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Abstract
The Variscan belt was formed when several continents interacted to finally form the superconti-
nent of Pangea. In this general context of continent interaction and orogen development this PhD was
projected and is presented in this document. The main objective of this thesis is to explore the origin,
the sources and the development of a high-grade metamorphic terrane involved in this orogenesis.
This PhD focuses on the Eclogitic Gneisses (Banded Gneisses), which are included in the high-pres-
sure and high-temperature (HP–HT) member of the Upper Allochthon of the Cabo Ortegal Complex
(NW Iberia), and also deals with the Eclogite band and the Cariño Gneisses, so that the general tec-
tonothermal evolution is better accounted for.
To address the objectives of this thesis, several methods have been applied. The first group of meth-
ods had the aim of recognising and describing the subject of study, from a macroscopic point of view
(cartography and detailed representation of geological sections) to a microscopic level (thin section
observation and description). When this recognition was carried out and the scientific literature was
handled, detailed isotopic experiments were performed. U–Pb, Lu–Hf and Sm–Nd radiogenic meth-
ods were applied to make provenance approaches of the detrital materials, to constrain the age of rock
formation, the primary sources of magmatic generation and the age and features of the high-grade
metamorphic development of the igneous rocks of the studied units.
This PhD document has been organised in an article format, where the majority of the results
are presented as articles, from which the PhD candidate is the corresponding author or co-author.
This thesis starts with a general introduction and with an exposure of the main objectives posed and
methods applied. Afterwards, a general review of the origin and evolution of the NW Iberian ter-
ranes, and a detailed study from a macroscopic to a microscopic point of view of the main formation
studied, together with a discussion of its tectonothermal evolution and an exhumation modelling is
presented. Then, a provenance approach together with a paleogeographic reconstruction proposal,
studying the detrital materials of the intermediate-pressure top member of the Upper Allochthon is
shown. These provenance studies were also applied to the HP–HT Banded Gneiss formation, mainly
to find out which continental fragment subducted in the Devonian. After this work, isotope geochem-
istry techniques were applied to several igneous rocks that registered the evolution of a long-lived
magmatic arc and the deformations attained by the HP–HT subduction-related metamorphic event.
Afterwards, a discussion integrating this subduction event in the general framework of the assembly
of Pangea is presented. Finally, this PhD finishes exposing an integrated discussion and the general
conclusions achieved.
The main conclusions derived from this PhD thesis are that the Upper Allochthon is a single ter-
rane and not a composite one, from which its metasedimentary rocks are derived from the Gond-
wana mainland, specifically from the West African Craton (WAC). The detritus that constitute the
metasedimentary rocks were deposited in a back-arc type basin between
c.
521–506 Ma. These sedi-
ments register the crustal growth events of the WAC and the formation of a volcanic arc system that
is considered to be the Cadomian arc. The initial activity of this arc is constrained between
c.
780 Ma
and
c.
590 Ma, defining the “proto-arc stage”, where mainly crustal recycling of the margin of Gond-
wana where it was built, took place. The “arc-stage” was developed between
c.
590 and 490 Ma where
broad mixing processes took place between the intruding depleted mantle (DM) derived magmas and
the old pre-existing crust. The igneous rocks registered part of the magmatic arc activity (D0 event).
The first record of acidic magma intrusion has an age of
c.
512 Ma and has diverse source signatures. It
seems that around
c.
498 Ma an arc-related high-temperature input gave as a result fractional crystal-
lisation of acidic rocks from DM-derived partial melts. The basic magmas started to intrude at
c.
505
Ma and had different sources, from magmas almost directly derived from the partial melting of the
depleted mantle, to isotopically enriched magmas, which could derive from the partial fusion of an
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