Trachytic Dome Research Articles

The Geological and Tectonic Evolution of Feni, Papua New Guinea

Feni is located at the southeastern end of the NW-trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, in northeastern Papua New Guinea. This island chain is renowned for hosting alkaline volcanics, geothermal activity, copper–gold mineralization, and mining. There is no agreed consensus on the tectonic and petrogenetic evolution of Feni. Thus, the purpose of our paper is to present the geology of Feni within the context of the regional tectonic evolution of the TLTF chain and offer a succinct and generic geodynamic model that sets the stage for our next paper. The methodologies used in this study include a critical review of published and unpublished literature in conjunction with our geological observations on Feni. The Pliocene-to-Holocene TLTF chain is a younger arc situated within the greater Eocene-to-Oligocene Melanesian Arc bounded by New Ireland to the west, the Kilinailau Trench and Ontong Java Plateau in the east, and the New Britain Trench to the south. The geological units mapped on Feni include a large volume of basaltic lava flow and trachyandesite stocks intruding a limestone and siltstone basement. Younger units include the trachyte domes, pyroclastic flow, and ash fall deposits. The major structures on Feni are normal or extensional faults such as the Niffin Graben. Feni magmatism is attributed to the petrogenetic processes of polybaric or decompression melting and crystal fractionation of magmas previously influenced by sediment assimilation, mantle wedge metasomatism, slab tears, slab melts, and subduction. Deep lithospheric normal faults provide the fluid pathways for the Feni alkaline magmas.

Interpretation of magnetic data using boundary analysis and inversion techniques: a case study from Gölcük/Isparta (Turkey) region

The Gölcük (Isparta) is on the southern side of the city of Isparta in the Mediterranean region of Turkey. The investigation of magnetic field strength variations over subterranean layers may reveal their locations on Earth?s surface and provide physical and geometrical characteristics. Magnetic studies were carried out around Gölcük caldera lake using proton magnetometers to identify subsurface volcanic structures. The acquired data were inverted using four different edge detection algorithms such as analytic signal, tilt angle, theta map, horizontal gradient. Afterwards, the results were used to determine the locations of the anomalous structures. We also used pseudo-gravity and reduction-to-pole techniques for interpretation. Additionally, the magnetic data were evaluated using the power spectrum technique and the results were compared with the 2D and 3D prismatic inversion outcomes. As a result, the boundaries and depth of the anomalous structures, such as the trachytic dome south of the Gölcük were determined for three different cross-sections and areas. The results show that the anomalous dome structures? average depth values vary between 225 m and 391 m in the region and the maximum depth of the Caldera reaches up to 1076 m.

Mineralogical and gemmological characteristics of garnets associated with xenoliths within trachyte dome, Hisarlıkaya (Ankara), Central Anatolia, Turkey

Garnet-bearing xenoliths are observed within a trachytic dome extrusion in the Hisarlıkaya region (Ankara). These garnets exhibiting greenish-reddish-dark brown colour and ranging in sizes up to 1 cm were examined in terms of mineralogical, geochemical, and gemmological characteristics. Mineralogical studies indicate that these garnets (And88-93 Grs7-12) are linked to solid solution series, which are dominantly andradite with lower content of grossular. According to major, trace, and rare earth element (REE) analysis, the representative garnet crystal shows high CaO, Fe2O3, Al2O3, MgO, MnO, V, W, and light rare earth elements (LREE) concentrations. These high concentrations might indicate the mobility of these elements during skarn formation related with contact metamorphism, material exchange occurring via hydrothermal fluids, site geometry of the crystal, ionic radius of cations, and charge balance. The Hisarlıkaya garnets display dodecahedron-trapezohedron crystal habits with good translucency, glassy, transparent features, and also optical isotropic character. Their refractive indices are high (>1.78), and specific gravities range from 3.66 to 3.67 g/cm3. These garnets are not suitable for use as gemstones since they cannot be cut or processed considering all the mineralogical properties of garnets.

