ARTICLES Geochronology of Bimodal Alkaline Volcanism in the Balcones Igneous Province, Texas

21 Pages·2009·0.96 MB·English

ARTICLES
Geochronology of Bimodal Alkaline Volcanism in the Balcones
Igneous Province, Texas: Implications for Cretaceous Intraplate
Magmatism in the Northern Gulf of Mexico Magmatic Zone
William R. Griffin, Kenneth A. Foland,1 Robert J. Stern, and Matthew I. Leybourne2
Department of Geosciences, University of Texas at Dallas, P.O. Box 830688,
MS FO21, Richardson, Texas 75083, U.S.A.
(e-mail: griffi[email protected])
ABSTRACT
Small-volumeLateCretaceousmonogeneticalkalinevolcanismalongthesouthernmarginofNorthAmericaresulted
in a broad igneous belt more than 1200 km long from the Trans Pecos region of west Texas to central Mississippi,
collectivelyforminganorthernGulfofMexicomagmaticzone(NGMMZ).Thelocusofigneousactivityisassociated
with the discontinuity separating MesoproterozoiccratoniclithosphereandJurassictransitionallithosphere,azone
approximating thesouthernmarginofLaurentia,thesubsurfacetrendofthePennsylvanianOuachitaorogenicbelt,
and the trace of the Miocene Balcones fault zone in Texas. Althoughpreviousstudieshaveattemptedtodetermine
the ages of igneous activity in the region, few well-constrained geochronologic data using modern high-resolution
techniques are available. We determined the age of eruption for the Balconesigneousprovince(BIP),a400-km-long
subsegment of the NGMMZ, using modern 40Ar/39Ar and U-Pb geochronology methods. Our results suggest that
previously reported 40K/40Ar and 40Ar/39Ar ages underestimate the age of igneous activity by as much as 17%. New
ages from this study, along with reevaluation of previous results, suggest that BIP igneous activity occurred intwo
discretephases,eachlastingonly2.6m.yr.andseparatedby2.7m.yr.:oldermaficvolcanismoccurredbetween81.5
and 84.1 Ma and younger felsic volcanism between 76.2 and 78.8 Ma. The total interval of 8 m.yr. for BIP igneous
activity is much shorter thanhad previouslybeeninferred.ThebestgeochronologicresultsareobtainedfromU-Pb
dating of zircon mineral (phonolites) separates and 40Ar/39Ar dating of phlogopite (nephelinites and basanites) and
amphibole (phonolites) separates.
Online enhancements: appendix tables.
Introduction
The southern margin of North America experi- 1997). The magmatism is constrained to have oc-
encedwidespreadalkalinemagmatismduringLate curred between 108(cid:2)5 Ma (Prairie Creek com-
Cretaceous time, ranging from west Texas to cen- plex, AR; Zartman 1977) and 66(cid:2)5 Ma (Jackson
tral Mississippi, collectively forming a northern Dome, MS; Merrill 1983). It is important to un-
Gulf of Mexico magmatic zone (NGMMZ; fig. 1; derstand the temporal history of a magmatic field
e.g., Byerly 1991). The timing and duration of this because time-space relationships provide testable
activity are poorly understood because of a lack of criteria to distinguish between various magmatic
modernhigh-resolutiongeochronologicdata(Baksi models(i.e.,mantleplumes,edgeconvection,lith-
osphericdelamination).Zartman(1977)recognized
ManuscriptreceivedJuly18,2008;acceptedAugust1,2009. that alkaline rocks are commonly emplaced after
1DepartmentofGeologicalSciences,OhioStateUniversity, cessation of orogeny or in anorogenic settings and
275 Mendenhall Lab, 125 South Oval Mall, Columbus, Ohio
that tectonic setting is often poorly understood in
43210,U.S.A.
thegenesisofalkalinerocksuites,contendingthat
2OceanExploration,GNSScience,Box30-368,LowerHutt
5040,NewZealand. accuratedatingisessentialforunderstandingtheir
[TheJournalofGeology,2010,volume118,p.1–21](cid:1)2010byTheUniversityofChicago.
Allrightsreserved.0022-1376/2010/11801-0001$15.00. DOI:10.1086/648532
1
2 W. R. GRIFFIN ET AL.
Figure 1. Filtered (40–250-km wavelengths passed) gravity map ofthe southernmarginofNorthAmerica(afterG.
R. Keller, pers. comm., 2008). The Laurentian margin defines the discontinuity between Mesoproterozoic Texas
cratoniclithosphereandJurassictransitionallithosphere.SegmentsofthelargernorthernGulfofMexicomagmatic
zonearelabeledasfollows:TPpTransPecos,BIPpBalconesigneousprovince,SWARpsouthwestArkansas,MU
p Monroe uplift, and JD p Jackson Dome. The thick dashed lines represent possible extensions of the Laurentian
margin; the thin dashed line represents the inferred track of the Great Meteor hot spot (after Duncan 1984).
origins. This is certainly true for the NGMMZ, km-long, 100-km-wide subsegment of the larger
where a mantle plume origin has been suggested NGMMZ.New40Ar/39ArandU-PbresultsfromBIP
(CoxandVanArsdale2002)andmagmatismispre- mineral separates are presented,evaluated,andin-
dictedtohavemigratedalongahotspottrackfrom tegratedwithresultsfrompreviousstudiestomore
W to E through the region. A review of the litera- tightly constrain the timing and duration of BIP
ture shows that alkaline volcanic complexes are magmatic activity. BIP magmatism resulted in
variable with respect to the duration of activity in 1200(EwingandCaren1982)intrusiveanderuptive
individual complexes, ranging from short-lived centers along the reactivated discontinuity be-
monogeneticfieldsthatcompletetheirentireerup- tween Grenville-age (1.1–1.3-Ga) Texas craton and
tive cycles over a few decades or less to long-lived Jurassic (∼160–180-Ma) transitional lithosphere of
polygenetic fields with repeated eruptive cycles the northern margin of the Gulf of Mexico basin.
