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Volume 68, 1938-39
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Petrofabric Investigations of Otago Schists. No. 2—Three Quartz-Albite-Sericite-Schists from Waipori.

By F. J. Turner, University of Otago.

[Read before the Otago Branch, November 9, 1937; received by the Editor, November 10, 1937; issued separately, June, 1938.]

Contents.

  • Introduction.

  • Megascopic Features of Fabric.

  • Petrography

  • Mica Fabric and Fold Structures.

  • Quartz Fabric.

  • Tectonic Considerations.

  • Literature Cited.

Introduction.

This is the second paper of a series in which structural features of Otago schists are being recorded. The rocks investigated were collected about four miles south of the old township of Waipori, eastern Otago (No. 4487, old copper mine, Reedy Cr., 1½ mls. W.S.W. of Trig. C.C., Tuapeka East Survey District* Nos. 4489, 4492, dam, Reedy Cr., ½ ml. N. of old copper mine). The laboratory work was carried out in the same manner as was described in the first paper (Turner, 1938).

Megascopic Features of the Fabric.

In the rocks of both localities schistosity is well defined and dips west-south-west at angles of 10° to 15°. This is typical of the schists of the Waipori district, for the schistosity here is usually subhorizontal though locally dipping as steeply as 45°. Lineation with a trend of 170° to 175° is especially noticeable in the more micaceous laminae. In No. 4487 there is also a coarser and less distinct lineation at 160°.

Quartzose veinlets ranging from 2 mm. to 20 mm. in thickness and usually spaced at intervals of 10 mm. to 20 mm. are ubiquitous, and invariably lie parallel to the schistosity. Traces of the principal lineation (170° to 175°) were occasionally noticed on the surface of coarser veins in specimen No. 4487. True foliation is obscure in Nos. 4489 and 4492 and altogether absent in No. 4487.

As is commonly the case in eastern Otago, the joints, all more or less vertical, fall into several distinct systems. The strongest and most regular have a strike of 75° or 80°, but there is also a prominent series the trend of which varies between 40° and 50°. Less numerous but very distinct and continuous joints were observed from 145° to 155°.

[Footnote] * See north-eastern portion of map accompanying Marshall, 1918.

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Fig. 1.—Diagrammatic representation of structural elements in No. 4487: ab plane = schistosity; b = main lineation; b′ = faint lineation; corrugated lines = early S-planes; stippled bands = quartz veins; the arrow points north.

In the field there is no indication that schistosity and bedding are not parallel. Indeed, in an old drive a few yards from the copper mine a band of what appears to be a fine-grained phyllite lies parallel to the schistosity of the enclosing-quartzo-feldspathic schists. However, the polished end-face of specimen No. 4487 (cut parallel to ac) shows a series of undulating S-planes cutting perpendicularly across the schistosity and giving rise to the indistinct second lineation. (at 160°) noted on the ab surface (see Fig. 1). In specimens 4489 and 4492, where metamorphism has reached a slightly higher grade, there is no trace of these S-planes in the hand-specimen, but their former existence is clearly indicated in micro-sections cut parallel to ac, for in these latter the original structure is only partially obliterated. While it is highly probable that the contorted S-planes correspond to the original bedding, it is also possible that they represent an early schistosity unrelated to bedding. In either case, however, the present subhorizontal schistosity cannot have developed as a simple structure parallel to the bedding. The interbedded “phyllite” at the copper mine must therefore be a phyllonite.

The megascopically observable features of the fabric may be summarised thus:—

Schistosity: dipping W.S.W. at 10° to 15°.

Principal lineation (b): 170°–175RR, horizontal.

Minor lineation (b′, = trace of early S-plane upon ab): 160°, horizontal.

Joint systems, in order of prominence: 75°–80°, 40°–50°, 145°–155°; all joints nearly vertical.

Quartzose veins: parallel to schistosity.

Phyllonite band: parallel to schistosity.

The three mutually perpendicular fabric axes are chosen thus: b = main lineation; a is parallel to schistosity and perpendicular to b; c = perpendicular to plane of schistosity (Fig. 1).

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Petrography.

