Journal of Volcanology and Geothermal Research 190 (2010) 192–202

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Journal of Volcanology and Geothermal Research

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Xenoliths from the sub-volcanic lithosphere of Mt Taranaki, New Zealand

Kerstin Gruender a, Robert B. Stewart a,⁎, Stephen Foley b

a INR, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealandb Institut für Geowissenschaften, Johannes Gutenberg Universtität, Becher-Weg 21, 55099 Mainz, Germany

⁎ Corresponding author.E-mail addresses: kerstin.gruender@gmx.net (K. Gru

r.b.stewart@massey.ac.nz (R.B. Stewart), foley@uni-mai

0377-0273/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.jvolgeores.2009.09.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 August 2008Accepted 22 September 2009Available online 3 October 2009

Keywords:xenolithmetasomatismMt TaranakiEgmont volcanolithospheresubduction

Mount Taranaki is located 140 km west of the Taupo Volcanic Zone and represents the most westerlyexpression of subduction-related volcanism on the North Island of New Zealand. Taranaki is a predominantlyhigh-K arc volcano but compositions range from basaltic andesite to andesite with minor dacite and basalt.The sub-volcanic basement under Taranaki is thought to comprise calc–alkaline plutonic and metamorphicrocks of the Median Batholith, overlain by a sequence of Cretaceous and Tertiary sediments. Taranaki lavascontain abundant xenoliths that represent samples of the upper to lower crust beneath the volcano. Thexenolith suite has been initially organised into six groups based on petrography, geochemistry and inferredgenetic relationships: supracrustal sedimentary rocks (1), mafic hornfels (2), garnet gneiss (3), granite andgranodiorite (4), finely banded amphibolitic gneiss (5) and gabbros and ultramafic rocks (6). Groups 1, 3 and4 are derived from the Median Batholith basement and Cretaceous–Tertiary sediments of the Taranaki Basinwhile Groups 2, 5 and some fine grained gabbros from Group 6 could either be derived from the MedianBatholith or be cognate xenoliths. Group 6 gabbros and ultramafic rocks are dominated by clinopyroxene,amphibole and plagioclase and are predominantly cumulate in origin.The Egmont xenoliths can also be classified into the Type I and Type II xenoliths defined by Frey and Prinz(1978). Type I dunite and wehrlite xenoliths are only present in basaltic andesite host rocks and are sourcedfrom depleted upper mantle whereas Type II xenoliths predominate in the more siliceous andesites and aresourced from the lower crust. The separate source depths for the two rock types can be explained by the “hotzone” model where the andesites have much greater interaction with the lower crust than the basalticandesites. Some xenoliths contain glass of rhyolitic to trachyitic compositions with up to 6% K2O thatrepresent partial melts of the sub-volcanic lower crust and may give rise to the andesite magmacompositions by mixing with lower crustal residual crystals. The widespread occurrence of amphibole in theEgmont xenolith suite reflects the fluid-rich environment of arc magma systems.

ender),nz.de (S. Foley).

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Xenoliths in volcanic rocks provide a window into the compositionand distribution of sub-volcanic lithologies (Graham, 1987; Grahamet al., 1988; O'Reilly et al., 1989; Graham et al., 1990) and sample thevertical extent of the magma plumbing system from mantle tosupracrustal rocks (Wysoczanski et al., 1995). Here we focus on thedeeper sourced xenoliths that provide information on lower crustaland mantle compositions beneath Egmont volcano, Taranaki, NewZealand.

Much data on sub-volcanic compositions are derived from alkalinerocks which appear to rise rapidly from their mantle source andfrequently sample the strata through which they pass (Griffin andO'Reilly, 1987; Wysoczanski et al., 1995; Chen and Arculus, 1995;Alletti et al., 2005). Frey and Prinz (1978) identified Type I and Type II

xenoliths; Type I comprise olivine-bearing ultramafic compositionswith Cr-diopside as the clinopyroxene and also contain Cr-spinels.They have low Al2O3, high Mg/Fe ratios and represent depleted uppermantle, which has undergonemultiple melt extractions (Kovács et al.,2004; Dessai et al., 2004). Texturally the Type I lithologies showevidence of deformation and recrystallisation in the presence ofdeformation lamellae, granoblastic textures and foliations (Griffin andO'Reilly, 1987; Alletti et al., 2005; Ghent et al., 2008).

In contrast, Type II are uppermost mantle/lower crustal rocks (Al-augite series of Wilshire and Shervais (1975)), which have cumulateor meta-igneous textures and are cumulate residues from partialcrystallisation of basaltic magmas at or near the base of the crust. TypeII lithologies generally do not exhibit strain or granoblastic textures.Both Type I and II xenoliths may show evidence of metasomatism(Kovács et al., 2004; Dessai et al., 2004).

A further group of deep crustal xenoliths is mafic and felsicgranulites and charnockites that are inferred to comprise much of thelower crust (Kempton et al., 1990; Rudnick and Fountain, 1995;Kovács and Szabó, 2005; Ghent et al., 2008). These have foliated to

Fig. 1. Partial Palaeozoic–Mesozoic map of New Zealand showing the distribution ofterranes. The Brook Street, Maitai, Dun Mountain ophiolite and Murihiku Terranes,together with part of the Median Batholith (which extends through to the Cretaceous),are fossil Carboniferous to Jurassic arcs. Egmont lies near the western boundary ofMedian Batholith basement.

193K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

granoblastic textures and show evidence of reaction between Ca-plagioclase and olivine to form spinel and clinopyroxene thatcharacterises granulite facies metamorphism (Alletti et al., 2005).Xenoliths in the alkaline basalts therefore record the passage ofmagma through depleted upper mantle, lower crustal cumulates,granulites and on through supracrustal lithologies.

The record of xenoliths from subduction zonemagmas is much lesscomprehensive and most examples of xenoliths in subductionvolcanics come from the western Pacific (Arai et al., 2007). Theyhave been used in arcs to provide clues about magma generation, pre-eruptive processes and interaction with the crust, for example theAleutian Arc (Conrad and Kay, 1984; Debari et al., 1987), the LesserAntilles (Arculus and Wills, 1980) and Mt Ruapehu in the TaupoVolcanic Zone of New Zealand (Graham, 1987; Graham and Hackett,1987; Graham et al., 1988, 1990), as well as characterising thecomposition of the mantle wedge (Arai et al., 2007).

Xenoliths in arc rocks are predominantly interpreted as cognate andsupracrustal in origin (Arculus and Wills, 1980; Graham et al., 1990).Because the lower crust/upper mantle zone that interacts with arcmagmas is generally hotter and contains a greater melt fraction thanthat encountered by alkaline basalts, the lower crustal lithologies aremore easily disaggregated and recorded predominantly as either singlecrystals or glomerocrysts (Arculus andWills, 1980; Stewart et al., 1996;Price et al., 1999, 2005; Annen et al., 2006; Price et al., 2008).

