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Four-stage building of the Cambrian Carion pluton (Madagascar)

M.O. M. RAZANATSEHENO1, A. NEDELEC2 *, M. RAKOTONDRAZAFY1,

J.G. MEERT3 and B. RALISON1

1 Department of Earth Sciences, BP 906, Antananarivo University, Madagascar

2 LMTG – OMP, Toulouse University – CNRS - IRD, 14 avenue Edouard Belin, 31400

Toulouse, France

3 Department of Geological Sciences, 241 Williamson Hall, Gainesville University, Florida

32611, USA

* corresponding author :

Anne Nédélec

tel : 00 33 5 61 33 25 76

fax : 00 33 5 61 33 25 60

nedelec@lmtg.obs-mip.fr

Key words: granitoid, Madagascar, anisotropy of magnetic susceptibility, zoned pluton,

magnetic reversal, diapir.

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SUMMARY

The 532 ± 5 Ma old Carion pluton is a dark, porphyritic ferro-potassic granitoid emplaced

near the late Pan-African Angavo mega-shear zone. A rough normal zoning from tonalitic to

granitic compositions can be recognized in the field. Steep magmatic foliations are evidenced

by K-feldspar megacryst preferred orientations. Microstructures are either magmatic or

typical of incipient solid-state deformation in near solidus conditions. Magnetic

susceptibilitity magnitudes (K) range from 11 to 111 x 10-3 SI in the pluton and can be

correlated to the petrography (highest K values in the tonalites ; lowest K in the granites ;

granodiorites in between). The susceptibility magnitudes display a complex zoning pattern.

Combined with the arrangement of magnetic foliation trajectories, it is possible to delineate

four nested sub-units, regarded as four magmatic pulses succesively emplaced from the west

to the east of the pluton. The four pulses are characterized by very similar magma

geochemistry, but variable magmatic differentiation. The highest degrees of magnetic

susceptibility anisotropies (up to 1.6) are observed along internal contacts between sub-units

and along the borders of the pluton. The magnetic lineations are also steeply plunging in some

places in each sub-units, possibly imaging the different feeder zones. Magma emplacement

occurred at the end of the activity of the Angavo shear zone, hence avoiding re-orientation of

the magmatic structures by the late Pan-African transcurrent tectonics. The diachronicity of

the four magmatic pulses is consistent with previously determined palaeomagnetic data,

because only the two older sub-units display a magnetic reversal sequence, whereas the two

youngest sub-units lack any reversion. Emplacement of these four magmatic batches was

responsible for a strain aureole and suggests a diapiric mode of ascent.

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INTRODUCTION AND GEOLOGICAL SETTING

Granitic plutonism is particularly active in collision orogens in which both the thermal

conditions necessary for magma genesis and the loci for magma ascent and emplacement in

the crust are present. In such collisional settings, granite magma production follows

continental suturing by a few tens of Mys and remains often voluminous until the last stages

of orogeny (Thompson & Connolly 1995). In a pre-drift reconstruction of Gondwana (Fig.

1a), Madagascar is located in the East African Orogen, a collisional orogen formed by the

closure of the Mozambique Ocean at the end of Neoproterozoic times (Stern 1994, Meert

2003). A number of granitoid plutons were emplaced in Madagascar at various stages. The

late-Pan-African history of Madagascar is dominated by transcurrent tectonics along major

shear zones, especially in the southern and eastern parts of the island (Martelat et al. 1997;

Ralison & Nédélec 1997; Nédélec et al. 2000).

This study focusses on the Carion pluton (Fig. 1b, c), located near the Angavo shear

zone, a lithospheric-scale transcurrent shear zone (Ralison & Nédélec, 1997), that has been

active at around 550 Ma, as can be inferred from the ages of spatially-related syntectonic and

deformed granitic rocks (Kröner et al. 1999 ; Grégoire et al. 2008). The Angavo shear zone is

associated with low-pressure granulitic parageneses and prograde charnockitisation, whose P

and peak T conditions were calculated at 790 (± 10)°C and 330 (± 30) MPa by Nédélec et al.

(2000). The Carion pluton itself crystallized at about 320 (± 10) MPa (i.e. at about 10 km

depth) after Al-in-hornblende barometric calculations using the calibration of Schmidt (1992).

