baddeleyite (zror) and zircon (zrsio) from anorthositic

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

Baddeleyite (ZrOr) and zircon (ZrSiO) from anorthositic rocks of theLaramie anorthosite complexo Wyoming: Petrologic consequences and U-Pb ages

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

Ansrn-c.crThe Zr-bearing minerals baddeleyite (ZrOr) and zircon (ZrSiOo) occur within plagro-

clase-rich (61-950/o plagioclase) cumulates of the Laramie anorthosite complex (LAC),southeastern Wyoming. In each of the examined samples, zircon is present as relativelycoarse (l-2 mm) interstitial grains, and baddeleyite occurs as small (0.05 mm) inclusionswithin cumulus plagioclase. Zircon crystallized between cumulus plagioclase crystals nearsolidus temperatures from highly fractionated, Zr-saturated liquids. The resultant shapeofzircon was controlled by the form ofthe remaining pore space. The origin ofbaddeleyitein the anorthositic rocks of the LAC is less well constrained. It may have crystallizedearlyfrom the anorthositic parental magmas at relatively low silica activities; however, thiswould require baddeleyite saturation at extremely low Zr concentrations in the parentalmagmas (<< 100 ppm).

Baddeleyite and zircon U-Pb ages reveal that several petrologically distinct intrusionswere emplaced and crystallized in the LAC over a relatively restricted 1-3 m.y. intervalaI ca. 1434 Ma. The 2o7Pb/2o6Pb ages obtained for the baddeleyite and zircon in each sampleare identical within error (+1-3 m.y.), and U concentrations are uniformly low (<240ppm), supporting a genetically related origin for the minerals. Two anorthositic layeredcumulates and a crosscutting, oxide-rich troctolite from the Poe Mountain anorthositehave crystallization ages that are identical within error: 1434.4 + 0.6, 1434.5 + 0.6, and1434.1 + 0.7 Ma, respectively. The only earlier period of anorthositic magmatism thatcan be identified from the Poe Mountain anorthosite is represented by a leucogabbroicxenolith (1436.2 + 0.6 Ma), which settled onto the floor of the magma chamber thatproduced the layered cumulates. A sample composed almost entirely of zoned, iridescent,plagioclase megacrysts from the Chugwater anorthosite yields a baddeleyite age of 1435.4+ 0.5 Ma, intermediate between the ages of the xenolith and the layered anorthositicrocks.

All the ca. 1.4-1.5 Ga anorthosites in North America, including the LAC, are locatednear or on Paleoproterozoic boundaries between Archean cratons and accreted Proterozoicisland arc terranes. These preexisting crustal structures appear to play a major role in theorigin and ascent of anorthositic magmas. The evidence for anorthositic magmatism at1.43 Ga in the LAC suggests that there may be a strong genetic link between these high-temperature mafic magmas and the regional production of anorogenic granites in thewestern and southwestern U.S., many of which have similar crystallization ages in theinterval of 1.43-1.44 Ga.

furnonucrroN

Recent studies have shown that zircon and baddeleyitecan be recovered from some anorthositic rocks in Pro-terozoic anorthosite complexes, and that precise U-Pbages can be determined from both minerals (Mclellandand Chiarenzelli, 1990; Doig, l99l; Higgins and vanBreemen, 1992;van Breemen and Higgins, 1993; Mar-tignole et al., 1993; Paces and Miller, 1993; Duchesne et

* Present address: P6trologie et G6odynamique ChimiqueCPI60/02, Universit6 Libre de Bruxelles, Avenue F.D. Roose-velt 50, 81050 Brussels, Belgium.

al., 1993; Owens el al., 19941, Amelin et al., 1994). Bad-deleyite, in particular, has proven to be a useful mineralfor U-Pb dating in mafic rocks because it has negligibleinitial common Pb, experiences little or no Pb loss, andrarely occurs as xenocrysts (Heaman and LeCheminant,1993). In conjunction with field relationships, this abilityfor precise dating potentially allows for detailed docu-mentation of the intrusive history of the mafic portionsof individual complexes, some of which contain as manyas 20 separate anorthositic intrusions [e.g., Harp Lake(Emslie, 1980); Nain (Yu and Morse, 1993)1. This maybe particularly useful where emplacement-related recrys-

0003-o04x/9 5 / | 1 I2-r3 r7 502.00 L3T7

t 3 1 8

tallization has obscured primary contacts between intru-sions. Because the relative stability of baddeleyite andzircon is related to silica activity (Butterman and Foster,1967), the textural and morphological relationships be-tween these two minerals and the rocks they crystallizedwithin may also help to constrain the crystallization pathof anorthosite.

The Laramie anorthosite complex (LAC), southeasternWyoming, contains the only major exposure of middleProterozoic anorthositic rocks in the western and south-western U.S. (Frost et al., 1993). The Nain and Grenvilleprovinces of Canada contain numerous anorthositic in-trusions of middle Proterozoic age (Ashwal, 1993), how-ever, intrusive rocks of the exposed Proterozoic crust inthe western U.S. are dominated by relatively highJevelgranitic batholiths with crystallization ages between 1.49and 1.40 Ga (Anderson, 1983). It has been proposed thatmafic magmatism at depth was the driving force for thelarge-scale crustal melting necessary to produce the gran-ites (Anderson and Bender, 1989), but direct ages of crys-tallization for coeval mafic rocks in this region are no-tably lacking. The age of the anorthositic rocks in theLAC is known only indirectly through the age of a cross-cutting monzonite (Frost et al., 1990). Thus, the purposesof this study are (1) to determine directly the ages ofcrystallization of a variety of anorthositic rocks from theLAC to limit the duration of middle Proterozoic maficmagmatism in southeastern Wyoming; (2) to investigatethe geochronological relationships among the LAC, othermiddle Proterozoic anorthosites, and similarly aged gra-nitic batholiths in North America; and (3) to investigatethe morphologic and textural characteristics of baddele-yite and zircon as guides to the crystallization history ofanorthositic rocks.

