Quartz and feldspar microstructures in metamorphic rocks

The shapes of quartz and feldspar grains in metamorphic rocks are among the most reliable criteria for determining parental rock types. Rational faces and elongate crystals of feldspar, especially with oscillatory zoning, indicate an igneous precursor, and residual faces and embayments in quartz indicate a volcanic precursor. Simple twinning in K-feldspar indicates a magmatic origin, and aligned crystals of feldspar indicate magmatic flow. K-feldspar phenocrysts are characterized by zonally arranged inclusions, whereas K-feldspar porphyroblasts are characterized by spherical inclusions of quartz and plagioclase, either arranged at random or in trails that reflect an overgrown foliation. The following microstructural criteria, preserved best in less-deformed migmatites, indicate anatectic leucosome: 1) crystal faces of feldspar may occur against quartz, 2) inclusion trails are absent, in contrast to grains of the same minerals in the mesosome, 3) overgrowths free of inclusion trails may occur on minerals with inclusion trails and 4) simple twinning may occur in K-feldspar, which appears to be diagnostic of crystallization of K-feldspar in a melt rather than in the solid state.