The Metamorphic and Plutonic Rocks of Lake Manapouri, Fiordland, New Zealand—Part II*
[Read before the Otago Branch, November 10, 1936; received by the Editor, November 11, 1936; issued separately, September, 1937.]
The Western Manapouri Province.
The Pegmatites and Aplites.
Distribution and Relations of Rock-types.
The Doubtful Sound Province.
Corelation within the Manapouri Region.
Correlation with other parts of Southern New Zealand.
Literature Cited in Part II.
The Western Manapouri Province.
The Western Manapouri Province lies within an injection-complex in which coarse grey gneisses are invaded by a “granitic” bathylith extending north and south far beyond the region mapped. Lit-par-lit injection is widespread, and there has probably been local minor contamination both of the host-rocks and the invading granites. In any one locality three distinct classes of rock can usually be recognised, viz., banded grey gneisses (constituting the host-rock of the injection-gneisses), intrusive “granites” and minor acid dyke-rocks. The petrography of these three classes will first be described, and their distribution, field-relations and petrogenesis will be discussed in later sections.
Forty specimens of the host-gneisses from the western province were sectioned and microscopically examined. For the most part they are dark-grey, rather poorly foliated coarse-grained rocks with average grain-sizes varying between 0.5 mm. and 5 mm. In many the crystals of mica and to a less extent hornblende show an obvious tendency toward parallel orientation, and distinct fissility results.
[Footnote] * This paper follows on directly from Part I, which appeared in Trans. Roy. Soc. N.Z., vol. 67, pt. 1, pp. 83–100.
On mineralogical grounds three fairly sharply defined divisions of these rocks may be distinguished, characterised respectively by the essential assemblages, plagioclase-biotite, plagioclase-hornblende-biotite and plagioclase-orthoclase-quartz-hornblende-biotite.
Plagioclase-biotite-gneisses. No. 2374 is typical of a group of rocks occurring around the upper portion of the West Arm, in which plagioclase and biotite are the only important constituents and quartz is lacking. The average grain-size is 1 mm. The approximate composition is acid oligoclase (An10–11) 60%, deep greenish-brown biotite 25%, epidote 10%, hornblende 3%, sphene 1% and accessory apatite, pyrite and nearly colourless acicular rutile. The amount of epidote is unusually high for rocks of this type, but the optical properties are typical and indicate a high iron-content: y—a = 0.41—0.46, sign negative, Z ∧ 001 cleavage = 27° in sections perpendicular to Y; colourless in all sections. The crystals are coarse, ragged and often intergrown at the margins with vermicular plagioclase. The sphene is in coarse pale-yellow grains with irregularly embayed or rounded outlines. Some of the grains of feldspar show marginal granulation and undulose extinction. In contrast with the above many of the plagioclase-biotite-gneisses from the North Arm contain between 5% and 15% of quartz, and a rather more calcic plagioclase averaging about An22. For example the composition of 2804 is medium oligoclase 60%, quartz 20%, deep yellowish-brown biotite 20% and accessory epidote, sphene, apatite and rare stout prisms of zircon. Rarely the biotite-content of these quartz-bearing gneisses is as low as 10% (e.g., 2461, 2464) and the composition of the rock is then closely similar to that of the trondhjemitic members of the invading granitic series except that potash-feldspar is completely lacking.
Plagioclase-hornblende-biotite-gneisses. No. 2389 (Fig. 1) is typical of a widely-distributed type of gneiss in which hornblende is the dominant dark constituent. The average grain-size is between 1 mm. and 1.5 mm., and the constituents are acid oligoclase (An17) 50%, hornblende 35%, epidote 10%, chloritised biotite 5%, sphene 1% and accessory apatite. The hornblende, as in all the rocks of this group, is a strongly pleochroic type with X = rather pale yellow, Y = deep olive-green, Z = deep-green with a tinge of blue, Z > Y > X. The grains of epidote are coarse, irregular, colourless and highly birefringent, and are often intergrown perfectly with vermicular plagioclase. The sphene too is coarse and irregular. In other rocks of the same type, biotite may be more plentiful (up to 20%), and the plagioclase more calcic (An35 in 2382, 2389) or occasionally rather richer in albite (e.g. 2463). In a few specimens quartz is present to the extent of about 10% of the total composition and hornblende and epidote are less plentiful than usual. The composition of 2454 for example is basic oligoclase 60%, quartz 10%, hornblende 15%, biotite 15% and small quantities of sphene and apatite.
Black amphibole-rich layers a few inches in thickness sometimes containing 80% of hornblende (2786) occasionally occur as local phases of the hornblende-gneisses. Here also may be included a specimen (2466) of black coarse-grained rather incoherent schist
interbedded with the gneisses on the eastern side of the North Arm, about a mile from the head. The composition is yellowish-brown biotite 45%, hornblende 45%, quartz 5%, oligoclase 5%, accessory coarse apatite and rare allanite. The hornblende, occurring in stumpy prismatic grains, is much paler in colour than in the typical hornblende-gneisses (X = very pale yellow, Y = pale yellowish-green, Z = rather pale bluish-green, Z > Y > X, Z ∧ c = 20°). An identical rock (2798), but with chlorite replacing biotite, is enclosed in the gneissic granite just outside the entrance to the West Arm on the southern shore of the lake.
Orthoclase-bearng Gneisses. In the vicinity of the North Arm and along the northern shore of the lake for some distance east of the entrance to the arm, many of the host-gneisses contain considerable quantities of potash-feldspar and approximate to hornblende-granite or adamellite in mineral composition. Plagioclase (acid to medium oligoclase) usually makes up about 40% or 50% of the rock and is accompanied by plentiful potash-feldspar (15% to 40%) and quartz (10% to 15%). Deep-green hornblende and yellowish-brown biotite together come to only 10% or 15% of the total composition, the former being the more plentiful of the two. Very coarse idiomorphic crystals of yellow sphene are often conspicuous even in the hand-specimen (2434). Epidote (1% to 3%) and apatite are constant minor constituents, while zircon, rutile and allanite are occasionally present. The potash-feldspar in these rocks is usually orthoclase with locally-developed microcline-and perthite-structures in parts of the larger crystals; it usually occurs as highly irregular interstitial masses, the largest of which may completely enclose grains of plagioclase and quartz (e.g. 2434), while on the other hand, small lakelets of orthoclase sometimes have also developed within the crystals of oligoclase (e.g. 2427). Myrmekitic growths of plagioclase and vermicular quartz are not uncommon at plagioclase-orthoclase junctions. There is no doubt that in some cases orthoclase-bearing gneisses have been produced in the Manapouri region as local modifications of plagioclase-hornblende-biotite-gneisses in the immediate vicinity of contacts with invading granitic or pegmatitic veins (e.g. 2802, 2803, 2782). In such rocks some of the orthoclase of microcline is very coarse—e.g. in 2803 and 2782 large single crystals in several instances completely enclose two or three grains of oligoclase—while commonly (e.g. 2802) rounded blebs of potash-feldspar are scattered through the larger plagioclases; there is always much ragged interstitial orthoclase in addition. On the other hand many of the orthoclase-bearing gneisses (e.g. 2808, 2434) represent a definite widely developed facies of the gneissic series apparently independent of proximity to invading granites. The possibility of large-scale introduction of potash-feldspar into rocks such as these is unlikely, as will be shown in a later section.
