Nd and Sr isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains Batholith, Alaskan and Canadian Cordillera

Nd and Sr isotopic ratios are reported from 15 samples of plutons of the northern Coast Mountains Batholith (CMB), between the Alexander-Wrangellia terrane and the Stikine terrane of southeastern Alaska. Samples of plutons that are part of the Late Cretaceous-Eocene CMB suite have a range in initial ε_Nd of -3.0 to -0.2 and ⁸⁷Sr/⁸⁶Sr of 0.70494-0.70607. Two plutons that are probably older than the bulk of the CMB plutons have present-day ε_Nd values of -6.8 and -2.6. The Late Cretaceous-Eocene plutons have Nd depleted-mantle model ages (t_DM) of 620-1070 Ma. These data indicate that the northern CMB must contain a significant component of old, evolved continental crust. The mid to Late Proterozoic t_DM ages of the CMB plutons are probably a result of a mixture of Early Proterozoic crustal material with younger, juvenile crust. -from Authors