PETROGRAPHIC, GEOCHEMICAL, AND ISOTOPIC CONSTRAINTS ON THE PROVENANCEOF THE EARLY PROTEROZOIC CHELMSFORD FORMATION, SUDBURY BASIN, ONTARIO

The early Proterozoic Chelmsford Formation, a coarse-grained turbidite unit in the Sudbury Basin, was deposited near the margin of the Super ior Province craton at about the time of Penokean orogenesis. Petrogra phy, geochemistry, and Nd and Pb isotopes are consistent with derivati on of the Chelmsford Formation from late Archean Superior Province roc ks. The framework mineralogy of medium sandstones is dominated by quar tz, plagioclase, and minor K-feldspar, consistent with derivation from plutonic rocks. The abundance of matrix (up to 40%), however, allows additional sources. The behavior of Al2O3 relative to Na2O, CaO, and K 2O constrains the source to have had, on average, a minor weathering h istory. Slightly elevated SiO2 and low Zr, relative to average plutoni c rocks, suggest that there was no more than a minor recycled sediment ary component in the source. High abundances and sympathetic variation s in ferromagnesians indicate a mafic component. REE and whole-rock Nd and Pb isotopes constrain the Chelmsford Formation to have been deriv ed from late Archean, LREE-enriched rocks without significant Eu anoma lies, and allow neither a significant component of Penokean mantle-der ived material nor early Archean crust. Pb isotopes in feldspar populat ions require a Superior Province source. Although the age, location, a nd facies of the Chelmsford Formation are consistent with deposition i n response to early Proterozoic active margin processes, the integrate d provenance data require the sediment source to have been dominated b y relatively fresh Superior Province rocks associated with basement up lift.