158 / 352
142
in response of the zircon exsolution process.
From the above-mentioned observations a solid-
sate recrystallisation process for the genesis of
domain II rims seems proved, but a genesis of
these rims by overgrowth cannot be ruled out
for the following reasons: (i) domain II rims
also show well defined combined oscillatory and
sector (“soccerball”) textures; (ii) this sample
is a leucosome formed by partial melting (and
therefore it crystallised from a fluid phase);
(iii)
176
Hf/
177
Hf
(t)
ratios in this sample and in the
previous sample (GCH-25) of the same zircon
domains have the same values within error, and
domain II rims from the previous sample were
interpreted as overgrowths (both samples are
leucosomes formed by the same partial melting
event from metasedimentary rocks from the
same formation). Furthermore, if a comparison
of
176
Hf/
177
Hf
(t)
ratios of cores and rims from
the same crystals is made, both processes are
evident. Analyses A011 (core) and A010 (rim;
Fig. 12) have clearly different
176
Hf/
177
Hf
(t)
ratios
(0.282532 and 0.282690 respectively) pointing to
a overgrowth genesis of the rim. Analyses A019
(core) and A020 (rim; Fig. 12) have very similar
176
Hf/
177
Hf
(t)
ratios (0.282629, 0.282694) pointing
to a very probable solid-state recrystallisation
genesis of the rim. From these observations it
seems reasonable to propose that both processes
were involved in the generation of domain
II Devonian rims. In the previous sample
(leucosome GCH-25) it was proposed that
overgrowth followed solid-state recrystallisation,
but from this sample this sequence of events does
not seem to have occurred in this order. Zircon
dissolution and re-precipitation can occur at the
prograde or the retrograde metamorphic path
and solid-state recrystallisation usually occurs
somewhere near the temperature peak (Harley,
2007). As the cracks observed in BSE images
seem to crosscut the entire rims (they have
been interpreted as a consequence of solid state
recrystallisation) it seems logical to propose that
overgrowths crystallised from the partial melt in
the prograde metamorphic path and that solid-
state recrystallisation occurred afterwards when
the system reached the temperature peak.
7.2. LEUCOSOMES