A Cretaceous back-arc basin in the Coast Belt of the northern Canadian Cordillera: evidence from geochemical and neodymium isotope characteristics ofthe Kluane metamorphic assemblage, southwest Yukon

The Coast Belt of the northern Cordillera in Canada is the locus of the bou ndary between accreted and ancient North American margin rocks. The largest exposure of metasedimentary rocks in the Coast Belt is the Kluane metamorp hic assemblage (KMA), a northwest-striking belt 160 km long of graphitic mi ca-quartz schist and gneiss with minor interfoliated olivine serpentinite. The KMA does not appear to correlate with other sedimentary or metamorphic rock assemblages in the Canadian Cordillera. To determine its tectonic sett ing and protolith provenance, we analyzed trace element, rare earth element s, and neodymium isotope compositions of the KMA, of the adjacent pericrato nic Aishihik metamorphic suite (AMS) of the Yukon-Tanana terrane, and of ad jacent slates of the Dezadeash Formation (DF), filling a Late Jurassic - Ea rly Cretaceous flysch basin. The epsilon Nd(0) values of analyzed KMA sampl es range from -1.4 to -5.6 and depleted mantle model ages (T-DM) range from 1.16 to 1.45 Ga. KMA samples are intermediate between more evolved AMS sam ples (average epsilon Nd(0) -25, T-DM = 2.6 Ga) and more juvenile DF sample s (epsilon Nd(0) = +1.9, T-DM = 0.95 Ga). The intermediate characteristics of the KMA samples cannot be linked to a known source region and are interp reted to reflect homogeneous mixing from predominantly juvenile and minor e volved sedimentary sources. A compatible tectonic setting is a back-arc bas in within influence of a continental source. Eastward subduction of the KMA beneath ancient North America collapsed the back-arc basin by latest Creta ceous time.