Ordovician A-type plutons in the Antigonish Highlands, Nova Scotia1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

Abstract

Previously unrecognized A-type granitoid and gabbroic rocks cover an area of ∼100 km2 in the central Antigonish Highlands in Avalonian northern mainland Nova Scotia. The granitoid rocks in this suite occur, together with minor gabbro, in separate mappable bodies named the Brora Lake, Haggarts Lake, West Barneys River, McGraths Mountain, and Leadbetter Road plutons and are the focus of this study. They are mainly alkali feldspar granite and quartz syenite with varied texture and modal mineralogy. Close spatial association, gradational contact relations, and magma-mingling relationships suggest that all of these rocks, including the gabbro, are coeval. Hence a U–Pb (zircon) age of 469.4 +/– 0.5 Ma obtained for a quartz syenite sample from the Brora Lake pluton is interpreted to indicate a Middle Ordovician age for all of these granitoid and gabbroic rocks. Most granitic and quartz syenitic samples have hypersolvus texture, characterized by strongly perthitic alkali feldspar, and some also contain interstitial granophyre, both features indicative of shallow emplacement. Mafic minerals include iron-rich calcic and calcic–sodic amphibole, hedenbergite, and in some samples fayalite. Most of the rocks have high concentrations of K, Na, P, Ti, and Fe and are alkalic, with agpaitic indices as high as 0.98. Discrimination diagrams indicate that they are A-type granitoid rocks formed in a within-plate extensional setting. Age correlatives of these A-type granitoid and gabbroic rocks are not yet known elsewhere in Avalonia. The available data suggest that this ca. 470 Ma magmatism occurred while Avalonia was a separate microcontinent in the Rheic Ocean.