Age and Chemostratigraphy of the Finlayson Lake District, Yukon: Age and Chemostratigraphy of the Finlayson Lake District, Yukon: Implications for Volcanogenic Massive Sulfide (VMS) Implications for Volcanogenic Massive Sulfide (VMS) Mineralization and Tectonics Along the Western Laurentian Mineralization and Tectonics Along the Western Laurentian Continental Margin

The Yukon-Tanana terrane in the Finlayson Lake district, Yukon, represents one of the ﬁ rst arc – back-arc systems that formed adjacent to the Laurentian continental margin in the mid-Paleozoic. Back-arc rocks contain many large and high-grade volcanogenic massive sul ﬁ de (VMS) deposits. This study integrates U-Pb zircon geochronology, lithogeochemistry, and Hf-Nd isotopes to establish precise controls on tectonomagmatic activity adjacent to the western Laurentian margin in the Late Devonian to Early Mississippian. High-precision chemical abrasion-(CA) ID-TIMS U-Pb zircon geochronology de ﬁ nes coeval arc (ca. 363.1 to 348Ma) and back-arc (ca. 363.3 to 355.0Ma) magmatism in the Finlayson Lake district that intruded continental crust of Laurentian a ﬃ nity (e.g., Snowcap assemblage). Ma ﬁ c and felsic rocks display geochemical and isotopic characteristics that are consistent with being formed from mixtures of depleted asthenosphere and enriched lithospheric mantle sources. These melts variably entrained Laurentian continental crust via high-temperature crustal melting due to basaltic underplating. The high-temperature back-arc felsic magmatism occurs at speci ﬁ c time periods coinciding with VMS deposits and supports previous genetic models for VMS mineralization that suggest elevated heat ﬂ ow and hydrothermal circulation were due to regional-scale rift-related magmatism rather than from local subvolcanic intrusions. The short timescales and transient nature of tectonomagmatic events in the Finlayson Lake district suggest that rapid and complex subduction initiation of oceanic and continental crust fragments facilitated coeval compression, extension, and magmatism in the arc and back-arc regions. We thus reevaluate the presently accepted tectonostratigraphic framework of the Finlayson Lake district and suggest revised interpretations that shed light on VMS depositional environments and a possible broader association with the ca. 358Ma Antler Orogeny. Results of this study have implications for incipient tectonics, magmatism, and mineralization along the western Laurentian continental margin and other orogenic belts globally.


Introduction
The Yukon-Tanana terrane represents the largest arcback-arc terrane in the northern Cordillera and is a critical component for deciphering the early tectonomagmatic and metallogenic evolution of the peri-Laurentian realm [1,2].
Rocks that comprise Yukon-Tanana terrane have geochemical, isotopic, and metallogenic links to the Laurentian continent (e.g., [3][4][5][6]) yet display magmatic, metamorphic, structural, and sedimentary characteristics that differentiated it from the neighboring platformal strata, thus defining an allochthon-parautochthon relationship [1,7].The Finlayson Lake district is the most stratigraphically intact segment of Yukon-Tanana terrane rocks in the northern Cordillera and has been the locus of research since the mid-1980s [4,8].There was increased interest in the area due to the discovery of numerous volcanogenic massive sulfide (VMS) occurrences in the mid-1990s [9][10][11], which has resulted in the discovery of >40 Mt of polymetallic ore, including the ~18.1 Mt Kudz Ze Kayah, ~1.5 Mt GP4F, ~10 Mt Kona, and 6.2 Mt Wolverine VMS deposits [12][13][14].Coincident mapping, geochemical, tracer isotope, and U-Pb geochronological work defined the chronology and chemostratigraphy of the Finlayson Lake district and illustrated that it formed as an evolving arc-back-arc system between ca.366 Ma and 345 Ma [15][16][17].
The geochronological work that underpinned much of the previous work was based on traditional, multigrain zircon fractions and air abrasion methods [18] prior to significant advancement in the field of U-Pb geochronology with the invention of chemical abrasion pretreatment techniques [19]; thus, issues remain with regard to the accuracy and resolution of various plutonic and volcanic events and the timing of VMS mineralization in the Finlayson Lake district (e.g., [20]).Advances in high-precision U-Pb geochronology have revolutionized our understanding of ore systems and have allowed refinement of the timing of volcanic and plutonic activity related to mineralization on relatively short time scales from 10 s to 100 s of thousands of years [21][22][23][24][25]. Refinement of the chemical abrasion pretreatment technique [19] and application of the EARTHTIME standard [26] to U-Pb zircon geochronology have increased precision and relative uncertainties to as low as ~0.1% for single crystals and ~0.02% for weighted means, compared to laser ablation methods that achieve closer to ~1% [27].In VMS districts, this precision provides the ability to define mineralized horizons on a regional scale and assess the timing of past volcanic eruptions and plutonic activity and their relationships to VMS mineralization [28,29].In the Finlayson Lake district, these high-precision geochronology methods have defined the ages of the Kudz Ze Kayah, GP4F, and Wolverine VMS deposits [30]; however, the age relationship to local plutons with respect to the proposed heat sources to the VMS systems remains unclear.
Magmatism is critical in the formation of VMS deposits, with magmatic heat interpreted to be the thermal driver of VMS-related hydrothermal circulation in the upper crust [31][32][33][34].While it remains uncertain if plutons and subvolcanic intrusions spatially associated with VMS camps provide enough heat to form large deposits [35,36], especially in light of recent models of incremental pluton assembly in arc settings [37][38][39][40], it is clear that high-temperature felsic volcanism is commonly associated with VMS mineralization in continental-dominated terranes [17,[41][42][43][44].This hightemperature magmatism is often expressed as high concentrations of high field strength elements (HFSE; e.g., Nb, Th, Y, and Zr) and rare earth elements (REE; [17,41,43,45]) and has been suggested that this is due to basaltic underplating of continental crust that provides higher than normal geothermal gradients, induces greater dissolution of HFSE-REE minerals in the crust during melting, and by association has the potential for the genesis of large VMS deposits [17,36,41,43].To test this underplating-crustal melting hypothesis, petrological (i.e., MLA-SEM) and radiogenic tracer isotope geochemical data is critical for validating and quantifying the relative abundance of crust versus mantle components involved in VMS-related felsic magma genesis; however, this requires both detailed lithostratigraphic and chronostratigraphic controls and VMS-bearing and VMS-barren stratigraphy with similar continental basement, to properly assess magmatic controls on VMS genesis (e.g., [42,46]).
In this contribution, we report new high-precision CA-ID-TIMS and LA-ICP-MS U-Pb zircon dates, whole-rock lithogeochemistry, MLA-SEM modal abundances, and Hf-Nd isotopes for the earliest arc-back-arc environments that formed immediately adjacent to the western Laurentian continental margin in the Late Devonian to Early Mississippian.Specifically, we report new results for back-arc rocks in the Big Campbell thrust sheet and arc rocks in the Cleaver Lake thrust sheets in the Finlayson Lake district of the Yukon-Tanana terrane.We present a revised interpretation of Yukon-Tanana terrane evolution that incorporates early and recent interpretations of the ultramafic rocks and their volcanic cover sequences in the district (e.g., [4,8,[47][48][49][50]).In this new context, our results constrain the timing and petrogenesis of upper-crustal plutonism and volcanism in the back-arc rift and provide constraints on the timing, duration, and host rock composition of VMS mineralization at or below the seafloor.In the arc regions, these new data increase temporal resolution for the earliest-known arc magmatism in the district and further constrain the longevity of the arc during a period of active back-arc extension.These data significantly contribute to our understanding of the nature and timescales of magma-rich arc rifting and associated VMS mineralization along an established and attenuated continental margin setting along the ancient Cordilleran margin of Yukon, Alaska, and British Columbia.

Previous Interpretation of the Finlayson Lake District
The Finlayson Lake VMS district of southeastern Yukon is a fault-bounded block of the Yukon-Tanana and Slide Mountain terranes that formed off the western continental margin of Laurentia in the mid-to late-Paleozoic (Figure 1; [8,16,49]).Rocks that comprise Yukon-Tanana terrane arc and back-arc assemblages are variably deformed and metamorphosed volcanic, plutonic, and sedimentary rocks that locally retain primary geological and geochemical characteristics; these rocks were deposited or intruded above a pre-to Late Devonian basement [1,5,7,16].The Jules Creek transform fault juxtaposes the Yukon-Tanana terrane adjacent to ophiolitic rocks of the Slide Mountain terrane [16], which were then together thrust above North American platformal strata along the Inconnu thrust in the Late Jurassic to Early Cretaceous [51].The current geographical configuration of the Yukon-Tanana terrane in central Yukon was attained in the Eocene following ~430 km of displacement along the Tintina strike-slip dextral fault system, which resulted in the offset sliver referred to as the Finlayson Lake district (Figures 1 and 2; [52]).
In the current interpretation of the structural geometry of the Yukon-Tanana terrane [16], Yukon-Tanana terrane rocks within the Finlayson Lake district are hosted in three distinct structural panels (from structurally deepest to shallowest): the Big Campbell, Money Creek, and Cleaver Lake thrust sheets (Figures 2 and 3; [16]).Upper Devonian and older metasedimentary rocks of the North River formation comprise the basement to both the Big Campbell and Money Creek thrust sheets, but the formation is not observed in the Cleaver Lake thrust sheet.The Big Campbell thrust sheet is bounded below by the post-Late Triassic Big Campbell thrust fault and above by the Money Creek thrust fault (Figures 2 and 4).Rocks in the Big Campbell thrust sheet comprise the basal Upper Devonian and older North River formation and the overlying Upper Devonian mafic and felsic metavolcanic and metasedimentary rocks of the Grass  Lakes group, which includes the Fire Lake, Kudz Ze Kayah, and Wind Lake formations (Figure 2).Late Devonian gabbroic to dioritic intrusions of the North Lakes intrusion and granitoid intrusions of the Grass Lakes plutonic suite cut the Grass Lakes group and then are all unconformably overlain by Lower Mississippian metaclastic and mafic to felsic metavolcanic rocks of the Wolverine Lake group (Figures 3 and 4).Metasedimentary rocks of the Money Creek formation and metabasalt of the Campbell Range formation, both originally thought to be Lower Permian, are interpreted to unconformably overlie the Wolverine Lake group; we revisit and substantially revise this interpretation below.The Grass Lakes and Wolverine Lake groups contain VMS mineralization felsic volcano-sedimentary stratigraphy that is interpreted to have formed in an evolving continental arc to back-arc basin tectonic setting that represents the earliest stages of mid-Paleozoic rifting on the western peri-Laurentian margin [7,16,30,47,48,53].
In the hanging wall of the Money Creek thrust fault, rocks comprising the Money Creek thrust sheet include the Upper Devonian North River formation that is overlain by Upper Devonian to Lower Mississippian felsic to intermediate metavolcanic and metasedimentary rocks (Waters Creek and Tuchitua River formations).These rock units are intruded by granitic rocks of the Early Mississippian Simpson Range plutonic suite, then capped by Mississippian to Lower Permian limestone, mafic metavolcanic, and metaclastic rocks (Figures 2 and 3; [3,4,16]).The rocks in the Money Creek thrust sheet are interpreted to have formed in a continental arc (±local back-arc basin) tectonic setting to the southwest of the Big Campbell thrust sheet and are presently not associated with any significant VMS Figure 2: Regional geologic setting of the Finlayson Lake district, Yukon-Tanana terrane, showing the spatial relationship to the Slide Mountain terrane and North American continental margin (modified after [6,54]).(a) Geological map of the district, where numbers indicate locations of prospective VMS deposits in the region.All geological units in legend are described in detail in Murphy et al. [16].Diamond symbols represent locations of geochronology (red fill) and lithogeochemistry/isotope (white fill) samples outside of detailed maps in Figures 4(a 4 Lithosphere mineralization [3,7,16]; however, several mineral occurrences are present [54].Rocks from the Money Creek thrust sheet were not investigated in this study and will not be further discussed.The Cleaver Lake thrust sheet structurally overlies the Money Creek thrust sheet and contains relatively undeformed and unmetamorphosed Late Devonian mafic and felsic volcanic rocks (Cleaver Lake formation) that overlie mafic and ultramafic rocks; these rock units are subsequently intruded by Early Mississippian granitoids of the Simpson Range plutonic suite [16,49,55].The Cleaver Lake thrust sheet was thrust above the Money Creek thrust sheet along the Cleaver Lake thrust fault after the Early Permian [16].Maficultramafic rocks were initially interpreted to have been Permian intrusions that were correlative to the Slide Mountain terrane, thereby constraining the age of thrusting to Permian [16]; however, in this contribution, we revise this interpretation as discussed below.The rocks in the Cleaver Lake thrust sheet are interpreted to have been generated in an intraoceanic arc and subduction complex to the southwest of the Money Creek and Big Campbell thrust sheets and are also not associated with any known VMS mineralization [3,6,7,16,45].
Slide Mountain terrane rocks are juxtaposed against Yukon-Tanana terrane rocks along the Jules Creek fault (Figure 2; [16]).North and east of the Jules Creek fault, these rocks comprise Early Mississippian to Lower Permian metasedimentary and metavolcanic rocks of the Fortin Creek Group, which are unconformably overlain by Lower Permian mafic metavolcanic rocks and metasedimentary rocks of the Campbell Range Formation and Lower to Middle Permian sedimentary rocks of the Gatehouse formation (Figure 2).The Slide Mountain terrane contains the Lower Permian mafic-type Ice VMS deposit and has been interpreted to form in a back-arc basin to mid-ocean ridge tectonic setting [56][57][58].

Previous Geologic Interpretation of the Big Campbell
Thrust Sheet 2.1.1.Grass Lakes Group.The Fire Lake formation has traditionally been interpreted to be the stratigraphically lowest volcanic unit overlying the metaclastic rocks of the North River formation [16,51].The original definition of the formation comprised mafic volcanic-volcaniclastic rocks with geochemical affinities equivalent to enriched midocean ridge basalt (E-MORB), back-arc basin basalt (BABB), alkalic ocean island basalt (OIB), and low-Ti island arc tholeiite (IAT) to boninite affinities [7,47,48]; these mafic volcanic rocks overlie mafic-ultramafic bodies that were originally inferred to be comagmatic intrusions, an interpretation that is revisited herein.Variable Nd isotopes (εNd 350 Ma = -0:3 to +8.5) and trace element signatures suggest they represent   [30,63], and [54]).Diamonds represent sample locations for U-Pb (red fill = this study; blue fill = [30]) and lithogeochemistry/isotopes (white fill) as in Figure 2. Fsp = feldspar; F = felsic; I = intermediate; carb = carbonaceous; Qtz = quartz.6 Lithosphere varying mixtures of back-arc mantle and arc mantle wedge with or without slab and/or continental crustal contributions that were generated during the onset of back-arc-related rifting along the Laurentian margin [47,48].The ~10 Mt Kona Cu-Co-Au VMS deposit is hosted by mafic volcaniclastic rocks (primarily boninitic) and fine-grained turbiditic metasedimentary rocks that are interpreted to have formed in a subaqueous setting [20,59].The age of this unit was originally thought to have been pinned by two Late Devonian 207 Pb/ 206 Pb dates (366:3 ± 10:2 Ma and 365:0 ± 1:3 Ma; [16]) for the North Lakes intrusion, which includes metadiorite-gabbroic rocks originally inferred to be comagmatic with Fire Lake volcanic rocks; we revisit and revise this interpretation below.
The Wind Lake formation overlies the Kudz Ze Kayah formation and consists of carbonaceous argillite; lesser mafic volcaniclastic, volcanic, and intrusive rocks; and minor felsic volcaniclastic rocks and quartzite.The total thickness of the Wind Lake formation is estimated at ~4500 m; mafic volcanic rocks are only present in the lowest 500 m and are otherwise dominated by carbonaceous argillite (Figure 4; [30,60]).The mafic rocks have alkalic, OIB-type geochemical signatures with near-chondritic Nd isotope signatures (εNd 350 Ma = -2:8 to +1.1; [61]), suggesting the magmas underwent varying degrees of crustal contamination following extraction from lithospheric or asthenospheric sources.The Wind Lake formation thus represents the cessation of felsic-dominant volcanism in the Grass Lakes group (Figure 4; [61]).

Previous Geologic Interpretation of the Cleaver Lake
Thrust Sheet.In the core region of the Finlayson Lake district, the Cleaver Lake thrust sheet is a klippe that contains two primary, relatively undeformed geological units: (1) the Upper Devonian Cleaver Lake formation and (2) Early Mississippian Simpson Range plutonic suite [16,55].The Cleaver Lake formation consists of both calc-alkaline and island arc tholeiitic basalt and rhyolite that are intruded by quartz-rich porphyritic rocks; the latter intrusions show local evidence for magma mingling with basaltic dikes and are interpreted to be the volcanic feeders to the extrusive 7 Lithosphere rocks [55].Geochemically, these rocks have arc-like signatures, and Nd isotopic signatures (εNd 350 Ma = -4:8 to +0.1; [6]) indicate this arc was built above a basement composed of both oceanic and continental rocks [3,6,45,48].Rocks in the Cleaver Lake formation have crystallization ages of 360:5 ± 1:9 Ma and 356:1 ± 0:9 Ma for felsic and mafic intrusive rocks, respectively, and indicate that the primary volcanic succession is likely coeval in age to the Grass Lakes group [4,7,16].The Simpson Range plutonic suite contains hornblende-and biotite-bearing monzodiorite, granodiorite, and quartz diorite, all with evolved geochemical signatures of arc affinity (εNd 350 Ma = -12:9 to -7.4; [3,6]); these granitoids exhibit cross-cutting relationships with rocks of the Cleaver Lake formation [3,55].U-Pb zircon dates for three granitoids in the Simpson Range plutonic suite include 354:9 ± 1:8, 348:4 ± 0:8 , and 345:9 ± 1:2 Ma [3,4,16] and define a clearly younger age than the Cleaver Lake formation.
In the southeastern Finlayson Lake district, the Cleaver Lake thrust sheet also includes the Klatsa metamorphic complex.The Klatsa metamorphic complex comprises bodies of serpentinized mafic-ultramafic rocks structurally imbricated with eclogite-facies sedimentary rocks and basalt and represents an Early Mississippian subduction complex [15,66].

