156 / 352
140
conserve their original
176
Hf/
177
Hf
(t)
ratios. Some
of the zircon textures favour this interpretation
for domain II zircon from this sample. Some
of the domain II zircon rims show badly
defined oscillatory textures (
e.g.
A056; Fig. 12)
and domain I zircon cores are surrounded by
alteration fronts (Fig. 12). In some cases domain
I cores show patchy chaotic textures, which are
interpreted as an extreme development of the
alteration fronts (
e.g.
A056, A020, A015; Fig. 12).
The process responsible for the development of
these textures is the exsolution of enriched REE
zircon into depleted REE zircon and xenotime
(with the addition of U–Th-silicates as a product,
Tomaschek
et al.
, 2003), following the reaction
(Zr,Hf,Y,REE)(Si,P)O
4
→ (Zr,Hf)SiO
4
+ (Y,REE)
PO
4
(Pan, 1997). This process is a consequence
of REE purge from the zircon structure triggered
by solid-state recrystallisation. Moreover, the
chaotic patchy cores are normally surrounded
by cracks (BSE images; Fig. 12) that could well
be the result of volume change in these cores
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Fig. 12.
BSE and CL images of representative zircons from the studied sample. Images without circles are BSE (back-scattered
electrons) images, and those with circles are CL (cathodoluminescence) images. Laser ablation pits for U–Pb analyses (red
line circles) have 30
μ
m diameters. Laser ablation pits for Lu–Hf analyses (green line circles) have 33
μ
m diameters. White
numbers are the reference number of the analysis, red numbers are the U–Pb age and its 2
σ
error (Ma) and the green
numbers are the
H
Hf values for the U–Pb age.
7.2. LEUCOSOMES