Art. VI.—Notes on some Rocks from Parapara, Bluff Hill, and Waikawa.
Communicated by G. M. Thomson, F.L.S.
[Read before the Otago Institute, 14th September, 1909.]
The description of a few isolated rocks can seldom advance geological knowledge very greatly, and this paper must be regarded mainly as an attempt to draw attention to some interesting regions of the Dominion. The day has gone by when descriptions of new types of igneous rocks can attract more than local interest, and petrologists are more and more tending to use description only as a handle for the discussion of theoretical questions. The two outstanding subjects of discussion are,—
The nature of igneous magmas, the history of their consolidation and their relations to neighbouring rocks, involving the theories of differentiation, admixture of igneous magmas, and assimilation of the walls of the magma-basins.
The mode of formation of the crystalline schists.
New Zealand is a country so rich in rock-types, and so well provided with natural sections, that it is not vain to suppose that much material may be found which may throw important light on these philosophical questions. Two such districts are outlined in the sequel.
The authors of Bulletin No. 3 of the Geological Survey describe certain basic facies of the Waitapu granite as occuring only at the contact with the carbonate rocks, and are led to the conclusion that it was owing to the introduction of the carbonate bases into the acid magma that these basic facies arose. This theory of assimilation or absorption has its home in France, and its most notable protagonists in Levy* and Lacroix.† It receives support from Finnish geologists, but is strongly opposed by the differentiation school of Rosenbusch and Brögger, with whom English petrologists are in large part inclined to agree. In America, however, the French have a strong following.‡
One of the difficulties in settling the question is the rugged and inaccessible nature of the country in Ariège described by Lacroix, while another lies in the complication of phenomena in regions of crystalline schists such as Adams describes. It suggested itself to me that the Parapara district would furnish an excellent case on which to test the rival theories, and Dr. Bell, Director of the Geological Survey, kindly furnished me with six specimens for preliminary study. These will now be described in detail.
The members of the igneous complex are represented by an acid and a basic type. The acid rock has the structure of an augen-gneiss and the mineral composition of a binary granite. The feldspars, which form the augen, are mostly untwinned and partially sericitised, and are referred to orthoclase, but there are also present microcline, and albite in microcline-microperthite. Graphic intergrowths of quartz and orthoclase are fairly abundant. Both dark and white mica are arranged in parallel flakes along the planes of foliation, the former in course of alteration to chlorite and rutile. Apatite is not abundant, but magnetite is more plentiful, and it is evident that the original iron-ore was titaniferous, since the magnetite shows minute outgrowths of a highly refringent platy mineral determined as anatase. There is, in addition, a brown mineral of which only one large prismatic crystal appears in the section, with a pleochroism from brown to yellow. The orientation is not favourable for study in convergent light, and the mineral is doubtfully referred to cero-epidote on account of its low obliquity of extinction. Although it is paler in colour than is usual in common orthite, it is too pleochroic for monazite.
The basic rock shows, in section, abundant common hornblende enveloping colourless patches, which are evidently pseudomorphs after feldspar. They consist chiefly of muscovite and epidote, but a little basic plagioclase has here and there escaped alteration. Magnetite is probably an original constituent, but much of iron is now present as pyrite.
Secondary alterations in the large hornblende plates are of two kinds: there is a decomposition to clinochlore and sphene, to be ascribed to shallow-seated alteration, and also a local separation of minute needles of rutile and
[Footnote] * Levy, MM. “Le granite de Flamandville, &c.,” Bull. Carte geol. Fr., tome v, No. 36, 1893–4. “Sur l'évolution des magmas de certains granites à amphibole,” Comptes Rendus, cxxi, p. 228, 1895. “Sur quelques particularités de gisement du porphyre bleu de l'Esterel,” Bull. Soc. geol. Fr., 3rd ser., xxiv, p. 123, 1896.
[Footnote] † Lacroix, A. “Les granites des Pyrenées et leur phénomènes de contact.” Bull. Carte geol. Fr., tome x, No. 64. 1898–99.
[Footnote] ‡ Cf. Adams, F. D., on the Structure and Relations of the Laurentian System in Eastern Canada. Q.J.G.S., lxiv, pp. 127–47. 1908.
linearly arranged granules of magnetite, giving rise to a rude schiller-like striation oblique to the prismatic cleavages. This latter alteration is very common in the hornblende of rocks that have suffered shearing, and seems to be due to an attempt on the part of magmatically formed brown horn-blende, rich in iron and titanium, to adapt itself to the altered conditions according to the volume law. From the presence of these striations and the strongly green colour of the hornblende, it appears original, and not a uralitic pseudomorph of pyroxene, and the rock is therefore an altered hornblende dolerite or gabbro.