New Stratigraphic and Structural Interpretations
In this study, we revise the stratigraphic and structural framework of the Finlayson Lake district presented above in light of our new results and the adoption of the reinterpretation of mafic and ultramafic bodies in the region presented by van Staal et al. [50], which follows early work by Tempelman-Kluit [49], Mortensen and Jilson [8], and Mortensen [4].Geological observations are presented here as a basis for reporting geochronological, geochemical, and isotopic data below and described further in the discussion.
3.1.Fire Lake and Kudz Ze Kayah Formations.Traditionally, the Fire Lake formation has been considered to be in stratigraphic succession between the North River and Kudz Ze Kayah formations, which contained mafic-ultramafic bodies that were interpreted as coeval intrusions.However, we have adopted the reinterpretation by van Staal et al. [50] that follows others [4,8,49], where the ultramafic-mafic slabs represent allochthonous slices of mantle and lower crust as opposed to intrusions [11,16].This reinterpretation requires that the base of the mafic-ultramafic bodies be a thrust fault, and the relationship of the Fire Lake formation to the North River and Kudz Ze Kayah formations therefore requires reevaluation.The Fire Lake formation and underlying ultramafic-mafic rocks define a new thrust sheet that overlies the North River formation and also possibly the Kudz Ze Kayah formation if the North Klippen is correlated with the Fire Lake thrust sheet (Figure 2).This reinterpretation indicates that (1) the Fire Lake formation no longer stratigraphically overlies the North River formation but instead is structurally juxtaposed against it (Figure 5), and (2) the mafic rocks of non-arc affinity (BABB, E-MORB, and OIB) between the Kudz Ze Kayah and North River for-mations, initially correlated with the Fire Lake formation [48,[67][68][69], are no longer in the same thrust sheet and thus not directly correlatable to the Fire Lake formation.The reassignment of these rocks to the Kudz Ze Kayah formation makes the formation a bimodal volcanic succession that directly overlies the North River formation, which includes BABB and E-MORB-affinity mafic rocks with juvenile isotopic signatures (εNd 350 Ma > +8; [48]).The placement of these mafic units into the Kudz Ze Kayah formation requires that only those volcanic and volcaniclastic rocks that overlie the occurrences of mantle ultramafic rocks or are demonstrably part of the Fire Lake thrust sheet are included in the Fire Lake formation (e.g., boninite and IAT affinity; Figure 2).
The Little Jimmy formation is the basal unit of the Wolverine Lake group and consists of mixed quartzofeldspathicpebble conglomerate, grit, sandstone, and carbonaceous phyllite (Figure 4(b); [16]); previously, the rocks of this unit have been mapped as unit "U-5l" [62].The Little Jimmy formation (ca. 357.6 to 356.4 Ma) is named after the Little Jimmy Lake, approximately 3.5 km directly south of the Wolverine mine site, and the type locality is immediately east of the regional unconformity between the Grass Lakes and Wolverine Lake groups (NAD83 9U 429446 mE, 6812106 mN; Figure 4(b)).
The Little Wolverine formation (ca. 356.4 to 355.0 Ma) stratigraphically overlies (and is locally intercalated with) the Little Jimmy formation and consists of felsic volcanic rocks, intercalated argillite, and high-level porphyritic intrusive rocks that host the ~6.2 Mt Wolverine Zn-Pb-Cu-Ag-Au deposit and other regional VMS prospects [16,17,71].The Little Wolverine formation is named after Little Wolverine Lake that is immediately southeast of Wolverine Lake (Figure 4(b)).The rocks in this unit comprise what has previously been termed "footwall" or "U-5f/qfp," "U-5cp," and "U-6FW" [45,62] and "Unit 1" within the Wolverine/Lynx zones [71].
The Go Creek formation stratigraphically overlies the Little Wolverine formation and is composed of rocks in the immediate hanging wall of the VMS deposits, including exhalites, iron formations, resedimented volcanic rocks, and carbonaceous argillite (Figure 4(b)).This unit is named after Go Creek, a watercourse that flows downhill through the Go Creek formation from the topographic highs in the Campbell Range basalts, joining with the Money Creek to the southeast (Figure 4(b)).Rocks of this unit have previously been classified as the "hanging wall" or "U-6HW" [45,62] and include both "Unit 2" and "Unit 3" within the Wolverine/Lynx zones [71].
The formation includes a diverse group of rocks that are distinct in lithology and geochemistry from the underlying Little Wolverine formation (e.g., [45]).
Lastly, the Jasper Creek formation lies stratigraphically above the Go Creek formation and caps the Wolverine Lake group (Figure 4; [16]).The Jasper Creek formation contains massive to pillowed basalts and minor greywacke and carbonaceous argillite.This formation is named after Jasper Creek, which originates at high levels in topography north-northeast of the southern end of Wolverine Lake (Figure 4(b)).Previous work has placed this unit into "U-6HW" [62] or "Unit 4" [71] and correlated it with the Lower Permian Campbell Range formation of Slide Mountain terrane.Although the age of the Jasper Creek formation has not been directly determined isotopically or paleontologically, recent mapping leads us to interpret that the Jasper Creek formation is in depositional continuity with the underlying Go Creek formation and does not correlate with the Campbell Range Formation north of the Jules Creek fault [16]; therefore, unconformities no longer exist between lithologic contacts (Figures 3, 4(b), and 5).

Significance of Mafic-Ultramafic Rocks and Age of Thrust
Faults.Several bodies of mafic-ultramafic rocks have been mapped near rocks in the upper part of the Money Creek thrust sheet.These occurrences were originally interpreted as intrusions related to the Lower Permian Campbell Range Formation (i.e., Slide Mountain terrane) that subsequently pinned the timing of major thrust faults in the Finlayson Lake district [16].Devine et al. [66], however, reinterpreted several of these mafic-ultramafic bodies as being klippen of the Cleaver Lake thrust sheet.We have adopted this interpretation and have reclassified mafic rocks originally mapped as intrusions of Early Permian Slide Mountain terrane to now being related to Late Devonian to Mississippian Yukon-Tanana terrane stratigraphy (see above regarding Jasper Creek formation; Figure 2).These discrete mafic-ultramafic bodies are interpreted to represent klippen Figure 5: Composite chronostratigraphic column for the Big Campbell and Fire Lake thrust sheets, zoomed in to the Late Devonian to Early Mississippian section (modified after [30,64]).Colored units indicate protolith rock types.Numbers correspond to VMS deposits in the district as in Figure 2. Vertical orange and red lines to the right of the figure indicate new U-Pb geochronology constraints from this study and Manor et al. [30].The zig-zag line represents an angular unconformity between the Grass Lakes and Wolverine Lake groups [51].GLPS = Grass Lakes plutonic suite; KZK = Kudz Ze Kayah; fm = formation; NL = North Lakes intrusion; BABB = back-arc basin basalt; BON = boninite; E-MORB = enriched midocean ridge basalt; N-MORB = normal midocean ridge basalt; IAT = island arc tholeiite; OIB = ocean island basalt.
of Devonian to Mississippian rocks that occur along an east-southeasterly trend between the Money Klippe and Klatsa metamorphic complex.As these rocks are no longer interpreted as intrusions, they provide no age constraints for the thrust faults that bound the Big Campbell, Money Creek, and Cleaver Lake thrust sheets; thrusting must have occurred post-deposition of the Money Creek formation but prior to intrusion of Early Cretaceous granitoids.Therefore, we propose that all three major thrust faults in the Finlayson Lake district can be reasonably interpreted as synthetic faults to the Jura-Cretaceous Inconnu thrust fault system that obducted Yukon-Tanana and Slide Mountain terranes onto the Laurentian continent (Figure 3).

Analytical Methods
Analytical procedures are outlined below and presented in detail in Supplementary Materials along with standard data and results from reproducibility monitoring (Tables S1,  S4, and S5 and Figure S7).Sample preparation, mineral separation and extraction, annealing, and imaging for U-Pb zircon geochronology samples were carried out at Memorial University of Newfoundland (MUN; Figures S3-S5).Laser ablation-(LA-) ICP-MS and chemical abrasion-(CA-) ID-TIMS analyses were performed at the Isotope Geology Laboratory at Boise State University, Idaho (BSU).Laser ablation was performed on each grain using a New Wave Research UP-213 Nd:YAG UV laser (213 nm), where ablated material was analyzed on a ThermoElectron X-Series II quadrupole ICP-MS for U-Th-Pb isotopic ratios and trace element concentrations.Following LA-ICP-MS analysis, individual zircon grains were plucked from the epoxy mount and chemically abraded following the same procedure as in Manor et al. [30].Isotopic determinations for CA-ID-TIMS analyses follow the methodology of Davydov et al. [72] and Schmitz and Davydov [73].U-Pb ratios were measured using an IsotopX PhoeniX-62 thermal ionization mass spectrometer at BSU. Sample preparation and measurement of major and trace element lithogeochemical data were performed at ALS Laboratories in North Vancouver, British Columbia, and Sudbury, Ontario; Geoscience Laboratories (Geo Labs) of the Ontario Geological Survey in Sudbury, Ontario, for transition metals, base metals, and semimetals; and Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia for trace elements on a subset of samples.At ALS, samples were fused with a lithium metaborate flux, digested in a HNO 3 -HCl mixture, and analyzed by ICP-AES for major elements and ICP-MS for trace elements.At Geo Labs, samples were digested on hot plates using a four-acid mixture and analyzed on a Perkin-Elmer Elan 9000 ICP-MS following the methodology of Burnham [74].At PCIGR, samples prefaced by 98DM-, P98-, P99-, and P00- [6,17,45,61] were processed using highpressure dissolution techniques [75] and analyzed on a Thermo Finnigan Element2 HR-ICP-MS.
Mineral liberation analysis (MLA) of polished thin sections of U-Pb TIMS samples was performed using a FEI Quanta 650 scanning electron microscope (SEM; CREAIT facility, MUN) equipped with MLA software.Each thin section was mapped to establish the modal abundances and varieties of minerals present in each rock.Quantitative modal abundances and false color thin section maps can be found in Table S6 and Figure S2, respectively.

