ORIGINAL PAPER

J. Dostal Æ A. K. Chatterjee Æ D. J. Kontak

Chemical and isotopic (Pb, Sr) zonation in a peraluminousgranite pluton: role of fluid fractionation

Received: 22 January 2003 / Accepted: 30 October 2003 / Published online: 20 January 2004� Springer-Verlag 2004

Abstract The Davis Lake pluton (DLP, �800 km2) ofsouthwestern Nova Scotia, Canada, part of the largeperaluminous South Mountain batholith of ca. 380 Ma(U/Pb zircon, Ar/Ar mica), consists of granite andsubordinate topaz–muscovite leucogranite that hostsgreisen tin-base metal mineralization. A new Pb–Pbisochron age for leucogranite from the most evolved partof the DLP indicates a crystallization age of378±3.6 Ma, coincident with other radiometric ages ofthe DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displaysa compositional zonation defined by lead and strontiumisotopic ratios, as well as some major elements (e.g., Si,F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn),and elemental ratios (e.g., K/Rb and Nb/Ta). The grei-sens and the leucogranites that host them are charac-terized by extreme radiogenic compositions for Pb andSr, and their chemical-isotopic trends are extensions ofthe trends displayed by the less evolved granites. Thecovariations of the isotopic ratios with several major andtrace elements and elemental ratios as well as the Pb–Pband Rb–Sr isochrones indicate that all phases of theintrusion originated from a homogeneous parentalmagma. The granitoid magma underwent extensivefractional crystallization of feldspars, minor biotite andaccessory minerals (monazite, apatite and zircon) in acompositionally zoned magma chamber that was sub-sequently accompanied by fluid fractionation, duringwhich time the internally derived fluorine-rich fluids

modified the Rb/Sr, U/Pb and Th/Pb ratios, leading todistinct variations of 87Sr/86Sr, 206Pb/204Pb, 238U/204Pband 232Th/204Pb isotopic ratios. These data thereforedocument the evolution of a granitic magma throughmagmatic (i.e., crystal fractionation), orthomagmatic(i.e., crystal-fluid fractionation) and hydrothermal (i.e.,fluid fractionation) stages that culminated in the for-mation of a tin-base metal deposit. The Pb isotope dataalso constrain the source region for the DLP as beingAvalonian basement that, by inference, must underliemuch of the Meguma Terrane.

Introduction

The isotopic abundances of lead in geological systemsare variable and reflect the system’s age, source reservoirand geologic history. In addition to age determinations,radiogenic Pb isotopes have been extensively used astracers for a wide variety of geological processes,including petrogenetic modeling (e.g., Zindler and Hart1986; Dickin 1997; Faure 2001). Since physicochemicalprocesses do not fractionate Pb isotopes from one an-other, they preserve a time-integrated record of the U/Pband Th/U ratios of a reservoir. These elemental ratioscan, however, be fractionated during various geologicalprocesses as a consequence of the contrasting geo-chemical behavior of these three elements. One suchprocess is fluid fractionation, which is known to lead toa distinct enrichment of some incompatible trace ele-ments (e.g., Tatsumi et al. 1986; Stolz et al. 1996) andcould, therefore, potentially modify Pb isotope system-atics.

A fractionated, peraluminous, F-rich granitic intru-sion (Davis Lake pluton, DLP) in southwestern NovaScotia, Canada, provides an ideal opportunity toexamine the effects of fluid fractionation on trace ele-ment composition and integrate this with Pb isotopicsystematics. Our previous studies of this pluton (Dostal

Editorial responsibility: T.L. Grove

J. Dostal (&) Æ A. K. ChatterjeeDepartment of Geology,Saint Mary’s University,Halifax, Nova Scotia,B3H 3C3, CanadaE-mail: jdostal@smu.caTel.: +902-420-5747Fax: +902-420-5261

D. J. KontakNova Scotia Department of Natural Resources,P.O. Box 698, Halifax,Nova Scotia, B3J 2T9, Canada

Contrib Mineral Petrol (2004) 147: 74–90DOI 10.1007/s00410-003-0547-x

and Chatterjee 1995, 2000) suggested that the concen-trations of several incompatible trace elements, includ-ing Ta, Sn, Rb and Li, in the F-rich granitic rocks areaffected by fluid fractionation, probably involving fluo-rine complexing. In this paper we expand on this earlierwork and (1) investigate the isotopic systematics ofU–Th–Pb and Rb–Sr in this zoned intrusion, (2) dem-onstrate their covariation with the major and traceelement chemistry, which suggests an originally homo-geneous magma, and (3) constrain the petrogenesis ofthe granitic rocks. In addition, isotopic data from theDLP are also used to obtain the age of the intrusion andinfer the source region for the magma and nature of thebasement beneath the intrusion’s metasedimentary hostrocks (Meguma Terrane).

Geological setting

The DLP is located in southwestern Nova Scotia,which is a part of the Meguma Terrane of the Cana-dian Appalachians (Fig. 1). The Meguma Terrane, themost easterly terrane of the northern Appalachians(Fig. 1), as well as the adjoining Avalon exotic terranewere accreted to North America (Laurentia) along theeastern margin of the Appalachians during continentalcollision in the early to middle Paleozoic (Williams andHatcher 1983). In particular, the Meguma Terrane wasthe last of the exotic terranes to be accreted to theAppalachians during the final closure of the TheicOcean.

The Meguma Terrane contains two dominant litho-tectonic units: (1) granitoid rocks of the Late DevonianSouth Mountain batholith (SMB), and (2) a thick(�10 km) sequence of Cambro-Ordovician flyshoidmetasedimentary rocks (Meguma Group), which hostthe SMB. The metasediments comprise metawackes and

metapelites that were deformed and regionallymetamorphosed to lower greenschist to upper amphib-olite facies during the Acadian Orogeny (ca. 400 Ma;Hicks et al. 1999) during accretion of the MegumaTerrane to ancestral North America (Keppie andDallmeyer 1987). The SMB, which covers an area ofapproximately 7,300 km2 (Fig. 1), is a post-tectonic,composite granitoid body that intrudes the MegumaGroup. It consists of 13 coalesced plutons that wereemplaced within a narrow time interval (<5 Ma) atabout 380 Ma (Clark et al. 1997; Kontak et al. 2003;Carruzzo et al. 2003, and references therein). Thecoincidence of ages for the SMB using different geo-chronometers (U–Pb (monazite, zircon), Rb–Sr, Ar–Ar)indicates that the batholith intruded, crystallized andquickly cooled to below 350 �C (i.e., the blocking tem-perature of argon in muscovite; McDougall and Har-rison 1999). The batholith was emplaced at a depth of10–12 km (Clarke et al. 1997) and was uplifted andunroofed in <20 Ma. The SMB granitoid rocks rangefrom megacrystic biotite granodiorite, with up to 20%biotite, to equigranular leucogranite, containing lessthan 2% biotite.

