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

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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 (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 axial planes; Vertical and striking due east; no consistent S, through crenulations to a layered crenu- rarely crenulation overprinting relationship to S2 lation cleavage is present (Fig. 2a). Mea- cleavage surements of modal composition, mineral F2, L2 and F2', L2' Intersection of S, with Often expressed as a crenulation lineation on S, chemistry, and grain size and shape were S2 and S2', respectively made in three regions of one thin section from this specimen — a region of uncrenu- TABLE 2. VARIATIONS IN GRAIN SHAPE AND SIZE DURING DEVELOPMENT lated Sj (Fig. 2b), one where there are low- OF CRENULATION CLEAVAGE amplitude crenulations of S! without obvi- ous mineralogical segregation, and one Mineral and parameter Structural regime with a moderately developed layering paral- Uncrenulated St Crenulated S, Mica Quartz-feldspar (Aim) (/¿m) domain of S2 domain of S2 lel to crenulation axial planes (Fig. 2c). In (fim) (/xm) the last of these, separate measurements were made in the mica-rich (M domains) 142 (29.6) 157 (33.6) Mica long axes 140 (31.2) 122 (24.8) and quartz-feldspar—rich (QF domains) Mica short axes 13 (8.4) 12 (7.7) 8 (5.1) 20 (12.3) layers parallel to the crenulation axial Quartz and feldspar long axes 168 (42.3) 177 (43.4) 162 (35.2) 177 (40.6) Quartz and feldspar short axes 68 (17.9) 59 (17.0) 42 (14.6) 80 (19.0) planes. Each domain or layer forms a whole limb of a relatively angular microfold; M Note: 300 measurements averaged for each, standard deviation of each data set given in paren- domains occupy the long limbs, and QF theses. domains occupy the short limbs. The data

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Figure 2. (a) Sketch (modified from photomi- crograph) illustrating nature of progression from uncrenulated S, (on left) with increasing micro- folding to a well-developed layered crenulation cleavage within a single thin section. The three regions from which the data of Tables 2, 3, and 4 were collected are shown, (b) Photomicrograph from left end of thin section illustrated in Figure 2a, showing uncrenulated microstructure and beginning of weakly crenulated region in upper right corner. Two small garnet porphyroblasts are visible in upper center. Plane-polarized light; width of field is 2.5 mm. (c) Photomicrograph from right end of thin section illustrated in Figure 2a. Plane-polarized light; width of field is 2.S mm.

in Tables 2, 3, and 4 do not apply directly from 2 to 10 mm and in amplitude from 0.5 crenulations become tighter and a crenula- to all of the examples studied from this to 5 mm, with most in the lower half of tion cleavage develops. Changes in grain area, but the relationships that they illus- these ranges. They are generally asymmetric size and shape accompany the earliest de- trate are certainly representative. and range from open with moderately velopment of crenulations, with quartz and Crenulations in range in wavelength rounded hinges to more angular as the feldspar grains becoming slightly smaller and more elongate parallel to the crenu- TABLE 3. MODAL COMPOSITIONS OF STRUCTURAL DOMAINS lated schistosity (Table 2). There is no Mineral Uncrenulated S , Crenulated S, M ica domain Quartz-feldspar mineralogical segregation at this stage (Table 3), and changes in mica grain size of S2 domain of S2 and shape are also very small (Table 2). Quartz 24.2 22.8 9.8 38.1 Undulose extinction and deformation Plagi ociase 12.8 13.2 6.2 11.3 lamellae in quartz, twinning in feldspar, Biotite 44.4 44.3 29.8 27.6 and kinking of mica are very rare in both Muscovite 17.8 18.5 53.1 22.8 uncrenulated and crenulated rocks, and no Opaque minerals 0.8 1.3 1.1 0.3 increase in the prevalence of these struc- Note: Percentage compositions obtained by point counting in thin ! section; at least 800 points tures accompanies the measured grain counted. Plagioclase identified by staining. elongation. Even in the more angular crenu- lation hinges, where the radius of curvature TABLE 4. PERCENTAGE COMPOSITION OF STRUCTURAL DOMAINS CALCULATED is smaller than the length of mica grains, FROM MODAL ANALYSES AND MINERAL CHEMISTRY intracrystalline deformation is rare. The microstructure at the early stages of Uncrenulated S i Crenulated S, Mica domain Quartz-feldspar development of a layered crenulation cleav- of S domain of S 2 2 age varies considerably with mineralogy.

