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JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 101, NO. B8, PAGES 17,425-17,445, AUGUST
of a mafic volcanic field in the central
Great Basin, south central Nevada G. M. Yogodzinski,T. R. Naumann,E. I. Smith,andT. K. Bradshaw 3 Centerof VolcanicandTectonicStudies,Department of Geosciences, Universityof Nevada,Las Vegas J. D. Walker
ABSTRACT. Evolutionof a maficvolcanicfield is investigated througha studyof Pliocene agerocksin theReveilleRangein southcentralNevada. Plioceneactivitybeganwith the eruptionof relativelyabundant hawaiite(episode1, 5-6 Ma), whichwasfollowedby trachytic volcanism (4.3 Ma) andby a secondepisodeof lower-volume hawaiiteandbasanite (episode2, 3.0-4.7Ma). Incompatible elementsindicateanasthenospheric source.Isotopically,episode2 basalts clusteraround87Sr/86Sr=0.7035 andœNd=+4.2, butepisode 1 samples varyto high 87Sr/86Sr (upto 0.7060)overa narrowrangeof end(+0.8to +4.5). Trachyticrocks(MgO--0.5%) areisotopicallyakinto theepisode1 basalts.Geochemical variationrequirestheadditionof a crustalcomponent (high87Sr/86Sr, Sr/Nd,Pb/La,lowœNd) to theepisode1 hawaiites and trachyticsamples, probablyby assimilation of carbonate-rich sedimentary wall rock. The volcanic field developedin at leasttwo eruptivecyclesof approximately equalduration.Basanites (deeper andlowerpercentage melts)appearonly in theyoungerepisode.Eruptiveepisodes wereapparently linkedto separatemeltingeventsin the mantle. Throughtime,basaltswereproducedin diminishingvolumesby lowerpercentage melting,magmageneration andstoragewasat greater depths,andmagmaascentwas at highervelocities.Spatially,themeltinganomalieswere largein thePliocenebutprogressively diminishedin sizesothatby Pleistocene time,volcanismwas restricted to a small area near the northern end of the initial outbreak.
1988; Farmer et al., 1989]. The assimilation of continental The studyof Miocene and Plioceneage mafic volcanicrocks crust is regardedby most workers to be of minor petrogenetic associated with Cenozoic crustal extension in the western importance [e.g., Leeman, 1982; Menzies et al., 1983; Fitton United States continuesto provide insight into a variety of et al., 1988], thoughthere are caseswhere crustalassimilation geologicallyimportant processesand systems. These include is thought to have been a primary control over Basin and lithospheric-scaletectonic features of rifting on continental Range basaltgeochemistry[e.g., Glazner et al., 1991; Glazner crust, important aspects of the crust-mantle geochemical and Farmer, 1992]. End-membersin the geochemicalspectrum of Basin and system,and clues to the natureof magmaticdifferentiationand in mafic volcanicrocksthat the genesis of igneous rocks in the continental rift Rangebasaltsare well represented environment [e.g., Leeman, 1982; Menzies et al., 1983; occur within the NNE trending zone of Pliocene and younger Fitton et al., 1988; Glazner et al., 1991; Glazner and Farmer, mafic volcanismthat extends from Death Valley on the south, to the PancakeRange and Lunar Crater Volcanic Field on the 1992; Bradshaw et al., 1993]. Previous and ongoing studies have shown that there is a north (Figure 1). This is the Death Valley-PancakeRange twofold geochemicaldivision among Pliocene and younger basaltzone of Vanimanet al.  andFarmer etal. [ 1989]. mafic volcanicrocks in the Basin and Range region [Menzies Basalts from the central part of this zone in the area around et al., 1983; Fitton et al., 1988; Orrnerod et al., 1988; CraterFlat (southernNevadaprovinceof Menzieset al. ) Rogers et al., 1995]. This twofold division is most often are among the most isotopically enriched in the region interpretedto reflect compositionallydistinct sourcesin the (87Sr/86Sr•- 0.707, œNd< -8.5 [seeFarmeret al., 1989; asthenosphericand lithospheric mantle [see also Leeman, Livaccariand Perry, 1993]) and haveall of the major andtrace
1970; Hedgeand Noble, 1971; Leeman, 1982; Fitton et al.,
element features that characterize basaltic rocks derived from
the lithospheric mantle (hypersthene-normative with low •Nowat Department of Geology,DickinsonCollege,Carlisle, FeO*, TiO2, Rb/Ba, and Ti/Hf and high La/Ta and Ba/Nb [see
Pennsylvania. 2Nowat Department of Geology,Universityof Idaho,Moscow. 3Now at House of Lords Committee Offices, London.
Fitton et al.,
In contrast, basaltic rocks from the
northernend of the zone, in the Reveille and Pancakeranges (including the Lunar Crater Volcanic Field), are isotopically
Range basalt zonefromVaniman etal. andFarmer etal. .Modified fromLuedke andSmith [1981.
1992]andfromtheCraterFlatareato the La/TaandBa/Nb[seeFittonet al., 1988, 1991]). Basaltsfrom FolandandBergman, the Reveille and Pancakerangearea are generallyregardedas
south [Vaniman etal.,1982;Farmer etal.,1989;Bradshaw
the asthenospherically derivedend-member in the region[e.g., andSmith, 1994].Twobroadly different themes aredeveloped. Fitton et al., 1988; Farmer et al., 1989].
In thispaperwe examinethe geologyandgeochemistry of Plioceneage mafic volcanicrocks in the Reveille Range (Figure1). We compare thePlioceneageReveilleRangerocks to Pleistoceneage basaltic rocks from the Lunar Crater Volcanicfield to the north[Bergman,1982; Lumet al., 1989;
First,thedataareinterpreted withinthecontext of a changing sourcechemistryfor the Pliocenevolcanic rocks, with emphasis on theaddition of a crustalcomponent to theoldest of the basaltsin the Reveille Rangearea. Second,the data are'
interpretedin the contextof volcanicfield evolution,with
YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
116 ø 15'
38 ø 10' -
38 ø 00'-
GEOLOGIC MAP OF REVEILLE RANGE MAFIC VOLCANIC ROCKS
Figure 2. Geologicmap of Pliocenevolcanicrocks in the Reveille Rangeof southcentralNevada (see also Figure 1). Mappingis modifiedslightlyfrom that of Naumannet al. [ 1991]. SeealsoMartin and Naumann .
emphasison time-space-compositional trendsas they relate to magmaevolutionin the mantle and crust.
Location, Volcanic Stratigraphy, and Petrography
rocks in the central Great Basin of the western United
This area lies along the axis of geophysicalsymmetryoutlined by Eaton et al.  for the central Great Basin, and is isolatedfrom the well-developedvolcanicfields of the Sierran Province/Western Great Basin to the west, and the transition zone/Colorado Plateau to the east [Leeman, 1970, 1982; Menzies et al., 1983; Fitton et al., 1988].
