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Volume 74, 1944-45
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Some Igneous Rocks from the New Plymouth Area.

[Read before the Wellington Branch, Royal Society, October 21, 1943; received by the Editor, March 22, 1944; issued separately, September, 1944.]

Summary.

The mineralogy and petrology of the andesites, dacites, and hybrid rocks of Paritutu, Mataora Island, and Ngataierua Point, have been investigated. In addition, a study has been made of a porphyrite encountered during boring operations at No. 1 Devon Well of New Zealand Petroleum Co., Ltd. In order to explain the distribution of, and structures displayed by these rocks, a cone-sheet theory has been enunciated in which it is assumed that Paritutu and the Sugar-loaf Islands are merely remnants of former steeply dipping cone-sheets, the igneous material having been injected upwards along tension cracks from a magma now represented by the porphyrite sill.

Introduction.

Lying approximately two miles west of the centre of New Plymouth are the Sugar-loaf Islands, composed of solid dacitic igneous material; and on the mainland adjacent to this ring of islands is a prominence, also dacitic, known as Paritutu (Pl. 14); this pinnacle, rising almost vertically to 505 feet from the sea-level, is usually included with the Sugar-loaf Islands, under the name of “Sugar-loaves.” The Devon Well is situated a mile and a half south-east of Paritutu, and the entire area dealt with lies within Paritutu Survey District.

According to E. de C. Clarke (1912, p. 14) the igneous rocks of this area include lava-flows, tuffs, and agglomerates belonging to the Pouakai Series (presumably Pliocene and Pleistocene). He considers that this series rests uncomformably upon the Onairo Series of sandstones, claystones, and limestones, whereas the lava flows and fragmental igneous material associated with the activity of the nearby extinct volcano Egmont, post-date the Pouakai Series. The present writer is concerned only with the igneous rocks of the Sugarloaves and the Devon Well, and no work has been carried out on the agglomerates and other fragmental rocks.

Structures.

In outcrops well displayed in the bay immediately north-east of Paritutu Trigonometrical Station, a system of joints is well developed. A series of major joint planes, slightly arcuate in trend, strike at approximately 35° with a dip to the north-west of 45°, and within these planes there is a noteworthy parallel arrangement of hornblende crystals with the long axes in the direction of dip. It is assumed that the hornblende crystals are parallel to flow lines and that the steeply dipping joint planes are parallel to flow surfaces. Therefore these joint planes are probably to be correlated with Cloos' (1922) longitudinal or S- joints (Spaltseite), which may be defined as steeply dipping joints, coinciding with the foliation. At this

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juncture it should be noted that the S - joints of Cloos are not to be confused with the s - planes of Sander (1930), though Balk (1937, p. 36) believes that Sander's very broad definition would include longitudinal joints.

A second series of joints not so well developed as the first, strike in approximately the same direction but dip to the south-east at 45°–50°—that is, perpendicular to the lineation or trend of the prismatic amphibole crystals. It would seem, therefore, that these joint planes are the equivalent of Cloos' cross or Q - joints (Querk-lufte). However, these joints, which must be regarded as equivalent to tear or tension fractures, are not open, nor are they filled with the usual veneers of hydrothermal or secondary minerals that are so common in these structures (Balk, 1937, p. 27). In places a slight orientation of crystals was observed along a plane striking at 120° and dipping at about 45°. On the north-eastern side of Ngataierua Point itself, well developed S- joints strike at 50°–60° and dip north-west at approximately 30°. In places these planes were observed to change their attitude gradually, and if traced some distance the dip increased until nearly vertical. A second set of planes also with a 50°–60° strike, the Q- joints, cross the former series and dip to the south-east at 45°. Again the orientation of the hornblende is a noticeable feature with the crystals oriented much as before—that is, approximately in a plane parallel to the S- joints but with a some-what steeper dip.

Owing to the weather conditions at the time of the writer's visit, it was possible to examine neither the sea-wall of Paritutu nor the outer ring of islands other than Mataora and Motu-o-Tamatea. From the Ngataierua Point, however, a poorly developed columnar structure was clearly seen on the sea-side of Paritutu; this development is roughly, vertical but in very many places the columns are bent and contorted as if movement of magma in a viscous or semi-solidified state had occurred.

On Mataora and Motu-o-Tamatea Islands similar poorly developed, vertical columnar structures were observed. Nearly east-west and nearly vertical jointing and shear-planes are prominent. Weathering solutions moving along these joints and shear surfaces have deeply altered the rocks on either side and hydrated iron oxides are liberally present. Seeps are numerous along these planes and hydrated iron oxides are at present being deposited. A most striking feature of all the outcrops in this locality is the size and abundance of the plagioclase phenocrysts; these commonly reach 10 mm. in length.

In the Devon Well, a porphyrite was encountered at a depth of 9,388 feet and drilling was continued in this material for a distance of 31 feet. Of course little can be said in regard to the structure of this mass, but the writer considers it to be a sill rather than a dyke or flow. This belief is supported first by the presence of a zone of indurated and slightly baked mudstones and siltstones for a distance of several hundred feet above the porphyrite; and secondly in the cores taken from the well at this point 80–90% of the hornblende crystals were observed to be oriented in a horizontal plane— that is, normal to the direction of the bore hole.

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Origin of Paritutu and the Sugar-Loaf islands.

It was E. de C. Clarke's (1912, p. 21) opinion that the igneous rocks of the “Sugar-loaves” were lavas. After some recent work the writer does not favour this view, for none of the field evidence is in support of it; on the contrary, most of the visible structural features such as major jointing and lineation, point to injection of the igneous rocks along steeply dipping planes. Any hypothesis that is advanced to explain the mode of emplacement of the igneous rocks must take into account the following points:—

1. As will be observed later, the chemical similarity between the porphyrite at the bottom of the Devon Well and the main mass of dacitic rocks of which Paritutu, Mataora Island, Ngataierua Point, and presumably the other islands are composed, is significant.

2. Hydrothermal solutions have been very active in both groups of rocks.

3. The geographic distribution, shape, and orientation of the Sugar-loaf Islands and Paritutu is of importance (Text fig. 1).

4. Drilling was undertaken at a point approximately 10 chains east of Paritutu, and this bore, after passing through a thin, superficial covering of Pouakai Series, went to a depth of nearly 3,000 feet in fairly homogeneous mudstones, without encountering any massive igneous material. If Paritutu was made up of dacite flows, their eastward extension would certainly be expected.

5. Known oil seepages have been observed in a number of places close to the Sugarloaf Islands and in all cases these were located on the outer periphery of the elliptical zone in which the islands and Paritutu lie. To the writer's knowledge, oil seepages are unknown within this zone, and if the hypothesis to be advanced is correct, then seepages are not to be expected here.

In the writer's opinion the sequence of events for the igneous rocks may be summarised as follows:—

An intrusion of porphyrite into Upper Tertiary sandstones and siltstones in the form of a sill or laccolith took place. The maximum thickness of this intrusion is not known, but the very minor contact effects that were brought about in the surrounding sediments seem to indicate either a relatively low temperature or very modest volume of magma or both. A cupola, or dome-like, upward extension of this magma reservoir may have developed as a result of the magmatic pressure overcoming the strength of the relatively thin crustal covering. Tension cracks would then have tended to open up in the overlying sediments in a fashion comparable to that postulated by E. M. Anderson (1924) to explain the development of cone-sheets in Mull (Text fig. 2). Up these tension cracks the andesitic magma has been injected with the development of a series of cone-sheets. Subsequent erosion has reduced these cone-sheets, the Sugarloaves being the remnants of the former continuous outcrops. In spite of the abundance of volatiles that appear to have existed in the porphyrite magma, andesites, and dacites, the relief of pressure on the magma chamber does not appear to have given rise to any explosive ejection of pumiceous or tuffaceous debris, unless, of course, these have been entirely removed by erosion. Nor is there any suggestion that with solidification of the gas-poor magma a collapse of the roof of the magma chamber has occurred, producing a small caldera.

A close study of the islands suggests the existence of at least two cone-sheets, although surface traces of additional ones may have

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Text Figure 1.

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Text Figure 2.

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been completely removed by erosion. If one allows a regular dip of 40°–50° for the cone-sheets a simple construction indicates the depth of the cone-sheet foci to be about 5,000 feet. At this point, therefore, a considerable upward extension of the magma chamber must be present for the upper surface of the porphyrite body was touched only at 9,388 feet in the Devon No. 1 bore, situated just two miles south-east of the centre of the cone-sheet structure (south-east corner of Text fig. 2).