In this paper we report on a suite of xenoliths from Egmontvolcano, Mt Taranaki, New Zealand which are both cognate and oflower crust/upper mantle origin. This study presents new petro-graphical data and major element analyses of minerals within thexenoliths and distinguishes possible origins and genetic relationships.The data also provide further constraints on subsurface geology of theTaranaki region and give unique insights into the sub-volcaniclithosphere and lower crust beneath the volcano.

2. Regional setting

Egmont volcano (Mount Taranaki) is centred on the TaranakiPeninsula in the western North Island of New Zealand and about140 km west of the Taupo Volcanic Zone (TVZ), the main region ofactive volcanism in New Zealand (Fig. 1). Egmont is the youngest offour volcanic centres in the region, forming a northwest–southeasttrending lineament with volcanism becoming progressively youngertowards the southeast. The oldest volcanic centre is Paritutu,including the Sugarloaf Islands near New Plymouth (1.75 Ma),followed by Kaitake (0.57 Ma), Pouakai (0.25 Ma) and Egmont(b0.12 Ma) (Neall, 1979; Neall et al., 1986). Volcanic rocks of Egmontare predominantly high-K andesites and basaltic andesites, withminor dacites and high-alumina basalts (Stewart et al., 1996). Theyoungest eruptives at the summit have the highest SiO2-content andthe lavas have also become progressively more K-rich with time(Neall et al., 1986; Price et al., 1992; Stewart et al., 1996; Price et al.,1999).

The volcanoes in the Taranaki region are located about 400 kmwest of the trench, are c. 250 km above a Wadati–Benioff zone(Adams andWare, 1977; Boddington et al., 2004; Reyners et al., 2006)and overlie 25 to 35 km thick continental crust (Stern and Davey,1987). However, the slab is only traceable to the southeastern part ofthe Taranaki region, c. 35 km east of Mt Taranaki (Stern et al., 2006) sois not clearly expressed beneath the volcanoes. In comparison, thevolcanoes in the TVZ overlie only c. 15 km of relatively hot continentalcrust and the Wadati–Benioff zone is at 80 km depth (Stern andDavey, 1987; Stratford and Stern, 2006). Recent seismic studies of theTaranaki region show a brittle–ductile transition zone at 10 km depthbeneath Mt Taranaki and 3D seismic velocity tomographic imaging ofthe Taranaki volcanoes shows a volcanic root system of around 5 kmin diameter, extending to a depth of c. 10 km (Sherburn and White,2005, Sherburn et al., 2006).

The Taranaki Peninsula is the onshore component of the TaranakiBasin and the upper 6 km of the crust comprises Cretaceous toTertiary sedimentary rocks (King and Thrasher, 1996). The deeperbasement geology of the Taranaki region has been extrapolated fromthe South Island and is considered to be part of the Median Batholith(Mortimer et al., 1999). This is based on information from oilexploration drill holes, seismic studies, magnetic anomalies andsome information from xenoliths in volcanic rocks (Wodzicki, 1974;Knox, 1982; Gamble et al., 1994; King and Thrasher, 1996; Mortimeret al., 1997; Sutherland, 1999). The Median Batholith is made up ofdiorites, gabbros and granitoid rocks (Challis et al., 1994; Rattenburyet al., 1998) and some igneous xenoliths have been reported to showsimilarities to rocks from the Median Batholith. Large sandstonexenoliths in the Pungarehu Formation in the western Taranaki regionhave been correlated to the Eocene–Oligocene Kapuni Groupsandstone at approximately 3.5 km depth beneath the volcano (Collenet al., 1985). However, no exploration wells close to Mt Taranaki havepenetrated basement rocks and proposed basement terrane bound-aries on the Taranaki Peninsula remain speculative (King andThrasher, 1996; Mortimer et al., 1997).

3. Materials and methods

Rock samples with xenoliths were collected from a variety of sitesalong beaches and on the mountain. Xenolith host rocks range frombasaltic andesite to dacite and are typically porphyritic with crystalcontents between 30 and 60% (Neall et al., 1986; Price et al., 1992;Stewart et al., 1996). All of thematerials are reworked fromprimary lavaflows and transported as debris flows and avalanches, from where it isagain reworked by fluvial or beach processes. There is therefore nostratigraphic control on the samples, except that where found in debrisavalanche deposits a minimum age can be ascribed. The focus in thispaper is on the spectrum of compositions represented by the xenolithsas representative of sub-volcano lithologies.

Table 1Representative olivine and spinel analyses, Egmont xenoliths. Rock type numbers are:1 = thermally altered sediment, 2 = amphibolitic gneiss, 3 = mafic hornfels, 4 = finegrained gabbro, 5 = granodiorite, 6 = hornblende gabbro, 7 = hornblende pyroxenegabbro, 8 = meta-hornblende pyroxenite, 9 = hornblende pyroxenite/pyroxenehornblendite, 10 = hornblendite, 11 = olivine hornblende clinopyroxenite, 12 =troctolite/clinopyroxenite, 13 = clinopyroxenite, 14 = olivine clinopyroxenite, 15 =wehrlite, and 16 = dunite. Ol = olivine, Cr-sp = chrome spinel, and Al–Mg-sp =spinel. FeO* is total Fe as FeO.

Rocktype

(11) (13) (12) (7) (15) (16) (15) (12)

Mineral ol ol ol (olivine inT952x3)

ol ol Cr-sp Al–Mg-sp

SiO2 37.36 38.82 39.40 39.72 40.71 40.78 0.04 0.01TiO2 nd nd nd nd nd 0.01 1.36 0.09Al2O3 0.01 nd nd nd 0.02 0.01 16.79 62.83Cr2O3 0.02 0.04 0.02 0.03 0.02 0.03 34.37 0.06FeO⁎ 23.25 20.21 17.47 16.39 12.02 9.24 35.25 19.61MnO 0.33 2.26 0.27 0.64 0.16 0.18 0.23 0.97MgO 38.79 39.59 43.22 43.83 47.41 49.36 10.56 16.19CaO 0.10 0.16 0.06 0.03 0.04 0.05 0.11 ndNa2O nd 0.01 nd 0.01 nd nd nd 0.03K2O nd nd nd nd nd nd 0.01 0.01ZnO 0.07 0.11 0 0.01 0.05 0.02 0.13 0.51NiO 0.09 nd 0.14 0.10 0.24 0.35 0.20 0.01V2O3 nd nd nd 0.01 nd 0.01 0.21 0.01Total 100.02 101.20 100.58 100.78 100.67 100.04 99.27 100.35

194 K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

Polished sections of the xenoliths were prepared and analysedmicroscopically. Mineral major element analyses were carried out byelectron microprobe (Jeol Superprobe JXA 8900) at Johannes Guten-berg University (Mainz, Germany) under the following analyticalconditions; acceleration voltage=20 kV for olivine, pyroxenes,spinel, Fe–Ti oxides and 15 kV for feldspars, glass, other minerals;beam current=8 nA for feldspars, 12 nA for all other minerals; beamdiameter=5 µm for feldspars and glasses, 2 µm for all other minerals.

