The conclusions put forward below are based on the petrofabric studies embodied in this paper, taking into account also the findings of the previous authors cited. Further work of the same nature will be necessary to verify the extent to which they may be applied, or have to be modified, in interpreting olivine fabrics in general.
(1) Flow in basic lavas containing suspended prismatic olivine crystals fails to produce preferred space-lattice orientation of olivine (Figs. 1–3) even though it may have given rise to pronounced fluxional arrangement of the accompanying laths of feldspar. This raises the possibility that dimensional parallelism of the feldspars in volcanic rocks may be governed by growth of crystals in the surfaces of flow of the viscous magma according to Sander's principle
Olivine (50 grains) in banded chromite-dunite, 7495, Dun Mt. Section perpendicular to trend of bands (broken line). Contours, 12, 8, 4, 2%.
Olivine (50 grains) in banded chromite-dunite, 7494, Dun Mt. Section perpendicular to trend of bands (broken line). Contours, 12, 8, 4, 2%.
Olivine (50 grains) in banded picotite-dunite, 7496, Glen South of Alt a'Chaoich. Skye. Section perpendicular to trend of bands (broken line). Contours, 12, 8, 4, 2%.
Olivine (50 grains) in fissile dunite, 7498, Glen south of Alt a'Chaoich. Skye. Section parallel to plane of fissility. Contours, 12, 8, 4, 2%.
Olivine (50 grains) in slightly fissile dunite, 7493, Dun Mt. Section approxi-mately parallel to fissility. Contours, 12, 8, 4, 2%; maximum concentration, 16% in Fig. 20.
Olivine (16 undulose grains) in fissile dunite, 7493, Dun Mt. Dots connected by straight lines represent observed positions of the indicatrix axis in any one crystal.
Olivine (50 grains) in black olivine-rock, 7497, south of Abhuinn Fiadh-innis, Rum. Section perpendicular to the plane of fissility (broken line) and approximately perpendicular to lineation. Contours. 12, 8, 4, 2%.
Olivine (50 grains) in harzburgite, 1380, Olivine Range, South Westland. Contours 12, 8, 4, 2%; maximum concentration 20% at × in Fig. 30; 16% in Fig. 28 and at Y in Fig. 30; 14% in Fig. 29.
Olivine (50 grains) in cataclastic dunite (7491), Anita Bay, Milford Sound. Section perpendicular to plane of fissility (broken line). Contours, 8, 4, 2%.
Olivine (50 grains) in nodule (7503) in basalt, Onepoto, Shoal Bay, Auckland. Contours, 8, 4, 2%. Maximum concentration in each case 10%. × = normal to the girdle (broken arc) of Fig. 40.
of Wegsamkeit, as well as by rotation of pre-existing crystals into the same surfaces.
(2) Settling of olivine crystals under the influence of gravity as they separate from basic magma under static conditions also fails to produce preferred orientation of the olivine space-lattice (Figs. 4–6). (3) The megascopically conspicuous regular banding of certain fresh chromite-rich dunites is believed to have originated during flow of a “magma” in an advanced state of crystallisation. A feature of the fabric, constantly associated with banding, is preferred orientation of olivine governed by external form of grain, in that individual crystals tend to be elongated parallel to the banding. Therefore it is probable that this dimensional orientation and any consequent preferred orientation of the space-lattice are also determined by flow prior to complete solidification of the magma, rather than by movement and actual deformation of grains after the mass had become completely crystalline. A high degree of penetrative intergranular movement, such as would necessarily affect a largely crystalline dunite “magma” in process of injection as pictured by Bowen (1928, p. 167), would be expected to give rise to preferred orientation of grains according to their external form, and this would also involve orientation of space-lattice if the grains in question conformed uniformly to a well defined crystallographic habit. The usual habit of magnesian olivines is a prismatic form flattened normal to b (α) and elongated parallel to c (β). Laminar flow of the largely crystalline mass (with its lubricating interstitial liquid) along approximately plane surfaces (such as is indicated by the regular subparallel layers of the banded dunites) should give rise to a fabric the dominant feature of which would be a strong concentration of α-axes normal to the plane of flow (Figs. 7, 10, 13). A secondary feature, depending upon elongation of the crystals parallel to β, is the tendency for β to lie in the flow-surface even in grains for which α departs from the ideal orientation, so that the β-girdle perpendicular to the α-maximum is more clearly defined than is the γ-girdle. This condition is shown in Figs. 14, 15. The opposite tendency for the γ-girdle to be more pronounced than the β-girdle, shown by one of the two banded dunites from Dun Mt. (Figs. 8, 9), cannot be explained in terms of the above-described mechanism, and is attributed to later deformation.
