Page 286 - Global Tectonics

SUBDUCTION ZONES 271

9.8 VOLCANIC AND have been interpreted as being derived by the fractional
crystallization of olivine from a primary magma origi-
PLUTONIC ACTIVITY nating 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 refl ect
magmas generated at depths greater than those that
Where subducting oceanic lithosphere reaches a depth result in tholeiitic rocks. Calc-alkaline magmas, repre-
of 65–130 km, volcanic and plutonic activity occurs, sented by andesite and basaltic andesite, are the most
giving rise to an island arc or an Andean-type continen- abundant of the volcanic series. Alkaline magmas
tal arc approximately 150–200 km from the trench axis exhibit the lowest abundance in island arcs and are more
(England et al., 2004). The thickness of arc crust refl ects common in continental rifts and intraplate environ-
both the age of the system and the type of crust on ments (Section 7.4.2).
which the arc forms. Relatively young island arcs, such Some island arcs exhibit spatial patterns in the distri-
as the 3–4 Ma active part of the Mariana volcanic arc, bution of the volcanic series. In the Japanese island arc
may be underlain by a crust of 20 km thickness or less. system, for example, a compositional trend of tholeiite/
Thin crust also generally occurs in settings where calc-alkaline/alkaline volcanic rocks is apparent with
extension is dominant, such as in the Mariana arc increasing distance from the trench. This trend may

system (Fig. 9.18b) (Kitada et al., 2006). Mature island reflect magmas derived from increasingly greater depths
arcs, such as those in the Neogene Japanese arc system, and/or differences in the degree of partial melting (Gill,
generally show crustal thicknesses ranging from 30 km 1981). A low degree of partial melting tends to concen-
to 50 km because they have been constructed on older trate alkalis and other incompatible elements into the
igneous and metamorphic rock (Taira, 2001). Conti- small melt fraction (Winter, 2001), and could lead to an
nental arcs, including the Andes and the Cascades, are increase in alkalinity away from the trench due to a
structurally the most complex of all arc systems greater depth of melting or a decrease in the availability
because of the numerous structural and compositional of water. However, there are many exceptions to this
heterogeneities that are intrinsic to continental litho- pattern in other arcs, indicating that differences in local
sphere. In compressional continental settings (e.g. Figs conditions strongly influence magma compositions.

9.18a, Plate 10.1 (between pp. 244 and 245)), where The Izu-Bonin–Mariana arc system (Fig. 9.1), for
substantial crustal thickening occurs, arc crust may example, shows compositional trends along the axis of
reach thicknesses of 70–80 km (Section 10.2.4). the arc. From 35°N to 25°N latitude, volcanoes that
The types of volcanic rocks that occur in the supra- form part of the Izu and Bonin arc segments are domi-
subduction zone environment generally form three nated by low and medium potassium rock suites (Fig.
volcanic series (Gill, 1981; Baker, 1982): 9.24). The Mariana segment is dominated by medium
potassium suites from 14°N to 23°N, and a shoshonitic
1 The low potassium tholeiitic series that is province is found between the Mariana and Bonin seg-
dominated by basaltic lavas associated with ments (Stern et al., 2003). This great spectrum of rock
lesser volumes of iron-rich basaltic andesites compositions reflects the diversity of processes involved

and andesites. in arc magmatism, including variations in the depth and
2 The calc-alkaline series, dominated by andesites degree of partial melting, magma mixing, fractionation,
(Thorpe, 1982) that are moderately enriched in and assimilation (see also Section 7.4.2). In general,
potassium, other incompatible elements, and these observations indicate that the three volcanic series
the light rare earth elements. In continental arcs form a continuum of rock compositions and do not
dacites and rhyolites are abundant, although correspond to absolute magma types or source
they are subordinate to andesites. regions.
3 The alkaline series that includes the subgroups Mature arc systems, and especially continental arcs,
of alkaline basalts and the rare, very high typically include large, linear belts of plutonic rock
potassium-bearing (i.e. shoshonitic) lavas. called batholiths. These belts are so common in conti-
nental crust that they are widely used as indicators of
In general, the tholeiitic magma series is well repre- ancient, now extinct convergent margins (Section 5.9).
sented above young subduction zones. These rocks Occasionally the term Cordilleran-type batholith is used