u–pb and sm–nd dating of high-pressure granulite- and upper amphibolite facies rocks from sw...

Precambrian Research 92 (1998) 319–339

U–Pb and Sm–Nd dating of high-pressure granulite- and upperamphibolite facies rocks from SW Sweden

X.-D. Wang, U. Soderlund *, A. Lindh, L. JohanssonInstitute of Geology, University of Lund, Solvegatan 13, S-223 62 Lund, Sweden

Received 4 September 1996; accepted 14 July 1998

Abstract

Geochronological data from different radiometric systems on high-grade metamorphic rocks in the SouthwestSwedish Granulite Region, southwestern Sweden, are generally analytically precise but sometimes in conflict witheach other. Age data from seven samples on which a variety of radiometric methods have been applied are presented.The age of peak metamorphism is determined by metamorphic zircons from two garnet amphibolites at c. 975 Mausing the conventional U–Pb isotope and Pb–Pb evaporation methods. Titanites from the same type of rocks giveages of c. 923±3 Ma and 945±2 Ma. The Sm–Nd data on mafic granulites and garnet amphibolites yield surprisinglyyoung ages between 945 and 900 Ma, similar to previous Sm–Nd ages. These ages are comparable to, or even youngerthan, the 40Ar–39Ar hornblende ages that range between 1030 and 930 Ma. Assuming that the Sm–Nd systematicsare entirely controlled by volume diffusion, the closure temperature of the Sm–Nd system in minerals from theserocks would be comparable to, or lower than, that of Ar diffusion in hornblende, i.e. around 500°C. © 1998 ElsevierScience B.V. All rights reserved.

Keywords: Ar-40/Ar-39; Granulite; Pb/Pb; Sm/Nd; Sweden; U/Pb

1. Introduction logical studies on metamorphic minerals have beenpresented lately, timing for the metamorphic evolu-tion of the SGR is still controversial due to seem-The southern part of the Eastern Segment iningly conflicting results.southwestern Sweden, referred to as the Southwest

Johansson et al. (1991) demonstrated that theSwedish Granulite Region (SGR, Johansson et al.,granulite facies metamorphism in the SGR was1991), comprises high-grade metamorphic rocksMeso- to Neoproterozoic (Sveconorwegian) in ageincluding garnet amphibolites and mafic granulites.and not Palaeo- to Mesoproterozoic, as previouslyThermobarometry on these rocks has giventhought (Hubbard, 1975; Gaal and Gorbatschev,temperatures of 680–800°C and pressures of1987). They obtained Sm–Nd mineral ages on0.85–1.17 GPa (Johansson et al., 1991; Wang andmafic granulites that yielded late SveconorwegianLindh, 1996). Although a number of geochrono-isochron ages of 907±12 and 916±11 Ma. Sinceno other radiometric data constraining the age ofmetamorphism were available at that time, the* Corresponding author. Fax: +46 46 121477;

e-mail: [email protected] Sm–Nd ages were interpreted as high-temperature

0301-9268/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved.PII S0301-9268 ( 98 ) 00084-9

320 X.-D. Wang et al. / Precambrian Research 92 (1998) 319–339

closure (Tc) ages at or near peak metamorphic 2. Geological settingconditions. Subsequent Sm–Nd ages on coexisting

The Precambrian basement of southernt Swedenmetamorphic minerals have also yielded ages inconsists mainly of the Svecofennian orogen (c.the same range, between c. 930 and 890 Ma1.9–1.75 Ga), the Transscandinavian Igneous Belt(Johansson and Kullerud, 1993; Johansson, per-(c. 1.85–1.65 Ga) and the Sveconorwegian orogensonal commnication).(c. 1.1–0.9 Ga, Fig. 1). The Eastern Segment ofWang et al. (1996) and Page et al. (1996)the Sveconorwegian orogen is bounded by a com-presented 40Ar–39Ar hornblende data yielding well-plex system of ductile deformation zones in thedefined plateau ages between 1030 and 930 Ma.east, the Protogine Zone, and a major ductileThe younger ages of c. 930 Ma were interpreteddeformation zone in the west, the Mylonite Zoneas coding ages associated with exhumation whereas(MZ, Fig. 1). The Southwest Swedish Granulitethe older ages were suggested to date an event ofRegion (SGR) in the southern part of the Easterncrustal thickening prior to peak metamorphism.Segment comprises variably deformed and mig-These 40Ar–39Ar ages are thus in conflict with thematized granitic to intermediate gneisses, meta-Sm–Nd ages, since Ar retention in hornblendebasic rocks and charnockites, metamorphosed inoccurs below approximately 500°C, much lowergranulite- and upper amphibolite facies (Hubbard,than the closure temperatures of Nd suggested by1975; Wikman and Bergstrom, 1987; JohanssonMezger et al. (1992) and others. Also the U–Pbet al., 1991; Johansson, 1993; Johansson andtitanite ages, ranging between 960 and 920 MaKullerud, 1993; Wang and Lindh, 1996; Moller(Johansson, 1990; Johansson and Johansson, 1993;et al., 1996a; Moller and Soderlund, 1997).Johansson et al., 1993; Connelly et al., 1996),Crystallization ages of moderately deformed igne-appear to be in conflict with the Sm–Nd data.ous rocks indicate that anorogenic magmatismHowever, the closure temperature for the U–Pboccurred during at least two main episodes;system in titanite has recently been suggested to1350–1400 Ma (Andersson, 1996; Ahall et al.,

be much higher (c. ≥700°C, Zhang and Scharer,1997) and c. 1250 Ma (Berglund and Connelly,

1996) than previous estimates (500–550°C, 1994). Geothermobarometry on mafic granulitesGascoyne, 1986; 500–670°C, Mezger et al., 1991; from regionally well distributed localities record>650°C, Mezger et al., 1993), which makes the temperatures of 700–800°C and pressures ofinterpretation of the titanite ages enigmatic. At 0.85–1.17 GPa (corresponding to depths betweenpresent, the U–Pb and Pb–Pb evaporation ages of 35 and 40 km). These estimates are considered to990–970 Ma of metamorphic zircons are consid- represent near peak metamorphic conditionsered as the ‘best’ age estimates for peak metamor- during the Sveconorwegian orogeny (Johanssonphism (Soderlund, 1996; Cornell et al., 1996). et al., 1991; Wang and Lindh, 1996). P–T estimates

