Petrofabric Analyses of Two Quartz-mica-piedmontite Schists from North-west Otago.
[Read before the Otago Branch, September 10, 1946; received by the Editor, September 16, 1946.]
Dr. F. J. Turner (1946) has recently suggested that the piedmontite-bearing quartz-mica schists of North-west Otago are derivatives of manganiferous cherts and that chemical processes (indirect componental movements) must therefore have played an important part in their deformation, since transformation of chert to schist has certainly involved very great increase in average grain-size. The writer has therefore undertaken petrofabric analyses of two typical rocks of this group, with a view to recording patterns of preferred orientation of quartz and mica developed during deformation of this kind.
Specimen No. 2671.
Specimen No. 2671 of the collection in the Geology Department, University of Otago, was collected from Kawarau Gorge by Dr. C. O.
Hutton (1940). In the hand-specimen bands of quartz with muscovite alternate with contorted layers rich in piedmontite. There is one well-developed sehistosity S2 (parallel to the a b fabric plane) which, however, is oblique to the general trend of the deformed piedmontiterich layers S1. Two lineations are present: a strong one which has been selected as the b fabric axis, and a weaker but still obvious lineation (denoted by b') which intersects b at 74°, and is conspicuous only on the piedmontite-rich surfaces. A diagrammatic sketch (Fig. 1) shows these features.
In thin section the quartz of the quartz-muscovite bands is uniformly coarser (about 0.5 mm. in diameter) than that of the piedmontite-rich layers (about 0.2 mm.)—a feature that seems to be fairly general in Otago piedmontite-schists, and is probably in some way connected with different recrystallisation behaviour of quartz in different local chemical environments.
In the ac section (Fig. 2) piedmontite prisms (seen end-on as equant granules) are concentrated in minutely corrugated streaks (S1), the trend of which is also followed by associated sharply crystallised micas. Although this micro-folding doubtless involved a high degree of shearing parallel to the limbs of the folds, the structures as figured in Figs. 2 and 3 are not to be regarded as pure
Fig. 2.—Sketch of micro-section (× 12) showing contorted configuration of piedmontiterich bands marking early S-surfaces (S1). Broken line S'1 indicates mean trend of S1 within measured area of section.
slip-folds, but rather as products of flexural slip, for the micas often lie sub-parallel to the crests and in no case clearly cut across them. Cutting obliquely across the contorted S1 lamination is the plane of megascopic schistosity (S2) marked in the micro-section by parallel orientation of mica (especially in the quartzose layers) and by local concentration of continuous mica streaks which cut across quartzose and piedmontite-rich layers alike.
Fig. 3.—Sketch of micro-section (× 40) showing contorted configuration of piedmontiterich bands (P) with mica flakes (M) tending to follow the fold crests rather than break through them.
Fig. 4 is an orientation-diagram showing distribution of optic axes of 300 quartz grains, measured in a number of separate traverses across the schistosity in the ac section. It shows a surprisingly low degree of preferred orientation compared with other Otago schists (cf. Turner, 1940), the only distinct feature being a weakly defined ac girdle. Separate plots of optic axes for small and for large grains of quartz respectively revealed no significant departure from the orientation pattern of the composite diagram (Fig. 4).
The poles of normals to (001) in 100 mica grains were plotted to give Fig. 5 which clearly shows the pronounced tendency for (001) to lie parallel to the ab fabric plane, i.e., to the megascopic schistosity S2. The maximum around the pole of S2 is, however, distinctly drawn out toward the pole of S1 (the mean trend of S1 in the measured area), showing that both sets of S-surfaces have influenced the preferred orientation of mica in this rock. The apparent incomplete girdle in Fig. 5 is probably due to the very pronounced tabular habit of the mica crystals and not to the existence of a true mica girdle in the fabric (cf. Turner and Hutton, 1941, p. 231).
At least two, and perhaps three, separate phases of deformation appear to be recorded in the fabric described above:—
Intense deformation, involving flexural slip, resulting in the present highly contorted configuration of the segregation bands S1; tectonic axis = b.
