Tectonic Implications of the Composition of Volcanic ARC Magmas

Abstract

Volcanic arc magmas can be defined tectonically as magmas erupting from
\nvolcanic edifices above subducting oceanic lithosphere. They form a coherent
\nmagma type, characterized compositionally by their enrichment in large ion
\nlithophile (LlL) elements relative to high field strength (HFS) elements. In
\nterms of process, the predominant view is that the vast majority of volcanic arc
\nmagmas originate by melting of the underlying mantle wedge, which contains
\na component of aqueous fluid and/or melt derived from the subducting plate.
\nRecently, opinions have converged over the key aspects of the physical model
\nfor magma generation above subduction zones (Davies & Stevenson 1992),
\nnamely:
\n1. that the mantle wedge experiences subduction-induced corner flow (e.g.
\nSpiegelman & MacKenzie 1987);
\n2. that the subduction component reaches the fusible part of the mantle wedge
\nby the three-stage process of (i) metasomatism of mantle lithosphere, followed
\nby (ii) aqueous fluid release due to breakdown of hydrous minerals at
\ndepth (e.g. Wyllie 1983, Tatsumi et al 1983) and (iii) aqueous fluid migration,
\nfollowed by hydrous melt migration, to the site of melting;
\n3. that slab-induced flow may be locally reversed beneath the arc itself, allowing
\nmantle decompression to contribute to melt generation (e.g. Ida 1983).
\nThe simplified model in Figure 1 highlights the physical and chemical processes
\nthat have been invoked as being important in controlling the composition of
\nvolcanic arc magmas. Magma compositions (coupled with experimental data
\non element behavior) can help us gain further understanding of these physical
\nand chemical processes. In this review, we first summarize knowledge of the
\nbehavior of elements in the subduction system. We then focus on compositional
\nevidence for the processes illustrated in Figure 1, which we group as follows:
\n1. derivation of the subduction component,
\n2. transport of the subduction component to the melting column,
\n3. depletion and enrichment of the mantle wedge, and
\n4. processes in the melting column.