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408 ANIKET CHAKRABARTY AND AMIT KUMAR SEN
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
JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010