Interesting Xenoliths from Whangarei Heads, Auckland, New Zealand.
[Read before Auckland Institute, June 9, 1937; received by the Editor, June 7, 1937: issued separately, December, 1937.]
Classification of Rocks Represented.
Xenoliths undoubtedly or probably igneous.
Noritic hornblende-gabbros; quartz-biotite-norite; hornblende-gabbros; hornblendites; fine-grained basic dyke rocks; hornblende-gabbro-aplites.
Xenoliths of uncertain nature: probably, igneous, but modified.
Garnetiferous diorites; garnetiferous hornblende-gabbro; hornblende-gabbros showing piezocrystallisation.
Xenoliths undoubtedly metamorphic.
Probably of sedimentary origin.
Tremolite-plagioclase-sehist: actinolite-plagioclase schist with subsidiary pyroxene; diopside-plagioclase-schist; hornblende-garnet-plagioclase-schist; biotite-pleonaste-garnet-plagioclase-schist; hornblende-biotite-plagioclase-hornfels.
Probably derived from basic igneous rocks.
Hornblende-plagioclase-gneisses and schists; hornblende-plagio-clase-schists with subsidiary diopside; hornblende-diopside-plagioclase-gneisses and schists.
Derivatives of impure calcareous rocks or basic tuffs.
Origin of Xenoliths.
Appendix: Whangarei Heads xenoliths and their comparison with rocks of Southern New Zealand.
By F. J. Turner, D.Sc., Otago University, Dunedin.
Origin of the rocks.
Comparison with rocks of Southern New Zealand.
Mention was made in 1925 in the New Zealand Geological Survey Bulletin on the Whangarei-Bay of Islands Subdivision of the occurrence of abundant xenoliths of hornblende-schist and hornblende-epidote-schist with hornblendites and garnet-bearing hornblende-gabbro in intrusive andesites near Whangarei Heads wharf and on the northern shores of McLeod's Bay of the same area (Ferrar, 1925, p. 57).
Since first noting these xenoliths when at work in this area for the Geological Survey, the present writer has continued to collect from these inclusions in the andesites. They are, however, so numerous and often present such macroscopic similarity amongst types that may in fact be very different, that further work will certainly add materially to the number of rock types identified. It is doubtful, however, if these will yield any important contribution to the knowledge of past batholithic injection and high-grade meta-
morphism in that area that is now possessed. For this reason it has been decided to publish the results so far obtained, which are based on the study of about 120 thin-sections.
The xenoliths occur in practically every major andesitie intrusion in the Whangarei Heads area, but are much more numerous and larger than elsewhere in dykes of garnet-bearing quartz-hornblende-mica-andesite that are exposed on the north shore of McLeod's Bay, especially in the most westerly one shown on the Geological Survey map, where they may reach a size as large as 8in. in diameter (see pl. 39, fig. 1). Most of the rock-types described herein have been collected from this last locality.
The particular interest of the xenoliths is that they afford information of a metamorphic substratum which underlies the substantially unmetamorphosed Mesozoic basement of the North Island, but which has not yet been discovered there in surface outcrops, in contrast with the position in the South Island, where metamorphic rocks of early Palaeozoic age are widely distributed.
Dr. F. J. Turner, of Otago University, very kindly examined the thin-sections and has assisted the writer very materially in a large number of difficulties that arose both in identification of one or two minerals and in classification of types and of their metamorphism. The reader may feel confident, therefore, that the conclusions reached are based on reasonably sound grounds.
The writer would extend to Dr. Turner most grateful thanks for his valuable help and for furnishing the appendix to this paper.
He wishes also to acknowledge with gratitude the courtesy of inhabitants of the Whangarei Heads district, who have allowed him access to their properties.
Classification of Rocks Represented.
Although there is room for some differences of opinion regarding the interpretation of individual rocks, it may be remarked that the writer's provisional conclusions regarding classification were supported in nearly every instance by Dr. F. J. Turner. This independent agreement suggests that the classification of the individual rocks adopted is likely to be substantially correct.
The xenoliths may be grouped as follows:—
Xenoliths Undoubtedly or Probably Igneous:
Xenoliths of Uncertain Origin—probably Igneous but Modified:
Xenoliths Undoubtedly Metamorphic:
A. Xenoliths Undoubtedly or Probably Igneous.
1. Noritic hornblende-gabbros (59, 112, 116, 121).*
These are coarse-grained somewhat granitic rocks, with, however, a tendency for the ferromagnesian minerals to occur as larger crystals enwrapped by smaller unoriented lath-like crystals of feldspar.
The ferromagnesian minerals generally are subequal in amount to feldspar, though they form about 75% of 116. As a rule pyroxene is a little in excess of the hornblende; in 112, indeed, the amount of this latter is very small. It is usually a brown variety with Z ∧ c′ about 12°, although in 116 much green hornblende accompanies and mantles the brown variety, generally with signs of reaction at the junction.
The pyroxenes are a very faintly coloured hypersthene and colourless monoclinic pyroxene in somewhat lesser amount than the other. This monoclinic pyroxene may show fine-scale diallage lamination and is often bordered or outgrown by hornblende with signs of reaction at the contact between the two minerals. Again, it commonly is minutely mottled by inclusions of brown hornblende, while, in a few instances, agglomerations of augite are enclosed by a border of hornblende. Such evidence demonstrates that the wellknown magmatic alteration of the pyroxene to brown hornblende has been in progress. Hypersthene may be enclosed as variously oriented grains in the brown hornblende or enwrapped by it, but this latter mineral is very rarely outgrown upon it. It sometimes shows the minute twinning lamination that is more characteristic of clinoenstatite or clinohypersthene.
The plagioclase present in various members of the group of rocks generally is about medium labradorite (Ab35 An65), but is as acid as Ab45 An35 in 112 and as basic as Ab20 An80 in 116. Accessory minerals are almost absent with the exception of rare iron-ore.
[Footnote] * The numbers are those of thin-sections in the possession of Auckland University College.
A type transitional to the hornblende-gabbros is exemplified by 51, a hornblende-rich gabbro with from 8% to 10% * of hypersthene. 2. Quartz-biotite-norite (105).
Macroscopically this is a greyish-black moderately coarse-grained holocrystalline rock. In thin-section it is seen to be constituted by about 75% of plagioclase (about Ab35 An65), about 5% of interstitial quartz, a little monoclinic pyroxene and an important proportion of hypersthene which often has been converted to a fibrous brownish-green uralitic hornblende. At other times this hypersthene is unaltered, especially when agglomerated, as it often is, into masses of small crystals. There is also about 8% of deep-brown to black very strongly absorbent biotite in flakes which often enwrap the hypersthene. Some of this latter shows minute twin lamination and a trace of intergrowth along the laminae with monoclinie pyroxene.
Each of the two sections prepared shows a crystal of olivine. One of these crystals is 5 mm. × 2.5 mm. in dimensions and encloses a small mass of pyroxene about 0.5 mm. across, which itself contains an inclusion consisting of plagioclase with pyroxene and some chlorite. The olivine is partially replaced by limonite; it is bordered by a narrow girdle of ferriferous serpentine, followed by a similar thin coat of tale in plates wrapped parallel to the borders of the olivine, and finally fringed by pyroxene.
3. Hornblende-gabbros (2, 21, 22, 29, 31, 45, 46, 48, 53, 64, 67, 71, 77, 82, 84, 97, 102, 104, 106, 111, 113, 120; also 39, 51, 96).
With hornblendites, these gabbros probably constitute the majority of the xenoliths. They are essentially hornblende-plagioclase rocks of very simple mineral constitution. The grain-size varies widely; the hornblende is often in very large stumpy crystals, occasionally exceeding 6 mm. in length (46, 104). It is usually greenish-brown to deep brown in colour. Its crystals are sometimes subhedral and then enwrapped by plagioclase, but in 45 they show poikilitic relations to the feldspar; in 71 they are concentrated into aggregates. The extinction angle Z ∧ c′ is 13°–16° in 64 and approximately 16° in 71, 77 and 84.
Nos. 29, 46, 53, 67 and 120 make a group of hornblende-gabbros which closely approach the hornblendites in abundance of their hornblende (60% to 70% or more) and frequently have their crystals of this mineral so closely crowded together as virtually to exclude feldspar from such aggregates. In general, however, the hornblende is subequal in amount to plagioclase, although, it falls as low as about 20% in 48 and 104, while in one rock (96) there is only about 5% of hornblende, the balance being plagioclase (Ab23 An77).
It is probable that the hornblende of these rocks, as in the noritic gabbros described above, is magmatically derived from earlier pyroxene, for grains of this latter mineral survive enclosed in the hornblende of 104. 51 indeed is transitional between the two groups, for it contains about 10% of pyroxene which is almost wholly hypersthene and repeats the relations to hornblende that were shown by the noritic hornblende-gabbros. The ragged crystals of hornblende
[Footnote] * Percentages such as this are based merely on estimates.
