A window into the Early to mid‐Cretaceous infrastructure of the Yukon‐Tanana terrane recorded in multi‐stage garnet of west‐central Yukon, Canada

Abstract

Amphibolite facies metasedimentary schists within the Yukon-Tanana terrane in the northern Canadian Cordillera reveal a two-stage, polymetamorphic garnet growth history. In situ U-Th-Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn-kinematic, inclusion-rich stage-1 garnet core; and (ii) an inclusion-poor, stage-2 garnet rim that crystallized with syn-to post-kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage-1 garnet growth initiated at 600°C, 8 kbar along a clockwise P-T path. Growth of the compositionally distinct, grossular-rich, pyrope-poor inner portion of the stage-2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage-1 core along a separate P-T loop, culminating at peak P-T conditions of~650-680°C and 9 kbar. Stage-2 metamorphism and the waning development of a composite transposition foliation (S T ) are dated at c. 118 Ma from monazite aligned parallel to S T, and inclusions in syn-to post-S T staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y-rich monazite that occurs within resorbed areas of both stage-1 and stage-2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near-isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below~4-5 kbar prior to cooling below 550°C. As most other parts of the Yukon-Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high-grade tectono-metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east-central Alaska and in the southeastern Canadian Cordillera.

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

“…In their pre-Paleogene confi guration, rocks of the Finlayson Lake district lie immediately northeast of a tectonic window that exposes an Early to mid-Cretaceous high-grade metamorphic infrastructural domain of the Yukon-Tanana terrane southwest of the Tintina fault (Staples et al, 2013;Fig. 2).…”

“…2). West of the Cretaceous domain, across the Australia Creek fault (Staples et al, 2013), Yukon-Tanana terrane rocks were transposed and metamorphosed in the Late Permian to Early Triassic and in the Early Jurassic Beranek and Mortensen, 2011;Fig. 2) before they were exhumed to upper crustal levels in the Early to Middle Jurassic (Johnston et al, 1996;Dusel-Bacon et al, 2002;Berman et al, 2007). Southeast of the Cretaceous domain, across the Stewart River normal fault, are sub-greenschist-facies, weakly deformed to undeformed Devonian and Mississippian volcanic and plutonic rocks that record Paleozoic and Mesozoic 40 Ar/ 39 Ar cooling ages (Knight et al, 2013).…”

“…Southeast of the Cretaceous domain, across the Stewart River normal fault, are sub-greenschist-facies, weakly deformed to undeformed Devonian and Mississippian volcanic and plutonic rocks that record Paleozoic and Mesozoic 40 Ar/ 39 Ar cooling ages (Knight et al, 2013). Both the Permo-Jurassic metamorphic domain to the west and the Devono-Mississippian rocks to the southeast were presumably the suprastructural "lid" from beneath which the Cretaceous metamorphic domain (essentially a core complex) was exhumed along the Australia Creek and Stewart River faults, respectively, in the mid-Cretaceous (Staples et al, 2013).…”

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

“…For example, in the Stewart River area, titanite and monazite ages suggest that Yukon-Tanana arc and Snowcap assemblage rocks were metamorphosed during deformation at relatively low-pressure conditions between 365 and 350 Ma and at medium-pressure conditions at ~ 265 Ma (Berman et al 2007). These rocks were further overprinted by regional Cretaceous plutonism and Jurassic-Cretaceous metamorphism during deformation and exhumation (Berman et al 2007;Staples et al 2013). In the St. Cyr area, the majority of 40 Ar/ 39 Ar muscovite cooling ages (Fallas et al 1998) (Erdmer 1987;Erdmer et al 1998;Perchuk et al 1999;Perchuk and Gerya 2005;Ghent and Erdmer 2011).…”

“…The Snowcap assemblage is a heterogeneous mix of psammitic, pelitic and calc-silicate schist, quartzite, marble and amphibolite (Colpron et al 2006a(Colpron et al , 2006bPiercey and Colpron 2009). These rocks are typically polydeformed and polymetamorphosed up to amphibolite-facies conditions (Colpron et al 2006a;Berman et al 2007;Piercey and Colpron 2009;Staples et al 2013). Mafic amphibolite in the Snowcap assemblage displays normal midocean ridge basalt (N-MORB), enriched mid-ocean ridge basalt (E-MORB), or ocean island basalt (OIB) signatures consistent with emplacement in a continental rift setting (Nelson and Friedman 2004;Colpron et al 2006b;Piercey and Colpron 2009).…”