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mantle origin of the lamproite magma of the Bokaro coal
eld is evidenced by the
presence of mantle-derived nodules and xenoliths of harzburgitic composition, their
primitive ma
c major element chemistry, enrichment of refractory trace
elements like Ni, Cr, Co, Sc and high mg value of the phlogopite xenocrysts or
phenocrysts associated with them. High concentration of REE (up to 1139.4 ppm)
and considerable LREE enrichment [(La/Lu)
n
from 16 to 560] in the samples
suggest their derivation from enriched mantle sources. The Bokaro lamproites have
87
Sr/
86
Sr ratio in the range of 0.70394
c-ultrama
0.71544. Available Sr isotopic data of the
Jharia, Raniganj and the Bokaro rocks are similar to those of lamproites (Bergman
1987). Despite the wide range of
87
Sr/
86
Sr ratio, it could be shown that these have
not been affected by contamination (low correlation coef
-
cient of the isotopic ratio:
0.243, with respect to SiO
2
) and are believed to re
ect source region characteristics.
It was therefore, suggested by Basu et al. that the source region of the Bokaro
lamproites was highly heterogeneous.
According to Basu et al. (1997), the K-rich rocks of Bokaro were produced by
partial melting of a phlogopite
bearing harzburgitic) mantle. Extreme enrichment
of incompatible elements like, K, Ti and LILE usually associated with phlogopite,
apatite, rutile, priderite etc. suggests that the parent liquid is a product of partial
melting of a metasomatized mantle source (Wilson 1989). The highly fractionated
rare earth pattern [(La/Lu)
n
up to 165] suggests derivation of the potassic rocks by
low degree of melting in the metasomatized mantle (Paul and Potts 1981).
-
4.1.4.2 Jharia Basin
According to Ghosh and Mukhopadhyay (1985), the Jharia basin is a part of an
east
west-trending chain of intracratonic Gondwana basin of eastern India. They
thought that at
-
oor,
followed by its early differentiation into a number of sub-basins. Later, there was
merging of the sub-basins into a single major basin because of a subsidence, which
was controlled by the development of a narrow intrabasinal graben. According to
them, the graben was bounded by contemporaneous normal faults. There was
sinking of inselbergs along with rising of basement ridges towards the later stages.
This was followed by post-depositional faulting and reactivation of old faults.
Later, there was rise of basement rocks forming horst and graben-like structures and
first there was initiation of a general downwarping of the basin
finally intrusion of dolerites,
lamproites and lamprophyres took place through
sedimentary country rocks.
Ghosh (1949) made petrographic studies of the intrusive rocks from the coal
fields of Jharia and Dhanbad petrographic province, and from Raniganj of Burdwan
district, West Bengal. He termed these rocks to be biotite lamprophyre, minette and
kersantite. He also reported the occurrence of leucite from both the areas (also see
Chatterjee 1974).
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