Xenoliths in a mafic dyke at Popes Harbour, Nova Scotia: implications for the basement to the Meguma Group

Abstract

A recently recognized series of northwest-trending mafic dykes cuts the Cambro-Ordovician Meguma Group along the eastern shore of Nova Scotia. Some of the dykes contain gneissic and (meta)plutonic xenoliths interpreted here as including fragments of the basement to the Meguma Group.Most of the gneissic xenoliths in the dyke exposed at Popes Harbour are sillimanite- and (or) kyanite-bearing quartz-poor metapelites and garnet + orthopyroxene-bearing tonalitic rock. Other enclaves include amphibolite, quartzite, gabbro, granite–granodiorite, and granitic pegmatite. Textural features suggest that the earliest paragneissic assemblage included poikiloblastic garnet (XFe = 0.55–0.65, XMg = 0.05–0.30, XCa = 0.02–0.16, XMn = 0.02–0.16), biotite, kyanite and (or) sillimanite, oligoclase, and quartz. Subsequent recrystallization at higher temperature and (or) lower pressure conditions is suggested by the replacement of kyanite by sillimanite. Mineral core composition data indicate early metamorphic conditions of ~450–600 MPa (gt–Al2SiO5–pl–qz and gt–opx–pi–qz barometers) and at least 600 °C (gt–bi and opx–gt thermometers), which lie close to the kyanite–sillimanite boundary and do not clearly distinguish the kyanite- and sillimanite-forming events. Other relatively high temperature and (or) lower pressure texturally overprinting assemblages include, in the pelites, (i) sapphirine (XMg = 0.75), spinel (XMg = 0.45–0.65), corundum, and rutile (all with anatectic(?) calcic plagioclase or ternary feldspar rims); and (ii) calcic rims on plagioclase and pyrope-rich overgrowths on garnet in both pelitic and tonalitic gneiss.The polymict assemblage of xenoliths in the Popes Harbour dyke testifies to the heterogeneity of the basement to the Meguma Group.