Terra Nova, 24, 7–15, 2012
Concordance and discordance between Nd model ages and U–Pb ages is still widely misunderstood in the geological community. In the SW Grenville Province, plutonic orthogneisses often yield TDM ages significantly older than U–Pb crystallization ages, but this can be understood in a model where a series of crustal Nd isotope evolution lines departs at intervals from the depleted mantle (DM) evolution line. Seven samples with concordant U–Pb and Nd model ages have initial ε Nd on the DM line, attributed to melting of juvenile crust generated in an Elzevirian back-arc rift zone. In contrast, plutons with discordant U–Pb and Nd model ages lie near crustal evolution lines defined by suites of basement gneisses on the flanks of the rift zone. This study shows that Nd data from large suites of geographically constrained, undated grey gneiss provide a crustal evolution framework for orogen-scale interpretation of more scattered but precisely dated plutons.