Lake George and Scottie Creek Assemblages

The quartz-rich compositions of many metasedimentary rocks in the Lake George assemblage in western Yukon-Tanana Upland (fig. 2), together with the presence of Archean and Proterozoic inherited zircons, and radiogenic strontium and neodymium isotopic compositions for Paleozoic and younger igneous rocks within the Lake George assemblage were used to infer a continental origin for the parautochthonous rocks (for example, Aleinikoff and others, 1986; Dusel-Bacon and others, 2004). Aleinikoff and others (1981) reported neodymium isotopic data that correspond to an epsilon neodymium (ε_NdT) value at time T (age of sample) before present of −19.9 for a sample of Early Mississippian augen gneiss from the Big Delta quadrangle, and Ruks and others (2006) reported ε_NdT values of −15.3 to −10.7 (number of samples [n] = 4) for samples from the Mount Burnham orthogneiss (of Mortensen, 1990) of the Fiftymile batholith in the western Stewart River map area (fig. 4). Todd and others (2019) reported epsilon hafnium (ε_HfT) values of −23.57 and −21.75 for zircons from a sample of Late Devonian orthogneiss and Early Mississippian augen gneiss, respectively, in the Lake George assemblage in the northeastern Tanacross quadrangle. All these isotopic data are consistent with magmas that have been highly contaminated by incorporation of continental crust in a continental-margin arc or within-plate setting.

The Lake George assemblage in eastern and east-central Alaska consists of amphibolite-facies pelitic and siliciclastic metasedimentary rocks and minor mafic and rare felsic metaigneous rocks, as well as large conformable bodies of peraluminous augen-bearing and biotite orthogneisses (Dusel-Bacon and Aleinikoff, 1985; Foster and others, 1994; Dusel-Bacon and others, 2006). Augen gneiss and non-augen bearing orthogneiss in the northeastern Tanacross quadrangle have been included in the Divide Mountain augen gneiss suite and the Lake George orthogneiss suite, respectively, by Wypych and others (2019a). The large body of metaplutonic rocks in the northwestern part of the Stewart River map area has previously been referred to as the Fiftymile batholith (Tempelman-Kluit and Wanless, 1980; Mortensen, 1986); however, subsequent mapping by Mortensen (1996) demonstrated that it is not a single body, but rather comprises two distinct metaplutonic units that are separated by a north-dipping Early Cretaceous extensional fault (figs. 2, 4, 5). The batholith south of, and in the footwall of, the fault consists of augen gneiss that is included in the Lake George assemblage, whereas the batholith in the hanging wall of the fault is included in the Yukon-Tanana terrane and correlated with the informal Simpson Range plutonic suite of Colpron and others (2016) (figs. 2, 4, 5). Dusel-Bacon and Aleinikoff (1996) correlated the footwall augen gneiss of the Fiftymile batholith with contiguous rocks in the eastern Tanacross quadrangle and with the Divide Mountain augen gneiss suite. To avoid confusion, in this report we refer to the northern batholithic body in the hanging wall of the extensional fault as the Sixtymile batholith, after the Sixtymile River that traverses it.

Augen gneiss from a large body in the southeastern Big Delta quadrangle in the western part of the Lake George assemblage (fig. 2), approximately 100 km west of our study area (fig. 2), yielded sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 371±3, 365±4, and 362±4 Ma (Day and others, 2003; Dusel-Bacon and others, 2004). Mafic gneiss associated with the dated augen gneiss in that body yielded SHRIMP U-Pb zircon ages of 369±3 Ma (Dusel-Bacon and others, 2004) and 369±6 Ma (Day and others, 2003), indicating bimodal Late Devonian magmatism in the Lake George assemblage. Eight other SHRIMP and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon ages ranging from 370.6±9.6 to 347.4±5.1 Ma have been reported for other bodies of augen-bearing Divide Mountain suite and augen-free Lake George orthogneiss suite samples in the Lake George assemblage in the northeastern Tanacross quadrangle (fig. 4; samples 43–50 of table 1) by Dusel-Bacon and Aleinikoff (1996), Dusel-Bacon and Williams (2009), and Todd and others (2019). LA-ICP-MS zircon ages of the Lake George assemblage were reported by Solie and others (2014, 2019) for a sample of granitic orthogneiss along the Alaska Highway within our study area (sample 71) and another sample just south of the southwestern part of our study area that was interpreted to be a felsic metavolcanic rock (sample 72) (table 1). The orthogneiss yields an age of 354.6±9.3 Ma (fig. 4) and the metavolcanic rock yields an age of 351.7±9.3 Ma (shown as stars on fig. 2).

The Scottie Creek assemblage, originally defined in the Scottie Creek 1:50,000-scale map of western Yukon, north of Beaver Creek, by Murphy and others (2009) is mapped in the southwestern and eastern parts of the Stewart River map area in western Yukon (Mortensen, 1996; Gordey and Ryan, 2005; Staples and others, 2013) and the northwestern part of the Stevenson Ridge map area (fig. 5). It is a package of mainly metaclastic rocks with minor marble that encloses a large body of granitic augen gneiss (fig. 4; Mortensen, 1996; Gordey and Ryan, 2005; Staples and others, 2013). The augen gneiss units in Yukon have been assigned by Colpron and others (2016) to the informal Grass Lakes plutonic suite, as defined in the Finlayson Lake mining district in southeastern Yukon, east of the Tintina Fault (fig. 1). Rocks of the Lake George and Scottie Creek assemblages are confined to the structural footwall of Early Cretaceous normal faults (figs. 4, 5) and have been interpreted to be gneiss domes, analogous to metamorphic core complexes of the western United States (for example, Hansen and Dusel-Bacon, 1998; Staples and others, 2013; Jones and others, 2017a; Ryan and others, 2017; Wypych and others, 2019a). Amphibolite-facies schist and gneiss of the Scottie Creek assemblage in the northeastern Stewart River map area are intruded by Mount Burnham orthogneiss (Mortensen, 1990; Gordey and Ryan, 2005; fig. 5). These rocks are exposed in a tectonic window, referred to as the Australia Mountain domain (Staples and others, 2013). A sample of the augen gneiss yielded an interpreted isotope dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon age of 363.8±1.5 Ma (Mortensen, 1990; sample 51, fig. 4, table 1). Ruks and others (2006) reported a considerably younger, but poorly constrained, ID-TIMS U-Pb zircon age of 347.5±0.7 Ma for a second sample from the same area. This sample was subsequently re-analyzed using SHRIMP U-Pb methods, however, and the age was revised to approximately 360 Ma (J.J. Ryan, Natural Resources Canada, oral comm., 2020; Yukon Geological Survey, 2020b; sample 52; fig. 4; table 1).

Because of the lithologic and age similarities between the Lake George and Scottie Creek assemblages, as well as the continuity of these rocks across the Alaska-Yukon border in the southern Tanacross quadrangle, we consider them to be parts of the same assemblage.

Fortymile River, Finlayson, and Snowcap Assemblages

The Fortymile River assemblage is exposed mainly in the Eagle quadrangle in eastern Alaska. It is made up of thoroughly recrystallized amphibolite-facies biotite gneiss, amphibolite, garnet amphibolite, mafic schist and gneiss, metapelite, quartzite, metachert, marble, and minor metarhyolite and metaporphyry, together with locally abundant feldspathic orthogneiss (primarily of granodioritic, tonalitic, or trondhjemitic composition) that form tabular bodies that are concordant with compositional layering in the host supracrustal rocks, as well as larger massifs (figs. 2, 4, 5) (Foster, 1976; Day and others, 2000, 2002; Werdon and others, 2001; Szumigala and others, 2002; Dusel-Bacon and others, 2006). A body of weakly foliated and epidotized, quartz-biotite-plagioclase granodioritic gneiss (shown in dark brown on fig. 2) crops out within interlayered amphibolite and quartz-biotite schist of the Fortymile River assemblage south of the Fortymile River at about 64°12′ latitude along the Taylor Highway (fig. 4). This unit was referred to as the Steele Creek Dome tonalite or Steele Creek Dome orthogneiss by Day and others (2000) and Day and others (2002), respectively. A SHRIMP U-Pb zircon age of 343±4 Ma (sample 42, fig. 4, table 1) was previously reported from this body (Day and others, 2002). Metasedimentary rocks and local intercalations of mafic and rare felsic metavolcanic rocks are intruded by mainly intermediate-composition metaplutonic bodies that appear to be intimately associated with the metavolcanic units.

The Fortymile River assemblage in Alaska can be mapped continuously with rock units that are assigned to the Finlayson assemblage in the southwestern Dawson map area and northwestern Stewart River map area on the most recent version of the digital Yukon Bedrock Geology compilation (Yukon Geological Survey, 2020b) (figs. 4, 5). The Finlayson assemblage was originally defined in the Finlayson Lake district in southeastern Yukon, east of the Tintina Fault (fig. 1) (Murphy and others, 2006). After restoration of motion on the Tintina Fault, the Finlayson Lake district would have been in part adjacent to rocks of the Yukon-Tanana terrane in western Yukon and eastern Alaska. The Finlayson assemblage, as defined in the Yukon Bedrock Geology compilation, mainly consists of metamorphosed and deformed mafic to felsic metavolcanic rocks, carbonaceous pelite, and metachert. Mapping by Mortensen (1996) and additional mapping by Mortensen in 1983­–2012 demonstrate that the rock units mapped as Finlayson assemblage in the Dawson map area consist mainly of metaclastic rocks that closely resemble those in the Fortymile River assemblage to the west but have less abundant metavolcanic units.

Farther to the southeast in the Stewart River map area, amphibolites of the Finlayson assemblage are interspersed with metaclastic rocks that have been assigned by Ryan and others (2013) to the Snowcap assemblage; the amphibolites are also interspersed with numerous bodies of marble, as well as small bodies and larger massifs of mainly intermediate-composition orthogneiss. The Snowcap assemblage is interpreted to comprise the oldest stratigraphic units in the Yukon-Tanana terrane. The Snowcap assemblage was originally defined in the Glenlyon area in central Yukon (fig. 1, locality C), south of our study area (Colpron and others, 2006b). There it consists of a Devonian and possibly older mainly metaclastic assemblage that contains 1- to 10-meter (m)-thick horizons of amphibolite and greenstone, on which the Finlayson assemblage volcano-sedimentary assemblage was deposited (Colpron and others, 2006b; Piercey and Colpron, 2009). Our own observations in numerous localities along the Yukon River in the Stewart River map area, however, agree with Ryan and others’ (2003) interpretation that at least some of the metaclastic rocks in this region are coeval with amphibolites (mafic metavolcanic rocks) of the Finlayson assemblage. Also, several samples of metaclastic rocks that are shown as Snowcap assemblage in compilation maps (Yukon Geological Survey, 2020b) that are closely associated with Finlayson assemblage amphibolites in the Stewart River map area have been shown to contain Late Devonian detrital zircons (Ryan and others, 2003; Cleven and others, 2019), a zircon population that is absent from the type Snowcap assemblage, thus supporting a depositional age for these rocks that coincides with deposition of the Finlayson assemblage. This indicates that the Finlayson assemblage, together with some of the metaclastic rocks presently mapped as Snowcap assemblage and the marble bodies in the Stewart River map area, are collectively correlative with the Fortymile River assemblage as mapped in eastern Alaska.

