A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.

A Guide to the Chemical Classification of the Common Volcanic Rocks

Geochemical discrimination of different magma series and their differentiation products using immobile elements

Naming materials in the magma/igneous rock system

Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.

Mountain belts and the new global tectonics

Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Ion substitution in the diopside-ferropigeonite series of clinopyroxenes

Japanese calderas may be classified into two types. The first type includes calderas formed by collapse following the eruption of a tremendous amount of pumice (calderas of Krakatau type of H. Williams). The second type includes those formed without the pumice eruption (calderas of Glencoe type of H. Williams). In both types, post-caldera eruptions build up cones or domes, usually inside the calderas but sometimes on their margin. Most of the pre-caldera lavas consist of rock series originating through fractional crystallization of basaltic magma. In the calderas of Krakatau type, the post-caldera lavas comprise a rock series formed through contamination of the same magma by granitic material, whereas in those of Glencoe type the post-caldera lavas represent an advanced stage of crystallization of the pre-caldera lavas. In the former type, the explosive eruption of the pumice probably shattered the roof of the magma reservoir and facilitated the contamination, whereas in the latter the subsidence took place along a simple system of ring fissures without fracturing of the subsided mass and without giving opportunity for contamination.

Formation of calderas and magmatic evolution

/pdf/review-of-pyroxene-relations-in-terrestrial-rocks-in-the-47ai3t3yak.pdf

Review of pyroxene relations in terrestrial rocks in the light of recent experimental works

Uranium and thorium contents in ultrabasic rocks

Hakone caldera, now 8 by 12 km in diameter, was formed by collapse of a center of a volcano probably 2700 m high. The collapse took place at two separate periods each of which was followed by periods of deep denudation. The central part of the caldera has been covered by a thick pile of lavas of post-caldera cones and domes. For the purpose of finding thermal spring, drilling to depths of a few hundred to one thousand meters was carried out at various points within the caldera except for its central region. The study of the drill cores revealed that the average amount of subsidence at points 2 and 3 km away from the base of the present caldera wall is 600 m and 1200 m respectively, and probably more than 1800 m in the middle of the caldera. Within the caldera, the pre-caldera lavas and pyroclastic rocks are either lacking or much thinner than would be expected. It is concluded therefore that the present topographic depression of the caldera owed its origin to both subsidence and denudation. It is inferred that the subsidence took place along a complicated system of concentric faults combined with tilting of individual fault blocks toward the middle of the caldera. The magma reservoir into which the fault blocks sank probably had a shape of a cupola with a diameter comparable to or a little smaller than the diameter of the caldera.

Hisashi Kuno

Papers

Ion substitution in the diopside-ferropigeonite series of clinopyroxenes

Formation of calderas and magmatic evolution

Review of pyroxene relations in terrestrial rocks in the light of recent experimental works

Uranium and thorium contents in ultrabasic rocks

Structure of Hakone caldera as revealed by drilling