Chemical Abrasion ID-TIMS and LA-ICP-MS U-Pb Zircon Geochronology
The CA-ID-TIMS U-Th-Pb data for zircon in the Finlayson Lake district provide significantly greater resolution than U-Pb dates obtained by LA-ICP-MS methods (Tables 1 and 2).
Results are presented as 206 Pb/ 238 U isotopic ratios for all the CA-ID-TIMS and LA-ICP-MS samples < 1 Ga; those grains with dates > 1 Ga are reported as 207 Pb/ 206 Pb dates.Presentation of the U-Pb dates follow the nomenclature of Schoene et al. [85] whereby the weighted mean U-Pb dates include uncertainties as ±X/Y/Z ð±2σÞ, where X is the internal error without all systematic errors, Y includes tracer calibration error, and Z includes uncertainty related to both tracer calibration and decay constant error.Unless otherwise noted, uncertainties below are reported as ±X as the dates originate from the same laboratory.High-precision CA-ID-TIMS dates are used in the text below to aid interpretations related to the primary magmatic crystallization history and tectonic evolution of the district; the LA-ICP-MS results for CA-ID-TIMS samples are provided in Supplementary Materials.Detailed U-Th-Pb CA-ID-TIMS and LA-ICP-MS geochronology results are reported in Table 1 and Table S2, respectively, and a summary table compares the results of each method in Table 2.The CA-ID-TIMS and LA-ICP-MS U-Pb results are displayed as Wetherill concordia diagrams in Figures 8 and 9 and Figures S7 and S8.The CA-ID-TIMS data for the Kudz Ze Kayah formation and Wolverine Lake group (except sample 17MM-004) were previously reported in Manor et al. [30].
5.1.Fire Lake Thrust Sheet 5.2.Fire Lake Formation.Zircon was separated from a felsic intrusion (17MM-047) that cuts mafic tuffs and lapilli tuffs 10 Lithosphere  S2), which is interpreted to be the crystallization age of this rock.A basal grit (17MM-004) from the Little Jimmy formation was sampled from the same sample locality as the detrital sample of Murphy et al. [16], immediately northeast of the angular unconformity that sep-arates the Grass Lakes and Wolverine Lake groups (Figures 4  and 6(a)).The CA-ID-TIMS results gave six concordant zircon fractions with a consistently low Th/U (0.2-0.4) and a weighted mean 206 Pb/ 238 U date of 357:658 ± 0:096 Ma (2σ; MSWD = 0:53; Figure 8(a); Table 1), which is inferred to represent the maximum depositional age of this rock.16 Lithosphere  .Each symbol represents one individual zircon analysis as its 206 Pb/ 238 U date with associated uncertainty (±2σ).Th/U for individual zircon grains, which correspond to a specific U-Pb analysis shown in (a).Analyses for the Wolverine Lake group and Kudz Ze Kayah formation are from Manor et al. [30], shown here as a compilation of new U-Pb dates in the district.Insets show individual 206 Pb/ 238 U dates (small symbols) and their calculated weighted mean date (±2σ), which define distinct periods of VMS mineralization in the district (vertical red bars).Symbols: circles = this study and from Manor et al. [30]; diamonds = historical U-Pb zircon dates from Mortensen [4], Piercey [151], Murphy et al. [16], and Piercey et al. [17].
A quartz-feldspar porphyritic intrusive rock (17MM-061) was sampled from a ~50 m thick intrusion in the upper 130 m of drill hole K16-372, which is interpreted to be near to the same stratigraphic level as the Kudz Ze Kayah VMS deposit.This sample contains medium-grained alkali feldspar phenocrysts and lesser quartz grains in a grey, glassy matrix (Figure 7(c)).Five of seven grains yielded concordant U-Th-Pb results, with Th/U between 0.5 and 1.3 and a weighted mean 206 Pb/ 238 U date of 361:71 ± 0:13 Ma (2σ; MSWD = 0:92; Figure 8(d); Table 1), interpreted as the crystallization age of this intrusive rock.Of the other two grains, one older grain (z3) gave an older 206 Pb/ 238 U date of 363:82 ± 0:34 Ma and is interpreted as a xenocryst; the other (z2) is significantly younger with a 206 Pb/ 238 U date of 349:03 ± 0:32 Ma but is much more discordant (~3%) relative to the other grains (<0.5%).This younger grain is interpreted to have not seen complete mitigation of Pb-loss through the chemical abrasion process.An additional feldspar-rich porphyritic intrusive rock (17MM-062) was sampled at the bottom of hole K16-372 (585 m depth).In contrast to 17MM-061, this rock is dominantly composed of plagioclase and quartz phenocrysts.Eight concordant zircon fractions yield moderate Th/U (0.6-0.8) and a weighted mean 206 Pb/ 238 U date of 361:901 ± 0:086 Ma (2σ; MSWD = 0:92; Figure 8(e); Table 1), which is interpreted to be the crystallization age of the sample.

Cleaver Lake Formation.
A quartz-porphyritic intrusive rock (18MM-108) was sampled from outcrop in the Cleaver Lake formation in the Cleaver Lake thrust sheet (Figure 7(h)), where a sharp contact with the Simpson Range granitoids is observed.The U-Th-Pb results gave eight concordant zircon fractions for CA-ID-TIMS, with moderate Th/U (0.4-0.6) and a weighted mean 206 Pb/ 238 U date of 363:185 ± 0:058 Ma (2σ; MSWD = 0:71; Figure 8(j); Table 1) that is interpreted to represent the crystallization age of this intrusive rock.
27 Lithosphere but the calc-alkalic rocks have negative Eu anomalies compared to a slight positive signature in the transitional rock (Eu/Eu * UCC = 0:8-1.0vs. 1.1, respectively).These signatures overlap with those of calc-alkalic to tholeiitic arc rocks from both the Cleaver Lake and Fire Lake formations [45].

Whole-Rock Hf-Nd Isotope Geochemistry
Whole-rock Hf and Nd isotopic compositions (n = 28) were measured for rocks in the Fire Lake, Big Campbell, and Cleaver Lake thrust sheets from the Finlayson Lake district (Tables 3 and 4; Figure 15).The results below are presented by thrust sheet, then in order of stratigraphically lowest to highest.In the Fire Lake thrust sheet, a Permian felsic dike that crosscuts mafic volcaniclastic rocks in the Kona VMS deposit hanging wall gave εHf i and εNd i of -4.7 and -7.3, respectively.In the Grass Lakes group, a mafic intrusive rock in the lowest stratigraphic levels of the Kudz Ze Kayah formation (17MM-056; previously mapped as Fire Lake formation) has a juvenile isotopic composition (εHf i = +1:5 ; εNd i = +2:4).Felsic rocks from the Kudz Ze Kayah formation (n = 8) gave a range of evolved isotopic compositions of εHf i = -13:7 to -2:8 and ε Nd i = -11:2 to -4.5.Rocks in the immediate hanging wall and footwall of VMS deposits (e.g., Kudz Ze Kayah and GP4F) have more chondritic isotopic compositions (εHf i = -7:0 to -2:9 ; εNd i = -7:3 to -4.9) relative to those rocks highest in the hanging wall (17MM-002), where εHf i and εNd i are -13.7 and -11.2, respectively.A mafic subvolcanic intrusive rock in the footwall of the Kudz Ze Kayah deposit (17MM-060) gave near-chondritic εHf i = +2:7 and εNd i = -0:5.Rocks of the Wind Lake formation were sampled either as felsic (17MM-001) or mafic (17MM-003) volcaniclastic rocks.The range of isotope values constrained to nearchondritic relative to the Kudz Ze Kayah formation, yielding εHf i from -1.6 to +2.7 and εNd i from -3.7 to -0.5, where the felsic tuff (17MM-001) has the most negative value.Granitoids from the Grass Lakes plutonic suite (n = 5) give the largest range of evolved isotopic compositions across the district (εHf i = -15:7 to -3:0 ; εNd i = -11:4 to -3.9).
Three samples from the Cleaver Lake thrust sheet include felsic volcanic rocks of the Cleaver Lake formation ðn = 2Þ and a hornblende granodiorite from the Simpson Range plutonic suite ðn = 1Þ.The Hf and Nd isotopic compositions for the Cleaver Lake formation are generally chondritic and correspond to the geochemical affinity of the rocks.Sample P99-39 is of tholeiitic affinity and yields juvenile isotope compositions ðεHf i = +4:8 ; εNd i = +0:25Þ relative to P99-45, which is of calc-alkaline affinity and gives evolved values ðεHf i = -4:0 ; εNd i = -4:8Þ.The Simpson Range plutonic suite granodiorite yields a much more evolved isotopic signature where εHf i and εNd i are -17.1 and -13.1, respectively.

Regional Geochemical Group Discrimination
Felsic and mafic rocks in the Finlayson Lake district have been subdivided into distinct groups based on their immobile major and trace element concentrations and isotopic compositions (Figure 12).The characteristics of each group are defined by least-altered, precursor rocks with distinct trends in immobile ratio-ratio diagrams (e.g., Figure 12).These groupings are also discussed in terms of stratigraphic packages in the Finlayson Lake district.There are multiple precursor rocks within each rock package, but for the scope of this paper, the details of the causes of these multiple populations are not fully evaluated.
In the Big Campbell thrust sheet, rocks of non-arc affinity (i.e., back-arc) are divided into felsic groups FN1, FN2, and FN3 and mafic group MN1 (Figure 12).Groups FN1 and FN2 comprise rocks of non-arc affinity (i.e., back-arc) and contain most volcanic and plutonic rocks in the Grass Lakes and Wolverine Lake groups (Figure 12).Group FN1 felsic rocks have high HFSE (e.g., Zr > 360 ppm) and REE, low Al 2 O 3 , similar TiO 2 , and less evolved εHf and εNd (>-8) compared to the FN2 group (εHf and εNd < -8).The FN3 group, however, contains Go Creek formation rocks that have arc-related geochemical affinities, despite being located immediately above FN1 and FN2 rocks in non-arc stratigraphy (Figures 4 and 5); these signatures have been attributed to either derivation from extrabasinal arc sources or significant crustal contamination of non-arc magmas [7,45].The group FN3 rocks have the lowest Zr (<150 ppm), HFSE, and REE contents of all felsic rocks in the district, Zr/Al 2 O 3 like group 28 Lithosphere  Coordinates for samples beginning with the prefix "17MM" are in NAD83; all other samples are reported as NAD27.# Groups presented as the geochemical group (e.g., FN2) following groupings detailed in the text and Figure 10.* * Hf and Lu concentrations from UBC-PCIGR measurements except for 17MM samples, which are from ALS (see text and Table S3).† † Initial Hf isotopic compositions calculated using λ   Coordinates for samples beginning with the prefix "17MM" are in NAD83; all other samples are reported as NAD27.# Groups presented as geochemical group (e.g., FN2) following groupings detailed in the text and Figure 10.* * Nd and Sm concentrations from UBC-PCIGR measurements except for 17MM samples, which are from ALS (see text and Table S3).† † Initial Nd isotopic compositions calculated using λ 30 Lithosphere FN2 rocks, distinct Th/Nb, but low Zr/Ti ratios most similar to arc-related rocks in the Cleaver Lake thrust sheet.Group MN1 contains all mafic rocks in the non-arc environment that are characterized by high Ti/V (>20) and Zr/Y (>4; e.g., N-MORB, E-MORB, and OIB; Figures 10(d) and 10(f)).
In the Cleaver Lake thrust sheet, felsic rocks of arc affinity are split into groups FA1 and FA2, which are typically designated as the Cleaver Lake formation (ca. 363 Ma) and Simpson Range plutonic suite (ca.354-349 Ma), respectively.Group FA1 felsic rocks have higher Zr, REE, Zr/ Al 2 O 3 , and Zr/Ti, lower TiO 2 , and less evolved εHf and ε Nd > -5 than group FA2.Arc-related mafic rocks in the Cleaver Lake formation comprise group MA1, which includes MORB to IAT-type mafic rocks that have similar Ti/V and Zr/Y to the non-arc mafic rocks.
In the Fire Lake thrust sheet, mafic rocks of arc affinity are separated into group MA2 (Figures 10(d) and 10(f)).This group contains mafic rocks of the Fire Lake formation that have boninite and IAT geochemical affinities that are characterized by low Ti/V (<20) and Zr/Y (<3; Figures 10(d) and 10(f)).

