Download A study of the ages of the Precambrian of Texas

JOURNAL
OFGEOPHYSICAL
RESEARCH
VOLUME67. NO. 10
SEPTEMBElt
1962
A Study of the Ages of the Precambrian of Texas
G. J. W•sssa•us•
California Institute o[ Technology,Pasadena
G. W.
Institute o/Geophysicsand Departme•t o• Geology
University o• California at Los Angeles
L.
T.
SILVER
California Institute o[ Technology,Pasadena
P.
T.
FLAWN
Bureau o[ EconomicGeology, University o/ Texas
Abstract. Age determinations
usingthe SrS7-RbS7,
Ar4ø-K4ø,
and Pb-U methodswere made
on samplesof muscovite,biotite, amphibole,microcline,and zirconfrom igneousand meta-
morphicrocksfrom the Franklin Mountains,Hueco Mountains,Pump StationHills, and
Carrizo and Van Horn Mountains. In addition ageswere determinedon a number of basement cores from Texas and New Mexico. The results show that a belt of rocks of varied
lithologyextendingfrom E1 Pasoto east of the Llano uplift are all of the sameage.The
generalage by the strontium and argon methodsis 1000 to 1090 m.y.; and by the leaduranium method on zirconsit is 1150 to 1200 m.y. This event is in the same time band as
the 'Grenville' orogenyin Canadaand the northeasternUnited Statesand possiblyshould
be consideredpart of the general 'Grenville' episode.All the data now available indicate
that the orogenicevent at about 1000to 1200m.y. is the mostwidespread
and pervasiveepisodeof Precambrianorogenyon the North Americancontinentfor which adequateevidence
has been presented.At least one and probably two older periodsof igneousactivity and
metamorphismoccurringat 1250 and 1400 m.y. are found in the northern regionsof the
Texas Precambrian basement. No evidence was found for any igneous event between
the early Paleozoicand the 1000-m.y.episode.
INTRODUCTION
(b) In the HuecoMountainsthe graniteis either
surroundedby unconsolidated
alluvial deposits
or in fault contactwith youngerbeds; general
regionalevidence,however,suggeststhat it is
pre-Bliss (Cambro-Ordovician)in age. (c)
Llano uplift area of central Texas and in a num- Rhyolitein the PumpStationHills is surrounded
alluvialdeposits,
and thereis
ber of separatedmountain uplifts in Trans- by unconsolidated
PecosTexas (Figure 1). Stratigraphicevidence no convincingstratigraphicevidencepertaining
to its age.On the basisof regionalgeologyearlier
as to the age of these rocks is as follows:
The purposeof this studyis to attempt to use
mineral agesto correlateigneousand metamorphic rocks'of Precambrianage in Texas.
Precambrianrocksin Texas crop out in the
1. Central Texas graniteswhich intrude meta-
sedimentaryrocksare overlainby Upper Cam-
investigatorssuggestedthat the rocks are Precambrian [King and Flawn, 1953; Masson,
brian strata (Hickory sandstonemember of the 1956]. (d) Metamorphicrocksand pegmatites
Riley formation [Cloud, Barnes, and Bridge, in the Carrizo and Van Horn Mountains are
1945]).
overlainby Permian(Wolfcamp)beds,but re2. In Trans-Pecos
Texasstratigraphicrelations gional stratigraphicevidencesuggeststhat the
differin the separatemountainranges.(a) In the rocks are pre-Van Horn sandstone(PrecamFranklinmountains,granite,volcanicrocks,and brian?), which is pre-Bliss (Cambro-Ordovimetamorphic rocks lie beneath the Cambro-
cian) in age.
OrdovicianBliss formation [Harbour, 1960].
4021
A number of chemical lead-uranium and lead-
4022
WASSERBURG,
WETHERILL, SILVER, AND FLAWN
N
E
M
W
E
X
I
C
I
S
O
ION
,1114
I
•1•1•
Sample
localities
and
numbers
I
$cal•'
I
.
ModifiedfromF,0 I. The Unlvers,I
V o! TelLO•
Publicolion
5:)01
Fig.1. Map showing
the exposures
of Precambrian
rocksin Trans-Peeos
Texasandthe
sample localities.
thorironagesup to 1100millionyearshavebeen clearlyshowthe existenceof plutonieigneous
determined on the rare-earth minerals from the activityat about1100m.y.in the south-central
BarringerHill pegmatitein the Llano region United States. The correlation of this occur[Bartell, 1917; Holmes,1931]. More recently reneewith 1000-m.y.plutonit activity in eastern
ageestimates
havebeenmadeon zirconsfrom North America some 1200 miles distant was not
madeby theseworkers.
the Llano regionand from well coresin west specifically
for agedetermiTexasby the 'Pb-a' technique
[Jaffe,Gottfried, In the presentstudysamples
Waring,and Worthington,1959]. Owingto the nation were obtainedboth from the exposedPremanydifficulties
both technicaland inherentin cambrian and from basement cores. Samples
thisnonisotopic
method,the resultswill not be were collected from the various localities with
the previousgeologic
studiesas a guide,and
considered in detail.
Modern age determinations
using Ar4ø-K
'ø, basementcoreswere selectedwith the study by
Sr87-Rb
87,Pb'ø•-U2.
•, and Pb'ø*-U
•5 methods
have Flawn [1956] as a guide,the purposebeingto
beenmadeby severalworkersonmineralsin the establish time correlations and compare the
methods.
graniticrocksandassociated
pegmatites
in the variousgeochronometric
Llano area [Aldrich,Wetherill,Davis, and Til- Results
ton, 1958;Goldich,Nier, Baadsgaard,
Hoffman,
The results for the exposedrocks are preand Krueger,1961; Zartman,1962,1963].
The da•a obtained by Aldrich et al., in the
sentedby area, after which the basementcore
Llano regionare summarized
in Table 1. They data are given.Thin sectionsof the rocksand
AGES IN THE
PRECAMBRIAN
OF TEXAS
4023
TABLE 1. Age Determinationsfrom the Llano Region
Locality
Petrick Quarry
(granite)
Mineral
Biotite*
Zircon*
(pegmatite)
Lone Grove
Pluton
Biotite]
•
Biotite, feldspar$
Ar4ø/K
4ø
Sr87/Rb87 pb2o6/u2s8pb2oT/u2s5 pb2o•/pb•o6
1060
1100
1(•)
li•(•
1060
.........
950
.........
990
1070
.........
(granite)
* After Aldrich, Wetherill,Davis, and Tilton [1958].
• After Goldich,
Nier, Baadsgaard,
Hoffman,andKrueger[1961].
•:After Zartman[1962a].Averageof severaldeterminations
by bothmethods.
All ageshavebeenrecalculatedfor the decayconstantsusedin this paper.
the mineralseparatesare describedin the ap-
tial white micrographicintergrowth. This rock
pendix.
also shows distinct
freshest material.
FRANKLIN
MOUNTAINS
iron oxide stains on the
A sampleof feldspar,TH-6, wastaken from a
Precambrian rocks are exposedin North microcline-perthite-quartz
pegmatitewhich was
FranklinMountainin thevicinityof E1Paso(see seen to cut the granophyric granite. This dike
Figure 2). These have been describedin a recent
has a thickness of about a meter. No. fresh mica
publicationby Harbour [1960]. He has pre- was found in this pegmatite.
senteda sedimentarysequence
in this area comIn addition to these materials a sample of
prisedof the followingunits in order of decreas- a friable tourmaline-biotite-plagioclase
schist,
ing age: (1) The Castnerlimestone,possibly TH-2, was collected. This material was not
equivalent to the Allamore limestone of the Van foundin the measured
sectionreportedby HarHorn area. Several diabase sills are included in
bour for a traversefurtherwest (seeFigure2).
the Castner. (2) The Mundy breccia,which The unit is about 3 meters, thick and lies berests unconformablyon the Castner. (3) The tween a thick section of Castner limestone on the
Lanoria quartzite.On top of this unit is (4) a west and a diabase sill to the east. The Castner
rhyoliteextrusive.(5) The youngestrock in the showsbeautifully developedstromatolitestrucarea underlyingthe Bliss sandstoneis a massive turesandmud cracksat thislocality.The top of
granite,whichis seento surroundand engulf the limestone here shows distinct contact metalarge blocks of the sedimentarysection.This morphic effectsfrom the granite. The diabase
granite showsa variety of phases,the larger sill containsabundantbiotite near the granite
exposuresbeing a pink-white biotite granite, contact. The origin of the schistlayer is not
whichis typicallyrather friable.Onephaseis a understood.
gray-whitebiotite granite, which locally conThe ages for the Sb87-Rb87and Ar•ø-K•ø
tains numerous reacted inclusions and shows a
methodson variousmineralseparates
are prejointingstructurewith the fracturesspacedat sentedin Table 2; the agesfor the zircons',
in
approximately 30 to 60 cm. These fractures all Table 3. An estimated
maximumerror is given
showiron oxidestainsand a pale green dis- for each of the ages. This estimate is based
colorationevenin a freshquarry face.We infer solely on the analytical data without considerthis alterationto be a deutericeffect.A sample ing possibleerrors in the decay constants.A
of this phase,TH-1, wastaken from the interior more detaileddiscussion
of errorsis presented
of a boulderof about 120-cmdiameter(see in the appendixunder analyticalprocedures.
Figure 2). Samplesof a third phase,a granoThe biotite and microclineseparatedfrom
phyric biotite-hornblende
granite,TH-5, were granite TH-1 gave the followingresults' the
collectedin FusselmanCanyon. This rock has strontiumageof the biotiteis considerably
less
a spottedappearanceproducedby the isolation than the argon age, and both are lessthan the
of dark gray microclinecrystalsby an intersti- strontiumageof the coexistingmicrocline.The 6
4024
WASSERBURG,
WETHERILL, SILVER,AND FLAWN
z
i•_
x
lu.i
'..
o..
AGES IN THE
PRECAMBRIAN
TABLE 2. Age Determinations from the
Franklin Mountains near E1 Paso, Texas
Age, m.y.
No.
TH-1
Rock
Granite
Mineral
K-Ar
Biotite
Rb-Sr
980 4- 20
Feldspar
930 4- 20
1040 4- 30
OF TEXAS
4025
and a microclinefrom a crosscuttingpegmatite
TH-6. Strontium ages were determined on the
feldspars,and an argon age was determinedon
hornblende[Hart, 1961].
The youngerpegmatitegivesa strontium age
of 1087 m.y., which is greater than the results
on the other feldspars.The differencebetween
the feldsparsfrom TH-6 and TH-5 is well out-
side of experimentalerror. The differencebe-
TH-2
Biotite
TH-5
schist
Granite
TH-6
Feldspar
Pegmatite Feldspar
Biotite
Hornblende
860 4- 20
710 4- 50
1000 4-20
970 4- 50
1090 4- 20
per cent differencebetweenthe feldsparand the
biotite is well beyond analytical error. This age
pattern is rather typical of almostall the data.
A biotite separated from the friable schist
TH-2 gave discordantages,the argo.nage being
considerablylessthan that obtainedon the biotite from TH-1. In both casesit was apparent
tween TH-6 and TH-1 feldsparsis just at the
limits of error. While this disagreement is
not large, it is certainly real and is yet an-
other exampleof the 'pegmatite-countryrock'
discrepancyreportedby previousworkers [Wassetburg,Wetherill, and Wright, 1959; Wetherill,
Davis, and Tilton, 1960; Gerling and Polkanov,
1958].
