Discovery of an alkaline orthogneiss in the eclogite-bearing Cellier Unit (Champtoceaux Complex, Armorican Massif): a new witness of the Ordovician rifting

Michel Ballèvre; Ramon Capdevila; Catherine Guerrot; Jean-Jacques Peucat

doi:10.1016/S1631-0713(02)01760-1

Discovery of an alkaline orthogneiss in the eclogite-bearing Cellier Unit (Champtoceaux Complex, Armorican Massif): a new witness of the Ordovician rifting
[Découverte d'un orthogneiss alcalin dans l'unité du Cellier (complexe de Champtoceaux, Massif armoricain) : un nouveau témoin du rifting ordovicien]

Michel Ballèvre ¹ ; Ramon Capdevila ¹ ; Catherine Guerrot ² ; Jean-Jacques Peucat ¹

¹ Géosciences Rennes, UMR 6118, université Rennes-1, 35042 Rennes cedex, France
² BRGM, service « Analyse et caractérisation minérale », BP 6009, 45060 Orléans cedex 2, France

Comptes Rendus. Géoscience, Volume 334 (2002) no. 5, pp. 303-311.

Résumés

Anglais
Français

An alkaline orthogneiss from the Lower Allochton of the Champtoceaux Complex has been identified by its mineralogy and its whole-rock chemistry, including trace elements and Sr and Nd isotopes. An U/Pb study of the zircons using the conventional method gives an upper intercept age of 481 +6/−5 Ma. The alkaline orthogneiss is thus a new witness of the Ordovician rifting in the Variscan Belt.

Dans l'allochtone inférieur du complexe de Champtoceaux (Massif armoricain) a été découvert pour la première fois un orthogneiss dont les caractéristiques minéralogiques et la composition chimique (majeurs, traces et isotopes du Sr et du Nd) montrent qu'il dérive d'un magma alcalin. Une étude UPb sur zircons par la méthode conventionnelle a fourni un âge par intercept supérieur de 481 +6/−5 Ma, qui est interprété comme l'âge de cristallisation du protolithe. L'orthogneiss alcalin est donc un nouveau témoin du rifting ordovicien.

Métadonnées

Reçu le : 2001-09-17
Accepté le : 2002-01-07
Publié le : 2002-01-01

DOI : 10.1016/S1631-0713(02)01760-1

Keywords: alkaline orthogneiss, Ordovician, Variscan Belt, Champtoceaux Complex, Armorican massif, France, Geomaterials
Mots-clés : orthogneiss alcalin, Ordovicien, Chaı̂ne varisque, complexe de Champtoceaux, Massif armoricain, France, Géomatériaux

Affiliations des auteurs :

Michel Ballèvre ¹ ; Ramon Capdevila ¹ ; Catherine Guerrot ² ; Jean-Jacques Peucat ¹

¹ Géosciences Rennes, UMR 6118, université Rennes-1, 35042 Rennes cedex, France
² BRGM, service « Analyse et caractérisation minérale », BP 6009, 45060 Orléans cedex 2, France

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     author = {Michel Ball\`evre and Ramon Capdevila and Catherine Guerrot and Jean-Jacques Peucat},
     title = {Discovery of an alkaline orthogneiss in the eclogite-bearing {Cellier} {Unit} {(Champtoceaux} {Complex,} {Armorican} {Massif):} a new witness of the {Ordovician} rifting},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {303--311},
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     volume = {334},
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Michel Ballèvre; Ramon Capdevila; Catherine Guerrot; Jean-Jacques Peucat. Discovery of an alkaline orthogneiss in the eclogite-bearing Cellier Unit (Champtoceaux Complex, Armorican Massif): a new witness of the Ordovician rifting. Comptes Rendus. Géoscience, Volume 334 (2002) no. 5, pp. 303-311. doi : 10.1016/S1631-0713(02)01760-1. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/S1631-0713(02)01760-1/

