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 by 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 com- positional 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 altera- tion, 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.