The prograde microfabric in a deformed quartzite sequence, Mount Isa, Australia

Tectonophysics, 19 (1973) 39-81 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE, MOUNT ISA, AUSTRALIA C.J.L. WILSON Gologisch

en Mineralogisch lnstituut der Rijksuniversiteit,

Leiden (The JVetherlands)

(Accepted for publication October 26, 1972)

ABSTRACT Wilson, C.J.L., 1973. The prograde microfabric in a deformed quartzite sequence, Mount Isa, Australia. Tectonophysics, 19 (1973): 39-81. The changes in quartz microfabric, which includes microstructure and c-axis patterns of preferred orientation, are examined in a prograde regional metamorphic sequence of pure quartzites: (1) initially undeformed, round and strain-free detrital quartz grains are modified with deformation and metamorphism to become flat and elongate in the foliation and contain &formation bands and lamellae (chlorite zone); (2) new strain-free grains appear which have c-axis orientations controlled by the old host grain (chlorite and biotite zone); (3) these new grains grow to eliminate old grains (biotite zone) and form a polygonal microstructure (biotite-cordierite zone); (4) abnormal growth of a few grains within the polygonal aggregate results in large grains with complex three-dimensional shapes (sillimanite zone). The quartz c-axis pattern accompanying these microstructures, changes successively from random to peripheral girdles, to crossed girdles, to random to either very strong girdles or maxima. The microfabric is also very dependent on the presence or absence of accessories such as mica. These microfabric observations are interpreted in the light of recent metallurgical theories and the results obtained from experiments performed on geological materials. It is believed that the variation in microfabric may be attributed to the operation of processes involving intracrystalline slip mechanisms or intracrystalline slip accompanied by recovery and recrystallization; these processes can occur simultaneously and are a reflection of different physical conditions existing in the same metamorphic sequence.

INTRODUCTION

In a deformed sequence of quartzite

at Mount Isa it has been possible to investigate

the

evolution of a coarse-grained metamorphic rock from an undeformed sediment. The aim of the microfabric investigation has been to examine the development of microstructure and preferred orientation in a prograde metamorphic sequence; these results are then interpreted in the light of recent metallurgical ideas and models and on the basis of results achieved in recent geological experiments. Although many geologists have recognized different orientation patterns at varying grades of metamorphism, m’any of which recur repeatedly in different tectonic and metamorphic environments, none have been related to differing physical conditions that may exist during the deformation and metamorphism through a comparison of microstructure.

40

V.J.L.. WILSOM

GEOLOGICAL SETTING

The Mount Isa region contains a sequence of deformed Precambrian quartz-rich sediments, volcanics, and dolomitic-rich sediments, which have been described by Carter et al. (1961), Bennett (1965) and Wilson (1972). The oldest of these Precambrian rocks are the Leichhardt metamorphics (Farquharson and Wilson, 1971), which consist of quartz-rich gneisses and acid metavolcanics. Progressing upwards is a highly faulted sequence which includes: (1) the Argylla Formation, a sequence of acid metavolcanics; (2) Mount Guide Quartzite, a sequence of quartz-rich sandstones with a few siltstone or shale horizons; (3) Eastern Creek volcanics, a sequence of basic volcanics with intercalated lenses of quartzite known as the Lena Quartzite; (4) Myally Beds, comprising siltstones, sandstones and conglomerates; and (5) the Mount Isa Group of dolomitic shales and siltstones. All the quartzites, however, used in this investigation underlie the Mount Isa Group and were collected from the Mount Guide Quartzite, Lena Quartzite, Myally quartzites and quart&es from within the Judenan Beds. The Judenan Beds are equivalent to the Myally Beds and outcrop to the West of the Mount Isa fault; this fault separates the area into two different portions of the same sequence differing only in their relative metamorphic grades. All the lowermost-grade quartzites have been taken from different units in the eastern portion of the area (Fig. 1). These units are similar in composition, have equivalent grain sizes and premetamorphic microfabric features. The quartzites west of the Mount Isa fault (Fig.2) are from identical units but differ in microstructure because of a higher metamorphic grade and degree of deformation. Specimens were also chosen beating the following points in mind: (1) There was a similarity in composition between specimens with quartz/mica ratios of > 90/10. (2) The grain size in a given thin section was essentially homogeneous, although this was not always possible in the higher-most-grade quartzites where there are complex threedimensional grain shapes. (3) Specimens were taken from limb areas of the folded units and from areas uncomplicated by second-generation fold structures or any sign of retrogression. In the area sampled it is possible to divide the structural history into three structural events (Wilson, 1970,1973): (1) Formation of first-generation folds. These are tight folds in which bedding (s) has been locally transposed into an orientation parallel to the axial planes (S,) of the folds. (2) Second-generation folds often coinciding with the peak of the prograde metamorphism. Second-generation folds are also confined to zones of intense deformation which overprint first-generation structures. These.zones include the Mount Isa fault and anumber of other almost parallel north-south trending zones (These are faults I and I.1 in Fig. 3 see also Wilson, 1973). (3) Local, open third-generation folds which are confined to the second-generation deformation zones.

PROGRADE MlCROFABRIC IN A DEFORMED QUARTZlTE SEQUENCE

41

Fig. 1. Simplifid geological map of the area to the east of the Mount Isa fault showing the location of the quartzite specimens.

42

C..I.L. WILSON

Fig.2. Simplified geological map of the area to the west of the Mount Isa fault showing the location of the quartzite specimens. The grid coordinates are the same as the Mount Isa Mines grid coordinate system.

The macroscopic structure at Mount Isa is complicated by large regional faults, many of which parallel the general north-south strike of all the units and accompany either the first- or second-generation deformations. The first-generation faults have irregular outcrop

PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE

Fig. 3. Distribution of the metamorphic Isa fault, fault I and fault III.

zones

43

in the Mount Isa area whichare separatedby the Mount

patterns, have been subjected to the regional metamorphism and are refolded by the second deformation event. The second generation of faults (see Fig.3) separate the area into four zones of different metamorphic grade each of which is characterized by the presence of similar deformation episodes and abundant quartzite units. The metamorphic subdivisions were established on the basis of mineral assemblages recognized in the pelitic rocks (Fig.3). They are zones which represent the first appearance of sillimanite, biotite-cordierite, biotite and chlorite, respectively. MICROSTRUCTURE

The microstructures of the quart&es can be conveniently divided into six main groups on morphological appearance. These subdivisions and their relationship to the metamorphic zones are shown in Fig.4. The demarcation between these main groups is not always clear, invariably because of accessory minerals, such as mica, which affect the grain growth of the quartz and therefore obscure the distinctsubdivisions.

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WILSON

Fig.4. Variation of quartz microstructure with metamorphic grade. The higher-grade continuations of the coarsening, polygonal and abnormal coarsening microstructures depend on the presence or absence of mica impurities.

Quartzites with dem’tal.grains These are found in the low-grade rocks (east and northeast of Mount Isa) where the quart&es are well sorted, equigranular, and composed of well-rounded elastic quartz grains. The mean grain size varies from 0.3 to 0.2 mm between different specimens, Undulatory extinction is almost ubiquitous in the quartz. Deformation lame&e are widespread, but not common, and deformation bands are very rare. Secondary quartz overgrowths are common in nearly all these quartzites with grains becoming interlocked so that they cannot be separated, as depicted in FigSA. There is no evidence of any subgrains within any of theundulatory extinction zones; they are inherited features predating any post-depositional changes such as secondary overgrowths. There is no obvious dimensional orientation of these quartz grams. The effect of deformation is evidenced by a development of undulose extinction in nearly all grams; gram boundaries become sutured or stylolitic (FigSB) but very few new deformation lamellae are observed in the weakly deformed quart&es. Further defomration results in the production of a pronounced dimensional preferred orientation commonly parallel to the slaty cleavage, Sr .

