Genesis of orbicular granitic rocks from the

Eur. J. Mineral. 2002,14,715-73 1

Genesis of orbicular granitic rocks from the Ploumanac'h Plutonic Complex (Brittany, France): petrographical, mineralogical and geochemical constraints SYLVIE DECITRE', DOMINIQUE GASQUET'.2*and CHRISTIAN MARIGNAP3

' CRPG-CNRS, BP 20, F-54501 Vandoeuvre-lès-Nancy Cedex, France ENSG-INPL, BP 40, F-54501 Vandoeuvre-lès-Nancy Cedex, France EMN-INPL, F-54042 Nancy Cedex, France *Corresponding author, e-mail: gasquet @crpg.cnrs-nancy.fr

Abstract: The studied orbicular body is found in the evolved high-K calc-alkaline La Clarté granite in the Ploumanac'h complex. The orbicules are composed of plumose K-feldspar around various nucleus. Reconstruction of the composition of the "orbicular magmas" and their crystallisation conditions indicate that the following conditions are required for the genesis of the orbicules: (1) a small (600 m3) insulated pocket of heated magma generated by the (re)melting of the cumulate Traouiéros granite; (2) a magma composition enriched in K-feldspar component which allows for a significant production of K-feldspar prior to the attainment of the Q-Ab-Or minimum and the final crystallisation of the magma; (3) a high confining pressure leads to a high crystallisation ratio prior to the exsolution of the water phase, which facilitates the development of radiated shells and (4) the existence in the melt of numerous "cold" germs (K-feldspar phenocrysts, with also homblendites, microdiorites and granites) that act to nucleate the crystallisation of the shells. The orbicules represent, thus, a "stockscheider" structure with endless "cold" walls. Key-words: orbicular granites, plumose feldspars, high-K calc-alkaline granite, Ploumanac'h.

Introduction Spectacular orbicular structures have been described at the margins of many plutonic complexes where they are always hosted in decametric to hectometric bodies (e.g. Sederholm, 1928; Goodspeed, 1942; Barrière et al., 1971; Barrière, 1972; Moore & Lockwood, 1973; Couturié, 1973; Elliston, 1984; Chauris et al., 1989; Piboule et al., 1989). These structures have been carefully reviewed by Leveson (1966) who proposed a standard terminology that is adopted here. Orbicules, crowded or sparsely scattered in their matrix, are spherical to ovoid (a few cm to 35 cm in size) with a core surrounded by a variable number of shells (up to several tens). They differ considerably by the nature of: (1) the surrounding plutons (gabbro, diorite, granodiorite, syenogranite, peraluminous granite); (2) the matrix between the orbicules (gabbroic, dioritic, granitic or pegmatitic); (3) the cores and their interna1 structures; and (4) the shells. Cores consist of plutonic rocks (granites, granodiorites, diorites, gabbros), metamorphic rocks (metapelites, amphibolites, peraluminous restites) or crystals (generally feldspars) and are currently interpreted as inert pre-existing nuclei on which nucleation started. The concentric shells are characterised by radially or tangentially oriented plagioclase andor K-feldspar associated with quartz and biotite andlor amphibole, the latter being more frequent in mafic environments. Features commonly associated with orbicuDOI: 10.1 12710935-1221/2002/0014-0715

les include xenoliths, layered or comb-layered igneous rocks, minera1 segregations, phenocrysts and proto-orbicules consisting of a core surrounded by a poorly defined shell. The genesis of orbicular rocks is still a matter of discussion, although their magmatic origin is not questioned. No single hypothesis provides a general explanation for their origin and numerous mechanisms have been proposed, reflecting the diversity of the orbicular settings and habits. Barrière (1972) proposed that the growth of shells corresponds to the rapid crystallisation of immiscible supersatured magmas convecting around nuclei. Piboule et al. (1989) suggested that the orbicular structure is controlled by adiabatic undercooling. The growth of successive shells may be also related to rapid diffusion-controlled crystallisation of the different components within the melt (Palmer et al., 1967; Meyer & Althen; 1991) or are formed by rhythmic supersaturation and crystallisation from a gel in a manner analogous to Liesegang rings (Leveson, 1966) or from a magma during differentiation (Aguirre et al., 1976). For Elliston (1984) the "magma" in which orbicules develop must have the diffusive and rheological properties of a concentrated macromolecular paste or gel of mixed hydrosilicates. The most common mechanism proposed by the authors involves diffusion. This paper is devoted to a particular orbicular structure outcropping in the Ploumanac'h plutonic complex (PPC), French Armorican Massif. The orbicules are made of radiat0935-1221/02/0014-O715 $7.65

8 2002 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart

S. Decitre, D. Gasquet, C. Marignac

Fig. 1. Geological rnap of the granitic Ploumanac'h cornplex (redrawn frorn Barrière, 1981) showing the site of the orbicular body (Gad Quarry). Note that the boundary between the La Clarté syenogranite and the Traouiéros monzogranite is gradual (transition granite).

ed K-feldspar-quartz shells surrounded by a granitic to pegmatitic matrix. Petrographical, mineralogical and geochemical data have been used to gain a better understanding of the genesis of that particular orbicular structure. The possible generalisation of the resulting mode1 is then discussed.

Geological setting The PPC is located on the northern Brittany Coast (Fig. 1). This late-Hercynian granitic epizonal complex is intrusive into (i) the Cadomian Perros-Guirec granite in the SouthEast and (ii) late Neoproterozoic orthogneisses in the East and South. The PPC is annular and can be subdivided (Fig. 1) into three main concentric units, from the inner (younger) to the outer (older) zone (Barrière, 1977): - the Ile Grande granite: a medium-grained two-mica-cordierite granite surrounding a fine-grained biotite granite; - a leucocratic fine-grained biotite monzogranite; - a gradual transition from the interna1 Traouiéros granite (a monzogranite, interpreted as a cumulate) to the externa1 La Clarté granite (a syenogranite, interpreted as a residual liquid) (Barrière, 1981). Several coeval mafic hectometre- to kilometre-size bodies and numerous schlieren are present in the Traouiéros granite. Pegmatitic pods are locally abundant in the syenogranite but are absent in the monzogranite. The La Clarté granite has been quarried for a long time. According to Albarède et al. (1 980), the three units, although coeval at around 300 Ma, were derived from inde-

pendent magma batches, originating from partial melting at different levels of the crust and upper mantle. The confining pressure at the time of emplacement of the Ploumanac'h pluton may be estimated from the parageneses of the metamorphic enclaves inside the pluton and of the contact aureole in the surrounding rocks. Barrière (1977) described a sequence of events in metapelites, with (i) static development of a K-feldspar-cordierite-andalusite assemblage, followed by (ii) appearance of a schistosity, together with replacement of andalusite by fibrolite. The initial assemblage is interpreted as corresponding to the lithostatic load at the time of the granite emplacement with a pressure of ca. 2 kbar. The orbicular facies, studied in this paper, outcrops in the La Clarté granite (Fig. 1). It was discovered in the western part of a 70 m depth excavation (Gad quarry) and was first described by Chauris et al. (1989). More recently, Decitre (1996) provided a new set of petrographical and geochemical data.

Petrography The observations were carried out at different scales: - In the Gad quarry, on the remnants of a large body, the

bottom of which was still at the outcrop in February 1996.