Si02 54.4 53.2 47.7 63.8 The more micaceous rocks usually develop Ti02 0.9 0.9 0.8 0.6 well-formed crenulations, and the layering AI2O3 18.2 18.5 26.4 16.7 results primarily from mineralogical CaO 0.6 0.6 0.3 0.5 changes in adjacent limbs of the crenula- FeO* 5.0 4.9 3.6 3.3 tions (Fig. 2b). In more psammitic rocks, MgO 10.0 10.6 7.6 6.1 well-formed crenulations are less common, K2O 6.2 6.2 7.8 5.0 and discrete mica-rich domains develop Na20 1.4 1.5 1.2 1.3 parallel to and continuous with the long Total 96.7 96.4 95.7 97.3 limbs of the microfolds in adjacent pelitic Note: Analyses were carried out by A. R. Milnes on the Cambridge Instruments Geoscan electron layers (Fig. 3). The mica in these domains microprobe at the CSIRO Division of Soils, Adelaide, Australia. The elements were determined using has the trace of (001) at a low angle to the analyzed mineral specimens as standards, and corrections to the raw data were made according to the domain boundary with the same sense to S2 method of Sweatman and Long (1969). as the deformed S, (Fig. 3). A third struc- '' Total iron calculated as FeO. ture, most common in the psammitic end

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bles 2 and 3, one can calculate the average number of mica grains or quartz and feldspar grains per unit area of thin section in the various microstructural regions. Each square millimetre contains about 340 mica grains in the uncrenulated region compared to 850 and 160 in M and QF domains, re- spectively. The corresponding figures per square millimetre for quartz and feldspar are 32, 24, and 35 grains, and there are 98, 544, and 73 muscovite grains and 244, 305, and 88 biotite grains per square millimetre, Figure 3. Photomicrograph of a more psam- respectively. Figure 4. Photomicrograph of one M domain mitic rock showing a narrow, vertical M domain and one QF domain showing the variations in Previous studies of layered crenulation grain size, shape, and mineralogy that are de- and some discrete mica grains subparallel to S2. The original orientation of S, is just north of east. cleavage (for example, Rast, 1965; Nichol- scribed in the text and enumerated in Tables 2 Note that the mica within the M domain is more son, 1966; Williams, 1972) have described and 3. Taken from the same specimen and the muscovite-rich than the remainder of the rock the differences in composition between the same region as the detailed measurements. and that mica (001) orientations within the do- layers equivalent to our M and QF do- Plane-polarized light; width of field is 1.3 mm. main are slightly oblique to it. Plane-polarized mains. However, only Williams was able to light; width of field is 1.3 mm. make a comparison of these compositions ite mentioned above is that FeO and MgO with that of the uncrenulated rock, and are lower in both domains than in the un- members, results from the appearance of then incompletely and somewhat indirectly. crenulated rock. This therefore represents a disseminated mica grains (both biotite and The specimen mentioned above, with the change in the bulk composition of the rock muscovite) with (001) either parallel to the gradation over a few centimetres from un- on a scale greater than two domains. In an crenulation cleavage (that is, domain crenulated St to a layered crenulation attempt to detail such a change, various boundaries) or at a low angle to it, again cleavage, thus offers an excellent opportu- combinations of the M- and QF-domain with the same sense as the deformed S,. The nity to document the changes in composi- compositions (all normalized to total microstructure which predominates in a tion, grain size, and shape which accom- 100%) have been calculated for compari- given specimen depends on the strain (as as- pany the layering development. son with the original rock (Table 5). With- sessed from the degree of apression of cre- Table 3 compares the mineralogy of un- out knowledge of volume changes on the nulations), as well as the mineralogy, with a crenulated and crenulated S, with both M scale of a domain, the appropriate combi- well-developed layering more common at and QF domains. The domain compo- nation cannot be determined, but the data higher strains except in the most psammitic sitions both differ from that of the original given in Figure 8 and the discussion below rocks. The latter merely develop a new mica rock, with the most striking change being indicate that a decreased weighting should preferred orientation subparallel to the the increase in the muscovite content of be given to M domains because of relative crenulation cleavage in interlayered psam- both domains. This is achieved at the ex- volume loss from them. It is also suggested mites (see below). pense of all other major phases in M do- by Figures 2b, 2c, and 8 that the most satis- Changes in grain size and shape of all mains and accompanies an increase in the factory weighting in the measured example components accompany the development quartz content of QF domains, primarily at will lie in the range from Vi(M + QF) to of mineralogical layering parallel to crenu- the expense of biotite. In order to assess V2C/2M + QF). This would indicate that lation axial planes (Fig. 4, Table 2). Mica, chemical changes related to these the proportion of Si02 and A1203 had in- quartz, and feldspar grains all have higher mineralogical changes, electron microprobe creased slightly and that the proportion of aspect ratios (length/width) and are smaller analyses were made of each mineral from MgO and FeO had decreased significantly. in M domains (micas by 45%, quartz by the same localities as the modal analyses. Again, without knowledge of bulk volume 25%) — mica by reducing all dimensions, Combination of the modes and mineral changes, the exact elemental movements but quartz and feldspar primarily by reduc- chemistry provides a good approximation cannot be determined uniquely. However, tion in their shorter dimension (normal to to the bulk compositions of the various if a small volume loss (10%) and a weight- S,). In QF domains, mica, quartz, and domains (Table 4). There is no detectable ing of V2 (%M + QF) is assumed, it can be feldspar grains are more equidimensional consistent difference between the major- shown that only MgO and FeO have been and larger than in the uncrenulated rock element compositions of the same mineral removed in significant quantities (that is, (mica by 75%, quartz and feldspar by in different domains, and thus the bulk >>10%) and that A1203 has remained 25%), and again, mica increases in all di- compositions of Table 4 reflect the almost unchanged. This last observation at mensions whereas quartz and feldspar in- mineralogical variations (Table 3). A con- least is consistent with our limited knowl- crease mainly in width (Table 2). By com- sequence of the increased proportion of edge of elemental movement during bining the grain-size and modal data of Ta- muscovite primarily at the expense of biot- metamorphic reactions (Carmichael, 1969).

