Art. XXXIV.—Additional Facts concerning the Distribution of Igneous Rocks in New Zealand: No. 2.
[Read before the Auckland Institute, 22nd December, 1919; received by Editor, 31st December, 1919; issued separately, 16th July, 1920.]
In these notes I wish to record a few identifications of rocks from various places, some possessing considerable interest; others are quite ordinary types, but none the less it may be useful to have their occurrences placed upon record.
Cleddau-Hollyford Saddle, South-West Otago.
This rock was gathered, with others, by Mr. R. W. Holmes, Engineer-in-Chief, New Zealand Public Works Department, from the Cleddau-Hollyford Saddle in 1890. In section it appeared to contain an unusual pyroxene, and after some study was sent to Dr. J. A. Thomson, Director of the Dominion Museum, who was so good, as to give the rock considerable attention, and who very kindly has handed over to me the results of his work. His observations corroborated and considerably amplified my own, but unfortunately sufficient information for exact determination is not available from the section, and Mr. Holmes was unable to find the remainder of the rock from which the chip for sectioning was originally taken. I am also greatly indebted to Dr. Thomson for other help, particularly for pointing out that a mineral in this same rock, carelessly identified by me without full investigation as apatite, is probably zoisite, a fact made almost certain by its high index of refraction, straight extinction, low polarization tints, distinct biaxial character with high optic axial angle, and, so far as I can judge from rather unsatisfactory tests, positive optical character.
The rock itself is a fairly coarse-grained dioritic type showing no outstanding characteristics in hand-specimen. Under the microscope it is seen to be a moderately typical norite but for two considerations: first, the plagioclase, which slightly exceeds the ferro-magnesian minerals in amount and with these latter comprises practically the whole of the rock, is somewhat acid, being in the main andesine-labradorite; secondly, there is distinct acidity, signalized by the presence of a little interstitial quartz. The chemical analysis given below, for which I am indebted to the Dominion Analyst, through the courtesy of Mr. P. G. Morgan, Director of the New Zealand Geological Survey, fully exemplifies these characteristics, and shows that chemically the rock is an intermediate type. Mineralogically, however, it is very closely allied to the norites, and for this reason I have classed it with them.
The structure is coarse and even-grained, the ferro-magnesians occasionally being recognizably ophitic to the plagioclase. Of the former group of minerals, one that would ordinarily be identified unhesitatingly as hyper-sthene is probably the most prominent, whilst pale-green augite and deep
Fig. 1.—Quartz-norite, Cleddau-Hollyford Saddle, illustrating the structure and the mineral relationships. The field of the photograph coincides with a portion of the diagrammatic sketch of the text-figure, and the identification of the minerals is possible from the labelling of that sketch. The enclosure of hypersthene by augite is evident at the left-hand end of the horizontal diameter, whilst the penetration of the same augite crystal by zoisite (dark) is also recognizable. The interstitial quartz is very obvious. In the lower right-hand quadrant it is just possible to recognize some fine lamellae of augite intergrown with a crystal of hypersthene-like pyroxene (dark). Crossed nicols. Magnification, 35 diameters.
Fig. 2.—Dolerite, Silverdale, Auckland. The general ophitic structure can readily be recognized, as also the bar-like growths of iron-ore. Crossed nicols. Magnification, 20 diameters. Fig. 3.—Basalt, near old wharf, Ti Point, Whangateau Harbour. A typical olivine phenocryst. fringed by a rim of oxidation, set in a groundmass in which augite plays an important part. Ordinary light. Magnification, 35 diameters.
Fig. 4.—Basalt, Ohakune ballast-pit. A central crystal of olivine (dark) with wide celyphitic rim of augite (mainly white). Some feldspar laths show up in the groundmass, but the olivine laths are here inconspicuous owing to clouding by haematite, whilst abundant vesicles also render the slide dark. Crossed +cols. Magnification, 21 diameters.
reddish-brown biotite are also very common. Reasons will be given later for the belief that probably a third pyroxene is also present. The hyper-sthene is in fairly idiomorphic crystals, and preceded the augite, whilst the biotite is later, for its large fresh crystals poecilitically enclose pyroxene, iron-ore, and even quartz grains. The pyroxenes all show imperfect schiller structure; they are occasionally fringed by a little bright grass-green hornblende, which from its association appears largely resorption-product, though perhaps in part outgrowth. Ilmenite is fairly abundant in coarse irregular masses, but magnetite also is common.