Discussion
Deciphering the mantle component (i.e., enriched or depleted) in ancient mafic rocks requires utilization of elements (and isotopes) that are immobile, insensitive to fractional crystallization, and reflective of the mantle source regions and crustal contamination and/or slab metasomatism (e.g., [94,95]).The immobile element ratios of Th/Yb, Nb/Yb, and TiO 2 /Yb are useful for delineating mantle sources as these ratios are generally reflective of incompatible enrichment of a mantle source (i.e., MORB-OIB array in Figures 11(d) and 11(e); [94]); however, the Th/Yb ratio is also sensitive to crustal contamination and/or slab fluid components as they typically have higher Th/Yb ratios at a given Nb/Yb ratio and distinctive trajectories in Th/Yb-Nb/Yb space (Figure 11(d); [61,[94][95][96][97]).The mafic rocks found in the back-arc region of the Finlayson Lake district that plot predominantly within the MORB-OIB array in Th/Yb-Nb/Yb space (Figure 11(d)) are interpreted to be derived from variably enriched mantle sources.For example, rocks from the lower Kudz Ze Kayah formation have relatively depleted BABB (~N-MORB) signatures (e.g., [48]), the upper Kudz Ze Kayah formation have weakly enriched (~E-MORB) signatures, and the Wind Lake formation has highly enriched (~OIB-like) signatures; metagabbros of the North Lakes intrusion and the lower Kudz Ze Kayah formation (samples 17MM-056 and 17MM-057) plot between E-MORB and OIB (Figure 11(d)).Mafic rocks from the arc regions, however, include IAT affinity rocks of the Cleaver Lake formation and boninite to IAT signature rocks of the Fire Lake formation that plot above the MORB-OIB array proximal to E-MORB and N-MORB, respectively (Figure 11(d)).The variation in incompatible element enrichment between these various rocks potentially reflects mixing and varying contributions from depleted, N-MORB-like asthenospheric melts and enriched lithospheric mantle-derived melts (e.g., [7]).The Hf-Nd isotope results for these rocks support this hypothesis.In particular, the back-arc mafic rocks typically have εH f 350 Ma = +0:8 to +2.6 and εNd 350 Ma = -0:8 to +2.2 (Figures 12 and 14), which, in the case of Nd data, overlaps with the limited dataset of Piercey et al. [61] (Figure 15); however, several E-MORB basalts in the lowest stratigraphic levels of the Kudz Ze Kayah formation overlap with juvenile εNd = +8:1 [48].Many of the samples with both near-chondritic and juvenile εHf and εNd have flat to positive Nb anomalies and the greatest incompatible element enrichment, indicative of rocks that have not seen slab metasomatism nor crustal contamination, suggesting that these isotopic values are a function of the incompatible element-enriched mantle source they were derived from.It is therefore suggested that (1) the nearchondrite Hf-Nd isotope signatures were generated from a mixture of depleted asthenospheric mantle and a component of preexisting and enriched lithospheric mantle [7,48,53] and (2) the juvenile Nd isotope signatures, particularly in the E-MORB samples (c.f., [48]), were generated from a nearly uncontaminated asthenospheric mantle source and erupted following little to no residence time in the lithospheric mantle.These geochemical and isotopic signatures are therefore distinct from other mafic rocks in the Finlayson Lake district that experienced crustal contamination, as evidenced by high Th/Yb and more evolved isotopic signatures (Figures 11(d)-11(f), and 15).
A subset of mafic rocks in the belt displays distinct geochemical ratios with Nb/La PM < 1, Nb/Th PM < 1, and elevated Th/Yb (Figures 12 and 14) indicative of either a subducting slab input or crustal contamination (Figure 11; [7]).Two boninite and IAT basalt samples from the Fire Lake and Cleaver Lake formations, respectively, have negative Nb anomalies and high Th/Nb, which are interpreted to be generated from the retention of Nb and enrichment of Th due to fluid mobilization from the subducting slab (e.g., Figures 11(d), 11(f), 14(c), and 14(d); [45,48,62,95,96]).These samples also overlap with rocks that have high εNd (+3 to +7; [48]) and are interpreted to have formed from primitive arc magmas that have not interacted with evolved continental crust.Further, the rocks with BABB signatures in the Kudz Ze Kayah formation also show negative Nb anomalies, slightly elevated Th/Yb, and high εNd = +8:5 (Figures 11(d) and 11(f); [48]), which is interpreted to indicate that back-arc affinity magmas were overprinted by a weak subduction signature.In contrast, other mafic rocks with back-arc affinities have high Th/Nb and Th/Yb, lower εHf and εNd (-1 to +3), and 1.0-1.4Ga model ages (Tables 3 and 4), which suggested that the mafic melts interacted with continental crust (e.g., [61]).
These data illustrate that the mafic rocks in the Finlayson Lake district reflect varying contributions of asthenospheric mantle and continental lithospheric mantle that were overprinted by subducting slab or evolved continental crust components.These results also suggest that different crustal architectures in both the arc and back-arc environment generated the variable isotopic and trace element compositions of these mafic rocks.Back-arc mafic rocks, the exception of the lowest Kudz Ze Kayah formation (i.e., BABB and E-MORB), interacted with old subcontinental lithospheric mantle and/or continental crust during ascent, whereas arc rocks did not.Therefore, some components of evolved lithosphere should be expected during the formation of felsic rocks in the back-arc, which is explored below.10.1.2.Petrogenesis of Felsic Rocks.Felsic rocks throughout the Finlayson Lake district have distinct geochemical and isotopic signatures that define them as arc or back-arc rocks (e.g., Nb/Y, Zr/Y, Zr/Ti, and SiO 2 ) and reflect varying contributions from evolved crust and juvenile mantlederived components (e.g., Nb/Ta ~9-18; εHf 350 Ma = -17 to + 4 ; εNd 350 Ma = -13 to +0.2; Figures 10,15,and 16;[6,7,17,45,48,61,70]).Arc rocks from the Cleaver Lake thrust sheet have highly variable isotopic values (εHf 350 Ma = -17 to +4; εNd 350 Ma = -13 to +0.2) and show distinct crystallization ages, including (1) calc-alkalic to tholeiitic volcanic rocks of the Late Devonian Cleaver Lake 32 Lithosphere formation (εHf 350 Ma = -4 to +5; εNd 350 Ma = -5 to and (2) calc-alkalic granitoids of the Early Mississippian Simpson Range plutonic suite (εHf 350 Ma = -17 ; εNd 350 Ma = -13; Figure 15).These suites of arc-related felsic rocks have a range in HFSE and REE contents from very depleted (tholeiitic rocks) to less depleted but significantly lower concentrations than the back-arc felsic rocks (e.g., Zr and REE sum < 200 ppm; Figures 11 and 16), while having isotopic signatures that reflect influence from the variable crustal basement during Late Devonian and Early Mississippian magmatism.Back-arc rocks, however, illustrate a significant decoupling of Hf-Nd isotopes and HFSE-REE contents, where these signatures define two geochemical groups that underwent differing crustal melting processes (Figures [15][16][17].There is a positive relationship between εHf and εNd and HFSE-REE contents with the VMS-proximal FN1 suite rocks having the higher values when compared to the VMS-distal FN2 rocks.These features are attributed to the variable contributions of mantle and crustal materials, degrees of crustal assimilation, and temperatures of melting and emplacement and are explored further below.The back-arc groups, FN1 and FN2, have isotopic and geochemical features that show evidence for contributions from both mantle and crustal sources; however, the degree of crustal melting and contributions from underplated basaltic magmas is variable (Figure 16).The variations in the basaltic/juvenile component are best recorded by (1) distinct Nb/Ta ratios (Figure 16), ( 2) bulk rock Hf-Nd isotopic compositions (Figure 15), and (3) Th/ U in zircon (Figure 9(b)).The Nb/Ta values for group FN1 are ~17 (i.e., mantle-like), whereas FN2 values are ~12 (i.e., crustal-like; [17,89,98,99]), and these also correspond with εHf and εNd values that are most juvenile (i.e., closest to chondrite-group FN1) and most evolved (group FN2; Figure 15), respectively.In zircon crystals, the highest Th/ U (>1) are associated with samples that have εHf-εNd greater than ~-8 and Nb/Ta ~17 (e.g., sample 17MM-031, group FN1).However, some samples have very wide ranges in zircon Th/U in a single sample, such as in plutonic samples 96DM-065 and 18MM-105 that gave large ranges of Th/U between 0.4-2.3 and 0.3-2.2,respectively (Figure 9(b)).These variations could reflect mixing between juvenile and evolved magmas in the chamber; however, equally viable is that it reflects potentially varying degrees of disequilibrium crystallization where zircon crystallization is not controlled by specific partition coefficients in the melt (e.g., [100]).Even with the variability in Th/U in some samples, there is a general trend at both the whole rock and mineral scale of more juvenile and higher temperature magmatic rocks proximal to VMS mineralization (i.e., the FN1 population).
Most felsic rocks in the Finlayson Lake district exhibit evidence for melting of Proterozoic to Archean crustal basement [3,4,6].The most common method of crustal melting, regardless of tectonic setting, involves a basaltic heat source as either intrusions or ponding of hot basaltic magmas that melt the overlying crust [101,102].In extensional settings, thinning lithosphere can accommodate upwelling asthenosphere into the rift regions leading to partial melting, basalt underplating of the crust, and crustal assimilation [103][104][105][106]. Here, we adopt this model to test and explain the variance in crust versus mantle Hf and Nd isotopic components in felsic rocks from the Finlayson Lake district (e.g., [17,36]).We employed a simple two-component mixing model for both isotopes and trace element ratios using the equation of Faure [107]: where R m x is the ratio of the mixture, R 1 x and R 2 x are the compositions of the mantle and crustal endmembers, C 1 and C 2 are the trace element concentrations of the mantle and crustal endmembers, and X is the mixing factor that defines the volume ratio of the mantle to crustal components.The variable X varies between 0 (0% of component C 1 ) and 1 (100% of component C 1 ).In these mixing models, the two endmembers were chosen as (1) C 1 = N-MORB basalt (i.e., asthenospheric mantle) and ( 2) C 2 = evolved Laurentian crust.The basaltic N-MORB component is defined based on the spatial association of most felsic rocks with basalts in the Finlayson Lake district and in the back-arc realm therein the asthenospheric component generally has N-MORB compositions (see previous section; Figures 11(d)-11(f)).The nature of the crustal basement directly beneath Yukon-Tanana terrane rocks in the Finlayson Lake district is enigmatic; however, regionally, the Neoproterozoic to Early Paleozoic Snowcap assemblage has been interpreted as the basement to Yukon-Tanana terrane [5], and we use this as a crustal endmember in our calculations.We utilized the average isotopic compositions of Snowcap assemblage rocks that were more evolved than our most evolved samples, since 100% assimilation of the crust is highly unlikely (e.g., [108]).
Isotopic mixing models indicate that rocks from the Finlayson Lake district formed from variable mixtures of depleted N-MORB mantle melts and crustal melts from the assimilation of preexisting Snowcap assemblage rocks (Figures 15,17,and 18).In the Cleaver Lake thrust sheet, arc rocks in the Cleaver Lake formation reveal between 9 and 21% Nd and Hf crustal components compared to 48-57% for the Simpson Range plutonic suite.In the Big Campbell thrust sheet, back-arc mafic rocks have isotopic compositions that indicate ~8-13% Nd and Hf contributions from the Snowcap endmember, whereas the felsic rocks vary from 17 to 45% Hf and 19 to 47% Nd from this endmember (Figure 15; Tables 3 and 4).The group FN1 rocks have the lowest contributions from the crustal endmember (17-28% Hf and 19-33% Nd), which also corresponds to the most elevated εHf-εNd, whereas group FN2 rocks have greater crustal contributions (24-45% Hf and 28-47% Nd) that correlate with the most evolved isotopic values (Figure 15).Therefore, it is evident that crust-mantle mixing was a dominant process in the formation of felsic rocks in the Finlayson Lake district and was particularly important in the differences in petrogenetic histories of the group FN1 and FN2 rocks.
The εHf-εNd compositions and HFSE-REE abundances for group FN1 rocks are elevated compared to group FN2 rocks; however, the trace element abundances but εHf and εNd are constant in each group, suggesting a decoupling of HFSE-REE from Nd and Hf isotopes.Twocomponent mixing lines using isotopes and trace element ratios also do not generate unique mixing curves with a single, evolved Snowcap endmember, which may indicate that an additional process beyond crustal contamination generated the high HFSE and REE and low 147 Sm/ 144 Nd ratios of group FN1 rocks (Figures 17 and 18).The uniquely high HFSE-REE could be explained by two hypotheses: (1) abundances are controlled by fractional crystallization, which would increase the trace element abundances but not further fractionate isotopic compositions (e.g., [43,109,110]) or (2) abundances are controlled by variable temperatures of crustal melting, which in turn facilitated variable dissolution of preexisting HFSE-REErich minerals into the melt (i.e., high temperature = increased dissolution; [17,42,43,45,109,111]).Typical fractionation processes would favor an increase in Zr with increasing SiO 2 in the felsic rocks; however, the FN1 rocks are trachytic to dacitic (i.e., lower SiO 2 ) and have the highest Zr relative to FN2 rhyolites, which contradicts this trend.Therefore, we prefer the temperature-dependent hypothesis to explain this variation.To test this, zircon saturation thermometry [111,112] was used to calculate temperatures at which zircon formed in rocks in the Finlayson Lake district.For this, we used the modified equation of Boehnke et al. [113]: where D zircon/melt Zr is the distribution coefficient of Zr between zircon and the silicate melt, for which a stoichiometric concentration of 497,646 ppm Zr and measured whole-rock Zr (in ppm) were used, T is the temperature in Kelvin, and M is the cation ratio expressed as ðNa + K + 2 • CaÞ/ðAl • SiÞ.Alteration was closely monitored by screening for M = 0:8-2, SiO 2 > 65 wt.%, and Na 2 O = 2-5% (Figure 19).The calculated zircon saturation temperatures indicate higher-temperature crustal melts (890-980 °C) contributed to the generation of group FN1 rocks, whereas distinctly lower-temperature melts (780-850 °C) generated rocks in group FN2; the temperature range for groups FN1 and FN2 overlaps with temperatures obtained for FP and QFP porphyritic rocks, respectively, in the Wolverine deposit [17].Interestingly, the higher zircon saturation temperatures are also associated with lower abundance of crustal components in the group FN1 rocks, which is interpreted to suggest that the dissolution efficiency of HFSE-REE-bearing minerals in the crust was far greater than those associated with lower-temperature FN2 melts (e.g., [42]).The SEM-MLA results also show that FN1 rocks have the highest modal abundances of HFSE-REE minerals (e.g., zircon, monazite, and xenotime; Table 5) and therefore directly relate to the HFSE-REE budget of the whole rock [110,114,115].Moreover, the increased mantle components in the group FN1 rocks likely provided higher temperatures required to increase dissolution efficiency of these minerals in the crust, which in turn led to higher HFSE-REE contents in the ascending melts that are observed in VMS-bearing stratigraphic horizons both in the Kudz Ze Kayah and Little Wolverine formations.[113], which show negligible variation from the original equations of Watson and Harrison [111] and Hanchar and Watson [112].Samples are parsed based on their alteration intensity, which greatly affects the accuracy of thermometry calculations.Large symbols contain M values of 0.8-2, Na 2 O = 2-5 wt.%, and SiO 2 > 65 wt.%, and large symbols with red borders are least-altered rocks.Small symbols contain Na 2 O < 2 wt.% and generally follow similar trends to their least-altered equivalents.The grey arrow represents the trajectory of increasing mantle-derived components in the formation of felsic rocks in the Finlayson Lake district (i.e., difference between group FN1 and FN2 compositions).34 Lithosphere VMS-bearing felsic rocks have been extensively studied [34,36,43,116,117].Similarly, U-Pb geochronology has provided constraints on the timing of magmatism and mineralization in some VMS districts globally [28,92,[118][119][120][121].Moreover, have been studies of shallow subvolcanic intrusions with close spatial and/or temporal ties to VMShosting stratigraphy, and these have also been implicated as the primary source of heat for the formation of VMS deposits [31][32][33][34]122].However, several authors have challenged this hypothesis and argue that these intrusions may not have been the direct heat source for mineralization but rather passive magmatic products in a regional, thermally anomalous geodynamic environment [17,35,36].
In the Finlayson Lake district, the Grass Lakes plutonic suite was previously suggested to be the heat driver for mineralization in the Kudz Ze Kayah formation [6].The new U-Pb dates for the Grass Lakes plutonic suite (ca.361.8 to 360.9 Ma; Figures 8 and 9) indicate that the intrusive rocks postdate the Kudz Ze Kayah VMS deposit and volcanism and therefore could not be the heat source for VMS mineralization.Additionally, while there is not a large plutonic suite associated with mineralization in the Wolverine Lake group, there are high-level syn-VMS porphyritic intrusions proximal to the Wolverine deposit that have been linked to regional extensional-related magmatism [17].Despite their variations in timing, these rocks have geochemical and isotopic signatures that suggest potential fertility for VMS mineralization.For example, the FN1 rocks within <100-200 m of the ca.362.8 Ma Kudz Ze Kayah deposit, the ca.355.2 Ma Wolverine deposit, and the ca.354.9 Ma Fisher zone have elevated HFSE-REE concentrations (e.g., Nb/Ta > ~15), zircon saturation temperatures (>890 °C), and εHf-εNd isotope signatures that are consistent with formation at high-temperatures to basaltic underplating (Figures 15, 16, and 19).In contrast, VMSbarren FN2 group rocks have distinct U-Pb dates outside of 2s uncertainty from the FN1 rocks, are HFSE-REE poor, and have more evolved εHf-εNd isotope signatures.The spatial association of VMS with magmas indicative of rifting and with evidence of high temperatures of emplacement and more juvenile signatures suggests that basaltic underplating association with back-arc rifting controlled the localization of VMS mineralization in the Finlayson Lake district.Rifting provided the ground conditions (i.e., synvolcanic faults) needed to focus fluid flow (e.g., [123]), whereas the basaltic underplate provided the essential heat needed to drive hydrothermal fluid circulation that formed both large and highgrade VMS deposits in the district (e.g., [17]).The underplating of rifts by basaltic magmas and associated crustal melting are processes likely recorded in many VMS districts globally (e.g., [36]), and further study of both bulk rock and mineral scale chemistry and isotopes is required to test these ideas and their validity.