No. 4487 is a rather fine-grained rock consisting of albite, epidote, chlorite, dusty graphite, less plentiful quartz and sericitic mica, and accessory sphene and apatite. As is usual in schists from the Chlorite Zone of Otago, the albite contains. no appreciable anorthite. The epidote mineral occurs in colourless idioblastic or subidioblastic prisms; much of it is clinozoisite but there are also many crystals with medium iron-content as indicated by their higher birefringence. The chlorite is a pale faintly pleochroic type; the majority of the flakes are negatively elongated and give yellowish-brown anomalous interference tints, but some are positively elongated and appear deep violet-blue between crossed nicols. The average grain-size is 0.1 mm. to 0.2mm.

Lineation parallel to b as seen in ab sections is the result of orientation of mica and much of the chlorite with long axes of the flakes in parallel position. Well defined foliation, the mean direction of which is perpendicular to the schistosity, is obvious in sections cut parallel to ac and ab. Individual foliae, usually 0.2 mm. to 2 mm. in width, are marked by concentration of particular minerals, especially albite + chlorite and epidote + graphite; sericite-rich foliae are less common. The trend of the foliation in the ab section crosses the main lineation at an acute angle and is parallel to the indistinct macroscopic lineation b′. In the ac section the foliae have been contorted by transverse shearing (Pl. 21, Figs. 11 and 12) and are cut by rather poorly-defined widely-spaced planes of shear, along which parallel flakes of mica have crystallised, thus giving rise to the present schistosity.

The quartzose veins are seen in section to consist principally of quartz, with albite, colourless iron-poor epidote and chlorite as minor constituents. The quartz is in interlocking equidimensional grains ranging from 0.1 mm. to 1 mm. in diameter and usually showing faint traces of undulose extinction. The grains of albite are smaller, never twinned, and usually aggregated into highly irregular streaks trending approximately parallel to b. As seen in the ac section the epidote prisms appear to lie perpendicularly to the vein-wall, but in bc sections they lie on transverse subparallel curved ares, though the individual crystals are not distorted. Chlorite occurs mainly as flakes of late origin margining the veins and parallel to the veinwalls. Micro-cracks parallel to ac were occasionally observed in bc sections. In the ab section of one of the thicker veins* there are a few prominent cracks parallel to b. and numerous well-defined cracks at 135° to b. The latter lie within 10° of macroscopic joints noted in the field at 145° to 155°.

[Footnote] * No. 5 on hand-specimen 4487, Geology Department, University of Otago.

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Nos. 4489 and 4492 are rocks that have suffered more complete shearing and recrystallisation during that stage of metamorphism in which the present schistosity developed. They are therefore of somewhat coarser grain than No. 4487, the schistosity is better defined, and the original foliation has been sufficiently obliterated to be invisible in hand-specimens. In sections cut parallel to ac, however, micaceous and feldspathic foliae that have developed parallel to the schistosity ab alternate with lenses in which the transverse arrangement of mica flakes parallel to the c axis of the rock is still perfectly preserved. The micas in such lenses show strong effects of deformation accompanying crystallisation (Pl. 21, Fig. 13). The rocks are much richer in mica than No. 4487, and consist of albite, sericite, chlorite, epidote, quartz, graphite, and accessory sphene and brown tourmaline. The grains of quartz and albite range from 0.2 mm. to 0.7 mm. in diameter, the latter mineral occasionally forming porphyroblasts with enclosed prismatic clinozoisite. The chlorite is positively elongated and gives anomalous violet-blue interference tints. Much of it seems to have recrystallised at a late stage as augen composed of rather coarse undeformed flakes oriented transversely with respect to the schistosity as seen in bc sections.

The quartzose veins cutting these rocks consist mainly of quartz and albite (15% to 20%), with coarse chlorite of late origin near the margins. Epidote is an accessory constituent only. In several veins the grains of albite are elongated perpendicularly to the veinwall. Attention may here be drawn to the invariable absence of mica from quartzose veins of the Waipori schists. On the other hand, mica is plentiful in what are believed to be much more ancient quartz veins in the schists from Patearoa described in the first paper of this series.

Mica Fabric and Fold Structures.