It is certainly possible that some of the metaclastic rocks in the Stewart River map area are indeed Snowcap assemblage equivalents that are older than the Finlayson assemblage and Fortymile River assemblage; however, the lithological composition of the Snowcap assemblage and the metaclastic component of the Fortymile River and Finlayson assemblages are not sufficiently different to allow them to be readily distinguished. On our compilation maps of the area (figs. 4, 5), we therefore label the metaclastic assemblages as “Snowcap? assemblage” to emphasize the uncertainty of which map units are stratigraphically interlayered with the Finlayson assemblage metavolcanic rocks and which possibly underlie them. Intermediate-composition orthogneiss bodies that are thought to be comagmatic with metavolcanic rocks of the Finlayson assemblage in Yukon have been grouped with the Simpson Range plutonic suite as defined in the Finlayson Lake district in southeastern Yukon (Colpron and others, 2016). They occur throughout both the Finlayson assemblage and the possible Snowcap? assemblage, so their presence or absence does not help distinguish between true Snowcap assemblage rock units that predate the Finlayson assemblage and similar rocks (Snowcap?) that are truly part of the Finlayson assemblage.

U-Pb zircon ages ranging from 355.0±1.8 to 343.7±1.4 Ma have been reported for intermediate to felsic orthogneisses associated with the Finlayson assemblage in the Stewart River map area (samples 60–65, fig. 4, table 1) (Yukon Geochronology Database; Yukon Geological Survey, 2020a). An older interpreted ID-TIMS U-Pb zircon age of 362.1±2.7 Ma was reported by Ruks and others (2006) for a body of granitic potassium-feldspar augen orthogneiss, informally called the Tenderfoot Creek orthogneiss, which is associated with Finlayson assemblage amphibolites and Snowcap? assemblage metaclastic rocks on the Stewart River approximately 10 km upstream from its confluence with the Yukon River (sample 58, fig. 4, table 1).

The Fortymile River assemblage, Finlayson and Snowcap? assemblages, and orthogneiss bodies within them are imbricated by numerous regional-scale thrust faults of probable Early Jurassic age, many of which are marked by discontinuous, lens-shaped, thrust-fault bounded bodies of serpentinized ultramafic rocks assigned to the Slide Mountain-Seventymile terrane (Mortensen, 1996; Gordey and Ryan, 2005). Mapping across the Alaska-Yukon border has shown that mixed metaclastic and mafic metavolcanic rocks of the Fortymile River assemblage intertongue, probably as an original facies change, with variably carbonaceous siliciclastic rocks of the Nasina assemblage (which are designated as sub-unit DMF3 of the Finlayson assemblage on the Yukon Bedrock Geology compilation [Yukon Geological Survey, 2020b]). Rocks of the Fortymile River, Finlayson, and Snowcap? assemblages are juxtaposed against structurally underlying Lake George and Scottie Creek assemblages across Early Cretaceous extensional faults (fig. 5).

An extensive neodymium isotopic database has been generated for rocks of the Finlayson assemblage in the Finlayson Lake district by Grant (1997) and Piercey and others (2003, 2004, 2006). Felsic rocks of the Finlayson assemblage in the Finlayson Lake district and associated metaplutonic rocks mainly yield evolved (crustal) neodymium isotopic signatures (ε_NdT = −12.9 to +0.1; Piercey and others, 2003), whereas mafic metavolcanic rocks and mafic dikes in the area have more juvenile isotopic signatures (ε_NdT = −0.3 to +8.5). Some of these mafic rocks are interpreted to represent juvenile arc lavas, whereas others are thought to be somewhat younger magmas related to initial rifting of the Early Mississippian arc and subsequent back-arc magmas (Piercey and others, 2004).

Nasina Assemblage

The Nasina assemblage is a major component of the Yukon-Tanana terrane in the central Stewart River and southwestern Dawson map areas in western Yukon and the northeastern part of the Eagle quadrangle in Alaska (Foster, 1976; Mortensen, 1988, 1990, 1996; Gordey and Ryan, 2005) (figs. 4–6). It mainly consists of strongly deformed and foliated fine-grained siliciclastic rocks, including quartz-mica schist and quartzite. Minor lithologies present include marble layers that are locally as thick as 100 m and 10 km in strike length, as well as rare occurrences of stretched pebble conglomerate and rare muscovite and quartz-muscovite schist layers that are interpreted to be derived from felsic airfall tuff. Several small bodies of moderately to strongly foliated biotite quartz monzonitic orthogneiss occur locally within the Nasina assemblage. Two of these bodies yielded Late Permian U-Pb zircon ages (samples 57 and 68; fig. 4, table 1) and are correlated with the regional Sulphur Creek orthogneiss suite. The Nasina assemblage has mostly been metamorphosed at upper greenschist facies (biotite and garnet grade). Two separate lithological facies in the Nasina assemblage are distinguished by Mortensen (1996) and additional mapping by Mortensen in 1983–2001 (fig. 4); these are a carbonaceous facies, which, in addition to the above-mentioned rock types, typically contains a substantial amount of carbonaceous material and generally weathers medium to dark gray, and a non-carbonaceous facies, which is lithologically similar but generally lacks the carbonaceous component. The Nasina assemblage intertongues with amphibolites of the Fortymile River assemblage in the southwestern Dawson map area just east of the Alaska-Yukon border (figs. 4–6). The Nasina assemblage was interpreted by Colpron and others (2006a) to represent a clastic sequence that was deposited in the Nasina basin, which is thought to have lain broadly in a back-arc position with respect to the Fortymile River assemblage arc.

The depositional ages for the Nasina assemblage are difficult to constrain, owing to negligible amounts of volcanic rocks that can be dated and analyzed geochemically, and the strong deformation and metamorphism that have largely obliterated any fossils that might have been present. Some information for the age of the rock units has been obtained from lead isotopic studies of syngenetic base metal occurrences in the Nasina assemblage (fig. 6), which yield middle Paleozoic lead model ages (Mortensen and others, 2006).

Klondike Assemblage

Supracrustal rocks of the Klondike assemblage are mainly exposed in the Klondike and Sixtymile mining districts in Yukon, as well as a large area that straddles the Alaska-Yukon border south of the Fiftymile batholith (fig. 4). The Klondike assemblage in the Stewart River and southwestern Dawson map areas in western Yukon comprises three main lithologies: (1) fine-grained quartz-muscovite schist and quartz (with or without feldspar) augen schist, (2) mafic schist and metagabbro, and (3) fine-grained, non-carbonaceous, quartz-mica schist and quartzite (Mortensen, 1988, 1990, 1996; Gordey and Ryan, 2005). Protoliths of these three middle to upper greenschist-facies lithologies are interpreted to be felsic volcanic and volcaniclastic rocks and associated subvolcanic rocks, mafic to intermediate volcanic rocks and associated mafic intrusions, and fine-grained metaclastic rocks, respectively. Small syngenetic base metal occurrences have been identified in many localities within the Klondike assemblage (Mortensen and others, 2006), indicating that much of the Permian volcanism occurred in a submarine setting.

Weakly to strongly foliated bodies of quartz monzonitic to granitic composition are also associated with the Klondike assemblage; these bodies are collectively termed the Sulphur Creek orthogneiss suite (Colpron and others, 2016), named for a specific body of that name in the southern Klondike district (figs. 4, 5; Mortensen, 1990; Colpron and others, 2016). Middle to late Permian U-Pb zircon ages have been determined for muscovite-quartz schist, interpreted to be metamorphosed felsic tuff, and for several of the quartz monzonitic to granitic metaplutonic bodies (samples 66 and 67; fig. 4, table 1; Mortensen, 1990, 1992; Ruks and others, 2006). Bodies of Sulphur Creek orthogneiss are intimately associated with the felsic metavolcanic units in the Klondike district, but bodies of similar age and composition are also widely distributed throughout the Finlayson and Snowcap? assemblages in Yukon and as rare occurrences in northern Tanacross and east-central Eagle quadrangles in Alaska (fig. 4).

Most contacts of the Klondike assemblage with other assemblages, as well as structures that locally imbricate the Klondike assemblage, are mapped as regional-scale thrust faults and locally are marked by small, fault-bounded bodies of serpentinized ultramafic rocks of the Slide Mountain-Seventymile terrane (fig. 4). Although original stratigraphic relations between the Klondike assemblage and older assemblages are poorly preserved, the presence of the Sulphur Creek orthogneiss bodies both within the Klondike assemblage and throughout the Finlayson and Snowcap? assemblages and, locally, the Nasina assemblage, demonstrates that the Klondike assemblage was originally built on top of the Finlayson, Snowcap?, and Nasina assemblages.

Mortensen (1990) and Beranek and Mortensen (2011) reported TIMS U-Pb zircon ages of 267 to 259 Ma for felsic metavolcanic rocks of the Klondike assemblage and the Sulphur Creek orthogneiss in the Klondike district.

Jones and others (2017b) report preliminary U-Pb zircon ages of approximately 262 Ma for a thin layer of felsic gneiss within metaclastic rocks and amphibolite of the Fortymile River assemblage in northern Tanacross quadrangle and approximately 257 Ma for a granitic pluton in south-central Eagle quadrangle that is interpreted to crosscut the main foliation in Fortymile River assemblage host rocks. Ruks and others (2006) determined ε_NdT values ranging from −14.0 to −7.1 for seven samples of Permian Sulphur Creek orthogneiss and felsic metavolcanic rocks of the Klondike assemblage, consistent with magmas that have been highly contaminated by incorporation of continental crust in a continental-margin arc or within-plate setting. Crustal contamination is also indicated by the common presence of older inherited zircon in many of the Klondike assemblage metavolcanic rocks and the Sulphur Creek orthogneiss suite (Mortensen, 1990).

Chicken Assemblage

The Chicken assemblage, as we herein rename it, has previously been described in the literature as the “Chicken metamorphic complex” (for example, Werdon and others, 2001; Dusel-Bacon and others, 2006, 2013). Werdon and others (2001) proposed the name “Chicken metamorphic complex” to describe a group of lower greenschist-facies mafic and minor felsic metavolcanic rocks, limestone, metagabbro, metadiabase, slate, quartz-mica phyllite, quartzite, and tonalitic gneiss that crop out along the eastern and southern margins of a large Late Triassic intrusion (Taylor Mountain batholith) in the southern Eagle and northern Tanacross quadrangles (figs. 2, 4, 5; Foster, 1970, 1976; Werdon and others, 2001; Dusel-Bacon and others, 2006, 2013). Another exposure of these rocks extends from the northwestern margin of the Taylor Mountain batholith outside of our study area in the south-central Eagle quadrangle (Foster, 1976; Dusel-Bacon and others, 2013; Day and others, 2014) (fig. 2). The low metamorphic grade of these rocks is markedly different from those of the adjacent Fortymile River and Lake George assemblages, which have experienced amphibolite-facies metamorphism. Because the term “metamorphic complex” implies much higher metamorphic-grade rocks (as in “core complex”) than those in this unit and results in a misconception about their metamorphic history, we hereby abandon that term and instead refer to them informally as the “Chicken assemblage,” retaining the location moniker of the nearby village of Chicken (figs. 2, 4, 5) and adjusting the name to match the other Paleozoic tectonic assemblages in the region. The type area for this assemblage is that shown and described for the “Chicken metamorphic complex” by Werdon and others (2001).

Recrystallized limestone interlayered with massive mafic metavolcanic rocks (greenstone) in the southeastern Eagle quadrangle yielded Paleozoic fossil crinoid columnals (Foster, 1969), and Dusel-Bacon and Harris (2003) described poorly preserved conodonts from the same outcrop area as having a late Paleozoic, possibly Mississippian, age. Foster (1976) originally tentatively correlated the Chicken assemblage with the Slide Mountain-Seventymile terrane because of the prominent mafic metavolcanic rocks that are common to both, but later corrected this correlation because the Chicken assemblage lacks peridotite, which is only present in the Slide Mountain-Seventymile terrane (Foster and others, 1994).