over10sto1100m.yr.(Nielsen1981;Fitton1987; Thisdiscontinuityisalsoapproximatedbythesub-
Barker 1996; Edwards and Russell 1999, 2000; surfacetrendofthePennsylvanian(∼300-Ma)Oua-
Ngounounoetal.2000;Ulrychetal.2000;Yongtao chitaorogenicbeltandtheMiocene-age(25–10-Ma)
andAnchun2003;GourgaudandVincent2004;Bai- Balconesfaultzone(Gallowayetal.1991)insouth
ley and Woolley 2005; Cook et al. 2005; Johansen centralTexas(fig.2).Low-degreepartialmeltsfrom
etal.2005;Lie´geoisetal.2005).Testingmodelsfor the focal zone (FOZO) and the FOZO (cid:1) high-m
the tectonic setting of alkalic provinces requires (HIMU)asthenosphericsourcesintheuppermantle
precise and accurate geochronology data. produced a suite of monogenetic volcanic rocksin
Thisstudy focusesonthegeochronologyofLate compositional range of melilite olivine nephelin-
CretaceousalkalinemagmatismintheBalconesig- ites, olivine nephelinites, nepheline basanites, al-
neousprovince(BIP)ofTexas(fig.2).Thisisa400- kali basalts, phonotephrites, tephriphonolites, and
JournalofGeology GEOCHRONOLOGY OF THE BIP 3
Figure 2. Map of the Balcones igneous province (after Griffin 2008). Numbered locations correspond to sample
locationswhereageswerederived,asdetailedintable3.Phonoliticoutcropsareindicatedbyanasterisknexttothe
location number.
phonolites (Griffin 2008; Griffin et al. 2009). On was isotopically dated by Baldwin and Adams
thebasisofthemajor,trace,andradiogenicisotope (1971).Tuffassociatedwiththeoutcropwasfound
compositions of BIP igneous rocks, we have sug- to interfinger with Upper Dessau (Upper Austin
gestedthatthelocationandstyleofmagmatismin group, Upper Santonian) strata. The Santonian
theBIPwereintimatelylinkedtoanimportantlith- stage is constrained between 83.5(cid:2)0.7 and
ospheric discontinuity (Griffin 2008; Griffin et al. 85.8(cid:2)0.7Ma(Gradsteinetal.2005).Fracturingof
2009); however, the temporalaspectsarelessclear Dessau strata near Pilot Knob was interpreted by
because of the paucity of modern, high-resolution Strong (1957) as evidence for the timing of volca-
geochronological data. Previous studies of the BIP nism,givingasomewhatolderagethantheaverage
used 40K/40Ar techniques to date a number of vol- 40K/40Ar whole-rock age of 79.5(cid:2)3 Ma (Baldwin
canic centers (Burke et al. 1969; Baldwin and Ad- and Adams 1971). Young et al. (1982) contended
ams 1971), but only a few ages using higher- thattheearliestmagmatismatPilotKnobwascon-
precision 40Ar/39Ar techniques are available temporaneous with the middleDessauFormation,
(Miggins et al. 2004), and no U-Pb zircon ages are andtheydiscussedareasproximaltothecenterthat
reported.AlthoughtheBIPcanbeseeninthefield werecoevalwiththeboundarybetweentheAustin
to consist of many low-volume, monogenetic vol- and Taylor groups.
canoesandplugs,existingradiometricagessuggest Stratigraphic relationships elsewhere in the BIP
protracted activity from 100–66 Ma, on the basis are less clear than at Pilot Knob. In a literature
of the stratigraphic relationshipsofpyroclasticde- review of the BIP, Matthews (1986)concludedthat
positsandfossiliferoussediments(Barker1996),to magmatism occurred between the Coniacian and
asshortas82–72Ma,onthebasisof40Ar/39Argeo- the Campanian stages (Austin and Taylor groups)
chronology (Miggins et al. 2004). and separated the province into threesegmentson
Stratigraphic relations are difficult to determine thebasisofvariationsoftheapparentstratigraphic
at most BIP volcanic centers because of poor ex- ages.Thenorthernsegmentincludedcentersinthe
posure. The stratigraphically best-studied outcrop eastern extreme of the BIP (including Pilot Knob),
is Pilot Knob, at the northeastern extreme of the centered on Travis County (fig. 2). Magmatism
BIP(fig.2,43;Strong1957;BarkerandYoung1979; there was contemporaneous with Santonian to
Ewing and Caren 1982; Young et al. 1982), and it Campanian strata (Austin andTaylorgroups;Mat-
4 W. R. GRIFFIN ET AL.
thews 1986); this corresponds to an absolute age Mexico began by rifting, attenuating the Ouachita
rangeof89.3(cid:2)1to70.6(cid:2)0.6Ma(Gradsteinetal. lithosphere,anditculminatedinseafloorspreading
2005). The central segment, southeast of Bexar duringmid-Jurassictime(∼160Ma),about80m.yr.
County (fig. 2), contains no exposures of igneous before the onset of BIP magmatism (Sawyer et al.
rock,butsubsurfacecentershavebeencutbydrill- 1991).Thediscontinuitybetweenthecratoniclith-
ing interpreted to be contemporaneous with Con- osphere of North America and attenuated transi-
iacian to Santonian strata of the Austin Group tional lithosphere on the NW flank of the Gulf of
(89.3(cid:2)1 to 83.5(cid:2)0.7 Ma; Gradstein et al. 2005), MexicooceanicbasinlocalizedtheLateCretaceous
stratigraphically older than the igneous centers in BIP magmatism (Griffin 2008; Griffin et al. 2009).
