The petrography of the dykes has already been dealt with to some extent by the present author (Speight, 1923. pp. 128–146) and analyses of a number have been given with a description of their general petrographical features. In this article the work is extended and the author must express his thanks and admiration to the Dominion Laboratory for the excellent analyses they have furnished.
Although there is general gradation between rocks lying at the ends of the trachyte to basalt series, yet certain types are more than ordinarily important and may be indicated as follows:—
1. Greenish trachyte, with phenocrysts of sanidine and microphenocrysts of soda-amphibole in a base of sanidine laths, aegirine-augite grains, and a little quartz. This is the most acid type.
2. Trachyte, brownish, pinkish, or grey-green in colour, with base composed of
i. stumpy sanidine and possibly anorthoclase-microlites with grains of aegirine-augite, and,
ii. long laths of sanidine and possibly anorthoclase showing either flow arrangement or laths in bundles of radiating sheaf-like forms,
both types containing phenocrysts of sanidine, andesine, or oligoclase-andesine, sometimes anorthoclase, phenocrysts of aegirine-augite, at times with microphenocrysts of the same mineral, occasional brownish hornblende, and very occasional olivine; tridymite occurs sparingly.
3. More basic trachytes, with light green-grey or deep green colour containing few phenocrysts.
4. Trachy-andesites, greenish or greyish in colour, frequently with brown hornblende and olivine.
5. Andesites, basic in character, and grading into 6.
6. Basalts, some acid and others very basic; in some of the basic forms hornblende is a notable constituent, and they appear to have some relation to the trachytes.
The hornblende is perhaps the most striking mineral in these rocks. It first of all occurs as large brownish or brownish green crystals and aggregations, the crystals ranging up to 10 cm. in length, but usually less than 1 cm.; they are sometimes unaltered, but occasionally are resorbed partially or completely. Another type of amphibole is a soda variety resembling arfvedsonite, which occurs sparingly as micro-phenocrysts in some of the more alkaline types. The augite which is most common is a greenish, slightly pleochroic aegirineaugite which occurs as phenocrysts, microphenocrysts, or disseminated through the base as grains and in stumpy crystal forms. A greyish, slightly purple, not appreciably pleochroic augite also occurs in subordinate amount. The olivine is sometimes unaltered but usually stained with iron oxides along cracks and marginally, and occasionally it is entirely replaced by iddingsite.
The feldspars are usually of the acid type, basic forms occurring only in the basic andesites and basalts. They frequently show zonal structure and denticulate margins, the last feature being specially characteristic of the base of the more acid rocks. Some of these denticulate feldspar laths have a lower index of refraction than balsam and may therefore be anorthoclase especially in the sodic rocks, but in other cases the index is higher than that of balsam and the minerals must be oligoclase or andesine, most likely the former.
It has not been found possible to section all the dykes, but fifty have been examined microscopically, and to the best of my knowledge no important type has been omitted. I quote the analyses kindly supplied by the Dominion Laboratory, and none previously published are listed here, but all that have appeared to date are quoted by the present author (loc. cit., pp. 130–1) in his paper on the Intrusive Rocks of Banks Peninsula. These recent analyses furnish a useful supplement to the list there given.
