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JOURNAL GEOLOGICAL SOCIETY OF INDIA 

Vol.76, October 2010, pp.403-413 

Enigmatic Association of the Carbonatite and Alkali-pyroxenite 

along the Northern Shear Zone, Purulia, West Bengal: 

A Saga of Primary Magmatic Carbonatite 

ANIKET CHAKRABARTY* and AMIT KUMAR SEN 

Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667 

*Now at: Department of Geology, Durgapur Government College, Durgapur – 713 214 

Email: senakfes@iitr.ernet.in 

Abstract: The Purulia carbonatite, ‘carbonatite’-‘alkali-pyroxenite’-‘apatite-magnetite rock’ association, is located at 

Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published 

literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated 

silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the 

area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support 

this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the 

carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb 

is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests 

probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, 

petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle 

decent. The 87 Sr/ 86 Sr ratio of the carbonatite and apatite separated from the carbonatite (~0.703) implies primary magmatic 

derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite 

(of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example 

where primary magmatic carbonatite is associated with the alkali-pyroxenite. 

Keywords: Carbonatite, Pyroxenite, Carbothermal residua, Mantle apatites, Liquid immiscibility, Northern Shear Zone. 

INTRODUCTION 

The genesis of carbonatite, a rare alkaline igneous rock, 

is still a matter of debate as there is no single mechanism 

which can unequivocally explain the formation of the 

carbonatitic melt. It can be formed both by the magmatic as 

well as hydrothermal process(es) (Mitchell, 2005). The 

existence of carbonatitic magma was established in the 

1960s, but since then it is debated whether carbonatite is a 

primary or a derivative magma. In other words, it is 

developed directly by partial melting of mantle peridotite 

or from silicate magma by fractional crystallization or liquid 

immiscibility process (Gittins, 1988, Le Bas, 1977). The 

derivative carbonatite magma is likely to be generated within 

the crust and hence relatively at lower pressure compared 

to a mantle derived magma. Much of these controversies 

are summarized by LeBas (1987), Twyman and Gittins 

(1987). Melting studies reveals there are three possibilities 

by which carbonatite can be formed, (1) direct partial 

melting of a metasomatized mantle (Wyllie and Hung, 1975; 

Wallace and Green, 1988; Wyllie and Lee, 1998); 

(2) immiscible separation at low or high pressure from 

carbonated silicate melts e.g. carbonated nephelinite (Koster 

van Groos and Wyllie, 1963; Le Bas, 1977; Kjarsgaard et 

al. 1995; Brooker, 1998) and (3) crystal fractionation of a 

carbonated alkali silicate melt (King; 1949; Veksler et al. 

1998). In recent years the genesis of the carbonatites 

achieved a new dimension with the advent of the concept of 

carbo(hydro)thermal carbonatites. Mitchell (2005) proposed 

a mineralogical-genetic approach in classifying the 

carbonatites in terms of the ‘petrological clan’. Two major 

groups of carbonatite can be classified on the basis of 

their petrological clan; these are (1) calcite or dolomite 

carbonatite (or both), these are primary carbonatites and 

genetically related to nephelinite, melilitite, kimberlite and 

other mantle-derived magmas (2) carbothermal residua 

derived from a wide variety of magmas. Carbothermal refers 

0016-7622/2010-76-4-403/$ 1.00 © GEOL. SOC. INDIA

404 ANIKET CHAKRABARTY AND AMIT KUMAR SEN 

to the low-temperature fluids derived from a fractionated 

magma dominated by CO 2 

also containing fluorine and H 2 

O 

in variable proportions. 

Occurrence of small bodies of carbonatite and alkaline 

rocks like nepheline-syenite is known since last two decades 

along 100 Km long Northern Shear Zone (NSZ) traced from 

Khatra in Bankura, West Bengal to Tamar in Jharkhand 

through Beldih, Medinitanr, Kutni, Chirugora, Sushina, and 

Tamakhun (Fig. 1) (Kumar et al. 1985; Basu, 1993, 2003; 