The other four rocks come from the contact aureole of the granite, one being an amphibolite intercalated among the stratified rocks. It is almost completely recrystallized, and in description it is desirable to make use of the terms introduced by Becke.* These are based on the belief that in crystalline schists the formation of the different minerals has proceeded simultaneously, since each may be found enclosed in the others, in opposition to the sequence of growth amongst minerals of igneous rocks. Owing, however, to the different powers of crystal-growth exhibited by the minerals, they differ widely in their development of crystal-faces, so that some appear moulded on others. The resulting structure is called “krystalloblastisch,” or, anglicé, “crystalloblastic,” and the apparent order of separation is termed “the crystalloblastic order.” Well-shaped (euhedral) crystals are termed “idioblastic,” those with irregular boundaries (anhedral) “xenoblastic,” and other structural terms used in the descriptions of igneous rocks are similarly adapted with the suffix “blastic.” In this amphibolite there are evidences of former igneous structure in the presence of original phenocrysts of feldspar. The rock is therefore “porphyro-blastic.” The minerals, especially the sphene, are frequently honeycombed with inclusions, and the crystalloblastic order appears to differ from that established by Becke, in that feldspar is idioblastic to sphene and hornblende; but this apparent difference may be due to the incomplete recrystallization of the feldspar. These three minerals are the most abundant constituents of the rock, but epidote, apatite, and pyrite are also present, and secondary sericite is forming in the feldspar, which is near andesine in composition. The mineralogical composition of the rock shows that it is an ortho-amphibolite, the large crystals of plagioclase that it was porphyritic, but to arrive at the nature of the original rock an analysis would be necessary.
The three stratified rocks are interesting as containing contact minerals, a class which must be common but has not yet been widely observed in New Zealand. One rock is a biotite-hornfels, containing biotite, quartz, plagioclase, feldspar, chlorite, and magnetite. The quartz forms a fine mosaic of polygonal grains, between the interstices of which are inserted abundant flakes of biotite, locally decomposed to chlorite.
The other two rocks are marbles with contact minerals. In one—the “complex-carbonate rock” of the Survey—there is little besides the large polygonal carbonate grains. White mica and sphene can be recognised, but small needles of a highly refringent and bi-refringent mineral lying in the carbonate defy determination in section, although they can scarcely be other than rutile. The other must have been an impure dolomitic rock, for, besides calcite, a colourless augite occurs very abundantly both in separate grains and in small aggregates. Octagonal cross-sections may occasionally
[Footnote] * Becke, F. Ueber Mineralbestand und Struktur der krystallinischen Sohiefer” Comptes Rendus ix Sess. Cong. Geol. Inter. Vienna, 1904, pp. 553–70.
be seen, but most of the crystals are rounded or irregular in shape, and are interrupted in crystallization by inclusions, so that the form is like a sponge. Sphene and quartz are found in the pores of the sponge, and also independently in the calcite. Finally there is a little pyrites.
This collection of rocks, interesting as it is in itself, throws little light on the question of assimilation. Certainly there are many points of resemblance with the Pyrenean granite-contacts, but the endomorphic modifications of the granite are too poorly represented to draw a parallel. A complete investigation would involve the detailed mapping of these basic facies with the rocks at their contact. Even then, the phenomena would admit of either interpretation, unless a thorough chemical examination of both classes of rocks conclusively proved the absorption of the earthy bases. Probably the truth will be found to lie between the extreme positions, a partial assimilation accompanying laccolitic differentiation. This paper will serve its purpose if it draws attention to this interesting question.
Part II.—Bluff Hill.
The rocks here described are the result of a few hours' collection along the shore south and west from Bluff Harbour. Immediately joining the harbour is a series of apparently bedded rocks, which are in some cases so dense as to resemble hornstones, but in other cases suggest sheared porphyritic igneous rocks. A section of one of the denser varieties proves the rock to be a fine-grained banded hornblende schist. Some of the bands are richer in hornblende, others in epidote and iron-ores, while a mosaic of polygonal quartz grains is common to both. Apatite is not rare, and rutile is very abundant in minute prismatic crystals with pyramidal terminations projecting out of quadrate magnetite crystals. A similar mineral occurs in more rounded crystals in the quartz, and is difficult to determine with certainty as rutile or sphene, but there are a few undoubted larger crystals of the latter. The hornblende, a common green variety, occurs in prismatic crystals mostly parallel to one another, more rarely oblique to the schistosity. Sometimes the crystals are so slim and needle-like as to simulate sillimanite needles. Cross-sections show that in addition to the prismatic faces the B pinacoids (010) are present, a feature rarely observed in hornblende schists. The epidote does not show clear crystalline form, but occurs in granular aggregates elongated in the same direction as the hornblende. The crystalloblastic order appears to be apatite, magnetite, rutile, sphene, hornblende, epidote, and finally quartz.
