Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

The Baie Verte Peninsula, western Newfoundland Appalachians, preserves evidence for Early to Mid Ordovician closure of the Taconic seaway, which led to obduction of the Baie Verte oceanic tract (BVOT) ophiolites onto the Laurentian (Humber) margin and Taconic orogenesis. The scarcity of Taconic radiometric ages (and predominance of Silurian (Salinic) data) from the Humber margin rocks (down-going plate) has been problematic, calling into question the intensity and existence of Taconic collisional orogenesis.  40 Ar/ 39 Ar and in situ U–Pb geochronology was undertaken on metamorphosed units from the Laurentian basement (Mesoproterozoic East Pond Metamorphic Suite), from the ca. 560 Ma Birchy Complex forming the leading edge of the Humber margin, and from the ca. 490 Ma ophiolitic rocks of the BVOT (Advocate Complex) in order to address this question. Our results confirm evidence of Taconic metamorphism along the Humber margin and at the base of the ophiolites. Ages obtained from the structural base of the Advocate Complex (481–465 Ma) are interpreted to reflect the timing of accretion and internal thickening of the ophiolite, whereas data from the underlying Birchy Complex (467–461 Ma) record the underthrusting and exhumation of the leading edge of the Humber margin along a subduction channel, penecontemporaneously with final obduction of the BVOT. A concordant ca. 465 Ma zircon age and REE data obtained from retrogressed eclogite of the East Pond Metamorphic Suite suggest that the parautochthonous Humber margin was locally subducted to eclogite-facies conditions during the Taconic collision and partly exhumed to amphibolite-facies conditions prior to a strong Silurian (Salinic) tectonometamorphic overprint. SOMMAIRE La peninsule de Baie Verte dans les Appalaches de l’ouest de Terre-Neuve a conserve des indices de la fermeture du bras de mer taconique, qui a mene a l’obduction des ophiolites de la bande oceanique de Baie Verte (BOBV) sur la marge laurentienne (Humber) et a l’orogenese taconique. La rarete des âges radiometriques taconiques (et la predominance des donnees siluriennes (saliniques)) provenant des roches de la marge de Humber (i.e. la plaque subductee) a ete problematique, mettant en question l’intensite et l’existence de la collision orogenique taconique. De la geochronologie  40 Ar/ 39 Ar et U–Pb in situ a ete realisee sur des unites metamorphises provenant du socle laurentien (la Suite Metamorphique d’East Pond d’âge Mesoproterozoique), du Complexe de Birchy date a ca. 560 Ma formant la partie frontale de la marge de Humber, and des roches ophiolitiques de la BOBV (Complexe d’Advocate) datee a ca. 490 Ma afin de confronter ce questionnement. Nos resultats confirment les indices de metamorphisme taconique le long de la marge de Humber et a la base des ophiolites. Les âges obtenus a la base structurale du Complexe d’Advocate (481–465 Ma) sont interpretes comme refletant la periode d’accretion et d’epaississement interne de l’ophiolite, tandis que les donnees du Complexe de Birchy sous-jacent (467–461 Ma) enregistrent le sous-charriage et l’exhumation de la partie frontale de la marge de Humber au sein d’un chenal de subduction, de facon penecontemporaine a l’obduction finale de la BOBV. Un âge concordant de ca. 465 Ma d’un zircon et les donnees de terres rares provenant d’une eclogite retromorphosee de la suite metamorphique d’East Pond suggerent que la marge de Humber parautochtone a ete localement subductee a des conditions du facies eclogitique durant la collision taconique et partiellement exhumee a des conditions du facies des amphibolites precedant la forte surimposition tectonometamorphique silurienne (salinique).