Implications for Tectonostratigraphy in the Late
Devonian-Early Mississippian Yukon-Tanana Terrane, Finlayson Lake District.Most magmatic rocks that comprise the Finlayson Lake district formed between ca.363.5 and 354.8 Ma (Figure 9; [30]) and were part of an east-dipping Japan-Japan Sea style arc and back-arc environment along the western Laurentian continental margin (e.g., [2,6,7,15,124]).This model has been scrutinized by van Staal et al. [50] who argued that the tectonic configuration of the Yukon-Tanana terrane was more like the New Britain-Solomon-New Hebrides arc system at the Australian-Pacific plate boundary (e.g., [125]).Our results allow testing of these models and will utilize the (1) stratigraphic evaluation of mafic rocks in the Fire Lake and Kudz Ze Kayah formations and (2) geological characteristics of the ca.360-358 Ma deformational event recorded in the Finlayson Lake district.An outcome of our work will be a series of testable models that will help guide future studies reconstructing the potential Late Devonian tectonic configuration of the Yukon-Tanana terrane and related rocks along the western Laurentian margin.10.2.1.Redefining Geological Relationships of Fire Lake and Kudz Ze Kayah Formations.Mafic magmatism in the Fire Lake formation was previously interpreted to represent arc-back-arc rift initiation on the western Laurentian continental margin that subsequently evolved into a regime dominated by crustal melting and within-plate magmatism in the Kudz Ze Kayah formation [16,45,48].This interpretation relied on three pieces of evidence: (1) ca.365 Ma dates for mafic intrusions interpreted to be comagmatic with the Fire Lake formation (i.e., North Lakes intrusion; [4,16]), (2) geochemistry of mafic rocks previously interpreted to represent the initiation and evolution of rifted continental arc (e.g., arc boninite and IAT to non-arc MORB to OIB; [47,48]), and (3) apparent stratigraphic continuity of mafic rocks of the Fire Lake formation and felsic rocks of the Kudz Ze Kayah formation [16].We will reevaluate the above in light of recent ideas (i.e., [50]) and new results and geological relationships.
Most mafic rocks in the Kudz Ze Kayah formation have chondritic isotopic compositions (εNd and εHf ~0) that have been interpreted as being mixtures of asthenospheric and enriched lithospheric mantle sources (±crustal/slab influence; see discussion above).The products of this magmatism were predominantly rocks with the most enriched E-MORB to OIB affinities; however, several mafic rocks stratigraphically overlie the North River formation that has primitive BABB to E-MORB affinities with juvenile εNd i values (+8.1 and +8.5; [48]).These isotopic compositions are significantly more juvenile than the results from the rest of the redefined Kudz Ze Kayah formation and, given their stratigraphic position, are interpreted to be one of the first products related to rift-related magmatism in the nascent continental back-arc basin.At this stage, early rift-related magmas were likely generated by high-temperature mantle melting where the depleted asthenospheric melts (e.g., BABB) interacted with metasomatized mantle from the mantle wedge as the back-arc opened (e.g., [95,126]).These primitive melts also contained an enriched E-MORB component that ascended through extensional structures in the continental crust with insufficient residence time in enriched lithospheric mantle and continental crust to influence the isotopic compositions.As rifting continued, 35 Lithosphere relatively deeper, incipient mantle melts would have had increased opportunity to interact with enriched lithospheric mantle and generated weakly alkalic rocks stratigraphically higher in the Kudz Ze Kayah formation (e.g., [94]), which then progressed upwards into a bimodal but felsic-dominated, within-plate volcanic sequence that was subsequently intruded by the North Lakes intrusion (ca.362.6 to 361.8 Ma) and the Grass Lakes plutonic suite (ca.361.9 to 360.9 Ma).These observations are interpreted to suggest that the Kudz Ze Kayah formation, as opposed to the Fire Lake formation (c.f., [48]), represents the pre-363.3Ma marker of bimodal magmatism and back-arc rift initiation in the Yukon-Tanana terrane.
In the redefined Fire Lake formation, the allochthonous ultramafic rocks are overlain by extrusive metavolcanic rocks with affinities common to primitive arcs (e.g., boninite and IAT; [48,68]).Such associations have been found in modern arcs and in many ophiolites and interpreted to have formed due to rapid rifting and exhumation of ultramafic rocks, as proposed for suprasubduction zone forearc [127][128][129] and back-arc environments [130][131][132].Using examples from modern arcs, we herein propose a revised petrogenetic and geodynamic setting for the redefined Fire Lake formation and associated ultramafic slab.Ultramaficmafic rocks and high-Ca boninites were generated in the forearc of the nascent arc system following subduction initiation, where rapid extension eventually led to a transfer of extensional stresses from the forearc to the back-arc region (e.g., [128]).Forearc extension and/or incipient back-arc formation thus facilitated deep exhumation of ultramafic mantle rocks in this model.Depleted boninites erupted above the ultramafic-mafic rocks, formed via decompression melting, due to rising asthenospheric melts that passed through a depleted, refractory mantle wedge that had previously extracted melt in the arc proper (e.g., IAT; [48,133]).These magmas would have depleted the mantle wedge prior to boninite formation and have left an ultradepleted residue that the boninitic rocks could have been derived from (e.g., [134]).The proposed forearc setting provides the most ideal scenario to explain the preservation of oceanic rocks in the Fire Lake formation and the juxtaposition of these rocks near continental affinity rocks, which requires that the Fire Lake formation and associated ultramafic rocks (i.e., the arc initiation suite) are thrust onto the inboard North River formation.
The proposed model for the Fire Lake formation implies that continental crust was absent in the formation of most rocks, in contrast to the suggestions of Piercey et al. [47] who suggested the boninites formed in an environment with stratigraphic continuity with continental crust (e.g., a Havre Trough-type setting in the Kermadec arc; [135,136]).The new observations and data presented here are not internally consistent with this model, and we suggest the Fire Lake formation formed in a separate oceanic domain proximal but outboard of the continental crust-floored Kudz Ze Kayah formation back-arc and were juxtaposed by thrusting.These observations require stratigraphic redefinition of both geological units and imply that the Kudz Ze Kayah formation, not the Fire Lake formation, records the onset of continental arc-back-arc rifting on the western Laurentian margin in the Late Devonian (pre-363.3Ma).10.2.2.Deformation, Uplift, and Erosion (ca.360-358 Ma): Implications for Tectonics in the Western North American Cordillera.The ca. 360-358 Ma deformation, uplift, and erosion are recorded by the angular unconformity between the Grass Lakes and Wolverine Lake groups [16,62]; the cause of the deformation and unconformity is poorly understood but broadly correlates with timing of the Late Devonian-Early Mississippian Antler Orogeny in the western United States (e.g., Figure 20; [137][138][139][140]).Evidence to support the nature and age of this unconformity includes the following: (1) quartzofeldspathic grits in the Little Jimmy formation have immature textures with angular feldspar laths, interpreted to be representative of deposition that was coincident with or shortly after seafloor volcanism; (2) zircon in this sample that yielded a maximum depositional age of ca.357.6 Ma indicates deposition coeval with magmatism in the Wolverine Lake group; (3) the unit has geochemical signatures identical to the underlying Kudz Ze Kayah and overlying Little Wolverine formations but has a very evolved isotopic signature in group FN2 (see above); (4) rocks below the unconformity are more highly deformed than those above, having well-developed cleavages, ductile fabrics, and locally exhibit isoclinal folding; (5) relatively undeformed granitoids cut folded granitic dikes, and both were subsequently uplifted and eroded; and (6) the unconformity cuts kilometers of stratigraphy observed as a beveled and tilted erosional feature from the Wind Lake formation down to the North River formation (Figure 2).
New constraints on volcanism in the Wind Lake formation and the new CA-ID-TIMS date of ca.357.6 Ma for the grit sample (Figures 8(a), 8(f), and 9(a)) indicate that the deformation period occurred between ca.360.9 Ma and ca.357.6 Ma.A similar style of unconformity has been recognized in the Money Creek thrust sheet between the Waters Creek and Tuchitua formations, but regional correlations beyond the Finlayson Lake district to other parts of the Yukon-Tanana terrane or North American continental margin are lacking.We interpret this to indicate that deformation at ca. 358 Ma was not more widely recognized in the northern Cordillera due to poor preservation of the backarc environments in the rock record.Further, this deformation is present during a period of sinistral transpression on the western Laurentian margin [57,137,[141][142][143][144][145][146], suggesting that it represents rarely preserved evidence that could be related to the Antler Orogeny in the northern North American Cordillera (Figure 20).If this is the case, the Antler Orogeny would have impacted the entire western North American continental margin during the Late Devonian to Early Mississippian (e.g., [140]), but much of the evidence in the northern Cordillera was then variably erased during younger deformation and metamorphic events.
Despite there being an unconformity between the Grass Lakes and Wolverine Lake groups, its timing suggests it may be unrelated to deformation in the region but due to the evolution of the subsequent rift-drift stage (e.g., [147]).Two types of Grass Lakes granitoids are present immediately 36 Lithosphere beneath the unconformity: (1) strongly deformed granite that preserves isoclinal folds interpreted to correlate with a transpressional deformation event (i.e., Antler Orogeny) and (2) a later, weakly foliated granite that crosscuts the strongly deformed above.The latter granitoid intrusion was dated at 357:3 ± 2:8 Ma [16] and, despite the large uncertainty, overlaps with the maximum depositional age and new CA-ID-TIMS date in the grits above the unconformity (Figure 8(a)).The younger cross-cutting granite is located below the unconformity and constrains the upper age limit of deformation, but it also establishes a distinct, post-transpressional magmatic event that preceded uplift and erosion.The ca. 357 Ma granite could also provide a local source for the Little Jimmy formation grits.Therefore, it appears that the erosional unconformity may be slightly younger and thus unrelated to the observed ca.360-358 Ma transpressional deformation event.
As a result of the "decoupled" nature of deformation and erosion, we propose that the basal Wolverine Lake group unconformity represents a rift-onset unconformity that quickly merged into a breakup unconformity, marking the outboard flank of the Slide Mountain ocean.Early rifting in the Grass Lakes group transitioned upwards into alkalic magmatism but was then aborted [7], then was succeeded by compressional deformation.Post-deformation extension quickly facilitated an abrupt, pre-rift uplift period on the rift flank of the new Wolverine Lake group basin as a regional isostatic response to upwelling mantle [147][148][149].The rapid change in topography on the continental back-arc resulted in an erosional surface (i.e., the basal Wolverine Lake group unconformity) that cut kilometers of stratigraphy and deposited sediment into the new low-lying basin between ca.357.6 and 356.4 Ma (i.e., Little Jimmy formation; Figures 4(b) and 9; e.g., [30]).Continental margin extension in the Wolverine Lake group back-arc then resumed for at least ~1.5 Myr before MORB-type basalts of the Jasper Creek formation were erupted.The Jasper Creek formation includes both E-MORB and N-MORB basalts with εNd i between -4 and +7 [57], which has been interpreted to suggest that Jasper Creek formation basalts have both lithospheric and asthenospheric mantle components [57].The presence of lithospheric components suggests a linkage to the continental lithosphere, and the asthenospheric component implies incipient back-arc spreading.Together with  [141] and Beranek et al. [137].The Antler Orogeny (orange) is related to sinistral transpression along the western Laurentian margin, and the Finlayson Lake district (red box; FLD) is interpreted to contain a previously unrecognized remnant related to this tectonism.AA = Arctic Alaska; CH = Chukotka; EK = Eastern Klamath terrane; FLD = Finlayson Lake district; NS = Northern Sierra terrane; PE = Pearya terrane; QN = Quesnellia; YT = Yukon-Tanana terrane.37 Lithosphere overlying stratigraphic and structural relationships, it suggests that the entire Wolverine Lake group stratigraphic section represents the evolution of crustal breakup, rifting, and the rift-drift transition following lithospheric rupture leading to incipient spreading (Figures 4 and 5; [45,53,71]).Further, these relationships fit with regional models for the Yukon-Tanana and Slide Mountain terranes the development of a back-arc to ocean spreading center on the western Laurentian continental margin.Our results illustrate, however, this Devonian-Mississippian evolution was much more complex than previously recognized (e.g., [7,16,48,124]).

