This work is protected by copyright and other intellectual property rights and duplication or sale of all or part is not permitted, except that material may be duplicated by you for research, private study, criticism/review or educational purposes. Electronic or print copies are for your own personal, non- commercial use and shall not be passed to any other individual. No quotation may be published without proper acknowledgement. For any other use, or to quote extensively from the work, permission must be obtained from the copyright holder/s. POLYOROGENIC EVOLUTION OF THE WESTERN PAYS DE LEON. FINISTERE. FRANCE H. ROPER PH.D. 1980 VOL.1: TEXT , BEST COpy . , .AVAILABLE . Variable print quality ABSTRACT 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. Th. WOK described in this thesis was carried out at the Departaent ot a..1.81, UDiTerlJiV ot ~eele, UDder the superY18ion ot Dr. ll.A.l1oach. !he WOK was supported by an B.E.R.C. Postgraduate Research Grant which i. gratetul1y aoDow1edpd. I should like to thaDk Dr. Roach tor drawing rq attentioZl to the rocks ot the western P~s de Lion, tor ~ usetul discussions, tor critically readiDg the JIS, and tor pel'lD1ssioDto use photograph1c material. I _ grateh1 to hooteuor GeXelling aDd other ._bers and tomer m_bers ot the ICeele Oeelogy Departllent tor their geod-hUllOuredencou;rasement, usetul 41s0US8iona, and technical &ssistance, particular tb.nlcB to the , tollowir:agl )(.A1ld.n, D.J(.AlderioD, !.!esly, J .cuee,. J .S.Daly,. lI... Duranco, D.W.Ealel, P.A.Jioyd, P.Greatbatch, J.Install, D.W.Kelsall, G.J.Lees, . R.G.Pa1'k, J .Pepper, G.Rowbothaaa,1I.".Stead, and J .A..Winchester. I .. also grateful tor usefUl discussions with the tollowinga at the Un1.versi. ti de Bretagne Occidentale in Brest with Protessor J. Didier, LJarriire, L.Cbauris, J-R.Darboux and J.Rolet, at the Univeniti de Bennes with hoot_sor J .Cogni, 1I.S••• Carpenter, S.X:.Haxaer, J-J .Peuoat, Ph.Vidal~and J-P.Brun, and elsewhere with C.J.D.AdaIaa, .A..J.!arber, • I • 6 B.CabaD1s, Y.Corre, J ."'.Gra3, J .A.Killer, J .'l'.llenout, L.B1chards and B.Reberis. I .. indebted to J'.G.ll1tchell tor collaboration on X'P-Ar datillg, and to R.T.T&110r tor pe~8sion to reter to his unpublished data. I should also like to thank: C.J .Gubeman, ](.J.Nichols, and Ie.O'Sullivan tor assistance in typing and preparation ot maps, and J.P Udal for tinanoial assistance. • CONTENTS (note: a more detailed list of contents is included at the beginning of each chapter) Chapter 1. INTRODUCTION A. Location and topography B. Geological setting C. Previous geological research Do Purpose and arrangement of the present study E. Geological mapping procedures Chapter 2. THE LANNILIS METAMORPHIC COMPLEX AND ASSOCIATED GRANITIC ROCKS A. Field relations and petrography B. Metamorphic and structural history C. Primary nature and pre-M2 relationships of the Lannilis Metamorphic Complex D. Post-M2 intrusions associated with the Lannilis Metamorphic Complex E. Summary of geological history of the Lannilis Metamorphic Complex and asseciated granitic rocks Chapter 3. THE PLOUGUERNEAU MIGMATITE COMPLEX AND ASSOCIATED GRANITIC ROCKS A. Introduction B. Diorite de Portsall C. Migmatites de Plouguerneau D. Granite de Landunvez E. Adamellite de Ste Marguerite and other late granitic intrusions F. Aspects of the structural geology of the Plouguerneau-Landunvez district G. Structural and magmatic evolution of the Plouguerneau-Landunvez migmatitic and granitic complex Chapter 4. THE PORSPODER LINEJJ1ENT A. Introduction B. Significance of the lineament as a major geological