The igneous rocks invading the gneisses of the Western Manapouri Province are acid types falling into two main groups, viz. (a) the components of the main bathylith, and (b) pegmatites, aplites and related rocks which, though widespread in their occurrence, are limited to minor veins and dykes. In areas of injection-gneiss rocks
of the first group also take the form of veins and thin sheets, but these are readily correlated on petrographic grounds with different facies of the main intrusion, and are thus distinguished from the pegmatites and aplites. These latter are described separately in the next section.
Thirty-eight representative sections of the granitic rocks have been examined. They are white, often gneissic, seldom coarse-grained rocks, having the following petrographic characters in common: abundance of quartz (15% to 40% of total composition), unformly acid composition of the plagioclase (sodic or medium oligoclase), relatively low content of biotite (typically 2% to 5%), absence of hornblende, almost universal presence of small quantities of coarse epidote and frequent presence of brown allanite among the accessories; effects of shearing are often pronounced. The rocks fall into two groups based on the content of potash-feldspar. In the first the proportion of this mineral falls below 5% and usually is as low as 1% or less, and the rocks in question are therefore classed as trondhjemites (cf. Goldschmidt, 1916, p. 76; Mackie, 1935, p. 296). In the second orthoclase or microcline is an essential constitutent, but in all except three out of thirteen specimens is subordinate to oligoclase. While some of these rocks contain sufficient potash-feldspar to be classed as adamellites or rarely even as potash-granites, the majority have too little for this and for want of a better term are simply grouped as oligoclase-granites.
Trondhjemites. The trondhjemites are rather fine-grained, white or greyish, usually distinctly gneissic rocks in which the flakes of biotite tend to be concentrated in thin, closely spaced subparallel layers. Where veins and sheets have been injected parallel to the foliation of the host-gneiss the trend of foliation in the trondhjemite usually coincides with that in the invaded rock.
Microscopically the structure is granitic, with an average grainsize of 1 mm. to 2 mm., but evidence of shearing is seen in frequently undulose extinction in grains of quartz, and sometimes in partial granulation of quartz and feldspar (e.g. 2371). The mineralogical composition is remarkably uniform: acid to medium oligoclase, 55%–80%, quartz 15% to 40%, potash-feldspar < 5%, biotite 3% to 5% (rarely as high as 8%, e.g. in 2365), minor epidote and accessories. The plagioclase typically has a composition near to An20 but sometimes is as sodic as An10–15 (2834, 2835) and rarely as calcic as An27 (2403). It tends to occur as subidiomorphic or allotrimorphic grains which often enclose small rather irregular lakelets of orthoclase or microcline (e.g. 2385, 2391); in 2371 the larger crystals of oligoclase enclose rounded quartzes about some of which very narrow peripheral zones of orthoclase have developed between the host-mineral and the quartz. In most rocks the potash-feldspar, either orthoclase or microcline, occurs as small, exceedingly irregular interstitial masses between the grains of quartz or plagioclase, and as sparsely-scattered antiperthitic lakelets in the larger grains of oligoclase; only rarely (e.g. 2421 and 2806) were a few coarser crystals, comparable in size with those of plagioclase, observed in addition. Myrmekite has sometimes developed along the junctions of the two varieties of feldspar (2388, 2791, 2810). Typically the biotite is a strongly pleochroic reddish-brown
type (occasionally green, e.g., in 2383), and in some sections (e.g. 2365, 2422) shows a definite tendency to develop around central coarse prisms of epidote (Fig. 3). Small quantities of epidote, usually less than 3%, are always present as coarse irregular grains the margins of which may occasionally be intergrown with vermicular quartz; in 2810 the habit of the epidote is definitely interstitial and the outlines of the grains exceedingly irregular in consequence. The mineral is colourless or rarely (2455) faintly pleochroic from pale yellow (Z) to colourless (X and Y), the optic sign always negative, and the birefringence medium to high (e.g. 0.028 in 2365; 0.040 in 2368). The larger grains frequently enclose rounded cores of allanite in crystalline continuity with the epidote but having different optic orientation (e.g. 2365, 2366, 2370), while in other sections allanite occurs as well-formed prismatic crystals sometimes reaching 1 mm. in length (e.g. 2810). A typical pleochroic scheme for allanite (2810) is—
X = nearly colourless,
Y = deep yellowish-brown,
Z = deep yellowish-brown,
Z > Y > X.
Sometimes, however, a greenish-brown tint comes in for the Y and Z vibration-directions. Muscovite is present in several sections—never in more than accessory amount—and conveys the general impression of having crystallised late, perhaps in some cases by reaction between the already-crystallised feldspar or biotite and a potassic residual magmatic liquid. Its occurrence in sections 2366 and 2791 is typical. In the first of these rocks, the flakes of white mica are small and sharply defined (mean length = 0.1 mm.), and tend to occupy interstitial spaces between the larger feldspars and to be concentrated especially in narrow streaks, in association with epidote and biotite, parallel to the foliation. Individual flakes may penetrate the margins of plagioclase grains and rarely may be scattered sparsely throughout the whole crystal. In 2791 muscovite (about 1% of the total composition) occurs (1) as independent coarse flakes, (2) as vermicular intergrowths with quartz or feldspar, developed along cracks between or cutting across grains of feldspar, and (3) as narrow borders partially surrounding flakes of biotite. Rather stout idiomorphic or subidiomorphic prisms of apatite occur in nearly every section, while rounded or embayed grains of sphene sometimes reaching 0.5 mm. in diameter are not uncommon (2368, 2380); smaller idiomorphic sphenes were also noted in some rocks (e.g. 2367). Stout small prisms of zircon with a somewhat dusty and corroded appearance were recorded in several sections, usually in association with the biotite. Pleochroic haloes have not developed in the latter mineral, however. Very minute slender needles of pale-yellow rutile are often enclosed here and there in the feldspar. In one section (2385) there are a few small rounded grains of pink garnet.
Oligoclase-granites and Adamellites. In hand-specimens the oligoclase-granites and adamellites are massive white rocks of typically granitic aspect, usually rather coarser than the trondhjemites and lacking the gneissic structure and slight fissility characteristic of those rocks.
The oligoclase-granites, which are the most abundant and widely-distributed rocks of this group, are exemplified by 2378 (bluffs at south side of head of West Arm) and 2861 (eastern side of entrance to North Arm). The essential constituents of these two rocks are acid to medium oligoclase 50% to 60%, orthoclase or microcline 20%, quartz 15% to 20%, biotite 3% and minor muscovite. In the adamellites (e.g. 2452, 2790, both from the North Arm) the proportions of oligoclase and orthoclase are subequal (about 30% or 40% of each). In two rocks only (2444, 2445, west side of North Arm ½ mile from head) the potash-feldspar is considerably in excess of the plagioclase and classification as true granites is warranted.
The minerals show much the same features as in the trondhjemites except that the orthoclase, or microcline as the case may be, is in much coarser crystals; the habit is still interstitial or enwrapping, however, and perthitic striping is not uncommon. Myrmekite was frequently noted at junctions between potash-feldspar and plagioclase. In almost every section there are small quantities of muscovite which shows the same tendency to develop along cracks in plagioclase or as rims to biotite, as in the trondhjemites. Intergrowths of muscovite and vermicular quartz are also common. Apatite is ubiquitously present as an accessory, allanite, sphene, zircon and rutile being less frequent. Crush-effects are displayed in nearly every specimen, while occasionally (e.g. 2459) shearing has been severe enough to produce mortar-structure throughout the section.
The Pegmatites and Aplites.
