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the relative contribution from lithospheric and sub-lithospheric sources to Kenyan
basalts was not determined, the similarity between melt generation rates between
the Birunga province and the Gregory Rift could be regarded as being largely
fortuitous.
Finally, the rates of melt generation in both the Birunga province and the
Kenyan Rift are at least an order of magnitude less than those of
flood basalt
0.4 km
3
/year, Stewart et al. 1996) and almost
two orders of magnitudes less than that for the Deccan volcanic region (Courtillot
et al. 1988; Ducan and Pyle 1988). Although this observation is not unexpected for
Birunga, it is surprising that the Kenyan Rift has such a low production rate,
considering the
provinces such as those in Parana (
*
factor of 2.5 proposed by Latin et al. (1993) against that inferred
for the Parana (1.5). It is possible that amounts of extension across the Kenyan Rift
are much less than estimated and or/the underlying plume is signi
β
cantly cooler
than that involved in the generation of continental
flood basalt provinces.
14.3 Tectonic Evolution of Silica-De
cient Potassic Rocks
from Brazil with Reference to Trinidade Plume
The signi
cance of the Trinidade Plume in relation to the genesis of potassic rocks
of Alto Paranaiba Igneous province was discussed by Gibson et al. (1994). They
thought that one of the major phases of plume activity and a major foci of this
upwelling was probably along the future site of the South Atlantic Rift (Willson
1992). White and McKenzie (1989) considered that the voluminous early Crete-
ceous magmatism of Parana-Etendeka and NE Brazil-Central Africa was related to
Tristan and St. Helana Plumes. According to Gibson et al. (1995), the recent geoid
anomaly maps of south Atlantic suggested that Trinidade (or Martin Vaz) plume
and other small-scale unnamed mantle plumes were present during Late Cretaceous.
Herz (1977)
first suggested that the alkali magmatism of this age in Brazil was
related to plume activity. He also realized the signi
cance of alkaline complexes in
the reconstruction of the movements of the South Atlantic plates during the breakup
of Gondwana supercontinent. Later Gough et al. (1980) suggested that plume-
related to magmatism in Brazil was traceable as early as 120 Ma ago on the South
American Plate. This observation was based on age dating of kimberlites form
Brazil, and the idea was however, debatable. The zircons form kimberlites occur-
ring both west (Pimenta Bueno, Rondonia) and east (Poco Verde, Minas Gerais)
yielded ages between 80 and 90 Ma. Much recent volcanic activity associated with
this plume occurred on the islands of Trinidade and Martine Vaz, when volcanism
took place 0.7 and 0.2
3 Ma ago, respectively. These islands lie 1,300 km off the
coast of Brazil and are linked by Trinidade
-
Vitoria seamount chain. The chain of
islands associated with the seamounts may represent the track of the Trinidade
mantle plume (Herz 1977; Grif
-
th and Campbell 1991; Gough et al. 1980). The
Eocene basaltic eruption (42
-
52 Ma; Cordiani 1970) in the continental shelf,
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