METAMORPHIC EVOLUTION OF HP-LT TECTONITES IN THE BRUNSWICK SUBDUCTION COMPLEX, NEW BRUNSWICK AND ITS IMPLICATIONS FOR THE SALINIC OROGENY IN NEW ENGLAND

The Silurian Salinic orogeny, dynamically distinct from the Early Devonian Acadian orogeny, is still contentious in New England, although the name and hypothesis of such an event derives from an unconformity recognised by Boucot in the Central Maine belt of northern Maine during the 1960's. The Brunswick complex comprises a well preserved association of latest Ordovician-Early Silurian forearc terranes, an association that is only partially preserved elsewhere. Its metamorphic core is represented by exhumed slices of underplated backarc oceanic and continental rocks. Underplating was sequential and started at c. 447 Ma with underplating of a seamount at epidote-blueschist depth (400oC-7Kb). Mineral zonation (act-bar-gln-act) suggests an anticlockwise hairpin P-T-t path reflecting subduction initiation beneath the c. 460 Ma Fournier ophiolite and subsequent subduction-related refrigeration. Accretion of the Spruce Lake block at c. 442 Ma imbricated the overlying Fournier ophiolite and partially extruded the intervening blueschists. The ophiolite slices above the blueschists were metamorphosed to greenschist conditions (360oC-4.8 Kb). Underplating of the Spruce lake block beneath the blueschist, led to metamorphism characterised by winchite ± Na-augite (350oC-6.2 Kb) or actinolite ± pumpellyite (350oC-5.5 Kb), but zonation (Ep & Am) and microstructures suggest a clockwise P-T path, which is also shown by the structurally underlying greenschist nappes of the Tetagouche block (380oC-5.7 Kb), which were underplated between 435-430 Ma. Both blocks are characterised by late-D1 biotite or stilpnomelane porphyroblasts, which are ascribed to Salinic collision with the Gander margin at c. 430 Ma. Tectonism in the accretionary wedge is directly linked to tectonism in the Matapedia forearc and the Fredericton foredeep. Llandovery exhumation of the Fournier ophiolite and its late Llandovery reburial beneath the onlapping Matapedia forearc relates to underplating of the Tetagouche block and subduction hinge-retreat. The blueschists and Spruce Lake nappes were exhumed during the Salinic Wenlock collision, but reburied beneath a latest Silurian/Early Devonian clastic wedge shortly thereafter. The latter represents the northwards advancing Acadian orogenic wedge.