Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia

Abstract

The Earn Group of central Yukon records the transition from a passive to an active margin along western Laurentia in the Late Devonian. Fine-grained clastic rocks and chert of the lower Earn Group contain late Early to Middle Devonian fossils and were deposited in an offshelf environment. The upper Earn Group comprises a mixture of sandstone and conglomerate, fine-grained siliciclastic rocks, and widespread crystal lithic tuff. Zircon from this succession are precisely dated using CA-ID-TIMS methods on igneous (ca. 363 Ma) and detrital (ca. 378-363 Ma) grains and confirmed by Frasnian to Famennian fossils. Abrupt, along-strike facies changes within the upper Earn Group of the Glenlyon–Tay River area occur across mapped faults that are inferred to have originated as syn-depositional extension faults in the Late Devonian. Occurrences of ca. 363 Ma tuff horizons within all facies of the upper Earn Group provide a temporal correlation across the area. Diorite plutons intrude lower Paleozoic rocks in the area and have U-Pb zircon crystallization dates of ca. 364 Ma. The diorite has calc-alkaline composition consistent with arc magmatism or crustal contamination. The Late Devonian magmatism in the Earn Group is coincident with onset of arc magmatism in the allochthonous Yukon-Tanana terrane, and extension related to rifting and opening of the Slide Mountain ocean in a back-arc setting. Magmatic rocks in the Earn Group of central Yukon thus represent part of a remnant continental arc and back-arc stranded behind the Slide Mountain ocean in the Mississippian.

U‐Pb Zircon Geochronology From the Northern Cordillera, Central Yukon, With Implications for Its Tectonic Assembly

“…Late Devonian metamorphism (370 ± 4 Ma; 2σ systematic error-17RAY-AP014A2) and magmatism (372 ± 5 Ma; 2σ systematic error-17RAY-AP016C1) recorded by our Cassiar terrane samples are consistent with magmatism reported elsewhere from the Cassiar terrane (Beranek et al, 2016;Cobbett et al, 2020;Colpron, Israel, & Friend, 2016;Tempelman-Kluit, 2012), and temporally overlap with low-pressure metamorphism and magmatism recorded in the Yukon-Tanana terrane (Berman et al, 2007;Colpron, Israel, & Friend, 2016). This thermal event may reflect rifting of Snowcap assemblage units of the Yukon-Tanana terrane from the Laurentian margin during the Late Devonian-Early Mississippian (e.g., Colpron, Mortensen, et al, 2006;Nelson et al, 2013).…”

“…17RAY-AP016C1 is a foliated and metamorphosed monzogranite within the Ingenika Group of the Cassiar terrane; it has a crystallization age of 372 ± 5 Ma (2σ systematic error; Figure 3b) and a peraluminous geochemical composition (Figures 3c-3h). Volcanic and plutonic rocks of similar age to 17RAY-AP016C1 are reported elsewhere in south-central Yukon from the Seagull group, which is associated with continental rifting and intra-plate magmatism (Beranek et al, 2016;Tempelman-Kluit, 2012), and the Earn Group, which is associated with backarc rifting and subduction (Campbell, 1967;Cobbett et al, 2020). The peraluminous composition of sample 17RAY-AP016C1 (Figures 3c-3h) is compatible with both of those settings.…”

Late Triassic to Jurassic Magmatic and Tectonic Evolution of the Intermontane Terranes in Yukon, Northern Canadian Cordillera: Transition From Arc to Syn‐Collisional Magmatism and Post‐Collisional Lithospheric Delamination

“…In southern Yukon, ca. 378–373 Ma volcanism is only documented in continental margin strata of the Earn Group, presently ∼250 km away northeast of the Tintina fault (Cobbett et al., 2021; Yukon Geological Survey, 2020a, 2020b, 2020c). Zircons within this age range are also documented in the Yukon‐Tanana terrane of southeast Alaska (>400 km south) but consistently have superchondritic εHf t values (Pecha et al., 2016).…”

Phanerozoic flooding of North America and the Great Unconformity

“… 38 , who reproduced the extent of Devonian subsidence and sedimentation in response to subduction. By the end Late Devonian, slab retreat ( 34 , 36 , 39 ) or oblique convergence ( 40 ) coincides with the marked cessation of marine sedimentation around northwest Laurentia and the end of the Kaskaskia I peak.…”