Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B

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Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B. Spörli and P.M. Black

IESE Report 1-2013.01 | November 2013 ISBN: [print] 978-0-9876566-4-3 [pdf] 978-0-9876566-5-0

Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B. Spörli*1, 2 and P.M. Black1 1 Geology Programme, School of Environment, University of Auckland 2 Institute of Earth Science and Engineering (IESE), University of Auckland *[email protected]

IESE Report 1-2013.01 | November 2013

This report was prepared by IESE as part of the DEVORA Project.

Disclaimer: While the information contained in this report is believed to be correct at the time of publication, the Institute of Earth Science and Engineering and its working parties and agents involved in preparation and publication, do not accept any liability for its contents or for any consequences arising from its use.

Copyright: This work is copyright of the Institute of Earth Science and Engineering. The content may be used with acknowledgement to the Institute of Earth Science and Engineering and the appropriate citation.

Photos, from left to right: 1) thin section of amphibolitic schist (plain polarized light). 2): Typical boulder of tuff with xenoliths. 3) Thin section of rodingite with garnet (black) replacing a crack-seal vein (x polarisers).

TABLE OF CONTENTS

ABSTRACT ...... 1 INTRODUCTION ...... 2 GEOLOGICAL FRAMEWORK ...... 2 THE AUCKLAND VOLCANIC FIELD ...... 2 BASEMENT ...... 7 Murihiku terrane ...... 7 Maitai terrane ...... 7 Caples/Pelorus terrane ...... 7 Waipapa terrane...... 8 Cenozoic cover sediments ...... 8 Northland Allochthon ...... 10 Tectonic history ...... 10 Present day structural state ...... 10 THE SOURCE VOLCANOES ...... 11 Glovers Park (St. Heliers) Volcano (Whakamuhu) ...... 11 Taylors Hill Volcano (Taurere) ...... 12 Source of fragment collections ...... 12 Mode of occurrence of the fragments ...... 12 DESCRIPTION OF XENOLITHS ...... 14 NON- OR LITTLE DEFORMED METABASITES ...... 14 GABBROS ...... 15 PORPHYRITIC VOLCANIC ROCKS ...... 16 Foliated metabasic rocks ...... 17 Opaque–rich coarse metabasites: ...... 26 Semi-opaque rich rocks ...... 30 SEDIMENTARY ROCKS ...... 31 Metamorphic sediments ...... 31 Non-metamorphic sandstones ...... 34 Metamorphic sandstone xenoliths from other volcanoes: ...... 34 SUMMARY AND DISCUSSION ...... 35 TYPES OF XENOLITHS IN THE AUCKLAND VOLCANIC FIELD ...... 35

THE ROCK ASSEMBLAGE EJECTED BY THE ST. HELIERS VOLCANOES ...... 35 Components ...... 35 Present state of the ejecta ...... 35 STRUCTURAL SYNTHESIS AND SEQUENCE OF EVENTS ...... 36 How many rock suites? ...... 36 Foliated metabasites ...... 36 Ductile, semi-mylonitic faulting in the foliated metabasites ...... 37 Non- or little- foliated metabasites ...... 37 Metasomatism of the metabasites ...... 37 Cataclastic faulting in the metabasites ...... 37 Veining in the metabasites ...... 37 Metamorphism of the metabasites...... 38 Metasediments ...... 38 CORRELATION WITH TERRANES AND STRATIGRAPHIC UNITS ...... 38 The metabasic rocks ...... 38 The metamorphic sandstones ...... 40 Red chert ...... 42 Source of the metamorphic metabasites and sediments ...... 42 ERUPTION MODEL ...... 43 FURTHER WORK ...... 44 The metabasites ...... 44 The metamorphic sedimentary rocks ...... 44 The non-metamorphic xenoliths...... 44 The enclosing lavas and tuffs ...... 45 Field work at Glovers Park ...... 45 Elsewhere in the Auckland Volcanic Field ...... 45 CONCLUSIONS ...... 46 ACKNOWLEDGMENTS ...... 47 REFERENCES ...... 48 APPENDICES ...... 54 APPENDIX I: list of samples in the Searle collection ...... 54 APPENDIX II: list of samples collected subsequently...... 60

ABSTRACT

We describe a collection of xenoliths ejected by phreato-magmatic eruptions and preserved in tuff from Glovers Park and Taylors Hill volcanoes in St. Heliers. These rocks are different from mantle-derived xenoliths and semi-assimilated crustal quartzose xenoliths within the monogenetic Auckland Volcanic Field in that they are a diverse suite of crustal rock fragments dominated by mafic schistose and non- schistose rocks ranging from a few mm up to 70 cm in diameter. Non-schistose mafic rocks include coarse grained peridotites, gabbroic to dioritic igneous rocks and basaltic lavas. Schistose rocks range from slightly foliated to highly transposed, with the most highly deformed showing up to three phases of syn-metamorphic deformation, including formation of ductile shear zones. A magmatic high temperature foliation was the first surface formed. Metamorphic minerals are dominated by amphiboles, feldspar, and epidote, but there is also garnet and some pumpellyite. The grade of metamorphism ranges from amphibolite to prehnite-pumpellyite facies. Metamorphism of these mafic rocks took place at a depth estimated to be less than 7 km. Metasomatism, including polygonization of feldspar lenses, is common and may be related to formation of hydrogrossular- bearing rocks resembling the rodingites found in serpentinite shear zones of the Dun Mountain belt and adjacent melanges of the South Island. The rock suite, deformation, metamorphism and metasomatism of the metabasic xenoliths suggest formation at a slow-spreading oceanic ridge. A suite of terrigenous clastics is generally at a lower metamorphic grade, in the prehnite-pumpellyite range. One specimen contains abundant calcite prisms identical to Atomodesma fragments seen in the Permian Maitai Group rocks of the South Island. We interpret this suite of rocks as fragments from a serpentinite melange adjacent to the Dun Mountain Belt of the Maitai terrane that lies hidden underneath the Auckland Volcanic Field. Because of their inferred high temperature /low pressure spreading ridge origin and subsequent accretion tectonics, the metabasic xenoliths have never resided at great crustal depths. Other rock types ejected are Miocene Waitemata Group and other low grade clastic rocks, and basaltic lavas from the host volcanoes. Pervasive cataclasite networks in the metabasite xenoliths indicate that these two volcanoes were mining a crustal fault zone at a depth of several hundred metres, at an unconformity between mafic basement rocks and overlying Cenozoic sediments. Lava coatings and other features of xenolith preservation record several stages of phreato-magmatic eruption. The 310 specimens lodged at Auckland University are a useful resource for further research about the nature of the crust below the Auckland Volcanic Field and the eruption mechanisms for these two volcanoes.

INTRODUCTION

Xenoliths brought up by volcanic eruptions can provide several types of information: a) they are samples of the lithosphere beneath the volcano; b) they can indicate the depth from which the magma plucked them; c) they may give clues to the processes by which the magma rose to the surface. We present detailed descriptions and a new interpretation of the whole range of xenoliths so far found in two volcanoes of the Auckland Volcanic Field, with special emphasis on samples of metamorphic basic igneous rocks. Some of these were already described by Searle (1959), but we have complemented his collections by additional field work, and some of these subsequent samples were examined by James (2007). Our synthesis has important implications for crustal structure below the Auckland Volcanic Field and the mechanism by which volcanoes mine xenoliths. We will first discuss the setting of the Auckland Volcanic Field (Fig.1) and the two volcanoes involved (Glovers Park and Taylors Hill, Figs. 2-5), then present details of selected samples of the basic metamorphic (metabasic) rocks and an important sandstone specimen, with additional brief descriptions of the remaining rock types. We then consider how individual samples are related to each other and where they may have been sourced. Finally the implications of these samples to mechanisms of magma ascent will be discussed. Since we have no information about the original orientations of the samples, orientations of structural and other features will be described as illustrated in the figures involved. An additional restriction is that structural features are generally only seen in two dimensions. For our descriptions of offsets on faults, we will therefore use the two-dimensional term ‘separation’ (e.g. Davies 1984 p. 273, Twiss & Moores 1992 p. 66), also because the orientation of the true movement vectors on these faults cannot be determined. In the schistose rocks, deformation phases recognized from cross-cutting patterns of structures will be labelled in the standard manner used for metamorphic tectonites (e.g. Hobbs et al. 1976, Passchier & Trouw, 1998): S1 - Sn for foliation surfaces, F1 - Fn for folds and, in the rare cases where a lineation is visible in three dimensions, L1 - Ln. The index number for fold phases is always one higher than the youngest preceding structural surface affected by that folding. D1 - Dn will be used to label deformation phases consisting of a number of such structural elements. Note that S1, F1, L1 and D1 merely denote the first event observed in a particular specimen and not necessarily the first phase experienced by that rock, because that first phase may have been obliterated by the subsequent deformations. Any primary layering (e.g. bedding, flow layering) if recognized, would be labelled S0. A structural synthesis will be attempted in the ‘Discussion’ section. The samples are lodged in the petrology collection of the Geology Programme, School of Environment, University of Auckland and are listed in Appendices I and II.

GEOLOGICAL FRAMEWORK

THE AUCKLAND VOLCANIC FIELD

The active Auckland Volcanic Field consists of ~50 alkali olivine basalt volcanoes (Hayward et al. 2011), and is very young, with the age of the oldest eruptions dated about ~250 Ka (Leonard et al. 2008, Lindsay & Leonard, 2009; Lindsay et al. 2011, Molloy et al. 2009). It is part of a group of intra-plate volcanic fields (Fig. 1 A) formed in an extensional setting distant from the subduction zone presently active further to the east (Huang et al. 1997). The volcanoes are mostly monogenetic (Smith 1989, 1992, Smith et al. 1993, Huang et al. 1997) although a few

appear to have erupted in the same, relatively brief time interval (Cassidy & Locke, 2004, Cassidy 2006, Cassidy & Locke 2010). The youngest, most productive, (Rangitoto, Fig. 2) had

Fig. 1: A) Tectonic setting of the Auckland Volcanic Field within northern New Zealand. Intraplate volcanic fields (orange) labelled in pink. Inset shows relationship to New Zealand tectonic setting. Green line: Maitai terrane (M) with the Dun Mountain Belt that generates the Junction Magnetic Anomaly. B) Gravity and magnetic anomalies of the Auckland area, dominated by the effects of the Dun Mountain Belt. Magnetic anomalies after Eccles et al. (2005). Note that the southern end of the lines marks the boundary of data acquisition, not the end of the anomalies. Takapuna gravity anomaly, contoured in gravity units (1 gravity unit (gu) = 0.1 milligal) after Williams et al. (2006). Recent shallow earthquakes after Davy (2008) and GeoNet. NW= Northwest Wairoa Fault, after Wise et al. (2003), BF = Beachlands Fault after Glading (1987). C) Schematic block diagram of basement structure and terrane subdivision under the Auckland region (not to scale) after Williams et al. (2006). Blue line marks position of in situ Maitai Group sediments. AVF = Auckland Volcanic Field

more than one episode of eruption (Needham et al. 2011, McGee et al. 2011) and may even have been active as far back as 1500 years ago (Shane et al. 2013). Timing of eruptions in the Auckland Volcanic Field was not even, with a possible flare- up indicated around 30 Ka ago (Leonard et al. 2008; Lindsay et al. 2011, Bebbington & Cronin, 2011). The source of the magma in these volcanoes lies in the lithospheric mantle (Huang et al. 1997, Horspool et al. 2006, McGee et al. 2013). Huang et al (1997) estimate that the magma source is at a depth of 80 to 140 km depth in a mantle area with low upwelling rates. At Motukorea (Fig. 2), magma was produced by melting of domains within relatively young

eclogite (McGee et al. 2012). Some of the volcanoes show clear compositional trends, with silica under-saturation decreasing in time (Smith et al. 1993, Huang et al. 1997, Smith et al. 2008) due to ponding at the asthenosphere/lithosphere boundary, dilution from a lithospheric mantle source (McGee et al. 2013) and subsequent rapid ascent (Smith 1992, Huang et al. 1997). Magma sources can be associated with local convection and are discrete batches from different depths and moving with different velocities (McGee et al. 2011). Because of urban noise and meagre instrumentation, embarrassingly little is known about the nature of the lithosphere below New Zealand’s largest city. Ashenden et al. (2008) estimate crustal seismic velocities from 2.70 km/sec to 6.20 km/s for the top 15 km of the crust. The crust/mantle boundary (Moho) under Auckland is at about 29 km depth (Horspool et al. 2006). There is no geophysical evidence for pooling of magma within the crust, although most volcanoes have near-surface ponds of lava predating the later scoria eruptions (Cassidy & Locke 2010). The field has an elliptical shape (Spörli & Eastwood 1997) possibly corresponding to a dome-shaped mantle structure located at the northward propagating tip of a major lithospheric fracture (Fig. 1). Within this ellipse, groups of three or more volcanoes form distinct alignments, indicating smaller scale control by fractures. There are some differences in the alignments seen by different authors: Kear (1964) emphasized the 010°, 050°, and 110° directions. After Magill et al. (2005), the 030o, 040o and perhaps NW-SE directions are important, while the analysis by von Veh and Nemeth (2009) produced main trends of ENE and NNE, with possible NNW to NW trends. Hayward et al. (2011) highlight NNE, NE and N-S alignments, with Glovers Park volcano lying on, and Taylors Hill near the only NW alignment. Since north-easterly alignments are common to all of these studies, these can be regarded as the most reliably established fracture controls. Indeed Bebbington and Cronin (2011) consider the spatial probability of vent opening to be strongly influenced by a structurally controlled NE-SW direction. However, von Veh and Nemeth (2009) also emphasize the importance of both the NE-SW and NW-SE fractures and the alignment of Rangitoto and Motukorea volcanoes with the NW – trending, active Wairoa Fault (see below) to be an important control for future eruptions. They also point out that some volcanoes are located at the intersection of differently oriented alignments. Two volcanoes thought to have erupted within a short time interval of each other appear to have tapped the same magma batch in the mantle (Rout et al. 1993): Wiri Mountain lies on a distinct alignment of three volcanoes trending NE. Crater Hill, 3.6 km to the NW, is part of less distinct NE-trending lineament (Rout et al. 1993 fig. 1). The most prominent alignment is at the south-eastern edge of the field in the Hampton Park-Styaks Swamp group (Fig. 2 and Hayward et al. 2011 p. 176) and strikes NNE. Aligned multiple vents indicating fissure eruptions along a NE striking fracture are particularly well developed at Crater Hill volcano (Houghton et al. 1999), and along a NNE- striking fracture on Mount Eden volcano (e.g. Hayward et al. 2011). One important point of these patterns is that the location of the volcanoes and their vents in the Auckland Volcanic Field are not controlled by one single direction of fractures but by a multi-directional network of defects. This probably is one of the reasons why the volcanoes in the Auckland Volcanic Field are distributed in a random, rather than a clustered pattern (Bebbington & Cronin, 2011, Le Corvec et al. 2013). The field may be in a transition of behaviour (McGee et al. 2011) that may also be expressed in a change from a youthful, simple elliptical array to a more irregular shape and could be continuing the northward propagation of volcanicity postulated by Hodder (1984) and Spörli and Eastwood (1997), as suggested by the following points: 1) seismic low velocity zones marking melt-producing regions in the mantle extend north of the field (Horspool et al. 2006); 2) the youngest volcano, Rangitoto (Fig. 2), lies in the north and has erupted lava volumes equivalent to the volume of all the other volcanoes, and 3) Rangitoto is also unique in producing at least two eruptions (Needham et al. 2011, Shane et al. 2013).

Shallow-seated controls on explosive basaltic eruptions in the Auckland Volcanic Field have been described by Houghton et al. (1999) at Crater Hill where simultaneous eruptions along a NE-striking, 600 m long dike from vents with contrasting eruption styles also changed their eruption mode rapidly with time, due to controls within the uppermost 80 m of the conduit. Main controls were differences in vesiculation rate due to geometrically caused changes in magma flow rates and presence or absence of water mainly controlled by magma ponding and coating of the vent. For Motukorea/Browns Island (Fig. 2), Bryner (1991) postulated downward excavation during phreato-magmatic eruptions. However there has also been deeper sourced change in eruption style due to arrival of magma from different mantle locations (McGee et al. 2012).

Fig. 2: Location of the St. Heliers volcanoes (red triangles) within the Auckland Volcanic Field. Blue triangles: other volcanoes in the field. Yellow rectangles: basement depths after Kenny et al. (2011, 2012) and Edbrooke et al. (1998). MRD: Mount Roskill drill hole, EP: Eden Park drill hole (E. Shalev, IESE, U. of Auckland, pers. com. 2013). Green rectangles: basement depths after Davy (2008). Broken red lines: sea floor faults after Davy (2008). t.c.: volcano with xenoliths of terrigenous clastics (greywacke).

