Abstract
This paper briefly reviews the magmatic manifestations of volcanic arcs, and aspects of how such magmas form. Common concepts about ‘how subduction zones work’ are reassessed – with particular emphasis on the atypical Cascades volcanic arc. This arc is associated with perhaps one of the warmest modern subduction zones, with accelerated slab dehydration and low flux of slab-derived fluids compared to most others. With minimized leverage of fluids, other aspects of arc magmatism become more evident. We see multiple distinct basaltic magma types, all represented by relatively primitive high-MgO end members that allow assessment of their conditions of formation. Dominant types include two distinct intraplate varieties: low-K tholeiite with similarities to MORB and another variety with affinities to many ocean island basalts. In addition, there is a spectrum of more typical calcalkalic basalts that range to fairly alkali-rich compositions (absarokites) as well as high-Mg basaltic andesites. These magma types are variably present throughout much of the arc and all are present coevally in a transect paralleling the Columbia River. Their complex spatial distributions may in part reflect along-strike differences in tectonic controls and/or dynamics of mantle convective flow. The Cascades arc is broader than most other arcs, with significant basaltic volcanism locally in frontal and back arc regions. Prominence of basaltic magmatism has emerged since the late Miocene, and may be related to changes in the regional tectonic regime over time. These mafic magmas provide energy to drive melting in the crust, and perhaps in the lithospheric mantle, that feeds the more prominent stratovolcanoes distributed along the arc axis. Thermobarometry of the most primitive basalts implies unusual relations between depth and composition, as well as conflicting views regarding melt generation. Some traditional interpretations concerning [1] the conditions of magma formation, [2] the role of subducting oceanic plates (± sediments) in forming volcanic arc magmas, and [3] the nature of magmatic source domains in general appear to be inconsistent with current observations to some degree. These topics are reviewed and their implications assessed regarding physical and tectonic aspects of subduction zones.
Highlights
It is well established that there is a causative relationship between ongoing subduction processes and volcanic arc magmatism
Most importantly, are the estimated conditions of melt equilibration with the mantle representative of the conditions at which melting occur, or points along much longer ascent paths? how does the nature of the source affect actual melting, as well as the accuracy of estimates for melt equilibration conditions? Theoretical models provide some sense of the relative differences in the thermal structures between subduction systems, and these have been compared with compositional features of arc lavas to evaluate how they may be linked to temperature
Such models are supported by the occurrence in some arc magmas of short-lived radioisotopes (e.g., 10Be; excess 238U, 226Ra) that must be recently added to arc magma sources, as well as fluid-mobile elements (FMEs; B, Pb, As, Sb, etc.), all of which are likely slab-derived to some degree
Summary
It is well established that there is a causative relationship between ongoing subduction processes and volcanic arc magmatism (cf. Gill, 1981; Stern, 2002; Tatsumi, 2005). It is essential to understand in greater detail what factors control magmatic diversity in subduction settings This need has promoted diverse investigations including: 1) systematic petrologic and geochemical studies of arc volcanic products at scales ranging from whole rocks to minerals to melt inclusions (MIs), 2) experimental studies of melting and crystallization of relevant materials, 3) development of suitable geochemical tracers to quantify involvement of particular materials proposed to contribute to forming or modifying arc magmas, 4) acquisition of geophysical data to image the physical properties of SZs and FIGURE 2 | Depth profiles of slab-surface temperature are shown for global subduction zones (gray shaded field for most) based on numerical models of Syracuse et al (2010), assuming full coupling between slab and mantle to a depth of 80 km. Many attempts have been made to define the dynamics of SZs, and to develop a global perspective of how they work
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