Origin and diagenesis of the Roque Nublo breccia,Gran Canaria (Canary Islands) — Petrology of roque nublo volcanics,II

The Pliocene Roque Nublo Series, the second of three major magmatic series on Gran Canaria (Canary Islands), consists of a lower sequence (200 m) of alkalic lavas (basanite to phonolite) and a thicker upper section (600 m) of interlayered lava and widespread breccia sheets: encompassing pyroclastic flow deposits, lahars and reworked epiclastic rocks. Components in the poorly sorted block — and ash — flow deposits are (unwelded) pumice, rock fragments, crystals, glass shards and, locally, bread-crust bombs. Some flow units are graded with fine-grained basal zones and lithic-rich lower and pumice-rich upper parts. Some have strongly grooved the underlying rocks, directions of these striations being independent of preexisting topography and are constant in direction for more than 5 km. The flows are thought to have been emplaced below minimum welding temperatures by collapse of eruption columns. They are similar in many respects to coarse-grained pyroclastic flow deposits found in andesite volcanoes. Glass of tephritic to phonolitic composition of clasts of the breccias is generally altered to «palagonite» and is partly replaced by clay minerals and zeolites (mainly chabazite and phillipsite). Palagonitization was a low temperature diagenetic process, resulting in the hydration of glass accompanied and followed by precipitation of zeolites and clay minerals. Electron-microprobe data suggest the following decreasing order of mobility of selected elements during palagonitization: Na, K, Al, Si, Ca, Mg, and Fe; Ti was assumed to be inert.     

Geochronology of Gran Canaria,Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km³ was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km³) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km³ of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km³. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.     

Ignimbrites of the Roque Nublo group, Gran Canaria, Canary Islands

 Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55% volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts for the intense zeolitic alteration in these deposits. Received: 5 November 1996 / Accepted: 3 March 1997     

Petrology of clinopyroxene-amphibole inclusions from the roque nublo volcanics,gran canaria,canary islands

Inclusions consisting of clinopyroxene, amphibole, Fe-Ti oxides and apatitc are abundant in the Roque Nublo volcanics, a unit of Late Tertiary age that is widespread on Gran Canaria Island. The unit includes alkalic basalts and breccias. Mafic minerals in several inclusions and in one basalt host have been analysed with the electron microprobe. Although the inclusions vary in size, texture and mineralogy, they show certain common teatures. The pyroxenes analyzed are all salites-augites and their position in the Ca-Fe Mg quadrilateral suggests that they are early formed representatives of the pyroxene crystallization trend characteristic of alkaliolivine basalt. The amphibole is invariably kaersutite. A common variety of inclusion is composed largely of kaersutite and titaniferous clinopyroxene. The kaersutite (TiO₂ 5.27%, K₂O 1.58%) is homogeneous, except for slight iron enrichment in the margins of crystals. The clinopyroxene is an hourglass-zoned, brownish titansalite, Ca 50 Mg 35 Fe 15, TiO₂ 3.08%, with a green core of Ca 49 Mg 38 Fe 13, TiO₂ 2.15%. Compositions of coexisting titanilerous magnetite and ilmenite, Usp 44 Mt 56 and Ilm 85 Hem 15, respectively, indicate they formed at approximately 975°C and pO, 10^?10.5 atm. In another type of inclusion and its host basalt, pyroxene relations are more complex. Inclusion pyroxene is markedly but diffusely zoned. Predominant is a green salite, Ca 47 Mg 38 Fe 15, TiO₂ 1.11%, which has small, patchy core zones of brownish. Ti-rich salite. Ca 48 Mg 35 Fe 17, TiO₂ 1.94%. Cores of crystals in the host basalt are Ca 47 Mg 41 Fe 12, TiO₂ 2.23%; rims are pale green, Cr-rich diopsidic augite, Ca 44 Mg 45 Fe 11, TiO₂ 1.32%, Cr₂O₃ 0.48%. This «reverse» Fe-Mg zoning is attributed to increasing partial pressure of oxygen as crystallization proceeded. Kaersutite similar to that mentioned above occurs in both the inclusion and its host, in which it is highly resorbed. The available field and analytical evidence strongly suggests that the inclusions and the associated basalts are genetically related. Resorption of the kaersutite at depth may have given rise to the alkalic basalts of the Roque Nublo series.     

