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The cadomian granites of Mancellia, northeast Armorican massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting
Precambrian Research, 51 ( 1991 ) 3 9 3 - 4 2 7 Elsevier Science Publishers B.V., A m s t e r d a m
393
The Cadomian granites of Mancellia, northeast Armorican Massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting Michael Brown a,l and Richard S. D ' L e m o s
b
a School o[Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK b Department qfGeology, Oxford Polytechnic, Gipsy Lane, Headington, Oxford, OX30BP, UK ( Received October, 23 1989; revised and accepted May 28, 1990 )
ABSTRACT Brown, M. and D'Lemos, R.S., 1991. The Cadomian granites of Mancellia, northeast Armorican Massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting. In: I. Haapala and K.C. Condie (Editors), Precambrian Granitoids--Petrogenesis, Geochemistry and Metallogeny. Precambrian Res., 51: 393-427. The North Armorican Shear Zone (NASZ) divides the Cadomian belt of the Armorican Massif of France into two major terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The NACT is a pastiche of displaced blocks and terranes (St. Brieuc Terrane, St. Malo Terrane and Mancellian Terrane) which results from the amalgamation of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone. In the NACT, the Neoproterozoic Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. The St. Malo migmatite belt is a syn-tectonic suite of anatectic migmatites derived through partial melting of the Brioverian succession. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that a sample of St. Malo diatexites comes from a population with a different mean than some samples of the Brioverian succession sandstones. Resulting homogeneous diatexites/anatectic granites of the core were emplaced syn-kinematically into Cadomian strike-slip shear zones. Thus the age of anatexis of ~ 540 Ma is also the date of strike-slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the NACT. The Mancellian granites, to the southeast of the St. Malo migmatites, were emplaced at ~ 540 Ma and are overlain unconformably by Cambrian sediments. Overall geochemistry and a variety of specific geochemical parameters show that the Mancellian granites are similar to the diatexites and anatectic granites of the St. Malo migmatite belt. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that the sample of Mancellian granites comes from a population with a different mean than the sample of St. Malo diatexites. Additionally, both suites of rocks are peraluminous, they have similar distributions of Rb, Sr and Ba, they have K / R b ratios around 225, and they plot largely within the VAG field using trace element discrimination diagrams for the interpretation of tectonic setting. Samples of the Brioverian succession sandstones, the St. Malo migmatites and the Mancellian granites all exhibit similar primordial mantle-normalized element patterns. Preliminary results of Nd and Sr isotope studies reveal a similar restricted range of values for eNd (between - 4 . 0 and - 7 . 3 ) and eSr (between - 3 and + 4 2 ) , and depleted model ages in the range 1.7 to 1.5 Ga (Tdmur) for both Mancellian granites and St. Malo migmatites, isotopic features which set these rock types apart from other Cadomian granites within the NACT. The St. Malo migmatite belt and the Mancellian granites are coeval at ~ 540 Ma and both developed by anatexis, at intermediate crustal depths, of the Brioverian succession, within an inverted, but only moderately overthickened, behindarc basin. In comparison with the St. Malo migmatites, the Mancellian granites represent farther-travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source.
t Present address: Department of Geology, University of Maryland at College Park, College Park, Maryland 20742, USA.
0301-9268/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
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Introduction
The pre-Mesozoic Armorican Massif of northwest France is subdivided into three major tectonic units by the North Arrnorican Shear Zone (NASZ) and the South Armorican Shear Zone (SASZ) (Gapais and Le Corre, 1980). North of the NASZ the major orogenic activity is Cadomian, especially in the east (Bal6 and Brun, 1989; Strachan et al., 1989; Brun and Bal6, 1990), although some Palaeozoic tectonothermal rejuvenation has occurred (D'Lemos et al., 1990). A Precambrian tectonostratigraphy for the Armorican Massif north of the NASZ was originally established around the Baie de St. Brieuc by Cogn6 (1959) and extended subsequently to the whole Massif (Cogn6, 1972). However, this tectonostratigraphy has proved ambiguous in respect of the unconformable relationship between supposed pre-Cadomian basement and the Neoproterozoic Brioverian succession cover on the east side of the Baie de St. Brieuc (Guerrot and Peucat, 1990; Shuffiebotham, 1990). Unequivocal pre-Cadomian ~2,000 Ma Icartian Gneiss basement is exposed only in the Channel Island of Guernsey, at La Hague and as rafts in the Cadomian Perros Guirec Complex of the Tr6gor (Roach et al., 1972; Leutwein et al., 1973; Calvez and Vidal, 1978; Auvray et al., 1980; Vidal et al., 1981 ). Basaltic rocks form a significant component of the Neoproterozoic Brioverian succession, especially in its lower part, and an understanding of the geochemistry of these rocks has contributed to the interpretation of the overall geotectonic framework of the Cadomian orogen as a series of volcanic arcs and intra-arc basins (Cabanis et al., 1987; Lees et al., 1987; Dupret et al., 1990). Intermediate and acid volcanic rocks occur within the upper parts of the Brioverian succession (Roach et al., 1986; Dupret et al., 1990). The deposition of the
M BROWN AN[) R.5. l)'1 EMt)S
Brioverian succession, which is predominantly an epiclastic sedimentary sequence, was followed closely by Cadomian deformation and metamorphism which culminated in the development of anatectic migmatites around St. Malo, and the emplacement of the late Cadomian post-tectonic plutonic complexes (Brown et al., 1990). The region has been the site of extensive Palaeozoic sedimentation and the Brioverian succession is overlain unconfbrmably by early Palaeozoic sediments which, at Ca~teret in Normandy, include limestones with late Lower Cambrian faunas (Dot6, 1972). Additionally, the late Cadomian post-tectonic Mancellian granites are overlain unconformably by the Red Conglomerate and arkoses of probable Cambrian age (e.g. Vire-Carolles Granite, Chauris, 1956; and Athis Granite, Graindor, 1953 ). Thc Precambrian-Cambrian boundary within the Armorican Massif is placed at ~ 540 ma (Pasteels and Dor6, 1982; see also Menning, 1989 ). Thus both the Mancellian granites and the St. Malo migmatite belt are regarded as Precambrian. Variscan reactivation in the north-east ARmorican Massif is (apparently?) limited, although granitoids related to the Variscan orogeny become an increasingly important feature of the geology further to the west and 4°Ar/ :~'%r muscovite ages from the St. Malo l'erranc indicate some post-Cadomian tectonothermal activity (D'Lemos et al., 1990). This paper is concerned with the petrogenesis of the late Cadomian Mancellian granites of the northeast Armorican Massif. The origin of these granite complexes has been related to crustal thickening during subduction - collision orogenesis by some workers (Graviou and Auvray, 1985, 1990; Graviou et al.. 1988). This is, however, difficult to reconcile with recent work around the Baie de St. Brieuc and the Baie de St. Malo (Strachan and Roach, 1990; Treloar and Strachan, 1990) which has
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
led to the development of a dominantly strikeslip related model for the Cadomian orogeny (Strachan et al., 1989). We test the hypothesis that the Mancellian granites and the anatectic rocks of the St. Malo migmatite belt have a c o m m o n origin related to basin inversion during sinistrally transpressive terrane accretion at ~ 540 Ma. Tectonic model for the Cadomian Belt of the northeast Armorican Massif
The NASZ separates the Cadomian Belt into two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT) (Strachan et al., 1989); see Fig. 1. In the NACT, the Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. Early Cadomian foliated plutonic
395
complexes locally form the basement to the Brioverian succession (Strachan et al., 1989; Shuffiebotham, 1990; Guerrot and Peucat, 1990). The Cadomian cycle occurred during some 275 Ma between ~ 700 Ma and ~ 425 Ma (Brown et al., 1990). However, in the CAT, although an angular unconformity separates the Palaeozoic succession from the Brioverian succession, the Brioverian sequence was not subjected to major orogenic activity until the Variscan orogeny (Gapais and Le Corre, 1980). Therefore, the NACT and the CAT are considered by Strachan et al. (1989) to have the status of separate Cadomian terranes. The NACT itself is a pastiche of variably displaced blocks and terranes which results from the juxtaposition of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a subduction zone. The terranes which comprise the NACT (see Fig. 1 ) are the St. Brieuc
Fig. 1. The principal terranes of the northeastern part of the Armorican Massif. The North Armorican Composite Terrane is composed of the St. Brieuc Terrane, the St. Malo Terrane and the Mancellian Terrane which are amalgamated along the Fresnaye Shear Zone (FSZ) and the Cancale Shear Zone (CSZ), respectively. The North Armorican Composite Terrane is separated from the Central Armorican Terrane by the North Armorican Shear Zone (NASZ). Light lines outline major geological features and heavy lines represent Cadomian shear zones or faults inferred to be located along Cadomian shear zones.
396
Terrane (SBT), the St. Malo Terrane (SMT) and the Mancellian Terrane ( M T ) (Strachan et al., 1989). The SBT is the most lithologically variable of the three terranes within the NACT. It includes various calc-alkaline gabbro-dioritetonalite-granite complexes emplaced into both Icartian basement gneisses (e.g. on Guernsey and at La Hague) and Brioverian volcanic and sedimentary sequences (e.g. on Jersey and around the Baie de St. Brieuc). These calc-alkaline plutonic complexes are reviewed by Brown et al. (1990). Cadomian metamorphism within the SBT varies from prehnitepumpellyite facies to amphibolite facies westwards around the south side of the Baie de St. Brieuc, a facies series similar to those described from Mesozoic arc environments (Miyashiro, 1973 ), and Cadomian deformation is both variable and heterogeneous (Strachan and Roach, 1990). The timing of deformation and metamorphism within the SBT predated ~ 570-560 Ma (Strachan et al., 1990). The SMT includes a number of migmatite belts and narrow strips of Brioverian metasediments (Brown, 1979; Strachan et al., 1989 ). The St. Malo migmatite belt is described further below. The SMT has been reworked heterogeneously by steep ductile shear belts (Brown, 1974, 1978; Treioar and Strachan, 1990) which were active synchronously with anatexis at ~ 5 4 0 Ma (Strachan et al., 1989: Treloar and Strachan, 1990 ). Shear criteria indicate a consistent sub-horizontal, sinistral strike-slip sense of m o v e m e n t (Gapais and Bal6, 1990; Treloar and Strachan, 1990). To the southeast of the SMT, the MT ("Domaine Mancellien" of Cogn6, 1964) is composed of mainly weakly deformed low-grade Brioverian metasediments and metavolcanics (Graindor, 1957; Dupret, 1974), into which have been emplaced the plutonic complexes of the Mancellian Batholith (Jonin, 1981; Graviou et al., 1988; Brown et al., 1990). The Mancellian granites are described further below.
M. BROWN A N D R . S D ' L E M O S
A number of recent models for the Cadomian Belt (e.g. Bal6 and Brun, 1983, 1989: Cabanis et al., 1987; Graviou et al., 1988; Dupret et al., 1990; Brun and Bal& 1990) attribute Cadomian events to tectonothermal activity in the hanging wall of a southerly dipping subduction zone, although Brun and Bal6 ( 1990 ) suggest that subduction may have been to the north. In these models, the rocks assigned by us to the SBT are interpreted as a "volcanic arc" and "back-arc basin" complex which was thrust southwest onto the facing continent. represented by our SMT and MT, at ~ 590-580 Ma. The resultant crustal thickening inferred is thought to have generated the post-tectonic formation of the migmatite belts of the SMT and the Mancellian Batholith of the MT. Dupret et al. (1990) suggest a subduction-related origin for the Mancellian Batholith, but invoke a second parallel subduction zone which developed along the line of the Cancale Shear Zone of Strachan et al. (1989) during closure and inversion of a marginal basin. In contrast. Strachan and Roach (1990) find no field evidence around the Baie de St. Brieuc which requires southwest verging thrusting, while Brown ( 1974, 1978) and Treloar and Strachan (1990) have shown that the migmatite belts are pre- to syn-tectonic and not post-tectonic with respect to the dominant shear zone deformation in the SMT. These workers (Strachan et al., 1989; Treloar and Strachan, 1990 ) have argued that the amalgamation of the SBT, SMT and MT occurred at ~ 540 Ma during displacement across a crustal-scale sinistral strike-slip belt defined by Cadomian shear zones (see Fig. 1 ). The St. Malo migmatite belt The SMT is made up of several belts of migmatite and anatectic granite, intercalated with belts of variably deformed medium-grade metasediment assigned to the Brioverian succession, and cut by numerous syn-anatectic shear zones, including the Fresnaye Shear Zone and
397
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
the Cancale Shear Zone which form the northwest and southeast boundaries of the terrane (see Fig. 1; Strachan et al., 1989; Treloar and Strachan, 1990). The main belt of migmatite and anatectic granite is the one which trends ENE through St. Malo (Fig. 2 ), and which has been the focus of much work during the past twenty years (e.g. Brown et al., 1971; Brown, 1973, 1974, 1978, 1979; Brun, 1975, 1977; Martin, 1977, 1979, 1980; Brun and Martin, 1978; Weber et al., 1985; Peucat and Martin, 1985; Peucat, 1986; Treloar and Strachan, 1990). To the west of the River Rance (see Fig. 2 ), the St. Malo migmatite belt comprises me-
tatexites--a stromatic migmatite produced by low to moderate degrees of partial melting (Brown, 1973), with the generation of up to ~ 30% melt, which is below the rheologically critical melt percentage (RCMP), according to Arzi (1978), Van der Molen and Paterson (1979) and Wickham (1987)--and diatexites--a schlieric or nebulitic migmatite produced by moderate to high degrees of partial melting (Brown, 1973 ), with the generation of greater than ~ 30% melt, which exceeds the RCMP and results in the distruption of the stromatic structure characteristic of a metatexite. These migmatites are clearly derived by in-
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Fig. 2. Generalized geological map of the northeastern part of the Armorican Massif, south of Upper Normandy. Heavy lines represent Cadomian strike-slip shear zones or younger brittle faults thought to lie along Cadomian strike-slip shear zones. I = Brioverian succession volcanics and sediments; 2 = belts of high-grade gneisses and migmatites thought to have been derived from the Brioverian succession; 3 = Late Cadomian post-tectonic plutonic complexes; 4 = Lower Palaeozoic successions, generally in synclines which often lie along the line of Cadomian strike-slip shear zones; 5 = Variscan plutonic complexes, generally dominated by granodiorite/granite; 6 = Mesozoic platform sequences of the Paris Basin.
398
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THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
Fig. 5. L-S teclonic fabric in slightly schlieric inhomogeneous diatexite, pnte de Grouin, north of Cancale, St. Malo migmadte belt. Plane of photograph approximately perpendicular to both schistosity and lineation.
from metasediments, metatexites, diatexites and anatectic granites. To the east of the River Rance (Figs. 2 and 4), the St. Malo migmatite belt comprises inhomogeneous diatexites, with characteristic schlieric structure, homogeneous diatexites, with nebulitic structure, and anatectic granites which represent diapiric rise of convectively homogenized and ponded anatectic melt, apparently synkinematically with sinistral shearing (Brown, 1974, 1978; Strachan et al., 1989; Treloar and Strachan, 1990). The eastern half of the St. Malo migmatite belt is illustrated in Fig. 4, which shows the homogeneous diatexite core to the belt. Linear structures within the migmatite belt plunge at a shallow angle between northeast and east (Brown, 1974, 1978 ),
399
so that, westward, the successive inhomogeneous diatexite and metatexite represent deeper structural levels. To the north, the metatexite structurally overlies the diatexite with a variably dipping shallow to moderate contact. To the south along the east bank of the River Rance, the diatexite is seen to structurally overly the metatexite along a steep contact. Thus, the diapiric diatexite core to the belt is overturned to the south or southeast, consistent with displacement across a top-to-thesouth shear zone, which may be represented in part by the pnte du Crapaud Shear Zone which occurs just south of the diatexite/metatexite contact along the River Rance section (Brun and Bal~, 1990; Treloar and Stracham 1990). To the east, the Port Briac Shear Zone is a high strain sinistral shear zone which separates the strongly deformed Brioverian sediments to the east from the strongly foliated homogeneous diatexite or anatectic granite of the core to the migmatite belt. The core has pierced its envelope in rising along the shear zone as well as having been displaced to the northeast, consistent with sinistral displacement. The diatexites o f p n t e du Grouin (Fig. 4) have a penetrative L-S tectonic fabric (Fig. 5) and this is variably developed throughout the diatexite south to its contact with the semi-pelites of the Brioverian metasediments at Port Briac (Figs. 4 and 6). In detail, the L-S tectonic fabric is seen to be a composite structure, with the local penetrative development of both C- and S-surfaces (Berth6 et al., 1980; Lister and Snoke, 1984), as shown in Fig. 7, as well as extensional shears, all of which indicate sinistral shear parallel to the stretching lineation (see also Treloar and Strachan, 1990). According to Gapais (1989), the widespread occurrence of pervasive grain-scale shear bands (C-S structures) is diagnostic of a retrograde deformation history, from high to medium temperature, especially as encountered in syntectonic plutons, which is consistent with our interpretation of the St. Malo diatexites. The fact that the emplacement of the anatec-
400
M. BROWN AND R.N. D'LEMOS
i!
Fig. 6. Contact between highly flattened Brioverian semi-pelites with an intense shear band fabric (in front) and homogeneous diatexite/anatectic granite with a strong L-S tectonic fabric, the planar component of which parallels the schistosity within the Brioverian and the linear component of which defines a strong stretching lineation which plunges gently to the NNE.
Fig. 7. Down-dip view of C-S fabric in strongly deformed homogeneous diatexite/anatectic granite, circa 2 m inside the contact with the Brioverian, which indicates sinistral shear parallel to the stretching lineation. tic core to the St. M a l o m i g m a t i t e belt can be s h o w n to be s y n c h r o n o u s with sinistral strikeslip s h e a r d e f o r m a t i o n ( B r o w n , 1974, 1978; S t r a c h a n et al., 1989; T r e l o a r a n d S t r a c h a n , 1990) c o n s t r a i n s the age o f the m a i n d e f o r m a t i o n w i t h i n the d i a t e x i t e s a n d the s h e a r
z o n e s t h e m s e l v e s to ~ 540 Ma, the age o f partial m e l t i n g as suggested by P e u c a t ( 1 9 8 6 ) . S t r a c h a n et al. ( 1 9 8 9 ) a n d T r e l o a r a n d Strac h a n ( 1 9 9 0 ) argue t h a t this also r e c o r d s the t e r r a n e a m a l g a m a t i o n to c r e a t e the N A C T . G r a n i t o i d sheets o f various types cut the
THE CADOM1AN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
metatexites and metasediments of the migmatite belt envelope, for example trondhjemite sheets occur in outcrops along the River Rance (Brown, 1974; Peucat, 1986) and granite sheets cut the metatexites in the west of the migmatite belt (Brown, 1974). Additionally, to the northwest of the St. Malo migmatite belt, leucogranites occur within the Brioverian metasediments north of Plage de Quatre Vaux and within both the pnte de la Garde and pnte de St. Cast Shear Zones (Brown and Roach, 1972a, b; Brown, 1974, 1978; Strachan et al., 1989; Gapais and Bal6, 1990; Treloar and Strachan, 1990).
The Mancellian granites The MT is composed of low-grade BrioverJan succession metasediments, usually only weakly deformed during the Cadomian orogeny, although high-strain zones, such as that at St. Pair, north of the Vire-Carolles Granite (Fig. 5), are interpreted as Cadomian shear zones by Strachan et al. (1989). These metasediments are intruded by the granite complexes and minor basic complexes of the Cadomian Mancellian Batholith (Jonin, 1981; Le Gall and Mary, 1982, 1983; Le Gall and Barrat, 1987). Inspection of the geological map (Figs. 1 and 2) shows that the Mancellian granites are emplaced either as diapirs between faults inferred to be located along Cadomian shear zones, or as elongate plutons along sets of more closely spaced faults inferred to be located along Cadomian shear zones. The location of synformal synclines of Lower Palaeozoic strata along the same faults which are inferred to be Cadomian shear zones, supports their Cadomian origin, although reconnaissance 4°Ar/39Ar muscovite age data suggest some Lower Palaeozoic tectonothermal reworking along these shear zones (R.D. Dallmeyer, pers. commun., 1990). This interpretation is consistent with published crustalscale cross-sections inferred from geophysical data (Matte and Hirn, 1988).
