Origin of rhyolites by anatectic melting of granitic crustal rocks

Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km². The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows:

-mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø.

-phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice.

-mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff.

-structurally transitional types between pumice and the granites in different state of melting.

-eutectic composition of the pumice glass for a pressure of 2 kb.

     

Sur l'éruption du Nyamulagira de décembre 1981 à janvier 1982: cône et coulée du Rugarambiro (Kivu,Zaïre)

The December 1981 — January 1982 eruption which started in the Christmas night on the SE side of Nyamulagira, gave the longest historical flow (26 km) representing the highest production rate of this volcano in this century (280×10⁶m³ of erupted magmas in 19 days). This eruption built Rugarambiro, a composite spatter cinder-cone. The ejected lava is a K-hawaiite (kivite) whose basicity decreased during the eruption (first emission: D.I. = 40; last products: D.I. = 35). This chemical evolution is reflected by:

--the modal composition of lavas. The first emissions are poor in ferromagnesian phenocrysts (olivine + clinopyroxene: 3%) and rich in plagioclase (12%); the contrary is observed in the last ejected lavas (livine + Cpx: 16%; plagioclase: 1%);

--the nature of the crystallizing minerals in the groundmass. In fact, only the first ejections include alkaline feldspars, nepheline and Tiphlogopite;

--the glassy phase composition which is more differentiated in the first lavas (D.I. 68–84) than in the last ones (D.I. 42–61).

A stratification of the Nyamulagira magmatic chamber is proposed where magmatic differentiation has probably occurred for fractional crystallization. Mineralogical thermobarometers lead to locate this magmatic reservoir at the depth of 6–7 km that we had already hypothesized. The presence of phenocrysts of bytownite, basic chrysolite, diopside and salite indicates a basaltic paragenesis which marks a hawaiitic magma chamber feeding.     

Geochemistry of some volcanic rocks from south-eastern sicily: Rare earth and other trace element distribution

Some trace element data for volcanic rocks found at different levels, from Tertiary to Holocene, in south-eastern Sicily (Iblean Plateau and Mt. Etna) are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types. Four groups of volcanic rocks have been recognized on the basis of their major element chemistry: 1) low-K tholeiites, 2) associated alkali basalts to nephelinites of the Iblean Plateau (Upper Pliocene to Lower Pleistocene), 3) the basal subalkaline lavas of Mt. Etna, and 4) the alkalic suite rocks that make up the bulk of the volcano. The distribution of Rb, Sr, Ni, Cr, Co, Cu, REE, Th and Sc suggests:

1. an origin of the Iblean magmas by a different degree of partial melting of a Rb-poor and possibly slightly hetereogeneous mantle;
2. quite distinct source compositions for the Etnean magmas, relative to those of the Iblean area, on the basis of their Rb and Sr contents;
3. an origin of the alkalic rocks of Mt. Etna from independently generated magma(s) rather than by crystal fractionation of the Etnean subalkaline magmas or of a magma having the geochemical features of the Iblean alkali basalts; evidence for this is given by the distribution features of the incompatible elements showing an origin for these rocks from compositionally different parent magmas and/or an evolution under widely variable environmental conditions;
4. the primary character for the chemical differences observed in some of the Etnean subalkaline rocks that can be accounted for by different physico-chemical conditions at their source rather than by crystal fractionation processes.

     

Petrology of the volcanic rocks of martinique,West Indies

Volcanological differences between the old and the recent lavas from Martinique, Lesser Antilles, are presented, showing that two volcanic series exist in this island:

Dash

a high-alumina basalt series generally mafic, line-grained, partly pillowed, with clinopyroxene-rich lavas which show iron enrichment tendancies en an A.F.M. plot;

a calc-alkaline (slightly potassic) series much more siliceous as a group, porphyric, predominantly sub-aerially erupted with orthopyroxene-rich lavas which show no iron enrichment.

