Chemographic relationships of biotite and cordierite in the McGerrigle thermal aureole, Gaspé, Quebec

Abstract

Abstract Biotite and cordierite occur in a 1‐km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes in XFe that can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the ‘breakdown’of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K‐feldspar and andalusite.The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. The KD of Fe‐Mg between the minerals decreased and apparently increased, respectively, during the two reactions.Biotite‐cordierite‐chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite‐cordierite‐andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been < 2 kbar.The results of this study suggest that the KD of Fe‐Mg between biotite and cordierite is a function of temperature, the Fe‐Mg exchange characteristics of the controlling continuous reaction and non‐ideal mixing of Fe and Mg.