Dusel-Bacon and others (2013) reported a SHRIMP U-Pb zircon age of 332.6±5.7 Ma for a 2-m-thick layer of strongly mylonitized gneiss of tonalitic composition within mafic metavolcanic rocks of the Chicken assemblage in the south-central Eagle quadrangle (table 1, location 41, shown as a star on fig. 2). The protolith of this rock unit is uncertain; it could have been a tonalitic sill or alternatively a dacitic volcanic flow. This relatively young age is different from any ages that have yet been obtained in the Yukon-Tanana terrane in this region.

Ladue River Unit

The Ladue River unit makes up an area in the eastern Tanacross quadrangle of dominantly greenschist-facies quartz-mica schist, mafic and felsic schist, amphibolite, and locally abundant felsic orthogneiss (fig. 4). The relatively low metamorphic grade contrasts with the typical amphibolite-facies mineral assemblages that characterize the adjacent Lake George assemblage. Foster (1970) mapped these rocks as a unit of uncertain Precambrian to Paleozoic age, and correlated these rocks with the Klondike schist unit of Cockfield (1921) across the border in Yukon based on broadly similar lithologies, especially the prominence of quartz-white mica-chlorite schist. As described above, the Klondike schist of Cockfield (1921) has since been shown to be Late Permian in age (Mortensen, 1990, 1992). Dusel-Bacon and others (2006) described these rocks as a new informal unit, termed the Ladue River unit (fig. 2), because of both a Late Devonian U-Pb zircon age obtained from a sample of augen gneiss in the unit, and a normal midocean ridge basalt (N-MORB) geochemical affinity for a metabasite sample in the unit that is completely different from the Klondike assemblage rocks. Dusel-Bacon and others (2006) proposed that the Ladue River unit is better correlated with the higher grade middle Paleozoic Lake George assemblage in which some metabasites have an N-MORB geochemical affinity.

Recent preliminary mapping and geochemical and geochronological studies in this area by Jones and others (2017a, b) and Twelker and others (2019) have determined that the late Permian Klondike assemblage does extend westward across the border into Alaska and forms thin thrust sheets that overlie the Ladue River unit, which is itself interpreted to be restricted to thin thrust sheets that have been thrust over the Lake George assemblage (figs. 4, 5).

Slide Mountain-Seventymile Terrane

The Slide Mountain-Seventymile terrane comprises allochthonous rocks of mainly oceanic affinity in numerous localities throughout southern and western Yukon and eastern Alaska (figs. 2, 4, 5; Keith and others, 1981; Foster and others, 1994; Dusel-Bacon and others, 2006; Yukon Geological Survey, 2020b). The Slide Mountain-Seventymile terrane consists mainly of massive, locally pillowed greenstone, mafic volcaniclastic rocks, radiolarian chert, minor gabbro and diabase, and serpentinized ultramafic rocks (peridotite), some of which have been interpreted to comprise an ophiolite sequence (for example, Keith and others, 1981; Mortensen, 1988; Nelson, 1993; Foster and others, 1994). In eastern and east-central Alaska, the Slide Mountain-Seventymile terrane occurs as several large, structurally imbricated klippen, including serpentinized peridotite, the largest three being the Salcha River klippe in the northwest corner of the Big Delta quadrangle (fig. 2), the Mount Sorenson klippe that spans the northwesternmost Eagle quadrangle and southwesternmost Charley River quadrangle (fig. 2), and the Wolf Mountain klippe astride the Taylor Highway in the northern Eagle quadrangle (fig. 4) (Foster and others, 1994). Somewhat smaller klippen of the Slide Mountain-Seventymile terrane occur as thrust-fault-bounded blocks in the vicinity of the abandoned Clinton Creek asbestos mine and Dawson in the southeastern Dawson map area (fig. 6) (Mortensen, 1988; Yukon Geological Survey, 2020b). Numerous small, isolated bodies of serpentinite, serpentinized ultramafic rocks, and greenstone occur throughout the Fortymile River and Nasina assemblages in the Eagle quadrangle (Foster, 1976) and southwestern Dawson map area (figs. 2, 4–6) (Mortensen, 1988; Yukon Geological Survey, 2020b) and it is probable that these bodies mark the traces of unmapped thrust faults. Rocks of the Slide Mountain-Seventymile terrane are typically weakly metamorphosed and generally do not show a recrystallization foliation. The ultramafic rocks commonly display shear fabrics that indicate these bodies are mantle tectonites (for example, van Staal and others, 2018). Many of the mapped bodies of the Slide Mountain-Seventymile terrane are composite, consisting of several imbricate thrust slices of differing lithology.

Age constraints for the Slide Mountain-Seventymile terrane are mainly from conodonts, radiolaria, and macrofossils in cherts and minor limestone associated with the greenstone. Permian and Mississippian fauna have been identified within rocks of the Seventymile terrane outside our study area in east-central Alaska (Dusel-Bacon and Harris, 2003), and equivalent metasedimentary rocks of the Slide Mountain terrane in Yukon (Murphy and others, 2006) and northern British Colombia (Nelson, 1993). A maximum Early Mississippian (Tournasian; ~351±4 Ma) age for the Seventymile terrane in Alaska is inferred based on fossil ages for the sedimentary rocks of the terrane; however, most of the fossils are middle to late Permian (Dusel-Bacon and Harris, 2003, and references therein). These young ages contrast with the older Mississippian to Permian fossil ages from the most extensive Slide Mountain terrane exposures in southern British Colombia (Klepacki and Wheeler, 1985; Orchard, 1985), the large and well-described Sylvester allochthon in northernmost British Columbia (fig. 1, locality G) (Nelson and Bradford, 1993) and the Germansen Landing area in central British Colombia (Ferri and others, 1994) (fig. 1, locality H).

Mafic and ultramafic igneous rocks of the Slide Mountain-Seventymile terrane are imbricated with thrust slices of weakly deformed dark gray siltstone, shale, and argillaceous limestone that have yielded Late Triassic conodont ages from many localities (for example, within the Mount Sorenson peridotite klippe in the northwestern Eagle quadrangle, along the northeastern margin of the Wolf Mountain greenstone klippe along the Taylor Highway at about 64°30′ in the east-central Eagle quadrangle, and in the vicinity of the Clinton Creek mine site in the southwestern Dawson map area (fig. 4) (Dusel-Bacon and Harris, 2003; Beranek, 2009; Beranek and Mortensen, 2011). These sedimentary strata commonly contain abundant Devonian to Mississippian and Permian detrital zircons (Beranek and Mortensen, 2011), and are interpreted to be synorogenic clastic rocks that were deposited during regional Late Triassic to Early Jurassic contractional deformation in the Yukon-Tanana terrane. In their discussion of Late Triassic conodonts in various allochthonous terranes and in the North American continental margin in the northern Cordillera, Dusel-Bacon and Harris (2003) interpreted the wide distribution of these fossils to indicate that these areas shared approximately similar warm, normal-marine conditions along the Late Triassic continental margin, but did not consider the Triassic rocks to represent an overlap assemblage, in the sense of draping across contacts between outboard allochthonous pericratonic and arc fragments and the ancient Pacific margin of Laurentia.

Rare exposures of felsic volcanic rocks are also spatially associated with Slide Mountain-Seventymile terrane units in several localities in Alaska. One of these is along the Taylor Highway in the easternmost Eagle quadrangle, another straddles the Alaska-Yukon border nearby (fig. 4), and another spans a larger area south of Mount Sorenson in the northwestern Eagle quadrangle (Foster, 1976). Foster and Keith (1968) and Foster (1976) mapped these felsic volcanic bodies as small areas of quartz-feldspar rock (unit | q), and quartzite and argillite (unit P q), respectively. Both described units contain a mixed lithology that includes phyllite, chert, limestone, and graywacke. The Permian age for unit P q of Foster (1976) resulted from Permian macrofossils being recovered from this rock unit at the Mount Sorenson locality. A body of weakly deformed and metamorphosed felsic volcanic rock is exposed in road cuts of this unit at the Taylor Highway locality. We tentatively interpret the rock to be a silicified felsic tuff based on our field observation of sparse sub-millimeter quartz or feldspar grains and narrow, crosscutting quartz veins, consistent with silicification. Foster (1976) interpreted the siliceous and felsic rocks to be metamorphosed silicified tuffs from the Paleozoic or Mesozoic. In thin sections of samples collected from the metamorphosed silicified tuff, including our sample 39 (fig. 4), the presence of sparse, 2- to 3-millimeter (mm)-long relict quartz and feldspar phenocrysts (app. 3, fig. 3.1D, E) confirms an igneous origin. The felsic rock unit is spatially associated with massive greenstone, sheared serpentinite, and dark gray argillite probably from the Late Triassic, and the association with greenstone and ultramafic rocks led to these rocks being included in the Slide Mountain-Seventymile terrane (Foster and others, 1994).

Lake George and Scottie Creek Assemblages

A sample of gneissic, potassium-feldspar augen-bearing, biotite quartz monzonite was collected from the Fiftymile batholith in the footwall of the Early Cretaceous extensional fault (sample 1; fig. 4, table 2). Six analyses of abraded and unabraded zircons were analyzed from this sample. The assigned age is based on the upper intercept age of a five-point regression (mean squared weighted deviation [MSWD]=0.16) that has upper and lower intercepts of 363.5±3.8 Ma and approximately 110 Ma, respectively (fig. 7A; table 2).

A second sample of coarse potassium-feldspar augen orthogneiss from this same body was collected from a locality 22 km to the west (sample 2; fig. 4, table 2). An age of 361.8±0.6 Ma is assigned to the sample based on a weighted average ²⁰⁶Pb/²³⁸U age of two partially overlapping concordant analyses (fig. 7B; table 2).

Fortymile River Assemblage

Weakly to moderately foliated, medium-grained, equigranular granodiorite of the Sixtymile batholith was sampled along the Sixtymile River near the north edge of the batholith (sample 3; fig. 4, table 2). Four zircon fractions were analysed. Two of these give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 347.5±0.4 Ma that is considered the best estimate for the crystallization age of the rock (fig. 7C; table 2).

A sample was collected from a narrow (approximately 5-m-thick) conformable band of granitic orthogneiss that contains potassium-feldspar augen as long as 3 centimeters (cm) in the longest dimension; the augen-bearing orthogneiss occurs within rocks of the Fortymile River and Finlayson assemblages, just above the Cretaceous extensional fault that separates the Fortymile River and Finlayson assemblage rocks from the underlying Lake George assemblage (sample 4; fig. 4, table 2). Rocks in this area are strongly metamorphosd to hornfels facies around several massive Late Cretaceous quartz monzonite intrusions and were previously mapped by Tempelman-Kluit (1974) as a separate rock unit comprising “chert and metachert with minor chloritic phyllite and marble” (p. 25–26). However, the rocks in the area package transition gradationally into more typical rocks of the Fortymile River assemblage away from the intrusions. Zircons recovered from the sample are similar to those in other samples of feldspar augen orthogneiss. Analyses of five fractions of strongly abraded zircons are all slightly discordant (fig. 7D). All give consistent ²⁰⁷Pb/²⁰⁶Pb ages, however, and a weighted average of these ages (360.5±3.2 Ma) is considered the best estimate for the crystallization age of the rock (table 2).