the northern subprovince (Matthews 1986). The The location of the BIP along this discontinuity is
southern segment is the locus of the most intense consistent with a number of different magmatic
magmatism,asindicatedbythehighest-concentra- mechanisms explored by Griffin (2008), including
tion BIP volcanoes and plugs, and is centered on edge convection (King and Anderson 1998), lith-
UvaldeCounty(fig.2).Matthews(1986)concluded osphericdelamination(i.e.,KayandKay1993),and
thatmagmatisminthesouthernsegmentwasstrat- lithospheric rifting (Corti et al. 2003). Although
igraphically younger than that in the central and strongly alkaline, silica-undersaturated intraplate
northernsegments,contemporaneouswithSanton- melts, such as those of the BIP, are commonly as-
ian and Campanian strata (Austin and Taylor sociatedwithcontinentalrifting(i.e.,Mitchelland
groups) to Maastrichtian strata (Escondido Forma- Platt1983).Griffin(2008)concludedthatnomajor
tion), corresponding to an age range of 89.3(cid:2)1 to rifting events occurred in the region during Late
65.5(cid:2)0.3 Ma (Gradstein et al. 2005). Our experi- Cretaceous time. Instead, stress regimes imposed
enceinthefieldwasthatthestratigraphicrelations on the region (Bird 2002) were sufficient to create
of volcanic horizons, as well as intrusive relation- small degrees of extension perpendicular to the
ships, were difficulttoobservebecauseofpoorex- lithosphericdiscontinuity,allowinglow-degreede-
posure,sowearguethatthebestmethodoffurther compression melting of volatile-enriched upper
definingtheageanddurationofmagmatisminthe mantle sources with FOZO and HIMU affinities.
BIP is by isotopic methods. BIPigneousactivityincludesshallowmarineerup-
This study provides new constraints on the age tionsofnephelinitesandbasanites,aswellasshal-
of the BIP by reporting results of 40Ar/39Ar and U- lowintrusionsofphonolite.Thediscontinuitywas
Pb analyses of high-purity amphibole, phlogopite, reactivated as late as Miocene time (25–10 Ma),
and zircon mineral separates. These data are used forming the Balcones escarpment (Galloway et al.
to address the followingquestions:(1)Whichmin- 1991);however,thereactivationoccurredlongafter
eralsandtechniquesprovidethemostreliableages BIP igneous activity ended.
of BIP igneous activity? (2) What was the timing
and duration of BIP magmatism? (3) Were the var-
Methods
iousBIPlithologiescomagmaticorconfinedwithin
discretetimeintervals?Answerstothesequestions Approximately45kgofbulkrockwascollectedat
can provide general insights regarding the differ- eachsamplinglocation.Allsampleswereprepared
ences between monogenetic and polygenetic mag- attheUniversityofTexasatDallasDepartmentof
matismandthefactorsinfluencingthedurationof Geoscience sample preparation lab by removing
activity in the region. weatheredsurfaceswitharocksaw,crushingthem
a steel-jaw crusher, washing and drying the rock
chips,andpowderingtheminaBicodiskmillpul-
Geologic Background
verizer to pass through a 60-mesh sieve, followed
The geological history of the BIP region is related by an 80-meshsieve.Powdersplitsbetween60–80
totectoniceventsassociatedwiththetectonicevo- and 80–100 mesh were preserved for zircon (felsic
lution of the southern margin of North America. rocks), amphibole, and nepheline-K-feldspar con-
This includes accretion of Mesoproterozoic (1.3- centrate (felsic rocks) and phlogopite (mafic rocks)
Ga) lithosphere resulting from the Grenville orog- separation.
eny, followed by the accretion of Pennsylvanian- Tensampleswereprocessedforzirconextraction
age (320–300-Ma) lithosphere associated with the (onemeliliteolivinenephelinite,twoolivineneph-
Ouachita orogeny (Galloway et al. 1991) and the elinites, one phonotephrite, and six phonolites).
opening of the Gulf of Mexico in Early Mesozoic The sieved materialforzirconseparationwascon-
time (∼200–160 Ma). The opening of the Gulf of centrated for heavy minerals on a shaker table,
JournalofGeology GEOCHRONOLOGY OF THE BIP 5
dried,andpassedthroughaFrantzisodynamicsep- sidedtape,in1#6-mmrows.Thegrainswerecast
arator. The 1.6-A (maximum) nonmagnetic split into 25#4-mm epoxy disks and polished to1-mm
was subjected to heavy-liquid separation in meth- finish. Transmitted light images were made on an
yleneiodineandhandpickedunderabinocularmi- opticalmicroscopetorevealthesurfacetexturesof
croscope. Zircon in the mafic rocks is very rare to the grains. The grain mount disk was then Au-
nonexistent,withonesampleyieldingasinglecrys- coatedbystandardsputter-coatingtechniques,and
tal that produced an age (although the degree of cathode luminescence (CL) images (fig. 3) were
uncertainty was high) consistent with that of madeusingaJEOL5600LLVscanningelectronmi-
phlogopite separates recovered from other mafic croscope to reveal the interior zonation of each
samplesinthearea.Otherzirconsfrommaficsam- grain. The grain mounts were then washed in a
ples yielded much older (xenocrystic) ages or were saturatedEDTAsolution,dried,andgold-coatedfor
discordant;theseneverthelessprovideinformation introduction to the SHRIMP-RG.
aboutBIPbasement.Zirconabundanceinthephon- Analysesofindividualzirconsbeganbyrastering
otephrite and phonolites ranges from abundant to a primary O2(cid:2) ion beam at an intensity of 4–6 nA
nonexistent. One sample yielded a large popula- for 120 s to remove the Au coat and surface con-
tion, two yielded moderate populations, two tamination from the analytical spot. The primary
yielded small populations, and one yielded no zir- ion beam was focused to approximately 20–40 mm
con. As was found in the mafic samples,thefelsic and rastered over the analytical spot for 12 min to
rocks carry rare zircons yielding much older (xen- produce secondary ions from an ablation spot ap-
ocrystic) ages, although compared with the mafic proximately 1–2 mm deep. Peaksfor90Zr16O,204Pb,
2
rocks, those make up a smaller proportion of the background (0.050 amu above 204Pb), 206Pb, 207Pb,
total populations recovered. Future work aimed at 208Pb, 238U, 232Th16O, and 234U16Oweresequentially
recovering zircon from the BIP should concentrate measured. An autocentering procedure performed
the effort on the felsic rocks only because the re- on guide peaks of variable or low abundance (i.e.,
sults obtained from mafic zircons were not useful 90Zr16O at 0.165 amu below the 204Pb peak) im-
2
in determining the age of eruption and cooling for proved peak center location reliability and elimi-
individualoutcrops.Baddeleyiteseparationwasnot nated isobaric interferences.
