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Geological Society, London, Special Publications; 1990; v. 51; p. 27-39;
DOI: 10.1144/GSL.SP.1990.051.01.03
© 1990 Geological Society of London

Armorican Massif

The geology of the Penthièvre crystalline massif: a reappraisal of the type-Pentevrian area, northern Brittany

M. M. Shufflebotham

Department of Geology, University College of Wales, Aberystwyth, Dyfed SY23 3DB, UK
Cambridge Arctic Shelf Programme, University of Cambridge, West Building, Gravel Hill, Huntingdon Rd, Cambridge CB3 ODJ, UK

Field evidence suggests a complex history for the Penthièvre crystalline massif (PCM) prior to the deposition of the local Brioverian supracrustal sequences, the Erquy Volcanic Formation and the Hillion Volcanic Formation. The PCM comprises three assemblages of igneous derivation: (a) the Yffiniac Group, consisting of amphibolites and banded orthogneisses which were deformed and metamorphosed to amphibolite facies during D1 and D2; (b) the Port Morvan Group, comprising Grey and Green Gneisses of trondhjemitic to quartz dioritic composition, the precursors of which intruded the Yffiniac Group and were deformed and metamorphosed during D3 and D4; and (c) the Jospinet Group of granodiorites, the St Maurice quartz diorite and, probably, the Fort de la Latte Complex (the latter two were emplaced during D5a). A shearing event (D5b) produced cataclastic fabrics in the basement, and the Hillion—Cesson Mylonite Belt, prior to the deposition of the Brioverian in the area.

The Erquy and Hillion Volcanic Formations were deposited in an ensialic basin upon a basement complex represented by the PCM. Island-arc and active continental margin magmatism occurred from before 670 Ma to as late as the Cambrian. This magmatism can be viewed as representing the effects of the Cadomian orogeny, during the early stages of which the local Brioverian sequences were deposited.