401
The rocks which form the bulk of the exposed level of the Mancellian Batholith vary between granodiorite (the majority) and granite; there is no evidence from either regional gravity or magnetic maps for the existence of substantial uncompensated masses of basic rock at depth. Jonin ( 1973, 1981 ) has grouped the granites into four types. The most commonly occurring is a grey granodiorite with small biotite schlieren and with minor cordierite and muscovite (type "Vire"), which has variants, such as porphyritic granodiorite, in the west of the Vire-Carolles Granite Complex, and minor leucogranodiorite and quartz diorite, in the east of the Vire-Carolles Granite Complex (type "Bois du Gast"). A white granodiorite (type "Louvignd") is distinguished both on colour and the absence ofcordierite, although it often contains small schlieren of biotite. The blue-grey granodiorite which occurs in the west of the Bonnemain Granite Complex (type "Lanhelin") is the most westerly component of the batholith. Finally, many of the complexes and the surrounding Brioverian succession metasediments are cut by small bosses of leucogranite. Basic complexes occur on the southern side of the Louvign6-Gorron Granite Complex, at Ern6e (Le Gall and Barrat, 1987), and also on the southern side of the Alexain-Deux Evailles-Izd Granite Complex, at Br6e (Le Gall and Mary, 1982, 1983). At Ernde, the basic complex comprises olivine gabbro, gabbro/ norite to diorite/quartz diorite and microdiorite with some sheets ofgranophyric granite, and at Br6e, the basic complex is made up of hornblende gabbro to quartz diorite with a narrow outer zone ofgranophyric granite. Although the relationship between the Ern6e Basic Complex and the Louvignd-Gorron Granite Complex cannot be established directly, Jonin has identified enclaves of microgabbro within the granodiorites of the granite complex. Le Gall and Barrat ( 1987 ) regard the Ern6e Complex as the surviving remnant of the mantle-derived magma which triggered crustal melting to give
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CaO Na20 KzO P20~ LOI Total
MgO
66.14 0.80 16.27 1.89 3.03 0.06 2.16 1.02 2.11 4.17 0.18 1.73 99.55
SiO2 TiO2 A1203 Fe203 FeO MnO
204
249 3.05 13.02 0.54 47.7
716 48 185 80 38 13 15 10 23 21 21 168 93 55 8 121 28 67 218
4.97
69.24 0.65 13.81 3.16 1.63 0.05 0.98 0.37 0.78 5.03 0.25 2.1~ 99.79
718
2t0 0.32 1.83 0.88 29,1
611 75 260 82 3 15 21 11 31 29 18 105 98 333 10 59 24 65 217
3.80
71.15 0.56 14.16 2.03 1.59 0.06 1.36 2.17 3.25 2.65 0.15 0.94 100.07
733
284 0.40 4.82 1.05 7/,3
1498 34 331 91 21 14 21 11 18 21 25 125 102 311 12 65 20 71 229
4.33
68.82 0.67 14.79 2.60 1.56 0.05 1.67 1.48 2.78 4.28 0.15 0,83 99.69
766
229 0.67 4.73 0.71 56.1
842 25 798 65 11 14 15 tl 15 23 22 120 131 178 9 84 21 67 206
4.11
70.70 0.63 14.01 1.89 2.00 0.04 1.56 1.09 3.02 3.31 0.16 1.14 99.55
761
250 0.62 5.06 0.50 81.17
972 37 134 77 29 17 12 11 19 33 25 119 802 19 8 76 24 70 203
4.54
68.52 0.70 14.94 1.95 2.33 0.05 1.85 1,12 3.17 3.59 0.17 1.38 99.78
205
232 0.68 4.39 0.78 44. -7
804 42 104 78 29 15 18 13 18 34 23 124 1308 183 7 102 23 80 185
5.27
67.06 0.75 15.46 2.08 2.87 0.05 2.11 1.08 3.11 3.46 0.I7 0,77 99.96
207
TABLE 1 Whole-rock geochemistry of representative rock types within the St. Malo migmatite belt
215 0.84 3,92 0,57 50.0
650 33 155 54 18 14 13 11 18 23 23 140 450 166 8 67 23 63 148
4.22
68.08 0.60 15.46 2.00 2.00 0.06 1.64 1.20 3.41 3.63 0.30 0.29 99.67
208
190 0.81 3.31 0.88 28.0
606 34 132 74 37 17 21 13 I5 35 2t 148 1385 183 9 87 24 79 161
5,25
65.36 0.79 16.29 2.12 2.81 0.06 2.13 1.31 3.34 3.39 0.25 1,68 99.54
210
206 1.(t4 4.02 0,5[! 405)
491 42 713 71 24 17 12 12 2t 28 19 127 702 122 10 102 24 79 176
4.88
67.44 0.68 15.54 3.04 1.66 0.05 1.81 0.87 3.15 3.15 0.26 1,89 99.55
704
2i3 0.50 2.35 0,69 29.9
538 30 290 66 67 19 18 11 18 15 20 115 122 229 8 76 26 60 144
4.70
66.69 0.64 16.36 2.63 1.86 0.06 1.83 1.81 3.18 2.95 0.32 0,~9 99.92
721
237 1.01 5.62 0.49 36."
770 74 662 35 18 24 21 25 37 17 26 138 116 137 13 75 43 86 324
5.48
66.48 0.71 15.30 2.58 2.61 0.07 1.40 1.72 3.42 3.94 0.32 1,13 99.69
757
226 0.69 4,06 0.75 405
729 40 562 60 25 16 18 11 20 17 17 124 490 179 7 80 24 65 175
4.44
68.30 0.64 15.38 2.27 1.95 0.05 1.73 1.12 3.34 3.37 0.15 1.50 99.80
759
215 0.68 3.33 0,55 49.2
590 43 310 67 18 t6 12 11 26 17 18 121 403 177 7 87 22 62 163
4.67
68.00 0.66 15.46 2.22 2.21 0.05 1.79 1.26 3.34 3.14 0.20 1.42 99.75
760
> z
z
©
231 0.65 4.23 0.62 55.9
K/Rb Rb/Sr Ba/Sr La/Y Ba/La
285 2.88 13.1 0.50 15.6
669 113 257 13 7 25 43 31 53 5 33 147 93 51 11 3 86 55 207
2.56
72.65 0.23 14,05 1.93 0.56 0.04 0.16 0.73 3.38 5.05 0.12 0,64 99.54
345
217 0.61 1.98 1.9 15.0
285 29 758 44 14 14 19 6 10 13 16 88 105 144 8 49 10 53 133
2.72
69.82 0.42 16.00 2.09 0.56 0.03 1.37 1.78 4.34 2.30 0.21 1,12 100.04
358
211 5.21 10.6 0.25 18.0
359 73 352 20 3 23 20 26 37 5 30 177 112 34 9 5 79 62 135
2.01
74.26 0.22 13.81 1.47 0.49 0.05 0.16 0.60 3.61 4.49 0.10 0.52 99.78
360
207 0.97 3.81 0.64 73.6
515 30 205 44 9 17 7 6 21 15 19 131 97 135 8 42 11 54 125
2.45
70.00 0.44 15.68 2.10 0.32 0.03 1.39 1.91 3.59 3.26 0.18 0.94 99.84
701
194 0.92 3.36 1.08 40.1
521 37 316 50 8 16 13 6 20 18 19 143 111 155 8 30 12 65 133
2.77
69.15 0.45 15.86 2.05 0.65 0.04 1.55 1.83 3.69 3.35 0.19 0.95 99.75
703
300 0.52 4.36 0.96 12.9
1054 155 252 31 11 14 82 11 57 8 31 127 109 242 27 32 85 40 331
2.45
71,86 0.38 14.83 1.72 0.66 0.03 0.72 1.06 3.14 4.59 0.14 0.82 99.97
720
288 0.86 4.77 0.10 630
630 25 154 30 6 13 l 5 8 11 22 114 91 132 7 18 10 40 100
1.76
71.91 0.29 15.31 1.39 0.33 0.03 0.95 1.45 3.40 3.95 0.19 0.88 100.07
758
129 3.24 0.51 -
34 0 ~98 17 0 14 0 9 1 2 9 217 144 67 1 1 8 8 23
0.32
74.12 0.11 15.28 0.32 0.0 0.04 0.00 0.37 5.02 3.37 0.71 0.Z6 100.11
211
105 4.24 0.28
19 6 277 18 1 15 0 8 14 3 9 288 110 68 2 0 8 44 32
0.71
74.22 0.06 15.36 0.71 0.0 0.08 0.02 0.34 4.32 3.63 0.66 0.61 100.00
212
331 0.61 4.95 1.10 797
797 8 163 23 1 13 1 5 7 8 27 98 106 161 6 8 10 30 73
1.33
73.27 0.21 14.92 1.33 0.0 0.03 0.52 1.12 3.67 3.91 0.20 0.81 100.00
708
242 0.17 0.88 0.79 22.3
245 35 554 20 2 15 11 4 29 7 13 47 112 279 2 15 14 31 107
1.78
73.53 0.28 15.11 1.47 0.28 0.03 0.62 2.18 4.58 1.37 0.09 0.71 100.23
740
239 0.13 0.42 0.17 64.0
128 25 164 23 0 16 2 3 22 6 4 41 119 305 1 16 12 29 95
1.64
71.64 0.34 16.00 1.44 0.18 0.02 0.69 3.10 4.68 1.18 0.10 (X~9 99.97
742
508 0.41 7.34 0.08 756.5
1513 0 415 26 3 7 2 3 16 4 40 84 103 206 1 7 24 7 37
0.43
75.51 0.07 14.05 0.43 0.0 0.01 0.12 0.72 2.81 5.14 0.19 0.68 99.73
745
Oxide data are quoted in weight percent oxide, elemental data are quoted in parts per million and values below the limit of determination are quoted as zero. Metasediments: 204 = Semi-pelitic schist, le Minihic sur Rance; 706 = Psammite layer in metatexite, St. Jacut de la Mer; 718 = Psammite enclave in diatexite, east of Rothn6uf; 733= Psammitic schist, north of St. Suliac sur Rance; 766=Psammitic schist, La Rance. Metalexite: 761 = n o r t h of St. Briac. Inhomogeneous diatexites: 205, 207, 208, 210=Montagne St. Joseph Quarry, east of St. Malo; 704=St. Jacut de la Mer; 721 =northwest of Cancale; 757=north of lie des H~bihens; 759 = north of St. Briac; 760 = north of St. Briac; 765 = north of N.D. du Guildo. Homogeneous diatexites/anatectie granites: 345 = northwest lle des H6bihens; 358=St. Jacut de la Mer; 360=Iie de la Colombi6re; 701 =St. Jacut de la Mer; 703=St. Jacut de la Mer; 720=northwest of Cancale; 758=west of St. Briac. Granitoid sheets: 211 = Pegmatite in Brioverian schists, north of N.D. du Guildo; 212 = Tourmaline leueogranite in Brioverian schists, north of N.D. du Guildo; 708=Granite in metatexite, St. Jacut de la Mer; 740, 742=Trondhjemites in psammitic schist east of Tr6gonde la Rance: 745=Alkaline granite in metatexite, southwest of Lancieux.
727 17 205 61 19 15 13 10 18 17 22 112 260 172 6 103 21 53 149
3.86
FezO3r
Ba Ce C1 Cr Cu Ga La Nb Nd Ni Pb Rb S Sr Th V Y Zn Zr
69.35 0.65 15.23 2.28 1.42 0.05 0.58 1.03 3.30 3.12 0.25 1,~5 99.80
SiO2 TiOz A1~O3 Fe203 FeO MnO MgO CaO Na:O K20 P205 LOI Total
765
TABLE 1 (continued)
%o
Z
©
N
E
©
g
-e
? z ©
t"
z
g
©
z
z
©
>
621 51 26 6 21 9 17 7 180 106 10 10 2 36 31 54 182
197 1.70 5.86 0,68 29.6
K/Rb Rb/Sr Ba/gr La/Y Ba/La
70.50 0.48 14.54 3.46 0.06 0.89 1.66 3.34 4.28 0.17 0.48 99.84
Ba Ce Cr Hf La Nb Nd Ni Rb Sr Sc Th U V Y Zn Zr
SiO2 TiO2 A1203 Fe203 T MnO MgO CaO Na20 K20 P205 LOI Total
1
189 1.65 5.97 0.41 48.2
675 42 23 9 14 11 17 8 187 113 15 7 4 33 34 57 177
70.49 0.51 14.61 3.48 0.05 0.91 1.65 3.34 4.26 0.20 0.50 99.99
2
224 1.61 6.26 0.53 36.5
620 27 26 6 17 10 14 7 159 99 10 7 3 35 32 47 171
70.63 0.48 14.37 3.42 0.06 0.81 1.63 3.39 4.29 0.18 0.45 99.71
3
229 1.76 5.23 0.58 59.7
836 56 50 8 14 12 28 10 122 160 21 7 3 71 24 74 250
65.77 0.92 15.52 5.64 0,07 1.73 2.84 3.60 3.36 0.22 0.43 100.09
4
234 0.78 5.22 0.78 29.1
814 42 57 6 28 13 25 15 122 156 19 12 3 84 36 76 246
65.79 0.93 15.40 5.69 0.07 1.85 2.70 3.53 3.44 0.20 0.45 100.06
5
234 1.00 5.51 0.81 28.8
749 54 62 5 26 I1 23 15 136 136 13 6 5 77 32 69 199
66.92 0.80 15.33 5.08 0.07 1.84 1.89 3.19 3.84 0.21 0.88 100.04
6
232 1,18 7.06 0.72 34.0
883 44 44 8 26 11 23 11 148 125 15 9 4 61 36 64 212
68.39 0.66 14.98 4.32 0.07 1.35 1.72 3.31 4.14 0.21 0.71 99.85
34
213 1.73 7.21 0.77 28.5
685 55 20 5 24 10 27 7 164 95 i0 7 6 22 31 39 166
71.80 0.36 14.41 2.75 0.03 0.69 1.34 3.46 4.20 0.20 0.83 100.08
40
TABLE 2 Whole-rock geochemistry of representative granites within the Mancellian Batholith
212 1.73 7.37 0,84 26,7
693 63 22 7 26 10 24 6 163 94 9 11 4 22 31 45 161
71,31 0.35 14.40 2.84 0.05 0.67 1.20 3.47 4.16 0.20 0.94 99.58
41
135 6.5J 3.41 -
140 17 13 5 0 8 3 0 267 41 8 0 3 9 13 18 45
74.53 0.12 14.13 1.16 0.03 0.12 0.41 3.82 4.33 0.42 0.74 99.81
10
226 1.43 6.1t 0.60 31.1
654 46 25 6 21 10 23 4 153 107 11 9 3 34 35 53 181
69.86 0.51 14.66 3.47 0.06 0.92 1.80 3.51 4.16 0.17 0.63 99.75
11
230 1.07 5.88 0.75 35.8
752 60 36 5 2l 11 25 5 137 128 16 10 2 51 28 54 197
68.20 0.66 15.26 4.19 0.04 1.16 2.38 3.68 3.79 0.18 0.58 100.13
12
226 1.0l 5,93 0.52 47.1
801 40 44 6 17 12 22 12 137 135 14 7 2 65 33 67 212
67.10 0.78 15.21 4.98 0.08 1.55 2.22 3.38 3.73 0.23 0.65 99.93
13
224 1.03 6.02 1.03 23.7
807 45 43 8 34 I1 24 8 138 134 12 9 3 54 33 59 217
67.60 0.72 15.19 4.48 0.07 1.35 2.33 3.58 3.72 0.20 0.75 100.00
14
g
Z > Z
o
-~ c~ -~
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIFOF FRANCE
405
~o.~¸
~e~eeeo~4ee~
I
I
•~ 0o 05
- - o m ~ o o m ~ O ~ N I
I
-.o e-
i
e~
-
d d ~ M d £ d M M d
~
"--
~
~
I
ca
dd4~d~Mdddd~e
~d d ~ ~ d 2_ M ~ M d d
~ d ~ d 2 d ~ M d d ~ e-
8
~ ," " ~ ~ I.,, c~ . -
'~
<
I
Q~
0
•~ - 6 II © ~ ~r~
~
406
rise to the granodiorites of the Mancellian Batholith. In contrast, the Br6e Basic Complex appears to have been emplaced into the contact aureole of the Alexain-Deux Evailles-Iz6 Granite Complex (Le Gall and Mary, 1982). If this latter interpretation is correct, it suggests that some of the basic rocks at least were emplaced after the main granitic components of the Mancellian Batholith. The Mancellian granites have developed contact aureoles in the surrounding Brioverian succession metasediments and are overlain unconformably by Cambrian strata (Chauris, 1956; Graindor, 1953). Although the granite complexes were probably emplaced sometime around the Precambrian-Cambrian boundary, the precise age of the Mancellian granites has proved difficult to determine, even though a variety of isotopic methods (Jonin and Vidal, 1975; Pasteels and Dor6, 1982; Autran et al., 1983) has been applied to the problem. A slightly younger age than that of 540 Ma for the Precambrian-Cambrian boundary and an episodic intrusive sequence are suggested by some of the Rb-Sr whole rock data of Jonin and Vidal ( 1975 ). An age of 525 + 6, derived from a composite R b - S r whole rock isochron, is indicated for leucogranites which form a volumetrically minor c o m p o n e n t of some of the granite complexes, and an age of 502 + 37 is derived for the Lanhelin blue granodiorite of the Bonnemain Granite (ages recalculated from data in Jonin and Vidal, 1975). Data from various parts of the Vire-Caroltes Granite (from Jonin and Vidal, 1975) give an age of over 600 Ma with a Sr~ which is impossibly low. Recently, Graviou et al. (1988) have presented a Rb-Sr whole rock errorchron based upon samples from the "Vire" type granodiorite using the published R b - S r analyses of Adams (1967) and Jonin and Vidal (1975), in combination with 14 unpublished data obtained by Chariot in collaboration with the BRGM. The resultant age of 551 + 2 8 Ma is certainly plausible and the Sr~ of 0.7077 _+ 15 is reasonable. However, the combination of ana-
M. B R O W N A N D R S. D ' L E M O S
lytical work from a 20-year period without regard either to the analytical imprecision ~J early isotopic determinations or to the lack ~f interlaboratory correlation leaves some uncertainty and, we presume, partly accounts for the high error on this age. Pasteels and Dore (1982) report two monazite ages. one of 547 + 10 Ma from the porphyritic granodiorite at the western end of the Vire-Carolles Granite and one of 542 + 9 from a leucogranite associated with the Vire-Carolles Granite, which are consistent with the Mancellian granites being coeval with the anatectic granites of the St. Malo migmatite belt. Thus we follow Pasteels and Dor6 (1982) and regard the age ot" the main components of the Mancellian granites as being ~ 540 Ma. This is consistent with recent 4°Ar/39Ar muscovite cooling ages of 527 and 521 Ma (D'Lemos et al., 1990)~
Geochemistry Analytical techniques and results For the St. Malo migmatite belt, the 29 samples listed in Brown (1974) were reanalyzed by X R F spectrometry in the Geochemistry Laboratories of the University of Keele using their standard analytical procedures (Table 1 )° For the Mancellian granites, 28 representative samples from a larger suite collected in 1987 were analyzed in the XRF Laboratory at the University of Nottingham using their standard analytical procedures (Table 2 ). In both cases several international standards were analyzed as a check on accuracy, which was satisfactory in both cases.
Comparative geochemisto, Table 3 gives the mean and standard deviation for three samples from sections through the Brioverian succession sandstones, one from the SBT (Binic) and two from the MT (Vi-
THE CADOMIANGRANITESOF MANCELLIA,NORTHEASTARMORICAN MASSIFOF FRANCE
407
TABLE 3 Average geochemical compositions of Brioverian succession sandstones (from Denis and Dabard, 1988 ), St. Malo diatexites and Mancellian granites
Number of analyses:
1
2
3
4
5
8(3)
10
10
17
22
Mean and standard
deviation:
k
cr
k
cr
x
cr
R
cr
X
o-
SiO2 TiO2 AI203 Fe,O 3 MnO MgO CaO Na20 K,O P_~Os Ba Ce La Nb Nd Ni Rb Sr Th V Y Zn Zr K/Rb Rb/Sr Ba/Sr Ba/La La/Y
67.34 0.68 14.35 5.19 0.29 1.68 1.09 3.44 2,30 0,30 787
3.70 0.09 1.04 1.16 0.60 0.87 0.87 0.53 0.47 0.54 213
69.52 0.56 14.33 4.81 0.05 1.26 0.31 3.18 2.89 0.13 479
4.79 0.18 1.74 1.44 0.01 0.58 0.19 0.40 0.62 0.05 126
72.43 0.53 12.65 5.27 0.04 1.01 0.20 2.46 1.97 0.09 408
2.94 0.09 0.99 0.82 0.01 0.40 0.06 0.47 0.44 0.02 170
69.1 l 0.54 15.35 3.77 0.05 1.42 1.29 3.41 3.57 0.21 642 51 20 13 24 18 129 155 10 61 32 63 176 230 1.15 4.84 0.69 39.6*
2.41 0.19 0.67 1.27 0.01 0.60 0.40 0.29 0.67 0.07 196 35 18 7 14 9 19 53 5 34 26 13 63 33 1.18 2.82 0.39 19.9
69.11 0.61 14.82 4.00 0.05 1.18 1.92 3.46 3.92 0.20 697 47 20 10 22 8 151 119 8 49 33 56 187 216 1.47 5.84 0.64 36.2
2.24 0.20 0.45 1.08 0.01 0.43 0.55 0.14 0.30 0.06 172 1l 7 2 6 4 31 26 2 19 6 13 43 31 1.17 0.97 0.18 10.5
68 152
11 49
143
52
66
27
l=Brioverian sandstones, Binic (8 analyses for oxides, 3 analyses for elements); 2=Brioverian sandstones, Vidouville; 3 = Brioverian sandstones, Athis; 4 = St. Malo diatexites; 5 = Mancellian granites. * Analysis 758 excluded.
the main types of Mancellian granites. The mean chemical composition of the St. Malo diatexites is compared with that for each of the Brioverian samples, and the mean chemical composition of the Mancellian granites is compared with that for the St. Malo diatexites, in all cases using a t-test (Davis, 1973 ). The ttest assesses the hypothesis that the mean of the population from which the first sample was drawn is the same as the mean of the parent population of the second sample. This hypothesis is posed against the alternative that the two
population means are not equal. Below we address the question: is the difference between the means of two samples significant at a particular level? At the 10% level of significance, for the comparison between the Brioverian succession sandstones and the St. Malo diatexites, the values of t for a two-tailed test with 23 degrees of freedom (sample B, 18 degrees of freedom for Ba, Rb and Sr) and 25 degrees of freedom (samples V and A) do not fall into either critical region for most oxides/elements consid-
408
M. BROWN ~tND RS. D'I,EMOS
ered (Table 4) and the null hypothesis cannot be rejected. Exceptions are Rb for sample B, CaO for sample V, and A1203 and CaO for sample A. Even at the 20% level of significance, the list of oxides/elements that falls into the critical region is only expanded to include K20 and Rb for sample A, still only CaO for sample V, and A1203, CaO, Na20, K20 and P205 for sample A. Thus we conclude there is no evidence to suggest that samples B and V come from populations having significantly different means than the St. Malo diatexites, except for K20 and Rb (sample B) and CaO TABLE 4 Values of t for the appropriate number of degrees of freedom 0') for the comparisons between the St. Malo diatexites (SMMB) and three samples of Brioverian succession sandstones (B, V and A), and between the St. Malo diatexites (SMMB) and the Mancellian granites (MG)
SiO2 TiO2 AI20~ Fe20~ MnO MgO CaO Na20
K20 P2Os Ba Ce La Nb Nd Ni Rb Sr Th v Y Zn Zr t (oe= 10%) t (c~=20%)
SMMB vs. B
SMMB vs. V
SMMB vs.-X
SMMB xs. MG
0.40 0.55 0.82 0.75 0.47 0.25 0.22 0.05 0.35 0.19 0.58
0.07 0.07 0.55 0.49 0.00 0.17 1.82 0.44 0.66 0.80 0.59
0.80 0.04 2, i 3 0.84 0.63 0.48 2.14 1.64 1.69 1.32 0.79
2.64 0.05
0.14
t.24
23 ~ 1.71 b 1.32 ~
25 1.71 1.32
25 1.71 1.32
0.00 0.20 0.54 0.1 I 0.00 0.27 0.73 0.13 0.40 0.09 0. t 7 0.09 0.00 0..35 0. t I 0.86 0.47 0.51 0.31 0.26 0.03 0.31 0.12 37 1.69 1.31
Brioverian succession samples: B=Binic: V=Vidouville; A=Athis. a 18 for Ba, Rb and Sr; ~ 1.73 for Ba, Rb and Sr; ': 1.33 for Ba, Rb and Sr.
