Evidences for brittle deformation events in eclogite-facies conditions: the case of the Mt. Emilius kilppe (Western Alps)
Résumé
Eclogitic rocks are crucial in tectonics as they provide key constraints
on both the evolution (P-T-t paths) and the deformation
modes sustained by the rocks in subduction zones. We herein focus
on eclogitized flattened mafic bodies remnants exposed within
granulites from the continental basement slice of the Mt. Emilius
klippe (Western Alps, Italy). These eclogites exhibit highly deformed
(up to mylonitic) garnetite and clinopyroxenite levels found
as clasts within meter-thick eclogitic shear zone formed close to
the metamorphic peak conditions under lawsonite bearing eclogite
facies conditions. The garnet-rich levels tend to behave in a
brittle fashion while deformation within clinopyroxene-rich levels is
mostly accommodated by creep, as evidenced by the presence of
elongated grains, subgrains boundaries and intense grain size reduction.
Especially, crystallographic preferred orientation (CPO)
measurements in garnet indicate a quasi-random distribution. In
most of the clinopyroxene levels nevertheless, the CPO is relatively
strong, with multiples of uniform distribution varying from
4 to 5.5 (value of 1 is random texture). This CPO is characterized
by a strong alignment of poles of the planes (001) parallel
to the lineation and poles of the planes (100) and directions of
the planes [010] distributed along girdles cross-cutting the foliation
plane. Based on a petrological study, from the outcrop to
the microscopic scale, we propose a tectono-metamorphic history
of the Mt. Emilius eclogites. This history related to the alpine
eclogitic event, allow to link the metamorphic P-T path and the
deformation processes in association with the chemical evolutions
that could lead to the formation of the Mt. Emilius brecciated
eclogites in the Arbole region. This model is consistent with the
alternating ductile and brittle events that developed in the same
metamorphic facies, at P~2.0-2.5 GPa and T~500-550C, closely
associated to metamorphic fluid circulations. Thus, our study attests
that the material along the subduction interface, at HP-LT
conditions, can locally be brittle where deformation is classically
envisioned as ductile.