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Progressive Metamorphism.
Establishment and Definition of the Metamorphic Zones: In his pioneer work upon progressive metamorphism in the Scottish Highlands, Barrow (1893 and 1912) established a sequence of metamorphic zones based upon the mineralogical changes observed to take place in sediments of pelitic composition, with gradually increasing metamorphism. Each zone was named after an index mineral, the first appearance of which defined the outer limit of the zone in question. The more recent investigation of Tilley (1925a) upon rocks south-west of the area studied by Barrow were also confined mostly to pelitic rocks. Tilley accepted the sequence of zones put forward by Barrow in 1912, except that he included the two outermost zones (Barrow's zones of clastic mica and digested clastic mica respectively) in a single zone characterised by chlorite. Modern workers (e.g., Tilley, 1925a; Elles and Tilley, 1930) now recognise for pelitic or psammo-pelitic rocks six zones characterised respectively by the following index minerals, in order of increasing metamorphic grade: chlorite, brown biotite, almandine, staurolite, kyanite, sillimanite. It is generally recognised, however, (e.g., Elles and Tilley, 1930, p. 622) that staurolite is formed only under rather special circumstances, so that the staurolite zone is not as clearly defined as the other five.
The effects of progressive metamorphism upon the green beds of the Scottish Dalradian have also been worked out recently (Phillips, 1930), and the mineral associations characteristic of all zones from that of chlorite to the kyanite-sillimanite boundary, as defined for pelitic rocks, have been determined.
From the petrological details given in the previous pages it is apparent that the rocks of the area between the Haast and Cascade Rivers also furnish an example of progressive metamorphism on a regional scale. Pelitic rocks are only rarely developed, however, and cannot therefore be used as the basis for zoning. By far the greater portion of the schists and gneisses of this region are of the quartzo-feldspathic type, derived ultimately from sandstones and grits of greywacke composition, and characterised by abundant quartz and sodic plagioclase, together with constant but small amounts of mica or chlorite. An attempt has therefore been made to establish a sequence of zones based upon the effects of progressive metamorphism of the quartzo-feldspathic rocks. In a later section the probable correlation with the zones as defined by Barrow and Tilley for pelitic rocks will be discussed.
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In the Haast-Cascade area three zones are recognisable on this basis, and are characterised respectively by chlorite, biotite, and oligoclase. This sequence of zones is thus somewhat comparable with that established by T. Vogt (1927) for the progressively metamorphosed sediments of the Sulitelma area of Norway. The indices of Vogt's zones are chlorite, biotite, almandine, and oligoclase.
Distribution of Metamorphic Zones: The approximate positions of the isograds which limit the three zones recognised above, as deduced from examination of several extensive critical sections, are shown upon the accompanying map. These lines are no more than approximations, since their exact determination would involve some months of additional field work in exceedingly difficult and inaccessible country.
The zone of chlorite occupies the greater part of the map, and continues south-east beyond its limits for many miles into the great belt of schists of low metamorphic grade, which underlie Central Otago and constitute the Maniototo Series of Professor Park. In the north-east corner of the area mapped, however, the complete chlorite zone is limited to a width of about 10 miles. Here there is relatively rapid transition from coarse quartz-albite-chlorite-schists near the biotite isograd, through the phyllites of the upper Haast Gorge, to slates, which appear in the vicinity of Haast Pass and are the least metamorphosed rocks of the chlorite zone.
The biotite zone constitutes a narrow belt of schists which flank the rocks of the chlorite zone on the north-west. They are well exposed along the southern side of the Haast Valley, between Thomas Bluff and the Landsborough Junction, and may also be observed on the south-eastern side of the Jackson Valley. In this latter locality the biotite zone is not fully developed, but is cut off sharply on the north-west by the Cascade-Jackson Valley Fault, which here constitutes the boundary between the zones of biotite and oligoclase.
The typical feldspar of the biotite zone, as defined above, is albite containing 5% to 7% of anorthite. As the metamorphic grade increases the plagioclase becomes richer in anorthite until the composition of oligoclase (Ab90An10 to Ab70An30) is attained. An oligoclase isograd has therefore been drawn, marking the first appearance of feldspar containing 10% of anorthite, and an oligoclase zone is recognised, including the area occupied by oligoclase-biotite schists, hornfelses, and gneisses. In those portions of the oligoclase zone where metamorphism has been most intense—e.g., the area of gneisses one mile inland from Mr J. Cron's homestead, near the mouth of the Haast River—basic oligoclase, oligoclase-andesine, and even in one case andesine of composition Ab60An40 may be developed. Normally, however, the composition of the feldspar of the oligoclase zone lies between Ab90An10 and Ab80An20.
The zone of oligoclase forms a wide belt of intensely metamorphosed rocks lying north-west of the biotite zone and stretching right to the coast. In the Haast Valley section the oligoclase isograd cuts the valley on its south side, not far east of Thomas Bluff, and from this point to the sea within the oligoclase zone there is a steady increase in intensity of metamorphism. Ten miles east of Thomas
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Bluff, however, there is a small isolated area in the vicinity of Douglas Creek which certainly lies well within the oligoclase zone, for the feldspar varies from medium oligoclase to oligoclase-andesine. The exact limits of this area have not been determined, but its approximate extent is indicated on the map.
The rocks of the oligoclase zone are also developed in considerable variety in the south-western portion of the map, between the sea coast and the Cascade-Jackson Valley Fault, which has here brought them into juxtaposition with some of the less metamorphosed of the schists of the biotite zone. Transition between the two zones, such as may be seen in the Haast Valley section, is therefore not apparent in this southern area. Indeed, the abrupt change in metamorphic grade at the outcrop of the fault is so conspicuous that in an earlier paper (Turner, 1930, p. 186) the rocks of the oligoclase zone (north-west of the fault) were provisionally separated as a distinct series from the quartz-albite-biotite-schists and the schists of the chlorite zone to the south-east.