Lithos 113 (2009) 555–569

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Geochemical and isotopic studies of the Lady of the Lake Intrusion and associatedtobacco root Batholith: Constraints on the genetic relation between Cretaceous maficand silicic magmatism in Southwestern Montana

Arindam Sarkar a,b, James G. Brophy a,b,⁎, Edward M. Ripley a,b, Chusi Li a, Sandra L. Kamo c

a Department of Geological Sciences, Indiana University, Bloomington, Indiana, IN 47405, USAb Judson Mead Geologic Field Station of Indiana University, Cardwell, Montana 59721, USAc Jack Satterly Geochronology Laboratory, University of Toronto, Canada

⁎ Corresponding author. Department of GeologicalBloomington, Indiana, IN 47405, USA. Tel.: +1 812 855

E-mail address: brophy@indiana.edu (J.G. Brophy).

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.06.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 June 2008Accepted 17 June 2009Available online 30 June 2009

Keywords:Southwest MontanaMafic and silicic magmatismFractional crystallizationCrustal meltingTobacco Root Batholith

Small volumes of alkalic mafic intrusions are spatially associated with Cretaceous to Early Tertiarygranodioritic to granitic intrusions in the batholithic province of southwestern Montana. The mafic rocksgenerally occur near the contacts of the Boulder, Pioneer, and Tobacco Root Batholiths with country rocks, buttheir genetic relation with the batholiths is uncertain. The Lady of the Lake Intrusion is a small layered bodycomposed of melagabbro and gabbro that occurs along the south-central margin of the Tobacco RootBatholith near its contact with Archean country rocks. A diorite unit, spatially distinct from the granodiorite/quartz monzonite of the Batholith intrudes the gabbroic rocks of the Lady of the Lake Intrusion. Zirconcrystals from the melagabbro and diorite units give U–Pb ages that are very similar to that of the TobaccoRoot Batholith at 74.88±0.17 Ma and 76.24±0.08 Ma, respectively. Mineral chemistry, whole rock major andtrace element compositions, and oxygen and sulfur isotope ratios have been utilized to evaluate the geneticrelation between the Lady of the Lake Intrusion, the diorite, and the Tobacco Root Batholith. No significantvariation in the composition of clinopyroxene is observed in different rock units of the Lady of the LakeIntrusion. Minor olivine with Fo64 in the melagabbro unit is interpreted to represent early crystallization inthe base of the intrusion. Whole rock major and trace element compositions, as well as results frommodelingusing the MELTS program, are consistent with the premise that the diorite was produced by fractionalcrystallization of the same magma that was parental to the gabbros of the Lady of the Lake Intrusion. Bothwhole rock chemistry and oxygen isotopes support the interpretation that the parental magma was anuncontaminated mantle-derived basaltic magma. In contrast, trace element and oxygen isotopes indicatethat the quartz monzonitic and granodioritic rocks of the Tobacco Root Batholith and the gabbroic rocksof the Lady of the Lake Intrusion are not related to the same parental magma. Elevated d18O values of quartz(9–10‰) and feldspar (8–9‰) from the Batholith are consistent with an origin involving partial melting ofArchean and Proterozoic country rocks that was initiated by heat supplied by basaltic magma, similar to thatwhich was parental to the Lady of the Lake Intrusion. d34S values of the melagabbro range from 0.1 to 0.7‰,and support the premise that the mantle-derived parental magma was uncontaminated. In contrast, d34Svalues of the gabbro and diorite units are elevated (0.6 to 6.5‰), and indicate that assimilation of countryrock S occurred, primarily via volatile transfer. Sulfur contents of the Lady of the Lake Intrusion are low(b1000 ppm) and no evidence suggests that S was transferred to crustal melts that produced the TobaccoRoot Batholith.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Cretaceous plutonic rocks associated with Laramide orogenicactivity in southwestern Montana occur some 200 km east of thelarge Idaho Batholith (Fig. 1). Specific bodies include the Boulder,Pioneer, Phillipsburg, Sapphire, and Tobacco Root Batholiths, the Flint

Sciences, Indiana University,6417.

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Creek pluton, and several smaller intrusions. The magmatism isunusual in that it occurs to the east of the Cordilleran-relatedCretaceous and Tertiary magmatic rocks located to the north andsouth. The Boulder Batholith is the largest of the intrusive bodies insouthwestern Montana and hosts the well-known Butte vein andporphyry Cu–Mo ore systems (e.g., Rusk et al., 2008). The Boulder,Pioneer, andTobaccoRoot Batholiths all containminor amounts of K-richmafic rocks (ultramafic tomafic) that are generally thought to representearly emplaced border phases of the batholiths themselves. Whetherthese mafic rocks are parental to the more silicic batholithic rocks via

Fig. 1. Geological index map showing Cretaceous batholiths in the southwest Montana area (after Hyndman and Myers, 1988; Foster and Fanning, 1997).

556 A. Sarkar et al. / Lithos 113 (2009) 555–569

fractional crystallization or if they represent melts that are geneticallyunrelated to the major rock types found in the batholiths has not beenresolved. From an ore mineralization standpoint, mafic rocks have beenproposed by Hattori (1993) and Keith et al. (1997) to be particularlyimportant for the generation of porphyry-style Cu mineralizationwherein mantle-derived sulfur in themafic rocks is transferred to silicicmagmas via fluids, a process recently confirmed by Thakurta et al.(2008). Thus, establishing the relationship between these southwesternMontana mafic border phases and their associated silicic plutons is animportant question not only from the standpoint of the origin of theplutons themselves, but also for the genesis of any associated sulfidemineralization that might be present.

Ideally, the most appropriate place to address such a questionwould be a plutonic body that is highly mineralized and also containsmafic border phase rocks. The Boulder Batholith (Fig. 1) is one suchexample but, unfortunately, the mafic rocks are poorly exposed andgenerally highly altered. An alternative plutonic body is the TobaccoRoot Batholith (Fig. 1) which is much less mineralized than theBoulder Batholith (e.g., Woodward, 1993; Zen, 1996) but does have awell-exposed and largely unalteredmafic border phase intrusion. Thisbody, known as the Lady of the Lake Intrusion is a small layeredintrusion that occurs along the margin of the Tobacco Root Batholithnear its contact with Archean country rocks. (Horn et al., 1991, 1992;Fig. 2). In this paper we present the results of mineralogic, chemical,and isotopic studies of the Lady of the Lake Intrusion, which help tofurther constrain the genetic association between mafic and inter-mediate to felsic plutonic activity in southwestern Montana.

2. Local geologic setting

The Lady of the Lake Intrusion is nearly 1 km wide and 2.5 km inlength (Fig. 2) and occurs between Archean quartzofeldpathic gneissto the east and a salient of the much larger Tobacco Root Batholith tothe west and south (Fig. 2). The intrusion is divided into three units:melagabbro, gabbro and diorite (Fig. 2). The diorite unit is intrusiveinto both melagabbro and gabbro (Fig. 2). The contacts betweenmelagabbro, gabbro, and diorite are sharp and lack chill margins.The contact between the predominantly granodioritic to quartzmonzonitic rocks of the Tobacco Root Batholith and the Lady of theLake Intrusion is not exposed. Compositional layering (pyroxene–amphibole-rich layers alternating with plagioclase layers) is com-

monly observed in the gabbro (Fig. 3A). The strike of the layeringparallels the contact with themelagabbro, and varies within N10°W toN15°W. Fan-bedding, cross-bedding and graded bedding are locallyobserved in the gabbro units as well (Fig. 3A). Melagabbro comprises~5 to 10% of the intrusion and represents an accumulation of pyroxenecrystals. The gabbro unit is much lighter in color (even in pyroxene-rich layers) due to the higher concentrations of plagioclase. Xenolithsof melagabbro and gabbro are found in the diorite (Fig. 3B and C), andare particularly abundant at the contact of the dioritewith rocks of theTobacco Root Batholith.

