The Geology of Siberia Hill and Mount Dasher, North Otago
With an Appendix
Notes on Alkali Feldspar, Phillipsite and Clinopyroxenes from Siberia Hill and Mount Dasher
By D. S. Coombs
[Received by the Editor, November 22, 1954.]
A small area in the Kakanur Ranges, west of Oamaru, includes a number of Pliocene lava residuals resting on a faulted, peneplained surface of semi-schist, and in places, a thin veneer of Late Cretaceous coal measures. The igneous rocks are alkali-rich basalts and basanites, an unusual example of which has been analysed. Large olivine nodules are commonly present. Volcanic agglomerates and scoriaceous material indicate the presence of eruptive centres nearby.
|General Account of the Aiea||348|
|The Igneous Rocks||352|
|Flow and Dyke Rocks||354|
|A. Siberia Hill—Crater||354|
|C. Mount Dashen||362|
|2. Richly zeolitic basanites||363|
|3. Olivine nodules||366|
|Note on the Nature of Olivine Crystals in the Lavas||368|
|Summary of Geological History of Aica||368|
|Appendix (by D. S. Coombs)—Notes on Alkali Feldspar, Phillipsite and Clinopyroxenes from Siberia Hill and Mount Dasher||369|
|Alkali feldspar from basanite||369|
|Clinopyroxene in zeolitic basanite||370|
This account records observations made in mid-January. 1954, during a visit to the Kakanui Range, west of Oamaru, for the purposes of mapping in detail and collecting from the more inaccessible parts of a volcanic mass occurring in the area known as Siberia Hill, and including the prominent points of Kattothyrst and Mount Dasher. I had already mapped the southern edges of this
mass in 1938 during the geological survey of Moeraki Subdivision, and the present paper completes the account of the volcanic rocks that extend northwards beyond the limits of that subdivision.
The field sheet for this investigation was based on the rather meagre information provided by Provisional 1-Mile Sheet S136 (Oamaru). Only one really reliable fixed point, namely Trig. C (G.R. 164582), is located in the area, and even this has a very limited visibility because it is situated on the south side rather than the summit of the hill. Pace and compass traversing was undertaken, but on the volcanic terrain, compass bearings were recorded with some misgiving, for differences of up to 30° were observed between forward and back sights.
Heights were recorded with a Paulin Altimeter checked back morning and evening on to Trig. C and from these, a fairly reliable set of form lines has been drawn to show the topography of the area.
A special word of thanks is due to Mr. Robert Gray and Mr. David Kane of the Dasher Station for providing horse transport and other amenities which enabled me to establish a base camp at Siberia Hill, some ten miles beyond the end of the road at the Dasher Station homestead. Without their assistance, this investigation would have been impossible in the time available.
I am also very grateful to Dr. D. S. Coombs of this Department for many observations and helpful comments connected with the petrology, including an appendix on the nature of certain minerals; to the Dominion Analyst for making a rock analysis; to Mr. D. Hamilton, also of this Department, for drawing my attention to a number of useful references; to Mr. N. J. W. Croxford for making universal stage measurements; to Mr. L. Seeuwen for photographic work; and to Mrs. Y. Sheddan for preparing the typescript.
General Account of the Area
The area dealt with in this paper totals about 12 square miles and its centre is situated about 22 miles south-west of Oamaru, that is, in Lat. 45° 10′ S., Long. 170° 32′ E. (see fig. 1). Most of the country, excepting the valleys, is over 3500 feet above sea level, and includes Siberia Hill (Trig. C. 4180ft), Crater (4350ft), Kattothyrst (4220ft) and Mount Dasher (4330ft). The area lies in the central portion of the Kakanui Ranges which stretch inland from the coast at Shag Point (as Horse Range), gradually rising north-westwards to Mount Pisgah (5394ft) and beyond to Mounts Nobbler (5092ft) and Alexander (4431ft). The Kakanui Ranges form the most easterly of the great tilted blocks of the Central Otago tectonic complex. The southern part, at least, of their steep south-western face is a fault-fold scarp, the Waihemo Fault Complex.
The basement rocks, where exposed at the surface in the Kakanui Ranges, consist of greywackes and semi-schists of the Chl. 1-3 sub-zones of Turner (1935, p. 344). In the course of the present investigation, these rocks were examined in passing, but no fossils were found that might have given some satisfactory mdication of their age. All known evidence suggests that they are Palaeozoic sediments metamorphosed in Late Palaeozoic or Mesozoic times.
Across this basement, a well-marked erosion surface known as the Cretaceous Peneplain has been cut, and upon this surface, where it has not been removed
Text-Fig. 1.—Locality map of Siberia Hill-Mount Dasher Region (in square). Black areas show distribution of Pliocene igneous rocks in North and East Central Otago. Contours approximate at 1000ft intervals.
by subsequent erosion, there lie remnants of a series of rocks ranging in age from Upper Cretaceous to about Lower Miocene. A second, better-preserved erosion surface, known as the Late Tertiary Peneplain, transects the Cretaceous Peneplain towards the western side of the Kakanui Ranges, and on it lie deposits ranging in age from Pliocene to Recent, including the igneous material.
In the small area under discussion, the two peneplain surfaces are almost coincident, the lower marked by a thin veneer of Late Cretaceous quartz-conglomerates resting upon the semi-schist, and the upper covered by Pliocene volcanic rocks and Recent deposits. The convergence of the two surfaces in the area east-north-east of Siberia Hill is clearly seen in Benson's section (1941, p. 212, fig. 2, section X-X).
The coastal terrain bordering the present area rises at first gently westwards along the surface of the stripped Late Tertiary Peneplain (Plate 12, fig. 2). The peneplain is preserved as interfluves strewn with basalt boulders, residuals of lava flows from the volcanoes that erupted during Early and Middle Pliocene times, mainly prior to the great deforming movements of the Kaikoura Orogeny. The relations between the volcanic eruptions and the orogenic movements are clearly seen along the borders of Shag Valley especially at Green Valley. Here volcanic rocks genetically related to and evidently coeval with the Siberia Hill volcanics as shown by Benson (1942a, p. 87 et seq.) are folded into a shallow
syncline where they abut against the scarp of the Waihemo Fault Complex which is, in this region, a fault-fold (see Benson 1941, p. 216).
The mass of Siberia Hill is seen from Oamaru to stand out clearly against the western horizon and rises abruptly in a series of lava terraces some 500-600 feet above the general level of the peneplain surface. The somewhat steep faces of Siberia Hill outline a narrow area about one and a quarter miles long and, on the average, a quarter of a mile wide, elongated in a west-north-west direction. The summit is rather flat except where broken by portions of columnar lava or of tabular laminated lava standing in places as pinnacles. To the west of Trig. C itself are small areas of dew ponds and Sphagnum bog, much disturbed by the rootings of wild pigs. Elsewhere, this small plateau is covered by tussock amongst which are large spalls and columnar fragments of lava making traverses very arduous.
To the south-west of and nearly 200 feet higher than Trig. C is a great tumbled mass of lava blocks known as the Crater. As considerable quantities of coarse volcanic agglomerate have now been found in this area, the name is probably not inappropriate. Beyond the Crater to the south-west, volcanic rocks in place and as scattered residuals extend for about a mile along the rising boundary-fence ridge leading to Trig. B (4681ft). It is here also that many remnants of Late Cretaceous quartz-conglomerates are found.
The Siberia Hill plateau also extends west-north-west as a tongue for about threequarters of a mile beyond Trig. C to a point here named West Pinnacle. Beyond this again, there is an abrupt fall to the semi-schist surface which forms a narrow ridge extending north-westwards to join Half-moon Spur. On either side of this connecting ridge, there is a rather steep fall of over 1000 feet to the valleys of the streams that drain this mountainous country. From the volcanic ridges above, there extend in many places down the steep slopes, great tongues formed of blocks of volcanic rock, most vividly seen on the slopes of Mount Dasher (Plate 14, fig. 1).
