Sedimentology, U-Pb Detrital Geochronology, and Hf Isotopic Analyses from Mississippian–Permian Stratigraphy of the Mystic Subterrane, Farewell Terrane, Alaska

RESEARCH

Sedimentology, U-Pb detrital geochronology, and Hf isotopic analyses from Mississippian–Permian stratigraphy of the Mystic subterrane, Farewell terrane, Alaska

Matthew A. Malkowski1 and Brian A. Hampton2 1DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES, STANFORD UNIVERSITY, STANFORD, CALIFORNIA 94305, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, NEW MEXICO STATE UNIVERSITY, LAS CRUCES, NEW MEXICO 88003, USA

ABSTRACT

The Farewell terrane of western Alaska is one of the more remote and understudied crustal fragments in the North American Cordillera. Although it is generally accepted that the oldest, Precambrian parts of the Farewell terrane originated along the Arctic margin (i.e., Siberia), the paleogeographic history of the Farewell terrane during much of the middle and late Paleozoic remains unknown. Here, we present new sedimentologic and provenance data from upper Paleozoic clastic strata of the Mystic subterrane, which represents the youngest part of the Farewell terrane. Sedimentary facies consist of high- and low-density sediment-gravity-flow deposits and are interpreted to represent a submarine fan depositional system. Sandstone modal composition trends show a relative abundance of lithic volcanic fragments (~65%) and subordinate occurrences of lithic sedimentary fragments (~15%) and chert (~13%). Laser-ablation–inductively coupled plasma–mass spectrometry analyses of detrital zircons reveal a bulk U-Pb age distribution of Precambrian–Paleozoic grains. U-Pb detrital zircon age spectra from Mississippian strata have a primary peak age between 400 and 325 Ma and secondary peak ages between 480 and 415 Ma and

2000 and 1800 Ma. Devonian–Mississippian zircons exhibit enriched eHf isotopic values (–3 to –35), whereas Ordovician–Silurian zircons have

both enriched (–5 to –25) and depleted (+5 to +14) eHf values. Age spectra from Permian strata show primary peaks between 320 and 275 Ma

and 460 and 415 Ma, with isolated occurrences of Precambrian-age zircons. Pennsylvanian–Permian zircons exhibit depleted eHf values (+2 to +14). Youngest peak ages support a Mississippian–Early Permian maximum depositional age for this part of the Mystic subterrane. Overall, provenance trends reflect primary detrital contributions from arc and recycled orogen source areas, which included both enriched and primitive magmatic sources. New U-Pb and Hf isotope analyses from the Mystic assemblage match most closely with magmatic source areas of the Alexander and Wrangellia terranes. Findings are consistent with a model where the Farewell terrane was proximal to both the Alexander and Wrangellia terranes by Mississippian–Permian time.

LITHOSPHERE; v. 6; no. 5; p. 383–398; GSA Data Repository Item 2014245 | Published online 17 June 2014 doi: 10.1130/L365.1

INTRODUCTION dinate volcanic rocks of the Mystic subterrane tica), intraoceanic settings in the Panthalassic (Figs. 1B and 1C). Parts of the Dillinger subter- Ocean, or represent displaced peri-Laurentian The Farewell terrane is a regionally exten- rane that outcrop along the Iditarod–Nixon Fork crustal fragments that developed in proxim- sive (~40,000 km2), Paleoproterozoic–Jurassic fault were originally referred to as the Minchu- ity to the western margin of ancestral North crustal fragment (Decker et al., 1994; Bundtzen mina terrane (Jones et al., 1987; Silberling et al., America (e.g., Nokleberg et al., 2000; Colpron et al., 1997; Bradley et al., 2003) that is located 1994; Patton et al., 1994). The oldest basement et al., 2007; Colpron and Nelson, 2009; Miller in the northernmost part of the North Ameri- rocks in the Farewell terrane are a Paleoprotero- et al., 2011). The Farewell terrane was initially can Cordillera in western Alaska (Fig. 1A). zoic–Neoproterozoic(?) metamorphic complex thought to have originated along the western The majority of the terrane crops out along the (Patton and Dutro, 1979; Bradley et al., 2003), margin of Laurentia (Coney et al., 1980; Box, southern boundary of the Denali fault and north- which makes up the northernmost exposure of 1985; Decker et al., 1994; Plafker and Berg, ern boundary of the Iditarod–Nixon Fork fault in the terrane, just north of the Iditarod–Nixon 1994); however, more recent paleontological western Alaska (Fig. 1B). As defined by previ- Fork fault (Fig. 1B). studies have made the case for an Arctic-margin ous regional studies (Decker et al., 1994; Bundt- One of the challenges with beginning to origin, which could include localities such as zen et al., 1997), the Farewell terrane is gener- understand the tectonic evolution of accretion- Siberia or northern Laurentia (Soja and Antosh- ally divided into a three-part geologic suite that ary plate margins, such as the North American kina, 1997; Blodgett et al., 2002; Dumoulin includes (1) latest Neoproterozoic (?)–Lower Cordillera, is in deciphering the origin and et al., 2002). Devonian carbonate-dominated strata of the subsequent paleogeographic history of its con- Beyond a proposed Arctic-margin origin, Nixon Fork subterrane, (2) Cambrian–Lower stituent terranes prior to accretion. Many of the very little is known about the Paleozoic tectonic Devonian mixed carbonate and clastic strata Paleozoic–Mesozoic terranes that make up the evolution and paleogeographic history of the of the Dillinger subterrane, and (3) Devonian– northern North American Cordillera have ori- Farewell terrane. Occurrences of Siberian floral Jurassic(?) clastic-dominated strata and subor- gins in circum-Arctic regions (e.g., Siberia, Bal- and faunal assemblages from lower Paleozoic

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A B C DZ - Hf isotope sample Ruby DZ - U-Pb sample 0 100km terrane Tuff - U-Pb age

Alaska B 64°N J biostrat. age (flora) biostrat. age (fauna) basalt unknown contact

Tintina faul T Denali faul Unconformity (?)

Innoko terrane

t Mt. Dall t conglomerate McGrath Quad. Talkeetna Quad. P Yukon Figure 2 Mystic assemblage A P Interbedded sandstone, Iditarod - Nixon Fork fault mudstone, and B M conglomerate This Study

62°N Lime Hills Quad. MYSTIC SUBTERRANE British N Columbia t Mixed siliciclastic faul D & carbonate units Denali Barren Ridge Limestone 0300 km S Terra Cotta 156°W 153°W Mtn. SS

Farewell terrane Mystic subterrane (Devonian - Jurassic) Arctic-Alaska terrane O Dillinger subterrane (Cambrian - Devonian)

Wrangellia composite terrane REWEL L TERRAN E Wrangellia terrane (Skolai arc) FA Nixon Fork subterrane (Neoproterozoic - Devonian) Carbonate platform Peninsular terrane Siliciclastic basinal facies Alluvium (Quaternary) C Alexander terrane DILLINGER SUBTERRANE Igneous rocks (Cretaceous - Paleogene) Intermontane belt Klinkit group Kahiltna Assem. or Kuskokwim Gr. (Jurassic - Cret.) Basement NIXON FORK SUBTERRANE OTHE R ROCKS NP Yukon-Tanana terrane Yukon-Tanana terrane (Neoprot.(?) - Mississippian) (metaseds.) Stikinia Quesnellia Metamorphic rocks (Neoproterozoic & Paleozoic)

Figure 1. Geologic and stratigraphic overview of the Farewell terrane. (A) Regional map of western North America showing the general location of the Farewell terrane and other Paleozoic terranes in the northern North American Cordillera. (B) Generalized geologic map of the Farewell terrane in western Alaska (modified from Bradley et al., 2003). Black boxes represent field localities (Mystic Pass and Farewell Lake–Sheep Creek) in the western Alaska Range. Refer to Figure 2 for detailed geology and sample localities. (C) Neoproterozoic–Mesozoic stratigraphy of the Farewell terrane with emphasis on Mississippian–Early Permian siliciclastic strata of the Mystic subterrane. Biostratigraphic age constraint from the Mystic subterrane is summarized from Reed and Nelson (1980), Mamay and Reed (1984), Blodgett et al. (2002), and Sunderlin (2008). U-Pb tuff age is from Bradley et al. (2007). Geologic time scale ages and divisions are from Gradstein et al. (2004). DZ—detrital zircon; SS—Sandstone.

