Yukon Crystalline Terrane Research Articles

Ancient continental margin assemblage in the northern Coast Mountains, southeast Alaska and northwest Canada

Geologic relations indicate that quartz-rich metasedimentary rocks in the northern Coast Mountains separate strata to the east that belong to the Stikine terrane from strata to the west of the Alexander, Wrangellia, and Taku terranes. The quartz-rich rocks structurally overlie western terranes along a mid-Cretaceous thrust fault and are overlain structurally (originally stratigraphically ) by strata of the Stikine terrace. These rocks are interpreted to be a continental margin assemblage that belongs to the Yukon Crystalline terrane. U-Pb and Nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of Proterozoic rocks.

Petrologic evidence for a terrane boundary in south‐central Circle Quadrangle, Alaska

Greenschist to amphibolite facies pelitic, mafic, and aluminous calc‐silicate schists from the south‐central Circle quadrangle, east‐central Alaska, are in contact with carbonaceous quartzite and calcareous phyllite recording diagenetic temperatures. Their mutual contact is best explained as a post‐metamorphic fault, indicating that the Yukon crystalline terrane was, at least in part, assembled after the peak of regional metamorphism. Mineral equilibria within the schists indicate metamorphism at minimum conditions of about 440° to 380°C at 0.3 GPa (3Kbar, biotite grade), 500°C at 0.5 GPa and XCO2 = 0.3 (garnet grade), and 550°C at 0.5 GPa and XCO2 = 0.2 (staurolite‐kyanite grade). Reflectance measurements on vitrinite and inertinite from the carbonaceous unit give temperatures of 170° to 230° ± 50°C.

An Eocene Rb–Sr isochron for rhyolite plugs, Skukum area, Yukon Territory

A new Rb–Sr whole-rock isochron for the Skukum rhyolite is 53.3 ± 1.1 Ma with an initial ratio of 0.7053. The rhyolite forms small plugs spatially and probably genetically related to the Skukum volcanic complex in the southwestern Yukon. They were intruded into granitic rocks of the Coast Plutonic Belt. The intrusions are coeval and part of an Eocene plutonic, subvolcanic, and volcanic suite correlated with the Bennett Lake cauldron complex. The initial 87Sr/86Sr ratio matches interpretations of the tectonic evolution of Yukon Crystalline Terrane and Coast Plutonic Complex.

A petrologic and Rb–Sr isotopic study of intrusive rocks near Fairbanks, Alaska

Epizonal granodiorite and granite plutons intrude the northwestern portion of the Yukon Crystalline Terrane near Fairbanks, Alaska. Mineralogic, major-oxide, and trace-element studies indicate that the plutons represent a comagmatic fractionation suite. A Rb–Sr whole-rock isochron for one pluton yields an age of 90.9 ± 0.9 Ma, in agreement with previous K–Ar dating of biotite and hornblende and an initial 87Sr/86Sr ratio of 0.71238 ± 0.00014. The elevated initial 87Sr/86Sr ratio suggests that the plutons may have had a different source from that of the belt of subduction-related, mid-Cretaceous plutonic rocks found farther to the southeast in the Yukon Crystalline Terrane. In contrast, they appear to have been formed at least in part by anatectic melting of Late Proterozoic or early Paleozoic crustal rocks. Comparison of aplite dike compositions with the synthetic granite system, estimates of average uplift rate, and K–Ar cooling ages of surrounding schists all indicate that the magmas that formed the plutons near Fairbanks intruded much higher into the crust after anatectic melting and crystallized at a relatively shallow depth of 3–5 km.

Review of radiometric data from the Yukon Crystalline Terrane, Alaska and Yukon Territory

The results of more than 20 years of geochronological studies in the Yukon Crystalline Terrane in east-central Alaska and the western Yukon Territory suggest at least six igneous and thermal (metamorphic?) events. Plutonism during Mississippian, Early Jurassic, mid-Cretaceous, Late Cretaceous, and early Tertiary times is indicated. Evidence also indicates that Mississippian, Early Jurassic, late Early Cretaceous, and late Cretaceous thermal (metamorphic?) events have affected parts of the terrane. The western part of the terrane was affected by a significant regional metamorphic event in late Early Cretaceous time, followed by a terrane-wide mid-Cretaceous plutonic event. The pattern of K–Ar ages allows division of the terrane into domains, bounded by northeast-trending lineaments.