The high-alumina basalt series is considered as having originated from a differentiation trend by fractionation of olivine, clinopyroxene and plagioclase. Lavas range from olivine basalt to tridymite-rich dacite. The calc-alkaline series probably derives from the contamination of the first suite but the occurence of hornblende-rich cumulates indicates the process of fractionation takes place too. Lavas range from orthopyroxene andesite and hornblende andesite to quartz-hornblende dacite and quartz-biotite dacite.     

The Erta Ale volcanic range (Danakil depression,northern afar,ethiopia)

The Erta Ale range is the main volcanic unit in the Danakil rift. It is located along the axis of the northern part of the Depression, in a zone clearly related to the Red Sea rift. The detailed study of the tectonics, volcanism and petrographic succession of the whole range allows one to draw the following conclusions.

1. A compl3te, tectonically controlled evolution is observed in the subaerial volcanism from simple fissure activity, to small shield volcanoes built along open fissures, and to more complex central volcanoes. The emitted products form a complete differentiation series from basalts to rhyolites, with a remarkably regular volume decrease from basic to silicic terms.
2. The close relationships existing between petrological and volcanological evolution suggest that the magma fractionation has been realized at relatively shallow depth, at the emplacement level of magmatic reservoirs. The importance of a time factor in the differentiation processes in volcanic conditions is clearly stressed.
3. All collected data strongly support the subcrustal origin of the whole volcanic series and the hypothesis of crustal separation with formation of new oceanic crust along the axis of the Northern Danakil Depression.

     

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.     

Liparitic volcanism of some regional volcanic structures in Kazakhstan,Middle Asia and Caucasus

Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:

1. Geo-structural zonation of the volcanic structures.
2. Dismembered Moho surface within the volcanic structures.
3. Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
4. Ignimbritic character of the liparitic volcanism.
5. Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.

Geo-structural and petrochemical zonations are likely governed in the regions studied by a deep-seated plutonic body.     

Evolution of the Vico lavas,Roman volcanic region,Italy

A petrochemical study of representative lavas from Vico, relative to their established stratigraphic succession, shows that the erupted magma was recurrent in composition. The predominant differentiation trend of the lava suite, illustrated by means of a principal component analysis (Le Maitre, 1968), leads to trachytic residual compositions which can be interpreted in terms of crystal fractionation processes under volcanic pressure regime.     

Distribution de l’uranium,du thorium et du potassium dans les roches volcaniques de l’Afar septentrional (Ethiopie)

Concentrations in uranium, thorium and potassium were determined by gamma spectrometry in more than 100 samples of volcanic rocks from the material obtained during the French-Italian expedition in Dankalia (Afar-Ethiopia) (December 1967 – February 1968). These samples were taken from two geographically distinct areas:

1. the danakile depression (Erta Ale range, Pierre Pruvost massif, Alayta, Afdera, Borawlu).
2. Ethiopian high lands (Adigrate — Axum).

The values we obtained give the following contents:

Dash

in the Erta Ale range, for basalts, K: 0,5%, U: 1,3 ppm, Th: 3,3 ppm, Th/U: 2,6.

for alkaline rhyolites, U: 4,1 ppm, K: 2,3%, Th: 15,2 ppm, Th/U: 3,7.

in the Pierre Pruvost massif, for acid rocks, K: 3,9%, U: 3,6 ppm, Th: 13,5 ppm, Th/U: 3,8.

in the Borawlu centre, K: 2,8%, U: 4,6 ppm, Th: 14,7, Th/U: 3,4 for the rhyolites, K: 0,6%, U: 0,9 ppm, Th: 3,1 ppm for the basalts.

on the Ethiopian high lands, for the Adigrate basaltic trap rocks, K: 0,6%, U: 0,6 ppm, Th: 1,1 ppm, Th/U: 1,7 and for the Axum zone of phonolitic pinnacles, K: 3,9%, U: 2,7 ppm, Th: 14,9 ppm, Th/U: 4,9.

The Th/K and U/K ratios are particularly high compared with the usual data, except for the Pierre Pruvost massif which seems to be different from the others. The difference of Th/U ratios for basic rocks and acid rocks is meaningful. For the magma which gave the volcanism of the studied area, one can think of an evolution more especially connected with transfers of volatile elements. The strong correlations existing between potassium, uranium and thorium probably indicate the presence of groups which are distinct from a geochemical point of view in the Dankalia lavas.     