About one and a quarter miles west of Trig. C stands Kattothyrst (4220ft), a lava residual rising some 200 feet above the semi-schist basement (Plate 13, fig. 1). There is good evidence as shown below that this is an original centre of eruption.
At the south-western extremity of Half-moon Spur, that is, about two and a quarter miles north-west of Trig C, stands the large residual of Mount Dasher (4330ft) (Plate 14, fig. 1). The lavas here, resting on the semi-schist basement are approximately 500 feet thick and contain numerous large olivine nodules.
Finally, about a mile north-east of Mount Dasher, at Camelback, is a large dyke of columnar lava cutting through the semi-schist. This, together with Mount Dasher, from which it is completely separated by semi-schist terrain, forms the north-western limit of the Pliocene igneous residuals, none of which are known to occur on the Kakanui Peak - Mount Pisgah watershed beyond.
The area south-west of a line joining the Crater and Dasherette (G.R. 135603) is drained by the headwaters of Deep Creek which flows north-west-wards parallel to the strike of the semi-schist and the major fault direction of the Waihemo Fault Complex. It later turns north-eastwards and follows the normal drainage pattern of the North Otago coastal area.
The remainder of the area is drained by north-east-flowing tributaries of Kakanui River (South Branch) and, to the south of Siberia Hill, by Kauru River.
The mature profile of many of the stream valleys is very striking. For instance, the headwaters of Deep Creek (Plate 13, fig. 2) lie in a very wide V-shaped valley with the stream itself entrenched to a depth of about 25 feet in a rock-floored terrace. A similar feature is observed in the headwaters of Waianakarua River (South Branch), south of the present area, which also flows parallel to the strike of the semi-schist terrain, but which, when it turns to the north-east to follow its course to the sea, enters a series of steep, youthful gorges. There is, in fact, a marked variation in the rate of erosion of the semi-schist dependant upon the direction of stream flow with respect to the strike of the rocks.
Another notable feature in this area is the nature of the weathering of the rocks. This is particularly the case with the volcanic rocks which for the most part remain perfectly fresh even on the tongue-like screes on the slopes of the lava residuals. It seems clear that most of the weathering is mechanical in nature and is probably effected largely by frost-wedging and heaving during the winter months.
Faulting. In the valleys east of Siberia Hill and on the higher country between and west of Kattothyrst and Mount Dasher, the semi-schist (Chl. 2 sub-zone) forms prominent strike ridges directed approximately north-westwards, that is, parallel to the Waihemo Fault Complex, the rocks dipping away to the north-east at angles up to 70° (Plate 13, fig. 1).
That the dislocating movements of the Waihemo Fault Complex extend into the Kakanui Ranges for a considerable distance north-east of the Shag Valley fault-fold scarp is suggested not only by the disturbed nature of the high country at the western extremity of the stripped peneplain surface, but also by the north-westerly direction of flow of the headwaters of Deep Creek, Waddell Creek, and Waianakarua River (South Branch). The two last-named streams lie to the south-east of the present area.
More direct evidence of a fracture even further to the north-east of the Shag Valley scarp is seen along the south-western side of the semi-schist ridge which connects the Siberia Hill plateau with Half-moon Spur. Assuming that the lavas were crupted onto a relatively smooth surface (Late Tertiary Peneplain), one notes that on Mount Dasher the base of these lavas is at an elevavation of about 3800 feet whereas no lavas are present on the neighbouring semi-schist pinnacle known as Dasherette (4010ft) to the north-east. There is thus a throw of at least 250 feet down to the south-west along a fault which lies between these two points. Moreover, a steep cuesta-like scarp of semi-schist facing south-westwards stretches from Dasherette towards Mount Evelyn (G. R.108648) about three miles to the north-west and is collinear with the steep south-west face of the Siberia Hill-Half-moon Spur connecting ridge.
A similar situation exists in the vicinity of Kattothyrst. Here the base of the lavas is at an elevation of 3950 feet, but no lavas are present on a small semi-schist knob (4040ft) about half a mile to the north-east. They are, however, present at just over 4000 feet elevation at West Pinnacle, a further half a mile to the east. The effects of the fault are not so clearly demonstrated south-west of the Crater though there is a marked topographic drop from the
Crater to the saddle leading to the Trig. B ridge. The elevations of the base of the lavas on Kattothyrst and Mount Dasher suggest that the throw of the fault is increasing to the north-west. I name this fracture the Dasherette Fault.
The Igneous Rocks
The Siberia Hill - Mount Dasher group of volcanic rocks forms the western margin of an area of similar material, the present outcrop of which is estimated by Benson (1942a, p. 87) to be approximately forty-three square miles in extent, occupying part of a tectonic region—the Moderately Deformed Region—of Eastern Otago. It is clear that this is only a fraction of the original extent of the volcanic material.
Within the present area, Benson (1942a) has already described material consisting of isolated specimens collected by Dr. J. Marwick and myself in 1938 and 1941 (Localities 88-91). The microscopic nature of these rocks shows quite clearly that they are related to the Pliocene lavas of the Eastern Otago Province (Benson, 1941 and 1944, p. 105) which occur, in the main, south-west of the Waihemo Fault Complex along Shag Valley, and not to the mid-Tertiary lavas of the North-East Otago Province. They do, in fact, rest upon the surface of the Late Tertiary Peneplain and were extruded immediately prior to the main movements of the Kaikoura Orogeny. In this connexion, it is interesting
to note the roughly linear arrangement of the Crater, Kattothyrst and Mount Dasher, all showing evidence of having been original sites of eruption, parallel to the line of the Waihemo Fault Complex and, in particular, to the Dasherette Fault. The actual age of the volcanics can be further delimited for no basalt pebbles are present in the Kurow conglomerates of probable Miocene age (Gage, pers. comm.). This is negative evidence for a Pliocene or post-Pliocene age for the basalts. Moreover, Couper (1953, pp. 14, 70) has recorded probable Taranakian microfloras from lignites interbedded with Second Period phonolites in the Waitati area north of Dunedin. It is with these Second Period rocks that Benson (1942a, p. 104) correlates the Waipiata dolerites, and by analogy, the Siberia Hill volcanics.
From an examination of the streams draining from the Kakanui Peak ridge, lying west of Deep Creek, it is fairly evident that no further volcanic remnants exist in this region west of Mount Dasher.
In view of the marked occurrence of alkaline rocks in the Siberia Hill and Mount Dasher residuals, it is perhaps of interest to note the presence in the rocks of the Chl. 2 sub-zone (or lower part of Chl. 1 sub-zone) of a large lensoid mass of crystalline marble, known as the Blue Mountain Formation. At the Blue Mountains (G.R. S146/310303), south-south-eastwards along the strike from Trig. B (Siberia), this marble is over 600 feet thick. The nearest known occurrence of the marble is in the headwaters of Waianakarua River (Middle Branch) two miles south-south-east of Conical Peak (G.R. S146/226417), that is, about 10 miles from the alkaline igneous rocks of Siberia Hill and Mount Dasher. However, even if these calcareous rocks lie below the surface in the present area, the lavas must have risen through a very considerable thickness of psammitic rocks before eruption.
Some of the most significant discoveries of the present investigation are the large masses of volcanic agglomerate in the vicinity of the Crater, Kattothyrst and Mount Dasher.
The most extensive of these occurs along the north-western slopes of the first-mentioned locality (G.R. 154576). Here a mass of agglomerate forms a prominent face about 10 feet high and 40 feet long in outerop at an elevation of 3970 feet near the head of the stream that leads into Deep Creek along the south side of Kattothyrst. The material is irregularly aggregated and consists of masses up to one foot across, chiefly of volcanic ejectamenta—broken basaltic columns, red and black scoriaceous material and, in one instance noted, a small volcanic bomb, pear-shaped and about eight inches in length. Also included are small angular fragments of semi-schist, rounded pebbles of quartz, and red baked clay. The quartz pebbles are of interest as indicating the former presence of a cover of Late Cretaceous coal measures between the Cretaceous and Late Tertiary Peneplains through and over which respectively the eruptions took place. These pebbles are also found in large numbers amongst the thin, weathered, basaltic residuals resting on the semi-schist ridge that leads towards Trig. B.
