Middle Ordovician disorganized arc rifting in the peri-Laurentian Newfoundland Appalachians: implications for evolution of intra-oceanic arc systems

<p>The Annieopsquotch Accretionary Tract comprises a tectonic collage of Ordovician peri-Laurentian terranes that formed during closure of Iapetus Ocean and imbricated beneath the composite margin of Laurentia. New zircon U–Pb geochronological data (<em>n</em> = 8) for felsic volcanic rocks representing three distinct tectonostratigraphic units (the Mary March Brook group, the Buchans Group and the Red Indian Lake Group) indicate deposition over a short interval between 465 and 462 Ma. These tectonostratigraphic units are characterized by submarine mafic and felsic magmatism, and sedimentation typical of intra-oceanic arc systems. New and compiled whole-rock geochemical (<em>n</em> = 315) and Sm–Nd isotopic (<em>n</em> = 33) data for mafic and felsic volcanic rocks indicate that these tectonostratigraphic units formed in tholeiitic island arc to calc-alkaline arc settings. Isotopic (ε<sub>Nd</sub> −11.2 to +6; T<sub>DM</sub> 1.0–2.4 Ga) and zircon inheritance data (0.5–3.4 Ga) are consistent with presence of Laurentian continental crust in the basement. Relationships within and between these and adjacent tectonostratigraphic units in the Annieopsquotch Accretionary Tract suggest that they represent imbricated slices of a submarine arc that experienced disorganized arc rifting, similar to sectors of the modern Tonga–Kermadec and Izu–Bonin–Mariana arcs. The recognition of disorganized spreading in the Annieopsquotch Accretionary Tract has significant implications for along-strike correlations within the Appalachian orogen, as the nature of the arc systems is expected to differ significantly along-strike. The presence of a clear isotopic and inheritance signature of peri-Laurentian continental crust within the Annieopsquotch Accretionary Tract submarine arc system is consistent with the discoveries of older arc and continental basement in modern intra-oceanic arc systems. </p>