(about 60%) in this rock occasionally have somewhat granulated margins, and, in such cases, grains of pyroxene, sometimes difinitely monoclinic in character, appear with the granulation products. The pyroxene may, however, occur also as aggregates of small grains which occasionally make a corona to serpentinous pseudomorphs that suggest original olivine. There is a very small amount of accessory iron-ore, and plagioclase (Ab17 An83) makes up the balance of the rock.
It is impracticable at times to determine precisely the variety of plagioclase present in the hornblende-gabbros; the data obtained show, however, that the majority have feldspar more calcic than Ab27 An73. In 45, 82 and 104 it is Ab15 An85, although in contrast it is as sodic as Ab30 An70 in 22. Zonal growth is occasional, and in 67, 84, 97 and 102 there is some granulation of this mineral, with development in a few of the rocks of typical stress twinning. This is shown, for example, in 84, where also the feldspar is almost excluded from local aggregations of hornblende, which itself exhibits some granulation. In 21 the feldspar crystals are elongated and show some degree of parallelism, although not to so marked a degree as in the hornblende gabbros that have been grouped together as showing piezocrystallisation.
As a rule accessory minerals are almost absent. Sometimes there are small amounts of iron ore and apatite, which are more abundant than usual in 53. Sphene is present in small amount in 29 and 120; in the latter rock it is rimmed by iron ore, while a small crystal of garnet 0.5 mm. across is enclosed in the hornblende.
In one or two rocks of this group (e.g. 31), as in others represented amongst the xenoliths, zeolite accompanies the feldspar as an alteration product of this latter. It possibly is a result of magmatic fluids emanating from the quartz-andesite host of the xenoliths, although it would appear likely if this were so that the development of zeolite would be more general than actually is the case.
Between the large abundant crystals of hornblende of 120 there is 20% of interstitial material which is not the usual feldspar, but consists largely of sheafs of weathered sub-parallel to sub-radiate needle-like crystals of what appears to be plagioclase. This encloses occasional small laths of obvious plagioclase and small crystals of hornblende. In this rock also the latter mineral is mainly a distinct greenish-brown, but has deep brown borders.
39 differs from the other rocks included in this group of gabbros both in texture and in type of hornblende. This latter has a reddishbrown tint for Z and an extinction angle Z ∧ c′ of 26°–28°; its large euhedral or subhedral crystals constitute about 75% of the rock and enclose occasional small crystals of augite. Surrounding the hornblende is a finely-crystallised matrix resembling that of a minor intrusion, for it is composed of small stumpy crystals of feldspar with a few regged ones of brownish-green hornblende and a little doubtful quartz. Were it not that zeolite has replaced much of the feldspar, one might have been tempted to believe that the xenolith had suffered partial disruption and minute invasion by the surrounding quartz-andesite.
These rocks are most abundantly represented in the xenoliths and fall into three groups:—
Hornblendites of normal type.
(a) Hornblendites of normal type (4, 20, 23, 42, 43, 66, 70, 87; also 80b and 101).
The majority of the rocks included here do not have particularly coarse texture, for in most of them the constituent crystals of hornblende are well under 2 mm. in diameter. In a few, however, notably 20 and 70, the grain-size is large and stout crystals well over half an inch in length are visible in hand-specimens. Frequently (e.g. in 23, 42, 43, 66, 70 and 81) the poorly-shaped crystals of hornblende are so abundant and so closely appressed as virtually, if not actually, to exclude feldspar; very seldom is this latter in as large an amount as 10%.
The common hornblende is a deep-brown variety with relatively high extinction angle (Z ∧ c′ is 18° in 4; 15° in 43; 18° in 66; 16° in 70; 15° in 87; as high as 22° in 20 and 23° in 23); in many, however, there is a faint tinge of green with the brown, leading to a brownish-green in 4 and to a grass-green with brown borders in 101. In occasional rocks it exhibits some schillerisation, which may be a heritage from original pyroxene, for, in addition, there are rare grains of what appears to be diopsidic pyroxene in the hornblende of 4.
Accessory minerals are practically absent from most of the rocks, with the exception of occasional iron ore and sphene. Small wisps of biotite may occur in negligible quantity, as in 4 and 20, and apatite is present with an increased amount of iron ore as large grains in 101.
The variety of the interstitial feldspar is seldom precisely determinable, for the mineral is usually in small granular crystals. There is some replacing zeolite in 20 and in 4 a little of the sheaf-like acicular (?) plagioclase noted earlier in the hornblende-gabbro No. 120 also appears.
In 101 there is a tendency for parallelism of the stumpy prisms of hornblende, and, in addition, near the contact with the andesite host, there are a few small pale-pink garnets not exceeding 1.3 mm. in largest dimension. In 80b there are somewhat larger garnets up to 3 mm. across and of deeper pink colour. Although the garnets appear free from any reaction, it is probable that they have been abstracted by the hornblendite magma from garnetiferous rocks through which it passed.
(b)Granulose hornblendites (36, 95, 117).
These bear much the same relation to the normal hornblendites that the granular hornblende-plagioclase rocks, described later and believed to be hornblende-gabbro-aplites, do to the hornblende-gabbros. As in the hornblende-gabbro-aplites, the texture is suggestive of metamorphic origin, but against this possibility must be set the variety of the hornblende and the abundance of the apatite.
36 is composed predominantly of stumpy prisms of brown barkevitic hornblende (Z ∧ c′=10°–12°) averaging about 0.12 mm. in diameter; there is a small percentage of plagioclase in widespread fairly large crystals (about 0.5 mm.) which wrap around adjacent hornblende. Apatite and magnetite are both plentiful and a needle of rutile was noted within a crystal of feldspar.
95 is a granular rock with grains about 0.08 mm. in diameter or stout prisms from 0.12 mm. to 0.15 mm. in length almost wholly of brownish-green hornblende (Z ∧ c′=15°–16°), which frequently is arranged with decided parallelism and may show poor schiller structure. There is an almost negligible amount of interstitial feldspar. Small grains of sphene are especially numerous, while apatite is rare and magnetite is only is small amount.
117 is an equigranular-type with grain-size about 0.25 mm. (see pl. 39, fig. 3). Its hornblende is a lighter shade of brown than usual and there is practically no feldspar. Apatite is, however, plentiful, though iron ore and sphene are only in minute quantity.
(c) Pyroxene-bearing amphibolites (90, 54).
In 90 subhedral crystals of brown hornblende enclose about 3% of pyroxene in small grains with random orientation, as well as a few others of plagioclase. There are a few flakes of reddish-brown biotite, a little apatite and rare sphene. About 15% of interstitial material occurs between the crystals of hornblende; it is largely the same zeolite as characterises many of the hornblende-gabbros, but there is also some of the sheaf-like (?) plagioclase noted earlier in hornblende-gabbro No. 120.
54 is an unusual type of coarse-grained rock with sub-equal amounts of pyroxene and greenish-brown hornblende in fairly large irregular crystals. Hypersthene and augite are both present, the former somewhat in excess of the latter. As in the noritic gabbros, the augite is intergrown with and minutely mottled by brown hornblende, while margined by a greener variety of this latter mineral; occasionally it encloses hypersthene. Both hornblende and hypersthene shown minute schiller inclusions, while the latter is usually enclosed or enwrapped by the hornblende, though not in manner that suggests that magmatic resorption has been operative. There can be no doubt, however, that most of the original augite has been converted magmatically to hornblende.
5. Fine-grained basic dyke rocks (9, 34, 49, 62).
These rocks constitute a more or less circumscribed group, though with many differences evident in the component members. Their igneous nature is demonstrated by the decided camptonitic character of 34 (see pl. 39, fig. 4) and to a less extent of 62, and by the apatite, which is acicular in the majority and prismatic in 34. 9 and 49 show approach to the others in general characters and have further evidence of dyke origin in the texture of their feldspar.
All are essentially aphyric hornblende-plagioclase rocks with from 35% to 40% of hornblende, which is either a greenish-brown or a somewhat pale brownish-green variety. In 49 and 62 it is accompanied by a few tiny flakes of brown boitite, and, in the latter rock, occasionally includes small remnants of colourless augite upon
which it is outgrown. In 34 it shows the sharply idiomorphic outlines characteristic of lamprophyres, and is present either as rare stout crystals up to 0.8 mm. in length or as numerous narrow prisms usually not exceeding 0.5 mm. in length. In the other rocks it is either in subhedral narrow prisms up to 1 mm. in length or in more ragged stumpier small crystals.
The feldspar is either in subhedral large laths, sometimes as much as 3 mm. in length (e.g. in 9), or more typically in laths which do not exceed 0.3 mm. in length; when small they have sharply-defined borders. Its variety usually approaches closely to Ab35 An65, although it is more sodic in 9 (Ab48 An52). Quartz was determined in all but 34 in amount varying from 5% to 10%.
Accessory minerals include apatite, which occurs in more or less numerous acicular crystals or, less commonly, in stout small prisms (34), and a little sphene and iron ore in 62. A little radial zeolite replaces some of the feldspar of 9 and 62.
