GEOCHEMISTRY OF THE BAY OF ISLANDS METAMORPHIC SOLE: GEODYNAMIC AFFINITY AND IMPLICATIONS FOR THE SUBDUCTION INITIATION

The Bay of Islands (BOI) complex, located on the west coast of Newfoundland, Canada, represents an ideal example of a fossil subduction system. The BOI complex is composed of oceanic lithospheric mantle and crust underlain by a metamorphic sole that were obducted onto the Laurentian margin during the mid-Ordovician Taconic orogeny. Metamorphic soles are metamorphosed slivers of oceanic crust that represent the first material entering the subduction channel at subduction infancy. Previous work demonstrated that most trace elements (e.g. REE and HFSE) are not susceptible to remobilization during metamorphism. As a result, trace element concentrations are inherited from the protolith and can be used as proxies to determine its magmatic history and geodynamic affinity. This study aims to characterize the protolith of the BOI metamorphic sole in terms of lithologies and geodynamic affinities by using immobile element fingerprinting. The timing of exhumation of the sole is investigated with in-situ U-Pb ages on titanite to further constrain the setting of subduction initiation.

The BOI metamorphic sole is composed of mafic granulites, garnet-clinopyroxene amphibolites, common amphibolites, and metasediments. Based on petrology and geochemistry, gabbroic and basaltic protolith can be distinguished within the metamorphic sole of mafic composition. Those metabasites represent the former upper oceanic crust at the subduction nucleation site. The rare earth elements (REEs) contents of the basaltic rocks indicate a predominantly N-MORB affinity suggesting that the subduction zone associated to the BOI complex possibly nucleated at an oceanic ridge or at an oceanic transform.