Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurrent...

Journal of African Earth Sciences xxx (2013) xxx–xxx

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Journal of African Earth Sciences

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Incipient charnockitisation due to carbonic fluid transfer related to latePan-African transcurrent tectonics in Madagascar; implicationsfor mobility of Fe, Ti, REE and other elements

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⇑ Corresponding author.E-mail address: [email protected] (A. Nédélec).

Please cite this article in press as: Nédélec, A., et al. Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurretonics in Madagascar; implications for mobility of Fe, Ti, REE and other elements. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013

Anne Nédélec a,⇑, Damien Guillaume a, Cécile Cournède a,b, Charley Duran a,c, Mélina Macouin a,Michel Rakotondrazafy d, Gaston Giuliani a

a GET, Observatoire Midi-Pyrénées, Université de Toulouse, CNRS, IRD, Franceb CEREGE, Université de Provence, Francec CERM, Université du Québec, Chicoutimi, Canadad Department of Earth Sciences, Université d’Antananarivo, Madagascar

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

Keywords:CharnockiteCO2

Magnetic susceptibilityMadagascarPan-African

Incipient charnockitisation of an A-type granite protolith has been recognized in a spectacular outcrop35 km north of Antananarivo. The charnockites form structurally-controlled dark greenish patches sug-gesting channelized fluid transfer, during and after partial reworking of the granite coeval with theAntananarivo virgation zone (ca 560 Ma) and/or the Angavo shear zone of late-Panafrican age (ca550 Ma). They are characterized by a significant increase of the bulk magnetic susceptibility. The graniticprotolith contains quartz, perthitic alkali feldspar, high-Ti hastingsitand accessory minerals (apatite, alla-nite, magnetite and zircon). The charnockitic granite contains quartz, perthitic alkali feldspar and ghost(altered) orthopyroxene crystals, as well as secondary low-Ti hastingsite surrounding orthopyroxene. Thelarge increase of magnetic susceptibility magnitudes is related to the formation of pockets of secondarymagnetite, spatially associated with quartz and other accessories, such as fluorine, calcite, bastnaesite,sphalerite, Ti-oxide and (Ca, REE)fluor-carbonates, in fluid percolation zones or in reaction rims aroundghost orthopyroxene. Fluid inclusions entrapped in quartz grains witness the presence of CO2-rich hydro-carbonic fluids of low salinity, that are more abundant in charnockites than in granites. It is suggestedthat the rocks underwent a rather long history of fluid percolation, leading to prograde and then retro-grade transformations. The corresponding metasomatic changes point to the mobility of Ti, Fe, Ca, Zn,F and REE. These changes are consistent with the CO2-rich nature of the percolating fluids.

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1. Introduction protoliths, and provided preliminary data witnessing the existence

Incipient prograde charnockites i.e., patches and veins of ortho-pyroxene-bearing granulite assemblages within amphibolite faciesgneisses (Pichamuthu, 1960; Hansen et al., 1984; Santosh et al.,1990), are generally regarded as evidence of influx of CO2-richand low-H2O activity fluids generally issued from the mantle andtransported through the lower crust (Touret and Huizenga, 2011;Touret and Nijland, 2013, and references therein). Well-known insouthern India (Hansen et al., 1987; Satish-Kumar and Santosh,1998), such conspicuous outcrops have been also reported else-where, including Sri Lanka (Hansen et al., 1987) and Madagascar(Rakotondrazafy et al., 2007). These latter authors observed incip-ient charnockitisation in a spectacular outcrop 35 km north ofAntananarivo. They also observed that the charnockites displaymuch higher magnetic susceptibilities than their granitic

of CO2 inclusions. However, their study was limited in scope. Here,magnetic properties, mineralogy and fluid nature are thoroughlyexplored from the same outcrop, in order to constrain the wholefluid transfer history and its consequences on chemical elementmobility. The studied outcrop provides a unique case of incipientcharnockitisation of an A-type granite protolith. Charnockitisationappears to be structurally controlled by a superimposed deforma-tion related to the late Pan-African (561–532 Ma) shearing eventsin northern central Madagascar (Grégoire et al., 2009; Nédélecet al., 2000). Magnetic properties and mineral reactions aredetailed to trace the history of fluid transfer and the relevantelement mobility. The nature of the fluids is determined by thestudy of fluid inclusions. Both magnetic and mineral data appearas useful tools to spot the main fluid channels. Therefore, this studymay have critical applications at different scales, and especially inthe interpretation of geophysical survey data and in miningexploration.

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2. Geological setting and field observations

The Precambrian basement of Madagascar represents the twothirds of the island and received a strong Panafrican metamorphicand magmatic overprint of the end of Neoproterozoic times. Thecentral part of Madagascar is everywhere intruded by so-calledstratoid (i.e. sheetlike) granites broadly conformable with theirgneissic country rocks. These granites are typical A-type graniticrocks likely emplaced in an extensional setting (Nédélec et al.,1995). They were dated at 630 Ma by the U–Pb zircon TIMSmethod (Paquette and Nédélec, 1998). They are characterizedby N–S striking foliations gently dipping to the west, and byWSW-trending lineations. This D1 structural pattern is ubiqui-tous in the gneisses, migmatites and granites of the Antananarivodomain and has been correlated with M1 metamorphic condi-tions at ca T = 700–750 �C and P = 400–500 MPa (Nédélec et al.,2000). Recent studies suggest that the D1 pattern may be relatedto tectonic events as early as ca 750 Ma (Tucker et al., this issue).Therefore, the age of the stratoid granites has remained an openquestion so far.

Later, this D1 pattern has been more or less deeply reworked bybroadly north-striking late Pan-African shear zones, that repre-sents the second most important tectonic event (D2) in centralMadagascar (Martelat et al., 2000; Grégoire et al., 2009). The corre-sponding D2 structural pattern is characterized by N–S steeply dip-ping to vertical foliations and N–S trending subhorizontallineations. The Angavo shear zone east of Antananarivo is one ofthese major D2 shear zones Ralison & Nédélec (1997) (Fig. 1a). Itis about 30 km wide and 800 km long. This shear zone was datedat ca 550 Ma after the age of synkinematic granites (Kröner et al.,1999; Raharimahefa and Kusky, 2010). The Carion pluton(Fig. 1b), emplaced at 532 ± 5 Ma (Meert et al., 2001) in its veryborder with nearly no orthogneissification structures (Razanatseh-eno et al., 2009), provides the youngest limit for the D2 event. TheAngavo shear zone was active in low-pressure granulitic condi-tions i.e., T = 750–800 �C and P = 300–350 MPa at the present out-crop level (Nédélec et al., 2000). Such high-grade conditions werelikely triggered by magma and fluid advection from deeper levels.The E–W striking Antanarivo virgation zone is another late-Pan-African structure possibly related to the same tectonic event orof slightly older age, as it was sealed by the Ambatomirantygranitic dykes (Fig. 1b and c) dated at 561 ± 4 Ma (Paquette andNédélec, 1998). The so-called Ambatomiranty granites are actuallymonzonitic to granitic and resemble the younger Carion plutonicrocks in composition (A. Nédélec, unpublished data). They are rec-ognized by their medium to dark grey colour and their fine-grainedsizes (<1 mm). These metaluminous ferropotassic rocks containamphibole, biotite and abundant accessories, especially titanite.