Fig.5. A. Quartzites with undufosmed~erical and. ol&& detrital quartz grains enclosed by irregular secondary overgrowths whi@ m@tain the sqmd optical -z&ntation as the enclosed detrit;rl w. The boundary bet&n the overgrOwtband_ the detriti-grain is generally outlined by very fme i&lusions. B. Deformed detrital quartz grains which ah& undilose extinc&on and have strongly sutied and .irree ular grain boundaries. (Scale 0.5 mm.)

PROGRADE MICROFABRIC

IN A DEFORMED QUARTZITE

SEQUENCE

45

Quartzites with old and new quartz grains

An apparent reduction in the grain size takes place with further deformation and increased grade of metamorphism. The grain size of the detrital quartz is reduced with the appearance of new grains. The quartz-quartz boundaries have a variable appearance being highly irregular and sutured. The grain boundary suturing becomes extreme where the rock consists of closely packed quartz grains while on the other hand, and often in the same specimen, detrital grains in areas of high matrix percentages of mica retain their detrital shapes with very little evidence of new grain development. The new grains are generally sharply delineated from their host by a distinct orientation difference and a sharp grain boundary which tends to be cuspate into the old grain, especially in the larger grains. The new grains are generally completely free of undulose extinction, unlike their hosts, and have three distinct modes of occurrence: (1) as small new grains along the margins of old grains; (2) as isolated new grains within old grains; or (3) as a series of new grains along the region of misorientation between two adjacent subgrains within an older host grain. Where new grains develop on the margin of a number of contiguous old grains it is often difficult to establish with which old grain the new grain may be associated (Fig.6A). It is only possible to identify the host to new grain relationship clearly in the case where isolated new grains are found within an old, or where an old grain is isolated from its neighbours (Fig.6B) by a mica or a rare dolomitic matrix. The average grain size of the new grains is approximately 0.05 mm, which is significantly smaller than the average grain size of the old host grains. In the very quartz-rich rocks, the new grains occur along the long margin of the elongate old grains and are commonly elongate themselves in one particular direction, which is usually parallel to the slaty cleavage. Along the shorter edge of these old grains which generally, but not always, lies perpendicular to the foliation, the new grains tend to have a polygonal shape, but are still characterized by sutured grain boundaries with gently curved interfaces, many of which are concave outwards. In many of these quartzites it becomes impossible to distinguish between the new grains and the relic portions of old grains, as illustrated in Fig.7A. The only positive identification of old grains is the presence of very fine deformation lamellae, deformation bands and strong undulose extinction. Many of these separate old grains (Fig.7B) may in fact belong to the one grain, but are now separated from one another by a set of new grains which occur on the boundaries of former deformation bands in the old grain. Coarsening

A marked change may be noted in the morphology of the quartz grains as one crosses from the biotite zone to the cordierite-biotite zone. The shape of’the grains becomes more equant and In some cases verges on polygonal; grain boundaries are clearly defined, irregular in outline, and have many gently curved faces, concave away from the centres of the grains and are smaller than the old grains (Fig.7C). The quartz is completely free of undu-

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t. I 1.. WILSON

PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE

41

lose extinction and deformation lamellae. Small amounts of muscovite may be contained on the quartz grain boundary margins as extremely fine-grained blades. In the more micaceous portions of these rocks quartz-mica-quartz boundaries are straight and the quartz grains have distinctly planar interfaces which bound regular-shaped grains. There is an apparent increase in grain size in these quartzites, from the quartzites with old and new grainc, but there is still a large number of relatively small grains; the grain size is not homogeneous in any portion of the rock but varies from 0.05 mm to 0.15 mm. The irregular shape and curved nature of these grain boundaries strongly suggest a migration of the grain boundary towards its centre of curvature, if it is analogous to a metal (Smith, 1948). This is particularly so in many of the larger grains whereas some of the smaller grains still have similar sizes and shapes to the new grams. Polygonal grains

The formation of aggregates of polygonal shaped, equant quartz grains, having smoothly curved to straight interfaces, represents another type of microstructure. The grain size varies according to the percentage of accessories, especially mica, present in the rock and ranges from 0.25 mm when there is less than 10% mica impurities to 0.06 mm with 20% mica. The shapes of these grains in any three orthogonal thin sections are polygonal, and sixsided grains appear to be most common, although grains with a greater or a lesser number of sides are also abundant (Fig.irD). Most grains are simple polygons with dihedral angles of approximately 120”. The only marked variation in shape and dihedral angle occurs where accessory micas are present. In general, the micas are smaller than the length of the edges of the quartz grams and occur between quartz-quartz boundaries. Here the dihedra. angle of (001) mica versus quartz-quartz is generally 180”. Occasionally more than one quartz grain may meet a large mica blade. In such cases the quartz-quartz interfaces, quartz-(001) mica interfaces have angles close to 90”. The ends of these mica blades are well defined and round, and are commonly partially included by quartz. Total inclusion of the smaller micas by quartz is observed, but is not common. In many of these quartzites the presence of large mica plates results in the formation not of polygonal shaped grains but a series of straightedged almost square- or rectangularshaped grains (c.f. Fig.8). Quartz-mica junctions are straight with the quartzes markedly elongate along this boundary in comparison to the shorter and irregular quartz-quartz -.----_-- ._.- -_.--_.-.. --. Fig.6. A. Old detrital quartz grains enclosed by new grains. The old grains now consist of several differ. ently oriented deformation bands, with the trace of the majority of these bands being east-west, parallel to the orientation of slaty cleavage in this particular specimen 36. Surrounding these old grains are smaller irregular shaped new grains completely free of any undulose extinction. (Scale 0.25 mm; crossed nicols.) B. A large old grain which appears to consist of two separate halves about a line of small new grains. The two ha!ves have slightly different orientations, both contain the same deformation bands, trending northeasterly, and fine deformation lamellae which lie perpendicular to the deformation bands. Surrounding the outer margin of this old grain are new grains. (Specimen 36; scale 0.1 mm; crossed nicols.)

Fig.7. A. ~n~tin~~shab~ area of old and new grams. (Specimen 12; scale 0.1 mm; crossed nicois.) 3. Relic of an old detrital grain surrounded by numerous new grains. The old grain contains poorly defined deformation bands and a series of very tine deformation lamellm, trending northeasterly. The new grains have a tendency to be elongate parallel to the boundary of the old grain, which k also parallel to the deformation bands. Many of the apparently separate old grains in such quartzites are differently oriented deformation bands of a much larger oid gain, but now separated by a number of new grains. ‘This photomicrograph illustrates an example of this, for another small old grain with similarly oriented deformation lamellae, presumably a retie of the same old grain, lies to the south of the large old grain; the grain containing the scale. (Specimen 37; scale 0.1 mm; crossed nicols.)

oblique

to the plane of the section.

(Specimen

16; scale 0.

I mm; crossed

nicols.)

C. Coarsening in specimen 14. The grain boundaries are highly irregular, the majority of grains are completely strain free except for a few isolated grains which possess broad deformation bands. (Scale 0.25 mm; crossed nicols.) D. Polygonal-shaped grains in a much larger aggregate of similar grains. Boundaries are planar or only very gently curved, mica inclusions are generally confined to grain boundaries, whereas small opaques and feldspar may be included by the quartz. Many of the curved and diffuse grain boundaries are

50

(:.J.L. WILSON

PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE

51

Fig.8. A and B. Effect of accessory mica on the development of microstructure in specimen 20. The quartz-mica boundaries are straight and many of the quartzes develop irregular shapes. Mica is generally included in the grain boundaries, but if it protrudes into the quartz grains it has characteristically rounded ends. C. Quartzite with polygonal shaped grains. (Specimen 38; scale 0.5 mm for all photomicrographs; crossed nicols.)

boundaries.

