Geology of Fanal Island (Motukino), Outer Hauraki Gulf, North Auckland

TANE 26,1980 GEOLOGY OF FANAL ISLAND (MOTUKINO), OUTER HAURAKI GULF, NORTH AUCKLAND by G.H. Browne and DA. Greig Department of Geology, University of Auckland, Private Bag, Auckland SUMMARY Fanal Island (Motukino) consists of thin flow banded rhyolite of presumed Pliocene age. The island represents the southern and western remnant of a cumulo-dome, whose northern portion has been eroded by the sea. The rocks of the island (Fanal Formation) are subdivided into a basal Flow Banded Member (new) and an upper Agglomerate Member (new). The distinction between the rhyolitic lithologies of Burgess and Fanal Islands is not considered as significant as previous authors. Two phases of concentric folding are recognised and their relationships described. Joint orientations are probably of igneous origin. INTRODUCTION AND GEOLOGICAL SETTING Fanal Island is the eastern-most and largest island of the Mokohinau group of islands, lying northwest of Great Barrier Island in the Hauraki Gulf, some 105 kilometres from Auckland City. The May 1979 A.U.F.C. scientific camp provided an opportunity to examine the geology of the island. The Mokohinaus form part of a long discontinuous chain of rhyolitic volcanics that outcrop within the Coromandel Volcanic Zone (Fig. 1). This northerly trending belt, extends from the Poor Knights Group in the north to the Aldermen Islands in the south, a distance of some 210 kilometres, and is equivalent to the Whitianga Arc of Ballance (1976) which was active some 6 to 3 m.y. ago. Kear (1964) had previously included Mayor Island within this rhyolitic chain, but Cole (1978) has argued that Mayor Island was part of a separate north-east trending tensional graben structure known as the Ngatoro Basin. PREVIOUS WORK The geology of the Mokohinau Islands was discussed by Fleming (1950). In particular he studied Burgess Island, with its lighthouse, in the north-west of the island group. He recognised four lithologic units: The Lighthouse Formation - andesites (youngest); 7 8 The Burgess Formation - bedded pyroclastics; The Mokohinau Formation - pale to orange, banded, glassy rhyolite overlain by coarse angular agglomerate; The Fanal Formation - pale to brownish rhyolites with well developed foliation (oldest). The island group was mapped on a scale of 1: 250,000 by Thompson (1960) who placed the Mokohinau and Fanal Formations within the Whitianga Group. He considered the Fanal Formation to be stratigraphically below the Mokohinau Formation, but noted that this relationship was uncertain. More recently, obsidian from Fanal Island has been studied with the aim of sourcing obsidian archaeological artifacts (Ward 1974, Leach and Funkhauser 1978). The geomorphology of the island is discussed in a separate paper (Browne 1980). In addition to the above published works, rock collections were made from Burgess Island during 1966, by Associate Professor P.M. Black, and these samples are lodged in the University of Auckland, Geology Department collections. STRATIGRAPHY Acidic rocks of the Mokohinau Islands are placed within the Minden Rhyolite Subgroup of the Whitianga Group. No landing was made on Burgess Island by the present authors. However from the descriptions of Fleming (1950), from hand and thin section examination of the samples collected by Professor Black, and from our own observations of the exposures on the south coast of Burgess Island from a boat, it is clear that the Mokohinau and Fanal Formations are more similar to each other than Fleming (1950) had supposed. Fleming had erected the Fanal Formation based upon: 1. the lack of vertical flow banding in contrast to the type locality of the Mokohinau Formation 2. the flow banding in the Fanal Island rhyolites was considered to be much thinner than those from Burgess Island. However at several localities on Fanal Island, vertical flow banded units were observed. Fleming's comments on the thinness of the foliation on Fanal Island seems to be valid. It is therefore justifiable to refine Fleming's criteria, and to separate the two formations solely on the basis of the thickness of the flow banded units. In all other respects however, the two formations have similar relationships. They both consist of flow banded glassy rhyolites, of light to brownish hues, that are capped either by glassy obsidian or by a coarse agglomerate. It is probably therefore that the Mokohinau and Fanal Formations were closely related, if not syn-igneous volcanic centres. Two members are recognised within the Fanal Formation. 