7110 Subduction zones, island arcs and active continental margins

volcanic arc In addition to basalt, dolerite and gabbro, sedi- depth backarc basin mentary rocks and splinters of continental crust [km] ocean deep sea trench continent carried with the subducted material experience 0 200 200 400 600 400 high-pressure metamorphism. However, the 600 800 mineral composition in these rocks is not easy to 800 identify as having formed under high-pressure

1000 1000 conditions. Many groups of minerals (garnets, 200 1200 amphiboles, micas, chloritoids, etc.) have special subducting members formed under high-pressure conditions plate 1400 which are chemically diff erent from their relatives 1400 formed under diff erent pressure-temperature con- 800 ditions. Only a detailed microchemical analysis of 400 1600 the minerals using an electron-microprobe can 1000 detect whether they were formed under high- pressure or normal conditions of metamorphism. Under conditions of high-pressure metamorphism, 1200 600 iron-magnesium silicates mostly have compositions similar to those of the Mg end-members because the small ion radius of magnesium supports denser

200 0 200 400 600 800 packing of atoms. distance from trench [km] Sediments contribute signifi cantly to the de- watering in a subduction zone although following depth deep sea trench volcanic arc [km] continental high-pressure metamorphism they continue to in- magmatic gap oceanic crust crust clude water-containing minerals such as phengite (a 0 type of mica), karpholite, chloritoid and lawsonite. 300 Water released in the subduction zone causes in- 20 Abukuma-type 300 metamorphism tense changes in the mantle above and, from a cer-

40 600 tain depth, melting processes which are responsible 600 for magmatism above a subduction zone. 60 lithospheric It is diffi cult to explain how rocks that have mantle been metamorphosed at great depths in subduc- 80 incipient tion zones are exhumed at the surface. Such high- formation of eclogite pressure metamorphic rocks occur in mountain 100 1100 ranges in lenses or in larger bodies within ophi- 1100 120 olite sequences and their adjacent rocks. Typically they occur within long narrow zones that contain 140 asthenosphere ophiolites and mark the suture zone of collisions between two continental masses. 160 50 km Commonly, high-pressure mineral associations are only preserved as relics. Th is is because high- pressure minerals adapt to decreasing pressure Fig. 7.25 Diagrammatic Oceanic crust is dewatered during subduction. and temperature conditions that accompany slow cross sections through Basaltic rocks contain abundant water because isostatic ascent. Only rapid uplift accompanied by subduction zones. Upper: interaction with seawater near the mid-ocean ridge relatively low temperatures or rapid cooling main- computer-modeled temperature distribution, forms water-containing minerals (zeolite, chlorite, tains high-pressure minerals in rocks. Investiga- in °C, of a subduction epidote, amphibole; Ch. 5). Under conditions of tions of textures in eclogites suggest that regions system and, lower: cross subduction metamorphism, the water-containing of extreme crustal extension above high-pressure section through an active minerals become unstable and are replaced. Water metamorphic rocks generate the conditions in continental margin show- set free by metamorphism of these minerals is which high-pressure minerals can be preserved; ing an estimation of the isograds (lines of the same partly bound in other water-containing but high- such conditions permit rocks from deep-seated temperature) (Schubert pressure resistant minerals (e. g., lawsonite, zoisite, subduction zones to rapidly reach the surface and Turcotte, 1975). glaucophane); however, the major portion is emit- (Platt, 1986). Processes associated with erosion by ted into the mantle above the subduction zone. water and ice occur much too slowly to remove the Subducting rocks are thus gradually dewatered and overlying rocks rapidly enough. that is the reason why eclogite, to a large extent, is Th e geologic setting of the Western Alps clearly a “dry” rock. illustrates this last point. Th e mineral assemblage

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