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Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the Hedmark Group, south-east Norway

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The Moelv Tillite is the Late Neoproterozoic Varanger glacial deposit recorded in the Hedmark Group, SE Norway. Paired U–Pb and Lu–Hf data collected on detrital zircons in the Rendalen Formation underlying the Moelv Tillite have identified an
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  Timing of Late Neoproterozoic glaciation on Baltica constrainedby detrital zircon geochronology in the Hedmark Group,south-east Norway B. Bingen, 1 W. L. Griffin, 2,3 T. H. Torsvik 1,4,5 and A. Saeed 2 1 Geological Survey of Norway, 7491 Trondheim, Norway;  2 Department of Earth and Planetary Sciences, GEMOC ARC National Key Centre,Macquarie University, NSW 2109, Australia;  3 CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia;  4 Institute for PetroleumTechnology and Applied Geophysics, NTNU, 7491 Trondheim, Norway;  5 School of Geosciences, University of the Witwatersrand, Private Bag3, PO WITS 2050, Johannesburg, South Africa Introduction Late Neoproterozoic glacial depositsare recorded on most continents(Evans, 2000). Some of these appar-ently formed at low latitude, implyingthat the Earth was affected by globalglaciations (the   Snowball Earth  hypothesis; Hoffman and Schrag,2002), or that the Earth was rotatingalong a highly oblique axis (the   highobliquity   hypothesis; Williams, 1975).The synchronicity of glacial deposits isone of the predictions of global glaci-ation models. Consequently large ef-forts are being made to improve thestratigraphy and geochronology of sequences containing glacial deposits(Brasier  et al. , 2000; Thompson andBowring, 2000; Bowring  et al. , 2003;Calver  et al. , 2004; Hoffman  et al. ,2004; Kendall  et al. , 2004; Xiao  et al. ,2004; Zhou  et al. , 2004). Availabledata give increasing evidence for atleast three major Late Neoproterozoicglaciations, the  c.  720 Ma Sturtian,  c. 630 Ma Marinoan and  c.  580 MaGaskiers events.The Neoproterozoic geological re-cord in Baltica includes a pair of diamictites on the Varanger peninsulain Finnmark in the foreland of theCaledonides (Fig. 1a; Edwards, 1984;Vidal and Moczydlowska, 1995; Har-land, 1997). The youngest one repre-sents a reference horizon for theVaranger glaciation (e.g. Hoffmanand Schrag, 2002). A diamictite hori-zon is also reported in sandstone-bearing nappes of the Lower andMiddle Allochthons of the Caledo-nides, the so-called   sparagmite nap-pes   (Kumpulainen and Nystuen,1985). Glacial deposits on Baltica arenot reliably dated. Consequently, cor-relation of the Varanger glacial depos-its with Neoproterozoic glacialdeposits recorded on other continentsis purely speculative.As part of a study on the Neopro-terozoicsedimentrecord of Baltica,weanalysedzirconsin10samplesofclasticsediments. An unanticipated but signi-ficant Late Neoproterozoic detritalcomponent was detected in one of thesamples giving a unique maximum ageconstraint of 620 ± 14 Ma for depos-itionoftheMoelvTillitesituatedintheHedmark Group of the Lower Alloch-thon (Fig. 1b). The data lead to adiscussion on the geochronology andpalaeogeography of the Varanger gla-ciation on Baltica. The Hedmark Group The  c.  3.5 km thick, weakly deformed,Hedmark Group is exposed in theOsen-Røa Nappe Complex of theLower Allochthon (Fig. 1a,b; Bjør-lykke  et al. , 1976; Nystuen andSæther, 1979; Nystuen, 1981, 1987;Vidal and Moczydlowska, 1995). It ischaracterized by important lateralvariations in thickness and facies.Marine turbidites of the Brøttum For-mation form the base of the group inthe south. In the north-east, the cor-relative >2 km thick Rendalen For-mation consists of fluviatile, braidedstream, arkosic sandstone and con-glomerate. The Brøttum and Renda-len Formations are