HOT FOREARC IN THE ORDOVICIAN BOUNDARY MOUNTAINS, MAINE

The most extensive exposures of pre-Silurian bedrock in western Maine and southern Québec lie in the Boundary Mountains and represent part of the Early Ordovician arc whose collision with the Laurentian margin initiated the Appalachian orogenic cycle. The Boundary Mountains comprise primarily the Chain Lakes massif and the Boil Mountain Complex. Consisting partly of boninitic mafic to ultramafic rocks, the Boil Mountain Complex originated as oceanic forearc crust. The Chain Lakes massif, a quartzofeldspathic metasedimentary diatexite of Laurentian provenance, appears to have been deposited on the boninitic crust ca. 479 Ma and intruded by tonalite and tholeiitic mafic magmas of the Boil Mountain Complex ca. 477 Ma. The source for the massif could have been either a rifted microcontinent or Laurentian detritus recycled from an accretionary wedge. The Chain Lakes massif melted at ca. 469 Ma following andalusite-sillimanite facies metamorphism. Given the relationship to boninitic crust and the short time span between deposition and metamorphism, the anatexis most likely occurred while the massif was in or near a forearc setting. Typical forearcs do not have elevated geothermal gradients, and low-pressure anatexis requires a major thermal perturbation. Slab break-off, as suggested for correlative events in Newfoundland, or ridge subduction could generate the hot forearc.