Mikko Nironen

The Svecofennian Orogen: a tectonic model

Abstract Recent geological studies have provided new concepts concerning the timing of the final break-up of the Archaean craton. Age data on detrital zircons from sedimentary rocks in Sweden and Finland show marked similarities and indicate pervasive homogenization of accumulated material. Geochemical and isotopic studies in Sweden and Finland suggest the existence of two volcanic belts, separated by an area dominated by sedimentary rocks. However, the ages of metamorphism and plutonism differ; in particular, the late (post-1.85 Ga) granitoid magmatism was more voluminous in Sweden than in Finland. The Svecofennian crust in Finland is anomalously thick (up to 65 km) and is characterized by a high-velocity lower crust of variable thickness. The evolution of the Svecofennian Orogen is modelled by opening of the pre-Svecofennian ocean at 1.95 Ga and progressive accretion of two arc complexes to the Archaean craton between 1.91-1.87 Ga. Initial accretion at 1.91 Ga led to thrusting of the slices of an island arc complex and Proterozoic marine strata onto the thickening craton margin. Subduction reversal with simultaneous anti-clockwise rotation of the converging plate, or convergence of another plate, resulted in subduction below the newly accreted arc complex. The tectonic style at the craton margin changed from thrusting to lateral displacement in the east (in Finland). In the west (in Sweden), the subduction zone retreated along a transform fault. A large sedimentary basin closed in the east 1.89 Ga ago when another arc complex collided against the accreted one. Collision resulted in lithospheric thickening, subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate, and large-scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material. The thick, dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance. Thus, the exhumation rate in this area was similar to the rate in areas of thinner crust. Subduction and sediment accumulation continued in the west until at least 1.87 Ga when large areas in the east were already stabilized. An intracratonic transpressional zone developed as the result of continued convergence. Subsequent extensional collapse along the transpressional zone caused melting of Svecofennian sedimentary rocks, migmatization and generation of the 1.84-1.82 Ga anatectic granites of southern Finland and central Sweden. The voluminous post-1.85 Ga granitoid magmatism in Sweden is tentatively associated with an east-west extension that is not related to the extensional collapse.

The Svecofennian orogen: a collage of microcontinents and island arcs

Abstract Based on an integrated study of geological and geophysical data, a tectonic model for the Palaeoproterozoic evolution of the Svecofennian orogen within the Fennoscandian Shield at the northwestern corner of the East European Craton is proposed. The Svecofennian orogen is suggested to have formed during five, partly overlapping, orogenies: Lapland-Savo, Lapland-Kola, Fennian, Nordic and Svecobaltic. The Svecofennian orogen evolved in four major stages, involving microcontinent accretion (1.92-1.88 Ga), large-scale extension of the accreted crust (1.87-1.84 Ga), continent-continent collision (1.87-1.79 Ga) and finally gravitational collapse (1.79 and 1.77 Ga). The stages partly overlapped in time and space, as different processes operated simultaneously in different parts of the plates. In the Lapland-Savo and Fennian orogenies, microcontinents (suspect terranes) and island arcs were accreted to the Karelian microcontinent, which itself was accreting to Laurentia in the Lapland-Kola orogeny. The formation of the Svecofennian orogen was finalized in two continental collisions producing the Nordic orogen in the west (Fennoscandia-Amazonia) and Svecobaltic orogen in the SSW (Fennoscandia- Sarmatia). The collisions were immediately followed by gravitational collapse.

1.88–1.87 Ga post-kinematic intrusions of the Central Finland Granitoid Complex: a shift from C-type to A-type magmatism during lithospheric convergence

Abstract The evolution of the Svecofennian Orogen spans from 1.95 Ga to 1.80 Ga with the main crust-forming activity having taken place 1.90–1.87 Ga ago. Subsequent deformation was largely concentrated to southern Finland where transpressional structures indicate lithospheric convergence until 1.80 Ga. Much of the Svecofennian Orogen in Finland is occupied by the Central Finland Granitoid Complex (CFGC) that mainly consists of I-type granodiorite and granite. These 1.89–1.88 Ga intrusive rocks are typically foliated and considered synkinematic. A distinct suite of post-kinematic, undeformed or slightly foliated plutons crosscut the synkinematic rocks. They are predominantly quartz monzonites and monzogranites, and have been divided into three types according to their petrographic, mineral, and chemical characteristics. Compared to the calc-alkaline synkinematic granitoids, the post-kinematic suite generally has an alkaline affinity, with higher Fe, Ti, K, Ba, Zr and Nb and lower Mg, Ca and Sr at a given SiO 2 content. The Type 1 post-kinematic plutons are peraluminous and contained a prominent sedimentary component in their source. The Types 2 and 3 plutons are marginally metaluminous to peraluminous. The Type 2 granites, especially those in the western CFGC, approach the classic 1.65–1.54 Ga rapakivi granites of southern Finland in their petrographic characteristics, magmatic association (tholeiitic mafic rocks) and elevated contents of incompatible elements, and hence have A-type characteristics. The Type 3 plutons contain a pyroxene-bearing margin (Type 3a) or contain pyroxene throughout (Type 3b). The Type 3b rocks have C-type (Charnockite magma type) affinities, and some plutons of this subgroup are alkaline. The Type 3a plutons appear to be transitional between the Type 2 and Type 3b plutons. The synkinematic magmatic episode in the CFGC area probably involved partial melting of intermediate high-K rocks in the lower crust, with a magmatic addition from a mafic underplate. This magmatic episode left a hornblende- and biotite-poor granulitic residue in the lower crust. Heat from the mafic underplate triggered partial melting of the granulite to produce the post-kinematic magmatism, and also introduced mafic material into the post-kinematic magmas. The post-kinematic magmatism took place over quite a short time period (ca. 15 Ma) during overall lithospheric convergence. It extended beyond the CFGC area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from C-type to A-type. The post-kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust. Partial melting of the mafic lower crust produced C-type magmatism soon after the first melting episode. The crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal A-type magmas were formed in the western CFGC.

Palaeoproterozoic accretionary processes in Fennoscandia

Abstract Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c. 1×106 km2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c. 1.89 and c. 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Mineral chemistry constraints on the evolution of the 1.88–1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

Abstract A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO2 (log fO2 ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites.