Geochronology of the felsic rocks in Orijärvi, southern Finland - implications for stratigraphy

Abstract

The Orijärvi area within the Paleoproterozoic Uusimaa belt contains volcanic and minor sedimentary formations, providing valuable insights into the oldest Svecofennian crust in southern Finland. Previously, felsic volcanic rocks from the Orijärvi, Kisko and Toija formations have been dated at 1895 ±3, 1878 ± 4 and 1878 ± 4 Ma, respectively. In the present study, the Sorvasto sample from the southern boundary of the Kisko formation yields a zircon age of 1885 ±5 Ma and a titanite age of 1800 ± 15 Ma. The zircon age falls within the interval between previously obtained age determinations and is coeval with the common Svecofennian crustal growth stage. The Kavasto sample from the western part of the area yields a zircon age of 1878 ± 6 Ma and a titanite age of 1796 ± 4 Ma. The zircon age corresponds to those obtained for similar rock types in the Toija and Ahdisto formations. Picritic interlayers occur within all these successions and are here interpreted as belonging to the same Toija formation. We tentatively infer that the Toija and Salittu formations, together with the overlying sedi -mentary rocks, can be traced to the West Uusimaa area in the east and the Turku area in the west. The titanite ages near 1.80 Ga reflect cooling or reheating events.

Headnote

ABSTRACT

The Orijärvi area within the Paleoproterozoic Uusimaa belt contains volcanic and minor sedimentary formations, providing valuable insights into the oldest Svecofennian crust in southern Finland. Previously, felsic volcanic rocks from the Orijärvi, Kisko and Toija formations have been dated at 1895 ±3, 1878 ± 4 and 1878 ± 4 Ma, respectively. In the present study, the Sorvasto sample from the southern boundary of the Kisko formation yields a zircon age of 1885 ±5 Ma and a titanite age of 1800 ± 15 Ma. The zircon age falls within the interval between previously obtained age determinations and is coeval with the common Svecofennian crustal growth stage. The Kavasto sample from the western part of the area yields a zircon age of 1878 ± 6 Ma and a titanite age of 1796 ± 4 Ma. The zircon age corresponds to those obtained for similar rock types in the Toija and Ahdisto formations. Picritic interlayers occur within all these successions and are here interpreted as belonging to the same Toija formation. We tentatively infer that the Toija and Salittu formations, together with the overlying sedi -mentary rocks, can be traced to the West Uusimaa area in the east and the Turku area in the west. The titanite ages near 1.80 Ga reflect cooling or reheating events.

Keywords:

geology, Paleoproterozoic, volcanic arc, zircon age, titanite age, compression, extension

Introduction

Constructing stratigraphic relations in Paleoproterozoic bedrock is difficult, especially in highly deformed terrains such as in southern Finland. Generally, all the supracrustal belts in this region show a complicated internal stratigraphy and structure, since the high metamorphic grade and deformation have largely obliterated the primary struc -tures. The Orijärvi area within the Uusimaa belt is one of the few locations in south -ernmost Finland (Fig. 1) where metamorphism took place in temperatures below anatexis, in contrast to the surroundings, where migmatites are common. The Orijärvi triangle (Ploegsma and Westra 1990) has escaped major deformation processes, being enveloped by two major shear zones, the Kisko and Jyly shear zones, which led to deformation partitioning into the low strain (Orijärvi) and high strain (West Uusimaa) domains (Skyttä et al. 2006; Fig. 2). These features make the Orijärvi area a suitable location for absolute age determinations in southern Finland and for studying the oldest Svecofennian stratigraphy and tectonism. The combined findings can be used to model a wider tectonic evolution of southern Finland.

Isotopic age determinations on volcanic rocks are scarce in southern Finland but the age of volcanism in the Orijärvi area is reasonably well known. However, because of the complexity of a volcanic arc, internal within-arc age variations are less well-known. In this study, we dated zircons and titanites from two felsic rocks in Orijärvi with the aim to fill the existing gaps that hamper the interpretation of stratigraphy and regional correlation of geological units. All the rock types are metamorphic. Therefore, the prefix 'meta-' is omitted.