A body of moderately foliated hornblende-biotite granodioritic gneiss occurs north of the Top of the World Highway along the Alaska-Yukon border (fig. 4) within an assemblage of mafic schist, amphibolite, marble, and carbonaceous metasedimentary rocks. The supracrustal assemblage into which this body was emplaced lies near a facies change between dominantly Nasina assemblage rocks to the east and Fortymile River assemblage to the west. The orthogneiss was sampled on a mine access road (sample 5; fig. 4, table 2). Two fractions of strongly abraded zircon give overlapping concordant analyses (fig. 7E) that have a weighted average ²⁰⁶Pb/²³⁸U age of 348.3±2.9 Ma, which gives the crystallization age of the sample.

A large body of hornblende-biotite granodiorite orthogneiss was sampled on the west side of the Yukon River (sample 6; fig. 4, table 2). The orthogneiss is interpreted to be in thrust contact with overlying, variably carbonaceous metasedimentary rocks and marble of the Nasina assemblage to the northeast and in uncertain, but presumably intrusive, contact with rocks of the Finlayson and Snowcap? assemblages to the southwest (Gordey and Ryan, 2005, and geologic mapping by Mortensen in 1989–1993). This orthogneiss body is compositionally similar to, and may be a southeastern extension of, the Sixtymile batholith (sample 3 above), but is separated from it in map view by a large area of overlying Late Cretaceous volcanic and sedimentary rocks (fig. 4). Five zircon fractions give slightly discordant analyses (app. 4, table 4.1; table 2; fig. 7F). Four of the fractions give overlapping ²⁰⁷Pb/²⁰⁶Pb ages and a weighted average of 351.9±1.9 Ma, which we consider the best estimate for the crystallization age of the sample. Two strongly abraded fractions of euhedral, pale yellow-brown titanite from this sample were also analysed (app. 4, table 4.1; fig. 7F, T1 and T2) in an attempt to constrain the age of the deformation and metamorphism that the gneiss has experienced. The titanite fractions yield concordant but imprecise analyses between about 339 and 332 Ma, indicating either that the titanite also has experienced substantial lead loss or was newly grown during a Middle or Late Mississippian metamorphic event (our preferred interpretation).

We collected a sample of granodioritic gneiss (sample 7; fig. 4, table 2; app. 1, fig. 1.1A) from the informal unit referred to as the Steele Creek Dome tonalite or Steele Creek Dome orthogneiss by Day and others (2002) about 5 km northwest of a previously analyzed sample (sample 42, table 1) that yielded a U-Pb zircon age of 343±4 Ma (Day and others, 2002). Seven strongly abraded zircon fractions were analyzed (app. 4, table 4.1; fig. 7G), and three of these give concordant but non-overlapping analyses and appear to be free of any inherited component. Fraction G gives the oldest ²⁰⁶Pb/²³⁸U age at 348.7±1.4 Ma, which we consider the best estimate for the minimum crystallization age of the sample and is in agreement with the age reported by Day and others (2002).

Foliated, garnet-bearing hornblende tonalite within Fortymile River assemblage amphibolites was sampled from a small body along the Fortymile River (sample 8; fig. 4, table 2; app. 1, fig. 1.1B). Five zircon fractions were analyzed (fig. 7H; app. 4, table 4.1), and two give overlapping concordant analyses that have a weighted average ²⁰⁶Pb/²³⁸U age of 355.0±0.5 Ma, which we interpret to be the crystallization age of the sample.

Quartz dioritic orthogneiss that forms a conformable band approximately 100 m thick within amphibolites of the Fortymile River assemblage is exposed along the north side of the Fortymile River, approximately 6.5 km east of the Alaska-Yukon border. Four fractions of zircon recovered from a sample from this body (sample 9; fig. 4, table 2; app. 1, fig. 1.1C) were analyzed and all yield concordant analyses (table 2; fig. 7I). Three of the analyses cluster on concordia and have a weighted average ²⁰⁶Pb/²³⁸U age of 348.3±1.2 Ma, which we interpret as the crystallization age of the sample (table 2).

A small body of mylonitic quartz-biotite-feldspar granitic augen gneiss with coarse augen (1–4 cm in length) of pink potassium-feldspar and polycrystalline quartz was sampled along the Taylor Highway (sample 10; fig. 4, table 2; app. 1, fig. 1.1D). All local outcrops and felsenmeer in this area consist of amphibolite or quartz-biotite schist. Five fractions of zircon give slightly to moderately discordant analyses with overlapping ²⁰⁷Pb/²⁰⁶Pb ages (fig. 7J); a weighted average ²⁰⁷Pb/²⁰⁶Pb age for the five fractions is 360.7±2.3 Ma, which is our interpreted crystallization age for the sample (table 2).

A body of strongly foliated, intensely folded, and mylonitic augen gneiss containing potassium-feldspar augen (1–4 cm in length) within Fortymile River assemblage amphibolites, located south of the Taylor Highway and east of the Chicken assemblage, was sampled for dating (sample 11; fig. 4, table 2; app. 1, fig. 1.1E). Six zircon fractions were analyzed (fig. 7K; app. 4, table 4.1); three of these are concordant and have a weighted average ²⁰⁶Pb/²³⁸U age of 354.5±2.1 Ma, which we interpret as the crystallization age of the sample (table 2).

A narrow (approximately 1- to 2-m-thick) band of rusty-weathering muscovite and quartz-muscovite schist is interlayered with biotite quartzite of the Fortymile River assemblage on the North Fork Fortymile River (sample 12; fig. 4, table 2; app. 1, fig. 1.1F). We interpret the schist to have been derived from a felsic tuff or cherty tuff within the Fortymile River assemblage. The sample yielded both euhedral grains and rounded and pitted or frosted grains that are clearly detrital. Eight fractions were selected from the euhedral zircon population; six of these yield slightly to moderately discordant analyses that have overlapping ²⁰⁷Pb/²⁰⁶Pb ages and a weighted average of these ages is 345.5±2.1 Ma (fig. 7L). We consider this to be the best estimate for the crystallization age of the sample.

Nasina Assemblage

A total of 15 samples of metamorphosed intrusive and volcanic rocks from within the Nasina assemblage were dated to better constrain the age of this assemblage (table 2). Ten of these samples are from thin (<1 m thick) conformable layers of felsic quartz-muscovite schist, locally containing small quartz and (or) feldspar augen, within the metaclastic rocks of the Nasina assemblage. Several of these conformable samples have gradational contacts with the enclosing clastic rocks and are interpreted to have been thin felsic airfall tuff layers; however, others have sharp contacts with the enclosing metasediments and could be either tuffs or thin porphyritic flows. The remainder of the samples are from orthogneiss or augen schist units that are interpreted to be intrusions that were emplaced into the metaclastic units; their ages give minimum depositional ages for the enclosing Nasina assemblage metaclastic rocks.

U-Pb zircon ages for thin metatuffaceous units within siliceous metaclastic rocks, such as in the Nasina assemblage, must be interpreted with caution because it is common for such units to be reworked to some extent during and immediately after deposition and mixing with detrital zircons from the host metasedimentary rocks may occur. Care was taken to avoid grains with rounded or frosted surfaces that may be detrital in origin.

A sample of pale gray quartz-muscovite schist (sample 13) was collected from a band of slightly rusty, buff- to pale-gray-weathered quartz muscovite schist approximately 5 to 8 m in thickness that occurs within an outcrop of medium-gray carbonaceous Nasina assemblage on the east side of Midnight Dome 3 km east of Dawson (fig. 6). This band has gradational contacts with the surrounding carbonaceous schist and is interpreted to be a felsic metatuff that was depositionally interlayered with the carbonaceous metaclastic rocks. A direct depositional age for the Nasina assemblage rocks in this area is provided by zircons recovered from a small amount of euhedral zircon consisting of stubby, square, prismatic grains and four strongly abraded fractions were analyzed (table 2). Fraction B yields a concordant age of 358.5±1.1 Ma (fig. 8A), which we interpret as a minimum crystallization age for the tuff protolith and depositional age for the clastic rocks of the Nasina assemblage in this area. The other three fractions each give much older ages and appear to contain a substantial inherited zircon component (table 2; app. 4, table 4.1). Two-point regressions from concordant fraction B through each of the discordant fractions give calculated upper intercept ages ranging from 3.91 to 2.07 Ga, indicating a wide range of ages for the inherited zircon component.

Three samples of felsic schist were dated from localities within the carbonaceous Nasina assemblage along the access road to the Clinton Creek mine (fig. 6). Several small occurrences of stratiform, syngenetic sedimentary exhalative, or SEDEX-type, lead-zinc-barium mineralization occur in this general area (fig. 6) and give broadly middle Paleozoic lead isotopic model ages (Mortensen and others, 2006). Pale to dark gray, fine grained, carbonaceous quartzite and quartz-muscovite schist are well exposed along much of the Clinton Creek mine access road (fig. 6). Sample 14 was collected from a thin (approximately 5-m-thick) layer of buff to slightly rusty-weathering quartz-muscovite schist (app. 2, fig. 2.1A) interlayered with the carbonaceous metaclastic rocks. We interpret it to be a felsic metatuff that was deposited along with the clastic rocks. Most of the zircons recovered from a sample of this material are euhedral, stubby, square prismatic grains that we interpret to be derived from the igneous protolith of the metatuff unit; however, a small number of zircons display subrounded, frosted textures, indicating they are detrital grains that were incorporated into the metatuff during deposition or subsequent reworking. Two of the euhedral zircon fractions (B and D) give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 360.8±1.0 Ma (fig. 8B), which we consider to be the crystallization age for the tuff protolith and the depositional age for the Nasina assemblage in this area. Fraction C gives a slightly younger ²⁰⁶Pb/²³⁸U age, reflecting minor post-crystallization lead loss, and the fourth fraction (A) gives an older age, indicating the presence of a minor inherited zircon component (table 2; app. 4, table 4.1).

Two small bodies of moderately foliated felsic metaporphyry also occur within Nasina assemblage rocks. Sample 15 (fig. 6), from a 2- to 3-m-thick band of quartz-feldspar metaporphyry that contains subhedral to euhedral feldspar and quartz augen as large as 1 cm in diameter in a very fine-grained felsic matrix (app. 2, fig. 2.1B), is exposed in a small, cleared area near the Clinton Creek mine access road. Abundant euhedral, prismatic zircon was recovered from a sample of the unit, and four fractions of strongly abraded euhedral zircon grains were analyzed. Fractions A and C give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 260.3±0.4 Ma (fig. 8C), which gives the crystallization age of the unit. One other fraction (D) has experienced minor lead loss and another (B) contains a small amount of inherited zircon (table 2; app. 4, table 4.1). Sample 16 (fig. 6), from a fine-grained, moderately foliated quartz-muscovite schist that contains sparse quartz augen as large as 4 mm in diameter, was intersected in a diamond drill hole at one of the SEDEX-type lead-zinc occurrences (Paterson, 1982), 1.9 km north of where sample 14 of this study (interpreted to be a fine-grained felsic metatuff) was collected. Zircons recovered from sample 16 are similar to those in sample 15. Three fractions give concordant analyses (fig. 8D). Two of these (A and D) give overlapping analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 257.8±0.4 Ma, which we interpret as the crystallization age of the sample and the unit in which it occurs. Fraction E is concordant with a slightly younger ²⁰⁶Pb/²³⁸U age, reflecting minor lead loss (app. 4, table 4.1). Fraction C contains a minor older inherited zircon component and fraction B (not shown on fig. 8D) contains a much larger inherited component (table 2).

