Phase transition in the subducted oceanic lithosphere and generation of the subduction zone magma |
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Authors: | ChunJing Wei YingHui Zhang |
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Institution: | (1) Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, School of Earth and Space Sciences, Peking University, Beijing, 100871, China |
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Abstract: | Two metamorphic processes, i.e. subsolidus dehydration and partial melting occurring in MORB, metasediments and peridotite
of subducted oceanic lithosphere are discussed on the basis of available experimental work and phase equilibrium modeling.
Phase diagrams of hydrous MORB show that in most cold subduction P-T (pressure-temperature) regimes a large portion of water in the basic layer has released below the onset of blueschist facies
(< 20 km), and at a depth (60–70 km) of transition from lawsonite blueschist to lawsonite eclogite facies through glaucophane
dehydration; only a smaller portion of water will escape from the slab through dehydration of lawsonite and chloritoid in
the depth range suitable for arc magma formation; and a very small portion of water stored in lawsonite and phengite will
fade into the deeper mantle. The role of amphibole for arc magma formation is still arguable. In cold subduction P-T regimes, the dehydration of chlorite and talc in Al-poor metasediments, and chloritoid and carpholite in Al-rich metapelites
at a depth around 80–100 km will make some contributions to the formation of arc magma. Comparatively, dehydration of serpentine
in hydrated peridotite occurs at depths of 120–180 km, playing an important role in the arc magmatism. Subduction of oceanic
crust along warm P-T regimes will cross the solidi at a depth over 80 km, resulting in partial melting under fluid-saturated and fluid-absent
conditions in the metasediments involving biotite and phengite, and in the basic rocks involving epidote and amphibole. The
melt compositions of the basic crust are adakitic at pressures < 3.0 GPa, but become peraluminous granitic at higher pressures.
Supported by National Natural Science Foundation of China (Grant No. 40525006) |
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Keywords: | dehydration partial melting subduction of oceanic lithosphere subduction zone magma |
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