Continental Basalts of the Boise River Group Near Smith Prairie, Idaho

JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 97, NO. B6, PAGES 9043-9061, JUNE 10, 1992
ContinentalBasaltsof the BoiseRiver Group Near SmithPrairie, Idaho
SCOTTK. VEI'FERl AND JOHNW. SHERVAI$
Departmentof GeologicalSciences,Universityof SouthCarolina,Columbia
The BoiseRiver Group of late Cenozoicage consistsof basalticlavas and intoresistedfluvial and
lacustrine
sediments
deposited
in drainages
of the BoiseRiverandBoiseRiver SouthFork from 1.8 to 0.2
m.y. Basaltsof the BoiseRiver Groupcanbe dividedintotwo groupsbasedon majorandtraceelement
chemistry.BoiseRiverGroup1 basalts(BRG 1), whichcomprise
thethreeoldestflowsin theSmithPrairie
area, are silica-saturated
elivinetholelitescharacterized
by low alkalis,Mg/Fe, and Ni, high concentrations
of highfieldstrength
elements
(HFSE),andenrichment
in theheavyisotopes
of Sr, Nd, andPb. They are
chemicallysimilarto basaltsof the SnakeRiver Plain, andbothunitsprobablyderivefrom a similar,
enrichedmantlelithosphere
source. SomeBRG 1 flows(LongGulch,Rock Creek)recordthe affectsof
crustalassimilation
andfractionalcrystallization.BoiseRiver Group2 basalts(BRG 2) are all youngerthan
BRG 1 (< 0.7 m.y.)andaretransitional
betweenelivinetholelites
andalkalielivinebasalts. BRG2 basalts
arecharacterized
by highalkalis,Mg/Fe, andNi, lowerHFSEconcentrations,
andisotopic
compositions
of
Sr andNd nearbulk earth. Theselavashavedistinctgeochemical
characteristics
and are not cogeneticwith
the older,BRG I flowsor with coevallavasof theSnakeRiver Plain. Chemicalvariationswithinindividual
flowsare consistent
with low-pressure
crystalfractionation
of the observed
phenocryst
phases(elivine +
plagioclase).
Chemical
variations
between
differentflowswithinthe samegroupcannotresultfromlowpressurefractionation
but canbe modeledby combined
high-pressure
pyrexonefractionation
andlowpressure
elivine+ plagioclase
fractionation.
ThehighMg numbers
andNi of theyounger
BRG2 basalts
are
not consistent
with enrichment
of the light field strengthelementsby crustalassimilation;
this enrichment
mustreflecta mantlesourceregioncharacteristic.
The traceclementandisotopicsystematics
of thissource
regionare similar(butnotidentical)to the asthenospheric
sourceinferredfor oceanislandbasalts. In
contrast,
chemical
andisotopicsystematics
of theBRG 1 basalts
implyderivation
fromancientsubcontinental
lithosphere
whichhasbeenisolatedfrom the asthenosphere
for at least 1.5 cons. The subcontinental
lithosphere
wasaffected
byanearly
enrichment
inRb/Sr
which
supported
growth
ofhighS?Sr/S6Sr.
A
secondenrichment
event,resultingin Fe-Ti metasomatism
of the lithosphere,
occurredlater in response
to
intrusionof partialmeltsfroma mantleplumerootedin the underlyingasthenosphere.
This is second
enrichment
eventis requiredby thehighNb/Rbratiosof theBRG 1 lavas,andof similarlavasof theSnake
River Plain. The transitionfrom saturatedelivine tholeiitesof BoiseRiver Group 1 to transitional-alkalic
lavasof BRG 2 in the SmithPrairieregionimpliesa time-dependent
changein mantlesourceregion,with
earlymagmas
derivedfroma shallowlithospheric
sourcegivingway to youngermagmasderivedfroma
deeperasthenospheric
source.Theeruption
of theseyounger
lavasalongtheflanksof theSnakeRiverPlain
coevalwitheruptionof saturated
tholeiites
of theSnakeRiverGroupto the southfurtherimpliesan axial
zonationto magmatism
in theSnakeRiver Province.The natureof thisaxialzonationsuggests
thatthe
deeperasthenospheric
moltswerepreferentially
tappedalongthe marginsof theSnakeRiverPlainbutwere
blockedfrom reachingthe surfacein the axial zone. Thesetime-space
relationships
are consistent
with a
modelin whichthinnedlithosphere
nearthe rift axisundergoes
extensivepartialmeltingwhichoverwhelms
anyasthenosphcric
meltsthatmayattempt
passage.
Thelithosphere
istoocoolto meltalongtherift margins
(wherepressure
release
by extensional
thinningis minor);subsequent
smallvolumes
of asthenospheric
melt
areableto traversethisregionmoreeasilyanderuptwithlittleor no lithospheric
contamination.
INTRODUCTION
have on basalt geochemistry is central to deciphering the
chemicalcharacteristics
of their sourceregions.
One of the main objectivesof studyingcontinentalbasaltsis to
The Boise River Group (BRG) consistsof basalticlavas and
identifythe chemicaland physicalcharacteritics
of their mantle intercalatedfluvial and lacustrinesedimentsdepositedin the
sourceregionsand to understandthe processesby which these drainages
of theBoiseRiverandBoiseRiverSouthFork during
sourceregionsevolved. Theseprocessesmay includechemical the late Plioccncand Plicstoccnc[Howard and Shervais, 1973;
enrichmentor depletionby magmaticor metasomaticevents, Howard et al., 1982]. The BRG lavas erupted through
partial melting, and assimilation. Characterization of these thickenedcontinentalcrust of the Idaho Batholith, some25 km
mantle sources is difficult, however, because a variety of northof the SnakeRiver Plain, and are generallycoevalwith
processesact to changethe compositionsof primary magmas basalticvolcanismin the SnakeRiver volcanicprovince. This
prior to eruption. These processes include fractional papercharacterizes
the chemicalandmineralogical
compositions
crystallization, magma mixing, and assimilation of crustal of BRG basaltsin the Smith Prairie region and addressestheir
materials. An understanding
of the effect theselater processes relationshipsto one anotherand to the more voluminouslavasof
Now at Departmentof Geology,CentenaryCollege, Shreveport,
Louisiana.
Copyright1992 by the AmericanGeophysicalUnion.
Papernumber92JB00209.
0148-0227/92/92JB-00209 $05.00
the SnakeRiver volcanicprovince.
Our datashowthatintrafiowchemicalvariationscangenerally
be modeledby low-pressurefractional crystallizationof the
observed phonetryst phases; crustal assimilation has had a
significanteffect on only two flows. Relationshipsbetween
flows requireeitherhigh-pressure
clinopyroxenefractionationor
distinctmantlesourceregions. Comparsionof thesebasaltswith
the Snake River Group basalts indicates the existence of
9043
9044
VETTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP,IDAHO
chemical hctcrogcnciticsin subcontinentalmantlebeneaththe
Snake River Plain area.
REGIONAL SE•O
0.7 m.y.) in the westernSRP consistsof canyon-fillingbasalt
flows erupted toward the northern margin of the SRP; these
lavas comprisethe Snake River Group of Malde and Powers
[1962], and correlate with the more abundantyoung volcanic
rocks of the eastern SRP [Leemah, 1952a].
The easternSRP is a downwarpedstructuraldepressionwith
The Snake River Plain (SRP) is a major elongatedstructural
and topographicdepressionin southernIdaho, extendingfrom only minor fault control along its margins [Mabey, 1982]. It
the Columbia Plateau in the west to the Yellowstone Plateau in
becomesprogressivelyelevatedtoward the east, culminatingin
the east.
The
SRP has formed
since
mid-Miocene
and is
characterized by bimodal basalt-rhyolitc volcanismwhich has
been activealmostcontinuouslyduringthe past 15 m.y. [Malde
and Powers, 1962; Mabey, 1982; Leemah, 1952a]. Rhyolitic
volcanismgenerally predatesbasalticactivity in a given area,
althoughboth types of volcanismmay be active concurrently.
Age relationshipsshow an castward progressionof volcanic
activity, with the current locuscenteredaroundthe Island Park
caldera-Yellowstone volcanic center [Christiansen and Blank,
1972; Armstronget al., 1975; Smith et al., 1977; Christiansen,
1982].
The SRP formstwo major segments,the NW trendingwestern
SnakeRiver Plain and a NE trendingeasternSnakeRiver Plain.
The western SRP is a graben like structure bounded by
northwest trending en echelon normal faults [MaMe, 1959,
1965; Mabey, 1982; Leeman, 1982a]. Volcanicactivity began
in the westernSRP about 15 m.y. ago and was predominately
rhyoliticwith minor basalticflows (the Idavadavolcanicgroup).
Major basalticactivityin the westernSRP beganabout11 m.y.
ago with the eruptionof the Idaho Group. The youngestunit in
this group, the Bruneau Formation, is coeval with the oldest
basaltsof the BoiseRiver Group. Youngvolcanicactivity ( <
the Yellowstone Plateau, which is the current locus of recent
volcanic activity. The oldestrhyolitic material in the eastern
SRP is about10 m.y. old, with minor intcrcalatedbasalticflows
at around 6 m.y. [Leemah, 1952a]. Pleistocene and Recent
volcanic activity was largely basaltic; the youngestof these
basaltsare correlativewith the SnakeRiver Groupof Malde and
Powers [1962].
SmithPrairieis an intermountain
plateaubuilt of late Pliocene
and Pleistocene
basaltsof the BoiseRiver Group,locatedalong
the BoiseRiver SouthFork (Figure 1). The area containsseven
basalt flows which eruptedonto granitic bedrock of the Idaho
batholith near the Boise River South Fork; additional flows are
found both upstream and downstream from Smith Prairie
[Howard and $hervais, 1973; $hervais and Howard, 1975;
Howard et al., 1982]. The ancestral Boise River South Fork
canyonwasperiodicallyfloodedby basaltfrom theseeruptions,
reentrenchedto a new, deeper level, and then flooded again.