3. Sampling and analytical methods

Sample locations are shown in Fig. 2. Petrographic observationswere conducted using standard transmitted and reflected lightmethods. Zircon was separated from the diorite and melagabbrosamples using standard heavy liquid and magnetic separationtechniques at the Jack Satterly Geochronology Laboratory, Universityof Toronto, Canada for U–Pb analysis by isotope dilution thermalionization mass spectrometry (ID-TIMS) methods. Zircon crystalsfrom the diorite were pre-treated by air abrasion (Krogh, 1982) toremove exterior surfaces that may have lost Pb. Zircon grains from themelagabbro, which were dated more recently, were pre-treated bychemical abrasion (Mattinson, 2005) modified by annealing them at930 °C for 60 h followed by an HF etch at 195 °C for 6 h. This is toensure removal of radiation-damaged zones that may have undergonePb loss due to alteration on crystal surfaces and internally within highU zones. All crystals were cleaned prior to dissolution in room-temperature HNO3 followed by ultracleanwater and acetone. A mixed205Pb–235U spike was added during sample loading into Teflondissolution bombs with concentrated HF; the mixture was placed inan oven at 200 °C for 5 days (Krogh, 1973). Anion exchange columnseparation was performed to isolate U and Pb, which were then driedin ~10 µl of 0.05 N H3PO4 and loaded directly onto out gassed rheniumfilaments with silica gel (Gerstenberger and Haase, 1997). Pb and Uwere analyzed with a VG354 mass spectrometer using a Daly collectorin pulse counting mode. All common Pb was assigned to the isotopiccomposition of the Pb procedural blank. A correction for excess 206Pbfrom Th decay and incorporated during zircon crystallization is madeassuming a Th/U of 4.2 in the magma. Dead time of the measuringsystem for Pb is 22.8 ns and 20.8 ns for U. The mass discrimination

Fig. 2. Generalized geologic map of the Lady of the Lake Intrusion, Montana showing: (A) main lithologic units; and (B) sample locations.

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correction for the Daly detector is constant at 0.05% per atomic massunit. Daly characteristics were monitored using the SRM982 Pbstandard. Thermal mass discrimination correction for Pb is 0.10% peratomic mass unit. Decay constants are those of Jaffey et al. (1971). Allage errors quoted in the text and table, and error ellipses in theconcordia diagram is given at the 95% confidence level. Plotting andage calculations are from Ludwig (2003).

Compositions of silicate minerals were determined by wavelengthdispersive X-ray analyses using a CAMECA SX-50 electron microprobeat Indiana University. Analytical conditions for the major elementswere 15 kV, 20 nA beam current, a peak counting time of 20 s, and a1 µm beam diameter size.

Whole rock major and trace elements were analyzed by X-rayfluorescence (XRF) using a Rigaku 3070 wavelength dispersive XRFspectrometer with a rhodium tube at the University of Cincinnati, Ohio.The detection limits were 0.02 wt.% for major elements with anestimated error of ±0.02 wt.%. Whole rock REE concentrations wereanalyzed by ICP-MS at the Geoscience Laboratories of the OntarioGeological Survey, Ontario, Canada. Samples were digested by refluxingat 120 °C for aweekwith a 10:1:1mixture of concentrated hydrofluoric,perchloric, and hydrochloric acids in closed screw-topped Savillex(tm)

Teflonbeakers, evaporating to incipientdrynesson ahotplate, dissolvingthe dried-down sample in 1.4% v/v hydrochloric acid +0.3% v/vperchloric acid, evaporating to dryness, and finally redissolving thesample in 10% v/v nitric acid containing trace hydrochloric andhydrofluoric acid. Recovery tests using a variety of certified referencematerials indicate that the method is capable of bringing most traceelement bearing accessory phases into solution, including zircon,monazite, xenotime, and spinel (Tomlinson et al., 1999; Burnham,2002). All the REE data are within an estimated error of ±2 ppm.S concentrationswere determined at Indiana University using a CS Eltrawith a resistance furnace temperature of 1450 °C. Multiple analyses ofstandardswith 128 and 1041 ppmshowed a rangewithin±17ppm.Thedetection limit of the combustion method is ~50 ppm. Minerals foroxygen isotopic measurements were drilled from polished sectionsusing a 0.75 mm diamond drill bit. Oxygenwas liberated from powdersof whole rock, pyroxene and plagioclase by reactionwith BrF5 at 650 °Cand from olivine at 700 °C (Clayton and Mayed, 1963). The oxygenwasthen converted to CO2 by reaction with a heated graphite disc. Isotopicratios were determined using a Finnigan MAT 252 mass spectrometerwith an analytical uncertainty of ±0.05‰ and a sample reproducibilityof ±0.2‰. Oxygen isotopic compositions are reported in standard δ

Fig. 3. Photographs showing field relationship of the various units of the Lady of theLake Intrusion. (A) Fanning in a modally layered gabbro (alternating layers rich inpyroxene and plagioclase). (B) Sharp contact between the melagabbro unit and thediorite with xenoliths of melagabbro within the diorite. (C) Sharp contact between thegabbro unit and the diorite with xenoliths of gabbro within the diorite.

558 A. Sarkar et al. / Lithos 113 (2009) 555–569

notation relative to Vienna Standard Mean Ocean Water (VSMOW).NBS-28 quartz has a δ18O value of 9.6±0.2‰ in our lab.

For sulfur isotopic analyses finely disseminated sulfide mineralswere drilled from polished slabs using a 0.5 mm tungsten carbide drillbit. Sample powders and small amounts of V2O5 were loaded into tincups and analyzed using Elemental-Analyzer-Continuous Flow Iso-tope Ratio Mass Spectrometry (Studley et al., 2002). Samples weremeasured using a Finnigan MAT252 isotope ratio mass spectrometer.Analytical precision is better than ±0.05‰, whereas sample reprodu-cibility is ±0.2‰. For samples that contained less than ~200 ppm Swhole rock powders were analyzed following procedures described inStudley et al. (2002). Sample reproducibility was ±0.4‰ for thewhole rock δ34S analyses. NBS-127 (BaSO4, δ34S=20.3‰) and IAEAstandards (S1=−0.3, S2=20.8‰) were used as standards (values onthe SO2 scale). Sulfur isotopic compositions are reported in standard δnotation relative to Vienna Canon Diablo Troilite (VCDT).

4. Results

4.1. Petrography

Hydrothermal alteration has been locally intense in the melagab-bro and gabbro units. Pyroxene and primary amphibole have beenconverted to mixtures of secondary amphibole and chlorite whereasplagioclase has been converted to albite, zoisite, and epidote. Muchfresher rocks that retain a primary mineral assemblage are locallypresent and attest to the strongly fracture-controlled fluid flow thatcaused the hydrothermal alteration.

4.1.1. MelagabbroSample M118 is the freshest melagabbro observed and contains

~20 vol.% of granular olivine, locally poikilitically enclosed in pyroxeneor primary amphibole (Fig. 4A). Olivine is also present in sampleM123, but most has been converted to an amphibole–chlorite mixtureand modal abundance is ~5 vol.%. Pyroxenes in samples M124 andM123 are prismatic to equidimensional (and subhedral), varying from~3 to 4.5 mm in length and width (Fig. 4B). In all melagabbro samplespyroxene has been replaced to various degrees by secondaryamphibole. Primary amphibole occurs as discrete grains and rarelyas rims on pyroxene (Fig. 4C). Interstitial plagioclase occurs inamounts up to ~15 vol.%. Magnetite occurs as a euhedral, early-formed mineral, in amounts up to 10 vol.%. Euhedral apatite occurswith interstitial plagioclase and is never found enclosed in olivine orpyroxene. Biotite also occurs as an interstitial mineral in amountsranging from 3 to 10 vol.%. Interstitial sulfide minerals are unevenlydistributed in themelagabbro, and never occur in amounts in excess of1 vol.%. Pyrrhotite, and lesser amounts of chalcopyrite are thepredominant minerals, and are frequently rimmed by magnetite.

4.1.2. GabbroPyroxene and plagioclase are the principal minerals in the gabbro.

Average modal proportions vary from ~50 to 55 vol.% pyroxene and 15to 30 vol.% plagioclase, with less than 15 to 25 vol.% each of primaryamphibole and biotite. Prismatic grains of pyroxene and plagioclaserange from 2 to 4 mm in length. Sample M100 is a pyroxene-rich layerwith less than 10 vol.% interstitial plagioclase. Plagioclase occursprincipally as anhedral, interpenetrating grains or interstitially topyroxene (Fig. 4D). All primary minerals show various degrees ofreplacement by secondary minerals as noted above. Sulfide mineralsoccur interstitially as in the melagabbro; overall the amounts are lessthan 0.3 vol.%, but in localized areas volume percents near one mayoccur.

