Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

24 Pages·2006·1.38 MB·English

JOURNALOFPETROLOGY VOLUME47 NUMBER9 PAGES1785–1808 2006 doi:10.1093/petrology/egl027
Leucocratic and Gabbroic Xenoliths from
Hualalai Volcano, Hawai‘i
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PATRICK J. SHAMBERGER AND JULIA E. HAMMER* e
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DEPARTMENTOFGEOLOGYANDGEOPHYSICS,UNIVERSITYOFHAWAII,1680EAST–WESTRD, m
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HONOLULU,HI96822,USA ttp
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RECEIVED APRIL 22, 2005; ACCEPTED APRIL 7, 2006; tro
ADVANCE ACCESS PUBLICATION JUNE 6, 2006 lo
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Adiverserangeofcrustalxenolithsishostedinyoungalkalibasalt Volcano, in particular, is noted for the abundance of rd
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lavas and scoria deposits (erupted (cid:1)3–5ka) at the summit of gabbroic and ultramafic xenoliths transported in the u
Hual(cid:1)aalai. Leucocratic xenoliths, including monzodiorites, diorites (cid:1)1800 AD Ka‘uu(cid:1)puu(cid:1)lehu alkali basalt lavas. A distinct rn
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and syenogabbros, are distinctive among Hawaiian plutonic rocks class of leucocratic, alkali feldspar-bearing xenoliths ls
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in having alkali feldspar, apatite, zircon and biotite, and evolved is hosted in alkali basalts erupted from summit rg
mineral compositions (e.g. albitic feldspar, clinopyroxene Mg- vents (Moore et al., 1987; Cousens et al., 2003). Such a
number 67–78). Fine-grained diorites and monzodiorites are differentiated plutonic rocks are rare for Hawaiian t S
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plutonic equivalents of mugearite lavas, which are unknown at volcanoes, and the xenoliths examined in this study are ria
Hual(cid:1)aalai. These xenoliths appear to represent melt compositions exceptional in that they are not associated with evolved ls D
fallingalongaliquidlineofdescentleadingtotrachyte—amagma alkalic magmas (Fodor, 2001). e
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type which erupted from Hual(cid:1)aalai as a prodigious lava flow and HighlyevolvedalkalicmagmaontheHawaiianislands artm
scoria cone at (cid:1)114ka. Inferred fractionating assemblages, tends to erupt in small volumes (Macdonald, 1963),
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MallEpLoiTntStomfoodrmeliantgio,npoyfrotxheenpeagreeonbtarroocmksetorfythaendleuwchocorlae-tircocxkennoolritmhss aGcacrocmiap,an19y8i8n;terFmreeydiaetteacl.o,m1p9o9s0it)i,onanladvaosc(cSuprenlagtleeri&n nt, U
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at (cid:1)3–7kbar pressure. This depth constraint on xenolith the post-shield alkalic period. However, none of iv
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formation, coupled with a demonstrated affinity to hypersthene- these generalizations is true of the Hual(cid:1)aalai trachyte. rs
normative basalt and petrologic links between the xenoliths and The emplacement of trachyte deposits on top of mafic ity
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the trachyte, suggests that the shift from shield to post-shield tholeiite lava and beneath capping alkalic lavas suggests f H
magmatism at Hual(cid:1)aalai was accompanied by significant deepen- that extreme magma differentiation occurred at the a
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ing of the active magma reservoir and a gradual transition from transition between the shield and post-shield magmatic a
tholeiitic to alkalic magmas. Subsequent differentiation of stages(Cousensetal.,2003).Mooreetal.(1987)suggested ii a
transitional basalts by fractional crystallization was apparently that the leucocratic xenoliths erupted from the summit t M
both extreme—culminating in >5.5km3 of trachyte—and rapid, vents were cumulate syenites related to the Pu‘u an
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at (cid:2)2.75 · 106m3 magma crystallized/year. Wa‘awa‘a (PWW) trachyte, which erupted at (cid:1)114ka. a
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Subsequent eruptions have produced exclusively mafic ib
magma, chiefly alkali olivine basalt (Moore et al., 1987). ra
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KEYWORDS:geothermobarometry; magma chamber; xenolith; cumulate; The intensive conditions and magmatic precursors o
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intensiveparameters to trachyte magma formation at Hual(cid:1)aalai are not well M
constrained because intermediate lavas are absent and a
INTRODUCTION the trachyte is virtually phenocryst-free, with the rch
