Nd isotopic and trace element constraints on the source of Silurian–Devonian mafic lavas in the Chaleur Bay Synclinorium of New Brunswick (Canada): Tectonic implications

• Upper Silurian–Lower Devonian basalts are rift-related continental tholeiites.
• Rifting was of limited extent.
• Continental tholeiites derived from Neoproterozoic subcontinental lithospheric mantle (SCLM).
• SCLM was common to both Avalonia and Ganderia.

Upper Silurian to Lower Devonian volcanic rocks of the Gander Zone, from the northern mainland Appalachians of northern New Brunswick, occur in the Chaleur Bay Synclinorium which forms the southeastern part of the Middle Paleozoic Matapedia cover sequence. These rocks, which are parts of shallow marine to subaerial sequences (Dalhousie, Dickie Cove and Tobique groups), were erupted in a continental rift environment between ca. 422 and 407 Ma. The volcanic rocks are mostly bimodal with mafic types corresponding predominantly to continental tholeiites inferred to be generated by partial melting of subcontinental lithospheric mantle (SCLM). The mafic rocks of all three groups have age-corrected ɛNd values ranging from + 3.4 to + 5.3, and depleted model mantle ages ranging from 0.65 to 0.95 Ma that are interpreted to represent mantle enrichment ages associated with ancient Neoproterozoic subduction. This Neoproterozoic SCLM shows no contributions from juvenile Silurian mantle, suggesting that rifting was of limited extent and did not result in the replacement of the old SCLM by upwelling juvenile asthenosphere beneath the rift. These Nd isotopic data do not support the generation of the volcanic rocks by slab break-off, which would have likely introduced a juvenile asthenospheric mantle source for some of the Silurian–Devonian basaltic rocks. The ranges in ɛNd values and depleted mantle ages in northern New Brunswick are similar to those recorded in penecontemporaneous mafic lavas in Avalonia suggesting that the Neoproterozoic SCLM was common to both Avalonia and Ganderia.