Version originale du texte intégral

Version abrégée

1 Introduction

La Chaı̂ne hercynienne comprend de nombreux témoins d'un magmatisme cambro-ordovicien. Leur étude géochimique révèle l'existence de magmas calco-alcalins, indiquant l'existence d'une zone de subduction active à cette époque [5], ou/et de magmas alcalins, parfois même hyperalcalins, suggérant, au contraire, l'existence d'un épisode de distension intracontinentale [8,32]. Savoir si ces deux types de magmas ont coexisté ou se sont succédé dans le temps et/ou dans l'espace est une question essentielle, qui requiert une analyse détaillée des orthogneiss. Dans ce cadre, cette note signale l'existence d'un orthogneiss alcalin, d'âge Ordovicien inférieur, dans le complexe de Champtoceaux.

2 Contexte géologique

Dans le Massif armoricain, de nombreux orthogneiss cambro-ordoviciens ont été identifiés [17]. Tel est en particulier le cas dans le complexe de Champtoceaux [1,7,24], un empilement d'écailles ultérieurement plissé en une vaste antiforme d'axe est–ouest (Fig. 1). Le parautochtone présente des metagrauwackes d'âge inconnu. Tant l'allochtone inférieur que l'allochtone moyen comprennent de grandes quantités d'orthogneiss leucocrates à grain fin (leptynites). Certains orthogneiss dérivent de granites, comme le montrent non seulement de rares volumes non déformés [20], mais aussi la présence de reliques de cornéennes, témoins du métamorphisme de contact de ces granites [25]. La présence d'un litage décimétrique à décamétrique dans la plupart des orthogneiss suggère cependant une dérivation à partir de volcanites ou d'intrusions hypovolcaniques. Dans l'allochtone inférieur (Fig. 1), les leptynites contiennent de nombreuses lentilles éclogitiques, qui dérivent de filons ou de sills doléritiques [13].

Figure 1
Simplified geological map of the Champtoceaux Complex (modified after [3]). The studied orthogneiss has been sampled at le Cellier, along the right bank of the Loire River.
Carte du complexe de Champtoceaux (modifiée d'après [3]). L'orthogneiss étudié a été échantillonné au Cellier, sur la rive droite de la Loire.

Jusqu'à présent, deux orthogneiss ont été datés dans l'allochtone inférieur du complexe de Champtoceaux. Le premier est un métagranite coronitique (la Picherais), pour lequel un âge Silurien (U/Pb sur zircons, intercept bas à 423±10 Ma) a été proposé [38]. Le second est un orthogneiss leucocrate (orthogneiss de Saint-Mars-du-Désert), qui a été daté de l'Ordovicien inférieur (U/Pb sur zircons, intercept haut à 485±11 Ma) [28]. L'affinité magmatique de ces deux orthogneiss n'a cependant pas été établie, de sorte que leur signification reste conjecturale.

3 Pétrographie et minéralogie

L'orthogneiss étudié a été récolté au Cellier, sur la rive droite de la Loire (Fig. 1), au bord de la voie de chemin de fer. L'unité du Cellier, qui affleure là dans de bonnes conditions, est essentiellement constituée de leptynites, avec de nombreuses lentilles d'éclogites. Les leptynites sont représentées par des faciès fins, avec parfois des niveaux d'épaisseur métrique de gneiss œillés (avec des porphyroclastes de feldspath potassique ayant jusqu'à 5 cm de longueur), intensément déformés. La plupart des orthogneiss (par exemple, CEL 22) contiennent, outre quartz et feldspaths, épidote, muscovite et grenat. Dans un niveau (CEL 23) ont été observés quartz, albite et feldspath potassique, ainsi que des quantités mineures d'une biotite fortement pléochroı̈que (environ 3 %) et d'une amphibole bleu–vert (environ 2 %). Les analyses à la microsonde électronique (Tableau 1) montrent que la biotite, pauvre en TiO₂, est ferrifère (Fig. 2). Quant à l'amphibole, il s'agit également d'une espèce ferrifère, en l'occurrence une hastingsite dans la classification internationale (Tableau 1). Un grenat calcique (X_Grs=0,36) est seulement rarement observé. Sphène, zircon, apatite, allanite, magnétite et pyrite sont également présents.