- On 5 to 15 m blocks lying in the quarry's stocks. - On a representative sample ( 4 5 x 2 5 I~O cm, Gad sample).

The Gad sample was sawn in four 2.5 cm thick slabs. One was carefully polished and served as reference in the definition of several lithofacies (Fig. 2), another was used to

Genesis of orbicular granitic rocks

Fig. 2. Representative Gad sample of the orbicular facies. DF, dark facies; 0,orbicular facies (n, nucleus of the orbicule; m, fine-grained matrix; p, pegmatite patch; s, shell of the orbicule); PO, proto-orbicular facies (h, hornblendite enclave; k, K-feldspar megacryst; m, matrix; po, proto-orbicule). The facies boundaries are marked for clarity. Note that the proto-orbicules and the K-feldspar megacrysts in the proto-orbicular facies are protruding through the boundary with the dark facies.

prepare petrographic thin sections and the two others served as source of geochemical samples. Observations were also completed around the Gad quarry. The La Clarté granite is poorly outcropping, but several other quarries are presently excavated in the La Clarté granite, allowing detailed examination. No other orbicular rocks were discovered, and the Gad orbicular body remains at present as isolated in the La Clarté granite.

1. The different facies of the orbicular body The following facies may be defined:

a. Schlieren These are mica-amphibole-rich stripes with gradua1 margins toward the La Clarté granite. They are several m in length, up to 10 cm thick. They consistently contain abundant coarse-grained K-feldspar (up to 5 cm), which make up to 30 to 40 volume %. The Fe-Mg minerals are mainly biotite, with subordinate amphibole; titanite, allanite and zircon are common accessory minerals. These structures are identical to those described as banded schlieren by Barrière (1977, 198 l), mainly in the Traouiéros and transition granites (Fig. 1). There is a thick sequence of these schlieren at the bottom of the orbicular body, and, from isolated blocks, it may be inferred that a schlieren layer coated the orbicular rocks in many (but not all) places. However, it must be emphasised that, at the kilometre scale around the orbicular body, schlieren are very rare in the La Clarté syenogranite, being only sporadically encountered in the quarries, whereas they are concentrated at the immediate vicinity of the orbicular body at the Gad quarry. It is common to observe, disseminated within these schlieren, small (< 10 cm) grey mi-

717

Fig. 3. Radiating plumose K-feldspar megacrysts of the orbicule shell, reminiscent of the K-feldspar in "stockscheider" pegmatites. Note the quartzo-feldspathic rim at the shell-matrix boundary.

crogranular mafic enclaves, with a flat ovoid shape that parallels the schlieren foliation. Such enclaves are particularly abundant in the schlieren associated with the orbicular body. They are either dioritic or homblenditic in composition, the latter being consistently rimmed by a biotite layer (a few millimetres thick).

b. Orbicular facies This is the main facies in the body, consisting of orbicules set in a granula matrix. The matrix is either fine-grained or pegmatitic, with al1 gradations, the pegmatitic areas commonly occurring as central patches within the inter-orbicular matrix (Fig. 2). (i) The orbicules are distinctly zoned, with a nucleus and a shell, and are cornrnonly ellipsoidal, close to spherical in shape. Some have a more complex morphology that mimics their irregularly shaped nucleus. The size of the nucleus is related to the thickness of the shell, in such way that a somewhat constant size of the orbicules is maintained at a given place. However, from one place to the other, the mean size of the orbicules may Vary between 11 and 7 cm; it seems that the size is comrnonly gradually changing. The nucleus is most often (around 90 9% of the occurrences) a homogeneous subhedral K-feldspar, 3-6 cm in size. Others include mafic nuclei (7 9%) that are similar to mafic enclaves, which are not rare in the La Clarté granite of the Gad quarry, and in the schlieren; and coarse-grained biotite-granite nuclei (3 %) that are clearly distinct from the La Clarté granite. The mafic nuclei are commonly ellipsoidal in shape, whereas the granite nuclei are more angular with smoothed boundaries. There is no systematic distribution of the different types of nuclei within the body. The shell is made up of radiating plumose K-feldspar megacrysts (Fig. 3), up to 5 cm in size, which grew orthogonally to the nucleus boundary; their habit is very reminiscent of K-feldspar in marginal pegmatites with a comb structure (so-called "stockscheider", Charoy, 1975), as reported by Chauris et al. (1989). These plumose K-feldspar crystals are systematically intergrown with quartz vermicules and de-

718


fine a micropegmatitic texture. Minor biotite, as minute euhedral crystals (100 pm), is also present. Where present, plagioclase (near to pure albite) is always related to late reequilibration. (ii) The fine-grained matrix is a mixture of intergrown quartz, K-feldspar, zoned plagioclase and biotite; myrmekitic textures are frequent. At the orbicule-shell boundary, there is commonly a mm-thick rim of coarser matrix (similar to the proto-orbicular matrix, see below). (iii) The pegmatitic patches of the matrix exhibit, by contrast, a definite sequence of crystallisation: K-feldspar + plagioclase + quartz. Biotite is commonly absent; where present, it is an early phase. The size of the patches is highly variable, as they tend to coalesce to produce larger pods. Thus, the ratio of fine-grained to pegmatitic matrix exhibits, within the orbicular body, large variations, from purely fine-grained to purely pegmatitic, with al1 intermediates. Where purely pegmatitic, it is obsewed that K-feldspar in the pegmatite grew from the radiating K-feldspar within the orbicules. There is a relationship between the size of the orbicules and both the abundance and the nature of the matrix such that the smallest orbicules are associated with restricted matrix of pegmatitic nature, whereas the largest orbicules are set in abundant fine-grained (millimetric) matrix, with about 75 vol. % occupied by the orbicules. The latter situation is by far the most frequent. One large (15 m) block showed a gradational decrease in the size of the orbicules with a gradational decrease in both the abundance of the matrix and the fine-grained to pegmatite volume ratio.

c. Proto-orbicular facies This facies is different from the orbicular facies in that: 1) the (proto)orbicules are less than 6 cm in size and exhibit almost exclusively K-feldspar nuclei (2 cm in size), with the exception of rare granitic nuclei; 2) isolated euhedral Kfeldspar (the same size as the feldspathic nuclei) and mafic enclaves (up to 4 cm) are independently set in the matrix; 3) the matrix is homogeneous and has a grain size (up to 5 mm) that is intermediate between the fine-grained matrix and pegmatite in the orbicular facies. Small thin laths of biotite (Bt A) coexist with the large macroscopic biotite laths (often present as clusters of two or more crystals, frequently associated with titanite: Bt B). The mafic enclaves are either dioritic, similar to the mafic nucleus in the orbicular facies; or composed of hornblendites, with a distinct rim of biotite (in association with the development of quartz droplets within the hornblende). d. High-Kfine-grained granite (dark facies) This facies is a homogeneous fine-grained (millimetric) rock, that is dark grey due to the relative abundance of biotite. It consists of an eutectoid aggregate of coeval K-feldspar, quartz and biotite, with subordinate plagioclase; myrmekitic textures are common. The boundaries with the other facies are smooth, and the proto-orbicules or the K-feldspar megacrysts in the proto-orbicular facies protrude through the boundary with the dark facies (Fig. 2). These features indicate that the contacts between the three facies are magmat-

/

ml

wl 1

Schlieren Dark facies proto-orbicdarfacies Orbicular facies Pegtnatite poci La Clan6 granite

2m

Fig. 4. Reconstructed morphology of the orbicular body (vertical section). The reconstruction is based on field observations and earlier publications (see text); the constraints from the proposed genetic mode1 (see Fig. 13) are also taken into account.

ic. In some instances, a few mafic enclaves (hornblendites) were observed in the dark facies.

e. Synplutonic dykes Some blocks in the Gad quarry stocks, which were apparently derived from the vicinity of the orbicular body, contain 1-m-thick zoned dykes that have fine-grained dark granitic cores and pegmatitic rims, with K-feldspar megacrysts and huge biotite laths. This dark rock is mineralogically quite similar to the dark facies in the orbicular body. As the contacts between the dykes and the surrounding granite are rather smooth, these dykes are better classified as synplutonic dykes (Pitcher, 1991).