The xenolith rock types were classified according to the IUGSrecommendations for igneous rocks (Le Maitre, 1989). Amphibolesand pyroxenes were classified using the IMA nomenclature (Leake,1997 andMorimoto, 1988) and amphibole analyses were recalculatedusing the procedure outlined by Schumacher (1997).

4. Results

4.1. Petrography and mineral chemistry of xenoliths

Xenoliths range in lithology and size from 10 mm up to 500 mm indiameter and are common in all Taranaki lavas. Glomerocrysts are

Table 2Representative amphibole (amph) and biotite (bio) analyses, Egmont xenoliths. FeO* is totalF and Cl. Rock type numbers as for Table 1.

Rock type (8) (5) (7) (2) (6

Mineral amph amph amph amph am

SiO2 52.85 48.32 47.57 46.82 45TiO2 0.48 1.05 1.44 1.66 1.Al2O3 4.03 6.64 7.13 7.61 9.Cr2O3 0.26 0.03 0.08 0.17 0.FeO⁎ 10.37 13.93 12.78 11.76 12MnO 0.29 0.54 0.28 0.18 0.MgO 18.08 14.36 14.75 15.58 14CaO 12.44 11.97 12.01 11.69 11Na2O 0.97 1.18 1.49 0.83 2.K2O 0.26 0.47 0.74 1.70 0.F nd 0.02 0.11 0.24 ndCl nd 0.06 0.05 0.01 0.Total 100.03 98.57 98.43 98.25 98O = F,Cl 0.02 0.06 0.10 0.Total−F,Cl 100.03 98.55 98.37 98.15 98

also ubiquitous in thin section and comprise similar mineralassemblages to those in the macroscopic xenoliths. The contactrelationships between xenoliths and host rock vary, and no systematictextures have been identified for specific xenolith rock types. Somehave sharp angular contacts to the host rocks, while others have areaction rim of amphibole and/or titanomagnetite and clinopyroxeneor show glass-filled fractures and melt areas within them.

Xenoliths are grouped primarily according to their mineralogy andtextures, taking into account some mineral chemistry and inferredgenetic relationships. Most xenolith types can be distinguished andgrouped macroscopically; only very fine grained xenoliths requiremicroscopic analysis because these can appear very similar in handspecimen (e.g. mafic hornfels and meta-sediments). Estimatedpressures, giving depth constraints, obtained using Al in hornblendegeobarometry (Hollister et al., 1987) were taken into account in thefinal grouping for amphibolitic gneiss, hornblende pyroxene gabbroand ultramafic rock types. To keep the list of xenolith groups short,some simplification was applied to the variety of rock textures found,for example the degree of foliation and grain size. Representativeanalyses for xenolith minerals are listed in Table 1 (olivine andspinel), Table 2 (amphibole and biotite) and Table 3 (clinopyroxene).

4.1.1. Supracrustal sedimentary rocksSedimentary type xenoliths typically occur as small (10–50 mm

diameter), angular clasts. Lithologies range from mature quartz-rich(95%) sandstones to less mature sediments containing up to 35% sub-rounded feldspars and clinopyroxene fragments in a clay-rich matrix.Coarse and fine grained rock types occur, with very fine grainedvarieties often being partially melted to form silica-rich (c. 80% SiO2)glass (Table 4) and Ca-rich clinopyroxenes. Similar glasses have beenreported from sedimentary xenoliths in Ngauruhoe lavas (Grahamet al., 1988). Creamy-white coloured and relatively unaltered, friablequartz sandstone xenoliths from the Kapuni Group occur in thePungarehu Formation debris avalanche. Kapuni Group quartz sand-stone does not outcrop in the Taranaki region and oil well data show itto occur at 3.6 km depth beneath the volcano (Collen et al., 1985).

4.1.2. Garnet gneissThese xenoliths are very rare and show a typical gneissose texture.

The mafic layers contain almandine garnet, together with amphiboleand biotite. Garnets are zoned with Mg-rich cores and more Fe-richrim compositions. The felsic layers are rich in quartz and feldspar.

4.1.3. Granite and granodioriteThese rock types are also rare as xenoliths and typically comprise

varying amounts of plagioclase (An52–56), alkali feldspar and quartz,

Fe as FeO. O= F, Cl is an adjustment for excess oxygen calculated due to the presence of

) (11) (9) (10) (9) (2)

ph amph amph amph amph bio

.84 41.62 40.25 41.59 40.96 37.1030 2.12 2.61 1.92 1.97 4.3232 12.00 13.35 13.91 14.24 14.5905 0.15 0.06 0.11 0.04 0.06.79 12.62 12.29 12.37 10.86 12.3544 0.21 0.22 0.17 0.10 0.16.83 14.32 13.19 13.11 14.08 16.92.22 11.89 12.03 11.87 12.18 0.0315 2.42 2.37 2.58 2.60 1.1041 0.80 0.79 0.70 0.97 8.39

nd 0.03 nd nd 0.2505 0.05 0.02 0.01 nd 0.05.39 98.20 97.20 98.33 98.00 95.3301 0.01 0.02 0.12.38 98.19 97.19 98.33 98.00 95.21

Table 3Representative clinopyroxene analyses, Egmont xenoliths. Mg# = magnesium number, En = enstatite. Rock type numbers as for Table 1. FeO* is total Fe as FeO.

Rock type (1) (3) (4) (2) (7) (9) (11) (12) (14) (15)

Mineral cpx cpx cpx cpx cpx cpx cpx cpx cpx cpx

SiO2 51.08 51.77 51.23 51.76 52.53 48.1 52.89 53.03 53.03 54.05TiO2 0.07 0.12 0.11 0.23 0.36 0.97 0.22 0.13 0.25 0.24Al2O3 0.65 1.07 1.57 2.04 1.64 6.30 1.42 1.35 1.95 1.89Cr2O3 nd 0.04 0.04 0.06 nd 0.02 0.07 0.02 0.56 0.69FeO⁎ 8.67 14.00 12.95 8.00 7.74 7.14 7.3 4.54 4.29 3.24MnO 0.55 0.11 0.34 0.30 0.41 0.12 0.33 0.54 0.12 0.06MgO 12.61 8.97 9.95 14.29 14.45 13.14 15.54 14.89 17.02 16.70CaO 24.88 23.66 23.61 22.43 22.55 23.47 21.32 24.88 22.13 23.20Na2O 0.13 0.23 0.45 0.29 0.42 0.33 0.37 nd 0.18 0.29K2O nd nd nd nd nd nd 0.01 nd nd ndNiO 0.01 0.02 nd 0.03 nd 0.02 nd nd 0.03 0.03Total 98.68 100.16 100.38 99.52 100.24 99.75 99.53 99.39 99.59 100.42Mg# 0.72 0.53 0.58 0.76 0.77 0.77 0.79 0.85 0.88 0.9En 0.36 0.26 0.28 0.4 0.4 0.36 0.43 0.41 0.47 0.45

195K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

with greenmagnesiohornblende andminor amounts of brown biotite.Accessory minerals include apatite, zircon and titanite.

