Neodymium and strontium isotopic characterization of the Wrangellia, Alexander, Stikine, Taku and Yukon crystalline terranes of the Canadian Cordillera.

Abstract

Nd and Sr isotopic data are reported from samples of the Alexander, Wrangellia, and Stikine terranes of the Canadian Cordillera. Initial ε(Nd) values range from 0 to +9.5, for Alexander, -0.5 to +6.7 for Stikine, and +1 to +7.3 for Wrangellia. Initial ⁸⁷Sr/⁸⁶Sr ratios for igneous samples of the terranes range from 0.70276-0.70663 (Alexander), 0.70369-0.70531 (Stikine), and 0.70323-0.70481 (Wrangellia). These data indicate that the terranes are isotopically very juvenile and probably resided in intra-oceanic environments until their accretion to North America. The origin of the terranes is envisioned to have occurred via island-arc volcanic processes, with accompanying erosion, sedimentary recycling, metamorphism and plutonism, with the critical characteristic that none of these processes involved more than trivial amounts of old, pre-existing continental crustal material. The terranes are thus exceptional when compared to many other Phanerozoic crustal regions that are composed predominantly of remobilized, pre-existing crust. Isotopic data are also presented for the Gravina belt, the Taku terrane, the Yukon Crystalline terrane (YCT), and the Coast Mountains batholith. The Gravina belt is isotopically similar to the Alexander terrane and most of the material in the belt was probably derived from Alexander. The Taku terrane is composed of juvenile and evolved crust and its relation to surrounding terranes is poorly understood. Nd data from the YCT indicate that it is partly composed of material derived from an early Proterozoic source. The proximity of the YCT to Stikine therefore raises important questions about the styles of accretion of the terranes. It is possible that the YCT was positioned next to Stikine by strike-slip faults, that there is a stratigraphic link between the terranes, or Stikine and other inboard terranes are thrust fragments that were emplaced on top of YCT. The YCT is isotopically and lithologically similar to the Yukon-Tanana terrane and may be the southern continuation of that terrane. Nd and U-Pb data for the CMB requires that it is composed of a Proterozoic crustal component and an isotopically juvenile component. It is likely that the Proterozoic component is YCT crust and the juvenile component is a combination of mantel-derived material and juvenile terrane crust. The style of crustal growth in the Cordillera, the sweeping up of juvenile terranes, may be an important analog to Precambrian crustal genesis.