4.1.3. DioriteDiorite consists of 50 to 60 vol.% plagioclase, 20 to 30 vol.%

pyroxene found in various stages of conversion to secondaryamphibole, 5 to 10 vol.% biotite, up to 5 vol.% quartz, 3 to 5 vol.%magnetite and minor amounts of orthoclase, amphibole and sulfideminerals. The grain size of the diorite is variable, with laths ofplagioclase ranging from less than 1.5 mm in length in fine-grainedvarieties to 3.0 mm in coarse-grained varieties. Pyroxene is prismaticto equidimensional (Fig. 4E), and may also occur as anhedral grainsthat are part of interstitial assemblages of pyroxene, feldspar, biotite,and quartz.

4.1.4. Adjacent Intrusive rocks of the Tobacco Root BatholithThe small part of the Tobacco Root Batholith that is in intrusive

contact with the Lady of the Lake Intrusion varies between a quartzmonzonite and a granodiorite and is mainly composed of quartz, sodicplagioclase, orthoclase, and microcline. Plagioclase is subhedral toeuhedral and in most cases zoned. Alteration of feldspar is minor withlocal conversion to sericite and epidote. Most of the quartz grains areinclusion free. Amphibolemay be present in amounts up to 5 vol.% and

Fig. 4. Photomicrographs of: (A) Olivine enclosed within pyroxene in themelagabbro unit; sampleM118, crossed polars. (B) Olivine enclosed in primary amphibole in themelagabbrounit; Sample M123, crossed polars. (C) Remnant pyroxene with secondary amphibole from the gabbro unit; Sample M147, crossed polars. (D) Remnant pyroxene with secondaryamphibole enclosed by interstitial plagioclase from the gabbro unit; Sample M147, crossed polars. (E) Lath-shaped interpenetrating plagioclase from the dioirite; Sample M109,crossed polars. (F) Euhedral to subhedral plagioclase and quartz with partially altered pyroxene from the diorite; Sample M149, crossed polars. P equals primary, S equals secondary.

559A. Sarkar et al. / Lithos 113 (2009) 555–569

is locally converted to chlorite, secondary biotite and epidote. Biotitemay also be present in amounts up to 5 vol.%, with less than 10%replaced by chlorite. Accessory minerals include magnetite, apatite,and rarely pyrite.

4.2. Geochronology

Mueller et al. (1996) reported an age of 75–77 Ma for the TobaccoRoot Batholith based on U–Pb ages from two different zircon crystalsseparated from a tonalite. According to Mueller the samples werecollected from the eastern part of the Tobacco Root Batholithic whichwould put them in close proximity to the Lady of the Lake intrusion. Aspart of our assessment of the potential genetic relationships betweenthe Tobacco Root Batholith, the Lady of the Lake Intrusion and thespatially associated (cross-cutting) diorite, we determined U–Pb

crystallization ages on individual zircon crystals from the melagabbroand diorite units.

A composite melagabrro sample yielded zircon crystals that areeuhedral, pale pink to colorless, 4-sidedprisms, generally ~150–400mmin length, and give concordant U–Pb data (Fig. 5A). Data obtained fromfour analyses (Table 1; analyses 1–4) of single, chemically-abradedgrains have a weighted mean 206Pb/238U age of 76.24±0.08 Ma (95%confidence; mean square of the weighted deviates (MSWD) is 0.56).This is interpreted as the best age estimate for emplacement of themelagabbro.

A composite diorite sample yielded zircon crystals that are euhedral,translucent, pale yellow, 4-sided (3:1 to 4:1 aspect) prismatic crystalsand multi-facetted, 2:1 prismatic grains. Some crystals containabundant spherical melt and rod-like (apatite?) inclusions, whilesome are completely devoid of inclusions. The inclusion-free zircon isoccasionallyobserved tomantle inclusion-rich zirconand this is taken as

Fig. 5. U–Pb concordia diagrams of zircon from the melagabbro (A) and diorite (B).

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an indication that inclusion-rich zircons from the source region wereincorporated into the magma, and were subsequently overgrown byinclusion-free zircon. Five fractions of zirconwere analyzed for Pb and Uand the results are presented in Table 1 (analyses 5–9) and Fig. 5B.Analyses 5, 6, and 8 were obtained from small (b200×60 mm andweighing b2 µg) crystals, and analyses 3 and 5 were obtained from

Table 1U–Pb data for zircon crystals from melagabbro and diorite associated with the Lady of the L

No. Weight U Th/U Pbtot PbCom 207Pb/204Pb

206Pb/238U

2σ 207Pb/235U

2σ

(µg) (ppm) (pg) (pg) measured

Melagabbro composite1 1.9 3342 na 71 0.7 369 0.011887 0.000022 0.07802 0.002 1.2 6001 1.4 109 0.6 441 0.011898 0.000023 0.07806 0.003 4.6 7194 1.0 467 0.5 2477 0.011892 0.000031 0.07790 0.004 3.7 2758 na 114 0.7 512 0.011907 0.000023 0.07783 0.00

Diorite composite5 3.8 170 0.63 8.1 1.0 38.6 0.011687 0.000031 0.0767 0.006 1.8 142 0.61 3.2 0.7 28.5 0.011669 0.000059 0.0772 0.007 11.0 170.55 0.27 234 4.3 649 0.11396 0.00030 3.116 0.008 1.3 50 0.41 35.9 0.6 702 0.48370 0.00205 14.160 0.059 14.8 287 0.27 1805 2.2 12019 0.37084 0.00146 13.250 0.05

Notes:Zircon grains from melagabbro have been chemically abraded; zircon from diorite have beePbtot —total amount of radiogenic Pb.Pbcom — common Pb assuming the isotopic composition of laboratory blank: 206/204 — 18Th/U calculated from radiogenic 208Pb/206Pb ratio and 207Pb/206Pb age assuming concordanCorrection for excess 206Pb assuming Th/U in magma of 4.2.Pb/U corrected for spike, fractionation, blank; 207/204 corrected for spike and fractionationDisc —% discordance for the given 207Pb/206Pb age.Uranium decay constants are from Jaffey et al. (1971).

relatively large (11.0 and 14.8 µg, respectively), multi-facetted crystals.All are single zircon analyses with the exception of analysis 5, whichcomprised 2 grains. Analyses 5 and 6 give overlapping, concordant datathat have a weighted mean 206Pb/238U age of 74.88±0.17 Ma(MSWD=0.27). Analyses 7–9 have 207Pb/206Pb ages of 2812.1±2.1,2923.3±4.3 and 3241.4±1.4Ma, and are 79%,16%, and 43% discordant,

ake Intrusion, Montana.

207Pb/206Pb

2σ 206Pb/238U

2σ 207Pb/235U

2σ 207Pb/206Pb

2σ % Disc Rho

Age (Ma) Age (Ma) Age (Ma)

024 0.04760 0.00010 76.18 0.14 76.28 0.23 79.5 5.1 4.2 0.732022 0.04758 0.00008 76.25 0.15 76.32 0.21 78.6 4.0 3.0 0.816023 0.04751 0.00004 76.21 0.19 76.17 0.21 75.0 2.2 −1.6 0.950022 0.04741 0.00008 76.31 0.15 76.10 0.21 69.7 4.0 −9.5 0.814

23 0.0476 0.0013 74.90 0.19 75.1 2.1 80 66 6.9 0.69447 0.0480 0.0027 74.79 0.38 75.5 4.4 97 136 23 0.7199 0.19828 0.00025 695.7 1.71 1436.6 2.34 2812.1 2.06 79 0.915 0.21232 0.00057 2543.3 8.9 2760.5 3.7 2923.3 4.3 16 0.7885 0.25913 0.00022 2033.3 6.9 2697.6 3.9 3241.4 1.4 43 0.111

n air abraded.

.221; 207/204 — 15.612; 208/204 — 39.360 (errors of 2%).ce.

.

Table 2Representative mineral compositions.

