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Guagua Pichincha, located 14 km west of Quito, Ecuador, is a stratovolcano bisected by a horseshoe-shaped caldera. In 1999, after some months of phreatic activity, Guagua Pichincha entered into an eruptive period characterized by the extrusion of several dacitic domes, vulcanian eruptions, and pyroclastic flows. We estimated the three-dimensional (3-D) P-wave velocity structure beneath Guagua Pichincha using a tomographic inversion method based on finite-difference calculations of first-arrival times. Hypocenters of volcano-tectonic (VT) earthquakes and long-period (LP) events were relocated using the 3-D P-wave velocity model. A low-velocity anomaly exists beneath the caldera and may represent an active volcanic conduit. Petrologic analysis of eruptive products indicates a magma storage region beneath the caldera, having a vertical extent of 7–8 km with the upper boundary at about sea level. This zone coincides with the source region of deeper VT earthquakes, indicating that a primary magma body exists in this region. LP swarms occurred in a cyclic pattern synchronous with ground deformation during magma extrusions. The correlation between seismicity and ground deformation suggests that both respond to pressure changes caused by the cyclic eruptive behavior of lava domes.

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 (

Situated in the fore-arc of the Northern Volcanic Zone (NVZ) of the Andes in Ecuador, Pichincha volcano is an active edifice where have been erupted unusual magmas as adakites and high-Mg andesites. The particular geodynamic setting of the ecuadorian margin (i.e. the flat subduction of the Carnegie Ridge) suggests that thermo-barometric conditions for the partial melting of the oceanic crust are accomplished beneath this volcano. Pichincha adakites possess all the geochemical and isotopic characteristics of slab melts described in various other arc settings. High-Mg andesites with geochemical characteristics close to those of adakites present strong enrichments in MgO that suggest that, once they were produced by ca. 10 % partial melting of the downgoing subducted slab, some adakites en route to the surface strongly interacted with the peridotitic mantle wedge. Adakitic magmas could then represent, as in many other arcs where slab melting occurs, the principal metasomatic agent of the mantle in the NVZ in Ecuador.

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

International audience; This paper presents 34 geochemical analyses, 24 Ar‐Ar ages, and two U‐Pb ages of igneous rocks from the back‐arc basins and submarine ridges in the Coral Sea‐New Caledonia region. The D'Entrecasteaux Ridge is a composite structural feature. Primitive arc tholeiites of Eocene age (34–56 Ma) are present along a 200 km length of the ridge and arguably were part of the initial line of subduction inception between Fiji and the Marianas; substantial Eocene arc edifices are only evident at the eastern end where Bougainville Guyot andesite breccias are dated at 40 ± 2 Ma. The South Rennell Trough is confidently identified as a 28–29 Ma (early Oligocene) fossil spreading ridge, and hence, the flanking Santa Cruz and D'Entrecasteaux basins belong in the group of SW Pacific Eocene‐Early Miocene back‐arc basins that include the Solomon Sea, North Loyalty, and South Fiji basins. The rate and duration of spreading in the North Loyalty Basin is revised to 43 mm/yr between 28 and 44 Ma, longer and faster than previously recognized. The direction of its opening was to the southeast, that is, parallel to the continent‐ocean boundary and perpendicular to the direction of coeval New Caledonia ophiolite emplacement. Medium‐ and high‐K alkaline lavas of 23–25 Ma (late Oligocene) age on the northern Norfolk Ridge are an additional magmatic response to Pacific trench rollback.

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].

Tungurahua, one of Ecuador's most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955±90 years ago, which left a large amphitheater and produced a ∼8-km3 debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ∼15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ∼10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km3 of volcanic products. Two periods of construction are recognized in Tungurahua's III history. From ∼2300 to ∼1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ∼1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition (andesite+dacite), and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916–1918. Given good age control and volumetric considerations, Tungurahua III growth's rate is estimated at ∼1.5×106 m3/year over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ∼3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ∼3000-year-old avalanche. The more frequent eruptive episodes of each century, characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano.

