ABSTRACT
The presence of LAYERING or BANDING in metamorphic rocks can be either primary or secondary in origin, and can occur on the macro-, meso- or microstructural scale. In metamorphosed sedimentary and volcanic sequences, original COMPOSITIONAL LAYERING is often still recognisable. This is due to primary chemical differences that influence the nature of metamorphic mineral assemblages that can develop. Such differences are often recognisable in thin section by virtue of mineral assemblage or grain size variations (Fig. 4.1). In low- to medium-grade pelitic and semipelitic sequences it is often possible to recognise fining-upwards sequences and to determine ‘way-up’ based on grain size variations or subtle changes in the proportion of quartz and phyllosilicate minerals present (Fig. 4.2). Compositional layering in greenschist facies semi-pelitic schist from Sierra Leone. Alternating layers are richer and poorer in biotite with respect to quartz. Note also the weak schistosity trending oblique to the layering from the top right to the bottom left. Scale = 1 mm (PPL). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315831626/67ff2ae0-2ee0-4d05-9e97-aa4df451c794/content/fig4_1_B.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/> A field photograph of grading in pelitic/semi-pelitic rocks, Snake Creek, Queensland, Australia. The pencil is 12 cm long; the younging direction (left to right) is shown by the symbol in the lower left corner. Platy layers represent micaceous (originally fine-grained clay-rich) tops to cycles. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315831626/67ff2ae0-2ee0-4d05-9e97-aa4df451c794/content/fig4_2_B.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/>