4.1.4. Mafic hornfelsThese xenoliths are light green to light brown in hand specimen

(very similar to some sedimentary xenoliths) and very fine grained tomicrocrystalline. In thin section they show a granoblastic texture andthe mineral assemblage clinopyroxene+calcic plagioclase (An80–93)+titanite+some sulphides, Fe–Ti oxides and minor amounts of wollas-tonite. Mineral grains are between 25 and 40 µm in diameter, but larger(400–800 µm) relict grains of plagioclase and clinopyroxene arepresentaswell (Fig. 2a). These are usually spongyor sieve texturedandare oftenembayed. Relict plagioclase has the same compositional range asplagioclase in gabbros (An80–90). Clinopyroxenes are diopsides, butdiffer from most other xenolithic clinoproxenes observed by having ahigher Fe content. Sulphides and Fe–Ti oxides occur mainly in areasclose to host rock contacts. Cross cutting sulphide veins occur in somesamples, as well as veins filled with plagioclase (An93).

4.1.5. Finely banded amphibolitic gneissThese xenoliths are typically very fine grained and strongly

foliated with alternating felsic and mafic layers. The individual layersare between 0.5 and 1 mm thick and irregular banding, schlieren andsmall folds are common (Fig. 2b).

Mafic layers are made up of 50–70% brown amphiboles (edenite,magnesiohastingsite and titanian magnesiohastingsite), 10–30%plagioclase (An58–75) and clinopyroxene (diopside to augite), and 5–

Table 4Representative glass analyses, Egmont xenoliths. Rock type numbers as for Table 1.FeO* is total Fe as FeO.

Glass type (11) (7) (1)

Trachytic Rhyolitic Rhyolitic

SiO2 63.25 73.94 80.47TiO2 0.612 0.267 0.1Al2O3 18.83 12.79 8.63Cr2O3 nd 0.071 0.047FeO⁎ 1.89 1.77 1.48MnO 0.166 0.097 ndMgO 0.39 0.039 0.037CaO 1.122 0.902 0.486K2O 5.54 5.32 4.88Na2O 6.39 3.4 2.21BaO 0.235 1.52 0.186Cl 0.207 0.086 0.136F 0.159 nd nd

98.791 100.202 98.662O=F,Cl 0.114 0.019 0.031Total 98.677 100.183 98.631

10% titanomagnetite. Some ilmenite and brown biotite are alsopresent. Larger amphibole grains are often embayed and decomposedto clinopyroxene, plagioclase and titanomagnetite. The felsic layersconsist of up to 50% plagioclase, 30–40% clinopyroxene, 5–10% apatiteand minor amounts of titanomagnetite and amphibole. Occasionallysulphide grains occur and the relatively thick felsic layers often haveclear glass in the centre of the layers and around mineral grains. Clearglass is sometimes also found in fractures that cut through thexenolith from the surrounding volcanic host.

4.1.6. Gabbros and ultramafic rocksThe xenoliths included in this group are the most common types

found in Mt Taranaki lavas. The term “gabbro” is here used to describemedium to coarse grained xenolith lithologies where the mainmineral phases are plagioclase, amphibole and clinopyroxene.Although amphibole may be more abundant than clinopyroxene itclearly replaces clinopyroxene and is not primary. Xenoliths aredescribed as “ultramafic” when containing less than 10% modalplagioclase.

Amongst all studied xenoliths, the most abundant rock type ismedium to coarse grained gabbro (~60%), followed by ultramaficrocks (~30%) that typically appear dark green to dark brown in handsample. Occasionally composite xenoliths occur, for example ultra-mafic types adjacent to or enclosed by a gabbroic xenolith. Texturesrange from completely non-foliated to strongly foliated and thefoliation is interpreted as a primary igneous feature rather thanmetamorphic because of the presence of cumulate textures andabsence of granoblastic textures or strain-induced recrystallisation.

4.1.6.1. Hornblende pyroxene gabbro. Although clinopyroxene is anessential mineral in the definition of gabbro, it is used here todifferentiate this xenolith type from hornblende gabbro. Hornblendepyroxene gabbro xenoliths occur as fine grained to coarse grainedvarieties. The modal mineral content is variable with 30–60%plagioclase (An80–90), 20–35% brown amphibole (mainly titanianmagnesiohastingsite), 10–35% clinopyroxene (diopside), 3–15% tita-nomagnetite with accessory minerals apatite and zircon. Textures aregabbroic and a very common feature inmany rocks is the replacementof clinopyroxene by amphibole. Clinopyroxene grains are mantled bybrown amphibole and often embayed and shaped irregularly (Fig. 2c).Some xenoliths contain small ultramafic inclusions. The minerals aretypically more coarse grained than their gabbro host and can befragmented. One particular xenolith, T95 2×3, shows even morecomplex textures which indicate metasomatism of an originalcumulate and subsequent decompression reaction of the amphibole.The xenolith contains oscillatory zoned plagioclase, as well asunzoned and normally zoned plagioclase and different sized

Fig. 2. Egmontxenolith fabrics and reaction textures. a)Relictplagioclase grainwithinassemblage of veryfinegrained cpx+plag+tmt inmafic hornfels (crosspolarised light). b) Irregularfolds and schlieren structures in amphibolitic gneiss (plane polarised light). c) Typical hornblende pyroxene gabbro, embayed cpx being replaced by brown amph (plane polarised light).d) Hornblende pyroxene gabbro T95-2x3, olivine crystal cluster with surrounding layer of px and tmt, rimmed by layer of bio and amph (plane polarised light). plag = plagioclase,ol = olivine, px = pyroxene, cpx = clinopyroxene, opx = orthopyroxene, amph = amphibole, bio = biotite, sp = spinel, tit = titanite, tmt = titanomagnetite, and ap = apatite.

196 K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

amphibole and clinopyroxene grains. Most clinopyroxenes aremantled by amphibole, but some larger amphibole grains also showreaction to clinopyroxene and titanomagnetite along their edges. Inthe same xenolith small patches of clear or brown glass are presentclose to some large amphiboles and a few crystal clusters of olivineoccur. The olivine is mantled by a layer of small clinopyroxenes,orthopyroxenes and titanomagnetite, which are in turn mantled bybiotite and amphibole (Fig. 2d).

