JOURNALOF GEOPHYSICALRESEARCH VOLUME67. NO. 10 SEPTEMBElt1962 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 determinationsusing the SrS7-RbS7,Ar4ø-K4ø, and Pb-U methodswere made on samplesof muscovite,biotite, amphibole,microcline, and zirconfrom igneousand meta- morphicrocks from the Franklin Mountains,Hueco Mountains,Pump Station Hills, and Carrizo and Van Horn Mountains. In addition ageswere determinedon a number of base- ment cores from Texas and New Mexico. The results show that a belt of rocks of varied lithologyextending from E1 Pasoto east of the Llano uplift are all of the sameage. The general age by the strontium and argon methodsis 1000 to 1090 m.y.; and by the lead- uranium method on zirconsit is 1150 to 1200 m.y. This event is in the same time band as the 'Grenville' orogenyin Canadaand the northeasternUnited States and 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 pervasiveepi- sodeof Precambrianorogeny on the North Americancontinent for which adequateevidence has been presented.At least one and probably two older periodsof igneousactivity and metamorphismoccurring at 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 HuecoMountains the graniteis either The purposeof this studyis to attempt to use surroundedby unconsolidatedalluvial deposits mineral agesto correlateigneous and metamor- or in fault contactwith youngerbeds; general phic rocks'of Precambrianage in Texas. regionalevidence, however, suggests that it is Precambrianrocks in Texas crop out in the pre-Bliss (Cambro-Ordovician)in age. (c) Llano uplift area of central Texas and in a num- Rhyolitein the Pump StationHills is surrounded ber of separated mountain uplifts in Trans- by unconsolidatedalluvial deposits, and there is PecosTexas (Figure 1). Stratigraphicevidence no convincingstratigraphic evidence pertaining as to the age of these rocks is as follows: to its age.On the basisof regionalgeology earlier 1. Central Texas graniteswhich intrude meta- investigatorssuggested that the rocks are Pre- sedimentaryrocks are overlainby Upper Cam- cambrian [King and Flawn, 1953; Masson, brian strata (Hickory sandstonemember of the 1956]. (d) Metamorphicrocks and pegmatites Riley formation [Cloud, Barnes, and Bridge, in the Carrizo and Van Horn Mountains are 1945]). overlainby Permian (Wolfcamp)beds, but re- 2. In Trans-PecosTexas stratigraphic relations gional stratigraphicevidence suggests that the differin the separatemountain ranges. (a) In the rocks are pre-Van Horn sandstone(Precam- Franklin mountains,granite, volcanic rocks, and brian?), which is pre-Bliss (Cambro-Ordovi- metamorphic rocks lie beneath the Cambro- cian) in age. OrdovicianBliss formation [Harbour, 1960]. A number of chemical lead-uranium and lead- 4021 4022 WASSERBURG,WETHERILL, SILVER, AND FLAWN N E W M E X I C I S O ION ,1114 I •1•1•Sample localities andnumbers I $cal•' I . Modifiedfrom F, 0 I. The Unlvers,IV o! TelLO•Publicolion 5:)01 Fig.1. Map showingthe exposuresof Precambrian rocks in Trans-PeeosTexas and the sample localities. thorironages up to 1100million years have been clearlyshow the existenceof plutonieigneous determined on the rare-earth minerals from the activityat about1100 m.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 age estimateshave been made on zirconsfrom North America some 1200 miles distant was not the Llano regionand from well coresin west specificallymade by theseworkers. Texasby the 'Pb-a' technique[Jaffe, Gottfried, In the presentstudy samples for agedetermi- Waring,and Worthington,1959]. Owingto the nation were obtainedboth from the exposedPre- manydifficulties both technicaland inherentin cambrian and from basement cores. Samples this nonisotopicmethod, the resultswill not be were collected from the various localities with considered in detail. the previousgeologic studies as a guide,and Modern age determinationsusing Ar4ø-K'ø, basementcores were selectedwith the study by Sr87-Rb87, Pb'ø•-U2. •, and Pb'ø*-U•5 methodshave Flawn [1956] as a guide,the purposebeing to beenmade by severalworkers on mineralsin