Radiometric age determinations from Finnish Lapland and their bearing on the timing of Precambrian volcano-sedimentary sequences Edited by Matti Vaasjoki Geological Survey of Finland, Special Paper 33, 255–279 , 2001. GEOCHRONOLOGY OF NORTHERN FINLAND: A SUMMARY AND DISCUSSION by Eero Hanski, Hannu Huhma and Matti Vaasjoki Hanski, E., Huhma, H. & Vaasjoki, M. 2001. Geochronology of northern Finland: a summary and discussion. Geological Survey of Finland, Special Paper 33, 255-279. 8 figures and one table. TheThe isotopic isotopic data data presented presented in in this this volume volume fromfrom northernnorthern Finland are summarized. The results show that ages from the Archean basement com- plexes fall mostly in the range of 2.68 to 2.83 Ga with ages higher than 3.0 Ga being found only at one location (Tojottamanselkä gneiss dome). The Paleoproterozoic magmatism occurred as distinct episodes which in this review are divided into the following age groups: c. 2.43-2.50 Ga intrusions and metavolcanics, 2.2 2.22 Ga Ga differentiated differentiated mafic mafic sills, sills, 2.14 2.14 Ga mafic Ga mafic intrusions, intru- sions,c. 2.05 c. Ga 2.05 intrusions Ga intrusions and dykes, and dykes, c. 2.0 Gac. 2.0 mafic Ga maficand felsic and felsicdyke rocks, dyke rocks, 1.91- 1.91-1.921.92 Ga felsic Ga felsic plutons plutons and and dykes, dykes, c. c.1.88 1.88 Ga Ga synorogenic synorogenic plutonism plutonism and volcanism, and c.c. 1.801.8 Ga Ga (postorogenic) (postorogenic) granitoids.granitoids. IsotopicIsotopic datadata on bulk detrital zircons can be used to distinguish quartzites from different stratigraphical levels: zircons in the Sodankylä and Kivalo Groups quartzites were derived exclusively from Neoarchean sources while the younger Lainio and Kumpu Group quartzites contain a significant zircon component from c. 1.9 Ga synorogenic plutonic rocks. Comparison and correlation of certain magmatic and sedimentological phases is made beyond the western and eastern borders of northern Finland and some problems in the available geochronological data and in their relation to geological observations are discussed. Key words (GeoRef Thesaurus, AGI): geochronology, stratigraphy, Precambrian, Archean, Paleoproterozoic, Peräpohja, Lapland, northern Fin- land. Eero Hanski, Geological Survey of Finland, P.O. Box 77, FIN-96101 Rovaniemi, Finland. E-mail: [email protected] Hannu Huhma, Geological Survey of Finland, P.O. Box 96, FIN- 02151 Espoo, Finland. E-mail: [email protected] Matti Vaasjoki, Geological Survey of Finland, P.O. Box 96, FIN- 02151 Espoo, Finland. E-mail: [email protected] INTRODUCTION This paper attempts to summarize age determin- the review is on the segment of northern Finland ations from northern Finland presented in this volume extending from the Peräpohja Schist Belt to the together with those published earlier on. The focus of Lapland Granulite Belt, that is the area from where the 255 Geological Survey of Finland, Special Paper 33 Eero Hanski, Hannu Huhma and Matti Vaasjoki dated rocks dealt with in this volume have been A compilation of zircon and baddeleyite ages on collected. igneous rocks from northern Finland is shown as The bulk of the available age determinations from histograms in Figure 1 together with comparative data northern Finland have been made over a period of tens from northern Sweden. It should be emphasized that of years using the conventional multi-grain U-Pb the errors of these age determinations have not been method on zircon. With the exception of a few newer taken into account in the histograms, but the ages are analyses, the sample size has been several milligrams simply divided into ten-million-year categories on the of handpicked zircon. Major problems encountered in face value basis. Thus several problematic age re- these datings are not purely analytical, but concern the sults, discussed in more detail later on, are included in discordancy and heterogeneity of the isotopic data. the data. Nevertheless, the histograms give a useful Reasons for scatter in multigrain conventional data overview on the general age distribution of the dated include primarily heterogeneous zircon populations igneous rocks and reveal notable peaks of magmatic (detrital or inherited zircons) and metamorphic ef- activity in northern Finland. In the following, the most fects. It is known that zircon will stay as a closed salient features of these data are discussed in a system at high temperatures (e.g., 800°C), but may chronological order starting from the oldest. Particular lose Pb if it experiences extended time periods at low attention is paid to their implications for regional temperatures, especially when abundant hydrother- correlation and to unsolved geochronological prob- mal fluid is available (Mezger & Krogstad 1997). In lems or discrepancies between the geological and addition to the above mentioned problems related to geochronological knowledge. It is namely true with zircon dating, there are occasionally difficulties in geochronological studies, as with any scientific re- determining the primary rock types in highly metamor- search, that apart from solving earlier problems new phosed and tectonized lithologies, i.e. whether a sam- discoveries often create new puzzles or alternative ple in question is truly magmatic with magmatic zircon interpretations not fitting perfectly, if at all, with the or contains a detrital component. previous concepts. GEOLOGICAL BACKGROUND Before plunging into the geochronology of northern eastern border of Finland over Central Lapland to the Finland, the geological background and especially the Finnish-Norwegian border records a sedimentary and stratigraphical nomenclature employed in two areas, volcanic evolution spanning hundreds of millions of the Peräpohja Schist Belt and the Central Lapland years. The general geology of the belt was recently Greenstone Belt, should be shortly made familiar to presented by Lehtonen et al. (1998) and the reader is readers less versed in the geology of northern Finland. referred to this work for more detailed information. The general stratigraphy of the Paleoproterozoic The lower parts of the supracrustal sequence were Peräpohja Schist Belt is well-established (Perttunen deposited unconformably on Archean basement et al. 1995). The supracrustal rocks are divided into gneisses with radiometric ages varying from 3.1 Ga two major units: the lower Kivalo Group containing (Kröner et al. 1981) to 2.6 Ga (Meriläinen 1976). The sedimentary rocks of the orthoquartzite-dolomite as- Paleoproterozoic stratigraphy of the belt is shown sociation and a few intervening mafic volcanic units, schematically in Figure 3 of Lehtonen et al. (1998) and and the upper Paakkola Group consisting of pelitic in Figure 3 of Hanski (2001, this volume). It is divided schists, black schists and minor mafic metavolcanics. into seven lithostratigraphic groups which are, from These two groups correspond, respectively, to the oldest to youngest, the Salla, Onkamo, Sodankylä, Jatulian and Kalevian rocks of the traditional classifi- Savukoski, Kittilä, Lainio and Kumpu Groups. cation of the Karelian rocks. They all are younger than The stratigraphic position of the volcanic rocks the c.2.43 2.44 Ga Ga mafic mafic layered layered intrusionsintrusions whichwhich were directly upon the Archean granite gneisses is taken to exposed and partly eroded before the deposition of the indicate that the volcanism of the Salla Group was lowermost sedimentary rocks of the schist belt. On the related to an initial stage of rifting of the Archean other hand, the minimum age for the deposition of the craton. The volcanic rocks of this stage are mostly Paakkola Group is provided by intersecting Haaparanta subaerially erupted andesites to rhyolites. Meta- Suite plutonic rocks dated at c. 1.89 Ga (Perttunen volcanics rocks forming the next unit, the Onkamo 1991). Group, have been discovered in a wide area extending The Paleoproterozoic Central Lapland Greenstone from Salla at the Russian border to near Kolari close Belt extending from the Salla-Kuusamo area at the to Sweden. In the type area (Onkamojärvi, Salla), the 256 Geological Survey of Finland, Special Paper 33 Geochronology of northern Finland: a summary and discussion Fig. 1. Histograms of zircon and baddeleyite ages obtained for igneous rocks from northern Finland coupled with zircon ages for igneous rocks from Norrbotten County, northern Sweden (literature source available from the first author). Some ages are labeled with a question mark due to large analytical errors or problems with the interpretation of these ages. 257 Geological Survey of Finland, Special Paper 33 Eero Hanski, Hannu Huhma and Matti Vaasjoki Onkamo Group metavolcanics are represented by Kittilä Group is tectonic, as a pronounced shear zone high-Mg basalts and andesites. At Möykkelmä, runs close to the western margin of the Sovasjoki komatiites of the Onkamo Group were erupted dis- gneiss dome. Ophiolitic serpentinite lenses are found cordantly on Archean granite gneisses (Räsänen et al. along this thrust zone (Hanski 1997). The Kittilä Group 1989) and were succeeded by andesitic and basaltic, includes massive and pillowed mafic lavas, hyalotuffs, highly amygdaloidal lavas. breccias and volcanic conglomerates as well as pelitic The volcanic activity manifested as the Salla and and chemical metasediments including the Porkonen Onkamo Groups was followed by a more tranquil banded iron formation, indicating deposition in a ma- period during which thick, wide-spread