A. The syenite
The syenite is the major igneous rock of the area. It forms the core of the Hurunui Peak ridge, which separates the valley of the Dove River from the Culverden depression, and extends across the Mandamus River nearly to the Hurunui River. Its outcrop is roughly oval, with a maximum length of four miles and a maximum width of half a mile. It is particularly well exposed in the gorge of the Mandamus River and that of Coal Creek and crops out at many places along the hill slopes.
In hand specimen (Fig. 2) the syenite is a dull white rock speckled with irregular grains of dark green ferromagnesian minerals and sometimes with lustrous plates of black biotite. The rock is markedly porous even on superficial examination, with numerous angular cavities (sometimes partly filled with yellow-brown natrolite) between the feldspar laths. This porosity accounts for the friable nature of the rock in most outcrops; in many places the rock disintegrates into its individual mineral grains on rubbing between the hands, and looks more like a poorly consolidated sandstone than a plutonic rock. Fresh outcrops have a speckled black and white appearance, but where the rock has been subjected to weathering, the ferromagnesian minerals have been largely removed and the rock has a rusty-white colour; large weathered outcrops seen from a distance show a distinctly reddish colour which often serves to differentiate them from neighbouring outcrops of greywacke. The syenite varies little in grain size and texture over its whole area; no pegmatitic phases were seen, and the
fine-grained selvages against the greywacke are never more than a few inches thick.
Under the microscope the feldspar is usually grey and turbid from beginning kaolinization. It is generally a fine-textured microperthite, although in some specimens the microperthite structure is scarcely detectible and the feldspar is best described as a cryptoperthite. The ferromagnesian minerals are aegirine, biotite and alkaline amphibole. Aegirine (X ∧ c = 8°) is the most abundant ferromagnesian mineral; sometimes the aegirine grains have a core of colourless or pale green augite. The biotite is very strongly pleochroic from yellow-brown to practically opaque. The alkaline amphibole is less common than the other ferromagnesian minerals and is absent from some thin sections; it often replaces pyroxene; it has extinction angle Z ∧ c = 20° and pleochroism × = greenish-brown, Y = reddish-brown, Z = dark brown, and is probably barkevikite. In some specimens a little riebeckite was seen replacing aegirine. Accessory minerals are apatite in very small amount; black opaque material, probably magnetite; and rare enhedral titanite. A little natrolite occurs in the interstices of the feldspar laths. A small amount of secondary calcite and a little opal is present in cavities. The mode of the analyzed specimen is given with the analysis (Table I).
Occasional variants of the syenite were found. One specimen collected from near the edge of the intrusion on the north side of Hurunui Peak was practically free from ferro-magnesian minerals and carried occasional grains (< 5%) of primary quartz. Within a few inches of the contact with the greywacke the syenite is fine-grained (average grain size of equigranular feldspar 0·15 mm.) and in hand specimen has the colour, texture, and jointing of fine-grained greywacke. A thin section of a specimen from this marginal facies at the contact in Coal Creek shows that it has a similar mineralogical composition to the main mass of the syenite except that the ferromagnesian mineral is entirely alkaline amphibole (Z ∧ c = 22°, × = pale yellow, Y = yellow, Z = bluish-green, γ–α = 0·012). This section also showed occasional grains of titanite, strongly pleochroic (yellow to reddish-brown).
The mineralogical and chemical composition of the rock shows that the rock is a typical syenite in the broad sense of the term. It is distinctly alkaline, as shown by the presence of aegirine and alkaline amphibole, and is comparable with the umptekites and nordmarkites. Indeed, the rock is a nordmarkite as this term is defined by Barth (1945, p. 85) “… Nordmarkite [is] a syenite or quartz-syenite containing alkali feldspar (perthite or anorthoclase) without determinable amounts of plagioclase.” It is very similar in composition to some of the type nordmarkites of the Oslo region.
The structural classification of this syenite intrusion is not clearcut. It is transgressive to the general strike (about N.–S.) of the country rock. It is possibly a stock, although the noncommittal term pluton is perhaps preferable.