SOURCE OF THE NORTHEASTERN IDAHO BATHOLITH - ISOTOPIC EVIDENCE FOR A PALEOPROTEROZOIC TERRANE IN THE NORTHWESTERN US

The northeastern portion of the Idaho batholith (NIB) intruded Protero zoic rocks of the Belt-Purcell supergroup between 50 and 90 Ma. Whole- rode Sm-Nd isotopic analyses of batholithic rocks yield depleted mantl e model ages (T-DM) between 1.72 and 1.93 Ga and values of epsilon(Nd) between -17.7 and -21.2, similar to associated metamorphic rocks and within the range for Belt-Purcell sedimentary rocks. Premagmatic zirco ns from one sample of the NIB were analyzed individually using the SHR IMP ion microprobe and yielded a single age population at 1.74 Ga. Thi s apparently single-aged source contrasts with the range of ages repor ted for zircons from sedimentary rocks of the Belt-Purcell supergroup and suggests that the batholith was not the product of melting Belt-Pu rcell sediments, nor was it significantly contaminated with these sedi ments. The source of the batholith, however, appears to be of appropri ate age and composition to be a major contributor of sediment to the B elt basin. In addition, the near coincidence of TDM and the age derive d from premagmatic zircons in one sample suggests the source of at lea st part of the batholith was extracted largely from 1.74 Ga depleted m antle, with little or no input from older rocks. If so, this crust may represent a possible continuation of crust of similar age and charact er exposed to the north in the Canadian cordillera and to the south in Nevada, Arizona, and southeastern California.