Comment on Tables.
Taking into consideration the grainsize, a glance at the column dealing with the amount of reconstitution reveals an almost constant degree of alteration resulting from stress between the upper south-western slopes of Mt. St. Mary and Parson's Rock Creek. Proceeding south-westwards along the section-line from the latter point, however, the intensity of reconstitution becomes less across the Parson's Rock Spurs; but when the Otematata River is neared increase again becomes apparent, until at the river itself, and for short distance on its south-western side, greywacke-schists are encountered, indicating an area of maximum shearing located along the general direction of the main channel.
The intensity of metamorphism begins to wane again southwestwards from the Otematata River, and with two or three exceptions remains fairly constant at the “slightly reconstituted” level until the Hawkdun Fault-block is reached. From here to the summit of the St. Bathans Range a slight general increase is noted.
C. Between St. Bathans Range and Dunstan Peak.
The localities to which reference is made are shown in Textfigure 1. Selected rocks typical of this part of the section are described below:—
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No. 2350.—Sheared, largely reconstituted greywacke, in handspecimen a greyish, medium to coarse-grained, poorly fissile rock. The section indicates considerable shearing, the quartz crystals frequently showing recrystallisation and granulation. Acid plagioclase, with some orthoclase, forms rounded crystals up to 11/4 mm. diameter, while the more basic feldspars have altered to dark, finely granular epidotic matter which has now become interstitial to the more resistant quartz and feldspar, and may possibly have the composition of clinozoisite. Green hornblende is present up to about 8%, but much of it has been replaced by actinolite, which is quite abundant (10%). There is also about 10% of stilpnomelane appearing both in the rock itself and in the quartz veins traversing it. Fine material consisting of minute granules of quartz and feldspar (albite) with numerous little sericite flakes forms the matrix in which the larger crystals are enclosed. Small clastic grains of sphene, ilmenite, and epidote, together with decomposing biotite and carbonaceous matter, constitute the accessories.
No. 2351.—Totally recrystallised rather fissile sheared grey-wacke. In section this is a thoroughly recrystallised sheared rock consisting of a fine mosaic of quartz and albite containing a few more or less rounded quartz and plagioclase relicts, as well as streaks and cluster of dark, granular epidotic matter. Some serieite and a little actinolite are present.
No. 2354 is a dark-grey fissile slate consisting of fine quartz, feldspar, sericite, chlorite, epidote and dark carbonaceous matter, enclosing occasional porphyroclasts of quartz or acid feldspar. Accessories are iron-ore, sphene and zircon. The carbonaceous matter is concentrated in separate layers, the others being relatively light coloured. Signs of strain-slip are evident.
No. 2355—A sheared, totally reconstituted greywacke (grey-wacke-schist) with distinct macroscopic foliation. The microsection shows a reconstituted rock with a few granulated relicts of quartz and clouded feldspar. The foliae are of two kinds: (a) greyish sheared quartz-albite aggregates with epidote sericite and some sphene enclosing a few almost totally recrystallised quartz and feldspar relicts; (b) clear quartz-albite aggregates containing irregular black patches of iron ore.
Nos. 2357–2360 are foliated quartz-albite-epidote-sericite-schists resembling those of Central Otago, but rather finer in grain. Nos. 2357 and 2358 are corrugated on a small scale. In No. 2357 the laminae of which the rock is composed are of two types. One consists of allotriomorphic quartz and albite grains (often with sutured margins) with a few granules of epidote and pieces of muscovite (often stained yellowish by iron oxide) and rare acicular crystals of stilpnomelane. In the other variety quartz and albite are relatively unimportant, the abundant minerals being epidote and muscovite. The epidote occurs here also as granules, while the muscovite (often stained greenish and yellowish by iron solutions) forms little elongated flakes. Chlorite is fairly plentiful in some parts. The parallelism and folding of the laminae is frequently emphasised by the presence of streaks of carbonaceous matter. In
addition, two or three slender prisms of tourmaline were observed cutting across the general direction of the foliation, while sphene is an unimportant local accessory.