On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline
basalts occur as glaucophane-eclogites, blueschists and greenschists in
isolated lenses and layers within metapelites. Whole-rock delta
O-18(SMOW) values of the metabasites show limited variations (10.4-12.0
parts per thousand) and no systematic differences among rock types and
metamorphic grades. This provides no argument for large-scale
blueschist-to-greenschist transformation driven by infiltration of
externally derived fluids. Metamorphic mineralogical changes should have
been triggered by internal fluids. Element variations in interlayered
blue- and greenschists can be attributed to magmatic fractionation.
Assemblages with garnet, clinopyroxene and glaucophane of a
high-pressure/low-temperature (HP-LT) metamorphism M1, and
NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a
medium-pressure/medium-temperature metamorphism M2 crystallized during
deformation Dl. Detailed core-rim zonation profiles display increasing
and then decreasing Al-IV in glaucophane of M1. NaCa-amphiboles of M2,
mantling glaucophane and crystallized in porphyroblasts, show first
increasing, then decreasing, Al-IV and Al-IV. Empirically calibrated
thermobarometers allowed P-T path reconstructions. In
glaucophane-eclogites of a metamorphic zone I, a prograde evolution to
M1 peak conditions at 400-500 degreesC/10-12 kbar was followed by a
retrograde P-T path within the glaucophane stability field. The
subsequent M2 evolution was again prograde up to > 600 degreesC at 8
kbar and then retrograde. Similarly, in metamorphic zones II and III,
prograde and retrograde paths of MI and M2 at lower maximal temperatures
and pressures exist. The almost complete metamorphic cycle during M2
signalizes that the HP-LT rocks escaped from an early erosion by a
moderate second burial event and explains the longlasting slow uplift
with low average cooling rates.