GENESIS OF THE EARLIEST (3.20-2.83 GA) TERRANES OF THE FENNOSCANDIAN SHIELD

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 <font face="Symbol">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.