Dykes of pegmatite are very widely distributed in the Central Manapouri Province. The rocks are normal potash-pegmatites showing few unusual features and composed essentially of about equal amounts of oligoclase (An10 to An18) and microcline, with about 20% or 25% of quartz. A few coarse hexagonal flakes of biotite are always present (X = pale yellow, Y = Z = deep sepia-brown to almost black), but white mica is rare (2419). Apatite is a common accessory, epidote (e.g. 2436, 2437) and sphene (2367, 2436) rather less so, while garnet (2387) rutile and allanite (2437, 2458) are rare. The microcline is usually coarse and sometimes has perthitic structure (e.g. 2392, 2787). The plagioclase tends on the whole to occur in smaller crystals than those of microcline, and often encloses lakelets or antiperthitic inclusions of the latter mineral (e.g. 2387, 2390). Quartz-feldspar intergrowths are rare (2787). In 2437 coarse masses of plagioclase ranging up to 6 mm. in diameter enclose numerous rather rounded fingers of quartz of variable optic orientation. Swarms of similar fingers or rod-like bodies of quartz with rounded ends also occur locally within microcline in the same rock. Myrmekite is sometimes well developed as outgrowths from grains of oligoclase adjacent to microcline (e.g. 2390, 2458). Partial or complete granulation of quartz, and to a less extent feldspar, is universal in the pegmatites, in some of which (e.g. 2393) the crushed quartz and the flakes of mica have been drawn out into ribbons between the larger crystals of microcline. Generally speaking, shearing in the pegmatites and aplites has been much more effective than in the “granites” and especially the gneisses that they invade.
The aplites are much more restricted in their distribution: one (2443) comes from the point at the north side of the entrance to Aweburn Bay, North Arm, while all the remaining specimens (2468, 2799–2802) were collected about half-way up the North Arm on its western side. These rocks are distinguished from the pegmatites by their generally finer texture, absence of biotite, and the presence of small rounded crystals of pink garnet dotting the otherwise white surface of the rock. The main constituents are oligoclase 40% to 60%, microcline 10% to 40%, quartz 20% to 30% and minor garnet. Muscovite, epidote and apatite are common accessories, while allanite, rutile, biotite and magnetite were noted in single sections. The microcline is usually interstitial and myrmekite has sometimes developed at its margins. The grain of the rocks varies a good deal even within the limits of a microsection; thus in 2800 occasional crystals of oligoclase reach 3 mm. in diameter though the average grain-size is about 0.05 mm.
No. 2443 is a soda-aplite very different from the garnet-aplites just described. It is an even-grained rock (average, 0.05 mm.) consisting of albite (An7) 65%, quartz 25%, muscovite 10% and accessory clinozoisitic epidote. Complete absence of potash-feldspar is noteworthy. The crystals of muscovite are relatively coarse, sharply defined and obviously primary.
Distribution and Mutual Relations of Rock-types.
(1) North Shore of Lake, East of North Arm.
The section from the entrance to the North Arm to the western end of the boulder-beach opposite the north-west corner of Pomona cuts across the trend of a belt of injection-gneisses striking between 5° and 15° W. of N. There is a uniform steep westerly dip, except towards the eastern end of the section, where the dip flattens and the gneisses are regularly corrugated by small-scale folds with north-south axes. These contorted rocks show microscopic evidence of more than usually strong cataclasis (2812).
The injection-gneisses consist of a dark schistose host-rock, invaded by more massive leucocratic trondhjemite or oligoclase-granite—the latter mainly near the mouth of the North Arm (e.g., 2861). The relative importance of these two principal phases varies considerably. Near the entrance to the arm the granitic component is dominant, but farther east it is rapidly displaced by the dark host-rock. Halfway along the section both are equally developed, but towards the eastern end the trondhjemitic phase of the injectiongneiss once more predominates. Where the intrusive component is in excess, the invaded gneiss occurs as blocks several feet in diameter, surrounded by “granite”; but where the host-rock predominates, bands and veins of trondhjemite have been injected along the foliation-planes of the original gneiss and a typical lit-par-lit gneiss is the result. Transgressive veins of trondhjemite are rather uncommon. Dykes of coarse pegmatite (2437) sometimes several feet in width cut irregularly across the injection-gneisses at many points.
Throughout much of the section the host-rock is a quartz-bearing oligoclase-biotite-gneiss (2433, 2811) sometimes carrying a little horn-blende. A coarse-grained orthoclase-bearing gneiss (2434) studded with conspicuous sphene is widely developed, however, for some distance east of the prominent sandy beach about 1 ½ miles from the entrance. The relation between this striking rock and the normal plagioclase-biotite-gneiss into which it seems to grade is not clear; both rocks are certainly invaded by trondhjemite and pegmatite.
(2) The Eastern Shore of the North Arm.
For about 1 ½ miles from the entrance the cliffs along the eastern shore of the North Arm afford fine exposures of coarse grey gneiss, intricately veined with massive dykes of pegmatite (2809) and trondhjemite (2806, 2810). The intrusive rocks are on the whole subordinate in quantity to the invaded gneiss, but locally this condition may be reversed. The host-rocks are coarse orthoclase-bearing oligoclase-quartz-hornblende-gneisses (2805, 2808) rather similar to the orthoclase-gneiss mentioned in the preceding paragraph. The strike of the foliation varies from 15° to 30° W. of N.
Further north, along the southern side of a prominent point opposite Aweburn Bay, oligoclase-granite becomes dominant, and at the point itself encloses large blocks of oligoclase-orthoclase-quartz-biotite-gneiss (2788) the potash-feldspar of which has been introduced from the enveloping granite-magma; small amounts of muscovite intergrown marginally with vermicular quartz are more definite evidence of contamination in these gneissic inclusions. North of the point the granitic rocks immediately give way once more to banded gneisses that continue for five or ten chains until the oucrop is obscured by a small beach. The rocks here exposed are mainly plagioclase-biotite-and plagioclase-hornblende-biotite-gneisses without orthoclase (2785, 2463). Leucocratic phases (2464) relatively poor in biotite, and thus approaching the trondhjemites in composition, and black schistose hornblende-biotite-rich or purely hornblendic types (2466, 2786) are developed locally as variants of the normal gneiss. The only definitely intrusive rocks in this locality are minor veins of pegmatite. For about half a mile further north banded gneisses (2461, 2462) of much the same type predominate, though here and there the section is interruped by masses of “granite.”
At a small point that juts into the lake at the northern end of a long sandy beach about one-third of a mile from the head of the arm, oligoclase-granite (2459) grading into adamellite (2452) becomes dominant and continues so to the head itself. At several places in this vicinity large blocks (sometimes 15 ft. in diameter) of plagioclose-hornblende-biotite-gneiss with conspicuous sphene (2454) are completely surrounded by rather gneissic granite. The strike of the foliation in the latter varies a good deal (15° W. of N. to 65° E. of N.), but the dip is always steep in contrast with the subhorizontal foliation of the enclosed blocks of gneiss. The inclusions are cut here and there by veinlets of pegmatite (2458) and of a trondhjemitic variant of the granite (2455), but there is no microscopic evidence of contamination by the granitic magma.
(3) Western Side of the North Arm.
Description of the full section is unnecessary. As on the opposite shore, oligoclase-granite and adamellite predominate near the head of the arm, while the host-gneisses become much more important towards the middle and southern end of the section. Two localities where exposures are particularly clear are worth describing in detail.