Explosive felsic volcanism on El Hierro (Canary Islands)

The Canary Islands consist of seven basaltic shield volcanoes whose submerged portion is much more voluminous than the subaerial part of each island. Like so many other volcanic oceanic islands, the indicative deposits of explosive felsic volcanism are not a common feature on the Canary archipelago. Hitherto, they have only been documented from the central islands of Gran Canaria and Tenerife, which are the largest volcanic complexes of the islands. On the other Canary Islands, the presence of felsic rocks is mostly restricted to intrusions and a few lava flows, generally within the succession in the oldest parts of individual islands. In this paper, we present a detailed stratigraphic, lithological and sedimentological study of a significant felsic pumice deposit on the island of El Hierro, referred here as the Malpaso Member, which represents the only explosive episode of felsic volcanism found on the Canary Islands (outside of Gran Canaria and Tenerife). The products of the eruption indicate a single eruptive event and cover an area of about 15 km². This work provides a detailed stratigraphic and chronological framework for El Hierro, and four subunits are identified within the member on the basis of lithological and granulometric characteristics. The results of this study demonstrate the importance of an explosive eruption in a setting where the activity is typified by effusive basaltic events. Given the style and the spatial distribution of the Malpaso eruption and its products, a future event with similar characteristics could have a serious impact on the population, infrastructure and economy of the island of El Hierro.     

The tahitites of gran canaria and haüynitization of their inclusions

Na-alkaline lavas of the eruptions related to two centers of emission (Montaña Arucas and Montaña Cardones), on the northern coast of the Grand Canaria island, are described in detail as well as their relationships with the other volcanic formations of the area. It is concluded that they are of Tyrrhenian age. The petrology and chemistry of these lavas of tahititic character are studied, as well as their genesis. Inclusions in tahitites from Gran Canaria have also taken into consideration, and the transformations they suffered and the processes involved are studied and discussed.     

The somma-vesuvius magma chamber: a petrological and volcanological approach

The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km³. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy.     

Genetic aspects of the Jan Mayen fissure volcano group on the mid-oceanic submarine Mohns Ridge,Norwegian Sea

During six recent expeditions, of which four were led by the author, to the mainly basaltic island of Jan Mayen (length 53.6 km; mean width 7 km; area 380 km²), evidence has been gathered for at least six distinct volcanic phases, coupled with rythmic magmatic variations in the oceanite-trachybasalt-trachyandesite-trachyte lava suite. There are also certain intermediate types and associate pyroclasts, and effusive or explosive uprise of these lavas through two fissure-swarms, intersecting at about 12°, produced a subaerial volcano-group of several hundred cones, elongate north-east — south-west on the north-west margin of a large submarine pedestal possibly capped by a drowned plane of marine erosion at 100–200 m below present mean sea level. These rocks appear to range in age between Tertiary and Recent. Jan Mayen grows from the north-west flank of the submarine Mohns Ridge close to its axial rift within a markedly seismic zone, at a likely junction of crustal fractures immediately north of a sharp east-west flexure in the rift which may indicate a major strike-slip fault. The lavas have affinities with corresponding lavas in Scottish Hebrides and with the basalt-trachyte associations on the islands of Ascension, St Helena, Tristan da Cunha and Gough on the mid-Atlantic Ridge. Both form and structure suggest the island mass has the configuration of a volcanic dome (or possibly two coalescent domes diverging slightly south-west) at least 70×30 km in area and about 1.5 km in height. In the mass are two distinct major volcanic foci: an earlier South Jan or Rudolftoppen « dispersed » or « plexiform » vent, ascribable to numerous «drilled out» fissure-intersections within an area of more than 25 km², and a later North Jan or Beerenberg central vent. A third focus of indeterminate relative age may lie beneath Straumflaket, in the shallow sea off South Cape. Magmas rose through individual fissures and their intersections, to form linear cones of tuff and lava, and extensive basalt floods. Most are vertical dikes but, in places, highly inclined sheets and sills tend to follow bedding and other planes of weakness in tuff and sometimes fed lava flows. Basaltic magma invaded a complex system of intersecting master fissures and subsidiary fractures in tuff near the surface, inflated the mass, distorted and generated local joint systems in the tuff and finally gave rise to meshworks of basaltic sheets in it. Following a long period of repeated fissure eruption, ten of the main basaltic throats at the South Jan dispersed focus, and one near the junction between North Jan and South Jan, were plugged by trachyte, after which there was volcanic quiescence with contemporaneous deep glacial, fluvial and marine erosion. During the subsequent resumption of volcanic activity the North Jan focus of central eruption rose to importance at the expense of the South Jan focus, which remained sealed by trachyte, but numerous small basaltic fissure volcanoes erupted on the seaward edges of the South Jan plateau and through the coastal platform beneath its cliffs, at or near sea level.     

A comparison of the recent eruptions of Mt. Pelée,Martinique and Soufrière,St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.     