Development of megascopic schistosity S2, probably by shearing parallel to S2 with b as tectonic axis.
Minor slip-movements resulting in slight wrinkling of piedmontite- or mica-rich bands parallel to b'.
The poor definition of the ac girdle in the quartz diagram (Fig. 4) is conceivably due, in part at least, to mutual superposition of these two later movements. It may well be due also to post-tectonic crystallisation, for judging from the general absence of undulose extinction and deformation lamellae in the quartz grains crystallisation has certainly outlasted direct componental movement.
Specimen No. 1489.
This specimen, collected by Professor James Park from a stream boulder in the Shotover River (Turner, 1933; Hutton, 1940), is an evenly laminated quartz-mica-piedmontite schist, in which the megascopic schistosity (= ab) coincides with the plane of lamination resulting from segregation of bands alternately rich in quartz and in piedmontite during metamorphism.
A strong lineation (b) is marked by parallel alignment of piedmontite prisms and of streaky aggregates of that mineral.
As in Specimen No. 2671, the quartz of the piedmontite-rich bands is noticeably finer in grain than that of bands composed mainly of quartz and subordinate mica. The same tendeney for slight elongation of grains of both types parallel to ab of the fabric was also noted. A striking feature of Specimen No. 1489 is the almost universal presence of deformation lamellae (“Boehm lamellae”) in both the large and small grains of quartz. These are particularly conspicuous in sections cut respectively parallel to the ab and ac planes of the fabric. In any grain the lamellae are marked by sheets of minute inclusions. Very rarely individual lamellae in one grain were seen to pass into corresponding subparallel shear fractures in an adjacent grain of different crystallographic orientation (Fig. 6). Undulose extinction, though distinct in most grains of quartz, is relatively weak compared with that recorded by other
writers (e.g., Hietanen, 1938; Fairbairn, 1941) in quartz showing deformation lamellae.
Fig. 7 shows the orientation of poles of (001) in 100 flakes of mica, measured in the ac section. There is a strong maximum at c, but the incomplete girdle (like that of Fig. 5) is probably apparent or exaggerated rather than real.
Fig. 8 is an orientation-diagram showing distribution of optic axes of 250 quartz grains measured in traverses perpendicular to b in a section parallel to ab. It shows an unusually high degree of preferred orientation compared with other Otago schists. There is not only a strongly developed ac girdle, but within this girdle the optic axes show a marked tendency to crowd into a sector extending about 50° on either side of the c fabric axis; in addition, there- are two strong maxima within this sector, separated by an area of weaker concentration adjacent to c itself.
Fig. 9 shows the orientation of normals to deformation lamellae in 100 grains measured in a section parallel to ac of the fabric. It is marked by a very strong preferred orientation of the deformation lamellae sub-parallel to two statistical S-planes, S2 and S3, inclined at angles of 58° and 66° to the schistosity S1 (=ab). This pattern is remarkably similar to that recorded by Fairbairn (1941, Fig. 1, p. 1268) for lamellae in quartz of a sheared quartzite from the Ajibik Formation, Michigan, U.S.A. The degree of preferred orientation of the lamellae in the Otago rock is noticeably higher than that of the optic axes measured in the same grains (Fig. 10), a fact which suggests that the observed orientation of the quartz lattice has been controlled by movements on the lamellae rather than by gliding parallel to prism faces. This is also supported by the fact that strong development of lamellae is associated with strong orientation of the optic axes of the quartz grains, as compared with weak orientation of quartz in Specimen No. 2671.
In each individual grain the angle between the normal to the observed deformation lamella and the optic axis was measured, and the results have been plotted to give Fig. 11. In most instances
Fig. 4.—No. 2671. Quartz, optic axes in 300 grains Contours 2, 1, 0.3%; maximum concentration 3%.
Fig. 5.—No. 2671. Muscovite, poles of (001) in 100 grains. Contours 4, 3, 2, 1%; maximum concentration 29%. S'1= mean trend of S1 in measured area; S2= megascopic schistosity.
Fig. 7.—No. 1489. Muscovite, poles of (001) in 100 grains. Contours 4, 3, 2, 1%; maximum concentration 28%.
