Volcanic And Plutonic Activity

Where subducting oceanic lithosphere reaches a depth of 65-130 km, volcanic and plutonic activity occurs, giving rise to an island arc or an Andean-type continental arc approximately 150-200 km from the trench axis (England et al., 2004). The thickness of arc crust reflects both the age of the system and the type of crust on which the arc forms. Relatively young island arcs, such as the 3-4 Ma active part of the Mariana volcanic arc, may be underlain by a crust of 20 km thickness or less. Thin crust also generally occurs in settings where extension is dominant, such as in the Mariana arc system (Fig. 9.18b) (Kitada et al., 2006). Mature island arcs, such as those in the Neogene Japanese arc system, generally show crustal thicknesses ranging from 30 km to 50 km because they have been constructed on older igneous and metamorphic rock (Taira, 2001). Continental arcs, including the Andes and the Cascades, are structurally the most complex of all arc systems because of the numerous structural and compositional heterogeneities that are intrinsic to continental lithosphere. In compressional continental settings (e.g. Figs 9.18a, Plate 10.1 (between pp. 244 and 245)), where substantial crustal thickening occurs, arc crust may reach thicknesses of 70-80 km (Section 10.2.4).

The types of volcanic rocks that occur in the supra-subduction zone environment generally form three volcanic series (Gill, 1981; Baker, 1982):

1 The low potassium tholeiitic series that is dominated by basaltic lavas associated with lesser volumes of iron-rich basaltic andesites and andesites.

2 The calc-alkaline series, dominated by andesites (Thorpe, 1982) that are moderately enriched in potassium, other incompatible elements, and the light rare earth elements. In continental arcs dacites and rhyolites are abundant, although they are subordinate to andesites.

3 The alkaline series that includes the subgroups of alkaline basalts and the rare, very high potassium-bearing (i.e. shoshonitic) lavas.

In general, the tholeiitic magma series is well represented above young subduction zones. These rocks have been interpreted as being derived by the fractional crystallization of olivine from a primary magma originating at relatively shallow mantle depths of 65-100 km. The calc-alkaline and alkaline series are encountered in more mature subduction zones, and appear to reflect magmas generated at depths greater than those that result in tholeiitic rocks. Calc-alkaline magmas, represented by andesite and basaltic andesite, are the most abundant of the volcanic series. Alkaline magmas exhibit the lowest abundance in island arcs and are more common in continental rifts and intraplate environments (Section 7.4.2).

Some island arcs exhibit spatial patterns in the distribution of the volcanic series. In the Japanese island arc system, for example, a compositional trend of tholeiite/ calc-alkaline/alkaline volcanic rocks is apparent with increasing distance from the trench. This trend may reflect magmas derived from increasingly greater depths and/or differences in the degree of partial melting (Gill, 1981). A low degree of partial melting tends to concentrate alkalis and other incompatible elements into the small melt fraction (Winter, 2001), and could lead to an increase in alkalinity away from the trench due to a greater depth of melting or a decrease in the availability of water. However, there are many exceptions to this pattern in other arcs, indicating that differences in local conditions strongly influence magma compositions. The Izu-Bonin-Mariana arc system (Fig. 9.1), for example, shows compositional trends along the axis of the arc. From 35°N to 25°N latitude, volcanoes that form part of the Izu and Bonin arc segments are dominated by low and medium potassium rock suites (Fig. 9.24). The Mariana segment is dominated by medium potassium suites from 14°N to 23°N, and a shoshonitic province is found between the Mariana and Bonin segments (Stern et al., 2003). This great spectrum of rock compositions reflects the diversity of processes involved in arc magmatism, including variations in the depth and degree of partial melting, magma mixing, fractionation, and assimilation (see also Section 7.4.2). In general, these observations indicate that the three volcanic series form a continuum of rock compositions and do not correspond to absolute magma types or source regions.

Mature arc systems, and especially continental arcs, typically include large, linear belts of plutonic rock called batholiths. These belts are so common in continental crust that they are widely used as indicators of ancient, now extinct convergent margins (Section 5.9). Occasionally the term Cordilleran-type batholith is used

Dacite and Rhyolite

Dacite and Rhyolite

O Izu - Bonin oShoshonitic province □ Marianas

Figure 9.24 Potassium-silica diagram for the mean composition of 62 volcanoes collected along the Izu-Bonin-Mariana arc system (modified from Stern et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union).

to describe large composite bodies of plutonic rock that were created above ocean-continent subduction zones. The majority of these batholiths are composed of hundreds to thousands of individual intrusions that range in composition from gabbro, tonalite and diorite to granodiorite and granite. Compositional similarities among many plutonic and some nearby volcanic rocks suggest that the former represent the crystallized residua of deep magma chambers that once fed shallow parts of the system. Their exposure in mature arcs results from prolonged periods of uplift and erosion.

