Petrogenesis and Implications of Calc- Alkaline Cryptic Hybrid Magmas from Washburn Volcano ...

41 Pages·2002·1.17 MB·English

JOURNALOFPETROLOGY VOLUME43 NUMBER4 PAGES663–703 2002
Petrogenesis and Implications of Calc-
Alkaline Cryptic Hybrid Magmas from
Washburn Volcano, Absaroka Volcanic
Province, USA
T. C. FEELEY1∗, M. A. COSCA2 AND C. R. LINDSAY1
1DEPARTMENTOFEARTHSCIENCES,MONTANASTATEUNIVERSITY,BOZEMAN,MT59717,USA
2INSTITUTEOFMINERALOGYANDGEOCHEMISTRY,UNIVERSITYOFLAUSANNE,BFSH2,1015LAUSANNE,
SWITZERLAND
RECEIVED JUNE 16, 2000; REVISED TYPESCRIPT ACCEPTED OCTOBER 18, 2001
Thepetrogenesisofcalc-alkalinemagmatismintheEoceneAbsaroka KEYWORDS:petrogenesis; magma mixing; calc-alkaline; Absaroka Vol-
Volcanic Province (AVP) is investigated at Washburn volcano, a canicProvince;40Ar/39Ardates
major eruptive center in the low-K western belt of the AVP. New
40Ar/39Aragedeterminationsindicatethatmagmatismatthevolcano
commencedasearlyas55Maandcontinueduntilatleast52Ma.
Although mineral and whole-rock compositional data reflect near
equilibrium crystallization of modal phenocrysts, petrogenetic mod-
elingdemonstratesthatintermediatecompositionmagmasarehybrids INTRODUCTION
formed by mixing variably fractionated and contaminated mantle- The widespread, but poorly understood Challis–
derived melts and heterogeneous silicic crustal melts. Nd and Sr Absaroka volcanic episode affected large areas of the
isotopic compositions along with trace element data indicate that northwestern USA in the Eocene following Laramide
silicic melts in the Washburn system are derived from deep-crustal crustalshortening(i.e.the‘Challisarc’;Armstrong,1978).
rocksbroadlysimilarincompositiontogranulite-faciesxenolithsin Volcanicrocksassociatedwiththiseventareparticularly
the Wyoming Province. Our preferred explanation for these features voluminous in the Absaroka Volcanic Province (AVP)
is that mantle-derived basaltic magma intruded repeatedly in the of northwestern Wyoming and southwestern Montana,
deep continental crust leading to fractional crystallization, silicic USA, where >20 000km3 of calc-alkaline to shoshonitic
meltproduction,andhomogenizationofmagmas,followedbyascent magmatic rocks are exposed in the Absaroka, Gallatin,
to shallow reservoirs and crystallization of new plagioclase-rich andBeartoothRanges(Fig.1;AbsarokaVolcanicSuper-
mineral assemblages in equilibrium with the intermediate hybrid groupofSmedes&Prostka,1972).BecausetheAbsaroka
liquids. The implications of this process are that (1) some calc- volcanic rocks have long been presumed to have calc-
alkaline magmas may only be recognized as hybrids on purely alkaline compositional affinities and across-strike en-
chemical grounds, particularly in systems where mixing precedes richments in KO similar to magmas erupted in some
2
andiswidelyseparatedfromcrystallizationinspaceandtime,and continental volcanic arcs (e.g. Dickinson & Hatherton,
(2)giventheroleascribedtocrustalprocessesatWashburnvolcano, 1967; Chadwick, 1970), early workers attributed their
the variation between rocks that follow calc-alkaline trends in the origintoshallowsubductionoftheFarallonplatebeneath
western AVP and those that follow shoshonitic trends in the east theNorthAmericanplateduringtheEocene(e.g.Lipman
cannot simply reflect higher pressures of fractionation to the east in et al., 1972). However, this interpretation is now con-
Moho-level magma chambers in the absence of crustal interaction. troversial for several reasons. First, spatial and temporal
Extended dataset can be found at http://www.petrology.
oupjournals.org
†∗Corresponding author. Telephone: [phone]. Fax: 406/994-
6923.E-mail:[email protected] OxfordUniversityPress2002
JOURNALOFPETROLOGY VOLUME43 NUMBER4 APRIL2002
studies have been unable to decipher any logical time- TECTONIC AND GEOLOGIC
transgressionofmagmaticactivitythroughoutthenorth-
SETTING
westernUSAduringtheEocene,withtheresultthatthe
‘Challis arc’ was much wider than any modern arc and Existing geochronologic information indicates that the
bulk of the magmas in the AVP were erupted between
oriented perpendicular to the present plate margin (Fig.