(sample V). Note that this does not mean that the St. Malo diatexites were derived from Brioverian succession sandstone similar in composition to the samples considered herein, but only that there is no basis in comparative geochemistry to reject such a model. At the 20% level of significance, for the comparison between the St. Malo diatexites and the Mancellian granites, the values of t lbr a twotailed test with 37 degrees of freedom do not fall into either critical region for any oxide/ element considered (Table 4) and the null hypothesis cannot be rejected. Thus we conclude there is no evidence to suggest that the samples being compared come from populations having different means. Note that we are not saying that the chemical composition of the Mancellian granites is the same as that of the St. Malo diatexites, but simply that they are not significantly different and, therefore, that a model which proposes a similar petrogenesis for the two samples cannot be rejected on the basis of comparative geochemist~,. In comparison with average compositions of granitic rocks from late Cadomian post-tectonic plutonic complexes from within the SBT (Table 5), neither the St. Malo diatexites nor the Mancellian granites show much similarity with the Cobo Granite or the L'Ancresse Granodiorite of Guernsey. The deeper crustal level exposed in south Guernsey, where lcartian basement is seen, and the scarcity of" Brioverian succession rocks on Guernsey, together with their isotope geochemistry, suggest that these late Cadomian granites might be derived from a different source (R.S. D'Lemos and M. Brown, unpubl, data). The late Cadomian granites are also associated with a volumetrically larger calc-alkaline plutonic complex, so that mantle-derived magma advecting heat into the crust to cause partial melting is a real possibility. However, the granites from Jersey, which is thought by Brown et al. ( 1990 ) to represent a higher structural level, are emplaced into Brioverian succession metasediments at very low grade and develop cordierite
409
THE CADOMIANGRANITES OF MANCELLIA,NORTHEASTARMORICANMASSIFOF FRANCE
TABLE 5 Average chemical composition of granitic rocks from late Cadomian post-tectonic plutonic complexes of the Channel Islands, St. Bricuc Terrane, North Armorican Massif, France
1
2
3
4
5
6
MgO CaO Na20 K,O P~()~ Ba ('e ka Nb Nd
76.01 0.01 13.36 1.39 0.05 0.26 0.47 4.72 4.62 0.03 688 38 19 11 I1
71.78 0.29 14.44 2.82 0.06 0.65 2.09 3.93 3.04 0.11 926 50 23 10 22
68.67 0.54 15.34 4.21 0.07 0.83 1.95 4.59 3.60 0.19 599 59 27 12 25
74.81 0.20 13.19 2,05 0.03 0.26 0.79 4,13 4,46 0,08 308 63 29 9 23
73.93 0.26 13.75 2.61 0.04 0.37 0.97 3.95 4.82 0.11 380 101 42 23 40
74.44 0.19 13.53 1.94 0.03 0.24 0.59 3.93 5.04 0.08 323 96 40 19 38
Ni
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
Rb Sr Th V Y Zn Zr K/Rb Rb/Sr Ba/Sr ka/Y Ba/La Rb-Sr Age
131 96 N.A, N.A, 14 14 86 293 1.36 7.17 1.36 36.2 4 9 6 ± 13
68 323 N.A. N.A. 22 21 138 371 0.21 2.87 1.05 40.3 -
123 169 N,A. 37 31 47 237 243 0.73 3.54 0.87 22.2 4 6 5 _+ 10
190 54 N.A. 14 34 22 173 195 3.52 5.70 0.85 10.6 438_+ 17
224 120 N.A. 19 47 20 190 179 1.87 3.17 0.89 9.05 550_+ 12
200 84 N.A. 16 43 19 165 209 2.38 2.85 0.93 8.08 4 8 3 ± 13
SiO2 TiO2 AI~O~ FQ,O~v
MnO
Guernsey: 1 = Cobo Granite ( D'Lemos, 1987 ); 2 = L'Ancresse Granodiorite (Brown and D'Lemos, unpublished data ). Jersey, North-west Granite Complex: 3 = Porphyritic Granodiorite (Bland, 1985 ); 4 = Coarse Granite (Bland, 1985 ); South-west Granite Complex: 5 = Porphyritic Granite (Bland, 1985 ): 6 = Coarse Granite ( Bland, 1985 ). N.A. = not analyzed.
hornfelses similar to those around the Mancellian Granite Complexes. Even so, there are few close similarities in geochemistry with the Mancellian granites, and a continental arc origin which involves both mantle and crustal sources is preferred (R.S. D'Lemos and M. Brown, unpubl, data). A comparison of mean chemical compositions suggests that the Mancellian granites have much closer similarities with the diatexites of the St. Malo migmatite belt than with the late Cadomian post tectonic granites of the SBT. A closer geochemical comparison between the Mancellian granites and the St. Malo migmatire belt is undertaken, largely using bivariate
plots. This is followed by a consideration of whether the established petrogenetic model for the migmatites, of anatexis of Brioverian succession metasediments, is applicable to the Mancellian granites. Geochemical plots are presented in pairs, consisting of one plot for the St. Malo migmatite belt and one plot for the Mancellian granites.
"S-type"granites Plots of Na20 vs. K20, A / N K vs. A/CNK and A / C N K vs. SiO2 are presented in Figs. 810. On a plot of Na20 vs. KzO, the Mancellian granites plot within the field encompassed by the St. Malo migmatites (Fig. 8). However,
410
M BROWN~kN[)RS. I)'L[LM()5,
MANCELLIAN GRANITES
ST. MALO MIGMATITE BELT
f
s.o
•
• 4.0~--
.
a
2-0 i
/,/'
[
j
In
4.0~-
°
I
oA~~
O
i
z
t
¢M3.0 Ft~
"
-
./ ! i
i
J"
I
i]
5.0~-
/
•
/-
2.ot-
i/
O
10
1.0
I
2.0
___
J ........
3.0
~
40
±
50
....
K20
10
i
2.0
_L...............J_ 3C
4 0
K20
Fig. 8. Plot of Na20 vs. K20 for representative samples from the St. Malo migmatite belt and the Manceltian gramtes. In this and subsequent figures, for the St. Malo migmatite belt, filled circles represent both inhomogeneous and homogeneous diatexites/anatectic granites, diamonds represent granitoid sheets which cut the metatexites and the metasediments, the triangle represents a bulk sample of metatexite and the inverted triangles represent metasediments, both from outside the metatexites and from enclaves from within the diatexites. In this and subsequent figures, for the Mancellian granites, filled circles represent the main granite types (type "Vire", type "Louvign6"" and type "Lanh61in'" ), diamonds reprcscnl leucogranites and squares represent samples from the Bois du Gast, at the eastern end of the Vire-Carolles Granite Complex. The large upright crosses represent the mean and standard deviation for three samples of Brioverian succession sandstones from Denis and Dabard (1988). B is the sample from Binic, V is the sample from Vidouville and 4 is the sample from Athis.
both the St. Malo migmatites and the Mancellian granites have higher K 2 0 contents than the Brioverian succession sandstones which may represent the composition of the protolith. Although the five metasediments sampled from within the migmatites are very variable in composition, it is noticeable that two of them have similar Na20 and K 2 0 contents to the samples of the Brioverian succession sandstones. Both suites of rocks are peraluminous using Shand's index (Fig. 9), but in terms of aluminum saturation index ( A / C N K ) vs. SiO2 (Fig. 10) the two suites only overlap at SiO2 > 70%, and below this value the St. Malo rocks trend to higher values of A / C N K , whereas the Mancellian rocks decrease marginally with decreasing SiO2 content. In terms
of Shand's index and the tectonic discrimination of granitoids developed by Maniar and Piccoli ( 1989 ), both suites of rocks overlap the peraluminous part of the Continental Arc Granitoids as well as falling within the field of Continental Collision Granites, although whether this reflects tectonic setting or source characteristics remains an open question. In terms of alkalis, the St. Malo suite of rocks shows minor overlap with the field of "S-type'" granites as discriminated within the Lachlan Fold Belt of southeastern Australia (White and Chappell, 1983, fig. 5), and the Mancellian granites show no overlap at all. This demonstrates the point made frequently by White and Chappell (e.g. 1988; see also ChappelI, 1984) that the sedimentary source of the Lachlan "S-
411
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMOR1CAN MASSIF OF FRANCE
ST. MALO M I G M A T I T E BELT Peraluminous
2.2 _ Metaluminous 2.0 1.8 ~1.6 Z ' ~ 1.4 1.2 1.0 0.8
I 0.8
1.0
I 1.2
I 1.4
I 1.6
I 1.8
I 2.0
A/CNK MANCELLIAN 2.2 - Metaluminous
GRANITES
Peraluminous
I
2.01 1.8 1.6 Z 1.4 1,2 !
1.0
0.8
1.0
1 1.2
I 1.4
I 1.6
I 1.8
I 2.0
A/CNK
Fig. 9. Plot of A/NK vs. A/CNK for the St. Malo rnigmatitebelt and the Mancelliangranites.
type" granites is unusual in its chemistry and casts doubt on this frequently used parameter to discriminate "S-type" from "I-type" granites. Furthermore, the genetic relationship between the "S-type" and "l-type" classification scheme of Chappell and White (1974) and the restite model for granite genesis (White and Chappell, 1977; Chappell et al., 1987; Chappell and Stephens, 1988; White and Chappell,
1988 ) with which it is intimately associated, is commonly neglected. It is clear from Fig. 10 that chemical compositions of the diatexites from the St. Malo migmatite belt could have been controlled by restite unmixing since the increase in A / C N K with decreasing SiO2 here could reflect increasing biotite. However, it is also clear from Fig. 10 that the restite unmixing model may not apply to those St. Malo
412
M. BROWNAND R,S. [)'LEM( IS ST. MALO
I
1.8 ,¢ Z 0
1.6
MIGMATITE
BELT
ST. MALO
MIGMATITE
BELT
0.9
•
•
0.8
• °1be
1.4
•
4'
°'
•
1.2
4
1,0
I.-
i I
I 62
I 64
I
l
I
66
68
70
I ..... L
72
74
0.4
4,
0.3 -
q,
__L__~,:'
76
0,2
SiO2
MANCELLIAN
•
ql
l
0.8
•
-
4,
o.,i
GRANITES
•
' 50
•
.l 150
100
__J__ 200
L~ . . . . . 250
; ......... ;!30
Zr
1.8
MANCELLIAN
GRANITES
1.6 ,,z'
0.9
1.4
Z
0~.2
o ddl8~
ee •
41, 41,
0.8
tt
0.7
1.0 i
1
0.8 0.6
%
L
0.6
&
Od
00.5 I
62
d 64
I 66
-.L-_ 68 SiO
l ~ 70
..... ~ .... 72 74
; .... 76
J
0.4 -
2
0. 3~-
Fig. 10. Plot of A / C N K ( a l u m i n i u m saturation index), corrected for P205, v s . SiO2 for the St. Malo rnigmatite belt a n d t h e Mancellian granites.
rocks which plot at values of SiO2 above 70% and does not apply to the whole suite of Mancellian granites since A / C N K decreases with decreasing SiO2. The distinction between the behaviour of the diatexites on the one hand and the Mancellian suite o f rocks on the other is interesting since it suggests that restite unmixing, perhaps by filter pressing, is a viable mechanism whilst convection homogenization has not occurred, but that once convection overturn has homogenized melt and residuum,
•
°2t olP 0,1
I
v
50
100
i
150
200
_J 250
.... ± . . . . . . 300
Zr Fig. 11. Plot of TiO2 v s . Zr for the St. Malo migmatite belt and the Mancellian
granites.
the system behaves as a "normal igneous" one and the chemical variation may be controlled by crystal fractionation rather than restite unmixing.
4 13
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. MALO MIGIMATITE BELT
ST. MALO MIGIMATITE BELT
300
200
O~
• •
•
•
•
200
100
_
I 62
I 64
I
I
I
I
I
I
66
68
70
72
74
76
lOO
8iO2
•
MANCELLIAN GRANITES
I
I 64
62
I
I
l
66
68
70
A
v
I 72
I
I
74
76
SiO 2 MANCELLIAN GRANITES 200 ¢,,
#
100
200
BB
eqP#°l, • I 62
1 64
I 66
I 68
I 70
l 72
I 74
I 76
__
lOO
SiO 2 Fig. 12. Plot of Rb vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
Trace element distributions
On plots of TiO2 vs. Zr (Fig. 11 ), both the diatexites from the St. Malo migmatite belt and the main types of granite within the Mancellian Batholith define steep positive covariation of strongly decreasing TiO2 with decreasing Zr and increasing SiO2 content. However, the absolute abundance of Zr in the Mancellian granites is higher at given TiO2 than in the diatexites, although values lie intermediate between the diatexites and the analyzed metasediments from the St. Malo migmatite belt (Fig.
I
1
1
I
1
I
I
62
64
66
68
70
72
74
v v
76
SiO2 Fig. 13. Plot of Sr vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
11 ). The granitoid sheets from within the St. Malo migmatite belt and the leucogranites and Bois du Gast granodiorites from the Mancellian Batholith lie on identical but shallower trends than either the diatexites or the main types of granites (Fig. 11 ). These features suggest that similar processes control the distribution of these elements in each suite. For the diatexites, Rb varies little with in-
414
m B R O W N A N [ ) R.S. I)'LEMt )5,
ST. MALO MIGMATITE BELT
•
1500
41,
1400 1300 1200 1100 1000 900
~
800 700
el
6OO 500 400
A
• I
300 200 IO0 __
1 62
I 64
1 66
_L~ 68
I 70
l 72
~1~ 74
I 76
SiO 2
MANCELLIAN GRANITE,S 900
| ooO
800
,,'It
700 600
~
500
III u
400
•
300 200 100
I 62
I 64
I 66
1 68
1__ 70
SiO 2
72
74
76
creasing SiO2, while the granitoid sheets have variable abundances according to type (Fig. 12). For the main types of granite within the Mancellian Batholith, Rb increases with increasing SIO2, while the leucogranites are variable and the Bois du Gast granodiorites have their own unique abundances. Sr decreases within the diatexites with increasing SiO2 while the granitoid sheets again exhibit variable abundances (Fig. 13), and Sr decreases within the Mancellian granites with increasing SiO:, except that the Bois du Gast granodiorites plot uniquely (Fig. 13). At given SiO2 content, Rb is slightly higher and Sr lower for the Mancellian granites than for the St. Mato diatexites (Table 1, Figs. 12 and 13). Ba has similar levels of abundance in both suites of rocks and decreases dramatically with increasing SiO2 content (Fig. 14). The covariation of Rb with Sr and Ba with Sr is shown on logarithmic plots in Figs. 15 and 17, respectively. Rayleigh fractionation generates straight line arrays of data points from comagmatic products of fractional crystallization (for a constant bulk distribution coefficient) on such bivariate logarithmic plots (Figs. 16 and 18). Variable separation of cumulus crystals and interstitial liquids within plutonic suites causes lateral scatter of the points around such arrays (McCarthy and Hasty, 1976). As with all coarsely crystalline plutonic suites, it is difficult to model trace element distributions quantitatively because their textures lead to uncertainty over the definite identification of liquidus minerals and, since the compositions are (indeterminate) mixtures of crystals and variably evolved residual liquid, no unequivocal samples of liquids are available (cf. McCarthy and Hasty, 1976; McCarthy and Groves, 1979). Additionally, silica contents in these rock suites extend up to values generally regarded as "high silica magmas". In very high silica systems it is Fig. 14. Plot of Ba vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
415
THE CADOM1AN GRANITES OF MANCELL1A, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. M A L O M I G M A T I T E BELT
500
100
II,
50
Rb
Sr
[
[
I
[
I
5
lo
50
100
500
MANCELLIAN GRANITES
500
,% 100
I
50
Rb
Sr
[
I
I
I
I
5
10
50
100
soo
Fig, 15. Logarithmic plot of Rb vs. Sr for the St. Malo migmatite belt and the Mancellian granites.
the structure and volatile content of the melt which are the dominant controls of trace element partitioning, whereas in less silicic magmas, partitioning between crystals and melt is governed primarily by crystal structure constraints. Partition coefficients for trace elements in high silica rhyolites are much larger than those generally accepted for less silicic compositions (Mahood and Hildreth, 1983; Nash and Crecraft, 1985). For example, elements consistently enriched in biotite over the
host liquid include Rb and Ba (Hanson, 1978 ), although the partition coefficient for Ba in particular shows substantial variation (Nash and Crecraft, 1985 ). Similarly, Ba substantially and Sr and Rb generally are enriched in K-feldspar over the host liquid (Nash and Crecraft, 1985 ), although Ba again shows substantial variation and Rb is enriched in K-feldspar over liquid rather than depleted in high silica systems (cf. Hanson, 1978 ). In plagioclase, Sr is strongly to very strongly enriched relative to the host liq-
416
M. BROWN
J
I
-
r
I
17
"-----4 0
1
-
-
-
-
AND
RS,
t ) : I , E M ( ~,~
~ ~0
/
3o
"-~--g~
PI
-~-
90
-
so./,/" ~/.-~
~"~"I~
90 f _ ~~ 90 90 90' 7~350
'
50
-
70 -
0
j
1
-
I
5
,
I 1
I
5
I 1
1
1
5
Fig. 16. Logarithmic plot of Rb vs. Sr to illustrate the effect of tractional crystallization of hornblende (Hb). c imopyroxene (Cpx), plagioclase (PI), K-feldspar (Kts) or biotite (Bt) on liquid compositions using limiting values ('or the distribution coefficients in granitoids from Nagasawa and Schnetzler ( 1971 ; Hb and PI), Philpotts and Schnetzler ( !070; Upx
and Kfs), and Nash and Crecraft ( 1985; Bt, Pl and/~'). Numbers represent the percentage of liquid which remains after each fractional crystallization step indicated by a cross tick on each fractionation line. Note how different distribution coefficients for the same mineral alter both the slope and the rate of change for fractionation by crystallization of thai mineral. uid (Hanson, 1978; Nash and Crecraft, 1985 ), Rb is always depleted but Ba m a y change in high silica systems from depleted to enriched (Nash and Crecraft, 1985). Given the problems inherent in cumulus crystal/residual liquid mixtures and the additional problems inherent in any trace element modelling of melting and fractionation trends in silicic igneous suites (determination o f partition coefficients, intrasuite variability o f partition coefficients, volatile transport o f trace metals, effects o f melt structure on elemental distributions etc. ), no quantitative modelling has
been attempted at this stage. Phases likely to be involved in controlling Rb, Sr and Ba distributions are biotite, plagioclase and K-feldspar. Qualitatively, fractional crystallization of biotite reduces both R b / S r and Ba/Sr ratios, plagioclase increases both R b / S r and Ba/Sr and K-feldspar increases Rb/Sr, but not as dramatically as plagioclase, and decreases Ba/Sr, but not as dramatically as biotite. It is clear that the distributions displayed in Figs. 15 and 17 for the St. Malo diatexites (filled circles ) and main Mancellian granites (filled circles) will allow only limited Rayleigh fractionation of
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. MALO
MIGMATITE
41 7
BELT
•T 1000
e,' 500
~v~ q°
100
50
Ba Sr
I
I
t
]_
I
5
10
50
100
500
MANCELLIAN GRANITES
1000
500
100
50
Ba
Sr
I
I
I
I
I
5
10
50
100
500
Fig. 17, Logarithmic plot of Ba vs. Sr for the St. Malo migmatite belt and the Mancellian granites.
biotite + plagioclase _+K-feldspar (see Figs. 16 and 18) and more detailed modelling is not warranted at this stage. The Mancellian leucogranites could be generated by feldspar + biotite fractionation from a parent similar to the main varieties of Mancellian
granites. However, the chondrite-normalized REE patterns (M. Brown and R.S. D'Lemos, unpublished data) show small to large positive Eu anomalies which suggest that the final crystallization of the leucogranites involved feldspar accumulation.
