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USGS Professional Paper 1662, Chapter 1

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USGS Professional Paper 1662, Chapter 1 Studies by the U.S. Geological Survey in Alaska, 2000 U.S. Geological Survey Professional Paper 1662 Geochemistry and Age Constraints on Metamorphism and Deformation in the Fortymile River Area, Eastern Yukon-Tanana Upland, Alaska By Warren C. Day, John N. Aleinikoff, and Bruce Gamble Abstract Natural Resources (ADNR)’s Division of Mining, Land, and Water to establish the baseline geology, as well as the geo- Determining the age of deposition for many of the major chemistry of the rocks, soil, and plants and the surface-water rock types within the Fortymile River area is a major challenge quality, of the Fortymile River mining district of east-central for understanding the geologic evolution of the Yukon-Tanana Alaska. As part of that effort, we present new data on the tectonostratigraphic terrane of east-central Alaska. Intense geochemical composition and age constraints for rocks of dynamic metamorphism and associated recrystallization, several the Fortymile River area. Reliable age data on the protoliths periods of ductile deformation, and poor outcrops conspire to of the metamorphic rocks are some of the most important conceal the age of the rocks in this important region. Definitive missing pieces of the puzzle in our understanding of the fossil evidence in the marble units has been obliterated; felsic geologic evolution of the Fortymile River area. To that end, gneissic horizons in the sequence could be either volcanic or we have obtained new geochemical data to help character- intrusive in origin. To help resolve some of these issues, we ize the bedrock units, and determined new U-Pb zircon ages present new geochemical data to further characterize the bed- on two critical units, within the Fortymile River area. These rock assemblage in the Fortymile River area and to address the new data, coupled with the pioneering work by Foster (1969, origin of the major rock units. In addition, new U-Pb-isotopic 1976) and Foster and others (1985), as well as the later work data on zircons from the early (pre-D1) Steele Creek Dome by Hansen and others (1991), Dusel-Bacon and others (1993, Orthogneiss and from a late tectonic (late-D2) epidote-bearing 1995), Hansen and Dusel-Bacon (1998), and Dusel-Bacon and leucogranite intrusion help constrain the youngest possible Cooper (1999), are critical in constraining the ages of bed- age for the supracrustal rocks, as well as the date of regional rock, as well as in dating the ductile deformation, in the vast D2 deformation. The Steele Creek Dome Orthogneiss crystal- Yukon-Tanana upland. The geologic time scale of Haq and lized at 343±4 Ma, indicating that the supracrustal sequence Van Eysigna (1998) as compiled by Wilson (2001) was used is partly at least Early Mississippian. The leucogranite crystal- for this report. lized during the waning stages of D2 deformation at 196±4 Ma. U-Pb data on this Early Jurassic leucogranite also indicate that the inherited zircons have a complex history, with analyses Previous Work yielding ages of 359±5 and 232±7 Ma. Our data confirm ear- lier 40Ar/39Ar ages that suggest an Early Jurassic date for the Historically, the Fortymile River corridor was an impor- intense regional D2 metamorphism and tectonism. However, tant entry point for mineral exploration, trapping, and com- although we have determined that the sedimentary and vol- merce during the early days of settlement of the Alaska-Yukon canic rocks are at least as old as Early Mississippian, the date region. Gold placer miners traveled from the Yukon River up of deposition for the major supracrustal rock packages in the the Fortymile River and established communities at Fortymile, Fortymile River area of the Yukon-Tanana terrane still remains Yukon Territory, as well as such settlements as Steele Creek a major question. and Chicken, Alaska. Mertie (1938) presented the first regional perspective on the geologic setting of the Yukon-Tanana upland, including the Fortymile River area. Yeend (1996) pro- Introduction vided an overview of the historical development of the region, as well as a characterization of the placer resources of the Fortymile River area. Foster (1969, 1976), Foster and O’Leary This research has resulted from a joint effort by the U.S. (1982), Wilson and others (1985), Dusel-Bacon and Hansen Geological Survey (USGS) and the Alaska Department of (1992), Mortensen (1992), Dusel-Bacon and others (1993), Geochemistry and Age Constraints on Metamorphism and Deformation in the Fortymile River Area, Eastern Yukon-Tanana Upland 5 Foster and others (1994), Dusel-Bacon and others (1995), and Geologic Setting Hansen and Dusel-Bacon (1998) reported on a series of geologic studies that have led to a better understanding