Plioceneand Pleistoceneage basaltsof the Reveille Range and PancakeRange(includingthe Lunar Crater Volcanic Field) Geologic mapping and K-At dating in the Reveille Range constitutethe largestvolume of Late Cenozoicmafic volcanic and adjacentareas[Naumannet al., 1991;Martin and Naumann,
YOGODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Table 1. RockNames,PartialNorms,Phenocryst Assemblages, andLocations for ReveilleRangeSamples Sample RockName neph* hyp* Phenocrysts? Latitude North Longitude West Elevation, feet(m) Episode 1 Basalts
*Nonnative compositions based onFe2+/ (Fe2++ Fe3+)= 0.80. tPhenocrystslistedin orderof decreasingabundance.
1995] indicate that mafic volcanism began in middle to late Miocene time with scatterederuptionsof volumetricallyminor basaltic andesite. The early basaltic andesitesoccur in the northwesternmostReveille Range (Figure 2) and in scattered locationsto the south. Near the town of Rachel (Figure 1) this unit has been dated at approximately14 Ma [Naumann et al., 1991]. Based on petrographicand chemical similarities, we anticipatea similar middle to late Miocene age for the early basaltic andesitesin the Reveille Range. Geochemicaldata on the Miocene rocks are not presentedhere, and theserocks will
Basalts of episode 1 are the most abundantof the Pliocene age volcanic rocks in the Reveille Range. They comprise a
minimumvolumeof approximately 8 km3(estimateof outcrop
volume) and were erupted from --52 vents located throughout the range (Figure 2). Most episode 1 basalts contain phenocrystsof olivine and plagioclase only (25-35 modal percent), but some also contain minor phenocrysts of clinopyroxene, Fe-Ti oxide, and occasionally biotite. Large phenocrysts(>5 mm) and/or megacrysts(>1 cm) of calcic feldspar (labradorite) are also common, and these sometimes not be considered further in this work. occur in glomerocrystictrains that range up to 15 cm in long Plioceneactivity in the Reveille Rangecommenced with the dimension. The common presence of biotite in the eruptionof a relatively large volume of alkalic basalt(5.1-5.9 groundmass of episode 1 basalts is notable. Alteration Ma), which was followedby trachyticvolcanism(4.3 Ma) and minerals include iddingsite and less commonly serpentineor finally by a seconderuptiveepisodeof lower volume basalt bowlingite (both after olivine) and calcite. (3.0-4.7 Ma, see Naumann et al.  for information on In the northeasternReveille Range, basaltsof episode 1 are
dates). These map units are shownin Figure 2 and will be overlainby two trachyticdome-likelava flows(--0.1 km3)and referredto throughoutthis paper as (1) the episode1 basalts, associatedpyroclastic surge deposits [see Naumann et al., (2) the trachytic rocks, and (3) the episode 2 basalts. 1990]. The trachytic lavas are sparsely phyric (<5 modal Petrographic information on these units is summarizedbelow and in Table 1.
percent) with phenocrysts of sanidine, plagioclase, greencoloredclinopyroxene,Fe-Ti oxides, and occasionallyapatite.
YOCR)DZINSKIET AL.: ORIGIN OF GREATBASIN BASALTS
c.,i o'• •
oo ,•o o
YOCA)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
ET AL.: ORIGIN OF GREAT BASIN BASALTS
Table 2c. Major andTraceElementAbundances in ReveilleRangeTrachyticRocks R8-1-16
TiO2 A1203 Fe203
1.11 16.11 9.82
0.44 15.93 6.86
0.47 16.94 6.74
0.51 17.28 7.11
0.45 17.35 6.85
0.51 17.23 7.20
Na20 K20 P205
5.53 4.28 0.30
5.98 5.33 0.17
6.25 5.58 0.11
6.50 5.24 0.24
6.22 5.83 0.16
6.58 5.68 0.18
Rb Th Ba $r La Ce Nd $m Eu
71.2 8.46 849 308 62.9 130 69.7 14.7 3.95
87.0 9.77 353 101 73.0 137 64.8 13.4 2.30
90.4 10.3 364 78 82.3 155 71.4 13.7 2.64
Tb Yb Lu Y
2.37 5.33 0.75 51.9
2.03 5.11 0.68 48.0
1.85 4.70 0.68 48.5
Nb Hf Zr $c Cr Ni
64 16.7 879 12.4 19
84 18.2 953 5.9 11
88 18.8 947 6.5 3
10.3 264 79
72.6 150 60.9 13.6 2.46 1.89 4.32 0.60 45.9
6.49 90 18.5
See Table 2a footnotes,
The groundmassin the trachytic rocks is dominated by standard analyses are in Table 3. Isotopic analysesfor feldspar, pyroxene, oxides, and colorlessto pale green glass. ReveilleRangerocksare presented in Table 4, alongwith new Exceptfor the presenceof calcite in the groundmass of some isotopicdata on Pleistocene-age basaltsof southernNevada samples, the trachytic rocks are mostly free of alteration CraterFlat area (traceelementdata for the CraterFlat samples minerals. are presentedin Bradshawand Smith, 1994). Basaltsof episode2 are the youngestvolcanicrocks in the Major Elements Reveille Range. They comprise a minimum volume of Pliocenebasaltsof episode1 (Table 2a) are mildly alkaline approximately 1 kin3andwereeruptedfrom14 ventslocated with silica-saturated and undersaturatedvarieties (Table 1). only in the northeasternpart of the range (Figure 2). Most episode2 basaltscontain phenocrystsof plagioclase,olivine, Episode1 basaltshave 45.7-49.4% SiO2, 1.0-2.2%K20, and total alkalis of 4.3-6.5% (K20+Na20, Figure 3 andTable 2a). clinopyroxene,and Fe-Ti oxides,but some samples(which we They are relativelyhigh in FeO* (9.6%-14.0%)andTiO2 (2.5classify below as basanites)lack phenocrystsof plagioclase. 3.6%) and have low-to-moderatecontentsof AI20 3 (15.1Episode2 basaltsare distinguished by the commonpresenceof 17.4%), MgO (2.6-6.1%), and CaO (6.6-9.4%). Normative large clinopyroxene crystals (phenocrysts or megacrysts) compositionsand total alkali contentsindicate that the rock and/or cognate xenoliths of medium-grain plagioclasename "hawaiite" is appropriatefor all episode1 basalts(Table clinopyroxene-oxidegabbro. At some locations, lavas of 1 andFigure 3; seealso MacDonaldandKatsura[ 1964] and Le episode 2 also contain phenocrystsand/or megacrystsof Maitre [ 1989]). amphibole and abundant ultramafic inclusions (dunite, Pliocene basalts of episode 2 (Table 2b) are nearly all harzburgite). Biotite doesnot occuras a phenocrystphasein nepheline-normative (Table 1). On average,episode2 basalts the episode 2 basalts and is less common and less well are therefore slightly more alkaline than episode1 (i.e., lower developed in the groundmassthan in basalts of episode 1. SiO 2, A120 3, and CaO andhigherTiO2, FeO*, Na20, andMgO), Alteration minerals are like those in episode 1 basalts,though but the major element similarities between the stratigraphic calciteappearsto be lesscommonin the episode2 samples. groups are generally more striking than are their differences. Geochemistry One exception is the subsetof episode2 sampleswhich are Major and traceelementdata for ReveilleRangevolcanic stronglyundersaturated(>5% normafivenepheline)with <45% rocksare presentedin Tables2a, 2b, and 2c. Replicateand SiO2 and/orhigh total alkali contents(Table 2b and Figure3).
