American Mineralogisl, Volume 71, pages 1085-1099, 1986
Origin of gabbro pegmatitein the Smartville intrusive
complex, northern Sierra Nevada,California
J,tNrBs S. Bnlno,*
Howano W. D,lv
Department of Geology, University of California, Davis, Davis, California 95616, U.S.A.
Arsrnlcr
Gabbro pegmatitesare common in cumulate olivine gabbrosin the Smartville complex,
northern California. The pegmatitesoccur as pods and segregationsand consist primarily
of calcic bytownite or anorthite and clinopyroxene,with minor orthopyroxeneand olivine.
Amphibole is common both as a late magmatic phase and as a subsolidus alteration of
magmatic amphibole and pyroxene. Many of the pegmatitesare zoned, with anorthositic
rims and pyroxenitic cores.Fine-grained,idiomorphic granular gabbro in some pegmatite
cores may be analogousto the quench aplites reported from many granitic pegmatites.
The igneousmineralogy and the mineral chemistry of five pegmatite pods are indistinguishablefrom that of their host gabbros.This suggeststhat these pegmatitesformed in
place and in equilibrium with their gabbro host. One pegmatite intrusive dike is more
mafic than its host gabbro, suggestingthat this pegmatite melt did not form in place. The
presenceof gabbro "aplites," the zoning of the pegmatites, and the high temperatures
calculated from coexisting pyroxenes support the idea that these pegmatites formed by
crystallization of a melt rather than by subsolidus replacement.High fluid pressuresare
suggestedby the compositions of coexistingolivine and plagioclase,by the presenceof late
magmaticamphibole, by the presenceof fluid inclusions,and by the restriction of subsolidus
hydrous alteration assemblageslargely to the cores of the gabbro pegmatites.
We proposethat the pegmatitesformed from a mafic intercumulus melt in the presence
ofa fluid phase.The evolution ofthe fluid phasemay have been enhancedby the drop in
confining pressurethat accompaniedre-emplacementof the cumulate gabbros as crystal
mushes.The disruption of the cumulate pile may have enhancedthe migration and accumulation of the intercumulus melt.
INrnooucrroN
Gabbro pegmatites are a common, but little studied,
constituent of mafic intrusive rocks. They have been reported from ophiolites (Moores, 1969; Evarts, 1977;
Lindsley-Griffin,1977;Allen, 1975; Wilson, 1959),layered mafic-ultramafic complexes(Hess, 1960; Taylor and
Forester,I 979), Alaska-typeultramafic rocks(Irvin e, 1974;
James, l97l), gabbroic plutons in orogenic areas(Snoke
et al., l98l; Smith et al., 1983; Lovering and Durrell,
1959; this study), and plutonic inclusions in calc-alkaline
arc volcanic rocks (Stern, 1979; Conrad and Kay, 1984).
The pegmatitesrange from kilometer-scale plutonic intrusions (Lindsley-Grifrn, 1977)to segregationsof a few
centimeter-sizedcrystalswithin a normal gabbro (this report). They commonly occur as pipes, sills, dikes, and
lenticular pods.
Most pegmatitesreported in the literature contain calcic
bytownite or anorthite, but their mafic mineralogy is more
diverse. In some pegmatites,amphibole is the principal
r Present address:Dept. of Mineral Sciences,National Museum of Natural History, Smithsonian Institution, Washington,
D.C. 20560,U.S.A.
l 0-l 085s02.00
0003-004x/86/09
mafic constituent (Irvine, 1974);in others, pyroxene pr€dominates(Hess, 1960).Pegmatitic ultramafic rocks consisting of various proportions of olivine, pyroxene, and
magnetite were reported by Cameron and Desborough
(1964)and Viljoenand Scoon(1985).Zoningtowardmore
felsic compositions is rare, but has been reported from
some gabbroic pegmatitesin the northern Sierra Nevada
(Lovering and Durrell, 1959).
The presenceof pegmatite is commonly consideredan
indication that a fluid phaseevolved in the magma (Jahns
and Burnham,1969). Ifone acceptsthis premise,the presenceofgabbro pegmatitein gabbrosfrom diversegeologic
environments might suggestthat the evolution of a fluid
phase is a fundamental part of the crystallization and
cooling history of these rocks. In spite of these implications for rocks generallyconsideredto be poor in volatile
species,few detailed studies of gabbro pegmatites have
been reported (cf. Cameron and Desborough, 1964; Viljoen and Scoon, 1985). In particular, little is known of
the geochemicalrelationship between gabbro pegmatites
and their host rocks. This paper reports the textures,mineralogy, and mineral chemistry of several small gabbro
pegmatitesand their olivine gabbro host rocks from the
1 0 85
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
1086
ofany composition are rare or absentin other rock types
within the zoned plutons.
Host gabbro
Petrography. The olivine gabbros are fresh rocks with
grain sizes rangrng from I to 5 mm. Igneous foliation,
defined by the preferred orientation of tabular minerals,
especiallyplagioclase,is commonly well developed.LenRANGE
soidal, discontinuous anorthosite layersare presentin some
places.The gabbro consistsof calcic bytownite or anorthite, clinopyroxene,and olivine (Fou.ro).Orthopyroxene
o
to 20Ml
is present in minor amounts in most samples.Most or--+-rJ
o ro20KM
thopyroxene occrus as thin reaction rims around olivine.
Someorthopyroxenein the reaction rims forms symplec6'
tic intergrowths with magnetite, suggestingsome olivine
9,
reaction occurred at subsolidus temperatures(Hag€erty,
1976;Ambler and Ashley, 1977).Chromian magletite or
magnetite is present in small amounts in most samples.
Pargasitic amphibole is a common late-crystallizing inL\
9.
D o vi s
terstitial phase.Someamphiboles are strongly zoned, and
I
compositions in single grains may range from pargasite
to actinolite. Biotite and apatite are minor accessoryminerals in some samples.
