The Problem with the Age of the Central Puolanka Group KEEPS Fighting Us
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Geologian tutkimuskeskus, Tutkimusraportti 207 – Geological Survey of Finland, Report of Investigation 207, 2014 Lauri, L. S., Heilimo, E., Leväniemi, H., Tuusjärvi, M., Lahtinen, R. & Hölttä, P. (eds) THE problem WITH THE age of THE Central Puolanka Group KEEPS fighting US by Asko Kontinen1, Hannu Huhma2, Yann Lahaye2 and Hugh O’Brien2 1 Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland 2 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail: [email protected] The Central Puolanka Group (CPG) along the western margin of the Palaeopro- terozoic Kainuu schist belt presents an unresolved problem in terms of its age of deposition, being inferred either as early Proterozoic 2.35–2.20 Ga (Laajoki 2005) or late Archean >2.70 Ga (Kontinen et al. 1996, Huhma et al. 2000). This state of affairs is badly hampering the modelling of the Archaean to Proterozoic tectonic transition in Kainuu and southern Lapland (cf. Laajoki 2005). The reason is that the CPG lacks easily datable syngenetic magmatic rocks. Our efforts to address the problem include U-Pb dating of detrital zircon grains from all the main units (from oldest to youngest: Puolankajärvi, Akanvaara and Pärekangas Fms) of the CPG within the Oikarila structure in the western-cen- tral part of the KSB (Fig. 1). The studied samples contain only Archaean zircons, in most cases dominated by ca. 2.70 Ga grains, which in several cases form the youngest, clearly detrital population. This also concerns samples from the rocks previously interpreted (Huhma et al. 2000) as ca. 2.70 Ga sodic rhyodacitic tuffs (Fig. 2) in the Pärekangas Fm, as these rocks also carry a wide scpectrum of grains from 2.7 Ga up to 3.7 Ga (Fig. 3). The heterogeneous zircon in the tuffs in fact supports their alternative interpretation (Laajoki & Wanke 2002) as albitized epi- clastic silts. However, given the occurrence of unmistakable mafic-intermediate lavas and lapilli tuffs in the Pärekangas stratigraphy below the sodic tuffs/silts, it is nevertheless possible that these rocks mix syngenetic 2.7 Ga volcanic with simi- larly aged and older epiclastic materials. Whether the mafic lavas and lapilli tuffs are indeed 2.7 Ga, as this option would require, remains an issue to be resolved. Recent mapping and exploration drilling within the Oikarila structure has re- vealed some useful new constraints. A 150-m-thick unexposed gabbro–wehrlite sill has been drill-intersected in the Pärekangas Fm within the Kaunisjoki val- ley. The distinct “Karjalite” character of the sill defines the host Pärekangas lavas, lapilli tuffs, sands and pelites as at least ca. 2.22 Ga in age (cf. Hanski et al. 2010). In the Varislahti area within the SE part of the Oikarila structure (Fig. 1), the granophyre of the 2.44 Ga (Fig. 3) Junttilanniemi layered gabbro intersects a se- quence of felsic and mafic metavolcanic rocks (Pitkälika rhyodacites and basalts), which is overlain by a thin layer of conglomerate-wacke (Soidensuu wacke). The wacke, which contains granophyre clasts and ca. 2.4 Ga zircons, both most 68 Geologian tutkimuskeskus, Tutkimusraportti 207 – Geological Survey of Finland, Report of Investigation 207, 2014 Current Research: 2nd GTK Mineral Potential Workshop, Kuopio, Finland, May 2014 probably derived from the Junttilanniemi gabbro-granophyre, which is in turn overlain by a >500-m-thick sequence of pillowed and massive amygdaloidal me- tabasalts (Varislahti basalts) that we find physically and chemically similar to the Greenstone 1 and correlative metabasalts in the Kuusamo and Koillismaa areas (Laajoki 2005 and references). Now, considering the tectonic-stratigraphic situa- tion within the Oikarila structure, it seems most likely that the Pitkälika and CPG rocks represent the framework into which the 2.44 Ga old Junttilanniemi gabbro intruded, and on which the Varislahti basalts extruded after an intervening pe- riod of erosion. Consequently, a >2.44 Ga age for the CPG is strongly indicated. It is notable here that the chemical compositions of the Pärekangas and Pitkä- lika basalts are close enough to allow a common magmatic lineage, whereas the Varislahti basalts seem distinct. The CPG schists in the