The thickness of the cone-sheets deserves brief comment. The outer sheet (Text fig. 2) appears to have a thickness of approximately 600 feet to 700 feet, whereas the supposed inner one would not exceed 200 feet to 250 feet. These sheets are much thicker than the well-known sheets of Ardnamurchan (Thomas, 1930, p. 176), where they do not exceed 50 feet, although the Beinn Chreagach Mhor sheet in Mull reaches a maximum width of about 200 feet (Bailey, 1924, p. 238). However, examples of very wide cone-sheets are not lacking, and the quartz porphyry sheet of Kudaru Hills, Nigeria, with a true maximum thickness of 2,800 feet, may be quoted (Bain, 1934, p. 211).

Mineralogy of the Paritutu-Devon Well Rocks.

Plagioclase.

Plagioclase feldspar is the dominant mineral in all of these rocks, and forms particularly conspicuous phenocrysts in the Paritutu-Sugar-loaf Islands group of dacites and andesites. Twinning on the albite and Carlsbad laws is commonly developed, but pericline twinning is much less usual. Twinning on the albite-ala B law was observed in one case, but other types of twinning have not been noted, although they may be present, for an exhaustive study of feldspar twinning was not made. The composition in all cases was determined with the universal stage following the methods of Nikitin (1936) and occasional check determinations by R.I. methods were carried out. Except for the occurrence of very calcic feldspar in curious hybrid rocks (P. 9339), the plagioclase is always andesine; in the Devon Bore porphyrite the feldspar is sodic andesine with an average composition of An30–33, while in the Paritutu group the composition is somewhat more basic, being about An34–50. In the hybrid rocks bytownite is the only plagioclase present, with a composition varying from An73–80.

In the normal dacites from the Paritutu area zoning on an elaborate scale is a conspicuous feature, and several types of zoning have been observed. First there is the phenocryst showing a gradual change from a rather calcic core or nucleus to a peripheral zone of sodic character. In the porphyrite from the Devon Well, several universal stage determinations indicated a range from An42 for the nucleus to An27 for the periphery, in an extreme case, while An30–An33 appeared to be the more usual range. A similar type of zoning is seen in the Paritutu group as well, which, following Larsen's (1938, p. 229) simplification of Homma's (1936) classification, has been termed normal. A much more complex development in zoning is a feature of the plagioclase phenocrysts of the Paritutu group. This consists of a fairly regular rhythmic type of zoning comparable to the oscillatory type which Larsen (1938, p. 229) states is the commonest arrangement in the San Juan andesites and allied lavas. In two particular

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phenocrysts the change in the percentage of anorthite was observed to vary as follows: central zone, An44 varying to An40 on the outer rim of this area; intermediate zone An44–An40; and for the peripheral zone the variation was more pronounced with An44–An34. In the hybrid type with very calcic plagioclase—viz., bytownite, zoning of this type is also present but in occasional crystals where this does not occur a very thin sodic envelope has been precipitated around the anorthite-rich core.

The cause of the development of the normal type of zoning is clear enough, and the course of crystallization can be readily determined from the albite-anorthite phase diagram (Bowen, 1913) but the normal oscillatory type, on the other hand, presents a more difficult problem. Phemister (1934) has advanced the theory of diffusion-reaction control, whereby crystals more calcic than the initial liquid separate, resulting in the production of sodic liquid surrounding the crystals. These crystals are then presumed to react with this sodic liquid causing a “concurrent rise of the An. Ab. ratio in the liquid both through reaction and through diffusion.” Finally a renewed deposition of calcic plagioclase results. Hills (1936, p. 50), in the present writer's view, correctly drew attention to a fallacy in Phemister's argument, particularly the latter's hypothesis that on account of a rise of temperature resulting from crystallization, reaction between plagioclase and the sodic enveloping liquid would produce a less sodic zone to the crystals. Hill's view (1936, p. 52) of the origin of what he terms oscillatory—normal zoning does seem to fit the case more accurately. He considered that crystallization, having commenced under conditions of equilibrium, proceeds gradually and produces, through diffusion, a crust surrounding the crystal itself, and a zone in the adjoining liquid that is enriched in soda. Concomitantly with this, the liquid beyond the sphere of reaction becomes supersaturated with lime. Crystallization of plagioclase is now slow or may have ceased altogether; but, when diffusion has increased the concentration of lime in the zone immediately adjacent to the crystals, calcic-rich material will again be precipitated.

When the twinning axes of measured plagioclase phenocrysts from some of the New Plymouth rocks are plotted on Nikitin's (1936, Pl. VII) useful diagram an important and constant angular variation from the standard Reinhardt-Nikitin curves was found to exist. This displacement of the poles was most noticeable in the hybrid rocks (P. 9339) of Mataora Island. The position of poles of a number of plagioclase phenocrysts twinned according to the albite law have been plotted in Text fig. 3 in order to show their position relative to the ⊥ (010) curve, and four points are also plotted for crystals in which pericline twinning is developed. This shift of the poles from the standard curves has been noted previously in a number of cases. In Otago, Benson and Turner (1940) considered a pronounced displacement of the twinning axes in the plagioclase of a series of mugearites as due to considerable orthoclase in solid solution.

Both Barth (1931) and Barber (1936) showed that intense heating would cause a shift or dispersion of the twin poles of plagioclase, although Barber (1936a) after some later work, somewhat reversed his opinions. However, a number of European workers—viz., Kohler

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(1941, 1942), Tertsch (1941, 1942), and Scholler (1942) have recently published their researches on this problem, and they indicate that the distinction must be made between high and low temperature plagioclases, while Scholler (1942) particularly points out that the difference in optical orientation of natural and artificially heated plagioclase is not due to any loss of alkali. Similar dispersion of the poles of the twinning planes has been described by Larsson (1940) in the plagioclases of volcanic rocks from northern Patagonia, and he considered (loc. cit. p. 367) that it is “a property characteristic of the plagioclase phenocrysts of intermediate effusive rocks. Such being the case, it is probable that the rapid fall of temperature from the intratelluric to the effusive stage is to be held responsible for the differences as against the plagioclases of the rocks crystallized during slow and tolerably continuous cooling.” Further, the effect of potash in the plagioclase is, Larsson believes, a problem that requires special investigations. Recently Benson (1944, pp. 74–75) has discussed similar variations in connection with plagioclases in the Tawhiroko sill, near Moeraki.

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Text-fig. 3.
Standard curves after Nikitin (1936) showing abnormal displacement of twin axes, ⊥ (010) (upper) and [010] (lower), in plagioclase phenocrysts from basified dacite (P. 9339), Mataora Island.

In the rocks from the New Plymouth area this angular shift of twin axes, as stated previously, seemed to be most noticeable in the bytownite phenocrysts of the hybrid or basified rocks, less noticeable in the dacites of Paritutu and environs, and apparently of no signific-

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ance in the plagioclase of the porphyrite of the Devon Well. As will be seen later volatile substances have been very abundant in the magmas that produced the Paritutu-Sugar-loaf Islands rocks; thus, in view of the possibility of high temperatures being produced here by oxidation of volatile substances on escape from the magma, the displacement of the poles of the twin axes from the standard curves of Nikitin, based on plagioclases from crystalline schists, pegmatites, and plutonic rocks, that is, low temperature plagioclases, is in agreement with the most modern theory.

Amphiboles.

A greenish-brown hornblende in stout acicular crystals is the most abundant mafic constituent of the porphyrite, and dacites of the New Plymouth area. Typical amphiboles have been separated from two rocks (viz. P. 9332, 9337) by electromagnetic and centrifuge methods, and the analyses made by Mr. F. T. Seelye are quoted in Table I (Nos. 1–2).

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Table I.
Analyses of Amphiboles.
1 2 3 4 5
SiO2 41.11 41.96 43.95 45.07 39.01
Al2O3 13.39 12.15 11.40 13.82 13.60
Fc2O3 4.94 6.98 4.69 1.82 5.25
FeO 9.89 9.75 9.02 10.58 7.42
MgO 11.63 10.36 13.73 12.74 11.73
CaO 11.81 11.29 11.12 11.68 12.05
Na2O 2.29 1.98 2.08 2.67 2.51
K2O 0.68 0.80 0.86 0.33 1.11
H2O+ 2.05 2.04 0.79 0.70 0.98
H2O— 0.03 0.11 0.19
TiO2 1.95 2.00 1.81 0.60 6.05
MnO 0.30 0.44 0.14 0.16 0.14
V2O3 0.06 0.06
Cr2O3 0.015 0.01
BaO 0.02 0.02 0.03
SrO 0.03 0.02
F 0.10 0.12 nd. nil
100.29 100.09 99.59 100.17 100.07
O for F 0.04 0.05
100.25 100.04
No. 1 No. 2
Sp. Gr. 3.22 ± 0.01 3.22 ± 0.01
α 1.666 ± 0.003 1.670 ± 0.003
β 1.680 1.684
γ 1.687 1.695
γ — α 0.021 0.025
γ ∧ c 70–78° 64–74°
2 ν 15–17° 10–12°
α Pale yellowish-green. Pale yellowish-green.
β Deep brownish-green. Deep brownish-green.
γ Deeper brownish-green. Deeper brownish-green.
Absorption γ > β > α γ > β > α
  • (1) P. 9332. Porphyrite, Devon Well, Paritutu S.D. Analyst: F. T. Seelye.