Between any two mica grains the quartzes are strongly elongate and generally

consist of single grains.

Abnormal coarsening Within many of the rocks with well-developed

polygonal microstructures

there appears

to be an abnormal growth of a relatively few isolated grains (Fig.9). This splits the microstructure into two clearly separate groups, namely the new coarse irregular grains and the remaining relatively unaltered ture” commonly

polygonal matrix grains; this gives rise to a “duplex struc-

described in the metallurgical

literature.

The quartz-quartz

boundaries

are generally gently curved being concave outwards and highly cuspate. The shape of the boundaries often is consistent with the idea that the grains are migrating towards their centres of curvature, which usually lie in the polygonal matrix area enclosing these abnor. mally large grains. Micas and other accessories such as feldspar, apatite and magnetite are no longer, or only rarely, confined

to grain boundaries.

They are completely

enclosed by the large quartz

Pig. 9. A. Superficially this microstructure resembles an aggregateof polygonal shaped grains except for the irregular quartz --quartz b:jundarics, whereas the quart7mica hour&r& are similar to the polygonal aggrcgatcs and are straight. Many of the micas arc either totally or partially included within the quartz grains. (Sprcimr!~ 2%) B. Abnormally large, irregular shaped grain in a matrix of polygonal shaped quartz grains.. Quartz-quartz boundaries between the polygonal grains are either straight or gently curved and boundaries meet at triple points to form 120” dihedral angles. (Specimen 17.)

PROGRADE ~ICROFABRIC

IN A DEFORMED QUARTZITE SEQUENCE

53

Fig.lO. Exaggerated grain growth in two adjacent thin sections from specimen 28. Many of the apparently separate grains are the one grain.

54

f:.J.L.

WILSOh

‘ROGRADE MICROFABRIC 1N A DEFORMED QUARTZITE SEQUENCE

Fig. 11. The effect of accessory minerals on exaggerated grain growth. A and B are two thin sections from the same handspecimen (27). C occurs as a layer of fine-grained quartzite 30 cm away from A and B. (Specimen 25.) It appears that the many small grains of microcline (stippled) and muscovite (hatched) appear to inhibit grain growth.

grains. The micas within such large quartz grains have pronounced rounded ends and have the same preferred orientation as those in the polygonal areas. Exaggerated grain growth This microstructure overlaps with the previous one, for the abnormal growth of these highly inequant irregular grains proceeds to such an extent in many of the high-grade quartzites that they develop into interlocking masses of irregular quartz grams in which no small polygonal grains are present. (Fig.10 and 11). In three dimensions (Fig.12) they are also very irregular in shape, for many grains which are seemingly separate in thin section are actually part of the same irregular grain. Thus, small grains with orientations similar to any one particular large grain were noted in many other areas in the same thin section. They generally lie well within the spread of c-axis orientations of the larger grain, l-4”, and many have identically oriented deformation features such as deformation lamellae ar.d undulose extinction if the quartzite has been affected by second-generation folding. Although no connection or juxtaposition is obvious, it is believed that many of these quartz grains are joined outside the section, similar to the irregular crystals which have been described from ice and the large secondary recrystallized grains found in metals. Most of these large irregular grains are free from substructure but a number of rocks from areas subjected to second-generation folding appear to have a well-formed regular substructure,

PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE

57

Fig. 13. Exaggerated grain growth in specimen 29, with the development of many new subgrains within these larger “secondary recrystallized” grains as a result of a second deformation of the rock. Abundant microcline inclusions are contained within the quartz grains. (Scale 4 mm; crossed nicols.)

as illustrated in Fig.13. This is a regular pattern of square to rectangular subgrains within broad zones of slightly differing orientation. The variation in the orientation of the c-axis between the subgrains within any one old grain is never more than 6”. Boundaries are generally hard to resolve and very closely spaced deformation lamellae are often fairly abundant in such rocks. l7ze variation of grain size in the sequence

The quartz grain size can be observed to both decrease and increase as the metamorphic grade increases, but the manner and rate of change in mean grain size varies quite considerably depending upon the slightest variation in the accessory content of the quartzite. This therefore places a very strong limitation on any attempt to obtain a quantitative definition for the rate of change of grain size of quartz deformation and/or metamorphic grade. The situation is also further complicated by the presence of sudden jumps in metamorphic grade across the major second-generation faults. Although a qualitative and semi-quantitative study shows that there is quite a strong relationship between microstructure and grain size in rocks with less than 10% accessory mica, no attempt has been made to define the

i-.J.L. WILSON

Fig.14. Variation of quartz grain size versus change in microstructure. The intermediate stages of microstructuraI development, coarsening and abnormal coarsening are not induded on this graph. Samples are from quartsites with more than 90% quarts. These changes also correspond to change in metamorphic grade: (1) undeformed detrital grains occur in the chlorite zone or lower; (2) old and-new grains occur in the chlorite and biotite zone; (3) polygonal grains occur in the biotite-cordierite zone; and (4) the exaggerated grains occur in the sihimanite zone. Them is an appreciable reduction of the grain size in the old grains with the appearance of the new grains which are generally smaller than the relic old grains. The grain size then increases during the period of coarsening which ultimately forms a polygonal microstructure. As a result of abnormal coarsening the grainsize in the rocks with exaggerated grain growth varies from 1 mm to 2 cm. The upper limit is not shown on the graph.

rate of change of grain size of the quartz with increase in metamorphic grade because of the complications outlined above. Instead Fig.14 shows the variation of gram see between the four main microstructural groups and excludes the intermediate stages of coarsening and abnormal coarsening. DISCUSSION OF MICROSTRUCTURE

Recognitionof microstnrcture

In 1960 Voll showed the possible application of physical metallurgical principles to a study of microstructure in silicate and carbonate rocks and detailed studies of such silicate rocks have recently received attention from such workers as Hobbs (1966), Kretz (1%6a, b, 1969), and Vernon (1968, 1970). The grain:&apes these authors Were concerned with are similar to those found jn annealed_metal aggregates, and consist of primary recrystallized polygonal grains. As a result of extensive study&e origin and nature of-thed_&ing forces for recrystallization are now understood (seeBeck and Hu, 1966). Although the fac-

