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 ele ment data from whole rock samples, and given a range of samples of varying degrees of alteration, pathways of the alteration sequences may be determin ed. The method may be used quantitatively if likely starting compositions a re known and aluminium immobility is assumed. Molar proportions of K, Na an d Al are plotted in the form of an Na/(Na + K) versus (Na + K)/Al graph. Mo delled plots of muscovitisation of an orthoclase/albite system are presente d, for comparison with actual data, so that actual compositions may be asse ssed in terms of feldspar muscovitisation. An example of the application of this technique to muscovitised granitic fault rocks in the external wester n Alps is presented, illustrating how the method is best used in conjunctio n with microstructural and field constraints on the fault rock evolution. T his yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation, and links specific alteration steps with particula r microstructural changes within the fault zone. We also apply this techniq ue to a previously published data set on basement shear zone geochemical ch anges from the Pyrenees where orthoclase albitisation provides an additiona l interest. Given that these reactions are fluid-induced, this work provide s further information on the effects of fluid flow in such fault zones in t erms of basement fault zone rheologies. (C) 2000 Elsevier Science B.V. All rights reserved.