	CEL 23	CEL 23A
	Pl	Pl	Kfs	Bt	Bt	Amp	Amp	Ttn	Apatite	Pl	Pl	Kfs	Kfs	Bt	Bt	Grt	Grt	Rt	Ttn
	clast	matrix	matrix	inclusion	matrix	matrix	matrix	matrix	matrix	clast	matrix	inclusion	matrix	matrix	matrix	core	rim	inclusion	matrix
	7	21	23	11	26	19	53	35	65	15	26	19	25	22	24	3	2	45	46
SiO₂	69.05	69.04	65.23	34.55	34.96	37.63	37.76	30.50	0.15	69.08	68.24	64.67	64.15	33.56	33.99	36.79	37.11	0.51	31.21
TiO₂	0.07	0.00	0.00	0.80	2.34	0.31	0.30	28.38	0.00	0.00	0.00	0.01	0.00	2.41	2.99	0.12	0.05	92.60	27.60
Al₂O₃	19.05	19.55	17.75	16.78	13.86	11.53	11.37	5.75	0.00	19.56	19.83	18.26	18.33	13.92	14.62	20.76	19.64	0.56	7.38
Cr₂O₃	0.08	0.00	0.03	0.00	0.00	0.00	0.03	0.00	0.00	0.01	0.02	0.00	0.00	0.00	0.09	0.01	0.00	0.00	0.00
Fe₂O₃^∗								2.69											1.30
FeO^∗	0.16	0.00	0.08	33.00	34.96	33.03	31.46		0.05	0.02	0.06	0.08	0.02	33.57	33.05	21.99	24.11	1.38
MnO	0.00	0.12	0.05	0.48	0.43	0.85	0.74	0.16	0.02	0.00	0.06	0.07	0.02	0.10	0.37	6.43	3.28	0.00	0.08
MgO	0.00	0.00	0.00	0.56	0.66	0.33	1.79	0.00	0.00	0.00	0.00	0.00	0.00	1.47	0.78	0.01	0.02	0.01	0.00
CaO	0.04	0.20	0.00	0.00	0.00	9.89	10.95	27.88	57.06	0.07	0.39	0.01	0.00	0.01	0.02	13.02	14.77	0.31	27.70
Na₂O	12.14	11.81	0.21	0.05	0.05	1.81	1.72	0.02	0.07	11.81	11.71	0.31	0.32	0.06	0.04	0.16	0.03	0.03	0.18
K₂O	0.09	0.12	17.54	9.61	9.57	2.66	2.42	0.00	0.01	0.23	0.11	17.25	17.41	9.61	9.25	0.00	0.02	0.04	0.00
P₂O₅									43.24
	100.68	100.84	100.89	95.83	96.83	98.04	98.54	95.38	100.60	100.78	100.42	100.66	100.25	94.71	95.20	99.29	99.03	95.44	95.45
O	8	8	8	22	22	23	23		12	8	8	8	8	22	22	24	24	2
Si	3.003	2.994	3.010	5.631	5.710	6.157	6.092	1.009	0.012	2.996	2.975	2.989	2.981	5.597	5.601	5.917	5.987	0.007	1.021
Ti	0.002	0.000	0.000	0.098	0.288	0.038	0.036	0.706	0.000	0.000	0.000	0.000	0.000	0.302	0.371	0.015	0.006	0.975	0.679
Al	0.976	0.999	0.965	3.221	2.666	2.224	2.162	0.224	0.000	1.000	1.019	0.995	1.004	2.734	2.837	3.937	3.735	0.009	0.284
Cr	0.003	0.000	0.001	0.000	0.000	0.000	0.004	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.012	0.001	0.000	0.000	0.000
Fe $^{3 +}$						0.438	0.514	0.067											0.032
Fe $^{2 +}$	0.006	0.000	0.003	4.498	4.776	4.120	3.775		0.003	0.001	0.002	0.003	0.001	4.682	4.554	2.958	3.253	0.016
Mn	0.000	0.004	0.002	0.066	0.059	0.118	0.101	0.004	0.001	0.000	0.002	0.003	0.001	0.014	0.052	0.876	0.448	0.000	0.002
Mg	0.000	0.000	0.000	0.136	0.161	0.081	0.435	0.000	0.000	0.000	0.000	0.000	0.000	0.365	0.192	0.002	0.005	0.000	0.000
Ca	0.002	0.009	0.000	0.000	0.000	1.734	1.893	0.988	4.792	0.003	0.018	0.000	0.000	0.002	0.004	2.244	2.553	0.005	0.971
Na	1.023	0.993	0.019	0.016	0.016	0.574	0.538	0.001	0.011	0.993	0.990	0.028	0.029	0.019	0.013	0.050	0.009	0.001	0.011
K	0.005	0.007	1.032	1.998	1.994	0.555	0.498	0.000	0.001	0.013	0.006	1.017	1.032	2.045	1.944	0.000	0.004	0.001	0.000
P									2.870
	5.020	5.006	5.032	15.664	15.670	16.039	16.048	2.999	7.690	5.006	5.013	5.035	5.048	15.760	15.580	16.000	16.000	1.014	3.000
															Alm	46.46	49.61
															Sps	15.02	7.51
															Prp	0.04	0.08
															Grs	32.39	35.49
															And	6.05	7.30

