Scott D. Anderson
Ph. D. Thesis
Structure, Metamorphism and U-Pb and 40 Ar/39Ar Geochronology of the Ming's Bight Group and the Paleozoic Tectonic Evolution of the Baie Verte Peninsula, Newfoundland.
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The Ming's Bight Group (MBG) comprises a sequence of Late Proterozoic to Early Ordovician continental margin siliciclastic rocks of Humber Zone tectonostratigraphic affinity which is surrounded by Early Ordovician volcanic arc and ophiolitic rocks of Dunnage Zone affinity on the eastern side of a major boundary (the Baie Verte-Brompton Line; BVL) on the Baie Verte Peninsula (BVP) in northwest Newfoundland. The apparently anomalous tectonostratigraphic setting of the MBG reflects the protracted Ordovician to Carboniferous tectonic history of the BVP, and the tectonic evolution of the Newfoundland Appalachians. The deformation history of the MBG, as constrained by overprinting relationships and geochronologic data, indicates a more or less progressive transition from sinistral transpressional deformation, to dextral transpresssion, to dextral transtension from Late Siluaian to Carboniferous time. Sinistral transpressional structures comprise a thick, south-verging zone of sinistral-reverse, oblique-slip shear that is bounded to the east by a wide, steeply east- and west-dipping zone of contemporaneous sinistral wrench shear. These structures are constrained to late- Early Silurian to early-Early Devonian time by a previously published U-Pb date of 420 ± 5 Ma from hydrothermal zircon in syn-tectonic quartz veins and a 40 Ar/39Ar date of 405 ± 4 Ma from hornblende which post-dates the earliest extensional structures in the MBG, respectively.. 40 Ar/39Ar ages of 386 and 388 ± 2 Ma from syn-tectonic hornblende in the zone of sinistral wrench shear are interpreted to record cooling subsequent to an amphibolite facies metamorphic peak, and provide a younger age limit for maximum burial during sinistral transpresssion along the BVL. These relationships indicate that, in contrast to the correlative Fleur de Lys Supergroup (FdLS), the MBG occupied a relatively high structural level of the Taconian (Early Ordovician) thrust stack, and remained at this level until it was overthrust high grade metamorphic rocks of the FdLS that were rapidly exhumed in the hanging of the BVL through Early Silurian time. Dextral transpressional structures were observed in a single shear zone in the thesis area, and are overgrown by titanite porphyroblasts with a U-Pb age of 388 ± 4 Ma. This shearing is tentatively correlated with a dextral transpressional shear zone exposed further west along the main trace of the BVL. Dextral transtensional structures are symmetrically opposed about the MBG. To the southeast, the MBG is bounded by a thick zone of penetrative, southeast-directed, normal-sense, non-coaxial shear which initiated during peak amphibolite facies metamorphism (T>600 °C; P>6 kbar), and continued to greenschist facies conditions.40 Ar/39Ar cooling ages of 377-382 ± 3 Ma from syntectonic hornblende from the footwall of this zone indicate extensional shearing through Middle to Late Devonian time. To the northwest, a series of narrow, high-angle, brittle-ductile, greenschist facies, dextral-normal oblique-slip shear zones bound the MBG. 40 Ar/39Ar muscovite cooling ages throughout the MBG cluster between 360 ± 3 and 368 ± 4 Ma, indicating that this deformation, and coeval rapid post-metamorphic cooling through the muscovite closure temperature, continued through to Early Carboniferous time. Based on the geometry of these extensional structures and the spatial patterns of cooling ages and cooling rates, the MBG is interpreted to be a symmetrical metamorphic core complex. These relationships indicate that the recently proposed models for Silurian extensional collapse of the Newfoundland Appalachians can't account for rapid post-metamorphic cooling in either the MBG or FdLS.
Supervisor: Rebecca Jamieson