M123 M147 M114 M123 M115 M114 M148 M123 M115 M123 M115 M114 M123 M115 M129 M123

MG G D MG G D TRB MG G MG G D MG G D MG

cpx cpx cpx pl pl pl pl amp (P) amp (P) amp (S) amp (S) amp (S) bio bio bio oliv

SiO2 49.98 51.78 52.88 48.48 56.95 58.78 62.5 42.34 41.55 49.02 48.44 51.48 37.28 37.15 37.94 36.96TiO2 0.69 0.22 0.18 – – – – 3.37 2.61 1.06 0.97 0.50 3.32 3.26 4.61 –

Al2O3 3.97 1.24 1.56 32.95 27.28 26.4 23.9 11.90 12.75 5.48 5.88 3.78 15.70 14.66 13.51 –

FeO 8.09 9.15 7.24 0.31 0.17 0.08 0.14 11.84 12.58 12.17 14.89 13.51 13.94 17.91 18.88 30.29MnO 0.27 0.52 0.31 – – – – 0.23 0.19 0.32 0.45 0.39 0.09 0.14 0.17 0.64MgO 14.37 13.40 14.79 – – – – 14.15 13.15 14.80 13.43 15.01 17.89 13.41 12.26 32.93CaO 21.55 22.25 21.71 15.2 8.76 8.4 4.87 11.47 11.93 12.00 12.15 12.03 0.00 0.01 0.05 0.03Na2O 0.36 0.44 0.45 2.73 6.26 6.86 8.81 2.17 2.12 0.91 0.89 0.55 0.76 0.18 0.08 –

K2O 0.00 0.06 0.01 0.01 0.11 0.17 0.15 0.97 1.13 0.46 0.58 0.26 8.46 9.71 9.33 –

Cr2O3 0.06 0.00 0.00 – – – – – – – – – – – – –

V2O3 0.10 0.09 0.06 – – – – – – – – – – – – –

H2O – – – – – – – 2.05 2.03 2.03 2.03 2.06 – – – –

Cl – – – – – – – – – – – – 0.04 0.00 0.00 –

Total 99.29 99.05 99.14 99.67 99.63 100.69 100.38 100.49 100.04 98.24 99.71 99.58 97.48 96.43 96.83 100.85Mg# 0.760 0.723 0.784 – – – – 0.680 0.651 0.684 0.617 0.664 0.562 0.428 0.394 –

An – – – 75.5 43.4 39.9 23 – – – – – – – – –

Fo – – – – – – – – – – – – – – – 65

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respectively. Assuming that each of the analyzed grains comprises twoage components, a primary Archean inherited core component and a74.88 Ma magmatic overgrowth, then the upper intercept age through

Table 4Trace element composition of the Lady of the Lake Intrusion and the Tobacco Root Batholit

Sample ID Lithologic unit Ba Mo Nb Zr Y Sr

M123 Melagabbro 303 b.d. 4.2 49 10 371M124 Melagabbro 337 b.d. 6.5 68 12 564M126 Melagabbro 351 b.d. 12.8 93 18 863M117 Melagabbro 487 b.d. 7.6 75 12 516M118 Melagabbro 240 b.d. 6.4 64 8 382M116 Melagabbro 165 b.d. 5.4 49 15 306M115 Gabbro 122 b.d. 13 106 12 534M147 Gabbro 667 b.d. 7.8 79 12 694M125 Gabbro 1138 b.d. 11.8 75 16 595M100 Gabbro 271 b.d. 7.2 66 8 283M109 Diorite 835 b.d. 19 171 8 834M114 Diorite 863 b.d. 19 182 11 1028M129 Diorite 843 3 11 135 12 682M133 Diorite 1033 b.d. 16.5 200 15 936M149 Diorite 1249 b.d. 18 174 16 922M144 Diorite 799 b.d. 19.8 212 13 809M148 T.R.Batholith – 29.2 167 70 917 220M143 T.R.Batholith – 25.1 182 75 1267 1.7M150 T.R.Batholith – 25.1 172 81 1073 b.d.

Values in ppm.

Table 3Major element composition of the Lady of the Lake Intrusion and the T.R. Batholith.

Sample ID Lithologic Unit SiO2 TiO2 Al2O3 FeO

M123 Melagabbro 43.53 1.51 10.11 21.63M124 Melagabbro 46.22 1.18 12.23 17.01M126 Melagabbro 43.11 1.58 14.48 16.6M117 Melagabbro 48.77 0.89 10.97 14.8M 118 Melagabbro 45.41 0.95 9.94 19.0M116 Melagabbro 43.59 1.65 9.89 18.13M115 Gabbro 50.37 0.76 11.60 14.4M147 Gabbro 49.42 1.14 12.77 13.2M125 Gabbro 52.94 1.16 11.45 11.23M100 Gabbro 52.23 0.59 8.10 11.12M109 Diorite 56.74 0.74 16.12 8.8M114 Diorite 54.22 0.96 16.58 9.7M129 Diorite 56.61 0.67 13.90 9.9M133 Diorite 54.79 0.86 15.78 8.5M144 Diorite 54.83 0.74 16.05 9.19M149 Diorite 54.27 0.96 16.82 10.0M148 T.R.Batholith 64.98 0.25 14.50 4.71M143 T.R.Batholith 67.55 0.27 15.44 2.6M150 T.R.Batholith 65.60 0.17 15.16 3.71

Results presented on an anhydrous basis (wt.%).

each of these data and anchored at 74.88 Ma will indicate anapproximate minimum source age of the inherited grain. The age isregarded as a minimum because the core component may have lost Pb

h.

U Rb Th Pb Cu Ni Co Cr V S

b.d. 12 b.d. 6 491 114 77 205 746 818b.d. 18 b.d. 7 310 90 62 187 493 12931.8 21 b.d. 4 187 61 49 142 456 621b.d. 41 b.d. 6 140 102 60 196 320 10532.8 25 b.d. 4 2 125 79 256 408 290b.d. 9 b.d. 3 179 101 65 274 511 2010.7 23 5 6 7 81 44 194 414 128b.d. 41 b.d. 3 190 54 51 132 398 592b.d. 49 b.d. 7 28 81 40 218 250 191b.d. 44 b.d. 2 57 169 53 647 135 4282.2 105 13 13 49 28 25 133 173 652.3 76 10 10 200 29 26 143 213 2971.8 57 6 25 204 45 24 178 163 4283.1 74 5 14 59 21 22 94 219 600.9 57 5 13 16 2 14 141 113 1383.6 93 13 11 37 35 27 155 206 7972 8 17 52 13 9 221 46 0.35 11230 5 18 40 8 6 136 38 0.25 13464 6 18 58 14 8 113 20 0.29 61

MnO MgO CaO Na2O K2O P2O5

0.25 12.02 9.12 0.91 0.75 0.160.23 10.15 9.39 2.10 1.29 0.18

7 0.18 8.48 10.33 3.28 1.43 0.460 0.25 11.65 9.05 1.62 1.78 0.222 0.25 13.42 8.53 1.38 1.04 0.07

0.24 12.50 11.70 1.07 0.85 0.395 0.22 9.36 9.64 2.71 0.82 0.075 0.20 8.66 9.49 2.89 1.98 0.19

0.28 10.29 9.08 0.66 2.63 0.270.23 14.60 9.86 1.98 1.21 0.08

7 0.13 3.86 6.55 6.40 0.41 0.189 0.17 4.38 6.20 4.66 2.75 0.293 0.15 4.94 6.30 4.06 3.16 0.278 0.15 3.87 6.17 6.03 3.52 0.26

0.15 4.40 6.54 4.87 3.02 0.215 0.16 3.95 6.30 4.51 2.69 0.29

0.09 1.43 3.32 8.05 2.56 0.103 0.06 0.74 2.79 8.02 2.40 0.09

0.11 1.17 3.29 7.90 2.79 0.10

562 A. Sarkar et al. / Lithos 113 (2009) 555–569

prior to incorporation into the melt. Analyses 7 and 8 are colinear withthe 74.88 Ma age and give an upper intercept age of ca. 2925 Ma, andanalysis 9 anchored at 74.88Magives anupper intercept age of 3260Ma.These ages are consistent with the ages of Archean rocks in the Tobacco

Fig. 6. Cross plots of (A) Al2O3 (B) MgO (C) CaO (D) Na2O versus SiO2 and (E) Ni (F) Cr (G)Tobacco Root Batholith.