Chakrabarty et al. 2009).These rocks are intruded within 

the Chandil formation of 1.5-1.6 Ga age and lies in the close 

proximity of the Chotanagpur Granite Gneissic Complex 

(CGGC). The alkaline rocks exposed in different areas along 

the NSZ are Beldih (carbonatite, alkali-pyroxenite, and 

apatite-magnetite), Kutni (carbonatite, apatite), and Sushina 

(nepheline syenite gneiss) (Singh et al. 1977; Ghosh Roy 

and Sengupta, 1988; Basu, 1988, 1989, and 1990). The 

exposure of the carbonatite is found only at Beldih (Purulia 

district, W.B.), the present area of investigation and 

henceforth will be termed as ‘Purulia carbonatite’. The 

Geological Survey of India had also reported other 

occurrences of carbonatite from Mednitanr and Kutni 

areas based on drill-core samples (Basu, 1993). This shear 

zone is also known for hosting different mineralization like 

Nb, apatite-magnetite, REE. The usual silicate rock 

association of magmatic carbonatite such as nephelinite, 

melilitolite, ijolites, urtites, kimberlites is absent in Beldih. 

Instead, the carbonatite is associated with the alkalipyroxenite 

and a large apatite-magnetite ore body; a case 

similar to the famous Khibina Complex of Russia (Zaitsev 

et al. 1998). Such similarity invokes a possible genesis of 

Beldih carbonatite as carbothermal residua of unknown 

parentage (Woolley and Kjarsgaard, 2008). In view of such 

possibilities, the genesis of the Purulia carbonatite and their 

possible genetic (?) lineage with the other silicate rocks 

present along the NSZ is important and leads to the present 

investigation. The investigation is based on detailed 

petrography and trace element geochemistry of the Purulia 

carbonatite. Three major rock types are found in this area 

and are carbonatite, alkali-pyroxenite and apatite-magnetite 

rock. 

Carbonatites 

PETROGRAPHY AND MINERALOGY 

The carbonatite is a light colored, medium-grained rock 

composed essentially of subhedral to euhedral calcite 

(~ 90% by volume, see Table 1 for detail modal compositions) 

with appreciable amounts of apatite (Table 1), 

altogether exhibiting mosaic texture. The accessory mineral 

Fig.1. (a) Regional geological map of the study area. (b) Generalized stratigraphic succession of the Singhbhum region showing the 

status of the Chandil Formation along with the rocks of Singhbhum Group. 

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010

ENIGMATIC ASSOCIATION OF CARBONATITE AND ALKALI-PYROXENITE, PURULIA, WEST BENGAL 405 

Table 1. Modal compositions of carbonatites (C), alkali-pyroxenite (Pxn) and apatite-magnetite (Apt-Mt) rocks (volume %). 

It must be noted that the calcite, apatite, biotite and albite are present as veins within alkali-pyroxenite along 

the zones of metasomatic alteration caused by the nearby carbonatite intrusion 

Sample No 1 2 3 4 5 1 2 3 4 5 1 2 

Rock Type 

Carbonatite (C) Pyroxenite (Pxn) Apt-Mt 

Minerals 

Calcite 90 90 86 94 84 3 5 × 8 2 

Apatite 4 3 5 5 6 1 3 × 2 1 95 97 

Amphibole 3 5 3 1 5 6 8 8 10.5 7 

Biotite 1 1 3 × 2 3 2 1 3.5 2 × × 

Clinopyroxene × × × 84 78 87 70 85.5 × × 

Magnetite 1 1 2 × 2 1 2 2 1 1 5 3 

Ilmenite 1 × 1 × 1 0.5 1 2 1 × × × 

Niobo-rutile × × × × × 0.5 × × 1 × × × 

Albite × × × × × 1 1 × 3 1.5 × × 

phases include amphibole, biotite, phlogopite, and ilmenite 

(Table 1). Overall, the mineral grains exhibit mosaic texture 

(Fig. 2a, b). Higher concentration of amphibole, biotite, and 

phlogopite at places resulting formation of the continuous 

and/or discontinuous bands of dark green color. Two 

varieties of amphibole grains are noticed; one with dark 

green colour showing dark brown pleochroism (magnesiokatophorite, 

Fig. 2b) and the other with light green colored 

amphibole showing pale-green to light green pleochroism 

(richterite). A detailed work on the alkali-amphiboles by 

Chakrabarty et al. (2009) has suggested a shallow depth of 

intrusion of this carbonatite and also advocated a sudden 

change in pressure during its emplacement. 

Alkali-pyroxenite 

The alkali-pyroxenite is a dark colored medium grained 

rock juxtaposed with the carbonatite. The subhedral 

clinopyroxene grains are making up about 85% of the rock 

by volume (Table 1) and give rise to hypidiomorphic texture. 

