Development of a layered crenulation cleavage in mica schists of the Kanmantoo Group near Macclesfield, South Australia P. C. MARLOW 1 Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia 5000, M. A. ETHERIDGE* J Australia ABSTRACT fer are likely to be available from the chem- from both microstructural and chemical ical reactions, strain history, volume viewpoints in an attempt to elucideate these Kanmantoo Group metasedimentary changes, and microstructural anisotropies. mechanisms. rocks are folded by a large, dextral, second-generation fold near Macclesfield, INTRODUCTION GEOLOGIC SETTING South Australia. In the hinge regions of this fold, pelitic schists are crenulated, which One of the fundamental problems of rock The rocks described form part of the gives rise to a variably developed, layered, deformation under metamorphic condi- Cambrian Kanmantoo Group of meta- axial-plane crenulation cleavage. The tions is the degree of interaction between sedimentary rocks (Thomson, 1969; Daily layered cleavage is produced by different mechanical and chemical processes at the and Milnes, 1972, 1973), which consist microstructural, mineralogical, and chemi- microscopic scale (compare Etheridge and predominantly of impure metasandstones cal changes on alternate limbs of asymmet- Hobbs, 1974). One of the most obvious re- and metasiltstones with interbedded mar- ric crenulations. The long limbs (mica or M sults of such interaction is the fine-scale bles, quartzites, calc-silicates, phyllites, domains) become enriched in muscovite at compositional layering so commonly paral- and pyritic horizons. The structures de- the expense of biotite, quartz, and feldspar lel to cleavages and schistosities, especially scribed in this paper are found predom- with a consequent large increase of A1203 those that develop by crenulation of a pre- inantly in the phyllites and andalusite and a smaller increase in KzO at the ex- existing planar structure. Descriptions of schists of the Backstairs Passage Formation pense of Si02, MgO , and FeO +Fe203. The such structures are numerous (for example, (Daily and Milnes, 1972) where it crops out compositional changes in the short limbs White, 1949; Rickard, 1961; Rast, 1965; in the hinge of the Strathalbyn anticline (quartz-feldspar or QF domains) are some- Nicholson, 1966; Talbot, 1964; Talbot and (Kleeman and Skinner, 1959). This struc- what complementary, but comparison with Hobbs, 1968; Williams, 1972), but there ture is a large second-generation fold plung- uncrenulated rock within 20 mm of these are only limited data that are useful in as- ing gently south and forming an asymmet- crenulations shows that the layering devel- sessing the mechanisms by which the com- ric fold pair with the adjacent Macclesfield opment involves a bulk chemical change, positional differentiation took place. syncline (Fig. 1). The deformational history primarily a depletion in MgO and FeO + Williams (1972) described the micro- and structural elements are summarized in Fe203. All mineral grains are finer and less structure and mineralogy of a "dif- Table 1. In the hinge of the Strathalbyn an- equidimensional in M domains and coarser ferentiated crenulation cleavage", and con- ticline, a divergent crenulation cleavage (S2) and more equidimensional in QF domains cluded that largely mechanical constraints is very well developed in the micaceous than in the equivalent uncrenulated rock. In led to removal of silica in solution from rocks, and a slaty cleavage in the addition, very little evidence of intracrystal- zones of high strain. Means and Williams metasedimentary rocks is parallel to either line deformation, recovery, or partial re- (1972, 1974) came to similar conclusions Sj or S2 (Table 1). The gradational devel- crystallization was found in a wide range of about the removal of salt from microfold opment of crenulation cleavage and layer- variably intensely crenulated rocks. limbs in experimentally deformed artificial ing described below occurs on a broad scale The crenulation cleavage probably de- salt-mica aggregates. The only theoretical across the major anticline and within veloped by a combination of (1) rotation of approach to the problem has used the mesoscopic F2 folds. All observations were existing grains accompanied by modifica- theory of equilibrium under nonhydrostatic made in sections normal to crenulation tion of their shape and size by diffusive pro- stress to predict that a two-phase aggregate axes. cesses, (2) migration of material, on the will tend to segregate into alternate scale of grains and domains, controlled by monomineralic layers normal to the MICROSTRUCTURE OF the deformation path and microstructural maximum compressive stress (De Vore, UNCRENULATED ROCKS anisotropies, and (3) nucleation and growth 1969). of grains with an orientation and shape It is evident from these studies that mate- The schists, metasiltstones, and meta- compatible with the strain history in their rial moves during formation of a crenula- sandstones consist predominantly of vicinity during nucleation and growth. It is tion cleavage and that the constraints on 1 shown that a pressure solution mechanism movement are related to stress and (or) The terminology in this paper is derived partly from Rickard (1961) and partly from Williams (1972). Cre- driven solely by differences in stress mag- strain heterogeneities on the scale of the re- nulations are the microfolds in a crenulated cleavage; nitude will not explain the range of micro- sultant layering. The problem remaining to crenulation cleavage is applied to the cleavage that is structural and mineralogical changes. More be solved is that of the mechanism(s) by subparallel to crenulation axial planes and that is usu- important controls on diffusive mass trans- which this material transfer takes place ally defined by alternating mica-rich (M domains) and quartz-feldspar-rich (QF domains) layers and (or) by under various conditions. This paper de- the mica (001) fabric that develops parallel to the do- * Present address: Department of Earth Sciences, scribes the stepwise formation of a crenula- mains. Genetic terms such as "strain-slip cleavage" and 1 Monash University, Clayton, Victoria 3168, Australia. tion cleavage and mineralogical layering "differentiated layering" are avoided. Geological Society of America Bulletin, v. 88, p. 873-882, 10 figs., June 1977, Doc. no. 70616. 873 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/873/3429550/i0016-7606-88-6-873.pdf by guest on 25 September 2021 874 MARLOW AND ETHERIDGE quartz, plagioclase (An20_30), biotite, and muscovite with minor andalusite, stauro- lite, garnet, opaque minerals, and tour- maline. In some of the schists, andalusite, staurolite, and (or) garnet may be major constituents, usually as porphyroblasts. These rocks were omitted from this study because of the complicating effects of the porphyroblasts on microscopic structures and processes. The composition of the metasedimentary rocks studied ranges from greater than 90% phyllosilicates (biotite and musco- vite), at the most pelitic end, to about 30% phyllosilicates in the coarser metasand- stones. In addition to this variation in the ratio of mica to felsic minerals, the biotite to muscovite ratio decreases toward the pelitic end members. On the microscopic scale, the finer-grained rocks are often well laminated parallel to schistosity (SJ, the parallelism being the result of extensive small-scale transposition of bedding (S0). On the outcrop scale, however, a small angle is usually discernible between S0 and S!. S, is defined by a strong preferred orien- tation of mica (001) and a weak dimen- sional orientation of the quartz and feld- Figure 1. Location and geologic maps of the Macclesfield-Strathalbyn area. The material for this spar, the latter being more pronounced as study was mainly collected from near the anticlinal hinge, south of the Macclesfield, Strathalbyn road. the mica content increases. TABLE 1. STRUCTURAL ELEMENTS AND DEFORMATIONAL HISTORY MICROSTRUCTURE OF CRENULATIONS AND Folding episode and Type Comments CRENULATION CLEAVAGE structural element There are two sets of crenulations in the Fi! So Bedding Usually at a low angle to S„ but fine-scale layer- ing may be transposed parallel to S, area (S2 and S2' in Table 1), both with sub- F„S, Schistosity or cleavage Defined by preferred orientation of (001) of mica vertical axial planes, one striking predomi- and by thin (<l-mm) laminations rich in nantly due north (S2), the other, due east highly oriented mica (S2'). Overprinting lelationships between F„ L, Mineral elongation Defined in pelitic rocks by elongation of mica these structures are somewhat ambiguous, within S, and andalusite grains and fanning of mica which suggests contemporaneity, but their (001) about L,; usually steeply pitching in S[ geometry is inconsistent with a conjugate and thus at a moderate angle to Bi shear pair origin. All observations recorded F„B, Axes of rare folds in Plunges shallowly south in general here were made on the north-striking set, S with S, as axial plane 0 whose axial planes are parallel to the axial and S -S, intersections 0 planes of the large folds in the area. The F2, S2 Crenulation cleavage or Always steep and striking close to due north layered schistosity in quantitative data in Tables 2, 3, and 4 were pelites; cleavage in collected from a single specimen in which some psammites the complete gradation from uncrenulated F2', S,' Crenulation