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and several carbonatite intrusive complexes. This indicates that these different types
of rocks were produced from compositionally varied ranges of mantle sources.
According to Gibson et al. low
143
Nd/
144
Nd and high
87
Sr/
86
Sr ratio along with
high concentration of incompatible trace elements and La/Yb ratios of different
rocks types indicate that the source rocks were in the sub-continental lithospheric
mantle. They thought that variable K
2
O content in the liquid and modal phlogopite
content was related to different carbonate and hydrous phases in the mantle. This
resulted in different CO
2
/H
2
O ratios in the source rocks during melting. The
presence of garent lherzolitic xenolith in Tres Ranchos kimberlite suggests that they
originated at great depth within the mantle. The kamafugitic rocks were probably
derived at relatively shallower mantle source, which might have undergone
metasomatism.
Gibson et al. considered that the mantle source rocks for the olivine lamproites in
Alto Paranaiba Igneous Province might have been metasomatized by introduction
of subducted materials. According to Gibson et al. low
143
Nd/
144
Nd and high
87
Sr/
86
Sr ratio along with high concentration of incompatible trace elements and La/
Yb ratios of different rock types of the Alto-Paranaiba Province indicate that the
subcontinental lithospheric mantle constituted the source rock.
Reference to different experimental studies (Chap.
13
) suggest that at least high
temperature was required to generate the kimberlitic melts (Tres Ranchos) and later
the kamafugitic melts at relatively lower temperature. The heat was provided by a
mantle plume. They thought that heat transfer by conduction and advection during
ascending asthenospheric melt might have been responsible for melting of volatile-
rich fusible component of the lithosphere.
Rogers et al. (1998) discussed volcanism of the Birunga complex comprising
four volcanoes. They estimated that these volcanoes yielded 2,000 km
3
of potassic
lava during the last 100 ky. This leads to an eruption rate of
0.02 km
3
per year.
They suggest that even if 50 % of the melt is trapped at depth as cumulates then the
production rate could be as high as 0.04 km
3
.
Rogers et al. (1998) concluded that the production rate of lava (0.04 km
3
)at
Birunga is low compared to the production of lava in the
*
flood basalt area of Parana
(0.4 km
3
per year, Stewart et al. 1996). According to them, signi
cantly low rate of
lava production and the lack of signi
cant plume contribution to the magmatism are
consistent with both the thick lithosphere inferred from the depth of magma source
and the remoteness of the Birunga province from the axis of the putative East
African mantle plume. They further thought that the control exerted by the litho-
sphere on the location of both extension and magmatism and on the composition of
the magmatism in the early stage of continental extension is dominant. They
thought that the timing of the magmatism is consistent with a model of conducting
heating of the lithosphere by the cooler outer part of the East African mantle plume.
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