In contrast to previous geochronological studies of garnet amphibolites are 680–750°C andthat utilized only one radiometric system for each 0.85–1.06 GPa, similar to the values from thesample, our objective has been to investigate granulite facies rocks ( Wang and Lindh, 1996).samples with as many radiometric systems as pos- Similar P–T estimates were obtained from high-sible. In this study we report combined U–Pb pressure granulite assemblages in decompressedzircon, Pb–Pb evaporation zircon, U–Pb titanite eclogites (Moller, 1998). The pressure estimatesand Sm–Nd data on metamorphic minerals from are thus regarded as minimum pressures.seven samples. 40Ar–39Ar hornblende ages from The approximately N–S trending Protoginesome of these samples were published in Wang Zone is strongly foliated, in part mylonitic, and iset al. (1996). This multi-isotopic approach has associated with aligned intrusions of syenites, gran-been utilized in order to: (1) compare age data ites and mafic dykes. In the southern part, thefrom different radiometric systems; (2) determine zone is about 25–30 km wide and dips steeply.the age of peak metamorphism; and (3) constrain There is an increase in metamorphic grade from

greenschist facies in the eastern Protogine Zone tothe cooling history of the SGR.

321X.-D. Wang et al. / Precambrian Research 92 (1998) 319–339

Fig. 1. Geological map of southern Sweden showing the major crustal domains. The framed area is shown in Fig. 2. Key: MZ=Mylonite zone, PZ=Protogine zone, SFDZ=Sveconorwegian frontal deformation zone, TIB=Transscandinavian igneous belt. B=Bastad, H=Helsingborg, M=Malmo, S=Stockholm, UH=Ulricehamn, UR=Ullared, V=Varberg. Modified after Wahlgrenet al. (1994).

high-P granulite facies in its western part and Kullerud, 1993; Johansson and Johansson,1993; Wang et al., 1996; Page et al., 1996; Cornell(Johansson, 1993; Wang and Lindh, 1996). The

Mylonite Zone comprises strongly deformed and et al., 1996). Zircon geochronology of granite andpegmatite dykes in the northern part of the SGR,retrogressed rocks (Johansson and Johansson,

1993). Thus, both the Protogine and Mylonite unaffected by deformation and metamorphism,yield ages of 956±7 Ma (Moller and Soderlund,zones must have been active during exhumation

of the SGR after the Sveconorwegian peak 1997) and 955±21 Ma (Andersson, 1996). Theseages are minimum ages of Sveconorwegian high-metamorphism.

Previous studies inferred that the high-grade grade metamorphism and deformation. Pre-tectonicdykes and granites have given intrusion agesmetamorphism and main gneissification in the SGR

are pre-Sveconorwegian in age (Hubbard, 1975; between c. 1.45 and c. 1.37 Ga (Soderlund, 1996;Andersson, 1996; Moller et al., 1996a; Ahall et al.,Larson et al., 1986, 1990; Ahall, 1995). However,

geochronological data of metamorphic minerals 1997). Altogether, these results demonstrate a per-vasive high-grade metamorphism and deformationdemonstrates a Sveconorwegian tectonothermal

overprint (Johansson et al., 1991, 1993; Johansson that is mainly Sveconorwegian.


Variably deformed, mafic granulites and garnet rons from the SGR have yielded similar metamor-phic ages between 933 and 882 Ma (A. Johansson,amphibolites are scattered in the high-grade

gneisses of the SGR. The mafic granulites are personal communication).generally fine- to medium grained and consist ofgarnet, clinopyroxene, ± orthopyroxene, horn- 3.2. U–Pb and Pb–Pb evaporation zircon

chronologyblende, plagioclase and quartz. The garnet amphib-olites are generally medium grained and consist ofgarnet, hornblende, plagioclase, quartz and locally Soderlund (1996) reported an age of

969±27 Ma of metamorphic zircons from aclinopyroxene. Some have thin felsic veins subpar-allel to the gneissosity. The rocks show equilibrium deformed pegmatite dyke, located at the coast c.

70 km south of Varberg (Fig. 1). Close totextures with only minor retrogression. In thenorthern part of the region, kyanite-bearing Ulricehamn, Cornell et al. (1996) dated zircons

from migmatitic veins in an amphibolite and in adecompressed eclogites occur (Moller andSoderlund, 1997; Moller, 1998). granite gneiss that yielded 970±9 and 970±27 Ma

(S ion probe analyses). A lower interceptIn the southern part of the Mylonite Zone, high-grade metamorphic rocks are almost completely age of 949±4 was obtained by combined zircon

and titanite U–Pb geochronology and was inter-retrogressed to garnet-free -9 amphibolites. The40Ar–39Ar hornblende and U–Pb titanite ages of preted as the crystallization age of an interboudin

pegmatite (Connelly et al., 1996). This pegmatitethese rocks are c. 920 Ma (Johansson andJohansson, 1993; Page et al., 1996). Also within was suggested to have formed during late extension

of more competent layers. West of Ulricehamn,the SGR, deformation-induced retrogression fromgranulite- to amphibolite facies rocks has been metamorphic zircons in c. 1.68 Ga igneous rocks

gave c. 1.0 Ga (Cornell et al., 1997). Further south,recognized (Moller et al., 1996a; Moller andSoderlund, 1997). However, mafic granulites and at Ullared (Fig. 1) decompressed eclogites have

recently been identified (Moller, 1998). Garnets ingarnet amphibolites that are equally deformed alsoco-exist in many outcrops, most likely due to local these rocks contain enclosed metamorphic zircons

that are 969±14 Ma and represents the maximumdifferences in fluid activity.age of peak metamorphism (Johansson et al.,1998). Within the same area, two post-tectonicfelsic dykes gave 955±16 Ma (Andersson, 1996)3. Previous geochronological studies constraining

the age of high-grade Sveconorwegian and 956±7 Ma (Moller and Soderlund, 1997),respectively. Based on the metamorphic andmetamorphismstructural relationships, the intrusive ages of thesetwo dykes constrain the minimum age of3.1. Sm–Nd mineral and whole rock geochronologySveconorwegian high-grade deformation andmetamorphism.The first radiometric data that demonstrated a

late Sveconorwegian age for the high-grade para-genesis were Sm–Nd mineral and whole-rock 3.3. U–Pb titanite chronologyisochron ages on mafic granulite-facies rocks,ranging from c. 920 to 880 Ma (Johansson et al., Titanite from a strongly deformed mafic rock in

the Mylonite Zone yielded an age of c. 920 Ma1991; Johansson and Kullerud, 1993). A lateSveconorwegian age of 893±5 Ma was also (Johansson and Johansson, 1993). The growth of

titanite was linked to deformation-induced retro-reported for the mineral assemblage of the Varbergcharnockite. The Sm–Nd ages were at that time gression of the high-grade rocks. Titanite from a

granite close to the Protogine Zone yielded aninterpreted as high-temperature closure ages of themineral assemblages at or near peak metamorphic almost concordant Sveconorwegian age of c.