BELr(wBl
Snoke et al. (1982) recognizeda suite ofcalc-alkaline
F.€ wesrenru
ll.'-liil.ott.*N BELr(EB)
ultramafic
to dioritic intrusive complexes in the Sierra
(
F
B
)
F E A T H E RR I V E R
S M A R T V I L L EC O M P L E X
Nevada and Klamath Mountains. On the basis of criteria
BELT
PERIDOTITE
I n t r u s i v eR o c k s ( S l l
laid out in their study and discussedin Beard (1985), the
(cB)
BELr
cer.rrnnl
E
x t r u s i v eR o c k s ( S V )
lTl
gabbroicplutons of the Smartville complex, including the
Fig. l. Locationof the Smartvillecomplexin the northern olivine gabbros that host the gabbro pegmatites, are part
ofthat suite. Arguments concerningthe genesis,tectonic
SierraNevada.The study areais outlined.
significance,and emplacementhistory of theseintrusives
are given in Snokeet al. (1982)and Beard(1985)and are
Smartville complex. We will propose that these pegma- beyond the scopeofthis paper. However, the conclusions
tites are not subsolidusfeatures,but mafic analoguesof ofthese studiesrelating to the olivine gabbrosare relevant
$anitic pegmatites(Jahnsand Burnham, 1969),perhaps to this investigation. First, the olivine gabbros in the
implying hypersolidusevolution ofa fluid phasein a mafic Smartville complex are mineralogically and geochemimagma.
cally identical to olivine gabbro cumulates found as xenoliths in volcanic rocks and in intrusive complexes in
Gnor,ocrc sETTTNGAND occuRRENCE oF
modern arcs (Beard, 1985; Beard and Day, I 984; also see
THE PEGMATITES
Chivas, 1977; Stern, 1979; Arculus and Wills, 1980).It
The Smartville complex is a rifted, Late Jurassic,en- appears,therefore,that the olivine gabbrosof the Smartsimatic arc located in the northern foothills of the Sierra ville zoned plutons and of the Sierra-Klamath intrusive
Nevada (Fig. l). It consistsof a carapaceof mafic to in- complexes, in general, are cumulate rocks that do not
termediatevolcanic rocks intruded by a plutonic and hyp- representliquid compositions. Secondly, the Smartville
abyssalcore (Moores,1972;Beardand Day, 1982, 1983; olivine gabbros,along with the other cumulate rocks in
Beard, I 985; Menzieset al., I 980; Xenophontosand Bond, the Sierra-Klamath intrusive complexes, lack cumulate
I 978; Day et al., I 985; Xenophontos, I 984).The intrusive structures and textures and commonly have brecciated
core of the Smartville includes a series of older meta- contactsor otherwise deformed wall rocks (James, l97l;
morphosed massive diabases,fine-grained diorites, and Snoke et al., l98l; Beard, 1985). Ultramafic dikes, ingabbros.Thesewere intruded by a sheeteddike complex cluding dunites, are not uncommon. The cumulate rocks
and a seriesofcoeval tonalite and gabbro-diorite plutons of the Sierra-Klamath intrusive complexes,including the
during intra-arc rifting @eard and Day, I 983; Beard, I 985) Smartville gabbros, are interpreted as deep-seatedcu(Fig. 2).
mulate piles that were mobilized and re-emplacedas crysThe gabbro-diorite plutons range from olivine gabbro tal mushesat a higher level in the crust.
intrusions, 200-1000 m in diameter, to elongateplutons,
25 km long, that are zoned from olivine gabbro in their Occurrenceand structure of the pegmatites
cores to quartz diorite at their margins. Gabbroic pegGabbro pegmatitesoccur in the olivine gabbrosas pomatites occur primarily in the olivine gabbros.Pegmatites diform or, rarely, tabular bodies less than I m in maxio: cB
I
I
1087
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
121'15
121"4 5'
39" 30
SWEDES
FLAT
PLUTON
1..- 879-136
65
EXPLANATION
rqiSt
Li$l s'.o
G r a n o d i o r i t eU n i t P o s t - N e v a d a ng r a n o d i o r i t e
p l u t o n s L o c a l l yg r a d a t i o n a lt o g r a n i t e
SMARTVILLE COMPLEX
V O L C A N I CR O C K S
ffi
U p p e r V o l c a n i cU n i t I n t e r m e d i a t ec o m p o s i t i o n
p y r o c l a s t i ca n d e p i c l a s t i cr o c k s
E
Lower Volcanic Unit Mafic flows and pillowedflows
I N T R U S I VR
EOCKS
l-l
T o n a l i t eU n i t B i o t i t e - h o r n b l e n d teo n a l i t ew i t h m i n o r
g r a n o p h y r i ct o n a l i t e
s,
gr
G r a n o p h y r i cT o n a l i t e U n i t . G r a n o p h y r i ch o r n b l e n d et o n a l i t e
C o n t a i n ss o m e n o n - g r a n o p h y r i cr o c k s .
75
sdc
D i k e C o m p l e x U n i t o f 1 0 0 %s h e e t e d a n d u n s h e e t e d f e l s i c
and mafic dikes
go
G a b b r o - D i o r i t eU n i t . Z o n e d g a b b r o - d i o r i t ep l u t o n s S o l i d p a t t e r n
ts olivinegabbro
lll:rl
Ffi
ffi
ffi
E
A
i: F-3
M a s s i v e D i a b a s e U n i t M a s s i v ed i a b a s ea n d a s s o c i a t e d
m e t a d i o r i t ea n d m e t a g a b b r o . D i k e a n d s i l l - l i k ef o r m s a r e
t a t e F o r m st h e c o u n t r vr o c k i n t h e C l a r k H i l l M i x e dZ o n e
:,r",il-f,,S
;ir-'l;:;F- 1
Jntoctttsl
conHenI
Nr"
M e t a g a b b r op l u t o n sc o e v a lw i t h t h e m a s s i v ed i a b a s e
S t e e p l yd i p p i n gt a u l t s ( F - 2 ,F - 3 , F - 4 )
\..rt-.4
T h r u s t F a u l t s( F - 1 Jh a t c h u r e so n u p p e r p l a t e
-'-
G e o l o g i cc o n t a c t s
(-
o
SCALE
|
2 3
r/
4Mi
|-1-.7.-r-l
Dike orientations
O | 2 3 4Km
I
I
I
I
,
Fig. 2. Geology of the Smartville intrusive complex. Pegmatites occur in olivine gabbro intrusions and in the olivine gabbro
coresof the larger gabbro-diorite plutons. Inset A (enlargementof loc. A) shows sample locations in the olivine gabbro core of the
PleasantValley pluton. Location B is where sample PS-I was collected.
