RETROGRESSION OF CORDIERITE TO KYANITE AND ANDALUSITE AT FISHTAIL LAKE, ONTARIO, CANADA

Cordierite alters to chlorite-sericite, called pinite. It also shows textural evidence indicating retrogression to kyanite, andalusite, and coarse flakes of chlorite. In common potash- bearing pelitic rocks, retrogression of cordierite results in the formation of sericite and chlorite. In gneisses containing sillimanite and cordierite, the breakdown of cordierite results in the association of the three aluminosilicates. The association of kyanite-andalusite in equilibrium probably suggests that the retrogression of cordierite took place close to the univariant line of kyanite-andalusite.