The DLP, the westernmost constituent body of theSMB, is exposed over an area of �800 km2. The pluton(Fig. 1) is dominated by biotite– and biotite–muscoviteleucomonzogranite (>95% of the pluton) and is lith-ologically zoned; evolved rocks, including leucogra-nites, occur in the southwestern part of the plutonwhereas more primitive rock types crop out in thenorthern part (Dostal and Chatterjee 1995). Evolvedtopaz–muscovite leucogranite (LG), which constitutesonly a minor portion of the DLP (<1%), hosts theEast Kemptville tin-base metal deposit (56·106 tonnesof ore containing 0.165 wt% Sn) in which mineraliza-tion is associated with quartz–topaz–cassiterite greisens(Kontak 1994; Halter et al.1998a, 1998b). The age of

Fig. 1 Generalized geologicalmap of southwestern NovaScotia with the Late DevonianSouth Mountain batholith, itsconstituent Davis Lake pluton(DLP) and Scrag Lake pluton(SLP), and the East Kemptvilletin deposit (EK). Inset displayseastern part of Canada and thenortheastern USA, the locationof the map and also thelithotectonic terranes of theCanadian Appalachians(terranes: M Meguma, AAvalon, G Gander, D Dunnage,H Humber; Williams andHatcher 1983). The terrane-bounding fault between theAvalon and Meguma terranes isthe Cobequid-ChedabuctoFault Zone (CCFZ)

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the mineralization and the host rock leucogranite havepreviously been determined at 366±4 Ma based on aPb–Pb isochron (Kontak and Chatterjee 1992), whichis younger than a recent 378±3 Ma Re/Os molybde-nite age for the ore zone (Kontak et al. 2003).

Petrography

The leucomonzogranite is medium to coarse grainedwith megacrysts of K-feldspar (microperthitic ortho-clase) enclosed in a groundmass composed of K-feld-spar, zoned plagioclase (An20–10), quartz and minoramounts of biotite (2–7%) and muscovite (4–8%).Accessory minerals include cordierite, garnet, ilmenite,zircon, monazite, apatite, fluorite and rare deutericcarbonate minerals. The proportions of biotite pro-gressively decrease towards the southwestern end of thepluton, culminating in leucogranite with only traces of

biotite. The leucogranite is a fine- to medium-grained,equigranular rock with average modal proportions of37% quartz, 30% plagioclase (albite An1), 19%K-feldspar, 8%muscovite and 5% topaz (Kontak 1990a).Accessory minerals include biotite, zircon, monazite,apatite, uraninite, Nb–Ta oxides, and ore minerals suchas cassiterite, sphalerite, chalcopyrite, pyrite and pyr-rhotite. Fluorite occurs as euhedral crystals in thegroundmass and in veinlets. Kontak (1990a, 1994) ar-gued that the major silicate phases of the leucogranite,including topaz, represent a primary assemblage, butthat the albite was the result of either late magmatic orpost-magmatic re-equilibration of a more calcic plagio-clase. The leucogranite is locally greisenized with fine-to medium-grained greisens forming zones of 10-cm to25-m thickness. These greisens are composed mainly ofquartz, topaz, albite, and mica with variable amountsof fluorite, sulfides, cassiterite and fluorapatite (Kontak1994; Halter et al. 1998a).

Fig. 2A Map of the Davis Lakepluton showing samplelocations. Inset shows asimplified geological map of thesouthwestern part of the plutonwith sample locations. The opentriangle represents the locationsof the samples 79-109, 7019,79-100, 7384, 7114-9 and 7101whereas the closed trianglecorresponds to the location of79-39, 79-114, 79-43, 35-135,35-84, 43-71 and 43-56. B Planmap of the East Kemptville tindeposit showing the trace of thegreisens (after Halter et al.1998a)

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Analytical procedures

The present investigation is based on 58 samples of leucomonzog-ranite (LMG), leucogranite (LG) and greisen (G) collected fromoutcrops and diamond drill holes located throughout the DLP(Fig. 2). The samples were analyzed for major (Ham et al. 1989) and25 trace (Li, Rb, Sr, Ba, Zr, Hf, Nb, Ta, Sn, rare-earth elements(REEs), Y and F) elements (Table 1). The description of analyticalmethods and the quality of these data are given in Dostal andChatterjee (1995). Based on replicate analyses, the precision is gen-erally between 2 and 10% for trace elements. Analyses of Pb, U andTh, as well as the lead isotopic ratios for 52 samples from the set,were done at the University of Alberta, Edmonton (Table 2); theanalytical procedures as well as their precision and accuracy arediscussed in Kontak and Chatterjee (1992), who reported data for12 LG and G samples from the set. The element analyses were doneby the isotope dilution method with a reproducibility of about 0.5%.The percentage uncertainty limits, at two standard deviations, forthe ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 0.021, 0.026and 0.031, respectively. These limits were determined by replicate

analyses of the US National Bureau of Standards SRM 981 andSRM982. In addition, the leachingmethodwas used for a Pb–Pb agedetermination (e.g., Cumming and Krstic 1987; Gariepy et al. 1990)of two leucogranites. The procedure is described in Kontak andChatterjee (1992). The Rb/Sr isotopic data were reported by Chat-terjee and Cormier (1991). Isochron regression with the mean squareweighted deviates (MSWD) value and age calculations follow theprocedure of Cumming (1969) and Cumming et al. (1972). The iso-topic and decay constants and other parameters employed are afterSteiger and Jager (1977) and Tatsumoto et al. (1973).

Geochemistry

Rocks of the DLP are all peraluminous and have SiO2

contents ranging between 68 and 77 wt% (with theexception of several greisens that have <60% SiO2) anda color index<10. The spatial distribution of silica andK/Rb (Fig. 3), taken as proxies for the major and trace

Table 1 Average compositions of granitic rock types of the Davis Lake pluton

LMG-1a LMG-2 LG G

K/Rb >150 50–150 <50 <50

n 17 16 13 12Aver. SD Aver. SD Aver. SD Aver. SD

SiO2 (wt%) 70.77 1.99 74.09 1.09 74.38 0.72 68.52 8.19TiO2 0.37 0.16 0.17 0.06 0.05 0.02 0.06 0.04Al2O3 15.06 0.60 13.36 0.55 14.22 0.64 15.29 1.75Fe2O3* 2.57 0.83 1.81 0.28 1.64 0.76 4.38 3.28MnO 0.05 0.02 0.05 0.01 0.08 0.04 0.11 0.07MgO 1.17 0.29 0.96 0.37 0.41 0.38 0.20 0.29CaO 1.26 0.38 0.57 0.31 0.48 0.28 0.60 0.13Na2O 3.32 0.26 3.51 0.36 3.48 1.03 1.48 1.77K2O 4.60 0.31 4.52 0.32 3.91 0.56 3.35 1.66P2O5 0.23 0.03 0.18 0.09 0.31 0.18 0.33 0.16LOI 0.55 0.14 0.61 0.14 0.85 0.39 1.13 0.79S 99.95 99.82 99.82 95.44Sn (ppm) 6.53 2.35 12.81 5.01 187.00 176.17 24,285 65,030Rb 199.5 35.3 425.8 89.4 904.3 179.2 1,073.2 579.9Ba 470.6 179.4 121.1 65.2 35.5 18.3 42.5 32.8Sr 102.4 40.4 27.0 15.5 43.1 58.0 29.9 42.0Li 65.4 16.4 133.5 48.0 377.4 103.4 879.2 529.4Ta 1.22 0.29 2.92 0.79 9.42 3.28 6.70 4.22Nb 9.82 1.70 14.13 2.87 24.08 4.84 68.58 144.9Hf 4.07 1.23 2.40 0.54 0.92 0.24 0.48 0.10Zr 149.6 46.72 85.88 18.76 32.46 10.31 22.42 7.22Y 29.82 4.14 47.88 13.52 13.17 5.83 7.87 6.15La 24.84 6.63 14.87 6.84 3.48 1.73 2.40 3.11Ce 55.65 14.26 34.43 15.07 8.79 4.30 6.25 8.71Pr 6.81 1.75 4.27 1.78 1.16 0.58 0.79 1.08Nd 26.16 7.26 16.12 6.88 4.22 2.12 3.09 4.02Sm 6.08 1.44 4.16 1.50 1.50 0.61 1.08 0.96Eu 0.88 0.35 0.35 0.26 0.05 0.03 0.03 0.03Gd 5.50 1.33 4.33 1.67 1.65 0.71 1.16 1.08Tb 0.83 0.17 0.84 0.34 0.37 0.13 0.27 0.23Dy 4.48 0.97 5.44 2.59 2.37 0.96 1.70 1.77Ho 0.79 0.19 1.10 0.64 0.42 0.24 0.31 0.40Er 2.05 0.56 3.18 2.04 1.24 0.83 0.86 1.25Tm 0.27 0.09 0.48 0.33 0.20 0.14 0.13 0.19Yb 1.61 0.56 3.20 2.29 1.41 1.11 0.89 1.36Lu 0.24 0.09 0.46 0.35 0.21 0.18 0.13 0.20F 625 126 1,565 662 4,743 1,313 15,148 5,604

aLMG-1, Leucomonzogranite with K/Rb=150–250; LMG-2, leucomonzogranite with K/Rb=50–150; LG, leucogranite; G, greisen;aver., average; SD, standard deviation; n, number of samples; *total Fe as Fe2O3

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element chemistry, respectively, define a distinct chemi-cal zonation or gradient that correlates with thepetrography of the granitic rocks (MacDonald et al.1992). The chemical zonation culminates in the south-western part of the DLP where the most evolved rockscontain mineralization.

The compositional zoning noted for SiO2 (Fig. 3A)and K/Rb (Fig. 3B) occurs for the incompatible traceelements including Rb, F, Li, U, Ta and Sn, and elementratios includingNb/Ta andLa/Sm (Dostal andChatterjee1995, 2000). It is important to emphasize that the ele-mental distribution suggests that the DLP is a normally

Table 2 U–Th–Pb isotopic and concentration data for the granitic rocks of the Davis Lake pluton

Sample U (ppm) Th (ppm) Pb (ppm) 238U/204Pb 232Th/204Pb 206Pb/Pb204±1d 207Pb/204Pb±1d 208Pb/204Pb±1d

LMG-1a

1006 1.725 7.491 19.72 5.612 25.18 18.627±0.00447 15.625±0.00484 38.988±0.0136539 1.974 11.16 17.86 7.091 41.42 18.678±0.00448 15.645±0.00485 38.917±0.013623000-1 2.985 9.348 23.67 8.057 26.08 18.722±0.00449 15.648±0.00485 38.570±0.013501018 2.142 12.37 22.3 6.152 36.71 18.783±0.00410 15.656±0.00570 38.680±0.017403029 2.671 n.a. 17.54 9.774 n.a. 18.884±0.00453 15.677±0.00489 38.717±0.013558028 3.383 11.22 21.73 9.988 34.23 18.884±0.00410 15.657±0.00570 38.700±0.017402345 2.136 10.23 16.92 8.077 39.97 18.933±0.00454 15.645±0.00485 38.465±0.013503025 2.789 13.3 20.24 8.832 43.52 18.945±0.00410 15.673±0.00580 38.556±0.0173014 1.576 8.4 18.02 5.632 31.02 19.192±0.00461 15.674±0.00486 38.653±0.013532350 2.744 11.64 15.87 11.18 49.01 19.608±0.00471 15.675±0.00486 38.561±0.011952328 2.806 10.95 16.48 11.02 44.44 19.644±0.00471 15.693±0.00486 38.567±0.013503009 3.684 11.22 18.3 13.15 41.37 20.103±0.00482 15.706±0.00534 38.768±0.013573005-1 3.678 5.115 18.09 13.28 19.09 20.309±0.00487 15.723±0.00487 38.573±0.013503000-2 2.914 11.17 17.14 10.92 43.25 19.162±0.00460 15.644±0.00532 38.527±0.011942334 2.995 11.43 17.37 11.16 44.02 19.630±0.00471 15.693±0.00534 38.600±0.01351LMG-27100 5.817 10.2 9.77 39.27 71.16 20.410±0.00492 15.722±0.00502 39.204±0.013891204 6.769 10.45 10.26 43.57 69.51 20.889±0.00501 15.748±0.00488 38.800±0.013583024 5.568 3.852 20.64 17.69 12.65 20.537±0.00493 15.742±0.00423 38.621±0.013521053 5.61 10.17 13.98 26.69 49.99 21.208±0.00460 15.784±0.00580 38.983±0.0175082-13 10.36 14.86 12.54 55.74 82.64 21.599±0.00518 15.782±0.00489 39.717±0.013903012 11.37 12.52 16.54 45.97 52.31 21.813±0.00480 15.819±0.00580 38.779±0.0174082-17 16.92 14.21 15.93 72.36 62.79 22.860±0.00549 15.861±0.00492 39.135±0.0137082-11 8.89 9.727 8.46 71.94 81.34 22.865±0.00503 15.868±0.00587 39.519±0.0138382-12 17.65 18.37 16.47 74.02 79.61 23.507±0.00564 15.894±0.00493 39.557±0.0138482-32 14.67 10.27 11.57 88.16 63.77 23.843±0.00572 15.918±0.00493 39.718±0.0139083-20 20.69 16.15 15.36 94.41 76.15 24.692±0.00543 15.956±0.00590 39.494±0.013822332 3.46 10.85 20.2 11.26 36.47 20.540±0.00431 15.738±0.00488 38.775±0.013571050 12.28 9.86 9.67 95.17 78.99 30.127±0.00633 16.255±0.00601 39.434±0.015771215 13.17 8.77 10.4 96.3 66.29 31.098±0.00653 16.308±0.00603 39.699±0.01588LG79-39 19.33 3.575 7.44 258.9 49.49 56.573±0.01130 17.676±0.00619 40.310±0.01410EK132b 19.2 4.43 9.63 143.5 34.21 27.189±0.00571 16.118±0.00580 39.017±0.0136EK105b 19.77 3.96 8.32 186.6 38.62 34.010±0.00816 16.470±0.00511 39.470±0.01368EK161b 18.97 3.86 5.13 351.6 73.92 51.925±0.01140 17.420±0.00645 39.871±0.0139579-114 16.85 2.882 6.82 213.8 37.79 42.419±0.01018 16.932±0.00525 39.928±0.0139779-109 18.67 13.89 9.21 151.4 115.1 27.742±0.01300 16.162±0.00800 40.639±0.0142279-100 12.85 6.80 2.76 443.5 242.5 49.459±0.01185 17.311±0.00640 42.625±0.014927019 6.965 20.41 4.01 152.4 461.4 38.480±0.00923 16.701±0.00518 46.413±0.016241061 6.847 10.63 8.94 52.23 83.8 23.052±0.00553 15.874±0.00492 39.024±0.013661054 13.1 14.84 4.40 252.4 295.5 38.601±0.00926 16.713±0.00518 41.953±0.0146879-43 22.7 4.136 7.88 340.4 64.1 75.639±0.01664 18.705±0.00655 41.745±0.01460G35-135b 23.28 4.32 5.62 356.5 68.35 42.344±0.00889 16.912±0.00609 39.466±0.01381EK016b 18.75 4.06 3.37 518.5 116.0 49.573±0.01190 17.328±0.00537 40.130±0.01405EK124Ab 20.12 3.21 2.61 822.7 135.6 64.062±0.01537 18.071±0.00560 40.629±0.014227384 29.74 26.4 21.48 146.7 134.6 56.162±0.01347 17.641±0.01350 48.001±0.0172835-84b 25.39 5.21 2.99 961.9 204.0 70.801±0.01490 18.440±0.00664 41.154±0.0144043-71b 26.08 5.05 3.00 1,177 235.5 94.134±0.01980 19.703±0.00709 42.279±0.0148043-56b 24.40 3.99 2.17 1,671 282.4 108.430±0.0228 20.466±0.00737 42.620±0.01492EK116b 23.29 4.29 2.35 1,369 260.6 97.137±0.02331 19.857±0.00616 42.298±0.014807114-9 37.79 46.29 26.96 98.85 125.1 24.354±0.00511 15.946±0.00574 40.337±0.014127101 21.47 24.83 13.29 156.9 187.5 48.211±0.01061 17.217±0.00619 46.085±0.01613