1088
BEARD AND DAY: ORIGIN OF GABBRO PEGMATITE
Fig. 3. Pegmatiteforms and structures.(a, upper left) Pegmatite se$egation.(b, lower left) Coarse-grainedanorthositic
gabbro (transitional gabbro). (c, upper right) Pegmatitepod
(outcrop wetted to enhance contrast). (d, lower right) Pegmatite dike from which sample PS-l was collected.
BEARD AND DAY: ORIGIN OF GABBRO PEGMATITE
mum dimension. In some, areasup to 50/oof the gabbro
is pegmatitic. Six host-pegmatitepairs were collected for
this study. One pair (PS-1)was collectedfrom the olivine
gabbro core of a small pluton just west of the northern
end of the PleasantValley pluton (Fig. 2). Five other pairs
(PS-2b,PS-4a,PS-4c,PS-5,879- I 36)werecollectedfrom
the olivine gabbro core of the PleasantValley pluton (Fig.
2).
The most common pegmatitesin the olivine gabbros
are irregular stringers and segregations(Fig. 3a). These
rangefrom clustersof a few crystals, I to 2 cm in size,to
meter-scalesegregationscontaining crystals up to l0 cm
long. The contacts between the pegmatitic segregations
and the host gabbroare gradational,marked by increasing
grain sizeand, lesscommonly, by a decreasingcolor index
toward the pegmatite. Anorthositic gabbro, consisting of
l- to 2-cm euhedral plagioclasecrystals enclosedin clinopyroxene,occurs in the transition zone between some
of the larger pegmatitesand their host gabbro (Fig. 3b).
There is no indication ofeither brecciation or chilling at
the contact betweenpegmatitesand their hosts.
Pegmatitepods are also common (Fig. 3c). They range
in size from 5 to at least 30 cm. Their contacts with the
host gabbrosare sharp and are marked by an increasein
plagioclasefollowed by an increasein grain size toward
the pegmatite.Someof the pods are elongateand may be
rods in three dimensions. Others have irregular shapes
and differ from the segregations,described previously,
mainly by having sharp contacts with their host. All of
the pegmatitescollected from the PleasantValley pluton
arepods, although PS-2b,PS-4a,and PS-4caresomewhat
irregular.
Pegmatitesills and dikes are rare. All of those observed
had widths of lessthan l0 cm and many were only a single
grain (l to 2 cm) wide. Sample PS-l was collected from
the dike shown in Figure 3d.
Most pegmatite pods and dikes are zoned, with anorthositic rims and pyroxenitic cores.In granitic pegmatites,
zoning has been shown to reflect the order of crystallization, with outer zones forming earlier and at higher
temperaturesthan the inner zones (Taylor et al., 1979).
The order of crystallization implied by the zoning of the
gabbro pegmatites is the same as that observed in the
transitional gabbrosshown in Figure 3b.
A few of the pegmatitescontain enclavesof finer-grained
gabbroic material. In the irregular segregations,this material has the texture and mineralogy ofthe gabbro host
and probably reflects the irregular contact between the
pegmatite and its host in three dimensions. In the pegmatite pods, however, the included material is a finergrained (0.2 to 2 mm), more nearly idiomorphic granular
gabbro consisting of plagioclaseand clinopyroxene. In
some cases,clinopyroxene crystals up to 2 cm long are
enclosedin the fine-grainedgabbro (Fig. a).
Granitic aplites having textures similar to the finegrained gabbro enclavesare common in the cores of granitic pegmatites.Granitic aplites form late in the crystallization history of pegmatitesand have been interpreted
1089
Fig. 4. Photomicrograph
of aplite from the coreof a small
gabbropegmatite.Note idiomorphicgranulartextureand gabPlag: plagioclase;
cpx: clinopyroxene;
ecpx:
broicmineralogy.
early-formed,
largeclinopyroxene
crystalfloatingin aplitic matrix.
as pressurequenchesof the pegmatitemelt brought about
by the catastrophicloss ofa fluid phasefrom the system
(Jahnsand Tuttle, I 963; Jahnsand Burnham, 19691'Taylor et al., 1979). We suggestthat the "aplitic" gabbro
enclavesin the coresof Smartville gabbropegmatitesmay
have formed in a similar fashion.
Petrography of the pegmatites
The igneousmineralogy of the gabbro pegmatitesis the
same as that of their host gabbros. Clinopyroxene and
calcic plagioclaseare the dominant phases.Orthopyroxene is less abundant than clinopyroxene and makes up
less than 5oloof most pegmatites. Olivine is present in
small amounts.Pargasiticamphibole occursas interstitial
crystals, as a partial replacement of clinopyroxene, and,
rarely, as monomineralic veins that cut the pegmatites.
Oxide minerals, biotite, and apatite are accessoryphases.
Petrographic differencesbetween the pegmatites and
their host gabbrosinclude the modal abundancesand habits of orthopyroxeneand olivine. Olivine is a minor phase
in most pegmatites,occurring only as relics within large
orthopyroxene crystals. Orthopyroxene is much more
abundant than olivine in the pegmatiteswhile the reverse
is true in the host gabbros. Orthopyroxene occurs primarily as reaction rims on olivine in the host gabbros
whereasdiscrete grains of orthopyroxene, as well as reaction rims, occur in the pegmatites.
Unlike the olivine gabbro host rocks, the pegmatitesand particularly the coresof the pegmatites-are altered.
Augite and pargasitichornblendeare partially replacedby
actinolite. Orthopyroxene is partially replaced by actinolite and/or chlorite. Plagioclaseis commonly fresh but
is locally sausseritizedor replaced by carbonate.Olivine
is partially replacedby tremolite, talc, chlorite, or serpentine.
