the flamanville granite (northwest france): an unequivocal...

GEOLOGICAL JOURNAL, VOL. 25,27 1-286 (1 990)

The Flamanville Granite (Northwest France): an unequivocal example of a syntectonically expanding pluton

J. P. BRUN, D. GAPAIS, J. P. COGNE lnstitut de Geologie/CA ESS CNRS, Universite Rennes 1-35042 Rennes Cedex, France

P. LEDRU BRGM-450 18 Orleans Cedex, France

and

J. L. VIGNERESSE Laboratoire de Tectonoph ysique, Universite de Nantes-44072 Nantes Cedex, France

The Flamanville pluton was emplaced during the upper Carboniferous in Palaeozoic sedimentary cover forming one limb of the Siouville syncline, a Hercynian kilometre-scale fold in Normandy, Northwest France. We review the geological environment of the pluton with special emphasis on the Siouville syncline. The features described include gravity data, metamorphic environment, fold patterns in the iontact metamorphic aureole, trajectories of principal strains in and around the pluton, finite strain and anisotropy of magnetic susceptibility within the pluton, and physical and kinematic aspects of pluton fabrics. The results emphasize that (1) the Flamanville pluton is syntectonic, ( 2 ) its emplacement involves lateral expansion of magma rather than spherical ballooning, and (3) granite fabrics do not reflect magmatic flow but are strain-controlled irrespective of grain-scale deformation processes and rheological state.

The example of the Flamanville granite further suggests ( I ) that grain-scale deformation features are not critical in distinguishing tectonic from magmatic origin of granite fabrics and (2) that pluton formation within the soft sediments of the shallow upper crust most likely results from lateral expansion of magma injected through the brittle crust rather than from ballooning of a diapiric body.

KEY WORDS Granite fabrics Pluton expansion Strain pattern Gravity modelling Magma ascent Variscan belt

1. INTRODUCTION

In his pioneering study of the Flamanville granite, Martin (1953) wrote: ‘Thus the granite expanded, each foliation plane in the plastic hornfels and flow-plane in the granite was distended in all directions within these planes, like the skin of a growing balloon’. More recent works (Ledru and Brun 1977; Brun 1981; Cogne 1988; Cogne and Perroud 1988) have confirmed this interpretation. This paper reconsiders the Flamanville pluton where a ‘ballooning effect’ (Ramsay 1975, 1981, 1989) is obvious.

Because the Flamanville pluton cuts across folded structures of the country rocks, i t has long been considered as a typical post-tectonic intrusion. However, analysis of the strain pattern in and around the pluton indicates that the emplacement was at least in part synchronous with regional deformation (Ledru and Brun 1977).

The present paper reviews the structural and geophysical data collected during the last fifteen years in an around the Flamanville pluton: fabrics, strain, metamorphism, gravimetry, and anisotropy of magnetic susceptibility. Some of these data have been published previously (Ledru and Brun 1977; Cogne 1988; CognC and Perroud 1988) but a large part was only described in unpublished theses (Ledru 1977; Brun

0072- I050190/04027 1-1 6$08.00 0 1990 by John Wiley & Sons, Ltd

272 J . P. BRUN, D. GAPAIS, J . P. COGNE, P. IEDRU AND J . 1.. VIGNERESSE

198 I ; Ciapais 1987). The discussion concerns kinematics of emplacement, their rclationsliips with regional defcmnation and the significancc of granite fabrics.

2 . THE PLUTON I N ITS GEOLOGICAL ENVIRONMENT

The Flamanville pluton is a small elliptical intrusion (7.4 X 4.5 kin). 11 is mostly homogeneous coarse-grained granodiorite with plagioclase and K-feldspar phenocrysls of centimetric size. MaGc xenoliths arc corninon near tlic external boundaries. A microgranitic fxies occupics tlic centre of the pluton (see Jerernine 1931 arid Martin 1953 for dctails).

Thc pluton is cmplaced within Cambrian to Devonian sedimciits of the southern limb of the Siouville syncline (Figure I ) . According to Graindor (196l), tlic Cambrian series consist of a thick pile of arkoscs followed by allcrnating pelites and sandstones. From bottom to top, the Ordovician is rcprcscnted by the Gres Armoricain ( a thick quartzite unit). the Schistes ;I Calymcncs, and the Gres de May. The Silurian consists of sandstones (Grcs culminants), black pelites, and cherts; the Devonian is a compositc formation made of liincstoncs, sandstones, and pelites. The Siouville synclinc is a plunging inclined fold (Turner and Weiss 1963) with a n ENE-WSW axial trcnd, verging towards the southeast. An axial plane clcavagc

0 5 10Km C a p de la HLGUF c-----t----l a riomanvrllc granite

F"1

n

Devonian

Silurion

Middle and upper Ordov,cron

Lower Ordovrciori

Cam br ran

Gneiss and mcashisk ,,~~~,

N

Iigiii-c 1 ( a1 Siniplilicil gcolugictil map of iiorttiwcrl C'otci11ii1. (b) Sclieinalic ci-oss-section oi' the Siouville syncline (after Graindoi 1961)