This historyof repeatedfilling and downcuttinghas resultedin
an invertedstratigraphy,with terracesof the older intracanyon
flows high abovethe river and benchesof youngerflows lower
on the canyon walls [$hervais and Howard, 1975; Howard et
al., 1982].
1 160
Holocene
or
-is•p.•
Basalt
of
Fall
Creek
Pleistocene
<
43045/
SAWTOOTH.•MOUNTA! N
••MORES
CREEK
Basalt of Smith Creek
I• Basalt
ofLong
Gulch
_
inc
4,.•
Smith
Prairie
Basalt
Basalt of Lava Creek
OUATERNARY
Plmstocene
Basalt
ofMores
Creek
OUATERNARY
Plmstocene
ORTERTIARYorPhocene
Basaltof RockCreek
Basaltof Anderson
Ranch
Steamboat Rock Basalt
TERTIARY
Phocene
Basalt
of
Lucky
Peak
Dan'
LUCKY
PEAK
LAVA
CREEK
•
C'
o LONG GI
•t•30 / --
Peak Dam
SLY4/c
SMITH
I
SMITH
CREEK
PRAIRIE
•
'•FALL
CREEK
/i/z,•
/
.
,'91• ,,,,
STEAMBOATROCK
/
ROCK CREEK
RANCH
0
10
20
/
Anderson Ranch Dam
KILOMETERS
1160
115030'
Fig. 1. Locationmap of the BoiseRiver Groupbasaltsnear Prairie, Idaho (modifiedfrom Howard et al. [1982]). P, town of
Prairie. Inverted star symbols, volcanoes,hatchuredline, approximateboundary of Snake River Plain and Sawtooth
Mountains.
VgTTER AND SHERVAIS:BASALTSOF BOISBRIVER GROUP,IDAHO
9045
Howard et al. [1982] publishedK-Ar agedeterminations
on
five basaltflowsof theBoiseRiverGroup(Table1). The older
flowsin the BoiseRiver Group(LuckyPeak,Steamboat
Rock,
An
and Long Gulch basalts)correlatewith the BruneauFormation
of the IdahoGroup[MaldeandPowers,1962]; youngerflows
(Lava Creek, Fall Creek, Smith Creek, Mores Creek, Smith
Prairie basalts)are less than 0.7 m.y. old and correlatewith
lavasof theSnakeRiverGroup[Howardet al., 1982]. The age
distinction into older and younger flows of the Boise River
Group correspondsto chemicalgroups 1 and 2 of the BRG,
which are defined below.
•D
pl
OCCURRENCE AND PETRO•PHY
OF TI-IE BASALT FLOWS
Avg Snake
Over 200 thin sectionsand polishedsectionswere studied
petragraphically, and 50 of these sections were selected for
detailedanalysesof phasecompositions.Phasecompositions
weredeterminedwith a CamecaSX-50 electronmicroprobeat
theUniveraityof SouthCarolinausingbothnaturalandsynthetic
mineralstandards,
15 kV accelerating
voltage,and25 nA beam
current;
data reduction
utilized
Cameca's
on-line
Ab
•
I
•-•<•
I
Group
2I
I
I •1
I
\
PAP
corrections.In mostsamples,20-30 spotswere analyzedfor
eachphase. Representative
mineralanalysesfrom eachflow are
showngraphicallyin Figure 2; completemineralchemicaldata
canbe requestedfrom the authors. Petrographyis discussed
belowunitby unit;representative
modalanalysesare givenin
Group 2
Di
Hd
Table 2.
Boise
River
Group
1Basalts
/
Steamboat
Rock
basalt.
The
Steamboat
Rock
basalt
isthe/
oldest
(1.$m.y.)
and
most
abundant
volcanic
unit
intheSmith
En
Prairieareaandactuallyformsmostof theplateau(Figure1).
The flow consista
of numerous
pahoehoe
flow unita,up to 20 m
thick, eruptedfrom a low, broad shieldvolcano3 km southeast
of the communityof Prairie. The SteamboatRock basaltattains
a maximumexposedthicknessof 180 m where the currentSouth
Avg
Snake
River
Plain
I
I
'l
I
I
I
I
I
IFs
Fig. 2. Plagioclase
andpyroxene
phasechemistry
of theBoiseRiver
Groupbasaltsas comparedto SnakeRiver Plainbasalts.
plagioclase
(An66.s0)
, olivine
(Fo76.ns)
, pyroxene
(Won•
• Enn2on
Fork BoiseRiver hasincisedadjacentto its ancestralcanyon. ), glass,and opaques.
Rock Creek Basalt. The Rock Creek basalt is a small flow
The baseof the Steamboat
Rockbasaltis markedby a wedgeof
pillowlavasandhyaloclastite
breccias
whichthickens
upstream; less than 10 m thick which overlies the Steamboat Rock flow
foresetbedsin the hyaloclastites
alsodip upstream.This wedge southof Prairie. The basaltis light gray in colorandcontains
of subaqueouslava represents a lava dam which formed a very rare phcnocrysta
of olivinc( < 1 mm) andplagioclasc
(<
temporaryreservoirin the river canyon.
10 mm)in a f'me-grained
intcrscrtal
groundmass
of plagioclasc,
The Steamboat
Rockbasaltconsists
of darkto mediumgray olivinc,glass,andopaques.No pyroxcncwas observeddueto
natureof thegroundmass.
basalt
withlarge
plagioclasc
phcnocrysts
( < 15mm;An7•o)
and thef'me-grained
smaller olivinc microphcnocrysts( < 5 mm; FoG ..) set in a
Basaltof LongGulch. The LongGulchflowis knownonly
medium
tocoarse-grained,
ophitic,
diktytaxitic
gro•dmass
of froma single,isolatedoutcropabovetheBoiseRiverSouthFork
TABLE1. K-Ar AgeDatesfor Someof theBoiseRiverGroupBasalts
Rock Unit
Field Number
Age, m.y.
Smith Prairie
HSP-181
0.20 + 0.15
Smith Prairie
HSP-186
0.26
Mores Creek
H79 Boise 4
0.44 + 0.20
Long Gulch
H79 Boise 1
0.68 + 0.25
Steamboat Rock
HSP-13
1.8
+ 0.30
Lucky Peak
H79 Boise 3
2.1
+0.50
from Howard et al. [1982].
+ 0.16
9046
VWrTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP,IDAHO
TABLE 2. AverageModal Compositions
of the BoiseRiver GroupBasalts
Flow
Plagioclase
Olivine
Pyroxene
Glass
Opaques
SteamboatRock
30 (10)
15
35
15
5
Rock Creek
60 (1)
30 (1)
0
8
2
Long Gulch
Smith Creek
15
5
10
0
30 (2)
20
0
30
10
Lava Creek
60 (15)
10 (5)
20
0
10
Unit 1
55 (20)
15 (10)
15
10
5
Unit 2
40
15
30
10
5
10
Smith Prairie
Unit 3
55 (10)
10 (5)
25
7
3
Fall Creek
45 (15)
7 (2)
5
40
3
Vnluewithin parentheses
is the percentof phenocrysts
for eachphasewithin the sample.
west of Prairie.
It consists of two flow units about 3 m thick
along the Boise River South Fork, where it forms a major
terrace200 m below the top of the SteamboatRock basalt. The
Smith Prairie basalt consistsof three distinct flow units: SPI:
largely
glassy
withraremicrophenocrysts
ofplagioclase
(An6•
' SP2, and SP3. Units SP1 and SP2 eruptedfrom the samevent
5ø
) andolivine
(Fo5•.54).
Groundmass
olivine
ismoreFe-rich (a cinder cone 5.5 km east of Prairie), whereas the SP3 unit
eruptedfrom a small shieldvolcano 1.5 km southof the earlier
•
unitcannot
becorrelated
witha ventbecause
of its vent. The terrace-forming part of the Smith Prairie flow is
isolatedoutcrop. Howard and $hervais[1973] interpretedit as characterized by a basal wedge of pillow lavas and
an old flow overlainby the SteamboatRock basalt,but its K-Ar hyaloclastites, similar to that found in the Steamboat Rock
age (0.68 m.y.) is younger than that of SteamboatRock (1.8 basalt. Thesesubaqueous
depositsare inferredto have formed
m.y. [Howard et al., 1982]). We now believe that the Long by lava flow into a lava-dammed reservoir [Shervais and
Gulch outcropis a terraceof youngerbasaltexhibitinginverted Howard, 1975; Howard et al., 1982].
stratigraphywith the adjacentSteamboatRock basalt;it may be
SP1basaltsare porphyriticwith largecorrodedphenocrysts
of
each, separatedby a thin gravel layer. The upperunit is a light
gray, fine-grained, aphyritic basalt;the lower unit is black and
related to the Rock Creek basalt.
plagioclase
(• 2 cm;An •0) andsmaller,
euhedral
phenocrysts
ofolivine
(• 2mm;
Fo80-53
• ) inanintersertal
groundmass
(0' 3
mm) of plagioclase(An
BoiseRiver Group 2 Basalts
Smith Creek basalt.
The Smith Creek basalt is an aa flow
which erupted from a cinder cone 2 km north of Prairie and
flowed south along Smith Creek. Smith Creek basalts are
mediumto light blue-grayin color and are nearlyaphyritic;the
fewobserved
phenocrysts
areolivine(( 5 ram;FOs3
•0) and
), olivine (Fo
), glass,opaques,
and
minor
sanidine
(Or•4•3.).
SP2
basagt•re
microporpyritic
withmicrophenocrysts
o•olivine
( ( 1ram;
Fos4
79
) and
plagioclase
( • 1 mm;An•. ) in a fine-grained,
subo•hitic
to
intersertal
matrix
ofplagio•ase,
pyroxene,
glass,
andopaques.