Diagrammatic Sketch (× 45)Illustrating Relations of Zoisite to Other Minerals, and General, Structure. (See Plate XXVIII, fig. 1.)
Au. =augite; Hy.= hypersthene; Pl. = plagioclase; Q. = quartz; Z.= zoisite.
Amongst the primary minerals I must finally mention the mineral identified at first by me as apatite, but recognized by Dr. Thomson as zoisite. It is in occasional rather allotriomorphic crystals up to 0.6 mm. in diameter, and one crystal is encased in a zone of iron-ore. In the instance portrayed by the figure above and the photomicrograph of Plate XXVIII, fig. 1, and in one other, the zoisite is enwrapped (by augite in the text-figure) in a manner that makes it certain that it was formed before the later stages of crystallization of the rock, and therefore is primary. The plagioclase includes occasional rutile needles.
An Undetermined Pyroxene.
A number of crystals of a pyroxene were observed in this rock, which in many respects greatly resembles hypersthene, but from other considerations I am inclined to believe must be referred to a closely allied mono-clinic pyroxene of a series either identical with or analogous to that of which the now well-known enstatite-augite is a member. These pyroxenes show a microscopic twinning lamination parallel to their elongation, and in addition are frequently intergrown with minute plates of obvious mono-clinic pyroxene approaching diopside, besides containing irregular inclusions of this latter mineral. The pleochroism distinctive of hypersthene
is general, the optical character is negative, and the elongation positive, whilst the optic axial angle is small. Most important of all, however, the extinction, whilst often nearly straight, reaches generally as high as 5° with the composition-planes of the lamellae, and in one or two crystals angles of over 15° are got with the cleavage. Were it that only one or two crystals showed the obliquity of extinction one might suspect some inaccuracy of observation, but at least eight in the slide show it. Further, even greater obliquity of extinction, with similar very finely microscopic twinning, is shown by nearly a dozen crystals of apparently identical hypersthene-like pyroxene in a noritic rock discovered at Milford, and described later in this paper (see p. 420).
In some instances the lamellae are here much broader than in others, whilst the extinction angles generally range up to 15°, but in one case up to as much as 42°. As a rule, the lamellae extinguish together in the one direction, but this is by no means invariably the case. In the Milford, as in the Cleddau-Hollyford rock, there is parallel intergrowth with fine lamellae of augite. It may be thought that the crystal in which the extinction angles of the lamellae reached 42£ exemplifies a similar parallel intergrowth with augite, but this is not so, for the lamellae are very distinct and comprise the whole mineral, which certainly is not an ordinary monoclinic pyroxene, for it is optically negative and has the distinct hypersthene-like pleochroism already noted. There is, however, one peculiarity of this crystal that may have a bearing upon the matter—namely, the optic axial angle appears to be large.
Only one further observational fact now remains to be mentioned in connection with this pyroxene, but it is of importance: the plane of the optic axes coincides with the composition-plane of the lamellar twin. Since the extinction is oblique, this plane must be at right angles to the plane of symmetry—that is, to the 100 plane—as in normal hypersthene. The lamellar twin appears not to follow the plane 110 frequent in the inter-growths of rhombic and monoclinic pyroxenes, for all the sections showing the lamination distinctly are approximately at right angles to the optic axial plane; nor is it the common 010 plane.* It is much more reasonably the 100 plane observed by Bowen in artificial clino-enstatite.† On this supposition, however, it is difficult to understand why the lamellae often appear all to extinguish in the same direction.
Enough data are available to show that a monoclinic pyroxene of unusual type is exhibited in the sections from the Cleddau-Hollyford and the Milford rocks. It approaches hypersthene, but differs from it in its oblique extinction, and it seems probable that it is a monoclinic member of a series embracing this latter. It is obvious that much of the mineral in the sections that is apparently hypersthene may really be this mono-clinic pyroxene viewed in sections lying near the orthopinacoid.