In curves (Figs. 2 and 3) representing the mica fabric, the angular distance is plotted between the trace of the (001) cleavage and b (for ab sections) or a (for ac sections) respectively. The ab curves for all three rocks (Fig. 2) show strong maxima at b, indicating that the mica flakes lie with their long axes parallel to b. A submaximum at 15° from b in the curve for No. 4492 corresponds to the second lineation b′, but in the other two rocks the ab mica curves give no hint of relation to the original foliation (b′c) even though the latter is clearly visible in one of the microsections (No. 4487). An additional maximum at 155° from b in the curve for No. 4487 seems to be related neither to the quartz fabric nor to the megascopic fabric of the rock as a whole.

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Fig. 2.—Orientation curves for mica in sections cut parallel to ab. The percentages refer to the number of crystals in which the trace of the (001) cleavage lies within 5° on either side of the particular angular distance from b.

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Fig 3.—Orientation curves for mica in sections cut parallel to ac. The relation between the early S-planes and the fabric axes in the sector within which crystals were measured in No. 4487 is indicated diagrammatically.

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Comparison of ac sections for Nos. 4487 and 4492 illustrate the way in which the mica has attained its present orientation. In the former the flakes of mica, many of which show effects of mechanical deformation, for the most part lie parallel to the sinuous boundaries of the contorted foliae in which they occur. The corresponding curve (Fig. 3), drawn from measurements of crystals in a restricted area of the section, shows also a slight tendency for mica to develop parallel to the schistosity. In the curve for No. 4492 (Fig. 3) the orientation of 600 flakes scattered over the whole section is represented. Metamorphism has here reached a more advanced stage, in which the original mica fabric has been partially destroyed and is becoming replaced by a fabric dominated by crystals parallel to ab. Here, too, most of the crystals of mica are obviously deformed by movements accompanying crystallisation.

Sharply crystallised post-tectonic micas are also present in all three rocks. The greater number lie parallel to ab. often margining the veins of quartz, but in some cases the orientation follows the old foliation.

Three stages in the evolution of the mica fabric may therefore be recognised:—

  • (1) Crystallisation of flakes parallel to the early foliation, i.e. to b'c.

  • (2) Deformation across the b tectonic axis giving rise to a subhorizontal schistosity. At the same time mica flakes were brought into alignment parallel to b, and in the surfaces of maximum differential movement approximately parallel to the plane of schistosity ab.

  • (3) Post-tectonic crystallisation of a small proportion of crystals parallel to already existing structures.

Sander (1930, pp. 243–262) has discussed the nature, origin and diagnostic criteria of the folded structures so commonly encountered in deformed rocks. He recognises two extremes, viz., shear-folds and flexure-slip-folds, together with intermediate types combining to some extent the characters of both (see also Knopf, 1933, pp. 464–467; Fairbairn, 1937, pp. 96–102). While the former originate by shearing parallel to the axial plane of the resultant folds, the typical flexure-slip-fold is produced by flexing of layers of varying competence, combined with a slipping movement between adjacent layers. The undulating structure of the early S-surfaces in the Waipori specimen No. 4487, while not constituting a series of pure shear folds, has nevertheless originated by subhorizontal shearing along rather widely spaced surfaces (ab) parallel to the axial planes of the folds (cf. examples from New York State figured by R. Balk, 1936, pp. 712, 715, 717). Such an origin is indicated by the following:—

  • (1) Orientation of the schistosity parallel to the axial planes of the folds (cf. Fairbairn, 1937, p. 100).

  • (2) The subhorizontal disposition of the axial planes. This is consistent with “folding” by non-homogeneous horizontal shear or by vertical compression. alternative seems hardly possible.

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  • (3) Absence of the structural criteria of flexure-folding as enumerated by Sander, e.g.: there is no sign of thickening of mechanically weak bands at the crests of folds as a result of migration of material during flexure, nor is there any relation between the size of the folds and the thickness of the individual bands.

  • (4) Comparison with the more strongly deformed rocks (Nos. 4489, 4492), in which the observed structures are obviously the result of shearing of greater intensity and along much more closely-spaced surfaces than in No. 4487. (Hence the well-defined schistosity and nearly complete obliteration of the early S-planes in the two former rocks.)

