Devolatilization reactions and subsequent transfer of fluid from
subducted oceanic crust into the overlying mantle wedge are important
processes, which are responsible for the specific geochemical
characteristics of subduction-related metamorphic rocks, as well as
those of arc magmatism. To better understand the geochemical fingerprint
induced by fluid mobilization during dehydration and rehydration
processes related to subduction zone metamorphism, the trace element and
rare earth element (REE) distribution patterns in HP-LT metamorphic
assemblages in eclogite-, blueschist- and greenschist-facies rocks of
the Ile de Groix were obtained by laser ablation inductively coupled
plasma mass spectrometry (LA-ICPMS) analysis. This study focuses on 10
massive basic rocks representing former hydrothermally altered mid-ocean
ridge basalts (MORB), four banded basic rocks of volcano-sedimentary
origin and one micaschist. The main hosts for incompatible trace
elements are epidote (REE, Th, U, Pb, Sr), garnet [Y, heavy REE
(HREE)], phengite (Cs, Rb, Ba, B), titanite [Ti, Nb, Ta, REE; HREE >
LREE (light REE)], rutile (Ti, Nb, Ta) and apatite (REE, Sr). The trace
element contents of omphacite, amphibole, albite and chlorite are low.
The incompatible trace element contents of minerals are controlled by
the stable metamorphic mineral assemblage and directly related to the
appearance, disappearance and reappearance of minerals, especially
epidote, garnet, titanite, rutile and phengite, during subduction zone
metamorphism. Epidote is a key mineral in the trace element exchange
process because of its large stability field, ranging from lower
greenschist- to blueschist- and eclogite-facies conditions. Different
generations of epidote are generally observed and related to the
coexisting phases at different stages of the metamorphic cycle (e.g.
lawsonite, garnet, titanite). Epidote thus controls most of the REE
budget during the changing P-T conditions along the prograde and
retrograde path. Phengite also plays an important role in determining
the large ion lithophile element (LILE) budget, as it is stable to high
P-T conditions. The breakdown of phengite causes the release of LILE
during retrogression. A comparison of trace element abundances in
whole-rocks and minerals shows that the HP-LT metamorphic rocks largely
retain the geochemical characteristics of their basic,
volcano-sedimentary and pelitic protoliths, including a hydrothermal
alteration overprint before the subduction process. A large part of the
incompatible trace elements remained trapped in the rocks and was
recycled within the various metamorphic assemblages stable under
changing metamorphic conditions during the subduction process,
indicating that devolatilization reactions in massive basic rocks do not
necessarily imply significant simultaneous trace element and REE
release.