Multi-stage garnet in the internal Brianconnais basement (Ambin massif, Savoy): New petrological constraints on the blueschist-facies metamorphism in the Western Alps and tectonic implications

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State: Public
Version: Final published version
Serval ID
serval:BIB_7CA36D6AB307
Type
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Institution
Title
Multi-stage garnet in the internal Brianconnais basement (Ambin massif, Savoy): New petrological constraints on the blueschist-facies metamorphism in the Western Alps and tectonic implications
Journal
Journal of Petrology
Author(s)
Ganne J., Bussy F., Vidal O.
ISSN-L
0022-3530
Publication state
Published
Issued date
2003
Peer-reviewed
Oui
Volume
44
Pages
1281-1308
Language
english
Abstract
Three types of garnet have been distinguished in pelitic schists from an
epidote-blueschist-facies unit of the Ambin and South Vanoise
Brianconnais massifs on the basis of texture, chemical zoning and
mineral inclusion characterization. Type-1 garnet cores with high Mn/Ca
ratios are interpreted as pre-Alpine relicts, whereas Type-1 garnet
rims, Type-2 inclusion-rich porphyroblasts and smaller Type-3 garnets
are Alpine. The latter are all characterized by low Mn/Ca ratios and a
coexisting mineral assemblage of blue amphibole, high-Si phengite,
epidote and quartz. Prograde growth conditions during Alpine D-1
high-pressure (HP) metamorphism are recorded by a decrease in Mn and
increase in Fe (+/-Ca) in the Type-2 garnets, culminating in peak P-T
conditions of 14-16 kbar and 500degreesC in the deepest parts of the
Ambin dome. The multistage growth history of Type-1 garnets indicates a
polymetamorphic history for the Ambin and South Vanoise massifs;
unfortunately, no age constraints are available. The new metamorphic
constraints on the Alpine event in the massifs define a metamorphic T
`gap' between them and their surrounding cover (Brianconnais and upper
Schistes Lustres units), which experienced metamorphism only in the
stability field of carpholite-lawsonite (T < 400degreesC). These data
and supporting structural studies confirm that the Ambin and South
Vanoise massifs are slices of `eclogitized' continental crust
tectonically extruded within the Schistes Lustres units and Brianconnais
covers. The corresponding tectonic contacts with top-to-east movement
are responsible for the juxtaposition of lower-grade metamorphic units
on the Ambin and South Vanoise massifs.
Open Access
Yes
Create date
01/10/2012 19:07
Last modification date
20/08/2019 14:38
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