The euhedral to subhedral calcite and apatite grains are at 

places giving rise to mosaic texture similar to that of 

associated carbonatite (Fig. 2c). Three distinct mineralogical 

assemblages viz. primary magmatic, vein-filling and post 

magmatic are observed in alkali-pyroxenite. Such 

mineralogical assemblages indicate micro scale metasomatic 

changes consanguineous to the carbonatite intrusion. The 

primary magmatic mineralogical assemblage is dominated 

by the diopsidic pyroxene (Fig. 2d) with significant aegirine 

component (Ae 34.42-37.29 

-Hd 12.67-13.32 

-Di 52.91-49.39 

) and sodiccalcic 

amphibole of magnesiokatophorite type. The 

alteration of the primary mineralogical assemblages by the 

late stage vein-filling assemblages indicates post magmatic 

alteration similar to the sodic fenitization during carbonatite 

intrusion. The alteration assemblage is represented by the 

amphibolization (katophorite/taramite) and biotitization of 

the magmatic or early pyroxene and that of the vein filling 

assemblage by the calcite-apatite-albite-biotite-ilmenitemagnetite. 

The alkali-pyroxenite is characterized by the 

presence of late stage apatite-calcite vein which give rise to 

carbo(hydro)thermal carbonatite (Chakrabarty, 2009). 

Apatite-magnetite 

In general, presences of number of apatite-magnetite 

lenses of variable dimensions have been reported along NSZ. 

These lenses are intruded mostly within the country rock 

represented by mica schist, calc-silicate, metabasite, 

quartzite and biotite gneiss which are often sheared (Basu, 

1993). Clayey alterations around the apatite-magnetite lenses 

are very common. 

In Beldih, the strike length of apatite bearing rock is 

about 300meters with a maximum width of about 60 m and 

shows tapering towards east and west. The rock is light 

coloured and dominantly composed of fluor-apatite (Table 

1, Fig. 2e). In general the apatite grains are surrounded by 

opaque of magnetite. However, at places remnant apatite 

grains under microscope are colorless and showing parallel 

extinction with imperfect basal cleavage which appears to 

be cross fractured (Fig. 2f). The shape of the apatite crystals 

or pseudomorphs is highly variable from elliptical to 

hexagonal prismatic (Fig. 2e). At places marginal overgrowth 

of the apatite grains are marked yellow colour under the 

microscope. This rock is extensively affected by alteration. 

In many places the meteoric water reacted and subsequently 

dissolved the apatite grains. Further, there are evidences of 

re-precipitation of secondary apatite giving rise to colloform 

texture (Fig.2f). 

ANALYTICAL TECHNIQUE 

Approximately fifty whole rock samples of different 



Fig.2. (a) Carbonatite showing the mosaic texture along with triple junctions at places. The bulk mineralogy is dominated by the calcite 

and apatite. Minute grains of magnetite are shown in the area marked by the circle. (b) Carbonatite with two varieties of 

amphiboles along with magnetite. (c) and (d) Part of the alkali-pyroxenite intruded by the late stage apatite-calcite vein. The 

apatite in the alkali-pyroxenite is anhedral compared to the associated carbonatite. The effect of metasomatic alteration is marked 

by the formation of albite, biotite and shown in the circular area. At places amphibole and biotite are indistinguishable due to 

intense metasomatic alteration. Most of the alteration is found to be associated with the calcite-apatite veins. (e) and (f) Apatitemagnetite 

rock showing subhedral apatite grains. The apatite grains are variable in size. Magnetite grains are relatively small and 

distributed irregularly within the rock. MK: Magnesiokatophorite; R: Richterite; Amph: Amphibole; Ab: Albite; Bt: Biotite; Px: 

Clinopyroxene; Cal: Calcite; Apt: Apatite and Mt: Magnetite. 



rock types were studied petrographically and twelve samples 

were selected for whole rock trace element analysis, five 

each of carbonatite, alkali-pyroxenite and two of apatitemagnetite. 

