Quartz and Feldspar Microstructures in Metamorphic Rocks

Vernon, R.H.

School of Earth Sciences, Macquarie University, Sydney NSW 2109, Australia

The shapes of quartz and feldpsar grains in metamorphic rocks are among the most reliable criteria for determining parental rock-types, with implications not only for history of metamorphic complxes, but also for ore exploration models. For example, crystal faces and elongate crystals of feldspar, especially if they have oscillatory zoning, indicate an igneous precursor, and residual crystal faces and embayments in quartz indicate a volcanic precursor. Simple twinning in K-feldspar appears to be indicative of a magmatic orgin, and evidence of magmatic flow is provided by aligned feldspar crystals. Though deformation and recrystallization tend to obliterate these features, many can be preserved, even at granulite facies conditions, especailly in zones of relatively low strain accumulation.

Quartz and plagioclase inclusions are useful for distinguishing between phenocrysts and porphyroblasts of K-feldspar in metamorphic terranes, and hence for determining parental rock-types. K-feldspar phenocrysts are chacracterized by zonally (crystallographically) arranged inclusions, especially of well-formed plagioclase crystals, whereas K-feldspar porphyroblasts are characterized by rounded quartz and plagioclase inclusions, either at random or arranged in trails reflecting an overgrown foliation.

Inclusions of quartz and feldspar tend to become rounded in metamorphic minerals, even where the boundary between the porphyroblast (e.g., staurolite) and the same minerals in the matrix is a crystal face. The reason crystal faces are not developed against the inclusions may be connected with the absence of fluid along the host-inclusion boundary, compared with its accumulation along the advancing porphyroblast-matrix boundary. In fact, the development of crystal faces generally in metamorphic rocks may involve fluid accumulation, either as a cause or a consequence.

Distinguishing between anatectic leucosome and quartz-feldspar aggregates formed in the solid state can be difficult, especially in the early stages of melting. The following microstructural criteria for anatectic leucosome may have general applicability. (1) Inclusion trails are absent, in contrast to grains of the same minerals in the mesosome. (2) Overgrowths free of inclusion trails may occur on minerals with inclusion trails (e.g., K-feldspar, cordierite). (3) Crystal faces of K-feldspar or plagioclase may occur against quartz. (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, though this may not apply if suitable feldspar nuclei were present before melting.

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