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9. FINAL DISCUSSION AND CONCLUSIONS
c.
480 Ma, and it is interpreted as the climax of
igneous intrusion, but an effect of the high-T arc-
related input cannot be discarded.
From the above-mentioned Cambrian–
Ordovician processes to the Devonian no activity
is registered. At this time interval the margin of
Gondwana was a passive one (already stated
in scientific literature) where the Gondwana-
directed subduction that formed the Cadomian
magmatic arc ended. At the Devonian, part of
this arc (which is now the bottom member of
the Upper Allochthon) underwent a HP–HT
metamorphic event. This event was induced
by the subduction of the margin of Gondwana
under another continental fragment. The
first zircon crystallisation climax due to this
metamorphism is shown by the orthogneisses
at
c.
395 Ma, which were very slightly affected,
probably because of their low competence. The
zircon crystallisation climax of the eclogites has
an age of
c.
393 Ma and most probably took
place near the metamorphic path temperature
peak. This lithology was severely disturbed by
the metamorphic event. The sediments do not
show any important zircon recrystallisation at
this time, but the leucosomes that they contain
were intensively re-melted (and probably new
leucosomatic production took place) at high-T
conditions. This second migmatisation was the
most important one and took place between 403
and 379 Ma, with a zircon crystallisation climax
at
c.
388 Ma (Fig. 2). It is noteworthy to mention
that this intense metamorphism induced zircon
recrystallisation in the different lithologies
heterogeneously. The sediments were almost
unaffected and only the leucosomes produced,
registered the effects of this metamorphism.
The orthogneisses were slightly affected and
the eclogites were the ones that best registered
the effects of this HP–HT metamorphic event.
It looks like this heterogeneity is related to the
competence of the lithologies, where a higher
competence allows a more effective isotropic
compression, and therefore a higher degree of
zircon recrystallisation.
The subducted ensemble experienced a
drastic and fast exhumation, from which its
progression probably favoured the development
of the important extensional detachment which
constitutes the actual contact between the Banded
and the Cariño Gneiss formations. The HP–HT
Upper Allochthon was most probably exhumed
as an ultra-high-P buoyant plume, returning
immediately over the Gondwana subducting
plate.