And Zircon (Zrsio) from Anorthositic Rocks of the Laramie Anorthositecomplexo Wyoming: Petrologic Consequencesand U-Pb Ages

Ameican Mineralogist, Volume 80, pages I3I7-1327, 1995

Baddeleyite(ZrOr) and zircon (ZrSiO) from anorthositic rocks of the Laramie anorthositecomplexo Wyoming: Petrologic consequencesand U-Pb ages

J.lvrns S. Scolrnsr* KrylN R. Cn*rnrRLArN Department of Geology and Geophysics,University of Wyoming, Laramie, Wyoming 82071, U.S.A.

Ansrn-c.cr The Zr-bearing minerals baddeleyite (ZrOr) and zircon (ZrSiOo) occur within plagro- clase-rich (61-950/oplagioclase) cumulates of the Laramie anorthosite complex (LAC), southeasternWyoming. In each of the examined samples,zircon is present as relatively coarse(l-2 mm) interstitial grains, and baddeleyiteoccurs as small (0.05 mm) inclusions within cumulus plagioclase.Zircon crystallized betweencumulus plagioclasecrystals near solidus temperatures from highly fractionated, Zr-saturated liquids. The resultant shape ofzircon was controlled by the form ofthe remaining pore space.The origin ofbaddeleyite in the anorthositic rocks of the LAC is lesswell constrained.It may have crystallizedearly from the anorthositic parental magmas at relatively low silica activities; however, this would require baddeleyite saturation at extremely low Zr concentrations in the parental magmas(<< 100 ppm). Baddeleyite and zircon U-Pb agesreveal that several petrologically distinct intrusions were emplaced and crystallized in the LAC over a relatively restricted 1-3 m.y. interval aI ca. 1434 Ma. The 2o7Pb/2o6Pbages obtained for the baddeleyiteand zircon in eachsample are identical within error (+1-3 m.y.), and U concentrations are uniformly low (<240 ppm), supporting a genetically related origin for the minerals. Two anorthositic layered cumulates and a crosscutting, oxide-rich troctolite from the Poe Mountain anorthosite have crystallization agesthat are identical within error: 1434.4 + 0.6, 1434.5 + 0.6, and 1434.1 + 0.7 Ma, respectively. The only earlier period of anorthositic magmatism that can be identified from the Poe Mountain anorthosite is representedby a leucogabbroic xenolith (1436.2 + 0.6 Ma), which settled onto the floor of the magma chamber that produced the layered cumulates.A sample composedalmost entirely of zoned, iridescent, plagioclasemegacrysts from the Chugwateranorthosite yields a baddeleyite ageof 1435.4 + 0.5 Ma, intermediate between the ages of the xenolith and the layered anorthositic rocks. All the ca. 1.4-1.5 Ga anorthosites in North America, including the LAC, are located near or on Paleoproterozoicboundaries between Archean cratons and accretedProterozoic island arc terranes.These preexisting crustal structuresappear to play a major role in the origin and ascent of anorthositic magmas. The evidence for anorthositic magmatism at 1.43 Ga in the LAC suggeststhat there may be a strong genetic link between these high- temperature mafic magmas and the regional production of anorogenic granites in the western and southwesternU.S., many of which have similar crystallization agesin the interval of 1.43-1.44 Ga.