  • (2) P. 9337. North-west extremity of Ngataierua. Point, Paritutu S.D. Analyst: F. T. Seelye.

  • (3) Green hornblende from andesite dyke cutting Potosi volcanics. (E. S. Larsen et al. 1937, p. 895, Table IV, column 3).

  • (4) Hornblende from zoisite-amphibolite, Nieripeivi, Vasterbotten. Analyst: N. Sahlbom. (T. Du Rietz, 1935, p. 179, Table XIV).

  • (5) Kaersutite, Leith Valley Quarry, Dunedin. Analyst: F. T. Seelye. (Benson, 1940, p. 286, Table II, analysis 11).

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For the sake of comparison, analyses 3, 4 and 5 have been included in Table I, and it has been impossible to find amphiboles that correspond more closely, for both of the New Plymouth hornblendes appear to be unusually high in alumina. Actually the New Plymouth hornblendes are not greatly dissimilar from some analyses of kaersutite, the notable difference is, of course, the high titanium content and the low value for water evolved at a temperature in excess of 105° C. of the latter. It was observed at a few cases that the normal hornblende developed a brown to reddish-brown colour, often zonally, and the most noticeable optical change accompanying this was a marked decrease in the angle Z ∧ c. These changes might possibly be due to an increase in the amount of TiO2 in the brown types, and if this is so, such amphiboles must have compositions approaching closely that of kaersutite.

Using Warren's (1930) extended amphibole formula, the analyses of the two analysed hornblendes have been calculated on the basis of 24 (O, OH, F) atoms to the unit cell (Table II).

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Table II.—Calculation of Formulae of Hornblendes.
Hornblende P. 9332.
Wt.% No. of Metal Atoms on Basis of 24 (O, OH, F).
SiO2 41.11 6.075 1.925 8.00
Al2O3 13.39 2.324 0.399 4.98
TiO2 1.95 0.213
Fe2O3 4.94 0.550 0.399
FeO 9.89 1.215 0.399 4.98
MgO 11.63 2.572 0.399
MnO 0.30 0.035 0.399
CaO 11.81 1.872 0.399
Na2O 2.29 0.461 2.45
K2O 0.68 0.124
H2O+ 2.05 2.022 2.07
F3 0.10 0.053

Formula: (OH, F)2 07 (Na, Ca, K)2 45 (Mg, Fe”, Fe”’, Ti, Mn, Al)4.98 [(Si, Al)8 O23].

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Hornblende P. 9337.
Wt.% No. of Metal Atoms on Basis of 24 (O, OH, F).
SiO2 41.96 6.229 1.771 8.00
Al2O3 12.15 2.120 0.349 4.91
TiO2 2.00 0.223
Fe2O3 6.98 0.784
FeO 9.75 1.212
MgO 10.36 2.290
MnO 0.44 0.053
CaO 11.29 1.791 2.50
Na2O 1.98 0.570
K2O 0.80 0.143
H2O+ 2.04 2.014
F2 0.12 0.053 2.06

Formula: (OH, F)2.07 (Na, Ca, K)2.45 (Mg, Fe”, Fe”’, Ti, Mn, Al)4.98 [(Si, Al)8 O23].

In both these hornblendes the Y, Z and (O, OH, F) groups appear to fit Warren's formula satisfactorily, and in both cases an

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important amount of aluminium replaces silicon in the tetrahedral chains. Warren (1930, pp. 508–509) has pointed out that when about one-quarter of the silicon is replaced by aluminium in hornblende, the figure for the (Na, Ca, K) group may increase to nearly 3μ0. However, this is not exactly the case with the New Plymouth amphiboles, for although there is a tendency for the (X) group to increase in value beyond 2μ0, in neither case is this increase as great as Warren (1930) contends in view of the important substitution of Si by Al ions. This apparent anomoly has been recognised in other common hornblendes by Deer (1938, p. 69), and the present writer agrees with Deer's conclusion that the value of the (X) group appears to be far more dependent upon the amount of alkalis in the hornblende composition than upon the Si/Al replacement.

Diopside.

The clinopyroxene in the porphyrite from the bottom of the Devon Bore is subordinate to the hornblende, and it usually occurs in small equant granules. Owing to the presence of a faint, but distinct, green colour in the mineral it was decided to separate it in the pure state and obtain an analysis. The separation was accomplished by centrifuging the rock powder (— 200, + 250 mesh) in methylene iodide and methylene iodide-tetrabromoethane mixtures.

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Table III.—Analysis of Clinopyboxenes.
1 2 3 4
SiO2 49.90 49.57 50.40 49.98
Al2O3 3.18 3.82 1.63 4.67
Fe2O3 3.10 2.00 2.06 0.25
FeO 7.65 6.59 9.74 9.89
TiO2 0.92 2.05 1.13 1.16
MgO 11.77 13.75 13.37 12.72
CaO 21.87 21.44 20.65 21.02
Na2O 0.71 0.69 0.66 0.53
K2O 0.15 0.08 0.23 0.16
H2O+ 0.45 0.10 0.51 0.20
H2O- 0.20 0.02 0.09
MnO 0.58 0.13 0.09 0.33
100.48 100.33 100.47 100.00
α: 1.684 1.690 1.691 1.695
β: 1.691 1.697 1.697 1.702
γ: 1.710 1.715 1.718 1.722
γ — α: 0.026 0.025 0.027 0.027
Pleochroism very poor very weak weak
2 V: 56° 52–54° 43° 43°
Z ∧ c: 44° 42° 51–66° 41°
Sp. Gr.: 3.33 3.34 3.38
(1)

Diopside from porphyrite, P. 9332, Devon Bore, Paritutu S.D. Analyst, F. T. Seelye.

(2)

Diopsidic augite, see Hutton, 1943, p. 354, Table I, column A. Total includes NiO, 0.04, V2O3, 0.045, SrO, 0.006.

(3)

Augite from dacite pumice, Komagataké, Japan, S. Kozu, 1934.

(4)

Augite from hypersthene olivine gabbro, Kangerdlugssuaq, East Greenland, Wager and Deer, 1939, p. 77, Table VII, column Ia.

Machatschki (1929) has proposed the general formula: XY (Si, Al)2 (O, OH, F)6 for monoclinic pyroxenes, in which X = Na, Ca, K, and Y = Mg. Fe, Mn, Al, Ti. Therefore on the basis of six (O, OH, F) atoms to the unit cell the clinopyroxene analysis has been recalculated (Table IV).

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Table IV.—Calculation of Formula of Clinopyroxene.
Wt. % No. of Metal Atoms on Basis of 6 (O, OH, F).
SiO3 49.90 1.886 0 114 2.00
0.026
Al2O3 3.18 0.140
TiO3 0.92 0.027
Fe2O3 3.10 0.084
FeO 7.65 0.240
MgO 11.77 0.662
MnO 0.58 0.018
CaO 21.87 0.885
Na2O 0.71 0.050
K2O 0.15 0.009

Formula: (Mg, Fe”, Fe”’, Ca, Mn, K, Na, Ti, Al)2 [(Si, Al)2 O6].

It will be observed that most of the aluminium is replacing silicon, while all of the titanium is required to bring up the Y group to 2 demanded by the structure.

The pyroxene is low in sesquioxides generally, and additional analyses of diopsidic types are presented for comparison. It is difficult to explain adequately the green colour that is developed, but possibly it is due to the FeO and Na2O, with the mineral showing a tendency towards aegirine-augite; the optical properties appear to have been little affected by the presence of a small amount of the aegirine molecule.

If Al2O3 Fe2O3, TiO2, and alkalies are neglected as has been suggested (Wager and Deer, 1939, Hess, 1941), the analysis may be recalculated in terms of the minals Wo, En, and Fs; this gives the following result: Wo 51, En 33, and Fs 16. Now if the values of 2 ν and γ ∧ c for this pyroxene are plotted on Wager and Deer's diagram (1939, p. 80) the composition would appear to be Wo 45, En 30, and Fs 25 (weight per cent.). In this case, therefore, the determination of chemical composition using 2 ν and the angle γ ∧ c is quite unreliable. Undoubtedly the minor constituents, particularly Fe2O3 and TiO2, must have a very considerable effect on the optical properties. However, it is most necessary that the angle γ ∧ c should be accurately measured, and for this the method of Nemoto (1938) is used when possible. According to Hess's classification the pyroxene belongs to the diopside-hedenbergite group of pyroxenes for the ratio (molecular per cent.) Wo: En+ Fs = 1:1 .02 and the pyroxene falls in the restricted field of the salite.

