Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 100

Fig. 4.4 Chondrite
normalized (Evensen et al.
1978) REE plots of
lamprophyric rocks from
Bokaro coalfield, Bihar (after
Basu et al. 1997)

10000

1000

100

10

1

0.1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

ranges between 0.70526 and 0.70595 (Rock et al. 1992). The Bokaro samples
encompass a much wider range of 87 Sr/ 86 Sr ratios.
In contrast to the preponderance of calc-alkaline lamproites, particularly minette,
in the Jharia and Raniganj basins of the Damodar Valley, the Bokaro coal

eld is
characterized by widespread occurrence of lamproites (leucite lamproite, olivine-
leucite lamproite and olivine lamproite). The minettes from Bokaro basin have
higher amount of alkali feldspar and phlogopite with higher abundances of TiO 2
(4.75 wt%) and Al 2 O 3 (7.14 wt%) but smaller amount of K 2 O (4.80 wt%) and MgO
(11.39 wt%) in comparison with minettes from the Jharia basin. The lamproites from
Bokaro are characterized by the presence of leucite and/or olivine, priderite and
show

flow differentiated layers. In general, the lamproites are much less abundant in
the other Gondwana basins of eastern India. Interestingly, a 2 m thick dyke in
Bokaro basin shows variation from leucite olivine lamproite to olivine lamproite.
The corresponding average K 2 O abundances are 5.46, 3.39 and 1.87 wt%, respec-
tively. High modal percentage of pseudomorphic olivine in the same samples
indicates their ultrama c composition.

In general, however, the mineralogical assemblage in Bokaro rocks is less
diverse than in the rocks of neighbouring Jharia basin (Rock et al. 1992). The
phlogopites of the lamprophyric suite from Bokaro have higher TiO 2 ,Al 2 O 3 , and
MgO compared to the phlogopite of Jharia (Rock et al. 1992). The mg-number
varies between 0.63 and 0.87. In the Mg/(Mg + Fe) Al 2 O 3 diagram (Scott

Smith 1989; Scott Smith and Skinner 1984), most of the Bokaro phlogopites plot
close to the lamproite

TiO 2 -

-

field and lots of them also lie in the vicinity of the minette-
lamproite interface. This re ects the grading of olivine lamproite to minette through

intermediate varieties.
The petrogenesis of the Gondwana lamproites from Raniganj and Jharia has
been discussed by Middlemost et al. (1988), Rock et al. (1992) and Paul and Sarkar
(1984). From the petrological data presented above, it is seen that the Bokaro
lamproites are more diverse, and include a signi cant proportion of lamproites. The