An example of compositional control of the celadonitic content of muscovite and the incoming of biotite in metapelites (Nurra, NW Sardinia)

The basement of the Nurra region is characterized by a prograde metamorphism ranging from chlorite to garnet-oligoclase zones. The muscovite of the paraschists shows, at the entry of biotite in the assemblages, a sharp variation in composition from celadonite-poor to celadonite-rich type. XRD work has been done on muscovite from 142 samples from the various metamorphic zones. Muscovites and chlorites have been analysed in 11 samples from the chlorite- biotite transition zone. The results of these analyses in reference to the A-K-Na-F system led to the conclusion that the 'jump' in the muscovite composition on the appearance of biotite in the paraschists does not depend on different pressure conditions of the metamorphism but, more probably, should be related to a significant variation of the bulk composition in terms of Al-saturation of the metasedimentary rock systems.