Groundmassphaseshave compositionssimilar to unit SP1.
Groundmasspyroxenesshowthe typicallimited compositional
plagioclase
(( 20mm;
An72.4
). Thegroundmass
isin•rsertal,
rangeWo•749
En32.33
butarehigherin Cathantheother
5
withplagioclase
(An?o
•0),olivine(Fo59.56)
, glass,andrare groundmasspyroxcncs(Figure2). SP3 basaltsare porphyritic
pyroxene
(Wo.... En. '•). Potassium
feldspar
(Or.... Ab.... ) with phenocrystsof plagioclase(• 5 mm; An._ .) and olivine (
isalsofound
as• stab•e•groundmass
phase.TheS• C•'•k 0.5-1ram;Fo81-73 ) inanintersertal
tointergran"•ar
groundmass
basalt is distinctive because it contains numerous granitic ofplagioclase
(An65
53),
olivine
(Fo?l
6
)'
pyroxene
(Wo
- 5
44-45
xenolithsin variousstagesof dissaggregation
nearits vent.
Lava
Creek basalt.
En.... ), sanidine(Or..Ab.•), glass,andopaques.
The Lava Creek basalt forms an aa flow
whicheruptedfrom an erodedcinderand agglutinatecone7 km
north of Prairie and flowed south along Lava Creek to the
northwest margin of the Steamboat Rock basalt. The Lava
Creek basaltsare dark gray to black in color, with phenocrysts
]T•'•Creekbasalt.*•Fhe'•Fall
Creekbasaltis considered
to be
(Ano), olivine(Fo•..7o),andpyroxene
(Wo..-aEn.... ). The
the youngest flow in the Smith Prairie region, basedon the
preservation
of its surfacefeatures;it is not in contactwith any
of the other flows. The Fall Creek basaltoriginatesat a large,
freshcindercone(Red Mountain)on the easternmarginof the
study area, 15 km northeastof Prairie (Figure 1). Near the
vent, the flow is typicallyaa; however,pahoehoelava becomes
dominant farther away. The basalts are characterized by
large p]agioclascphcnocrysts
arc commonlycorroded.
microphenocrysts
ofplagioclase
(( 2 mm;An?s.6s)
andolivine
ofolivine( • 5 ram;Fo•s7s
) andplagioclase
( • 10ram;An
72
) set
inafrae-grained,
in'•rgranular
groundmass
ofplagioclas•e
Smith Prairie
basalt.
The Smith Prairie
basalt overlies
the.
Steamboat Rock basalt and the basalts of Smith Creek and Lava
Creek. This basalthasa maximumexposedthicknessof 120 m
(0.5 ram; Fo
.) set in a dark gray, glassy to aphanitic
•roundmass
o•gioclase
(An , olivine
(Fo?),pyroxene
(Wo43.44
En•8.4•)
,alkali
feldspar,
g69'
a•ss,
and
opaques?9'.
4
VETTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP,IDAHO
]VIAlOR AND TRACE EL•
GEOC•$TRY
9047
Trace Elements
AnalyticalTechniques
Divisionof the BoiseRiver basaltsinto two groupsbasedon
major elementchemistryis supportedby trace elementdata
Whole rock compositionswere determinedon 80 selected
samplesof basalt representing every major flow unit in the
SmithPrairie region;the resultsare reportedin Table 3. Major
elementswere determinedby a fusedglassbeadtechniqueusing
the Cameca SX-50 electron microprobeat the University of
SouthCarolinaand naturalglassstandards.Detaileddescription
of this techniqueis given by Vetteret al. [1988]. Trace element
concentrationswere determinedby X ray fluorescence(XRF)
analysisat the Universityof SouthCarolinausingan automated
PhilipsPW-1400 wavelengthdispersiveXRF spectrometerand
U.S. GeologicalSurvey(USGS) rock standards.In addition,30
representative
sampleswere analyzedfor the rare earthelements
(REE) and other trace elements by instrumental neutron
activation analysis (INAA) at NASA- JohnsonSpace Center,
Houston. Analyticalerrors are -t,1-2% for major elementsand
(Figure 4 and Table 3). Normal BRG 1 basalts(SteamboatRock
flow) have high field strengthelement (HFSE) and large ion
lithophileelement(LILE) concentrations
similarto SRP basalts.
HFSE concentrations
rangefrom Zr = 223 to 307 ppm, Nb =
18 to 48 ppm, and Ta = 1.2 to 1.48 ppm. LILE eoneentrations
rangefrom Rb = I to 20 ppm, Sr = 329 - 404 ppm, and Ba =
411 to 758 ppm. SteamboatRock basaltsalso have low Rb/Sr
ratiosof 0.003 - 0.06. Evolved BRG 1 basalts(Rock Creek and
Long Gulch flows) are higher in HSFE and LILE, with Zr =
314 to 476 ppm, Nb = 34 - 61 ppm, Ta = 2.06 to 3.28 ppm,
Rb = 22 to 39 ppm, Sr = 346 to 362 ppm, and Ba = 581 to
1330 ppm. EvolvedBRG 1 basaltshavea wider rangeof Rb/Sr
+5-10% for trace elements.
= 20 - 41 ppm; Ta = 1.4 - 2.3 ppm), but they are significantly
enrichedin LILE (Rb = 15 -68 ppm; Sr = 324- 526 ppm; Ba
= 470- 2161 ppm). BRG 2 basaltshave Rb/Sr ratios'of 0.06 0.14. One sample(HSP-77) is unusual,with Zr = 342 ppm, Sr
= 969 ppm, and Ba = 1130 ppm.
All of the samplesare light REE (LREE) -enrichedand show
Major Elements
Two distinct chemical groups are defined by the major
elementdata. BRG 1 basaltsare characterizedby low Mg # (=
100*Mg/[Mg+Fe], molar), which rangefrom 49 to 31 and have
lowerSiO2 , A120
alkalis
thantheBRG2 basalts
(Figure
3 , and
3). BRG 2 basaltsare characterizedby higherMg # (60 to 46),
ratios (0.063 - 0.111).
BRG 2 basalts have HFSE concentrationsthat are lower (Zr
= 142 - 208 ppm) or the sameas "normal"BRG 1 basalts(Nb
no Eu anomalies; however, the REE concentrations of the
chondrite-normalized
REE paRemsare distinct(Figure 5). The
BRG
1 basalts have
La concentrations
of 67x-
162x chondrite
and an averageLa/Lu_ (chondrite-normalized)
ratio of 5.7-6.4,
higher
SiO2,A1203,
andalkalis,
andlowerTiO2andFeOthan similarto or slightly
•igherthanSRPbasalts.TheBRG2
the BRG 1 basalts(Figure3). All of the flows from both groups basalts have lower REE concentrationscompared to BRG 1,
show an overall increasein FeO, TiO,2 SiO•,• (slight), and alkalis with La concentrationsof 48x - 70x chondriteand an average
with decreasingMg# (Figure 3).
BRG 1 lavas decreasein La/Lu ratio of 5.5.
AI 2 O3 with decreasingMg #, but BRG 2 lavasgenerallyincrease
The enrichment of BRG 1 lavas in HFSE and depletion in
in A120
Mg #. Differencesin K2 O LILE comparedto the BRG 2 basaltsresult in crossingpatterns
3 with decreasing
concentrationrelative to Mg/Fe are especiallyevident (Figure when incompatibleelement contentsare plotted on a spider
3). The Steamboat Rock basalt ("normal" BRG 1) has diagramnormalizedto primordialmantle(Figure 6). The BRG
consistently
low K•O (0.5 to 0.7 wt %) at Mg # between40 and 1 and BRG 2 basalts have similar Nb concentrations,however,
50,while
evolved
•RG1lavas
have
high
K20(1.0to2.5wt%)
the BRG 1 basalts are enriched in the HFSE (Zr, Hf, Sm, and
at Mg # of 30 to 40. In contrast,BRG 2 lavasare characterized Ti), the LREEs (La, Ce), and the HREE (Yb). BRG 2 basalts
byhighK20(0.7to2.46wt %)atMg# of45to60(Figure
3). are enrichedin LILE (Rb, Sr, Th) and depletedin HFSE and
As a result, BRG 2 basalts are distinct from both SRP and
REE comparedto BRG 1.
Craters of the Moon (COM) basalts. "Normal" BRG 1 basalts
(SteamboatRock flow) are generallysimilarto olivine tholelites
oftheSnake
RiverPlain(butlowerinSiO2),
whereas
evolved Radiogenic IsotopeCompositions
BRG 1 lavas (Long Gulch, Rock Creek) mimic trends seenin
COM
lavas.
hypersthene, and 0-5 % normarive nepheline; they are best
Six Boise River Group basaltswere selected for isotopic
analysisof Sr, Nd, and Pb (Table 4). The isotopic analyses
were performedby R. Carlsonof the DepartmentTerrestrial
Magnetism, Carnegie Institute of Washington. Analytical
proceduresare reported by Carlson [1984]. Six flows are
represented:
Steamboat
RockBasalt(BRG 1), SmithPrairieunit
SP2, Smith Prairie unit SP3, Fall Creek, Smith Creek, and Lava
Creek (all BRG 2). These data are reconnaissancein nature,
classified as alkali olivine basalts.
and more data are needed before definitive
Thesedifferencesin major elementchemistryare reflectedby
the normarive mineralogy. The BRG 1 basalts are Hy normative, with 11-20%
normative olivine and 0 to 25%
normarivehypersthene,and are classifiedas olivine tholeiites.
The BRG 2 basalts are transitional between ne - and hy
-normative, with 5-22% normative olivine, 0-27% normative
The classification of the BRG
conclusions can be
2 lavasas alkali olivine basaltsis supportedby their high alkali drawn from the isotopicresults.
