153 / 352
137
rocks. The presence of newly formed Grt (Fig. 2)
leads one to suppose that the silicate fluid from
which this leucosome crystallised had sufficient
temperature to contain Grt forming cations.
Therefore this partial melting is considered as to
have occurred at high-T conditions. The very low
Th/U,
176
Lu/
177
Hf and
176
Yb/
177
Hf ratios of domain
II zircon (with respect to domain I cores) implies
that domain II zircon is highly depleted inThand
HREE. Therefore these Devonian overgrowths
most probably grew coherent or shortly after a
rare element partitioning mineral, probably Grt.
Despite having discarded a solid-state
recrystallisation process to explain the formation
of domain II rims, domain I cores are normally
surrounded by lobulated sinuous rims, which
are dark in CL images and bright in BSE images
(Fig. 5). As atoms with higher atomic number
(Z) produce stronger scattering, these rims are
brighter in BSE images probably because their
heavy atomcontents are considerably higher than
the rest of the zircon. These rims are interpreted
as alteration fronts, formed by the rare element
purge triggered by a solid-state recrystallisation
(these fronts are not always visible in BSE
images). These observations lead one to conclude
that the Banded Gneiss metasedimentary rocks
underwent partial melting at high-T conditions,
triggering solid-state recrystallisation processes
in the sedimentary inherited zircon (domain I
Cambrian and older zircon) and when the partial
melt cooled sufficiently domain II overgrowths
formed between 377 and 403 Ma, with a
crystallisation climax at
c.
387 Ma.
7.2. LEUCOSOMES