In the New Plymouth rocks, diopside has usually resulted from magmatic resorption of hornblende and concomitantly with this reaction there has been precipitation of magnetite. A study of ratios MgO:FeO in the analysed amphibole (Table I, anal. 1), and diopside clearly shows the later formed pyroxene to be the less ferriferous mineral, the magnetite representing the excess over that required in the reaction.

Leonhardite.

A fibrous zeolite which the writer has interpreted as leonhardite, is an important constituent of the Devon Well porphyrite (P. 9330–9333). It occurs in an irregular, patchy manner, replacing andesine, and frequently the plagioclase is entirely replaced by the zeolite. Rarely it forms radiate aggregates when it appears to have been

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one of the last minerals to crystallize. The mineral was carefully separated from the rock by the usual centrifuge methods and the analysis was made by F. T. Seelye. In Table V the New Zealand mineral is compared with seven others obtained from the literature and a study of this table will reveal some distinctive features in analysis No. 1. The mineral has a slightly higher percentage of silica than the other minerals, but this is offset by a rather low figure for alumina. The figure for combined water is below that in any of the other analyses, and this property appears to be reflected in the refractive indices.

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Table V.—Analyses of Leonhardite and Laumontite.
1 2 3 4 5 6 7 8
SiO2 54.10 52.13 50.64 50.82 50.96 52.34 52.24 53.04
Al2O3 20.44 23.04 21.86 20.06 21.60 22.27 22.14 22.94
Fe2O3 1.70 0.20 2.18 0.03
TiO2 0.11 nil
MgO 0.45 trace 0.74 0.02 trace
CaO 8.65 11.85 12.18 12.14 11.27 10.83 10.55 9.67
Na2O 2.60 0.14 0.42 0.31 0.32
K2O 0.55 1.34 0.22 0.18 0.43
MnO 0.05 nil
H2O+ 9.32 12.64 12.01 14.87 16.04 14.06 15.14 14.64
H2O- 2.13 12.64 1.58
100.10 100.00 100.77 99.97 100.40 99.60 100.81 100.31
α 1.502* 1.508 1.505 1.504 1.511
β 1.512 1.514 1.518
γ 1.514 1.524 1.513 1.516 1.522
γ — α 0.012 0.016 0.008 0.012 0.011
2 ν 35–38° 25–35°
Sp. Gr. 2.38 2.2–2.3 2.23 2.283
(1)

Leonhardite from porphyrite P. 9332, Devon Well, Paritutu S.D. Analyst: F. T. Seelye.

(2)

Laumontite from vughs in Enoggera granite. Whitehouse, 1937, p. 541. Analyst: Queensland Government analyst.;

(3)

Laumontite, Southern Oregon. McClellan, 1926. Analyst: V. E. Shannon.

(4)

Laumontite from North Table Mountain, Golden, Colorado. Henderson and Glass, 1933.

(5)

Laumontite from Nova Scotia. Walker and Parsons, 1922.

(6)

Laumontite from Hawaiian Islands. Dunham, 1933.

(7)

Laumontite from granodiorite at Zsidóvár. Takats, 1936.

(8)

Laumontite (= hypostilbite) from Snizort, Skye. Heddle, 1901, p. 91, analysis No. 5. Analyst: Scott.

The analysis has been recalculated on the basis of the formula (Ca, Na)7 Al12 (Al, Si)2 Si26 O80. 25H2O, proposed by Berman (1937, p. 357) for laumontite, FeO, Fe2O3, etc., having been neglected in this calculation.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Table VI.—Recalculation of Analysis of Leonhardite.
Wt.% Metal Atoms on Basis of 80 Oxygens.
SiO2 54.10 27.65 26.00 26.00
Al2O3 54.10 27.65
1.65 1.98
0.33
CaO 8.65 4.73 7.86
Na2O 2.60 2.76
K2O 0.55 0.37
H2O 11.45 19.52 19.52

Formula: (Ca, Na, K)7.80 Al12 (Al, Si)1.98Si26 O80 19.52 H2O.

[Footnote] *± 0.001

[Footnote] † ± 2°

[Footnote] ‡ ± 0.02

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It will be noted that the theoretical figures are not obtained for the (Na, K, Ca), group and for total H2O, but otherwise the formula fits reasonably well, and since the ratio Al2O3: (CaO+Na2O+K2O) = 0.98 and following this O: (Al+Si) = 1.95, the mineral is a true zeolite. There appears to be a considerable substitution of Ca by Na and K, as was found by Hey (1937) for the thomsonite group. Berman (1937, p. 374) states that considerable variation is usually shown by these ions in laumontite, and that a range from an almost pure Ca minal to a member with a ratio Ca: Na = 5: 2 may occur. The New Zealand mineral, therefore, appears to be a more sodic, less calcic member of the laumontite group than has hitherto been described.

In his discussion of this mineral group Winchell (1933, p. 392) states that laumontite very readily loses about one-eighth of its water, when it is called caporcianite, leonhardite or β-leonhardite. Walker and Parsons (1922) state that on exposure to air one molecule of water is quickly lost. In the New Zealand mineral, however, the amount of water, even when total water is employed, is approximately one-fifth less than that usually recorded for laumontite, and on this basis alone the use of the term leonhardite seems justified. The refractive indices appear to be too low compared with those usually quoted for laumontite but are comparable with the data given for laumontite No. 3 in Table V, and by Larsen (in Winchell, 1933, p. 391). It is of interest to note that Larsen and Berman (1934, p. 153) later classify this latter zeolite with α 1.506, β 1.512 and γ 1.517, as leonhardite. In addition the maximum extinction angle Z ∧ c for the New Zealand zeolite is 40°, almost equal to the figure of 44° determined by Larsen and Berman (loc. cit.) for a leonhardite. Finally Walker and Parsons (1922) have shown that, due to the loss of water from laumontite, there is fundamental change in optical orientation resulting in an increase in the extinction angle on prismatic cleavage plates, to 40° which is much greater than the maximum extinction angle on the clinopinacoid of fresh laumontite.

Chabazite.

A mineral, tentatively determined as chabazite, is an important accessory constituent of a curious hybrid-type of rock (P. 9340) outcropping on Mataora Island. It occurs in idiomorphic to subidiomorphic, elongated, and sometimes triangular grains; the latter are generally simply twinned and appear as if they are sectors of pseudo-hexagonal crystals. The refractive index is approximately 1.486, and the birefringence about 0.006. The optic axial angle does not appear to exceed 20–25° and in some cases the mineral is uniaxial or very nearly so; a positive sign is general. If the author is correct in identifying this mineral as chabazite, then, according to data given by Winchell (1933, p. 385), it seems to be a fully hydrated calcium-chabazite.

Anhydrite.

The anhydrous form of calcium sulphate is a minor accessory constituent in the porphyrite where it occurs closely associated with other secondary products such as calcite, chlorite and zeolites. Anhydrite usually crystallizes in subidiomorphic, rarely idiomorphic grains up to 1.0 mm. in length. Two perfect cleavages, and one

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slightly less perfect, and polysynthetic twinning on (101) are characteristically developed. The optic-axial angle determined for four grains was found to be 36° in every case while the optic sign was positive. The axial angle in this case appears to be lower than that usually recorded for anhydrite, for Winchell (1933, p. 98), Larsen and Berman (1934, p. 107), and Rogers and Kerr (1942, p. 216) quote a figure of 42°. Refractive indices determined by the immersion method gave:—

α = 1.573 (1.570)*
β = 1.579 (1.576)
γ = 1.618 (1.614)
γ — α = 0.045 (0.044)

In the literature available to the writer, no mention was found of the occurrence of anhydrite in igneous rocks. Gypsum, on the other hand is not uncommon, particularly in hydrothermally altered andesite pyroclastics and flows (Larsson, 1940, pp. 317–385). This occurrence of anhydrite in association with very hydrous minerals such as zeolites is of considerable interest, and in order to advance any explanation for its occurrence, the phase relationship of the different calcium sulphates requires to be understood. Recently Posnjak (1938) has published the results of his study of the system CaSO4-H2O in which he states that “owing to the fact that the only dissociation pressure curve that gypsum has, does not reach the value of the vapour pressure of its saturated solution until the temperature of 97° ± 1° is attained, gypsum in contact with its solutions between 42 and 97.5° represents a true metastable system, which in the absence of anhydrite nuclei may persist indefinitely.” Posnjak has shown by experiment that gypsum in contact with its solution at 75° remains unchanged after long periods; but, if anhydrite is added to this, then the solid phase consists only of anhydrite. From this summary, therefore, we may assume that the late hydrothermal solutions from which anhydrite crystallized must have been at a temperature at least in excess of 42° C., and that early development of embryonic crystals of β CaSO4 or anhydrite has prohibited crystallization of any gypsum.

