Differentiation associated with discrete crenulation cleavages

Differentiation, both mineralogical and chemical, plays a significant part in the development of discrete crenulation cleavages in low to medium grade metamorphic rocks. These cleavages are therefore not iron stained fractures and microfaults as was previously hypothesized. They are thin differentiated zones (seams or mica films) which truncate a crenulated pre-existing fabric. Their mineralogy is white mica-opaques-iron oxides particularly where the host rock mineralogy is white mica-quartz/feldspar-opaques-iron oxides-(chlorite/biotite). The chemistry of discrete crenulation cleavages can be extremely variable both along individual cleavages and between different cleavages in the same rock (due to variable amounts of Fe and Ti oxides along the cleavages). However, all the cleavages analysed show a consistent decrease in SiO₂ and increases in Al₂O₃ and K₂O with respect to the host rock. This chemical differentiation is due largely to the observed mineralogical variations which are related to the development of the cleavages. The differentiated nature, the microfabrics and the microtextural relations of discrete crenulation cleavages suggest that they are ‘solution planes’ characterized by relatively insoluble minerals. Solution and removal of quartz along the cleavages resulting in a passive concentration of phyllosilicates, is considered responsible for the development of this variety of cleavage.