GBor,ocv oF THE L.1,ru.*nn ANoRTHosrrE coMpLEx

The unmetamorphosed and undeformed72l km'? LACoutcrops in the southern Laramie Mountains, southeast-ern Wyoming (Fig. l). Only a portion of the LAC is pres-ently exposed; west-dipping Laramide (Late Cretaceousto early Tertiary) thrust and high-angle reverse faultstruncate the eastern margins, and shallowly dipping, earlyPaleozoic sediments unconformably overlie the westernmargins. The LAC intruded Archean gneisses of the Wy-oming province in the north and Proterozoic supracrustalrocks in the south (Fig. l). It probably intruded along theCheyenne belt (Frost et al., 1993), the ca. 1.78 Ga colli-sional zone that separates the Archean rocks from theaccreted Proterozoic island arc terranes in the south(Karlstrom and Houston, 1984).

The LAC consists of a central mass of anorthositic rocks(550 km'z) that was intruded by dioritic and troctoliticrocks of the Strong Creek complex (Mitchell, 1993), mon-zonitic rocks of the Maloin Ranch pluton (Kolker andLindsley, 1989; Kolker et al., 1990, 1991), the Sybilleintrusion (Fuhrman et al., 1988), and the Red Mountainpluton (Anderson et al., 1987, 1988), and granitic rocksof the Sherman batholith (Geist et al., 1989) Grg. l).

SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES

Estimates of emplacement pressures for the monzoniticrocks (equilibria among pyroxenes, olivine, and quartz)yield midcrustal pressures of about 3 kbar in the northand 4 kbar in the south (Fuhrman et al., 1988; Kolkerand Lindsley, 1989). Three major composite anorthositicintrusions are contained within the LAC: the Poe Moun-tain, Chugwater, and Snow Creek anorthosites (Figs. Iand 2).

The Poe Mountain anorthosite consists of (l) a centralcore of pervasively recrystallized megacrystic anortho-site, (2) a marginal layered series containing a 5-7 kmthick package of layered and laminated anorthositic andleucogabbroic cumulates, (3) a suite of igneous xenoliths,ranging in composition from leucogabbro to anorthosite,and (4) a series of late, undeformed troctolitic intrusions(Fig. 2) (Scoates, 1994). The central core area ofthe PoeMountain anorthosite contains mainly homogenous, re-crystallized anorthosite of uniform plagioclase composi-tion (-Anou) with little evidence of primary igneousstructures or textures. Where zones of relatively weak re-crystallization occur, the protolith appears to be com-posed of coarse-grained (2-4 cm) megacrysts of plagio-clase. The surrounding marginal layered series containstwo distinct layered zones: an inner anorthositic layeredzone (ALZ) and an outer leucogabbroic Iayered zone(LLZ).The overall shape of the intrusion is domical. Lay-ering is steepest along the outer margins of the PoeMountain anorthosite, dipping 60-90", and layering shal-lows considerably toward the central core, dipping 30-60". The contact between the two layered zones is con-tinuous along strike, across several Laramide faults, for-20 km. Each layered zone is, in turn, subdivided intodistinct layered sections (lower, middle, and upper) onthe basis of layering characteristics and cryptic geochem-ical variations (Scoates, 1994) (Fig. 2). The base of thelowermost ALZ cannor be identified in the field becausethe effects of high-temperature recrystallization that wereproduced during late emplacement deformation (La-france et al., 1994) are pervasive in the interior of thePoe Mountain anorthosite. The uppermost portions ofthe upper LLZ are missing because of subsequent em-placement of the Sybille intrusion (Fig. 2). Well-devel-oped igneous layering and a prominent plagioclase lam-ination are characteristic of anorthositic cumulatesthroughout the entire layered series. Rocks in the layeredseries are composed of cumulus plagioclase (Anor-rr), oi-kocrystic clinopyroxene, inverted pigeonite, olivine, il-menite, and magnetite, with interstitial apatite and sul-fides and late biotite.

A suite of igneous xenoliths, ranging in compositionfrom anorthosite to leucogabbro, is found throughout thelayered series. These xenoliths are mineralogically similarto the layered series and consistently have more calcicplagioclase compositions and lower initial Sr isotopic ra-tios than their host cumulates (Scoates and Frost, 1994;Scoates, 1994). They therefore appear to represent a sep-arate and distinct product of anorthositic magmatism inthe LAC. Small troctolitic (plagioclase * olivine) intru-

Fig. 1. Geologic map of the southern Laramie Mountains ofsoutheastern Wyoming, showing the location of the Laramre an-orthosite complex and the inferred trace of the Cheyenne beltthrough the l,aramie Mountains. PMa : Poe Mountain anor-thosite, Ca : Chugwater anorthosite, and SCa : Snow Creekanorthosite.

sions occur throughout the Poe Mountain anorthosite.The largest truncates the southwestern extension of thelayered series (Fig. 2). Coarse-grained troctolite is alsofound associated with massive ilmenite in the Sybille irontitanium oxide deposit (Bolsover, 1986; Epler, 1987),where angular inclusions of anorthosite indicate that an-orthosite was completely solidified prior to intrusion oftroctolite.