On the other hand the mica-fabric in the contorted bands of No. 4487 is non-homogeneous with reference to ab the plane of shearing. Thus while the mica flakes in the limbs of the folds show a strong tendency towards orientation parallel to the S-surface at the point in question, there is only a slight tendency for a tangential arrangement of flakes at the crests. In a pure shear-fold there should be a homogeneous fabric and the micas as seen in the ac section should lie with their long axes parallel to a, the direction of shearing (Sander, loc. cit. pp. 251, 261). Since the mica crystals as seen in ab sections define b the tectonic axis of the deformation under discussion, it can hardly be argued that the present orientation of the micas is a result of post-tectonic crystallisation in the contorted S-surfaces (cf. Fig. 1). The mica-fabric is therefore inconsistent with a hypothesis of pure shear-folding.

Finally it may be noted that the folded structure discussed above is quite unrelated to the secondary flexure-slip-folding (with axial planes parallel to bc) not infrequently seen in schists from various parts of Otago. For example the “crumpled schists” described by J. B. Mackie (1936, pp. 136, 140) from the eastern end of the Dunstan Range exhibit the features of typical flexure-slip-folds as described by Sander.

The Quartz Fabric.

Throughout the main portion of each specimen albite greatly predominates over quartz, and it was therefore usually impossible to measure sufficient quartz grains to construct orientation curves except in sections cutting the ab quartz veins. The following observations therefore refer for the most part to the quartz fabric as developed in veins of this type.

(a) Fabric in Specimen No. 4487. In all 660 grains were measured in ac sections cut from three separate veins (numbered 1, 2 and 4 respectively on the section block). The three orientation curves agree sufficiently well to allow combination into a composite curve (Fig. 4) in which five maxima (D, E, G, H and J) may be recognised. All three of the component curves drawn for the individual veins show maxima corresponding to E, G, H and J, while D is represented on the curves for veins 1 and 2.

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Fig. 4.—Composite orientation curve for quartz in sections cut parallel to ac. The percentages refer to the number of quartz crystals having Z' lying within 5° on either side of the angular distance from a. Specimen No. 4487.

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Fig. 5.—Orientation curves for quartz in sections of two quartz veins cut parallel to bc. Maxima are lettered to correspond with those in the ac and ab curves (Figs. 4 and 6).

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Curves drawn for sections cutting two veins (1 and 3) parallel to bc are closely similar to each other (Fig. 5). There are strong maxima at 57° and 116° from b in one case compared with 60° and 120° in the other. A submaximum at 40° is also present in both curves, but is much more definite in that for vein 3.

Curves for ab sections of two veins (3 and 5) and a quartz-rich folia in the schist agree in that all possess strong maxima at 125° to 132° from b (Fig. 6). A prominent double maximum at 45° to 60° in the case of vein 3 corresponds to one at 40° to 50° in the curve representing the quartzose folia. A further maximum at 105° (i.e. perpendicular to b′) is well developed in curves drawn for vein 5 and the folia, but absent from the third curve.

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Fig. 6.—Orientation curves for quartz in sections of two quartz veins and a quartz-rich folia cut parallel to ab (all in specimen No. 4487). Maxima in the curve for Vein 3 are lettered to correspond with those in ac and bc curves (Figs. 4 and 5).

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Fig. 7.—Stereographie projections upon ab. ac and bc showing positions of maximum concentration (solid circles) of quartz axes (Z) in Vein 3, specimen No. 4487. The ab and ac projections are plotted directly from corresponding curves (Figs 4 and 6). In the bc projection the maxima have been graphically determined from the other two. Lettering corresponds with that in Figs. 4, 5 and 6.

The fabrics of the quartz veins are thus far from simple and are not all identical. From the close similarity of the curves concerned it may be assumed, however, that the same type of fabric prevails in veins 1 and 3 and in the one quartz-rich folia measured. In correlating the maxima present on ab, bc and ac curves, the most definite results should be obtained by comparing the ab and bc curves for vein 3 and the composite curve for ac sections. Several correlations of these three curves are possible, but that summarised in Table I is quite the most satisfactory and further accords well with the fabric data determined for the other two rocks. The calculated maximum for bc shown in the third column is the value graphically determined by correlating appropriate maxima on the curves for ab and ac sections.* Agreement with the actual maxima for the bc curves (fourth column) is very close. In the absence of a maximum parallel to b in the curves for ab sections, the maximum at G in the ac curve must be due to slight concentration of grains with their optic axes (Z) parallel to the c fabric axis. This is not indicated in the two bc sections measured, but a corresponding feature does show clearly in an orientation curve for quartz in a bc section of specimen No. 4492.