A perusal of the list given herewith indicates the presence of trachytoid rocks with a moderately high percentage of soda, which is accounted for by the presence of oligoclase and andesine and by aegirineaugite. The basic rocks do not show any special abnormality in composition, except a fairly high percentage of titanium. The water content of the specimens submitted is very high considering that all were obtained from rock cuttings and every care was taken to get as fresh a sample as possible; in some cases some amount of decomposition appeared to be inherent in the rocks as if it contained some percentages of unstable mineral.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|No. 1||No. 2||No. 3||No. 4||No. 5||No. 6||No. 7|
|Water lost above 105° C.||1.00||1.79||1.38||2.32||1.14||0.44||1.45|
|Water lost below 105° C.||2.54||2.22||3.12||4.98||1.96||1.22||0.92|
|CO2||trace||nt. fd.||nt. fd.||trace||nt. fd.||trace||trace|
|NiO||nt. fd.||nt. fd.||nt. fd.||0.01||0.01||0.03||0.02|
|ZrO2||0.06||nt. fd.||trace||nt. fd.||nt. fd.||nt. fd.||nt. fd.|
|Cr2O3||nt. fd.||nt. fd.||nt. fd.||nt. fd.||nt. fd.||nt. fd.||0.05|
|Cl||nt. fd.||nt. fd.||nt. fd.||trace||nt. fd.||nt. fd.||nt. fd.|
No. 1. Trachyte, Dyer's Pass ridge, I″. 4. ″2. 3. (4), Toscanose.
No. 2. Trachyte, Cass Peak, (I). II. 5. 2. ″4., Akerose.
No. 3. Soda Trachy-Andesite, Dyer's Pass ridge, II. 5. 2. 4., Akerose.
No. 4. Basic Andesite, occurring as an intrusion in No. 1., Dyer's Pass ridge, II. 4″. 3. 4., Tonalose.
No. 5. Basalt, Mt. Ada, II (III). 5. 3. 4., Andose.
No. 6. Feldspar Basalt, near Hoonhay Dyke, (II) III. 5. 3°. 4°., Camptonose.
No. 7. Dolerite. 4th dyke down cutting from Coopers Knobs, ″III. 5. 3″. 4., Camptonose.
[Footnote] * Amount of SrO determined spectrographically in the CaO by S. H. Wilson.
The most acid trachyte encountered is the soda-amphibole trachyte of Kennedy's Bush (Speight, 1908, p. 176), and it should be noted that this rock contains a considerable amount of soda-augite in addition to the micro-phenocrysts of the amphibole.
The most common type of trachyte is brown in colour, and there is also a frequent occurrence of a stain of oxide of iron which occasionally appears to be developed along spheroidal cracks and ordinary joints. A typical example is the large dyke (fig. 3) on the summit of the Dyer's Pass ridge. Crystals of feldspar show plainly in the hand specimen, and under the microscope they prove to be andesine with very occasional sanidine. The base is composed of short stumpy laths of sanidine, and there are as well a fair number of forms with denticulate margin and nearly straight extinction, both forms having a lower index of refraction than balsam, so anorthoclase is probably present in addition to sanidine. In between the laths are numerous grains of magnetite, some of which are no doubt derived from the augite. Towards the margin of the dyke the texture of the base becomes progressively finer, and this applies to the part with platy jointing, while at the margin of the dyke there is a selvage which is definitely glassy. In this facies the feldspar phenocrysts become more numerous and are apparently entirely of andesine; greenish crystals of aegirine-augite also occur, and the glass has developed well-defined perlitic cracks. This facies is the chilled margin of the mass and the increased proportion of phenocrysts is noteworthy. Analysis No. 1 is from this dyke. This is similar in all respects with the analysis of a dyke from Heathcote also cutting the Summit Road (Speight, 1923, pp. 130–1). I have little further comment to make on this rock except that the slides of specimens taken from the summit of the ridge show very little hornblende, no mica, and much more aegirine-augite as phenocrysts and microphenocrysts, and also there is an approach to a bostonitic habit in the base (fig. 5).
The intersection of Castle Rock, the large dyke that forms the crest of the ridge on the western side of Heathcote Valley, shows variations which are characteristic of the main dyke. First of all the interior is definitely trachytic, the feldspars being chiefly sanidine and occasional andesine; the microphenocrysts are of aegirine-augite; and the base of long laths of feldspar showing flow structure and also radiating sheaf-like forms (fig. 6), among which are small grains of greenish grey augite and magnetite. The feldspars have lower index of refraction than balsam and most of them show straight extinction, but there are a number which have an angle of extinction for albite, so in all probability both are present.
