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| Titel |
Pressure variations during ultra-high pressure metasomatism? |
| VerfasserIn |
J. C. Vrijmoed, Y. Y. Podladchikov |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030294
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| Zusammenfassung |
| Incomplete reactions or apparent disequilibrium between minerals in metamorphic rocks
from high pressure (HP) and ultra-high pressure (UHP) terrains are commonly observed in
field and thin section. Here we show an example of a peridotitic body enclosed in migmatitic
felsic Proterozoic gneiss at the Svartberget locality in the northernmost UHP terrain of the
Western Gneiss Region (WGR) in Western Norway.
The WGR represents the basement of Baltica which became metamorphosed during the
Caledonian Orogeny. In the highest grade parts of the WGR metamorphic temperatures were
between 600-800 -C for several Ma, pressures reached the diamond stability field, fluids were
available and yet well known pressure sensitive reactions such as from spinel- to
garnet-peridotite went only locally to completion in the studied peridotite. Previously
reported field observations, major, trace and isotope geochemistry, geochronology,
mineral-chemistry and textures suggest that this peridotite body became metasomatised
by supercritical fluids from the host rock gneiss during the Caledonian Orogeny.
Contrasting pressure estimates, incomplete reactions and preserved compositional
gradients in minerals from this body may indicate very rapid exhumation, very sluggish
kinetics of diffusion, metasomatically disturbed equilibrium at low temperatures or a
combination of the three. These possibilities call for a re-evaluation of existing methods in
geothermobarometry.
Alternatively we may consider that pressure variations existed from the grain up to the
outcrop scale during the metasomatism of the peridotite at UHP conditions. The mechanical
responses to volume changes that are involved in chemical reactions in rocks may control the
progress of reactions significantly (Schmid et al.1, 2009). Our main results, obtained from
numerical modelling, show that pressure variations may be generated, and maintained on the
geological time scale, as a result of the mechanical response of the rocks during melting
reactions in confined space. |
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