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Crystalline xenoliths present in some Hawaiian lavas exception of sparse nepheline. Cousens et al. (2003) 9
offer the possibility of studying crystallization environ- suggested that trachyte was derived by shallow (3–7km) , 2
0
ments and processes that are otherwise inaccessible (e.g. crystallization of an alkalic parent magma, based on Pb 10

Fodor&Vandermeyden,1988;Gaffney,2002).Hual(cid:1)aalai isotope similarities between trachyte and alkalic basalt,
(cid:1) The Author 2006. Published by Oxford University Press. All
*Corresponding author. Telephone: [phone]. Fax: 808–956– rightsreserved.ForPermissions,pleasee-mail:journals.permissions@
5512.E-mail:[email protected] oxfordjournals.org
JOURNALOFPETROLOGY VOLUME47 NUMBER9 SEPTEMBER2006
the spatial distribution of leucocratic xenoliths solely even larger volumes ofcoevaltrachyte may underlie the
around summit vents, and the absence of peridotite capping alkalic basalts has been suggested on geophysi-
xenoliths within trachyte deposits. However, a shallow calevidence.Shallowsubsurfacetrachyteisimplicatedin
origin for these differentiated magmas contrasts with causingalow-amplitudegravityhighdisplacedfromthe
a model of post-shield magma differentiation in a rift zone (Kauahikaua et al., 2000), and generating a
deep-rooted ((cid:1)20km) magma chamber that is generally pronounced aeromagnetic low over the summit and rift
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accepted forMaunaKea(Frey etal.,1990).The Mauna zone regions (Moore et al., 1987). Direct evidence of o
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Kea model is supported by major and trace element laterallyextensiveburiedtrachyteisprovidedbysamples n
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trends requiring clinopyroxene fractionation. ofnepheline-bearingtrachyterecoveredfromwaterwell a
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This study examines xenoliths collected from young drill-holesattheNWtipofthemainriftzone,andblocks ed
((cid:1)3–5ka) spatter cones and ramparts at the summit in a maar deposit on the SE flank (Cousenset al., 2003). fro
of Hual(cid:1)aalai to better understand the formation of All the dated trachyte samples are younger (92.0±6.0 to m
trachyte and the evolving magmatic system of Hual(cid:1)aalai 107±9.8ka; Cousens et al., 2003) than the youngest http
during the shield to post-shield transition. The principal known tholeiite (<133ka; Moore & Clague, 1992), ://p
goals are to (1) determine which xenoliths, if any, suggesting that trachyte formed at the interface between e
represent liquid compositions; (2) relate xenoliths to the shield and post-shield stages of basaltic volcanism. tro
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magmatic affinities characterizing the various stages Intermediate-composition extrusives are significant by g
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of Hawaiian volcanism; (3) investigate the intensive their absence from Hual(cid:1)aalai. No lavas with compositions .o
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plizroaptieorntieosfotfhtehesymenaog-mxeantioclitshyss;temanddu(r4in)gevthaleuactreysttahle- b((cid:1)et6w1e–e6n4whtaw%aiSitieO()(cid:1)h4a5v–e5b0eewntrec%ognSiizOed2)atatnhdisvtroalccahnyote. ford
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possibility that the xenoliths constrain the conditions Plutonic mafic and ultramafic xenoliths representing u
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of trachyte formation. The primary data presented are several of the volcano’s magmatic stages are common a
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detailed petrography and compositional analyses of in the capping basalts of the flanks and summit of .o
mineralsandwhole-rocks.Theintensivethermodynamic Hual(cid:1)aalai. Most notable are the Ka‘uu(cid:1)puu(cid:1)lehu cobble beds rg
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parameters (e.g. P, T, PH2O) of trachyte differentiation of dunite, wehrlite and olivine clinopyroxenite, with t S
are then investigated using MELTS calculations, minor gabbro, troctolite, anorthosite and websterite e
clinopyroxene thermobarometry and basalt phase (Baloga et al., 1995; Guest et al., 1995; Kauahikaua ria
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equilibria. Finally, we propose a conceptual model of et al., 2002). Phase equilibria and fluid inclusion studies D
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the shield to post-shield transition that differs from the indicate that at least some of these xenoliths crystallized p
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Cousens et al. (2003) model of shallow differentiation at at moderate pressure (>2.5kbar; Roedder, 1965; rtm
Hual(cid:1)aalai,yetisconsistentwiththeMaunaKeamodelof Bohrson & Clague, 1988). The mafic xenoliths are e
