61 / 352
in zircon; Abati
et al.
, 1999, 2007). These xeno-
liths were transformed in intermediate-P granu-
lites (
c.
800 °C and 9.5 kbar) during susequent
prograde metamorphism at
c.
498 Ma (U-Pb in
monazite; Abati
et al.
, 1999). The mineral assem-
blage is formed by spinel-garnet-orthopyrox-
ene-cordierite-sillimanite-biotite, being devel-
oped during a counter-clockwise metamorphic
path (Abati
et al.
, 2003) (Fig. 19a). The garnet
crystals exhibit thick overgrowth rims surround-
ing previous core domains developed at high-T
and low-P conditions (Figs. 19b,c). Count-
er-clockwise P-T paths are generally associated
with magmatic arcs where intense underplating
generates strong heating at low pressure. Subse-
quent subduction of sections of the magmatic arc
itself explains the drastic pressure increase, which
occurred shortly after the gabbro intrusion. Still
during the high-T evolution, the Monte Caste-
lo massif was affected by a new intermediate-P
shearing dated at
c.
482 Ma (Abati
et al.
, 2007). A
similar evolution can be recognized in the Corre-
doiras Unit. The large Corredoiras Orthogneiss
Massif recrystallized under intermediate-P gran-
ulite facies and includes large xenoliths of pel-
itic granulites, strongly migmatised, from which
identical counter-clockwise P-T paths have been
inferred (González Cuadra, 2007). U-Pb dating
in monazite obtained from these migmatitic xe-
noliths yielded an age of 484-493 Ma (Abati
et
al.
, 1999), a similar range to that obtained in the
Monte Castelo Unit.
Following the two first deformation phases,
the tectonothermal evolution of the IP Upper
Units is characterized by a new prograde tec-
tonic event with collisional nature (D
3
), which
seems to be separated in time from the previous
arc-related event. Therefore, the IP Upper Units
are equivalent to an old basement deformed first
during D
1
and D
2
and affected long afterwards by
reworking associated to a new thickening event
D
3
. This evolution has been better identified
in the SE part of the Órdenes Complex, in the
Corredoiras Unit. There, the orthogneiss massif
characteristic of this unit and its large xenoliths
of migmatised pelitic granulites appear deformed
by a generation of prograde shear zones (Fig.
20). These shear zones are up to 1 km thick and
their associated foliation (S
3
) exhibits N-S trend-
ing stretching lineation and consistent top-to-
the-N kinematic indicators. When D
3
affects the
metapelitic rocks, S
3
may have mylonitic charac-
ter and is defined by a prograde mineral assem-
blage with garnet-staurolite-biotite, preferential-
ly for the case of pelitic granulites formed during
the previous cycle (González Cuadra, 2007) (Fig.
20). These shear zones have been interpreted as
D
3
thrusts repeating slices of granulitic basement
deformed and metamorphosed in Cambrian
times (González Cuadra, 2007). There are not
isotopic geochronological data in the Corredoi-
ras Unit to constrain the age of the D
3
event in
the IP Upper Unit. However, D
3
shear zones are
clearly intersected at high angle by the Corredoi-
ras Detachment (Fig. 20), which separate the IP
Upper Units from the HP-HT Upper Units in the
SE of the Órdenes Complex (Díaz García
et al.
,
1999b; González Cuadra
et al.
, 2007) (Fig. 20).
An
40
Ar/
39
Ar age of
c.
375 Ma dating the activity
in the Corredoiras Detachment (Dallmeyer
et al.
,
1997), can be taken as a minimum age for D
3
.
Equivalent shear zones to those described in
the Corredoiras Unit have been described in the
O Pino Unit. Thrusting in this case is associat-
ed with recumbent folds vergent to the E-NE
and accompanied by metamorphism under gar-
net-bearing amphibolite facies conditions (Gó-
mez Barreiro
et al.
, 2006; Gómez Barreiro, 2007).
S
3
in this particular case was dated at
c.
397 Ma
(
40
Ar/
39
Ar dating on hornblende; Gómez Bar-
reiro
et al.
, 2006). This is a reference chronology
for D
3
, which is thought to represent rework-
ing of an old metamorphic basement in a much
younger collisional setting. The Ponte Carreira
and Corredoiras extensional detachments cut D
3
structures and consequently have been integrat-
ed in the gravitational collapse that followed D
3
crustal thickening.
High Pressure and High Temperature Upper
Units
The lower part of the Upper Units consists of
a HP-HT metamorphic belt. The protoliths are
similar to those described in the IP Upper Units,
but the intensity of deformation and metamor-
phism hinders the preservation of primary sedi-
mentary or igneous features, that only in few cas-
es can be recognized. The succession of the main
deformation and metamorphic events are also
comparable, regardless of their intensity, as well
45
3. GEOLOGICAL FRAMEWORK