to 150 MPa and similar to 1000 degrees C with high water contents (similar to 5 wt.%). We propose the existence of an andesitic magma body located at similar to 7-12 km depth that is frequently intruded by more primitive, hydrous magmas from a deeper source. The initial crystallization of amphibole from the hydrous primitive magma seems typical of El Reventador, as well as the historically recurrent and regular periods of eruptive activity lasting several years. This eruptive behaviour coupled with the fractionation and mixing processes inferred from the 2002 and 2004-05 petrologic data suggest that deep magmatic recharge at El Reventador is frequent, and is probably responsible for the high frequency of eruptions.

Ice-capped Chimborazo is one of the large composite Ecuadorian volcanoes whose recent eruptive activity is poorly known. This paper presents the characteristics and the ages of a newly discovered Holocene sequence of pyroclastic deposits on the east and north sides of the cone. Lying upon a moraine of the Late-Glacial period, the most complete section of similar to 4.5 m in thickness is located 5 km from the present summit crater. It consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils. Based on held study, most flow deposits were assessed as surge layers, and six radiocarbon analyses obtained from charcoal fragments and paleosoils indicate that eruptions occurred at quite regular intervals between about 8000 and 1000 years ago. The first two and most potent events generated thick lahars over the north and west flanks of the cone. Surface textures of volcanic clasts were analysed by scanning electron microscopy. Blocky and blocky/fluidal vitric clasts indicate fragmentation during vulcanian explosions of a quite solidified shallow magma body. In addition, aggregates either cemented at a cooling stage (with surface fluidal textures), or consisting of fine particles (moss-looking aggregates), form a large part of the surge deposits. These characteristics indicate powerful explosions and intense fragmentation due to phreatic water reaching the conduit, probably from the ice-cap. Since the last eruption occurred between the early part of the 5th century (similar to AD 420) and the end of the 7th century, these results highlight that Chimborazo is a potentially active volcano. Given its dominating presence over the densely populated Ambato and Riobamba basins, and owing its large ice-cap, Chimborazo should be considered a dangerous volcano.

International audience; The Iliniza Volcanic Complex (IVC) is a poorly known volcanic complex located 60 kin SSW of Quito in the Western Cordillera of Ecuador. It comprises twin peaks, North Iliniza and South Iliniza, and two satellite domes, Pilongo and Tishigcuchi. The study of the IVC was undertaken in order to better constrain the role of adakitic magmas in the Ecuadorian arc evolution. The presence of volcanic rocks with an adakitic imprint or even pristine adakites in the Ecuadorian volcanic arc is known since the late 1990s. Adakitic magmas are produced by the partial melting of a basaltic source leaving a gamet rich residue. This process can be related to the melting of an overthickened crust or a subducting oceanic crust. For the last case a special geodynamic context is required, like the subduction of a young lithosphere or when the subduction angle is not very steep; both cases are possible in Ecuador. The products of the IVC, made up of medium-K basaltic andesites, andesites and dacites, have been divided in different geochemical series whose origin requires various interactions between the different magma sources involved in this subduction zone. North Iliniza is a classic calc-alkaline series that we interpret as resulting from the partial melting of the mantle wedge. For South Iliniza, a simple evolution with fractional crystallization of amphibole, plagioclase, clinopyroxene, magnetite, apatite and zircon from a parental magma, being itself the product of the mixing of 36% adakitic and 64% calc-alkaline magma, has been quantified. For the Santa Rosa rhyolites, a slab melting origin with little mantle interactions during the ascent of magmas has been established. The Pilongo series magma is the product of a moderate to high degree (26%) of partial melting of the subducting oceanic crust, which reached the surface without interaction with the mantle wedge. The Tishigcuchi series shows two stages of evolution: (1) metasomatism of the mantle wedge peridotite by slab melts, and (2) partial melting (10%) of this metasomatized source. Therefore, the relative ages of the edifices show a geochemical evolution from cale-alkaline to adakitic magmas, as is observed for several volcanoes of the Ecuadorian arc.