The Chugwater anorthosite (Fig. 2) consists of interfin-gered lenses of recrystallized anorthosite and undeformedmegacrystic anorthosite and troctolite (Frost et al., 1993).There are no laterally continuous, layered cumulates inthe Chugwater anorthosite, and well-zoned, iridescentplagioclase megacrysts are common. Plagioclase compo-sitions are uniformly more calcic in the Chugwater an-orthosite (>Anro) than in the core of the Poe Mountainanorthosite (-An*). The smaller Snow Creek anorthositeoccurs in the west-central LAC (Fig. 2) and consists ofrecrystallized megacrystic anorthosite with abundantzoned, iridescent plagioclase megacrysts (Mitchell, I 993).Contact relationships between the Poe Mountain anor-

l 3 l 9

Fig.2. Simplified geologic map of the northern Laramie an-orthosite complex. The four geochronological sample sites fromthe Poe Mountain anorthosite are shown as solid circles. ALZ:anorthositic layered zone, andLLZ: leucogabbroic layered zone.The contact between the ALZandLLZ is shown as a solid line,and the contacts between layered sections within the two layeredzones (lower, middle, and upper in the ALZ, and lower andupper in the LI-Z) are shown as dashed lines. The "contact zone"denotes the area where pervasive emplacement-related recrys-tallization overprinted the primary magmatic contacts betweenthe Poe lriountain, Chugwater, and Snow Creek anorthosites.

thosite to the north and the Chugwater and Snow Creekanorthosites are not easily identifiable in the field becauseall primary igneous structures in the area have been over-printed by the effects of high-temperature emplacement-related deformation. The approximate locations of intru-sion contacts in this area are marked by a gradationalcontact zone in Figure 2.

Intrusive relationships indicate that anorthosite is theoldest lithology in the LAC (Frost et al., 1993). In thenorthern LAC, the Poe Mountain anorthosite was intrud-ed by monzonitic rocks of the Sybille intrusion that, intum, were intruded by the Red Mountain pluton (Figs. Iand 2). Frost et al. (1990) determined the uranium-leadzircon age for a fayalite monzonite in the Red Mountainpluton to be 1439 1l Ma, subsequently revised to 1431.3+ 1.3 Ma (C. D. Frost, personal communication). In ac-cordance with field relationships, this is a minimum agefor intrusive activity in the northern part of the LAC and


r320

a minimum age for the anorthositic rocks. The only pre-vious study that attempted to determine directly the ageof anorthositic rocks in the LAC used Rb-Sr isotopic sys-tematics (Subbarayudu, 197 5). No meaningful isochronswere derived, however, because of the complex, open-system crystallization history of the anorthositic rockswith respect to Sr isotopes, which is the result ofcrustalcontamination and magma mixing (Scoates and Frost,r994).

Sl.tvrpr,rnc STRATEGv AND ANALvTTCAL TEcHNreuEs

Anorthositic rocks are predominantly plagioclase-richadcumulates. Textural evidence suggests that they typi-cally include only minor amounts of trapped interstitialliquid that could be enriched in incompatible elements.Thus, whole-rock samples contain only minor amountsof U-rich trace minerals suitable for U-Pb geochronology.In the Poe Mountain anorthosite, this is reflected by lowzr contenrs in both the ALZ (16-73 ppm) and ttJe LLZ(12-106 ppm) (Scoates, 1994). We processed > 100 kg ofeach sample in this study to extract sufficient quantitiesof U-rich minerals for analysis. Each of the five samplesreported in this study, ranging in plagioclase abundancefrom 6l-950/0, contained both baddeleyite and zircon.

Hearly minerals were concentrated from crushed sam-ples by standard density and magnetic separation proce-dures. No cores in baddeleyite or zircon were visible withthe binocular microscope under high-power magnifica-tion (50 x ). Small inclusions were visible in some of theconcentrated baddeleyite and zircon, but grains selectedfor analysis were optically devoid of inclusions. The grainswere cleaned in cold 2N HNO3 prior to dissolution. Bothbaddeleyite and zircon were dissolved in concentratedHF and HNO, in teflon microvessels following the pro-cedure ofParrish (1987) and converted to chlorides. Ali-quots of sample solutions were spiked with a mixed'z3sU/208Pb spike. Pb and IJ were purified in anion-exchangecolumns using HCI chemistry modified from Krogh(1973). The total Pb blank decreased significantly duringthe course of this study, from 80 to 5 pg. The blank com-position was 206Pb/204Pb : 19.09, 2o7Pb/2o4Pb : 15.652,and 208Pb/204Pb : 38.31 and was used in corrections forblank contributions. The Pb and U samples were loadedonto single rhenium filaments for isotopic analysis bythermal emission mass spectrometry. The HrPOo-silicagel technique was used for Pb, and U was loaded withH3PO. and graphite and run as metal ions. The isotopicanalyses were conducted at the University of Wyomingon a multiple-collector, VG Sector mass spectrometer.Mass discrimination factors for Pb and U were deter-mined by multiple analyses of NBS SRM 981 and U-500,respectively, and were 0.094 t 0.0560/o per atomic massunit for Pb and 0 + 0.060/o per atomic mass unit for U.The program PBDAT (Ludwig, 1988a) was used to re-duce the raw mass spectrometer data, correct for blanks,and calculate uncertainties. Much of the data from theanalyzed fractions overlaps concordia, and the 207Pbl206Pb

SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U.Pb AGES

ages of analyzed fractions from each sample are nearlyidentical. Therefore, the weighted average 207Pbl206Pb ages(weighted by the inverse variance ofthe individual ages)were calculated rather than linear regressions. Theweighted average ages and uncertainties were calculatedusing ISOPLOT (Ludwig, 1988b), and errors are quotedat 2o.

