hornblende- and phlogopite-bearing gabbroic xenoliths from...
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JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 PAGES 219–241 2002
Hornblende- and Phlogopite-BearingGabbroic Xenoliths from Volcan San Pedro(36°S), Chilean Andes: Evidence for Meltand Fluid Migration and Reactions inSubduction-Related Plutons
F. COSTA1∗, M. A. DUNGAN1 AND B. S. SINGER2
1SECTION DES SCIENCES DE LA TERRE, UNIVERSITE DE GENEVE, 13 RUE DES MARAICHERS, 1211 GENEVA,
SWITZERLAND2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WISCONSIN–MADISON, 1215 W. DAYTON ST.,
MADISON, WI 53706, USA
RECEIVED JUNE 19, 2000; REVISED TYPESCRIPT ACCEPTED AUGUST 7, 2001
KEY WORDS: Tatara–San Pedro; gabbroic xenolith; hornblende; phlogopite;Two groups of gabbroic xenoliths (I and II) containing largemelt migrationproportions of late-crystallized hornblende (up to 50 vol. %) and
Na-rich phlogopite (up to 15 vol. %), were brought to the surface
by a late Holocene eruption of Volcan San Pedro, the youngest
edifice of the Tatara–San Pedro Volcanic Complex (36°S, ChileanINTRODUCTION
Andes). Group I are inferred to be fragments of partially solidifiedHornblende-bearing gabbroic xenoliths found at sub-Holocene plutons because they contain residual interstitial glass,duction-related volcanoes have been inferred to reflectwhereas exsolution and deformation textures in Group II indicatedifferentiation processes that have operated on magmasthat they are fragments of pre-Quaternary plutonic basement. Onof these arcs (Itinome-gata, Japan, Aoki, 1971; Lesser
the basis of textural relations plus the mineral and whole-rockAntilles, Arculus & Wills, 1980; Adak, Aleutian arc,
compositions of both groups of xenoliths, we suggest that hornblendeConrad et al., 1983; Conrad & Kay, 1984; DeBari et al.,
and phlogopite with high mg-numbers and Cr contents have formed 1987; Medicine Lake, California, Grove & Donnelly-by reactions between refractory cumulus minerals (olivine, Cr-spinel, Nolan, 1986; Japan, Yagi & Takeshita, 1987; Martinique,pyroxenes or plagioclase) and evolved melts ± aqueous fluids that Lesser Antilles, Fichaut et al., 1989; Arenal volcano, Costamigrated through partly solidified crystalline frameworks. Thus, the Rica, Beard & Borgia, 1989; Mt. St. Helens, Cascades,hydrous minerals are not early-crystallized phases in the basaltic Heliker, 1995; Calbuco volcano, Southern Chile, Hickey-magmas from which the cumulus minerals precipitated. The high Vargas et al., 1995; compilation from various sites, Beard,proportions of hornblende in many subduction-related gabbroic 1986). Many such xenoliths have been interpreted asplutons and xenolith suites compared with its paucity in basaltic crystal fractionation residues, and their mineral abund-or basaltic andesitic lavas may be partially explained by multistage ances and compositions have been used to constrainplutonic crystallization histories involving reaction and migration of fractional crystallization models of arc magmas. Theevolved melt ± aqueous fluids that either could have originated presence of hornblende in these gabbroic xenoliths andwithin the cumulus pile of the mafic intrusion or were derived certain apparently refractory compositional char-
acteristics (e.g. high Cr contents) have led to inferencesexternally, from broadly contemporaneous felsic magmas.
Extended dataset can be found athttp://www.petrology.oupjournals.org∗Corresponding author. Present address: ISTO, 1A rue de la Ferollerie,45071 Orleans, France. Telephone: +33-238255213. Fax: +33-238636488. E-mail: [email protected] Oxford University Press 2002
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002
that (1) hornblende formed as an early-crystallizing min- low-grade metavolcanic and metasedimentary rocks in-truded by two shallow-level granitoid plutons dated ateral in water-bearing mafic magmas (e.g. Conrad & Kay,6·2–6·4 Ma (Davidson & Nelson, 1994; Nelson et al.,1984; Yagi & Takeshita, 1987; Beard & Borgia, 1989),1999). Gabbroic rocks are not observed at the surface,and (2) fractionation of near-liquidus hornblende con-although gabbro and olivine gabbro (including troctolitetributes to the calc-alkaline differentiation trend (Caw-and norite) with anhydrous mineralogy are the mostthorn & O’Hara, 1976; Beard, 1986; Grove & Kinzler,common xenolith lithologies and the sources of minor1986; Yagi & Takeshita, 1987). Despite the plausibility(<10 vol. %) but widespread xenocrystic contaminationof a relation between hornblende stability in water andin mafic to intermediate lavas. The TSPC (>55 km3)alkali-rich, high- fO2 mafic magmas (e.g. Sisson & Grove,consists mainly of basaltic andesitic lavas, although erup-1993) and early silica enrichment along the calc-alkalineted magmas range from primitive basalt to high-SiO2trend, the high modal abundances of hornblende in arc-rhyolite that define calc-alkaline medium- to high-Krelated gabbros (xenoliths and plutons) are in markedtrends (e.g. Singer et al., 1997; Dungan et al., 2001). Daciticcontrast to the rarity of hornblende-bearing basaltic orlavas at the TSPC typically contain minor hornblendebasaltic andesitic magmas erupted from subduction-zonephenocrysts that are rarely accompanied by biotite, butvolcanoes. This could be explained by the instabilityno lava with <65 wt % SiO2 contains hornblende pheno-of amphibole at low pressure in mafic magma, or bycrysts.protracted closed-system crystallization reactions in gab-
broic plutons. However, the association of hornblendeand Na-rich phlogopite in the San Pedro gabbroic xeno-
Volcan San Pedro and xenolithsliths suggests that reactions in mafic plutons might beThe principal phases of volcanic construction at Volcantriggered also by migrating water-rich melts and aqueousSan Pedro (1·5 km3) are divided into a main cone-buildingfluids, as has been found in experiments involving inter-stage comprising andesitic and dacitic lavas, and a lateactions between evolved liquids and mafic cumulates (e.g.event triggered by sector collapse of the eastern flank.Prouteau et al., 2001; Costa et al., in preparation).The latter comprises an explosive eruption that producedWe propose that two groups of hornblende- and phlo-air-fall dacitic tephra (Singer & Dungan, 1992; Singer etgopite-bearing gabbroic crustal xenoliths from the Holo-al., 1995) followed by extrusion of a succession of lavascene Volcan San Pedro (Tatara–San Pedro Volcanicfrom the eastern flank: (1) 0·2 km3 of biotite–hornblendeComplex, Chilean Andes) are distinct in terms of agedacite containing up to 10% mafic xenoliths plus minorand origin, but that both suites are the result of multistagequenched mafic inclusions (QMI); (2) 0·5 km3 of two-differentiation histories involving migration of evolvedpyroxene dacite with abundant QMI; (3) 0·1 km3 of two-melts± aqueous fluids through cumulate piles. Reactionspyroxene andesite with rare QMI. The last volcanicbetween early-crystallized refractory minerals (olivine,activity at this cone consisted of 0·2 km3 of basalticCr-spinel, pyroxenes or plagioclase) and percolating meltsandesitic magma erupted from the summit crater. Theand fluids have produced substantial proportions of horn-majority of the xenoliths are gabbroic (22 samples),blende (up to 50 vol. %) plus Na-rich phlogopite (up toalthough scarce granites and metamorphic rocks15 vol. %) with features such as high Cr2O3 contents,(hornfels) similar to exposed basement are also present.which have been interpreted elsewhere as indicatingSmall xenoliths are rounded to subrounded, but largerearly crystallization from hydrous basaltic magmas. Thefragments (up to 45 cm in diameter) tend to be angular.processes of melt and aqueous fluid migration and re-The observation that the xenoliths are found exclusivelyaction-replacement proposed for the San Pedro gabbrosin the first lava flow following structural failure of thecould be analogous to those described in tholeiitic in-east flank of Volcan San Pedro suggests that they aretrusions (e.g. Muskox intrusion, Canada, Irvine, 1980;fragments of the conduits or upper parts of the marginsSkaergaard intrusion, Greenland, McBirney, 1995; Bush- of the San Pedro magma chamber that were shattered
veld Complex, South Africa, Mathez, 1995; Stillwater during the eruption (in a similar fashion to the 18 MayComplex, USA, Boudreau, 1999) or those caused by 1980 Mount St. Helens eruption; Heliker, 1995).interactions between mafic and felsic magmas (e.g. Sha,1995).
TEXTURES, AND MINERAL ANDGLASS COMPOSITION OF THE
GEOLOGICAL SETTING XENOLITHSThe Quaternary Tatara–San Pedro Complex (TSPC; Major and minor element compositions of minerals and36°S, 71°51′W) is a long-lived frontal arc volcanic centre glass were determined by electron microprobe (CA-(930 ka; Holocene) of the Southern Volcanic Zone of the MECA SX-50; see the Appendix). Mineral names, struc-
tural formulae and end-members were determinedAndes (Fig. 1). Exposed basement lithologies are mainly
220
COSTA et al. GABBROIC XENOLITHS, VOLCAN SAN PEDRO
(2) Group II have subsolidus exsolution and de-formation textures (19 samples). These observations and40Ar/39Ar data from two xenoliths (Fig. 3) indicate thatthey are fragments of the pre-Quaternary, plutonic base-ment of the volcano. These have been further subdividedinto: (a) Group IICL, which are mainly clinopyroxeneleuconorites; (b) Group IIHN, which are hornblendenorites.
