Scott Secord's Honours Thesis Abstract

Thesis Title: 
The Petrography and Geochemistry of the Grassy Pond Sill

The Grassy Pond sill lies in the Caribou Lake greenstone belt of the Wabigoon subprovince of the Superior province. It was exposed in a Landore Resources Ltd. mineral exploration trench about twelve kilometres east of Armstrong, Ontario. It is composed of a megracrystic plagioclase (individual crystals 1-15 cm across) anorthosite intruding mafic pillowed to massive volcanic rocks; this occurrence provides an excellent opportunity to add to the limited research into Archean megacrystic anorthosites. The Grassy Pond sill is c omposed of primarily plagioclase megacrysts of high anorthite content (An65 to An85) with associated mafic matrix composed of replacement minerals, chiefly amphibole, with calcite and epidote group minerals. A cogenetic relationship between all three units is suggested as the spatially associated mafic volcanic rocks and gabbros share similar mineral assemblages and chemical signatures. Detailed field mapping at outcrop scale and petrographical and geochemical work was performed in order to accurately determine the relationships between the units. The contact between the anorthosite and gabbro is often gradational (decreasing concentration of plagioclase megacrysts and increasing coarse grained mafic matrix). The gabbro and volcanic rocks have a much different relationship. The gabbro has intruded the volcanic rocks along selveges and through more massive sections and lacks well defined chill margins. This relationship suggests that the volcanic rocks might have been warm at the time of intrusion. This could imply that gabbros and anorthosites have intruded their own volcanic pile. These relationships coupled with the similar trend in geochemical signature suggest that the anorthosites and gabbro served as the subvolcanic source for the associated volcanic rocks. The previously unclassified Grassy Pond sill shares similar lithologies and geochemical pattern with the Bad Vermilion Lake calcic megacrystic anorthosite complex, as well as other Archean anorthosites and therefore is recognised as an Archean calcic megacrystic anorthosite.