178 / 352
162
of this rock crystallised (the majority of the
available Hf in a melt is incorporated into zircon,
and Lu into other minerals,
i.e.
the matrix). So,
the only way in which these domain II zircon
achieved its higher
176
Hf/
177
Hf
(t)
is growing in
equilibrium with the matrix at
c.
395 Ma, assisted
by a melt phase (another way is a solid state Hf
isotope exchange between matrix and domain
II, a very unlikely possibility owing the grate
and proofed immobility of Hf in zircon, Hoskin
& Schaltegger, 2003; Gerdes & Zeh, 2009 and
that no inter-element isotopic fractionation is
expected). The geological event in which this
could happen is most probably the high-grade
metamorphism that transformed the basic rock
into an eclogite. This interpretation is favoured
by the low
176
Lu/
177
Hf,
176
Yb/
177
Hf andTh/U ratios
of the
c.
395 Ma (domain II) zircon (Fig. 42a,b &
Appx. 4). These low ratios require that domain
II zircon grew together with, or subsequent to, a
HREE andTh fractionating phase, most probably
metamorphic garnet (Zeh
et al.
2010). These
low ratios can also be achieved by over-growing
domain II Zrn from a depleted Th and HREE
fluid phase. This scenario is not expected because
the first fluids to form should be enriched in
these incompatible elements.
As commented in section 7.3, the protolith of
thisrockwasacumulateoftroctolitecomposition.
This composition, very differentiated from a
normal gabbroic composition, explains why
the magmatic crystallisation age is younger (
c
.
482–473 Ma) than of those co-genetic eclogites
with gabbroic protoliths (eclogite GCH-19;
c.
505–485 Ma).
7.3. ECLOGITES