Two. zircon concentratesof differing uranium
content were extracted from the granophyric
granite TH-5 for Pb-U analysis.The lechniques
followed are described by Silver, McKinney,
Deutsch, and Bolinger [1962]. The analytical
results are given in Table 3, and the parent
daughterratios are plotted in Figure 3 in a consectionsof the rocks,or in the grain mountsof cordia diagram [Wetherill, 1956]. A straight
the separatedminerals, which might indicate line drawn through these points intersectsthe
that the biotite was unsuitable for age determi- concordiacurve at about 1200 m.y. It has been
nation.
shown by Silver and Deutsch [1961] that disSamples of hornblendeand microcline were cordant U-Pb systemsin cogeneticzirconsmay
separated from the granophyric granite TH-5
generatelinear patternson a concordiadiagram.
in the field that the rocks were not 'fresh.' Nonetheless no characteristics were observed in thin
TABLE
3.
West Texas and Llano Zircons
Apparent Ages, m. y.
Sample
pb•o6/u•8
pb•o7/u•5
pb•oT/pb•o6
West Texas Samples
Micrographic granite (TH-5), Franklin Mrs., E1 Paso Co.
Fraction
Fraction
A
B
880 4- 20
770 4- 20
950 4- 20
860 4- 20
1095 -V 20
1080 4- 20
1065 4- 20
1005 4- 25
925 4- 20
1105 4- 20
1050 4- 25
970 4- 20
1175 4- 25
1140 4- 20
1065 4- 20
745 4- 20
875 4- 25
1220 4- 40
950 4- 25
990 4- 15
1070 4- 25
970 4- 20
1020 4- 20
1120 4- 20
Rhyolite porphyry (TV-13), Pump Station Hills,
Hudspeth Co.
Fraction
Fraction
A
B
Granite well core, Phillips Puckett //lc, Pecos Co.
Muscovite-oligoclase-quartzschist (TV-1), Van Horn
Mrs., Hudspeth Co., fine fraction
Llano Samples
Town Mr. granite, Petrick quarry [Tilton et al., 1957]
Town Mr. granite, Granite Mr. quarry, [Silver et al.,
in preparation]
Constants:•3s = 1.537 X 10-•ø;X•35= 9.72 X 10-•ø;(atomsU•S•)/(atomsU TM)= 1/137.8.
WASSERBURG, WETHERILL,
4026
SILVER, AND FLAWN
0.21
0.20
0.19
0.18
0.17
0.16
0.15
0.14.
0.13
0.121.
2
1.3
1.4
1.5
t.6
1.7
1.8
1.9 2.0
Z.I
Z.Z
23
2.4
pb2O7/u
235
Fig. 3.
The Pb•øø/U•S
and Pb2ø?/U•ratiosfor samplesof zirconsfrom the Precambrian
of
Texas.
Throughoutthis paperthe linearpattern of the
cloudingand in generalappearsunaltered.A
accumulated zircon data is used as an argument
strontiumage of 1045 m.y. was determinedon a
sample (TV-14) (Table 4).
for contemporaneity.
The intersectionof the
patternwith the concordia
curveis interpreted
as an originalage of crystallizationfo.rthe zircons.A more completediscussion
of this intersectionis givenin a later section.The Pb2ø•-Pb
'•ø•
age must be taken as minimal assumingany
(nonfractionating)form of lead loss.
The resultsobtainedin this area by the various methods indicate an event at about 1100
PUMP STATION •ILLS
A sampleof a granophyricrhyolite porphyry
was collectedfrom this locality. This rock is ex-
tremely tough and resistantbut in thin section
shows considerablealteration. Samples of the
other phasesthat have been reportedwithin the
Pump Station Hills area [Masson, 1956] were
not collected.A 2.5-cm cube of rhyolite, TV-13,
was crushedto pass 80 mesh, and a split was
m.y. Ito.wever,they scatter far outsideof experimentalerror. The only visibleevidencefor
a postemplacement
metamorphism
is the altera- taken for whole-rock strontium analysis. Antion and recentweatheringpreviouslydescribed. other strontium age was determinedon a sample
The block faulting in this area doesnot appear
to have causedmore generalmetamorphiceffects, but post-Paleozoic
intrusive activity is
evident in the regionand its local effectscannot be estimated.
of microclineseparatedfrom another specimen.
The results,given in Table 4, are in goodagreeTABLE 4. Age Determinations from the
Hueco Mountains and Pump Station Hills
I-IuEco MOUNTAINS
NO.
A samplewascollectedfrom a low-lyingknob
of red porphyritic hornblendegranite from the
foothills of the Hueco Mountains.
No
fresh
pegmatitescontainingmica were found in lhis
locality. The graniteshowsa tendencyto granular disintegration,presumablydue lo weathering. The granitecouldbe described
as'inelastic.'
In thin sectionthe microclineshowsonly slight
Locality
TV-14
Hueco
Mrs.
TV-13
Pump
Rock
Mineral
Age, m.y.,
Rb-Sr
Granite
Feldspar 1050 • 60
Rhyolite
Feldspar 1060 4- 40
Station
Hills
Whole
rock
1020 4- 60
AGES IN THE
PRECAMBRIAN
TABLE 5.
ment and indicate a strontium age of 1060 m.y.
Two zircon separates of differing uranium
content were obtained from a hand specimen
of this porphyry.The resultsare givenin Table
3 and Figure 3. The two fractions give Pb2ø7Pb2ø•agesof 1175 and 1140 m.y. and lie closeto
OF TEXAS
4027
Age Determinations from
the Carrizo
Mountains
Age, m.y.
No.
Rock
Mineral
K-Ar
the sameline as the Franklin Mountain samples TV-10-a Schist
Biotite
(Figure 3). These results clearly indicate the TV-10-b Amphibo- Hornlite
blende
Precambrianage of the rocks in the Pump StaTV-12-b
TV-12-c
tion Hills.
This area is known to be structurally more
complexthan any of the others investigatedin
this work. It is bounded to the north by the
Hillside fault and is possibly involved in the
Streeruwitz overthrust in this region. Flawn
[King and Flawn, 1953] has describedthe Carrizo Mountain group as a seriesof steeplytilted
metasedimentaryand metaigneousrocks. The
sedimentary rocks were intruded by rhyolites
and basicdikes,and the sectionwassubsequently
deformed.The rhyolites,markedly deformedand
sheared,are describedas metarhyolites.Toward
the northwest of the map area (Figure 4) the
metarhyolite is converted into a mylonite. The
metasedimentary rocks are a relatively lowgrade metamorphicassemblage,although there
is distinct evidencefor a polymetamorphichistory.
Samplesof metarhyolite TV-12-c and an amphibolite schist TV-12-b were collected from
the northwestern part of the area, where the
rhyolite showsprofound evidenceof cataclasis
and has a strong planar structure and linearion.
The schist.is only locally present and presumably is a metaigneousrock. A 2.5-cm cube of
the metarhyolite was crushedto. pass 80 mesh,
•070 m.y.
In addition to these materials, samplesof a
fine-grainedhornblende-biotite-garnet
schistTV10a and a hornblende-epidoteschist TV-10b
were collected from the southern part of the
area, farther from the locus of the overthrust.
TV-10b occursin a small dike-like body about
13 cm thick. Biotite was extracted from TV-10a,
and hornblende from TV-10b.
The results shown
in Table 5 indicatean argonageof 900 m.y. for
580 -v 20
900 + 20
960 4- 60
rock
1070 :k 70
the hornblendeand a distinctly younger result
on the biotite. The strontium age on the biotite
is much less than the argon age.
These measurementshave a pattern quite
similar to that obtained in the Franklin
Moun-
tains, but becauseof the polymetamorphichistory in the Carrizo Mountain area it is rather
difficult to make any simplestatements,and the
conclusions.
must be regarded as tentative. It is
evident that these rocks are at least 1000 m.y.
old. The apparentage may reflectthe effectsof
recrystallization during shearing, and the true
age may be considerablyhigher. It is also impossibleto draw any conclusions
about the time
of thrusting from thesedata. If the biotite from
TV-12-b lost its radiogenicstrontiumat the time
of shearing, the time of shearing would be
greater than 960 m.y., but whether complete
loss would occur under these conditions is not
known. Furthermore, the shearingneed not have
been concurrent with the thrusting. Although
experiencewith deformedrocks of this type is
very limited, we tentatively suggestthat the
whole-rock strontium age on the rhyolite is the
most reliable estimateof the age of the rhyolite.
stron-
tium age determination.A biotite separatewas
obtained from the schist.The results,given in
Table 5, indicate an age in the neighborhoodof
780 + 20
Biotite
Whole
rhyolite
CARRIZO •¾[OUNTAINS
and a fraction was taken for a total-rock
Schist
Meta-
Rb-Sr
VA>•
I-Ioa>•
MO•?NTA•NS
A thick sectionof middle-grademetamorphic
rocks intruded by pegrnatitesis exposedin the
Mica Mine area of the Van Horn Mountains,
an area describedby Flawn [1951, 1956]. These
rocks are unconformably overlain by Permian
(Wolfeamp) rocks. The metamorphicsequence
is comprisedof a feldspathie quartzite, a feldspathic muscoviteschist, and biotite schist and
amphibolite.The pegmatitesoccur in bodiesof
varied shapethat conformto and cut acrossthe
foliation
of the host rock.
A detailed comparisonof the various mineral
4028
WASSERBURG, WETHERILL,
SILVER, AND FLAWN
30 o
Kc
Ph
(
"-.,
Qal
04'
Ph'
J
Qal
"•'/
"'IIiii'i!:.
:Cl
:
ß
.,,-,/
p.CCq
00'
Ph'
..'
/
Qal
:
/
105000'
I04ø55
.After Plote1,The University
SCALE
of Texos Publicorion No. ,5501
EXPLANATION
j Qal
Alluvialdeposits
Intrusiveigneousrocks
p•Cq
Huecolimestone
Van Horn sandstone
Metarhyolite
Limestone
Quartzoo
feldspaJhic
rocks
Chlorite-mico schisl
+TV12
Cox sandstone and
Campogrande
Granodiorite
Phyllite
Samplelocalityand number
limestone
Amphibolite
Mixed units
Fig. 4. Geologicmap of the Carrizo Mountains (after Flawn), showingthe samplelocalities
Fig. 5. Geologic map of Mica Mine area in northwest Van Horn Mountains (after Flawn),
showing sample localities.
4030
WASSERBURG, WETHERILL,
ageswas undertaken at this locality becauseof
the wide variety of minerals in the various
lithologies and the relative freshness of the
rocks. The locality provides an excellent opportunity to compare the ages of several coge-
netic metamorphicminerals and a presumably
late-stage pegmatite. The sample localities are
shown in Figure 5.
A sample (TV-1) of the biotitie muscovitealbite-oligoclaseschist was collected.This rock
is quite friable, presumablybecauseof the high
mica content. Mineral separates of muscovite,
biotite, and zircon were made,and in additiona
total-rock sample was obtained from a 4-cm
cube.Argon and strontium ageswere determined
on the mineral separates,and a strontium age
was determined on the total-rock sample. The
zircons are variable in form and color and show
markedly irregular forms due to corrosion.
The general suite has the appearance of
detrital origin modifiedby reactionin the metamorphic environment.
Hornblende sampleswere obtained from two
amphibolites, TV-2 and TV-6. These samples
are of very high purity, aad the argon agesare
unaffected by any biotite or K-feldspar impuffties.
A sample of a coarse-grainedbiotite schist
TV-5 was collected,and the biotite separatewas
analyzed. The results of these analyses (TV-1,
TV-2, TV-5, TV-6) on the metamorphicrocks
are presentedin Table 6. Except for the TV-1
biotite sample, all the ages are in excellent
agreement,the spread being just larger than
the analytical error. The TV-1 biotite has a
TABLE 6.
SILVER, AND FLAWN
rather low argon age and a distinctly low strontium age.
The maximum age obtained on these meta-
morphic rockswas 1030 m.y. by the strontiumrubidium method on a muscovite from TV-1.
The total rock age on TV-1 was somewhat
low compared with the muscovite result. The
zircon concentrateyieldeda very discordantset
of ages (see Table 3), the Pb'•ø•/Pb
2ø•age being
the highestobservedamongthe zirconsstudied.
