Geology and U–Pb geochronology of the Klondike District, west-central Yukon Territory

Abstract

Geological mapping and U–Pb geochronology of the Klondike District provide new information on the nature and evolution of the Yukon–Tanana terrane (YTT) in western Yukon. The area is underlain by a sequence of thrust panels of regional extent. A continuously mappable sequence of interlayered metasedimentary and metavolcanic rocks is intruded by a variety of deformed metaplutonic rocks within two of these thrust sheets. Layering in the metasediments and metavolcanics is considered to be at least in part transposed stratigraphy. Small bodies of greenstone and altered ultramafic rocks thought to be part of the Slide Mountain terrane occur discontinuously along the thrust faults.U–Pb age determinations indicate that the uppermost thrust panel (assemblage I), which underlies much of the Klondike District, consists largely of metamorphosed, mid-Permian felsic plutonic, subvolcanic, and tuffaceous rocks. Beneath assemblage I is a second thrust panel (assemblage II), also of large areal extent, of mid-Paleozoic or older metasedimentary and mafic and felsic metavolcanic rocks, intruded by a large body of latest Devonian – Early Mississippian granitic augen orthogneiss. U–Pb analyses of zircon from the orthogneiss reflect both lead loss and a significant inherited zircon component. A third structural unit (assemblage III), which consists mainly of carbonaceous schist and phyllite, crops out in the northern part and along the southwestern edge of the study area, where it underlies both assemblages I and II.The earliest stage of deformation and metamorphism that affected the area (F1) produced the pervasive recrystallization fabric characteristic of all of the metamorphic rocks in assemblages I, II, and III, and occurred between mid-Permian and Late Triassic time. Thrust faulting, presumed to be northerly or northeasterly directed, postdates Late Triassic but predates mid-Cretaceous. The second phase of deformation (F2) was either synchronous with or later than thrust faulting. Monazite ages for the augen orthogneiss indicate that at least local metamorphism and (or) deformation lasted until Early Cretaceous time.Close similarities between composition, U–Pb ages, as well as timing and style of deformation, documented in the Klondike District and observed elsewhere in the YTT in southeastern Yukon and east-central Alaska suggest that much of the YTT either evolved as a single entity or else shared a very similar history.

Magmatic flow and emplacement foliations in the Early Jurassic Aishihik Batholith, southwest Yukon: Implications for northern Stikinia

“…The Yukon-Tanana Terrane was imbricated along regional scale thrust faults prior to intrusion of the LLPS, but after or during intrusion of Early Jurassic plutons thought to be correlative with the APS. The thrust faults are characterized by mafic and ultramafic rocks thought to represent ophiolite of the Slide Mountain Terrane (Mortensen, 1990(Mortensen, , 1992. In British Columbia, Early Jurassic arc volcanism began to wane by 187 Ma (Marsden and Thorkelson, 1992), although contraction, during which the Cache Creek Terrane was thrust westward over Stikinia along the Nahlin and King Salmon faults, did not occur until the Middle Jurassic (Marsden and Thorkelson, 1992;Ricketts et al" 1992).…”

“…Recent studies have suggested that some plutons included in the APS intrude rocks of the Yukon-Tanana Terrane (Mortensen, 1990(Mortensen, , 1992. These findings raise two key questions: (1) Are plutonic bodies of the APS allochthonous with respect to the Nisling and Yukon-Tanana Terranes?…”

Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, Yukon–Tanana Terrane, Yukon, Canada

“…The Finlayson assemblage is unconformably overlain by the Mississippian to Early Permian Klinkit assemblage, consisting of variably metamorphosed mafic to intermediate calcalkaline volcanic and volcaniclastic rocks with minor alkali basalt, limestone/marble and conglomerate (Simard et al 2003;Nelson and Friedman 2004;Murphy et al 2006;. The Klondike assemblage, the youngest assemblage in the Yukon-Tanana terrane, consists of Middle to Late Permian felsic calc-alkaline metavolcanic and metaplutonic rocks and minor mafic rocks (Mortensen 1990;Dusel-Bacon et al 2006).…”

Wandering gravel-bed rivers and high-constructive stable channel sandy fluvial systems in the Ross River area, Yukon Territory, Canada

“…1) is underlain by rocks of the YukonTanana terrane, an assemblage of polydeformed metamorphic rocks of Palaeozoic age (Mortensen, 1990(Mortensen, , 1992(Mortensen, , 1996Colpron et al, 2006). According to Mortensen (1992), these consist mainly of pelitic to quartzo-feldspathic metasedimentary schist and gneiss, with minor marble and mafic to felsic metavolcanic, and metaplutonic rocks.…”

“…According to Mortensen (1992), these consist mainly of pelitic to quartzo-feldspathic metasedimentary schist and gneiss, with minor marble and mafic to felsic metavolcanic, and metaplutonic rocks. The Yukon-Tanana was juxtaposed with other terranes (i.e., Slide Mountain) by regional scale thrust faulting in the Early Mesozoic, so that it now structurally overlies the ancient continental margin of North America (Mortensen, 1990;Colpron et al, 2006;Colpron and Nelson, 2009). The Yukon-Tanana terrane is unconformably overlain by post-accretionary, Mid-to Late-Cretaceous sedimentary and volcanic rocks (Mortensen, 1996).…”