The metamorphic petrology of the Southern Brittany Migmatite Belt, France
The Southern Brittany Migmatite Belt developed during the
Ligerian Orogeny within a narrow time interval 403 Ma to 372
Detailed field mapping of several key localities within the
belt (Port Navalo, Port Blanc and Roguedas, around the Golfe
du Morbihan, and Ville-es-Martin at St. Nazaire) has revealed
a heterogeneous suite of high grade gneisses, Al-silicate
rich gneisses (morbihannites), low to high melt fraction
metatexites and diatexites.
Careful petrography and mineralogy has allowed the
establishment of reaction histories for each rock type which
have been utilised in constructing segments along a P-T path.
Reverse zoned rims and the replacement of garnet by Crd
and/or Bt + PI suggests the following generalised reactions
have occurred: Grt + Bt + S11 + Qtz~ Crd + He + Um + Kfs +
H 0 and Grt --=;>Bt + PI + Qtz.
tarnet-cordierite gneisses record an early prograde event.
Growth zoned garnet cores and a sequence of inclusions, from
the garnet core to the garnet rim, of Qtz + 11m + Ky, PI + St
+ Rt + Ky + Bt and PI + St + Rt + Sil + Bt constrain the
prograde evolution and suggest the crossing of the simplified
reactions Ms + Chl~ St + Bt + Qtz and St + Qtz ~Grt + Ky +
A detailed evaluation of the available thermometers and
barometers, equilibrium considerations, the stability
relations of biotite, and petrographic analysis have enabled
the construction of a tightly constrained P-T path.
It is suggested that the prograde P-T path is the result of
a series of sediments deposited in an ensialic marginal basin
with a higher than normal geothermal gradient which has been
tectonically buried by overthrusting during basin closure.
The retrograde near isothermal decompression path is
interpreted to be the result of the development of a rising
anatectic granite diapir which has dragged its thermal
envelope, the migmatites, to shallow crustal levels during