9 Flow Banded Member (new) 120 m + The Flow Banded Member consists of pinkish-brown to cream coloured, finely laminated rhyolite with flow bands spaced 0.3 to 3 cm apart. Scattered plagioclase phenocryts within a fine grey coloured matrix are common in hand specimen. Flow units are commonly folded on a meso- and macro-scopic scale. Higher in the sequence the rhyolite grades into a two metre thick black to green/grey flow banded obsidian with phenocrysts of plagioclase. This upper obsidian layer is interpreted as the chilled equivalent of the stratigraphically lower glassy rhyolite unit. The type section of the Flow Banded Member follows Fleming's original description for the Fanal Formation, sensu lato at the south-west landing (Fleming 1950, p 266). Agglomerate Member (new) 1.5 m + Both conformably and unconformably overlying the Flow Banded Member, the Agglomerate Member consists of angular to subangular obsidian clasts ranging from 1 to 40 cm in size. The larger xenoliths show no preferred orientations, and are set within a fine brecciated matrix. The agglomerate forms a discontinuous carapace over much of the island. The type section is designated to be the large outcrop immediately south of the summit (Fig. 2). A small, 4 m thick outcrop of a white ?kaolinitised sequence was observed at 'the Gap' in the centre of the island. From it's stratigraphic position it would appear to be the weathered equivalent of the Flow Banded Member. PETROLOGY Thin section microscopy of the Flow Banded Member shows a glassy spherulitic banding, separated by a coarser grained crystal band (Fig. 3). The flow bands appear to contain small iron titanium oxides, with dark green, needle-like amphiboles which have a sub- parallel orientation to the banding. The coarser grained bands contain quartz crystals with 'sutured' auhedral boundaries. Small equidimensional, pale brown biotite occur irregularly.The flow bands are often disturbed by large fayalite phenocryts, with or without alteration rims of iddingsite. The feldspars in the ground-mass are probably of alkali type. STRUCTURE The gross structure of the island consists of an arcuate foliation pattern which delineates the south and western margin of a cumulo- dome. Flow banding in the rhyolite dips dominantly to the west, with angles that are generally greater for the coastal cliff exposures than 10 Fig. 2. Angular to sub-angular obsidian blocks, within a finer grained matrix. Type locality of the Agglomerate Member, immediately south of the summit. Outcrop viewed in section, looking toward the north. Tape measure scale is 50 cm. atop the island (Fig. 4). Variations to this overall pattern occur occasionally, such as in the west of the island where a volcanic pipe is exposed (Fig. 5). The major volcanic centre was probably a short distance to the north-east of the present island. The whole of the northern portion of the volcanic dome has been subsequently eroded by the sea (Fig. 6). Folding A geometric classification of folds within the Flow Banded Member has been attempted from 14 folds, photographed in section. The method used follows that of Ramsay (1967), which relates a measurement of the thickness of the fold limbs (t «x / tc ; Fig. 7A) Analysis of folds from Fanal Island using this method indicates that three classes of fold are present (Fig. 7 B): - dominantly Class 1 C whereby the dip isogons are weakly convergent. In these types the curvature of the inner fold surface always exceeds that of the inner arc. - a less important Class 1 B in which the curvature of the inner fold surface is always less than that of the outer arc. Dip isogons are always perpendicular to the surfaces of the folded layer, and the thickness of the layers is always constant. - less important Class 3 in which the relationship of arc curvature is the same as in Class 1 B, but where the dip isogons are divergent. Neither of Ramsay's Class 1 A or Class 2 were observed. Both mesoscopic and microscopic open and closed concentric folds were recognised. Examples are shown in Figs. 8 and 9. The concentric folding has axial surfaces inclined at moderate to high angles, with fold axes plunging predominantly to the north-west and the south-east, at angles generally less than 30° (Fig. 10). It is probably that the maximum compression was in the direction of magma flow to the southwest, and the preferred north-west—south-east orientation of the fold axes would therefore parallel the intermediate axes, normal to the infered flow direction. Small crenulation folds were developed within the thickness of one or two flow banded layers and are designated as F with a horizontal to moderately inclined axial surface S. In places the F folds have been refolded by a later open set of folds that has affected several metres of stratigraphic thickness, and are designated F with a near vertical axial plane S . The S axial planes have been rotated to be sub-parallel to the S axial surface. It is considered that both F and F sets were closely related, which both occurred prior to solidification of the magma. Faulting A northerly trending fault was mapped in the centre of the island. 12 LEGEND: ^7 Flow banding ^ Joint orientation Vertical jointing *" Fold axis & plunge Fault 7X Weathered ?kaolinitised outcrop N 70 ^7 7S 51:> J7 300 X 02 27 metres "20 0 > SO -J4S Fig.

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