THE PROVENANCE AND TECTONIC HISTORY OF DASHWOODS

In the northern Appalachians, the composite Dashwoods terrane forms the basement to the peri-Laurentian Early Ordovician to Silurian Notre Dame arc. The southern Dashwoods is intruded by the Early Ordovician to Late Silurian arc and slab breakoff plutons and was subjected to polyphase deformation and high-grade metamorphism during the Taconic orogeny with a moderate Salinic overprint. The crystalline basement of Dashwoods is not exposed, as such U-Pb zircon provenance of the pre-Middle Ordovician paragneiss is investigated herein to constrain the provenance of Dashwoods and paleogeography of the Laurentian margin. SHRIMP analysis yielded low Th/U ratio metamorphic zircon rims ranging from ca. 395 to 500 Ma. These dates are consistent with existing constraints on the Taconic and Salinic metamorphism. The sample yielded abundant detrital zircon grain cores ranging from ca. 546 to 1853 Ma (YSG 563±14 Ma; YGC 926±14 Ma). Presence of abundant Tonian dates differentiates Dashwoods from the adjacent Humber Margin in Newfoundland, and Hebridean and Grampian terranes in the British Isles. Detrital provenance of Dashwoods is most similar to the Baie Verte margin in Newfoundland, and Tyrone Complex and Dalradian Supergroup in Ireland. Although it has been recently suggested that Dashwoods is a peri-Gondwanan terrane, the similarity in U-Pb zircon provenance, combined with other geological evidence such as evidence of Ediacaran hyperextension, support the origin of Dashwoods and Baie Verte margin near the Laurentian Rockall promontory. Ordovician to Carboniferous dextral displacement along the Baie Verte – Brompton Line and Cabot Fault resulted in significant translation of Dashwoods and the associated, allochthonous Baie Verte margin along the Laurentian margin, leading to their eventual emplacement outboard of the autochthonous Humber margin.