TABLE 5. VARIOUSLY WEIGHTED COMBINATIONS OF NORMALIZED MICA-DOMAIN (M) AND QUARTZ-FELDSPAR (QF) COMPOSITIONS FOR COMPARISON WITH ORIGINAL UNCRENULATED ROCK COMPOSITION

Normalized Normalized V2 (M + QF) V2 (% M + QF) '/2 (V2 M + QF) V2 (Vi M + QF) V2 ('/5 M + QF) Original M domain QF domain rock

Si02 49.8 65.6 57.7 59.2 60.4 61.6 63.0 56.3 Ti02 0.8 0.6 0.7 0.7 0.7 0.7 0.7 0.9 AI2O3 27.6 17.2 22.4 21.4 20.6 19.8 18.9 18.8 CaO 0.3 0.5 0.4 0.4 0.4 0.4 0.5 0.6 FeO* 3.7 3.4 3.6 3.5 3.5 3.5 3.4 5.2 MgO 8.3 6.3 7.3 7.1 6.8 6.8 6.6 10.4 K2O 8.2 5.1 6.6 6.4 6.1 5.9 5.6 6.4 Na20 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.4 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

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Figure 5. Photomicrograph of sharp, tight crenulation hinges illustrating the lack of intra- crystalline deformation structures in the mica — despite the large finite strains and variable strain histories involved. Note also the variations in mica grain shape and biotite/muscovite ratio from crenulation limbs to hinges, and the ab- sence of mica with (001) parallel to crenulation axial planes. Plane-polarized light; width of field

Figure 7. Photomicrograph of a crenulation in adjacent highly pelitic and psammitic layers. De- spite the absence of quartz and feldspar in the pelitic layer, changes in the mica grain shape and biotite/muscovite ratio, without any visible intracrystalline deformation, are apparent. The amplitude of the crenulation dies out rapidly in the psammitic layer, again without any intracrystalline defor- mation evident. Plane-polarized light; width of field is 3.2 mm.

Evidence of intracrystalline deformation sitions of the maximum concentrations in and recovery is very rare in all microstruc- the symmetry plane are asymmetric relative tural positions, and there are no partially to both the deformed S, orientation within recrystallized grains. The quartz is optically the domain and the domain boundary (S2). clear, almost totally without undulose ex- A case that illustrates many of the above tinction or deformation bands, and in more features particularly clearly is shown in macaceous regions (for example, M do- Figure 7. A virtually pure mica layer has mains), isolated single grains surrounded by been crenulated, and the variations in grain mica are the rule, rather than aggregates shape and (to a lesser extent) mineralogy, that would suggest a recrystallization stage together with the sharp changes in S, orien- in the microstructural development. tation without intracrystalline deformation, Plagioclase is clear and untwinned, and are evident. Note also that the crenulation biotite and muscovite contain only rare, dies out in the psammitic layer without any very open kinks. The last of these observa- grain deformation apparent. tions is particularly striking in regions such Where the S2 layering is only weakly de- as that illustrated in Figure 5, where large, veloped, M and QF domains are about the rapid changes of S, orientation are accom- same width and have nearly parallel plished with very little intragranular de- boundaries. However, where the layering is formation. Quartz c-axis orientations were more pronounced, the M domains are rela- measured in the uncrenulated region, QF tively narrower (Fig. 8), and ultimately domains, and M domains of the specimen form an anastomosing network around that was analyzed in detail above. In the highly elongate QF domains. In such struc- uncrenulated region and in the QF domains tures, the M domains define the cleavage, the preferred orientation patterns appear to and mica (001) within these domains is very be close to random (Figs. 6a, 6b), but in the strongly oriented parallel (or at a very low M domains, where quartz grains are fewer angle) to them. The mica (almost exclu- and more elongate, a moderate preferred sively biotite) in the QF domains has its in- orientation is developed (Fig. 6c). This pat- herited S, orientation and becomes more tern has monoclinic symmetry; the po- equidimensional and less well oriented without any apparent internal deformation, ^ Figure 6. Equal-area stereographic proj- producing a number of (001) planes at high ections of the distribution of quartz c axes in var- angles to crenulation axes. ious microstructural domains: (a) uncrenulated The ultimate development of these struc- S,, (b) QF domains, (c) M domains. Each dia- tures in the pelitic rocks produces a layered gram is the result of measurements of 300 c axes. schistosity, with little or no remnant of the The distributions were contoured on the lower pre-existing S,. It is similar in most respects hemisphere using a counting-cell area of 3% of to the strongly layered structure described the hemisphere area, by means of the computer- program SUBPLOT on the Burroughs 6700 above, except that mica (001) preferred computer at Monash University. Contours repre- orientations are parallel to the layering (S2 sent 1%, 2%, and 3% per 1% area, and the in both domains (weak to moderate in QF plane of projection is perpendicular to crenula- domains, strong in M domains), and the tion axes. anastomosing structure is enhanced. There

