Evidence for a ridge subduction event in the Ordovician rocks of north-central Maine

Modern and past ridge subduction events are characterized by the intrusion of midocean-ridge basalt (MORB) magmas into an overlying accretionary prism. The field relationships and trace-element geochemistry of Ordovician matic igneous rocks of the Weeksboro-Lunksoos and Munsungun anti-clinoria of north-central Maine indicate that they resulted from such an event. The Bean Brook gabbro intrudes the Hurricane Mountain melange and other related sedimentary strata of continental derivation. The gabbro and associated Dry Way volcanics have MORB trace-element chemistries, while the Bluffer Pond and Stacyville volcanics are more enriched (E-MORB), all of which indicate derivation from a mid-ocean ridge. On petrogenetic diagrams, mafic samples plot in MORB fields, or when Th is used as a discriminator, in arc fields along trends that originate from MORB fields and extend toward the composition of upper continental crust. These trends are consistent with the presence of silicic and metasedimentary xenoliths in the Dry Way volcanics and Bean Brook gabbro, and indicate the magmas were not subduction products, but were contaminated by Th-rich upper continental crust. The nearby Chain Lakes Massif likely represents the basement to the Chain Lakes microcontinent, and the geographic relationship and ages of the massif, Hurricane Mountain, melange, and Dry Way-Bean Brook magmatic rocks indicate northwest-directed subduction (modern coordinates) on the southeastern margin of the Chain Lakes microcontinent, within the Early to Middle Ordovician Taconic ocean. Subduction at this boundary probably terminated because of the ridge subduction, analogous to Neogene California.