One important, and highly controversial, area of research centers on the origin of the magmas supplying volcanic and plutonic complexes. Certainly the generation of the magmas must be linked in some way to the Benioff zone, as there is a very strong correlation between its depth and the systematic variation in volcanic rock composition and elemental abundances. Early models (e.g. Ringwood, 1975) suggested that the magmas were derived from melting of the top of the descending oceanic slab. However, this idea subsequently was rejected as a general model, in part because thermal models indicate that subducted lithosphere rarely becomes hot enough to melt (Peacock, 1991). In addition, petrologic and mineralogic evidence (Arculus & Curran, 1972) and helium isotope ratios (Hilton & Craig, 1989) indicate that the parental magmas originate by partial melting of asthenospheric mantle immediately overlying the descending plate. Karig & Kay

(1981), Davidson (1983), and Hilton & Craig (1989), among others, demonstrated that certain isotopic ratios require a large contribution from continent-derived sediments. Consequently, sediments from the trench must be carried down the subduction zone and incorporated into the asthenospheric melt (Plank & Lang-muir, 1993). Most authors have concluded that the igneous crust of the subducting lithosphere contributes only very small amounts of melt, except, possibly, in special circumstances where young, hot lithosphere is subducted or warmed by mantle flow (Plate 9.3 between pp. 244 and 245). In this latter case, distinctive melt compositions such as adakites may be produced (Johnson & Plank, 1999; Yogodzinski, 2001; Kelemen et al., 2003).

A major problem of arc magmatism is the source of the heat required for melting the asthenosphere above the descending slab. It was originally believed that this was derived solely by shear heating at the top of the slab. However, this is unlikely because the viscosity of the asthenosphere decreases with increasing temperature, and at the temperatures required for partial fusion the asthenosphere would have such a low viscosity that shear melting could not occur. Ringwood (1974, 1977) suggested that partial melting takes place at a relatively low temperature because of the high water vapor pressure resulting from the dehydration of various mineral phases in the downgoing slab. Indeed, the greater the amount of water present, the more the melting tem

perature of the mantle is reduced (Stolper & Newman, 1994). Thus, water acts as a primary agent that drives partial melting beneath arcs.

It is thought that as much as half of the water carried down into a subduction zone is released below the forearc region, partly into the crust, and also into the mantle producing serpentinite (Fig. 9.19) (Bostock et al., 2002). Most of the water carried to great depths is sequestered in hydrous minerals in altered and metamorphosed crust, including serpentinite. With increasing pressure, hydrous basalt and gabbro are metamorphosed progressively to blueschist, then amphibolite, and then eclogite (Section 9.9). At each transformation water is released. Serpentinite is particularly effective in transporting water to great depth, but the extent to which the subducting lithosphere is serpentinized is unclear. Fast spread oceanic crust is thought to contain little or no serpentinite, but slow spread crust is known to contain some, perhaps as much as 10-20% (Carlson, 2001). However, as described above (Section 9.4), the lithospheric mantle in the downgoing slab may be hydrated to a depth of tens of kilometers as a result of the normal faulting associated with the bending of the plate as it approaches the subduction zone.

A generalized model of arc magmatism begins with the subduction of hydrated basalts beneath continental or oceanic lithosphere (Fig. 9.3). As the slab sinks through the mantle, heat is transferred to it from the surrounding asthenosphere and the basalt in the upper part of the slab begins to dehydrate through a series of metamorphic mineral reactions (Sections 9.4, 9.9). Sediments that have been subducted along with the basalt also dehydrate and may melt due to their low melting temperatures. The release of metamorphic fluids from the slab appears to be quite rapid, possibly occurring in as little as several tens of thousands of years (Turner & Hawkesworth, 1997). By contrast, the recycling of subducted sediment into the upper mantle may be slow (2-4 Ma). As heat is transferred to the slab, temperature gradients are established such that the asthenosphere in the vicinity of the slab becomes cooler and more viscous than surrounding areas, particularly near the upper part of the slab. This more viscous asthenosphere is then dragged down with the slab causing less viscous mantle to flow in behind it, as indicated in Fig. 9.3. It is the interaction of this downwelling mantle with aqueous fluids rising from the sinking slab that is thought to produce partial melting of the mantle. In addition, some melts may result from the upwelling ofhot mantle material within the mantle wedge (Sisson & Bronto, 1998). If hot material rises quickly enough so that little heat is lost, the reduction in pressure may cause pressure release or decompression partial melting (see also Section 7.4.2).

A detailed study of the depth to the zone of seismic-ity and, hence, to the lithospheric slab directly beneath arc volcanoes has shown that, although these depths are consistent within a particular arc, they vary significantly from arc to arc within a range of 65-130 km (England et al., 2004). Surprisingly these depths correlate not with the age or rate of underthrusting of the subducting lithosphere but inversely with the vertical rate of descent of the slab. England & Wilkins (2004) suggest that a high rate of descent increases the rate of flow in the mantle wedge and hence the rate at which hot mantle is drawn towards the corner of the wedge. This would produce higher temperatures, and hence melting, at a shallower depth than in the case of slow rates of descent.