Nédélec et al. (2000) suggest that the virgation zone is a sinis-tral transpressive structure and regard it as the result of a distinctevent. Here, we consider that both the virgation zone and the Ang-avo shear zone represent structures collectively attributed to theso-called late Pan-African tectonic events (D2), that are character-ized by the formation of ductile shear zones and their nearby inter-ference deformation features. Carbonic fluid transfer in the Angavoshear zone is suggested by ubiquitous charnockites. Charnockiteswere also recognized locally along the Antatanarivo virgation zoneand north of this zone (Fig. 1c), where they were mapped followingNE–SW trending directions (Dottin et al., 1967). The Angavo shearzone remained hot for about 20 Myrs, and then cooled at 15 �C/kmuntil 520 Ma, and at 6 �C/km until 480 Ma, as evidenced by Ar–Arages (Grégoire et al. (2009). Lateral heat transfer from the shearzone and age rejuvenation occurred at a distance as far as 60 km,but no more as evidenced by Ar–Ar ages of (Meert et al., 2003). Thisis also the distance over which the D1 structures appear to have

Please cite this article in press as: Nédélec, A., et al. Incipient charnockitisationtonics in Madagascar; implications for mobility of Fe, Ti, REE and other element

been more or less ductilely reworked by the D2 event (Nédélecet al., 2000; Grégoire et al., 2009).

The studied outcrop lays precisely in an area where the graniticfoliations were reorientated due to the late-Pan-African D2 shear-ing event, forming interference folds such as the Andranovelonadome and its lateral steep limbs (Fig. 1c). Its coordinates areX = 492.879 and Y = 826.877. Most of the surface outcrop displaysa fine-grained (grain sizes �1 mm), well-foliated, grey granite, lo-cally turning to pink in colour. The pink granite appears as a minordifferentiate of the grey granite. Reworked foliations strike NE–SWand dip at a high angle. Ductile D2 shear zones locally deform themain foliation, without any change in the grain size or rock compo-sition. It is worth to notice that no remelting of the granite oc-curred during D2. Charnockites occurs as dark greenish patcheswith rather well-defined boundaries. Due to their colour, it wasnot possible to recognize any foliation in the field in these patches.The patches are generally ten to a few tens of centimetres in widthand several tens of centimeters up to 1 metre in length. Their ver-tical extents cannot be estimated, because the outcrop is a largesubhorizontal bed rock. However, these incipient charnockitepatches appear to be structurally controlled and related to the duc-tile D2 event, because they look either at a high angle with thegranite foliation with a typical spacing of several tens of centime-ters, or parallel to the foliation; also, they sometimes developpedin local shear zones (Fig. 2).

3. Material and methods

3.1. Sampling

One large block (MG235) was picked in the field, straddling thecontact between grey granite and charnockite. This block wascored in the laboratory to get two granitic samples (235-G) andtwo charnockitic ones (235-C). The other samples were cored di-rectly from the outcrop and oriented in the field. They representeight granite samples and thirty-six charnockite samples. Fromthese eight granite samples, four were taken from the pink granite(AF006).

3.2. Mineral studies

Thin sections were used for optical and electronic microscopy,and for in situ mineral analyses with the electronic microprobe.BSE (back scattered electron) imaging and EDS analyses were ded-icated to the study of reactional zones and to the analyses of thesmallest accessory phases using the JEOL 6360 scanning electronmicroscope of the GET laboratory (Toulouse). Operating conditionsare as follows: accelerating voltage 20 kV, magnification 40–1000,spot size 50 lm. Other minerals were analysed using the electronicmicroprobe CAMECA SX50 of the GET laboratory. Operating condi-tions were 15 kV and 20 or 10 nA (respectively for anhydrous orhydroxylated minerals), with a beam width of 1–5 lm.

3.3. Magnetic studies

All fifty-eight samples were used for magnetic susceptibilitymeasurements in the GET laboratory with a Kappabridge suscep-tometer (KLY-3S, Agico) working at a low alternative inductivefield with a sensitivity of 2 � 10�7 SI, allowing anisotropy discrim-ination below 0.2%. The bulk susceptibility magnitude K is given by(K1 + K2 + K3)/3. Other scalar parameters are P (=K1/K3), the totalanisotropy, L (=K1/K2), the linear anisotropy, F (=K2/K3) and T(=[2 ln(K2/K3)/ln(K1/K3)] � 1), the shape parameter of Jelinek(1981). The long axis of the magnetic ellipsoid, K1, corresponds

due to carbonic fluid transfer related to late Pan-African transcurrent tec-s. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.005


a

b

c

Fig. 1. Geological map of Madagascar (with location of studied outcrop).

A. Nédélec et al. / Journal of African Earth Sciences xxx (2013) xxx–xxx 3

to the magnetic lineation, whereas the short axis, K3, defines thepole of the magnetic foliation.


Thermal magnetic curves of representative samples were ob-tained using a CS2 apparatus coupled to the KLY-2 Kappabridge



Fig. 2. Incipient charnockites developped either perpendicular (a) or parallel (b) tothe granite foliation, or sometimes in relation with local oblique shear zones (c).


isntrument. Four samples (two granites and two charnockites)were further investigated by performing Isothermal RemanentMagnetization (IRM) experiments, hysteresis parameters determi-nations and First Order Reversal Curves (FORC) measurements. Allthese measurements were carried out with a Vibrating SampleMagnetometer (VSM) at the IPGP laboratory (Paris, France). Anaveraging time of 100 ms was used and 100 minor loops for eachFORC diagram were measured. The smoothing factor was set at 5for all the FORC diagrams.

3.4. Fluid inclusion study

Twenty-two doubly-polished thick sections were prepared fromselected granitic and highly-charnockitised samples. They wereexamined by standard microscopy using an Olympus BX51 micro-scope before undertaking microthermometric measurements atGéosciences Environnement Toulouse (GET, Toulouse, France) fol-lowing the procedures outlined by Roedder (1984) and Shepherd


et al. (1985) using a Linkam THMSG-600 heating–freezing stageconnected to a programmable thermal control unit. The stagewas calibrated at CO2 triple point (–56.6 �C) and critical point(31.1 �C) using pure CO2-bearing natural fluid inclusions in quartzfrom Camperio, Switzerland (Mullis et al., 1984) and at H2O triplepoint (0.2 �C) and critical point (375.13 �C) using H2O syntheticfluid inclusions in quartz supplied by SynFlinc. The analytical errorwas constant at 0.2 �C for temperatures near or below room tem-perature, and 1.0 �C for higher temperatures. Cryogenic experi-ments were carried out before heating runs to avoid the risk ofdecrepitating the inclusions.

