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1.
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 To;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.  相似文献   

    2.
    Jan Mayen is a small isolated Arctic island located on or very near the junction of the southern end of Mohns ridge and the central part of the Jan Mayen fracture zone. The alkaline petrology and tectonic setting of Jan Maven are similar to some other volcanic islands along the mid-Atlantic ridge, particularly the Westmann Islands of Iceland. Both the Westmann Islands and Jan Mayen are underlain by a relatively thick oceanic(?) crust, about 15 km thick, and recent eruptions were initiated and accompanied by earthquakes with loci from 25 to 30 km deep. The northern hall of the island is dominated by Mt. Beerenberg, a major central volcano composed mainly of alkali olivine basalt. The oldest exposed lavas are of upper Pleistocene age. Flank eruptions on Beerenberg in 1732, 1818 and 1970, together with historic and cartographic evidence for similar eruptions in the 16th century and the middle of the 19th century, suggest an eruption frequency of 150 years ± 75 years. A three-station seismograph network, six surface tilt-measuring sites, and a gravity and levelling profile comprise the main features of the present surveillance system.  相似文献   

    3.
    The eruption commenced on July 7th 1963 with activity at the summit crater which had been dormant for at least 50 years. Production of lava spatte r characterised the opening stages of the eruption, and although hot lava blocks avalanched down the north-eastern slope no flows were produced. In August a crater opened at a height of approximately 1,000 metres at the head of a north-west trending fissure, the site of the 1960 eruption. Intermittent lava fountaining up to a height of 600 feet took place at the crater which was active throughout the remainder of the eruption, and viscous steep-sided tongues of «aa» lava flowed from it. A new east-west trending fissure 200 feet deep and 400 feet wide opened in September at a height of approximately 240 metres and extended up the slope to a point approximately 660 metres above sea level. From this fissure lavas of more fluid character though identical in mineral composition to tongues issuing from the flank crater flowed into the sea until mid November when activity at the fissure ceased. Whilst the fissure was active gas issued from a vent located immediately beyond it’s uper end. The slopes above the anchorage at Tematu were the site of subsidiary activity. Four small fissures opened at heights of up to 180 metres above sea level from mid-October to February 1964 producing short tongues of «aa» lava which flowed into the water. Emission of small ash clouds at sporadic intervals was noted at a crater situated in the highest fissure during a visit in December, 1963. There was a change from activity of «Strombolian type» with associated production of lava flows at the flank crater from November 1963 when the proportion of ash emitted increased. Ash emission became the predominant type of activity throughout the remainder of the eruption. Although the interval between successive outbursts lengthened progressively during 1964 the activity reached a climax on April 8th when the ash column attained a height of 30,000 feet, the maximum recorded during the course of the eruption. There was also an increase in July culminating in the production of a dense ash cloud 15 miles in diameter on the 26th. The activity entered a new phase in July 1964 when fissures producing lava tongues opened not only on the northern slopes but on the east side of the volcano as well. Activity continued on the opposite side to the north-west quadrant in which it had previously been localised when a fissure with a small crater at it’s head appeared in September on the south-east slopes a few hundred metres above sea level. The infrequency of outbursts during 1965 suggests that the present cycle of activity is waning, and that the volcano will soon become quiescent once more. Structures of interest in the lava flows include channels and tunnels. Hypersthene andesite was produced simultaneously with tholeiitic olivine bearing basalt during the opening stages of the eruption although the lavas produced later were all of the latter type. It is suggested that the hypersthene andesite was formed by magmatic differentiation of an olivine-bearing basalt parent magma, the lighter more acid fraction being tapped first at the beginning of the eruption. Such differentiation could account for similar basalt-andesite associations in older volcanic sequences within the central area.  相似文献   