Fig. 8.—No. 1489. Quartz, optic axes in 250 grains. Contours 4, 3, 2, 1%; maximum concentiation 7%.
Fig. 9.—No. 1489. Quartz, poles of deformation lamellae in 100 grains. Contours 4, 3, 2, 1%. Maximum concentration 14%. S1=megascopic schistosity. S2 and S3= statistical S-planes.
Fig. 10.—No. 1489. Quartz, optic axes of the 100 grains used in Fig. 9. Contours 4, 3, 2, 1%. Maximum concentration 8%.
this angle lies between 10° and 30° (with a maximum concentration between about 18° and 25°), but there is a marked tendency for a small percentage to lie between 80° and 90° as well.
For recent discussion of the origin of deformation lamellae in quartz the reader is referred to Sander (1930, pp. 177, 178), Hietanen (1938, pp. 31–38), and Fairbairn (1941). The observations of all these writers agree with those recorded in this paper in one important respect, viz.; the angle between the optic axis and the pole of the deformation lamellae in any one grain most commonly lies between about 10° and 30°. Sander attributes the lamellae to translation gliding on planes approximately to a flat rhombohedron; Hietanen to gliding on (0001) with subsequent “change of front” in the deformed grain; Fairbairn to translation on irrational surfaces subparallel to (0001) with a rational glide-line (m:r). The strong incidence of the angle (optic axis: lamella pole) within the range 10° to 30°, and rarity of values between 0° and 5° seems to support a hypothesis of gliding on irrational surfaces subparallel to a flat rhombohedron rather than surfaces subparallel to the basal place. The distinct sub-maximum at 85° to 90° indicates that deformation lamellae show a limited tendency to develop subparallel to faces in the prism zone.
The exceptional feature brought out by the fabric analysis of this rock is the pattern produced by the orientation of the deformation lamellae. This pattern is one usually interpreted as being due to flattening, thus:—
However, it is seen that the mica and piedmontite orientations bear no relation to a flattening movement in this direction; moreover, the strong lineation and schistosity indicate that the original fabric was a B-tectonite due to intersecting shear planes (parallel to b [=B]) and/or rotation parallel to b. The orientation of the piedmontite suggests that the latter has glided, and that it has not been passively rotated to bring the long dimension parallel to b (cf. Hutton, 1942).
The orientation of optic axes in quartz shown in Fig. 13 could conceivably be the result of gliding parallel to a prism, the deformation lamellae being secondary effects only; but this seems hardly
likely in view of the much sharper concentration of lamellae poles than of optic axes (see discussion on deformation lamellae above).
A possible mechanism of evolution of the fabric of Specimen 1489 is as follows :—first, development of a B-tectonite fabric involving strong preferred orientation of all three main constituents by deformation in which recrystallisation (indirect componental movement) was a major factor; and, finally, a slight compression parallel to the schistosity, giving the present quartz orientation but not affecting the mica and piedmontite grains.
The writer is very greatly indebted to Dr. F. J. Turner for teaching him the universal stage technique and also for much helpful criticism in the compilation of this paper. Thanks are also due to Dr. C. O. Hutton for correcting the proofs, and to Professor W. N. Benson for allowing the writer the use of the Geology Department, Otago University, and the apparatus therein.
Fairbairn, H. W., 1941. “Deformation Lamellac in Quartz from the Ajibik Formation, Michigan.” Bull. Geol. Soc. Amer., Vol. 42, pp. 1265–1278.
Hietenan, Anna, 1938. “On the Petrology of Finnish Quartzites.” Govt. Press, Helsinki.
Hutton, C. O., 1940. “Metamorphism in the Lake Wakatipu Region, Western Otago, New Zealand.” D.S.I.R. Geol. Mem., No. 5.
—— 1042. “Piedontite-bearing Quartz Schists from Black Peak, Northwestern Otago.” N.Z. Journ. Sci. and Tech., Vol. XXIII, No. 6B, pp. 231B–232B.
Sander, B., 1930 “Gefügekunde der Gesteinc,” Vienna, J. Springer.