Bulletin of the Geological Society of Finland, Vol. 88, 2016, pp 85–103, https://doi.org/10.17741/bgsf/88.2.003
The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy
Mikko Nironen1*, Irmeli Mänttäri1 and Markku Väisänen2
1 Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland 2 Department of Geography and Geology, FI-20014 University of Turku, Finland
Abstract
Because of the relatively low metamorphic grade, low strain and well-preserved early structures in volcanic and volcaniclastic rocks, the Orijärvi triangle (in the center of the larger Orijärvi area) is one of the few locations in the Svecofennian orogen of southern Finland where lithostratigraphy has been determined. The geochemistry of the picritic- basaltic metavolcanic rocks of the Salittu Formation, located in the northeastern part of the Orijärvi area, has been characterized but otherwise the bedrock and structures have been barely touched. After remapping we give an interpretation of structural evolution, provide new age data to constrain duration of volcanism at Salittu, and refine the stratigraphy in the Orijärvi area. The original stratigraphy is visible at Salittu: metabasalt overlies migmatitic gneisses,
and metapicrite is on top. The rocks were folded during early Svecofennian D1–D2
deformations, and the large synformal structures developed as D2–D5 interference
structures formed during late Svecofennian D5 deformation. The structural pattern at Salittu is much the same as in the Orijärvi triangle. The new age data, combined with earlier published data, constrains the Salittu volcanism at ca. 1875 Ma. The stratigraphy in the Orijärvi area consists of the early (1.90–1.89 Ga) volcanic Orijärvi Formation, overlain by the sedimentary Vetio Formation, the volcanic Kisko Formation, the volcanic-sedimentary Ahdisto Formation, the volcanic Toija Formation, and on top the Salittu Formation, all emplaced at 1.88–1.87 Ga. We propose a model in which the Orijärvi Formation represents magmatism at the margin of a microcontinent, and the overlying package represents sedimentation and magmatism
above a subduction zone during an initial stage of microcontinental accretion. D1 deformation occurred in an advanced stage of accretion, after emplacement of the volcanic rocks of the Salittu Formation.
Keywords: metavolcanic rocks, gabbros, volcanism, structural geology, stratigraphy, U−Pb, zircon, Paleoproterozoic, Orijärvi, Salittu, Finland
*Corresponding author (e-mail: [email protected])
Editorial handling: Jussi S Heinonen ([email protected]) 86 M. Nironen, I. Mänttäri and M. Väisänen
1. Introduction southern Svecofennia of Finland where reliable structural evolution and lithostratigraphy can The Svecofennian bedrock in the Uusimaa belt in be determined. Väisänen and Mänttäri (2002) southernmost Finland is characterized by rather presented the stratigraphy of the supracrustal rocks high strain and high grade metamorphism, reaching in the Orijärvi area (including Salittu), with four granulite grade e.g. in the West Uusimaa Complex formations: Orijärvi (oldest), Kisko, Toija, and (Fig. 1; Parras, 1958). There are also areas of lower Salittu (youngest), with an age span from 1.90 Ga to amphibolite facies assemblages such as the Orijärvi 1.88 Ga. The Salittu Formation (SFm) differs from area. Because Orijärvi is an old mine area, early the other ones by the characteristic metapicrites geological studies were focused on ore mineralogy (metapicrite occurs in the Toija formation as well). and alteration of the rocks around the mines. Eskola The geochemistry of the picritic-basaltic volcanism (1914) presented petrology of rocks in the larger at Salittu is studied in a companion paper (Nironen, Orijärvi area and described also an ultramafic rock submitted). at Salittu which he considered a plutonic peridotite. Based on remapping of the area where the The structural interpretation of the western rocks of the SFm occur we give an interpretation Uusimaa belt by Tuominen (1957) included two of structural evolution. In addition, we provide shear zones, later called the Kisko and Jyly shear additional age data to earlier published ages to zones (Fig. 1). In the 1970’s and 1980’s the research constrain the duration of volcanism at Salittu, projects by the Free University of Amsterdam led provide and interpretation of stratigraphy at Salittu, to a series of papers on metamorphism (Schreurs, and refine the stratigraphy in the Orijärvi area. 1984; Schreurs, 1985; Schreurs and Westra, 1986; Blom, 1988) and structures (Bleeker and Westra, 1987; Ploegsma and Westra, 1990; Ploegsma, 2. Geological setting 1991) in the central Uusimaa belt. Ploegsma and Westra (1990) called the triangular area bordered The Paleoproterozoic tectonic evolution in by Kisko and Jyly shear zones the Orijärvi triangle. Fennoscandia, including the Svecofennian orogeny, They considered it a low-strain area that preserved has been modeled basically in two ways. In the first early (D1) folds and was protected from regional semi-continuous, protracted accretion of island arcs deformation during later deformation events (D2, to the Archean craton led to southwestward growth D3). More recently Skyttä et al. (2006) and Pajunen of the orogenic belt (Gorbatchev and Bogdanova et al. (2008) described profoundly the structures 1993); in a refinement of this model continuous in the Orijärvi triangle and presented slightly long-lived subduction occurred with repeated different interpretations of structural evolution. retreating and advancing stages (Hermansson et al., The rocks at Salittu, to the northeast of the Orijärvi 2008; Stephens and Andersson, 2015). The second triangle were studied as well: Schreurs et al. (1986) model involves accretion of microcontinents were the first to consider the ultramafic rocks at (Nironen, 1997), and the orogeny consisted of Salittu to be of volcanic origin and they presented separate orogenic stages, including microcontinent basic geochemistry of these rocks. Since the 1980’s accretion (1.92–1.87 Ga), continental extension tentative interpretations of structures at Salittu (1.86–1.84 Ga), continental collision (1.84–1.79 have been given by Skyttä and Mänttäri (2008) and Ga), and finally orogenic collapse and stabilization Pajunen et al. (2008). (1.79–1.77 Ga: Lahtinen et al., 2005). According Because of the relatively low metamorphic to the second model the Svecofennian orogen in grade, low strain and well-preserved early southern Finland may be divided into Central structures in volcanic and volcaniclastic rocks, Svecofennia with a microcontinent in the core, and the Orijärvi area is one of the few regions in Southern Svecofennia with another microcontinent The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 87
Figure 1. Geological Metapicrite map of the Laidike Orijärvi area, Mafic metavolcanic rock modified from Intermediate and felsic metavolcanic rock Salli (1955) and Migmatitic intermediate Fig. 3 Väisänen and and felsic gneiss Mänttäri (2002). Metapelite OFm = Orijärvi Formation, Gabbro Salittu KFm = Kisko Tonalite and Formation, granodiorite SFm TFm = Toija Granite Formation,
SFm = Salittu Jyly shear zone Formation. Inset shows location SFm KFm of the Orijärvi TFm area (thick black line) within the Kisko shear zone Central Svecofennia Uusimaa belt Kisko Vetio (UB). WUC = Southern Svecofennia West Uusimaa 60 014' Complex. OFm WUC Orijärvi 0
6 km 23 40' UB 50 km Helsinki
6 km Nironen et al. Fig. 1
Figure 2. Aeromagnetic map of the Orijärvi area (GTK database).
