Hugues Raimbourg 氏 (地球惑星科学専攻 学振研究員)

2006.01.25

タイトル：　「Eclogitization processes and consequences
　　　　　　　　　　　　　　　 for high-pressure rocks exhumation」

要旨：
　　The granulite-facies anorthositic unit on northwestern Holsnoy, within the
Bergen Arcs, Norway, was buried and partially eclogitized at depth during the
caledonian orogenesis. The protolith and the high-pressure rocks and structures
were remarkably preserved throughout exhumation, enabling us to study in detail
eclogitization processes and their consequences for exhumation and deep crustal
movements.
　　Eclogite-facies deformation is very consistent at the scale of the studied
zone (5*5 km), and results therefore from the action of large-scale tectonic
stresses, rather than local stresses related to metamorphic reactions. When the
unit is restored to its original position at depth within the caledonian slab,
the top-to-the-east sense of shear is interpreted as resulting from the the
thrusting of kilometer-scale crustal slivers. The eclogite-facies shear zones,
which enable the mechanical decoupling between the light crustal slivers and the
dense underlying mantle, play therefore a crucial role in the initiation of
exhumation.
　　The relevance of such a conceptual model is assessed through the
unidimensional channel-flow model. Within this model, all material properties
are integrated into a single adimensional parameter, the exhumation number,
which describes the competition between reactions-related density increase
(impeding exhumation) and viscosity decrease (promoting exhumation).
The evolution of the exhumation number through the partially eclogitized domain
controls the circulation over the entire channel, and the burial of
high-exhumation-number crust leads to the formation of a backward flow starting
in the partially eclogitized zone. We show that exhumation rates of the order of
the plate convergence velocities are only reached in the transitional regime
when the nature of the buried crust drastically changes.
　　Both the field and mechanical analyses stress the importance of progressive
metamorphic reactions to exhumation, and call therefore for a detailed study of
how spatially and temporally eclogitization propagates and progresses.
Eclogite-facies metamorphism was divided in two successives phases, based on the
different garnet generations: first, inherited granulitic garnet were fractured
and reequilibrated from their boundaries (crystal or fracture rims); then
eclogite-facies minerals crystallised in the fractures and as overgrowths on
inherited garnets. We show that the reequilibration of inherited garnets,
achieved through Fe2+Mg-1 exchange, while equilibrium eclogite-facies garnets
compositions are notably richer in Ca, is kinetically controlled by the
inefficient intergranular transport properties of Ca compared to Fe2+ and Mg.
On the contrary to the diffusional reequilibration of granulitic garnet that
lasted for several My, the strong compositional gradients between
eclogite-facies and reequilibrated garnets show that rapid exhumation followed
the crystallisation of eclogite-facies minerals. The resulting evolution near
metamorphic peak conditions is therefore strongly asymmetrical, as penetrative
granulitic garnet reequilibration evidences low rates of final prograde motion,
while well-preserved compositional gradients attest to the much higher uplift
rates of the first steps of exhumation.