Impact of textural anisotropy on syn-kinematic partial melting of natural gneisses: an experimental approach.
Résumé
Partial melting of continental crust is a strong weakening process controlling its rheological behavior and
ductile flow of orogens. This strength weakening due to partial melting is commonly constrained experimentally
on synthetic starting material with derived rheological law. Such analog starting materials are preferentially
used because of their well-constrained composition to test the impact of melt fraction, melt viscosity and melt
distribution upon rheology. In nature, incipient melting appears in particular locations where mineral and water
contents are favorable, leading to stromatic migmatites with foliation-parallel leucosomes. In addition, leucosomes
are commonly located in dilatants structural sites like boudin-necks, in pressure shadows, or in fractures within
more competent layers of migmatites. The compositional layering is an important parameter controlling melt flow
and rheological behavior of migmatite but has not been tackled experimentally for natural starting material. In this
contribution we performed in-situ deformation experiments on natural rock samples in order to test the effect of
initial gneissic layering on melt distribution, melt flow and rheological response. In-situ deformation experiments
using a Paterson apparatus were performed on two partially melted natural gneissic rocks, named NOP1 & PX28.
NOP1, sampled in the Western Gneiss Region (Norway), is biotite-muscovite bearing gneiss with a week foliation
and no gneissic layering. PX28, sampled from the Sioule Valley series (French Massif Central), is a paragneiss
with a very well pronounced layering with quartz-feldspar-rich and biotite-muscovite-rich layers. Experiments
were conducted under pure shear condition at axial strain rate varying from 5*10-6 to 10-3 s-1. The main stress
component was maintained perpendicular to the main plane of anisotropy. Confining pressure was 3 kbar and
temperature ranges were 750C and 850-900C for NOP1 and PX28, respectively. For the 750C experiments
NOP1 was previously hydrated at room pressure and temperature. According to melt fraction, deformation of
partially molten gneiss induced different strain patterns. For low melt fraction, at 750C, deformation within the
initially isotropic gneiss NOP1 is localized along large scales shear-zones oriented at about 60 from main stress
component 1. In these zones quartz grains are broken and micas are sheared. Melt is present as thin film (20
m) at muscovite-quartz grain boundaries and intrudes quartz aggregates as injections parallel to 1. For higher
melt fraction, at 850C, deformation is homogeneously distributed.
Domaines
Planète et Univers [physics]
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