T A N E 26,1980

G E O L O G Y O F F A N A L I S L A N D ( M O T U K I N O ) , O U T E R H A U R A K I G U L F , N O R T H A U C K L A N D

by G . H . Browne and D A . Gre ig Department of Geology, University of Auckland, Private Bag, Auckland

S U M M A R Y

Fana l Island (Motukino) consists of th in 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 F low Banded Member (new) and an upper Agglomerate Member (new). The distinction between the rhyolit ic lithologies of Burgess and Fana l Islands is not considered as significant as previous authors. Two phases of concentric folding are recognised and their relationships described. Jo int orientations are probably of igneous origin.

I N T R O D U C T I O N A N D G E O L O G I C A L S E T T I N G

Fana l Island is the eastern-most and largest island of the Mokohinau group of islands, l y ing northwest of Great Barrier Island in the H a u r a k i Gulf , some 105 kilometres from A u c k l a n d C i t y . The M a y 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 wi th in the Coromandel Volcanic Zone (Fig. 1). This northerly trending belt, extends from the Poor K n i g h t s Group in the north to the Aldermen Islands in the south, a distance of some 210 kilometres, and is equivalent to the Whit ianga A r c of Ballance (1976) which was active some 6 to 3 m.y. ago.

Kear (1964) had previously included M a y o r Island wi th in this rhyolit ic chain, but Cole (1978) has argued that M a y o r Island was part of a separate north-east trending tensional graben structure known as the Ngatoro Bas in .

P R E V I O U S W O R K

The geology of the Mokohinau Islands was discussed by F leming (1950). In particular he studied Burgess Island, w i th its lighthouse, in the north-west of the island group. H e recognised four lithologic units :

The Lighthouse Formation - andesites (youngest);

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The Burgess Formation - bedded pyroclastics; The Mokohinau Formation - pale to orange, banded, glassy

rhyolite overlain by coarse angular agglomerate; The Fana l Formation - pale to brownish rhyolites wi th well

developed foliation (oldest). The island group was mapped on a scale of 1: 250,000 by

Thompson (1960) who placed the Mokohinau and Fana l Formations within the Whit ianga Group. H e considered the Fana l Formation to be stratigraphically below the Mokohinau Formation, but noted that this relationship was uncertain.

More recently, obsidian from Fana l Island has been studied wi th 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 Univers i ty of Auck land , Geology Department collections.

S T R A T I G R A P H Y

Ac id ic rocks of the Mokohinau Islands are placed within the Minden Rhyol ite Subgroup of the Whit ianga Group.

N o landing was made on Burgess Island by the present authors. However from the descriptions of F leming (1950), from hand and th in 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, i t is clear that the Mokohinau and Fanal Formations are more similar to each other than Fleming (1950) had supposed. F leming had erected the Fana l Formation based upon:

1. the lack of vertical flow banding in contrast to the type locality of the Mokohinau Formation

2. the flow banding i n the Fana l 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 val id . 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 a l l other respects however, the two formations have similar relationships. They both consist of flow banded glassy rhyolites, of l ight 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 w i t h flow bands spaced 0.3 to 3 cm apart. Scattered plagioclase phenocryts within a fine grey coloured matr ix 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 wi th 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 F low 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 + B o t h 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 G a p ' in the centre of the island. F r o m it ' s stratigraphic position it would appear to be the weathered equivalent of the F low Banded Member.

P E T R O L O G Y

T h i n section microscopy of the F low Banded Member shows a glassy spherulitic banding, separated by a coarser grained crystal band (Fig. 3). The flow bands appear to contain small iron t i tanium oxides, w i th dark green, needle-like amphiboles which have a sub-parallel orientation to the banding.

The coarser grained bands contain quartz crystals w i th 'sutured' auhedral boundaries. Smal l equidimensional, pale brown biotite occur irregularly.The flow bands are often disturbed by large fayalite phenocryts, w i th or without alteration rims of iddingsite. The feldspars in the ground-mass are probably of a lkal i type.

S T R U C T U R E

The gross structure of the island consists of an arcuate foliation pattern which delineates the south and western margin of a cumulo-dome. F low banding in the rhyolite dips dominantly to the west, w i th angles that are generally greater for the coastal cliff exposures than

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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).

Fo ld ing A geometric classification of folds within the F low 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 / t c ; F i g . 7A) Analys is of folds from Fana l 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 i n which the curvature of the inner fold surface is always less than that of the outer arc. D i p 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. B o t h 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.