Late Cretaceous age of the Hutshi, Mount Nansen, and Carmacks groups, southwestern Yukon Territory and northwestern British Columbia

Volcanic rocks of the Hutshi, Mount Nansen, and Carmacks groups occur in the southwestern Yukon where they unconformably overlie the Yukon Crystalline Terrane and deformed strata of the Whitehorse Trough. The volcanic rocks are faulted and tilted, locally altered, and largely postorogenic. The more basic Carmacks volcanics locally overlie intermediate to acid rocks of the Mount Nansen Group, but are mostly in isolated exposures northwest of the Hutshi and Mount Nansen volcanics.Hutshi – Mount Nansen volcanics of the Miners Range are porphyritic, partly vesicular calc-alkaline andesite flows and flow breccias intruded by calc-alkaline alkali-rich rhyolite and two-feldspar andesite dikes. A low-greenschist metamorphic overprint affects most rocks.Carmacks volcanics, near Carmacks, are flows, epiclastic breccias, and sintered tuffs interbedded with immature volcanic sandstone. One analysed breccia clast is calc-alkaline andesite, but the flows are potassic basalt, trachybasalt, and tristanite.Recent assignments of the Hutshi – Mount Nansen and Carmacks volcanic suites to early and mid-Tertiary ages, respectively, are incorrect as all are late Cretaceous. K–Ar dates for a Hutshi – Mount Nansen whole rock and plagioclase of 72.4 ± 2.5 and 69.1 ± 2.6 Ma and Carmacks whole rocks and biotite of 73.1 ± 2.5, 67.9 ± 2.3, and 68.0 ± 2.2 Ma are concordant among themselves and agree with a Rb–Sr whole-rock date of 72.4 ± 2.1 Ma for rhyolite from the Hutshi Group in northern British Columbia.This widespread late Cretaceous volcanic episode has typical subduction-related volcanic arc chemical polarity: calc-alkaline to alkaline from active trench towards stable craton. There is a dearth of documented early to mid-Cenozoic rocks in the Yukon.

Evolution of a high-level, high-silica magma chamber: the Pattison pluton, Nisling Range alaskites, Yukon

The Pattison pluton is a high-level, high-silica Tertiary alaskite intruding rocks of the Yukon Crystalline Terrane. The pluton is roughly circular in form with a diameter of about 18 km. The intrusion is cut by a series of shallowly dipping aplite dikes. Mineralogically, the alaskite body and the aplite dikes are very similar, but the pluton is divided into a series of phases based on texture and "stratigraphic" position: a fine-grained upper border phase, a medium-grained, graphic, strongly miarolitic, porphyritic phase, and a lower, coarser grained phase, contacts being gradational.No strong major element trends are found within the pluton (including the aplite dikes) because of eutectic crystallization. The trace elements, particularly Ba, Sr, Rb, Zr, and the REE, however, do show a liquid line of descent, with the upper border phase as the least differentiated and the aplite dikes as the most differentiated phases. Rayleigh fractional crystallization of quartz, K-feldspar, and plagioclase (and accessory allanite) in eutectic proportions readily accounts for the observed trends.The parental magma was water undersaturated (< 1.2% H2O). Crystallization of anhydrous phases led to water-saturated conditions late in the pluton's history. Volatiles migrated to and concentrated in the upper parts of the chamber where the melting temperature was depressed, causing the pluton to crystallize from the bottom up. The volatile pressure eventually exceeded the confining pressure, resulting in failure of the surrounding rocks, retrograde boiling, vertical extension, and lateral injection of residual magma caused by filter pressing to form the aplite dikes. The escaping volatile phase resulted in quartz veining and Mo mineralization.

Rb–Sr whole-rock age of the Klondike Schist, Yukon Territory

Seven samples of metarhyolite from a measured section of the Klondike Schist, in the Yukon Crystalline Terrane, 5 km southwest of Dawson City, give a whole-rock Rb–Sr isochron age of 202 ± 11 Ma with an initial 87Sr/86Sr ratio of 0.7140 ± 0.0001. The high initial ratio suggests that this is a metamorphic age, recording the time of shearing of the metarhyolites. An estimated minimum age of formation of the protolith of approximately 260 Ma was obtained by rotating the isochron to an assumed primary initial 87Sr/86Sr ratio of 0.707.The siliceous cataclasites of the Klondike Schist may have formed as a forearc wedge of arkose with intercalated volcanics. The unit was intensely sheared in a subduction zone, prior to obduction onto the ancient North American continental margin. The results of this isotopic study suggest an age of formation of 260 Ma and an age of cataclasis of 202 Ma. The metarhyolites remained closed to Rb–Sr isotopic equilibration during obduction, though the thermal event is recorded in K–Ar isotopic data reported by other authors. The results of this study are consistent with theories of formation of this assemblage.

Terranes and suture zones in east central Alaska

East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well‐bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head‐on, their leading edges lie along a multiple set of high‐angle faults that outline microterranes in accretion zones.

Zircon ages for the Pelly Gneiss and Klotassin granodiorite in western Yukon

Two samples of Pelly Gneiss with different field relations and post-crystallization histories give different U/Pb ages that are Devonian and Permian (375 and 276 Ma). A sample of Klotassin granodiorite gave an age of 192 Ma confirming its Early Jurassic age. These Devonian, Permian, and Jurassic plutonic rocks in Yukon Crystalline Terrane lack equivalents in the adjacent Omineca Belt where sedimentary strata accumulated at these times. This suggests that the Yukon Crystalline Terrane is allochthonous with respect to the Omineca Belt.