Jyly shear zone Major shear Kisko shear zone zones (solid line) and extent of the rocks of the Salittu Formation (broken line) are shown. Nironen et al. Fig. 2 88 M. Nironen, I. Mänttäri and M. Väisänen
(Fig. 1, inset). The difference between these conditions in the West Uusimaa Complex during accretionary units is that whereas both display peak of the late Svecofennian metamorphism structural and metamorphic evolution attributed (1.83–1.81 Ga). According to Schreurs (1985), the to microcontinental accretion, in Southern increase in metamorphic grade and development Svecofennia this evolution is overprinted by another of the West Uusimaa Complex was a thermal, one, attributed to continental collision. virtually isobaric event, and therefore Schreurs Magmatism and metamorphism during the and Westra (1986) suggested that the whole region, Svecofennian orogeny in Southern Svecofennia including the Orijärvi area, represents one crustal may be divided into the early Svecofennian (1.90– level. However, Skyttä et al. (2006) noted an abrupt 1.87 Ga) and the late Svecofennian (1.84–1.79 change in metamorphic grade across the Jyly Ga) periods. The early Svecofennian period was shear zone and interpreted that the Jyly and Kisko followed by an intra-orogenic phase of minor shear zones acted as block margins during the late magmatism (Bergman et al., 2008; Väisänen et al., Svecofennian period, with reverse (east-side-up) 2012). The late Svecofennian granitic magmatism dip-slip movement in the Jyly shear zone. and associated high-temperature, low-pressure type metamorphism overprinted the early Svecofennian magmatism, metamorphism and deformation in 3. Rock types a wide belt called the Late Svecofennian Granite- Migmatite zone (Ehlers et al., 1993) that covers The SFm consists of pictitic and basaltic southernmost Finland. metavolcanic rocks. The extent of the SFm in The supracrustal rocks in the Uusimaa belt the Orijärvi area is about 20 km (Fig. 1), and are mainly metavolcanic and volcanic-derived considering the metapicrite occurrences farther east metasedimentary rocks, with epiclastic schists the total length exceeds 30 km. In the aeromagnetic and gneisses as minor constituents. They were map the metavolcanic rocks of the SFm show up as metamorphosed during the early Svecofennian positive anomalies (Fig. 2). period at amphibolite facies conditions (Pajunen The metabasalt is generally compositionally et al., 2008). The lower amphibolite facies rocks in banded (Fig. 4a) but breccia structures are rather the Orijärvi triangle include andalusite-cordierite common, and pillow structure occurs locally. The schists; this area was largely preserved from the late metapicrite varies from homogeneous to fragmental Svecofennian deformation and metamorphism. type that probably represents volcanic breccia. Skyttä et al. (2006) explained the preservation of the Inclusions of metabasalt in the metapicrite (Fig. 4b) early Svecofennian structures by strain partitioning show that picrite erupted after basalt and dragged into high-strain zones (the Jyly and Kisko shear fragments of the basalt into the lava. The boundary zones; Figs. 1 and 2). of the metapicrite and metabasalt is usually The metamorphic grade increases from the tectonized but sharp where a primary contact can be Orijärvi triangle, and most of the rocks at Salittu found (Fig. 4c). are migmatitic (Fig. 3). Peak regional metamorphic In addition to the metavolcanic rocks, the pressure of 3 kbar (Latvalahti, 1979) and supracrustal sequence at Salittu consists of felsic- temperature of 600° C (Schumacher and Czank, intermediate gneisses and mafic schists (Fig. 3). 1987) have been estimated for the Orijärvi triangle. The gneisses are migmatitic, with leucosome Schreurs and Westra (1986) suggested an increase in veins usually parallel to compositional layering. peak metamorphic temperature from 550–650° C Quartz-feldspar gneiss is light-colored and faintly in the Orijärvi triangle to 700–825° C in the West layered (Fig. 4d). As the gneiss does not contain Uusimaa Complex at 3–5 kbar pressure. Mouri aluminosilicate porphyroblasts, it is probably et al. (2005) estimated 750–800° C and 4–5 kbar volcaniclastic in origin. Quartz-feldspar gneiss The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 89
70
75
55 60 60 78 60 80 85 A2362 Arpalahti
67 58
Ruona Salittu Aromäki
70 58 80
A2282 85 Sorttila 73 70 Lake Enäjärvi
45 Laari 53
0 40
23 37' 58 60 017' Fig. 4c 2 km
Salittu Formation Other Svecofennian Svecofennian supracrustal rocks intrusive rocks Metapicrite Mafic schist Gabbro Foliation Intermediate gneiss Granodiorite, Metabasalt (migmatitic) tonalite Axial trace Biotite gneiss (migmatitic) Granite (Fn+2) Quartz-feldspar gneiss Shear zone, (migmatitic) fault Intermediate and felsic volcanic rock Metapelite
Figure 3. Geological map of the Salittu area, modifi ed from Salli (1955), Schipper (1981), and Väisänen and Mänttäri (2002). Location of Figure 6 is shown by rectangle. Sample sites for age dating are shown by red stars.Nironen The foliation formet al.lines Fig.show orientation 3 of the prominent foliation
(Sn or Sn–Sn+1 in supracrustal rocks). grades into cordierite-bearing biotite gneiss, At Sorttila, within the Orijärvi triangle indicating an increasing proportion of weathered area (Fig. 5), the metavolcanic rocks are much material in the protolith. Th ere are also gneisses of similar to the metavolcanic rocks at Salittu. Th e intermediate composition with some hornblende rocks surrounding the metavolcanic rocks are as well as diopside- and amphibole-bearing intermediate, faintly layered schists, interpreted mafi c schists; these rocks are likely of volcanic to have a volcanic or volcanic-sedimentary origin or volcanic-sedimentary origin. Garnet-bearing (tuff s or redeposited tuff s). Farther south the schists biotite paragneiss occurs scatteredly and represents contain andalusite porphyroblasts indicating epiclastic sedimentary rocks that are scarce at Salittu. sedimentary origin. 90 M. Nironen, I. Mänttäri and M. Väisänen
a b
c d
N
e f
Figure 4. Outcrop photographs from the Salittu area. a. Foliation in metabasalt (Sn, solid white line), parallel to