The assemblage of diverse xenoliths discussed in the present report was collected from two volcanoes (Fig. 3), Glovers Park (St. Heliers) volcano (Whakamuhu) and Taylors Hill (Taurere). The two centres can be linked by a NNW -trending line (azimuth 351o, length 1.93 km) which, however, does not appear to closely match any accepted volcano alignment. The

Fig. 3: Map of the St. Heliers volcanoes, based on Searle (1981, Fig.6.6) other volcanoes have not produced such assemblages of xenoliths, except for Motukorea /Browns Island (Fig. 2) where tuffs contain fragments interpreted as Waipapa Group greywacke, Waitemata Group sandstones and young surficial sediments (Bryner 1991, Hayward et al. 2011). While more detailed studies of these inclusions are necessary, the presence of ‘chipwacke’ (angular fragments of dark mudstone floating in a sandstone matrix, see Schofield 1974) would support an assignment of the basement rock to the Waipapa terrane (Bryner sample AU 43356). Rounded greywacke clasts on Motukorea are inferred to have been plucked from basal Waitemata Group conglomerates (see below) which may have provided the lowest aquifer for initiation of phreato-magmatic eruption. A conglomerate origin is supported by Bryner’s samples AU 43356 and AU43358 which have a coating of Waitemata sediment. Very rare, unaltered greywacke-type xenoliths were collected by Prof. Searle at Mount Wellington and Mangere Mountain and will be briefly described in the section on greywacke xenoliths from the St Heliers volcanoes. Distinctive white quartz xenoliths with pyroxene reactions rims from Mount Eden, Mount Wellington and Mangere Mountain were interpreted by Searle (1962) as due to partial assimilation of greywacke-type basement rocks. A different xenolith population, only consisting of olivine -rich inclusions, occurs in tuffs from Pupuke volcano (Fig. 2). These have been interpreted to have been formed at 1250o C and 11 kb pressure in the upper mantle (Brothers 1960, Brothers & Rodgers 1969, Rodgers & Brothers 1969, Rodgers et al. 1975). Abundant crystals and aggregates of olivine are also present in large bombs near the tholoid of Mangere Mountain (Fig. 2, KBS observation June 2012).

BASEMENT

The volcanic field is underlain by Permian to Mesozoic north-westerly striking basement terranes either formed at or accreted to the margin of east Gondwana in the Mesozoic, consisting of the Murihiku terrane in the west, the Maitai terrane in the middle and the Caples/Waipapa terrane in the east (Figs. 1 A, C). These rocks are unconformably overlain by Cenozoic sediments (Edbrooke et al. 1998, Edbrooke 2001), at various depths below Auckland (Fig. 2 and Kenny et al. 2011, 2012).

MURIHIKU TERRANE A belt of very low grade metamorphic (up to zeolite facies) Mesozoic volcaniclastic rocks (Briggs et al. 2004), along the west coast of the North Island (Fig. 1 A) was deposited in a deep water fore-arc basin and has simple synclinorial structure (Spörli 1978). The nearest outcrops are 55 km to the south of Auckland at Port Waikato, but Murihiku rocks were reached at a depth of 340 m in a drill hole ~20 km to the SW of the Auckland Volcanic Field (Waterhouse 1989).

MAITAI TERRANE This unit is of particular importance for this report, as it is the source of the New Zealand-wide Junction Magnetic Anomaly (Eccles et al. 2005) which passes under the Auckland Volcanic Field (Figs. 1 A,C) and is associated with the large Takapuna Gravity Anomaly (Williams et al. 2006). In the South Island the Maitai terrane can be seen to consist of the ophiolitic Dun Mountain Belt which is overlain by the Permian to Triassic Maitai Group sediments. This belt is flanked to the east by the serpentinite-bearing Patuki and Croisilles mélanges. In the North Island, the only exposure of Dun Mountain Belt is at the Wairere serpentinite Quarry near PioPio (O’Brien and Rodgers 1973 a, 1974), 190 km south of Auckland. Here a 50 m wide NE-striking, steeply dipping zone of serpentinite with partially metasomatised blocks of gabbro is faulted against (Murihiku?) greywackes in the west and Cenozoic sediments in the east, indicating reactivation of the belt in the Cenozoic. In the Auckland area, the Takapuna gravity anomaly (Fig. 1B) of Williams et al. (2006), coinciding with the Junction Magnetic Anomaly, delineates the largest ultramafic massif of the Dun Mountain Belt in New Zealand. Here the magnetic anomaly can be subdivided into a number of NW-SE trending linear sub-maxima (Eccles et al. 2005). At least the western four of these (Fig. 1B) are interpreted to be serpentinite shear zones within the ultramafic massif. Lithologies in the Maitai terrane will be considered in more detail in the ‘Discussion’ section.

CAPLES/PELORUS TERRANE In the South Island the Caples/Pelorus terrane lies outboard (east and north, see Fig. 1 A) of the re-curved Maitai terrane (Bishop et al. 1976, Cawood 1986, 1987). It consists of greywacke-type rocks, including red and green volcaniclastics. Atomodesma-bearing limestones are associated with basic igneous rocks. Caples/Pelorus terrane is not exposed in the Auckland area but the terrane has been recognized in Northland (Jennings 1991, Black 1994) where its metamorphism shows an east-to-west increase from prehnite-pumpellyite to pumpellyite- actinolite facies. It is reasonable to assume that it also lies hidden beneath the Auckland Volcanic Field, unless it has been sheared out by later tectonic movements (e.g. Spörli & Aita 1995).

WAIPAPA TERRANE This is the major terrane to the east of the Auckland Volcanic Field, with the nearest exposures at Islington Bay, Motutapu, 3.63 km ENE from the summit of Rangitoto volcano and on Motuihe Island, 4.1 km ENE from Motukorea /Browns Island volcano (Fig. 2). Late Jurassic terrigenous clastic greywacke-type sandstone/mudstone sequences dominate (Adams & Maas, 2004). Main strikes of bedding are N-S to NNW-SSE with moderate to steep westerly dips (Spörli 1978, Spörli et al. 1989). Regularly interspersed seams of basic volcanics, red cherts and red and green argillites were originally interpreted as part of a single stratigraphic succession with the greywackes (Schofield 1974, 1979) but study of radiolarian faunas in the cherts and siliceous argillites (Spörli et al. 1989) revealed that they are consistently older than the surrounding terrigenous clastics, i.e. that they mark tectonic thrust repetitions in the sequence (e.g. Fig. 1C). Such structures are typical of an accretionary prism, with the basic volcanics, cherts and siliceous argillites representing the sliced-off upper parts of the incoming ocean floor upon which the terrigenous clastics had been deposited from the Gondwana margin. Metamorphism is mostly in the prehnite pumpellyite facies with the easternmost units in the attaining zeolite facies only (Black 1989, Nishimura & Black 2004).

CENOZOIC COVER SEDIMENTS These units generally have low angle dips or are horizontal and overlie the metamorphic Palaeozoic/Mesozoic basement rocks on a sharp unconformity. They consist of the Late Eocene to Oligocene Te Kuiti Group, the Miocene Waitemata Group (Fig. 4) and a complex, irregular assemblage of Late Miocene to Holocene marine, coastal sand and terrestrial sediments (Hayward 1993, Edbrooke et al. 1998, Edbrooke 2001). Te Kuiti Group is only exposed south of the Auckland Volcanic Field and typically has basal coal measures followed by a calcareous unit which is overlain by various sections of sandstone and mudstone (Edbrooke 2001). The base of the turbidite-dominated Waitemata Group is another prominent erosion surface, which can have rugged topography (Kenny et al. 2011, 2012) and cuts down-section towards the east so that, on Motutapu and Motuihe Islands and to the east (Fig. 2), Waitemata Group directly overlies the metamorphic rocks of the Waipapa terrane. In contrast to this, in the Mount Roskill drill hole (Fig. 2), below 23 m of Holocene deposits and 452 m of Waitemata Group, there were 117 m of Te Kuiti Group, including basal coal measures (Edbrooke et al. 1998). From geophysics, Williams et al. (2006) consider that the ultramafic source body of the Takapuna gravity anomaly has a flat, low angle, top erosional surface at about 500 m depth capped by Cenozoic sediments. The faults in the Auckland area delineate a central, NNW trending graben about 50 km wide (Kenny et al. 2011, 2012), bounded by a concealed horst in the west and the exposed Waipapa terrane in the east (Fig. 2).

Fig. 4: Typical exposures of the Miocene Waitemata Group in the sea cliffs and shore platforms north of Glovers Park Volcano. A) East-dipping turbidite beds overlain by a slide horizon with contorted beds at Tamaki Head. B) View north of the shore platform at the west end of Ladies Bay: a sandstone bed outlines a faulted, east-verging anticline; Rangitoto volcano in the background. Note the sharp cross- fault in the foreground with two people for scale. C) Normal fault plane with striations (parallel to yellow sense arrow), looking south, just east of Achilles Point. D) East-verging fold with thrust fault in its core, in turbidites directly underneath the volcano.

NORTHLAND ALLOCHTHON This important tectonic unit, which was emplaced in the late Oligocene/Early Miocene and contains rocks from Late Cretaceous to Late Oligocene, is not present in the volcanic field. The nearest exposures, at the southern end of the allochthon, are 19 km to the NW of the Auckland Volcanic Field, near Dairy Flat (e.g. Schofield 1989)

TECTONIC HISTORY Within the tectonic framework of the SW-Pacific and New Zealand, the development of the Auckland region can be summarized as follows: 1) accretion of the Palaeozoic-Mesozoic terranes (Murihiku, Maitai, Caples and Waipapa) onto eastern Gondwana. 2) Cessation of subduction, uplift and erosion of the terranes with subsequent deposition of Cenozoic sedimentary sequences and extensional faulting during rifting of New Zealand away from Gondwana from 84 Ma to 52 Ma (Gaina et al. 1998) with reduction of crustal thickness to 29 km. 3) Establishment of a new subduction system to the NE of the area, initiated by obduction of Cenozoic shelf sequences and underlying/adjacent ocean floor in the form of the Northland Allochthon (Ballance & Spörli 1979, Hayward 1993) in the Late Oligocene and culminating in the formation of a Miocene volcanic arc and the Waitemata turbidite basin in the region. 4) Transfer of the subduction zone to its present location about 400 km to the east, leaving the Auckland/Northland area in an intraplate setting (e.g. Smith 1989) and leading to formation of the Auckland Volcanic Field. The ~ 250 Ka span of activity in this field covers the last two major ice ages (e.g. Denton et al. 2010). Sea levels were high in the 250-200 Ka interval and in the last interglacial (~130 Ka). The 30 Ka flare up took place during a very low sea level interval just before the termination of the last ice age. Therefore Glovers Park and Taylors Hill volcanoes and a number of others in the field erupted when sea level was low and the area was well inland from the coast. The present sea coast exposure in the vicinity of the volcanoes (Figs. 2, 3) is entirely due to postglacial sea level rise.

PRESENT DAY STRUCTURAL STATE From geophysics and regional analogues (Fig. 1C), units in the Maitai terrane under the Auckland Volcanic Field are interpreted to be dipping steeply to the east, on the overturned eastern limb of the synclinorium affecting the Murihiku terrane (Eccles et al. 2005). This structure is probably due to Early Cretaceous back-folding within the continental margin of Gondwana in the last stages of accretion, but no doubt has been further modified by Cenozoic shearing (Spörli & Aita, 1995) The nearest faults suspected of being active include the NNW-trending North Wairoa Fault (Wise et al. 2003) which can be traced over a distance of 35 km and separates two of the basement fault blocks inherited from Late Cretaceous/Cenozoic rifting, and the smaller Beachlands Fault (Glading 1987) 5 km to the east (Fig. 1B). In addition, Davy (2008) postulates a NNE-striking Wynyard Fault and two faults trending N-S and NW in the Auckland CBD harbour area (Fig. 2) which displace the present day sea floor. He suggests that NNE-striking faults are controls on nearby volcano alignments. Recent tectonic earthquakes have occurred under Rangitoto Volcano and in the Tamaki estuary (Fig. 1B). Three magnitude 3 and one magnitude 2 earthquakes occurred in 2005 on Waiheke Island along a NNW trending alignment, 20 km NE of Glovers Park volcano (Sherburn et al. 2007). In 2011, an earthquake of magnitude 2.9 (GeoNet) occurred 2.9.km WNW of Glovers Park volcano at a depth of 9 km (Fig. 1B). Two magnitude 3 earthquakes occurred on Motutapu Island in April 2013 (GeoNet and Fig. 1B). All the active faults in the Auckland area may be spatially related to the western shoulder of the active Hauraki Rift (Fig. 1A). It has been suggested that there may be a structural link between the rift and the Auckland Volcanic Field via a deep detachment fault (Spörli & Eastwood, 1997).

THE SOURCE VOLCANOES

GLOVERS PARK (ST. HELIERS) VOLCANO (WHAKAMUHU) This volcano produced a simple inland tuff eruption without any lavas extruding (Searle 1981, Hayward et al. 2011). The 200-300 m diameter, slightly E-W elongated explosion crater today underlies the Glover Park sport facilities. Cliff retreat during post-eruption sea level rise has left the northern rim of the tuff cone perched about 30 m up in a cliff (Fig. 5), from which the xenoliths and blocks of tuff have dropped down to the shore platform in the underlying contorted Waitemata Group sediments, where they can be most easily sampled (Searle 1959). Large blocks of basement material (“several feet in diameter’) were first noted by Fox (1902) and attributed to the “Matai slates.” The age of the eruption is estimated at > 45 Ka with a large uncertainty, based on Rotoehu tephra (> 45 Ka) present within 2 m of the surface in drill core from Glover Park (Lindsay & Leonard 2009). Glovers Park volcano is underlain by Waitemata Group that is exposed in the main sampling area north below the volcano. It consists of interbedded turbiditic sandstones and mudstones which have been affected by eastwards-verging folds/thrusts (Figs. 4 B and D) similar to those described elsewhere in the Auckland area (Spörli & Rowland, 2007) and are cut by later steeply dipping extensional faults (Fig. 4 C). There are also some localised submarine slide deposits (Fig. 4 A).

Fig. 5: Cliff exposure of Glovers Park Volcano tuffs. For location see Fig. 3. Some of the smaller dark cavities mark former locations of xenoliths that have fallen out of the cliff.

TAYLORS HILL VOLCANO (TAURERE) In contrast to Glovers Park volcano, Taylors Hill has a variety of volcanic rocks. A tuff ring was formed first which then was occupied by a scoria cone with a number of spatter cones, and two small lava flows were produced (Searle 1981).This is another of the volcanoes in the Auckland Volcanic Field with a large uncertainty in its age. Best estimate is 32-34 Ka because of an anomalous paleo-magnetic signature (Cassidy & Locke 2004, 2010) similar to that of the better dated Wiri Mountain and Crater Hill volcanoes (Fig. 2) indicating that they all erupted during the same geomagnetic excursion (Cassidy 2006, Cassata et al. 2008, Lindsay et al. 2011). This would indicate that Taylors Hill is ~10 Ka younger than Glovers Park volcano and perhaps was part of the 30 Ka volcanic flare-up in the Auckland Volcanic Field.

SOURCE OF FRAGMENT COLLECTIONS Initial collections both from Taylors Hill and Glovers Park volcanoes were made by Prof. E Searle (e.g. Searle 1959) and are the most comprehensive in terms of lithologies sampled (Appendix I). However the richest concentration of fragments is on the shore platform north below Glovers Park (Fig. 3, ‘main sampling area’), where inclusions eroded out from the tuff cliff have been concentrated by wave action. Some care needs to be exercised in order to not pick up loose contact metamorphic and igneous rocks from northern Coromandel Peninsula (Fig. 1 A) that had been unloaded at West Tamaki Head for building purposes. We therefore only collected samples below the tuff cliff, preferring tuff- and lava-coated xenoliths and avoiding light coloured, coarse-grained, massive igneous rocks. The dark contact metamorphic rocks from Coromandel can only be recognised in thin section and their metamorphic minerals. From 1999 to 2004, graduate classes under the supervision of KBS collected further samples (Appendix II) in the framework of structural geology studies. These were complemented by a dissertation (James 2007), followed by some subsequent collections of samples.

MODE OF OCCURRENCE OF THE FRAGMENTS The largest fragments are found in the main sampling area. One angular block measured 70 cm in largest diameter, confirming the report by Fox (1902) of blocks ‘several feet in diameter.’ The inclusions are mostly angular, but there are also some with rounded shapes (Fig. 6). While the fragment usually are in sharp contact with the surrounding tuff (Figs. 6 A and B), some are encased in lava (Figs. 6 C-F), fitting a classification as cored bombs or composite clasts after White and Houghton (2006). Swarms of very small xenoliths, down to 1 mm or less in diameter, occur in the lavas and sometime are associated with fine bubble layers (Fig. 6 F), possibly indicating that the fragments induced incipient magma rupture due to a transition from a viscous to a brittle state (e.g. Cordonnier et al. 2012). On Motukorea Island there are also basement and Waitemata Group inclusions encased in lava. Bryner (1991) concluded that this interaction of magma with the surrounding rocks occurred just below the basal conglomerate of the Waitemata Group.