It has been dated at 532 (± 5) Ma by SHRIMP zircon age (Meert et al. 2001b). Hence, it was

slightly younger than the transcurrent tectonics along the Angavo shear zone, therefore

representing one of the latest stage of Pan-African magmatism in Madagascar. On the

geological map of Besairie (1969), it is classified as a post-tectonic pluton.

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Since the pioneer work of Ellwood & Whitney (1980), anisotropy of magnetic

susceptibility (AMS) has been proven the most efficient tool in determining the internal fabric

of granitic plutons, as recently reviewed by Bouchez (1997 ; 2000 and references therein).

This method is here applied to the Carion pluton. Preliminary AMS results were already

provided by Nédélec et al. (2000). These authors observed well-defined magnetic/magmatic

foliations and steeply plunging magnetic/magmatic lineations, in sharp contrast with the

subhorizontal N-S lineations of the nearby Angavo shear zone, hence confirming the late- to

post-tectonic character of the pluton. Here, we provide a more detailed AMS study of the

pluton and its wall rocks. Combined with field geology and petrology data, it reveals the

internal structure of the pluton. These data are in turn used to infer the magma emplacement

mode. In addition, they highlight previous observations related to former palaeomagnetic

results and Ar-Ar geochronology of Meert et al. (2001b).

FIELD AND PETROLOGY DATA

The Carion pluton extends over ~370 km2 and its elliptical shape is deeply indented to the

north (Fig. 1c). Country rocks are made of migmatites, pink biotite granites, hornblende

gneisses and charnockites, that all belong to the Ambatolampy Group of the Graphite System

(Besairie 1969; Hottin 1976). On the maps, foliations of the country rocks were drawn

parallel with the massif contours, except in the northern indentation, where they seem to be

crosscut by the intrusion, hence the post-tectonic setting first proposed by Lautel (1953). The

pluton area is rather hilly (Fig. 2a), with good outcrops (some of them used as quarries), due

to the fact that the granitic rocks are less weatherable than their gneissic host rocks. The

Carion pluton does not show any contact metamorphism at its margins, evidencing the lack of

thermal contrats between the magma and its host rocks. When observed, contacts are not

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sharp. There is often a transition zone up to 1 km in width, where the granitic magma formed

conformable sheets within the country rocks. To the northwest, the magma was intruded as a

sinuous veining network in the nearby migmatites (Fig. 2e), suggesting that the country rocks

were still hot and soft. Rafts and xenoliths of migmatites of various sizes can be observed in

the southern margin of the pluton (Fig. 2d). Hence, the pluton contours drawn in the maps

represent a simplification.

The first detailed petrographic account of the Carion Massif was given by Hottin

(1975). This author underlined the foliated nature of the granite, but failed to recognize the

mainly magmatic nature of the foliation, and, for this reason, regarded the intrusion as an

orthogneissified body, a contention that is no longer tenable at the light of microscopic

observations (Fig. 3). As observed by Hottin (1975), the main rock type is a mesocratic

(medium to dark grey) porphyritic granitoid containing K-feldspar megacrysts with an

average length of 3 cm. The feldspar preferred orientation is responsible for the conspicuous

magmatic foliation observed in the field (Fig. 2b). Apart from the megacrysts, rocks are

medium- to coarse-grained, with an average grain-size of about 5 mm. Scarce mafic

microgranular enclaves are globular in shape and no more than a few tens of centimetres in

length. Feldspar megacrysts were mechanically introduced from the host granitic magma into

these enclaves before their complete crystallization, pointing to their co-magmatic nature (Fig.

2f).

Towards the margins of the intrusion, feldspar megacrysts are smaller and less

abundant, and plagioclase prevails over alkali feldspar, hence a tonalitic composition. In the

centre of the pluton, rocks are lighter in colour (light grey to pink), due to less abundant ferro-

magnesian minerals and to the pink colour of the K-feldspar megacrysts ; they have a granitic

composition. A pink leucogranite sometimes forms magmatic dykes or larger hectometric

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stocks, but its porphyritic texture is very similar to the texture of the main petropgraphic type

(Fig. 2c).

Mineral abundances and compositions are determined in thin sections and by electron

microprobe analyses. Feldspars, both as megacrysts and as interstitial grains, constitute about

60% of the rocks. The K-feldspar megacrysts are made of perthitic orthose or microcline (Fig.