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109I
ponents within a single sample typically vary by I to 4
mol0/0.For most pegmatite and host pairs, within-sample
variability (two sigrna)exceedsany compositional difference between host and pegmatite. The clinopyroxene in
pegmatitePS-1, however, is considerablymore calcic and
less ferroan than clinopyroxene in the host.
Orthopyroxene. Quadrilateral orthopyroxene compositions are Enrr-rrFsr,-rrWo1-3in the pegmatites and
Enr-rrFsro-roWor-,in the host gabbro (Table 4, Fig. 5).
Total quadrilateral components in most orthopyroxenes
vary by I to 3 mol0/0.The within-sample variation (two
MrNnur- cHEMrsrRy
sigma) of quadrilateral components exceedsany compoOlivine, plagioclase,augite, orthopyroxene, and amsitional differencebetween pegmatite and host.
phibole were analyzed using a Kevex energy-dispersive
Amphibole. Selectedamphibole analysesare given in
spectrometeron an ARL-EMx-svelectron microprobe loTable 5. Amphiboles from PS-2b,PS-4a,and PS-4cpairs
cated in the geologydepartment at the University of Caland from 879-136 pegmatitewere analyzed.The amphiifornia, Davis. Operatingconditions were I 5-keV filament
bole in all of these samples is characterizedby strong
voltageand 300-nA beam current. Beam diameter was 3compositional zoning. Optically, the zoning is manifested
l0 pm for the mafic phasesand l5-30 pm for plagioclase. by a color changefrom brown or olive-greencoresthrough
Unless otherwise noted, tabulated analysesare averages a zone of green amphibole to green or blue-greenactinofthree to twelve severalspots from one to three grains. olitic rims. Figure 6a is a plot of A-site alkalis vs. Si. Ti,
MAGrcrv (Colby, 1968) was used to reduce the data. All
like the total alkalis, variesinversely with the SiO, content
minerals except amphibole were analyzedfor all six host of the amphiboles (Fig. 6b). These data show that the
and pegmatite pairs.
composition of the amphibole ranges from paragasitic
Plagioclase.Plagioclaseanalysesare summarizedin Tahornblendeto actinolite (Leake, I 978). The concentration
ble l. In all casesthe plagioclaseis very calcic, ranging in
of samplesin the high- and low-Si regionsof the diagrams
composition from An* to Ann, in both the gabbros and
largely reflects sample bias becausecores and rims were
pegmatites.Plagioclasecontainsbetween0.3 and 0.5 wolo preferentially analyzed.A microprobe traverseacrossone
FeO. KrO was below the detection limits (0.2 wto/o)of the grain in sample879-136 suggeststhat the complete range
energy-dispersivesystem.Although core-to-rim zoning was of compositions is present.
not detectedoptically, anorthite content varied by 1.5 to
DrscussroN
4.4 molo/oin most samples,suggestingsome inhomogehost
Equilibrium
between
and pegmatite
neity. Plagioclasecompositions in the pegmatite and host
are the same for all of the analyzedpairs within the limits
pegmatite
pods collected from the
gabbro
Each of the
of sample variation.
PleasantValley pluton containsminerals that areidentical
Olivine. Olivine compositionsare summarizedin Table
in composition, within the limits of observed variation,
2. Olivine is unzoned and rangesbetween Foro and For*
to the minerals in the host rock. The mineralogical simin the pegmatitesand between Fo, and Fo, in the host
ilarities within eachpegmatiteand host pair are especially
gabbro. CaO and NiO contents of the olivine were below
striking given the considerablevariation of mineral comdetection limits of the nos (less than 0.2 and 0.3 wt0/0, positionswithin the host gabbropluton. For example,host
respectively).MnO content rangesfrom 0.5 to 0.7 wto/0. gabbro samplesPS-5and PS-2bwere collectedwithin 200
In five of the samples studied, the difference in olivine
m ofone another yet olivine and plagioclasein PS-5 (Fo,
compositionsbetweenthe pegmatitesand the host gabbro
and Anor)are considerablymore mafic than in PS-2b(Fo'o
is lessthan 0.6 molo/oFo. PegmatitePS-1, however, conand Anrr). The similarity of host and pegmatite mineral
tains olivine considerablymore magnesianthat that in its
chemistry, therefore, suggeststhat equilibrium between
host gabbro.
host and pegmatite was attained on a local scaleand that
Clinopyroxene. Clinopyroxene analyses are given in
the pegmatitesformed in place.This interpretation is conTable 3, and quadrilateral compositions are plotted on
sistentwith the lack of intrusive contactsor intrusive form
Figure 5. Clinopyroxene quadrilateral compositions are of the pegmatite pods.
Enor-orFsr-,rWoor_0,
in the pegmatites and Enor_orFs,o_,0- In contrast, the mineral phasesin the pegmatite dike,
Woor_o,in the host gabbros. With the exception of peg- PS-I, are considerablymore mafic than those in the host
matite PS-l (EnorFsrWoor),the rangesfor pegmatite and
gabbro (olivine; Fo^ vs. Forr; plagioclase,An* vs. Anrr;
gabbro are virtually identical. The AlrO, content of the
clinopyroxene, EnorFsrWoo,vs. EnooFs,:Wooriorthopyclinopyroxene rangesfrom 2.2 to 3.0 wt0/0.CrrOr, NarO,
roxene, En^ vs. Enrr). SamplePS-l is the only pegmatite
and MnO contents are less than 0.3 wto/oand approach studied that has an intrusive form and may have crystaldetection limits (0.2 wto/o)in many cases.TiO, content
lized from material that had equilibrated elsewherein the
rangesfrom 0.3 to 0.5 wto/0.The total quadrilateral comcumulate pile.