boundary C. Function of the lineament as a locus for major granitic intrusions D. Deformation and other features associated with the Lineament E. Displacement associated with the lineament F. Analogous structures elsewhere in the Massif Armoricain Chapter 5. GEOCHRONOLOGY OF THE NW PAYS DE LEoN A. Introduction B. Uranium-lead dating C. Rubidium-strontium whole-rock dating D. Rubidium-strontium mineral dating E. Previous K-Ar dating F. New K-Ar dating of metamorphic amphiboles from the NW Pays de Leon G. Conclusions Chapter 6. GEOLOGICAL RELATIONSHIPS IN THE LE CONQUET DISTRICT OF THE SW PAYS DE LOON A. Introduction B. Previous geological work in the SW Pays de L'eon C. Complexe Metamorphique du Conquet D. Post-metamorphic intrusions E. Structural, metamorphic and magmatic evolution of the SW Pays de L~on Chapter 7. GEOCHRONOLOGY OF THE SW PAYS DE LEON: A NEW INTERPRETATION A. Introduction B. Stratigraphic setting of the SW Pays de Leon C. Geochronology of the SW Pays de L~on D. Conclusions ~6 Chapter 8. THE M2 METAMORPHISM IN THE WESTERN PAYS DE LEON: A CROSS-SECTION THROUGH A HIGH-GRADE METAMORPHIC BELT IN THE HERCYNIDES A. Introduction B. Lithologies affected by the M2 metamorphism c. Pre-M2 metamorphic episodes D. The M2 metamorphism E. Metamorphism attributable to the M2 episode innareas adjacent to the western Pays de Leon F. Metamorphic reactions and physical conditions during the M2 episode G. Regional context and geotectonic significance of the M2 metamorphism in the Pays de Leon Chapter 9. CONCLUSIONS A. Summary of present work B. Geological evolution of the Western Pays de L~on c. Possible further work LIST OF TEXT FIGURES Fig. 1-1. Massif Armoricain: location diagram. 1-2. pays de Leon and adjacent areas: location map. 1-3. Geological map of the Massif Armoricain. After Cogne (in Debelmas, J., 1974). 1-4. Geological map of the Northern part of the Massif Armoricain. After Roach and Brown (in Brown, 1974). 1-5. Geological map of NW Finistere. After Carte G~ologique de la France, 1:320.000, 1970. 1-6. Geological map of the Pays de Leon. After Chauris, 1967. 1-7. Geological map of the Massif Granitique de L'Aber-Ildut and adjacent areas. After Chauris, 1966b. 1-8. Structural map of Les Abers district. From Cogne and Shelley, 1966. 1-9. Geological map of the NW Pays de Leon. After Carte G~ologique de la France, 1:80.000, 1966, 1972. 2-1. Location map for Lannilis district. 2-2 Geological map of L'Aber-Wrac'h. After 1:80.000 map of France, 1966, 1972. 2-3. Geological map of L'Aber-Benoit. 2-4. Schematic sections of the Lannilis Metamorphic Complex. 3-1. Location map of NW coast of the Pays de Leon. 3-2. Geological map of the Portsall area. 3-3. Schematic cross-section through Diorite de Portsall. 3-4. Geological map of the Guiss~ny area. 3,-.5. Intrusive contacts at Carrec Cros. 3-6. Intrusive contacts at lIe Tariec. At end. 3-A. Geological map of the NW coastal sector of the Pays de Laon. At end. 3-B. Structures in the Adamellite de Ste Marguerite and associated lithologies. Fig. 4-1. The Porspoder Lineament. 4-2. Porspoder Lineament: L'Abe~-Beno1t sector. 4-3. D4 structures at Porspoder. 4-4. Quartz c-axes in a mylonite. After Cogne and Shelley, 1966. 4-5. Major transcurrent faults in West Brittany. After Rolet, 1977. 5-1. Location of samples used for U-Pb and Rb-Sr geochronology of the NW Pays de Leon and adjacent areas. After various authors. 5-2. Nicolaysen diagram for samples from NW Pays de L~on and adjacent areas. After Adams, 1967a, and Leutwein et aI, 1969a. 5-3. Locations of hornblende and sphene samples used for K-Ar dating.