U-Pb and Hf Isotope Analysis of Detrital Zircons from the Banks Island Assemblage (Coastal British Columbia) and Southern Alexander Terrane (Southeast Alaska)

U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska)

Clare J. Tochilin1, George E. Gehrels1,*, JoAnne Nelson2, J. Brian Mahoney3 1DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA 2BRITISH COLUMBIA GEOLOGICAL SURVEY, VICTORIA, BRITISH COLUMBIA, V8W 9N3, CANADA 3DEPARTMENT OF GEOLOGY, UNIVERSITY OF WISCONSIN, EAU CLARE, WISCONSIN 54702, USA

ABSTRACT

The Banks Island assemblage consists of regionally metamorphosed and deformed metasedimentary rocks that occur in a narrow belt between the Alexander and Wrangellia terranes in coastal British Columbia. In an effort to evaluate potential correlations with adjacent terranes, we conducted U-Pb and Hf isotope analyses on detrital zircons from quartz-rich metasedimentary rocks of the Banks Island assemblage and from Paleozoic strata of the southern Alexander terrane. U-Pb data from the Banks Island assemblage provide maximum depositional ages ranging from Ordovician to Permian; complementary Hf data demonstrate proximity to a continental landmass during accumulation of most strata. These ages and Hf isotope compositions are quite different from the more juvenile signatures of arc-type rocks that characterize the southern Alexander terrane, but they strongly resemble values previously reported from the northern Alexander terrane. We accordingly suggest that the Banks Island assemblage formed as part of the northern Alexander terrane and was offset southward by ~1000 km to now reside adjacent to the southern portion of the terrane. At least some of this motion was accommodated along the Early Cretaceous Kitkatla shear zone, which now forms the inboard margin of the Banks Island assemblage. Deformation and metamorphism of the Banks Island assemblage occurred prior to this sinistral motion, however, as crosscutting dikes yield U-Pb ages as old as ca. 156 Ma. Our data provide support for a circum-Arctic origin of the Alexander terrane, as suggested by many previous workers. U-Pb and Hf data for Ordovician and older rocks of the Banks Island assemblage and the northern Alexander terrane are similar to values from the Timanides, whereas Silurian–Lower Devonian rocks yield values shared with Caledonian rocks in Baltica and northern Greenland. Banks Island assemblage strata of suspected late Paleozoic age are more juvenile, perhaps recording motion of the Alexander terrane from the circum- Arctic into the paleo-Paciﬁ c realm.

LITHOSPHERE; v. 6; no. 3; p. 200–215; GSA Data Repository Item 2014168 | Published online 2 April 2014 doi: 10.1130/L338.1

INTRODUCTION southeastern Alaska. Woodsworth and Orchard GEOLOGIC SETTING (1985) and Wheeler et al. (1991) proposed that The Banks Island assemblage occurs as the Banks Island assemblage is part of the Alex- The Banks Island assemblage is an enig- metasedimentary pendants within the western ander terrane to the east, while van der Heyden matic assemblage in the Canadian Cordillera Coast Mountains batholith in coastal British (1989, 1992) suggested that these rocks were because it consists of quartz-rich (mature) Columbia (Fig. 1; Wheeler and McFeely, 1991; associated with the Wrangellia terrane to the metasedimentary rocks and has evolved Nd-Sr Boghossian and Gehrels, 2000; Gehrels and west. Gehrels and Boghossian (2000) and Bog- isotopic signatures (Boghossian and Gehrels, Boghossian, 2000; Gehrels et al., 2009). Most hossian and Gehrels (2000) raised the possibility 2000), which are in contrast to arc-type rocks exposures consist of highly deformed and meta- that the Banks Island assemblage is a fragment and juvenile Nd-Sr isotopic signatures of adja- morphosed metaclastic quartzite interbedded of the Yukon-Tanana terrane, displaced from cent terranes (Samson and Patchett, 1991). Con- ε with marble and subordinate metapelite (Rod- inboard of the Alexander terrane (Fig. 1), based tinental afﬁ nities are also suggested by the Hf(T) dick, 1970; Baer, 1973; Hutchison, 1982). Prior on the abundance of highly folded metaclastic values of plutonic rocks intruding the Banks to this study, the only constraint on protolith age quartzite and marble in both assemblages. Island assemblage, which are more evolved than was provided by the Late Jurassic–Early Creta- This study presents U-Pb ages and Hf isoto- plutonic rocks intruding the adjacent Alexander ceous age of surrounding plutons (van der Hey- pic data for 21 metasedimentary samples from terrane (Cecil et al., 2011). den,1989, 1992; Butler et al., 2006; Gehrels et the Banks Island assemblage and 10 sandstone Inboard of the Banks Island assemblage, al., 2009; Cecil et al., 2011). samples from the southern Alexander terrane in there are rocks of the southernmost Alexan- Several correlations have been proposed an effort to evaluate potential correlations and der terrane (Figs. 1 and 2). In contrast to the between the Banks Island assemblage and displacement histories. We also report U-Pb quartz-rich metasediments of the Banks Island other terranes in coastal British Columbia and ages on ﬁ ve crosscutting quartz diorite dikes to assemblage, the Alexander terrane consists pri- place a minimum age on the deformation and marily of arc-type volcanic, plutonic, and volca- metamorphism that affect rocks of the Banks niclastic rocks of Neoproterozoic–Silurian age, *Corresponding author: [email protected] Island assemblage. and Devonian–Triassic conglomerate, shale,

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

and Patchett, 1991). Additionally, Hf isotopic Skagway analyses of plutonic rocks intruding rocks of Coast 138ºW the Alexander terrane yield juvenile ε values Mountains Hf(T) batholith 58ºN ranging from +13 to +9 (Cecil et al., 2011). W-N Several theories have been proposed for the SEM origin of the Alexander terrane. Suggested ori- Chugach Juneau gins include the Klamath region of northern Cal- terrane SEAK ifornia (Jones et al., 1972), a location similar to BIA its current position followed by accordion-style 58ºN Gravina belt & W-S rifting and re-accretion (Churkin and Eberlein, Tyaughton-Methow 1977), the Austral-Asia region (Gehrels and S H Saleeby, 1987), and the peri-Gondwana/Appa- Petersburg lachian region (Wright and Wyld, 2006). Many Alexander terrane workers now favor an origin for the terrane in the Arctic realm on the basis of geologic, geo- Wrangellia terrane chronologic, paleomagnetic, and paleontologic K D data (Soja, 1994; Bazard et al., 1995; Gehrels et al., 1996; Blodgett et al., 2002; Soja and Kru- Banks Island Ketchikan tikov, 2008; Grove et al., 2008; Blodgett, 2010; assemblage M T N Miller et al., 2010, 2011; Colpron and Nelson,

W 2009, 2011; Beranek et al., 2013a, 2013b). º

Stikine terrane 128 East of the Alexander terrane, rocks are Bowser assigned to the Yukon-Tanana terrane, which Prince Rupert basin consists of Proterozoic–Lower Paleozoic Taku terrane quartz-rich metaclastic rocks overlain unconformably by Middle–Upper Paleozoic metavol- Queen canic and metasedimentary rocks (Gehrels et Port Houghton & Endicott Arm Charlotte al., 1992; Gehrels, 2001). Quartz-rich layers in assemblages Islands this terrane are similar lithologically to quartz- (Yukon-Tanana) ites in the Banks Island assemblage. Nd-Sr signatures of these rocks also point to a con- 52ºN Figure 2 Tracy Arm tinental afﬁ nity for this terrane (Samson and assemblage Patchett, 1991), as do the abundant Paleopro- (Yukon-Tanana) Pacific Bella Coola Ocean terozoic detrital zircons that occur in all assemblages of the Yukon-Tanana terrane (Gehrels et M DZ samples al., 1991, 1992; Gehrels, 2001). Strike-slip fault Directly west of the Banks Island assem- 52 124ºW Thrust fault ºN blage is the Wrangellia terrane, which con- Trace of Coast sists of Middle- to Upper Paleozoic arc-type Shear Zone metavolcanic and metasedimentary rocks overlain by Triassic–Jurassic volcanics (Wheeler ? et al., 1991). Like the Alexander terrane, these 0150 km rocks have juvenile Nd-Sr signatures, charac- teristic of assemblages formed in a volcanic arc Figure 1. Sketch map showing location of the Banks Island assemblage setting (Samson and Patchett, 1991). At present, relative to other ﬁ rst-order assemblages in the Canadian-Alaskan Cor- dillera (adapted from Wheeler and McFeely, 1991). Samples from the there are no available U-Pb or Hf data available Alexander terrane in southern and central SE Alaska are as follows: H— from detrital zircons of the Wrangellia terrane. Halleck Formation, S—Saginaw Bay Formation, K—Karheen Formation, The Alexander and Wrangellia terranes have D—Descon Formation, T—Tah Bay sandstone, M—Moira Inlet conglom- been together since late Paleozoic time (Gard- eratic sandstone, N—Nehenta Formation. On inset map: W-N—northern ner et al., 1988), forming the Insular terrane of portion of Wrangellia; W-S—southern portion of Wrangellia; SEM—Saint Monger et al. (1982). This composite terrane Elias Mountains portion of Alexander terrane; SEAK—SE Alaska portion of Alexander terrane; BIA—Banks Island assemblage. DZ—detrital zircon. may have been accreted to the western margin of North America during mid-Cretaceous time, resulting in the widespread metamorphic and magmatic events of this age observed limestone, rhyolite, and basalt (Woodsworth with a subordinate population of ca. 3.0–1.0 Ga throughout coastal British Columbia and south- and Orchard, 1985; Gehrels and Saleeby, 1987; grains (Gehrels et al., 1996; Gehrels and Bog- east Alaska (Monger et al., 1982; Crawford et Gehrels et al., 1996; Gehrels and Boghossian, hossian, 2000). Nd-Sr signatures in the southern al., 1987). McClelland and Gehrels (1990), 2000). Detrital zircons from strata of the Alex- Alexander terrane are very juvenile, as would be McClelland et al. (1992), and van der Heyden ander terrane yield U-Pb ages of 480–420 Ma, expected for an oceanic volcanic arc (Samson (1992) suggested that mid-Cretaceous colli-