To the west of the Crater, at the boundary-fence corner (G.R. 153572), there occur several large, isolated, boulder-like masses of volcanic agglomerate about six feet in diameter (Plate 13, fig. 5). These masses contain large blocks
of columnar and rounded basaltic material up to four feet across, pieces of semi-schist up to two inches long, rounded quartz pebbles and several olivine nodules, the last-named not having been seen in the lower outcrop described earlier. The matrix of the agglomerate is dark, yellowish-brown in colour, and amongst the weathered debris on one of the masses, a large angular piece of quartz seven inches across was found. (Plate 13, fig. 6.)
Sporadic outcrops of agglomerate occur at about the same elevation along the western slopes of the Crater as one proceeds along the boundary fence towards Trig. C. There is a marked increase in the amount of fragmentary semi-schist in these outcrops, and here also are numerous blocks of basaltic material, considerably more weathered than the lavas in place at the Crater itself, and containing inclusions of semi-schist and olivine nodules.
The occurrence of this mass of agglomerate and the presence in it of extremely coarse material and of relatively delicate scoriaceous constituents clearly suggests the location of an eruptive centre on the western side of the Crater.
Another small mass of weathered tuffaceous and scoriaceous material associated with a dyke containing small pieces of semi-schist is seen at the north base of the volcames that form Kattothyrst. The semi-schist basement in the immediate vicinity of the dyke is noticeably whiter in colour than usual.
Finally, a small patch of volcanic agglomerate occurs on the south side of a small stream that drains into Kakanui River (South Branch) opposite Half-moon Hut. (G.R. 142605.) The largest fragments in this agglomerate are only about three inches across and contain a much higher proportion of semi-schist than in the other known outcrops of agglomerate described.
It is of interest to note that Benson (1942a, p. 112) recorded the presence of very massive and extensive bedded tuffs and agglomerates associated with the basaltic flows near the head of Shag Valley between Locs. 74 (Hill west of Pigroot Hut, 94 chains S.e. of Trig. L. Swmburn SD) and 75 (Beside road immediately west of Round Hill, Swmburn S.D.) and suggested that they might indicate the proximity of an important explosive vent. These localities lie immediately west and abreast of Siberia Hill across the Waihemo Fault Complex.
Flow and Dyke Rocks
A. Siberia Hill–Crater
This is the largest mass of volcanic rocks in the area and extends eastwards beyond the boundary of the detailed map along the ridge top for about three miles to Three Brothers Rocks (G.R. 221583). There is marked terracing of the lavas, possibly representing distinct flows, along the north-eastern face of Siberia Hill.
These rocks appear to form the lowest portion of the volcanic pile and are exposed along the eastern, northern and southern flanks. There is also a small occurrence on the western flank, but this is evidently much thinner than that on the east.
Atlantites, which may also be termed ankaramite-basanites, contain a predominant amount of ferromagnesian minerals, among which augite exceeds olivine in amount (see Benson, 1942a, p. 107). Typical examples may be cited. The first (11360) is from the small pyramidal mass, about thirty feet high, east of Siberia Hut (G.R. 181576) (Plate 12, fig. 2, extreme left), formed of reclining columns
Text-fig. 3.—Geology Sketch Map of Siberia Hill-Mount Dashei Region. Form lines drawn with aid of Pauhn Altimetei. Other topographical data and [ unclear: ] lines taken from Lands and Survey Department Provisional 1-Mile Sheet S 136 ( [ unclear: ] ). Boundaries of various lava flows are approximate only, and the extent of the screes of volcanic [ unclear: ] is not indicated.
dipping at 35° N.W. It has a most marked effect on the compass needle. In section, there are crystals of titanaugite (< 1.5 mm) and olivine (< 3.0 mm) set in a groundmass consisting mainly of idiomorphic prisms of augite with some microlites of feldspar, much magnetite and interstitial clear base, probably nepheline.*. There is some evidence of fluxional arrangement of the components.
The second (11383) is taken from the prominent rocky point about forty chains N.N.E. of Trig. C, probably portion of a northerly-trending dyke. The rock consists of idiomorphic crystals of olivine (< 1.0 mm) showing alteration to iddingsite along their peripheries, set in a groundmass of idiomorphic titanaugite prisms (< 0.2 mm), magnetite (< 0.05 mm) and a considerable amount of clear base, again probably nepheline.
A specimen from the west side of the mass (11380), about a quarter of a mile south of West Pinnacle, shows in section crystals of idiomorphic olivine (< 2.0 mm, 2V∝ = 96 — 82°) with undulose extinction and undoubtedly xenocrystic in nature derived from olivine nodules, and zoned titanaugite (< 0.8 mm, 2Vγ = 57°) with numerous inclusions. The groundmass consists mainly of idiomorphic prisms of titanaugite, tiny feldspar microlites in moderate amount, magnetite grains and interstitial clear base much of which is probably nepheline, but some, which does not take the stain, is probably a soda-rich alkah feldspar. Much of the nepheline forms sieve-crystals with the pyroxenes. There are also numerous patches of strongly pleochroic biotite and many needles of apatite.
These form the bulk of the western portions of the volcanic mass and appear to rest on the atlantites. At the extreme south-west end along the boundary-fence ridge leading to Trig. B, there are exposed fine-grained basalts with well-marked flow lamination (11372-11374). In thin section, they are fluidal feldspar-basalts with small phenocrysts of olivine (< 0.8 mm) and feldspar (< 0.7 mm) with dominantly simple twinning, set in a groundmass of feldspar microlites, idiomorphic prisms of pyroxene and magnetite grains. There are occasional clots of pyroxene. Benson (1942a, pp. 100, 102) also recorded a feldspar-basalt (5769) collected by Dr. Marwick from the western extremity of the volcanic mass. (Loc. 89b.) This rock he distinguished from that forming the extensive residual of Trig. H, Highlay S. D, by the presence in it of biotite flakes.
At West Pinnacle, the basalts stand in vertical polygonal columns each about three feet across (Plate 13, fig. 3). The rock 11361, 11369) is coarse grained and has a markedly warty surface. It is pilotaxitic in texture and contains numerous idiomorphic crystals of olivine (< 4.5 mm) partly altered to iddingsite. Less common are phenocrysts of titanaugite (<3.0 mm), much corroded and embayed. Many of these phenocrysts completely enwrap feldspars, laths of which (< 0.75 mm) make up the bulk of the rock. The feldspars (An60) mostly exhibit simple twinning though a few are multi-twinned and show quite strong zoning. Also included in the finer material in the groundmass are idiomorphic grains of pyroxene and magnetite and small needles of apatite, together with a small amount of clear untwinned material which is probably nepheline. There
[Footnote] * The identification of the interstitial material is based on Shand's methylene blue staining test (1939).
are also occasional patches of zeolite, in one case containing an embedded aggregate of idiomorphic pyroxene prisms, similar to the example from Round Hill, Upper Shag Valley, figured by Benson (1942a, p. 106, fig. 2). The present rock from West Pinnacle may best be described as an olivine-feldspar-basalt, though much of the olivine may be xenocrystic in nature.
Similar fluidal basalts occur along the southern portion of the mass at South Pinnacle (G.R. 161574) (11346, 11347), the latter specimen consisting mainly of laths of feldspar (< 1.5 mm) with large iddingsitized olivines (< 3.0 mm). There occur also the remains of a quartz pebble, in process of reaction, surrounded by idiomorphic prisms of diopsidic augite arranged normally to the surface of the quartz nucleus. There are also numerous tiny pockets of dark green material, probably celadonite.
