Metamorphic Zones in North-west Otago
[Read before the Otago Institute, April 16, 1935; received by the Editor, May 4, 1935; issued separately, March, 1936.]
In a recent account of the metamorphism of the Te Anau Series, one of the writers (Turner) has subdivided the Chlorite Zone of the Otago metamorphic region into subzones of progressive metamorphism based upon the degree of reconstitution shown by rocks at different stages in the transition between greywacke and quartz-albite-epidote-chlorite-schist. This scheme was applied to the district northwest of Lake Wakatipu, where the Te Anau rocks grade into schists of the Central Otago type.
For the past two years the other writer (Hutton) has been engaged in a detailed study of an area of 400 square miles lying between the upper arm of Lake Wakatipu and the lower end of Lake Wanaka, where he has independently established the existence of subdivisions of the Chlorite Zone comparable with those developed further northwest. Since the detailed results of Hutton's work will not be published for some time, it is considered advisable to record in this preliminary note the sum of the available data as to variation in metamorphic grade throughout the northwest corner of Otago. In addition to the two areas referred to above, the accompanying map includes an extensive region lying north and east of the head of Lake Wakatipu, where the distribution of the zones is based partly upon material collected by various members of the Otago Section of the New Zealand Alpine Club and partly upon the writers' own observations1.
The four divisions of the Chlorite Zone are designated in order of increasing metamorphic grade by the symbols Chl. 1 to Chl. 4. The first three have already been defined by Turner, and need not be mentioned further. The Chl. 4 subzone, however, is developed only in the more easterly portion of the map in the area studied by Hutton, whose observations are recorded below.
The characteristic features of the schists of Chl. 4 Subzone as distinguished from those of Chl. 3 are their much coarser grain (averaging 0.25 mm-0.4 mm.) and strong foliation, the thickness of the quartz-albite foliae frequently being as much as 8 to 10 mm. Mineralogically the rocks are completely reconstituted and all traces of clastic structures have been obliterated.
The following equilibrium assemblages of minerals have been observed in this subzone:—
(a) Quartzo-feldspathic schists:
[Footnote] 1 Cf. F. J. Turner, Schists from the Forbes Range and Adjacent Country, Western Otago, Trans. Roy. Soc. N.Z., vol. 64, pp. 161–174, 1934.
(c) Micaceous schists:
(d) Ferruginous schists:
(e) Manganiferous schists:
The typical Central Otago schists described by earlier writers as the Wanaka and Maniototo Series belong without known exception to Subzone Chl. 4. The less metamorphosed rocks of Chl. 3 and Chl. 2, occurring only along the borders of the Otago schists region, are those to which names such as Kakanui Series and Cecil and Walter Peak Series have previously been applied. In the Chl. 1 subzone where only incipient stages of metamorphism are reached, the rocks have been classed as belonging to the Te Anau Series, into which they undoubtedly merge in north-west Otago.
Attention is drawn to the fact that the distribution of the subzonal boundaries, originally determined by orogenic movements which controlled the metamorphism of the rocks in question, have since been modified by the effects of subsequent dislocations, some of which are no older than the later half of the Tertiary Era. The accompanying map shows only defiuitely established faults and thrusts, notably that which has caused the involvement of Tertiary strata among the schists for many miles north of Bob's Cove, Lake Wakatipu. Rapid changes in metamorphic grade, as shown by the distribution of the four divisions of the Chlorite Zone, indicate the probable existence of yet other major dislocations. For example, the westward prolongation of Chl. 4 north of the Upper Dart is perhaps due to an east-west fault along the upper portion of the Dart Valley. Again the narrow outcrop of Chl. 3 along the eastern shores of Lake Wakatipu suggests that the upper arm of the lake is located along a fault zone. Proved faults and thrusts in the district surrounding Bob's Cove, Lake Wakatipu, have undoubtedly caused profound modifications of the distribution of the Chl. 3 and Chl. 2 subzones, as seen in the map.