attempted but should be in future studies. Data were processed off-line at the end of each
Ninemaficand11felsicsampleswereprocessed analytical session, using SQUID 1.02 (Ludwig
for 40Ar/39Armineralseparation.Crushedsplitsfor 2001),byJ.WoodenattheSUMAC,usingthemeth-
phlogopite, amphibole, and nepheline-K-feldspar ods of Williams (1997) and Ireland and Williams
concentrate separation were washed to remove (2003).Commonleadcorrectionswereachievedby
fines, dried, and passed through a Frantz isody- model age curve fitting of the uncorrected analyt-
namic separator; magnetic splits 0.3–0.4 and 0.4– ical data to Stacey and Kramers’s (1975) lead evo-
0.5 A were targeted for phlogopite and amphibole, lution model. Results were calibrated relative to
respectively. Further concentration was achieved thezirconstandardR33,a419-Maquartzdioriteof
byheavy-liquidseparationinBromoformandhand- the Braintree complex (Black et al. 2004). Initial
picking under a binocular microscope. Phlogopite calibrationoftheinstrumentwasperformedbyan-
was targeted in the mafic rocks, even though rare alyzing four or five R33 grains at the beginning of
inmostsamples,becausenoothersuitablemineral the analytical run for each new grain mount,with
targets exist for 40Ar/39Ar geochronology. Amphi- calibration checks performed throughout the ses-
bole was targeted for separation in the felsic rocks sion by analysisofan R33 grain aftereveryfouror
because it is the most common suitable mineral fiveunknowns.Zirconsfromsixphonolitesamples
for 40Ar/39Ar analyses. Nepheline-K-feldspar con- were analyzed during analytical sessions in 2004–
centrates were recovered from the 1.6-A nonmag- 2006.Relativetotheacceptedvalueof419Ma,18
neticFrantzsplit.Futureworkrequiringmicasep- analyses of R33 produced an average of 418.9(cid:2)
arates should focus on the mafic rocks, and work 2.1 Ma during 2004, 15 analyses produced an av-
requiring amphibole separates should focusonthe erage of 419.0(cid:2)2.7 Ma during 2005, and 15 anal-
felsic rocks of the BIP. ysesproducedanaverageof418.9(cid:2)3.5Maduring
Zircons were analyzed at the USGS sensitive 2006(tablesA1,A2,availableintheonlineedition
high-resolution ion microprobe reverse geometry or from the Journal of Geology office). Analytical
(SHRIMP-RG) instrument housed at the Stanford spots were chosen on individual grains according
University Microanalysis Center (SUMAC). Zir- to zircon morphology, the surface texture as re-
cons were mounted on glass slides, using double- vealed in the reflected light images, and the zo-
Concordiaage(Ma) (cid:2)81.18.9 (cid:2)77.21.6(cid:2)77.92.3(cid:2)77.01.4(cid:2)77.91.5HiU(cid:2)75.91.7(cid:2)78.62.7(cid:2)77.92.5(cid:2)76.82.0(cid:2)75.723HiU(cid:2)77.41.9 (cid:2)76.633(cid:2)77.01.8(cid:2)771.4(cid:2)79.31.7(cid:2)77.71.9(cid:2)77.31.5(cid:2)778290 (cid:2)78.88.4(cid:2)3.6416Discordant
6 98
onnt
Errororrelatioefficie .154 .184.158.524.377.676.255.246.346.264.331.563.243 .107.322.351.335.289.303.309 .291.077.393
cc
Pb-corrected 207235206238Pb/UPb/U(%error)(%error) .90(8.9).1115(1.4) .08(5.6).0121(1.0).09(9.5).0122(1.5).08(1.7).0120(.9).08(2.5).0122(.9).08(1.2).0126(.8).08(4.4).0118(1.1).09(7.0).0123(1.7).08(4.7).0122(1.6).08(5.0).0120(1.3).08(3.4).0118(1.1).08(1.7).0124(.9).08(5.1).0121(1.2) .06(13.3).0120(1.4).08(3.7).0120(1.2).08(2.6).0121(.9).08(3.2).0124(1.1).08(4.2).0121(1.2).08(3.3).0121(1.0)1.05(4.5).1288(1.4) 1.62(3.2).1640(.9).07(14.2).0131(1.1)1.82(2.0).1711(.8)
204 206b/Pberror) 8(8.8) 8(5.5)9(9.4)7(1.4)4(2.3)8(.9)6(4.3)4(6.8)7(4.4)4(4.8)0(3.2)4(1.4)3(4.9) 2(13.2)9(3.5)0(2.5)5(3.0)6(4.0)4(3.2)4(4.3) 9(3.0)8(14.2)3(1.8)
