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The Birchy Complex of the Baie Verte Peninsula, northwestern Newfoundland, comprises an assemblage of mafic schist, ultramafic rocks, and metasedimentary rocks that are structurally sandwiched between overlying ca. 490 Ma ophiolite massifs of the Baie Verte oceanic tract and underlying metasedimentary rocks of the Fleur de Lys Supergroup of the Appalachian Humber margin. Birchy Complex gabbro yielded a Late Ediacaran U–Pb zircon ID–TIMS age of 558.3 ± 0.7 Ma, whereas gabbro and an intermediate tuffaceous schist yielded LA–ICPMS concordia zircon ages of 564 ± 7.5 Ma and 556 ± 4 Ma, respectively. These ages overlap the last phase of rift-related magmatism observed along the Humber margin of the northern Appalachians (565–550 Ma). The associated ultramafic rocks were exhumed by the Late Ediacaran and shed detritus into the interleaved sedimentary rocks. Psammite in the overlying Flat Point Formation yielded a detrital zircon population typical of the Laurentian Humber margin in the northern Appalachians. Age relationships and characteristics of the Birchy Complex and adjacent Rattling Brook Group suggest that the ultramafic rocks represent slices of continental lithospheric mantle exhumed onto the seafloor shortly before or coeval with magmatic accretion of mid-ocean ridge basalt-like mafic rocks. Hence, they represent the remnants of an ocean – continent transition zone formed during hyperextension of the Humber margin prior to establishment of a mid-ocean ridge farther outboard in the Iapetus Ocean. We propose that microcontinents such as Dashwoods and the Rattling Brook Group formed as a hanging wall block and an extensional crustal allochthon, respectively, analogous to the isolation of the Brianconnais block during the opening of the Alpine Ligurian–Piemonte and Valais oceanic seaways. SOMMAIRE Le complexe de Birchy de la peninsule de Baie Verte, dans le nord-ouest de Terre-Neuve, est constitue d’un assemblage de schistes mafiques, de roches ultramafiques et de metasediments qui sont coinces entre des massifs ophiolitiques d’ascendance oceanique de la Baie Verte au-dessus, et des metasediments du Supergroupe de Fleur de Lys de la marge de Humber des Appalaches en-dessous. Le complexe de gabbro de Birchy a donne une datation U-Pb sur zircon ID-TIMS correspondant a la fin de l’Ediacarien, soit 558,3 ± 0,7 Ma, alors qu’un gabbro et un schiste tuface intermediaire montrent une datation LA-ICP-MS Concordia sur zircon de 564 ± 7,5 Ma et 556 ± 4 Ma, respectivement. Ces datations chevauchent la derniere phase de magmatisme de rift observee le long de la marge Humber des Appalaches du Nord (565-550 Ma). Les roches ultramafiques associees ont ete exhumees vers la fin de l’Ediacarien et leurs debris ont ete imbriques dans des roches sedimentaires. Les psammites de la Formation de Flat Point susjacente ont donne une population de zircons detritiques typique de la marge laurentienne de Humber des Appalaches du Nord. Les relations chronologiques et les caracteristiques du complexe de Birchy et du groupe de Rattling Brook adjacent, permettent de penser que ces roches ultramafiques pourraient etre des ecailles de manteau lithospherique continental qui auraient ete exhumees sur le plancher oceanique peu avant ou en meme temps que l’accretion magmatique de roches mafiques basaltiques de type dorsale medio-oceanique. Par consequent, elles seraient des vestiges d’une zone de transition ocean-continent formee au cours de l’hyper-extension de la marge de Humber avant l’apparition d’une dorsale medio-oceanique plus loin au large dans l’ocean Iapetus. Nous proposons que des microcontinents comme de Dashwoods et du groupe de Rattling Brook ont constitues respectivement un bloc de toit et un allochtone crustal d’extension, de la meme maniere que le bloc Brianconnais a ete isole lors de l’ouverture des bras oceaniques alpins de Ligurie-Piemont et de Valais.

geoscience canada

Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis