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

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 An_57-63; B = dioritic, with An_41-47) 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. Ductile thrusting was not an important mechanism in the formation of the sheets' narrow geometry but 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. -from Author