418
M, BROWNCNI)
I
~
)S
30
1
so/t o
50 -~
70
~--~L~9o
.......
i
70~ 90 0/7"70
90
PI
-
D'LfM
'
1
~'~--
5
R~
4-. . . . . . .
i
4
90
2q
9o
50
i 4 f
//
/
/// ,,/<70
/
1
,,"/45 5
/ •
/"
Ba
4-/ /
,//
i
,,/
/
//
4
/
/
S so
90-
/
t
] / /
/___l 5
!
5
_t
Fig. 18. Logarithmic plot of Ba vs. Sr to illustrate the effect of tractional crystallization of hornblende (lib), clmopyroxcnc (Cpx), plagioclase (Pl), K-feldspar (K~'), or biotite (Bt) on liquid compositions using limiting values for thc distribu-tion coefficients in granitoids from Nagasawa and Schnetzler ( 1971; lib and Pl), Philpotts and Schnetzler ( 1970: (7~.~ and Kfs), and Nash and Crecraft ( 1985; Bt, PI and K/~). Numbers represent the percentage of liquid which remains after each fractional crystallization step indicated by a cross tick on each fractionation line. Note how different distribution coefficients for the same mineral alter both the slope and the rate of change for fractionation by crystallization or that mineral.
K / R b ratios are essentially the same for both suites of rocks at 230 + 33 for the diatexites and 216 + 31 for the Mancellian granites (Tables 1, 2 and 3), but show a very small increase with increasing SiO2in the St. Malo suite and a very small decrease with increasing SiO2 in the Mancellian suite (Fig. 19). The effect of fractional crystallization of biotite on the K / R b ratios in rocks of this range of SiO2 contents will be to reduce it, while the effect of K-feldspar could be either to reduce it slightly or to increase it slightly depending upon the SiO2 content and the distribution coefficient for Rb. Plagioclase will not affect the K / R b ratio.
Tectonic interpretation using trace element discrimination diagrams The introduction of a set of trace element discrimination diagrams for the tectonic interpretation of granitic rocks in 1984 (Pearce et al., 1984) has led to their widespread use in the identification of the tectonic setting of ancient granite suites. A majority of the St. Malo and Mancellian samples plot within the field of Volcanic Arc Granites as discriminated by Pearce et al. (1984), for example on the plot of Rb vs. Y + N b (Fig. 20). The high Rb content of some samples takes them across into the "syn-Collision Granite field". The tendency for
419
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. M A L O
MIGMATITE
ST. M A L O
BELT
MIGMATITE
BELT
!
600 500
syn-COLG
500 ¢~400 re 3OO
B= ~,
100
lle•
5O
200
Rb
100 _~
I
62
64
I ~__J
66
68 70 SiO2
I
I
I
72
74
76
MANCELLIAN
10
f
GRANITES
VAG
5
400 I=
ORG
10
.Q 300
Y+Nb
50
100
MANCELLIAN
GRANITES
/ ~200
L__
500
ee
100 II 62
I 64
I 66
I 6B
I 70
I 72
I 74
I 76
WPG
syn-COLG
_
SiO 2
100
Fig. 19. Plot of K / R b vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
50 Rb
the Mancellian granite samples to plot close to the boundary with the Within Plate Granite field and for a small n u m b e r of the St. Malo diatexites to plot inside that field, may reflect the slightly peraluminous nature of the melts and their volatile contents, reflected in P2Os contents, since these factors promote enrichment of rare metals (Webb et al., 1987 ). However, what is most interesting is that these suites of rocks classify as VAG when the St. Malo migmatites are clearly derived by anatexis of Brioverian succession metasediments and the Mancellian granites could have an essentially similar origin. This suggests that the discrimination being made is between the tectonic environment of the provenance source area to the
10
//
/
j/ ORG
VAG I
I
5
10
I
Y+Nb
50
100
Fig. 20. Rb vs. Y + N b discriminant plot of Pearce et al. (1984) for the St. Ma[o migmatite belt and the Mancellian Granites.
protolith sediment (in this case the Brioverian succession metasediments), rather than the magmatic environment in which these suites formed, in this case an inverted basin rather
420
M BROWN
,AND R ~ ; I ) ' I _ E M ( ) S
Brioverian sedimentary rocks I -~ ~oo ~ - ~ l ~
~oo
I I,
,/ ,
'~"
100
0 rr ] l [ [ J I ~ _ _ l . k ~ k i _i_J R b B a T h U K T a N b L a C e S r N d P Hf Z r S m T i T b
GS \ /
50
i Y i/0
Fig. 21. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements tbr three samples of Brioverian succession sandstones (data from Denis and Dabard, 1988 ).
t,--
V
IE
,
~
-
!O
loo ~O
50
E
than a true volcanic arc. We conclude that this "discrimination" diagram simply may not work! Brown et al. (1984) use primordial mantle normalized element patterns and selected H F S / L I L element ratios to discriminate primitive, calcic arc granitoids with low LIL and HFS element abundances from normal, calcalkaline continental arc granitoids with enhanced LIL element abundances and low HFS/ LIE ratios and mature, alkali-calcic granitoids with high levels of LIL and HFS elements and higher H F S / L I L ratios. Primordial mantle normalized element patterns for three samples of Brioverian succession sandstones (data from Denis and Dabard, 1988) and selected samples from the St. Malo migmatite belt suite of rocks and the Mancellian granite suite are shown in Figs. 21, 22, 23 and 24. All of these patterns are similar to those from normal continental arcs as defined by Brown et al. ( 1984 ). That the Brioverian succession sediments have such a potential provenance source is consistent with their generally epiclastic character and deposition in basins of limited extent, as suggested by the work of Denis (1987) and Denis and Dabard ( 1988 ). Patterns for the inhomogeneous and homogeneous diatexites from the St. Malo migmatite belt are similar to those from the Vire-Carolles, Bonnemain, Louvign6-Gorron and Alexain-Deux Evailles-
a.. "~ I 0 0 O rr
50
100 50
~-
Ms->\,\
/~\ \ / ,q,
10
~_
/MS
5
t I_1 J J i_
I I
RbBaTh U K TaNbLaOeSrNd
HPf Z r S m ~ - - ~ , ~ T b '7
Fig. 22. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements for selected samples from the St. Malo migmatite bell MS=metasediment: MT=metatexite; ID = inhomogeneous diatexites; H D = homogeneous diatexites; GS=granitoid sheets.
Iz6 Granite Complexes in Mancellia. The pattern for the Bois du Gast granodiorites can be matched with something intermediate between the two patterns from granitoid sheets within the St. Malo migmatite belt. These data strongly support a c o m m o n origin for these two suites of rocks. The patterns from the two teucogranite samples reflect strong fractionation and these isolated bosses within the Mancel-
If
421
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMOR1CAN MASSIF OF FRANCE
LEUCOGRANITES
~oo~ 5o- ',~¢
-I loo -so
~ /'~1~
v-o 21A
100
100 m ¢-
100
50
50
"1:3 i_ O
10
10
E
5
v' O
#
~ooo
V
" - -~p~
0.1
0.1
0.5
0.5
5 J RbBaTh
100
--
0
l
I
I
U
K TaNbLaCeSrNd
i
i
J
I
I
J
I
I
I
P HfZrSmTi
I
J Tb Y
Fig. 24. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements for selected leucogranites from the Mancellian Batholith.
5O
I I I I RbBaTh
U
I
I I
K TaNbLaCeSrNd
P HfZrSmTiTb
Y
Fig. 23. Plots of rock/primordial mangte (normalizing values from Wood, 1979) for a range of elements for selected samples from the Mancellian granites: V-C G=Vire-Carolles Granite: B G=Bonnemain Granite; L-G G = Louvign6-Gorron Granite; A-DE-I G= AlexainDeux Evailles-lz6 Granite. lian Batholith may represent residual liquids from the m a i n granite magma. Discussion The C a d o m i a n orogeny involved active continental margin processes during up to 275 Ma from ~ 7 0 0 - 6 5 0 Ma to ~ 4 2 5 Ma (Brown et al., 1990). Within this period of time we can recognize two major periods of magmatic activity. The first includes the early C a d o m i a n foliated plutonic complexes (Power et al., 1990a) and the Lower Brioverian basaltic vol-
canic formations (Lees et al., 1987; Dupret et al., 1990). The second period includes late Cad o m i a n post-tectonic complexes and the Upper Brioverian andesite-rhyolite volcanic formations (Dupret et al., 1990). Rocks developed during the first period of magmatic activity are confined at the present level of erosion to the SBT. Rocks developed during the second period of magmatic activity occur within all three terranes of the NACT, but as discussed above, those of the SBT are geochemically distinct from the St. Malo migmatire belt samples and the Mancellian granite samples. Our preferred tectonic model for the Cadomian orogeny (Brown et al., 1990) involves left oblique plate convergence, i.e. southerly or southwesterly subduction across a northeast to east-northeast oriented trench, which has caused fore-arc, intra-arc and back-arc regions of the orogenic belt to behave as semi-independent terranes relative to the bordering continental (the Central Armorican Terrane) and oceanic (the Cadomian Ocean to the north or northwest) lithospheric plates. This deformation is transpressional, the terranes are b o u n d e d by the subduction zone on the ocean-
422
ward side and are both dissected by and bounded by strike-slip faults through the forearc, intra-arc and back-arc regions. Structural trends in the SMT are east-northeast (Brown, 1978) and reflect transverse compression across the trend of the arc. These terranes were shortened perpendicular to the trend of the subduction zone and migrated as coherent bodies variable distances sub-parallel to the subduction zone between the bounding strikeslip faults. The transpression is dated at ~ 540 Ma by the syn-kinematic emplacement of diatexite, anatectic granite and leucogranite into shear zones in the SMT (Brown, 1978; Strachan et al., 1989; Treloar and Strachan, 1990 ). Anatexis at mid-crustal levels within the SMT requires a high geothermal gradient, which we believe was generated during behind-arc basin extension and subsequent basin inversion prior to 540 Ma ago. There may be similarities between the tectonic setting of the St. Malo migmatite belt and the Variscan migmatites and granites of the Pyrenees (e.g. Wickham and Oxburgh, 1986, 1987). The overall similarity between the chemistry of the St. Malo migmatite belt and the Mancellian granites and their similar age argue for a substantially c o m m o n origin. The MT is interpreted as a former behind-arc basin (Brown et al., 1990; Dupret et al., 1990), and a shallower geothermal gradient through the crust during extension and accumulation of the Brioverian succession is likely to have been present, followed by thickening of the hotter crust during basin inversion following the start of the transpressional event. Thus, anatexis in the lower part of the Brioverian succession is a probability within the postulated tectonic models for the Cadomian orogeny (Strachan et al., 1989; Brown et al., 1990; Dupret et al., 1990). There remains to be considered the possible role of mantle-derived magmas in the generation of the Mancellian Batholith. The volumetrically minor basic complexes at Ern6e and Br6e have a calc-alkaline geochemistry and are
~1 BROWN AND R S. I)'I3(M~ ~",
likely to have had an ultimate origin in the mantle wedge, even though the chemistry of some members of the suite may reflect subsequent processes. However, there is no geophysical evidence to suggest that there are substantial masses of basic rocks beneath the exposed level of the MT. We conclude that the role of mantle-derived magma in the generation of the Mancellian Batholith was minor. Initial results of S m - N d and Rb-Sr isotopic studies (R.S. D'Lemos and M. Brown. unpubl. data) show that the Mancellian granites and the St. Malo migmatites share similar isotopic characteristics, having similar eNd ( - 4 . 0 to - 7.3 ) and cSr ( + 3 to + 42 ) values and model ages ( T d m u r ) in the range 1.7-1.5 Ga. These features set the SMMB and the MG apart from the post-tectonic Cadomian granites which occur in the St. Brieuc Terrane at La Hague and in the Channel Islands. This isotopic evidence further points to a genetic link between the SMMB and the MG, and is interpreted to reflect fundamental differences in the isotopic characteristics of the source region and magmatic evolution of the SMT and MT in comparison with the SBT. In some models for the Cadomian orogeny, the migmatite belts of the SMT and the granites of the MT are regarded as being the result of thermal relaxation following thrusting dur* ing subduction-collision orogenesis (Bal6 and Brun, 1983: Graviou and Auvray, 1985; Graviou et al., 1988). The arguments against the thrusting model are presented in more detail by Strachan and Roach (1990) and Treloar and Strachan (1990). However, we note that neither the regional low grade of metamorphism at the present erosion level nor the normal average thickness of crust in this region (Bois and Courtillot, 1988), when combined with the absence of significant physiographic relief, allow any substantial overthickening during arc-continent collision. Accordingly, we prefer the terrane amalgamation model in which oblique collision of crustal blocks within the hanging wall of a major subduction zone in
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
the interval 580-530 Ma has inverted a behind-arc basin. Imbrication of this inverted basin during transpressional orogeny at ~ 540 Ma has juxtaposed two different structural levels along the Cancale Shear Zone, represented by the MT and the SMT. The major terrane boundary, which separates the SBT from the SMT, is the Fresnaye Shear Zone. This separates two crustal blocks of fundamentally different geology within an overall model for the Cadomian Belt of southerly-directed subduction beneath the Armorican Massif.
Conclusions (1) The St. Malo migmatite belt and the Mancellian granites are interpreted to be coeval at ~ 540 Ma and both developed by anatexis at intermediate crustal depths of the Brioverian succession. The higher geothermal gradient required for crustal anatexis was generated during behind-arc basin extension while the Brioverian sediments were deposited and preserved during subsequent basin inversion to allow melting to occur at intermediate depths in the crust. (2) The syn-kinematic emplacement of the homogeneous diatexite/anatectic granite core to the St. Malo migmatite belt along the Port Briac Shear Zone/Cancale Shear Zone is inferred to date the age of transpressional accretion of the St. Malo terrane to the Mancellian terrane, although these two terranes may simply be different structural levels of an imbricated behind-arc basin. By analogy, the Fresnaye Shear Zone and related shear zones which occur on the northwestern side of the St. Malo migmatite belt are inferred to be of the same age and thus the docking of the St. Brieuc terrane with the St. Malo terrane occurred prior to, and transpression continued synchronously with, the culmination of anatexis at ~ 540 Ma. This particular terrane boundary may well be the more important, since the geology across it changes more strikingly than across the
423
boundary between the St. Malo terrane and the Mancellian terrane. ( 3 ) The comparative geochemistry of the St. Malo migmatites and the Mancellian granites allows a similar petrogenesis for both suites of rocks, viz. anatexis of Brioverian succession metasediments. In the case of the Mancellian granites, it is postulated that partial melting substantially in excess of the R C M P was followed by convective overturn and homogenization prior to emplacement as diapirs between Cadomain shear zones or elongate plutons along sets of more-closely spaced shear zones. The St. Malo migmatites, across the Cancale Shear Zone, represent a deeper structural level where both migmatitic source and anatectic granite product are found together and where the genetic link between the two can be demonstrated unambiguously. As the frontispiece of Read's ( 1957 ) book suggests: "Per migma ad magma." (4) The interpretation of tectonic setting using trace element discrimination diagrams should be made with caution since it could be the source of the granites which is being identiffed, that is the tectonic setting of the provenance source area to the protolith sediment for an "S-type" granite, rather than the tectonic setting of the magmatism. It is clear in the case of the St. Malo migmatites that this is the case and it is our contention that the VAG classification of the Mancellian granites reflects the source characteristics rather than the tectonic setting. We conclude that this "discrimination" diagram simply may not work!
Acknowledgements We thank our colleagues for many years of stimulating discussion, in particular G.M. Power, R.A. Roach, R.A. Strachan, C.G. Topley and P.J. Treloar. Geochemical analyses were provided by T.S. Brewer (University of Nottingham) and G.J. Lees (University of Keele). Funding by Kingston Polytechnic and Oxford Polytechnic is acknowledged. This pa-
424
per is a contribution to IGCP Project 217 "Proterozoic Geochemistry". We thank one anonymous reviewer, N.M. Halden, G.M. Power, R.A. Strachan and P.J. Treloar for comments on an earlier version of the manuscript. References Adams, C.J.D., 1967. A Geochronological and Related Isotope Study of Rocks from North Western France and the Channel Islands (United Kingdom). Unpublished D. Phil. thesis, University of Oxford, Oxford, 259 pp. Arzi, A.A., 1978. Critical phenomena in the rheology of" partially melted rocks. Tectonophysics, 44:173-184. Autran, A., Beurrier, M., Calvez, J.-Y.. Cocherie, A.. Fouillac, A.-M. and Rossi, P., 1983. Age et origine des granitoids du batholite mancellien (Normandie, France). Principaux Rdsultats Scientifiques et Techniques du B.R.G.M., Resumes, 57. Auvray, B., Chariot, R. and Vidal, P., 1980. Donnecs nouvelles sur le Proterozoique inferieur du domainc Nord-Armoricain (France): ~ge et signification. Can. J. Earth Sci., 17: 532-538. Bale, P. and Brun, J.P., 1983. Les chevauchments cadomtens de la Bate de St.-Brieuc (Massif Armoricain ). C.R. Acad. Sci., Paris, 297: 359-362. Bale. P. and Brun, J.P., 1989. Late Precambrian thrust and wrench zones in northern Brittany (France). J. Struct. Geol., I 1: 391-405. Bcrthd, D., Choukroune, P. and Jdgouzo, P., 1980. Orthogneiss, mylonite and noncoaxial deformation of granites: the example of the South Armorican Shear Zone. J. Struct. Geol., 1: 31-42. Bland, A.M., 1985. The Geology of the Granites of Western Jersey, With Particular Reference to the South-West Granite Complex. Unpublished PhD Thesis, Oxford Polytechnic, Oxford, 419 pp. Bois, C. and Courtillot, V., 1988. The French ECORS Program. Deep Seismic Profiling of the Crust and Evolution of the Lithosphere. Eos, Trans. Am. Geophys. Union, October 25 1988: 977,989. Brown, G.C., Thorpe, R.S. and Webb, P.C., 1984. The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources. J. Geol. Soc., London, 141: 413-426. Brown, M., 1973. The definition of metatexis, diatexis and migmatite. Proc. Geol. Assoc., 84: 371-382. Brown, M., 1974. The Petrogenesis of the St. Malo Migmatite Belt, North-Eastern Brittany, France. Unpublished PhD Thesis, University of Keele. Brown, M., 1978. The tectonic evolution of the Precambrian rocks of the St. Malo region, Armorican Massif, France. Precambrian Res., 6:1-21.
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Pearce, J.A., Harris, N.B.W, and Tindle, A.G., t 984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 25: 956-983. Peucat, J.J., 1986. Behaviour of Rb-Sr whole rock and U-Pb zircon systems during partial melting as shown in migmatitic gneisses from the St. Malo Massif, NF Brittany, France. J. Geol. Soc., London, 147: 876-886. Peucat, J.J. and Martin, H.. 1985. Are Rb-Sr thin slab migmatite ages always meaningful? Neues,!abrb, Mineral., Abh., 152:1-2 l. Philpotts, J.A. and Schnetzler, (-'.C:., 1970. Phenocrystmatrix partition coefficients lor K, Rb. Sr and Ba. with applications to anorthosite and basalt genesis. Ge~chim. Cosmochim. Acta, 34: 307-322. Power, G.M., Brewer, T.S., Brown, M. and Gibbons, W., 1990a. Late Precambrian foliated plutonic complexes of the Channel Islands and La Hague--Early CadomJan plutonism. In: R.S. D'Lemos, R.A. Strachan and C.G. Topley (Editors), The Cadomian Orogeny. Geol~ Soc. Spec. Publ., 51:215-229. Power, G.M., Brewer, T.S. and D'Lemos, R.S., 1990b. The post-tectonic plutonic complexes of La Hague, Manche, N. France. In: R.S. D'Lemos, R.A. Strachan and C.(L Topley (Editors), The Cadomian Orogeny. Geol. Soc. Spec. Publ., 51: 261-272. Read, H.H., 1957. The Granite Controvers'~. Murby. London, 430 pp. Roach, R., Adams, C., Brown, M., Power, G. and Rya~, P., 1972. The Precambrian stratigraphy of the Armorican Massif, NW France. Proc., 241h Int. Geol~ Congr... Montreal, Section 1, pp. 246-252. Roach. R.A., Topley, C.G., Brown, M. and Shufflcbotham, M.M., 1986. Brioverian Volcanism and Cadomian Plutonism in the Northern Part of the Armor~ ican Massif. Pre-Confercncc Excursion Guide t~, I.G.C.P. 217 Meeting, Keyworth, Notts., i.K~, April 1-5, 1987, 90 pp. Shuffiebotham, M.M., 1990. The geology of the Pcnthibvre crystalline massif: a reappraisal of the typepentevrian area, northern Brittany. In: R.S. D'Lemos. R.A. Strachan and C.G. Topley (Editors), The ( a domian Orogeny. Geol. Soc. Spec. Publ., 5 l: 27-39. Strachan, R.A. and Roach, R.A.. 1990. Tecto~lic evolution of the Cadomian belt in north Brittany In: R.S. D'Lemos, R.A. Strachan and C.G. Topley (Editors), The Cadomian Orogeny. Geol. Soc. Spec. Publ., 5 ! 133-150. Strachan, R.A., Treloar, P.J., Brown, M. and D'Lemos, R.S., 1989. Cadomian terrane tectonics and magmatism in the Armorican Massif. J. Geol. Soc., t,ondon, 146: 423-426. Strachan, R.A., Dallmeyer, R.D. and D'Lemos, R.S., 199~). Timing of Cadomian tectonothermal activity, north Brittany, France: evidence from 4°Ar/39Ar mineral ages. Geological Association of Canada--Mineral Association of Canada Annual Meeting, Program with Abstracts, 15: A125.