Planar arrays ofsecondary fluid inclusions are present
in some samplesof plagioclase.A reconnaissancestudy
ofeleven polished thick sectionssuggeststhat fluid inclusions are restricted to plagioclaseand are abundant only
where there is independent evidence for late-stagealteration. Only a few of the fluid inclusions are as large as 2
pm in diameter and most were much smaller. Bubblesare
mostly lessthan l0o/oby volume of the inclusion, consistent with low temperaturesof entrapment, and daughter
minerals are rare.
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1093
BEARD AND DAY: ORIGIN OF GABBRO PEGMATITE
P-Z conditions of pegmatite formation
The pressuresunder which the pegmatitesformed can
be estimatedfrom the metamorphic history of the Smartville complex and from the regional associationand crystallization history of the gabbro-diorite plutons. Temperaturescan be estimatedfrom pyroxeneand, qualitatively,
from amphibole chemistry.
The pegmatitesprobably formed at relatively low pressures,almost certainly lessthan 5 kbar and possibly near
I kbar. Beiersdorfer(1983)and Xenophontos(1984)suggestedthat the regionalprehnite-pumpellyite faciesmetamorphism ofthe Smartville volcanic pile occuned at pressures between I and 3 kbar, although the low-pressure
limit for such metamorphism is poorly constrained
(Schitrmanand Liou, 1980). The associationof the gabbro-diorite plutons with a hypabyssaldike complex and
with severalplutons ofgranophyric tonalite also suggests
emplacement at shallow levels (Beard and Day, 1983;
Beard, 1985). The AlrO, content of amphiboles from
monzodioritesin the gabbro-diorite plutons suggestspressuresof 3 t 2 kbar (Beard, 1985;Hammarstrom andZ,en,
r 985).
Experimental work on a wide range of basaltic compositionshasshownthat amphiboleis a hypersolidusphase
under water-saturatedconditions at pressuresgreaterthan
800-1000 bars (Yoder and Tilley, 1962; Eggler, 1972;
Gilbert et al., 1982).The relationship betweenthe uppertemperature stability limit of amphibole and the basalt
solidus is a function of basalt composition and oxygen
fugacity, but under all conditions studied, if the melt is
water saturated,amphibole crystallizesif PHroexceeds2
kbar. The textures of the pargasitic amphiboles in the
gabbro pegmatites(i.e., replacementof clinopyroxene,interstitial crystals) suggestthat they formed at conditions
just above and, perhaps,below the solidus (Wones and
Gilbert, 1982). The late crystallization of amphibole in
the gabbrosand pegmatitesis consistentwith low-pressure
crystallization.
The temperaturesat which the pegmatitesformed can
be estimated from the compositions of coexisting clinopyroxeneand orthopyroxeneand from amphibole chemistry. Table 6 gives pyroxene temperaturescalculatedusing the pyroxene geothermometer of Lindsley (1983).
Temperatureswere calculatedusing the l-atm thermometer. Correction factors are approximately *4'lkbar for
clinopyroxene and +3.5'lkbar for orthopyroxene. Oneatmosphere clinopyroxene temperatures from the pegmatitesrangefrom 970 to l0l6.C with a mean of 992C,
whereasthose from the host gabbros range from 970 to
1020"Cwith a mean of l00l'C. Given the errors associated with the thermometers(inhomogeneityof pyroxene,
Fe3+calculations,analytical error, uncertainty in the thermometer), thesevalues can be consideredidentical.
Orthopyroxene temperatures are considerably more
variable and generally lower than those given by clinopyroxene.Pegmatitetemperatures(exclusiveof PS-I ) range
from 830 to 1030"Cwith a mean of 948"C.Host gabbro
. PEG
O HOST
Wo
O
^o'1
odd
En
Fs
80
En
Wo
70
En,
compositions.
Fig. 5. Orthopyroxene
andclinopyroxene
werecalculatedfrom atomicratiosof
Fs,and Wo components
Mg, Fe, and Ca. En, Fs, and Wo werenot correctedfor the
presence
of additionalcomponents.
temperaturesrange from 800 to 1005"C with a mean of
906t. Orthopyroxene in pegmatite sample PS-l yields a
temperatureof 700"C,much lower than any other sample.
Five samples,including two pegmatitesand three host
gabbros,yield orthopyroxenetemperatureslessthan 900"C.
In all five cases,the analyzedorthopyroxene is from orthopyroxene-magnetitereaction rims around olivine, and
the low temperaturesclearly reflect the subsolidusorigin
of the reaction rims. For such samples,the two-pyroxene
thermometer is not valid, and the clinopyroxenetemperature may representa minimum estimate.
The sevenof twelve sampleswhoseorthopyroxene and
clinopyroxenetemperaturesare in agreement,within the
by Lindsley ( 1983),
100"limits of combinederror suggested
give temperatures ranging from 975 to 1022C with a
mean of 990'C. The temperaturesgiven by the two-pyroxene thermometer are l0o higher than the l-atm basalt
solidus (Wright and Okamura, 1977) and approximately
100" above the l-kbar water-saturatedsolidus.
Oxygen-isotopedata from pegmatite sample 879-136
are consistent with high temperatures of equilibration.
Plagioclase,clinopyroxene,and whole-rock samplesshow
'80/'60 enrichments(relative to SMOW) of 6.79, 5.93,
and 6.49V*, respectively (A. Williams, written comm.).
These data are typical magmatic values, providing no
evidencefor either high-temperatureor low-temperature
alteration. The fractionation between plagioclase(Anro)
and clinopyroxene suggestsan apparent temperature of
562'C (Matthews et al., 1983, p. 640), consistentwith
closure of the minerals with respect to oxygen exchange
at lower temperaturesthan crystallization of the pyroxenes.
Amphibole compositionsin the gabbrosand pegmatites
place only qualitative constraints on late magmatic and
postmagmatictemperatures.Experimental work on a variety of basalt compositions has shown that amphiboles
with Ti, Al, and A-site alkali contents similar to the pargasitic amphibolesanalyzedhere form at temperaturesof
between 700 and 1000'C at 5 kbar water pressure(Helz,
1973,1976:Gilbert et al., 1982).