Figure 2
Mineral chemistry of biotite from the Cellier orthogneiss.
Composition chimique de la biotite de l'orthogneiss du Cellier.

4 Géochimie

L'analyse chimique de l'échantillon CEL 23 (Tableau 2) montre que la roche étudiée est une roche magmatique n'ayant pas été significativement altérée, sauf peut-être pour le Cs. La roche est riche en Si, relativement pauvre en Al et Fe, et pauvre à très pauvre en Ti, Ca, Mg et P. L'échantillon est relativement riche en alcalins, avec un rapport K₂O/Na₂O voisin de 1,1. Ces caractéristiques sont celles d'un granite à feldspaths alcalins, riche en potassium, faiblement saturé en alumine (A/CNK ca 1) et ferrifère (Mg/(Fe + Mg) < 0,1). Dans le détail, les éléments en traces montrent que le protolithe de l'échantillon CEL 23 était un granite (ou une rhyolite) de type A1 [10,11].

SiO₂	TiO₂	Al₂O₃	Fe₂O₃	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅	LOI	Total
75.66	0.13	12.23	1.68	<0.03	0.09	0.36	4.07	4.44	0.03	1.04	99.73
Cs	Rb	Ba	W	Th	U	Ta	Nb	Pb	Sr	Zr	Hf
0.911	109.7	181	0.75	17.85	4.29	3.88	48.89	9.8	34.3	304	9.06
Sn	Be	Y	Ga	V	Co	Zn	Cu	Cr	Ni	La	Ce
2.81	2.3	41.3	24.8	2.47	0.56	38	5.1	13.3	2.34	64.06	130.3
Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
14.58	52.74	10.42	0.447	8.53	1.27	7.97	1.56	4.27	0.67	4.55	0.61

Le rapport ε_Nd à 480 Ma est d'environ +1 (Tableau 3), en accord avec les valeurs mesurées pour des roches similaires en Galice, dans le Nord-Ouest de l'Espagne [33]. Le magma alcalin pourrait être issu d'un manteau appauvri, avec éventuellement une faible composante crustale à partir d'une source à 2 Ga, telle qu'elle est connue dans le Nord du Massif armoricain [14]. Le rapport initial du Sr à 480 Ma (0,702) est compatible avec le composant mantellique défini ci-dessus, mais peut également résulter d'une perturbation du système Rb/Sr durant le métamorphisme.

