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Fluid backflow in subduction zones (eclogite-blueschist transformation in subduction channels)
F. van der Straaten
Subproject within the SFB 574 "Volatiles and Fluids in Subduction
Fig 1: Retrogressive growth of glaucophane with relics of omphacite
included. Thin section under transmitted light from Tianshan.
Descending oceanic plates in subduction zones dehydrate and the
released fluids are involved in the deep fluid cycle of subduction zones.
For a better understanding of this cycle it is necessary to get information I)
about the amount and composition of the fluid that will enter the mantle
wedge, II) how much may escape during upflow along the slab-wedge
interface and III) how much is captured in the subduction channel. The aim
of the study is to identify the sources of the fluid, which is flowing through
the hydrous subduction channel. The study is focused on metabasic rocks,
which underwent prograde dehydration to "dry" eclogite-facies rocks during
burial in a subduction zone but experienced subsequently retrograde
blueschist-facies rehydration during the ascension in a subduction channel.
Key samples are partly rehydrated eclogites, which show sequences of
increasing blueschist-facies overprinting.
Fig 2: Hydrous reaction (1). Omphacite and garnet break down to form
glaucophane and epidote. Inclusions of omphacite in garnet, glaucophane
and epidote. Thin section from Tianshan.
Fig 3: Growth of glaucophane and titanite from omphacite and rutile (reaction 03). Thin section under transmitted light from Syros.
Fig 3: Growth of glaucophane and titanite from omphacite and rutile
(reaction 03). Thin section under transmitted light from Syros.
Retrogressive hydrating reactions during the eclogite-blueschist transition
Subducted oceanic plates underwent a typical HP-LT-Metamorphism,
which is well understood. The predominant basic composition of the
descending plate is transformed into mineral assemblages of the blueschist
facies and in the following of the eclogite facies with several dehydrating
reactions. The rocks become denser, lose volatiles. In rare cases, in relation
to the mass of the subducted plate, some slices of the slab could break
away and would be transported, e.g. in the subduction channel back to the
surface (Gerya et al., 2002). What happens to this former eclogitic rocks
when they pass a volatile rich regime in the water saturated mantle wedge
developed as a circulating subduction channel?
Hydrous reactions were observed during the retrogressive metamorphism
from the eclogite facies back to the blueschist facies. The islands Sifnos and
Syros from the Cycladic archipelago in Greece and the Tianshan Mountains in
NW-China are examples of previous subduction zones and maybe relics of
subduction channels. The outcrops conform to tectonic melanges.
Eclogitic rocks in these areas contain usually the assemblage of Grt -
Omp - Rt - Bar - Ep - Carb. Although the rocks are mostly blueschists,
some show relics of omphacite and garnets indicating eclogitic facies.
Inclusions in garnet describe the prograde mineral assemblages. Inclusions
of epidote, quartz, barroisite or glaucophane and pseudomorphs of epidote,
mica and quartz from lawsonite in the core of garnet describe the mineral
assemblage during the prograde blueschist facies metamorphism. Towards
the rim of garnet inclusions of paragonite, omphacite and rutile are the main
inclusions. Omphacite apparent in the matrix and replaced by glaucophane
and/or epidote in addition with omphacite included in garnet indicates a
retrogressive p-t path from the eclogite facies back to the blueschist facies.
Following hydrous reactions are visible in the samples:
At eclogite facies to epidote-blueschist facies:
- Omp (>Jd60) + Grt + Qtz + H2O ⇒ Gln + Ep
- Jd + Prp + Grs + H2O + CO2 ⇒ Pg + Gln + Dol
- Omp + Rt + Qtz + H2O ⇒ Gln + Ttn
During epidote-blueschist facies:
- Jd + Grt + H2O ⇒ Gln + Pg
- Grt + Omp + H2O ⇒ Gln + Tr (Ca-Na-Amph)
- Omp (Di + Jd) + Qtz ⇒ Di + Ab
From epidote-blueschist facies to albite-epidote-blueschist facies:
- Jd + Qtz ⇒ Ab
- Grt + H2O ⇒ Chl
- Omp + Gln + Qtz ⇒ Win + Ab
- Pg + Gln + H2O ⇒ Ab + Chl
Albite-epidote-blueschist facies to greenschist facies:
- Gln + Ep + Qtz + H2O ⇒ Ab + Chl + Tr/Act
Eclogite-facies pillow basalt, partly overprinted to blueschist.
Entrance to the town of ZhouSu
Eclogite surroundet by blueschist (Syros, Greece).
Bay of Vroulidia (Sifnos, Greece).
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