Two samples of felsic metaigneous rock within non-carbonaceous Nasina assemblage schists were dated from localities on the Yukon River upstream from the Alaska-Yukon border (fig. 6). Sample 17 is from a rusty-weathered muscovite-quartz schist that occurs as a 2- to 3-m-thick band within brown-weathering quartz-muscovite schist, minor quartzite, and rare marble, approximately 51 km upstream from the border. We tentatively interpret this sample as a felsic metatuff. Zircons recovered from the sample consist mainly of pale pink, subhedral grains, some of which have smoothly rounded surfaces that are thought to result from magmatic resorption. Eight fractions of zircon were analyzed (app. 4, table 4.1). Two of these (B and G) give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 347.7±0.7 Ma (fig. 8E), which is interpreted as the crystallization age of the sample. Three other analyses (A, H, and J) are slightly discordant and give slightly younger U-Pb ages. Three fractions (C, D, and E) yield strongly discordant analyses (fig. 8E) and regressions through the two concordant analyses and each of these single analyses give calculated upper intercept ages ranging from 2.08 to 1.55 Ga, indicating a range of Paleoproterozoic to Mesoproterozoic inherited zircon components were present in each of these fractions (app. 4, table 4.1).

A second sample from along the Yukon River (sample 18) is from a 4- to 5-m-thick band of pale- to medium-green-weathering quartz-muscovite-chlorite schist that contains small, scattered quartz eyes. The schist occurs within a separate structural slice of non-carbonaceous Nasina assemblage schist approximately 36 km upstream from the Alaska-Yukon border (fig. 6). This rock unit could be either a metamorphosed crystal tuff or a fine-grained metaporphyry. Zircons recovered from the sample consist of euhedral, stubby, square, prismatic grains. Four zircon fractions were analyzed (app. 4, table 4.1). Three of these yield non-overlapping concordant analyses, interpreted to reflect variable post-crystallization lead-loss effects that were not completely removed­ by air abrasion. The oldest concordant fraction (D) has a ²⁰⁶Pb/²³⁸U age of 349.4±0.7 Ma, which we interpret as a minimum crystallization age for the rock unit. One zircon fraction (B) gives a strongly discordant analysis, and a regression through this analysis and that of fraction D yields a calculated upper intercept age of 3.00 Ga, indicating the presence of Neoarchean inherited zircon components in this sample (fig. 8F).

Another sample (19) was collected along the Top of the World Highway 29 km northwest of Dawson (fig. 6). The sample comes from a thin layer of felsic schist that occurs within medium gray, moderately carbonaceous Nasina assemblage quartz-muscovite schist and minor marble, immediately above a thrust contact that emplaces Nasina assemblage over Permian Klondike assemblage rocks. The layer is approximately 3 m thick and displays gradational contacts with the enclosing Nasina assemblage schists. We interpret the unit to be a felsic tuff that was depositionally interlayered with the Nasina assemblage metaclastic rocks. A moderate amount of zircon was recovered from sample 19, and comprises mainly fine, subhedral to euhedral grains. Five strongly abraded multigrain fractions were analyzed (app. 4, table 4.1) and all give concordant analyses. Three of these (fractions A, C, and E) form a cluster of overlapping analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 254.5±1.4 Ma (fig. 8G), which we interpret as the best estimate for the age of the sample. Two other fractions give somewhat younger ²⁰⁶Pb/²³⁸U ages, reflecting minor post-crystallization lead loss.

Medium to dark gray, carbonaceous quartz-muscovite schist that contains rare interlayers of gray fine-grained quartzite, marble, and chlorite schist occurs in an east-west-trending, south-dipping band that crosses the Yukon River at Dawson and underlies the Klondike River valley to the east (fig. 4). It is bounded to the north and south (structurally below and above, respectively) by felsic schists of the Permian Klondike assemblage. The Nasina assemblage package in this area is structurally imbricated, and both internal and bounding thrust faults are marked by thin to very thick lenses of massive greenstone and variably sheared serpentinized ultramafic rocks (fig. 4). Zircons recovered from a leucogabbro dike that intrudes serpentinized peridotite southwest of Midnight Dome yield a U-Pb crystallization age of 264±4 Ma (fig. 4, sample 73; van Staal and others, 2018). Because the ultramafic rocks are in fault contact with the Nasina assemblage, this age does not directly constrain that of the Nasina assemblage.

We collected a sample from an approximately 10-m-thick layer of hornblende-biotite granodioritic gneiss that is parallel to compositional layering in carbonaceous Nasina assemblage metaclastic rocks with minor marble on the west shore of the Yukon River, just across from Dawson (sample 20; figs. 4, 6). Zircons from the granodioritic gneiss display complex U-Pb systematics (fig. 8H). One strongly abraded fraction (D) gives a concordant ²⁰⁶Pb/²³⁸U age of 348.6±0.7 Ma, which we interpret as a minimum crystallization age for the intrusive protolith of this sample. Fraction E gives a slightly younger ²⁰⁶Pb/²³⁸U age of 344.5±1.0 Ma, probably reflecting minor post-crystallization lead loss that was not completely removed by abrasion. Three other multigrain zircon fractions (A, B, and C) give a range of much older ages, indicating the presence of abundant inherited zircon cores (app. 4, table 4.1). Two-point chords through each of these discordant fractions and concordant fraction D give calculated upper intercept ages ranging from 3.13 to 2.31 Ga, indicating a range of Neoarchean to Paleoproterozoic ages for the inherited zircon component (fig. 8H).

Sample 21 (figs. 4, 6) is from a conformable body of granodioritic gneiss at least 20 m thick that intrudes carbonaceous Nasina assemblage metaclastic rocks and minor chlorite schist interlayers; it was sampled from an outcrop on the Yukon River about 36 km downstream from Dawson. The gneiss unit contains subhedral to strongly flattened potassium-feldspar augen as large as 2 cm in diameter. Abundant zircon was recovered from a sample of this unit that comprises mainly medium- to coarse-grained, euhedral, stubby prismatic grains with no visible cores. Six strongly abraded multigrain zircon fractions were analyzed (app. 4, table 4.1). Two of these (F and G) give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 357.3±0.9 Ma (fig. 8I), which we interpret as the crystallization age of the unit. Fraction H gives a slightly younger age, reflecting lead loss, and four other fractions (A, B, D, and E; D and E plot outside of fig. 8I) give slightly to strongly discordant analyses that yield older U-Pb and Pb-Pb ages, indicating that inherited zircon cores were present in at least some of the grains in each fraction. Two-point regressions through the two concordant fractions and each of the two strongly discordant fractions give calculated upper intercept ages of 1.91 and 1.70 Ga (fig. 8I), indicating that the age of the inherited zircon component was predominantly Paleoproterozoic.

A large body of biotite orthogneiss that contains sparse potassium-feldspar augen intrudes carbonaceous Nasina assemblage schist and quartzite near the south edge of the Klondike district (figs. 4, 6). Sample 22, collected from this body, yielded abundant, clear, euhedral, prismatic zircon grains. Five strongly abraded fractions were analyzed (app. 4, table 4.1; fig. 8J). Fraction C is concordant with a ²⁰⁶Pb/²³⁸U age of 260.2±0.5 Ma. We interpret this as a minimum crystallization age for the sample. Fraction C, together with two other fractions (B and D) that give slightly older ages, define a three-point chord with a calculated upper intercept age of 1.69 Ga, indicating that a Mesoproterozoic inherited zircon component is present in these two discordant fractions (fig. 8J). Two other zircon fractions (A and E) fall below this regression line, and likely reflect a combination of inheritance and post-crystallization lead loss.

Zircon samples were collected from Nasina assemblage metarhyolite at five localities in the eastern Eagle quadrangle; sample 23 is from felsic metatuff within the non-carbonaceous facies of the Nasina assemblage and samples 24–27 are from the carbonaceous facies of the Nasina assemblage (figs. 4, 6). Sample 23 was collected from blocks derived from a layer of slightly rusty-weathered quartz-muscovite schist (app. 2, fig. 2.1C) in a borrow pit along the Taylor Highway. The sample yielded a small amount of pale pink, euhedral, stubby, prismatic zircon grains. Six strongly abraded multigrain fractions were analyzed (app. 4, table 4.1). None of the analyses is concordant (fig. 8K). Fractions A and B give identical ²⁰⁷Pb/²⁰⁶Pb ages with a weighted average of 360±11 Ma. We interpret this as an approximate depositional age for the sample. The other four analyses give variably discordant analyses (fig. 8K), indicating the presence of minor older inherited zircon components. Two of the analyses form a discordia line with a lower intercept of approximately 360 Ma, similar to the interpreted depositional age, and an upper intercept of 1.79 Ga, indicating a Paleoproterozoic average age for at least some of the inherited zircon components.

Metarhyolite sample 24 has a weakly developed foliation and contains quartz and feldspar phenocrysts (app. 2, fig. 2.1D); it was collected from rubble along the Taylor Highway within the carbonaceous Nasina assemblage. A moderate amount of pink to pale yellow, stubby to elongate, euhedral zircons were recovered from the sample and four strongly abraded fractions were analyzed (app. 4, table 4.1). All fractions give concordant analyses (fig. 8L). Two of these (A and B) give overlapping ²⁰⁶Pb/²³⁸U ages with a weighted average of 256.5±0.5 Ma, which we interpret as the depositional age of the sample. Two other fractions give slightly younger ²⁰⁶Pb/²³⁸U ages, reflecting very minor lead-loss effects that were not completely avoided by air abrasion of the zircons.

Sample 25 is a fine-grained metarhyolite containing rotated feldspar and strained quartz phenocrysts (app. 2, fig. 2.1E) that was collected along the Taylor Highway from a felsic block within the carbonaceous Nasina assemblage unit. Six strongly abraded fractions of stubby, euhedral, prismatic zircons were analyzed. One of these (E) gives a concordant analysis with a ²⁰⁶Pb/²³⁸U age of 253.3±1.0 Ma (fig. 8M). We interpret this as a minimum crystallization age for the sample (table 2). The other analyses are very slightly to strongly discordant, indicating the presence of older inherited zircon components. Three of the analyses (B, F, and G), together with concordant fraction E, define a discordia line with a calculated upper intercept age of 1.44 Ga, indicating a Mesoproterozoic age for some of the inherited zircon components (fig. 8M).

Metarhyolite sample 26 has wavy crenulation cleavage containing thin folia of white quartz and rounded augen of white quartz and orange, iron-stained feldspar (app. 2, fig. 2.1F). The felsic metarhyolite has sparce, scattered dark, chip-like patches <1.5 cm in length composed of quartz and a dark mineral (graphite?) that likely were derived from graphitic quartz layers above and below the approximately 20- to 30-m-thick felsic metavolcanic layer. Folding may have thickened the metarhyolite layer, consistent with the folding observed at mesoscopic and microscopic scales. Sample 26 yielded a distinctly bimodal zircon concentrate, with approximately half of the recovered grains forming clear, colorless to pale pink, stubby, euhedral prismatic grains and the rest occurring as rounded, frosted pinkish grains that are clearly detrital. Seven fractions of the euhedral population were analyzed after strong abrasion (app. 4, table 4.1). None of the analyses is concordant; however, six of them range from 2.6 to 4.9 percent discordant and give overlapping ²⁰⁷Pb/²⁰⁶Pb ages with a weighted average of 267.0±2.7 Ma (fig. 8N). We consider this to be a reasonable estimate for the crystallization age of the euhedral zircon component in the sample. One of the fractions gives a slightly older ²⁰⁷Pb/²⁰⁶Pb age and appears to contain a minor older inherited zircon component (table 2).

