Dark-coloured bands rich in mafic and aluminous minerals, e.g. biotite, sillimanite, and garnet, which are found between multiple coarse-grained quartzofeldspathic veins in regionally metamorphosed (see regional metamorphism) pelites and psammites. The melanosomes represent planar regions which have undergone extreme shortening by dissolution (or melting), and where removal of the quartz and feldspar components of the original rock along non-penetrative cleavage planes developed as a result of high shear stresses imposed during the regional metamorphic event. The mafic and aluminous components of the melanosome represent the undissolved or unmelted mineral residue left over from the original rock.
Subjects: Earth Sciences and Geography.