Tectonics and magmatism in the northern Antigonish Highlands, Nova Scotia

Mapping in the Antigonish Highlands of Nova Scotia has resulted in a complete re-interpretation of the geological history of the region and its significance in the Avalon Zone. This study defines three new groups; the Georgeville Group (Precambrian) and the Iron Brook and MacDonald Brook Groups (both Cambrian).The Georgeville Group records a progressively deepening depositional environment, from basalt and interbedded marbles at the base giving way to greywackes and shales. Basalts have ocean-floor affinities, but also show some island-arc characteristics. The Georgeville Group may have been deposited in a marginal basin, possibly related to the Precambrian island arc volcanism in southeastern Cape Breton. Possible correlatives include the Mona Complex in Anglesey, Wales. In the late Precambrian the rocks were polydeformed, metamorphosed (greenschist facies) and post-tectonically intruded by gabbro and alaskite.Cambrian rocks unconformably overly the Georgeville Group and consist of either fluviatile to shallow marine sediments (Iron Brook Group) or laterally equivalent interbedded sediments and volcanic rocks (MacDonald Brook Group). Basalts are alkalic, and probably erupted in continental extensional tectonic regime. The succession is similar to other Avalonian rocks of Cambridge age. Thrusting and isoclinal folding, probably during the Taconic Orogeny, is attributed to local transcurrent fault movement, possibly during the closure of the Iapetus Ocean.Part 2 of this study focused on the late Precambrian intrusion of a water-rich gabbro (appinite) into marble and basalt of the Georgeville Group. Contamination of the wet magma by the host rocks probably resulted in supersolidus mobility of elements with variations in X(CO(,2)) in the magma and in the fluid phase. Trends closely mimic those expected from crystal fractionation and may be attributed to variable partitioning between H(,2)O-rich silicate melt, CO(,2)-rich silicate melt and a CO(,2)-rich vapour. Alteration trends in the host basalt are virtually opposite to those in the intrusive rock suggesting exchange between them. The extent of alteration defines the edge of the transport system. These processes may account for the occurrence of felsic dykes near the contact zones and for a nearby stock of alaskite. More generally, super-solidus mobility due to variations in X(CO(,2)) may be an important mechanism of fractionation, causing compositional gradients in silicic magma chambers similar to those caused by crystal fractionation. This type of mobility may also be significant in the genesis of bimodal or mixed alkalic-tholeiitic suites