3a). Plagioclase are nearly unzoned, with a usually restricted composition in the range An26–

20. Quartz is present as large crystals, sometimes elongate parallel to the magmatic foliation

(Fig. 3b). The grains often display chessboard patterns due to both prismatic and basal

subgrain boundaries, attesting for incipient solid-state deformation in near solidus conditions.

However, the lack of recrystallized grains testifies that no significant deformation occurred

after the solidification of the magma. Ferro-magnesian silicate minerals are hornblende,

actually edenite or magnesio-hornblende after the classification of Leake et al. (1997), and

biotite. They often form clusters with accessory minerals (Fig. 3c). Biotite is ubiquitous (Fig.

3b-d), whereas hornblende is rare or absent in the most evolved (granitic) rocks. Whatever the

rock type, hornblende and biotite contain a relatively high magnesium fraction (XMg = 0.6-

0.7) and are also characterized by a realtively high fluorine content (XF = 0.3-0.5), as can be

seen in Table 1. Accessory minerals are ilmenite, magnetite, titanite, apatite and zircon. The

iron oxide compositions were determined by electron microprobe analysis (Table 1). Ilmenite

is the most abundant accessory mineral and forms aggregates, together with titanite and

apatite, in the insterstitial spaces surrounding the large feldspar and quartz grains (Fig. 3a, c).

Ilmenite crystals sometimes contain a few haematite exsolution lamellae. Magnetite is less

abundant than ilmenite and is also observed as inclusions in feldspars and ferro-magnesian

silicates, pointing to its earlier crystallization in the magma.

Whole-rock compositions of seven representative samples are given in Meert et al.

(2001a). The darkest tonalites from the eastern and western sides have silica contents in the

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61-63 % range and total iron oxide contents of about 6 %, whereas the most evolved rock, a

pink granite from the southern central area of the pluton, has a silica content of about 70 %

and an iron oxide content of 2.6 %. Other samples display major element contents in between

these endmembers. All analyzed rocks are metaluminous and follow the sub-alkaline

magmatic trend in the R1-R2 diagram of La Roche et al. 1980 (Fig. 4a). More precisely, they

are highly potassic (shoshonitic) and ferriferous magmatic rocks (Meert et al. 2001a :

Rakotondrazafy et al. 2001). On the basis of field and geochemical data, the Carion pluton

had been regarded so far as a simple normally zoned pluton (Fig. 4b).

AMS DATA

Material and methods

Internal fabrics of granitic rocks are generally difficult to measure through classic techniques

based on direct observation. For instance, lineations are difficult to determine in the field even

in the case of a well foliated porphyritic granite like Carion granite. AMS measurements

provide scalar parameters as well as foliation and lineation maps that, combined to

petrographic and microstructural data, enable to refine the internal structure of the Carion

pluton and to infer its emplacement history. Preliminary structural data on the Carion pluton

was based on 10 sampling sites in the pluton and 7 sites in its wall rocks, that were part of an

extensive AMS and metamorphic study in an area extending over 150 by 70 km around

Antananarivo city (Nédélec et al. 2000). This first data set was augmented by 48 sites in the

plutons and 25 in the country rocks, hence a total of 58 sampling sites within the Carion

pluton and 32 sites in the surroundings (Fig. 1c). A sampling station is characterized by 2 to

11 (median value : 4) oriented cylinder specimens (22mm in long and 25mm in diameter).

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AMS was determined using a Kappabridge KLY 2 susceptometer (AGICO, Brno, Czech

Republic) with a detection limit of about 4 x 10-8 SI. The magnetic susceptibility magnitude

(or bulk magnetic susceptibility) is the arithmetic mean defined by K = 1/3 (k1 + k2 + k3),

where k1, k2 and k3 are the respective magnitudes of the principal axes of the AMS ellipsoid

(Nagata, 1961). The anisotropy degree is given by P = K1/K3, and the linear and planar

anisotropies are L = K1/K2 and F = K2/K3 respectively. The results are presented in Table 2.

Averages of magnetic foliations and lineations from each site can be seen in the projection

diagrams of Figure 5. Foliations and lineations are generally well-defined in most sites, with

average F values (1.14) comparable to average L values (1.10). In a few cases, some

scattering of data is observed, corresponding to average angular departures from the mean

axis a(Ki) > 5° (Table 2). This is attributed to the porphyritic nature of the rocks, with large

feldspar megacrysts that can locally disturb the mineral fabric at the specimen scale.