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 201

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

Kitkatla Shear Zone number of grains of variable size. Unfortunately, zircon yields for most Banks Island assemblage Coast quartzites were low, and grains were generally Mountains Prince Rupert small (less than ~50 μm in length). batholith Zircon grains were mounted with fragments of our Sri Lanka primary zircon standard and Banks Island Absalom R33 secondary standard for U-Pb, as well as assemblage 128°W 87JBM09 Mud Tank, Temora, FC-1, 91500, R33, and Alexander Plesovice standards for Hf analysis (Gehrels et terrane al., 2008; Woodhead and Hergt, 2005; Sláma et 11TC01 al., 2008; Cecil et al., 2011). For U-Pb analy- Gravina belt sis, individual grains were selected at random 11TC02 11TC03 and analyzed by laser ablation–multicollector– inductively coupled plasma–mass spectrometry Yukon- 11TC04 & 05 Tanana (LA-MC-ICP-MS), as described by Gehrels et Union & 11JN1009 terrane Banks & al. (2008). Grains too small for a 30 μm spot 11TC07 were analyzed with a 12 μm beam diameter Stikine 04GJP54 07GJ201 using ion counters for Pb isotopes. terrane 54°N Images of zircon grains were acquired prior 11TC09-10 11TC14 to U-Pb analysis using a Hitachi 3400N scan- 11TC13 ning electron microscope (SEM) equipped with 11TC11 & 12 assemblage a Gatan Chroma cathodoluminesence (CL) boundary 07GJ208 system at the University of Arizona (www.geo- arizonasem.org). CL imaging was conducted on Coast all igneous and most detrital samples to identify shear 11TC15 & 16 11TC17 grains suitable for U-Pb analysis and to char- zone 11TC18,19, acterize the internal structures and domains of 07GJ231, each grain. In two Banks Island assemblage 10JN0201, 02 detrital samples (11TC15 and 11TC16), nearly Sample 126°W all of the zircon grains were very small (less than 11TC20 50 μm in length) and highly altered. For these Bella samples, high-resolution backscattered electron Bella (BSE) imaging provided a clearer display of each grain’s internal structure. These CL and

05025 Bella Coola BSE images were used to guide the placement of analytical locations on each zircon grain, as km 52°N many grains contained complex zoning patterns, inherited cores, or high levels of alteration. Figure 2. Sketch map of the Banks Island assemblage and sample localities (adapted from The U-Pb data from many of the Banks Wheeler and McFeely, 1991). Island assemblage metasedimentary samples are complicated due to the presence of zircon grains that yield ages similar to the crosscut- sional accretion was preceded by initial juxta- graphic position of the metasedimentary sam- ting dikes, or ages that are older but highly position during mid-Jurassic time, followed by ples is shown on Figure 3, based in large part on discordant. The young grains are interpreted to extension to form the Late Jurassic–Early Cre- the ages of detrital zircon grains (Figs. 4 and 5) be igneous in origin because they have mod- taceous Gravina basin (Fig. 1). and cross-cutting dikes (Figs. 6 and 7). Data erate U concentration, low U/Th, euhedral to from the Banks Island assemblage are reported subhedral shape, and typical oscillatory CL Samples and Methods in Tables DR1 and DR21, whereas data from the zoning, and they are coeval with the known or Alexander terrane are reported in Tables DR3 suspected age of nearby dikes or plutons. Such Twenty-one samples of metasedimentary and DR4. grains are interpreted to have been derived rocks, ﬁ ve samples of quartz diorite/tonalite Samples were processed using standard from narrow dikelets of igneous rock that from crosscutting dikes, and one sample of methods of zircon separation (Gehrels, 2000a; were not recognized during sample collection. quartz dioritic orthogneiss were collected from Gehrels et al., 2008). Samples of quartz diorite These young ages are retained in Table DR1 outcrops of the Banks Island assemblage for and of sandstones from the Alexander terrane (see footnote 1) but are omitted from discus- U-Pb and Hf isotope analysis. Ten additional yielded typical zircon separates, with a large sions of detrital ages. samples of sandstone were collected and ana- Several Banks Island assemblage samples lyzed from the southern Alexander terrane for 1GSA Data Repository Item 2014168, Tables DR1– yielded analyses that are analytically discordant comparison. Sample locations are shown on DR4, is available at www.geosociety.org/pubs/ft2014 from grains that have high (e.g., >500 ppm) U .htm, or on request from [email protected], Figures 1 and 2, and locations and descriptions Documents Secretary, GSA, P.O. Box 9140, Boulder, concentrations, high (e.g., >5) U/Th, anhedral are provided in Table 1. The interpreted strati- CO 80301-9140, USA. form, and irregular zonation in CL images. Such

202 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

TABLE 1. LOCATIONS AND DESCRIPTIONS OF SAMPLES ANALYZED FROM and acquiring less than 60 s of data in cases THE BANKS ISLAND ASSEMBLAGE AND FROM THE ALEXANDER TERRANE OF SE ALASKA where the laser pit intersected the bottom of Sample Location Lat. Long. Description the grain. (°N) (°W) For interpretation of the Hf data, analyses Banks Island igneous samples that are near depleted mantle are referred to as 11TC03 Banks Island 53.44111 129.97748 Quartz-rich orthogneiss juvenile, and the magma from which the zircons 11TC05 Pitt Island 53.40513 129.84851 Quartz diorite crystallized must have consisted largely of juve- 11TC09 Labiniere Island 52.99842 129.53704 Quartz diorite dike crosscutting 11TC10 nile crustal material. Analyses that are farther 11TC12 Aristazabal Island 52.78245 129.28756 Quartz diorite dike crosscutting 11TC11 from depleted mantle are referred to as evolved, 07GJ201 Gil Island 53.09518 129.26845 Quartz diorite crosscutting garnet-bearing schist and the magmatic source materials are inter- 07GJ208 Aristazabal Island 52.68090 129.03918 Foliated quartz diorite with bluish quartz preted to have consisted largely of older crustal phenocrysts materials (e.g., Reimann et al., 2010; Bahlburg Banks Island detrital samples et al., 2011). 11TC01 Banks Island 53.57634 130.27612 Thin interbedded quartzite and laminated siltstone RESULTS FROM THE BANKS ISLAND 11TC02 Ralston Islands 53.46604 129.95819 Quartz-rich layers interbedded with thin ASSEMBLAGE marble layers 11TC04 Pitt Island 53.40513 129.84851 Mudstones and quartz-rich metasediments 11TC07 Banks Island 53.16442 129.78989 Alternating quartz-rich and mudstone layers U-Pb and Hf of Igneous Samples 11TC10 Labiniere Island 52.99842 129.53704 Thinly bedded calcareous siltstone, quartzite, marble U-Pb analyses of igneous zircons from ﬁ ve 11TC13 Princess Royal Island 52.94947 129.19446 Centimeter-scale quartz-rich layers interbedded with marble crosscutting dikes yield Late Jurassic–Early 11TC14 Princess Royal Island 52.95525 129.19623 Interbedded quartzite, siltstone, and marble Cretaceous ages (Table DR1). The ages (and 11TC15 Higgins Pass 52.48158 128.72826 Thinly bedded quartzite and marble 2σ uncertainties) for these dikes, from oldest to 11TC16 Higgins Pass 52.47613 128.7515 Folded quartzite layers within massive youngest, are 156.2 ± 2.5 Ma (11TC09), 149.6 marble ± 1.4 Ma (11TC03), 144.4 ± 2.8 Ma (11TC05), 11TC17 Nowish Islands 52.50909 128.42728 Interbedded quartzite, siltstone, and marble 11TC18 Swindle Island 52.44433 128.48626 Interbedded quartzite and mudstone; 121.5 ± 1.7 Ma (07GJ201), and 116.0 ± 2.3 Ma no marble (11TC12). Outcrop photos of these dikes are 11TC19 Swindle Island 52.45781 128.4786 Interbedded quartzite and marble shown in Figures 6A (sample 11TC05), 6B 11TC20 Athlone Island 52.18798 128.32777 Interbedded quartzite and laminated (sample 11TC09), and 6C (sample 07GJ201). siltstone These ages are generally younger eastward 04GJ54 Trutch Island 53.02046 129.59639 Quartz-rich layers interbedded with thin marble layers across the study area, and they are a good match 07GJ231 Jorkins Point 52.44698 128.48085 Quartzite-clast sedimentary breccia for the ages of surrounding plutons (Gehrels et 87JBM09 Dolphin Island 53.90326 130.27723 Metamorphosed sandstone al., 2009). Our sample of quartz dioritic orthog- 11JN10-09 Union Passage 53.37303 129.45838 Interbedded quartzite and marble neiss (07GJ208) yields concordant analyses that Union Union Passage 53.38278 129.45828 Interbedded quartzite and marble result in a weighted mean age of 359.1 ± 3.9 Ma Absalom Absalom Island 53.83823 130.60582 Conglomeratic sandstone σ 10JN0201 Jorkins Point 52.44704 128.48102 Sandstone matrix of quartzite-clast (2 ; Table DR1). sedimentary breccia Hf isotopic analyses from crosscutting dikes 10JN0202 Jorkins Point 52.44573 128.48555 Interbedded quartzite and marble ε − yield Hf(T) values that range from +8 to 10 (with Alexander terrane detrital samples one grain at −32) for grains that are interpreted Moira Prince of Wales Island 55.09892 132.08046 Conglomeratic sandstone to record the age of crystallization (Fig. 7; Table Descon Heceta Island 55.73667 133.49833 Coarse sandstone ε DR2). As shown on Figure 7, the Hf(T) values Saginaw Kuiu Island 56.86255 134.19151 Coarse arkosic sandstone ε for these dikes overlap with the Hf(T) values for Karheen 1 Heceta Island 55.795 133.310 Pebbly sandstone plutons that intrude the Banks Island assemblage Karheen 2 Heceta Island 55.58166 133.29167 Pebbly sandstone (from Cecil et al., 2011) and also extend to con- Karheen 3 Heceta Island 55.79410 133.31084 Pebbly sandstone Tah Prince of Wales Island 54.86361 132.34929 Coarse arkosic sandstone siderably more negative values. Inherited grains Halleck 1 Keku Islets 56.93013 134.13426 Pebbly sandstone from sample 11TC05, which are commonly seen Halleck 2 Keku Islets 56.92311 134.13044 Pebbly sandstone ε as cores in CL images, yield variable Hf(T) values Nehenta Gravina Island 55.16333 131.78333 Pebbly sandstone (Fig. 7).