The single BRG 1 lava (HSP-14, SteamboatRock unit) is
concentrations (Na 2 O + K 2 O = 3.0 - 5.7 wt %) and by the
occurrenceof stablegroundmass
olivine and potassiumfeldspar; enriched in the heavy isotopes of Sr and Nd relative to the
It should be noted that the ne-normative
nature of the BRG 2
hypothetical
"bulksilicateearth"component
of Zindlerand Hart
basaltsis due to an enrichment in alkalis, not to a lack of silica: [1986], andplotswithinthe array def'med
by lavasof the Snake
the BRG 2 basaltsare actually higher in silica than either the River Plain (Figure 7) [Leemanand Manton, 1971; Menzieset
BRG 1 or the SRP basalts.
al., 1983' Lum et al., 1989]. This array falls on a mixing curve
9048
V•i-rm< AND$HERVAI$:
BASALTS
OFBOISERIVERGROUP,IDAHO
TABLE3a.WholeRock- MajorElement
Concentrations
oftheBoise
RiverGroup
Basalts
Sample
sio
2
TiO2 AI2O•
FeO
MnO MgO
CaO
Na2O
Mg #
K:O
P2Os Total
3.43
2.92
2.40
1.21
1.18
0.88
11.00
9.89
9.81
10.32
9.85
10.29
10.00
9.66
10.30
10.15
10.26
2.40
2.40
2.58
1.99
2.66
2.43
2.42
2.43
2.36
2.43
2.63
0.33
0.4
0.61
0.28
0.69
0.44
0.58
0.65
0.41
0.46
0.5
0.54 99.18
0.5 99.60
0.46 100.95
0.3 99.30
0.69 99.12
0.54 99.60
0.64 98.16
0.42 99.30
0.44 99.31
0.76
99.27
0.71 99.48
42.8
7.86
8.1
3.27
3.01
1.73
1.75
1.01
1
99.55
100.59
37.2
9.42
7.91
8.79
8.88
8.50
9.56
8.82
9.09
8.35
2.73
1.81
2.31
2.67
2.32
2.78
2.91
2.55
2.50
1.36
1.07
1.65
1.69
1.68
I;3!
1.78
1.58
1.63
0.33
0.38
0.16
0.39
0.36
0.33
0.48
0.39
0.43
99.64
99.00
99.39
99.80
100.43
100.93
99.66
98.22
99.77
8.24
8.85
8.15
8.38
3.21
2.31
3.18
3.31
2.14
1.52
2.21
2.22
0.43
0.11
0.41
0.45
99.40
99.27
99.86
99.80
2.92
2.16
2.66
2.85
1.52
1.16
1.46
1.41
0.40
0.12
0.40
0.34
99.72
99.66
100.98
99.71
3.30
2.42
2.79
2.94
2.73
3.10
2.83
3.02
2.62
2.77
2.73
2.87
2.38
2.96
2.91
2.85
2.86
2.07
2.95
2.84
2.22
2.19
1.16
1.71
1.63
1.30
1.88
1.70
1.89
1.46
1.36
1.37
1.65
1.10
1.68
1.51
1.60
1.45
1.05
1.82
1.45
1.12
0.43
0.19
0.34
0.44
0.34
0.28
0.29
0.47
0.50
0.35
0.37
0.29
0.23
0.47
0.31
0.39
0.38
0.11
0.46
0.41
0.12
99.81
99.17
99.64
99.51
99.04
99.16
99.43
100.07
100.05
101.16
99.81
100.07
99.58
99.92
99.34
99.52
99.64
100.05
101.68
100.53
99.72
BRG
1:LOng
Gulch
46.4
3.13
3.15
14.5
14.7
14.93
14.99
0.25
0.21
45.6
45.4
46.7
45.3
46.6
45.6
44.5
46.7
46.3
44.7
45.7
3.12
3.05
3.02
3.19
3.08
3.11
3.28
2.92
3.23
3.21
2.79
15.8
15.5
15.0
15.1
15.1
14.9
14.8
15.2
14.9
15.0
15.4
14.24
14.43
13.89
14.73
13.38
14.24
14.69
13.67
14.38
14.85
13.94
0.16
0.16
0.13
0.16
0.18
0.21
0.20
0.15
0.15
0.20
0.21
HSP-33
48.9
HSP 34
HSP-46
HSP-14
HSP-17
HSP-90
HSP-21
HSP-15
HSP 16
HSP-18
HSP-19
HSP 91
SSP-81
3.73
6.05
7.07
9.04
99.45
99.61
30.8
41.8
Steamboat Rock
5.97
7.91
7.54
7.91
6.93
7.82
7.10
7.46
6.86
7.51
7.34
49.4
49.2
48.9
48.0
49.5
46.3
49.3
46.0
47.4
48.4
Rock Creek
HSP-53
HSP-121
46.6
47.9
3.00
3.14
15.8
15.6
15.02
15.06
0.24
0.25
HSP-7
HSP-2
SSP-34
HSP-100
HSP-1
SP-4
SSP-43
SSP-46
HSP-101
47.2
46.8
48.9
48.0
48.4
47.7
49.5
46.3
48.2
2.56
2.70
2.07
2.25
2.39
2.54
1.91
2.37
2.44
15.8
17.1
16.2
15.8
17.3
16.3
17.3
15.8
16.0
12.68
13.68
11.49
12.13
12.72
12.97
10.30
12.58
12.44
0.17
0.16
0.16
0.15
0.16
0.18
0.11
0.18
0.20
4.99
4.79
36.2
BRGZ' Smith Creek
7.34
7.38
7.64
7.83
6.63
7.27
6.58
7.47
7.51
50.8
49.0
54.2
53.5
48.2
5O.O
53.3
51.4
51.8
Lava Creek
SSP-76
SSP-57
SSP-58
SSP-59
49.1
49.4
49.2
48.1
1.99
2.01
1.97
1.95
16.7
17.5
17.1
17.3
11.41
11.12
11.42
11.94
0.18
0.14
0.18
0.18
HSP-49
48.4
HSP-51
49.3
HSP-48
49.2
HSP-47
48.4
1.91
2.00
1.85
1.84
17.0
17.8
16.8
16.6
10.31
10.39
11.65
!1.30
0.11
0.12
0.17
0.11
HSP-76
49.4
HSP-95
48.4
HSP-89
49.2
HSP-27
48.4
1.91
1.80
1.92
1.98
1.73
2.01
1.92
1.96
1.90
1.80
1.87
1.93
1.69
2.03
1.94
1.96
1.82
1.79
2.07
1.88
2.05
17.3
16.3
17.3
16.3
16.2
16.8
16.8
17.0
17.2
17.1
16.4
17.3
16.4
16.9
16.6
16.5
16.4
16.5
17.6
16.7
17.3
9.99
10.41
10.30
10.58
10.49
11.77
10.39
10.56
10.90
10.89
10.31
10.40
10.73
10.96
10.60
10.53
10.13
10.53
10.64
10.52
10.69
0.16
0.13
0.12
0.18
0.11
0.13
0.1!
0.17
0.18
0.17
0.11
0.12
0.13
0.13
0.14
0.11
0.17
0.16
0.17
0.18
0.13
5.98
6.26
6.02
6.00
Smith Prairie:Unit
6.85
6.78
7.37
7.52
HSP-39
47.7
HSP-188
48.8
HSP-73
49.6
HSP-82
49.2
HSP-93
47.6
HSP-43
48.9
HSP-96
48.4
HSP-87
49.7
HSP-104
47.9
HSP-24
48.5
HSP-25
48.3
HSP-26
48.5
HSP-38
48.3
HSP-40
48.8
HSP-117
49.7
HSP-132
48.6
HSP-129
49.9
6.15
8.40
6.66
7.91
8.55
6.14
6.58
6.61
8.45
7.83
8.58
6.51
8.86
7.10
7.76
7.83
8.26
8.90
6.83
8.18
6.98
48.4
47.3
1
10.33
9.82
9.49
9.38
Smith Prairie:
48.3
50.1
54.2
53.8
53O
54.3
Unit 2
8.90
9.90
9.36
9.19
9.84
8.28
9.14
9.24
9.26
9.99
9.68
9.38
10.10
9.14
9.30
9.13
9.87
10.07
9.42
9.83
9.21
52.3
59.0
53.6
57.2
59.2
48.2
53.O
52.8
58.0
56.2
59.7
52.7
59.6
53.6
56.6
57.0
59.2
60.1
53.4
58.1
53.8
VETTERAND SHERVAIS:BASALTSOF BOISERIVER GROUP,IDAHO
9049
TABLE 3a. (continued)
Smith Prairie:
Unit 2
Sample
$io
TiO
HSP-169
49.4
1.92
HSP-187
49.4
1.99
HSP-186
48.9
1.83
HSP-11
46.2
2.20
HSP-12
47.9
2.36
HSP-59
48.7
2.15
HSP-52
HSP-67
47.9
48.5
1.97
1.98
HSP-64
49.2
2.08
Al O
FeO
MnO
MgO
6.55
6.09
6.46
7.18
6.85
17.6
17.3
17.3
15.8
16.3
10.41
12.30
11.60
13.34
11.62
0.16
0.20
0.18
0.19
0.17
17.0
16.8
17.4
16.7
16.9
17.1
18.0
17.0
16.9
17.2
17.1
10.62
11.59
10.56
10.58
10.52
10.78
10.02
10.45
10.55
10.33
10.43
0.17
0.13
0.11
0.19
0.11
0.16
0.15
0.11
0.10
0.13
0.15
CaO
Na O
9.28
8.35
8.64
9.14
9.77
3.16
2.83
3.07
3.00
2.87
K 0
P0
Total
Mg #
1.80
0.43
100.72
52.9
2.01
0.28
100.73
2.05
1.09
1.37
0.45
0.41
0.59
100.48
98.52
99.86
46.9
49.8
49.0
51.2
Smith Prairie: Unit 3
HSP-61
48.7
1.99
HSP-66
47.6
1.99
HSP-63
50.0
2.02
HSP-69
SSP-26
49.6
47.9
1.97
1.95
SSP-21
48.8
1.94
HSP-72
49.5
1.98
6.31
7.35
6.54
6.60
6.87
6.70
5.72
6.54
6.66
6.65
7.27
9.66
9.45
9.61
9.62
9.35
9.96
9.62
9.56
9.40
9.69
9.84
2.96
2.66
2.76
2.90
2.91
2.83
3.01
2.83
2.98
2.89
2.11
1.69
0.49
99.75
51.4
1.35
0.33
99.51
53.1
1.52
0.34
99.38
52.4
1.69
0.43
100.03
52.7
1.78
1.62
0.45
0.52
99.61
99.21
53.8
52.5
1.90
0.52
100.98
50.4
1.60
1.59
1.53
0.36
0.54
0.44
100.07
98.52
99:61
1.16
0.08
99.67
52.7
52.9
53.5
55.4
1.01
0.29
0.37
0.31
0.45
0.20
99.33
100.55
99.38
99.38
98.79
49.4
53.5
50.1
51.5
47.6
Fall Creek
HSP-113
47.6
2.34
HSP-110
49.6
1.76
HSP-111
47.8
2.16
HSP-109
48.7
1.69
HSP-112
46.6
2.44
16.1
17.4
16.1
17.4
15.6
13.11
10.56
12.80
10.73
13.55
0.15
0.18
7.18
6.81
8.88
8.82
2.72
2.97
0.15
0.16
7.20
6.39
8.97
8.47
2.88
3.00
0.20
6.90
9.29
2.83
2.09
1.01
2.40
1.12
betweenthe "depletedmid-oceanridge basalt (MORB) mantle"
(DMM) and "enrichedmantle2" (EM2) componentsof Zindler
South
Carolina.
