Origin and geodynamic evolution of the Archean crust of eastern Finland

Abstract

The Archaean crust of eastern Finland is composed of three main lithological units: -Gneissic basement. It was originally composed of tonalites, trondhjemites and granodiorites emplaced at 2.86 G.a. and 2.62 G.a. and subsequently transformed into grey gneisses. The parental magmas originated from the mantle in a two-stage process: (i) formation of a basaltic crust, (ii) melting of this crust transformed into garnet-bearing amphibolite. Field, petrological and geochemical data argue in favour of a subduction-like process rather than an underplate accretion model. -Greenstone belt (Kuhmo-Suomussalmi). The lower volcanic sequence of this greenstone belt is composed of mafic and ultramafic lavas with komatiitic and tholeiitic affinities and was emplaced at about 2.65 G.a. The uprising of a mantle diapir initiated the breaking of the preexisting sialic crust (grey gneisses) and induced the emplacement of the greenstones in a proto-oceanic rift geodynamic environment. -Calc-alkaline magmatism. The calc-alkaline magmas were emplaced as volcanic rocks in the greenstone belt 2.5 G.a. ago (Luoma acid volcanics), or as K-feldspar phenocryst granodiorites in the immediate vicinity of the KuhmoSuomussalmi greenstone belt 2.5 G.a. ago (Suomussalmi and Arola augen gneisses). The younger plutons were emplaced as pink leucogranites 2.41 G.a. ago. The origin of this magmatism is thought to be correlated to the late tectonic evolution of the Kuhmo-Suomussalmi greenstone belt. The high density (d ~ 3.3) lavas of the lower volcanic sequence were emplaced over a low density (d -2.7) sialic crust. This created an inverse density gradient and generated a gravity instability that initiated the subsidence (sagduction) of the greenstone belt with respect to its basement. This later was carried down in the vicinity of the belt and underwent partial melting, thus generating the calc-alkaline magmas.It must be pointed out that, in the course of time, petrogenetic processes have changed from ensimatic to ensialic, implying a major reworking of the preexisting crustal materials.

3D structure and physical properties of the Kuhmo Greenstone Belt (eastern Finland): Constraints from gravity modelling and seismic data and implications for the tectonic setting

“…The geodynamic models for the Kuhmo Greenstone Belt included subduction-related magmatic arc or marginal basin settings (Piirainen, 1988;Taipale, 1983Taipale, , 1988 or crustal downsagging (Martin et al, 1984). Luukkonen (1992) proposed intracontinental rift model, in which he explained the origin and evolution of the belt by the mantle plume arising from a thermal boundary layer deep within mantle, or from the core-mantle boundary.…”

“…The main theories include subduction-related magmatic arc or marginal basin settings (Piirainen, 1988;Taipale, 1983Taipale, , 1988, crustal downsagging (Martin et al, 1984) and intracontinental rift model (Luukkonen, 1992).…”

Palaeoproterozoic dome-forming structures related to granulite-facies metamorphism, Jequié block, Bahia, Brazil: petrogenetic approaches

“…The temperature and plasticity increase at the deep crustal level results in diapiric ascent of plutonic rocks and in downward motion of the supracrustal formations (sagduction; Gorman et al, 1978). Consequently, most plutonites are located in domes while greenstone belts appear in basin domains as described in the Archean of the Dharwar craton in south India (Choukroune et al, 1995;Bouhallier et al, 1995;Chardon et al, 1998) or in the Baltic Shield (Martin et al, 1985;Martin, 1987). Similar structures appear in the Jequié granulitic block of the São Francisco craton (Bahia State NE Brazil) (Barbosa, 1990; Barbosa and Sabaté, 2002;Fornari and Barbosa, 1994).…”

U-Pb zircon geochronology of selected Archaean migmatites in eastern Finland

“…The Kuhmo-Suomussalmi-Tipasjärvi belt (referred to here as the Kuhmo greenstone belt; Fig. 2) has been variably attributed to intracontinental rifting (Martin et al, 1984;Luukkonen 1992Luukkonen , 2001, aborted rifting preceded and followed by subduction (Piirainen, 1988), island arc magmatism (Taipale 1983(Taipale , 1988, and back-arc rifting (Gaál & Gorbatschev, 1987). In addition, Kontinen et al (2007) have suggested that the Nurmes paragneisses (Fig.…”

“…In the Kuhmo district, the migmatitic TTG gneisses or associated banded amphibolites have been considered as the oldest rocks in the area (cf. Martin et al, 1984;�uukkonen, 1992). Previously reported age determinations from the migmatites are based on conventional multigrain U-Pb analyses (�uukkonen, 1985) and wholerock Rb-Sr method (e.g.…”