A SYNTECTONICALLY EXPANDING PLUTON 273

i ITE CRYSTALLINITY

6 m m < w < 5 m m

w c 5mm

S la ty c leavaae

Figure 2. (a) Map of illite crystallinity index within pelitic rocks. w is the width of the illite [001] diffraction peak measured at half height. Crystallinity contours are based on the analysis of 40 samples. (b) Map of cleavage type. Geological contours on

(a) and (b) correspond to the granite and the Gres Armoricain (see Figure la for further details)

is observed within the pelitic horizons with dip varying from 50" to the North to subvertical to the South. The cleavage varies strongly in type and intensity throughout the area. A slaty cleavage occurs along the northern limits of the syncline and around the pluton (Fourmarier et ul. 1962; Ledru and Brun 1977) (Figure 2b). Elsewhere in the syncline, the cleavage is less well developed. According to local lithology, difrerent types of spaced cleavages occur, including fracture cleavage, crenulation of the microstratification, and pressure solution seams. To the south and southeast of the pluton, the cleavage disappears completely and a cleavage front can be mapped within the Cambrian formations (Figure 2b). It is worth noting that slaty cleavage affects the Devonian whereas the lower Cambrian rocks remain completely uncleaved.

Metamorphic conditions contemporaneous with cleavage development have been estimated using illite crystallinity measured on 40 pelite samples. The width of the illite [OOI] diffraction peak (measured at half height of the peak) is used as a crystallinity index (see Le Corre 1975). It ranges between 4 and 9 mm, which indicates low-grade metamorphism (Dunoyer de Segonzac 1969; Le Corre 1975). Peak widths contoured on Figure 2a provide a thermal image of the Siouville syncline and the pluton neighbourhood. The eastern low-grade zone corresponds to the cleavage front. The occurrence of a low-grade zone within the inner part of the syncline, above the Grks Armoricain, suggests that this competent unit acted as a thermal screen during cleavage development.

The overall consistency observed between the cleavage type and illite crystallinity maps suggests a strong thermal control on cleavage development. As previously proposed by Fourmarier et ul. (1962), this is a strong argument in favour of a direct relationship between granite emplacement and regional deformation.

A strong contact metamorphic zone is observed around the pluton with a width varying between 0.5 and 1 .Okm (see Martin 1953; Saleeb-Roufaiel 1962).

3. SHAPE OF THE PLUTON

The geological map shows that the pluton is basically elliptical in shape but is pinched to the east where the contact cuts across the Grtts Armoricain. From northwest to south, lithologic boundaries are smoothly bent into conformity with the pluton contact.

1 km - c

I.'igurc 3 . Maps of thc Bouguer :inonialy and ol' depth contours ( i n k i n ) of pluton/country rocks iiitcrfacc obtained after inversion of' gravity data

NNE

IkmL 2 krn

ESE - _ -- -- - --- - - -_ W N W ,I'

Figure 4. ('ross-sections of the Flamanville pluton and Siouville syncline. Daslied lines represent cleavage in country rocks and planar fabrics within pluton. Blonk layer is thc (ires Arnioricain. The shape uf the pluton is deduced from gravity (Figure 3 )

and geological (Figures I . 5. and 6) data


The shape of the pluton at depth was deduced using a dense network of gravity data with a measurement spacing of approximately 500m. The Bouguer anomaly map (Figure 3a) shows a low amplitude anomaly from +3 to -7 mgals whose contours are slightly shifted to the east with respect to the geological contours of the pluton. The shift is in part due to a regional anomaly effect which can be estimated using the l i l ,000,000 gravimetric map of France. After the usual corrections, gravimetric data were inverted (Vigner- esse 1983) in order to deduce the shape of the pluton at depth. Variations of rock densities measured on samples have been taken into account. The results show a flat asymmetric cone with an elliptical horizontal section. The maximum depth, slightly more than 3 km, is located near the eastern constricted termination. A series of independent tests (Bott and Smith 1958; Durbaum 1974) has confirmed that the maximum depth of the body responsible for the anomaly is between 3.5 and 4.2 km (for details, see Vigneresse in press).

Both structural and gravimetric data have been used to draw two geological cross-sections parallel respectively to the long and short axes of the elliptical shape of the pluton (Figure 4).

4. DEFORMATION IN AND AROUND THE PLUTON

4a. Folds

Second order folds along the southern limb of the Siouville syncline are open upright folds with kilometric wavelengths and with E-W to WSW-ENE striking and westerly plunging axes (Figure 5). The traces of axial planes show a slight curvature when approaching the pluton. The sediments are strongly folded in the zone of contact metamorphism and the folds are upright with slightly plunging axes. Their wavelengths are metric to decametric in scale and their shapes range from concentric to similar (class 1B to 2 according to Ramsay 1962). Along the northern, western, and southern boundaries of the pluton, fold axes are always almost parallel to a stretching lineation.

A curved and doubly plunging syncline occurs around the western border of the pluton (Figure 5a). A precise cross-section of this structure has been drawn using structural data collected within an iron mine (Dielette mine) (Saleeb-Roufaiel 1962) (Figure 5b). As the trend is almost perpendicular to the regional folding, this syncline allows us to estimate the variation in horizontal shortening due to pluton emplacement (Figure 5c). The shortening is around 80 per cent near the contact and drops down to 50 per cent within a distance of 500m.

