164 / 352
148
their protoliths were gabbros or basalts (Mendía,
2000). Ordoñez Casado
et al.
(2001) discussed
that as basalts may only contain inherited but
not co-magmatic zircon, the protolith must have
been gabbroic rocks. Peucat
et al.
(1990) gave
a
207
Pb/
206
Pb age of 480 ± 5 Ma for these rocks
(TIMS technique) and interpreted this age as the
age of HP–HT metamorphism. Ordoñez Casado
et al.
(2001) dated the magmatic domains of
zircon from this formation at 495 ± 11 Ma
(average of eight analyses on five crystals) and
the metamorphic domains (rims) at 386 ± 41
Ma (average of four analyses on three crystals;
SHRIMP technique).
The Ky-rich eclogites from the Eclogite Band
formation (represented by sample GCH-22,
section 7.3.3) are richer in Mg and Al and have
lower REE and trace element contents than the
common N-MORB type eclogites (Gil Ibarguchi
et al.
, 1990). Presumably, they originated
from plagioclase-rich cumulates of troctolitic
composition with spinel-bearing protoliths (Gil
Ibarguchi
et al.
, 1990). Mendía (2000) stated
that, from a geochemistry point of view, the
eclogites from the Eclogite Band formation are
co-genetic, where the Ky-rich eclogites would
represent cumulates from the tholeitic series,
rich in plagioclase contents and with Sr and Eu
positive anomalies. Whole-rock Nd experiments
show
H
Nd
(t=480)
values of
c.
+1 for this lithology
(Bernard-Griffiths
et al.
, 1985; Peucat
et al.
,
1990). The eclogitic metamorphism was
interpreted to be Early Ordovician, as this type
of eclogites was dated at 480 Ma by the TIMS U–
Pb zircon aggregate method (Bernard-Griffiths
et al.,
1985; Peucat
et al.,
1990). Retrogression
under amphibolite-facies conditions has been
dated between 410 and 430 Ma by Van Calsteren
et al.
(1979; K–Ar on amphibole concentrates).
Ordoñez Casado
et al.
(2001) analysed five
magmatic domains on five zircon grains which
yielded a mean age of 469 ± 9 Ma, and four spots
from high luminescent rim domains of three
zircon grains yielded an average age of 380 ±
14 Ma (SHRIMP technique). Whole-rock Nd
experiments show values of
H
Nd
(t=o)
= -0.5 and
H
Nd
(t=472)
= -1.04 (Ordoñez Casado
et al.,
2001).
7.3. Eclogites
Four eclogites have been studied, two from the
Eclogite Band formation and the other two from
the Banded Gneiss formation (Cabo Ortegal
Complex, NW Iberian Massif; Appx. 1).
The intention of studying these rocks is to
better constrain what we know about the age
of crystallisation, the nature of the sources and
the development of the mafic igneous activity of
the magmatic arc that was built at the margin of
Gondwana in the Neoproterozoic–Cambrian.
Studying these rocks will also give important
information about the age and conditions of
the metamorphic event that transformed the
protoliths of these rocks into eclogites.
In the Eclogite Band formation three types of
eclogites have been described (Mendía, 2000):
(i) the common MORB-type eclogites (which
constitute a
c.
70% of the formation outcrops
and is represented in this PhD by sample GCH-
19, section 7.3.1), (ii) the Ky-rich eclogites
(which constitute a
c.
25% of the outcrops and
is represented by sample GCH-22, section
7.3.3) and (iii) the Fe–Ti rich eclogites (which
constitute a
c.
5% of the outcrops and have not
been studied in this work).
The eclogites from the Banded Gneiss
formation (also called eclogite facies gneisses,
see chapter 4) are found as variably sized
boudins within the meta-sedimentary rocks of
this formation. They are generally retrograded
and they are represented by the sample GCH-
20 (section 7.3.2). There is also an eclogite the
outcrop of which is intimately related to an
igneous felsic rock by a mingling process (see
chapter 4) and is represented by sample GCH-
23 (section 7.3.4; the felsic rock is represented by
sample GCH-14, described in section 7.4.4).
Previous studies (Bernard-Griffiths
et al.
,
1985; Peucat
et al.
, 1990; Gil Ibarguchi
et al.
,
1990) have shown that the common eclogites
from the Eclogite Band formation (represented
by sample GCH-19, section 7.3.1) have N-MORB
type compositions, unfractionated HREE an
LREE depleted patterns,
H
Nd
(t=480)
values of +10
and
87
Sr/
86
Sr ratios of 0.7032–0.7052; and that
7.3. ECLOGITES