54 and 38Ma, following or possibly overlapping the
1). On the basis of this distribution and geochemical
latestphasesofLaramideforelandthrusting,whichended
arguments some workers now consider the Challis–
at>55Maatthislatitude(Loveetal.,1975;Armstrong,
Absaroka volcanic episode as resulting mainly from
1978; Feeley et al., 1999; Hiza, 1999). Eruption of the
regional lithospheric extension and resultant de-
Absarokarocksalsoappearstotemporallycoincidewith
compression melting (Dudas, 1991; Hooper et al., 1995;
theonsetofregionalcrustalextensioninthenorthwestern
Morris et al., 2000). Second, the origins of magmatic
USA. Evidence supporting this contention comes from
rocks in the AVP are poorly understood because there
abundantisotopicagesofmetamorphicandigneousrocks
existfewdetailedstudiesaimedatdecipheringthepetro-
exposed in the cores of extensional complexes in Idaho
logicevolutionofmagmaseruptedfromindividualerupt-
and Washington State that indicate tectonic elevation
ive centers. This lack of detailed studies has led to
and denudation at >45–50Ma (Burchfield et al., 1992),
uncertainty on a number of issues ranging from the role
of crustal interaction during differentiation of the calc- andinterbeddedtuffsinsynextensionalbasin-filldeposits
thatrangefrom46Matoyoungerthan30Ma(Janecke&
alkaline magmas to the significance of the across-strike
Snee,1993).Nevertheless,althoughtemporallyassociated
KO enrichments in the AVP.
2 with regional crustal extension, there is little evidence
Toaddresssomeoftheseissuesandprovideareference
withintheAVPforthepresenceofmajorearlyTertiary
point for future studies we present results from our
tectonic extensional faults. Several large-displacement
investigation of Washburn volcano, one of the principal
extensional structures involving the volcanic rocks are
eruptive centers for calc-alkaline magmas in the AVP
present, but these are generally regarded as features
and the type locality for the Washburn Group of the
relatedtoeast-directedgravitationalslidingoffthegrow-
Absaroka Volcanic Supergroup of Smedes & Prostka
ing volcanic highland (Hauge, 1985).
(1972; Fig. 1). Overall, this work represents the first At present, the AVP covers >23000km2 in north-
detailed study of a calc-alkaline (sensu stricto) eruptive
western Wyoming and southwestern Montana (Fig. 1).
center in the AVP. The goals of this paper are (1) to
It has been tentatively correlated with other Eocene
document the ages and ranges of major element, trace
volcanic rocks in southwestern Montana, the Challis
element, and isotopic (Sr and Nd) compositions of
volcanicrocksofIdaho,andtheColvilleigneouscomplex
magmatic products at Washburn volcano, (2) to place
of NE Washington, although these fields are not linked
constraints from phenocryst compositions on the crys-
by outcrops (Fig. 1; Armstrong, 1978; Dudas, 1991).
tallization history of Washburn rocks, (3) to evaluate
Undoubtedly,muchoftheoriginalextentofthevolcanic
the possible roles of different petrologic processes in
field is eroded or covered by Miocene and younger
producingthespectrumofcompositionsobserved,and(4)
volcanic rocks of the Snake River Plain–Yellowstone
tousetheinformationin(1)–(3)todevelopageneralized Plateau fields, and continuity with volcanic rocks of
model for the evolution of calc-alkaline magmas at similaragemaybeobscured.However,apparentregional
Washburn volcano and in the AVP. geochemical trends within the AVP, such as increasing
The origin of intermediate composition rocks at the KO contents eastward, suggest it may be separate.
