TECTONICS, VOL. 18, NO. 6, PAGES 977-1003 DECEMBER 1999 Paleozoic rocks of northern Chukotka Peninsula, Russian Far East: Implications for the tectonicsof the Arctic region BorisA. Natal'in,1 Jeffrey M. Amato,2 Jaime Toro, 3,4 and James E. Wright5 Abstract. Paleozoicrocks exposedacross the northernflank of Alaskablock the essentialelement involved in the openingof the the mid-Cretaceousto Late CretaceousKoolen metamorphic Canada basin. domemake up two structurallysuperimposed tectonic units: (1) weaklydeformed Ordovician to Lower Devonianshallow marine 1. Introduction carbonatesof the Chegitununit which formed on a stableshelf and (2) strongly deformed and metamorphosedDevonian to Interestin stratigraphicand tectoniccorrelations between the Lower Carboniferousphyllites, limestones, and an&site tuffs of RussianFar East and Alaska recentlyhas beenrevived as the re- the Tanatapunit. Trace elementgeochemistry, Nd isotopicdata, sult of collaborationbetween North Americanand Russiangeol- and texturalevidence suggest that the Tanataptuffs are differen- ogists.This paperpresents the resultsof one suchstudy from the tiatedcalc-alkaline volcanic rocks possibly derived from a mag- ChegitunRiver valley, Russia,where field work was carriedout matic arc. We interpretthe associatedsedimentary facies as in- to establishthe stratigraphic,structural, and metamorphicrela- dicativeof depositionin a basinal setting,probably a back arc tionshipsin the northernpart of the ChukotkaPeninsula (Figure basin. Orthogneissesin the core of the Koolen dome yielded a 1). The ChegitunRiver regionexposes an Ordovicianto Lower Devonian(between ~369 and ~375 Ma) U-Pb zirconage which is Carboniferouscarbonate and metacarbonate,shale, phyllite, and similarto the agesof the Tanataptuffs as well as graniticplutons thin-beddedturbidite sequencewith rare interbeddedtuffs that formedwithin a Devonianactive continentalmargin of northern were metamorphosedand deformedduring the Cretaceous.This Alaska. The stratigraphyof the Chegitununit is similarto that of sequenceis in fault contact with a mid-Cretaceousto Late the Novosibirskcarbonate platform which overlies the Late Cretaceoushigh-grade metamorphic complex, the Koolendome. Precambrian Bennett-Barrovia block. The basement of the block The geologyof the high-gradecomplex was discussedby the is exposedin Chukotkawhere ortogneissin the ChegitunRiver Bering Strait GeologicalField Party (BSGFP) [1997]. valley yielded Late Proterozoic(•-650 to 550 Ma) U-Pb ages. Stratigraphiccorrelations between Paleozoic sequences in the These two tectonic units form the shelf of the Chukchi and East ChegitunRiver region and similar successionsin Chukotka,in Siberian Seas and may continue into northern Alaska as the the various islands of the Chukchi and East Siberian Seas,and in Hammond subterrane. The deep-waterTanatap unit can be northernAlaska bear on an importantunsolved problem for this tracedalong the southernboundary of the Bennett-Barroviablock region:the presenceor absenceof a large Precambrianblock on from the Novosibirsk Islands to northern Alaska. This basin was which theselower Paleozoiccarbonate platform depositsaccu- pairedwith a Devonianmagmatic arc that existedfarther to the mulated. south. The northernmargin of the Bennett-Barroviablock col- The Paleozoicframework of the regionsheds light on the ge- lidedwith NorthAmerica in the Late Silurianto Early Devonian. ometryand sizeof the Chukotka-ArcticAlaska microplate whose In Chukotka,during Middle to Late Carboniferoustime the re- Early Cretaceousdisplacement is thoughtto be responsiblefor constructedDevonian arc-trench system at the southernedge of the openingof the CanadaBasin (Figure 1) (see Lawyer et al. the Bennett-Barrovia block collided with an unknown continental [1990] for a review of the differentmodels proposed for the object,fragments of which now occurto the southof the South openingof the CanadaBasin). In this paperwe investigatethe Anyui suture.Triassic to Cretaceousdeformation strongly modi- correlation of Late Proterozioc and Paleozoic rocks of the fied the Paleozoicunits. Our resultsprovide new constraintson Chegitun River valley with rocks of similar ages in the the geometryand Paleozoichistory of the Chukotka-Arctic Siberian/northernAlaska sector of the Arctic region,the deposi- tional setting of these strata, and the geochemistryand geochronologyof Devonian magmatism on the Chukotka 1 Departmentof Geology,Istanbul Technical University, Istanbul, Peninsula. These data are then used to discuss the Paleozoic Turkey. tectonic evolution of northeastern Russia and northern Alaska. 2 Departmentof Geological Sciences, New Mexico State University, We will address the Cretaceous thermal evolution and structural Las Cruces. 