Thermal Alteration of Eocene Sediments.
The various contact-alterations of the invaded sediments are described in order of their age. The Danian rocks in the area considered (Benson, 1943, p. 127, fig. 3) are not seen in contact with igneous rocks. Lower Eocene Kurinui Beds are in contact with dolerite on the shore three-quarters of a mile east of Hillgrove station and beyond induration show little mineralogical change. Specimen (5863) is a siltstone consisting of angular grains (< 0.1 mm. across) of quartz and a little acid plagioclase set in a dominant turbid argillaceous matrix containing glauconite (< 0.15 mm.) a few biotite flakes, abundant obscure casts of radiolaria, and less abundant foraminifera and sponge-spicules. In (5862 and 5864) calcite is the chief mineral in all but the small argillaceous patches. It forms turbid interlocking granules (< 0.5 mm.) containing poikilitically much angular quartz, rounded grains of magnetite and scanty plagioclase; glauconite is represented by limonitic pseudomorphs. It is possible that heating by the adjacent magma may have facilitated induration by recrystallisation in the sediments, but it is evident that the temperature did not reach or exceed the point (rather more than 500° C. at low pressure such as existed here) where reaction between quartz and calcite could commence according to Goldschmidt (1912, p. 6), though Bowen (1940, p. 13) indicated there are uncertainties inherent in this estimate.
The Middle Eocene Bortonian Mudstone invaded by the dolerite at Tawitiatauka shows to a slight degree the effects of induration and change described below for the similar Tahuian sediments.
Upper Eocene Tahuian mudstones are invaded by dolerite at Moeraki Point and at several places from Tawhiroko Point southwards. Their contact effects were observed at the first locality by von Haast (1872, 1877) and Hutton (1887), who thought that the igneous rocks were flows. The nomenclature of such contact-altered rocks has recently been discussed by Tomkeieff (1940, pp. 56–8, 62–3) and is here reviewed. The term buchite was apparently used by Möhl (1873) for the glassy product of sandstone vitrified at its contact with thin basic and ultrabasic sills, and containing relics of the original quartz grains with small, new formed crystals later recognised to be cordierite; mullite, augite, rhombic pyroxene, magnetite, colourless microlites, and tridymite (?), also trichites and
globulites and numerous gas-pores, have also been noted in such rocks. The term buchite has been used in this sense by several authors including Rosenbusch (1908, pp. 1295–6), Harker (1904, pp. 245–6, 1932, pp. 70–1) and Tomkeieff (loc. cit.), the last noting the presence of new-formed crystals of plagioclase and sanidine suggesting “a certain amount of transfusion between the magma and the sediments,” a view which has been supported by Reynolds (1936) and others. Harker (1932), quoting Lemberg's analyses, stated that such semi-vitreous buchites may contain 3–5% of water rising to 10–12% in the purely glassy material. Prohaska (1886) and Flett (1908) extended the connotation of buchite to included vitrified shales and phyllites. Holmes (1920) thereunder included also phyllitic and other rocks vitrified by the effects of friction in mylonitised crush-belts, and Thomas (1922) vitrified aluminous clays from which sapphire, spinel, anorthite and mullite might crystallise. Jugovics (1933), however, restricted the use of the term to more or less vitrified arenaceous rocks as originally proposed, and applied the old term basalt-jasper to the altered argillaceous rocks, as did Richarz (1924, p. 689) and Rosenbusch (1908, p. 1298) who, however, included vitrified greywackes under this term. Tomkeieff (loc. cit.), remarking that this term is self-contradictory and is not used in Britain, made the alternative suggestion (a) to use buchite in its original sense and porcellanite for the products of thermally altered argillaceous rocks, or (b) to give to these terms a textural rather than a chemical significance, using buchite for vitreous or semi-vitreous material whether arenacous or argillaceous, and porcellanite for unvitrified but finely crystalline contact products of such sediments, with hornfels for their less finely crystallised products. Since the amount of argillaceous material in the Moeraki mudstones is variable, though usually small, and some of them are rather calcareous, the latter alternative is here adopted. Though no sharp distinction has been observed between porcellanite and buchite (von Haast used the name porcellain-jasper to cover both), the porcellanite is “a compact rock of light colour with the appearance of unglazed porcelain” (cf. Holmes, 1920), and is formed by the induration of mudstone making a semi-conchoidally fracturing rock within a few feet of the intrusion; the buchitic character appears nearer to the intrusive mass, where the curving fractures assume smaller radius, and a vitreous lustre is observable in hand-specimens, while under the microscope an isotropic base is observable which may finally make up the greater part of the rock, though enclosing small residual and often corroded quartz grains, traces of micas, which may be residual, and perhaps scanty new-formed feldspar microlites.