In a 6 ft. splinter of this dyke the features are similar except that the augite phenocrysts are definitely purplish, and olivine occurs unaltered and again replaced at times by iddingsite. The base is composed of microlites, some with straight edges and others with denticulate margins both with index less than balsam and with extinction angles of sanidine and albite. The texture is definitely bostonitic and it is probable that anorthoclase is present as well;
there are also frequent patches of tridymite. As noted previously (loc. cit., pp. 136–8) this dyke is very variable in character both along the strike and across it.
The type of trachyte mentioned first occurs as dykes at Giants Causeway, further west near the 4 mile post, on the eastern side of the Sugarloaf, on the ridge leading up to Mt. Ada, and elsewhere.
In addition to these normal trachytes there are others morebasic in character, and it is possible that they might be assigned to the trachy-andesites, andesine occurring as phenocrysts more commonly than sanidine. These rocks are usually grey or greenish grey in colour, an excellent example being that quarried on the north side of the road passing over the shoulder of the spur leading up to Cass Peak. This is very vesicular, sometimes showing in the hand specimen white crystals of feldspar and very occasionally small crystals of hornblende and also olivine. Under the microscope it shows phenocrysts of andesine and microphenocrysts of sanidine. The base is holocrystalline and consists chiefly of feldspar laths from .3 to .4 mm. in length, some with straight edges and others markedly denticulate; the index of refraction of some of the laths in about that of balsam, some a little lower, and the angle of extinction measured with the length is generally straight or nearly so, but some of the laths are twinned and show an extinction angle which with the index of refraction just below balsam shows them to be albite-oligoclase. The untwinned laths show straight extinction and are no doubt sanidine, but the denticulated margins suggest the presence of anorthoclase, though it was not determined for certain (fig. 7). In between the laths are numerous short crystals and laths of aegirine-augite, green-grey in colour and frequently stained with decomposition products of iron-oxide. Neither hornblende nor olivine appeared in any of the slides made. The chemical composition of this rock is given in analysis No. 2, which shows that it has a fairly high percentage of alkalies with dominant soda and a moderate amount of lime, which is in keeping with the absence of a basic feldspar. Composition, mineral content, and texture therefore indicate that the rock should be called a trachyte. It should be noted that the high percentage of water shown in these analyses reduces considerably the importance of other constituents in the percentage composition. The dyke which occurs just east of the main trachyte dyke on Dyer's Pass ridge (fig. 3b) might possibly be placed in this category, but the composition is definitely more basic, and it will be best to assign it to next section.
The rock just referred to belongs here. It occurs just east of the large trachyte dyke on Dyer's Pass ridge referred to earlier, marked b in fig. 3. In the hand specimen it is dark grey in colour but with a suggestion of very dark green as it occurs in the road cutting. On the joint surfaces it shows brown owing to a stain of iron oxide. It is generally slightly vesicular and in parts markedly so. Under the microscope it appears to be composed chiefly of
prismoids of feldspar with denticulate margins and of lengths from .3 to .4 mm., and with index of refraction slightly higher than that of balsam. The extinction angle is very low, so they must be oligoclase, with probably more acid outgrowths, the whole texture being strongly reminiscent of bostonite (fig. 8). Some few of the prisms of the base have an index of refraction lower than that of balsam and with approximately straight extinction, so some sanidine or anorthoclase is probably present. In between the feldspar laths are small grains and stumpy crystals of aegirine-augite, sometimes stained brown with oxide of iron, and there are grains of magnetite as well as some brownish alteration product. In this base are very occasional microphenocrysts of feldspar with refractive index higher than balsam, an extinction angle, as observed, of andesine, but the feldspar may really be more basic. The chemical composition of the rock is given in analysis No. 3. The importance of soda as compared with potash and the high percentage of water are the two most striking features. The former is reflected by the amount of albite in the norm, and the lime is reflected in the amount of anorthite. The rock has certain relations to andesite on account of the oligoclase in the base, and