furnonucrroN al., 1993;Owens el al., 19941,Amelin et al., 1994).Bad- Recent studieshave shown that zircon and baddeleyite deleyite, in particular, has proven to be a useful mineral can be recovered from some anorthositic rocks in Pro- for U-Pb dating in mafic rocks becauseit has negligible terozoic anorthosite complexes, and that precise U-Pb initial common Pb, experienceslittle or no Pb loss, and agescan be determined from both minerals (Mclelland rarely occurs as xenocrysts(Heaman and LeCheminant, and Chiarenzelli, 1990; Doig, l99l; Higgins and van 1993).In conjunction with field relationships,this ability Breemen,1992;van Breemenand Higgins, 1993; Mar- for precise dating potentially allows for detailed docu- tignole et al., 1993; Pacesand Miller, 1993; Duchesneet mentation of the intrusive history of the mafic portions of individual complexes,some of which contain as many * Present address: P6trologie et G6odynamique Chimique as 20 separateanorthositic intrusions [e.g., Harp Lake CPI60/02, Universit6 Libre de Bruxelles,Avenue F.D. Roose- (Emslie, 1980);Nain (Yu and Morse, 1993)1.This may velt 50, 81050 Brussels,Belgium. be particularly useful where emplacement-relatedrecrys- 0003-o04x/95 / | 1I2-r3 r7502.00 L3T7 t318 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES tallization has obscuredprimary contacts betweenintru- Estimates of emplacement pressuresfor the monzonitic sions. Becausethe relative stability of baddeleyite and rocks (equilibria among pyroxenes, olivine, and quartz) zircon is related to silica activity (Butterman and Foster, yield midcrustal pressuresof about 3 kbar in the north 1967), the textural and morphological relationships be- and 4 kbar in the south (Fuhrman et al., 1988; Kolker tween thesetwo minerals and the rocks they crystallized and Lindsley, 1989).Three major composite anorthositic within may also help to constrain the crystallization path intrusions are contained within the LAC: the Poe Moun- of anorthosite. tain, Chugwater, and Snow Creek anorthosites (Figs. I The Laramie anorthosite complex (LAC), southeastern and 2). Wyoming, contains the only major exposure of middle The Poe Mountain anorthosite consistsof (l) a central Proterozoic anorthositic rocks in the western and south- core of pervasively recrystallized megacrystic anortho- westernU.S. (Frostet al., 1993).The Nain and Grenville site, (2) a marginal layered series containing a 5-7 km provinces of Canada contain numerous anorthositic in- thick packageof layered and laminated anorthositic and trusions of middle Proterozoic age (Ashwal, 1993), how- leucogabbroiccumulates, (3) a suite of igneousxenoliths, ever, intrusive rocks of the exposedProterozoic crust in ranging in composition from leucogabbroto anorthosite, the western U.S. are dominated by relatively highJevel and (4) a seriesof late, undeformed troctolitic intrusions granitic batholiths with crystallization agesbetween 1.49 (Fig. 2) (Scoates,1994). The centralcore areaofthe Poe and 1.40Ga (Anderson,1983). It hasbeen proposed that Mountain anorthosite contains mainly homogenous,re- mafic magmatism at depth was the driving force for the crystallized anorthosite of uniform plagioclasecomposi- large-scalecrustal melting necessaryto produce the gran- tion (-Anou) with little evidence of primary igneous ites (Anderson and Bender, 1989),but direct agesof crys- structuresor textures.Where zonesof relatively weak re- tallization for coeval mafic rocks in this region are no- crystallization occur, the protolith appears to be com- tably lacking. The age of the anorthositic rocks in the posed of coarse-grained(2-4 cm) megacrystsof plagio- LAC is known only indirectly through the ageof a cross- clase. The surrounding marginal layered seriescontains cutting monzonite (Frost et al., 1990).Thus, the purposes two distinct layered zones:an inner anorthositic layered of this study are (1) to determine directly the ages of zone (ALZ) and an outer leucogabbroic Iayered zone crystallization of a variety of anorthositic rocks from the (LLZ).The overall shapeof the intrusion is domical. Lay- LAC to limit the duration of middle Proterozoic mafic ering is steepest along the outer margins of the Poe magmatism in southeasternWyoming; (2) to investigate Mountain anorthosite, dipping 60-90", and layering shal- the geochronologicalrelationships among the LAC, other lows considerably toward the central core, dipping 30- middle Proterozoic anorthosites,and similarly agedgra- 60". The contact between the two layered zones is con- nitic batholiths in North America; and (3) to investigate tinuous along strike, across several Laramide faults, for the morphologic and textural characteristicsof baddele- -20 km. Each layered zone is, in turn, subdivided into yite and zircon as guides to the crystallization history of distinct layered sections (lower, middle, and upper) on anorthositic rocks. the basis of layering characteristicsand cryptic geochemical variations (Scoates,1994) (Fig. 2). The base of the GBor,ocv oF THE L.1,ru.*nn ANoRTHosrrE coMpLEx lowermost ALZ cannor be identified in the field because The unmetamorphosedand undeformed72l km'?LAC the effectsof high-temperaturerecrystallization that were outcrops in the southern Laramie Mountains, southeast- produced during late emplacement deformation (La- ern Wyoming (Fig. l). Only a portion of the LAC is pres- france et al., 1994) are pervasive in the interior of the ently exposed;west-dipping Laramide (Late Cretaceous Poe Mountain anorthosite. The uppermost portions of to early Tertiary) thrust and high-angle reverse faults the upper LLZ are missing becauseof subsequentem- truncate the easternmargins, and shallowly dipping, early placement of the Sybille intrusion (Fig. 2). Well-devel- Paleozoic sediments unconformably overlie the western oped igneous layering and a prominent plagioclaselam- margins. The LAC intruded Archean gneissesof the Wy- ination are characteristicof anorthositic cumulates oming province in the north and Proterozoic supracrustal throughout the entire layered series.Rocks in the layered rocks in the south (Fig. l). It probably intruded along the seriesare composed of cumulus plagioclase(Anor-rr), oi- Cheyennebelt (Frost et al., 1993),the ca. 1.78 Ga colli- kocrystic clinopyroxene, inverted pigeonite, olivine, il- sional zone that separatesthe Archean rocks from the menite, and magnetite, with interstitial apatite and sul- accreted Proterozoic island arc terranes in the south fides and late biotite. (Karlstrom and Houston, 1984). A suite of igneous xenoliths, ranging in composition The LAC consistsof a central massof anorthositicrocks from anorthosite to leucogabbro,is found throughout the (550 km'z) that was intruded by dioritic and troctolitic layeredseries. These xenoliths are mineralogically similar rocks of the StrongCreek complex (Mitchell, 1993),mon- to the layered series and consistently have more calcic zonitic rocks of the Maloin Ranch pluton (Kolker and plagioclasecompositions and lower initial Sr isotopic ra- Lindsley, 1989; Kolker et al., 1990, 1991), the Sybille tios than their host cumulates (Scoatesand Frost, 1994; intrusion (Fuhrman et al., 1988), and the Red Mountain Scoates,1994). They therefore appear to representa sep- pluton (Anderson et al., 1987, 1988), and granitic rocks arate and distinct product of anorthositic magmatism in of the Sherman batholith (Geist et al., 1989) Grg. l). the LAC. Small troctolitic (plagioclase* olivine) intru- SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES l3l9