(4) In the absence of visible banding in dunites poor in chromite, there is often no way of determining whether a simple orientation pattern, with a high concentration of a-axes and a fairly even spread of β and γ in the girdle normal to this, has originated by laminar flow prior to complete solidification of the magma or by deformation of the already solid mass. If magmatic flow is the controlling process, the space-lattice orientation should be accompanied by dimensional orientation of crystals with their long axes aligned in the surfaces of flow, and distinct fissility perpendicular to the α-maximum might be obvious in the hand-specimen. However, according to Ernst (op. cit.) dimensional orientation of olivines elongated at right angles to α is also conspicuous in schists whose fabric is of deformational origin.
(5) The fabrics of three Hebridean peridotites have been described in this paper. One (7496) is a banded type with a simple
fabric believed to have been determined by magmatic flow as discussed under (3) above. The second is a fissile non-banded rock (7498, Figs. 16–18) with a closely similar fabric which is therefore attributed to the same general process. The third rock (the black peridotite, 7497, from Rum) has a rather weakly defined fabric of quite a different type, the main features of which are an α-girdle normal to the plane of fissility, and a β-maximum perpendicular to the α-girdle and approximately parallel to a weak megascopic lineation (Figs. 25–27). This pattern is just what would be expected if, during flow of a largely but not completely crystalline mass, the crystals rotate with their long axes (β) aligned in the flow surface and normal to the direction of flow. The presence of a megascopic lineation strongly supports the possibility that such a mechanism has indeed controlled development of the fabric in this particular instance. Of the four Hebridean peridotites investigated by Phillips (1938) two have simple fabrics dominated by an α-maximum (Phillips, op. cit., 3, 4, 7, 8); in one other the α-maximum and β-γ-girdle are accompanied by a definite concentration of γ in the latter (Figs. 1, 2); the fourth fabric shows no preferred orientation of olivine (Fig. 6). Both Phillips' results and those of the writer may be explained on the assumption that the fabrics of Hebridean peridotites are due to flow of largely but not completely crystalline olivinerich magmas, and have not been influenced to any extent by post-crystallisational deformation, of the rocks concerned. This is much the same conclusion as was arrived at by Phillips (op. cit., p. 134), except that the present writer would emphasise his opinion that deformation of crystals has played little or no part in development of the observed preferred orientation.
(6) The great steeply inclined dunite sills of Nelson and South Westland have been injected along zones of major dislocation during profound orogeny. The rocks concerned may therefore be expected to bear the imprint of post-crystallisational deformation superposed upon a fabric determined by flow prior to complete consolidation. The two effects will often be difficult to separate since both owe their origin to the same set of deforming forces acting upon the intrusive mass. (a) In specimens 1380 and 7505 there is strong preferred orientation of the olivine space-lattice but no recognisable dimensional orientation, so that the fabrics may be assumed with some confidence to have been determined mainly by deformation after complete solidification of the rocks in question. The orientation pattern of 1380 (Figs. 28–30) is marked by strong mutually perpendicular concentrations of α, β and γ respectively. The α-maximum is most pronounced and is somewhat elongated in a plane perpendicular to the mean trend of γ. If, as appears likely both on theoretical grounds and from fabric studies of Andreatta and of Ernst on olivine-tectonites, α tends to be oriented at right angles to the principal slip-plane of the fabric, then the direction in which γ is concentrated is the b fabric axis about which an incipient α-girdle is in process of development. This would point to β (crystal axis c) as the preferred glide-direction within a glide-plane (010). This conclusion is borne out by the fabric of 7505 (Figs. 31–33) in which the α-girdle is fully developed in a plane normal to a strong γ-maximum; in this
rock, too, micro-fractures inclined at high angles to the mean trend of γ (b fabric axis) are conspicuous and could be interpreted as ac fractures such as are commonly present in girdle-tectonites.
