TIMING OF ORDOVICIAN VOLCANISM IN NORTHEASTERN MAINE: RETHINKING REGIONAL TECTONIC MODELS

The first plate tectonic era bedrock mapping of three Cambro-Ordovician belts in northeastern Maine and recent geochronological and geochemical analyses offer new information about the closing of Iapetus and collision of Laurentia and Ganderia. The Munsungun-Winterville (MW) and Weeksboro-Lunksoos Lake (WL) belts are particularly valuable as they provide details about the Ganderia-Laurentia suture zone for which there is almost no information in New Brunswick.

• The MW belt, long considered the leading edge of Ganderia, is, instead, peri-Laurentian. The suture, the Red Indian Line or Mckwe’jit Line, is buried beneath Silurian and Devonian cover rocks that separate MW and WL.
• Coeval arc volcanism (485-460) in MW, WL, and the Miramichi (MI) belt is inconsistent with models of a northwestward-migrating single arc. We propose that there were two simultaneously active arcs above subduction zones of opposite polarity – NW- directed subduction beneath a MW arc and SE-directed subduction beneath an arc comprising the WL and MI belts.

The new data also reveal details of orogen-parallel asynchronous variations within the belts that are separated by major faults.

• MI: Middle to Late Ordovician Tetagouche Group back-arc volcanic rocks (470-455Ma) in northern New Brunswick are separated by the Woodstock-Catamaran fault from Early Ordovician (~480 Ma) Meductic Group arc rocks to the southwest. These are in turn isolated by plutons from mostly younger (-474-465 Ma) MI arc rocks in Maine.
• WL: The Cambro-Ordovician WL, containing 481-485 Ma back-arc and arc volcanic rocks, is separated by a NW-striking fault from the newly recognized Number Nine Mountain terrane. This belt includes 470 Ma arc volcanic rocks; 433 Ma (Salinic) volcanic rocks; and a hitherto unrecognized Silurian mélange.
• MW: Two episodes of arc volcanism are recognized in the Munsungun segment of this belt. An older event (457-454 Ma) restricted to its southeastern area, and a younger event (450-455 Ma) confined to the northwest, possibly indicating advancing NW-directed subduction