to trachyte by the presence of the more acid feldspar, even if in small amount. There is a close resemblance in composition to other undoubted trachy-andesites of the area, such as those at Evans Pass, Castle Rock, and Hoonhay (see Speight, loc. cit. pp. 130–1) and the group is extremely well represented. They are usually rocks of light colour, grey or greenish grey, as a rule, and show freely crystals of hornblende which reach 1 cm. in length, though they are usually less. Under the microscope the phenocrysts are hornblende, augite, plagioclase, olivine in that order of importance, the plagioclase being either andesine or labradorite. The hornblende is brown with a faint tinge of green, at times resorbed on the margins; the augite is grey and then again faint purplish in colour, the olivine fresh or altered to iddingsite, either as crystals up to 1 mm. in diameter or as smaller grains between the feldspar of the base; microphenocrysts of sanidine also occur. The groundmass is composed of microlites of sanidine either quadratic in section or in lath-shaped forms, and there is an admixture of forms with higher index of refraction than that of balsam and with the extinction of oligoclase-andesine, and frequently with denticulate margins. In among them lie stumpy forms of greenish augite and grains of magnetite.
The rocks are related to the ciminite of Washington, but they have too much soda in relation to the potash, too much olivine, and the feldspar is somewhat too acid as well; it will therefore be best to call them merely trachy-andesites. They furnish another instance of the association of hornblende with rocks of this class.
A good example (13 ft.) can be seen on the rock cutting east of Heathcote Valley, another (10 ft.) on the saddle east of the Sugarloaf, the latter showing numerous inclusions of large hornblende with and without matrix attached, as well as a foreign block of granitoid rock 12 inches in diameter. This inclusion is formed chiefly of brown hornblende but contains some diallage as well, a little
biotite, and frequent crystals of apatite and ilmenite, and a subordinate amount of granular feldspar, the granular form being apparently the result of pressure; some of this is andesine, and none was observed with a lower index than that of balsam. This rock is in all probability a diorite or diorite gneiss and may be the deep-seated equivalent of the trachy-andesitic material of the dykes just referred to.
Basic Andesites and Basalts.
Typical andesites are apparently not represented but basic andesites verging on basalts occur freely. They are found first of all as narrow dykes, dark in colour, and frequently as one of the members of a multiple dyke. A case illustrating this is furnished by the assemblage associated with the large trachyte dyke on the Dyer's Pass ridge about half a mile west of the pass. In the hand-specimen this is dark coloured with a fine-grained base in which crystals over 1 cm. in length show up occasionally; the rock is deeply weathered, a feature which applies in other cases. Under the microscope the phenocrysts prove to be labradorite with very occasional augite; the groundmass is composed of feldspar laths the great majority of which have the low extinction angle corresponding with oligoclase and often have denticulate margins, but there are microlites as well without such borders and with the extinction-angle of andesine. If only one plagioclase can be present in such a groundmass they must all be andesine. There are as well grains of magnetite and patches of material with no effect on polarised light, in all probability a glass. Numerous cavities also occur lined with fibrous chalcedony marginal to a brownish aggregate occupying the interior with an index lower than that of balsam, and having some effect on polarised light at minute points, which are occasionally arranged in definite parallel lines in one section of the amygdule and with a similar arrangement but oriented differently in another section. Analysis No. 4 is of this rock. It shows a high percentage of water, and when this is allowed for the composition becomes that of a basic andesite; the norm, too, shows a very high percentage of quartz, no doubt accounted for by the presence of chalcedony.
Similar rocks marginal to trachytes occur on the crest of the ridge at the head of Heathcote Valley, and with some glass in the base as well as numerous vesicles full of yellowish green alteration products. The microlites of the base appear in two forms, one lathshaped and the other more quadrate, both with index higher than that of balsam. These occurrences appear to be restricted to only short lengths of trachyte dykes, and they appear to have no close relations in composition to them, that is, they are in no sense composite dykes arising from differentiation of a common magma.
Andesite forms the complex dyke east of the Heathcote Valley saddle and occurs much decomposed on the side of the road crossing the ridge leading up to Mt. Ada and near Rhodes's Bush on the south side of Cass Peak. An undecomposed rock also occurs on the southern side of the Sugarloaf as a moderate-sized dyke (6 ft.) cutting the road twice. This contains a few phenocrysts of plagioclase
Fig. 1.—Multiple Dyke, Heathcote Valley; trachy-andesite (A) in the middle, with margins of basic rock, 18 in. on left (B) and 5–7 feet on right (C) the basic rock being probably intruded along the walls of the trachytoid dyke.
Fig. 3.—Multiple Dyke on bdgo west of Dyer's Pass; trachyte (A) in middle; darkcoloured tracchy-andesite (B) on left; basic andesite (C) on middle right following up junction of the cross with the platy jointing of the trachyte. The marginal trachytes (D) on both sides show platy jointing, and that on the right contains basic andesite (E), which no doubt owes its position as well as the increased thickness of the platy trachyte (D) on that margin of the main dyke to faulting. On the extreme left (F) is a basalt which shows fluting of the internal part on the external platy facies, while (G) is country rock.
Fig. 5.—Trachyte, Heathcote, showing phenocryst of sanidine in base composed of feldspar laths, many of which show denticulate borders, the texture being trachytic with an approach to bostonitic.
Fig. 7.—Basic Trachyte, Cass Peak, showing ground-mass of sanidine and albite-oligoclase laths, many being denticulate; the texture is bostonitic.
In a hyalopilitic base with lath-shaped microlites of andesine and grains of augite; and andesites of this grade occur on the cutting leading down from Coopers Knobs to Gebbies Pass. One of these is composed of phenocrysts of labradorite in a base of oligoclase microlites, augite grains and broken-comb forms of ilmenite and patches of brownish glass as well as amygdules of fibrous chalcedony. Other cases of this type show oligoclase microlites with denticulate margin in the base, and in one case the phenocrysts are of labradorite and augite in a base of andesine microlites often arranged with radial grouping and at times showing flow structure, and in between the laths are grains of augite and patches of alteration products green and brown in colour, the amount of F.M. mineral present being unimportant.
A special case of basic andesite occurs as a dyke on the north-east face of Witch Hill just east of the Giants Causeway. The rock is green-grey in colour and under the microscope shows phenocrysts of feldspar up to 1 cm. in diameter, slightly purplish augite, occasional olivine in grains up to 0.1 cm. in diameter, and grains of brownish-black magnetite. The base consists of laths of oligoclaseandesine usually showing flow arrangement, grains and short crystals of greenish to purplish augite, and grains of magnetite. There appears to be a very occasional feldspar which has an index of refraction slightly lower than that of balsam, but by far the great majority of the microlites have a higher index. Amygdules of chalcedony and of tridymite also occur as well as small patches of a greenish anisotropic mineral.
These rocks grade into undoubted basalts, and probably some of them might be classed as such, but it is difficult without analyses to say definitely at times what group they should be assigned to. One of these border-line cases is the basalt with groovings referred to elsewhere in this paper. This is an even-grained rock, without phenocrysts, and composed of laths of oligoclase, grains and short crystals of augite, and grains of magnetite. Sections taken with orientation parallel to the walls of the dyke show no difference in composition or texture from sections taken at right angles. Segregations of coarser texture, most composed of feldspar, also occur.
Undoubted basalts form the 12 ft. dyke on the east side of Heathcote Valley saddle, the core of the multiple dyke near mile post 4, and a dyke cutting the road on the south side of Mt. Ada. The last of these is dark in colour with feldspar and olivine showing occasionally in the hand-specimen. The feldspar proves to be labradorite, but it often has a more acid border. The F.M. minerals are faint violet augite and olivine in grains and aggregations up to 2 mm. in diameter, the latter sometimes fresh and again partly replaced by iron oxides. The holocrystalline base is composed of feldspar laths, augite grains, small olivines and grains of magnetite. The feldspar which makes up the bulk of this rock is of two types, the first, and more common, has approximately straight extinction and has denticulate border, while the other has an extinction angle corresponding to basic andesine, both with index of refraction higher
than that of balsam. If only one feldspar can be present it must be andesine, but if two can co-exist, then there appears to be oligoclase in addition, the extremely large proportion which shows the extinction angle of oligoclase being in support of the contention that both are present. No appreciable difference in texture or composition of the inside from the outside of the dyke was disclosed in slides selected from various parts, although the hand-specimens did suggest a difference.
Analysis No. 4 is of this rock. The most striking feature is the amount of normative albite, which seems to bear out the hypothesis that oligoclase is present in the base.
Some of the rocks which look like andesites in the hand-specimen prove to be undoubted basalts when examined microscopically. A good example of this is the dyke (6 ft.) which crosses the road a few yards south of the Hoonhay dyke and which presents a facies strongly reminiscent of the olivine andesites or feldspar basalts so characteristic of the effusive rocks of the area. In this there are numerous phenocrysts of labradorite 1 cm. in length; olivines up to 2 mm. in diameter with the usual alteration products; somewhat occasional augite; in a base composed of microlites of andesinelabradorite frequently with denticulate margin, grains of augite, small brown olivines, and grains of magnetite. Its composition is given in analysis No. 5, which shows that it is definitely a basalt.
The most basic dyke judged microscopically is the fourth down the cutting from Coopers Knobs and 12 ft. in thickness. The phenocrysts take up fully 50 per cent. of the slide and consist of labradorite, angite, and olivine in sub-equal proportions. The augite is light brown in colour, the olivine sometimes fresh and sometimes seamed and bordered with iron oxide. The base is holocrystalline and composed of short laths of labradorite, grains of augite, olivine, and magnetite; needles of apatite and alteration products of iron-oxide occur as well. Analysis No. 7 gives the composition of this rock.
I have left to the last all reference to the hornblende basalts which present certain interesting features. This type has been referred to by Bartrum (1917, p. 416) and by the present author (1923, p. 144), the rock referred to being a dyke in Sumner Valley which does not appear to cut the Summit Road. An excellent example can be seen in a dyke (6 ft. 6 in.) on the rock cutting east of Heathcote Valley (fig. 4). In the hand-specimen this rock is grey with a slight tint of green; it is very vesicular and shows crystals of hornblende up to 5 mm. in length, also very occasional feldspars in a base which shows well-marked flow structure. Under the microscope the phenocrysts appear as brown hornblende, sometimes resorbed; light purplish augite in sporadic crystals and in concentrations; olivine, fresh and stained with iron-oxide; very occasional microphenocrysts of plagioclase with higher index of refraction than balsam; grains of faintly brownish magnetite (?titaniferous); all in a base of lath-shaped microlites and more quadratic forms of andesine, twinned and untwinned, with definite flow arrangement, forms of greenish grey augite and grains of magnetite. This basalt is entirely different in facies from the other basalts cut by the road.
It is not intended in the present article to refer in particular to the rocks into which these dykes have been intruded. It will be sufficient to say that they consist of flows of feldspar basalt or basic andesite and ordinary basalts, coarse agglomerates of the same materials, and finer ash beds, some of the last simulating at times the andesitic flow macroscopically. Their age is late Tertiary, perhaps even early Pleistocene.
I have also in addition to acknowledgements mentioned earlier to record my indebtedness to Miss Thelma Kent, A.R.P.S., for the micro-photographs, and to Mr G. Stokell for material assistance also in connection therewith.
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