On the low shelf south-east of the Crater are large spheroidal or ovoidal masses of basalt up to five feet in diameter. These exhibit perfect spheroidal weathering, an unusual form in the present region In section (11370), however, there seems little, apart from a rather higher content of ferromagnesian constituents, to distinguish these rocks from other feldspar-basalts.
Along the upper eastern slopes of Siberia Hill, feldspar-basalts are also exposed. For instance, at a point a quarter of a mile east-south-east of Trig. C, they form vertical columns each 4 ½ feet across of rather coarse texture (11338). The phenocrysts are mainly feldspar in laths (< 1.5 mm) showing simple twinning and ranging in composition from An58 to An62. A few phenocrysts of titanaugite (< 3.0 mm) show hour-glass zoning and most of these possess a narrow outer corroded zone (< 0.1 mm thick) impregnated with magnetite grains The groundmass consists of smaller feldspar laths and idiomorphic grains of colourless augite. There is a remarkably large amount of finely divided magnetite, numerous tiny needles of apatite, and a few flakes of pleochroic biotite. Some very large olivine crystals (< 4.5 mm) are present and may be xenocrystic.
About 200 yards north of Trig C, the basalt (11378) is very similar to the one just described, but displays almost perfect fluidal arrangement of the feldspars.
Benson (1942a, pp. 100, 111) has already noted the presence of an abnormal probably zeolitized, olivine-nephelinite at the Crater (Loc. 89a, 5768). A fresh specimen (11350) collected from the tumbled angular blocks forming the crest of this prominence shows in section crystals of olivine (< 1.0 mm) partially iddingsitized, set in a groundmass of purplish titanaugite prisms with fluidal arrangement, subidiomorphic grains of nepheline (< 0.05 mm), magnetite grains and needles of apatite. The rock in section is, in fact, almost identical with the basic olivine-nephelinite described by Benson (1942a, p. 110) from a small conical hill three miles east of Middlemarch (Loc. 50, 5649). No biotite was seen though Benson reported this constituent in his material from the Crater (5768).
Just to the west of Crater are irregular vertical columns in place, the rock of which (11351) proves on sectioning to be closely similar to that from the Crater itself and also to material (11371) collected about a quarter of a mile distant from the Crater on its southern slopes. The only observable difference in 11351 is the appreciable amount of feldspar laths in the groundmass It might thus be more appropriate to call this rock an atlantite, but the high proportion
“Biotite Atlantite, rich in Anorthoclase” (5766), 15 chains south of Siberia Hill, Trig. C, Kakanui S.D. (Benson, 1942a, pp. 112, 113) (= Sanidine-basanite of this paper). F. T. Seelye Anal.
Feldspar Olivine Basalt (5762), Mount Difficulty, Trig. D, Kauru S.D. (Benson, 1942a, pp. 112,113). F. T. Seelye Anal.
Feldspathic Olivine Basalt from Hill west of Pigroot Hut, 94 chains S.E. of Trig. L, [ unclear: ] S.D. (Williamson, 1939, p. 65, Analysis No. 5.) F. T. Seelye Anal.
Fine-grained iddingsitic olivine basalt from Trig. H, Highlay S.D. (not Swinburn S.D.–see Benson, 1942a, p. 99). (Williamson, 1939, p. 65, Analysis No. 2.) F. T. Seelye Anal.
Westerwaldite, from The Stöffel, Westerwald, Germany. Average of two analyses quoted by Johannsen, 1938, p. 204.
Average composition of Late Tertiary basaltic igneous rocks in regions peripheral to Dunedin District. (Benson, 1944, p. 104, Analysis No. 14.)
Nepheline-basanite from Fernando Noronha (Campbell Smith & Burri, 1933, p. 430). S. Parker Anal.
of nepheline revealed by Shand's test and the association with the olivinenephelinite of the Crater shows that it must be grouped with the latter.
About 30 chains south-south-west of Trig. C, there occurs a small residual of rather unusual form, in which the rocks project upwards as a series of sharp spikes. In section (11376), the rock is seen to be composed of large numbers of idiomorphic crystals of olivine (< 2.0 mm) and titanaugite (< 0.75 mm) in approximately equal proportion set in a groundmass of idiomorphic augite prisms and magnetite grains with much interstitial nepheline. Here again, as with 11371 above, it is not easy to draw the distinction between nephelinite and atlantite.
No other nephelinites are known in the immediate vicinity, the nearest being that reported by Benson at Kauru Hill (1942a, pp. 110, 116).
4. Sanidine-basanites. (11331,11348,11349,11377,11379.) (See Analysis No. 1.)
These rocks which form the eastern summit of Siberia Hill, are characterized by their marked abundance of alkali feldspar, which often predominates over the ferromagnesian constituents. An isolated specimen collected by Dr. J. Marwick from a point 15 chains south of Trig. C (i.e, Loc. 90, 5766) has been analysed and identified as an anorthoclase-rich biotite atlantite (Benson, 1942a, pp. 100, 109, 113, Analysis No. 11). This type of rock has now been recognized in quite a number of places in the locality mentioned. It frequently contains small olivine and pyroxene nodules and weathers with a pale-grey crust slightly rusty on its outer surface.
A typical specimen is 11377 taken from horizontal columns about 10 chains S.W. of Trig. C. The rock contains occasional crystals of olivine and pyroxene of nodule origin, but is otherwise fairly even-grained. The bulk of the rock consists of feldspar mostly exhibiting simple twinning but also showing some multiple twinning with very fine laminae (for nature of feldspar, see Appendix). In this groundmass are scattered microphenocrysts of olivine, idiomorphic prisms of titanaugite (< 0.1 mm) and numerous granules of magnetite (<0.05 mm). Flakes of pleochroic biotite (< 0.4 mm) are plentiful and needles of apatite are also present (Plate 15, fig. 2). Staining suggests the presence of much interstitial nepheline, but appreciable quantities of analcime also occur. There is a considerable quantity of greenish mineral present, probably celadonite.
In one specimen (11348) taken from about half a mile east of the Crater, a vein of quite unweathered feldspar about 1 mm thick transects the rock and merges with the groundmass.
Such rocks in which feldspar is so abundant scarcely seem to correspond with the definition of an atlantite in which dark minerals are regarded as predominating over light, although in hand specimen the rocks are quite dark. I have therefore distinguished them from more usual basanites as “sanidine-basanites”. There is a strong resemblance in texture and mineral content to the rock named westerwaldite by Johansen (1938, p. 203), the chief difference apparently lying in the higher sanidine content of the present material. Chemically also, the westerwaldite has a rather lower FeO/MgO ratio and less alkalies (cf. Analyses Nos. 1 and 5).
These are stratigraphically the youngest rocks in the volcanic sequence and represent a late differentiate of the basic magma that gave rise to the Pliocene volcanics.
This very prominent point so like a volcanic neck in appearance, lies to the west of Siberia Hill and is separated from the latter by a shallow, rounded saddle in semi-schist (Plate 12, fig. 1). The igneous rocks forming the prominence are about 260 feet thick, and the summit consists of a short north-south ridge formed of columns of fine-grained atlantite with olivine nodules (11355). These columns dip at 30° W. and form the western slope, while the eastern face is steep and rugged. Olivine nodules are frequent throughout the mass. At the base at the north end, as has already been noted, there is a small mass of weathered tuffaccous and scoriaceous material associated with a dyke of columnar lava containing small pieces of semi-schist. The dyke rock (11357) consists of idiomorphic crystals of olivine and titanaugite, the latter with well-marked hour-glass zoning, set in a very fine-grained, almost glassy groundmass of augite, magnetite, and microlites of feldspar. This groundmass is even more fine grained where it comes in contact with flakes of semi-schist. A good deal of zeolitic alteration appears to have taken place, but the rock is essentially an ankaramite.
