Russian Journal of Earth Sciences
Vol. 5, No. 2, April 2003
Genesis of the earliest (3.20-2.83 Ga) terranes
of the Fennoscandian shield
S. B. Lobach-Zhuchenko, V. P. Chekulaev, N. A. Arestova, A. B. Vrevsky,
and A. V. Kovalenko
Abstract
Our study of magmatic rocks provides grounds for discussing the successive phases
and
geodynamic conditions of continental crust generation between 3.2-2.83 Ga within
the Fennoscandian
shield. Three tectono-magmatic phases, 3.2-3.1 Ga, 3.0-2.92 Ga, and 2.92-2.83 Ga,
are established. During
the 3.2-3.1 Ga phase, voluminous intrusives were emplaced, creating the large Vodlozero
"sialic core."
Positive
e Nd(t) values for mafites and,
partly, granitoids point to a weighty contribution of juvenile material
derived from a depleted mantle. At the same time, Nd isotope composition for a number
of granitoid massifs
and zircon ages suggest the presence of an earlier (as old as 3.5 Ga) crustal
component.
Endogenic processes that occurred during the second, 3.0-2.92 Ga phase, have
been recorded in
southeastern and western Karelia and are inferred to have occurred in the Kola Peninsula
as well. During this
phase, oceanic plateaus and island arcs were formed near, to be accreted onto, the
western and eastern
margins of the ancient Vodlozero core. Simultaneously, the central part of the Vodlozero
"sialic core" was
the locus of emplacement of gabbronorite-diorite intrusions and purely dioritic bodies,
as well as vigorous
tonalite-granodiorite magmatism, to form the Vodlozero domain, the oldest on the
shield. In the western
Karelian domain, rocks younger than 2.92 Ga are exposed at the current erosional
surface. The presence of
ancient material in western Karelian crust is pinpointed by Nd model ages for granitoids
and volcanites and
by a detrital zircon age from granite.
The third, 2.92-2.83 Ga phase entailed further reworking of ancient terranes
and initiation of new
sialic cores. At the northern margin of the Vodlozero domain and within the western
Karelian one, a system
of rift-related features came into being, eventually to evolve into bimodal greenstone
belts largely dominated
by mafic and ultramafic volcanites. The Kola province provided the stage for inception
of rift-related
greenstone belts with their associated komatiite-tholeiite (2.92-2.87 Ga) series
followed by the basalt-andesite-dacite
(2.88-2.79 Ga) series. Apparently, these belts in their present-day form, just
like those of the
western and eastern margins of the Vodlozero domain, result from tectonic juxtaposition
of rock
assemblages that originated from a variety of geodynamic settings.
Archean continental crust of the Fennoscandian shield is shown to have formed through
both
progressive addition of sialic crust over time, mainly at convergent boundaries of
ancient plates, and via
reworking of ancient fragments, which involved input of juvenile material resulting
from rising mantle
plumes. In all likelihood, ascending mantle plumes are responsible for the formation
of accretionary and
collisional orogens, whereas the coeval magmatism at active plate margins was due
to subduction (in the
context of the plate tectonic mechanism). Generation of rift-related structures and
associated magmatism
may have been driven by rising mantle plumes. Brittle deformations leading to rifting
and associated with
the ascent of mantle plumes did not result in break-up of the young continental crust.
The main outcome of
this mechanism was massive inflow of high-temperature magma into the lithosphere
forming within-crust
layers of "asthenosphere," in which granite melts originated, eventually to migrate
to shallower levels.