Petrographic analyses of the samples was used to observe anddescribe the fluid inclusions within quartz in granite and char-nockitised samples, in order to understand the history of fluid cir-culations. Microthermometry was used to explore phasestransitions within the fluids between �100 �C and high tempera-ture (max + 600 �C); fusion of solids (gaz, hydrates and aqueousphases), eutectic and total homogeneisation of the inclusions.

4. Petrography and mineral chemistry

4.1. Microscopic observations

The grey granite protolith is made of quartz, perthitic alkalifeldspar, green amphibole and accessory minerals (apatite, allanite,magnetite and zircon). Plagioclase is absent or only present as late-magmatic albite rims around some perthitic alkali feldspars. Elon-gate crystals of amphibole enable to trace the magmatic foliationplane (Fig. 3a). This granite is very similar to the hypersolvus A-type granites described in the area by Nédélec et al. (1995). Thepink granite appears as a leucocratic differentiate, lacking theamphibole and slightly richer in quartz.

The charnockitized granite contains quartz, perthitic alkali feld-spar and ghost (altered) orthopyroxene crystals. This pyroxene is no-where devoid of alteration and was recognized mainly owing to itstypical habitus. The distribution of the pyroxene crystals does notseem homogeneous. Rather, they seem to have grown in patches ata close distance from the primary amphibole (Fig. 3b), but not in con-tact with it. The initial magmatic foliation is less obvious than in thegranite, but the primary texture is still recognizable and no meltingcan be evidenced. In some cases, the primary amphibole is still pres-ent. Elsewhere, a secondary bluish amphibole is recognized (Fig. 3c).The charnockites generally contain a larger amount of iron oxidesthan the granites (respectively: 1–5% and less than 1%, after imageanalysisof thin sections). Some of them display a (sub)hedral habitas in the granite, whereas others display xenomorphic tortuousshapes and seem to have developped along grain boundaries. Theyare usually associated with quartz and other accessory mineralssuch as allanite (Fig. 3d). Such a conspicuous association is commonin charnockites, but very rare in the granite.

4.2. SEM observations and EDS analyses

Bastnaesite, (La, Ce, Nd)CO3F, has been evidenced in the grainboundaries and feldspar cleavages in the grey granite (Fig. 4a and b).

In the charnockites, ghost orthopyroxene is replaced by a mix-ture of calcite and amphibole (Fig. 4c–e). Graphite, bastnaesite andsphalerite were also evidenced in symplectitic reaction zones be-tween altered orthopyroxene and bluish secondary amphibole(Fig. 4f and g). Tiny crystals of fluorite and calcite were also locallyobserved in these reaction zones. The secondary amphibole oftencontain small quartz inclusions (Fig. 4c and f).

Primary oxides are (sub)hedral magnetite with typical exsolu-tion lamellae of ilmenite (Fig. 4h). Xenomorphic oxides are mainlyTi-poor magnetite (Fig. 4i). A Th-rich allanite, sphalerite and a REE-



Fig. 3. (a) Microphotograph of the granitic protolith, characterized by pronounced magmatic foliation and lineation, due to the preferred orientation of magmaticferrohastingsite; (b) detailed microphotograph of charnockitized granite, with new orthopyroxene crystals and preserved (although compositionally-modified) amphibole;(c) black-and-white image of the charnockite (mafic silicates are in black, felsic silicates in white), showing the inhomogeneous distribution of orthopyroxene crystals inpatches surrounding the magmatic amphibole. (d) Magnetite-rich pocket with allanite in charnockite AF1B2; scale bar = 0.5 mm.


fluorocarbonate mineral, such as parisite ((Ce, La, Nd)2Ca(CO3)3F2)or synchisite ((Ce, La, Nd)2Ca(CO3)2F), were recognized in associa-tion with this late iron oxide in a charnockite sample (Fig. 4i–k). Apeculiar magnetite-rich patch in the pink granite appears spatiallyassociated with two generations of a symplectitic association of Ti-oxide (rutile or anatase) and calcite (Fig. 4l). A (Ca, REE)fluorocar-bonate phase is also present in this symplectite (Fig. 4m and n).

4.3. Mineral chemistry

Microprobe amphibole analyses are presented in Table 1. Thegreen primary amphibole is an iron-rich hastingsite (XMg = 0.01).Amphibole compositions are plotted in Fig. 5. The bluish secondaryamphibole is very similar to the primary hastingsite in composi-tion, but lacks Ti (Fig. 5a): it contains only 0.01–0.02 atom per for-mula unit (apfu) instead of 0.17–0.22 apfu in the primaryamphibole. It also contains higher amounts of Fe3+ (Fe2O3 up to10 wt.% i.e., Fe3+ up to 1.2 apfu), compensated by lesser amountsof Na (Fig. 5 b). Finally, the secondary amphibole also displays lessCl (Fig. 5c), but there is no change in F contents compared to theprimary hastingsite (Table 1), despite the very iron-rich nature ofthe amphiboles. These features show that the secondary amphi-bole does not contain any riebeckite component and rather is anoxidized hastingsite.

Analyses of secondary calcite are presented in Table 2. Despitesome contamination by small including silicates, the calcite ap-pears to contain a few percents manganese and iron.

5. Rock magnetic properties

5.1. Susceptibility magnitudes

Charnockites correspond to much higher susceptibility magni-tudes than granites, as already pointed by the preliminary workof Rakotondrazafy et al. (2007). The present study confirms thisobservation using a larger number of samples. The average valueof K, the susceptibility magnitude, is 9197 � 10�6 SI for the greygranites, with respect to an average of 27,983 � 10�6 SI for thecharnockites (Table 3). The pink granites display slightly lower val-ues (average K = 8114 � 10�6 SI) than the grey granites (averageK = 9919 � 10�6 SI), in agreement with their more leucocratic char-acter. However, all rocks are in the field of magnetite-bearing gra-


nitic rocks (Bouchez, 2000). Indeed, all stratoid granites studied sofar in the Antananarivo domain have proven to show large suscep-tibility magnitudes due to their high magnetite content. Charnock-itic rocks display a large increase of their magnetic susceptibilitymagnitudes (three times higher in average), suggesting the forma-tion of abundant secondary magnetite.

5.2. Magnetic mineralogy

Thermomagnetic curves (Fig. 6) confirm the presence of magne-tite in all rock types. The very clear decrease of the magnetic sus-ceptibility at 580 �C corresponds to the Curie temperature of puremagnetite. Unsurprisingly, the pink granite contains haematite inaddition to magnetite as shown by the existence of another phasewith a higher Curie temperature.

