By D. S. Coombs, University of Otago
Note on the Occurrence of Further Porphyritic Rocks in River Gravels of South Canterbury and North Otago
Just before his recent departure from New Zealand, Mr Amies collected a series of pebbles of altered porphyritic rocks from river gravels at three localities in South Canterbury and North Otago—
(a) from along the Hakataramea River for about seven miles upstream from the junction with the Waitaki,
(b) from Kakanui River mouth,
(c) from the North Branch of the Waianakarua River, where porphyrite pebbles are particularly large and abundant near Glencoe (pers. comm.).
It seems appropriate to record brief petrographic descriptions of the rock types represented, namely, relatively unsheared metaporphyrites, severely crushed metaporphyrites, and lastly keratophyres. Professor W. N. Benson has earlier collected pebbles of metaporphyrite from both the North (8070a) and South (8064) branches of the Waianakarua River (pers. comm.) and he has generously suggested that his collection also be recorded in this note.
Unsheared metaporphyrites are represented by 8619 and 8622 from Kakanui Beach, and by all the Waianakarua specimens (e.g. 8064, 8070a, 8627, 8628). They contain large white or very pale green feldspar phenocrysts averaging 5 to 10 mm. in diameter, set in a fine-grained purple-tinted groundmass. In thin section the phenocrysts are seen to be thickly studded with granules and prisms up to 0·1 mm. long of epidote (usually an iron-poor type), together with finely divided sericite and sometimes a little pale green positive chlorite. In general, albite twinning is still discernible in the host plagioclase which was determined as albite, an3–4 (determinations by universal stage methods). The twin planes are bent and sometimes broken, and occasionally the feldspar has completely recrystallized as a fine mosaic of albite grains. Some granules of new albite, twinned and water-clear, were found to have the same composition, an3–4, as the phenocrysts. The groundmass contains the minerals already mentioned and in addition much finely divided “leucoxene” and skeletal or dusty reddish iron ore. Chlorite is more common in the groundmass than in the phenocrysts. Although virtually all the present groundmass minerals are metamorphic in origin, their grain size is so small that the igneous structure of the rock can be recognized without difficulty.
8064 (South Branch) is interesting in that it contains several clusters of quartz grains in part surrounded by very crudely radiating skeletal rods of iron ore and rod-like aggregates of minutely granular light brown material. It is possible that these are relics of coronas formed about small quartzose xenoliths caught up in the original magma. 8070a (North Branch) shows some comparable features.
Severely crushed metaporphyrites. Nos. 8624 and 8629 from Hakataramea River represent this group. They are visibly sheared both in hand specimen and in thin section and s-planes are locally thrown into small-scale contortions. There is no trace of relict twinning in the feldspar phenocrysts, which have been replaced by pseudomorphous aggregates of epidote, sericite, positive chlorite and granular albite. In the groundmass, pseudomorphs after small feldspar laths can still be recognized, although with difficulty. Tiny stringers of dusty “leucoxene” and reddish iron ore are abundant and small veinlets and lenticles of quartz are not uncommon, these latter sometimes
containing water-clear albite and relatively coarse chlorite and sericite as well. Similar veinlets are present in the unsheared metaporphyrites described above, but they are rare. Scattered aggregates of positive chlorite in 8629 may possibly be pseudomorphous after augite.
8626 and 8630, also from Hakataramea, retain recognizable twinning in the phenocrysts, being transitional towards the unsheared type in this respect. They are notable also for containing pumpellyite. 8630a contains many irregular aggregates (0·5–1 mm.) of pale green positive chlorite fringed with dusty reddish iron ore.
Keratophyres. 8620 and 8621 from Kakanui Beach are classified as keratophyres. They are very hard, almost flinty rocks in which the feldspar phenocrysts, less than 3 mm. long, are small and scattered as compared with those of the metaporphyrites. The groundmass has trachytie texture. Both phenocrysts and groundmass feldspars tend to show fine and often blotchy twinning rather reminiscent of anorthoclase, but all grains determined were optically positive, composition albite, an3 4 They contain sparse inclusions of calcite, epidote and rare pumpellyite. In contrast to the metaporphyrites the groundmass contains only very subordinate epidote together with minor interstitial or finely intergrown quartz, a little sericite, and dusty iron ore and lencoxene. The sections are crossed by a few fine joint-filling quartz-veins, but apart from slight distortion and fracturing of feldspar phenocrysts, there is no evidence of severe crushing.
Comments. Disregarding the keratophyres, the rocks described above differ from the Waihao porphyrites chiefly in their less sheared condition, although the Hakataramea examples are comparable to some of the Waihao ones in this respect. Further, the mineralogical composition seems less varied, and relict augite, so abundant in some of the Waihao rocks, has not been found. The usual mineral assemblage is albite-epidote-sericite-chlorite with iron ore and leucoxene, and occasionally pumpellyite. Evidently ancient bodies of porphyrite, as yet unmapped except at Waihao, are rather widely distributed in the general area considered. It should be possible to locate them by tracing the river pebbles to their sources.