Fig.2. Simplifiedgeologic map of the northernLaramie anorthositecomplex. The four geochronologicalsample sites from Fig. 1. Geologicmap of the southernLaramie Mountains of the PoeMountain anorthosite are shown as solid circles. ALZ: southeasternWyoming, showing the location of the Laramrean- anorthositiclayered zone, andLLZ: leucogabbroiclayered zone. orthosite complexand the inferredtrace of the Cheyennebelt The contactbetween the ALZandLLZ is shownas a solidline, : throughthe l,aramieMountains. PMa PoeMountain anor- andthe contacts between layered sections within thetwo layered thosite, : : Ca Chugwateranorthosite, and SCa SnowCreek zones(lower, middle, and upper in the ALZ, and lower and anorthosite. upperin theLI-Z) areshown as dashed lines. The "contact zone" denotesthe areawhere pervasive emplacement-related recrys- tallization overprintedthe primary magmaticcontacts between sions occur throughout the Poe Mountain anorthosite. the Poelriountain, Chugwater, and SnowCreek anorthosites. The largest truncates the southwesternextension of the layered series (Fig. 2). Coarse-grainedtroctolite is also found associatedwith massiveilmenite in the Sybille iron thosite to the north and the Chugwater and Snow Creek titanium oxide deposit (Bolsover, 1986; Epler, 1987), anorthositesare not easily identifiable in the field because where angular inclusions of anorthosite indicate that an- all primary igneousstructures in the areahave been over- orthosite was completely solidified prior to intrusion of printed by the effectsof high-temperatureemplacement- troctolite. related deformation. The approximate locations of intru- The Chugwateranorthosite (Fig. 2) consistsof interfin- sion contacts in this area are marked by a gradational geredlenses of recrystallizedanorthosite and undeformed contact zone in Figure 2. megacrysticanorthosite and troctolite (Frost et al., 1993). Intrusive relationships indicate that anorthosite is the There are no laterally continuous, layered cumulates in oldest lithology in the LAC (Frost et al., 1993).In the the Chugwater anorthosite, and well-zoned, iridescent northern LAC, the Poe Mountain anorthosite was intrud- plagioclasemegacrysts are common. Plagioclasecompo- ed by monzonitic rocks of the Sybille intrusion that, in sitions are uniformly more calcic in the Chugwater an- tum, were intruded by the Red Mountain pluton (Figs. I orthosite (>Anro) than in the core of the Poe Mountain and 2). Frost et al. (1990) determined the uranium-lead anorthosite(-An*). The smaller Snow Creekanorthosite zircon age for a fayalite monzonite in the Red Mountain occurs in the west-central LAC (Fig. 2) and consists of pluton to be 1439 1l Ma, subsequentlyrevised to 1431.3 recrystallized megacrystic anorthosite with abundant + 1.3 Ma (C. D. Frost, personalcommunication). In ac- zoned, iridescentplagioclase megacrysts (Mitchell, I 993). cordancewith field relationships, this is a minimum age Contact relationships between the Poe Mountain anor- for intrusive activity in the northern part of the LAC and r320 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U.Pb AGES a minimum agefor the anorthositic rocks. The only pre- ages of analyzedfractions from each sample are nearly 207Pbl206Pb vious study that attempted to determine directly the age identical. Therefore,the weightedaverage ages of anorthositic rocks in the LAC used Rb-Sr isotopic sys- (weighted by the inverse variance ofthe individual ages) tematics (Subbarayudu, 1975). No meaningful isochrons were calculated rather than linear regressions.The were derived, however, becauseof the complex, open- weighted averageages and uncertainties were calculated system crystallization history of the anorthositic rocks using ISOPLOT (Ludwig, 1988b), and errors are quoted with respect to Sr isotopes,which is the result ofcrustal at 2o. contamination and magma mixing (Scoatesand Frost, r994). ZrncoN AND BADDELEyITE MoRpHoLocrEs In four of the five samplesexamined, zircon is far more Sl.tvrpr,rnc STRATEGvAND ANALvTTCALTEcHNreuEs abundant than baddeleyite (on the order ofthousands to Anorthositic rocks are predominantly plagioclase-rich one), although the total abundanceof zircon is typically adcumulates.Textural evidence suggeststhat they typi- <0.2o/oof the whole rock. All analyzed zircon crystals cally include only minor amounts of trapped interstitial were colorless,anhedral fragments.Texturally, zircon oc- liquid that could be enriched in incompatible elements. curs as a late-crysallizing phase interstitial to cumulus Thus, whole-rock samples contain only minor amounts plagioclase(Fig. 3A). Contacts between zircon and other of U-rich trace minerals suitable for U-Pb geochronology. minerals are sharp and devoid of secondary alteration In the Poe Mountain anorthosite, this is reflectedby low products, indicating that the original morphology of zir- zr contenrs in both the ALZ (16-73 ppm) and ttJeLLZ con was not modified subsequentto crystallization. In (12-106 ppm) (Scoates,1994). We processed> 100kg of thin section, zircon is typically I mm in diameter, with each sample in this study to extract sufficient quantities the largestgrains up to 1.5mm across.The analyzedgrains of U-rich minerals for analysis.Each of the five samples were much smaller (0.25-0.50 mm) and were probably reported in this study, ranging in plagioclaseabundance fragments of larger grains. Poikilitic zircon is common from 6l-950/0,contained both baddeleyiteand zircon. (Fig. 38) and is further textural evidencefor late intersti- Hearly minerals were concentratedfrom crushed sam- tial crystallization. Euhedral zircon was not found in any ples by standard density and magnetic separationproce- of the mineral separates.Many of the grains selectedfor dures.No coresin baddeleyiteor zircon were visible with analysis were characterizedby minute rippleJike struc- the binocular microscope under high-power magnifica- tures across their outer surfaces,observable under both tion (50 x ). Small inclusions were visible in some of the moderate- to high-power magnification (20-50 x ) with a concentratedbaddeleyite and zircon, but grains selected binocular microscopeand a scanningelectron microscope for analysis were optically devoid of inclusions. The grains (SEM). These structures may be a manifestation of non- were cleanedin cold 2N HNO3 prior to dissolution. Both planar (skeletal)growth, perhaps reflecting rapid growth baddeleyite and zircon were dissolved in concentrated from a late-stagezircon supersaturatedmelt (Bossart et HF and HNO, in teflon microvesselsfollowing the pro- al.. 1986). cedure ofParrish (1987) and converted to chlorides. Ali- Baddeleyiteis presentin all samplesand exhibits mor- quots of sample solutions were spiked with a mixed'z3sU/ phological and textural characteristicsmarkedly different 208Pbspike. Pb and IJ were purified in anion-exchange from those ofzircon. It occurs as small euhedral to sub- columns using HCI chemistry modified from Krogh hedral grains within cumulus plagioclase (Fig. 3C and (1973). The total Pb blank decreasedsignificantly during 3D) and is not observed interstitially with zircon. The the courseof this study, from 80 to 5 pg. The blank com- occurrence of baddeleyite is not related to fractures or position was 206Pb/204Pb: 19.09, 2o7Pb/2o4Pb : 15.652, healed fractures in plagioclase,indicating that it did not and 208Pb/204Pb: 38.31 and was usedin correctionsfor form by secondaryalteration processes.No rims of zircon blank contributions. The Pb and U sampleswere loaded were observed,optically or with the SEM, on baddeleyite. onto single rhenium filaments for isotopic analysis by Average grain size is about 0.05 mm, and the largestob- thermal emission mass spectrometry. The HrPOo-silica servedgrains are 0.2-0.3 mm in diameter. Baddeleyiteis gel technique was used for Pb, and U was loaded with commonly concentratedin selectplagioclase grains with- H3PO. and graphite and run as metal ions. The isotopic in a given thin section. It is restricted to the cores of analyseswere conducted at the University of Wyoming plagioclasegrains and does not occur in the relatively on a multiple-collector, VG Sector mass spectrometer. potassic marginal areasthat contain alkali feldspar exso- Mass discrimination factors for Pb and U were deter- lution lamellae. Where multiple grains are observedin a mined by multiple analysesof NBS SRM 981 and U-500, singleplagioclase grain, there is typically no preferredori- respectively,and were 0.094 t 0.0560/oper atomic mass entation, although examples of parallel orientation were unit for Pb and 0 + 0.060/oper atomic mass unit for U. observed. Analyzed baddeleyite grains were translucent The program PBDAT (Ludwig, 1988a) was used to re- brown, and many grains displayed the well-developed duce the raw mass spectrometerdata, correct for blanks, polysynthetic twinning that is characteristic of baddele- and calculate uncertainties. Much of the data from the yite (Smith and Newkirk, 1965; Heaman and Le- analyzedfractions overlapsconcordia, and the 207Pbl206PbCheminant. 1993). SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES t32l