Magnetic susceptibility magnitudes

Magnetic susceptibility magnitudes (K) range from 11 to 111 x 10-3 SI, with an average at 62

x 10-3 SI in the Carion pluton (Table 2). These large magnetic susceptibilities are typical of

magnetite-bearing granitoids. Indeed, Rochette et al. (1992) recognized that magnetite-free

granitoids are characterized by K values generally less than 0.5 x 10-3 SI. The susceptibility

magnitudes of Carion granitoids are in the highest observed range for granitic rocks

(Bouchez, 2000). Ilmenite, that is abundant in the Carion granitoids, can also have contributed

efficiently to the susceptibility magnitudes, as this oxide has a rather high susceptibility

(Borradaile & Henry 1997, Diot et al. 2003). Thermomagnetic curves (K vs temperature)

obtained from representative samples show a fast susceptibility decrease at about 580°C, the

Curie temperature of pure magnetite (Fig. 6). This is evidence of the ferrimagnetic nature of

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these rocks, where magnetite is indeed the main susceptibility carrier in addition to ilmenite,

hornblende and biotite.

Outside the limits of the Carion pluton, the country rocks have generally lower

susceptibilities (Fig. 7a). Within the pluton, the magnetic susceptibility map displays a pattern

more complicated than the expected simple normal zoning. In paramagnetic (magnetite-free)

granites, the susceptibility magnitudes are directly proportional to the iron contents of the

rocks, thus providing a reliable petrographic indicator (Gleizes et al. 1993). In ferrimagnetic

rocks, the magnetic susceptibility magnitude is less straightforward to use, but it often

displays some relationships with the petrography, as it mainly depends from the amounts of

magnetite (in addition to other iron-bearing minerals). In intermediate to felsic metaluminous

rocks such as the Carion pluton, it is expected that the amount of Fe-Ti oxides decreases as a

result of magmatic differentiation. The susceptibility map confirms that a differentiated area

does exist in the centre of the pluton, corresponding to the pink granite type. Other minor

areas with relatively low suceptibilities are also correlated to the most felsic granites. In the

Carion pluton, the susceptibility values display a zoning pattern (Fig. 7a), where the lowest

values correspond to the previously recognized granitic core and the highest values

correspond to the tonalitic compositions near Manjakandriana city. However, the zoning

pattern is not a simple concentric one, such as the first pattern suggested from whole-rock

geochemistry (Fig. 4b). This complex zoning pattern is explained by comparison with the

foliation map (Fig. 7c). Indeed, the whole set of scalar and directional susceptibility data is

consistent with the recognition of four sub-units, as discussed hereafter.

The magnetic fabrics of magnetite-bearing plutonic rocks has been demonstrated to

parallel the shape-preferred orientation of magnetite grains (Archanjo et al. 1995; Grégoire et

al. 1998). Moreover, in the case of the Carion pluton, the consistency of the magnetic

foliation with the mineral foliation due to the preferred orientations of feldspar megacrysts has

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been checked locally in the field (Nédélec et al. 2000). The magnetic fabrics is therefore a

good proxy for the mineral fabrics. In the Carion pluton, foliations are rather steep and

arranged in patterns that locally crosscut each other and help to delineate four nested sub-units

inside the pluton (Fig. 7c). The emplacement order of the sub-units is derived from the

crosscutting relationships of the foliation trajectories. The sub-units are numbered 1 to 4 from

east to west according to their nesting relationships, hence their temporal relationships. The

steepest dips generally coincide with external or internal contacts. Foliation trajectories are

rather consistent with the limits of the pluton, with the exception of the northern central

indentation, where they seem to be at a high angle with the pluton contour. Actually, in this

place, foliations trajectories outside the pluton can also be traced inside and parallel the

contact between sub-units 2 and 3. The low-susceptibility leucocratic granite core is included

in sub-unit 3 that presents the widest range of susceptibilites, likely recording the largest

magmatic differentiation. However, there is some overlap of magnetic susceptibility

magnitudes in the four sub-units, due to their very similar petrographic nature. Sub-units 1

and 2 display a lateral zoning due to an evolution from high to medium susceptibilities, that

seems to witness a magmatic differentiation from tonalitic to granodiortic compositions. Sub-

unit 4 display only high susceptibilies: it is mainly made of tonalites and represents the last

emplaced magmas.