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deepening magma storage accompanying the transition interpreted as cumulates formed in alkalic and tholeiitic t, U
fromtheshieldstagetothepost-shieldstage(e.g.Clague, Hual(cid:1)aalai magma chambers and fragments of the n
1987; Frey et al., 1990). underlying crust (Bohrson & Clague, 1988; Chen-Hong ive
et al., 1992). rs
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Geologic background f H
Hual(cid:1)aalai Volcano is located on the western coast of the SUMMIT XENOLITHS aw
itshlaenidslaonfdH,aHwuaai‘li(cid:1)aa(lFaiigl.a1st).eTruhpetethdiridny(cid:1)ou1n8g0e0stAvDolc(aBnaoloogna Lsmeuacllo-vcoralutimceantedphgraababnrodicspxaettneorlidthepsoasritesc(cid:1)o3n–s5pikcauoinusagine aii at M
etal.,1995;Guestetal.,1995;Kauahikauaetal.,2002).A near the summit (Fig. 1b; Moore & Clague, 1991). The
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thin layer of alkalic basalts, transitional basalts and xenolithsare unevenlydistributedatthe surface overan o
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less common hawaiite comprises (cid:1)97% of the subaerial area of about 2km in diameter around 19(cid:3)4103800N, L
edifice;theoldestbasaltsare(cid:1)25ka(Mooreetal.,1987). 155(cid:3)5202800W(Fig.1c), having erupted fromatleastfive ib
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Holocene eruptions occurred primarily along Hual(cid:1)aalai’s vents of similar age (Moore & Clague, 1991). ry
NWand SSEriftzones,although athirdpoorly defined The 264 xenoliths examined in this study range from o
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rift zone, containing less than 5% of the exposed vents, lessthan10gtonearly4kg,andaretypicallyroundedto M
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extends to the north (Moore et al., 1987). The oldest sub-angular.Many are coated by aweathered, vesicular rc
extrusivesexposedonHual(cid:1)aalaiconstitutetheprodigious rind of host lava, and some are iron-stained along h 1
volume of trachyte ((cid:1)5.5km3 magma) of the (cid:1)1.5km internalfractureplanes.Theyareclassified petrographi- 9, 2
diameter Pu‘u Wa‘awa‘a pumice cone and (cid:1)5km long cally as syenogabbro, diorite, monzodiorite, anortho- 0
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Pu‘u Anahulu lava flow (Moore et al., 1987), erupted at site, gabbronorite, olivine–gabbronorite, hornblende– 0

113.5±3.2ka (Cousens et al., 2003). The trachyte flow gabbronorite and poikilitic gabbro; the gabbronorites
escaped burial by subsequent lavas because of its dominatethesamplepopulationbothbyvolumeandby
extraordinary thickness (>275m). The possibility that mass (Fig. 1c). Modal and textural diversity were
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Fig.1. MapofHual(cid:1)aalaiindicatingtrachyteandsummitxenolith-bearingvents.(a)Hual(cid:1)aalai(Hua)islocatedonthewesternflankofHawai‘i, L
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surroundedbymorerecentlavaflows(b)fromMaunaLoa(ML).TrachyteexposuresonHual(cid:1)aalaiincludethePu‘uWa‘awa‘aobsidianandpumice ra
coneandthe Pu‘uAnahulutrachyteflow.Trachyteis alsofoundinwell holesandinmaardepositsasdescribedin Cousensetal.(2003).(c) ry
Xenolithsincludegabbronorites (GN), olivine–gabbronorites (OGN), diorites and monzodiorites (D),and syenogabbros (SG). Blackquadrants o
indicate which lithologies were present at each site. Xenolith componentry (mass and modal proportions) were determined at two sites (x). n M
GeologicalunitsandagesfromtheGeologicalMapofHual(cid:1)aalaiVolcano,Hawai‘i(Moore&Clague,1991).
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characterized in 84 thin sections of representative PETROGRAPHY 9, 2
xenoliths (Table 1). Mineralogical and petrographic The vast majority of xenoliths comprise two principal 0
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analysis was performed on 13 xenoliths representing series based on textural, modal and compositional 0

the principle lithologies and textural variations. Major affinities:(1) Leucocraticxenoliths,including monzodiorites,
and minor element whole-rock analyses were obtained diorites and syenogabbros (all alkali feldspar-bearing
from 15 xenoliths and two trachyte lava samples. lithologies); and (2) Gabbroic xenoliths, consisting of
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Table 1: Modal composition (vol.%) and textural measurements of select Hual(cid:1)aalai summit xenoliths
Leucocraticxenoliths Gabbroicxenoliths
SampleID: HM06 HM01a HM43 HM02a HM12 HM19 HM45 HM44 HM47 HM50a HM53
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Lithologies: MD MD D D D D SG GN GN HG OGN ow
Plagioclase 64 73 61 67 59 76 72 68 62 24 35 nlo
Alkalifeldspar 13 9 7 6 1 1 1 — — — — ad
