Nd isotope analyses are presented for granitoid rocks from the western part of Frontenac Terrane in the Grenville Province of Ontario. The depleted mantle model (TDM) ages show no correlation with the silica content of the rocks, but instead correlate with geographical location, suggesting that the TDM ages are indicative of regional crustal formation age and do not result from mixing between sources with different provenance ages. Based on these observations, we identify a new crustal age boundary that follows the Desert Lake – Canoe Lake fault and the Rideau Lake fault and, hence, a new juvenile crustal block (Westport domain). This domain is identified as part of the ensimatic back-arc rift zone that formed the juvenile segment of the Central Metasedimentary Belt in Ontario. However, additional sampling along the Ottawa River suggests that the juvenile Westport domain does not extend into Quebec. Instead, a narrower ensialic rift zone is represented by the Marble domain in Quebec. Based on comparison with the Taupo volcanic zone and the northern Red Sea as modern analogues, we suggest that the transition from a wide ensimatic rift zone in Ontario to a narrow ensialic rift in Quebec was accommodated by transtensional motion along a zone of diffuse shear east of Ottawa.

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