strata of the Nixon Fork and Dillinger subter- when it was transported to the northwestern Pan- of Pennsylvanian–Permian clastic strata in the ranes have led some to propose that the Farewell thalassic Ocean and eventually to the western upper part of the Farewell terrane (i.e., Mystic terrane originated as a displaced (rifted) frag- margin of Laurentia and northern Cordillera. subterrane) are thought to represent part of the ment of the Siberian platform (Soja and Antosh- Bradley et al. (2007) reported Precambrian foreland basin associated with the Browns Fork kina, 1997; Blodgett et al., 2002). Dumoulin U-Pb detrital zircon ages from isolated lower- orogen (Bradley et al., 2003). In this study, we et al. (2002) reported faunal occurrences that most Paleozoic clastic units of the Nixon Fork present new sedimentologic and provenance suggest a paleogeographic link with the Arctic terrane, which support detrital contributions from data from Mississippian–Early Permian strata Alaska-Chukotka microplate during the early Siberian cratonic sources. Permian 40Ar/39Ar of the Mystic subterrane (Fig. 1) that provide Paleozoic. Based on previous workers’ sugges- plateau ages (284–285 Ma) from Paleoprotero- initial constraints on the late Paleozoic depo- tion of a Siberian affinity as well as Uralian-aged zoic–Neoproterozoic(?) basement metamorphic sitional setting and paleogeographic history of metamorphism (Bradley et al., 2003), Colpron rock of the Farewell terrane have been inter- the Farewell terrane. Results from this investi- and Nelson (2009) suggested that the Fare- preted to record a late Paleozoic orogenic event gation suggest a revised paleogeographic model well terrane originated in the “Northwest Pas- (referred to as the “Browns Fork orogen”) coeval wherein the Mystic subterrane was in proximity sage” (i.e., late Paleozoic Uralian seaway) and with the Pennsylvanian–Permian Uralian orog- to the Alexander and Wrangellia terranes during remained marginal to Siberia until the Permian, eny (Bradley et al., 2003). Thick successions the late Paleozoic.

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GEOLOGIC AND STRATIGRAPHIC with isolated fine-grained intervals containing east-central McGrath quadrangle (Farewell OVERVIEW Permian plant fossils that have a mixed Siberia- Lake–Sheep Creek area; Fig. 2). Laurentia affinity (Mamay and Reed, 1984; At Shellabarger Pass, the base of the Mys- With the exception of regional mapping Sunderlin, 2008). Bradley et al. (2003) sug- tic subterrane is thought to be marked by either projects and isolated paleontologic and geo- gested that the Mount Dall conglomerate repre- a late Early Devonian (Emsian) limestone unit chronologic studies (e.g., Reed and Nelson, sents the foreland basin (referred to as the Dall (Gilbert and Bundtzen, 1984; Blodgett and Gil- 1980; Bundtzen et al., 1997; Bradley et al., basin) that was associated with the Pennsylva- bert, 1992; Blodgett et al., 2002) or a thin (~10 m) 2003, 2007; Sunderlin, 2008), very little work nian–Permian Browns Fork orogeny. nonmarine red-bed sequence consisting of sand- has been carried out that focuses on the detailed The most extensive tracts of Mississippian– stone and conglomerate with coal and fossilized sedimentology, stratigraphy, and provenance of Permian strata of the Mystic subterrane lie south wood and plant debris (Reed and Nelson, 1980). siliciclastic units from the upper part of the Fare- of the Denali fault in the northwestern Talkeetna Basal units are overlain by a 125–250-m-thick well terrane (Mystic subterrane). Upper Paleo- quadrangle (Mystic Pass region) and in the east- succession of sandstone, siltstone, and shale that zoic rocks of the Mystic subterrane include a central part of the McGrath quadrangle (Fare- contains late Middle and early Late Devonian Mississippian–Pennsylvanian succession of well Lake–Sheep Creek area; Figs. 1B and 2). fossils (Reed and Nelson, 1980). Up section, interbedded sandstone, siltstone, mudstone, and Isolated occurrences of age-equivalent rocks in there is a 60–90-m-thick reefoid limestone that isolated conglomerate (Fig. 1C). These strata the Farewell terrane also crop out in the White is overlain by a phosphatic chert unit (informally have been assigned the map unit “Pzus” from Mountains region (southwestern McGrath referred to as the “blackball chert”), which con- the Talkeetna quadrangle (Reed and Nelson, quadrangle), and in the central part of the Lime tains radiolarians of Late Devonian age (Famen- 1980) and “PDs” from the McGrath quadrangle Hills quadrangle (Fig. 1B). Although strata in nian; Reed and Nelson, 1980). Pillow basalts are (Bundtzen et al., 1997). In the northwestern these localities are thought to roughly overlap interbedded within some of these strata and have Talkeetna quadrangle, “Pzus” strata are over- in age with Mississippian–Permian units in the been inferred to be middle or late Paleozoic in lain by an additional coarse-grained siliciclastic Mystic Pass and Farewell Lake–Sheep Creek age (Reed and Nelson, 1980). Devonian rocks of unit referred to as the Mount Dall conglomer- localities, exposures are sparse and very lim- the Mystic subterrane are overlain by Mississip- ate (Reed and Nelson, 1980). The Mount Dall ited. This study focuses on Mississippian–Per pian–Permian siliciclastic strata (this study) and conglomerate has been interpreted to represent mian stratigraphy exposed in the northwestern capped by Permian coarse clastic strata of the fluvial-deltaic sedimentation (Sunderlin, 2008) Talkeetna quadrangle (Mystic Pass region) and Mount Dall conglomerate (Fig. 1C).

A 152°30′ W 152°25′ W B 154°55′ W 154°50′ W

55 62°29′ N Fault nzona River 62°40′ N To Denali + 80 MYS-05

80 + FSC-02 70 Mystic Pass Mystic Pass Valley 65 k MYS-01 MYS-03 Cree Fork + 85 West

Sheep

62°25′ N

N 70 N 0 1 km 62°36′ N 0 1 km

Pillow basalt (Triassic?) Glacier/ice Devonian limestone Fault Mt. Dall conglomerate Barren Ridge Limestone Detrital zircon sample Quaternary alluvium (Permian) Mystic assemblage + Detrital zircon + Hafnium sample Tertiary volcanics (Mississippian–Permian) Terra Cotta Mtn. Sandstone

Figure 2. Geologic map and sample localities from (A) the Mystic Pass region in the western Talkeetna quadrangle and (B) the Farewell Lake–Sheep Creek locality in the McGrath quadrangle. The Mystic Pass region is considered the type locality for Mississippian–Early Permian strata of the Mystic assemblage. Figure is modified from Reed and Nelson (1980) and Bundtzen et al. (1997), respectively.