4h. Finite strain

Because it is now generally accepted that cleavage is parallel to the A , A z principal plane of the finite strain ellipsoid, the map of cleavage trajectories (Fig 6a) describes the strain pattern on a regional scale. In the northern part of the Siouville syncline, cleavages are dipping to the north and display a southward steepening fan-like pattern with a mean E-W trend. In the vicinity of the pluton, the cleavage bends around the contact. At the junction of these two domains, to the northeast of the pluton, trajectories define a triangular zone (Ledru and Brun 1977) which indicates an interference between the regional strain field and the local strain field due to granite emplacement (see numerical simulations in Brun and Pons 1980, and theoretical analysis in Brun 1983). Along the northwestern and southern borders of the pluton, trajectories are oblique to the contact and parallel to the internal foliation of the pluton. The internal foliation (Martin 1953) defines inward dipping concentric trajectories.

The stretching lineation (parallel to the A, axis of the finite strain ellipsoid) can be easily measured in the zone of contact metamorphism using deformed metamorphic blobs, fossils and ooliths in metasedi- ments, and stretched minerals in pegmatite and aplite dykes. The lineation always trends nearly parallel to the contact (Figure 6b). Its plunge is always lower than 40" and defines a culmination to the east and a saddle in the central part of the doubly plunging syncline described before (Figure 5a).

276 J . P . BRUN, D. GAPAIS, J . P . COGNE, P. LEDRU A N D J . L . VIGNERESSE

S E + /

N W .Sea l e v e l / + ~ I

. -. , " " ,

Pluton contuct , _I

1 5 0 % c

Dis tance f r o m p luton contact ( m )

Figure 5 . Aspects of folding around the pluton. ( a ) Map of axial plane traces (croasea, anticlines; dashcd lincs, syrtclines). (b) Section acrosa DI' Ihc wcstcrn syncline. (c ) Variations of amounts of radial shortening across the wcstcrn synclinc, deduced from

rcstoration ofcross-section (b). (a) and (b), modified after Saleeh-Roufaiel(1962)

I 7 krn i_i

C L E A V A G E - F O L I A T I O N

a V

Figure 6. (a) Map ol'clcavagc and foliation trajcctorics in and around the plutoii. (b) Map ofstretching lineationsaitd bedding4eavage intersection lineations around the pluton. Numbers refer to foliation dip (a ) arid lineation plunge (b)


(Left) - 50"

- 40"

~ 30" ~ 20"

- 10" Plunge - 0"

- 100

Stretching lineation ( A,)

in the pluton E

Foliation - cleavage trajectories

Sites of AMS and strain measurement

I '

0 3 1 In( ki / kj)

0 2

0 1

0 0

In(ki/kj)=O025+0 155 In(Xi

r=O 957 In( .. - h i I hj) -

0 1 2

2 5

2 0

., L..,

1. *~. ~. - -- STRAIN INTENSITY

SHAPE OF THE STRAIN ELLIPSOID ' ' .

/'

, . a * k 2 in the pluton

Stretching lineation in Contry rocks

A B C D E F G t i I

Figure 7. Variations of finite strain in and around the pluton. (a) Map of the western closure of the pluton showing stretching lineations in (open arrows) and around (solid arrows) pluton. Numbers are lineation plunges. Letters A to I refer to sites of anisotropy of magnetic suceptibility (AMS) measurements. (b) Correlation between ratios of principal axes of AMS (K,/K;) and finite strain

ellipsoid (A/A,) measured from deformed enclaves. (c) Finite strain and AMS variations from site A to site I

A study of finite strain and anisotropy of magnetic susceptibility (AMS) was carried out at 9 sites along the inner western border of the pluton (Figure 7a). The finite strain ellipsoid was estimated using axial ratios of deformed mafic enclaves measured in the field. AMS was measured on standard palaeo- magnetic cylindrical specimens (25mm in diameter, 22mm in length) using a Schonsted spinner mag- netometer. Mean AMS tensors were determined from about 10 measurements per site (CognC 1988; Cognk and Perroud 1988). An excellent positive correlation is found between strain ratios and principal susceptibility ratios (Figure 7b). Orientation and inclination of K3 axes and of poles to foliation are always very similar, thus indicating that the K1K2 principal plane of the AMS is nearly parallel to the foliation. The shape of the strain ellipsoid is of flattening type with K values ( K = ( A J A 2 - l)/(A2/A3 - 1); Flinn 1962) ranging between 0.0 and 0.5 (Figure 7c). Strain intensity shows a minimum value around r = 2.3(r = A , / A2 + &/A3 - 1; Watterson 1968) in the central part of the western closure of the pluton (sites C, D, E; Figure 7a) and increases towards both the north (Site A) and the south (Site I) up to maximum values of r = 3.4 (Figure 7c). The total shortening shows a similar variation as A3 reaches its highest values at sites C, D, E, and F (40-45 per cent shortening) and lowest values at sites A and 1(57 per cent shortening). Note the discrepancy between the 80 per cent radial shortening accumulated in country rocks (Figure

278 J . P . BRUN, D. GAPAIS, J . P. COGNE, P. LEDRU AND J. L. VIGNERESSE

5c) and the 40 per cent shortening recorded at the neighbouring site C in the granodiorite (Figure 7c). This could be because enclaves deformed only after a certain critical degree of granite crystallization (Brun and Gapais 1986) and have therefore minimized the total strain. Moreover, a significant amount of pluton deformation has been taken up by brittle failure associated with dyke injection and stoping (Martin 1953).