4.1.6.2. Hornblende gabbro. Hornblende gabbro xenoliths are lesscommon than hornblende pyroxene gabbro xenoliths and contain 40–50% plagioclase (An59–74), 40–60% green (magnesiohornblende toedenite) or brown (edenite) amphibole, 5–15% titanomagnetite andrare ilmenite. Accessory mineral phases are apatite, titanite andsulphides. In some xenoliths, areas with different grain sizes occurand in one of these glass films occur around grains in the transitionarea between fine and coarser grained zones.

4.1.6.3. Hornblendite and clinopyroxenite. The majority of theseultramafic xenoliths are hornblende pyroxenites and pyroxenehornblendites. Modal contents of either mineral range between 20and 60% with additional 8–20% titanomagnetite. Some xenoliths

contain up to 5% plagioclase, predominantly present as small,interstitial grains. Fine grained and coarse grained cumulate rocktextures occur and one particular hornblendite xenolith is made up oflarge amphibole crystals up to 10 cm long. In some hornblendite andclinopyroxenite xenoliths, clinopyroxene and amphiboles appear asan irregular intergrowth, in others clinopyroxene and titanomagne-tite are poikilitically enclosed by amphibole (Fig. 3a). Chemically,amphiboles are titanian pargasite and pargasite; the coarse grainedvarieties have the highest Mg numbers observed and also show thehighest Al2O3 contents. The clinopyroxene is diopside.

4.1.6.4. Meta-hornblende pyroxenite. This type includes an unusualxenolith with textures that resemble hornblende pyroxenites but isquite different in terms of mineral assemblage, chemistry andmicrotextures. In hand specimen the xenolith is mostly very finegrained and light green with a visible network of irregular felsic veins,some of which contain apparently brecciated larger crystals ofamphibole and clinopyroxene. In thin section these have beenpartially broken down to form smaller amphibole and clinopyroxenecrystals. The light green areas in the rock comprise medium to coarsediopsidic clinopyroxenes (Fig. 3b) that have been partially orcompletely replaced by brown amphibole and in parts also replaced

Fig. 3. Metasomatic reaction textures and troctolite fragment in Egmont xenoliths. Abbreviations as in Fig. 2. a) Amph poikilitically enclosing subhedral cpx and tmt in hornblendepyroxenite (plane polarised light). b) Meta-hornblende pyroxenite, showing coarse grained cpx partially replaced by amph and partially broken down to finer grained px towardsthe more felsic areas (plane polarised light). c) Troctolite patch within fine grained clinopyroxenite. Plag shows even polysynthetic twinning (cross polarised light). d) Green Al-sp“spots” within clinopyroxenite. Al-sp is enclosed by poikilitic plag, ap and cpx (plane polarised light).

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by small subhedral orthopyroxene and clinopyroxene. Amphibolesare mainly magnesiohornblende, but some smaller grains are edenite.The felsic veins contain plagioclase (An56) and some clinopyroxeneand titanomagnetite. In some parts of the xenolith, clear glass witheuhedral to subhedral small ortho- and clinopyroxene, amphibole andplagioclase crystals is present.

4.1.6.5. Olivine-bearing ultramafic xenoliths (olivine hornblende clinopyr-oxenite and troctolite).Olivine-bearing rocks are rarely found as xenolithsin Mt Taranaki lavas and only in association with other xenolith typessuch as gabbros and hornblende pyroxenites. An exception is olivine-bearing xenoliths in basaltic andesite and these are described as aseparate group.

Olivine hornblende pyroxenite contains up to 30% modal olivine,but the olivine is not evenly distributed. Instead, medium grained andinclusion-rich clinopyroxenes in hornblende pyroxenite have beenpartially replaced by amphiboles and in places grade into a finergrained assemblage comprising inclusion-free clinopyroxene, olivineand titanomagnetite. One xenolith is zoned with a fine grained corecontaining olivine, clinopyroxene and amphibole surrounded bycoarser grained clinopyroxene and amphibole towards the host rockcontact. The finer grained clinopyroxenes have less CaO and slightlyless Al2O3 than medium and coarse grained clinopyroxenes in same

xenolith. Olivines are themost iron rich (Fo73–75) amongst the studiedrocks and contain the least amounts of NiO (b0.01wt.%) of thoseanalysed.

Another xenolith containing olivine is troctolitic, occurring asirregular shaped but discrete coarser grained areas within very finegrained clinopyroxenite (Fig. 3c). Green Al-spinel occurs throughoutthe surrounding clinopyroxenite in “spots” and larger, subhedralgrains are enclosed poikilitically by plagioclase. In contact with theclinopyroxenite, a greyish rim of clinopyroxene and apatite isdeveloped (Fig. 3d). Oxide phases in troctolite are magnetite andilmenite. Within the clinopyroxene-rich parts of the rock, ilmeniteand sulphides occur. Plagioclase in troctolite and around spinel isalmost pure anorthite (An98–100). Clinopyroxenes are Ca-rich diopsideand have high Mg numbers (0.83–0.86), but relatively low Cr2O3 (0–0.07 wt.%); especially compared to wehrlite and pyroxenite xenolithsfrom basaltic andesite lavas. Olivines are Fo77–78, very Ca-rich(N0.15wt.%), with very low NiO concentrations.

4.1.6.6. Xenoliths in basaltic andesite. These xenolith types have beengrouped separately because they solely occur in basaltic andesite hostrocks. Basaltic andesite containsmany angular fragments of other andes-ite lithologies, together with abundant small, olivine-bearing ultramaficxenoliths, xenocrysts and olivine-clinopyroxene crystal clusters.

Fig. 4. Composition of glasses from xenoliths showing three distinct compositionalgroups. High silica partial melts are from sediment xenoliths, gabbroic and gneissicxenoliths contain glasses of rhyolite composition while trachytic glasses occur inultramafic hosts. A glass inclusion in cpx from a basaltic andesite is also trachytic.

Table 5Estimated crystallisation pressures of amphiboles in Egmont xenoliths from totalnumber of Al cations per formula unit (based on 23 oxygen), using the geobarometer ofHollister et al. (1987).

Xenolith type Hollister et al.