(b) The following sequence of events is suggested in explanation of the fabrics of 7494 (Figs. 10–12) and 7495 (Figs. 7–9):—Laminar flow in the still partially liquid peridotite “magma” gives rise to a dimensional orientation of olivine crystals with which a strong concentration of α-axes normal to the flow-surfaces (s) may be correlated. As deformation of the intrusive mass continues after it has completely solidified, slip-movements develop principally in the original flow-surfaces, with which, moreover, the best marked crystallographic glide-plane (010) already coincides in many of the grains of olivine. However, before gliding can occur in any given grain it is necessary that the grain should rotate until the preferred glide-direction (β) is brought into approximate coincidence with the direction of slip-movement (a fabric axes) for the rock as a whole. Therefore one of the component movements accompanying the earliest stages of deformation of the solid rock is a rotation of both β and γ about the normal to s, with resultant concentration of β and γ in two mutually perpendicular directions in s. This condition is exemplified by Figs. 10–12, the fabric in this case being more weakly defined than is usual in West Coast peridotites. A further complication in the fabric would be introduced if there were several sets of slip-surfaces inclined at low angles to s, or if the latter were somewhat curved by external rotation about the b axis of the fabric. Both possibilities are commonly encountered in deformed rocks and would have the same effect upon the orientation of the olivine space-lattice—namely, spreading of the α-and β-maxima in the plane normal to the b axis of the fabric (direction of concentration of γ). A strongly developed fabric of this type is shown in Figs. 7–9, in which it will be noticed that whereas the γ-maximum is elongated within s, the β-maximum is much less distinct and is dispersed not only in s but also in the plane of elongation of the α-maximum.
(e) The strongly defined unique orientation pattern of Figs. 19–21 cannot be related to the megascopic fissility-planes of the rock in question on any such hypothesis as those put forward above. It would seem that the fissility is due to persistence of original surfaces of magmatic flow, and that the space-lattice orientation is the expression of a subsequent deformation under a new set of kinematic conditions. The unusually sharp γ-girdle is interpreted as a B-girdle such as is commonly developed in many tectonite fabrics, and the β-maximum on this assumption represents a concentration of β-axes parallel to B. Further interpretation is somewhat speculative, for more than one gliding mechanism could give rise to an orientation pattern of this one type, e.g.:
(1) glide-plane = (010) (i.e., ⊥ α), glide-direction = γ;
(2) glide-plane = (100) (i.e., ⊥ γ), glide-direction = α.
On account of the sharpness of the γ-girdle and wide range of orientation of both α and β, gliding parallel to γ (theoretically the most probable glide-direction in olivine) in several crystallographic glide-planes, of which (010) is specially favoured, would appear the most likely orienting mechanism.
(7) In most peridotites having orientation patterns showing what is believed to be the influence of deformation subsequent to complete solidification [cf. (6) above], undulose extinction is conspicuous in the constituent grains of olivine. Since this property is considered to be an expression of combined flexural gliding and incipient rupture of the space-lattice, its origin is almost certainly bound up with the process of deformation by which the orientation pattern was imprinted upon the fabric as a whole. It has been suggested above that in many of the investigated rocks olivine grains, originally oriented by magmatic flow so as to bring α normal to the flow-surfaces (s), have subsequently rotated until β approaches the principal direction of slip for the deforming rock. As long as the opposing force of friction on the intergranular boundary surfaces did not exceed a limiting value, this rotation could be in the nature of differential movement of the individual grains. Once the friction exceeded this limiting value, intragranular flexural gliding on space-lattice surfaces inclined at high angles to s (the principal slip-surfaces of the rock fabric) could achieve the same result. For example, in the case of a grain with α approximately normal to s, flexural gliding on (001) with γ (crystal axis a) as glide-direction could rotate both β and γ within s. Deformation of the individual grains is therefore pictured as a complex process involving flexural gliding on (001) with γ as glide-direction, rupture and some differential movement on (100), and finally (when β is brought sufficiently close to the principal glide-direction for the fabric as a whole) translation-gliding on (010) parallel to β. It has long been recognised that some such process as flexural gliding, or a combination of gliding movements on two mutually perpendicular sets of crystallographic glide-planes, must be assumed in order to account for the high degree of preferred orientation of minerals that is commonly attained during comparatively slight deformation that cannot have involved extensive rotation of individual grains (e.g., cf. Schmidt, 1932, pp. 173–176; Eskola, 1939, pp. 297, 298, 304, 305). Finally attention is drawn to the single case (7493, Figs. 19–21) where gliding parallel to γ on various crystallographic planes, but especially on (010), is thought to have been the most influential process in development of the olivine fabric; as might be expected, undulose extinction is here more strikingly shown by the olivine grains than in any other rock investigated.