Within the pluton, the stretching lineation given by KI always plunges to the left when seen from the centre of the pluton (Figure 7a), with increasing amounts of stretch from north to south (Figure 7c). Within the country rocks, the stretching lineation plunges to the left to the north of site D and to the right to the South (Figure 7a and c). Thus, the relative positions of A, in the country rocks and in the pluton are almost perpendicular along the southern margin of the pluton. In contrast, they are nearly parallel along the northern margin of the pluton.

The paleomagnetic study has also shown that the granodiorite carries a thermoremanent magnetisation which, although consistent with the known Carboniferous magnetic field direction, may be significantly modified by AMS towards the flattening plane A,A2 (CognC 1988) (see Van Der Voo and Klootwijk 1972; Cognd 1988 and Cogne and Perroud 1988 for further details).

4c. Fabrics M>itliin the pluton

Strains recorded by the fabrics within the pluton increase gradually towards the contact. Outcrops of late microgranitic material located in the thickest area of the pluton, basically show an isotropic texture (the area with no foliation trajectories on Figure 6a). Away from the margins of the pluton, the granodiorite generally shows a weak foliation marked by alignment of feldspar phenocrysts, amphiboles, and biotites. The quartm-feldspathic matrix around the phenocrysts does not show evidence of significant plastic deformation. Where present, mafic enclaves show in general no evidence of internal strain (Jeremine 1931). As crystallization proceeds, enclaves are expected to maintain a high crysta1:melt ratio with respect to the more leucocratic granodioritic magma. Consequently, the features described above suggest that the crystals did not act as a stress supporting framework during foliation development. However, the amount of melt present cannot be precisely estimated.

The fabrics increase in intensity in the vicinity of the margins of the pluton. On outcrop scale, a foliation and a lineation are well developed and are defined by preferred orientations of phenocrysts, micas, hornblende crystals, and mafic enclaves (Figures 8, 9, and 10). The foliation cuts across mafic enclaves without refraction. This attests to a negligible viscosity contrast between enclaves and granodiorite and reflects homogeneous strain of both components in a bulk solid state. In contrast, fragments of hornfelsed country rocks have locally suffered substantial amounts of rigid rotation without significant internal straining after their incorporation in the magma (Figure 9). This attests to a high viscosity contrast between these xenoliths and the granite and indicates that granite emplacement and deformation occurred in a partially molten state. Thus, we infer that the observed strains have accumulated during and after magma crystallization, once the amount of melt fraction had dropped down to less than about 35-30 per cent (see Arzi 1978). Further evidence for bulk solid state deformation near the margins of the pluton are: ( I ) The foliation affects shortened and locally folded pegmatite or aplite veins emplaced along radial extensional joints (Figures 8 and 9). (2) Incipient C-S fabrics (Berthe et al. 1979) are locally present (Figure 1Oc). The localization of such shear instabilities is controlled by interactions between phenocrysts and matrix (Figure 10d) (Gapais 1989), which suggests that the bulk rheology was dependent on the framework formed by the most resistant crystals (Jordan 1987; Gapais 1989).

Thz above features are consistent with a combination of ductile deformation and of radial cracking of the peripheral cortex of the pluton (the early-emplaced magma) during expansion.

On a microscopic scale, the deformation of the granodiorite at the margins of the pluton has the following characteristics (Figure 10):


Figure 8. Detailed structural map of contact area, site B, Figure 7a ( A I L , principal plane). (1) Foliation trace, (2) narrow and often incipient dextral shear bands, (3) joints, (4) radially oriented aplite veins, (5) hornfelsed country rocks. Elongate black spots are deformed mafic enclaves (one of them is substantially offset by a set of shear bands, see Figure 1Oc). Note (i) continuity between cleavage in country rocks and granite fabric, (ii) obliquity between contact and both foliation and shear bands, and (iii) continuity of granite fabric across aplite veins. Both, contact-foliation obliquity and attitude of C-S fabric indicate dextral strike-slip component

1. The feldspar phenocrysts are often euhedral, with little evidence of plastic strain. In contrast, primary amphiboles and biotites are generally deformed.

2. The grains within the quartzo-feldspathic matrix are basically strain-free. Quartz shows no lattice preferred orientations. Strongly lobate quartz grain boundaries attest to extensive migration recrystal- lization processes (Gapais and Barbarin 1986). The extensive development of myrmekites, concentrated within high strain zones (C surfaces and phenocryst boundaries) but present throughout the whole aggregate, reveals that diffusion processes are largely involved in the deformation of the feldspathic

280 J . P. BRUN, D. GAPAIS, J. P. COGNE, P. LEDRU A N D J . L. VIGNERESSE

3 m.

