Nd and Sr Isotopic Characterization of the Wrangellia Terrane and Implications for Crustal Growth of the Canadian Cordillera

Abstract

Nd and Sr isotopic data are reported from samples of most of the major lithologic units of the Wrangellia terrane exposed on Vancouver Island and the Queen Charlotte Islands. Initial $$\epsilon_{Nd}$$ values range from +1.0 to + 7.3 and initial $$^{87}Sr/^{86}Sr$$ ratios range from 0.70323 to 0.70481. These ratios are similar to those of modern island arcs and demonstrate the unevolved, juvenile nature of this terrane. These data indicate that Wrangellia probably resided in an intra-oceanic environment until its accretion to North America. Nd crustal residence ages for clastic sedimentary rocks of Wrangellia (mean $$T_{CR} = 0.77 \pm 0.33 Ga, T_{DM} = 0.57 \pm 0.35 Ga$$) are much closer to their stratigraphic ages than is the case for the ages of many other Phanerozoic sedimentary rocks, thus providing strong evidence for Phanerozoic crustal addition to North America. Early Proterozoic crustal components cannot comprise more than about 6% of Wrangellia; therefore the terrane consists of -94% new, mantle-derived crust. The amount of new crust in Wrangellia is very similar to the amount determined for two other large Cordilleran terranes, Alexander and Stikine. The accretion of juvenile terranes to a continent appears to be an important mechanism of crustal growth.