A magmatic sheet origin for thin metagabbroic anorthosite units in the Fishog subdomain of the southern Central Gneiss Belt, Grenville Province, Ontario

Abstract

Thin (tens of metres wide by tens of kilometres long) sheets of metagabbroic anorthosite are a common and intriguing feature of the Central Gneiss Belt (CGB), southwestern Grenville Province. Their origin, however, as either tectonic remnants of originally much larger parent masses or as moderately strained magmatic sheets is controversial because regional high strain has largely obliterated original contact relationships and modified primary textures. The origin of three sheets of metagabbroic anorthosite within the Fishog subdomain of the CGB is examined. Field relationships and geochemical analyses indicate that the sheets are composed of two distinct subunits (A = anorthositic, with An57–63; B = dioritic, with An41–47) that may be genetically related but have been contaminated by a significant crustal component during intrusion. Grenvillian deformation was accommodated by the sheets through early ductile flow and subsequent brittle rupturing during folding that was accompanied by the intrusion of pegmatite veins. Field observations and the results of Robin's method of strain analysis applied to representative samples of the least strained of these sheets, the Head River unit (HRU), show that internal strain was low. Preserved contact relationships of gabbroic anorthosite magma into dioritic country rocks and of granitic sheets into the HRU can be used to infer that ductile thrusting was not an important mechanism in the formation of the sheets' narrow geometry but that it simply modified the shape of magmatic sheets. A magmatic sheet hypothesis is proposed for the origin of meta-anorthositic sheets and associated rocks of the Fishog subdomain and may serve as a viable model for the precursors to some of the more highly strained sheets of meta-anorthositic gneiss found throughout the CGB.