Detrital Zircon Provenance of Cambro-Ordovician Inliers in the Northern Maine Appalachians: Relocating the Laurentia–Ganderia Boundary in an Accretionary Orogen

The boundary between peri-Laurentian terranes and those derived from Gondwana or Baltica, the Mekwe’jit Line in Newfoundland, is a significant tectonic feature in the Northern Appalachians. In northern Maine, this boundary has traditionally been placed northwest of a group of Ordovician inliers, long interpreted as the leading edge of Gondwana-derived Ganderia. Mélange and slate of the Chase Brook Formation serve as basement to the Munsungun and related inliers and are unconformably overlain by Darriwilian to early Katian (469–452 Ma) arc and rifted-arc volcanic successions. Detrital zircon spectra from three sandstone samples from the Chase Brook Formation demonstrate clear Laurentian provenance, comparable to the peri-Laurentian Chain Lakes massif to the southwest. New conodont data confirm a primarily Early Ordovician age for the Chase Brook Formation. Together, these data indicate a peri-Laurentian affinity for the Munsungun and Winterville inliers. Conversely, a detrital zircon spectrum from the Cambrian Grand Pitch Formation of the Weeksboro–Lunksoos Lake inlier, farther southeast, confirms its Gondwanan affinity and close relationship to the Ganderian Miramichi inlier in Maine and New Brunswick. Similarly, detrital zircon from the Mars Hill conglomerate in the recently recognized Mars Hill inlier also exhibits a Ganderian distribution, though its provenance more closely resembles the Massabesic Gneiss, a unit farther south in the orogen. These results imply a relocation of the Mekwe’jit Line south of the Munsungun inlier, substantially enlarging the Laurentian contribution to the Northern Appalachian orogen. This reinterpretation is also consistent with previous paleomagnetic interpretations that placed the Munsungun and Winterville inliers close to the Laurentian margin during Middle to Late Ordovician time.