Cristobalite.

Crisobalite has been recognised in most of the rocks of Paritutu, Mataora Island, and Ngataierua Point. It occurs in irregularly shaped, colourless “pools” up to 0.3 mm. in diameter but usually averaging 0.1 mm., or rarely as strings of subidiomorphic grains through the plagioclase phenocrysts. In the “pools” or clear areas of cristobalite no definite idiomorphism of the mineral is apparent but in some cases they bear some similarity in appearance to the bead-like crystals figured by Greig, Merwin, and Shepherd (1933, p. 66, fig. 1). The refractive index is not noticeably distinct from that of a paraffin oil mixture of 1.485. The birefringence is very low and in very thin portions of the slide it can be detected only by the use of a mica plate; further, multiple twinning is not developed. From the point of view of refractive index alone, the writer believes the determination as cristobalite is satisfactory.

[Footnote] *Figures in brackets from Winchell (1933, p. 98).

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The occurrence of cristobalite in patches and vesicle-like aggregates points to a late period of crystallization and, therefore, a rather low temperature for deposition. The presence of cristobalite, the high-temperature modification of silica, in circumstances that indicate a low temperature of crystallization, is not uncommon. Foshag (1926, p. 16) has described its occurrence lining cavities in the obsidians of Obsidian Cliff, and it has been recognised as the mineral attached to the walls of lithophysae and other cavities in rhyolitic rocks from Southwestern Yellowstone Park by Howard (1939); the latter writer also observed megascopic crystals of cristobalite along joint cracks where the rocks were appreciably altered, probably by siliceous solutions. In their investigation of the San Juan lavas Larsen et al (1936) recognised the importance of cristobalite (and tridymite); they found it to be the most common silica mineral in the gas cavities of basalts, but less common in the andesitic types of lavas, while its presence in the groundmass of rhyolites was confirmed by X-ray powder photographs (Hurlbut, 1936). Again Fenner (1938, pp. 45–47) has observed the crystallization of cristobalite from fumarolic exhalations on the shattered andesites of Falling Mountain near Katmai Pass, Alaska, while Rosicky (1928) records crystals of cristobalite, together with other minerals, occurring in cavities of andesites in southern Silesia. Kuno's (1933) investigation of the silica minerals in the groundmass of a series of basic igneous volcanic rocks from north Idu District, Central Japan, has shown that cristobalite is commonly present; and he is also of the opinion that some “tridymite” of earlier writers is in reality cristobalite. A study of a series of intermediate volcanic rocks from the Keli region in the Middle Caucasus by Ustiev (1934, p. 164) has shown the existence of dacites containing both tridymite and cristobalite, about 5% of these constituents being present, though up to 10% has been recorded in some cases. The author is of the opinion that these minerals have originated by solution of the glassy mesostasis by magmatic gases at moderately low temperatures.

The experiments of Greig, Merwin, and Shepherd (1933) have made it very clear how readily cristobalite may crystallize as a metastable phase at relatively low temperatures, and very low pressures provided water is present. The presence of cristobalite in vesicles and cavities in lavas is itself a clear indication of a relatively low temperature of crystallization, while Fenner's evidence from Falling Mountain is most significant. In view of these data it is believed that the crystallization of cristobalite in the Paritutu dacites has been facilitated by the late aqueous solutions which were apparently present in abundance as indicated by the occurrence of zeolites and deuteric carbonates.

Petrology of the Paritutu-Devon Bore Rocks.

Devon Bore Rocks.

Macroscopically these rocks are light coloured to pale grey, with plentiful phenocrystic feldspar and hornblende, the latter in prisms up to 10 mm. in length. In the 31 feet or so of core the igneous material is fairly homogeneous; although, adjacent to the igneous-sedimentary contact, a slight increase in the amount of dark con-

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stituents is apparent (Pl. 15, fig. 1). In most specimens there is a pronounced orientation of the amphibole crystals within a plane normal to the axis of the bore, and there is also a rather marked parallelism within that plane.

A study of a thin section of the rock from the deepest part of the well (P. 9331, 9332) shows that very considerable hydrothermal alteration has occurred, principally in the plagioclase (Pl. 15; fig. 2A). The rocks are all holocrystalline with a very finely crystalline groundmass. Plagioclase occurs in two generations, and although the feldspar is so replaced by zeolite that little else but irregular relicts remain, pronounced zonary structure, and twinning on Carlsbad and albite laws were observed. The alteration products and inclusions are often themselves zonally arranged. The composition is, in the main, about An30-An33 with nuclei as basic as An42 and peripheral zones sometimes as sodic as An27. Hornblende is the most important coloured constituent present and occurs briefly as phenocrysts (Pl. 15; fig. 2B); the optical properties of this mineral have been fully dealt with in the previous section on mineralogy. Crystals commonly show repeated twinning, and zoning was observed in some cases. Where this latter structure is present a pale nuclear zone is commonly separated from a pale periphery by an intervening narrow dark band. Inclusions are common, usually pyrite, magnetite, apatite, anhydrite, or zeolitized feldspar, but swarms of colourless needle-like bodies oriented parallel to the vertical crystallographic axis were also noted; these needles have a refractive index lower than the amphibole, while the birefringence is about 0.010. Diopsidiç augite is subordinate to amphibole, and occurs in crystals, rarely as large as 1.5 mm. Only very rarely did the hornblende and pyroxene show a reaction relation to one another. Leonhardite has almost completely replaced the phenocrystic and groundmass feldspar and in addition has crystallized as nests and irregular veinlets. A most unusual feature is seen in the occurrence of anhydrite to the extent of about 1 ½–2% (Pl. 15, fig. 2A). This constituent is sometimes closely associated with other secondary products such as calcite, zeolites, and chlorite. The latter mineral usually crystallizes in tuft-like aggregates commonly enclosed in pools of leonhardite, and in addition to its close association with other secondary minerals, it appears to be developing from the amphibole.

Iron ores include allotriomorphic grains of magnetite and cubes of pyrite. Calcite, and granules and prismatic crystals of apatite up to 0.2 mm. in length are minor but important constituents; not infrequently apatite has crystallized at the same time as the anhydrite with which it may be closely associated. Sphene is present as aggregates of allotriomorphic grains, sometimes clearly derived from iron-ore, while again it has been noted as distinct wedge-shaped crystals embedded in calcite or anhydrite.

The rocks between the level of P. 9331 and 9332 at 9408 feet and P. 9326 at the igneous-sedimentary rock contact at 9388 feet vary but little. The most noticeable feature is the gradual change from a finely crystalline groundmass to a microcrystalline one adjacent to the actual contact, and as a result the rocks close to the contact appear to be more strongly porphyritic. In addition, some minor changes in the accessory constituents may be noted. The percentage

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of anhydrite in rocks adjacent to the contact seems to be less important than in the deeper rocks, whereas pyrite is much more plentiful in the former (P. 9326–9327). Quartz, often in sharply idiomorphic crystals showing development of hexagonal pyramids and prisms, and not exceeding 5%, occurs in some sections (P. 9326–9328; 9333); it has clearly crystallized late in the sequence as it is associated with the secondary minerals. Augite, which was free from alteration in P. 9331, occurs in the upper zones only as irregular relicts embedded in green serpentinous products. Calcite is important in some sections (P. 9333), and rarely glomeroporphyritic clots of hornblende, magnetite, and pyrite were noted (P. 9333).

The microcrystalline mesostasis appears to be composed chiefly of finely divided chlorite, zeolite and leucoxene; possibly some feldspar and quartz are present.

The sedimentary rock lying immediately above the porphyrite is an indurated gritty mudstone or argillite consisting of angular fragments of quartz and feldspar never exceeding 0.1 mm. in diameter. The somewhat gritty bands alternate with bands of very fine grain-size (clay-grade) that have less than 5% of angular fragments of quartz and feldspars. The feldspars can barely be recognised as such on account of almost complete alteration to sericite, kaolin or clinozoisite. Lamellar twinning may sometimes be observed in remnants of relatively unaltered plagioclase. Quartz is usually clear and fairly free from the mechanical effects of pressure, although occasional grains show undulatory extinction. Other relict grains observed include biotite, now much chloritized, and muscovite. Pools of chlorite and serpentinous material probably indicate completely altered ferromagnesian constituents and large grains of clinozoisite and iron-poor epidotes may be either allogenetic or represent the anorthite molecule of the altered plagioclase in the mudstone itself. The fine-grained base of groundmass is composed of an intimate mixture of kaolin, quartz, sericite, chlorite, clinozoisite, dusty iron-ore or carbonaceous material and leucoxene; plagioclase is almost certain to be present, although any specific identification was not possible.