The degree of anisotropy (P) ranges from 1.05 to 1.64 (Fig. 7b), with an average of

1.25. This is rather high for granitoids that are almost free of solid-state deformation

microstructures, despite the fact that magnetite-bearing granites are known to be generally

more anisotropic that magnetite-free granites (Bouchez 2000). The lowest values characterize

the granitic core of sub-unit 3. High values characterize the eastern contact between the pluton

and its country rocks. In addition, high anisotropies are also observed inside the pluton and

correspond to the previously inferred contact of sub-unit 3 against sub-unit 2. These highly

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anisotropic zones possibly record the forcible intrusion of magma of sub-unit 3 with respect to

the more crystallized unit 2 inside the pluton, as well as some ballooning effect of the first

emplaced magma againt its wall rocks. Linear and planar anisotropies are both rather high,

but with a predominance of the planar anisotropies (1.14 in average for F, against 1.10 for L)

suggesting the predominance of flattening with respect to constrictional strain.

Magnetic lineations are most valuable to evidence mineral lineations that are generally

difficult to determine precisely in plutonic rocks . In the Carion pluton, magnetic lineation

plunges are rather steep in agreement with local field observations of the mineral lineation.

Each sub-unit contains at least one zone where lineation plunges are higher than 60° (Fig. 7d).

Such zones are generally regarded as the indication of magma-feeding zones (Amice &

Bouchez 1989 ; Naba et al. 2004).

INTERPRETATION

Four-pulse emplacement of the Carion magmas

The Carion pluton displays a complex susceptibility zoning, that cannot fit the simple normal

zoning derived from preliminary geochemical studies. Moreover, the foliation pattern

suggests that the pluton is made of four sub-units consistent with the distribution of

susceptibility values. These sub-units are regarded as four successive magmatic pulses

emplaced in a nested fashion. Each sub-unit contains at least one zone where highly plunging

lineations likely indicate the location of a magma feeder zone.

Nested granites are not uncommon, but sometimes correspond to unrelated magmas

with independent emplacement kinematics (Bouchez & Diot, 1990). In the Carion pluton, the

four magmatic pulses represent very similar magmas of intermediate composition that

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underwent some variable magmatic differentiation (either at depth or at emplacement level).

Only in the case of sub-unit 3 did magmatic differentiation reach a proper granitic

composition. Nevertheless, the whole pluton is made of co-genetic magmas as deduced from

petrographic observations, very similar mineral compositions (Table 1) and whole-rock

geochemistry (Fig. 4a). The magmas were likely tapped from the same source or from the

same magma chamber at depth. The Carion pluton is different from some other cases where a

complex distribution pattern of susceptibiliy magnitudes corresponds to bimodal intrusions

with mixing/mingling interactions (e.g. Déléris et al., 1996 ; Asrat et al., 2003). Indeed, the

recurrence of petrogenetic processes leading to the incremental building of large plutons has

been recently admitted even in the absence of conspicuous internal contacts (Glazner et al.

2004).

Chronology of magma emplacement

The crosscutting relationships between foliation patterns of the four sub-units enable to

determine their emplacement order. Nevertheless, the time lag between two successive sub-

units remains difficult to estimate. Due to the warm environment evidenced by amphibolite to

granulite facies conditions in the country rocks, the older sub-units may have remained highly

ductile and even only partly crystallized until the emplacement of the younger sub-units.

However, the time lag between emplacements of sub-units 2 and 3 may have been longer than

in the other cases, because these sub-units are separated by a highly anisotropic area (Fig. 7b).

The relative chronology of the four sub-units is consistent with the palaeomagnetic

data of Meert et al. (2001b). The Carion pluton provided a good-quality palaeomagnetic pole

dated at 509 (± 11) Ma. It was demonstrated that the remanence was a TRM carried by nearly

pure magnetite, because of oven unblocking temperatures in the 560-600°C range. The 3-axis

IRM experiments confirmed the lack of haematite. In addition, the palaeomagnetic samples

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exhibit either a dual polarity, or a single reverse polarity, with an apparent spatial bias (Fig.