2
volcano is best explained by mixing between variably AccordingtocorrelationsadvancedbySmedes&Pro-
fractionated and contaminated mafic magmas and stka (1972) for Eocene rocks exposed in Yellowstone
heterogeneous silicic partial melts of Archean granulite- National Park, three groups make up the Absaroka
faciesrocksinthedeepcrust.Despitechemicalevidence Volcanic Supergroup. In ascending stratigraphic order
formixing,thereisverylittletexturalormineralchemistry these are (1) the Washburn Group, (2) the Sunlight
evidence in thehybrid magmas to supportthe model. A Group, and (3) the Thorofare Creek Group. The areal
scenario where hybrid magmas produced in the deep distribution of the three groups, illustrated in Fig. 1, in
crust ascend to shallow reservoirs and crystallize low- part reflects the prevalent thinking at the time of the
pressure mineral assemblages dominated by plagioclase study of Smedes & Prostka (1972) that volcanic activity
is preferred to explain these relationships. These results migrated SE along two subparallel belts of intrusive–
have important implications for the interpretation of extrusivecenters:the‘K-poor’(calc-alkaline)westernbelt
igneous textures and their bearing on rock-forming pro- and the ‘K-rich’ (shoshonitic) eastern belt (Chadwick,
cesses and the significance of across-strike geochemical 1970). However, subsequent stratigraphic and geo-
variations in the AVP. chronologic studies in eastern and southern areas of the
664
FEELEYetal. PETROGENESISOFCALC-ALKALINEMAGMATISM
Fig.1. MapoftheAbsarokaVolcanicProvinceshowingthestratigraphicunitsofSmedes&Prostka(1972).Blackareasrepresentlocationsof
principalventcomplexesandintrusivecentersdiscussedbyChadwick(1970).Thickdashedlineshowstheapproximatedivisionbetweenwestern
K-poor (calc-alkaline) and eastern K-rich (shoshonitic) belts (after Chadwick, 1970). Single dot-dashed line is the boundary of Yellowstone
National Park. Inset shows the locations of early-to-middle Eocene magmatic fields (after Chadwick, 1985; Holder & Holder, 1988; Dudas,
1991;Norman&Mertzman,1991;Wheeleretal.,1991;Luedke,1994).Numbersreferto:1,SlokoVolcanicProvince;2,FrancoisLakeigneous
complex; 3, Colvilleigneous complex; 4, Clarno volcanics; 5,Challis Volcanic Province; 6, AbsarokaVolcanic Province; 7, Montana Alkalic
Province;8,BlackHills.DiagonallyruledfieldshowsinferredextentofArcheancratonicWyomingProvince(Dutch&Nielsen,1990).
volcanic sequence are incompatible with the no- of these complications we illustrate the inferred location
menclatureestablishedinYellowstoneNationalPark(e.g. of the KO dividing line of Chadwick (1970) and the
2
Brown, 1982; Eaton, 1982; Sundell & Eaton, 1982; stratigraphic subdivisions of Smedes & Prostka (1972) in
Hague, 1985; Decker, 1990; Hiza, 1999). Even within Fig. 1 because this facilitates comparison with previous
the park itself, Smedes & Prostka (1972) encountered studies. Our work also demonstrates that mafic magmas
major difficulties in establishing regional stratigraphic erupted at Washburn volcano and the Electric Peak–
subdivisions because many of their formational units Sepulcher Mountain eruptive center (Lindsay & Feeley,
consist of monotonous sequences of andesitic vol- 1999, and this study) are not especially potassium rich.
caniclastic rocks, which are difficult to distinguish at Werecognize,however,thatthestratigraphyanderuptive
differentlocalities.Furthermore,Hiza(1999)hasrecently historyoftheAVPareundoubtedlymorecomplexthan
argued that mafic magmas (i.e. <56 wt % SiO) are originally envisioned (e.g. Hiza, 1999).
2
characteristically potassic throughout the AVP and that Seismic refraction studies indicate that the crust be-
if only these compositions are considered, little regular neaththeAVPatpresentis>45–50kmthick(Prodehl&
geographicvariationinmagmachemistryexists.Inspite Lipman,1989).ExposedbasementrocksincludeArchean
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JOURNALOFPETROLOGY VOLUME43 NUMBER4 APRIL2002
crystallinerocksoftheWyomingProvince(Fig.1),which The samples examined in this study were chosen
are mainly granitoid gneisses that intruded high-grade accordingtothestratigraphicunitsofProstkaetal.(1975;
metasedimentary and metavolcanic rocks at >2·8 Ga, Fig 2) from well-exposed, vertical stratigraphic sections
and shallow marine carbonate and clastic sedimentary dominated by vent-facies rocks. Temporal and spatial