The nature and origin of magnetic anomalies over the Gölcük caldera; Isparta; South-Western Turkey

For the first time; ground magnetic survey was performed which data were inverted by means of total horizontal derivative; horizontal gradient analytic signal; and hyperbolic tilt angle techniques to identify subsurface volcanic structures around Isparta city (South-Western Turkey). Here; Golcuk volcanism took place at the apex of the Isparta Angle at the intersection of the Lycian and Antalya nappes. It initiated between 4.0—4.7 my ago mainly as lava extrusions and ended with phreatoplinian eruptions during Quaternary time. The study area is covered by authochtonous and allochthonous units that are intruded by Pliocene and Quaternary Golcuk volcanics and also overlain by pyroclastic fall and flow deposits. The boundaries were revealed for the buried volcanic structure from the edge detection methods. The geometry of the trachytic dome southwestern of the Golcuk Lake and its downward continuation were studied by 2D modelling with the control of the power spectrum depth results applied to the focused anomaly. The azimuthally-averaged logarithmic power spectra plot indicates that the downward continuation of source depth of the trachytic dome reaches up to 850 m. The forward inversion results indicate that the horizontal size of the model for this trachytic dome is 1250 m beneath the surface while it’s surface extension is only about 400 m.

Volcanology and geochemical study of the volcanic rocks of the Bafmeng area (Mount Oku, Cameroon Volcanic Line)

Detailed volcanological and petrographic descriptions, and petrogenetic studies of lavas from Bafmeng were performed, with the aim to establish their origin, magmatic evolution, and the tectonic environment. Bafmeng area is located on the NW flank of Mount Oku, an important volcanic massif along the Cameroon Volcanic Line in Central Africa. Four types of eruptive dynamisms are successively manifested; (1) an effusive dynamism (basaltic flows); (2), an extrusive dynamism (rhyolitic and trachytic domes); (3) a basaltic explosive dynamism (Strombolian cones) and (4) hydromagmatic eruption (maar). The lavas form a sodic alkaline series with three distinctive types: ultra-basic type (basanite), an intermediate type (benmoreite and trachyte), and an acid type (rhyolite). The mineralogical composition is made up of olivine, augite, plagioclase and opaque minerals for basic lavas. Acidic lavas are made up of sanidine, quartz, clinopyroxene, opaque minerals, and aegyrine. The peridotite has as minerals; plagioclases, spinel, olivine, and pyroxene The lavas show enrichment in LREE as compared to HREE with negative anomalies in Ba, Eu, Sr in trachyte and rhyolite and a positive anomalies in Nb, La, Zr, Dy. The major process responsible for the emplacement of the Bafmeng felsic lavas is partial melting of spinel-garnet rich peridotite, followed by contamination and assimilation. Mafic lavas are derived from the partial melting of the asthenospheric mantle. The tectonic context based on geochemistry indicates that, the regional development is related to within plate setting. Keywords : Cameroon Volcanic Line, Mount Oku, Bafmeng, Peridotite, Eruptive dynamism.

Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)

A 70-m-thick, 2200-m-long (51 × 106 m3) trachytic lava flow unit underlies the Puy de Cliergue (Mt. Dore, France). Excellent exposure along a 400-m-long and 60- to 85-m-high section allows the flow interior to be accessed on two sides of a glacial valley that cuts through the unit. We completed an integrated morphological, structural, textural, and chemical analysis of the unit to gain insights into eruption and flow processes during emplacement of this thick silicic lava flow, so as to elucidate the chamber and flow dynamic processed that operate during the emplacement of such systems. The unit is characterized by an inverse chemical stratification, where there is primitive lava beneath the evolved lava. The interior is plug dominated with a thin basal shear zone overlying a thick basal breccia, with ramping affecting the entire flow thickness. To understand these characteristics, we propose an eruption model that first involves processes operating in the magma chamber whereby a primitive melt is injected into an evolved magma to create a mixed zone at the chamber base. The eruption triggered by this event first emplaced a trachytic dome, into which banded lava from the chamber base was injected. Subsequent endogenous dome growth led to flow down the shallow slope to the east on which the highly viscous (1012 Pa s) coulee was emplaced. The flow likely moved extremely slowly, being emplaced over a period of 4–10 years in a glacial manner, where a thick (>60-m) plug slid over a thin (5-m-thick) basal shear zone. Excellent exposure means that the Puy de Cliergue complex can be viewed as a case type location for understanding and defining the eruption and emplacement of thick, high-viscosity, silicic lava flow systems.