Sample 27 was collected from an outcrop of metarhyolite along King Solomon Creek, just west of the Taylor Highway (fig. 6). The sample is a mylonitized metarhyolite with rotated and rounded quartz and feldspar phenocrysts in a fine-grained matrix of quartz and sericite (app. 2, fig. 2.1G). The sample yielded a small amount of pale pink, euhedral, stubby, prismatic zircon grains. Seven strongly abraded fractions were analysed (app. 4, table 4.2). Two of these fractions (E and G) give overlapping ²⁰⁶Pb/²³⁸U ages that have a weighted average of 256.5±0.5 Ma (fig. 8O), which we interpret as the best estimate for the crystallization age of the sample. All the other fractions give slightly older ²⁰⁷Pb/²⁰⁶Pb ages, indicating both minor inheritance of older inherited zircon components and variable effects of post-crystallization lead loss.

Ladue River Unit

A body of biotite orthogneiss, locally retrograded to greenschist facies, that contains scattered potassium-feldspar and quartz augen (<2 cm in length) was sampled in the east-central part of the Tanacross quadrangle (sample 28, fig. 4, table 2; app. 3, fig. 3.1A). Only a small amount of zircon was recovered, and most of the coarser grains contain visible cloudy inherited cores. Clear tips were broken from grains with visible cores, and then strongly abraded. Three of the resulting fractions yield concordant analyses (fig. 9A; table 2; app. 4, table 4.1). The oldest of these gives a ²⁰⁶Pb/²³⁸U age of 362.9±1.4 Ma, which we interpret as the minimum crystallization age of the sample.

Klondike Assemblage

Nine separate samples of metavolcanic and meta-intrusive rocks mapped as the Klondike assemblage were collected for U-Pb dating in this study. Six of these are from layers of rusty-weathered, pyritic and locally baritic, muscovite and quartz-muscovite schist that are interpreted to be extrusive (probably tuffaceous) in origin. Three samples are quartz-muscovite (±chlorite) schist that contain abundant subhedral quartz and (or) feldspar augen and are interpreted to have been crystal-rich flows or subvolcanic sills. Most zircons recovered from Klondike assemblage samples contain abundant clear bubble- and tube-shaped inclusions and many show evidence of magmatic resorption of zircon prior to final crystallization of the magma. Many of the zircon fractions analysed contain a high proportion of common lead (presumably from the abundant inclusions) and some analyses are therefore somewhat imprecise.

Very fine-grained, rusty-weathered, quartz-muscovite schist was collected from an approximately 10-m-thick band exposed in a roadcut on the Top of the World Highway 17 km west of Dawson (sample 29; fig. 4, table 2; app. 3, fig. 3.1B). The sample contains fine, clear, resorbed quartz eyes and minor barite. This is one of several bands of felsic schist in the vicinity that are interlayered with tan- to pale-brown-weathering quartz-muscovite-chlorite schist of probable sedimentary or volcaniclastic origin, as well as chloritic schist (probable mafic metavolcanic protolith) and narrow conformable bodies of metadiabase and metagabbro. Six fractions were analysed; all analyses are concordant or nearly so (app. 4, table 4.1; fig. 9B). The best estimate for the crystallization age for this sample is given by the total range of ²⁰⁶Pb/²³⁸U ages of 255.7±0.5 Ma for three fractions with overlapping concordant analyses.

A second felsic metatuff sample consisting of rusty-weathered, variably pyritic muscovite schist was collected from another roadcut on the Top of the World Highway 12 km east of sample 29 (sample 30; fig. 4, table 2). Five fractions were analysed (fig. 9C; app. 4, table 4.1), one of which is concordant with a ²⁰⁶Pb/²³⁸U age of 255.1±0.5 Ma, interpreted as the minimum crystallization age for the sample (table 2). Fractions A and E have suffered post-crystallization lead-loss and fractions C (not shown on fig. 9C) and D contain substantial components of inherited lead.

A conformable layer approximately 4 m thick of blocky-weathering, weakly to moderately foliated, fine-grained, quartz-muscovite schist with abundant quartz and potassium-feldspar augen as large as 3 mm in diameter occurs within more typical fine-grained, tan to pale gray-green quartz-muscovite schist of the Klondike assemblage along the Top of the World Highway (sample 31; fig. 4, table 2). This unit likely originated as a porphyritic sill. Three fractions were analysed (fig. 9D; app. 4, table 4.1), and all analyses are concordant. Two of these give overlapping analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 254.0±0.4 Ma, which is interpreted as the crystallization age of the sample.

Another felsic metatuff was sampled in the northeastern part of the Klondike district (sample 32; fig. 4, table 2). The sample is from a band of rusty-weathering muscovite and quartz-muscovite schist that is interlayered with a sequence of non-carbonaceous, tan-weathering micaceous quartzite. Four fractions of zircon were analysed (fig. 9E; app. 4, table 4.1), two of which fall on or near concordia at about 255 Ma. The best age estimate is given by the weighted average ²⁰⁶Pb/²³⁸U age for these two fractions at 253.1±0.7 Ma (table 2).

An outcrop of very rusty-weathered, pyritic muscovite-quartz schist (felsic metavolcanic protolith) was sampled in the central part of the Klondike district (sample 33; fig. 4, table 2). Eight zircon fractions were analyzed (fig. 9F; app. 4, table 4.1); however, none of the analyses is concordant. Four of the analyses are moderately discordant and define a linear array (MSWD=0.70) with calculated lower and upper intercept ages of 261.3±1.6 Ma and 1.99 Ga, respectively (fig. 9F). One fraction (not shown in fig. 9F) is highly discordant but falls close to this regression line. Three of the fractions (A, D, and E) analyzed plot below the line, probably reflecting minor lead loss that was not completely removed by the abrasion process. We interpret the calculated lower intercept as the best estimate for the age of the sample.

A sample of felsic augen schist, which is interpreted to be a deformed and metamorphosed porphyry, was collected from a large outcrop west of an access road into the headwaters of Moose Creek in the southwestern Dawson map area (sample 34; fig. 4, table 2). This unit occurs within a thrust-fault-bounded sequence of felsic metavolcanic rocks in the northern part of the Sixtymile district. The sample is from a conformable band of augen schist approximately 3 m thick. It consists of subhedral to euhedral potassium-feldspar augen as large as 1 cm in diameter in a matrix of moderately to strongly foliated quartz-muscovite (±chlorite) schist. The surfaces of some of the grains recovered from the sample show evidence of magmatic resorption. Eight fractions were analysed (fig. 9G; app. 4, table 4.1). Three of these give overlapping concordant analyses with a weighted average ²⁰⁶Pb/²³⁸U age of 256.6±0.3 Ma, which is the best estimate for the crystallization age of the unit (table 2).

Another metaporphyry body (sample 35; fig. 4, table 2) is from a fine-grained, approximately 5-m-thick layer of quartz-augen-bearing quartz-muscovite schist within a thick package of felsic schists in the northwestern Stewart River map area. Six fractions were analysed (fig. 9H; app. 4, table 4.1), and all contained high levels of common lead, presumably related to abundant inclusions in most of the zircons. Two fractions give overlapping concordant analyses that have a weighted average ²⁰⁶Pb/²³⁸U age of 253.5±1.2 Ma, which we interpret as the crystallization age of the sample.

Sample 36 (fig. 4, table 2) is from an outcrop in the southern Sixtymile district, south of the Sixtymile River. It is from a body of rusty quartz-muscovite schist that is interpreted to form a window through a thrust fault that places rocks of the Finlayson assemblage (or Fortymile River assemblage) above felsic schists of the Klondike assemblage. Only a small amount of poor-quality zircon was recovered from this sample. The zircons in this sample are mainly euhedral, but all contain abundant clear inclusions. Seven fractions of strongly abraded zircons were analyzed (fig. 9I; app. 4, table 4.1), and four of these yield concordant, although imprecise, analyses. The ²⁰⁶Pb/²³⁸U age of the oldest concordant analysis (B), at 256.2±1.8 Ma, is tentatively considered a reasonable estimate of the minimum crystallization age of the rock unit (table 2).

An undeformed biotite-muscovite plagioclase-phyric granodiorite dike (sample 37; fig. 4, table 2) was collected from a 30-cm-thick body that appears to cut a package of biotite quartzite and minor interlayered marble and rusty-weathering metasedimentary rocks of the foliated Fortymile River assemblage. The location is approximately 6.5 km upstream from the Taylor Highway bridge over the Fortymile River. The dike has a porphyritic texture formed by 2-mm-long plagioclase phenocrysts within a fine-grained matrix of biotite, muscovite, and quartz (app. 3, fig. 3.1C). Three fractions of strongly abraded zircons were analyzed; two of these are single grains and fraction D consists of two grains. The analyses fall on or below concordia between approximately 262 and 200 Ma (fig. 9J; app. 4, table 4.1). Single-grain fraction A is concordant at 262.8±1.5 Ma (fig. 9J), which we tentatively interpret as the minimum crystallization age of the sample. The analyses from the other two fractions appear to mainly indicate the effects of post-crystallization lead loss. These results are puzzling, because emplacement of the dike appears to have been post-tectonic, and igneous rock units elsewhere in the Nasina and Fortymile River assemblages that yield middle and late Permian crystallization ages have experienced the same high degree of strain as the enclosing rock units. Sample 37 may be analogous to a sample from a granitic pluton approximately 34 km farther west for which Jones and others (2017b) reported a U-Pb zircon age of approximately 257 Ma; Jones and others (2017b) interpreted the granitic pluton to crosscut the main foliation in Fortymile River assemblage host rocks.

Chicken Assemblage

We separated zircon from a body of moderately foliated plutonic rock of tonalitic composition located in a roadcut on the Taylor Highway approximately 4 km east of the village of Chicken (sample 38; fig. 4, table 2, app. 4, table 4.2). This body is in apparent intrusive contact with weakly foliated mafic metavolcanic rocks of the Chicken assemblage. Zircons from the sample are euhedral and have no visible inherited cores. LA-ICP-MS analyses were carried out on 20 grains and the results are shown in figure 10A. Eighteen of the grains give overlapping concordant analyses that have a weighted average ²⁰⁶Pb/²³⁸U age of 319.4±1.1 Ma, which we interpret to be the crystallization age of the sample. Two grains give slightly older concordant ages, and we interpret the grains to be xenocrysts.

Seventymile Terrane

Zircons recovered from metamorphosed silicified tuff sample 39 (app. 3, fig. 3.1D), which crops out along the Taylor Highway north of latitude 64°30′ (fig. 4), are small, euhedral, stubby prismatic grains. Fifteen grains were analyzed by LA-ICP-MS methods (app. 4, table 4.2). All but one of the grains yield concordant analyses (fig. 10B). Eleven of these yield a tight cluster on concordia with a weighted average ²⁰⁶Pb/²³⁸U age of 259.0±0.8 Ma (fig. 10B, right column), which is interpreted to be the crystallization age of the rock. Three analyses give slightly younger ages, reflecting minor post-crystallization lead loss, and one analysis gives an older and strongly discordant age, which we interpret to be from a xenocryst in the sample.