(8) When under high confining pressure a rock is strained beyond the elastic limit, it may deform plastically by combined plastic deformation of individual grains and recrystallisation through the agency of aqueous pore-solutions. In this way a cooling mass of peridotite, intimately penetrated by magmatic waters containing silica or carbon dioxide, may by continued deformation be converted to a schistose antigorite-serpentine. In the absence of water, however, recrystallisation of olivine or crystallisation of new minerals like serpentine or talc appears to be impossible. And when deformation advances beyond the stage where internal adjustment to the stress system can be achieved by movements on crystallographihc glide-planes and intergranular boundary surfaces, rupture of the olivine grains and differential movement of the fragments so produced become the essential factors in a deformation that may now be described as cataclastic. The ultimate product of a process of this sort is a fine-grained cataclastic dunite such as 7491, with a distinct
schistosity (s-planes of shearing) marked by alternation of bands of different grain-size and by dimensional parallelism of the elongated crystal fragments. Lack of preferred orientation of the olivine space-lattice is a characteristic feature of the fabric (Figs. 37–39).
(9) Mineralogically the olivine-nodules from New Zealand Tertiary and Pleistocene basalts are identical with lherzolites and harzburgites of plutonic origin. The constituent minerals include enstatite, chrome-diopside, picotite and chromite, none of which has been recognised in the enclosing basalts. In two of the three specimens used for fabric analysis (5100, 6655; Figs. 43–48) the orientation pattern, though very weak, conforms to a type here attributed to deformation of a solid rock, while the third fabric (7503; Figs. 40–42) with its pronounced α-girdle and absence of dimensional orientation of grains must have originated in a like manner. This conclusion is supported by prevalence of undulose extinction in all three cases. It is therefore suggested that all three nodules are fragments torn from masses of solid peridotite, which have formed under plutonic conditions but have never flowed in the form of injected bodies of largely crystallised peridotite magma. If these parent peridotites were formed as accumulations of early crystals separating from the primitive basaltic magma at sufficient depth for the conditions of temperature and pressure to be quite different from those obtaining during later crystallisation of the basalts themselves, the load of overlying rock and magma might well be sufficient to give rise to the weak deformation recorded in the fabric. The European nodules described by Ernst (op. cit.) on the other hand include rocks with strongly developed fabrics in which well developed girdles appear to indicate deformation of a more pronounced nature, especially in that the olivine of such rocks often shows strongly developed undulose extinction and translation-lamellae. The petro-fabric evidence, taken in conjunction with the widespread occurrence of olivine-rich nodules in basalts in many parts of the world, points to deep-seated solid accumulations of olivine crystals and in some cases perhaps intrusions of peridotites as the source of the nodules in question. As Phillips has pointed out (op. cit., p. 134) it is not necessary to assume the existence of a deep-seated metamorphic terrane as a source of olivine-nodules. Nor, however, can they be pictured as mere local aggregations of crystals separating from the basaltic magma just prior to eruption.
(10) The following characters of peridotite fabrics are considered as criteria of origin by flow in a largely but not wholly crystalline magma: s-planes marked by alternation of layers respectively rich and poor in chromite, or by parallel orientation of elongated olivine grains; concentration of α normal to s-surfaces of flow, and fairly even spread of β and γ within the plane of flow, the β-girdle sometimes being sharper than the γ-girdle; any concentration of β or γ within the plane of flow must be accompanied by microscopic or megascopic lineation due to dimensional parallelism of elongated grains as seen in sections parallel to the s-planes in question.
(11) Characteristic criteria of origin of a peridotite fabric by deformation of the solid rock include the following: presence of a complete or incomplete α-girdle at right angles to any visible flow
surfaces, accompanied by concentration of γ normal to the α-girdle and parallel to the b axis of the fabric; less commonly presence of simple mutually perpendicular maxima for α, β and γ respectively; occasionally development of a γ-girdle at right angles to a β-maximum which is parallel to b of the fabric; conspicuous undulose extinction and sometimes translation-lamellae normal to γ; cataclastic structure, schistosity due to shearing, dimensional parallelism of crystal fragments and lack of preferred orientation of the olivine space-lattice.