940 Ma (Johansson, 1990). Two titanite ages ofconditions (Johansson et al., 1991; Johansson andKullerud, 1993). Three additional Sm–Nd isoch- 929±22 and 960±45 Ma from a gneiss close to


Fig. 2. A schematic map of the study area and a summery of geochronological data (this study) from the sample localities. Ar–Arhornblende data is from Wang et al. (1996). The cities in the study area: H=Helsingborg, B=Bastad.

sample 9223 (this study, Fig. 2) have been interpre- the younger to the cooling and uplift of the region.The spread in ages was suggested to be related toted to be associated with the high-grade metamor-

phism (Johansson et al., 1993). Connelly et al. local differences in deformation and/or fluid flow(Page et al., 1996). A single 40Ar–39Ar muscovite(1996) reported titanite ages of 949±4 Ma from

granite gneisses near Ulricehamn. analysis gave 904±6 Ma, that combined with agroup of hornblende ages around 930 Ma, resultedin an average cooling rate of 5°C Ma−1 from3.4. Ar–Ar chronology500°C to 350°C (Page et al., 1996).

Page et al. (1996) and Wang et al. (1996)together reported thirty 40Ar–39Ar hornblende agesof 1030–930 Ma obtained from mafic granulites 4. Analytical proceduresand garnet amphibolites in the SGR. The olderages were suggested to correspond to an earlier The minerals used for geochronology were sepa-

rated using a Wilfley table, magnetic separator andSveconorwegian event of crustal thickening and


heavy liquids. The final selection of grains to be according to Table 1. The uncertainty for147Sm/144Nd ratio was estimated to 0.5% (2s). (cf.analyzed was carried out by hand-picking under a

microscope. Johansson and Kullerud (1993). The programI 90 (Provost, 1990) was used for calculat-Zircon and titanite fractions were abraded using

the method described by Krogh (1982). Chemical ing the isochrons. The two sigma error (2s) of theisochron ages was multiplied by the ‘normalizedseparation of U and Pb essentially followed Krogh

(1973). The samples were spiked with separate mean residue’ (square root of the MSWD) whenthe normalized mean residue was larger than 1.208Pb and 233–235U tracers except for analysis

number 5 in sample 8705 and fractions 4 and 5 in This precaution was taken in order to avoid tooprecise ages (Provost, 1990).sample 9007; these were spiked with a mixed

208Pb–233–235U tracer. U and Pb were extractedusing standard anion-exchange techniques. HBrand HNO3 separation steps were added for titanite 5. Analytical results(Corfu and Stott, 1986). Pb was loaded on singleRe filaments with H3PO4 and silica-gel; U was Four mafic granulites and three garnet amphibo-

lites were selected for geochronological study usingloaded on double Re filaments with HNO3.Isotopic ratios were measured on a Finnigan MAT five methods: conventional U–Pb and Pb–Pb evap-

oration systematics on zircon, U–Pb on titanite,261 mass spectrometer at the Laboratory forIsotope Geology at the Swedish Museum of Sm–Nd on whole rock and mineral separates and

40Ar–39Ar on hornblende. The results are listed inNatural History, Stockholm. The decay constantsapplied for the U–Pb system are those recom- Tables 1–3 and are summerized in Table 4 (see also

Fig. 2). The 40Ar–39Ar data are published in Wangmended by Steiger and Jager (1977). The calcula-tions of the corrected isotope ratios, the error et al. (1996). Chemical data of minerals and P–T

determinations on these rocks were reported bypropagation (2s) and the ages were made usingthe programs by Ludwig (1980, 1991). Pb blank Wang and Lindh (1996). The coordinates of the

sample locations are given in the Swedish nationallevels, run with each batch of samples, variedbetween 7 and 30 pg. grid (SNG) and are listed in Table 4.

The Pb–Pb evaporation analyses were carriedout on the same mass spectrometer and followed 5.1. 8705 — garnet amphibolitethe procedures described by Kober, 1986, 1987).Only data from the high temperature runs Sample 8705 (Fig. 2) consists of garnet, horn-

blende, plagioclase, quartz and opaques. It is fine(>1400°C) with constant Pb-isotope ratiosbetween the evaporation–deposition steps were to medium grained and weakly foliated. Zircon

and rutile occur in interstices and as inclusions inconsidered for geochronological evaluation. Moredetails are given in Soderlund et al. (1996). hornblende and garnet [Fig. 3(a) and (b)]. Three

different types of zircon were distinguished bySm–Nd analyses were performed at theMineralogical–Geological Museum in Oslo. The microscopic examinations in a highly refracting

liquid combined with backscatter electron and cath-procedures of Sm–Nd analysis and calculationsfollow the methods described by Johansson and ode-luminescence techniques. These types are: (1)

transparent and yellowish brown, prismatic zirconsKullerud (1993). Nd ratios were normalized to146Nd/144Nd=0.7219. During the time these analy- that display resorption features [Fig. 3(c) and (d)];

(2) transparent, colourless, multifaceted zirconsses were run, the BCR-1 standard was analyzedfour times resulting in 143Nd/144Nd=0.512631±8 which display core-rim relationships [Fig. 3(f )]; (3)

transparent, colourless to pink, multifaceted zircons(2s). Based on the reproducibility of the standard,errors of ±0.000020 (2s) for the 143Nd/144Nd ratio that are homogenous in backscatter/cathode-lumi-

nescence images [Fig. 3(f )]. Zircons of type (1) andwere used for calculation. In case the analyticalerror of the 143Nd/144Nd ratio is higher than (2) are interpreted as igneous or inherited, type (2)

being rimed by metamorphic overgrowths, whereas±0.000020 (2s), we used the measured value


Table 1Sm–Nd mineral and whole rock analyses from mafic granulite and garnet amphibolite samples

Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd±2c

8705 (garnet – amphibolitc)grt 1.37 2.28 0.3661 0.513330±7hbl 8.70 31.01 0.1708 0.512161±6plg 2.50 13.74 0.1107 0.511786±6wr 1.63 24.76 0.1328 0.511921±2

9015 (gamet amphibolite)grt 0.42 0.9691 0.2645 0.512797±64hbl 0.46 1.92 0.1444 0.512050±8titanite 135.64 559.05 0.1478 0.512087±6wr 3.16 18.91 0.1373 0.512010±2

9007 (garnet amphibolite)grt 1.06 2.60 0.2479 0.512818±21hbl 2.39 21.95 0.0663 0.511843±9wr 5.67 24.02 0.1437 0.512203±5

9238 (mafic granulite)grt 2.47 3.68 0.4066 0.513683±7cpx 4.06 17.34 0.1414 0.512121±6hbl 12.92 53.47 0.1461 0.512104±5plg 0.60 6.21 0.0585 0.511569±7wr 3.96 18.17 0.1318 0.512033±6


9290 (mafic granulite)grt 6.94 7.99 0.5254 0.514507±4cpx 13.34 43.17 0.1867 0.512509±5plg 1.63 6.81 0.1451 0.512249±5wr 16.35 80.17 0.1233 0.512126±5


grt=gamet, cpx=clinopyroxene, hbl=hornblende, plg=plagioclase, wr=whole rock.

type (3) is interpreted as metamorphic. Type (2) conventional single-grain U–Pb analysis. Fivesingle-grain analyses of abraded crystals plot closeand (3) can only be separated in backscatter/

cathode-luminescence images [Fig. 3(e) and (f )]. to the concordia and yield intercept ages of974±25 and 303±428 Ma (Fig. 4). Their weightsZircons of type (2) and (3) were selected for


Table 2U–Pb zircon and titanite analytical data

No. Fraction Weight U Total Isotopic ratios Apparent ages (Ma)(mm) (mg) (ppm) Pb

(ppm) 206/204b 206/238c 207/235c 207/206c 206/238 207/235 207/206

Sample 8705, garnet amphibolite, zircona1 >150 7.3 154 26.5 394 0.16810±0.5% 1.65382±1.6% 0.07135±1.4% 1002±5 991±10 968±292 >150 8.7 138 21.5 787 0.16239±0.4% 1.60322±1.3% 0.07160±1.1% 970±4 971±9 975±233 >150 15 141.5 20.9 1385 0.15627±0.3% 1.53325±0.9% 0.07116±0.7% 936±3 944±5 962±154 >150 4.6 150.9 27.1 211 0.16898±0.9% 1.69601±2.7% 0.07280±2.3% 1007±8 1007±17 1008±475 >150 5.5 229.4 37.9 342 0.15747±0.6% 1.55968±2.5% 0.71834±2.3% 943±5 954±15 981±47

Sample 9015, garnet amphibolite, zircon1 75–100 79.0 58.8 10.7 3970 0.18316±0.5% 2.02617_+0.7% 0.08027±0.4% 1084±5 1124±5 1204±92 100–150 115 41.3 8.76 3353 0.20104±0.3% 2.38700_+0.4% 0.08611±0.2% 1181±3 1238±3 1341±43 >150 119 64.8 15.8 8230 0.21968±0.6% 2.77798±0.7% 0.09171±0.3% 1280±7 1350±5 1462±6

Sample 9015, garnet amphibolite, titanite1 (L) 150–200 149 49.3 8.75 618 0.15382±0.7% 1.48848±1.3% 0.07018±0.9% 922±6 926±8 934±202 (B) 150–200 415 79.4 13.9 676 0.15389±0.4% 1.49810±0.9% 0.07060±0.7% 923±3 930±5 946±143 (B) 100–150 222 225.8 36.7 2243 0.15404±0.3% 1.4889.1±0.7% 0.07010±0.6% 924±3 926±4 931±13S(1–3) 923±3 927±4

Sample 9007, garnet amphibolite, titanite1 (L) 150–200 1208 30.5 8.1 187 0.12098±0.3% 1.18229±2.0% 0.07088±1.8% 736±2 792±11 954±382 (B) 150–200 1560 39.8 13.2 233 0.15795±0.2% 1.56973±1.0% 0.07206±0.9% 945±2 958±7 988±183 (B) 100–150 786 40.8 13.4 231 0.15763±0.2% 1.57169±1.5% 0.07232±1.3% 944±2 959±8 995±284 (L) 150–200 205 32.8 11.6 163 0.15872±0.4% 1.52705±2.0% 0.06978±1.9% 949±4 941±13 922±405 (B) 150–200 109 33.9 11.6 158 0.15699±0.5% 1.53739±2.6% 0.07102±2.5% 940±5 945±16 958±51S(2–5) 945±2 953±5

aSingle grain zircon analyses.bMeasured ratio corrected for mass fractionation (correction factor 0.1% per AMU for Pb).cCorrected for mass fractionation (Pb with 0.1% per AMU for Pb, U according to analyses with 233−235U spike), blank and commonlead (Stacey and Kramers, 1975).L and B=light and brownish titanite fraction, respectively.

vary from 4.6 to 15 mg (Table 2). Uranium and to type (2) zircons. These analyses have beenomitted as they lack relevance for the interpreta-lead contents are low, 138–154 and 21–27 ppm,

respectively. Similar types of zircons were also tion of the metamorphic evolution.The Sm–Nd analyses of garnet, hornblende,investigated by the Pb–Pb single-grain evaporation

technique. The 207Pb/206Pb ages of zircon Z1–Z5 plagioclase and whole-rock yield an isochron ageof 920±15 Ma. The initial 143Nd/144Nd ratio at c.are 979±9, 972±4, 968±3, 959±5 and

956±6 Ma (Table 3), respectively, and overlap the 900 Ma is 0.511122±22 and the MSWD is 0.11(Fig. 5 and Table 1). This age is significantly youn-U–Pb intercept age of 974±25 Ma. Z1 and Z3

gave only one evaporation step that make these ger than the zircon ages obtained both by theconventional U–Pb and the Pb–Pb evaporationages ambiguous to interpret. Some of the zircons

analyzed with the conventional U–Pb technique as methods. The Sm–Nd isochron age is also youngerthan the 40Ar–39Ar hornblende age of 948±4 Mawell as the Pb–Pb evaporation method gave much

higher ages, indicating an origin that corresponds for this sample ( Wang et al., 1996).