Along the western base of Kattothyrst, the columns are. in places, horizontal. A specimen from this locality (11352) shows phenocrysts of diopsidic augite (< 0.8 mm, 2Vγ = 54°) and fewer olivine crystals (2V∝ = 94 — 84°). Some of the augites are zoned and twinned. The groundmass consists of idiomorphic prisms of augite, small feldspar microlites with fluxional arrangement, magnetite grams, and a small amount of clear base, probably nepheline. Olivine nodules, some markedly corroded and embayed, are also present. An interesting, partially resorbed xenocryst of enstatite. 3.7 mm in length. surrounded by zonally arranged reaction rims of intergrown olivine and augite, was observed. This rock is transitional between the rock types ankaramite and atlantite. In this connexion, it is noted that Benson (1942a, p. 104) identified as ankaramite a specimen collected by me from the east base of Kattothyrst in 1938 (Loc. 88, 5746). This rock contains numerous small crystals of fresh olivine (< 0.6 mm) and a few of augite (< 1.0 mm). Idiomorphic prisms of titanaugite (< 0.05 mm) together with a small amount of magnetite and laths of labradorite (< 0.06 mm) make up most of the groundmass.
At the south-west end of Kattothyrst are two large dykes forming wall-like remnants. The more southerly dyke, with horizontal columns about 8 feet long, is directed north-west—that is, parallel to the regional strike of the semi-schist. The other and much larger dyke expands towards the west into a tholoid about 20 feet in diameter, consisting of radiating columns each about 6-10 inches across (Plate 13, fig. 4.) These contain many olivine nodules. The centre of the mass has been exposed by erosion, and a rusty coating on the surface of the rock causes this portion to stand out conspicuously against the black background of the tholoid making it visible from a considerable distance. The rock (11354) is a typical ankaramite, many of the olivine crystals in it probably being xenocrystic in nature.
The section also shows a most interesting inclusion, elliptical in cross-section, and about 4.5 mm in its longer diameter. It consists of an outer zone (< 0.5 mm thick) of augite prisms with much brown interstitial glassy material and grains of magnetite. Then follows a zone (about 0.25 mm thick) of augite and interstitial glass, but no magnetite. The inner zone of what may be termed the shell
of the inclusion is up to 0.2 mm thick and consists of diopsidic augite prisms, many of them elongated normally to the surface of and projecting into the central portion of the inclusion. This central portion consists first of a zone of somewhat variable width (< 0.3 mm) of clear isotropic material (n < C.B) into which the diopsides project. Finally, there is what may be called the nucleus which has a thin outer layer (< 0.1 mm thick) of pale brownish-green material in which are numerous skeletal crystals. This material also appears within the nucleus proper enclosing isolated clear areas. The inner portion of the nucleus consists of a mosaic of quartz crystals together with some typical arrowheads of tridymite identified as such by Dr. D. S. Coombs. The whole inclusion is evidently the result of reaction of the magma with a quartz pebble derived from the remnants of the Upper Cretaceous coal measures.
A rather unusual rock (11353) occurs south of the above-mentioned dyke. It is unusual because it weathers to a friable, crumby debris. In section, the phenocrysts (?) appear to have been removed and replaced by zeolitic material, faintly brown in colour under ordinary light. The groundmass consists of titanaugite prisms (< 0.2 mm), magnetite grains and interstitial clear base. No olivine was observed, but a corroded fragment of picotite (0.3 mm) was seen.
Although the lava at the south end of Kattothyrst can nowhere be seen in true contact with the semi-schist, there is a most noticeable change in the attitude of the basement rocks. To the south, the semi-schist strikes at 323° T, then changes quite abruptly near the contact with the igneous material to 342° T, maintaining a dip of 55° N.E.
In summary, the evidence provided by the presence of tuffaceous and scoriaceous agglomerate, fragments of semi-schist in the lavas that are in place, and disturbance and alteration of the adjacent semi-schist basement, indicates that Kattothyrst is also the site of original eruption in this area.
C. Mount Dasher
This outstanding residual, elliptical in plan, lies north-west of the Siberia Hill-Crater mass, at the south-western extremity of Half-moon Spur. (Plate 14, fig. 1.) The summit of the mountain may be described as flat-topped, though in its length of about 18 chains, it is quite narrow and rises almost 100 feet from the north-eastern end to the highest point at the south-west. Down its steep faces there spread like gigantic tongues, screes of uncemented basaltic blocks extending in places as far as Deep Creek, some 1800 feet below. The material in these great screes has an angle of rest between 45° and 50°, yet so heavy and stable are the blocks and so closely do they interlock that climbing such steep faces is done with relative case.
So far as is known, no geological examination or collection of material has been carried out previously on Mount Dasher. Williamson (1939, p. 21a) photographed the prominence from the Kakanui Peak ridge to the north-west and referred to it as an “igneous cone”, but evidently did not visit the locality.
The lava mass itself rests, as do the others described above, on the semi-schist basement, and is almost 500 feet thick at its south-western end. There are probably three or four flows present, but it is difficult to define these owing to the large amount of scree.
From the top to the bottom of the lava material, except near the northern end, one is struck by the great abundance of olivine nodules, either in place on
the lower slopes or as a series of deep avoid pits in the weathered igneous rock exposed on the summit.
On the south-western face of Mount Dasher are numerous salient dykes of columnar lava, and at the extreme southern end may be seen two flows, one above the other, of compact, fine-grained atlantite (11367), forming prominent buttresses and overhanging cliffs.
The major portion of the igneous rocks on Mount Dasher appear to fall within or close to this classification.
Those along the base of the igneous pile on the north-west side (11340, 11343, 11344) are all rich in olivine nodules and show considerable quantities of clear interstitial material, probably nepheline, forming in many cases characteristic “sieve” crystals with augite prisms. Specimen 11344, in fact, contains so much nepheline as to place it more properly in the nephelinites. In it the nepheline is aggregated into clots, which appear on the weathered surface as whitish-grey patches. It is in this particular rock also that one finds the largest olivine nodules, up to 10 inches in diameter.
The summit of the mountain also consists of atlantites (11335, 11365, 11364) with a pitted surface marking the former positions of olivine nodules since weathered out. In the specimen of blocky columnar lava from the south-west summit (11365), there is a considerable amount of dark brownish glass in the groundmass.
Finally, the rock which forms the overhanging bluffs and cliffs on the southern face of the mountain (11367) is similar to the other atlantites except that it contains numerous pockets of sheaf-like zeolitic material. One section also shows a large crystal of picotite (2.00 × 1.00 mm) with its edges deeply corroded and rimmed with feldspar.
2. Richly zeolitic basanites. (11333, 11334, 11339, 11342, 11362, 11363, 11368.)
(See Analysis No. 8.)
Near the north-eastern end of Mount Dasher, there occurs a very peculiar group of rocks, characterized in thin section chiefly by considerable quantities of dendritically crystallized mesostasis, large titanaugites and feldspar laths, and a paucity of olivine. On weathering, outcrops, even in place, have a rusty coating which causes them to stand out very clearly against the dark-grey or black weathering crusts of the surrounding rocks (atlantites and ankaramites). In hand-specimen also, the feldspar laths are clearly seen where the matrix has been etched out.
In thin section, for example in 11363 taken in place from near the north summit of Mount Dasher (at 4190 feet), the rock is seen to be composed of large, faintly pleochroic titanaugites (2V = 50–58°; < 3.5 mm long), feldspar laths (An42; < 2.0 mm long), and large square crystals of magnetite (< 0.5 mm) set in a brownish matrix packed with dendritic crystallites in the form of needle-like rays with secondary rays at right angles to the first. Much of this material is strongly pleochroic, reddish brown to pale brown and apparently is barke-vikite. Less strongly coloured crystallites with large extinction angle are plentiful. In other specimens (e.g, 11368) in which crystallization of the matrix has proceeded further, it is fairly clear that these latter skeletal crystals
are titanaugites. In this slide barkevikite is absent but skeletal iron ores occur. Acicular apatite is a constant constituent. Also, as shown in the photomicrograph of one of these rocks (11339), there is often marked ophitic structure developed in the phenocrysts (Plate 15, fig. 1.)