An analysis of the Cleddau-Hollyford quartz-norite by the Dominion Analyst gave the results tabulated in column A below. In columns B and C the analyses of somewhat similar rocks are quoted from Iddings.‡
[Footnote] * See H. Rosenbusch, Mikroshopische Physiographie der Mineralien und Gesteine, vol. 1, pt. 2, p. 149, 1905.
[Footnote] † N. L. Bowen, The Ternary System Diopside-Forsterite-Silica, Am. Journ. Sci. (4), vol. 38, pp. 207–64, 1914.
[Footnote] ‡ J. P. Iddings, Igneous Rocks, vol. 2, p. 218, 1913.
The norms and classifications calculated by the C.I.P.W. quantitative method are also appended.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
Norms and Classification.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
A. Quartz-norite, Cleddau-Hollyford Saddle, New Zealand. (Dominion Analyst.)
B. Quartz-gabbro, Carrok Fell, England.
C. Gabbro, Druim an Eidhne, Isle of Skye.
The norm agrees well with the observed mineral composition, although it is difficult to account for the apatite shown by the norm and indicated by the chemical analysis: apatite appears to be practically absent from the section examined.
Another more typical coarse norite was amongst the material collected by Mr. Holmes from the Cleddau-Hollyford Saddle. The plagioclase (labradorite) and ferro-magnesian minerals are subequal; of the ferro-magnesians, hypersthene and augite are both plentiful, the former the more so; it is often finely lamellar as in the quartz-norite, but the extinction appears here to be straight. A brownish-green hornblende clouded by dust-like iron-ore occurs in large crystal which poecilitically enclose the pyroxenes and some feldspar. Some of the augite is intergrown with the amphibole; it is occasionally lamellar, passing to diallage. The hypersthene
is largely converted to a finely fibrous, tufted, almost colourless aggregate bordered by green; it is referred with some diffidence to tremolite and actinolite.
Wairau Creek, Milford, Auckland.
I found a very-well-rounded boulder about 5 in. in diameter included in the sandstones of the Waitemata beds at the mouth of Wairau Creek, a little north of Lake Takapuna, and quite unaccompanied by other boulders or pebbles. It is a very coarse norite with plagioclase (medium labradorite) a little more plentiful than the ferro-magnesian minerals, which are hypersthene (including an allied pyroxene dealt with on p. 417) and less abundant diallage. The hypersthene is being converted into talc, and particularly adjacent to cracks there is chloritization and the entrance of fine pyrite.
Swinging-basin, Silverdale, Auckland.
This occurs as coarse angular blocks several feet in diameter, unearthed during excavation for the swinging-basin at the Silverdale (Wade) wharf on the Weiti Stream, which flows to the east coast about twenty miles north of Auckland. Though apparently the actual outcrop is not laid bare, yet this cannot be far distant, unless—a suggestion their comparative freedom from weathering would negative—the blocks come from the formerly overlying Waitemata beds now removed by erosion from the stratigraphically lower hydraulic limestone of this area, but which contain large blocks of andesite and perhaps other igneous types not far distant on Whangaparaoa Peninsula. The site of discovery of the dolerite is immediately adjacent to where McKay maps one of his serpentine dykes* (which, however, I was unable to locate during my brief visit), and the idea suggested itself that there might be some connection between the intrusion of the two rocks. Professor W. N. Benson remarks anent this point, after examination of the dolerite section, “It does not contain albite, and is more like the post-peridotitic than the pre-peridotitic of my New South Wales dolerite dykes.”†
Macroscopically the rock is a very dark finely holocrystalline type with inconspicuous glistening lath-like feldspars and prominent chloritized ferro-magnesian mineral. In section the most striking feature is the ophitic structure; the mass consists largely of large labradorite laths, between which is faintly pleochroic partially chloritized pink augite in somewhat columnar crystals, and a profusion of irregular iron-ore growths and skeleton crystals enwrapped by the pyroxene (see photomicrograph, Plate XXVIII, fig. 2). The feldspars are fairly fresh, but a little secondary calcite and a very great deal of chlorite developed from the augite are present.