To constrain the trace elemental concentrations 

of the carbonatites, routine analyses of major elements (see 

Table 2) of the whole rock were carried out on Li-tetraborate 

pellets at IMP (Institute for Mineralogy and Petrology), ETH 

Zurich following the procedure given by Nisbet et al. (1979) 

and Dietrich et al. (1984) using WD-XRF of Axios by 

PANalytical. The trace element analyses (see Table 3) were 

carried out by an inductively coupled plasma emission mass 

spectrometry (ICP-MS, Perkin Elmer, Elan 6000e) using a 

VG Elemental Plasma Quad instrument at Wadia Institute 

of Himalayan Geology (WIHG), Dehradun, India. The 

details of the analytical procedure have been given by Rathi 

et al. (1996). The precision of the measurements is generally 

better than ±5% for concentrations =1 ppm. 

GEOCHEMISTRY 

Carbonatite 

Most of the major elements in terms of their oxides do 

not show any substantial variations except for one sample 

(No. 2, Table 2) where the SiO 2 

content goes up to 5%. 

Expectedly all the carbonatites are essentially enriched in 

CaO and P 2 

O 5 

(Table 2). The major element geochemistry 

confirms they are calcico-carbonatites or sovites. Selected 

trace elements (Table 3) of the carbonatite when plotted in 

a multi element spider variation diagram shows that they 

are enriched in all the incompatible elements with respect 

to the primitive mantle (Sun and McDonough, 1989) 

(Fig. 3a), except for Rb. The Purulia carbonatite is 

characterized by the lower concentration of Th, Nb and U, 

whereas the trend of other incompatible elements (Ba, U, 

REEs and Y) matches well with that of the world-average 

of the calico-carbonatite of Woolley and Kempe (1989). 

Such Nb depletion (~20-44 ppm.) is in general characteristics 

of carbo(hydro)thermal carbonatite (Mitchell, 2005). The 

Sr concentration of the carbonatite is much higher than the 

world-average of calico-carbonatite. Replacement of Ca by 

Sr, in Ca bearing minerals like calcite and apatite resulted 

higher proportion of Sr in carbonatite. Chondrite-normalized 

REE patterns of the carbonatites (Fig. 3b) shows that they 

are enriched in LREE compared to the HREE which in 

general a characteristics of carbonatites. The average (La/ 

Yb) (avg) 

(=53.74) and REE (ΣREE = 1633.57) concentration 

of carbonatites are falling well within the range of calicocarbonatite 

(Woolley and Kempe, 1989). 

Alkali-pyroxenite 

The alkali-pyroxenite is also characterized by the relative 

enrichment of all incompatible elements. The major 

Table 2. Bulk rock major element compositions of the carbonatite (C) 

Sample No 1 2 3 4 5 

Rock type C C C C C 

SiO 2 

1.04 5.28 1.09 1.16 2.14 

TiO 2 

0.25 0.01 0.27 0.03 0.14 

Al 2 

O 3 

0.06 0.08 0.06 0.07 0.07 

Fe 2 

O 3 

2.63 1.19 2.60 1.39 1.95 

MnO 0.40 0.15 0.40 0.19 0.29 

MgO 1.64 0.53 1.61 0.73 1.13 

CaO 49.91 49.11 48.70 50.63 49.10 

Na 2 

O 0.25 0.12 0.26 0.19 0.21 

K 2 

O 0.01 0.01 0.02 0.01 0.01 

P 2 

O 5 

3.23 0.53 3.21 4.88 2.96 

NiO 0.01 0.00 0.01 0.00 0.01 

H 2 

O 0.00 0.00 0.00 0.00 0.00 

CO 2 

0.00 0.00 0.00 0.00 0.00 

LOI 37.81 42.21 38.07 36.90 41.28 

Total 97.23 99.23 96.30 96.17 99.28 

Fig.3. (a) Primitive mantle normalized spider plots of the studied 

rocks. Studied carbonatite is compared with the world 

average of calicocarbonatites. (b) Chondrite normalized 

REE plot of the studied rocks. The normalizing values are 

from Sun and McDonough (1989). 