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Clinopyroxene 2 8 17 20 20 11 17 18 22 52 14 d
Orthopyroxene 7 <1 — <1 <1 3 — 10 10 <1 28 fro
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Olivine — — — — 1 — <1 <1 1 — 21 h
Biotite 4 <1 4 <1 4 <1 5 <1 <1 1 — ttp
Amphibole — — <1 — — — <1 1 <1 20 — ://p
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Apatite 2 1 <1 <1 2 3 1 <1 <1 — <1 tro
Zircon <1 — <1 <1 <1 — — — — — — lo
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Fe–Tioxides y
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Magnetite 3 6 7 <1 8 3 3 2 4 3 — x
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Ilmenite 2 2 4 6 5 4 1 1 2 <1 <1 rd
Hematite <1 — — <1 — — — — — — <1 jou
Fe–sulfide — — <1 — — — — — — — 1 rna
Quartz 3 — — — — — — — — — — ls.o
Re-normalizedternarymodes rg
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Plagioclase 80 89 90 92 99 99 98 t S
Alkalifeldspar 16 11 10 8 1 1 2 e
Quartz 4 — — — — — — rials
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Grainsizes
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Plagioclase a
sqrt(l·w)[mm] 0.2 0.4 1.6 0.4 1.3 0.2 2.2 0.7 0.6 0.4 0.2 rtm
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l2/sw (10..42) (10..37) (11..95) (10..55) (11..46) (10..62) (33..12) (20..19) (20..28) (10..56) (10..51) nt, U
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Clinopyroxene: ive
2sqsrt(l·w)[mm] (00..11) (00..22) (11..89) (00..23) (00..44) (00..11) (11..10) (11..15) (00..88) (00..12) rsity o
l/w 1.6 1.7 1.8 1.6 1.5 1.8 1.5 1.5 1.4 1.4 f H
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LGiNth,ogloagbibesrofnoollroitwei;nHgGIU,GhoSrncblalessnidfiec–agtiaobnbsrcoh;eOmGeN(,LeolMivianietr–egaebtbarl.o,n2o0r0it2e);.AM,Da,nomrothnozsoidteios;ritPeG;,Dp,odiikoilriittiec;gSaGbb,rsoy;enHoGgNab,bro; ii a
hornblende–gabbronorite. t M
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gabbronorites,olivine–gabbronoritesandahornblende– Grain size and textural variations among diorites L
gabbro. All members of these two series are holocrystal- typically exceed differences between monzodiorites and ib
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line.Allcontactswithhostlavasaresharp,andminerals diorites(Table1),andbecausethelithologicdistinctionis ry
along boundaries are not compositionally zoned or somewhatarbitrary,theterm‘dioriticxenoliths’includes o
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thermally altered. Poikilitic gabbros, anorthosites and both types. Dioritic xenoliths are generally fresh and M
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hornblende-rich gabbronorite vein material are petro- have allotriomorphic textures (Fig. 2a). Anhedral rc
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graphically and compositionally dissimilar to the princi- plagioclase grains interfinger at consertal boundaries, 1
pal series, and are not discussed further. and are pervaded by spongy alkali feldspar (Fig. 2b). 9, 2
Leucocratic xenoliths contain abundant plagioclase Clinopyroxene and orthopyroxene both form distinct 0
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and clinopyroxene, and minor alkali feldspar, biotite, subhedral grains with no visible exsolution lamellae. 0

magnetite, ilmenite, apatite, ±orthopyroxene, ±olivine, Rare prismatic plagioclase phenocrysts ((cid:1)1–2mm) are
±amphibole (tr.), ±zircon (tr.) (Table 1). A single normally zoned. Fe–Ti oxides occur as isolated amoe-
monzodiorite (HM06) contains minor quartz (3vol.%). boid interstitial grains, inclusions in feldspars and
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Fig.2. Opticalphotomicrographs(a,c,d,e,g),andback-scatteredelectron(BSE)images(b,f,h)ofrepresentativexenolithtextures.Lithologies h
includediorite(D),monzodiorite(MD),syenogabbro(SG),gabbronorite(GN)andolivinegabbronorite(OGN).(a)Fine-andmedium-grained 1
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diorites and monzodiorites share similar allotriomorphic textures. (b) Alkali feldspar is found as spongy exsolution blebs in plagioclase. , 2
(c) Subhedral plagioclase laths form orthocumulate textures in syenogabbros. (d) Both diorites and monzodiorites share contacts with 0
gabbronorites. The contact is distinct, marked by fine-grained diorite and truncated gabbronorite grains. (e) Gabbronorites are composed of 10
denselypacked,roundedplagioclaseandclinopyroxenegrains.(f)Gabbronoritescontainanumberofexsolutionandlate-stagereactionfeatures,
includingpyroxeneexsolution,Fe–Tioxidelamellaeinpyroxenesandorthopyroxene-rimmedolivine.(g)Olivine–gabbronoriteshavepolygonal
granulartextures.(h)Crystallizedmeltinclusionsingabbronoritesaredominantlyolivineandorthopyroxene.alkfs,alkalifeldspar;ap,apatite;bt,
biotite;cpx,clinopyroxene;kaer,kaersutite;ol,olivine;opx,orthopyroxene;ox,Fe–Tioxide;pl,plagioclase;qz,quartz;vo,voidspace.
