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| Titel |
Quantitative estimates of reaction induced pressures: an example from the Norwegian Caledonides. |
| VerfasserIn |
Johannes C. Vrijmoed, Yuri Y. Podladchikov |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078185
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| Zusammenfassung |
| Estimating the pressure and temperature of metamorphic rocks is fundamental to the
understanding of geodynamics. It is therefore important to determine the mechanisms that
were responsible for the pressure and temperature obtained from metamorphic rocks. Both
pressure and temperature increase with depth in the Earth. Whereas temperature can vary due
to local heat sources such as magmatic intrusions, percolation of hot fluids or deformation in
shear zones, pressure in petrology is generally assumed to vary homogeneously
with depth. However, fluid injection into veins, development of pressure shadows
around porphyroblasts, fracturing and folding of rocks all involve variations in
stress and therefore also in pressure (mean stress). Volume change during phase
transformations or mineral reactions have the potential to build pressure if they
proceed faster than the minerals or rocks can deform to accommodate the volume
change. This mechanism of pressure generation does not require the rocks to be under
differential stress, it may lead however to the development of local differential
stress.
The Western Gneiss Region (WGR) is a basement window within the Norwegian
Caledonides. This area is well known for its occurrences of HP to UHP rocks, mainly
found as eclogite boudins and lenses and more rarely within felsic gneisses. Present
observations document a regional metamorphic gradient increasing towards the
NW, and structures in the field can account for the exhumation of the (U)HP rocks
from ~2.5 to 3 GPa. Locally however, mineralogical and geothermobarometric
evidence points to metamorphic pressure up to 4 GPa. These locations present an
example of local extreme pressure excursions from the regional and mostly coherent
metamorphic gradient that are difficult to account for by present day structural field
observations.
Detailed structural, petrological, mineralogical, geochemical and geochronological study at
the Svartberget UHP diamond locality have shown the injection of transitional fluids from
host rock felsic gneiss into a peridotite body and subsequent UHP metasomatism of the
peridotite by these fluids. It thus provides direct evidence that the host rock gneiss was at
higher pressure than the peridotite in order to drive the fluids into the ultramafic
body.
Here we quantify with a numerical model the magnitudes of pressure variations due to
volume change of reaction. We use the 2D geometry of the Caledonian orogen along a profile
across the WGR at the start of UHP metamorphism. The effects of varying viscosity,
temperature, activation energy, horizontal convergence rate and rate of reaction are
investigated.
For typical effective viscosities of 1022 Pas and volume change of 13% per 100.000 years
local pressure variations up to 1.5 GPa can be sustained. It can therefore be concluded that
significant pressure variations can be maintained in rocks on the geological timescale. In the
case of the WGR in Norway, it suggests that the local extreme pressure excursions from the
coherent metamorphic gradient can be explained by the mechanism of pressure generation as
a result of volume change of reaction. |
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