ZrncoN AND BADDELEyITE MoRpHoLocrEs

In four of the five samples examined, zircon is far moreabundant than baddeleyite (on the order ofthousands toone), although the total abundance of zircon is typically<0.2o/o of the whole rock. All analyzed zircon crystalswere colorless, anhedral fragments. Texturally, zircon oc-curs as a late-crysallizing phase interstitial to cumulusplagioclase (Fig. 3A). Contacts between zircon and otherminerals are sharp and devoid of secondary alterationproducts, indicating that the original morphology of zir-con was not modified subsequent to crystallization. Inthin section, zircon is typically I mm in diameter, withthe largest grains up to 1.5 mm across. The analyzed grainswere much smaller (0.25-0.50 mm) and were probablyfragments of larger grains. Poikilitic zircon is common(Fig. 38) and is further textural evidence for late intersti-tial crystallization. Euhedral zircon was not found in anyof the mineral separates. Many of the grains selected foranalysis were characterized by minute rippleJike struc-tures across their outer surfaces, observable under bothmoderate- to high-power magnification (20-50 x ) with abinocular microscope and a scanning electron microscope(SEM). These structures may be a manifestation of non-planar (skeletal) growth, perhaps reflecting rapid growthfrom a late-stage zircon supersaturated melt (Bossart eta l . . 1986).

Baddeleyite is present in all samples and exhibits mor-phological and textural characteristics markedly differentfrom those ofzircon. It occurs as small euhedral to sub-hedral grains within cumulus plagioclase (Fig. 3C and3D) and is not observed interstitially with zircon. Theoccurrence of baddeleyite is not related to fractures orhealed fractures in plagioclase, indicating that it did notform by secondary alteration processes. No rims of zirconwere observed, optically or with the SEM, on baddeleyite.Average grain size is about 0.05 mm, and the largest ob-served grains are 0.2-0.3 mm in diameter. Baddeleyite iscommonly concentrated in select plagioclase grains with-in a given thin section. It is restricted to the cores ofplagioclase grains and does not occur in the relativelypotassic marginal areas that contain alkali feldspar exso-lution lamellae. Where multiple grains are observed in asingle plagioclase grain, there is typically no preferred ori-entation, although examples of parallel orientation wereobserved. Analyzed baddeleyite grains were translucentbrown, and many grains displayed the well-developedpolysynthetic twinning that is characteristic of baddele-yite (Smith and Newkirk, 1965; Heaman and Le-Cheminant. 1993).

SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES t 32 l

U-Pb nrsur.rsThe U-Pb isotopic compositions of l5 baddeleyite and

four zircon fractions (Table l) were determined from fivesamples: four from the Poe Mountain anorthosite (Fig.2), including one from each of the two layered zones (ALZand LLZ), a crosscutting troctolite, and a leucogabbroicxenolith, and one sample from the Chugwater anortho-site. Between 40 and 120 grains ofbaddeleyite and zirconwere dissolved for each analysis. The concentrations ofU and Pb are extremely low, ranging from 43 to 238 ppmU and l0 to 55 ppm Pb for baddeleyite, and 9 to 127ppm U and 3 to 34 ppm Pb for zircon. These concentra-tions are typical of Proterozoic anorthosites (e.g., Higginsand van Breemen, 1992; Martignole et al., 19931, vanBreemen and Higgins, 1993; Owens et al., 1994) and areconsistent with the crystallization of baddeleyite and zir-con from U-poor parental magmas. Even with these lowconcentrations, 206Pb/201Pb values (corrected for fraction-ation, spike composition, and blank contribution) are ex-tremely high, ranging from 3680 to 285 360 (Table l),thus indicating very low contributions of initial Pb.

The U-Pb data cluster near concordia (0-1.10/o discor-dance), demonstrating the closed-system behavior ofbaddeleyite and zircon with respect to Pb loss or U gain.There is no discernible difference within error betweenbaddeleyite and zircon 2D7Pb/2o6Pb ages within each sam-ple. Because of this, we consider the weighted average207Pbl206Pb ages of combined baddeleyite and zircon tobe the best estimate of crvstallization aee.

Oikocrystic anorthosite (PM468) - ALZ, P oe Mountainanorthosite (latitude 4146' 03', longitude 105"21'58')

Sample PM468 is from a well-laminated anorthosite(930/o plagioclase) containing abundant, regularly spacedclinopyroxene, olivine, and iron titanium oxide oiko-crysts. Stratigraphic reconstructions place it 2240 m be-Iow the ALZ/LLZ contact. The data from three badde-leyite fractions and one zircon fraction are nearlyconcordant (0. 6-0.90lo discordance), with 207Pbl206Pb agesof 1433.9-1434.8 Ma for baddeleyite and 1434.8 Ma forzircon (Fig. 4,A.). The weighted average 207Pb/2o6Pb age of1434.4 + 0.6 Ma is considered the best estimate for thecrystallization age of PM468.

Laminated leucogabbro (SR247)-LLZ, Poe Mountainanorthosite (latitude 4144' 27', longitude 10522' l9')

Sample SR247 is from a well-laminated leucogabbrooccurring 3240 m stratigraphically above PM468. TheU-Pb data for three baddeleyite fractions are nearly con-cordant (0.3-0.80/o discordance), and the zircon fractionis concordant (Fig. 4B). They yield a weighted average201Pb/206Pb age of 1434.5 + 0.6 Ma. Thus, the crystalli-zation ages of anorthositic cumulates from both the ALZand LLZ agree within uncertainty.

