PLATE ACCRETION OF THE ORDOVICIAN BAY OF ISLANDS OPHIOLITE AND HOT SUBDUCTION/SUBCRETION DURING THE EVOLUTION OF A FORE-ARC R-TR-TR TRIPLE JUNCTION AS THE TACONIC ARC ENCROACHED UPON LAURENTIA

The early Ordovician (c. 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic Taconic arc that collided in the mid-Ordovician (c. 470 Ma). Part of whose early remnants of the subduction system occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Late Cambrian Coastal-Lush’s Bight fore-arc-arc system. The Bay of Islands ophiolite formed by the fore-arc accretion along a subsequent suprasubduction-zone fore-arc spreading center normal to the subduction zone system at a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the hot subcretion and metamorphism of MORB and OIB basalt and mafic gabbroic rocks that were part of the descending slab at about starting at ~10 kbar and 850-900°C . They were quickly attached to the base of the overlying ophiolite during progressive slab flattening. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Metamorphic protoliths during progressive oceanic slab flattening transitioned form mainly mafic oceanic lithosphere to mixtures of mature mafic and metasedimentary complexes as the margin was approached. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation sheeted dike complexes and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the lower greenschist-facies and obducted, oceanic and continental margin rocks trend WSW (close to the obduction direction).