6. Hornblende-gabbro-aplites (18, 25, 33, 40, 55, 72, 108).
These are granulose hornblende-plagioclase rocks, some of which, notably 18, have a texture identical with that of granoblastic metamorphic rocks (see pl. 39, fig. 5). Though inclined to regard them as aplites, for the reason that some of them appear to be related to the hornblende-gabbros, the writer was doubtful as to their true nature and must thank Dr. F. J. Turner for deciding the issue in favour of an igneous origin.
All of the rocks have the composition of hornblende-gabbros, but show markedly equigranular texture over the greater part if not the whole of their extent in thin-sections. Usually in such cases there is a tendency towards aggregation of the stumpy prisms or granular crystals of hornblende, sometimes to the virtual exclusion locally of feldspar (e.g. 40, 72).
The hornblende is usually a rich-brown variety with little trace of green, except in 55 and 72 where it is greenish-brown; in 18 X = pale-yellow, Y = deep-brown with a tinge of red, Z = deep-yellowish-brown; X < Y = Z; Z ∧ c′=12°. The proportion of this mineral to plagioclase varies as in the hornblende-gabbros; the two minerals are generally in sub-equal amounts, though in 108 there is only about 15% to 20% of feldspar interstitial to the hornblende, which here exhibits many elongated prisms (up to 2 mm. in length) in addition to the general stumpy ones.
The variety of feldspar is seldom determinable with precision; it approximates to calcic labradorite. In 25 it is Ab27 An73 and occurs in sub-parallel leucocratic bands which give a gneissose structure to the rock. It may be outgrown marginally by a less calcic variety and is sometimes accompanied by a little isotropic zeolite (e.g. in 55).
In 25 there is a little quartz with the feldspar; in addition a few flakes of brown biotite were noted as well as abundant apatite and iron ore. In general, indeed, iron ore is more prominent than in the corresponding gabbros, though, as in these latter, sphene is usually rare. In 55 there is about 6% of golden-yellow phlogopite and rare needles of rutile appear in this mineral and in hornblende.
33 is an unusual type of rock. It has a decided tendency towards porphyritic texture in the presence of phenocrysts and glomeroporphyritic masses of aggregated sub-granular or bluntly prismatic crystals of hornblende, as well as of hornblende-fringed aggregates of a mineral doubtfully determined as tremolite. This mineral is faint-yellow in colour, is crowded with minute crystallites and is usually in sheafs of somewhat divergent fibres or blades. It has high birefringence, high optic axial angle, positive elongation and is doubtfully negative in optical character. There is no clue to the origin of these aggregates they may represent original pyroxene.
Hornblende makes up about 55% to 60% of the rock and is often zoned; it is green centrally but brown at the borders, and generally contains numerous dust-like inclusions. It is accompanied, in addition to plagioclase (approximately Ab35 An65), by about 2% of brown biotite and a little finely-granular iron ore.
While the general mass of the rock is sub-granular in texture. both hornblende and plagioclase sometimes are relatively elongated and are then involved in decided flow structure around the borders of the phenocrysts or phenocrystic aggregates.
B. Xenoliths of Uncertain Nature: Probably Igneous but Modified.
1. Garnetiferous diorites (1, 13, 19, 28, 37, 44) and porphyrite (122).
In hand-specimen these are fairly light-coloured fine-grained dioritic types with obvious and plentiful small pink garnets.
In thin-section a somewhat unusual feature is the idiomorphie crystallisation of the two main minerals plagioclase and hornblende, more especially of the latter (see pl. 40, fig. 6). The hornblende forms from 15% to 35% of the various rocks and frequently is a variety close to barkevikite with small extinction angle (Z ∧ c′=10°–12°) and raddish-brown tint for Y. In several thin-sections it is rimmed by a green variety. The plagioclase appears to vary in its content of anorthite; in 1 it is andesine-labradorite, but in others it is more calcic, reaching Ab35 An65 in 13. It commonly is zoned and very generally is replaced along irregular cracks by zeolite, which is also plentiful elsewhere and sometimes has radial habit. In 13 and some of the other rocks there is often a border of clear plagioclase unaffected by zeolitisation about the enclosed zeolitised plagioclase and of more sodic nature. Dr. Turner * suggests that the development of zeolite from earlier plagioclase and subsequent growth of the rim of later more sodic plagioclase are both the result of the penetration into the xenoliths of fluids from the andesitic magma that enclosed these foreign rock fragments.
It is possible also that a certain amount of quartz present in most of the rocks was also introduced during zeolitisation, for occasionally this quartz is sharply idiomorphic. Some introduced silica certainly is present, for chalcedony is sometimes recognisable (e.g. in 37).
[Footnote] * Private communication.
In some of the thin-sections there is a very little pale-green diopside, usually in granules, though in small crystals outgrown by hornblende in 28. Aggregations of granules of this mineral are common in 19, where they constitute nearly 8% of the rock, and to a less extent in 28. These granules may be enwrapped by the brown hornblende, but more often are associated with a small quantity of greenish hornblende. As will be mentioned later, pyroxene is also associated with the garnets that characterise the rocks.
Accessory minerals include needles of apatite, which are frequent in 28 and 44, and rare sphene and zircon, one prism of this latter mineral reaching a length of 0.35 mm. in 28.
The garnet varies in its abundance; it is in large amount in 13 (about 12%) and to a less extent in 37, but only moderately abundant in the others. Its crystals are usually well under 1 mm. in width, although in 37 they are much larger and reach 4 mm. in diameter; often a number of small granular crystals are massed together.
The garnets generally enclose a few small fragments of brown hornblende, while the larger crystals of this latter mineral in turn may be moulded on garnet. Central reaction is occasionally shown by crystals of this mineral in 28, 37 and 44. In 28, for example, a border of solid garnet in several instances surrounds a wide core of reaction products. In one instance these latter include probable pyroxene and dusty iron ore and in another colourless pyroxene, some pale-brown hornblende, granular garnet and a little feldspar. Other cores consist of granular garnet interwoven with the ragged borders of remnant-crystals of brown hornblende.
In 37 there is interesting zonal growth shown by a crystal of garnet which is approximately a rhombic dodecahedron in form. A central core of solid garnet 1 mm. across is enclosed in a zone 0.3 mm. wide which includes little blebs and irregular small strings of brown hornblende; the whole is then covered by an outer coat 1.4 mm. wide of solid garnet.
These facts show clearly that reaction has been proceeding largely at the expense of hornblende and pyroxene, and that the garnet certainly separated out during the crystallisation of the rock as a whole. The dioritic rock that encloses it appears to be that of a smaller intrusion; it appears likely, therefore, that some assimilation or marginal reactions have occurred during injection of the magma leading to the crystallisation of the garnets as a visible result.
It may be mentioned here that one or two hornblendic xenoliths were observed from which the central cores had been etched out by weathering, while the marginal portions remained plastered to the andesite of the dykes, and are so enriched in garnet that one gains a very strong, if perhaps erroneous, impression that the andesite magma has reacted with such xenoliths to produce the garnets. This impression is not borne out by the microscope, for a thin-section (122) of one of these garnet-rich xenoliths proves it to be comparable exactly with the garnetiferous diorites already described, except that the numerous sharply-idiomorphic large
crystals of deep-brown hornblende are enclosed in a fairly finely-crystallised leucocratic groundmass in which the plagioclase appears as stout rectangular sections accompanied by abundant zeolite. A solitary small grain of pale-green augite was noted. There is no suggestion either in this rock or in the xenoliths taken as a whole that there has been marginal reaction between the material of the xenoliths and the quartz-andesite magma of the dykes from which they were collected, so that these garnetiferous hornblende rocks must have derived their garnets before they were incorporated in the andesite magma. Discussion of this point is carried further on a later page.
The writer had hoped from his present study to be able to come to some definite conclusion regarding the origin of the numerous garnets in the andesitic dykes themselves. In his earlier work upon these latter rocks, he wrote regarding the garnet: “No conclusion was reached as to whether or not it has crystallised from the magma, but appearances support the view that it has so crystallised.” (See under Ferrar, 1925, p. 66). No more definite conclusion has proved to be possible.
The writer had envisaged the possibility that the crystals of this mineral were strewed from garnetiferous xenoliths by attack upon these latter by the andesite magma. The garnets in the quartz-andesite not infrequently are in well-crystallised rhombic dodecahedra; similar crystals were observed macroscopically only in one or two xenoliths which, unfortunately, were too incoherent for sectioning, but approached hornblendites in composition. Garnetiferous types indeed are very rare amongst the xenoliths as a whole, and microscopical evidence makes it appear most unlikely that such rarity is a consequence of earlier xenoliths with garnets having been attacked by the enclosing magma with greater readiness than those lacking them. One has to conclude, therefore, that the andesitic magma did not obtain its garnets from any rocks represented by the xenoliths now visible. In the event of their having crystallised direct from the andestic magma, it is possible that this may have resulted from contamination of the magma during injection, although the writer was unable to recognise any suggestion that this has occurred. It is an intriguing fact that, wherever the andesitic rocks of Whangarei Heads area contain garnets, they also include xenoliths of the same range of types as are described in this paper. Where the xenoliths are especially abundant the garnets are correspondingly so in the andesite.