Fig. 3. Representative photomicrographs ofzircon and bad-deleyite morphologies in anorthositic rocks of the Poe Mountainanorthosite. (A) Interstitial zircon, (B) poikilitic zircon, (C andD) baddeleyite inclusions in plagioclase.

Troctolite (PM622) - Poe Mountain anorthosite (latitude4146' 45', longitude 105'21'06')

Sample PM622 is from a coarse-grained, oxide-apatite-rich troctolite (610/o plagioclase) associated with massiveiron titanium oxide ore in the Sybille deposit (Bolsover,1986; Epler, 1987; Lindsley and Frost, 1990). Theweighted average 207Pb/2o6Pb age of four nearly concor-dant fractions (three baddeleyite and one zirconl 0.6-0.90lodiscordance) is 1434.1 + 0.7 Ma (Fig. aQ. Localizedshearing of the enveloping anorthositic cumulates and in-clusions of anorthosite xenoliths in the undeformed troc-tolite indicate that the anorthosite was consolidated priorto intrusion ofthe troctolite. There are, however, no re-solvable differences between the ages ofthe layered seriescumulates and this crosscutting troctolite.

Leucogabbroic xenolith (SR246)-Poe Mountainanorthosite (latitude 4l' 44' 16', longitude 10525' 45' )

Sample SR246 is from a leucogabbroic xenolith (830/oplagioclase), with a minimum length of 50 m and an in-ternal fabric defined by plagioclase lamination that is

r322 SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES

TABLE 1. U-Pb results for baddeleyite and zircon fractions separated from anorthositic rocks of the LAC

Concentration

Description' Com-mon

Measured atomic ratios-- Ages (Ma)

Frac- Weighttion (mg)

U Pb Pb '6Pb/(ppm) (ppm) (pS)t zo4Pbf, ,GPb/238U

26Pbl 8rPbl 2otPbl2o?Pbl2sl) 4?Pb/r6Pb ruu 45U mPb$ Rho Dll

B (nm1) 0.397B (d-1) 0.139B (m2) 0.149z (d-2) 0.464

B (nml) 0.447B (m1) 0.437B (m3) 0.201Z (nml) 0.724

B (m1) 0 422B (m2) 0.348B (m3) 0.268z (d-2) 0.408

B (d-1) 0.546B (d-3) 0.179z(d-2) 0.668

B (nml) 0.521B (ml) 0.447B (d-1) 0.258B (m2) 0.298

49 1264 1590 21

1 1 0 3 1

7 1 1 760 1468 169 3

1431.1't429.71428.91430.4

0.98 0.350.960.98 0.66

0.92 0.740.99 1.070.96 1.080.98 0.9s

1 0 1422378

1 2 R

1882444

324302951

41Q I

232

415455020

192 4686 20

182 4390 26

76 1843 10

127 34

109 26238 558 1 1 9

120 28

PM468: Poe Mountain anorthosite, ALz1 3138 0.2481(0.45) 3.093(0.46) 0.090406(0.09) 1429.032776 0.2476(0.29) 3.088(0.31) 0.090437(0.10) 1426.235447 0.2475(0.271 3.085(0.28) 0.090396(0.07) 1425.63153s 0.2479(0.40) 3.091(0.41) 0.090437(0.07) 1427.4

SR247: Poe Mountain anorthosite, LLZ81 64 0.2488(0.40) 3.101(0.41 ) 0.090405(0.07) 1432.2 1433.03680 0.2476(0.37) 3.087(0.40) 0.090423(0.14) 1426.3 1429.6

29668 0.2481(0.25) 3.094(0.26) 0.090441(0.06) 1428.7. 1431.213819 0.2492(0.461 3.105(0.s0) 0.090382(0.18) 1434.3 1434.0

PM622: Poe Mountain anorthosite, oxide-rich troclolite4794 0.2481(0.48) 3.092(0.49) 0.090407(0.10) 1428.s 1430.8

79812 O.2476(0.4s1 3.086(0.53) 0.09037s(0.29) 1426.3 1429.2165750 0.2475(0.241 3.086(0.25) 0.090415(0.06) 1425.7 1429.241176 0.2474(0.40) 3.082(0.44) 0.090359(0.16) 1424.8 1428.2

SR246: Poe Mountain anorthosite, LLZ xenolith228640 0.2492(0.92) 3.1 1 0(0.33) 0.090513(0.06) 1434.2 1435.125158 0.2499(0.30) 3.118(0.31) 0.090478(0.09) 1438.0 1437.08649 0.2484(0.40) 3.100(0.40) 0.090s08(0.07) 1430.4 1432.8

BM1 36: Chugwater anorthosite, iridescent megacry3tic anorthosite2516 0.2480(0.42) 3.092(0.46) 0.090447(0.18) 1428.0 1430.0

285360 0.2471(0.36) 3.082(0.37) 0.090450(0.06) 1423.5 1428.128224 0.2472(0.25) 3.084(0.26) 0.090476(0.07) 1424.3 1428.8

156380 0.2476(0.35) 3.088(0.35) 0.090478(0.06) 1425.9 1429.8

1434.1(1.7) 0.98 0.601434.8(1 .9) 0.94 0.851433.9(1.4) 0.97 0.831434.8(1 .3) 0.99 0.75

1434.1(1.4) 0-98 0.341434.5(2.7) 0.94 0.841434.9(1.2) 0.97 0.671433.6(3.5) 0.93

14i]4.1(1 .9) 0.98 0.631433.s(5.6) 0.84 0.741434.3(1 .1) 0.97 0.851433.1(3.1) 0.93 0.82