L'étude des laves du volcan Antisana (Equateur) révèle la présence de laves calco-alcalines, enrichies en Sr, Nb, Ti et P par rapport à des laves calco-alcalines normales. Certaines laves montrent également un appauvrissement net en terres rares lourdes, typique des adakites. Cependant, les conditions nécessaires à la genèse d'adakites n'étant pas réunies sous l'Antisana, nous proposons que ces roches à caractère adakitique soient issues de la fusion partielle d'un manteau hydraté, préalablement métasomatisé de façon hétérogène par des liquides adakitiques issus de la plaque plongeante. (Résumé d'auteur)

In Ecuador, Volcan Mojanda, previously thought to be a single edifice, consists of two contemporaneous volcanoes, Mojanda and Fuya Fuya. Despite their proximity and contemporaneity, these volcanic centres continuously showed contrasting eruptive dynamics and geochemistry. Andesitic lava flows form the main part of basal Mojanda (Moj I). Following caldera collapse, a small andesitic stratocone (Moj II) was built, consisting mainly of basic andesite lava flows, scoria flow deposits and a thick summit series of vitric breccias. This cone was partly destroyed by phreatoplinian eruptions that led to the formation of a small, summit caldera. Fuya Fuya grew on the western flank of basal Mojanda and was contemporaneous with Mojanda II. Its activity began with andesitic and dacitic viscous lava flows and domes (FF I) and continued with a period of intense pyroclastic activity (FF II), during which two voluminous Plinian airfalls of rhyolitic pumice (R1 and R2) were erupted. Later, the activity of Fuya Fuya became effusive with the building of an intermediate andesitic edifice, the San Bartolo cone (FF III). Subsequently, a Mount St. Helens-like collapse event occurred in Fuya Fuya which was responsible for the loss of a large part of FF III and the western part of Mojanda. This avalanche (unit FF IV) was accompanied by voluminous pyroclastic flows and was followed by the construction of a dacitic dome complex (FF V) within the avalanche caldera. Both the Fuya Fuya and Mojanda magmatic suites show adakitic characteristics. This adakitic character is more marked for the Fuya Fuya volcanics than for the Mojanda rocks, suggesting different sources. Nevertheless, several arguments suggest that both volcanoes are related in their development and magmatic evolution. For example, the older rhyolitic Plinian deposits (R1) of Fuya Fuya contain juvenile andesitic clasts that have petrogenetic affinity with the Mojanda suite, a characteristic also observed in the ash flow deposits emitted after the avalanche event in Fuya Fuya.

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.

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.

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 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.

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)

Recently, [Garrison and Davidson (2003)][1] questioned the possibility that the adakites of the Northern volcanic zone of the Andes were generated by slab melting. The conclusions of the authors, involving a lower-crustal source for adakites, are in clear contradiction with ideas already proposed

At Santa Maria Volcano (New Hebrides island arc), extensive ash and scoria flow deposits overlie the mainly effusive, pre-caldera cone. Hydromagmatic features characterize these deposits, the composition of juvenile clasts ranges from basalt to acid andesite/dacite (SiO2= 51–63.6%) with a dominant basaltic composition. The stratigraphic position of this pyroclastic series and its spatial distribution around a 8.5 km × 6 km wide caldera provide evidence of a relationship between this series and the caldera formation. In addition, these pyroclastic deposits are co-genetic to parasitic cones and lava flows developed along faults concentric to the caldera. Both series result from a compositionally layered magma reservoir, the subordinate differentiated magmas being the result of fractional crystallization from the basalts. A model of caldera formation which implies a large hydromagmatic eruption at the central vent and minor magma withdrawal by flank eruptions is proposed. This model emphasizes the importance of mafic hydroclastic eruptions in the caldera forming event and contradicts a model implying only quiet subsidence, a process often proposed for the formation of calderas in island are volcanoes of mainly mafic composition.

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.