of The Fortymile River area is underlain by several the regional geologic setting. lithotectonic stratigraphic packages (see Dusel-Bacon and In 1997, the USGS initiated a joint research project others, 1995; Hansen and Dusel-Bacon, 1998). This chap- with the ADNR to assess the effects of placer mining ter addresses the geochemistry and geochronology of the on the water quality of the Fortymile River (Gough and “Taylor Mountain assemblage” of Hansen and Dusel-Bacon others, 1997; Wanty and others, 2000), as well as to estab- (1998), one of the most widespread lithotectonic assemblages lish the geologic framework of the area (Day and others, in the Yukon-Tanana tectonostratigraphic terrane of east-cen- 2000), along with the geochemistry of soil, rock, selected tral Alaska (fig. 1). In their report on the bedrock geology of vegetation, and surface waters in the area (Crock and the area, Day and others (2000) followed the terrane termi- others, 1999, 2000). Wanty and others studied the effects nology established by Hansen and Dusel-Bacon (1998) and of suction dredging in the Fortymile River and bulldozer- referred to the medium- to high-grade metamorphic rocks operated placer mining on water quality in the area. The of the area as belonging to the Taylor Mountain assemblage. reconnaissance geologic study by Day and others (2000) However, because of the potential confusion that can arise outlined the bedrock types and compositions, as well as when discussing both the Taylor Mountain batholith, which the polyphase deformational history, of the area. Their is a Late Triassic granitoid body, and metamorphic rocks of study showed that the earliest recognized ductile deforma- the “Taylor Mountain assemblage,” which extends far beyond tion (D1) resulted in isoclinal F1 folding, a strong regional the Taylor Mountain area, we, like other workers (Cynthia schistosity (S1), and an L1 mineral lineation that is locally Dusel-Bacon, written commun., 2001), now refer to these preserved. The second episode of ductile deformation (D2) rocks as the “Fortymile River assemblage.” was associated with intense regional high-grade metamor- The Fortymile River assemblage is made up of high- phism (Dusel-Bacon and others, 1995), tight to isoclinal grade metamorphic, polydeformed supracrustal rocks that F2 folding with coaxial L2 lineations, and a weak S2 cleav- were intruded by tonalite, trondhjemite, ultramafic rocks, age that is axially planar to the F2 folds. The L2 lineations and monzodiorite-diorite-quartz diorite. The metasedimen- occur both as stretching and mineral lineations that, where tary and metavolcanic(?) supracrustal rocks include biotite preserved together, are coaxial to the F2 folds. The young- schist and siliciclastic metasedimentary rocks, quartzite, est recognized folding event (D3) folded the earlier ductile marble, and metagabbro and hornblende-biotite schist of fabric elements about north- and south-plunging, eastward- basaltic composition (Dusel-Bacon and Cooper, 1999). The vergent open F3 folds, which have neither any recognizable ages of the protoliths of the Fortymile River assemblage are axial planar cleavage nor mineral lineations. High-angle poorly constrained. The intense regional dynamic recrys- brittle faults represent the latest episode of deformation tallization that accompanied high-grade metamorphism (Newberry and Burns, 2000; Szumigala and others, 2000a), (Dusel-Bacon and others, 1995) has almost completely variously uplifting the region along north- to northeast- destroyed primary textures within the supracrustal rocks, trending block-bounding faults. and so standard fossil studies are almost hopeless. Foster New research by the ADNR’s Division of Geological (1976) assigned the metasedimentary rocks to the Paleozoic, and Geophysical Surveys, as reported by Szumigala (2000), using as evidence sparse, poorly preserved crinoid stems at a Szumigala and others (2000a, b), and Werdon and others locality to the west of the Fortymile River area. (2000), is leading to a clearer understanding of the detailed Although the primary ages of the protoliths and early bedrock geology in the Fortymile River area. Their map pat- (pre-Cretaceous) intrusive rocks are poorly known, cooling terns show continuous horizons of mafic metavolcanic rocks ages for the peak metamorphic event have been determined interlayered with biotite schist and gneissic rocks, all of by using both K-Ar (Wilson and others, 1985) and 40Ar/39Ar which have undergone an early episode of deformation (D1 of (Cushing, 1984; Hansen and others, 1991) techniques. Wilson Day and others, 2000). They recognized that this early epi- and others (1985) reported ages of 175 to 182 Ma on biotite sode of deformation formed large-scale isoclinal to recum- and hornblende in amphibolite and gneiss from the Wade bent folds that were
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