Acid Andesites
Previous observations on the Hornblende-Andesites of Fiji
These rocks were first described by Wichmann[[112]] from specimens obtained by Kleinschmidt from the mountain of Mbuke Levu in Kandavu. These Kandavu rocks had a microfelsitic base, the porphyritic brown hornblende having usually black borders in which a change into epidote was observed. Rhombic pyroxene was only noted as an occasional constituent of a rock from Ono. Renard[[113]] described these rocks from the vicinity of Ngaloa Harbour in Kandavu and remarked that bronzite was of more common occurrence than the monoclinic pyroxene. In the groundmass were numerous felspar and augite microliths, whilst there was a porphyritic development of plagioclase, hornblende, biotite, and pyroxene. The hornblende phenocrysts played an important part in the rock-composition, being surrounded by a black zone of magnetite or bordered by a bacillary aggregate of small pyroxene prisms, parallel and colourless or greenish, with extinction-angles of 40°. There was often also a development of biotite in the heart of the mineral, the whole hornblende section being sometimes thus transformed.
Mr. Eakle[[114]] has more recently described the hornblende-andesites from Mbuke Levu in Kandavu. As the result of his examination of a collection of volcanic rocks made by the Agassiz expedition in Viti Levu, Kandavu, Mbenga, Totoya, Malolo, Yasawa group, and in several other small islands, he inferred that the hornblende-andesites are much more limited in their occurrence in Fiji than the augite-andesites; whilst hypersthene-andesite was only represented in the collection from Vomo-lailai near Waia in the Yasawas. The specimens from Waia had a microfelsitic base with pseudomorphs of hornblende and some augite. Mr. E. C. Andrews[[115]] in his account of his collection of volcanic rocks, made mostly in the Lau Group and Taviuni, makes no special reference to hornblende-andesites, the andesites being mainly augitic, rhombic pyroxene also occurring as a common porphyritic constituent.
It may be inferred from the above and from my own observations in Vanua Levu below given that hornblende-andesites have a relatively limited distribution in Fiji. They are not generally distributed as in the case of the augitic and basaltic andesites; but are confined to certain localities in Viti Levu,[[116]] Vanua Levu, Ovalau, Kandavu, Ono, Malolo, Yasawa Islands, etc.
The occurrence of quartz-andesites or dacites in Fiji.—In connection with the existence of these rocks in Vanua Levu, it is noteworthy that except in Mr. Eakle’s paper there is no reference in any of these writings to the occurrence of quartz-andesites in Fiji. Wichmann expressly states that the rocks he examined were free from quartz, and that up to his time (1882) no quartz-bearing younger eruptive rocks were known from the South Seas. Mr. Eakle in 1899 described a holo-crystalline andesite with a felsitic aspect from Malolo and another similar looking rock from Vatu Mbulo, in the same sub-group of the Fijian Islands, showing quartz both in the phenocrysts and in the microcrystalline groundmass, concerning which he observed that it was perhaps more of a dacite than an andesite. Dacites were found by me in 1884 in the island of Fauro in the Solomon group,[[117]] and it is probable that they are of more frequent occurrence in the Pacific than has been generally supposed. As shown immediately below, they are represented in Vanua Levu; and the extent of their distribution in the island depends on the limits we assign to the definition of the term “dacite.”
If we restrict the term to a hornblende-andesite carrying porphyritic quartz and displaying a microfelsitic groundmass, such rocks, though they form some of the highest peaks in the Ndrandramea district, namely Ngaingai and Wawa-levu, would not be very frequent in Vanua Levu. If, however, a microfelsitic groundmass is alone necessary to constitute a “dacite,” the great majority of the acid andesites of the island would fall under this designation.[[118]] This has long been a controverted point in petrology. If I adopted the last procedure, my general classification for the andesites would fall into confusion and many rocks without any quartz would be included in the dacites.
In the Synopsis it will be seen that my classification of the andesites is as far as concerns the great groups based on the mineral and not on the structural characters. There are three sub-classes closely allied to each other, the hypersthene-andesites, the hornblende-hypersthene-andesites, and the quartz-hornblende-hypersthene-andesites or dacites, which cannot be distinguished at their boundaries by their petrological characters or by their different modes of occurrence. These groups of rocks which include all the acid andesites of the island will now be dealt with.
The Acid-Andesites of Vanua Levu
(Comprising the hypersthene-andesites; hornblende-hypersthene-andesites;