Volcanology and petrology of Nisyros Island (Dodecanese,Greece)

Nisyros is a totally volcanic island located at the eastern limit of the quaternary calc-alkaline island arc system of the South Aegean Sea. Its age is rather young since K/Ar dating has given an age of 0.2 m.y. B.P. for one of the oldest outcropping products of this volcano. The volcanological evolution of Nisyros has tentatively been reconstructed as follows:

1. after a period (from 3.0? to 0.2 m.y. B.P.) of submarine activity, evidenced by the presence of pillow-lavas and hyaloclastites, the volcano grew above sea level;
2. effusive and explosive subaerial activity from different vents built up a complex stratovolcano, probably around 0.2 m.y. B. P.;
3. at the end of an intense explosive activity (between 0.2 m.y. B.P. and Present) the top of the volcano collapsed forming a caldera which is still perfectly preserved. A post-caldera activity with eruptions of huge and viscous domes and lava flows of uniform composition, both inside and outside the caldera, concluded this stage of the volcano evolution;
4. in historical times, spectacular phreatic explosion craters formed on the caldera floor;
5. presently, a large area of the caldera floor is affected by a considerable hydrothermal activity. The hypothesis is formed that Nisyros volcano is not yet extinct.

Four small volcanic islets — Yali, Stronjili, Pakia and Perigusa — located a few miles on the North and West of Nisyros, although volcanologically independent of one another, are composed of products which are attributable, from the petrologic point of view, to the Nisyros magma. The volcanic rocks of Nisyros and of its neighbouring minor islands as well as the volcanics of the coeval volcanoes of the South Aegean Sea arc (Aegina, Milos, Santorini, etc.) belong to a typical orogenic calc-alkaline series (from basic andesites through andesites-dacites-rhyodacites to strongly silicic rhyolites) with normal K₂O contents. The potassium contents of these rocks are compatible with the depth of 150 km (as inferred from geophysical data) for the inclined seismic zone underneath the active volcanic arc. The existence of a top-caldera as well as the occurrence of a huge amount of xenoliths (hornblende-rich cumulates and contact-metamorphic calcareous rock derivatives) suggest the presence of a magma chamber at a relatively shallow depth beneath the volcano. The rhyolitic obsidians of Yali can be considered as residual liquids from the Nisyros rhyodacites, thus representing the end-members of a fractionation process. Volcanological and petrological arguments are in favour of fractional crystallization as the most probable genetic process for the calc-alkaline differentiation series of Nisyros and of its neighbouring minor islands. However, the lack of any rock with a high alumina basalt composition makes it difficult to define exactly the nature of the parent magma. According to recent geophysical data, continental collision is already in progress at the Hellenic trench. Therefore, Nisyros and the other active volcanoes of the South Aegean Sea arc are approaching the senile stage. What would follow could be a transition to shoshonitic magmatism as a consequence of the deepening of the lithospheric slab under the Aegean microplate. The limited extension and the relatively short-lived calc-alkaline activity of the South Aegean Sea arc could be related to the particular geodynamic pattern of the Mediterranean area which is characterized by a microplates mosaic between the two converging African and Eurasian major plates.     

Pétrologie de la Série Volcanique de l’île de Fayal (Açores)

Four volcanic units have been distinguished on the islanf of Fayal. In order of decreasing age, these are:

the eastern rift, and products of the activity preceding the collapse of the caldera visible at the summit of the stratovolcano, characterized by an alkaline series: basalt-hawaiite-mugearite-trachyte;

the products of the explosive and postcaldera activity where only evolved lavas occur (benmoreites and trachytes);

the recent basaltic activity of the Horta region;

the western fissural activity — recent and historical.

