TACONIC ARC-CONTINENT COLLISION CONFIRMED IN THE NEWFOUNDLAND APPALACHIANS

The Taconic orogeny is generally considered to represent a collision between the Laurentian margin and outboard arc(s). Although evidence for penetrative Taconic tectonism is generally strong in deformed Notre Dame arc rocks, the scarcity of Taconic radiometric ages in the Humber margin rocks (down-going plate) of the Canadian Appalachians has been problematic, calling into question the intensity and nature of Taconic orogenesis by many workers and even shedding doubt on whether the Taconic orogeny was due to an arc-continent collision. The large number of available radiometric ages indicate that the predominant amphibolite facies metamorphism of the Fleur de Lys Supergroup was Salinic, not Taconic, suggesting to some that Taconic suturing was incomplete.

Our new ⁴⁰Ar/³⁹Ar ages of amphibole and micas from the Birchy Complex combined with in-situ U-Pb zircon ages of eclogite in the Fleur de Lys rocks, confirm Taconic metamorphism (477-460 Ma) in the footwall of the obducted ophiolites of the Baie Verte oceanic tract. The c. 558 Ma Birchy Complex is interpreted to have been part of the leading edge of the Humber margin. It comprises a base of Latest Neoproterozoic rift-related gabbro and basalt overlain or structurally interlayered with Palaeozoic coticule rocks and dark shales and siltstones. The latter locally contain actinolite-fuchsite clasts and single fuchsite grains, which are thought to represent the alteration products of ultramafic knockers and chromite minerals respectively that were incorporated in an Early Ordovician foredeep sequence, partially transformed into mélange during ophiolite overthrusting. In Newfoundland, the stark contrast between the intensity of the Taconic in the Notre Dame arc with respect to most of the Humber margin is probably due to a combination of several factors: 1) renewed post-Taconic tectonism, which caused emplacement of the arc and ophiolites further over the weakly metamorphosed Taconic foreland, burying most of the Taconic Humber margin rocks beneath the overriding allochthons, 2) locally strong Salinic overprint possibly guided by thermal weakening and 3) large strike-slip translations, juxtaposing weakly metamorphosed segments of the Humber margin with segments of the Notre Dame arc characterised by high grade metamorphism.