TECTONIC HISTORY OF THE BUCHANS GROUP: EVIDENCE FOR LATE TACONIC ACCRETION OF A PERI-LAURENTIAN ARC TERRANE AND ITS REIMBRICATION DURING THE SALINIC OROGENY

The Buchans Group comprises an imbricated peri-Laurentian continental arc sequence that hosts the world-class Buchans VMS ore bodies. The overall structure of the Buchans Group is more complex than the simple Ordovician antiformal thrust stack proposed by others. Reevaluation of the tectonostratigrapy suggests it is a composite terrane comprising several tectonically juxtaposed, unrelated units with a complex structural history under sub-greenschist to upper amphibolite facies conditions. The tectonothermal history is consistent with the polyphase Ordovician to Silurian deformation observed in the surrounding rocks of the Annieopsquotch accretionary tract (AAT). Two new U/Pb zircon ages obtained for the upper part of the Buchans Group (462±3 and 465±3 Ma) support the existing stratigraphic model. However, they significantly expand the age range of volcanism and demand modification of existing tectonic models. The footwall of the Hungry Mountain Thrust, which separates the Buchans Group from Notre Dame Arc plutons, is marked by an amphibolite facies ophiolite belt that preserves a complex tectono-metamorphic history including early eclogitic, garnet-cpx mineral assemblages. Unravelling the metamorphic history of this ophiolite belt sheds light on the nature of accretionary processes in the AAT during the Middle Ordovician Taconic Orogeny. Portions of the Buchans Group lithologically resemble Silurian volcanic rocks associated with the c. 430 Ma Topsails Igneous Suite. Hence, the probable involvement of Silurian rocks during thrusting suggests that the structure of the Buchans Group results from the superposition of Taconic and Salinic south-southeast directed thrust and fold belts, consistent with the regional tectonic history.