148 / 352
132
partial melting undergone by the HP–HT Upper
Allochthon. To do so an investigation was carried
out studying zircon from several orthogneisses,
eclogites and leucosomes from the Eclogite Band
and Banded Gneiss formations from the HP–HT
Upper Allochthon of the Cabo Ortegal Complex
(NW Iberia).
7.2. Leucosomes
Concerning the Upper Allochthon HP–HT
metamorphic event, the initial hypothesis is
that this event took place at
c.
400–390 Ma. But
from the five meta-sedimentary rocks analysed
(chapter 6) no Devonian zircon recrystallisation
or new zircon formation was found (with the
exception of sample GCH-02 where one analysis
gave a Devonian age). Taking into account that
the meta-sedimentary rock zircon grains have
very small rims (making them very difficult
to analyse with the LA–ICP–MS actual spatial
resolution) and that only the zircon cores were
analysed (because those experiments aimed for
provenance information) the HP–HT Devonian
event cannot be discarded. To advance in what
we know about this eclogitic event, or about the
expected post-eclogitic migmatization event,
and also about the age of the regional foliation
development, three leucosomes were analysed
from the Banded Gneisses meta-sedimentary
rocks, the eclogitic metasedimentary unit of the
Cabo Ortegal Upper Allochthon.
There are no previous studies concerning
the Banded Gneisses leucosomes, but a Ms
concentrate from the metasedimentary rocks
was analysed with the
40
Ar/
39
Ar method. This
experiment, published by Peucat
et al.
(1990) gave
an age of 375.9 ± 1.4 Ma, and most probably dates
the last stages of regional foliation development
of the Banded Gneiss formation.
The abundance of leucosomes in the Banded
Gneisses metasedimentary rocks shows that this
formation underwent abundant partial anatexis.
This abundant leucosomatic production could
be a response to temperature input due to the
igneous activityof theNeoproterozoic–Cambrian
magmatic arc system, or to the temperature input
duringtheHP–HTDevonianmetamorphicevent,
or to a post-eclogitic isothermal decompression
during the exhumation of the Banded Gneisses
(and most probably of the entire HP–HT Upper
Allochthon). These leucosomes are displayed as
small pods within the gneisses, called “
augen
”
gneisses by Vogel (1967), or as thin more or less
continuous veins within the gneisses, giving its
planar or plano-linear character. The abundance
of leucosomes is a characteristic feature of these
gneisses and is the reason why this formation is
called the Banded Gneisses.
7.2. LEUCOSOMES