Paleoproterozoic Mafic and Ultramafic Volcanic Rocks in the South Savo Region, Eastern Finland

Home , Metavolcanic rock, South Savo, Virtasalmi

Development of the Paleoproterozoic Svecofennian orogeny: new constraints from the southeastern boundary of the Central Finland Granitoid Complex Edited by Perttu Mikkola, Pentti Hölttä and Asko Käpyaho Geological Survey of Finland, Bulletin 407, 63-84, 2018

PALEOPROTEROZOIC MAFIC AND ULTRAMAFIC VOLCANIC ROCKS IN THE SOUTH SAVO REGION, EASTERN FINLAND

Jukka Kousa, Perttu Mikkola and Hannu Makkonen

Kousa, J., Mikkola, P. & Makkonen, H. 2018. Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland. Geological Survey of Finland, Bulletin 407, 63–84, 11 figures and 1 table.

Ultramafic and mafic volcanic rocks are present as sporadic interlayers in the Paleo- proterozoic Svecofennian paragneiss units in the South Savo region of eastern Finland. These elongated volcanic bodies display locally well-preserved primary structures, have a maximum thickness of ca. 500 m and a maximum length of several kilometres. Geo- chemically, the ultramafic variants are picrites, whereas the mafic members display EMORB-like chemical compositions. The picrites, in particular, display significant compositional variation in both major and trace elements (light rare earth and large-ion lithophile elements). These differences may have been caused by differences in their magma source, variable degrees of crustal contamination and post-magmatic alteration, as well as crystal accumulation and fractionation processes. The volcanic units are interpreted to represent extensional phase(s) in the development of the sedimentary basin(s) where the protoliths of the paragneisses were deposited. The eruption age of the volcanic units is interpreted to be 1.91–1.90 Ga.

Appendix 1 is available at: http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_407_appendix_1. pdf

Electronic Appendix is available at: http://tupa.gtk.fi/julkaisu/liiteaineisto/bt_407_elec- tronic_appendix.xlsx

Keywords: volcanic rocks, picrite, ultramafic composition, mid-ocean ridge basalts, extension tectonics, Paleoproterozoic, Svecofennian, Finland

Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland E-mail: [email protected]

https://doi.org/10.30440/bt407.4 Editorial handling by Pentti Hölttä and Asko Käpyaho. Received 30 July 2017; Accepted 29 June 2018

63 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

1 INTRODUCTION

Picrite is a variety of high-magnesium basalt rich pillow lava structures (Pääjärvi 1975, Kousa 1985). in olivine phenocrysts, which can make up to 40% Gaál and Rauhamäki (1971) also observed ultramafic of the rock, such as in arc picrites from Solomon lavas at the same time near Savonlinna, approxi- Islands (Schuth et al. 2004). Svecofennian (meta) mately 30 km east of Rantasalmi. Since their ini- picrites have been described around the Central tial discovery, most of the ultramafic volcanic units Finland Granitoid Complex (e.g., Lehtonen et al. have received little attention, with the exception of 2005), southern Finland (Nironen et al. 2016b) and some of the occurrences that have been involved in Sweden from the Skellefte area (Berge 2013 and in target-scale ore exploration, e.g., the Ni miner- references therein). The nomenclature used for alisation at Rantala (Makkonen 1984) and Ylänne these ultramafic rocks varies considerably, includ- (Laitakari 1985). ing at least the following: cortlandite, ultramafic This article provides a brief overview of the dif- rock/lava/volcanic rock, metaperidotite, metapicrite ferent occurrences of picrites and associated mafic and komatiite. volcanic rocks in the South Savo region, focusing on The presence of ultramafic volcanic rocks in South geochemical data that have been accumulated over Savo has been known since the early 1970s, when the last 30 years in various research projects. The Korsman (1973) and his bedrock mapping group data also include the results of two single-grain age found ultramafic olivine-bearing rocks among the determinations. In addition, we discuss the possi- more common amphibolites. A volcanic origin both ble stratigraphic significance of these volcanic units for the amphibolites and some of the ultramafic and their tectonic implications. rocks was evident from their locally well-preserved

2 GEOLOGICAL SETTING

The bedrock of the South Savo region is located on ages of paragneisses vary from 1.92 to 1.90 Ga (e.g. the southeast flank of the Central Finland Granitoid Huhma et al. 1991, Lahtinen et al. 2002, 2009, 2017, Complex and mainly consists of Paleoproterozoic Mikkola et al. 2018b). Both of the migmatite suites Svecofennian paragneisses, which are divided into locally contain mafic to ultramafic volcanic rocks two major units: the Pirkanmaa migmatite suite (Fig. 1), which have been interpreted to represent of the Western Finland supersuite and the Häme extensional stages in the development of the dep- migmatite suite of the Southern Finland super- ositional basin (e.g. Lahtinen et al. 2009, 2017). In suite (Luukas et al. 2017). The latter unit in the the study area, the ultramafic variants are assigned study area has also been referred to as the Saimaa to the Pahakkala suite when occurring in the Häme area (Kähkönen 2005). The contact between the migmatite suite and the Ala-Siili lithodeme when Pirkanmaa and Häme migmatite suites in the South found in the Pirkanmaa migmatite suite. Based on Savo region is tectonic and marked by the Mikkeli the locally well-preserved pillow and lava breccia shear zone, which also represents a step-like structures (Korsman 1973, Pääjärvi 1975, Kousa change in the degree of metamorphism (Korsman 1985, Viluksela 1988, Pekkarinen 2002), the suites et al. 1988, Mikkola et al. 2018a). In addition to the are thought to represent submarine eruptions, main domain in the western part of the study area, although according to Makkonen (1996), some rocks belonging to the Pirkanmaa migmatite suite of the ultramafic bodies are probably subvolcanic are also present within the Häme migmatite suite sills. as smaller segments bound by fault and thrust con- The Jäppilä–Virtasalmi block (Fig. 1, Pekkarinen tacts (Fig. 1). 2002), also known as the Pieksämäki block Protoliths of the paragneisses have been inter- (Korsman et al. 1988), shows up as a distinct anom- preted as turbiditic greywackes deposited in a aly pattern on both aeromagnetic and electromag- passive margin setting following the collision of netic maps. This complex migmatite block has been the older Svecofennian arc system at 1.91 Ga with regarded as a part of the Southern Finland plutonic the Karelian craton. The maximum depositional suite (Luukas et al. 2017), but based on new age

64 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland data (~1910 Ma) it can be correlated with the older present within the paragneisses as smaller tectonic Svecofennian magmatic rocks occurring along the slivers. margin of the Archean Karelian Craton (Kousa et The paragneisses are intruded by granodiorites al. 2018). The rock types in the Jäppilä–Virtasalmi and quartz diorites belonging to the Central Finland block consist of hornblende gneisses, amphibolites Granitoid Complex and Southern Finland Plutonic and paragneisses, in addition to a complex collection suite. The ~1885 Ma age of these intrusions defines of originally intrusive, but intensively deformed, a minimum age for the deposition of the parag- often migmatized gneisses varying in composition neiss protoliths and eruption of the volcanic inter- from granodiorite to gabbro (Vorma 1971, Fig. 1). In layers within them (Korsman et al. 1984, Vaasjoki this paper, the unit name Maavesi suite is used to & Kontoniemi 1991, Pekkarinen 2002). Rocks of the cover the entirety of the rock types in the Jäppilä– Pirkanmaa migmatite suite also host smaller, less Virtasalmi block (Fig. 1). This name will also be used evolved intrusions varying in composition from in the Bedrock of Finland - DigiKp map database ultramafic to diorite and being potentially prospec- and the associated Finstrati database for geological tive for Ni and Cu ores (Makkonen 1996, Barnes et units. The contact zone between the Maavesi suite al. 2009). and the paragneisses of the Häme migmatite suite Evidence for the existence of a younger sed- typically hosts amphibolites, often diopside banded, imentation phase in the area is provided by the carbonate rocks, calc-silicate-bearing quartz 1885 ± 6 Ma age obtained for a granitoid clast in feldspar gneisses and intermediate metavolcanic the Haukivuori conglomerate (Korsman et al. 1988). rocks forming the Virtasalmi suite (Vorma 1971, This now spatially limited unit has been preserved Pekkarinen & Hyvärinen 1984, Pekkarinen 2002, on top of a down-warped bedrock block (ibid.), Luukas et al. 2017). Volcano-sedimentary rocks although originally, the unit was presumably sig- interpreted as part of the Virtasalmi suite are also nificantly wider.

3 METHODS AND MATERIAL

Analytical data from a total of 180 outcrop and drill mass spectrometry (ICP-MS) was used for REE and core samples were utilized in this study. These data other trace elements. The analytical methods are are listed in the Electronic Appendix, and repre- described in Appendix 1, and the analytical method sentative chemical compositions of volcanic rocks applied to each sample is indicated in the Electronic from each subarea are presented in Table 1. Due to Appendix. the extended period of data gathering, the analyses Two single-grain age determinations were per- were carried out using several analytical methods formed using a Nu Plasma AttoM single collector and laboratories. The main elements were in all LA-ICP-MS in GTK’s isotope laboratory in Espoo. cases determined using X-ray fluorescence (XRF), The method is described Appendix 1. and for most samples, inductively coupled plasma

4 RESULTS

4.1 Field observations and petrography

4.1.1 Ala-Siili lithodeme ing a N–S-trending positive Bouguer and magnetic anomaly (Mikkola et al. 2014). Based on the geo- 4.1.1.1 Ala-Siili physical data, the volcanic sequence in the area is The type area of the Ala-Siili lithodeme of the approximately 5 km long and 500 m wide. As the Pirkanmaa suite (Mikkola et al. 2016) is poorly drilling profile only transected the eastern con- exposed and practically all of the material used in tact, only a minimum thickness of 350 m can be this study is from a single drilling profile target- deduced from it. The volcanic sequence is located

65 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen Fig. 1. Bedrock map of South Savo with the locations described in this study. Map modified from Finland - DigiKP.

66 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Fig. 2. A drill core from the Ala-Siili area displaying compositional variation, which suggests a supracrustal origin, despite intensive deformation and alteration. Box width 1 m. in a sheared contact zone of a quartz diorite intru- migmatite suite in the south and the volcanic rocks sion in the west and Pirkanmaa migmatite suite of the Makkola suite in the north. The contact coin- paragneisses in the east (Fig. 1). cides with the Leivonmäki shear zone (Mikkola et al. The Ala-Siili lithodeme consists of picritic vol- 2018a). Mikkola et al. (2016) assigned the picrites of canic rocks that are intensively sheared and altered. this area to the Ala-Siili mafic volcanic lithodeme. Thus, most of the primary structures have been Picrites in the Haapasuo profile (Mikkola & destroyed, but locally, volcaniclastic features can Niemi 2015) occur among the metasediments of be observed. Small-scale mineralogical variation, the Tammijärvi greywacke lithodeme. Although probably representing original layering, can also strongly serpentinized (Fig. 3), compositional be seen (Fig. 2), suggesting a supracrustal, rather and structural variations indicate that they were than subvolcanic, origin. Mineralogically, the pic- originally volcanic rocks. In addition to serpentine, rites are typically lepidoblastic biotite-amphibole most of the serpentinites contain olivine as a main schists, locally with abundant chlorite and epidote. mineral together with various proportions of talc, Tshermakite is the most common amphibole, with actinolite, biotite, and chlorite. cummingtonite occurring in some of the samples. In the outcrops west of Haapasuo and in the Drill cores also intersect interlayers of serpentinite Harjujärvensuo profile, the picrites are variably (<5 m) and intermediate volcanic rock (<20 m). sheared uralite porphyrites (Fig. 4), which are interpreted as supracrustal in origin based on tex- 1.1.1.2 Leivonmäki tural variations (Mikkola et al. 2018d). The main In the Leivonmäki area, picrites have been observed minerals are hornblende, biotite, and plagioclase, in two drilling profiles and a few outcrops, although and the porphyritic texture is typically caused by the this area is also mostly poorly exposed. The picrites presence of rounded biotite-hornblende aggregates. are located near the contact between the Pirkanmaa

67 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

Fig. 3. A) and B) Drill core samples of serpentinized picrites from the Haapasuo drilling profile. Core diameter 42 mm.

4.1.2 Pahakkala suite metavolcanic rocks

4.1.2.1 Loukee–Airikka area The Loukee–Airikka metavolcanic formation is formed by a ca. 14-km-long and less than 1-km- wide, N–S-trending, discontinuous chain of mafic- ultramafic bodies in paragneisses of the Häme migmatite suite, forming part of the Pahakkala suite (Fig. 1). From north to south, the separate bodies are named Airikka, Kukkulakangas and Loukee. Pekkarinen (2002) used the name Halmelampi for the last one. In addition to outcrops, the volcanic rocks are accessible in drill cores from a profile at Loukee, consisting of three holes transecting the ca. 400-m-thick metavolcanic formation. Fig. 4. Uralite porphyrite from the Leivonmäki area. The main rock type of these bodies is dark green, Scale bar with cm scale. usually more or less schistose amphibolite composed of hornblende and plagioclase with occasional diopside- and epidote-bearing bands and nodes. In places, amphibolite is garnet bearing. Locally preserved lava breccia and pillow lava structures

68 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Fig. 5. Drill core samples of variably altered and deformed picrites from the Loukee area. Core diameter 42 mm.

indicate a primary submarine volcanic nature for suite are known to occur in the Rantasalmi and Juva the amphibolites. areas (Fig. 1, Korsman 1973). Well-preserved pillow Approximately one-third of the Airikka–Loukee lava and volcanic breccia structures exist, especially formation is composed of picrites with green to pale in outcrops of the Hukkionsuo, Koivikkomäki and green clinoamphibole (tremolite-actinolite), chlo- Pirilä areas (Pääjärvi 1975, Kousa 1985, Viluksela rite, and serpentine as the main minerals. Locally, 1988, Makkonen & Ekdahl 1988). The mineral biotitization is intensive. Secondary alteration and compositions of the Rantasalmi amphibolites and deformation have generally resulted in highly vari- picrites are almost identical to those in the Loukee– able mineral assemblages and structures of the Airikka area. In spite of the good exposure in some picrites, as demonstrated in Figure 5. A common areas, the contacts between the Pahakkala suite and diagnostic feature of this rock type is the presence the surrounding metagreywackes are mostly cov- of partly serpentinized olivine porphyroblasts up to ered by glacial overburden. 1 cm in size. In most cases, the ultramafic rock is The mafic metavolcanic rocks are fine-grained, homogeneous, fine grained and massive, especially usually dark green, fragmented lavas, locally show- in the Airikka area. Locally in the Kukkulakangas ing pillow or pillow breccia structures and their area, the ultramafic rock type shows pillow lava deformed variants. Mineralogical variation exists structures and in the Loukee area, lapilli-sized according to the metamorphic grade. Mafic pillow fragments and amygdales can be observed in a lavas and their highly deformed banded variants few outcrops. The southernmost observations of have an amphibolitic mineral composition with the highly schistose picrite-amphibolite associa- green or brownish green hornblende, some pale tion intruded by granites are from the Ylänne area tremolite-actinolite, and granular andesine plagio- approximately 15 km southwest of Loukee (Fig. 1). clase as the main constituents. The pale green bands Based on drill core observations from Loukee, and cavity fillings are mainly composed of diopside the paragneisses represent the original eruptional and plagioclase. Sphene, epidote, apatite, chlorite, environment of the picrites. In addition to tuffite carbonate and opaque minerals are common acces- interbeds, the picrites contain calcite-rich inter- sory minerals. Rare tourmaline, zircon and scapolite beds, further supporting a submarine eruption are also present (Viluksela 1988). environment. The picrites are cut by amphibolite Picrites occur as greyish green, fine-grained, and fluorite granite dykes and quartz veins. conformal lava flows among the amphibolites or sometimes as obvious dykes intersecting them. The 4.1.2.2 Rantasalmi–Juva area thickness of the lava flows and dykes is typically Several mafic and ultramafic metavolcanic rock several metres. Picrites are commonly fragmented. bodies surrounded by, or intercalated with, metape- Some massive beds in the Koivikkomäki area show lites and metapsammites of the Häme migmatite both bottom and top breccia structures, indicating

69 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen an extrusive origin. In outcrops of the Pahakkala area, pillow structures have also been observed (Kousa 1985). The presence of reddish brown olivine porphyroblasts is a reliable distinguishing feature of the picrites in the field. These porphyroblasts attain a diameter of 10 mm in the most magnesium- rich lavas. Spinifex-like pseudofeatures have been observed in some Pahakkala and Ylänne outcrops, but indisputable spinifex textures have not been recognized. Variation in the whole-rock chemis- try of picrites results in mineralogical differences within single lava beds, such as differences in the amount of pale green tremolite-actinolite and green Fig. 6. Well-preserved pillow lava structures in the hornblende. Pahakkala type area. Length of the scale bar 12 cm. Other occurrences of picrites in the Juva area include those of Levänomainen, Myllynkylä, Pakinmaa, Rantala and Rautjärvi (Makkonen 1996). clinoamphibole, olivine). The textures varying from The Rautjärvi picrite is composed of a distinctly harrisitic to cumulate coupled with chemical and layered ultramafic rock. Six separate layers can be mineral compositional variations (whole-rock MgO, distinguished in outcrop, varying in thickness from FeO and En of orthopyroxene; Makkonen 1992) sug- 0.4 m to more than 10 m. The observed mineral- gest multiple lava flows or magma injections. ogy is totally metamorphic (mainly orthopyroxene,

4.2 Geochemistry

Table 1 presents representative analyses of 12 vol- and the most effective plot for distinguishing the canic rock samples from the studied locations. two suites is TiO2 vs. P2O5 (Fig. 7F). As the Al2O3 All analyses are listed in the Electronic Appendix. concentrations are similar in both suites, the lower

According to the IUGS classification (Le Bas 2000), TiO2 concentration of the Ala-Siili lithodeme results picrite is a volcanic rock with SiO2 <52 wt%, MgO in it having a significantly higher Al2O3/TiO2 ratio.

>18 wt% and Na2O+K2O = 2–3 wt% or if MgO is An exception to the above are the four samples from

12–18 wt%, Na2O+K2O must be <3 wt%. Of these, Oravakallio (1 picrite, 3 mafic volcanic rocks), which we have only applied the volatile free SiO2 and consistently plot in the area occupied by samples MgO contents of analyses normalised to 100 wt%, from the Ala-Siili lithodeme. Picrites and the mafic because locally intensive biotitization has affected volcanic rocks of the Pahakkala suite follow the the K2O contents of our samples. This alteration is same differentiation trend. especially evident in several samples from the Ala- On the R1-R2 plot of De la Roche et al. (1980), Siili lithodeme. Of the 180 samples, 106 are consist- the picrite samples of the Pahakkala suite plot ent with the picrite classification in terms of SiO2 along the boundary between tholeiite and picritic and MgO (Fig. 7). A subset of twenty-four samples rock fields and the Ala-Siili samples predomi- from several locations also meet the definition of nantly plotting in the tholeiite field (Fig. 8A). The komatiite, having SiO2 <52 wt%, MgO >18 wt%, TiO2 samples of mafic volcanic rocks mainly classify as

<1 wt%, and Na2O+K2O <1 wt% (Le Bas 2000). basalts on the same diagram. On the TAS diagram When plotted against MgO, most of the elements (Fig. 8B), the samples mainly plot in the basalt form variably well-defined differentiation trends field. On the classification diagram of Hanski et

(Fig. 7), and certain differences between the studied al. (2001), the differences in Al2O3/TiO2 ratio result units can be observed. Picrite samples representing in the majority of the samples from the Ala-Siili the Ala-Siili lithodeme display lower FeOt, CaO, TiO2 lithodeme and Oravakallio locality plotting in the and Ni concentrations than the samples from the Al-undepleted komatiite field, whereas those from Pahakkala suite, although overlap exists. Picrites the Pahakkala suite mainly plot in the Ti-enriched from Ala-Siili display on average higher P2O5 con- komatiite field and partly in the picrite field (Fig. centrations than those from the Pahakkala suite, 8C). A small number of samples, mainly from the

70 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Loukee–Airikka area, have higher TiO2 values than more strongly fractionated REE patterns, but do not the main group. On the Jensen cation plot of Fig. significantly differ in other chemical aspects from 8D, the picrite samples fall in the komatiite and the other samples. Three samples from Porttiaho komatitic basalt fields, whereas the mafic volcanic location display higher REE abundances, but like rocks are tholeiitic basalts. The samples from the the anomalous samples from Loukee do not dif- Ala-Siili lithodeme and the Oravakallio locality form fer significantly in other respects from the main a slightly Mg-richer trend than the majority of the population. The samples representing the Ala-Siili samples from the Pahakkala suite. On the Zr vs. Ti lithodeme have on average a more fractionated REE classification plot (Pearce 1982), the samples from distribution and higher LREE levels, and some of the Pahakkala suite mainly plot in the MORB field, them display negative Eu anomalies. It should be whereas those from Ala-Siili lithodeme plot in the noted that the data set does not include REE data island arc field or below it (Fig. 8E). The exceptions from the Oravakallio locality. are again the Oravakallio samples, which plot in the In the primitive mantle-normalized spidergram same group with the Ala-Siili lithodeme samples. (Fig. 10), the samples from the Pahakkala suite do On the diagram of Wood (1980), the samples from not display significant anomalies that would char- the Ala-Siili lithodeme plot in the field of continen- acterize the whole group, but on average, the mafic tal arc basalts, whereas the samples representing volcanic samples display higher concentrations of the Pahakkala suite mainly plot in the EMORB field, trace elements than the picrites. The samples from with the most significant exception again being the the Ala-Siili lithodeme have significantly higher Th Oravakallio samples. and U concentrations, although enrichment of the Chondrite-normalized REE patterns of the sam- latter can partly be due to alteration. Clear negative ples from the Pahakkala suite show mainly weak Nb anomalies and small Ti anomalies are observ- LREE enrichment and relatively unfractionated REE able in most of the samples from Ala-Siili. Some patterns (Fig. 9). On average, the mafic volcanic of the samples from the Ala-Siili location display rocks have higher REE abundances than the pic- strong positive Sr anomalies, but the lithodeme also rites. Two samples from Loukee display higher and includes samples with negative Sr anomalies.

71 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

Table 1. Representative analyses from the studied locations. All chemical data are listed in the Electronic Appendix.

Sample N4432014R1 N4312015R4 JPK1-1984-88.5 PHK1-1 PR03-1 PR08-1 105.00-106.00 99.00-100.00 Rock Chlorite-biotite- Antophyllite- Plagioclase Ultramafic Ultramafic Mafic type amphibole schist hornblende porphyrite volcanic rock volcanic rock volcanic rock schist Unit Ala-Siili Ala-Siili Pahakkala suite Pahakkala suite Pahakkala suite Pahakkala suite Area Ala-Siili Leivonmäki Pahakkala Pahakkala Pirilä Pirilä

SiO2 % 48.60 46.40 47.54 44.61 39.57 46.95

TiO2 0.51 0.45 1.97 0.81 0.52 1.49

Al2O3 9.38 9.54 17.00 8.38 5.97 12.73

Fe2O3t 9.06 9.82 13.45 10.25 11.91 15.47 MnO 0.13 0.14 0.17 0.18 0.17 0.24 MgO 20.30 19.30 4.89 14.92 19.19 9.83 CaO 6.02 5.63 9.07 15.53 14.06 8.97

Na2O 0.96 0.78 5.21 0.59 0.03 2.42 K2O 2.08 1.68 0.23 0.14 0.02 0.10

P2O5 0.18 0.14 0.17 0.06 0.05 0.14

C ppm 912 n.a. 7900 n.a. n.a. n.a. Ba 458 425 190 811 n.a. 17 Cl 95 163 n.a. n.a. n.a. n.a. Co 58.2 62.3 n.a. 70.0 85.0 64.0 Cr 1499 2389 140 1698 2169 372 Cu n.a. 40 180 106 117 70 Hf 1.3 1.2 n.a. 1.4 0.9 2.7 Nb 3.6 4.3 n.a. 4.2 2.7 7.1 Ni 873 656 150 807 892 240 Rb 65.3 53.2 n.a. n.a. n.a. n.a. S n.a. 98 400 663 16412 97 Sc 20.5 27.7 n.a. n.a. n.a. n.a. Sr 93 36 280 327 30 160 Ta 0.22 n.a. n.a. 0.20 0.20 0.41 Th 2.4 2.0 n.a. 0.3 0.1 0.5 U 0.5 0.7 n.a. 0.2 0.1 0.2 V 141.0 162.0 420.0 188.0 129.0 292.0 Y 10.1 10.9 40.0 16.0 10.0 29.0 Zn 79 81 110 94 653 104 Zr 59.2 52.2 120.0 40.0 27.0 86.0

La 12.8 9.4 8.8 4.2 1.3 6.3 Ce 26.0 20.9 23.0 9.7 3.9 15.9 Pr 3.2 2.6 n.a. 1.5 0.7 2.5 Nd 12.60 9.97 16.20 6.65 3.34 11.60 Sm 2.59 2.13 4.60 1.91 0.99 3.53 Eu 0.79 0.50 1.64 0.81 0.29 1.42 Gd 2.39 2.08 n.a. 2.39 1.27 4.41 Tb 1.96 1.79 7.30 2.46 1.35 4.77 Dy 0.34 0.29 0.89 0.42 0.22 0.78 Ho 0.39 0.35 n.a. 0.55 0.30 1.07 Er 1.14 1.03 n.a. 1.43 0.80 2.91 Tm 0.15 0.15 n.a. 0.21 0.11 0.45 Yb 1.05 0.93 2.50 1.30 0.72 2.70 Lu 0.15 0.14 0.28 0.19 0.11 0.43 n.a. = not analysed

72 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Table 1. Cont.

Sample 10.5-JPK-87 N5112015R8 N5112015R8 MIK-2.1 MIK-3.3 Porttiaho 125.00-126.00 158.00-159.00 Rock Ultramafic Mafic lava Ultramafic Mafic Ultramafic Mafic type volcanic rock lava volcanic rock volcanic rock lava Unit Pahakkala suite Pahakkala suite Pahakkala suite Pahakkala suite Pahakkala suite Pahakkala suite Area Oravikallio Loukee-Airikka Loukee-Airikka Mikkeli Mikkeli Other

SiO2 % 48.60 46.40 43.30 47.75 45.09 45.72

TiO2 0.54 3.32 0.89 1.77 0.87 1.39

Al2O3 9.09 14.10 9.98 14.03 9.53 9.23

Fe2O3t 9.07 14.90 11.70 16.17 12.70 12.30 MnO 0.16 0.14 0.17 0.22 0.19 0.20 MgO 17.00 5.12 15.60 6.17 20.38 14.19 CaO 6.54 8.04 10.04 10.56 8.23 10.39

Na2O 0.89 2.98 1.50 2.54 0.94 1.63 K2O 1.92 0.78 0.45 0.36 0.09 1.59

P2O5 0.21 0.72 0.07 0.16 0.07 0.45

C ppm 400 3600 1700 400 300 n.a. Ba 540 294 30 60 n.a. 455 Cl n.a. 148 67 n.a. n.a. n.a. Co n.a. 33.4 68.7 n.a. n.a. 53.0 Cr 2070 68 1432 120 2030 1261 Cu 70 278 164 170 120 32 Hf n.a. 5.2 1.2 n.a. n.a. 5.3 Nb n.a. 21.1 3.8 n.a. n.a. 20.2 Ni 540 38 678 90 1110 195 Rb n.a. 19.9 10.2 n.a. n.a. 75.0 S 580 3048 9222 130 340 n.a. Sc n.a. 60.3 44.3 n.a. n.a. n.a. Sr 150 399 177 220 30 161 Ta n.a. 0.94 <0.2 n.a. n.a. 1.34 Th n.a. 1.4 <0.5 n.a. n.a. 8.1 U n.a. 0.8 <0.2 n.a. n.a. 3.2 V 160.0 521.0 215.0 400.0 250.0 250.0 Y n.a. 43.4 13.1 20.0 10.0 26.0 Zn 90 182 79 110 80 114 Zr 90.0 222.0 44.1 130.0 60.0 227.0

La n.a. 22.7 3.8 n.a. n.a. 42.8 Ce n.a. 54.9 9.1 n.a. n.a. 93.8 Pr n.a. 7.5 1.3 n.a. n.a. 14.4 Nd n.a. 34.80 6.42 n.a. n.a. 58.69 Sm n.a. 8.51 1.95 n.a. n.a. 11.98 Eu n.a. 2.65 0.69 n.a. n.a. 3.45 Gd n.a. 9.29 2.45 n.a. n.a. 9.91 Tb n.a. 8.99 2.77 n.a. n.a. 6.48 Dy n.a. 1.46 0.42 n.a. n.a. 1.37 Ho n.a. 1.74 0.57 n.a. n.a. 1.17 Er n.a. 4.80 1.59 n.a. n.a. 2.69 Tm n.a. 0.65 0.22 n.a. n.a. 0.36 Yb n.a. 4.19 1.43 n.a. n.a. 2.04 Lu n.a. 0.61 0.20 n.a. n.a. 0.31 n.a. = not analysed

73 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

Fig. 7. Plots of MgO versus selected elements (A–E) and TiO2 vs. P2O5 plot (F). Main elements normalised to 100% on a volatile-free basis.

74 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Fig. 8. Chemical classification diagrams applied to volcanic rocks from South Savo. A) R1-R2 plot of De la Roche et al. (1980), B) TAS (total alkali silica) diagram of Le Bas et al. (1986), C) Al2O3 vs. TiO2 diagram of Hanski et al. (2001), D) Jensen cation plot (Jensen 1976), E) Zr vs. Ti plot of Pearce (1982), F) Zr/117-Nb/16-Th diagram of Wood (1980).

75 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

Fig. 9. Chondrite-normalised REE diagram for A) picrite samples and B) mafic volcanic rocks. Anomalous samples from Porttiaho and Loukee locations are separated from the Pahakkala suite, for which the main population is shown as “Other locations”. Normalization values for chondrite from Boynton (1984).

76 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland

Fig. 10. Primitive mantle-normalised spidergram for A) picrite samples and B) mafic volcanic rocks. Anomalous samples from Porttiaho and Loukee locations are separated from the Pahakkala suite, for which the main population is shown as “Other locations”. Normalization values for primitive mantle from McDonough & Sun (1995).

77 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

5 AGE DETERMINATIONS

Sample HVM-83-1.5 is a picrite from the Maivala only produced a limited amount of zircon. Eight location (Makkonen 1992, 1996). Atypically for spots from seven zircon grains were analysed. Two ultramafic rocks, the sample contains relatively of the analyses were discarded due to high common abundant zircon, which was utilized for in situ iso- lead. One of the analyses yielded a Neoarchean age tope analysis from a thin section. Eleven spots from (207Pb/206Pb = 2744 ± 24 Ma), whereas the remain- 10 zircon grains were analysed, but one spot was ing five analyses gave 207Pb/206Pb –ages from 1718 discarded due to its high common lead content (Fig. to 1809 Ma (Fig. 11). These five spots contain high 11). Two analyses from a large zircon grain yielded concentrations of both Th (407–12089 ppm) and U 207Pb/206Pb ages of 2083 and 2031 Ma, with the latter (774–6088 ppm). The Neoarchean crystal is inter- being discordant. Eight analyses from smaller crys- preted as being captured from the paragneisses in tals are concordant and the obtained ages cluster at the area during the emplacement of the dyke, as its 1840 Ma, yielding a weighted average of 1841 ± 18 age is similar to those obtained for zircon from the Ma. The analysed large crystal probably represents surrounding paragneisses (Mikkola et al. 2018b). xenocrystic material captured from the sediments The younger ages represent metamorphic event(s), on which the lava erupted. The younger ages are but the small number of analyses together with interpreted as metamorphic. their large scatter prevents further interpretations. Sample A2438 is from an 8-m-wide amphibolite dyke cutting picrites at Loukee. Mineral separation

Fig. 11. Concordia diagrams for zircon from samples HVM-83-1.5 (A) and A2438 (B). Note that the two oldest dates from sample HVM-83-1.5 are from the same zircon crystal and the single Neoarchean age obtained from sample A2438 is not shown. Errors drawn at the 2σ level.

6 DISCUSSION

6.1 Geochemistry

The scatter in chemical composition displayed nation during ascent or emplacement is provided by the studied volcanic rocks has several possible by the inherited zircon grains in the samples used explanations. Post-crystallization alteration is evi- in dating, which most likely originated from the dent, especially in the case of the Ala-Siili location, sediments on which the lavas erupted. In the case as strong biotite alteration is clearly reflected in of mafic and ultramafic magmas, cumulation pro- elevated K2O concentrations. Evidence for contami- cesses have to be taken into account. For example,

78 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland the Sr-enriched samples from the Ala-Siili loca- the picrites (see below). A partial analogue to the tion have higher than average CaO concentrations, units studied here is the 1877–1852 Ma magma- suggesting the accumulation of clinopyroxene. It tism in the Skellefte district, where LREE-enriched, is possible that some of the picrite samples are Al-undepleted, continental arc-type mafic to ultra- cumulates from a basaltic magma (e.g. Makkonen mafic volcanic rocks erupted contemporaneously 1996, Barnes et al. 2009), while some of the basic with mafic to ultramafic rocks with a MORB affinity volcanic samples are fractionated members of a pic- (Berge 2013). Berge (2013) proposed a model where ritic magma (Nironen 2017). Moreover, the observed the REE and Al enrichment as well as the over- difference in the MgO/FeOt ratio of the picrites of all continental arc signature was superimposed on the two suites (Fig. 7C) could be a result of oli- “normal” Al-depleted ultramafic magma by con- vine fractionation and accumulation from a single tamination with felsic crustal material. This model parental magma. could explain certain compositional features of the Especially the samples of the Pahakkala suite Ala-Siili lithodeme, e.g., the LILE, LREE enrich- display rather typical EMORB-type trace element ment and negative Nb anomalies. However, con- characteristics, indicating formation in an exten- tamination alone cannot modify the Pahakkala-type sional environment. The initial values for the picrites into Ala-Siili-type picrites, as the latter εNd

Pahakkala suite samples are close to +4, suggesting have, for instance, higher Mg# and P2O5. Both of a depleted mantle source for the parental magma these would be lower if the magma had been con- (Makkonen & Huhma 2007). Certain compositional taminated with felsic material. Thus, differences in characteristics of the Ala-Siili lithodeme, e.g. the the melting depth and temperature are required, in negative Nb and Ti anomalies, Zr/Ti ratio and frac- addition to different degrees of contamination, to tionated REE pattern, point to arc-type magmatism. explain the compositional differences between the These rocks are not included in the calc-alkaline Ala-Siili lithodeme and Pahakkala suite. It is also Makkola suite (Mikkola et al. 2018c), as, based possible that magmas forming the Ala-Siili litho- on drill core observations, the rocks of the Ala- deme originated from lithospheric mantle enriched Siili lithodeme erupted on the paragneisses of the during an earlier subduction event. Pirkanmaa migmatite suite (Mikkola et al. 2018d) Comparison with the mafic volcanic rocks from and the contact between the Pirkanmaa migmatite other parts of the Western Finland supersuite and Makkola suites has been interpreted to be tec- reveals that the picrites relatively abundant in tonic (Mikkola et al. 2018a). As indirect evidence, our study area are a rarity further west (Lahtinen the lack of similar compositions in the Makkola at al. 2017). Only one locality in central parts of suite favours the current interpretation of the Ala- the Pirkanmaa migmatite suite in the data set of Siili lithodeme as part of the Pirkanmaa migmatite Lahtinen et al. (2017) contains rocks with picrite suite. compositions, whereas the remaining are tholeiitic Based on field observations, the mafic to ultra- basalts variably displaying within-plate basalt and mafic volcanic rocks erupted on the protoliths of MORB affinities. More abundant picrite observa- the paragneisses on the sea bottom. A plausible tions have been reported from the Western Finland explanation is that they represent extensional supersuite further west, where narrow interbeds phase(s) in the development of the sedimentary exist in both mafic metavolcanic rocks and parag- basins in which the paragneisses were deposited. neisses (Lehtonen et al. 2005, Kousa & Lundqvist One possible explanation for the compositional 2000, Kontoniemi & Mursu 2006). The picrites from differences is that the Pahakkala suite represents the Salittu area (Nironen 2017) in South Finland a purely extensional phase and the Ala-Siili litho- resemble those from the Pahakkala lithodeme, e.g. deme marks a transition to arc-type magmatism. FeOt >10%, high CaO, TiO2 and Ni, low P2O5 and However, it should be noted that claiming that the weakly fractioned REE patterns. Additionally, on Ala-Siili lithodeme formed in an arc setting would the classification diagram of Hanski et al. (2001), mark the discovery of a completely new episode of samples from Salittu plot in the picrite field, the arc magmatism between the older (1930–1910 Ga) majority close to the boundary of the Ti-enriched and younger (1895–1875 Ma) Svecofennian mag- komatiite field, and on the diagram of Pearce (1982) matism (Kähkönen & Huhma 2012, Kousa et al. in the MORB field, akin to the samples from the 2018, Mikkola et al. 2018c and references therein) Pahakkala suite. due to the time constraints for the eruption of

79 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

6.2 Age of the picrites

Due to their ultramafic metamorphic mineral associated metatholeiites in the South Savo region assemblages, the picrites do not contain minerals have earlier been interpreted to represent extru- suitable for dating them directly. When zircon is sive counterparts of these intrusions (Makkonen present, it is either metamorphic or inherited, as 1992, 1996, 2015, Makkonen & Huhma 2007). Hill demonstrated by the samples of this study used for et al. (2005) and Barnes et al. (2009) concluded dating. The maximum depositional age of the parag- that the parent magmas to the intrusions and the neisses, 1900–1920 Ma (Lahtinen et al. 2002, 2017, extrusive picrites had a common mantle source, Mikkola et al. 2018b), is also the maximum eruption but the magmas ascended through different routes age of the picrites. The age is further constrained by into the upper crust and the intrusive phases were the observation that the paragneisses do not con- more contaminated by crustal material than the tain zircon grains derived from the active arc vol- extrusive phases. This interpretation is supported canic phase in the vicinity of the study area, which by the initial εNd values of the picrites and associated is dated at 1895–1875 Ma (Mikkola et al. 2018c), mafic volcanic rocks being close to that of depleted and hence 1900–1910 Ma probably represents the mantle and differing distinctly from those of the approximate age of deposition. An indisputable gabbro intrusions (Makkonen & Huhma 2007). minimum age for the deposition of the paragneisses However, the now available age constraints on the and eruption of the picrites is given by the grani- South Savo picrites suggest that they predate the toid intrusions at ~1885 Ma (Vaasjoki & Kontoniemi emplacement of the above-mentioned intrusions 1991, Pekkarinen 2002), thus all ages younger than more clearly than previously interpreted (Hill et this must be regarded as metamorphic. al. 2005). Similar ~1.91 Ga eruption ages have been The synorogenic 1885 Ma gabbros and peridotites interpreted for mafic volcanic rocks interbedded occurring in South Savo area have been studied in with paragneisses in other parts of the Western detail due to their Ni and Cu sulphide ore potential Finland Supersuite (Lahtinen et al. 2017). (see references in Peltonen 2005). The picrites and

6.3 Implications for areal geology and stratigraphy

The division of the metasedimentary units in our could be used as an argument for separating the study area has been ongoing since the tectono-met- Häme and Pirkanmaa migmatite suites, despite the amorphic interpretation of Korsman et al. (1988), fact that the paragneisses do not differ significantly who recognized a fundamental difference between from each other in their composition or deposi- the northern and southern parts of the “Savo schist tional age (Lahtinen et al. 2002, 2009, Mikkola et belt”. The northern part consists of many fault- al. 2018b). This interpretation would also require separated blocks along the Raahe–Ladoga shear dismissing the Ala-Siili-type chemical characteris- zone, where the metamorphic peak conditions were tics of the samples from Oravakallio locality, which reached at about 1880 Ma, whereas the southern is located clearly within the Häme migmatite suite part, the Rantasalmi–Sulkava area, is character- (Fig. 1) and thus belongs to the Pahakkala suite. ized by zones of progressive metamorphism with its Furthermore, as the compositional differences at peak at 1830–1810 Ma. In the current unit division least partly result from a variable degree of crustal (Nironen et al. 2016a, Luukas et al. 2017), the Savo contamination and fractional crystallization, the schist belt is divided into the Savo supersuite in the separation or fusing of these units is not possible north, the Pirkanmaa migmatite suite in the west without further studies. and southwest, and the Häme migmatite suite in Our interpretation of the eruption age of the the south, with the last one roughly corresponding picrites does not differ significantly from the ages to the Rantasalmi–Sulkava area of Korsman et al. obtained for the tonalites and associated volcanic (1988). The metamorphic ages obtained for zircon units from the Viholanniemi area in the Jäppilä- in this study, despite a significant scatter, are in Virtasalmi block (Fig. 1, 1.91 Ga; Kousa et al. 2018). line with the areal metamorphism of this subarea. According to Pekkarinen (2002), these tonalites The chemical differences displayed by the picrites cut the amphibolites of the Jäppilä–Virtasalmi of the Pahakkala suite and the Ala-Siili lithodeme block, which have been interpreted as volcanic in

80 Geological Survey of Finland, Bulletin 407 Paleoproterozoic mafic and ultramafic volcanic rocks in the South Savo region, eastern Finland origin. Ultramafic rocks have not been reported in by metagreywackes. Makkonen and Ekdahl (1988) connection with these amphibolites and similar suggested a stratigraphic succession for the Pirilä tonalites have not been reported from the Häme area, in which the rock units are, from oldest to migmatite suite. Thus, the amphibolites of the youngest, mica schists, iron formations, felsic vol- Jäppilä–Virtasalmi block and the picrites of this canic rocks, intermediate volcanic rocks, mafic vol- study seem to represent different tectonic stages, canic rocks and picrites. with the former being associated with the latest The different locations of the mafic and ultra- activity of the Savo arc (Kousa et al. 2018) and the mafic units in the above-mentioned stratigraphic latter representing a slightly younger extensional interpretations seem confusing at first, but it must phase before the onset of calc-alkaline volcanism be taken into account that each of them is based on and magmatism at 1895 Ma (Kähkönen & Huhma studies in relatively small areas. Nevertheless, it is 2012, Mikkola et al. 2018c). Thus, our interpretation possible that mafic to ultramafic volcanism occurred is the same as that of Lahtinen et al. (2017) made several times during the evolution of the sedimen- from areas further west. tary basin(s). We suggest that the picrite units in Several interpretations of the stratigraphy in the South Savo region do not represent a distinct the South Savo region have been offered over the marker horizon, which could be used to solve the years. In the Virtasalmi area, including the Loukee– stratigraphy of the associated sedimentary rocks. Airikka area, Hyvärinen (1969) regarded mica This is even more evident when the ultramafic and schists and intercalated black schists as the oldest, mafic intrusive rocks showing chemical similarities being overlain by quartz-feldspar gneisses, diop- to the picrites of this study, but ca. 20 Ma younger side gneisses, carbonate rocks and amphibolites, in age (Makkonen 1996, Hill et al. 2005), are taken which are topped again by mica schists. According into account. Furthermore, Salittu picrites from to Simonen (1982), the stratigraphic sequence in South Finland have been interpreted as 1.88–1.87 the Mikkeli area starts with diopside amphibolites Ga old (Nironen et al. 2016b), i.e. postdating the and quartz-feldspar gneisses, which are followed peak of the Svecofennian orogeny in Finland at 1.88 by mica schists, whereas Gaál and Rauhamäki Ga. It is thus evident that mantle-derived mafic to (1971) proposed that the mafic and ultramafic vol- ultramafic melts were generated in at least three canic rocks in the Haukivesi–Savonlinna area were stages during the development of the Svecofennian deposited between two separate sedimentary units, orogeny in Finland. i.e. they are underlain by metapelites and overlain

7 CONCLUSIONS

Volcanic rocks displaying picritic chemical compo- Compositional differences in the here-studied sitions are widespread but low-volume constituents volcanic rocks in both major and trace elements in the paragneiss dominating the bedrock of the (light rare earth and large-ion lithophile elements) South Savo region. can be explained by differences in their magma These ultramafic rocks are spatially associated source, degree of crustal contamination, post-mag- with mafic volcanic rocks displaying EMORB-like matic alteration and accumulation and fractionation chemical affinities. processes. Based on locally observable pillow lava, lava brec- The picrites were erupted during the extensional cia and amygdale structures, some of the picrites phase(s) of the basin(s) that the surrounding par- are volcanic rocks, whereas cross-cutting relation- agneisses were deposited in. ships with surrounding rocks indicate that the some The best estimation for the eruption age of the of them originated as subvolcanic dykes. picrites is 1.91–1.90 Ga.

81 Geological Survey of Finland, Bulletin 407 Jukka Kousa, Perttu Mikkola and Hannu Makkonen

ACKNOWLEDGEMENTS

Mauri Luukkonen is acknowledged for Figure 2 and ples. Reviews by Dr Tuomo Törmänen and Professor Jouko Ranua for Figures 3 and 5. Hannu Huhma Eero Hanski helped in the overall improvement of and the whole staff of GTK’s isotope laboratory are the original manuscript. thanked for the work on the age determination sam-

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