Interpretation of isotopic ages and 87Sr/86Sr initial ratios for plutonic rocks in the Whitehorse map area, Yukon

Sixteen new K–Ar dates and four new Rb–Sr isochrons help define four plutonic suites in the Whitehorse map area, Yukon. The Triassic(?) suite, defined on stratigraphic evidence, is the southern extension of the Yukon Crystalline Terrane and is correlative with plutonic suites in the Intermontane Belt in British Columbia. The mid-Cretaceous (~100 Ma) suite in the Intermontane Belt in the Whitehorse map area is time equivalent to plutonic suites in the Omineca Crystalline Belt to the east. Late Cretaceous (~70 Ma) and Eocene (~55 Ma) suites include volcanic and subvolcanic as well as plutonic phases and are correlative with continental volcano–plutonic suites near the eastern margin of the Coast Plutonic Complex. The predominance of the mid-Cretaceous suite in the Intermontane Belt in Whitehorse and adjacent map areas in Yukon and northern British Columbia suggests that this area has undergone posttectonic magmatism more characteristic of the Omineca Crystalline Belt than of the Intermontane Belt elsewhere in the Canadian Cordillera.87Sr/86Sr initial ratio determinations suggest that the southern extension of the Yukon Crystalline Terrane in the western part of the Whitehorse map area and in northern British Columbia includes Precambrian crust separated from the North American craton by Paleozoic oceanic crust of the Intermontane Belt.

Rb/Sr Ages and a profile of initial Sr87/Sr86 ratios for plutonic rocks across the Yukon Crystalline Terrane

Nine Rb/Sr apparent ages are reported for igneous rocks of the Yukon Crystalline Terrane. The oldest age (144 m.y.) is from the Triassic? Klotassin quartz diorite and is thought to be a hybrid age that probably reflects the effects of younger intrusives on rocks at least 190 m.y. old. Five ages of about 100 m.y. presumably reflect the cooling of the Coffee Creek quartz monzonite. K/Ar ages for this event are slightly younger than the Rb/Sr ages, suggesting slow cooling. Rb/Sr ages of 53 and 67 m.y. were obtained for the Ruby Range batholith and an age of 61–67 m.y. for the Nisling Range alaskite. The Rb/Sr ages obtained generally confirm recently determined K/Ar ages. There is a regional decrease in initial Sr87/Sr86 ratios, southwestward across the Yukon Crystalline Terrane. This may mean that Precambrian rocks extend under the Yukon Crystalline Terrane, but are absent under the adjoining Coast Plutonic Complex.

The Yukon Crystalline Terrane: Enigma in the Canadian Cordillera

The Yukon Crystalline Terrane, an area of Proterozoic and Paleozoic metamorphic rocks intruded by Mesozoic and Tertiary plutons and covered by Tertiary volcanic strata, has been one of the least understood tectonic elements in the Canadian Cordillera. It incorporates characteristics of the Omineca Crystalline Belt, Intermontane Belt, and Coast Plutonic Complex. The metamorphic rocks are not part of a single time-stratigraphic assemblage as the term “Yukon Group” has implied. They can be divided into gross lithologic assemblages which are correlated with Proterozoic and Paleozoic strata of the Omineca Crystalline Belt, and the metamorphic rocks of the Yukon Crystalline Terrane are therefore the continuation of the Omineca Crystalline Belt. Four plutonic suites are recognized in the Yukon Crystalline Terrane. The two oldest (Late Triassic and mid-Jurassic) are related to similar plutons in the Intermontane Belt and mark its continuation through the central part of the Yukon Crystalline Terrane. The mid-Cretaceous suite in the northern part of the Yukon Crystalline Terrane is a westward continuation of plutonic rocks of the Omineca Crystalline Belt. Early Eocene alaskite marks the superposition of Coast Plutonic Complex igneous rocks on the southern part of the Yukon Crystalline Terrane. Tertiary tholeiitic basalts, which cover part of the Yukon Crystalline Terrane, lie on an eroded plateau whose relief and physiography approximate the present terrane.

Potassium–Argon Age Determinations of Metamorphic and Plutonic Rocks in the Yukon Crystalline Terrane

Forty-four new potassium–argon age determinations on minerals of metamorphic and igneous rocks from the Yukon Crystalline Terrane define the timing of the three most recent thermal events affecting this region. The oldest, 160 to 170 Ma ago, involved weak retrograde metamorphism of igneous and metamorphic rocks and coincides with the intrusion of batholiths of pink quartz monzonite. The next event, 90 to 100 Ma ago, reflects the emplacement of batholiths of the Coffee Creek quartz monzonite suite. The youngest thermal episode, 50 to 60 Ma ago, marks the time when the Nisling Range alaskite, with its porphyry dyke swarms and explosive acid volcanic rocks, was emplaced and when the K–Ar system of the Ruby Range Batholith was thermally reset. The data provide a younger limit to the age of the oldest Mesozoic plutonic rocks, the Klotassin suite, but they do not define its time of emplacement.