Microscopic deformation mechanisms associated with mica film formation in cleaved psammitic rocks

At Islesboro, Maine, cleavage is well developed in low greenschist-facies siltstones and interbedded pelites of early Paleozoic age. The siltstones contain a spatial sequence of mica film structures that corresponds to increasing intensity of mesoscopic cleavage. In the most weakly foliated rocks, cleavage is defined by the preferred orientation of individual mica particles. In siltstones displaying slightly higher strain, these particles are accompanied by short, discontinuous mica film segments, thought to have formed by the recrystallization of early pore-space layer silicates. In moderately cleaved rocks, these segments link up to form lengthened mica films by a process thought to include intergranular fracture, transgranular fracture and layer silicate crystallization. In strongly cleaved rocks, the lengthened mica films become longer and thicken appreciably by solution transfer of quartz and residual accumulation of layer silicates and opaque minerals. Layer silicate crystallization is evident at all stages of mica film development, but is especially marked by the growth of decussate mica inside late-stage, thick mica-rich layers. This sequence of mica film development is probably characteristic of fine-grained psammitic rocks, and may not necessarily occur in carbonate-rich or mica-rich rocks.