Oxygen
wasextracted
using
theCIFatechnique
and Hart [1986].
spectrometer.
•slSo
compositions
relative
toSMOW
range
from
However, the current Rb/Sr ratio of this
basalt (0.003) is too low to support the production of the
observedSr isotopiccomposition(0.707113) from primordialSr
sincethe Archcan. The BRG 2 lavas have Sr and Nd isotopes
closeto "bulk silicateearth" in compositionand plot within the
field of oceanislandbasalts(OIBs) on a Sr-Nd correlationplot
(Figure 7). The mantle source of these lavas could be an
unfractionated
primordialreservoirbut more likely representsa
[Borthwickand Harmon, 1982] andanalyzedon a VG504 mass
5.23 to 6.54 (Table5). Most samplesarewell withintherange
of primary mantle oxygen compositionsinferred from MORB
[e.g., Kyser et al., 1981, 1982]. These data are consistentwith
the derivationof thesemagmasdirectlyfrom the mantle,with
littleor no crustalinput. Samplesfromtwo flows(SmithCreek,
LongGulch)areslightly
enriched
in•SlSO
(6.3to6.54),which
may reflectassimilation
of a crustalcomponent
or variationsin
the oxygencompositionof the mantlesourceregion[Kyseret
Pb from all six samplesplots abovethe northernhemisphere al., 1981, 1982]. These flows are also highestin Ba, which
oceanicregressionline in the so-calledDupal anomaly,withinor supportsderivation of the heavy O signature from a crustal
mixture of various mantle reservoirs.
closelyadjacentto the field definedby lavas of the SnakeRiver
Plain (Figure 8). The singleBRG 1 lava (HSP-14) has the least
source.
radiogenic
2ø6pb/2ø4pb
butthemostradiogenic
2ø7pb/2ø4pb,
indicatinga higher mu reservoir. The BRG 2 lavas clusternear
the C3 componentof Carlson [1984], which he interprets to
representan ancientenrichedlithospherereservoir. The Pb data
suggestinvolvementof continentalcrust or lithospherein the
evolution of these lavas, either by direct assimilationor, more
likely, by the incorporation of a subcontinentallithosphere
componentinto their sourceregion. The latter caseseemsmore
likely becausewe can see no evidencefor a crustalsignaturein
the major or trace elements.
OxygenIsotope Compositions
Oxygen isotopecompositionswere determinedfor 13 whole
rock samplesusingthe facilitiesof D. Stakesat the Universityof
PETROGF2qETIC MODELING
The observedgeochemistry
of the BoiseRiver Groupbasalts
shows that there is a distinction between the BRG 2 and the
olderBRG 1 basalts.Chemicalanalyses
of BoiseRiver Group
lavas form quasi-linear arrays on element-elementvariation
diagramswhich define the liquid - line - of- descentfor each
basaltunit. Variationswithina singleeruptiveunit (formation)
are mostlikely dueto low-pressure
fractionalcrystallization,
but
high-pressurefractionalcrystallization,assimilationof crustal
material,and the influx of new magmabatchesmay also be
important. Differencesbetweenunitsmay be due to crystal
fractionationfrom a commonparent magmaat high or low
pressures,assimilation
crustalmaterials(with or withoutcrystal
fractionation),variabledegreesof meltingin a commonmanfie
9052
VETTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP, IDAHO
500
52
50
400
f ß..x.'o:f
48
300
• 4•
2OO
o¸
COM
t
100
42
I
30
35
4o
45
50
O••)+]•eO)
4.0
....
55
60
t
I
35
3O
65
* 1o0
B
I
!
I
45
i
,
I
I
50
O•go/•)+•eo)
7O
! .........
I
40
i
!
55
60
65
* • oo
,
i V '
!
'
6t
3.5
3.0
or_e m•l
ß
2.0
1.5
3O
I o0
30
I
I
35
[
I
40
[
I
45
I
I
50
I
I
55
I
I
60
65
600
0VisO/MsO+FeO)* • 00
3.0
500
2.5
400
2.0
3OO
200
0.5
'
[
35
30
'
•
40
'
•
45
'
[
50
Ov[•so+]•eo)
0.0
I
30
I
35
t
I
40
I
I
45
(M••]•eO)
i
I
50
I
I
55
I
I
60
'
I
'
55
i
t
60
65
* • oo
Fig. 4. Traceelement
variation
diagrams
of (a) Zr, (b)Rb,and(c)$r
65
versusMg #. Symbols
andfieldsimilarto Figure3.
* • oo
Fig. 3. Major elementvariationdiagramsof weight percent(a) SiO., !xb)
2
TiO
, and
(c) K2 O versusMg # (MgO/MgO+FeO).
Solidsymbols,
.2
.
.....
'
BoiseRiver Group 2 basalts(Fall Creek, rovertedtriangle;SrmthPrairie,
circles; Lava Creek, squares;Smith Creek, triangles); open symbols,
BoiseRiver Group 1 (SteamboatRock, circlesand squares,Long Gulch,
Rock Creek); SRP, Snake River Plain basalts; COM, Craters of the
Moons.
source region, or compositionaldifferences between distinct
mantle sourceregions. These processesmay act in concertto
producea variety of effects.
In this sectionwe assesspossiblerelationshipsbetween and
within each unit usingquantitativepetrogeneticmodels. Major
element variations were modeled with a least squaresmixing
approachusingthe program GENMIX [LeMaitre, 1981]. Low
pressure crystal fractionation was modeled using observed
phenocrystcompositionsin the chosenparentmagma(olivine
and plagioclase only). The effect of high-pressurecrystal
lO
f
I
I
La Ce
I
Boise
River
Group
Lavas
I
Nd
I
I
I
Sm Eu
I
I
Tb
I
I
I
I
I
I
Yb Lu
Fig. 5. Chrondrite-normalized
REE abundances
of theBoiseRiver
Groupbasalts.
Opencircles,BoiseRiverGroup1 basalts;
solidlines,
BoiseRiverGroup2; dottedfield,Cratersof Moons;dot-dashed
line,
averageSnakeRiver Plainbasalt.
VETTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP, IDAHO
!
i
i
i
[
i
i
[
i
i
i
i
i
i
i
0.5135
i
,
i
9053
,
1 oo
0.5130
o
BULK
EARTH
,.•0.5125
•
2
o
0.5120
BRG 2
0.5115
0.701
r.•
,
•
0.702
,
t
,
0.703
•
•
0.704
0.705
,
•
0,706
,
t
,
0.707
t
0.708
,
0.709
87Sr/•6Sr
4
Boise River Group Lavas
'
'
I
RbBaTh
I
I
I
I
KNbLaCoSrNd
I
I
I
I
I
I
I
P HfZrSmTiTbYb
I
I
I
Y
Fig. 6. Spiderdiagramof averagetrace elementabundancesnormalized
to primordialmantle[Wood,1979]. Solidcircles,BoiseRiver Group 1
basalts;solid lines, BoiseRiver Group 2 basalts;dot-dashline, Snake
River Plain basalts.
Fig. 7. Sr-Nd anticorrelationdiagramshowingselectedBoiseRiver
Group basalts. Fields: MORB, mid oceanridge basalts;OIB, ocean
islandbasalts;SRP,SnakeRiver Plainbasalts. Solidcircles,BoiseRiver
Group2 basalts;opencircle, BoiseRiver Group1 basalts.
Fractional Crystallization
Intrafiow variations. Intrafiow chemical variations were
fractionation was tested using a clinopyroxene composition
producedexperimentallyby Thompson[1975] at 8 kbar from a
primitive SRP composition. Crustal assimilation was tested
using two alternative assumptions: (1) bulk assimilation of
crustal material and (2) assimilationof a partial melt derived
from crustalmaterials. Bulk assimilationwas modeledby using
an average granite xenolith from the Smith Creek basalt flow.
Assimilationof a crustalpartial melt was modeledusingan SRP
rhyolite thoughtto have formed by crustal anatexis [Leemun,
1982b; Honjo and Leeman, 1987].