The age pattern for the zircon stronglysuggests
that they retaineda.t least part of their original
predetrital daughter products. The great discordance indicates that at least some of the zir-
cons originated considerably more than 1220
m.y. ago. This is reflected in the location of the
point representingthe TV-1 zircons.in the concordia diagram in Figure 3. If the detrital
sources of the other minerals in TV-1
were riot
significantlydifferent from those of the zircons,
we must conclude from the whole-rock
strontium
age of 970 m.y. that the strontium of this system
was
mixed
with
a
common
strontium
reservoirat the time of metamorphism.
Samples of four pegmatites were collected:
TV-3, TV4, TV-118, TV-252. TV-3 muscovite
was obtained from a single crystal of 17-cm
diameter. A cleavagefragment of microclinewas
taken from the same outcrop.A fraction was
ground to pass40 mesh (total F); the remainder
was ground, and the --28 +40 mesh fraction
was taken for analysis.The strontium and rubid-
ium contents of these two feldspar fractions
are slightly different. The resulting ages are in
good agreement.The argon age of the coexist-
Age Determinations from the Mica Mine Locality near Van Horn, Texas
Age, m.y.
No.
Rock
TV-1
Muscovite
Mineral
schist
Muscovite
Biotite
Whole rock
K-Ar
980 q-20
940 q- 20
TV-2
Amphibolite
Hornblende
1000 q- 20
TV-5
Biotite
Biotite
1020 q-20
TV-6
TV-3
Amphibolite
Pegmatite
990 q- 20
1020 q- 20
TV-4
TV-118
TV-252
Pegmatite
Pegmatite
Pegmatite
Hornblende
Muscovite
Feldspar (sized)
Feldspar (total)
Muscovite
Muscovite
Biotite
schist
1090 q- 20
1010 q- 20
Rb-Sr
1030 q- 30
890 q- 30
970 q- 50
990 q- 30
1090
1050
1040
1060
1090
970
qqqqqq-
20
30
30
20
20
20
AGES IN THE
PRECAMBRIAN
ing mica agrees with the strontium ages of the
feldsparsbut is significantlylessthan the st.rontium age of the mica. The feldsparagesand the
muscovite strontium age agree just within experimental error. The agreement of the two
feldsparagesindicatesthat there is no significant
fractionationof parent or daughteron the scale
of the sampling.
Muscovite (TV-4) was obtainedfrom crystals
of 2-cm diameter from a graphic granite phase
of a pegmatite. TV-118 was collected during
previous field work by Flawn. The argon age
was first determined, and since it gave a result
significantly higher than previous samplesthe
strontium age was also.measured.It also yielded
a high value of 1090 m.y., which is not different
from that obtained on the other pegmatitic
muscovites.
In one locality (TV-252) pegma.titicbiotite
crystals were found. They have a diameter of
OF TEXAS
4031
appears to be preservedin the micas and amphiboles.
BASEMENT CORES
The minerals dated on the exposedPrecambrian rocks of the Trans-Pecosregion indicate
that all thesemineralswere formed or recrystallized at about 1100m.y. This result was obtained
on a variety of lithologiesrepresentinga wide
spectrum of metamorphic and igneousactivity.
Theseeventsappearto be contemporaneous
with
the metamorphismand plutonismof the Llano
area some 500 miles distant. To delineate the oc-
currence of this activity further it was necessary to use samplesof basementcores.Samples
were chosenfrom wells lying between the Van
Horn Mountains and the Llano uplift and adjacent regionsto test the continuity of the 1100m.y. event.
For this purpose cores of granite were obtained from Pecos,Schleicher,Williamson, and
pegmatite as thin (0.1 to 0.2 cm) plates along Coleman counties.Their petrographyhas been
fractures crosscuttingthe quartz and feldspar. describedby Flawn [1956]. The well locations
The biotite is a late-stage phenomenonand and data are given in Figure 6 and Table 7.
shows some iron stains around the borders of
Enough of the PecosCounty core was availthe crystals'. Fresh elastic material was care- able t.o permit a zircon analysis.However, feldfully selectedfor analysis. The argon age is in spar separatesmade from this sample had ungoodagreement,with the other argonresults,but favorableRb-Sr ratios,and so no other analyses
•ø• age obtained on
the strontium age is distinctly low, by 7 per were possible.The Pb•ø7-Pb
about
3 cm and occur in a coarse microcline
the zirconswas 1070m.y. The data plot in the
cent.
concordia diagram close to the same line as the
previous samples.
The feldspar separates obtained from the
probable age of eraplacementof the pegmatites
is 1090 m.y. These pegmatite results are defi- other cores had favorable Rb-Sr ratios and were
nitely greater than the ages of the geologically used for age determination.The coresshowconolder metasedimentary rocks (970 to 1030 siderablefracturingand somealteration,but in
m.y.). Although the discrepancyis not la.rge,it the samplespresentedhere the microclineapappearsto be distinctlygreaterthan the analyti- pears fresh and only slightly cloudedby alteracal error. This is an additional example of the tion. The feldsparin the SchleicherCounty core
'pegmatite-countryrock' discrepancy.Assuming showsmore staining and cloudingthan that in
diffusion to be the cause of relative daughter the other samples.The biotite in some of the
lossthis would suggestthat pegmatiticmaterials cores was typically altered and shredded and
other than biotite have a characteristically did not appearsuitablefor dating purposes.
The ages obtained for these samples range
smaller D/a 2 value, possibly reflecting larger
From these results on the pegmatitic mate-
rials we conclude
units
that
the minimum
in the mosaic structure.
The
and most
biotites
in
both the schistsand the pegmatitestend on occasion to give low results, particularly by the
Sr-Rb method. The preceding data show that
during middle-grademetamorphismall the Sr•Rb87and Ar4ø-K4ø
clocksin the parent minerals
were completely reset during recrystaIlization,
and no evidenceof the premetamorphichistory
from 960 to 1050 m.y. This spread is beyond
experimentalerror and is comparableto the dispersion exhibited by the feldspars from the
Franklin
Mountains.
Two corefragmentsfrom the ColemanCounty
well represented a coarse-grainedleucogranite
and a fine-grainedor aplitic phase.The results
are in excellent agreement.
4032
WASSERBURG, WETHERILL,
NEW
,r
, I
SILVER, AND FLAWN
MEXICO
- •rExAs
.....
.
..... ' ....
'V•'O,
!o
"!
....w.•
(•26'•'
.............
•%
I '-,,,'' .., 'AMARILLO
"
I///V
C
! ""
.....': _..,,,TERRAN.E..'."'-.;
....,, E'
•x
! ,/'"
'"'-.""..
CRATON
,/"i,•'"•.
......
.....
ARBUCKLE
•.•
...............
' "-'
............
.:_C.;.'3;i's
..........
,,
:\. !," ..........
• -.•--"
........
' RED
RWER
MOBILE
BELT
• •,•o...o....©
..........
i'•, _•>••'•'"••,•2o
......................
©i"•:'e'•
.......................
.... •
•'
.,<,,•
<
,--
-
-..<•I...'"
......
''"-....
...•
....
• ......
j.'
ß•
'---......
•,"•.•.
,•,
'•C•;•-•'"'"'",...
TEXAS
CRATON
•.•
'•,;
........
,•*,,..............
,•oœc•*./
©,240
I
,'"'"',
•%",
I'EL
,ASO
......•i.... N_E_W.._..M_•_X.!C_O
...... J
FRANKLIN
MTNS
/ STATION
TEXAS
•UMP
1090
•HUECO
/HILLS-1060
MTNS
io5o
•
TEXAS
•ORT WORTt
", •-'-.,'
\.....
'(•)
050
............
•..•.•
CRATON
>'.-•-..•
-SAN
ANGELO
v'4/V
ß I10
ß.•r•
(• •oo)
...•,•"' •'%....
•,
%,
z
/•
"•/•r•Tn3'•
•'{•'•'
'AUST•N
'SAN
ANTONIO
Fig. 6. Map of part of Texas, showingthe samplelocationsand the various terranesas designated by Flawn [1956]. Well coresare numbered as in Table 7. The best age estimate for
each locality is given.
TABLE 7. Age Determinations on Drill Core Samples
Age, m.y.
Locality
Rock
Terrane
Mineral
Alkaline granite Texas eraton
A. Aplite dike
Texas craton
B. Granite
Texas craton
3. Williamson Co., Texas Granite
Texas craton
Feldspar
Feldspar
Feldspar
Feldspar
960 q- 30
1050 q- 30
1040 •: 40
1010 + 70
Texas craton
Feldspar
Texas
Biotite
1240 q- 60
1400 q- 40
1. SchleicherCo., Texas
2. Coleman Co., Texas
K-Ar
Rb-Sr
Gneiss
5. Eddy Co., N.M.
6. Roosevelt Co., N.M.
7. Roosevel• Co., N.M.
8. Potter Co., Texas
Granite
Hornblende
Biotite gneiss
Rhyolite
Granite
craton
Hornblende
Whole
9. Lubbock Co., Texas
10. Crosby Co., Texas
Rhyolite
Amphibolite
Biotiteamphibole
30
1180
1260
1190
1235
rock
Panhandle volcanic Feldspar
Red River
qqqq-
70
40
70
120
mobile
belt?
11. Cottie Co., Texas
1390 •
Panhandlevolcanic Feldspar
Panhandle volcanic Feldspar
Texas craton?
Red River mobile
belt
Hornblende
1320 + 30
Biotite
1400 •- 40
Feldspar
1120 q- 30
Gneiss
12. Giles Co., Tenn.
Granite
AGES IN THE
PRECAMBRIAN
OF TEXAS
4033
In addition to these samples several cores from the rhyolite porphyries,and the strontium
were obtainedfrom other areasto gain informa- ageswere measured.The results are 1180 and
tion on the agesin otherpartsof the basement 1235 m.y.
The Potter County sampleis a micrographic
granite having some fine-grained phases.The
lithologicdivisionsof the Texasand southeast feldsparsare parfly sericitizedand clouded,the
New Mexico basement on the basis of petro- degreeof alteration being similar to that of the
graphicstudyof the availablebasement
well Roosevelt CounW sample. The strontium age
coresand samples.
He distinguished
the follow- was determined on a total-rock sample taken
ingunits:Texascraton,Van Hornmobilebelt, from a 3-cm cube.The result was 1190 m.y. For
Red River mobilebelt, Panhandlevolcanicter- a felspar separate from this rock a strontium
rane,Fishermetasedimentary
terrane,Swisher age of 1260 m.y. was obtained.
as described
by Flaton [1956].
Flawn oufiined a number of structural and
gabbroic
terrane,andWichitaigneous
province. Red River mobile belt. Cores were obtained
He alsoattemptedto relatethesedivisions
to from Crosbyand Cottle counties,Texas.Flawn
oneanotherin time by meansof lithologiechar- indicated that the assignmentof the Crosby
acteristics,
structural
behavior,
andtheavailable County sampleis uncertainand that it may beagedeterminations,
andhepresented
a tentative long to the Texas eraton. Both coresare quite
fresh. Some serieitizationof the plagioelaseoccurs, but it is confinedto occasionalfractures.
for this study a numberof favorablesamples Hornblende was obtained from the Crosby
were selectedfor investigation.These samples, County coreand biotite from the Cotfie County
describedbelow, are grouped accordingto core.Biotite, mostly chloritized,is presentin the
Crosby County sample. The ages for the two
Flawn's divisions:
Texas craton. In addition to the granite samplesare 1320 and 1400 m.y., respectively.
correlation of these subdivisions.