    4.
    Equilibria calculations of high-temperature volcanic gases from lava lakes are carried out on the basis of best volcanic gas samples. The equilibrium gas composition at temperatures from 800° to 1400°K and pressures up to 25 kilobars (in ideal gas system) was calculated using the free energy minimization model as well as the Newton-Raphson methods. It is shown that the juvenile «magmatic gas » of basaltic magma consists of three components: H2O, SO2, CO2; the water vapor being about 60%. The increase of temperature under constant pressure results in the increase of the SO2 concentration and in the simultaneous decrease of H2S. Under the same conditions the ratios CO/CO2 and H2/H2O are found to increase. Methane cannot be a component of «magmatic gas» corresponding to the elemental composition of basaltic lava gases. The calculated values of \(P_{O_2 } \) are in good agreement with the experimental data obtained from direct measurements of \(P_{O_2 } \) in lava lakes and experiments with basaltic melts.  相似文献   

    5.
    The Superior volcanic field occupies approximately 8,000 square kilometers of central Arizona in the zone between the southern Basin and Range Province and the Colorado Plateaus Province. The primary structural elements of an eruptive center in the western part of this field are: 1) volcanic plateau, 2) ring fracture zone, and 3) resurgent caldera core. A northwest trending graben controls the location of three small subsided blocks, the Willow Springs cauldron (2 km diameter), the Black Mesa cauldron (4 km diameter), and the Florence Junction cauldron (8 km diameter), which were centers for rhyolite ash and lava eruption. These late features are superimposed on a much larger volcano-tectonic structure, the Superstition resurgent cauldron which subsided at an earlier stage following the extrusion of quartz latite welded tuff. The history of the volcanic center is as follows: An early ring of dacite domes of up to 900 meters in relief formed a semi-circular are 7 km in diameter on the western margin of the caldera. The last phases of dome building were contemporaneous with the extrusion of a vast quartz latite welded tuff (22.6 m.y.). The plateau formed by the welded tuff collapsed to a maximum depth of 800 meters along a northwest trending graben which is the locus of three small cauldrons. These late cauldrons were the source of rhyolitic magma which produced non-welded ash flows, lava (21 m.y.), and a thick sequence of epiclastic breccias. The rhyolitic volcanism was followed by intrusion of domes and extrusion of glassy lavas (20 m.y.) of quartz latite composition in a 270° are 16 km in diameter concentric to the arc of older dacite domes. Following deposition of the epiclastic breccia and intrusion of the ring fracture dikes was the extrusion of mafic lava (18 m.y.) into low places in the graben. The mafic lava composition ranges from basalt to basanite.  相似文献   

    6.
    The Managalase Plateau in north-east Papua is a faultbounded block of fractured basement metabasalt and basic plutonic rock overlain in the east by some thirty small volcanic centres of late Pleistocene to Recent age which include rhyodacite ash cones and trachybasalt and basaltic latite lava. To the north andesitic strato-volcanoes of Pleistocene and Recent age occur along the margin of the Cape Vogel geosyncline, whereas to the south a series of basaltic latite, alkali basalt and ultra-alkaline lava of Pliocene age margin the Papuan basic-ultrabasic belt. There is a transition, probably by fractional crystallization from alkali basalt through trachybasalt towards trachyandesite, but the basaltic latite and ultraalkaline lava are aberrant offshoots, possibly owing to contamination by phyllite. The « orogenic andesite » shows disequilibrium relations among the minerals including complexly zoned and twinned plagioclase, and common xenocrysts and xenoliths. Their chemistry can be simulated by mixtures of rhyodacite with either alkali basalt or trachybasalt and they are richer in both magnesia and potash than other calc-alkali rock suites. The rhyodacites, however, are of normal cale-alkali character.  相似文献   

    7.
    New K-Ar dates on Ischia volcanic rocks are reported in order to reconstruct the temporal evolution of volcanic activity and to estimate the rate of associated energy release. The most probable age of eruption of the « green tuff » pumice flows forming the horst in the central part of the island is 0.74 (±0.09) m.y. The formation of the horst occurred in the time span 0.7–0.3 m.y. The subsequent volcanic activity was episodic. The oldest formations (0.37–0.31 m.y.) were formed by lava lake activity with small energy content, whereas formations younger than 0.1 m.y. were formed by eruptions characterized by higher explosivity and energetic content. No ages in the interval 0.31 (±0.02) ?0.10 (±0.07) have been found. Geological considerations also suggest that volcanic activity was very weak or absent in Ischia during this time interval. The different energies and characteristics of the eruption at Ischia in relation to the tectonic pattern are used to evaluate the volcanic risk for the island.  相似文献   

