K-feldspar augen in felsic gneisses and mylonites—deformed phenocrysts or porphyroblasts?

Abstract

A review of mesostructural and microstructural evidence favours the hypothesis that K-feldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation. In most examples, the augen appear to represent variably deformed megacrysts in former granitoids, as they commonly have similar distributions and shapes of inclusions and many of them show simple twinning; megacrysts in granitoids show abundant evidence of a phenocryst origin. Some augen may represent phenocrysts that grew in migmatite melt leucosomes and later were deformed, and others may have been phenocrysts in felsic volcanic or pyroclastic rocks, although megacrysts are uncommon in these rock-types. A porphyroblastic origin is opposed by the typical zonal distribution of inclusions and the common evidence of plastic deformation and partial recrystallization of the augen. The use of K-feldspar megacrysts as indicators of K metasomatism or granitization is of doubtful validity.

Key Words:

• K-feldspar
• augen
• felsic gneiss
• mylonite
• phenocryst
• porphyroblast