1436.4(1 .2)143s.6(1 .7)1436.3(1.4)

1435.0(3.4)1435.0(1 .2)143s.6(1 .3)1435.6(1 .2)

* 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, including sample, spike, and blank.+ The ePb/ePb corrected for fractionation, spike composition, and blank contributions.$ Errors of the e?Pb/26Pb ages are quoted at 2d in parentheses.ll D = discordance in percent.

oblique to the fabric in the envelopingLLZleucogabbros.It also contains several iridescent plagioclase megacrysts,which are not found in the layered cumulates of eitherthe AW or LW. The weighted average 207Pbl206Pb age ofthree analyzed fractions (two baddeleyite and one zircon)is 1436.2 + 0.6 Ma (Fig. 4D). The leucogabbroic xenolithis resolvably older than the LLZ cumulates that containit, and thus the occurrence of a distinct earlier phase ofanorthositic magmatism in the LAC has been identified.

Megacrystic anorthosite (BMf 36) -Chugwater

anorthosite (latitude 4134' 20', longitude 10523' 52')

Sample BMl36 is from a megacrystic anorthosite (950/0plagioclase) from the Chugwater anorthosite. It occurs ina relatively unrecrystallized lens, 100 m wide, within per-vasively recrystallized anorthosite and may represent ei-ther a late intrusive phase of the Chugwater anorthositeor a low-strain window in the recrystallized anorthosites.It is compositionally distinct from the Poe Mountain an-orthosites to the north and is composed of abundant ir-idescent plagioclase megacrysts, up to 4 cm in diameter,showing strong oscillatory zoning, normal and reversezoning, and prominent resorption surfaces. In contrast tothe other four samples in this study, baddeleyite is far

more abundant than zircon in BMl36. The weighted av-erage2o'Pb/2o6Pb age of four baddeleyite fractions is 1435.4+ 0.5 Ma (Fig. aD. This age overlaps within error theyounger ages of the Poe Mountain layered series samplesand the older age of the xenolith; the megacrystic anor-thosite may be the same age as the layered series cumu-lates to the north or as much as 2 m.y. older.

OnrcrN oF zrRcoN AND BADDELEyITE rN THELlnnvrrn ANoRTHosrrEs

In mafic rocks, baddeleyite and zircon are most com-monly found in late interstitial regions or coarse pegma-toids (e.g., Heaman and LeCheminant, 1993). The resultsof this study, however, indicate a separate crystallizationhistory for baddeleyite and zircon in anorthositic rocksof the LAC. The co-occurrence of baddeleyite and zirconin anorthositic samples has rarely been reported. Amelinet al. (1994) provided U-Pb data for both baddeleyite andzircon from a gabbro-anorthosite (sample 7/90) in theKorosten complex, Ukraine, and showed that the 'zo?Pb/206Pb ages of fractions ofboth minerals are identical with-in error. Although the physical settings of these mineralsin the sample are not noted, their morphological descrip-tions are strikingly similar to those described in this study.

SCOATES AND CHAMBERLAIN: BADDELEYITE AND ZIRCON U-Pb AGES r323

> 0.249

$ o.zcs

0 251

0 ?50

F 0.24e

p ozar

0.247

0 2463.07 3 .08 3 .09 3 10 3 l l 3 12 3 13 3 .14

20?Pb/2r5fI3 06 3.07 3.08 3 09 3.10 3. l l 3.12 3.13 3. t4

2o7Pbl235tJ

0.251

0.250

F o 2 4 e

p o,z+r

o.247

o.2463.07 3.08 3 .09 3 .10 3 . l l 3 . 12 3 .13 3 . r 4

207Pb/2r5fI3 06 3 07 3.08 3.09 3 l0 3.11 3.r2 3.13 3.r4

207Pb135tJ

0.25r

0.250

o.247

0.2463.06 3.07 3.08 3 09 3 10 3 r l 3,12 3,13 3 t4

20?Pb/2r5fl

Fig. 4. Concordia plots for U-Pb data from analyzed baddeleyite and zircon fractions. Each ellipse represents the result oftheanalysis of a single fraction. Solid line ellipses : baddeleyite, and dashed line ellipses : zircon. (A) PMa68: anorthosite, ALZ, PoeMountain anorthosite; @) SR247: leucogabbro, LLZ, Poe Mountain anorthosite; (C) PM622: coarse-grained oxide-rich troctolite,ALZ,Poe Mountain anorthosite; @) SR246: leucogabbroic xenolith, LLZ,Poe Mountain anorthosite; (E) BM136: megacrysticanorthosite, Chugwater anorthosite.

p o.z+l

$' o.zcs

The textural interpretation of zircon in the LAC an-orthosites is relatively straightforward. The obvious in-terstitial and poikilitic habit ofzircon indicates that zir-con crystallized near or at solidus temperatures in the lastavailable pore spaces ofa nearly consolidated plagioclase-rich cumulate. The resultant shape of zircon was strongly

influenced by the shape ofpreexisting plagioclase grainsand the shape of the remaining pore space. The opticallack of cores and resorption surfaces, the extremely lowU and Pb concentrations, and the extremely high ,06Pb/20aPb values in zircon indicate that it is unlikely to beinherited. Similar anhedral zircons from anorthosite were

SR247: Poe Mountain ano(hositeL L Z . . . " t ' o

's- / ./ weighred 207Pb/206pb average:L-' 1434.5 +l- 0.6 Ma

B. (MSWD = 0.64)

PM468: Poe Mounrain anonhositeALZ

z K il:tl*jl1:,'l;ftPb average:

A. (MSWD = 0.67)

0.247

o.246

o25r

0.250

p o.zts

H o.z+s

o.247

0.246

PM622: Poe Mountain anorthositeoxide-rich troctolite

% [:'L'l'".9-'Li"fiftn' ave.tge :

c. i.::'- (M,swD = 0.32)

SR246: Poe Mountain anorthositeleucogabhroic xenolith

tqzs -y'-/'? weightect 2o7Pbfocnpb average:

1436.2 +l- 0.6 MaD. (MSWD = 0.46)

BM1 36: Chugwater anorthositemegacrystic anorthosite

Weighred 20?Pb/2o6Pb average :1435.4 +/- 0.5 Ma(MSWD = 0.39)

t324

t433


Anorthosite (8M136) *-----ra+

---------a-+ Troctolite (PM622)

Recent experimental work may help to resolve the originof baddeleyite in anorthositic rocks of the LAC. Rawlinget al. (1995) reported that baddeleyite and intermediate-composition plagioclase are not compatible at 1100 "C,at both I atm and 5 kbar, and therefore a true inclusionorigin seems to be precluded. No matter what the exactmechanism of origin of baddeleyite, the similarity be-tween the 2oiPb/2o6Pb ages of baddeleyite and zirconindicates that the interval of time between the crystal-lization of the two minerals was within the determinedanalytical error.

Eupr.lcnunNT AI\D cRysrALLrzATroN HrsroRyOF THE LIN-I,VTTN ANORTHOSITES

The baddeleyite and zircon U-Pb ages that we reportspan a relatively restricted interval of geologic time in themiddle Proterozoic: a maximum interval of 3.4 m.y. anda minimum interval of 0.8 m.y. (Fig. 5). Crystallizationages of both the layered cumulate samples of the PoeMountain anorthosite and the crosscutting troctolite areindistinguishable within error (< I m.y.).The two layeredcumulate samples are separated by over 3000 m of pla-gioclase-rich cumulates. If I m.y. was the maximum timefor the crystallization of -3000 m of adcumulate rocks,and crystallization was continuous, then a minimumaverage rate of accumulation of -0.3 cm/yr is implied.Our estimate is comparable to that of Morse (1979a, 1986)for the crystallization of the plagioclase-rich Kiglapait in-trusion (average modal feldspar : 73o/o; Morse, 1979b)in the Nain plutonic suite, Labrador. He estimated thatthe crystallization time for the 8400 m thick LayeredGroup in the Kiglapait intrusion was about 106 yr, withan average accumulation rate of about 1 cm/yr.

The leucogabbroic xenolith in the LLZ, with a crystal-lization age of 1436.2 + 0.6 Ma, is the only sample fromthe Poe Mountain suite with a resolvably different age(Fig. 5). However, even the age difference between thexenolith and the rest of the Poe Mountain anorthosite isIimited to a minimum of 0.5 m.y. and a maximum of 2m.y. Numerous other compositionally and isotopicallysimilar igneous xenoliths occur within the ALZandLLZof the Poe Mountain anorthosite (Scoates and Frost, 1994;Scoates, 1994), and it is possible that some of these maybelong to this earlier 1436 Ma phase of anorthositic mag-matism. The xenoliths may have been derived from apreexisting intrusion near or at the level of emplacementof the LAC (-3-4 kbar), or they may have been en-trained in ascending magma and derived from depthsomewhere along the magma conduit.

Field relationships indicate that the anorthositic rocksare the oldest lithology in the LAC (Frost et al., 1993).Each of the samples in this study has crystallization agesolder, outside of error, than the only existing, precise U-Pbage from the LAC, 1431.3 + 1.4 Ma from the Red Moun-tain pluton (C. D. Frost, personal communication). Be-cause the Red Mountain pluton is the youngest of themonzonitic intrusions in the northern LAC, it appearsthat the majority of the intrusions in the LAC, displaying

1435 r436 743'I

F-------++ Irucogabbro (SR247)l P M a I

'------rt--+ Anorthosite (PM468)

Xenolith(SR246) F-----O+

1433 1434 1435 1436

Age (Ma)t431

Fig. 5. Comparison of U-Pb ages determined in this studyfor anorthositic, leucogabbroic, and troctolitic rocks of theLaramie anorthosite complex.

also reported by Martignole et al. (1993) from the Ri-vi€re-PentecOte anorthosite in the east-central Grenvilleprovince, Canada. These observations have importantimplications for U-Pb geochronology because there is lit-tle morphological and compositional difference betweenthe zircons analyzed in our study and the demonstrablymetamorphic zircons from the Adirondack anorthosites(Chiarenzelli and Mclelland, 1993). Thus, it is evidentthat care is required when interpreting the significance ofU-Pb data from anhedral zircon, and textural constraintsin thin section should always be considered.

Although the nearly identical U and Pb concentrationsand 207Pbl206Pb ages ofzircon and baddeleyite are strongevidence that baddeleyite is igneous in origin and relatedto the crystallization history of the anorthositic rocks inthe LAC, the mechanism of formation of baddeleyite re-mains unclear. The two most commonly proposed originsfor baddeleyite in mafic rocks are late crystallization fromhighly fractionated interstitial liquids (Heaman andLeCheminant, 1993) and exsolution from ilmenite (Nas-lund, 1987; Mclelland and Chiarenzelli, 1990; Loferskiand Arculus, 1993). Neither mechanism is appropriatefor the origin of baddeleyite in the Laramie anorthositesbecause this baddeleyite occurs in early-crystallized pla-gioclase and is not associated spatially with discrete grainsof ilmenite.