Traceelementswere modeledusingtwo approaches,
Rayleigh
fractionationand assimilation-fractionalcrystallization(AFC
[DePaolo, 1981]). The Rayleigh fractionationmodelsassume
that the percent fractionationcalculatedby the least squares
major element modeling is applicable. AFC models used
compositionsrepresentingupper and lower crustal material as
the assimilantand primitiveSRP basaltsas the parentalmagma
[Rogers, 1977; Taylor and McLennan, 1985; Honjo and
Leeman, 1987; Lum et al., 1989]. Trace element f'itsto models
are presentedas "percenterror", definedas 100 * (true value calculated value) / (true value), where the "true value" is the
concentration in parts per million in the sample whose
composition
we are trying to matchin the model.
modeledusingthe mostprimitive samplefrom eachflow (low
SiOn, high Mg #, Ni, and Cr) as the parent composition.
ResUlts
oftheintrafiow
leastsquares
calculations
aregiven
in
Table6. The evaluation
of thefit of eachmodelis determine
by
thesumsof thesquares
of theresiduals
(Er2);modelsare
assumed
tohaveacceptable
fitswhenEr2 < 1.0andallmajor
elements
are withintheiranalyticaluncertainty.Notethatwhere
the extentof fractionalcrystallizationbetweentwo flow unitsis
small (less than a few percent), the calculated solutions are
sensitive to analytical uncertainties and represent only
approximatesolutions.
The leastsquares
solutions
for intrafiowvariationsuggest
that
shallowcrystalfractionation
or heterogeneous
distributionof the
observed
phenocrysts
(01ivine+ plagioclase)
is responsible
for
the chemical variations observed within most of the flows.
The
results
of theintrafiow
calculations
showgoodfits(Er2 = .12
to .93) (Table 6). The range in fractionation is 1-20%, but
withina givenflow it is typically< 10%. The Steamboat
Rock
basaltrequires10-18% dominatedby plagioclaseand olivine.
The BRG 2 basalts results require 2-13% fractionation
dominatedby olivine.
Chemical variations within the Smith Prairie flow units SP1
and SP3 samplessuggest< 10% low-pressurefractionationof
TABLE 4. IsotopicAbundances
tbr theBoiseRiver GroupBasalts
Flow
Sample
S7Sr/S6Sr
143Nd/144Nd 2ø6pb/2ø4pb 2ø7pb/2ø4pb 2øSpb/2ø4pb
BRG1
Steamboat Rock
HSP-14
0.707113
0.512458
Unit 2
HSP-95
0.705128
Unit 3
Lava Creek
HSP-52
SSP-34
SSP-57
0.705215
0.706244
0.75594
0.512641
0.512547
0.512601
Fall Creek
HSP-111
0.705977
0.512602
18.438
15.701
38.93
18.601
15.635
39.08
18.603
18.648
18.642
15.591
15.669
15.674
39.23
18.558
15.620
39.02
BRG2
Smith Prairie
Smith Creek
38.87
38.96
9054
Vm'rF• AND SHF.
RVAIS:BASALTS
OF BOISERIVERGROUP,IDAHO
15.8
Least squaresmixing models using a primitive SRP basalt
[Lure et al., 1989] as the parent magma wcrc only partially
successful. Low-pressure fractionation models using the
primitive SRP basaltparent magmawcrc unableto reproduce
15.7
15.6
anyoftheobserved
BRGbasalt
compositions
(l;r2 = 15to6.7;
Table 7). A combinationof high-pressurepyroxcnc and lowpressureolivinc + plagioclascfractionationresultsin acceptable
15.5•
solutions
fortheBRG1 basalts
(l;r2 = .67 to 1.1)with3455
MORB
high-pressureand 35 55 low-pressurefractionationnccdcdto
form primitive Steamboat Rock basalt from the SRP parent
magma(Table 7). However, BRG 2 basaltscannotbc relatedto
a primitive SRP parent by any combinationof high- and low-
15.3
15.2
17.0
18.0
19.0
20.0
22.0 pressurecrystal fractionation(Table7), consistentwith the
21.0
differencesin radiogenicisotopes.
To summarizethe above modeling, major and trace clement
compositionspreclude any fractionationrelationshipbetween
BRG 1 (SteamboatRock Unit) and BRG 2 basalts,based on the
poor fit of least squaresmodel solutions. BRG 1 basaltscould
result from combinedhigh- and low-pressurefractionationof a
primitive SnakeRiver Plain basalt. The BRG 2 basaltscan bc
relatedto a primitiveBRG 2 parent(SmithPrairie flow SP2) by
a combinationof high-pressureand low-pressurefractionation,
but cannotbc derivedfrom a primitive SnakeRiver Plain basalt
206pb/204Pb
41.0
•39.0
/ MORB
•o 3•.0
parent.
Assimilation
37.0
36.0
•?.o
I
I
[
•8.o
I
•9.o
]
I
20.0
i
I
2•.o
•
22.0
2o6pb/2O4pb
When interpretinggeochemicaldatafor continentalbasaltsthe
possibility must be considered that observed elemental
abundanceshave been affected by crustal assimilation. Three
Fig. 8. Pbisotopes
showing
BoiseRiverGroupbasalts.Fieldsand modelsare consideredhere to test for possiblecrustal
symbolssameas Figure7.
contamination of the Boise River basalts' bulk assimilation,
assimilation of a crustal anatectic melt, and assimilation-
fractionalcrystallization(AFC) models[Taylor, 1980; DePaolo,
olivine with minor plagioclase. Smith Prairie unit SP2,
•98q.
however, requires a combinationof about 2055 low-pressure
fractionationof olivine + plagioclasealong with approximately
TABLE 5. OxygenIsotopeCompositionfor BoiseRiver GroupBasalts
115%high-pressurefractionationof pyroxene, suggestingthat
more than one flow unit is includedin this map unit. The Long
Flow
Sample
/•lSO
Gulchbasaltis the only other flow wherethe observedintraflow
variationscan not be modeledby low pressurefractionationof
BRG1
olivine and plagioclase;at least21555pyroxenefractionationis
HSP-14
6.03
Steamboat Rock
alsorequired(Table6).
HSP-33
6.48
Interflow Variations. Relationships between flows were
Long Gulch
modeledto determineif any flowscouldbe relatedto a common
parent magma. Two potentialparent magmaswere tested'the
BRG2
mostprimitive BRG 2 sample(a Smith Prairie basaltfrom flow
Smith Prairie
unit SP2) and a primitive SRP basalt [Lum et al., 1989]. A
HSP-48
5.23
Unit 1
primitiveBRG 2 basaltparentis considered
possiblesincethese
HSP-49
5.82
samplesare the mostprimitiveof all the observedsamplesin the
HSP-104
5.82
Unit 2
area. Age constraintsprohibit a direct geneticlink betweena
HSP-40
5.64
primitiveBRG 2 parentbasaltandBRG 1 basaltsbut do not rule
out similar mantle sources.
unit
3
Least squares mixing models of low-pressure fractional
crystallizationusinga primitive BRG 2 parentmagmaproduce
HSP-52
6.02
HSP-61
5.91
SSP-59
5.82
poorfitsforall BRGlavas.(l;r
2 = 6.15
to2.8). Modelswhich
combinehigh-pressure
pyroxenefractionation
with low-pressiire
Smith Creek
olivine+ plagioclase
fractionation
produce
goodfits(l;r: =
0.26 to 0.64) for derivation of BRG 2 flows from this type
parent magma (Table 7). Combinedhigh- and low-pressure
fractionationmodelsfail to reproducethe observedSteamboat
Rock
basalt
compositions
(BRG
1)from
thissame
SP2-type
parentmagma(l;r = 3.3).
Fall Creek
HP-1
6.3
SSP-34
6.54
SSP-43
6.3
HSP-100
5.79
VETTERAND SHBRVA!S:
BASALTS
OF t{OISBRIVERGROUP,IDAHO
9055
9056
VE•rER ANDSHERVAIS:
BASALTS
OFBOISERIVER(3ROUP,
IDAHO
TABLE 7. Selected
MajorElementLeastSquares
Calculations
for InterflowVarations
Parent
Daughter
Daughter,
Olivine
PlagioclasePyroxene Residual
Sum
%
Model
(8kbar)
of Squares
15.2
2.8
0.8
I
BoiseRiver Group 1
HSP-104
HSPo104
LongGulch
LongGulch
63.2
50.6
11.2
8.1
25.6
26.2
HSPo104
Steamboat
Rock
69.6
8.1
22.0
HSP-104
SteamboatRock
52.7
5.0
23.7
SmithCreek
SmithCreek
LavaCreek
Lava Creek
Fall Creek
Fall Creek
78.2
61.7
78.4
58.4
72.8
58.9
6.5
4.1
9.3
5.3
8.5
6.0
15.4
18.0
12.2
15.8
18.7
20.6
BoiseRiver Group 2
HSPo104
HSPo104
HSPo104
HSP-104
HSP-104
HSPo104
Model
6.5
18.7
3.3
14.6
3.0
0.6
4.7
0.3
3.3
0.6
33.7
11.0
1.1
16.1
20.5
2
BoiseRiver Group 1
PrimSRP
PrimSRP
LongGulch
LongGulch
55.7
27.7
13.7
6.8
30.7
31.9
Prim SRP
Prim SRP
Prim SRP
Prim SRP
SteamboatRock
SteamboatRock
RockCreek
Rock Creek
62.9
31.7
52.4
23.6
10.5
5.0
16.0
7.0
26.6
29.7
31.7
32.4
Smith Creek
68.5
9.2
21.9
SmithCreek
Lava Creek
Lava Creek
33.4
66.3
28.3
4.5
12.8
5.2
27.6
20.9
27.8
76.1
38.6
89.7
50.7
79.1
7.6
4.2
3.3
2.5
9.1
39.7
64.3
32.4
4.9
11.2
5.5
16.3
25.2
7.0
19.2
11.8
22.9
24.5
28.8
BoiseRiver Group 2
Prim SRP
PrimSRP
Prim SRP
Prim SRP
33.6
36.9
10.9
0.7
15.2
1.0
12.0
34.5
1.2
18.2
38.8
2.4
Smith Prairie
Prim SRP
Prim SRP
Prim SRP
Prim SRP
Prim SRP
Prim SRP
Prim SRP
Prim SRP
Unit 1
Unit 1
Unit 2
Unit 2
Unit 3
Unit 3
Fall Creek
Fall Creek
9.2
32.0
1.8
6.1
27.6
1.8
9.7
32.5
1.7
10.5
33.3
1.0
Bulk assimilation. Interactionbetweenthe BRG parent lavas [Leeman, 1952b; Honjo andLeeman, 1987]. The rhyolitic
and the Idaho batholith may have resulted in the bulk
assimilationof graniticmaterial. Granitexenolithswithinthe
Smith Creek basalt show that this processis at least locally
important. The effect of bulk assimilation of granite on
primitive SRP and BRG 2 basaltswas modeledusingleast
squares mixing techniques. Based on major element
magmasare thus anatecticmelts of continentalcrust and
representa silicicend-member
whichmay be assimilated
by
basalticmagmas.In our modelswe selected
a rhyolitefromthe
Magic Reservoir[HonjoandLeeman,1987]asthe silcicendmember.