In the Northern Volcanic Zone of the Andes, the Cayambe Volcanic Complex consists of: (1) a basal, mostly effusive volcano, the Viejo Cayambe, whose lavas (andesites and subordinate dacites and rhyolites) are typically calc-alkaline; and (2) a younger, essentially dacitic, composite edifice, the Nevado Cayambe, characterized by lavas with adakitic signatures and explosive eruptive styles. The construction of Viejo Cayambe began > 1.1 Myr ago and ended at similar to 1.0 Ma. The young and still active Nevado Cayambe grew after a period of quiescence of about 0.6 Myr, from similar to 0.4 Ma to Holocene. Its complex history is divided into at least three large construction phases (Angureal cone, Main Summit cone and Secondary Summit cone) and comprises large pyroclastic events, debris avalanches, as well as periods of dome activity. Geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite. On the contrary, field observations, and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex. The adakitic signature of Nevado Cayambe magmas is related to partial melting of a basaltic source, which could be the lower crust or the subducted slab. However, reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab. Thus, the Viejo Cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab-derived melts (adakites), whereas the Nevado Cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis. This temporal evolution can be related to the presence of the subducted Carnegie Ridge, modifying the geothermal gradient along the Wadati-Benioff zone and favouring slab partial melting.

Previous studies on propagating rifts suggested that segmentation of a spreading axis could represent the superficial mark of mantle behavior (Sinton et al., 1983; Nicolas, 1989; Gente et al., 1995). The study of North–South Propagating Spreading Center (NSPSC) from the North Fiji Basin (NFB) brings new insights to this debate. Basalts from the central part of the propagator have more variable incompatible and isotopic ratios then those from its northern tip. A model of dynamic partial melting of a thermally and slightly geochemically and isotopically heterogeneous mantle is proposed. Beneath the central segment, the partial fusion starts deeper (ca. 30 km) and reaches a higher rate (ca. 22%). Further open system differentiation occurs within shallow permanent magma reservoirs along most of the central segment. Below the segment closest to the tip of the propagator the partial fusion starts shallower (ca. 25 km) and stays at a lower rate (ca. 16%). The maximum of differentiation occurs close to the propagator, in small, periodically disconnected, magma bodies resulting in the production of ferrobasalts close to the tip. In order to explain these variations, the presence of an asthenospheric diapir focused beneath the central part of the NS-PSC is proposed. The petrogenetic processes of propagating spreading centers of mature oceanic basins or back-arc basins are identical.

Compositional variations (major- and trace-element) of Pleistocene to Present rocks along the volcanic New Hebrides Central Chain (Southwest Pacific) are presented and discussed, using a new set of analyses. We focus in particular on the anomalous Mg-, K-, LILE- and LREE-rich and Al and Si-poor character of the basalts from Aoba and Santa Maria volcanoes (and, to a lesser extent, from Ambrym volcano), all located in the area where the D'Entrecasteaux Zone collides with the arc. In addition, we note a compositional transition from 'normal' arc basalts to boninite-related high-Mg andesites at the southern termination of the NHCC. The peculiar composition of the rocks of the volcanoes facing the D'Entrecasteaux Zone has traditionally been related, more or less directly, to the collision-subduction occurring in this area. We present an alternative hypothesis, of a strong geochmeical anomaly under Santa Maria and Aoba volcanoes, unrelated to this collision. In this model, a westward and upward invasion of DUPAL-type enriched mantle, corroborated by the pattern of intermediate seismicity, occurs under this part of the arc, in strong contrast to the usual Pacific-type MORB mantle source present under all other volcanoes of the NHCC. Such a DUPAL-type source may explain the anomalous geochemistry of the resulting volcanic products. (Résumé d'auteur)

Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI), Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs. The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower — phreatomagmatic — deposits from the Siwi sequence also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization. During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.

Ambrym, in the New Hebrides arc, has been considered as an effusive, basaltic volcano. The present paper discusses the general structure of this edifice, which consists of a basal shield volcano topped by an exceptionally large tuff cone surrounding a 12-km-wide summit caldera. Dacitic pyroclastic flow deposits are exposed in the lower part of the tuff series; they grade upward into composite sequences of bedded surtseyan-type hyaloclastites, ash flow deposits, and fallout tephra which are essentially basaltic in composition, in such a way that the tuff cone may be considered as mainly basaltic. The relationship between the eruption of pyroclastics and the collapse event precludes a classical model of caldera formation at a basaltic volcano in which “quiet” subsidence (i.e. Kilauea type) is the dominant mechanism. Interpretation of the tuff series implies intervention of external water and suggests both explosive and collapse mechanisms. A model of caldera formation which assumes an enlargement of the ring fracture during a first plinian and dacitic, then essentially hydrobasaltic eruption is proposed.