Oikarila structure are in the west in an abrupt fault contact with the quartzitic, pelitic and mafic gneisses of the Kalpio Complex/ Oulunjärvi shear zone. Just west of the separating Raappana Fault, the gneisses include a <100-m-wide and >700-m-long body of quartz-albite gneiss (Fig. 1) with a similar sodic rhyodacite composition to the Pärekangas albite-rich felsites. The Petäjäniemi gneiss body, although faulted-mylonitic for its contacts, appears to represent a dyke into the host mafic and metasedimentary gneisses. As the dyke contains a rather homogeneous, apparently magmatic zircon population dated at ca. 2.70 Ga (Huhma et al. 2000), it appears to define the host Kalpio gneisses as correspondingly older. In the light of the Petäjäniemi evidence and Laajoki’s (2005) view of the Kalpio gneisses as lithodemic derivatives of schists in lower parts of the CPG, this would mean that the latter rocks should also be older than ca. 2.70 Ga. In a summary of the presently available evidence, we feel comfortable in only proposing that the CPG was most probably deposited somewhere between 2.72 and 2.45 Ga. Although this represents some progress, the keys to full resolution of the problem are still missing. References Hanski, E., Huhma, H. & Vuollo, J. 2010. SIMS zircon ages and Nd isotope systematics of the 2.2 Ga mafic intrusions in northern and eastern Finland. Bulletin of the Geological Society of Finland 82, 31–62. Huhma, H., Kontinen, A. & Laajoki, K. 2000. Age of the metavolcanic-sedimentary units of the central Puolanka Group, Kainuu schist belt, Finland. In: Eide, E. (ed.) 24. Nordiske Geologiske Vintermøte, Trondheim 6.–9 Januar 2000. Geonytt 1, 87–88. Kontinen A., Huhma, H. & Laajoki, K. 1996. Sm/Nd isotope data on the Central Puolanka Group, Kainuu Scist Belt, Finland; constraints for provenance and age of deposition. In: Kohonen, T. & Lindberg, B. (eds) The 22nd Nordic Geological Winter Meeting, Turku – Åbo, 8–11 January 1995. Abstracts, p. 95. Laajoki, K. 2005. Karelian supracrustal rocks. In: Lehtinen, M., Nurmi, P.A., Rämö, O.T. (eds) Precambrian Geology of Finland − Key to the Evolution of the Fennoscandian Shield. Amster- dam: Elsevier B.V., 279–342. Laajoki, K. & Wanke, A. 2002. Kainuu Belt. In: Laajoki, K. & Wanke, A. (eds) Stratigraphy and sedimentology of the Palaeoproterozoic Kainuu, Kuusamo and Peräpohja belts, northern Finland. Res Terrae A22, 18–73. 69 Geologian tutkimuskeskus, Tutkimusraportti 207 – Geological Survey of Finland, Report of Investigation 207, 2014 Lauri, L. S., Heilimo, E., Leväniemi, H., Tuusjärvi, M., Lahtinen, R. & Hölttä, P. (eds) Fig. 1. Simplified geological map of the western central part of the Kainuu schist belt with locations of the Oikarila structure (black quadrangle) and lithostratigraphic units discussed in the text. AR Archaean rocks, mainly TTG gneisses, PjF Puolankajärvi Fm, AvF Akanvaara Fm, PkF Pärekangas Fm (Kalpio mica and quartzite gneisses with the same colour as PjF and AvF, respectively), Nsp Nuottasaari serpentinite. JAT Jatuli rocks, MJAT Marine Jatuli rocks. LKA Lower Kaleva rocks, UKA Upper Kaleva rocks, JOC Jormua Ophiolite Complex, Kajaani Gr c. 1.8 Ga pegmatite-granite. 70 Geologian tutkimuskeskus, Tutkimusraportti 207 – Geological Survey of Finland, Report of Investigation 207, 2014 Current Research: 2nd GTK Mineral Potential Workshop, Kuopio, Finland, May 2014 Fig. 2. Photographs of typical Pärekangas sodium-rich felsic (SiO2 63–69 wt%, Na2O 5–9 wt%) tuffs or silts. (Left) Field photograph of sample HAQ-157, for which detrital zircon age data are presented below in Fig. 3. (Right) Scanned image of a diamond-sawn slab (height ca. 7 cm) from sample 14A- ATK-90 (chemically nearly identical with HAQ-157). The main minerals in both these near mica- free rocks are actinolitic amphibole, quartz and albite. Fig. 3. Concordia diagrams for zircon U-Pb data. (Left) LA-MC-ICPMS analyses of zircons from Pärekangas sodic tuff or silt (photo of the sampled rock in Fig. 2) at Haapalanmäki. The young- est cluster of 22 technically good grains gives an intercept age of 2716 ± 8 Ma. (Right) SHRIMPII analyses of zircons from the Junttilanniemi granophyre. The older data points give the magmatic crystallization age of 2444 ± 4 Ma. The one close to 1.7 Ga is from a metamorphic grain domain. 71.