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6-

U. § O -c -c T3 í¡ 4- 5 i

2-

20 30 40 50 Angle between domain boundary and S, in M domain Figure 8. Graph of the ratio of QF domain width to M domain width plotted against the angle between S, in the M domain and the domain boundary. This angle is a measure of the degree of appression of a crenula- tion. The graph indicates that M domains become preferentially narrower with increasing deformation.

Figure 9. Histograms showing the orientation of the trace of mica (001) in adjacent psammitic (a) and pelitic (b) layers in a strongly crenulated rock. The orientation of the trace of the crenulation axial plane is marked, and the initial S, orientation would have been approximately perpendicular to this prior to crenulation.

90 60 30 0 30 60 90 Angle between mica (001) and crenulation axial plane trace

is still no evidence of intracrystalline de- This specimen is from the hinge of a small and that even a combination of them is not formation of any of the phases in either F2 fold, and the psammite cleavage is paral- entirely satisfactory. We shall begin by domain. lel to the crenulation cleavage. However, in summarizing the important aspects of the the more common limb areas, some cleav- data, and we shall then discuss the vital MICROSTRUCTURE OF age refraction is observed, and the psam- question of postdeformational (mimetic) ef- THE PSAMMITES mite and crenulation cleavages may be as fects on the microstructure. This will be fol- much as 30° apart. lowed by an outline of possible stress and

In rocks with less than 30% to 40% Specimens containing both the S, and S2 strain histories on the scale of the mi- mica, formation of crenulations is appar- slaty cleavages are rare. They consist of crofolds and finally by the discussion of de- ently inhibited by the absence of a well- mica (predominantly biotite) of two main formation mechanisms. developed Sj in which folds can nucleate. orientations — parallel to S[ and S2, respec- Where such rocks are interlayered with pe- tively — in a granoblastic aggregate of ap- Summary of Critical Data lites containing weakly to moderately de- parently undeformed quartz and plagio- veloped crenulations, they retain the Sj clase. In all our examples the earlier cleav- 1. The layering or domainal structure cleavage little modified, and none of the age is dominant. that defines the crenulation cleavage is minerals shows significant intracrystalline closely related to microfolds in a pre- deformation or recrystallization. However, DISCUSSION existing schistosity. M domains occupy the in the hinge of F2 folds, where crenulation long limbs and QF domains occupy the cleavage development is advanced in the pe- One of the main aims of this paper is to short limbs of a set of usually asymmetric lites, many of the psammites have a mica present data that enable a comprehensive microfolds. (001) preferred orientation subparallel to evaluation of established microscopic-scale 2. Considerable variations in the grain the crenulation axial planes (S2) in the pe- deformation mechanisms. We hope to show shape of each mineral are related to crenu- lites. The mica orientation relationships for that at least some of the data are difficult to lation cleavage development, and grains of one such specimen are given in Figure 9. reconcile with each of these mechanisms all shapes have few if any preserved defor-

