Chemical changes and the development of quartz preferred orientation in zones of crenulation cleavage, Anglesey

Crenulation cleavage is well developed in rocks of the New Harbour Group of Anglesey, North Wales. Modal and chemical analyses, and a study of the crystallographic orientation of quartz grains, indicate significant differences between the microlithons and the zones of cleavage. The reduction in quartz content and the increase in muscovite observed in the zones of cleavage is accompanied by enrichment in TiO2, Al2O3, Fe2O3, MnO, MgO, K2O and P2O5. Conversely SiO2, CaO and Na2O are depleted. Mass balance calculations suggest at least a 26% volume decrease in the cleavage zones. -rich layers in the microlithons become attenuated when traced into the cleavage zones and the quartz grains are reduced in size and assume a platy habit. The orientation of the quartz c-axes is weakly preferred within the microlithons but strongly preferred within the cleavage zones with the development of a single point maximum oriented normal to the plane of cleavage in which the quartz plates lie. This is interpreted as the result of the dissolution of quartz during the formation of the crenulation cleavage with the development of lenticular shapes due to the crystallographic control of the solution rates. Mechanical orientation of the lenses parallel to the cleavage is the likely origin of the preferred crystallographic orientation.