Fabric Analysis of Foliated Pomona Island Granite.
For investigation of the microfabric of the Pomona Island granites a well foliated specimen from the southern contact zone was selected.* The megascopic data (cf. Fig. 3a) are as follows:—
Foliation (ab) vertical; strike 5° E. of N. Rift (direction of easiest cleavage) parallel to foliation.
Lineation (b) dips southward at 45°. Cross joints (ac) strike 95° E. of N. and dip northward at 45°.
Joints subparallel to bc well developed; strike 95° E. of N.
Polished surfaces of the hand-specimen show that the foliation is due principally to aggregation of quartz into discontinuous foliae about 0–5 mm. to 1 mm. in thickness, which are more clearly defined on the bc than on the ac surface. Flattened aggregates of flaky chlorite and granular iron-ore of xenolithic origin, not exceeding 1 cm. in length, lie with their greatest dimensions parallel to the foliation. As seen on ah and bc surfaces they are arranged along “flow lines” parallel to b thus clearly defining the latter direction.
Fig. 3.—A. Diagram showing structural features of granite (represented by specimen No. 4525) halfway along east coast of South Arm; invaded amphibolite and gneiss indicated by dotted areas (the actual exposures are often 100 yds. in width). Joints in granite are indicated by full lines; broken lines represent lineation. Foliation vertical, parallel to plane of the page.
B. Section block showing relative positions of a, b and c axes in relation to lineation and foliation. Directions in A and B correspond.
In section the principal constituents are sodic oligoclase, quartz, microcline and chlorite. Both feldspars build up a mosaic of small
grains averaging 0.2 mm. in diameter, in which larger tabular crystals (1 mm. to 1–5 mm.) are enclosed; the latter tend to lie parallel to the foliation. Quartz occurs mainly as relatively coarse crystals (0–5 mm. to 2 mm.) concentrated in streaks and lenticles trending parallel to ab, but is also represented by small grains in the feldspar mosaic. Undulose extinction is almost universal and is developed on a perfect scale. Closely spaced cracks and lines of dusty inclusions oriented parallel to ac are clearly observable in ab and be sections.
Fig. 5.—Orientation curve for quartz in be section of specimen No. 4525 (500 grains)
Fig. 6.—Orientation curves for quartz in ac section of specimen No. 4525; broken curve represents 450 grains in a single rock section; full curve represents 900 grains in two different sections, including that upon which the broken curve is based.
Sections perpendicular to each of the three fabric axes were cut (Fig. 3B) and the quartz fabric was analysed following the method described by the writer in previous papers (Turner, 1936, 1938). This involves measurement of the angle Z° to b (or a) for a large number of quartz grains in each section, using an ordinary petrographic microscope fitted with a simple mechanical stage. The results are graphically represented in Figs. 4 to 6. The main features of the fabric are clearly indicated and are summarised below:—
(1) A pronounced minimum parallel to b (Figs. 4 and 5).
(2) Strong concentration of the quartz axes (Z) in the ac plane (Figs. 4 and 5).
(3) A second minimum parallel to c (Fig. 6). This is borne out by the prominence and sharpness of the maximum in the ab curve (Fig. 4) compared with the less pronounced and broader zone of concentration perpendicular to b in the bc curve (Fig. 5). Prevalence of grains showing low double refraction in the be section also indicates that the majority of the grains are oriented with their optic axes not greatly inclined to a.
(4) Presence of three maxima in the ac concentration zone, situated respectively at angular distances of 25° and 145° from a and parallel to a itself. To test the validity of these maxima a second ac section was cut and the component quartz grains measured. The two sets of results showed a very close correspondence as will be seen in Fig. 6 by comparing the broken curve (representing a single section) with the full curve (combining the results obtained from the two sections).
Figs. 7a and 7b show the positions of the deduced maxima in stereographic projections upon ab and ac respectively. In Fig. 7c a contoured orientation diagram, such as might correspond to the orientation curves of Figs. 4 to 6, has been constructed in stereographic projection. The positions of contours are not represented accurately, but their general pattern has been deduced broadly from consideration of the orientation curves. Thus the points where the a axis is cut by contours corresponding to the percentages 2, 4, 6, 8, 10 and 12 the ac curve of Fig. 5 were constructed stereographically in the contoured diagram. Some degree of elongation of one or both of the principal maxima toward the negative end of the b axis is indicated by the form of the bc curve, and is shown accordingly in Fig. 7c. The reconstructed figure though obviously far from exact does indicate the main features of tectonic significance, viz. the positions of the main maxima and minima, the broken ac girdle, and the general pattern of the quartz orientation. Further it allows rapid comparison with the standard contoured diagrams constructed from results obtained with a universal stage.
Fig. 7.—(a) Stereographic projection upon ab showing the points of maximum concentration of quarts ‘axes’ (Z) in Pomona Island granite, No. 4525.
(b) Stereographic projection upon ac showing relative positions of S1, S, and Sa, In specimen No. 4525.
(c) Stereographic projection upon ab with hypothetical contoursrs indicating the type of orientation that could account for the data recorded on the orientation curves in Figs. 4, 5 and 6 (quarts in specimen No. 4525).
In the previous section it was shown that the lineation in the granites of the southern marginal zone was developed by flow of the partially crystallised magma parallel to the lineation itself. The quartz fabric just described however, is not obviously related to such a moverdent arid must have originated during continued deformation after cessation of magmatic flow. The universal undulose extinction exhibited by the grains, now generally interpreted as a result of gliding in crystalline grains on (1010) in a direction parallel to the vertical crystallographic axis (Fairbairn, 1937, p. 38), is itself an indication that the present orientation of the grains is due to “plastic” deformation of the solid rock and not to “viscous” deformation of a partly crystalline magma. From the orientation diagrams it follows that the direction of gliding was perpendicular to the lineation. On one interpretation it may have occurred in two principal planes (S1 and S3 of Fig. 7b) inclined respectively at 25° and 145° to the foliation (S2), and
probably also to some extent in the plane of foliation itself. Sub-horizontal compression acting more or less perpendicularly to the foliation in the closing phases of orogeny could give rise to such a movement in S1 and S3 and, need not have involve much actual transport (cf. Sander, 1934, p. 42). On the other hand, following Schmidt's interpretation of the significance of maxima in B-tectionite girdles, the orientation diagrams just described may indicate operation of a compressive force acting parallel to the a fabric axis (cf. Schmidt, 1932, fig. 49; Phillips, 1937, p. 598). In either case there is an intimate relation between the linear and foliated structures produced by magmatic flow, and the quartz fabric resulting from later deformation of the solid rock, though the actual directions of the movements concerned in the earlier and the later phases are mutually perpendicular. Fairbairn (1937, pp. 112, 113) summarises other instances where the fabrics of foliated intrusive rocks show that the latest movements occurred across the flow-lines after solidification, thus giving rise to a B-tectonic girdle perpendicular to the principal lineation.
In conclusion, attention may be drawn to the similarity between the reconstructed contoured diagram representing the quartz fabric of the Manapouri granite, and the orientation diagrams for quartz in certain foliated pegmatites discussed by Sander (1930, pp. 184, 185; D.30, 31, 33, pp. 307, 308).