2. Garnetiferous hornblende-gabbro (7).
The only thin-section available was cut from a chip which represented a nest of garnets enclosed in a gabbroid xenolith. Very few other xenoliths of this type were seen; two others at most may have belonged here, but they were too incoherent for sectioning.
In thin-section the texture is coarsely granitic and garnets as much as 3 mm. across constitute about 40% of the rock along with about 35% or 40% of deep-greenish-brown hornblende, the balance being mainly plagioclase. The hornblende is crowded with minute dust-like grains of iron ore. The plagioclase was not accurately
determined. The maximum extinction angle in the zone ⊥ to 010 that was obtained did not exceed 38°, but the optical character appears definitely to be negative, so that the variety is probably bytownite.
There is a small quantity of strongly pleochroic hypersthene in small prisms which mainly are in close association with the garnet, sometimes enclosed in it along with magnetite and other minerals, but more often in the border zone about the garnet with plagioclase, fairly plentiful iron ore and a little hornblende.
In addition to the pyroxene and iron ore, plagioclase and sometimes brown hornblende may be enclosed in the garnet. In more than one instance a central crystal of this latter mineral fringed by small crystals of magnetite and prisms of pyroxene is surrounded by a zone of plagioclase and magnetitie with hornblende in relatively large (up to 0.5 mm.) ragged crystals. Finally the whole is enclosed by an almost continuous thick outer coat of garnet. In other examples the central core is connected with the surrounding gabbro by fissures in which plagioclase and hypersthene are prominent.
The writer had been inclined to regard the mineral association described as indicating that the garnets had crystallised as a result of magmatic reactions at the expense of earlier hornblende and plagioclase. Dr. Turner,* however, emphasises the fact that any central cores are generally connected by fissures with the gabbro around the garnets, and suggests that such hypersthene-bearing cores, as well as the rim with similar ferriferous hypersthene which surrounds the garnet, are both a result of reaction at the expense of this latter. He writes: “Almandine must be low in the reaction-series, judging from its stability in granitic liquids; hence garnets of this composition caught up in gabbro magma would produce hypersthene or some such pyroxene by a reversal of the reaction that is discussed by Alling (1936, p. 267) whereby in gabbro magmas hypersthene commonly yields garnetiferous reaction rims.”
The garnets must have been derived from earlier garnetiferous rock into which the gabbro magma was intrusive, for evidence from the xenoliths as a whole makes it clear that no reaction occurred when fragments of the resulting garnet-bearing gabbro were enclosed in the andesitic magma that gave rise to the present host of the xenoliths.
3. Hornblende-gabbros showing piezocrystallisation (57, 58, 61, 63, 69, 73, 114). See pl. 40, fig. 7; pl. 40, fig. 8.
Although very different from normal banded gabbros, these coarse-grained rocks show imperfect gneissose structure owing to a tendency for their elongated, often sub-parallel crystals of hornblende to be concentrated in imperfectly-defined bands which alternate with feldspathic layers. Even where the elongated crystals of hornblende are replaced, as in 57 and 58, by stumpy prisms, the same parallelism is apparent. It is suggested that this feature is the product of piezocrystallisation, or, in other words, that the gabbro magma has crystallised during the operation of lateral stress.
[Footnote] * Personal communication.
Mineralogically the rocks are identical with the hornblende-gabbros already described and, in all probability, represent a marginal stress-affected phase of these latter as Weinschenck (1916, p. 57) suggested is the case for certain of the central Alpine granites.
The hornblende typically is a brown variety in which any tint of green is faint, though in 57 and 58 the relation between the two colours is reversed and the green tint is dominant. In 58 the mineral encloses a grain of hypersthene 0.15 mm. across, which is itself narrowly margined by feldspar. As in the gabbros described earlier, hornblende and plagioclase are subequal in amount in most of the rocks of this group, although in very different proportions in others. Similarly the accessory minerals are unimportant in all but 58. In this latter rock, however, sphene and iron ore, often in close association one with the other, are each present in amount approximating 3%, while stout prisms of apatite are not infrequent.
The plagioclase varies in different members of the group from optically positive calcic labradorite to an optically negative more highly calcic type (Ab10 An90 in 61). In one or two of the thinsections (e.g. 63, 114) it shows typical stress-twinning. In some parts of thin-section 58 a feature shown by some of the gabbros reappears, for, while the feldspar in general is coarsely crystallised, in some areas it is in stout rectangular crystals well under 0.1 mm. in length, with which, in addition to a little zeolite, there are some of the sheaf-like aggregates of (?) plagioclase that already have been noted in the hornblende-gabbro 120 and other rocks.
Nos. 57 and 73 are very doubtfully included here. 57 is strongly gneissose in texture and has zonally-grown feldspar (approximately Ab33 An67) in almost equigranular crystals; in many respects it shows strong resemblance to the hornblende-schists and gneisses described below. 73, on the other hand, exhibits alliance to the granulose hornblende-plagioclase rocks that have been classed as gabbro-aplites, but has evidence of the incidence of stress during its crystallisation in the tendency to parallelism of the more elongated of its stout prisms of brown hornblende.
C. Xenoliths Undoubtedly Metamorphic.
I. Probably of Sedimentary Origin.
1. Tremolite-plagioclase-schist (32).
Only one very small xenolith of this rock was found; in section it is largely granoblastic in texture.
On one margin of the thin-section, against the contact with the enclosing andesite, there is a band of closely-crowded prisms of somewhat pale brownish-green hornblende about 0.15 mm. in length. The remainder of the rock is composed essentially of tremolite and plagioclase. The amphibole is in diversely-oriented stumpy prisms, from 0.15 mm. to 0.2 mm. in length, which are aggregated locally into lensoid bands or patches to the virtual exclusion of feldspar. Elsewhere the tremolite is in much smaller crystals and is in much less amount than the plagioclase.
In the tremolite bands and to a less extent elsewhere, there is 5% or 6% of colourless mineral material which is much more refractive
than the tremolite and is in prisms which usually are well under 0.05 mm. in length. All but one or two of the very large number of prisms tested have positive elongation; all have straight extinction. Nearly all show very low interference tints, but a few give tints which denote birefringence near to 0.015. One large oval colourless grain of similar very high refractive index gave a definite negative uniaxial figure and undoubted uniaxial figures were also given by two other small crystals. A few others of similar appearance, however, gave biaxial figures; in one instance negative optical character was clearly demonstrated, while the sharp curvature of the isogyre indicated that the optical axial angle is small. It is undoubted, therefore, that two minerals are represented in this highly refractive material. It may be suggested that the uniaxial mineral is vesuvianite; if so, it is in very small quantity, for prisms giving negative elongation are exceedingly rare. The main mineral represented is probably an epidote near clinozoisite in constitution.
A few minute shreds of very pale brown to almost colourless mica, with birefringence a little above 0.050, are wedged in with the tremolite in association with the epidote and appear to be bleached biotite. In addition, in the more feldspathic portions of the rock, a very small amount of colourless refractive mineral in minute ragged crystals accompanies the tremolite, sometimes outgrown from this latter. Occasional well-defined prisms give an extinction angle of 40°, while the optical character is biaxial and positive, so that the mineral appears certainly to be diopside.
The more feldspathic parts of the rock have fairly fine-grained granoblastic texture and appear to have a little quartz with the feldspar. There are also some larger crystals of plagioclase at least as calcic as An75, which enclose grains of other minerals including what appears to be more sodic plagioclase, judged from the imperfect refractive tests practicable. Adjacent to the tremolite-rich parts of the thin-section, the plagioclase is in much larger crystals than in the granoblastic areas and is more or less poikiloblastic.
The presence of the brownish-green hornblende near the borders of the xenoliths naturally raises the question as to whether or not this results from the alteration of earlier tremolite by volatile constituents coming from the andesitic magma in which the xenolith was immersed. All that can be said in this connection is that there is a certain amount of justification for believing that this may be so, because, close to the contact, the hornblende is brownish-green and similar to that of the andesite, but shows progressive loss of colour towards the colourless tremolite within.
The mineral constitution of this tremolite-plagioclase-schist indicates that it is a calc-schist developed by high-grade thermal metamorphism of an impure magnesian limestone.
