Pb isotope constraints on the provenance and diagenesis of detrital feldspars from the Sudbury Basin, Canada

Residues after HF leaching of detrital feldspar grains from the Early Proterozoic Chelmsford Formation of the Sudbury Basin, Ontario record isotopic compositions very close to the initial Pb isotopic composition of their Late Archean, Superior Province source. These grains have been recrystallized to albite and K-feldspar. Residues of Chelmsford feldspar grains lie within the range of reported Pb-206/Pb-204 composition of feldspars from plutons of the adjacent Abitibi subprovince of the Superior Province, with a steep Pb-207/Pb-206 slope of 0.35 +/- 0.15 (3.7 +/- 0.7 Ga). Leaches are more radiogenic than residues, but leach-residue pairs do not record in situ U (and Th) decay since either the Late Archean time of feldspar crystallization in the Superior Province or the Early Proterozoic time of recrystallization in the Chelmsford Formation. Instead, leach-residue pairs lie along an average Pb-207/Pb-206 slope of 0.250 +/- 0.036, representing mixing trends. Most whole-rock samples from the Chelmsford Formation lie close to a 2.7 Ga dope, collinear with detrital feldspar compositions. We interpret the Pb data from the leaches as recording the passing of altering fluids. These fluids likely carried Pb derived from rocks or minerals with relatively high U/Pb, Th/Pb and Th/U within the sedimentary basin itself. The residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the Superior Province.