Figure 9. Line drawing of outcrop. from Martin (1953), showing granite pod radially injected in a pull apart structure within country rocks (southern pluton margin). Note ( i ) continuity between fabrics of intrusion and of country rocks, (ii) sharp contacts between granite and country rocks, (iii) rigid rotation of country rock xenolith, central part of the Figure (outlined by rotated internal fabric and by folding of cross-cutting aplite vein). The anticlockwise rotation of this xenolith is consistent with a sinistral strike slip component. The granite fabric is subperpendicular to the expected injection ( fow) direction (NS), but is consistent with

the stretch direction (EW), although country rocksigranite relationships indicate injection in a molten state

phase. Both these features demonstrate that ductile deformation within the pluton stopped at temperatures of at leas1 500-550°C (Simpson 1985; Gapais and Barbarin 1986; Gapais 1989).

3. The shear bands are made of fine-grained aggregates of myrmekitic feldspars, quartz, amphiboles, and biotites (Figure 10d) with no evidence of intense plastic deformation although shear strains accumulated in these zones can be very large. This feature, combined with the lack of substantial plastic deformation of phenocrysts further suggests that the fine grain size within shear bands is likely to be a primary feature resulting from crystallization within high strain rate zones which probably contained residual melt.

5. DISCUSSION AND CONCLUDING REMARKS

As recognized by Martin (1953), the Flamanville pluton was emplaced with a strong component of lateral expansion. The generally shallow plunging stretching lineation all around the pluton (Figure 6b) demonstrates a perimeter increase and therefore the ballooning. However, the expansion here is not spherical (cJ. balloon model of Ramsay 1989). Several lines of evidence show that the pluton has expanded preferentially towards the West (Figure 11). The 3D shape indicates that the deepest part of the pluton is located to the east. The constricted eastern closure suggests that the Gr&s Armoricain and lower Cambrian units were more difficult to intrude than the upper units of the Ordovician, Silurian, and Devonian which are dominantly pelitic. The obliquity between foliation trajectories and pluton contacts (Figure 6a), together with shear criteria (Figures 8, 9, and 11) indicate wrenching shear components dextral to the north (Figure 8) and sinistral to the south (Figure 9). The doubly plunging syncline bordering the western contact separates

A SYNTECTONICALLY EXPANDING PLUTON 28 1

Figure 10. Aspects of granite fabrics in the contact area (Figure 8 and site B, Figure 7a). (a) Typical aspect of the granite foliation; scale bar is 5 mm. (b) General aspect of microstructures; feldspars (F) show well-developed zoning, are weakly deformed, and have often myrmekitic boundaries; quartz (Q) shows poorly-developed internal structures and lobate grain boundaries; biotites ( B ) and hornblendes show evidence of plastic deformation (e.g. curved cleavage in B); scale bar is 5 mni. (c) C-S structures indicating dextral transcurrent shear (see Figure 8) (a set of shear bands offset a basic xenolith); scale bar is 5cm. (d) Microstructural aspect of shear bands (arrowed). They are very narrow, fine-grained (associations of quartz, myrmekitic feldspar, biotite, and hornblende), and are often localized at the vicinity of well-developed feldspar phenocrysts. Straight segments of feldspar boundaries are often subparallel to shear bands. Note the combined occurrence of poorly-deformed phenocrysts and very sharp grain size reduction

across shear bands. Scale bar is 0.5 mm

282 J. P. BRUN, D. GAPAIS, J. P. COGNE, P. LEDRU A N D J . L. VIGNERESSE

these two wrenching boundaries. Pluton expansion attests to a dominantly ductile behaviour of the low-grade sedimentary cover during emplacement.

Figure 11. Sketch showing dominantly westwards expansion of the pluton with induced reverse sense of strike-slip shear component along northern and southern margin

Figure 12 presents a model of emplacement of the Flamanville pluton in its crustal environment. The critical feature is that it is a very small pluton which must nevertheless have travelled through about 15 km or more of brittle crust before it was emplaced within the shallow sedimentary cover.

Diapirism is one of the more favoured hypothesis to explain the rise of granitic magma through the crust. Estimates of the physical parameters which control diapirism can be obtained from the simplified sphere model (Grout 1945; Fyfe 1971; Marsh 1978; Schmeling et LiI. 1988). The spherical equivalent of the Flamanville pluton would have a volumc of about 550kmj. Such a buoyant sphere, with a density contrast of 0.1 to 0.2 with surrounding rocks (Ramberg 1970; Fyfe 1971), would develop differential stresses at its top in the range of a few bars to tens of bars (Berner et ul. 1972). Starting from the lower crust, the diapir has to cross the brittle-ductile transition zone where the crustal strength is maximum, in the range of a few Kbars according to lithology, fluid pressure, temperature gradient, strain rate, and tectonic regime (Figure 12a) (see Kirby 1985). Whatever the exact values of these parameters, the differential stress at the top of the sphere can never be large enough to deform the brittle crust. On the other hand, the hot blob model (Marsh and Kantha 1979) is not a convenient alternative because the amount of heat available within so small a sphere will never be sufficient to soften a conduit across the whole brittle crust.