At its northern end the entrance to Aweburn Bay is defined by a sharp promontory where coarse grey gneisses, striking at 65° to 70° W. of N. and dipping southward at low angles (20° or so), are invaded by massive dykes of white “granite.” The latter rather exceeds the host-rock in total bulk and builds up a massive framework between and around the large isolated blocks of gneiss. The invaded rocks (2441, 2447, 2448, 2451, 2783, 2784) consist of oligoclase, microcline, quartz, biotite and hornblende, often with coarse epidote as a less important constituent. Very coarse sphene is usually present and occasionally may make up 1% to 2% of the microsection (2447). Sections cut from specimens immediately adjacent to veins of pegmatite or granite show that in some cases the potash-feldspar has been introduced into the gneiss (e.g., in 2782), but it is uncertain how far general contamination of this type has taken place. Local chloritisation of biotite, development of yellow rims to grains of epidote and recrystallisation of small interstitial patches of stilbite (2451) are attributed to hydrothermal activity in connection with intrusion of granites or pegmatites. The main intrusive rock is sufficiently rich in potash-feldspar to be classed as a typical potash-granite (2444, 2445), but immediately north of the promontory it passes into oligoclase-granite and trondhjemite. Minor intrusions include pegmatites and the soda-aplite 2443.
About halfway along the west side of the arm not far below a prominent waterfall is a small island lying a few chains south of a sandy cove on the main shore. On the island steeply-dipping plagioclase-biotite-and plagioclase-hornblende-biotite-gneisses (2804, 2803) striking 30° E. of N. are intricately veined with dykes of garnet-aplite (2799–2802) a foot or so in width. A micro-section (2802) of the actual junction between gneiss and aplite shows clearly that potash-feldspar has been introduced locally into the latter from the aplite magma. At the southern side of the cove the gneisses are orthoclase-bearing types (2470, 2789) consistently carrying allanite and occasionally stilbite (2470). These are here invaded irregularly by sparse dykes of pegmatite, garnet-aplite (2468) and adamellite (2790), and a chain or so away on the opposite side of the cove by massive trondhjemite which continues as the dominant rock for some distance further north. The strike of the gneisses at the cove is 40° E. of N. and the dip about 25° to the north-west.
(4) The West Arm.
Where the West Arm cuts across the injection-complex, the host-rocks are plagioclase-hornblende-biotite-gneisses (2369, 2373, 2379, 2382, 2389) and plagioclase-biotite-gneisses (2374, 2375, 2386), ortho-clase-bearing types invariably being absent. Epidote is more plentiful than in corresponding rocks from other parts of Lake Manapouri, but quartz which elsewhere is commonly present is here consistently
absent. On the whole the intrusive rocks preponderate over the invaded gneisses in the vicinity of the West Arm. Fine-grained rather gneissic trondhjemites are ubiquitous (2365, 2366, 2368, 2370, 2371, 2380, 2383–2385, 2388, 2391), but near the head of the arm oligoclase-granite (2394) outcrops in the bluffs along the south side. Pegmatites (2367, 2387, 2390, 2401) are everywhere abundant but aplites so far have not been observed. Localities where the hostgneisses are well developed are: north shore at either end of the long beach at the mouth of the Oonahburn; south shore around the headland immediately opposite the Oonahburn Valley. In many parts of the West Arm, gneissic trondhjemite seamed with pegmatite, and locally enclosing blocks of the invaded gneiss, outcrops continuously for long distances: e.g. along the northern shore near the head of the lake, and for a distance of a mile or so along the southern shore west of the entrance to the arm.
East of the West Arm the southern shore of the lake has not been examined in detail, but as far as the entrance to the South Arm oligoclase-granite and adamellite sometimes enclosing blocks of hornblendic or biotitic gneiss appears to be the principal rock along this stretch of coast (2796, 2797).
The following generalisations relating to distribution and mutual relationships of rock types within the Western Manapouri Province have been established and are illustrated in the sections just described:—
On the whole the gneisses and the invading “granites” (trondhjemites, oligoclase-granites, etc.) are equally extensive in the Western Manapouri Province. Dykes of pegmatite cutting both “granites” and gneiss are everywhere numerous, but aplites have so far been recorded only along the western side of the North Arm.
Though the strike of the gneisses varies considerably and often rapidly, it usually lies between 50° W. of N. and 30° E. of N. and the dip is generally at high angles. Towards the heads of the North and West Arms, and again on the north shore of the lake some three miles east of the entrance to the North Arm, the dip flattens considerably however.
The most widely distributed gneisses are those consisting essentially of plagioclase, hornblende and biotite, or plagioclase and biotite respectively; quartz-bearing varieties of these are common except in the West Arm where they have not been recorded. The coarse orthoclase-bearing gneisses also are not known to occur in the West Arm.
The intrusive “granites” are mainly trondhjemites and oligoclase-granites, differing from each other mainly with regard to the amount of potash-feldspar present. The oligoclase-granites occur principally in the vicinity of the North Arm and along the southern shore of the lake between the entrances of the West and South Arms, but examples are also known near the head of the West Arm. Adamellites and potash-granites are exposed only in the North Arm and are regarded as the potassic extreme of the trondhjemite series. From the above it will be seen that the granitic rocks
carrying 10% or more of potash-feldspar tend to be concentrated towards the centre of the injection-complex while trondhjemites are more frequent towards the eastern and western margins.
Mechanical effects of shearing are often noticeable in the rocks of the Manapouri Province. These are least marked in the gneisses, more obvious in the intrusive “granites,” and always very pronounced indeed in the pegmatites.
The compound bathylithic mass of trondhjemite and oligoclase-granite has the essential features of a synchronous bathylith as recently described by Browne (1931, pp. 114, 115). From the rapid alternation of “granite” and host-gneiss through the Western Manapouri Province, it is clear that only the upper portion of the intrusion with numerous roof-pendants is exposed in this area.
The structure and mineralogical composition of the host-gneisses in the injection-complex have been determined by temperature-stress conditions during metamorphism. The constant association oligoclase-hornblende-biotite shows that reconstitution was effected at high temperature; and though shearing-stress of rather low magnitude would account for the structural features of the rocks in question, the frequent presence of considerable quantities of epidote apparently in equilibrium with oligoclase indicates that the dynamic factor was important. Like the gneisses of Holmwood Island the rocks are therefore considered to be products of regional metamorphism of high—though not the highest—grade. While the rising bathylith may have contributed to some extent to the maintenance of high temperature during the closing stages of metamorphism, from field and petrographic evidence alike it is almost certain that by the time the rising magma had effected contact at any particular level with the host-rock, the latter had already assumed its present structure and composition. This agrees with Professor W. R. Browne's view that all synchronous bathyliths examined by him had been injected “after regional metamorphism had altered the sedimentary rocks but while conditions of strong dynamic pressure still prevailed” (Browne, 1931, p. 123).
Reconstitution of the gneisses has been so complete that the only trace of their original structure still preserved is the regular banding that is especially conspicuous in localities where intrusive rocks are absent or unimportant. This banding with its variable more or less northerly strike and generally steep dip is suggestive of bedding of a folded sedimentary series and is here interpreted as such. The quartz-bearing plagioclase-hornblende-biotite-and plagioclase-biotite-gneisses as well as some of the rocks that contain orthoclase might reasonably be regarded as the equivalents of greywackes. On the other hand those gneisses that contain no quartz are perhaps more likely to be derivatives of basic and semibasic igneous rocks, while the coarse orthoclase-bearing rocks (such as 2434) certainly have the appearance of orthogneisses and the mineral composition of hornblende-granites.