PROGRADE MICROFABRIC

IN A DEFORMED

QUARTZITE

SEQUENCE

59

tors which determine the mobility of the grain boundary of these recrystallized grains are, however, not so clearly understood, they have been reviewed by Lticke and Stiiwe (1963) and Aust (1969). Recrystallization is used throughout this paper to refer to new grains which have developed as a result of the migration of high-angle grain boundaries. Recovery is the result of the movement of dislocations to form sub-boundaries (polygonization) or even large-angle boundaries in the form of subgrains (Beck, 1954) with central regions of very low dislocation densities. The term primary recrystallization is used in most metallurgical literature to mean isothermal recrystallization of cold worked materials, the process obeying certain kinetic laws. These laws were discussed by Burke and Turnbull (1952) for isothermal annealing and can be represented empirically by either the increase of average grain size with time or by percentage of material recrystallized versus time. The kinetics for recrystallization involve an incubation period after which new grains begin to grow at a number of sites. In most of the ciassical studies of recrystallization (cf. Beck, 1954), both primuty and secondary recrystullization are described in terms of such a process, namely the nucleation of new grains and their growth. This assumes that primary recrystallized grains form at the onset of the release of the stored energy of plastic deformation and are independent of any pre-existing stages. In much of the recent metallurgical literature (cf. Li, 1966; Sellars and Tegart, 1968; Hu, 1969) many workers now believe recovery processes, which involve the formation of subgrains, may act as nucleation sites for subsequent recrystallization either by a coalescence of adjacent subgrains or a subgrain rotation mechanism. These then grow by the migration of high-angle boundaries. In this paper such a process will be distinguished from the classic nucleation ideas for primary recrystallization. It is believed that subgrains form as a result of recovery, and these recovered subgrains act as the nuclei for the primary recrystallized grains. Therefore no true nucleation stage exists. Secondav recrystallized aggregates which have been recognized by metallurgists are characterized by “discontinuous grain growth” or “coarsening” of relatively few grains which become very large, and absorb the finer polygonal primary recrystallized matrix. This microstructure is characterized by a marked grain-size contrast between the coarser grains and the matrix and has been called “duplex structure” (Beck, 1954). Very few detailed descriptions exist in the geological literature of secondary recrystallization (some ex amples are Harris and Rast, 1960; Ramsay, 1962), although many of the rocks described by Behr, 1961; Carstens, 1966; and Wenk, 1965, 1966, could possibly be examples of sub. a phenomenon. i%e evolution of microstnrcture at Mount Isa The microstructures described above resemble those of many metals or ceramics (cf. Heuer et al., 1968; Farag et al., 1968); the only fundamental differences arise from the type of material, inherited inhomogeneities such as sedimentary features, anisotropy in the

shape of the quartz detritus, and the presence of matrix material. Generally metallurgists and ceramists are only concerned with extremely pure material with known quantities and type of second-phase impurities, a variable which cannot be accurately controlled in geological environments. The deformed detrital grains appear to be flat and elongate in the plane of the foliation. The likelihood that this ~mension~ orien~tion of the quartz is a primary feature accentuated by solution parallel to cleavage (Rlessman, 1964) cannot be eliminated, although there is no strong evidence for such a mechanism. The rotation of constituent grains into a plane monocular to the direction of ln~~urn shorten~g, has been considered an import~t mechanism by a number of authors, including Sorby (1853), Flinn (1965) and Ramsay (1967). Although the latter theory is probably applicable to the quartzites in which there is a high percentage of matrix material, it is unsatisfactory in pure quartzites because spaces must open at grain boundaries during the deformation. Instead a mechanism employing intracrystalline slip (Bishop and Hill, 195 1) is probably more likely to account for their elongate shape, for here the strain is homogeneously distributed and hence the strain undergone by an in~vidual grain is approx~ately the same as that experienced by the whole of the deformed body. The formation of new grains and subgrains has been illustrated in many metallurgical works both during cold working and hot working con~tions, as a result of either recrystallization or recovery. An electron microscope study would also be necessary to establish whether the observed new grains in the Mount Isa quartzites do in fact develop from recovered grains by a process involving subgrain coalescence and rotation (Li, 1962 and Hu, 1963). In the present study evidence of a distinct nucleation stage was not obvious. Similar observations have been recorded by geologists in many natural quartzites (F.C. Phillips, 1937; Fellows, 1943; Riley, 1947; W.J. Phillips, 1965; and Hara et al., 1966) and in experimentally deformed quartz (Hobbs, 1968). The marked grain-size reduction of the old grains which accompanies the development of the new grains is similar to observations that have been made in metals by Wonsiewicz and Chin (1970). Here single crystals of cop per were observed to divide into four ~fferen~y oriented regions by a process involving surface friction during plastic deformation, These new individuals were then capable of deforming as separate old grains on five independent slip systems. The growth of the new quartz grains results in the formation of the cuspate and apparently migrating gram boundaries found in the quartzites of the coarsening stage; and here old and new grains are often indistinguishable. This probably corresponds to the phenomenon described by metallurgists as solid state grain adjustment (Smith, 1948), and is generally an unstable sta8e in the microst~c~r~ development. The larger grains tend to have concave boundaries whereas the smaller grains have convex boundaries. In the smaller grains the boundary will migrate towards its centre until it disappears, whereas the larger grains, with concave boundaries, will tend to grow in size. As the number of grains per unit volume decrease the average grain size increases. This grain boundary migration results in grain shapes which approach an equilibrium condition (Kretz, 1966a and b) with strain

PROGRADE MICROFABRIC IN A DEFORMED QUARTZITE SEQUENCE

61

free polygonal grains, triple junctions, and 120’ interfacial angles. This also closely corresponds to the appearance of the quartz grains described in the polygonal stage of microstructural development. The abnormal coarsening and exaggerated grain growth observed in the quartzites appears to be equivalent to the process of secondary recrystallization widely recognized by metallurgists (Beck, 1954; Dunn and Walter, 1966). A prerequisite for secondary recrystallization is the formation of a stable primary recrystallized matrix structure. Much of the strain accumulated during the deformation of the Mount Isa Quartzite was probably removed by recovery and recrystallization. However, the grain shapes in the higher-grade quartzites, for instance the presence of equilibrium polygonal microstructures and abnormally coarse growing grains, may also suggest that there was some post-tectonic movement and adjustment of grain boundaries (involving local diffusion) after the syntectonic recrystallization. To distinguish between the two is difficult, for metallurgists have demonstrated that comparable microstructures can be developed both during processes of hot working (Farag et al., 1968, p.63) and of annealing (Beck, 1954). Evidence in favour of thy former at Mount Isa is the presence of late syn-kinematic cordierite porphyroblasts, and other metamorphic minerals, in some of the schists interbedded with the quartzites (Wilson, 1970, 1972). For in such cases the mica foliation is flattened around the porphyroblasts and the quartz is completely strain-free. As there is no evidence for rapid changes after the deformation, the temperature gradient and time were probably sufficient to allow some grain adjustment in a solid state to occur under essentially annealing conditions. Unfortunately the contribution of such grain-boundary adjustment under a subsequent annealing condition cannot be estimated from the observed microstructure. But it was probable that some local diffusion on the scale of a few grains may have occurred which was responsible for the growth and adjustment of many grains. QUARTZ C-AXIS ORIENTATION PATTERNS

The specimens selected for the study of preferred orientation were chosen on the basis of position in the metamorphic sequence and on the particular type of microstructure developed. The location of the specimens are indicated in Fig.1 and 2. All c-axis measurements have been rotated into their geographical orientation to provide a common basis for comparison and contoured according to the method proposed by Kamb (1959b)*. Quartzites with detrital grains

Two distinct patterns of c-axis orientation may be recognized in the quartzites with de* In all orientation diagrams, unless otherwise stated, the equitorial plane is the horizontal plane; data are plotted on the lower hemisphere; N on the diagrams points to north. In the explanatory text, E is the number of points expected to fall within a given test area A, which is the area of the counting circle; D is the standard deviation from the expected density of a uniform population, and the counter intervals are 217.

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C.3 .L. WILSON

Fi 15. Pattern of c-axes for quartz in the quart&es .containm# detritai grains. Spec-imen~ 1,Z *slid3 am comparative@ undeformed 300 gra&s measured in each, except 4, and all are contdumd accordhg to Kamb (1959bj: A = 0.029; E = 8.7; u = 2.9; con* irrtervals: 0,2,4,6u. I, maxima = 7.80; 2, maxima = 7.90; ?, maxima = 7.60; 4 (500 gains, A = 0.018, B = 8.6, s = 2.9) maxims = 9.h; 5, maxima = 8.60; 6, maxima = 8.6~; 7, maxiha = IO&u. trital grains; both of which caabg directly cor@ated witi the mkro+uctwe. Theaideto any of-*e formed quartzes, show in @J.{S, ti@go ~8 ~~:~~ thin sections measured, and non9 is ieprgdugW~~T&wpattsmr 81% charwte+tie ofgrains showing very good secondary over@rowthstid occiu?ing as tarpsinterlocking @ins.