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mation substructures visible in the optical tensive annealing recrystallization, espe- will be shown below, this conclusion is of microscope. cially because such recrystallization must considerable significance to mechanistic 3. Similarly, grain-size variations are have been very advanced to remove all evi- discussions. dence of intracrystalline deformation even found without any preserved evidence of Stress and Strain History strain-induced recrystallization (for exam- in the lowest-amplitude crenulations where of Crenulations ple, deformed and partially recrystallized strain must have been least. Such advanced relics or discrete fine-grained aggregates of recrystallization would be expected to pro- The first stage in the crenulation process quartz similar in size to individual grains in duce more equiangular and equidimen- involves folding (generally asymmetric) of the uncrenulated rock). sional aggregates, particularly in areas the pre-existing S! schistosity. This must 4. The mineralogy and chemistry of both where the driving forces (that is, strain produce heterogeneities of stress and strain domains and of the bulk crenulated rock energy) for recrystallization were highest. along S, (Ramsay, 1967; Dieterich, 1969; differ in quite complex ways from those of This is not the case in these rocks. In addi- Hobbs, 1971, 1972). In the early stages of the uncredulated rock. The most notable tion, the limited data on mica deformation development, where crenulations are change is an increase in the proportion of and recrystallization indicate that kinked rounded, this heterogeneity is gradational, muscovite in layered regions as a whole. grains in the hinges of microfolds would but as the folds become tighter and more This is accompanied by a small increase in have kink-band boundaries at low angles to angular, discrete short and long limbs (in the proportion of A1203 and a marked de- microfold axial planes and that recrystalli- the asymmetric case) form that must have crease in the proportions of FeO and MgO. zation along such boundaries would pro- had different stress and strain histories. The 5. For each of the two domains and the duce grains with (001) near those axial earliest recognizable variations in grain uncrenulated region, the average number of planes (Etheridge and others, 1973; shape and mineralogy along crenulated S, grains of each mineral per unit area of thin Etheridge and Hobbs, 1974; T. H. Bell and surfaces are related systematically to this section is different. M. A. Etheridge, unpub. data). No such stress and (or) strain variation, and it is thus 6. In the more psammitic rocks (less grains were found in these rocks. considered to be fundamental to their de- than about 35% mica), crenulations do not The role of mimetic crystallization is velopment. occur, and a new mica orientation develops more difficult to determine, especially be- Because in comparison to stress, strain in by the appearance of disseminated mica cause the term lacks a precise, generally ac- rocks is easier to interpret from microstruc- grains parallel to S2. In rocks with weakly cepted definition. We regard mimetic crys- tures and is more readily related to fold developed S2, the orientation distribution of tallization to include all processes in a non- shapes, we shall discuss its role in some de- undeformed mica (001) is bimodal, with deforming environment that give rise to tail first. In addition, the progressive spatial one group parallel to St and the other paral- overgrowths on existing grains, partial so- development of crenulations in individual lel to S2. lution and pseudomorphous replacement of specimens may provide some clue to strain Any proposed mechanism(s) for produc- existing grains, and growth of new grains history. Reliable strain markers are absent ing the crenulation cleavage in these rocks which mimic the microstructure of the host from the rocks in our study, but the follow- must be compatible with all of these data. crystal. The mineralogical and chemical ing observations provide some information. However, before considering possible variations can hardly be ascribed to any 1. In the long limbs (M domains), mechanisms, some evaluation must be such processes, unless muscovite replaces grain-shape changes indicate that there was made of the importance of postdeforma- biotite to varying extents in different mi- a significant component of finite shortening tional microstructural effects. crostructural environments after crenula- normal to the deformed S, wherever it tion formation, and we consider this most makes an angle of less than about 40° to POSTDEFORMATIONAL unlikely. Similarly, to produce or even crenulation axial planes (S2). MICROSTRUCTURAL CHANGES preserve the delicate variations in grain size, 2. In the short limbs (QF domains), shape, and preferred orientation by these where S, remains at a high angle to crenula- Any microstructural study of metamor- processes in a static environment either re- tion axial planes throughout, the grain- phic tectonites must attempt to evaluate the quires particularly selective overgrowth and shape changes are consistent with limited effects of postdeformational processes. For solution without external controls, or a finite shortening subparallel to S,. example, one of the features that is com- close reproduction of the pre-existing mi- 3. The relative intensity of these grain- mon to many slates and almost all schists crostructure. The only mechanism that we shape changes suggests that the amount of and gneisses is that most of the mineral can envisage to accomplish the first of these finite strain in the long limbs is greater than grains (especially the phyllosilicates) are op- alternatives is that suggested by Oertel that in the short limbs at all stages where tically undeformed (neglecting the effects of (1970) for increasing mica preferred orien- they are readily distinguishable. deformation that obviously postdates for- tation in a slaty lapillar tuff by mimetic 4. The long limbs (M domains) become mation of the metamorphic foliation). It is overgrowth. However, this mechanism re- narrow relative to the short limbs (QF do- essential to interpretation of the micro- quires an increase in mica grain size and is mains) as the crenulations become more structures of these rocks to determine if (1) thus wholly inapplicable to the M domains appressed (Fig. 8). this is the result of postdeformational ad- in these rocks. The second alternative — 5. In low-amplitude crenulations of a pe- justments (for example, annealing recrystal- that of reproduction of a previous, lite with interlayered psammites, there is lization or mimetic crystallization) or (2) deformation-induced microstructure — is little or no evidence of strain in the psam- the deformation took place without large- obviously irrelevant to discussion of defor- mites on the microscopic scale. Changes of scale production and movement of lattice mation mechanisms operative during de- grain shape and orientation in the psam- defects, especially dislocations. Since we are velopment of the layered crenulation cleav- mites do accompany the advanced stages of always looking at the end product of the age. crenulation cleavage development in the sum of whatever processes were involved, We therefore consider that the observa- pelites, however. this is generally not easy to determine. Our tions recorded above effectively document 6. In most cases, there appear to be little present example is no exception. the microstructural changes that took place variation in strain along either domain However, the very complex relationships during the formation of a layered crenula- within any one pelitic layer and no dis- of grain size and shape to microstructural tion cleavage, with little or no change being placement along boundaries between do- position are difficult to rationalize with ex- due to postdeformational adjustments. As mains.

