187 / 352
171
are generally observed around domain I zircon
textures. Around these bright rims, darker rims
are observed, which correspond to domain II
and show darker (due to higher U content) faded
oscillatory textures.
Domain I zircon is interpreted to represent
magmatic protolith zircon. Their textures have
been considered to have grown during high-T
anatexis (Vavra
et al.
, 1996), to high-T subsolidus
growth (Schaltegger
et al.
, 1999) and more
recently to have grown from partial melts in
ultrahigh-T terrains (
e.g.
Kelly & Harley, 2005).
Nevertheless this protolithic zircon is related
to the crystallisation of the orthogneiss in an
arc development environment at
c
. 496 Ma (the
entire Banded Gneiss formation is arc-related).
Domain II textures have the same
176
Hf/
177
Hf
(t)
ratios than domain I textures. This implies that
these textures are not overgrowths, but instead
they are a consequence of a
c.
402 Ma solid-state
recrystallisation (
e.g.
Zeh
et al.
, 2010). This is due
to the high immobility of Hf in Zrn, basically
because Hf has the same ionic radius than Zr
(155
p
m).The quantities of Lu and Yb HREE with
respect to Hf (lower
176
Lu/
177
Hf and
176
Yb/
177
Hf
ratios) and Th with respect of U (lower Th/U
ratios) of domain II with respect of domain I
are lower. This is explained by the recrystallising
event that purged from the crystal structure the
cations with ionic radii significantly different
from those of Zr and Hf (Hoskin & Schaltegger,
2003).
Other observations that favour this
interpretation are the small bright rims that
are normally observed between both domains.
These rims are interpreted as alteration fronts,
where trace elements are enriched in detriment
to depletion of domain II darker rims. This trace
element partitioning is an effect of the solid-
state recrystallisation, where purging of non-
essential structural cations takes place (Hoskin
& Schaltegger, 2003). Another observation is
the textures of the cores of many of the domain
II zircon grains. These cores (top row images;
Fig. 54) have chaotic patchy dark textures, they
are surrounded by bright alteration fronts, and
have a high amount of inclusions. These cores
are only present in zircon grains where domain
II textures are well developed and seem to be
the result of extreme cation purge from the
protolithic domain I zircon. The inclusions that
these cores present are probably exsolutions of
rare mineral phases (apatite, xenotime, U–Th-
silicates...) from the Zrn structure.
Solid-state recrystallisation can take place
at any stage during metamorphism, but it
usually dominates the near-peak T region of the
metamorphic P–T path (Harley
et al.
, 2007).
7.4. ORTHOGNEISSES