GROUP I xenoliths: olivine–hornblendenorites and melanoritesThese samples consist of olivine, orthopyroxene, horn-blende, plagioclase and phlogopite forming a medium-grained (1–5 mm) crystal network with interstitial daciticto rhyolitic glass (SiO2 >67–72 wt %; K2O >3·7–8·6 wt %) bounded by euhedral crystal faces suggestingthat the glass is residual from crystallization and not dueto partial melting (Fig. 4a). Cr-spinel (Ulv0·01–0·31; Cr2O3
>10–18 wt %) occurs as inclusions in olivine, ortho-pyroxene, hornblende and rarely in plagioclase cores.Olivine (Fo86–76; NiO Ζ0·05−0·4 wt %) is typicallyresorbed and surrounded by hornblende, orthopyroxeneand phlogopite that formed in reaction relationship witholivine (Fig. 4b). Less commonly, euhedral olivineshowing no reaction is in contact with glass (Fig. 4a).
Fig. 1. Simplified geological map of Central Chile showing the location Rare clinopyroxene (Wo46–45En46–44Fs9–11; Cr2O3 Ζ0·3–of the Tatara–San Pedro Volcanic Complex. Μ, main Quaternary 0·8 wt %) is also resorbed and has largely been replacedvolcanic centres. Pz, Palaeozoic rocks; Mz, Mesozoic rocks. Grey
by hornblende. Plagioclase is euhedral, whether it occursshaded areas indicate Tertiary plutons. Figure modified from Hildreth& Moorbath (1988) and from Dungan et al. (2001). The location of as free crystals or as inclusions within orthopyroxene,Tertiary plutons is from Mapa Geologico de Chile (Servicio Nacional hornblende or phlogopite. Most plagioclase crystals con-de Geologıa y Minerıa, 1982).
sist of a normally zoned core (An86–78) surrounded by anormally zoned rim (An45–26) with an >35 An mol %gap between cores and rims (Fig. 5). The high Fo andfollowing Morimoto et al. (1988) for pyroxenes; Leake etNiO contents of olivine, the high Wo and Cr2O3 contentsal. (1997) for amphiboles; Rieder et al. (1998) and Deerof clinopyroxene and the high An contents of plagioclaseet al. (1962) for micas; Deer et al. (1992) for olivine,cores suggest that these minerals are near-liquidus crys-plagioclase and apatite; and Stormer (1983) for spinel andtallization products of a water-bearing basaltic magmailmenite. Tables with the complete electron microprobe(e.g. Gaetani et al., 1993; Sisson & Grove, 1993).analyses of spinel, ilmenite, pyroxenes, olivine, plagioclase
Subhedral to euhedral orthopyroxene, hornblendeand glass can be downloaded from the Journal of Petrology(magnesiohastingsite) and phlogopite are late-crystallizingWeb site at http://www.petrology.oupjournals.org.minerals, as they are commonly in contact with interstitialNone of the samples shows evidence of low-temperatureglass (Fig. 4a). Despite this, the three minerals are char-hydrothermal alteration. Partial melting along grainacterized by high mg-numbers [mg-number = 100Mg/boundaries or post-entrainment modification as a result(Mg+ Fet) in mols, where Fet is total iron] ranging fromof interaction with the host lava are minimal. On the77 to 82, and Cr2O3 contents from <0·1 to 1·2 wt %basis of textures and modal mineralogy (Table 1 and(Tables 2–4). The Cr2O3 concentrations in these threeFig. 2), we have divided the xenoliths into two groupsminerals vary irregularly within and between crystals,[nomenclature after Streckeisen (1976) and LeMaitre
(1989)]: and they are as high as, or higher than those of clino-pyroxene (Fig. 6). This is in accord with the textural(1) Group I are olivine–hornblende norites and me-
lanorites (three samples) with interstitial glass bounded relations indicating that orthopyroxene, hornblende andphlogopite are the products of reactions between cumulusby euhedral crystal faces. These samples were probably
dislodged from partly solidified crystal-rich zones of an minerals (olivine, Cr-spinel and clinopyroxene) andliquid. Most phlogopite is characterized by Na2O contentsactive conduit or reservoir system.
221
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002
Tab
le1
:M
odes
,na
mes
,an
dm
ain
text
ural
feat
ures
ofth
exe
nolith
s
Sam
ple
Ro
ckty
pe
Gra
insi
zeM
od
alm
iner
alo
gy
(vo
l.%
)D
efo
rmat
ion
Mic
rofr
actu
res
Ol
Cp
xO
px
Hb
lP
lP
hl
Ph
l(lt
)O
paq
ue
Gla
ss
Gro
up
I:p
art
iall
ycry
sta
lliz
ed
xe
no
lith
s
Hx1
4bO
ln
ori
tem
ediu
m28
·20·
08·
011
·537
·21·
60·
00·
912
·7n
on
o
Hx1
4kO
ln
ori
tem
ediu
m22
·90·
020
·721
·125
·37·
90·
01·
40·
6n
on
o
Hx1
4nO
lm
elan
ori
tem
ediu
m20
·60·
316
·833
·123
·43·
00·
00·
32·
7n
on
o
Gro
up
II:
xe
no
lith
sw
ith
su
bso
lid
us
tex
ture
s
Gro
up
IICL
Hx1
2aC
px
leu
con
ori
tem
ediu
m0·
08·
414
·11·
768
·10·
70·
24·
02·
7P
ln
o
Hx1
2bH
bl
leu
con
ori
tem
ediu
m0·
42·
313
·013
·858
·50·
01·
64·
06·
5P
ln
o
Hx1
4aC
px
leu
con
ori
tefi
ne–
med
ium
0·6
8·8
16·1
3·6
65·9
0·1
0·8
4·3
0·0
Pl
Op
x,H
bl,
Ph
l,o
paq
ue
Hx1
4cH
bl
no
rite
med
ium
–co
arse
0·0
0·0
11·7
23·1
50·0
0·3
9·4
1·0
4·6
Pl
Op
x,P
l
Hx1
4dC
px
leu
con
ori
tem
ediu
m0·
06·
110
·75·
270
·80·
00·
02·
64·
6P
lO
px,
Hb
l,P
hl,
op
aqu
e
Hx1
4eC
px
leu
con
ori
teco
arse
7·5
0·0
13·4
8·3
68·5
1·8
0·0
0·6
0·0
Pl,
Ph
ln
o
Hx1
4hC
px
leu
con
ori
tem
ediu
m–c
oar
se3·
81·
514
·51·
370
·21·
93·
83·
00·
0P
lH
bl
Hx1
4iC
px
leu
con
ori
tem
ediu
m–c
oar
se0·
04·
913
·98·
366
·80·
62·
72·
80·
0P
lH
bl,
Ph
l,o
paq
ue
Hx1
4jC
px
leu
con
ori
tem
ediu
m0·
09·
319
·22·
262
·30·
91·
94·
30·
0P
ln
o
Hx1
4lH
bl
no
rite
med
ium
0·0
0·0
0·6
37·9
50·8
0·0
2·1
6·4
2·2
Pl
no
Hx1
4mH
bl
leu
cog
abb
rofi
ne–
med
ium
0·0
7·5
7·2
16·3
65·5
0·0
0·0
3·4
0·0
no
no
Hx1
4qC
px
leu
con
ori
tem
ediu
m0·
53·
516
·78·
155
·90·
011
·73·
70·
0P
lH
bl,
Ph
l(1
mm
wid
e)
Hx1
4sH
bl
no
rite
med
ium
0·0
0·2
8·0
28·0
57·0
0·0
2·7
3·1
1·2
Pl
Op
x,P
l,H
bl
Hx1
4uC
px
leu
con
ori
tem
ediu
m0·
03·
025
·90·
164
·13·
90·
02·
90·
0P
lH
bl
Hx1
4wC
px
leu
con
ori
tem
ediu
m–c
oar
se0·
00·
67·
24·
581
·31·
40·
51·
33·
2P
lO
px,
Hb
l,P
hl
Hx1
4yH
bl
leu
con
ori
tem
ediu
m–c
oar
se0·
50·
05·
810
·455
·315
·013
0·1
0·0
Pl
Op
x,H
bl,
Ph
l
Hx1
4xH
bl
gab
bro
fin
e0·
00·
00·
030
·261
·41·
50·
06·
90·
0P
ln
o
Gro
up
IIHN
Hx1
4vH
bl
no
rite
coar
se22
·60·
06·
031
·635
·54·
00·
00·
40·
0P
l,O
px,
Ph
ln
o
Hx1
4zH
bl
mel
ano
rite
med
ium
–co
arse
10·3
0·0
9·7
48·1
25·3
3·9
0·0
2·7
0·0
Pl,
Op
x,P
hl
Op
x,P
l,H
bl,
Ph
l
Mo
des
bas
edo
nco
un
tin
g>
2000
po
ints
exce
pt
for
fin
e-g
rain
edro
cks
(>10
00p
oin
ts).
Ave
rag
eg
rain
size
:co
arse
>5m
m;
med
ium
1–5
mm
;fi
ne
<1m
m.
Ro
ckn
ames
follo
wS
trec
keis
en(1
976)
and
LeM
aitr
e(1
989)
.M
iner
alsy
mb
ols
afte
rK
retz
(198
3).
Ph
l(l
t)re
fers
tola
tep
hlo
go
pit
e.
222
COSTA et al. GABBROIC XENOLITHS, VOLCAN SAN PEDRO
Table 2: Ranges of mg-numbers and Cr2O3 (wt %) contents of the mafic minerals from the three
groups of xenoliths
Ol Cpx Opx Hbl Phl
Group I
mg-number 86–76 84–81 82–77 80–72 82–77
Cr2O3 n.a. Ζ0·9 Ζ0·5 Ζ1·2 Ζ0·6
Group IICL
mg-number 81–72 81–73 80–65 77–64 81–70
Cr2O3 n.a. Ζ0·2 Ζ0·2 Ζ0·7 Ζ0·4
In microfractures
mg-number — — 78–73 77–64 76–70
Cr2O3 — — Ζ0·2 Ζ0·2 Ζ0·2
Group IIHN
mg-number 79–78 89–81 81–77 80–72 84–77
Cr2O3 n.a. Ζ0·4 Ζ0·2 Ζ0·6 Ζ0·4
mg-number = 100Mg/(Mg + Fet), in mols, where Fet is total iron. Mineral symbols after Kretz (1983). n.a., not analysed.