Sample	Sm	Nd	¹⁴⁷Nd/¹⁴⁴Nd	¹⁴³Nd/¹⁴⁴Nd	error × $10^{- 6}$	$ϵ_{Nd}$	$ϵ_{Nd}$	$𝐓_{DM}$
	(ppm)	(ppm)			$(\pm 2 σ m)$	(0 Ma)	(480 Ma)	(Ga)
CEL 23	10.27	52.93	0.1173	0.512 457	4	−3.6	1.3	1.10
CEL 23	10.13	51.92	0.1180	0.512 437	3	−4.0	0.9	1.13
Standard AMES	–	–	–	0.511 972	3
	Rb	Sr	⁸⁷Sr/⁸⁶Sr	⁸⁷Sr/⁸⁶Sr	error $\times 10^{- 6}$	$𝐈_{Sr}$
	(ppm)	(ppm)			$(\pm 2 σ m)$	(480 Ma)
CEL 23	110	34.3	9.33	0.766 221	6	0.7024
Standard NBS 987	–	–	–	0.710 252	7
Duplicate NBS 987	–	–	–	0.710 256	6

5 Géochronologie

Une étude UPb sur zircons de l'échantillon CEL 23 a été effectuée par la méthode conventionnelle (Tableau 4). Les zircons, nombreux, appartiennent aux types S22, P3 ou P5 dans la classification de Pupin [34], types qui sont généralement observés dans les magmas calco-alcalins ou alcalins de haute température. Les zircons ont été séparés et abrasés [18,19,26], leur analyse étant réalisée sur un spectromètre Finnigan Mat 261. Les six fractions s'alignent sur une discordia, dont l'intercept supérieur, à 481±6 Ma (Fig. 4), est interprété comme l'âge de cristallisation du protolithe magmatique de l'orthogneiss alcalin.

Fraction	Number of		Weight (μg)	Concentrations	Atomic ratios	Age
	grains			U (ppm)	radio. Pb (ppm)	com. Pb (pg)	²⁰⁶Pb/²⁰⁴Pb	²⁰⁸Pb/²⁰⁶Pb	²⁰⁶Pb/²³⁸U	²⁰⁷Pb/²³⁵U	²⁰⁷Pb/²⁰⁶Pb	207/206
		[1]				[2]	[3]	[4]	[4]	[4]	[4]	(Ma)
1	3g	m5, op	67	1468	79.9	189	1792	0.0903	$0.055 19 \pm 84$	0.4307±64	0.056 60 ± 22	476
2	3g	m1, tr	26	597	35	48	1160	0.1235	$0.057 77 \pm 20$	0.4508±18	0.056 60 ± 10	476
3	3g	m1, tr	19	926	59.8	46	1534	0.115	$0.064 05 \pm 72$	0.5008±56	0.056 71 ± 16	480
4	4g	m1, tr	25	606	41.8	41	1528	0.1438	$0.066 73 \pm 36$	0.5219±30	0.056 72 ± 10	481
5	3g	m5, op	22	2859	152	104	2017	0.0998	$0.053 46 \pm 40$	0.4178±32	0.056 68 ± 8	479
6	3g	m5, op	26	1676	87.7	360	423	0.0655	$0.054 25 \pm 28$	0.4236±26	0.056 63 ± 16	477

Figure 4
Concordia plot for the UPb analyses of zircons from the Cellier orthogneiss (sample CEL 23). Numbers refer to the fractions shown in Table 4.
Diagramme concordia UPb pour les zircons de l'orthogneiss du Cellier (échantillon CEL 23). Les chiffres correspondent aux fractions identifiées dans le Tableau 4.

6 Conclusion

Les gneiss à biotite-hastingsite de l'unité du Cellier (allochtone inférieur du complexe de Champtoceaux) dérivent d'intrusions superficielles (ou éventuellement de volcanites) alcalines, ayant cristallisé aux environs de 480 Ma (Ordovicien inférieur). Il s'agit donc d'un nouveau témoin du magmatisme alcalin, antérieurement identifié dans le Massif armoricain en Choletais [22,36] et dans la région de Concarneau [15,37]. Ce magmatisme est le témoin du rifting ordovicien qui affecte largement tout le domaine occupé ultérieurement par la Chaı̂ne hercynienne.

