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Extended abstract
NW Iberia includes a rather complete section
of a Variscan suture, where different terranes
with continental or oceanic affinities appear with
clear structural relationships. Three groups of ter-
ranes, namely Upper, Ophiolitic and Basal units
and a frontal tectonic mélange appear in Galicia,
in Cabo Ortegal, Órdenes and Malpica-Tui com-
plexes. They constitute a huge allochthonous pile
thrust over the Iberian parautochthonous and
autochthonous domains, which represent the
section of the Gondwanan margin that escaped
continental subduction during the Variscan cy-
cle (Schistose Domain of Galicia-Trás-os-Montes
and Central Iberian Zone).
In the Upper Units,
c.
12000 m of terrigenous
sediments (Órdenes Series) intruded by large
massifs of Cambrian (
c.
500 Ma) I-type calc-al-
kaline granitoids (Corredoiras Orthogneiss)
and tholeiitic gabbros (Monte Castelo Gabbro),
are considered to represent a section of a mag-
matic arc built up in the periphery of Gondwa-
na during Neoproterozoic-Cambrian times. Nd
model ages from the Cambrian topmost turbid-
itic series (Ares-Sada greywackes) are relatively
young (720-1215 Ma) and suggest proximity to
some Avalonian terranes. The uppermost part of
this terrane was affected by metamorphism rang-
ing between the greenschist facies and the inter-
mediate pressure granulite facies conditions (IP
Upper Units), dated at 496-484 Ma. The IP Upper
Units can be considered a relic section preserving
the Cambrian tectonothermal activity associated
with peri-Gondwanan arc activity. That activity
was caused by magmatic underplating and fol-
lowed by accretion of arc slices, which developed
counter-clockwise P-T path evolution. However,
the lower part of this terrane shows a completely
different tectonothermal record, as it was affect-
ed by intense high-P and high-T metamorphism
(HP-HT Upper Units; Cedeira and Capelada
units in the Cabo Ortegal Complex, Sobrado
Unit in the Órdenes Complex). This event devel-
oped extensive recrystallization under eclogite
and granulite facies conditions, the peak pres-
sures being in the range of ultra-high-P meta-
morphism (proved minimum pressure at 22 Kb,
with some indicators of higher values). The high/
ultra-high-P metamorphism (
c.
400 Ma) was fol-
12
3. GEOLOGICAL FRAMEWORK
lowed by drastic and fast exhumation coeval with
partial melting (
c.
390 Ma). The development of
extensional detachments (
c.
375 Ma), recumbent
folds and thrusts drove the exhumation of this
high-P complex later on.
True MORB ophiolites derived from typical
oceanic lithosphere are unknown in the Variscan
suture of Galicia. On the contrary, the mafic-ul-
tramafic sequences preserved in NW Iberia were
generated in supra-subduction settings, their
mafic rocks showing island-arc tholeiitic compo-
sition. There exist two critical oceanic to transi-
tional crust-forming events: a Middle-Late Cam-
brian phase (
c.
500 Ma, Lower Ophiolitic Units,
Vila de Cruces and Bazar unit), and a Middle De-
vonian phase (
c.
395 Ma, Upper Ophiolitic Unit,
Careón, Purrido and Moeche units). Both types
of mafic units were accreted beneath the Upper
Units during Variscan convergence, the Upper
Ophiolites first (
c.
391-377 Ma), and then the
Lower Ophiolites (
c.
367 Ma). Due to their buoy-
ant nature, many Devonian ophiolites escaped
from early-Variscan subduction, so they are the
most common ophiolites preserved in the Vari-
scan suture across Europe. The mafic-ultramafic
sequence of the Bazar Unit has been interpreted
as peri-Gondwanan oceanic lithosphere accreted
beneath the magmatic arc system at 480 Ma. This
unit is the only one in NW Iberia showing low-P
granulite facies conditions, which has been linked
to mid-ocean ridge subduction under Gondwana
and the opening of an asthenospheric window.
The Basal Units consist of a thick sequence
of Ediacaran-Early Ordovician terrigenous
metasedimentary rocks intruded by Cambrian
to Ordovician granitoids (calc-alkaline to per-
alkaline) and minor mafic igneous rocks. These
units are considered to outline the most exter-
nal part of the Gondwanan paleomargin during
the Late Devonian. However, detrital zircon age
populations along with (very old) Nd model ages
obtained from well-preserved turbiditic series
(ranging between 1782 and 2223 Ma) indicate a
paleoposition further to the East than the arc-re-
lated section preserved in the Upper Units. The
Basal Units also represent a continental paleo-
subduction zone affecting a wide and continuous
section of the margin at the onset of the Vari-
scan collision (
c.
370 Ma). Subduction polari-
ty was to the W (present-day coordinates) and