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The contact between the base of the Mystic erate, and fossil-leaf–bearing siltstone. A Perm- SEDIMENTOLOGY AND STRATIGRAPHY subterrane and top of the Dillinger subterrane ian age for the Mount Dall conglomerate is based has been reported as a conformable contact on occurrences of plant fossils (Zamiopteris) and Upper Paleozoic siliciclastic units from the (Bundtzen et al., 1997). However, at the basin brachiopods (Mamay and Reed, 1984; Sunderlin, northwest quadrant of the Talkeetna quadrangle scale, debate remains over the fundamental 2008), as well as conglomerate clasts that con- (Mystic Pass region) and the Farewell Lake– nature of this contact and the stratigraphic inter- tain Pennsylvanian to Early Permian conodonts Sheep Creek area in the east-central McGrath val where it occurs. In the western Talkeetna (Bradley et al., 2003). Boulders and cobbles of quadrangle (Fig. 2) consist of faulted and iso- quadrangle, the basal part of the Mystic subter- the Mount Dall conglomerate consist primarily of clinally folded successions of interbedded rane is thought to be locally underlain by car- chert, limestone, and sandstone, all of which may sandstone, siltstone, mudstone, and subordinate bonate and siliciclastic strata of the Dillinger have been sourced from the Dillinger subterrane conglomerate (Figs. 2 and 3). We present new subterrane. Decker et al. (1994) suggested a and older units of the Mystic subterrane (Bradley lithologic and sedimentologic data from these Middle Devonian angular unconformity as the et al., 2003). A conformable relationship has been strata, which include the “Pzus” and “PDs” contact between the Mystic and Dillinger sub- proposed for Mississippian–Permian units and map units from the Talkeetna quadrangle and terranes; however, local conformable relation- overlying strata of the Mount Dall conglomer- McGrath quadrangle, respectively (Reed and ships have also been reported in some localities ate based on the similarity in ages between these Nelson, 1980; Bundtzen et al., 1997). Collec- (Blodgett and Gilbert, 1983; Patton et al., 1994). units (Reed and Nelson, 1980). With the excep- tively (and informally), we refer to these Mis- The nature of this contact is not well docu- tion of work by Sunderlin (2008) on the Mount sissippian–Permian units as the “Mystic assem- mented and warrants further investigation. Dall conglomerate, there have been no studies to blage.” Based on extensive exposure of the Strata of Mississippian–Permian age in the date that have documented detailed sedimentol- Mystic assemblage in the northwest part of the Mystic subterrane consist primarily of interbed- ogy, stratigraphy, provenance, and structural rela- Talkeetna quadrangle, the Mystic Pass field area ded sandstone and mudstone, with isolated occur- tionships between the Mississippian–Permian (Figs. 2B and 3A) is considered the type locality rences of conglomerate. Biostratigraphic age con- stratigraphic succession and overlying rocks of for the Mystic assemblage. straints for these units include Upper Mississippian the Mount Dall conglomerate within the Farewell and Middle Pennsylvanian echinoderms and fora terrane. The focus of this study is on the Missis- Mystic Assemblage minifers (identified by A.K. Armstrong in Reed sippian–Permian clastic strata that overlie basal and Nelson, 1980). Overlying rocks of the Mount Devonian strata of the Mystic subterrane and Lithologic Descriptions Dall conglomerate consist of a >1500-m-thick underlie Permian rocks of the Mount Dall con- Sandstone, siltstone, and mudstone (subor- succession of Permian-aged sandstone, conglom- glomerate (Fig. 1C). dinate conglomerate). The Mystic assemblage is

Denali LMA LMA LMA fault

UMA

1 m

Figure 3. Outcrop photos of the Mystic assemblage in the Mystic Pass field area. (A) Regional view to the north-northwest of mapped exposures of the lower (LMA) and upper (UMA) parts of the Mystic assemblage that outcrop south of the Denali fault. (B) Inclined, tabular beds of sandstone, siltstone, and mudstone that make up the lower part of the Mystic assemblage. Note the true thickness of 1 m for the tabular sandstone bed in the lower-middle part of the photo.

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characterized by interbedded gray to black, fine- lated occurrences of mud rip-up clasts occur near Conglomerate units occur sporadically to coarse-grained sandstone, siltstone, mudstone, the base of sandstone beds. From base to top, throughout the Mystic assemblage and are both and subordinate conglomerate (Figs. 3B, 4A, and sandstone beds are commonly massive (Sm) to matrix and clast supported (Fig. 4). Individual 4B). Individual beds of fine-grained sandstone, horizontally laminated (Sh) and fine upward into beds are lenticular to tabular, range from 1 to siltstone, and mudstone are thin (5–30 cm thick) ripple cross-laminated, fined-grained sandstone, 4.5 m in thickness, and commonly exhibit an and laterally extensive (>100 m; Fig. 4B). Beds massive siltstone, and mudstone (Fm; Figs. 4A upward transition from massive conglomerate of medium- to coarse-grained sandstone range and 4B). Flame structures, load structures and (Gm) into fine- to medium-grained massive in thickness from 0.5 to 2 m thick (Fig. 4A) convolute laminations are common in sandstone sandstone (Sm). Pebble clasts are rounded to and can be traced laterally for >100 m. Nearly and mudstone beds (Figs. 4B and 4C). Rare subrounded and range from 1 to 10 cm in diam- all beds are tabular and show little evidence of occurrences of nondescript bioturbation were eter. Conglomerate units are dominated by vol- large-scale erosional scour; however, sole marks observed near the base and top of mudstone and canic and chert pebbles with rare occurrences of are common at the base of sandstone beds. Iso- siltstone beds. sandstone and limestone clasts.

7 A LEGEND Ripple cross- laminations 6 Convolute laminations

Planar 5 laminations

Mudstone rip-up clasts High-density cl: claystone 4 turbidite (R3) sl: siltstone fs: fine-gr. ss ms: medium-gr. ss 3 cs: coarse-gr. ss p: pebble conglom.

1 2 1 m Low-density turbidites

1 TD

TC T 0 A 0 cl sl fs ms m cl sl fs ms cs p B CD convolute laminae

Sm TA

7 cm

Sh TB

Figure 4. Sedimentologic observations from the Mystic assemblage. (A) Outcrop photo and corresponding measured stratigraphic section showing examples of both low- and high-density sediment-gravity-flow deposits. Note 1.5 m Jacob Staff for scale. Black arrow represents direction of stratigraphic up. The basal part of the section (left half of photo) consists of low-density turbidite deposits. High-density turbidite deposits are characterized by clast-supported conglomerate (right half of photo). (B) Thin-bedded, low-density turbidite deposits showing normal grading from medium- to coarse-grained sandstone up section into massive siltstone and mudstone. (C) Sandstone facies commonly include horizontal laminations (Sh) and massive bedding (Sm), which are interpreted to represent Ta and Tb turbidite divisions of Bouma (1962). Note coin for scale (~2.4 cm in diameter). (D) Soft-sediment deformation (convolute laminae) and mudstone rip-up clasts (arrows) in sandstone bed (coin for scale).

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P (1.3%) A B Qp (0.4%) K (0.9%) Q Qm (3.5) Lm (0.1%) Chert (C) Qm Monocrystalline Lv quartz (Qm) C Polycrystalline (12.8%) quartz (Qp) RECYCLED RECYCLED Plagioclase OROGEN OROGEN feldspar (P) Ls CONTINENTAL (Dis.) (15.0%) Potassium BLOCK Lv feldspar (K) (Trans.) (65.8%) Lithic MAGMATIC metamorphic (Lm) ARC Lithic (Undis.) F L volcanic (Lv) Lt F Lt Lithic sedimentary (Ls) Lm N = 31 Ls

Lv C P Qm Lv Lv Lv Lv C C P Ls C Ls Ls Lv C LsLs C C Lv 100µm Lv 100µm Lv 100µm Ls

Figure 5. Summary of sandstone modal composition and photomicrographs from the Mystic assemblage. (A) Relative abundances of framework grains of 31 samples from the Mystic assemblage. Provenance fields are from Dickinson et al. (1983). (B) Percentage breakdown of parameter occurrences from the Mystic assemblage. (C) Photomicrographs showing pervasive occurrences of lithic volcanic (Lv), chert (C), and lithic sedimentary grains (Ls). White bar = 100 μm for scale. A summary of point-count parameters is available in Appendix 1B. Q = Qm+Qp+C; F = P+K; L = Ls+Lm+Lv; Lt = L+Qp+C.