Fig. 6: Modes of occurrence of the inclusions; all examples are from the main sampling area (Fig. 3): A) Fallen block of bedded tuff with two sizes of dark metabasic rock inclusions. B) Fallen block of coarser tuff on the beach: Light-coloured fragments are Waitemata Group sandstones (W); note rounding of some inclusions. Dark fragment on lower left is Glovers Park lava. Hand lens for scale. C) Cored bomb encased in tuff. D) Lava inclusion with angular fragments of Waitemata Group making a jigsaw pattern. E) Rounded meta-gabbroic fragment in lava. F) Very small xenoliths associated with bubble planes (arrowed) in a recycled composite fragment. Paired yellow arrows show sense of lava flow-shearing along bubble/fracture surfaces.

SUMMARY OF XENOLITH TYPES Rock types: Rock types: Number of Examples Main groups Sub-groups specimens METABASITES 253 Schistose/foliated 148+ AU58777 (Fig. 12), AU58775 (Fig. 13) Garnet- bearing 4 AU58784 (Fig. 15), Searle 4992/5197 (Fig. 19) Non-schistose 22 AU58774 (Fig. 9) Pyroxenites 2 Searle 5114/5233 (Fig. 7) Gabbros/diorites 16 AU62889, AU6290, AU63059 (Fig. 8) Rodingites 3 Searle 4992/5197 (Fig. 19) Cataclasites 19 Fig. 13 E, Fig. 14 E and specimens in Fig. 21 METASEDIMENTS 8 Ordinary greywacke 6 AU63067, Searle 5138/5257 With fossils 1 Searle 5155/5275a (Fig. 23) Chert 2 AU63053 (Fig.22) CENOZOIC 14 AU63038 + Searle rock 5149 (Fig. 24) SEDIMENTS ENCASED IN TUFF 15 AU63065 (Fig. 6 A), Figs. 6 B and C LAVA-COATED 30 Several lithologies* 4 Searle rock 4997 (Fig. 6 F) Metabasites 11 Searle rock 4988 (Fig. 6 E), AU58782 (Fig. 16 A) Cenozoic sediments 9 AU63038 + Searle rock 5149 (Fig. 24) Table 1: Rock sub-group marked with * are lava-dominated samples with many small xenoliths. 148+ indicates a minimum number of available samples, due to uncertainties where the boundary between schistose versus non- schistose is to be located. The Searle specimens are listed as ‘rock specimen number/thin section number’. 13

DESCRIPTION OF XENOLITHS

In the following we will give brief descriptions of the main groups of xenoliths found (also see Table 1), with a number of typical examples selected for more detailed treatment. The more recently collected rock samples will be identified by the presently used system of Auckland University numbers (AU54989 - AU55008, AU58774 - AU58784 and AU62885 - AU63071). However, because the system was different at the time of the collections by Prof. Searle, we will add the prefix “Searle” to the blank numbers 4974 – 5017 and 5103 – 5158 listed in Appendix I. Note that the numbers of the Searle thin sections (5179 - 5278) are different from those of their rock samples. Where quoted in the text without comment the rock number will precede the thin section number, e.g. Searle 5114/5233.

NON- OR LITTLE DEFORMED METABASITES

These rock types have not been described before and range from pyroxenites (e.g. Searle thin section 5208, Searle 5114/5233 (Fig. 7)) to gabbro-diorite (e.g. Searle 4974/5179, Searle 4977/5182, Searle thin section 5186, Searle 4981/5187, Searle 4982/5188, Searle 5007/5215, Searle thin section 5220, Searle 5116/5235, Searle 5154/5274, AU62889 (Fig. 8), AU62910, AU63059), but also include porphyritic volcanics (e.g. AU58774 (Fig. 9), AU58778)

Pyroxenite (Searle thin section 5233): This rock has a coarsely crystalline fabric, dominantly consisting of clinopyroxene crystals up to ~3 mm in size (Figs. 7 A and B). The crystal cleavage patterns of the pyroxenes give a rough indication of their orientation (Figs. 7 B, C, D) suggesting the presence of a fabric. Further studies are necessary to determine whether or not this fabric is due to the mantle flow recognized by Christensen (1984) in the ultramafics on Dun Mountain. Metamorphic actinolite occupies the interstices between the crystals (Fig, 7 D) but also partially replaces some clinopyroxenes. Talc occurs in veins and irregular masses. The whole rock is intensively disrupted by a rhomboidal network of cataclasites (Figs. 7 A and B), which at the smallest scale has exploited the cleavage planes of the clinopyroxenes (Figs. 7 C and D). The cataclasites occur in two forms: 1) rich in opaque mineral and 2) without opaques. Some of the clinopyroxene crystals have subsequently been kinked (Fig. 7 B), probably during the cataclastic faulting. In the hand specimen (Searle 5114), the cataclasites show up as reddish rusty seams.

Fig. 7: Pyroxenite: Thin section Searle 5233 from the main sampling area (Fig. 3): A) Scan of the thin section in plane polarized light. Note the intricate network of dark cataclastic seams. B) Tracing of the scan in (A). Clinopyroxene crystals have been colour-coded by their orientation. C) Micro-photograph (plane polarized light) showing transverse section of clinopyroxene crystal (py) disturbed by cataclastic seams. D) Micro-photograph (plane polarized light) with clinopyroxene crystals (py), interstitial metamorphic amphibole and cataclastic microfaults (f).

GABBROS

Gabbros are some of the coarsest rocks in the collection, with visible crystals up to several mm in diameter. Although the minerals can define some layering (e.g. AU62910) there is little development of tectonic foliations. All samples show strong alteration, with the clinopyroxenes and amphiboles minerals replaced by often acicular actinolite. AU63059 has a well-developed fault striations, but without development of fibrous minerals (Fig. 8 A). Cataclastic faulting is also visible in thin sections (Figs. 8 B, C). The cataclasites are post-dated by talc veins.

Fig. 8: Gabbro: A) Rock sample displaying a rusty fault surface with striations. B) And C) black and white scans of thin sections in transmitted light. Cc = cataclasite, Tlcv = talc veins. B) Xtl = outline of crystals, mostly mafic, and replaced by fibrous amphibole. There are some feldspar clusters (not shown). The talc vein marked replaces a cataclasite fault. C) A = amphibole, F = Feldspar. The talc vein marked replaces a coarse-grained granular cataclasite. Note the faint sub-horizontal layering.

PORPHYRITIC VOLCANIC ROCKS

These may be derived from more than one source. Some are non-schistose but strongly altered and faulted (Fig. 9), others show incipient development of a foliation.

Fig. 9: Sample AU58774 from the main sampling area. Mafic phenocrysts (m) some rounded, some with intact crystal outlines, are totally altered to talc in a groundmass of feldspar laths and mafic crystals. Cc1: Early cataclasite, postdating epidote veins. Cc2: Cataclastic faults, associated with brecciation and talc veining

FOLIATED METABASIC ROCKS Most of these xenoliths are derived from mafic rocks and many show relict igneous textures. There appears to be a complete gradation between finely foliated, almost phyllonitic schist (AU62982 (Fig. 10 B), Searle 5246, Searle 5248) and coarsely foliated types (Searle 5216, AU62885, part of AU62900 and of AU58783). One sample (AU58784) contains layers of garnet. Crenulation or stretching lineations are visible only on a few samples (Fig. 10 C). Rock colour ranges from greenish grey to black. Multiple sets of veins are common, with early veins often folded (Fig. 10 A) and the younger veins interacting with cataclastic faulting. For those samples that have a fine foliation macroscopically visible in 3D on their surfaces, we use the collective term “mafic schist”, whereas the term “metabasite” is used for more massive and coarsely layered rocks (see appendices).

Fig. 10: Macroscopic examples of schists. A) Relatively massive foliated rock with epidote veins. The thickest vein has a sinistral fold. B) Well foliated schist. Note curvature of schistosity due to late post-metamorphic deformation. C) Platy black schist. The schistosity surface shows a faint lineation. Shearing of the foliation can be seen on the cut face.

Fig. 11: Macroscopic examples of veining and faulting in metabasites: A) faulting of epidote veins. CF= conjugate faults extending a schistosity-perpendicular vein. See Figure 10 A for a view of the schistosity surface marked. B) Three phases of faulting and veining (I-III) in coarse metabasite. C) Interaction of veining, folding and faulting in a schistose metabasite. 1: isoclinally folded quartz or albite vein contributing to formation of metamorphic layering. 2: foliation-parallel prehnite/calcite vein, deformed by open (kink?) fold (3), hidden in this view. 4: straight quartz/carbonate/ veins, displaced by movement (5) along schistosity-parallel vein (2). 6: latest vein phase.

Coarser grained metabasite (amphibolite) with veins and faults (Sample No. AU58777): The rock sample is macroscopically massive, fine- grained and dark grey to black with a greenish tinge. A few discontinuous segments of irregular thin veins are visible. The thin section (Fig. 12) shows the following main sequence: 1) formation of the igneous/metamorphic rock (schist); 2) formation of veins; 3) cataclastic faulting. 1) Schist: Aside from a few clinopyroxenes (Fig. 12 A), no original features are preserved. Feldspar-rich layers are interspersed with amphibole-rich layers (Figs. 12 A and B), forming a foliation containing isoclinal folds (e.g. Fig. 12 A, below location (B)) which have to be designated as F2. Therefore the foliation was formed in two phases, S1 and S2. Titanite crystals outline some S1 surfaces (e.g. in the F2 fold at loc. (1), Fig. 12 A). F3 folds are more open (Fig. 12 C), and have no schistosity developed along their axial planes (Fig. 12 C). Many are asymmetric as minor folds on larger folds (Fig. 12 A, location (2)). The S1 – F2 – F3 relationship is also well presented at Loc. (1) in Figure 12 A. There are some epidote-rich layers (Fig. 12 A, upper left), patches of coarser amphibole crystals and some pumpellyite. Amphiboles can be present as porphyroclasts and some of these have a sigmoidal shape. Their apparent movement sense (e.g. Fig. 12 B) may indicate a shear zone process, but could also be due to non- rotational transposition of S1 by S2 (e.g. Passchier & Trouw, 1998 p. 88, Johnson 1999 p. 1712). 2) Veins: Seemingly sub-parallel prehnite veins up to 1 mm thick, almost perpendicular to S1/S2 (Figs. 12 A and D) are largely fault repetitions of one vein (e.g. Fig. 12 D). Some of these have younger centres of talc (Loc. (3), Fig. 12 A). Separate talc veins are also present (between locs. (B) and (D), Fig. 12 A). Some of the prehnite crystals display kinked cleavages. 3) Cataclastic faults: There are at least three phases of dark-seamed cataclasite: Phase 3a), sub-horizontal faults (e.g. just above loc. (3), Fig. 12 A), showing conflicting separations of top to the right and top to the left (loc. (3), Fig. 12 A), either because of reactivation during a later fault phase (3 c) or because of movement of differently oriented vein segments at a high angle to the thin section. Phase 3 b, thick seams of cataclasite dipping steeply to the right, one with down- to the right separation (Fig. 12 D) that have grown by aggregation of thinner seams. Phase 3 c, thin faults with top-to-the left separation and thin vertical faults with down-to-the right separation that have acted as conjugate faults (e.g. loc. (4), Fig. 12 A) implying shortening sub- perpendicular to the metamorphic foliation.

Fig. 12: A) Tracing of thin section scan. Ringed letters refer to micro-photos B-D (plane polarized light) in this figure and features mentioned in the text. Metamorphic structures are indexed in red. B) S1 sigmoidal amphibole porphyroclast (AP) and example of foliation (fsp = feldspar). Arrows indicate apparent shear sense on S2. Note that the microphotograph is rotated 45° anticlockwise compared to (A). C) Open F2 fold affecting S1/S2 foliation and truncated by a cataclastic fault zone. amph = amphibole-rich portion of rock. D) Cataclastic faulting: a prehnite vein (magenta lines) is repeated by a thin phase 3 b fault (blue lines) which is itself cut by a phase 3 c fault.

Finer- grained amphibolitic schist (AU58775): The rock specimen has a dark, fine- grained ground mass with a schistosity (Fig 13 A), a crenulation lineation (Fig. 13 B) and is cut by prominent yellow- green epidote veins. All of these are affected by later kink folds and faults. The thin section (Figs. 13 C -F) reveals the following main sequence: 1) formation of the igneous/metamorphic rock (schist); 2) formation of epidote veins; 3) kink folding; 4) cataclastic faulting; 5) further veining. 1) Schist: The main part of the section is made up of a green actinolitic schist with thin foliation laminae (Figs. 13 C, D) and some feldspar-rich markers here designated as S1. Isoclinal folds of S1 (Fig. 13 D) must be F2, with axial surfaces S2 mostly parallel to S1. S2 also occurs as a fanning pattern of actinolite crystals in the isoclinal folds (Fig. 13 D), indicating that D2 is an important metamorphic event. If this fold is due to the same deformation as the lineation in Figure 13 B, the thin section must cut the fold at a very low angle to its axis, i.e. the actual cross- section is much less elongated. But the straightness of the lineation may also indicate a certain amount of stretching of the crenulation axes. 2) Epidote veins: these veins can be parallel or at high angle to the foliation (Figs. 13 A, B and C). A thick foliation-parallel vein has a reaction rim against the enclosing schist along its edge (Fig. 13 C) and lacks any S1/S2 texture. 3) Kink folding (F3): These folds have axial planes at relatively high angles to S1/S2 and without any development of schistose fabric. Opposing fold vergences in the thin section (Fig. 13 C, indexed folds) mark a conjugate couple of F3 folds, indicating shortening along the foliation. 4) Cataclasites: An intricate network of very thin cataclasite faults, semi-opaque in the thin section, is in part sub-parallel to the foliation but also develops around the F3 axial planes (Figs. 13 C-F). It is particularly well developed in the thick, foliation-parallel epidote vein which appears to have reacted to deformation in a more brittle manner than the amphibole-rich part of the rock (Figs. 13 A and F). The cataclastic faulting has exploited the geometry of the right hand F3 kink fold, but by-passes its complexities in the centre of the thin section. Discontinuous chlorite veins (Fig. 13 E) either predate or (most likely) formed during formation of the cataclasites. 5) Albite veins: At a/c in Fig. 13 C, one of these veins cross-cuts the cataclasites. The late albite vein shown in Figure 13 E follows the prominent down-to-the right cataclasite fault. It and two other veins are side-stepping or offset along the foliation and are also slightly ptygmatic, possibly indicating a later shortening across of the S1/S2 foliation.

Fig. 13: A) and B): photographs of the specimen: ep = epidote; x marks the same spot near a faulted epidote vein; FR = possible fault repetition of the thick epidote vein. The yellow rhomb in (B) = position of the thin section traced in (C). In (A), the sample rests on the thin section face; in (B), it sits on the face shown in (A). C) Tracing of thin section, location shown in (B). Letters in rectangles locate the three micro-photos. An albite vein cross-cutting cataclasite is shown at a/c. Micro-photos (D) - (F): fsp = feldspar, chl = chlorite vein, alb = albite vein, CC = cataclasite. D) Example of an F2 fold. E) Vein interaction with a cataclasite fault zone. F) Cataclastic disruption of the bedding- parallel epidote vein. cz = contact zone of the vein. 20

Interlayered thinly foliated and coarser amphibolite (AU62900): The rock specimen (Figs 14 A and B) is one of the few examples where a layering of contrasting lithologies is macroscopically visible. The layering is discontinuous, either due to isoclinal F2 folding and/or disruption in a broken formation (in the sense of Raymond (1984)). The thin section (Fig. 14 C) details the tectonic juxtaposition of the two lithologies and indicates the following sequence of events: 1) formation of the amphibolite; 2) first phase of cataclastic faulting; 3) second phase of cataclastic faulting; 4) chlorite veining. 1) Amphibolite: The contact shown in Figure. 14 C is between the units as coloured blue and white in Figure 14 B; the coarsest unit (green) has not been sectioned. In the coarser grained part (blue), the foliation consists of alternating amphibole and feldspar laminae averaging about 100 µm in thickness (Fig. 14 D). In the finer grained part (white), amphiboles are dominant in the foliation, which attains thicknesses of about 50 µm, but there are also some epidote-rich layers (Fig. 14 C). Both parts also contain some thicker feldspar layers up to a few mm in thickness (Fig. 14 D), that outline the structural development: The main foliation is S1/S2 (Fig 14 D). In many cases (S3) is also parallel to it, but has no mineral expression. 2) First phase of cataclastic faulting (CC1): Dominant cataclasite development occurred sub- parallel to the foliation (Fig. 14 C) and was more intensive in the more finely foliated part where it has reactivated S1/S2/S3, as for instance at location 1 (Figure 14 C), where two adjacent schist packages bounded by cataclasite show opposite shear senses from sigmoidal foliation patterns, representing two sheared-out limbs of an F3 fold. The thickest cataclasite seam is along the contact between the two lithologies and part of it has a matrix-supported breccia texture (Fig. 14 E). 3) Second phase of cataclastic faulting (CC2): These faults have much thinner deformation zones and sometimes are devoid of cataclasite (Fig. 14 E). They are at high angle to the foliation and often form conjugate couples (Fig. 14 C) indicating shortening across the foliation. 4) Chlorite veining: These veins either occur as distinct veins opening on pre-existing faults mostly at high angle to the foliation but there are also irregular replacement patterns: Figure 14 F and the zone extending from it in Figure 14 C.