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8

11

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12

Fig.16. Pattern of c-axes for quartz in the quartzrtes contaming 300 old and new grains. All are contoured according to Kamb (1959b): A = 0.029, E = 8.7, o = 2.9; contour intervals: 0, 2,4,6u. 8, maxirna = 11.30; 9, maxima = 8.60; 10, maxima = 9.3a; 11, maxima = 9.60; 12, maxima = 11.3~.

The deformed detrital grains on the other hand have quite distinct, reproducible patterns; within any one specimen, they are generally partial girdles with broad maximas lying in planes inclined at a high angle to the foliation and lineation. The strongest pattern of preferred orientation, specimen 7 (Fig. 15) occurs in the most highly deformed of these rocks where there are very elongate detrital quartz grains. Quartzites with old and new grains

In rock with both old and new grains the pattern of c-axis orientations (Fig.16) does not differ significantly from the pat:em found in the deformed detrital grains. Both patterns appear to be unrelated to the orientation of any surface such as bedding or slaty cleavage and both are characterized by large volumes parallel to the lineation devoid of c-axis. So that the c-axis tends to lie at high angles to the foliation. The only significant modification of the previous pattern is the presence of minor crossed girdles, especially noticeable in specimens 8,9 and 10 (Fig.16) where there is a shift of the maxima out of one particular plane into two slightly inclined planes. In a number of these specimens it was possible to establish the relationship between the old and new grain c-axis patterns. Only grains that were effectively isolated, generally against another mineral phase such as dolomite or muscovite, or those in which the relationship of new grain to a single old grain was unambiguous, were measured. In Fig. 17 old

C.J.L. WILSON

Fig. 17. Relationship between the taxes of old host quartz grains and the c-axes of the new quartz grains, and the combination of both old and new grains. Each diagram is contoured according to Kamb (1959b); contour intervala: 0,2,4,6o. The hlato~ains show freqriency, F, of anples, 8, between c-axes of new grain and c-axes of t&e immedialqly ..i .adjacent old grain. Superimposed on the histogram is a curve showing the theoreticaI‘distribu-tion-between two tines, one of fixed snd one of random orientation. 4 Combined, 300 grains. A = 0.029;E = 8.7; u = 2.9. Maxima = 11.00.4 Old, 150 grains. A = 0.057; E= 8.5;~~ 2.8. Maxima = 7.417.4 New, 150grains.A= O.O57;E= 8.5;0= 2.8. Maxima = 9.90. 11Combined,200grains.A=0.043;E~8.6;u=2.9.Yaxima=8.7u. 11Old,100gains.A=0.083; E = 8.3; u = 2.8. Maxima = 8.40. 11 New, 100 grains. A = 0.083; E = 8.3; u = 2.8. Maxima = 8.70.

and new quartz grains have been separated and the resulting preferred orientations of c-axes are compared. There is a strong suggestion that the c-axis pattern of the new grains reflects the pattern of the old grains, but the maxima for the new grains are not as clearly defmed as the old host grains. There is also quite a distinct angular relationship existing-between old and new c-axis when plotted on a frequency diagram: most fall between the intervals of 30-SO’, and there appears to be a,sharp decrease in the numbers which he at a high angle to the oid grain. A comparison has been illustrated Hth the distribution of a random population (Plumrner, 194O,p.83)expresaedas a frequencypercent. The angle between the old and new c-axis does not exhibit any =_tendency, towarda randomness and there is a strong resemblance to the experimental resul& of Hobbs (I9@. Hobbs attributes this tendency for new grains to form30-50” from the old-grain, to-a host-control phenomenon. A .similar result has also been established in the. quartz of .some deformed gneisses by Ransom (197 1).

65

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N

N

13 Fig.18. Patterns of c-axes for quartz in the quartzites containing coarsening. 300 axes, all are contoured according to Kamb (1959b); A = 0.029, E = 8.7, IJ= 2.9, contour intervals: 0, 2,4,6u. 13, maxima = 8.60; 14, maxima = 9.60; 15, maxima = 8.30.

The strength of the crossed-girdle pattern is more noticeable (Fig.18) with a strong tendency away from the peripheral and marginal crossed-girdle patterns observed in the rocks containing the numerous old grains. Associated with these c-axis patterns is a disappearance of the large pole-free areas, with a tendency for the crossed girdles to disappear also. Polygonal grains The c-axis patterns (Fig. 19) of the polygonal-shaped quartz grams are characterized by a random or very weak preferred orientation with small pole-free areas, very low symmetry and low maximum concentrations. The highest major contour interval is 40; with very lo. cal small irregular maximas of 60, the 40 is only just above what Kamb (1959b) considers to be random (that is 30).

17

18

Fig.19. Pattern of c-axes for quartz in the quart&es containing polygonal grain aggregates. In 16 and 17 300 c-axes were measured and contoured according to Kamb (1959b); A = 0.029, E = 8.7, u = 2.9. Contour intervals 0, 2,4,6a. 16, maxima = 8.60; 17, maxima = 7.90; 18 (600 grains, A = 0.015, E = 8.9, u = 3.0) maxima = 7.80.

(1J.L.. WIL.SON

Fig.20. Histograms to Illustrate the random property that may be observed in c-axis distributions in a stereographic projection. (a) Distribution in 4 diagrams each of 50,100,200,300,500 randomly oriented space directions (poles) obtained with the aid of an electronic computer. (b) Examples from Mount Isa specimens which show different but characteristic microstructures. Distributions comparable with the random distributions in (a) are only observed in the quartzites with undeformed detrital grains, polygonal shaped grains and abnormally coarse grains.

To establish whether these patterns were truly random each specimen was measured as two separate diagrams which were then compared. No reproducibility was obtained between any of the partial diagrams; the totals shown in Fig.19 are the sum of the partials. To further test the random property of these diagrams, the area occupied by each contour interval was calculated and plotted as a frequency percentage against contour interval (Fig.20).* These results were then compared with results obtained using random numbers to generate random-pole diagrams. A significant property of ail random diagrams was the very high concentration of poles between 20 and 40 with small areas higher or lower than these limits. Where a preferred orientation existed, the large area between 2a and 40 disappeared and this area of pole occupation has been split between the higher contour values or an area of no poles. ,The histograms showing frequency of area between contours to contour interval for rocks with pdygonal gram aggregates closely conform to a random pattern. *The area was calculated during the plotting of the diagrams using an 1A.M. 360/50 computer and an I.B.M. 1403 printer. The frequency percentage wascalculated as the number of grid points faIlIng between contour ir?tervals against the total number of grid points over a 2O-diameter net, which totalled approximately 4,900 grid points..

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Fig.21. Pattern of c-axes for quartz in the quartzites containing abnormally coarse grains. 300 grains measured in each. Contoured according to Kamb (1959b);A = 0.029, E= 8.7; u = 2.9; contour intervals: 0, 2,4,6, 80. 19, maxima = 7.2~; 20, maxima = 7.90; 21, maxima = 8.90; 22, maxima = 8.20; 23, maxima = 9.30; 24, maxima = 10.30; 25, maxima = 12.00; 26, maxima = 11.00.

Abnormal coarsening The c-axis pattern for rocks containing both polygonal grains and abnormally large grains

is illustrated in Fig.21. They are characterized by extremely strong maxima, unrelated to the prominent foliation, and have larger pole-free areas than those found in the polygonal grained aggregates.

