When did subduction start—And how did it evolve?
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Adakite
Eclogitization
Ultramafic rock
A finite-difference numerical program has been constructed to model the thermal structure of a subduction zone, including the combined effects of frictional heating and the heat sink created by dehydration of the subducted oceanic crust. Frictional heating is assumed to begin at the surface and to decrease linearly to zero at the depth at which dehydration begins. Dewatering of the oceanic crust occurs in the 80-125 km depth range and removes approximately 50 cal/gm of oceanic crust dehydrated. In young (<60 m.y. old) island arcs, isotherms are sufficiently elevated by frictional heating to melt the slab, perhaps accounting for the early tholeiitic volcanism recognized in some arcs. In old subduction zones, however, the frictional heating term must be increased to the equivalent of > 10 Kb shear stress (at the surface) before isotherms below the dehydration zone are elevated above the level predicted by conduction models. For lower stresses, the slab at km depth never becomes hotter than 600°C and thus does not melt. Alternative heat sources for arc volcanism are convection of asthenosphere above the slab and lowering of the melting point of overlying peridotite by release of water from dehydration of the subducted oceanic crust. In either case, the released water is probably a major transporter of Si, Na, K, Rb, Sr, Ba, and REE from the oceanic crust to the overlying wedge.
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Asthenosphere
Eclogitization
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The petrogenesis of adakites holds important clues to the formation of the continental crust and copper ± gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given that oceanic crust has copper concentrations about two times higher than those in the continental crust, whereas the high oxygen fugacity in the subduction environment promotes the release of copper during partial melting, slab melting provides the most efficient mechanism to concentrate copper and gold; slab melts would be more than two times greater in copper (and also gold) concentrations than lower continental crust melts and normal arc magmas. Thus, identification of slab melt adakites is important for predicting exploration targets for copper- and gold-porphyry ore deposits. This explains the close association of ridge subduction with large porphyry copper deposits because ridge subduction is the most favorable place for slab melting.
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Fractional crystallization (geology)
Petrogenesis
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The timing and mechanism of the closure of the Palaeo‐Asian Ocean are problematic and controversial. To help resolve these problems, we report geochronological, geochemical, and isotopic data from mid‐Triassic adakitic intrusions in the Eastern Tianshan, NW China. U–Pb dating shows that the adakitic intrusions formed at 243–234 Ma. These mid‐Triassic adakitic intrusions are characterized by high Sr/Y and La/Yb ratios, low Yb and Y, and positive ε Nd (t) (+3.12 − +4.71) and low ( 87 Sr/ 86 Sr) i (0.703956–0.704487) values. However, they have high K and low A/NK values (1.16–1.71), relatively low MgO (0.43–1.81 wt%) contents, and Mg # (44–61), low abundances of compatible elements (Cr = 4.09–16.69 ppm, Ni = 2.02–8.03 ppm), which are different from the typical slab‐melting adakites. These features indicate that they were derived from the partial melting of the relatively depleted thickened lower crust. Our new geochronological, geochemical, and isotopic data integrated with the established amalgamation of nearby arcs lead us to conclude that the adakitic intrusions were most likely generated by partial melting of the tectonically thickened (>40 km) juvenile Dananhu intra‐oceanic arc and, therefore, the Kanguer branch of the Palaeo‐Asian Ocean closed since ca. 234 Ma.
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Island arc
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Adakite
Solidus
Volcanic arc
Underplating
Eclogitization
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Adakite
Underplating
Obduction
Eclogitization
Convergent boundary
Continental Margin
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Adakite
Convergent boundary
Underplating
Volcanic arc
Andesites
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Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and the potential for oceanic crust (including metasediments) to transport water deeper into the mantle.
Adakite
Amphibole
Eclogitization
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Adakite
Amphibole
Island arc
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Adakite
Andesites
Protolith
Lile
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