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
Topley, C.G., Brown, M., D'Lemos, R.S., Power, G.M. and Roach, R.A., 1990. The northern igneous complex of Guernsey, Channel Islands. In: R.S. D'Lemos, R.A. Strachan and C.G. Topley (Editors), The Cadomian Orogeny. Geol. Soc. Spec. Publ., 51: 245-259. Treloar, P.J. and Strachan, R.A., 1990. Cadomian strikeslip tectonics in NE Brittany. In: R.S. D'Lemos, R.A. Strachan and C.G. Topley (Editors), The Cadomian Orogeny. Geol. Soc. Spec. Publ., 51: 151-168. Van der Molen, I. and Paterson, M.S., 1979. Experimental deformation of partially-melted granite. Contrib. Mineral. Petrol., 70:299-318. Vidal, P., Auvray, B., Chariot, R. and Cogn6, J., 1981. Precadomian relicts in the Armorican Massif: their age and role in the evolution of the western and central European Cadomian--Hercynian Belt. Precambrian Res., 14: 1-20. Webb, P.C., Brown, G.C. and Tindle, A.G., 1987. Geochemical evolution in a range of UK granites: implications for heat production and rare metal enrichment. In: The Origin of Granite: A Symposium. The Royal Society of Edinburgh and the Royal Society of London, Unpubl. Abstract. Weber, C., Barby, P. Cuney, M. and Martin, H., 1985. Trace element behaviour during migmatization. Evidence for a complex melt-residuum-fluid interaction
427
in the St. Malo migmatitic dome (France). Contrib. Mineral. Petrol., 90: 52-62. White, A.J.R. and Chappell, B.W., 1983. Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia. Geol. Soc. Am., Mem., 159:21-34. White, A.J.R. and Chappell, B.W., 1988. Some supracrustal (S-type) granites of the Lachlan Fold Belt. Trans. R. Soc. Edinburgh, Earth Sci., 79:169-181. Wickham, S.M., 1987. The segregation and emplacement of granitic magmas. J. Geol. Soc., London, 144:281297. Wickham, S.M. and Oxburgh, E.R., 1986. A rifted tectonic setting for Hercynian high thermal gradient metamorphism in the Pyrenees. Tectonophysics, 129: 53-59. Wickham, S.M. and Oxburgh, E.R., 1987. Low-pressure regional metamorphism in the Pyrenees and its implications for the thermal evolution of rifted continental crust. In: E.R. Oxburgh, B.W.D. Yardley and P.C. England (Editors), Tectonic Settings of Regional Metamorphism. Phil. Trans. R. Soc., London, A, 321 : 219-242. Wood, D.A., 1979. A variably-veined sub-oceanic upper mantle: genetic significance for mid-ocean ridge basalts from geochemical evidence. Geology, 7: 499-503.
393
The Cadomian granites of Mancellia, northeast Armorican Massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting Michael Brown a,l and Richard S. D ' L e m o s
b
a School o[Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK b Department qfGeology, Oxford Polytechnic, Gipsy Lane, Headington, Oxford, OX30BP, UK ( Received October, 23 1989; revised and accepted May 28, 1990 )
ABSTRACT Brown, M. and D'Lemos, R.S., 1991. The Cadomian granites of Mancellia, northeast Armorican Massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting. In: I. Haapala and K.C. Condie (Editors), Precambrian Granitoids--Petrogenesis, Geochemistry and Metallogeny. Precambrian Res., 51: 393-427. The North Armorican Shear Zone (NASZ) divides the Cadomian belt of the Armorican Massif of France into two major terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The NACT is a pastiche of displaced blocks and terranes (St. Brieuc Terrane, St. Malo Terrane and Mancellian Terrane) which results from the amalgamation of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone. In the NACT, the Neoproterozoic Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. The St. Malo migmatite belt is a syn-tectonic suite of anatectic migmatites derived through partial melting of the Brioverian succession. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that a sample of St. Malo diatexites comes from a population with a different mean than some samples of the Brioverian succession sandstones. Resulting homogeneous diatexites/anatectic granites of the core were emplaced syn-kinematically into Cadomian strike-slip shear zones. Thus the age of anatexis of ~ 540 Ma is also the date of strike-slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the NACT. The Mancellian granites, to the southeast of the St. Malo migmatites, were emplaced at ~ 540 Ma and are overlain unconformably by Cambrian sediments. Overall geochemistry and a variety of specific geochemical parameters show that the Mancellian granites are similar to the diatexites and anatectic granites of the St. Malo migmatite belt. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that the sample of Mancellian granites comes from a population with a different mean than the sample of St. Malo diatexites. Additionally, both suites of rocks are peraluminous, they have similar distributions of Rb, Sr and Ba, they have K / R b ratios around 225, and they plot largely within the VAG field using trace element discrimination diagrams for the interpretation of tectonic setting. Samples of the Brioverian succession sandstones, the St. Malo migmatites and the Mancellian granites all exhibit similar primordial mantle-normalized element patterns. Preliminary results of Nd and Sr isotope studies reveal a similar restricted range of values for eNd (between - 4 . 0 and - 7 . 3 ) and eSr (between - 3 and + 4 2 ) , and depleted model ages in the range 1.7 to 1.5 Ga (Tdmur) for both Mancellian granites and St. Malo migmatites, isotopic features which set these rock types apart from other Cadomian granites within the NACT. The St. Malo migmatite belt and the Mancellian granites are coeval at ~ 540 Ma and both developed by anatexis, at intermediate crustal depths, of the Brioverian succession, within an inverted, but only moderately overthickened, behindarc basin. In comparison with the St. Malo migmatites, the Mancellian granites represent farther-travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source.
t Present address: Department of Geology, University of Maryland at College Park, College Park, Maryland 20742, USA.
0301-9268/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
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Introduction
The pre-Mesozoic Armorican Massif of northwest France is subdivided into three major tectonic units by the North Arrnorican Shear Zone (NASZ) and the South Armorican Shear Zone (SASZ) (Gapais and Le Corre, 1980). North of the NASZ the major orogenic activity is Cadomian, especially in the east (Bal6 and Brun, 1989; Strachan et al., 1989; Brun and Bal6, 1990), although some Palaeozoic tectonothermal rejuvenation has occurred (D'Lemos et al., 1990). A Precambrian tectonostratigraphy for the Armorican Massif north of the NASZ was originally established around the Baie de St. Brieuc by Cogn6 (1959) and extended subsequently to the whole Massif (Cogn6, 1972). However, this tectonostratigraphy has proved ambiguous in respect of the unconformable relationship between supposed pre-Cadomian basement and the Neoproterozoic Brioverian succession cover on the east side of the Baie de St. Brieuc (Guerrot and Peucat, 1990; Shuffiebotham, 1990). Unequivocal pre-Cadomian ~2,000 Ma Icartian Gneiss basement is exposed only in the Channel Island of Guernsey, at La Hague and as rafts in the Cadomian Perros Guirec Complex of the Tr6gor (Roach et al., 1972; Leutwein et al., 1973; Calvez and Vidal, 1978; Auvray et al., 1980; Vidal et al., 1981 ). Basaltic rocks form a significant component of the Neoproterozoic Brioverian succession, especially in its lower part, and an understanding of the geochemistry of these rocks has contributed to the interpretation of the overall geotectonic framework of the Cadomian orogen as a series of volcanic arcs and intra-arc basins (Cabanis et al., 1987; Lees et al., 1987; Dupret et al., 1990). Intermediate and acid volcanic rocks occur within the upper parts of the Brioverian succession (Roach et al., 1986; Dupret et al., 1990). The deposition of the
M BROWN AN[) R.5. l)'1 EMt)S
Brioverian succession, which is predominantly an epiclastic sedimentary sequence, was followed closely by Cadomian deformation and metamorphism which culminated in the development of anatectic migmatites around St. Malo, and the emplacement of the late Cadomian post-tectonic plutonic complexes (Brown et al., 1990). The region has been the site of extensive Palaeozoic sedimentation and the Brioverian succession is overlain unconfbrmably by early Palaeozoic sediments which, at Ca~teret in Normandy, include limestones with late Lower Cambrian faunas (Dot6, 1972). Additionally, the late Cadomian post-tectonic Mancellian granites are overlain unconformably by the Red Conglomerate and arkoses of probable Cambrian age (e.g. Vire-Carolles Granite, Chauris, 1956; and Athis Granite, Graindor, 1953 ). Thc Precambrian-Cambrian boundary within the Armorican Massif is placed at ~ 540 ma (Pasteels and Dor6, 1982; see also Menning, 1989 ). Thus both the Mancellian granites and the St. Malo migmatite belt are regarded as Precambrian. Variscan reactivation in the north-east ARmorican Massif is (apparently?) limited, although granitoids related to the Variscan orogeny become an increasingly important feature of the geology further to the west and 4°Ar/ :~'%r muscovite ages from the St. Malo l'erranc indicate some post-Cadomian tectonothermal activity (D'Lemos et al., 1990). This paper is concerned with the petrogenesis of the late Cadomian Mancellian granites of the northeast Armorican Massif. The origin of these granite complexes has been related to crustal thickening during subduction - collision orogenesis by some workers (Graviou and Auvray, 1985, 1990; Graviou et al.. 1988). This is, however, difficult to reconcile with recent work around the Baie de St. Brieuc and the Baie de St. Malo (Strachan and Roach, 1990; Treloar and Strachan, 1990) which has
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
led to the development of a dominantly strikeslip related model for the Cadomian orogeny (Strachan et al., 1989). We test the hypothesis that the Mancellian granites and the anatectic rocks of the St. Malo migmatite belt have a c o m m o n origin related to basin inversion during sinistrally transpressive terrane accretion at ~ 540 Ma. Tectonic model for the Cadomian Belt of the northeast Armorican Massif
The NASZ separates the Cadomian Belt into two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT) (Strachan et al., 1989); see Fig. 1. In the NACT, the Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. Early Cadomian foliated plutonic
395
complexes locally form the basement to the Brioverian succession (Strachan et al., 1989; Shuffiebotham, 1990; Guerrot and Peucat, 1990). The Cadomian cycle occurred during some 275 Ma between ~ 700 Ma and ~ 425 Ma (Brown et al., 1990). However, in the CAT, although an angular unconformity separates the Palaeozoic succession from the Brioverian succession, the Brioverian sequence was not subjected to major orogenic activity until the Variscan orogeny (Gapais and Le Corre, 1980). Therefore, the NACT and the CAT are considered by Strachan et al. (1989) to have the status of separate Cadomian terranes. The NACT itself is a pastiche of variably displaced blocks and terranes which results from the juxtaposition of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a subduction zone. The terranes which comprise the NACT (see Fig. 1 ) are the St. Brieuc
Fig. 1. The principal terranes of the northeastern part of the Armorican Massif. The North Armorican Composite Terrane is composed of the St. Brieuc Terrane, the St. Malo Terrane and the Mancellian Terrane which are amalgamated along the Fresnaye Shear Zone (FSZ) and the Cancale Shear Zone (CSZ), respectively. The North Armorican Composite Terrane is separated from the Central Armorican Terrane by the North Armorican Shear Zone (NASZ). Light lines outline major geological features and heavy lines represent Cadomian shear zones or faults inferred to be located along Cadomian shear zones.
396
Terrane (SBT), the St. Malo Terrane (SMT) and the Mancellian Terrane ( M T ) (Strachan et al., 1989). The SBT is the most lithologically variable of the three terranes within the NACT. It includes various calc-alkaline gabbro-dioritetonalite-granite complexes emplaced into both Icartian basement gneisses (e.g. on Guernsey and at La Hague) and Brioverian volcanic and sedimentary sequences (e.g. on Jersey and around the Baie de St. Brieuc). These calc-alkaline plutonic complexes are reviewed by Brown et al. (1990). Cadomian metamorphism within the SBT varies from prehnitepumpellyite facies to amphibolite facies westwards around the south side of the Baie de St. Brieuc, a facies series similar to those described from Mesozoic arc environments (Miyashiro, 1973 ), and Cadomian deformation is both variable and heterogeneous (Strachan and Roach, 1990). The timing of deformation and metamorphism within the SBT predated ~ 570-560 Ma (Strachan et al., 1990). The SMT includes a number of migmatite belts and narrow strips of Brioverian metasediments (Brown, 1979; Strachan et al., 1989 ). The St. Malo migmatite belt is described further below. The SMT has been reworked heterogeneously by steep ductile shear belts (Brown, 1974, 1978; Treioar and Strachan, 1990) which were active synchronously with anatexis at ~ 5 4 0 Ma (Strachan et al., 1989: Treloar and Strachan, 1990 ). Shear criteria indicate a consistent sub-horizontal, sinistral strike-slip sense of m o v e m e n t (Gapais and Bal6, 1990; Treloar and Strachan, 1990). To the southeast of the SMT, the MT ("Domaine Mancellien" of Cogn6, 1964) is composed of mainly weakly deformed low-grade Brioverian metasediments and metavolcanics (Graindor, 1957; Dupret, 1974), into which have been emplaced the plutonic complexes of the Mancellian Batholith (Jonin, 1981; Graviou et al., 1988; Brown et al., 1990). The Mancellian granites are described further below.
M. BROWN A N D R . S D ' L E M O S
A number of recent models for the Cadomian Belt (e.g. Bal6 and Brun, 1983, 1989: Cabanis et al., 1987; Graviou et al., 1988; Dupret et al., 1990; Brun and Bal& 1990) attribute Cadomian events to tectonothermal activity in the hanging wall of a southerly dipping subduction zone, although Brun and Bal6 ( 1990 ) suggest that subduction may have been to the north. In these models, the rocks assigned by us to the SBT are interpreted as a "volcanic arc" and "back-arc basin" complex which was thrust southwest onto the facing continent. represented by our SMT and MT, at ~ 590-580 Ma. The resultant crustal thickening inferred is thought to have generated the post-tectonic formation of the migmatite belts of the SMT and the Mancellian Batholith of the MT. Dupret et al. (1990) suggest a subduction-related origin for the Mancellian Batholith, but invoke a second parallel subduction zone which developed along the line of the Cancale Shear Zone of Strachan et al. (1989) during closure and inversion of a marginal basin. In contrast. Strachan and Roach (1990) find no field evidence around the Baie de St. Brieuc which requires southwest verging thrusting, while Brown ( 1974, 1978) and Treloar and Strachan (1990) have shown that the migmatite belts are pre- to syn-tectonic and not post-tectonic with respect to the dominant shear zone deformation in the SMT. These workers (Strachan et al., 1989; Treloar and Strachan, 1990 ) have argued that the amalgamation of the SBT, SMT and MT occurred at ~ 540 Ma during displacement across a crustal-scale sinistral strike-slip belt defined by Cadomian shear zones (see Fig. 1 ). The St. Malo migmatite belt The SMT is made up of several belts of migmatite and anatectic granite, intercalated with belts of variably deformed medium-grade metasediment assigned to the Brioverian succession, and cut by numerous syn-anatectic shear zones, including the Fresnaye Shear Zone and
397
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
the Cancale Shear Zone which form the northwest and southeast boundaries of the terrane (see Fig. 1; Strachan et al., 1989; Treloar and Strachan, 1990). The main belt of migmatite and anatectic granite is the one which trends ENE through St. Malo (Fig. 2 ), and which has been the focus of much work during the past twenty years (e.g. Brown et al., 1971; Brown, 1973, 1974, 1978, 1979; Brun, 1975, 1977; Martin, 1977, 1979, 1980; Brun and Martin, 1978; Weber et al., 1985; Peucat and Martin, 1985; Peucat, 1986; Treloar and Strachan, 1990). To the west of the River Rance (see Fig. 2 ), the St. Malo migmatite belt comprises me-
tatexites--a stromatic migmatite produced by low to moderate degrees of partial melting (Brown, 1973), with the generation of up to ~ 30% melt, which is below the rheologically critical melt percentage (RCMP), according to Arzi (1978), Van der Molen and Paterson (1979) and Wickham (1987)--and diatexites--a schlieric or nebulitic migmatite produced by moderate to high degrees of partial melting (Brown, 1973 ), with the generation of greater than ~ 30% melt, which exceeds the RCMP and results in the distruption of the stromatic structure characteristic of a metatexite. These migmatites are clearly derived by in-
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398
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THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
Fig. 5. L-S teclonic fabric in slightly schlieric inhomogeneous diatexite, pnte de Grouin, north of Cancale, St. Malo migmadte belt. Plane of photograph approximately perpendicular to both schistosity and lineation.
from metasediments, metatexites, diatexites and anatectic granites. To the east of the River Rance (Figs. 2 and 4), the St. Malo migmatite belt comprises inhomogeneous diatexites, with characteristic schlieric structure, homogeneous diatexites, with nebulitic structure, and anatectic granites which represent diapiric rise of convectively homogenized and ponded anatectic melt, apparently synkinematically with sinistral shearing (Brown, 1974, 1978; Strachan et al., 1989; Treloar and Strachan, 1990). The eastern half of the St. Malo migmatite belt is illustrated in Fig. 4, which shows the homogeneous diatexite core to the belt. Linear structures within the migmatite belt plunge at a shallow angle between northeast and east (Brown, 1974, 1978 ),
399
so that, westward, the successive inhomogeneous diatexite and metatexite represent deeper structural levels. To the north, the metatexite structurally overlies the diatexite with a variably dipping shallow to moderate contact. To the south along the east bank of the River Rance, the diatexite is seen to structurally overly the metatexite along a steep contact. Thus, the diapiric diatexite core to the belt is overturned to the south or southeast, consistent with displacement across a top-to-thesouth shear zone, which may be represented in part by the pnte du Crapaud Shear Zone which occurs just south of the diatexite/metatexite contact along the River Rance section (Brun and Bal~, 1990; Treloar and Stracham 1990). To the east, the Port Briac Shear Zone is a high strain sinistral shear zone which separates the strongly deformed Brioverian sediments to the east from the strongly foliated homogeneous diatexite or anatectic granite of the core to the migmatite belt. The core has pierced its envelope in rising along the shear zone as well as having been displaced to the northeast, consistent with sinistral displacement. The diatexites o f p n t e du Grouin (Fig. 4) have a penetrative L-S tectonic fabric (Fig. 5) and this is variably developed throughout the diatexite south to its contact with the semi-pelites of the Brioverian metasediments at Port Briac (Figs. 4 and 6). In detail, the L-S tectonic fabric is seen to be a composite structure, with the local penetrative development of both C- and S-surfaces (Berth6 et al., 1980; Lister and Snoke, 1984), as shown in Fig. 7, as well as extensional shears, all of which indicate sinistral shear parallel to the stretching lineation (see also Treloar and Strachan, 1990). According to Gapais (1989), the widespread occurrence of pervasive grain-scale shear bands (C-S structures) is diagnostic of a retrograde deformation history, from high to medium temperature, especially as encountered in syntectonic plutons, which is consistent with our interpretation of the St. Malo diatexites. The fact that the emplacement of the anatec-
400
M. BROWN AND R.N. D'LEMOS
i!
Fig. 6. Contact between highly flattened Brioverian semi-pelites with an intense shear band fabric (in front) and homogeneous diatexite/anatectic granite with a strong L-S tectonic fabric, the planar component of which parallels the schistosity within the Brioverian and the linear component of which defines a strong stretching lineation which plunges gently to the NNE.
Fig. 7. Down-dip view of C-S fabric in strongly deformed homogeneous diatexite/anatectic granite, circa 2 m inside the contact with the Brioverian, which indicates sinistral shear parallel to the stretching lineation. tic core to the St. M a l o m i g m a t i t e belt can be s h o w n to be s y n c h r o n o u s with sinistral strikeslip s h e a r d e f o r m a t i o n ( B r o w n , 1974, 1978; S t r a c h a n et al., 1989; T r e l o a r a n d S t r a c h a n , 1990) c o n s t r a i n s the age o f the m a i n d e f o r m a t i o n w i t h i n the d i a t e x i t e s a n d the s h e a r
z o n e s t h e m s e l v e s to ~ 540 Ma, the age o f partial m e l t i n g as suggested by P e u c a t ( 1 9 8 6 ) . S t r a c h a n et al. ( 1 9 8 9 ) a n d T r e l o a r a n d Strac h a n ( 1 9 9 0 ) argue t h a t this also r e c o r d s the t e r r a n e a m a l g a m a t i o n to c r e a t e the N A C T . G r a n i t o i d sheets o f various types cut the
THE CADOM1AN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
metatexites and metasediments of the migmatite belt envelope, for example trondhjemite sheets occur in outcrops along the River Rance (Brown, 1974; Peucat, 1986) and granite sheets cut the metatexites in the west of the migmatite belt (Brown, 1974). Additionally, to the northwest of the St. Malo migmatite belt, leucogranites occur within the Brioverian metasediments north of Plage de Quatre Vaux and within both the pnte de la Garde and pnte de St. Cast Shear Zones (Brown and Roach, 1972a, b; Brown, 1974, 1978; Strachan et al., 1989; Gapais and Bal6, 1990; Treloar and Strachan, 1990).