U-Pb and Hf of Metasedimentary Samples

grains are interpreted to be metamorphic in ori- Hf analyses were conducted utilizing the Most Banks Island assemblage metasedi- gin, and they are accordingly omitted from dis- analytical methods described by Cecil et al. mentary samples yield one dominant peak of cussions of detrital ages (but they are reported (2011) and Gehrels and Pecha (2014), and full Paleozoic age and scattered older grains (Fig. 4; in Table DR1). Hf analytical data can be found in Tables DR2 Table DR1). Because we have no stratigraphic In contrast, U-Pb analyses from strata of the and DR4. It should be noted that the uncertain- information about the units sampled, we order Alexander terrane are not complicated by the ties for many analyses of Banks Island assem- samples in the following discussion, and on Fig- presence of such young grains, and data inter- blage zircons are large because of the small ures 3 and 4, in terms of the age of the peak in pretation is straightforward. These age data are size of most zircon grains. This required using probability density of the main age group. Given reported in Table DR3 (see footnote 1). a small laser spot (30 μm) for most analyses that this main age group generally contains the

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 203

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

Alexander terrane Banks Island Saint Elias Central Southern Assemblage Mountains SE Alaska SE Alaska 201 RIFT 11TC13 ENVIRONMENT Nehenta Triass 11TC20 Union 11JN10-09 252 ?? STABLE Perm MARINE

Disturbance? 299 ?? Halleck 1 & 2 Penn Klawak 323

Miss 07GJ208 11TC02 STABLE 11TC17 359 MARINE 87JBM09, 11TC01 ?? Devon Karheen 1, 2, & 3 Icefield Assemblage KLAKAS OROGENY 419 Saginaw Tah Sil 443 Goatherd Assemblage MAGMATIC Ordo Descon ARC

485 Donjek ?? Assemblage Moira 04GJ54 11TC18 Banks WALES OROGENY Camb 11TC07 11TC15 11TC14 11TC19 541 11TC10 10JN0201 10JN0202 MAGMATIC 07GJ231 ARC (Ma) Protero ??

plutonic felsic mafic shale & limestone conglomerate sandstone chert rock volcanic volcanic siltstone rocks rocks Figure 3. Schematic columns showing stratigraphy and interpreted sample positions for the Banks Island assemblage (this study), strata of the Alexander terrane in SE Alaska (Gehrels and Saleeby, 1987), and strata of the Alexander terrane in the Saint Elias Mountains (Beranek et al., 2012, 2013a, 2013b). Stratigraphy of the Banks Island assemblage is interpreted largely from the U-Pb geochronology and Hf isotope data in this study.

youngest grains from each sample, this essen- (1) Two samples (JN0201 and 07GJ231; gray (3) Three samples (11TC07, Banks, and 04GJ54; tially orders the samples in terms of maximum shading on Figs. 4 and 8) contain only Pre- purple shading on Figs. 4 and 8) are domi- depositional age. cambrian grains. nated by 450–438 Ma grains (Fig. 4) that Samples are further divided into ﬁ ve differ- (2) Six samples (JN0202, 11TC10, 11TC19, yield highly evolved Hf isotopic signatures ent groups on the basis of observed peak ages 11TC14, 11TC15, and 11TC18; blue shad- (Fig. 8) and also contain signiﬁ cant popula- (Fig. 4) and Hf isotopic compositions (Fig. 8). ing on Figs. 4 and 8) are dominated by 500– tions of Paleoproterozoic grains (Fig. 4). The primary distinguishing characteristics of 450 Ma grains (Fig. 4) that yield moderately (4) Four samples (11TC01, 87JBM09, 11TC17, each group are as follows: evolved Hf isotope compositions (Fig. 8). and 11TC02; red shading on Figs. 4 and 8)

204 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH >500 Ma (n=198) <400 Ma (n=248) 450-438 Ma (n=274) 500-450 Ma (n=367) 438-400 Ma (n=337) Detrital Zircon Age (Ma) Age Detrital Zircon 478 Ma 434 Ma 442 Ma 409, 367, 307 Ma 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 the Banks from grains zircon detrital for diagrams distribution age Composite 5. Figure fac- of zigzags) enhanced by right (to of older grains with proportions Island assemblage, the divisions to combined according Samples are grains. younger to relative of four tor for indicated analyses are and number of constituent The peak ages 4. in Figure shown distribution. age each Banks (93) Union (55) 11TC13 (78) 11TC13 (92) 11TC17 11TC07 (83) 11TC07 11TC10 (91) 11TC10 JN0202 (65) 11TC20 (30) 11TC20 (91) 11TC02 11TC01 (60) 11TC01 11TC18 (9) 11TC18 JN0201 (92) 04GJ54 (98) 11TC15 (18) 11TC15 (70) 11TC14 07GJ231 (64) 11TC19 (112) 11TC19 87JBM09 (94) 11JN1009 (85) Detrital Zircon Age (Ma) Age Detrital Zircon 372 Ma 306 Ma 375 Ma 434 Ma 431 Ma 443 Ma 462 Ma 437 Ma 439 Ma 388 Ma 416 Ma 479 Ma 447 Ma 467 Ma 462 Ma 486 Ma 1119 Ma 477 Ma 1019 Ma 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Figure 4. Normalized age distribution diagrams for detrital zircon grains from the Banks from grains zircon detrital for diagrams distribution age Normalized 4. Figure proportions all samples, For sample. each for noted peaks are Main age Island assemblage. (locations of cut- grains younger to relative of two factor enhanced by are of older grains of the dominant clus- is based on the peak age Sample order zigzags). black by shown offs sample. each for shown are The number of analyses and the peak age sample. in each ter

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 205

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

A B

C D

E F

Figure 6. Photographs of typical lithic types in the Banks Island assemblage. (A) Crosscutting quartz diorite dike with age of 144.4 ± 2.8 Ma (sample 11TC05). (B) Crosscutting quartz diorite dike with age of 156.2 ± 2.5 Ma (sample 11TC09). (C) Crosscutting quartz diorite dike with age of 121.5 ± 1.7 Ma (sample 07GJ201). (D) Typical interbedded quartzite and marble of the Banks Island assemblage (sample 11TC14). (E) Typical folds in the Banks Island assemblage. (F) Sedimentary breccia sampled as 07GJ231 and 10JN0201.

206 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

20 DM average 15 uncertainty

10 x x x xxx x xx x xx xxxx x Figure 7. Hf isotope compositions of crosscut- 5 xx x x xx x ting quartz diorite dikes, of young analyses in x x xx CHUR x xxx metasedimentary assemblages, and of sur- 0 xx rounding plutons (from Cecil et al., 2011). DM— x xxxxx x depleted mantle, CHUR—chondritic uniform xxx -5 xx x reservoir. Overlap of the three data sets sup- xxx xx ports the interpretation that some metasedi- xxx -10 x x mentary samples contain veins, dikelets, or pods of igneous material. Average uncertainty of all analyses shown is 3.6 epsilon units (at 2σ). -15 11TC03 11TC05 Gray arrow shows interpreted crustal evolution trajectory assuming present-day 176Lu/177Hf = 11TC09 0.0115 (Vervoort and Patchett, 1996; Vervoort et -20 11TC12 al., 1999). 07GJ201 -25 tal evolution x Detrital average crus Plutons

-30 Epsilon Hf Detrital Zircon Age (Ma) -35 0 100 200 300 400 500 600 700 800 900 1000

20 average 15 uncertainty

10 DM 5

0 CHUR

-5 Figure 8. Hf isotope data from metasedimentary -10 units of the Banks Island assemblage. Samples are divided according to the scheme shown in Figure 4. Probability plots are from Figure 5 -15 (with heights of older grains enhanced by 2× Peak Ages relative to younger grains). Average uncertainty -20 <400 Ma of all analyses shown is 3.4 epsilon units (at 2σ). 438-400 Ma DM—depleted mantle, CHUR—chondritic uni- rustal evolution 450-438 Ma form reservoir. Gray arrow shows interpreted -25 age c 500-450 Ma crustal evolution trajectory assuming present- aver 176 177 Epsilon Hf >500 Ma day Lu/ Hf = 0.0115 (Vervoort and Patchett, -30 1996; Vervoort et al., 1999).

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Detrital Zircon Age (Ma)