Turning now to the intrusive rocks, the pegmatites and aplites, almost invariably much richer in potash than the associated
“granites,” are without exception the youngest intrusions. This favours the hypothesis that the parent magma was essentially trondhjemitic and poor in potash, that the potash-content of the residual liquids increased during differentiation, and that the order of intrusion of the various magmatic components of the injection-complex was from less to more potassic types. The implication that the rocks of the oligoclase-granite group are in general rather younger than the trondhjemites is supported by the typically much more marked primary gneissic structure in the trondhjemites than in the oligoclase-granites, and by the tendency for the latter rocks to occur in the central portion rather than at the margins of the injection-complex. From the evidence afforded by the structural peculiarities of the intrusive rocks, it seems, however, that the parent magma may well have been richer in potash than the average trondhjemite. The strongly foliated structure of these latter rocks is undoubtedly in the main protoclastic (cf. Harker, 1932, pp. 298–300). The parallel disposition of the micas, the tendency for the plagioclase in many rocks to occur in rather rounded crystals, and perhaps to some extent the incipient granulation of quartz or feldspar, have developed as a result of differential movement during the later stages of consolidation of the rocks in question. From this point of view their low microcline-content is highly significant and is probably due not only to lack of potash in the original magma, but in addition to squeezing off of the residual potassic liquids at a late stage, to form pegmatites elsewhere (cf. Barrow, 1892; Harker, loc cit.). Now the oligoclase-granites though not generally so distinctly gneissic as the trondhjemites, on the whole show more pronounced granulation of quartz and feldspar, while the pegmatites are nonfoliated but invariably the most strongly sheared of the intrusive rocks. These facts coupled with general lack of cataclastic structure in the host-gneisses indicate renewal of shearing at a late stage in the igneous cycle after intrusion of the pegmatites; movement was then localised especially along the numerous recently-injected dykes whose heated condition and perhaps incomplete state of consolidation rendered them weaker than the surrounding rocks. This recrudescence of dynamic influences may well have been responsible too for some of the crushing observed in the trondhjemitic gneisses.
The extent to which assimilative reaction between magma and gneiss has taken place can be estimated only from careful consideration of the petrographic and field evidence. The observed field-relations of the rocks in question appear at first sight to favour extensive assimilation. In many of the cliff-sections gneiss seamed with dykes of pegmatite and trondhjemite gives place to “granite” enclosing great blocks of gneiss with undisturbed strike and dip, and this in turn passes gradually into granite in which remnants of the host-rock are completely lacking. This condition is well shown towards the head of the North Arm, where the enclosed blocks may themselves be cut by veins of “granite” emanating from the surrounding rock. Such exposures are apparently comparable with wellknown occurrences of migmatites such as those of Finland (e.g., cf. Sederholm, 1934, p. 41, fig. 24). Nevertheless, it must be remembered that slow mechanical wedging of blocks of the host-gneiss in the
roof of the rising bathylith and simultaneous squeezing out of veinlets into the roof-pendants could also account for the field-relations without assuming large-scale assimilation. The critical evidence must therefore be that afforded by the petrography of the rocks under consideration.
Instances of undoubted contamination of gneiss by reaction with granitic magma have been recognised in a number of localities, but are generally local. The usual result is introduction of potash-feldspar into gneiss immediately adjacent to veins of pegmatite or into blocks enclosed in oligoclase-granite or adamellite (see pp. 235, 236). Occasionally secondary muscovite and epidote have also developed in the rocks thus affected. The local presence of allanite in a few of the orthoclase-bearing gneisses is also strongly suggestive of the influence of magmatic emanations (e.g., in 2470 and 2789—small cove south of waterfall, halfway down west side of North Arm). A further example is furnished by injection-gneisses exposed in a channel of the Spey River about four miles from its mouth. These consist of white trondhjemite enclosing streaky bands of dark oligoclase-quartz-biotite-gneiss with allanite and interstitial microcline, probably representing contaminated relicts of the original host-rock.
On the other hand the evidence against general widespread contamination of the gneisses is convincing. Extensive masses of orthoclase-bearing gneiss are known in several localities in the vicinity of the North Arm. But it is scarcely conceivable that these owe their content of potash-feldspar to wholesale contamination when the local “granites” are usually poorer in potash than the invaded rocks, while on the other hand blocks of gneiss enclosed in the magma elsewhere often contain no potash-feldspar whatever. Around the upper half of the West Arm the gneisses, though intimately injected with trondhjemite and pegmatite, are obviously uncontaminated. Quartz and potash-feldspar here are absent and the abundant hornblende shows no trace of replacement by biotite. This applies even to specimens taken from the immediate vicinity of the intrusive veins.
The petrography of the granitic rocks shows conclusively that their composition has been almost unaffected by assimilative reaction. A biotite-rich marginal phase of the trondhjemite is sometimes developed within a a few millimetres of the invaded rock, but otherwise there is no suggestion of hybridisation near the contacts which always are sharply defined. The universally low percentage of biotite (the only dark silicate of importance) in the “granites” is itself strong evidence against contamination, for the invaded gneisses are mostly rich in hornblende and biotite, and the latter mineral should, therefore, be plentiful in hybrid rocks. Thus the crystals of epidote usually found in the “granites” cannot be xenocrysts derived from the gneisses; indeed such a possibility is prohibited by the presence in many of the epidotes of rounded cores of magmatic allanite. In most of the granitic rocks the epidote is a primary product of magmatic crystallisation (cf. Bartrum, 1917, pp. 419–421) and in some cases has obviously crystallised before the biotite, but the conditions controlling its development are not clear.
Taken together, the field and petrographic evidence is against the possibility that there has been important assimilation of gneiss by granitic magma in the Western Manapouri Province. In the writer's opinion the reason probably lies in the low content of volatiles in the invading magma as indicated by the petrography of the trondhjemites and oligcolase-granites. During assimilative reaction bebetween basic rock and acid magma as discussed by Nockolds (1933), equilibrium between altered xenolith and contaminated liquid is established and maintained by a process of diffusion in which the volatile constituents play an all-important part. During injection of the granitic magmas in the Manapouri area, and especially in the case of magmas of trondhjemitic composition, the volatiles are believed to have been progressively squeezed away from the partly crystalline mass. Under these conditions diffusion at the contacts would be ineffective. Where there are definite petrographic indications of assimilative reaction in this region the intrusive rock is usually a pegmatite or less commonly an adamellite or potash-granite, representing magmas richer in volatiles than the trondhjemite-magma. Even here, however, the total quantity of liquid magma in contact with gneiss seems to have been insufficient to effect complete permeation and consequent large-scale contamination of the host-rocks.
Doubtful Sound Province.
The rocks exposed in the gorge of the Dashwood and around Wilmot Pass have not been examined in detail on account of the heavily forested nature of the country, but sufficient material was collected to warrant separation of this region as part of a third geological province. The principal rocks recorded are intensely deformed banded gneisses, invaded here and there by veins and sheets of coarse pegmatite (also greatly sheared). The strike ranges between 50° W of N. and 35° E. of N, the dip being variable. Granitic rocks comparable with those further east have not been observed, but a single specimen of norite apparently intrusive into the gneiss was collected from an outcrop on the Deep Cove track about 1 ½ miles north-west of Wilmot Pass.