The two last units are characterized by exclusively basaltic, frequently picritic, eruptions. The lava groups cannot be distinguished by chemical criteria and have thus been treated as a single suite. Ninety samples have been analysed by X-ray fluorescence, and the mineralogy of 6 representative specimens has been determined by microprobe. The data were used to work out the evolution of the lava. The series is shown to have been produced by crystal fractionation under moderate water pressure from an alkali basalt. Moderate fractionation of amphibole during the last stages allow the liquids to remain weakly undersaturated from initial basalts until final trachytes. Mineralogical and chemical diversity between the most evolved lavas, benmoreites and trachytes, is an evidence of the strong influence ofpH₂O and/orfO₂ on the composition of such residua.     

Merapi volcano (Java,Indonesia) and Merapi-type Nuée ardente

A particular nuée ardente type (Merapi-type avalanche nuée) has been defined at the Merapi volcano because of its prominent role in the recent activity of the volcano: gravity plays a significant role during the eruption. However, some other eruption styles occur too producing surges and ashfalls. Three types of tephra, deposited in a very short time-span (15 years) are compared: chemistry and mineralogy are similar, but grain-size analyses are different. There is no vesicular glass, and it is concluded that there is an absence of new magma. This example shows clearly the variety of volcanic styles, with similar chemistry in a very short period. Avalanche nuées from collapsed domes or flows are separated into two types:

1. Merapi-typesensu stricto, without any fresh glass, derived from a wholly solidified dome.
2. Arenal-type, containing pumiceous glass, derived from a dome, the interior of which is still liquid.

     

Ignimbrite sequence on Gran Canaria

The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:

1. A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase (Or_15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe/Ti-oxides. The commonest groundmass minerals are anorthoclase and alkali-amphibole, with minor quartz and aegirine.
2. A middle unit of comenditic and pantelleritic ignimbrites characterized by anorthoclase (Or_20–32) and amphibole. Phenocryst minerals restricted to individual flows are Fe/Ti-oxides (several comendites), clinopyroxene, biotite, and sphene. The commonest groundmass minerals are anorthoclase and Tiaegirine, with lesser katophorite, arfvedsonite and quartz.
3. An upper unit of trachvtic and phonolitic ignimbites and lava flows (normative ne rarety exceeding 10%) with nepheline phonolite lava flows becoming increasingly abundant upwards. The ignimbrites have mostly anorthoclase (Or_30-04), and biotite, with rarer Fe/Ti-oxides, hornblende, and clinopyroxene. The commonest groundmass minerals are anorthoclase, aegirine, and alkali-amphiboles, and in some flows nepheline.

The change from Na-rich to K-rich anorthoclase upwards in the sequence supports the conclusion, based on over 50 new stratigraphically controlled chemical analyses that the Na₂O/K₂O-ratio decreases within the sequence. possibly as a result of crystal iractionation processes and this effect is independent of probable loss of Na on post-eruptive crystallization. While hydroxyl-bearing phenocryst minerals are absent from all rocks called lava in the field, they are ubiquitous in the ignimbrites, indicating the importance of P_u2o in the generation of suspension-type cruptions. Compositional gradients must have been particularly pronounced in the small magma chambers that existed beneath Gran Canaria, resulting in a wide range of compositionally zoned or mixed deposits.     

Dikes of the McClure mountain — Iron mountain alkalic complex,Fremont county,Colorado, U.S.A.

The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types:

1. Relatively homogeneous calcitic or dolomitic carbonatites with few accessories.
2. Composite carbonate — hematitic potash feldspar bodies which, with diminution of carbonate, grade into feldspathic Th-RE veins.
3. Carbonatites with abundant barite and/or fluorite, and rarer species. The Goldie carbonatite also contains a complex suite of aluminofluorides.

Many carbonatites and Th-RE veins emit, when broken, a fetid gas consisting of a mixture of fluorinated hydrocarbons of the C₅ and C₆ types along with F₂, HF, and F₂O. The fluorine has been derived from structurally degraded radioactive fluorite; the hydrocarbon gases appear to represent primary inclusions. Within the dike halo are two small breccia pipes consisting of fenitized, locally derived Precambrian gneiss blocks with minor fragments of transported biotitized lamprophyre in a subordinate calcitic matrix also containing aegirine, crocidolite, potash feldspar and hematite. The abundance of the lamprophyres, the great width of the dike halo, the lack of ring structure within the complex, the restriction of carbonatite to dikes, and the subordinate fenitization effects all combine to suggest that the McClure Mountain — Iron Mountain complex is now exposed at a relatively deep niveau, possibly near the mesozone — catazone boundary.     