P% 8 057788495167 6877589 197
207 ( .05 .05.05.04.04.04.04.05.04.04.05.04.04 .03.04.04.04.04.04.05 .07.03.07
ples 238206U/Pb(%error) 8.97(1.4) 82.95(1.0)82.22(1.5)83.24(.9)82.26(.9)79.61(.8)84.40(1.1)81.49(1.7)82.22(1.6)83.43(1.3)84.67(1.1)80.33(.9)82.79(1.2) 83.52(1.4)83.24(1.2)82.59(.9)80.79(1.1)82.39(1.2)82.85(1.0)7.76(1.4) 6.10(.9)76.55(1.1)5.85(.8)
usProvinceSam Total207TotalPb/238206206U/PbPb(%error)(%error) 8.89(1.2).07(2.3) 82.17(1.0).06(1.8)83.01(1.4).05(3.6)83.25(.9).05(1.5)82.09(.9).05(1.7)79.61(.8).05(.9)84.10(1.1).05(2.4)81.96(1.7).05(4.7)82.35(1.6).05(4.5)83.09(1.3).05(2.9)84.71(1.1).05(3.2)80.27(.9).05(1.3)82.44(1.2).05(2.9) 81.97(1.3).05(3.3)83.15(1.2).05(3.1)82.41(.9).05(1.6)80.58(1.1).05(2.3)82.02(1.2).05(2.0)82.62(1.0).05(2.4)7.72(1.4).06(2.1) 6.07(.9).08(1.3)75.84(.9).05(2.7)5.87(.8).07(1.1)
conesIgneo 204Pb-corrected 208232Pb/Thagej(1;Ma) 614(74) 79(1)85(4)78(1)79(1)80(1)78(6)89(4)79(2)77(2)82(2)81(1)80(2) 76(2)71(2)78(1)77(2)76(2)79(2)665(66) 914(33)77(8)1060(26)
edfromBal 204Pb-corrected 207206Pb/Pbagej(1;Ma) 558(192) 234(126)450(208)85(34)70(55)137(21)128(100)387(153)182(102)(cid:2)34(116)241(74)19(33)63(118) (cid:2)618(361)144(82)50(59)73(72)(cid:2)22(97)117(75)580(93) 982(62)(cid:2)362(368)1130(36)
mZirconGrainsRecover 207208Pb-correctedPb-corrected 206238206238Pb/UPb/Uageagejj(1;Ma)(1;Ma) 684.2(8.3)685.3(8.6) 76.9(.8)76.6(1.3)77.1(1.1)76.6(1.3)77.0(.7)76.6(1.0)77.9(.7)77.7(1.0)80.4(.7)80.7(.9)75.8(.8)75.9(.9)78.0(1.3)76.9(1.6)77.7(1.3)77.6(1.8)77.0(1.0)76.7(1.2)75.4(.9)74.8(1.0)79.9(.7)79.5(1.0)77.4(.9)76.7(1.3) 77.8(1.0)77.3(1.9)76.8(.9)77.5(1.0)77.6(.7)77.5(.9)79.3(.9)79.6(1.0)78.0(.9)78.6(1.6)77.3(.8)77.1(.9)786.7(10.5)785.0(10.4) 978.6(8.5)984.0(9.0)84.5(.7)84.2(.8)1013.1(7.5)1015.2(7.7)
d
sultsfro Pb-correcte 206238Pb/Uagej(1;Ma) 681.5(8.9) 77.3(.8)77.9(1.2)77.0(.7)77.9(.7)80.5(.7)75.9(.8)78.6(1.3)77.9(1.2)76.8(1.0)75.7(.8)79.8(.7)77.4(.9) 76.7(1.1)77.0(.9)77.6(.7)79.3(.8)77.8(.9)77.3(.8)781.3(10.2) 978.7(8.4)83.7(.9)018.0(7.3)
Re 204 1
MP-RG 232hTh/238Um) 18.39 452.13841.02451.73051.36151.7698.1942.96711.6051.9120.90861.64361.52 722.3781.54621.1270.96032.1360.7511.22 45.5686.4853.47
Tp 22976 123485 633575 1
RI (p 2 217 3 1 1
H m) 8 892884459335 3174762 386
S Up 4 082876294654151739484536 2973256182775 83911
p 1 114 2 1
b (
U-P Comm.206Pb(%) .45 1.35.10.00.18.14.49.29.12.15.39(cid:2).08.38 .45.28.15.25.21.38(cid:2).17 .44(cid:2).05.19
Table1. Sample,grainandspotno. G-UV01:1.1G-UV03:1.12.13.14.15.16.17.18.19.110.111.112.1(cid:2)GUV05:1.12.13.14.15.16.17.1G-UV06:1.12.13.1
6
(cid:2)76.51.9HiU(cid:2)79.31.9(cid:2)73.93.2(cid:2)73.14.5…(cid:2)79.85.6(cid:2)78.32.0(cid:2)75.31.9HiTh/U(cid:2)78.924(cid:2)76.84.0(cid:2)82.7120(cid:2)75.91.9(cid:2)78.92.0HiU(cid:2)73.13.2Discordant(cid:2)78.43.6(cid:2)78.22.4(cid:2)74.64.1(cid:2)7.262.0(cid:2)80.52.3(cid:2)77.51.9 (cid:2)81.22.2(cid:2)76.21.9(cid:2)94.02.6 Discordant(cid:2)782300(cid:2)78321
7
36993 742138900214868483 653 033
97162…388917879678246918 956 262
34202 233342133502122224 131 744
..... .................. ... ...
2)2)2)2)1)2)5)3)3)2)2)6)4)2)3)2)4)4)3)5)8)3)4)2) 4)3)4) 8)4)4)
1.1.1.2.3.4.3.1.1.1.1.2.4.1.1.1.2.1.2.1.2.1.1.1. 1.1.1. 1.1.1.
(((((((((((((((((((((((( ((( (((
964548527830183649227161 797 192
122111221222312212221222 214 399
111111111111111111111111 111 822
000000000000000000000000 000 211
........................ ... ...
(3.2)(2.4)(5.6)(31.6)(13.8)…(14.7)(3.3)(3.3)(3.1)(2.9)(9.4)(23.2)(3.3)(3.2)(2.1)(33.5)(5.0)(17.9)(6.2)(10.4)(4.3)(6.5)(2.5) (7.1)(3.6)(8.5) (2.5)(2.9)(3.3)
78857 888899188859789788 889 806
00000 000000200000000000 000 211
..... .................. ... 4.1.1.