From the limited number of cores available
samples
fromPecos,
Williamson,
Sehleicher,
and
Coleman countiespreviously described,cores DrscvssroN 0r SRS'-RBs* AND AR•ø-K • RESULTS
fromEddyandRoosevelt
counties,
New Mexico,
Comparisonof the resultsobtainedon the outwererun. The micrographic
granitefrom Eddy crop materials, where there is more control than
Countyyieldedmicrocline
suitablefor strontium in the well cores,indicates several regularities.
analysis.Whereasthe biotite in this rock is The pegmatitic muscovites,although showing
severelyaltered,all the feldsparsappearrather some scatter, have the highest strontium and
fresh.The age obtainedwas 1240 m.y.
The sample from RooseveltCounty is describedby Flawn as a quartzdioritegneiss.
This
rock lies beneath a breeeiatedgranite found at
a shallowerdepth in the samewell. The rock is
argon age in a locality. The strontium ages on
microclineare usually in agreementwith those
on muscovitein the one locality where there
are comparisons.
The argon ageson amphiboles
appear somewhatlow; relative to the highest
quitefreshand containsboth biotiteand horn- strontium ages they indicate a retentivity of
blende. An argon age was determinedon the about 90 per cent. Biotites, particularly finehornblende,
and a strontiumageon the biotite. grained material (_•0.5 mm), yield rather low
The resultsare in very goodagreementat 1400 argon ages and even more disturbedstrontium
m.y.
Panhandle
ages.
volcanic
terrane.
Cores
from
Conversely, data of other workers [Aldrich,
RooseveltCounty,New Mexico,andPotter and Wetherill,Davis, and Tilton, 1958; Tilton, WethLubbock counties,Texas, were obtainedfrom erill, Davis, and Bass, 1960] have shownthat it
this terrane.
is also commonfor the strontium age of biotite
The samplesfrom Rooseveltand Lubbock to be greater thaa• the argon age. From the
countiesare rhyoliteporphyries.All the feldspars present observationswe concludethat no regular relationshipis to be expectedbetweenthe
and clouding.The LubbockCounty sampleex- two agesin this mineral,in contrastto potassium
hibits distinct flow structure. The alteration of
feldspar where the argon age is invariably less
the feldsparsis somewhat
lessthan in the previ- than (or equal to) the strontium age.
The strontium and argon ages from all the
ous sample.Feldsparseparateswere obtained
in these rocks show characteristic discoloration
zt03•i
WASSERBURG, WETHERILL,
SILVER, AND FLAWN
outcrop samples are illustrated in the histo- must be partly due to differential weathering
grams of Figure 7. These results show a wide and to differencesin the nature of the crystal
dispersionover a range of about 350 m.y. The mosaicand in the particular geometriclocation
maximum age is 1090 m.y., and 77 per cent of of the crystal. The dispersionon a regionalscale
the results are above 950 m.y. The maximum of a synchronousevent is a result of both the
age in each of the five outcrop localitieslies be- local effects and the grosserdifferencesin geotween 1050 and 1090 m.y. All but one of the logic history. The superpositionof several epiages significantlyless than 1000 m.y. are from sodesof metamorphism (in the general sense)
at different times and intensities at different
biotites' showing obviously discordant ages.
As previous experiencein this field indicates, localitieson a primary regionalevent produces
pegmatitic feldspar and muscovite apparently dispersion.This may involve the local 'quenchgive the best measureof the absoluteage by the ing' of a particular site or mineral early in a
strontium and the strontium and argon methods, regional episode of considerableduration, in
respectively.The scatter in the ages is due to. which case the dispersionshould give informatime resettingmechanismsin which the daugh- tion about the time scale for a single-episode
ter-parent ratio is diminishedin a mineral.
regional metamorphism, or it may involve one
The nature of the resetting mechanismsis or more completely independent later events.
somewhatdifferent for each mineral speciesin Severalexamplesof two eventsclearly separated
responseto the particular decayschemeand the in time have been found [Tilton, Davis, Wethphysical-chemical
characteristics
of the. species. erill, and Aldrich, 1958; Campston,Je#ery, and
The mechanisms'are poorly understood, and Riley, 1960; Wetherill, Kuovo, Tilton, and Gast,
there are few a priori criteria for choosingmate- 1962]. These results are well known and were
rials for dating. The general effects, however, elucidated by comparing the Pb-U ages of zirare apparent in all the mineral speciesand are conswith the Sr87-Rb
87and Ar4ø-K4ø
agesof the
manifest on a very local scale (hand specimen, other minerals and by studying the Sr87-Rb
•
outcrop, and locality) as well as on a regional ages in whole-rock samplesand in the mineral
scalefor rocks that are the product of synchro- components.In the present discussionwe wish
nousprimary events.The very local manifesta- to emphasizethe situation where the 'events' are
tions of age dispersionof one mineral species not so disparate in time.
8
03
500
600
"'
Z
700
800
900
1000
1100
1200
1500
MOO
1500
1500
1400
1500
•3•o'
1400
1500
ALL STRONTIUM AGES OF OUTCROP SAMPLES
500
600
700
ALL
800
ARGON
900
AGES
I100
I000
OF
OUTCROP
1200
SAMPLES
o Indicates outcrop sample
600
MAXIMUM
7oo
AGE
800
FOR
ALL
950
•o•
LOCALITIES
•oo
AND
o
CORES
Fig. 7. YIistogramsshowingthe age patterns by the strontium and argon methodson all the
outcrop materials studied. The lowest diagram shows the maximum ages obtained at each
locality by either method.
AGES
IN
THE
PRECAMBRIAN
OF TEXAS
4O35
As a consequence
of theseresettingprocesses, in a comparatively small time interval. Both the
for regionM-scalephenomenawe may expect to episodicand continuousdiffusionmodels of lead
obtain a time band with the age distributionde- loss would give a primary age of 1150 to 1200
pendent on the details and time scale of the m.y. for both the Pump Station Hills and the
processes,
as hasbeendiscussed
in more detail by Franklin Mountains, which is greater by about
Wasserburg [1961]. The fact that the maximum 10 per cent than the maximum age obtainedby
agesby the strontium and argon methodsat all the other methods. This effect is manifest in
the outcrop localitieslie between 1050 and 1090 some other suites of data comparing zirconm.y. strongly suggeststhat all the rocks are of uranium ages with the strontium and argon
the sameageand that the dispersionobservedto ages.The reasonfor this discrepancyis not unbe commonto them, despite the diversity of derstood.
their occurrence,is dominantly the product of
REGIONAL CONSIDERATIONS
local phenomena,although some of the other
effectsmay be present.Theseage measurements From the comparisonof the results by the
are indistinguishablefrom the results obtained various dating methods we conclude that the
by previousworkerson the Llano graniteslAid- Franklin Mountains granite, Hueco Mountains
rich, Wetherill, Davis, and Tilton, 1958; Goldich granite, Pump Station Hills rhyolite, Carrizo and
et al., 1961; Zartman, 1962, 1963].
Van Horn metamorphic rocks and pegmatites,
and the Llano granites originated nearly conU•AN•u•-LE•
A•Es oN Z•coNs
temporaneously,possiblywithin a period of 100
The Pb2ø7/Pb
2ø•ages on the zirconsare equal m.y. The different lithologiesand modes of octo or greater than the maximum ages obtained currence,therefore, representa wide variety of
by the strontium and argonmethods.The zircon geologicenvironmentsin which there has been
isotope ratios from the Trans-Pecos and the magmatic activity ranging from shallow intruLlano localities lie close to a well-defined common
sion and vulcanism to deep-seatedplutonic emline in the concordiadiagram (Figure 3), suggest- placement and middle-rank metamorphism.
ing approximate,if not precise,contemporaneity. These contemporaneousevents represent the
It is probablethat the zircondata representa dis- manifestations of a single diversified orogenic
tinct spreadin agesamongthe variousareasbut period.
0.21
0.20
Apl)alachi•n
-A•
ironda
:kZircons
0.19
0.18
o.•?
•0.16
0.15
0.14
0,13
,.
/,,•.:--.,-TEX.•,S
ZIR
._,ON
C-lORD
0'121.2 I.•
._
1.4
1.5
1.6
17
18
1.9 20
2.1
2.•
2.5 2.4
pb207/U
235
Fig. 8. Concordia diagram illustrating the location of some Appalachian Grenville zircons.
The best-fit straight line for the Texas zircons is shown for reference.
4036
WASSERBURG, WETHERILL,
SILVER, AND FLAWN
Various workers,includingFlawn [1956] and
Bass [1960], have used lithologiccharacterand
distinctionsin lithologiccharacterto map the
eastern Tennesseeremains open. No basement
coresare available from this intervening region.
Through the generouscooperationof Manuel
concealed basement and to make time-tectonic
Bassone sampleof cuttingsfrom a well which
correlations.The resultspresentedaboveclearly
indicate the difficultiesinherent in using lithologic characteras the principal basisof classification in time-tectonicproblems.
The zircon ages determined on the granite
core from PecosCounty and the strontiumages
on coresfrom Schleicher,Williamson,and Coleman countiesindicatethat theserocksoriginated
contemporaneously
with all the outcropsstudied.
This finding is highly suggestiveof an almost
continuousbelt of magmatic and orogenicactivity extending from E1 Paso to beyond the
Llano-Burner region--more than 500 miles.The
time of this orogenywas about 1090m.y. by the
penetrated granophyric granite was obtained
from south central Tennessee(see Figure 9). A
strontium age determinationon a feldsparsepa-
rate gave a result of 1120 m.y. (see Table 7).
This result is in agreementwith an earlier independent determinationby Bass (personalcommunication).
There is a scarcity of basementcores containing mineralssuitablefor dating in tile intervening region.The availability of suchcoresin
the future will be of great value in clarifying
this matter.
The widespreadoccurrenceof the 'Grenville'
event (about 1000 to 1100 by Sr-Rb) in north-
Sr87-Rb87and Ar'ø-K •ømethods and between 1150
eastern North America has been recently disand 1200 m.y. by the Pb-U method on zircons. cussedby Tilton, Wetherill, Davis, and Bass
These agesare very similar to thoseobtainedby [1960]. Silver [1961], has pointed out that
previous workers on the Grenville orogenicbelt evidence for 1150 to 1200-m.y. magmatic acin the northeastern United States and Canada
tivity appearsintermittently from Californiato
[see Tilton, Wetherill, Davis, and Bass, 1960]. Greenland.To illustrate the geographicaldistriSomeof the agesin this regionare complicated bution of these ages we have compiled the
by a Paleozoicmetamorphism,but wheresatis- available data, including our results, those of
factory comparisons
are availableby the stron- Tilton et al. [1960], and somerecentagesfrom
tium and argonmethodsthe resultsare directly easternCanada,southwestGreenland (Illimauscomparablewith the data presentedfor Texas. saq), westernUnited States,and the Lake SuSince the zircon ages are less affectedby the perior regionby other workers [compilationby
younger metamorphism we have plotted the Lowdon, 1961; Moorbath, Webster, and Moravailablezircondata for 'Grenville'samples
from gan, 1960; Silver, 1960,1961; Silver et al., 1962;
the northeastern United States and the best-fit
Goldichet al., 1961; Fairbairn, Bullwinkel,Pinchord from the zircon results from Texas on a
son, and Hurley, 1959]. These data are shown
concordia diagram (Figure 8). Uranium-lead in Figure 9, together with some data insystemsin Ontario and Quebecare generally dicatingolderterranes,to delineatethe boundsimilar but contribute a somewhat broader disary of the Grenville event. These do not
persionto the data. All the resultslie essentially representcompletecoverageof the older rocks,
on a commonline with a rather goodfit. The as- and none of the Phanerozoic orogenies are
sembledage data from these geographically represented.The availabledata suggestthat the
widespread regions indicate that the Grenville Grenville orogenicevent is the most widespread
orogenyand the episodeof intrusion,extrusion, and pervasiveepisodeof Precambrianorogeny
and metamorphism found in Texas lie in the and magmatic activity on the North American
same time band. Detailed discussion of the discontinentfor which recognizableevidenceis pre-
persionof U-Pb systemdata for the type Gren- served.
ville of Ontario and the Adirondack-AppalaTHE I)ISCREPANCY BETWEEN METHODS
chianswill be discussed
by Silver (in preparation). It is our opinionthat the orogenyfoundin
Some uncertainty remainsabout the time of
Texas shouldbe considered
part of the general the Grenville orogeny,in particular about the
'Grenville'orogenic
episode.
