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6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
acidic and basic magmas (protolith ages of
c.
506–484 Ma) and Cariño Gneisses were not.
All these observations lead one to suppose that
they were probably deposited in the same back-
arc basin at
c.
521–506 Ma. The first sediments
deposited were those of the Banded Gneiss
formation, formed by the mixture of the old
components from the WAC and abundant arc
related
c.
590–520 Ma sediments. The Cariño
Gneisses protoliths deposited afterwards, filling
the same basin with WAC sediments and
c.
560–
510 Ma arc-derived sediments.
Between the time of deposition (
c
. 512–506
Ma) and high-grade metamorphism (
c
. 400–
390 Ma), both formations constituted a part of
the margin of Gondwana, that changed from
a volcanic arc to a passive margin setting at
around the Cambrian–Ordovician boundary.
The different metamorphic conditions reached
by both units indicate that the HP–HT member
of the Upper Allochthon was affected by a deep
subduction at
c.
400–390 Ma. Different structures
andmetamorphic recrystallizations related to this
collision have also been described in the IP upper
units. In this case, the intensity of deformation
was much lower and only limited shear zones
and amphibolite facies Barrovian metamorphism
weregenerated.Thedifferentiatedtectonothermal
evolution recorded in the Cariño and Banded
gneiss formations, could have been favoured by
the previous structure of the Gondwana margin,
achieved during the Cambrian–Ordovician
extension. It has been published by other authors
that in the Cambrian–Ordovician transition,
lithospheric extension induced crustal necking
and stretching, compartmenting the Gondwana
lithosphere into several continental micro-
blocks. During the Devonian collision the thicker
sections of the margin were affected by limited
accretion and thickening, developing Barrovian
metamorphism. The adjacent thinned sections
experienced profound subduction directed to
the North, favoured by zones of inherited crustal
weakness, developing the HP–HT subduction-
related metamorphic conditions of the Banded
Gneiss formation and its HP–HT equivalents
along the Variscan belt. The new provenance
data presented in this PhD is conclusive on the
origin of the Banded Gneiss formation, whose
eclogite facies evolution can only be explained as
generated during subduction of the Gondwana
margin directed to the North.
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