[Footnote] * The graphical method is described in the first paper of this series.

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Table I.
Maximum in ac curve. Correlated Maximum in ab curve for vein 3. Estimated Maximum in bc curve. Observed Maxima in bc curves of veins 1 and 3.
D (40° from a) 60° from b 124° from b 115°,   120°
E (60° " ") 132° " " 63° " " 58°,     60°
H (120° " ") 132° " " 117° " " 115°,   120°
J (140° " ") 45° " " 38° " " 35°,     40°

In Fig. 7 stereographic projections on the ab, ac and bc planes show the positions of the quartz maxima as calculated from the composite ac curve and the ab curve for vein 3.

(b) Fabric in Specimen No. 4489. In this rock measurements were confined to three sections, parallel respectively to ab, bc and ac, cutting quartz-rich veins lying parallel to ab. The orientation curves are complex (Fig. 8) and the only satisfactory correlation

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Fig. 8.—Orientation curves for quartz in veins, specimen No. 4489. Correlation of maxima in the three curves is indicated by the lettering.

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Fig. 9.—Stereographic projections upon ab, ac and bc showing positions of maximum concentration (solid circles) of quartz axes (Z) in No. 4489. The ab and ac projections are plotted directly from corresponding curves in Fig. 8. The maxima in the bc projection were determined graphically from the other two. Lettering corresponds with that in Fig. 8.

of the various maxima is that summarised in Table II below. In this the third column gives the positions of maxima in the bc curve as calculated graphically from data recorded in the curves for ab and ac. In Fig. 9 the same correlation is represented in the form of stereographic projections upon ab, ac and bc.

Table II.
Maximum in ac curve. Correlated Maximum in ab curve. Estimated Maximum in bc curve. Observed Maximum in bc curve.
— — — 25°
Q (28° from a) 110° from b 58° from b
E (58° " ") 125° " " 67° " " 50°–65°
J (140° " ") 50° " " 44° " "
D (40° " ") 50° " " 138° " " 138°
K (155° " ") 150° " " 160 " " 165°

The point X in the ab curve is correlated with the slight concentration of quartz axes parallel to a in the ac curve. This is borne out by the minimum at 90° to b in the bc curve.

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(c) Significance of the Quartz Fabric. The quartz veins must obviously be younger than the schistosity since this latter structure has determined their orientation parallel to ab. The fabric of quartz veins will therefore be determined by late movements operating upon an original growth fabric. In movements of this type, in an already schistose rock, shearing parallel to the direction of schistosity may be assumed to predominate. Consequently the present orientation of quartz grains in sections cut parallel to ab should give some indication of the directions in which movement took place during the latest stages of metamorphism. Comparison of the ab curves for veins in both rocks described above shows the existence of maxima at angular distances of 45°, 50°-60°, 90°, 110° and 125°—132° from the positive end of the b axis (measured in an anticlockwise direction). The maxima that occur most consistently are those at 45°, 50°-60° and 125°-132°, and it will be noted that the position of the points D, E and J by which they are determined are almost identical in the two rocks. They may have originated as the result of shearing movements operating parallel to ab in directions inclined at 45°, 50°-60° and 125°-132° to b, i.e. across N.E.-S.W. and N.W.–S.E. tectonic axes respectively.

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Fig. 10.—Orientation curves for quartz in ac sections of No. 4492 and Vein 1, No. 4487. Note the maxima Y parallel to a. Lettering corresponds to that in Figs. 4 and 8.

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Disturbance of the original growth-fabric has been so great that the latter is difficult to reconstruct. Certain significant features may still be detected however, viz.:—

(1) Very strongly marked minima parallel to b in all bc curves.

(2) Occasional development of submaxima parallel to a in ac curves (e.g. for vein 1 in No. 4487; also in No. 4489). In the ac curve for a single vein measured in No. 4492 there is a strong maximum parallel to a (Fig. 10).

(3) Presence in ab curves of certain veins from Nos. 4487 and 4489, of maxima perpendicular to b and b′ (i.e. at 90° and 105°–110°). These are especially marked in a vein of unusual thickness (vein 5 in 4487) in which later movements have been insufficient toallow development of the maxima at 40°–60° so consistently shown by all other curves (Fig. 6).

(4) Occurrence of submaxima parallel to c in several ac curves and in a single bc curve (No. 4492).

