A Preliminary Account Of The Syenite And Associated Rocks Of The Culverden District
The occurence of syenite in the Mandamus River, near Culverden, North Canterbury, has long been known. It was recorded by Haast in 1870, and described as an augite syenite by Hutton in 1889, in a paper on the eruptive rocks of N.Z. Professor Speight mentions it in a paper on the Hurunui Valley published in 1919. No mention has, however, been made in the literature of the small intrusion of gabbro in the same district, and the minor intrusions associated with these rocks are also worth comment.
The syenite itself outcrops in an irregular elliptical form, extending from near the Hurunui River in a north-easterly direction across the Mandamus River and forming the greater part of the Hurunui Peak ridge which bounds the Hurunui-Waiau depression in this area. Its greatest length is about three and a-half miles, and the greatest width about a mile, the total area of outcrop being somewhat over two square miles. It is well exposed over a total height of 2,000ft from the summit of Hurunui Peak to the bed of the Mandamus River. It is best studied in the Mandamus River, which cuts right across the intrusion in a narrow rock-walled gorge, exposing contacts with the country rock on both sides.
It is difficult to classify this body in any of the standard types of intrusions. Professor Speight speaks of it as a sill, but the strike of the country rock. although variable, is generally about north, transverse to the direction of elongation of the body. It may represent an offshoot from a larger body of igneous rock at depth. Its present outcrop is due to its occupying the crest of an eroded anticline. The anticline is of late Tertiary age, but the date of intrusion was much earlier, probably Cretaceous.
The gabbro occurs a short distance to the north-east of the syenite intrusion, and is exposed for about half a mile in the bed of a small unnamed stream entering Dove River just below a hut and yards belonging to Cascade Station; I will refer to it as Hut Creek. It was not seen away from the stream bed and evidently has no great area of outcrop. It was actually localised by the tracing of pebbles of the rock from where they were first found in the Mandamus River, the rock being highly characteristic and recognisable at a glance.
Numerous smaller intrusions are common in the country rock adjoining the syenite and gabbro, and diminish in abundance away from these rocks. The farthest that these intrusions have been found away from the syenite and gabbro is two miles. These smaller intrusions appear to be mainly sills, although their dip is more or less vertical, as they are generally accordant to the bedding of the, argillite and greywacke which they intrude They are generally not very thick, few being more than 10ft. In composition they are mainly trachytic, but a few lamprophyric types occur. A number of these sills are multiple intrusions.
In hand specimens the syenite is a white rock with numerous dark crystals of ferromagnesian minerals. Weathered outcrops often show a distinct reddish colour easily recognisable at a distance. Much of the syenite is very friable, due to the interstices between the feldspar laths not being filled, and some of it can be crushed between the hands. This leads to rapid weathering and disintegration, and the syenite peebles survive but a short time in the river gravels. Under the microscope the rock is seen to consist of 80–90% feldspar and 10–15% of ferromagnesian minerals. When fresh the feldspar is found to be anorthoclase or microperthite, but in most samples it is thoroughly kaolinized and indeterminable. Neither quartz nor nepheline has been recognised.
The ferromagnesian minerals are augite, often accompanied by biotite. The augite is colourless but rimmed with green, evidently due to the formation of aegirine towards the end of crystallization. In some specimens the pyroxene is entirely aegirine. Some magnetite is generally present.
The gabbro is a dark-coloured coarse-grained rock, crystals of augite up to 1 cm. long being clearly visible on weathered surfaces. Augite, olivine and labradorite are the main constituents, biotite and iron ore in subsidiary amount, and numerous crystals of apatite as accessories. The structure is simple, both olivine and augite showing idiomorphic outlines to the feldspar, which generally surrounds them as tabular prisms of smaller size. The augite is a pale lilac-brown
titaniferous variety, markedly pleochroic. The olivine is colourless and is often partly decomposed. Biotite is not everywhere present, and is generally grouped about the iron ore.
The most common rock type of the smaller intrusions is a grey porphyritic trachyte, with phenocrysts of orthoclase in a groundmass of feldspar laths and small crystals of hornblende. Generally a small percentage of quartz is present. Most sections show secondary calcite.
Chemical analyses show that the syenite and the trachyte are very similar in composition. The gabbro is probably a local differentiate of the same magma which gave rise to the syenite.
The age of these intrusions can be stated with some degree of certainty. They are intrusive into greywacke and argillite, which is unfossiliferous except for indeterminable plant remains. Hutton, however, recorded a specimen, of Taeniopteris from this locality. On this slender evidence and on lithologic similarity the greywacke and argillite may be considered to be Triassic or possibly Jurassic. Pebbles of these intrusive rocks occur in the basal Tertiary conglomerates of this area, which are probably Lower Eocene in age. The obvious inference is that these rocks were intruded during the post-Hokonui orogeny, probably towards the end of the orogeny, as they appear to be less shattered and broken than the country rock. The most probable age is Lower Cretaceous.
The nearest occurence of similar rocks is in the Inland Kaikoura Mountains, on Mount Tapuaenuku. During a trip through the middle Clarence Valley about a year ago, from Kekerangu to Hanmer, I collected numerous pebbles of igneous rocks from the Swale, Mead and Dee Rivers, and from the types therein represented was able to match these Culverden rocks almost identically. The rocks from the middle Clarence valley show a wide variety, much wider than is indicated by Thomson's preliminary description of 1913, and that area is undoubtedly a fine field of work for a geologist interested in igneous intrusions. Thomson considered these rocks to be Clarentian in age, on what appear to be fairly good grounds.
The other occurrence with which these intrusives may be compared is the small outcrop of syenite on the tip of Onawe Peninsula, in Akaroa Harbour. The Onawe syenite show distinct resemblances to the Mandamus syenite both chemically and mineralogically. There is no real evidence for dating the Onawe syenite, and I suggest that it is coeval with the other intrusions discussed, i.e., about Middle Cretaceous.