Abstract

Metamorphic differentiation is widely believed to result from processes that are either solely chemical or dominantly chemical and subordinately mechanical. Three occurrences of mineralogically layered mylonites derived from nonlayered parent rocks (metadolerite: Beartooth Mountains, Montana; biotite-amphibole-plagioclase gneiss: New London County, Connecticut; and granitic augen gneiss: Lower Engadine, Switzerland) indicate that metamorphic differentiation may also result from processes that are dominantly mechanical. Mechanical metamorphic differentiation is probably restricted to deep-seated faults or to finite zones of intense penetrative movement, whereas chemical and chemical-mechanical metamorphic differentiation are likely to be regional in extent.

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