Again, as illustrated by 11339, there is marked similarity in texture to a sakalavite forming part of a pillow-lava suite at Ziaret Khodor (Bassit) in North-West Syria (Dubertret, 1953, p. 143, No. 1427, 1427a, Pl. 5, figs. 1–4), though the rocks are quite different in chemical composition. Coarse feldspars and pyroxenes are set in a groundmass of skeletal crystals like those of the Mount Dasher rocks.
Although the amount of olivine in these rocks is extremely small and in some cases it could not be detected (a marked contrast with the associated atlantites and ankaramites rich in olivine nodules), there are some slides that contain scattered, pale, serpentinous pseudomorphs after olivine. In 11334, moreover, there are tiny relicts of fresh olivine partially surrounded by serpentine (See Appendix.)
In 11342 (the analysed specimen), overall zoning of the plagioclase from An52 to An30 was noted. The most sodic zone, An30, is confined to the extremities of a crystal.
A zeolite, identified as phillipsite (see Appendix) occurs in pools and also apparently in the groundmass, interstitial to the plumose augite, barkevikite and minor iron ore, where it is possibly accompanied by alkali feldspar and perhaps feldspathoid. The mineral takes the methylene blue stain rather unevenly.
The vertical extent of this rock type is at least 50 feet (the lowest specimen found in place was 11368 at 4140 feet elevation, and the highest was 11363 at 4190 feet). Where seen in place it forms regular columns vertical at the base (11368) and sloping downwards at 45° in a south-easterly direction near the north summit of the mountain (11363).
It is difficult to understand the reasons for the formation of such an unusual type of rock and for the almost complete absence from it of olivines which occur in such profusion in the remaining portions of the igneous mass. The presence of the plumose crystallites naturally suggests rapid cooling of the magma, yet the phenocrysts of titanaugite and feldspar are very much larger than those seen in the associated ankaramites and atlantites. It is perhaps of interest to note that Lacroix (1893, p. 484) recorded that olivine nodules are generally found in the greatest numbers in dykes of small thickness, in compact basalts, and on the margin of quickly chilled flows.
It seems most probable that this particular rock occupied a subsidiary magma chamber from which the olivines had almost completely precipitated, and that before complete and undisturbed crystallization had taken place, the rock was suddenly chilled.
There are a number of points of interest in the analysis of this rock (Analysis No. 8). Although the silica content is moderate, the relatively high amount of soda and potash causes marked undersaturation as indicated by the Niggli quartz index (qz). On the other hand, the C.I.P. W. Norm places this rock in the dosalic Class II in contrast to most of the other analysed igneous rocks from the Moderately Deformed Region of Eastern Otago which fall in the salfemic Class III (cf. Analysis No. 6). In this way, the Mount Dasher basanite compares closely
|II.6 2.4.||II” 6”.2 4.||“II”. 6.2” (3)4.||II.“6 “3.4.||II.6 2.”4.|
|8. Zeolitic basanite from north end of Mount Dasher (11342) (G.r. 131601). Dominion Laboratory Analysis Aa. 1641. J. A. Ritchie Anal.|
|9. Nepheline basanite pegmatoid, Auckland Domain (Washington, 1917, pp. 570, 571). H. S. Washington Anal.|
|10. “Monchiquite” (?) from Fernando Noronha. (Campbell Smith & Burn, 1933, p. 421.). J. Jakob Anal.|
|11. Trachybasalt from Tristan da Cunha (Campbell Smith. 1930, p. 83) E.d. Mountain Anal.|
|12. Nepheline-monzonite from Serra de Monchique, Portugal. (Pereira de Sousa, 1927. p. 329.) Raoult Anal.|
with the nepheline basanite pegmatoid from the Auckland Domain described by Marshall (1907, p. 366; 1912, p. 307) and analysed by him and by Washington (1917, p. 571) (see Analysis No. 9). However, Benson (1942b, p. 175 and Fig. 3A) has made further observations on the Auckland material which show it to be a coarsely crystalline rock with large crystals of olivine, a mineral that is quite subordinate in quantity in the Mount Dasher rock. Benson (1942b, p. 176) has also pointed out the difficulties in naming this type of rock, an even more difficult task in the present case because of the dendritic crystallization of the mesostasis. In this connection, one may note that rocks of very similar composition from Fernando Noronha, an island off the east coast of Brazil, have been described as monchiquite by Prior (see Campbell Smith & Burri, 1933, p. 420) (Analysis No. 10). Another from Tristan da Cunha (see Analysis No. 11) is classed as a trachybasalt, while a third from the Serra de Monchique, Portugal (see Analysis No. 12) is termed a nepheline-monzonite.
There are also close comparisons to be made with analyses of basalts containing haüyne from Tahiti and the Bohemian Mittlelgebirge (see Washington, 1917, p. 573) though no trace of this mineral was seen in the Mount Dasher rock.
The “monchiquite” from Fernando Noronha (Campbell Smith & Burri, 1933, p. 420 et seq., Plate, fig. 4) is especially interesting as it possesses a glassy ground-mass crowded with extremely minute hair-like crystals thought to be of hornblende and augite. The phenocrysts, too, are of titanaugite and barkevikitic hornblende, olivine being absent. The major differences in the analyses lie in the considerably greater amounts of TiO2 and H2O± in the Fernando Noronha rock.
In spite of the considerable quantity of zeolite (phillipsite) observed in the Mount Dasher basanite, it is probable that much of the high alkali content is absorbed in nepheline and alkali feldspar which may consequently be more abundant in the mesostasis than observation indicates.
Since nepheline could not be positively identified and the zeolite is so abundant in the Mount Dasher rock, I prefer to call this rock a zeolitic basanite.
3. Olivine nodules.
These are not, of course, confined to Mount Dasher, but they are most abundantly developed there. The most striking examples are observed along the base of the lavas on the north-west face of the mountain. (Plate 14, Figs. 2, 3.) Here at an elevation of 3850 feet, a very fine-grained, columnar atlantite or nephelinitc (11344) contains large ellipsoidal masses, 9-10 inches in their longest diameter, of olivine and other minerals (e.g., 11345). This diameter lies parallel to the bedding planes of the igneous rock which cut across the lava columns in a direction 195° T and dip at 15° E. These nodules, the largest of which weigh (by estimation) about 8-10 kilogrammes, are by far the largest recorded in New Zealand,* though evidently quite small compared with some from other places, for Lacroix as long ago as 1893 (p. 484) stated that some
[Footnote] * Records of olivine nodules in New Zealand are few in number and, to my knowledge, are as follows:—Shrewsbury (1892, p. 368), in basaltic tuff at Northcote, Auckland; Thomson (1906, 1907), in the Kakanui Mineral [ unclear: ] , North Otago; and Benson (1942a, p. 98) and Turner (1942, p. 293) from Pliocene basalt at Kokonga, Central Otago. Mr. H. E. Fyfe (pers. comm) informs me that small nodules occur in tuffaceous material on Mount [ unclear: ] , South Auckland, and small examples are known from the Dunedin [ unclear: ] rocks.
Fig. 1.—Panotama looking north-eastwards from
B. ridge Mount Dashet with Mount
immediately behind at
Hill-Crater mass on right. The Crater is the dark area on the
Fig. 2.—Panotama looking south-eastwards from [ unclear: ] Hut (G. R.177376) showing surface of stopped Late [ unclear: ] peneplam. Note small [ unclear: ] on extreme left foreground ( [ unclear: ] —11360). [ unclear: ] surface on extreme [ unclear: ] falls [ unclear: ] to Shag Valley and is due to [ unclear: ] along Wathemo Fault [ unclear: ] .
Fig. 1.—View south-eastwards from Mount
showing steeply dipping semi-schist
Chl. 2 sub-zone Kattothyrst in
middle distance and the
Fig. 2.—Headwaters of Deep Creek west of Mount [ unclear: ] looking towards [ unclear: ] B. Note mature valley sides and entrenched stream.
Fig. 3.— [ unclear: ] feldspar basalt (11361) at West [ unclear: ]
Fig. 4.—Dyke [ unclear: ] about 20 feet in diameter formed by columns of [ unclear: ] (11354) 6-10 inches across. west side of Kattothyrst.
Fig. 5.—Large (6ft) blocks of volcanic agglomerate south-west of the Crater.
Fig. 6—Volcame [ unclear: ] south-west of the Crater showing (at left) large block of basalt and (above hammer) [ unclear: ] fragments of quartz 7 inches across.
Fig. 1.—Mount Dasher from summit of
. Note screes of
Dark rounded mass in left foreground is dyke
on west face of
Fig. 2.—Olivine nodules (11345) up to 10 inches across in atlantite (11344) at base of [ unclear: ] on north-west face of Mount Dasher. Note differential weathering.
Fig. 3.—Olivine nodule 6 inches across in [ unclear: ] north end of Mount [ unclear: ]
basanite (11339) from north side of Mount Dasher. White laths of
) in ophitie relation with
of titanaugite. These and black magnetites in a groundmass of skeletal
X 50 diameters.
Fig. 2.— [ unclear: ] of [ unclear: ] (11377) from Table Top [ unclear: ] Hill [ unclear: ] crystals, of augite and magnetite in a groundmass of [ unclear: ] with needles of apatite. X 120 diameters.
nodules “… peuvent atteindre et měme dépasser une centaine de kilogrammes.”
The present nodules are similar in all respects to those described so fully by Lacroix (1893) and others. In hand-specimen they are seen to be composed of a granular mass of green olivine crystals each up to 3 mm in diameter, intermingled with grey or pale-green diopsidic augite in lesser amount. Weathering of the olivines is relatively rapid, and most nodules either possess a rusty surface in which the pyroxene crystals stand out (Plate 14, Figs. 2, 3), or are eroded, leaving a deeply pitted surface well displayed along the summit of Mount Dasher. The contact of the nodules with the basaltic material appears always to be sharp.
In section (e.g., 11345), the nodules are seen to be composed in the main of highly magnesian olivines and lesser amounts of pale-green chrome diopside, enstatite, and grains of picotite, in other words, of typical peridotite composition.
Lacroix considered (1893, p. 483 et seq.) that the olivine nodules are “homoeogenous” (derived from the same magma as the enclosing basaltic rocks), “allomorphous” (torn from masses already consolidated in depth), and “antilogous” (formed by basic differentiation or original heterogeneity).
This view has been confirmed by Chudoba & Frechen (1941) in their studies of the lavas in the Siebengebirge and the Eifel. There the earlier separated olivines were more magnesian than the latter, and these authors also state that the formation of olivine nodules is most favoured in “dry” laccolithic foci so frequently found in the Rhenish lava province. Turner & Verhoogen (1951, pp. 140, 172) also favour the view that olivine nodules have originated by gravitational accumulation of olivine in basaltic magmas”… at depths sufficiently great to allow continuous crystalline masses with the mineral composition and fabric of peridotites to be built up.”
More recently, Harumoto (1952, p. 82) has considered the origin of olivine nodules in melilite-nepheline-basalts from Nagahama, Japan, and indeed, in nepheline-basalts in general. He considers them to have been derived from a gabbroic magma, but points out that the residual liquid, if any, from such a magma cannot be a nepheline-basalt. There seems little reason, therefore, to substitute a gabbroic parent magma for the more usually accepted source, namely, a material of wehrlitic or lherzolitic composition.
As Thomson (1916, p. 134) says: “The fact that the inclusions themselves form a differentiation series comparable with that which would have been expected had the various basic volcanic rocks enclosing them consolidated under plutonic conditions, suggests that they are actually fragments from such a series which has arisen from the differentiation and consolidation in depth of a portion of the primary basic magma.”
In the present area, it seems probable that this fragmentation was assisted by the extensive faulting that has occurred. In this connexion, it should be recalled that although the major orogenic movements of the Kaikoura Orogeny took place after the extrusion of the Pliocene basalts, the Waihemo Fault Complex is directed along a zone of weakness in which movements have occurred at least since Middle Cretaceous times.
In a recent paper, Ross, Foster & Myers (1954, p. 732) conclude that the remarkable likeness between olivine nodules and dunites indicates that both have
had a similar origin. This they consider to have been the earth's peridotite shell, the dunites having been brought up by profound orogenic processes and the nodules by eruptive processes. They consider (p. 733) that a mode of direct derivation from basaltic rocks is unlikely, and in support of this they state that “The dunites here considered do not show any relation to basalts, and in general occur in regions quite free from other igneous activity.” This contrasts with their record on p. 731 of nodule-bearing basalts occurring in “close proximity” to the dunite areas of New Zealand. However, in the majority of cases where olivine-nodules have been recorded in New Zealand no association with dunite is known.
Note on the Nature of Olivine Crystals in the Lavas
Universal stage measurements (by Mr. N. J. W. Croxford) on some of the olivine crystals show that the large grains (< 5 mm in length) are highly magnesian in character. For example an ankaramite, 11352, from the west base of Kattothyrst, has 2V∝ = 94 — 84°, or Fa6-Fa23, and 11380, an atlantite from the south side of West Pinnacle, has 2V∝ = 96 — 82°, or Fa2-Fa33). In many cases crystals exhibit undulose extinction. They are also frequently corroded and embayed and often associated with corroded crystals of picotite. The smaller olivine crystals in the groundmass, however, are more ferriferous (e.g., 11369, 2V∝ = 82–77°, or Fa33–Fa44) (cf. also Benson, 1942a, p. 109). These facts suggest that the large crystals are xenocrystic, rather than phenocrystic in nature, and derived from the peridotitic material from which the olivine nodules originated. This is borne out by the highly magnesian character of the olivines in the nodules, for example, in 11345. The small olivine crystals in the groundmass, however, are probably direct crystallization products of the magma.
Summary of Geological History of Area
In Upper Cretaceous times, the gradual depression of the Cretaceous Peneplain cut in greywackes and semi-schists led to the deposition, first of quartzose coal measures and then a sequence of marine formations which, in the coastal portions of North Otago, extend in age up to Lower Oligocene or even Lower Miocene. Extensive erosion in Upper Eocene times, corresponding with a period of crustal unrest and outbreak of igneous activity in the Oamaru district, and later during the Miocene, removed most of these sediments from the present area. Here and there are thin remnants of the quartzose coal measures, pebbles of which frequently occur as xenoliths in the Pliocene volcanic rocks.
Prior to the major movements of the Karkoura Orogeny, extensive vulcanism took place in North and East Central Otago. In the Siberia Hill-Mount Dasher region, the earliest phase of eruptive activity is represented by coarse volcanic agglomerates and tuffs. These were probably the products of the initial explosions that preceded the outpouring of lavas from a series of fissures or isolated centres aligned along the zone of the Dasherette Fault. The earliest lavas appear to have been atlantites and ankaramites which form the bulk of the flows and dykes on Kattothyrst, Mount Dasher and the eastern slopes of Siberia Hill. Then came an extensive flow of feldspar-basalt which is still largely preserved on Siberia Hill, and is absent from the other volcanic residuals, as also are the later eruptives. Next in the sequence is the small nephelinite flow
at the Crater and along the west side of Siberia Hill, and finally, the youngest preserved lavas, the sanidine-basanites around the summit of Siberia Hill itself.
The zeolitic basanites of Mount Dasher are probably intermediate in age between the atlantites at the base of the lavas and the atlantites forming the summit of the mountain.
Notes on Alkali Feldspar, Phillipsite and Clinopyroxene from Siberia Hill and Mount Dasher.
By D. S. Coombs
Alkali feldspar from basanite.
Comparatively little information has been published about the nature of the alkali feldspar phases that appear in the groundmass of certain rocks of basaltic habit. Observations on micropoikilitic feldspar in a sanidine-basanite, 11377, from Siberia Hill are therefore perhaps worth recording, even though the data are incomplete and inferences tentative.