2. Morphology of the orbicular body Our observations allow us to reconstruct a "normal" sequence of zones for the La Clarté granite towards the centre


719

7 Schlieren

I

-

T

I

1

La ClafiB granite

Dark facies

I

u

'

u

Proto-orbicular facies

i

-.---------

1

T

I 1

70

80 Megacrysl

90

0Matrix 0Plumose shell

Orbicular facies i

,010

Pegmatite patch XOr

[IIIIID Euhedral Kfs

Fig. 5. Variation of the orthoclase content in the K-feldspar of the La Clarté granite and the different orbicular facies.

of the orbicular body: 1) a schlieren zone in the granite; 2) a proto-orbicular zone; 3) a dark-facies zone; and 4) the main orbicular zone. Typically, the thickness of the three first zones is 10-20 cm. However, it seems that there is a large variation from the top to the bottom of the body. In the preserved (1996 in situ observation) bottom part, the schlieren and proto-orbicular zones are well developed (up to 0.5-1 m each) and the dark facies is absent; whereas towards the top, the proto-orbicular facies and the schlieren are apparently fading and the dark facies may be in direct contact with the La Clarté granite. Chauris et al. (1989) and Chauris (pers. comm.) observing the orbicular body at the time of the discovery (i.e., observing the very top of the body), described a series of up to ten spherical bodies, several cubic metres each, apparently not connected, which graded towards the bottom into two separate lenses. By contrast, the preserved bottom (1996) shows the existence of a unique elliptical section, 7 m in the greater axis lying in the N40 "E direction. According to Chauris the orbicules in the spherical bodies were smaller than in the main body, and no dark facies was found rimming the spheres. From the quarry data, a minimum volume of 500-600 m3 may be inferred for the orbicular body as a whole. Finally, from al1 the observations and data presented above, we derive a possible morphology of the orbicular bod y ( ~ prior ) to the destruction by the quany work (Fig. 4). This body could have been fed by a synplutonic dyke similar to those observed in the Gad quarry's stocks. It may be speculated that the first magmatic injection at the origin of the orbicular body was channelled by the thick sequence of schlieren which is now observed al1 around the lower part of the body. This will be further discussed in a following section.

Mineralogical data Minerals from the La Clarté granite and the orbicular facies were analysed with a CAMECA SX50 electron microprobe at the Henri Poincaré University in Nancy (standards are natural minerals; 15 kV, 10 nA; 8 s counting time; PAP correction).

In the La Clarté granite, the orthoclase (Or) content of Kfeldspar is restricted to the 88-90.5 range (in mole %) in contrast with K-feldspar in the orbicular body (Fig. 5): - In the proto-orbicular facies, the K-feldspar megacrysts range from Or81.5 to 01-90, K-feldspar from the orbicular shells ranges from Or70 to Or96 and that in the matrix is Or88 to Or96. - In the orbicular facies, the K-feldspar from the shells of the orbicules ranges from Or88 to Or94, whereas that in the fine-grained matrix is Or81-0r95. - In the dark facies, the Or content of K-feldspar span the Or8 1-0195 range. - In the pegmatites, K-feldspar exhibits a restricted range of compositions, between Or93 and Or95. The K-feldspar megacrysts in the schlieren exhibit a broad range of compositions, Or76-Or96 with a mode around Or88.

2. Plagioclase In the La Clarté granite, plagioclase is zoned, with anorthite (An) 20-22 cores and An 12-14 rims. In the orbicular body, plagioclase is more Ab-rich: - In the proto-orbicular matrix, plagioclase is in the An124 range. - In the fine-grained orbicular matrix, plagioclase is zoned, with An20-21 cores and pure albite rims. - In the dark facies, plagioclase has restricted compositions in the An12-16 range. - In the pegmatites, plagioclase is homogeneous, around An18-21, similar to the cores in the fine-grained matrix.

3. Biotite Biotite is unzoned. Unaltered biotite compositions (representative analyses in Table 1) are plotted in the Al,,, vs. Mg diagram (Fig. 6) which allows to discriminate granitic compositional trends (Nachit et al., 1985). Practically al1 the analysed biotites plot in the subalkaline domain which roughly corresponds to the High-K calc-alkaline domain (Le Maitre, 1989). The compositions of biotites from the orbicular body (excluding biotite BtB from the proto-orbicular facies) are diverse, with considerable overlap between the different facies. They contrast with biotite from the La Clarté granite, which has a homogeneous composition that is distinctly more magnesian, slightly less aluminous (Fig. 6a), and lower in Ti (see Table 1). They also contrast with biotite from the granite nucleus in orbicule cores, the latter displaying a great spread in Al content at a constant Mg content (and being also distinct from the biotites in the La Clarté granite) (Fig. 6a). The biotite rimming mafic enclaves in the proto-orbicular facies is by far the most Mg-rich and Al-poor, the less magnesian being close to the mean composition of biotite from the cumulative Traouiéros granite (wet analysis of


720

Table 1. Representative analyses of biotites and amphiboles from the La Clarté granite and the orbicular body. Biotites (base: 22 [O]) Sample Anal.

Amphiboles (base: 23 [O])

1

10 96-3v

11 96-3v

12 96-3

13 96-3v

14 96-14

141d 1146 22 94 58b 26 138b 142b mean -

32b

7

96d

55

161d

1 96-13

2 96-3

3 BI7

4 Bt2

5 Bvl

6 Bvl

7 BI6

8 BI6

9

1

A1203 Fe0 Mn0 Mg0 Ca0 Na20 K20 Total Si Aliv Ti Alvi Fe Mn Mg Ca Na K

Biotltes 1, La Clarté granite; 2, schlieren; 3, dark facies; 4, proto-orbicular facies (K-feldspar shell); 5-8: orbicular facies. 5, granite nucleus; 6, K-feldspar shell; 7, fine-grained matrix (Ba0 = 0.16%); 8, pegrnatite patch; 9: mean biotite from the dark facies. Amphiboles: 10 - 11: hornblendite enclave. 10, frorn enclave core; I l , frorn enclave reaction rim; 12, schlieren; 13, granite nucleus; 14, La Clarté granite.

minera1 separate from Barrière, 1977) (Fig. 6a). It contrasts with the biotite in the schlieren, which has the same low Al content, but is distinctly poorer in Mg (Fig. 6a). The biotite BtB in the proto-orbicular facies (Fig. 6c) has a composition overlapping that of the schlieren, suggesting that it could be xenocrysts derived from the schlieren (in agreement with its occurrence as clusters and association with titanite). Biotite in the orbicular body displays the following compositional trends: - In the orbicular facies (Fig. 6b), biotite from orbicule shells has a rather uniform composition that contrasts with biotite from the fine-grained matrix; the latter is often less magnesian and more aluminous; biotite from the pegmatitic patches is more magnesian than most biotite from the fine-grained matrix, and slightly less aluminous than biotite from both the shells and the matrix. - In the proto-orbicular facies (Fig. 6c), biotite from the shells of the proto-orbicules and biotite BtA have basically the same compositions (with some biotite BtA being more magnesian) that encompass the field for biotite from the orbicular facies. - Most biotite from the dark facies (Fig. 6d) encompasses the field of biotite in the orbicular facies, particularly the biotite from the orbicule shells. However, some biotite from the dark facies is enriched in Mg, spreading along a trend toward the compositions of the biotite rimming hornblendite enclaves in the proto-orbicular facies and an

isolated analysis corresponds to the field of the schlieren biotite.