The petrography of the gneisses is illustrated below by descriptions of typical specimens:—
No. 2404 (Dashwood Gorge, 2 miles south of Wilmot Pass) is a medium-grained non-schistose rock the gneissic appearance of which is occasioned by a pronounced streaky foliation. Rounded white grains of feldspar 1 mm. to 2 mm. in diameter stand out clearly on the broken surface of the hand-specimen. The composition as shown by the microsection is medium oligoclase 70%, quartz 20%, partially chloritised red-brown biotite 8%, coarse irregular epidote 2% and accessory apatite. The grains of quartz, though showing little sign of granulation, are lensoid in shape, greatly elongated and tend to be drawn out around the equidimensional rounded grains of feldspar.
Where the track commences to descend on the Doubtful Sound side of Wilmot Pass the main gneisses are coarsely foliated rocks of igneous aspect in which a coarse-grained relatively light-coloured component alternates irregularly with streaks and bands of finer, more homogeneous dark-green material. The light-coloured phase is illustrated by 2413 and 2780, coarse streaky rocks dotted with conspicuous round feldspars 2 mm. to 3 mm. in diameter separated by a dark micaceous matrix. The principal mineral is andesine (60%), the grains of which have suffered considerable rotation involving bending of twin-lamellae and development of undulose extinction. They are set in a mashed streaked and granulated matrix of quartz (30%), biotite (5%), and minor epidote, chlorite and sericite. The quartz has been drawn into long ribbons twisted around and between the larger feldspars, while the hornblende and biotite have been reduced to intricately twisted, shredded masses, partially replaced by pale-green optically positive chlorite. Occasional zircons were noted in 2413. The dark component of the gneiss (2781) while much finer in grain likewise shows pronounced effects of dynamic metamorphism. The main constituents are untwinned plagioclase (probably oligoclase or andesine), deep blue-green hornblende, brown biotite and epidote; granular sphene is very plentiful for an accessory constituent. The section is reminiscent of some of the hornblende-biotite-oligoclase-schists that occur in association with the granite and contaminated epidiorite of Pomona Island (Part I). A rather infrequent phase of the gneiss in the vicinity of Wilmot Pass is a biotite-plagioclase-quartz-gneiss (2415) containing minor muscovite and epidote (Fig. 5). The biotite is an intensely pleochroic red-brown variety and makes up 40% of the total composition; it is strongly oriented and tends to enwrap the larger augen of plagioclase. The latter is an acid andesine. In many respects the rock is analogous to certain of the Holmwood Island gneisses of the eastern province (e.g., 2792).
Further towards the head of Deep Cove the gneisses are rather finer in grain and the effects of shearing are not quite so conspicuous as in the rocks around Wilmot Pass. No. 2405 is a fine oligoclase-biotite-hornblende-gneiss with minor epidote and quartz, exactly comparable with the commoner gneisses of the Holmwood Island group. No. 2410 is unusual in that it contains rounded fractured grains of colourless garnet showing incipient replacement by chlorite. The composition is plagioclase 60%, quartz 20%, garnet 7%, partially chloritised red-brown biotite 7%, deep blue-green hornblende 5% and accessory magnetite and sphene. The feldspar is a basic andesine and shows incipient replacement by calcite and sericite along cracks caused by shattering; many of the grains are as much as 2 mm. in diameter. Quartz occurs as large nests of equidimensional undulose grains (0.2 mm.). The garnet is colourless in section and in hand-specimen is yellowish-brown indicating a member of the grossularite-andradite series.
The gneisses of the Doubtful Sound province, like those of the Holmwood Island group, are considered to be products of intense regional metamorphism of greywackes and tuffaceous rock containing
basic igneous material.* Their present cataclastic condition has been superimposed on the original crystalloblastic structure during deformation subsequent to the main metamorphism, and is subsequent also to intrusion of both pegmatites and gabbroid rocks.
The pegmatites of the Doubtful Sound Province are illustrated by the following examples.
No. 2409 (2 miles north-west of Wilmot Pass) is a coarse-grained biotite-rich rock with occasional large crystals of allanite clearly visible in the hand-specimen. The section consists principally of coarse microcline (70%), quartz (20%) and biotite (8%). The grains of quartz typically are rounded against the microcline and often are completely enclosed by the latter mineral. Albitic plagioclase occurs as highly irregular streaks intergrown in perthitic fashion with the microcline and also as sparsely scattered small independent crystals largely replaced by ragged muscovite. Apatite and iron-ores are accessory constituents. No. 2408 from the same locality is a rather similar rock the structure of which has been modified by intense shearing (cf. Fig. 6). The main constituent is orthoclase locally showing microcline-structure and always enclosing ragged streaks of intergrown albite. The quartz has been completely granulated and streaked out by post-crystalline deformation. Sericitised oligoclase makes up about 10% of the total composition of the rock. Streaky aggregates of hornblende, biotite, epidote and apatite represent fragments of the invaded gneiss that have been incorporated mechanically into the pegmatite during deformation. The accessory constitutents of the rock include secondary epidote, stout prisms of apatite and occasional small square prisms of colourless cassiterite.
Pegmatites of a different type are illustrated by 2407, 2778, and 2779 (2 miles north-west of Wilmot Pass). These are coarse-grained rocks of pegmatitic aspect with plentiful well-formed crystals of black allanite conspicuously developed in the hand-specimen. The microsections indicate that their origin involved replacement of the invaded gneiss rather than purely mechanical intrusion, and they differ further from the definitely intrusive pegmatites in the rarity or absence of potash-feldspar.
The composition of 2407 is albite-oligoclase 50%, quartz 30%, biotite 10%, muscovite 6%, allanite 3%, apatite 1%, orthoclase 1% and minor colourless epidote. The quartz is undulose and partially granulated and the biotite (a yellowish-brown type) is to some extent twisted and shredded as a result of shearing. The crystals of feldspar have been less affected mechanically, but enclose a good deal of secondary sericite and epidote. Much of the muscovite has crystallised at a late stage fringing and partially replacing biotite; some is intergrown on a coarse scale with quartz. Apatite in the form of stout hexagonal prisms is unusually plentiful. The section shows numerous prisms of allanite, some of which are 3 mm. in length, apparently but little affected by shearing (Fig. 7). The birefringence
[Footnote] * The marbles of Doubtful Sound occur not far beyond the limit of the area examined by the writer, and probably belong to this same series of rocks.
is low and the pleochroism very intense from pale yellow (X) to deep reddish brown (Y and Z); rhomb-shaped end-sections perpendicular to b give an extinction-angle of 40°. as measured from the trace of the (001) cleavage to X (the bisector of the obtuse angles of the rhomb). One crystal cut perpendicularly to X has an irregular border in which the pleochroism is from smoky green (Y) to brownish-yellow (Z) and the double refraction (γ-β)=0.01, an unusually high figure. Narrow borders of colourless epidote also surround some crystals.
In 2778 the structure clearly indicates replacement, though slightly complicated by late shearing. Large crystals of oligoclase and biotite, 3 mm. or 4 mm. across, have grown in a fine-grained equiangular mosaic of oligoclase, biotite and quartz, aggregates of which may be completely enclosed in the larger crystals. There are also nests of coarse strained quartz that have obviously been introduced from a magmatic source. Apatite is very plentiful in parts of the section, and a few small grains of allanite were noted (though the mineral is conspicuous in the hand-specimen). There are also a number of idiomorphic colourless grains of cassiterite, the largest of which is 0.5 mm. in diameter. Section 2779 taken from the same outcrop as 2778 is composed almost entirely of coarse biotite and quartz, with small enclosed quartz-biotite-oligoclase aggregates and local clusters of apatite.