Interpretation of Depositional units are commonly capped by siltstone and PROVENANCE Environment mudstone deposits, reflecting suspension fallout Submarine fan system. The Mystic assem- during waning flow conditions, which are inter- Compositional Data blage exposed in the northwest Talkeetna quad- preted to represent Td divisions of the Bouma rangle (Mystic Pass) and east-central McGrath sequence (Bouma, 1962). All Bouma sequences Petrographic and compositional data was quadrangle (Farewell Lake–Sheep Creek area) are interpreted to represent sedimentation of obtained for 31 sandstone samples collected is interpreted to represent deposition of low- to low-density turbidity currents. from the Mystic assemblage in the northwest- high-density sediment gravity flows in a sub- We interpret the Mystic assemblage to ern Talkeetna quadrangle (Fig. 5). Standard marine fan environment (e.g., Middleton and have been deposited in a basinal (deep-water) petrographic thin sections were cut and stained Hampton, 1973). Upward fining in individual submarine fan setting. Thinly bedded tabular for plagioclase and potassium feldspar. Thin beds of sandstone and conglomerate reflects sandstone and mudstone units are interpreted as sections were analyzed according to the modi- normal grading and is interpreted to represent unconfined, low-density flows, which are com- fied Gazzi-Dickinson point-counting method event-based sedimentation characterized by mon in mid- to outer-lobe depositional environ- (Dickinson, 1970; Ingersoll et al., 1984). Modal initial traction transport that was followed by ments (cf. Mutti and Ricci Lucchi, 1975; Ricci compositions were determined by identifying waning energy conditions, which allowed for Lucchi, 1975; Mutti, 1977). Thick-bedded con- 400 grains from each thin section. Point-count final suspension settling of siltstone and mud- glomerate units, which display tabular to len- parameters and raw point-count data are avail- stone (Kuenen and Migliorini, 1950). Normally ticular bedding geometries, are interpreted to able in the GSA Data Repository, Appendix graded, clast-supported conglomerate and mas- represent confined to unconfined, high-density 1A,1 and recalculated data are available in sive coarse-grained sandstone are interpreted sediment gravity flows that were deposited in Table 1. Recalculated data are based on pro- to reflect rapid sedimentation associated with more proximal parts of submarine fans (e.g., cedures defined by Ingersoll et al. (1984) and

the R3 and S3 divisions of high-density sedi- channel to channel-lobe transition zones). Dickinson (1985). ment gravity flows (Lowe, 1982). Massive, Although we do not rule out a shallow-shelf horizontally laminated, and ripple cross-lami- submarine depositional model for these strata, 1GSA Data Repository Item 2014245, Appendices nated sandstone beds are interpreted as Ta, Tb, there is no direct evidence that these strata were 1A–3B, is available at www.geosociety.org/pubs /ft2014.htm, or on request from editing@geosociety and Tc divisions of partially preserved Bouma directly adjacent or proximal to a shelf-margin .org, Documents Secretary, GSA, P.O. Box 9140, sequences, respectively (Bouma, 1962). These depositional system. Boulder, CO 80301-9140, USA.

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The modal composition of the Mystic TABLE 1. RECALCULATED MODAL PERCENTAGES OF SANDSTONE FROM THE MYSTIC ASSEMBLAGE assemblage is characterized by predominantly Q-F-L (%) Qm-F-Lt (%) Qm-P-K (%) Lv-Lm-Ls (%) lithic fragments (L), with subordinate amounts Sample ID QFLQmF Lt Qm PK Lv Lm Ls of quartz (Q) and relatively minor occurrences MYN 080709-01 41940198252550960 4 of feldspar (F) (Q 17%, F 2%, L 81%; Figs. 5A MYN 080709-02 134821495235523921 7 and 5B). The total quartz composition consists MYN 080709-03 126821693184636960 4 primarily of chert (C), with relatively minor MYN 080709-04 124852495324523970 3 amounts of monocrystalline quartz (Qm; Fig. MYN 080709-05 96852692283834940 6 5C). Feldspar grains are relatively rare, with MYN 080709-07 185783593384121940 6 plagioclase feldspar (P) being more abundant MYN 080709-08 104863494422335920 8 MYN 080709-09 32563559147183490010 than potassium feldspar (K) grains (Qm 62%, MYN 080709-10 2027932966338 077023 P 23%, K 15%; Fig. 5B). Lithic fragments MYN 080709-1128172619488 4 885015 consist primarily of volcanic types (Lv), with MYN 080709-12 2617341957525 084313 lithic sedimentary grains (Ls) making up a rela- MYN 080709-13 2517551958214 580020 tively smaller overall occurrence (Lv 81%, Lm MYN 080709-15 14185219764 72973126 0%, Ls 19%; Figs. 5B and 5C). Lithic volcanic MYN 080709-16 2217731967320 775025 MYN 080709-17 24076309792 8 072028 fragments are characterized almost entirely by MYN 080709-18 2317621977517 871029 a mafic to intermediate fine-grained volcanic MYN 080709-19 31168519483 41377023 groundmass (Fig. 5C). Metamorphic fragments MYN 080709-20 2926962927123 681019 are all but absent in these samples (statistically MYN 080709-21 18479649060301081019 making up less than 1 grain per sample). MYN 080709-22 1728122975736 771029 MYN 080709-23 18379639169141777023 MYN 080709-24 1318641968218 078022 Summary of Modal Compositional Trends MYN 080709-25 11 387339550252582018 Compositional data from the Mystic assem- MYN 080709-26 17380839073171078022 blage suggest that detritus was derived from a MYN 080709-28 15184519478 91375025 source rich in volcanic rocks. Source areas also MYS 080709-04 4196319783 8 883017 contributed chert and sedimentary rock frag- MYS 080709-07 1218831968119 075025 ments, which are reflected as minor detrital con- MYS 080709-08 619311985050 075025 01 DZ 11 288629277 61677023 tributions. Comparing our compositional data 03 DZ 1018951947913 865035 with the provenance fields of Dickinson et al. 05 DZ 1937873906926 578022 (1983), the majority of the Mystic assemblage Note: Q—quartz; F—feldspar; L—lithics; Qm—monocrystalline quartz; Lt—L+Qp+C; P—plagioclase feldspar; plots with sandstone derived from undissected K—potassium feldspar; Lv—lithic volcanic; Lm—lithic metamorphic; Ls—lithic sedimentary. arc sources (~84% of total samples), with minor contributions from recycled orogen sources (~16% of total samples; Fig. 5A). It is worth not- ing that while lithic volcanic fragments make up ods of crushing, grinding, Wilfley table, heavy diagrams account for each age and its uncer- a majority of overall framework grains (>65%) liquid, and a Frantz magnetic separation. A tainty (for measurement error only) as a normal in the Mystic assemblage, feldspar is only a large split of these grains (typically thousands of distribution and sum all ages from a sample into minor component (<3%) in these samples (Fig. grains) was incorporated into a 2.54 cm (1 in.) a single curve. Composite age-probability plots 5B). This could be explained by a combination epoxy mount together with fragments of the Sri were made from an in-house Excel macro (pro- of several scenarios such as breakdown/weath- Lanka (SL) zircon standard. The mounts were vided by the Arizona LaserChron Center) that ering of detrital feldspar during sedimentary sanded down to a depth of ~20 μm, polished, normalizes each curve according to the number transport or diagenetic breakdown/dissolution imaged, and cleaned prior to isotopic analysis. of constituent analyses, such that the area under of in situ feldspar after deposition. In summary, A detailed description of analytical methods, each curve is the same. the sandstone compositional data from the Mys- as well as data and concordia plots, is available tic assemblage indicate primary contributions in the GSA Data Repository, Appendix 2A (see U-Pb Age Distribution from arc sources and secondary contributions footnote 1). The Mystic assemblage of the western from a recycled orogen source. The resulting interpreted U-Pb ages are Alaska Range shows numerous occurrences of shown on age histograms and relative age- concordant Precambrian–Paleozoic age zircons U-Pb Detrital Zircon Geochronology probability diagrams (Fig. 6) using the routines (Fig. 6), with Paleozoic (Pz) age grains being in Isoplot (Ludwig, 2008). The age-probability more abundant than Precambrian (Pc) grains Methods Zircon separates were obtained from four medium-grained sandstone samples (MYS-01, TABLE 2. LIST OF LOCATIONS AND DESCRIPTIONS OF DETRITAL ZIRCON SAMPLES MYS-03, MYS-05, and FSC-02) from the lower Latitude Longitude and upper parts of the Mystic assemblage in the Sample ID Location (°N) (°W) Description western Alaska Range (for detailed locations of MYS-01 Mystic Pass 62°38.495152°29.788 Fine- to medium-grained sandstone samples in the Talkeetna and McGrath quadran- MYS-05 Mystic Pass 62°39.573152°31.560 Fine- to medium-grained sandstone gles, see Table 2 and Figure 2). Individual zir- MYS-03 Mystic Pass 62°38.216152°29.777 Fine-grained sandstone con crystals were extracted by traditional meth- FSC-02 Sheep Creek 62°26.141153°53.018 Fine- to medium-grained sandstone