A vein of coarser grain separates the hornblendic and epidotic bands, and contains, besides hornblende, epidote, and quartz, a few large twinned basic feldspars. The absence of albite in the rock is surprising if it is derived from an igneous or pyroclastic rock. Untwinned feldspar is always difficult to recognise in the presence of quartz, but an examination of as many of the colourless grains as possible in convergent light gave only uniaxial figures. From the absence or relative paucity of feldspar the rock has more affinities with a para-hornblende-schist than with an ortho-hornblende-schist. It presents many points of resemblance to the “green schists” among the Old Lizard Head series of Cornwall.
A dyke of a dark porphyritic rock crosses the schist formation at an angle approaching 45o. In section this rock also proves to be quite schistose, but, unfortunately, the directions of schistosity of the dyke and the intruded schists were not compared in the field. The phenocrysts consist of large
compact hornblende crystals, often white in the centre, and smaller anhedral feldspars. In section the hornblende presents rhomboidal shapes with ragged outlines, and is seen to be a strongly pleochroic variety of common hornblende, with occasional tremolitic cores. Not seldom some crystallographically discontinuous hornblende crosses the main phenocryst, a feature often observed in uralite. The feldspars give more rounded outlines, and are turbid, and filled with calcite. They include also hornblende, magnetite, and epidote. The groundmass consists of elongated, often needle-like hornblende prisms, with associated biotite flakes, slightly elongate feldspars, probably albite, showing Carlsbad twinning, magnetite grains, and occasionally a green epidote. The rock is thus a porphyroblastic hornblendeschist derived from a basic igneous dyke-rock.
The next three specimens to be described come from a small headland about half a mile round the coast to the south-west, just beyond the mouth of the harbour. There is here an interesting complex of coarse holocrystalline rocks. Three elements may be distinguished, a dark dioritic rock (No. 1), which appears to vein a much lighter dioritic rock (No. 2), occasionally enclosing both the latter and a coarse hornblendic rock (No. 3) as xenoliths. The dark diorite (No. 1) is often gneissose, while the walls of light diorite (No. 2) which surround it are quite massive.
In section, all three rocks present a similar assemblage of minerals, but the relative amounts of iron-ores, hornblende, and feldspar vary very considerably. Hornblende forms the predominate mineral of No. 3, and is a brown-green variety with a fine schiller-structure in the centre, surrounded by a margin of green hornblende. Besides the hornblende, there is a limited amount of a basic feldspar and iron-ores. The hornblende shows a fair approach to idiomorphism, but the larger iron-ores, probably ilmenite, are moulded both on hornblende and feldspars. Minute octohedra of magnetite are abundantly included in the two last-named minerals, but are probably of secondary formation.
In No. 1, iron-ores and hornblende of the same nature as above described are abundant, but feldspar predominates, and besides these minerals a little apatite and pyrite are found. A very peculiar zoning is observable in the feldspar; there are only two zones, separated by a boundary of the most irregular nature. The cores are in some instances as basic as labradorite, but the exteriors are so acid that there is a marked difference of relief between the two on lowering the condenser of the microscope, and the Becke effect may be easily observed. The Carlsbad and albite twinning of the cores does not persist in the exteriors, as in the case of albitization recently described by Bailey and Grabham* in the quartz dolerites of the central valley of Scotland, but it appears probable that something of similar nature has taken place here.
The structure differs from that of No. 3 in that the hornblende is distinctly ophitic to the feldspar. This observation is in accord with recent theories of the dependence of structure on eutectic relations,† since in the former rock the hornblende is in excess, and in the latter the feldspar predominates. But caution must be used in describing rocks as much altered as these, for, though in No. 2 the hornblende is moulded on the feldspar
[Footnote] * Bailey, E. B., and Grabham, G. W. “Albitization of Basic Plagioclase Feldspars.” Geol. Mag. Dec. V, vol. vi, p. 250. 1909.
[Footnote] † Vogt, J. H. L “Physikalische-chemische Gesetze der Kristallisationfolge in Eruptiogesteine.” Isch. min. u. petr., Mitt. xxiv, p. 437. 1905.
it is also included within it, a structure already noted typical of the crystalline schists. There is also evidence for a slight cataclasis in the section.
A still paler variety of hornblende may occasionally be seen in this rock, occurring in almost colourless fibrous forms with beautiful polysynthetic twins. Further study would be necessary to prove whether this is due to bleaching, to recrystallization of coloured hornblende with separation of iron-oxides, or to uralisation of previously existing pyroxenes.