Assimilationof a partial melt alonecan notproduceany of
the
observedinterflow variations. Not only are the residuals
compositions,
bulkassimilation
of granitecannotproducethe
intraflow variations of the BRG lavas' negative amountsof significantly
higher
thanacceptable
values
([;re = 2.9- 56),but
granite
arerequired,
andthecalculated
error[;re = 3 to32 someflowsrequirenegativeamountsof anatecticrhyolite(Table
whentheBRG2 parentis usedand[;re = 57 to 15when 8). Assimilationof an anatecticrhyolite combinedwith both
high-andlow-pressurefractionationproducesacceptableleast
primitiveSRP is used.
solutions
([;re = 0.3 to 0.4) forthemajorelement
Assimilationof anatecticmelts. Evolvedand hybridlavas squares
which occurnear the marginsof the SnakeRiver Plairiandin compositions,but are inconsistentwith the trace element
the vicinityof sili½iceruptivecentershavemineralogical
and concentrations.Trace elementconcentrationswere modeledby
geochemical
characteristics
whichresultfrom the mixingof Rayleighfractionationusingthe resultsof the major element
rhyolitic magmaswith basalt. The rhyolitic volcanismis modeling(assimilationeffectscalculatedbeforefractionation).
consideredto originatefrompartial meltingof crustalmaterial The errorsrangefrom 35% - 110% for the incompatibletrace
due to the influx of heat from the ascendingbasaltic magma
elementsand 196% to 514% for the compatibleelements.
VETTERAND SHERVAI$:BASALTSOF BOISEI•IVER GROUP,IDAHO
oo
•
•n•
•
9057
9058
Vu!'ruR AND SHERVAIS:BASALTSOF BOISERIVER GROUP,IDAHO
AFCprocesses. Assimilation-fractionalcrystallization(AFC)
models[Taylor, 1980; DePaolo, 1981] were appliedto the Boise
SteamboatRock basalt, while similar to lavas of the Snake River
Plain, cannotrelatedto primitiveSRP basaltsby AFC processes.
River basalts in order test for the effects of crustal assimilation
Evolved BRG 1 basalts (Rock Creek and Long Gulch flows)
independent of major - element modeling. A variety of have higher Zr concentrations, higher incompatible trace
representative
upperand lower crustalcompositions
were tested elementratios,and lower Mg# comparedto the SteamboatRock
becausethe potentialcontaminantsare poorly known [Rogers, flow. These characteristics are consistent with formation of the
1977; Taylorand McLennan,1985]. To minimizethe effectsof Rock Creek and Long Gulchlavasby AFC processesinvolving
crystal fractionation,the AFC modelswere determinedusing a parent magma similar to the Steamboat Rock basalt in
incompatibletrace elements(Zr, Nb, Rb, Ta, La, Th)whose composition(Figure9). This interpretation
is supportedby the
partition coefficientsare < < 1 for the observedphenocryst slightly
higher
d•sO
valueofoneLongGulch
sample
(Table
5).
phases.
However,moreisotopicdataare neededto testthismodel.
AFC modelspredicta rapid increasein Zr concentration
with
little or no changein incompatibletraceclementratios,as shown Mantle Melting Models
in Figure 9. The BRG 2 basalts have nearly constant Zr
concentrations(150 to 190 ppm) with large variations in the
The petrogeneticmodelingpresentedabove showsthat.basalt
ratios Nb/La, Rb/Nb, and Rb/La, which are muchhigher than suitesof the BoiseRiver Group (BRG 1 and BRG 2) cannotbe
either the BRG 1 or the SRP lavas. The wide range in these related to one another by crustal processes such as crystal
ratios, and their high absolute values, preclude any AFC
fractionation, crustal assimilation, or combined assimilationrelationship between the BRG 2 basalts and primitive SRP fractional crystallization, althoughwithin suitevariations are
basalts (Figure 9), consistentwith observeddifferences in consistentwith theseprocesses(primarily crystal fractionation).
isotopiccomposition.
This impliesthat differencesbetweenBRG 1 basaltsand BRG 2
The SteamhostRock basalt(normal BRG 1) has a range in Zr basalts are either inherited from chemically distinct source
concentrations(223 to 307) which are generallyhigherthan in regionsor reflect variable partial meltingof chemicallysimilar
BRG 2, and low, nearly constant Nb/La, Rb/Nb, and Rb/La sourceregions. We appliedmodalbatchmeltingcalculationsto
ratios; theseratios are also lower than primitive SRP basalts. the "primordialmantle"of V•bod[1979] to determineif the BRG
The high Zr may result from AFC processes,however, the i and 2 basaltswere generatedfrom a similar sourceor from
lower incompatibleelementratios cannot, suggestingthat the chemicallydistinctsources. Figure 10 illustratesthree melting
curvesfor Nb (a high field strengthelement)and Rb (a low field
strengthelement)in relationto datafor the BoiseRiver Group
2.5
basalts. These curves are basedon sourcecompositionswith
(curve A) 2x unfractionated primordial mantle, (curve B)
2.0
fractionated mantle with Nb = 4x primordial and Rb = 2x
•
c1
primordial, and (curve C) fractionated mantle with Nb = 4x
1.5
primordialand Rb = 0.Sx primordial. Fractionalcrystallization
pathsfollow straightlines at a constantratio from the primary
_•.•C3
v•
0.5
ß o cl
......
0.0
C4
250
C2,C5
350
•
Data for the BRG 2 basalts lie between curves A and B,
[]
o ,s•'=•-L•---•---•.
- .
..........
150
50
melts.
450
550
ppm
suggestingderivation from a sourcesimilar to unfractionated
primordialmantlewhich is slightlyenrichedin Nb relativeto Rb
(high HF$/LFS cation ratio). A range in sourcecompositions
may be required to explain the spread in the data. The
calculations indicate that less than 10% melting of the
7O
2.0
6O
5O
1.5
[]
•
oI'•q• C5
oB -
c2,0
•40
[]
C1
o,c2,0,cs
o
•3o
•
©v
A
C4
2o
0.5
lO
i
0.0
50
!
i
i
150
i
i
!
I
250
i
i
i
I
350
i
i
i
I
450
i
i
i
I
i
i
550
Z• ppm
Fig. 9. Assimilation-fractional
crystallization
(AFC) modelsof the Bois•
River Groupbasalts.C1, uppercrust[TaylorandMcLennan,1981];C2,
lower crust[Taylorand McLennan,1981]; CS, sili½i½
granulite[Honjo
andLeeman,1987]; C4, rhyolite[HonjoandLeeman,1987];C5, marie
granulite[Rogers,1977];A, primitiveSRPbasalt[Lureet al., 1989];B,
primitiveSteamboat
Rockasparent. SymbolssameasFigure3.
0
10
20
30
40
50
60
70
80
90
100
Rb ppm
Fig. 10. Melt model for BoiseRiver Group basalts. Melting curves
represents:curveA, unfractionated
mantlewith 2x Primordialmantle
values;curve B, fractionatedmantlewith Nb 4x primordialmanfieand
Rb 2x primordial mantle; curve C, fractionatedmantlewith Nb 4x
primordialmantleandRb 0.Sx primordialmantle.
VETTERAND SHERVAIS-BASALTSOF BOISERIVER GROUP,IDAHO
unfraetionated
primordialsourceis necessary;differentassumed
source concentrationswould yield different percentagesof
partial melting, but the relative concentrationsof Nb and Rb
would remain the same. The high Rb concentrationsoverall in
the BRG 2 basaltssuggestthe possibility of Rb addition to the
source,butthelowinitialS7Sr/S6Sr
requiresthatthisRb
enrichmentis a recentphenomenon.We postulatethat this Rb
enrichment may be related to the subductionin Cascadia, as
suggested
be Carlsonand Hart (1987) for basaltsof the Oregon
Plateau.
The BRG 1 basaltsare extremely low in Rb but contain Nb
concentrationsin the same range as the BRG 2 basalts. These
data cannot be explained by the melting of an unfractionated
primordial source or a subduction-enrichedlithosphere. As
shownin Figure 10, a highly fraetionatedsourcewith Nb/Rb -8x primordialis neededto explainthe SteamboatRock basalts.
An important result of this modeling is that the BRG 1
(SteamboatRock) and BRG 2 basalts cannot be related to one
another by variable melting of a similar source region;
chemically as well as isotopically distinct source regions are
requiredfor each.