Fig. 2. Mineral modes of the xenoliths compared with other gabbroic xenoliths from subduction-related volcanoes. Group IICL xenoliths havesimilar modes to other xenoliths, whereas Group I and Group IIHN have less common compositions. Data sources: Mt. Pelee xenoliths (Fichautet al., 1989), Mt. St. Helens (Heliker, 1995), Medicine Lake (Grove & Donnelly-Nolan, 1986), Calbuco (Hickey-Vargas et al., 1995) and LesserAntilles [including quartz-bearing gabbros of Arculus & Wills (1980)]. Figure modified from Arculus & Wills (1980).
(2–3·4 wt %; Table 4 and Fig. 7) that are higher than solvus between phlogopite and aspidolite (synonymouswith the sodium phlogopite end-member; Rieder et al.,those for most biotite or phlogopite reported in the
literature. Such high sodium contents have been in- 1998).On the basis of the data discussed above, we proposeterpreted by Costa et al. (2001) as the result of (1)
crystallization from a liquid with high Na2O contents, a two-stage crystallization sequence for these xenoliths:(1) Cr-spinel + olivine + clinopyroxene ± plagioclase(2) crystal-chemical effects, as incorporation of Na in
biotite or phlogopite is enhanced by high Mg/Fe of the crystallized from a water-bearing basaltic magma; (2)reaction occurred between the mafic minerals with anmica (Volfinger et al., 1985), and (3) the presence of a
223
JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002
(Fig. 5), which reflect that some samples have mainlyanorthite-rich plagioclase (Hx14e, Hx14y) whereas inothers, plagioclase is more albitic (Hx14a). Olivine(Fo81–72; NiO Ζ0·25 wt %) is commonly anhedral andsurrounded by rims of orthopyroxene, Fe–Ti oxide sym-plectites and occasionally by small flakes of phlogopite,which we refer to as late phlogopite (Table 1). Rare Cr-spinel inclusions (Ulv0·23; Cr2O3 >18 wt %) are presentin olivine. Anhedral to subhedral diopsidic to augiticclinopyroxene (Wo40–48En40–44Fs10–17) and orthopyroxene(mg-number 80–65) containing exsolution lamellae com-monly occur as clusters interstitial to plagioclase crystals.Cr-poor magnetite (Ulv0·04–0·44) and ilmenite (Ilm0·74–0·96)are typically exsolved, and occur as euhedral inclusionsin pyroxenes, or as anhedral oikocrysts surrounding an-hedral plagioclase, pyroxenes and occasionally horn-blende and phlogopite. Anhedral apatite is the onlyaccessory mineral, and it occurs between grain boundariesof plagioclase crystals or inside phlogopite. Halogen con-tents and ratios in apatite are variable between samples(Table 5, Fig. 8), which suggests that they crystallizedfrom melts with different halogen compositions (e.g.Boudreau, 1995).
Fig. 3. Results of 40Ar/39Ar analyses of two hornblende separates Subhedral to anhedral hornblende and phlogopite are(samples Hx14v and Hx14z) of Group IIHN xenoliths. Incremental
late-crystallizing minerals, as they typically occur as smallfurnace heating 40Ar/39Ar analyses were performed at the Universityof Geneva following the methods described by Singer & Pringle (1996). (<1 mm) poikilitic crystals surrounding resorbed, an-Both hornblendes show comparable, but discordant apparent age hedral pyroxene, olivine and, in contrast to Group Ispectra. The presence of phlogopite in the mineral separate could xenoliths, partly resorbed plagioclase (Fig. 4c). Horn-explain the first high K/Ca steps, but afterwards the K/Ca remains
blende is commonly magnesiohastingite, but rare tscher-low (0·04–0·05) and within the values of hornblende obtained byelectron microprobe analyses (0·03–0·08). Determining the cause(s) of makitic hornblende and magnesiohornblende are alsothe discordant 39Ar release spectra is beyond the scope of this paper, present in sample Hx14y (Table 3). Hornblende andbut as the samples are xenoliths, heat from the host lava may have
phlogopite have mg-numbers (Table 2) that overlap withpartially degassed the hornblendes. We interpret these 40Ar/39Ar dataas indicating that the xenoliths are certainly older than 1 Ma and may those of pyroxene and olivine, whereas their Cr2O3be up to 8 Ma. The total fusion age is>5 Ma, which is>1 my younger contents are typically higher (Fig. 6). As previously arguedthan the age of the Huemul and Cerro Risco Bayo plutons that form for Group I xenoliths, we interpret the high mg-numbersthe basement of the TSPC (6·2–6·4 Ma; Nelson et al., 1999).
and Cr2O3 contents of the hydrous minerals as indicationsof reactions between Mg-rich, Cr-bearing minerals (oli-vine, pyroxenes and Cr-spinel) and water-rich evolvedevolved water-rich liquid that produced hornblende +liquid, and not as evidence for crystallization from maficorthopyroxene + phlogopite, accompanied or followedmagma. However, in contrast to Group I, plagioclase isby plagioclase + apatite crystallization. The question ofresorbed in most xenoliths of this group. This could bewhether these reactions are due to progressive closed-explained by an increase in the water contents of thesystem crystallization or were triggered by ingress of aninterstitial melt (e.g. Sisson & Grove, 1993). The Na2Oevolved water-rich liquid is addressed in the Discussion.contents of phlogopite are lower (>1–2 wt %; Table 4and Fig. 7) than those of Group I xenoliths, but are nonethe less higher than in most biotite or phlogopite analyses
GROUP IICL xenoliths: clinopyroxene reported in the literature (Costa et al., 2001). Lowleuconorites with subsolidus textures proportions (Ζ6·5 vol. %) of rhyolitic glass (SiO2
>71–74 wt %; K2O >5·7–6·7 wt %) are present ex-The majority of these samples are characterized byclusively along resorbed plagioclase–plagioclase, plagio-mosaic, seriate textures, with subsolidus textural re-equi-clase–orthopyroxene and plagioclase–hornblende grainlibration along grain boundaries between plagioclaseboundaries, indicating that glass is a secondary partialcrystals, and between plagioclase and pyroxenes (i.e.melting product that formed after xenolith entrainmentconstant grain boundary dihedral angles between crystals;rather than as a primary residual melt as is the case forHunter, 1987). Plagioclase is dominantly normally zoned,
and there are compositional modes at An85−80 and An60–55 Group I xenoliths.
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Fig. 4. Photomicrographs of the xenoliths. Group I: (a) vesiculated interstitial SiO2-rich glass (>66–72 wt %) in contact with euhedralorthopyroxene (Opx), hornblende (Hbl) and phlogopite (Phl). [Also note the olivine (Ol) crystal (Fo82) in contact with the silica-rich glass.](b) Hbl enclosing resorbed Ol, but plagioclase (Pl) is euhedral. (Note also Phl surrounding anhedral Ol.) Group IICL: (c) poikilitic Hbl surroundingresorbed Pl crystals, many of which are bent or cracked (black arrows). This textural relation suggest that Hbl crystallized after deformation anddissolution of Pl. (d) Subvertical microfracture cutting across Pl and filled with Hbl, and Phl. (Note that where the microfracture intersectspyroxenes, larger Hbl crystals are present.) Group IIHN: (e) large poikilitic Hbl surrounding resorbed Pl and Ol, suggesting a reaction relationbetween the interstitial liquid, Pl and Ol to produce Hbl. The bent twins of the Pl crystal in the upper right corner of the picture (black arrow)indicate that reaction took place after deformation of the cumulate pile. (f ) Poikilitic Phl surrounding anhedral Ol and subhedral Pl.
Fe–Ti oxides, hornblende and phlogopite (and oc-Deformation and microfracturingcasionally also orthopyroxene) cut across all mineralsPlagioclase commonly shows bent twins and microcracksexcept poikilitic hornblende and phlogopite. Where(Fig. 4c), whereas hornblende and phlogopite do notmicrofractures intersect pyroxene–plagioclase contacts,display textural evidence of deformation other than kink-both minerals are resorbed, and anhedral, and they arebands in phlogopite. In many samples, discontinuous
microfractures (Ζ0·5 mm in width; Fig. 4d) filled with mantled by a rim of hornblende or phlogopite (Fig.
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Fig. 5. Histograms of plagioclase composition of the three xenolith groups. Plagioclase of Group I xenoliths that is not included in otherminerals shows a bimodal composition, with cores at An85–75 and rims at An45–30. Those Pl included in Opx, Hbl and Phl lack the compositionalgap and have similar compositions, suggesting that the three minerals co-crystallized. Most samples of Group IICL have Pl of An60–40 composition,except for samples Hx14y and Hx14e, which have Pl with higher An contents (>An88−80). Group IIHN xenoliths have mainly Pl with high Ancontents.
magnetite (Ulv0·07–0·76) and ilmenite (Ilm0·89). Phlogopite is4d). Orthopyroxene, hornblende and phlogopite fillingalso present as oikocrysts that include resorbed olivine,microfractures have mg-numbers and Cr2O3 contents thatorthopyroxene and plagioclase, and occasionally it alsooverlap with or are lower than those of poikilitic mineralsoccurs inside hornblende oikocrysts (Fig. 4f ). Rare(Table 2). These compositional and textural observationsanhedral apatite is present along plagioclase grainsuggest that the microfractures hosted evolved water-boundaries.bearing melts or aqueous fluids that reacted with pyr-
Late-crystallized hornblende (magnesiohastingite) andoxenes, olivine and plagioclase to produce the poikiliticphlogopite have mg-numbers and Cr2O3 contents (Tablehydrous minerals with mg-numbers and Cr2O3 contents2) that overlap with, or are higher than those of olivinethat are higher than those hosted by microfractures.and pyroxenes (Fig. 6). As in Group IICL xenoliths,resorbed plagioclase inside hornblende could be ex-plained by an increase in the water content of the
GROUP IIHN xenoliths: hornblende norites interstitial melt before or during hornblende (and phlo-with subsolidus textures gopite) crystallization-reaction. Phlogopite has extremelySamples from this group are texturally heterogeneous high Na2O (>1·5–5 wt %), approaching the compositionand are characterized by large anhedral hornblende of aspidolite (Table 4 and Fig. 7).oikocrysts ([1 cm; Fig. 4e) that surround resorbed olivine Plagioclase and orthopyroxene inside hornblende oiko-(Fo79–78; NiO Ζ0·20 wt %), diopsidic to augitic clino- crysts are deformed (Fig. 4e), whereas hornblende oiko-pyroxene (Wo42–48En45–47Fs6–11) and orthopyroxene (mg- crysts do not show textural evidence of deformation, andnumber 81–77). Plagioclase (An88−80, occasionally An50) phlogopite occasionally shows kink bands. From theseis also resorbed when it occurs as inclusions in hornblende. textural relations, we infer that deformation occurred
before crystallization of the hydrous minerals.Oxide minerals are Cr-spinel (Ulv0·2; Cr2O3 >9 wt %),
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Fig. 6. Concentrations of Cr2O3 wt % of mafic minerals from the three groups of xenoliths. It should be noted that the Cr2O3 contents of Hbl,Phl, and occasionally Opx (only in Group I xenoliths) are as high as or higher than those of clinopyroxene (Cpx).
