The geology of the Morlaix - St Michel-en-Greve region, N.W. Brittany, France.
Detailed mapping of the area between Morlaix and St. Michel-en-
Greve has been carried out within the northwest part of the Massif
Armoricain and three distinct tectono-metamorphic areas have been recognised.
In the western area a medium-high grade metamorphic complex comprised
of an older basement complex of amphibolites and metasediments, originally
Brioverian supracrustals, and Cadomian granitic orthogneisses is recorded
These units have been truncated by numerous syn-tectonic and post-tectonic
Hercynian granites and this area is termed the St. Pol-de-Leon Metamorphic
In the central part of the area a restricted basin of Devonian-Lower
Carboniferous metasediments and basic metavolcanics is recognised. The
Morlaix Basin lithologies are of low metamorphic grade and are in contact
with the St. Pol-de- Leon Metamorphic Complex across a major tectonic
lineament termed the Carantec Shear Belt (CSB). A new stratigraphy of the
basinal sequence is presented and the timing of peak Hercynian deformation
is established at c. 350 M. y. using available geochronological and field
In the eastern part of the area a low-grade metamorphic terrain
composed of Brioverian metabasic volcanics and metasediments occurs, known
as the Petit Tregor. This succession is truncated by a multi-component
Cadomian intrusive complex which has undergone major, heterogeneous deformation.
The recognition of a major shear belt the Locquirec Shear Belt and
numerous smaller shear zones within the Moulin de la Rive Orthogneiss Complex
has resulted in the re-appraisal of the whole stratigraphy of the Petit
Tregor area. Field evidence shows that deformation in the Petit Tregor
is somewhat earlier than in the Morlaix Basin and St. Pol-de-Leon
Metamorphic Complex and occurred in the period late Cadomian-early
Hercynian. Discussion is then given to the timing of regional
deformation events in the light of evidence from the study area.