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

Gabbro — quartz diorite inclusions, angular to rounded and up to 20 cm in size, have been found as accidental fragments in a mud flow of the Okata basalt group, O-shima Island and in a tuff breccia, Hakone. New analyses are represented for seventeen inclusions and three pyroxenes. It is reasonable to conclude from petrographic and chemical features that the olivine gabbro inclusions were produced by crystal settling from a quartztholeiite magma at the early stage of fractionation within a magma reservior. On the other hand, gabbro and quartz gabbro inclusions are fragments of a small intrusive body within the Tertiary volcanic formation and consist of various amounts of cumulus phases and liquid. Quartz diorite inclusions are also fragments of a plutonic equivalent, but represents a strongly-differentiated liquid phase of the quartz-tholeiite magma.

Gabbro — quartz diorite inclusions from Izu-Hakone Region, Japan

https://www.jstage.jst.go.jp/article/jgg1949/22/4/22_4_559/_pdf

Hisashi Kuno

Papers

Gabbro — quartz diorite inclusions from Izu-Hakone Region, Japan

Paleomagnetism and K-Ar Ages of Successive Lava Flows (4)

Mafic and ultramafic inclusions in basaltic rocks and the nature of the upper mantle

Frequency Distribution of Rock Types in Oceanic, Orogenic, and Kratogenic Volcanic Associations

Occurrence of Porphyritic Pigeonite in “Weiselbergite” from Weiselberg, Germany