The stratigraphy of the external Humber zone of the western Newfoundland Appalachians records protracted Neoproterozoic–Early Cambrian rifting, followed by development of a passive margin that persisted until late Early Ordovician (Arenigian) time (ca. 475 Ma). However, adjacent metamorphic rocks, derived from the Laurentian margin and preserved in the Dashwoods subzone, were deformed, overthrust by ophiolites, and intruded by arc plutons by 488 Ma. The adjacent Notre Dame subzone also records isotopic evidence of interaction with the margin by 488 Ma. We propose that a microcontinent (Dashwoods microcontinent) was rifted from Laurentia during the Early Cambrian after an earlier, Neoproterozoic opening of the Iapetus Ocean, and was separated from the margin by a narrow oceanic tract (Humber seaway). Attempted subduction of the Dashwoods microcontinent prior to 488 Ma was followed by closure of the Humber seaway in the Taconian orogeny.