Where sufficient partial melting occurs, probably 10 ± 5% (Pearce & Peate, 1995), the melt aggregates and begins to rise toward the base of the crust. As the magma moves into the crust it differentiates and may mix with either new, crust-derived melts or older melts, eventually forming the magmas that result in the calc-alkaline and alkaline series (Fig. 9.25). In the context of continental arcs, the generation of crust-derived melts appears to be common because the melting point of continental crust may be low enough to result in partial melting. Many of the Mesozoic Cordilleran-type batholiths of western North America (Tepper et al., 1993), the Andes (Petford & Atherton, 1996), West Antarctica (Wareham et al., 1997), and New Zealand (Tulloch & Kimbrough, 2003) contain chemically distinctive plutons that are thought to have originated from the partial melting of the lower continental crust. Tonalites, which are varieties of quartz diorite (see also Section 11.3.2), may be produced if the melting occurs at relatively high temperatures (~1100°C). Granodiorite may be produced if the melting occurs at cooler temperatures (~1000°C) and in the presence of sufficient quantities of water. Melts that move through a thick layer of continental crust may become enriched in incompatible elements before reaching the surface. These magmas also may lose some of their water content and begin to crystallize, with or without cooling. This latter process results in volcanic rocks that are characteristically fractionated, porphyritic, and wet. With time, the crust is thickened by overplating and underplating (Fig. 9.25).

d-Backarc-txi-Active arc-x- Forearc

Figure 9.25 Idealized section through an island arc illustrating the numerous processes involved in its construction. Similar processes may operate beneath Andean-type arcs (redrawn from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

Figure 9.25 Idealized section through an island arc illustrating the numerous processes involved in its construction. Similar processes may operate beneath Andean-type arcs (redrawn from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

Compression of the arc, such as that which occurs in the Chilean Andes (Fig. 9.18a), results in deformation that assists the thickening of arc crust (Section 10.2.4).

The mechanisms by which melts are transported through the mantle and crust are the source of a great deal of controversy. In general, transport processes operate on at least two different length scales (Petford et al., 2000): the centimeter- to decimeter-scale segregation of melt near its source region and the kilometer-scale ascent of magma through the lithosphere to its final site of emplacement. Melt segregation from along grain boundaries probably involves porous flow mechanisms, assisted by ductile and brittle deformation. The ascent of melt appears to involve complex, nonvertical pathways from sources located at different depths. Schurr et al. (2003) identified regions of low Q (Section 9.4) from P-wave arrivals beneath the central Andes that reveal a variety of possible sources and ascent pathways for metamorphic fluids and partial melts (Plate 9.4(top) between pp. 244 and 245). A seismic reflection profile across the central Andes (Fig. 10.7) suggests the presence of fluids, including partial melt, at 20-30 km depth beneath the volcanic arc (ANCORP Working Group, 2003).

Measurements of disequilibria between short-lived uranium series isotopes in island arc lavas have suggested that melt ascent velocities from source to surface can be extremely rapid (103 m a-1) (Turner et al., 2001). Such rates are much too fast for ascent to occur by grain-scale percolation mechanisms. Instead, melts probably separate into diapirs or form networks of low density conduits through which the flow occurs, either as dikes or as ductile shear zones. There is general agreement that deformation greatly enhances the rate of magma ascent. Laboratory experiments conducted by Hall & Kincaid (2001) suggest that buoyantly upwell-ing diapirs of melt combined with subduction-induced deformation in the mantle may create a type of channelized flow. Predicted transport times from source regions to the surface by channel flow range from tens of thousands to millions of years. It seems probable that a range of mechanisms is involved in the transport of magma from its various sources to the surface.

The emplacement of plutons and volcanic rock within or on top of the crust represents the final stage of magma transport. Most models of magma emplacement have emphasized various types of deformation, either in shear zones (Collins & Sawyer, 1996; Saint Blanquat et al., 1998; Brown & Solar, 1999; Marcotte et al., 2005) or in faults (e.g. Section 10.4.2), fractures and propagating dikes (Clemens & Mawer, 1992; Daczko et al., 2001). Some type of buoyant flow in diapirs also may apply in certain settings (e.g. Section 11.3.5). Various mechanisms for constructing plutons and batholiths are discussed by Crawford et al. (1999), Petford et al. (2000), Brown & McClelland (2000), Miller & Paterson (2001b), and Gerbi et al. (2002), among many others.

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  • yvonne
    Why volcanic and plutonic activity occur subducting reach depth of 65130km?
    6 months ago

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