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6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
different ages, preserved in two distinct terranes with
continental or transitional characteristics. These
high-P events were probably generated during two
stages of collision between Gondwana and Laurussia,
separated in time by the opening of a long, but
ephemeral, pull-apart basin at c. 395 Ma, with gener-
ation of oceanic crust that presently represents the
most common ophiolites in the Variscan belt (Sanchez
Martınez et al., 2011; Arenas et al., 2014a,b).
The younger high-P event is characteristic of an
allochthonous terrane located below the ophiolites,
currently described in NW Iberia as the basal units.
Dated at c. 370 Ma (Rodrıguez et al. , 2003; Abati
et al. , 2010), this high-P event generated low to inter-
mediate-T mineral assemblages, with extensive devel-
opment of C-type eclogites, blueschists, high-P
metapelites and orthogneisses (Gil Ibarguchi &
Ortega Girones, 1985; Arenas et al., 1995, 1997; Gil
Ibarguchi, 1995; Lopez-Carmona et al. , 2013, 2014).
According to their structural position below the ophi-
olites, the basal units were interpreted early on as a
section of the Gondwanan margin affected by sub-
duction (Arenas et al. , 1986). This subduction was
directed to the North and it can be considered as the
first s.s. Variscan event that affected the margin of
Gondwana (Martınez Catalan et al., 1996; Dıez
Fernandez et al. , 2011, 2012b).
The interpretation of the older high-P event is more
problematic. It is a high- P and high-T (HP–HT) event
that appears in the bottom member of a thick
allochthonous terrane, which is located above the
ophiolites and is described as the Upper Allochthon
(or upper units) in the NW Iberian Massif. This high-
P event has been repeatedly dated at c. 400–390 Ma
(Ordo~nez Casado et al., 2001; Fernandez-Suarez
et al. , 2007) and generated B-type eclogites, high-P
granulites, high-P gneisses and related high-P mineral
assemblages in gabbros and ultramafic rocks. The top
member of the Upper Allochthon has continental
crust-active margin equivalence, but does not show
high-P assemblages. Instead, it is only affected by
intermediate-pressure (IP) metamorphism. The upper
units have been traditionally interpreted as a single
terrane, because they appear above the ophiolites
(Arenas et al. , in press). However, considering the
tectonothermal differences between the top and bot-
tom members of the Upper Allochthon and the fre-
quent cryptic character of many sutures, doubts arise
in relation to the single or composite character of this
terrane. Preliminary data in the IP upper units indi-
cated a Gondwanan provenance for this terrane
(Fernandez-Suarez et al. , 2003). However, no infor-
mation existed dealing with the provenance of the
HP–HT upper units, and so it was not possible to
know if this continental–transitional subducted
margin represents a section of the Gondwanan
margin or a part of the northern colliding continent,
Laurussia. Answering this question will ensure a
better understanding of the Variscan belt and conse-
quently of the characteristics of the Pangea assembly.
It is also important to consider the original linear
trend of the belt, and that the allochthonous terranes
are rather continuous between Iberia and the Bohe-
mian Massif, with systematic presence of the two
high-P events described in NW Iberia (Ballevre et al. ,
2014).
To improve knowledge about the origin and prove-
nance of the HP–HT upper units, a two-step experi-
ment was carried out based on the study of
geochronology and isotopic geochemistry of detrital
zircon (U–Pb, Lu–Hf) and whole-rock samples (Sm–
Nd). The first step consisted of the investigation of
the provenance characteristics of the siliciclastic series
of the IP upper units, using very well-preserved and
representative samples of the Cabo Ortegal Complex,
in the NW Iberian Massif (Fig. 1b,c). This study,
recently published (Albert et al. , 2015), confirmed the
Gondwanan provenance of the IP upper units pro-
viding solid basis for comparison. The investigation
of the provenance of the HP–HT upper units of the
same complex is the objective of this article. Data
derived from this type of investigation should defini-
tively enable one to know if the upper units are in
fact a single or a composite terrane, and consequently
clarify the Gondwanan or Laurussian provenance of
the subducted margin represented by the HP–HT
upper units.
GEOLOGICAL FRAMEWORK
The Iberian Massif has been divided into several
zones according to stratigraphic, structural, metamor-
phic and magmatic characteristics (Lotze, 1945, 1950;
Julivert et al. , 1972; Farias et al. , 1987). The Canta-
brian (CZ), West Asturian–Leonese (WALZ) and
Central Iberian (CIZ) zones define an autochthonous
domain representing a large section of the Gond-
wanan margin involved in the Variscan deformation
(Fig. 1a). They are overlain by an allochthonous
thick pile which is represented by the Galicia–Tras–
os–Montes Zone (GTOMZ). A similar zone distribu-
tion can be followed along the Variscan belt
(Martınez Catalan, 2011; Fig. 1a). The lowest unit in
the GTOMZ is considered the Lower Allochthon or
Parautochthon. Above it, five allochthonous com-
plexes, structured as late Variscan synforms, are
observed. In Galicia, they are the Cabo Ortegal,
Ordenes and Malpica–Tui complexes (outcropping in
an East–West direction), together with the Braganca
and Morais complexes in the Tras–os–Montes
region of Portugal. They are nappe piles constituted
of three main groups of terranes, from bottom to
top: basal, ophiolitic and upper units (Martınez Cat-
alan et al., 2009; Arenas et al., in press). It is
assumed that all these zones have been condensed
and piled up, thrust from more external positions in
relation to the Gondwanan mainland to form what
today is known as the Iberian Massif.
© 2015 John Wiley & Sons Ltd
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