We therefore propose that the granitic magma has risen through the brittle crust along a narrow channel (Figure 12c and d) (see Martin 1953; Pitcher 1979; Castro 1987). Preexisting steeply dipping faults are possible candidates for magma percolation, especially if they correspond to active discontinuities.

I n thc Flamanville region, where Variscan deformations are rather weak, local partial melting of the lower crust is likely to be associated with some basic intrusion coming from the mantle (Figure 12b).


Ductile lower crust

Magma Dyke chamber injection

Pluton formation

Figure 12. Model of ascent and emplacement of the Flamanville pluton. (a) Postulated type of strength profile. (b) Partial melting and magma chamber formation in the ductile lower crust. (c) Dyke injection into the brittle crust. (d) Pluton formation and expansion

into the soft shallow sedimentary cover

A certain amount of diapiric rise across the low-strength lower crust could be involved, leading to the formation of a magma chamber beneath the brittle-ductile transition zone. In our model, the Variscan compression enhanced the opening of some basement faults, thus allowing the upward escape of a small amount of magma (Figure 12c). The magma must have had a very low viscosity (in the range of lo6 to lo8 Pas, Gapais et al. in prep.) so that the flow rate has been high enough to prevent solidification into the conduit. Magma can thus reach the sedimentary cover in a subliquid state (a mush with more than about 35-30 per cent melt fraction; see Arzi 1978).

Because of magma advection, the cold but soft country rocks and the solidifying peripheral envelope of the intrusion undergo substantial horizontal extension. Early emplaced magma and adjacent country rocks are strongly pushed aside and can locally break up, thus leading to radial injection of magma, in a somewhat similar way as observed for pillow-lavas.

Pluton expansion is often interpreted as a ballooning effect and consequently considered as an evidence for diapirism. The Flamanville example emphasizes that pluton expansion is basically a final feature which does not require any particular mode of ascent.

Sc. Emplucement and regionul tectonics

The analysis of cleavage types and fronts (Figure 2b), cleavage-foliation trajectories (Figure 6), and their comparison with isograds of low grade metamorphism (Figure 2a), demonstrate a partial synchronism between regional deformation and pluton emplacement. This indicates that cross-cutting relationships must be handled with care. The Flamanville pluton which cuts across the Siouville syncline does not postdate regional tectonics but is at least in part synchronous (Ledru and Brun 1977). This also indicates

284 .I. P. BRUN, D. GAPAIS, J . P. COGNE, P. LEDRU A N D J . L. VIGNBRESSE

that mapping of cleavage types, cleavage fronts, and cleavage trajectories is a powerful tool to deduce the relationships between granite emplacement and tectonics. The interference between pluton expansion and regional deformation is very sensitive to kinematic environment. In the present example, expansion has preferentially occurred towards the west, in a direction roughly perpendicular to that of the regional shortening whose intensity is very weak. In contrast, plutons can expand parallel to the bulk convergence direction in situations of large regional strains (Brun and Pons 1980). Thus, plutons provide good chrono- logical and kinematic indicators to analyse strains and progressive deformation within orogenic belts (see Lagarde 1989; Lagarde et al. 1990).

5d. Plij~sical and dynamic sign$cunce of gruriite jubrics

The above results are critical to the discussion of a two-fold problem which has been addressed many times in the literature (see Balk 1937; Pitcher and Berger 1972; Berger and Pitcher 1970; Pitcher 1979; Hutton 1988) and is still under debate (see Paterson et al. 1989): the diapiric or tectonic origin of internal fabrics on the one hand, and the magmatic or solid-state rheology during fabric development on the other. The Flarnanville example underlines the following:

1. The internal foliation of the pluton reflects expansion-induced strains irrespective of the associated temperature and relative amount of melt that may be inferred (see Figure 9). Some authors make a sharp distinction bctween magmatic flow and solid-state fabrics (e.g. see discussion by Paterson rt (11. 1989). This has an important genetic implication. Indeed, it suggests that magmatic fabrics would mimic flow lines within a material with a liquid-type behaviour (Balk 1937) in contrast to solid-state fabrics which reflect finite strain. Thc Flamanville example emphasizes that such a distinction ic very confusing and unjustified (see also Gapais and Barbarin 1986; Hutton 1988) because granite fabrics are concentric whereas flow directions arc expected to be more or less radial, close to expansion directions (see Figure 9).

2. The kinematics deduced from shear sense indicators (Figure 8) are consistent with local wrenching components during granite emplaccmcnt (Figure 10). This feature has to be interpreted in the light of three characteristics of the Flamanville pluton, (a) the regional deformation is quite moderate and does not involve any component of transcurrent shear, (b) thc pluton is rather small and emplaced in a rather cold regional environment, which implies very rapid cooling rate down to low temperatures (in any C ~ S C S less than a time range of the order of 100,000 years, see Spera 1980), (c) the fabrics indicate that the pluton behaved as a rigid body for temperatures of at least 550-500°C. Thus, observed C-S fabrics purely relate to pluton dynamics. This rules out the popular inference that solid-state unstablc flow should always have a tectonic origin (see review by Paterson rt al. 1989). It further emphasizes that, within syntectonic plutons, the transition bctwcen intrusion-induced fabrics and purely tectonic fabrics has no basic reason to coincide with the magmatic-solid-state transition. The latter is obviously directly controlled, in space and time, by the thermal evolution within the pluton, whereas the former, which reflects the relative ratio between internal forces and tectonic forces, is clearly linked to bulk crustal mechanics operating during emplacement.