The questionof the age of the orogenyin Texas. It is manifestedin
continuity of this orogenyin the buffed base- part by the dispersionof the strontiumand arment betweenWilliamsonCounty,Texas,and gon agesand by the differencebetweenthem and
AGES IN THE
ß
450-600
o
GRENVILLE
x
1200-1800
ß
> 2000
PRECAMBRIAN
OF TEXAS
4037
Fig. 9. The occu•ence of 1000- to 1200-m.y. events in North America. Open circles represent 1000- to 1200-m.y. events; crosses,events between 1200 and 2000 m.y.; squares, events
greater than 2000 m.y. The solid circles represent the 525-m.y. ages found in the Wichita
Mountains.
the Pb-U age as determinedfrom the concordia the maximum strontium and argon agesof 1090
construction. As was seen from the data shown
m.y. Up to this point we have usedthe common
earlier, all the Pb-U ages on the zircons from linear relationshipexhibitedby the zirconsand
Texas show varying degrees of discordance. the characteristic age pattern given by the
Nevertheless,all these results lie near a single strontium and argon methodsto argue for constraight line on the concordiadiagram with
temporaneiW.However, the precisespecification
upper intercept of 1150 to 1200 m.y. and an ap- of the age must remain in questionbecauseof
proximate lower intercept of 200 to 400 m.y. the differences between the methods. This effect
From the point of view of episodicloss [Wether- is present in a similar comparisonof the differill, 1956] this impliesthat thesezirconsrepresent ent methods in the 'Grenville' of the eastern
systemswith • primary age of 1150 to 1200 m.y. United States and Canada, where the intercept
which were subjectto a reductionin their Pb/U
on the concordia plot (see Figure 8) is again
1150 to 1200 m.y., whereas the oldest stronratio 200 to 400 m.y. ago. The continuousdiffusion loss model [Tilton, 1960] for a 1150- to tium and argon ages on associatedminerals in
1200-m.y. primary age generates an almost this area are about 1070 m.y.
identical line over the range for which the data
Possibleexplanationsof this discrepancyare
are shownwith no inferred secondepisodicevent, an error in the U =• half-life superimposedon
and therefore the data do not distinguishbe- the 'normal' pattern of lead loss,a lossof intertween these mechanismsof discordance.The age mediatedaughter (possiblyRn2•2)from the U •
1150 to 1200 m.y. is significantly greater than chain superimposedon the 'normal' pattern of
4035
WASSERBURG, WETHERILL,
SILVER, AND FLAWN
leadloss,on a differentialresponseof the various constant of U 23•has been discussedby Aldrich
and Wetherill[1958], who have emphasizedthat
the best determinationof this decay constantis
the experimentby Fleming, Ghiorso,and Cundisplacedto the right of the curve characterized ningham[1952]. The experimentalerror quoted
by the original choice of Xu•'5 and Xu•'8. For by these workers is 2.2 per cent. Aldrich and
small changesthe shapeof the curve will not be Wetherill concluded that the uncertainty in
drastically altered. It can be seen that the ku•s•is at least 2 per cent. An increaseof the
constant fractional
loss of an intermediate
decayconstantby this uncertaintyfactorwould
daughterproductin the U TMchain would give a essentially eliminate the discrepancybetween
decreased
Pb•ø6/U
TMratio for a given age. Such the zircon and the strontium and argon ages as
changeswill decreasethe age determinedby the presentedin this work. There is no independent
intersection of a line with the concordia curve.
evidence, however, to justify such a change.
This is illustrated in Figure 10 for a 2.2 per cent A considerationof the changesin the Rbs7and
to accountfor the
increasein •v •. The best-fit line for the Texas K 4ødecayconstantsnecessary
zircon data is also shown on the graph. It is differencein the ages indicates that these are
evident that the intercept on concordiais widely unlikely.
In this paper we have chosento use the geoshifted by this change,the 1200-m.y. intercept
on curve A changingto a l100-m.y. intercept on logically determineddecay constantfor Rb8•
[Aldrich, Wetherill, Tilton, and Davis, 1956].
curve B. This is due to the small angle of intersectionof the episodiclossline and the concordia There are several recent determinations of this
curve. It is difficult to evaluate the possibility decayconstantby countingmethods[Flynn and
Glendenin, 1959; MeNair and Wilson, 1961;
of a constant fractional loss of intermediate
Egelkraut
and Leutz, 1961; Beard and Kelly,
daughter,but it seemsreasonableto expectsuch
lossesto be dependenton the detailedhistory of 1962]. They show a wide range of values,and it
each sample and hence to be rather irregular. is not possibleto chooseoneof them asdefinitive.
The regularlossrequiredfor the samplesreported As a matter of convenience we have continued
to use the value ;t = 1.39 X 10-• yr-• for prehere would be surprising.
The experimental uncertainty in the decay sentingthe age data. When a definitivedetermiminerals to loss of daughter products.
The effect of increasingthe decay constantfor
U •'5 would be to generate a concordia curve
O2O
O•,l9
018
017
J3016
015
014
013
0 121.
2
15
14
15
16
17
18
19
20
21
22
25
24
pb207/U235
Fig. 10.
The graph showsthe displacementof the concordiacurve for a 2.2 per cent increasein
ku TM The straight line representsthe best fit to the Texas zircon data.
AGES IN THE PRECAMBRIAN
OF TEXAS
4039
nation of this constant becomesavailable, all the extensivegranitic intrustion [Flawn, 1956,
the strontium ages calculatedusing this value p. 68]. The metamorphicrocks of the Carrizo
will have to be slightly revised by the same Mountains and Van Horn Mountains which
factor.
Flawn assignedto the Van Horn Mobile belt were
If the decayconstantis significanfiydecreased interpreted as youngerthan the Texas craton bebelow the value used here, the strontium and cause they appeared to have been deformed
lead agesreportedin this paper will be brought against a stable massto the north. The new data
closerto concordance.
This would in turn imply developedin this study indicate that the metaa lower argon retentivity in the micas.Such a morphic rocks of Flawn's Van Horn mobile belt
change is not suggestedby our evaluation of are the sameage as someof the rocksassigned
present counting results. If the decay constant to the Texas craton, namely about 1100 m.y.
is increasedby asmuchas 6 per cent [Flynn and Moreover, rocks in the Pump Station Hills,
Glendenin,1959], the age measurementswould Hueco Mountains, and Franklin Mountains west
indicate that radiogenicstrontiumis preferen- of Flawn's Texas craton are of this sameage.
tially lost in both feldsparsand micas in comIn the following discussionthe comparative
parison with argon in micas.
ages in the various terranes are evaluated by
Althoughzirconsvery commonlydisplaydis- restricting considerationto the data obtainedby
cordantages,they are alsomore likely to pre- the Rb-Sr method, which provides the most
serveevidenceof their originalage during some completelocality coverage.
metamorphicprocesses
becauseof the peculiariThe micrographicgranite from Eddy County,
ties of the two uranium decay schemes.It is New Mexico, was assignedto the Texas craton,
thereforepossiblethat the differencein agesre- but the measuredstrontium age of 1240 m.y. is
ported by the strontiumand argonmethodsand distincfiy older than that obtainedon the preby the zircon lead-uranium method is due to the vious samples. This result is particularly infact that the zirconsform partially closedsys- teresting, since on lithologic grounds the matems under higher temperature (or meta? terial is almost identical with many of the
morphic) conditionsthan the feldspars,micas, granites of the craton. The metadiorite from
and hornblendes.Thus, for example, in deep- Roosevelt County, New Mexico, comesfrom a
seated rocks the zircons could possibly begin rather complex area that was assignedto the
registering time before other minerals dur- Texas craton.The ageobtainedfor this material,
ing a cooling period. This could possi- 1400 m.y., is the oldest found in our study.
bly account for the observations. There is
Similar older ages of 1320 and 1400 m.y. are
some difiqculty in applying this argument found in metamorphic rocks in cores from
to such shallow intrusions
as occur in the
Crosby and Cottle counties,respectively.These
Franklin Mountains and Pump Station Hills. It
localities have been assignedby Flawn to the
is possiblethat strontiumand argonlossesunder Red River mobilebelt. Previouswork by Tilton,
normal near-surfaceconditionsare responsible Wetherill, and Davis [1962] on rocks from
for this effect or that a more recent undetected
the Arbuckle Mountains also givesagesof 1400
thermal event caused the losses.
m.y. by the strontium,argon,and lead methods.
The availabledata do not permit any concluSamplesof rhyolites from the Panhandlevolsionsabout the causeof this discrepancybe- canicterrane in RooseveltCounty, New Mexico,
tween the methods.It must be resolvedby fu- and LubbockCounty, Texas,and a granite from
ture studies.
Potter County, Texas, give ages of 1180, 1235,
and 1260 m.y., respectively.All these ages are
P•EGIONAL GEOLOGICALIMPLICATIONS
greater than the 1090 m.y. that appearsso comThe relationshipof the measuredagesto the monly farther south acrossthe Texas craton.
basement provinces of Flawn [1956] is comNo evidenceof the younger igneousevent of
plicated. Flawn's fundamentalbasementelement 500 m.y. found in the Wichita Mountains,Oklais the Texas craton, which was definedboth on homa [Tilton, Wetherill, and Davis, 1962], has
a lithologicbasisas consistinglargely of granitic beenfound in Texas.Roth [1960] has reported
plutonic rocks and structurally on the basis of a whole-rockAr4ø-K•ø
ageof 650 m.y. on a rhyoits apparent behavior as a stable element after lite core from Deaf Smith County, Texas. The
4040
WASSERBURG, WETHERILL,
result reported by Roth is at best a minimum
age, since it is well known that argon is lost
SILVER, AND FLAWN
terrane to the south.Theserocksare part of the
larger cratonic structure comprising all the
buried basementstudied,the southernpart of
this province being a dominantly granttic terrane that appearsto be a southwesternexten-
from feldspars.The age he reported was unsubstantiatedas to analytical accuracyand is
an isolatedresult of questionablequality. Such
a measurementcannot be seriouslyconsidered sion of the Grenville belt. To the north this
evidenceof a youngerevent.
The rhyolites in the Panhandle volcanic terrane pose a very puzzling problem. They include extrusive rocks which Flawn interpreted
as extruded
on rocks of the Texas craton and
therefore younger than the 1100-m.y. granite
rocks. The new data presentedhere indicate
that they are older. Petrographicallythey are
similar to the younger 500-m.y. suite in the
Wichita Mountains, but they are also petrographically similar to the older Precambrian
rhyolites in Missouri. The samples from the
Panhandle volcanic terrain suggestthat this
terrain may be a true time unit. Sinceit is made
up almost exclusively of extrusive rocks, it
must rest on an older (earlier than 1250-m.y.)
basement.
All the resultsare illustrated in the histogram
of Figure 7. There is little doubt that the ages
of 1090 and 1400 m.y. are well resolved.The
ages ranging from 1180 to 1260 m.y. are significantly greater than the 1090-m.y. results.It
is not clear whether these 1090-m.y. results
represent a single event or a degradedor dispersedolder event. The older ages(1100 to 1200
m.y.) obtained on zircons from the so-called
1090-m.y.rocks also suggestthe possibilitythat
the true age of these rocks is about 1200 m.y.
If so, the distinctionbetweenthe 1090- and the
1180- to 1260-m.y. ages becomesuncertain. It
may be noted that all the 1180- to 1260-m.y.
ages are the same to within analytical error.