4. Amphibole (Fig. 7, Table 1) Al1 the analysed amphiboles are K-rich pargasitic hornblendes. They are unzoned. Amphibole from the hornblendite in the proto-orbicular facies is the more magnesian of al1 the facies of the body, with a slight tendency to be less magnesian and more sodic when it is close to the biotite-rich rim of the enclave (reaction rim). The amphibole from the schlieren is similar. Both kind of amphiboles are in marked contrast with the amphibole from the granite nucleus of the orbicules, which, in turn, is very different from the distinctly less magnesian amphibole of the La Clarté granite.

Bulk-rock compositions 1. Samples and methods Several facies of the orbicular bodies were analysed. Two slabs of the large Gad sample were cored for the finegrained matrix (m, ca. 200 g) and sawn for the orbicul: shell (s) and the dark facies (DF) (both more than 500 g).

72 1


A

Proto-orbicular facies A Shell

Traouiéros granite Granodiorite nucleus Mafic inclusion Schlieren

O Dark facies

Shell Matrix

2 1.4

1.6

1.8

2.0

2.2

2.4

. 1.4

2

~ 1.6

l

l 1 1.8

1 2.0

1

1 2.2

1

1 2.4

1

1v1y 1 1

6. Biotite composition in the Al,,, vs. Mg diagram of Nachit et al. (1985) for a) the different facies of the PPC; b-d) the orbicular rocks.

They are thought to be representative. A huge sample (ca. 2 kg) from the surrounding schlieren (S) was also analysed. In addition, the La Clarté granite (LCg) from the Gad quarry, a granite nucleus (n) from the orbicular facies and the finegrained central facies in a synplutonic dyke (SPD) were also analysed. Major and trace elements were determined by ICP-AES and ICP-MS at CRPG-CNRS Nancy (France). Analytical uncertainties are estimated at 2 % for major elements and at 5 or 10 % for trace-element concentrations (except REE) higher or lower than 20 ppm, respectively. Precision for REE is estimated at 5 % when chondrite normalised concentrations are >10, and at 10 % when they are lower.

2. Major elements Our analyses (Table 2), together with older analyses from the la Clarté and Traouiéros granites taken for reference in Barrière (1977), were plotted in the Q-P and the A-B diagrams (Debon & Le Fort, 1988) (Fig. 8,9). These diagrams were chosen because they allow representation of chemical analyses in a realistic mineralogical frame and are therefore particularly suitable for testing mixing models in petrological studies. The Q-P diagram is a representation of the classical Quartz-Plagioclase-Orthoclase triangle. The A-B diagram, with al1 the quartz, plagioclase, orthoclase projected at the origin, expresses in detail the balance between the alu-

minous minerals (excess alumina, parameter A) and the ferromagnesian charge (parameter B). It is apparent that the compositions of the orbicular rocks are significantly different from those of the Ploumanac'h granites, and from the granite nucleus as well. The granite nucleus is different from the La Clarté granites, but is rather similar to the cumulate Traouiéros granite.

3. REE data Consideration of the REE data (Table 2 and Fig. 10) reinforces the preceding observations. - The LREE and HREE patterns for the orbicule shells and fine-grained matrix in the orbicular facies, the dark facies, and the synplutonic dyke are very similar (30 < La/ Yb c 45) although with different total REE contents: (33 < ZREE < 385 ppm) except for europium; the latter exhibits a marked positive anomaly in the orbicule shells and the dark facies (Eu/EuX= 1.7 1 and 1.17, respectively), a flat pattern in the fine-grained matrix and a negative anomaly in the synplutonic dyke. - Compared to the dark facies, for instance, the La Clarté granite is richer in REE, with a distinct negative Eu anomaly and the HREE slightly more fractionated. - The granite nucleus composition is different from both dark facies and La Clarté granite compositions.


722

Table 2. Bulk-rock analysed (1-9) or calculated (10-12) compositions. Total iron as Fe,O,, LOI: loss on ignition. Analyses 1 facies

n

2 s

3

4

m

DF

5 SPD

6 LCg

7 LC

8

9

1

Recalculations 10 11

12

S

---------

SiO2 Ai203

Fe2@ Mn0 Mg0 Ca0 Na,O

K2O TiO, pz05

LOI Total

1-9: Analyses 1-9: representative analyses of the main facies (1-8, lhis study: 9, frorn Barrière1977). 1-3: orbicular facies. 1 (n). granite nucleus; 2 (s). orbicule shell; 3 (m). fine-grainedmatrix; 4 (DF), dark facies; 5 (SPD), syn-plulonic dyke; 6 (LCg). La Clartégranite (Gad Quarry); 7 (LC), common La Clartégranite; 8 (S). schlieren; 9 (T). Traouiéros granite. 10-12: recalculations 10-11: 'dark facies' (DF) compositionaccording to 0.72 shell (col. 2) + 0.28 matrix (col. 3) 11: "dark facies' (DF)composition according to 0.70 shell (col. 2) + 0.28 matrix (col. 3) + 0.02 biotite (col. 9 in table 1). 12: syn-plutonic dyke (SPD) compositionaccording to 0.70 La Clart6 granite (col. 7 for major elements; col. 6 for trace elements) + 0.295 dark facies (col. 4) + 0.005 schlieren (col. 8).