Table 3Pb–Pb evaporation analytical data

Zircon Evaporation Number of Evaporation 206Pb/204Pb ±1s 207Pb/206Pb ±1s 207Pb/206Pb Age±2s (Ma)No. steps block (ratios) temperature (°C)

Sample 8705:1 1 2(19) 1430 70 200±14800 0.07197±16 979±9

S 1 2(19) 979±92 1 47 1410 21 200±500 0.07205±6 968±4

2 46 1420 32 800±2600 0.07189±3 970±43 46 1420 73 800±8200 0.07147±4 965±34 37 1420 72 000±9600 0.07182±14 975±85 94 1430 48 200±3050 0.07201±4 978±36 8 1450 55 000±8600 0.07163±34 968±19

S 1–6 30(278) 972±43 1 38 1430 68 900±8700 0.07156±5 968±3

S 1 4(38) 968±34 1 10 1440 100 000±25000 0.07130±17 962±10

2 10 1440 100 000±25000 0.07111±28 957±163 48 1450 100 000±25000 0.07112±7 959±6

S 1–3 7(68) 959±55 1 9 1430 100 000±25000 0.07121±28 959±17

2 18 1440 100 000±25000 0.07108±10 956±73 10 1450 78 000±18000 0.07121±21 958±164 29 1460 97 000±14500 0.07101±12 954±7

S 1–4 7(67) 956±6

Sample 9015:1 1 10 1500 43100±3700 0.07265±25 991±15

S1 1(10) 991±152 1 10 1500 22100±2800 0.07250±13 982±9

2 10 1540 20000±2600 0.07259±26 982±16S1–2 2(20) 982±9

The isotopic ratios are the measured values. Ages and errors of each evaporation step and zircon are calculated as weighted meansand errors for the 207Pb/206Pb and 206Pb/204Pb block ratios. Age calculation was carried out by repeated testing in Monte Carloloopes of the measured isotope ratios (program designed by M. Hedberg, Stockholm). Statistical outliers are excluded from agecalculation according to Dixon (1950). The ages are not corrected for fractionation (cf. Kober, 1987).

Table 4Summary of geochronological data

Sample no. U–Pb zircon Pb–Pb zircon U–Pb titanite Sm–Nd wr+min. 40Ar−39Ar hornblende Sample locations

8705 974±25 955–980 920±15 948±8 629 455/138 2609015 975±17 980–990 c. 925 945±80 974±5 627 855/136 2009007 c. 945 900±42 933±4 627 915/139 1609238 920±14 958±5 621 750/136 6359286 930±5 623 680/137 8659290 903±10 623 350/153 73592117 909±14 969±7 624 790/133 200


Fig. 3. Scanning electron microscope images of zircons from the rock sample 8705. Zircons occur as inclusions in (a) garnet; (b)hornblende; (c) yellow–brown, transparent, corroded prismatic zircon; (d) transparent, elongated and colourless zircon; (e) transpar-ent, colourless, multifaceted zircon with a spherical shape; (f ) cathode-luminescence images of polished zircons showing two smallhomogeneous crystals and two larger zircons that display core-rim relationships. Rt=rutile. Zr=zircon.


ourless, oval and multifaceted [Fig. 6(b)].Backscatter electron images show that the majorityare homogeneous [Fig. 6(c)].

Three zircon fractions of different sizes wereselected and abraded in order to minimize theeffect of recent Pb loss. The concentration of Uand Pb in these zircons are very low, 41–65 ppmand 9–16 ppm, respectively (Table 2). The threefractions are strongly discordant, yielding upperand lower intercept ages of 1798±37 and975±17 Ma (MSWD=1.8, Fig. 7). The discor-dance, is most likely due to the presence of anolder zircon component in the analysed zirconfractions.

Several large, colourless, rounded and multifac-Fig. 4. U–Pb concordia diagram. Conventional analyses of eted zircons were selected and dated by the Pb–Pbsingle crystals from garnet amphibolite sample 8705. evaporation method. Only two of the mounted

crystals gave intensities high enough for data col-lection, probably due to low Pb concentrations.The two crystals analyzed yielded 207Pb/206Pb agesof 991±15 and 982±9 Ma (Table 3).

All three titanite fractions, one with light yellowcrystals and two with dark brown varieties, plotconcordantly. All analyses overlap within errorand yield an age of 923±3 Ma (Fig. 8, Table 2).The brown titanites are more U-rich than theyellow variety.

The Sm–Nd analyses of whole-rock, garnet,hornblende and titanite give an isochron age of945±80 Ma. The initial 143Nd/144Nd ratio is0.511161±78. The MSWD is 0.18 (Fig. 9 andTable 1). The large error of 80 Ma is due to thelarge analytical error in the garnet analysis.

Fig. 5. Sm–Nd isochron diagram for garnet amphibolite sample8705. Wr=whole rock, Grt=garnet, Hbl=hornblende, Plg=plagioclase.

5.3. 9007 — garnet amphibolite

Sample 9007 (Fig. 2) is a fine-grained, foliated5.2. 9015 — garnet amphiboliterock with thin felsic veins subparallel to thegneissosity. It consists of garnet, hornblende, pla-Sample 9015 (Fig. 2) is a fine- to medium-

grained, veined garnet amphibolite with garnet, gioclase quartz, opaques and minor clinopyroxene.The titanites range in colour from brown to lighthornblende, plagioclase, quartz, opaques and

minor clinopyroxene. The garnets range in yellow. The U and Pb contents are 30–41 ppmand 8–13 ppm, respectively (Table 2). Four of fivegrainsize between 1 and 8 mm. The sample con-

tains titanite that vary in colour from dark brown fractions plot on the concordia, whereas one frac-tion of light yellow titanites is much more discor-to light yellow. Both titanite and zircon occur in

interstices and as inclusions in garnet and horn- dant (Fig. 10). The titanites in this fraction wereprobably not completely dissolved, which resultedblende [Fig. 6(a)]. Most of the zircons are col-


Fig. 6. Scanning electron microscope images of zircons from garnet amphibolite sample 9015. (a) Zircon and titanite occur as inclusionsin garnet; (b) transparent, clear and colourless, multifaceted ovoid zircon; (c) cathode-luminescence image showing homogeneous,rounded zircon. Rt=rutile, Tin=titanite, Zr=zircon.

in fractionation between U and Pb during the two 5.4. Samples 9238, 9286, 9290 and 92117 — maficevents of spiking (separate U- and Pb-tracers were granulitesused). The remaining four fractions yield a late