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clinopyroxene, and, in coarser dioritic xenoliths, as and plagioclase grains of the gabbronorite truncated at
localized symplectite concentrations. Biotite also occurs the contact (Fig. 2d). The reverse mineral cross-cutting
interstitially. Small (<0.1mm) euhedral apatite crystals relationships are notobserved. Biotite and amphibolein
are common inclusions in both sodic feldspars and gabbronorites are more abundant near the contact with
clinopyroxene. In coarser xenoliths (e.g. HM43), apatite diorite,suggestingthatthesemineralsaresecondary;the
prisms extend up to 3.5mm in length. grain size of the diorite matrix decreases by (cid:1)50%
D
Syenogabbros are characterized texturally as ortho- within (cid:1)1mm of the contacts. These contact relations o
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cumulates, in which large ((cid:1)3–9mm) tabular calcic and grain size variations indicate that diorite magma n
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plagioclaselaths form a loosebut contiguous framework interacted with near-solidus or subsolidus gabbronorite, a
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filling 55–70vol.% (Fig. 2c). Except for subhedral placing a relative age constraint on these magma types. ed
plagioclase phenocrysts, all mineral grains are anhedral. fro
Sodic plagioclase crystals contain patchy exsolved alkali m
feldspar.Plagioclasephenocrysts,Fe–Tioxidesandsodic ANALYTICAL TECHNIQUES http
plagioclase are enclosed within clinopyroxene oikocrysts X-ray fluorescence (XRF) spectrometry was performed ://p
up to (cid:1)1cm diameter (Table 1). using the University of Hawai‘i Siemens 303AS fully e
Gabbroic xenoliths contain abundant plagioclase automated, wavelength dispersive, XRF spectrometer. tro
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and clinopyroxene, and lesser orthopyroxene and Fe– Samples were crushed in a tungsten carbide (WC) g
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Ti oxides. Gabbronorite also contains minor amounts hydraulic splitter. Visibly altered and oxidized chips .o
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of olivine (<1vol.%), as well as amphibole and biotite were removed. The remaining chips were rinsed in fo
((cid:1)0–2vol.% combined); olivine–gabbronorite and deionized water and ground in either a WC ball mill or rd
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hornblende–gabbro contain substantial olivine and smallWCswingmillintoafinepowder.Duplicatefused u
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hornblende ((cid:1)20vol.%), respectively (Table 1). Plagio- buttons and a pressed powder pellet were prepared for a
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clase and clinopyroxene crystals, together composing samples following methods similar to Norish & Hutton .o
75–85vol.%, are nearly equant and form a closely (1969) and Chappell (1991). Samples were analysed for rg
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packed mesocumulate texture (Fig. 2e) in which majorandtraceelements(Sc,V,Cr,Co,Ni,Zn,Rb,Sr, t S
orthopyroxene and biotite-rimmed Fe–Ti oxides are Y, Zr, Nb, Ba, Pb and Th). Analytical uncertainty is e
interstitial. The interstitial and enclosing orthopyroxene estimatedfromrepeatanalysisofstandardsandiswithin ria
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constitutes (cid:1)10vol.% of the gabbronorites. Amphibole (cid:1)1% relative for major element oxides, except Na O D
2
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occurs as small blebs ((cid:1)10mm) within clinopyroxene (<9%). Minor element oxides TiO2, MnO, K2O and pa
grains, except in the hornblende–gabbro, where it rims P2O5 are accurate within (cid:1)5% relative. Analysed rtm
clinopyroxene. Plagioclase and clinopyroxene crystals standards W-1 and BHVO-1 are reported alongside e
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in gabbronorites contain abundant Fe–Ti oxides. Rare XRF results in Table 2. t, U
spherical, multi-phase inclusions ((cid:1)75vol.% olivine, Electron microprobe (EMP) analysis was performed n
(cid:1)20vol.% orthopyroxene, ±clinopyroxene, ±biotite, using the University of Hawai‘i CAMECA SX-50 five- ive
±magnetite)inplagioclaseappeartobecrystallizedmelt WD spectrometer electron microprobe. Accelerating rs
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inclusions (Fig. 2h). voltage was maintained at 15kV and beam current o
Gabbronoritesdisplayevidenceoflate-stagemagmatic between 10–30nA in a beam-regulated mode. Minerals f H
and subsolidus reactions (Fig. 2f). Most clinopyroxene susceptible to volatile loss (feldspars and biotite) were aw
glarmaienlslaec,oanstawinellfianseelpoanrgaallteelFoer–tThoipoyxriodxeebnleadeexssaolliugtnioend adnefaolycusesdedatsplootwe(r5–b1e0ammmcudriaremnetster()1.0–O2l0ivninAe), wFiteh–Tai aii at M