compositional layering, is deformed by tight, sinistral Fn+1 folding which is folded by open folding (Fn+2). Fn+1 and Fn+2 axial traces are shown by broken and dotted white lines, respectively. A composite dike crosscuts all structures. Shaft of hammer (length 60 cm) points to north.Nironen b. Inclusion et of metabasaltal. Fig. in metapicrite.4 Length of code bar 12 cm. c. Primary contact (dotted white line) between metabasalt of fragmental type (upper part of figure) and metapicrite. Dark
alteration bands occur at the base of the metapicrite. Sinistral folding (Fn+1) of banding (Sn) is visible. Shaft of hammer
points to north. d. Quartz-feldspar gneiss. Foliation parallel to compositional layering (Sn) and leucosome veins are
tightly folded (Fn+1; axial trace shown). e. Intrusive contact of Arpalahti tonalite with metabasalt. f. Detail of the intrusive
contact. Note how the almost undeformed tonalite crosscuts foliation (Sn) in the metabasalt. The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 91
Gabbroic bodies occur within and adjacent to Sn 85 the metavolcanic rocks. Th e largest body within the Dn anticline metavolcanic rocks at Aromäki (Fig. 3) is medium- Sn+1 87 82 grained and unfoliated. Gradational contacts to Fn+2 axial 88 metapicrite and similar geochemistry (Nironen, trace 87 submitted) suggest that the Aromäki gabbro is Fault synvolcanic with respect to metapicrite. In contrast, the body at Laari consists of fi ne- to medium- grained, foliated intrusive rock that varies in 85 composition from gabbro to diorite and is crosscut o 48 45 by the metapicrite. 23 32' 78 46 60 o 17' An elongate body in the northern part of the 70 80 Lake 77 Ahdisto Salittu area (Fig. 3), here named as the Arpalahti 77 tonalite, crosscuts the metabasalt of the SFm (Figs. 85
4e, 4f). Th e tonalite is homogeneous, medium- 1 km grained, generally foliated but less foliated at the ends of the elongate body. A sample (A2362) was Metapicrite Gabbro taken from the tonalite for dating. Another sample Metabasalt Granite (A2282) was taken from a foliated, medium-grained Intermediate metavolcanic rock granodiorite, here named as the Laari granodiorite, Felsic metavolcanic rock occurring within the metapicrite (Fig. 3). Metapelite / Biotite gneiss A large, elongate granite body occupies the southern part of the study area and crosscuts all the Figure 5. Geological map of the Sorttila supracrustal rocks (Fig. 1). It is medium- to coarse- areaNironen within the Orijärvi et al.triangle, Fig. based 5 on grained and almost unfoliated except for a few shear observations of this study and GTK bedrock database. zones. In addition, small granite bodies crosscut the ductile structures in the metavolcanic rocks but some of these granites are deformed by faults (Fig. 6). the adjacent quartz-feldspar gneiss, and is therefore
considered to defi ne nD foliation.
Sn foliation and leucosome veins in the gneisses
4. Structure are tightly to isoclinally folded (Fn+1). A faint, steeply
dipping to vertical Sn+1 foliation can be identifi ed in Th e Salittu Formation trends approximately ENE– the axial plane in the mica-bearing layers (Fig. 4d).
WSW (Fig. 1). A small area in the center of the Th e F n+1 fold axis plunges vary from subhorizontal to n Salittu area, with good outcrop density and easily steep, and a mineral lineation (L n+1) is parallel to the distinguishable rock types, was studied in detail fold axis. Fn+1 folding is visible also in the metabasalt to understand the structure (Fig. 6).Th e quartz- (Fig. 4a). feldspar gneiss and biotite gneiss exhibit a weak Th e F n+1 folding is deformed by upright, open compositional layering, and leucosome veins are to tight folding (Fn+2) with NNW–SSE to NNE– (sub)parallel to the layering. Generally the amount SSW striking subvertical axial planes (Figs. 4a and of mica is small in the gneisses but a schistosity (Sn) 6). Based on interpretation of the structures in the can be seen in more mica-bearing layers (mainly in area of Figure 6, the synformal structure at Laari the biotite gneiss). Compositional layering in the results from interference of two, almost orthogonal metabasalt has the same orientation as layering in foldings Fn+1 and Fn+2. 92 M. Nironen, I. Mänttäri and M. Väisänen
65 70 80 65
Lake Figs. 4e, 4f 67 Tiekslammi 77 78 78 82 68 85 82 80 70 60 85 80 85 80 70 60 75 Fig. 4d 83 83 83 Fig. 4b 10 Fig. 4a 75 55 Road 65 82 75 65 85 77 55 75 35 82 85 45 85 N 75 80 85 53 60 85
65 70 0 Quarry 23 37'30" 500 m 60 018'12"
Metapicrite Gabbro Sn Fn+1 fold axis
Metabasalt Tonalite Fn+1 axial trace, syn- form, antiform Fn+2 fold axis Biotite gneiss Granite Fn+2 axial trace Quartz-feldspar Fault gneiss
Figure 6. Structural interpretation of the Ruona-Arpalahti area. Nironen et al. Fig. 6 At Sorttila, the structural sequence is correlative narrow lense within the metabasalt. Since Sn+1 to that at Salittu. Th e intermediate metavolcanic foliation is visible within this lense, the structure schist displays a rather weak Sn foliation, and is interpreted as a Dn anticline. In the northeastern compositional layering in the metabasalt is parallel part of Sorttila, Sn compositional layering in the to Sn (Fig. 5). Rather open Fn+1 folding deforms Sn metabasalt is tightly folded, with axial plane in in the schist and compositional layering in the NNE–SSW direction. Th is folding is interpreted as metabasalt, with development of a schistosity in Fn+2. the axial plane. In the southern part of the area the Th e sharply crosscutting contact of the intermediate schist crops out as an E–W directed Arpalahti tonalite to the foliated (Sn) metabasalt The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 93
