Preferred Orientation in Quartz Ribbon Mylonites C.J.L. WILSON School of Earth Sciences, Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia ABSTRACT Australia, and an upper greenschist or lower amphibolite facies quartzite from Risfjallet, Sweden. Quartz mylonites composed of elongate ribbon quartz without appreciable recrystallization at grain boundaries were examined GEOLOGICAL SETTING and contrasted. One was from a lower greenschist fades environ- ment at Mount Isa, Australia, in which the c-axis preferred orienta- The Mount Isa mylonite occurs as quartzite lenses within pelitic tion of the ribbons is either a pronounced orthorhombic distribu- schists, which are localized in a zone of intense deformation known tion or a small-circle distribution (with a small opening angle) as the Mount Isa fault zone (Wilson, 1973a). On the margin of the about the normal to the foliation and lineation. The other was an fault zone, the quartzites are generally massive with no obvious upper greenschist or lower amphibolite facies mylonite from layering, whereas in the center the quartzites have a well-developed Risfjallet in the Swedish Caledonides, in which the c-axis preferred planar layering within which is a prominent lineation. Other orientation of the ribbon is a maximum lying close to the foliation quartzite lenses are transitional between these two types, with only and normal to the lineation. Variation in preferred orientation can a poor foliation and lineation. Schist and quartzite in the fault zone be accounted for by temperature and (or) strain-rate differences, are not metamorphosed to grades higher than lower greenschist with basal-slip mechanisms predominant at lower temperatures facies, and the schist consists of white mica, chlorite, and deformed and prismatic slip (and possibly other slip systems), together with detrital quartz grains. The pelitic rocks overlying the fault are diffusion-controlled processes, predominant at higher tempera- characterized by white mica, quartz, and minor biotite, whereas the tures. Key words: quartz, mylonite, deformation, preferred orienta- underlying rocks are dolomitic, with occasional pure pelitic mem- tion, structural geology. bers containing white mica, chlorite, and quartz (Croxford and Jephcott, 1972). INTRODUCTION The Risfjallet mylonite comes from the allochthonous Seve-Koli complex in the eastern thrust belt of the central Swedish Quartz mylonites exhibiting similar microstructural features Caledonides. In the Marsfjallet area (Trouw, 1973; Glass, in from different metamorphic environments generally exhibit differ- prep.), the nappe complex can be divided internally into tectonic ent patterns of c-axis preferred orientations. If mylonitic rocks belts bounded by low-angle thrust faults at which mylonite zones composed only of ribbon (Spry, 1969, p. 294) quartz grains (al- are developed; the Risfjallet mylonite is one of these zones and also though there may be recrystallization at deformation band bound- coincides with a metamorphic boundary. The mylonite has a low- aries and grain boundaries) are considered, then three basic quartz dipping foliation bearing a strong quartz lineation, and it consists c-axis patterns consistently occur in different areas. These are (1) of mylonitized gneiss and schist (quartz, 90 percent; biotite, 3 per- an orthorhombic pattern with two maxima lying in a peripheral cent; muscovite, 6 percent; almandine, 2 percent; kyanite and girdle 30° to 60° from the foliation plane, or a crossed-girdle pat- minor apatite; rutile and opaque minerals; chlorite absent). The tern (Christie, 1963; Hara, 1971; Hietanen, 1938; Johnson, 1957, rocks that overlie the Risfjallet mylonite carry premylonitization 1960; Shelley, 1971); (2) a maximum or small-circle girdle normal assemblages belonging to the kyanite-almandine-orthoclase zone of to the foliation and lineation (Beavis, 1961; Christie, 1963; the amphibolite facies of this area; the rocks that underlie the Hobbs, 1966; Phillips, 1965a, 1965b); and (3) a maximum lying Risfjallet mylonite are lower grade, carrying premylonitization as- within or close to the foliation and normal to the lineation (Balk, semblages that belong to the staurolite-almandine zone. Myloniti- 1952; Behr, 1961; Christie, 1963; Crampton, 1963; Gan- zation postdates the metamorphic culmination, but during gopadhyay and Johnson, 1962; Hara and others, 1973; Hofmann mylonitization the metamorphic grade was still at least upper and Korcemagin, 1973; Ross, 1973; Sander, 1950; Wenk, 1973). greenschist facies. This is evidenced by the growth of biotite and Except for the papers cited above, and the very few that have some almandines during or just following the deformation that possibly been overlooked, literature