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1095
BEARD AND DAY: ORIGIN OF GABBRO PEGMATITE
Table 5. Selected amphibolespot analyses
879-136
PS-2b
sio,
Tio,
Al,o3
FeOCaO
MnO
Mgo
KrO
Naro
cl
Total
Pegmatite
Pegmatite
Host
core
Peg.
core
Core
42.16
2.05
11.58
11.58
11.87
0.15
14.84
1.24
1.97
0.26
97.71
42.83
2.25
11.31
12.25
12.38
0.22
14.19
1.31
1.66
0.30
98.71
44.28
2.65
9.45
12.91
12.20
n.d.
14.49
1.01
1.34
n.d.
98.33
46.36
1.03
7.93
12.52
12.59
n.d.
14.94
0.78
1.31
n.d.
97.47
52.37
0.42
3.69
11.92
12.82
n.o.
16.96
o.32
o.12
n.d.
98.63
6.25
1.75
o.27
0.23
1.44
0.02
3.28
1.88
0.23
0.57
15.91
6.30
1.70
0.26
0.25
1.51
0.03
3-11
1.95
0.25
0.47
15.83
6.50
1.50
0.14
0.29
1.59
6.82
1-18
0.20
0.11
1.54
7.47
0.53
0.09
0.05
1.42
3.17
1.92
0.19
0.38
15.67
3.28
1.99
0.15
0.37
15.63
3.61
1.96
0.06
0.03
15.22
Peg.
Core
Core
44.69
2.68
9.18
12.50
12.05
0.24
14.10
1.05
't.42
0.26
98.18
51.95
0.31
3.03
11.84
12.30
n.d.
17.23
0.19
o.24
n.o.
97.O2
43.88
2.63
10.37
12.85
12.09
0.26
13.29
1.36
1.57
0.34
99.07
48.92
0.75
6.56
11.77
't2.10
n.d.
16.51
0.56
0.93
n.d.
98.09
42.88
2.65
10.45
11.86
12.29
0.23
14.25
't.26
1.43
0.21
97.53
43.19
2.88
10.41
13.02
12.04
0.18
13.68
1.27
1.17
0.30
98.15
49.68
0.61
4.74
12.49
11.61
n.d.
17.10
0.57
0.78
n.d.
97.57
6.44
1.57
o.23
0.29
1.59
0.03
3.01
1.90
0.26
0.43
15.72
7.70
0.93
0.18
0.08
1.42
6.37
1.63
0.19
0.30
1.47
0.03
3.15
1.96
0.24
0.41
15.75
6.39
1.61
0.21
0.32
1.61
0.02
3.02
1.91
0.24
0.34
15.67
7.23
0.77
0.04
0.07
1.52
Cations per 23 oxygens
JI
A|v
AIVI
Ti
Fe
Mn
Mg
Ca
K
Na
Total
6.58
1.42
0.17
0.30
1.54
0.03
3.08
1.90
0.20
0.41
15.63
7.53
0.48
0.04
0.04
1.43
3.78
1.91
0.03
0.07
15.25
3.56
1.87
0.10
0.26
15.48
3.71
1.81
0.11
0.22
15.46
A/ote.'Analysesin weight percent. Peg. : psgmatite; inter. : intermediatebetween core and rim; n.d. : not determined; FeO- : total Fe as FeO.
Presenceof a silicate melt
Becausesomegabbro pegmatitesappearto form at subsolidus temperatures(Bow et al.,1982), it is important to
ascertainwhether the Smartville pegmatitesformed in the
presenceof a melt. Pyroxenethermometry indicates that
they formed above the basalt solidus, but this does not
rule out the possibility that the pegmatitesare high-temperature, subsolidus replacements of refractory cumulates.The zoning in the pegmatitessuggests,but does not
demonstrate,crystallization from a melt. The diferences
in chemical composition implied by the higher orthopyroxene: olivine ratio in the pegmatitescompared to the
host gabbros rules out isochemical replacement,but the
differencescould have developed by subsolidusreaction
with a silica-rich aqueousfluid.
The strongestevidencefor the presenceof a mafic silicate melt during pegmatite formation are the gabbroic
"aplites" found in the coresof somepegmatitepods.These
"aplites," as suggestedearlier, may be analogousto aplites
in granitic pegmatitesformed by catastrophicloss of fluid
from the magma. If the aplite analogy is valid, then (l)
the mineralogy of the gabbro aplites should resemblethe
mineralogy of the pegmatitesmore closely than the host
rock, (2) the aplites should show evidenceofhaving formed
during or after the late stagesof pegmatiteformation, and
(3) the aplites should show some evidenceof the quench
mechanism by which they formed.
The gabbroaplites,asnoted earlier, consistsofpyroxene
and plagioclaseand lack olivine. Thus, they resemblethe
olivine-poor pegmatitesmore closelythan the olivine-rich
host gabbros. Large clinopyroxene crystals included in
someaplites(Fig. 4) suggestthat the aplitesformed slightly
later than the closelyrelatedpegmatites.Someof the large
clinopyroxene grains included in the aplites are broken,
suggestingsome high-energy mechanism for aplite formation that would be consistentwith catastrophicloss of
a fluid phase.The absenceof high-temperaturepargasitic
amphibole in the gabbro aplites is consistent with crystallization under the fluid-poor conditions that would follow catastrophicloss ofa fluid phase.
Fluid phase
In granitic systems, the presenceof a fluid phase is
commonly considerednecessaryfor pegmatiteformation.
Conversely,the presenceof pegmatite is consideredevidence for the presenceof a free fluid phase (Jahns and
Burnham, 1969;Jahns,1982).However,direct evidence
for the presenceof a fluid phase at magmatic conditions
is commonly elusive. Somelow-pressurepegmatitescontain cavities,and gem-bearingpegmatitesin southernCalifornia show evidenceof late crystallization directly from
an aqueous-fluidphase(Jahnsand Burnham, 1969).Other
indications ofan aqueous-fluidphasearethe simultaneous
crystallization and corrosion of a mineral phaseat different placeswithin a pegmatite and fluid inclusions in pegmatite minerals. We have found no direct evidence for
the presenceof a fluid phase associatedwith the silicate
melt, but some aspectsof the mineralogy and the crystallization of the pegmatites suggestthat the activity of
HrO was elevated.
