141 / 352
6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
125
conclusive about the origin of the subducted conti-
nental margin, as subduction of either the Gond-
wanan margin to the North or the Laurussian
margin to the South could explain this tectonic evolu-
tion. However, the new provenance data presented in
this paper 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.
CONCLUSIONS
The sedimentary protoliths of the eclogitic gneisses
of the Cabo Ortegal Complex (Banded Gneisses) has
a Gondwanan provenance. They were deposited in a
sedimentary basin located in the periphery of West
Africa. This implies that the margin of Gondwana,
which developed a Cambrian
–
Ediacaran continental
–
magmatic arc and a subsequent transition to a pas-
sive margin, subducted during the Devonian below
another continent located to the North, most proba-
bly Laurussia. This tectonothermal event represents
a first collision related to the final assembly of Pan-
gea, as has been recently proposed (Arenas
et al.
,
2014a).
Due to the strong bimodal character of the U
–
Pb
detrital populations and the high amount of ages
near to the sedimentation age (Max. Dep.
Age
=
c.
521 Ma, Min. Dep. Age
=
c.
506 Ma) the
basin where these sediments deposited was probably
a back-arc type basin. The Lu
–
Hf data shows that
at this time the Cambrian
–
Ediacaran population
was formed in a continental
–
magmatic arc and
strongly suggests, due to timing and due to its
WAC provenance, that the arc system was the
Cadomian arc. The igneous activity of the arc lasted
at least until Middle Cambrian in the Iberian sector
of the Gondwanan margin. Comparing this study
with a previous one on the Iberian IP Upper
Allochthon (Albert
et al.
, 2015), it seems reasonable
to propose that the siliciclastic series involved in the
HP
–
HT Upper Allochthon deposited first (due to its
isotopic heterogeneity between samples and high
amount of basic and acid intrusions). The sedimen-
tary protoliths of the IP Upper Allochthon depos-
ited shortly after than the HP
–
HT Upper
Allochthon (due to its homogeneity and scarce
igneous intrusions) in the same basin. The NW Ibe-
rian IP Upper Allochthon scarcely registers Edi-
acaran arc igneous activity, probably because when
erosion and sedimentation of the detrital series took
place the topographic relief of the Ediacaran part of
the arc was dismantled.
Considering the original linear character of the
Variscan belt and the presence of a rather continuous
equivalent HP
–
HT unit from the Iberian Massif to
the Bohemian Massif, we suggest that the conclusions
presented in this article can be extrapolated to the
entire Variscan realm.
ACKNOWLEDGMENTS
This work was supported by the Ministerio de Econ-
omıa y Competividad (Spanish project CGL2012
–
34618). R. Albert is thankful to the Universidad
Complutense de Madrid which provided him with a
pre-doctoral grant. The authors are grateful to A.P.
Willner and J. Payne who performed insightful
reviews of the manuscript and to M. Brown for excel-
lent editing work. The authors are also thankful for
the technical and laboratory support provided by
J.M. Fuenlabrada and J.A. Hernandez Jimenez
(TIMS Laboratory, UCM).
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–
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