Russian Journal of Earth Sciences
Vol. 5, No. 4, August 2003
Layered intrusions as transitional magma chambers above the local
plumes of
large igneous provinces: Evidence from the Early Paleoproterozoic province of a
siliceous highly magnesian rock series in the Baltic Shield
E. V. Sharkov
Abstract
It is shown, using the example of the Baltic large
Early Paleoproterozoic igneous province of the siliceous,
high-Mg (boninite-like) series (SHMS), that the layered
intrusions had been intermediate-depth magma chambers
beneath thick lava flows. Operating in these chambers had
been the processes of the accumulation of periodically entered mantle-derived
magmas, their crystallization differentiation, and the
mixing of the fresh and evolved magmas. As a result, the magmas
uprising from these chambers to the surface, are usually
modified to various degrees, and primary mantle magmas are
extremely rare. The origin of the SHMS magmas is believed to
have been linked with large-scale assimilation of the
lower-crustal material by the hot mantle-derived magmas as the new magma
chamber that had originated at the crust-mantle boundary
"floated'' upward. This floating seems to have operated by the
mechanism of zone refinement, that is, by way of the melting of the
roof's rocks with the simultaneous crystallization of the bottom. This
"floating'' ceased when the magma chamber reached to the essentially
sialic upper crust, because a layer of light granite magma with
its own convection system originated in the upper part of the
chamber.
Similar positions are occupied by layered intrusions in
large igneous provinces (LIP) that had originated after 2
billion years ago, when a change in tectonic and magmatic
activity occured. These intrusions had been formed as a result of
solidification of intermediate magma chambers residing at
different depths, the uppermost of which had resided immediately
beneath lava plateaus and the lowermost at the crust-mantle
boundary, forming up the lower continental crust from below
(underplating). In all cases the LIP systems originated,
irrespective of their composition, above protuberances (local plumes) on the
surface of the superplumes, where the mantle material
experienced melting. The character of the magma system evolution
was controlled by the structure of the lithosphere, the heights
of the plume rises, and the temperature of the mantle-derived magma. It
is shown that the magmatic systems of the Moon highlands had been,
apparently, of the same structure as those of the Earth.