Fig. 7. Phlogopite composition (atoms per formula unit) from the three groups of xenoliths are characterized by high Na/K. (See text fordiscussion.) For comparison are also shown the biotite compositions of the host dacite lava, and those of low-pressure (Ζ0·3 GPa) water-bearingcrystallization experiments of basaltic to andesitic composition. Data sources: SG, 1993 is Sisson & Grove (1993); RC, 1996 is Righter &Carmichael (1996).
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Table 3: Representative hornblende analyses
Group I Group IICL Group IIHN
wt % n-1-17 n-III-2-5 14a-VII-1p e-III-1-5p w-VIII-1p y-I-1p y-V-4v w-IX-3v v-II-7 z-I-3
SiO2 43·61 43·31 42·67 45·10 42·50 42·47 43·26 42·66 42·40 42·02
TiO2 2·22 3·54 3·50 1·03 3·34 1·99 0·17 2·91 3·47 3·20
Al2O3 11·32 11·13 10·41 10·60 10·88 11·82 10·93 9·83 12·63 12·13
Cr2O3 0·64 0·06 0·01 0·41 0·00 0·36 0·04 0·10 0·27 0·25
FeO∗ 9·31 8·99 13·51 10·11 12·75 11·71 10·95 11·94 9·35 9·17
MnO 0·15 0·08 0·22 0·05 0·17 0·24 0·31 0·16 0·10 0·17
MgO 15·96 15·91 13·54 16·71 13·85 14·62 16·08 15·07 15·46 15·97
CaO 11·60 11·63 11·37 11·01 11·66 11·46 11·50 11·22 11·26 11·20
Na2O 2·81 2·97 1·98 2·53 2·15 2·29 2·10 2·29 3·00 2·92
K2O 0·36 0·34 0·89 0·27 0·65 0·43 0·44 0·64 0·42 0·43
F 0·09 0·10 0·10 0·08 0·00 0·00 0·00 0·00 n.a. 0·10
Cl 0·02 0·02 0·05 0·13 0·09 0·10 0·13 0·20 n.a. 0·08
F=O 0·04 0·04 0·04 0·04 0·00 0·00 0·00 0·00 — 0·05
Cl=O 0·01 0·00 0·01 0·03 0·02 0·02 0·03 0·04 — 0·02
Tot 98·08 98·06 98·20 97·96 98·01 97·46 95·87 96·98 98·36 97·58
Si 6·24 6·22 6·22 6·37 6·19 6·14 6·27 6·22 6·05 6·03
Al 1·91 1·88 1·79 1·76 1·87 2·01 1·87 1·69 2·12 2·05
Cr 0·07 0·01 0·00 0·05 0·00 0·04 0·00 0·01 0·03 0·03
Ti 0·24 0·38 0·38 0·11 0·37 0·22 0·02 0·32 0·37 0·35
Fe3+c 0·65 0·43 0·74 1·17 0·66 0·97 1·31 0·94 0·64 0·82
Mg 3·41 3·42 2·94 3·52 3·01 3·15 3·48 3·28 3·29 3·42
Fe2+c 0·47 0·65 0·91 0·03 0·89 0·44 0·02 0·52 0·47 0·28
Mn 0·02 0·01 0·03 0·01 0·02 0·03 0·04 0·02 0·01 0·02
Ca 1·78 1·79 1·77 1·67 1·82 1·77 1·79 1·75 1·72 1·72
Na 0·78 0·83 0·56 0·69 0·61 0·64 0·59 0·65 0·83 0·81
K 0·06 0·06 0·16 0·05 0·12 0·08 0·08 0·12 0·08 0·08
mg-no. 75·4 76·0 64·1 74·7 65·9 69·0 72·4 69·2 74·7 75·6
∗Total iron as Fe2+.The first letter or number of the analysis label indicates the sample. p, poikilitic mineral; v, mineral in microfractures; n.a.,not analysed; c, calculated. Structural formula (23 oxygens and OH+ F+ Cl= 2, and cations= 13 – Na – Ca – K) calculatedas in Leake et al. (1997). mg-number = 100Mg/(Mg + Fet), in mols, where Fet is total iron.
GROUP I xenoliths: olivine–hornblendeWHOLE-ROCK CHEMICALnorites and melanoritesCOMPOSITIONSCompared with the mean basaltic composition of theIt is well established that the bulk-rock compositions ofTSPC (Table 6), the xenoliths have higher MgO (>20–many plutonic rocks are not representative of liquids,21 wt %) and Ni (446–643 ppm), and lower SiO2 (>46–but are the result of varying degrees of mineral ac-47 wt %) and incompatible elements (e.g. K2O, Zr) (Fig.cumulation or/and melt ± fluid migration (e.g. McBir-9). This, together with the high modal proportions ofney, 1995). In the following sections we describe theolivine, suggests that the low incompatible elementmajor, minor and trace element compositions of theabundances are mainly due to accumulation of olivine.xenoliths with respect to the mean composition of 10Ratios of elements that are not affected by olivine ac-basalts that we think are representative of mafic liquidscumulation, such as P/Zr (9–10), or Rb/Y (1·1–1·5) fallof the TSPC (see the Appendix for the analytical methods
and precision of the analyses). within the range of TSPC basalts (Fig. 10).
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Table 4: Representative phlogopite analyses
Group I Group IICL Group IIHN
wt % b-tr1-1 k-tr2-6 n-5-1 a-VI-6-1p e-I-3p y-V-1p w-V-5p a-VI-2-1v w-IV-6v v-III-8 z-II-3
SiO2 37·16 38·92 37·80 39·40 36·87 36·90 36·42 38·77 37·08 42·40 42·02
TiO2 2·56 1·05 1·49 4·05 0·73 1·30 4·78 3·70 5·11 1·94 2·35
Al2O3 16·13 16·46 16·30 13·27 16·18 16·11 14·63 13·90 13·83 17·32 16·30
Cr2O3 0·08 0·33 n.a. 0·05 0·12 0·29 0·08 0·04 0·06 0·08 0·19
FeO∗ 10·11 9·04 9·51 12·09 9·23 10·63 11·21 12·69 10·76 8·35 9·02
MnO 0·02 0·08 0·06 0·09 0·09 0·08 0·18 0·17 0·09 0·06 0·08
MgO 19·68 21·55 20·30 18·09 21·17 19·65 17·63 17·82 17·86 20·89 20·56
CaO 0·03 0·02 0·04 0·01 0·04 0·02 0·00 0·01 0·11 0·03 0·00
Na2O 2·21 3·36 2·58 1·24 1·92 1·13 1·36 1·23 1·21 4·08 1·94
K2O 7·53 5·70 7·03 8·51 8·25 8·98 8·63 8·54 8·26 5·06 7·83
BaO 0·47 0·25 0·30 0·16 0·07 0·28 0·47 0·18 0·21 0·37 0·14
F 0·18 0·06 0·13 0·16 0·00 0·00 0·00 0·10 0·38 0·13 0·14
Cl 0·02 0·05 0·07 0·12 0·08 0·14 0·20 0·09 0·17 0·06 0·11
F=O 0·08 0·03 0·05 0·07 0·00 0·00 0·00 0·04 0·16 0·05 0·06
Cl=O 0·00 0·01 0·02 0·03 0·02 0·03 0·04 0·02 0·04 0·01 0·02
Tot 96·10 96·83 95·58 97·14 94·72 95·50 95·54 97·18 94·93 96·92 97·29
Si 5·39 5·50 5·47 5·69 5·40 5·42 5·38 5·62 5·49 5·43 5·49
Ti 0·28 0·11 0·16 0·44 0·08 0·14 0·53 0·40 0·57 0·21 0·25
Al 2·75 2·74 2·78 2·26 2·79 2·79 2·55 2·37 2·41 2·87 2·72
Cr 0·01 0·04 — 0·01 0·01 0·03 0·01 0·00 0·01 0·01 0·02
Fe∗ 1·23 1·07 1·15 1·46 1·13 1·31 1·39 1·54 1·33 0·98 1·07
Mn 0·00 0·00 0·01 0·01 0·01 0·01 0·02 0·02 0·01 0·00 0·00
Mg 4·25 4·54 4·38 3·90 4·63 4·30 3·89 3·85 3·94 4·38 4·35
Ca 0·00 0·00 0·01 0·00 0·01 0·00 0·00 0·00 0·02 0·00 0·00
Na 0·62 0·92 0·72 0·35 0·54 0·32 0·39 0·35 0·35 1·11 0·53
K 1·39 1·03 1·30 1·57 1·54 1·68 1·63 1·58 1·56 0·91 1·42
Ba 0·03 0·01 0·02 0·01 0·00 0·02 0·03 0·01 0·01 0·02 0·01
Mg/Fet 3·47 4·25 3·81 2·67 4·09 3·30 2·80 2·50 2·96 4·46 4·06
mg-no. 77·6 81·0 79·2 72·7 80·3 76·7 73·7 71·5 74·7 81·7 80·3
Na/K 0·45 0·90 0·56 0·22 0·35 0·19 0·24 0·22 0·22 1·23 0·38
∗Total iron as Fe2+.The first letter or number of the analysis label indicates the sample. p, poikilitic mineral; v, mineral in microfractures; n.a.,not analysed. Structural formula calculated with 22 oxygens. mg-number = 100Mg/(Mg + Fet), in mols, where Fet is totaliron.