3. The microstructures of the Flamanville pluton are basically defined by undeformed phenocrysts sur- rounded by a quartzofeldspathic matrix made of strain-free grains. At pluton margins, these features are associated with bulk solid-state flow. This emphasizes that microstructural indicators classically used to distinguish betwecn magmatic and solid-state flow (e.g. strain-free feldspar phenocrysts and quartz matrix) are not unequivocal criteria (see also Gapais and Barbarin 1986).

ACKNOWLEDGEMENTS

J. L. Lagarde and D. Hutton helped to clarify many ideas discussed in this paper. D. Hutton is further acknowledged for his constructive review of the manuscript.


REFERENCES

Arzi, A. A. 1978. Critical phenomena in the rheology of partially melted rocks. Tectonophysics, 44, 173-184. Balk, R. 1937. Structural behaviour of igneous rocks. Geological Society ofAmerica Memoir, 5, 1-1 17. Berger, A. R. and Pitcher, W. S. 1970. Structures in granitic rocks: a commentary and a critique on granite tectonics. Proceedings

Berner H., Ramberg, H., and Stephanson, 0 .1972. Diapirism in theory and experiment. Tectonophysics, 15, 197-218. Berth4 D., Choukroune, P., and JBgouzo, P. 1979. Orthogneiss, mylonite and non-coaxial deformation of granites: the example

Bott, M. H. P. and Smith, R. A. 1958. The estimation of the limiting depth of gravitating bodies. Geophysical Prospecting, 6,

Brun, J. P. 1981. Instabilites gravitaires et deformation de la croGte continentale. These d’Etat, Universite de Rennes. Brun, J. P. 1983. Isotropic points and lines in strain fields. Journal ofStructuru1 Geology, 5, 321-327. --_ and Gapais, D. 1986. Mecanismes physiques, champs de deformation et cinematique de mise en place des granites syntec-

-__ and Pons, J. 1980. Strain patterns of pluton emplacement in a crust undergoing non-coaxial deformation, Sierra Morena,

Castro, A. 1987. On granitoid emplacement and related structures. A review. Geologische Rundschuu, 76, 101-124. Cogne, J. P. 1988. Strain-induced AMS in the granite of Flamanville and its effects upon T R M acquisition. Gcwphysicul Journcrl,

___ and Perroud, H. 1988. The anisotropy of the magnetic susceptibility as a strain gauge in the Flamanville granite, NW

Dunoyer De Segonzac, G. 1969. Les mintraux argileux dans la diagenese, passage au metamorphisme. Memoirm clu Service de

Durbaum, H. J. 1974. On the estimation of the limiting depth of gravitating bodies. Geologisches Jahrbuclz, E2,23-32. Flinn, D. 1962. On folding during three dimensional progressive deformation. Quurtcrly Journal of the Geologicul Society, London,

Fourmarier, P., Pareyn, C., and Dore, F. 1962. Observations complementaires au sujet de I’influence du granite sur les deformations

Fyfe, W. S. 1971. Some thoughts on granitic magmas. In: Newall, G. and Rast, N. (Eds), Mechunisni oflgneous Intrusion. Gallery

Gapais, D. 1987. Les orthogneiss: structures, mecanismes de deformation et analyse cintmatique. These d’Etat, Universite de Rennes. ___ 1989. Shear structures within deformed granites: mechanical and thermal indicators. Geology, 17, 11441 147. _-_ and Barbarin, B. 1986. Quartz fabrics transition during cooling of syntectonic granite (Hermitage Massif, France). Tectono-

Graindor, M. J. 1961. Geologie du Nord-Ouest du Cotentin. BullrJtin du Service dc lu Curte GPologique clr. Frunce, 262, 1-75. Grout, F. F. 1945. Scale models of structures related to batholiths. American Journal cfScience, 243A, 260-284. Hutton, D. W. 1988. Granite emplacement mechanisms and tectonic controls: inferences from deformation studies. Transactions

Jeremine, E. 1931. Quelques nouvelles donnees chimiques et mineralogiques sur le granite et les roches metamorphiques de Flamanville.

Jordan, P. J. 1987. The deformational behaviour of bimineralic limestone-halite aggregates. Tectonophysics, 135, 185-198. Kirby, S. H. 1985. Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localisation of

strain along shear zones. Tectonophysics, 119, 1-27. Lagarde, J. L. 1989. Granites tardi-carboniferes et deformation crustale: I’exemple de la Meseta marocaine. Memoires et Documents

rlir CAESS, UniversitP dc Rennes, 26, 1-342. ___ , Ait Omar, S., and Roddaz, B. in press. Structural characteristics of granitic plutons emplaced during weak regional deformation.

