PB ISOTOPIC VARIATION OF CRETACEOUS ARC CRUST IN THE PACIFIC NORTHWEST, U.S.A. AND CANADA

The Pb isotopic composition of detrital K-feldspar can serve as a useful indicator of sedimentary provenance when expressed as ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb values measurable by LA-ICPMS methods. In order to assess the provenance of the Late Cretaceous Nanaimo Group on Vancouver Island we have constructed a database of Pb isotopic data from potential basement source regions in the Pacific Northwest (southern Coast Mountains, Northern Cascades, Okanogan batholith, and Idaho/Boulder batholith). This database includes new results from 63 basement K-feldspar samples, as well as previously reported results from 84 basement K-feldspar samples and 166 age-corrected whole rock basement samples. Samples from the Coast Plutonic Complex and Northern Cascades show a relatively limited range of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb values, 0.81–0.84 and 2.02–2.05, respectively. The Okanogan batholith of eastern Washington and western Idaho yield a slightly expanded range of ²⁰⁷Pb/²⁰⁶Pb values (0.80-0.85) and ²⁰⁸Pb/²⁰⁶Pb values that plot 0.02 units higher. In contrast, Pb isotopic values for the Idaho batholith that is underlain by evolved continental crust define a significantly wider range of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb from 0.79 to 0.88 and 1.99 to 2.16, respectively. Boulder batholith K-feldspar Pb values define a more limited range with ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb from 0.86 to 0.92 and 2.11 to 2.24. We have compared our database results with Pb isotope values measured from two sandstone samples of the Nanaimo Group in British Columbia. Samples from stratigraphically lower Nanaimo Group formations overlap most closely with Pb isotope values for the more proximal Coast Plutonic batholith and Northern Cascades. In contrast, samples from stratigraphically higher Nanaimo Group formations reflected a wide range of Pb isotope values more in line with the more extraregional Okanogan and Idaho batholiths. This local to extraregional transition is in excellent agreement with ⁴⁰Ar/³⁹Ar results from detrital K-feldspar and U-Pb ages from coexisting detrital zircon.