YOC•DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Table 3. Data Precision and Accuracy INAA Replicate
INAA USGSStandard BHVO-1 (n=4)
Meanppm Th Ba Ta Hf La Ce Sm Eu
6.38 768 5.27 8.97 62.6 123 11.0 3.19
4.1% 3.8% 4.8% 6.1% 6.1% 4.0% 5.8% 3.0%
1.13 140 1.24 4.68 15.7 39.4 6.47 1.97
16.3% 11.5% 7.6% 1.7% 2.8% 11.4% 2.3% 5.9%
XRF USGSStandard SCO-1 (n=6)
Accepted 1.08 139 1.23 4.38 15.8 39.0 6.20 2.06
Yb Lu Sc Cr Co Rb Sr Nb
2.47 0.36 16.5 37.8 31.5 na na na
2.9% 3.4% 5.7% 5.1% 10.4% na na na
1.81 0.28 30.9 293. 43.2 na na na
3.8% 19.8% 1.2% 2.5% 2.1% na na na
2.02 0.40 31.8 289 45.0 na na na
Y Zr Ni
na na na
na na na
na na na
na na na
na na na
na na na na na na na na
na na na na na na na na
na na na na na na na na
na na na na na 115 168 12.4
na na na na na 4.2% 6.6% 2.2%
na na na na na 112 174 11.0
34.8 157 38
2.2% 10.2% 17.8%
26.0 160 27
*Analyticalprecisionis two standard deviations expressed as a percentage of themeanfor repeatanalyses of samples andstandards. Othervaluesare in partsper million. Acceptedvaluesfor the primaryINAA standard(NIST 1633fly-ash)are Th (24.7 ppm),Ba (1420),Ta (2.00), Hf (7.40), La (84.0), Ce (175), Sm (17.0), Eu (3.70), Tb (2.50), Yb (7.40), Lu (1.12), Sc (39.0), Cr (196), Co (43.0).
These lavas are petrographicallydistinct in that plagioclaseis generallynot an abundantphenocrystphase(Table 1). These featuresjustify the rock name "basanite"to distinguishthem from the less alkaline and more plagioclase-phyrichawaiite basalts.
The Reveille Range trachytic rocks (55-62% SiO2) have total alkalis of 9.8-12.3% (Na20+K20) and contain 3-11% normativenepheline(Figure 3 and Table l c). Comparedto the Reveille Rangebasalts,the trachyticrockshave K20 contents thatare higherby a factorof 4 (K20--4.3-5.8%). The trachytic rocks are moderateto high in A1203 (15.9-17.4%) and FeO* (6.7-9.8%), variably low in TiO 2 (0.44-1.11%) and CaO (2.04.6%), and very low in MgO (0.4-1.2%). The Reveille Range trachyticrocks resembletristanitesand otherevolvedrocksof the alkaline ocean island basalt (OIB) igneous series [e.g., Wilkinson,
An extendedplot of OIB-normalized concentrations (Figure 7) shows again that the Reveille Range basanites have relatively high concentrations of incompatible elements compared to the hawaiites. Episode 2 hawaiites have, on average, higher incompatible element concentrationsthan episode 1 hawaiites, but these differencesare small compared to the distinctivebasanites. Figure 7 also pointsout the broad similarity in interelement ratios among all Reveille Range
basalts. The only clear exception to this is the relative concentrationof Pb (i.e., Pb/La, Pb/Ce) which is substantially higher in episode 1 hawaiites than in any of the episode 2 samples(hawaiitesor basanites). The rare earth element characteristicsof the Reveille Range basalts are consistent with other incompatible element features. All of the Reveille Range basaltsshow an overall
light REE-enriched pattern (Figure 8) with uniformly low abundancesof heavy REE (Yb, Lu) at approximately8 times chondritic. On average, the episode 2 basaniteshave the highest concentrations of light REE at 100-155 times chondritic. The episode 1 hawaiites have light REE concentrationsof 56-130 times chondrites,and the episode2 hawaiites fall within a relatively restricted compositional range with light REE at 66-100 times chondrites(see also La/Yb versusMgO in Figure 6). In the Reveille Rangetrachyticrocks,concentrations of the most incompatibleelements are significantlyhigher than in the associatedbasalts(Figure 7). The highestconcentrations are in Zr (-926 ppm), Hf (-18 ppm), Rb (-83 ppm),and K20 (5.4 wt %) which range between 2.5 and 3.8 times the
Incompatible element concentrations (Tables 2a-2c) and interelement ratios are broadly similar for episode 1 and episode2 hawaiites,episode2 basanites,averageOIB, and for Pleistoceneage basaltsof the Lunar Crater Volcanicfield. This is clear from the average compositions plotted on extended mid-ocean ridge basalt (MORB)-normalized incompatible element diagrams (Figure 4), and from ratio-ratio plots of all the available samples (Figure 5). The similarity of the Reveille Rangeand Lunar Crater data to averageOIB contrasts with that of Pleistoceneage basaltsfrom Crater Flat, which have higher concentrationsof Ba, Th, and light rare earth elements(LREE) but lower relative concentrations of high field concentrationsin the basalts. Concentrationsof La (73 ppm), strengthelements(HFSE), especiallyTa and Ti (Figures4 and Nb (82 ppm), Y (49 ppm), and Yb (4.8 ppm)are 1.4-2.4times 5). higherthan in the basalts. The trachyticrockshave negative Important differences among Reveille Range basalts are Eu anomalies(Eu/Eu* = 0.54 - 0.83), but their REE patternsare revealed by their incompatible and compatible element otherwise parallel to those of the basalts at higher (Figure 9). In contrast,concentrations of Ba in concentrations. Nearly all the basanites have higher concentrations concentrationsof Sr, La, Th, Zr, and Ta (relative to MgO) than the trachyticrocks (•-420 ppm) are similar to or slightlylower the hawaiites (Figure 6), and in this way the episode 2 than in the basalts, and concentrationsof Sr (-160), P205 lower basanites are similar to the youngest basalts of the Lunar (0.19 wt %), and TiO2 (0.56 wt %) are all substantially Crater Volcanic Field. (Table 2 and Figure 7).