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(Pz 59%, Pc 41%). The majority of Paleozoic age grains have ages between 480 and 415 Ma MYS-03 (n = 99DZ/48Hf) 10 DM (peak occurrences between 425 and 450 Ma), 400 and 325 Ma (peak occurrences between 0 335 and 380 Ma), and 320–275 Ma (peak CHUR 298 15 177 6 Hf 0.01 occurrences between 280 and 300 Ma) (Fig. 6). Epsilon Hf –10 17 Lu/ Precambrian peak ages are primarily Paleo –20 cluster n=22 proterozoic and fall between 1.8 and 2.0 Ga 30 (14%). There are also isolated occurrences of 5 youngest ages Neoproterozoic (5%), Mesoproterozoic (7%), 25 266 ± 4 and Archean age grains (5%) (Fig. 6). Based on 20 266 ± 7 271 ± 6 age spectra and populations of youngest grains, 425 15 274 ± 14 we have subdivided the Mystic assemblage into

# of grains 10 275 ± 2 a lower member and an upper member. 5 Lower Mystic assemblage. Two samples from the lower part of the Mystic assemblage

FSC-02 (n = 110DZ/53Hf) (MYS-01 and MYS-05) contain a range of 10 DM Paleozoic- and Precambrian-age detrital zir- 0 cons. A primary Paleozoic peak age for sample CHUR 15 MYS-01 occurs at 349 Ma, while secondary 177 Hf 0.01 –10 176 Lu/ Epsilon Hf Paleozoic and Precambrian peaks occur at 435 –20 Ma and 1872 Ma (Fig. 6). The three youngest overlapping concordant grains are Mississip- 5 youngest ages pian (Serpukhovian) in age (324 ± 2 Ma, 325 ± 440 278 ± 4

Upper Mystic assemblage 7 Ma, and 325 ± 12 Ma). Sample MYS-05 has 280 ± 5 a primary Paleozoic peak at 335 Ma, with sec- 20 281 ± 6 15 283 ± 12 ondary Paleozoic and Precambrian peaks at 380, 282 284 ± 5 450, and 1870 Ma (Fig. 6). The three youngest # of grains 10 overlapping concordant grains from this sample 5 are Pennsylvanian (Bashkirian–Moscovian) in age (307 ± 7 Ma, 313 ± 3 Ma, and 314 ± 3 Ma). MYS-05 (n = 89DZ/56Hf) 10 DM Upper Mystic assemblage. Two samples from the upper part of the Mystic assemblage 0 (FSC-02 and MYS-03) also contain both Paleo- CHUR 15 177 –10 6 Hf 0.01 zoic- and Precambrian-age detrital zircons. A Epsilon Hf 17 Lu/ primary Paleozoic peak age for sample FSC-02 –20 occurs at 440 Ma, while a secondary Paleo- 5 youngest ages zoic peak occurs at 282 Ma (Fig. 6). The three 307 ± 7 313 ± 3 youngest overlapping concordant grains in this 314 ± 3 sample are Early Permian (Artinskian) in age 335 325 ± 11 (278 ± 4 Ma, 280 ± 5 Ma, and 281 ± 6 Ma). 15 325 ± 5 Sample MYS-03 has a primary Paleozoic peak 380 # of grains 1870 10 at 298 Ma, with a secondary Paleozoic peak at 450 5 425 Ma (Fig. 6). The three youngest overlapping concordant grains from this sample are Middle

MYS-01 (n = 85DZ) Permian (Rodian–Wordian) in age (266 ± 4 Ma, 349 266 ± 7 Ma, and 271 ± 6 Ma). 5 youngest ages 324 ± 2 Maximum depositional age. Although age 325 ± 7 constraints are sparse for much of the Mystic 15 435 325 ± 12 subterrane, Upper Mississippian–Middle Penn- Lower Mystic assemblage 10 332 ± 6 # of grains 1872 sylvanian marine fossils have been used to con- 5 333 ± 6 strain the age of rocks that make up the Mystic assemblage (Reed and Nelson, 1980). While 0 100020003000 it has been shown that the youngest concor- Age (Ma) dant detrital age in a sample can be a statistically robust and valid approach to constrain Figure 6. U-Pb ages and eHf isotope concentrations of detrital zircons from the upper and lower parts of the Mystic assemblage. Detrital zircon (DZ) ages are shown as histograms (gray bars) and maximum depositional ages (Dickinson and

relative probability functions (area under the black curve). eHf values are depicted by gray diamonds Gehrels, 2009), here we rely on the youngest (error bars on e values are 1σ). DM—depleted mantle; CHUR—chondritic uniform reservoir. Hf graphical age peak as a conservative, first-order constraint on the maximum depositional age of

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each sample. Based on the youngest peak ages characterized primarily by negative eHf values were derived from igneous rather than meta- from the Mystic assemblage, we report an early (–3 to –43; Fig. 6). Ordovician–Silurian grains morphic source areas. A summary and plot of Middle Mississippian maximum depositional that cluster between 415 and 480 Ma show a U-Th data are available in GSA Data Reposi- age for the lower part of the assemblage and an range of both positive (+5 to +15) and nega- tory, Appendix 2A (see footnote 1).

Early Permian maximum depositional age for tive eHf values (–3 to –25; Fig. 6). Precambrian the upper part of the assemblage. detrital zircons, including those that fall in the Comparison with Potential Source Areas

Bradley et al. (2007) reported zircon ages 1800–2000 Ma age range, typically have eHf from two detrital samples as well as U-Pb ther- values that range between –10 and +10 (Fig. 6). Late Paleozoic paleogeographic models mal ionization mass spectrometry (TIMS) ages Lower Mystic assemblage. One sample for the Farewell terrane include restored loca- from a 1-m-thick ash-fall tuff from the Mystic from the lower part of the Mystic assemblage tions along the northwestern margin of Lauren- subterrane. One of the detrital samples was col- (MYS-05) contains Ordovician–Silurian and tia (e.g., Plafker and Berg, 1994), marginal to lected from map unit “Pzus” (Reed and Nelson, Devonian–Mississippian detrital zircons with Siberia in the Uralian seaway (Colpron and Nel-

1980) located near Surprise Glacier, just a few negative eHf values. Ordovician–Silurian zir- son, 2009), or more generally, as part of a zone

kilometers north of Mystic Pass. This sample con eHf values range from –3 to –12, whereas of convergence that extended from the Urals to yielded an interpreted Mississippian maximum Devonian–Mississippian values range from –10 offshore the Cordilleran margin of North Amer- deposition age based on the youngest concor- to –43 (Fig. 6). Pervasive occurrences of nega- ica (Bradley et al., 2003). Given the wide range

dant grain age of 335 Ma (Bradley et al., 2007). tive eHf values from Paleozoic detrital zircons of magmatic source areas along these regions, a The other two samples were also collected from suggest they were derived from enriched, conti- detrital zircon provenance investigation should map unit “Pzus,” near Pingston Creek, several nental magmatic source areas. Proterozoic- and ideally involve a comprehensive examination

kilometers west of Mystic Pass. The detrital zir- Archean-age zircon clusters have eHf values that of the entire Cordillera and all Arctic margin con sample yielded a youngest concordant grain range from –10 to +8 and –10 to +5, respectively crustal provinces. Such an approach is beyond age of 293 Ma; however, the age of the underly- (Fig. 6), and suggest a combination of magmatic the scope of this study given the limited num- ing tuff deposit is interpreted to be Late Triassic, sources that included depleted mantle reservoirs ber of samples. However, we do provide a gen- at 223 Ma (Bradley et al., 2007). and more enriched continental sources. eral summary of potential Paleozoic magmatic Upper Mystic assemblage. Two samples sources from northwestern Laurentia and from Hf Isotopic Analyses of Detrital Zircon from the upper part of the Mystic assemblage Arctic terranes that experienced magmatism (FSC-02 and MYS-03) contain clusters of Ordo- during the middle and late Paleozoic (Fig. 7). Methods vician–Silurian detrital zircons that have both This summary focuses on magmatic events