rocks ranging in age from Cambrian to Cretaceous variations in bulk chemistry for lava flows within the
(Ruppel,1972;Wooden&Mueller,1988).Deep-tomid- Washburnvolcanicsequenceareshownschematicallyin
crustal lithologies are represented by mafic to silicic Fig. 3. In Fig. 3 (and subsequent figures) we designate
Archean granulite-facies xenoliths carried to the surface with different symbols lava flows exposed on Mt. Wash-
by Eocene alkalic magmas in the Crazy, Bearpaw, and burn and those in the SW Washburn Range to the west
Highwood Mountains (Dudas et al., 1987; Collerson et of the Grand Loop Road because these have different
al., 1989; Joswiak, 1992), and late Cenozoic basaltic compositional ranges. Lava flows and dikes in the SW
magmas of the Snake River Plain (Leeman et al., 1985). Washburn Range consist of a crudely bimodal package
Many of these xenoliths are similar to granulite-facies ofolivine+pyroxenebasalticandesitesandamphibole-
rocksworldwideinthattheyhaverelativelylowcontents bearing dacites, whereas dikes, stratigraphically higher
of Rb, U, and heavy rare earth elements (HREE), al- lava flows, and the Sulphur Creek stock to the east and
thoughtheyarelightrareearthelement(LREE)enriched NE on Mt. Washburn are predominantly olivine +
(Leeman et al., 1985; Joswiak, 1992). pyroxene basaltic andesites and pyroxene ± amphibole
andesites. Included in this latter sequence are the stra-
tigraphicallyhighestexposedlavaflowsonMt.Washburn
thatProstkaetal.(1975)designatedaspartoftheLangford
Stratigraphy
Formation of the Thorofare Creek Group of Smedes &
Washburnvolcanoisamajorcalc-alkalineeruptivecenter Prostka (1972; Figs 2 and 3). Although these flows were
in the AVP and is the largest Eocene volcanic center originally interpreted as younger (middle to upper Eo-
exposed in Yellowstone National Park. It is the primary cene) and erupted from vents much more distal than
source area for the Lamar River Formation, the eastern otherunits atthevolcano, ourworkshows thattheyare
member of the Washburn Group of Smedes & Prostka comparable in age and composition with other flows on
(1972). The Lamar River Formation is particularly well Mt. Washburn. We therefore consider all exposed units
known to visitors of Yellowstone National Park because to be derived from the same or very similar magmatic
initarepreservedthefamousuprightfossilforests(Dorf, systems.
1964). Washburn volcano has been previously mapped
by Schultz (1962; 1:30000) and Prostka et al. (1975;
1:62500; Fig. 2).
The eroded northern flank of Washburn volcano in GEOCHRONOLOGY
thevicinityofMt.Washburn,andHedgesandDunraven To ascertain the timing of magmatism at Washburn
PeaksintheSWWashburnRange,consistsof>1300m volcano we determined 40Ar/39Ar ages from phenocryst
ofvolcanicventfaciesstrata,mainlyoftheLamarRiver and groundmass samples of the stratigraphically highest
Formation, that include dikes, lava flows, flow breccias, and lowest exposed lava flows together with a biotite
and debris flow deposits that dip up to 30° away from separate from the Sulphur Creek stock (Table 1). The
the primary vent region (Fig. 2; Prostka et al., 1975). data are graphically presented in the form of 40Ar/39Ar
The lava flows and dikes are largely pyroxene basaltic agespectraand39Ar/40Arvs36Ar/40Arisochrondiagrams
andesites and andesites, although numerous amphibole- in Fig. 4. All errors on ages and intercepts reported in
bearing dacitic lava flows are present near the base Fig.4are2(cid:1).Theerrorsonindividualsteps,graphically
of the sequence. With increasing distance from Mt. represented by the width of rectangular boxes on the
Washburn and Hedges and Dunraven Peaks the vent- age spectrum diagrams, also represent a 2(cid:1) level of
faciesrocksgradeintoalluvial-facieslithologiesconsisting confidence. The sample localities are shown in Fig. 2.
ofepiclasticvolcanicconglomerateandbreccia,volcanic Details of the analytical procedures are described in the
sandstone and siltstone, and ashfall tuff deposits. The Appendix and a summary of the results is presented in
southern flank of Washburn volcano is truncated by Table 1. The full dataset may be downloaded from the
the northern segment of the Yellowstone Caldera fault, Journal of Petrology Web site at http://www.petrology.
exposing the interior of the volcano (Fig. 2). Here, oupjournals.org.