C.J.L. WILSON

Fii.22. Pattern of c-axes for-quartz in the quartxjtes exhibiting.an exaggwatedgrein grow*. 300 grains measured in each extipt numbers -29 and 34,.&itound accoidhgto I&nb (1959b);A = 0.029, E = 8.7, u = 2.9, contour intwvak 0, 2,4,6,&. 27, maxima= 10.30; 28, maxima= 15;4u; 29, a cpmbiaation ofmeas~mments from three orthogorapl sections, totaling 331 c-axes. A = O.U26,‘= 8.8, u = 2.9. khimr = 9;6u. 30, maxima = 10.00; 34, number of c-axes 257; 35, maxima = 11.70. A = 0.034, E = 8.7, u = 2.9. Maxima 9.60.

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Specimens 19 and 20 have patterns very similar to that of the polygonal grained aggregates, and the microstructure confirms this as they are composed predominantly of polygonal grains with occasional abnormally large grains. Exaggerated grain growth

This group of rocks is characterized by either very strong single maxima or has broad girdles containing small submaxima (Fig.22). There is no systematic relationship to the bedding or the slaty cleavage, which are parallel to one another in all specimens. The patterns of c-axis orientations recognized are: (1) girdles with c-axes parallel to the foliation as in 30; (2) girdles lying at a high angle to the foliation as in 27 and 34; (3) point maxima close to the foliation as in 29,3 1,32,33 or 35, or at a very high angle to the foliation as in 28. The effect of accessory mineral phases on the quark c-axis patterns of preferred orientatiO?l

The effect of accessories, such as feldspar and more importantly mica, on the quartz c-axis patterns of preferred orientation in rocks with microstructures varying from detrital to polygonal grain aggregates is not markedly noticeable. The patterns of preferred orientation obtained are reproducible in adjacent mica-rich and mica-poor rocks, and the microstructure is modified to a very minor extent. The effect of accessories, especially mica, in rocks showing abnormal coarsening is to restrict the grain growth and hence preserve the previous pattern of preferred orientation. This is especially noticeable in a number of adjacent specimens. In 27 (Fig.22) a quite distinct girdle pattern containing three very high maximas exists, whereas 25 (Fig.21) has a much more diffuse girdle and the c-axes are distributed over a greater percentage of the net area. In the case of 25 (Fig.21) the microstructure on first appearances is superficially that of a polygonal grained aggregate (cf. Fig. 1 lC), but a closer examination reveals that many of the quartz-quartz grain boundaries are highly curved, whereas the quartz- microcline and quartz-muscovite boundaries, which are numerous, tend to be straight. It is the accessory microcline and muscovite grains which have restricted abnormal grain growth and have thus affected the ultimate pattern of preferred orientation. In general, most of the rocks exhibiting exaggerated grain growth have probably had their quartz c-axis patterns of preferred orientations modified by accessories to a minor extent. Quark-quark

relationships in the quarkites with exaggerated grain growth

In the extremely coarse-grained quartzites it became necessary to record the position (Fig.10, 11 and 12) and orientation of each individual grain because of their extremely

complex shapes, for many apparently separate grains were observed to have orientations similar to other nearby grains. These separate grains were presumed to be one grain, but

C.J.L. WILSON

2

F% .”

25 1

!tF 127

Fig.23. Hiatogrags sbwingfreq~ncy c-axis of the adjaceht grain.

7

34

Y3l

of adjacent quartz grains whose c-axes occur et an& 6, to the

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connected outside the thin section. This method therefore eliminated the possibility of falsifying the maxima by duplicate measurements. During these measurements of the orientation of the quartz c-axis orientations, it appeared that there was no strong inhomogeneity in orientation of the c-axes. Preliminary A.V.A. diagrams on a small number of grains were prepared, but not presented here. These showed that no strong inhomogeneity existed on the scale of several adjacent thin sections. This inhomogeneity can also be illustrated on a frequency-distribution diagram of percent versus the angle between each gram and its nearest neighbours (Fig.23). In all cases the deviation from a random distribution is not significant. Similar diagrams (“neighbour-grain statistics”) have been prepared by Wenk (1965). In these there were very notable distributions of c-axes around certain angles. This is also reflected in the A.V.A. diagrams which have a strong inhomogeneity, with a tendency to form “super-individuals”. THE ORIGIN OF PREFERRED

ORIENTATIONS

Considerable information on preferred orientations in experimentally deformed rocks has emerged since the early work of Griggs, Turner and colleagues (see Griggs and Handin, 1960), but still the picture is obscure and many interpretations still need to be based on analogy with metals. Many geological experiments have involved the use of quartz because of its ubiquitous occurrence in metamorphic terrains, and its relatively simple structural and optical properties in comparison the other rock-forming minerals. All the basic experimental work on quartz (Carter et al., 1964; Raleigh, 1965; Green, 1966; Hobbs, 1968; Green et al., 1970; Tullis, 1973) has involved sets of experiments designed for conditions of creep, cold and hot working, with or without annealing; the preferred orientations developed in these experiments and others have been attributed to two different kinds of processes, namely intracrystalline and recrystallization mechanisms. In tracrystalline mechanisms

Experiments in which single crystals of known crystallographic orientation have been deformed, have established that deformation occurs by slipping or twinning on particular crystallographic planes and in certain directions, only when a critical shear stress in the direction of slip is exceeded (S&mid, 1925). The slipping produces a rotation of the axes of the specimen relative to the axes of the crystal and this rotation ultimately brings different slip planes into more favourable positions for slipping. Von Mises (1928) and subsequently Taylor (1938) have described the general deformation of aggregates of randomly oriented grains which may achieve any imposed shape change by shear on five independent slip systems provided there is no volume change. The Von M&s-Taylor criteria is based on the concept of minimum (internal) work, and involves finding amongst all possible combinations of five independent slip systems, those in which the sum of the glide shears is a minimum (Taylor, 1956). This assumes that the strain is homogeneously distributed and

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that the strain undergone by an individual grain is the same as that experienced by the ag gregate. Taylor’s original treatment assuming homogeneous strain was largely dismissed because: (1) the work of Barrett and Leverson (1940) showed that Taylor’s predictions were not true for at least one-third of the grains they studied; and (2) it is generally observed that the strains in deformed aggregates depart greatly from homogeneity. The first of these objections was more or less eliminated by the work of Bishop and Hill (1951) who considered the other combinations neglected by Taylor and the correction of Taylor’s work produces predictions which are largely in agreement with Barrett and Leverson’s (1940) observations (Bishop and Hill, 1951; Kocks, 1970). The second of these objections has been recently removed by Wonsiewicz and Chin (1970) who have demonstrated that the Taylor approach is also capable of handling the case of inhomogeneous strain within grains. The Taylor approach (or the Bishop and Hill approach) has also been strengthened by its application by Chin et al. (1967a and b) and Chin and Mammel(1967), and it now stands as a physically realistic theory which enables an analytical approach to the plastic deformation of aggregates (Chin, 1969; Kocks, 1970). It has recently been applied by Siemes (1970) and Saynisch (1970) to predict preferred orientations in galena and zincblende, respectively. Preferred orientations developed in carbonate rocks without recrystallization have also been recognized by Griggs et al. (1960); this again is attributed to intracrystalline mechanisms such as twinning and translation gliding. Although much of the older geological literature ascribes patterns of preferred orientations to slip mechanisms, many of these rocks are strongly recrystallized, and therefore any pm-existing preferred orientation would have been modified by grain growth. Also many of these postulated slip mechanisms have not been veriffed experimentally (Carter et al., 1964; Christie and Green, 1964; Christie et al., 1966; Heard and Carter, 1968; Bdta and Ashbee, 1969; Ave’ Lallement and Carter, 1971; and Hobbs et al., 1972). If a polycrystalline aggregate is to sustain a large plastic strain by deformation on less than five independent slip systems then a general change of shape cannot occur by glide alone (Van Mises, 1928) without discontinuous behaviour at grain boundaries. Then other mechanisms such as cross-slip (Hirth and Lothe, 1968), and dislocation climb will-be involved; these processes generally occur at elevated temperatures or low strain rates. This has been formulated and treated in detail by Groves and Kelly (1963,1969) who have considered the strains in non-metals which can be produced by climb, by climb in conjunction with unrestricted glide and by climb in conjunction with glide on specific crystallographic planes. Recrystallization Recent geological interpr&atirms have accounted,for observed patterns of preferred orientations by a mechanism of recrystallization where the pattern can be correlated with different elastic properties in individually oriented-grains in a stress field. The theoretical basis

