Local volume change vs overall volume constancy during crenulation cleavage development in low grade rocks

The role of volume change in crenulation cleavage development involving possible pressure solution of quartz and mica is investigated taking into account two end-member crenulation cleavage evolutionary models, involving (1) mica loss from microlithons and (2) immobile mica. Mathematical relationships derived for both models are used to generate data sets for comparison with measured mineralogical proportions for two samples of crenulation cleavage from the Singhbhum district, eastern India. The development of crenulation cleavage is considered to be under overall volume constancy if the average volume loss from cleavage domains is counterbalanced by average volume gain in adjoining microlithons. A test based on χ2 statistics obtained from comparison of expected values from model calculations and observed proportions of mineral phases in microlithon and cleavage domains is proposed to accept or reject the null hypothesis of overall volume constancy during crenulation cleavage development. An application of the test to the two examples of crenulation cleavage from India suggests that mica was relatively immobile, local volume loss in cleavage domains was about 6–19%, and there was an overall volume constancy on a decimetre scale during crenulation cleavage development.