Anorthosite, igneous; phaneritic, intrusive rock, from Dana Black Granite Ltd in River Valley, Ontario. Anorthosite is primarily composed of feldspar which creates its bluish-grey colour. This rock was donated by Tom Palangio of Wipware Inc. in honour of Dr. John A. Franklin, an Earth Sciences professor, eminent Engineering Geologist and president of the International Society of Rock Mechanics.
This rock was part of the River Valley pluton; a 100 square kilometer body of anorthosite and gabbroic rocks. The pluton is entirely within the Grenville Province but its western margin is controlled by a series of thrust faults associated with the Grenville Front Tectonic Zone. Pb-Pb whole rock isotope studies date the primary crystallization of the rock to 2560 ± 155 million years ago.
Anorthosites are produced from basaltic magma. The crystals of iron and magnesium rich (mafic) minerals olivine and pyroxene settle out of the mixture falling to the bottom of the magma chamber in the mantle. Plagioclase feldspar forms at the same time. These crystals float and eventually move into the crust as plutons of anorthosite.