Cutting across the sedimentary rocks close to the intrusive contact are narrow planes, more or less horizontal but slightly oblique to the bedding. At first glance the effect is as if the sediment were current bedded. In thin section, however, the planes are represented by strings or lines of dark material that appears most probably to consist of hydrated iron or manganese oxides or both. It is suggested that these planes are shear-surfaces formed either by submarine slumping of the wet sediments or by pressure produced by intrusion of the porphyrite magma from below.

The chief effect of the intrusive porphyrite has been to weld the argillite slightly, for drilling has proved that as the porphyrite body is approached the apparently homogeneous sediment becomes much harder.

The Igneous-Sedimentary Rock Contact.

It is most surprising that the porphyrite magma has effected only very minor thermal changes in the argillite, causing merely some induration and perhaps slight bleaching of the sediment. At the actual contact of the two rocks no re-crystallization of the sedimentary

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material has occurred, although islands and tongues of sediment are seen within the porphyrite itself. The slight variations from the normal porphyrite are shown in Pl. 15, fig. 3, and these variations extend over a distance of only about 3–4 mm. This contact may be considered to consist of three narrow zones in addition to the argillite material. First extending down from the actual contact for approximately 1.0 mm. is a zone of almost crypto-crystalline material similar to [ unclear: ] the porphyrite mesostasis except that it is fairly free from phenocrysts, is not so altered, and surrounds numerous unaltered islands of argillite. This grades imperceptibly down into the next zone of approximately the same width, which is more comparable with the normal porphyrite from the bottom of the bore. Following this is a narrow band of rather abnormal composition with abundant phenocrystic hornblende and idiomorphic to subidiomorphic pyrite crystals. From this zone downwards only minor changes in the porphyrite have been observed, and these have been mentioned previously.

The Rocks of Paritutu.

The rocks outcropping on and around Paritutu are all rather light grey, strongly porphyritic, and with large, prominent, tabular phenocrysts of plagioclase and acicular crystals of hornblende.

In thin section, the porphyritic nature of the rock is emphasised because the feldspar and amphibole crystals are set in a hyalopilitic groundmass of microlites of feldspar and augite with a small residuum of colourless or rarely pale brown glass. In some cases (P. 9336) the phenocrysts increase in number to such an extent that the groundmass is more correctly termed mesośtasis. Crystalline material usually predominates over glass, though in rare cases (P. 6658) glass is predominant.

Plagioclase occurs in idiomorphic tabulae up to 5.0 mm. in length and these are usually flattened parallel to 010. In the groundmass they occur as minute microlites or at the most narrow laths. Zoning is most pronounced and twinning on albite, pericline, and combined Carlsbad-albite laws is usual. The composition of the plagioclase does not vary notably from rock to rock, but owing to zoning the composition in individual crystals varies from An34-An50; usually the composition averages about An45. A rhythmic type of zoning is often developed in the plagioclase phenocrysts, but particulars of this have been discussed earlier in this paper. Inclusions of glass and portions of groundmass are common, and these may be of irregular shape or definitely globular; the arrangement is either haphazard or strictly zonal, but more commonly the latter, although in most cases the marginal zone or periphery of the phenocrystic plagioclase is entirely free of any inclusions. Although inclusions of hornblende, apatite, and glass occur in most of the andesine phenocrysts of a particular rock, some phenocrysts in the same rock may be completely devoid of inclusions; again in some rocks all of the phenocrystic plagioclase is entirely free of inclusions. It is difficult to understand the manner by which some plagioclase crystals in crystallizing during the intratelluric period incorporated some of the magmatic liquid while others did not.

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Amphibole, the most important mafic constituent in these rocks, usually occurs in prismatic crystals up to 4.0 mm. in length. It is a strongly pleochroic type of hornblende with colour varying from greenish-brown through yellowish-brown to a warmer brown tint; the latter, however, nowhere approaches the colour of lamprobolite (Rogers, 1940, p. 828). Slight zoning is evident in most cases, and is usually shown by the development of a green peripheral zone surrounding a brown or greenish-brown central portion [ unclear: ] (Text-Fig. 4B); in some cases curious greenish-brown blotches occur in brown crystals. A poor hour-glass structure was noted in some cases. Twinning is uncommon, but if developed, is parallel to the orthopinacoid. Resorption has occurred to a different degree in different specimens, varying from merely a narrow border zone surrounding the hornblende to examples where there has been complete reaction with the magma involving removal of the amphibole. The products of resorption appear to be plagioclase, diopside, and magnetite; as the resorption becomes important, the amount of clinopyroxene in the rock increases considerably.

Usually the products of resorption are closely associated with one another or with the partially resorbed amphibole, but this relationship is not quite so clear in those rocks where the resorption of amphibole is complete or nearly so. In all rocks where resorption is important the groundmass has a minimum amount of glass, but in one glassy type (P. 6658) resorption of the amphibole, or of biotite which is present in this case, has not occurred (Text-Fig. 4B). Larsen and Irving's work (1937) on somewhat similar rocks from the San Juan region, Colorado, has clearly shown comparable features in the resorption of hornblende, and they point out that the absence of resorption in rocks with a glassy groundmass clearly indicates that much of the resorption took place after the lavas had been erupted. The evidence of the Paritutu rocks certainly seems to support this contention. Clinopyroxene is not an abundant constituent and occurs chiefly in close association with resorbed hornblende although to a minor extent dispersed throughout the rock as minute granules or prismatic crystals. Its occurrence as phenocrysts up to 2.0 mm. in length, is rare, and only in a glassy type (P. 6658) are they at all well developed (Text-Fig. 4B). In all cases, the pyroxene is a diopsidic type with very faint green tint and in some examples extremely delicate pleochroism. Slight zoning was noticed in a few cases and lamellar twinning is uncommon. Only rarely was any alteration noticeable, but in one case (P. 9341) a pale green negative chlorite occurring in the form of minute spherules appeared to have developed from the pyroxene. Biotite is an unusual constituent and was observed in only one rock, a glassy type from the cliffs on the north side of Paritutu, where it constitutes less than 5% of the rock. Here the mica occurs in plates of slightly rounded outline up to 2.0 mm. in diameter. Resorption has never proceeded beyond the development of a narrow peripheral zone of fine iron oxide dust and a colourless prismatic or granular mineral, probably pyroxene (Text-Fig. 4B). Resorption is not so extensive as that seen in the San Juan lavas described by Larsen and others (1937, p. 900).

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Pleochroism is intense, varying from very pale straw yellow (X) to a very deep rusty-brown (Z).

Cristobalite was recognised in several of these rocks—e.g., P. 9334–9336, both in the groundmass, where it occurs as minute grains, and in what appear to have been steam or fluid cayities; in the latter case the grains rarely exceed 0.6 mm. in diameter. The grains are colourless and irregular and lack the twinning so characteristic of cristobalite. The birefringence is very faint and refractive index determinations exclude the possibility of the mineral being tridymite or opal. Although cristobalite usually crystallises in the vesicles of lavas, it does not seem to be so common as a constituent of the groundmass or mesostasis, but recently, however, Hurlbut (1936) has proved by X-ray methods the existence of cristobalite in the groundmass and as intergrowths with the feldspar of spherulites in some rhyolitic rocks.

The accessory minerals in this group of rocks are apatite, magnetite, and zircon. The apatite generally forms stout idiomorphic hexagonal prisms; not uncommonly the apatite crystals have a central smoky coloured zone, just sensibly pleochroic in some cases. Zircon is rare and was noted in only one case (P. 9341). Iron-ore occurs in rather ragged irregular grains, less commonly in sub-rectangular crystals that rarely exceed 1.5 mm. in diameter. Finely granular iron-ore occurs throughout the groundmass and magnetite dust is important locally in association with clinopyroxene surrounding resorbed amphibole. Microchemical tests on the magnetite dust have shown a rather low percentage of titanium, possibly not in excess of 1 ½–2%.

On account of the amount of visible cristobalite present in a number of these rocks, it seems more appropriate to classify them as dacites. In one specimen (P. 9341), however, with no visible quartz or cristobalite in thin section the norm calculation shows 14.75% quartz. Therefore all of the rocks of the Paritutu group are considered dacites.

At the north-west extremity of Ngataierua Point several specimens were collected that differ somewhat from the Paritutu group, although, of course, very distinctly related thereto. In the hand specimen they are similar to the Paritutu group but contain rather more dark minerals. Microscopically they are also similar but differ in the following points:—(1) Phenocrysts of plagioclase are not so abundant. (2) Hornblende is far less resorbed than in the previous group, the amphibole phenocrysts being surrounded by only narrow zones of finely granular magnetite and a very minor augite. (3) Augite is rare throughout the rocks.

The Hybrid Rocks of Mataora Island (P. 9339, 9342).

These rocks are darker in colour than the Paritutu types and not so coarsely porphyritic.

The base is made up of microlites of feldspar, much fine granular augite, magnetite, cristobalite, and a residuum of almost colourless glass.

The plagioclase occurs in coarse idiomorphic phenocrysts, commonly twinned on several laws, and zoned. The phenocrysts show a curious type of alteration in which most of the centres of the crystals

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Picture icon

Text-fig. 4.
(A) Basified dacite (P. 9339) from the east side of Mataora Island showing zonally altered bytownite phenocrysts, heavily resorbed hornblende, and clear areas of cristobalite. X 26.
(B) Glassy dacite (P. 6658) from the seaward side of Paritutu. Phenocrysts of andesine are clear and almost unaltered; hornblende, slightly zoned, and biotite, show only faint resorption effects. X 26.

are replaced by a microcrystalline aggregate of pale yellow material with low birefringence and irregular granules of what are believed to be oxidised siderite (Text-Fig. 4A). In many cases, very narrow peripheral zones of the plagioclase crystals are water-clear and devoid of any inclusions (Text-Fig. 4A), and an intermediate clouded zone is seen in some crystals; others are completely free from any inclusions at all. It is considered that much of the pale yellow, poorly birefringent material is glass, perhaps in some cases somewhat devitrified. The cloudy intermediate zone, on the other hand, consists of feldspar, magnetite, shreds of glass and diopsidic pyroxene; other granules are present but the identity of these is not certain. In the main, zoning in the plagioclase is not so pronounced as in the feldspar of previous groups, although the oscillatory types of zoning were also observed here. A few crystals have been noted with corroded margins but lacking zoning.

The unusual feature of these rocks is the composition of the plagioclase, which is in no case less calcic than An73, and examples with up to 80% of the anorthite molecule, particularly in the core of the phenocrysts, have been recorded.

Both hornblende and pyroxene occur as large phenocrysts. The amphibole has more pronounced brown colour in these rocks with a pleochroism that follows the scheme:—

α = yellow.
β = brown.
γ = deep olive brown.

Resorption has occurred extensively, producing broad peripheral zones of finely granular magnetite, and augite (Text-Fig. 4A). In

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some instances where resorption has proceeded nearly to completion, a type of sieve-structure is developed in addition to alteration around the peripheral zone; this consists of the skeletal remains of amphibole phenocrysts sieved with rarely twinned, new formed plagioclase, diopsidic pyroxene, and magnetite. Complete resorption of the amphibole has occurred in some instances. Phenocrystic clinopyroxene, in crystals up to 2.0 mm. in length, is a common constituent. Except for its abundance and phenocrystic character, the diopsidic augite is similar to that in the previous groups of rocks. Accessory constituents are similar to those in the dacites—viz., apatite, cristobalite, and iron ores; some celadonic material and ferriferous calcite is also present.

On the west side of Mataora Island an extremely altered type (P. 9340) has been observed. This is a crushed feldspathic rock with the plagioclase averaging about An75. Amphibole and pyroxene are absent, but very abundant carbonate with the γ refractive index varying from 1.658–1.667 is present. Microchemical tests indicate considerable iron in the ferrous state but no manganese or magnesium. From Winchell's table (1933, p. 70) it seems that this carbonate must contain up to approximately 7–10% FeCO3 in solid solution. The refractive index varies considerably and there is marked zonary banding in the ferriferous calcite; this is particularly noticeable when oxidation of the carbonate takes place, the resultant limonite being precipitated in a zonary fashion. A zeolite tentatively referred to as fully hydrated calcium chabazite is an important minor cónstituent of these hydrothermally altered hybrid rocks. It is intimately associated with carbonate, and the idiomorphic form of the mineral suggests a late date for the period of crystallization.

Scraps of intensely pleochroic biotite are scattered throughout the rock: these are occasionally much altered to limonite but show no sign of resorption or chloritization. Rare patches of micaceous quartzo-feldspathic schist with intensely pleochroic biotite were also observed; these patches are interpreted as xenoliths of pelitic sediment that have been caught up and recrystallized by hot andesitic magma in the deeper zones of the magma chamber where temperatures have been higher than appear to have existed in the porphyrite.

Cristobalite, coarse crystals of apatite (up to 0.3 mm. in diameter) and iron-ore are accessory constituents.

Petrochemical Discussion.

As stated earlier, it is held that the andesites and dacites are genetically connected with the sill of porphyrite that was located in the Devon No. 1 Bore at a depth of 9388 feet. It is the writer's contention that magma from the chamber now occupied by this porphyrite was injected upwards along a series of tension cracks or planes caused by a doming-up of the magma chamber. A study of analyses and norms A, B, and C in Table VII makes it clear that except for slight increase in the amount of SiO2, represented mineralogically as cristobalite in B and C, these rocks are chemically similar. In addition, in Table VII the chemical compositions of the New Plymouth rocks are compared with those of similar types. In the case of the porphyrite (analysis A) it will be noted that whereas soda is fairly high, potash is very low; an important amount of water occurs, the latter

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being due almost entirely to the presence of a considerable amount of leonhardite. The occurrence of anhydrite is, of course, reflected in the small amount of SO3 recorded in the analysis. It should be noted that although nearly 3% of quartz occurs in the norm, this was not recognised as a modal constituent.

The analyses B and C seem to place these rocks definitely in the dacite group; thin section study of P. 9341 (analysis B), however, did not reveal any quartz or cristobalite so that the 14.75% of quartz in the norm probably represents cristobalite in the groundmass. These two analyses (B and C) are closely alike, and for comparison, analyses F, G, and H have been included.

The hybrid rocks of Mataora Island, however, fall into a slightly different category. It has been observed that these rocks, though comparable in most respects with the andesites and dacites, contain an extremely basic plagioclase, An73-An80, and in addition some specimens contain an important amount of xenolithic material. An analysis of one of these types (analysis D) has been made, and it will be noted that CaO is higher and Na2O lower than in any of the associated rocks.

The occurrence of large phenocrysts of bytownite, sometimes partly resorbed and set in an environment not dissimilar to that of the associated dacites, suggests that they did not crystallize from magmas in which they are now found. In dealing with a very similar problem, Larsen et al (1938, pp. 252–256) come to the conclusion that although some of the crystals may have originated by floating fròm a lower, or settling from a higher layer of magma of different composition, it is more probable that they are the result either of the mixing of two partly crystallized magmas or of reaction between andesitic magma and country rocks. Unfortunately the present author has little definite evidence to offer on this issue; but it must be pointed out that although some reaction has occurred between andesite magma and the mudstone country rock, as shown by the recrystallization of xenoliths as quartz-biotite aggregates, this reaction could not bring about precipitation of very basic plagioclase. It seems more probable that the bytownite xenocrysts have originated by the straining away of early formed phenocrysts from a basic magma, and subsequent incorporation of these in the dacite or andesitic magma. However, it should be noted that in this district there are no known igneous rocks more basic than andesites.

Table VIII.
Depth in Feet. H2O K2O Na2O
8755–8767 1.34 3.09 2.55
9021–9037 2.96 3.05 2.32
9383 0.72*
9385 0.65*
9387 (1 foot above sill) 0.72 2.17 2.72

All of the igneous rocks of the Paritutu area, and particularly those on Mataora Island, have been affected by late magmatic solutions and vapours. These agents are believed, as has been shown earlier, to have brought about the distribution and crystallization of cristobalite. To this must be added the extensive zeolitization of the feldspars that has occurred in the sill rocks and in the hybrid or

[Footnote] * Analyses by C. Osborne Hutton, remainder by F. T. Seelye.

*
Picture icon

An aerial view of Paritutu (centre), and Mataora, Pararaki, and Tamatea Islands. Ngataienrna Point is to the left of Paritutu.

Picture icon

Fig. 1.
A section of the core obtained at 9,388 feet in No. 1, Devon Well, showing the junction of the porphyrite (left) and mudstone (right). ¾ natural size.
Fig. 2A.
Porphyrite (P. 9331) showing zeolitized plagioclase, with hornblende and anhydrite set in a base of zeolite and altered feldspar. X 26.
Fig. 2B.
Porphyrite (P. 9327) one inch below the mudstone-porphyrite contact, showing phenocrystic hornblende, and zoned and zeolitized plagioclase. X 26.
Fig. 3.
A section across the mudstone-porphyrite contact (P. 9326), illustrating the absence of significant reciystallization of the sedimentary material. A concentration of pyrite is evident on the left. X 26.