Interpretation 1. Evidence for several "orbicular magmas" Field and chemical evidence shows that the orbicules developed in situ and, consequently, that the orbicular facies did evolve from a magma (hereafter called "orbicular magma"). The evidence is (i) the orbicules lack the features indicative of transport from a deeper source into their present setting: they are never deformed, disrupted or broken with healing; (ii) the orbicules are not sedimented; and (iii) the orbicular matrix is not likely to represent an independent granite magma carrying already formed orbicules since its composition

is irrealistic (flat REE pattern, Fig. 10). This "orbicular magma" was intrusive into the La Clarté granite at an early stage of its crystallisation. The observed cauliflower-like contacts are evidence that more than Ca. 30 % (and probably more) of the La Clarté magma was still not crystallised (Fernandez & Gasquet, 1994). The "orbicular magma" evolved according to the sequence: l ) crystallisation of the orbicules around pre-existing K-feldspar (as inferred from their Na-rich compositions) and mafic or granitic enclaves - the melt at this time was crystallising along the K-feldspar-quartz cotectic; 2) quick solidification of the surrounding magma, yielding the finegrained matrix - the melt then reached the K-feldspar-

723


La Clarté granite

+ Schlieren 0

Granodiorite nucleus

A

Rim

A

Core

)>

Enclave

0.2 0.3 Fig. 7. Amphibole composition in the Na+K (in A site) vs. Mg number [Mg/(Mg+Fe)] diagram.

quartz-plagioclase minimum point. This interpretation is consistent with the composition of matrix biotite being somewhat more evolved than that of biotite trapped in the orbicular shells (see above). It is proposed that the quick solidification of the melt resulted from the unmixing of a fluid phase, which yielded the pegmatite patches. It was shown by Hort (1998) that, upon fluid release (i.e., increase of the liquidus temperature), the melt experiences intense crystal-

lisation in a very short time. This process explains the observed spatial relationships between the fine-grained and pegmatitic matrices, which record the progressive collection of magmatic fluids through interconnected drains (the pegmatite patches) into discrete bodies (the pods devoid of fine-grained matrix). That the magmatic fluids remained trapped within the orbicular pocket, in place of escaping through overpressuring and breaking up, may be explained by the physical state of the surrounding La Clarté granite at the time of crystallisation of the orbicular magma. As discussed above, the La Clarté granite was still partially molten at that moment and, consequently, was able to sustain the transient overpressuring associated with fluid unmixing in the orbicular pocket. Indirect evidence is given by the biotite compositions. Biotite of pegmatitic patches is distinctly less magnesian than the earlier biotite that was trapped within the orbicular shell (Fig. 6). This is to the contrary of what would be expected if the pegmatite patches had evolved from a residual liquid (a distinctly Fe-rich composition is expected), but may be compared with the behaviour of hydrothermal biotites in the porphyry copper settings that evolved from magmatic fluid, which are systematically and significantly more magnesian than the primary magmatic biotites (e.g., Beane & Titley, 1981). This is because extraction of the iron from a melt is less efficient when the melt is less saturated in aluminium (Candela, 1990). By contrast, the homogeneous coarse-grained matrix in the proto-orbicular facies may be interpreted as resulting of the slow crystallisation of an unmixed melt which reached

orbicule matrix

E 4 granite nucleus

+

La Clarté granite

Fig. 8. The orbicular facies in the Q-P chemical nomenclature diagram (Debon & Le Fort, 1988). TOM: theoretical "orbicular melt" (75% shell + 25% matrix); see text. Abbreviations: gb, gabbro; mzgb, monzogabbro; mz, monzonite; s, syenite; dq, quartz diorite; mzdq, quartz monzodiorite; mzq, quartz monzonite; sq, quartz syenite; to, tonalite; gd, granodiorite; ad, adamellite; gr, granite. Variables in number of cations. Sil3-(K+Na+2Ca/3) = f(%quartz); K-(Na+Ca): plagioclase - orthoclase balance. Trend 1 is for the mixture of orbicular shells and matrix in the orbicular facies, leading to "orbicular magma". Trend 2 is for the mixture of an "orbicular magma" that is close in composition to the dark facies with the La Clarté granite.

S. Decitre, D. Gasquet, C. Marignac the K-feldspar-quartz-plagioclase ternary minimum. This in turn would mean that the proto-orbicular and orbicular facies evolved from independent batches of magmas. The dark facies may be indicative of the "orbicular magma" that was devoid of any germs to crystallise orbicules. Indeed, compared to the orbicular or proto-orbicular matrices, this facies is markedly enriched in K-feldspar, as would be expected of a magma crystallising orbicules. Petrographical (eutectoid texture) and mineralogical (homogeneous composition of the main biotite stock) evidence points to rapid crystallisation of the dark-facies melt. Thus, the dark facies would represent a third independent batch of magma. The question of the apparently similar synplutonic dykes being representative of the feeder dykes for the orbicular body will be addressed in a following section.

Q

s

A = 6 3 , B=575 B = Fe+Mg+Ti

IV

Metaluminous diorite .

H

O

1 \ gabbro Fig. 9. The orbicular facies in the A-B diagram of Debon & Le Fort (1988). Domains 1, II, III: peraluminous rocks with muscovite > biotite, biotite > muscovite, biotite, respectively.Domains IV and V: metaluminous rocks with, respectively, biotite + amphibole Ipyroxene, pyroxene Iamphibole Ibiotite. Symbols as in Fig. 8. Point TOM is the mixture of orbicular shells and matrix in the orbicular facies, leading to "orbicular magmas" (see text). Trend 1 is for the mixing of such "orbicular magmas" with biotite, leading to dark facies composition. Trend 2 indicates that the synplutonic dyke composition may be obtained by the mixing of "orbicular magma" and La Clarté magma, with some minor mafic component in addition (see text). Diorite and gabbro are means from Barrière (1977). -75

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 10. Chondrite normalised (Evensen et al., 1978) REE patterns for the orbicular facies and related rocks. Symbols as in Fig. 8.

2. The nature of the "orbicular magmas" Since the orbicular facies results from the crystallisation of an "orbicular magma", it is possible to derive an estimate of its composition by the proper combination of orbicule and fine-grained matrix compositions. The direct analysis of a bulk sample is impossible because of: (i) the strong heterogeneity of the rock (a representative sample would be in the 1 m3 order...) and (ii) the various nuclei must be avoided. Petrographical examination indicates that a rough estimate of the "orbicular melt" may be obtained using a 25 % finegrained matrix to 75 % orbicular shell mixture This may be compared with the composition of the dark facies (deemed to represent a distinct batch of a similar "orbicular melt"). As clearly seen in the Q-P diagram (Fig. 8), dark facies plots very close to the 1 to 4 ratio (cJ:TOM) of the fine-grained matrix and the orbicular shells, respectively. However, as seen in the A-B diagram, which takes into account the ferro-magnesian components (Fig. 9), the dark facies is enriched in biotite relative to a natural mixture of the orbicule shells and the fine-grained matrix, in agreement with petrographical observations. These considerations are reinforced by mass-balance calculations. Using the REE data (which are not sensitive to the biotite), the REE pattern of the dark facies, with its pronounced positive europium anomaly, is well reconstructed by mixing 72 % of the orbicular shells and 28 % of the fine-grained matrix (Table 2, col. 10, and Fig. 11). Furthermore, very good agreement is obtained for the major elements if the dark facies composition is reconstructed by mixing 70 % of orbicular shells, 28 % of fine-grained matrix and 2 % of the biotite of the dark facies (Table 2, col. 11). The discrepancies are commonly on the order of the analytical errors. The agreement is equally good for the REE (Fig. 11). For the trace elements selected in Table 2, the agreement is commonly good and is excellent for Ba, Rb and Sr (Fig. 1 1); the exceptions (Nb, Zn) may be ascribed to the lack of data for the biotite (this minera1 being commonly the bearer of such trace-elements: Neiva et al., 1998). We interpret these data by assuming that the "orbicular magmas" resulted from a variable content in ferro-magnesian component of an essentially quartzo-feldspathic meit enriched in potassium. The latter would be well approximat-


&synplutonic dyke (m) &orbicular magma (c)

725

pressed as slight differences in the biotite content and in the Mg number of biotite). Although granitic in composition, these "orbicular magmas" are quite different from all the granites from the PPC. Moreover, they basically differ from most granite magmas elsewhere, being characterised by a pronounced positive europium anomaly (Fig. 11).