2.9)2.1)5.5)31.5)13.4) 14.3)3.1)3.1)2.9)2.6)9.0)22.8)3.1)3.0)1.7)33.4)4.8)17.8)6.0)10.0)4.2)6.4)2.2) 6.9)3.3)8.3) 1.8)2.6)3.0)
(((((…(((((((((((((((((( ((( (((
41432 256739296981155542 090 552
57623 788406777792176469 563 915
44434 444455144425445444 444 066
00000 000000100000000000 000 100
..... .................. ... ...
2)2)2)2)1)2)5)3)3)2)2)6)4)2)3)2)4)4)3)5)8)3)4)2) 4)3)4) 8)4)4)
1.1.1.2.3.4.3.1.1.1.1.2.4.1.1.1.2.1.2.1.2.1.1.1. 1.1.1. 1.1.1.
(((((((((((((((((((((((( ((( (((
653052706866765280302602 974 304
718666281212441147698567 800 577
3.9.0.6.7.4.0.1.5.8.1.3.6.4.1.9.7.7.1.1.5.2.9.2. 8.4.8. 3.7.7.
878888888788788787888878 786
2.9)1.6)1.8)6.0)13.4)11.4)14.3)3.1)2.6)2.1)1.8)9.0)9.3)2.9)2.5)1.7)7.5)3.6)7.5)4.2)10.0)2.6)3.5)2.2) 4.0)2.5)3.0) 1.8)1.7)1.9)
(((((((((((((((((((((((( ((( (((
556546555556655555556555 555 167
000000000000000000000000 000 100
........................ ... ...
2)2)2)8)1)2)5)3)3)2)2)6)6)2)3)2)1)4)2)5)8)3)4)2) 4)3)3) 8)4)4)
1.1.1.1.3.3.3.1.1.1.1.2.2.1.1.1.2.1.2.1.2.1.1.1. 1.1.1. 1.1.1.
(((((((((((((((((((((((( ((( (((
610752704076081236702432 937 383
709562280132430150668317 393 567
3.9.9.4.7.0.0.1.5.8.1.3.2.4.1.9.5.8.0.1.5.2.9.2. 8.3.7. 3.7.7.
877888888788888787888878 786
2)2)4)2)4) 0)2)7)2)2)3)0)1)3)1)0)6)5)4)8)3)6)2) 3)3)3) 9)3)7)
(((11 2((((16(((1(1(((2( ((( 322
7785808(0(…4(746481832((17673834(936(7171765(82 818094 4(4(4(
59 6 90 5 5 3 787
1 9 577
3 1
0)1)2)2)4) 1)3)3)0)1)9)8)3)1)1) 0)2)4)1)2)3)2) 9)0)8) 2)6)4)
75323 4777690774 1542055 680 356
((193 3((((14((( 141211( 1(2 (((
33(((…(2019((795 ((((((8 (6( 179
35857 8237064979 9972515 645 957
(cid:2) 135 511(cid:2) 281 8777821 5 6 767
91 49 22 4(cid:2) (cid:2) 1 1
(cid:2)(cid:2) 1 (cid:2) (cid:2)
1.1)1.2)1.0)1.6)2.3)2.6)3.0)1.0)1.0)1.2)1.0)2.1)2.1)1.1)1.0)1.2)1.8)1.2)1.8)1.3)2.2)1.1)1.1)1.0) 1.3)1.0)1.7) 29.1)10.8)11.2)
(((((((((((((((((((((((( ((( (((
432473954977492610579793 201 748
6.0.9.5.2.9.6.8.5.1.8.6.7.5.9.0.5.2.9.8.4.7.0.7. 1.6.4. 0.7.3.
787777777877777878777787 879 188
677
1
(1.0)(.9)(1.0)(1.4)(2.3)(2.6)(2.9)(1.0)(1.0)(1.0)(.9)(2.0)(2.1)(.9)(1.0)(1.0)(1.6)(1.2)(1.7)(1.2)(2.1)(1.0)(1.1)(1.0) (1.1)(1.0)(1.3) (28.3)(10.4)(10.7)
704459922171489990128963 536 382
6.1.9.5.3.8.9.8.5.2.8.6.6.5.8.0.4.2.9.8.3.7.0.7. 1.6.4. 8.0.3.
787777777877777878777787 879 898
577
1
(.9)(.9)(1.0)(1.6)(2.2)(3.2)(2.8)(1.0)(1.0)(1.0)(.9)(2.0)(3.6)(.9)(1.0)(1.0)(1.7)(1.2)(1.8)(1.2)(2.1)(1.0)(1.1)(.9) (1.1)(1.0)(1.3) (26.0)(10.1)(10.4)
593017833890899034527655 221 221
6.0.9.4.3.5.9.8.5.1.8.7.3.5.8.1.3.2.8.8.4.7.0.7. 1.6.4. 7.7.3.
787777777877877878777787 879 088
677
1
745252297383377768898363 801 169
894511220931182152232502 644 844
...............1......... ..1. ...
159554695787745927422730 764 901
2176 0579 30151211722 116 265
795 2 05 929 1 1 3 2 332
1 1 1 1
676972647212456156835763 924 758
593233256015518992894390 712 330
8031 7726 176 4 2 730 489 11
21 11 1 1 1
711744618498158340874241 198 200
210452012101711001331142 303 322
(cid:2)..1..(cid:2).1.(cid:2)...(cid:2)..1.1........1.(cid:2)... ... 1.(cid:2)..