fine-grained biotite tonalite of the Sulphur Creek stock Plagioclase and amphibole phenocrysts and a sample
intrudes stratigraphically low Lamar River Formation offine-grainedgroundmasswereseparatedfromadacitic
volcanic rocks. This stock is similar in composition and lava flow at the base of Hedges Peak (MW971), the
age (see below) to the Eocene volcanic rocks and rep- stratigraphicallylowesteruptiveunit,todatetheinitiation
resentsashallowintrusionrelatedtoWashburnvolcano. of volcanic activity at Washburn volcano. Theoretically,
666
FEELEYetal. PETROGENESISOFCALC-ALKALINEMAGMATISM
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667
JOURNALOFPETROLOGY VOLUME43 NUMBER4 APRIL2002
Fig.3. SchematiccompositestratigraphiccolumnforWashburnvolcanocombiningtotalthicknessesofstratafromtheSWWashburnRange
andMt.Washburnareas.Lavaflowsareindicatedbysolidpatterns;clasticunitsareindicatedbystipplepatterns(excludingbrecciatedautoclastic
lavaflowtopsandbases).Geochemicaldatapanelsshowthecompositionalvariationsofmagmaswithstratigraphicposition.Opensymbolsare
forsamplesfromtheSWWashburnRangeandfilledsymbolsareforsamplesfromMt.Washburn.Circles,squares,andtrianglesareforbasaltic
andesitic,andesitic,anddaciticcompositionrocks,respectively.Note:(1)reinterpretationof‘LangfordFormation’flowsonMt.Washburnas
lateWashburnvolcanounitsbasedondatapresentedinthisstudy;(2)bimodalassemblageofdaciticandbasalticandesiticlavasinlowerpart
ofsectionfromSWWashburnRange;(3)dominantlyandesiticlavasinupperpartofsectionfromMt.Washburn.
Table 1: Summary of 40Ar/39Ar results from the Washburn volcano
Sample Material Plateauage %39Ar (steps) Isochronage 40Ar/36Ar Totalfusion
K i
(Ma) (Ma) age(Ma)
MW971 amphibole — — — — 59·3
MW971 groundmass — — 55·2±0·6 275±4 54·1
MW971 plagioclase — — — — 54·7
MW9746 biotite 53·5±0·4 78·9(10of15) 52·6±0·2 323±6 61·6
MW9743 groundmass — — 51·9±0·8 258±6 44·4
Allerrorsgivenareat±2(cid:1).AnalyticaldetailsdescribedintheAppendix.
all three samples should yield identical ages because all possibly with some K-rich phyllosilicate contamination
were at high temperatures immediately before extrusion consistent with the high K/Ca ratio in the first step.
and cooled rapidly once emplaced. The amphibole Data from the amphibole sample failed to plot on a
sample yielded an 40Ar/39Ar spectrum with evidence of statistically meaningful isochron, precluding an in-
excess argon in the initial heating steps, and most of the dependent evaluation of the isotopic ratio of trapped
gas released defined ages of >60Ma (Fig. 4a). The argonandtheageofthesample.Aninternallydiscordant
integratedtotalfusionage(roughlyequivalenttoaK–Ar 40Ar/39Ar age spectrum was obtained from the plagio-
age) is 59·3Ma; however, no age plateau was obtained. clase, which has an integrated fusion age of 54·7Ma
ThecorrespondingK/Caratiosdeterminedfromthestep (Fig. 4b). Intermediate heating steps defined ages of
heating data indicate a relatively homogeneous sample, >50Ma, but the last 30% of gas released defined ages
668
FEELEYetal. PETROGENESISOFCALC-ALKALINEMAGMATISM
Fig.4. (a)–(c),(e)and(g)showapparentagespectrafor40Ar–39Arincrementalheatingexperimentsforgroundmassandmineralseparatesfrom
Washburnvolcanorocks.Widthsofrectangularboxesindicateestimatedanalyticalerror(±2(cid:1))foreachstep.(d),(f)and(h)showcorresponding
36Ar/40Arvs39Ar/40ArisochrondiagramsforthestepArcompositionsmeasured.Theisochronageswithuncertainties(indicated)arecalculated
fromthebest-fittinglinesthroughcollinearstepcompositionsfollowingthemethodofYork(1969).