Smal l crenulation folds were developed within the thickness of one or two flow banded layers and are designated as F wi th 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.

Faul t ing A northerly trending fault was mapped in the centre of the island.

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LEGEND:

N

m e t r e s

70

300

7 X

^ 7 Flow banding ^ Joint orientation

Vertical jointing *" Fold axis & plunge

Fault Weathered ?kaolinitised outcrop

^ 7

51 :>

02 X

> 0

7S

"20

J 7

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SO

-J4S

Fig. 4. Geological map of Fanal Island. Lower hemisphere, equal area stereoplot is from the large outcrop immediately south of the summit. It indicates a conjugate fault pair with compression (single arrow head) and extension (double arrow head) indicated.

Fig. 5. Volcanic pipe exposed on the western side of the island. View looking toward the east. Cliff exposure is approximately 60 m high.

The only other mesoscopic faulting was observed at the locality immediately south of the summit, where a normal fault (119/54 N E ) and a conjugate fault involv ing two normal faults were recorded (Fig. 4).

Jo int ing Vert i ca l and high angle jo int ing is characteristic of the Fana l

Formation. Three dominant joint sets were recognised-the strongest wi th a pole 035°/10 w i t h a less well defined pole of 315°/15, and a th i rd set w i th a westerly pole (Fig. 11). Cross cutt ing relationships of jo int planes were not found to be consistant at successive outcrops, making time sequential phase relationships inconsistant. The joints probably are a direct result of the cooling mechanisms (see for example H i l l s 1965).

G E O L O G I C A L H I S T O R Y Fana l Island represents the western and southern margin of a

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rhyolitic cumulo-dome. If the conclusions of F leming (1950) and Thompson (1960) concerning the age relationships of the Mokohinau and Fanal Formations are val id (a criterion not tested in this study) then Fanal Island marks the commencement of volcanism at the Mokohinau centre, probably in the Pliocene (1.8 to 5 m.y BP) The sudden cooling of the acid magma on Fanal Island is shown by a brecciated carapace (Agglomerate Member) consisting of angular obsidian clasts. The chil l ing was complete at the periphery where massive obsidian lacking any supporting matrix was formed.

Continued volcanism not preserved on Fanal Island, is represented by a latter acid igneous phase (represented by the Mokohinau Formation) together wi th subsequent intermediate volcanism (Burgess and Lighthouse Formations) now only preserved on Burgess and associated islands.

A C K N O W L E D G E M E N T S

We would like to thank Mr F. Brook who kindly cut the thin section, to Associate Professor P.M. Black for her interest in the project, and to Dr K.B. Sporli who kindly read the manuscript and suggested many improvements.

R E F E R E N C E S

Ballance, P.F. 1976: Evolution of the Upper Cenozoic magmatic arc and plate boundary in northern New Zealand. Earth and Planetary Science Letters 28: 356-370.

Browne, G . H . 1980: Geomorphic features of Fanal Island. Tane 26: (this issue). Cole, J.W. 1978: Tectonic setting of Mayor Island volcano (note). New Zealand Journal of

Geology and Geophysics 21: 645-647. Fleming, C A . 1950: The geology of the Mokohinau Islands, North Auckland.

Transactions of the Royal Society of New Zealand 78: 255-268. Hayward, B.W. 1976: Geology of the Whitianga Group, Great Mercury Island - part 1.

Coroglen Subgroup stratigraphy. Tane 22:5-14. Hills, E.S. 1965: "Outlines of structural geology" (3rd Ed) Methuen, London. 182p. Kear, D. 1964: Volcanic alignments north and west of New Zealand's central volcanic

region. New Zealand Journal of Geology and Geophysics 7: 24-44. Leach, B .F . & Fankhauser, B. 1978: The characterisation of New Zealand obsidian

sources by thermoluminescence. Journal of the Royal Society of New Zealand 8: 331-342.

Ramsay, J . G . 1967: "Folding and fracturing of rocks" McGraw-Hill Inc. 568p. Thompson, B.N. 1960: Sheet 2B "Barrier" (1st Ed) Geological Map of New Zealand,

1:250,000. New Zealand Department of Scientific and Industrial Research, Wellington.

Ward, G.K. 1974: A paradigm for sourcing New Zealand archaeological obsidians. Journal of the Royal Society of New Zealand 4: 47-62.

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