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Nd-isotope ratios indicate that approximately contemporaneous and widespread
LREE-enrichment occurred in the subcontinental lithospheric mantle. Hill (1991)
considered that the uplift taking place in the Alto Paranaiba Igneous Province
during pre- and postmagmatic episodes agreed well with the time constraints for the
plume impact. Gibson et al. further considered that uplift of the cool lithosphere up
to 20 Ma before magmatism probably re
ect the ascent of the plume through the
asthenosphere, and 20 Ma post-magmatic uplift may re
ect heating and displace-
ment of the underlying lithosphere.
Gibson et al. further added that transfer of heat by conduction and advection by
asthenospheric source melts into the subcontinental lithosphere probably coincided
with the impact of the plume. The melts produced due to the plume activity should
mobilise into subcontinental lithosphere enriched in volatile-rich segments. The
enriched nature of horizons should enhance their predominance in the
final melts,
which consolidated to produce K-rich ma
c rocks of the Alto Paranaiba, Igneous
Province. According to Gibson et al. the enriched isotopic signature of these vol-
canic rocks may indicate that they were probably derived due to Trinidade plume
activity.
14.4 Structural Control and Tectonic History of Potassium-
Rich Volcanic Province of Asia
14.4.1 The East Indian Rift Zone
The tectonic history of the Jharia basin (also see Fig. 4.3, Chap.
4
) was studied by
Ghosh and Mukhopadhyay (1985). This Gondwana basin is intracratonic and
located in eastern India. The Jharia basin is a part of the E
W-trending Damodar
Valley Coal Fields. The eastern fringe of the basin lies about 175 km northwest of
Kolkata. The Gondwana Supergroup in the Jharia basin is characterized by the
Talchir formation (upper Carboniferous to Permian) at the basement followed by
-
fluviatile Permian sediments comprising the Barakar, the Barren Measure and the
Raniganj formations. According to Ghosh and Mukhopadhyay, the basin has an
oval shape and the dip of the sedimentary beds is around 5
. The sediments are
intruded by numerous dykes and sills of dolerite and lamproitic rocks. The dolerites
are post-tectonics with reference to different movements in the basin. The lam-
proites are found to be displaced by many faults. For example, towards northeast of
Mohuda, the lamproite dykes show evidence of shearing, where they were em-
placed along fault planes. Transverse geological sections of the western part of the
Jharia basin is shown in Fig.
14.9
a
10
°
-
e.
In rare instances the dykes cut across pre-existing faults. On the basis of above
observations they considered that the time of emplacement of lamproites over-
lapped with that of late stage faulting. The movement was related to reactivation of
early contemporaneous basement faults and the emplacement of lamproitic rocks
took place after the deposition of at least the major part of Gondwana sediments in
-
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