The Mancellian granites The MT is composed of low-grade BrioverJan succession metasediments, usually only weakly deformed during the Cadomian orogeny, although high-strain zones, such as that at St. Pair, north of the Vire-Carolles Granite (Fig. 5), are interpreted as Cadomian shear zones by Strachan et al. (1989). These metasediments are intruded by the granite complexes and minor basic complexes of the Cadomian Mancellian Batholith (Jonin, 1981; Le Gall and Mary, 1982, 1983; Le Gall and Barrat, 1987). Inspection of the geological map (Figs. 1 and 2) shows that the Mancellian granites are emplaced either as diapirs between faults inferred to be located along Cadomian shear zones, or as elongate plutons along sets of more closely spaced faults inferred to be located along Cadomian shear zones. The location of synformal synclines of Lower Palaeozoic strata along the same faults which are inferred to be Cadomian shear zones, supports their Cadomian origin, although reconnaissance 4°Ar/39Ar muscovite age data suggest some Lower Palaeozoic tectonothermal reworking along these shear zones (R.D. Dallmeyer, pers. commun., 1990). This interpretation is consistent with published crustalscale cross-sections inferred from geophysical data (Matte and Hirn, 1988).
401
The rocks which form the bulk of the exposed level of the Mancellian Batholith vary between granodiorite (the majority) and granite; there is no evidence from either regional gravity or magnetic maps for the existence of substantial uncompensated masses of basic rock at depth. Jonin ( 1973, 1981 ) has grouped the granites into four types. The most commonly occurring is a grey granodiorite with small biotite schlieren and with minor cordierite and muscovite (type "Vire"), which has variants, such as porphyritic granodiorite, in the west of the Vire-Carolles Granite Complex, and minor leucogranodiorite and quartz diorite, in the east of the Vire-Carolles Granite Complex (type "Bois du Gast"). A white granodiorite (type "Louvignd") is distinguished both on colour and the absence ofcordierite, although it often contains small schlieren of biotite. The blue-grey granodiorite which occurs in the west of the Bonnemain Granite Complex (type "Lanhelin") is the most westerly component of the batholith. Finally, many of the complexes and the surrounding Brioverian succession metasediments are cut by small bosses of leucogranite. Basic complexes occur on the southern side of the Louvign6-Gorron Granite Complex, at Ern6e (Le Gall and Barrat, 1987), and also on the southern side of the Alexain-Deux Evailles-Izd Granite Complex, at Br6e (Le Gall and Mary, 1982, 1983). At Ernde, the basic complex comprises olivine gabbro, gabbro/ norite to diorite/quartz diorite and microdiorite with some sheets ofgranophyric granite, and at Br6e, the basic complex is made up of hornblende gabbro to quartz diorite with a narrow outer zone ofgranophyric granite. Although the relationship between the Ern6e Basic Complex and the Louvignd-Gorron Granite Complex cannot be established directly, Jonin has identified enclaves of microgabbro within the granodiorites of the granite complex. Le Gall and Barrat ( 1987 ) regard the Ern6e Complex as the surviving remnant of the mantle-derived magma which triggered crustal melting to give
5.26
F%O3 r
177 0.20 0.97 0.65 50,1
229 0.81 5.44 1.39 25.9
K/Rb Rb/Sr Ba/Sr
Ba/La
La/Y
752 58 637 62 17 9 15 8 22 29 11 76 104 381 7 73 23 72 246
1012 56 144 81 32 17 39 13 23 36 27 151 1648 186 12 93 28 105 191
3.84
73.98 0.59 12.65 2.50 1.21 0.07 1.25 2.21 2.99 1.62 0.12 0,87 100.06
706
Ba Ce CI Cr Cu Ga La Nb Nd Ni Pb Rb S Sr Th V Y Zn Zr
CaO Na20 KzO P20~ LOI Total
MgO
66.14 0.80 16.27 1.89 3.03 0.06 2.16 1.02 2.11 4.17 0.18 1.73 99.55
SiO2 TiO2 A1203 Fe203 FeO MnO
204
249 3.05 13.02 0.54 47.7
716 48 185 80 38 13 15 10 23 21 21 168 93 55 8 121 28 67 218
4.97
69.24 0.65 13.81 3.16 1.63 0.05 0.98 0.37 0.78 5.03 0.25 2.1~ 99.79
718
2t0 0.32 1.83 0.88 29,1
611 75 260 82 3 15 21 11 31 29 18 105 98 333 10 59 24 65 217
3.80
71.15 0.56 14.16 2.03 1.59 0.06 1.36 2.17 3.25 2.65 0.15 0.94 100.07
733
284 0.40 4.82 1.05 7/,3
1498 34 331 91 21 14 21 11 18 21 25 125 102 311 12 65 20 71 229
4.33
68.82 0.67 14.79 2.60 1.56 0.05 1.67 1.48 2.78 4.28 0.15 0,83 99.69
766
229 0.67 4.73 0.71 56.1
842 25 798 65 11 14 15 tl 15 23 22 120 131 178 9 84 21 67 206
4.11
70.70 0.63 14.01 1.89 2.00 0.04 1.56 1.09 3.02 3.31 0.16 1.14 99.55
761
250 0.62 5.06 0.50 81.17
972 37 134 77 29 17 12 11 19 33 25 119 802 19 8 76 24 70 203
4.54
68.52 0.70 14.94 1.95 2.33 0.05 1.85 1,12 3.17 3.59 0.17 1.38 99.78
205
232 0.68 4.39 0.78 44. -7
804 42 104 78 29 15 18 13 18 34 23 124 1308 183 7 102 23 80 185
5.27
67.06 0.75 15.46 2.08 2.87 0.05 2.11 1.08 3.11 3.46 0.I7 0,77 99.96
207
TABLE 1 Whole-rock geochemistry of representative rock types within the St. Malo migmatite belt
215 0.84 3,92 0,57 50.0
650 33 155 54 18 14 13 11 18 23 23 140 450 166 8 67 23 63 148
4.22
68.08 0.60 15.46 2.00 2.00 0.06 1.64 1.20 3.41 3.63 0.30 0.29 99.67
208
190 0.81 3.31 0.88 28.0
606 34 132 74 37 17 21 13 I5 35 2t 148 1385 183 9 87 24 79 161
5,25
65.36 0.79 16.29 2.12 2.81 0.06 2.13 1.31 3.34 3.39 0.25 1,68 99.54
210
206 1.(t4 4.02 0,5[! 405)
491 42 713 71 24 17 12 12 2t 28 19 127 702 122 10 102 24 79 176
4.88
67.44 0.68 15.54 3.04 1.66 0.05 1.81 0.87 3.15 3.15 0.26 1,89 99.55
704
2i3 0.50 2.35 0,69 29.9
538 30 290 66 67 19 18 11 18 15 20 115 122 229 8 76 26 60 144
4.70
66.69 0.64 16.36 2.63 1.86 0.06 1.83 1.81 3.18 2.95 0.32 0,~9 99.92
721
237 1.01 5.62 0.49 36."
770 74 662 35 18 24 21 25 37 17 26 138 116 137 13 75 43 86 324
5.48
66.48 0.71 15.30 2.58 2.61 0.07 1.40 1.72 3.42 3.94 0.32 1,13 99.69
757
226 0.69 4,06 0.75 405
729 40 562 60 25 16 18 11 20 17 17 124 490 179 7 80 24 65 175
4.44
68.30 0.64 15.38 2.27 1.95 0.05 1.73 1.12 3.34 3.37 0.15 1.50 99.80
759
215 0.68 3.33 0,55 49.2
590 43 310 67 18 t6 12 11 26 17 18 121 403 177 7 87 22 62 163
4.67
68.00 0.66 15.46 2.22 2.21 0.05 1.79 1.26 3.34 3.14 0.20 1.42 99.75
760
> z
z
©
231 0.65 4.23 0.62 55.9
K/Rb Rb/Sr Ba/Sr La/Y Ba/La
285 2.88 13.1 0.50 15.6
669 113 257 13 7 25 43 31 53 5 33 147 93 51 11 3 86 55 207
2.56
72.65 0.23 14,05 1.93 0.56 0.04 0.16 0.73 3.38 5.05 0.12 0,64 99.54
345
217 0.61 1.98 1.9 15.0
285 29 758 44 14 14 19 6 10 13 16 88 105 144 8 49 10 53 133
2.72
69.82 0.42 16.00 2.09 0.56 0.03 1.37 1.78 4.34 2.30 0.21 1,12 100.04
358
211 5.21 10.6 0.25 18.0
359 73 352 20 3 23 20 26 37 5 30 177 112 34 9 5 79 62 135
2.01
74.26 0.22 13.81 1.47 0.49 0.05 0.16 0.60 3.61 4.49 0.10 0.52 99.78
360
207 0.97 3.81 0.64 73.6
515 30 205 44 9 17 7 6 21 15 19 131 97 135 8 42 11 54 125
2.45
70.00 0.44 15.68 2.10 0.32 0.03 1.39 1.91 3.59 3.26 0.18 0.94 99.84
701
194 0.92 3.36 1.08 40.1
521 37 316 50 8 16 13 6 20 18 19 143 111 155 8 30 12 65 133
2.77
69.15 0.45 15.86 2.05 0.65 0.04 1.55 1.83 3.69 3.35 0.19 0.95 99.75
703
300 0.52 4.36 0.96 12.9
1054 155 252 31 11 14 82 11 57 8 31 127 109 242 27 32 85 40 331
2.45
71,86 0.38 14.83 1.72 0.66 0.03 0.72 1.06 3.14 4.59 0.14 0.82 99.97
720
288 0.86 4.77 0.10 630
630 25 154 30 6 13 l 5 8 11 22 114 91 132 7 18 10 40 100
1.76
71.91 0.29 15.31 1.39 0.33 0.03 0.95 1.45 3.40 3.95 0.19 0.88 100.07
758
129 3.24 0.51 -
34 0 ~98 17 0 14 0 9 1 2 9 217 144 67 1 1 8 8 23
0.32
74.12 0.11 15.28 0.32 0.0 0.04 0.00 0.37 5.02 3.37 0.71 0.Z6 100.11
211
105 4.24 0.28
19 6 277 18 1 15 0 8 14 3 9 288 110 68 2 0 8 44 32
0.71
74.22 0.06 15.36 0.71 0.0 0.08 0.02 0.34 4.32 3.63 0.66 0.61 100.00
212
331 0.61 4.95 1.10 797
797 8 163 23 1 13 1 5 7 8 27 98 106 161 6 8 10 30 73
1.33
73.27 0.21 14.92 1.33 0.0 0.03 0.52 1.12 3.67 3.91 0.20 0.81 100.00
708
242 0.17 0.88 0.79 22.3
245 35 554 20 2 15 11 4 29 7 13 47 112 279 2 15 14 31 107
1.78
73.53 0.28 15.11 1.47 0.28 0.03 0.62 2.18 4.58 1.37 0.09 0.71 100.23
740
239 0.13 0.42 0.17 64.0
128 25 164 23 0 16 2 3 22 6 4 41 119 305 1 16 12 29 95
1.64
71.64 0.34 16.00 1.44 0.18 0.02 0.69 3.10 4.68 1.18 0.10 (X~9 99.97
742
508 0.41 7.34 0.08 756.5
1513 0 415 26 3 7 2 3 16 4 40 84 103 206 1 7 24 7 37
0.43
75.51 0.07 14.05 0.43 0.0 0.01 0.12 0.72 2.81 5.14 0.19 0.68 99.73
745
Oxide data are quoted in weight percent oxide, elemental data are quoted in parts per million and values below the limit of determination are quoted as zero. Metasediments: 204 = Semi-pelitic schist, le Minihic sur Rance; 706 = Psammite layer in metatexite, St. Jacut de la Mer; 718 = Psammite enclave in diatexite, east of Rothn6uf; 733= Psammitic schist, north of St. Suliac sur Rance; 766=Psammitic schist, La Rance. Metalexite: 761 = n o r t h of St. Briac. Inhomogeneous diatexites: 205, 207, 208, 210=Montagne St. Joseph Quarry, east of St. Malo; 704=St. Jacut de la Mer; 721 =northwest of Cancale; 757=north of lie des H~bihens; 759 = north of St. Briac; 760 = north of St. Briac; 765 = north of N.D. du Guildo. Homogeneous diatexites/anatectie granites: 345 = northwest lle des H6bihens; 358=St. Jacut de la Mer; 360=Iie de la Colombi6re; 701 =St. Jacut de la Mer; 703=St. Jacut de la Mer; 720=northwest of Cancale; 758=west of St. Briac. Granitoid sheets: 211 = Pegmatite in Brioverian schists, north of N.D. du Guildo; 212 = Tourmaline leueogranite in Brioverian schists, north of N.D. du Guildo; 708=Granite in metatexite, St. Jacut de la Mer; 740, 742=Trondhjemites in psammitic schist east of Tr6gonde la Rance: 745=Alkaline granite in metatexite, southwest of Lancieux.
727 17 205 61 19 15 13 10 18 17 22 112 260 172 6 103 21 53 149
3.86
FezO3r
Ba Ce C1 Cr Cu Ga La Nb Nd Ni Pb Rb S Sr Th V Y Zn Zr
69.35 0.65 15.23 2.28 1.42 0.05 0.58 1.03 3.30 3.12 0.25 1,~5 99.80
SiO2 TiOz A1~O3 Fe203 FeO MnO MgO CaO Na:O K20 P205 LOI Total
765
TABLE 1 (continued)
%o
Z
©
N
E
©
g
-e
? z ©
t"
z
g
©
z
z
©
>
621 51 26 6 21 9 17 7 180 106 10 10 2 36 31 54 182
197 1.70 5.86 0,68 29.6
K/Rb Rb/Sr Ba/gr La/Y Ba/La
70.50 0.48 14.54 3.46 0.06 0.89 1.66 3.34 4.28 0.17 0.48 99.84
Ba Ce Cr Hf La Nb Nd Ni Rb Sr Sc Th U V Y Zn Zr
SiO2 TiO2 A1203 Fe203 T MnO MgO CaO Na20 K20 P205 LOI Total
1
189 1.65 5.97 0.41 48.2
675 42 23 9 14 11 17 8 187 113 15 7 4 33 34 57 177
70.49 0.51 14.61 3.48 0.05 0.91 1.65 3.34 4.26 0.20 0.50 99.99
2
224 1.61 6.26 0.53 36.5
620 27 26 6 17 10 14 7 159 99 10 7 3 35 32 47 171
70.63 0.48 14.37 3.42 0.06 0.81 1.63 3.39 4.29 0.18 0.45 99.71
3
229 1.76 5.23 0.58 59.7
836 56 50 8 14 12 28 10 122 160 21 7 3 71 24 74 250
65.77 0.92 15.52 5.64 0,07 1.73 2.84 3.60 3.36 0.22 0.43 100.09
4
234 0.78 5.22 0.78 29.1
814 42 57 6 28 13 25 15 122 156 19 12 3 84 36 76 246
65.79 0.93 15.40 5.69 0.07 1.85 2.70 3.53 3.44 0.20 0.45 100.06
5
234 1.00 5.51 0.81 28.8
749 54 62 5 26 I1 23 15 136 136 13 6 5 77 32 69 199
66.92 0.80 15.33 5.08 0.07 1.84 1.89 3.19 3.84 0.21 0.88 100.04
6
232 1,18 7.06 0.72 34.0
883 44 44 8 26 11 23 11 148 125 15 9 4 61 36 64 212
68.39 0.66 14.98 4.32 0.07 1.35 1.72 3.31 4.14 0.21 0.71 99.85
34
213 1.73 7.21 0.77 28.5
685 55 20 5 24 10 27 7 164 95 i0 7 6 22 31 39 166
71.80 0.36 14.41 2.75 0.03 0.69 1.34 3.46 4.20 0.20 0.83 100.08
40
TABLE 2 Whole-rock geochemistry of representative granites within the Mancellian Batholith
212 1.73 7.37 0,84 26,7
693 63 22 7 26 10 24 6 163 94 9 11 4 22 31 45 161
71,31 0.35 14.40 2.84 0.05 0.67 1.20 3.47 4.16 0.20 0.94 99.58
41
135 6.5J 3.41 -
140 17 13 5 0 8 3 0 267 41 8 0 3 9 13 18 45
74.53 0.12 14.13 1.16 0.03 0.12 0.41 3.82 4.33 0.42 0.74 99.81
10
226 1.43 6.1t 0.60 31.1
654 46 25 6 21 10 23 4 153 107 11 9 3 34 35 53 181
69.86 0.51 14.66 3.47 0.06 0.92 1.80 3.51 4.16 0.17 0.63 99.75
11
230 1.07 5.88 0.75 35.8
752 60 36 5 2l 11 25 5 137 128 16 10 2 51 28 54 197
68.20 0.66 15.26 4.19 0.04 1.16 2.38 3.68 3.79 0.18 0.58 100.13
12
226 1.0l 5,93 0.52 47.1
801 40 44 6 17 12 22 12 137 135 14 7 2 65 33 67 212
67.10 0.78 15.21 4.98 0.08 1.55 2.22 3.38 3.73 0.23 0.65 99.93
13
224 1.03 6.02 1.03 23.7
807 45 43 8 34 I1 24 8 138 134 12 9 3 54 33 59 217
67.60 0.72 15.19 4.48 0.07 1.35 2.33 3.58 3.72 0.20 0.75 100.00
14
g
Z > Z
o
-~ c~ -~
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIFOF FRANCE
405
~o.~¸
~e~eeeo~4ee~
I
I
•~ 0o 05
- - o m ~ o o m ~ O ~ N I
I
-.o e-
i
e~
-
d d ~ M d £ d M M d
~
"--
~
~
I
ca
dd4~d~Mdddd~e
~d d ~ ~ d 2_ M ~ M d d
~ d ~ d 2 d ~ M d d ~ e-
8
~ ," " ~ ~ I.,, c~ . -
'~
<
I
Q~
0
•~ - 6 II © ~ ~r~
~
406
rise to the granodiorites of the Mancellian Batholith. In contrast, the Br6e Basic Complex appears to have been emplaced into the contact aureole of the Alexain-Deux Evailles-Iz6 Granite Complex (Le Gall and Mary, 1982). If this latter interpretation is correct, it suggests that some of the basic rocks at least were emplaced after the main granitic components of the Mancellian Batholith. The Mancellian granites have developed contact aureoles in the surrounding Brioverian succession metasediments and are overlain unconformably by Cambrian strata (Chauris, 1956; Graindor, 1953). Although the granite complexes were probably emplaced sometime around the Precambrian-Cambrian boundary, the precise age of the Mancellian granites has proved difficult to determine, even though a variety of isotopic methods (Jonin and Vidal, 1975; Pasteels and Dor6, 1982; Autran et al., 1983) has been applied to the problem. A slightly younger age than that of 540 Ma for the Precambrian-Cambrian boundary and an episodic intrusive sequence are suggested by some of the Rb-Sr whole rock data of Jonin and Vidal ( 1975 ). An age of 525 + 6, derived from a composite R b - S r whole rock isochron, is indicated for leucogranites which form a volumetrically minor c o m p o n e n t of some of the granite complexes, and an age of 502 + 37 is derived for the Lanhelin blue granodiorite of the Bonnemain Granite (ages recalculated from data in Jonin and Vidal, 1975). Data from various parts of the Vire-Caroltes Granite (from Jonin and Vidal, 1975) give an age of over 600 Ma with a Sr~ which is impossibly low. Recently, Graviou et al. (1988) have presented a Rb-Sr whole rock errorchron based upon samples from the "Vire" type granodiorite using the published R b - S r analyses of Adams (1967) and Jonin and Vidal (1975), in combination with 14 unpublished data obtained by Chariot in collaboration with the BRGM. The resultant age of 551 + 2 8 Ma is certainly plausible and the Sr~ of 0.7077 _+ 15 is reasonable. However, the combination of ana-
M. B R O W N A N D R S. D ' L E M O S
lytical work from a 20-year period without regard either to the analytical imprecision ~J early isotopic determinations or to the lack ~f interlaboratory correlation leaves some uncertainty and, we presume, partly accounts for the high error on this age. Pasteels and Dore (1982) report two monazite ages. one of 547 + 10 Ma from the porphyritic granodiorite at the western end of the Vire-Carolles Granite and one of 542 + 9 from a leucogranite associated with the Vire-Carolles Granite, which are consistent with the Mancellian granites being coeval with the anatectic granites of the St. Malo migmatite belt. Thus we follow Pasteels and Dor6 (1982) and regard the age ot" the main components of the Mancellian granites as being ~ 540 Ma. This is consistent with recent 4°Ar/39Ar muscovite cooling ages of 527 and 521 Ma (D'Lemos et al., 1990)~
Geochemistry Analytical techniques and results For the St. Malo migmatite belt, the 29 samples listed in Brown (1974) were reanalyzed by X R F spectrometry in the Geochemistry Laboratories of the University of Keele using their standard analytical procedures (Table 1 )° For the Mancellian granites, 28 representative samples from a larger suite collected in 1987 were analyzed in the XRF Laboratory at the University of Nottingham using their standard analytical procedures (Table 2 ). In both cases several international standards were analyzed as a check on accuracy, which was satisfactory in both cases.
