THE COBEQUID HIGHLANDS, NOVA SCOTIA, CANADA: EXTENDING THE AVALONIAN GEOLOGICAL RECORD BACK TO THE EARLY NEOPROTEROZOIC

The Cobequid Highlands of northern mainland Nova Scotia occupy a unique position along the most outboard part of Avalonia, with the southern boundary marked by the Cobequid fault. New mapping combined with U-Pb zircon geochronology and chemical data show that the oldest rocks in the highlands, located in a faulted-bounded belt along the southernmost margin, are mafic metavolcanic rocks interbedded with quartzite, metawacke, and minor marble and ironstone of the Folly River and Gamble Brook formations, with maximum depositional ages of ca. 1 Ga. The mafic volcanic rocks have chemical characteristics of continental tholeiite with εNd(t) values of +4.6 to +7.1. Associated gabbroic bodies are MORB-like with εNd(t) of +6.9 to +7.7. Minimum age of both formations is constrained by the ca. 765–735 Mount Ephrairn plutonic suite which intruded them. The Mount Ephrairn suite consists of calc-alkaline gabbro/diorite to granite that show more evolved εNd(t) (+0.2 to +1.7). It likely records the initiation of subduction along the margin of Rodinia, related to its eventual breakup. The age of the Mount Ephrairn suite is similar to that of the Burin Group in the Newfoundland Avalon but not its petrological characteristics. Calc-alkalic, arc-related volcanic and plutonic rocks with more typical Avalonian ages and εNd(t) values between -1.5 and +3.7 form the most extensive units in the Cobequid Highlands. Ages are mainly ca. 610 to 585 Ma with a few older ages between 640 and 620 Ma; many of the dated samples also contain a significant inherited ca. 680–650 Ma zircon component suggesting the existence of older volcanic/plutonic rocks similar in age to the Stirling belt in Cape Breton Island. The 610–585 Ma ages are similar to those from Avalonian volcanic and plutonic rocks in the Antigonish Highlands to the east and in the Boston area, but lack the typical Avalonian Ediacaran-Cambrian cover sequence. Small bodies of within-plate syenite to alkali-feldspar granite and gabbro along the southern margin of the highlands yielded zircon ages of 482–480 Ma, slightly older than similar Ordovician plutonic units in the Antigonish Highlands. The Silurian Wilson Brook formation, a fossiliferous succession of sedimentary rocks similar to the Arisaig Group in the Antigonish Highlands, unconformably overlies the older rocks of the Cobequid Highlands.