Download Oceanic crust is dewatered during subduction. Basaltic rocks

7
110
Subduction zones, island arcs and active continental margins
depth
[km]
0
volcanic arc
ocean
200
backarc basin
deep sea trench
400
continent
200
600
400
600
80
0
800
1000
10
00
200
1200
subducting
plate
1400
14
00
80
0
400
1600
10
00
12
00
600
0
200
depth
[km]
200
deep sea trench
600
800
distance from trench [km]
volcanic arc
magmatic gap
oceanic
crust
400
continental
crust
0
20
40
60
300
Abukuma-type
metamorphism
300
600
600
lithospheric
mantle
80
incipient
formation of eclogite
100
1100
1100
120
140
160
asthenosphere
50 km
Fig. 7.25 Diagrammatic
cross sections through
subduction zones. Upper:
computer-modeled
temperature distribution,
in °C, of a subduction
system and, lower: cross
section through an active
continental margin showing an estimation of the
isograds (lines of the same
temperature) (Schubert
and Turcotte, 1975).
Oceanic crust is dewatered during subduction.
Basaltic rocks contain abundant water because
interaction with seawater near the mid-ocean ridge
forms water-containing minerals (zeolite, chlorite,
epidote, amphibole; Ch. 5). Under conditions of
subduction metamorphism, the water-containing
minerals become unstable and are replaced. Water
set free by metamorphism of these minerals is
partly bound in other water-containing but highpressure resistant minerals (e. g., lawsonite, zoisite,
glaucophane); however, the major portion is emitted into the mantle above the subduction zone.
Subducting rocks are thus gradually dewatered and
that is the reason why eclogite, to a large extent, is
a “dry” rock.
In addition to basalt, dolerite and gabbro, sedimentary rocks and splinters of continental crust
carried with the subducted material experience
high-pressure metamorphism. However, the
mineral composition in these rocks is not easy to
identify as having formed under high-pressure
conditions. Many groups of minerals (garnets,
amphiboles, micas, chloritoids, etc.) have special
members formed under high-pressure conditions
which are chemically different from their relatives
formed under different pressure-temperature conditions. Only a detailed microchemical analysis of
the minerals using an electron-microprobe can
detect whether they were formed under highpressure or normal conditions of metamorphism.
Under conditions of high-pressure metamorphism,
iron-magnesium silicates mostly have compositions
similar to those of the Mg end-members because
the small ion radius of magnesium supports denser
packing of atoms.
Sediments contribute significantly to the dewatering in a subduction zone although following
high-pressure metamorphism they continue to include water-containing minerals such as phengite (a
type of mica), karpholite, chloritoid and lawsonite.
Water released in the subduction zone causes intense changes in the mantle above and, from a certain depth, melting processes which are responsible
for magmatism above a subduction zone.
It is difficult to explain how rocks that have
been metamorphosed at great depths in subduction zones are exhumed at the surface. Such highpressure metamorphic rocks occur in mountain
ranges in lenses or in larger bodies within ophiolite sequences and their adjacent rocks. Typically
they occur within long narrow zones that contain
ophiolites and mark the suture zone of collisions
between two continental masses.
Commonly, high-pressure mineral associations
are only preserved as relics. This is because highpressure minerals adapt to decreasing pressure
and temperature conditions that accompany slow
isostatic ascent. Only rapid uplift accompanied by
relatively low temperatures or rapid cooling maintains high-pressure minerals in rocks. Investigations of textures in eclogites suggest that regions
of extreme crustal extension above high-pressure
metamorphic rocks generate the conditions in
which high-pressure minerals can be preserved;
such conditions permit rocks from deep-seated
subduction zones to rapidly reach the surface
(Platt, 1986). Processes associated with erosion by
water and ice occur much too slowly to remove the
overlying rocks rapidly enough.
The geologic setting of the Western Alps clearly
illustrates this last point. The mineral assemblage
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