inexcessof60Ma.Theplagioclasedataalsofailedtoplot calculated making no assumptions about the isotopic
on a statistically meaningful isochron. The groundmass composition of the trapped argon, we consider this age
sample also has an internally discordant 40Ar/39Ar age asthemostreliableestimatefordatingtheemplacement
spectrum,withaintegratedfusionageof54·06Ma(Fig. of lava flow MW97-01. Moreover, the isochron age is
4c). Apart from the initial heating step, the K/Ca ratios similar to the integrated fusion ages of both the ground-
are consistent with argon degassing from a relatively mass and the plagioclase. This interpretation suggests
homogeneoussample.Isotopicdataobtainedfrominter- that the amphibole age of >59Ma is unreliable, and it
mediate heating steps, representing >50% of the total isconsistentwithnopublishedreportsofsucholderuptive
argon released, plot on an isochron defining a sample ages being described from the AVP. We therefore con-
age of 55Ma (Fig. 4d). Because the isochron age is sider the amphibole data as anomalously old, probably
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JOURNALOFPETROLOGY VOLUME43 NUMBER4 APRIL2002
because of extraneous argon. A similar result was ob- porphyritic to partially glomeroporphyritic lavas and
tained in the biotite data of sample MW9746 described dikes. In thin section the rocks are hypocrystalline with
below. intersertaltopilotaxiticgroundmasstextures.Phenocryst
The 40Ar/39Ar age spectrum obtained for the ground- contentsrangefrom44to5%(byvolume)withthetotal
mass of sample MW9743 is internally discordant with decreasingwithincreasingSiO contentsuntil>63wt%
2
anintegratedfusionageof44·4Ma(Fig.4e).Thehighly SiO (Fig.5;Table2).Daciticrockshavewidelyvarying
2
variableK/Caratiosareconsistentwithargondegassing phenocryst contents. Changes in mode with bulk com-
from reservoirs of strongly varying retentivity and com- position are regular throughout the suite. Basaltic–
position.Whenplottedonanisochrondiagram,however, andesiticrockscontainplag[cpx>ol±opx;andesitic
the data from heating steps making up >97% of the rocks contain plag > cpx > opx ± amph ± ol; and
argon released define a relatively precise (MSWD = dacitic rocks contain amph + plag ± cpx ± opx
3·9)ageof51·9Mafortheeffusionofthislava(Fig.4f). ± bio (Fig. 5). All rocks also contain Fe–Ti oxide
Taken together, the isochron ages from the groundmass microphenocrysts.Glomerocrystsarecommonofcpx±
samplesofthestratigraphicallylowestandhighestsamples Fe–Ti oxides, ol + cpx ± Fe–Ti oxides ± plagioclase
indicate that the >1km thick accumulated Washburn ± opx, and plag + amph ± Fe–Ti oxides. Mineral
volcanic pile was constructed in >3my. inclusion patterns and the occurrence of minerals in
BiotitefromtheSulphurCreekstocksampleMW9746 the glomerocrysts, together with textural features and
yielded an 40Ar/39Ar age spectrum with evidence of compositionsofthephenocrystsdescribedbelow,suggest
extraneous argon in the first few heating steps with an the following generalized crystallization sequences. Ba-
integrated fusion age of 61·6Ma and a well-defined age saltic andesitic magmas crystallized olivine followed by
plateauof53·5±0·4Ma(Fig.4g).However,anisochron Fe–Ti oxides, clinopyroxene + plagioclase, and ortho-
with these data (MSWD = 3·2) indicates a slightly pyroxene. Andesitic magmas crystallized plagioclase fol-
younger age of 52·6 ± 0·2Ma (Fig. 4h). The isochron lowedbyclinopyroxene+orthopyroxene,Fe–Tioxides
ageof52·6Maisourpreferreddateforthesample,and and then amphibole in some cases. Dacitic magmas,
the 40Ar/36Ar ratio of the trapped argon component in which pyroxenes are rare or absent, precipitated
(323)issignificantlygreaterthanpresent-dayatmosphere plagioclase followed by Fe–Ti oxides, amphibole, and
(295·5)andisconsistentwith‘excess’argonaspreviously then biotite when present. Groundmass assemblages in-
inferred for the amphibole in sample MW971. clude glass (partially to pervasively devitrified), plagio-
On the basis of the 40Ar/39Ar data presented here clase, clinopyroxene, and orthopyroxene. Zircon and
we interpret magmatism at Washburn volcano to have apatitearecommonaccessoryphasesintheintermediate
commenced as early as 55Ma and possibly continued and silicic lavas. Xenocrysts, identified by non-equi-
until at least 52Ma. These ages bracket the 53·4 ± 0·3 librium compositions (see below) or magmatic reaction
age reported by Hiza (1999) for a dacitic block and ash textures,arepresentbutrare.Furthermore,weidentified
flowatthebaseofSepulcherMountain,located>50km no clear petrographic evidence for mixing or mingling
to the NW of Mt. Washburn (Fig. 1). The sequence of between compositionally disparate magmas, such as the
rocks exposed on Sepulcher Mountain represents the presence of undercooled blobs of mafic magma that are
type section for the Sepulcher Formation, the western frequently found in many andesitic to dacitic rocks
member of the Washburn Group of Smedes & Prostka (Bacon, 1986; Wilcox, 1999).