(Fig. 4f) indicates that the tonalite intruded during mode with beam di- ameter of 20 μm, pulse a late stage or after Dn. The tonalite is foliated (Figs. frequency of 10 Hz, and beam energy density of 2 and 6) but it is difficult to assess whether this 2.07 J/cm2. A single U–Pb measurement included foliation correlates with Sn or Sn+1 in the supracrustal 30 s of on-mass background measurement, followed rocks because the two are parallel in many places. by 60 s of ablation with a stationary beam. Masses The form of the tonalite body suggests that it was 204, 206 and 207 were measured in secondary deformed during Dn+2 (Fig. 1). electron multipliers and 238 in the extra high mass An unfoliated composite (mafic) dike was Faraday collector. Ion counts were converted and found to sharply crosscut the foliation and folding reported as volts by the Nu Plasma timeresolved in the metabasalt (Fig. 4a). The subvertical dike is analysis software. 235U was calculated from the signal 238 235 oriented parallel to Fn+2 fold axial plane, suggesting at mass 238 using a natural U/ U=137.88. Mass 204 emplacement during Dn+2. number 204 was used as a monitor for Pb. Raw Numerous faults at Salittu appear as data were corrected for background, laser induced discontinuities in lithology (Fig. 6) and as linear elemental fractionation, mass discrimination, and depressions in topography. They are exposed on drift in ion counter gains and reduced to U–Pb few outcrops where they occur as narrow mylonitic isotope ratios by calibration to concordant reference zones. Larger shear zones were interpreted on zircon of known age, using protocols adapted from the basis of the aeromagnetic data (Fig. 2) and Andersen et al. (2004) and Jackson et al. (2004). discontinuities in lithology. Standard zircon GJ-01 (609 ± 1 Ma; Belousova et To the north of Salittu, at Laidike (Fig. 1), there al., 2006) and an in-house standard zircon A1772 is tight folding with E-W axial trace. The relative age (2711 ± 3 Ma/TIMS; 2712 ± 1 Ma/SIMS; Huhma of this folding is open: it may be Fn+1 folding, or it et al., 2012) were used for calibration. In addition, may be the youngest deformation in the area. A382 (1877±2 Ma; Huhma et al., 2012) was measured as a quality control sample to check the calibration (1882 ± 6 Ma; MSWD of concordance 5. U−Pb zircon dating = 4.8; Mean 207Pb/206Pb age = 1880 ± 5 Ma; MSWD = 0.95; n=25/25. MSWD = Mean Square Weighted 5.1. Analytical methods Deviation). Age related common lead (Stacey and Kramers, 1975) correction was used when the Zircon for LA-MC-ICPMS (Laser Ablation analysis showed common lead contents above the Multi-collector Inductively Coupled Plasma Mass detection limit. The calculations were done off-line, Spectrometry) U−Pb dating were selected by hand- using an interactive spreadsheet program written picking after heavy liquid and Frantz magnetic in Microsoft Excel/ VBA by Tom Andersen (Rosa separation. The chosen grains were mounted in et al, 2009). To compensate for drift in instrument epoxy resin and sectioned approximately in half and sensitivity and Faraday vs. electron multiplier gain polished. Back-scattered electron images (BSE) during an analytical session, a correlation of signal and cathodo luminescence (CL) images were taken vs. time was assumed for the reference zircons. A using SEM (Scanning Electron Microscope) to target description of the algorithms used is provided in the spot analysis sites on mineral grains. Rosa et al (2009). U–Pb dating analyses were performed using The age calculations and plotting of the U−Pb a Nu Plasma HR multicollector ICPMS at the isotope data were performed using the Isoplot/ Geological Survey of Finland in Espoo using a Ex 3 program (Ludwig, 2003). All the ages were technique very similar to Rosa et al (2009) except calculated with 2σ errors and without decay that Analyte G2 193 nm laser laser microprobe constant errors. Data-point error ellipses in the was used. The analyses were made in static ablation figures are at the 2 σ level. 94 M. Nironen, I. Mänttäri and M. Väisänen
Table 1. LA-MC-ICPMS zircon U-Pb data, Laari granodiorite and Arpalahti tonalite.
$JHV 5DWLRV 6DPSOH 3E 3E 3E 3E 3E 3E 1) 'LVF 8 3E 3E 3E I σ σ σ σ σ σ ρ VSRW 3E 8 8 3E 8 8 SSP PH DVXUHG $/DDULJUDQRGLRULWH6DOLWWX $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $BD ( $$USDODKWLWRQDOLWH6DOLWWX $D ( $E ( $D ( $D ( $E ( $D ( $E ( $D ( $E ( $D ( $D ( $E ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $D ( $E ( $D ( $E ( $D ( $E ( $D ( $D ( $D ( $D ( $D ( $D ( $OOHUURUV DUHLQVLJPD OHYHO (UURUFRUUHODWLRQ LQ FRQYHQWLRQDOFRQFRUGLDVSDFH $JHGLVFRUGDQFH DW FORVHVW DSSURDFKRIVLJPDHUURU HOOLSVH WR FRQFRUGLD 3HUFHQWDJHRI FRPPRQ 3E LQ PHDVXUHG 3EFDOFXODWHG IURP WKH 3E VLJQDO DVVXPLQJ DSUHVHQWGD\ 6WDFH\ .UDPHUV PRGHO WHUUHVWULDO3ELVRWRSH FRPSRVLWLRQ ,Q %6(&O LPDJHVDOO WKHDQDO\VHG]LUFRQGRPDLQVLQVDPSOH$DUHWH[WXUDOO\VLPLODUDQGLQVDPSOH$WKH[[DUHIHUV%6(GDUNHUFHQWHUDQG[[EWKHSDOHURXWHUULPGRPDLQV The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 95
5.2. Results Zircon in the A2362 Arpalahti tonalite is transparent to translucent and stubby. Th e BSE Th e zircon population from the A2282 Laari and CL images usually show BSE-darker/CL- granodiorite is very homogeneous consisting mostly paler center and BSE-paler/CL-darker outer/ of prismatic (length : width = 2‒6) and transparent rim domains. In the CL images, mixed type grains. Magmatic zoning is visible especially in the (oscillatory and sector zoning) growth zoning is rather dark Cl images (Fig. 7). Twenty zircon grains visible (Fig. 8). Altogether 35 zircon domains from were dated using LA-MC-ICPMS (Table 1). On 27 grains were U-Pb dated (Table 1). Ages from the concordia diagram (Fig. 7a), the U-Pb data plot the center and outer/rim zircon domains are coeval in a tight cluster defi ning a 1876 ± 8 Ma age with a within the error limits. On the concordia diagram high MSWD of concordance. Th erefore the mean (Fig. 9), all the U-Pb data plot in a tight cluster Pb‒Pb age of 1871 ± 9 Ma (Fig. 7b) is considered determining an age of 1878 ± 5 Ma for the tonalite the best age estimate for the timing of granodiorite crystallization. crystallization.
A2282 Laari granodiorite
01a:1.84 Ga 20a:1.87 Ga 04a:1.87 Ga 200 m
A2362 Arpalahti tonalite
01a:1.89 Ga 21a:1.88 Ga 14a:1.88 Ga 16a:1.89 Ga 01b:1.90 Ga 21b:1.87 Ga
Figure 7. Representative BSE and CL images of U–Pb dated zircon grains from Salittu. The analysis spot sites on the BSE images, corresponding analysis numbers and resulted 207Pb/206Pb ages are indicated. See Table 1. Nironen et al. Fig. 7 96 M. Nironen, I. Mänttäri and M. Väisänen
1970 0.42 A2362 Arpalahti tonalite 1950
1930 0.38 age (Ma)
1910 U
2000 Pb 238 1900 206 1890 0.34 Pb/ Pb/
206 1800 1870 207
1700 1850 0.30 Concordia Age = 1878 ± 5 Ma (2σ, decay-const. errs ignored) 1830 MSWD (of concordance) = 1.0; n=35/35
0.26 1810 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 1790 207Pb/235U
Figure 8. U-Pb age data of the Laari granodiorite. a) Concordia plot. b) 207Pb/206Pb age distribution and mean age.