U-Pb nrsur.rs The U-Pb isotopiccompositions of l5 baddeleyiteand four zircon fractions (Table l) were determined from five samples:four from the Poe Mountain anorthosite (Fig. 2), including one from eachof the two layered zones(ALZ and LLZ), a crosscuttingtroctolite, and a leucogabbroic xenolith, and one sample from the Chugwater anorthosite.Between 40 and 120grains ofbaddeleyite and zircon were dissolved for each analysis. The concentrations of U and Pb are extremely low, ranging from 43 to 238 ppm U and l0 to 55 ppm Pb for baddeleyite,and 9 to 127 ppm U and 3 to 34 ppm Pb for zircon. Theseconcentra- tions are typical of Proterozoic anorthosites(e.g., Higgins and van Breemen, 1992; Martignole et al., 19931,van Breemenand Higgins, 1993;Owens et al., 1994)and are consistentwith the crystallization of baddeleyite and zircon from U-poor parental magmas.Even with theselow concentrations,206Pb/201Pb values (correctedfor fractionation, spike composition, and blank contribution) are extremely high, ranging from 3680 to 285 360 (Table l), thus indicating very low contributions of initial Pb. The U-Pb data clusternear concordia(0-1.10/o discordance), demonstrating the closed-system behavior of baddeleyiteand zircon with respectto Pb loss or U gain. There is no discernible difference within error between baddeleyiteand zircon 2D7Pb/2o6Pbages within each sample. Becauseof this, we consider the weighted average 207Pbl206Pbages of combined baddeleyite and zircon to be the best estimate of crvstallization aee.

Fig. 3. Representativephotomicrographs ofzircon andbad- Oikocrystic anorthosite (PM468) - ALZ, Poe Mountain deleyitemorphologies in anorthositicrocks of thePoe Mountain anorthosite(latitude 4146'03', longitude105"21'58') anorthosite.(A) Interstitialzircon, (B) poikilitic zircon,(C and plagioclase. Sample PM468 is from a well-laminated anorthosite D) baddeleyiteinclusions in (930/oplagioclase) containing abundant, regularly spaced clinopyroxene, olivine, and iron titanium oxide oiko- Troctolite (PM622) - Poe Mountain anorthosite(latitude crysts. Stratigraphic reconstructionsplace it 2240 m be- 4146'45', longitude105'21'06') Iow the ALZ/LLZ contact. The data from three badde- SamplePM622 is from a coarse-grained,oxide-apatite- leyite fractions and zircon one fraction are nearly rich troctolite (610/oplagioclase) associated with massive concordant(0. 6-0.90lo discordance), with 207Pbl206Pb ages iron titanium oxide ore in the Sybille deposit (Bolsover, of 1433.9-1434.8Ma for baddeleyiteand 1434.8Ma for 1986; Epler, 1987; Lindsley and Frost, 1990). The zircon (Fig. 4,A.).The weighted 207Pb/2o6Pb average ageof weighted average207Pb/2o6Pb age of four nearly concor- 1434.4+ 0.6 Ma is consideredthe best estimatefor the dant fractions (threebaddeleyite and one zirconl 0.6-0.90lo crystallization age of PM468. discordance)is 1434.1 + 0.7 Ma (Fig. aQ. Localized shearingof the enveloping anorthositic cumulatesand inclusions of anorthosite xenoliths in the undeformed troc- Laminated leucogabbro (SR247)-LLZ, Poe Mountain tolite indicate that the anorthosite was consolidatedprior anorthosite (latitude 4144' 27', longitude 10522' l9') to intrusion ofthe troctolite. There are, however, no re- Sample SR247 is from a well-laminated leucogabbro solvable differencesbetween the agesofthe layered series occurring 3240 m stratigraphically above PM468. The cumulates and this crosscuttingtroctolite. U-Pb data for three baddeleyite fractions are nearly concordant (0.3-0.80/odiscordance), and the zircon fraction Leucogabbroicxenolith (SR246)-Poe Mountain is concordant (Fig. 4B). They yield a weighted average anorthosite (latitude 4l' 44' 16', longitude 10525' 45' ) 201Pb/206Pbage of 1434.5 + 0.6 Ma. Thus, the crystalli- SampleSR246 is from a leucogabbroicxenolith (830/o zation agesof anorthositic cumulates from both the ALZ plagioclase),with a minimum length of 50 m and an in- and LLZ agreewithin uncertainty. ternal fabric defined by plagioclase lamination that is r322 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES

TABLE1. U-Pb results for baddeleyiteand zircon fractions separated from anorthositic rocks of the LAC