Fig. 14: Metabasite folds and surfaces are indexed in red, by their deformation phase. A) View of the rock specimen. Note the contrast between dark green-grey and light grey layering. Colour rectangles match colour pattern in (B). Brown patches are irregular weathering seams. B) Tracings of orthogonal faces cut in the specimen shown in (A) where the specimen is resting on the left hand cut face of (B) and x marks a spot on the other face. BF = broken formation. C) Tracing of thin section. Letters in circles show locations of micro-photos D-F. Darkest green background shows zone of thinnest foliation. (1) marks the sheared-out fold discussed in the text. Micro-photos D-F (plane polarized): Fsp = feldspar layer, Mf = amphibole rich schist, CC = cataclasite, Chl = chlorite. D) Folded feldspar layer showing S1/S2/S3 relationship and offset by a cataclastic fault. E) Thick fragmental cataclasite cut by thinner fault, both replaced by chlorite veins. F) Partial and irregular replacement of cataclasite by chlorite.

Garnet-bearing metabasite with polygonised feldspar (Sample AU58784): The following sequence of events can be recognized: (D2-D4) formation of the igneous/metamorphic rock; (D5) feldspar polygonization; (D6) formation of talc veins; (D7) cataclastic faulting; (D8) further veining. The rock specimen (Figs. 15 A - C) shows metamorphic layering (S1/S2) outlined by clinopyroxene- and amphibole- rich layers. There are two garnetiferous layers, one captured by the thin section (Figs. 15 B and D) and a thicker, discontinuous layer (Figs. 15 A, B and C). The S1/S2 layering describes two F3 folds with rounded hinges (Fig. 15 B), arbitrarily labelled “antiform” and “synform”, without formation of an axial plane foliation. Some terminations of the thicker garnet layer (Fig. 15 C) represent a mushroom interference pattern (Twiss & Moore 1992 p. 257: Type 2): refolding of an isoclinal F2 fold by an F3 fold. The highest “antiform” has been displaced by a top-to-the left ductile shear zone. The associated drag fold would be F4 (Fig. 15 C). Irregular patches of yellowish white material (Figs. 15 A, B, D) consist of polygonised feldspar (Fig. 15 F). Since they cross-cut all the metamorphic features, they must belong to a D5 event not associated with any folding or foliation development. The main cataclastic fault recognized in the rock specimen (Figs. 15 A-D) is associated with a pervasive network of minor faults (Fig. 15 D). Drag folds associated with the main fault are F7. Reconstructed offsets of the garnet layers indicate an up-to-the right separation on this fault. This implies shortening ~ parallel to the axial surfaces of the F3 folds. Since the original orientation of the specimen is not known, the nature of the fault cannot be determined. If the axial planes of the F3 folds were originally sub-horizontal, as in Fig. 15 B and C, the fault was a thrust. If they were steeply dipping, it was a normal fault. The thin section (Fig. 15 D) reveals feldspar-rich layers, in addition to the mafic layers mentioned above. In a few places, S1 oblique to the main layering (S1/S2) is preserved. No fold corresponding to S1 has yet been seen. D4 top-to-the left shearing is visible above the garnet- bearing layer and also at location (1) in Figure 15 D, where some shears have been reactivated by D7 top-to-the right cataclastic faulting. A complex vein/fault sequence is shown in Fig. 15 E and consists of up to three phases of D6 talc veins both parallel and oblique to the metamorphic layering (and possibly some prehnite veins), followed by D7 cataclastic faulting which in turn is post-dated by a D8 set of clear veins ((quartz or feldspar or both? ) . Figure 15 F shows a typical andradite garnet and the pervasive feldspar polygonization.

Fig. 15: A) and B): rock specimen. C) Structural interpretation. D) Tracing of thin section scan. Location (1) shows reactivation of D4 top-to-the left ductile shear by D7 top-to-the right cataclastic faulting. E) Detail of cross-cutting veins and faults. D8 = quartz (+albite?) vein. F) Microphoto, plane polarized light: G = andradite garnet. Note contrast between dusty (saussuritized?) metamorphic feldspars (Fsp) and clear polygonised (metasomatic?) feldspars (pFsp).

OPAQUE–RICH COARSE METABASITES: Specimens of this type are blackish and consist of alternating light and dark coloured layers. In AU58782 (Fig. 16), the light coloured layers mainly consist of feldspar crystals up to 0.1 mm in size and a few euhedral crystals of clinopyroxene. The material in the dark layers is very fine grained and opaque even in thin section. A slight brownish tinge appears where the thin section is thinner than normal. There are no relics of other minerals visible. A sinistral fold without any associated axial plane foliation is visible in Figure 16 B. An important question is whether the dark layers are primary or represent replacement of a pre-existing material such as fine grained amphibole. The layering is cut by a swarm of cataclasite faults. No veins were detected in this specimen.

Fig. 16: Opaque-rich metabasite AU58782: A) Lava-coated xenolith showing a saw-cut face. B) Black and white scan in transmitted light of thin section from the xenolith. Note sinistral fold at S.

Fig. 17: Altered cataclasite (AU54990): A) View of the specimen, with dark cores of pyroxenite in dense disrupted matrix. B) Thin section with crossed polarisers. Clinopyroxene (Py) replaced by amphibole (Am) and surrounded by granular augite replacing a cataclastic fabric. Note extensional fracturing in clinopyroxene crystal below G.

Schist with grid-shaped alteration zones (rock Searle 4984, thin section 5190): This fine grained, dark metabasite dominantly consists of amphiboles arranged in a pronounced planar fabric with a few interspersed feldspar-rich layers. There are rare feldspar porphyroclasts. Rootless isoclinal folds indicate that the fabric is at least an S2 foliation. It is further deformed by a system of ~ 5 mm wide (F3) kink zones perpendicular to the foliation. Flexural slip on these has allowed opening of saddle reef- like cavities (Davies 1984 p. 359, Twiss & Moores 1992 p. 251) in which feldspar has been precipitated (Fig. 18 D). Together with foliation- parallel feldspar layers these form a sub-orthogonal alteration network visible on the polished surface of the rock specimen (Fig 18 A). Veins of prehnite and chlorite are also present. One set is ptygmatically folded on the S2 foliation, the other cuts across the F3 kink folds, although the veins change direction in response to pre-existing structural discontinuities.

Fig. 18: (rock Searle 4984, Thin section Searle 5190) A) view of the face on the rock sample cut perpendicular to schistosity, with the grid pattern emphasized by polishing of albite. Paper clip is 32 mm long. B and C) examples of kink folds (outlined) with saddle reef development in the thin section, plane polarized light. The white mineral is albite. Note the veins cross cutting the structures. D) Sketch to explain migration of solution from thinned limbs of folds to the thickened hinges for forming saddle reefs (grey).

Whitish silica-rich rock with shear planes (Searle 4992/5197, Searle 5111/ 5230, and Searle 5134/5253): All three rock samples are remarkably similar. Samples Searle 4992 and 5134 have symmetrical patterns of fabric in their cut faces indicating that they are derived from one single rock. Sample Searle 5111 is also similar, but definitely is not a cut off the other two. It may have been broken off from the same rock during collection in the field. In sample 4992 (Fig. 19 A) a lighter coloured material on the left is in sharp contact with a darker lithology containing even darker spots and with a reaction rim along the contact. The lighter coloured material is traversed by a system of seams made up of bundled strands. The thickest of these are parallel to the contact; others form a complex network with many at a high angle to the contact. The corresponding thin section (Searle 5134), sampling the lighter coloured lithology (Fig. 19 B) shows that the seams consist of quartz aggregates and in part follow fold- like structures (also see Fig. 19 C) which are mostly dextral. The seams also produce patterns reminiscent of metamorphic transposition of an earlier fabric by a later one (Fig 19 B at T).Various shear planes displace the seams. Late quartz veins (V) are segmented and in one case appear to form an en echelon pattern along a more competent layer. The main mass of the rock consists of an aggregate of quartz and fibrous material that appears to be foliated, often parallel to the axial planes of the folds. XRD analysis shows that the latter is talc. The eye- shaped spots in the darker lithology (Fig. 19 F) are augen of fibrous chlorite surrounded by a reaction rim containing a different chlorite. Thin sections Searle 5230 and Searle 5253 display spectacularly zoned quartz veins (not shown in the figures). All three thin sections contain minute disperse semi-opaque spots, that on closer examination turn out to be garnets, identified by XRD as hydrogrossular. In some cases these are concentrated in layers and seams (e.g., Fig 19 B at OP). In Figures 19 D and E such a layer is cross-cut by a clear quartz vein. These rocks are similar to the metasomatic rodingites which occur in the serpentinites of the Patuki melange of the northern South Island (Coleman, 1966), as illustrated in Fig. 19 G, and in the northernmost exposure of the Dun Mountain Belt in the North Island (O’Brien & Rodgers 1973 b). In our sample, the rock replaced by metasomatic processes appears to be a schistose metamorphic tectonite.

Fig. 19: A) Saw-cut face of rock Searle 4992. Scale units are 1 cm. Note network of seams and, on right hand portion of the specimen, contact between lighter and darker rock types, and dark spots. B) Sketch of corresponding thin section (Searle 5197) in the portion with seams. Q+P: main rock mass of quartz + talc, OP: Semi-opaque seams consisting of hydrogrossular garnet, Q: quartz-rich zone with relic of a crack seal vein (CK), T: transposition fabric, V: clear quartz veins. C) Fold in the same thin section, crossed polarisers. Note schistosity-like fabrics, indicated by red lines. D) Seam of garnets (black) crosscut by quartz veins. Thin section Searle 5253, crossed polarisers, enlargement in E outlined by box. Note talc/quartz material with schistosity-like fabric (red lines), paralleled by quartz veins, in the upper left hand corner. The rest of the section is more quartz-rich. E) Enlarged view of part of the seam in (D), plane polarized light, showing high relief garnet crystals. F): ’eye’ of chlorite in thin section Searle 5253, crossed polarisers. Dark spots are hydrogrossular garnets. G) Yellowish lens of rodingite in sheared serpentinites of the Patuki mélange, KBS photo, Whangaroa Quarry, NW Nelson (e.g. Mortimer 2011). 28

Very cataclastic metabasite (rock Searle 5000, thin section Searle 5206): This sample (Fig. 20) illustrates cataclastic faulting prior to quartz veining and prior or simultaneous with talc veining. It, and the next three specimens shown in Figure 21, illustrates that cataclastic fault zones in the source area of the volcanoes were wide enough to provide decimetre-size fragments.

Fig. 20: Rock Searle 5000: P 1 and P 2 identify the same faces in Figures A and B. A) Saw-cut face showing cataclastic texture. Dark layer near bottom is the least altered metabasite sheared by faults (example identified by arrow). Irregular white patches are talc seams. Steep dark line in the middle of the face is a quartz vein. B) Same specimen, showing face P 2 with striations parallel to yellow line.

SEMI-OPAQUE RICH ROCKS This type of rock is quite common in the collection. Internal structures always show strong disruption, mostly in a style suggesting cataclastic fault zones. The dark material mainly consists of small augite granules which appear to have metasomatically replaced the cataclastic matrix.

Fig. 21: A) Grey variety (Specimen AU62977) Note the layers of relict disrupted fragments of altered metabasite floating in the dark matrix. B) Thin section of A with feldspar- rich, altered metabasic fragments and albite/quartz veins. Dark groundmass consists of very fine augite grains. C) Reddish variety (saw-cut face on specimen Searle 5002) with sigmoidally deformed fragment in a down-to-the left shear zone.

SEDIMENTARY ROCKS

METAMORPHIC SEDIMENTS Sandstone from Glovers Park volcano (AU63067): This lithic sandstone rich in volcanic fragments and somewhat similar to the sandstone from Taylors Hill described below, is probably also part of a basement sequence.

Sandstone from Taylors Hill (Rock: Searle 5138, thin section: Searle 5257): This is a very lithic sandstone with the clastic assemblage dominated by volcanic rock fragments. Other components are feldspar, clinopyroxene, titanite, and possibly quartz. No bedding or cleavage is visible in the section, but the following fault/vein sequence can be recognized: 1. relatively thick granular cataclastic faults, 2. bundles of thick prehnite/calcite veins, 3. faults with thin deformation zones, 4. quartz veins with crack-seal textures, 5. chlorite veins.

Siltstone from Taylors Hill (Rock: Searle 5144. Thin section: Searle 5263): A very fine-grained lava-coated clastic rock with thin faint laminations and a few lenticels outlined by opaque minerals (current bedding?). The matrix is highly recrystallised. There is a very thin recrystallised contact zone along the contact with the lava.

Red chert (AU63053, Fig. 22): A small reddish brown inclusion in tuff from Glovers Park Volcano is a non-calcareous and siliceous mudstone, i.e. a chert. Close inspection with the hand lens reveals portions of the rock with sub-mm round, semi- translucent spots which may represent recrystallised Radiolarians (“rads?” in Fig. 22). There is one small quartz vein (qv) and many very thin black seams which may be manganiferous.

Fig. 22: Red chert (AU63053)

Calcareous sandstone with shell fragments (Rock Searle 5155 / thin section Searle 5275 a): This specimen of clastic sediment from Glovers Park volcano is special in that it contains numerous angular calcite platelets (Fig. 23 A). These are generally the same size as the other clastic grains. Calcite crystals in the platelets form a prismatic fabric at right angle to the long edges of the fragments, indicating that these are prismatic layers of bivalves. The platelets make up 20 to 50% of the sandstone, and although very angular, appear to be graded together with the other grains, which are mainly feldspar and clinopyroxene. The thin section tracing (Fig. 23 B) shows alternating sandstone and mudstone layers that are disrupted by closely spaced cataclastic faults. There are two types of sandstone: a fine grained variety that contains the shell fragments and a coarser variety without shell fragments. The faults all extend the bedding and may be linked to one bedding-parallel, top-to-the right shear (at (a) in Fig. 23 B). Folding is indicated at (b) and (c). The clastic fabric shows no development of cleavage, except for some slight suturing of grains by pressure solution seams. Prehnite/calcite veins, mostly at high angles to the bedding, predate the cataclastic faults, as is shown by truncation on a braided fault pair at (d), drag on faults at (e) and fracturing of the crystals in the vein fill. Calcite / chlorite veins mostly at low angles to the bedding postdate the network of cataclastic faults. The slabbed rock specimen (Fig. 23 C) shows the same lithologies and structural relationships. A mudstone body at the top right is faulted against the other rocks. Irregular, somewhat diffuse, shear zones that often exploit the mudstone horizons and show some folding at (a) and (b), are only well visible at this scale.

Fig. 23: A) Portion of thin section in plane polarised light, showing shell fragments, clinopyroxene heavy minerals and calcitic veins. B) Tracing of a thin section scan. Note trends of bedding and strong disruption indicated by distribution of lithologies. C) Tracing of slabbed face of specimen oriented approximately similar to (B). Note diffuse shears predating narrow cataclasites. The irregular distribution of lithologies suggests a broken formation fabric.

NON-METAMORPHIC SANDSTONES These are mostly homogeneous; light grey to reddish sandstones with no or little faulting and similar to sandstones in the Waitemata Group. The specimens in Figure 24 are both rounded, possibly due to abrasion during volcanic excavation or, less likely, to pre- eruption erosional reworking of the sediments. The rock in Figure 24 B is stained red, indicating a small amount of baking by the lava, as is also suggested by the thin alteration rim at the contact with the lava.

Fig. 24: Lava-coated examples of non-metamorphic sandstones, probably from the Waitemata Group. L = Lava, S = sandstone, C = contact alteration rim. Note the rounding of these fragments and red colour of Searle 5149.

METAMORPHIC SANDSTONE XENOLITHS FROM OTHER VOLCANOES: These samples are also part of the Searle collection described in this report and are included here because of their significance. A lava-coated specimen from Mount Wellington (Rock: Searle 5137. Thin section: Searle 5256) has no veining, but displays well preserved mm- to cm- thick laminar beds of sandstone and mudstone. One or two beds are graded and some have subtle sole marks. The thickest mudstone horizon has lenticular cross-bedding of coarser material. There are some clastic intrusions. Small faults contracting the bedding may have formed relatively shortly after sedimentation. The clastic suite is dominated by very angular feldspar and opaques. There is much matrix material that appears to have been recrystallised. One specimen from Mangere Mountain (Rock: Searle 5152; thin section: Searle 5271, mislabelled 5272 in the original list) is lava-coated, contains interstitial calcite and has cm– size angular fragments of mudstone floating in a sandstone matrix, i.e. it fits the definition of a chipwacke (e.g. Schofield 1974).

