ABSTRACT

The early Proterozoic Chelmsford Formation, a coarse-grained turbidite unit in the Sudbury Basin, was deposited near the margin of the Superior Province craton at about the time of Penokean orogenesis. Petrography, geochemistry, and Nd and Pb isotopes are consistent with derivation of the Chelmsford Formation from late Archean Superior Province rocks. The framework mineralogy of medium sandstones is dominated by quartz, 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 K₂O constrains the source to have had, on average, a minor weathering history. Slightly elevated SiO₂ and low Z , relative to average plutonic rocks, suggest that there was no more than a minor recycled sedimentary component in the source. High abundances and sympathetic variations in ferromagnesians indicate a mafic component. REE and whole-rock Nd and Pb isotopes constrain the Chelmsford Formation to have been derived from late Archean, LREE,-enriched rocks without significant Eu anomalies, and allow neither a significant component of Penokean mantle-derived material nor early Archean crust. Pb isotopes in feldspar populations require a Superior Province source. Although the age, location, and facies of the Chelmsford Formation are consistent with deposition in response to early Proterozoic active margin processes, the integrated provenance data require the sediment source to have been dominated by relatively fresh Superior Province rocks associated with basement uplift.