P (±1 kbar)

Granodiorite 0–3.0Meta-hbl–pyroxenite 0–3.0Hbl–gabbro (green amph) 1.3–3.5Phenocrysts in andesite 3.8–6.8Hbl–px–gabbro 4.6–7.2Phenocrysts in basaltic andesite 5.6–7.9Ol–hbl–pyroxenite 5.3–8.7Hornblendite and px–hornblendite 7.2–8.6Hbl–pyroxenite (fine grained) 7.5–8.3Hbl–pyroxenite (coarse grained) 8.4–9.2

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Type I (Frey and Prinz, 1978) pyroxenites and dunites are rare andonly found in basaltic andesites. The most primitive xenoliths found inEgmont lavas are dunite and wehrlite and their mineral assemblage ofolivine+Cr-diopside+Cr-spinel confirms that they are Type I. Dunitecomprises 95% olivine (Fo90), 2–4% clinopyroxene (diopside) and 1–2%small Cr-spinel inclusions in clinopyroxene. Wehrlite and olivinepyroxenite have the same mineral assemblages, but different modalabundances (up to 20% clinopyroxene in wehrlite and up to 10% olivinein olivine pyroxenite) and Cr-spinel is not always present. Olivine inwehrlite is Fo87 and Fo82 in olivine clinopyroxenite. The olivines arehighly magnesian (FoN80) and also have high NiO contents (N0.25%).Olivine in dunite has the highest NiO contents (0.35–0.4wt.%), followedby c. 0.25wt.% NiO in wehrlite and between 0.1% and 0.2wt.% NiO inolivine clinopyroxenite. Clinopyroxenes in dunite and wehrlite havethe highest Mg numbers (N0.88) and also the highest Cr-contents (upto 0.70wt.% Cr2O3) in the Egmont xenoliths. Cr-contents in clinopyr-oxenes from olivine pyroxenite and pyroxenite are lower, as are NiOand Fo-contents in olivines in the same xenoliths. Olivine and clino-pyroxene react to form pargasitic amphibole in the amphibole-bearingxenoliths. Multiple generations of amphibole and clinopyroxene arepresent in some rocks, reflecting a complex history of metasomatism.

Fine grained gabbroic xenoliths consist of up to 50% Fe-richclinopyroxene, 45% plagioclase and 5% titanite. The mineral assem-blages, textures and chemistries are nearly the same as for mafichornfels xenoliths, but lack relict mineral grains and sulphides. Fur-ther, plagioclase in the mafic hornfels is An80–90 whereas it is An64–69

in fine grained gabbro.

4.2. Glass in xenoliths

Clear or brown glass occurs in some xenolith samples (Table 4). Itcan be found in fractures that cross-cut xenoliths, in areas close to thecontact with the volcanic host rock, between mineral grains (olivinehornblende pyroxenite, meta-hornblende pyroxenite), as light brownfilms around grains in the contact area between fine and coarsegrained areas within one rock type (e.g. hornblende gabbro) or infelsic rock layers (finely banded amphibolitic gneiss). One thermallymetamorphosed sedimentary xenolith contains up to 50% clear glasswith small euhedral crystals of plagioclase and clinopyroxene. Thisglass has the highest SiO2 concentrations (~80wt.% SiO2) of allanalysed glasses. Most glasses are rhyolitic in composition and richin K2O (4.1–6.2wt.%); the total range of silica is from 62.9 to80.6wt.% SiO2, with most analyses falling between 69 and 74wt.%SiO2. Trachytic glass occurs as inclusions in phenocrysts in basalticandesites and interstitial glass between mineral grains in olivinehornblende pyroxenite (Fig. 4).

4.3. Pressure conditions of xenoliths

The geobarometer based on total Al contents in amphibole wasapplied to amphibole-containing xenoliths (Hollister et al., 1987). Thebarometer is experimentally calibrated to the mineral assemblagequartz+K-feldspar+plagioclase+biotite+Fe-Ti oxide+titaniteover a pressure range of 2–8 kbar with an estimated error of±1 kbar. The required conditions for this geobarometer are not metfor Taranaki rocks as K-feldspar was not identified microscopically inthe xenoliths and quartz was only present in some; the calculatedpressures may therefore be overestimated by up to 1.5 kbar (Hollisteret al., 1987; Johnson and Rutherford, 1989; Anderson and Smith,1995). The results show the range of pressure estimates for thedifferent xenolith groups (Table 5).

5. Discussion

The xenolith suite from Egmont volcano comprises a diverse set ofrocks that represent the variations in lithology of the crustal sequence

beneath the volcano. Most are Type II xenoliths (Frey and Prinz, 1978)and show textural evidence of metamorphism and metasomatismoverprinting original magmatic cumulate textures. A small groupwithin the suite is Type I and almost exclusively found in volcanics ofbasaltic andesite composition. The xenolith suite can be grouped intofour components that represent different crustal levels beneathEgmont volcano (Fig. 5).

5.1. Components of the crust beneath Egmont volcano

5.1.1. Supracrustal rocksSeismic studies and drilling of oil exploration wells have showed

that the uppermost 6 km of the Taranaki Basin is filled with asequence of Tertiary sedimentary rocks. Xenoliths from these unitsare largely unmodified sedimentary lithologies and sub-volcanics thatcan be identified in either regional surface outcrops or oil well drillcores. They include Kapuni Formation quartz sandstones, Tertiaryquartzofeldspathic sandstones and siltstones together with andesitevolcanics from the upper part of the volcano. Fine grained sedimentxenoliths show evidence of thermal alteration in both inorganic andorganic constituents (Collen et al., 1985) and rarely partial melting toproduce melts with up to 80% silica (Table 4, Fig. 3).

5.1.2. Upper to mid-crustal basementThese comprise regionally metamorphosed and non-metamorphic

basement to the Cretaceous and Tertiary sediment cover. Thebasement below 6 km depth is considered to comprise plutonic and

Fig. 5. Al(IV) in hornblende from xenoliths, andesite and basaltic andesite. See text for descriptions of components 1–4. Hornblende in ultramafic xenoliths (4) has the highest Al(IV)and therefore inferred pressurewhile gabbroic lithologies (3) have intermediate Al(IV) or pressure and lowpressures are indicated for plutonic andmetamorphic lithologies (1 and2).

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metamorphic rocks of the Median Batholith (e.g. King and Thrasher,1996; Mortimer et al., 1997). Granodiorite, granite and gneissxenoliths are the only studied rocks that contain quartz andsignificant amounts of biotite and lithologies are similar to thosedescribed from the Median Batholith (Wodzicki, 1974; Challis et al.,1994; Rattenbury et al., 1998). Gneiss from the Median Batholith alsocontains almandine garnet, which is commonly found in regionalmetamorphosed sedimentary rocks, and only occurs in the gneiss rocktype from the Egmont xenolith suite. Further, low Al- and Ti-contentsin amphiboles from granodiorite xenoliths indicate crystallisation orrecrystallisation at lower crustal pressures than most other amphi-bole-bearing xenoliths. This is consistent with pressure estimatesfrom the Al in amphibole geobarometer that yields values up to 3 kbaror 10–12 km depth. The granodiorite, granite and gneiss xenoliths aretherefore interpreted as fragments of basement Median Batholithrocks.