0.42 A2282 Laari granodiorite 0.40
0.38 2060 2020
U 0.36 1980 238 1940 Pb/ 0.34 206 1820 0.32 1780
1740 Concordia Age = 1876 ± 8 Ma 0.30 (2σ, decay-const. errs ignored) MSWD (of concordance) = 13; n=20/20 0.28 4.4 4.8 5.2 5.6 6.0 6.4 6.8 207Pb/235U
Figure 9. Concordia plot of the Arpalahti tonalite. The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 97
TableTable 2. 2. Interpretations Interpretations of of deformation deformation events, events, their their styles styles and and orientations orientations of of foliations foliations and and axial axial traces traces in in the the Orijärvi Orijärvi area. area.
Ploegsma & Westra 1990 Skyttä et al. 2006 Skyttä & Mänttäri 2008 Pajunen et al. 2008 Present study Early Svecofennian (1.90–1.87 Ga)
D1 (subhorizontal, D1 (subhorizontal, D1 (subhorizontal, DA (horizontal, Dn (subhorizontal, thrusting) thrusting) thrusting) thrusting) thrusting? - upright)
D2 D2 (upright, ENE-WSW) D2 DB-DC-DD (extension - Dn+1 (upright) compression - transpression) Late Svecofennian (1.84–1.79 Ga)
D3 (subhorizontal, DG-DH (upright, E-W - extension) horizontal, extension)
D3 (upright, E-W) D4 (upright, ENE-WSW) DH (transpression, upright, NE-SW)
D4 (upright N-S & NE, D5 (NE, NW shear DI (upright, N-S) Dn+2 (upright, N-S) NW shear zones) zones) Post-Svecofennian (1.53 Ga) D3 (upright, N-S)
6. Discussion According to Skyttä and Mänttäri (2008), the area east of the Jyly shear zone (including the Salittu
6.1. Structural correlation area) is dominated by upright D4 structures with ENE–WSW axial planes. D4 deformation started Skyttä et al. (2006) presented a structural sequence by N–S contraction, and counterclockwise rotation for the Orijärvi area with two early Svecofennian of contraction caused strain localization into the and two late Svecofennian deformation events subvertical D5 Kisko and Jyly shear zones that were
D1–D2 and D3–D4, respectively (Table 2). In their formed after 1820 Ma. sequence S1 is a weak schistosity subparallel to In the structural scheme of southern Finland bedding and possibly associated with thrusting by Pajunen et al. (2008) the western part of the during D1; D2 deformation occurred at 1.88–1.87 Uusimaa belt is characterized by late Svecofennian
Ga Ma and caused the penetrative S2 foliation dome-and-basin interference structure (DG+H±I, see with ENE–WSW trend and development of a Table 2).
D2 synform in the northern part of the Orijärvi Väisänen and Skyttä (2007) found evidence for triangle (Orijärvi sub-area in their terminology, contrasting movement senses e.g. in the Jyly shear including Sorttila). Subhorizontal D2 structures zone. They proposed that the late Svecofennian were refolded into upright folds in the southern oblique-slip reverse faulting (sinistral, east-side- part of the Orijärvi triangle during D3 deformation. up) caused a positive flower structure in the Jyly The Kisko and Jyly shear zones and the different shear zone, followed by extensional displacement crustal sections in the present erosion surface are (east-side-down) in brittle conditions. Väisänen the result of E–W contraction during retrograde and Skyttä (2007) tentatively suggested such an
D4 deformation that continued in brittle regime. extension at 1.79–1.77 Ga. Later Skyttä and Mänttäri (2008) refined the The occurrence of metabasalt inclusions deformational sequence over a larger area by adding in the metapicrite (Fig. 4b) and the rock type a crustal extension stage D3 to the beginning of distribution in Figures 5 and 6 imply that the late Svecofennian deformation, at 1835–1825 Ma. original stratigraphic succession is essentially 98 M. Nironen, I. Mänttäri and M. Väisänen
preserved at Salittu and Sorttila: metapicrite overlies 2008). Neither did we find evidence of large- metabasalt, and metavolcanic schists and gneisses scale, open, upright D4 folding with ENE–WSW are below these rocks. The oldest structure at Salittu trending axial planes which was supposed to is the penetrative composite layering Sn that can characterize the Salittu area; however the tight be identified in the gneisses and in the metabasalts. folding at Laidike, with E–W trending axial planes,
Preservation of the original stratigraphy suggests is possibly D4 deformation. Fn+2 folding with N–S that the volcanic sequence remained flat-lying trending axial planes corresponds to F5 and DI during Dn deformation. However, the style of Dn is of Skyttä and Mänttäri (2008) and Pajunen et al. unclear; it may have been thrusting (see Table 2). Dn (2008), respectively. anticline at Sorttila may result from upright, rather Structural interpretation of the Orijärvi area open Fn folding during a late stage of progressive Dn is shown in Figure 10. The structure at Sorttila was deformation. Sn and Sn+1 at Sorttila are the same as interpreted as D2 synform by Skyttä et al. (2006)
S1 and S2 of Skyttä et al. (2006). We interpret that Sn but the ovoidal form of exposed metavolcanic and Sn+1 at Salittu may be correlated with S1 and S2 as rocks suggest the effect of 5F folding. We interpret well. The leucosome veins in the gneisses probably that the synforms at Laari and Sorttila are the result developed along S0/S1 surfaces during early stages of of two almost orthogonal folding phases, early
D2 deformation, and were folded during progressive Svecofennian (D2) and late Svecofennian (D5). If
D2. this interpretation is correct, the Orijärvi triangle
We did not find evidence of D3/DH crustal was not totally preserved from late Svecofennian flattening that was suggested to have initiated late deformation (cf. Ploegsma and Westra, 1990). Svecofennian deformation (Skyttä and Mänttäri
D2 synform, antiform
D5 synform, antiform
?
?
6 km ? ?
