A new concordia age for the 'forearc' Bay of Islands Ophiolite Complex, Western Newfoundland utilizing spatially-resolved LA-ICP-MS U-Pb analyses of zircon

Abstract

Advancements of zircon U-Pb dating techniques using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide an opportunity to precisely constrain the age of the Bay of Islands Ophiolite Complex (BOIC) in the Newfoundland Appalachians. New LA-ICP-MS U-Pb dates are presented for a large population of relatively simple magmatic zircon grains from trondhjemites using selective domain analyses and a 3% discordance filter in five samples collected from a single ophiolite massif, with four of the samples from a single pluton. All samples were derived from plagiogranite plutons intruded just below the gabbro-sheeted dike contact of the Blow Me Down Mountain massif, the massif sampled for three of the four prior single sample ID-TIMS age studies of the ophiolite. A highly consistent series of LA-ICP-MS concordant ages from the five samples collected yield a composite concordia age of 488.3 ± 1.5 Ma, which we propose for the revised igneous age of the ophiolite massif. This new age for the BOIC is based on 127 individual LA-ICP-MS zircon analyses and is slightly older and with lower uncertainty than the previous consensus age. We review previous pioneering ID-TIMS U-Pb dates, ages, intercept assumptions, and uncertainties for the BOIC in light of this new formation age. Legacy ages and uncertainties differ somewhat when data are reduced with modern recommended techniques and without geological interpretations that fix lower intercepts. The ca. 488.3 Ma BOIC age indicates its formation along a younger supra-subduction forearc trench-orthogonal spreading center that rifted the older Coastal Complex forearc ophiolitic assemblages reported to be formed at ca. 500-508 Ma. This rifted forearc is analogous to many modern ophiolitic forearcs. The BOIC age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites, indicating temporal BOIC spreading center linkages within the Baie Verte Oceanic Tract.