Cassiterite has previously been recorded as a detrital mineral in Recent gravels (Morgan, 1927, p. 21) and Tertiary marine sand-stones (Hutton and Turner, 1936, pp. 259, 261) in Fiordland, but as far as the writer is aware this is the first time it has been observed as a primary constituent of an igneous rock in this region.
Basic Intrusive Rocks.
A single specimen of dark non-foliated quartz-mica-norite (2406) was collected about 1 ½ miles north-west of Wilmot Pass. The rock is of medium grain and in its original unaltered condition (shown in parts of section 2406a) consisted of pyroxene 50%, plagioclase 30%, biotite 10%, greenish-brown hornblende 5%, opaque iron-ore 4%, quartz 1%, abundant accessory apatite and rare sphene. The pyroxene includes equal amounts of schillered pale-pink hypersthene and pale-green diopsidic augite, often also with schiller structure. Compact brownish-green hornblende may build up magmatic reaction-rims around the grains of pyroxene or iron-ore. The biotite is intensely pleochroic from yellow to deep reddish-brown and is sometimes very coarse. The plagioclase is basic andesine (about Ab55 An45). an unusually sodic type for such a basic rock as this. Throughout most of section 2406 and in parts of 2406a the pyroxenes are now represented largely by-pseudomorphs of fibrous or acicular colourless tremolite which are fringed with a narrow border of deep blue-green hornblende where they adjoin crystals of plagioclase. The magmatic hornblende also shows a tendency to be replaced by tremolitic amphibole. The plagioclase is unaltered. As in the norite described by Benson and Bartrum (1935, pp. 143, 144) from Preservation Inlet the development of secondary tremolite has not accompanied any recognisable cataclasis, though the surrounding gneisses have suffered severe deformation.
Doubtfully included under this heading is a dark, fine-grained, non-fissile rock of dioritic appearance invading or interlaminated with the gneisses on the north-west side of Wilmot Pass. In section the rock is even-grained (average grain-size=0.5 mm. to 1 mm.) and consists of hornblende (50%), basic andesine 45%, iron-ore 5%, accessory apatite and small amounts of secondary chlorite. The hornblende is rather pale in colour (X=yellow, Y=brownish-green, Z=bluish-green), but here and there contains cores of a darker brownish variety. Schiller-inclusions of iron-ore are sometimes developed in the larger crystals, suggesting the possibility of origin by replacement of pyroxene.
Correlation within the Manapouri Region.
In each of the three provinces considered separately above, the oldest rocks are steeply-dipping regularly banded gneisses in which a plagioclase feldspar is constantly associated with biotite, hornblende or both these minerals, while other constituents such as quartz, potash-feldspar, epidote or sphene are often plentiful. These gneisses are believed to be the highly metamorphosed equivalents of a series of rocks essentially sedimentary, but containing interstratified tuffaceous beds or basic lavas and probably locally including more acid igneous rocks. The general similarity as regards chemical and mineralogical composition, major structure, and relation to intrusive rocks warrants correlation of the gneisses throughout the whole of the Lake Manapouri region.
On the other hand two distinct series of granitic rocks are recognised. In the Western Manapouri province the great compound intrusion of trondhjemite and oligoclase-granite with its accompanying pegmatites has all the characters of a synchronous bathylith (Browne, 1931, pp. 114, 115), and is considered to have been squeezed up immediately after the metamorphism of the invaded gneisses while folding movements were still in progress. The less extensive Pomona Island granite is interpreted as a subsequent bathylith (cf. Browne, 1931, p. 116), the intrusion of which took place under totally different physical conditions after compressional movement had almost ceased. The petrography of the Pomona Island granite points to consanguinity with the trondhjemites and oligoclase-granites of the western province, and its relatively high content of potash-feldspar implies a highly differentiated condition such as might be expected in a subsequent bathylith.
The Beehive epidiorites and the altered norite near Wilmot Pass though separated by a distance of about twenty miles are correlated on petrographic grounds. In the Wilmot Pass rock mineralogical reconstitution and structural modification are not so far advanced as in the epidiorites of the eastern province. This difference is probably due in part at least to the metamorphic and contaminating influence of the Pomona Island granite in the latter district. There is some uncertainty as to the exact age of the basic intrusions. In the eastern province they invade the gneisses and are themselves invaded and modified by the Pomona Island granite; but whether
they precede or are later than the trondhjemitic intrusions is not clear. It seems safe to assume that intrusion of the basic magma was subsequent to reconstitution of the gneisses to their present condition, for, in spite of the well-known susceptibility of basic rocks to metamorphism, the epidiorites and especially the Wilmot Pass norite are obviously not isogradic with the gneisses which they invade. On this assumption it is probable that the basic rocks are later than the trondhjemites and this hypothesis is therefore tentatively put forward. On these grounds they might well be differentiation-products complementary to and immediately preceding the Pomona Island granites. Their occurrence as relatively small masses on either side of the great trondhjemite injection-complex of the Western Manapouri Province accords with this hypothesis.
The sequence in the Manapouri region is provisionally summarised thus:—
Tertiary marine sandstones, conglomerates, etc.
Pomona Island granite.
Beehive epidiorite, and norite of Wilmot Pass.
Trondhjemites, oligoclase-granites and adamellites of the western injection-complex.
The rocks of group (1) were strongly metamorphosed and folded, and invaded by the trondhjemite magma during the later stages of folding. The rocks of groups (3) and (4) post-date the main compressive movement, but have themselves been affected locally by still later shearing.
Correlation with other parts of Southern New Zealand.
To-day Fiordland is perhaps less known geologically than any other area of comparable size in New Zealand. Professor James Park (1921) has given a general account of the geology of the whole region based upon such data as were available at that time, while the recently published work of Professor Benson and his collaborators includes the results of detailed investigations in the extreme south-west around Preservation and Chalky Inlets (Benson, 1933; Benson, Bartrum and King, 1934; Benson and Bartrum, 1935; Benson and Keble, 1936). Dr. J. G. Williams' researches upon the granites and schists of Stewart Island (Williams, 1934), and the earlier work of Wild (1912) on the noritic rocks of Bluff also bear upon the geology of Fiordland. In the following paragraphs the results obtained in the Manapouri district are compared briefly with those published in the works referred to above.
In Professor Park's account (Park, 1921, pp. 33, 34) the metamorphic rocks of Fiordland are divided, mainly on lithological grounds, into three series, viz., in ascending order, the Dusky Sound, Maniototo and Preservation Inlet Series. In the south-west Professor Benson and his co-workers have shown clearly that with progressive metamorphism the graptolite-bearing Ordovician slates of Park's Preservation Inlet Series pass gradually into schists and gneisses comparable with rocks included by Park in his Maniototo Series (Benson and Bartrum, 1935, especially pp. 133–135). They have also demonstrated conclusively the closely-folded structure of the
rocks around Preservation and Chalky Inlets. If the same structure continues further north in the vicinity of Dusky Sound there is no necessity to assume a pre-Ordovician age for the Dusky Sound Series; indeed the possibility that part at any rate of the latter may be Upper Ordovician has been put forward (Benson, 1933, p. 414). The gneisses described in the present paper are petrographically similar to certain rocks widely prevalent in the Dusky Sound Series (Speight, 1910; Park, 1921, pp. 35, 36), though absence of pyroxene in the Manapouri gneisses indicates a rather lower grade of metamorphism than prevailed further south-west. On the other hand there are few analogies between the gneisses of Lake Manapouri and even the more strongly metamorphosed of the rocks from Preservation Inlet or Stewart Island. This difference, though partly due to variation in conditions of metamorphism, is essentially connected with the chemical composition of the rocks concerned. In the south-west pelitic rocks are widely distributed and hornblendic rocks rare; but around Lake Manapouri hornblende-gneisses for the most part represent psammitic and tuffaceous rather than pelitic sediments. The Manapouri gneisses are therefore correlated with the Dusky Sound Series. Their occurrence on the islands to the east of the granitic intrusions provides an interesting exception to Professor Park's generalisation to the effect that the Dusky Sound Series is confined to a relatively narrow strip west of the plutonic rocks of the main divide. No further evidence as to the age of these highly metamorphosed rocks has been discovered in the Manapouri district. Attention is drawn, however, to the possibility that they may even include rocks stratigraphically equivalent to the Te Anau Series. for the latter have certainly been affected by regional metamorphism further to the north-east (Turner, 1935, especially pp. 347, 348); also the Te Anau Series consists mainly of rocks which would give hornblendic gneisses if subjected to high-grade metamorphism.
