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the Damodar Valley coal
fields. The lamproitic dykes cut across Upper Permian
sedimentary beds in the Jharia basin and the middle Triassic Panchet beds occurring
in the adjoining Raniganj basin. Ghosh and Mukhopadhyay found that the
emplacement of the lamprotic rocks were restricted to the basinal area only. These
dykes and sills occur extensively within the Gondwana sediments and are usually
con
ned to the Precambrian framework of the basin. There is only one example of
the occurrence of lamproitic dykes in the Dughda high, which is very close to
basinal border. According to them the subsurface data show that the maximum
concentration of the lamprotic intrusives coincides with the zones of maximum
subsidence in the central part of the Jharia basin and the volume of the lamproites
increase with depth. They found that in the other Gondwana basins of the Damodar
Valley, the volume of lamproites is larger in the Barakar formation.than in the
higher formations. The magnetic anomaly data of the Jharia basin suggested the
presence of large intrusive bodies within the central axis of the Jharia basin. The
occurrence of lamproitic intrusives within the Gondwana sediments is maximum in
the Jharia and Raniganj basins towards the eastern part of the Damodar lineaments.
The volume of the intrusives decreases towards the western part of the basin and is
minimum in the north Karanpura basin.
Although the emplacement of the lamproites was later than the development of
the basin, the spatial correlation between the total mass of the intrusives and the
amount of subsidence suggests a close genetic relation between the development of
the basin and the igneous activity at deeper levels. The close spatial association of
the basin and lamproite emplacement and the time-lag between the two events
could be explained in the following way. The development of contemporaneous
normal faults and the intrabasinal graben no doubt indicates a crustal tension during
the development of the basin. The normal faulting, and the crustal tension asso-
ciated with it, continued even after the deposition of the Raniganj beds, which are
the youngest beds of the Jharia basin. It is suggested that during the late phase of
such crustal
filled with lower
Gondwana sediments, did the resulting late fractures reach the mantle. Propagation
of the fractures up to the mantle should cause a sudden release of pressure, which in
turn would lead to a lowering of the temperature of the mantle solidus. If the
existing geothermal gradient was not far below the solidus, melting would be
initiated (Yoder 1952; Uffen 1959; Uffen and Jessop 1963).
tensions, when the Jharia basin was already
14.4.2 Tectonic Setting of K-Rich Rocks from Indonesian
Archipelago
The Indonesian Archipelago (for tectonic setting of Indonesian archipelago and
Benioff zone contours see Fig.
4.5
a) has been evolving since the late Palaeozoic, by
an evolving system of island arcs (Whitford et al. 1979; Edwards et al. 1993). The
fossil arcs are now represented by a series of linear volcano-plutonic igneous
complexes with associated sedimentary deposits now representing the Indonesian
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