Four mafic granulite-facies rocks (9238, 9286,Sveconorwegian age of 945±2 Ma.9290 and 92117) come from widely separatedThe Sm–Nd analyses of whole-rock, garnet andlocalities (Fig. 2). Samples 9238 and 9286 consisthornblende do not define an isochron. The garnet

and whole-rock reference line corresponds to an of garnet, clinopyroxene, hornblende, plagioclase,quartz and opaques. The clinopyroxene is partlyage of 900±42 Ma (Fig. 11, Table 1). The horn-

blende from the same sample was dated by the replaced by secondary hornblende, whereas thehornblende is partly replaced by secondary biotite40Ar–39Ar method to 933±4 Ma ( Wang et al.,

1996). in sample 9286. Samples 9290 and 92117 are made


Fig. 9. Sm–Nd isochron diagram for garnet amphibolite sampleFig. 7. U–Pb concordia diagram. Conventional U–Pb analytical 9015. Wr=whole rock, Grt=garnet, Hbl=hornblende, Ttn=data of zircons from garnet amphibolite sample 9015. titanite.

Fig. 10. U–Pb concordia diagram for titanite analyses ofFig. 8. U–Pb concordia diagram for titanite analyses of different different size-fractions from garnet amphibolite sample 9007. Lsize-fractions from garnet amphibolite sample 9015. L and B and B are the light yellow and brown titanites, respectively.are the light yellow and brown titanites, respectively.

sample. The Sm–Nd ages of samples 9238 and92117 are younger than the 40Ar–39Ar hornblendeup of garnet, clinopyroxene, orthopyroxene, horn-

blende, plagioclase, quartz and opaques. No meta- plateau ages from the same samples; 958±5 and969±7 Ma, respectively ( Wang et al., 1996,morphic zircon was found in these samples.

Sm–Nd analyses of whole-rocks and mineral Table 4).separates define isochron ages of 920±14 Ma,903±10 Ma and 909±14 Ma for samples 9238,9290 and 92117, respectively [Fig. 12(a), (c) and 6. Discussion and interpretation(d) and Table 1]. The Sm–Nd isotope data ofsample 9286 do not define an isochron. Garnet 6.1. U–Pb zircon and titanite dataand whole-rock reference line yields 873±40 Ma[Fig. 12(b)]. The suspiciously young age could be In the SGR, the peak metamorphic temperatures

(c. 700–800°C) exceeded the closure temperaturedue to secondary alteration for this particular


any rock dated in the region (Johansson et al.,1993; Welin, 1994; Lindh, 1996). This age is,however, obtained from very discordant fractionsand both the lower and upper intercept ages mustbe interpreted with caution. The lower interceptage of 975±25 Ma is within error identical to thePb–Pb evaporation ages of 980–990 Ma.

The titanite data of sample 9007 and 9015 plotat or near the concordia. The three fractions fromsample 9015 and fractions 2–5 from 9007, give206Pb/238U ages of 923±3 and 945±2 Ma, respec-tively. The interpretation of the two titanite agesis dependent on the closure temperature of theU–Pb titanite system. Recent studies suggest thatthe Tc is >700°C (Zhang and Scharer, 1996) and

Fig. 11. Sm–Nd isochron diagram for garnet amphibolite 660–700°C (Scott and St-Onge, 1995), thus sig-sample 9007. Wr=whole rock, Grt=garnet, Hbl=homblende. nificantly higher than previous estimates of aroundThe dashed line is an age reference line.

500–600°C (Gascoyne, 1986; Mezger et al., 1991).Accepting a high Tc for titanite, these ages shoulddate titanite growth during retrogression, approxi-(Tc) for all isotopic systems, except for the U–Pb

zircon system. The ability of zircon to retain its mately 30–60 Ma after the peak of metamorphism.If, on the other hand, a lower Tc of 500–600°C isoriginal isotope signature, even during granulite

facies metamorphism and anatexis, has been considered, the titanite ages likely date coolingafter peak metamorphism. The interpretation ofdemonstrated in previous studies ( Kroner et al.,

1987a,b; Mezger and Krogstad, 1997). This implies these ages are thus enigmatic and it is emphasizedthat the Tc is controlled by a number of parametersthat zircon that formed at any time during a high-

grade metamorphic event generally records the that must be taken into account in each case(Cherniak, 1993; Lee, 1995).time of zircon growth.

The upper intercept age of 974±25 Ma of A further complication is that titanite occursboth as inclusions in garnet and in intersticessample 8705, together with the Pb–Pb evaporation

ages of c. 970 Ma (Zl, Z2 and Z3), are interpreted between other minerals (sample 9015). Therefore,the obtained titanite age could represent a mixingto date growth of metamorphic zircon close to the

peak of high-pressure granulite metamorphism. age of older titanite enclosed in garnet and youngertitanite in the matrix. On the other hand, if theSoderlund (1996) reported a similar Pb–Pb evapo-

ration age of 969±27 Ma, obtained from zircons titanites in the garnet were considerably older thanthe obtained age and made up a significant partin a c. 1410 Ma old, strongly deformed pegmatite

dyke. Those zircons showed characteristics typical of the analyzed sample one would not expect toget concordant data.of a metamorphic origin, distinctly different from

the c. 1410 Ma old, igneous zircons. The Pb–Pbevaporation ages of 959±5 and 956±6 Ma (crystal4 and 5) of sample 8705 are significantly younger 7. Reinterpretation of previous Ar–Ar hornblende

ages from granulites and garnet amphibolitesand possibly date a later stage of metamorphiczircon growth (more details are given in Soderlundet al., 1996). In view of these new isotope data, previous

published 40Ar–39Ar hornblende ages must beDue to the complex nature of the zircons insample 9015, the linear array of the data points is reevaluated. The older 40Ar–39Ar hornblende ages,

that range from c. 1030 to 960 Ma, were suggestedinterpreted to represent a mixing line (Fig. 7). Theupper intercept age of 1798±37 Ma is older than correspond to an event of Sveconorwegian crustal


Fig. 12. Sm–Nd isochron diagrams for four mafic granulite samples from the SCR. Wr=whole rock, Grt=garnet, Cpx=clinopyro-xene, Hbl=hornblende, Plg=plagioclase. The dashed line in B is an age reference line.

thickening and the younger ages around 930 Ma in these rocks are in equilibrium with pyroxene,garnet, plagioclase and quartz, which implies thatto date cooling and uplift of the high-grade rocks

(Page et al., 1996; Wang et al., 1996). The inter- hornblende recrystallized during high-grade meta-morphism ( Wang and Lindh, 1996).pretation that the older 40Ar–39Ar hornblende ages

(>c. 960 Ma) date an event of crustal thickening, The scatter in 40Ar–39Ar hornblende ages donot show any distinct geographic trend (Fig. 4 inmust be reconsidered since: (1) the 40Ar–39Ar

hornblende ages are in the same range as, or older Wang et al., 1996). Hornblende from granulitesand amphibolites from SW Sweden may not havethan, the zircon ages and significantly older than

the titanite ages obtained from the same sample any unique Tc. Harrison and Gerald (1986) andOnstott and Peacock (1987) proposed that micro-(Table 4); (2) hornblende that was formed during

an event of crustal thickening and escaped subse- textures like exsolution in hornblende and thecomposition of hornblende may affect the retentionquent recrystallization, should become isotopically

reset since the retention temperature of Ar (c. of Ar and thus the Tc. Small differences in Tc mayinduce large age differences in high-grade terrains500°C) is far below the estimated peak metamor-

phic temperatures (c. 700–800°C); (3) hornblende undergoing very slow cooling. However, horn-


blende in our samples are homogenous in backscat- Nakamura, 1995). The suggested Tc rangesbetween 500 and 900°C. Compared to the temper-ter images and the 40Ar–39Ar data do not allow

any correlation with the Fe/(Fe+Mg) ratios atures recorded by the Fe–Mg exchange thermom-eter of Ellis and Green (1979), higher closure( Wang et al., 1996). An alternative explanation

may be incorporation of variable amounts of temperatures of 850–900°C for the Sm–Nd systemin garnet were suggested by Cohen et al. (1988)excess Ar due to recrystallization during high-

grade metamorphism. The excess Ar may be intro- and Jagoutz (1988). This view was based on theassumption of a similar diffusion rate of Sm andduced by a local fluid, causing the c. 100 Ma

scatter in the 40Ar–39Ar hornblende data. Step- Nd to that of Fe and Mg in garnet. A high closuretemperature is supported by data of high-gradewise heating technique may not have removed all

the excess Ar from the hornblende, therefore yield- metamorphic rocks in terrains with a short coolinghistory, showing similar ages from differenting false plateaus (Cumbest et al., 1994).

In comparison with the U–Pb zircon and titanite radiometric systems (Gebauer, 1990; Paquetteet al., 1994). For rocks consisting only of garnetages, the younger 40Ar–39Ar hornblende ages of

c. 930 Ma are interpreted to date cooling of the and pyroxene, like eclogite, the diffusion of Sm–Ndin garnet is mainly controlled by clinopyroxene,high-grade rocks through c. 500°C.which causes the diffusion to cease at a hightemperature (Becker, 1993; Kalt et al., 1994).Mezger et al. (1992) proposed a lower closure8. The closure temperature of the Sm–Nd system in

garnet temperature of about 600±30°C for the Sm–Nddiffusion in garnet in terrains which have experi-enced a slow cooling rate. This Tc was assessed byThe closure temperature of the Sm–Nd isotopic

system in garnet has been the subject of many comparing their Sm–Nd data with other geochro-nological data from the same rocks or the samepapers (cf. Table 5; Humphries and Cliff, 1982;

Cohen et al., 1988; Jagoutz, 1988; Mezger et al., geological units. A Tc around 600°C is furthersupported by the results of Bohlen et al. (1985),1992; Paquette et al., 1994; Hensen and Zhou,

1995; Schmadicke et al., 1995; Maboko and Mezger et al. (1990) and Cosca et al. (1991).

Table 5Closure temperature estimates of the Sm–Nd isotopic system in garnet

Rock type Tectonic setting Garnet composition Tc (°C) Ref.

Granulites Bergen Arc, Western Norway Alm0.31–0.35 Py0.43–0.49 Grs0.15–0.23 900 Cohen et al.(1988)

Eclogitic xenolith Kimberlite province of Nzega Alm0.36–0.38 Py0.25–0.33 Grs0.30–0.36 850 Jagoutz (1988)Granulite-facies rocks Granulite domain in SE Madagascar — 850 Paquette et al.

(1994)Mafic granulites Paleozoic granulite-facies terrane, — >700 Hensen and Zhou

Søstrene Island, Australia –750 (1995)Amphibolite- Archean Pikwitonei Granulite Alm0.62–0.70 Py0.22–0.34 Grs0.04–0.02 600 Mezger et al.and granulite-facies rocks domain, Canada; (1992)

Grenville orogen in Ontario, Alm0.41–0.64 Py0. 12–0.43 Grs0.09–0.29Canada and New Yary, USA

Granulite andAnorthosite Uluguru granulite complex — 500 Maboko andof Eastern Tanzania Nakamura (1995)

Granulites Scourian gneiss complex, Sutherland Pyrope 480 Humphries andCliff (1982)

Grossular 700 Humphries andCliff (1982)

Mafic granulites Southwesten Sweden Alm0.52–0.66 Py0. 12–0.26 Grs0.17–0.29 500 This study


Recently, Maboko and Nakamura (1995) pro- metamorphism. Providing this age constrain isapplicable also to the southern part of the SGR,posed an even lower closure temperature of c.

500°C, by comparing their Sm–Nd age of it strongly argues against a high closure temper-ature of the Sm–Nd system in garnet. We infer618±16 Ma with an 40Ar–39Ar hornblende age of

628±3 Ma and a U–Pb zircon age of 695±4 Ma that diffusion of Sm–Nd invariably ceased some30–80 million years after growth of metamorphicfrom a nearby granulite and a meta-anorthosite,

respectively (Maboko et al., 1989; Muhongo and zircon. A high Tc for the Sm–Nd system is also inconflict with the 40Ar–39Ar hornblende ages (c.Lenoir, 1994). However, the meta-anorthosite was

recently redated to c. 625 Ma using U–Pb monazite 930 Ma) and the 40Ar–39Ar muscovite age(904±6 Ma), interpreted as cooling ages (Pageand zircon chronology (Moller et al., 1996b). This

age is comparable within error with both the et al., 1996). At present, the geochronological datasupport a Tc of around 500°C or lower for theSm–Nd isochron age of 618±16 Ma (Maboko

and Nakamura, 1995) and was interpreted as the Sm–Nd system.An alternative explanation for the youngage of the granulite metamorphism. Moller et al.

