NA-BIOTITE IN DALRADIAN PELITIC SCHISTS, ANGUS

Na-biotites have been found within pelitic schists in lower amphibolit e facies Dalradian metasediments. These micas are optically very simil ar to normal K-biotites which they partially replace, although the Na- biotites do contain a more prominent 001 cleavage. The formation of th e Na-biotites is linked to the local infiltration of a Na-rich fluid d uring retrogression and was dependent on late deformation opening flui d pathways within existing biotite porphyroblasts. Micas of such unusu al compositions (up to 3 wt% Na2O) have not previously been reported f rom pelitic rocks.