Central Finland Granitoid Complex Research Articles

Evidence of crustal growth during the Svecofennian orogeny: New isotopic data from the central parts of the Paleoproterozoic Central Finland Granitoid Complex

The Central Finland Granitoid Complex (CFGC) is the Paleoproterozoic core of the Svecofennian orogeny composed mainly of plutonic rocks varying from diorites to granodiorites and granites. Common mixing and mingling textures, synplutonic dykes, and mafic magmatic enclaves highlight the simultaneous nature of mafic, mantle derived magmatism and more evolved felsic magmatism in the CFGC. New U-Pb geochronological data support crustal growth during 1.90–1.87 Ga for the CFGC. Both Sm-Nd (εNd -1.7–+2.5) from whole rocks and Lu-Hf (εHf -8–+9) from zircons point towards relatively juvenile sources for the plutonic rocks without Archean crustal component. On the whole, isotopic evidence collectively points to crustal growth with maturing arc characteristics at 1.90–1.87 Ga, and a possible amalgamation of island arcs during the Svecofennian orogeny. In a larger scale, the CFGC was 1.9 Ga ago located in the active margin of the Great Proterozoic Accretionary Orogen when supercontinent Columbia (Nuna) formed.

Hiekkapohja hydrothermal system – ore mineral, lithogeochemical and paleomagnetic evidence from the Paleoproterozoic Central Finland Granitoid Complex

The Paleoproterozoic Svecofennian Central Finland Granitoid Complex (CFGC) has been regarded as an area of low mineralisation potential. The Hiekkapohja area, 20km north-east of the town of Jyväskylä, host a concentration of variable metalliferous showings. Samples from mineralised boulders and outcrops display variable combinations of anomalously high concentrations of Cu, Mo, Zn, Pb, W, Pb, Ag, As, and Au. The area is composed mainly of peraluminous and ferroan granitoids. The dominant porphyritic Hiekkapohja granodiorite (~1.88 Ga) is cross-cut by the equigranular Soimavuori granite of similar age. The porphyritic Lehesvuori granite on the western side of the study area represents marginally older (~1.89 Ga) magmatism. The paragenetic sequence of the ore minerals shows that the Hiekkapohja area has been affected by at least two separate stages of hydrothermal activity. The first mineralisation stage was widespread, crystallising typically chalcopyrite, pyrrhotite, sphalerite, galena, arsenopyrite, magnetite and Ag-bearing minerals. After the first stage, a low temperature oxidising phase formed hematite and marcasite. The second mineralisation stage enclosed low temperature minerals, such as marcasite and native Ag and Ag-minerals, as inclusions inside chalcopyrite, pyrite, pyrrhotite, sphalerite, and arsenopyrite. The mineralised samples typically display signs of K-metasomatism and less commonly signs of propylitic alteration. During the second mineralisation stage the fluid flow was controlled by the dominant 120°–135° trending shear zones. Both the hydrothermal activity and the regional geology indicate that porphyry type ore forming processes have occurred in the Hiekkapohja area. Paleoproterozoic resetting of the remanent magnetisation is further evidence for the role of the hydrothermal system.

Full-Scale Crustal Interpretation of Kokkola\u2013Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.

Petrography, geochemistry, and geochronology of the Sc-enriched Kiviniemi ferrodiorite intrusion, eastern Finland