Hf isotopic data were collected from three positive and negative eHf values ranging from that overlap with prominent detrital zircon age out of the four sandstone samples (MYS-03, –26 to + 15 (Fig. 6). Both samples also con- populations from the Mystic assemblage in the MYS-05, and FSC-02) for which there were tain Pennsylvanian–Early Permian zircons with Ordovician–Silurian (peak ages at 425, 435, and

U-Pb age data. The Hf analyses account for entirely positive eHf values (+1 to +12; Fig. 6). 440 Ma), Devonian–Mississippian (peak ages

~53% of zircon grains analyzed for U-Pb Occurrences of positive eHf values for clusters at 335, 349, and 380 Ma), and Pennsylvanian– geochronology and represent both the lower of Pennsylvanian–Early Permian zircons sug- Permian (peak ages at 282 and 298 Ma) (Fig. 6). (MYS‑05) and upper (MYS-03 and FSC-02) gest depleted mantle reservoir sources, whereas Precambrian-age zircons are present in the parts of the Mystic assemblage. These analy- Ordovician–Silurian detrital zircons were likely Mystic assemblage and are primarily Paleo ses were conducted at the Arizona LaserChron sourced from a combination of depleted man- proterozoic, with peak ages of ca. 1870 Ma Center (ALC) at the University of Arizona. A tle and enriched continental sources. Samples (Fig. 6). Given the relatively small percentages description of the analytical methods and the Hf MYS-03 and FSC-02 contain clusters of Paleo- of these ages, it is likely that these grains have analytical results can be found in the GSA Data proterozoic-age zircons that have both positive been recycled from sedimentary or metasedi-

Repository, Appendix 3A (see footnote 1). and negative eHf values (+1 to –15). FSC-02 mentary sources. It is important to note that has clusters of Mesoproterozoic and Neopro- 2000–1800 Ma igneous zircons occur in both

Hf Isotopic Distribution terozoic zircons with positive and negative eHf the Laurentian and Siberian cratons (as well as Hf isotopic data from Paleozoic and Pre- values that range from +10 to –5 (Fig. 6.). Pre- others) and do not represent a unique population

cambrian zircons of the Mystic assemblage cambrian eHf values suggest a combination of for provenance comparisons (Prokopiev et al., show a wide range of both positive and negative magmatic sources that included enriched con- 2008; Condie et al., 2009; Safonova et al., 2010).

eHf values (+15 to –32; Fig. 6). Approximately tinental sources and depleted mantle reservoir

65% of eHf values are reported from Paleozoic- sources. Uralian Source Areas age zircon grains, and the remaining ~35% of Uralian magmatism was nearly continuous

eHf values are from Precambrian grains. The eHf DISCUSSION along the Arctic margin throughout much of the values are reported in the context of the four pri- Paleozoic, but it has been divided into pulses mary U-Pb detrital zircon age clusters from the Comparison of U-Pb ages and Hf isotopic that occurred at 460–420, 415–395, 365–355, Mystic assemblage (275–320 Ma, 325–400 Ma, values of detrital zircons from the Mystic 345–330, 320–315, and 290–250 Ma (Fershtater 415–480 Ma, and 1800–2000 Ma). assemblage with ages and geochemistry from et al., 2007). Ordovician–Silurian grains from Pennsylvanian–Early Permian detrital zir- possible magmatic source areas provides a the Mystic assemblage overlap with magmatic cons with ages that cluster between 275 and powerful provenance approach to better under- events associated with early arc activity in the 320 Ma are dominated almost entirely by posi- stand the late Paleozoic tectonic history of the Uralides, which peaked between 460 and 420 Ma

tive eHf values (+2 to +14), whereas Middle Farewell terrane. Low U-Th ratios (values <8) (Fershtater et al., 2007). Plutonic and volcanic Devonian–Mississippian detrital zircons with from detrital zircon grains within the Mystic rocks of this age are most abundant in the Tagil ages that cluster between 325 and 400 Ma are assemblage suggest that the majority of grains megazone and Platinum belt of the northwest

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Urals (Fershtater et al., 2007). Magmatism during this time was characterized by mafic to ultra- Potential Magmatic Sources Isotopically primitive magmatism mafic primary melts associated with an island- arc setting (Fershtater et al., 2007). Isotopically enriched magmatism Stikinia Devonian–Mississippian subduction-related Klinkit group Variable isotopic signatures granites were emplaced during two episodes of YTT Unknown isotopic composition magmatism at ca. 370–350 Ma and 335–315 Ma and yielded relatively primitive ranges of isotopic Uralides signatures (eNd: –4.1 to +4.1 and –0.9 to +6.6, Skolai arc Sicker group respectively; Bea et al., 2002). The timing of Wrangellia Klakas Descon these magmatic episodes corresponds with Mis- arc arc sissippian age populations in the Mystic assem- Alexander DGS BGS blage; however, the isotopic signatures do not match well with the primarily enriched isotopes upper MA (eHf: –10 to –30) observed in the detrital zircons. εHf signatures of lower MA Subduction processes are thought to have detrital zircon trends ceased in the Urals by the end of the Pennsyl- 300 400500 vanian (Bea et al., 2002; Brown et al., 2006). +15 upper MA However, the generation of intracontinental granite bodies continued through the Permian 0 CHUR and transitioned from initial intermediate to juvenile magmatism (e : –2.2 to +6.4) to more -15 Nd enriched magmatism (eNd: –12 to +5) during Epsilon Hf the latest Permian (Bea et al., 2002). Finally, it -30 should be noted that much of the Pennsylvanian n=137 of 213 in the Urals was characterized by a lull in mag- 16 matism from 315 to 300 Ma (Fershtater et al., 2007), which overlaps, in part, with one of the 12 primary age populations from the upper Mystic assemblage. 8 Relative probability # of grains Peri-Laurentian Source Areas 4 Potential peri-Laurentian sources for the Mystic assemblage include rocks from Stikinia lower MA and Yukon-Tanana terrane and to a lesser extent, 0 Quesnellia and the Slide Mountain assemblage. CHUR Devonian to Mississippian peak ages from the -15 Mystic assemblage overlap with magmatism of

Epsilon Hf the Middle to Late Devonian Ecstall cycle (390– -30 365 Ma), Late Devonian–earliest Mississippian n=114 of 245 Finlayson cycle (365–357 Ma), Early Mississip- pian Wolverine cycle (357–342 Ma), and Late 16 Mississippian Little Salmon cycle (342–314 Ma), which are associated with Yukon-Tanana 12 and related terranes (Colpron et al., 2006; Relative Piercey et al., 2006; Nelson et al., 2006). Middle 8 probability to Late Devonian felsic igneous rocks have been # of grains 4 reported from parts of the Endicott and Tracy Arm assemblages of the Yukon-Tanana terrane in southeast Alaska and British Columbia 300 400500 Age (Ma) (McClelland et al., 1991; Gehrels et al., 1992; Currie, 1994; Gehrels, 2001) and from as far

Figure 7. Summary of U-Pb detrital zircon ages and eHf isotope composition from the Mystic assem- north as east-central Alaska and western Yukon blage presented in the context of Cordilleran and circum-Arctic magmatic source areas. Refer to (Dusel-Bacon et al., 2004, 2006). Although text for a summary of previous studies that helped defined age ranges for potential magmatic less voluminous, Devonian magmatism also source areas (DGS—Donjek Glacier suite, BGS—Barnard Glacier suite). Note that the lower Mystic assemblage (MA) also includes 25 zircon ages from age-equivalent strata reported by Bradley et al. occurs in continental margin rocks of western Laurentia in the Selwyn basin, Cassiar terrane, (2007). For clarity, 1σ error bars of eHf values are not shown here but have been included in Figure 6. YTT—Yukon-Tanana terrane; CHUR—chondritic uniform reservoir. and the western Kootenay terrane of southeast British Columbia (Nelson et al., 2006; Paradis et al., 2006).