aLMG-1, Leucomonzogranites with K/Rb=150–250; LMG-2, leucomonzogranites with K/Rb=50–150; LG, leucogranites; G, greisens;n.a., not analyzedbData from Kontak and Chatterjee (1992)

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zoned intrusion and that part of it is hidden by Megumametasediments such that the core of the intrusion iscoincident with the area of mineralization and maximumhydrothermal alteration. The three lithologic variants(LMG, LG, G) discussed above have distinct chemicalcompositions and so are discussed separately below.

The LGM have high K2O (3.7–5 wt%), K>Na andcompositionally correspond to low-Ca granites. Theserocks can be subdivided according to their K/Rb ratiosinto two groups (Fig. 4). The first group (LMG-1),

Fig. 3A, B Outline of the Davis Lake pluton showing spatialvariations of A SiO2 (wt%), and B K/Rb ratios in the graniticrocks. The zonation shows gradual evolution from the northernpart composed of LMG-1 rocks to the southwestern partcontaining leucogranites. Closed dots represent the sample sites;the values refer to the SiO2 and K/Rb ratios, respectively. The opentriangle represents an average for six samples (79-109, 7019, 79-100,7384, 7114-9 and 7101) and the closed triangle is an average ofseven samples (79-39, 79-114, 79-43, 35-135, 35-84, 43-71, 43-56).Solid lines for SiO2 are contour lines that have been computer-generated using software Surfer 7.02; those for K/Rb ratios arehand-drawn contour lines

Fig. 4 Variations of Rb, F, Li(ppm) and U/Pb relative to K/Rb ratio in the granitic rocks ofthe Davis Lake pluton. CrossesLMG-1 (leucomonzograniteswith K/Rb=150–250),open circles LMG-2(leucomonzogranites with K/Rb=50–150), open trianglesLG (leucogranites), x Ggreisens. The trend has beensubdivided into the followingthree groups, as discussed in thetext, to reflect the dominantpetrological process thataccounts for the chemistry ofthe samples: X-Fr (crystalfractionation), XF-Fr (crystal-fluid fractionation), and F-Fr(fluid fractionation)

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with higher K/Rb ratios (150–250), includes biotite-bearing rocks with lower SiO2. The second group(LMG-2), with lower K/Rb ratios (50–150), comprisesmore evolved muscovite–biotite-bearing granites. Rel-ative to LMG-1, these granites are also lower in TiO2,FeOt, MgO, CaO and several trace elements, particu-larly Ba, Sr, La and Zr, but are enriched in F, Li, U,Sn, Ta and Rb, and have higher U/Pb values. The REEpatterns of LMG-1 samples (Fig. 5) exhibit (La/Yb)Nbetween 10 and 14, (Gd/Yb)N between 2 and 4, andnegative Eu anomalies (Eu/Eu*�0.4–0.6). These REEdata are similar in abundance and fractionation tothose of the granites of the SMB. The REE patterns ofLMG-2 samples (Fig. 5) have more pronounced rela-tive depletion of Eu (Eu/Eu*�0.1–0.3) and less frac-tionated heavy REEs.

The LG and G are characterized by K/Rb ratios<50(Fig. 4). Hence, they occupy the most evolved part ofthe DLP (Fig. 3B). The LG are more fractionated thanthe LMG-2 rocks and usually have >72 wt% SiO2. TheLG have a chemical composition similar to peralumi-nous Li–F-rich granites (e.g., Cuney et al. 1992), as wellas some volcanic rocks including ongonites (Kovalenkoand Kovalenko 1984), the Macusani glass (Pichavantet al. 1987) and some of the topaz rhyolites (Christiansenet al. 1986). Relative to LMG-2 rocks of a given SiO2,they are more evolved and have higher F, Sn, Li, U, Taand Rb concentrations and U/Pb ratios (Fig. 4). The Grocks are highly evolved chemically and formed fromvariable degrees of fluid:rock interaction of LG rocksduring the late magmatic-early hydrothermal stage ofthe pluton’s evolution (Kontak 1994; Halter et al. 1998a,1998b). Where fluid:rock ratios were low, the G rocksare marginally altered and resemble LG. Compared toother rocks of the pluton, including LG, the G areenriched in Rb, Li, Sn, U, F and related elements (Fig. 4),and depleted in Zr, Ba, Sr and REEs. Where alteration ismost intense and the G are essentially mineralized

quartz–topaz rocks, the samples are in fact depleted inmany trace elements, including Rb and Li (Fig. 4).

The normalized REE patterns of LG and G samples(Fig. 5) are relatively unfractionated (i.e., low (La/Yb)N)compared to LMG samples and, in addition, have lowerREE concentrations and larger negative Eu anomalies(Eu/Eu* down to 0.01). The concave inflections at Nd inthe normalized patterns are common among the evolvedrocks from the DLP. The concave inflection for Nd istypical of evolved granites that have undergone mona-zite fractionation (Yurimoto et al. 1990) and the frac-tionation from LMG to LG rocks is similar to thatmodeled for the Macusani volcanic rocks using mona-zite (Montel 1986).

The continuous variation trends for many major andtrace elements (e.g., Zr versus TiO2) suggest that LMG-2and LG were generated by extensive fractionation ofLMG-1 parent magma. The process involved crystalli-zation of major mineral phases (mainly K-feldspar, butalso plagioclase and biotite) and accessory minerals,particularly monazite and zircon (Dostal and Chatter-jee 1995). A decrease of Ba and Pb accompanied byan increase of Rb from LMG-1 to LG illustrates the

Fig. 5 Chondrite-normalized rare-earth element patterns of theaverage of LMG-1 (average of 17 samples), LMG-2 (average of 16samples), LG (average of 13 samples) and G greisen (average of 12samples). Normalizing values after Sun and McDonough (1989).Note the development of the inflections in the profiles at Nd and Tbin the more evolved samples (LG, G), as discussed in the text

Fig. 6 Variations of Ba and Rb relative to Pb (ppm) in graniticrocks of the Davis Lake pluton. The associated greisens are notplotted on this diagram. Crosses LMG-1 (leucomonzogranites withK/Rb=150–250), open circles LMG-2 (leucomonzogranites withK/Rb=50–150), open triangles LG (leucogranites), x greisens

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predominant role of K-feldspar during fractionation(Fig. 6).

The fractional crystallization process alone, however,cannot explain some characteristics of the DLP,including several geochemical trends that differ fromthose produced by crystal-melt equilibria. For example,unlike fractional crystallization trends, LMG-2, LG andG samples display a negative correlation of K/Rb withRb (Fig. 4) that is comparable to the pegmatite-hydro-thermal trend of Shaw (1968). The low values of the K/Rb ratio in many rocks (compared to the crustal averageof 240; Shaw 1968) and the correlation of this ratio withF and Li (Fig. 7) imply an involvement of fluid phases,and probably fluid fractionation. Fluid interaction isalso supported by the presence of albite and topaz in theLG rocks, which are typical phases in such rocks(Christiansen et al. 1986; Cuney et al. 1992). Fractionalcrystallization accompanied fluid fractionation, sincesamples that have the lowest K/Rb ratios are among themost chemically evolved (Figs. 3B and 4). Correlationsof the enriched trace elements (e.g., Li, U, Rb, Ta) with

F (Fig. 7) indicate that fluid fractionation probably in-volved fluorine complexing. Thus, fluid fractionationappears to be an integral part of the late-stage evolutionof the fractionated granitic melts, leading to crystalli-zation of highly evolved LG and attending greiseniza-tion of the LG, where fluid:rock ratios were highest(Kontak 1994; Halter et al. 1998a, 1998b), and Snmineralization.

In order to assess the extent of the role of crystalversus fluid fractionation, we present numeric modelsfor the two dominant types of crystal fraction-ation—equilibrium and Rayleigh—for the petrogeneticrelevant elements Rb, Sr, Ba, Li and F. These calcu-lations used the average LMG-1 as a starting compo-sition (Table 1) and extracted the average modalmineralogy of this rock (Qtz=30%, Plg=35%,Kfeld=25%, Biot=10%) as well as solid-liquid parti-tion coefficients (KD) from the literature (Philpotts andSchnetzler 1970; Bea et al. 1994; Ren et al. 2003) togenerate LMG-2 and LG. Based on these calculations,Raleigh fractionation gives appropriate values for Rb

Fig. 7 Variations of Li, La, Rb,Zr, Ta (ppm) and TiO2 (wt%)versus F (ppm) in granitic rocksof the Davis Lake pluton.Crosses LMG-1(leucomonzogranites with K/Rb=150–250), open circlesLMG-2 (leucomonzograniteswith K/Rb=50–150), opentriangles LG (leucogranites),x greisens

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and Ba after 30–60% (LMG-2) and 60–85% (LG)crystallization. However, appropriate values for Li andF cannot be reproduced in either model (see alsoLondon 1997). Therefore, the behavior of F and Li,and also Nb/Ta fractionation (Dostal and Chatterjee1995) and the degree of Sn enrichment (Table 1)strongly suggest that a fluid phase was involved inelemental partitioning and enrichment.

In summary, the element zonation and enrichmentresulted from a combined crystal-fluid fractionationprocess. The initial stages of the DLP magma weredominated by crystal fractionation, and the transitionfrom LMG-1 to LMG-2 and LG lithologies can be ac-counted for by Rayleigh-type fractionation. However,toward the latter stages of crystallization to form LGrocks and beyond (i.e., formation of G), the role of afluid phase became important, as manifested by thedramatic increase in F, Li, Sn, etc. Although fractionalcrystallization models and solid-liquid partition coeffi-cients can accommodate the variable chemistry andmodal mineralogy of the DLP rocks, the enrichment ofthe latter elements necessitates the involvement of a fluidphase.

Lead isotope ratios

The lead isotope data are used to (1) determine the ageof the pluton and the initial Pb isotope compositions ofthe rocks, (2) evaluate the variations of isotopic ratiosand their relationship with incompatible trace elements,and (3) constrain the source composition and infertectonic implications.

Geochronology (Pb–Pb, Rb–Sr) and initialPb isotope ratios

The measured 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pbisotopic ratios for representative DLP rocks, togetherwith the U/Pb and Th/Pb isotopic ratios and the ele-mental abundances of U, Th and Pb are given in Ta-ble 2. Average values for various rock types of thepluton are reported in Table 3. Of note, the LMG-2rocks are distinctly more radiogenic than the LMG-1rocks, and the LG and G samples have extremelyradiogenic compositions.

The whole-rock Pb isotopic data, including samplesanalyzed by Kontak and Chatterjee (1992), are plottedon a conventional uranogenic lead diagram in Fig. 8Band yield a regressed line corresponding to an age of370±3.5 Ma and an MSWD of 3.09 (at 2r). Two LGsamples were selected for more detailed investigation,having been subjected to leachate analysis in order togenerate an isochron. The results of the analysis areshown in Fig. 9A, where an age of 378±3.6 Ma isdefined by regression of the data (Table 4). Thus, thetwo Pb–Pb isochron ages indicate an average age of ca.375 Ma for the DLP and greisens. These Pb–Pb ages arein agreement with a whole-rock Rb–Sr age of375±3 Ma (with an MSWD=1.88 and initial 87Sr/86Srratio of 0.7054±0.0023) for the DLP (Fig. 9B). Collec-tively, these ages are also in agreement with 40Ar/39Armica ages for the DLP of ca. 370 Ma (Reynolds et al.1981). Thus, there appears to be triple concordance ofthe Pb–Pb, Rb/Sr and Ar/Ar ages for the DLP thatindicates a cogenetic history for all phases of the intru-sion. In addition, the isochrons for Pb–Pb and Rb–Sr

Table 3 Average U–Th–Pb isotopic and concentration data for the granitic rock types of the Davis Lake pluton

LMG-1a LMG-2 LG G

K/Rb >150 50–150 <50 <50

n 15 14 11 12Aver. SD Aver. SD Aver. SD Aver. SD

U (ppm) 2.68 0.65 10.94 5.25 15.95 5.31 23.25 6.81Th 10.36 2.13 11.45 3.58 8.13 5.92 11.14 13.88Pb 18.75 2.29 13.70 3.94 6.78 2.34 7.71 8.37206Pb/204Pb 19.21 0.53 23.28 3.39 42.28 15.38 61.01 26.60207Pb/204Pb 15.67 0.03 15.89 0.18 16.92 0.82 17.91 1.43208Pb/204Pb 38.66 0.15 39.25 0.40 41.00 2.16 41.80 2.75238U/204Pb 9.33 2.52 59.47 29.37 231.5 112.8 651.0 536.3232Th/204Pb 37.10 8.77 59.01 24.82 136.0 138.5 155.3 78.21206Pb/204Pbi 18.65 0.42 19.75 2.26 28.44 11.15 22.34 10.80207Pb/204Pbi 15.64 0.02 15.69 0.12 16.17 0.59 15.82 0.58208Pb/204Pbi 37.96 0.25 38.07 0.27 38.47 1.07 38.91 2.50238U/204Pbi 17.94 8.92 83.38 56.30 350.21 189.7 745.4 466.8232Th/204Pbi 24.07 7.81 56.13 20.00 146.2 121.7 173.0 135.7

aLMG-1, Leucomonzogranite with K/Rb=150–250; LMG-2,leucomonzogranite with K/Rb=50–150; LG, leucogranite; G,greisen; Aver., average; SD, standard deviation; n, number ofsamples; 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, measured Pbisotopic ratios (Table 2); 238U/204Pb and 232Th/204Pb, calculatedisotopic ratios (Table 2); 206Pb/204Pbi,

207Pb/204Pbi and208Pb/204Pbi, initial isotopic ratios assuming age of 372 Ma;238U/204Pbi and

232Th/204Pbi, initial isotopic ratios assuming the

initial Pb isotopic compositions at 372 Ma for the granitic magmacorrespond to that of the galena from the Davis Lake pluton(Kontak and Chatterjee 1992). The initial Pb isotopic ratios of thegalena are 18.141, 15.603 and 38.209 for the 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios, respectively. The greisen aver-ages include also additional data from Kontak and Chatterjee(1992)

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in the DLP (Figs. 8B and 9) indicate that all rocks,including LG and G, originated from a homogeneousparental magma and that intrusive and hydrothermalmineralizing events within the whole pluton are essen-tially synchronous. Finally, the data also suggest that forthese samples there has been no post-crystallizationdisturbance of these isotopic systems.

Whereas uranogenic lead defines a strong colinearvariation in Fig. 8B, the thorogenic lead displays somescatter with two groupings apparent (Fig. 8A). Exam-ination of several petrogenetic indicators (e.g., traceelements) versus thorogenic lead indicates no correla-tion that might readily account for the distribution ofthe lead data. This indicates, therefore, that a simplemodel of accessory mineral fractionation controlling Uand Th partitioning cannot be invoked as a plausiblemechanism. Alternatively, either fractionation ordecomposition of a mineral phase that specificallycontrols the partitioning of Th relative to U (alsothorogenic versus uranogenic leads) depending on theintensive parameters may have occurred. However,

detailed mineralogical/petrographic investigation of LGrocks (Kontak 1990a and our unpublished data) didnot reveal the presence of a potentially suitable phase(e.g., thorite). In fact, the most 208Pb-enriched samplescorrespond to areas of most evolved rock with super-imposed hydrothermal activity, suggesting that thismay have affected the thorogenic lead. We explorethis possibility in more detail below when addressingthe spatial variation of the initial Pb isotopic com-position of the samples (Fig. 10).

The initial Pb isotopic ratios for the pluton, as de-fined by the intersection of the Pb–Pb regression line andStacey and Kramers’ (1975) time-integrated curve(238U/204Pb=l=9.74), are 18.143 and 15.603 for206Pb/204Pb and 207Pb/204Pb, respectively. These isotopicratios are nearly identical to the isotopic composition ofgalena of 18.141 and 15.603, respectively, obtained formaterial from fresh granodiorite of the SMB (Chatterjeeand Ham 1991) and, therefore, also suggest similarsources. These values are also similar to the calculatedinitial Pb isotopic ratios for the LG and G at EastKemptville (18.143, 15.601, respectively; Kontak andChatterjee 1992) obtained from intersection of a Pb–Pbregression line and Stacey and Kramers’ (1975) time-integrated curve. Again, these data point to a homoge-neous reservoir, in this case the magma from which theDLP evolved. The calculated (initial) 238U/204Pbi and232Th/204Pbi ratios for the DLP rocks (Fig. 10) assumethat the initial Pb isotopic composition at 375 Ma forthe Davis Lake granitic magma was the same as theabove-mentioned values.

Spatial variations of the Pb isotope ratios

The calculated 238U/204Pbi,232Th/204Pbi and present-day

206Pb/204Pb ratios in the DLP show well-defined con-tinuous zoning (Fig. 10; also the present-day 87Sr/86Sr,232Th/204Pb and 238U/204Pb ratios display similar varia-tions). Whereas the spatial variation of U/Pb and Th/Pbshow increasing trends, the gradients and, therefore, thetrends are not as dramatic and well developed as withthe isotopic ratios. The isotopic ratio gradients increasefrom LMG-1 samples towards the southwestern part ofthe pluton, and culminate in the areas of most evolvedrock where LG and G samples are from. As alreadynoted, several elements, including Si, Rb, U, Li, La, Taand F and element ratios (e.g., K/Rb, Nb/Ta), displaysimilar spatial variations (Fig. 3B and Dostal andChatterjee 2000) and correlate with these isotopic ratios(Figs. 11 and 12).

The spatial variation of the initial 206Pb/204Pb(Fig. 10D), calculated for 375 Ma, shows that someanomalously elevated values occur in the most evolvedpart of the DLP, in particular in LG and G samplesfrom the mineralized areas. These same samples are theones with elevated thorogenic versus uranogenic leadcompared to most samples in the Pb–Pb plot of Fig. 8A.There is no database to test for the possibility of

Fig. 8A, B Standard Pb–Pb plots showing variations of208Pb/204Pb versus 206Pb/204Pb (A) and 207Pb/204Pb versus206Pb/204Pb ratios (B) in granitic rocks of the Davis Lake pluton.Significance of the field labeled wall rock contamination in A isdiscussed in the text. In B, regression of the data yields an age of370±3.5 Ma (2r). Note that the data of Kontak and Chatterjee(1992) have been included in the age calculation in B

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contaminant lead coming from the wall rock here, butwe suggest that such contamination is the most likelycause, given that this area is the one most affected by

hydrothermal processes. In light of this, it can also benoted that Kontak and Dostal (1992) documented metalenrichment (e.g., Sn, Zn, Cu) of the metasedimentarycountry rocks around the East Kemptvilledeposit, which they attributed to infiltration of miner-alizing fluids. Similarly, contamination of the graniteby Pb derived from the metasedimentary rocks issuggested.

Spatial variations of isotopes and trace elements ingranitoid plutons are generally associated with texturallyand mineralogically distinct facies of rocks within thepluton. Three models have recently been invoked toexplain such variations. The first postulates that thesevariations are due to two distinct intrusions (e.g., Kistleret al. 1986; Eichelberger et al. 2000; Schmitt et al. 2001)whereas, according to the second hypothesis, the zoningreflects roof-wards assimilation of crust in the magmachamber prior to crystallization (e.g., Johnson 1989;Tegtmeyer and Farmer 1989). The third model invokesclosed-system fractional crystallization (convectivefractionation; Wolff et al. 1990; De Silva 1991).

Fig. 9 A Leach-residue data fortwo leucogranite samples (1054and 79-100) on a conventionalPb–Pb diagram. The curved partof the line represents the two-stage growth curve (l=9.7) ofStacey and Kramers (1975).Regression of the data yields anage of 378±3.6 Ma, which iscomparable (within error limits)to the Rb–Sr age ofleucomonzogranite. B Rb/Srisochron plot for whole-rocksamples (LMG) of the DavisLake pluton. Regression of thedata yields an age of375±3 Ma. Note that thesamples of leucogranites andgreisens fall off the 375-Mareference line and, therefore, arenot plotted

Table 4 Pb isotopic data for two leucogranites

Sample 206Pb/204Pbm207Pb/204Pbm

208Pb/204Pbm

79-100 49.459 17.311 42.625L1

a 56.861 17.698 44.644L2 127.270 21.512 51.438L3 47.728 17.204 42.132L4 21.985 15.721 38.738L5 24.249 15.932 38.6951054 38.601 16.713 41.953L1 54.205 17.554 44.666L2 43.928 17.030 45.168L3 41.171 16.877 44.471L4 21.921 15.806 39.077L5 21.757 15.797 38.945

aL1, L2, L3, L4, and L5 refer to progressive leachate steps usingprogressively stronger acid solutions (see text and Kontak andChatterjee (1992) for discussion); m, measured

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Regarding the first hypothesis, there is no field evi-dence for two intrusions in the DLP. In fact, the smoothchemical gradients within the DLP (Figs. 3 and 10)indicate that a single, chemically zoned magma chamberonce existed. Furthermore, the Sr and Pb–U–Th isotoperatios correlate with several trace elements (e.g., Rb, F,Li, La and Sn; Fig. 11), suggesting that the spatialvariations of the isotopic ratios are an integral part ofthe geochemical evolution of the DLP.

The second hypothesis, involving progressive crustalcontamination, is inconsistent with the stable isotopedata (i.e., oxygen, sulfur) for the East Kemptville areawhere Kontak (1990b) has demonstrated that the LGand G units were magmatically dominated and there is

Fig. 10A–D Outline of the Davis Lake pluton showing spatialdistributions of calculated 238U/204Pbi (A), calculated

232Th/204Pbi(B), 206Pb/204Pbm (C), and calculated 206Pb/204Pbi (D; present-day232Th/204Pb and 238U/204Pb ratios are not plotted, but they showa similar pattern). Note the similarities in zoning patterns of theSiO2 and K/Rb (Fig. 3) and the isotope ratios, and theprogressive increase and zonal arrangement in these ratiossurrounding the East Kemptville tin deposit in the southwest.The open triangle represents an average for six samples (79-109,7019, 79-100, 7384, 7114-9 and 7101) from Table 2 and threeadditional samples from Kontak and Chatterjee (1992). Theclosed triangle is an average of seven samples (79-39, 79-114,79-43, 35-135, 35-84, 43-71, 43-56) and two additional samplesfrom Kontak and Chatterjee (1992). Closed dots represent thesample sites; the values refer to the isotope ratios. Solid lines arehand-drawn contour lines

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no evidence for a systematic external input, although wedid note above that there was some modification of thePb isotopes via a hydrothermal fluid. This is consistentwith the conclusions of other workers (Longstaffe et al.1980; Kontak et al. 1988, 1991; Clarke et al. 1993a),based on a larger database of oxygen isotopes for theSMB, that contamination was not a significant processin the evolution of the batholith. In addition, the well-defined Pb–Pb isochrons for the DLP rocks, includingLMG, LG and G, along with similar initial Sr and Pbisotope ratios for the DLP rocks, preclude that thezonation was produced by contamination by uppercrustal material.

The third hypothesis, invoking extensive fractionalcrystallization accompanied by fluid fractionation andprobably associated with fluorine complexing, can ac-count for the isotopic and elemental variations in theDLP. The distinct variations of the present-day

206Pb/204Pb, 232Th/204Pb and 238U/204Pb ratios correlatewith incompatible trace elements (Fig. 11) as well aswith fluorine (Fig. 12), and support a combined crystaland fluid fractionation process. In fact, Dostal andChatterjee (1995) suggested that the DLP was formed by

Fig. 11A, B Variations of A present-day 238U/204Pb isotopic ratioversus La (ppm), and B present-day 232Th/204Pb isotopic ratioversus Rb (ppm) in granitic rocks of the Davis Lake pluton.Crosses LMG-1 (leucomonzogranites with K/Rb=150–250), opencircles LMG-2 (leucomonzogranites with K/Rb=50–150), opentriangles LG (leucogranites), x greisens. The arrows indicate vectorsof fractionation

Fig. 12A–C Variations of the present-day 206Pb/204Pb (A),232Th/204Pb (B), and 238U/204Pb (C) isotope ratios versus F (ppm)in the granitic rocks and associated greisens of the Davis Lakepluton. Note the restricted range in isotopic composition of theLMG-1, whereas the LMG-2, LG and G samples show the greatestspread. The 232Th/204Pbi and 238U/204Pbi isotope ratios displaysimilar variations as the present-day 232Th/204Pb and 238U/204Pb.Crosses LMG-1 (leucomonzogranites with K/Rb=150–250), opencircles LMG-2 (leucomonzogranites with K/Rb=50–150), opentriangles LG (leucogranites), x greisens

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extensive crystal and fluid fractionation of an F-richperaluminous granitic melt in a vertically/symmetricallyzoned magma chamber. The Sr and Pb isochrons sup-port the fluid inclusion and stable isotope conclusions(Kontak 1994) that the DLP rocks were the products ofclosed-system fractionation during which the circulationof the internally derived fluids was essentially confinedto the pluton. Stable and Pb isotope evidence appear toeliminate convective cooling of the granites by externalfluids as the main heat-loss mechanism. This would al-low higher than subsolidus temperatures to be main-tained for a longer time period under the thermalblanket of a cover and would also facilitate extensivefractionation of the pluton. The compositional zonationand isochrons of the Pb and Sr isotopic systematics inthe pluton support a model where extensive fractionalcrystallization, accompanied by fluid fractionation andfluorine complexing, eventually led to the formation ofgreisens and Sn mineralization. The internally derivedfluids rich in F, U and Rb fractionated the Rb/Sr, U/Pband Th/Pb ratios that, in turn, produced distinct varia-tions of 87Sr/86Sr, 206Pb/204Pb, 238U/204Pb and232Th/204Pb.

Potential source and tectonic implications

The basement of the Meguma Terrane is not exposedso its character is controversial (Nance 1987; Owenet al. 1988; Clarke et al. 1988; Greenough et al. 1999).The lead isotopic signatures of granitic rocks havebeen used to characterize their crustal source (Mi-chard-Vitrac et al. 1981; Ayuso and Bevier 1991;Tomascak et al. 1996) and, therefore, the data re-ported here can also be used to constrain the natureof the basement underlying the Meguma Group andthe SMB that it hosts.

The Meguma Terrane is in tectonic contact with theAvalon Terrane; the common boundary, the Cobequid-Chedabucto Fault Zone (Fig. 1), is a south-dippingthrust with the Meguma Terrane overlying the AvalonTerrane (Nance 1987; Keppie and Dallmeyer 1987). TheAvalon crust has a distinct Pb isotopic compositionrelative to the Laurentia crust (Ayuso and Bevier 1991;Whalen et al. 1994; Tomascak et al. 1996), but there is arather limited amount of Pb isotope data on rocks of theMeguma Terrane (e.g., Kontak and Chatterjee 1992).The compositions and ages of the lower crustal xenolithsfrom lamprophyre dykes intruding the Meguma Terraneconfirm that they are a part of the Avalon basement sothat the Meguma Terrane is, at least in part, underlainby Avalonian basement (Greenough et al. 1999). How-ever, detrital zircon studies (Krogh and Keppie 1990;Keppie et al. 1998) show contrasting provenance forAvalonian and Meguma sedimentary rocks. This showsthat the Meguma sediments were not deposited onAvalonian basement, but are probably allochthonousover it. Prior to this study, Avalonian xenoliths haveonly been found proximal the Avalon-Meguma bound-ary, and thus the aerial extent of Avalonian basementbeneath the Meguma Terrane was unknown and gen-erally inferred to be localized to the northeast part of theterrane (Greenough et al. 1999). Pb isotope data fromthe DLP, located far from the terrane boundary (Fig. 1),can be used to document whether Avalonian basement is

Fig. 13 Variations of the initial 207Pb/204Pb versus 206Pb/204Pbisotope ratios in the granitic rocks (372 Ma) of the Davis Lakepluton. SMB Field of leached K-feldspars from the SouthMountain batholith (our unpublished data), Avalonia field ofK-feldspars from granitoid rocks of Avalon Terrane of coastalMaine and southern New Brunswick (Ayuso et al. 1988; Ayusoand Bevier 1991), Laurentia field of K-feldspars from granitoidrocks of Laurentia of northern Maine and New Brunswick(Ayuso et al. 1988; Ayuso and Bevier 1991); lower crustalgranulite xenoliths (mafic and metasedimentary) from inferredAvalonian basement of the Meguma Terrane (Tangier granuliticxenoliths; Greenough et al. 1999; our unpublished data). Shownfor reference are the Stacey and Kramers (1975) growth curve(SK) and the growth curves for different reservoirs: UC uppercrust, OR orogene, Mn mantle of Doe and Zartman (1979) andZartman and Doe (1981). Star Model mantle composition at372 Ma calculated according to Doe and Zartman (1979) andZartman and Doe (1981)

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present even in the southwestern part of the terrane,thereby suggesting that the Meguma Terrane representsa thrust sheet over Avalonian basement.

In order to examine possible sources of the graniticrocks, the initial lead isotope data for granitic rocks ofthe DLP, as well as leached K-feldspars from the SMB(our unpublished data) are plotted on a 207Pb/204Pbversus 206Pb/204Pb diagram (Fig. 13), together withavailable lead isotopes for K-feldspars from granitoidrocks of the northern Appalachians (Ayuso et al. 1988;Ayuso and Bevier 1991; Whalen et al. 1994). The datashow that K-feldspars from granitic rocks derived fromLaurentian crust have distinct Pb isotopic signatures(northern group of Ayuso and Bevier 1991;207Pb/204Pb207Pb/204Pb ratios (>15.60; southern groupof Ayuso and Bevier 1991). The data imply, therefore,that the crustal sources of the granitic rocks of the DLP,as well as the SMB, differ from the Laurentian crust, butare isotopically similar to the Avalonian crust of coastalMaine and southern New Brunswick (Ayuso et al. 1988;Ayuso and Bevier 1991; Whalen et al. 1994). As alreadynoted, the presence of Avalon-like crust underlying theMeguma Group in southern Nova Scotia suggests thatthe Meguma Terrane represents a thrust sheet overAvalon basement (Greenough et al. 1999). Alternatively,the available data are also consistent with a model of aunique crustal block beneath southern Nova Scotia withan Avalon-like isotopic signature. However, several linesof evidence favor the former model whereby theMeguma Group of the southern Nova Scotia is under-lain by bona fide Avalonian basement. The lead isotopiccomposition of the granulite xenoliths derived fromAvalonian basement overlaps the field for the SMB andDLP (Fig. 13). Clarke et al. (1993b, 1997) and Ebertzet al. (1991) also suggested that the granulite xenolithsfrom the Avalonian basement compositionally resemblethe source of the SMB granitic rocks. Likewise, Srand Nd isotope data on the SMB (Clarke et al.1988) and structural analyses of the two terranes (Nance1987; Keppie and Dallmeyer 1987) further support thismodel.

Conclusions

The Davis Lake pluton consists of biotite–muscoviteleucomonzogranite and subordinate-evolved topaz–muscovite leucogranite, which contains quartz–topaz-cassiterite greisen. Pb–Pb and Rb–Sr isochrons indicatecrystallization at ca. 375 Ma from a single homogeneousmagma. The well-defined isochrons for all the DLProcks, as well as an agreement of Pb–Pb isotopicages with Ar/Ar mica, Rb/Sr whole-rock and Re/Osmolybdenite ages, imply that the Sn mineralization isgranite-related and that intrusive and hydrothermalmineralizing events within the pluton are essentiallysynchronous. Pb isotopic ratios show that the radiogenicisotope systems in the DLP closed immediately after

emplacement and solidification. The isotope data alsoindicate that whereas most of the fluids were internallyderived from the granitic magma, some late-stage fluidsinfiltrated from the country rock and locally modifiedthe Pb isotopic composition of some hydrothermallyaltered rocks.

The granitic rocks of the DLP were generated byextensive fractional crystallization of major mineralphases (mainly K-feldspar, but also plagioclase andbiotite) and accessory minerals, particularly monaziteand zircon, in a compositionally zoned magma chamber.The correlation of F with incompatible trace elementsand Pb and Sr isotopic ratios in the DLP suggest thatfluid fractionation, associated with fluorine complexing,accompanied fractional crystallization and played a keyrole in developing chemical variation trends in the DLP,particularly during the final stages of the pluton evolu-tion. The zonal distribution of Sr and Pb isotopic ratiosaccompanied by the distinct isochrones for the DLProcks is likely the result of the fractionation of Rb–Srand U–Th–Pb systems by F-rich fluids. These fluidsfractionated Rb/Sr, U/Pb and Th/Pb ratios, leading todistinct variations of the 87Sr/86Sr, 206Pb/204Pb,238U/204Pb and 232Th/204Pb ratios. Such a process is alsoconsistent with the zonation displayed by some majorelements (e.g., Si), incompatible trace elements (e.g., Rb,Li, U, Ta, and Sn), and element ratios (e.g., K/Rb andNb/Ta).

The parent magma of the DLP was derived fromlower crustal metasedimentary basement rocks bymelting during the terminal stages of the AcadianOrogeny. The source of the DLP magma has Pb isotopecharacteristics comparable to the Avalon basement incoastal Maine and southern New Brunswick This sup-ports the model that portrays the Meguma Terrane as athrust sheet that overrode the Avalon Terrane.

Acknowledgements This work was funded by the Nova ScotiaDepartment of Natural Resources through Federal-ProvincialMineral Development Agreements and the Natural Sciences andEngineering Research Council of Canada (Discovery Grant).We thank Victor Owen and the journal reviewers, R.A. Ayuso andR. Walker, for their constructive comments.

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