Chapter 10 Distribution and significance of basaltic eruptive centres: São Miguel, Azores

Abstract Basaltic volcanism occurs at all the active volcanic systems of São Miguel. For the last 30 ka, the eruption of basaltic magma has been particularly significant in the areas between the three polygenetic volcanoes – Sete Cidades, Fogo and Furnas – and two basaltic fields have developed: the Picos Fissural Volcanic System (PFVS) and the Congro Fissural Volcanic System (CFVS). About 5 ka ago volcanic activity at CFVS ended abruptly and since then almost all basaltic activity has been concentrated at the PFVS, where about 30 eruptions have taken place. Despite the absence of eruptive activity, the Congro system represents one of the most active seismic areas in the archipelago. It shows episodes of volcanic deformation that are indicative of magma ascent, which halted beneath the volcanic system. Two historical eruptions are associated with the PFVS. The 1563 eruption at Pico do Sapateiro was of basaltic composition, whereas that of the 1652 eruption was, for almost two centuries, also considered to be basaltic. This event had, however, a Vulcanian style and involved the production of large amounts of fine ash and the growth of three trachyte domes with associated coulées. It was accurately described in contemporary historical accounts.

Structural study of the Foréké-Dschang trachytic dome (Mount Bambouto, West Cameroon): An anisotropy of magnetic susceptibility (AMS) approach

The Tertiary trachytic dome of Foréké-Dschang is located on the southwestern slope of Mount Bambouto. This dome, made of porphyritic and microlitic lavas which enclose sanidine, pyroxene and oxides, is dated at 16–8.8Ma and was emplaced in gneisses belonging to the Pan-African basement. The mean magnetic susceptibility magnitude Km values of the trachytic dome show a ferromagnetic behavior in 79% of the stations, likely due to the presence of more or less maghemitized titanomagnetite based on thermomagnetic and hysteresis data.The Foréké-Dschang dome is elliptical and strikes NE–SW, parallel to a border fracture. The Pan-African host gneisses display N70° striking and steeply dipping foliations crosscut by the dome. Some Tertiary sinistral reworking of these foliations may have induced the opening of a local tension gash in agreement with the regional model suggesting that the N30° Cameroon Volcanic Line opened as a mega-tension gash with respect to the N70° Adamawa fault zone. Two petrographic types are recognized in the trachytic dome, aphyric (domain I) or porphyritic (domain II). The magnetic foliations show an outward-dipping concentric pattern in domain I. A flat lying zone with the highest magnetic anisotropy (18%) is regarded as overlying the vent. The organization of magnetic fabrics suggests that domain II was emplaced before domain I. As a whole, the Foréké-Dschang trachytes represent a multilobate dome emplaced in a pulsatory manner.

Inner structure of the Puy de Dôme volcano: cross-comparison of geophysical models (ERT, gravimetry, muon imaging)

Abstract. Muon imaging of volcanoes and of geological structures in general is actively being developed by several groups in the world. It has the potential to provide 3-D density distributions with an accuracy of a few percent. At this stage of development, comparisons with established geophysical methods are useful to validate the method. An experiment has been carried out in 2011 and 2012 on a large trachytic dome, the Puy de Dôme volcano, to perform such a comparison of muon imaging with gravimetric tomography and 2-D electrical resistivity tomography. Here, we present the preliminary results for the last two methods. North–south and east–west resistivity profiles allow us to model the resistivity distribution down to the base of the dome. The modelling of the Bouguer anomaly provides models for the density distribution within the dome that are directly comparable with the results from the muon imaging. Our ultimate goal is to derive a model of the dome using the joint interpretation of all sets of data.

Possible Late Pleistocene volcanic activity on Nightingale Island, South Atlantic Ocean, based on geoelectrical resistivity measurements, sediment corings and 14C dating

Tristan da Cunha is a volcanic island group situated in the central South Atlantic. The oldest of these islands, Nightingale Island, has an age of about 18 Ma. In the interior of the island, there are several wetlands situated in topographic depressions. The ages of these basins have been unknown, and their genesis has been debated. Aiming towards the reconstruction of the geomorphological history of these basins, we conducted geoelectrical resistivity measurements to map the subsurface topography, extracted peat and sediment cores and dated the onset of sedimentation applying the radiocarbon method. The irregular shapes of the basins and the lack of clear erosional features indicate that they are not eruption craters and were not formed by erosion. Instead, we regard them as morphological depressions formed between ridges of trachytic lava flows and domes at a late stage of the formation of the volcanic edifice. The onset of sedimentation within these basins appears to have occurred between 24 and 37 ka with the highest situated wetland yielding the highest ages. These ages are very young compared to the timing of the main phase of the formation of the island, implying volcanic activity on the island during the Late Pleistocene.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Post-collisional potassic-rich volcanism of Gölcük Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Gölcük flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Gölcük lavas were derived from a metasomatized lithospheric mantle source containing phlogopite–amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Gölcük lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte–trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte–trachyandesites of the extracaldera volcanics. The Sr–Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Gölcük volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Gölcük lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19–19.54, 207Pb/204Pb = 15.64–15.67, 208Pb/204Pb = 39.12–39.49) indicate an enriched mantle source region.