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 207

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

ε are dominated by 438–400 Ma grains (Fig. 4) TABLE 2. U-Pb PEAK AGES AND AVERAGE HF(T) older grains. Samples with younger than 400 that yield moderately juvenile Hf isotope VALUES FOR SAMPLES FROM THE BANKS Ma peaks ages have moderately juvenile Hf iso- ISLAND ASSEMBLAGE, ALEXANDER TERRANE IN ε compositions (Fig. 8). SE ALASKA, AND ALEXANDER TERRANE IN THE tope compositions (Fig. 8) with an average Hf(T) (5) Four samples (11JN10–09, Union, 11TC20, SAINT ELIAS MOUNTAINS value of +7.0 for younger than 500 Ma grains and 11TC13; shown with green shading on Sample U-Pb peak age Average (Table 2). ε Figs. 4 and 8) contain signifi cant popula- (Ma) Hf(T) value Absalom is an additional sample that yields tions of younger than 400 Ma grains (Fig. 4) Banks Island assemblage a single dominant age group of 138–100 Ma with mostly juvenile Hf isotope composi- <400 Ma (peak ages) 409–367–307 +7.0 (age peak of 117 Ma) and subordinate groups of tions (Fig. 8). 438–400 Ma (peak ages) 434 +6.6 168–150 Ma (age peak of 160 Ma) and 452–427 Characteristics of samples in each of these 450–438 Ma (peak ages) 442 –12.5 Ma (peak age of 437 Ma; Table DR1). This unit groups are described in more detail in the fol- 500–450 Ma (peak ages) 478 –0.3 consists of granite-cobble conglomerate (with lowing. Alexander terrane in Saint Elias Mountains rounded clasts to 30 cm in diameter), which is The oldest interpreted group consists of two Middle Devonian 420 +4.9 not recognized elsewhere in the Banks Island samples (JN0201 and 07GJ231) that lack Paleo- Silurian–Lower Devonian 443 –7.3 assemblage. zoic grains and yield a nearly continuous distri- Cambrian–Ordovician 477 +5.8 Hf isotope compositions were also deter- bution of ages between 1.8 and 1.0 Ga (black Alexander terrane in SE Alaska mined for zircon grains in the detrital samples curves, Fig. 4). When the two age distributions Devonian 432 10.7 that yield anomalously young U-Pb ages. These are combined, there are three main age groups Ordovician 463 11.1 analyses are plotted on Figure 7 for comparison of 1.82–1.58 Ga, 1.53–1.30 Ga, and 1.19–0.98 Note: U-Pb peak ages are the peak in age prob- with Hf isotopic data from dikes and plutons ability from all younger than 500 Ma ages from each Ga and two smaller groups of 2.87–2.54 and ε ε that intrude the metasedimentary rocks. The group. Average Hf(T) values are the average Hf(T) 2.02–1.87 Ga (Fig. 5). These samples also dif- values of all younger than 500 Ma grains analyzed similarity of ages and Hf isotope signatures of fer from the others in terms of lithology—they from strata belonging to each group. Banks Island as- the three sets of zircons lends support to the are from a sedimentary breccia that consists of semblage analyses are from this study. Alexander ter- interpretation that the anomalously young zir- rane in Saint Elias Mountains data are from Beranek clasts of quartzite and subordinate marble and et al. (2013a, 2013b). Alexander terrane in SE Alaska con grains were derived from narrow dikelets amphibolite (Fig. 6F). Clasts reach up to 80 cm data are from this study. of igneous rock that were not recognized during in length and are hosted in a quartz-rich sandy sample collection. matrix. Sample 07GJ231 consists mainly of quartzite clasts, whereas sample JN0201 con- 450 Ma samples. Collectively, these samples Results from the Alexander Terrane in sists mainly of matrix sandstone; there appears yield a single dominant age group of 476–418 Southeast Alaska to be no signifi cant difference in age distribu- Ma, with an age peak of 442 Ma (Fig. 5). The tion between the clasts and matrix. Hf isotope one sample from this group that has been ana- U-Pb analyses have been conducted on sand- compositions are juvenile to evolved for older lyzed for Hf isotopes (sample 11TC07) yields a stones of Ordovician (n = 2), Silurian (n = 1), ε than 1.6 Ga and younger than 1.2 Ga grains and range of Hf(T) values for Precambrian grains and Devonian (n = 4), Permian (n = 3), and Triassic somewhat more juvenile for 1.6–1.2 Ga grains highly evolved values for early Paleozoic grains (n = 1) age. Many of these samples were ana- ε (Fig. 8). (Fig. 8). The average Hf(T) value for the younger lyzed previously by isotope dilution–thermal The next youngest samples are those with than 500 Ma grains is −12.5 (Table 2). ionization mass spectrometry (ID-TIMS; results maximum depositional ages between 500 and Four samples are shown in red shading reported by Gehrels et al., 1996), and several 450 Ma (blue curves, Fig. 4). Four of these on Figure 4 (11TC01, 87JBM09, 11TC17, have also been analyzed by LA-ICP-MS (results samples (JN0202, 11TC10, 11TC14, 11TC15) 11TC02), with peak ages between 438 and 400 reported by Grove et al., 2008). U-Pb data from also contain grains of ca. 800 Ma, whereas two Ma. These samples yield a dominant age group all analyses are summarized on Figure 9, and samples (11TC19 and 11TC18) do not. Col- of 471–381 Ma with a peak age of 434 Ma (Fig. reported in Table DR3. Hf isotope analyses lectively, these six samples yield a dominant 5). Hf isotopic compositions for the dominant have been conducted on six of the 10 samples, age group of 542–428 Ma (peak age of 478 Paleozoic grains are moderately juvenile (Fig. with results shown on Figure 10 and reported in ε Ma), a subordinate age group of 870–710 Ma 8), with an average Hf(T) value of +6.6 (Table 2). Table DR4. (peak age of 798 Ma), and scattered older than Finally, four samples (11JN10-09, Union, As shown on Figure 9, all samples are domi- 1.0 Ga grains (Fig. 5). Figure 6D is an exam- 11TC20, and 11TC13) yield age peaks between nated by peaks of Ordovician–Silurian age, ple of banded quartzite and marble, typical of 388 and 358 Ma (green curves, Fig. 4). In one which is not surprising given the abundance most Banks Island assemblage outcrops, from sample (11TC20), this is the dominant age of plutonic and volcanic rocks of this age in sample 11TC14. Hf isotope compositions from group, whereas another sample (11TC13) con- the southern Alexander terrane (Gehrels and these samples are mainly juvenile to somewhat tains a younger age peak of 306 Ma, and two Saleeby, 1987). Ordovician strata yield age evolved for Precambrian grains and moderately samples (11JN10–09 and Union) contain older peaks of 558 Ma (Neoproterozoic) and 463 Ma, ε − evolved (average Hf[T] of 0.3) for younger than age peaks of 458 Ma and 409 Ma, respectively whereas Silurian, Devonian, and Permian strata 500 Ma grains (Fig. 8; Table 2). (Fig. 4). This group is accordingly different yield nearly identical age peaks of 432–429 Ma. Next youngest are three samples (11TC07, from the others in that there is signifi cant varia- Devonian strata also have minor proportions of Banks, and 04GJ54) that yield maximum depo- tion in the age distribution of the four constitu- Precambrian grains that are commonly pinkish sitional ages between 450 and 438 Ma. These ent samples. When combined, the samples yield in color, rounded in morphology, and smaller samples also contain a signifi cant fraction of three age groups of 475–394 Ma (peak age of in size. These grains were oversampled during Precambrian ages (purple curves, Fig. 4), partic- 409 Ma), 392–350 Ma (peak age of 367 Ma), grain selection and are shown with expanded ularly Paleoproterozoic, in contrast to the Meso- and 324–280 Ma (peak age of 307 Ma) (Fig. vertical scale on Figure 9. Permian strata have proterozoic to Neoproterozoic grains in the 500– 5). Most of these samples also contain scattered additional age peaks of 366 and 288 Ma and a

208 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

signiﬁ cant proportion of older grains. Our one 417 Ma sample of Triassic strata yields only one age peak of 417 Ma, which is not surprising given that the conglomeratic sandstone rests unconformably on a trondhjemite pluton of similar age. Hf data for detrital zircons from these strata Triassic (92) are shown on Figure 10. Unfortunately, few 431, 366, & 288 Ma analyses could be conducted on Precambrian grains from the Devonian strata because of their Permian (326) small size. The Hf isotope compositions for younger than 600 Ma grains are mostly juvenile, 432Ma which is consistent with the juvenile Hf signature of zircon grains from nearby plutons (Cecil ε Devonian (420) et al., 2011; Fig. 10). The average Hf(T) values for detrital grains are +11.1 for Ordovician 429 Ma strata and +10.7 for Devonian strata (Table 2).

Comparison with Alexander Terrane in Saint Elias Mountains Silurian (91) U-Pb geochronologic and Hf isotopic data 558 & 463 Ma are also available from the northern Alexander terrane in the Saint Elias Mountains of northern British Columbia and Yukon (Fig. 1). Beranek Ordovician (170) et al. (2013a) reported data from Cambrian– Ordovician strata of the Donjek assemblage, 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 and Beranek et al. (2013b) reported data from Detrital Zircon Age (Ma) Silurian–Lower Devonian and Middle Devonian Figure 9. Normalized age distribution diagrams of U-Pb ages from Ordovician, Silurian, Devonian, strata of the Icefi eld assemblage (Figs. 1 and 3). Permian, and Triassic strata from the Alexander terrane in southeastern Alaska. Abundance of older The data from strata of known age in the than 800 Ma grains (to right of zigzags) is exaggerated by a factor of 20 relative to younger than Saint Elias Mountains provide critical insights 800 Ma grains. into the terrane affi liation of the Banks Island assemblage, and the ages of the metasedimentary units sampled. As shown on Figures 11 and 12, both data sets show generally similar patterns of U-Pb ages and Hf isotope compositions. Of par- ticular signifi cance is the pronounced negative 20 average excursion of Hf isotope compositions displayed Igneous uncertainty by zircons from strata of Silurian–Early Devo- 15 Detrital nian age from the Saint Elias Mountains and from strata with 450–438 Ma peak ages in the 10 Banks Island assemblage (Fig. 12). As reported DM on Table 2: (1) Saint Elias Mountains strata of 5 Cambrian–Ordovician age and Banks Island assemblage strata with 500–450 Ma peak ages 0 yield peak ages of 477 and 478 Ma (respectively) CHUR ε and moderately juvenile average Hf(T) values of +5.8 and −0.3 (respectively); (2) Saint Elias -5 Mountains strata of Silurian–Early Devonian age and Banks Island assemblage strata with peak -10 average crustal evolution ages of 450–438 Ma yield similar peak ages of Epsilon Hf Detrital Zircon Age (Ma) 443 and 442 Ma and moderately evolved average ε values of −7.3 and −12.0 (respectively); and 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Hf(T) (3) Saint Elias Mountains strata of Middle Devo- Figure 10. Hf isotope composition of detrital zircon grains from Ordovician through Triassic strata nian age and Banks Island assemblage strata of the Alexander terrane in SE Alaska. Also shown are Hf isotope data for nearby early Paleozoic with 438–400 Ma peak ages yield similar peak plutons (Cecil et al., 2011). Average uncertainty of all detrital analyses is 2.6 epsilon units (at 2σ). ε ages of 420 and 434 Ma and average Hf(T) values DM—depleted mantle, CHUR—chondritic uniform reservoir. Gray arrow shows interpreted crustal of +4.9 and +6.6 (respectively). 176 177 evolution trajectory assuming present-day Lu/ Hf = 0.0115 (Vervoort and Patchett, 1996; Ver- The remarkable similarity of these patterns voort et al., 1999). lends strong support to previous interpretations