In the southern New Hebrides arc, magmatic and tectonic processes are closely linked. Between 21°S and 22°S, a “normal” broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc, whereas south of 22°S, at the southern termination of the arc, a high-Mg andesite suite appears, with the more mafic endmembers having mineralogical and compositional affinities to high-Ca boninites. During the last 2 Ma, this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the N-S spreading axis of the North Fiji Basin. In this region, which has been dominated for several million years by fast transform motions, the low magma production rates in this section of the arc, and the unusual boninite-related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones, abnormally small depth-to-slab distances beneath volcanoes, and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back-arc spreading axis. The on-going collision between the Loyalty Ridge and the arc may also contribute to these unusual magmatic characteristics, as presently only a very small amount of convergence occurs along the southernmost segment of the trench. In such a complex tectonic setting, with a spreading axis propagating into an intra-oceanic arc, and major transform plate motions, generation of high-Ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the Hunter Ridge, the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis. The high-Mg andesite suite probably results via a combination of fractional crystallization from such high-Ca boninitic parental magmas and subsequent magma mixing processes, operative all along the southern termination of the arc.

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 volcanic arcs, two main types of magmatism are recognized: (1) widespread calc-alkalic magmatism, generated by hydrous partial melting of a metasomatized mantle wedge, and (2) less common adakitic volcanism produced by subducted-slab melting. The Cayambe volcanic complex in Ecuador shows a progressive temporal change from an older calc-alkalic volcano (Viejo Cayambe) to a younger adakitic edifice (Nevado Cayambe). This evolution may be related to the unusual geodynamic setting of the Ecuadorian Andes, controlled mainly by the subduction of the Nazca plate, including the Carnegie Ridge thickened oceanic crust. The Viejo Cayambe magmas appear to be generated from a mantle-wedge source, slightly metasomatized by slab-derived melts. Conversely, the Nevado Cayambe magmas imply either stronger and more advanced interactions between slab melts and mantle peridotite, or that adakitic magma could reach the surface due to higher degrees of slab melting. Fractional crystallization, assimilation, and mixing processes also contributed to lava diversity. We propose that the magmatic evolution of the Cayambe volcanic complex is due to increasing efficiency of interaction between slab melts and the mantle wedge, because of higher degrees of slab melting in response to the subduction of the Carnegie Ridge.

Detailed surveys (bathymetry, magnetism, seismicity and focal mechanism solutions) recorded on the junction between the southern New Hebrides Arc and the North Fiji Basin enlighten the geodynamic complexity of the area. The presently active c. N-S-oriented spreading structures of the North Fiji Basin are superimposed on ancient ones, which appeared during a N135°E spreading episode active before 3 Ma. A recent southward extension of the New Hebrides Arc occurred c. 2 Ma ago. Structural and geophysical consequences of both events partly obscure the present arc-ridge junction. For example, back-arc troughs do not exist to the south of the former arc termination. N45°E structural directions, present all over the studied area, are interpreted as older transform faults related to the N135°E spreading axis, which also left recognizable NW-SE magnetic anomalies. However, the New Hebrides Arc/North Fiji Basin junction remains geodynamically unstable, due to the concomitance of convergent, strike-slip and divergent movements. Their respective crustal expressions, i.e. arc volcanism, lateral displacements and seafloor spreading, are examined and considered in their regional environment. A provisional model of the arc-ridge junction that accounts for most of the parameters analysed above is presented.

Matthew and Hunter, the two southernmost active volcanoes of the New Hebrides island arc (southwest Pacific) differ markedly from the other (mainly tholeitic) Quaternary volcanoes of this arc. Geodynamically related to the New Hebrides subduction zone, they also lie close to the southern limb of the active expanding ridge of the North Fiji Basin. Both volcanoes are made up of acid, medium-K, calcalkaline orogenic andesites, containing a variety of inclusions (pyroxene- and gabbroic cumulates, as well as doleritic cognate inclusions). This paper presents the first systematic petrographic and chemical study of these volcanics and their inclusions. Trace-element geochemistry and rare-earth element modelling suggest that the two volcanoes developed from successive batches of similar parental magmas, originating from limited partial fusion of garnet peridotite in the mantle wedge. Various degrees of fractional crystallization ot these batches led to the formation of three volcanic suites: Hunter (little fractionated), West-Matthew (moderately fractionated) and East-Matthew (highly fractionated). Moreover, on Matthew island, no correlation exists between the degree of fractionation and the eruptive chronology, the youngest edifice (West-Matthew) being less evolved than the older on (East-Matthew). (Résumé d'auteur)