3. The synplutonic dyke Contrary to the hypothesis of the synplutonic dyke being akin to the (presently unknown) feeder dyke(s) for the orbicular pocket, the measured composition is very different from that of the "orbicular magmas" (Table 2, Fig. 8,9, 10). However, as seen in the Q-P diagram (Fig. 8), the synplutonic dyke composition may be interpreted as a 3 to 1 mixture of the La ClartC and the "orbicular" magma. Moreover, the A-B diagram (Fig. 9), shows clearly that the mixture must involve a mafic component - a gabbro of the mafic complex in the PPC being well suited. Finally, consideration of the REE patterns (Fig. 10, 11) indicates that the synplutonic dyke is enriched in HREE in comparison to the orbicular fac i e or ~ the La ClartC granite but is very similar to the TraouiCros granite (and the granite nuclei in the orbicular facies). Nevertheless, all our attempts to reconstruct the composition of the synplutonic dyke by mixing "orbicular melt" and La ClartC granite (as suggested by most geochemical data: Fig. 8, 9, 11) and PPC gabbro or schlieren (to provide the missing HREE) failed to give good agreement with both major and trace-element patterns in the synplutonic dyke. Finally, the best fit was obtained by an awkward combination of the common La ClartC granite for major elements and the La ClartC granite from the Gad quany for the trace elements; it is given in Table 2 (col. 12) and shown in Fig. 11. Therefore, we feel that the composition of the synplutonic dyke is not well explained by our modelling. Nevertheless, it would seem that the synplutonic dykes are somewhat related to the "orbicular magmas", although it is difficult to take them as the direct feeders of the observed orbicular body in the Gad quarry. Fig. 1 1. Modelling REE and trace elements patterns: a) REE patterns of calculated (c) dark facies and syn-plutonic melts, compared to the measured (m) compositions; b) Rb-Ba diagram, showing: (i) the excellent agreement of orbicular rocks with the model; (ii) the consistency of a mixing model of orbicular and La ClartC melts for the synplutonic dyke; (iii) the similarity of granite nucleus and TraouiCros compositions; c) La vs. B = Fe+Mg+Ti diagram, showing the same relationships as in b) for the orbicular rocks and granite nucleus; however, the synplutonic dyke appears more ferromagnesian and REE-rich than the theoretical model deduced from major and trace (see b) element data. TOM: theoretical "orbicular melt" (75 % shell + 25 % matrix).

ed by the "72 % shell - 28 % matrix" mixture model, whereas the former could be related to interaction with mafic rocks or melts, as suggested by the "refractory" biotites "floating" in the dark-facies melt. This is reflected in the variable Fe-Mg content of the "orbicular magmas" (ex-

Discussion and conclusion 1. Origin of the "orbicular magmas" From petrographical and geochemical data (e.g. REE), it may be concluded that the "orbicular magmas" cannot be derived from the La ClartC granite magma by fractional crystallisation. Furthermore, a test of fractional crystallisation was carried out on different melt compositions using the classical tools (Rollinson, 1996), with the result that no granitic magma and, indeed, no mafic or intermediate magma from the PPC could be a good parental melt for the "orbicular magmas". Thus, the latter were necessarily produced by the melting of a yet unknown source. A key point is the marked positive europium anomaly which characterises the "orbicular magmas". Together with the pronounced potassium enrichment, this fact points to the melting of a K-feldspar-rich source.


the orbicules represent a "stockscheider"structure with endless "cold" walls. This statement implies that the observed nuclei were indeed "cold" germs. The time constant for the heating of K-feldspar or mafic enclaves (calculated from equation (2) in the following section, with a thermal diffusivity on the order of 1.10-6 m2.s-') is in the 1 O2 to 2.1O2 s range. Supposing that these "cold" germs were supplied from the close vicinity (see below for a discussion), they should have been heated from 720 "C to 720+AT, the temperature of the intrusive "orbicular melt". Considering a AT range of 50 to 100 "C, it should have taken from 300 to 400 s (K-feldspar) or 800 to 900 s (mafic enclave) to heat the potential nuclei. This may be considered sufficient a time to promote plumose crystallisation around the nuclei (heterogeneous nucleation). On the other hand, since this time is comparatively short, the temperature of the intrusive "orbicular melt" must have been very close to its liquidus temperature in order to allow immediate crystallisation around the nuclei. However, the rate of dyke propagation for a granite melt with the viscosity of the "orbicular magma" is on the order of 3 lo-' m.s-I (Petford, 1996), which gives no more than 10 to 30 m of propagation during 300 s to 900 s. This means that the germs could not have been incorporated in the "orbicular melt" in the proposed source area located several hundred of metres (or even, a few kilometres) below (see Fig. 12). Therefore, the "cold" germs must have originated in the vicinity of the orbicular body. A rough calculation indicates that the orbicular body (assuming a volume of 600 m3) contained lo5 orbicules, i.e., that about 9.104 K-feldspar crystals and lo4 mafic enclaves must have occur close to the present setting of the orbicular body. It is evident that they could not have come from the La ClartC magma, because: (i) the K-feldspar of the La ClartC granite is less Na-rich that many K-feldspar nuclei; and (ii) the content of mafic enclaves in the La ClartC granite is too low. We suggest that most nuclei came from the disruption of a schlieren zone, the remnants of which are seen at the bottom of the orbicular body or wrapping around the orbicular facies. This is probably because schlieren are related to an early cumulate crystallisation (Barrikre, 1981). The schlieren origin is likely for the K-feldspar (with Na-rich compositions comparable to those found in the schlieren, Fig. 5) or for the mafic enclaves, very similar to those found in the schlieren, where they still are abundant - and would, indeed, explain the presence of biotite xenocrysts from the schlieren in the proto-orbicular facies (BtB). The comparatively rare granite nuclei were possibly extracted from the Traouitros granite at a greater distance. Consequently, the spatial coincidence between the occurrences of a unique massive schlieren aggregate in the La ClartC granite and the orbicular body, far from being fortuitous, was the clue for the formation of the orbicules. The schlieren were the site for the collection of the migrating "orbicular melt", first as a sill followed by dispersion of the schlieren by ballooning into the present-day orbicular pocket (Fig. 13). The existence of three facies in the orbicular body and their relationships may be explained by considering three successive batches of "orbicular melts", with only the third, corresponding to the "normal" orbicular facies, being really voluminous (Fig. 13).

727

During the emplacement of the "orbicular melts", desagregation of the schlieren could have taken place, and "cold" K-feldspar and mafic enclaves could be incorporated into the "orbicular melts". This is reasonable for the first batch of "orbicular melt" (OM 1, at the origin of the protoorbicular facies). As for the second batch (OM2, at the origin of the dark facies), it is devoid of orbicules because it invaded the early OM1 chamber with few, if any, interactions with the schlieren, due to the limited volume of new melt. However, the model in Fig. 13 raises a problem for the third batch (OM3, at the origin of the main orbicular facies): the OM3 batch was necessarily charged in "cold" seeds before the arrival into the crystallising OM1 chamber. It is possible that the OM3 melt was first collected in a small chamberjust below the present observation level that was littered with "cold" germs (in the same way as OM 1 in Fig. 13), and was subsequently injected in the OM 1 chamber (Fig. 13). If this is true, then, the ballooning OM3 chamber was already crystallising orbicules.