5.1.3. Mid to lower crustal cumulates and granulitesThis group comprises mafic hornfels, fine grained gabbro and

finely banded amphibolitic gneiss xenoliths. The mineral assemblagein mafic hornfels includes wollastonite, which can form duringthermal or contact metamorphism as well as in amphibolite togranulite regional metamorphism (Deer et al., 1992). Its occurrenceindicates the presence of carbonate in the protolith. The granoblastictexture with randomly oriented anhedral to subhedral grains andrelict minerals in the mafic hornfels is consistent with thermal ratherthan regional metamorphism. Clinopyroxenes are intermediatebetween diopside and hedenbergite, which is common for highgrade metamorphosed mafic igneous rocks (Deer et al., 1992). Theabsence of amphibole in the mineral assemblage and the presence ofrelict plagioclase (An80–90) and clinopyroxene suggest an anhydrousgabbroic protolith, possibly a medium grained to fine grainedcumulate gabbro that had been metasomatised by CO2-bearing fluids.It could be associated with the Median Batholith, but could also befrom the Egmont volcanic–plutonic system. Fine grained gabbrofound as xenoliths in basaltic andesite has essentially the samemineralogy as this mafic hornfels, apart from a slightly coarser textureand lacking relict grains, and has a similar mineral chemistry. Mafichornfels may therefore be the more intensively metamorphosedequivalent to the fine grained gabbros.

Finely banded amphibolitic gneiss xenoliths comprise mafic (clin-opyroxene+amphibole) and plagioclase (An58–75) layers. Clinopyrox-enes are augites and diopsides and the latter are likely reaction products

of amphibole breakdown. Amphibole compositions encompass a simi-lar range to those in andesites and gabbros and show evidence ofdecomposition to a plagioclase+clinopyroxene+titanomagnetite as-semblage. Biotite, orthopyroxene and amphibole mantling clinopyrox-ene indicate probable metasomatism by hydrous fluids and reactionwith the high silica melts that are present (Kovács et al., 2004). Onexenolith is cut by fractures that offset the mafic–felsic layering by a fewmillimetres. In many places, these fractures contain clear glass andeuhedral pyroxene and plagioclase crystals. The presence of fracturessuggests high differential stress causing brittle deformation in the lowercrust that has facilitated entrainment of wall rock in magmas (O'Reillyet al., 1989). Estimated pressures from total Al in amphibole range fromapproximately 2.5–7.5 kbar and are comparable to results fromphenocrysts in andesites, hornblende gabbros and hornblende pyrox-ene gabbros (Fig. 5). We interpret this petrographic type as originallycumulate gabbro in the mid to lower crust that has been subject to atleast amphibolite facies metamorphism and associated deformation.Later metasomatism and reaction with siliceous partial melts formedorthopyroxene and biotite and late metasomatism also involvedsulphides.

5.1.4. Gabbros and ultramafic xenolithsHornblendite and hornblende pyroxenite xenoliths contain

amphiboles and clinopyroxenes that have high Al2O3 contents.Pargasitic amphibole occurs both as a cumulus and postcumulusphase and amphibole-alteration of clinopyroxenes is also observed.Residual clinopyroxene surrounded by fine grained hornblendeindicates that previously the rock was more clinopyroxene-rich andthe clinopyroxene has reacted to form amphibole. Modal abundancesand mineral chemistry of plagioclase and clinopyroxene in gabbroicxenoliths are comparable to those of phenocryst/xenocryst assem-blages in Egmont andesites and these gabbros can be regarded ascumulates from andesite magmas that have stalled at various levels inthe crust. Pressure estimates from Al in hornblende (Table 5) point tocrystallisation at mid-crustal pressures. Most of these cumulatexenoliths contain evidence of hydrous metasomatism in widespreadreaction of clinopyroxene to amphibole. Most amphiboles in the rockalso show signs of decompression reaction to clinopyroxene andtitanomagnetite plus melt which, forms glass containing fine,elongated clinopyroxene and plagioclase crystals.

The meta-hornblende–pyroxenite appears to have originally beena coarse to medium grained pyroxenite or hornblende pyroxenite.Metasomatism has replaced the original clinopyroxene with fine

200 K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

grained orthopyroxene and clinopyroxene. Earlier formed amphibolesalso started to break down to form clinopyroxene and titanomagne-tite along felsic veins similar to those observed in finely bandedamphibolitic gneiss xenoliths.

For the most part the gabbros and ultramafic xenolith types arecognate cumulate xenoliths, which have also been described fromother arcs, for example the Aleutians (Conrad and Kay, 1984) and theLesser Antilles (Arculus and Wills, 1980). However, the foliatedgabbroic rocks in this category may be an exception and an origin inthe Median Batholith cannot be entirely excluded, as it also comprisesarc-related calc–alkaline plutonic rocks. An example from a drill holein the Tasman Sea about 70 km southwest of Mt Taranaki has beendescribed as hornblende diorite and correlated with igneous rocksfrom the Rotoroa Complex (within theMedian Batholith) in the SouthIsland of New Zealand (Wodzicki, 1974). The petrographic descriptionof this rock resembles those referred to as hornblende gabbro in thisstudy. Hornblende gabbros found as xenoliths could therefore includemafic granulites from both earlier magmatism and earlier under-plating in the Egmont magmatic system and differentiating thecumulates from granulites is difficult (Chen and Arculus, 1995;Wysoczanski et al., 1995).

5.2. Comparison with fossil arcs

There are few exposures of fossil arc systems in which thecomplete architecture of the structures and petrological relationshipsof the mantle to supracrustal sequence are exposed. The most wellknown are the Kohistan section in northern Pakistan (Bignold et al.,2006; Garrido et al., 2006, 2007; Dhuime et al., 2007, 2009) theTalkeetna arc in south-central Alaska (Pflaker et al., 1989; DeBari andSleep, 1991; Greene et al., 2006) Darb Zubayda in Saudi Arabia (Quick,1990) and the Hokkaido section in Japan (Takashima et al., 2002).

The two best described sections are the Kohistan Arc and theTalkeetna Arc complexes. The Cretaceous Kohistan section is inter-preted as an oceanic arc that was sutured to Asia at about 100 Ma(Bignold et al., 2006; Dhuime et al., 2009). This Arc complex can besubdivided into six units (Dhuime et al., 2009). The Jijal Complex isthe stratigraphically lowest unit with a lower ultramafic and uppermafic part which spans the mantle/crust boundary. It comprisesdunites, wehrlites and pyroxenites grading up to garnet-bearinggabbros or “garnet granulites” with some younger granitic intrusives(Dhuime et al., 2007; Garrido et al., 2007). The Patan-Dasumetaplutonic complex comprises a thick sequence of metabasicrocks (gabbros and gabbronorites) metamorphosed and deformedunder amphibolite facies conditions (Bard, 1983) and represents themain constructional phase of the oceanic arc. Lithologies representoriginal laccoliths interlayered with volcanic/volcaniclastic units orremnants of oceanic crust (Bard, 1983; Bignold et al., 2006). The Jaglotand Utror–Chalt meta-sediments and metavolcanics, and Yasin Groupvolcaniclastics, were emplaced in arc-related basins, also during themain phase of arc construction. The Chilas complex, comprisingdominantly mafic (gabbronorite) intrusives represents post-sutureintra-arc rifting (Garrido et al., 2006) while the Kohistan Batholithformed post-suture when the arc was continental and activity ceasedwhen the arc was obducted during the onset of the collision betweenIndia and Asia (Garrido et al., 2006).

