METRES - Bookshelf Collection - The University of Auckland

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anhedral clinopyroxene crystals, opaques and alkali feldspar constitutes between
52 and 93% of the bulk composition of all sections. Small anhedral clinopyroxene
crystals and opaques, of less than 0.3mm diameter, fill the angular interstices
between the plagioclase microlites. The often indistinguishable colourless alkali
feldspar occurs as a late crystallising phase in cavities between the plagioclase
and clinopyroxene crystals.
Coarseness and groundmass constituents is highly variable, with microlites
generally less than 0.3mm in length, although 19987, with subophitic texture,
contains some up to 0.6mm long.
basalt.
Rock type for all flows and dykes examined is considered to be an olivine
(b) Tuff Beds
Three samples from the tuff beds were examined. Colours vary from
whitish through red-brown to dark brown, though all appear to be of similar
composition. The whitish sample, however, consists of what seems to be highly
weathered pumiceous material, with common euhedral to anhedral opaques
and rare quartz grains. Some relatively unweathered darker scoriaceous fragments
are also present.
The other two samples consist largely of whitish and grey to red-brown rock
fragments, often very glassy and vesicular, and common feldspar phenocrysts
and abundant colourless to white glassy shards. The remaining fragments
consist of both black and red-brown glass in varying amounts, minor black ores,
and rare quartz grains. The red-brown glass shows a "grain" or sheen similar to
that found in some of the obsidians of Mayor Island (pers. obs.).
(c) Te Kokowai Intrusion
One fresh specimen (19984) of the Te Kokowai Intrusion was sectioned and
examined. The modal analysis is presented in Table 1. In hand specimens the
sample is indistinguishable from the flows and dykes. It is light grey, vesicular,
and with golden-brown phenocrysts of altered olivine, of maximum dimension
1.5mm, scattered throughout.
In thin section the rock is porphyritic although the phenocrysts, mostly
iddingsite pseudomorphs after olivine, comprise only 5% of the bulk composition.
Stubby subhedral poikilitic plagioclase phenocrysts are rare.
The groundmass has an intergranular to intersertal texture of which 70%
is indeterminable fine grained material of less than 0.01mm diameter. Slender
plagioclase microlites, 0.05 to 0.2mm long, are the coarsest and most common
groundmass mineral indentifiable, with very fine grained crystals of clinopyroxene
rarely discernible. Opaques are in minor proportions, and small
patches of alkali feldspar occur as late crystallising groundmass cavity fillings.
GEOLOGICAL HISTORY
During an undetermined period in Pliocene and/or lower Pleistocene, the
Whakau Volcanics were erupted onto a terrestrial surface. The nearest centre of
activity appears to have been off Ngatoka Puta Bay where a considerable