CONTRASTS BETWEEN SM-ND WHOLE-ROCK AND U-PB ZIRCON SYSTEMATICS IN THETOBACCO ROOT BATHOLITH, MONTANA - IMPLICATIONS FOR THE DETERMINATION OF CRUSTAL AGE PROVINCES

Proper documentation of the extent and age of crust in the western US is critical for constraining a variety of geologic problems ranging fr om the growth rate of continents to Precambrian continental reconstruc tions. 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 Id aho a group of Late Mesozoic to Cenozoic, dioritic to quartz monzoniti c 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 (E(Nd) = -17.9 to -19.1) and Sm-Nd model age (T-DM = 1.63 to 1.90 Ga) typica l of this group. The TRB, however, intruded Archean crust (similar to 3.3 Ga, epsilon(Nd) = similar to -35), rather than the presumed Proter ozoic crust intruded by the other plutons. The Archean heritage of the TRB is confirmed by the presence of premagmatic zircons which range f rom 2.2 to 3.0 Ga. The combination of U-Pb zircon and Nd model ages su ggest that the batholith was derived from both Archean and Proterozoic crustal sources, as well as an similar to 80 Ma mantle component. Thi s contrasts with a sample from the northern Idaho batholith which exhi bits concordancy between its Sm-Nd and premagmatic zircon systems at s imilar to 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 northwester n US where Archean and Proterozoic crust is poorly exposed and dispers ed over a large area.