Conclusions
Yukon-Tanana terrane rocks in the Finlayson Lake district, Yukon, represent early manifestations of mid-Paleozoic arc-back-arc magmatism immediately adjacent to the Laurentian continental margin which led to a rift-drift transition and the onset of the opening of the Slide Mountain ocean.When interpreted in the context of new structural and stratigraphic interpretations, our integrated geochronological, geochemical, and isotopic results indicate that the back-arc rocks formed during back-arc rifting in the Late Devonian to Early Mississippian (ca.363.4 to 355.0 Ma), punctuated by a complex period of compressional deformation, postcompressional magmatism, uplift, and erosion between ca.360 and 358 Ma.The Cleaver Lake arc contains coeval Late Devonian volcanic rocks (ca.363.2 Ma) and multiple pulses of granitic plutonism between 352 and 348 Ma.The rocks in both the Grass Lakes and Wolverine Lake groups were generated above the continental crust of Laurentian affinity, which we interpret as rocks similar to the Snowcap assemblage that makes up the basement to Yukon-Tanana terrane throughout Yukon, Alaska, and British Columbia.Geochemical and isotopic results from felsic rocks in both rock packages indicate that crustal melting of the Snowcap assemblage occurred at different temperatures and correlates with variable degrees of assimilation.High-temperature (890-980 °C) crustal melting facilitated low degrees of assimilation for Nd and Hf (<~35%) that generated rocks with group FN1 geochemical signatures (e.g., εHf and εNd > -8); this group contains rocks proximal to VMS mineralization.Low-temperature signatures (780-850 °C), however, correspond to increased assimilation (>40%), evolved isotopic signatures (εHf and εNd < -8), and group FN2 geochemistry.These geochemical distinctions suggest that rocks directly hosting mineralization in the Kudz Ze Kayah and Wolverine VMS deposits were formed from high-temperature crustal melting from an increasingly mantle-derived component, interpreted to result from extensive basaltic underplating during an active period of slab rollback and mantle upwelling.
The conditions favorable for VMS generation were achieved through a series of complex tectonomagmatic configurations beginning in the Late Devonian.We propose that rapid and complex subduction initiation of various oceanic and continental crust blocks, which originated from remnants of an attenuated Laurentian margin, facilitated rapid arc mobility and extension in the back-arc regions.Rapid extension and magmatism in the Kudz Ze Kayah and Wolverine back-arc basins were punctuated by a period of transpressional deformation between ca.360 and 358 Ma, possibly related to a rarely preserved segment of the Antler Orogeny.We propose that the deformation was separate from later uplift and erosion that define the regional angular unconformity between the two units, an unconformity interpreted as a rift-onset and overlapping breakup unconformity that marks the outboard flank and beginning of the continental margin rifting that led to the rift-drift transition and formation of the Slide Mountain ocean.This study highlights the importance of new, high-precision CA-ID-TIMS U-Pb zircon geochronology and its utility in placing tectonomagmatic processes within tight temporal chronostratigraphic frameworks.Further work in the Finlayson Lake district has the potential to define the earliest Late Devonian history (i.e., pre-Kudz Ze Kayah back-arc) of the Yukon-Tanana terrane and its relationship with the Laurentian continental margin, which can then be translated across the entire North American Cordilleran margin.