“Sugarloaf,” Matakana-Leigh Road, Rodney County.
The “Sugarloaf” is a conical knob a little west of the main road from Matakana to the Omaha, and about two miles from Matakana. It is mapped as igneous by Cox in his report on the geology of the Rodney and Marsden
[Footnote] * A. McKay, On the Occurrence of Serpentine Dykes in Cretaceo-Tertiary Strata near the Wade, Auckland, Rep. Geol. Explor. during 1883–84, pp. 99–101, 1884.
[Footnote] † Letter dated 13th September, 1918.
Counties,* and is in fact a basalt neck breaking through the massive concretionary greensands of the Waitemata series, which are well exposed near by. The basalt is a fairly typical holocrystalline type, rich in coarse idiomorphic phenocrysts of olivine and pale-greenish augite, with numerous smaller intermediate crystals of plagioclase showing good fluxional arrangement. The groundmass is largely of small feldspar laths enclosing a moderate number of pyroxene grains and small magnetite crystals.
Ti Point, Whangateau Harbour, Rodney County.
This occurrence is referred to as a dyke and also mapped by Cox in the paper just quoted. Several sections were cut from various parts of the mass forming the small peninsula of Ti Point. There is often good fluxional banding shown in the field, but the differences in section are unimportant, except that flow structure is better shown in some sections than others. There are resemblances to the Auckland basalts, but the chief differences lie in the more sharply idiomorphic nature of the large very abundant olivine phenocrysts, and the greater proportion of augite in the finely holocrystalline groundmass (see photomicrograph, Plate XXVIII, fig. 3). Feldspar scarcely occurs in the first generation, and augite is unimportant. The olivine is almost invariably oxidized on its margins, and stained thereby a deep reddish. This phase of oxidation accompanying extrusion is even better exemplified by a basalt from Ohakune, next to be described.
Mr. H. G. Cousins, Director of the Teachers' Training College, Auckland, closely studied this rock some years ago, and very kindly has allowed me to read his unpublished thesis thereon. He shows that the eruption has burst through the Waitemata beds, and concludes, mainly from considerations of denudation, that the period of extrusion was probably Upper Miocene. I entirely agree with him that it was probably not contemporaneous with that of the basalts of Auckland, but much earlier.
Public Works Department Ballast-pit, Ohakune.
I am indebted to Mr. F. E. Mason, of the Seddon Memorial Technical College, for specimens of this rock. My sections are decidedly interesting, and deserve comment. Much of the material is highly scoriaceous, and stained a deep chocolate-red through oxidation.
The phenocrysts are coarse glomeroporphyritic groups of colourless augite and a few olivine crystals. In the groundmass are but a few augite crystals, with subequal parts of plagioclase laths and another mineral comprising the main portion. This other mineral is also lath-like, is greatly stained by haematite as a result of oxidation of contained ferrous compounds during the final phases of the eruption, and seems certainly to be olivine, for it has straight extinction, very high refraction and birefringence, and a high optic axial angle. The optical character was not satisfactorily determined. Fine iron-ore is moderately abundant. Occasionally, as the photomicrograph (Plate XXVIII, fig. 4) depicts, the olivine phenocrysts are enwrapped more or less celyphitically by a rim of augite.
[Footnote] * S. H. Cox, Geology of the Rodney and Marsden Counties, Rep. Geol. Explor, during 1879–80, pp. 13–39, 1881.
South-east Flank of Mount Hikurangi, Whangarei District.
A greyish porphyritic rock with idiomorphic phenocrysts of hypersthene, coarse plagioclase from the acid end of the series, and rare brownish-green hornblende. There are in addition numerous smaller lath-like feldspars, arranged with noticeable parallelism, and some fairly large sporadic iron-ore crystals, enclosed with the major phenocrysts in a somewhat lithoidal irresolvable matrix in which magnetite specks furnish the only recognizable mineral, and which appears to be densely cryptocrystalline. The hyper-sthene is not plentiful, and is only in small crystals in the two sections cut, though occasional crystals reaching as much as 1¼in. in length can be gathered in the field.