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basified rocks of Mataora Island. The abundance of sideritic calcite in the latter rocks is also believed to be due to hydrothermal solutions. It is possible that the source of some of this water lies in the thick series of fine-grained sedimentary rocks that has been invaded by porphyritic or dioritic magma.

Determinations of hygroscopic water in several specimens of the fine-grained mudstones taken from the Devon Bore, indicated a relatively “dry zone” close to the sill contact as shown in Table VIII. However, in view of the very slight contact effects produced by the sill, a low temperature for this magma seems necessary.

On account of the abundance of soda in the porphyrite, soda metasomatism of the surrounding mudstones was suspected. In view of this, alkali determinations were made in three cases by Mr. Seelye, but, as the figures in Table VIII show, nothing significant appears to have occurred; the apparent decrease in K2O in the sediments adjacent to the sill cannot be correlated with any absorption of K2O by the porphyrite, which is abnormally low in this constituent.

Age of the Igneous Rooks.

Clarke (1912, p. 21) has grouped the dacites and andesites of Paritutu and the Sugarloaf Islands together with the neighbouring tuffs and agglomerates that cover most of Paritutu and adjoining survey districts, as the Pouakai Series; further he has suggested that there was a strong resemblance between the Pouakai rocks and the andesitic agglomerates, breccias, etc. of Miocene age in the Whangaroa Subdivision. Marshall (1907, pp. 375–376) on the other hand, believed that the Pouakai rocks were older than the basaltic types of Auckland yet younger than the rhyolites of the Central Plateau of the North Island.

The Devon No. 1 bore, after penetrating about 280 feet of tuffs, passed down through a thick series chiefly of fine sandstones and siltstones ranging downwards in age from Waitotaran or Urenian to Mahoenui. It is into sediments belonging to the latter group that the porphyrite laccolith or sill has been injected, and, if the author's conesheet theory is correct, the dacite sheets or dykes have penetrated through the entire sequence. The sedimentary rocks from the bore are particularly poor in tuffaceous or other igneous material, and only at about the 6,500 feet mark is there a very poor development of slightly tuffaceous mudstones. Thus here is clear evidence that the volcanicity at Paritutu did not contribute any debris to these Taranakian sediments, and therefore the age of the massive igneous rocks of the New Plymouth area appears to be post-Waitotaran, or at least post-Urenuian; that is, early Pliocene or latest Miocene. The presence of water-sorted tuffs and rounded, worn boulders in the igneous debris adjacent to Paritutu supports Clarke's belief that there was a considerable time interval between the deposition of the agglomerates and tuffs, and the emplacement of the dacites. It is held by the present writer that the Pouakai tuffs post-date the period of conesheet development, although it is possible that surface manifestations of these intrusions may have given rise to a minor amount of tuffs, etc. Thus the Pouakai tuffs proper, in the vicinity of Paritutu, were deposited in a shallow sea from which rose concentric rings of islands,

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these being the remnants of the cone-sheets brought into relief by. erosion of the soft Waitotaran sediment during post-Waitotaran, but pre-Pouakai times. The source of the Pouakai tuffs is probably to be looked for in the Pouakai and Kaitaki Ranges.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Table VII.—Chemical Composition and Affinities of the Igneous Rocks of the Paritutu Area.
A B C D E F G H
SiO2 53.65 60.21 60.65 51.49 53.34 60.71 62.48 64.27
Al2O3 17.83 18.74 19.55 16.61 15.95 18.53 18.07 16.87
Fe2O3 2.65 2.65 2.39 5.35 2.43 2.02 2.61 3.13
FeO 3.73 1.20 1.16 3.46 4.93 2.16 1.97 2.01
MgO 2.99 1.69 1.16 3.69 5.23 1.26 1.34 1.85
CaO 7.09 6.26 7.40 8.04 7.96 5.66 4.67 4.63
Na2O 5.51 4.06 4.27 3.42 5.42 4.82 4.69 3.97
K2O 0.90 1.91 1.98 1.21 0.63 1.93 2.16 1.68
H2O+ 2.88 1.35 0.87 1.98 2.75 2.45 0.52 1.20
H2O- 0.37 0.86 2.41 0.09 0.12
CO2 0.37 trace 1.04 nt. fd.
TiO2 0.73 0.58 0.35 0.81 1.21 0.77 0.60 0.38
P2O5 0.26 0.39 0.25 0.63 0.22 0.28 0.08
ZrO2 nt. fd. nt. fd. nt. fd.
V2O3 0.02 0.015 0.035
S 0.29 0.03 0.08 trace 0.03
SO3 0.62
MnO 0.10 0.09 0.08 0.22 trace 0.17 0.06
NiO nt. fd. nt. fd.
Cr2O3 trace? trace 0.015
BaO 0.13 0.21 0.09 0.09
SrO 0.07 0.065 0.05 trace
Cl 0.01 trace 0.11 trace
100.26 100.31 99.86 100.36 100.57 100.53 99.80 * 100.20
Less O for Cl 0.02
100.34
Sp. Or. 2.65 2.65 2.67 2.84 2.702
Normative composition
Q 2.85 14.75 12.48 9.10 13.14 15.06
or 5.34 11.30 11.68 7.18 3.89 11.12 12.79
ab 42.57 34.34 36.15 27.68 45.59 40.35 39.82
an 23.39 27.26 28.36 27.04 17.24 23.63 21.68
th 1.10 0.13
di 6.31 0.93 6.26 3.83 14.95 1.94
wo 0.12
hy 8.45 3.77 8.45 3.42 2.30 4.19
ol 5.41
mt 3.31 2.48 3.02 7.66 3.48 2.55 3.71
il 1.39 1.11 0.61 1.54 2.28 2.89 1.22
ap 0.60 0.94 0.60 1.34 0.67
pr 0.55 0.09
hm 0.94 0.48 0.32
(cc) 0.84 2.36
hi 0.18
A.

Porphyrite (P. 9331) from 9408 feet Devon No. 1 bore, New Plymouth, Paritutu S.D. Analyst: F. T. Seelye.

B.

Dacite (P. 9341) 2 chains south of Paritutu, Paritutu S.D. Analyst: F. T. Seelye.

C.

Dacite, seaward face of Paritutu. Analyst: J. S. Maclaurin (Bull. N.Z. Geol. Surv. no. xiv, p. 23, analysis 11).

D.

Hybrid rock or basified-dacite (P. 9339), Mataora Island, Paritutu S.D. Analyst: F. T. Seelye.

E.

Kersantite, Washington, 1917, p. 526, analysis 14.

F.

Diorite porphyrite, Washington, 1917, p. 260, analysis 66.

G.

Dacite, see Washington, 1917, p. 256, analysis 27.

H.

Average of 19 dacites, Johannsen, 1932, vol. ii, p. 398.

[Footnote] † The summation in Washington and in original publication is incorrect.

[Footnote] * Additional constituents 0.07%.

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In their work in the Mokau area, Henderson and Ongley (1923, pp. 55–56) drew attention to the marked resemblance between the rocks of Whareorino§ and Paritutu; presumably the dacites of Pehimatea (P. 3093, 3096, 3099) should be included in this comparison also. The age of these lavas is not definitely known, as no contact with Tertiary beds has been observed; however, Henderson and Ongley infer that an upper Miocene age is probable on account of the widespread occurrence of andesitic agglomerates, and crystal tuffs in the Mohakatino Series, in particular in the lowermost beds. The apparent absence of any Tertiary beds beneath the Whareorino lavas is curious, and suggests that the greywacke block on which the lavas rest had been a rigid elevated block for some considerable time. However, the relation of these igneous rocks to the Mohakatino Series appears to be quite uncertain, and there is no evidence to show whether the greywackes were always devoid of a Tertiary covering or whether such rocks were stripped off before the period of volcanicity at Whareorino. The age of the Whareorino volcanics then can only be fixed between wide limits; they are pre-Mohakatino but post-Jurassic or post-Taranakian. If the rocks are comparable with those at New Plymouth then a Pliocene or perhaps Pleistocene age is more likely.

Acknowledgments.

The author desires to thank Mr. van Asche, of New Zealand Aerial Mapping, and Mr. D. O. Haskell, Works Department, for permission to publish the excellent photograph of Paritutu. Thanks are also due to Messrs. G. Barnwell and A. Weymouth, of the New Zealand Petroleum Company, for making available for study material from the Devon No. 1 bore.

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[Footnote] § A recent cursory petrological examination of these rocks (P. 9238–9242) has shown that they are dacites and very similar to those of Paritutu. So far cristobalite has not been recognised, but quartz is present in most of the sections examined, and in some examples may be an important constituent (P. 9341); biotite is rather more plentiful than in the rocks of the Paritutu group.

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