The IRM-acquisition curves (Fig. 7) for both granites and char-nokites are similar and display mostly one soft component (B1/2 = 22–63 mT), except for the granite sample AF2a (B1/2 = 63 mT(74%) and 20 mT (26%)), that contain a mixture of two componentsidentified using the fitting method of Kruiver et al. (2001).

Hysteresis parameters are useful to determine the domain-struc-ture of magnetite (see Dunlop, 2002a, 2002b). Both granites andcharnokites hysteresis parameters fall clearly into the multidomain(MD) grain size region of the classical Day plot (Day et al., 1977) indi-cating that the magnetite is very large (likely >50 lm and up to100 lm after microscopic observations). The magnetite from thecharnockites seems to be larger as shown by slighty lower Mrs/Msand higher Hcr/Hc ratio (Fig. 8a). The large size of magnetite in bothgrey granites and charnockites is also clear in FORC diagrams (seeFig. 8b) (Roberts et al., 2000; Muxworthy and Dunlop, 2002).

In conclusion, large magnetite grains seem to dominate theremanence and the susceptibility signal and differences betweengrey granites and charnockites and differences between bothrock types are attributed to a higher magnetite content in thecharnockites.

5.3. AMS data

All rocks are characterized by a high anisotropy degree (P) oftheir magnetic susceptibility. At first sight, the granites (averageP = 1.55) seem slightly more anisotropic than the charnockites(average P = 1.42). Actually, this difference is due to the pink gran-ites (average P = 1.83) that display significantly higher values than



Fig. 4. (a) Detail SEM image of perthitic feldspar with minor bastnaesite (white) in grain boundary and cleavages SEM image of granite MG235G2. (b) EDS spectrum ofbastnaesite (+ K-spar). (c) Ghost orthopyroxene and secondary amphibole in charnockite MG235C1. (d) Detail of secodnary calcite replacing orthopyroxene. (e) EDS spectrumof secondary phases replacing orthopyroxene. (f) Secondary amphibole with quartz inclusions and reaction rim. (g) EDS spectrum of sphalerite. (h) Primary Fe-Ti oxide withexsolution lamellae in charnockite AF1B1. (i) Interstitial secondary oxides (magnetite + ilmenite) spatially associated with a large allanite crystal and small (Ca, REE)F-carbonate grains (white): cf Fig. 3d. (j) EDS spectrum of Th-rich allanite. (k) EDS spectrum of (Ca, REE)fluor-carbonate. (l) Fe-oxide and quartz aggregate (pipe ?) with reactionrims containing TiO2 crystals and insterstitial calcite in pink granite AF6A1. (m) Detail of reaction rim with TiO2, calcite, (Ca, REE)F-carbonate (center, white) and bastnaesite(top center, small fracture infilling, white, in the reaction rim). (n) EDS spectrum of (Ca, REE)fluor-carbonate.


Please cite this article in press as: Nédélec, A., et al. Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurrent tec-tonics in Madagascar; implications for mobility of Fe, Ti, REE and other elements. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.005


Table 1Amphibole representative analyses and structural formulae on the basis of 13 cations + (Ca, Na, K).

Amphiboles Type I Type II (retrograde after pyroxene)Rock Granite Charnockite

Sample MG235G AF212 MG235C AF1B2

SiO2 37.77 38.63 37.87 38.20 38.05 38.27 38.30 38.38 37.85 38.60 38.82 38.57 37.72TiO2 1.32 1.49 1.67 1.34 1.79 0.00 0.00 0.15 0.05 0.13 0.01 0.19 0.11Al2O3 9.40 9.46 9.15 9.33 9.62 9.44 10.00 10.16 9.91 10.39 9.41 10.24 11.36Cr2O3 0.00 0.05 0.00 0.06 0.04 0.04 0.00 0.09 0.06 0.00 0.05 0.00 0.02Fe2O3 (c) 4.35 7.70 6.11 6.11 5.91 9.42 8.12 8.16 9.18 8.31 7.75 10.67 9.49FeO (c) 28.24 27.34 27.89 27.65 27.44 25.45 26.41 26.17 25.79 26.16 27.10 24.59 25.01MnO 0.50 0.59 0.61 0.64 0.80 1.06 0.66 0.90 0.89 0.89 0.54 1.13 0.94MgO 0.27 0.16 0.11 0.17 0.11 0.14 0.13 0.13 0.25 0.25 0.23 0.20 0.13CaO 10.17 10.07 9.96 10.10 9.85 10.43 10.37 10.41 10.54 10.49 10.53 9.91 10.50Na2O 1.81 1.79 1.93 1.69 2.00 1.21 1.40 1.38 1.45 1.39 1.37 1.68 1.35K2O 1.71 1.63 1.61 1.75 1.55 1.51 1.70 1.70 1.70 1.88 1.75 1.60 1.74NiO 0.03 0.01 0.00 0.01 0.02 0.00 0.00 0.00 0.06 0.06 0.11 0.00 0.01F 0.68 0.96 0.43 0.39 0.34 0.61 0.81 0.92 0.59 1.02 0.77 0.93 0.57Cl 0.33 0.41 0.43 0.36 0.28 0.05 0.09 0.10 0.10 0.06 0.03 0.17 0.08H2O(c) 1.39 1.30 1.50 1.54 1.59 1.52 1.42 1.37 1.52 1.35 1.46 1.37 1.56O@F 0.29 0.40 0.18 0.16 0.14 0.26 0.34 0.39 0.25 0.43 0.32 0.39 0.24O@Cl 0.07 0.09 0.10 0.08 0.06 0.01 0.02 0.02 0.02 0.01 0.01 0.04 0.02Sum Ox% 97.59 101.09 98.99 99.09 99.18 98.87 99.05 99.61 99.68 100.54 99.58 100.83 100.34