YOCK)DZINSKI • AL.:ORIGINOFGREATBASINBASALTS
YOC•DZINSKI ET AL.: ORIGIN OF GREATBASIN BASALTS 13.0
SiO2 Figure 3. Total alkalis versusSiO2 for Plioceneage volcanicrocksof the Reveille Rangecomparedwith Pleistoceneage basaltsfrom CraterFlat and the LunarCraterVolcanicField. Reveille Rangerocksdesignated here as basanites(solid circles) are distinguishedby their relatively low SiO2 and high alkali contentsand by
petrographicfeatures,especiallythe absenceof plagioclasephenocrysts (see text and Table 1). Reveille Rangedata are from Table 2 and unpublishedUniversityof Nevada,Las Vegas,data. Crater Flat samplesare Red Cone and Black Coneanalysesfrom Bradshawand Smith[ 1994]. LunarCraterVolcanicField dataare from
Bergman andKargel. AVERAGE
100.0 -I ,•
Sr, Nd, and Pb Isotopes
Isotopic compositionsof episode 1 and episode2 basalts are somewhat variable considering the overall similarity in their incompatibleelementfeatures. Episode1 hawaiitesshow
• episode #I, g2 hawaiites e•g2 basanites
a widerangein 87Sr/86Sr (0.7043 - 0.7061),whereasthe range in episode 2 (basanites and hawaiites) is more restricted (0.7035 - 0.7037, Figure 10). Neodymium isotopesare also more variable in basaltsof episode I than episode2, though the differencesare not as great as for Sr (Figure 10). A similar patterncan be seenin the Pb isotopedata; mostof the episode
2 basaltsform a linear trendimmediatelyabovethe mantle referenceline at high 2o6pb/2OnPb(..-19.2), whereas the episode1 basaltsscatterfrom the mantle trend toward high 2O7pb/2O4pb andhigh A7/4Pb(Figure11). The trachyticrocks
1.0 i Rb
in the Reveille Rangeare also variable(high 87Sr/86Srand K
2O7pb/2O4pb) and in this regardare like the episode1 hawaiites (Figures 10 and 11).
Figure 4. MORB-normalizedincompatibleelementdiagram. Normalizing values and plotting order are from Sun and McDonough[ 1989]. (a) Comparisonof averagePlioceneage Reveille Range hawaiites (episode 1 and episode 2) to the averagePliocene age basanite,averagePleistoceneage basalt fromthe LunarCraterVolcanicField [Bergman, 1982; Kargel, 1987], and average ocean island basalt (OIB from Sun and McDonough). (b) Comparisonof averagePlioceneage Reveille Range hawaiites (episode 1 and episode 2) to the .averageRed Cone and Black Cone analysesfrom Crater Flat [Bradshawand Smith, 1994]. Notice the similarity among OIB, Lunar Crater, and all the Reveille Rangedata (Figure4a), and contrast those relatively smooth patterns with the spiky patternproducedby the CraterFlat data (Figure 4b).
YOC,ODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
(MORB & OIB)
al., 1988]. The presence of the OIB-type incompatible element signaturein Pleistoceneage basaltsof the Lunar Crater Volcanic field [Bergman, 1982; Foland et al., 1987; Lum et al., 1989] indicates that the mantle source of mafic volcanism in this part of central Nevada has remainedlargely unchanged for the past 5-6 Myr. In general, the isotopic data for these rocks are consistent with the incompatible element data and the asthenospheric
source interpretation. The episode 2 samples form a tight
clusteraround87Sr/86Sr-0.7035and œNd---+4, and mostof the
11e_Rang_e __ [
Rb / Ba 140
I[• episode #1 hawaiites
[C)episode #2hawaiites Lunar Crater
Pb isotopesfor the episode2 rocks fall on a trend immediately above and parallel to the northern hemispherereference line (Figures 10 and 11). Isotopic characteristicsof Pleistocene age basaltsin the Lunar Crater Volcanic field [Bergman, 1982; Foland et al., 1983; Foland and Bergman,1992] are like those of episode 2 and are thus consistent with the asthenospheric source interpretation. There are, however, isotopic features of the episode 1 hawaiites (5-6 Ma) that probably do not reflect geochemical variation within the asthenosphericsource. Specifically, the wide variation in Sr isotopesrelative to end in the episode1 samplescontrastswith the tight cluster of the episode2 data. The flat trend in the episode1 data (shallownegativeslope)on the Nd-Sr isotopediagram (Figure 10) is toward a high Sr/Nd component and, in this regard, is unlike well-correlatedtrends that are generally seen in largely mantle-derived volcanic systems(e.g., southernNevada area basaltsin Figure 10). The absence of this radiogenic Sr signature from the younger basaltsin the area (episode2, Lunar Crater) arguesthat is not a feature of the asthenosphericsource and must therefore have been acquired when episode 1 basalts moved through the lithosphere.
L•episode g2 basanites basalts _
The high87Sr/86Sr andhighSffNd featuresof the radiogenic
Ti / Hf
component in the episode 1 hawaiites are relatively well Figure 5. Incompatibleelement ratio-ratio plots comparing constrainedby the data array and concave-downwardcurvature Pliocene Reveille Range basalts to Pleistocenebasalts from of the mixing line and are similar to Sr-Nd componentsin Crater Flat and the Lunar Crater Volcanic Field (locations in modemmarinesedimentand/oraverageuppercrest(Figure 12). Figure 1). (a) K/Ba versus Rb/Ba; note the similarity of the The high SffNd requirementis particularlyimportantbecauseit Reveille Range the Lunar Crater basalts to oceanic basalt and disqualifiesenrichedlithosphericmantle (or basaltsfrom such the distinctivecharacter(high Ba relative to Rb and K) of the mantle) as likely sourcesof this component(Figure 12). We Crater Flat data. (b) La/Ta versus Ti/Hf; note again the believe therefore that the episode 1 basaltsacquiredan upper similarity of the Reveille Range the Lunar Crater basalts to oceanic basalt and the distinctive character (low relative Ti and crustal componentthrough assimilation of wall rock in a Ta) of the Crater Flat data. Reveille Rangedata are from Table 1. Lunar Crater data are from Bergman  and Kargel . Crater Flat data are Red Cone and Black Cone analyses from Bradshaw and Smith [ 1994]. Oceanic basalt data are from Hofmann and White . Data regardedas "suspect"by Hofmann and White (in parenthesesin their Table 1) are excluded.
shallowmagmachamber.Folandet al.  and Folandancl Bergman  showedthat Sr and Nd isotopesin basaltsof the Reveille Rangeand PancakeRange-LunarCrater Volcanic
Fieldarestronglycorrelated with oxygenisotopes (•5180),and they too arguefor assimilation of crustby the older(Pliocene age) rocksin the area. Case
in Episode 1 Hawaiites
Petrogenesis Asthenospheric Basalts With From the Upper Crust
Pliocene age basalts of episode 1 and episode 2 in the Reveille Range have incompatibleelement concentrationsand interelementratiosthat are nearly identicalto thoseof average OIB (Figures5 and 6). The incompatibleelementdatatherefore indicate that the predominantsourcefor Pliocenebasaltsin the Reveille Range was asthenosphericmantle [see also Fitton et
The isotopic data presented here and from Foland and Bergman  indicate that the crustal contaminant in the
episode1 basaltshadhigh 87Sr/86Sr, Sr/Nd,and•5180. It must additionally have had relatively low concentrationsof most other incompatible elements,becausealthough the episode 1 lavas have relatively radiogenicSr, their overall incompatible elementprofile is nearly the sameas that for the episode2 and Lunar Crater samples(Figures5 and 6). One possibility is that the episode 1 lavas were contaminated by limestone or some other carbonate-rich rock.