Root Mountains (e.g., detrital zircon ages between 3 and 4 Ga, Muelleret al., 2004). Our data suggest that the crystallization ages of the Lady ofthe Lakemelagabbro (~76Ma) anddiorite (~75Ma),which appear to berelated via fractional crystallization (see below) are essentially identical

Ba (H) Rb versus Zr content for the melagabbro, gabbro, and the diorite units, plus the

563A. Sarkar et al. / Lithos 113 (2009) 555–569

to that of the Tobacco Root Batholith (75–77 Ma), thus confirming thepossibility of a genetic relationship between the Lady of the LakeIntrusion as a whole and the Tobacco Root Batholith.

4.3. Geochemistry

4.3.1. Mineral compositionsRepresentative mineral compositions of plagioclase, pyroxene,

amphibole, biotite and olivine from the melagabbro, gabbro and dioriteunits are given in Table 2. Additional mineral compositional data areprovided as an electronic supplement. The compositions of plagioclasecores from the melagabbro and gabbro units overlap. The anorthitecontent of plagioclase in mol% varies between 52 and 78, between 46and 68 and between 31 and 40 in the melagabbro, gabbro and dioriteunits, respectively. The compositions of pyroxene from themelagabbro,gabbro, and diorite units are similar and within the augite range. Thecore composition of pyroxene from the melagabbro and gabbro unitshave similar Mg-numbers (molar MgO/(MgO+FeO)) that rangebetween 0.71 and 0.80. Primary amphibole is high in Al2O3 (11.5 to13wt.%), K2O (0.7 to 1wt.%) and TiO2 (2 to 3.5wt.%) and low in SiO2 (41to 42wt.%) compared to secondary amphibole. Compositions of primaryamphibole are very close to magnesio-hornblende whereas thecompositions of the secondary amphibole are close to hornblende. Themagnesium content and Mg-numbers of biotite in the melagabbro unitare higher than those of biotite from the gabbro and diorite units (14 to17wt.%MgOversus 11 to 13.5wt.%MgO,Mg#of 0.54 to 0.57 versus 0.42to 0.43). Olivine is present in samples M118 and M123 and shows a Fomole percent range between 64 and 65.

4.3.2. Major and trace element compositionsWhole rock major and trace element analyses of nineteen samples

from themelagabbro and gabbro units of the Lady of the Lake Intrusion,the diorite, and the Tobacco Root Batholith are listed in Tables 3 and 4and illustrated in Fig. 6A to F. Loss on ignition values vary between0.47 wt.% in fresh melagabbro and 2.31 wt.% in a pyroxene-rich layer ofthe gabbrowith ~50% secondary amphibole and chlorite. Because of thesimilarity in LOI and degree of alteration in all analyzed rocks, all of thesamples are normalized to 100% on an anhydrous basis for comparativepurposes. The SiO2 contents of themelagabbro, gabbro anddiorite rangefrom 43 to 49 wt.%, 51 to 53.5 wt.% and 54 to 57 wt.%, respectively. Thelow SiO2 content of the melagabbro reflects the presence of olivine,biotite (3 to 10 vol.%), apatite (2–4 vol.%),magnetite (~4 to 10 vol.%) andprimaryamphibole (15 to 35vol.%). HarkerdiagramsutilizingMgO, CaO,Al2O3, andNa2O versus SiO2 are illustrated in Fig. 6A toD. The threeunitsdefine moderately linear trends that display increasing Al2O3 and Na2O

Table 5Concentrations of REE (chondrite normalized) in the Lady of the Lake Intrusion and the Tob

Sample ID Lithologic unit La Ce Pr Nd Sm Eu

M117 Melagabbro 71.35 59.38 49.55 41.08 25.59 17.16M123 Melagabbro 42.77 38.40 35.18 32.15 22.26 15.97M124 Melagabbro 53.16 46.87 41.30 37.07 24.15 18.35M100 Gabbro 54.16 43.94 36.34 29.27 18.36 12.90M125 Gabbro 92.45 76.96 66.11 54.83 32.92 21.18M147 Gabbro 75.39 59.32 47.16 38.97 23.79 16.33M115 Gabbro 68.55 57.75 44.94 35.70 20.67 13.43M133 Diorite 141.13 102.75 76.78 58.13 30.92 21.70M109 Diorite 146.19 105.37 75.98 55.92 28.77 19.01M129 Diorite 112.68 79.79 57.03 42.87 23.64 16.97M114 Diorite 135.81 101.32 76.22 57.98 31.54 24.60M144 Diorite 138.39 103.86 75.17 56.75 29.49 20.19M113 T.R.batholith 84.90 56.47 39.10 28.13 14.46 10.951045 JLS T.R.batholith 85.87 57.45 39.75 28.87 15.64 11.55937 JLS T.R.batholith 118.23 95.99 72.26 53.55 27.23 17.16M148 T.R.batholith 108.45 73.14 51.77 37.37 19.54 13.25M150 T.R.batholith 93.77 62.17 43.56 31.23 16.21 12.41M143 T.R.batholith 79.42 51.88 34.87 24.48 12.31 10.29

N: normalized. Values in ppm.

and decreasing MgO and CaO as a function of increasing SiO2. The lowSiO2 content of the melagabbro supports the hypothesis that itrepresents, at least in part, an ultramafic cumulate. The relationshipbetween the melagabbro, gabbro and diorite are generally consistentwith fractional crystallization of a parental basaltic magma (gabbro) toproduce derivative intermediate magmas (diorite) and a correspondingfractionation residue (melagabbro). Significantly, the Tobacco RootBatholith plots along an extension to higher SiO2 content (64 to 68wt.%)of the trends defined by the Lady of the Lake units, supporting theinterpretation that it might have originated by the same process (oreven event) that produced the Lady of the Lake diorite. Fig. 6G to Hshows the abundances of selected compatible (Cr, Ni) and incompatible(Ba, Rb) trace elements plotted against Zr, which shows generallyincompatible behavior. The results show steadily decreasing Ni and Crand steadily increasing Ba and Rbwith increasing Zr. The general trendsdefined by Ni and Cr are curvilinear and concave upwards while thosedefined by Ba and Rb are linear. Both of these trends are generallyconsistent with a fractional crystallization origin for the diorite (e.g.,Allegre et al., 1977). Significantly, the trace element compositions of theTobacco Root Batholith samples plot far off the trend of the Lady of theLake Intrusion data.We interpret the trace elements of the Tobacco RootBatholith samples to indicate, unlike themajor elements, that theywerenot produced by the same process (or events) as the Lady of the Lakediorite.

Chondrite-normalized REE patterns of the melagabbro and gabbrounits of the Lady of the Lake Intrusion, the diorite, and samples of theTobacco Root Batholith are listed in Table 5 and illustrated in Fig. 7,using the chondrite normalization values of Boynton (1984). Allsamples show distinctive enrichment in LREEs. The diorite is enrichedin all REE relative to the melagabbro and gabbro. The Tobacco RootBatholith and the diorite display essentially identical REE abundancesand overall patterns. The roughly linear relation observed on a plot ofLa/Sm versus Zr (index of fractionation) for the melagabbro, gabbroand diorite (Fig. 8) again supports a cogenetic origin via fractionalcrystallization. A genetic relationship between the Lady of the LakeIntrusion and the Tobacco Root Batholith is inconsistent with the REEdata as shown by the high La/Sm ratio compared to the Lady of theLake trend.

Sulfur concentrations (Table 4) for all but two samples are lessthan 1000 ppm and considerably below reasonable values required tosaturate parental magmas with sulfide (e.g., mafic magma near QFMwhere sulfide is the predominant melt species, Li and Ripley, 2005).Melagabbro samples tend to show the highest S concentrations (1293to 201 ppm with an average of 713 ppm). The S contents of gabbro(128–592 ppm) and diorite (60–428 ppm) are similar, whereas the

acco Root Batholith.