Broad mantles of optically monoclinic feldspar surround large plates of andesine, zoned An52-35 according to Nikitin-method determinations. In at least some cases the contact between the two phases is a sharp one. Both core and mantle are thoroughly sieved with copious tiny prisms of titanaugite, subordinate olivine, granular magnetite and hair-like needles of apatite. After crushing to pass 250 mesh sieves, a concentrate of material with the approximate specific gravity range 2.50-2.63 was made with heavy liquids. Greenish alteration products and composite grains were removed with a Cook Magnetic Separator, and feldspathoids by digestion in hydrochloric acid followed by sodium carbonate. The refractive index α of the residue varied from 1.525 to 1.536 and higher, about half the grains having α 1.525, to 1.529, and over 20% having γ greater than 1.54 (including some obvious plagioclase contaminant). The optic axial angle was variable, 2Vα = 40° to 60°, suggesting membership of Tuttle's sanidine-anorthoclase cryptoperthite series. Angles of 65–68° are recorded by Benson (1942a, p. 109) for alkali feldspar from the same locality. The maximum of the 201 reflection in an X-ray powder photograph, taken after homogenization at 900° C. corresponds to the composition Or37 (Ab + An)63, with broadening suggesting a range from Or45 to Or30.. The method used is that of Bowen and Tuttle (1950) as discussed by the writer (Coombs, 1954). This orthoclase content is higher than would have been predicted from the refractive indices assuming that lime-free alkali feldspars were involved, and in fact α for high-temperature albite is only 1.527. In the plagioclase series 2% An raises the indices by about 0.001. On this basis it can be suggested that the Siberia Hill alkali feldspar is lime-bearing with a composition of the order Or45Ab51An4 where α = 1.525, but with 10 or 20% An and about 30% Or where α reaches or exceeds 1.529.
Powder lines such as 111 are not doubled, confirming the optical evidence that there is no appreciable departure from monoclinic symmetry, this being a distinction from phenocrystic lime-bearing anorthoclase and potash oligoclase as described by Mountain (1925) and others. Very slight departures from monoclinic symmetry, however, would not be detected by this method. In the terminology of Laves (1952, p. 570) the Siberia Hill feldspars could be termed K-barbierite and calcic K-barbierite. Less precisely, the names soda-sanidine and
soda-lime-sanidine could be applied, the term anorthoclase being restricted by most writers to triclinic alkali feldspars of appropriate composition.
The zeolitic basanites from Mount Dasher contain minute pools of phillipsite, making up nearly 5% of the rocks, and in some cases at least, for example 11342, it is an important constituent of the voluminous mesostasis. It has not been possible to determine the extent to which it is accompanied there by alkali feldspar and feldspathoid, but the chemical analysis suggests that these must be present.
The identification of phillipsite rests on a 19 cm X-ray powder photograph virtually indistinguishable from one of well-formed crystals of phillipsite (α = 1.493; δ = 1.497) from Collingwood, Melbourne, Australia. Optical properties of Mount Dasher phillipsite 11334 are as follows:—
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α = 1.492; δ = 1.496; 2Vδ = 45.70°; r < v.
It is interesting to note that a member of the phillipsite group was found by Holmes (1942, pp. 203, 205) and Bannister (1942, p. 222) to be a significant member of kalsitite-bearing pegmatoid segregations in the potassic ultrabasic lava mafurite from south-west Uganda, representing a late, hydrothermal phase in the cooling history. Presumably the same applies to the Mount Dasher phillipsite In the mesostasis it is most intimately associated with the dendritic augite and barkevikite, and although phillipsite can hardly be regarded as of strictly magmatic crystallization, the rock is fresh in appearance and there is nothing to suggest that the phillipsite is other than a direct crystallization product of constituents contained within the magma itself.
In the sanidine-basanite, 11377, phillipsite also occurs (α = 1500; δ = 1.505), this time confined to cavity-fillings where it is accompanied by greenish birefringent material. A powder photograph of the zeolitic fraction of this rock confirms the presence of both phillipsite and analcime. In contrast to the phillipsite, the areas of analcime poikilitically enclose titanaugite, olivine, magnetite and apatite within the body of the rock in precisely similar fashion to that of sanidine and presumed nepheline in the same rock. The analcime has the appearance of truly magmatic crystallization.
Clinopyrorene in zeolitic basanite.
Brownish titanaugite from the zeolitic basanite 11334 has β = 1.709.1.712, 2Vδ = 55–56°, suggesting a moderate degree of iron enrichment comparable to that found by Murray (1954) in his study of clmopyroxenes from Garbh Eilean Sill, Shiant Isles, and other minor basic intrusions of mildly alkaline type. It should be noted, however, that Murray found that such pyroxenes have appreciably higher indices for a given iron content than have those, usually less titaniferous, of tholeiitic origins. In the analysed rock 13342, zoned hourglass crystals of brownish clinopyroxene have β = 1.710 — 1.723, 2Vδ = 53–56°. The members of higher index are presumably well into the ferroaugite field with iron contents up to about Fe40 (Ca + Mg)60. They thus approach the greenish ferroaugite (β = 1.728) separated by Murray from an orthophyric vein from Portrush, Antrim, where the trend of iron enrichment in the pyroxenes is carried far beyond that recorded for the Garbh Eilean Sill.
Compositions inferred from the optical properties of the scattered olivines in 11334 are Fa10 and Fa42-48. The former value is in such marked contrast to
the others and to the ferriferous nature of the pyroxene, that it is considered to represent xenocrystic material comparable to that of the olivine nodules. Even the more fayalitic olivines had ceased to crystallize before the iron-rich clinopyroxenes. In the analysed rock 11342 only rare pseudomorphs after olivine are found.
The iron enrichment of the pyroxene and the abundance of zeolite emphasise that the Mount Dasher zeolitic basanite is a late product of fractionation. even if on a rather aberrant trend. In this respect some analogy can be drawn with zeolitie pegmatoid veinlets in the Clarendon basanite and the upper members of the Waihola theralite sill elsewhere in the Eastern Otago Pliocene Petrographic Province.
Bannister, F. A, 1942 Kalsilite, a polymorph of KalsiO4 from Uganda Min Mag vol. 26, pp. 218-224, Pl 7.
Benson, W. N., 1941. The Basic Igneous Rocks of Eastern Otago and Then Tectonic Envnonment Part I Trans. Roy. Soc. N. Z. vol. 71, pp. 208-222, Pl 36
—— 1942a. Ibid. Part II Op cit. vol. 72, pp. 85-118
—— 1942b. Ibid Part III. Op cit. vol. 72, pp. 160-185
—— 1944 Ibid, Part IV. B.Op cit., vol. 74. 71-123
Bowen, N. L. and Tuttle, O. F. 1950. The System NaAlSi3O8-Kalsi2O8-H2O Journ Geol vol. 58, pp. 489-511.
Brown. D. A., 1938. Moeraki Subdivision. 32nd Ann Rep. N. Z. Geol Suri. pp. 9-12
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Coombs, D. S, 1954. Ferriferous orthoclase from Madagascar. Min Mag. vol. 30, pp. 409-427.
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—— 1912. Nephelinite Rocks in New Zealand. Trans. N. Z. Inst., vol. 44. pp. 304-307
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—— 1907. Inclusions in some Volcanic Rocks. Geol. Mag. dec. 5, vol. 4, pp. 490-500
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—— 1942. Preferred Orientation of Olivine Crystals in Peridotites, With Special Reference to New Zealand Examples. Trans. Roy. Soc. N.Z., vol. 72, pp. 280-300, Pls. 26-33
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—— 1952. Optical Studies on Alkali Feldspars. Amei. Journ. Sci., Bowen Volume, pp. 553-567.
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D. A. Brown, M. Sc., Ph.D., D.I.C.
D. S. Coombs, M. Sc., PhD.
Department of Geology
University of Otago
P.O. Box 56