Tectonic Implications of the Composition of Volcanic ARC Magmas

Volcanic arc magmas can be defined tectonically as magmas erupting from volcanic edifices above subducting oceanic lithosphere. They form a coherent magma type, characterized compositionally by their enrichment in large ion lithophile (LlL) elements relative to high field strength (HFS) elements. In terms of process, the predominant view is that the vast majority of volcanic arc magmas originate by melting of the underlying mantle wedge, which contains a component of aqueous fluid and/or melt derived from the subducting plate. Recently, opinions have converged over the key aspects of the physical model for magma generation above subduction zones (Davies & Stevenson 1992), namely: 1. that the mantle wedge experiences subduction-induced corner flow (e.g. Spiegelman & MacKenzie 1987); 2. that the subduction component reaches the fusible part of the mantle wedge by the three-stage process of (i) metasomatism of mantle lithosphere, followed by (ii) aqueous fluid release due to breakdown of hydrous minerals at depth (e.g. Wyllie 1983, Tatsumi et al 1983) and (iii) aqueous fluid migration, followed by hydrous melt migration, to the site of melting; 3. that slab-induced flow may be locally reversed beneath the arc itself, allowing mantle decompression to contribute to melt generation (e.g. Ida 1983). The simplified model in Figure 1 highlights the physical and chemical processes that have been invoked as being important in controlling the composition of volcanic arc magmas. Magma compositions (coupled with experimental data on element behavior) can help us gain further understanding of these physical and chemical processes. In this review, we first summarize knowledge of the behavior of elements in the subduction system. We then focus on compositional evidence for the processes illustrated in Figure 1, which we group as follows: 1. derivation of the subduction component, 2. transport of the subduction component to the melting column, 3. depletion and enrichment of the mantle wedge, and 4. processes in the melting column.