concentrations of SiO2 (>46–47 wt %), and generallyGROUP IICL xenoliths: clinopyroxene higher MgO (>7·6–10·7 wt %) and Al2O3 (>21·8–leuconorites with subsolidus textures 23 wt %) (Fig. 9). Minor and trace element abundances
are highly variable. Most xenoliths have concentrationsThe major element compositions of most of these xeno-of incompatible elements (e.g. K2O, Zr, Y) that rangeliths are comparable with those of high-alumina basaltsfrom those of the TSPC basalts to much lower valuestypical of subduction zones (e.g. Gust & Perfit, 1987),(Fig. 9). Concentrations of compatible elements (e.g. Srwith >49–52 wt % SiO2, >5·2–8·2 wt % MgO andand Ni) of most xenoliths are, however, within the ranges>17·2–19 wt % Al2O3 (Table 6). Samples Hx14e, Hx14h
and Hx14y stand out from the rest by their lower defined by TSPC basalts (Fig. 9), and thus their low
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Table 5: Representative apatite analyses
Group IICL Group IIHN
wt % 12a-1c 14a-1c e-1c 4w-1c y-4c z-1c z-4r-5p
SiO2 0·10 0·14 0·05 0·25 0·05 0·05 0·13
FeO∗ 0·66 0·57 0·63 0·75 0·43 0·42 0·71
CaO 53·83 53·79 53·47 53·51 53·71 53·94 52·61
Ce2O3 0·41 0·21 0·11 0·27 0·10 0·11 0·07
Na2O 0·06 0·13 0·16 0·23 0·21 0·20 0·50
P2O5 41·03 41·61 41·67 41·41 41·94 42·02 41·92
SO3 0·00 0·00 0·00 0·17 0·06 0·06 0·10
MnO 0·13 0·08 0·08 0·09 0·09 0·10 0·03
SrO 0·00 0·00 0·00 0·00 0·00 0·00 0·00
F 1·42 0·79 0·45 0·55 0·19 0·54 0·18
Cl 1·50 1·55 2·29 2·63 2·17 1·90 2·77
F=O 0·60 0·33 0·19 0·23 0·08 0·23 0·08
Cl=O 0·34 0·35 0·52 0·59 0·49 0·43 0·63
Tot 98·29 98·16 98·19 99·03 98·39 98·67 99·33
Si 0·03 0·02 0·01 0·04 0·01 0·01 0·02
Fe∗ 0·09 0·08 0·09 0·11 0·06 0·06 0·10
Ca 9·77 9·73 9·70 9·65 9·70 9·70 9·52
Ce 0·03 0·01 0·01 0·02 0·01 0·01 0·00
Na 0·02 0·04 0·05 0·07 0·07 0·06 0·16
P 5·88 5·95 5·97 5·90 5·99 5·97 5·99
S 0·00 0·00 0·00 0·02 0·01 0·01 0·01
Mn 0·02 0·01 0·01 0·02 0·01 0·01 0·00
Sr 0·00 0·00 0·00 0·00 0·00 0·00 0·00
F 0·38 0·21 0·12 0·15 0·05 0·14 0·05
Cl 0·22 0·22 0·33 0·38 0·31 0·27 0·40
OHc 0·40 0·57 0·55 0·48 0·64 0·59 0·55
Total 16·84 16·85 16·83 16·83 16·85 16·84 16·82
Cl/F 0·57 1·05 2·73 2·57 6·18 1·87 8·17
∗Total iron as Fe2+.The first letter or number of the analysis label indicates the sample. Structural formula calculated with 25 O, OH, F, Cl.c, calculated.
incompatible element concentrations cannot be solely of some incompatible elements (e.g. Y, Zr), althoughK2O (0·34–2·48 wt %) and Rb (10–80 ppm) are highlyexplained by plagioclase or olivine accumulation, and
suggest that loss of interstitial melt rich in incompatible variable. Their Ni and Sr concentrations are within thevalues or higher than those of the TSPC basalts, andelements is a more plausible explanation. Some xenoliths
show a positive Eu anomaly (Eu/Eu∗ up to 1·75) when thus accumulation of olivine + plagioclase may be acontributing factor for the low abundances of somenormalized to primitive mantle (McDonough et al., 1992).
This could be due to plagioclase accumulation, although incompatible elements in these three xenoliths. Ratios ofincompatible elements (e.g. K/P, P/Zr, Rb/Y) of allit could also occur if the parent liquid initially had a
positive Eu anomaly or, as will be discussed later, by loss xenoliths of this group are highly variable, ranging frommuch higher to lower than those of the TSPC basaltsof interstitial liquid with a negative Eu anomaly. Samples
Hx14e, Hx14h and Hx14y also have low concentrations (Fig. 10). Such a large range of incompatible element
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Cr2O3 contents) indicate that they are probably the resultof reactions between early-crystallized mafic minerals(Cr-spinel, olivine and clinopyroxene) and an evolved,water-rich liquid. Reactions between clinopyroxene orolivine and liquid to produce hornblende, and betweenolivine and liquid to produce orthopyroxene have beenreported in low-pressure (Ζ0·3 GPa) crystallization ex-periments using basaltic to andesitic starting compositions(e.g. Holloway & Burnham, 1972; Heltz, 1973; Sisson &Grove, 1993; Grove et al., 1997; Moore & Carmichael,1998). However, co-crystallization of hornblende, ortho-pyroxene and phlogopite in reaction relationship witholivine or clinopyroxene has never been reported. Inparticular, the high Na2O contents of the phlogopite arean uncommon compositional feature, which points to amore complex differentiation history for these xenoliths(Costa et al., 2001). Recent experimental work (Prouteauet al., 2001; Costa et al., in preparation) involving inter-Fig. 8. Apatite halogen compositions of Group II xenoliths. Apatite
from some samples has subequal contents of Cl (1·4–1·6 wt %) and F actions between evolved water-rich liquids and mafic(1·4–1·7 wt %), whereas that from other samples has higher Cl minerals (e.g. forsteritic olivine) have co-crystallized horn-(1·4–2·9 wt %) than F (0·2–0·9 wt %). The field of layered intrusions
blende, orthopyroxene and phlogopite as reaction prod-includes analyses from Skaergaard, Jimberlana, Dufek, Munni Munni,Penikat, Great Dyke, Mt. Thirsty, Ora Banda Sill, Windimurra, Still- ucts, and thus an alternative possibility to closed-systemwater and Bushveld (not below major platinum group element bearing crystallization is that these minerals in the San Pedrozones). References have been given by Boudreau (1995). Figure redrawn xenoliths are due to ingress of a differentiated melt (e.g.from Boudreau (1995). OHc indicates calculated OH from structural
dacitic) by displacement of the mafic interstitial liquidformula.in equilibrium with olivine, clinopyroxene and high-anorthite plagioclase. This process could explain severalratios further indicates that processes other than mineraltextural and mineralogical features of the xenolithsaccumulation (e.g. migration of interstitial melt and aque-such as the compositional gap between plagioclase coresous fluids) have contributed to bulk compositional vari-(An85–70) and rims (An45–20) [a detailed discussion of traceations among the xenoliths.element composition of plagioclase has been given byCosta (2000)], and the coexistence of forsteritic olivineand rhyolitic glass, both of which are atypical of closed-GROUP IIHN xenoliths: hornblende noritessystem crystallization (e.g. Brophy et al., 1996), and arewith subsolidus texturesmore characteristic of mixing or mingling between felsicCompared with the mean basaltic composition of theand mafic magmas (e.g. Feeley & Dungan, 1996). In theTSPC these samples have low concentrations of SiO2 next section we derive some constraints on the amount(>45 wt %) and incompatible elements (e.g. K2O, Zr),and composition of the reacting liquid.and high MgO (>13·3–16·5 wt %), Ni (226–335 ppm)
and Ca/Na values (5·2–6·3), and thus olivine and plagio-clase accumulation could partly explain their low in- Mass-balance constraints on the amount and compositioncompatible element abundances (Fig. 9). However, ratios of the reactive meltof incompatible elements such as K/P (7–25) and Rb/ To test the hypothesis of melt migration, we have under-Y (2–3) range from those of the TSPC basalts to higher taken least-squares mass-balance calculations using the(Fig. 10), suggesting that apart from mineral ac- composition of the cumulus minerals (high-anorthitecumulation, migration of interstitial melt and fluids are plagioclase, clinopyroxene, Cr-spinel and olivine) and aimportant processes for understanding the petrogenesis representative dacitic composition of Volcan San Pedroof these xenoliths. as an analogue for the replacive interstitial melt. The
results show that (Table 7): (1) the residuals (R2) arelow (<1) and thus do not preclude the melt migration
DISCUSSION hypothesis; (2) before reaction the three xenoliths con-Melt migration and reaction in Group I sisted of large amounts of olivine (42–45 wt %), so thatxenoliths for sample Hx14n the calculated amount of olivine before
reaction is more than twice the observed amount; (3)Textural relations and compositions of orthopyroxene,hornblende and phlogopite (e.g. high mg-number and the amount of clinopyroxene that was consumed
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Table 6: Whole-rock major and trace element analyses
Group I: olivine norites Group IICL: Cpx and Hbl leuconorites
Hx14b Hx14k Hx14n Hx12a Hx12b Hx14a Hx14c Hx14d Hx14e Hx14h Hx14i Hx14j
X-ray fluorescence (wt %)
SiO2 46·4 45·8 47·3 52·0 51·7 50·9 52·2 52·3 46·2 46·9 51·1 50·4
TiO2 0·54 0·64 0·58 1·30 0·81 1·00 0·47 0·91 0·26 0·70 1·06 0·92
Al2O3 11·72 10·63 10·94 18·17 18·02 17·75 18·90 18·96 23·41 21·59 18·86 17·30
Fe2O3∗ 13·25 13·10 11·73 9·72 8·93 9·78 7·72 8·85 6·54 8·91 8·95 9·95
MnO 0·20 0·19 0·18 0·16 0·15 0·16 0·13 0·16 0·10 0·11 0·15 0·20
MgO 19·6 21·4 20·8 5·47 7·26 7·09 7·95 5·21 9·74 7·64 5·91 8·17
CaO 5·26 5·07 5·44 8·77 9·10 9·20 8·17 9·26 11·59 10·24 10·16 10·40
Na2O 2·16 2·06 1·67 3·63 3·57 3·25 3·11 3·72 1·57 2·25 3·16 2·62
K2O 0·56 0·52 0·71 0·72 0·49 0·29 0·78 0·37 0·34 0·57 0·35 0·23
P2O5 0·13 0·13 0·11 0·35 0·12 0·19 0·12 0·24 0·08 0·09 0·09 0·04
Sum 99·9 99·6 99·5 100·3 100·2 99·6 99·6 100·0 99·8 99·0 99·8 100·2
X-ray fluorescence (ppm)
Nb 3·0 3·0 2·0 4·9 2·0 2·7 1·7 1·5 0·9 0·2
Zr 56 56 53 84 50 38 34 35 11 14
Sr 381 333 316 603 692 675 672 647 725 734 685 616
Zn 106 109 94 83 78 85 57 74 65 81
Ni 446 643 621 39 107 21 161 133 46 64
Cr 707 1065 1308 63 160 72 189 213 67 219
V 109 109 142 195 188 157 46 193 265 214
Ce 19 12 9 33 19 23 12 14 6 13
Ba 181 167 186 350 165 245 106 222 188 123
La 8 3 6 15 7 8 3 6 3 3
Y 8 9 17·8 10·5 10·6 5·3 11·5 4 4·3 7·5
Rb 12 10 14·8 9·9 3·8 27·4 6·2 9·5 14 3
Ga 11 11 20 17 19 17 20 15 19 17
ICP-AES (ppm)
La 6·4 6·5 14·3 6·0 9·5 4·1 5·4 4·9 3·4
Ce 15·5 12·4 34·8 15·2 22·5 10·0 11·1 10·2 7·0
Pr 2·1 1·6 4·5 2·1 3·0 1·3 b.d. b.d. b.d.