the establish time correlations and compare the graniticrocks and associatedpegmatites in the various geochronometricmethods. Llano area [Aldrich,Wetherill, Davis, and Til- Results ton, 1958;Goldich, Nier, Baadsgaard,Hoffman, and Krueger,1961; Zartman,1962, 1963]. The results for the exposedrocks are pre- The da•a obtained by Aldrich et al., in the sentedby area, after which the basementcore Llano regionare summarizedin 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 Mineral Ar4ø/K4ø Sr87/Rb87 pb2o6/u2s8pb2oT/u2s5 pb2o•/pb•o6 Petrick Quarry Biotite* 1060 1100 ......... (granite) Zircon* 950 990 1070 (pegmatite) Biotite]• 1(•) li•(• ......... Lone Grove Biotite, feldspar$ 1060 ......... Pluton (granite) * After Aldrich, Wetherill,Davis, and Tilton [1958]. • After Goldich,Nier, Baadsgaard,Hoffman, and Krueger[1961]. •:After Zartman[1962a]. Average of severaldeterminations by both methods.All ageshave beenre- calculatedfor the decayconstants used in this paper. the mineral separatesare describedin the ap- tial white micrographicintergrowth. This rock pendix. also shows distinct iron oxide stains on the freshest material. FRANKLIN MOUNTAINS A sampleof feldspar,TH-6, wastaken from a Precambrian rocks are exposed in North microcline-perthite-quartzpegmatite which was FranklinMountain in thevicinity of 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 com- In addition to these materials a sample of prisedof the followingunits in order of decreas- a friable tourmaline-biotite-plagioclaseschist, ing age: (1) The Castnerlimestone, possibly TH-2, was collected. This material was not equivalent to the Allamore limestone of the Van foundin the measuredsection reported by Har- Horn area. Several diabase sills are included in bour for a traversefurther west (seeFigure 2). the Castner. (2) The Mundy breccia,which The unit is about 3 meters, thick and lies be- rests 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 developedstromatolite struc- area underlying the Bliss sandstoneis a massive turesand mud cracksat this locality.The top of granite,which is seento surroundand engulf the limestone here shows distinct contact meta- large blocks of the sedimentarysection. This morphic effects from the granite. The diabase granite showsa variety of phases,the larger sill containsabundant biotite near the granite exposuresbeing a pink-white biotite granite, contact. The origin of the schistlayer is not whichis typicallyrather friable.One phase is a understood. gray-white biotite granite, which locally con- The ages for the Sb87-Rb87and Ar•ø-K•ø tains numerous reacted inclusions and shows a methodson variousmineral separates are pre- jointingstructure with the fracturesspaced at sentedin Table 2; the agesfor the zircons',in approximately 30 to 60 cm. These fractures all Table 3. An estimatedmaximum error is given show iron oxide stains and a pale green dis- for each of the ages. This estimate is based colorationeven in a fresh quarry face.We infer solely on the analytical data without consider- this 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 grano- The biotite and microclineseparated from phyric biotite-hornblendegranite, TH-5, were granite TH-1 gave the followingresults' the collectedin FusselmanCanyon. This rock has strontiumage of the biotiteis considerablyless a spottedappearance produced by the isolation than the argon age, and both are less than the of dark gray microclinecrystals by an intersti- strontiumage of the coexistingmicrocline. The 6 4024 WASSERBURG,WETHERILL, SILVER,AND FLAWN z i•_ x lu.i '.. o.. AGES IN THE PRECAMBRIAN OF TEXAS 4025 TABLE 2. Age Determinations from the and a microclinefrom a crosscuttingpegmatite Franklin Mountains near E1 Paso, Texas TH-6. Strontium ages were determined on the feldspars,and an argon age was determinedon Age, m.y. hornblende [Hart, 1961]. No. Rock Mineral K-Ar Rb-Sr The youngerpegmatite gives a strontium age of 1087 m.y., which is greater than the results TH-1 Granite Biotite 980 4- 20 930 4- 20 on the other feldspars.The differencebetween Feldspar