Figure 1 :
Figure 1: Paleozoic to early Mesozoic terranes of the North American Cordillera of Canada and the United States of America (modified after [150]).Note the terranes are superimposed on the present-day geographic configuration of North America.AZ = Arizona; CA = California; B.C. = British Columbia; ID = Idaho; MT = Montana; NV = Nevada; NWT = Northwest Territories; OR = Oregon; UT = Utah; WA = Washington; W = Whitehorse; D = Dawson City.

Figure 3 :
Figure 3: Composite chronostratigraphic columns for the Finlayson Lake district with locations of VMS prospects, U-Pb zircon and fossil ages, and petrogenetic affinities (modified after [16]).New U-Pb zircon dates obtained in this study are shown.Late Devonian to Early Mississippian stratigraphy in the Big Campbell thrust sheet has been adjusted to correspond to new age constraints.KMC = Klatsa Metamorphic Complex; NR = North River formation; Penn = Pennsylvanian.

Figure 9 :
Figure 9: Summary of CA-ID-TIMS U-Pb zircon geochronological results for the Finlayson Lake district (a), with associated Th/U (b).Each symbol represents one individual zircon analysis as its206 Pb/ 238 U date with associated uncertainty (±2σ).Th/U for individual zircon grains, which correspond to a specific U-Pb analysis shown in (a).Analyses for the Wolverine Lake group and Kudz Ze Kayah formation are from Manor et al.[30], shown here as a compilation of new U-Pb dates in the district.Insets show individual 206 Pb/ 238 U dates (small symbols) and their calculated weighted mean date (±2σ), which define distinct periods of VMS mineralization in the district (vertical red bars).Symbols: circles = this study and from Manor et al.[30]; diamonds = historical U-Pb zircon dates from Mortensen[4], Piercey[151], Murphy et al.[16], and Piercey et al.[17].

Figure 12 :
Figure 12: Geochemical group discrimination diagrams.(a) Al 2 O 3 /TiO 2 vs. Zr/TiO 2 .Symbols represent lithology, which does not compose any trends with geochemical signature.Colors represent distinct geochemical groups based on distinct Zr/Al 2 O 3 ratios.Small circles are data from literature sources as in Figure 10.(b) Yb C vs. La/Yb C , for felsic volcanic rocks only; data is normalized to C1 chondrite (C; [91]).Petrochemical discrimination fields (FI to FIV) from Hart et al. [34].The black arrow shows a general trend of isotopic compositions within each group, and blue shaded regions indicate endmembers.

Figure 16 :
Figure 16: Nb/Ta versus (a) Zr and (b) sum of REEs.Reference lines are shown for Nb/Ta where values are ~12 for crust and ~17 for chondrite (i.e., mantle;[89,98,99]).Box and whisker plots for each geochemical variable (Nb/Ta, Zr, ΣREE) indicate the interquartile range of data in each geochemical group for back-arc (BA) and arc (A) rocks; open circles are outliers in the population.Note the significant increase in trace element concentrations and Nb/Ta ratios for group FN1 relative to group FN2 back-arc rocks.Colored shaded regions represent samples that also have complementary Hf-Nd isotopic compositions, which are also distinct between groups FN1 and FN2.Arc rocks in groups FA1 and FA2 also show a temporal distinction in both trace elements and isotopes, indicative of a transitional environment during the Late Devonian-Early Mississippian.

Figure 17 :
Figure 17: Isotope and trace element ratio mixing curves: (a) εNd 350 Ma vs. 147 Sm/ 144 Nd; (b) εNd 350 Ma vs. Th/Nb; (c) εNd 350 Ma vs. Zr/Yb; (d) εHf 350 Ma vs. La/Yb.Endmembers are N-MORB (star), using trace element values from Sun and McDonough[91] and depleted mantle isotopic compositions as in Figure15, and Snowcap assemblage (grey region;[5,157,158]).Dashed lines indicate location of samples from platformal strata of the North American ancenstral margin from Yukon-Northwest Territories (blue;[159]) and BC-Alberta (orange;[160]).Graticules on mixing curves are shown as 10% increments.Symbols and colors as in Figure10.Data that fall significantly off the mixing curves are interpreted to be results of high-temperature crustal melting (see text for details).

Figure 18 :
Figure 18: Trace element ratio mixing curves: (a) Zr/Nb vs. La/Sm; (b) Zr/Nb vs. Th/Yb.The binary mixing scenario of N-MORB and Snowcap assemblage endmembers (as in Figure15) is shown as mixing curves with 10% increments.However, this scenario does not necessarily correspond to all rocks in the Finlayson Lake district, and different mixing lines are likely possible where geochemical signatures of mafic melts are mixtures of N-MORB and OIB or E-MORB.

Figure 19 :
Figure19: Zircon saturation thermometry calculations (T Zr ( °C)) relative to Nb/Ta.Temperatures were obtained using the equation of Boehnke et al.[113], which show negligible variation from the original equations of Watson and Harrison[111] and Hanchar and Watson[112].Samples are parsed based on their alteration intensity, which greatly affects the accuracy of thermometry calculations.Large symbols contain M values of 0.8-2, Na 2 O = 2-5 wt.%, and SiO 2 > 65 wt.%, and large symbols with red borders are least-altered rocks.Small symbols contain Na 2 O < 2 wt.% and generally follow similar trends to their least-altered equivalents.The grey arrow represents the trajectory of increasing mantle-derived components in the formation of felsic rocks in the Finlayson Lake district (i.e., difference between group FN1 and FN2 compositions).

Table 1 :
Chemical abrasion-ID-TIMS U-Pb zircon isotopic data for rocks in the Finlayson Lake district.

Table 2 :
Summary of U-Pb geochronology results.

Table 3 :
Whole-rock MC-ICP-MS Lu-Hf isotopic results for rocks in the Finlayson Lake district, Yukon.

Table 4 :
Whole-rock MC-ICP-MS Sm-Nd isotopic results for rocks in the Finlayson Lake district, Yukon.

Table 5 :
Modal abundances of accessory minerals in felsic volcanic and plutonic rocks in the Finlayson Lake district.