Geological setting

The Svecofennian orogen was formed between 1.95 and 1.75 Ga (e.g. Lahtinen et al. 2005). It forms the Proterozoic basement in Finland, parts of Sweden and Norway, and the Lake Ladoga area in northwestern Russia (Gaál and Gorbatschev 1987; Fig. 1A). To the south and southeast, the basement is covered by Paleozoic sedi -mentary rocks (Soesoo et al. 2020). In the major Svecofennian Province in Finland, two sub provinces can be distinguished: the Northern and Southern Svecofennia Subprovinces (Kohonen et al. 2021; Fig. 1), which were amalgamated during early Svecofennian tectonism (Lahtinen et al. 2005). The Southern Svecofennia Subprovince is further divided into the Häme and Uusimaa volcano-sedi mentary belts (Nironen 2017a).

The Orijärvi area within the Uusimaa belt is a non-mig -matised, lower metamorphic grade area within the high grade anatectic late Svecofennian granite-migmatite zone of southern Finland (LSGMZ in Fig. 1; Ehlers et al. 1993). The Orijärvi area contains volcaniclastic and other sedimentary rocks with well-preserved primary structures that were meta -mor phosed in the andalusite-cordierite stability field (Eskola 1914; Ploegsma and Westra 1990; Skyttä et al. 2006). The area comprises four defined lithostratigraphic formations (Fig. 2): the lowermost Orijärvi formation (fm), Kisko fm, Toija fm and the uppermost Salittu fm (Väisänen and Mänttäri 2002). According to the interpretation of the authors, the Orijärvi and Kisko formations represent growth of the volcanic arc, whereas the Toija and Salittu formations are related to the subsequent rifting stage. This model, based on field relations and the chemostratigraphy of volcanic rocks, has not been fully verified by absolute age deter minations. The lithostrati graphic subdivision (Väisänen and Mänttäri 2002) was modified by Nironen et al. (2016) with the addition of two new formations, the Ahdisto and Vetio formations.

Zircons from rhyolite in the Orijärvi fm (low in the stratigraphic sequence) and the Kisko dacite (high in the stratigraphic sequence) were dated using the thermal ioniz a -tion mass spectrometer (TIMS) at 1895.3 ± 2.4 Ma (later referred to as 1895 ±3 Ma) and 1878.2 ±3.4 Ma (later referred to as 1878 ± 4 Ma), respectively (Väisänen and Mänttäri 2002). Zircons from rhyolite in the Toija fm yielded a secondary-ion mass spectrometry (SIMS) age of 1878 ±4 Ma (Väisänen and Kirkland 2008). In addition, zircons from the synvolcanic Orijärvi granodiorite are dated at 1891 ± 13 Ma (TIMS; Huhma 1986), 1898 ± 9 Ma (SIMS; Väisänen et al. 2002) and 1892 ± 4 Ma (laser ablation mass spectrometer (LAMS); Kara et al. 2018). Within 2 sigma errors, these ages are similar to the age of the Orijärvi rhyolite sample (Fig. 2).

Sampling sites and samples

Two felsic rock samples were collected for uranium-lead (U-Pb) isotope analysis. The Sorvasto sample from the south -ern boundary of the Kisko fm was collected from a road cut along the Finnish Route Number 186 near Sorvastonlammi. This outcrop is part of felsic volcanics extending in the east- west direction for about 8 km across the whole Orijärvi triangular area between the Kisko and the Jyly shear zones (Fig. 2). In the same rock suite, approximately 500 metres east of Sorvasto, a visibly similar rock type was previously geo chemically identified as rhyolite (sample 87.3MV95 in Väisänen and Mänttäri 2002). The outcrop sampled for this study consists of a coarsely layered rock with thick homogene ous tuff layers alternating with volcanic breccia layers. The breccia contains 1-20 cm fragments, whose com -position is evidently similar to that in the matrix, only some -what finer-grained. The sample was taken from a homoge -neous layer (Fig. 3A).

Based on thin section examination, the Sorvasto sample is fine- and even-grained, with an average grain size of 0.1 mm. These grains are predominantly anhedral to subhedral. There are a few larger subhedral fragments ranging in size from 0.5 to 2 mm. The main minerals are quartz, plagioclase (oligo -clase), K-feldspar and biotite. K-feldspar and plagioclase occur in approximately equal amounts. The larger fragments are predominantly quartz. Variations in mica content impart layering to the rock (Fig. 4A and B). Small feldspar grains are difficult to distinguish under the optical microscope. Therefore, thin sections were also analysed and imaged with scanning electron microscope (SEM) (Fig. 5A).

