Date of Metamorphism.
In his paper on the metamorphic rocks of South Westland, Turner (1933) carefully summarises such evidence as is available to indicate the date of progressive dynamothermometamorphism which affected as a combined unit the rocks of that area and of Central Otago, and was later followed by movements which led to retrogressive changes in certain of these rocks. He clearly states two alternative hypotheses that are possible, namely:
(a) That there was an early period of intense metamorphism affecting rocks now represented in Palaeozoic and Mesozoic conglomerates followed by a later, Lower Cretaceous, period yielding the schists of Otago, Westland, and Nelson.
(b) That there was a single Palaeozoic period of intense metamorphism.
In his paper Dr. Turner favours the second hypothesis, and points out that the subsequent retrogressive effects evidenced by many schists could well have been introduced by the Early Cretaceous (post-Hokonuian) orogeny.
It cannot be claimed that the Ururoa Point conglomerates yield evidence at all conclusive in its bearing on this question, though it does focus attention on one aspect of the problem, namely, that, if the first hypothesis be assumed correct, erosion of exposed Palaeozoic metamorphic rocks resulting from the earlier period of pressure and batholithic invasion was proceeding in Jurassic times. It becomes desirable, therefore, to determine if possible the probable date of this “early” metamorphism, and, in this connection, Dr. Turner's (1933) demonstration of the gradual metamorphic transition from the schists of Central Otago to those of South Westland assumes considerable importance, for, under the theory discussed, it involves the necessity of referring the South Westland schists, in conjunction with those of Central Otago, to the later period of metamorphism. The unconformity separating the schists of the two periods would thus have to be below the South Westland schists.
Fig. 1.—Soda-granite (U 26). Essentially large crystals of albite enwrapped by micropegmatite. Nicols crossed. × 32 diams.
Fig. 2.—Hornblende - augite - andesite (U.25). Large semi-opique phenocrysts of hornblende crowded by dust-like magnetite occur with smaller clear ones of shattered feldspar in a microcrystalline groundmass × 32 diams.
Fig. 3—Quartz-sericite-chlorite-epidote-clinozoisite-schist (U.4) Black opaque portions are enrichments of magnetite and are associated with abundant epidote To the right of central quartz-rich band there is chlorite arranged transverse to schistosity. Nicols crossed. × 32 diams.
Fig. 5—Granoblastic quartz - albite - chlorite-muscovite hornfelsic schist (U 16) A large poikıloblastic crystal of muscovite occupies much of upper half Ragged chlorite (grey) with associated iron-ore, quartz (clear) and weathered albite (grey) also are visible. × 32 diams
Fig. 6.—Quartz - oligoolase - mica -? cordierite-schist (U.13) A large poikıloblastic pinite replacement of ? cordierite is enwrapped by blotite (dark) with grams of quartz and a little feldspar × 29 diams
Fig. 7.—Blastoporphyritic andesine-hornblende-quartz-biotite-schist (U.2). Porphyroblasts of cloudy andesins are enclosed in a mesostasis in which sub-parallel prisms of hornblende are conspicuous × 32 diams
In Preservation Inlet area, the Ordovician sediments show metamorphism comparable with that of the South Westland rocks further north, and, if, as appears reasonable, the metamorphism of the two sets of rocks was one and the same event, the date of the assumed earlier period of pressure and igneous invasion must have been pre-Ordovician.
This provisional conclusion admittedly rests on evidence that is not conclusive, but its implications, nevertheless, are worth consideration: they are that, in the area supply débris to the Ururoa Point conglomerates, either the sediments representing the whole of Ordovician and subsequent pre-Triassic time had been removed by the Jurassic, or else the assumed Lower Cretaceous metamorphism so prominently evidenced in these rocks in the South Island did not affect them in the north, for metamorphic rocks comparable with those of the South Island are not there known, whether in situ or in Upper Cretaceous or Tertiary conglomerates.
Neither alternative appeals to the writer, and he therefore prefers to regard the schists of the Ururoa Point pebbles as a product of an epoch of metamorphism certainly prior to the Upper Triassic. but not pre-Ordovician. The date of this epoch must be placed in advance of that of the Ururoa Point beds because the Kawhia section indicates substantial conformity in the Upper Triassic and Jurassic sequence. Certain similarities drawn attention to above by Dr. Turner between the schists described in this paper and those of South Westland make it appear, therefore, that the major progressive metamorphism of the Otago and South Westland region must similarly be pre-Triassic, a view which conflicts with the belief of Ongley (1933) that it took place at the end of the Jurassic. Evidence from the Preservation Inlet area noted above indicates that this metamorphism must be later than the Ordovician.
In conclusion, it is perhaps desirable to point out that obviously the retrogressive metamorphic effects exhibited by some of the rocks of the Ururoa Point pebbles cannot be due to the early Cretaceous orogeny by which Turner (1933) provisionally explains similar changes in his South Westland schists.