This last feature is considered to represent a survival of the original fabric in which growth of quartz crystals took place perpendicularly to the vein-wall, i.e. with Z parallel to c. Items (2) and (3) above are interpreted as characters imposed upon the growth fabric by the influence of crystals in the vein-walls oriented with Z perpendicular to b and b′.* If the veins are younger than the schistosity, i.e. than the development of the directions b and b′ in the rock fabric, no other interpretation is possible especially as the maximum perpendicular to b′ (the older of the two lineations) is much more prominent in the vein fabric than is that perpendicular to b.

Tectonic Considerations.

The following principal stages are recognised in the tectonic history of the Waipori schists as deduced from the fabric data recorded in this paper:—

(1) Development of a vertical set of S-planes in a sedimentary series, probably by isoclinal folding; direction of tectonic axis (b′), 20° W. of N.

(2) Horizontal shearing giving rise to the present schistosity (ab) and the main lineation (b); contortion followed by almost complete obliteration of early S-planes; local development of phyllonitic bands along zones of most intense movement; direction of teetonic axis (b), 5° W. of N. The principal joint-system with a trend of 75°–80° E. of N. is correlated with this movement.

(3) Growth of horizontal quartzose veins parallel to the schistosity and located especially along surfaces of strong differential movement.

(4) Late deformations of minor intensity affecting the quartz fabric profoundly, but having no influence on the mica fabric.

[Footnote] * Growth from anisotropic blastetrices (of. Fairbairn, 1937, pp. 118, 119).

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Fig. 11.—No. 4487, section perpendicular to b, showing contected early S-planes (probably the original bedding). Magnification: 10 diameters.
Fig. 12.—No. 4487, section perpendicular to b. showing early S-planes cut by quartzose parallel to ab. Magnification 10 diameters.
Fig. 13.—No. 4489, section perpendicular to b. In the centre of the photograph mica oriented subparallel to c preserves the direction of the early S-planes (Probably the original bedding). Magnification: 10 diameters.

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(a) Trend of tectonic axis, N.W.–S.E.; correlated features of the fabric are the prominent N.E.–S.W. joints (40°–50°) and the maximum at 125°–132° from b in ab curves for quartz.

(b) Trend of tectonic axis, 35° E. of N. Corresponding to this movement are the maxima between 45° and 60° to b in ab curves for quartz.

There is nothing in the fabric to indicate which of these last two movements is the earlier.

In Table III a comparison is given between the directions of the tectonic axes at corresponding stages in the metamorphism of the Waipori schists and the three rocks from Patearoa described in my last paper. The close agreement between the two sets of data supports my previous conclusion that in the Patearoa rocks the contorted bc quartz veins are relics of an ancient structure that preceded development of the present schistosity and lay approximately at right angles to the latter.

Table III.
Stage of Metamorphism. Waipori. Patearoa.
(1) Close folding 20° W. of N. 15°—25° W. of N.
(2) Development of Schiostosity (main metamorphism) 3° W. of N. 15°–25° W. of N.
(3) Late movements—(a) N.W. N.W.
(b) 35° E. of N. 10°—25° E. of N.

Literature Cited.

Balk, R., 1936. Structural and Petrologic Studies in Dutchess County, New York, Part I, Bull. Geol. Soc., Amer., vol. 47, pp. 685–774.

Fairbairn, H. W., 1937. Structural Petrology (Revision of Introduction to Petrofabric Analysis, 1935), Queen's University, Kingston, Canada.

Knoff, E., 1933. Petrotectonics, Amer. Jour. Sci., vol. xxv, pp. 433–470.

Mackie, J. B., 1930. A Geological Traverse from the Waitaki River to Dunatan Peak, Otago, Trans. Roy. Soc. N.Z., vol. 66, pt. 2, pp. 125–142.

Marshall, P., 1918. The Geology of the Tuapeka District, N.Z. Geol. Surv. Bull., no. 19.

Sander, B., 1930. Gefügekunde der Gesteine, Vienna, J. Springer.

Turner, F. J., 1938. Petrofabric Investigations of Otago Schists, No. 1: Quartz and Mica Fabrics of three Schists from Patearoa, Trans. Roy. Soc., N.Z., vol. 67, pt. 4, pp. 443–462.

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