The Kavasto sample was collected from the northwestern part of the study area, from within the formally defined Kisko fm (Fig. 2). In this area, several rock types occur: mafic and intermediate volcanic rocks, pelitic sedimentary rocks, pla -gio clase porphyrites (samples KII-30 and KII-31 in Väisänen and Mänttäri 2002) and felsic rocks. The previously analysed 1878 ± 4 Ma dacite (sample 20MV96 in Väisänen and Mänttäri 2002) occurs about 2 km east-northeast of the present locality. Approximately 200 m south of the sampled outcrop is an occurrence of a relatively rare rock type: layered fragment-bearing picrite (Fig. 3C; sample 272MV95 in Väisänen and Mänttäri 2002). Contacts with other rock types are not exposed. The present sampled outcrop is a layered felsic rock with alternating 5-20 cm light-coloured layers and darker, thinner, more fine-grained layers. The sample was taken from a lighter layer (Fig. 3B).

Based on examination of the thin section, the Kavasto sample is extremely fine-grained, with a grain size of around 0.01 mm (Fig. 4C and D). It is relatively even-grained, with grain morphologies mostly subhedral to anhedral. The main minerals are quartz, plagioclase (oligoclase) and biotite. Plagi oclase and quartz are present in roughly equal propor -tions, with micaceous bands defining the banding. Optical mineral identification was verified with SEM mineral analyses and images (Fig. 5B). In the field, the rock was interpreted as a felsic volcanic or volcaniclastic rock but for further discussion, see section 'Older zircons in the samples'.

Analytical methods

The separation of the zircons and titanites was performed with conventional methods of crushing, grinding, panning and removal of magnetite with a hand magnet. After the heavy liquid (methylene iodide) and Franz magnetic separator runs, the grains were hand-picked, cast in an epoxy mount and polished.

The backscattered electron (BSE) imaging of the zircons and titanites was carried out with two instruments. The Sorvasto sample was imaged using an Apreo S micro scope by Thermo Fisher Scientific Inc. at the Department of Physics and Astronomy, University of Turku. The Kavasto sample was imaged with a desktop scanning electron microscope Phenom XL by Thermo Fisher Scientific Inc. at Åbo Akademi

University, Geohouse, Turku. Thin sections were imaged and minerals were analysed with the same instrument.

The zircon U-Pb isotope age determinations were con -ducted at the Finnish Geosciences Research Laboratory (SGL) facilities, Geological Survey of Finland, Espoo. The analyses were performed using a Nu Plasma AttoM laser ablation-single collector-inductively coupled plasma-mass spectrometer (LA-SC-ICPMS). Ablation was performed with a Teledyne Excite laser ablation system that was con -nected to the AttoM device. During analysis, samples were ablated in He gas (gas flow = 0.4 and 0.1 l/min) in a HelEx ablation cell (Müller et al. 2009). Argon was mixed into He aerosol before entering to plasma (Ar gas flow = 0.95 l/min). The laser spot size was 25 µm in diameter and pulse fre -quency 5 Hz. The energy of the laser was 50% of 5.0 mJ, producing an on-sample fluence of 1.25 J/cm².

Pre-ablation consisted of 10 pulses using a 35 µm spot, fol lowed by 20 seconds with the laser off to sweep clean the cell prior to on-mass background measurement. Ablation for analysis then followed with a 30 second stationary beam. 235U was calculated from the counts measured at mass number 238U, using the natural ratio of 238U/235U=137.88. The raw data from the analyses were corrected for the background, mass discrimination, laser-induced elemental fractionation and drift in ion counter gains, and reduced to U-Pb isotope ratios by using concordant calibration standard zircons with known ages: GJ-01 (609 ± 1 Ma; Belousova et al. 2006), A382 (1877±2 Ma; Huhma et al. 2012) and A1772 (2712 ± 1 Ma; Huhma et al. 2012). The standards were run at the beginning and end of each sample, as well as during the analytical sessions at roughly regular intervals between every ten sample analyses.

The data reduction for the zircon U-Pb raw data was performed with Glitter 4.4.4 software (Achterberg et al. 2001), which includes visualisation of isotope data and allows the user to calibrate standards and optimise the selection of each analysis based on signal and time. Outliers in the raw data were filtered with Glitter. Further data reduction included error propagation and common Pb corrections, which were calculated with an in-house (SGL) Microsoft Excel spread -sheet written by Yann Lahaye and Hugh O'Brien.