Gd Tb Dy Ho Er Tm Yb Lu (La/Sm)N

16.34 12.66 10.71 9.11 8.57 7.81 7.85 7.55 2.7915.05 11.50 9.68 8.09 7.49 6.48 6.41 6.02 1.9216.31 12.28 10.27 8.90 8.17 7.16 7.08 6.61 2.2011.80 9.30 7.78 6.60 6.01 5.52 5.65 5.50 2.9521.00 16.37 13.82 11.66 10.96 9.78 9.95 9.50 2.8115.42 11.84 9.76 8.41 7.91 7.22 7.37 6.89 3.1713.20 10.19 8.55 7.34 6.85 6.33 6.41 6.24 3.3218.38 13.65 11.12 9.51 8.92 8.12 8.47 8.11 4.5616.80 12.49 10.34 8.86 8.52 7.84 8.09 7.86 5.0815.25 11.79 10.32 9.29 8.95 8.49 8.56 8.39 4.7719.28 13.99 11.74 10.06 9.56 8.67 8.85 8.60 4.3117.27 12.95 10.93 9.50 8.92 8.21 8.47 8.14 4.698.71 7.03 6.16 5.68 5.87 5.77 6.17 6.24 5.879.02 7.24 6.41 5.84 6.05 6.14 6.89 6.93 5.49

15.99 12.51 10.85 9.78 9.81 10.00 11.00 11.40 4.3412.22 9.96 8.78 8.05 8.39 8.43 9.28 9.47 5.559.66 7.85 6.80 6.20 6.35 6.33 6.84 6.77 5.797.43 6.01 5.33 4.93 5.09 5.12 5.55 5.71 6.45

564 A. Sarkar et al. / Lithos 113 (2009) 555–569

granodiorite samples of the Tobacco Root batholiths show valuesbetween 61 and 134 ppm.

Fig. 8. La/Sm versus Zr for samples from the melagabbro and gabbro units of the Lady ofthe Lake Intrusion, diorite, and the Tobacco Root Batholith.

4.3.3. Stable isotopesOxygen isotope data for all rock types in the area of the Lady of the

Lake Intrusion are listed in Table 6 and illustrated in Fig. 9. Pyroxenesfrom the melagabbro and gabbro units have d18O values that rangebetween 5.3 and 5.8‰ and between 5.1 and 5.9‰, respectively. Thesevalues are within the range considered to be “normal” for mineralsthat crystallized from uncontaminated mantle-derived magma (Eileret al., 1993). d18O values of plagioclase from the same units varybetween 6.2 and 6.9‰ and between 6.1 and 8.4‰, respectively. Biotiteand primary amphibole in the melagabbro and gabbro units rangefrom 4.7 to 5.2‰ and from 4.4 to 5.2‰, respectively. Magnetite wasnot analyzed in this study, but values of 3–5‰ are required to explainthe whole rock d18O values that range between 5.2 and 5.6‰ andbetween 5.4 and 6.9‰ for the melagabbro and gabbro units,respectively. d18O values of pyroxene and plagioclase from the diorite

Fig. 7. Chondrite-normalized REE abundance patterns of (A) melagabbro and gabbrounits of the Lady of the Lake Intrusion. (B) Diorite and the Tobacco Root Batholith.Normalization factors from Boynton (1984).

unit range from 5.5 to 6.9‰ and from 6.5 to 7.9‰, respectively.Plagioclase–pyroxene Δ (d18Oplag–d

18Opyr) values range between 1.0and 1.6‰. d18O values of quartz in the diorite are characterized byelevated values from 8.0 to 9.8‰. Whole rock values of diorite samplesfall in a restricted range from 6.1 to 6.9‰. Feldspar and quartz from theTobacco Root Batholith samples have d18O values between 7.4 and8.2‰ and between 8.3 and 9.8‰, respectively. Quartz-feldspar Δvalues range from 0.9 to 2.0‰. Whole rock d18O values of the TobaccoRoot Batholith range from 7.1 to 8.9‰.

Results of S isotope analyses are also presented in Table 6. Valuesof samples from the melagabbro range between 0.1 and 0.7‰, similarto those considered to be “normal” for S derived from upper mantle(0±2‰, e.g. Ripley, 1999). Gabbro samples show a wider range ofd34S values, from 0.6 to 6.5‰. Four sample of diorite also showelevated d34S values from 3.1 to 4.9‰.

5. Discussion

5.1. Mineral and bulk rock chemistry

Wehypothesize that themelagabbro, gabbro, anddioriteunitsmaybegenetically related through fractional crystallization as supported by themajor and trace element data presented above. On the basis of thecompositional data for the Tobacco Root Batholith it cannot be related tothe more mafic Lady of the Lake units via fractional crystallization.Oxygen isotope data are consistent with this conclusion and will bediscussed more fully below. Mineral composition variations lead tofurther insightswith respect to a possible genetic relationship among theLadyof the Lakeunits via crystal fractionation. Pyroxene compositions aresimilar in the melagabbro, gabbro, and diorite units, with no observabletrends (Fig. 10A). Biotite compositions are more Mg-rich in themelagabbro units (Fig. 10C), but this is readily explained as a result ofFe–Mg exchange between trapped liquid and abundant pyroxene in themelagabbro (e.g. Li and Naldrett, 2000). The textures and compositions(Fig. 10B) of feldspar in the diorite are distinctly different from those inthe melagabbro and gabbro units. Euhedral plagioclase in the diorite ismuchmore sodic (An31–40) than is interstitial plagioclase in the gabbroicunits (An78–43). All of these are consistentwith a fractional crystallizationorigin for the diorites from a parental magma similar in composition tothe gabbro. The similarity in compositions of pyroxene, plagioclase, andamphibole in the gabbroic rocks suggests that the difference between themelagabbro and gabbro units is related primarily to crystal sorting andthe distribution of trapped liquid. The presence of modal layering,fanning, andcross-bedding in thegabbroicunit is indicativeof convection

Table 6Oxygen and sulfur isotope compositions of the Lady of the Lake Intrusion and the Tobacco Root Batholith.

Sample ID Rock unit δ34S δ18Opx δ18Opl δ18Owr δ18Oqtz δ18Oamp δ18Obt Δ(pl-px) Δ(qtz-pl)

1045 JLS T.R.Batholith 8.7406 JLS T.R.Batholith 8.9DFH 1056 T.R.Batholith 7.6DFH 149A T.R.Batholith 7.4DH 277 T.R.Batholith 7.7M113 T.R.Batholith 8.2 7.1 9.8 1.6M127 T.R.Batholith 2.9 7.8 9.8 2.0M143 T.R.Batholith 7.4 7.4 8.3 0.9M145 T.R.Batholith 8.2 9.3 1.1M148 T.R.Batholith 8.2 7.5 9.5 1.3M150 T.R.Batholith 7.5 7.1 8.5 1.0M116 Melagabbro 0.5 6.9 5.0M117 Melagabbro 0.7 5.8 6.7 5.4 4.8 5.1 0.9M118 Melagabbro 0.4 5.3M123 Melagabbro 0.5 5.3 6.2 5.3 4.6 4.7 0.9M124 Melagabbro 0.1 6.2 5.2 5.2M126 Melagabbro 0.5 6.3 5.6 5.1M146 Melagabbro 5.4 6.7 1.3M97 Gabbro 2.0 6.1 5.0M100 Gabbro 6.5 5.9 4.4M106 Gabbro 5.5M108 Gabbro 1.7 5.9 8.4 6.9 2.5M115 Gabbro 3.4 8.1 6.6 5.1M119 Gabbro 6.5 6.7M120 Gabbro 4.4 5.2M121 Gabbro 3.5 6.6 5.0M122 Gabbro 5.6 6.7 4.5 4.7 1.1M125 Gabbro 1.2 6.4 5.5 4.7M128 Gabbro 4.7 5.8 6.4 0.6M130 Gabbro 0.6 5.4M142 Gabbro 5.4M147 Gabbro 2.7 5.1 6.7 5.8 4.7 1.6M153 Gabbro 5.5 6.5 1.0M107 Diorite 7.8 8.8M109 Diorite 5.5 6.5 6.3 1.0M110 Diorite 4.7 6.1 7.6 9.7 1.5M111 Diorite 4.5 6.9 7.9 8.6 1.0M114 Diorite 6.0 7.1 6.9 8.5 1.1M129 Diorite 3.1 6.7 6.1M131 Diorite 6.9 9.8M133 Diorite 7.8 6.3M144 Diorite 5.5 7.1 6.8 1.6M149 Diorite 4.9 6.2 7.3 8.0 1.1Arch-1 Quartzo feldspathic gneiss 7.9Arch-2 Quartzo feldspathic gneiss 7.7Arch-3 Quartzo feldspathic gneiss 8.4Car-11 Quartzo feldspathic gneiss 9.0Car-12 Quartzo feldspathic gneiss 7.0

NotesDFH-149A, 277 and 1056 are from the PhD thesis collection of David Hess (Hess, 1967). Sample locations are unknown.Car-11 and 12 collected along the South Boulder Road near the Indiana University Geologic Field Station, Cardwell, MT.

565A. Sarkar et al. / Lithos 113 (2009) 555–569

in the chamber, and is consistent with the premise that the melagabbrounitwasderivedvia accumulationof greaterquantities ofdensepyroxeneand olivine toward the base of the chamber.

As a further check on a possible fractionation association betweenthe diorite and gabbro units we have utilized the program MELTS(Ghiorso and Sack, 1995) to evaluate crystallization sequences andderivative liquid compositions. Our starting composition is assumed tobe 15% of melagabbro (M118) and 85% of gabbro (M147), whichrepresent the weighted average of the map units, adjusted to 0.5 wt.%H2O (Table 7). Runswere undertaken at theNNObuffer at 2 kbpressure.Olivine compositions (Fo64–67) provided a further constrain on the FeO/MgO ratio of the parent magma. Utilizing themagma–olivine FeO–MgOKd of 0.33 (Kd=molar (FeO/MgO)oliv/(FeO/MgO)liq)determined byRoeder and Emslie (1970) we estimated an FeO/MgO ratio in the parentmagma of ~0.4 (Table 7). Equilibrium crystallization using MELTSgenerates an assemblage of clinopyroxene (40 wt.%), plagioclase (40%),minor olivine (9%), and trace orthopyroxene (2%). This assemblagecloselymatches the bulkmineralogy of the Lady of the Lake Intrusion. In

addition, fractional crystallization produces a residual liquid similar tothat of the diorite at 57 wt.% crystallization (Table 7). This supports thenotion that the diorite unit may represent an evolved liquid thatintruded rocks that formed earlier from a similar parental magma. Ouranalyses also indicate that a mass of mantle-derived magmamore thantwice the size of the Tobacco Root Batholith would be required togenerate that body via fractional crystallization. Given the small volumeof themafic rocks in the Ladyof the Lake Intrusion this seemsunfeasible.This conclusion corroborates the conclusion from the trace element datathat the Tobacco Root Batholith cannot be derived from magmas thatproduced either the gabbroic or dioritic rock types of the Lady of theLake Intrusion.

5.2. Stable isotope systematics

Having concluded that: (1) the melagabbro–gabbro–diorite asso-ciation of the Lady of the Lake Intrusion most likely representsfractional crystallization of a mantle-derived mafic magma, and:

Fig. 9. Oxygen isotopic variations in the melagabbro and gabbro units of the Lady of theLake Intrusion, the diorite and the Tobacco Root Batholith.

Fig. 10. Binary variation diagrams of (A) Mg# of pyroxene (B) An# of plagioclase and(C) Mg# of biotite versus whole rock SiO2.

566 A. Sarkar et al. / Lithos 113 (2009) 555–569

(2) the Tobacco Root Batholith is not a fractionation product of thesame or similar mafic magmas, we address two additional questions.First, the oxygen isotopic data for minerals and whole rocks of theLady of the Lake Intrusion (Fig. 9) clearly indicate the presence of non-mantle values which implies the operation of additional processesbeyond simple fractional crystallization. Second, if the Tobacco RootBatholith is not related to mafic magmas such those preserved in theLady of the Lake Intrusion thenwhat is its origin? These questions areturned to now.

Oxygen isotope data may be used to constrain magma sources anddegree of contamination, as well as the extent of interaction withexternally derived fluids. Mantle-derived magmas are generallycharacterized by d18O values between 5 and 6‰ (e.g. Eiler, 2001).Crustally derived granitic rocks reflect the isotopic composition of thesource from which they were derived, and generally range from 7 to15‰ (in line with the high-18O nature of sedimentary and metase-dimentary rocks). d18O values of Archean quartzofeldspathic gneissesin the Tobacco Root Mountains range from 7 to 9‰ (Table 6).Assimilation of high or low-18O rock types by mantle-derived maficmagmas may significantly perturb the mantle signature, and for thisreason oxygen isotope ratios may serve as sensitive indicators ofmagma contamination (e.g., Taylor and Sheppard, 1986). Open systeminteraction between water and fluids may also lead to anomalousoxygen isotopic signatures in igneous rocks (e.g., Criss and Taylor,1986). Hydrothermal alteration may lead to either elevated ordepressed d18O values depending on the isotopic composition of thewater involved and the temperature of alteration reactions. Low andhigh d18O magmas may be generated by the assimilation ofhydrothermally altered rocks. Slow cooling of plutonic rocks mayalso lead to oxygen isotope reequilibration governed principally bydiffusion (e.g., Eiler et al., 1993). Because mineral–mineral fractiona-tion factors are high at low temperatures what appear to be elevatedd18O values may be generated in minerals characterized by highdiffusivities. Because several processes may contribute to the produc-tion of oxygen isotopic values that vary from those expected as a resultof crystallization from an uncontaminated mafic magma at high

temperatures, care must be taken in the interpretation of d18O valuesof minerals in igneous rocks.

The d18O values of melagabbro samples are all within the rangeexpected for minerals that crystallized from uncontaminated mantle-derived magma. Plagioclase is an interstitial mineral whose abun-dance does not exceed 10 vol.% in the melagabbro unit. At 500 °C thefractionation between plagioclase and pyroxene is ~3‰ (Chiba et al.,1989); taken with low modal abundance of plagioclase the observedd18O values that range from 6.2 to 6.9‰ are consistent with diffusiveexchange during slow cooling (e.g. Eiler et al., 1993). Whole rock d18Ovalues that range between 5.2 and 5.6‰ confirm the conclusion thatthe magma from which the melagabbro crystallized was notcontaminated by high-18O country rocks.

Whole rock d18O values of gabbro samples show a wider range,between 5.4 and 6.9‰. The whole rock values in excess of 6‰ reflectthe presence of relatively high-18O plagioclase. The “normal” d18Ovalues of pyroxene, biotite, and primary amphibole from the gabbrounit are not suggestive of magma contamination by high-18O country

Fig. 11. Closed system (A) quartz–feldspar fractionation computed from a simple twocomponent mass balance, illustrating that oxygen isotopic exchange during slowcooling could produce the observed Δ(qtz–felds) values found in the Tobacco RootBatholith. Isotopic values produced during closed system exchange are constrained tolie between the Xqtz and Xfelds=1 lines. (B) Plagioclase–pyroxene plot illustrating thatmost samples from the melagabbro and gabbro units are consistent with closed systemexchange during cooling. Samples from the diorite suggest equilibration with anexternally derived fluid of δ18O~8–9‰ at a temperature between 450 and 600 °C. Wesuggest that the fluid may have been derived from the Tobacco Root Batholith.

Table 7Compositions of magmas related to the crystallization of the gabbro and diorite units.

Magma composition 1 2 3 4

SiO2 46.21 50.00 49.43 55.89TiO2 0.97 1.15 1.12 1.29Al2O3 10.11 12.92 12.50 15.95Fe2O3 17.59 12.19 13.00 2.07MnO 0.25 0.20 0.21 0.38MgO 13.66 8.76 9.49 2.82CaO 8.68 9.60 9.47 5.42Na2O 1.40 2.93 2.70 4.61K2O 1.06 2.00 1.86 4.05P2O5 0.07 0.20 0.18 0.42

1. Melagabbro composition; normalized to 0.5 wt.% H2O.2. Gabbro composition; normalized to 0.5 wt.% H2O.3. Estimated parental magma composition determined by mixing 15% composition 1and 85% composition 2.4. Calculated magma composition after 57% fractionation of magma 3 determined byMELTS (Ghiorso and Sack, 1995) at 2 kbars and NNO.

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rocks. Because of the relative high modal abundance of plagioclase,local elevated values are not easily explained as a result of diffusiveexchange during slow cooling. However, kinetically governed oxygenisotopic exchange with an external fluid can explain the elevated d18Ovalues of some plagioclase and mantle signatures of ferromagnesianminerals. The similarity in the d18O values is consistent with thevariability in local water/rock ratios that is also manifested in theirregular distribution of secondary minerals in hydrothermally alteredzones of the Lady of the Lake Intrusion.

