Metamorphism at a late Mesozoic accretionary margin: a study from the Coast Belt of the North American Cordillera

Abstract

The late Mesozoic and Cenozoic metamorphic evolution of the western North American continental margin is recorded in a belt of homogeneous metapelitic rocks, the Kluane metamorphic assemblage (KMA), in the northern Coast Belt of Yukon Territory. A record of Late Cretaceous medium‐pressure and ‐temperature (c. 7 kbar, 500 °C) metamorphism, M1, is preserved in Ca‐rich garnet and Na‐rich plagioclase cores in rocks that were little affected by later events. M1 was synchronous with mylonitization and is attributed to accretion of the KMA to the ancient continental margin. Isothermal decompression during rapid uplift was followed by early Eocene emplacement of the Ruby Range Batholith (RRB), part of a magmatic arc produced by subduction of the Kula plate. The intrusion of the RRB led to a contact metamorphic overprint, M2, producing a 5–6 km wide aureole in which the grade ranges from subgarnet zone to garnet–cordierite–K‐feldspar zone. Pressure and temperature estimates for M2, calculated from mineral equilibria, are 3.5–4.5 kbar and 530–720 °C, generally consistent with the stability limits of the observed mineral assemblages. Comparison of mineral assemblages and P–T conditions in the KMA with those in the Mclaren Glacier metamorphic belt in Alaska does not support the correlation of the two metamorphic sequences. This weakens the hypothesis proposing 400 km of dextral slip along the Denali fault zone.

Thermotectonic History of the Kluane Ranges and Evolution of the Eastern Denali Fault Zone in Southwestern Yukon, Canada

“…(c) Chronology of local and regional geologic events. References for timeline are the following: 1 Miller et al (); 2 Cobbett et al (); 3 Ingram and Hutton (), Gehrels et al (), and Lowey (); 4 Eisbacher (), Cole et al (), Miller et al (), and Riccio et al (); 5 Erdmer and Mortensen (), Mezger et al (), and Israel et al (); 6,15,16 Eisbacher and Hopkins (), Ridgway et al (), and Ridgway and Sweet (); 7 Nokleberg et al (), Rubin et al (), and Dusel‐Bacon et al (); 8 Haeussler et al (); 9 Falkowski et al (); 10 Finzel et al (); 11,12,13,14 Dodds and Campbell (); 17 Denton et al ().…”

“…Rapid cooling from ~70–45 Ma within the Intermontane terranes and northeast of the EDFZ, both directly adjacent to and ~ 100 km southeast of our study area, is recorded by low‐temperature thermochronometry data (Figures and a; Enkelmann et al, ; Falkowski & Enkelmann, ). Exhumation of the Kluane Schist also occurred post 55‐Ma (Mezger et al, ). Spatial migration of contractional deformation from the Kluane Ranges to the adjacent Intermontane terranes in Phase II may be due to strength contrasts facilitated by arc magmatism in the Intermontane terranes and a lack thereof in our study area (Figures c and b; Enkelmann et al, ; Ingram & Hutton, ).…”

“…The present-day EDFZ occupies the ancient suture zone between the Insular and Intermontane terranes, which has been the locus of long-lived erosion and tectonism (Nokleberg et al, 1985;Ridgway et al, 2002). In southwestern Yukon, Jurassic-Late Cretaceous back-arc basin flysch and metamorphic rocks of the Dezadeash Formation and Kluane Schist outcrop as a NW-SE trending,~225 km belt that parallels, and is cut by, the EDFZ (Figure 7; Eisbacher, 1976;Erdmer & Mortensen, 1993;Mezger et al, 2001;Israel et al, 2011). The timing of peak amphibolite facies metamorphism of the Kluane Schist is~80-70 Ma and interpreted to record accretion of the Insular and Intermontane terranes (Figure 9c; Erdmer & Mortensen, 1993;Mezger et al, 2001;Israel et al, 2005;Israel et al, 2011).…”

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

“…Data include andalusite-bearing (Likhanov et al, 2001(Likhanov et al, , 2004, sillimanite-bearing (Brommer, 1998;Hansen et al, 1987;Knudsen, 1996;Qi et al, 2004;Schmid & Wood, 1976;St-Onge, 1984;Viruete et al, 2000;You et al, 1993) and kyanite-bearing (Engi et al, 1995;Hauzenberger et al, 2004;Perkins, 1991;Schmidt et al, 2011;St€ aubli, 1989) metapelites; and (c) P-T conditions of CaOdeficient metapelites determined by the GB geothermometer (Holdaway, 2000) in concert with the present GBAQ geobarometer. Data are the same as in (b); (d) P-T conditions of thermal contact aureoles (Barboza & Bergantz, 2000;Homam, 2005;Mezger et al, 2001) determined by the GB-GBAQ geothermobarometers; and (e) P-T conditions of regional thermal contact aureoles (Holdaway et al, 1988) determined by the GB-GBAQ geothermobarometers. Al 2 SiO 5 phase diagram: the sold lines are from Holdaway and Mukhopadhyay (1993), whereas the dashed line is from Pattison (1992).…”

Crustal architecture of SW Yukon, northern Cordillera: Implications for crustal growth in a convergent margin orogen

“…The Kluane metamorphic assemblage has escaped terrane designation [ Mezger et al , 2001a], while the Dezadeash Group to the southwest has been included in the Gravina‐Nutzotin terrane, an Upper Jurassic‐Lower Cretaceous arc‐marginal volcano‐sedimentary sequence [ Rubin and Saleeby , 1991]. The Gravina‐Nutzotin rocks were deposited unconformably across the Wrangellia and Alexander terranes, the two main terranes that together define the Insular Belt (Figure 1).…”