serie NOVA TERRA nº 48

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5. PROVENANCE OF THE UPPER ALLOCHTHON
89
Provenance of the Variscan Upper Allochthon (Cabo Ortegal Complex,
NW Iberian Massif)
R. Albert a,⁎, R. Arenas a, A. Gerdesb,c, S. Sánchez Martínez a, J. Fernández-Suárez a, J.M. Fuenlabrada d
a Departamento de Petrología y Geoquímica and Instituto de Geociencias (UCM-CSIC), Universidad Complutense de Madrid, Spain
b Institut für Geowissenschaften, Petrologie und Geochemie, J.W. Goethe Universität, Frankfurt am Main, Germany
c Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland, South Africa
d CAI de Geocronología y Geoquímica Isotópica, Universidad Complutense de Madrid, Spain
a b s t r a c t a r t i c l e i n f o
Article history:
Received 28 May 2014
Received in revised form 17 October 2014
Accepted 20 October 2014
Available online 15 November 2014
Handling Editor: I. Safonova
Keywords:
Detrital zircons
Provenance
Isotope geochemistry
Variscan Upper Allochthon
Cabo Ortegal Complex
This study focuses on the provenance of the uppermost unit of the Upper Allochthon of the Variscan belt with
combined U–Pb and Lu– Hf zircon (LA-ICPMS) and Sm– Nd whole-rock analyses. This unit is represented in the
Cabo Ortegal Complex (NW Iberia) by the metasiliciclastic Cariño Gneisses which overlap ophiolitic units that
represent perigondwanan oceanic domains. The data set indicates a maximum depositional age of c. 510 Ma
and a c. 1.73 Ga Sm–Nd model age, typifying a late or post Pan-African (or Cadomian) and Eburnean events,
which entailed abundant input of juvenile material involving broad mixing with older crustal sources. The
Mesoproterozoic activity is scarce and scattered and therefore unlikely to represent a major crust generation
pulse in the source area of the siliciclastic unit. The data set also records an Archean orogenic pulse in its source
area followed by a long lasting crust reworking process, where the Eburnean juvenile materials intruded. These
dataareinterpretedasindicativeofaWestAfricaCratonprovenance,wherethesiliciclasticseriesfromwhichthe
CariñoGneissesarederivedwereprobablydepositedinaback-arctypebasinwheredetrituswasmostlysourced
by the continent rather than the magmatic arc.
© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction
The Variscan–Appalachian–Alleghenian orogeny was formed when
the Pangea supercontinent assembled during Carboniferous and Early
Permian. Part of the European branch of this orogen, the Variscan belt,
is well preserved in the Iberian Massif. This Massif exhibits an almost
complete section of the southern part (present day coordinates) of the
orogen from the most external zones (Autochthon) to the internal
sectionswhicharerepresentedbytheParautochthonandtheallochtho-
nous complexes.
Our understanding of the sources and provenance of the rock series
involved in the Variscan Orogeny has significantly increased in the last
few years mostly driven by U–Pb and Lu–Hf zircon (LA-ICPMS) and
Sm–Nd whole rock studies. In the NW Iberian Massif most of the prov-
enance studies have been focussed on sedimentary series located below
the ophiolitic units, either in the autochthonous domain (Cantabrian
zone, CZ, West-Asturian–Leonese zone, WALZ, and Central Iberian
zone, CIZ, Fig. 1a, Fernández-Suárez et al., 1999; Gutiérrez-Alonso
et al., 2003; Martínez Catalán et al., 2004; Fernández-Suárez et al.,
2013; Shaw et al., 2014 ), the Parautochthon (Díez Fernández et al.,
2012b ) or the basal units (Díez Fernández et al., 2010). These studies
suggest as a main conclusion that the rock series were formed at the pe-
riphery of the Gondwanan margin, between the Nubian platform and
the periphery of West African Craton (WAC), although their exact
location is still a matter of debate. The key for paleogeographic recon-
structions is usually focussed on the presence or absence of a Stenian–
Tonian zircon population in Ediacaran and post-Ediacaran rock series.
Rocks containing this population are usually placed in front of the
Sahara Craton or the Nubian Shield ( Díez Fernández et al., 2010;
Fernández-Suárez et al., 2013; Shaw et al., 2014), whereas those lacking
or having an extremely low quantity of it are considered to have a
proximal WAC provenance (Abati et al., 2010a, 2012). Nevertheless,
the real importance of the isotopic sources has not been completely
evaluated in NW Iberia as there are almost no previous studies integrat-
ing U–Pb/Lu–Hf (zircon) and Sm–Nd (whole rock) data systematics.
This kind of studies is absent in metasedimentary rock series and only
very few works have presented some of these combined results for
metaigneous rocks (i.e. Sánchez Martínez et al., 2011).
The Upper Allochthon or upper units is located above the ophiolitic
units, which are considered to represent the Variscan suture in the
NW Iberian Massif. This issue poses first order questions, not only
about tectonic settings but also about provenance and sources of the in-
volved rockseries. In the Upper Allochthon, two works were carried out
separately publishing U–Pb detrital zircon and Sm–Nd whole rock data
on metagreywackes (Fernández-Suárez et al., 2003; Fuenlabrada et al.,
Gondwana Research 28 (2015) 1434–1448
⁎ Corresponding author.
E-mail address: (R. Albert).
1342-937X/© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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