Figure 6. Incompatibleelements(Sr, La, Zr, Th, Ta, La/Yb) versusMgO for alkalic basaltsof the Reveille Range and for the youngestLunar Crater Volcanic Field basalts(Qb-1 units from Bergman ). Hawaiite samplesfrom episode 1 (open squares)and episode 2 (open circles) generally have similar incompatible element concentrationsrelative to MgO. Episode2 basanites(solid circles) have relatively high concentrationsof incompatibleelementsand high La/Yb comparedto MgO. Data from Table 2 and Bergman [ 1982]. AVERAGE
Figure 7. Average incompatibleelement concentrations for Reveille Range basaltsnormalizedto average oceanislandbasaltof Sun and McDonough[ 1989]. Average episode2 basaniticsamplesincludesepisode2 basanitesand episode 2 hawaiites with relatively high incompatibleelement concentrations(see Figure 6). Notice the similarity for ratiosamongmost elementsexceptPb which is relatively high in only the episode1 average(e.g., high Pb/Ce in episode 1 hawaiites).
ET AL.: ORIGIN OF GREAT BASIN BASALTS
I I I, ,I EPISODE #1 HAWAIITES
100 •_[:• X•
lSX• [I-] R9-1-661 [AR8-1-281
I 'R9-1-47 I Is R8-1-30 I
I*RS-l-ll I .
/o R8-1-1I .
I I Sm Eu
I •0 Yb Lu
Figure 8. Chondrite-normalizedrare earth element concentrationsfor Reveille Range basalts(Table 1). Samplesplotted here were selectedto show the full rangeof light rare earth elementconcentrationwithin each stratigraphic-geochemical grouping(see also Figure 6). Normalizing valuesare the Leedy Chondrite: La (0.378), Ce (0.976), Nd (0.716), Sm (0.230), Eu (0.0866), Tb (0.0589), Yb (0.249), Lu (0.0387). See also Masuda et al. .
Carbonates may have high concentrations of Sr but will (15.1-17.4%) contents in episode 1 basalts, suggestingthat generally have low concentrationsof other incompatible DSr = 1.0 +/- 0.15 is appropriate. elements. Carbonatesalso have high 5180 and Sr/Nd and Strontiumisotopesin the episode1 hawaiitesalso appearto therebyhave all of the featuresof the putative contaminant[cf. be correlatedwith SiO2 content(Figure 14), and this may also reflect an AFC control over 87Sr/86Sr. There are, however, no Veizer, 1983; Shaw, 1985]. The carbonatecontaminantinterpretationis supportedby clear correlations between 87Sr/86Sr and other general the observation that episode 1 samples with higher Sr indicatorsof crystal fractionation(e.g., decreasingMgO, CaO, concentrations alsohavehigher87Sr/86Sr.Specifically,a plot CaO/A120 3, increasingBa, Th, K), but this may in part be of 87Sr/86Sr versusSr concentration (Figure13) showsthatthe becauseisotopic analysesare available for hawaiite samples episode 1 samplesare scatteredarounda mixing line between that are all similarly evolved (MgO 4.5-6.1%). an evolved basalt composition(Sr=550 ppm, 87Sr/86Sr= Mixing calculations indicate that the observed shift in
0.7035) and a carbonate assimilant (Sr=850 ppm,87Sr/ 87Sr/86Sr (from 0.7035 to 0.7060) requires 10-40% 86Sr=0.7085). The binary mixing line is equivalentto an assimilation of a carbonate that contains 850 ppm Sr with assimilation-fractional crystallization (AFC) model wherein 87Sr/86Sr=0.7085and Sr/Nd = 85 (Figure 12). This is a the bulk distributioncoefficientfor Sr is 1.0 (bulk Dsr=l.0 [see substantial amount of contamination, and it implies that DePaolo, 1981]). The data are somewhat scattered but are radiogenic Sr mobilized in the wall rock reactions was largely encompassed by AFC calculationsusing Dsr between efficiently incorporatedinto the basalticmelt. A large amount 0.85 and 1.15 (Figure 13). There is no systematicchange in of Sr may have been liberatedin wall rock reactionswherein Sr concentration over the rangeof MgO (2.7-6.1%) and A1203 Ca-Mg carbonates(with variably high Sr) are replacedby CaMg silicates(with relatively low St). Minerals formed in a melt-carbonate reaction zone (e.g., 1000 : I I I ,I I I wollastonite,garnet, Ti-Al-rich pyroxenes,nepheline) are not observedin the episode 1 basalts,nor do thesebasaltsshow Reveille RangeTrachytes shiftsin major elementcompositionthat might be anticipated • R9-1-62 as a consequenceof basalt-limestoneinteraction (e.g., Ca O R8-1-41 enrichment,Si-A1 depletion [see Wyllie, 1974]). Detailed
studies of basalt-limestone interaction indicate, however, that
thesepetrologicconsequences of limestoneassimilationwill be produced in only a verylocalizedpartof a magmaticsystem. Specifically,Baker and Black  andJoesten found small veins and apophysesof strongly Ca-rich hybridized basalt in melt-limestone reaction zones, but they concluded that these melts were producedin only very small volumes becauseelementsliberatedby carbonatebreakdown(mostly Ca) were readily accommodated by crystallizationwithin the reactionzone [seealso Wyllie, 1974].
Figure 9. Chondrite-normalizedrare earth element reemphasizedtwo importantpoints initially made by Bowen concentrations for Reveille Range trachyticrocks compared [1922, 1928] in his classic treatment of wall rock assimilawith basalts(data from Table 1).
tion. Theseare, first, that the sluggishtransferof heat from the
YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
tightclusterof episode 2 data(hawaiites andbasanites) andthe"fiattrend"of theepisode I basalts towardhigh 87Sr/86Sr relativeto œNa-NoticealsothattheReveilleRangetrachytic rocksscatter to highrelative87Sr/86Sr valuesand in this way are isotopicallyakin to the episode1 basalts.ReveilleRangeand CraterFlat data are from Table 2. Data for southern Nevadaareabasalts(< 7 m.y. old) are from Farmeret al. [ 1989], Coleman , Walker and Coleman, Feuerbachet al., , Hoffine , Ormerod , and unpublishedUniversityof Nevada,Las Vegas,Universityof Kansasdata. Modem marinesedimentdata are from Ben Othman et al. [ 1989].
YOCK)DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
Reveille Range [• episode#1 Oepisode#2 hawaiite •) episode#2 basanite
Sr=-550ppm Nd=26 ppm (Sr/Nd=21)
-5 I mixing with
• .•_ "X •
Carbonate End-member Sr=850ppm Nd=10ppm
Lithospheric End-member St=1400ppm Nd=97ppm
X•l •Nd=85) .
