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decompression melting takes place. First, there is generation of basaltic magma by
partial melting of the ultrama
c mantle, and formation of lava pool below the
mechanical boundary layer (MBL) immediately below the region of rifting.
Thompson et al. (1990) suggested that potassic ma
c magmas are generated in the
sub-continental MBL. They think that eruption of the liquids along the axis of
continental rift is much more consistent with pure-shear than simple-shear models
of continental extension. The emplacement of strongly potassic magmas along
major strike-slip faults is an evidence in support of the view that the latter is
generated at the base of continental lithospheric plates rather than terminating in the
lower crustal detachment zones. This model suggests the cause of magmatism is
probably a combination of both extension of the continental lithosphere (McKenzie
1989) and heating from below by progressively hotter asthenosphere. McKenzie
proposed that the sub-continental lithospheric mantle is semi-continuously being
in
ltrated by small partial melts from the asthenosphere, which introduced volatiles
and incompatible elements.
While describing the genesis of ultrapotassic lava from Tibetan Plateau, Turner
et al. (1996) did not share the view of McKenzie and argued that this mechanism
should equally enrich elements of similar incompatibility and therefore, does not
provide a mechanism for producing the observed fractionation of Nb, Ta and Ti
relative to rare earth. Thus, this mechanism does not explain the negative Nb
Ta
and Ti anomalies as observed in case of ultrapotassic rocks in the Tibetan Plateau
and elsewhere. According to Turner et al. such anomalies are characteristics of
upper mantle sediments into the source region during a subduction event (e.g. Hergt
et al. 1989), which might also explain Archaean Pb model ages. They further add
that Rb/Ba is probably strongly fractionated during sediment subduction resulting
in having low ratio of these two elements. Otherwise, such a low Rb/Ba ratio of the
majority of ma
-
c lavas could not be explained due to the subduction of upper
crustal sediments. As negative Nb/Ta and Ti anomalies are characteristics of island
arc magmas, they suggested that ancient subduction processes were responsible for
the metasomatism of the Tibetan sub-continental lithospheric mantle. While dis-
cussing the genesis of post-collision potassic volcanism on the Tibetan plateau,
Turner et al. (1996) concluded that convective thinning of the lithospheric mantle
brought cooler and insulated zones of the lithospheric mantle into direct contact
with asthenospheric temperatures such that any metasomatized peridotite, would
undergo partial fusion, as its solidus should be lowered due to addition of volatiles.
They think that the thermal time constant of the lithosphere is such that gradual
conductive heating will continue over a period of
*
10
-
15 Ma. Thus, their model
agrees with the observation that eruptions continue from
13 Ma into this century
and there is seismic evidence for the presence of nearly 7 % partial melt over the
80 km thick slab of the upper mantle beneath Tibet.
Voluminous potassic magmatism in the Alto Paranaiba Igneous Province
(Chap.
4
) was described by Gibson et al. (1995) from the margin of the San
Fransiscan Craton of Brazil during the Late Cretaceous period. This produced wide
variety of rock types including madupite, kamafugitic rocks, olivine-rich madupitic
lamproite and kimberlite. There is also occurrence of minor amounts of melilitite
*
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