Partitioning of deformation within high-strain metapelitic zones and preservation of early high- pressure metamorphic structures : Insights from phase relation modelling along a cold alpine geothermal gradient
Abstract
The influence of shearing and fluid transfer on phase relationships is investigated in a alpine eclogitic massif of metapelitic composition, deformed at different crustal levels, from blueschist to greenschist metamorphic conditions. Phase diagram sections are commonly used to study phase relationships as a function of pressure and temperature in closed-systems. Here, an open system model without mass element transfer is required to discuss the interaction between fluid flow and mineral phase evolution occurring during deformation. Mapping phase relations as a function of fluid flow variation, from underdeformed to mylonitic metapelitic sequences provides a model coupling metamorphism, deformation and fluid circulation. Predictions of phase composition and modal proportion of P-T-MH20 phase diagrams (using Perplex calculations) are in good agreement with natural observations. They suggest that the distribution of shearing throughout the massif leads to the preservation or destabilisation of the HP-assemblages it contains. Thermodynamic modelling of open systems illustrates how the development of small-scale (mm-cm) textural heterogeneities in high-strain zones, such as large porphyroblasts of albite, have preferentially been controlled by the amount of H20 brought by shearing, rather than by the effective bulk rock composition. Such effects of Def versus MH20 would have a profound impact on the control of rheological properties for the bulk rock, and ultimately the processes of deformation leading to the exhumation of the Internal Briançonnais massifs. Keywords: High-strain zone, strain partitioning, channelized fluid-flow, High-Pressure-metapelites, P-T-MH20 phase diagram, exhumation
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2007
- Bibcode:
- 2007AGUSM.V53A..04G
- Keywords:
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- 3654 Ultra-high pressure metamorphism;
- 3902 Creep and deformation;
- 3924 High-pressure behavior;
- 8124 Earth's interior: composition and state (1212;
- 7207;
- 7208;
- 8105);
- 8162 Rheology: mantle (8033)