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 209

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

10 DM 5

Figure 11. Hf isotope composition of detrital zir- 0 CHUR cons from the Banks Island assemblage and the -5 Alexander terrane in the Saint Elias Mountains. Data from the Saint Elias Mountains are from Beranek et al. (2013a, 2013b). DM—depleted -10 mantle, CHUR—chondritic uniform reservoir. Gray arrow shows interpreted crustal evolution 176 177 -15 trajectory assuming present-day Lu/ Hf = 0.0115 (Vervoort and Patchett, 1996; Vervoort et Banks Island Saint Elias al., 1999). -20 Assemblage strata <400 Ma 438-400 Ma Middle Dev -25 Epsilon Hf 450-438 Ma Sil-Lower Dev 500-450 Ma Camb-Ordo -30 average crustal evolution >500 Ma

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Detrital Zircon Age (Ma)

15 DM 10

0 CHUR Figure 12. Hf isotope composition for young grains shown on Figure 11 and also data from -5 the Alexander terrane of SE Alaska. Data from the Saint Elias Mountains are from Beranek -10 et al. (2013a, 2013b). DM—depleted mantle, CHUR—chondritic uniform reservoir. Gray arrow shows interpreted crustal evolution trajectory -15 assuming present-day 176Lu/177Hf = 0.0115 (Ver- voort and Patchett, 1996; Vervoort et al., 1999). -20 on average crustal evoluti

-25 Banks Island Saint Elias SE Alaska Assemblage strata strata Epsilon Hf 438-400 Ma Middle Dev -30 450-438 Ma Sil-Lower Dev 500-450 Ma Camb-Ordo

350 400 450 500 550 600 650 700 750 800 850 900 Detrital Zircon Age (Ma)

210 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

(e.g., Wheeler and McFeely, 1991) that the Banks 443 Ma Island assemblage is a portion of the Alexander terrane. These patterns also suggest that Banks Saint Elias Mountains Island assemblage strata with 500–450 Ma peak (Silurian-Lower Devonian) ages are Cambrian–Ordovician, strata with 450– 4x 438 Ma peak ages are Silurian–Lower Devonian, 431 Ma and strata with 438–400 Ma peak ages are Middle Southeast Alaska Devonian. The stratigraphic assignments shown (Silurian-Lower Devonian) for Banks Island assemblage strata on Figure 3 20x are based largely on these interpretations. These 434 Ma patterns also ﬁ t a simple pattern geographically, with maximum depositional ages that generally Banks Island Assemblage (450-438 Ma age peaks) are younger northward. 2x It is interesting that a similar negative deﬂ ec- ε 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 tion in Hf(T) values is not apparent in Lower Devonian strata from the Alexander terrane in Detrital Zircon Age (Ma) ε SE Alaska (Fig. 12), where average Hf(T) values Figure 13. Normalized age distribution diagrams of U-Pb ages from strata of known or suspected from Lower Devonian strata are all quite juve- Silurian–Early Devonian age from the Banks Island assemblage and the Alexander terrane of SE nile (Table 2). Instead, increased involvement Alaska and the Saint Elias Mountains. Peak ages are indicated for each distribution. The relative of continental material during Early Devonian abundance of older than 650 Ma grains is exaggerated by the factors indicated. time is apparently recorded by the presence of small/round zircon grains of Precambrian age in the Lower Devonian strata. This may be a more distal expression of the same continental 351 Ma involvement given that Precambrian grains in Yukon-Tanana terrane Silurian–Lower Devonian strata of the Banks (n=1078) Island assemblage, Saint Elias Mountains, and southern Alexander terrane all have generally similar ages (Fig. 13), but grains in the Banks Island assemblage and Saint Elias Mountains 441 Ma are larger and more abundant.

Comparison with Yukon-Tanana Terrane

Comparison with data from the Yukon- Tanana terrane is relevant given the suggestion Banks Island Assemblage by Gehrels and Boghossian (2000) that the two (n=1424) assemblages may be related. This possibility was based on the presence in both assemblages 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 of deformed and metamorphosed quartz-rich Figure 14. Normalized age distribution diagrams of U-Pb ages from the Banks Island assemblage metaclastic rocks. Similarities with the Yukon- and the Yukon-Tanana terrane. Peak ages are indicated for each distribution. The relative abun- Tanana terrane can now be tested by compari- dance of older than 600 Ma grains (to right of zigzags) is enhanced by a factor of two. Ages from son of U-Pb ages from both assemblages; Hf the Yukon-Tanana terrane have been generated from metasedimentary rocks of Neoproterozoic(?) data from the Yukon-Tanana terrane are not yet through Carboniferous(?) age in southeast Alaska and British Columbia (Gehrels et al., 1992; Geh- rels and Kapp, 1998; Gehrels and Boghossian, 2000; Gehrels, 2000b, 2001). available. As shown on Figure 14, U-Pb ages from the two assemblages are quite different, with dissimilarities in both dominant Paleozoic age peaks (351 Ma for Yukon-Tanana terrane Comparison with Circum-Arctic Regions sented a model for early Paleozoic formation of vs. 441 Ma for Banks Island assemblage) and the Alexander terrane along the Arctic margin of in Precambrian age distributions. These data Comparison with circum-Arctic assem- Baltica and northeastern Laurentia. do not support primary connections between blages is appropriate given the paleontologic U-Pb and Hf data from the Banks Island the Banks Island assemblage and the Yukon- (Soja, 1994; Blodgett et al., 2002; Blodgett, assemblage and the Alexander terrane are Tanana terrane. 2010; Soja and Krutikov, 2008), paleomag- compared with several data sets from circum- netic (Bazard et al., 1995), and detrital zircon Arctic regions on Figure 15. Included for Comparison with the Wrangellia Terrane (Gehrels et al., 1996; Grove et al., 2008; Miller comparison are U-Pb and Hf data from detri- et al., 2010, 2011; Beranek et al., 2012, 2013a, tal zircons in Neoproterozoic clastic strata of Comparisons of geochronologic and Hf iso- 2013b) data, which suggest that the Alexander the Timanides (Kuznetsov et al., 2010), early topic data are not yet possible because there are terrane may have formed in this area. Colpron Paleozoic plutonic rocks in East Greenland no published data available from rocks that are and Nelson (2009, 2011) have provided excel- (Rehnström, 2010), detrital zircons in Creta- known to belong to Wrangellia. lent recent syntheses of these relations and pre- ceous strata near northern Greenland (Rohr et

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 211

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

20 tial accretion of the outboard terranes (McClel- land et al., 1992; van der Heyden, 1992). An 15 earlier phase of deformation and metamorphism has also been proposed in coastal Brit- 10 ish Columbia on the basis of geochemical data, DM which indicate that Late Jurassic–Early Creta- ceous plutons intruding the Banks Island assem- 5 blage were generated during a period of sig- CHURC nifi cant crustal thickening (Girardi et al., 2012). 0 ε The moderately negative Hf(T) values for the dikes (Fig. 7) indicate that considerable crustal -5 melting accompanied this Late Jurassic–Early Cretaceous deformation. -10 Depositional Chronology -15 Given that fossils have not been recovered -20 Banks Island Assemblage from Banks Island assemblage protoliths, and SE Alaska that stratigraphic relations are obscured by the Saint Elias Mountains -25 deformation, metamorphism, and widespread Timanides intrusive bodies, our U-Pb geochronologic and Epsilon Hf Greenland plutons Hf isotopic data provide critical constraints on -30 Greenland basins average crustal evolution Baltica the ages of deposition. Our proposed strati- Caledonides graphic order (Fig. 3) is based largely on the peak age of the youngest signifi cant group of 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 detrital zircons in each sample (Fig. 4), which Detrital Zircon Age (Ma) provides a maximum depositional age (e.g., Figure 15. Hf isotope composition for detrital zircons from the Banks Island assemblage and the Dickinson and Gehrels, 2009). Samples are Alexander terrane of SE Alaska and the Saint Elias Mountains, compared with data from vari- accordingly assigned to groups with similar ous circum-Arctic regions. DM—depleted mantle, CHUR—chondritic uniform reservoir. Gray arrow maximum depositional ages, with additional 176 177 shows interpreted crustal evolution trajectory assuming present-day Lu/ Hf = 0.0115 (Vervoort information provided by Hf isotopic data. and Patchett, 1996; Vervoort et al., 1999). Assignment of stratigraphic ages (Fig. 3) is aided by comparison of U-Pb ages and Hf isotope compositions from metasedimentary rocks al. [2010] data for the Sverdrup basin and Rohr dilleran orogen; (2) the depositional age of vari- of the Banks Island assemblage and fossilifer- et al. [2008] data for the Wandel Sea basin), ous Banks Island assemblage metasedimentary ous strata of the Alexander terrane in SE Alaska detrital zircons from Permian strata of Baltica assemblages; (3) potential correlations among and the Saint Elias Mountains. For Banks Island (Andersen et al. [2011] data for the Oslo rift), the Banks Island assemblage and strata of the assemblage rocks with peak ages between 500 and early Paleozoic plutons of the Scottish Alexander terrane in SE Alaska and the Saint and 400 Ma, the key to this correlation is the Caledonides (Appleby et al., 2010). Elias Mountains; and (4) possible displacement recognition of negative Hf isotopic excursions This comparison supports the conclusions histories of the Alexander terrane. Each of these in Silurian–Lower Devonian strata of the Saint of Beranek et al. (2013a, 2013b) and Colpron dimensions is described in turn next. Elias Mountains and in Banks Island assemblage and Nelson (2009, 2011), that U-Pb ages and strata with 450–438 Ma peak ages (Fig. 12; Hf isotope signatures of detrital zircons from Deformational Chronology Table 2). This correlation is supported by simi- the Banks Island assemblage and Alexander larities in the age distributions of detrital zircons terrane share similarities with data from cir- Crystallization ages of 156–116 Ma for from 450 to 438 Ma Banks Island assemblage cum-Arctic regions. The strongest similarities crosscutting dikes indicate that the main phase strata and from Silurian–Lower Devonian strata are in the timing of Neoproterozoic and early of deformation and metamorphism of rocks of of both the Saint Elias Mountains and southern Paleozoic magmatism, and the broad range of the Banks Island assemblage occurred prior to SE Alaska (Fig. 13). ε Hf(T) values for early Paleozoic grains. Sig- Late Jurassic–Early Cretaceous time. This sig- Banks Island assemblage strata with younger nifi cant differences are in the somewhat more nifi cantly predates the main phase of deforma- than 400 Ma peak ages are interpreted to be at ε evolved Hf(T) values for 800–600 Ma grains tion in the Coast Mountains, which is tradition- least in part of Permian age (Fig. 3), on the basis and the more juvenile Hf isotopic signature of ally interpreted to record initial mid-Cretaceous of the similarity with U-Pb ages from Permian most early Paleozoic grains in the Banks Island accretion of the Alexander and Wrangellia ter- strata in central SE Alaska (Fig. 16), and the assemblage and Alexander terrane. ranes with inboard terranes (e.g., Monger et al., occurrence of one sample (11TC13) with a peak 1982; Crawford et al., 1987). A likely possibility age of 306 Ma and individual ages as young as DISCUSSION is that this deformation is related to mid-Jurassic 256 Ma. deformation observed within the Alexander ter- Finally, one of our samples (Absalom) Our data yield new insights into: (1) the chro- rane in SE Alaska (McClelland and Gehrels, yields a peak age of 119 Ma, which indicates nology of deformation in this portion of the Cor- 1990), which may record an earlier phase of ini- that this unit is signifi cantly younger than the