SUMMARY AND DISCUSSION

TYPES OF XENOLITHS IN THE AUCKLAND VOLCANIC FIELD

Apart from cognate clusters of olivines and pyroxenes probably precipitated from the magma during its ascent, there are three types of xenoliths produced by the vents in the Auckland Volcanic Field: 1) mantle derived ultramafic xenoliths at Lake Pupuke (Brothers & Rodgers 1969, Rodgers & Brothers 1969, Rodgers et al. 1975) indicating conditions at 80 km+ depth; 2) the quartzose inclusions described by Searle (1962), which, because of their partial resorption, must originate from some deeper levels of the crust and 3) the xenoliths described in the present report and by Bryner (1991) from Motukorea /Browns Island, which must have originated at shallower depths (see ‘Eruption Model’ below).

THE ROCK ASSEMBLAGE EJECTED BY THE ST. HELIERS VOLCANOES

COMPONENTS Glovers Park and Taylors Hill volcanoes have ejected a heterogeneous suite of xenoliths dominated by mostly foliated (schistose) metabasic rocks. Additional lithologies are low grade metamorphic sedimentary rocks, fragments of cataclasites, less deformed volcanic rocks and non-metamorphic sedimentary fragments. The metabasic rocks range from peridotites, gabbros and diorites to amphibolitic schists and none contain primary quartz. The non- (or less) foliated metabasites include coarse ultramafics (Fig. 7), gabbros (Fig. 8) and various finer-grained volcanic lithologies (Fig. 9). All of them display extensive recrystallisation by amphiboles, are veined and cut by cataclastic faults. Some have incipient schistosity, with phenocrysts changing into porphyroclasts. Metasediments are mostly greywacke-type terrigenous sandstones, but there is also one sample of red chert (Fig. 22) similar to the red ocean floor cherts common in the Waipapa terrane (Fig. 1 C and Spörli et al. 1989) but also sporadically present in the Caples and Maitai terranes. All the metamorphic xenoliths are cut by various phases of veins and are characterized by networks of cataclastic microfaults. Most veins predate or were formed during this faulting, but post-faulting veins occur in some samples, e.g. AU62889 and 62910 (Figs. 8 B and C, talc), AU5775 (Fig 13 C, albite), AU62900 (Fig. 14 C, chlorite), AU58784 (Fig. 15 E, quartz (+albite?)), Searle 5000 (Fig. 20, talc), Searle 5515 (Fig. 23, calcite/chlorite). The non-metamorphic sedimentary rocks can in many cases be correlated with the Waitemata Group sediments underlying the volcanoes, but there may also be fragments from the older tertiary sequences in the region. Their definite identification would contribute to knowledge about the stratigraphic section traversed by the ascending magma

PRESENT STATE OF THE EJECTA The xenoliths range from a few mm up to 70 cm in diameter, however the most common size is in the 2 to 5 cm range. Most are angular but some are also rounded. Some fragments were coated by basaltic lava before they were ejected. Very weak lithologies were intruded by magma along checkerboard patterns probably delineated by joints. Of the non-metamorphic sandstones, only one (rock Searle 5149, Fig. 24 B) shows a hint of an alteration rim caused by the surrounding magma.

STRUCTURAL SYNTHESIS AND SEQUENCE OF EVENTS

HOW MANY ROCK SUITES? A fundamental question regarding this assemblage of xenoliths is whether they represent a contiguous outcrop or come from separate locations along the track of the magma ascending to the vents of each of the two volcanoes. Monogenetic volcanoes generally are fed by dikes or plugs (Kiyosugi et al. 2012). In both cases it is likely that the horizontal extent of the area, and therefore the geological units sampled by the ascending magma, will be limited in size. It could be possible that, in a horizontally layered lithosphere, the magma may nevertheless be vertically sampling a number of lithological units. However the tectonic model of the region under the Auckland Volcanic Field (Fig. 1 C) suggests that the tectonic units in the basement are steeply dipping, again restricting the geological variety sampled. The association of amphibolite grade, in part schistose, metabasites with non- metamorphic sedimentary rocks, many of which can be correlated with the Miocene Waitemata Group, indicates the presence of an unconformity between these lithologies. We therefore conclude that the xenoliths brought up by the St. Heliers volcanoes are from a contiguous body of rock dominated by metabasics, unconformably overlain by Cenozoic sedimentary rocks.

FOLIATED METABASITES The foliated metabasics display a consistent structural sequence of deformations. Foliation development can be subdivided into at least two phases (S1, S2), associated with isoclinal folding, development of ductile shears and porphyroclasts. This is followed first by open folding without generation of schistosity or cleavage (F3, Fig. 12 C) and then by kink folding (Fig. 13 C, F3) that in some cases appears to be associated with the next phase of deformation: pervasive cataclastic faulting. This faulting is complex and consists of multiple sub-phases (Fig. 14). Fault rocks include fine-grained, often anastomosing seams of various thickness (Figs. 7, 8 B and C, 12 A and D, 13 C, 14 C, 15 D) and breccia textured zones (Figs. 9, 13 D-F, 14 E (phase CC1)), In contrast to quartzofeldspathic schists, where the first-formed foliation usually arises from metamorphic/structural segregation differentiation of quartz and/or feldspar along pressure solution cleavage planes during initial folding (e.g. Hobbs et al. 1976, Craw 1998), the earliest foliation in the metabasites studied for this report generally shows no evidence of such solution transfer (Figs. 12 B; 14 D, and Searle 5108/5227). Because the original rocks were all igneous, these surfaces may either represent a primary flow foliation (S0) formed in a partially molten stage and/or an S1 foliation due to very high temperature deformation. Exceptions to the above are samples AU 54998, AU 62997 and AU63001 (Fig. 11 C), which shows some possible segregation textures. Formation of veins started late in foliation development and also postdates it. It had its peak after kink folding and before cataclastic faulting, but a few veins postdate the faults. After the albite veins which contribute to formation of the schistosity in sample AU 63001(Fig. 11 C), epidote veins, both parallel and oblique to the foliation are some of the earliest veins (Figs. 10 A, 11 A, 13 A, B). Prehnite veins then follow (Fig. 12 A) and may indicate a step down to lower grade metamorphism. Talc veins mostly predate (e.g. Figs. 7 B, 12 A, 15 E) but also may be syn- (Fig. 8 C) cataclastic faulting. Veins post-dating cataclastic faulting can be filled with quartz/calcite (Fig. 11 C), albite or quartz (Figs. 13 C and E, 15 E) or chlorite (Fig. 14 C).

DUCTILE, SEMI-MYLONITIC FAULTING IN THE FOLIATED METABASITES Shears with sigmoidal porphyroclasts and deflection of schistosity can be associated with the second phase foliation S2 (Fig. 12 B). They can also occur later in the deformation (Figs. 15 C, D), but all of them predate cataclastic faulting. Searle 5017/5225 is another example.

NON- OR LITTLE- FOLIATED METABASITES These rocks are clearly different from the juvenile lava fragments in the tuffs in that they have suffered significant alteration, veining, and some deformation. It is likely that all of them are closely linked to the more schistose metabasites.

METASOMATISM OF THE METABASITES The metabasites can show various intensities of metasomatic recrystallisation. The replacement of a calcite-bearing cataclasite by chlorite in sample AU62900 (Fig. 14 F) is an example. Andraditic garnet in sample AU58784 (Fig. 15) is probably due to an early phase of metasomatism predating F2 folding, whereas the polygonization of the feldspar postdates all the high grade deformation but not the cataclastic faulting. Replacement of the mafic minerals in the foliated metabasite AU58782 (Fig. 16) and of the cataclasites shown in Figure 21 by semi- opaque minerals is probably in part a manifestation of metasomatism as are the grid-patterned feldspar zones in sample Searle 4984 (Fig. 18) and the irregular replacement of much of specimen AU54990 (Fig. 17) by granular augite. In the rodingite of specimens Searle 4992, 5111 and 5134 (Fig. 19) a schistose metamorphic tectonite has been completely overprinted by metasomatic talc, quartz and hydrogrossular garnet.

CATACLASTIC FAULTING IN THE METABASITES All the metabasic rocks display networks of cataclastic faults at the scale of a thin section and in the coarser-grained rocks, cataclastic deformation can cause incipient brecciation of crystals (Fig. 7 C, D). This indicates that the source rock mass was pervasively faulted, possibly being located within a master fault zone. In some cases more than one phase of faulting can be detected (Figs. 12 A and D, 13 E, 14 E, 21 C). Styles of the micro-fault zones varies from thin single-strand seams to bundles of discrete seams (Figs. 12and 15) and to thicker breccia-textured faults (Figs. 8 C, 9, 13 F, 14 E, 21). Fault braiding is common (Figs. 12 A loc. 4, 15 D). Some faults have internal zones richer in semi-opaque minerals (Figs. 7 A, B). Cataclasite faults often exploit pre-existing schistosity surfaces (Figs. 13 C and E, 14 C) and can be associated with formation of scratch striations (Fig. 20). The cataclasites can exceed 5cm in thickness (Fig. 21) and some of these fault rocks may predate metasomatism (Figs. 14 F, 18 B, 19 B, 21).

VEINING IN THE METABASITES Epidote veins are amongst the earliest veins (Figs. 11 A and B, 13). Some feldspar veins were formed early enough to be affected by schistosity development (Fig.11 C). Prehnite veins, followed by talc veins, are younger but predate the cataclasites (Figs 12 A and D). Chlorite/ quartz/albite veins mostly postdate the cataclastic faulting. In the very schistose rocks, the late veins from intricate side-stepping patterns (Fig. 13 C, E). Calcite veins are quite rare and are always of a late phase.

METAMORPHISM OF THE METABASITES While some of the specimens, e.g. AU58777 (Fig. 12) and AU58775 (Fig. 13), only display obvious evidence of dynamic metamorphism, others, e.g. AU58784 (Fig.15), have been affected by an often complicated combination of dynamic and static recrystallisation. Metamorphism in the metabasics is complex and is mainly amphibolite facies, but can range down to prehnite/pumpellyite facies or lower. On the one hand clinopyroxenes are irregularly replaced by large amphiboles, on the other there is tectonically controlled actinolite occupying axial planes and fanning cleavages in F2 folds (Fig. 13 D). These rocks probably underwent metamorphism in an environment similar to that postulated for the Patuki Volcanics of the Northern South Island (Sivell & Waterhouse 1984 b), with a dominance of hydrothermal alteration under a steep (hundreds of degrees per km) geothermal gradient.

METASEDIMENTS Except for one specimen of red chert (Fig. 22), all other rocks are terrigenous clastics with a volcaniclastic composition. Generally, the sandstones are non-calcareous except the shell fragment-bearing specimen from Glovers Park volcano (Fig. 23) and interstitial calcite in thin section Searle 5271 from Mangere Mountain (location see Fig. 2). Graded bedding and current laminations are present in the xenolith from Mount Wellington (thin section Searle 5256). Large angular fragments of argillite occur in thin section Searle 5271 from Mangere Mountain. Some sandstones are cut by cataclastic fault zones and veins (thin sections Searle 5275 (Fig. 23) and Searle 5257), others are not (thin section Searle 5263). Metamorphism up to prehnite- pumpellyite grade is indicated by the veins and/or recrystallisation of the rock matrix. There is no evidence for formation of any cleavage. These sedimentary rocks do not seem to have experienced the extensive metasomatic processes seen in the metabasics.

CORRELATION WITH TERRANES AND STRATIGRAPHIC UNITS

THE METABASIC ROCKS The position of the two St. Heliers volcanoes in relation to the geophysical anomalies in Auckland (Fig. 1 B) and the assemblage of xenoliths (pyroxenites, amphibolitic meta-igneous rocks, rodingites) places the source somewhere in the ophiolitic part of the Maitai terrane (Dun Mountain Belt and adjacent melanges). At its type locality in Nelson, South Island, the Dun Mountain ophiolite consists of partially serpentinized harzburgite, dunite, pyroxene, peridotite with pyroxenite, diallage and rodingite veins, as well as some altered gabbro in the higher portions (Johnston 1981). It is weakly to strongly layered and contains minor amounts of wehrlite, plagioclase-bearing pyroxenite, and pods of chromitite. Because of its porphyroclastic, strongly lineated texture (Christensen 1984) it is interpreted as mostly consisting of mantle tectonites with minor cumulates. The igneous Lee River Group is in fault contact to the west (above) and includes tholeiitic gabbros, spilitic basalt, albite dolerite and sediments. Alteration of the igneous rocks in the Lee River Group includes augite replacement by amphibole, saussuritization of plagioclase and in the deeper portions, replacement of feldspar by hydrogrossular garnet. The ophiolite is bounded to the east by the Patuki and Croisilles mélanges which reach widths of up to 4.5 km and consist of metasomatised blocks of ultramafics, basic volcanics and sediments in a sheared matrix of mostly serpentinite but also including some sediments. Shear zones in the Dun Mountain ultramafics either consist of recrystallised foliated serpentinite or serpentinite cataclasite and in the Nelson area show consistent down-to-the west sense of shear, opposite to 38

those in the Patuki melange (Jugum & Norris, 2004). Metasomatised sandstone blocks with abundant fragments of Atomodesma bivalve shells occur in the Croisilles mélange (Dickins et al. 1986). Some parts of the Patuki melange have been interpreted as coherent sequences of basalts and sediments tectonically intruded by thin bands of ultramafic rocks (Sivell & Rankin 1982 fig. 1, Sivell & McCulloch 2000). The sediments include sandstone, argillite, red mudstone and siltstone (Sivell & Rankin, 1982) and the volcanics are metamorphosed in the prehnite- pumpellyite facies. The basalts in the Patuki melange have been interpreted as the equivalent of the (ocean ridge?) phase preceding the supra- subduction stage within the Dun Mountain ophiolite and the sediments correspond to, but are finer grained than, the Maitai Group sediments overlying the ophiolite (Jugum et al. 2007, Jugum et al. 2008) In the southern part of the South Island, the Dun Mountain ophiolite can be up to 4 km thick (Sinton 1980). Maitai Group with layers of Atomodesma fragments stratigraphically overlies the ophiolite on the west side but also underlies it as a tectonically disrupted unit on the east side (Cawood 1986, 1987). The irregular Upukerora breccia at the base of the sedimentary cover contains some red siltstone beds. The ophiolite units mostly dip steeply and are overturned to the south and west. Development of schistosity is only rarely mentioned in descriptions of the Dun Mountain ophiolite and the adjacent units. Schistose epidote amphibolites occur in the melanges of the northern South Island (Sivell & Waterhouse 1984 b), where they form bodies up to several hundred metres in diameter (Sivell & Waterhouse 1984 a, fig. 1). Their formation is attributed to submarine hydrothermal metamorphism and deformation at a spreading ridge in the transition between greenschist and amphibolite facies conditions. For the southern South Island, Cawood (1986) states that “the ultramafic and plutonic sequences are extensively serpentinized and tectonised.” Melanges have kernels of serpentinite in a schistose matrix. While some of Maitai Group sediments develop a fracture cleavage and the metamorphic grade ranges from zeolite to prehnite- pumpellyite, the igneous rocks of the Dun Mountain Ophiolite display pumpellyite- actinolite facies metamorphism which is thought to be due to hydrothermal fluids active before deposition of the Maitai Group sediments (Cawood 1987) and there is little evidence for metamorphic changes during the subsequent tectonic history. Unfortunately there is yet no published detailed structural description of any serpentinite shear zones in the Dun Mountain ultramafics or of any of the tectonised amphibolites in the adjacent melanges. The metabasic rocks we describe are undoubtedly part of the Dun Mountain Belt or associated melanges that cause the Junction Magnetic Anomaly passing under the Auckland Volcanic Field. According to the majority of interpretations of these tectonic units elsewhere in New Zealand (e.g. Sinton, 1980, Sivell & Mc Culloch 2000, Jugum & Norris 2004, Jugum et al. 2007, 2008) they must therefore be ocean floor rocks formed at or near a spreading ridge. Descriptions of rocks from or near presently active spreading ridges should therefore indicate the significance of such deformed and altered metabasites. While fast-spreading ridges are dominated by basic igneous rocks with plutonic textures (MacLeod &Manning 1996), slow-spreading ridges produce and preserve less primary igneous material and therefore can contain larger bodies of deformed metabasites, including schistose rocks (Schroeder & DePan 2006). The slowest spreading ridges are non-volcanic and often expose the sole of low-angle detachment faults tectonically denuding mantle peridotites and gabbro plutons (Schroeder & DePan 2006). On the slow-spreading Mid-Atlantic Ridge, gabbros initially developed high temperature magmatic foliations (probably similar to S1 of the metabasic xenoliths described in this report) and lineations which are post-dated by brittle deformation associated with hydrothermal alteration (Gaggero & Cortesogno 1997). Metamorphism and deformation can be extremely discontinuous and heterogeneous especially near the intersection with fracture zones. Berger-DeBrodt et al. (2006) describe granulite facies deformation localised on margins of gabbro intrusions and in gabbroic veins and continuing within these zones down- temperature into greenschist facies. Transform faults can also be sites for schistose rocks: from

the Vema Fracture Zone in the Atlantic, Honnorez et al. (1984) describe complete gradations from gabbroic rocks into mylonitic and gneissic amphibolites accompanied by serpentinite and rodingite. In the same fracture zone, Peive et al. (2001) noted a range of deformations from hot plastic to schistose (with considerable folding) and through the brittle-ductile transition, due to transfer of the site from a spreading ridge position into an active transform fault zone. Metasomatic alteration by invading fluids is mentioned for all stages of development of such ocean floor rocks (e.g. Mc Leod & Manning 1996, Cortsogno et al. 2004, Schroeder & DePan 2006), can range from magmatic conditions to zeolite facies (Blackman et al. 2005) and can occur under static conditions as well as during deformation (Dick et al. 2000). A conclusion that the basic rocks ejected by the St. Heliers volcanoes come from a piece of ocean floor generated at a slow spreading ridge would be in agreement with the interpretation of analogous rocks from the northern end of the South Island (e.g. Sivell & Mc Culloch 2000). The close spatial association of highly deformed schistose rocks with their practically undeformed protolith is also typical of ocean floor rocks formed at a slow spreading ridge (Gaggero & Cortesogno 1997, Schroeder & DePan 2006). The high grade alteration indicated by clinopyroxene and hydrogrossular assemblages also fits into this scenario. It is possible that such a ridge segment could be traversed by minor faults associated with a transform structure, increasing the likelihood of forming schistose rocks. Although there is much evidence for metasomatism, including formation of rodingite, amongst the xenoliths collected so far, there has been no serpentinite, which is often associated with such rocks. This however may be a preservational bias, as serpentinite may be much less likely to survive the violent processes of magma ascent and eruption.