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for the development of such recrystallization fabrics are the Kamb (1959a, 1961) and MacDonald (1960) theories. These theories may be inapplicable to many plastically deformed crystals or rocks, for they disregard the possibility of annealing recrystallization which depends upon the formation and subsequent growth of new grains through a dissipation of the stored energy of deformation. When deformed crystalline metals are annealed at temperatures higher than those required for recovery, the remaining stored energy of deformation is released through the migration of high-angle boundaries (Shewmon, 1966; Cordon and Vandermeer, 1966), resulting in the growth of new unstrained grains at the expense of old deformed host grains. The ultimate result of this grain-boundary migration is the formation of a polycrystalline structure by the process of primary recrystallization. Hu (1969) has presented strong evidence that orientation of new recrystallized grains in heavily rolled copper bears a strong relationship to the host grain. But subsequent anisotropic boundary mobility then controls the pattern of preferred orientation that develops. The primary recrystallization-preferred orientation then depends on an oriented-growth mechanism. The role of this host-control phenomenon has also been noted by a number of geological workers in the experimental field, both for calcite (Griggs et al., 1960) and quartz (Hobbs, 1968); it has also been described in natural quartz aggregates by Ransom (1971). It has been established in metals (Beck et al., 1950; Beck and Hu, 1966; Hu, 1969) and in experimentally deformed quartz (Hobbs, 1968) that many of these new grains are oriented at an angle of high mobility. Such grains have a greater volume of favourably oriented material for growth than grains of other orientations and therefore grow to dominate the aggregate and in so doing establish a new preferred orientation. Such a process of oriented growth during primary recrystallization is favoured by many metallurgists (see review by Beck and Hu, 1966) over the hypothesis of oriented nucleation, as the latter hypothesis cannot explain why only restricted orientations should nucleate. The pattern of preferred orientation developed by secondary recrystallization generally differs significantly from the preferred orientation in the surrounding primary recrystallized matrix grains and appears to be highly dependent on the matrix-preferred orientation. The nuclei for secondary recrystallization are generally present in the primary recrystallized matrix and the driving force required for their boundary migration has been reviewed by Dunn and Walter (1966) and Walter (1969). In metals with strong primary re. crystallization-preferred orientation there may be little modification of this stable pattern of preferred orientation by secondary recrystallization. However, in metals with a weak matrix-preferred orientation there is a high average angle of misorientation, high average mobility and high average driving force for the secondary nuclei. This boundary migration also depends on a number of other factors which include the surface energy of the boundaries between the matrix and secondary grains, on the boundary curvature, its angle of misorientation and the possible concentration of solute atoms in the boundary. A large proportion of the published metallurgical information on secondary recrystallization has been derived from two-dimensional grain studies where particular preferred

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orientations are believed to be controlled by a specimen surface energy grain selection mechanism (Mee, 1968). Many of these experimental methods may not be applicable to geological conditions. The only exceptions are for rocks rich in mica (cf. Fig.8); these rocks may be affected by differences in surface energies between the mica and quartz. A special case of secondary recrystallization development is that described by Taguchi and Sakakura (1966) where impurities may be responsible for the inhibition of the growth of certain orientations, but may enhance the growth of others. This only occurs if a special relationship exists between the impurity grain and the potential secondary grain. Another origin for secondary recrystallization textures is that suggested by Beck et al. (1950). In this case the secondary grain is believed to develop from a small fraction of the primary grains which are misoriented with respect to the general primary texture, and their growth capacity is due to this misorient&ion. The preferred orientation developed then would be similar to the oriented growth hypothesis used to account for priinaryrecrystallized preferred orientations. However, it is generally believed, in conformity with Beck’s ideas, that scattered throughout the primary-recrystallization structure are some small grains whose orientation relative to the primary structure is such that they possess a maximum grain boundary energy. These grains therefore have high average mobility and high average driving force, and consequently, maximum capacity for grain growth. Furthermore, most of these grains may from their beginning be several times larger than the surrounding primary grain. i%e evolution of preferred orientation at Mount Isa

in the quartz&s with detrital quartz grains the change from random c-axis patterns to peripheral girdles is probably a result of processes similar to that described by Bishop and Hill (1951). Therefore the grains will rotate as each grain within the quart&e undergoes deformation by crystallographic slip, satisfying-the Von Mises-Taylor criteria. It has been established that there is a strong host control of the c-axis pattern of preferred orientation of the new recrystallized.grains. The pattern of preferred orientation reflects that of the old c-axes, with lower concentrations of c-axes in the vicinity of any strong maxima found in the c-axis pattern of the. old grains. This appears to be a significant and distinguishing feature of most recrystallized metals (c.f. S&R, 1968) and could be accounted for by a subgrain coalescence and rotation mechanism. This was firstproposed by Li (1962) and illustrated by Hu (1963, 1969) to account for the development of recrystallized grains and referred to by Cahn (1%9) as the polygoniqztion model uf nu&ation. A similar host control has also been observed to exist in the single crystal quptz deformation experiments of Hobbs (1968). In Hobb’s syntectonic experiments, no obvious nucleation stage was observed and new grains appeared to develop from subgrains of a different orientation from the host, most-lying between 30’ and 50’ from the host c-axis. Hobbs also concluded from hydrostatic and stress an&&g experiments that the growth of the. newly recrystallized grains was anisotropic, and that-new grains with c-axis oriented within 10” of the host c-axis remain small. New grains with c-axes oriented between 20” and

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40” to the host c-axis grew fastest. In the Mount Isa rocks it could not be established whether anisotropic growth of the new grams took place or not. However, during these microstructural developments old grains and possibly some new grains, not suitably oriented for grain growth, would be eliminated and a new pattern of preferred orientation would develop. Also a stable pattern of preferred orientation would not be established until grain growth and solid-state grain adjustment had reached an equilibrium state (cf. Kretz, 1966a, b; and Vernon, 1968, 1970). This is a probable explanation for the large variation in quartz c-axis patterns observed in the coarsening stage, where there are still many old grains together with their host-controlled new grains. For the coarrerring stage is intermediate between the beginning of grain growth, where c-axis patterns are either peripheral or crossed girdles, and the formation of the polygonal microstructure where the quartz appears to have closely approached equilibrium shapes and has a random c-axis preferred orientation. It is also possible that the initial preferred orientation of the sediments may contribute to this random preferred orientation, had these rocks been subjected to a slightly different strain history, but this was not evident in the material studied. The development of the irregular, abnormally large quartz grains has been attributed to secondary recrystallization. But an interpretation of their patterns of preferred orientations is beset with the same difficulties encountered by the metallurgists. The major difficulty i: the establishment of a relationship between the matrix-preferred orientation and what cartrols the boundary mobility for growth of a potential secondary. The relationship of stress and strain to the microfabrics

