Résumé
Les domaines de Saint-Brieuc et de Guingamp (Massif armoricain) occupent une place centrale dans les reconstitutions de l'orogène cadomien car ils furent fortement déformés et métamorphisés aux environs de 580-590 Ma. En outre, les roches métamorphiques sont largement développées dans ces domaines, qui contiennent les roches de plus haut degré de la chaîne cadomienne. La caractérisation de leur évolution métamorphique devrait donc apporter des contraintes significatives sur l'évolution géodynamique de l'orogène cadomien. Le domaine de Saint-Brieuc a été affecté par une déformation progressive qui se traduit essentiellement par un raccourcissement NW-SE, tandis que le domaine de Guingamp est caractérisé par le développement d'un dôme extensif asymétrique. Le métamorphisme régional est caractérisé par un accroissement d'intensité vers le sud. Ce gradient est mis en évidence par les différences d'assemblages minéralogiques dans des roches de composition semblable (en particulier les métabasaltes et les métagrauwackes) et les variations progressives de composition chimique des phases. Les pressions estimées tant pour la Formation de Lanvollon (3-5 kbar) que pour les Migmatites de Guinguamp (environ 6 kbar) montrent qu'une phase d'épaississement crustal prit place au début de l'orogenèse cadomienne. Malgré ces pressions faibles à modérées, de fortes températures ont été atteintes dans ces roches (jusqu'à 650-700°C), indiquant l'existence durant l'épaississement d'un gradient géothermique élevé. De volumineuses intrusions gabbro-dioritiques (Saint-Quay et Squiffiec) se mirent en place durant cet épisode de déformation, aux environs de 580-590 Ma, et ces intrusions perturbent localement le gradient régional. D'autres intrusions gabbro-dioritiques (Saint-Brieuc) eurent lieu après la déformation majeure et le métamorphisme régional, aux environs de 540-530 Ma. Ces données suggèrent que le métamorphisme cadomien ne résulte pas de la relaxation thermique après une collision continent - continent, car aucune relique de haute pression - basse température n'a été observée. Un gradient géothermique initial élevé et/ou l'advection de chaleur par les magmas semblent nécessaires au développement du métamorphisme. Ces deux cas sont compatibles avec un modèle de raccourcissement d'un arc volcanique.
Abstract
The Late Proterozoic Cadomian belt is well preserved in northern Brittany Previous studies have identified several domains with different tectono-thermal histories (see, for example, Strachan et al., I 989; Brun and Balé, 1990; Dallmeyer et al., 1991; Egal et al., 1996), which amongst the Saint-Brieuc and Guingamp domains (Fig. 1) represent a key area for understanding the Cadomian orogeny because they are the only ones affected by the major phase of deformation occuring at 580-590 Ma. Metamorphic rocks are extensively developed in both domains, which also contain the highest-grade rocks of the Cadomian belt. Consequently, characterization of their metamorphic evolution should provide significant constraints on the geodynamic evolution of the Cadomian belt. Main lithologies and structures. The Saint-Brieuc domain comprises two main formations: the Lanvollon Formation essentially consisting of strongly deformed mafic flows and sills and some felsic volcanics, dated at about 590 Ma (Egal et al., 1996), with minor interbedded sedimentary layers and ore bodies; and the Binic Formation, which conformably overlies the Lanvollon formation and consists of turbiditic sediments with a strong volcanogenic contribution (Denis, 1988). The Saint-Brieuc domain is characterized by a steeply dipping foliation (Figs. 2 and 3) which results from a progressive déformation during a NW-SE shortening. The Guingamp domain is made up of migmatitic gneisses and abundant leucogranites and pegmatites; rare metabasites are present within the migmatites. As a whole, the structure of the Guingamp domain contrasts with that of the Saint-Brieuc domain (Fig. 3), being characterized by a flat-lying foliation and by the late development of an extensional asymetric dome (Le Goff et al., 1994). Mineral parageneses within metabasites and metagreywackes. The regional metamorphism is documented by the change in mineral assemblages for similar bulk-rock compositions, especially in the metabasites (Fig. 4) and metagreywackes (Fig. 6). Mineral parageneses within the Lanvollon metabasites reveal an increasing metamorphic grade towards the south and southwest: transitional assemblages between the greenschist and amphibolite facies are observed in the Hillion peninsula, whereas epidote-bearing amphibolites, then