Comparative geochemisto, Table 3 gives the mean and standard deviation for three samples from sections through the Brioverian succession sandstones, one from the SBT (Binic) and two from the MT (Vi-
THE CADOMIANGRANITESOF MANCELLIA,NORTHEASTARMORICAN MASSIFOF FRANCE
407
TABLE 3 Average geochemical compositions of Brioverian succession sandstones (from Denis and Dabard, 1988 ), St. Malo diatexites and Mancellian granites
Number of analyses:
1
2
3
4
5
8(3)
10
10
17
22
Mean and standard
deviation:
k
cr
k
cr
x
cr
R
cr
X
o-
SiO2 TiO2 AI203 Fe,O 3 MnO MgO CaO Na20 K,O P_~Os Ba Ce La Nb Nd Ni Rb Sr Th V Y Zn Zr K/Rb Rb/Sr Ba/Sr Ba/La La/Y
67.34 0.68 14.35 5.19 0.29 1.68 1.09 3.44 2,30 0,30 787
3.70 0.09 1.04 1.16 0.60 0.87 0.87 0.53 0.47 0.54 213
69.52 0.56 14.33 4.81 0.05 1.26 0.31 3.18 2.89 0.13 479
4.79 0.18 1.74 1.44 0.01 0.58 0.19 0.40 0.62 0.05 126
72.43 0.53 12.65 5.27 0.04 1.01 0.20 2.46 1.97 0.09 408
2.94 0.09 0.99 0.82 0.01 0.40 0.06 0.47 0.44 0.02 170
69.1 l 0.54 15.35 3.77 0.05 1.42 1.29 3.41 3.57 0.21 642 51 20 13 24 18 129 155 10 61 32 63 176 230 1.15 4.84 0.69 39.6*
2.41 0.19 0.67 1.27 0.01 0.60 0.40 0.29 0.67 0.07 196 35 18 7 14 9 19 53 5 34 26 13 63 33 1.18 2.82 0.39 19.9
69.11 0.61 14.82 4.00 0.05 1.18 1.92 3.46 3.92 0.20 697 47 20 10 22 8 151 119 8 49 33 56 187 216 1.47 5.84 0.64 36.2
2.24 0.20 0.45 1.08 0.01 0.43 0.55 0.14 0.30 0.06 172 1l 7 2 6 4 31 26 2 19 6 13 43 31 1.17 0.97 0.18 10.5
68 152
11 49
143
52
66
27
l=Brioverian sandstones, Binic (8 analyses for oxides, 3 analyses for elements); 2=Brioverian sandstones, Vidouville; 3 = Brioverian sandstones, Athis; 4 = St. Malo diatexites; 5 = Mancellian granites. * Analysis 758 excluded.
the main types of Mancellian granites. The mean chemical composition of the St. Malo diatexites is compared with that for each of the Brioverian samples, and the mean chemical composition of the Mancellian granites is compared with that for the St. Malo diatexites, in all cases using a t-test (Davis, 1973 ). The ttest assesses the hypothesis that the mean of the population from which the first sample was drawn is the same as the mean of the parent population of the second sample. This hypothesis is posed against the alternative that the two
population means are not equal. Below we address the question: is the difference between the means of two samples significant at a particular level? At the 10% level of significance, for the comparison between the Brioverian succession sandstones and the St. Malo diatexites, the values of t for a two-tailed test with 23 degrees of freedom (sample B, 18 degrees of freedom for Ba, Rb and Sr) and 25 degrees of freedom (samples V and A) do not fall into either critical region for most oxides/elements consid-
408
M. BROWN ~tND RS. D'I,EMOS
ered (Table 4) and the null hypothesis cannot be rejected. Exceptions are Rb for sample B, CaO for sample V, and A1203 and CaO for sample A. Even at the 20% level of significance, the list of oxides/elements that falls into the critical region is only expanded to include K20 and Rb for sample A, still only CaO for sample V, and A1203, CaO, Na20, K20 and P205 for sample A. Thus we conclude there is no evidence to suggest that samples B and V come from populations having significantly different means than the St. Malo diatexites, except for K20 and Rb (sample B) and CaO TABLE 4 Values of t for the appropriate number of degrees of freedom 0') for the comparisons between the St. Malo diatexites (SMMB) and three samples of Brioverian succession sandstones (B, V and A), and between the St. Malo diatexites (SMMB) and the Mancellian granites (MG)
SiO2 TiO2 AI20~ Fe20~ MnO MgO CaO Na20
K20 P2Os Ba Ce La Nb Nd Ni Rb Sr Th v Y Zn Zr t (oe= 10%) t (c~=20%)
SMMB vs. B
SMMB vs. V
SMMB vs.-X
SMMB xs. MG
0.40 0.55 0.82 0.75 0.47 0.25 0.22 0.05 0.35 0.19 0.58
0.07 0.07 0.55 0.49 0.00 0.17 1.82 0.44 0.66 0.80 0.59
0.80 0.04 2, i 3 0.84 0.63 0.48 2.14 1.64 1.69 1.32 0.79
2.64 0.05
0.14
t.24
23 ~ 1.71 b 1.32 ~
25 1.71 1.32
25 1.71 1.32
0.00 0.20 0.54 0.1 I 0.00 0.27 0.73 0.13 0.40 0.09 0. t 7 0.09 0.00 0..35 0. t I 0.86 0.47 0.51 0.31 0.26 0.03 0.31 0.12 37 1.69 1.31
Brioverian succession samples: B=Binic: V=Vidouville; A=Athis. a 18 for Ba, Rb and Sr; ~ 1.73 for Ba, Rb and Sr; ': 1.33 for Ba, Rb and Sr.
(sample V). Note that this does not mean that the St. Malo diatexites were derived from Brioverian succession sandstone similar in composition to the samples considered herein, but only that there is no basis in comparative geochemistry to reject such a model. At the 20% level of significance, for the comparison between the St. Malo diatexites and the Mancellian granites, the values of t lbr a twotailed test with 37 degrees of freedom do not fall into either critical region for any oxide/ element considered (Table 4) and the null hypothesis cannot be rejected. Thus we conclude there is no evidence to suggest that the samples being compared come from populations having different means. Note that we are not saying that the chemical composition of the Mancellian granites is the same as that of the St. Malo diatexites, but simply that they are not significantly different and, therefore, that a model which proposes a similar petrogenesis for the two samples cannot be rejected on the basis of comparative geochemist~,. In comparison with average compositions of granitic rocks from late Cadomian post-tectonic plutonic complexes from within the SBT (Table 5), neither the St. Malo diatexites nor the Mancellian granites show much similarity with the Cobo Granite or the L'Ancresse Granodiorite of Guernsey. The deeper crustal level exposed in south Guernsey, where lcartian basement is seen, and the scarcity of" Brioverian succession rocks on Guernsey, together with their isotope geochemistry, suggest that these late Cadomian granites might be derived from a different source (R.S. D'Lemos and M. Brown, unpubl, data). The late Cadomian granites are also associated with a volumetrically larger calc-alkaline plutonic complex, so that mantle-derived magma advecting heat into the crust to cause partial melting is a real possibility. However, the granites from Jersey, which is thought by Brown et al. ( 1990 ) to represent a higher structural level, are emplaced into Brioverian succession metasediments at very low grade and develop cordierite
409
THE CADOMIANGRANITES OF MANCELLIA,NORTHEASTARMORICANMASSIFOF FRANCE
TABLE 5 Average chemical composition of granitic rocks from late Cadomian post-tectonic plutonic complexes of the Channel Islands, St. Bricuc Terrane, North Armorican Massif, France
1
2
3
4
5
6
MgO CaO Na20 K,O P~()~ Ba ('e ka Nb Nd
76.01 0.01 13.36 1.39 0.05 0.26 0.47 4.72 4.62 0.03 688 38 19 11 I1
71.78 0.29 14.44 2.82 0.06 0.65 2.09 3.93 3.04 0.11 926 50 23 10 22
68.67 0.54 15.34 4.21 0.07 0.83 1.95 4.59 3.60 0.19 599 59 27 12 25
74.81 0.20 13.19 2,05 0.03 0.26 0.79 4,13 4,46 0,08 308 63 29 9 23
73.93 0.26 13.75 2.61 0.04 0.37 0.97 3.95 4.82 0.11 380 101 42 23 40
74.44 0.19 13.53 1.94 0.03 0.24 0.59 3.93 5.04 0.08 323 96 40 19 38
Ni
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
Rb Sr Th V Y Zn Zr K/Rb Rb/Sr Ba/Sr ka/Y Ba/La Rb-Sr Age
131 96 N.A, N.A, 14 14 86 293 1.36 7.17 1.36 36.2 4 9 6 ± 13
68 323 N.A. N.A. 22 21 138 371 0.21 2.87 1.05 40.3 -
123 169 N,A. 37 31 47 237 243 0.73 3.54 0.87 22.2 4 6 5 _+ 10
190 54 N.A. 14 34 22 173 195 3.52 5.70 0.85 10.6 438_+ 17
224 120 N.A. 19 47 20 190 179 1.87 3.17 0.89 9.05 550_+ 12
200 84 N.A. 16 43 19 165 209 2.38 2.85 0.93 8.08 4 8 3 ± 13
SiO2 TiO2 AI~O~ FQ,O~v
MnO
Guernsey: 1 = Cobo Granite ( D'Lemos, 1987 ); 2 = L'Ancresse Granodiorite (Brown and D'Lemos, unpublished data ). Jersey, North-west Granite Complex: 3 = Porphyritic Granodiorite (Bland, 1985 ); 4 = Coarse Granite (Bland, 1985 ); South-west Granite Complex: 5 = Porphyritic Granite (Bland, 1985 ): 6 = Coarse Granite ( Bland, 1985 ). N.A. = not analyzed.
hornfelses similar to those around the Mancellian Granite Complexes. Even so, there are few close similarities in geochemistry with the Mancellian granites, and a continental arc origin which involves both mantle and crustal sources is preferred (R.S. D'Lemos and M. Brown, unpubl, data). A comparison of mean chemical compositions suggests that the Mancellian granites have much closer similarities with the diatexites of the St. Malo migmatite belt than with the late Cadomian post tectonic granites of the SBT. A closer geochemical comparison between the Mancellian granites and the St. Malo migmatire belt is undertaken, largely using bivariate
plots. This is followed by a consideration of whether the established petrogenetic model for the migmatites, of anatexis of Brioverian succession metasediments, is applicable to the Mancellian granites. Geochemical plots are presented in pairs, consisting of one plot for the St. Malo migmatite belt and one plot for the Mancellian granites.
"S-type"granites Plots of Na20 vs. K20, A / N K vs. A/CNK and A / C N K vs. SiO2 are presented in Figs. 810. On a plot of Na20 vs. KzO, the Mancellian granites plot within the field encompassed by the St. Malo migmatites (Fig. 8). However,
410
M BROWN~kN[)RS. I)'L[LM()5,
MANCELLIAN GRANITES
ST. MALO MIGMATITE BELT
f
s.o
•
• 4.0~--
.
a
2-0 i
/,/'
[
j
In
4.0~-
°
I
oA~~
O
i
z
t
¢M3.0 Ft~
"
-
./ ! i
i
J"
I
i]
5.0~-
/
•
/-
2.ot-
i/
O
10
1.0
I
2.0
___
J ........
3.0
~
40
±
50
....
K20
10
i
2.0
_L...............J_ 3C
4 0
K20
Fig. 8. Plot of Na20 vs. K20 for representative samples from the St. Malo migmatite belt and the Manceltian gramtes. In this and subsequent figures, for the St. Malo migmatite belt, filled circles represent both inhomogeneous and homogeneous diatexites/anatectic granites, diamonds represent granitoid sheets which cut the metatexites and the metasediments, the triangle represents a bulk sample of metatexite and the inverted triangles represent metasediments, both from outside the metatexites and from enclaves from within the diatexites. In this and subsequent figures, for the Mancellian granites, filled circles represent the main granite types (type "Vire", type "Louvign6"" and type "Lanh61in'" ), diamonds reprcscnl leucogranites and squares represent samples from the Bois du Gast, at the eastern end of the Vire-Carolles Granite Complex. The large upright crosses represent the mean and standard deviation for three samples of Brioverian succession sandstones from Denis and Dabard (1988). B is the sample from Binic, V is the sample from Vidouville and 4 is the sample from Athis.
both the St. Malo migmatites and the Mancellian granites have higher K 2 0 contents than the Brioverian succession sandstones which may represent the composition of the protolith. Although the five metasediments sampled from within the migmatites are very variable in composition, it is noticeable that two of them have similar Na20 and K 2 0 contents to the samples of the Brioverian succession sandstones. Both suites of rocks are peraluminous using Shand's index (Fig. 9), but in terms of aluminum saturation index ( A / C N K ) vs. SiO2 (Fig. 10) the two suites only overlap at SiO2 > 70%, and below this value the St. Malo rocks trend to higher values of A / C N K , whereas the Mancellian rocks decrease marginally with decreasing SiO2 content. In terms
of Shand's index and the tectonic discrimination of granitoids developed by Maniar and Piccoli ( 1989 ), both suites of rocks overlap the peraluminous part of the Continental Arc Granitoids as well as falling within the field of Continental Collision Granites, although whether this reflects tectonic setting or source characteristics remains an open question. In terms of alkalis, the St. Malo suite of rocks shows minor overlap with the field of "S-type'" granites as discriminated within the Lachlan Fold Belt of southeastern Australia (White and Chappell, 1983, fig. 5), and the Mancellian granites show no overlap at all. This demonstrates the point made frequently by White and Chappell (e.g. 1988; see also ChappelI, 1984) that the sedimentary source of the Lachlan "S-
411
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMOR1CAN MASSIF OF FRANCE
ST. MALO M I G M A T I T E BELT Peraluminous
2.2 _ Metaluminous 2.0 1.8 ~1.6 Z ' ~ 1.4 1.2 1.0 0.8
I 0.8
1.0
I 1.2
I 1.4
I 1.6
I 1.8
I 2.0
A/CNK MANCELLIAN 2.2 - Metaluminous
GRANITES
Peraluminous
I
2.01 1.8 1.6 Z 1.4 1,2 !
1.0
0.8
1.0
1 1.2
I 1.4
I 1.6
I 1.8
I 2.0
A/CNK
Fig. 9. Plot of A/NK vs. A/CNK for the St. Malo rnigmatitebelt and the Mancelliangranites.
type" granites is unusual in its chemistry and casts doubt on this frequently used parameter to discriminate "S-type" from "I-type" granites. Furthermore, the genetic relationship between the "S-type" and "l-type" classification scheme of Chappell and White (1974) and the restite model for granite genesis (White and Chappell, 1977; Chappell et al., 1987; Chappell and Stephens, 1988; White and Chappell,
1988 ) with which it is intimately associated, is commonly neglected. It is clear from Fig. 10 that chemical compositions of the diatexites from the St. Malo migmatite belt could have been controlled by restite unmixing since the increase in A / C N K with decreasing SiO2 here could reflect increasing biotite. However, it is also clear from Fig. 10 that the restite unmixing model may not apply to those St. Malo
412
M. BROWNAND R,S. [)'LEM( IS ST. MALO
I
1.8 ,¢ Z 0
1.6
MIGMATITE
BELT
ST. MALO
MIGMATITE
BELT
0.9
•
•
0.8
• °1be
1.4
•
4'
°'
•
1.2
4
1,0
I.-
i I
I 62
I 64
I
l
I
66
68
70
I ..... L
72
74
0.4
4,
0.3 -
q,
__L__~,:'
76
0,2
SiO2
MANCELLIAN
•
ql
l
0.8
•
-
4,
o.,i
GRANITES
•
' 50
•
.l 150
100
__J__ 200
L~ . . . . . 250
; ......... ;!30
Zr
1.8
MANCELLIAN
GRANITES
1.6 ,,z'
0.9
1.4
Z
0~.2
o ddl8~
ee •
41, 41,
0.8
tt
0.7
1.0 i
1
0.8 0.6
%
L
0.6
&
Od
00.5 I
62
d 64
I 66
-.L-_ 68 SiO
l ~ 70
..... ~ .... 72 74
; .... 76
J
0.4 -
2
0. 3~-
Fig. 10. Plot of A / C N K ( a l u m i n i u m saturation index), corrected for P205, v s . SiO2 for the St. Malo rnigmatite belt a n d t h e Mancellian granites.
rocks which plot at values of SiO2 above 70% and does not apply to the whole suite of Mancellian granites since A / C N K decreases with decreasing SiO2. The distinction between the behaviour of the diatexites on the one hand and the Mancellian suite o f rocks on the other is interesting since it suggests that restite unmixing, perhaps by filter pressing, is a viable mechanism whilst convection homogenization has not occurred, but that once convection overturn has homogenized melt and residuum,
•
°2t olP 0,1
I
v
50
100
i
150
200
_J 250
.... ± . . . . . . 300
Zr Fig. 11. Plot of TiO2 v s . Zr for the St. Malo migmatite belt and the Mancellian
granites.
the system behaves as a "normal igneous" one and the chemical variation may be controlled by crystal fractionation rather than restite unmixing.
4 13
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. MALO MIGIMATITE BELT
ST. MALO MIGIMATITE BELT
300
200
O~
• •
•
•
•
200
100
_
I 62
I 64
I
I
I
I
I
I
66
68
70
72
74
76
lOO
8iO2
•
MANCELLIAN GRANITES
I
I 64
62
I
I
l
66
68
70
A
v
I 72
I
I
74
76
SiO 2 MANCELLIAN GRANITES 200 ¢,,
#
100
200
BB
eqP#°l, • I 62
1 64
I 66
I 68
I 70
l 72
I 74
I 76
__
lOO
SiO 2 Fig. 12. Plot of Rb vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
Trace element distributions
On plots of TiO2 vs. Zr (Fig. 11 ), both the diatexites from the St. Malo migmatite belt and the main types of granite within the Mancellian Batholith define steep positive covariation of strongly decreasing TiO2 with decreasing Zr and increasing SiO2 content. However, the absolute abundance of Zr in the Mancellian granites is higher at given TiO2 than in the diatexites, although values lie intermediate between the diatexites and the analyzed metasediments from the St. Malo migmatite belt (Fig.
