The geology of the Antigonish Highlands, Nova Scotia

Abstract

The Antigonish Highlands consist predominantly of late Precambrian rocks of the Georgeville Group. The stratigraphy of the Georgeville Group records the progressive development of a marine basin. Subaerial to shallow marine volcanic-dominated sequences are overlain by a thick sequence of turbidites with a volcanic arc source area. The geochemistry of the volcanic rocks indicates that there are at least three distinct contemporaneous volcanic rock types: (i) a calc-alkalic basaltic andesite; (ii) a continental within-plate tholeiitic-alkalic basalt; and (iii) volcanic arc-relatedfelsic volcanic rocks. The geochemistry, stratigraphy, and geochronology suggest that the Georgeville Group is synorogenic, genetically associated with subduction, and was deposited in an intra-arc or peri-arc basin. Geochronological data constrain the depositional age to ca. 618-610 Ma. Deformation of the Georgeville Group by thrusts, easterly vergent isoclinal folds, and north-south and east-west upright folds is associated with westward telescoping and closure of the basin. Synchronous movement along major northeast-trending strike-slip faults may have accompanied this deformation and may have continued into the Cambrian. Intrusions by ca. 615-610 Ma appinitic gabbro and granite are spatially and temporally associated with movement along major faults. Cambrian-Lower Ordovician rocks of the Antigonish Highlands were deposited in a pull-apart basin associated with a dextral transgressional stage of the late Precambrian Avalonian-Cadomian Orogeny. Two groups of rocks were deposited: the predominantly sedimentary Iron Brook Group and the predominantly volcanic McDonalds Brook Group. The rocks were deposited in a continental to shallow marine environment. The geochemistry of the volcanic rocks is consistent with a continental within-plate tectonic environment and is genetically related to the development of the pull-apart basin. Cambrian plutonic rocks (granite and gabbro) display geochemical characteristics consistent with a transgressional environment. The basin was deformed by thrusts and isoclinal folds probably in the Middle Ordovician. Deformation is possibly related to sinistral motion on northeast-trending faults. Late Ordovician to Early Devonian rocks of the Arisaig Group consist of mafic to felsic volcanic rocks overlain by a thick sequence of fossiliferous siliciclastic rocks, and were deposited in a continental to shallow niarine environment. The geochemistry of the volcanic rocks is consistent with a continental rift-related tectonic environment. These rocks were deformed by faults and thrusts, probably in the Middle Devonian. Upper Devonian-Carboniferous plutonic rocks are associated with the transgressional stages of the Acadian Orogeny and may be related to the docking of the Meguma Terrane