serie NOVA TERRA nº 48

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8. TWO STAGE COLLISION
GR Letter
Two-stage collision: Exploring the birth of Pangea in the Variscan terranes
Ricardo Arenas a,⁎, Rubén Díez Fernández a,b, Sonia Sánchez Martínez a, Axel Gerdes c,d,
Javier Fernández-Suárez a, Richard Albert a
a Departamento de Petrología y Geoquímica and Instituto de Geociencias (UCM, CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain
b IDL, Departamento de Geociências, ECT, Universidade de Évora, Apartado 94, 7001-554 Évora, Portugal
c Institut für Geowissenschaften, Mineralogie, J.W. Goethe Universität, Frankfurt am Main, Germany
d Department of Earth Sciences, Stellenbosch University, Private Bag X1,Matieland 7602, South Africa
a b s t r a c t a r t i c l e i n f o
Article history:
Received 24 June 2013
Received in revised form 1 August 2013
Accepted 4 August 2013
Available online 31 August 2013
Handling Editor: R.D. Nance
Keywords:
Assembly of Pangea
Allochthonous Variscan terranes
NW Iberian Massif
The Variscan suture exposed in NW Iberia contains a stack of terranes including two allochthonous units with
continental affinity and Gondwanan provenance (Upper and Basal Units), separated by an ophiolite belt where
the most common units show protolith ages at c. 395 Ma. Recent Lu–Hf zircon data obtained from these
ophiolites indicate interaction between the gabbroic magmas and old continental crust. Hence, the ophiolites
could not have originated in a deep ocean basin associated with a mature mid-ocean-ridge or intraoceanic
subduction. The tectonothermal evolution of the continental terranes bounding the suture zone records two
consecutive events of deep subduction. The Upper Units record an initial high-P/ultra-high-P metamorphic event
that occurred before 400–390 Ma, while the Basal Units were affected by a second high-P/low-to-intermediate-T
metamorphic event dated at c. 370 Ma. Continental subduction affected the most external margin of Gondwana
and developed inasettingof dextralconvergencewithLaurussia.Developmentofthetwohigh-Peventsalternated
with the opening of an ephemeral oceanic basin, probably of pull-apart type, in Early Devonian times. This ephem-
eral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the
Variscan suture. Current ideas for the assembly of Pangea advocate a single collisional event between Gondwana
and Laurussia in the Carboniferous. However, the new evidence from the allochthonous terranes of the Variscan
belt suggests a more complex scenario for the assembly of the supercontinent, with an interaction between the
collidingcontinentalmarginsthatstartedearlierandlastedlongerthanpreviouslyconsidered.Basedonmodernan-
alogs of continental interaction, the development of complex collisions, as here suggested for Gondwana and
Laurussia during the assembly of Pangea, could have been the norm rather than the exception throughout Earth
history.
© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction
It is broadly accepted that the assembly of Pangea occurred in
Carboniferous and Early Permian times, after a long stage of continental
convergence that ended with the closure of the Rheic Ocean (e.g. Nance
et al., 2010 and references therein) and the collision of Gondwana and
Laurussia (Stampfli and Borel, 2002; Murphy and Nance, 2008). This
collision resulted in the formation of the Variscan–Appalachian–
Alleghanian orogen, which extends from Europe to eastern North
America and contains key information for reconstructing the amalgam-
ation history of the supercontinent. In the Variscan belt, the oldest
tectonothermal events are preserved in a complex suture zone that can
be traced from the Iberian Peninsula to the Bohemian Massif ( Fig. 1).
This belt is affected by several oroclinal folds, but the original geometry
was broadly linear (Matte, 2001; Martínez Catalán, 2011; Weil et al.,
2012). The suture zone is made up of a stack of allochthonous terranes
with ophiolites and high-P (HP) and ultra-high-P (UHP) metamorphic
rocks. One of the most distinctive features of the Variscan belt is the
presence of two different events of HP metamorphism that appear to
haveoccurredrelativelycloseintime,butwereseparatedbythedevelop-
ment of oceanic basins. This evolution is unusual in large collisional belts,
whose tectonothermal evolution is commonly interpreted as re flecting a
single HP or UHP metamorphic event associated with subduction of one
of the colliding continental margins (Platt, 1986; Beaumont et al.,
2009). In the Variscan belt, both HP events and the development of
some of the oceanic domains occurred after the earliest Devonian and
are thus broadly coeval with the initial stages of the assembly of Pangea.
Thispaperpresentsashortsummaryofthe originandtectonothermal
evolution of the allochthonous terranes involved in the Variscan suture,
and proposes a conceptual model to integrate part of this history in the
context of Pangea formation. The geological section exposed in the NW
Iberian Massif is taken as an example and described in some detail, but
Gondwana Research 25 (2014) 756–763
⁎ Corresponding author. Tel.:+34 639601919,+34 915442535.
E-mail addresses: (R. Arenas),
(R. Díez Fernández), (S. Sánchez Martínez),
(A. Gerdes), (J. Fernández-Suárez),
(R. Albert).
1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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