serie NOVA TERRA nº 48

as the isotopic geochronology obtained for them.

The second metamorphic event is the one show-

ing HP-HT characteristics. The tectonic fabrics

and mineral assemblages of the first Cambrian,

arc-related, metamorphic event were in this case

affected by strong reworking, and they have been

almost completely obliterated. However, the ini-

tial existence of this first event still can be detect-

ed, mainly due to the presence of relict monazites

in high grade gneisses whose age has been deter-

mined by U-Pb.

Equivalent HP-HT units are known in the Va-

riscan Belt outlining the Variscan suture, both

in the Armorican Massif, French Massif Cen-

tral and the Bohemian Massif (Ballèvre

et al.

2014). In some sectors the presence of mineral

assemblages including coesite suggests that ul-

tra-high-P conditions were reached at least local-

ly (Lardeaux

et al.

, 2001). Peak pressures calcu-

lated so far for the NW Iberian Massif are close to

ultra-high-P values, although evidence of coesite

is still missing.

Lithologies and tectonothermal evolution

In the western sector of the Órdenes Complex,

the Fornás Unit (Figs. 2 and 4) is mainly con-

stituted by migmatitic garnet amphibolites with

few mineral relicts of previous high-P granulites

(Cpx-Grt-Pl) and coronitic metagabbros. Two

kilometric inclusions of metaperidotites and gar-

net-bearing pyroxenites, and dispersed layers of

paragneiss (Grt + Ky + Bt + Pl + Qtz + Kfs + Rt)

can also be found in this unit (Gómez Barreiro,

2007).

In the eastern part of the Órdenes Complex,

the HP-HT Upper Units are represented by the

Sobrado, Melide and Belmil units, formed by

similar lithological ensembles but variable thick-

ness and structural complexity (Figs. 2, 4 and

20). The Sobrado Unit shows the most complete,

though much thinned, HP-HT succession. It in-

cludes three tectonic slices, the intensity of HP

metamorphism of which increases from top to

bottom (Arenas and Martínez Catalán, 2002).

The lower slice is formed by serpentinized ultra-

mafic rocks, eclogites and Cpx-Grt rocks without

3. GEOLOGICAL FRAMEWORK

Fig. 23.

P-T paths indi-

cating the metamorphic

evolution followed by

some characteristic HP-

HT lithologies from the

Sobrado Unit (Órdenes

Complex). Boxes in the

diagram are indicative of

peak conditions. Isople-

ths of jadeite content in

clinopyroxenes

coexis-

ting with albite (Xalb=1)

and quartz, and stabili-

ty curves of the Al

SiO

polymorphs are also

shown. The P-T paths

were calculated using

Cpx-Grt thermobarome-

try (Díaz García

et al.,

1999b).