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Any interpretation of the strain history of with crenulation development. Grain-shape examples, metamorphic grade is at least as the crenulation process must be compatible and grain-size changes in a deforming rock high as in the rocks described above (Ver- with all of these observations, and this can be accomplished by four mechanisms; non and Ransom, 1971; Ross, 1973; Bell allows us to reject the two simplest his- they are (1) slip and (or) twinning, (2) selec- and Etheridge, 1973), and the strain is tories. First, solely simple shear in the M tive addition of material to and (or) re- higher without complete recrystallization. domains parallel to the domain boundary is moval of material from different parts of In addition, the preservation of undulose inconsistent with observations 2 and 4 existing grains (for example, Coble creep, extinction and subgrain structures in a wide above, since both require a component of pressure solution), (3) the complete re- range of deformed rocks indicates that re- shortening perpendicular to domain moval of existing grains and the growth of crystallization is relatively inefficient under boundaries. Second, flexural slip (kinking) new grains of a different shape, and (4) slic- long-term static conditions. In this paper on the deformed schistosity surface is in- ing (fracturing) of grains by essentially brit- we have described examples with a very compatible with almost all the observa- tle processes. wide range of strains, from just detectable tions. The absence of displacement of pre-S2 If intracrystalline deformation has been a to near-isoclinal crenulations, and yet there (for example, fine laminations and mica major contributor to grain-shape changes, is almost no optical evidence of intracrystal- grains) surfaces across domain boundaries recovery and recrystallization must have line deformation. We suggest that this is suggests that there is little difference in the been particularly efficient in order to re- due to the limited role of dislocation pro- pure flattening components normal to do- move all optical evidence (for example, un- cesses rather than complete recrystalliza- main boundaries in adjacent domains, un- dulóse extinction and subgrains in quartz, tion. In addition, there is no evidence in less there are differential volume changes kinks in mica, twinning in plagioclase, and these rocks of the substantial grain-size re- (see below). Also observation 5 above indi- partially recrystallized relicts of all miner- duction that commonly accompanies early cates that the overall shortening of pelitic als). In other examples where gradational stages of recrystallization, especially in layers during at least the early stages of cre- changes in microstructure have been re- polymineralic rocks. nulation is small. corded (especially mylonite zones, as The penetrative brittle fracture necessi- On this basis, we suggest a combination studied by Christie [1963], Ransom [1969], tated by mechanism 4 is also unlikely at this of three separate components of an and Bell and Etheridge [1973], evidence of metamorphic grade, so the changes in grain idealized crenulation strain history: (1) intracrystalline deformation is widespread, shape must have been primarily due to progressive simple shear in M domains, (2) especially at lower strains. In some of these mechanisms 2 or 3, or to a combination of progressive flattening in both domains with them. shortening at a high angle to the domain If it can be shown that substantial new boundary, and (3) preferential volume re- mineral crystallization has taken place dur- duction of M domains (Fig. 10). We do not ing crenulation, then mechanism 2 above mean that these three components are suc- has not been the sole one operative. The cessive steps in the path, but observation 5 mineralogical evidence suggests that such suggests that there is some change in their mineral growth has occurred. In the exam- relative importance with time, with com- ple from which the modal data were col- ponents 2 and 3 dominating at higher lected, a 65% volume loss from M domains strains. In addition, bulk volume changes is necessary to explain the increased pro- are probable but not estimable, with an portion of muscovite if there has been no overall reduction being most likely. new muscovite growth. However, the Likely stress variations around the mi- grain-size data and the calculations of crofolds are much more difficult to assess. numbers of grains per unit area of thin sec- Stephansson and Berner (1971) and Hobbs tion require a volume loss of greater than (1972) calculated the stress distributions 80%. None of the other mineralogical data around mathematically modeled folds, and supports such a large volume reduction, it is evident that both differential and mean and the general appearance of the layering stresses are likely to be higher in most limb does not seem consistent with it either. The regions, except in instances where pure only possible conclusion is that nucleation buckling is involved (Stephansson, 1974). and growth of muscovite has taken place However, stress distribution on the grain- during the crenulation process in M do- size scale will be complex (Raj and Ashby, mains and possibly within QF domains 1971). also. In summary, therefore, we suggest that In addition, the variations of muscovite the strain in the long limbs (M domains) of Figure 10. A highly schematic illustration of grain size and shape with microstructural the various strain components suggested to con- the microfolds will be higher and that they position are closely related to those of all tribute to crenulation cleavage development; a —* will undergo a preferential volume reduc- b = simple shear parallel to domain boundary in the other minerals. It is thus evident that, tion (Holcombe, 1973). Nothing conclusive M domain; b —» c = flattening perpendicular to during formation of the crenulation cleav- can be inferred about stress distribution. domain boundary in both domains; c —» d = vol- age, controls on the nucleation and growth ume reduction of M domain without loss of con- of new mineral grains existed that dictated Deformation Mechanisms tinuity on domain boundary. In the rocks, there their shape, size, and location. If the miner- is probably some simple shear in QF domains (of als other than muscovite are essentially If we assume that postdeformational ad- opposite sense to that in M domains), the flatten- modified relict grains, then the same con- justments have had little or no effect on the ing may not be perpendicular to the domain trols must have affected their modification. boundaries, and some volume change may take grain shape, size, and mineralogical var- We would thus suggest that the deforma- place in QF domains. Note, however, that some iations described above, they must have change in length of a material line originally tion accompanying crenulation cleavage taken place during the deformation. They parallel to S, is envisaged in both domains, unlike formation took place by a combination of are therefore primarily the result of the the strain histories of Williams (1972) and Hol- (1) modification of the shape, size, and stress and (or) strain variations associated combe (1973). orientation of existing grains (especially