Si 6.330 6.250 6.271 6.304 6.260 6.304 6.299 6.275 6.206 6.253 6.361 6.218 6.109Ti 0.166 0.181 0.207 0.166 0.222 0.000 0.000 0.019 0.006 0.016 0.002 0.023 0.014Al IV 1.670 1.750 1.729 1.696 1.740 1.696 1.701 1.725 1.794 1.747 1.639 1.782 1.891Al VI 0.186 0.055 0.057 0.119 0.126 0.138 0.237 0.232 0.122 0.237 0.178 0.164 0.278Cr 0.000 0.006 0.000 0.008 0.005 0.006 0.000 0.012 0.008 0.000 0.007 0.000 0.002Fe3+ 0.548 0.938 0.762 0.758 0.731 1.167 1.005 1.004 1.132 1.013 0.955 1.294 1.157Fe2+ 3.958 3.700 3.862 3.815 3.776 3.506 3.633 3.578 3.537 3.544 3.714 3.315 3.387Mn 0.071 0.081 0.086 0.090 0.112 0.148 0.092 0.125 0.124 0.122 0.075 0.155 0.129Mg 0.067 0.038 0.026 0.043 0.026 0.035 0.033 0.031 0.062 0.060 0.055 0.049 0.031Ca 1.825 1.746 1.768 1.786 1.736 1.840 1.827 1.823 1.852 1.820 1.848 1.711 1.822Na(B) 0.175 0.254 0.232 0.214 0.264 0.160 0.173 0.177 0.148 0.180 0.152 0.289 0.423Na(A) 0.413 0.309 0.388 0.325 0.375 0.227 0.275 0.262 0.312 0.257 0.282 0.236 0.245K 0.365 0.336 0.341 0.368 0.325 0.317 0.357 0.355 0.356 0.388 0.365 0.330 0.359Ni 0.004 0.002 0.000 0.002 0.003 0.000 0.000 0.000 0.009 0.008 0.014 0.000 0.002F 0.359 0.489 0.225 0.204 0.175 0.318 0.421 0.474 0.305 0.520 0.399 0.475 0.290Cl 0.092 0.112 0.121 0.100 0.078 0.013 0.026 0.027 0.028 0.016 0.007 0.047 0.021OH 1.548 1.399 1.653 1.696 1.746 1.669 1.553 1.499 1.667 1.464 1.593 1.478 1.689Sum cations 17.779 17.645 17.728 17.693 17.700 17.544 17.632 17.617 17.668 17.645 17.647 17.566 17.604

XMg = Mg/(Mg + Fe2+) 0.017 0.010 0.007 0.011 0.007 0.010 0.009 0.009 0.017 0.017 0.015 0.014 0.009XF = F/(F + Cl + OH) 0.180 0.245 0.113 0.102 0.088 0.159 0.211 0.237 0.153 0.260 0.200 0.234 0.145

c: Calculated.


the grey granites (average P = 1.36). Such a difference may be dueto the presence of haematite (Nédélec et al., 1994). All rocks havelinear anisotropies (L) slightly more pronounced than their planaranisotropies (F), hence average negative T parameters (Jelinek,1981) indicative of slightly prolate magnetic susceptibility ellip-soids (see Table 3).

In these high-susceptibility rocks, magnetic lineations and foli-ations mainly reflect preferred orientations of magnetite andusually mimic the mineral lineations and foliations (Grégoireet al., 1998). The magnetic lineations (K1) of all samples clusteredat 237/13 and 237/8 respectively in the granites and in charnock-ites and the only difference appears a slightly better clustering inthe granites (Fig. 9). In both cases, magnetic foliation poles arearranged around a zone axis corresponding to the best lineation,although foliations possibly dip at a higher angle in average inthe charnockites. These structural features are typical of rocksdeformed in a constrictional regime, that appears to be consistentwith the development of more or less tightened interferencefolding during D2 reworking.

6. Fluid inclusion chemistry

6.1. Petrography of fluid inclusions

Microscope observation of the samples shows that fluid inclu-sions are present in both granite and charnockitised samples as


primary aqueous fluid inclusions (Fig. 10a and b) and trails cross-cutting the quartz crystals (Fig. 10c and d). Primary fluid inclusionsshow evidence of having being strongly affected by deformation(Tarantola et al., 2010) and most of them have probably leakedor the fluid has evolved. Fluid inclusions in the trails are three-phased in both samples and are much more abondant in the char-nockitised samples. They show an aqueous liquid, a liquid gaz anda vapour at room temperature with almost the same relative ratios.They show regular forms (negative crystal or ovoid shape) andrange in size from less than 1 lm to 15 lm. Consequently, it is sug-gested that a primary aqueous fluid is present in the granite and afluid event was recorded by secondary fluid inclusions in bothtypes of samples.

6.2. Microthermometry of fluid inclusions

Microthermometry has been performed on 28 CO2 fluid inclu-sions from granite samples and 8 fluid inclusions from charnocki-tised samples. Other fluid inclusions were too small formicrothermometric analyses, thus statistical interpretation of thedata was not possible. Microthermometry has not been performedon aqueous inclusions as they are strongly affected by thedeformation.

The melting temperature of the gas phase (Tm gas) is close tothe CO2 triple point (�56.6 �C) with measurements ranging downto �58 �C (Fig. 11a), in both granite and charnockitised samples.



0

0.1

0.2

0.3

0.4

0

0Fe2O3 (wt % calculated)

TiO

2 (w

t %)

Cl (

wt %

)

Granite : hb I

Charnockite : hb II

Na2O (wt %)

Na2O (wt %)

0

0.5

1

1.5

2

0.5 1 1.5 2 2.5

0 0.5 1 1.5 2 2.5

2 4 6 8 10 12

b

c

TiO

2 (w

t %)

0

0.5

1

1.5

2a

Fig. 5. Amphibole composition diagrams.

Table 2Representative compositions of calcite in charnockite MG235C.

Oxides Calcite

CaO (wt.%) 55.86 47.06 44.66MgO 0.29 0.58 0.53Al2O3 0.10 0.24 0.31SiO2 1.02 1.60 1.70MnO 2.24 3.14 4.86FeO 1.09 3.22 3.25


Eutectic and ice melting temperatures of the aqueous phase couldnot be measured probably because of the very small size of thefluid inclusions, a very low amount of the aqueous phase and theformation of a clathrate. Thus the composition of the aqueousphase could not be explored from these data. Clathrate meltingtemperatures (Tm Clath.) were measured for 10 fluid inclusionsbetween 7.7 and 9.4 �C in granite samples corresponding to salin-ities varying between 1.5 and 8.7 wt.% eq. NaCl (Zhang and Frantz,


1987). Tm Clath. was measured for only one fluid inclusion at8.4 �C in a charnockitised sample (Fig. 11b) corresponding to asalinity of 5.8 wt.% eq. NaCl (Zhang and Frantz, 1987). Totalhomogenisation of the CO2 fluid inclusions in both granite andcharnockitised samples always occurred at temperatures rangingfrom 250 to 375 �C, with a peak at 260 �C (Fig. 11c).

The analytical results indicate that a mixed aqueous-carbonicfluid, trapped in the homogeneous field of the H2O–CO2 system,is abondant in the charnockitised samples and also present, butless abondant, in the granite samples. The carbonic phase is pureCO2 in a few fluid inclusions where Tf gas is measured very closeto the CO2 triple-point at �56.6 �C. In other fluid inclusions, theTf gas ranging from �57.2 to �58 �C is in agreement with theobservation of a small amount of N2 in some fluid inclusions withRaman spectroscopy (Rakotondrazafy et al., 2007). The aqueousphase was difficult to study by microthermometry because fluidinclusions are rather small and volume of aqueous phase is alsosmall, but the clathrate melting temperatures allow to calculatea low salinity of the aquous phase between 1.5 and 8.7 wt.% eq.NaCl.