From
consideration
of earlier
discussions we
tentatively concludethat the 1180- to 1260-m.y.
results represent a single event at about 1250
m.y. and that this is dist.inctfrom the 1090-m.y,
and the 1400-m.y.events.The 1320-m.y.age is
from a hornblende.The comparisons
presented
earlier indicate that this result may be low by
several per cent. We thus assignthe 1320- to
1400-m.y. samplesto a singleepisodeat about
1400 m.y. The rocks of this age may represent
an o.rogenic belt extending to the Arbuckle
Mountains. It now appearsthat Flawn's Texas
craton is composedof older rocks,1250 to 1400
m.y., to the north, and a younger 1090-m.y
larger cratonic structure contains older rocks of
1250- and 1400-m.y. age. The southern bound-
aries are undefined.The precedingassignments
are partly arbitrary, and they must be looked
on as tentative
until more materials
have been
studied and the dating methods better understood.
In the oversimplified
picture presentedabove,
we may appear to have assumedthat the orogenic events must actually be associatedwith
very sharply definedtimes. The problem of the
dispersionof ages and the existence of time
bands makes such a simplified interpretation
doubtful. More data on adequatesamplesmay
clarify the question of the width of the time
bands.Our knowledgeof the episodesof igneous
and metamorphic activity in the Phanerozoic
showsclearly that in certain regionsthere were
long intervals of time that were almost densely
populated with orogenies.Certain events may
have been more widespread and indicate a
greater intensity of activity, but they were not
alwayswidely separatedin time. If we consider
the age distribution in the Phanerozoictranslated back in time by 1000 m.y., we would see
that the events which are resolvedstratigraphically and sometimesgeochronologically
in the
Phanerozoicmay (within the experimentalerror
and natural dispersion) merge in the translated assemblage,and from this point of view
could appear more as a continuousdistribution.
Our understandingof Phanerozoichisiory indicates that we need not always expect to find
sharply defined,widely spacedorogenicepisodes
in the Precambrian.The only evidencesuggesiing a change in orogenic phenomena between
the
Phanerozoic
and the Precambrian
is the
fact that in North America where many age
measurements
have
been
made
there
is no
evidencefor an orogenythat occursbetweenthe
early Cambrian (about 500 to 550 m.y.) and
the Grenvillianat about 1100 m.y. The present
study again fails to indicate any event in this
wide (some500-m.y.) time gap. Less complete
data on other continentsindicate a gap of at
least between650 and 1000 m.y. The existence
AGES IN THE
PRECAMBRIAN
of this period of orogenicquiescence
is a most
remarkable fact deserving considerableattention.
The outcrop and well core data for all the
samplesstudiedindicatesomerelationships
dif-
OF TEXAS
4041
was completed,the charcoaltraps were warmed
and the samplewas.mixedwith Ar• tracer which
had been introducedpreviously.The gaseswere
then purifiedover hot CuO and passedthrough
a cold trap into a titanium furnace.After puri-
fication the argon residue was transferred to
the mass spectrometerusing a charcoalsample
tube. The samples were run on a 60ø sector
massspectrometerwith a 6-inch radiusof curvastructural criteria in the absenceof a reliable ture. This instrumenthas two collectors,one of
stratigraphy may yield equivocalresults in in- which is a normal Faraday cup and the other an
terpreting geologichistory. Flawn emphasized electronmultiplier. Ar'ø/Ar• data were taken on
ferent from those based on the regional lithologic-structuralinterpretation of the basement
as presentedby Flawn [1956]. It is evidentthat
a classificationbased solely on lithologic and
this earlier and pointed out that, in the absenceof age data, many of the time-tectoni('
conclusionsare decidedly tentative and conjectural. Severaldifficultiesare inherentin such
an approach.One is the fact that most rock
types occur throughout the geologiccolumn,
and, consequently,a lithologictype is not necessarily a time type. This includesmuch finer
correlations such as those based o.n accessory
minerals and mode of occurrence,which sometimes appear to be more exclusivecriteria.
Perhaps the greatest difficulty with all our
studiesis that a fundamentalunderstandingof
orogenicprocessesand driving forces is very
incomplete,and our ability to make consequential predictionsin this field is singularlylacking.
We in no way imply that agemeasurements
will
provide the critical answers,but in favorable
circumstancesthey may permit a clearer interpretation of simultaneity (of crystallization)
and possiblyadd mo.reknowledgeto the morphologyo.forogenesis,
so that somemore fundamental insight may provide a basis for understanding.
the simplecollector,Ar•6/AF8 data on the multiplier.
Discrimination was monitored by running
normalargonsamples.SamplesTV-3M, TV-SBi,
and TV-6Hbd were each run in duplicate for
argon; the maximum differenceobservedwas 2
per cent. The Ar• tracers were made by a
pipetting techniquefollowingP. Hurley and J.
ZShringer (personal communication). The particular spike line used in this laboratory was
designedand testedby Dr. M. Lanphere,R. E.
Zartman, and G. J. Wasserburg.This apparatus
consists of a 2-liter
reservoir
and a double
pipette isolatedby two stopcocksand two mercury cutoff reservoirs(Figure 11). The pipette
to
so
system
,-.,4cc//
APPENDIX
Analytical Procedures
The analytical procedures were essentially
those reported in previous papers. Argon was
extracted from the minerals by fusion in molybdenum crucibles with a radio-frequency
heater. All the sampleswere heated to a temperature of 1400øC,producinga fluid melt. One
sample of biotite was heated to produce a
sinteredmat, and a split of the same material
was heated to 1400øC. The a'•'gonyields were
the same. The gasesproduced during heating
were frozen out continuously on charcoal at
liquid nitrogen temperature. After the fusion
cm
6 ( trocer
)
2 liters
Fig. 11. Argon spike pipette. Drawing not to
scale. An aliquot of the spike is taken by raising
the
left-hand
column
to
one
of the
two
cutoff
positions. The right-hand column is lowered below
the cutoff and the gas is equilibrated. The aliquot
is then isolated by raising the right-hand column.
The fractional depletion per release is about 2 X
10-=.
WASSERBURG, WETYiERILL,
4042
TABLE 8.
Sample
SILVER, AND FLAWN
Analytical Data for Rb-Sr and K-Ar Ages
K, %
6.19
Ar4O*,
10-s mole/g
Ar4ø*/Ar4ø Rb, ppm
1. 416
O. 97
6'(•
1.i•
o'•
0.937
0.2195
0.90
2274
304.9
TH-1
Tin1
TH-2
TH-5
TH-5
TH-6
(Bi)
(F)
(Bi)
(Hb)
(F)
(F)
TV-1
(M)
8'•'•
1.•
0'•
244.2
TV-1
TV-1
TV-2
TV-3
TV-3
TV-3
TV-4
TV-5
TV-6
TV-10a
TV-10b
TV-12b
TV-12c
TV-13
TV-13
TV-14
TV-118
TV-252
(Bi)
(Rk)
(Hb)
(M)
(F)
(total F)
(M)
(Bi)
(Hb)
(Bi)
(Hb)
(Bi)
(Rk)
(F)
(Rk)
(F)
(M)
(Bi)
7..
1.•4o
o..
44.o
85.42
0'•
s.•s
o.6iil4
.071
o'g•
0..
Drill
243.5
Sr87', ppm
Sr87*/SrS7
8.35
1.253
O. 684
0.90
0.36
0.095
15.02
1. O01
1.557
0.3274
0.95
0.50
0.61
0.14
1.781
1. 736
2.791
1.326
0.46
0.49
0.84
0.65
.........
o'i•
3483
.........
.........
10•'
4..0
422.8
663.0
o..a
5.92
o.o•841
1.014
o.7.
0.94
o..s
0.0•4s
o.so
6•]
5
o'•:•
.........
l•'a
o'•io
o'i•
.........
.........
.........
10a.•
a74.•
lS4.s
lS0..
0
I
0
0
3
3
o. 097
0.26
0.13
0.11
0.93
0.76
s'•;
7.•0
.•g•
1.so•
0'•
o.•s
s7•..
.7.s
441
576
724
760
796
60
Cores
1.
Schleicher Co.,
Texas (F)
2A. Coleman Co.,
Texas (F)
2B. Coleman Co.,
Texas (F)
3. Williamson Co.,
Texas (F)
5. Eddy Co., N.M.
(F)
6. Roosevelt Co., N.M.
(Hb)
(Bi)
7. Roosevelt Co., N.M.
(P)
8. Potter Co., Texas
(P)
(Rk)
9. Lubbock Co.,
Texas (F)
10. Crosby Co.,
Texas (Hb)
11. Cottle Co.,
Texas (Bi)
12. Giles Co.,
Tenn. (F)
.........
471.1
1.782
0.33
.........
676.7
2.807
0.57
.........
736.8
3.026
0.26
.........
346.9
1. 382
0. 093
.........
256.7
1.265
0.15
1.19
0.433
0.95
.........
•i•'•
.........
291 .S
1.359
0.11
.........
.........
•.s
280.6
2.669
1.322
0.26
0.12
.........
234.6
1.150
0. 077
O. 869
O. 295
O. 87
. . .
.
.
.
ß
.
.........
314.1
1.745
0.36
.........
182.3
0.810
0.41
.
AGES IN THE
PRECAMBRIAN
has a sufficientlylarge volume to overcomethe
problems associatedwith capillary tubes. The
double pipette permits the removal of two different spike aliquots, dependingon the sample
size. The ends of the pipette are tapered to a
2-mm bore and are ground flat. Aliquots of the
spikeare transferredby expandinginto the sample systemand freezing on charcoal.The whole
apparatus is built in a portable frame and can
be conveniently moved after cutting the connection to the sampleline.
Potassium analyses were done on a PerkinElmer flamephotometerusinga lithium internal
standard.In addition, all the hornblendeanalyseswere doneby isotopedilution.The resultsby
the two methodsagreed to within experimental
error. The estimated maximum error in the Ar•ø/
K • ratio is taken to be 2.5 per cent.
The atomic abundanceof K 4øwas taken,to be
1.19 X 10-4, and the decay constantsused are
ke - 0.585 X 10-•ø yr -•, k • - 4.72 X 10-•ø yr -•.
The isotopedilution proceduresfor the strontium and rubidium analysesare essentiallythose
TABLE 9.
OF TEXAS
4043
developedat the Department of Terrestrial
Magnetismof the CarnegieInstit.utionof Washington.We shouldparticularlylike to thank the
membersof that group for aiding us in setting
up the procedures.
Sampleswere run by a surface ionization
technique on a 60ø sector mass spectrometer
with a 12-inch radius of curvature, with an
electronmultiplier as the collector.Discrimination wasmonitoredby runningnormalrubidium
and strontium.
The rubidium and strontium tracer solutions
were calibratedby mixing with known normal
rubidium and strontium solutions. The normal
rubidium solutions were made with spectro-
scopicallypure RbC1 purchasedfrom Johnson
and Matthey and Co., Ltd., and with RbC1
purifiedfrom C.P. gradesaltby the RbICl• procedureas describedby Archibald [1932]. Both
salts were analyzed by a conventionaloptical
spectrographicmethod for cationic impurities
and were found to contain less than 0.01 per
cent for the maximum impurity. Rubidium and
West Texas Zircon Analyses
Radiogenic Lead
Observed Lead Isotope Ratios
Isotopic Composition,
atom %
pb•oo/pb•o4 pb•oo/pb•o* pb•.oo/pb•.OS pb•.OO
Pb•o*
pb•os
Concen-
trations,
ppm
Pb *,d
U
Micrographic granite
(TH-5), Franklin
Mrs., E1 Paso Co.
Fraction
Fraction
A
B
905.7
711.5
11.141
10.544
5. 459
4.277
81.80
79.38
6.16
5.93
12.05
14.49
50.4
71.8
330
527
Rhyolite porphyry
(TV-13), Pump Station Hills, Hudspeth Co.