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Latest Devonian–Early Mississippian inter- present in the eastern part of Stikinia (Diakow Brandon et al., 1986; Dodds and Camp- mediate to mafic igneous rocks record a hall- and Rogers, 1998). However, volcanism in the bell, 1988; Andrew et al., 1991; Parrish and mark episode of magmatism in the Yukon- Stikine terrane is thought to have ceased by lat- McNicoll, 1992; Sluggett, 2003). More recent Tanana terrane in British Columbia, the Yukon, est Carboniferous time (Gunning et al., 2006). geochronologic studies from the Sicker Group and parts of east-central Alaska (e.g., Dusel- Pennsylvanian igneous rocks from the western on Vancouver Island make the case for mag- Bacon and Aleinikoff, 1985; Aleinikoff et al., margin of Stikinia exhibit enriched mid-ocean- matic pulses during the Late Devonian–Mis- 1986; Mortensen, 1990; McClelland et al., ridge basalt (E-MORB) geochemical signatures sissippian (370–339 Ma) and Pennsylva- 1991; Gehrels et al., 1992; Johnston et al., 1996; (Logan, 2004) and are interpreted to have devel- nian–Permian (reported ages of ca. 295 Ma, Alldrick, 2001; Szumigala et al., 2002; Day oped within an intra-arc or back-arc rift setting. 309 Ma, and 313.5 Ma; Ruks and Mortensen, et al., 2003; Colpron et al., 2006; Dusel-Bacon Permian igneous rocks have been reported from 2007; Ruks et al., 2010). Additionally, stud- et al., 2004, 2006; Mihalynuk et al., 2006; Mur- the Taku terrane in southeast Alaska (Gehrels, ies by Israel et al. (2014) documented a Late phy et al., 2006; Nelson et al., 2006; Roots et al., 2002) and exhibit E-MORB to enriched within- Devonian age (363.5 Ma) for gabbros from 2006). Late Devonian–Mississippian magma- plate geochemical signatures (Stowell et al., both Wrangellia and the Alexander terranes, as tism has also been recorded in the Quesnel, 2000). Geochemical analyses from the Klinkit well as a 353.8 Ma age for a felsic tuff from the Slide Mountain, and Stikine terranes in south- Group show that volcanic rocks have very Station Creek Formation on Wrangellia. These western Yukon and western British Columbia minor crustal inheritance and reflect primitive recent findings indicate that igneous source

(e.g., Mortensen, 1990; Currie, 1994; Gunning (eNd: +6.7 to +7.4) isotopic signatures (Simard rocks for most of the Devonian–Mississippian et al., 1994; Greig and Gehrels, 1995; John- et al., 2003). detrital zircon ages in the Mystic assemblage ston et al., 1996; Gunning et al., 2006). Isolated The age of the Klinkit Group overlaps with are present in the Wrangellia terrane and per- occurrences of Early Mississippian metavol Pennsylvanian–Early Permian ages in detrital haps the Alexander terrane as well. canic and metaplutonic rocks are present within zircon populations from the Mystic assemblage. Pennsylvanian–Permian plutonic suites (320– the Endicott Arm assemblage from southeast However, Klinkit cycle magmatism is primarily 285 Ma) have been reported from the Skolai Alaska (Gehrels and Kapp, 1998). recorded by mafic volcaniclastic rocks, which arc and Strelna metamorphics of Wrangellia Devonian–Mississippian felsic to interme- are unlikely to be a significant zircon source. (Nokleberg et al., 1986; Aleinikoff et al., 1988; diate plutonic and volcanic rocks occur in the Consequently, most of the Klinkit magmatic Dodds and Campbell, 1988; Gardner et al., Iskut and Scud River areas and Semenof Hills of cycle actually corresponds to a decline (or 1988; Beard and Barker, 1989; Plafker et al., Quesnellia (Gunning et al., 1994; Brown et al., near absence) in the frequency of Late Pennsyl 1989). Recent studies of magmatic rocks in the 1996; Logan et al., 2000; Logan, 2004; Simard vanian–Early Permian igneous ages in the Alexander and Wrangellia terranes refined the and Devine, 2003; Simard, 2003; Gunning et al., Yukon-Tanana terrane (Nelson et al., 2006). timing of magmatism for the Barnard Glacier 2006). Some of the Early Mississippian plutonic suite as ca. 301–307 Ma and that of the Donjek complexes in northern Stikinia are thought Insular Terrane (Wrangellia and Alexander) glacier suite to be 284–291 Ma (Beranek et al., to record relatively primitive arc magmatism Source Areas 2014). Pennsylvanian arc-related igneous rocks

(Logan et al., 2000; Logan, 2004). Middle to Previous work has demonstrated that the of the Skolai group have positive eHf values (eNd:

Late Mississippian-age igneous rocks are rare in Wrangellia and Alexander terranes have been +5.2 to +7.3; eHf: +10.6 to +11.4) suggestive of the northern parts of the Yukon-Tanana terrane, proximal to each other since late Paleozoic time more depleted mantle sources (Greene et al., as only isolated occurrences have been reported (Gardner et al., 1988), and together formed a 2009). Geochemical values from the Barnard

in association with the Fortymile River assem- composite terrane that has been referred to as and Donjek Glacier suites (eNd: –0.5 to +6.8 and blage of east-central Alaska (Dusel-Bacon et al., the Insular terrane (Monger et al., 1982). Ordo- +2.1 to +6.1, respectively) suggest mixing of 2006). Much of the Late Mississippian magma- vician–Silurian detrital zircons from the Mystic both depleted and slightly more enriched mantle tism along the northern Cordillera is recorded assemblage overlap closely in age with igneous sources (Beranek et al., 2014). Late Pennsyl- in the southeastern parts of the Yukon-Tanana rocks that are associated with the Descon and vanian–Permian magmatism and primarily terrane in mafic to felsic plutonic rocks of the Klakas arcs of the Alexander terrane in southeast juvenile isotopic signatures are both compat- Wolf Lake–Jennings River and Glenlyon areas Alaska (Gehrels and Saleeby, 1987; Gehrels, ible with detrital zircon trends from the Mystic of northern British Columbia and southern 1990; Gehrels et al., 1996). Cambrian–Ordo- assemblage. Yukon (Nelson and Friedman, 2004; Colpron vician volcanic rocks also occur in the Donjek et al., 2006; Mihalynuk et al., 2006; Roots et al., assemblage in the Saint Elias Mountains of SW Up-Section Trends in Provenance 2006). Late Mississippian intermediate to fel- Yukon (Dodds and Campbell, 1988; Mihalynuk sic igneous rocks have also been reported from et al., 1993; Beranek et al., 2012). Ordovician– Detrital zircon ages from the lower parts of northwestern Stikinia (Mihalynuk et al., 1994; Silurian igneous rocks of the Alexander terrane the Mystic assemblage cluster in two distinct

Gunning et al., 2006). show relatively juvenile eNd values (+3.3 to +12; Paleozoic age groups that include Ordovician– Grains of Pennsylvanian–Permian age over- Samson et al., 1989; Gehrels et al., 1996; Cecil Silurian ages (peak ages at 435 and 450 Ma) lap with the timing of the Klinkit magmatic et al., 2011). This age population and isotopic and Devonian–Mississippian ages (peak ages at cycle (314–269 Ma) associated with the Yukon- signature have also been reported from detrital 335, 348, 380 Ma; Fig. 6). Ordovician–Silurian Tanana terrane (Colpron et al., 2006; Dusel- zircon data sets from middle to late Paleozoic- detrital zircons overlap closely with plutonic Bacon et al., 2006; Nelson et al., 2006; Roots age rocks in the Banks Island assemblage and and volcanic source areas that are associated et al., 2006). Pennsylvanian igneous rocks Alexander terrane (e.g., Beranek et al., 2012; with the Descon and Klakas arcs of the Alex- have been documented in the Stikine River Tochilin et al., 2014). ander terrane in southeast Alaska. Late Devo- region of Stikinia (Mihalynuk et al., 1994; Isolated occurrences of Devonian-age igne- nian–Early Mississippian detrital zircon ages Gunning et al., 1994, 2006), and Late Pennsyl- ous rocks have been reported from the Sicker overlap with plutonic and volcanic source areas vanian–age felsic volcanic rocks (308 Ma) are Group of southern Wrangellia (Muller, 1980; from both Insular (Alexander and Wrangellia)