on preferred orientation of produced the mylonite (Glass, in prep.); a slightly higher grade quartz in mylonites gives little insight into the relationship between cannot be excluded (that is, lower amphibolite facies). preferred orientation, microstructure, and tectonic conditions; the three main reasons for this are that (1) authors fail to describe the MICROSTRUCTURE OF THE MOUNT ISA MYLONITE microstructure of samples for which they present diagrams of pre- ferred orientation; (2) many authors do not separate measurements A complete progression of microstructural changes from weakly of old deformed grains from measurements of new recrystallized deformed sedimentary strata to strongly deformed and recrystal- grains; and (3) some authors omit a description of the tectonic and lized quartz blastomylonites is displayed, and the quartz micro- (or) metamorphic environment in which the mylonites developed. structural changes are similar to those described from the Moine Quartz mylonite with different preferred orientations, but with thrust zone (Christie, 1963, p. 397). However, a progressive some similarities in microstructural features, are described and change from the margins to the center of the zone, as suggested by compared herein, and their patterns of c-axis preferred orientations McLaren and Hobbs (1972), cannot be demonstrated because of are interpreted in the light of some experimental studies of quartz. the lenticular and discontinuous nature of the quartzite bodies They are a lower greenschist facies quartzite from Mount Isa, within the fault zone. Geological Society of America Bulletin, v. 86, p. 968-974, 7 figs., July 1975, Doc. no. 50713. 968 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/968/3433752/i0016-7606-86-7-968.pdf by guest on 28 September 2021 PREFERRED ORIENTATION IN QUARTZ RIBBON MYLONITES 969 Two areas of microstructural development are found in these absence of recrystallization. The shape and grain size of these rela- quartzite lenses: (1) from the margins, and (2) from the center of tively undeformed quartz grains suggest that the original grains the fault zone, where there is a variable amount of recrystallization. were probably similar to those immediately outside the fault zone The relationship between amount of strain and microstructure (see Wilson, 1973b, Fig. 5), although now these grains are elongate cannot be established with certainty because there are no strain and reduced to as little as one-half of their former diameter in markers except detrital quartz grains. In quartzite that has under- sections normal to the foliation and lineation. There is also evi- gone intense deformation (and recrystallization), outlines of origi- dence that water-assisted diffusive or transport processes are re- nal grains cannot be identified; therefore, any quantitive estimate sponsible for some modification of grain shape in the mylonite after of strain is unobtainable. deformation. Deformed detrital grains in quartzite from the eastern margin of In the central part of the fault zone, the quartzites have a foliated the Mount Isa fault zone show undulatory extinction (Fig. 1A), appearance defined by the grain shape of the ribbon quartzes and off-basal deformation lamellae (Fig. IB), and quartz grains sepa- show varying degrees of recrystallization. In regions with little or rated by well-defined surfaces across which there are small dis- no recrystallization, it is often difficult to distinguish boundaries of placements (now healed) of deformation lamellae and deformation the original quartz grains from deformation band boundaries, be- bands (displacements of as much as 20 /jl); there is a noticeable cause the quartz occurs as distinct ribbons, commonly with ser- 0*2 mm i Figure 1. A. Quartzite from eastern margin of Mount Isa fault zone, with detrital quartz grains that contain abundant banded extinction features (White, 1973); these include areas of undulose extinction and deformation bands. Old grains are noticeably elongate and illustrate beginnings of foliation development in these quartzites. B. Deformation lamellae and undulose extinction in quartzite from eastern margin of Mount Isa fault zone. Figure 2. Mount Isa mylonite from center of fault zone, with quartz ribbons in section normal to foliation and lineation. Ribbons occur as optically distinct elongate areas separated from one another by curved or serrated boundaries or by regions composed of numerous fine recrystallized grains. Individual ribbons generally consist of numerous elongate and rectangular subgrains and fine new grains. Most of narrow ribbons are probably highly complex deformation bands that closely parallel mylonitic foliation. Lines show variation in c-axis orientation.