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. Quartz and plagioclase inclusions are useful for distinguishing between phenocrysts and porphyroblasts of K-feldspar in metamorphic terranes. K-feldspar phenocrysts are characterized by zonally arranged inclusions, whereas K-feldspar porphyroblasts are characterized by spherical inclusions of quartz and plagioclase, either at random or arranged in trails that reflect an overgrown foliation. Inclusions of quartz and feldspar tend to be spherical in metamorphic porphyroblasts (e.g., staurolite), even where the boundary between the porphyroblast and quartz or feldspar in the matrix is a rational face, which may be due to absence of fluid along the host-inclusion boundary, compared with its accumulation along the advancing porphyroblast-matrix boundary. The following microstructural criteria, preserved best in less deformed migmatites, indicate anatectic leucosome. (1) Crystal faces of K-feldspar or plagioclase 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 (e.g., K-feldspar, cordierite). (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.