In No. 2 the feldspars predominate greatly over the darker minerals. They give evidence of considerable crushing in strain shadows, bent twin lamellæ, and local cataclasis. Often irregularly disposed grains are included in the larger crystals. Both coarse and fine albite lamellation may be observed, while pericline twinning is rare. Probably more than one variety of feldspar is present, and, indeed, the larger crystals show gradual zoning from the centre to periphery, but the prevailing species appears to be andesine. Delicate colourless needles are often abundant in the feldspars, and seem to be sillimanite. Beyond these the feldspars show little sign of alteration with the exception of rare grains of epidote or zoisite, and flakes of chlorite which have probably migrated from the ferromagnesian minerals along cracks. The hornblende and iron-ores occur in small patches of individuals with such ragged contours that the structural relations to the feldspars cannot be made out. The cores of the hornblende crystals generally consist of a paler variety in optical continuity with the green exteriors, so that the former presence of pyroxene is suggested. This small collection of rocks is yet sufficient to show that there is a considerable diversity of rocks in Bluff Hill. It would be exceedingly valuable to have a detailed examination of this area made, as the phenomena observed are probably common to a large part of Stewart Island, if not also in the Sounds region, and Bluff Hill is a much more easily accessible region. The relative ages of the different rocks, the mode of origin of the foliation, whether arising during or after consolidation, the origin of the rock-variations and “basic secretions,” whether by pure differentiation or differentiation combined with absorption as in Skye, are a few of the problems presented for solution. Probably many analogies will be found with the Lizard district of Cornwall, on which the Geological Survey of Great Britain is shortly issuing a detailed memoir.
The last rock is an isolated beach-pebble picked up at Waikawa, and possesses interest as being a type not hitherto recorded in New Zealand. The other beach-pebbles consist mainly of microgranites derived from the Triassic conglomerates, but from its soft nature it is more probable that this rock has come from a neighbouring intrusion. It shows a rich mineralogical association, consisting of amphiboles, biotite, muscovite, clinozoisite, epidote, two varieties of chlorite, talc, magnetite, and pyrite, but hornblende is so abundant as to render the designation “lustremottled hornblende rock” applicable.
The hornblende occurs chiefly in large pœcilitic plates of a pale-brown variety, passing gradually to a pale-green on the exteriors. The minerals enclosed pœcilitically are feldspar and secondary minerals, partly after feldspar and partly after some ferromagnesian, while there are chloritic areas with sphene, which may simply represent alterations of a hornblende.
The feldspars are often rounded, and show good albite twinning with rarer pericline lamellæ. The extinction angles are characteristic of basic labradorite. Of the secondary minerals the chief is tremolite, oriented in crystallographic continuity with its host. This structure, described by me* in the amphibolite of Glendalough, Ireland, is commonly ascribed to the uralitisation of pyroxene, but may also arise, as shown at Glendalough, from the amphibolitisation of olivine. Probably both methods have operated here. Occasionally small fibres of tremolite oriented independently of the host occur in these areas. The feldspar inclusions are sometimes wholly or partly replaced by clinozoisite associated with muscovite and a feebly birefringent chlorite. The green margins of the hornblende plates contain similar inclusions, and, in one case, twinned clinochlore and small prisms of pale actinolite.
Only one large crystal of biotite is seen in the section, and it includes hornblende.
Outside the large pœcilitic plates, feldspars altering to clinozoisite and muscovite are not rare, but there is a large development of recrystallized actinolitic hornblende fibres pointing in all directions, but most often distinctly aggregated in bundles, and interspaced with most of the secondary minerals included in the large plates. There are, in addition, some areas consisting of finely divided talc, into which fibres of hornblende project. It is difficult to be sure of talc in the presence of sericitic muscovite, since there is no certain microscopic method of discriminating between these minerals, unless recourse be had to microchemical tests; but the mineralogical association supports the general appearance. These areas do not contain clinozoisite or epidote, which are associated with muscovite in the altered feldspars, and contain hornblende, which does not occur in the latter. Assuming the presence of talc, these areas are then pilite† after olivine, and the rock is derived from a hornblende olivine dolerite such as Harker‡ describes from Anglesey in association with hornblende-peridotites.
[Footnote] * Thomson, J. A. “The Hornblendic Rocks of Glendalough and Greystones.” Q.J.G.S., lxiv, p. 475. 1908.
[Footnote] † Becke, F. “Eruptiogesteine aus der Gneissformation des niederösterreichischen Waldviertels.” Isch. min. u. petr., Mitt. v, p. 147. 1883.
[Footnote] ‡ Harker, A. “The Bala Igneous Rocks of Carnarvonshire,” pp. 92, 97.