Geochemical and U-Pb and Sm-Nd Isotopic Constraints on the Evolution of the Paleoproterozoic Ylitornio Nappe Complex, Northern Fennoscandia
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Bulletin of the Geological Society of Finland, Vol. 91, 2019, pp 75–100, https://doi.org/10.17741/bgsf/91.1.003 Geochemical and U-Pb and Sm-Nd isotopic constraints on the evolution of the Paleoproterozoic Ylitornio nappe complex, northern Fennoscandia Raimo Lahtinen1*, Hannu Huhma1, Laura S. Lauri2 and Mohammad Sayab1 1 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland 2 Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi,, Finland Abstract The Ylitornio area in northern Finland is a typical example of a poorly outcropping, multiply deformed, complex Precambrian terrain. U–Pb, Sm-Nd and geochemical data from metavolcanic, metasedimentary and plutonic rocks combined with structural data define the lithodemic units and tectonostratigraphy of the area. The Mellajoki, Martimo, Hosiojoki, Kierovaara and Uusivirka suites form the Yli- tornio nappe complex. Although main thrust stacking had occurred between 1.92– 1.89 Ga, later reverse faulting probably continued until 1.78 Ga. The Väystäjä bimodal volcanic rocks (2.09–2.05 Ga) from the Martimo suite are plume-related EMORB-OIB rocks formed during a continent breakup. The Kaskimaa greywacke of the Martimo suite is inferred to represent sedimentation in a deepening rift basin in the continental margin. The coeval Hosiojoki felsic rocks with A-type affinity and the Kierovaara suite granites (2.00–1.98 Ga) represent dry melting of a dominantly 2.1–2.05 Ga contaminated mafic underplate, and hydrous melting of both the mafic underplate and a refractory lower crust of Archean age, respectively. The older rocks of the Uusivirka suite are early-collisional (1.92–1.91 Ga) and derived from poorly mixed local sources, whereas younger psammite-pelite associations (Ristivuoma) are syn-collisional (1.91– 1.89 Ga) and derive from multiply recycled and thoroughly mixed sources. These rocks were deformed during continued collision with the development of foreland fold- and thrust belt. Keywords: Svecofennian orogen, detrital zircon geochronology, Nd isotopes, sedimentary rocks, volcanic rocks, granitoids, *Corresponding author (e-mail: [email protected]) Editorial handling: Alexander Slabunov ([email protected]) 76 Lahtinen, Huhma, Lauri and Sayab 1. Introduction 2005; Lahtinen et al., 2015b). Also the occurrence of orthoquartzites, earlier correlated with quartzites Polydeformed Precambrian cratonic shield areas of Jatulian age (2.30–2.06 Ga), have a young typically lack vertical bedrock exposures hampering maximum deposition age of < 1.92 Ga (Lahtinen 3D field observations that are necessary to resolve et al., 2015b). Previously defined lithostratigraphic the structural evolution and stratigraphic record units in northern Finland are informal, often of the bedrock (e.g., Sayab, 2009; Lahtinen et al., discontinuous, and separated by regional-scale 2015a). Moreover, paleo-glaciation in Finland faults and thrust planes. Thus, many of these units has mostly carved the landscape along with the were re-assigned as lithodemic units in the Bedrock deposition of Quaternary sedimentary cover, of Finland−DigiKP database (see also Nironen, leaving limited windows for studying the bedrock. 2017). (Figure 1) Consequently, stratigraphic and chronologic The lithostratigraphy of the Peräpohja Belt matching offers a tremendous challenge both (PB; Fig. 1) is extensively studied and rather well in the lateral and vertical extent in such regions. known at least locally. Two major lithostratigraphic The central part of northern Finland is a typical units, the Kivalo and Paakkola groups, have been example of such a poorly outcropping, yet multiply proposed (Perttunen, 1985; Perttunen & Hanski, deformed and metamorphosed complex. In general, 2003; Kyläkoski et al., 2012; Ranta et al., 2015). the area may be divided into different parts where In these studies, both Väystäjä and Martimo were the migmatitic Paleoproterozoic Central Lapland included in the PB but as discussed later, we prefer Granitoid Complex (CLGC) occurs in the center, to separate them from the PB. Metamorphic grade surrounded by Paleoproterozoic supracrustal belts (Perttunen & Hanski, 2003; Hölttä & Heilimo, in the north, east and south, and the Pajala shear 2017) varies from greenschist facies in the southwest zone in the west (Fig. 1). (PB) to lower amphibolite facies more to the north Supracrustal belts in northern Finland have (PB, Martimo, Väystäjä), and finally reaches upper been divided into stratigraphic units and shield- amphibolite facies in the Hosiojoki and Mellajoki wide correlations have been made using traditional suites (Figs. 1 and 2; Bedrock of Finland−DigiKP). stratigraphic names such as Sumi, Sariola, Jatuli, The