Figure 10. Structural interpretation of the Orijärvi area. Rock types as in Figure 1. Nironen et al. Fig. 10 The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 99
6.2. Metamorphism and structural domains of zircons from a felsic metavolcanic rock of evolution the Toija Formation, and interpreted it to represent the crystallization of the rock. They attempted to
As noted by Blom (1988), the S1-S2 foliation is in date the SFm and although they did not get a direct places orthogonal to the metamorphic isogrades, age they concluded that the formation was deposited showing that the late Svecofennian thermal pulse between 1878 and 1875 Ma. overprinted the early Svecofennian structural grain. The Arpalahti tonalite gives the minimum
At Salittu the segregation of melt parallel to S1 age of for the Salittu metabasalt, and the Laari foliation is interpreted to be an early Svecofennian granodiorite most probably crosscuts the feature. The implication is that the gneisses metapicrite. Considering the error limits, the represent a deeper section of the crust than rocks in 1874 ± 8 Ma and 1878 ± 5 Ma ages for the Laari the Orijärvi triangle; the rise in metamorphic grade granodiorite and Arpalahti tonalite, respectively, are is not related to the late Svecofennian thermal event. the same as the ages obtained from the Kisko and The Orijärvi area is within the Late Toija formations. As the dacite from the upper part Svecofennian Granite-Migmatite zone (Ehlers of the Kisko Formation yielded an 1878.2 ± 3.4 Ma et al., 1993) but we could not find migmatization age, the timing of picritic-basaltic volcanism may be at Salittu that was explicitly late Svecofennian. estimated to ca. 1875 Ma. The large granite body around Salittu (Fig. 1) is A belt of metapelite (metagreywacke) occurs late Svecofennian because it crosscuts in places between the Orijärvi and the Kisko Formations, early Svecofennian structures (Fig. 3). The small, in the southern limb of the Orijärvi D2 synform undeformed granite bodies that occur along faults (Fig. 1; Väisänen and Mänttäri, 2002; Skyttä et al.,
(Fig. 6) are similar to granites in D5 shear zones (cf. 2006). The Vetio site within this belt was studied in Skyttä and Mänttäri, 2008, Fig. 2). The composite detail by Skyttä et al. (2006), Pajunen et al. (2008), mafic dike at Salittu shows that magmatism during and Sayab et al. (2015). Detrital zircons in the
D5 deformation was not only granitic. metapelite are older (2.72, 2.12–1.93 Ga; Claesson Ploegsma and Westra (1990) interpreted that et al., 1993) than zircons in the volcanic rocks in the folding (F3) with N–S trending axial planes governs Orijärvi and Kisko Formations. We interpret that the large-scale structure at Salittu, and Pajunen et al. the metapelite represents a sedimentary basin that
(2008) came to a similar conclusion (their FI). These developed after the earlier magmatic event at 1.90– interpetations appear to be valid because the form 1.89 Ga, and sediments from a large area were mixed of the large granite body is curved and the granite in the basin. Since the stratigraphic position of the contains deformation zones, suggesting large-scale belt is known within the Orijärvi triangle, we here
D5 deformation (Fig. 10). name it Vetio Formation. The metasedimentary rocks as well as the 6.3. Stratigraphy and age overlying intermediate metavolcanic rocks at correlation Sorttila (Fig. 5) were previously considered to be the uppermost part of the Kisko Formation (Väisänen The oldest ages in the Southern Svecofennia are 1898 and Mänttäri, 2002). Since the lower part of the ± 9 Ma for the Orijärvi granodiorite (Väisänen et al., Kisko Formation consists solely of metavolcanic 2002) and 1895.3 ± 2.4 Ma for a synplutonic rhyolite rocks, we consider that these rocks form a separate at Orijärvi (Väisänen and Mänttäri, 2002). Moreover, formation within the Orijärvi triangle, here named Väisänen and Mänttäri (2002) obtained 1878.2 ± 3.4 as Ahdisto Formation. The stratigraphic position of Ma age from a dacite from the upper part of the Kisko the Ahdisto Formation is correlative to the quartz- formation. Subsequently Väisänen and Kirkland feldspar gneisses and biotite gneisses at Salittu. (2008) attained 1878 ± 4 Ma concordia age from core Field evidence suggests that the Toija Formation 100 M. Nironen, I. Mänttäri and M. Väisänen
D1 Metapicrite and a unit of ultramafic metavolcanic rock in the 1874±8 Ma SFm upper part. Uppermost are the ultramafic-mafic 1879±5 Ma Mafic meta- volcanic rock metavolcanic rocks of the SFm, crosscut by the Intermediate 1878±4 Ma Arpalahti tonalite and the Laari granodiorite. KSZ AFm metavolcanic rock In the stratigraphic column (Fig. 11) rocks TFm Felsic meta- 1878±4 Ma volcanic rock of 1890–1880 Ma age are missing which is somewhat strange considering that this was a KFm Metapelite period of voluminous crust formation during Syn-accretion Carbonate rock the Svecofennian orogeny. The ca. 20 Ma age gap VFm Banded iron between the Orijärvi and Kisko Formations is ? ? formation somewhat loosely constrained because no age data OFm Gabbro 1895±3 Ma exists from the bottom of the Kisko Formation but Tonalite and 1898±9 Ma granodiorite at present we take the age gap to be real. Pre-accretion The ca. 1875 Ma picritic-basaltic volcanism of Figure 11. Stratigraphy of the Orijärvi area (not in SFm gives the maximum age of D deformation. If scale), and related tectonics. Ages from Väisänen and 1 Mänttäri Nironen(2002), Väisänen et et al.,al. (2002), Fig. Väisänen 11 and the presented stratigraphy and structural correlation Kirkland (2008) and this study. Broken line represents are correct, deformation of the stratigraphic package a tectonic discordance. OFm = Orijärvi Formation, overlying the Orijärvi Formation experienced D1 VFm = Vetio Formation, KFm = Kisko Formation, TFm deformation at an advanced stage of accretion. = Toija Formation, AFm = Ahdisto Formation, SFm = Salittu Formation, KSZ = Kisko shear zone. See text for Isotopic evidence suggests an evolved crust explanation. in southernmost Finland with an age of ≥ 2.0 Ma (Lahtinen and Huhma 1997), and therefore a ≥ 2.0 Ma microcontinental core is assumed conformably underlies the Salittu Formation to have existed to the south of the Orijärvi area (Väisänen and Mänttäri, 2002). Therefore both the during accretion in the models of Nironen (1997) Ahdisto and Toija Formations are stratigraphically and Lahtinen et al. (2005). In line with these below the Salittu Formation but as they are models we propose the following evolution: the separated by the Kisko shear zone, their mutual Orijärvi Formation represents magmatism and position is unknown. sedimentation at the margin of this older