G-UV15:1.12.13.14.15.16.17.18.19.110.1011.1012.1013.1014.1015.1016.1017.1018.1019.1020.1021.1022.1023.1024.10G-UV16:1.12.13.1G-UV26:1.12.13.1
7
8 W. R. GRIFFIN ET AL.
Table2. 40Ar/39ArResultsfromAmphibole,Phlogopite,andNepheline-K-FeldsparMineralSeparatesfromBalcones
Igneous Province Samples, with Applicable Notes
39Ar
wt% t t released t
int p ic
Sample Lithology Mineral Runno. mg K (Ma) (Ma) (%) (Ma) MSWD
G-UV03 Phonotephrite Amphibole 71C47 6.73 .2 77.8 77.4(cid:2)1.2 67 79.8(cid:2)1.7 26
G-UV05 Phonolite Amphibole 72B21 9.5 .9 76.9 76.2(cid:2).6 82 76.0(cid:2).5 5
G-UV10 Phonolite Amphibole 77A2 1.41 1.1 77.9 78.0(cid:2)1.3 96 77.3(cid:2).8 1
G-UV10 Phonolite Amphibole 77A2 77.6(cid:2).7 66
G-UV16 Phonolite Amphibole 77A3 5.89 .96 78.6 78.4(cid:2).8 97 77.8(cid:2).7 2
G-UV16 Phonolite Amphibole 77A3 78.1(cid:2).4 60
G-410 Phonolite Amphibole 72B17 4.36 1.2 71.6 76.3(cid:2).4 49 76.4(cid:2).7 5
G-UV26 Phonolite Amphibole 77A5 5.78 1.1 77.9 77.2(cid:2).6 97 76.9(cid:2).6 2
G-UV26 Phonolite Amphibole 77A5 77.0(cid:2).3 71
G-UV30 Olivinenephelinite Biotite 77A6 3.88 5 83.4 83.5(cid:2).5 75 83.9(cid:2).5 2
G-UV33 Nephelinebasanite Biotite 77A7 5.3 5.5 83.1 83.5(cid:2).5 86 83.6(cid:2).6 2
G-UV34 Nephelinebasanite Biotite 77A8 .37 5.5 81.5 81.5(cid:2)1.6 85 81.3(cid:2)2.8 2
G-UV03 Phonotephrite Nepheline-K-feldspar 71C48 5.13 4.3 73.1 74.4(cid:2).4 56 74.0(cid:2).8 60
concentrate
G-UV03 Phonotephrite Nepheline-K-feldspar 71C49 10.4 3.9 73.0 74.8(cid:2).4 55 73.5(cid:2)1.1 204
concentrate
Note. t pintegrated(ortotal-gas)age;derivedfromthesummationofallfractionsoftheincremental-heatinganalysis.t p
int p
plateau age; derived from the incremental-heating age spectrum. All quoted age uncertainties are at the 2j level. t p isotope-
ic
correlationage;derivedfromthe36Ar/40Arversus39Ar/40Arregression.ThereliabilityofthelistedagemustbeevaluatedtheMSWD,
amounts of Ar, number of gas fractions used, and general age spectrum results. These regressions are included to augment and
illustratethegeneralreleasepatternbutgenerallydonotprovidemorereliableagesthantheagespectra.
nationofeachzirconasrevealedinCLimages(fig. standard (PM-1) that has a 40Ar/39Ar age of 165.3
3).Spotswerechosentoavoidcracksandespecially Ma; an uncertainty of (cid:2)1% is assigned to thisage
dark (high-uranium) orbright(low-uranium)zones in order toallowforuncertaintiesinthestandards
withineachgrain.AnalyticalresultsforU-Pbanal- againstwhichPM-1wascalibrated.Theageforthis
ysesofzirconsfortheBIPsamplesaresummarized monitor was determined by simultaneous cross-
in table 1. calibration with several monitors, including the
The 40Ar/39Ar measurements were performed in FishCanyonTuffbiotitestandard(FCT-3),withan
the Radiogenic Isotopes Laboratory at Ohio State age of 27.84 Ma. All factors and constantsare pro-
University. The general procedures have been de- vided in table A3, available in the online edition
scribed previously (see Foland et al. 1993 and ref- or from the Journal of Geology office.
erencestherein),exceptfortheuseofanewnoble- Threekindsof40Ar/39Aragesarereportedintable
gas mass analysis system. Sized aliquots of 2. The total gas age (t ) represents an integrated
int
phlogopiteandamphiboleseparateswereirradiated agederivedbysummingallgasfractions.Totalpla-
under a high neutron flux for about 25 h in the teau age (t ) is determined by heating the sample
p
McMaster Nuclear Reactor at McMaster Univer- to a given temperature and deriving an apparent
sity in Hamilton, Ontario. The aliquots were sub- agefromthegasfractionreleased.Aseriesofheat-
sequently heated incrementally to successively ing steps at increasing temperature produces a se-
higher temperatures, using a custom-built, resis- ries of gas release increments through which iso-
tance-heating, high-vacuum, and low-blank fur- chrons are drawn. Meaningful plateau ages must
nace. Step-heating was continuous, with ramp have the following characteristics: (1) plateau gas
times from one temperature to another of about 1 ages must be well-defined plateaus by contiguous
min and with dwelltimesofabout30 minateach release of 150% of total 39Ar, (2) plateau gas frac-
temperature. These incremental-heating fractions, tions must form well-defined isochrons, (3) iso-
typically about 20–25 steps, were analyzed by chron ages and plateaus must be concordant, and
static-gas mass analysis with a MAP 215-50 mass (4) 40Ar/36Ar must not differ significantly from the
spectrometer. atmospheric value of 295.5(cid:2)0.5 (Lanphere 1999).