overlain by shaleand limestone of the Biri Formation,and proximal fluviatile sandstone toconglomerate of the Ring Formation.The <30 m thick Moelv Tillite capsthe Rendalen, Biri and Ring forma-tions, with a possible hiatus. It iscomposed of glacial diamictite andglaciomarine laminated shale with ice-dropped stones. The Moelv Tillite iseverywhere covered by a conformable,transgressive, generally 30–40 mthick, shale horizon, the Ekre Forma-tion. Where exposed, the transitionbetween the diamictites and shale ischaracterized by a gradual decrease inthe grain size of large clasts and ABSTRACT The Moelv Tillite is the Late Neoproterozoic Varanger glacialdeposit recorded in the Hedmark Group, SE Norway. PairedU–Pb and Lu–Hf data collected on detrital zircons in theRendalen Formation underlying the Moelv Tillite have identifiedan uncommon 677 ± 15 to 620 ± 14 Ma population, thatconstrain the deposition of the Moelv Tillite to be youngerthan 620 ± 14 Ma. The youngest detrital zircons may bederived from granite magmatism related to the 616 ± 3 MaEgersund dolerite magmatism, situated in the western part ofthe Sveconorwegian orogen. The Moelv Tillite, which is notoverlain by a cap carbonate, possibly correlates with the  c  .580 Ma Squantum-Gaskiers glacial deposits of Avalonia. Avail-able palaeomagnetic data for the Late Neoproterozoic suggestthat Baltica was located at intermediate to high latitudebetween 620 and 555 Ma. Terra Nova, 17, 250–258, 2005 Correspondence: Bernard Bingen, Geolo-gical Survey of Norway, Leiv Eirikssonsvei 39, 7491 Trondheim, Norway. Tel.: 0047 7390 4240; fax: 00 47 7396 1620; e-mail:bernard.bingen@ngu.no. 250    2005 Blackwell Publishing Ltd doi: 10.1111/j.1365-3121.2005.00609.x  matrix (Bjørlykke  et al. , 1976). Theyoungest exposed Vangsa ˚s Formationis overlain by a fossiliferous transgres-sive Cambrian sequence. Results Sampling and U–Pb analyses Sample B01104 is a typical fluviatilefacies of the Rendalen Formationcollected in Hanestad (Fig. 1a). Theoutcrop is poorly bedded arkose,showing common green to red shaleinterbeds and cobbles. The sample is apoorly sorted coarse-grained ( c. 1 mm) arkose (SiO 2  ¼  81.3 %) show-ing subangular clasts in a silt matrix.Detrital zircons are prismatic to roun-ded, and variably abraded and frag-mented because of transportation.Analytical work on zircon was per-formed at the Department of Earthand Planetary Sciences, MacquarieUniversity. Non-magnetic zirconswere mounted in epoxy. They wereimaged and analysed for Si, Zr, Hf,and Y in an electron microprobe.More than 95% of the zircons displaymagmatic prism-parallel oscillatoryzoning. U–Th–Pb geochronologicalanalyses were performed with laserablation inductively coupled plasmaquadrupole mass spectrometry (IC-PMS) on 73 crystals (Table 1). Ana-lytical procedures, standardization,data reduction, error propagationand common-Pb corrections are sum-marized in Belousova  et al.  (2001) andJackson  et al.  (2004). The age selectedfor geological interpretation is the 207 Pb/ 206 Pb age for zircons older than1000 Ma and the  238 U/ 206 Pb age foryounger zircons.All but two of the analysed zir-cons yield concordant to near-concordant U–Pb analyses rangingfrom 1839 ± 54 to 985 ± 18 Ma(65 grains, 2 r  uncertainty) and from685 ± 28 to 611 ± 26 Ma (six grains;Fig. 2a). The probability density plotof ages show peaks at 1606 ± 36,1483 ± 35, 1302 ± 57, 985 ± 18,677 ± 15and620 ± 14 Ma(Fig. 2b).The peak at 1.48 Ga largely domi-nates the distribution. The young-est peak is defined by the weightedaverage  206 Pb/ 238 U age of overlappinganalyses on two grains. The grainat 611 ± 26 Ma is a non-abra-ded, U-poor (24 ppm), weakly oscil-latory-zoned, prismatic (80  ·  250  l m)crystal and the other at 623 ± 16 Mais a rounded fragment (100  l m indiameter) of a probably large crystalshowing oscillatory zoning and amedium U content (167 