block The refined stratigraphy of the Orijärvi area that was approaching the subduction zone in the is shown in Figure 11. The Orijärvi Formation is overriding plate; and the overlying volcanic package lowermost, with felsic and mafic metavolcanic was formed above the subduction zone when the rocks and interlayers of marble, metapelite and block started to accrete to another microcontinental iron formation; stratigraphic interpretation by block (i.e. accretion of Southern Svecofennia and Latvalahti (1979) differs from that of Colley & Central Svecofennia). Because of the proposed Westra (1987). The synvolcanic granodioritic duality (pre-accretion vs. accretion sequences) and gabbroic intrusions are shown. The overlying a tectonic discordance is inferred in Figure 11 metasedimentary unit is the new Vetio Formation, between the Orijärvi and Vetio Formations. The with metapelite containing interlayers of mafic metapelites of the Vetio Formation represent metavolcanic rock. The Kisko Formation consists sediments of the forearc basin and the metavolcanic of mafic to felsic metavolcanic rocks. The new rocks of the Kisko and Toija Formations are Ahdisto Formation between the Kisko and Salittu volcanic rocks of the growing magmatic arc. The Formations consists of metasedimentary rocks metavolcanic rocks of the Salittu Formation at the base and intermediate metavolcanic rocks represent a rifting episode in the magmatic on top. The Toija Formation consists of mafic and arc (Väisänen and Mänttäri, 2002; Nironen, felsic metavolcanic rocks, with interlayers of marble submitted). The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 101
7. Conclusions volcanic unit (Orijärvi Formation), overlain by a mainly sedimentary unit (Vetio Formation), the The Salittu formation consists of pictitic and basaltic Kisko Formation, a volcanic-sedimentary unit metavolcanic rocks. The original stratigraphy is (Ahdisto Formation), the Toija Formation, and on visible at Salittu although slightly obscured by top the Salittu Formation; these were all emplaced deformation: metabasalt overlies migmatitic at 1.88–1.87 Ga. We propose a model in which gneisses and metapicrite is on top. The early the Orijärvi Formation represents magmatism
Svecofennian D1 foliation is visible both in the at the margin of a microcontinent, and the metavolcanic and metasedimentary rocks. The overlying package sedimentation and magmatism rocks were folded during the early Svecofennian above a subduction zone during an initial stage
D2 deformation, and the large synformal structures of microcontinental accretion. First identified
developed as D2–D5 interference structures during deformation occurred in an advanced stage of
late Svecofennian D5 deformation. The structural accretion after the emplacement of the volcanic pattern at Salittu is much the same as in the Orijärvi rocks of the Salittu Formation. triangle although the metamorphic grade is higher. The rise in metamorphic grade from the Orijärvi Acknowledgements triangle to Salittu is the result of early Svecofennian reverse faulting in the Jyly shear zone, not related to The reviews by Taija Torvela and Esa Heilimo the late Svecofennian thermal event that caused the improved the manuscript. Thanks to Y. Lahaye for development of the West Uusimaa Complex. help and keeping the LA-MC-ICPMS working, The new age data, together with earlier P. Simelius for rock crushing, M. Saarinen for published data constrains the Salittu volcanism at mineral separation and L. Heikkinen for mount ca. 1875 Ma. The stratigraphy in the Orijärvi area preparation. is refined to consist of the early (1.90–1.89 Ga)
References
Andersen T, Griffin W.L., Jackson S.E., Knudsen T.-L. & Pearson Blom, K.A., 1988. Subsolidus migmatization in high-grade meta- N.J., 2004. Mid-Proterozoic magmatic arc evolution at the tuffs (Kurkijärvi, southwest Finland). Lithos 21, 263–278. southwest margin of the Baltic Shield. Lithos 73, 289–318. http://dx.doi.org/10.1016/0024-4937(88)90032-1 http://dx.doi.org/10.1016/j.lithos.2003.12.011 Claesson, S., Huhma, H., Kinny, P.D. & Williams, I.S,. 1993. Belousova E.A., Griffin W.L. & O’Reilly S.Y., 2006. Zircon Svecofennian detrital zircon ages - implications for the crystal morphology, trace element signa- tures and Hf Precambrian evolution of the Baltic Shield. Precambrian isotope composition as a tool for petrogenetic modeling: Research 64, 109–130. examples from Eastern Australian granitoids. Journal of http://dx.doi.org/10.1016/0301-9268(93)90071-9 Petrology 47, 329–353. Colley, H. & Westra, L., 1987. The volcano-tectonic setting and http://dx.doi.org/10.1093/petrology/egi077 mineralization of the early Proterozoic Kemiö–Orijärvi– Bergman, S., Högdahl, K., Nironen, M., Ogenhall, E., Sjöström, Lohja belt, SW Finland. In: Pharaoh, T.C., Rickard, D. H., Lundqvist, L. & Lahtinen, R., 2008. Timing of (eds.) Geochemistry and Mineralization of Proterozoic Palaeoproterozoic intra-orogenic sedimentation in the Volcanic Suites. Geological Society, London, Special central Fennoscandian Shield: evidence from detrital zircon Publication 33, 95–107. in metasandstone. Precambrian Research 161, 231–249. http://dx.doi.org/10.1144/GSL.SP.1987.033.01.08 http://dx.doi.org/10.1016/j.precamres.2007.08.007 Ehlers, C., Lindroos, A. & Selonen, O., 1993. The late Bleeker, W. & Westra, L., 1987. The evolution of the Mustio Svecofennian granite‑migmatite zone of southern gneiss dome, Svecofennides of SW Finland. Precambrian Finland ‑ a belt of transpressive deformation and granite Research 36, 227–240. emplacement. Precambrian Research 64, 295–309. http://dx.doi.org/10.1016/0301-9268(87)90022-2 http://dx.doi.org/10.1016/0301-9268(93)90083-E 102 M. Nironen, I. Mänttäri and M. Väisänen
