Russian Journal of Earth Sciences
Vol. 3, No. 1, March 2001
Kola alkaline province in the Paleozoic:
evaluation of primary mantle magma
composition and magma generation conditions
A. A. Arzamastsev, F. Bea,
V. N. Glaznev, L. V. Arzamastseva,
and P. Montero
Abstract
This paper is an attempt to estimate quantitatively the chemical
composition and volume of magma, the reactivation of which resulted in the formation
during the Paleozoic of the northeastern part of the Baltic Shield in the Kola alkaline
province. In contrast to the known models, a gravity-based method of 3-D density
modelling was used to evaluate the extent of alkaline magmatism in the province and,
accordingly, to estimate the volume of mantle magma produced during the Paleozoic
cycle of activity. During the early period of study, along with sample collection
and
geochemical analyzes, the deep structure of the province was investigated, and 3-D
density models were derived for all alkaline intrusions of the province to a depth
of
22.5 km. The next step included the high-precision determinations of trace element
concentrations in the rocks using an ICP-MS method as a basis for calculating the
weighted mean concentrations of these elements in the rocks of the province, deriving
models for mantle rock melting, and estimating the geodynamic consequences of these
mantle processes.
The calculations showed that the total volume of the Paleozoic magma
produced in the NE part of Fennoscandia had been 15000
2700 sq. km. The
calculated composition of the magma that might have been melted from the mantle of
an intermediate composition revealed that a significant amount of incoherent elements
must have been added to the primary mantle material. It is shown with a high degree
of
certainty that the primitive magma was produced in the Kola Province because of the
low melting degree of the substrate (0.3-0.5%) whose composition answered to a
phlogopite ( amphibole) bearing garnet lherzolite residing
at a depth of a garnet
mantle. The enrichment degree of this substrate was calculated to have been 3 times
higher than the mean contents of incompatible elements in the primitive mantle.
It is shown that the process of magma generation involved a significant portion
of the NE Fennoscandian lithosphere ca. 400 km across and 120 km in depth,
that is,
covered the entire depth of the mantle garnet lherzolite. This areal estimate agrees
with
the area of Paleozoic igneous rocks in the region. The estimated depth correlates
with
the PT conditions of the mantle xenoliths found in the dikes and breccia pipes of
the
region.