Peculiarities of the Rb–Sr Isotope System in Granites: Comparative Study Results for Fresh and Altered Plagioclase Varieties from the Raumid Pluton Leucogranites (Pamirs)

Plagioclase varieties from leucogranites of the Raumid pluton (Pamirs) were studied by isotope Rb–Sr method. The studied varieties are: (1) fresh, crystal-clear magmatic oligoclase and (2) altered by deuteric saussuritization, turbid plagioclase. Comparison of data obtained for the coexisting varieties shows that saussuritization of the leucogranites led to Rb and Sr content and 87Rb/86Sr growth, and (87Sr/86Sr)0 distortion. This imply gain of alkaline and alkaline earth elements during saussuritization. The case of the Raumid pluton suggests a possibility of the gain from country rocks.