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mica), essentially by diffusive mass-transfer simply that variations in strain magnitude the rock and any fluid phase present as af- processes accompanying bodily rotation, or rate on the grain and domain scales are fected in differing manners in adjacent do- (2) migration of material, on the scales of not necessarily related systematically to var- mains by any facet of the deformation. grains and domains, controlled by the de- iations in stress magnitude. It is thus concluded that the mass transfer formation path and microstructural an- The contribution of processes driven by involved in crenulation cleavage formation tisotropies, and (3) nucleation and growth variables other than differential stress is likely to be controlled by a wide range of of grains with an orientation and shape should also be evaluated. If any of these effects of the deformation and not simply compatible with the stress and (or) strain processes are independent of stress and po- by differences in stress magnitude between history and microstructural anisotropy in tentially faster than pressure solution, then microstructural sites. This is quite consis- their locality during their nucleation and they will dominate, as Elliott pointed out. tent with Elliott's (1973, p. 2659 and fol- growth. For example, deformation is usually ac- lowing) concept of a deformation map with The relative contribution of these is companied by metamorphism, which fields dominated by either diffusive mass difficult to assess, as are the precise commonly involves bulk removal or addi- transfer or dislocation flow. The main dif- mechanisms and controls. This is primarily tion of a fluid phase. The driving forces for ference lies in the nature of the controls on because so little is known about the this long-range mass transfer will be related this mass transfer. mechanisms of metamorphic reactions and to variables such as temperature gradients, crystal growth, and about the nature and total pressure, fluid partial pressures, and CONCLUSIONS role of diffusing phases in a deforming envi- the chemical potentials of the various ronment. phases involved in the metamorphic reac- The formation of a layered crenulation One class of mechanisms that could con- tions taking place. It is feasible that these cleavage is accompanied by chemical, tribute significantly to the deformation could contribute significantly, even where mineralogical, grain shape, grain size, and suggested above has been treated by Elliott stress differences are very small. If so, what preferred orientation changes. These (1973) under the heading of "diffusion flow is likely to control the direction of mass changes are related spatially to pairs of laws." Elliott considered a range of stress- transfer as evidenced in many rock struc- limbs of asymmetric microfolds to produce controlled diffusion mechanisms (for tures? In any rock developing a metamor- mica-rich domains in the long limbs and example, pressure solution, Herring- phic foliation, diffusional anisotropics will quartz-feldspar-rich domains in the short Nabarro creep, Coble creep) and showed result from grain elongation and the con- limbs. In symmetric folds, the changes pro- that strain rate is linearly dependent on sequent grain-boundary preferred orienta- duce mica-rich domains in the limbs and stress for each. This led him to suggest a tion. We therefore have potential mass quartz-feldspar-rich domains in the hinges. general diffusion flow law for lower-grade transfer arising from the solely "metamor- The progressive development of crenula- metamorphic rocks, but he was unable to phic" driving forces mentioned above, and tion cleavage over short distances in these define precisely the diffusion path or the na- a self-propagating diffusional anisotropy rocks enables comparison between both ture of the diffusing species. However, he from the earliest stages of mineral and domains and the uncrenulated rock. Both suggested that grain-boundary diffusion grain-boundary preferred orientation. The domains change chemically, mineralogi- dominated over volume diffusion and mass diffusional antisotropy is self-propagating cally, and microstructurally without any transfer in solution via pores for several because it controls mineral nucleation and optical evidence of intracrystalline defor- examples figured in his paper. growth and the modification of existing mation by dislocation processes (gliding Durney (1972, 1974) also derived a rela- grains such that the