The Kiviniemi mafic intrusion, near the eastern margin of the Paleoproterozoic Central Finland Granitoid Complex, is both spatially and temporally associated with post-kinematic Fe-Ti-P-enriched Svecofennian orogenic mafic magmatism. The main rock types in this small (~ 15 ha) intrusion are garnet-bearing fayalite ferrodiorite, leucoferrodiorite, ferromonzodiorite, and pyroxene diorite. The garnet-bearing fayalite ferrodiorite and leucoferrodiorite contain 50–281 ppm Sc, 275–5600 ppm Zr, and 58–189 ppm Y (n = 42), delineating a mineralized deposit some 2.5 ha in extent. Overall, these rocks show an evolved (iron-enriched) tholeiitic character; low values of Ni (< 20–40 ppm), Cr (< 20 ppm), and Cu (< 20–80 ppm); and high contents of Zn (213–700 ppm). The rock-forming minerals in the ferrodioritic rocks are (ferro)hedenbergite, plagioclase (~ An40), ferropargasite and ferroedenite, almandine garnet, and fayalite (Fo1–4). Accessory minerals include zircon, ilmenite, fluorapatite, biotite, pyrite, pyrrhotite, potassium feldspar, grunerite, and clinoferrosilite. Some relict cumulate textures have been preserved, but primary magmatic features have largely been overprinted by strong recrystallization and corona formation. The main carriers of Sc are amphibole, clinopyroxene, and apatite. The remarkably strong enrichment of Sc in ferromagnesian silicates and apatite, rather than in specific Sc-minerals, implies magmatic enrichment. Post-kinematic mafic intrusions in central Finland constitute a bimodal association with co-existing granitoid counterparts. The Kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts, indicative of contemporaneity of the two magmas. This conclusion is in accord with the coincident U-Pb zircon ages for the ferrodiorite, at 1857 ± 2 Ma (multigrain ID-TIMS) and the megacrystic granite, at 1860 ± 7 Ma (single-crystal LA-MC-ICP-MS). The initial εNd value of the ferrodiorite and the granite are + 0.1 and − 2.5, respectively. These Nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some Archaean crustal material into the granite in the course of magmatic evolution. The resource estimation calculated for Kiviniemi intrusion by using 40 g/t Sc cut off value is 13.4 Mt of rock with an average grade of 162.7 g/t scandium, 1726 g/t zirconium, and 81 g/t yttrium.

3D nanopetrography and chemical imaging of datable zircons by synchrotron multimodal X-ray tomography

Zircon is the most widely used mineral in petrochronology and provides key information about magmatic and crustal differentiation history of plutonic rocks, transport paths of clastic material ‘from source to sink’ and significantly contributes in the reconstruction of enigmatic planetary-scale tectonic episodes since the Archaean. However, detailed textural analysis of this accessory mineral has always been hampered by two-dimensional (2D) analytical limitations. With the advancements in X-ray nanotomography technology, it is now possible to non-destructively, yet digitally, cut, visualize, compare and quantify internal textures within zircons, their growth and zoning patterns and chemical distribution of trace elements in three dimensions (3D). We present a novel multimodal approach of using a synchrotron radiation nanobeam to perform 3D nanopetrography of < 100 µm zircons at ~100 nm resolution, demonstrating the capabilities of the technique by analysis of Paleoproterozoic zircons from the Central Finland Granitoid Complex. The integrated X-ray absorption, diffraction and fluorescence tomography revealed sector and oscillatory zoning patterns in 3D as well as differences in zoning pattern between trace elements, in addition to lattice parameters and inclusion composition within zircons. The multimodal synchrotron nanotomography elucidates the 3D nanopetrography and trace element composition of submillimeter-sized zircons in unprecedented detail.

Geochemistry, U-Pb geochronology and Sm-Nd data from the Paleoproterozoic Western Finland supersuite – A key component in the coupled Bothnian oroclines

The Western Finland (WF) supersuite, characterized by supracrustal rocks and intruding plutonic rocks, forms the core component of the coupled Bothnian oroclines in the central part of the Svecofennian orogen. We present here geochemical-isotope data, detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the WF supersuite in the Pirkanmaa (PB) and Pohjanmaa (PoB) belt representing the southernmost limb of the southern and the hinge zone of the northern Bothnian orocline, respectively. The Tampere belt (TB) in the north and Häme belt (HB) in the south bound the PB.Basaltic to picritic volcanic rocks in the WF supersuite, with a proposed eruption age at ca. 1.91Ga (1.92–1.90Ga), show locally pillow-structures and are typically associated with black shales±cherts±carbonate-bearing rocks. Based on geochemistry and Sm-Nd data a variable MORB-WPB-type origin is evident and an evolution scenario from earlier WPB-type (rifting) to more MORB-affinity (ocean/sea opening) is proposed. Detrital zircon age distributions from sedimentary rocks with ≥1.91Ga maximum depositional age in the TB, PB and HB have been compared and typically the age distributions of <2.2Ga zircons in different areas favor comparable source characteristics. The major metamorphic event at 1885–1875Ma in the WF supersuite is well dated. Although an earlier metamorphic stage at 1.92–1.91Ga is locally probable most of the <1.94Ga overgrowths in the WF supersuite are considered pre-depositional and recycled. Two models are presented to explain the hypothesis of similar pre-1.91Ga evolution in the WF supersuite and lowermost parts of the TB and HB, and contemporaneous arc magmatism at 1.90–1.88Ga in the Skellefte district (SD), Central Finland Granitoid Complex (CFGC), TB and HB: a) all the 1.90–1.88Ga volcanic rocks have formed in one linear arc system from the SD to the HB; b) there have been two separate arcs SD-CFGC-TB and HB of similar age with double-plunging subduction zones. In both models, orogen parallel shortening and buckling is geometrically viable mechanisms for the origin of oroclinal bending, but slab rollback models should also be evaluated in further studies.

Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm–Nd, Lu–Hf, O)

The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92–1.85Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U–Pb dating and oxygen isotopes, followed by analyzes for Lu–Hf by LA–MC–ICP–MS.The AC plutonic rocks (1.87–1.85Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1–2.0Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced “subduction-like” Nb–Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite±rutile partly prevailing in the residue (Nb–Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb–Ba–Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks±amphibolites is proposed. The CFGC is characterized by both I-type and A-type (CFGC/A) intermediate and felsic granitoids. The I-type granitoids are divided into two groups at ≥1885Ma and ≤1882Ma, where the latter overlap in age with the CFGC/A granitoids. Both I-type CFGC and CFGC/A granitoids are interpreted to have formed from mixing of Paleoproterozoic SCLM-derived melts with crustal melts from hydrous and dry intermediate-felsic igneous sources, respectively. The geochemistry, dominantly δ18O values below 6.5‰ in zircons and TDM (2.11–2.42Ga) of the CFGC granitoids favor the occurrence of older crust (ca. 2.1–2.0Ga) in their genesis. The BZ granitoids are similar in age but more juvenile with TDM ages between 1.94Ga and 2.16Ga. The 1.92Ga granodiorite in the BB is correlated with juvenile gneissic tonalites and granodiorites found from the AP boundary.We suggest that the present high-velocity lower crust under the CFGC is composed of melt-extracted granulites (crustal source age≥2.0Ga) and mafic cumulates which both formed during 1.90–1.88Ga arc magmatism. The ≤1.88Ga stage represents the end of compression/transpression and is followed by 1.87–1.86Ga buckling, forming the Bothnian Oroclines.

Shatter cones at the Keurusselkä impact structure and their relation to local jointing

AbstractShatter cones are the only distinct meso‐ to macroscopic recognition criterion for impact structures, yet not all is known about their formation. The Keurusselkä impact structure, Finland, is interesting in that it presents a multitude of well‐exposed shatter cones in medium‐ to coarse‐grained granitoids. The allegedly 27 km wide Keurusselkä impact structure was formed about 1150 Ma ago in rocks of the Central Finland Granitoid Complex. Special attention was paid in this work to possible relationships between shatter cones and local, as well as regionally occurring, fracture or joint systems. A possible shatter cone find outside the previously suggested edge of the structure could mean that the Keurusselkä impact structure is larger than previously thought. The spacing between joints/fractures from regional joint systems was influenced by the impact, but impact‐induced fractures strongly follow the regional joint orientation trends. There is a distinct relationship between shatter cones and joints: shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends. Planar fractures (PF) and planar deformation features (PDF) were found in three shatter cone samples from the central‐most part of the impact structure, whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features. PDF occurrence is enhanced within 5 mm of shatter cone surfaces, which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation. Some shatter cone surfaces are coated with a quasi‐opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins. The vein‐filling is composed of two mineral phases, one of which could be identified as a montmorillonitic phyllosilicate. The second phase could not be identified yet. The original composition of the fill could not be determined. Further work is required on this material. Observed joints and fractures were discussed against findings from Barringer impact crater. They show that impact‐induced joints in the basement rock do not follow impact‐specific orientations (such as radial, conical, or concentric).