in two distinct orientations, suggesting crystallographi- oxides and amphiboles generally occur as small grains
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cally controlled (i.e. subsolidus) exsolution (Fleet et al., and required a focused spot (1–3mm diameter). When no
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1980).Olivinegrainsinthegabbronoritesarecommonly analysed, Na spectra were counted first. Counting time L
rimmed by orthopyroxene and rounded magnetite for all analyses was at least 30s on peaks, and lasted up ib
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blebs (Fig. 2f), indicating the incipient stages of olivine to60sforminorelements.Calibrationswereperformed ry
breakdown to form orthopyroxene and magnetite on natural and synthetic mineral standards. Reported o
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(Johnston & Stout, 1984). Olivine–gabbronorites are concentrations were calculated using a PAP correc- M
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fine grained (Table 1) and are the only xenoliths to tion procedure (Pouchou & Pichoir, 1988). Analytical rc
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exhibit a polygonal, annealed texture (Fig. 2g). accuracy was determined by comparing repeat analyses 1
of mineral standards (collected both before and after 9, 2
analysis of xenolith samples) against their published 0
Contact relations compositions.Major elements deviate bylessthan (cid:1)1% 10

Several xenoliths contain both gabbronorite and diorite relative, whereas minor and trace elements deviate by
lithologies.Gabbronoriteoccursascentimetre-scaleclots less than (cid:1)10%. Repeated analysis of pyroxene,
in a diorite matrix (e.g. HM01a/b), with clinopyroxene olivine, phlogopite and Fe–Ti oxide mineral grains in
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1
BHVO- 19 178 26 396 10.16<.22< 47 300 302 131 29 103 130 as<
Standards W-1 (3) .5246.108 .1511 .1129 .016 .664.1099 .225.064.013 .10076 7 96 22 189 22.22 .68 46 128 266 158 35 86 80 eported
r
AH10 (1) .4854.068 .2055 .628 .009 .771.1578 .081.022.004 .10072 .01 5 40 8 477 4.15<.20< 45 719 145 104 29 32 147 mitsare Downlo
Gabbronorites AH04aHF01HM44HM47 (2)(2)(2)(1) ....4382424049504937....511513085084 ....1259123918851881 ....18251802853919 ....026026012013 ....501588757766....954102112501251 ....308287179175....116076021018....106237005005 ....998710026999510047 ....05054004003(cid:4)(cid:4)(cid:4) 514644 2631913434 589288 576745448453 251342....21181515<<<....25252020<<<< 34415754.25356974< 190191160168 5134927072 22182421 1421176362 31228596 centrationsbelowdetectionli http://petrology.oaded from
n x
lavas Syenogabbros 11AH02aHM08AHX01HM45AH07 (2)(2)(2)(2)(1) .....52885135498248204563.....233283308201255 .....17011535149319991959 .....10431338152010141402 .....012019020013016 .....346266275408568.....6366666811110901 .....483436435302290.....127222196073083.....126098121018026 .....9993999510029995610063 .....006015002003018(cid:4)(cid:4)(cid:4)(cid:4) 4751581519 203585501127138 5677842017 549565566887785 526191522.....1528341516<<<....202223422<<<< 4045333346...3132322334<<< 1444051125281 10381002823307244 111012169 6192486286 2026253272 traceelementsinppm.Traceelementcoe-richdioriticxenoliths. at Serials Department, Ufordjournals.org
Table2Whole-rockcompositionsofselectedxenolithsandtrachyte: TrachyteDioriticxenoliths 11HM09HM10SampleID:PA13PWW13HM06HM19HM01a (majors):(2)(2)(2)(2)(2)(2)(2)n .......627255705486544554255329SiO64292.......039035211213198216242TiO2.......1827174516771653176517161663OAl23.......tot45245988090411341142977OFe23.......028030009013010015014MnO.......043034335343175187371MgO.......061058483666455515686CaO.......654777490538550484493NaO2.......495500229101192281145KO2.......016016113114059071122PO25.......10041992599941002999801004910040SUM.......1310210110020004064008(cid:4)L.O.I. 133148—51615153Nb 9851055—370525362273Zr 5359—76627256Y 4127—536472502569Sr 119131—4164615Rb......8584—16224315<Th...77—2021320<<<Pb 515—24262830Co......3030—31303132<<<<<<Cr..3434—1332427140<<V 396316—4959231115599Ba 86—126812Sc 132195—48465249Zn 1918—20282625Ni SamplesanalysedbyX-rayfluorescence.Majorsreportedinwt%,1(detectionlimit).TraceelementsnotanalysedforsampleHM06.F niversity of Hawaii at Manoa Library on March 19, 2010
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JOURNALOFPETROLOGY VOLUME47 NUMBER9 SEPTEMBER2006
the xenoliths yielded 2s variations similar to repeat individual crystals. The majority of plagioclase crystals
analyses of mineral standards, indicating remarkable are homogeneous. Exceptions are normally zoned and,
homogeneity of these phases. Thus, mineral composi- in these cases, core compositions are reported. The
tions are reported as averages of several spots on complete electron microprobe dataset is available as
Electronic Appendix 1, available at http://www.
petrology.oupjournals.org/.