Table 3. Whole rock trace elements analyses of representative carbonatites (C), alkali-pyroxenites (Pxn) and apatite-magnetite rocks (Apt-Mt) 

Sample No. 1 2 3 4 5 1 2 3 4 5 1 2 

Rock type C C C C C Pxn Pxn Pxn Pxn Pxn Apt-Mt Apt-Mt 

La 390.26 434.44 424.24 423.43 460.58 109.32 121.01 129.03 154.95 114.98 397.17 422.12 

Ce 666.20 730.01 735.35 725.19 833.28 248.15 270.46 302.50 347.46 256.13 891.85 953.88 

Nd 295.16 336.39 341.24 329.79 373.82 113.10 114.86 138.68 152.96 111.45 509.84 537.76 

Sm 41.71 45.26 48.09 46.21 48.29 18.62 19.62 23.25 24.67 18.87 82.24 86.64 

Eu 11.40 12.33 13.27 12.77 12.58 5.01 5.27 6.34 6.55 5.20 22.43 24.04 

Gd 30.59 33.03 35.25 33.87 34.69 12.63 13.42 15.73 16.82 12.79 54.31 57.60 

Tb 4.07 4.45 4.66 4.59 4.52 1.76 1.87 2.21 2.36 1.79 6.96 7.45 

Dy 17.48 18.43 19.53 19.28 19.06 7.75 8.15 9.32 10.09 7.71 25.27 26.64 

Ho 3.68 3.76 3.99 3.97 3.81 1.54 1.69 1.85 2.03 1.58 4.25 4.46 

Er 7.44 7.65 7.89 7.97 7.87 3.02 3.33 3.58 3.97 3.08 7.50 7.91 

Tm 0.93 0.93 0.94 0.99 0.93 0.35 0.40 0.40 0.46 0.36 0.65 0.67 

Yb 5.27 5.36 5.37 5.57 5.39 2.04 2.35 2.27 2.59 2.11 3.20 3.34 

Lu 0.65 0.66 0.66 0.68 0.68 0.26 0.30 0.28 0.31 0.26 0.34 0.36 

Sc 1.24 1.18 2.05 1.47 1.79 21.50 20.16 19.93 17.70 19.96 10.85 11.30 

Y 84.18 86.97 89.63 90.12 87.93 32.55 36.45 38.99 43.60 34.28 77.58 81.70 

Nb 21.13 28.03 43.78 19.60 44.49 209.65 325.16 314.81 336.85 261.34 95.74 146.24 

Th 2.63 2.89 5.08 4.47 5.51 2.89 3.03 4.42 3.34 2.32 4.11 5.19 

Sr 8609.33 8592.00 8401.46 8699.16 8807.14 1720.81 1936.99 1616.02 2181.60 1868.77 3353.00 3486.94 

Rb 0.85 1.16 0.54 1.31 2.22 52.54 42.96 77.27 77.12 70.46 0.23 0.30 

Ba 1483.54 1515.18 1428.83 1434.86 1488.88 850.63 771.00 1019.28 1025.95 1089.15 618.16 658.56 