Fraction
Fraction
A
B
351.4
528.4
8. 413
9. 616
3.268
3. 654
77.71
77.77
6.09
5.99
16.20
16.24
61.6
54.1
313
299
459.5
9. 482
4.852
82.98
6.16
10.86
32.4
205
177.5
6.244
2.894
81.50
6.53
11.97
42.0
329
Granite well core,
Phillips Puckett
# lc, PecosCo.
Total
zircon
Muscovite-oligoclasequartz schist
(TV-1), Van Horn
Mrs., Hudspeth Co.
Zircon fraction,
pass 200 mesh
Common
Pb
correction
206/204
17.9
207/204
15.65
208/204
38.0
4044
WASSERBURG,WETHERILL, SILVER, AND FLAWN
chlorine were determinedin duplicate on the
Johnsonand Matthey material.Rubidiumwas
of the massspectrometer.
For a 10 per cent enrichment the maximum error is estimated as 6
discussion
of analytideterminedas the perchlorateafter a butyl percent.A morecomplete
alcohol-ethyl
acetateextraction.The chloride cal errors is given by Zartman [1963].
The decayconstantusedfor calculating
the
was determinedby precipitatingAgC1from a
AgNO•solution.
The RbC1wasfoundto be strontiumagesis ,• • 1.39X 10-• yr-L The isousedin this
stoichiometricto within better than 0.1 per cent. topicratiosfor normalstrontium
ss• 0.0843.
The normal strontium solutionswere made work are Sr•/Srss-- 0.1194,Srs•/Sr
withspectroscopically
pureSrC08(Johnson
and The ratio Sr•/Sr • givenin the last columnof
SF• to total SF•
MattheyandCo.,Ltd.)andreagent
grade
SrC08 Table 8 is the ratio of radiogenic
in
the
experiment.
(Merck).The cationic
impurities
of bothsalts
werenegligible.
The saltswereanalyzed
in du-
Thin Sections
o/ Rocksand
plicateby the following
procedures.
Strontium Descriptionoy
Mineral Separates
wasprecipitated
as the sulfatefromacidsoluVan Horn Mountains
tioncontaining
50 per centby volumeC•H50H
TV
1.
Quartz
and albite-oligoclase
(79'%),mus(denatured),
washed
in 95 per centC•H5OH,
and filtered; the precipitatewas driedto con- covite (15%), potassiumfeldspar(5%), biotite
(1%), apatite(tr.), zircon(tr.). Paralleloriented
stantweightat 700øC.The saltswerealsocon- platesof muscoviteand pale-brownbiotite occur
verteddirectlyto thesulfate.
Theresults
showed in a quartz-feldsparmosaic.Grain size: 0.1-0.3
that the SrCO•was stoichiometric
to within 0.1
mm. Fabric: crystalloblastic.
Rock: biotitic mus-
per cent.
covite albite-oligoclaseschist. Muscovite separate
The interlaboratory
standards
MIA-Std microclineand B-3203 biotite were analyzed.The
results obtained on MiA-Std are 6.78 ppm
(--40 •-80 mesh):98.5%musc.,15%qtz.,bio,plag.
Biotite separate(--40 -]-80 mesh): 89'%bio, 6%
muse.,1% partly chloritizedbiotite,4% opaques,
feldspar, quartz.
Sr8•* and 1004ppm Rb. The resultsobtained TV 2. Plagioclase(56%), hornblende(40%),
(4%), apatite (tr.). Blue-green
previously
by L. T. Aldrich(personal
com- magnetite-ilmenite
hornblende in sheavesand small prismatic grains,
munication)are 6.90 ppm Sr•* and 1010ppm
Rb. The results obtained on B-3203 Bi are
locallypoikiloblastic,
formsa mosaicwith plagioclase(oligoclase-andesine).
Grain size: 0.2-0.3ram,
1.701ppmSF•* and440.6ppmRb.G.W.Weth- hornblendeup to 2 mm. Fabric: crystalloblastic.
erill (personal
communication)
hadpreviouslyRock: amphibolite.]¾ornblendeseparate (--80
•100 mesh): 97% hbd. 0.2% muse.,0.8% opaques,
obtained
1.707ppmSr•* and439ppmRb. Our 2%
plagioclaseand unidentifiedinclusions.
valuesare in goodagreement
with the data re-
TV 3. Pegmatitic perthitc containingshreds
portedby theseworkers.
The dataon B-3203 of muscovite,small grainsof quartz, and a trace
Bi are somewhatdifferentfrom the resultsby
of apatite.Feldsparquite fresh.Someslightdis-
P. Hurley (personal
communication).
coloration.
TV 5. Quartz (48%), plagioclase(25%), bio-
The CarnegieInstitutionrubidiumtracercon- tite (25%), muscovite(2%), garnet (tr.), hemacentrationwas determinedby us. Our results tite (tr.), apatite (tr.), zircon (tr.). Dark green
agreed
to within0.5percentof thevaluemea- biotite and poikiloblasticsodic(?) plagioclaseocsuredpreviously
by Wetherillet al. (personalcurs in a mosaicof strained quartz. Plagioclaseis
communication).
partly to completelyaltered to sericite.Grain
size: 0.2-3 mm. Fabric: crystalloblastic.Rock:
Duplicate
analyses
weredoneonseveral
sam- biotite schist.Biotite separate(--40 -]-80 mesh):
ples,including
thefollowing.
EddyCounty,
New 96% bio., 1% muse.,1% opaques,2% plag. and
Mexico,feldspar:256.6ppm Rb, 1.274ppm qtz.
TV 6. Plagioclase(51%), hornblende(45%),
SF•*. TH-1 biotite: 2278 ppm Rb, 8.269 ppm
magnetite-ilmenite
(4%),biotite(tr.), apatite(tr.).
Sr•*. TV-1 biotite: 438.5 ppm Rb, 1.574ppm Prismaticto large poikiloblasticblue-greenhornSF•*. Theseresultsare in goodagreement
with blendegrainsoccurin an aggregate
with incipiently
sericitizedplagioclase(oligoclase-andesine).
Grain
the first measurements
reportedin Table 8.
size: 0.2-0.5, hornblendeup to 2 mm. Fabric:
The estimatedmaximumerror in the Sr•*/
crystalloblastic.
Rock: amphibolite.Hornblende
Rb• ratioistakenas2 per centfor highlyradio- separate
(--40 -]-80 mesh): 96% hbd., 3.6%
genic
samples.
Forsamples
onlyslightly
enrichedopaques,0.4% apatite.
in radiogenic
strontium
the errorsareconsider- TV 1œB. Plagioclase (45%), biotite (30%),
hornblende(10%), epidote(7%), microcline(2%),
ablyamplified,
owing
tochanges
indiscrimination
AGES IN THE PRECAMBRIAN
OF TEXAS
4045
sphene (2%), magnetite-ilmenite (2%), quartz ? size: phenocrystsup to 3-5 mm, groundmassintergrowth 0.2-0.4 mm. Fabric: porphyritic. Rock:
(1%), apatite (1%). Plagioclase,partly altered to
epidote, occursas broken twinned subhedraand rhyolite porphyry.
TV. 14. Microcline microperthite (80%), plagiomosaic fragments. Green biotite forms a foliated
mass and includes hornblende prisms and grains clase and quartz (15%), amphibole (5%), epidote
of epidote. Sphene occursas abundant clusters of
(tr.), magnetite-ilmenite (tr.), zircon (tr.). Microsmall grains. A few large grains of microcline are cline microperthite occurs as large grains and as
smaller grains in an intergranular aggregate with
present, and quartz (?) occurs as a granular aggregate around some feldspar grains. Grain size: sodic plagioclase and quartz. Quartz occurs as
round grains in feldspar and in amphibole. The
average 0.1-0.5 mm, larger grains up to 2 mm.
Fabric: relict hypidiomorphic granular-crystallo- amphibole is a deeply colored green to brown
blastic. Rock: hornblende-epidote-biotite schist dehornblende that occurs in large skeletal grains.
Grain size: perthire I cm ñ, smaller grains 0.1rived from alteration of an igneousrock, probably
a diorite. Biotite separate (--100 -]-150): 73%
5 mm. Fabric: panallotriomorphic granular. Rock:
bio., 2% hbd., 7% chlor., 2% qtz. and feldspar, hornblende granite.
12% opaques,4% epidote.
Franklin
Mountains
TV 12-C. Plagioclase and microcline (10%),
TH 1. Microcline microperthite (70%), sodic
quartz (5%), groundmass(85%), calcite (tr.), seriplagioclase (10%), quartz (10%), biotite (8%),
cite-muscovite (tr.), sphene (tr.), zircon (tr.).
Large grains of microcline and plagioclase sub- sericite (1%), fluorite (1%), magnetite-ilmenite
(tr.), zircon (tr.). Microcline microperthite occurs
hedra (phenocrysts) together with fragments of
quartz mosaic occur in a crushed and sheared in large grains; plagioclase, as twinned subhedra.
groundmassof alkali feldspar and quartz contain- Deeply colored brown biotite forms large irregular skeletal grains and also occursin small plates.
ing sericite fibers. Quartz mosaic fragments may
It is partly altered to a fibrous mass that may be
be fragments of broken quartz veins. Other quartz
a mixture of bleached biotite, chlorite, and sericite.
veins occur parallel to the flow structure or foliation in the groundmass. Grain size: groundmass Fluorite is in large grains connectedby thin veinlets. Zircon is abundant.
Grain size: 0.5-5.0 mm.
0.01-0.10 mm, phenocrysts up to 2 mm. Fabric'
Fabric: hypidiomorphic granular. Rock: mediumrelict prophyritic-cataclastic.Rock: shearedrhyoto coarse-grained biotite granite. Biotite separate
lite porphyry or metarhyolite.
(--20 -t-40 mesh): 82% bio., 4% hbd., 9%
Carrizo Mountains
opaques,5% qtz., plag., microc.
TH 2. Plagioclase (60%), biotite (26%), tourTV 10A. Quartz and albite-oligoclase (74%),
maline (8%), sphene (2%), ilmenite (2%), semibiotite (14%) partly altered to chlorite (20%),
opaque titanium (?) mineral (2%), microcline
muscovite (7%), garnet (2%), apatite (tr.), zir(tr.), pyrite (tr.), apatite (tr.). Plagioclase (oligocon (tr.), iron oxides (1%), tourmaline (tr.).
Thinly laminated schist with alternate coarser- clase?) and biotite occur in a poikiloblastic mosaic. Foliation is well developed. Plagioclase is
grained quartz-rich laminas and finer-grainedmicarich laminas. Orientation
of both micas is transpartly altered to sericite. Large poikiloblasts of
verse to lamination and does not show good plagioclase and pink-brown tourmaline enclose
orientation. Grain size: 0.1-0.5 mm. Fabric: crystalloblastic laminated. Rock' garnet-biotite schist.
Biotite separate (--100 •-150): 76% bio., 16%
chlor., 3% qtz. and plag., 1% hbd., 1% musc.,
3% opaques.
TV lOB. Itornblende (34%), plagioclase and
quartz (57%), epidote (6%), opaques(2%), sphene
and apatite (1%), biotite (tr.). Blades of hornblende randomly oriented in a finer-grained mosaic of plagioclase and epidote. Separate clots of
epidote and of hornblende occur irregularly
through the rock. Magnetite in irregular coarseaggregates. Grain size: 0.1-2 mm. Fabric: Rock:
amphibolite. Itornblende separate: 88% hbd., 10%
iron stained hbd., 2% opaques.
Hueco Pump Station and Pump Station Hills
TV 13. Feldsparphenocrysts(15-20%), largely
altered to clay (?) and rounded and corroded
quartz phenocrysts (75-80%) occur in a granophyric groundmass.The groundmasscontainssericite, chlorite, and biotite (?) as well as finely
intergrown quartz and alkali feldspar. Biotite (?)
occurs as tiny plates. Zircon (tr.) is present. Grain
other rock constituents. The semiopaque mineral
may be mixed sphene and leucoxene. Grain size:
poikiloblasts 1.0-2.0 mm., groundmass 0.05-0.20
mm. Fabric: crystalloblastic-poikiloblastic. Rock:
tourmaline-biotite-plagioclase schist. This is an
interesting rock which shows disequilibrium relations. Growth of poikiloblasts may be a reflection
of near-by granitic intrusion. Biotite separate (--40
-•80 mesh): 75% bio., 24% bio. with opaque inclusions,1% qtz. and feld. No chlorite observed.