1 Introduction

Identifying the nature and age of the pre-orogenic magmatism in the Variscan Belt is a prime tool for unraveling its plate tectonic development before the final collision during the Carboniferous. The Cambro-Ordovician age of a large part of the pre-orogenic magmatism has been recognised during the past years. The geochemistry of the Cambro-Ordovician felsic magmatism presents either calc-alkaline affinity, indicative of subduction processes (e.g., [5]), or alkaline (and even peralkaline) affinities, suggesting continental rifting (e.g., [8,32]). Whether the two types of magmas occur synchronously in different parts of the Variscan Belt or diachronously in the same unit requires detailed knowledge of their age.

In the Armorican Massif, pre-orogenic orthogneisses are found in a large area, including the Leon in its northwestern part, a few bodies in its northern and central parts, as well as numerous bodies in the South-Armorican Domain. The petrology of most of these bodies is poorly known. This paper represents a contribution towards a better understanding of the felsic orthogneisses from the Champtoceaux Complex. Previous work in this Complex led to the recognition of (i) a poorly defined Silurian age (423±10 Ma) for a coronitic metagranite [38], and (ii) Early Ordovician (485±11 Ma) protoliths for the Saint-Mars-du-Désert orthogneiss [28]. The magmatic affinity of both bodies has not been investigated.

2 Geological setting

The Champtoceaux Complex [1,7,24] consists of several stacked units, later refolded by a large-scale, east–west trending antiform (Fig. 1). The Parautochton displays monotonous metagreywackes of unknown age. Both the Lower Allochton and the Middle Allochton contain large amounts of fine-grained, leucocratic, orthogneisses. In the Lower Allochton, the Cellier Unit is characterised by orthogneisses displaying numerous, well-preserved, eclogite lenses.

Evidence for the nature of the orthogneisses of the Cellier Unit before the ductile deformation is rarely preserved. Metre-scale undeformed pods with a well-preserved granitic texture occur at la Picherais (Fig. 1) within strongly sheared gneisses [20]. Most outcrops or quarries show strongly deformed gneisses that present a faint to well-defined layering at decimetre to decametre scale. In particular, layers of augen-gneisses (with up to 5 cm long K-feldspar porphyroclasts) up to a few metres in thickness are present. Dark microgranular enclaves have been recognised in the undeformed pod at la Picherais, but are not found in the deformed gneisses. Other evidence in favour of the intrusive character of part of the protoliths of the gneisses is the discovery of relics of hornfelses, at Casson and Campbon (Fig. 1) [25]. Nevertheless, the lack of homogeneity and the obvious layering argues in favour of volcanic or high-level intrusions rather than huge plutons.

The eclogite lenses from the Cellier Unit most probably derive from doleritic sills or dykes, as recorded by peculiar texture in undeformed samples [13]. This is consistent with their restricted bulk-rock chemistry, basaltic whole-rock compositions similar to N-MORBs to P-MORBs [27]. During high-pressure metamorphism, the metadolerites have been converted into a fine-grained assemblage of garnet–omphacite–amphibole–rutile [13].

The age relationships between the eclogites and the surrounding gneisses cannot be determined accurately on the field. Relics of high-pressure parageneses are rarely found in the orthogneisses, where they consist of high-Si phengite and grossular-rich garnet [3,21]. In addition, the garnet corona around biotite in the undeformed pod at la Picherais, once thought to develop during a granulite-facies event [20], is now regarded as a witness of an incomplete reequilibration during the eclogite-facies event.