V 11.24 9.46 12.94 15.23 14.47 345.54 311.35 329.97 320.01 326.31 14.62 15.46 

U 6.28 7.86 10.90 5.94 11.66 1.82 1.93 1.92 1.16 1.61 4.92 5.17 

Ga 19.63 19.96 19.25 19.29 22.12 27.87 24.46 29.85 29.00 30.62 8.64 9.28 

Pb 5.41 8.69 4.92 7.16 5.62 8.00 3.66 7.25 8.23 4.91 11.00 15.00 

Li 0.41 0.50 0.48 0.53 0.73 13.00 11.37 16.33 16.64 15.55 0.24 0.36 

ΣREE 1474.84 1632.70 1640.48 1614.31 1805.50 523.55 562.73 635.44 725.22 536.31 2006.01 2132.87 

La/Yb 74.05 81.05 79.00 76.02 85.45 53.59 51.49 56.84 59.83 54.49 124.12 126.38 

Ba/La 3.80 3.49 3.37 3.39 3.23 7.78 6.37 7.90 6.62 9.47 1.56 1.56 

Nb/Pb 3.91 3.23 8.90 2.74 7.92 26.21 88.84 43.42 40.93 53.23 8.70 9.75 

Nb/Th 8.03 9.70 8.62 4.38 8.07 72.54 107.31 71.22 100.85 112.65 23.29 28.18 

Y/Ce 0.13 0.12 0.12 0.12 0.11 0.13 0.13 0.13 0.13 0.13 0.09 0.09 

Th/U 0.42 0.37 0.47 0.75 0.47 1.59 1.57 2.30 2.88 1.44 0.84 1.00 

Y/Ho 22.88 23.13 22.46 22.70 23.08 21.14 21.57 21.08 21.48 21.70 18.25 18.32 

difference observed with that of the carbonatite is the 

higher concentration of Rb and Nb (Fig. 3a). The higher 

concentration of Rb in alkali-pyroxenite can be attributed 

to the presence of pyroxene and mica. On the other hand, 

Nb enrichment is mainly due to the presence of niobo-rutile 

in alkali pyroxene (Chakrabarty, 2009). The most noticeable 

feature of the alkali-pyroxenite is the higher concentration 

of Sr. This is mainly due to the presence of late stage 

carbo(hydro)thermal carbonatite veins of apatite-calcite 

during metasomatic alteration of the primary magmatic 

alkali-pyroxenite. The chondrite normalized REE plot 

(Fig.3b) reveals that the trend is very similar with that of 

the carbonatite. However, both the ΣREE (596.65) as well as 

(La/Yb) (avg) 

(37.53) are lower compared to the carbonatite. 

Apatite-magnetite 

Minor differences are noticed in the primitive mantle 

normalized spider variation diagram between the apatitemagnetite 

rock and carbonatite (Fig.3a). The chondrite 

normalized REE pattern (Fig.3b) reveals that the apatitemagnetite 

rock is characterized by the highest ΣREE (>2000) 

concentration compared to the carbonatite and alkalipyroxenite 

and particularly in MREE, a common trend for 

apatite associated with carbonatites (Brassinnes et al. 2005; 

Bühn et al. 2001). But the HREE concentration of this rock 

is lower compared to the carbonatite. 

DISCUSSION 

It has already been pointed out in the ‘Introduction’ that 

genesis of carbonatite, in general, is an enigma and can be 

explained by more than one mechanism. Two basic questions 

are to be answered in petrogenesis of Purulia carbonatite 

are (i) the process and/or processes responsible for the 



formation of the rocks of the Beldih area i.e. carbonatites, 

alkali-pyroxenites and apatite-magnetite and (ii) the genetic 

lineage among them. In addition, the Purulia carbonatite is 

intruded within in a Precambrian terrain and the country 

rocks have suffered amphibolite to greenschist facies of 

metamorphism. It is likely that the carbonatite might have 

undergone deformations post-dating its intrusion. In that 

case, the primary magmatic mineralogy, texture and 

geochemistry are likely to get perturbed. The different 

aspects of genesis of the Purulia carbonatite are discussed 

in details here under. 

Field and Petrographic Signatures 

The carbonatite here is characterized by the mosaic 

texture with prominent triple junction (Fig.2a) and 

dominantly consists of calcite and apatite, along with 

accessories like amphibole, biotite and magnetite. Such 

mineralogical assemblage even under highest grade of 

metamorphism does not greatly change the mineralogical 

assemblages of carbonatite as exemplified by the 

carbonatites from East India, Bull’s Run carbonatite in 

Natal, some Ontario examples and those of the Canadian 

Cordillera gneissic rocks (Natarajan et al., 1994; Viladkar 

and Subramaian, 1995; Scogings and Forster, 1989; 

Moecher et al. 1997; Pell and Höy, 1989). So, the presence 

of triple junction can be attributed to the rheological 

changes/readjustment particularly in response to post 

crystallization deformational activities. Presence of apatite, 

magnetite and accessory silicates are in well agreement 

with the genesis the Purulia carbonatite by magmatic 

process. Moreover, the presence of two different amphiboles 

in the Purulia carbonatite is indicative of sudden pressure 

variation rather than the metamorphic amphiboles. In 

case of metacarbonatite one should expect presence of 

tremolite, actinolite etc. but not the sodic-calcic amphiboles. 

The above evidences point towards the primary magmatic 

origin of the Purulia carbonatite (Chakrabarty et al. 2009). 

The most convincing evidence that the Purulia carbonatite 

is of magmatic origin is the effect of alkali-metasomatism 

in the associated alkali-pyroxenite. The alkali-pyroxenite, 

present in association with the carbonatite, is dominantly 

consists of diopsidic pyroxene along with accessory 

amphibole of primary magmatic origin. Addition of albite, 

biotite, phlogopite and numerous calcite veins are the 

result of alkali metasomatism during carbonatite intrusion. 

Alteration of alkali-pyroxene to alkali-amphibole is 

also caused by this process. These evidences support the 

intrusive (as a dyke) nature of the carbonatite and 

also relatively late stage formation after the alkalipyroxenite. 