TH 5. Microcline microperthite (75%), myrmekite (10%), biotite and amphibole (8%),
quartz (7%), magnetite-ilmenite(tr.), apatite (tr.),
zircon (tr.). Microcline microperthite occurs as
large grains. Micrographic intergrowths are rare.
Ferromagnesian minerals occur as intergranular
•nests.' Grain size: microperthite up to 1 cm, interstitial grains 0.2-1.0 mm. Fabric: granophyric.
Rock' biotitic hornblende granite. I-Iornblende separate (--60 -]-200): 69% hbd., 14% bio., 1.5%
microcl., 2% qtz., 8% zircon, 5.5% opaques.
Basement
Cores
The majority of cores are described by Flawn
4046
WASSERBURG, WETHERILL,
[1956]. The page referenceto the core description
is given by the number (FXXX).
1. Schleicher Co., Texas. Humble No. I Spencer
6909-10 feet (F-196).
2. Coleman Co., Texas. Naylor No. 1 Stone
5335-38 feet (F-140).
3. Williamson Co., Texas. Shell and Sinclair No.
1 Purcell 9474-79 feet (F-202).
4. Pecos Co., Texas. Phillips No. 1-C Puckett
14924-30 feet (F-189).
16459 feet (F-216).
6. Roosevelt Co., New Mexico. Spartan No. 1-36
State 7240-42 feet (F-232). Biotite separate (--40
•80 mesh): 91% bio., 8% hbd., 1% opaques,feldspar, sphene.
7. Roosevelt Co., New Mexico. Shell Saunders
No. 1 8650 feet (F-231). 86% hbd., 6.5% bio., 2.7%
alkaline feldspar, 2% quartz, 2.8% opaques,plagioclase, apatite, zircon.
8. Potter Co., Texas. Shell Bivins Ranch No.
1-207 located
1000 feet from west line and 660 feet
from north line, section 207, block 2, AB and M
Survey, micrographic granite.
9. Lubbock Co., Texas. Farris No. 1--11 778-83
feet (F-175).
10. Crosby Co., Texas. Humble No. 1 Irvin
9947 feet (F-147). tIornblende separate (--80 •100
mesh): 98% hbd., 0.2% feldspar, 1.8% opaques,
epidote, sphene, and zircon.
11. Cottle Co., Texas. General Crude No. 13-1
Swenson5719-20 feet (F-144). tIornblende separate
(--80 -]-150 mesh): 95% hbd., 3% biotite, 1.6%
feldspar, 1.4% apatite, opaques.
12. Giles Co., Tennessee. California Company
No. I E. W. Beeler NE, NE, section 4-15S-29E
•/• mile SW of Campbellsville 5620-5750. Cuttings.
Massive granophyric porphyry. Microcline phenocrysts. Contains a blue sodium amphibole (0.5%)
plagioclase (tr.), the remainder being quartz
and microcline.
Acknowledgments. We should like to acknowledge the help of Robert L. Harbour of the U.S.
Geological Survey, who generouslyaided us in our
collecting trip to the Franklin Mountain area.
Some of the core samples were made available
to us through the kindness of Dr. 1•. G. Swanson
of the Shell Oil Company, Dr. Milton Williams
of the Humble Oil and Refining Company, and
Dr.
James
F. Johnson
ogy by radioactive decay, Ann. Rev. Nuclear
$ci., 8, 257-298, 1958.
Aldrich, L. T., G. W. Wetherill, G. L. Davis, and
G. R. Tilton, Radioactive ages of micas from
granitic rocks by I•b-Sr and K-A methods,
Trans. Am. Geophys. Union, 39, 1124-1134,
1958.
Aldrich, L. T., G. W. Wetherill, G. R. Tilton, and
G. L. Davis, Half life of Rbs7,Phys. Rev., 103,
1045-1047, 1956.
5. Eddy Co., New Mexico. Richardson and
Bass No. 1 Cobb-Federal
SILVER, AND FLAWN
of the Sinclair
Oil
Com-
pany.
We should particularly like to thank Professor
Manuel Bass, whose enthusiastic interest encouraged this work and who gave freely of his time
and knowledge in discussingthe problems.
This work was supported by the National Science Foundation grants NSF-G19084 and NSFG15945, and the Atomic Energy Commission contract AT(11-1)-208.
REFERENCES
Aldrich, L. T., and G. W. Wetherill, Geochronol-
Archibald, E. H., The Preparation o• Pure Inorganic Substances, John Wiley & Sons, New
York, 1932.
Barrell, J., Rhythms and the measurements of
geologictime, Bull. Geol. $oc. Am., 28, 858-871,
1917.
Bass, M. N., Grenville boundary in Ohio, J. Geol.,
68(6), 673-677, 1960.
Beard, G. B., and W. It. Kelly, Nuclear Phys., in
press, 1962.
Cloud, P. E., V. E. Barnes, and J. Bridge, Stratigraphy of the Ellenburger group in central Texas
--a progress report, Univ. Texas Publ. 4301.
133-161, 1943, 1945.
Compston, W., P.M.
Jeffery, and G. H. Riley,
Age of emplacement of granites, Nature, 186,
702-703, 1960.
Egelkraut, K., and tI. Leutz, Z. Physik, 161, 13-19,
1961.
Fairbairn, H. W., H. J. Bullwinkel, W. H. Pinson,
and P.M. Hurley, Age investigations of syenites
from Coldwell Ontario (abstract), Bull. Geol.
$oc. Am., 69(12), p. 1543, 1959.
Flawn, P. T., Pegmatites of the Van Horn Mountains, Texas, Econ. Geol., 46(2), 163-192, 1951.
Flawn, P. T., Basement rocks of Texas and southeast New Mexico, Univ. Texas Publ. 5605, 1956.
Fleming, E. tI., Jr., A. Ghiorso, and B. B. Cunningha.m, The half life of U 2•, Phys. Rev., 88,
642, 1952.
Flynn, K. F., and L. E. Glendenin, Phys. Rev.,
116, 744-748, 1959.
Gerling, E. K., and A. A. Polkanov, Problems of
absolute ages of the Precambrian of the Baltic
shield, Geokhimiya, 8, 695, 1958.
Giletti, B. J., S. Moorbath, and R. St. J. Lambert, A geochronological study of the metamorphic complexes of the Scottish Highlands,
Quart. J. Geol. $oc. London, 117, 233-272, 1961.
Goldlob, S.S., A. O. Nier, H. Baadsgaard, J. tI.
Hoffman, and H. W. Krueger, The Precambrian
geology and geochronologyof Minnesota, Minn.
Geol. Survey Bull. 41, 1961.
Harbour, R. L., Precambrian rocks at north Franklin Mountain, Texas, Bull. Am. Assoc. Petrol.
Geologists, 44(11), 1785-1792, 1960.
Hart, S. R., The use of hornblendesand pyroxenes
for K-Ar dating, J. Geophys. Research, 66(9),
2995-3001, 1961.
Holmes, A., Bull. Natl. ResearchCouncil, 80, 330334, 1931.
Jaffe, H. W., D. Gottfried, C. C. Waring, and
H. W. Worthington, Lead-alpha age determina-
AGES IN THE
PRECAMBRIAN
tions of accessory minerals of igneous rocks,
U.S. Geol. Survey Bull. 1097B, 131-133, 1959.
King, P. B., and P. T. Flawn, Geology and mineral deposits of Precambrian rocks of the Van
Horn area, Texas, Univ. Texas Publ. 5301, 1953.
Lowdon, J. A., Age determinations by the Geo-
logical Survey of Canada, Geol. Survey Can.
Paper 61-17, 127 pp., 1961.
Masson, P. //., Age of igneous rocks at Pump
Station //ills, //udspeth Co., Texas, Bull. Am.
Assoc. Petrol. Geologists, 40(3), 501-518, 1956.
McNair, A., and l:I. W. Wilson, Phil. Mag., 6,
563-573, 1961.
Moorbath, S., R. K. Webster, and J. W. Morgan,
Absolute age determination in southwest Greenland, Medd. Grpnland, 162(9), 5-14, 1960.
Roth, R., Swisher gabbroic terrane of the Texas
Panhandle, Bull. Am. Assoc. Petrol. Geologists,
4J(11), 1775-1784, 1960.
Silver, L. T., Age determinations on Precambrian
diabase differentiates in the Sierra Ancha, Gila
County, Arizona (abstract), Bull. Geol. Soc.
Am., 71 (12), 1973, 1960.
Silver, L. T., Major gabbroic magmatic events and
geochronology, J. Geophys. Research, 66(8),
2560, 1961.
Silver, L. T., and S. Deutsch, Uranium-lead method
on zircons, Ann. N.Y. A cad. Sci., 91, 279-283,
1961.
Silver, L. T., C. R. McKinney, S. Deutsch, and
J. Bolinger, Precambrian age determinations in
the western San Gabriel Mountains, California,
J. Geol., in press,1962.
Tilton, G. R., Volume diffusion as a mechanism
for discordant lead ages, J. Geophys. Research,
65, 2933-2945, 1960.
Tilton, G. R., G. L. Davis, G. W. Wetherill, and
L. T. Aldrich, Isotopic ages of zircon from granites and pegmatites, Trans. Am. Geophys. Union, 33, 360-371, 1957.
Tilton, G. R., G. W. Wetherill, and G. L. Davis,
OF TEXAS
4047
Mineral ages from the Wichita and Arbuckle
Mountains.
Oklahoma.
and
the
St.
Francis
Mountains, Missouri, J. Geophys. Research,
67 (10), 1962.
Tilton, G. R., G. W. Wetherill, G. L. Davis, and
M. N. Bass, 1000-•n.y.-old minerals from the
eastern United States and Canada, J. Geophys.
Research, 65(12), 4173-4179, 1960.
Tilton, G. R., G. W. Wetherill, G. L. Davis, and
C. A. Hopson, Ages of minerals from the Baltimore gneiss, Bull. Geol. Soc. o[ America, 67,
1469-1474, 1958.
Wanless, R. K., J. A. Lowdon, R. D. Stevens,
R. J. Traill, J. Y. I-I. Rimsaite, and S. Abbey,
Age determinations by the Geological Survey of
Canada, Geol. Survey Can. Paper 61-17, 127 pp.,
1961.
Wasserburg, G. J., Crustal history and the Precambrian time scale, Ann. N.Y. Acad. Sci., 91,
583-594, 1961.
Wasserburg, G. J., G. W. Wetherill, and L. A.
Wright, Ages in the Precambrian terrane of
Death Valley, California, J. Geol., 67(6), 1959.
Wetherill, G. W., Discordant uranium lead ages,
Trans. Am. Geophys. Union, 37, 320-326, 1956.
Wetherill, G. W., G. L. Davis, G. R. Tilton, Age
measurements
on
minerals
from
the
Cutler
batholith, Cutler, Ontario, J. Geophys.Research,
65, 2461-2466, 1960.
Wetherill, G. W., O. Kouvo, G. R. Tilton, and
P. W. Gast, Age measurements on rocks from
the Finnish Precambrian, J. Geol., 70, 74-88,
1962.
Zartman, R. E., Thesis, California In•.itute of
Technology, 1963.
Zartman, R. E., and G. J. Wasserburg, A geochronologic study of a granitic pluton from the
Llano uplift, Texas (abstract), J. Geophys. Research,67, 1664,1962.
(Manuscript received May 7, 1962;
revised June 19, 1962.)