Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera: Ancient crust bounded by juvenile terranes

Abstract

Nd isotopic data are reported for 52 samples from the crustal region between the Alexander‐Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic‐Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, a terrane of probable Early Permian to Late Triassic age, and four assemblages of metamorphic rocks that occur to the west of and within the Coast Mountains batholith. The Gravina belt has εNd(T) values that range from −1.1 to +8.3, similar to values of the underlying Alexander terrane, and consistent with the interpretation that it is a juvenile belt that formed in a back‐arc or intra‐arc basin within the Alexander terrane. Mid‐Cretaceous plutons that were emplaced into the Gravina belt have εNd(T) values of +4.4 to +5.7 and were probably produced by mantle‐derived melts that incorporated some Alexander terrane crust. The Taku terrane has εNd(0) values that range from −5.5 to +3.3, with corresponding depleted‐mantle model (TDM) ages of 440 to 1430 Ma. A mid‐Cretaceous pluton intruding the Taku terrane has an εNd(T) value of +5.1, a value indistinguishable from those determined for Cretaceous plutons intruding the Gravina belt. Metamorphic rocks east of and structurally overlying the Taku terrane are divided into the Tracy Arm assemblage, εNd(0)=−26 to 0, TDM=800–2450 Ma; the Endicott Arm assemblage, eNd(0)=−10 to −1.3, TDM=950–1500 Ma; the Port Houghton assemblage, εNd(0)=−9.4 to +1.1, TDM = 550–1500 Ma; and the Ruth assemblage, εNd(0) = −9.4 to +2.0, TDM=650–1300 Ma. These isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. The metamorphic rocks from these assemblages are lithologically very similar to rocks of the Yukon‐Tanana (YTT) terrane of eastern Alaska and Yukon Territory and have such similar U‐Pb detrital zircon ages and Nd isotopic compositions to YTT rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander‐Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander‐Wrangellia terrane.