Concentration

Description' Com- Measuredatomic ratios-- Ages (Ma) mon Frac- Weight U Pb Pb '6Pb/ 26Pbl 8rPbl 2otPbl tion (mg) (ppm) (ppm) (pS)t zo4Pbf, ,GPb/238U 2o?Pbl2sl) 4?Pb/r6Pb ruu 45U mPb$ Rho Dll PM468:Poe Mountain anorthosite, ALz B (nm1) 0.397 49 12 101 1 0.090406(0.09)1429.0 1431.1 1434.1(1.7) 0.98 0.60 3138 0.2481(0.45)3.093(0.46) 't429.7 B (d-1) 0.139 64 15 42 32776 0.2476(0.29)3.088(0.31) 0.090437(0.10) 1426.2 1434.8(1.9) 0.94 0.85 B (m2) 0.149 90 21 23 35447 0.2475(0.2713.085(0.28) 0.090396(0.07) 1425.6 1428.9 1433.9(1.4) 0.97 0.83 z (d-2) 0.464 110 31 78 3153s 0.2479(0.40)3.091(0.41) 0.090437(0.07) 1427.4 1430.4 1434.8(1.3) 0.99 0.75 SR247:Poe Mountain anorthosite, LLZ B (nml) 0.447 71 17 12R 8164 0.2488(0.40)3.101(0.41 ) 0.090405(0.07)1432.2 1433.0 1434.1(1.4) 0-98 0.34 B (m1) 0.437 60 14 188 3680 0.2476(0.37) 3.087(0.40) 0.090423(0.14) 1426.3 1429.6 1434.5(2.7) 0.94 0.84 B (m3) 0.201 68 16 24 29668 0.2481(0.25)3.094(0.26) 0.090441(0.06) 1428.7. 1431.2 1434.9(1.2) 0.97 0.67 Z (nml) 0.724 93 44 13819 0.2492(0.4613.105(0.s0) 0.090382(0.18) 1434.3 1434.0 1433.6(3.5) 0.93 PM622:Poe Mountain anorthosite, oxide-rich troclolite B (m1) 0 422 192 46 324 4794 0.2481(0.48)3.092(0.49) 0.090407(0.10) 1428.s 1430.8 14i]4.1(1.9) 0.98 0.63 B (m2) 0.348 86 20 30 79812 O.2476(0.4s13.086(0.53) 0.09037s(0.29) 1426.3 1429.2 1433.s(5.6) 0.84 0.74 B (m3) 0.268 182 43 29 165750 0.2475(0.2413.086(0.25) 0.090415(0.06) 1425.7 1429.2 1434.3(1.1) 0.97 0.85 z (d-2) 0.408 90 26 51 41176 0.2474(0.40) 3.082(0.44) 0.090359(0.16) 1424.8 1428.2 1433.1(3.1) 0.93 0.82 SR246:Poe Mountain anorthosite, LLZ xenolith B (d-1) 0.546 76 18 41 228640 0.2492(0.92) 3.11 0(0.33) 0.090513(0.06) 1434.2 1435.1 1436.4(1.2) 0.98 0.35 B (d-3) 0.179 43 10 QI 25158 0.2499(0.30)3.118(0.31) 0.090478(0.09) 1438.0 1437.0 143s.6(1.7) 0.96 z(d-2) 0.668 127 34 232 8649 0.2484(0.40)3.100(0.40) 0.090s08(0.07) 1430.4 1432.8 1436.3(1.4) 0.98 0.66 BM136: Chugwateranorthosite, iridescent megacry3tic anorthosite B (nml) 0.521 109 26 415 2516 0.2480(0.42)3.092(0.46) 0.090447(0.18) 1428.0 1430.0 1435.0(3.4) 0.92 0.74 B (ml) 0.447 238 55 45 285360 0.2471(0.36) 3.082(0.37) 0.090450(0.06) 1423.5 1428.1 1435.0(1.2) 0.99 1.07 B (d-1) 0.258 81 19 50 28224 0.2472(0.25) 3.084(0.26) 0.090476(0.07) 1424.3 1428.8 143s.6(1.3) 0.96 1.08 B (m2) 0.298 120 28 20 156380 0.2476(0.35) 3.088(0.35) 0.090478(0.06) 1425.9 1429.8 1435.6(1.2) 0.98 0.9s * Minerals analyzed: B : baddeleyite,Z: zircon. Magnetic properties and side angle of Franz magnetic splitter in parentheses. -- Atomic ratios corrected for fractionation, spike composition, blank contribution, and common Pb (Stacey and Kramers, 19751;2o percent errors in oarentheses. t Total common Pb, includingsample, spike, and blank. + The ePb/ePb corrected for fractionation,spike composition, and blank contributions. $ Errors of the e?Pb/26Pbages are quoted at 2d in parentheses. ll D = discordancein percent. oblique to the fabric in the envelopingLLZleucogabbros. more abundant than zircon in BMl36. The weighted av- It also contains severaliridescent plagioclasemegacrysts, erage2o'Pb/2o6Pbage of four baddeleyitefractions is 1435.4 which are not found in the layered cumulates of either + 0.5 Ma (Fig. aD. This age overlaps within error the the AW or LW. The weighted average207Pbl206Pb age of younger agesof the Poe Mountain layered seriessamples three analyzedfractions (two baddeleyiteand one zircon) and the older age of the xenolith; the megacrysticanor- is 1436.2 + 0.6 Ma (Fig. 4D). The leucogabbroicxenolith thosite may be the same age as the layered seriescumu- is resolvably older than the LLZ cumulates that contain lates to the north or as much as 2 m.y. older. it, and thus the occurrenceof a distinct earlier phase of anorthositic magmatism in the LAC has been identified. OnrcrN oF zrRcoN AND BADDELEyITE rN THE Llnnvrrn ANoRTHosrrEs -Chugwater Megacrystic anorthosite (BMf 36) In mafic rocks, baddeleyite and zircon are most com- anorthosite (latitude 4134' 20', longitude 10523' 52') monly found in late interstitial regions or coarsepegma- SampleBMl36 is from a megacrysticanorthosite (950/0 toids (e.g.,Heaman and LeCheminant, 1993).The results plagioclase)from the Chugwateranorthosite. It occurs in of this study, however, indicate a separatecrystallization a relatively unrecrystallizedlens, 100 m wide, within per- history for baddeleyite and zircon in anorthositic rocks vasively recrystallizedanorthosite and may representei- of the LAC. The co-occurrenceof baddeleyiteand zircon ther a late intrusive phase of the Chugwater anorthosite in anorthositic sampleshas rarely been reported. Amelin or a low-strain window in the recrystallizedanorthosites. et al. (1994) provided U-Pb data for both baddeleyiteand It is compositionally distinct from the Poe Mountain an- zircon from a gabbro-anorthosite (sample 7/90) in the orthosites to the north and is composed of abundant ir- Korosten complex, Ukraine, and showed that the 'zo?Pb/ idescentplagioclase megacrysts, up to 4 cm in diameter, 206Pbages of fractions ofboth minerals are identical with- showing strong oscillatory zoning, normal and reverse in error. Although the physical settingsof theseminerals zoning, and prominent resorption surfaces.In contrast to in the sample are not noted, their morphological descrip- the other four samples in this study, baddeleyite is far tions are strikingly similar to those described in this study. SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES r323

