Biotite chloritization process in hydrothermally altered granites

Abstract

Chloritization of biotite in a hydrothermal environment was studied in two granitic rocks of the Massif Central (France). Four types of transformation were observed: (1) Mg-chlorite growth upon (001) biotite planes in contact with quartz; (2) biotite pseudomorphosed by a corrensite—residual biotite assemblage; (3) corrensite—residual biotite pseudomorphosed by a ferromagnesian chlorite; and (4) biotite pseudomorphosed by chlorite. These transformations induce an expansion of the site of the ferromagnesian minerals which can be observed by the corrosion of neighbouring minerals and fracturation of the rock around the new chlorites (open fractures or chloritic veinlets). If Al is considered as a strictly inert component, the reaction biotite → chlorite produces a decrease of the ferromagnesian mineral volume of ∼ 13% (Ferry, 1979). This is not in agreement with petrographic observations of the rocks studied here, therefore writing of a chloritization reaction must take into account a volume increase. In this case Al does not behave as an inert component but as a mobile one at the scale of the microsystem biotite—neighbouring crystals. Albitization of plagioclase which occurs simultaneously with the chloritization process is the Al furnisher. In addition, Fe and Mg must be brought into the microsystem during the transformation of biotite into chlorite, this explains why the Mg ( Mg+Fe) ratio is not the same between biotite, chlorite growths and chlorite pseudomorphs. Because the bulk-rock composition is enriched in these components, the hydrothermal fluids must contain Fe and Mg before they flow into the rock. Chloritization in propylitized granites is not an isochemical process.