Geology Reference
In-Depth Information
c rocks
signify their mantle origin. However the N-isotope and the N contents are too high.
Accordingly, a multi-stage process may be suggested. Subduction of crustal rocks
The Mg, Cr and Ni concentrations of many K-rich ma
c and ultrama
first into asthenospheric mantle, then melting of the rocks to generate a K-rich melt
and subsequent hybridization of the melt with lithospheric mantle. Subsequently,
during rifting, the metasomatized mantle underwent decompression melting.
Petrological experiments on oceanic crust samples characterize recycling of
potassium from mid-ocean ridge basalts and sediments (with high Sr isotopic ratio).
Metasomatism could develop directly and continuously from subducted potassium-
bearing crust from shallow levels to a maximum depth of 300 km. The analogy
between mantle processes and chromatographic fractionation in the laboratory has
been suggested by Navon and Stolper (1987) to explain large scale mantle meta-
somatism. The method of mantle metasomatism by the process of Navon and
Stolper should induce metasomatism within a few centimetres at
the most.
Numerous
field-oriented studies of Alpine massif, ophiolites and xenoliths show
that a common mode of transport in the upper mantle is through the conduits
formed by hydraulic fracturing. Most probable method of
fluid migration may be
related to the presence of non-percolating
fluid, which should increase pore pres-
sure, approaching lithostatic pressure. Isolated pockets of
fluid should nucleate
microcracks and with the increase in strain microcracks would interact to produce
echelon of crack arrays, which should extend and coalesc further to form fracture
system.
In case of potassic volcanism in the regions associated with continental rifting,
the following sequence of events should take place: (1) impinging of a mantle
plume below a lithospheric plate, (2) continental rifting associated with lithospheric
heating, which causes continental extension and (3) generation of K-rich magma
from the lithospheric mantle boundary layer. Emplacement of K-rich lava along the
regions of continental rift axis is associated with pure shear rather than simple-sheer
model. The eruption of K-rich liquids along major strike-slip faults suggest their
genesis at the base of the continental lithospheric plate.
Search WWH ::
Custom Search