The titanites were analysed at the SGL facilities, using the same equipment and workflow as for the zircons. The dia -meter of laser spots was 40 µm, with 50 µm pre-ablation with a pulse frequency of 5 Hz. For the Kavasto sample, the laser energy was 45% of 5.0 mJ and fluence 1.40 J/cm². Argon gas flow was 0.94 l/min. For the Sorvasto sample, the laser energy was 35% of 5.0 mJ and fluence 2.54 J/cm². Two known titanite calibration standards were used for quality control during both analytical sessions: MKED-1 (Spandler et al. 2016) and an in-house sample A1756 (1857 Ma, un -published). The standards were run at the beginning and end for each sample, as well as during analytical sessions at roughly regular intervals between every ten analyses.

The same data reduction procedure was used for the ti -tanite U-Pb raw data as for the zircons. However, the com -mon Pb correction was slightly modified to avoid over-cor -rection of the data. Age calculations and plotting of the final U-Pb isotopic data were carried out using the Isoplot/Ex4.15 program (Ludwig 2012).

Results

Zircon data

Sorvasto sample

The zircons in the Sorvasto sample range from prismatic to subhedral grains, with average lengths of 80-100 µm. The grain colour varies from transparent to light brown. The BSE image (Fig. 6A) shows that many of the grains are fractured. Faint oscillatory zoning and core domains are apparent (Fig. 6A, grain 87). Most of the grains are slightly meta-mict.

A total of 43 analyses were performed on 38 zircons (Table 1). Two analyses were rejected because of high dis -cordance and high common Pb. Nine analyses resulted in 207Pb/206Pb ages =1900 Ma and are interpreted as recording inheritance. One of the analyses shows a younger age, likely due to lead loss, and was therefore omitted from the cal -culations (cf. Corfu 2013). The rest of the analyses form an elongated cluster upon and above the concordia line, yielding an upper intercept age of 1884.4 ± 5.0 Ma (95% conf., MSWD = 0.33; Fig. 7A). The age is similar to the 207Pb/206Pb weighted average age of 1884.5 ±5.1 Ma (2s, MSWD = 0.33; Fig. 7B), which we prefer. This age is hereinafter referred to as 1885 ± 5 Ma and interpreted as the crystallisation age, which represents an eruptive event.

Kavasto sample

The morphology of the zircon grains from Kavasto varies from a few prismatic grains to anhedral and rounded grains, with lengths ranging greatly between 40 and 120 µm. Many grains show faint oscillatory zoning as well as core domains. The grains are occasionally fractured and/or metamict, but less than in the Sorvasto sample zircons (Fig. 6B).

A total of 65 analyses were performed on 52 grains in two sessions (Table 2). One analysis was rejected due to dis -turb ance during the analysis and high discordance. Multiple age popu lations can be identified from the dataset. Forty-two analyses resulted in 207Pb/206Pb ages between 2937 and 1904 Ma (Fig. 7C and D). Many of these ages originate from the middle parts of zircon grains, which occasionally display separate core areas. Some of the older ages in turn originate from the edges of zircon grains. Some grains also display similar old ages from the middle of the grain. We interpret these zircons as inherited.

The upper intercept age of the main group of 15 analyses is 1877.5±5.6 Ma (95% conf., MSWD = 1.3; Fig. 7C). This group yields a similar 207Pb/206Pb weighted average age of 1877.6 ± 5.7 Ma (2s, MSWD = 1.4; Fig. 7D). These are hereinafter referred to as 1878±6 Ma. The six youngest ages are interpreted to have suffered lead loss and were omitted from calculations (cf. Corfu 2013).

Titanite data

Sorvasto sample

The titanite grains in the Sorvasto sample have rough edges, and some grains are broken. The grains are euhedral in shape, and some are slightly elongated, 100-250 µm in size (Fig. 8A). A total of eight spots on eight grains were analysed (Table 3). The analyses are concordant but one of t hem shows an about 30 Ma younger age, presumably due to lead loss. The seven analyses form a cluster, which yields a concordia age of 1802 ± 13 Ma (2s, MSWD = 0.33; Fig. 9A). The 207Pb/206Pb weighted average age for the same analyses is 1800±15 Ma (2s, MSWD = 0.19; Fig. 9B).

Kavasto sample

The titanite grains in the Kavasto sample are mostly light brown and larger than the zircon grains, varying in size from 100 to 200 µm. The shapes vary from euhedral to rounded and broken. All grain types were selected for U-Pb analyses. The BSE images of the selected grains are shown in Fig. 8B.

A to t al of 30 a naly s e s wer e per fo rmed on 2 8 tit a nite grai ns (Table 4). One analysis was rejected due to high common Pb value and discordancy. Two of the analyses are clearly older at about 1.84 Ga, while four grains form a cluster with a 207Pb/206Pb weighted average age of about 1.82 Ga. The main group of 22 analyses yields a concordia age of 1796±4.1 Ma (2s, MSWD = 5.6; Fig. 9C) and a similar 207Pb/206Pb weighted average age of 1795.6 ±4.1 Ma (2s, MSWD = 0.32; Fig. 9D). These are hereafter referred to as 1796± 4 Ma. The youngest grain has apparently suffered from lead loss and was omitted from calculations.

Discussion

Zircon age data

The present study investigated two new locations for age determinations on felsic volcaniclastic rocks from the Sorvasto and Kavasto sites in the Orijärvi area. The results provide significant clarification and confirmation regarding the local and regional stratigraphic positions of the dated rock units. Some analyses are reversely discordant, which is at -tributable to the matrix effect, i.e. the difference between the reference material and the unknowns (e.g. Marillo-Sialer et al. 2016). However, this only slightly detracts from the reliability of ages, since we use both the U-Pb and 207Pb/206Pb calculations, which are almost identical. We prefer the 207Pb/206Pb ages because of lack of U-Pb fractionation effect and lower Pb loss effect compared to the concordia ages.

The Sorvasto sample from the lower part of the Kisko fm yields zircon 207Pb/206Pb and U-Pb ages of 1885±5 Ma. The age obtained here is younger than that of the Orijärvi fm (1895 ± 3 Ma) and suggests that the Kisko fm overlies the Orijärvi fm, as assumed in the previous study (Väisänen and Mänttäri 2002).

The Kavasto sample provides more information on the northwestern part of the Orijärvi triangle. The age of 1878 ± 6 Ma is interpreted as the magmatic crystallisation age (see section 'Older zircons in the samples' for further discussion). The obtained age corresponds to the previously dated dacite from the northern part of the Kisko fm (1878 ±4 Ma; Väisänen and Mänttäri 2002), which was redefined as the Ahdisto fm by Nironen et al. (2016). The rhyolite from the Toija fm also yielded a similar age of 1878 ± 4 Ma (Väisänen and Kirkland 2008). All three formations, which share broadly similar ages, also exhibit many common lithological features, including the spatial association with rare picritic volcanic rocks, which have only been observed at higher stratigraphic levels. Further east, a tonalitic intrusion dated at 1878 ± 5 Ma in -trudes the Salittu metabasalts, which are interbedded with picrites. Similar ultramafic volcanics also occur in Kavasto, which indicates that 1878 ± 5 Ma represents a minimum age for the Kavasto sample as well.

Older zircons in the samples

The dated samples contain zircons older than the preferred crystallisation ages. This is especially true for the Kavasto sample, where nearly 75% of the analysed spots are older, while in the Sorvasto sample, about 20% are regarded as inherited. These proportions are probably arbitrary because of biased sampling, separation and selection of analysis spots. In Kavasto, the largest and most transparent grains proved to belong to older populations, while the smaller, slightly meta -mict grains are younger. Since larger, higher-quality grains were preferentially chosen for analysis, this potentially skewed the ratio of older to younger populations.

The Sorvasto sample contains older grains with 207Pb/ 206Pb ages between 1.92-1.90 Ga. These ages form a slightly reversely discordant cluster with an upper intercept age of 1.91 Ga (Fig. 7), probably originating from a single 1.91 Ga source. Additionally, there is a reversely discordant grain dating to 1.97 Ga.

The Kavasto sample resulted in ages ranging between 3.16 and 1.90 Ga, though these do not form distinctly separate clusters, except for the 2.14 Ga and 1.92-1.90 Ga analyses (Fig. 7). Such a plethora of ages suggests a sedimentary origin for these grains, either (1) through contamination with sedi -ments while in transit to the surface or (2) during depos -i t i o n b y derivation from a source terrain with zircons of this age range. Preservation of inherited zircons is controlled by the temperature of the magma, subdivided into cold and hot magmas. Zircons can survive in cooler magmas but dissolve in hotter ones (Miller et al. 2003). Bea et al. (2007), in their case study on the Cambro-Ordovician rocks from the Central Iberian Zone, examined calc-alkaline granites and metavolcanic rocks from 18 samples and found that 70-95% of zircons were inherited. As the magmas were regarded as having been relatively hot, they concluded that, in addition to the temperature, the rate at which magma gen -erates and ascends also affects the preservation of inherited zircons.

In the present case, option 1 could account for the older grains, which were entrained in the magma ascending through a thick volcanic arc with sediment intercalations. Recently, Salminen and Kurhila (2023) studied detrital zircons from metasedimentary rocks in southern Finland, and the zircon populations resemble those in this study. In a related study, Claesson et al. (1993) analysed detrital zircons from the Svecofennian domain and included an Orijärvi sample from Sorvastonlammi, close to the Sorvasto sample. Their Orijärvi sample contained one 2.74 Ga grain and Proterozoic ages between 2.1 and 1.93 Ga. Although we did not find these age populations in the Sorvasto sample, they are present in the Kavasto sample. If the older zircons were sourced from exposed provenance areas during deposition (option 2), the layered Kavasto rock type is a mixed rock (tuffite), and the older grains are part of the sedimentary portion of the tuffite. Nevertheless, the small amount of biotite and the absence of K-feldspar in the Kavasto thin section suggest a volcanic origin for the sample.

In total, there are three options for interpreting the Kavasto ages. First, the magma may have contained all the older zircons, in addition to the 1878 Ma grains that crystal -lised from the magma. Second, the rock could represent a tuffite, where the younger grains crystallised from the magma while the older grains resided in the sediment. Third, all the zircons may have derived from an external source area of diverse ages, with 1878 Ma representing the maximum de -position age. Since the maximum deposition and minimum ages would be the same, we consider the igneous origin of the 1878 Ma grains the most likely alternative.

Titanite age data

The titanite ages from both Sorvasto and Kavasto are close to 1.80 Ga. The Kavasto sample also contains two older titanite grains at 1.84 Ga (red in Fig. 9D) and four grains at 1.82 Ga (yellow in Fig. 9D). However, most of the analyses yielded the preferred age of 1.80 Ga. Previously, a titanite sample was collected from the northern part of the Kisko fm, east of the Kavasto sampling site (now the Toija fm). The ti tanite from a dacitic rock sample yielded a concordant TIMS age of 1798 ± 3 Ma (Väisänen and Mänttäri 2002). They in -terpreted that this approximately 1.8 Ga age refers to reheat -ing of the crust or cooling through the blocking temperature for titanite. The behaviour of titanite in magmatic and meta -morphic systems is well studied but complex, since its closure temperature is dependent on several factors. It can be as high as 700 °C but titanite is highly reactive and its closure tem -perature varies with pressure and the availability of fluids (Frost et al. 2001). Additionally, titanite commonly has high concentrations of common Pb, which complicates reliable age calculations (Kirkland et al. 2018).

The peak metamorphism in Orijärvi took place at about 600 °C and 3 kbars (Latvalahti 1979; Schumacher and Czank 1987). Therefore, the difference of c. 80 million years be -tween the zircon and titanite ages of the present samples is problematic. We propose that the titanite ages reflect cooling, and the titanite was primarily crystallised directly from magma but was recrystallised during the peak metamorphism (possibly around 1.82 Ga). Subsequently, as temperatures steadily decreased, the titanite isotope system was closed at 1.80 Ga. The 1.84 and 1.82 Ga ages observed in the present study, along with the older 1.85-1.84 Ga ages found by Torvela and Kurhila (2022), can be explained by the ability

of titanite to retain an older isotopic signature in some grains or subgrains within individual grains (Kohn 2017). An alter -native explanation is that the reheating of the crust at 1.80 Ga led to partial recrystallisation of the titanite, since magmatism of that age is common in southern Finland (e.g. Rutanen et al. 2011). Recrystallisation of zircon at 1795 Ma was also detected within the Jyly Shear Zone (Väisänen and Kirkland 2008).

A simi lar case is o bserve d in Ga rpen berg , cen tra l S we den, where volcanic rocks are about 1895 Ma in age, while ti tan -ites give ages that are about 40 million years younger, at roughly 1.86 Ga. This younger age is interpreted to reflect the age of metamorphism (Jansson and Allen 2011).

Implications for the timing of volcanism and the superposition of the supracrustal units

Previously, the age relations of the formations in the Orijärvi triangle were poorly constrained, leaving a 17-million-year age gap between the Orijärvi fm and the northern part of the Kisko fm (Väisänen and Mänttäri 2002; Nironen et al. 2016). The present age determination from the Sorvasto sample (1885 Ma) in the lower part of the Kisko fm apparently reduces this gap, allowing for a subdivision of the formations into three consecutive age groups (Figs 10 and 11). No major discordances were found in the previous investigations (Väisänen and Mänttäri 2002; Skyttä et al. 2006; Nironen et al. 2016).

The same rock types and ages observed in the Toija and the uppermost Kisko (Ahdisto) formations call into question the justification for assigning them to different formations, when they are situated so closely to one another, even though separated by the Kisko Shear Zone. We propose that all three of these locations are part of the same formation, which we designate here as the Toija fm, as first introduced by Väisänen and Mänttäri (2002). However, the location of the boundary between the Kisko and Toija formations within the Orijärvi triangle remains obscure (Fig. 10). The quartz-feldspar and biotite gneisses located east of the Jyly Shear Zone, below the Salittu fm, are correlated with the Toija fm as defined in this study (formerly the Ahdisto fm, as per Nironen et al. (2016)). This correlation supports the interpretation that the Toija fm is wider than previously interpreted. Thus, the present Toija fm is present both to the west and east of the Kisko Shear Zone and to the east of the Jyly Shear Zone.

The picrites mainly occur at higher stratigraphic levels within the Salittu fm. The ultramafic volcanic rocks can be followed for at least 40 km from Toija village towards the east-northeast, based on information published in the Suomusjärvi and Lohja geological maps as well as positive anomalies in aeromagnetic maps (Salli 1955; Laitala 1994; Nironen et al. 2016).

The Salittu fm (Schreurs et al. 1986; Nironen 2017b) is apparently overlain by a thick sequence of pelitic sedimentary rocks, which now are migmatitic garnet-cordierite mica gneisses (e.g. Schreurs and Westra 1986). Within these sedi -ments, picritic intercalations occur (Salli 1955; field obser -va tions by the authors). This strongly suggests that sedi -mentation and ultramafic volcanism are approximately coeval and belong to the same extensional event.

All the ages combined, Figs 10 and 11 present a revised formation map and lithostratigraphic column of the Orijärvi area, respectively. These representations are derived from available field relations and age determinations. In case of the Salittu fm, the age is inferred from field relations (Väisänen and Mänttäri 2002; Nironen et al. 2016).

Regional correlation

The present data allow to discuss regional stratigraphy and tectonics also outside the Orijärvi area. However, the cor -relation is only tentative due to significant differences in metamorphic grade and structural evolution (e.g. Ploegsma and Westra 1990).

The lowermost units in the Orijärvi area include the Orijärvi granodiorite, located in the core of a regional upright antiform, and the Orijärvi fm on its northern limb (Figs 10 and 12). On the southern limb of the antiform is the 1891 ± 4 Ma Kuovila felsic tuff, which is coeval with the Orijärvi fm (Skyttä et al. 2005). The antiformal structure, with an east- west fold axis (see Skyttä et al. 2006 for more details), and similar rock types extending both east and west from Orijärvi suggest potential correlation (A in Fig. 12). The rock types at Kemiö are similar: bimodal volcanic rocks, clastic sedi -mentary rocks and marbles. Moreover, a felsic volcanic rock at Kemiö was dated at 1888 ± 11 Ma (Reinikainen 2001). However, the extent of the Kisko fm outside the Orijärvi area remains unknown.

The extent of the Toija fm is greater than previously inferred. Features typical of the Toija fm, such as bimodal c. 1878 Ma volcanism with transitional mafic rocks of mid-ocean ridge (MORB) affinity and incipient ultramafic vol canism, have been observed throughout the Orijärvi area and probably beyond. This MORB-type volcanism, especially with associated ultramafic variants, is relatively rare in southern Finland, having been found in only a few places

(Nironen 2017b). These occur in extensional tectonic settings, which probably represent the same event across different regions (cf. Kara et al. 2021). To the east, across the Jyly Shear Zone, the felsic gneisses might correspond to the Toija fm (Ahdisto; Nironen et al. 2016). In the West Uusimaa granulite area (Schreurs and Westra 1986), pelitic migmatites, felsic gneisses, mafic volcanic rocks, picrites and minor marbles dominate the supracrustal rocks. The presence of picrites, which are distinctive due to their rarity, indicates that, at least in the northern part of the West Uusimaa area, these rocks can be correlated with the Toija and Salittu formations (B in Fig. 12). To the west of Orijärvi, the geological con -tinuity is disrupted and obscured by abundant late-orogenic granites and migmatites (e.g. the Perniö granite; Selonen et al. 1996). However, in the Turku area, within the Pargas and Turku groups, mafic volcanic rocks occur among pelitic migmatites, which also possess transitional MORB-type com -positions, resembling those of the Toija fm (Väisänen and Mänttäri 2002). Moreover, there are extensive marble de -posits in Pargas. The Turku granulite area resembles the West Uusimaa area in terms of protolith and granulite facies meta -morphism, and may be coeval, as previously inferred by Väisänen and Westerlund (2007). This tentative regional correlation is shown in Fig. 12.

The Toija and Salittu formations, along with the overlying sediments, were formed during the same tectonic event,

namely the 1878 Ma extension. After the arc growth (the Orijärvi and Kisko formations), tectonism switched to ex -tension, initiating basin subsidence. Firstly, bimodal volcan -ism and carbonate rocks deposited and in the later stage incipient ultramafic volcanism occurred, concomitant with felsic volcan ism. As rifting intensified, magmatism progressed entirely to the mantle-derived tholeiitic mafic and ultramafic volcanism of the Salittu fm (cf. Nironen 2017b). Continued extension deepened the basin(s) and led to the simultaneous filling of the basin(s) with erosional sediments, accompany -ing the wan ing mantle-derived ultramafic magmatism that manifests as interlayers within the sediments.

The model invoking short-term alternating compressional and extensional episodes during subduction in the Svecofennian orogeny was presented by Hermansson et al. (2008). This model was further utilised by Kara et al. (2020, 2021, 2022), who modelled the enriched MORB (E-MORB)-type and within plate lava (WPL)-type magmatism in the Tampere and Pirkanmaa/Häme belts. Extensional episodes were identified at 1.92-1.90 Ga, 1.89 Ga and 1.86 Ga. In the present study, we introduce the concept of a 1.88 Ga (1878 Ma) extensional stage in southernmost Finland. However, this extensional stage must have been short-lived, since the nearby 1876 ± 4 Ma intermediate mafic dyke intrudes the orogenic de -formation (Skyttä et al. 2006).

Conclusions

1. Two U-Pb zircon and two titanite age determinations from the Orijärvi area are presented.

2. The Sorvasto sample from the southern boundary of the Kisko formation is dated at 1885 ± 5 Ma and the Kavasto sample from the western part of the Orijärvi triangle at 1878±6 Ma.

3. The results of the titanite age determinations (c. 1.80 Ga) correspond to the previous results from the Orijärvi area and plausibly reflect lower temperature cooling or re -heating during this period.

4. The Kavasto sample is coeval with the previously dated Toija and Ahdisto formations. These formations, all as -sociated with ultramafic volcanic rocks, are now com -bined and named as the Toija formation.

5. The Toija formation, representing incipient rifting, and the Salittu formation, representing a more intense rifting stage, were formed during the same extensional event. It is proposed that similar rock types formed during the same event throughout the Turku and West Uusimaa areas.

Data availability statement All the data used here are published in this article.

Acknowledgements We thank Alv ar Soes o o and P e ter Sorjon en -Ward f or their reviews and comments, which greatly helped to enhance the manuscript. Arto Peltola is thanked for preparing the epoxy mounts and thin sections, and Sören Fröjdö and Ermei Mäkilä for their assistance with SEM imaging and analysis. We also thank Nils Jansson, Ville Järvinen, and Jarmo Kohonen for their advice. This study was partly funded by the K. H. Renlund Foundation to Teemu Vehkamäki and Jaakko Kara. The pub -lication costs of this article were partially covered by the Estonian Academy of Sciences.

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