D
o
w
n
lo
ANALYTICAL RESULTS a
d
e
Whole-rock compositions d
Petrographically defined lithologies are distinguished fro
m
by discrete bulk-rock MgO contents: gabbronorites h
((cid:1)7.5–8.0 wt %), syenogabbros ((cid:1)4.0–6.0 wt %) and ttp
dioritic xenoliths ((cid:1)1.5–4.0 wt %; Table 2; Fig. 3). ://p
e
Diorites have SiO2 contents between those of trachyte tro
and alkalic basalt lavas, and are the crystalline equi- lo
g
valentsofmugearitelavas(Fig.4).Notably,dioriteshave y
bhaigshaltAs(l(cid:1)2O53–/1C6awOt%raMtiogsOb)estpwaenenar2a.n2geanindM3g.5O. Asimlkaillaicr .oxford
to the gabbronorites, but are otherwise dissimilar (e.g. jo
u
Al2O3, FeO*; Fig. 3). All leucocratic xenoliths are rn
a
classified as ‘alkalic’ according to the scheme of ls
Macdonald&Katsura(1964),whereasthegabbronorites .o
rg
are sub-alkalic (Fig. 4). a
Trace-elementconcentrationsvarybetweenlithologies t S
e
but are relatively constant within a lithology (Table 2). ria
Gabbronorites have lower concentrations of incompa- ls
tibletraceelements(Nb,Zr,Y,Rb)thanotherxenoliths, D
e
as well as alkalic, transitional and tholeiitic Hual(cid:1)aalai pa
lavas (Clague et al., 1980; Hammer et al., 2006). rtm
Syenogabbros have more variable trace-element con- en
tents than other lithologies, corresponding with signifi- t, U
cant variations in mineralogy (e.g. higher Sr correlates n
iv
with larger plagioclase mode). Dioritic xenoliths have e
rs
higher incompatible element (Nb, Zr, Y) concentrations ity
than alkalic, transitional or tholeiitic Hual(cid:1)aalai lavas o
(Clague et al., 1980; Hammer et al., 2006). Dioritic f H
a
xenoliths are depleted in Sc and enriched in Ba relative w
a
toHual(cid:1)aalaibasalts,andhavesimilarSrconcentrationsto ii a
the most evolved Hual(cid:1)aalai basalts. t M
a
n
o
Mineral compositions a
L
The compositional similarity of the dioritic whole-rocks ib
ra
is mirrored by similarities in constituent mineral ry
compositions (Table 3). The plagioclase, clinopyroxene o
n
M
a
Fig. 3. Whole-rock MgO variation diagrams. Analyses of xenoliths rc
and two new trachytes are compared against previously analyzed h
Hual(cid:1)aalai lavas (Macdonald, 1968; Clague et al., 1980; Moore et al., 19
1987; Wolfe & Morris, 1996; Cousens et al., 2003). Also shown are , 2
fields for the Laup(cid:1)aahoehoe series of Mauna Kea (West et al., 1988; 0
1
Frey et al., 1990; Wolfe & Morris, 1996), the H(cid:1)aaw(cid:1)ıı series of Kohala 0

(Feigenson&Spera,1983;Spengler&Garcia,1988;Wolfe&Morris,
1996), and the Honolua series of West Maui (Macdonald, 1968;
Sinton,unpublisheddata).Theseseriesrepresenttypicalevolvedalkalic
lavaseruptedfromlatepost-shieldstageHawaiianvolcanoes.
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SHAMBERGERANDHAMMER LEUCOCRATICAND GABBROIC XENOLITHS
14
Dioritic Xenoliths
12 ( Fe-rich Diorites)
Syenogabbros Trachyte
O 10 Gabbronorites D
K2 Trachyte benmoreite ow
n
aO + 2 68 mugearite A LTKHAOLLIECIITIC loaded fro
N hawaiite m
% basanite http
wt 4 Transitional ://p
Alkalic e
tro
2 picrobasalt Tholeiitic lo
andesite g
y
basaltic .o
0 basalt andesite xfo
40 45 50 55 60 65 rd
jo
wt% SiO u
2 rn
a
ls
Fig.4. Totalalkalisvssilicadiagramshowingxenolithsamplesinthecontextofthetholeiitic–alkalicaffinitydividingline(Macdonald&Katsura, .o
1964)andfieldsrepresentingpreviouslypublisheddataforHual(cid:1)aalaiwhole-rocks,includingtrachytes(Mooreetal.,1987,Clague&Bohrson,1991; rg
a
Cousensetal.,2003)andalkalic(Moore,unpublisheddata),transitional(Hammeretal.,2006)andtholeiiticbasalts(Clague,unpublisheddata). t S
e
and orthopyroxene in the mafic gabbroic xenoliths are However, olivine–gabbronorite contains both clino- ria
ls
correspondingly enriched in anorthite, enstatite and pyroxene (En Wo Fs ) and orthopyroxene D
52–56 38–42 5–6
e
diopside components, respectively. (En80–81Wo3Fs16–17) that are more magnesian than pa
pyroxenes in other gabbronorites. rtm
Feldspars
e
Monzodiorites and diorites contain plagioclase of Fe–Ti oxides nt, U
similar composition (An17–23Or5–1 and An15–37Or4–13, Titanomagnetite (Usp12–30) and hemoilmenite (Ilm65–90) n
respectively; Fig. 5; Table 3). Matrix plagioclase in in leucocratic xenoliths are poor in Cr2O3 (<0.7 wt %) ive
syenogabbro (HM45) is slightly more anorthitic (An and Al O (<3 wt %). These compositions are similar rs
32– 2 3 ity
40Or3–5); plagioclase phenocrysts are normally zoned to titanomagnetite (Usp12–20, Mg-number 7–13) and o
and more An-rich (An41–67Or1–4). Plagioclase in gab- hemoilmenite (Ilm76–85, Mg-number 14–24) in gabbro- f H
bronorites are also normally zoned, but is more norites (Table 3). Fe–Ti oxides in olivine–gabbronorite aw
