Prograde P–T evolution and partial melting of a Variscan high- temperature eclogite, French Massif Central, Haut-Allier
Résumé
A detailed analysis of the petrographic relations and mineral chemical zoning combined
with the use of calculated psuedosections is a powerful tool to unravel cryptic, yet
important details of the P–T evolution of rocks, with a potential impact on the
interpretation of the geodynamic context.
A mafic kyanite-bearing eclogite from the Haut-Allier displays garnet-rich layers that
have intriguing similarities and differences with respect to the surrounding eclogites.
Chemical zoning of garnet inside and outside the layers is similar, but the layers are
enriched in rutile and ilmenite, and lack kyanite, common in the eclogite. Three
successive metamorphic stages, M1, M2 and M3, are characterized by assemblages
comprising garnet1-omphacite- kyanite, garnet2-plagioclase, and amphiboleplagioclase,
respectively, and define a clockwise P–T path. These events occurred at
the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼
22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and hightemperature
amphibolite facies (M3; < 9 kbar, 750– 850 °C), respectively. Phasediagram
modelling of garnet growth zoning and mineralogy of the inclusions reveal a
prograde M1 stage, first dominated by burial and then by near-isobaric heating.
Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in
terms of a decompression during M2. High- pressure partial melting is predicted for
both the M1 temperature peak and M2, due to focussed influx of H2O-rich fluids or
external melts, probably associated with localized deformation. It is inferred that
subsequent melt loss resulted in the formation of the garnet- rich layers. Despite the
absence of clear accumulations of crystallised melts in the eclogite, high-pressure
melting may he linked to the existence of HP trondhjemitic melts, described elsewhere
in the Massif Central. M3 testifies to further strong decompression associated with
limited cooling. The preservation of garnet growth zoning indicates the short-lived
character of the temperature increase, decompression and cooling cycle. We argue that
such P–T evolution is compatible with the juxtaposition of the asthenosphere against
the subducted crust prior to exhumation driven by slab rollback.