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| Titel |
Multi-stage barites in partially melted UHP eclogite: implications for fluid/melt activities during deep continental subduction in the Sulu orogenic belt |
| VerfasserIn |
Songjie Wang, Lu Wang |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103796
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| Publikation (Nr.) |
 EGU/EGU2015-3212.pdf |
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| Zusammenfassung |
| Barite (BaSO4) is well-known from deep-sea sedimentary environments but has received less
attention to its presence in high-grade metamorphic rocks. Recently, barite in ultrahigh
pressure (UHP) eclogite has drawn increasing attention from geologists, especially in the
Dabie-Sulu orogen, since it is an important indicator for high-salinity fluid events, thus aiding
in further understanding HP-UHP fluid / melt evolution. However, its formation time and
mechanism in UHP eclogite are still controversial, with three representative viewpoints: (1)
Liu et al. (2000) found barite-anhydrite-coesite inclusions in zircon and interpreted them
to have formed by UHP metamorphic fluids; (2) Zeng et al. (2007) recognized
isolated barite within K-feldspar (Kfs) and Quartz (Qz) surrounded by radial cracks
in omphacite, and interpreted Kfs+Qz to be reaction products of potassium-rich
fluid/melt and coesite, with the barite formed by prograde metamorphic fluids; (3) Gao
et al. (2012) and Chen et al. (2014) found barite-bearing Multiphase Solid (MS)
inclusions within garnet and omphacite and assumed that the barite formed by phengite
breakdown possibly caused by eclogite partial melting during exhumation, though no
direct evidence were proposed. The controversy above is mainly due to the lack
of direct formation evidence and absence of a clear link with the metamorphic
evolution of UHP eclogite along the subduction-exhumation path. We report detailed
petrological and micro-structural analyses revealing four types of barites clearly
linked with (1) the prograde, (2) earlier stage of partial melting and (3) later stage of
crystallization differentiation, as well as (4) high-grade amphibolite-facies retrogression of a
deeply subducted and partially melted intergranular coesite-bearing eclogite from
Yangkou Bay, Sulu Orogen. Round barite inclusions (type-I) within UHP-stage garnet
and omphacite are formed by internally buffered fluids from mineral dehydration
during prograde metamorphism. Zr-in-rutile thermometry shows their formation
temperature to be 586-664 oC at 1.5-2.5 GPa. Barite-bearing MS inclusions with
Ba-bearing K-feldspar (type-II) connected by Kfs+Pl+Bt veinlets of in-situ phengite
breakdown and thin barite veinlets along grain boundaries (type-III) are products of
phengite breakdown and induced fluid flow during exhumation. These barites have
witnessed the gradational separation process of melt/ fluid from miscibility on/above the
second critical endpoint during UHP metamorphism, to immiscibility along the
exhumation path of the subducted slab. Associated reactions from pyrite to hematite
and goethite with the type-III barite ring surrounding the pyrite provide evidence
for a local high oxygen fugacity environment during eclogite partial melting and
subsequent melt/fluid crystallization processes. Moreover, large grain barite aggregations
(type-IV) modified by amphibole+albite symplectite are most likely formed by
release of molecular and hydroxyl water from anhydrous minerals of eclogite during
high-grade amphibolite-facies retrogression. The growth of multi-stage barites in UHP
eclogite further advances our understanding of fluid/melt transfer, crystallization
processes along the subduction-exhumation path of the partially melted eclogite,
broadening our knowledge of melt/fluid evolution within subduction-collision zones
worldwide.
REFERENCES
Chen Y.X., et al., 2014, Lithos, 200, 1-21.
Liu J.B., et al., 2000, Acta Petrologica Sinica 16(4), 482-484.
Zeng L.S., et al., 2007, Chinese Science Bulletin, 52(21), 2995-3001.
Gao X.Y., et al., 2012, Journal of Metamorphic Geology, 30(2), 193-212. |
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