0 251 PM468:Poe Mounrain anonhosite SR247: Poe Mountain ano(hosite ALZ LLZ ..."t'o 0 ?50

> 0.249 F 0.24e

$ o.zcs p ozar

z 's- 207Pb/206pb 0.247 K average: 0.247 / ./ weighred average: il:tl*jl1:,'l;ftPb L-' 1434.5+l- 0.6 Ma A. (MSWD= 0.67) B. (MSWD = 0.64) o.246 0 246 3.07 3.08 3.09 3 10 3 ll 3 12 3 13 3.14 3 06 3.07 3.08 3 09 3.10 3.ll 3.12 3.13 3.t4 20?Pb/2r5fI 2o7Pbl235tJ

o25r 0.251 PM622:Poe Mountain anorthosite SR246:Poe Mountain anorthosite oxide-richtroctolite leucogabhroicxenolith 0.250 0.250

p o.zts F o24e

H o.z+s p o,z+r tqzs -y' -/ o.247 ave.tge: '? weightect2o7Pbfocnpb % [:'L'l'".9-'Li"fiftn' o.247 average: i.::'- 1436.2+l- 0.6 Ma c. (M,swD= 0.32) D. (MSWD = 0.46) 0.246 o.246 3.07 3.08 3.09 3.10 3.ll 3.12 3.13 3.r4 3 06 3 07 3.08 3.09 3 l0 3.11 3.r2 3.13 3.r4 207Pb/2r5fI 207Pb135tJ 0.25r BM136: Chugwateranorthosite megacrysticanorthosite 0.250

p o.z+l

$' o.zcs

20?Pb/2o6Pb o.247 Weighred average: 1435.4+/- 0.5 Ma (MSWD = 0.39) 0.246 3.06 3.07 3.08 3 09 3 10 3 rl 3,12 3,13 3 t4 20?Pb/2r5fl

Fig. 4. Concordia plots for U-Pb data from analyzedbaddeleyite and zircon fractions. Each ellipse representsthe result ofthe analysisof a single fraction. Solid line ellipses: baddeleyite,and dashedline ellipses: zircon. (A) PMa68: anorthosite, ALZ, Poe Mountain anorthosite; @) SR247: leucogabbro,LLZ, Poe Mountain anorthosite; (C) PM622: coarse-grainedoxide-rich troctolite, ALZ,Poe Mountain anorthosite; @) SR246: leucogabbroicxenolith, LLZ,Poe Mountain anorthosite; (E) BM136: megacrystic anorthosite, Chugwateranorthosite.

The textural interpretation of zircon in the LAC an- influenced by the shape ofpreexisting plagioclasegrains orthosites is relatively straightforward. The obvious in- and the shape of the remaining pore space.The optical terstitial and poikilitic habit ofzircon indicates that zir- lack of cores and resorption surfaces,the extremely low con crystallizednear or at solidus temperaturesin the last U and Pb concentrations,and the extremely high ,06Pb/ available pore spacesofa nearly consolidatedplagioclase- 20aPbvalues in zircon indicate that it is unlikely to be rich cumulate. The resultant shapeof zircon was strongly inherited. Similar anhedralzircons from anorthosite were t324 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES t433 1435 r436 743'I Recent experimental work may help to resolve the origin of baddeleyitein anorthositic rocks of the LAC. Rawling et al. (1995) reported that baddeleyite and intermediate- Anorthosite (8M136) *-----ra+ composition plagioclaseare not compatible at 1100 "C, at both I atm and 5 kbar, and therefore a true inclusion ------a-+ Troctolite (PM622) origin seemsto be precluded. No matter what the exact mechanism of origin of baddeleyite, the similarity be- F------++ Irucogabbro(SR247) tween the 2oiPb/2o6Pbages of baddeleyite and zircon lPMa I indicates that the interval of time between the crystal- '------rt--+ Anorthosite (PM468) lization of the two minerals was within the determined analytical error. Xenolith(SR246) F-----O+ Eupr.lcnunNT AI\D cRysrALLrzATroN HrsroRy OF THE LIN-I,VTTN ANORTHOSITES

