BONINITES IN NE APPALACHIAN NOTRE-DAME SUBZONE OPHIOLITES: TECTONIC MODELS FOR IAPETUS TERRANES

Boninites are high-Mg andesites found in forearcs, typically forming the basement to arc volcanos, and are common in NE Appalachians ophiolites. Tectonic models for boninite genesis include subduction initiation, and slowed convergence and slab rollback leading to arc splitting and/or forearc extension and spreading. The common thread is rapid extension of the overriding plate and upwelling of the mantle leading to volatile-fluxed (<1% fertile arc component) decompression melting of depleted mantle rocks (~20% prior melt extraction). Boninites occur in some old accreted terranes (Mt Chagnon, Orford, 504Ma, Qu&eacute;bec; Hawley Fm., 496 Ma, Massachusetts), but are absent from the arc-related Coastal Complex (505 Ma, Newfoundland) and(?) the Belvidere Mtn. Ophiolite (505Ma, Vt.). Thus, several arc-related terranes developed along the continental margin of Laurentia only 45-85 Ma after the opening of Iapetus (590-550Ma). We infer that the oldest boninites were associated with Taconian subduction initiation, the "infant"arc model. Boninites are also common in an extensive group of younger ophiolites including: Betts Cove (BC, 489 Ma), Bay of Islands (BOI, ~484 Ma), and Thetford Mines (TM, 480-478 Ma). In the BOI, boninite sills underplate previously solidified gabbros with mature backarc basin basalt signatures. BC and TM have cumulates derived from boninites, and lavas and sheeted dyke complexes of boninitic composition, indicating seafloor-spreading in a SSZ-forearc context. BC boninite lavas are overlain by undepleted tholeiitic lavas, feeder sills, and debris flows that alternate (500m scale) with calc-alkaline andesitic to rhyolitic pyroclastic deposits, lavas and turbidites. Alternation of proximal tholeiitic/calc-alkaline deposits implies vertical zonation of the mantle; and suggests that undepleted mantle welled up in response to extension and slab rollback as a new mature arc volcano developed. The TM ophiolite experienced intra-oceanic detachment (477Ma) and was emplaced against continental rocks (~470 Ma) immediately after formation. Boninites in these younger ophiolites were produced in a major period of SSZ extension in re-entrants, possibly linked to docking of Taconian arcs along Laurentian promontories.