(1972). Because these rocks are compositionally and The Sulphur Creek stock is tonalitic to quartz dioritic
petrographicallyidenticaltorocksexposedatWashburn with plag > qtz > bio > cpx = opx > Fe–Ti oxide >
volcano (Lindsay & Feeley, 1999), we concur with the
amph. Texturally, the stock is fine grained, phaneritic,
opinion of Smedes & Prostka (1972) that rocks within
and subophitic in that late-crystallizing anhedral quartz
theSepulcherandLamarRiverFormationswereerupted
partiallyencloseselongateplagioclaseandbiotitegrains.
from similar and nearly contemporaneously active vol-
canic centers. In addition, our results are also consistent
with the suggestion of Hiza (1999) that the oldest rocks
in the AVP are calc-alkaline lavas at present exposed in Olivine
the northwestern part of the field. Olivinegenerallyoccursasequant,euhedraltosubhedral
phenocrystsinbasalticandesiticlavas.Inthemajorityof
these samples the phenocrysts range in size from about
4·0to0·5mm,althoughpopulationsinindividualsamples
PETROGRAPHY AND SILICATE
typicallyhaveamuchnarrowerrange.Inafewsamples
MINERAL CHEMISTRY the size variation is continuous from a maximum of
Washburn igneous rocks investigated in this study are 1·0mm to a minimum of 0·1mm. Additionally, a few
generally non- to slightly vesicular (<3 vol. % vesicles), andesitic and dacitic rocks contain small amounts of
670
FEELEYetal. PETROGENESISOFCALC-ALKALINEMAGMATISM
8 9 9 2 2 5 5 8 9 3 7 0
7 2 6 5 4 0 8 9 3 2 1 6
W9 va 64· 15· 4· 0· 0· 3· 2· 4· 3· 0· 99· 80 04 48 10 13 55 49 32 9 6 19 4
M La D 1 20 7 1
2 7 1 3 3 4 6 6 7 2 8 7
8 1 9 2 4 0 5 8 1 1 1 6
OP98 Dike D 64· 15· 4· 0· 0· 4· 2· 2· 4· 0· 98· 78 140 63 15 1356 38 800 142 10 6 21 3
1
1 9 1 2 1 8 1 3 8 5 9 7
7 0 2 8 5 0 6 0 8 8 1 2
W9 va 64· 16· 4· 0· 0· 4· 3· 1· 3· 0· 99· 79 15 53 9 72 79 26 76 13 5 17 5
M La D 1 13 6 1
6 3 8 5 3 4 9 3 6 7 1 9
s 7 6 1 8 5 0 1 3 9 9 2 8
rock MW9 Lava A 62· 16· 4· 0· 0· 3· 2· 4· 2· 0· 97· 89 63 30 8 1585 81 535 162 14 6 18 3
s
u
oigneo OP9875 Lava A 57·94 15·11 7·00 0·67 0·10 6·54 6·18 1·97 3·36 0·20 99·07 149 312 107 18 1137 35 675 135 14 6 13 3
n
a 4 5 7 9 8 2 8 3 6 8 0 6
c 7 4 9 9 6 1 7 9 8 3 2 3
nvol OP98 Lava A 57· 14· 6· 0· 0· 6· 6· 1· 3· 0· 99· 146 355 136 21 1088 30 668 132 13 7 14 2
r
u
Washb OP9861 Lava A 57·10 17·49 7·36 0·81 0·10 6·92 3·93 1·87 4·01 0·23 99·81 168 33 14 22 1219 29 757 141 16 6 14 5
datafor MW979 Lava BA 56·91 14·50 7·48 0·69 0·17 7·06 7·79 1·75 2·97 0·20 99·52 155 471 174 18 1063 29 648 128 13 5 7 5
modal MW9715 Dike BA 56·46 17·44 7·68 0·82 0·11 7·30 3·87 2·11 3·29 0·22 99·31 179 35 13 21 1061 30 748 140 15 5 13 5
d
n 4
a 71 62 07 51 70 14 83 08 67 97 23 82