    8.
    The influence of volcanic processes on magmatic differentiation can be evidenced by the study of some of the most typical volcanoes of post-orogenic magmatism of Central Italy. It has been recognized that a close relationship exists between degree and type of differentiation on one hand, and structure and evolution of volcanic edifices as well as shape of their magmatic chambers on the other. The effect of the structural features of volcanic apparata on the magmatic differentiation is often so strong as to obliterate the original genetic characters of the magma. It was seen that, in Central Italy, magmas of «atlantic» affinities differentiating from basalt to trachyte, can turn to magmas of strong « mediterranean » affinities in the more superficial volcanic environments.  相似文献   

    9.
    The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits.It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.  相似文献   

    10.
    Geology of the peralkaline volcano at Pantelleria,Strait of Sicily   总被引：1，自引：1，他引：1  
    Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode.The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level.Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13–6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.  相似文献   

    11.
    Hyaloclastites are volcaniclastic rocks generated by non-explosive granulation of volcanic glass which takes place when basaltic magmas are quenched by contact with water; hyaloclastites are common products of deep submarine basaltic central volcanoes (seamounts and guyots). We suggested (Honnorlz, 1966) calling hyalotuffs the actual pyroclastic rocks which are generated by phreatomagmatic and phreatic explosions taking place when basaltic volcanoes crupt in shallow waters; hyalotuffs are restricted to shallow subaqueous conditions since no volcanic explosion can occur in deep seas. The distinction between hyaloclastites and hyalotuffs is therefore a useful tool when reconstructing the paleogeography of ancient submarine volcanic edifices and the mechanism by which their lavas were emplaced. We propose using two sets of morphometric parameters to discriminate the hyaloclastites from the hyalotuffs. The granules making up these two volcaniclastic rock types plot in different areas of either 1) a ternary diagram the apexes of which represent the grain planarity (P), convexity (V) and concavity (C) % («roundness» according to Szadeczky-Kardoss); or 2) a binary diagram relating the number of grain corners (N) to their planarity (P) %.  相似文献   

    12.
    Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-39Ar/40Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-39Ar/40Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-39Ar/40Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km3) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km3) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km3) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.  相似文献   

    13.
    On August 1, 1952, a new volcano named Bárcena was born on Isla San Benedicto, which is located about 300 nautical miles off the west coast of Mexico. A pyroclastic cone nearly 1100 feet above sea level was formed by August 2. By mid-September cone formation had ceased and a small lava plug capped the magma conduit in the crater. After a period of quiescence from mid-September until early November activity resumed and blocky, soda trachyte lava formed two domes in Bárcena crater during November and early December. On December 8 lava flowed through the base of the volcano and formed a delta nearly one-half mile out to sea by the end of February, 1953. All activity, except solfataric steaming, stopped by this date. Volcanic density flows («nuées ardentes ») descended the cone during the period of cone formation. As the expulsion of ash and steam decreased in early September, 1952, the exterior of the cone is believed to have been furrowed by these avalanches. Volcán Bárcena has an index of explosiveness of about 90 per cent, the highest of any known oceanic volcano in the eastern Pacific Ocean. Calculations indicate that about 10,500 million cubic feet (300 million cubic meters) of tephra and lava were erupted during the life of Bárcena.  相似文献   

    14.
    A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.  相似文献   

    15.
    A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities. Aim of this survey was to complete the island structural pattern relatively to the shallower structures. Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ>2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm3) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm3). The shorter wavelength residual gravity field (λ<2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T>200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney. A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data.  相似文献   