3. A metallurgical analogue The sequence of crystallisation in the orbicular pocket bears striking similarities with that in steel ingots, in which metallurgists describe the inwards transition from "columnar" to "equiaxed" solidification as a consequence of undercooling (ThCvoz, 1998). Columnar solidification is a monodirectional (dendritic) crystallisation proceeding from the walls of the ingot inwards, and is equivalent to the plumose growth of K-feldspar in shells. At a critical undercooling rate, the solidification becomes equiaxed, producing interlocked equidimensional crystals - clearly equivalent to the matrix in the proto-orbicular facies. It is interesting to note that the columnar-equiaxed transition occurred in the orbicular facies just before the melt-fluid phase separation at the origin of the fine-grained matrix, as recorded by the "equiaxed" rims at the shell-matrix boundaries. The columnar-to-equiaxed transition is a complex phenomenon, depending upon several coupled factors (growth rate of the columnar front, thermal gradient, growth undercooling of the equiaxed grains) (ThCvoz, 1998). Numerical modelling for steel ingots shows that the transition is controlled by the cooling rate: an increase in cooling rate allows an earlier attainment of the critical undercooling for equiaxed nucleation, thus promoting the equiaxed zone (ThCvoz, 1998). This is qualitatively consistent with the observations at Ploumanac'h. The small OM1 pocket must have undergone faster cooling than the comparatively larger OM3 chamber (Fig. 13). Therefore, it follows that OM1 gave way to the proto-orbicular facies, characterised by reduced orbicules, i.e. by an earlier columnar-to-equiaxed transition during the course of crystallisation. In addition, the small size of the orbicules, i.e., the reduced crystallisation ratio, shows that the "proto-orbicular melt" remained water undersaturated during most of the equiaxed crystallisation. On the other hand, in the OM3 chamber, the "orbicular melt" underwent the columnar-to-equiaxed transition later in the crystallisation sequence which explains the limitation of equiaxed zone to

728


in liquidus temperature due to the Kfs-enriched composition of the "orbicular melt" at a,, = 1 (Johannes & Holtz, 1996, p. 39), the To temperature is finally estimated at 855 "C. With these figures, the cooling time of the magmatic chambers may be evaluated using equation (1): we find 22 days for OM1 and 134 days for OM3. On the other hand, rough calculation of viscosities for the "orbicular melt" close to the liquidus temperature and for the "La ClartC magma" with a fraction of crystals (a) of 0.2-0.3 at T = 720 "C with T* the reduced temperature (T* = T-Te,/To-T,,); T is (Holtz et al., 1999), show that they are consistent with the the temperature of the sphere at the time t, To its initial tem- observed field relationships (cauliflower structures of the orperature, and T,, the temperature of the liquid medium bicular body, see above).The growth time of the orbicules (here, T,, = 720 "C and To is close to the liquidus tempera- may be calculated with the growth rate of a K-feldspar in its ture for the "orbicular melt", see above). This calculation as- water undersaturated melt provided by the experimental data sumes that no thermal gradient was present in the sphere; of Fenn (1977): for an undercooling of 135 "C, the measured rate was of 4.10-6cm.s-I. With this value, a proto-orbicule (2 this a legitimate hypothesis, as no crystallisation gradient of the orbicules (neither in number, nor in size, of the orbicu- cm long K-feldspar) would grow in less than 6 days, which is les) is observed from the walls towards the centre of the consistent with the estimated 22 days for the solidification of the OM1 chamber (note that reduced growth rates for the Kbody. The time constant -c is given by: feldspar - for instance, 1/60 the rate for pure K-feldspar melt, as proposed by Jambon, 1996 - would result in a time far lonwith m the mass of the sphere, R the radius, p the density and ger than the estimated solidification time). According to the A the thermal diffusivity of the surrounding liquid (here, p calculation, at the end of the growth of the proto-orbicules, = 2.3 kg.m-3and A = 5.10-7m2.s-I).The cooling time is cali.e., at the time of the columnar-to-equiaxed transition, the culated as corresponding to the attainment of the water-satu- temperature in the OM1 chamber was lowered to 838 "C. On rated solidus temperature (Ts); consequently, it is a maxi- the other hand, in the OM3 chamber, the growth time of a true mum life time for the "orbicular melts". The recalescence orbicule (5 cm long K-feldspar) would be less than 15 days, phenomenon (Lesoult, 1998) is neglected. Together with the and the temperature would be lowered to 848 "C. This value is spherical approximation, this means that the results yield close to the temperature calculated in the OM1 chamber, and only an order of magnitude. The calculation is done for a is therefore consistent with the "orbicular melt" undergoing sphere with a volume of 600 m3, approximating the OM3 the columnar-to-equiaxed transition at the end of the orbicule chamber, and a sphere of 40 m3, approximating the OM1 growth in OM3. The exsolution of magmatic water in OM3 chamber (see Fig. 13). It is now necessary to get estimates of that was responsible for the quick crystallisation of the "orbicboth Ts and To.The liquidus temperature (TO) depends upon ular melt" did occur after some time of equiaxed crystallisatithe composition, including the water content of the melt. on following the end of the orbicule growth (see above) and The solidus temperature (Ts) is of complicated estimation, consequently the crystallisation time of the OM3 chamber because it is known from a fluid inclusion study (Decitre, was certainly much shorter than the full 134 days calculated 1996) that the "orbicular melts" contained CO,, with the above. Finally, taking in consideration the fact that the three consequence of reducing a,, of the melt and increasing Ts batches of "orbicular magmas" were coeval, it may be con(Johannes & Holtz, 1996). To get an estimate of a,, and Ts, cluded from these calculations that the emplacement and crya knowledge of the water content of the melt is crucial. The - stallisation of the orbicular body at Ploumanac'h occurred in latter may be roughly estimated from: (i) the maximum sol- a very short interval of time, likely no more than a few weeks. ubility of water in a melt with the composition of the "orbic- It is interesting to note that these short crystallisation times exular melt" (ca. 5 wt% H20, see Johannes & Holtz, 1996, p. plain why the orbicules and proto-orbicules are matrix-sup65) and (ii) the melt fraction at the time of water saturation. ported and were not accumulated at the bottom of the orbicuThe latter may be estimated for the OM3 melt, since it expe- lar body. Calculation of settling velocities using Stokes' law rienced water saturation close to the end of the orbicule indicates that these velocities were comprised between 0.03 growth, i.e., for a melt fraction of 0.28 (see above). Then an and 0.3 mm per day, allowing no more than a few mm (protoinitial water content of 4 wt% is derived. At temperatures orbicules) to a few cm (orbicules) of total vertical displacearound 800 "C and at a pressure of 2 kbar (the confining ment during the crystallisation of the different magmatic pressure at the time of granite emplacement, see above), the chambers. water mole fraction of a fluid in equilibrium with a C02bearing fluid-saturated rhyolite melt containing 4 wt% H 2 0 is X,, = 0.6 (Tamic et al., 2001, their Fig. 6). Then, the cor- 4. General considerations responding solidus temperature for the minimum composition in the quartz-albite-orthoclase system (Johannes & Finally, it appears that the orbicular body in the La ClartC Holtz, 1996, p. 36) is 750 "C at 2 kbar. For the same water granite formed due to a unique combination of favourable content, the liquidus temperature of the minimum composi- conditions, limiting the chances of finding other similar tion in the quartz-albite-orthoclase system is 750 "C (Johan- bodies in the future within the PPC. Nevertheless, if the nes & Holtz, 1996, p. 52); taking in account the 105 "C shift same ingredients are brought together, it may be possible a rim around the orbicules, due to the coeval attainment of water saturation for that melt. A more quantitative assessment may be gained by comparing the cooling time of the orbicular pockets and the growth time of the orbicules. The cooling time may be approximated by considering the pockets as spheres, and using the formula for the cooling of a sphere in a liquid medium:


that some kind of orbicular body occurs elsewhere. The Aber Ildut granitic complex, to the west, is very similar to the PPC in terms of its granite nature (high-K calc-alkaline granite with cumulative facies and schlieren), contribution of mafic magmas, style of intrusion and time of emplacement. Therefore, it is not surprising that an orbicular body, similar to the La Clartt one, was discovered inside the Aber Ildut granite (Barrikre et al., 1971). It is therefore interesting to consider the generality of our results concerning the generation of orbicular rocks.

a. Critical parameters for the genesis of PPC-type orbicular bodies From the results of the present study, the following conditions are thought to be relevant to the generation of orbicules and orbicular~rocksfrom a distinct "orbicular magma", in the same way as in the PPC. - An "orbicular melt", characterised by a composition enriched in a feldspar component (K-feldspar for a "PPC type", plagioclase for the most common orbicules worldwide). A composition enriched in a feldspar relative to the ternary minimum in the Q-Ab-Or system (Ehlers, 1972) means the possibility of a significant production of K-feldspar or plagioclase before the attainment of the ternary minimum and final crystallisation. Such compositions could result from the (re)melting of feldspar-rich cumulates or cumulative rocks, yielding a distinct geochemical signature such as a marked positive Eu anomaly. - A small isolated pocket of high-temperature magma (close to the liquidus) contrasting with a colder magmatic surrounding. The high temperature would normally result from the very generation process of the "orbicular melt". - The existence in the "orbicular melt" of numerous "cold" germs for the nucleation of the crystallisation of the shells. Otherwise, either a true "stockscheider" (not unlike the lateral pegmatites in the synplutonic dykes at Ploumanac'h) or an "eutectoid granite" (similar to the dark facies in the Ploumanac'h orbicular body) would have formed. Considering the thermal constraints, these seeds must be provided in the close vicinity of the future orbicular body A "cold" magmatic environment, able to yield "cold" xenocrysts or enclaves, is a favourable setting. - Finally, a high confining pressure that permits a high crystallisation ratio prior to the exsolution of the water phase (e.g., Cline & Bodnar, 1991) which allows the development of the radiated shells. Depending on the pressure and the initial water content, magmatic fluids may be exsolved earlier or later in the crystallisation course. Early exsolution results in freezing of the orbicular matrix and prevents the sedimentation of the orbicules at the bottom of the orbicular chamber. The development of pegmatite patches and pods within the orbicular matrix would be the normal result of this process. b. Are single-layered orbicules representative of the PPC-type? A review of the published occurrences of "simple" orbicules, with a single radiating layer of feldspar (* cotectic min-

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erals), independently of their claimed genetic processes, shows the following. - The few published reconstructed compositions of "orbicular magmas" (Barrikre et al., 197 1 ; CouturiC, 1973) are consistently "exotic" relative to their plutonic host-rocks, and characterised by strong relative enrichment in feldspar components. In the other cases, simple consideration of the orbicule to matrix ratio yields similar conclusions (Sederholm, 1928; Goodspeed, 1942; Moore & Lockwood, 1973; Chauris et al., 1989; Piboule et al., 1989; Elliston, 1984). Yet, the compositions of the "orbicular magmas" are similar to the compositions of the hosting magmatic suites -just as the orbicular body at Ploumanac'h is similar to the high-K calc-alkaline granites of the PPC. - The orbicular bodies are, as a rule, of very limited extent and superheating is frequently called for (Barribre et al., 1971; Piboule et al., 1989; Meyer & Altherr, 199 1). - Germs are very variable in nature: magmatic rocks of varied origin (xenoliths or coeval disrupted magmas) and compositions, restites (Barribre et al., 1971; Couturit, 1973), metamorphic rocks (Elliston, 1984). As a whole they bear the characteristics of "cold" germs, and geological evidence would favour a proximal origin. - In most orbicular rocks the orbicules are matrix-supported and there are pegmatite patches, suggesting that rapid crystallisation due to fluid phase separation occurred. Thus, this rapid crystallisation prevented the sedimentation of the orbicules. As discussed above, this is favoured by a high confining pressure. The radius of orbicules depends upon the size of the orbicular chamber and the initial temperature of the melts, controlling in turn the cooling rates and the attainment of the columnar-to-equiaxed transition. For large orbicular pockets, it may be speculated that the transition would be more or less coeval with water exsolution followed by the final fast crystallisation of the residual melt, thus producing a contrasted granulometry typical of many orbicular rocks worldwide. Thus, the critical parameters for the PPC orbicular body are. indeed met world-wide for the single-layered orbicular bodies, with either plagioclase or K-feldspar as components of the radiating shell.

c. Orbicules with multi-layered slzells In several occurrences, however, orbicules with multi-layered shells have been described. These orbicules (e.g. in Finland: Elliston, 1984; and references therein) with numerous rhythmic shells, contrasted mineralogical contents and truncated features are more difficult to explain by the PPC model. Yet, even in these cases, some features of the orbicular rocks are akin to the PPC model: radiated feldspathic shells, pegmatitic patches in a fine-grained matrix and, finally, very high water content in the melt. Therefore, the possibility of a common origin remains. The idea of successive fresh batches of "orbicular magma" is appealing, and was previously proposed by Barribe (1972) and Elliston (1984). In this hypothesis the main difficulty is to maintain a thermal contrast between the orbicules and the new "orbic-

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ular melt". A possibility could be that the new, fresh, "orbicular batches" were distinctly superheated relatively to the cooling first batches of the same (or other) "orbicular melts".

d Final comment Although each of the conditions that may lead to the formation of a PPC-like orbicular body is frequently met independently in plutonic environments, their combination is not likely to be frequent. As may be deduced from the study of the PPC, the crucial factor is the feeding of the "orbicular melt" in "cold" seeds together with a restricted range (around the liquidus) of the melt temperature, which is a severe limiting condition. Thus, the paucity of orbicular rocks in the geological record is readily explained. These conditions do not depend on a specific composition however, which explains why orbicular rocks cover such a large spectrum of compositions.

Acknowledgements: Thanks are due to Mr Chi at La ClartC for the easy access to the Gad Quarry, to R. Capdevila and E. Hallot for providing analyses of the Ploumanac'h massif, to A.M. Boullier for providing the first orbicular sample, and to H. Combeau and G. Lesoult for fruitful discussions. K. Gillis greatly improved the English text. Two anonymous reviewers and C. Chopin are thanked for their constructive criticism.

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Received 29 November 2000 Modified version received 14 December 2001 Accepted 22 January 2002