THE METAMORPHIC SANDSTONES Assignment of small sedimentary basement xenoliths to a specific terrane will always be difficult because of the limited information such samples can provide. However, this is somewhat easier for the shell fragment-bearing calcareous sandstone (thin section Searle 5275 a, rock Searle 5155, Fig. 23) in our collection. Although there are Cretaceous shell fragment deposits derived from Inoceramus group fossils in allochthonous clastic rocks of Northland (Isaac et al. 1994 p. 53) and north-eastern North Island (e.g. Kenny 1984), these can be excluded with reasonable certainty for the following reasons: 1) mostly derived from gigantic shells, the fragments in the Cretaceous sediments are usually much larger (Fig. 25) than the 0.2-0.5 mm range in our specimen (Fig. 23 A). 2) There are no Cretaceous sedimentary rocks known in the area of the Auckland Volcanic Field. The nearest suitable units would be in the Northland Allochthon, which terminates 19 km to the north of Auckland (see ‘Geological Framework’ above). 3) Rich accumulations of much smaller Atomodesma shell fragment are typical for the Permian rocks of some basement terranes in the South Island that will be further considered below. 4) These terranes are concentrated near to or are part of the Maitai terrane that hosts the Junction Magnetic Anomaly (Fig. 1A). The location of our sample is also closely related to this anomaly (Fig. 1 B).

Fig. 25: Inoceramus shell fragment bearing sandstone from the north-eastern North Island (KBS collection) to illustrate the difference in size from the Atomodesma fragments in Fig. 23 A.

In the South Island, Atomodesma shell layers occur in the Permian Maitai Group overlying the Dun Mountain ophiolite (Landis 1980, Johnston 1981, Cawood 1986, 1987), i.e. to the west and south of the ophiolite (see Fig. 1A), in sedimentary slivers within melanges adjacent to the Dun Mountain ophiolite (Dickins et al. 1986) and in the Caples/Pelorus terrane (Bishop et al. 1976, Turnbull 1980, Dickins et al. 1986) to the east and north of the ophiolites. Cawood (1986) attributes an anomalous occurrence of large tracts of Maitai Group to the north of the Dun Mountain ophiolite in north-western Southland, South Island to sinistral shearing. The Tramway Formation (and equivalents) of the Maitai Group is the main unit that contains prominent horizons of Atomodesma shell fragments (e.g. Landis 1980, Cawood 1986), but they also occur in the other formations, e.g. Wooded Peak Formation (Landis 1980), near the base of the group. The clinopyroxene-dominated heavy mineral suite in our sample may indicate erosion off an ultramafic body and therefore would support a derivation from low in the Maitai Group overlying the Dun Mountain Ophiolite. The arguments put forward above and an association with xenoliths of schistose metabasites and rodingites make it most likely that the shell fragment-bearing sandstone ejected by Glover Park volcano is derived from a sliver in a melange on the east side of the Dun Mountain Ophiolite under the Auckland Volcanic Field. A much less likely explanation would be that it came from a belt of Caples/Pelorus terrane hidden under Auckland. The remaining sandstone xenoliths must so far be regarded as of undetermined provenance. All of them are volcaniclastic, but the terrigenous sandstones of the relevant terranes in the Auckland area (Murihiku, Caples/Pelorus and Waipapa) are also all volcaniclastic. The Permian Maitai Group of the South Island is also quartz-poor and volcanogenic (Landis 1980, Stratford et al. 2004) but is much more carbonate-rich. An assignment of all the metamorphic clastics to the Maitai Group is entirely possible but cannot be proven. While horizons of angular argillite fragments (chipwacke) are common in the Waipapa terrane sandstones, such sedimentary features can also occur in the other terranes. Therefore the argillite flakes seen in the sandstones ejected in the Auckland Volcanic Field are not necessarily a definite indicator of Waipapa terrane affinity.

RED CHERT Red siliceous mudstones and cherts are an important but minor component of the Waipapa terrane (Fig. 1 C and Spörli et al. 1989). However they also occur in parts of the Caples/Pelorus terrane in the South Island (Bishop et al. 1976), and in the melanges adjacent to the Dun Mountain ophiolite (Sivell & Rankin 1982). In the Maitai Group there are also rare red mudstones (Stratford et al. 2004). It is therefore difficult to decide definitely from which of these terranes our single sample could be derived. A source in a melange is most easily reconciled with the other evidence presented. Additional tectonic complexity would have to be contemplated, should the chert be part of the Waipapa terrane.

SOURCE OF THE METAMORPHIC METABASITES AND SEDIMENTS If the assignment of ‘chipwacke’ type greywacke xenoliths from Motukorea (Bryner 1991) to the Waipapa terrane is correct, the eastern boundary of the ophiolitic terranes must lie between the St. Heliers volcanoes and Motukorea (Fig. 2). The steepest gravity gradient of the Takapuna Anomaly occurs under the two volcanoes (Fig. 1 B), also suggesting proximity of such an eastern boundary. If the structure of the Dun Mountain Belt under Auckland is similar to that of all the known exposures, the Permian Maitai Group should be on-lapping onto the west side of the ophiolites (Fig. 1 C), so their anomalous occurrence on the east side must be due to tectonics. This, together with the predominance of schistose metabasites, suggests that the xenoliths of the St. Heliers volcanoes are not derived directly from the Dun Mountain ophiolite, but from a serpentinite melange on its eastern boundary analogous to the Patuki melange of the South Island. If our model of the tectonic situation (Fig. 1 C) is correct, the magma was traversing steeply dipping units through the basement, i.e., it was rising at low angle to the unit boundaries. Thus the number of different units would to a certain extent be restricted. In the Cenozoic cover rocks, dipping at low angles, more differences in lithology may be traversed in a shorter distance. Complications may be introduced into such a simple model by the possible occurrence of metamorphic terrigenous clasts at Mount Wellington and Mangere Mountain (Fig. 2) which should be located deeper in the steeply dipping ophiolite. This could indicate that some of the serpentinite shear zones generating the maxima in the Junction Magnetic Anomaly are more profound structures, imbricating boundary melanges into the ophiolite. As Mount Wellington, Mangere Mountain, the St. Heliers volcanoes and Motukorea appear to lie on a NE striking line, an alternative, extremely speculative, interpretation could be that this line represents a fault along which material from the surrounding sedimentary terranes has been dragged in. An even more complicated model would have to be constructed if the sandstones are derived from more than one terrane. Cenozoic tectonic disruptions of the ophiolite as seen at Dun Mountain (Johnston 1981) or at Piopio in the North Island (O’Brien & Rodgers 1973 a) could be responsible for such complications. Obviously such models can only be substantiated or otherwise with more collections of xenoliths from the Auckland Volcanic Field.

ERUPTION MODEL

The occurrence of the xenoliths in tuffs of the Glovers Park and Taylors Hill volcanoes clearly indicates the phreato-magmatic nature of the eruptions. The association of metamorphic basement rocks with non-metamorphic Cenozoic sedimentary rocks, including Waitemata Group sandstones, that all appear to have undergone the same interaction with the ascending magma suggests that they were mined from a region near the basal unconformity of the Cenozoic sediments onto the basement rocks, analogous to the processes proposed by Bryner (1991) for Motukorea. The depth to the unconformity is difficult to gauge. It must be at least 268 m, but could be as deep as ~600 m (Fig. 2). It may be possible to further refine the depth of origin of individual xenoliths by P/T studies of the latest post–cataclasite veins. According to this model, the interception of the ascending magma with the base of a groundwater reservoir hosted by the permeable Cenozoic rocks triggered a phreato-magmatic eruption. The question arises whether the source area of the eruption had the shape and structure of a dike or was plug-like, as illustrated for the San Rafael field, Utah by Kiyosugi et al. (2012) and was associated with a diatreme structure as exposed in the Ngatatura volcanics (Fig. 1 A) of South Auckland (Heming 1980). In any case, such a scenario would indicate that the metabasic xenoliths, because they were initially formed in high temperature/low pressure regime at or near a spreading ridge and were subsequently involved in Mesozoic accretion and final exhumation at the Gondwana margin, have never resided at great crustal depth. Glovers Park volcano ejected numerous fragments of juvenile lava (Fig. 6 B) either representing the tips of individual fingers of magma that reached the aquifer or sub-events where the eruption excavated down into the magma reservoir. While most xenoliths are directly embedded in the tuff (Figs 6 A, B), others are coated by lava (Figs. 6 C and D, 16 A) or completely encased in larger fragments of lava (Figs. 6 E and F). In some specimens, magma appears to have infiltrated the country rocks along joints, leading to jigsaw-puzzle patterns of breccia fragments (Fig. 6 D). The smallest fragments (down to mm size) are encased in the larger lava fragments. Here it can be seen that lava flowage, non-violent out-gassing, and some solidification occurred after inclusion of the xenoliths and before the eruption (Fig. 6 F) indicating that they are recycled juvenile clasts in the sense of White and Houghton (2006). It is still an open question whether blockage due to this solidification was in part a trigger for the eruption. The non-coated basement fragments were most probably directly blasted from in-place basement during the explosive phase. While most of the xenoliths, especially the largest and the smallest ones, are angular in shape (Figs. 6 A and F, 10) having mostly been cut out along joints or schistosity, others are rounded. Rounding can occur both in the weak Waitemata sediments (Figs. 6 B, 24) as well as on more resistant metabasic fragments (Fig. 6 E). Such rounding could either be due to abrasion during eruption or be inherited from processes predating inclusion in the magma. Rounded xenoliths on Motukorea are undoubtedly fragments of a Cenozoic basal conglomerate (Bryner 1991). The xenolith/magma/tuff relationships listed above can either be due to a set of phases of magma intrusion aborted before a successful eruption or may represent stages in one single eruption event. What the appropriate scenario is for the St. Heliers volcanoes needs to be clarified by further studies. Whichever version is the correct one, the following simplified sequence of events would have to apply: 1) magma ascent into basement rock and spalling off of fragments, with some rounding occurring; 2) further ascent of magma, degassing, and some solidification; 3) rise through the basal Cenozoic unconformity and intrusion into fractures in the sedimentary rocks; 4) Interaction with the aquifer leads to an explosive phreato-magmatic eruption excavating larger blocks of basement and overlying sedimentary rocks and rounding some fragments; 5) The initially unsorted fragments are sorted to some extent during ballistic transport.

Because we have not made a study of it, the sedimentology of the tuff deposit at Glover’s park volcano will not be discussed here. However this will be an important aspect for further deciphering the eruption mechanisms of this volcano.

FURTHER WORK

This report is only an overview of the xenoliths so far collected from the St. Heliers volcanoes, and because of the large number of samples, had to be somewhat cursory, to illustrate the main rock types. There is therefore the potential for much more in-depth work, not only on the collection, but also in the field, as set out below. Some of the aims of such further studies could be: 1) determination of how many protoliths there are in the subsurface and how they correlate with units known from the surface; 2) better control on P/T conditions of mineral formation in the metabasites and metasediments; 3) improved determination of the structural history stored within the xenoliths. 4) In the metabasites distinguish between ocean floor processes and those formed later; 5) a better estimation of the crustal depth interval sampled by the magma; 6) obtain more detailed information on the eruption mechanisms

THE METABASITES In order to pin down the source terrane(s) of the xenoliths more closely, detail their tectono-metamorphic history and allow comparison with the Maitai terrane elsewhere, the following are needed: • Better mineral determinations by microscope and microprobe work • Much more detailed analysis of textures and textural development • Thin sectioning of additional samples, with special techniques for small specimens • Geochemical analyses: bulk rock chemistry of metabasites, trace element and isotope analysis. Trace element and isotope studies on minerals • Age determinations on suitable minerals in the metabasites • Study of cataclasites by thin section work and geochemistry • Detailed mineralogical analyses of veins, with special emphasis on the post- cataclastic veins (also with fluid inclusion work), in case they indicate conditions of magma ascent

THE METAMORPHIC SEDIMENTARY ROCKS To facilitate more exact correlation of these samples with possible source terranes and detail their history, the following additional studies could be done: • Describe and compare sedimentary structures seen in thin section • Thin section and microprobe analysis of detrital suite • Geochemistry (including isotope geochemistry) of the detrital suite • Whole rock geochemistry • Where present, analyse cataclasites by thin section work and geochemistry • Detailed mineralogical analyses of veins, with special emphasis on the post- cataclastic veins (also with fluid inclusion work), in case they indicate conditions of magma ascent.

THE NON-METAMORPHIC XENOLITHS In order to gain more insight into the stratigraphy of the Cenozoic cover beds under the Auckland Volcanic field additional work could: • Distinguish Waitemata from non-Waitemata rocks, including study of microfossils

• Obtain more detailed thin section analysis

THE ENCLOSING LAVAS AND TUFFS • Thin section and geochemical work on lava coatings and juvenile ejecta to obtain more information on the processes of magma formation and propagation • Geochemical work on the tuffs, to compare them to the juvenile lavas • Study of lava textures and structures in relation to xenoliths to further understanding of the lava flow dynamics during the transport of the xenoliths • Study phenocrysts in the lavas and search for possible cognate and mantle inclusions in order to understand the full range of inclusion transport

FIELD WORK AT GLOVERS PARK • Search for more xenoliths, especially of ultramafics, gabbros, serpentinite and metamorphic sedimentary rocks to complete the range of rock types known to have been ejected. The following studies would lead to a better understanding of the eruption mechanisms: • Analyse the sedimentology of the tuffs to determine mechanisms of deposition • Make a shape analysis of juvenile ejecta • Analyse size and shape variations of xenoliths

ELSEWHERE IN THE AUCKLAND VOLCANIC FIELD Work that could be done to further elucidate the relevance of the St. Heliers xenoliths to processes in the Auckland Volcanic Field overall: • Search for more inclusions of the type seen at the St. Heliers volcanoes elsewhere in the Auckland Volcanic Field • Make a detailed study of the xenoliths of Motukorea (see Bryner 1991) • Study olivine and other inclusions at Mangere Mountain. • Update the work of Searle (1962) on quartzose inclusions with pyroxene reaction rims • Update the work by Brothers (1960), Brothers and Rodgers (1969), Rodgers and Brothers (1969), Rodgers et al. (1975) on mantle inclusions at Pupuke volcano.

