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197
gabbroic or basaltic protolith from 505 to 485 Ma
(zircon crystallisation climax at
c
. 485 Ma).
From the second sample studied (GCH-20,
section 7.3.2) it was concluded that the protolith
mafic rock was crystallised almost directly from
a DM source, and that a HP–HT metamorphic
event induced extreme zircon solid-state
recrystallisation of the protolith zircon between
372 and 410 Ma, with a zircon crystallisation
climax at 390 Ma.
From the third sample studied (GCH-22,
section 7.3.3) it was concluded that the age
of crystallisation of the protolith material is
constrained between 482 and 473 Ma, with a
zircon crystallisation climax at 473 Ma. This
protolithic zircon overgrew at high-T conditions
at
c
. 395 Ma together with, or subsequent to,
metamorphic Grt.
From the fourth sample studied (GCH-
23, section 7.3.4) it was concluded that the
protolithic zircon crystallised at around
c
. 508
Ma. This mafic rock was intruded by a felsic rock
(GCH-14, section 7.4.4) at
c
. 500 Ma, inducing a
high-T input in the mafic rock, which triggered
zircon pseudomorphic alteration at
c
. 489 Ma,
with no presence of a fluid phase.
The four eclogites have been represented
together in Fig. 93, which combines the
H
Hf
(t)
data of the four samples and the U–Pb combined
density diagram of these four and of the detrital
zircon of the HP–HT metasedimentary rocks
(Banded Gneisses). This combined figure shows
a zircon crystallisation climax for the Cambrian–
Ordovician eclogite population at
c
. 480 Ma and
a relative abundance peak at
c
. 505 Ma. Both ages
are interpreted as the ages of the protolith mafic
generation, but the possibility of a generation
at
c
. 505 Ma and a metamorphic event at
c
. 480
Ma cannot be discarded or proved. The sources
of the mafic protolith rocks are varied. Two
samples (GCH-19 and GCH-20) show that their
protoliths are almost directly derived from a
depleted mantle source, most probably by direct
partial fusion of it. The geological environment
in which these primitive and juvenile protolithic
rocks are enclosed is a back-arc type basin where
the oceanic crust is generated as the basin widens
and evolves. This environment fits perfectly
with a juvenile source protolith origin for these
eclogites. The other two samples (GCH-22
and GCH-23) have
H
Hf
(t)
and
H
Nd
(t)
values not
compatible with a juvenile source derivation
for the genesis of their protoliths. It seems that
the only possible ways to explain a non-juvenile
derivation of a basic rock are: (i) as a partial
fusion of an enriched mantle component, or (ii)
as an important contamination of a depleted
mantle partial melt with continental crust
material. Both explanations are probable, since
in a magmatic arc system, contamination is
very possible due to the intrusion of juvenile
materials into continental crust ones, the same
way as an enriched mantle component is also
very probably present in this geological context.
This enriched component could be, for example,
the subcontinental mantle of Gondwana, which
evolved separately from the depleted mantle
since the Mesoproterozoic (Nd model ages and
the average of Hf zircon model ages fall in the
Mesoproterozoic Era).
The zircon crystallisation climax is
c
. 393 Ma
for the Devonian eclogite population. This age
is interpreted to be the age of maximum zircon
crystallisation due to a HP–HT metamorphic
event. This metamorphism triggered solid-
state recrystallisation of inherited zircon and
also zircon overgrowths at high temperature
conditions, most probably near the metamorphic
path temperature peak.
7.5.3. Orthogneisses
From the first sample studied (GCH-01,
section 7.4.1) it was concluded that the magmatic
protolith zircon crystallised between 518 and
466 Ma with a crystallisation climax at
c
. 496 Ma.
A solid-state recrystallisation of the protolithic
zircon triggered by a high-grade metamorphic
event took place between 410 and 381 Ma, with a
crystallisation climax at
c
. 402 Ma.
From the second sample studied (GCH-
03, section 7.4.2) it was concluded that the
protolithic zircon crystallised between 510 and
450 Ma, with a crystallisation climax at
c
. 485
Ma, and that a posterior recrystallisation event
altered the zircon textures without triggering any
rare element purge from the zircon structures.
7.5. DISCUSSION