The Early to Mid-Jurassic Talkeetna Arc accreted in the LateJurassic to Middle Cretaceous and is thought to have developed inoceanic crust (Pflaker et al., 1989). It also comprises 6 units; a basalresidual mantle harzburgite interfingering with an overlying pyrox-enite unit that in turn interfingers with the succeeding basalgabbronorite. The gabbronorite is overlain by a lower crustalgabbronorite that exhibits modal layering. The upper part of thesequence comprises mid-crustal plutonics (gabbros, diorites andtonalities) succeeded by basaltic to rhyolitic supracrustal volcanics(Greene et al., 2006). In both the Talkeetna and Kohistan Arcs,

obduction has exposed relatively complete arc sections from mantleto crust of mature arc sequences.

Both fossil arcs therefore exhibit a similar overall architecture of anuppermost mantle sequence of ultramafic rocks, succeeded by lowercrustal gabbroic rocks, lower to mid-crustal layered gabbroic unitsand mid-crustal mafic to silicic intrusives. The upper part of each Arccomplex comprises upper crustal volcanics.

The xenolith lithologies identified from Egmont are similar to thelithologies identified from the fossil Kohistan and Talkeetna arcs.Ultramafics are represented by peridotites, pyroxenites and duniteand the Al-in-hornblende data indicate the highest pressures offormation for these rock types (Fig. 5). Lower to mid-crustal rocks aredominated by gabbroic compositions (hornblende gabbros) withhornblendites and hornblende pyroxenites. Thus the Egmont ultra-mafic and gabbroic xenoliths can be interpreted as representing thelithologies of the crust/mantle boundary to mid crust under thevolcano. There are also superficial similarities with the fossil arcs inthe presence of meta-igneous lithologies (amphibolitic gneisses, somewith garnet, and siliceous plutonic rocks) but these are inherited fromthe older Median Batholith rocks that form the basement. BecauseEgmont represents the crustal section of a single volcano, isolatedfrom themain arc in a rear arc setting of a relatively young arc, and thevolcanic history spans only 0.13 Ma, the crustal section at Egmontdoes not exhibit the degree of arc maturity shown by the Kohistan andTalkeetna Arcs and thick volcanics and volcaniclastics are also lackingat Egmont.

5.3. Magmatic processes and crustal structure

The Egmont xenolith suite represents a cross section of the sub-volcanic crustal structure beneath Taranaki. Basaltic andesites haveuniquely sampled depleted upper mantle (Type I xenoliths) whichsuggests they are sourced from deep within the magmatic plumbingsystem. Andesites, however, carry a xenolith cargo dominated by typeII cumulates and granulites from the lower crust. Pressure estimatesfrom Al-in-hornblende geobarometry are broadly consistent withlower pressures recorded from the Type II rocks (Fig. 5). This clearlyindicates different source areas for the two magma compositions.

The model of a lower crustal “hot zone “ for arc magma genesisproposed by Annen et al. (2006) is a useful concept with which toexamine the processes involved. In this model underplating by basalt ofthe base of the crust raises the geotherm sufficiently over time to causemelting of lower crustal and previously underplated material. Thebasaltic andesites in the Egmont system appear to originate from thelower part of the “hot zone”, within the lithospheric mantle. In contrastthemore silicic andesiteswere sourced fromthe lower crust, as reflectedin the composition of their xenolith cargo. A contributing factor to themore silicic composition of the andesites is generation of silicic partialmelts from the underplated lower crust. Price et al. (1999, 2005) haveargued that andesites and rhyolites are formed by this mechanism anddiffer only in that andesites carry a large “crystal cargo” compared withthe low crystal content of rhyolites. The groundmass glass compositionsof Egmont andesites (Platz et al., 2007) are of similar composition to theglassesmeasured in the xenoliths, consistentwith the Price et al. (2005)model. One outcome of these processes is progressive “andesitisation”of the middle crust as arc systems mature.

6. Conclusions

Xenoliths in the Egmont suite show widespread and pervasivereaction of anhydrous minerals with fluids to form hydrous mineralssuch as amphibole and apatite and reaction with siliceous partialmelts to form orthopyroxene. The mineralogy was further over-printed by decompression reaction of the hydrous minerals, partic-ularly amphibole, as magmas rise to the surface.

201K. Gruender et al. / Journal of Volcanology and Geothermal Research 190 (2010) 192–202

The Egmont xenolith lithologies tell a broadly similar story to thatfrom mantle and lower crustal xenoliths in alkali basalts in intraplatesettings of entrainment of upper mantle peridotites and lower crustalcumulates and granulites. The most significant difference is in thedegree of hydrous metasomatism in the Egmont xenoliths wherepervasive formation of amphibole occurs, reflecting the more fluid-rich environment of arc magma systems.

The overall crustal section defined by the Egmont comprises, fromtop to bottom, supracrustal sediments and high level cumulates, mid-crustal metamorphosed basement, lower crustal cumulates andgranulites and depleted upper mantle. Supracrustal xenolith litholo-gies are consistent with the known Tertiary sedimentary sequences inthe Taranaki Basin. Type I xenoliths are only present in basalticandesite host rocks and are sourced from depleted upper mantle,suggesting that this is also where the basaltic andesites accumulate.Type 2 xenoliths predominate in the more siliceous andesites andreflect accumulation in the lower crust. The separate source depths forthe two rock types can be explained by the Annen et al. (2006) “hotzone”model, where the andesites have much greater interaction withthe lower crust than the basaltic andesites. Xenolith textures,compositions and partial melt relationships are also consistent withthe Price et al. (2005) model which proposes that the andesites arederived from mixing of siliceous partial melts in the lower crust withcrystals residual in the source rocks. Such processes progressively“andesitise” the middle crust.

Acknowledgements

This paper is based on the Diplomarbeit of K. Gruender and wassupported by a Johannes Gutenberg University Masters Scholarshipand the DAAD (Deutscher Akademischer Austauschdienst). Theauthors acknowledge the long standing interest John Gamble hashad in Taranaki xenoliths and express their appreciation of helpfuldiscussions in the early stages of this project. Insightful discussionswith Richard Price also helped shape this paper. We would like toacknowledge the thoughtful and constructive reviews of the manu-script by Ian Graham and an unknown referee.

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