INSTITUTE OF SEISMOLOGY UNIVERSITY OF HELSINKI REPORT S-62 LITHOSPHERE 2014 EIGHTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE IN FENNOSCANDIA PROGRAMME AND EXTENDED ABSTRACTS edited by Olav Eklund, Ilmo Kukkonen, Pietari Skyttä, Pia Sonck-Koota, Markku Väisänen, David Whipp Åbo Akademi University, Turku, November 4-6, 2014 Turku 2014 LITHOSPHERE 2014 Symposium, November 4-6, 2014, Turku, Finland 107 Late Svecofennian mafic magmatism in southern Finland Markku Väisänen1, Charlotta Simelius1,2, Hugh O’Brien3, Mira Kyllästinen1 and Jussi Mattila4 1 Department of Geography and Geology, 20014 University of Turku, Finland 2 Pöyry Finland Oy, Vantaa, Finland 3 Geological Survey of Finland, FI-02151 Espoo, Finland 4 Posiva Oy, Eurajoki, Finland E-mail: [email protected] We present single-grain zircon U-Pb laser-ablation data from gabbroic rocks in three locations in southern Finland. In Ruissalo, Turku, the mafic intrusions are found within the Airisto shear zone with ages of c. 1.831.82 Ga. In Muurla, the c. 1.81 Ga granite cuts across the c. 1.83-1.82 Ga gabbro. In Salittu, Karjalohja, the 1.831.82 Ga mafic dyke crosscuts the picrite. These findings indicate heat transfer from the mantle to the crust during the lateorogenic stage. Keywords: lateorogenic, mafic magmatism, heat source, shear zones, U-Pb, zircon, LA-MCICP-MS 1. Introduction The Svecofennian orogeny in southern Finland has been divided into two main orogenies: the 1.89-1.87 Ga Fennian orogeny (synorogenic) and 1.84-1.80 Ga Svecobaltic orogeny (lateorogenic) intervened by the poorly known intra-orogenic period (Lahtinen et al., 2005). The lateorogenic stage is characterised by dextral transpression (Ehlers et al., 1993), culmination of high temperature metamorphism (Korsman et al. 1984, Väisänen et al. 2002, Mouri et al. 2005) and, consequently, large amount of lateorogenic anatectic granites (Figure 1). The onset of anatectic granite magmatism has recently found to have started at c. 1.85 Ga, i.e., during the intra-orogenic period (Kurhila et al. 2005, Kurhila et al., 2010, Väisänen et al., 2012). Until now, only one occurrence of lateorogenic mafic intrusion has been described (1838 ± 4 Ma Jyskelä gabbro; Pajunen et al., 2008) Figure 1. Geological map of SW Finland, modified after the 1:5 million map by the Geological Survey of Finland (1999: www.gtk.fi). Symbols on different colours and shades are: D=diabase dykes, G=gabbros, LG=lateorogenic granites, M=migmatitic mica gneisses, R=rapakivi granites, SG=synorogenic granitoids, V=volcanic rocks. Thick black lines=shear zones. Study targets are shown by numbers on white background; 1=Turku, Ruissalo; 2= Muurla, road cut; 3= Karjalohja, Salittu. LITHOSPHERE 2014 Symposium, November 4-6, 2014, Turku, Finland 108 In this study we present new single-grain zircon ages from lateorogenic gabbroic rocks from three locations in southern Finland (Figure 1) from different geological environments. We also preliminary discuss some features of their geochemical compositions. The U-Pb analyses were performed using the LA-MC-ICP-MS techniques in the Finnish Geosciences Research Laboratory (SGL) at the Geological Survey of Finland, Espoo. 2. Study targets and their U-Pb zircon ages 2.1 Ruissalo, Turku In the western end of the Ruissalo Island, in the Saaronniemi Camping Place area, the N-S trending Airisto shear zone is exposed in two parallel sub-vertical shear zones located c. 300 m from each other. The shear zones are characterised by mylonites, brittle fracture zones and also pseudotachylites. Mafic intrusions, also affected by shearing, are found within the shear zones but not outside the zones. Two samples (RUIS1 and RUIS2) were taken from the gabbroic rocks located within the two branches of the shear zone (Simelius 2013). Both of the samples gave, within errors, the same 1.83-1.82 Ga concordia ages. No older inherited ages were found in the data set. 2.2 Muurla, highway E-18 road cut Dark mafic intrusions and pink granites are exposed along a c. 400 m long road cut of the E18 road at Muurla. Dikes from the granitic bodies cut across the gabbro. Both of these rock types are affected by narrow sub-horizontal shear zones. Two samples were taken from the gabbro and the granite (samples MGB and MGR; Kyllästinen, 2014). The gabbro gave a concordia age of c. 1.83-1.82 Ga. The granite was slightly younger with a c. 1.81 Ga age. No older zircon populations were found from either of the samples. 