8). Whereas sites from the country rocks and sites from plutonic sub-units 1 and 2 display

mixed polarities with reversal sequences from normal to reverse, the younger parts of the

pluton, thus the latest cooled areas, i.e. the granitic core of sub-unit 3 and the sub-unit 4, only

display the reverse component. Curie runs on samples that carried the reverse, normal and

mixed directions showed nearly reversible heating and cooling curves with well-defined Curie

temperature in the magnetite range, poiting to the same type of remanence in all samples, that

is mainly a TRM carried by magnetite. Assuming parallel and rather slow cooling paths for all

sub-units, a realistic assumption in such a metamorphic environment, the lack of dual polarity

in the youngest parts of the plutons is evidence that these areas were slightly warmer than the

others, hence too warm to register the change from "normal" (negative) to "reverse" (positive)

inclination of the magnetic field. Nevertheless, the lack of any significant difference in normal

and reverse polarity directions suggests that the different plutonic sub-units (and their

respective coolong paths) were separated only by relatively short time intervals. Indeed, the

motion of Gondwana during the 530-490 Ma interval was quite rapid (Meert et al. 2001b) and

a 5 to 10 Myr resolution would have been possible. Therefore, it is concluded that the whole

Carion pluton was built in less than 5 Myrs.

Pluton-related strain vs tectonic strain

At the time of Carion pluton emplacement (532 ± 5 Ma), transcurrent tectonics along the

Angavo shear zone was in its waning stage or already finished, as the Carion pluton is nearly

devoid of any solid state deformation and its structures did not come into parallelism with the

foliations and lineations of the shear zone. Moreover, a strain aureole with a maximum

thickness of 3 kilometres can be traced in many places around the pluton. This aureole is

responsible for a conspicuous foliation triple point to the south (Fig. 1c). Formation of such a

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triple point requires that the wall rocks were still ductile at the time of pluton emplacement

(Guglielmo 1993). Indeed, new Ar-Ar ages from the Angavo shear zones point to

temperatures well above 500°C at that time (Grégoire et al. 2008). In addition, a higher

degree of anisotropy is observed in the country rocks along the western border of the pluton

(Fig. 7b) and is also regarded as the result of pluton-related strain. The strain aureole is not

observed along the whole northern border of the pluton. There, foliations in the wall rocks

only come into parallelism only with those of the last sub-unit, in agreement with its last-

emplaced character. In the nearby Angavo shear zone (further East from the pluton), steeply

dippping foliations are striking to the north and lineations are also trending to the north with

subhorizontal to gentle plunges (Nédélec et al. 2000). Close to the Carion pluton, foliations

display variable dips and lineations are more scattered (Fig. 7c-d). These observations are

consistent with pluton-related strain around a post-tectonic intrusion.

The pluton-related strain aureole can have resulted from a ballooning effect. However,

the rather steep lineations observed in the pluton also suggest that the different magmatic

pulses may have been emplaced as individual diapirs. Diapiric emplacement cannot be

advocated for granitic magmas emplaced in the brittle upper crust (Petford 1996). Here, due

to the sustained high temperature conditions in the country rocks, diapiric emplacement was

possible. The first diapirically-emplaced sub-unit would have favoured the emplacement of

the following magma batches, because of additional heat advection and country rock

softening due to magma ascent and emplacement. Then, in agreement with the model of

sequential diapirism of Weinberg (1997), the subsequent magma diapir rises into and across

the former one, finally building a nested intrusion.

CONCLUSIONS

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The Carion pluton, east of Antananarivo, was emplaced as four nested magmatic sub-units

identified by their magnetic foliation patterns, the complex zoning of magnetic susceptibility

magnitudes and the location of the highest magnetic anisotropies along some internal

contacts. Their relative chronology of emplacement is consistent with previous paleomagnetic

data showing that cristallization of the pluton covered a time interval sufficient for at least one

magnetic reversal, resulting in a dual polarity (from normal to reverse) in the oldest parts of

the pluton and a single reverse polarity in the youngest parts of the pluton. Steep lineations

and lack of solid-state deformation point to magma ascent and emplacement occurring in the

waning stage of the transcurrent tectonics along the Angavo shear zone. A pluton-related

strain aureole is possibly due to the diapiric ascent of the sequential magmas batches.

ACKNOWLEDGMENTS

The Madagascan authors received financial support from the French Ministry of Cooperation

through the Campus project "Geology of the Precambrian basement of Madagascar and its

mineralisations". J. Meert received support from a NSF grant EAR98-05306. J.L. Bouchez, K.

Benn and an anomymous reviewer contributed to the substantial improvement of the

manuscript. C. Cavaré-Hester, P. Lespinasse, F. de Parceval and A.M. Roquet are warmly

acknowledged for technical assistance.