87Sr/ 86Sr Figure 12. Neodymium-Srisotopecorrelationdiagramfor ReveilleRangebasaltswith binary mixing lines between low 87Sr/86Srepisode2 basalts (hawaiites and basanites)and end-memberswith compositionsof
lithospheric basalt(high 87Sr/86Sr, low Sr/Nd)andcarbonate (high 87Sr/86Sr, highSr/Nd). Noticethatthe downwardcurvatureof thecarbonate mixingline andthetrendtowardhigh 87Sr/86Sr relativeto œNdmatchthe episode1 hawaiitetrend well. Crosseson mixing lines are at incrementsof 10%. magma to the reaction zone means that energy for wall rock assimilation will generally come from crystallization within
In the case of limestone assimilation,
this means that Ca
liberatedby carbonatebreakdownwill generallybe crystallized the reaction zone itself and, second, that elements mobilized as clinopyroxene within or near to the basalt-limestone by melt-wall rock reactionswill generallybe accommodated by reaction zone [Joesten, 1977; Baker and Black, 1980]. So solid solution in minerals crystallizing in the reaction zone. basaltic lavas erupted from a magma system that interacted
0,708 AFC model: [ (~binary m••_._•• / Ma/Mc= 0.35 I
I-! episode#1 hawaiites
¸ episode #2hawaiites
0.703•Basalt Parent in AFC models
'' I 4OO
'' ! '' 500
''' ' I 600
'''' I 1000
' ' 1200
Sr (ppm) Figure 13. Sr isotopesversusSr concentrationand assimilation-fractional crystallizationmodeling (AFC [DePaolo, 1981]) of episode1 basaltsfrom the Reveille Range. Parameter"r" in AFC calculationsis the mass assimilateddivided by the mass crystallized (Ma/Mc). Crosseson AFC curves are at incrementsof 10% crystallization.
YOCK)DZINSKIET AL.: ORIGIN OF GREATBASINBASALTS
48.0E'"'' I, ,',, I .... I .... '1.... I,, •, I .... =]
Figure 14. Sr isotopesversusSiO2 content for Reveille Rangebasalts. Note the generalincreasein 87Sr/86Srwith increasingSiO2 among Reveille Range hawaiites. Data from Tables 2 and 4.
extensivelywith limestoneare generallynot expectedto show anomalousbehaviorin CaO or othermajorelements. For our purposesit is perhapsmost interestingthat the strongly hybridized basalts analyzed by Baker and Black  were enrichedin CaO by approximately a factorof 2.4 (overtheunhybridized basalts),but wereenrichedin Sr by up to a factorof 15 [seeBaker and Black, 1980, Table II]. This
16 18 20
Figure 15. Strontium isotopes versus A120 3 contentfor Reveille Range basalts compared with hypothetical Cafeldsparwith high 87Sr/86Sr.The trendof the mixingline fails to passthroughthe episodeI data indicatingthat mechanical incorporation of Ca-feldspar (i.e., plagioclase megacrysts) with high 87Sr/86Sr cannot explain the Sr isotopic composition of the episode I samples. Strontium concentrationin feldsparusedin mixing calculationsis 1200 ppm and is similar to the Sr concentrationin a labradorite megacrystanalyzedby Bergman [ 1982]. Only unreasonably high Sr concentrationsfor plagioclase (>10,000 ppm) will bendthe mixing line to passthroughthe episode1 data.
profound enrichment in Sr over CaO in the small-volume
hybridizedmelts providesclear and tangibleevidencethat Sr compared to other incompatibleelements (i.e., a carbonateliberatedin carbonatebreakdownis preferentiallyexcluded rich wall rock). from the Ca-silicate minerals that crystallize in a basaltlimestone reaction zone. We conclude that in the case of the
episode1 hawaiitesin the Reveille RangesuchdisplacedSr may have beenefficientlyincorporatedinto the basalticmelts. Assumingthat significantSr may be liberatedby meltlimestoneinteraction,we turn to the questionof how that Sr wasincorporated into theepisode1 hawaiitemagmas.Muchof the radiogenicSr diffusingaway from the wall rock reaction zone may have been scavengedby feldsparscrystallizingin the cumulate mush. Pieces of this mush zone in the form of
Geologic and Petrographic Carbonate
The olivine + plagioclasephenocrystassemblagein the episode 1 basalts contrasts with the higher pressure phenocryst-megacryst-xenolith assemblage present in younger basalts in the area (see petrographicdescriptions above). The phenocrystassemblagein episode1 hawaiitesis therefore consistentwith evolution in a low-pressuremagma chamber in the upper crust. The explosive eruption of the highly evolved trachytic rocks isotopically akin to the
megacrysticand large phenocrystic plagioclaseare commonin the episode 1 basalts. Aluminum-87Sr/86Sr mixing relationshipsindicate, however, that mechanicalincorpora- episode1 hawaiites(87Sr/86Sr> 0.7040) probablymarkedthe
coolingand deathof that high level magmasystem. We know thereforethat among Pliocene-Pleistocene basalts in the area, the episodeI hawaiiteswere eruptedin the largest If radiogenicSr was not carriedinto the basalticmelt by volume (see geologic map in Figure 2), they resided in the plagioclasecrystals, then it must have been transferredfrom shallowest magma chamber (plagioclase-olivine-dominated the reaction zone to the melt largely by diffusion. Other phenocryst assemblage,see Table 1), and they eventually elementsthat may moveefficientlyby diffusiondo not appear, cooledto producethe mostevolvedmelts (the trachyticrocks). however,to have been as stronglyaffectedas does Sr. There All of these features are consistent with the model that the are no clear relationships betweenBa or Ba/La and 87Sr/86Sr, episode I hawaiitesappearto have experiencedcontamination and correlations between K or K/La and 87Sr/86Sr are weak in the upper crust whereasyoungerbasaltsin the area (i.e., despitethe fact that the diffusivemobilityof K is thoughtto be episode2 and Lunar Crater)do not. Petrographic support of the assimilation model is also relatively high in systemswhere basalt is assimilatingcrustal rocks [Watson, 1982; Watson and Jurewicz, 1983]. The Pb present. Small phenocrysts of biotite appearin some episode data do show a trend toward higher Pb/La with increasing I hawaiitesbut not in basaltsof episode2 (Table 1). Biotiteis phasein the episode1 basalts 87Sr/86Sr,but one samplenonetheless hasvery radiogenicSr also well-developedgroundmass and also low relative Pb (Figure 16). We conclude therefore but is much lesscommonin basaltsof episode2. The presence that the episode I hawaiiteswere altered by assimilationof a of groundmasscalcite in the Reveille basaltsmay also be wall rock that had high concentrationsof Sr (and possiblyPb) significant. We have interpretedcalcite in episodeI basalts tion of feldspar from the mush zone cannot accountfor the shift in Sr isotopecompositionsobservedin the episode 1 samples(Figure 15).
