![Thumbnail: [Volume 82, 1954-55 page 226]](/journals/rsnz/images/rsnz_82/thumbnails/rsnz_82_02_0265_0226_ac_02.gif)
Volcanic Rocks
Rhyolite, 2139, 2244, 2280, 2322–8, 2360–1; Arawhero Point, Totara Point, Te Angina, Ohaua Point, Parapara. These rocks show fine flow structure with a microspherulitic and microgranular groundmass. Plumose axiolites and large spherulites enclosing crystals of microperthite have been developed in some cases. (Plate 10, Fig. 1).
Dacite, 2163–4, 2206–10, 2228, 2245, 2329–37, 2365–6, 2381–2; Ururoa Point, Urawitiki Point, Parikawau, Totara Point, Parapara, Ohaua Point, Waiharakeke.
Trachyte, 2283; Te Angina.
Quartz-keratophyre, 2140, 2165; Arawhero Point, Ururoa Point.
Keratophyre, 2141–2, 2166–8, 2171, 2213, 2215, 2232, 2246, 2285–9. 2239–42, 2385; Arawhero Point, Ururoa Point, Urawitiki Point, Parikawau, Totara Point, Te Angina, Ohaua Point, Waiharakeke.
Pebbles of keratophyre are abundant, and contain numerous phenocrysts of albite and a few large crystals of oligoclase set in a fine-grained trachytic ground-mass (Plate 10, Fig. 2). Although most of the albite is unusually fresh there is nothing to indicate replacement of an earlier formed plagioclase, and this suggests that the albite was primary in origin. Rare phenocrysts of colourless augite also occur, but the original ferromagnesian phenocrysts are mostly represented by pseudomorphs of chlorite. The extremely fine-grained nature of the groundmass renders it difficult to determine any variation in the feldspar laths,
![Thumbnail: [Volume 82, 1954-55 page 227]](/journals/rsnz/images/rsnz_82/thumbnails/rsnz_82_02_0266_0227_ac_02.gif)
but they were identified as albite in rare instances. Small grams of epidote, calcite, chlorite and magnetite are very abundant.
Andesite, 2172–5, 2217–8, 2233–5, 2249–69, 2292–5, 2349–51, 2367–9, 2386–7; Ururoa Point, Urawitiki Point, Parikawau, Totara Point, Te Angina, Parapara. Ohaua Point, Waiharakeke.
The andesites contain phenocrysts of oligoclase or andesine feldspar, colour-less augite or a pale-green diopsidic variety usually very altered, and occasionally hypersthene. The groundmass has a pilotaxitic texture consisting of laths of feldspar, mainly oligoclase, with small patches of calcite, granular epidote, chlorite, and magnetite dust. Many of the pebbles, particularly those from Totara Point, are highly amygdaloidal and the cavities have been filled with a variety of secondary material including quartz, fibrous epidote, chalcedony, chlorite, calcite and zeolites (Plate 10, Fig. 3).
Meta* andesite, 2177–8, 2237, 2240, 2271–2, 2296, 2370–1; Ururoa Point, Parikawau, Totara Point, Te Angina, Parapara.
These rocks appear to be andesites which have undergone alteration by a process of soda metasomatism. In certain sections (2177, 2272, 2370) the original feldspar has been partially replaced by analcite (Plate 10, Fig. 4) and in others (2178, 2237, 2271, 2371) albitisation is indicated by a narrow rim of clear albite (Ab90An10) or albiclase (Ab85An15) surrounding the weathered original feldspar (Plate 10, Figs. 5 and 6). In both cases the original feldspar is either oligoclase or andesine and usually it has undergone considerable alteration to chlorite and epidote. Occasionally the host feldspar has been completely replaced by albite, which in turn has been partially replaced by orthoclase (2240, 2296; Plate 11, Fig. 1). A similar sequence of feldspathisation affecting an intrusion of granodiorite is described by Leedahl (1952, pp. 52–58) who considers it is due to potash metasomatism accompanying or following albitisation.
The meta-andesites also contain a few crystals of weathered oligoclase, about Ab75, An25, which appear to have escaped secondary replacement, and rare phenocrysts of a pale green diopsidic augite undergoing alteration to epidote, chlorite, calcite, and iron-ore. One notable section (2178) contains chloritic pseudomorphs in which cross-fractures are so outstanding as to suggest replacement of original olivine. The groundmass is pilotaxitic in texture and consists of tiny microlites of feldspar (albiclase to oligoclase) with abundant chlorite, epidote, and iron-ore.
[Footnote] * The term meta- is defined by Holmes (1920, p. 154) as “a prefix used before the names of igneous rocks to signify that the mineral and chemical composition of the latter have been modified by alteration.”