1433 1434 1435 1436 t431 The baddeleyite and zircon U-Pb agesthat we report interval geologictime in the Age (Ma) spana relatively restricted of middle Proterozoic: a maximum interval of 3.4 m.y. and Fig. 5. Comparisonof U-Pb agesdetermined in this study a minimum interval of 0.8 m.y. (Fig. 5). Crystallization for anorthositic,leucogabbroic, and troctolitic rocks of the ages of both the layered cumulate samples of the Poe Laramieanorthosite complex. Mountain anorthosite and the crosscuttingtroctolite are indistinguishablewithin error (< I m.y.).The two layered cumulate samplesare separatedby over 3000 m of pla- also reported by Martignole et al. (1993) from the Ri- gioclase-richcumulates. If I m.y. was the maximum time vi€re-PentecOteanorthosite in the east-centralGrenville for the crystallization of -3000 m of adcumulate rocks, province, Canada. These observations have important and crystallization was continuous, then a minimum implications for U-Pb geochronologybecause there is lit- averagerate of accumulation of -0.3 cm/yr is implied. tle morphological and compositional difference between Our estimateis comparableto that of Morse(1979a, 1986) the zircons analyzedin our study and the demonstrably for the crystallization of the plagioclase-richKiglapait in- metamorphic zircons from the Adirondack anorthosites trusion (averagemodal feldspar : 73o/o;Morse, 1979b) (Chiarenzelli and Mclelland, 1993). Thus, it is evident in the Nain plutonic suite, Labrador. He estimated that that care is required when interpreting the significanceof the crystallization time for the 8400 m thick Layered U-Pb data from anhedral zircon, and textural constraints Group in the Kiglapait intrusion was about 106yr, with in thin section should always be considered. an averageaccumulation rate of about 1 cm/yr. Although the nearly identical U and Pb concentrations The leucogabbroicxenolith in the LLZ, with a crystal- and 207Pbl206Pbages ofzircon and baddeleyite are strong lization ageof 1436.2 + 0.6 Ma, is the only sample from evidence that baddeleyite is igneousin origin and related the Poe Mountain suite with a resolvably different age to the crystallization history of the anorthositic rocks in (Fig. 5). However, even the age difference between the the LAC, the mechanism of formation of baddeleyitere- xenolith and the rest of the Poe Mountain anorthosite is mains unclear. The two most commonly proposedorigins Iimited to a minimum of 0.5 m.y. and a maximum of 2 for baddeleyitein mafic rocks are late crystallization from m.y. Numerous other compositionally and isotopically highly fractionated interstitial liquids (Heaman and similar igneousxenoliths occur within the ALZandLLZ LeCheminant, 1993) and exsolution from ilmenite (Nas- of the Poe Mountain anorthosite(Scoates and Frost, 1994; lund, 1987; Mclelland and Chiarenzelli,1990; Loferski Scoates,1994), and it is possible that some of these may and Arculus, 1993). Neither mechanism is appropriate belong to this earlier 1436 Ma phaseof anorthositic mag- for the origin of baddeleyite in the Laramie anorthosites matism. The xenoliths may have been derived from a becausethis baddeleyite occurs in early-crystallizedpla- preexistingintrusion near or at the level of emplacement gioclaseand is not associatedspatially with discrete grains of the LAC (-3-4 kbar), or they may have been en- of ilmenite. trained in ascending magma and derived from depth On the basis of the observed textural relations only, somewherealong the magma conduit. baddeleyite in the LAC may have crystallized prior to Field relationships indicate that the anorthositic rocks plagioclase.This would imply lower silica activities in the are the oldest lithology in the LAC (Frost et al., 1993). early crystallization of the anorthosites,prior to late sat- Each of the samplesin this study has crystallization ages uration ofzircon at elevatedsilica activities. It would also older, outsideof error, than the only existing,precise U-Pb imply early saturation of baddeleyite in the anorthositic agefrom the LAC, 1431.3 + 1.4 Ma from the Red Moun- parental magmasat exceedinglylow Zr concentrations.If tain pluton (C. D. Frost, personal communication). Be- Al-rich gabbroic dikes in the LAC can be consideredas cause the Red Mountain pluton is the youngest of the representativeof parental magmas, this implies Zr con- monzonitic intrusions in the northern LAC, it appears centrationsin the l0-60 ppm range(Mitchell et al., 1995). that the majority of the intrusions in the LAC, displaying SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES t325 a wide range of compositions, crystallized over a 3-7 area with crystallization agesin this interval. The Sr and m.y. interval. Nd isotopic compositions of the anorthositic rocks and associatedAl-rich gabbros are consistent with crystalli- RrcroN,ll, sETTrNc oF THE zation from mantle-derived magmas that were contami- 1.43 Ga Lmlpnn ANoRTHosrrE coMpLEx nated by crust during ascent (Scoatesand Frost, 1994; The LAC is one of over 50 intrusions with crystalli- Mitchell et al., 1995). If heat from the parental magmas zation agesbetween 1.49 and 1.40 Ga contained in a at depth produced the crustal melting required for granite major 4800 km long belt that extends from Labrador to production, then the general absenceof exposed mafic California(Emslie, 1978; Anderson, 1983; Anderson and magmatism associatedwith these batholiths may be an Morrison, 1992;Gower and Tucker, 1994).The vast ma- artifact of crustal structure (Frost et al., 1993). The LAC jority of theseintrusions are granitic batholiths of A-type may have been allowed to ascendto higher crustal levels affinity, commonly referred to as anorogenic granites along the reactivated Cheyennebelt. (Anderson, 1983). However, five intrusions containing anorthositeare known: the 1.43 Ga LAC; the 1.49 Ga AcrNowr,Bnclrnr.crs Wolf River batholith of Wisconsin (Van Schmus et al., This paper representsa portion of J.S.S.'sPh.D. dissertation at the 1975);and three intrusions in Labrador, the 1.45 Ga Harp University of Wyoming. We extend thanks to J.N. Mitchell, D.H. Lind- for helping to collect the large samples,C.D. Frost Lake complex(Krogh and Davis, 1973),the 1.46Ga Mi- sley, and A. Greene for use of the clean laboratory facilities and mass spectrometerat the chikamau intrusion (IGogh and Davis, 1973), and the University of Wyoming, and S. Swapp and K. Krugh for cheerfully op- 1.42 Ga Mistastin intrusion (Emslieand Stirling, 1993). erating the SEM at the University of Wyoming B.R. Frost, C.D. Frost, As has been observed in the LAC, each of these anor- D.H. Lindsley, R.F.J. Scoates,and D. Weis reviewed versions of this thositic intrusions is near or along collisional zones be- paper and contributed many useful and insightful comments.Joumal re- views by P. Mueller and J. Mcl-elland significantly improved the clarity tween Archean cratons and Paleoproterozoic orogens of the final version. J.S.S.wishes to thank J. Michot for extending an (Gower and Tucker, 1994). This suggeststhat the pres- invitation to visit the Universit6 Libre de Bruxelles,where the revisions ence of preexisting crustal structures (lithospheric weak- to this paper were completed.Funding for this project was obtained from nesses)facilitates the generation and the ascent of anor- NSF grant EAR-9218360 to B.R. Frost and C.D. Frost and earlier gtrants C.D. Frost (EAR-9017465) and D.H thositic magmas. Reactivation of these terrane boundaries for field work to B.R. Frost and Lindsley(EAR-8618480 and EAR-8816040). may have occurred in responseto either regional com- pressionor extension.Nyman et al. (1994) argued on the Rnrnnrxcns crrED plutons basisof kinematic data from 1.4 Ga in the south- Amelin, Y.V., Heaman, L.M., Verchogliad, V.M., and Skobelev, V.M. western U.S. (Colorado, Arizona, and New Mexico) that (1994) Geochronologicalconstraints on the emplacementhistory ofan a major orogenic event involving contractional or anorthosite-rapakivigranite suite: U-Pb zircon and baddeleyitestudy transpressionaltectonism and magmatism occurred be- of the Korosten complex, Ukraine. Contributions to Mineralogy and tween 1.4 and 1.5 Ga. Conversely, Gower and Tucker Petrology,116, 41 l-419 Anderson, I.C., Frost, 8.R., and Lindsley, D.H. (1987) Crystallization (1994) arguedthat, although orogenesismay have contin- conditions ofthe Red Mountain syenite.Eos, 68,442-443. ued up to 1.45 G4 a major rifting event extending from Anderson,I.C., Geist,D.J, Frost,B.R., and Lindsley,D.H. (1988)Rare Wyoming to Baltica occurred at 1.43 Ga and detached earth element depletion during late-stagefractionation, Red Mountain large sections of crrrst from the southwesternGrenville pluton, Iaramie anorthositecomplex. Eos, 69, 524. (l Proterozoicanorogenic g,ranite plutonism ofNorth province. Anderson, J.L. 983) We suggestthat an extensionalor transtension- America. In GeologicalSociety of America Memcir, 161, 133-154. al environmefi at 1.43 Ga was responsible for the em- Anderson, J.L., and Bender,E.E. (1989)Nature and origin ofProterozoic placement of anorthositic rocks of the LAC along the A-tpe ganitic magmatism in the southwesternUnited States.Lithos, Cheyennebelt (Fig. l). This is consistentwith the lack of 23, 19-52. (1992) The role of anorogenicgranites a superposedregional strain field on the magmatic fabrics Anderson, J.L., and Morrison, l. in the Proterozoiccrustal developmentof North America. In K.C. Con- of the anorthosites (Scoates,1994), the preservation of die. Ed., Proterozoic crustal evolution, p. 263'299. Elsevier, Amster- magmatic microstructures even in strongly recrystallized dam. rocks (implying small stresses)(Lafrance et al., 1994),and Ashwal, L.D. (1993) Anorthosites,422 p. Springer-Verlag,Berlin. the relatively weak fabric development in the surround- Bolsover, L.R. (1986) Petrogenesisofthe Sybille iron-titanium oxide deposit, Laramie anorthosite complex, Laramie Mountains, Wyoming, ing pelitic and metabasic rocks and granitic gneisses(Grant 78 p. M.S. thesis, State University of New York, Stony Brook, New and Frost, 1990). York. Andersonand Bender(1989) and Hoffman (1989)pro- Bossart,P.J., Meier, M., Oberli, F., and Steiger,R.H. (1986) Morphology posed that the mafic melts responsiblefor the formation versusU-Pb systematicsin zircon: A high-resolutionisotopic study of population Variscan dike in the Central Alps. Earth ofanorthosite were also responsiblefor causingthe large a zircon from a and Planetary ScienceLetters, 78,339-354. amounts of crustal melting and the formation of the 1.49- Butterman, W.C., and Foster,W.R. (1967)Zfcon stability and the ZrOl 1.40 Ga granitic batholiths in the midcontinental U.S. In SiO, phasediagram. American Mineralogist, 52, 880-885. the western and southwesternU.S., the majority of these Chiarenzelli, J.R., and Mclelland, JM. (1993) Granulite facies meta- granitic batholiths have crystallization agesbetween l 43 morphism, paleo-isothermsand disturbance of the U-Pb systematics ofzircon in anorogenicplutonic rocks from the Adirondack Highlands. (summarized Anderson, 1983;Anderson and 1.44Ga in Journal of Metamorphic Geology, I I, 59-70. and Bender. 1989). The anorthositic rocks of the LAC Doig, R. (1991)U-Pb zircon datesof Morin anorthositesuite rocks, Gren- are the only significant exposure of mafic rocks in this ville Province, Quebec.Joumal ofGeology, 99,729-738. t326 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U.Pb AGES