    16.
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    17.
    Mafic and ultramafic xenoliths are well represented within a large basaltic lava field of Stromboli. These basalts, known as San Bartolo lavas, show a high-K calc-alkaline (HKCA) affinity and were erupted <5 ka BP. Xenoliths consist of olivin-gabbro, gabbronorite, anorthosite, dunite, wehrlite and clinopyroxenite. Thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0.17–0.24 GPa, at temperatures ranging from 940 to 1,030°C. These materials interacted with hydrous ascending HKCA basaltic magmas (with temperatures of 1,050–1,100°C) at pressures of about 0.2–0.4 GPa. These pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within HKCA basaltic lavas. Gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier HKCA Strombolian basalts.Dunite and wehrlite show porphyroclastic-heterogranular textures, whereas the clinopyroxenite exhibit a mosaic-equigranular texture typical of mantle peridotites. These ultramafic materials are in equilibrium with more primitive basaltic magmas (under moderately hydrous and anhydrous conditions) at pressures of 0.8–1.2 GPa, which is below the crust-mantle transition, located at about 20 km depth under Stromboli.Major and trace element distributions indicate comagmatism between the host basaltic lava and the mafic and ultramafic inclusions. REE patterns for mafic nodules are relatively regular and overlap the field of basaltic lavas (HKCA). They show moderate to high LREE enrichments and moderate enrichments in HREE relative to chonrites. Spider diagrams also show significant similarities between the lavas and the mafic-ultramafic xenoliths as well.During their ascent, primitive Strombolian magmas may be stored in upper-mantle regions where they interact with peridotitic materials and partly differentiate (to give dunite and wehrlite) before migrating to upper crustal levels. In this region, hydrous basaltic magmas (with estimated water contents of 2–3.5 wt%) are stored in the subvolcanic environment, and are allowed to crystallize the gabbroic materials before reaching the surface under nearly anhydrous conditions.An erratum to this article can be found at  相似文献   

    18.
    Fluid motions are important in virtually all volcanic processes. Attempts to understand the mechanism of volcanic activity or the origin of magmas generally require knowledge of fluid dynamics. The use of fluid dynamics is illustrated by considering the Reynolds numbers of some volcanic fluid flow systems. The physics of high Reynolds number buoyant plumes is found to be important in situations ranging from the rise of eruption columns in the atmosphere to the replenishment of basaltic magma chambers. Application of theoretical and experimental work on plumes enables eruption rates to be deduced from eruption column heights and new hypotheses on the origin of some magmatic ores to be put forward. The influence of Reynolds number on the behaviour of lava is also discussed with application to the origin of Archaean komatiite lavas. Komatiite lavas are argued to have flowed in a turbulent manner whereas modern basalt lavas nearly always flow by laminar shear. The turbulent character of komatiites seems to provide an explanation for the origin of associated nickel-sulfide mineralization in komaiites by melting and assimilation of sulfide-rich sediment. This hypothesis depends on komatiite flow having had a high Reynolds number.  相似文献   

    19.
    Three composite cones have grown on the southern edge of the previously existing Atitlán Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlán, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolimán are mantled by more silicic lava flows. On Tolimán, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlán’s rocks and late lavas from Tolimán have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.  相似文献   

    20.
    The results of petrological and volcanological investigations of the Assab area (Ethiopia) are reported. Fissure activity — which produced basaltic lava flows and several spatter cones — and central activity — represented by a cumulus dome and two explosive craters — have been recognized. The area is characterized by E-W and NE-SW tectonic trends, whereas the NNW-SSE « Eritrean trend » is absent. Transverse tectonics is limited to the blocks bordering the Danakil Depression, and never extends into the Depression itself. Mineralogical composition and chemical data point to an alkaline nature of the Assab lavas, which have been classified as: picritic basalts tending to ankaramites; alkali olivine basalts; hawaiites; and all the rock types ranging from mugearites to trachytes. Two rock groups have been identified which could be due to crystal fractionation processes controlled by different degree of oxidation. The petrological difference between the rocks from Afar proper and those from the Danakil block (unquestionably alkaline rock types in the Danakil block, and transitional rock types in Afar) is emphasized.  相似文献   

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