The pre-Mesozoic metamorphic pattern of the External Massifs, composed
of subunits of different metamorphic histories, resulted from the
telescoping of Variscan, Ordovician and older metamorphic and structural
textures and formations. During an early period, the future External
Massifs were part of a peri-Gondwanian microplate evolving as an active
margin. Precambrian to lower Palaeozoic igneous and sedimentary
protoliths were reworked during an Ordovician subduction cycle
(eclogites, granulites) preceding Ordovician anatexis and intrusion of
Ordovician granitoids. Little is known about the time period when the
microcontinent containing the future External Massifs followed a
migration path leading to collision with Laurussia. Corresponding
rock-series have not been identified. This might be because they have
been eroded or transformed by migmatisation or because they remain
hidden in the monocyclic areas.
Besides the transformations which originated during the Ordovician
subduction cycle, strong metamorphic transformations resulted from
Variscan collision when many areas underwent amphibolite facies
transformations and migmatisation. The different subunits composing the
External Massifs and their corresponding P-T evolution are the
expression of different levels in a nappe pile, which may have formed
before Visean erosion and cooling. The presence of durbachitic magmatic
rocks may be the expression of a large scale Early Variscan upwelling
line which formed after Variscan lithospheric subduction. Late Variscan
wrench fault tectonics and crustal thinning accompanied by high thermal
gradients triggered several pulses of granite intrusions.