Duchesne,J.C., Scharer,U., and Wilmart, E. (1993) A l0 Ma period of clasefrom anorthositesin the Stillwater Complex, Montana: knplica- emplacementfor the Rogaland anorthosites,Norway: Evidence from tions for the origin of the anorthosites.Contributions to Mineralogy U-Pb ages.Terra Nova, 5, 64. and Petrology, 114, 63-7 8. Emslie, R.F. (1978) Anorthosite massifs,rapakivi granites,and late pro- Ludwig, K.R. (1988a) PBDAT for MS-DOS, a computer program for terozoic rifting of North America. PrecambrianResearch, 7, 6l-98. IBM-PC compatiblesfor processingraw Pb-U-Th isotopedata, version - (1980) Geology and petology ofthe Harp Lake complex, Central 1.03. U.S. GeologicalSurvey, Open File Report 88-542. labrador: An example of Elsonian magmatism. Geological Survey of -(1988b) ISOPLOT for MS-DOS, a plotting and regressionpro- CanadaBulletin, 293,136 p. gram for radiogenic-isotopedata, for IBM-PC compatible computers, Emslie, R.F., and Stirling, J.A.R. (1993) Rapakivi and relared$anitoids version 1.04. U.S. Geological Survey, Open File Report 88-557. of the Nain Plutonic Suite: Geochemistry, mineral assemblages,and Manignole, J., Machado, N., and Nantel, S. (1993) Timing of intrusion fluid equilibria. Canadian Mineralogist, 3 l, 821-847. and deformation of the Riviere-Pentecdteanorthosite (Grenville Prov- Epler, N.A. (1987) Experimental study ofFe-Ti oxide ores from the Sy- ince).Journal ofGeology, l0l, 652-658. bille pit in the Laramie anorthosite,Wyoming, 67 p. M.S. thesis,State Mclelland, J.M., and Chiarenzelli, J. (1990) Isotopic constraintson em- University of New York, Stony Brook, New york. placementage of anorthositic rocks of the Marcy massif, Adirondack Frost, B.R., Frost, C.D. Lindsley, D.H., Scoates,J.S., and Mitchell, J.N. Mts., New York Journal of Geolory,98, 19-41. (1993) The Laramie anorthosite complex and Shermanbatholith: Ge- Mitchell, J.N. 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