CONCLUSIONS

• The suite of xenoliths ejected by Glovers Park and Taylors Hill volcanoes come from almost identical sources, 1.93 km apart, possibly part of a contiguous, approximately N-S striking tectonic unit. • They are dominated by an assemblage of metabasics: mostly amphibolitic schist, but ranging from pyroxenite to basalt, rodingites, and including numerous cataclasites. An association of highly schistose with non-schistose xenoliths of the same lithology is typical. Terrigenous low-grade metamorphic sandstones are rare but younger non- metamorphic clastic rocks are common. • Non-metamorphic xenoliths are mostly derived from the underlying Miocene Waitemata Group. Fragments from older Cenozoic rocks may also be present but have yet to be identified. • Most of schistose metabasic rocks have a magmatic or very high temperature, tectonic, first foliation, followed by formation of at least one further, tectonic, foliation under amphibolite facies conditions followed by lower grade metamorphism accompanied by open folding and kink folding, overprinted by veins and cataclastic faults, with some veins postdating the cataclastic faulting. • Metasomatic recrystallisation is quite common and is associated with formation of rodingites which may have been encased in serpentinite. • One terrigenous sandstone rich in shell fragments and cut by cataclastic microfaults is very likely correlative with the Permian Maitai formation that in Nelson overlies the Dun Mountain ophiolite. • The provenance (s) of the other terrigenous sandstone and one red chert sample is (are) not yet certain, although possible 'chipwackes’ from Motukorea and Mangere Mountain may be derived from the Waipapa terrane on the east side of the Auckland Volcanic Field. • The assemblage of metamorphic xenoliths, the position of the two volcanoes relative to the Junction Magnetic Anomaly and the Takapuna Gravity Anomaly, and the anomalous occurrence of Maitai sandstone, makes a derivation from a melange on the eastern side of the Dun Mountain Belt (analogous to the Patuki melange of the South Island) most likely. • The occurrence of pyroxenites, gabbros and prevalence of schistose metabasites derived from them, but associated with minor equivalent non-schistose volcanic rocks, suggests an origin at a slow spreading ocean ridge with possible influence of minor transform faulting. • Because of their ocean floor character and their subsequent tectonic history, the source rocks of the metabasic xenoliths never resided at great crustal depths. • The association of the metamorphic rocks with non-metamorphic sedimentary xenoliths, all occurring both as lava coated and non-coated fragments suggests that the source lay near the unconformity of the Cenozoic sediments on the Maitai terrane basement and that in some cases the magma infiltrated the country rocks, spalling off fragments before being swept up in a phreato-magmatic eruption. • Quartzose and ultramafic xenoliths elsewhere in the Auckland Volcanic Field sample a lower part of the crust and the mantle respectively and need to be re-investigated by modern methods. • The 310 xenoliths in the Auckland University collection provide ample opportunity for further research on the nature of the crust and the nature of magma ascent and eruption in the Auckland Volcanic Field to complement assessment of volcanic hazard.

ACKNOWLEDGMENTS

Collecting and report-writing by the students in KBS’s structural geology / tectonics graduate classes from years 1999 to 2004 laid the groundwork for this study. We would like to thank Jennifer Eccles for doing initial scanning of thin sections and organisation of samples. Elaine Smid, Madison Frank and Isabelle Chaillou brilliantly tackled the complex task of curating the final sample set. Neville Hudson patiently organised access to the Searle collection and the eventual storage of the whole sample set. Pat Browne kindly let KBS use his microscope during the report-writing stage. Hamish Campbell is thanked for advice on the shell fragment- bearing xenolith. Jan Lindsay made useful suggestions both during and at completion of the writing of this report. Elaine Smid and Madison Frank set up the manuscript for printing. The DEVORA project provided financial support.

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APPENDICES

APPENDIX I: LIST OF SAMPLES IN THE SEARLE COLLECTION Locations: G = Glovers Park volcano, T= Taylors Hill volcano

NOTE: Rocks 5012- 5017, 5103-5104 and their associated thin sections appear to have been mixed up and in part lost during curating prior to this study.

Macroscopic and thin section observations have been added to the original descriptions Rock Thin Remarks: Macroscopic rock description in black Thin section description in red, where necessary Location Section Mb. = Metabasite, fol. = foliation, foliated. Arrows (→) show time sequence of structures Gabbro or diorite, incipient fol., 3 mm quartz vein with (actinolite?) 4974 5179 G Mb. fol. , with disrupted epidote vein needles 4975 5180 G Mb. dark, fine- grained, massive with faulted vein Schistose amphibolite with cataclasite and talc vein 4977 5182 G Mb. coarse-grained, with shears Amphibolitised gabbro, brecciated in cataclasite 4978 5183 G Mb.? Kink folds and veins Cataclasite with semi-opaque minerals Mb.? Dark, Fe-rich and sheared, with slickensides 4979 5184 G Cataclasite with semi-opaque minerals (gabbro?) 4980 5185 G Mb. coarse, fol. , with veins Thin section missing Non-schistose coarse gabbro with talc veins. Incipient schistosity with xxxx 5186 G Rock not found actinolite 4981 5187 G Mb. with fol. -parallel veins Gabbro with talc veins and semi-opaque veins Cataclastically deformed gabbro, amphiboles with metamorphic rims, 4982 5188 G Mb. coarse-grained talc portions, kinked crystals Pyroxenes + altered amphiboles with metamorphic rims, talc veins, 4983 5189 G Pyroxenite, coarse-grained epidote veins Fig. 17: Mb. Fine-grained, dark. Grid pattern 4984 5190 G Amphibolite with feldspar saddle reef segregations alteration Lava-coating: vesicular basalt, various ~ 5 mm 4985 5191 G Mostly olivine xenoliths, one feldspar aggregate xenoliths, bubble trains Mb. coarse, with 2 phase veining, folded 4986 5192 G Schistose, with transposed folds epidote(?) vein, layered late vein 54

Mafic or sedimentary schist? Dark, fine-grained 4987 5193 G Quartz foliae. Chert? Prehnite veins with kink folds Fig. 6 E: Lava-coating: basalt with numerous 4988 5194 G variously sized xenoliths Mafic schist, cataclastic, with 2 phase veining, 1st 4989 5195 G Many talc veins, some gabbro textures phase folded Coarse feldspar + pyroxenes, crystals kinked. Inter-crystal actinolite 4990 5196 G Mb. coarse, veined shears (= incipient schistosity), part of shear zone? Chlorite veins→ cataclasite→talc veins→quartz veins Original description: schist, green, ex tuff. Feldspar/amphibole rock (gabbro?), sinistral shearing (schist?) fold-like 4991 5197 G Rock missing? structures, prehnite veins→cataclasite Fig. 19: Whitish rock with 'cleavage', c.f. samples 4992 5198 G Rodingite with schist structures, garnet 5111, 5134 Contact coarse (diorite?)/ fine-grained Mb. incl. 4993 5199 G Non-schistose, much chlorite, primary contact? actinolite 4994 5200 G Mb. fine-grained, massive, with veins Non-schistose, euhedral amphibole, cataclasites 4995 5202 G Lava-coating: Basalt with xenoliths

4996 5203 G Lava-coating: Basalt with 2-3 mm xenoliths Pyroxene (?) xenolith with bulbous (reaction?) rim Fig. 6 F: Lava-coating: Basalt with 1-5 mm 4997 5204 G Resorbed pyroxene phenocrysts (xenoliths), calcite amygdules xenoliths Lava-coating: vesicular basalt with some very 4998 5205 G small xenoliths 4999 5201 G Lava-coating: Basalt with xenoliths

Igneous fabric (gabbro). Incipient shearing/schistosity. Pyroxene 5000 5206 G Fig. 20: Cataclastic Mb. replaced by amphibole. 2-phase talc veins +chlorite. Best example of zoned veins Porphyroclastic shear texture, large parts replaced by talc (post- 5001 5207 G Mb. coarse foliation, with late veins deformational?). Pyroxene porphyroblasts, late pumpellyite (or epidote?) needles. Late cross-cutting talc veins Pyroxenite cataclasite, granulated, replacement of mafics with 5208 G Pyroxenite amphiboles Fig. 21 C: Lava-coated cataclastic semi-opaques, 5002 5209 G Granulated cataclasite→quartz veins sigmoidal clast between 2 shears 5210 G Same rock as TS 5209?

Non-schistose; very good multiphase veins. Broken vein surrounded by 5003 5211 G Mb. coarse-grained, massive (diorite?), with veins feldspar. Early veins→ perpendicular cataclasite→talc veins→perpendicular cataclasite Quartz/Epidote rock, contact of epidote vein with Epidote + quartz. Some veins show fluid inclusion trails, some have 5004 5212 G host rock crack-seal type structure. 5005 5213 G Cataclasite. Semi-opaques with disrupted material Cataclasite post-dated by quartz (?) veins Thinly foliated, F /F , kinks, talc, some ptygmatic veins, albite(?) 5006 5214 G Mb. with veins 2 3 veins, epidote veins. Gabbro? Transformation of crystals into porphyroclasts by shearing. 5007 5215 G Mb., coarse, with tremolite Post-kinematic and post-cataclasite talc

5008 5216 G Mb. with round fold F2/F3 etc. Amphibolite (thick section), 2 feldspar-rich layers, porphyroclasts with 5009 5217 G Mafic schist perpendicular older foliation. Post-schistosity prehnite veins parallel to schistosity, pools of talc material Mafic schist with extremely disrupted and kinked 5010 5218 G S shear sense indicators +good folds. Catclasite→kinks→cataclasite veins 2 Mb.? Very strong 2-phase veining (incl. calcite 5011 5219 G Highly cataclastic , semi-opaque with talc veins vein?) Semi-schistose gabbro, sigmoidal, folds amphibolitised, 2+ phase talc xxxx 5220 G xxxxxxxxxxxxxxxxxxxxx veins Similar to thin section 5217. Green internally structured vein → quartz 5012 5224? G Mb.? Gabbroic, non-schistose, with faulted vein (?) en-echelon veins. Similar to rock AU62998? 5013 5222? Massive igneous rock Cataclasite with fold-like structures → en echelon chlorite veins. Cataclasite of Mb.→chlorite veins, polygonized quartz (?). Same as thin 5014 5221? Semi-opaque, dark disrupted rock. section 5222? Lava-coated, layered (fol.?) rock similar to 5015 ? AU62995 Massive igneous rock, fine network of intricate 5016 ? veins Fig. 10 C: Mb. Intensely foliated schist with 5017 5225? Mylonitic amphibolite. Epidote-albite → tremolite (?) veins isoclinal mylonitic folds. 5103 ?

5104 ?

5105 5223 G Original description: Tremolite Amphibole crystals, non -schistose, ghost textures 56

5106 ?

5107 ?

5108 5227 G Mb. (epidiorite). Massive , with offset veins Fold -like structures. F0/F1? Very weak incipient S1/S2 5109 5228 G Mb. massive, with disrupted veins Very finely foliated. Kink folds and epidote veins Semi-schistose Mb. with fold-like structures (mushroom interference 5110 5229 G Mb. with many very thin veins patterns). Talc veins→prehnite veins→cataclasite→quartz/chlorite veins Rodingite with spectacular brown veins →2 phase quartz veins with 5111 5230 G Grey dense quartz schist with late veins large vugs coated with comb quartz. 5111 5230 G Second section: same as above

Quartz and/or albite? Epidote layers and shear sense indicators. Quartz 5112 5231 G Quartz/epidote, intensely veined rock polygonized, with intergrowths 5113 5232 G Fine-grained rock Semi-opaque cataclasite, no Mb. textures 5114 5233 G Fig. 7: Pyroxenite, massive, with alteration veins Metamorphic amphiboles. Cataclastic deformation Lenticular schist structure. S /S (?) along amphibole layers. Feldspar 5115 5234 G Mb. coarse grained 0 1 resists deformation, with feldspar-healed fractures Gabbro, non-schistose, mafic minerals replaced by actinolite. 5116 5235 G Mb. with two vein phases Chlorite/quartz vein→ cataclasite 1→cataclasite 2→tremolite vein. 5117 5236 G Undetermined sedimentary (siliceous?) rock Probably not sedimentary. Semi-opaque cataclasite 5118 5237 G Mafic schist with kink folds

5119 5238 G Epidotised (sample missing) Semi-opaque cataclasite? Breccia fragments, one veined rock fragment Non-schistose. Replacement of amphiboles. Talc veins→epidote 5120 5239 G Mb. (amphibolite), massive, with veins (?)veins Cataclasite with sheared ghost Mb remnants. Later quartz 5121 5240 G Mb.? semi-opaque with shears (?)veins→perpendicular, stepped chlorite shears Cataclasite with very fine grained shear zone with folds. Chlorite-rich 5122 5241 G Original description: Greywacke? Sub-schist Mb fragments. Late laminated quartz (?) veins. 5123 5242 G Original description: Greywacke? Cataclasite with Mb fragments. Late chlorite/prehnite(?) veins Cataclasite, replacement of fragments by prehnite. Irregular late 5124 5243 G Original description: Greywacke? calcite/quartz (?)/chlorite veins. Calcite unstrained Cataclasite, replacement of Mb. fragments by prehnite. Calcite 5125 5244 G Original description: Greywacke? fragments. Late calcite/quartz vein. Laminated zone of maximum shear 5126 5245 G Mb. massive, with many very thin veins Amphibolitic schist. Pre-schistosity veins →epidote veins→cataclasite+ 57

kinks → quartz veins with crack seal textures Very finely foliated schist (phyllonite). F folds. Epidote veins →talc 5127 5246 G Mb. fine-grained, with disrupted veins 2 veins→ cataclasite → crack seal quartz or albite veins Feldspar-dominated, with fold -like structures and shear lenses. Mafics 5128 5247 G Mb. coarse- grained with disrupted veins finely foliated; large opaques, talc veins and pools. Chlorite pools Mb. schistose, with faulted vein, similar to 5129 5248 G AU63001 Somewhat spotty schist, fold -like structure. Cataclasite→talc 5130 5249 G Mb. fine-grained, with cross-cutting veins vein→cataclasite Strongly cataclastic Mb., fol. with fold-like structures. Epidote vein 5131 5250 G Mb. fine-grained →cataclasite+ quartz/chlorite vein Finely laminated amphibolitic schist. S /S , kinks, boudined layers, talc 5132 5251 G Mb. strongly faulted 1 2 material, cataclasites Rodingite with very finely fibrous vein-like internal curved features. 5134 5253 G Quartzose schist like samples 4992 and 5111 Late ptygmatic veins of polygonized quartz Very finely foliated amphibolitic schist, very little feldspar. S /S Mb. fol. with prehnite, 2 phase veins, 1st phase 1 2 5135 5254 G pumpellyite? Prehnite veins→ kinks→ cataclasite→ ptygmatic talc kinked veins → quartz veins Coarse-grained Fe-rich red rock with folds, contains Cataclasite? Semi-opaques + talc with very good shear zones. Syn- 5136 5255 T tremolite shearing (?) folds Fine grained, straight bedding. No cleavage or veining. Burrow-like 5137 5256 T Greywacke Mt Wellington sandstone lenses Sandstone with very angular feldspar+volcanic clasts. Prehnite 5138 5257 T Greywacke Taylors Hill veins→cataclasite →quartz-chlorite veins 5141 5260 T Vesicular basalt

Feldspar(?) replaced by epidote. Mafics: unstrained amphibole, non- 5142 5261 T Epidote/sphene rock, with cross-cutting veins schistose. Epidote veins→cataclasites→polygonal quartz veins Cataclasite + prehnite-bearing lenses, one folded chlorite/sphene lens. 5143 5262 T Epidote/sphene rock Late chlorite veins Lava-coating of fine-grained (non-Waitemata?) 5144 5263 T Lava: very fluidal, pyroxene phenocrysts/xenoliths sedimentary xenolith The xenolith could be greywacke siltstone: Dark blebs = flattened 5263 B T Lava-coating of sediment. burrows? No faulting and veins 5145 5264 T Lava-coating: Vesicular basalt with xenoliths 2 lithic xenoliths, 1 lava; all have one -grain thick alteration rims

5146 5265 T Lava-coating of thin platy xenolith Aggregation of lava fragments? Lava at contact very vesicular. Main xenolith: quartzose (?) sandstone with angular grains, incl. glauconite? Many very small xenoliths: 5147 5266 T Lava-coating of low density, sandy xenolith Sandstones have no matrix, no fossils or veins. Dark rims of translucent needles Fig. 24 B: Lava-coating of reddish stained Feldspathic sandstone with lithic (sedimentary and volcanic) clasts, one 5149 5268 G sediment fossil tube, one foram Lava-coating: Small xenoliths and large Similar to thin section 5268 above. The cryptocrystalline xenolith has a 5150 5269 T phenocrysts in basalt with bubble trains thin alteration rim and coarse (prehnite?) crystals in the centre Lava-coating: xenolith in basalt (very small Very good contact with crypto-recrystallised coarser igneous rock. The 5151 5270 T sample) replacement mineral is fibrous Quartz/epidote -bearing coarse-grained Mb. with Coarse gabbro with epidote zones. Cataclasite→ talc veins → veins of 5154 5274 G veins polygonised quartz Calcareous sandstone with shell fragments, pyroxene heavy minerals, 5155 5275a G Fig. 23: Greywacke calcite-chlorite veins Groundmass coarser and less fine grained than other lavas. Much 5155 5275b G 5275 b: Basalt chlorite alteration Greywacke in contact with thick cataclasite. Greywacke is 5156 5276a G Greywacke cataclastically deformed and replaced by fine crystals. Cataclasite→ quartz-calcite veins 5156 5276b G Basalt Similar texture to thin section 5275 b 5277a G Basalt

Fine-grained cataclasite with angular fragments of re-crystallised ?igneous? rocks mostly replaced by prehnite, also mosaic quartz. Fold- 5157 5277b G Initial description: Greywacke like shear sense indicators→ quartz/prehnite/calcite veins→ cleavage- like chlorite seams 5158 5278 G Greywacke with disrupted veins Thin section missing?

