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ly migmatitic (Arenas, 1988; Arenas et al., 1997).
Some 40Ar/39Ar ages obtained in the high-P as-
semblages have provided minimum age estima-
tions for the high-P metamorphism (c. 370-365
Ma; Rodríguez et al., 2003; López Carmona et al.,
2014). However, U-Pb data obtained from met-
amorphic zircon rims from orthogneisses of the
Agualada Unit pointed to an age of c. 372 Ma,
which is somewhat older but probably closer to
the real age of the high-P metamorphic event
(Abati et al., 2010). In the same unit, the migma-
tization following the high-P event has been dat-
ed at c. 351-346 Ma using monazites extracted
from paragneisses (Abati and Dunning, 2002).
The thermal structure of the Basal Units seems
to reflect the original layout of a continental
subduction zone. In the LMG, peak pressures
reached during the high-P event increase to the
West, from the less pressurized Forcarei Unit to
the relatively deeper Lalín and Santiago units
up to the deepest Malpica-Tui Unit. Such layout
suggests westward subduction polarity (present
coordinates; Martínez Catalán et al., 1996). The
Malpica-Tui and Santiago units reached eclogite
facies conditions, while the Lalín and Forcarei
units did not exceed the blueschist metamor-
phic facies. Blueschist facies conditions were also
reached in the Ceán and Lamas de Abade units,
located upwards in the original stacking, but the
metamorphic conditions recorded in the Cercio
Unit during the high-P event are still not clear.
According to the higher metamorphic conditions
reached by the UMG, these units are interpreted
as a piece of the Lower Sequence subducted to
greater depths and subsequently thrust onto the
Upper Sequence, most probably during still ac-
tive continental subduction (Arenas et al., 1997).
The original subduction zone has been calcu-
lated to dip c. 18° using thermal modeling (Alcock
et al., 2005). The continental subduction complex
was later affected by thermal inversion, which
caused extensive migmatization in the Lalín,
Agualada and Espasante units. Such inversion
was fuelled by conductive heat transfer from the
overlying mantle wedge, in part during exhuma-
tion of the subduction complex. Remnants of this
mantle wedge are still preserved in the ultramafic
section preserved in the Campo Marzo Unit (Fig.
2). The reaction history deduced for the high-P
pelitic schist from the Ceán Unit, according to
thermal modeling in the MnNCKFMASHTO
system, is shown in Fig. 9 (López Carmona et al.,
2013). Pressure peak conditions in these schists
are calculated in c. 22 Kbar, but higher pressure
values were attained in sectors of the Malpica-Tui
(Rodríguez et al., 2003), and probably in the
Agualada and Espasante units. The schists of the
Ceán Unit also show the regional thermal inver-
sion characteristic of the Basal Units, as can be
observed in Figs. 9a, 9b and 9c.
Structural evolution
The regional structure of the Basal Units was
first described in theÓrdenes Complex (Martínez
Catalán et al., 1996). More recent works have ex-
panded the study to the Malpica-Tui Complex
(Díez Fernández, 2011; Díez Fernández et al.,
2011, 2012d, 2012e) or have provided detailed
microstructural descriptions (Llana-Fúnez, 2002;
Gómez Barreiro et al., 2010a).
The record of the first deformation that affected
the Basal Units (D1) appears strongly overprinted
by subsequent events and with relict character.
Remnants in the metasedimentary rocks occur
as an internal schistosity within albite porphy-
roblasts (Si ) grown during decompression. Si
consists of an aligned mineral assemblage of
garnet-phengite-chlorite-rutile-clinozoisite (S1).
This high-P foliation and its associated stretch-
ing lineation have been also preserved as variably
sized resisters in the Malpica-Tui Unit, such as
eclogite boudins and pods of eclogitic gneisses.
The stretching lineation associated to S1 trends
NE-SW, whereas shear sense indicators suggest
a consistent top-to-the-NE kinematics in pres-
ent coordinates. Such kinematics were developed
during westward subduction of the Gondwanan
margin, thus supporting that the continental
convergence had a dextral component at this
stage (Díez Fernández et al., 2012d, 2012e).
In advanced stages of D1, sections of the
continental margin located inboard reached the
subduction zone. Progressive accretion of thicker
and more buoyant sections of the margin locked
the subduction complex. Thrusting of the UMG
(Agualada and Espasante units) above the LMG
probably occurred during this stage (Martínez
Catalán et al., 1996). P-T-t paths obtained in the
Agualada, Santiago and Lamas de Abade units
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3. GEOLOGICAL FRAMEWORK