1096
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
1.0
PS2b
+ 8?9-136
OO PS4a
tr r PS4c
A
q
O1
i<
++
04
o
?
oE}
z
o
+'
00
600
625
650
6?5
?00
750
Si (cations)
PSzb
| 879-136
OO PS4a
tr a PS4c
A
q
03
F
775
portedby Arculusand Wills (1980)(Beardand Day, 1984;
Tables I and 2).
The presenceof subsolidus hydrous alteration assemblagesin the gabbro pegmatitesis strong evidencefor the
presenceofa fluid phase.As noted earlier, this alteration
is commonly restrictedto the coresof the pegmatites,but,
in some samples,involves the entire pegmatite.Evidence
of that alteration is commonly lacking in the host gabbro.
Fluid inclusions in plagioclaseappear to be associated
preferentiallywith the alteration in the pegmatites.Hence,
either a fluid phasewas present locally during pegmatite
crystallization and cooling, or fluids migrated preferentially through the podiform pegmatitesas they cooled.
Finally, ifaplites in granitic pegmatitesform by a mechanism involving fluid loss and if the analogy with the
gabbroicaplitesproposedearlier is complete,the presence
of a fluid phasein the gabbro pegmatitesis implied.
fnnpr,rc,q.rroNs FoR pEGMATITE FoRMATToN
Granitic pegmatites
Jahnsand Burnham (1969)and Jahns(1982)proposed
a model for the formation of granitic pegmatitesthat has
won widespreadacceptance(Cernf, 1975; IJebel, 1977).
r
r
r
r
ggr
r
r
I
The central premise of the model is that granitic peg6.00
6.25 6.50 6 75
? 00
7 25
? 50
7 75
matites form from a silicate melt in the presenceof an
Si (cations)
aqueous-fluid phase. Evolution of the fluid may result
Fig.6. Amphibolechemistrycalculated
on a 23-oxygen
basis. from crystallization in a magma that was originally un(a)A-sitealkalicationsversusSi. (b) Ti cationsversusSi. Open dersaturated,although some pegmatites, such as those
symbolsarethe hostgabbro;closedsymbolsarepegmatite.
formed during partial melting of metamorphic rocks, may
be saturatedwith respectto HrO from the outset. Most
of the pegmatitepresumably forms from the constituents
The presenceof magmatic amphibole in the gabbro of the silicate melt. Although the model provides for endpegmatitesindicates that the silicate melt was not anhy- stagecrystallizationdirectly from the fluid phase,the prindrous. One might arguethat the restriction of amphibole cipal role ofthe aqueousfluid is to enhancethe transport
crystallization to conditions near the solidus indicatesun- of material and to increasethe growth ratesof the crystals.
dersaturationof HrO. On the other hand, the pressuresat In many pegmatites,there is evidence for gravitational
which the pegmatitesformed, as indicated by the regional separationof the fluid phase from the silicate melt. The
metamorphic grade and the close temporal and spatial pegmatitemay form alongthe hangingwall ofadike where
associationwith hypabyssalrocks, may have been quite fluid is concentrated while aplites are crystallizing in a
Iow, perhapsas low as I kbar. The crystallization of mag- fluid-poor region along the footwall. Other aplites form
matic amphibole at low lithostatic pressurerequires high at low temperaturesin the coresof the pegmatites(Jahns
water activity in the melt. Decreasingthe activity ofwater
and Tuttle, I 963; Taylor et al., 1979) when the fluid phase
raisesthe minimum pressureat which amphibole can be is lost catastrophicallyduring the last stagesof pegmatite
stablein a basalticmelt (Gilbert et al., 1982).We conclude formation.
that the presenceof magmatic amphibole in the gabbroic
pegmatitesand their host rocks implies high water activ- Mafic pegmatitesin other gabbroic intrusions
ities and is consistent with water saturation during the
Cameron and Desborough (196a) suggestedthat peglast stagesof crystallization.
matitic dunite pipes in the Bushveld Complex formed as
Arculus and Wills (1980) noted that the compositions the result of subsolidusmetasomatic replacementof the
of plagioclaseand olivine in cumulate gabbro xenoliths gabbroic host. Bow et al. (1982) interpreted pegmatitic
in andesitesfrom the LesserAntilles are substantially more pods associatedwith the Howland reef of the Stillwater
calcic and magnesian,respectively,than plagioclaseand Complex as subsolidus replacements,based on textural
olivine in layered mafic complexes.They attributed this evidence. Gabbro pegmatite dikes in the Duke Island
differenceto modification of the plagioclaseliquidus and Complex (Irvine, 1974) and the Skaergaard Complex
solidusinduced by high water pressures(Johannes,1978). (McBirney and Noyes, 1979) were interpreted as filterPlagioclaseand olivine compositions in the Smartville pressed,water-saturated intercumulus liquids. Viljoen and
pegmatitesand the host gabbros are similar to those re- Scoon(1985) suggestedthat extremely Fe-rich pegmatites
0.1
oQ+
E+;
ro9'7
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
('C)calculatedat 1 atm
Table6. Pyroxenetemperatures
PS-1
PS-2b
PS-4a
PS-4c
PS-5
879-136
Pegmatite
Host
Pegmatite
Host
Pegmatite
Host
Pegmatite
Host
Pegmatite
Host
Pegmatite
Host
53.4
50.7
48.2
49.1
50.0
49.8
49.3
51.4
50.4
52.3
49.7
50.3
En
4.8
10.4
11.9
1 1. 1
10.8
9.7
10.1
9.2
9.2
6.8
10.6
9.2
41.8
38.9
39.9
39.8
39.2
40.6
40.6
39.4
40.4
40.8
39.7
40.5
Average
Average pegmatite
(except PS-1)
Average host
2 Pyroxene
Orthopyroxene
Clinopyroxene
En
FS
997
700
950
830
830
1030
980
990
800
960
1005
930
870
906
990
1002
948
906
1000
1030
970
990
1015
975
970
1020
995
1005
1000
990
81.6
77.6
76.4
76.1
75.3
77.5
75.6
76.2
75.2
79.7
76.1
78.4
17.9
20.1
21.7
2',t.9
21.5
19.9
21.5
17.7
22.O
17.7
21.5
19.6
1.0
2.3
1.9
2.0
3.2
2.6
2.9
1.7
2.8
2.6
2.4
1.9
n.a.