APPENDIX II: LIST OF SAMPLES COLLECTED SUBSEQUENTLY. (all from Glovers Park Volcano, main sampling area)

A) Collections studied by J. James (2007) AU T = thin Field numbers Remarks Numbers sections 54989 LB 01 Metabasite, dark, massive, with disrupted veins. Similar to AU63070. (1 core) 54990 LB 02 Fig. 18: Metabasite, splotchy green /grey. (2 cores) 54991 LB 03 Metabasite, massive, with some veins. (2 cores) 54992 LB 04 Metabasite, dark, fine-grained massive, multiphase veins, earliest (epidote veins) faulted 54993 LB 05 T Metabasite, dark, fine-grained, massive with intricate, fine veining. (1 core) 54994 LB 06 Metabasite, dark, fine-grained, massive. (main sample+ fragments) 54995 LB 07 Metabasite, dark, fine-grained, massive, with cross-cutting veins 54996 LB 08 Metabasite, dark, coarse-grained, with veins 54997 LB 09 Metabasite, fine-grained with 2 phases of veining 54998 LB 10 Metabasite with thick (segregation?) laminae and a lineation 54999 LB 11 Metabasite, dark, coarse-grained, with veins 55000 LB 12 Metabasite or greywacke? Grey rock with veins 55001 LB 13 Metabasite, foliated and lineated, with epidote vein 55002 LB 14 Metabasite with multiphase veins 55003 LB 15 Metabasite or greywacke? Grey rock with veins 55004 LB 16 Metabasite, dark, fine-grained, foliated with kink-lineation, folded epidote vein 55005 LB 17 Metabasite, dark, massive, with veins 55008 LB 18 Metabasite, dark, fine-grained, foliated and lineated 58774 LB-D5 T Fig. 9: Metabasite, with veins and faults (no rock?) 58775 LB-V9 T Fig. 13: Metabasite with large faulted epidote vein 58776 LB-X2 T Metabasite, foliated, with veins 58777 LB-D2 T Fig. 12: Metabasite, semi-schistose, with disrupted veins 58778 LB-D6 T Metabasite, massive, fine-grained, dark, with epidote vein 58779 LB-C1 T Metabasite coarse-grained with 2 phase veining 60

58780 SPB 1 T Metabasite , massive, fine-grained, dark, with multiple vein sets, incl. epidote veins 58781 LB00 (L)-KBS-15 T Metabasite, layered, opaque-rich, disrupted veins, one vein ends abruptly in fold 58782 LB00-JOE-1 T Fig. 16: Metabasite, lava coated, strongly foliated, opaque-rich 58783 LB00-ANI T Metabasite, foliated with lineation, disrupted veins 58784 HAW LB00 T Fig. 15: Metabasite with garnet layers, polygonised feldspar, folds

B) Remainder of subsequent collections AU T= thin Field numbers Remarks numbers section (s) 62885 LB-1 T Metabasite, fine-grained, foliated with veins 62886 LB-3 T Metabasite (also labelled LB3-LB X3) 62887 LB-IV (LB-4) T Metabasite (hand specimen labelled LB VI) 62888 LB-4 L5 Metabasite 62889 LB-C2 T Fig. 8 B: Metabasite, coarse, with veins 62890 LB-C3 Metabasite, coarse, with veins 62891 LB-C4 Metabasite, coarse 62892 LB-C6 Metabasite, coarse, with veins 62893 LB-D1 T Metabasite, massive with a thick epidote vein 62894 LB-D3 T Metabasite, veins, including a pumpellyite (?) vein 62895 LB-D4 Metabasite, massive, with veins 62896 LB-D7 Metabasite, massive 62897 LB-D8 T Metabasite with multi-phase veins, earliest veins faulted 62898 LBEI T Metabasite, with 2-phase veining 62899 LB-F1 T Metabasite, with brecciated vein 62900 LB-F2 T Fig. 14: Metabasite, 1 cm fine-grained/coarse-grained layers , fine-grained part is schistose 62901 LB-IN T Metabasite with much opaque material, lava-coated 62902 LB-V2 T Metabasite, fractured veins 62903 LB-V3 T Figs. 10 A, 11 A : Metabasite with epidote veins 62904 LB-V4 T Metabasite 61

62905 LB-V5 Metabasite 62906 LB-V6 Metabasite with epidote veins 62907 LB-V7 Metabasite 62908 LB-V8 Metabasite, contact of fine-grained with coarse grained 62909 LB-VT 1 Mafic schist with lineations 62910 LB-X4 T Fig. 8 C: Metabasite, coarse, strongly foliated 62911 LB00-AM-1 (L) Metabasite, coarse 62912 LB00-AM-2 (L) Metabasite, coarse, with folded/faulted veins 62913 LB00-AM-3 (L) Metabasite, coarse 62914 LB00-AM-4 (L) Metabasite, intensively veined (multiphase) 62915 LB00-AM-5 (L) Metabasite, fine-grained with green epidote vein 62916 LB00-AM-6W T Mafic schist, with epidote layers, crenulated foliation, later veins 62917 LB00-AM-7 (L) Metabasite, coarse 62918 LB00-AM-8 (L) Metabasite, coarse, with at least two phases of veins 62919 LB00-AM-9 (L) Metabasite, veined 62920 LB00-AM-10 (L) Mafic schist, with one major fold, a crenulation lineation and veins 62921 LB00-AM-11 (L) Metabasite, fine-grained, massive, with veins 62922 LB00-AM-12 (L) Metabasite, with folded vein 62923 LB00-AM-13 (L) Metabasite very dense and massive 62924 LB00-AM-14 (L) Metabasite, foliated, with veins 62925 LB00-AM-15 (L) Metabasite, with two phases of veins 62926 LB00-AM-16 (L) Metabasite, fine-grained, with epidote veins 62927 LB00-AM-17 (T) Metabasite, tuff-coated 62928 LB00-AM-18 (T) Metabasite 62929 LB00-AM-19 (T) Metabasite, foliated and veined 62930 LB00-AN-2 (L) Metabasite, coarse-grained, with two phases of veining 62931 LB00-AN-3 (L) Metabasite, massive, with three phases of veining. Oldest vein thickest and folded 62932 LB00-AN-4 (L) Metabasite, with intensive, fine veining 62933 LB00-AN-5 (L) Metabasite, coarse, with disrupted vein

62934 LB00-AN-6 (L) Metabasite, with crenulated veins 62935 LB00-AN-7 (L) Metabasite, with gridded vein pattern 62936 LB00-AN-8 (L) Metabasite (or sedimentary?), with sub-parallel veins. 62937 LB00-AN-9 (L) Metabasite, coarse, with a few veins 62938 LB00-DNB-5 Metabasite, foliated, with multiphase veins; first veins faulted 62939 LB00-HAW-1 (L) Metabasite 62940 LB00-HAW-2 (L) Metabasite 62941 LB00-HAW-3 (L) Metabasite 62942 LB00-HAW-4 (L) Metabasite, fine-grained, with folded vein 62943 LB00-HAW-5 (L) Metabasite 62944 LB00-HAW-6 T Metabasite, foliated and folded. ?garnet? 62945 LB00-HAW-7 (A) T Metabasite, foliated with folded feldspar layers. ?garnet? (Listed as 7HAW LB00 (A) ) 62946 LB00-HAW-8 (L) Metabasite or clastic? 62947 LB00-HAW-9 (L) Metabasite, coarse, with veins 62948 LB00-HAW-10 (L) Metabasite, fine-grained, with veins 62949 LB00-HAW-11 (L) Metabasite? 62950 LB00-HAW-12 (L) Metabasite, coarse 62951 LB00-HAW-13 (L) Mafic schist, fine-grained 62952 LB00-HAW-14 (L) Metabasite, lineated, containing a sigmoidal vein 62953 LB00-HAW-15 (L) Metabasite, coarse-grained, 62954 LB00-HAW-16 (L) Metabasite, dark, fine-grained, foliated and lineated 62955 LB00-HAW-17 (L) Metabasite, with disrupted veins 62956 LB00-HAW-18 (L) Metabasite, with thick (1 cm) vein (quartz?) 62957 LB00-HAW-19 (T) Metabasite 62958 LB00-HAW-20 (T) Metabasite, with veins 62959 LB00-HAW-21 (T) Mafic schist 62960 LB00-HAW-22 Metabasite, fine-grained, with faulted epidote veins 62961 LB00-JOE-2 (L) Lava-coated sediment (Waitemata Group?) 62962 LB00-JOE-3 (L) Lava-coated, fine-grained dark, semi-opaque material with disrupted veins

62963 LB00-JOE-4 (L) Metabasite, fine-grained, with two phases of veins 62964 LB00-JOE-5 (L) Metabasite, fine-grained, with two phases of veins 62965 LB00-JOE-6 (L) Metabasite, with epidote vein 62966 LB00-JOE-8 (L) Metabasite, with multiphase veins; early phase veins folded (kinked) 62967 LB00-JOE-9 (L) Metabasite, coarse, with veins 62968 LB00-JOE-10 (L) Metabasite, with 2- phase veining. One faulted, thick epidote vein 62969 LB00-JOE-11 (L) Metabasite, with 2-phase veining 62970 LB00-JOE-12 (L) Metabasite, foliated with 2-phase veining 62971 LB00-JOE-13 (L) Metabasite, with thick (3 cm) epidote vein 62972 LB00-JOE-14 (L) Metabasite, coarse, with veins 62973 LB00-JOE-15 (T) Contact xenolith/tuff 62974 LB00-JOE-16 (T) Undetermined, rounded xenolith 62975 LB00JR T No rock. Thin section: metabasite 62976 LB007-DEI-B T Possibly same as LB00JOE1 / AU58782 62977 LBJ07 A T Figs. 21 A, B: Disrupted veins in semi-opaque, fine-grained matrix (similar to LB00-JOE3?) 62978 LBJ07 B T Metabasite, coarse-grained /fine-grained 62979 LB00-JDE-1 (L) T Metabasite, coarse, with a folded vein termination 62980 LB00-JDE>10 T Lava-coated, fine-grained semi-opaque material identical to LB00-JOE3 62981 LB00-KBS-2 Tuff with xenoliths of metabasite and Waitemata Group sandstone 62982 LB00-KBS-3A (T) T Fig. 10 B: Mafic schist, with folds and veins 62983 LB00-KBS-4 (T) Metabasite(?) with tuff coating 62984 LB00-KBS-5 (T) Metabasite with veins 62985 LB00-KBS-6 (T) Metabasite with veins 62986 LB00-KBS-7 (T) Metabasite (?) 62987 LB00-KBS-8 (T) Metabasite (?) 62988 LB00-KBS-9 (T) Metabasite with tuff coating 62989 LB00-KBS-10 (T) Metabasite (?) with tuff coating 62990 LB00-KBS-11 (T) Metabasite (?) with tuff coating 62991 LB00-KBS-12 (T) Metabasite (?) with tuff coating

62992 LB00-KBS-13 (L) Metabasite, coarse, with incipient lineation, multiphase veining 62993 LB00-KBS-15 (L) Metabasite, fine-grained, slaty (?) 62994 LB00-KBS-16 (L) Metabasite, with multi-phase disrupted veins 62995 LB00-KBS-17 (L) Lava coated, yellow weathering, foliated, disrupted rock 62996 LB00-KBS-18 (L) Metabasite with incipient lineation 62997 LB00-KBS-19 (L) Metabasite, coarse grained, with feldspar (and/or quartz?) segregations 62998 LB00-KBS-20 (L) Fig. 11 B: Metabasite, coarse grained, with 2-phase veining,; 1st phase faulted 62999 LB00-KBS-21 (L) Metabasite 63000 LB00-KBS-22 (L) T Metabasite, coarse, foliated 63001 LB00-KBS-23 (L) T Fig. 11 C: Mafic schist, with shear planes and folds and multi-phase veins, some faulted 63002 LB00-KBS-24 (L) Undetermined rock (similar to LB00-JOE3? / AU62962) 63003 LB00-NIGL T Metabasite, foliated, lineated, with multi-phase veins 63004 L00-PB00 (X) T Metabasite (?) 63005 LB00-PMB-1 Lava-coated mafic schist, 63006 LB00-SR-1 (L) Metabasite 63007 LB00-SR-2 (L) T Metabasite, foliated, with crenulated vein 63008 LB00-SR-3 (L) Metabasite, fine-grained, massive, with veins 63009 LB00-SR-4 (L) Metabasite, foliated, with thick vein 63010 LB00-SR-5 (L) Metabasite, altered, intensely veined 63011 LB00-SR-6 (L) Metabasite, fine-grained, massive, with 2-phase veining 63012 LB00-SR-7 (L) Metabasite with veins 63013 LB00-VT-2 (L) Metabasite with epidote veins 63014 LB00-VT-3 (L) Metabasite, with epidote veins 63015 LB00-VT-4 (L) Metabasite, coarse grained , with veins 63016 LB00-VT-5 (L) Metabasite, fine-grained, with veins 63017 LB00-VT-6 (L) Metabasite, with faulted epidote vein 63018 LB00-VT-7 (L) Metabasite, fine-grained, foliated, with incipient lineation 63019 LB00-VT-8 (L) Metabasite, coarse, with lineation and veins 63020 LB00-VT-9 (L) Metabasite

63021 LB00-VT-10 (L) Metabasite, fine-grained, with veins 63022 LB00-VT-11 (L) Metabasite, coarse grained, with one vein 63023 LB00-VT-12 (L) Metabasite, with disrupted vein 63024 LB00-VT-13 (L) Metabasite, coarse, with disrupted veins 63025 LB00-VT-14 (L) Metabasite, fine-grained, foliated, with disrupted veins 63026 LB00-VT-15 (T) Mafic schist, weathered 63027 LB00-WSS-1 Metabasite, fine-grained, foliated, with veins 63028 LB00-WSS-2 Metabasite, with epidote veins 63029 LB00-WSS-3 Metabasite (?), intensely veined 63030 LB00-WSS-4 Metabasite, fine-grained, finely veined 63031 LB00-WSS-5 Metabasite(?), with highly sheared veins 63032 LB00-WSS-6 Undetermined grey rock with veins (metabasite?) 63033 LB00-WSS-7 Metabasite, fine-grained, lineated, with 2-phase veining, 1st stage crenulated 63034 LB00-WSS-8 T Metabasite, coarse, with multiple vein phases 63035 LB00-WSS-9 Metabasite, foliated, with veins 63036 LB00-WSS-10 Metabasite, with one thick (1 cm) disrupted epidote (?) vein 63037 LB00-WSS-11 Tuff with many lava fragments and one shell (?) fragment 63038 LB00-WSS-12 T Fig. 24 A: Lava coated sandstone (Waitemata Group?) or tuff? 63039 LB00-WSS-13 Tuff 63040 SPB-L003 Metabasite, with lineation and intensely crenulated veins 63041 SPB-L004 T Metabasite, with intensely disrupted veins cut by non-disrupted veins 63042 SPB-L005 Lava-coated sandstone (Waitemata Group?) 63043 SPB-L00 T1/6 Metabasite, fine-grained, with earlier epidote(?) vein, displaced by later veins 63044 SPB-L00 T2/7 Metabasite with coating of tuff which includes larger Waitemata Group fragments 63045 SPB-L00T3/8 Mafic schist? 63046 SPB-L008 T Mafic schist, foliated and lineated (Hand specimen labelled LB SPB 00) 63047 LB04-1 Fine grained sedimentary xenoliths (Tertiary?) 63048 LB04-2 Fine grained sedimentary xenoliths (Tertiary?). In the same container as AU63047 63049 LB04-3 Fine-grained grey xenolith with tuff coating

63050 LB04-4 Rusty weathering inclusion with tuff coating 63051 LB04-5 Rusty weathering inclusion with tuff coating 63052 LB04-6 Rusty weathering inclusion with tuff coating 63053 LB04-7 Fig. 22: Fine-grained red xenolith, veined. Basement chert 63054 LB04-8 Fig. 6 D: Lava-coated Waitemata Group fragments in a jigsaw pattern 63055 LB04-9 Lava coating: lava with small (rounded) , mostly sedimentary xenoliths 63056 LB04-11A Undetermined dense, massive rock 63057 LB04-11B Undetermined dense, massive rock 63058 LB04-12 Tuff? Or sandstone xenolith? 63059 LB04-13 Fig. 8 A: Coarse-grained mafic igneous rock with slickensides. 63060 LB04-14 Tuff with various xenoliths, including lava 63061 LB04-15 Fig. 6 C: Tuff with lava-coated inclusion 63062 LB04-16 Lava-coated coarse sandstone (Waitemata Group?) 63063 LB04-17 Sandstone (xenolith?) 63064 LB04-18 Sandstone xenolith with tuff coating 63065 LB20-1 Fig. 6 A: Tuff with xenoliths, large sample 63066 LB20-3 T Metabasite, intensely faulted 63067 LB20-4 T Clastic sediment 63068 LB20-5 T Metabasite, coarse-grained 63069 LB20-6 T Metabasite, intensely veined Metabasite, massive, fine-grained, with crenulation lineation, with multi-phase veins, the earliest are disrupted, 63070 LB05May09 folded 63071 LBO5 T Metabasite, non-schistose; good pattern of 2-phase veining