Physical properties of Vilppula drill cores and petrographic analysis of associated breccias in Keurusselkä impact structure, central Finland

The Mesoproterozoic deeply eroded Keurusselka impact structure in central Finland is situated within the ∼1860–1890 Ma Central Finland Granitoid Complex. An estimate for the original size of the structure is 30 km, yielding a 5 km wide central uplift with insitu shatter cones and shock metamorphic features in quartz. Petrophysical and rock magnetic properties of the three shallow drill cores (V-001, V-002 and V-003) in the vicinity of the central uplift are determined in order to assess the dimensions of the central uplifts magnetic anomalies. The drill core lithologies consist of schists (metagraywackes), metavolcanic rocks, gneisses and breccia. Petrophysical properties of the drill core rocks show average densities (D) of 2644–2752 kg/m3, susceptibilities (κ) of 160–761 × 10−6 SI and natural remanent magnetization (NRM) of 3–306 mA/m and Koenigsberger Q ratios of 0.1–10. Rock magnetic measurements with temperature dependence of susceptibility (κ-T) curves and hysteresis indicated mostly paramagnetic behaviour. However, a fraction of fine-grained ferromagnetic minerals (pyrrhotite and magnetite) was detected from all lithologies. Breccia veins cutting the parautochthonous subcrater floor show lower values of petrophysical properties (D, κ, NRM, Q) and this could be related to the impact event. Amphiboles and micas in the breccia are strongly altered and replaced by secondary chlorite. Chloritization may indicate widespread impact-induced hydrothermal alteration of the target rocks or it may be related to regional tectonic shearing. However, planar deformation features in quartz, found from shatter cones in the central uplift area, are decorated with fluid inclusions indicating that alteration by post-impact processes was present.

Rock magnetic and paleomagnetic study of the Keurusselkä impact structure, central Finland

Abstract–There are 31 proven impact structures in Fennoscandia—one of the most densely crater‐populated areas of the Earth. The recently discovered Keurusselkä impact structure (62°08′ N, 24°37′ E) is located within the Central Finland Granitoid Complex, which formed 1890–1860 Ma ago during the Svecofennian orogeny. It is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism, e.g., planar deformation features in quartz. New petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented. The suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility. The presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization. Four different remanent magnetization directions were isolated: (1) a characteristic Svecofennian target rock component A with a mean direction ofD = 334.8°,I = 45.6°, α95 = 14.9° yielding a pole (Plat = 51.1°, Plon = 241.9°,A95 = 15.1°), (2) component B,D = 42.4°,I = 64.1°, α95 = 8.4° yielding a pole (Plat = 61.0°, Plon = 129.1°,A95 = 10.6°), (3) component C (D = 159.5°,I= 65.4°, α95 = 10.7°) yielding a pole (Plat = 21.0°, Plon = 39.3°,A95 = 15.6°), and (4) component E (D = 275.5°,I = 62.0°, α95 = 14.4°) yielding a pole (Plat = 39.7°, Plon = 314.3°,A95 = 19.7°). Components C and E are considered much younger, possibly Neoproterozoic overprints, compared with the components A and B. The pole of component B corresponds with the 1120 Ma pole of Salla diabase dyke and is in agreement with the40Ar/39Ar age of 1140 Ma from a pseudotachylitic breccia vein in a central part of the structure. Therefore, component B could be related to the impact, and thus represent the impact age.

A case study of lateral spreading: the Precambrian Svecofennian Orogen

AbstractWe have studied the crustal structures of the Palaeoproterozoic Svecofennian (c. 1.9 Ga) Orogeny with the help of large scale seismic reflection surveys (FIRE 1–3), preliminary structural field work and geological and geophysical databases. The central part of the orogen is occupied by the Central Finland Granitoid Complex, which comprises two suites of granitoid rocks and associated mafic and volcanic rocks. The complex and the surrounding supracrustal belts are cut and deformed by numerous shear zones and faults; here divided into six groups. The most prominent reflections are usually shear zones or faults on outcrops. The granitoid complex is interpreted as a deep, lower-level section of an old core complex, where the younger granitoid intrusions form the basins and older granitoid intrusions and associated volcanic rocks form the horsts. The upper–middle crust detachment zone is exposed at the northeastern edge of the complex and middle crust is exposed in the migmatitic domes at northern and western margins.The seismic reflection sections display a frozen image of orogenic thickening and lateral spreading. The decoupling of the upper, middle and lower crust during spreading resulted in the formation of layered superstructure–infrastructure of the crust.