2.3 Salittu, Karjalohja The Salittu formation within the Orijärvi area comprises mafic and ultramafic (picritic) volcanic rocks interpreted to have formed during rifting of the volcanic arc. The absolute age of the formation is unclear, but it overlies the c. 1.88 Ga volcanic arc rocks. In the Salittu village, the picrite is excavated for industrial raw material by Nordkalk Oy Ab. On the more mafic portion of the quarry wall, a lighter coloured mafic dike, c. 50-100 cm wide, crosscuts the picrite. The sample Salittu from the dike has a geochemical analysis number 127.2MV96 in Väisänen and Mänttäri (2002). The mafic dike yielded a concordia age of c. 1.83-1.82 Ga. The zircons also contained two older ages of c. 1.86 Ga and 2.1 Ga but we interpret these as inherited zircons. 3. Geochemical data Because the geochemical data on the 1.83-1.82 Ga mafic rocks are very limited, consisting only of the dated samples, only very general comments can be stated at this stage. One of the observations is that, although of the same age, the compositions of the Ruissalo, Muurla and Salittu intrusion are not similar. The Ruissalo samples show an enriched character resembling the intra-orogenic mafic rocks in SW Finland (Väisänen et al., 2012), whereas the Muurla and Salittu samples are more primitive (Figure 2) with higher Mg-numbers, Ni and Cr contents, but lower P, F, LREE contents. LITHOSPHERE 2014 Symposium, November 4-6, 2014, Turku, Finland 109 Figure 2. K2O vs SiO2 diagram after Peccerillo and Taylor (1976). Symbols are: triangle=Ruissalo, square=Muurla, sphere=Salittu. For comparison a sample from the shoshonitic 1815 Ma Urusvuori monzogabbro (Rutanen et al., 2011) is shown with star symbol. Ruissalo samples fall within the highK field whereas Muurla and Salittu fall within the tholeiitic field. 4. Discussion and conclusions The present lateorogenic data and the data from the earlier intra-orogenic mafic intrusions (Mänttäri et al., 2006, Väisänen et al., 2012, Nevalainen, 2014) show that the mantle-derived magmatism was active throughout the Svecofennian orogeny in southern Finland. However, these findings are quite recent and their geodynamic implications are yet ambiguous since the important data on field relations, geochemistry and isotopic geology is still mostly lacking. Nevertheless, we propose that mafic intrusions with ages between c. 1.86-1.81 Ga are quite common throughout southern Finland and that more of such findings will emerge. The Ruissalo intrusions are located within the steeply dipping Airisto shear zone and we thus infer that the early shearing played important role in the intrusion mechanism of the mafic magmas of the Ruissalo area. In general, shear zones may have acted as pathways to the mafic magmas to ascend through the thick, hot, ductile and partially molten crust during the lateorogenic stage. A gently dipping shear zone was also detected close to the Muurla gabbro (Aho et al., 2014), which supports this idea. The heat source for the high temperature metamorphism in southern Finland is controversial. Schreurs and Westra (1986) proposed mafic intraplating, Korsman et al. (1999) mafic underplating and Lahtinen et al. (2005) evoked extensional tectonics. Kukkonen and Lauri (2009) modelled the heat budget and proposed that radioactive decay of the earlier granitoids led to crustal melting during the lateorogenic stage. Väisänen et al. (2012) emphasised the role of the mafic magmatism in transferring external heat from the mantle to the crust. There are now evidences from different sources, as discussed above, that the crust was intruded by mantle-derived magmas during the high-grade metamorphism at 1.85-1.81 Ga. Although the number of the so far discovered mafic intrusions and their areal extent is still quite low, they nevertheless inevitably show that mantle-crust interaction took place at the time. Our hypothesis is that the incremental heat flow from the mantle to the crust, combined with radioactive decay, gradually increased the crustal temperatures high enough to cause wide-spread crustal melting and the formation of migmatites and granites during the lateorogenic stage. The tectonic setting/settings for this is/are, however, uncertain, since the mantle-derived magmatism spans over the proposed episodic Svecofennian intra-orogenic, lateorogenic and postorogenic stages. 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