2. Actinolite-plagioclase-schist with subsidiary pyroxene (50).
This is a strongly schistose rock with a little over 50% of pale green actinolitic hornblende which is streaked out in long narrow parallel prisms generally from 1 mm. to 2 mm. in length, but sometimes as much as 3 mm. (see pl. 40, fig. 9). Pale-green diopside occurs in important, though much less, quantity (10%) in stumpy
irregular crystals usually concentrated into somewhat lensoid layers about 1 mm. in maximum width. In a few instances it is fringed by a narrow margin of deep-bluish-green hornblende. Plagioclase (Ab30 An70) showing occasional zonal growth and a little quartz form most of the balance of the rock and are in moderately large granular crystals. There is also about 10% of an opaque material which has more or less the appearance of leucoxene, though it certainly is not this mineral. It is usually in irregularly-shaped masses which are more abundant in certain parts of the thin-section than others; occasionally it assumes an imperfect stout lath form (see pl. 40, fig. 9). A little biotite also occurs in one lamella of the rock and there is sphene in plentiful ubiquitous grains. Rare large grains of apatite and a few small prisms of zircon were also noted. Iron ore is sparse except in the “leucoxenic” patches, where it is very plentiful in minute grains.
The “leucoxenic” material is of very doubtful origin. In most instances it is a dense opaque irresolvable product which is white in reflected light and is crowded with minute grains of iron ore. In patches that are at all resolvable into their constituent elements, a few minute garnets can be detected under high magnification along with feldspathic material, a moderate number of small grains of iron ore and a few tiny shreds of green hornblende. In one patch which is more coarsely granular than most of the others, but which has the same general appearance as they, recognisable garnets are numerous and larger than elsewhere; with them there are feldspar, some grains of iron ore and rare ragged shreds of a hornblende which shows pleochroic change of tint from deep-grass-green to strong yellow. The same type of pseudomorphous material appears in other types of schistose rocks represented amongst the xenoliths, so that discussion of its origin will be deferred to a later page.
The actinolite-plagioclase-schist described above is one of a series of calc-schists represented in the xenoliths and, like others of the series, is derived essentially by contact metamorphism of calcareous sediment. Its structure indicates that some regional stress accompanied the phase of igneous injection.
3. Diopside-plagioclase-schist (10).
This is a markedly schistose rock consisting dominantly of plagioclase with about 40% of diopside, a little brown hornblende and a few small flakes of brown biotite. Sphene is plentiful in small grains and subhedral crystals in some of the more richly pyroxenic bands, but is not associated with iron ore as is characteristically the case in the amphibole-schists or amphibolites. Iron ore does not exceed 2%; zircon is rare and there are only occasional tiny elongated prisms of apatite.
The pyroxene tends to be concentrated in layers which alternate with more feldspathic ones, and is in long irregular prisms which may exceed 2 mm. in length and largely lie parallel to the schistosity. It shows faint pleochroic change of tint from pale-green to faint-brownish-green in sections more or less at right angles to c′.
In one lensoid band a little over 4 mm. in maximum width, there is only about 20% of ferromagnesian mineral; this latter consists, however, of hornblende and diopside in subequal proportions, the former sometimes outgrown from the latter.
The plagioclase (Ab25 An75) is fairly coarsely crystallised and is usually in grains, though sometimes in long crystals which may exhibit zonal growth at their margins. With it there is a small amount of orthoclase which appears to be intergrown cryptoperthitically with some soda feldspar on a minute scale.
One lamella of a duplicate thin-section shows moderately abundant fairly finely granular pale-brown garnet; occasional larger grains as much as 1.5 mm. in maximum dimension may appear in other layers. A small amount of zeolite accompanies the feldspar and accessory apatite is present in moderate amount. The biotite is in small, fairly plentiful, reddish-brown flakes associated with the hornblende.
The mineral constitution of the rock shows that, like others grouped with it in the scheme of classification adopted, it has been derived from an impure calcareous sediment by metamorphism which dominantly has been thermal.
4. Hornblende-garnet-plagioclase-schist (107).
Macroscopically this is a most distinctive type with its long flattened columnar crystals of hornblende as much as 17 mm. in length which are arranged in sub-parallel layers alternating with fine-grained feldspathic matrix in which streaks of finely-granular pink garnet are obvious (see pl. 39, fig. 2). Within the layers the columns of hornblende have a more or less random arrangement.
Viewed microscopically the rock is seen to consist of from 12% to 15% of green hornblende, its large elongated crystals ragged at their edges and containing small grains of feldspar in moderate number. The garnet is a fairly bright pink and is generally aggregated into clusters of grains 0.1 mm. or less in diameter accompanied by tiny granules of the same mineral, the whole comprising about 6% of the rock. There is a small quantity of diopside near the margins of the hornblende crystals.
Sphene is fairly plentiful and usually encloses iron ore. Apatite also is prominent in grains of moderate size; zircon is present, but is rare.
The main base of the rock is weathered feldspar (Ab30 An70) in grains from 0.15 mm. to 0.2 mm. in diameter, which have between them a small amount (4%) of interstitial quartz.
This rock is another high-grade derivative of a calcareous sediment. Its structure suggests that its alteration has been effected by contact metamorphism.
5. Biotite-pleonaste-garnet-plagioclase-schist (6).
This schist was found as a small xenolith in a dyke at the shore of the mainland approximately north of High (McGregor's) Island, Taurikura, about 3 ½ miles south-east from where most of the other xenoliths were collected. It is a strongly schistose, fairly fine-grained type showing small garnets in hand-specimen.
The thin-section shows that the rock is made up of about 80% of plagioclase, which is usually in somewhat elongated poorly-shaped crystals generally well under 0.3 mm. in length, though at times exceeding 1 mm. Its variety could not be determined precisely owing to lack of crystals giving satisfactory interference figures at right angles to a bisectrix. It is, however, not less calcic than An60. Sometimes it is outgrown by a more sodic variety and is commonly mottled in-curious manner by intergrowth with what appears to be orthoclase, for in one or two instances clear feldspar, with refractive index very much less than that of the plagioclase associated with it was intergrown in minute micrographic manner with this latter. A little radial zeolite occurs with the feldspar, but quartz is absent, as is expectable in view of the presence of a spinel.
The schistosity is demonstrated by the parallelism of the more elongated crystals of plagioclase and, more particularly, by the striking parallelism of narrow bands enriched in reddish-brown biotite and of lines of crystals of spinel. The garnets cut across the schistosity; they are pale-pink in colour, irregular in form and small, not exceeding 1.5 mm. in diameter, and are present in some-what small number. The biotite (about 10%) is in flakes averaging from 0.3 mm. to 0.4 mm. in length, but may also occur in narrow wisps nearly 2 mm. in length. Not infrequently it is converted to a green phase preparatory to complete chloritisation. Rare tiny prisms of zircon are present in the biotite and elsewhere; a few grains of sphene occur sporadically, while iron ore is almost limited to a few large grains up to 0.9 mm. in largest dimension. There are a few small grains of epidote, one with a central core of allanite. In addition, one or two small grains of a deep-blue pleochroic mineral with absorption × < Z occur with the altered biotite and probably represent either an amphibole or tourmaline. The spinel (about 5%) is mainly concentrated in indefinite almost linear streaks; it is a deep-green variety near pleonaste and shows square sections usually from 0.06 mm. to 0.08 mm. in length of side. It commonly is associated closely with the green semi-chloritised biotite.
In a number of the lamellae of the schist there is a small quantity of an unknown colourless highly refractive mineral in numerous minute needles seldom more than 0.2 mm. in length, though attaining as much as 0.6 mm. These are associated with tiny rhombic or square cross-sections of what appears to be the same mineral. The birefringence was not accurately determinable on account of the immersion of the mineral in others, but is not less than 0.012. Its elongation is positive, and it has straight extinction. The writer had been inclined to regard this mineral as wollastonite, but Dr. Turner* has pointed out that the universal positive elongation shown could not be attained with wollastonite elongated parallel to the b axis, as would have to be the case in this instance. He suggests instead that the mineral is anthophyllite and points out that the association plagioclase-biotite-spinel-anthophyllite agrees moderately well with Class 4 of Goldschmidt's classification of “hornfels” (see Harker, 1932, p. 99). This class includes the association cordierite-anorthite-enstatite.
[Footnote] * Private communication.
Dr. Turner notes that, in the corresponding silica-deficient rock, spinel would take the place of cordierite, while in the present schist, anthophyllite has taken that of enstatite, possibly on account of stress accompanying the high temperature that is evidenced by the spinel, or for other reasons. He states further “The presence of garnet is not incongruous, for almandine lies half-way between the enstatite and cordierite points on the Goldschmidt triangle. Biotite and albite, as pointed out by Tilley and others, constantly enter into all rocks of this type.” Wollastonite, he points out, would not enter into such an association. He concludes “The rock is a high-temperature derivative of calcareous-argillaceous sediment low in silica. It is essentially a contact-rock, but has suffered sufficient stress during its metamorphism to give it schistose structure. Such stress may explain the development of garnet and anthophyllite.”
6. Hornblende-biotite-plagioclase-hornfels (30).
Only one small xenolith of this rock was found. It is a strongly poikiloblastic and granoblastic rock with average grainsize of about 0.05 mm. for its matrix. Ragged porphyroblasts of greenish-brown hornblende, varying in size from 0.3 mm. to 3 mm. in greatest dimension, are common and are densely crowded with grains of plagioclase, a few small flakes of biotite and a variable number of small crystals of magnetite, as well as rare grains of clinozoisite. In one part of the only thin-section available, such spongy crystals of hornblende alternate with small poikiloblastic porphyroblasts of deep-brown biotite, which is scarce in other parts of the section. Magnerite is very plentiful in small euhedral crystals and there are a number of small crystals of apatite and, very rarely, of zircon.
The feldspathic basis may form large local areas free from coloured constituents. Elsewhere it is subequal in amount to these latter and constitutes about 65% of the whole rock. It appears wholly to be plagioclase in small granular crystals which are relatively seldom twinned. Symmetrical extinction angles on 010 show that it is not less calcie than Ab32 An68; it may even he richer in the anorthite molecule, for interference figures appear to give negative optical character.
This rock is a typical contact type derived from a somewhat calcareous argillaceous sediment.
II. Probably Derived from Basic Igneous Rocks.
1. Hornblende-plagioclase-gneisses and schists.
These rocks constitute two groups: the first is more or less allied in characters to the piezocrystallised hornblende-gabbros, having similar, but more definite, gneissose texture. In this group the hornblende is greenish-brown and different from the green variety that characterises the other hornblende-schists. Quartz also is in negligible amount, if not absent, although in the second group of rocks included here it commonly is abundant.
Group (a). Hornblende-plagioclase-gneisses or amphibolites (38, 41, 52).
These are coarse-grained rocks of very simple mineralogical composition in which greenish-brown hornblende is slightly in excess of plagioclase, or else subequal to it in amount. 38 is very well banded, with the layers enriched alternately either in hornblende or plagioclase (see pl. 40, fig. 11). Both in this rock and in 52, the hornblende is in columnar crystals, as much as 3 mm. in length, which show parallelism in their arrangement. In 41, however, the texture is more granular.
The plagioclase is a basic labradorite of positive optical character, and is somewhat seldom twinned. It was not determined accurately in 38, but in 41 and 52 it is Ab35 An65. It is accompanied by a zeolite in 38 and 52, and in 52 by some interstitial quartz. The accessories include sphene, iron ore and apatite. The first two are in very small amount in all but 52, where they are in close association one with the other and together amount to about 4% of the rock. Apatite is in relatively large grains in 38 and 52, one grain in 38 being as much as 1.5 mm. × 0.6 mm. in dimensions.
In 52 much of the leucocratic material is moderately finely crystallised and consists of fairly long laths of plagioclase enclosed in sheaf-like aggregates of acicular (?) plagioclase such as have already been described for the hornblende-gabbro No. 120. It is perhaps possible that this finely crystallised phase represents liquid introduced during mechanical disintegration of the xenolith in the andesite magma.
Group (b). Hornblende-plagioclase-schists (74, 78, 80a, 91, 92a, 92b, 98, 100, 103).
These schists are characterised by a hornblende with distinctly greener tints than that in the last-described rocks; it is in poorly-shaped elongated crystals sub-parallel to the schistosity, which is marked in all but the somewhat granular rocks 92a and 103.
Usually the hornblende is subequal in quantity to the colourless minerals; in 92b, however, it is present only to the extent of about 20%. In 78 also it does not exceed 35%, while in this rock there is also a most puzzling occurrence of a finely granular feldspathic veinlet, 0.3 mm. or less across, transverse to the schistosity, and containing a small amount of diopside similar to that of the diopside-bearing hornblede-schists next to be described. It does not appear reasonable to regard this veinlet as a product of localised shear. It is more likely to be a minute dyke of injected igneous material; yet the diopside present is not that commonly crystallised from igneous magma.
Quartz is abundant in several of the schists of this group (e.g. 80a and 91), though always in less amount than feldspar; it is practically absent, however, from 78, 92b, 100 and 103 and only in small interstitial amount in 92a.
Sphene and iron ore characteristically are abundant in close association one with the other; in 78 they are unusually plentiful and together constitute about 8% of the rock. Sometimes the sphene is fringed by iron ore. but at other times (e.g. in 92b and 103) the relation is reversed. In 91 the two minerals make thick lensoid granular aggregates 2 mm. and more in length. Away from the
sphene the iron ore is generally in large wide-spaced irregular grains. Apatite is usually plentiful. Epidote is present in 78, 91, 100 and 103, though abundant only in 100 (about 2%) and 103 (5%). It is usually in yellowish grains; in 103, however, it is colourless and in 91 it is in minute elongated crystals.
The determination of the feldspar is usually handicapped by infrequency of crystals suitable for application of tests. It ranges from acid labradorite (Ab30 An79) in 74 to Ab15 An85 in 78, 92a and 92b, and to Ab10 An90 in 98. Occasionally it exhibits zonal growth and at other times may show mottling as a result of intergrowth with another feldspar. In 78, indeed, several sections nearly at right angles to bisectrices give extinction angles that suggest the presence of a plagioclase relatively rich in soda. The plagioclase is usually in granular crystals of moderate size; in 103, however, it is in large crystals which poikiloblastically enclose hornblende and sphene.
2. Hornblende-plagioclase-schists with subsidiary diopside (16, 35, 56, 68, 76, 81, 115 and ? 109).
All of the rocks grouped here are moderately coarse-grained and schistose with the exception of 81, a granoblastic type, and all but 56 and 109 are very closely allied one to another. They are very abundant amongst the xenoliths. 56 shows alliance to the hornblende-plagioclase gneisses or amphibolites and 109 correctly should be separated out as a distinct type.
The typical rock of the group contains hornblende in which fairly deep green tints prevail over brown, and which occurs in fairly well-elongated, though usually ragged, crystals, and in amount subequal to that of the leucocratic constituents, although falling to 40% in 35 and 68. In 35 a little of the hornblende is notably poikiloblastic.
The pyroxene is a faintly pleochroic pale-green diopside and is in more or less rare scattered irregular small crystals in 16, 68 and 115. In 35, 56 and 81, however, its amount is greater and its crystals may be larger, so that these rocks show transition to the hornblende-pyroxene-plagioclase-schists. A few grains of yellow epidote appear in 16. Sphene generally is plentiful, though not so in 56, 81 and 115. As in the hornblende-plagioclase-schists it typically is associated with iron ore, illustrating again its development from earlier ilmenite. In 68 the associated sphene and iron ore show the same coarse lenses that appeared in 91 of the other group of hornblende-schists. At other times the iron ore may occur in grains as large as 0.7 mm. in diameter with tiny grains of included sphene; it is absent from 16. Apatite is seldom prominent, though almost always present in small quantity.
In most of the rocks quartz accompanies the plagioclase, although it appears to be absent from 16 and 56 and is in very small amount in 81. Otherwise it is always an important constituent, though subordinate on the whole to the feldspar. In certain layers, however, large grains of quartz may be in excess of the plagioclase.
The feldspar is typically in granular crystals rarely with zonal growth; in 35 and 76 a little zeolite occurs with it. Its variety is seldom obtainable accurately; in 56 it is Ab40 An60 and in 115 Ab35 An65, while in other thin-sections it is seldom less calcic than sodic labradorite.
No. 56 has the structure of the hornblende-plagioclase-gneisses described in the last section and differs from them only in having a moderate amount of diopside, which they lack. Its hornblende is green with a border of brown to the elongated ragged crystals, which show only a slight tendency towards parallelism in their arrangement. There appears to be no quartz, while the sphene and iron ore are mainly in a few narrow strings as much as 4 mm. and more in length.
109 is placed here for want of a better location. It has very close resemblance to the hornblendites, but is separable from them by virtue of the presence of the diopside that is characteristic of most of the schists and by its abundant sphene (10%). The rock consists essentially of brownish-green hornblende either in stumpy variously-oriented prisms about 0.4 mm. in length, or in large irregular somewhat spongy crystals often crowded with small grains or aggregates of sphene.
Only about 2% of diopside is present and forms granular fringes to large aggregates of hornblende upon which occasionally it is outgrown.
About 8% of plagioclase occurs in the spaces between the large crystals or aggregates of hornblende and typically is in crystals as much as 1.5 mm. or more across. It may be mottled in the same manner as that of other schists in this group, and is optically positive and not less calcic than An40, though its exact variety could not be determined.
Iron ore is absent but for rare small remnants in the sphene, apatite was noted in occasional prisms and grains and a single small flake of biotite was observed in a crystal of hornblende.
3. Hornblende-diopside-plagioclase-gneisses and schists (3, 11, 14, 17, 26, 27, 47, 75, 85, 119).
Most of the rocks of this group are coarsely and strongly schistose and are abundant amongst the xenoliths. 14, 26 and 75, however, are not schistose but gneissose in texture, while 47 is a coarsely subgranular type with a closer intermixture of diopside and hornblende than appears in other members of these schists.