Nd 7·6 7·1 19·0 8·7 12·1 4·8 4·7 6·1 4·6
Sm 2·1 1·9 4·7 2·6 3·2 1·2 1·2 1·6 1·3
Eu 0·63 0·59 1·39 1·04 1·32 0·42 0·57 0·85 0·80
Gd 1·5 1·7 3·7 2·3 2·5 1·0 b.d. 1·5 1·5
Dy 1·4 1·6 3·1 1·9 2·1 b.d. b.d. 1·6 1·4
Ho 0·26 0·35 0·60 0·41 0·38 0·18 b.d. 0·33 0·34
Er 0·8 0·9 1·4 1·1 0·9 b.d. b.d. b.d. b.d.
Tm 0·11 0·13 0·20 0·15 0·13 b.d. b.d. b.d. 0·13
Yb 0·6 0·8 1·2 0·8 0·8 b.d. b.d. b.d. b.d.
Lu 0·10 0·11 0·18 0·11 0·11 b.d. b.d. 0·10 0·11
mol
Ca/Na 2·35 2·37 3·14 2·33 2·46 2·73 2·53 2·40 7·11 4·38 3·10 3·83
K/P 8·2 7·6 12·3 3·9 7·8 2·9 12·4 2·9 8·1 12·0 7·4 10·9
P/Zr 10·1 10·1 9·1 18·2 10·5 — — 27·6 10·3 11·2 35·7 12·5
Rb/Y 1·5 1·1 — 0·8 0·9 0·4 5·2 0·5 2·4 3·3 — 0·4
Eu/Eu∗ 1·03 — 0·98 0·98 — 1·27 — 1·38 1·14 — 1·65 1·75
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COSTA et al. GABBROIC XENOLITHS, VOLCAN SAN PEDRO
Group IICL: Cpx and Hbl leuconorites Group IIHN Basalts of TSPC
Hx14l Hx14m Hx14q Hx14s Hx14u Hx14w Hx14y Hx14x Hx14v Hx14z Mean 2�
X-ray fluorescence (wt %)
SiO2 49·6 48·9 49·4 49·2 51·7 49·4 46·1 49·1 44·9 45·8 50·7 0·6
TiO2 1·32 1·20 0·78 0·87 0·94 1·31 0·12 1·41 0·28 0·34 1·92 0·05
Al2O3 18·29 19·04 18·16 18·19 17·66 18·31 21·76 18·98 17·43 19·50 17·75 0·88
Fe2O3∗ 10·68 10·90 9·80 9·60 9·60 10·74 6·87 10·04 9·78 9·50 9·30 0·34
MnO 0·16 0·15 0·16 0·18 0·16 0·16 0·11 0·18 0·14 0·14 0·15 0·02
MgO 6·07 5·70 8·24 7·98 7·59 6·11 10·75 5·19 16·45 13·31 8·15 0·60
CaO 9·76 10·57 10·09 9·90 8·94 9·79 10·57 9·69 8·99 9·24 8·86 0·69
Na2O 3·45 3·38 2·88 3·00 3·40 3·53 1·33 4·10 1·37 1·72 3·27 0·13
K2O 0·69 0·41 0·80 0·57 0·37 0·68 2·48 0·52 0·26 0·47 0·85 0·14
P2O5 0·22 0·15 0·02 0·12 0·13 0·22 0·06 0·31 0·07 0·04 0·17 0·05
Sum 100·3 100·4 100·3 99·6 100·5 100·2 100·2 99·5 99·7 100·1 100·1 0·17
X-ray fluorescence (ppm)
Nb 4·5 2·4 1·0 1·2 1·6 2·1 1·0 3·9 1·0 1·4 2·7 1·1
Zr 79 42 7 20 32 40 23 70 30 28 83 28
Sr 562 633 594 591 592 868 586 693 573 632 614 86
Zn 78 75 68 84 72 32 75 88 73 78 77 7
Ni 45 34 95 78 91 36 237 21 335 226 109 31
Cr 95 69 212 229 204 76 161 51 252 305 252 103
V 234 244 208 212 154 58 33 242 57 91 197 21
Ce 25 17 7 11 18 12 9 35 13 10 23 7
Ba 213 178 171 171 247 180 206 213 88 140 233 43
La 12 6 1 3 4 4 4 13 2 4 11 3
Y 17·6 12 6·6 8 10·6 6·7 2·9 23·4 3·5 4·2 14·9 1·6
Rb 15·4 6·1 39·5 14·1 6·4 11 79·5 7·2 7·2 14·3 16·1 6·3
Ga 20 20 18 17 20 19 14 21 13 15 19 1
ICP-AES (ppm)
La 7·5 3·3 4·2
Ce 15·5 6·7 7·9
Pr 1·9 b.d. b.d.
Nd 9·5 3·6 4·1
Sm 2·7 1·0 1·2
Eu 0·88 0·36 0·45
Gd 2·3 b.d. b.d.
Dy 2·10 b.d. b.d
Ho 0·37 0·16 0·18
Er 1·0 b.d. b.d
Tm 0·14 b.d. b.d
Yb 0·9 b.d. b.d
Lu 0·13 b.d. b.d
mol
Ca/Na 2·73 3·01 3·38 3·18 2·53 2·67 7·66 2·28 6·32 5·18 2·6 0·2
K/P 6·0 5·2 76·1 9·0 5·4 5·9 78·6 3·2 7·1 22·4 9·7 3·1
P/Zr 12·2 15·6 12·5 26·2 17·7 24·0 11·4 19·3 10·2 6·2 8·7 3·8
Rb/Y 0·9 0·5 6·0 1·8 0·6 1·6 27·4 0·3 2·1 3·4 1·1 0·4
Eu/Eu∗ — 1·05 — — — — — — — — — —
∗Total iron as Fe3+.b.d., below determination. (For methods and precision of the analyses, see the Appendix.)
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Fig. 9. Major and trace element variation diagrams of the three groups of xenoliths. Also shown is the mean composition of 10 TSPC basalts.Black arrows indicate the effects of accumulation of Pl and Ol to the TSPC composition. Ni concentration is that of Ol analysis n-V-1c. TheSr concentration of An88 and An60 is taken from the isotope dilution analyses of Pl from samples Hx14v (Sr 1130 ppm) and Hx14a (Sr 1030ppm), respectively. (See text for discussion.)
(4·5–6·5 wt %) is also significant; (4) the quantity of liquid. No experimentally determined stoichiometry forthe phlogopite reaction was found in the literature, soreactive liquid is almost the same for all three xenoliths
(30–33 wt %). As an alternative means of obtaining a we assumed that the quantity of liquid consumed bythis reaction equals the observed modal proportion ofrough estimate of the amount of interstitial melt that
could have been displaced, we have used the proportions phlogopite. Lastly, we have estimated that plagioclaserims are one-third of the plagioclase in the xenoliths.of post-cumulus minerals (hornblende, orthopyroxene,
phlogopite, plus plagioclase rims) and glass present in The calculations (Table 8) give similar results to thoseobtained from mass-balance constraints, i.e. (1) largethe xenoliths. However, as hornblende, orthopyroxene
and phlogopite are the products of reactions that con- amounts of olivine and clinopyroxene were consumedby the reactions and (2) the amount of liquid that wassumed liquid and minerals, their observed modal abund-
ances do not directly correspond to the porosity at consumed is 32–33 wt %. Although the calculationsmight not be very accurate because the stoichiometrythe time of melt migration. As an approximation, the
proportions of liquid and minerals that participated in of the reactions depends on the liquid composition, itillustrates how estimating the amount of ‘trapped melt’the reactions were taken from the literature: for the
hornblende reaction we have used the stoichiometry or porosity in cumulate rocks by considering only theamount of post-cumulus minerals could be misleading if(wt %) determined by Sisson & Grove (1993): 100 Hbl=
22 Ol + 38 Cpx + 42 liquid. For the orthopyroxene there are reactions involved. For example, if we take theamount of post-cumulus minerals and glass as rep-reaction we have estimated the stoichiometry (wt %)
suggested by Kelemen (1990): 100 Opx = 60 Ol + 40 resentative of ‘trapped melt’ the values vary between 44
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COSTA et al. GABBROIC XENOLITHS, VOLCAN SAN PEDRO
Fig. 10. Variation diagrams of element ratios of the three groups of xenoliths. The legend is the same as in Fig. 9. (See text for discussion.)
and 58 wt %, up to 1·8 times higher than the calculated anomalies could be due to loss of interstitial liquid richamount (32–33 wt %). in REE with a negative Eu anomaly.