In June 1989 a diving cruise by the submersibleNautile was carried out on the North Fiji Basin spreading ridge. The objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes. This operation was the third cruise of the French-Japanese Starmer project following the two cruises of the R.V.Kaiyo in 1987 and 1988. Six dives along the spreading axis between 16°58′S and 17°00S show that the axial graben consists of alternating N15—trending horsts and grabens. Extinct hydrothermal sites have been observed all along the graben. These consist of fossil chimneys, oxide staining and dead shells. At 16°59′S an active chimney has been discovered and called the “White Lady” because of its almost exclusively anhydrite composition. This chimney expels a peculiar water characterized by low chlorinity and a 285°C maximum temperature. In the northern extremity of the N15 axis a wide fossil hydrothermal site has been explored and sampled. It is located in an area cut by N15, N140 and N60-trending faults and fissures.

Detailed studies by submersible were carried out in the axial zone of the Red Sea Rift near 18"N during the Soviet Red Sea expedition of the Oceanological Institute of the Academy of Sciences (December 1979-March 1980). The initial bathymetric, magnetic and seismic surveys established the general organization of the symmetric tectonic steps (1-3) descending towards the axial rift. The 45 km wide inner floor of the rift was explored during 21 dives. It is occupied by 100-300 m high, young pillowed volcanoes, isolated or grouped to form elongated hills, frequently cut by open fissures except in the zone of most recent extrusion. The 42 samples collected are typical plagioclase k olivine 5 clinopyroxene & spinel, more or less porphyritic mid-ocean ridge basalts whose compositions were mainly controlled by polybaric fractionation of plagioclase, olivine and minor clinopyroxene. They have been separated into porphyritic and sub-alfhyric groups using modes andmineralogical criteria. Mineral-liquid equilibria, crystal zonation, andmodal proportions indicate some magma mixing but probably only of closely related magma batches within each described group, as can occur inside a single magma chamber. Crystal accumulation is believed to have played a significant role in only a few porphyritic samples. Three sub-groups (from less to more evolved; (a) FeO*/MgO 1.49) were distinguished on the basis of glass and whole-rock major element chemistry. Glass compositions follow the multisaturated cotectic-like curve for MORB-type basalts and show a general evolution very comparable to what is observed on the Mid-Atlantic Ridge near 36"N, but are less diverse than in the FAMOUS area. 87Sr/86Sr, 143Nd/144Nd, and zo6Pb/204Pb data for 4 samples show strong similarities to those from the Mid-Atlantic and East Pacific Ridges, and indicate no continental contamination despite the fact that they have been produced during recent continental break-up and ocean opening. 206Pb/204Pb values, Th/Ta vs. Th/Tb correlations, and rare earth element patterns allow recognition of three different groups of samples, indicating that the Red Sea Rift near 18"N is fed by a heterogeneous mantle source. The chondrite-normalized LREE

Rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12-km section of the southern Juan de Fuca Ridge. Xenoliths comprise ≪ 1% of the dredge contents and range in size from 1 cm3 (glomerocryst) to 240 cm3. The xenoliths have ophitic to intersertal texture with 5–50% interstitial glass of ferrobasaltic composition more evolved than the host lava. On the basis of texture and mineralogy, the xenoliths have been subdivided into three types: type I, plagioclase + olivine + glass; type II, plagioclase + augite + glass ± olivine; and type III, plagioclase + augite + olivine + glass ± pigeonite (partially inverted) + Fe-Ti oxides. Mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type I is the least evolved and type III is the most evolved. Application of a graphical pyroxene geothermometer to augite in xenolith types II and III yields crystallization temperatures of 1100°–1200°C and to host-lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100°–1150°C. Coexisting titanomagnetite-ilmenite pairs in type III xenoliths yield temperature estimates of 1000°–1070°C and log f02 = −9.7 to −10.8. We infer that the xenoliths represent the partially crystalline “mush” boundary zone of a magma chamber based on the abundance of interstitial glass, zonation of mineral grains in the most crystalline samples, and coherence of chemical trends between interstitial glass, glass inclusions, and mineral phases. The evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber. The erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts.