Litho¢ 6, 1973, 337-.48 Microstructure of mylonites and their descriptive terminology T. H. BELL & M. A. ETHERIDGE Bell, T. H. & Etberidge, M. A. 1973: MicrmmJctute of mylonites and their descriptive terminolosy. La'tTjm6, 337-48. The r n i ~ transition from amphiboli~ facies, ~ country ro~ with increasing deformation, to • free-grained phyllitic mylonite is described in derail. All micrmtnmm~ (e.g. p o l ~ t i o n , subs~=~, .err,tea ~ bound- aries, kink bands, wminaWept~) are ittributed to ductiledeformation,recovery and recrysmll~tion prm:emes. The similarity of these 8trucuxr~ to other de- scribed mylonites susgesm that brittle deformation is not widmprmd in mylonitic rocks, and that terminology with brittle connomtimm d~ould be discontinued. A new definition for the term 'mylonite' is proposed. T. H. ~4: & M. ,4. E ~ , Z~partm~t of ~ o g y and M~r-toO, U~xn~'y of A~u'~, A~t,~, s.A. s001, A~ura~ De~ptte the fact that microetructures similar to those occurring in mylonites have been produced by experimental plastic deformation and recrystalliza- tiot~ (Carter et al. 1964, Pls. 9, 10) most subsequent workers (e.g. Higgirm 1971, Belliere 1971, Krupiclm & Sassano 1972) have attributed mylonitic microstructures to brittle processes. This is probably due mainly to the continued use of Lapworth's (1885) definitions and terminology(cf. Christie 1963, Verhoogen et al. 1970, Higgins 1971). Thus, concepts of brittle de- formation, such as 'crushed', 'dragged' and 'ground out', introduced by Lapworth (1885), are inherent in the descriptive terms my/ore're, catacku/s, flas~ and ~u~ar. The aint of this paper is to show that brittle deformation is unnecessary for the fo:nmation of a typical mylonite. It is further suggested that the ductile processes recoTnized in this suite of rocks are dominant in many mylonites ,'cL Vernon & Ransom 1971, Ross 1973), and that the processes envisaged ~y Lapworth (1885) are of little importance. With this in mind, the term '~nylonite' is redefined in the hope that its genetic connotatio~m will be lessened. The microstructural :ransition from amphibolite facies, gneissic country rock, with increasing deformation, to a fine-grained phyllitic mylonite is described. It is then shown that most of the microstructures can be attri- buted to ductile processes, although they are indistinguishable from typical m::lonite structures described elsewhere (Teall 1918, Waters & Campbell 1%5, Christie 1963, Higgirm 1971).