212 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

terrane formed along the paleo-Arctic margin of 20x Baltica, Greenland, and northeastern Laurentia, Alexander Terrane built in part on older continental crust and blan- Permian (n=326) keted by detritus from the Timanides and Cale- donides, whereas the southern part of the terrane 4x formed farther offshore in a juvenile magmatic arc and was shielded from signifi cant cratonal input. Younger Paleozoic strata of the Banks Banks Island Assemblage Island assemblage and Alexander terrane may <400 Ma (n=248) record migration of the terrane out of the Arctic realm and into the Cordilleran realm (e.g., Col- 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 pron and Nelson, 2009, 2011). Detrital Zircon Age (Ma) Figure 16. Normalized age distribution diagrams of U-Pb ages from Banks Island assemblage strata CONCLUSIONS with younger than 400 Ma peak age and Permian strata from the Alexander terrane. Ages older than 650 Ma (to right of zigzags) are enhanced by factor of 20 for Alexander terrane and by factor of Metasedimentary rocks of the Banks Island four for Banks Island assemblage. Peak ages of young grains are 409, 367, and 307 Ma for the Banks assemblage yield detrital zircon ages ranging Island assemblage and 429, 366, and 288 Ma for the Alexander terrane. from ca. 3.3 Ga to ca. 250 Ma, but most samples are dominated by a single dominant group with peak ages between 486 and 306 Ma (Fig. Paleozoic and early Mesozoic strata that char- Alexander terrane during early Paleozoic time, 4). The peak ages of these dominant groups are acterize the Banks Island assemblage (Fig. 3) and it has been displaced southward, to a posi- used to order the samples by maximum deposi- and Alexander terrane (Gehrels and Saleeby, tion outboard of the southern Alexander terrane, tional age, and stratigraphic ages can be inferred 1987). One possibility is that these strata by ~1000 km of sinistral displacement. This on the basis of correlation with strata of known belong to the Gravina belt, which consists of possibility was fi rst suggested by B. Mahoney age in the Alexander terrane along strike to the Upper Jurassic–Lower Cretaceous strata that (2011, verbal commun.). At least some of this north. Similarities in Hf isotopic patterns for occur along the inboard (eastern) margin of the sinistral displacement may have occurred along Cambrian–Ordovician through Middle Devo- Alexander terrane (Gehrels and Berg, 1994) the Kitkatla shear zone (Fig. 2), which is known nian strata in the Banks Island assemblage and and are known to contain plutonic clasts of Cre- to have been active during Early Cretaceous the Saint Elias Mountains are striking and raise taceous age (Kapp and Gehrels, 1998; Gehrels, time (Butler et al., 2006; Chardon et al., 1999). the possibility that the Banks Island assemblage 2001). An alternative is that that these strata This timing and sense of offset are consistent was originally located adjacent to the northern accumulated in a pull-apart basin along the with previous suggestions of large-scale sinis- portion of the Alexander terrane. Approximately Kitkatla shear zone, given that the sample was tral motion along this segment of the Cordil- 1000 km of sinistral displacement is needed to collected from metasedimentary rocks within leran margin (Monger et al., 1994; Plafker and bring the Banks Island assemblage adjacent to this fault system (Fig. 2). The Kitkatla shear Berg, 1994; Chardon et al., 1999; Gehrels et al., the southern portion of the terrane—some of zone is known to have experienced signifi cant 2009; Angen et al., 2012). this displacement may have occurred along the sinistral motion during Early Cretaceous time Kitkatla shear zone, which is known to have (Butler et al., 2006; Chardon et al., 1999). Origin of the Banks Island Assemblage experienced sinistral displacement during Early and Alexander Terrane Cretaceous time (Butler et al., 2006; Chardon et Correlation with Rocks of the Alexander al., 1999). Terrane Given that our geochronologic and Hf isotopic The abundance of Precambrian detrital zir- data from the Banks Island assemblage are very cons and the highly evolved Hf isotope compo- The similarities in U-Pb ages and Hf isoto- similar to data from the Saint Elias Mountains sitions of early Paleozoic zircons in the Banks pic compositions noted previously indicate that (Beranek et al., 2013a, 2013b) and share similari- Island assemblage and Saint Elias Mountains Lower Paleozoic metasedimentary rocks of the ties with a variety of circum-Arctic assemblages, indicate that these regions were located near an Banks Island assemblage may be directly cor- we concur with Soja (1994), Bazard et al. (1995), ancient cratonal region during early Paleozoic relative with less-deformed strata of the Alexan- Gehrels et al. (1996), Blodgett et al. (2002), time. Based on similarities in U-Pb ages and der terrane in the Saint Elias Mountains. In con- Soja and Krutikov (2008), Grove et al. (2008), Hf isotopic data, as well as abundant paleomag- trast, Lower Paleozoic strata in the Banks Island Blodgett (2010), Miller et al. (2010, 2011), Col- netic and faunal data (as recently summarized assemblage and Saint Elias Mountains differ pron and Nelson (2009, 2011), and Beranek et by Colpron and Nelson, 2009, 2011; Beranek et from the more juvenile units of the Alexander al. (2012, 2013a, 2013b) that the Banks Island al., 2012, 2013a, 2013b), it is plausible that the terrane in southeast Alaska, which (1) include assemblage–Alexander terrane may have formed Banks Island assemblage and northern Alexan- abundant volcanic and plutonic rocks and lack in proximity to the Arctic margin of Baltica der terrane were located along the paleo-Arctic quartz-rich sandstones, (2) contain less abun- and eastern Laurentia. As shown on Figure 15, margin of Baltica, Greenland, or northeast Lau- dant Precambrian detrital zircons, and (3) yield however, all portions of the Alexander terrane rentia during Cambrian–Ordovician through more juvenile Hf isotope compositions for early also record the presence of more juvenile early Early Devonian time. The northern portion Paleozoic zircon grains (Figs. 11 and 12). Paleozoic magmatism than is recorded in any of the Alexander terrane and the Banks Island These comparisons raise the possibility that of the circum-Arctic assemblages. Our data are assemblage were apparently also near a juvenile the Banks Island assemblage was located adja- accordingly consistent with a model in which the volcanic arc that extended suffi ciently far from cent to the Saint Elias Mountains portion of the Banks Island assemblage and northern Alexander the continental margin that some regions (e.g.,

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 213

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 TOCHILIN ET AL.

