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Séminaire CENTRELLA Stephen, Université de Münster

Jeudi 29 Mars 2018, Salle -107M
Stephen Centrella, Institute of Mineralogie, University of Münster, Germany

The Bergen Arcs is located in the western coast of Norway near the city of Bergen. This PhD project has been focussed only on the Lindås Nappe because it represents the most important nappe in terms of volume in the whole Bergen Arc system. The Lindås Nappe is mainly composed of rocks belonging to a Proterozoic anorthosite-mangerite-charnockite-granite. During the Greenvillian Orogeny, these rocks experienced a granulite-facies metamorphism at pressures less than 10kbar and around 800-850°C. During the Caledonian orogeny the Lindås Nappe has been affected by a partial eclogite and amphibolite facies. Both metamorphic facies occur along brittle fracture where the fluid enters and reacts into the shear zone through the granulite.
This region provides a natural laboratory to better understand metamorphic reactions from a regional to millimetre scale. Because the interface between these three metamorphic facies is very sharp, it provides an exceptional site to determine the mechanism of metamorphic reactions.
The work of this doctoral thesis shows that the relationship between volume-density and mass transfer is important in metamorphism. The study of the transition from granulite to the hydrated amphibolite shows that the bulk rock composition doesn’t change significantly during the hydration event for major and trace element. In this example, the shape of the parent mineral (garnet and clinopyroxene) is preserved supposing a pseudomorphic replacement. Because the system stays isochemical, it supposes a redistribution of element with the pseudomorphic replacement of the individual garnet and clinopyroxene grains. For major and trace element, there is a clear correlation between the gain and the loss during garnet replacement and the corresponding losses and gains for the clinopyroxene. However, the molar volumes of the hydrated phases are greater than the anhydrous phases in the granulite. This implies that stress should be generated during hydration. Because there is no fracture in the system, it proof that the stress is compensated by another mechanism. The loss of mass during amphibolitisation seems to compensate this stress generation.
Using another sample for the granulite-amphibolite transition located in the same region, we can see a reaction gaining density and losing volume whereas a second, losing density but gaining volume. This happened within a single crystal of clinopyroxene. Even if the two reactions differ by density, volume and paragenesis, the mass behaves in the same way. The only difference found between them is the presence of a couple stress-strain for the reaction losing volume and gaining density. Associated to this, the estimated thermodynamic pressure is 15kbar higher than the rock showing that the stress redistribution seems to influence thermodynamic equilibrium.

This relationship between two reactions balancing volume, density with a similar mass behaviour has been seen also at outcrop scale. The outcrop presents the transition granulite-eclogite-amphibolite. The system is isochemical except a gain in LOI for eclogite and amphibolite. Eclogitisation of the granulite gains density but loses volume whereas amphibolitisation of the granulite, loses density but gains volume. Using thermodynamic modelling, it appears that eclogite and amphibolite can be stable at the same temperature with the same fluid composition (H2O-CO2) but differ by the pressure. The thermodynamic pressure for the eclogite is around 10kbar higher than the amphibolite. We discuss the eventual way of interpreting this pressure because the gain of volume associated to amphibolitisation should generate a stress in the system. This might influence the thermodynamic equilibrium as described in the previous example.

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