Thus, two types of CO2-rich fluids are identified both in graniteand charnockitised samples: a low salinity H2O–CO2 fluid and alow salinity H2O–CO2–N2 fluid, these fluids being much moreabondant in the charnockitised samples.

7. Interpretation and discussion

7.1. Nature of the percolating fluids

The fluid inclusion study evidenced CO2-rich fluids present inall rocks types, but much more abondant in charnockitised sam-ples. This CO2 likely triggered orthopyroxne formation in the incip-ient charnockite patches as commonly observed elsewhere(Hansen et al., 1987; Srikantappa et al., 1992; Satish-Kumar andSantosh, 1998, among others). Indeed, influx of CO2-rich fluid iswell-known to promote the development of anhydrous granulitefacies parageneses (Touret, 1971; Newton et al., 1980; Santoshand Omori, 2008). The replacement of amphibole-bearing assem-blages by orthopyroxene-bearing assemblages will increase theH2O-content in the fluid phase. Because pyroxene did not remainstable in our case, it is expected that fluids changed in compositionand/or properties (T, fO2). Indeed, Endo et al. (2012) establishedthat orthopyroxene stabilization requires XFe3+ < 0.03. Thus, high-er oxygen fugacity conditions would result in orthopyroxene-freeassemblages. Composition of the secondary amphibole is indica-tive of an increasing oxygen fugacity (as discussed hereafter), thusdecreasing temperature or increasing O2-fugacity possibly explainthe disappearance of the orthopyroxene and the formation of thesecondary amphibole.

The mineral phases crystallized in reactional sites provide con-straints for the fluid evolution. Formation of retromorphic amphi-bole after pyroxene required some water influx. The lack of Ti inthis secondary amphibole is evidence of a lack of O2� substitutingfor OH� in the relevant crystallographic site, hence it is consistentwith both Ti-mobility and/or less oxidizing conditions (Hawthorneet al., 1998). However, identification of secondary amphibole asoxidized hastingsite precludes the latter explanation. Graphitedeposition is generally indicative of both fO2 and temperature de-crease during retrogression of granulites (Bento dos Santos et al.,2011; Huizenga and Touret, 2012). Indeed, the upper graphite sta-bility limit is under the QFM buffer when T > 650 �C and logfO2 > �18, whereas it lays above the QFM at lower temperaturesand lower fO2. In the studied samples, graphite is a rare accessorymineral formed in some reaction zones surrounding altered ortho-pyroxene. As decreasing fO2 seems unlikely during the fluid evolu-



Table 3AMS scalar and directional data.

Scalar data Directional data

K (10�6 SI) K1 K2 K3 P L F T K1 (lineation) K3 (foliation pole)

Azimuth Plunge Azimuth Plunge

GRANITESAF002A1 9143 10,826 9033 7569 1.43 1.20 1.19 �0.01 247 6 350 64AF002A2 7673 8621 7752 6646 1.30 1.11 1.17 0.18 246 5 345 60AF002B1 8294 9449 8253 7180 1.32 1.14 1.15 0.01 232 23 335 29AF002B2 12,856 14,408 13,192 10,970 1.31 1.09 1.20 0.35 227 24 337 37AF006A1 247 5974 4310 3208 1.86 1.39 1.34 �0.05 246 10 102 78AF006A2 7171 10,019 6565 4929 2.03 1.53 1.33 �0.19 242 13 116 68AF006B1 12,057 15,867 11,383 8921 1.78 1.39 1.28 �0.15 230 9 96 77AF006B2 8731 11,203 8123 6866 1.63 1.38 1.18 �0.31 223 14 6 73MG235-G1 11,487 13,908 10,979 9573 1.45 1.27 1.15 �0.27MG235-G2 10,064 11,709 9754 8730 1.34 1.20 1.12 �0.24

(n = 10)Mean 8772 11,199 8934 7459 1.55 1.27 1.21 �0.07Std. deviation 3548 2948 2549 2264 0.26 0.14 0.08 0.21

