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mantle temperature and pressure, which favour inter-connection of inter-granular
melts and indicate that this mechanism may be the most likely condition for porous-
medium
flow of inter-granular
buoyancy and deformation in upwelling mantle and rising diapirs. Numerous
flow. There are driving forces that may direct the
field-
oriented studies of alpine massif, ophiolites and xenoliths show that a common
mode of transport in the upper mantle is in conduits formed by hydraulic fracturing.
Such fracturing occurs even in rocks with large melt fraction; the process is
generally accompanied by plastic deformation. The process of hydraulic fracturing
in the mantle, requires melt pressures in excess of the peridotite yield stress, which
is estimated by Nicolas (1986) to be <50 MPa. Davies (1999) considered that
during subduction of a lithospheric slab along a continental margin, as the tem-
perature and pressure are increased,
fluids are released because of the breakdown of
fluid should increase by pore
pressure, which may approach the lithostatic pressure. A greater amount of de-
fluid-bearing phases. The presence of non-percolating
fluidization may result in higher pore-pressure. This may ultimately lead to the
generation of intermediate earthquake. He further considers that ductile creep can
also increase pore-pressure even in cool systems with
finite permeability and
negative volume change due to de-volatilization. Faulting may be initiated along
well-healed faults that were present earlier prior to subduction. Isolated pockets of
fluid should nucleate micro-cracks and under compression fracture should form due
to coalescence of dilating tensile micro-cracks. The fluid should flow locally to
open under higher con
ning pressure. With increase in strain micro-cracks would
interact to produce echelon crack arrays, which should extend and coalesce further
to form fracture system.
Foley (1992) discussed about vein-plus-wall-rock melting mechanism in the
lithosphere and the origin of potassic magmas. He suggested remobilization of
several generations of veins, which should explain the occurrence within a
restricted space and time of rocks bearing chemical characteristics which are gen-
erally thought to indicate contrasting tectonic setting (e.g. central Italy). According
to him the generation of ultra-potassic rocks is explained by melting of vein-rich
segments (i.e. with high vein/wall-rock ratio). Further dilution of the vein com-
ponents by wall-rock, supplemented by asthenospheric melt in advanced cases,
leads to the production of more voluminous lavas. These lavas bear incompatible
element signatures reminiscent of the ultrapotassic rocks.
15.13 Potassic Volcanism Associated with Rift and Tectonic
Processes
'
Formation of rift systems in the earth
s crust may be related to ascent of mantle
plumes. With the rise of such a plume and presence of two opposite convective
limbs, tensional forces are created. This causes thinning and stretching of the crust.
As a consequence of the emplacement of the plume, asthenoshere upwelling and
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