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.70926 to 0.70999, 143Nd/144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for 206Pb/204Pb, 15.685–15.701 for 207Pb/204Pb, and 39.048–39.076 for 208Pb/204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87Sr/86Sr ranges from 0.70665 to 0.70745, 143Nd/144Nd ranges from 0.51234 to 0.51238, 206Pb/204Pb ranges from 18.924 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved.

Pleistocene eruptive chronology of the Gölcük volcano, Isparta Angle, Turkey. Chronologie des épisodes volcaniques pléistocènes du volcan Gölcük, Angle d’Isparta, Turquie

In the Eastern Mediterranean region, the Isparta volcanic belongs to the post-collisional alkali-potassic to ultrapotassic magmatism active since the Miocene in this part of the Anatolian peninsula from Afyon to Isparta. In the so-called Isparta Angle (IA) the magmatism is contemporaneous with an extensional regime intiated during Late Miocene and active throughout the Pliocene and Quaternary. Previous K/Ar dating performed on lavas suggested that potassic-ultrapotassic magmatism occurred between 4.7 to 4 Ma. However, a more recent (Quaternary) activity of the Golcuk volcano is evidenced by the present-day morphology and field evidence although it remained undated and poorly studied so far. Field mapping and new radiometric data indicate that the main volcano-forming stages of the Golcuk volcano consist of three main eruptives cycles. (1) Cycle I, represented by more than 200m-thick pyroclastic flow deposits occasionally separated by paleosoils and corresponding to caldera-forming ignimbritic eruptions. (2) Cycle II, consisting of tephriphonolite lava dome-flows extruded throughout the caldera and currently found along the rim of the present crater. (3) Cycle III made up of tuff-ring deposits related to several phreatoplinian eruptions of a maar-type volcanic activity. This youngest cycle ends with trachytic domes protruding within the maar crater. Unspiked 40K/40Ar dating on mesostasis was performed on lavas (tephriphonolites and trachytic domes), and complemented by preliminary 40Ar/39Ar data on tephra deposits (sanidine). Our preliminary results show that the entire activity of Golcuk volcano took place during the Pleistocene and was disconnected from the older Pliocene volcanism. This volcanic activity can be considered as a new volcanic cycle, starting (Cycle I) around 200 ka with major explosive, regional-scale, events represented by at least six ignimbrites sheets. Cycle II occurred between 115 ± 3 ka to 62 ± 2 ka with probably some associated tephra deposits. Tuff-ring of Cycle III formed from 72.7 ± 4.7 ka to 24 ± 2 ka. The associated phreatoplinian eruptions have almost entirely destroyed the previously formed flow-dome. This latest activity corresponds to several volcanic crises as illustrated by the two domes protrusions separated by about 30 ka. The volcanic history of Golcuk ceased around 24 ka ± 2 ka, but the periodicity of eruptive events appears to be long and complex. Currently, the volcano is at rest, but there is no doubt that the Isparta town (more than 120 000 people) built on top of the most recent tephra falls is exposed to a major volcanic hazard in the future.

Volcanic history and 40Ar/ 39Ar and 14C geochronology of Terceira Island, Azores, Portugal

Seven new 40Ar/ 39Ar and 23 new radiocarbon ages of eruptive units, in support of new geologic mapping, improve the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores and define an east-to-west progression in stratovolcano growth. The argon ages indicate that Cinco Picos Volcano, the oldest on Terceira, completed its main subaerial cone building activity by about 370–380 ka. Collapse of the upper part of the stratovolcanic edifice to form a 7 × 9 km caldera occurred some time after 370 ka. Postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera. The southern portion of Guilherme Moniz Volcano, in the central part of the island, began erupting prior to 270 ka and produced trachyte domes, flows, and minor pyroclastic deposits until at least 111 ka. The northern part of Guilherme Moniz Caldera is less well exposed than the southern part, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44 ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano. The Pico Alto Volcanic Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and our new radiocarbon ages suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000 years BP. Santa Barbara Volcano is the youngest stratovolcano on Terceira, began erupting prior to 29 ka, and has been active historically.