The strain these regionally metamorphic rocks have experienced is clearly evident in thn: low-grade quartzites and comprises a flattening of the detrital quartzes perpendicular to the foliation, with maximum elongation in the direction of the lineation. A conclusion that may be drawn from this observation is that the pole of the foliation and the lineation are principal strain axes,* of maximum shortening and elongation, respectively. The orientation of both old and new grains appears to be related to this strain, with the formation of strong peripheral girdles about the lineation, the e3-axis of the strain ellipsoid. Hobbs (1968), in his experimental studies of quartz, was able to effectively isolate the effect of stress and strain, and showed that in the stress-annealing and hydrostatic annealing experiments the c-axis of the new grain always developed at a high angle to ul, and that old and new c-axes lay in a plane containing ul. An attempt was made to establish whether such a relationship existed in the low-grade quartzites where distinct new grains could be observed, but the results were inconclusive. In the polygonal grained quartz&s and in the stage of exaggerated growth evidence for the strain is not obvious. Instead an idea of the strain is obtained from the preferred *The principalstrain axes are q, c2 and e3 which are, respectively, the shortest, intermediate, and longest principal axes of the strain ellipsoid. The principal stresses are 01, 02 and 03 which are, respectively, the greatest, intermediate and least principal axes.

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orientation of the mica in the accompanying interbedded schists and the hornblende lineation in some interbedded amphibolites. It is believed by some workers (Rutten, 1955; Brace, 1960) that a strong slaty cleavage forms normal to ul. Other ideas for the origins of such cleavage include that of Dieterich (1969) where the cleavage is parallel to the plane of the strain ellipsoid (containing e2 and eg) but not necessarily normal to ui ; or that cleavage forms as a slip surface on planes of highshear-stress or strain (Becker, 1907; Sander, 1930). In these coarser grained rocks there is no evidence of slip having taken place parallel to the cleavage; if such a phenomenon has taken place it may well have been modified by recrystallization. A strong mica-preferred orientation has been observed (Wilson, 1970) and this could be accounted for by either of the first two mechanisms, and to distinguish between the two is not possible here. It is believed that the strain in this regional sequence comprises a considerable shortening perpendicular to the foliation, Si , and elongation parallel to the mineral and stretching lineatlon which is sometimes parahel to the first-generation fold axes. The relationship of stress to strain is more obscure but from some dolomite measurements (Rosengren, 1968; Wilson, 1970) it appears that ui is probably oriented normal to the foliation and would correspond to the shortest principal strain axis. The plane of the schistoslty so defined-was the e3e2-plane of the local strain ellipsoid. As the detrital grams are destroyed by grain growth it could not be established if the magnitude of this strain was the same throughout the sequence. CONCLUSIONS

The variation observed in preferred-orientation at Mount Isa is found as discrete patterns which show a direct cormapondence to accompanying changes in microstructure and is also dependent on changes in metamorphic grade. In the least deformed quart&es the quartz c-axis patterns are random, but with increasing deformation and metamorphic grade the patterns change successively.to peripheral .girdkx, crossed girdles, random strong ginlIes and maxima. The particular pattern of preferred orientation developed at a particular metamorphic grade may be controhed by compositional changes, especially the percentage of mica present. In interpreting this microfabric data it is believed that intmgram&u slip with recovery and recrystallization were operative processes during the deformation of the Mount Isa rocks. The low-grade rocks, which contain elongate and flat old detrital grains, with numerous deformation lamellae at&b&s, are beheved to have developed by aprocess of intragranular slip and, rotation similar to that described by Bishop and HiIl(195 l), while many of the new grains which accompany this deformation have been interpreted as a recovery phenomenon and bear a strong orientation nIationahip to the host grains from which they developed. These recovered grains are believed to act as nuclei for.& development of a primary recrystallization-type of microstructure: polygon&sti~ m&e! ofnucleation. During this early strge of new grain development there appears to be a strong host control and also an apparent relationship to the strain which governs the quartz c-axis pattern of preferred orientatiorrdeveloped.

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The role of a host control during the coarsening and polygonal stage is not obvious. But there may have been a very strong matrix control over the preferred-orientation development during abnormal coarsening and the development of the extremely coarse-grained higher-grade quartzites, similar to that found during the secondary recrystallization of met&. These changes in microfabric do not necessarily represent individual progressive steps, but are believed to be contemporaneous with one another and reflect different physical conditions existing in different portions of the same sequence during the one deformation. The mechanism responsible for the deformation may be equivalent to a hot working condition where different processes are responsible for the dissipation of the stored energy of deformation in areas of differing temperature conditions. It has therefore been the attempt of this study to show that by utilizing observations on quartz microfabric the effects of the most prominent deformation mechanisms on random detrital quartz grains may be rationalized. Some of these mechanisms play a more significant role than others depending on initial composition, deformation temperature and initial orientation distribution. It is obvious that no single mechanism can explain all of the observations, not even most of them. Only when all the competing mechanisms are viewed in perspective is there hope for a reasonably simple understanding of a complex problem.

ACKNOWLEDGEMENTS

This work was carried out in the Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T., while the writer held an A.N.U. Research scholarship. I gratefully acknowledge the facilities made available by the A.N.U., and also Mount Isa Mines Limited who gave invaluable assistance during the period of fieldwork. I am also grateful to Dr. K.R. Rosengren for the computer programs used to compile the data presented here. I am indebted to Dr. B.E. Hobbs for the stimulating discussions we had during the preparation of this work and for constructive criticism of the final manuscript. A travel grant from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) enabled the author to discuss the content of this paper with colleagues in America. This grant is also gratefully acknowledged.

REFERENCES Aust, K.T., 1969. Interface migration. In: R.C. Gifkins (Editor), Inrerfuces Conference Melbourne 1969. Butterworths,London, pp. 307-334. Avk Lallemant,H.G. and Carter, N.L., 1971. Pressure dependence of quartz deformation lamellae orientations. Am. J. Sci., 210: 218-235. Btita, R.D.and Ashbee, K.H.G., 1969. Slip systems in quartz. Am. Mineral., 54: 1551-1582. Barrett. C.S. and Leverson, L.H., 1940. The structure of aluminum after compression. Trans. A.Z.M.E., 137: 112-126.

Beck, P.A., 1954. Annealing of cold worked metals. Adv. Phys., 3: 245-324. Beck, P.A. and Hu, H., 1966. The origin of recrystallization textures. In: H. Marolm (Editor), Recrystallization, Grain Growth ond Textures. Am. Sot. Metals, Metals Park, Ohio, pp. 393-433.

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APPENDIX Specimens indicated in Fig.1 and 2 with the number which corresponds to specimens found in the collection of the Department of Geophysics and Geochemistry, the Australian National University. Localities are also given of specimens whose localities are not shown in Fig.1 and 2. The grid coordinates are the same as the Mount Isa Mine grid coordinates (see Bennett, 1965; Wilson, 1972). 1 = 8059

2= 3= 4= 5= 6= 7= 8= 9= 10 = 11= 12 = 13 =

7940 1947 7936a 7929 7943 7927 7909 7869 786.5 7830 7887 7868

14 = 15 = 16 = 17= 18 = 19 = 20 = 21= 22 = 23 = 24 = 25 = 26 =

8082 8069 7838 7931a 7716 7886 7992 7888 7895 7850 7956b 7957d 7905

27 = 28 = 29 = 30 = 31= 32 = 33 = 34 = 35 = 36 = 37 = 38 =

7957b 7962 7841 7839 7851 7966 7972b 7972a 7973 7945 (11500E 2780083 7950 (48000E 495008) 7953 f 6000W 2500s)