Cassiar Terrane

Alkaline and peralkaline volcanic rocks and associated syenitic intrusions in the Cassiar terrane (fig. 1) are interpreted to have erupted during early stages of rifting that led to the opening of the Slide Mountain Ocean back-arc basin (for example, Mortensen, 1982). Medium- to coarse-grained quartz syenite from a large body of the syenite (sample 40) was sampled approximately 53 km due south of the village of Ross River (fig. 1) for U-Pb dating (locality C on fig. 1). Abundant coarse, stubby, pink zircons were recovered from the sample. Three strongly abraded fractions were analyzed along with four unabraded fractions (app. 4, table 4.1). The analyses form a linear array with discordance ranging from 0.8 to 2.0 percent. Three fractions cluster together at the upper end of this array; two overlap completely but the third analysis falls slightly to the right of the others, indicating a possible inherited zircon component. A regression through six analyses, omitting the rightmost analysis, gives calculated upper and lower intercept ages of 361.2 ±1.0 Ma and approximately 66 Ma, respectively (fig. 10C). We interpret the upper intercept age to be the best estimate for the emplacement age of the syenite body.

Fortymile River, Finlayson, and Snowcap? Assemblages

Most geochemical data from our study and previous work in the study area are from metavolcanic rocks from the Fortymile River assemblage, Finlayson assemblage, and Snowcap? assemblage and from orthogneiss bodies that are associated with these assemblages. On the Zr/TiO₂ versus Nb/Y plot (fig. 11A), two augen gneiss samples and a granodiorite gneiss from the Fortymile River assemblage plot in the granodiorite field; U-Pb zircons ages for augen gneiss (table 3) span the 361 Ma Late Devonian–Early Mississippian boundary and for granodiorite are Early Mississippian. Undated metarhyolite sample 75 (photograph and photomicrographs shown in app. 1, fig. 1.1G), which is interpreted to be Mississippian because of the predominance of igneous rocks of this age within the unit (Day and others, 2002; this study), plots as andesite or tonalite. However, this more mafic composition is questionable because the loss on ignition value for this sample is 6 percent, indicating alteration, and the normalized SiO₂ concentration is 77.6 percent (table 3). Tonalitic compositions are appropriately indicated for two Early Mississippian tonalite gneiss samples from the Fortymile River assemblage. We correlate our tonalite gneiss samples with the Steele Creek Dome orthogneiss unit of the Fortymile River assemblage, first described by Day and others (2000, 2002; equivalent to “tonalitic orthogneiss” unit of Szumigala and others, 2002), based on their field occurrence, mineralogy, and normalized SiO₂ contents (64.2–69.5 percent). All our Fortymile River assemblage samples plot in the volcanic arc field on the niobium versus yttrium diagram (fig. 11B). As expected, the tonalitic gneiss samples plot lowest in the arc field and farthest away from the values for average upper continental crust (red star, fig. 11B).

Trace elements are arranged in order of decreasing incompatibility during mantle melting on the primitive-mantle-normalized multielement plot of the felsic and intermediate-composition metaigneous rocks from our study (fig. 12). In general, at a given SiO₂ content or other fractionation index, felsic rocks that formed in within-plate (extensional) settings have higher total REE and HFSE contents and lower compatible trace-element concentrations (titanium) than do arc rocks and average upper continental crust (fig. 12A). Within-plate rocks are represented in figure 12A by rhyolite from the Yellowstone Plateau volcanic field (Hildreth and others, 1991), which were generated in an unambiguous intracontinental setting. Because the chemistry of continental-margin arc magmas is quite variable and depends on local crustal composition and thickness (Hildreth and Moorbath, 1988), we include trace-element plots of both a continental-margin arc emplaced on thick (70–80 km) crust (central Andes, Chile; Lindsay and others, 2001) and on thinner (~40 km) crust composed of accreted terranes (Mount Mazama, Crater Lake, Oregon; Bacon and Druitt, 1988). Primitive-mantle-normalized multielement patterns for the continental-margin arc of Chile are similar to that of average continental crust and higher in HFSE contents than the felsic rocks of the continental-margin arc of Crater Lake (fig. 12A).

Primitive-mantle-normalized plots of Early Mississippian Fortymile River assemblage metarhyolites (Fig 12B) and Late Devonian to Early Mississippian augen gneisses (fig. 12D) all have patterns with distinctive negative Nb-Ta and Eu-Ti anomalies characteristic of continental-margin arcs. The augen gneisses resemble the thicker crust (Chile) analog, and the rhyolites resemble an intermediate position between the analogues of thicker crust and the thinner crust of accreted terranes (Crater Lake) (fig. 12A). Early Mississippian Fortymile River assemblage intermediate-composition orthogneisses also show two different trace-element patterns: a granodiorite gneiss sample has higher HFSE and REE contents compared to two Fortymile River assemblage tonalite gneiss samples that have flatter, more primitive-arc-like patterns (fig. 12C). The primitive-mantle-normalized plots of the two Fortymile River assemblage tonalite samples resemble those of the Crater Lake analog or a continental arc basalt (fig. 11A of Dusel-Bacon and others, 2006) because of their weakly negative slopes, niobium and tantalum troughs, and absolute abundances. However, the shape of these patterns is also similar to that of average upper continental crust (fig. 12A), albeit abundances of HFSE and light REE are distinctively lower in our tonalite samples relative to average upper continental crust. This similarity between arc and upper continental crust trace-element compositions is not surprising, given that much of the Earth’s crust was derived from erosion of orogenic arcs.

We speculate in the “Regional Tectonic Assemblages” section that much of what is currently interpreted to be Snowcap assemblage in the Stewart River map area (Yukon Geological Survey, 2020b) probably consists mainly of metasedimentary and lesser metavolcanic rocks that, together with the Finlayson assemblage rock units in this area, correlate with the Fortymile River assemblage in eastern Alaska, rather than with the pre-Devonian Snowcap assemblage as defined elsewhere in Yukon. To test this hypothesis, we compare trace-element compositions among felsic metaigneous rocks from the Fortymile River assemblage, Snowcap? assemblage, and Simpson Range plutonic suite (on niobium versus yttrium plots), and between mafic metavolcanic rocks (mostly amphibolites; SiO₂ <59 percent) from the Fortymile River assemblage in Alaska (Dusel-Bacon and Cooper, 1999; and app. 5, tables 5.2 and 5.3 of this paper; Wypych and others, 2017, 2018) and a suite of mafic rocks that are assigned to the Finlayson and Snowcap assemblages by Ryan and others (2018) (on thorium-halfnium-niobium diagrams). The niobium versus yttrium plot of Pearce and others (1984) shows a clear overlap of volcanic arc-like compositions of felsic gneiss from the Fortymile River assemblage (overlapping data from our study shown in fig. 11B), felsic gneiss and metavolcanic rocks from the Snowcap? assemblage, and many felsic orthogneiss samples from the Simpson Range plutonic suite (fig. 13A, B).

We compare thorium-halfnium-niobium ratios of mafic amphibolites from the Fortymile River assemblage (fig. 13C) with those of the Finlayson and Snowcap? assemblages and the Simpson Range plutonic suite (fig. 13D). The majority of amphibolite samples from the Fortymile River assemblage, Finlayson assemblage, and the Simpson Range plutonic suite plot in the calc-alkalic basalt field. Fewer Fortymile River assemblage samples plot as normal- (N-MORB) and enriched (E-MORB) basalts, in addition to three outliers from the Fortymile River assemblage that plot as primitive arc tholeiites, one sample from the Simpson Range plutonic suite that plots as ocean island basalt, and a few Finlayson assemblage samples that plot outside but near the ocean island basalt field. In contrast, with the exception of three samples that plot in the calc-alkalic field, trace-element ratios in amphibolites from the Snowcap? assemblage have mostly non-arc, E-MORB to ocean island basalt geochemical signatures (fig. 13D), like those in the type Snowcap assemblage in the Glenlyon area (fig. 1, locality D) (Piercey and Colpron, 2009). Amphibolites from the Dorsey Complex in northern British Columbia, which is correlated with the type Snowcap assemblage, also has MORB to ocean island basalt geochemical signatures (Nelson and Friedman, 2004). These observations, although based on a small dataset, indicate that most of the Snowcap? assemblage metasedimentary rocks that are associated with the analyzed amphibolites in the Stewart River map area in Yukon are not equivalent to the Finlayson assemblage or the Fortymile River assemblage.

Lake George Assemblage

Although no new samples from the Lake George assemblage were collected in this study, we describe previously published geochemical results from the Lake George assemblage to compare them with results for samples from the Yukon-Tanana terrane. Primitive-mantle-normalized multielement plots of Late Devonian to Early Mississippian Lake George assemblage augen gneiss (fig. 12D) reveal that all have similar patterns with distinctive negative Nb-Ta and Eu-Ti anomalies characteristic of arc rocks derived from, or containing, a large component of continental crust. The gray field in figure 12D shows the multielement plots of previously analyzed Lake George assemblage augen gneiss in the southeastern Big Delta (n=10) and southwestern Eagle quadrangles (n=1) in Alaska (Dusel-Bacon and Aleinikoff, 1985; gneiss bodies shown on fig. 2). Also shown are plots of four Lake George assemblage augen gneiss samples from the northeastern Tanacross quadrangle (data from Dusel-Bacon and Aleinikoff, 1985, and app. 5, table 5.1 of this report). Two of the samples from the northeastern Tanacross quadrangle have been dated by U-Pb zircon geochronology (356±2 Ma and 350.4±5.6 Ma, samples 43 and 45, respectively; table 1; Dusel-Bacon and Aleinikoff, 1996, and Dusel-Bacon and Williams, 2009). The previously analyzed Lake George assemblage augen gneiss samples from the northeastern Tanacross quadrangle (Dusel-Bacon and Aleinikoff, 1985) (fig. 12D) have the highest trace-element contents of any of the middle Paleozoic rocks in the study area and a pattern intermediate between that of felsic rocks from the thick continental-crust arc and average upper continental crust analogs (fig. 12A). On primitive-mantle-normalized multielement plots, the four augen gneisses from the Tanacross quadrangle plot above or within the top of the field of previously analyzed Lake George assemblage augen gneisses from the southeastern Big Delta and southwestern Eagle quadrangles.

On the tantalum versus ytterbium discrimination diagram of Pearce and others (1984) (fig. 14), we plot all the analyzed augen gneiss and one non-augen-bearing felsic orthogneiss from the Lake George assemblage and correlative parautochthonous North American units that have been dated by U-Pb zircon geochronology. Three augen gneisses and one orthogneiss from the study area along with three samples from west of the study area in the Big Delta and Healy quadrangles (fig. 2) plot in a tight cluster in a corner of the volcanic arc granite field, overlapping with the tantalum and ytterbium ratios of average upper continental crust (red star, fig. 14). The samples also plot adjacent to the within-plate and anomalous ocean-ridge-type granite field. U-Pb zircon igneous crystallization ages of the samples range from 373±3 to 350±6 Ma (fig. 14)—showing a consistent trace-element composition for about 20 million years.