Corrections for interfering reactions producing Isotope correlation, or reverse isochron, age (t ) is
ic
Ar from K, Ca, and Cl were made using factors derivedbyplotting36Ar/40Arversus39Ar/40Ar.With
determined from interference monitors that were the ordinate intercept representing the inverse of
irradiatedalongwiththemineralaliquots.Theage theatmosphericArratioandtheabscissaintercept
monitor used was an intralaboratory muscovite representingthe39Ar/40Arreleasedduringtheheat-
JournalofGeology GEOCHRONOLOGY OF THE BIP 9
Figure 3. Cathode luminescence images of typical zircon grains from Balcones igneous province samples. Dashed
circlesrepresenttheionablationspotsfromwhichageswerederived.Zircongrainsexhibitednormalcompositional
banding (G-UV03 grains 1, 9; GUV05 grains 1, 3), as well as complex compositional banding, although both types
produced concordant ages.
ing steps, ages are derived from the slope of the 289(cid:2)6, 271(cid:2)18, 295(cid:2)40, 296(cid:2)6, and 282(cid:2)
isochron. The t agesprovidethemostmeaningful 7).Analysesofthephonoliteamphibolesproduced
p
resultsbecausetheymustadheretothepreviously t ages ranging from 71.6 to 77.9 Ma and an age
int
defined criteria and are derived from a pooled age of 77.8 Ma from the phonotephrite (fig. 4; table 2);
of multiple step-heating increments.Gasfractions errors are not calculated for t ages. The t ages
int p
released at the highest- and lowest-temperature rangedfrom76.2(cid:2)0.6to78.4(cid:2)0.8Ma(2j)forthe
stepstendtoproducehigheruncertaintiesthanthe phonolite amphiboles, and the age was 77.4(cid:2)1.2
intermediate heating steps and can be eliminated Ma(2j)forthephonotephriteamphiboles.Plateaus
from the plateau age calculation (e.g., Lanphere rangeinwidth(total39Arrelease)fromalowof49%
1999). The t ages are not as precise as the t ages (phonolite sample G-410; all other phonolites
ic p
becausetheyarerelatedtohowwelltheisochrons 160%) to 97% in amphibole from the felsic rocks.
fit the data. The tint ages are the least preferredbe- The tic ages range from76.4(cid:2)0.7 (MSWDp5) to
cause they are derived from the summation of all 77.8(cid:2)0.7(MSWDp2)forthephonolites,andthe
gasreleasefractionsanddonotexcludethehighest- age was 79.8(cid:2)1.7 (MSWDp26) for the phono-
andlowest-temperatureheatingstepsthattypically
tephrite (fig. 4; table 2).
contain the greatest uncertainty.
Phlogopite separated from two nepheline bas-
anites produced t ages of 81.5 (initial
int
40Ar/36Arp300(cid:2)7) and 83.1 Ma (initial
Results
40Ar/36Arp299(cid:2)11), t ages of 81.5(cid:2)1.6 and
p
40Ar/39Ar Geochronology. Amphiboles were sepa- 83.5(cid:2)0.5 Ma (2j; these plateau ages consist of
rated from five phonolite and one phonotephrite 75%–86% total 39Ar release), and t ages of
ic
sample, phlogopites were separated from one oli- 81.3(cid:2)2.8 (MSWDp2) and 83.6(cid:2)0.6
vine nephelinite and two nepheline basanites, and (MSWDp2)Ma.Phlogopiteseparatedfromtheol-
a nepheline(cid:1)K-feldspar concentrate was separated ivinenepheliniteproducedat ageof83.4Ma(ini-
int
from a phonotephrite. Sample locations are given tial40Ar/36Arp298(cid:2)6),a t age of83.5(cid:2)0.5Ma
p
in table 3 and figure 2. Detailed results for the 11 (2j),andat ageof83.9(cid:2)0.5Ma(MSWDp2;fig.
ic
analysesare listed in tableA3andshowninfigure 4; table 2).
4.Resultsaresummarizedintables2and3.Results Nepheline separates were prepared for dating;
forthe11analysesarethatallsamplesyieldedini- however, the presence of K-feldspar in the more
tial 40Ar/36Ar, close to atmospheric (298(cid:2)5, evolved phonolites made the separation difficult,
310(cid:2)9, 300(cid:2)3, 298(cid:2)6, 300(cid:2)7, 299(cid:1)11, owingtothecommonphysicalcharacteristics(sim-
t
n
a
d
r
6,co 2, p
MA MA (cid:2)1edis MA(cid:2)2. grou
U-Pb(Ma) Xenocryst(cid:2)77.26.61W (cid:2)77.78.74W Single-grain83.64onexenocryst,on (cid:2)77.32.99W(cid:2)76.21.9,81.2(cid:2)94.02.6Nocoherentage Xenocryst
s
u
pl
k
Aroc
40K/oler(Ma)
40 wh
e
4039Ar/Aroncentrat(Ma) (cid:2)4.4 (cid:2)8.4 (cid:2)9.6(cid:2)8.29
nce c 74. 74. 80.80.4
vi
o
r
P
nesIgneous 4039Ar/Armineral(Ma) (cid:2)77.41.2amphibole (cid:2)76.2.6amphibole (cid:2)78.01.3amphibole(cid:2)77.6.7amphibole (cid:2)78.4.8amphibole(cid:2)78.1.4amphibole(cid:2)76.3.4amphibole(cid:2)77.2.6amphibole(cid:2)77.0.3amphibole(cid:2)83.5.5phlogopite(cid:2)83.5.5phlogopite(cid:2)81.51.6phlogopite (cid:2)72.41.15K-feldspar
o
c
al
B
omthe Fig.2ocation 13 35 6 7 7 1213 13 49 23 23 27 30 31 36365
r l
f
s
t
ul
s ee
Re nitnit
AllGeochronology Lithology OlivinenephelinitePhonotephrite PhonotephritePhonolite Olivinenephelinite Phonolite Phonolite PhonolitePhonolite Phonolite Phonolite Phonolite Phonolite Olivinenephelinite Nephelinebasanite Nephelinebasanite MeliliteolivinebasaMeliliteolivinebasaPhonolite
of
ail
et 225
D 000
000
3. e 0103 0305 06 10 10 1516 16 26 26 30 33 34 CB-CB-CB-
ble mpl UVUV UVUV UV UV UV UVUV UV 410 UV UV UV UV UV 01-01-01-
Ta Sa G-G- G-G- G- G- G- G-G- G- G- G- G- G- G- G- M-M-M-
10