Contrasts between SmNd whole-rock and UPb zircon systematics in the Tobacco Root batholith, Montana: implications for the determination of crustal age provinces

Abstract

Proper documentation of the extent and age of crust in the western US is critical for constraining a variety of geologic problems ranging from the growth rate of continents to Precambrian continental reconstructions. The secondary isotopic systematics of granitoids have been one of the principal means used to characterize continental crust in areas where the basement is covered. In southwestern Montana and eastern Idaho a group of Late Mesozoic to Cenozoic, dioritic to quartz monzonitic batholiths (e.g., Tobacco Root, Idaho, Pioneer, Boulder, etc.) share a limited range of Paleoproterozoic Sm-Nd depleted mantle model ages. The Tobacco Root batholith (TRB) has a Nd isotopic composition ( ϵ Nd = −17.9 to −19.1) and SmNd model age ( T DM = 1.63 to 1.90 Ga) typical of this group. The TRB, however, intruded Archean crust (∼3.3 Ga, ϵ Nd = ∼ −35), rather than the presumed Proterozoic crust intruded by the other plutons. The Archean heritage of the TRB is confirmed by the presence of premagmatic zircons which range from 2.2 to 3.0 Ga. The combination of U-Pb zircon and Nd model ages suggest that the batholith was derived from both Archean and Proterozoic crustal sources, as well as an ∼80 Ma mantle component. This contrasts with a sample from the northern Idaho batholith which exhibits concordancy between its Sm-Nd and premagmatic zircon systems at ∼1.74 Ga. These data point to the difficulties that can occur if crustal age provinces are defined solely on the basis of Nd model ages of younger plutons, particularly in areas such as the northwestern US where Archean and Proterozoic crust is poorly exposed and dispersed over a large area.