Whole rock d18O values of the diorite are in the 6–7‰ range andsuggest that either the parental magmawas contaminated by high-18Ocountry rocks or that exchange with fluids has resulted in the elevatedd18O values. The presence of inherited zircon in the diorite indicatesthat magma contamination by Archean, and potentially Proterozoic,crust must have occurred. Small (b~10%) degrees of contamination byArchean crust cannot be detected via d18O analyses; however the ~1.5to 2‰ ranges in the d18O values of pyroxene, plagioclase, and quartzare not consistent with crystallization from a uniformly contaminatedmelt. Rather the values are more in line with variable amounts ofoxygen isotopic exchange with a fluid during hydrothermal alteration.The whole rock d18O values of the diorite must reflect the expectedlow d18O values of magnetite, biotite, and unexchanged pyroxene. Theelevated values of quartz may in part reflect its low abundance andexchange during slow cooling. However, whole rock d18O valuessuggest that quartz d18O values in excess of 9‰ and plagioclase d18Ovalues near 8‰ must indicate open-system exchange with ahydrothermal fluid. Δ (plagioclase–pyroxene) values (Fig. 11, fractio-nation factors of Chiba et al., 1989) suggest that equilibration with afluid of d18O values ~8 to 9‰ occurred between 450 and 600 °C.Oxygen isotope values of the diorite samples are most easilyinterpreted as the result of exchange with a hydrothermal fluid,most likely derived from the Tobacco Root Batholith.

The d18O values of quartz and feldspar from the Tobacco RootBatholith samples are consistent with equilibration of small domainsduring slow cooling at temperatures between 350 and 475 °C (Fig. 11,fractionation factor of Matsuhisa et al., 1979). The Δ (quartz–feldspar)values are not suggestive of kinetically dominated exchange with anexternal fluid. Elevated d18O values of samples from the Tobacco RootBatholith are in linewith a derivation involvingmelting of crustal rocks.They are also consistent with an origin involving themixing of a mantlecomponentwith a crustalmelt as proposed byMueller et al. (1996). Theelevated d18O values indicate that the Tobacco Root Batholith could nothave been produced solely from fractional crystallization of a mantle-derived magma. Even with extensive fractionation of low-18O mineralssuch as magnetite and olivine fractional crystallization of basalticmagmas cannot lead to the production of granitic residues with d18Ovalues in excess of ~7‰ (e.g., Chalokwu et al., 1999).

Sulfur isotope compositions of sulfide minerals from the melagab-bro units are in the “normal” mantle range, and corroborate theoxygen isotope data that indicate the magma fromwhich the mineralsin the melagabbro accumulated was uncontaminated. The d34S valuesof the associated gabbro fall within a wider range, and are suggestiveof the involvement of externally-derived sulfur. There are two possibleinterpretations of the gabbro d34S values. One is that the magma fromwhich the melagabbro and gabbro formed was contaminated bycountry rock sulfur after the accumulation of melagabbro. Selectivecontamination of this type could be accompanied by sulfur derivationaccompanying dehydration reactions in contact rocks via reactionssimilar to (Ripley, 1981):

FeS2 þ 3H2O þ 2:5C ¼ FeSþ H2S þ 1:5CO2 þ CH4 ð1Þ

Because of the relatively low S content of the primary magma(b~1000 ppm), the amount of fluid assimilation need not be large toaccount for the elevated d34S values of the gabbro. The d34S value ofthe gabbro would be a strong function of the d34S value and S contentof the country rock-derived fluid, but fluid to magma ratios as low as0.005 are sufficient to account for gabbro values near 4‰ if the fluidwas characterized by a d34S value near 10‰ and a mole fraction of Sspecies of ~0.1. The amount of oxygen contributed by the fluid in thisscenario is insufficient to significantly alter the primary d18Osignatures of the magma. A second alternative that is in line with

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the variable d18O values that reflect local exchange with a hydro-thermal fluid is that sulfide-forming reactions accompanied theintroduction of fluid derived from the Tobacco Root Batholith.However, sulfide textures are not indicative of an origin related tohydrothermal processes, but are more in line with crystallization ofinterstitial liquid. In either case S isotope values are stronglysupportive of the premise that externally derived S has been involvedin theminor amount of sulfide formation in the gabbro. The S contentsand d34S values of sulfide minerals in the diorite are similar to thosefound in the gabbros, and similar interpretations apply.

6. Conclusions

Mafic rocks of the Lady of the Lake Intrusion with compositionsranging from melagabbro to diorite occur near the contacts of theTobacco Root Batholithwith Archean country rocks. Chemical and stableisotopic compositions, plus the absence of evidence for inheritedzircons, support the hypothesis that the parental magma of the Ladyof the Lake Intrusion was an uncontaminated mantle-derived magma.Different rock types in the Lady of the Lake Intrusion are related to eachother by the crystallization of olivine and pyroxene, and accumulation inthe Lady of the Lake Intrusion chamber. The diorite may have beenderived via fractional crystallization of a similar parental magma in astaging chamber, perhaps modified by incorporation/assimilation ofArchean/Proterozoic country rock as evidenced by the zircon inheri-tance. Zircon U–Pb crystallization ages of 76.24±0.08 Ma for themelagabbro and 74.88±0.17 Ma for the diorite are similar to the zirconU–Pb ages of 75 to77 Ma for the Tobacco Root Batholith reportedpreviously by Mueller et al. (1996). On the basis of chemical data, theLady of the Lake Intrusion and the Tobacco Root Batholith do not sharecommon parental magmas. Chemical and isotopic data support theconclusion that the rocks of the Tobacco Root Batholith formed viapartial melting of high-18O country rocks. Although our data aresupportive of the origin of the Tobacco Root batholiths from crustalmelting, we cannot rule out the possibility that the crustal melt wascontaminated by small amounts of mafic, mantle-derived magma, asproposed by Mueller et al. (1996). The multiple stages of Cretaceousmagmatism recorded in the area of the Lady of the Lake Intrusion areconsistent with the partial melting of Archean and Paleproterozoiccountry rocks being driven by underplating of primitive mafic magma.The Lady of the Lake Intrusion represents the emplacement of maficmagma generated in a subduction zone that was minimally contami-nated by country rocks. Intrusion of mafic magma similar to that whichproduced the Lady of the Lake Intrusion is proposed to have led to thegeneration of siliceous partial melts derived from country rocks and theformation of the Tobacco Root Batholith.

Thakurta et al. (2008) have demonstrated how fractional crystal-lization of high-fO2 arc mafic magmas can lead to the transfer of sulfurto derivative dioritic/andesitic magmas. Our data clearly show that theLady of the Lake Intrusion is not related to the Tobacco Root Batholithvia fractional crystallization and hence sulfur accumulation via thisprocess was not tenable. The process of mixing mantle-derivedmagma and crustal partial melts proposed by Mueller et al. (1996)could have led to sulfur enrichment of the felsic melt if the S content ofthe mafic magma were high. Sulfur isotope values of gabbro anddiorite samples indicate that assimilation of country rock sulfur bymagma involved in the formation of the Lady of the Lake Intrusion didoccur. However, the sulfur content of the parental magma to the Ladyof the Lake Intrusion appears to have been too low to havesignificantly elevated the sulfur concentration of crustal partialmelts. We suggest that if mafic magmas were to be important sulfursources for potential mineralization in the Tobacco Root Batholith,then a greater degree of assimilation of country rock sulfur than thatrecorded by the Lady of the Lake Intrusion would have been required.Our data also do not support the premise that sulfur has been leached

from the Lady of the Lake Intrusion by fluids generated by the TobaccoRoot Batholith.

Acknowledgements

Wewould like to thank Erika Elswick, CraigMoore, PaulMueller andJohn Dilles for thoughtful reviews of the earlier versions of this paper.Craig Moore and Steve Studley of the Indiana University Stable IsotopeResearch Facility are thanked for assistancewith various phases of stableisotopic analyses. We also thank Curtis Williams for his assistance infieldwork. This researchwas supported byUS-NSFgrants (EAR0608645and EAR 0710910), by a grant from the “Charles J Vitaliano” fund and asummer research grant from Indiana University.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.lithos.2009.06.022.

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