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Mafic sills are abundant In the early Palaeozoic Halifax, White Rock and Torbrook Formations exposed along the northwestern margin of the South Mountain batholith in southwestern Nova Scotia. Field observations, petrography, and geochemistry indicate that the sills are of at least two ages. SIIIb of the older type are restricted to the Halifax Formation, and were regionally metamorphosed and folded together with their host rocks during the Acadian Orogeny. These sills are generally 1 m or less in width and are inferred to be approximately penecontemporaneous with their host rocks (Ordovician). Sills of the younger type intruded all three formations, including the Early Devonian Torbrook Formation, but apparently predated the South Mountain batholith, and hence at least in part are early to middle Devonian in age. They are generally larger than the older sills and concordant with Acadian structural trends but not demonstrably folded. Sills of both types are essentially basaltic in composition, but are extensively altered. Wide ranges in chemical composition largely reflect this alteration which partly obscures chemical differences between the two types. Both are tholeiltlc transitional to alkalic, but the older sills appear to have more alkalic tendencies. Both types were emplaced in a continental within-plate environment, thus placing constraints on tectonic models for southwestern Nova Scotia during the early Palaeozoic.   RESUME  Les fiIons-couches abondent dans les formations de Halifax, de White Rock et de Torbrook (Paleozolque inferieur) le long de la bordure nord-ouest du batholite South Mountain au sud-ouest de la Nouvelle-Ecosse. Les observations de terrain, la petrographie et la geochimie indiquent que les flions-couches se partagent en au moins deux groupes chronologiques. Les plus anciens filons-couches se retrouvent seulement dans la formation de Halifax et, avec la roche note, ont subi le metamorphisme regional et le plissement cause par l'orogenie Acadienne. Ces filons-couches mesurent generalement 1 m ou moins de largeur et on croit qu'ils ont ete mis en place 3 peu pres en meme temps que la roche hote (Ordovicien). Les filons-couches plus jeunes recoupent les trois formations, y inclus la formation de Torbrook (Devonien) , male comme lis semblent antecedents au batholite South Mountain lis dateraient done, au moins en partie, du debut au milieu du Devonien. lis sont generalement de plus grande dimension que les anciens filons-couches et s'accordent avec les lignes structurales Acadiennes sans toutefois paraftre plisses. Les deux types de filons-couches sont essentiellement de composition basaltique mals sont profondement modifies. La grande variation des compositions chimiques reflete cette modification qui obscurcit lee differences chimiques entre les deux types. Les deux types de filons-couches furent mis en place au sein d'une plaque continentale ce qui impose des limites aux modeles tectoniques du sud-ouest de la Nouvelle-Ecosse pour le Paleozoique inferieur.  [Traduit par le Journal]

Geochemistry and tectonic implications of mafic sills in Lower Paleozoic formations of southwestern Nova Scotia