southern SE Alaska) were shielded from signifi - from an integrated O, U-Pb and Hf isotope study of zir- and Kroner, A., eds., Earth Accretionary Systems in con: Contributions to Mineralogy and Petrology, v. 160, Space and Time: Geological Society of London Special cant cratonal infl uence. p. 115–132, doi:10.1007/s00410-009-0469-3. Publication 318, p. 273–307. Less geochronologic and isotopic informa- Baer, A.J., 1973, Bella Coola Laredo Sound Map areas, British Colpron, M., and Nelson, J., 2011, A Palaeozoic NW Passage tion is available from the Banks Island assem- Columbia: Geological Survey of Canada Memoir 372, and the Timanian, Caledonian and Uralian connections 122 p., 1:250,000 scale map. of some exotic terranes in the North American Cordillera, blage and Alexander terrane following Early Bahlburg, H., Vervoort, J.D., DuFrane, S.A., Carlotto, V., in Spencer, A.M., Embry, A.F., Gautier, D.L., Stoupakova, Devonian time, although Middle Devonian strata Reimann, C., and Cardenas, J., 2011, The U-Pb and Hf A.V., and Sørensen, K., eds., Arctic Petroleum Geology: in the Saint Elias Mountains and strata with peak isotope evidence of detrital zircons of the Ordovician Geological Society of London Memoir 35, p. 463–484. Ollantaytambo Formation, southern Peru, and the Ordo- Crawford, M.L., Hollister, L.S., and Woodsworth, G.J., 1987, ages of 388–306 Ma in the Banks Island assem- vician provenance and paleogeography of southern Crustal deformation and regional metamorphism blage yield mainly juvenile Hf isotope composi- Peru and northern Bolivia: Journal of South American across a terrane boundary, Coast Plutonic Complex, tions (Figs. 11 and 12). The displacement scenario Earth Sciences, v. 32, p. 196–209, doi:10.1016/j.jsames British Columbia:Tectonics, v. 6, p. 343–361, doi:10.1029 .2011.07.002. /TC006i003p00343. proposed by Colpron and Nelson (2009, 2011), Bazard, D.R., Butler, R.F., Gehrels, G.E., and Soja, C.M., 1995, Dickinson, W.R., and Gehrels, G.E., 2009, Use of U-Pb ages where the Alexander terrane was moving from Paleomagnetism of the Early Devonian Karheen For- of detrital zircons to infer maximum depositional ages mation, southeast Alaska: Implications for Alexander of strata: A test against a Colorado Plateau Mesozoic the paleo-Arctic into the paleo-Pacifi c realm terrane paleogeography: Geology, v. 23, p. 707–710, database: Earth and Planetary Science Letters, v. 288, during this time, appears consistent with the doi:10.1130/0091-7613(1995)023<0707:EDPDFT>2..CO;2. p. 115–125, doi:10.1016/j.epsl.2009.09.013. available data. The ca. 359 Ma orthogneiss that Beranek, L.P., van Staal, C.R., Gordee, S.M., McClelland, W.C., Gardner, M.C., Bergman, S.C., Cushing, G.W., MacKevett, E.M., Israel, S., and Mihalynuk, M.G., 2012, Tectonic signifi - Jr., Plafker, G., Campbell, R.B., Dodds, C.J., McClelland, intrudes rocks of the Banks Island assemblage cance of Upper Cambrian–Middle Ordovician mafi c vol- W.C., and Mueller, P.A., 1988, Pennsylvanian pluton stitch- (sample 07GJ208) may have formed within a canic rocks on the Alexander terrane, Saint Elias Moun- ing of Wrangellia and the Alexander terrane, Wrangell magmatic arc that was active at this time. tains, northwestern Canada: The Journal of Geology, Mountains, Alaska: Geology, v. 16, p. 967–971, doi:10.1130 v. 120, p. 293–314, doi:10.1086/664788. /0091-7613(1988)016<0967:PPSOWA>2.3.CO;2. The next phase recorded in the study area Beranek, L.P., van Staal, C.R., McClelland, W.C., Israel, S., and Gehrels, G.E., 2000a, Introduction to detrital zircon studies is the regional deformation and metamorphism Mihalynuk, M.G., 2013a, Baltican crustal provenance for of Paleozoic and Triassic strata in western Nevada and Cambrian–Ordovician sandstones of the Alexander ter- northern California, in Soreghan, M.J., and Gehrels, G.E., that affect all rocks of the Banks Island assem- rane, North American Cordillera: Evidence from detrital eds., Paleozoic and Triassic Paleogeography and Tecton- blage. Constraints on this tectonism demonstrate zircon U-Pb geochronology and Hf isotope geochemis- ics of Western Nevada and Northern California: Geologi- only that it occurred between ca. 306 Ma (the try: Journal of the Geological Society of London, v. 170, cal Society of America Special Paper 347, p. 1–17. p. 7–18, doi:10.1144/jgs2012-028. Gehrels, G.E., 2000b, Reconnaissance geology and U-Pb geo- maximum depositional age of deformed rocks) Beranek, L.P., van Staal, C.R., McClelland, W.C., Israel, S., and chronology of the western fl ank of the Coast Mountains and ca. 156 Ma (the oldest crosscutting dike). Mihalynuk, M.G., 2013b, Detrital zircon Hf isotopic com- between Juneau and Skagway, southeastern Alaska, in We postulate that this tectonism may have been positions indicate a northern Caledonian connection Stowell, H.H., and McClelland, W.C., eds., Tectonics of for the Alexander terrane: Lithosphere, v. 5, p. 163–168, the Coast Mountains in SE Alaska and Coastal British related to mid-Jurassic deformation recorded doi:10.1130/L255.1. Columbia: Geological Society of America Special Paper within the Alexander terrane (e.g., McClelland Blodgett, R., 2010, Upper Ordovician–Middle Devonian paleo- 343, p. 213–234. biogeographic signals from shelly faunas of the Alex- Gehrels, G.E., 2001, Geology of the Chatham Sound region, and Gehrels, 1990) and inferred to exist dur- ander terrane, southeast Alaska: Geological Society of southeast Alaska and coastal British Columbia: Cana- ing generation of the Late Jurassic–Early Cre- America Abstracts with Programs, v. 42, no. 5, p. 573. dian Journal of Earth Sciences, v. 38, p. 1579–1599, taceous plutons that intrude the Banks Island Blodgett, R.B., Rohr, D.M., and Boucot, A.J., 2002, Paleozoic doi:10.1139/e01-040. links among some Alaskan accreted terranes and Sibe- Gehrels, G.E., and Berg, H.C., 1994, Geology of southeastern assemblage (Girardi et al., 2012). ria based on megafossils, in Miller, E.L., Grantz, A., and Alaska, in Plafker, G., and Berg, H.C., eds., The Geology Finally, our sample “Absalom” records accu- Klemperer, S., eds., Tectonic Evolution of the Bering of Alaska: Boulder, Colorado, Geological Society of mulation of plutonic-clast conglomerate during Shelf–Chukchi Sea–Arctic Margin and Adjacent Land- America, The Geology of North America, v. G1, p. 451– masses: Geological Society of America Special Paper 467 (replaces U.S. Geological Survey Open-File 88-659). Early Cretaceous time. These strata may have 360, p. 273–290. Gehrels, G.E., and Boghossian, N.D., 2000, Reconnaissance accumulated in part of the regionally extensive Boghossian, N.D., and Gehrels, G.E., 2000, Nd isotopic signa- geology and U-Pb geochronology of the west fl ank of Gravina belt, or in a more restricted pull-apart ture of metasedimentary pendants in the Coast Moun- the Coast Mountains between Bella Coola and Prince tains between Prince Rupert and Bella Coola, British Rupert, coastal British Columbia, in Stowell, H.H., and basin along the sinistral Kitkatla shear zone. Columbia, in Stowell, H.H., and McClelland, W.D., eds., McClelland, W.C., eds., Tectonics of the Coast Moun- Tectonics of the Coast Mountains, Southeastern Alaska tains, Southeastern Alaska and British Columbia: Geo- ACKNOWLEDGMENTS and British Columbia: Geological Society of America logical Society of America Special Paper 343, p. 61–76. Special Paper 343, p. 77–87. Gehrels, G.E., and Kapp, P.A., 1998, Detrital zircon geochronol- This work was supported by National Science Foundation Butler, R.F., Gehrels, G.E., Hart, W., Davidson, C., and Craw- ogy and regional correlation of metasedimentary rocks awards EAR-0947094 and EAR-1032156 and a Geological Sur- ford, M.L., 2006, Paleomagnetism of Late Jurassic to in the Coast Mountains, southeastern Alaska: Canadian vey of Canada–British Columbia Geological Survey partner- mid-Cretaceous plutons near Prince Rupert, British Journal of Earth Sciences, v. 35, p. 269–279, doi:10.1139 ship (the GEM/Cordilleran Multiple Metals initiative). Nicky Columbia, in Haggart, J.W., Enkin, R.J., and Monger, /e97-114. Giesler, Ken Kanipe, Chen Li, Clayton Loehn, Mark Pecha, J.W.H., eds., Paleogeography of the North American Gehrels, G., and Pecha, M., 2014, Detrital zircon U-Pb geochro- Gayland Simpson, Chelsi White, and Intan Yokelson provided Cordillera: Evidence For and Against Large-Scale Dis- nology and Hf isotope geochemistry of Paleozoic and invaluable assistance with sample preparation, imaging, and placements: Geological Association of Canada Special Triassic passive margin strata of western North America: analysis. Thanks go to Don Willson, Taiya Gehrels, and James Paper 46, p. 171–200. Geosphere, v. 10, p. 49–65, doi:10.1130/GES00889.1. Gehrels for very capable logistical support during the fi eld Cecil, M.R., Gehrels, G.E., Ducea, M.N., and Patchett, P.J., 2011, Gehrels, G.E., and Saleeby, J.B., 1987, Geologic framework, work. This paper was reviewed by Maurice Colpron, Cees van U-Pb-Hf characterization of the central Coast Mountains tectonic evolution, and displacement history of the Alex- Staal, and an anonymous reviewer. batholith: Implications for petrogenesis and crustal ander terrane: Tectonics, v. 6, p. 151–173, doi:10.1029 architecture: Lithosphere, v. 3, p. 247–260, doi:10.1130 /TC006i002p00151. REFERENCES CITED /L134.1. Gehrels, G.E., McClelland, W.C., Samson, S.D., and Patchett, Andersen, T., Saeed, A., Gabrielsen, R.H., and Olaussen, S., Chardon, D., Andronicos, C.L., and Hollister, L.S., 1999, P.J., 1991, U-Pb geochronology of detrital zircons from 2011, Provenance characteristics of the Brumunddal Large-scale transpressive shear zone patterns and dis- a continental margin assemblage in the northern Coast sandstone in the Oslo Rift derived from U-Pb, Lu-Hf placements within magmatic arcs: The Coast Plutonic Mountains, southeastern Alaska: Canadian Journal of and trace element analyses of detrital zircons by laser Complex, British Columbia: Tectonics, v. 18, p. 278–292, Earth Sciences, v. 28, p. 1285–1300, doi:10.1139/e91-114. ablation ICMPS: Norwegian Journal of Geology, v. 91, doi:10.1029/1998TC900035. Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., p. 1–19. Churkin, M., Jr., and Eberlein, G.D., 1977, Ancient borderland and Orchard, M.J., 1992, Geology of the western fl ank Angen, J.J., van Staal, C., and Lin, W.S., 2012, Structural geol- terranes of the North American Cordillera: Correlations of the Coast Mountains between Cape Fanshaw and ogy of the Alexander terrane in the vicinity of Porcher and microplate tectonics: Geological Society of America Taku Inlet, southeastern Alaska: Tectonics, v. 11, p. 567– Island, northwestern British Columbia: British Colum- Bulletin, v. 88, p. 769–786, doi:10.1130/0016-7606(1977) 585, doi:10.1029/92TC00482. bia Ministry of Energy and Mines, Geological Fieldwork 88<769:ABTOTN>2.0.CO;2. Gehrels, G.E., Butler, R.F., and Bazard, D.R., 1996, Detrital 2011, Paper 2012, p. 135–147. Colpron, M., and Nelson, J., 2009, A Palaeozoic Northwest zircon geochronology of the Alexander terrane, south- Appleby, S.K., Gillespie, M.R., Graham, C.M., Hinton, R.W., Passage: Incursion of Caledonian, Baltican and Siberian eastern Alaska: Geological Society of America Bulle- Oliver, G.J.H., and Kelly, N.M., 2010, Do S-type granites terranes into eastern Panthalassa, and the early evolu- tin, v. 108, p. 722–734, doi:10.1130/0016-7606(1996)108< commonly sample infracrustal sources? New results tion of the North American Cordillera, in Cawood, P.A., 0722:DZGOTA>2.3.CO;2.