Geophysical prospecting on the Santorini Islands

A program of geophysical research was carried out as a preliminary stage of study of the Santorini volcanic group. This area is of remarkable geothermal and volcanological interest, and the definition of a volcanological structural model is the starting point for an understanding of the local geodynamic processes. Gravity, magnetic and geoelectrical data proved that:

1. the core of the volcanic edifice consists of a sedimentary-metamorphic basement;
2. the basement is tectonically disturbed and a linear tectonic system produces a graben-type structure in the middle part of the area.

     

Les grandes étapes d’évolution d’un volcan andésitique composite: Exemple du Nevado de Toluca (Méxique)

The Nevado de Toluca, in the middle of the Mexican volcanic belt, has been built by two very dissimilar phases. The first one that lasted more than one million years is mainly andesitic. Numerous massive and autobrecciated lava flows of this phase pass outwards into thick conglomeratic formations. The volume of this primitive volcano represents the essential part of the Nevado. After an intense periode of erosion, the second phase is of very short duration (about 100.000 years) and is dacitic in nature. Three main episode can be distinguished:

1. Eruption of important ash and pumice pyroclastic flows related to caldera collapse above a shallow magmatic reservoir.
2. Extrusions of several dacitic domes within and outside the caldera with numerous associated «nuées ardentes» surrounding the volcano.
3. Plinian eruption leading to widespread pumiceous air-fall and to the opening of the present crater inside the caldera. Extrusion of a new small dacitic dome and late phreatic explosions.

This second sequence of events can be interpreted as the progressive emptying of the crustal magmatic chamber without refilling by a new magma supply. The most recent activity in the area is represented by monogenic cones and flows of basic andesites outside the central vent system of the Nevado.     

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.

     

Gases and water isotopes in a geochemical section across the Larderello,Italy, geothermal field

Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by

1. Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;
2. Constant CO₂ (95±2 percent); near constant H₂S (1.6±0.8), N₂ (1.2±0.8), H₂ (2±1), CH₄ (1.2±1), and no O₂ in the dry gas;
3. Presence of numerous, straight chain and branched C₂ to C₆ hydrocarbons plus benzene in amounts independent of CH₄ contents with highest concentrations in the deeper wells;
4. Oxygen-18 contents of steam increasing south to north from ?5.0‰ to ?0.4‰ with little change in deuterium (?42±2‰).

These observations are interpreted as showing:

1. Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;
2. Synthesis of CH₄ from H₂ and CO₂ with CO₂ and H₂ produced by thermal metamorphism and rock-water reactions;
3. Extraction of C₂ to C₆ hydrocarbons from rock organic matter;
4. Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ～220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.

     

Numerical and experimental models on the formation mechanism of collapse basins during the Green Tuff orogenesis of Japan

The geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:

1. Dome-shaped uplift with a mean diameter of 30 km.
2. Collapse of the central part of the domes forming basins with a mean diameter of 10 km.
3. Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust.

In finite element analyses performed as plane strain problems, earth's crust is assumed to be an elasto-plastic homogeneous layer and to undergo sinusoidal vertical displacement at the base of the layer due to an ascending magma reservoir. These analyses reveal that the diameter of the dome is proportional to the depth of the magma reservoir rather than to its size. The magma reservoir is estimated at 12 ～ 24 km in depth. Scale model experiments using powdered material were performed in order to reproduce a collapse basin. These three-dimensional models are reduced to a scale of 1:200,000 th of the natural size. The results of experiments show that radial and concentric cracks are produced on top of the dome and a central part encircled by concentric cracks collapses to form a basin. The boundary of the collapsed portion forms a steep cliff with a height of about 2 mm. This is equivalent to 400 m in natural size and is nearly similar to field observations.