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2005-Bingen-Etal-Terranova.Pdf doi: 10.1111/j.1365-3121.2005.00609.x Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the Hedmark Group, south-east Norway B. Bingen,1 W. L. Griffin,2,3 T. H. Torsvik1,4,5 and A. Saeed2 1Geological Survey of Norway, 7491 Trondheim, Norway; 2Department of Earth and Planetary Sciences, GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Australia; 3CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia; 4Institute for Petroleum Technology and Applied Geophysics, NTNU, 7491 Trondheim, Norway; 5School of Geosciences, University of the Witwatersrand, Private Bag 3, PO WITS 2050, Johannesburg, South Africa ABSTRACT The Moelv Tillite is the Late Neoproterozoic Varanger glacial the Sveconorwegian orogen. The Moelv Tillite, which is not deposit recorded in the Hedmark Group, SE Norway. Paired overlain by a cap carbonate, possibly correlates with the c. U–Pb and Lu–Hf data collected on detrital zircons in the 580 Ma Squantum-Gaskiers glacial deposits of Avalonia. Avail- Rendalen Formation underlying the Moelv Tillite have identified able palaeomagnetic data for the Late Neoproterozoic suggest an uncommon 677 ± 15 to 620 ± 14 Ma population, that that Baltica was located at intermediate to high latitude constrain the deposition of the Moelv Tillite to be younger between 620 and 555 Ma. than 620 ± 14 Ma. The youngest detrital zircons may be derived from granite magmatism related to the 616 ± 3 Ma Terra Nova, 17, 250–258, 2005 Egersund dolerite magmatism, situated in the western part of The Neoproterozoic geological re- Introduction The Hedmark Group cord in Baltica includes a pair of Late Neoproterozoic glacial deposits diamictites on the Varanger peninsula The c. 3.5 km thick, weakly deformed, are recorded on most continents in Finnmark in the foreland of the Hedmark Group is exposed in the (Evans, 2000). Some of these appar- Caledonides (Fig. 1a; Edwards, 1984; Osen-Røa Nappe Complex of the ently formed at low latitude, implying Vidal and Moczydlowska, 1995; Har- Lower Allochthon (Fig. 1a,b; Bjør- that the Earth was affected by global land, 1997). The youngest one repre- lykke et al., 1976; Nystuen and glaciations (the ÔSnowball EarthÕ sents a reference horizon for the Sæther, 1979; Nystuen, 1981, 1987; hypothesis; Hoffman and Schrag, Varanger glaciation (e.g. Hoffman Vidal and Moczydlowska, 1995). It is 2002), or that the Earth was rotating and Schrag, 2002). A diamictite hori- characterized by important lateral along a highly oblique axis (the Ôhigh zon is also reported in sandstone- variations in thickness and facies. obliquityÕ hypothesis; Williams, 1975). bearing nappes of the Lower and Marine turbidites of the Brøttum For- The synchronicity of glacial deposits is Middle Allochthons of the Caledo- mation form the base of the group in one of the predictions of global glaci- nides, the so-called Ôsparagmite nap- the south. In the north-east, the cor- ation models. Consequently large ef- pesÕ (Kumpulainen and Nystuen, relative >2 km thick Rendalen For- forts are being made to improve the 1985). Glacial deposits on Baltica are mation consists of fluviatile, braided stratigraphy and geochronology of not reliably dated. Consequently, cor- stream, arkosic sandstone and con- sequences containing glacial deposits relation of the Varanger glacial depos- glomerate. The Brøttum and Renda- (Brasier et al., 2000; Thompson and its with Neoproterozoic glacial len Formations are overlain by shale Bowring, 2000; Bowring et al., 2003; deposits recorded on other continents and limestone of the Biri Formation, Calver et al., 2004; Hoffman et al., is purely speculative. and proximal fluviatile sandstone to 2004; Kendall et al., 2004; Xiao et al., As part of a study on the Neopro- conglomerate of the Ring Formation. 2004; Zhou et al., 2004). Available terozoic sediment record of Baltica, we The <30 m thick Moelv Tillite caps data give increasing evidence for at analysed zircons in 10 samples of clastic the Rendalen, Biri and Ring forma- least three major Late Neoproterozoic sediments. An unanticipated but signi- tions, with a possible hiatus. It is glaciations, the c. 720 Ma Sturtian, c. ficant Late Neoproterozoic detrital composed of glacial diamictite and 630 Ma Marinoan and c. 580 Ma component was detected in one of the glaciomarine laminated shale with ice- Gaskiers events. samples giving a unique maximum age dropped stones. The Moelv Tillite is constraint of 620 ± 14 Ma for depos- everywhere covered by a conformable, ition of the Moelv Tillite situated in the transgressive, generally 30–40 m Correspondence: Bernard Bingen, Geolo- Hedmark Group of the Lower Alloch- thick, shale horizon, the Ekre Forma- gical Survey of Norway, Leiv Eirikssons thon (Fig. 1b). The data lead to a tion. Where exposed, the transition vei 39, 7491 Trondheim, Norway. Tel.: 00 discussion on the geochronology and between the diamictites and shale is 47 7390 4240; fax: 00 47 7396 1620; e-mail: palaeogeography of the Varanger gla- characterized by a gradual decrease in [email protected]. ciation on Baltica. the grain size of large clasts and 250 Ó 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 3, 250–258 B. Bingen et al. • Timing of Late Neoproterozoic glaciation ............................................................................................................................................................. a Permian Oslo Paleorift Varanger peninsula Devonian Old Red Sandstone Vestertana Group Upper + Uppermost Allochthons Mortensnes Formation Tromsø Osen-Røa nappe complex Hedmark Group Lower + Middle Allochthon continental margin of Baltica b Hedmark Group Autochthon + parautochthon platformal sediments South Northeast of cratonic Baltica Precambrian rocks Vangsås Fm Vangsås Fm of cratonic Baltica Ekre Fm Ekre Fm B01104 collected in Moelv Fm Moelv Fm Hanestad, Rendalen kommune, Ring Fm Biri Fm UTMWGS84 coordinate: zone 32 Biri Fm Osdalen Fm x: 601531 Biskopås Fm Atna Fm y: 6858950 100 km B01104 B01104 <620±14Ma Brøttum Fm Bergen Fennoscandian >2000 m 60° shield Rendalen Fm 5° Oslo Stockholm >2000 m Sveconorwegian orogen Egersund dykes Fig. 1 (a) Simplified tectonostratigraphic map of Scandinavia showing the location of the Hedmark Group and sample locality. (b) Simplified stratigraphic columns for the Hedmark Group in the southern and north-eastern parts of the Osen-Røa Nappe Complex (Bjørlykke et al., 1976; Kumpulainen and Nystuen, 1985; Nystuen, 1987; Vidal and Moczydlowska, 1995). matrix (Bjørlykke et al., 1976). The were mounted in epoxy. They were Fig. 2a). The probability density plot youngest exposed Vangsa˚ s Formation imaged and analysed for Si, Zr, Hf, of ages show peaks at 1606 ± 36, is overlain by a fossiliferous transgres- and Y in an electron microprobe. 1483 ± 35, 1302 ± 57, 985 ± 18, sive Cambrian sequence. More than 95% of the zircons display 677 ± 15 and 620 ± 14 Ma (Fig. 2b). magmatic prism-parallel oscillatory The peak at 1.48 Ga largely domi- zoning. U–Th–Pb geochronological nates the distribution. The young- Results analyses were performed with laser est peak is defined by the weighted ablation inductively coupled plasma average 206Pb/238U age of overlapping Sampling and U–Pb analyses quadrupole mass spectrometry (IC- analyses on two grains. The grain Sample B01104 is a typical fluviatile PMS) on 73 crystals (Table 1). Ana- at 611 ± 26 Ma is a non-abra- facies of the Rendalen Formation lytical procedures, standardization, ded, U-poor (24 ppm), weakly oscil- collected in Hanestad (Fig. 1a). The data reduction, error propagation latory-zoned, prismatic (80 · 250 lm) outcrop is poorly bedded arkose, and common-Pb corrections are sum- crystal and the other at 623 ± 16 Ma showing common green to red shale marized in Belousova et al. (2001) and is a rounded fragment (100 lmin interbeds and cobbles. The sample is a Jackson et al. (2004). The age selected diameter) of a probably large crystal poorly sorted coarse-grained (c. for geological interpretation is the showing oscillatory zoning and a 207 206 1 mm) arkose (SiO2 ¼ 81.3 %) show- Pb/ Pb age for zircons older than medium U content (167 ppm). Both ing subangular clasts in a silt matrix. 1000 Ma and the 238U/206Pb age for crystals have typical magmatic Th/U Detrital zircons are prismatic to roun- younger zircons. ratios of 0.7–0.8. The moderate U ded, and variably abraded and frag- All but two of the analysed zir- content of these grains, the occurrence mented because of transportation. cons yield concordant to near- of prism-parallel zoning, the concor- Analytical work on zircon was per- concordant U–Pb analyses ranging dant behaviour and the absence of formed at the Department of Earth from 1839 ± 54 to 985 ± 18 Ma significant metamorphic overprint in and Planetary Sciences, Macquarie (65 grains, 2r uncertainty) and from the rock suggest that the U–Pb sys- University. Non-magnetic zircons 685 ± 28 to 611 ± 26 Ma (six grains; tematics have not been affected by Ó 2005 Blackwell Publishing Ltd 251 .................... Timing of Late Neoproterozoic glaciation 252 Table 1 LA–ICPMS U–Pb data on detrital zircons, sample B01104, Rendalen Formation. Concentrations Ratios, common-lead corrected Ages à 207 206 207 235 206 238 208 232 207 206 206 238 No.* U (ppm) Th (ppm) Pb/ Pb ±2r Pb/ U±2r Pb/ U±2r Pb/ Th ±2r Pb/ Pb (Ma) ±2r Pb/ U (Ma) ±2r Conc§ (%) 57 24 19 0.0594 0.0053 0.834 0.072 0.0994 0.0026 0.0306 0.0026 581 190 611 26 105 58 167 122 0.0610 0.0013 0.853 0.015 0.1014 0.0017 0.0313 0.0005 640 44 623 16 97 ..................... 27 111 52 0.0627 0.0014 0.903 0.019 0.1044 0.0020 0.0329 0.0007 699 48 640 18 92 59 74 54 0.0587 0.0025 0.875 0.033 0.1076 0.0020 0.0333 0.0010 556 90 659 18 119 44 101 47 0.0627 0.0016 0.956 0.023 0.1107 0.0023 0.0343 0.0008 697 52 677 20 97 56 18 12 0.0624 0.0037 0.964 0.055 0.1120 0.0032 0.0338 0.0018 686 124 685 28 100 52 272 5 0.0720 0.0014 1.638 0.028 0.1651 0.0029 0.0518 0.0016 985 40 985 18 100 47 37 4 0.0690 0.0026 1.644 0.051 0.1713 0.0035 0.0518 0.0050 898 76 1019 20 113 .....................
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