The large ranges of ratios of incompatible elementsinvolving Rb and K suggest that an aqueous fluid hasalso modified the bulk-rock composition of some xeno-Textural, mineralogical and compositionalliths. For example, many samples have Rb/Y valuesevidence for melt and fluid migration insimilar to or lower than those of the TSPC basalts,GROUP II xenolithswhereas others have very high Rb/Y. As Y is highly
The low concentrations of incompatible elements (e.g. compatible in apatite (e.g. Pearce & Norry, 1979), theZr) of most xenoliths are not correlated with high con- low Rb/Y values of some xenoliths could be explainedcentrations of compatible elements (e.g. Sr, Ni), suggesting
by loss of interstitial liquid after apatite crystallization.that loss of evolved interstitial liquid rather than mineralHowever, the high Rb/Y of other xenoliths suggests thataccumulation is responsible for the low incompatiblethey have gained Rb with respect to Y. Decoupling ofelement abundances. Only six samples have major andK and Rb from the rest of incompatible elements cantrace element abundances that could be partly explainedbe produced by the involvement of an aqueous fluidby accumulation of plagioclase (Hx14w) or plagioclasephase, as fluid–melt partition coefficients of K and Rb+ olivine (Hx14e, Hx14h, Hx14v, Hx14y and Hx14z)are much higher than those of Y and Zr (e.g. Keppler,(Fig. 9). However, the large range of P/Zr values cannot1996). Accordingly, the high Rb and K2O concentrationssimply be the result of melt loss from a crystal pileand the high Rb/Y (or low P/Rb) of some xenolithsconsisting of plagioclase, pyroxenes or olivine, as the(Fig. 10) could be explained by the arrival of an aqueouspartition coefficients of P and Zr in these minerals arefluid phase rich in alkalis (e.g. K, Rb, Na) that dissolved<0·1 (e.g. Rollinson, 1993). Thus, we propose that ex-into the remaining melt. Further evidence for the arrivalpulsion of interstitial liquid occurred both before andof a fluid is the variable halogen contents of apatite.after apatite crystallization. Samples with high P/Zr lostFluid addition to a melt can be recorded as high Cl/Fmelt after apatite crystallization, whereas xenoliths within apatite (e.g. Boudreau & McCallum, 1989) becauseP/Zr values within the range of the TSPC basalts (butCl tends to partition into the fluid phase, whereas Fwith low Zr and P2O5 concentrations) lost melt beforeremains in the melt (e.g. Candela & Piccoli, 1995; Vil-apatite crystallization (Fig. 10). Xenoliths that lost meltlemant & Boudon, 1999). Apatite from one xenolithbefore apatite crystallization are commonly those that
show positive Eu anomalies, which suggests that the Eu (Hx12a) has low Cl/F (Fig. 8), whereas the rest have
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Table 7: Least-squares mixing model using the cumulate mineralogy (Group I xenoliths) and a dacitic
composition of Volcan San Pedro
Hx14b Calc. mixa Residual Hx14k Calc. mix Residual Hx14n Calc. mix Residual Dacite (H23)
SiO2 47·1 47·1 0·00 45·9 45·9 0·03 47·5 47·4 0·03 63·5
TiO2 0·55 0·36 0·20 0·64 0·38 0·26 0·58 0·34 0·24 0·64
Al2O3 11·90 11·88 0·02 10·66 10·67 0·00 10·98 10·94 0·03 16·70
FeO∗ 12·10 11·88 0·22 13·14 12·91 0·24 11·77 11·61 0·15 5·00
MnO 0·20 0·17 0·03 0·19 0·18 0·01 0·18 0·17 0·01 0·09
MgO 19·9 19·9 0·03 21·5 21·5 0·05 20·9 20·9 0·02 2·28
CaO 5·34 5·24 0·10 5·09 5·00 0·09 5·46 5·42 0·04 4·79
Na2O 2·19 1·82 0·37 2·07 1·64 0·42 1·68 1·75 0·07 4·34
K2O 0·57 0·82 0·25 0·52 0·75 0·23 0·71 0·80 0·09 2·45
P2O5 0·13 0·06 0·06 0·13 0·06 0·07 0·13 0·06 0·07 0·19
Cr2O3 0·10 0·75 0·64 0·15 0·93 0·78 0·19 0·59 0·40 0·00
R2 0·73 0·98 0·26
Cumulate minerals (wt %)
Spl (n-1-27) 4·5 5·5 3·4
Fo83 (n-V-1c) 41·8 44·8 43·3
An80 16·6 13·5 14·4
Cpx (n-III-2r) 3·9 5·7 6·5
Total of crystals 66·8 69·6 67·6
Dacite (wt %) 33·2 30·4 32·4
aCalc. mix is wt % of cumulate + wt % of dacite.∗Total iron as Fe2+.R2, squared sum of the residuals; Spl, spinel; Fo, forsterite; An, anorthite; Cpx, clinopyroxene.
much higher Cl/F, suggesting that it has crystallized brittle deformation, melt migration occurred mostlythrough microfractures. The remaining interstitial meltfrom melts enriched in Cl by fluid addition. Because itwas enriched by aqueous fluids (and alkalis), which causedis not apparent from Fig. 9, it is worth noting that samplesdissolution of olivine, pyroxenes and plagioclase andthat lost melt before apatite crystallization also havecrystallization of poikilitic hornblende and Na-rich phlo-apatite with high Cl/F (e.g. Hx14y and Hx14e), sug-gopite with high mg-numbers and Cr2O3. The availablegesting that fluid arrival might have post-dated meltstable isotope analyses (sample Hx14h, whole rock:migration.�18O = 5·4; sample Hx14v: hornblende, �18O = 5·3;The bent laths and microcracks displayed by the plagio-bulk rock, �D = −62, all values relative to SMOW; B.clase and the microfractures filled with hornblende, phlo-S. Singer, unpublished data, 1993) suggest that the fluidsgopite, orthopyroxene and magnetite (Fig. 4c–e) arethat fluxed the xenoliths were magmatic and not meteoricinterpreted as textural evidence of expulsion of interstitial(e.g. Taylor & Forester, 1979).liquid by compaction of a crystal-rich magma. Micro-
fractures commonly cut across bent plagioclase crystals,suggesting that rock deformation changed from ductileto brittle (e.g. Kronenberg & Shelton, 1980), or thatthe behaviour of plagioclase changed from plastic to
IMPLICATIONS OF THE PRESENCEcataclastic (e.g. Hacker & Christie, 1990). In manysamples poikilitic hornblende contains plagioclase with OF HORNBLENDE ANDbent twins (Fig. 4c and e). Thus, deformation of the PHLOGOPITE IN SUBDUCTION-crystal pile and expulsion of interstitial liquid seem to
RELATED GABBROIC ROCKShave occurred before arrival of aqueous fluids. Perhapsduring initial ductile deformation of the crystal matrix, Most of San Pedro gabbroic xenoliths have significant
proportions of hornblende and phlogopite, either as smallmelt migrated through the pore spaces, and later, during
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COSTA et al. GABBROIC XENOLITHS, VOLCAN SAN PEDRO
Table 8: Estimate of the mineral and liquid proportions consumed by reactions (Group I xenoliths; values are
all wt %)
Calculated mineralogy before
Mode reaction
Hx14b Hx14k Hx14n Hx14b Hx14k Hx14n Hx14b Hx14k Hx14n
Ol 32·0 24·7 22·4 Liquid + Ol = Opx∗ Ol 39·9 42·5 40·5
An80 22·8 14·8 13·8 Ol 5·3 13·1 10·7 Cpx 4·4 7·7 12·5
An40 11·1 7·2 6·7 liquid 3·5 8·7 7·1 An80 22·8 14·8 13·8
Opx 8·8 21·8 17·8 liquid + Cpx + Ol = Hbl†Spl 1·4 2·2 0·4
Cpx 0·0 0·0 0·3 Ol 2·7 4·7 7·5 Total 68·5 67·2 67·1
Hbl 12·2 21·5 33·9 Cpx 4·4 7·7 12·2
Phl 1·6 7·3 2·8 liquid 5·1 9·0 14·2 Calculated amount of replaced melt
Spl 1·4 2·2 0·4 An40 11·1 7·2 6·7
Glass 10·1 0·5 2·1 Total of reacted minerals and liquid Phl 1·6 7·3 2·8
Ol 8·0 17·8 18·1 liquid 8·7 17·7 21·3
Cpx 4·4 7·7 12·2 Glass 10·1 0·5 2·1
Tot min 12·4 25·5 30·3 Total 31·5 32·8 32·9
liquid 8·7 17·7 21·3
∗Opx reaction, 100 Opx=60 Ol + 40 liquid (in wt %). Estimated from Kelemen (1990).†Hbl reaction, 100 Hbl= 22 Ol + 36 Cpx + 42 liquid (in wt %). From Sisson & Grove (1993).Mineral symbols after Kretz (1983). (See text for discussion.)