significant contamination in the uppercrust,and as largely alteration-related (see petrographic descriptionsexperienced above),but in somecases,groundmass calciteis intergrown thatyoungerbasaltsin the area(episode2 andLunarCrater alongstraightcrystalboundaries with biotiteandfeldsparin basalts)generallydid not [see also Folandand Bergman, areas where there is no hint of alteration (aside from the
presence of calcite). The textures suggest thatsomeof the groundmass carbonate couldmagmatic or deuteric, thoughon Discussion thispointthetexturalevidence is probably notconclusive. Perhapsmostimportantly,quartzo-feldspathic xenoliths Alternatives to the Carbonate Assimilation Model andxenocrysts are not seenin the ReveilleRangeepisode1 Thegeochemical variation in theepisode 1 lavasconstrains hawaiites, evenin the presence of cleargeochemical evidence component in for crustalcontamination.In this regard,the Reveille Range fairlytightlythenatureof thehigh87Sr/86Sr theserocks and would appearto eliminatethe mantle as a source for thissignature. TheEMII enriched mantle LakeMeadAreaandcertainMioceneagehawaiitesin theBasin potential
basalts are unlike the Pliocene Colville Mesa basalts of the
component, whichis widespread in OIB of certainwestern a Nd-Srarray 1995]. In these areas,geochemical evidencefor crustal PacificIslands[Zind!erandHart, 1986],produces that falls directly over the episode 1 data. However, the assimilation is supported by abundant petrographic evidence absence of this isotopic signature from episode 2 and Lunar in the form of gneissicxenolithsand quartz xenocrysts, Craterbasalts(whichhaveincompatible elementcharacterispresumably from the deepcrust. Sr did Finally,it is important to recognize thatPaleozoic andlate ticslike thoseof episode1) arguesthattheradiogenic Precambriancarbonatesconstitutea great thicknessof the not come from the asthenosphericsource. Enriched
andRangeof Mexico[Feuerbach et el., 1993;Luhret el.,
uppermost crustbeneath central Nevada (6-12km[Langenheimlithosphericmantle beneaththe westernUnited States haslowSr/NdandSr-Ndisotope ratios thatproduce a and Larson, 1972]) and that they are therefore the most generally trendon theNd-Srisotope correlation diagram probablesourceof uppercrustalcontamination in young steepnegative basalts in the area. Most of the limestone section has (e.g.,CraterFlatdatain Figure10)whichis unlikethetrend bytheepisode 1data.In addition, thehigh878r/86Sr probably undergone diegenesis andmaynottherefore have produced basalts of episode 1 show no signs of having acquired the particularlyhigh Sr concentrations (aragonite is Sr-rich, calcite and dolomite are not), but the great thicknessof
distinctive trace element characteristics of Great Basin
mantle(seeFigures4 and5). miogeoclinal carbonate in south-central Nevadaat leastraises lithospheric Assimilation of plagioclase-rich rocksin thedeepcrustmay the possibility for the preservation or formation of Sr-rich carbonatehorizons beneath the Reveille Range.
We concludethat availablegeologicand petrographicdata
be another alternative to the carbonateassimilation model
outlinedabove. Geochemically, an anorthosite wouldhavethe
requiredof the assimilant (high87Sr/86Sr, providebroadsupport to theidea,developed on thebasisof characteristics geochemistry, thattheepisode1 basalts in theReveilleRange Sr/Nd, 8180, low concentrationsof other incompatible
YOC•DZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
elements), but with no a priori evidence for an anorthosite singleeruptiveepisodeand that this episodewas 1.0-1.5 Myr body beneath central Nevada, this can only be an ad hoc in duration. interpretation. Glazner et al.  and Glaznerand Farmer Becausethe episode2 basaltsimmediatelyoverlie but are  arguethat extensiveassimilationof gabbroicrocksin isotopicallydistinctfrom the trachyticrocksand the basaltsof the deepcrustmay accountfor diverseisotopiccharacteristics episode1, we believethat the episode2 basaltsrepresent the in basalts from the Mojave area of southeasternCalifornia. beginningof a new eruptiveepisodefor the volcanicfield. It Indeed, some Mojave basaltsfollow a Sr-Nd isotopictrend is unlikely that episode2 basaltscould have reoccupiedthe similar to that seen in episode1 hawaiitesfrom the Reveille shallow episode 1 storage system without also showing Range. There is, however,no clearrelationshipbetweenSr-Nd isotopic evidence of crustal contamination, so we conclude isotopes and •5180in the Mojavebasalts[ Glazier et al., 1991, that episode2 eruptionswere fed from a separatestorage Figure 10] and in this way they contrastthe Reveille Range- location. The widespread occurrence of clinopyroxene and megacrystsin episode2 basaltsindicatesthat LunarCraterrocks[ Folandet al., 1991; FolandandBergman, phenocrysts 1992]. Furthermore,the unusualelement-isotope correlations this storagelocationwas deeperthan duringepisode1 time. seenin the Mojave basalts[Glazier et al., 1991, Figure 12] are This is confirmed,at least in part, by the presenceof mantleabsentfrom the episode 1 hawaiites. We concludetherefore derived xenoliths (dunites, harzburgites) and amphibole that the assimilationprocessthat has operatedin the genesis megacrysts in some of the episode 2 basanites. These of the Mojave basalts is unlike that which has effected the xenoliths provide good evidence that their host basaltswere episode1 hawaiitesin the Reveille Range. storednearthe crust-mantle boundaryand wereeruptedrapidly Postmagmaticalteration might also explain radiogenic Sr without a significant period of storage within the shallow
and high •5180 in the episode1 basalts. One possibilityis
The presenceof basanitesamongonly the episode2 rocks may provide further insight into the development of the volcanicfield. Specifically,the episode2 basanitesare more alkaline (>5% normative nepheline) than the hawaiites,
waters have carried dissolved carbonate
cracks and other openingsin the episode 1 basalts(samples were not leachedin acid prior to isotopicanalysis). Episode1 basaltsare slightly older, so pedogenicprocesseswould have had moretime to operateon them thanon the youngerepisode 2 rocks. Recall, however, that all of the Reveille Range
basaltsare Pliocenein age, so if pedogenicprocesses have affected basalt compositions, it is surprising that the alterationaffectsare so clearlypresentin the episode1 rocks (4.5-6 m.y. old) but are completelyabsentfrom the episode2 rocks(3-4.5 m.y. old). A secondpossibilityis that alteration in the episode 1 sampleswas producedby a hydrothermal systemestablishedfollowing the eruption of the episode 1 basaltsand trachyticrocks. If a hydrothermalsystemwere not established following the eruption of the relatively small volume episode 2 basalts, then those basalts would have escaped alteration.
The problem though, with any hydrosphericinterpretation for the origin of the crustalcomponentin episode1 basalts,is that it requiresthat all of the contaminantSr and Nd be carried in the relatively small amount of (apparently alterationrelated) calcite that is presentin the rocks. The large amount of contaminantSr and Nd required by massbalance (10-40%, see Figure 12), and the small amountof calcite present(less than -3%) therefore argue strongly against a hydrospheric origin for the observedisotopic shifts. In general, we believe that a magmaticorigin is far more likely to mobilize the large amountof Sr and Nd requiredto explain the observedisotopic variation.
Geologic, petrographic,and geochemicalevidenceoutlined above indicates that between 5 and 6 m.y. ago, episode 1 hawaiites in the Reveille Range were contaminatedby wall rock assimilation in an upper crustal magmatic plumbing system.This high level magma system is interpretedto have cooled and died 4.5 m.y. ago with the explosive eruption of trachytic lavas and pyroclastic surges in the northeastern Reveille Range. The evidence therefore indicates that the episode 1 hawaiites and the trachytic rocks were part of a
indicating a greaterdepthof melting(higherpressure melts [O'Hara, 1968; see also Takahashi and Kushiro, 1983; Klein
and Langmuir, 1987]).