ppm). Bothcrystals have typical magmatic Th/Uratios of 0.7–0.8. The moderate Ucontent of these grains, the occurrenceof prism-parallel zoning, the concor-dant behaviour and the absence of significant metamorphic overprint inthe rock suggest that the U–Pb sys-tematics have not been affected by StockholmBergenOsloTromsø100km60°5°Upper+UppermostAllochthonsDevonianOldRedSandstonePermianOsloPaleorift a Osen-RøanappecomplexHedmarkGroupLower+MiddleAllochthoncontinentalmarginofBalticaPrecambrianrocksofcratonicBalticaAutochthon+parautochthonplatformalsedimentsofcratonicBaltica B01104Varanger peninsulaVestertana GroupMortensnes Formation Egersund dykes SveconorwegianorogenFennoscandianshield B01104 collected inHanestad,Rendalen kommune,UTM coordinate: WGS84 zone 32x: 601531y: 6858950 Rendalen Fm>2000 m B01104<620±14MaBrøttum Fm>2000 mAtna FmBiskopås FmOsdalen FmBiri FmBiri FmRing FmMoelv FmMoelv FmEkre FmEkre FmVangsås FmVangsås Fm SouthHedmark GroupNortheast b 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 NappeComplex (Bjørlykke  et al. , 1976; Kumpulainen and Nystuen, 1985; Nystuen, 1987; Vidal and Moczydlowska, 1995). Terra Nova, Vol  17 , No. 3, 250–258 B. Bingen  et al  .  •  Timing of Late Neoproterozoic glaciation .............................................................................................................................................................   2005 Blackwell Publishing Ltd  251  -12-8-404812 Hf 170050070090011001300 T  (Ma) 1500 150013001100900700 0.060.080.103579 238206 U/Pb 207206 Pb/Pb 0.0150.020Average 2 error 176177 Lu/Hf0.010Sveconorwegianmagmatism inSveconorwegianorogen Sample B01104,Hemark Group, Rendalen FormationLaser ablation ICPMS data 73 detrital zircons2 error ellipses ac  37 detrital zircons : zircon derived from granitoid. : zircon derived from dolerite“Gothian” magmatismIdefjorden, Kongsberg and Bamble sectors b  71 detrital zircons 620   ±   14620±14 Maweightedaverage of2 crystals985   ±   18677   ±   15677±15 Maweightedaverage of3 crystalsAnalyses discarded for age probability density diagram1606   ±   39 Ma1302   ±   571483   ±   351.5 Ga magmatismTelemark sectorDetrital zircons“Gothian” sedimentsIdefjorden sector  ennosc ndi nshieldm gm tism Fennoscandianshieldmagmatism D e p l e t e d  mant l e  CHUR T  (Ma) 5007009001100130015001700 Fig. 2  U–Pb and Lu–Hf data on detrital zircons from sample B01104 of the Rendalen Formation. (a) Concordia diagram. (b) Ageprobability density diagram following Ludwig (2001). (c) Initial  e Hf vs. time diagram. Zircons derived from probable granite anddolerite sources are distinguished according to trace element content (U, Th, Y, Yb, Lu and Hf), following the method of Belousova  et al.  (2002). Initial  e Hf values were calculated using  k 176 Lu  ¼  1.865  ·  10 ) 11 yr ) 1 (Scherer  et al. , 2001). Chondriticreservoir (CHUR) and depleted mantle follow Griffin  et al.  (2000). Regional data in the Sveconorwegian orogen andFennoscandian shield are from Patchett  et al.  (1981), Vervoort and Patchett (1996) and Andersen  et al.  (2002, 2004).Mesoproterozoic zircon populations of sample B01104 are situated between the growth vectors ( 176 Lu/ 177 Hf   ¼  0.015) of the 1.6– 1.5 Ga   Gothian   juvenile magmatism exposed in the central part of the Sveconorwegian orogen and the Palaeoproterozoicmagmatic rocks exposed in the east of the Sveconorwegian orogen and in the Fennoscandian shield. The Hf isotopic signature of zircon is consistent with a Baltica provenance for the Rendalen Formation. Timing of Late Neoproterozoic glaciation  •  B. Bingen  et al  . Terra Nova, Vol  17 , No. 3, 250–258 ............................................................................................................................................................. 254    2005 Blackwell Publishing Ltd
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