Fig.4. (a) to (d) Comparison of the apatites of apatite magnetite rock with the hydrothermal apatites (Apatite A) and magmatic mantle 

apatites (Apatite B) of Reilly and Griffin (2000) with respect to selected trace elements. Most of the studied apatites are similar 

to the Apatite B representing magmatic apatites. However, the concentrations of the selected trace elements are mostly low 

compared to the Apatite B. (e) Covariation of U and Th of the studied apatites with that of the Apatite A and B respectively. The 

plot shows that the studied apatite is falling within the domain of Apatite A but the lower U concentration and thus lower U/Th 

ratio is comparable to the magmatic mantle apatites of type B. 



Chemical Signatures 

In general, the metacarbonatites are indistinguishable 

with the marble owing to the similar response of limestone 

and carbonatite under metamorphic conditions at 

amphibolite or even higher grade. Presence of diagnostic 

minerals such as pyrochlore in metacarbonatite can help its 

identification. But there are carbonatites which does not 

contain pyrochlore (Le Bas, 2002). Purulia carbonatite is 

characterized by low value of Nb, Th and Pb compared to 

the world-average of calico-carbonatite (Woolley and 

Kempe, 1989). Absence of carrier mineral phase like 

pyrochlore in the Purulia carbonatite substantiates this 

observation. There are many carbonatite of primary 

magmatic origin having Nb concentration below the 

detection limit or even absent e.g. Eden Lake carbonatite, 

Manitoba, Canada (Chakhmouradian et al. 2008); Turiy 

massif, Kola Peninsula, Russia (Dunworth and Bell, 2001). 

The REE enrichment of all the studied rocks follows the 

sequence: apatite-magnetite>carbonatite>alkali-pyroxenite. 

It must be noted that the original ΣREE of the alkalipyroxenite 

would have been much lower if the rock was 

not hydrothermally altered by carbonatite causing formation 

of numerous late stage apatite-calcite veins within it. 

Enrichment of LREE over HREE is indicated by high 

(La/Yb) CN 

ratios, which range from 50 to 58 in carbonatite 

and ~90 in apatite-magnetite rock. It has been already 

established that the apatites of the apatite-magnetite rocks 

are fluorapatites (Ghosh Roy and Sengupta, 1988; Basu, 

1990, 2003) which represent the magmatic mantle apatites 

(O’Reilly and Griffin, 2000). Slight MREE enrichment over 

the LREE and HREE along with (La/Yb) CN 

(~90) below 

100 is again very common for the early fluorapatites (Bühn 

et al. 2001). Moreover, the similarities between the apatitemagnetite 

rock with that of the magmatic mantle apatites of 

Apatite B (Fig. 4) indicate that these apatites are primary 

mantle derivatives rather than the hydrothermal apatites of 

Apatite A (O’Reilly and Griffin, 2000). 

Immiscible Melt, Carbothermal Fluid or Discrete 

Carbonatitic Magma? 

The petrologists are divided into two groups in 

interpreting the origin of the Sr-Ba rich and HFSE depleted 

carbonatites. Some workers (e.g. Cooper and Reid, 2000; 

Xu et al. 2003) believed that such carbonatites are bona 

fide member of the primary magma of mantle origin while 

others ascribed the formation by metasomatic reworking of 

the wall rocks or direct fractional crystallization from Ca- 

Sr-Ba bearing carbothermal fluids and hence termed them 

as carbothermal residua (Mitchell, 2005; Kjarsgaard and 

Fig.5. (a) and (b) Selected pairs of trace element ratios for 

carbonatite, alkali-pyroxenite and apatite-magnetite to find 

out the genetic link between these rocks. The plot reveals 

completely opposite trends for both the immiscible 

separation from a parent silicate melt as well as derivation 

of the carbonatite as carbothermal residua. The block arrow 

indicates where the hypothetical immiscible carbonate 

magmas (a) and carbothermal fluids (b). (see discussion 

for further details) 