On the basis of the observed textural relations only,baddeleyite in the LAC may have crystallized prior toplagioclase. This would imply lower silica activities in theearly crystallization of the anorthosites, prior to late sat-uration ofzircon at elevated silica activities. It would alsoimply early saturation of baddeleyite in the anorthositicparental magmas at exceedingly low Zr concentrations. IfAl-rich gabbroic dikes in the LAC can be considered asrepresentative of parental magmas, this implies Zr con-centrations in the l0-60 ppm range (Mitchell et al., 1995).

a wide range of compositions, crystallized over a 3-7m.y. interval.

RrcroN,ll, sETTrNc oF THE1.43 Ga Lmlpnn ANoRTHosrrE coMpLEx

The LAC is one of over 50 intrusions with crystalli-zation ages between 1.49 and 1.40 Ga contained in amajor 4800 km long belt that extends from Labrador toCalifornia (Emslie, 1978; Anderson, 1983; Anderson andMorrison, 1992; Gower and Tucker, 1994). The vast ma-jority of these intrusions are granitic batholiths of A-typeaffinity, commonly referred to as anorogenic granites(Anderson, 1983). However, five intrusions containinganorthosite are known: the 1.43 Ga LAC; the 1.49 GaWolf River batholith of Wisconsin (Van Schmus et al.,1975); and three intrusions in Labrador, the 1.45 Ga HarpLake complex (Krogh and Davis, 1973), the 1.46 Ga Mi-chikamau intrusion (IGogh and Davis, 1973), and the1.42 Ga Mistastin intrusion (Emslie and Stirling, 1993).As has been observed in the LAC, each of these anor-thositic intrusions is near or along collisional zones be-tween Archean cratons and Paleoproterozoic orogens(Gower and Tucker, 1994). This suggests that the pres-ence of preexisting crustal structures (lithospheric weak-nesses) facilitates the generation and the ascent of anor-thositic magmas. Reactivation of these terrane boundariesmay have occurred in response to either regional com-pression or extension. Nyman et al. (1994) argued on thebasis of kinematic data from 1.4 Ga plutons in the south-western U.S. (Colorado, Arizona, and New Mexico) thata major orogenic event involving contractional ortranspressional tectonism and magmatism occurred be-tween 1.4 and 1.5 Ga. Conversely, Gower and Tucker(1994) argued that, although orogenesis may have contin-ued up to 1.45 G4 a major rifting event extending fromWyoming to Baltica occurred at 1.43 Ga and detachedlarge sections of crrrst from the southwestern Grenvilleprovince. We suggest that an extensional or transtension-al environmefi at 1.43 Ga was responsible for the em-placement of anorthositic rocks of the LAC along theCheyenne belt (Fig. l). This is consistent with the lack ofa superposed regional strain field on the magmatic fabricsof the anorthosites (Scoates, 1994), the preservation ofmagmatic microstructures even in strongly recrystallizedrocks (implying small stresses) (Lafrance et al., 1994), andthe relatively weak fabric development in the surround-ing pelitic and metabasic rocks and granitic gneisses (Grantand Frost, 1990).

Anderson and Bender (1989) and Hoffman (1989) pro-posed that the mafic melts responsible for the formationofanorthosite were also responsible for causing the largeamounts of crustal melting and the formation of the 1.49-1.40 Ga granitic batholiths in the midcontinental U.S. Inthe western and southwestern U.S., the majority of thesegranitic batholiths have crystallization ages between l 43and 1.44 Ga (summarized in Anderson, 1983; Andersonand Bender. 1989). The anorthositic rocks of the LACare the only significant exposure of mafic rocks in this

t325

area with crystallization ages in this interval. The Sr andNd isotopic compositions of the anorthositic rocks andassociated Al-rich gabbros are consistent with crystalli-zation from mantle-derived magmas that were contami-nated by crust during ascent (Scoates and Frost, 1994;Mitchell et al., 1995). If heat from the parental magmasat depth produced the crustal melting required for graniteproduction, then the general absence of exposed maficmagmatism associated with these batholiths may be anartifact of crustal structure (Frost et al., 1993). The LACmay have been allowed to ascend to higher crustal levelsalong the reactivated Cheyenne belt.

AcrNowr,Bnclrnr.crs

This paper represents a portion of J.S.S.'s Ph.D. dissertation at theUniversity of Wyoming. We extend thanks to J.N. Mitchell, D.H. Lind-sley, and A. Greene for helping to collect the large samples, C.D. Frostfor use of the clean laboratory facilities and mass spectrometer at theUniversity of Wyoming, and S. Swapp and K. Krugh for cheerfully op-

erating the SEM at the University of Wyoming B.R. Frost, C.D. Frost,D.H. Lindsley, R.F.J. Scoates, and D. Weis reviewed versions of thispaper and contributed many useful and insightful comments. Joumal re-views by P. Mueller and J. Mcl-elland significantly improved the clarityof the final version. J.S.S. wishes to thank J. Michot for extending aninvitation to visit the Universit6 Libre de Bruxelles, where the revisions

to this paper were completed. Funding for this project was obtained from

NSF grant EAR-9218360 to B.R. Frost and C.D. Frost and earlier gtrants

for field work to B.R. Frost and C.D. Frost (EAR-9017465) and D.H

Lindsley (EAR-8618480 and EAR-8816040).

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MaNuscnrrr RECETvED Ocronsn 24, 1994Mewuscnrsr AocEPTED Jurv 6, 1995


baddeleyite (zror) and zircon (zrsio) from anorthositic

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