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The relationship between low-grade metasedimentary and high-grade paragneissic Precambrian rock units in the Bras d'Or terrane of central Cape Breton Island is a long-standing geological problem. Wholerock geochemical and Nd isotopic data from representative components of these units in the Kellys Mountain area, the Glen Tosh formation and Kellys Mountain Gneiss, show strong similarity. Major and trace element characteristics suggest that protolith sediments for both units were wackes derived from felsic igneous sources and deposited at an active continental margin. Samarium-Nd isotopic data in combination with previously published detrital zircon ages show that the sediments contained an ancient (ca. 2 Ga) end-member of recycled continental crust mixed with a juvenile component of Late Neoproterozoic age. The similarities suggest that the rocks represent the same sedimentary unit at different grades of metamorphism, implying a major crustal break under the area of mixed dioritic and granitic rocks that separates the gneissic and lower grade rocks. Whole-rock chemical and Sm-Nd isotopic composition of a Late ABStrACt rÉSUMÉLe lien entre les unités lithologiques précambriennes métasédimentaires faiblement métamorphisées et celles paragneissiques fortement métamorphisées du terrane Bras d'Or et du centre de l'île du Cap Breton est un problème géologique de longue date. Les données isotopiques du Nd et géochimiques sur roche totale d' éléments représentatifs de ces unités dans le secteur du mont Kellys, la formation de Glen Tosh et le gneiss du mont Kellys révèlent une similarité prononcée. Les caractéristiques des éléments majeurs et des éléments traces laissent supposer que les sédiments protolithiques des deux unités étaient des wackes provenant de sources ignées felsiques qui se sont déposés le long d'une marge continentale active. Les données isotopiques du samarium-néodyme combinées avec les datations sur zircon détritique précédemment publiées révèlent que les sédiments comportaient un membre extrême ancien (environ 2 Ga) de croûte continentale recyclée mélangée avec un jeune élément de l' époque du Néoprotérozoïque tardif. Les similarités notées permettent de penser que les roches constituent la même unité sédimentaire à des niveaux différents de métamorphisme, ce qui suppose une rupture crustale majeure sous le secteur des roches dioritiques et granitiques mixtes séparant les roches gneissiques et moins métamorphisées.

Abstract--An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near-continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments. Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink-banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock-metamorphic pressures ≤25 GPa.The age of the BCS is not determined. The low depth to diameter ratio of the crater, coupled with anomalously high shock-metamorphic pressures recorded at its exposed rim, may be a result of significant post-impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS.

Late Neoproterozoic (680 Ma to 560 Ma) Avalonian rocks in the Mira terrane of southeastern Cape Breton Island have been metamorphosed inhomogeneously, at very low grade to low grade, as recorded in mafic volcanogenic rocks (metabasite). Local occurrences of higher grade (amphibolite-facies) rocks are limited to aureoles surrounding Devonian plutons. At the lowest grades, metabasite contains primary igneous textures and relict plagioclase and pyroxene. Chlorite, prehnite, and sporadically developed pumpellyite indicate that the rocks have reached prehniteactinolite facies conditions. Higher grade rocks contain actinolite and epidote indicative of greenschist-facies conditions. Pumpellyite was not observed to occur with actinolite, indicating that pumpellyite-actinolite facies was not attained. Relict pyroxene is diopsidic in composition, and along with mineral textures and chemographic projection analysis, suggests that clinopyroxene was an active participant in equilibria at the lowest grades. Calculation of the P-T position of equilibria involving diopside with chlorite, epidote, prehnite, and pumpellyite yields P-T values consistent with very low-grade conditions and with the P-T conditions determined using actinolite, chlorite, prehnite, and pumpellyite. With the exception of the youngest (ca. 560 Ma) rocks, the samples show the full range of P-T conditions from prehnite-actinolite to greenschist facies. The ca. 560 Ma samples record only very low-grade (prehniteactinolite facies or lower).Prehnite-actinolite-to greenschist-facies assemblages suggest that metamorphism occurred at temperatures between 250 °C and 350 °C with pressures less than ca. 3 kbar (300 MPa). The P-T calculations indicate that pressures were somewhat lower, at about 2 kbar (200 MPa). These conditions imply relatively high geothermal gradients (at least 50 °C/km), consistent with burial metamorphism in a volcanic-arc setting. Metamorphism in the area as a whole was an ongoing process throughout the formation and amalgamation of the components of the Mira terrane, rather than the product of a discrete orogenic event. RÉSUMÉLes roches avaloniennes du Néoprotérozoïque tardif (680 à 560 Ma) dans le terrane de Mira, au sud-est de l'île du Cap-Breton, ont subi un métamorphisme hétérogène et produit des roches très faiblement à faiblement méta-morphosées, comme l'attestent les roches volcanogéniques mafiques observées dans la région (métabasite). Les cas isolés de roches nettement plus métamorphosées (faciès amphibolitique) se limitent aux auréoles qui ceinturent des plutons du Dévonien. S'agissant des roches plus faiblement métamorphosées, la métabasite contient des roches à texture ignée primaire, des plagioclases reliques et du pyroxène. La chlorite, la prehnite et la pumpellyite minéralisée de façon éparse présentes, indiquent que les roches ont atteint le stade d'un faciès à prehnite-actinolite. Les roches plus métamorphosées contiennent de l'actinolite et de l'épidote, ce qui traduirait la présence d'un faciès à schistes verts. La ...

At Kellys Mountain, Cape Breton Island, Nova Scotia, the late Neoproterozoic Glen Tosh formation (a low-grade metapsammite–metapelite unit of the George River Metamorphic Suite) has been intruded by diorite, granodiorite, and granite plutons, and the diorite hosts a narrow contact metamorphic aureole. New mapping and sampling in the contact aureole reveals that the metasedimentary rocks have reached amphibolite-facies metamorphism resulting in the development of neoformed biotite, muscovite, cordierite, ilmenite, garnet, andalusite, sillimanite, monazite, and spinel within the meta-pelite, a mineral assemblage also found in the Kellys Mountain Gneiss as a result of low-pressure regional metamorphism. Neoformed minerals and the disappearance of foliation defines a contact metamorphic aureole within 300 m of the pluton contacts. Petrographic and microprobe analyses of equilibrium assemblages in metapelitic units of the contact aureole yielded metamorphic pressures of 250 MPa, implying an intrusion depth of ∼9 km, with temperatures ranging from 365 to 590 °C. The presence of earlier-formed andalusite and garnet indicates the rocks may have initially undergone a low-pressure regional metamorphic event prior to contact metamorphism. Monazite in the contact aureole was dated using in-situ U–Pb methods and yielded an age of 480.9 ± 3.7 Ma, interpreted as the time of formation of the contact metamorphic aureole.