3 Departmentof Geologicaland Environmental Sciences, Stanford historyof the ChegitunRiver valley metamorphicrocks in a later University,Stanford, California. paper. 4 Nowat Departmentof Geologyand Geography, West Virginia University,Morgantown. 5 Departmentof Geology and Geophysics, Rice University, Houston, 2. Tectonic Setting of the Study Area Texas. Prior to the now widely acceptedhypothesis of the counter- clockwise rotation of the Chukotka-Arctic Alaska block for 60 ø Copyright1999 by theAmerican Geophysical Union. abouta pole at the McKenzie delta (Figure 1) [e.g., Tailleur and Papernumber 1999TC900044 Brosge,1970], the idea of a Hyperboreanplatform underlain by 0278-7407/99/1999TC900044512.00 Precambrianbasement in the centralpart of the Arctic Oceanwas 977 978 NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA NATAL'IN ET AL.: PALEOZOIC ROCKS OF NORTHERN CHUKOTKA 979 proposedby Shatsky[ 1935] to explainthe sinuosityand disposi- igneousand sedimentaryprotoliths metamorphosed to upperam- tion of fold belts(e.g., Chukotkafold belt, the BrooksRange) and phiboliteand locally granulitegrade. The age of metamorphism high-grademetamorphic rocks in the Arctic region. Eardley was previouslyestimated to rangefrom Archean[Shuldiner and [1948] referred to this Precambrianplatform as "Ancient Nedornolkin,1976; Nedornolkin, 1977] to Paleozoic [Gnibidenko, Arctica•" Americangeologists proposed the existenceof an area 1969; Bunker et al., 1979] or with an additionalepisode in the called "BarroviaLand" to providea northemsource region for Mesozoic [Gelman, 1973; Natal'in, 1979], but it is now known to Paleozoicand Mesozoicclastic rocks in northemAlaska [e.g., haveoccurred during mid-Cretaceous time on both the Chukotka Tailleur, 1973]. However,if a rotationalorigin for the Chukotka- Peninsula [BSGFP, 1997] and on the Seward Peninsula [Arnatoet Arctic Alaska block abouta pole in the McKenzie delta is as- al., 1994; Arnatoand Wright, 1998]. sumed, the North American craton is a suitable source for these Paleozoicrocks and high-grademetamorphic complexes on sediments. Zonenshain et al. [1990] defined a microcontinent, the Chukotka Peninsulacomprise part of a broader tectonic "Arctida," as one of several Precambriancratons, such as the East provincein the RussianFar Eastand northernAlaska: the former Europeanand Siberiancratons, which was later disruptedowing Chukotka-ArcticAlaska microplate. The southernboundary of to the openingof the oceanicbasins in the Arctic Ocean. •engiir this ancientmicroplate is definedby the NeocomianSouth Anyui and Natal'in [1996] supportedthe necessityof an old continental suture in northeast Russia and the Late Jurassic to Neocomian block but definedit with a shapeand size differentfrom that of Kobuk suturein Alaska (Figure 1). For a review of the tectonics Arctida, They namedit after the Bennettmassif which is located of northeastAsia, seethe work of Parfenovand Natal'in [1985], in the northwesternpart of the East SiberianSea [Vinogradovet Zonenshainet al. [1990], Fujita and Cook [1990], and •enggir al., 1974] but notedthat it may be contiguouswith Precambrian and Natal'in [19%]. For a review of the tectonics of northern crustin the northeasternpart of the Chukchi Sea [Grantz et al., Alaska, see the work of Moore et al. [ 1994, 1997b]. 1990]. In this paperwe usethe term Bennett-Barroviablock for this Precambrian block. It should be stressed that the Bennett- Barrovia block is not the same as the Arctic Alaska terrane 3. Lithotectonic units of the Chegitun River Valley [Newmanet al., 1977;Moore et al., 1994].The latteris a younger tectonicelement not assembleduntil Devoniantime (seebelow). From the southeastto the northwest,three principal fault- The Chegitun River valley exposescarbonate and metacar- boundedlithotectonic units are exposedin the ChegitunRiver bonate rocks which we correlate with the early to middle valley (Figure3): (1) the High-Gradeunit, which consistsof Ul> PaleozoicNovosibirsk carbonate platform. The Novosibirskcar- per amphibolitefacies metamorphicrocks along the northern bonateplatform covers the Bennett-Barroviablock as definedby flank of the Koolen metamorphicdome; (2) the Tanatap unit, •engb'r and Natal'in [1996]. This platform includesrocks ex- which consistsof Devonianto Lower Carboniferousgreenschist posedon the Novosibirsk(New Siberian)Islands of the East faciesmetasedimentary rocks; and (3) the Chegitununit, which SiberianSea, on Wrangel Island, and in severallocalities of consists of virtually unmetamorphosedOrdovician through northernAlaska including the SewardPeninsula and the western Lower Devonianshallow marine carbonate rocks. It was previ- and centralBrooks Range (YM and HM, Figure 1). The wide ously believed that the high-grade rocks representedthe geographicdistribution of rocksof