Woolley, 2008) rather than the bona fide carbonatites. To 

establish the process(es) involved in the genesis of the 

carbonatite and their associates with the help of a set of 

trace element pairs, particularly those are having contrasting 

partitioning behaviors is now well known (Chakhmouradian 

et al. 2008 and references there in). Published experimental 

data indicate that the immiscible carbonate fraction should 

have higher Ba/La and Nb/Th ratios at a comparable 

Nb/Pb value relative to its conjugate silicate fractions 

(Chakhmouradian et al. 2008) where as a carbothermal fluid 

should have much higher Y/Ce and Nb/Th ratios 

(Chakhmouradian et al. 2008). In both the cases the Purulia 

carbonatite is showing the reverse trend (Fig.5). The Ba/La 

and Nb/Th ratios of the Purulia carbonatite is much lower 



compared to the associated alkali-pyroxenite (Fig.5a) thus 

the possibility of a primary silicate magma derivative and 

subsequently immiscible separation from the same can be 

ruled out. On the other hand the lower Y/Ce ratio relative to 

the alkali-pyroxenite is also uncharacteristic of 

carbothermally derived carbonatites (Fig.5b). Thus the origin 

of the Purulia carbonatite from a primary carbonatitic magma 

seems to be a logical conclusion. It must be noted here that 

the alkali-pyroxenite is metasomatized and bulk of the REEs 

are introduced during carbonatite induced alkalimetasomatism 

(Chakrabarty, 2009). However, experimental 

evidences suggest that a carbothermal fluid should have 

higher Ho and U relative to its parental melt ultimately giving 

rise to sub chondritic Y/Ho and Th/U ratio. This is true for 

the U concentrations of the studied carbonatite which is 

almost two times higher than the alkali-pyroxenite and the 

Y/Ho ratio of both the carbonatite and associated alkalipyroxenite 

is near identical. Moreover, it is well established 

that the Purulia carbonatite was formed at shallow depth 

under low P-T condition (Chakrabarty et al. 2009). Thus 

possibility of carbothermal derivation is certainly an open 

issue of arguments. However, the most convincing evidence 

comes from the 87 Sr/ 86 Sr ratios of the carbonatite (0.70332 

to 0.70340) as well as the apatites separated from this 

carbonatite (0.70336 to 0.70339) which are lying well within 

the range of mantle values (Chakrabarty, 2009) indicating 

the genesis of the Purulia carbonatite from a primary mantle 

derived carbonatitic magma. 

CONCLUSION 

The association of the carbonatite, alkali-pyroxenite and 

apatite-magnetite at Beldih, Purulia is a typical example of 

alkaline-carbonatitic activity along the Northern Shear Zone. 

Though such association, particularly the carbonatite and 

alkali-pyroxenite, in other parts of the globe advocate origin 

from carbothermal residua, our observation contradicts such 

conclusion. The plot of Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios 

nullifies the genesis of the Purulia carbonatite by 

hydrothermal process and/or immiscible separation from 

primary silicate magma. Based on field, petrographic and 

geochemical data, our work successfully explains the genesis 

of this carbonatite from a primary carbonatitic magma. The 

87 Sr/ 86 Sr ratio of both the bulk rock as well as apatite also 

strengthens the primary magmatic signature of the Purulia 

carbonatite. Moreover, the apatite-magnetite rock associated 

with the carbonatites is essentially mantle derived magmatic 

fluorapatites. Such association, point towards a possible 

prevalence of extensional tectonic regime prior to the 

formation of the Northern Shear Zone. However, further 

information on the stable isotopic composition (C, O & H) 

and more detailed geochemistry of the apatite-magnetite rock 

will be useful in verifying our inference. In a larger context, 

the present study may be useful in depicting the geodynamic 

evolution in this part of the Indian shield which is yet to be 

properly understood. 

Acknowledgements: AC acknowledges MHRD, CSIR 

(India) and ESKAS (Swiss Federal Commission scholarship, 

Switzerland) for providing scholarships for carrying out the 

work as part of his Doctoral programme. Help rendered by 

Prof. Christoph A. Heinrich, Prof. Albrecht von Quadt 

(ETHZ, Switzerland) and Dr. Pulok Mukherjee (WIHG, 

Dehradun) are thankfully acknowledged. The authors 

acknowledge the Director, Wadia Institute of Himalayan 

Geology, Dehradun for providing the ICP-MS facility. The 

funding provided by the CSIR, New Delhi for research project 

(no. 24(0286)/05/EMR-II) is gratefully acknowledged. 

Authors express sincere thanks to the anonymous reviewer 

for constructive suggestions to improve the manuscript. 

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(Received: 23 October 2009; Revised form accepted: 28 April 2010)

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