The Geochronology And Geochemistry Of Plagiogranites And Metamorphic Sole Rocks From The Forearc Bay Of Islands Complex And Coastal Complex Ophiolites And Arc Twillingate Granite, Newfoundland

LA-ICP-MS U-Pb zircon analyses for five cogenetic plagiogranites from the Northern Appalachian Bay of Islands Ophiolite Complex (BOIC) yield comparable ages from two nearby plutons allowing a new composite age of 488.3 ± 1.5 Ma. This age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites indicating they formed along a contiguous set of supra-subduction zone spreading centers at high angles to the westward (present-day reference frame) verging trench. The spreading centers were offset by left-lateral offset transform faults that rifted an older Coastal Complex (CC)-Lushs Bight forearc basement. Five plagiogranite plutons in the CC yield a set of older composite ages ranging from 514.3-502.7 Ma indicating prolonged forearc magmatism. A new composite age of 504.3 ± 1.8 Ma from three co-genetic samples from the Twillingate arc batholith overlaps youngest CC forearc plagiogranite magmatism and establishes the oldest age of voluminous silicic magmatism in the juvenile Notre Dame arc. It also provides a minimum age for the intruded arc tholeiitic and boninitic ophiolitic basement of Sleepy Cove volcanics, a correlative of undated Lushs Bight volcanics and sheeted dikes terrane to the west. Metamorphic sole subcreted to the base of BOIC ophiolite upon subduction initiation was examined to establish a high-temperature metamorphism age. A prior 40Ar-39Ar amphibole age of 469 Ma is thought to be incorrect, based on ~20 Ma of cooling of the 488.3 Ma BOIC, which would be incapable of producing sole granulites and amphibolites. A new LA-ICP-MS zircon rim metamorphic age of 491.8 ± 6.0 Ma from a sample immediately below the ophiolite-sole contact indicates the sole formed near-simultaneously with overlying BOIC ophiolite. Petrogenetic modeling of the BOIC plagiogranites indicates they are part of the tholeiitic magma series and derived by fractionation of primitive MORB-like BOIC forearc basalts within a SSZ forearc. They result from, in part, deep-crustal fractional crystallization of primitive magmas, followed by post-cumulate equilibrium crystallization, migration, and reaction with mid-crustal gabbroic crystal-rich mushes, and final segregation to shallow plutons just below the sheeted dike that accounts for significant variability in Al2O3 and their trace element enrichments.