Concordant thermoluminescence and 238U– 230Th ages for a trachytic dome (Grand Sarcoui) from the Chaı̂ne des Puys (French Massif Central)

This paper presents the results of thermoluminescence (TL) and 238U– 230Th datings of a trachytic dome from the Chaı̂ne des Puys in the French Massif Central, the Grand Sarcoui dome. TL dating was performed on quartz xenocrysts extracted from the phreatomagmatic tephra that initiated the eruptions. Red TL and blue TL give consistent results with an average age of 12.6±3 ka (2 σ). A 238U– 230Th isochron age is calculated using data obtained on groundmass and zircon: this corresponds to a crystallization age of 12.8±1.4 ka (2 σ). The excellent agreement between the ages derived from both methods indicate that the interval between zircon crystallization in the magma chamber and eruption was probably very short (a few centuries at most). This and other geochemical arguments suggest that the differentiation (through fractional crystallization accompanied by crustal contamination) of the trachytic magmas of the Chaı̂ne des Puys took place rather quickly in small and separate shallow reservoirs, only a few km apart. The eruption of trachytic magma was restricted to a period between 13 and 9 ka, near the end of eruptive activity in the central part of the Chaı̂ne des Puys.

From seamount to oceanic island, Porto Santo, central East-Atlantic

The uplifted and deeply eroded volcanic succession of Porto Santo (central East-Atlantic) is the product of a wide spectrum of dynamic processes that are active in shoaling to emergent seamounts. Two superimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous submarine to subaerial phreatomagmatic explosions, pyroclastic fragmentation being subordinate. The lower basaltic and the upper mugearitic to trachytic sections are dominated by redeposited tephra and are called 'lapilli cone aprons'. Vertical growth due to accumulation of tephra, voluminous intrusions, and minor pillowed lava flows produced ephemeral islands which were subsequently leveled by wave erosion, as shown by conglomerate beds. Periods of volcanic quiescence are represented by abundant biocalcarenite lenses at several stratigraphic levels. The loose tephra piles became stabilized by widespread syn-volcanic intrusions such as dikes and trachytic to rhyolitic domes welding the volcanic and volcaniclastic ensemble into a solid edifice. Shattering of a submarine extrusive trachytic dome by pyroclastic and phreatomagmatic explosions, accentuated by quench fragmentation, resulted in pumice- and crystal-rich deposits emplaced in a prominent submarine erosional channel. The dome must have produced an island as indicated by a collapse breccia comprising surf-rounded boulders of dome material. Subaerial explosive activity is represented by scoria cones and tuff cones. Basaltic lava flows built a resistant cap that protected the island from wave erosion. Some lava flows entered the sea and formed two distinct types of lava delta: 1. closely-packed pillow lava and massive tabular lava flows along the southwestern coast of Porto Santo, and 2. a steeply inclined pillow-hyaloclastite breccia prism composed of foreset-bedded hydroclastic breccia, variably-shaped pillows, and thin sheet flows capped by subhorizontal submarine to subaerial lava flows along the eastern coast of Porto Santo.The facies architectures indicate emplacement: 1. on a gently sloping platform in southwestern Porto Santo, and 2. on steep offshore slopes along high energy shorelines in eastern Porto Santo.Growth of the pillow-hyaloclastite breccia prism is dominated by the formation of foreset beds but various types of syn-volcanic intrusions contributed significantly. Submarine flank eruptions occurred in very shallow water on the flanks of the hyaloclastite prism in eastern Porto Santo. The island became consolidated by intrusion of numerous dikes and by emplacement of prominent intrusions that penetrate the entire volcanic succession. Volcanic sedimentation ended with the emplacement of a debris avalanche that postdates the last subaerial volcanic activity.