anisotropy is enhanced. flow). tionship for mass transfer in solution that A possible example of this has been de- The chemical and microstructural falls into the above category, but his scribed by Etheridge and others (1974), changes are attributed to a combination of theoretical treatment has been criticized by who grew mica with a strong preferred preferred nucleation and growth of new Paterson (1973). Durney attributed a wide orientation from its chemical components grains, shape and size modification of exist- range of rock structures to such a mass- during quite rapid deformation. In all cases, ing grains, and rotation of grains, all ac- transfer process, as have Plessman (1964), stresses were less than 1.2 kb, and the de- complished dominantly by diffusive mass- Williams (1972), Durney and Ramsay gree of preferred orientation was shown to transfer processes. (1973), and Gray and Durney (1974). be a function of finite strain but not of Even though no quantitative data are There are several objections to this appli- stress magnitude. available, we suggest that the driving forces cation of solely stress-induced flow mech- How does this relate to the formation of for this diffusive mass transfer arise from anisms to the development of metamorphic a layered crenulation cleavage? We believe the chemical reactions and the strain and microstructures in general and to this cren- that the strain and volume-change anisot- volume-change heterogeneities, rather than ulation cleavage in particular. The first ob- ropics established in the early stages of solely by variations in stress magnitudes as jection stems from the very small different- crenulation are important controls on the suggested by Elliott (1973), Durney (1972), ial stresses likely in geologic situations and chemical and mineralogical changes ob- and others. The directions of mass transfer the heterogeneity of stress on the grain-size served. These changes require mass transfer will be a function primarily of the micro- scale. Elliott (1973, p. 2647) inferred that a on the scale of grains and domains, and the structural, physical, and chemical aniso- reasonable geologic normal stress is 3.5 kb; preferred directions of mass transfer will be tropies arising from the pre-existing schis- however, it is the differential stress that controlled primarily by the structural tosity and the developing domainal struc- drives the diffusive processes, and this is anisotropies (for example, the pre-existing ture. We suggest that this group of controls likely to be an order of magnitude smaller. schistosity and the domains as they devel- supplements those of classical pressure so- The stress heterogeneities necessitated by op) and any other deformation-induced lution as defined by Durney (1972) or El- maintaining continuity in a deforming physical or chemical variations (for exam- liott (1973). polycrystalline material (Raj and Ashby, ple, pressure or diffusivity differences be- 1971) will further compound the problem tween domains). Thus, material could mi- ACKNOWLEDGMENTS of achieving a realistic deformation rate for grate either along or between domains as a the aggregate at small differential stresses. result of these controls. For example, effec- Part of the work for this paper was car- This is not to say that there is no functional tive mass transfer from one domain to ried out with the support of an Adelaide relationship between stress and strain, but another could occur if equilibrium between University Research Grant Scholarship for

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Marlow. Many people have contributed by Bull., v. 84, p. 2645-2664. Ramsay, J. G., 1967, Folding and fracturing of helpful discussion or by reading and crit- Etheridge, M. A., and Hobbs, B. E., 1974, Chem- rocks: New York, McGraw-Hill, 568 p. ical and deformational controls on the re- Ransom, D. M., 1969, Structural and metamor- icizing the manuscript at various stages, crystallization of mica: Contr. Mineralogy phic studies at Broken Hill [Ph.D. thesis]: especially Bruce Clark, Jim Granath, Vol- and Petrology, v. 43, p. 111-124. Canberra, Australian Nad. Univ. ker Hirsinger, Bruce Hobbs, Roye Rutland, Etheridge, M. A., Hobbs, B. E., and Paterson, Rast, N., 1965, Nucleation and growth of Vic Wall, Richard Groshong, and Win M. S., 1973, Experimental deformation of metamorphic minerals, in Pitcher, W. S., Means. Rod Holcombe is acknowledged single crystals of biotite: Contr. Mineralogy and Flinn, G. 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