3 Petrography and mineralogy

The leucocratic gneisses from the Cellier Unit can be divided into two broad categories. Most gneisses are « calc-alkaline» gneisses, containing epidote, garnet, biotite and muscovite as key phases (CEL 22). Rare examples of « alkaline» gneisses contain a strongly pleochroic biotite and a blue-green amphibole. The studied samples (CEL 23 and CEL 23A) have been collected along the right bank of the Loire River, close to the village of le Cellier, where the railway section provides a nice exposure of the Champtoceaux Complex. The studied samples are fine-grained leucocratic gneisses, displaying a well-developed foliation. Microscopic examination reveals that quartz and feldspar are modally dominant phases (33% Qtz, 37% Pl and 24.5% Kfs) and that foliation is defined by minute grains of biotite (3%) and amphibole (2%).

Two types of plagioclase grains are found, with larger, weakly strained, grains displaying albite twins that are surrounded by a fine- to very-fine grained matrix consisting of minute plagioclase, microcline and quartz grains. Ferromagnesian minerals include abundant biotite lamellae with a very strong pleochroism (from pale yellow–brown to dark brown), prismatic amphibole crystals with a strong pleochroism (from pale yellow–green to blue–green to green), and very rare garnet (<0.5%) and clinozoisite (ca 0.1%). Minor variations in modal proportions show that biotite and amphibole coexist in some layers, whereas biotite ± garnet is stable in other layers. Titanite (ca 0.3%), zircon, apatite, allanite, magnetite and pyrite are also present.

Microprobe analyses have been performed with the Microsonde Ouest (Brest) using the following analytical conditions: 15 kV acceleration voltage, 20 nA sample current, 10 s counting time. Representative mineral compositions are given in Table 1. Plagioclase grains are close to albite end-member, whether they are large blasts (with no core to rim variation) or fine-grained matrix. The K-feldspar composition is also close to end-member. Biotite grains are Al-poor and Fe-rich, thus plotting close to the annite end-member (Fig. 2). In addition, their TiO₂ content is relatively low. Amphibole grains are calcic varieties, which plot in the IMA classification in the hastingsite field, close to the hastingsitic hornblende field. Garnet is an almandine–grossular–spessartine solid solution, with minor core-to-rim zonation (decreasing spessartine and increasing almandine and grossular). The high grossular content (up to 36 mol per cent) is a characteristic feature of the garnets from the orthogneisses of the Cellier Unit [21]. Rutile is rarely observed (sample CEL 23A), and is rimmed by titanite, the latter being also present in the matrix. Titanite shows a relatively high Al + Fe³⁺ content (up to 0.34 pfu), with Al ⪢ Fe³⁺.

4 Geochemistry

The chemical composition of sample CEL 23 (Table 2) is that of a magmatic rock that has not been altered significantly, except perhaps for Cs. The rock is rich in Si, relatively poor in Al and Fe, and poor to very poor in Ti, Ca, Mg and P. The sample is relatively rich in alkali, with a K₂O/Na₂O ratio of 1.1. The above characteristics are those of a high-K alkali feldspar granite, slightly alumina-saturated (A/CNK ca 1) and ferriferous (Mg/(Mg + Fe) < 0.1). Sample CEL 23 is thus a moderately fractionated alkaline granite. The low concentrations in Cs and Rb, the negative anomalies in Ba, Sr, Eu, P and Ti as well as the concave shape of the HREE (Fig. 3) indicate fractionation of feldspars, amphibole, apatite, titaniferous magnetite and probably biotite. The most remarkable geochemical characteristic of sample CEL 23 is the lack of significant anomalies for Ta, Nb and Pb (Th/Ta = 4.6, La/Nb = 1.31 and Ce/Pb = 13.3), which testify of the alkaline nature of the sample. The low value of the Ti/Nb ratio is consistent with this observation [2]. The Nb/Y, Ta/Yb, Rb/(Y + Nb), Rb/(Yb + Ta), Ga/Al values and the relations between Rb, Hf and Ta are typical of acid intraplate magmas [16,29,39]. More specifically, the low values of the Yb/Ta, Y/Nb and Ce/Nb ratios, as well as the Nb-rich composition in the ternary diagrams Nb/Y/(Ce or Ga), indicate that the protolith of sample CEL 23 was an A1-type granite (or rhyolite), i.e., was derived from a mantle source similar to those of the oceanic-island basalts (OIB) [10,11]. The lack of major crustal contamination (because of the minor anomalies in Ta, Nb and Pb) implies a two-stage evolution, first fractionation of an underplated magma in a deep-seated chamber followed by high ascent velocity through dyking, preventing major crustal contamination.