epidote-poor or epidote-absent amphibolites characterize the Lanvollon metavolcanics towards the west. This increase is also revealed by systematic variations in the chemical composition of coexisting phases, such as the Tï content of calcic amphiboles coexisting with either ilmenite or titanite (Fig. 5). Higher grade parageneses are found in the Yffiniac Formation, where garnet- or clino-pyroxene-bearing amphibolites are observed, and in the amphibolites interlayered within the Guingamp migmatites. Metagreywackes are less abundant than metabasites, and are used to define two isograds (Fig. 6). The cordierite-in isograd (documented in the Binic Formation to the south of the pluton) marks the extent of the contact aureole due to the Saint-Quay Diorite, whereas the staurolite-in isograd (documented in the Légué area) parallels the regional trend deduced from the Ti content of calcic amphiboles. Strongly deformed leucosomes and sillimanite-biotite melanosomes in the Guingamp migmatites are thought to result from partial melting, but the early stages of the P-T evolution are only rarely preserved in the form of kyanite inclusions within garnet. P-T conditions of the metamorphism. A summary of the geothermo-barometric estimations available in the area studied is shown on Figure 7. P-T estimations for the Binic and Lanvollon formations are difficult to obtain because of the lack of diagnostic assemblages. Nevertheless, the detailed study of the staurolite-bearing metagreywackes from the Légué area indicates pressures of the order of 3-5 kbar (Hébert and Ballèvre, 1993). Despite low to medium pressures, high temperatures were achieved in these rocks (up to 500-600°C), indicating a high geothermal gradient. The Guingamp migmatites developed aI about 6 kbar, unambiguously indicating that crustal thickening took place in the early stages of the Cadomian orogeny This reinforces the model of the tectonic evolution of the Cadomian belt, according to which the boundary between the Saint-Brieuc and Guingamp domains was a major early thrust (Balé and Brun, 1989). Finally, the highest P-T conditions are recorded in the Yffiniac Formation (9 ± 1 kbar, 700 ± 50°C according to Hébert, 1994), but their meaning remains uncertain. Timing of gabbro-dioritic intrusions with respect to metamorphism. Two generations of gabbro-dioritic intrusions are distinguished. The first generation (Saint-Quay Squiffiec) was synchronous with the main ductile deformation and is associated with local perturbations ofthe regional temperature graduent (e.g. the Crd-in isograd around the Saint-Quay Diorite). These early plutons are dated at about 580-590 Ma (Vidal et al., 1972; Dallmeyer et al., 1991; Egal et al., 1996). The second generation (Saint-Brieuc) overprints the regional deformation and metamorphism, develops a narrow contact aureole, and is dated aI about 540-530 Ma (Hébert et al., 1993). Geodynamic setting of the Cadomian metamorphism The data suggest that the Cadomian metamorphism does not result from a thermal relaxation following continent - continent collision because no early high pressure - low temperature (i.e. blueschist or eclogite facies) relics have been found An initial high geothermal gradient and/or heat advection arising from synkinematic magmatic intrusions seem to be more suitable models. Both cases are compatible with the shortening of a volcanic arc. The proposed tectonic evolution within this overall frame, is as follows. Firstly, a major volcanic (Lanvollon Formation) then sedimentary (Binic Formation) episode reflects the existence of an extensional period at about 590 Ma, interpreted as the opening of a basin in a back-arc position (Chantraine et al., 1988). The basin was probably located either on an active continental margin (Cabanis et al., 1986) or an early volcanic arc (Dissler et al., 1988), of wich reworked rocks are found in the Cesson metaconglomerate (Barrois, 1895; Guetmt and Peucat, 1990). Secondly, thrusting of the Lanvollon-Binic Formations over the Guingamp greywackes is associated with shortening of the basin. This period, dated at about 580 Ma, constitutes the major tectonothermal event. Partial melting then developed in tire footwall of the major thrust. Intrusion of gabbrodioritic stocks took place both during and after the main tectonothermal event. Finally, the exhumation of the Guingamp migmatiteS was at least partly accomodated by some extensional faulting (Le Golf et al., 1994) and occurred earlier than the latest gabbro-dioritic intrusions (the Saint-Brieuc Diorite at about 530-540 Ma) (Hébert et al., 1993; Hébert, 1995).
Dernière mise à jour le 28.07.2015