Quantifying orthoclase and albite muscovitisation sequences in fault zones

A new graphical method is described for assessing the degrees of alteration of orthoclase and albite feldspars to muscovite. The method uses major element data from whole rock samples, and given a range of samples of varying degrees of alteration, pathways of the alteration sequences may be determined. The method may be used quantitatively if likely starting compositions are known and aluminium immobility is assumed. Molar proportions of K, Na and Al are plotted in the form of an Na/(Na+K) versus (Na+K)/Al graph. Modelled plots of muscovitisation of an orthoclase/albite system are presented, for comparison with actual data, so that actual compositions may be assessed in terms of feldspar muscovitisation. An example of the application of this technique to muscovitised granitic fault rocks in the external western Alps is presented, illustrating how the method is best used in conjunction with microstructural and field constraints on the fault rock evolution. This yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation, and links specific alteration steps with particular microstructural changes within the fault zone. We also apply this technique to a previously published data set on basement shear zone geochemical changes from the Pyrenees where orthoclase albitisation provides an additional interest. Given that these reactions are fluid-induced, this work provides further information on the effects of fluid flow in such fault zones in terms of basement fault zone rheologies.