A completely new 11thological classification is made of the Lannilis Metamorphic Complex and the Plouguerneau Mi~atites, which lie south and North respectively of the Porspoder Lineament in the north-west Pays de Leon. The sequence of metamorphic, magmatic and tectonic episodes is analysed and a correlation between the two complexes is suggested. The metasedimentary Iithologies are thought to be derived from Brioverian semi-peli tes, and the meta-igneous rocks from post-Brioverian intrusions. The later granitic rocks associated with the Plouguerneau Migmatites are described and classified in detail for the first time. The structures which define the Porspoder Lineament itself are also described. This feature may have originally been an intra-continental fault of transform type. Existing geochronological data and new K-Ar mineral dates are used to fit the geological evidence from the NW Pays de Leon to the absolute time scale. The importance of the Hercynian orogeny is emphasised; the main metamorphism and migmatisation are attributed to the Bretonic (Devono-Carboniferous) phase. A study has also been made of the Le Conquet Metamorphic Complex in the SW Pays de Leon. An entirely new interpretation of the field relations and geochronology of the lithologies previously known as 'Gneiss de Brest', 'Gneiss de Lesneven', and 'Granodiorite des Renards' is presented. A single major intrusion, the Granodiorite de Brest, is recognised. It was emplaced into the already deformed and metamorphosed Briverian during the latter stage~ stages of the late-Precambrian/Cambrian Cadomian orogeny, probably at about 550 my. Other intrusions (Granodiorite de Pont-Cabioc ~ and doleritic Filons de Kermorvan) were also emplced prior to the main regional metamarphism (the Bretonic phase of the Hercynian orogeny) which produce~taurolite- and sillimanite-bearing assemblages in the associated (originally Br1over1an) metasediments. This sequence of events, and later phases of folding and granite emplacement, are tentatively correlated with events in the NW Pays de Leon and in the supracrustal terrain of central Finistere. A special study has also been made of the Bretonic regional metamorphism of the whole of the western Pays de Leon. Observed assemblages suggest a geothermal gradient intermediate between the Barrovian and Abukuma types. Attention is drawn to the long and complex history (Lower Proterozoic or Pentevrian, Cadomian and Hercynian) represented by the rocks of the western Pays de Leon.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:587057 |
| Date | January 1980 |
| Creators | Roper, H. |
| Publisher | Keele University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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