This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.

One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust

The continental crust has an andesitic composition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg# andesites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previously depleted, subsequently metasomatised mantle peridotite. However, they are more likely produced by reaction between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in modern island arcs, but may have been more common in the past, may be produced in specific environments (such as “ridge subduction”), may be more common among plutonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.

Genesis of high Mg# andesites and the continental crust

The continental crust has an andesitic compo- sition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg#e an- desites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previ- ously depleted, subsequently metasomatised mantle pe- ridotite. However, they are more likely produced by reac- tion between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in mod- ern island arcs, but may have been more common in the past, may be produced in specific environments (such as "ridge subduction"), may be more common among plu- tonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.

http://rd.springer.com/content/pdf/10.1007%2FBF00311004.pdf

Genesis of high andesites and the continental crust

This paper presents results of modelling reaction between peridotite and fractionating tholeiitic basalt in simple and complex silicate systems. Reactions are outlined in appropriate binary and ternary silicate systems. In these simple systems, the result of reactions between 'basalt' and 'peridotite' may be treated as a combination of Fe-Mg exchange and mass transfer reactions at constant Fe/Mg. Fe-Mg exchange in ternary and higher-order systems is nearly isenthalpic, and involves a slight decrease in magma mass at constant temperature. Mass transfer reactions, typically involving dissolution of orthopyroxene and consequent crystallization of olivine, are also nearly isenthalpic in ternary and higher-order silicate systems, and produce a slight increase in the magma mass at constant temperature. The combined reactions are essentially isenthalpic and produce a slight increase in magma mass under conditions of constant temperature or constant enthalpy. Initial liquids saturated in plagioclase-I-olivine will become saturated only in olivine as a result of near-constant-temperature reaction with peridotite, and crystal products of such reactions will be dunite. Liquids saturated in clinopyroxene + olivine will remain on the cpx-ol cotectic during reaction with peridotite, but will crystallize much more olivine than clinopyroxene as a result of reaction, i.e., crystal products will be clinopyroxene-bearing dunite and wehrlite rather than olivine clinopyroxenite, which would be produced by cotectic crystallization. The Mg/Fe ratio of crystal products is 'buffered' by reaction with magnesian peridotite, and dunites so produced will have high, nearly constant Mg/Fe. Production of voluminous magnesian dunite in this manner does not require crystal fractionation of a highly magnesian olivine tholeiite or picrite liquid. Combined reaction with ultramafic wall rock and crystal fractionation due to falling temperature produces a calc-alkaline liquid line of descent from tholeiitic parental liquids under conditions of temperature, pressure, and initial liquid composition which would produce tholeiitic derivative liquids in a closed system. Specifically, closed-system differentiates show iron enrichment at near-constant silica concentration with decreasing temperature, whereas the same initial liquid reacting with peridotite produces silica-enriched derivatives at virtually constant Mg/Fe. Reaction between fractionating basalt and mafic to ultramafic rock is likely to be important in subduction-related magmatic arcs, where tholeiitic primary liquids pass slowly upward through hightemperature wall rock in the lower crust and upper mantle. Although other explanations can account for chemical variation in individual calc-alkaline series, none can account as well for the characteristics snared by all calc-alkaline series. This process, if it is volumetrically important on Earth, has important implications for (Phanerozoic) crustal evolution: sub-arc mantle should be enriched in iron, and depleted in silica and alumina, relative to sub-oceanic mantle, acting as a source for sialic crust It is probable that inter-occanic magmatic arcs have basement similar to alpine peridotite, in which suboceanic mantle has been modified by interaction with slowly ascending basaltic liquids at nearly

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma I. Phase Relations, the Origin of Calc-alkaline Magma Series, and the Formation of Discordant Dunite

The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydritebearing pumices

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome

H. Sakai

Papers

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome