High-pressure rocks in the Variscan belt of Western Europe: the Malpica-Tui Complex (NW Iberian Massif)
Abstract
Pods and relicts of high-pressure (HP) rocks are the best
record of the subduction of the north Gondwana margin at
the onset of the Variscan collision. Across Western Europe,
blueschist-facies terranes are restricted to scarce and
relatively small areas, whereas eclogite-facies terranes are
more abundant.
The Malpica-Tui complex (MTC) is the westernmost
exposure of HP rocks in the NW Iberian Massif, and in the
Variscan belt of Western Europe. It comprises two
tectonically juxtaposed units separated by an extensional
detachment: (i) an upper unit consisting of rocks in the
blueschist-facies, and (ii) a lower unit in the medium
temperature eclogite-facies conditions. Assuming a
northwest-directed component of subduction, in present
day coordinates, the characteristics of each sequence
suggest that the upper unit would occupy an oceanward
position compared to the lower unit before the Variscan
collision. Thus, the lower unit is interpreted as a slice of
continental crust, whereas the upper unit may represent a
transitional to oceanic crust of the same continental margin.
The MTC preserves evidence of late Devonian HP
metamorphism varying from eclogite (P~26 kbar and
T~650 °C) to blueschist-facies conditions (19-22 kbar and
460-560 °C). Petrological analysis involving P-T-X
pseudosections in the (Mn)NCKFMASHTO chemical
system on the HP rocks reveals a P-T evolution
characterised by a subisothermal decompression to
~10 kbar, 480 °C in the blueschist-facies rocks and 650 °C
in the eclogites, followed by cooling to ~5 kbar at 380 °C
and 500 °C, respectively. New 40Ar/39Ar data indicate a
minimum age of ~370 Ma for the subduction-related HP
metamorphism. Subsequent decompression to pressures
of about 10 kbar started at ~360 Ma and was
contemporaneous with thrust-and-fold nappe tectonics and
intrusion of early Variscan granodiorites dated at ~350-
340 Ma. Final, “post-nappe”, exhumation is interpreted to
have taken place from ~345-335 Ma to 320 ± 5 Ma, which
is the age of the syntectonic igneous rocks emplaced in the
autochthon of the MTC. These ages support the
equivalence of the HP rocks from NW Iberia and their
counterparts in the southern Armorican Massif.
From a methodological point of view, modelling
calculations of H2O and Fe2O3 on the metamorphic
evolution of blueschist-facies rocks reveals trends that may
have general applications in the investigation of rocks with
similar composition: (i) subduction-zone metamorphism
may occur in H2O-undersaturated conditions induced by
the crystallization of a significant modal amount of
lawsonite, although the transition from lawsonite blueschist
facies to amphibolite-greenschist facies may involve
significant hydration, principally as a result of lawsonite
breakdown. (ii) The analysed values of Fe2O3 may not
reflect the oxidation state during the main metamorphic
evolutionary stage and are probably easily modified by
superficial alteration, even in apparently fresh samples