The basanites also have higher
incompatible element concentrations than the hawaiites (relative to MgO, Figure 6), and assumingthat the mantle source of Reveille Range hawaiites and basanites was
compositionally similar (an assumption that is strongly supportedby the isotopic data presentedhere), these higher incompatibleelementconcentrations imply a lower percentage melting for the basanites. Other compositionalfeaturesof the basanites, including steeper REE patterns (higher La/Yb, Figure 6) and higher NAB.0 (Table 2), also point to a low percentagemelting origin for the basanitescomparedto the hawaiites.
of basanite in the Reveille
Range argues further that these basalts were produced by smaller percentage melting than the associated hawaiites which were produced in relatively large volumes, especially during episode1 time. The seculartrend toward deeper and lower volume melting can be extended into the Pleistocene
with the formation
youngestbasaltsin the Lunar Crater Volcanic field (Qb-3 units from the Lunar Crater Volcanic Field [see Scott and Trask, 1971]). These basaltsoccur in very small volumes, they are more stronglyalkaline (9-14% normatire nepheline)than the Reveille Range basanites, they have high MgO and high incompatible element contents (Figure 6), and they have relatively steep REE patterns(high La/Yb, Figure 6). The youngestLunar Crater basalts also contain an abundantand diversesuiteof both type I and type II megacrystsand nodules [Bergman, 1982]. These data clearly require that the youngest Lunar Crater basaltswere formedby low percentagemeltingof a relatively deep mantle sourceand that they traveledfrom the mantle to the surfaceat relatively high velocities. Comparisonof the Reveille Rangedata with the mappingof Scott and Trask  in the Pancake Range suggeststhat through time, there have been systematic shifts in the geographicalshift distributionof eruptionsacrossthe region. In its early history (3-6 Myr ago), the volcanicfield covereda
-YOGODZINSKI ET AL.: ORIGIN OF GREAT BASIN BASALTS
eruptiveepisodes(first the hawaiites,then the basanites).The
duration of the episode 2 episode is, however, poorly constrained becausethereare relativelyfew radiometricages
[• 5-6Ma(episode #1) ::":"• 3-4.5 Ma(episode #2)
and becausemany of the 3-4.5 Myr old basaltsare locatedin ............ ........
thePancakeRangewherethe Pliocenestratigraphy is lesswell established.It maybe thatthe lengthsof the eruptiveepisodes
have changedwith time, such that as the volcanic field aged, eruptive episodes that produced the younger and smaller volume basalts were shorter than those that produced older,
1 <3Ma(Lunar Crater field)..,.
:-:?: ........ •: .'-' .½. 5>::
largervolumeepisodes.Only a moredetailedknowledgeof the Pliocene stratigraphyin the PancakeRange will allow us to addressthis aspectof the volcanicfield history. Overall, the volcanic field appearsto have developedin responseto spatially and temporallydiscretemelting eventsin the mantle. The possible role of lithospheric extension or delamination in triggering these melting events cannot, however,be evaluatedwithout a substantialknowledgeof local and regional structural/tectonic events at a resolution of approximately 1-2 Myr. The idea that the formation of small volume mafic volcanic fields may be coupled to specific tectonic events therefore appears to be beyond our current understandingof tectonic events for most parts of the Basin and Range.
•. • }T.
':'"•::•:• . t . .N..
•e e]He•ange •.._.
,:.-....... ....... "A•.;
Acknowledgments. Helpful reviewsby Todd Housh,Britt Hill, and AssociateEditor William Melson are gratefully acknowledged. This manuscriptalso benefited from discussionswith Richard Carlson and Erik Christianson. A helpful review of an early version of this manuscriptwas provided by Terry Plank. Thanks also to Shirley Morikawa and Alex Sanchez for their assistancein the laboratory aspectsof this study. Pat Braught provided valuable assistancein preparing camera-readycopy. This researchwas supportedby the NevadaNuclearWaste ProjectsOffice. We thank Carl Johnsonof that office for his support.
• ß . . ..::•:
• ..... .
Baker,C.K., andP.M. Black,Assimilation andmetamorphism at a basaltlimestone contact, Tokatoka, NewZealand,Mineral.Mag.,43, 797807, 1980.
BenOthman,D., W.M. White,andJ. Patchett, The geochemistry of
Figure 17. Time-space patterns for volcanism in the Reveille and Pancake rangesfrom approximately6 Ma to present. Basedon mappingpresentedhere (Figure 2) and from Scott and Trask [ 1971].
Bowen,N.L., Thebehaviour of inclusions in igneous magmas, J. Geol., 30, 513-570, 1922. broad area that encompassed what are today the Reveille and N.L., TheEvolutionof IgneousRocks,332 pp.,PrincetonU n iv. Pancakeranges. By the end of the Plioceneand in Pleistocene Bowen, Press,Princeton,N.J., 1928. time, the areaover whichbasaltswereeruptinghad retreatedto Bradshaw,T.K., andE.I. Smith,Polygenetic Quaternary volcanism at a small area in the north which today is marked by the CraterFlat, Nevada,J. Volcanol.Geotherm.Res.,63, 165-182, 1994. distributionof the youthfulconesof the Lunar Crater Volcanic Bradshaw,T.K., C.J. Hawkesworth,and K. Gallagher,Basaltic volcanismin the southernBasinand Range:no role for a mantle field (Figure 17). It appearsthat the initial melting anomaly plume,Earth Planet.Sci. Lett., 116, 45-62, 1993. that producedthe volcanicfield was large and that subsequent Coleman,D.S., and J.D. Walker, Geochemistryof Mio-Pliocene melting episodeswere smaller and centeredat the northernend volcanic rocksfromaroundPanamint Valley,DeathValley a re a, California,in Basin and Range Extensional TectonicsNear the of the initially large outbreak. Thesegeneraltime-spacetrends Latitudeof Las Vegas,Nevada,editedby B. Wemicke,Mern. Geol. were also noted by Naumann et al. [ 1991] and Foland and Soc.Am., 176, 391-411, 1990. Bergman . DePaolo, D.J., Trace element and isotopic effects of combined Available age information indicates that in the Reveille wallrockassimilationand fractionalcrystallization, Earth Plant. Sci. Range the volcanic field developed in at least two eruptive Lett., 53, 189-202, 1981. episodeswhich were apparentlyboth 1.0-1.5 Myr long. The Eaton, G.P., R.R. Wahl, H.J. Prostka,and M.D. Kleinkopf, Regional gravity and tectonic patterns: their relation to late presenceof both hawaiite and basanitewithin the episode2 Cenozoic epeirogeny and lateral spreading in the western sequence,and the relatively young age for the only basanite Cordillera,in CenozoicTectonicsand RegionalGeophysics of the
sample that has been dated (3.0 Ma [see Naumann et al., 1991]), suggeststhat episode 2 may itself be two separate
WesternCordillera, edited by R. B. Smith and G. P. Eaton, Mem. Geol.Soc.Am., 152, pp. 51-92, 1978.
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Conference on High Level RadioactiveWasteManagement, Martin, M.W., and T.R. Naumann,TertiaryGeologyof the Reveille American NuclearSociety, LaGrange Park,Ill., 2366-2371,1992. Range Quadrangle,NorthernReveille Range,Nye County,
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