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Jharia, Ranganj, andBokaro basins and two lamprophyres from the Ranganj basin. All
the samples have been characterized petrographically by Gupta et al. (2002).
As there is a large variation in the isotopic composition of the geologic reservoir
(Clayton 1981), to increase the database on ultrapotassic rocks, Jia et al. (2003) also
included a lamproite from dyke swarms at Chelima in the Andhra Pradesh district
of south India, which were emplaced in the Cuddapah basin on the eastern margin
of the Archean Dharwar craton. They intrude a rock sequence of quartzite and
shales intercalated with limestone, dolomite, and cherts emplaced at about
1,200 Ma ago (Bergman and Gas 1984).
Several petrographic and geochemical studies have been conducted on the
Gondwana ultrapotassic suites (Rock et al. 1992) and Basu et al. (1997). To
summarize, the rocks are characterized by variable modal proportions of major
phases, principally micro-phenocrysts of Ti
olivine
pseudomorphed by talc and serpentine. The following minor minerals have been
reported: aegirine, amphibole, apatite, both primary and secondary ankerite and
Mg
Ti-rich phlogopite, and Mg
-
-
feldspar, hematite, ilmenite,
perovskite, priderite, pyrite, rutile, titanomagnetite, and zeolites (Rock et al. 1992).
Least altered rocks feature typical
-
calcite, chlorite, Cr
-
spinel, diopside,
fluorite, K
-
c
minerals and micas, globular structures, and lack of felsic phenocrysts. They are
distinct from kimberlites in having euhedral rather than rounded olivine, and in the
presence of amphibole and feldspar.
lamprophyric textures, with euhedral ma
5.1.1 Nitrogen Content of Gondwana Potassic Rocks
15
N values for the Gondwana ultrapotassic suite (Jia et al.
2003) is from +1.6 to +8.7
The total range of
δ
with and average of +3.8 %. Ranges for the different
basins overlap, and lamprophyres have similar N
‰
isotope compositions to lam-
proites. Nitrogen content averages 107 ppm. Results for phlogopite separates from
these ultrapotassic rocks yield a similar range of
-
15
N values from 1.4 to 5.2 %.
δ
15
N=
15
N
rock
-
δ
15
N
phlogopite
)
Bulk rock
—
phlogopite N
isotope fractionations (
δ
-
Δ
0.8 to + 0.7 %. The Proterozoic lamproite from Cuddapah is also
15
N-enriched
(Table
5.1
and Fig.
5.1
). The Sr isotopic characteristics as a function of 1/Sr is
shown in Fig.
5.2
. The co-variation of K
2
O and N content of ultrapotassic rocks
from Damodar Valley region is shown in Fig.
5.3
, whereas chondrite-normalized
spidergrams for the same rocks (Table
5.1
) are shown in Fig.
5.4
.
are
-
5.1.2 Major and Trace Elements
Lamproites from the Raniganj and Jahira basins are characterized by extremely high
contents of TiO
2
(4.2
6.2 wt%)
over a range of Mg # from 80 to 75, in keeping with the compositions of lamproites
10.1 wt%), K
2
O (2.4
7.6 wt%), and P
2
O
5
(2.3
-
-
-
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