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related to the oceanic crust subduction directed
to Gondwana, responsible for the formation of
the Cambrian–Ordovician magmatic arc system.
The six orthogneisses have been represented
together in Fig. 94, which combines the
H
Hf
(t)
data of the six samples and the U–Pb combined
density diagram of all six 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 orthogneiss population at 498 Ma
with a long tail of ages down to
c
. 460–470 Ma
and a relative abundance peak at
c
. 512 Ma.
These ages are interpreted as the ages of the
protolith felsic generation, but the possibility of a
generation at
c
. 512 Ma and a later metamorphic
event cannot be discarded or proved, but from the
above-mentioned discussions it seems probable.
If this event took place it was most probably a
high-T metamorphic event related to the activity
of the magmatic arc system. The sources of the
felsic protolith rocks are varied. At around
c
.
512 Ma the sources (as viewed in Fig. 94) are
diverse, as would be expected from a felsic arc-
related magmatism. At around c. 498 Ma (the
zircon crystallisation climax) the sources are
also varied, but some of the orthogneiss zircon
analyses plot on the MORB-DM evolution array
(such as GCH-01 and GCH-03). This implies
that the rocks, from which the mentioned
juvenile zircon analyses belong to, were almost
directly derived from a depleted mantle source.
As a felsic rock cannot be a direct product of
the partial fusion of the mantle, they must be
the result of fractional crystallisation of a basic
magma, or more likely the result of the partial
fusion of a DM derived mafic rock.The magmatic
arc geological environment in which these rocks
are related, could explain the high temperature
conditions necessary for partially melting mafic
rocks. For these reasons it is concluded that from
the acidic igneous rocks studied, the magmatic
activity of the arc systemstarted at around 512Ma
and finished around 470–460 Ma, and that this
activity induced high–T related metamorphism
that partially melted previously generated
igneous rocks within the same magmatic arc.
The zircon crystallisation climax is
c
. 395 Ma
for the Devonian orthogneiss population. This
age is interpreted tobe the age ofmaximumzircon
crystallisation due to a HP–HT metamorphic
event. This metamorphism triggered solid-state
recrystallisation of inherited zircon at high
temperature conditions, most probably near
the metamorphic path temperature peak. It is
noteworthy to mention that the orthogneisses
are the igneous lithologies that were less affected
by this HP–HT metamorphic event.
7.5. DISCUSSION