Late CdrboniferouS plutons, Morocco. Journul uf Structurul Geology. Le Corre, C. 1975. Analyse comparte de la cristallinite des micas dans le Briovkrien et le Paleozoi’que centre armoricain: zoneographie

et structure d’un domaine epizonal. Bulletin de la SociPtP GPologique rlr. France, (7), t. XVII, 547-553. Ledru, P. 1977. Influence cle la mise en place du granite de Flaniunville SUY 11, dP’veloppement de Iu schistositl duns le synclinal de

Siouville, Diplome d’Etudes Approfondies, Universite de Rennes. ___ and Brun, J. P. 1977. Utilisation des fronts et des trajectoires de schistosite dans I’etude des relations entre tectonique

et intrusion granitique: exemple du granite de Flamanville (Manche). Comptes Rendus de I’AcadPmic~ des Sciences, Puris, 258, 1199-1202.

Marsh, B. D. 1978. On the cooling of ascending andesitic magma. Philosophical Transactions uf the Royal SocicJty, London, A 288, 61 1-625.

___ and Kantha, L. H. 1979. On the heat and mass transfer from an ascending magma. Eurth und Planetary Science Letters, 39,435-443.

Martin, N. R. 1953. The structure of the granite massif of I’lamanville, Manche, North-West France. Quarterly Journal of the Giwlogical Society, London, 108,311-341.

Paterson, S. R., Vernon, R. H., and Tobisch, 0. T. 1989. A review of criteria for the identification of magmatic and tectonic foliations in grdnitoi’ds. Journul ofStructura1 Geology, 1 I , 349--364.

Pitcher, W. S. 1979. The nature, ascent and emplacement of granitic magmas. Journal of the Gmlogicul Society. London, 136,

___ and Berger, A. R. 1972. The Geology of Donegal: a Study qfGranite Emplacement and Unroofing. Wiley-Interscience, London.

of the Geologist’s Association, London, 81,441461.

of the South Armorican Shear zone. Journal of Structural Geology, I , 3 1 4 2 .

1--10.

toniques. In: Societe Geologique de France (Ed.), 11 emf, Rlunion des Sciences de la Terre, Clermont-Ferrund, 27 pp. (abstract).

Southern Spain. Journal ($Structural Geology, 3.21 9-229.

92,445-453.

France. Physics of the Earth and Planetary Interior, 51,264270.

Iu Carte GPologique d’Alsuce Lorruinc, 29, I-- 320.

118,385433.

mineures de roches dans le Massif Armoricain. Menioires de I’AcurIhie Roycile de Belgiqur, XXXIII, 4, 1-66.

Press, Liverpool. 201-206.

physics, 125, 357-370.

($the Royul Society of Edinburgh Eurth Sciences, 79, 245-255.

Biilletin de la Socidl Frcinquise de MinPralogie, 54, 2546.

627-662.

286 J. P. BRUN, D. GAPAIS, J . P. COGNE, P. LEDRU A N D J . L. VIGNERESSE

Ramberg, H. 1970. Model studies in relation to intrusion of plutonic bodies. In: Newal, G. and Rast, N. (Eds), Mechanism sf

Ramsay, J . G. 1962. The geometry and mechanics of formation of similar type folds. Journul of Geology, 70,309- 327. -__ 1975. The structure of the Chindamora batholith. fYth Anmu/ Krpurt ofthr RP,WJ~& Instirute of Africun geoiogy. Uniwrsiry

___ 1981. Emplacement mechanics of the Chindamora batholith, Zimbabwe. Journulof Structural Geology, 3,93. ___ 1989. Emplacement kinematics of a granite diapir: the Chindamora batholith, Zimbabwe. Journal of Structural Geology,

Saleeb-Roufaiel, G. S. 1962. Contribution a I'etude du gisement ferrifere de Dielette (Manche). Sciences de lu Terre, Me'moires,

Schmelling If., Cruden A. R., and Marquart G. 1988. Finite deformation in an around a fluid sphere moving through a viscous

Simpson, C . 1985. Dcformation of granitic rocks across the brittle-ductile transition. Journal of Structural Geology, 7,503-5 I I . Spera, F. 1980. Thermal evolution of plutons: a parametcrized approach. Science, 207,299-301. Turner, F. J . and Weiss, I,. E. 1963. Structural Anulysis rfMetumorphic Twtonites. McGraw-Hill, New York. Van Der Voo, R., Klootwijk, C. T. 1972. Paleomagnetie reconnaissance study of the Flamanville granite with a special reference

Vigneresse. J. L. 1983. Enracinement des granites armoricains cstimc d'apris la gravimetrie. Bulletin de lu SociPtP Ce'ologiyue et

___ in press. Forme en profondeur du massif de Flamanvillc (Cotentin). C'omptrs Rmdus de I ' AcadPmie des Sciences. Paris. Watterson, J . 1968. Homogeneous deformation of the gneisses of Wcsterland, south-west Greenland. M e d d c M x r om Gronlund,

igneou.u Intrusion. Special issue of the Geological Journal, 2, 26 1-286.

oj1md.v. 8 I pp.

11, 191 210.

2.

medium: implications for diapiric ascent. Trctonup/iysic.v, 149, 17-34,

to the anisotropy or its susceptibility. Geologic. Mijnhouit, 51, 609-617.

MinPralogiyut~ d c ~ Breragne, XV, I , I - 15.

75, I 77.