With the exception of the epidiorites and norites the plutonic rocks of Manapouri have little in common with those previously described from Fiordland. Hornblende-diorites which seem to be the dominant igneous rocks further north (Park, 1921, p. 43) are not represented in the area under discussion unless the hornblendie gneisses might be considered equivalent to these rocks; nor is there the wide variety of tonalites, granodiorites and more basic rocks that border the granitic bathyliths in the vicinity of Preservation Inlet. The trondhjemites and oligoclase-granites described in this paper resemble certan of the soda-tonalites of Preservation Inlet only in that oligoclase is associated with abundant quartz and only minor orthoclase in all these rocks (Benson and Bartrum, 1935, pp. 136–139). But the soda-tonalites are easily distinguished by their much higher content of biotite and the more basic composition of the plagioclase.
There are also important differences with regard to the tectonic conditions accompanying intrusion. Professors Benson and Bartrum (1935, p. 133) have brought forward evidence indicating that regional metamorphism of the Preservation Inlet sediments took place after the intrusion of the granites. On the other hand at Manapouri injection of the trondhjemite and oligoclase-granite magmas occurred
Fig. 1.—Hornblende (black) and coarse epidote in gneiss (2839) from West Arm: note grain of sphene enclosed by epidote. × 45.
Fig. 2.—Orthoclase-bearing biotite-plagioclase-gneiss (2789), North Arm. × 45.
Fig. 3.—Trondhjemite (2368) showing biotite crystals growing round epidote. × 45.
Fig. 4.—Biotite drawn out to enwrap partially recrystallised plagioclase in gneiss (2780) near Wilmot Pass. × 45.
Fig. 5.—Biotite-plagioclase-quartz-gneiss (2415), near Wilmot Pass. × 45.
Fig. 6.—Streaked-out biotite and minor hornblende in plagioclase-biotite-gneiss (2413) near Wilmot Pass. × 45.
Fig. 7.—Coars allanite in pegmatite (2407); biotite on the top margin of the photograph. × 45.
Fig. 8.—Norite (2406), Doubtful Sound Track; a large crystal of pyroxene has been replaced completely by colourless tremolite bordered with blue-green hornblende, but still retains the original schillerisation structure; black grains are iron-ore; apatite plentiful. × 45.
immediately after regional metamorphism of the invaded rocks while strong dynamic influences still prevailed. The Pomona Island granite, though petrographically not unlike some of the Preservation Inlet rocks, has been shown to be still later than the trondhjemites with reference to the compressive phase of the orogeny. In Stewart Island, as in the Western Manapouri Province, the granite magma rose up under dynamic conditions and failed to produce extensive assimilation-effects near the contacts with the invaded gneisses and schists; but petrographically there is little resemblance between the rocks concerned.
Petrographic analogies with the rocks of Preservation Inlet and Stewart Island are not entirely lacking however. They include relatively unimportant features such as widespread development of myrmekite and intergrowths of muscovite with quartz or plagioclase as a result of late-magmatic reactions facilitated by shearing. Perhaps more significant from the genetic point of view is the prevalence of accessory allanite in the granites of Manapouri and Preservation Inlet, for this may well indicate consanguinity. Obviously a number of distinct intrusive masses are present in Fiordland, representing separate injections of magma under different tectonic conditions and at more than one stage in a complex orogenic cycle.
Correlation of the basic plutonic rocks is more certain. The distribution of mica-norites and gabbros in Fiordland has recently been summarised by Benson and Bartrum (1935, p. 144), while Hutton (1936, pp. 29–31) has described additional allied types from glacial boulders that must have been transported from the northern border of the Fiord Region. The norite from near Wilmot Pass is petrographically similar to many of these rocks in that it contains plentiful primary biotite. Further it closely resembles the norite of Long Sound (Benson and Bartrum, 1935, p. 143) in the presence of secondary tremolite partly replacing the primary hornblende and pyroxene. It is probable then that the norites and gabbros of Fiordland belong to a single intrusive series. In the Darran Range the norites are veined with granite, but the Long Sound norite is considered by Benson and Bartrum to be later than the granites in that district. These facts agree with the writer's observations in the vicinity of Manapouri, where the basic plutonic rocks are certainly older than the Pomona Island granite, but are believed to be younger than the trondhjemites and allied rocks of the western injection-complex. In the writer's opinion the Fiordland norites are not connected genetically with the peridotites and associated basic and ultrabasic rocks of South Westland and the Lake Wakatipu district. Genetic connection with the gabbro of Bluff is not unlikely, however, for here too a series of hornblende-schists is invaded by the gabbros, which in turn are cut locally by dykes of granite. *
In conclusion, the rocks of Manapouri have been compared with the boulders described by Mackie (1935) from Triassic and Jurassic conglomerates in eastern Otago. These include several trondhjemites rather similar to but not identical with those of Manapouri: they
[Footnote] * Personal communication from Mr. H. Service. See also paper by Service in this volume.
never contain primary epidote, and rather coarse magnetite is often present. The oligoclase-quartz-biotite-gneisses described by Mackie (1935, pp. 292, 293) resemble certain of the Manapouri gneisses, though here too there is not complete petrographic identity. While the ultimate source of the eastern Otago boulders remains unknown, it can at least be said that in Triassic and Jurassic times this region was being supplied with detritus including rocks generally similar to certain of the gneisses and “granites” to-day exposed in Fiordland [compare Professor Bartrum's conclusions as to the source of plutonic and metamorphic rocks in the Jurassic conglomerates of Kawhia (Bartrum, 1935, pp. 106, 107)]. In this connection Professor Benson has kindly permitted the writer to record his recent discovery of a pebble of quartz-biotite-muscovite-schist in a Jurassic conglomerate near Papatowai, Catlins district. This rock is similar to but not identical with rocks described by Benson and Bartrum (1935, p. 122) from Long Sound, Fiordland.
I take this opportunity of thanking the Murrell brothers of Lake Manapouri for hospitality and assistance during field work, and especially Mr. Burton Murrell for loan of a boat and for local geological information. Thanks are also due to Drs. L. H. Briggs and C. M. Focken and Messrs. C. O. Hutton and J. B. Mackie for field assistance, to Professor W. N. Benson for encouragement and advice during the preparation of this paper, and to Professor Bevan Dodds of the University Dental School for use of photomicrographic apparatus.
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