麻豆传媒

听

Cambrian-Ordovician metasediments of the Meguma Group, Nova Scotia, form the lowest exposed rocks of the most outboard of the terranes accreted to the northern Appalachian Orogen. The turbiditic sandstones and slates of the Goldenville Formation and overlying Halifax Formation escaped all but the last tectonic phase, the Early to Late Devonian Acadian Orogeny. Clues to the early geologic history of the Meguma Group remain in southern Nova Scotia, in Mahone Bay and the Tancook and LaHave Islands.

Metamorphic grade in greenschist (locally sub-greenschist) facies, and rocks retain original sedimentary structures. A lack of higher-grade metamorphic overprinting makes the area an excellent laboratory for studying early stages of the Acadian Orogeny. Field studies, with petrographic, electron microprobe, back-scatter electron image, XRD composition, XRD illite-muscovite crystallinity, and 40Ar/39Ar techniques provide new data that reveal details of the early history. This history can be described in three phases.

Diagenesis and very-low-grade metamorphism caused reactions that produced a restricted mineralogy of quartz, chlorite, muscovite, albite, graphite and accessory phases. Reactions were mainly controlled by the lithochemistry of the sediments, temperature as a function of depth, and restricted fluid flow. XRD illite-muscovite crystallinity data show that local areas retain diagenetic-lower anchizone characteristics. Detrital muscovite gives an age of 597 Ma; diagenesis is tentatively dated at 484 Ma.

Regional metamorphism affected the Meguma Group between 395-388 Ma, causing buckling of competent layers, cleavage, and tight folds ranging from the centimetre to the kilometre scale. The timing of metamorphism, based on 40Ar/39Ar ages from muscovite separates and whole rock samples subjected to rigorous step-heating schedules, is younger than earlier estimates. Combined with recent adjustments to the Early Proterozoic time scale, the new dates reported here resolve the outstanding problem of metamorphism in 417-400 Ma rocks of the Torbrook Formation.

Late-stage deformation, locally illustrated by reactivated folding and hydrothermal alteration, is dated at 376 Ma, and has affected XRD illite-muscovite crystallinity signatures in the areas of the study.

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Supervisor: Rebecca Jamieson