LOCAL VOLUME CHANGE VS OVERALL VOLUME CONSTANCY DURING CRENULATION CLEAVAGE DEVELOPMENT IN LOW-GRADE ROCKS

The role of volume change in crenulation cleavage development involvin g possible pressure solution of quartz and mica is investigated laking into account two end-member crenulation cleavage evolutionary models, involving (1) mica loss from microlithons and (2) immobile mica. Math ematical relationships derived for both models are used to generate da ta sets for comparison with measured mineralogical proportions for two samples of crenulation cleavage from the Singhbhum district,eastern I ndia. The development of crenulation cleavage is considered to be unde r overall volume constancy if the average volume loss from cleavage do mains is counterbalanced by average volume gain in adjoining microlith ons. A test based on Chi(2) statistics obtained from comparison of exp ected values from model calculations and observed proportions of miner al phases in microlithon and cleavage domains is proposed to accept or reject the null hypothesis of overall volume constancy during crenula tion cleavage development. An application of the test to the two examp les of crenulation cleavage from India suggests that mica was relative ly immobile, local volume loss in cleavage domains was about 6-19%, an d there was an overall volume constancy on a decimetre scale during cr enulation cleavage development. (C) 1998 Elsevier Science Ltd. All rig hts reserved.