Abstract

The most important petrological problem relating to the development of island arc systems is the origin of the basalt-andesite-dacite-rhyolite volcanic suite. Characteristics of the suite are reviewed. They include distribution of active volcanoes relative to Benioff zones, chemical composition and fractionation trends of magmas, the evolutionary development of island arcs in time, often comprising an earlier tholeiitic stage followed by a later calcalkaline stage, and the trend of magmas to become richer in potassium and other incompatible elements, the greater the height of the volcano above the Benioff zone.

Recent researches in experimental petrology suggest three fractionation controls which might be responsible for various characteristics of the suite; amphibole-controlled fractionation, eclogite-controlled fractionation and direct partial melting of mantle pyrolite under conditions of high water vapour pressure. The operation of these processes is reviewed. Amphibole-controlled fractionation is limited to depths smaller than 100 km and causes a calcalkaline trend among residual liquids but with little change in K/Na and rare earth element abundances. Eclogite-controlled fractionation likewise causes a calcalkaline trend but is accompanied by increase of K/Na ratios and fractionation of the rare earth elements. This process probably occurs in the 100–150 km depth interval. Partial melting of the mantle under high PH2O is shown to cause the formation of basaltic magmas in the depth interval, 70–100 km. Upon rising, these magmas fractionate towards basaltic andesite and andesite compositions via the crystallization mainly of olivine. Resulting liquids display a tholeiitic differentiation trend.

A model for the operation of these processes in the island arc environment is proposed. Amphibolite in the subducted oceanic crust becomes dehydrated at depths of 70–100 km under subsolidus conditions. The water produced causes partial melting to occur in the pyrolite wedge above the Benioff zone. Magmas thus produced differentiate under high PH2O to produce the early tholeiite stage of arc development. As the oceanic crust is sub-ducted to depths of 100–150 km, a high PH2O is maintained by the dehydration of serpentinite and derivative high pressure hydrated magnesium silicates. Partial melting of the quartz eclogite oceanic crust produces rhyodacite magmas. These react with overlying mantle pyrolite to form pyroxenite. Diapirs of pyroxenite rise upwards from the Benioff zone, partially melting to form magmas which fractionate by eclogite crystallization (80–150 km) and amphibole crystallization (30–100 km) thereby producing the calcalkaline phase of arc development.

The residual, refractory eclogite and peridotite in the lithosphere plates which sink below about 150 km have become irreversibly differentiated and never again are able to participate in the formation of basaltic magmas at mid-oceanic ridges. The complementary differentiate is, ultimately, the continental crust, which grows through time by the accretion of island arcs and by the addition of the andesitic volcanic suite, derived as discussed above. It is estimated that about 30–60% of the volume of the mantle has passed through this process of irreversible differentiation.

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