214 www.gsapubs.org | Volume 6 | Number 3 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021 Banks Island assemblage | RESEARCH

Gehrels, G.E., Valencia, V.A., and Ruiz, J., 2008, Enhanced of Petroleum Geologists Bulletin, v. 94, p. 665–692, Soja, C.M., and Krutikov, L., 2008, Provenance, depositional precision, accuracy, effi ciency, and spatial resolution of doi:10.1306/10200909036. setting, and tectonic implications of Silurian polymic- U-Pb ages by laser ablation–multicollector–inductively Miller, E.L., Kuznetsov, N., Soboleva, A., Udoratina, O., Grove, tic conglomerates in Alaska’s Alexander terrane, in coupled plasma mass spectrometry: Geochemistry M.J., and Gehrels, G.E., 2011, Baltica in the Cordillera?: Blodget, R., and Stanley, G., eds., The Terrane Puzzle: Geophysics Geosystems, v. 9 (Q03017), 13 p. Geology, v. 39, p. 791–794, doi:10.1130/G31910.1. New Perspectives on Paleontology and Stratigraphy Gehrels, G.E., Rusmore, M., Woodsworth, G., Crawford, M., Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J., from the North American Cordillera: Geological Society Andronicos, C., Hollister, L., Patchett, J., Ducea, M., But- 1982, Tectonic accretion and the origin of two major of America Special Paper 442, p. 63–75. ler, R., Klepeis, K., Davidson, C., Friedman, R., Haggart, metamorphic welts in the Canadian Cordillera: Geol- van der Heyden, P., 1989, U-Pb and K-Ar Geochronometry of J., Mahoney, B., Crawford, W., Pearson, D., and Girardi, ogy, v. 10, p. 70–75, doi:10.1130/0091-7613(1982)10<70: the Coast Plutonic Complex, 53N to 54N, British Colum- J., 2009, U-Th-Pb geochronology of the Coast Moun- TAATOO>2.0.CO;2. bia, and Implications for the Insular-Intermontane tains batholith in north-coastal British Columbia: Con- Monger, J.W.H., van der Heyden, P., Journeay, J.M., Evench- Superterrane Boundary: Vancouver, British Columbia, straints on age and tectonic evolution: Geological Soci- ick, C.A., and Mahoney, J.B., 1994, Jurassic–Cretaceous Canada, University of British Columbia, 392 p. ety of America Bulletin, v. 121, p. 1341–1361, doi:10.1130 basins along the Canadian Coast Belt: Their bearing van der Heyden, P., 1992, A Middle Jurassic to Early Tertiary /B26404.1. on pre–mid-Cretaceous sinistral displacements: Geol- Andean–Sierran arc model for the Coast Belt of Brit- Girardi, J., Patchett, J., Ducea, M., Gehrels, G., Rusmore, M., ogy, v. 22, p. 175–178, doi:10.1130/0091-7613(1994)022 ish Columbia: Tectonics, v. 11, p. 82–97, doi:10.1029 Woodsworth, G., Pearson, D., Manthei, C., and Wetmore, <0175:JCBATC>2.3.CO;2. /91TC02183. P., 2012, Elemental and isotopic evidence for granitoid Plafker, G., and Berg, H.C., 1994, Overview of the geology Vervoort, J.D., and Patchett, P.J., 1996, Behavior of hafnium genesis from deep-seated sources in the Coast Moun- and tectonic evolution of Alaska, in Plafker, G., and and neodymium isotopes in the crust: Constraints from tains batholith, British Columbia: Journal of Petrology, Berg, H.C., eds., The Geology of Alaska: Boulder, Colo- crustally derived granites: Geochimica et Cosmochi- v. 53, no. 7, p. 1505–1536, doi:10.1093/petrology/egs024. rado, Geological Society of America, Geology of North mica Acta, v. 60, no. 19, p. 3717–3733, doi:10.1016/0016- Grove, M., Gehrels, G., Cotkin, S., Wright, J., and Zou, H., 2008, America, v. G1, p. 989–1021. 7037(96)00201-3. Non-Laurentian cratonal provenance of Late Ordovician Rehnström, E.F., 2010, Prolonged Paleozoic magmatism in the Vervoort, J.D., Patchett, P.J., Blichert-Toft, J., and Albarede, F., eastern Klamath blueschists and a link to the Alexander East Greenland Caledonides: Some constraints from 1999, Relationships between Lu-Hf and Sm-Nd isotopic terrane, in Wright, J., and Shervais, J., eds., Ophiolites, U-Pb ages and Hf isotopes: The Journal of Geology, systems in the global sedimentary system: Earth and Arcs, and Batholiths: A Tribute to Cliff Hopson: Geologi- v. 118, p. 447–465, doi:10.1086/655010. Planetary Science Letters, v. 168, p. 79–99, doi:10.1016 cal Society of America Special Paper 438, p. 223–250. Reimann, C.R., Bahlburg, H., Kooijman, E., Berndt, J., Gerdes, /S0012-821X(99)00047-3. Hutchison, W.W., 1982, Geology of the Prince Rupert-Skeena A., Carlotto, V., and Lopez, S., 2010, Geodynamic evo- Wheeler, J.O., and McFeely, P., 1991, Tectonic Assemblage Map Area, British Columbia: Geological Survey of Can- lution of the early Paleozoic western Gondwana mar- Map of the Canadian Cordillera and Adjacent Parts of ada Memoir 394, 116 p. gin 14°–17°S refl ected by the detritus of the Devonian the United States of America: Geological Survey of Jones, D.L., Irwin, W.P., and Ovenshine, A.T., 1972, Southeast- and Ordovician basins of southern Peru and north- Canada Map 1712A, scale 1:2,000,000. ern Alaska: A Displaced Continental Fragment?: U.S. ern Bolivia: Gondwana Research, v. 18, p. 370–384, Wheeler, J.O., Brookfi eld, A.J., Gabrielse, H., Monger, J.W.H., Geological Survey Professional Paper 800B, p. B211– doi:10.1016/j.gr.2010.02.002. Tipper, H.W., and Woodsworth, G.J., 1991, Terrane Map B217. Roddick, J.A., 1970, Douglas Channel–Hecate Strait Map of the Canadian Cordillera: Geological Survey of Can- Kapp, P.A., and Gehrels, G.E., 1998, Detrital zircon constraints Area, British Columbia: Geological Survey of Canada ada Map 1713A, scale 1:2,000,000. on the tectonic evolution of the Gravina belt, southeast- Paper 70-41, 56 p., 1:250,000 scale map. Woodhead, J.D., and Hergt, J.M., 2005, A preliminary ern Alaska: Canadian Journal of Earth Sciences, v. 35, Rohr, T.S., Andersen, T., and Dypvik, H., 2008, Provenance appraisal of seven natural zircon reference materials for p. 253–268, doi:10.1139/e97-110. of Lower Cretaceous sediments in the Wandel Sea in situ Hf isotope determination: Geostandards and Geo- Kuznetsov, N.B., Natapov, L.M., Belousova, E.A., O’Reilly, Basin, North Greenland: Journal of the Geological analytical Research, v. 29, p. 183–195, doi:10.1111/j.1751 S.Y., and Griffi n, W.L., 2010, Geochronologic, geochemi- Society of London, v. 165, p. 755–767, doi:10.1144/0016 -908X.2005.tb00891.x. cal and isotopic study of late Neoproterozoic clastic -76492007-102. Woodsworth, G.J., and Orchard, M.J., 1985, Upper Paleozoic strata along the NE margin of the east European cra- Rohr, T.S., Andersen, T., Dypvik, H., and Embry, A.F., 2010, to Lower Mesozoic strata and their conodonts, west- ton: Implications for plate tectonic models: Gondwana Detrital zircon characteristics of the Lower Cretaceous ern Coast Plutonic Complex, British Columbia: Cana- Research, v. 17, p. 583–601, doi:10.1016/j.gr.2009.08.005. Isachsen Formation, Sverdrup Basin: Source con- dian Journal of Earth Sciences, v. 22, p. 1329–1344, McClelland, W.C., and Gehrels, G.E., 1990, The Duncan Canal straints from age and Hf isotope data: Canadian Journal doi:10.1139/e85-137. shear zone: A right-lateral shear zone of Jurassic age of Earth Sciences, v. 47, p. 255–271, doi:10.1139/E10-006. Wright, J.E., and Wyld, S.J., 2006, Gondwanan, Iapetan, along the inboard margin of the Alexander terrane: Samson, S.D., and Patchett, P.J., 1991, The Canadian Cor- Cordilleran interactions: A geodynamic model for the Geological Society of America Bulletin, v. 102, p. 1378– dillera as a modern analogue of Proterozoic crustal Paleozoic tectonic evolution of the North American 1392, doi:10.1130/0016-7606(1990)102<1378:GOTDCS>2 growth: Australian Journal of Earth Sciences, v. 38, Cordillera, in Haggart, J.W., Enkin, R.J., and Monger, .3.CO;2. p. 595–611, doi:10.1080/08120099108727994. J.W.H., eds., Paleogeography of the North American McClelland, W.C., Gehrels, G.E., and Saleeby, J.B., 1992, Sláma, J., Kosler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Cordillera: Evidence For and Against Large-Scale Dis- Upper Jurassic–Lower Cretaceous basinal strata along Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, placements: Geological Association of Canada Special the Cordilleran margin: Implications for the accretionary L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett, Paper 46, p. 377–408. history of the Alexander-Wrangellia-Peninsular terrane: M.N., and Whitehouse, M.J., 2008, Plesovice zircon—A Tectonics, v. 11, p. 823–835, doi:10.1029/92TC00241. new natural reference material for U-Pb and Hf isoto- Miller, E.L., Gehrels, G.E., Pease, V., and Sokolov, S., 2010, pic microanalysis: Chemical Geology, v. 249, p. 1–35, MANUSCRIPT RECEIVED 6 OCTOBER 2013 Paleozoic and Mesozoic stratigraphy and U-Pb detri- doi:10.1016/j.chemgeo.2007.11.005. REVISED MANUSCRIPT RECEIVED 20 JANUARY 2014 tal zircon geochronology of Wrangel Island, Russia: Soja, C.M., 1994, Signifi cance of Silurian stromatolite-sphinc- MANUSCRIPT ACCEPTED BY THE SOCIETY 16 FEBRUARY 2014 Constraints on paleogeography and paleocontinental tozoan reefs: Geology, v. 22, p. 355–358, doi:10.1130/0091 reconstructions of the Arctic: American Association -7613(1994)022<0355:SOSSSR>2.3.CO;2. Printed in the USA

LITHOSPHERE | Volume 6 | Number 3 | www.gsapubs.org 215

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/3/200/3040154/200.pdf by guest on 02 October 2021