Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front

Abstract

In situ sensitive high-resolution ion microprobe monazite geochronology and garnet isopleth thermobarometry reveal a previously unrecognized Middle Jurassic to earliest Cretaceous mid-crustal tectonometamorphic event in the eastern part of the Yukon-Tanana terrane (Finlayson Lake district, southeast Yukon) in the northern Canadian Cordillera. Intersection of garnet end-member compositional isopleths applied to single-stage, growth-zoned garnet records progressive garnet growth from 550 °C and 6.1-6.6 kbar to 600 °C and 7.5 kbar. Monazite textures, chemical zoning, and in situ U-Pb ages record a single protracted episode of monazite growth from ca. 169 to 142 Ma coeval with the development of transposition fabrics and the late stages of garnet growth. This event post-dates widespread Early Jurassic exhumation of Yukon-Tanana terrane rocks west of the Tintina fault in west-central Yukon, which were previously ductily deformed and metamorphosed in the Permo-Triassic. The lack of evidence for Permo-Triassic ductile deformation and high-grade metamorphism within the Finlayson Lake district, and its position east of the Permian arc center and west of Permian blueschists and eclogites, suggests this eastern part of the terrane occupied the cool forearc at this time. These data indicate younger, more protracted mid-crustal orogenesis in the northern Cordillera than was previously recognized, with deformation and metamorphism migrating toward the foreland and downwards in the Middle Jurassic to Early Cretaceous, in part contemporaneous with and analogous to that in the southeastern Canadian Cordillera.

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

“…Moreover, the age of thermotectonism in the study area appears younger than in other parts of the region affected by the Big Sky orogeny, suggesting propagation of peak metamorphism and deformation from NW to SE over 40-80 m.y. These results are similar to lateral growth and propagation patterns observed in other major collisional orogens (e.g., Jamieson et al, 2011;Staples et al, 2014).…”

Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

“…See text for interpretation. Arc and arc proximal: 1—Saint Elias Mountains, Yukon (this study); 2—Eastern Coast Mountains, British Columbia [ Gehrels et al ., ]; 3—Western Coast Mountains [ Gehrels et al ., ]; 4—Blue Mountains, Oregon [ LaMaskin et al ., , and references therein]; 5—Sierra Nevada batholith, California [ Ducea , ]; 6—Alexander‐Wrangellia‐Peninsular composite terrane forearc and arc, Alaska [ MacKevett , ; Trop and Ridgway , ]; 7—Coast belt, British Columbia [ Greig , ]; 8—Franciscan Complex, California [ Wakabayashi and Dumitru , ]; 9—Sierra Nevada arc and host rocks, California [ Cao et al ., ]; 10—Chugach terrane (McHugh complex) strata, Alaska [ Amato and Pavlis , ]; 11—Gravina belt strata, southeastern Alaska and British Columbia [ Yokelson et al ., ]; Retroarc: 12—Omineca belt, Yukon [ Allan et al ., ; Bailey , ; Staples et al ., ]; 13—Intermontane belt, Yukon and British Columbia [ Evenchick et al ., ; White et al ., ]; 14—Omineca and Foreland belts, Alberta and British Columbia [ Colpron et al ., , Evenchick et al ., ; Pană and van der Pluijm , , and references therein]; 15—Luning‐Fencemaker belt, Nevada [ Wyld , ; DeCelles , ]; 16—Sevier hinterland, Idaho, Utah, and California [ Hoisch et al ., ; Cruz‐Uribe et al ., ]; 17—Whitehorse trough, Yukon [ Colpron et al ., ]; 18—Bowser basin, British Columbia [ Evenchick et al ., ]; 19—Alberta foreland basin [ Poulton , , Raines et al ., , and references therein]; and 20—Montana foreland basin [ Fuentes et al ., , and references therein]. Geological timescale of Cohen et al .…”

“…Southwest directed thrusting within Stikinia and Cache Creek terrane (Figure ) is considered to indicate Middle Jurassic linkages with southwest vergent folding and thrusting in the parautochthonous Omineca belt of southern British Columbia [ Evenchick et al ., ]. By late Middle to early Late Jurassic time, some hinterland structural styles had switched from southwest to northeast vergent and likely record the cratonward propagation of the retroarc thrust system in Yukon and southeastern British Columbia [e.g., Crowley and Brown , ; Colpron et al ., ; Gibson et al ., , ; Staples et al ., , ]. Peak Barrovian metamorphism associated with this retroarc deformation occurred ~170–160 Ma in southern Omineca belt of British Columbia [e.g., Parrish , ; Evenchick et al ., ], and regional orogenic gold veins that are characteristic of very late to postpeak metamorphic conditions [ Goldfarb and Groves , ] were emplaced into the northern Omineca belt of Yukon by the early Late Jurassic (Figure ) [ Allan et al ., ; Bailey , ].…”

Tectono-Metamorphic Evolution of the Snowcap Assemblage, Yukon-Tanana Terrane, West-Central Yukon

“…The YTT re-accreted to the Laurentian margin in the Upper Permian to mid-Cretaceous (Beranek et al, 2010;Berman et al, 2007;Colpron et al, 2007;Beranek and Mortensen, 2011). The accretion was accompanied by diachronous metamorphism reaching amphibolite facies, recorded during the Permo-Triassic, Lower Jurassic, Middle to Upper Jurassic, and Lower to mid-Cretaceous (Dusel-Bacon et al, 1995;Berman et al, 2007;Staples et al, 2013Staples et al, , 2014Staples et al, , 2016Clark et al, 2016). Plutons of Triassic, Jurassic, and Cretaceous ages have intruded the YTT near the end of its accretion (Woodsworth et al, 1991;Logan and Mihalynuk, 2014).…”

“…The third event peaked at 7.8 kbar and 595 °C at 195 to 187 Ma (U-Pb monazite : Berman et al, 2007;Staples et al, 2014). Similar to M2, the cause of this event is poorly constrained.…”