The interdependence of seismic and volcanic phenomena: Some space — Time relationships in seismicity and volcanism

Research is described aimed at detecting significant relationships between patterns of earthquakes and volcanic eruptions. A large data file of seismic and volcanic events has been assembled on punched cards, and two computer programmes used to relate earthquakes to active or potentially active volcanic centres, and deep earthquakes to succeeding shallower ones and volcanic eruptions after the manner suggested byBlot. It is concluded that recognisable sequences occur in some areas, but that the process is neither universal nor invariable. The presence of random events or events due to other causes almost always obscures patterns of seismicity which seem to be related to volcanic eruptions. The «Blot process» appears to operate in some cases but is probably a second order phenomenon: primarily, correlated sequences of seismic and volcanic events result from periods of tectonic instability and perhaps increased tensional conditions which affect very wide areas of the Earth’s surface for periods of several months to several years at a time.     

New K-Ar ages,chemical analyses and magnetic data of rocks from the islands of santa maria (azores), porto santo and madeira (madeira archipelago) and gran canaria (Canary Islands)

K/Ar-determinations, major and trace element chemical analyses and magnetic data are reported for rocks from Santa Maria (Azores), Madeira and Porto Santo, and Gran Canaria. Based on these data, the age of the basement of Santa Maria is believed to have formed between about 5.2. and 4.6 m.y.; the unconformably overlying pillow complex interbedded with fossiliferous calcarenites about 3.8 to 3.3 m.y. with the capping subaerial basalt being part of the same magmatic phase. The major erosional phase levelling the basement is thus approximately synchronous with the major Pliocene regression (R2) on Gran Canaria (Lietz andSchmincke, 1975), possibly indicating a widespread eustatic event. The upper part of the submarine, partly fossiliferous series of Porto Santo was dated as ca. 12 to 13 m.y., and the Quaternary age for the major basalt formation in Eastern Madeira (Watkins andAbdel Monem, 1971) is confirmed. The ages of several formations on Gran Canaria were slightly revised. Chemical differences between basement (shield) and later posterosional series on Gran Canaria are re-emphasized by the new data, while such differences are much less pronounced between the basement and younger series on Santa Maria.     

Volcanic ash hazard in the Central Mediterranean assessed from geological data

Volcanic ash produced during explosive eruptions can have very severe impacts on modern technological societies. Here, we use reconstructed patterns of fine ash dispersal recorded in terrestrial and marine geological archives to assess volcanic ash hazards. The ash-dispersal maps from nine Holocene explosive eruptions of Italian volcanoes have been used to construct frequency maps of distal ash deposition over a wide area, which encompasses central and southern Italy, the Adriatic and Tyrrhenian seas and the Balkans. The maps are presented as two cumulative-thickness isopach maps, one for nine eruptions from different volcanoes and one for six eruptions from Somma-Vesuvius. These maps represent the first use of distal ash layers to construct volcanic hazard maps, and the proposed methodology is easily applicable to other volcanic areas worldwide.     

Pumice eruptions of the lesser Antilles

Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue.     

Phreatic eruption clouds: the activity of la Soufrière de Guadeloupe,F.W.I., August — October, 1976

From August to October, 1976, La Soufrière de Guadeloupe was observed, and recorded with an automated sequence camera and numerous handheld cameras. During the period of observation, the nature of volcanic activity ranged from mild steam emission to moderately energetic phreatic eruptions. Background fumarolic activity (steam emission) was characterized by the emission of generally tephra-free steam clouds 50 to 150 m above the summit. The clouds rose buoyantly above the vent and were blown downwind at prevailing wind velocities. Phreatic eruptions were well-documented on September 22, October 2, and October 4. In the latter two eruptions, small bursts of tephra-laden steam erupted at intervals of 30 to 45 min, and rose from 350 to 500 m above the summit. In the largest observed eruption, that of October 2, the steam and tephra cloud rose to a maximum height of 600 to 650 m in 20 min. A white vapor cloud and a medium gray, tephra-laden cloud were erupted simultaneously from the summit vent and both were surrounded by a vapor collar: the clouds were thoroughly mixed within 1 km downwind of the summit. The concurrent growth of clouds from separate vents (summit and flank) implies a common source. Simultaneous eruption of tephra-free and tephra-laden clouds from the same vent is puzzling and implies: (i) lateral changes in the degree of alteration of dome rocks along the elongate vent, hence erodability of the dome lavas, or (ii) differences in the gas velocities. These «mixed» clouds moved westward, downwind and downslope as a density current, along the watersheds of the R. Noire and R. des Pères with an approximate velocity of 10 to 25 m/sec. Upon reaching the sea the clouds continued to move forward, but at a decreased velocity, and spread laterally, having left behind the restrictions of valley walls. A thin gray veneer of moist tephra, ranging from several cm thick near the dome to less than 1 mm thick several km downwind, was deposited along a narrow corridor southwest of the summit. Tephra from the phreatic eruptions consisted mostly of hydrothermally altered lithic, mineral, and glass fragments derived from dome lavas; no fresh (juvenile) pyroclasts were present in the tephra. Absence of juvenile tephra at La Soufrière supports the view that activity was due to groundwater circulating in a vapor-dominated geothermal system, probably driven by a shallow heat source. At La Soufrière, most vapor-dominated systems are located in elevated areas of groundwater recharge where groundwater movement is downward and outward. The sporadic phreatic eruptions may be related to the rate of recharge of meteoric waters within the dome, the decrease in pore pressure during fortnightly tidal minimums or both. Whatever the triggering mechanism, vapor-dominated fluids eroded vent walls during phreatic eruptions and carried out fine-grained, hydrothermally altered, pre-existing dome material as tephra.     