Eskola, P., 1914. On the petrology of the Orijärvi region in Pajunen, M., Airo, M-L., Elminen, T., Mänttäri, I., Niemelä, southwestern Finland. Bulletin de la Commission R., Vaarma, M., Wasenius, P. & Wennerström, M., 2008. Géologique de Finlande 40, 67–102. Tectonic evolution of the Svecofennian crust in southern Gorbatschev, R. & Bogdanova, S., 1993. Frontiers in the Baltic Finland. In: Pajunen, M. (Ed.) Tectonic evolution of the Shield. Precambrian Research 64, 3–21. Svecofennian crust in Finland – a basis for characterizing http://dx.doi.org/10.1016/0301-9268(93)90066-B bedrock technical properties. Geological Survey of Hermansson, T., Stephens, M. B., Corfu, F., Page, L. M., & Finland, Special Paper 47, 15–160. Andersson, J., 2008. Migratory tectonic switching, Parras, K., 1958. On the charnockites in the light of a highly western Svecofennian orogen, central Sweden: metamorphic rock complex in SW Finland. Bulletin de la Constraints from U/Pb zircon and titanite geochronology. Commission Géologique de Finlande 181, 1–137. Precambrian Research, 161, 250–278. Ploegsma, M., 1991. A pilot Rb-Sr dating of the Suomusjärvi http://dx.doi.org/10.1016/j.precamres.2007.08.008 ultramylonite: evidence for major post-Svecofennian Huhma, H., Mänttäri, I., Peltonen, P., Kontinen, A., Halkoaho, deformation in SW Finland. Bulletin of the Geological T., Hanski, E., Hokkanen, T., Hölttä, P., Juopperi, H., Society of Finland 63, 3–13. Konnunaho, J., Layahe, Y., Luukkonen. E., Pietikäinen. Ploegsma, M. & Westra, L., 1990. The Early Proterozoic Orijärvi K., Pulkkinen, A., Sorjonen-Ward, P., Vaasjoki, M. & triangle (southwest Finland): a key area on the tectonic Whitehouse, M., 2012. The age of the Archaean greenstone evolution of the Svecofennides. Precambrian Research 47, belts in Finland. Geological Survey of Finland, Special 51–69. Paper 54, 74–175. http://dx.doi.org/10.1016/0301-9268(90)90030-T Jackson S.E., Pearson N.J., Griffin W.L. & Belousova E.A., Rosa D.R.N., Finch A.A., Andersen T. & Inverno C.M.C., 2009. 2004. The application of laser ablation inductively U−Pb geochronology and Hf isotope ratios of magmatic coupled plasma-mass spectrometry to in-situ U–Pb zircon zircons from the Iberian pyrite belt. Mineralogy and geochronology. Chemical Geology 211, 47–69. Petrology 95, 47–69. http://dx.doi.org/10.1016/j.chemgeo.2004.06.017 http://dx.doi.org/10.1007/s00710-008-0022-5 Lahtinen, R. & Huhma, H., 1997. Isotopic and geochemical Salli, I., 1955. Geological map of Finland 1:100 000, pre- constraints on the evolution of the 1.93–1.79 Ga Quaternary rocks. Sheet 2023 Suomusjärvi. Geological Svecofennian crust and mantle. Precambrian Research 82, Survey of Finland. 13–34. Sayab, M., Suuronen, J.-P., Hölttä, P., Aerden, D., Lahtinen, R. http://dx.doi.org/10.1016/S0301-9268(96)00062-9 & Kallonen, P., 2015. High-resolution X-ray computed Lahtinen, R., Korja, A. & Nironen, M., 2005. Paleoproterozoic microtomography: A holistic approach to metamorphic tectonic evolution. In: Lehtinen, M., Nurmi, P.A., Rämö, fabric analyses. Geology 43, 55–58. O.T. (Eds.) Precambrian Geology of Finland – Key to http://dx.doi.org/10.1130/G36250.1 the Evolution of the Fennoscandian Shield. Elsevier B.V., Schipper, N., 1981. The geology of the Salittu area. Internal Amsterdam, 481–532. report, Department of Geology, Petrology, and Mineralogy, http://dx.doi.org/10.1016/S0166-2635(05)80012-X Free University, Amsterdam. 1–44. Latvalahti, U., 1979. Cu-Zn-Pb ores in the Aijala-Orijärvi area, Schreurs, J., 1984. The amphibolite–granulite facies transition southwest Finland. Economic Geology 74, 1035–1059. in West Uusimaa, S.W. Finland. A fluid inclusion study. http://dx.doi.org/10.2113/gsecongeo.74.5.1035 Journal of Metamorphic Geology 2, 327–341. Ludwig, K.R., 2003. Isoplot/Ex 3.00. A geochronological toolkit http://dx.doi.org/10.1111/j.1525-1314.1984.tb00593.x for Microsoft Excel. Berkeley Geochronology Center. Schreurs, J., 1985. Prograde metamorphism of pelites, garnet- Special publication No. 4. biotite thermometry and prograde changes of biotite Mouri, H., Väisänen, M., Huhma, H. & Korsman, K., 2005. chemistry in high-grade rocks of West Uusimaa, southwest Sm–Nd garnet and U–Pb monazite dating of high-grade Finland. Lithos 18, 69–80. metamorphism and crustal melting in the West Uusimaa http://dx.doi.org/10.1016/0024-4937(85)90011-8 area, southern Finland. GFF 127, 123–128. Schreurs, J. & Westra, L,. 1986. The thermotectonic evolution of http://dx.doi.org/10.1080/11035890501272123 a Proterozoic, low pressure, granulite dome, West Uusimaa, Nironen, M., 1997. The Svecofennian orogen: a tectonic model. SW Finland. Contributions to Mineralogy and Petrology Precambrian Research 86, 21–44. 93, 236–250. http://dx.doi.org/10.1016/S0301-9268(97)00039-9 http://dx.doi.org/10.1007/BF00371326 Nironen, M. The Salittu formation in southwestern Finland Schreurs, J., van Kooperen, P. & Westra, L., 1986. Ultramafic II: Arc rifting and picritic-basaltic volcanism in the metavolcanic rocks of early proterozoic age in West- Paleoproterozoic Svecofennian orogen. Submitted to Uusimaa, SW Finland. Neues Jahrbuch für Mineralogie – Bulletin of the Geological Society of Finland. Abhandlungen 155, 185–201. The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy 103
Schumacher, J.C. & Czank, M., 1987. Mineralogy of triple- and Tuominen, H.V., 1957. The structure of an Archean area: Orijärvi, double-chain pyriboles from Orijärvi, southwest Finland. Finland. Bulletin de la Commission Géologique de American Mineralogist 72, 345–352. Finlande 177, 1–32. Skyttä, P. & Mänttäri, I., 2008. Structural setting of late Väisänen, M. & Mänttäri, I., 2002. 1.90–1.88 Ga arc and back- Svecofennian granites and pegmatites in Uusimaa belt, arc basin in the Orijärvi area, SW Finland. Bulletin of the SW Finland: Age constraints and implications for crustal Geological Society of Finland 74, 185–214. evolution. Precambrian Research 164, 86–109. Väisänen, M. & Skyttä, P., 2007.Late Svecofennian shear zones in http://dx.doi.org/10.1016/j.precamres.2008.04.001 southwestern Finland. GFF 129, 55–64. Skyttä, P., Väisänen, M. & Mänttäri, I., 2006. Preservation of http://dx.doi.org/10.1080/11035890701291055 Palaeoproterozoic early Svecofennian structures in the Väisänen, M. & Kirkland, C.I., 2008. U-Th-Pb zircon Orijärvi area, SW Finland – Evidence for polyphase strain geochronology on igneous rocks in the Toija and Salittu partitioning. Precambrian Research 150, 153–172. Formations, Orijärvi area, southwestern Finland: http://dx.doi.org/10.1016/j.precamres.2006.07.005 Constraints on the age of volcanism and metamorphism. Stacey, J.S. & Kramers, J.D., 1975. Approximation of terrestrial Bulletin of the Geological Society of Finland 80, 73–87. lead isotope evolution by a two-stage model. Earth and Väisänen, M., Mänttäri, I. & Hölttä, P., 2002. Svecofennian Planetary Science Letters 26, 207–221. magmatic and metamorphic evolution in southwestern http://dx.doi.org/10.1016/0012-821X(75)90088-6 Finland as revealed by U-Pb zircon SIMS geochronology. Stephens, M. B., & Andersson, J., 2015. Migmatization Precambrian Research 116, 111–127. related to mafic underplating and intra- or back-arc http://dx.doi.org/10.1016/S0301-9268(02)00019-0 spreading above a subduction boundary in a 2.0–1.8 Ga Väisänen, M., Eklund, O., Lahaye, J., O’Brien, H., Fröidö, accretionary orogen, Sweden. Precambrian Research, 264, S., Högdahl, K. & Lammi, M., 2012. Intra-orogenic 235–257. Svecofennian magmatism in SW Finland constrained by http://dx.doi.org/10.1016/j.precamres.2015.04.019 LA-MC-ICP-MS zircon dating and geochemistry. GFF 134, 99–114. http://dx.doi.org/10.1080/11035897.2012.680606