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- 20 -

Figure captions

Figure 1. (a) Gondwana reconstruction at 550 Ma after Lawver et al. (1998). (b) The East African

Orogen at 550 Ma after Abdelsalam & Stern (1996), Nédélec et al. (2000) and Reeves & de Wit

(2001) ; A, BR, M, PC : Achankovil, Bongolava-Ranostsara, Moyar and Palghat-Cauvery shear

zones ; E.L. : Enderby Land. (c) Location of AMS sampling sites.

Figure 2. (a) Characteristic dome outcrop in Carion pluton. (b) Typical porphyritic Carion granitoid

with conspicuous foliation. (c) Contact between prophyritic grey granitoid and and pink granite at site

DG52. (d) Elongate xenolith of migmatite at site DG 5. (e) Veining network of porphyritic granitoid in

wall rock migmatite at site DG 9. (f) Microgranumlar mafic enclave within a granitic host at site DG

5.

Figure 3 (c) Microcline megacryst (Mi), biotite (Bi) and quartz (Q) in sample DG 62 (crossed polars).

(d) Elongate quartz grain (Q), slightly perthitic K-feldspar (FK) and biotite (Bi) in sample DG60

(crossed polars). (e) Cluster of hornblende (Hb) and magnetite (Mg) along the boundary of a

subautomorphic plagioclase (Pl) in sample DG 52 (crossed polars). (f) Quartz (large undeformed

grain), biotite and associated magnetite in sample DG 60 (crossed polars).

Figure 4. (a) Plot of Carion whole-rock compositions in the R1-R2 diagram of la Roche et al. (1980).

(b) Location of analyzed samples and inferred normal concentric zoning.

Figure 5. Projection diagrams (lower hemisphere) for all Carion pluton and country rock sampling

sites : open symbols : AMS results for each specimen ; solid symbols : averages for each site.

Figure 6. Thermomagnetic curves (K vs temperature) of representative samples from the

Carion pluton.

- 21 -

Figure 7. (a) Magnetic susceptibility map and location of the four sub-units. (b) Magnetic anisotropy

degree map : values are presented as (P-1) x 100 for sake of clarity. (c) Magnetic foliation map and

projection diagram of foliation poles (lower hemisphere ; contour lines at 4 and 8 for the pluton and at

2, 4 and 8 % for the country rocks). (d) Magnetic lineation map and projection diagram (lower

hemisphere ; contour lines at 4 and 8 for the pluton and at 2, 4 and 8 % for the country rocks).

Figure 8. Location of the mixed and single polarity sites of Meert et al. (2001b) with respect to the

here-defined four sub-units

- 22 -

Table captions

Table 1. Representative mineral compositions and structural formulae.

Table 2. AMS data from the Carion pluton and its country rocks; see text for significance of K, P, F

and L.

Bur

Nadj fault system

Nadj fault system

Kenya

Bur

Kenya

EthiopiaSomalia

Aswa fault

Ang

avo

Ang

avo

E.L.PC

M

A PC

M

A

EastAntarctica

Dharwarcraton

EastAntarctica

India

Antongilcraton

Arabian-NubianShield

SriLanka

SriLanka

Congocraton

1000 km

Tanzania

Aswa fault

Moz

amb i

q ue

b el t

Moz

amb i

q ue

b el t

ductile shear zones

ophiolitic sutures

Pan-African orogens

unreworked Archaean cratons

BRBR

Carion

E-ANTARCTICAE-ANTARCTICA

WESTAFRICAWEST

AFRICA

AMAZONIAAMAZONIA

CONGO INDIA

KALA-HARI

CONGO

KALA-HARI

EASTGONDWANA

WESTGONDWANA

WESTGONDWANA

AUSTRALIA

INDIA

Madagascar

AUSTRALIA

Fig.1b

Cratons > 1Ga

Pan-African belts

a b

c

LacMantasoa

MantasoaMantasoa

5 km

ManjakandrianaManjakandriana

I k o p a

47°45ʼE8°45ʼS

Fig 1

DG58

DG24

DG61

DG35

DG42 DG36DG37

DG62 DG38

DG39DG40

DG45

DG43DG46

DG44

DG41

DG52DG53

DG57

DG54DG55

DG56

DG48 DG49DG50

DG17

DG14

DG13DG12

DG2

DG1DG15 DG10

DG4

DG3DG5

DG6

DG9

DG16DG18

DG19

DG20

DG21DG34

DG22

DG23

DG51DG59

DG73

DG72DG69

DG68

DG67

DG25

DG26DG27

DG28

DG30

DG31 DG32

DG33DG29

DG66

DG64

DG7

DG8

DG63

DG60DG65

DG71

TV40

TV39

TV37

TV43

TV44

TV36

TV42

MG58MG149

MG147

MG59

TV31

TV32

TV41

TA1

MG148

DG58

DG24

DG61

DG35

DG42 DG36DG37

DG62 DG38

DG39DG40

DG45

DG43TV38TV38 DG46

DG44

DG41

DG52DG53

DG57

DG54DG55

DG56

DG48 DG49DG50

DG17

DG14

DG13DG12

DG2

DG1DG15 DG10

DG4

DG3DG5

DG6

DG9

DG16DG18

DG19

DG20

DG21DG34

DG22

DG23

DG51DG59

DG73

DG72DG69

DG68

DG67

DG25

DG26DG27*

DG28

DG30*

DG31 DG32

DG33DG29

DG27DG27

DG30DG30

DG66

MG57MG57

DG64

DG7

DG8

DG63

DG60DG65

DG71

TV40

TV39

TV37

TV43

TV44

TV36

TV42

MG58DG59* MG149

MG147

MG59

TV31

TV32

TV41

TA1

MG148

CarionCarion

Carion plutonQuaternary sediments

Country rocksFoliation trajectories after Besairie (1969)

ba

dc

1 m1 m1 m

pinkgranitepink

granitepink

graniteporphyritic

greygranitoid

porphyriticgrey

granitoid

porphyriticgrey

granitoid

porphyriticgranitoid

porphyriticgranitoid

porphyriticgranitoid migmatitemigmatitemigmatite

migmatite

migmatite

migmatitegranitoid

granitoid

granitoid

granitoid

granitoid

granitoid

fe

Fig. 2

mafic enclavemafic enclavemafic enclave

granitic hostgranitic hostgranitic host

K-sparK-sparK-spar

NN

a b

dc

0.5mm 0.5mm

Mg Bi

Mg

Pl

Hb

Q

Q

Fig. 3

0

1000

2000

Syenogranite

Monzo-nite

MonzograniteMonzogranite

Tonalite

Granodiorite

Diorite

Gabbro

Calc-a

l ka

l ine

t ren

d

1000 2000 3000

Su

ba

lka

l ine

t rend

GC3GC3

GC24

GC14

GC25

GC12

GC6

a b

granite

granodiorite

tonalite

Alk

al ine t rend

R1 = 4Si - 11(Na + K) - 2(Fe + Ti)

R2 = 6Ca + 2Mg + Al

Fig .4

GC20GC20

DG 72

DG 73

DG 59

DG 1 DG 2 DG 3 DG 5 DG 7 DG 8 DG 10DG 9

DG 12 DG 13 DG 14 DG 15 DG 18 DG 19 DG 20 DG 21

DG 22 DG 23 DG 27 DG 28 DG 30 DG 34

MG 58 MG 59

country rocks

TAN 1 TV 31 TV 32 TV 39 TV 40

DG 6 DG 17 DG 25 DG 26DG 24

DG 29 DG 32 DG 37DG 36DG 33 DG 35DG 31

DG 4 DG 27

DG 30

DG 38 DG 39 DG 41 DG 43 DG 45DG 44DG 42DG 40

DG 46 DG 48 DG 50 DG 53

DG 55

DG 54DG 51 DG 52DG 49

DG 56 DG 57 DG 59 DG 61

DG 63

DG 62DG 60DG 58

DG 64 DG 65 DG 67 DG 71DG 69DG 66

TV 36

TV 41 TV 43 TV 44TV42

TV 37MG 57 MG 149 TV 38MG 148

K1

K2

K3

Fig. 5

Carion pluton

DG 16

600

Fig. 6

DG 51

MG 57

TV 41

0

100

200

300

400

500

1000 200 300 400 500 600 700

Temperature (°C)

K (

x 10

-6)

SI

K (

x 10

-6)

SI

K (

x 10

-6)

SI

0

40

80

120

120

0

40

80

160

200

Carion Manjakandriana

Ambatomanga

RRRR

RR

RR

RR

RR

RR

RR

RR

RR

4

1

2 3

Fig. 8

N N N N

N N

N N

N N

Carion Manjakandriana

Ambatomanga