Pukala intrusion, its age and connection to hydrothermal alteration in Orivesi, southwestern Finland

The Pukala intrusion is situated in the Paleoproterozoic Svecofennian domain of the Fennoscandian Shield in the contact region between the Central Finland Granitoid Complex and the Tampere Belt. The acid subvolcanic intrusion, which is in contact or close to several altered domains, mainly consists of porphyritic granodiorite and trondhjemite. The Pukala intrusion was emplaced into volcanic sequence in an island-arc or fore-arc setting before or during the early stages of the main regional deformation phase of the Svecofennian orogeny. On the basis of the geochemical data, the Pukala intrusion is a peraluminous volcanic-arc granitoid. After crystallisation at 1896±3 Ma, multiphase deformation and metamorphism caused alteration, recrystallisation, and orientation of the minerals, and tilted the intrusion steeply towards south. The 1851±5 Ma U-Pb age for titanite is connected to the late stages of the Svecofennian tectonometamorphic evolution of the region. Several hydrothermally altered domains are located in the felsic and intermediate metavolcanic rocks of the Tampere Belt within less than one kilometre south of the Pukala intrusion. Alteration is divided into three basic types: partial silica alteration, chloritesericite±silica alteration, and sericite alteration in shear zones. The fi rst two types probably formed during the emplacement and crystallisation of the Pukala intrusion, and the third is linked to late shearing. Intense sericitisation and comb quartz bands in the contact of the intrusion and the altered domain at Kutemajarvi suggest that the hydrothermal system was driven by the Pukala intrusion.

Magmatic zircon oxygen isotopes of 1.88–1.87 Ga orogenic and 1.65–1.54 Ga anorogenic magmatism in Finland

Oxygen isotope ratios of igneous zircon from magmatic rocks in Finland provide insights into the evolution and growth of the Precambrian crust during the Svecofennian orogeny. These data preserve magmatic δ18O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88–1.87 Ga Central Finland granitoid complex (CFGC) range from 5.50‰ to 6.84‰, except for three plutons in contact with the adjacent greenstone and metasedimentary belts (δ18O(Zrc) = 7.60‰–7.78‰). There is a systematic variation in δ18O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60±0.23‰ (σ) in the northeast to 5.90±0.40‰ in the west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen isotope composition of the 1.65–1.54 Ga rapakivi granites and related rocks in southern Finland show a decreasing trend from north to south, independent of their emplacement age. The southern anorogenic granite group has an average δ18O in zircon of 6.14±0.07‰ and the northern anorogenic group has an average δ18O in zircon of 8.14±0.59‰. This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic.

Nd isotope variation across the Archaean-Proterozoic boundary in the North Ladoga Area, Russian Karelia

In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleoproterozoic crust of the Svecofennian Orogen in the area north and west of the lake Ladoga (the North Ladoga Area) 24 samples of mostly granitoid rocks, collected along the 100 km long profile across the inferred suture, were analysed for their Nd isotopic composition. Ten previously published Nd data were incorporated in the dataset. It was established that gneisses from so-called “mantled domes” north of the suture have very low εNd values and represent reworked Archaean basement. North of the Kirjavalahti Dome, the presence of Archaean basement under Kalevian sedimentary cover is registered by abundant Archaean cores in zircons, revealed by a SHRIMP study, and by low εNd values in the rocks of the 1874±13 Ma-old Alattu dyke complex. The transition from Archaean to Proterozoic crust is registered by a shift from very low to Bulk-Earth-type εNd values and occurs within a 10-20 km-wide zone north of the inferred suture. The rocks of the Svecofennian Orogen south of the suture are characterized by relatively uniform initial Nd isotopic composition (average εNd value at 1880-1860 Ma +0.7, n = 17). In terms of Nd isotopic composition, this domain, the Lahdenpohja Domain, is comparable to the Svecofennian terranes to the west: the Central Finland Granitoid Complex and the Accretionary Arc Complex of Southern Finland. The Primitive Arc Complex of Central Finland has, in contrast, significantly more juvenile Nd isotopic composition.

Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

A metamorphic event in the Vammala Migmatite Belt (VMB) at ~1.92 Ga, revealed by SHRIMP U-Pb analyses of both zircon overgrowths and monazite ,i s interpreted as post-depositional and is correlated with the development of the early high-grade schistosity .N either this Early Svecofennian deformation and metamorphism, nor the associated complex folding, is present in the overlying Ta mpere Schist Belt (TSB) sequence, consistent with the VMB being part of a pre-1.91 Ga basement complex. The ~1.92 Ga event provides a maximum deposition age for the TSB ,c onfirm ing earlier age estimates. Earlier stratigraphic correlations between parts of the VMB and TSB ,a nd associated tectonic interpretations, can no longer be sustained.The crustal thickening seen in the VMB ,a nd previously attributed to arc accretion at ~1.89 Ga, is now attributed to accretion of a large Svionian marginal basin during the ‘Early Svecofennian’ orogenic phase at ~1.92 Ga.This is of similar age to the deformation and metamorphism associated with collision in the Lapland-Kola Orogen to the north of the Karelian Province. The well-known post-TSB orogenic phase was also identified in the VMB by a monazite age of 1881±6 Ma. A granitoid intrusion gave an emplacement age of 1888±5 Ma, comparable to the age of granitoid clasts in the upper part of the TSB succession. The detrital zircon data are interpreted to suggest that deposition of the precursor VMB sediments probably took place soon after an earlier pre-depositional metamorphism at ~1.98 Ga, which affected igneous source complexes dated at ~1.99 Ga and ~2.01 Ga. Mafic rocks in the southern part of the VMB ,a nd probably also the Haveri basalts, represent a renewed episode of extensional magmatism, which might correlate with the 1.96–1.95 Ga Jormua and Outokumpu ophiolites. A pre-1.96 Ga older stage basin has an expression in Sweden and complexes of similar age occur in the concealed Palaeoproterozoic basement south of the Gulf of Finland. Similar rocks, deformed and metamorphosed before ~1.96 Ga, might be present beneath the Central Finland granitoid complex and the late Svecofennian granite-migmatite zone ,a nd were possibly more local sources for both the younger stage Svionian basin sediments and the post-1.91 Ga Bothnian Basin sediments. The TSB and other post-accretionary volcanic sequences, and the associated plutonism, are interpreted to reflect a ~40 m.y .e xtensional period, inboard of the contemporaneous active margin, between orogenic phases at ~1.92 Ga and ~1.88 Ga.This interpretation provides a more satisfactory explanation of the major heat input to the crust over a very wide area than does the arc accretion hypothesis.The tectonic evolution of the Svecofennian Province has strong similarities to that of the Palaeozoic Lachlan Fold Belt in eastern Australia.

Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex II; Sources and Magmatic Evolution

The 188--187Ga post-kinematic granitoids of the Central Finland Granitoid Complex (CFGC) provide a key geochemical link to understanding granite formation in Paleoproterozoic orogenic and post-orogenic terrains. Thickness of the crust and intra-crustal differentiation processes played an important role in the formation of three granitoid types that shortly followed the peak of the Svecofennian orogeny. In the eastern CFGC, pyroxene-bearing plutons with C-type geochemical affinities predominate. These were formed from a mixture of low- to moderate-degree partial melts (30%) of mafic mantlederived (basaltic, 49% SiO2) source rocks and partial melts of pre-existing mafic granulite lower crust at depth. In the western CFGC, high-silica, iron-rich, fluorite-bearing plutons with A-type granite characteristics predominate. A higher thermal gradient, thinner upper and lower crust, and significantly more shallow Moho depth resulted in higher proportions of crustal melts (03--0 4v s 0 1--02 in the eastern CFGC) incorporated into the partial melts of a mafic mantle-derived source. A geochemical model focusing on the J and Honkajoki plutons of the post-kinematic suite is presented, constraining the nature of the source rock(s), the degree of partial melting, proportions of partial melts and fractionation processes involved.

Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex I; Radiogenic Isotope Constraints and Implications for Crustal Evolution

Post-kinematic granitoids of the Central Finland Granitoid Complex INTRODUCTION (CFGC) were emplaced into the Palaeoproterozoic crust of central In many Precambrian shield areas, extensive PalaeoFinland shortly after the >1·89 Ga peak of the Svecofennian proterozoic granitoid magmatism marks the mature stage orogeny. They are found as discordant, generally non-foliated plutons of crustal accretion that led to rapid growth of juvenile with Aand C-type granite characteristics and a bimodal (mafic– terranes around pre-existing Archaean cratons (Patchett felsic) magmatic association. Conventional and ion microprobe et al., 1981; Van Schmus & Bickford, 1981; Patchett & U–Pb data show that the post-kinematic magmatism commenced Bridgwater, 1984; Huhma, 1986; Sadowski & Bettenin the northeastern CFGC at >1·885 Ga and gradually moved court, 1996). On a global scale, these processes led to the to the west, where the plutons are dated at >1·870 Ga. Radiogenic establishment of the Palaeoproterozoic supercontinent isotope (Nd, Sr, Pb) composition of the post-kinematic plutons NENA (Northern Europe–North America; Gower et al., indicates homogeneous initial ratios: Nd (at 1·875 Ga) >0, Sr/ 1990; Rogers, 1996) the scattered remnants of which Sr >0·703, and Stacey & Kramers-type Pb with low longcurrently constitute a substantial fraction of the conterm Th/U (>2). These help to constrain the protolith history of tinental crust of the northern hemisphere. the post-kinematic magmatism. Nd and Pb isotopic data for In the Finnish part of the Fennoscandian (or Baltic) Palaeoproterozoic granitoids, Pb isotopic data for associated sulphShield (Fig. 1), the evolution of the >1·9 Ga Svecoides, and Nd and Pb isotopic composition of the classic >1·6 Ga fennian orogen involved several magmatic events (e.g. Finnish rapakivi granites reveal several distinct crustal domains in Korsman et al., 1999). Emplacement of early orogenic the Finnish Svecofennian orogen, and indicate that the central part (1·93–1·91 Ga) tonalites was followed by predominant of the Svecofennian orogen consists of several terranes (microsynorogenic (1·90–1·87 Ga) magmatism characterized by continents) that were amalgamated in Palaeoproterozoic times. The I-type tonalite, granodiorite, and granite and arc-type CFGC region is the oldest (>2·0 Ga) among these terranes. volcanism. At 1·84–1·81 Ga, late-orogenic (mainly Stype) granites were emplaced as a broad belt across southernmost Finland. These and associated mafic rocks have been ascribed to crustal shortening (Vaisanen &

Variation of radiogenic heat production rate in 2.8–1.8 Ga old rocks in the central Fennoscandian shield

We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( 120 km×500 km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere. The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield. Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m −3 in the Archaean domain, 1.3±0.6 μW m −3 in the Höytiäinen autochthonous Proterozoic domain, 1.2±0.5 μW m −3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m −3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m −3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m −3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m −3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres. Well-defined systematic variations of heat production rate values with either SiO 2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary). Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations. A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m −2) from radiogenic heat sources is calculated as 26 mW m −2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m −2, respectively.

Ion microprobe uranium‐lead dating of zircons from the Lappajärvi impact crater, western Finland

Abstract— The lake Lappajärvi impact crater lies in Paleoproterozoic Svecofennian metasedimentary rocks, on the western side of the Central Finland granitoid complex (∼1.9 Ga). Two conflicting ages have been reported for the meteorite impact: an age of 77.3 ± 0.4 Ma on the basis of Ar‐Ar whole‐rock data from impact melt samples and a paleomagnetic age of 195 Ma. During studies on impact crater indicator minerals at Lappajärvi, zircons with an atypical appearance were found in suevite boulders. These zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe U‐Pb dating, allowing a new and independent age estimate for the impact event at Lappajärvi. Four spot analyses on two black‐coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1.8 Ga for the source rock. Seventeen analyses were done on three dull zircon grains showing patchy impact‐related partial recrystallization. Most of these data fell fairly well on a single discordia line with intercept ages of 73.3 ± 5.3 Ma and 1854 ± 51 Ma. However, five of the data spots near the lower intercept end fell on the younger side of the line. This was interpreted to indicate post‐impact loss of lead. Importantly, the new ion microprobe U‐Pb age of 73.3 ± 5.3 Ma is in a very good agreement with the previously reported Ar‐Ar age.

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

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.