Geology Reference
In-Depth Information
4.1.4.3 Raniganj Basin
Bulk rock analyses of the lamproites from Mohanpur, Poidih and Sudamdih
(Table
4.2
) show the following features: (i) extremely low SiO
2
,Al
2
O
3
and high
P
2
O
5
, TiO
2
and K
2
TiO2 contents, (ii) higher concentration of some minor and trace
elements such as Ce, Y, La, Th, Nd, Rb, Sr, Ba and Zr, which are higher almost by
one order of magnitude than those of the average basalts (Frey et al. 1978).
From the mineral assemblages and bulk compositions, Gupta et al. (1983)
considered that the rocks studied by them cannot be termed as kersantite, minette or
camptonite (Nockolds et al. 1978), but they should belong to the group
'
glimme-
rites
, as described by Holmes (1937), which consist mainly of mica with some
other minor constituents. Since the rocks are also high in apatite and ankerite, they
may be designated as
'
belonging to the lamproitic
family. Analyses of these rocks are summarized in Table
4.2
. From a study of
combined Rb
'
carbonated apatite glimmerites
'
-
Sr, Sm
-
Nd and U
-
Th
-
Pb isotopic systems of the potassic rocks of
the Jharia and Raniganj coal
fields, it was also shown by Middlemost et al. (1988)
and Rock et al. (1992) that these are generated from an enriched mantle source.
Geochemical and experimental studies of rock samples from three localities of
the Jharia and Raniganj coal
fields were made by Gupta et al. (1983) to elucidate the
genesis of these rocks. These samples were from drill cores at Sudamdih
(S, Dhanbad district, Bihar), Mohanpur (M-1, M-2, M-3, Burdwan district, West
Bengal) and from a dyke (P) at Poidih (also from Burdwan district). All these
localities, included in Damodar Valley, are situated within 200
280 km west-
northwest of Calcutta. Microscopically, all the samples are characterized by vari-
able proportions of phenocrystal phlogopite and apatite and microphenocrystal
ankerite and chromian spinel in the groundmass of phlogopite, apatite, ankerite,
rutile, pyrite and devitri
-
ed glass. In addition, priderite microphenocrysts occur
rarely in the Mohanpur rocks as accessory minerals. Phlogopite is always the most
important constituent, whereas modal percentage of apatite is 20
30 %, ankerite
-
8
18 % and spinel less than 5 %.
-
4.1.5 Leucite-Bearing Rocks of Indonesia
4.1.5.1 K-Rich Rocks of Sunda Arc
The Banda Arc (Whiteford and Jazek 1979) and its westerly continuation, the Sunda
Arc occurs in the southeast margin of the Asian lithospheric plate (Varne 1985,
Fig.
4.5
a). The arc has been squeezed due to northward movement of the Indo-
Australian plate and the westward movement of the Paci
c plate. According to Varne,
northerly subduction of the Indo-Australian plate during the Neogene time led to the
collision of the plate northwest of Australia with the arc. It is thus, tightly bounded to
the south by the continental lithosphere, and there was back-arc thrust occurring north
of the arc from Bali to Wetar during subduction following the collision.
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