On the niobium versus yttrium discrimination diagram of Pearce and others (1984)—a trace-element plot comparable to the tantalum versus ytterbium plot—a large number of Lake George assemblage augen gneiss samples from the Big Delta and Eagle quadrangles (n=15) (fig. 2), the northeastern Tanacross quadrangle (n=22), the Fiftymile batholith (n=2), and the Mount Burnham body (n=8) in the Stewart River map area in Yukon all plot in a small area in the corner of the volcanic arc granite field near the composition of average upper continental crust (red star), with a smaller proportion in the within-plate and anomalous ocean-ridge-type granite field (fig. 13E) (Dusel-Bacon and Aleinikoff, 1985, and app. 5, table 5.1 of this report; Ryan and others, 2018; Wypych and others, 2019b). The strong crustal and arc trace-element signature for the Lake George assemblage augen gneiss across the Yukon-Tanana Upland indicated by the tantalum versus ytterbium and niobium versus yttrium discrimination plots is consistent with the continental arc pattern shown in the primitive-mantle-normalized multielement plot (fig. 12D). One sample (18RN213; Wypych and others, 2018) of non-augen bearing felsic gneiss (from buried float of mostly augen gneiss) plots within the within-plate granite field (fig. 13E). This sample may have been a late-stage differentiate of the augen gneiss or unrelated to it. Other felsic to intermediate-composition (68–80 percent SiO₂) non-augen bearing orthogneiss in the Lake George assemblage in the Tanacross quadrangle (n=34; Wypych and others, 2017, 2018, 2019b) (fig. 13F) also cluster near the average upper continental crust composition but span an even larger area of the volcanic arc granite field, relative to Lake George assemblage augen gneiss; two samples plot in the within-plate granite field.

The within-plate trace-element characteristics shown by a small number of felsic Lake George assemblage orthogneiss samples also are evident in some of the associated mafic metaigneous rocks. Geochemical trace-element analyses of amphibolites in the Big Delta and northeastern Tanacross quadrangles by Dusel-Bacon and Cooper (1999) showed that 7 of 11 samples plot as within-plate basalt in Ti-Zr-Y and Nb-Zr-Y ternary diagrams. Similar results are also shown for samples that could be plotted on the Th-Hf-Nb discriminant diagram (triangles on fig. 13G) (Dusel-Bacon and Cooper, 1999, and app. 5, table 5.3 of this report). In addition to the five amphibolites (metabasalt or metagabbro) that plot within or near the within-plate granite field, indicating magma generation in an extensional environment, three amphibolites plot as E-MORB and two as calc-alkalic arc basalt. A much larger dataset for Lake George assemblage amphibolites from localities differing from those of Dusel-Bacon and Cooper (1999) in the northern and eastern Tanacross quadrangle (n=32; circles on fig. 13G) reported by Wypych and others (2017, 2018) plot mainly in the calc-alkalic arc field with a minority plotting as primitive arc tholeiites or E-MORB (fig. 13G).

West of our study area, Late Devonian (approximately 369 Ma) U-Pb zircon igneous crystallization ages from two samples of amphibolite associated with Lake George assemblage augen gneiss in the Big Delta quadrangle, and an interlayered boundary between dated Late Devonian to Early Mississippian siliceous metaigneous rocks and undated mafic schist from the Totatlanika Schist in the northeastern Healy quadrangle in the Alaska Range (fig. 2), indicate bimodal Devonian to Mississippian mafic and siliceous magmatism (Day and others, 2003; Dusel-Bacon and others, 2004, 2006). A within-plate origin is indicated by trace-element data for most of the metabasites and peralkaline rhyolites that are associated with syngenetic volcanic-hosted massive sulfide and SEDEX-type deposits in the Alaska Range and Big Delta quadrangle, respectively (Dusel-Bacon and others, 1998, 2004, 2012).

Ladue River Unit

Augen gneiss that we analyzed from the Ladue River unit has primitive-mantle-normalized abundances that fall within the field of Lake George assemblage augen gneiss from the Big Delta and Eagle quadrangles and are intermediate between those of augen gneiss from the Lake George assemblage in the Tanacross quadrangle and Fortymile River assemblage groups (fig. 12D). Niobium and yttrium concentrations for a more extensive sample suite of Ladue River unit felsic metaigneous rocks from the Tanacross quadrangle (Wypych and others, 2019b) form a tight field surrounding the composition of average upper continental crust and span the boundary between the volcanic arc granite and the within-plate granite plus anomalous ocean-ridge-type granite fields in figure 13H, similar to the concentrations of Lake George assemblage augen gneiss (fig. 13E). However, four amphibolites from the same unit have thorium, halfnium, and niobium concentrations that indicate a calc-alkaline arc protolith, two amphibolites plot as E-MORB (Wypych and others, 2019b), and one greenstone plots as N-MORB (Dusel-Bacon and others, 2006; table 3) (fig. 13I). These results are similar to amphibolite from the Fortymile River assemblage (fig. 13C), but different from those from the Lake George assemblage in that none have the ocean island basalt signature of some amphibolites in the Lake George assemblage (fig. 13G).

Nasina Assemblage

Two samples of Late Devonian to Early Mississippian Nasina assemblage metarhyolite were analyzed as part of our zircon geochronology and geochemical study (table 3). On the Zr/TiO₂ versus Nb/Y diagram (fig. 11A), one Nasina assemblage metarhyolite plots in the rhyodacite/dacite field and the other metarhyolite sample (sample 14), plots in the trachyandesite field. Sample 14 has a normalized SiO₂ content of 82.5 percent, which may indicate modification of trace elements as well as silicification. Niobium versus yttrium concentrations from the metarhyolites fall in the volcanic arc granite field (fig. 11B). Primitive-mantle-normalized multielement plots of the two samples (fig. 12B) have patterns similar to those of continental-margin arcs developed on continental crust of intermediate thickness (fig. 12A).

Klondike and Nasina Assemblages

Five of the nine Permian geochemical samples from our study are metarhyolites from within the Nasina assemblage, one sample is from the Klondike assemblage, and another is an undeformed granitic dike that intrudes the Fortymile River assemblage (table 4). Two other analyzed samples of Late Permian metarhyolite (sample 39 and a sample from a nearby locality, indicated as sample ~39) are from the Slide Mountain-Seventymile terrane in the eastern Eagle quadrangle.

On the Zr/TiO₂ versus Nb/Y diagram (fig. 11A), most of the Permian felsic metavolcanic samples from the Klondike and Nasina assemblages plot within or close to the boundary of the rhyolite (granite) compositional field, consistent with their high normalized SiO₂ contents (77.8–81.7 percent; table 4). The high SiO₂ contents probably reflect some degree of silicification of the primary rhyolitic composition. The Permian Nasina assemblage metarhyolite sample with the highest niobium content (sample 24, table 4) plots within the comendite/pantellerite field. The undeformed Permian granitic dike sample (sample 37) plots near the rhyolite field, just inside the syenodiorite field, and has a lower normalized SiO₂ content (73.1 percent) relative to the Permian metavolcanic rocks. The niobium and yttrium concentrations for most of the Permian felsic metavolcanic samples from the Nasina assemblage carbonaceous unit fall in either the upper right corner of the volcanic arc granite field or the adjacent within-plate granite field (fig. 11B).

Metarhyolite from the Klondike assemblage, along with the undeformed granitic dike, plot in the volcanic arc granite field. Incorporation of continental crust into the melt of the Permian felsic metavolcanic rocks from the Nasina assemblage is indicated by the proximity of their niobium and yttrium contents to that of average upper continental crust (fig. 11B). The lower trace-element contents of the undeformed Permian dike and the Klondike assemblage metarhyolite indicate more primitive (less crustally contaminated) compositions. The more primitive trace-element signature of the Klondike assemblage sample indicates that its high normalized SiO₂ content (81.7 percent) is a result of silicification, rather than a highly evolved original composition.

The chemically evolved character of the Permian Nasina assemblage metarhyolite samples is shown by their primitive-mantle-normalized multielement patterns (fig. 12E). Their moderately high HFSE and REE abundances, steep negative slopes, and pronounced negative niobium and tantalum anomalies resemble those of the continental-margin arc of Chile that developed on thick continental crust; however, their pronounced negative titanium and europium anomalies and high incompatible element contents are comparable with those of the within-plate (Yellowstone) example (fig. 12A). Two of the Nasina assemblage metarhyolites (samples 24 and 26) exhibit a noticeable depletion in light REE (La, Ce, Pr, and Nd) relative to the other three Nasina assemblage samples (fig. 12E). The LREE depletion may have resulted from separation of accessory phases such as allanite, monazite, or chevkinite, into which the LREE are preferentially partitioned during the differentiation of silicic magmas (for example, Miller and Mittlefehldt, 1982). Klondike assemblage metarhyolite and the undeformed Permian granitic dike that cuts the Fortymile River assemblage have the lowest trace-element abundances of the Permian rocks (fig. 12F), and their primitive-mantle-normalized patterns are similar to average upper continental crust and the Crater Lake-type continental-margin arc; their low titanium and europium contents are also similar to those found in the within-plate-type Yellowstone metarhyolite (fig. 12A).

The niobium and yttrium concentrations of our single analyzed Klondike assemblage metarhyolite sample (fig. 11B) is consistent with the compositions of Klondike assemblage metarhyolites reported by Piercey and others (2006, and data repository no. 2 therein), Ryan and others (2018), and Mortensen and others (2019) and has similar compositions to those of Klondike assemblage felsic metavolcanic samples from outside our study area (fig. 15A); all plot in the volcanic arc granite field. Felsic and intermediate-composition metavolcanic rocks in the Sulphur Creek orthogneiss suite, long considered co-magmatic with the Klondike assemblage (for example, Mortensen, 1990) vary widely in niobium and yttrium contents, but similarly plot consistently in the volcanic arc granite field (fig. 15B). Niobium and yttrium concentrations of samples from both the Sulphur Creek orthogneiss suite and the Klondike assemblage are centered on the composition of average upper continental crust (red star in fig. 15B). Trace-element contents of mafic metavolcanic rocks of the Klondike assemblage (Piercey and others, 2006) and a smaller dataset of more mafic metaplutonic rocks of the Sulphur Creek orthogneiss suite (Ryan and others, 2018) have thorium-halfnium-niobium ratios that primarily plot as calc-alkalic arc basalts, with a few samples plotting in the N-MORB and E-MORB fields (fig. 15C).

Milidragovic and others (2016) discussed the geochemical characteristics of the Klondike assemblage metavolcanic rocks and associated Sulphur Creek orthogneiss suite, based mainly on data reported by Ryan and others (2018). Milidragovic and others (2016) presented primitive-mantle-normalized trace-element plots of their data and determined that the Sulphur Creek orthogneiss suite includes metamorphosed tonalite, granodiorite, and granite that can be grouped into (1) strongly LREE-enriched rocks, (2) LREE-enriched rocks, and (3) heavy and middle REE-depleted rocks. Intermediate to felsic metavolcanic rocks (SiO₂ >60 percent) of the Klondike assemblage are indistinguishable in their incompatible trace-element chemistry from the LREE-enriched plutonic rocks. Trace-element primitive-mantle-normalized plots of Klondike assemblage basaltic metavolcanic rocks were also grouped by Milidragovic and others (2016) into (1) tholeiitic with flat REE and high HFSE profiles, (2) LREE-enriched with substantial HFSE depletion, and (3) calc-alkaline metavolcanic rocks with LREE enrichment and HFSE depletion (primarily Nb, Ta, and Ti). This variety of geochemical signatures, like those shown in figure 15C, probably reflects a complex arc setting.

Slide Mountain-Seventymile Terrane

The two Permian metarhyolite samples from within the metasedimentary component of the Slide Mountain-Seventymile terrane body that straddles the Taylor Highway north of latitude 64°30′ (sample 39 and a nearby sample locality along the Taylor Highway approximately 1.5 km south of sample 39; fig. 4) plot near the rhyolite-comendite field boundary (fig. 11A) and plot just inside the volcanic arc granite field, close to the average composition of upper continental crust (fig. 11B). They have primitive-mantle-normalized multielement plots (fig. 12F) like that of a continental-margin arc developed on thick crust (fig. 12A).