atios, MW9 Lava BA 55· 14· 8· 0· 0· 7· 7· 1· 2· 0· 98· 166 507 158 23 1008 27 694 121 13 4 12 3
r
2
c 7 2 0 8 5 3 5 6 2 7 3 0
otopi MW9 Lava BA 55·6 14·6 8·2 0·7 0·1 7·9 7·3 1·7 2·8 0·2 99·5 188 324 96 21 945 27 676 124 14 5 11 5
s
i
ndNd OP9873 Lava BA 55·14 14·51 8·28 0·77 0·14 8·74 7·08 1·52 2·73 0·23 99·14 201 422 116 26 738 28 620 109 16 5 9 1
a
Sr 77 3 1 5 7 3 6 5 8 1 2 1
ent, MW9 Lava BA 55·1 15·3 8·7 0·7 0·1 8·4 6·3 1·5 2·7 0·2 99·4 185 497 58 24 917 41 652 116 14 4 12 4
m
6
e 1
raceel MW97 Lava BA 54·59 14·16 8·88 0·75 0·15 8·79 8·05 1·41 2·62 0·23 99·62 185 421 117 26 709 25 599 108 14 5 7 3
t 2
element, ge MW971 Lava BA 54·52 15·49 8·87 0·82 0·14 8·33 6·27 1·74 2·62 0·21 99·00 184 206 63 28 941 32 642 126 16 5 14 3
Major burnRan MW9717 Lava BA 54·32 14·09 9·07 0·75 0·14 8·69 8·21 1·53 2·47 0·23 99·50 199 420 114 25 748 27 623 109 15 4 8 3
Table2: SWWash MW9713 Lava BA 52·39 14·98 9·98 0·90 0·16 8·58 8·65 1·25 2·58 0·18 99·65 179 415 134 26 589 22 463 104 17 5 6 6
n:
o
Sample: Rocktype: Classificati XRFwt% SiO2 OAl23 TOFe23 TiO2 MnO CaO MgO OK2 ONa2 OP25 ∗Total XRFppm V Cr Ni Sc Ba Rb Sr Zr Y Nb Pb Th
671
JOURNALOFPETROLOGY VOLUME43 NUMBER4 APRIL2002
8
7
MW9 Lava D — 2·3 — — 0·2 12·6 — — 15·0
2
8
8
9 e 3 0 5 8
OP Dik D — 4· — — 1· 9· — — 14·
1
1
7
MW9 Lava D 10·3 2·4 39 69 23 4·5 1·32 0·38 1·2 0·21 5·0 — 2 0·3 1·5 — 3·1 0·7 1·9 — — 7·6
6
7
MW9 Lava A — 12·1 8·5 — 1·8 3·6 — — 26·1
5
7
OP98 Lava A 0·2 12·8 13·9 3·7 0·8 — — — 31·4
4
7
OP98 Lava A 19·0 0·8 30 51·5 17·0 4·2 1·21 0·39 1·7 0·22 3·40 — 0·40 — 6·4 8·1 6·1 0·3 — — — 20·9
1
6
OP98 Lava A 18·7 0·7 33 57·0 21·0 4·8 1·42 0·46 1·5 0·24 3·60 0·27 — 2·6 11·6 2·1 0·5 1·6 — — — 18·4
9
7
MW9 Lava BA 4·4 9·7 10·1 5·3 0·4 — — — 30·0
5
1
7
9
W e 4 5 0 8 4 1
M Dik BA 0· 25· 6· 0· 1· — — — 34·
7 2 2 1
4 2 3 4 9
1 7 6 6 5
7 5 1 5 1
MW9 Lava BA 23·3 0·9 24·3 46·7 19·0 3·68 1·23 0·51 1·50 0·19 2·82 0·21 0·77 0·70 0·51 0·70 0·51 7·6 10·4 12·9 1·3 0·7 — — — 32·9
2
7
MW9 Lava BA 1·8 21·6 10·1 3·4 0·4 — — — 37·4
3
7
OP98 Lava BA 8·3 9·3 14·2 1·5 — — — — 33·3
7
7
MW9 Lava BA 2·0 13·6 7·6 0·6 0·2 — — — 23·9
6
1
7
MW9 Lava BA 5·2 20·6 13·6 1·2 1·5 — — — 42·1
2
1
7
d e MW9 Lava BA 6·1 27·4 5·7 2·5 1·6 — — — 43·4
e g
u n 7
n a 1
conti burnR MW97 Lava BA %) 8·6 20·0 10·4 2·4 1·3 — — — 42·7
Table2: SWWash MW9713 Lava BA 28·0 0·7 15·7 31·0 14·6 3·20 1·19 0·56 1·60 0·22 2·94 0·25 0·73 0·705847 0·511890 0·705744 0·511844 ysts(vol. 9·2 21·2 2·9 0·2 0·9 — — — 34·4
Sample: Rocktype: Classification: INAAppm Sc Cs La Ce Nd Sm Eu Tb Yb Lu Hf Ta U 8786Sr/Sr)(m 143144Nd/Nd)(m 8786Sr/Sr)†(i 143144Nd/Nd)(i Modalphenocr ol plag cpx opx oxide amphibole bio qtz Total
672