lainthosr.thAitilcka(lAi nf5e0l–d8s1pOarr1–c2o)mthpaonsitiinontsheinleuaclolcrlaeuticcoxcreantoic- a6r0e–6e3n,richheemdoiinlmbenoitthe CCrr-(tnituamnobmerag6n2et–i6te7)Cra-nndumMbegr aii at M
xenoliths are similar (An Or ). (titanomagnetite Mg-number 16–20, hemoilmenite
1–4 55–77 a
n
Mg-number 23–26). o
Pyroxenes and olivine a
L
Clinopyroxenes in monzodiorites (En Wo Phlogopite ib
47–49 41–43 ra
Fs9–10), diorites (En40–49Wo40–44Fs8–15) and syenogab- Phlogopite in diorite and monzodiorite is magnesian ry
bros (En Wo Fs ) span similar compositional (Mg-number 60.2–78.7) and Ti-rich (1.9–8.0 wt % o
46–49 43–45 8–9 n
ranges (Fig. 6). Orthopyroxene is slightly more Fe-rich TiO ),withmoderatefluorine(0.8–3.1wt%F)andlow M
in monzodiorites (En68–70Wo1–2Fs28–30) than in diorites chlor2ine (0.2–0.4 wt % Cl) contents. Phlogopite in arc
(En70–71Wo2Fs27–28). Olivine grains in syenogabbros syenogabbros is titaniferous (6.3–7.7 wt % TiO2) and h 1
(Fo69.8–71.2) have higher forsterite contents than those lower in fluorine (0.76–0.88 wt % F) than that in 9, 2
in diorites (Fo66.4–66.6), but concentrations of minor monzodiorite and diorite xenoliths (Table 3). 01
elements (MnO and CaO) are identical. Clinopyroxene 0

(En Wo Fs ), orthopyroxene (En Wo Amphibole
49–52 38–43 7–10 73–80 2–3
Fs18–24) and olivine (Fo73.3–79.9) in gabbronorites Calcic amphibole blebs in diorites (Mg-number 59.5–
are more magnesian than those in any syeno-xenoliths. 61.7) are less magnesian than their clinopyroxene hosts
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JOURNALOFPETROLOGY VOLUME47 NUMBER9 SEPTEMBER2006
IlmPhlog .402..357427..014123..003001..58397..044010..175193.003.030.917.310.027..963999.778 MagIlm ..647502..558025..1288073..672407..029041..371658 ..961989 ergrain.m,ilmenite;edusingthe
pilat
Alk-FsMag .650.449..193127.006..017865.034..000128.051.402.104 ..994939 .25.614 gabbronorite) 2OlPlag ..388503 .315 ..195031.029..409002..0031365.366.016 ..996996.786 .668.09 for(1–4)pointsmag,magnetite;(Wo)arecalcul Downloaded
no,Hawai‘i HM19(diorite) PhlogCpxOpxPlag ....370520532635...804029021....141091038223...001000001....124918187023...008045087....146148247001....001206107368....022061001852....926000000159.085.029....1000991992999...675732692..115279..41622.176.90 HM53(olivine– IlmAmphCpxOpx ...411513540....455406084055....029115258149...004040020....469119547124....072012014028....489130161292...116220139...211046005...137000000 .051....983974997996...657837804..53167..42028 Reportedvaluesareaveragesoclase;alk-fs,alkalifeldspar;Ferrosilite(Fs)andwollastinite http://petrology.oxfordjo from
(cid:1)Table3Representativemineralcompositions(wt%)forselectedsummitxenolithsfromHualaalaiVolca: HM06(monzodiorite)HM12(diorite) CpxOpxPlagAlk-FsMagIlmPhlogCpxOlPlagAlk-FsMagIlm .........521529630651384510366595641SiO2........TiO0380214743685170798644842............AlO11604023019110001312921025219030101523........CrO00100100500100100100300123.............totFeO843199031019870568122846289022007811459.........MnO031064028038008033070045076.............MgO152241000000106191167147330002000175377.........CaO209089438037000210005680024........NaO0550028394030270577082452........KO0000001161059040001051292.F152.Cl031.............Total9949921003993942960991993993999988949990.....Mg#756676708748665...Fs104298103...Wo41718417....An2091832712....Or6662160767 HM45(syenogabbro)HM47(gabbronorite) 11CpxOlPlagAlk-FsMagIlmPhlogAmphCpxOpxOlPlagMag ..........511376575641367403513536384519SiO2........TiO0786954717565280730335112...........AlO21526619029101514212123213530236423.......CrO00000500100100100102323.............tot745253033009837468131125779163224040829FeO..........MnO039080059120012018022044045037.............MgO147360004001147334146120156266388002238..........CaO2180048180190011162091040041227........NaO0566582430342490410024302........KO0000541319281050000010322.F076..Cl023025.............Total99299999899095798610029779959981001995947.......Mg#769711663628776739751...Fs8695237...Wo43340321...An39610602...Or3177218 Analysisbyelectronmicroprobe.SeeElectronicAppendix1forcompletereportingofmineralanalyses.12analysisfromrim;analysisfromcore.cpx,clinopyroxene;opx,orthopyroxene;ol,olivine;plag,plagitotphlog,phlogopite;amph,amphibole.Mg-number(Mg#)=100Mg/(MgMnFe),atomicbasis.*þþQUIlFprojection(Andersen.,1993).Anorthite(An)andorthoclase(or)calculatedonatomicbasis.etal at Serials Department, University of Hawaii at Manoa Library on March 19, 2010 urnals.org
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