The hornblende generally is a brownish-green variety and is in large long crystals attaining as much as 5 mm. in length (e.g. 27), and possessing ragged margins. In the manner usual in such rocks, leucocratic layers alternate with others enriched in ferromagnesian mineral. This latter is mainly hornblende, which is in wide bands alternating with narrow diopsidic ones; in some rocks the diopside is mainly in the more leucocratic layers. This latter mineral characteristically is in small stout crystals, although in 14 and more particularly in 119, which is characterised by actnolite in place of the usual variety of hornblende, its crystals are more elongated and may reach 3 mm. in length. Its tints show pleochroic change from
pale-green to a faint greyish-pink or very pale brownish-green. It is typically in much less quantity than hornblende, upon which it is sometimes outgrown, although in a few rocks (e.g. 74 and 119) the two minerals have subequal proportions.
In 3, 14 and 119 there are a few small flakes of brown biotite; rare crystals of this mineral were observed even in hand-specimens of 3.
Grains of pale-yellow epidote are present in moderate number in the leucocratic lamellae of 11, though scarce in hornblendic ones. A few appear also in 3 and 17 and colourless grains of clinozoisitic epidote occur rarely in 119. Sphene and iron ore are generally plentiful, commonly in close association one with the other; sometimes their aggregates form large lenses (e.g. 14). Where not with sphene, the iron ore is in large wide-spaced grains; in 47, however, it forms swarms of tiny grains adjacent to the contact of the xenolith with the enclosing andesite.
In 14, 85 and 119 the same “leucoxenic” pseudomorphs reappear as have been noted from rocks already described. In addition apatite is generally present in all rocks of the group, though in small amount; zircon is scarce and was noted only in 3 and 11.
Numbers of small grains of pale-pink garnet occur in 14 and 119 in connection with the “leucoxenic” patches; larger crystals are, however, rare and do not exceed 0.8 mm. in diameter.
Plagioclase constitutes most of the light-coloured portion of the rocks; quartz generally is absent or in very small amount, except in 3, where about 15% is present, and in 27, where it is a very important constituent of certain layers. A small amount of zeolite was noted in 47 and 119.
As a rule the feldspar is in fairly small granular crystals, which are somewhat infrequently twinned and occasionally show zonal growth (3, 47, 119). The average variety is a labradorite about Ab40 An60 (3, 11, 75, 119 and 27). In 14, 26 and 36 it approximates Ab25 An75, and in 85 is even more calcic (? Ab20 An80) and has negative optical character.
As remarked earlier, 14, 26 and 75 are gneissic rather than schistose in texture, while 47 is granular. Apart from this major textural difference, however, other characters separate 14 and 26, along with 85 and 119, from others of the group.
In 14 wide finely-granoblastic areas of hornblende with a somewhat lesser amount of feldspar alternate with more coarsely crystallised irregular lensoid layers in which feldspar is more plentiful than elsewhere and diopside almost entirely replaces hornblende, being present as elongated crystals as much as 1.7 mm. in length. In addition, however, there are subgranular patches where there is fairly complete intermixture of the two ferromagnesian minerals, and where both may be more or less poikiloblastic, especially the diopside. In this rock also there are large aggregates of sphene with a little iron ore as well as the “leucoxenic” pseudomorphs with associated garnet that already have been mentioned.
Apart from its content of diopside, 26 shows resemblance to the piezocrystallised hornblende-gabbros, for it has similar gneissose structure and the same type of brownish hornblende, which is present in stout prisms from 0.6 mm. to 1 mm. in length. These are arranged with good parallelism in broad hornblendie bands which alternate with others in which diopside is the dominant ferromagnesian mineral. Feldspar (Ab25 An75) is almost excluded from some of the coarser layers, though where the grain-size is smaller it is subequal in amount to the ferromagnesian minerals.
85 is a more leucocratic type than the others, and has excellent schistosity evidence in the manner in which the narrow columnar crystals of green hornblende are streaked out with perfect parallelism intermixed with granular crystals of diopside in almost equal quantity. The hornblende is mottled by intergrowth with the diopside, and minute grains of this latter occur also in the feldspathic lamellae. In addition, one or two layers of the rock are enriched in iregular patches of the “leucoxenic” material that already has been mentioned.
119 differs from other members of the group in that its amphibole is near actinolite. It is subequal in amount to the diopside; both are in columnar crystals as much as 3 mm. in length and together are somewhat in excess of the plagioclase. Sphene is abundant and iron ore is practically absent except in the “leucoxenic” pseudomorphs. The pale-green faintly pleochroic diopside is concentrated in bands which alternate with hornblendic ones and is often fringed by a narrow border of what appears to be a deep-grass-green hornblende. There is a very considerable quantity of the “leucoxenic” material that characterises 14 and 85 of this group of schists, the closely allied actionolite-plagioclase-schist No. 50 and the hornblende-diopside-epidote-schist No. 118 described on a later page.
Although some account was given of these pseudomorphs during description of No. 50, it is desirable at this stage to discuss them in more detail. Generally they form irregular patches in layers that otherwise are feldspathie, or else are wedged between the larger crystals of ferromagnesian mineral. Occasionally they have more or less regular rectangular outlines. In other respects also they have the general characters that have been noted for No. 50. Even in minute detail many of them parallel the similar masses of 50.
In 14 they are small rounded patches in which there are rare tiny grains of a pinkish garnet, abundant granules of greenish hornblende and rare tiny flakes of biotite enmeshed with abundant granular magnetite in a basis of feldspar. Very much the same type of material is represented in the majority of the pseudomorphs of 119. Wherever granules of hornblende are abundant, garnet is scarce and larger grains of sphene may appear in fair number. In addition, however, many of these masses in 85 and 119 show numerous grains of identifiable garnet with others which are colourless and very highly refractive and possibly may be clinozoisite. In 85 the proportion of garnet is often difficult to determine because there are also grains of a similar pink very highly refractive unidentified mineral which is not isotropic but appears to give very low interference
tints. Nevertheless, both in this rock and in 119, granules of garnet are abundant in many of the pseudomorphs, where they are associated with magnetite, feldspar and grains of hornblende in small number. A final, but rarer, type of pseudomorph is one consisting of swarms of tiny grains of iron ore immersed in feldspar.
It is possible that certain peculiarities of 119 throw light upon the origin of these unusual reaction products. In this rock there are occasional grains of a pale clinozoisitic epidote which very often are enclosed in crystals of hornblende. These grains always have a reaction-border of greater or less depth, which consists in most instances of some unknown opaque grey material, which is white in reflected light and shows a minutely interwoven or sub-radiate structure. Tiny vermicular fibres of this material penetrate the margins of the epidote and crowded granules of magnetite fringe the outside of the whole mass. The appearance suggests strongly that these products are closely comparable with the adjacent “leucoxenic” pseudomorphs.
It is likely, therefore, that this “leucoxenic” material, as it has been called herein for want of a better designation, is a product of reaction upon earlier epidote. It is significant that the hornblende-diopside-epidote-schist No. 118 demonstrates clearly that epidote is very scarce in the layers enriched in the “leucoxene,” though abundant elsewhere; another important fact shown by this rock is that the “leucoxenic” masses occur only in half of the thin-section.
It may be suggested, therefore, that if such reaction as has been postulated did actually occur, it was limited to more superficial portions of the xenoliths and did not affect the cores of larger examples. It would appear reasonable to ascribe such reaction to the andestite magma that rafted up the xenoliths, but it must be pointed out that any effects of this magma are found to be very slight when the xenoliths are considered as a whole. Nevertheless reaction certainly has proceeded in the metamorphic rocks that contain these pseudomorphs and apparently followed the main metamorphism of such rocks. The most reasonable hypothesis seems to be, therefore, that such reaction was due to the inclusion of the xenoliths in the andesitic magma, but that these latter were included in deeper, larger and therefore hotter bodies of magma than were the xenoliths that have been little if at all affected.
Origin of schists and gneisses of Class II.
In this class are included hornblende-plagioclase-schists and gneisses with or without subsidiary diopside. Many correspond very closely with classical examples of amphibolites; such as those of the Lizard, Cornwall, which Harker (1932, p. 281) classes as typical products of high-grade regional metamorphism of earlier basic igneous rocks. In others, however, certain facts suggest an origin from basic tuffaceous material. This is especially the case for several of the hornblende-plagioclase-schists with subsidiary diopside in which the quartz characterises particular layers.
Schistosity is so generally prevalent that high-grade stress must have been operative during the metamorphism. It is necessary, however
Fig. 1—Boulder of andesite with included xenoliths from north Shore of McLeod's Bay, Whangarei Heads. The [ unclear: ] is [ unclear: ] . in diameter. Fig. 2—Hornblende-garnet-plagioclase-schist (107). The finery granular greyish material in right halt of figure below the long column of hornblende (dark) is garnet. X [ unclear: ] diams. Fig. 3—Granular hornblendite (117). [ unclear: ] of the white areas are gaps in the section. × 31 [ unclear: ] . Fig. 4—Fine-grained basic dyke rock of comptonitie texture ( [ unclear: ] 34). Hornblende is the [ unclear: ] mineral. × 31 diams. Fig. 5—Hornblende-gabbro-aplite (18). The white mineral is plagioclase: the other is hornblende. × 45 diams.
