Assembly of the Annieopsquotch Accretionary Tract, Newfoundland Appalachians: Age and Geodynamic Constraints from Syn‐Kinematic Intrusions

Abstract

The Annieopsquotch Accretionary Tract (AAT) comprises several ophiolites and arc-back-arc igneous complexes that were accreted to the Dashwoods microcontinent during the Ordovician Taconic orogeny. The Lloyds River Fault Zone, which separates the AAT from the Dashwoods microcontinent, yielded 40 Ar/ 39 Ar hornblende ages of ca. 470 Ma. The fault zone was intruded syn-kinematically by the shoshonitic Portage Lake monzogabbro and the Pierre's Pond suite, which gave U/Pb zircon ages of Ma plus Ma and Ma, respectively. The Otter 462 ‫ע‬ 2 464 ‫ע‬ 2 459 ‫ע‬ 3 Pond granodiorite intruded syn-kinematically into the Otter Brook Shear Zone, which separates the Annieopsquotch ophiolite belt from the structurally underlying ophiolitic Lloyds River Complex. It yielded a U/Pb zircon age of Ma. The Buchans arc and its continental basement were accreted to the Lloyds River Complex prior to 468 468 ‫ע‬ 2 Ma. Syn-kinematic plutons have arc affinity, with Nd ranging between Ϫ0.9 and Ϫ6.8, and are coeval with the adjacent Notre Dame Arc. Our data thus suggest the majority of the AAT was accreted to the Dashwoods microcontinent by 468 Ma, when consanguineous, dominantly arclike plutons intruded within the AAT and adjacent Notre Dame Arc. The Portage Lake monzogabbro and Otter Pond mafic suite are more mafic than Notre Dame Arc plutons of similar age because of their intrusion into the thin, mafic crust of the AAT and ascent along shear zones. Our data indicate the formation and subsequent accretion of ophiolites and arc-back-arc complexes occurred within a very short time span (5-10 Ma). The sources of AAT syn-orogenic magmatism are diverse and include melting of subarc mantle during slab breakoff, lithospheric mantle, and lower crust. The Ordovician Appalachian margin of Laurentia grew by the accretion of oceanic terranes and intrusion of mantle-derived magma. Recycling of continental crust by rifting and subsequent collision played an important part of the tectonic evolution of the AAT.

From Large Zones to Small Terranes to Detailed Reconstruction of an Early to Middle Ordovician Arc–Backarc System Preserved Along the Iapetus Suture Zone: A Legacy of Hank Williams

“…Partial closure of the backarc basin led to underplating of the TremadocianDapingian infant arc and backarc terranes beneath composite Laurentia between ca. 473 and 468 Ma (Lissenberg et al 2005b). This was followed by addition and imbrication of Darriwilian arc terranes during collision with the peri-Gondwanan Popelogan -Victoria arc.…”

“…On-going convergence in the Taconic Seaway places Lloyds River marginal basin under compression, initiating subduction at an old rifting scar at the Dashwoods -Iapetus oceanic crust interface. Lloyds River marginal basin terranes are underplated beneath Dashwoods, forming an accretionary tract (Lissenberg et al 2005b). C. The Floian arc (Mink Lake Formation, Gullbridge Tract) is accreted to the composite Laurentian margin accompanied by slab break-off and stitching magmatism in the Annieopsquotch accretionary tract (AAT).…”

“…Proximity to continental margins has been documented in many ophiolite belts; however, in contrast to most ophiolites, the Annieopsquotch belt was structurally underplated beneath the composite margin of Laurentia rather than obducted (Figs. 1, 2; Lissenberg et al 2005b). The changes in chemistry and isotopic characteristics along strike suggest that this supra-subduction zone spreading centre actually propagated into Laurentian continental crust farther to the north at the latitude of Moreton's Harbour Group ( Fig.…”

“…In addition, Darriwilian arc -backarc complexes escaped the pre-Darriwilian deformation history that affected the Floian arc -backarc complexes (ca. 470 Ma; Lissenberg et al 2005b). …”

Time-Transgressive Salinic and Acadian Orogenesis, Magmatism and Old Red Sandstone Sedimentation in Newfoundland

“…This thrust represents the backstop of the Annieopsquotch accretionary tract (Fig. 2) and is a fundamental structure between it and the igneous rocks of the adjacent Notre Dame arc (van der Velden et al 2004;Lissenberg et al 2005;Zagorevski et al 2009;van Staal et al 2009). The Harry's River volcanic sequence was previously included in the Ordovician Buchans Group (Thurlow and Swanson 1987).…”