The grey mudstone is almost unaltered within three feet of the base of the Moeraki Point intrusion, and in the middle of the narrow (14ft) strip of sediment between the bottom of the Tawhiroko sheet and its basal spur. The latter rock (5744) is a calcareous mudstone with casts of foraminifera, radiolaria and sponge-spicules. Within two feet of the intrusive sheet (5894) or four of the basal spur (5745) conchoidal fracture is well developed, and sparse residual grains (< 0.02 mm.) of quartz with a little feldspar, biotite and
sericite occur in a dominant isotropic matrix still showing the outlines of the casts of these siliceous organisms, but rendered turbid by dust-like iron-ore and argillaceous material. The refractive index of this glass, according to Dr. Hutton, is usually 1.470 ± 0 003, but ranges from 1.465 to 1.475. A similar rock (5893) forms the margin of the huge slab of altered mudstone 5–10 feet thick and over 50 yards long standing almost vertically in the dolerite of the Main Moeraki sheet south of Okahau Point. The fragment of mudstone (5873a) included in the dolerite in the centre of the Tawhiroko sill is still more turbid, and its very abundant radiolarian casts are filled with carbonates. Similar to this is a sample (5928) obtained four inches from the basal spur of this sheet, differing from the above in the tawny colour of the isotropic material resulting from a greater content of dust-like haematite, and the rather greater proportion of small residual quartz grains. The possibility that hot magmatic and connate water acting on the finely divided quartz grains in this rock might have developed opal, and that such might be the isotropic matrix of this rock suggested investigation, the results of which are indicated below:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|5928||Quartz (a)||Quartz Glass (a)||Opal (a)||Buchite|
|B||3.0%||Nil||Nil||0–12%. Av. 6%||3–5% (b)|
|C||1.47–1.48 (d)||1.544–1.553||1.458||1.450||1.47–1.48 (e)|
A Solubility on boiling finely ground material in 10% NaOH for two hours.
B Water given off above 110° C.
C Refractive index or indices.
D Specific gravity.
(a) Data from Doelter (1914).
(b) After Lemberg cited by Harker (1932).
(c) Kindly determined by Dr. S. N. Slater.
(d) Kindly determined by Dr. C. O. Hutton, who notes that 1.470 ± 0.003 measured in Na light is the usual figure, though one flake gave the higher value.
(e) Data from Tomkeieff (loc. cit. p. 56). R.1. = 1.472–1.497 in Holmes (1936).
It would appear from the above that buchite may be accepted as the nature of this material (5928). The slight variation in properties will probably be due to the variation in content of lime. The rock is transversed by many irregular cracks containing calcite.
A small chip (5839) enclosed in a thin tachylytic dyke invading the Tawhiroko sheet (see Benson, 1944, p. 96, fig. 7) may represent a less rapidly cooled product of semi-fused material which has taken up material from the magma, for though it contains minute angular or embayed relics of quartz grains, the groundmass seems to have devitrified into a mass of skeletal crystals of alkaline feldspar with a general parallel elongation, and occasional flakes of biotite.
Contrasting with these is a more calcareous rock, which was probably plastic when taken up as an inclusion into (5833) the dolerite near Matiaha Head (see Benson, 1943, p. 135, fig. 8). This
has been converted into an exceedingly finely granular calc-silicate rock, pale green in colour, in which it is just possible to recognise an abundance of tiny (< 0.03 mm.) prisms of diopside in a colourless matrix containing granules of quartz and microlites (< 0.05 mm.) of alkaline feldspar. Casts of radiolaria are still obscurely indicated. Since the grain-size of this rock is so small the names calc-silicate hornfels or porcellanite previously applied to it (Benson, 1943) may with advantage be replaced by calc-flinta. The presence of the calc-silicate mineral diopside is indicative of metamorphism at a relatively high temperature, which its complete enclosure in basalt would also indicate.