crystals filling microfractures, or as large poikilitic post- Arculus et al., 1976, Bogoslof Volcano, Alaska; Luhr &Carmichael, 1985, Cerro la Pilita, Mexico; Peterson &cumulus crystals that can make up >50 vol. % of the
rock. In this respect they are not unusual, and a survey Rose, 1985, Ayarza caldera, Guatemala; Rose, 1987,Santa Marıa Volcano, Guatemala). Apart from the pos-of the literature shows that the majority of subduction-
related gabbroic xenoliths and plutons have important sibility that the hydrous minerals are produced byreactions triggered by protracted closed-system crys-amounts of hornblende, and occasionally phlogopite. In
some localities, at least one generation of hornblende is an tallization, the petrological, mineralogical and geo-chemical characteristics of the San Pedro xenolithsearly-crystallizing mineral (Ulmer et al., 1983, Adamello
batholith, Italy; Sisson et al., 1996, hornblende gabbro suggest that the large proportions of hornblende withhigh mg-numbers and Cr2O3 in gabbroic rocks can be thesill, California), whereas in others it is a late phase in a
reaction relation with other minerals (Smith et al., 1983, result of reactions between early-crystallized refractoryminerals (olivine, Cr-spinel, pyroxenes and plagioclase)Peninsular Ranges batholith, California; Regan, 1985,
Coastal batholith of Peru; Whalen, 1985, Uasilau–Yau and evolved melt± aqueous fluids that percolate throughcrystal-rich mafic magmas, and do not necessarily implyYau Intrusive Complex, New Britain; Himmelberg et al.,
1987, Yakobi intrusion, Alaska; Beard & Day, 1988, hornblende crystallization from water-rich maficmagmas. These melts and aqueous fluids could be derivedSmartville Complex, California; DeBari & Coleman,
1989, Tonsina Complex, Alaska; Springer, 1989, Pine from within the cumulate pile of the intrusion, as has beendocumented for the Skaergaard intrusion (e.g. McBirney,Hill Complex, California; Kepezhinskas et al., 1993,
Kamchatka; DeBari, 1994, Fiambala intrusion, Ar- 1995) or from broadly contemporaneous felsic magmas(e.g. Sha, 1995). Interactions between partially solidifiedgentina; Tepper, 1996, Chilliwack batholith, Washing-
ton; Roberts et al., 2000, Querigut Complex, French mafic cumulate piles and invading water-rich silicicmagmas may provide a means of stabilizing substantiallyPyrenees). The high proportions of hydrous minerals in
gabbroic plutons and xenolith suites is in marked contrast larger quantities of amphibole and mica (e.g. >50%mode) in subvolcanic reservoirs or plutons than closed-with the rare occurrences of hornblende phenocrysts in
basalts or basaltic andesites in arc volcanoes world wide system evolution of either mafic or silicic magmas. Partialmelting and recycling of these hydrous and alkali-en-(Sigurdsson & Shepherd, 1974, Kick’em-Jenny Volcano,
Lesser Antilles; Arculus, 1976, Grenada, Lesser Antilles; riched plutonic facies could have important implications
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JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002
Arculus, R. J. (1976). Geology and geochemistry of the alkali basalt–for magma evolution at long-lived subduction-relatedandesite association of Grenada, Lesser Antilles island arc. Geologicalvolcanic centres. Assimilation of partially or completelySociety of America Bulletin 87, 612–624.solidified metasomatically modified cumulates could lead
Arculus, R. J. & Wills, K. J. A. (1980). The petrology of plutonic blocksto complex and apparently uncoupled major and trace and inclusions from the Lesser Antilles island arc. Journal of Petrologyelement trends in the affected lavas (e.g. Upper Placeta 21, 743–799.San Pedro sequence of the TSPC, Dungan et al., 2001). Arculus, R. J., DeLong, S. E., Kay, R. W., Brooks, C. & Sun, S. S.
(1976). The alkalic rock suite of Bogoslof Island, eastern AleutianPartial melting of such gabbroic plutons could also leadarc, Alaska. Journal of Geology 85, 177–186.to the generation of second-stage silicic magmas (e.g.
Beard, J. S. (1986). Characteristic mineralogy of arc-related cumulateFeeley et al., 1998).gabbros: implications for the tectonic setting of gabbroic plutonsand for andesite genesis. Geology 14, 848–851.
Beard, J. S. & Borgia, A. (1989). Temporal variation of mineralogyand petrology in cognate gabbroic enclaves at Arenal volcano, CostaCONCLUSIONSRica. Contributions to Mineralogy and Petrology 103, 110–122.
A detailed petrographic and compositional study of two Beard, J. S. & Day, H. W. (1988). Petrology and emplacement ofgroups of gabbroic xenoliths from the Tatara–San Pedro reversely zoned gabbro–diorite plutons in the Smartville complex,
northern California. Journal of Petrology 29, 965–995.Complex has shown that these xenoliths record multistageBoudreau, A. E. (1995). Fluid evolution in layered intrusions: evidencecrystallization histories involving migration of melts and
from the chemistry of the halogen-bearing minerals. In: Thompson,aqueous fluids. Reaction of early-crystallized refractoryJ. F. H. (ed.) Magmas, Fluids, and Ore Deposits. Mineralogical Associationminerals (Cr-spinel, olivine, pyroxenes and plagioclase)of Canada, Short Course Series 23, 25–45.
with percolating, evolved melts and aqueous fluids Boudreau, A. E. (1999). Fluid fluxing of cumulates: the J-M Reef andtriggered crystallization of significant proportions of horn- associated rocks of the Stillwater Complex, Montana. Journal of
blende and Na-rich phlogopite with compositional sig- Petrology 40, 755–772.Boudreau, A. E. & McCallum, I. S. (1989). Investigations of thenatures (e.g. high mg-numbers and Cr2O3) that might
Stillwater Complex: Part V. Apatites as indicators of evolving fluidotherwise be attributed to crystallization from primitivecomposition. Contributions to Mineralogy and Petrology 102, 138–153.basaltic magmas. This implies that the much higher
Brophy, J. G., Dorais, M. J., Donnelly-Nolan, J. & Singer, B. S. (1996).abundance of mafic hydrous minerals in subduction-Plagioclase zonation styles in hornblende gabbro inclusions from
related gabbros compared with arc basalts or basaltic Little Glass Mountain, Medicine Lake volcano, California: im-andesites may be explained by the complex differentiation plications for fractionation mechanisms and the formation of com-histories of plutonic crystal-rich magmas. position gaps. Contributions to Mineralogy and Petrology 126, 121–136.
Candela, P. A. & Piccoli, P. M. (1995). Model ore-metal partitioningfrom melts into vapor and vapor/brine mixtures. In: Thompson, J.F. H. (ed.) Magmas, Fluids, and Ore Deposits. Mineralogical Association of
Canada, Short Course Series 23, 101–127.ACKNOWLEDGEMENTSCawthorn, G. R. & O’Hara, M. J. (1976). Amphibole fractionation inWe would like to thank A. Wulff and M. Rhodes for the
calc-alkaline magma genesis. American Journal of Science 276, 309–329.X-ray fluorescence analyses, P. Voldet for the inductively Conrad, W. K. & Kay, R. W. (1984). Ultramafic and mafic inclusionscoupled plasma atomic emission spectrometry analyses, from Adak island: crystallization history, and implications for theF. Ramos for the thermal ionization mass spectrometry nature of primary magmas and crustal evolution in the Aleutian
arc. Journal of Petrology 25, 88–125.analyses, Y. Vinzce and T. Thon-That for the 40Ar/Conrad, W. K., Kay, S. M. & Kay, R. W. (1983). Magma mixing in39Ar analyses, and F. Parat for the electron microprobe
the Aleutian arc: evidence from cognate inclusions and compositeanalyses of apatite. We are also grateful to J. Barclay forxenoliths. Journal of Volcanology and Geothermal Research 18, 279–295.making available to us her unpublished experimental
Costa, F. (2000). The petrology and geochemistry of diverse crustaldata and for discussions about occurrences of hornblende- xenoliths, Tatara–San Pedro Volcanic Complex, Chilean Andes.bearing mafic lavas. Reviews of an earlier version of the Ph.D. thesis, University of Geneva.manuscript by J. Davidson, A. McBirney and P. Ulmer Costa, F., Dungan, M. A. & Singer, B. S. (2001). Magmatic Na-richare acknowledged. Comments by Tom Sisson and Rich- phlogopite in a suite of gabbroic crustal xenoliths from Volcan
San Pedro, Chilean Andes: evidence for a solvus relation betweenard Arculus helped to clarify our views on hornblendephlogopite and aspidolite. American Mineralogist 86, 29–35.stability in subduction-related mafic magmas. Fieldwork
Davidson, J. P. & Nelson, S. T. (1994). Tertiary magmatism in thewas supported by a grant from the Swiss Academy ofsouthern Andes: the basement to the Tatara–San Pedro volcanicNatural Sciences, and by research grants 20-42124-94complex, 36°S. Congreso Geologico Chileno Abstracts 7, 1316–1320.
and 20-49730-96 of the Swiss National Fonds. DeBari, S., Kay, S. M. & Kay, R. W. (1987). Ultramafic xenolithsfrom Adagdak volcano, Adak, Aleutian islands, Alaska: deformedigneous cumulates from the Moho of an island arc. Journal of Geology
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Singer, B. S., Dungan, M. A. & Layne, G. D. (1995). Textures and strument at the University of Lausanne. Analyses wereSr, Ba, Mg, Fe, K, and Ti compositional profiles in volcanic
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Major and trace elements were determined at the Uni-for the Sierra Nevada batholith. Contributions to Mineralogy and Petrology
126, 81–108. versity of Massachusetts by standard techniques (see
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Rhodes, 1988). The 2� relative precisions are as follows: Inductively coupled plasma-atomicSiO2, 0·6%; TiO2, 0·5%; Al2O3, 1%; Fe2O3, 0·5%; MnO, emission spectrometry analyses6%; MgO, 1·2%; CaO, 0·6%; Na2O, 5%; K2O, 1·5%; Rare earth element analyses were performed at theP2O5, 4·5%. Rb, 10%; Sr, 1%; Zr, 1%; Nb, 6%; La, University of Geneva. Details of the methods have been4%; Ce, 12%; Y, 2·5%; V, 2%; Cr, 1·8%; Ni, 2%; Zn, given by Voldet (1993). The relative 2� precisions range
between 5 and 10% depending on the concentration of1%.the element.
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