Martimo-Mellajoki area (Fig. 1) manifests Ludicovi, and Kaleva (e.g., Meriläinen, 1980; complex Paleoproterozoic tectonic evolution and Lehtonen et al., 1998; Ojakangas et al., 2001; five near-orthogonal deformation successions (D1 Laajoki, 2005; Hanski et al., 2005; Hanski & to D5) were identified (Lahtinen et al., 2015a; Sayab Melezhik, 2012; Ranta et al., 2015). According et al., 2017). The earliest deformation (D1) involved to the “Guidelines and Procedures for Naming east-directed thin-skin thrusting. D2 deformation Precambrian Geological Units in Finland” by is characterized by N-S shortening with the develop- Strand et al. (2010) the use of these traditional terms ment of pervasive E-W striking foliation and should be restricted to broad, informal discussion associated F2 upright folds with varying plunges due only. Recent detrital zircon geochronological data to the effects of later deformation events (D3, D4 (e.g. Lahtinen et al., 2010, 2013, 2015b,c; Ranta and D5). These two (D1+D2) deformation events et al., 2015) and an inferred new Paleoproterozoic combined with low areal topography challenge the plate boundary in northern Fennoscandia (Lah- lithostratigraphic approach and mainly prevents tinen et al., 2015b) also make the use of these its use. In the Pajala shear zone, rock units occur as terms problematic, especially in shield-wide blocks bounded by steeply oriented deformation correlations. The old stratigraphic division is further zones, which complicates the interpretation even complicated by the recognition of ca. 1.99 Ga further. A-type felsic tuffs/arkosites on both sides of the In the study area (Fig. 2) thrust sheets are aulacogen (CLGC, R in Fig. 1) (Hanski et al., imbricated and stacked, and overprinted by Geochemical and U-Pb and Sm-Nd isotopic constraints on the evolution of the Paleoproterozoic Ylitornio nappe complex 77 Figure 1. Simplified geological map of the study area modified from the Bedrock of Finland – DigiKP database. Sample points refer to samples included in this study (Data from Ranta et al., 2015 and Lahtinen et al., 2015b). A720 (star) will be discussed. CLGB = Central Lapland Greenstone Belt. R = Rovaniemi supersuite samples; P = Paskavaara suite samples; U = Uusivirka suite samples. Inset figure shows the location of the study area in the Fennoscandian shield. V = Väystäjä; K = Kiiminki; O = Otanmäki; S = Siilinjärvi; T = Tohma järvi; Vi = Viinijärvi; Nä = Näläntöjärvi; PB+TB+HB = Pirkanmaa Belt occurs in the middle and Tampere (TB) and Häme (HB) belts occur north and south of it, respectively. 78 Lahtinen, Huhma, Lauri and Sayab a sequence of later deformation events (Lahtinen has a pre-collisional (>1.92 Ga) history of multiple et al., 2015a). The eastern part of the study area has rifting and depositional stages (Lahtinen et al., been a focus of detailed studies due to the recent 2015b). The western boundary of the CLGC discovery of the extensive gold-mineralized Rompas is marked by the approximately N−S trending zone (Fig. 2; Vanhanen et al., 2015; Molnár et Paleoproterozoic Pajala shear zone (Kärki et al., al., 2016). Ranta et al. (2015) have studied the 1993; Baltic-Bothnian megashear by Berthelsen metasedimentary and igneous rocks from the study & Marker, 1986), which is inferred to coincide area and the surroundings using zircon U–Pb with a cryptic suture separating two colliding dating and Sm–Nd analysis. Lahtinen et al. (2015b) Archean continental units (Norrbotten and published 1130 U–Pb analyses on zircons from Karelia) (Lahtinen et al., 2015b). The occurrence of 25 samples across the Pajala shear zone. a plate boundary is also favored based on different In addition to existing data, new U–Pb data anisotropic structures between the Norrbotten and from four metasedimentary rock and five granitoid Karelia Archean mantle lithospheres (Vecsey et samples, and Sm-Nd and geochemical data from al., 2014). The Pajala shear zone proper is mainly metavolcanic, metasedimentary and plutonic a younger overprinting structure which was multiply rocks are presented. Instead of a lithostratigraphic reactivated by both lateral and vertical movements. approach, we will apply lithodemic units and The Karelia province includes the Archean tectonostratigraphy to untangle the evolution of Pudasjärvi complex (Fig. 1; Mutanen and Huhma, this polyphase deformed and metamorphosed area. 2003), intruded by 2.44 Ga layered intrusions We have used geochemisty, and U-Pb and Sm-Nd (Iljina & Hanski, 2005; Lauri et al., 2012). isotope data to define the units and the tectonic The Paleoproterozoic supracrustal rocks of the setting of rocks within units. By combining multiple Peräpohja Belt were deposited in both continental datasets, we propose a framework of a fold-and- and marine environments on top of the Pudasjärvi thrust system, comprising the Mellajoki,