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The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during the Middle to Late Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of the composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites (481 to 477 Ma) with organized ridges indicated by the presence of well-developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This Laurentian crust fragment then formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc (473 to 468 Ma) underwent extensive rifting indicated by organized spreading in the Lloyds River backarc basin, which was floored by juvenile backarc ophiolitic crust (472 Ma). The establishment of the Darriwilian arc (467 to 462 Ma) was in part coeval with yet another stage of rifting. Darriwilian magmatism is characterised by significant along-strike variability, ranging from continental to primitive calc-alkaline arc to tholeiitic backarc-like magmatism. The diversity of Darriwilian magmatism can be attributed to fragmentation and magmatic reworking of Laurenian-derived basement along strike in the same arc undergoing disorganized spreading. The development of the AAT is interpreted to be similar to that of the modern Izu – Bonin – Mariana arc in the western Pacific.SOMMAIRELa bande d’accretion d’Annieopsquotch (AAT) est constituee d’un empilement de chevauchements de l’Ordovicien precoce a moyen, et de terranes d’arc et d’arriere-arc qui se sont accretes a la marge composite laurentienne japetienne a l’Ordovicien moyen a tardif.  Les faits geologiques releves portent a penser que les terranes constitutifs de l’AAT se sont constitues a l’exterieur de la marge laurentienne dans un arc d’extension qui a subi de multiples episodes de rifting avant son accretion.  L’initiation du magmatisme de l’AAT a mene au developpement de zones d’ophiolites de supra-subduction du Tremadocien au Floien (481 Ma a 477 Ma), avec des cretes ordonnees mises en evidence par la presence de complexes de tapis de dikes bien developpes.  Ce centre d’extension s’est propage a travers un fragment de la croute laurentienne, et l’a ultimement separe de la marge composite laurentienne.  Et, du Floien au Darriwilien, ce fragment de croute laurentienne a servi de substratum au magmatisme d’arc de l’AAT.  Au Floien (473 Ma a 468 Ma), cette zone d’arc a subi un important rifting, comme l’indique la distension ordonnee du bassin d’arriere-arc de Lloyds River, lequel a servi de semelle a une croute ophiolitique d’arriere-arc (472 Ma).  La mise en place de l’arc au Darriwilien (467 Ma a 462 Ma) a coexiste pour un temps avec un autre episode de rifting.  Le magmatisme darriwilien est caracterise par une variabilite de composition importante parallelement a sa direction, passant d’une composition d’arc continental a celle d’arriere-arc primitif calco-alcalin jusqu’a une composition de magmatisme de type tholeiitique d’arriere-arc.  La diversite du magmatisme darriwilien peut etre attribuee a la fragmentation et au remaniement magmatique de la croute d’origine laurentienne parallelement a la direction d’un meme arc subissant une distension desordonnee.  Nous proposons que le developpement de l’AAT a ete similaire a celui de l’arc moderne Izy–Bonin–Marianne du Pacifique occidental.

geoscience canada

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