Fig. 6—Garnetiferous diorite (13). The dark mineral is hornblende. Above the large idiomorphi [ unclear: ] crystal of hornblende is an [ unclear: ] uhedral one of plagioclase marginally outgrown by a more sodic feldspar. To the right of it is another large crystal of plagioclase in great part replaced by zeolite. A crystal of garnet (light grey with dark border) is on the right, and zeolite appears around the gaps in the section (white). × 31 diams. Fig. 7—Hornblende-gabbro showing piezocrystallisation. A relatively fine-grained, sub-granular local phase of 61. × 31 diams. Fig. 8—Hornblende-gabbro showing piezocrystallisation (63). Plagioclase appears white and hornblende grey or black. × 31 diams. Fig. 9—Actinolite-plagioclase-schist (50). The dark material represents dense “leueoxenic” pseudomorphs described in the text. × 31 diams. Fig. 10—Biotite-pleonaste-garnet-plagioclase-schist (6). Garnet is seen below; above this, on the right, is biotite in process of conversion to chlorite (almost black). The small euhedral black crystals are pleonaste. × 31 diams. Fig. 11—Hornblende-plagioclase-gneiss (38). × 31 diams.
to consider the metamorphism evidenced by the xenoliths as a whole rather than that suggested by an individual group of rocks represented therein, and many demostrate clearly that they have been subjected to high-grade contact metamorphism. It is probable, therefore, that the schists and gneisses now discussed, in common with other metamorphic types amongst the xenoliths, have been produced by contact metamorphism assisted by severe stress. Flett and Hill (1912) indeed came to a somewhat similar conclusion regarding the comparable rocks of the Lizard and Meneage district.
III. Derivatives of Impure Calcareous Rocks or Basic Tuffs.
1. Hornblende-epidote-schist (5).
This rock is exactly comparable with quartz-bearing hornblende-plagioclase-schists described under Class II, except that it has 5% or more of pale-yellow granular epidote and that sphene is in somewhat less amount than in the other rocks. About 45% of hornblende is present in sub-parallel prisms and there is 20% of quartz along with weathered plagioclase and a moderate amount of zeolite. Some of the plagioclase poikiloblastically encloses small grains of quartz. Unfortunately it was not found possible to determine the variety of this feldspar.
2. Hornblende-diopside-epidote-schists (12, 86, 118).
No. 12 is a schistose rock which shows considerable variation in the two thin-sections prepared. Some lamellae of 12a consist mainly of bluish-green hornblende in long parallel columnar crystals, with coarse masses of sphene associated with a little iron ore, and a little epidote. Between such lamellae there are thin lensoid ones of quartz and plagioclase. There is also a layer from 3 mm. to 5 mm. in width composed of granular yellowish epidote and quartz in subequal proportions, with 6% of diopside, a little plagioclase and occasional large lenses of sphene and iron ore.
The thin-section 12b shows very good alternation of narrow lamellae with either columnar hornblende [ unclear: ] stumpy crystals of diopside dominant one over the other, with quartz and plagioclase in slightly less amount, some epidote, abundant separate grains of sphene and some ovoid aggregates of sphene and iron ore which may be as much as 2.5 mm. in length. At the edge of the thin-section there is the remnant of a band, now only 5 mm. in width, which is specially rich in epidote (about 25%) in grains about 0.1 mm. in diameter; with this there are diopside in somewhat smaller quantity, a few fairly large aggregates of sphene, rare hornblende, a moderate amount of quartz and plagioclase (Ab40 An60) and, as the enwrapping medium, crystals of scapolite as much as 6 mm. × 2.5 mm. in dimensions. Tests of refractive index and birefringence indicate that the scapolite is near mizzonite in composition. A solitary grain of brownish garnet 0.7 mm. across was also noted in this layer.
Three small chips of 86 were collected and sectioned, but give very varied proportions of the constituent minerals. All, however, show 30% or over of granular yellowish epidote in lensoid bands as much as 4 mm. in width, along with a less proportion of bluish-green
hornblende, faintly pleochroic diopside and abundant quartz and plagioclase. In other leucocratic layers, however, quartz may be unimportant, although finely granular epidote may still be prominent. In some bands rich in slender elongated crystals of hornblende, the epidote is greatly reduced in amount, while the plagioclase (approximately Ab40 An60) is not granular as elsewhere, but is in relatively large crystals which enclose the other minerals and are accompanied by large nests of sphene and iron ore. In one of the thin-sections (86.1) there are a few grains of zonally-grown allanite separable from garnet by imperfect biaxial interference figures.
118 is another schistose rock in which leucocratic layers, which may have diopside as the dominant ferromagnesian mineral, alternate with belts in which abundant bluish-green hornblende is streaked out in ragged sub-parallel prisms with a little plagioclase, a small quantity of diopside, very abundant sphene (8%) and a slightly less percentage of epidote. Both sphene and epidote are most plentiful in such hornblendic layers; there may indeed be very little epidote elsewhere than in them.
While rare in the hornblendic areas, quartz is very plentiful in some of the pyroxenic leucocratic layers and may there be in excess of the plagioclase (approximately Ab40 An60). In one wide band of this type there is a large quantity of a very dense phase of the “leucoxenic” material described and discussed on an earlier page. With it there are rare grains of what appears to be allanite.
Origin of the hornblende-epidote-and hornblende-diopside-epidote-schists.
The amount of quartz present suggests that these schists may have been derived from impure calcareous sediments or basic tuffs. It is possible, however, that the somewhat localised occurrence of the epidote observed may be a result of the addition of carbonate to certain parts of original basic rocks as a consequence of weathering, which also could have introduced silica. On the other hand the individual minerals show a degree of segregation into particular bands that recalls the lamination of an original sediment. The occurrence of scapolite in 12b indicates that pneumatolytic processes have been at work and is in support of the conclusion that the highgrade metamorphism attained is essentially the result of contact action, even though regional stress has been important.
Origin of the Xenoliths of Whangarei Heads Area.
Description of the various types of xenoliths has brought forward evidence of the presence of unmetamorphosed plutonic rocks, which occur with others of similar mineralogical character, though with structures which show that the rocks were influenced by stress during their crystallisation. With these essentially igneous rocks there are many others which demonstrate clearly metamorphism of very high grade.
The strongly-marked schistosity of so many of the rocks is proof that stress was very important during the metamorphism, and indeed the mineral constitution of a large number of them agrees well with that expected in high grades of regional metamorphism.
Nevertheless the occurrence of pleonaste with garnet, plagioclase and biotite in 6 and that of scapolite in 12b, as well as the hornfelsic nature of several of the rocks, indicate just as definitely that contact metamorphism has been operative. In addition a very large number of the schists agree very closely with cale-schists as generally described.
The general conclusion that emerges appears to be that an earlier series of calcareous sediments with basic igneous rocks and tuffs was invaded by batholithic masses of gabbroid nature during a period of severe regional stress, which must have concluded before the final consolidation of the basic magma, for most of its crystallised products show little if any effect of stress. Marginal portions, however, do show such effect in imperfect gneissic banding. As Weinschenck (1916, p. 57) has pointed out, however, such stress could not be transmitted into interior liquid portions of the intrusive magma.
The rocks that crystallised from these major injections are largely hornblende-gabbros, noritic hornblende-gabbros and hornblendites. There is every reason to believe that the hornblende in all cases has been derived magmatically from earlier pyroxene. As pointed out on an earlier page, the gabbro magma passed through garnetiferous rocks from which it derived garnets which are found in occasional xenoliths of hornblende-gabbro.
It is of interest to note that a land-mass which was composed in part of metamorphic rocks was in existence near Whangarei as late as the early Tertiary, for at Mangapai conglomerates probably of this age contain actinolite-feldspar-gneiss in addition to sheared gabbros and other plutonic types (Bartrum, 1924, p. 345).
High-grade schists also occur as xenoliths at Silverdale, about 20 miles north of Auckland City, where a fragment of diopside hornblende-plagioclase-schist was found in an intrusion of serpentine which is probably of early Tertiary age (Turner and Bartrum, 1929, p. 898).
In addition, the writer recently described a varied series of schists found in Jurassic conglomerates at Kawhia on the south-west coast of Auckland, and pointed out the convergence of evidence in favour of the existence in the North Island, beneath the cover of Mesozoic and younger rocks, of a metamorphic basement of early Palaeozoic age and analogous with that in the South Island (Bartrum, 1935). The present paper adds to our knowledge of this buried metamorphic basement.
Dr. F. J. Turner, of Otago University, very kindly furnished the writer with comments upon the xenolithic rocks described in this paper. A number of his suggestions already have been incorporated in the text, and in the appendix below are added comparisons that he has drawn between the rocks represented in the Whangarei xenoliths and corresponding types from southern New Zealand with which he is so familiar.