Figure 3
Diagram normalised to the primitive mantle of sample CEL 23.
Diagramme normalisé au manteau primitif de l'échantillon CEL 23.

The ε_Nd ratios at 480 Ma are ca +1 (Table 3), suggesting that the alkaline magma is derived from a depleted mantle (DM). Nevertheless, classical DM is more depleted at 480 Ma with ε_Nd ratios close to +9. This suggests that the T_DM model ages of 1.1 Ga measured are not significant, but indicate a mixing between a magma derived from a normal DM at 480 Ma, with an old crustal component (stricto sensu older than 1.1 Ga), probably ca 2 Ga old, as known in northern Brittany [14]. The ε_Nd values are in agreement with those reported for similar rocks in Galicia, NW Spain [33]. The initial Sr ratio at 480 Ma (0.702) is compatible with the mantle component previously defined, but could also result from an alteration of the RbSr system during metamorphic processes.

5 Geochronology

Numerous zircons have been extracted from sample CL 23. They are generally subhedral; when euhedral, they are of the S22, P3 or P5 types [34], which are generally found in high temperature alkaline and calc-alkaline magmas. Analytical procedures are from Krogh [18] and Parrish [26], populations have been abraded following Krogh [19]. Analyses have been performed on a Finningan Mat 261 mass spectrometer using an electron multiplier system in dynamic mode. Pb and U blanks are lower than 15 and 1 pg, respectively. Final ratios are corrected from fractionation, blank, and initial common lead, using the two-stage model of Stacey and Kramers [35]. Errors are at two sigma (95% confidence level). The high U content (Table 4) explains probably the relative high degree of discordancy observed (14 to 30%) (Fig. 4). All the fractions have a restricted range of ²⁰⁷Pb/²⁰⁶Pb ages with a weighted average of 479±2 Ma (using [23]). The discordia defines an upper intercept with an age of 481 +6/−5 Ma after the Davis regression programme [9]. The age of 481±6 Ma is interpreted as the magmatic stage of this alkaline rock.

6 Discussion and conclusion

The above data allow two main conclusions.

1. The biotite–hastingsite gneisses from the Cellier Unit (Champtoceaux Complex) derive from high-level intrusions (or volcanics) of typical alkaline affinity. The magmas crystallised during the Early Ordovician (at about 480 Ma). This Early Palaeozoic magmatism is largely represented in the Armorican Massif [17], either in units that have been slightly deformed during the Variscan Orogeny, like in the Choletais [22,36] or as intensely strained rocks, like in the Champtoceaux Complex [28, and this study], the ‘Golfe du Morbihan’ [30], or the Concarneau area [4,15,37]. Alkaline protoliths are now identified in the Concarneau area [37], the Choletais area [36] and the Champtoceaux Complex (this study).

2. As usual, orthogneisses are poor recorders of the metamorphic evolution. Mineral parageneses within the biotite–hastingsite gneisses indicate nearly complete reequilibration at epidote–amphibolite facies conditions. Potential relics of the eclogite-facies event in the alkaline orthogneisses are coexisting grossular-rich garnet and rutile (cf. [6]).

3. Alkaline orthogneisses are known in several eclogite-bearing units from the Variscan Belt (e.g. [12,33, this study]). Accordingly, continental subduction in the Variscan Belt mainly took place at the expense of a continental crust, which was significantly thinned through intracontinental rifting of Cambro-Ordovician age. This represents a major contrast with the Alpine Belt, where continental subduction involves burial of the passive palaeomargins, where syn-rifiting magmatism is either poorly developed or lacking.

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