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

Abstract

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 Briançonnais block during the opening of the Alpine Ligurian–Piemonte and Valais oceanic seaways.SOMMAIRELe complexe de Birchy de la péninsule de Baie Verte, dans le nord-ouest de Terre-Neuve, est constitué d’un assemblage de schistes mafiques, de roches ultramafiques et de métasédiments qui sont coincés entre des massifs ophiolitiques d’ascendance océanique de la Baie Verte au-dessus, et des métasédiments du Supergroupe de Fleur de Lys de la marge de Humber des Appalaches en-dessous. Le complexe de gabbro de Birchy a donné une datation U-Pb sur zircon ID-TIMS correspondant à la fin de l’Édiacarien, soit 558,3 ± 0,7 Ma, alors qu’un gabbro et un schiste tufacé intermédiaire montrent une datation LA-ICP-MS Concordia sur zircon de 564 ± 7,5 Ma et 556 ± 4 Ma, respectivement. Ces datations chevauchent la dernière phase de magmatisme de rift observée le long de la marge Humber des Appalaches du Nord (565-550 Ma). Les roches ultramafiques associées ont été exhumées vers la fin de l’Édiacarien et leurs débris ont été imbriqués dans des roches sédimentaires. Les psammites de la Formation de Flat Point susjacente ont donné une population de zircons détritiques typique de la marge laurentienne de Humber des Appalaches du Nord. Les relations chronologiques et les caractéristiques du complexe de Birchy et du groupe de Rattling Brook adjacent, permettent de penser que ces roches ultramafiques pourraient être des écailles de manteau lithosphérique continental qui auraient été exhumées sur le plancher océanique peu avant ou en même temps que l’accrétion magmatique de roches mafiques basaltiques de type dorsale médio-océanique. Par conséquent, elles seraient des vestiges d’une zone de transition océan-continent formée au cours de l’hyper-extension de la marge de Humber avant l’apparition d’une dorsale médio-océanique plus loin au large dans l’océan Iapétus. Nous proposons que des microcontinents comme de Dashwoods et du groupe de Rattling Brook ont constitués respectivement un bloc de toit et un allochtone crustal d’extension, de la même manière que le bloc Briançonnais a été isolé lors de l’ouverture des bras océaniques alpins de Ligurie-Piémont et de Valais.

Geological significance of 40Ar/39Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

“…The formation of double-vergent nappe structures and regional-scale metamorphism is closely related to emplacement of mafic to felsic plutons of the Connemara complex (Leake 1989), a calc-alkaline magmatic province that formed during the middle Ordovician GrampianTaconic orogeny (ca. 474-463 Ma) along the eastern Laurentian margin (e.g., Dewey and Ryan 2016;Friedrich et al 1999a;van Staal et al 1998van Staal et al , 2013. The country rocks for the Connemara complex are metasedimentary units of the Neoproterozoic Dalradian Supergroup, which represents the rift-to-drift transition related to breakup of Rodinia and the formation of the Iapetus ocean (e.g., Soper et al 1999).…”

Ode to Field Geology of Williams: Fleur de Lys Nectar Still Fermenting on Belle Isle

“…These rocks have since been inculcated to define the western metamorphic zone of the Central Mobile Belt of the Appalachian fold belt in Newfoundland exposed on the Burlington Peninsula, along the eastern edge of White Bay and inland farther to the southwest (Baird 1951;Neale and Nash 1963;Neale and Kennedy 1967;Williams 1969;Church 1969;Kennedy 1971;Hibbard 1983;Cawood and Nemchin 2001;van Staal et al 2013; Fig. 1).…”

“…The FdL sequence was elevated to Fleur de Lys Supergroup (FdLS) by Church in 1969(see Hibbard 1983, for an excellent detailed historical account), and is interpreted as a deformed late Neoproterozoic to early Paleozoic clastic sequence deposited along a passive continentocean transition zone flanking the Iapetus Ocean and its stable Laurentian continental shelf of Mesoproterozoic (Grenville-age) basement. This continental foundation is well exposed to the west of White Bay as the Long Range Inlier, the northeasternmost basement inlier of the Appalachian Orogen (Williams 1969;Bird and Dewey 1970;Bursnall and de Wit 1975;Hibbard 1983;Waldron et al 1998;Cawood and Nemchin 2001;Heaman et al 2002;van Staal et al 2009avan Staal et al , 2013van Staal and Barr 2012).…”

“…Subsequently, the Laurentian passive margin was tectonically 'drowned' as a foreland basin in response to its collision with outboard arc-related Iapetus oceanic lithosphere. This resulted in the cessation of carbonate sedimentation on the continental shelf and the input of clastic detritus from the central Appalachian orogen Stevens 1969, 1974;Stevens 1970;Bird and Dewey 1970;Dewey and Bird 1971;Strong and Williams 1972;Cawood and Nemchin 2001;van Staal et al 2009avan Staal et al , 2013, and references therein).…”

“…1) underwent multiple episodes of rifting, as evidenced by rift-related magmatism between 615 and 580 Ma and between 565 and 550 Ma, in the process opening Iapetus and a small early Ordovician seaway, the Taconic seaway (e.g. Waldron and van Staal 2001;van Staal et al 2013). This resulted in an outer passive Humber Zone of ultra-thinned continental crust with mafic volcanic rocks directly underlain by old continental mantle lithosphere, similar to seaward-dipping volcanic rocks and continental mantle material at the Continent-Ocean Boundary (COB) zones of the North and South Atlantic margins (e.g.…”