CHARNOCKITESAF001B1 29,167 35,243 28,982 23,277 1.51 1.22 1.25 0.06 228 16 318 3AF001B2 54,990 69,261 52,836 42,872 1.62 1.31 1.23 �0.13 229 21 139 0AF003A1 26,717 31,772 26,389 21,991 1.44 1.20 1.20 �0.01 211 25 341 54AF003A2 17,644 20,710 16,824 15,399 1.34 1.23 1.09 �0.40 206 14 98 50AF003B1 53,119 63,163 54,273 41,921 1.51 1.16 1.29 0.26 246 6 343 51AF003B2 50,863 63,256 50,268 39,065 1.62 1.26 1.29 0.05 230 34 353 38AF004A1 32,900 40,911 29,944 27,846 1.47 1.37 1.08 �0.62 232 4 53 86AF004A2 28,845 34,710 28,411 23,415 1.48 1.22 1.21 �0.02 234 9 121 67AF004B1 25,072 30,202 23,437 21,576 1.40 1.29 1.09 �0.51 51 2 317 64AF004B2 25,754 31,680 24,491 21,091 1.50 1.29 1.16 �0.27 57 7 304 73AF005A 20,637 23,844 20,965 17,102 1.39 1.14 1.23 0.23 218 15 347 67AF005B1 14,750 18,193 14,274 11,784 1.54 1.27 1.21 �0.12 227 12 95 72AF005B2 17,850 21,058 17,660 14,833 1.42 1.19 1.19 �0.00 227 14 64 76AM001A1 10,059 12,252 9367 8558 1.43 1.31 1.09 �0.50 245 5 349 70AM001A2 15,192 18,884 13,846 12,847 1.47 1.36 1.08 �0.61 247 4 341 50AM001B 36,109 40,357 35,913 32,058 1.26 1.12 1.12 �0.01 38 3 302 65AM002A 34,133 39,428 31,604 31,366 1.26 1.25 1.01 �0.93 233 18 329 17AM002B1 18,948 21,668 19,391 15,786 1.37 1.12 1.23 0.30 230 6 135 43AM002B2 53,345 59,346 53,069 47,621 1.25 1.12 1.11 �0.02 29 19 133 36AM003A 55,473 71,781 53,340 41,299 1.74 1.35 1.29 �0.07 228 18 332 38AM003B 31,448 39,101 29,252 25,991 1.50 1.34 1.13 �0.42 84 5 176 24AM004A 25,163 30,283 24,659 20,546 1.47 1.23 1.20 �0.06 235 16 140 18AM004B1 29,131 36,900 29,423 21,070 1.75 1.25 1.40 0.19 242 15 335 13AM004B2 12,484 14,623 12,265 10,564 1.38 1.19 1.16 �0.08 237 14 330 10AM005A1 33,040 39,395 32,071 27,653 1.42 1.23 1.16 �0.16 73 25 180 32AM005A2 10,584 12,764 10,515 8473 1.51 1.21 1.24 0.05 72 27 174 22AM005B1 20,657 24,250 20,576 17,144 1.41 1.18 1.20 0.05 231 7 328 43AM005B2 16,541 19,994 16,497 13,132 1.52 1.21 1.26 0.09 228 11 339 61AM006A 27,108 31,287 26,890 23,148 1.35 1.16 1.16 �0.01 51 2 319 45AM006B1 39,689 46,840 39,067 33,160 1.41 1.20 1.18 �0.05 239 18 351 49AM006B2 21,314 23,734 20,917 19,292 1.23 1.13 1.08 �0.22 254 23 1 35AM007A1 13,944 16,552 13,063 12,217 1.35 1.27 1.07 �0.56 256 17 166 1AM007A2 19,223 21,943 18,681 17,044 1.29 1.17 1.10 �0.27 243 18 341 21AM007B1 13,112 15,684 12,463 11,188 1.41 1.26 1.11 �0.36 250 5 340 1AM007B2 16,377 19,069 15,825 14,237 1.34 1.21 1.11 �0.28 75 3 345 9AM008A1 38,889 47,628 35,297 33,741 1.41 1.35 1.05 �0.74 244 15 341 25AM008A2 33,168 39,714 31,009 28,782 1.38 1.28 1.08 �0.54 240 15 146 14AM008B1 35,922 41,450 35,888 30,428 1.36 1.15 1.18 0.07 67 4 336 2AM008B2 46,152 53,511 47,440 37,504 1.43 1.13 1.26 0.32 234 13 327 12AM009A1 26,216 32,520 24,656 21,474 1.51 1.32 1.45 �0.33 167 17 74 9AM009A2 23,059 26,930 23,651 18,596 1.45 1.14 1.27 0.30 174 14 81 12AM009B1 32,399 36,331 31,891 28,974 1.25 1.14 1.10 �0.15 243 11 340 34AM009B2 16,483 19,337 16,438 13,674 1.41 1.18 1.20 0.06 258 9 10 67AM010A1 21,013 23,329 21,990 17,720 1.32 1.06 1.24 0.57 81 11 345 29AM010A2 21,743 27,551 18,867 18,810 1.46 1.46 1.00 �0.98 248 9 351 57AM010B 56,417 66,779 55,438 47,032 1.42 1.20 1.18 �0.06 253 15 349 23MG235-C1 25,021 28,768 24,861 21,433 1.34 1.16 1.16 0.01MG235-C2 15,325 16,937 14,997 14,041 1.21 1.13 1.07 �0.30

(n = 48)Mean 27,983 33,352 27,289 23,308 1.42 1.22 1.17 �0.15Std. deviation 12,885 15,745 12,746 10,382 0.12 0.08 0.08 0.33


tion, formation of graphite is mainly taken as evidence of decreas-ing temperature at the end of the fluid percolation history.


Conversely, graphite scarcity is an indirect evidence of the high(T > 650 �C) temperature of the magnetite-depositing fluids.



cooling

heating

heating

heating

cooling

cooling

a

b

c

Mag

netic

sus

cept

ibilit

y, K

(µ S

I)M

agne

tic s

usce

ptib

ility,

K (µ

SI)

Mag

netic

sus

cept

ibilit

y, K

(µ

SI)

Charnockite

Pink granite

Grey granite

Temperature (°C)

Temperature (°C)

Temperature (°C)

Fig. 6. Thermomagnetic curves.

200 400 600 8000

1

IRMSIRM

mT

0.2

0.4

0.6

0.8

0

charnockite AF5MG235C

granite AF2AMG235G

Fig. 7. Isothermal Remanence Magnetization (IRM) acquisition curves normalizedby maximum value.


REE-mobility during incipient charnockite formation was al-ready recognized by Stähle et al. (1987) and Ravindra Kumar(2004). From a more general point of view, REE-mobility is possibleat high temperatures as complexes with carbonates and especiallyfluorides. REE-mineral precipitation from CO2-rich fluids has beenrecognized by Smith and Henderson (2000) in the BayanObo ?tul?> world’s largest REE-deposit. The stability conditionsof assemblages containing bastnaesite, calcite, fluorite and otherREE-fluor-carbonates depend on pressure, temperature and theactivities of Ca2+, CO2�

3 and F in the fluid phase (Williams-Jones


and Wood, 1992; Ngwenya, 1994). Indeed, it is worth to notice thatthe secondary amphiboles display a relatively high F content foriron-rich end-members (XF in the range 0.15–0.26 forXMg � 0.01–0.02, see Table 1), poiting that they cristallized inequilibrium with a fluid of high HF activity. Such a fluid composi-tion is well-known to favour REE-mobility (Salvi and Williams-Jones, 2006).

Charnockites evidence significant iron enrichment from thefluid phase, that is in other cases usually correlated with a highCl content in the fluid, but may be also controlled by pH, redoxconditions and temperature of the fluid after Smith et al. (2012).These authors point to higher iron contents with higher fluid tem-peratures, and this is the retained interpretation here. The ubiqui-tous sphalerite is indicative of transportation of Zn, likely as acomplexed form with HS�. H2S is regarded as completely misciblewith a CO2-rich hydrocarbonic fluid (Smith et al., 2012). Therefore,metal transportation is likely efficient in such a fluid phase, even atlow salinity conditions i.e., in the absence of chlorides.

Finally, the formation of new Ti-oxides is evidence of some Ti-mobility in the fluids of interest, at least in the beginning of thefluid percolation history. Ti may derive from teh destabilizationof the primary (magmati)c amphibole. Ti is generally regarded asan immobile element under usual metamorphic conditions. Never-theless, Ayers and Watson (1993) show that Ti-solubility increaseswith increasing temperatures and decreasing pressures i.e., in con-ditions that are precisely those of the Late-Pan-African HT-LP gran-ulitic D2 event.

7.2. Chronology of the fluid transfer and reactions

Fluid percolation began during the D2 reworking of the graniticprotolith, as suggested by field observations and because granitesand charnockites do not show any difference in their AMS signa-tures corresponding to D2-reworking. This fluid was carbonic-richand at a temperature high enough to ensure some mobility of Feand Ti. Iron oxide precipitation likely occurred at the same timethan orthopyroxene formation. Then, the fluid temperature de-creased (and possibly some changes of fugacity and in water con-tent also occurred) and this late evolution was responsible for theretrograde transformation of pyroxene in calcite and for the forma-



Fig. 8. (a) Day plot of hysteresis ratios with SD (single domain), PSD (pseudo-signle domain) and SD (single domain) fields for magnetite and SD-MD mixing lines followingDunlop (2002a). (b) FORC diagram of grey granite MG235G sample (smoothing factor SF = 5). (c) FORC diagram of the charnokite MG235C2 sample (SF = 6).