Geología y vulcanología de La Palma y El Hierro, Canarias Occidentales

Geología y vulcanología de La Palma y El Hierro, Canarias Occidentales

A morphotectonic study of an extensional fault zone in a magma-rich rift: the Baringo Trachyte Fault System, central Kenya Rift

The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N–S-trending linked extensional fault network. This fault system bounds to the west the 8 km deep Baringo Basin which itself lies within the axial valley of the central Kenya Rift. It mainly affects a middle Pleistocene trachytic dome (510 ka), the so-called Baringo Trachyte (BT). A morphotectonic study of the 10 km long BT master fault and associated downthrow geometries provides constraints on the evolution of a magma-type rift fault system from an initial stage of crack opening through to propagation. A model of radial fault growth is proposed in order to account for the longitudinal segmentation of the main fault escarpment from the median part to the tips. The small-scale half-graben geometry developed in the median high-strain zone is progressively accommodated laterally by both flexure and related narrow compensation grabens. The resulting crack swarms are well-developed at the free southern tip zone. Both the spatial distribution of rock-breaking products and their relations to the immediate hangingwall provide further evidence for this hypothesis. Well-developed screes and other gravity-driven structures (slumps) preferentially occur along the median part of the Baringo Trachyte Fault Escarpment, probably as earthquake-induced features. The hangingwall fault zone shows an asymmetrical triangular-shape with a maximum width of about half the length of the main scarp. This zone of maximum deformation and subsidence appears to be laterally controlled by two major, conjugate, transverse basement discontinuities lying with a conjugate geometry. Its internal architecture is dominated by antithetic westerly-dipping normal faults bounding discrete half-grabens, locally infilled by syn-tectonic volcaniclastics. Chronological data on hydrothermal silica filling open cracks on the BT footwall suggest that the master fault evolution occurred from 345 to 198 ka, as the possible result of at least four major normal faulting earthquakes.

The mineralogy and geochemistry of a subvolcanic alkaline complex from the Cameroon Line: The Nda Ali massif, southwest Cameroon

The Nda Ali massif is one of the 60 Tertiary pluntonic anorogenic complexes known as ‘granites ultimes’ of the Cameroon Line. It forms a plutono-volcanic complex comprising a gabbro-diorite-monzonite-syenite suite and associated basanite-trachyte-phonolite volcanic dykes and lava flows. The mineralogical and geochemical data indicate: 1. i) Alkaline affinity of both plutonic and volcanic rocks. 2. ii) Early development of peralkalinity in syenites marked by the presence of aegirine (only one sample contains normative acmite); its crystallization after alkali amphibole indicates that the high activity of sodium may be the controlling factor. 3. iii) Penecontemporaneity of trachytes and basanite dykes with plutonic rocks; both are chemically similar. The other lavas form a later volcanic sequence with both undersaturated (tephri-phonolite to phonolites) and saturated (only represented by trachyte domes) trends. 4. iv) Evolution from an undersaturated alkaline magma near syeno-monzonitic or basanitic composition; both rocks have 100× Rb Ba ratios (6.7 and 6.5) in the range of those obtained for mantle liquids. This evolution is dominated by fractional crystallization, the process involving feldspars, pyroxenes, amphiboles and oxides together with apatite for plutonic rocks and sphene for the volcanic sequence. 5. v) Moderate and negligible crustal contamination, respectively, for plutonic rocks and associated dykes and for the undersaturated volcanic trend. Isotopic data, not yet available, are needed here to quantify the effect of crust involvement.

Geological setting and tectonic significance of Mississippian felsic metavolcanic rocks in the Pelly Mountains, southeastern Yukon Territory

The central Pelly Mountains in southeastern Yukon Territory consist of imbricate thrust sheets, which have undergone syn- and post-thrusting deformation and metamorphism. The local geology is further complicated by the intrusion of Late Cretaceous batholiths, and by strike-slip faulting related to the Tintina Fault, a major northwest-trending transcurrent fault of latest Cretaceous or early Tertiary age. This faulting disrupts the northeast edge of the study area.Upper Devonian and Mississippian strata are present in at least two of the structural packages, but the Mississippian metavolcanic rocks occur only in the lowermost package. Rb–Sr geochronology indicates a mid-Mississippian age for the igneous suite. The volcanic rocks consist of volcaniclastic material with minor interbedded flows, and were deposited in a submarine environment. Several coeval and cogenetic syenite and trachyte domes and small stocks are the remains of vent areas. Although the volcanic rocks are all highly altered and show evidence of widespread chemical mobility, trace element data indicate that the rocks are metaluminous trachytes, most closely resembling peralkaline volcanics generated in extensional environments. This suggestion of a predominantly extensional tectonic setting in mid-Mississippian time in the Pelly Mountains is consistent with recent tectonic syntheses for the area.