DISCUSSION
9059
unsupportedby the current Rb/Sr ratios. BRG 2 basalts are
higher in LFS elementsand lower in HFS elementsrelative to
BRG 1 and have higher Rb/Sr ratios but are characterizedby
lowerS?Sr/S6Sr
ratiosandepsilon
Ndvalues
nearbulkearth.
These characteristicsrequire complexhistoriesfor the mantle
sourceregionsof thesebasalts.
Basedon isotopicdata, the SRP basaltsourceis consideredto
be enrichedlithosphericmantlewhich hasbeenisolatedfrom the
asthenosphere since at least 1.5 Ga [Leeman, 1982a]. This
conclusionis also consistentwith our singleBRG 1 basaltdatum
(HSP-14). However, our modeling shows that the BRG 1
basaltswere derivedfrom a mantlewith extremelyhigh ratiosof
HFS/LFS elements, ratios which are inconsistentwith subdue-
tion enrichmentof depletedlithosphereor w;•ththe assimilation
of continental crust. We suggesttb.at the high HFS/LFS ratios
of the BRG
1 basalts are consistent with a mantle source which
has been affected by Fe-Ti metasomatism from basanitic or
alkalic melts [e.g., Ehrenburg, 1979; Smith and Ehrenburg,
1984; Nielsen and Noller, 1987]. This Fe-Ti metasomatismwas
superimposed
uponan earlier (andprobablymucholder) event
which produced the high Rb/Sr ratios and low Sm/Nd ratios
nt-t,t-•a•-v tt• at,lat-•vt-tht- 87•./86•. on•l 143M•l/144M•l
ratios
obse• today. This •o-s•ge enrichment
h•to• is n•essa•
to expla• the TiO/K O ratiosof the BRG 1 basalts,which am
Origin of the BoiseRiver GroupMagmas
highrelative
to:fl•od
basalts
withhighS7Sr/S•Sr
[e.g.,
Hawkesworthet al., 1984]. Hawkesworthet al. [1984] aRfibute
thehighS7Sr/•Sr
floodbasalts
topotassie
metasomatism
ofthe
Two mantle source regions are commonly proposed for
continentalbasalts [e.g., Thompsonand Morrison, 1988]: (1)
subcontinental lithosphere, which has been isolated from
chemical exchange with the asthenosphere for hundreds of
millionsof years, or (2) the underlyingasthenosphere,
which is
chemically similar to the source region of mid-ocean ridge
basalts (MORB) and ocean island basalts (OIBs). In addition,
crustal assimilation can modify magmas from either source
during their ascent to the surface [Carlson, 1984]. Recent
studieshave arguedthat flood basaltsform from mantleplumes
which have trace elementsystematicssimilar to oceanicbasalts
[Richardset al., 1989; Richardsand Griffiths, 1989; Campbell
and Griffiths,1990; Duncan, 1991]. Thesemodelssuggestthat
large volumefloodbasalteruptionsresultfrom the mushrooming
of risingplumeheadswithin the asthenosphere,
with subsequent
volcanism (the "hot spot track") derived from the less
voluminous plume tail [Richards et al., 1989; Richards and
Griffiths, 1989; Campbelland Griffiths, 1990; Duncan, 1991].
In contrast,a lithosphericmantlesourcehas been suggestedfor
basalts of the Snake River Plain, based on trace element ratios
that differ from OIB [Fitton et al., 1991] and heavy elementenriched isotopic signatures and old model ages [Leeman,
1982a; Doe et al., 1982]. This requires the enrichmentof a
previously depleted ancient subcontinentallithosphere. The
enrichmentprocessis commonly attributed to fluid migration
from subduetingoceaniclithosphereduring the Proterozoic or
Phanerozoic[Carlson, 1984; Hawkesworthet al., 1984; Zindler
and Hart, 1986; Ormerod et al., 1988; Menzies, 1989].
The most significantcontrastbetweenthe BRG 1 and BRG 2
basalts is the pronounced decoupling of trace element
concentrationsand isotopiccompositions. BRG 1 basalts(and
Snake River Plain basalts) are characterized by low
concentrationsof LFS incompatibleelements (K, Rb, Sr, Th)
relative to the HFS elements (Ti, Zr, Nb, Hf, REE), and low
sourcemantle(K-fichtefiteandphlogo•temodaHy),probablyas
the resultof subduetion-defiv•fluid migration. In contrast,the
BRG 1 basalts,whichhavehigh a7Sr/S•Sr
ratios, are
characterized
byhighTiO:/K:O,
highNb/Rb,andhighLa/Rb
ratiossimilarto basaniticmeltsproducedby smalldegreesof
pa•ial meltingof the asthenosphere
[e.g., Hawkesworthet al.,
1984]. Thesemeltsprobablyreflect passageof a plume •il
ben•th the S•
[e.g., Richar• et al., 1989].
BRG 2 basaltsare similar chemically and isotopically to
oc•nic islandbasalts(OIBs) and may be defiv• from a s•flar
asthenosphefic
sourceregion. Onemajorcontrastb••n
BRG
2 basaltsand normalOIB lavasis in Pb isotopecompositions,
which (in BRG 2) are similarto "Dupal" OIBs. This may be
dueto pa•ial ass•flation of •thosphefiematerialdung ascent,
or to a young subduction enrichment event in the shallow
asthenosphere
(Caseadiasubduetion?).The high Rb, Ba, and
Rb/Nb
ratios
of the BRG
2 lavas
are not due to crustal
contamination, as shown by (1) the low Sr isotopic
compositions,(2) high Mg g, Cr, and Ni, (3) mantleoxygen
isoto• com•sitions (5.8 •0.3 •r mfl), and(4) the absenceof
mixing trends betweenthe BRG 2 lavas and typical crustal
com•sitions.
Crustal Assimilation
Can BRG 1 and BRG 2 be related by crustalassimilation?
No. Mixing calculations,traceelementsystematics,
and our
limited isotopicdata all showthat BRG 1 and BRG 2 cannotbe
relatedby crustalassimilation. The clearestevidencefor this is
the contrastbetweentraceelementenrichments
and isotopic
compositions.
BRG 2 lavas,whichare higherthanBRG 1 lavas
in
SiO
, K O, Rb,andBa,havelowerS7Sr/S•Sr
andhigher
143
1
Nd/•dNdaratios
than
BRG
1. This
istheopposite
ofwhat
we
would expect if theselavas were related by assimilationof
Rb/Srratiosbuthaverelatively
highS7Sr/S6Sr
andnegative crustal material, especiallythe Precambriancrustal material
epsilonNd values. The Sr isotopecompositions
in particularare known to underlie the $RP. Additional evidenceagainst
9060
VETTER AND SHERVAIS:BASALTSOF BOISERIVER GROUP, IDAHO
forming BRG 2 lavas from BRG 1 magma by crustal between different flows within the same group cannot result
assimilation is listed above, e.g., mantle oxygen isotope from low-pressure fractionation but can be modeled by
high-pressure
pyroxenefractionation
andlow-pressure
compositionsin most flows; high Mg #, Cr, and Ni (since combined
assimilation
forcescrystallization);
failureof mixing-assimilation olivine + plagioclase
fractionation.
Differencesin the majorandtraceelementcompositions
of
modelsdiscussed
in the sectionon petrogenetic
modeling.
This does not mean that assimilation of crustal materials did
BRG I and BRG 2 basaltscannotresult from crustalprocesses:
not occur, however. Some flows in each group (e.g., Rock the observedvariationsmustrepresentmantleprocesses.Partial
CreekandLong Gulchbasaltsin BRG 1; SmithCreekbasaltin meltingmodelsindicatethatthetwo groupsformedfromdistinct
BRG 2) exhibitchemicaland isotopiccharacteristics
which may mantlesources.The highNb/Rb ratiosin the BRG 1 lavas,and
be attributed to crustal assimilation.
These characteristics
in similarlavasof the SnakeRiver Phin, requirethat the source
include low Mg #, Cr, Ni, high incompatible element regionof thesemagmasunderwenta recentepisodeof Fe-Ti
metasomatism.
The compositional
characteristics
of thisevent
concentrations
(especiallyBa), andhigherthannormaloxygen
isotoperatios. It is importantto notethat theseflowsdo not
definemixingtrendsbetweenthe BRG 1 andBRG 2 basaltsbut
that flows from each group remain distinctregardlessof the
are consistentwith the additionof small volume asthenospheric
melts. Fe-Ti metasomatism
of the lithospherealongthe rii• axis,
and the extra heat neededto causeextendedlithosphereto melt,
extentof assimilation.This further supportsour argumentthat
probablyresultedfrom the passageof plumetail beneaththe
BRG 1 and BRG 2 must come from distinct mantle source
SRP.
regionsand that they cannotbe related by assimilationof
continentalcrustof any age.
TemporalandAxial Zonations
The transition from saturated olivine tholeiites of BRG 1 to
transitional-alkalic lavas of BRG 2 in the Smith Prairie region
Acknowledgments.
This manuscript
benefitedgreatlyfrom reviewsby
William Leeman, S.P. Reidel, and Richard Carlson and editorial
commentsfrom RodeyBatizaandAllen Glazner. We alsowish to thank
Richard Carlson for providing selectedradiogenic isotopeanalyses,
M.M. Lindstrom for assistancewith the neutron activation analyses,
D.S. Stakesfor assistance
with the oxygenisotopes,and K.A. Howard
the use of unpublishedmap data. Field work was supportedby a
impliesa time-dependent
changein mantlesourceregion,with for
Chevron field scholarshipto Vetter.
earlymagmas
derivedfroma shallowlithospheric
source
giving
way to youngermagmas
derivedfroma deeperasthenospheric
sourcethroughtime. The eruptionof theseyoungerlavasalong
the flanks of the Snake River Plain coeval with eruption of
saturated
tholeiitesof the SnakeRiver Groupto the southfurther
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(ReceivedMay 11,1990;
revised December 13,1991;
acceptedJanuary29,1992.)