990
n.a.
n.a.
1023
978
980
n.a.
978
1005
960
n.a.
988
/Vofe.'Compositionsare recalculatedand temperaturescalculatedafter Lindsley(1983).n.a. : not applicablebecauseorthopyroxeneis a subsolidus
ohase.
in the Bushveld Complex representhighly fractionated,
fluid-saturated intercumulus melts concentratedin areas
disturbedby the late-magmaticdisruption of the Bushveld
cumulate pile. In all of these instances,the presenceof
pegmatiteis usedto infer the presenceof an aqueousfluid
phase.As noted by Viljoen and Scoon(1985),the major
differencebetweengranitic and gabbroicpegmatitesis that
granitic peg;matitesapparently form only from silicate melts
while at least some gabbroic pegmatites may form by
subsolidusprocesses.
Formation of gabbro pegrnatitesin the
Smartville complex
Any model for the formation of the gabbro pegmatites
in the PleasantValley pluton must account for the following observations: (l) The mineral chemistry of pegmatite and host rock pairs suggeststhat the pegmatites
were in equilibrium with their hosts and formed in place.
(2) The presenceof gabbro aplites, the high apparenttemperatures recorded by pyroxene thermometers, and the
zoning of the pegmatitessuggestthat the pegmatitescrystallized from a melt. (3) The scarcity of olivine and the
abundanceof orthopyroxenein the pegmatitesrelative to
the gabbros suggeststhat the pegmatites contain more
SiO, than the host gabbros.(4) Severallines ofevidence
suggestthat the pegmatitesformed in the presenceof elevated fluid pressures.
Replacementmodels for the origin of the pegmatites
would fail to account for the apparent equilibrium between pegmatiteand host and would be inconsistentwith
evidencesuggestingthat the pegmatitescrystallized from
a melt. Filter pressingmay account for the segregationof
the melt from which the pegmatite dikes such as PS-l
formed, but the other pegmatitesformed from a melt in
local equilibrium with their gabbro hosts and must have
formed essentially in place. Although these pegmatites
may be slightly enriched in SiOz relative to their hosts,
they do not show the extreme fractionation of the Fe-rich
pegmatitesin the Bushveld Complex (Viljoen and Scoon,
I 985).
We proposethat the gabbro pegmatitesin the Pleasant
Valley pluton formed in place from a fluid-enriched intercumulus melt by a mechanism analogousto that proposed by Jahns and Burnhan (1969) for granitic pegmatites. The Jahnsand Burnham model accountsfor several
important characteristicsof the Smartville pegmatitesincluding (l) the presenceof a silicate melt, (2) the presence
of aplitic gabbrosin the cores of some pods, and (3) the
inferred presenceofan aqueous-fluidphase.Their model,
however, doesnot addresssome of the problems peculiar
to the formation of the Smartville gabbro pegmatites.First,
the Smartville pegmatitesare scarcelyfractionated relative to their host rocks. It is not possible, therefore, to
appeal to fractional crystallization as a mechanism for
concentratingthe volatile species.Nonfractional (e.g.,eutectic) crystallization,however,might have led to the evolution of a fluid phase.Second,judging from their mineral
chemistry, the composition of the Smartville pegmatites
more closely resemblestheir cumulate host rocks than
that of any of the more-fractionated rocks in the Smartville zoned plutons. Finally, the Jahns-Burnham model
does not suggesta mechanism for the segregationof a
fluid-saturated melt from its host.
The loss of confining pressure accompanying the reemplacementof the olivine gabbro cumulates may have
played an important role in the evolution of a fluid phase.
Fluid saturation may have occurred in the following manner: (l) Prior to their remobilization, the gabbro cumulatesbeganto cool and to equilibrate with an intercumulus
melt in a magna chamber at unknown depth. (2) Continued crystallization would causean increasein the volatile content of the remaining liquid. (3) As the cumulates
1098
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
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Reviews by J. A. Speer and two anonymous reviewers greatly Haggerty, S.E. (1976) Oxidation of opaque mineral oxides
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(1985) An empirical equaof J.S.B.'sPh.D. dissertationat University of California, Davis. Hammarstrom, J.M., and Znt,E-an.
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igneous
calcic
tion
g,rants
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(Beard)and the National ScienceFoundation (EAR78-03640and
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(1976) Phaserelations of basalts in their melting ranges
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were remobilized and intruded to a higher level of the
crust, the drop in pressure would decrease the solubility
of the volatile species in the remaining melt and a fluid
phase may evolve.
The reasons why the Smartville pegmatites-as well as
certain other pegmatites formed from intercumulus melts,
e.g., Skaergaard (McBirney and Noyes, 1979) and Duke
Island (Irvine,1974)-arc so mafic are unclear. One might
speculate that that the mafic compositions are a reflection
of complex interactions involving resorption and back
reaction between the intercumulus melt and the cumulus
crystals during cooling.
The mechanism by which intercumulus melt segregates
prior to pegmatite crystallization is not known. Perhaps,
as suggestedby viljoen and Scoon (1985), disruption of
a cumulate pile facilitates the migration and accumulation
of any remaining intercumulus melt. Profound disruption
of the Smartville olivine gabbro cumulates certainly must
have occurred during their remobilization and emplacement to higher levels of the crust. Even in the relatively
well-studied granitic pegmatite systems, little is known
about the mechanisms of melt separation. We have observed that pegmatitic granites are common in schlieren,
among other places, suggesting that disruption or flow in
a magma can facilitate the segregation of a fluid-saturated
melt.
BEARDAND DAY: ORIGIN OF GABBROPEGMATITE
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