I
1
1
I
1
I
I
62
64
66
68
70
72
74
v v
76
SiO2 Fig. 13. Plot of Sr vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
11 ). The granitoid sheets from within the St. Malo migmatite belt and the leucogranites and Bois du Gast granodiorites from the Mancellian Batholith lie on identical but shallower trends than either the diatexites or the main types of granites (Fig. 11 ). These features suggest that similar processes control the distribution of these elements in each suite. For the diatexites, Rb varies little with in-
414
m B R O W N A N [ ) R.S. I)'LEMt )5,
ST. MALO MIGMATITE BELT
•
1500
41,
1400 1300 1200 1100 1000 900
~
800 700
el
6OO 500 400
A
• I
300 200 IO0 __
1 62
I 64
1 66
_L~ 68
I 70
l 72
~1~ 74
I 76
SiO 2
MANCELLIAN GRANITE,S 900
| ooO
800
,,'It
700 600
~
500
III u
400
•
300 200 100
I 62
I 64
I 66
1 68
1__ 70
SiO 2
72
74
76
creasing SiO2, while the granitoid sheets have variable abundances according to type (Fig. 12). For the main types of granite within the Mancellian Batholith, Rb increases with increasing SIO2, while the leucogranites are variable and the Bois du Gast granodiorites have their own unique abundances. Sr decreases within the diatexites with increasing SiO2 while the granitoid sheets again exhibit variable abundances (Fig. 13), and Sr decreases within the Mancellian granites with increasing SiO:, except that the Bois du Gast granodiorites plot uniquely (Fig. 13). At given SiO2 content, Rb is slightly higher and Sr lower for the Mancellian granites than for the St. Mato diatexites (Table 1, Figs. 12 and 13). Ba has similar levels of abundance in both suites of rocks and decreases dramatically with increasing SiO2 content (Fig. 14). The covariation of Rb with Sr and Ba with Sr is shown on logarithmic plots in Figs. 15 and 17, respectively. Rayleigh fractionation generates straight line arrays of data points from comagmatic products of fractional crystallization (for a constant bulk distribution coefficient) on such bivariate logarithmic plots (Figs. 16 and 18). Variable separation of cumulus crystals and interstitial liquids within plutonic suites causes lateral scatter of the points around such arrays (McCarthy and Hasty, 1976). As with all coarsely crystalline plutonic suites, it is difficult to model trace element distributions quantitatively because their textures lead to uncertainty over the definite identification of liquidus minerals and, since the compositions are (indeterminate) mixtures of crystals and variably evolved residual liquid, no unequivocal samples of liquids are available (cf. McCarthy and Hasty, 1976; McCarthy and Groves, 1979). Additionally, silica contents in these rock suites extend up to values generally regarded as "high silica magmas". In very high silica systems it is Fig. 14. Plot of Ba vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
415
THE CADOM1AN GRANITES OF MANCELL1A, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. M A L O M I G M A T I T E BELT
500
100
II,
50
Rb
Sr
[
[
I
[
I
5
lo
50
100
500
MANCELLIAN GRANITES
500
,% 100
I
50
Rb
Sr
[
I
I
I
I
5
10
50
100
soo
Fig, 15. Logarithmic plot of Rb vs. Sr for the St. Malo migmatite belt and the Mancellian granites.
the structure and volatile content of the melt which are the dominant controls of trace element partitioning, whereas in less silicic magmas, partitioning between crystals and melt is governed primarily by crystal structure constraints. Partition coefficients for trace elements in high silica rhyolites are much larger than those generally accepted for less silicic compositions (Mahood and Hildreth, 1983; Nash and Crecraft, 1985). For example, elements consistently enriched in biotite over the
host liquid include Rb and Ba (Hanson, 1978 ), although the partition coefficient for Ba in particular shows substantial variation (Nash and Crecraft, 1985 ). Similarly, Ba substantially and Sr and Rb generally are enriched in K-feldspar over the host liquid (Nash and Crecraft, 1985 ), although Ba again shows substantial variation and Rb is enriched in K-feldspar over liquid rather than depleted in high silica systems (cf. Hanson, 1978 ). In plagioclase, Sr is strongly to very strongly enriched relative to the host liq-
416
M. BROWN
J
I
-
r
I
17
"-----4 0
1
-
-
-
-
AND
RS,
t ) : I , E M ( ~,~
~ ~0
/
3o
"-~--g~
PI
-~-
90
-
so./,/" ~/.-~
~"~"I~
90 f _ ~~ 90 90 90' 7~350
'
50
-
70 -
0
j
1
-
I
5
,
I 1
I
5
I 1
1
1
5
Fig. 16. Logarithmic plot of Rb vs. Sr to illustrate the effect of tractional crystallization of hornblende (Hb). c imopyroxene (Cpx), plagioclase (PI), K-feldspar (Kts) or biotite (Bt) on liquid compositions using limiting values ('or the distribution coefficients in granitoids from Nagasawa and Schnetzler ( 1971 ; Hb and PI), Philpotts and Schnetzler ( !070; Upx
and Kfs), and Nash and Crecraft ( 1985; Bt, Pl and/~'). Numbers represent the percentage of liquid which remains after each fractional crystallization step indicated by a cross tick on each fractionation line. Note how different distribution coefficients for the same mineral alter both the slope and the rate of change for fractionation by crystallization of thai mineral. uid (Hanson, 1978; Nash and Crecraft, 1985 ), Rb is always depleted but Ba m a y change in high silica systems from depleted to enriched (Nash and Crecraft, 1985). Given the problems inherent in cumulus crystal/residual liquid mixtures and the additional problems inherent in any trace element modelling of melting and fractionation trends in silicic igneous suites (determination o f partition coefficients, intrasuite variability o f partition coefficients, volatile transport o f trace metals, effects o f melt structure on elemental distributions etc. ), no quantitative modelling has
been attempted at this stage. Phases likely to be involved in controlling Rb, Sr and Ba distributions are biotite, plagioclase and K-feldspar. Qualitatively, fractional crystallization of biotite reduces both R b / S r and Ba/Sr ratios, plagioclase increases both R b / S r and Ba/Sr and K-feldspar increases Rb/Sr, but not as dramatically as plagioclase, and decreases Ba/Sr, but not as dramatically as biotite. It is clear that the distributions displayed in Figs. 15 and 17 for the St. Malo diatexites (filled circles ) and main Mancellian granites (filled circles) will allow only limited Rayleigh fractionation of
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. MALO
MIGMATITE
41 7
BELT
•T 1000
e,' 500
~v~ q°
100
50
Ba Sr
I
I
t
]_
I
5
10
50
100
500
MANCELLIAN GRANITES
1000
500
100
50
Ba
Sr
I
I
I
I
I
5
10
50
100
500
Fig. 17, Logarithmic plot of Ba vs. Sr for the St. Malo migmatite belt and the Mancellian granites.
biotite + plagioclase _+K-feldspar (see Figs. 16 and 18) and more detailed modelling is not warranted at this stage. The Mancellian leucogranites could be generated by feldspar + biotite fractionation from a parent similar to the main varieties of Mancellian
granites. However, the chondrite-normalized REE patterns (M. Brown and R.S. D'Lemos, unpublished data) show small to large positive Eu anomalies which suggest that the final crystallization of the leucogranites involved feldspar accumulation.
418
M, BROWNCNI)
I
~
)S
30
1
so/t o
50 -~
70
~--~L~9o
.......
i
70~ 90 0/7"70
90
PI
-
D'LfM
'
1
~'~--
5
R~
4-. . . . . . .
i
4
90
2q
9o
50
i 4 f
//
/
/// ,,/<70
/
1
,,"/45 5
/ •
/"
Ba
4-/ /
,//
i
,,/
/
//
4
/
/
S so
90-
/
t
] / /
/___l 5
!
5
_t
Fig. 18. Logarithmic plot of Ba vs. Sr to illustrate the effect of tractional crystallization of hornblende (lib), clmopyroxcnc (Cpx), plagioclase (Pl), K-feldspar (K~'), or biotite (Bt) on liquid compositions using limiting values for thc distribu-tion coefficients in granitoids from Nagasawa and Schnetzler ( 1971; lib and Pl), Philpotts and Schnetzler ( 1970: (7~.~ and Kfs), and Nash and Crecraft ( 1985; Bt, PI and K/~). Numbers represent the percentage of liquid which remains after each fractional crystallization step indicated by a cross tick on each fractionation line. Note how different distribution coefficients for the same mineral alter both the slope and the rate of change for fractionation by crystallization or that mineral.
K / R b ratios are essentially the same for both suites of rocks at 230 + 33 for the diatexites and 216 + 31 for the Mancellian granites (Tables 1, 2 and 3), but show a very small increase with increasing SiO2in the St. Malo suite and a very small decrease with increasing SiO2 in the Mancellian suite (Fig. 19). The effect of fractional crystallization of biotite on the K / R b ratios in rocks of this range of SiO2 contents will be to reduce it, while the effect of K-feldspar could be either to reduce it slightly or to increase it slightly depending upon the SiO2 content and the distribution coefficient for Rb. Plagioclase will not affect the K / R b ratio.
Tectonic interpretation using trace element discrimination diagrams The introduction of a set of trace element discrimination diagrams for the tectonic interpretation of granitic rocks in 1984 (Pearce et al., 1984) has led to their widespread use in the identification of the tectonic setting of ancient granite suites. A majority of the St. Malo and Mancellian samples plot within the field of Volcanic Arc Granites as discriminated by Pearce et al. (1984), for example on the plot of Rb vs. Y + N b (Fig. 20). The high Rb content of some samples takes them across into the "syn-Collision Granite field". The tendency for
419
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
ST. M A L O
MIGMATITE
ST. M A L O
BELT
MIGMATITE
BELT
!
600 500
syn-COLG
500 ¢~400 re 3OO
B= ~,
100
lle•
5O
200
Rb
100 _~
I
62
64
I ~__J
66
68 70 SiO2
I
I
I
72
74
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MANCELLIAN
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ee
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Fig. 19. Plot of K / R b vs. SiO2 for the St. Malo migmatite belt and the Mancellian granites.
50 Rb
the Mancellian granite samples to plot close to the boundary with the Within Plate Granite field and for a small n u m b e r of the St. Malo diatexites to plot inside that field, may reflect the slightly peraluminous nature of the melts and their volatile contents, reflected in P2Os contents, since these factors promote enrichment of rare metals (Webb et al., 1987 ). However, what is most interesting is that these suites of rocks classify as VAG when the St. Malo migmatites are clearly derived by anatexis of Brioverian succession metasediments and the Mancellian granites could have an essentially similar origin. This suggests that the discrimination being made is between the tectonic environment of the provenance source area to the
10
//
/
j/ ORG
VAG I
I
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Fig. 20. Rb vs. Y + N b discriminant plot of Pearce et al. (1984) for the St. Ma[o migmatite belt and the Mancellian Granites.
protolith sediment (in this case the Brioverian succession metasediments), rather than the magmatic environment in which these suites formed, in this case an inverted basin rather
420
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Fig. 21. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements tbr three samples of Brioverian succession sandstones (data from Denis and Dabard, 1988 ).
t,--
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,
~
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than a true volcanic arc. We conclude that this "discrimination" diagram simply may not work! Brown et al. (1984) use primordial mantle normalized element patterns and selected H F S / L I L element ratios to discriminate primitive, calcic arc granitoids with low LIL and HFS element abundances from normal, calcalkaline continental arc granitoids with enhanced LIL element abundances and low HFS/ LIE ratios and mature, alkali-calcic granitoids with high levels of LIL and HFS elements and higher H F S / L I L ratios. Primordial mantle normalized element patterns for three samples of Brioverian succession sandstones (data from Denis and Dabard, 1988) and selected samples from the St. Malo migmatite belt suite of rocks and the Mancellian granite suite are shown in Figs. 21, 22, 23 and 24. All of these patterns are similar to those from normal continental arcs as defined by Brown et al. ( 1984 ). That the Brioverian succession sediments have such a potential provenance source is consistent with their generally epiclastic character and deposition in basins of limited extent, as suggested by the work of Denis (1987) and Denis and Dabard ( 1988 ). Patterns for the inhomogeneous and homogeneous diatexites from the St. Malo migmatite belt are similar to those from the Vire-Carolles, Bonnemain, Louvign6-Gorron and Alexain-Deux Evailles-
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/MS
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Fig. 22. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements for selected samples from the St. Malo migmatite bell MS=metasediment: MT=metatexite; ID = inhomogeneous diatexites; H D = homogeneous diatexites; GS=granitoid sheets.
Iz6 Granite Complexes in Mancellia. The pattern for the Bois du Gast granodiorites can be matched with something intermediate between the two patterns from granitoid sheets within the St. Malo migmatite belt. These data strongly support a c o m m o n origin for these two suites of rocks. The patterns from the two teucogranite samples reflect strong fractionation and these isolated bosses within the Mancel-
If
421
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMOR1CAN MASSIF OF FRANCE
LEUCOGRANITES
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i
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I
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I
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Fig. 24. Plots of rock/primordial mantle (normalizing values from Wood, 1979) for a range of elements for selected leucogranites from the Mancellian Batholith.
5O
I I I I RbBaTh
U
I
I I
K TaNbLaCeSrNd
P HfZrSmTiTb
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Fig. 23. Plots of rock/primordial mangte (normalizing values from Wood, 1979) for a range of elements for selected samples from the Mancellian granites: V-C G=Vire-Carolles Granite: B G=Bonnemain Granite; L-G G = Louvign6-Gorron Granite; A-DE-I G= AlexainDeux Evailles-lz6 Granite. lian Batholith may represent residual liquids from the m a i n granite magma. Discussion The C a d o m i a n orogeny involved active continental margin processes during up to 275 Ma from ~ 7 0 0 - 6 5 0 Ma to ~ 4 2 5 Ma (Brown et al., 1990). Within this period of time we can recognize two major periods of magmatic activity. The first includes the early C a d o m i a n foliated plutonic complexes (Power et al., 1990a) and the Lower Brioverian basaltic vol-
canic formations (Lees et al., 1987; Dupret et al., 1990). The second period includes late Cad o m i a n post-tectonic complexes and the Upper Brioverian andesite-rhyolite volcanic formations (Dupret et al., 1990). Rocks developed during the first period of magmatic activity are confined at the present level of erosion to the SBT. Rocks developed during the second period of magmatic activity occur within all three terranes of the NACT, but as discussed above, those of the SBT are geochemically distinct from the St. Malo migmatire belt samples and the Mancellian granite samples. Our preferred tectonic model for the Cadomian orogeny (Brown et al., 1990) involves left oblique plate convergence, i.e. southerly or southwesterly subduction across a northeast to east-northeast oriented trench, which has caused fore-arc, intra-arc and back-arc regions of the orogenic belt to behave as semi-independent terranes relative to the bordering continental (the Central Armorican Terrane) and oceanic (the Cadomian Ocean to the north or northwest) lithospheric plates. This deformation is transpressional, the terranes are b o u n d e d by the subduction zone on the ocean-
422
ward side and are both dissected by and bounded by strike-slip faults through the forearc, intra-arc and back-arc regions. Structural trends in the SMT are east-northeast (Brown, 1978) and reflect transverse compression across the trend of the arc. These terranes were shortened perpendicular to the trend of the subduction zone and migrated as coherent bodies variable distances sub-parallel to the subduction zone between the bounding strikeslip faults. The transpression is dated at ~ 540 Ma by the syn-kinematic emplacement of diatexite, anatectic granite and leucogranite into shear zones in the SMT (Brown, 1978; Strachan et al., 1989; Treloar and Strachan, 1990 ). Anatexis at mid-crustal levels within the SMT requires a high geothermal gradient, which we believe was generated during behind-arc basin extension and subsequent basin inversion prior to 540 Ma ago. There may be similarities between the tectonic setting of the St. Malo migmatite belt and the Variscan migmatites and granites of the Pyrenees (e.g. Wickham and Oxburgh, 1986, 1987). The overall similarity between the chemistry of the St. Malo migmatite belt and the Mancellian granites and their similar age argue for a substantially c o m m o n origin. The MT is interpreted as a former behind-arc basin (Brown et al., 1990; Dupret et al., 1990), and a shallower geothermal gradient through the crust during extension and accumulation of the Brioverian succession is likely to have been present, followed by thickening of the hotter crust during basin inversion following the start of the transpressional event. Thus, anatexis in the lower part of the Brioverian succession is a probability within the postulated tectonic models for the Cadomian orogeny (Strachan et al., 1989; Brown et al., 1990; Dupret et al., 1990). There remains to be considered the possible role of mantle-derived magmas in the generation of the Mancellian Batholith. The volumetrically minor basic complexes at Ern6e and Br6e have a calc-alkaline geochemistry and are
~1 BROWN AND R S. I)'I3(M~ ~",
likely to have had an ultimate origin in the mantle wedge, even though the chemistry of some members of the suite may reflect subsequent processes. However, there is no geophysical evidence to suggest that there are substantial masses of basic rocks beneath the exposed level of the MT. We conclude that the role of mantle-derived magma in the generation of the Mancellian Batholith was minor. Initial results of S m - N d and Rb-Sr isotopic studies (R.S. D'Lemos and M. Brown. unpubl. data) show that the Mancellian granites and the St. Malo migmatites share similar isotopic characteristics, having similar eNd ( - 4 . 0 to - 7.3 ) and cSr ( + 3 to + 42 ) values and model ages ( T d m u r ) in the range 1.7-1.5 Ga. These features set the SMMB and the MG apart from the post-tectonic Cadomian granites which occur in the St. Brieuc Terrane at La Hague and in the Channel Islands. This isotopic evidence further points to a genetic link between the SMMB and the MG, and is interpreted to reflect fundamental differences in the isotopic characteristics of the source region and magmatic evolution of the SMT and MT in comparison with the SBT. In some models for the Cadomian orogeny, the migmatite belts of the SMT and the granites of the MT are regarded as being the result of thermal relaxation following thrusting dur* ing subduction-collision orogenesis (Bal6 and Brun, 1983: Graviou and Auvray, 1985; Graviou et al., 1988). The arguments against the thrusting model are presented in more detail by Strachan and Roach (1990) and Treloar and Strachan (1990). However, we note that neither the regional low grade of metamorphism at the present erosion level nor the normal average thickness of crust in this region (Bois and Courtillot, 1988), when combined with the absence of significant physiographic relief, allow any substantial overthickening during arc-continent collision. Accordingly, we prefer the terrane amalgamation model in which oblique collision of crustal blocks within the hanging wall of a major subduction zone in
THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
the interval 580-530 Ma has inverted a behind-arc basin. Imbrication of this inverted basin during transpressional orogeny at ~ 540 Ma has juxtaposed two different structural levels along the Cancale Shear Zone, represented by the MT and the SMT. The major terrane boundary, which separates the SBT from the SMT, is the Fresnaye Shear Zone. This separates two crustal blocks of fundamentally different geology within an overall model for the Cadomian Belt of southerly-directed subduction beneath the Armorican Massif.
Conclusions (1) The St. Malo migmatite belt and the Mancellian granites are interpreted to be coeval at ~ 540 Ma and both developed by anatexis at intermediate crustal depths of the Brioverian succession. The higher geothermal gradient required for crustal anatexis was generated during behind-arc basin extension while the Brioverian sediments were deposited and preserved during subsequent basin inversion to allow melting to occur at intermediate depths in the crust. (2) The syn-kinematic emplacement of the homogeneous diatexite/anatectic granite core to the St. Malo migmatite belt along the Port Briac Shear Zone/Cancale Shear Zone is inferred to date the age of transpressional accretion of the St. Malo terrane to the Mancellian terrane, although these two terranes may simply be different structural levels of an imbricated behind-arc basin. By analogy, the Fresnaye Shear Zone and related shear zones which occur on the northwestern side of the St. Malo migmatite belt are inferred to be of the same age and thus the docking of the St. Brieuc terrane with the St. Malo terrane occurred prior to, and transpression continued synchronously with, the culmination of anatexis at ~ 540 Ma. This particular terrane boundary may well be the more important, since the geology across it changes more strikingly than across the
423
boundary between the St. Malo terrane and the Mancellian terrane. ( 3 ) The comparative geochemistry of the St. Malo migmatites and the Mancellian granites allows a similar petrogenesis for both suites of rocks, viz. anatexis of Brioverian succession metasediments. In the case of the Mancellian granites, it is postulated that partial melting substantially in excess of the R C M P was followed by convective overturn and homogenization prior to emplacement as diapirs between Cadomain shear zones or elongate plutons along sets of more-closely spaced shear zones. The St. Malo migmatites, across the Cancale Shear Zone, represent a deeper structural level where both migmatitic source and anatectic granite product are found together and where the genetic link between the two can be demonstrated unambiguously. As the frontispiece of Read's ( 1957 ) book suggests: "Per migma ad magma." (4) The interpretation of tectonic setting using trace element discrimination diagrams should be made with caution since it could be the source of the granites which is being identiffed, that is the tectonic setting of the provenance source area to the protolith sediment for an "S-type" granite, rather than the tectonic setting of the magmatism. It is clear in the case of the St. Malo migmatites that this is the case and it is our contention that the VAG classification of the Mancellian granites reflects the source characteristics rather than the tectonic setting. We conclude that this "discrimination" diagram simply may not work!
Acknowledgements We thank our colleagues for many years of stimulating discussion, in particular G.M. Power, R.A. Roach, R.A. Strachan, C.G. Topley and P.J. Treloar. Geochemical analyses were provided by T.S. Brewer (University of Nottingham) and G.J. Lees (University of Keele). Funding by Kingston Polytechnic and Oxford Polytechnic is acknowledged. This pa-
424
per is a contribution to IGCP Project 217 "Proterozoic Geochemistry". We thank one anonymous reviewer, N.M. Halden, G.M. Power, R.A. Strachan and P.J. Treloar for comments on an earlier version of the manuscript. References Adams, C.J.D., 1967. A Geochronological and Related Isotope Study of Rocks from North Western France and the Channel Islands (United Kingdom). Unpublished D. Phil. thesis, University of Oxford, Oxford, 259 pp. Arzi, A.A., 1978. Critical phenomena in the rheology of" partially melted rocks. Tectonophysics, 44:173-184. Autran, A., Beurrier, M., Calvez, J.-Y.. Cocherie, A.. Fouillac, A.-M. and Rossi, P., 1983. Age et origine des granitoids du batholite mancellien (Normandie, France). Principaux Rdsultats Scientifiques et Techniques du B.R.G.M., Resumes, 57. Auvray, B., Chariot, R. and Vidal, P., 1980. Donnecs nouvelles sur le Proterozoique inferieur du domainc Nord-Armoricain (France): ~ge et signification. Can. J. Earth Sci., 17: 532-538. Bale, P. and Brun, J.P., 1983. Les chevauchments cadomtens de la Bate de St.-Brieuc (Massif Armoricain ). C.R. Acad. Sci., Paris, 297: 359-362. Bale. P. and Brun, J.P., 1989. Late Precambrian thrust and wrench zones in northern Brittany (France). J. Struct. Geol., I 1: 391-405. Bcrthd, D., Choukroune, P. and Jdgouzo, P., 1980. Orthogneiss, mylonite and noncoaxial deformation of granites: the example of the South Armorican Shear Zone. J. Struct. Geol., 1: 31-42. Bland, A.M., 1985. The Geology of the Granites of Western Jersey, With Particular Reference to the South-West Granite Complex. Unpublished PhD Thesis, Oxford Polytechnic, Oxford, 419 pp. Bois, C. and Courtillot, V., 1988. The French ECORS Program. Deep Seismic Profiling of the Crust and Evolution of the Lithosphere. Eos, Trans. Am. Geophys. Union, October 25 1988: 977,989. Brown, G.C., Thorpe, R.S. and Webb, P.C., 1984. The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources. J. Geol. Soc., London, 141: 413-426. Brown, M., 1973. The definition of metatexis, diatexis and migmatite. Proc. Geol. Assoc., 84: 371-382. Brown, M., 1974. The Petrogenesis of the St. Malo Migmatite Belt, North-Eastern Brittany, France. Unpublished PhD Thesis, University of Keele. Brown, M., 1978. The tectonic evolution of the Precambrian rocks of the St. Malo region, Armorican Massif, France. Precambrian Res., 6:1-21.
m BROWN AND RS. D'I,EM()S
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THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN MASSIF OF FRANCE
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