Carbonatite-alkalic complex of Panwad-Kawant,Gujarat, and its bearing on the structural characteristics of the area

Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.     

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:

The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.

The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).

Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T^o;fO₂ andpH₂O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO₂) between the magmas, mixing involves:

mechanical transfer of phenocrysts from one component to another, in both directions;

volatile transfer to the basaltic xenoliths, with chemical exchanges.

It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.     

Characteristics of widespread pyroclastic deposits formed by the interaction of silicic magma and water

We have recognized a type of pyroclastic deposit formed by the interaction of water and silicic magma during explosive eruptions. These deposits have a widespread dispersal, similar to plinian tephra, but the overall grain size is much tiner. Several deposits studied can be associated with caldera lakes or sea water and water/magma interaction is proposed to account for the fine grain size. Several examples have been studied, including the Oruanui Formation, N.Z., and the Askja 1875 deposit. Both show little downwind decrease in median diameter, a downwind decrease in sorting (σ_φ) (more evident in the Askja deposit) and coarse tail grading. The Askja example has base surge deposits near source and some Oruanui members show multiple thin beds near source; both are common features of phreatomagmatic deposits. Isopachs of the Askja deposit indicate a source under Lake Oskjuvatn in Askja Caldera and those of the Oruanui indicate a source under the NW part of Lake Taupo. In terms of dispersal area, volume and calculated eruption column heights, these deposits are similar to plinian. However, their extreme fragmentation due to magma/water interaction, superimposed on fragmentation imparted by carlier vesiculation, gives a much finer and more complex grain size distribution than plinian counterparts. The field of phreatomagmatic equivalents to plinian pumice deposits was unoccupied onWalker’s (1973) classification of explosive volcanic eruptions. Such deposits are the phreatomagmatic analogue of plinian deposits and the name « phreatoplinian » is proposed.     

Peralkaline acid volcanic rocks of oceanic islands

Occurrences of peralkaline acid volcanic rocks on oceanic islands are reviewed. Peralkaline differentiates are usually associated with mildly alkaline or transitional basalts and often with related sodic intermediate rocks. A compositional gap between basaltic and salic rocks is not invariably present. Although a comenditic end member is more usual in the oceanic suites, pantellerites are particularly well developed on Socorro Island and also on Gran Canaria where they form extensive ignimbrite sheets. There may be a genetic distinction between peralkaline rocks of islands which lie near the crests of oceanic rises and those which are built on broad submarine plateaux.     

Small scale arcuate intrusions on Saint Helena,South Atlantic

Sixteen arcuate intrusions have been emplaced at extremely high levels into the basaltic shield volcanoes of Saint Helena. These intrusions are of special interest because of their small size and modes of emplacement. The arcuate masses are of three distinctive types:

1. Irregular, steeply inward-dipping, basic sheets with diameters of 150 m to 450 m infill tensional fractures originating at depths of about 500 m beneath the volcano surface.
2. Strongly curved sheets from 25 m to 750 m in diameter are cross-sections of inclined intrusions which in three dimensions resemble single sticks of celery. These intrusions, varying from basalt to trachyte in composition, are infilled tensional fractures originating at « point » pressure sources, inclined to the horizontal, at depths of about 500 m below the surface.
3. Salic intrusions with near-vertical sides and gently inclined roof-infillings have outer diameters of 350 m to 1070 m. Pressure exerted onto the flanks of the volcano by domed, convex upwards, areas of a magma chamber roof, at a depth of about 2 km, caused near-vertical ring fractures to form. Formation of a sub-horizontal cross fracture and subsequent intrusion of magma produced the « roof-infillings » by updoming the overlying basalts or sinking of the enclosed block, or combinations of the two processes. Two intrusions of this third type are multiple.

     

Volcan ceboruco: A major composite volcano of the mexican volcanic belt

Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO₂=58–61%) to acid dacite (SiO₂=68%) in composition. The rate of increase of K₂O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.