Granitesn = 8best line: 237/13zone axis of foliation poles: 239/10

N Na b

Charnockitesn = 46best line: 237/8zone axis of foliation poles: 242/10

K1

best linebest polezone axis

K3

Fig. 9. Projection diagrams of magnetic lineations and foliation poles.


tion of secondary amphibole assemblages, that occurred in staticconditions at the end and/or immediately after the D2 event.Decreasing temperature induced Ti-oxide precipitation, sphaleriteprecipitation, and local formation of graphite. Finally, REE-fluor-carbonates, bastnaesite and fluorite formed due to still decreasingtemperature, depending of the evolution of Ca- anf F-activities inthe aqueous carbonic fluid.


7.3. Fluid sources: a link with Ambatomiranty magmatism ?

Carbonic fluids responsible for incipient charnockitisation mayhave a metasedimentary origin, either from metasediments con-taining graphite (by reaction of graphite with H2O: Huff and Nab-elek, 2007) or from the prograde decarbonation of carbonatemetasediments, but a deeper origin from the upper mantle is



Fig. 10. Primary altered aqueous fluid inclusions in granite (a) and in charnokite (b). Trails of secondary mixed aqueous-carbonic fluid inclusions, trapped in thehomogeneous field of the H2O–CO2 system in granite (c) and in charnokite (d).


generally preferred (Touret and Nijland, 2013). Due to the domi-nantly orthogneissic and granitic nature of the Antananarivo do-main in the studied area and the likely absence of any calc-silicate layer at depth, a metasedimentary origin is dismissed anda magmatic and/or mantle origin is favored. Fluids of magmaticderivation issued from crystallizing carbonatites or alkaline to per-alkaline magmas at depth constitute a potential source of carbonicand/or aqueous-carbonic fluids. Such fluids are well-known fortheir ability to carry iron and HFSE (e.g., Kresten and Morogan,1986; Smith and Henderson, 2000; Salvi and Williams-Jones,2006). In the studied area, a possible link with emplacement ofthe Ambatomiranty dykes is suggested by the close vicinity of suchdikes and by their NE–SW strikes, parallel to the D2-reworked foli-ations of stratoid granites (Fig. 1c).

7.4. An East-Gondwana perspective

Carbonic fluid transfer likely occurred on very broad scale inMadagascar, as carbonic fluids of deep juvenile origin were alsoevidenced in relation with the major shear zones of southern Mad-agascar by Pili et al. (1997,1999). Yoshida and Santosh (1994) re-viewed incipient charnockites in southern India and Sri Lankaand reported that they were induced by CO2-rich fluids infiltrationthrough faults and shears during late Pan-African tectonic events.They suggested that magmas of upper mantle and lower crustalderivation were the potential carriers of the CO2 volatiles. In addi-tion, they stated that incipent charnockitisation took place afterthe latest regional metamorphism, likely in an extensive fault-con-trolled setting at ca 550 Ma. In the present study, we establishedthat incipient charnockitisation began in relation with the so-called D2 Pan-African event and as early as 561 Ma (the age ofthe Antananarivo virgation zone). Advection of magmas and car-bonic fluids also triggered the peak granulite metamorphism atca 550 Ma in the Angavo shear zone. However, it is not possibleto determine the total duration of the fluid metasomatic processes.More structural, isotopic and geochronological data are required toassess the age of fluid infiltration in the whole East Gondwanarealm.


7.5. Dating implications

Percolation of CO2-rich fluids able to dissolve and reprecipitatezircons and monazites may have important consequences for geo-chronology (Touret and Nijland, 2013). This may provide an expla-nation for a recent age of 540 Ma obtained from a stratoid granitenorth of the present study area (Tucker et al., 2012), that does notappear consistent with the structural data. Indeed, ages from manyplaces in Madagascar where CO2 influx related to Pan-African shearzones occurred have to be considered carefully and might reflectmetasomatic effects, rather than peak metamorphism ormagmatism.

8. Conclusions

Prograde incipient charnockitisation is usually observed as aconsequence of localized CO2 percolation in high grade rocks (e.g.Touret, 1971; Newton et al., 1980; Srikantappa et al., 1992). In thisstudy, we have a peculiar protolith (an A-type granite), that pro-vides a case so far undescribed, where the initial (magmatic) para-genesis is dominated by quartz, perthitic alkali-feldspar andferrohastingsite. Two CO2-rich hydrocarbonic fluids are identified(with and without N2). This transformation occurred at the endof a partial reworking of the D1 structures by a D2 late-Pan-Africanshearing event dated at ca 550 Ma.

It is suggested that the rocks underwent a rather long history offluid percolation, leading to prograde and then retrograde transfor-mations. The corresponding metasomatic changes include mobilityof Ti and a gain of Fe, Zn, Ca, F and REE. These changes are indeedconsistent with the carbonic to hydro-carbonic nature of the fluidinclusions. The iron enrichment is responsible for the formation ofnew magnetite grains, hence a higher magnetic susceptibility ofthe charnockitized samples. Metasomatic processes at the regionalscale can therefore be monitored by aeromagnetic surveys, a mostvaluable tool for the study of geological structures and the recog-nition of potential ore bodies (Martelat et al., Eddy, this issue).

Finally, our observations confirm the importance of high grademetamorphism and associated carbonic fluid percolation during



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1011

Tm (Gas) °C

charnockite granite

n= 36

0

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-58.2 -58 -57.8 -57.6 -57.4 -57.2 -57 -56.8 -56.6 -56.4

7.4 7.6 7.8 8 8.2 8.4 8.6 8.8 9 9.2 9.4 9.6 Tm (Clath.) °C

0

1

2

3

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5

250

260

270

280

290

300

310

320

330

340

350

360

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380

390

Th °C

n=15

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n= 11b

c

Fig. 11. (a) Histogram of melting temperatures of gas ice (Tm gas) in fluidinclusions from granite and charnockitised samples. (b) Histogram of meltingtemperatures of clathrate (Tm clath.). (c) Histogram of total homogenizationtemperatures (Th).


the late-Panafrican history of Madagascar, as in other parts of East-Gondwana.

Acknowledgments

This contribution received funding from the INSU (Institut Na-tional des Sciences de l’Univers)-3F (Failles, fluides, flux) program.C. Cavaré-Hester, S. Gouy, J.F. Mena, F. and P. de Parceval arethanked for technical assistance. Comments by both reviewers(J.E. Martelat and J. Touret) and by the invited editor (B. Moine)contributed to improve the manuscript.

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Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurrent...

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