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20 and peri-Laurentian (Yukon-Tanana, Stikinia, and Quesnellia) terranes. However, none of the 15 peri-Laurentian terranes includes compatible magmatic sources for zircon populations of 10 Ordovician–Silurian age present in the Mystic DM assemblage. The timing of magmatism in the 5 Urals also overlaps, albeit somewhat variably, CHUR with both age populations but consists of Devo- 0 nian–Mississippian igneous rocks that are far less enriched than isotopic signatures observed –5 in the Mystic assemblage. Epsilon Hf The upper units of the Mystic assemblage con- –10 tain two primary Paleozoic age clusters of detrital zircons that include Ordovician–Silurian (peak –15 ages at 440 and 425 Ma) and Pennsylvanian– n –20 Early Permian (peak ages at 298 and 282 Ma) Permian (?) – Banks Island age populations. The Late Pennsylvanian–Early assemblage, n=72 –25 Permian populations are perhaps the most diag- average crustal evolutio Permian – upper Mystic nostic trend from the Mystic assemblage. With assemblage (this study), n=101 the exception of the Wrangellia and Alexander –30 terranes, there is a general paucity of known magmatic ages from all of the source regions that are compatible with the range of Pennsyl- Permian (?) – Banks Island assemblage vanian–Permian ages in the Mystic assemblage. n=248 This population of detrital zircon ages, as well as their depleted isotopic composition, overlaps closely with sources in the Wrangellia and Alexander terranes. Additionally, a comparison with Permian-age units from the Alexander ter- Permian – Alexander terrane rane and the associated Banks Island assemblage n=326 (Tochilin et al., 2014) reveals similar trends in

both age populations and eHf values to that of the Relative Probabilit y upper Mystic assemblage (Fig. 8). Although magmatism documented in the Urals and peri-Laurentian terranes is correla- tive with some of the Paleozoic detrital zircon Permian – upper Mystic assemblage (this study) populations observed in the Mystic assemblage, n=209 only the Wrangellia and Alexander sources can account for all of these trends. Thus, provenance trends from the Mystic assemblage are 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 best explained by a scenario in which the Fare- Age (Ma) well terrane was receiving primarily arc-derived detritus from exhumed parts of the Wrangellia Figure 8. Comparison of U-Pb ages and Hf isotopes of detrital zircons from the Mystic assemblage with results from Permian strata of the Alexander terrane (Halleck Formation) and Banks and Alexander terranes during Pennsylvanian– Island assemblage (Tochilin et al., 2014). The projected average crustal evolution assumes present- Permian time and perhaps as early as the Mis- day 176Lu/177Hf = 0.0115 (Vervoort and Patchett, 1996; Vervoort et al., 1999). DM—depleted mantle; sissippian. CHUR—chondritic uniform reservoir.

Tectonic and Paleogeographic Implications consistent with a model where the Farewell ter- As previously mentioned, the Browns Fork rane was in the Panthalassic Ocean (along the orogeny recognized by Bradley et al. (2003) Given the size and scale of the Farewell ter- outboard margin of the Slide Mountain Ocean) was based on a ca. 285 Ma metamorphic age rane and the fact that we have only just begun and adjacent to components of the Insular ter- in the Kuskokwim Mountains (proposed hinter to understand the basic geologic framework of rane during the Mississippian–Pennsylvanian land) and the occurrence of a thick succession western Alaska, it is difficult at this stage to con- (Fig. 9). This is also somewhat consistent with of Pennsylvanian–Permian coarse-grained clas- fidently summarize in detail the Neoproterozoic the hypothesis put forth by Bradley et al. (2003) tics comprising the Dall basin (proposed fore- to late Paleozoic paleogeographic history of the for the Browns Fork orogen. While the precise land basin) in the Mystic subterrane. The Mystic Farewell terrane. However, this study serves location and basin setting of the Farewell terrane assemblage and Mount Dall conglomerate may as a small contribution and offers a revised during this time are debatable, it was most likely partially overlap in age; however, their strati- interpretation of the tectonic history and paleo proximal to, and receiving detrital contributions graphic relationships and/or structural juxta- geography of the Farewell terrane. Our data are from, the Wrangellia and Alexander terranes. position have not been verified. Beranek et al.

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Pennsylvanian–Early Permian (c. 300–285 Ma) Mystic assemblage overlap with magmatic OM source areas from the Wrangellia, Alexander, and Yukon-Tanana terranes and in part with Stikinia and Quesnellia. Pennsylvanian–Per 60° SIB mian zircon populations in the upper part of AX FW/MYS Taimyr the Mystic assemblage overlap in age with Angayucham FW/MYS magmatic sources from the Wrangellia and AA Alexander terranes, which also correspond to WR times of minimal zircon production in potential BAR KAZ source rocks from Yukon-Tanana and related ST Slide Urals terranes. Maximum depositional ages from the YT Mountain Mystic assemblage overlap in part with previ- Ocean BAL ously reported Mississippian–Pennsylvanian fossil age ranges for the Mystic assemblage and QN EK LAU extend the upper age limit of these strata to at least the Early Permian. Provenance trends from 0° NS the Mystic assemblage are best explained by a ian paleogeographic model wherein the Farewell FW/MYS – Siberia model Alleghen Paleo- terrane was in the Panthalassic Ocean (along the FW/MYS – This study Tethys outboard margin of the Slide Mountain Ocean) Subduction zone proximal to, and receiving detritus from, arc and AFR recycled orogen sources of the Alexander and Collisional belt SAM Wrangellia terranes during Mississippian–Early ARB 30° Ocean spreading Permian time.

Figure 9. Pennsylvanian–Early Permian paleogeographic map showing a reinterpreted location of ACKNOWLEDGMENTS the Farewell terrane (Mystic subterrane) (FW/MYS) in the Panthalassic Ocean and proximal to peri- This research was supported in part by the National Science Laurentian terranes along the western margin of the Slide Mountain Ocean. Paleogeographic map Foundation (EAR-1119550 awarded to Hampton) and gradu- ate student research grants awarded to Malkowski from the is modified from Nelson et al. (2013). Dashed FW/MYS depicts a previously proposed late Paleozoic Geological Society of America and the American Association paleogeographic locality along the Siberian margin (Colpron and Nelson, 2009). New provenance of Petroleum Geologists. Detrital zircon analyses were con- data from the Mystic assemblage support a revised paleogeographic model where the Farewell ducted under the direction of George Gehrels, Mark Pecha, terrane was in a basinal setting that was proximal to parts of the Alexander (AX) and Wrangellia and staff at the University of Arizona LaserChron Center (WR) terranes during Mississippian–Early Permian time (depicted by solid FW/MYS). ST—Stikinia, (ALC), which is supported by the National Science Founda- tion (EAR-1032156 and EAR-0929777). We thank Dan Bradley YT—Yukon-Tanana, QN—Quesnellia, EK—eastern Klamath terranes, NS—northern Sierra terranes, for field assistance while in Alaska and Kraig Koroleski for AA—Arctic Alaska, OM—Omulevka, SIB—Siberia, KAZ—Kazakhstania, BAR—Barentsia, BAL—Bal- laboratory assistance. This manuscript benefited from discus- tica, LAU—Laurentia, SAM—South America, AFR—Africa, ARB—Arabia. sions with Robert Blodgett, Dwight Bradley, Julie Dumoulin, Kaz Fujita, and Duncan Sibley, as well as insightful reviews by Dwight Bradley, Maurice Colpron, and John Goodge.

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