(1996b) reconsidered the U–Pb zircon age of Sm–Nd ages could be that diffusion was not onlycontrolled by volume diffusion but some additional695±4 Ma (Muhongo and Lenoir, 1994) as the

age of the anorthosite intrusion whereas the 40Ar diffusional process. The closure-temperatures con-cept (Dodson, 1973) is based on the assumption–39Ar hornblende age of 628±3 Ma (Maboko

et al., 1989) was considered to be too old due to that the redistribution of isotopes is controlled byvolume diffusion only. Alternative diffusionalexcess Ar. Moller et al. (1996b) thus favoured a

high closure temperature of 700–750°C for the models emphasizing the role of modal compositionand grain boundary fluids have been proposed bySm–Nd system in garnet.

In the present study, we essentially follow the Jenkin et al. (1995) and Eiler et al. (1992).Romer and Smeds (1996) argued that garnetsmethodology utilized by Mezger et al. (1992) and

Maboko and Nakamura (1995) to assess the clo- formed during breakdown of plagioclase inheritedof their Sm–Nd isotopic composition partly fromsure temperature for the Sm–Nd system in garnets.

The Sm–Nd mineral isochrons from samples 8705, the plagioclase. This resulted in a too low initial143Nd/144Nd ratio and consequently too young9015, 9238, 9290 and 92117 (4 to 5 point isochrons)

yield ages between 940 and 900 Ma that are indis- age. We find such a hypothesis less likely sinceexcluding the garnet in regression of the sampletinguishable from the garnet whole-rock ages

[Figs. 5, 9 and 12(a), (c) and (d)] and compare data does not significantly affect the Sm–Nd ages[Figs. 5 and 12(c) and (d)]. Alternative explana-well with those obtained by Johansson et al. (1991)

and Johansson and Kullerud (1993). Compared tions of the Sm–Nd systematics are the scope ofcontinued research in the SGR.with the metamorphic zircon ages of 975±17 and

974±25 Ma, Sm–Nd data from the same rocksyield ages which, invariably, are c. 30–80 millionyears younger. These ages are similar to or younger 9. T–t history of the southwest Swedish granulite

regionthan the 40Ar–39Ar hornblende ages and onlyslightly older than the 40Ar–39Ar muscovite age ofc. 900 Ma. The geochronological data presented in this

study, combined with earlier published P–T esti-If a high closure temperature of ≥750°C for theSm–Nd system in garnet is applicable also to our mates (Johansson et al., 1991; Wang and Lindh,

1996), allow some speculation about the coolingsamples, the Sm–Nd ages should be similar to thezircon ages of c. 975 Ma. The occurrences of c. history of the southern SGR.

Evidence for a prograde evolution in the studied955 Ma old, post-tectonic, granite and pegmatitedykes in the northern SGR (Andersson, 1996; area is restricted to a single sample (9015) that

preserve growth zoning in 2 mm large garnetsMoller and Soderlund, 1997) are considered as areliable lower age limit for Sveconorwegian peak (pyrope and grossular increase and almandine and


spessartine decrease from core to rim, Fig. 5 in the matrix. The interpretation of titanite ages isfurther complicated by ambiguous closure temper-Wang and Lindh, 1996). This zoning was consid-

ered to reflect garnet growth during increasing ature. Assuming a Tc of c. 700°C (Zhang andScharer, 1996) instead of c. 550°C (Mezger et al.,pressure and temperature. P–T estimates of

680–800°C and 83–11.7 kbar have been considered 1991) increases the cooling rate by as much as4°C Ma−1 (from c. 5 to 9°C Ma−1), if using horn-to reflect peak metamorphic conditions (Johansson

et al., 1991; Wang and Lindh, 1996). The mineral- blende and muscovite ages, their correspondingclosure temperatures and assuming a titanite ageogical evidences of retrogression in our samples

are replacement of pyroxene by hornblende, biotite of c. 940 Ma.Metamorphic zircon enclosed in garnet with areplacing hornblende and garnet with thin retro-

gressed rims ( Wang and Lindh, 1996). prograde growth zoning have also been foundin kyanite eclogites at Ullared further north inGeochronological data that constrain the cool-

ing history of rocks in the southern SGR includes the SGR (Moller, 1998). Ion-probe (N,Stockholm) analyses of these zircons gavethe zircon ages of 974±25 (sample 8705) and

975±17 Ma (sample 9015) which date the maxi- 969±14 Ma (Johansson et al., 1998), an age iden-tical to our zircon ages. The eclogites at Ullaredmum age of peak metamorphism (discussed

below). Cooling through approximately 550°C and demonstrate that metamorphic zircons formedduring prograde metamorphism, most likely asso-500°C occurred at c. 945–925 (samples 9007 and

9015) and c. 930 Ma according to U–Pb titanite ciated with breakdown and recrystallization ofprimary magmatic, Zr-bearing phases.and 40Ar–39Ar hornblende systematics (Page et al.,

1996; Wang et al., 1996). The single 40Ar–39Armuscovite analysis indicates cooling through c.350°C at 904±6 Ma (Page et al., 1996). These Acknowledgmentsages and temperature estimates will yield a mini-mum average cooling rate of 5–7°C Ma−1. This work was supported by the Swedish

There are, however, a number of restrictions to Natural Science Research Council (NFR). Wethe calculated cooling rate. In the calculation thank C. Moller, L.M. Page and P-O. Persson forabove, the maximum age of peak metamorphism discussions. P-O. Persson is especially thanked bywas assumed to be constrained by the age of the Wang Xiangdong for introducing him to the ana-zircons. Zircon enclosed in garnet (Fig. 3) indicate lytical procedures of U–Pb geochronology and tothat some zircons formed prior to, or contempora- mass spectrometer analysis at the Laboratory forneously with garnet during increasing pressure. Isotope Geology at the Swedish Museum ofHowever, some of the analyzed zircons are proba- Natural History, Stockholm. The manuscript bene-bly derived from the matrix and may have formed fited from constructive comments from J.N.at a later stage of metamorphism. This is indicated Connelly, K. Mezger, J.S. Daly, C. Moller andby the significantly lower Pb–Pb evaporation ages E.J. Essene.of 959±5 and 956±6 Ma for two of the analyzedcrystals in sample 8705. Since the location of theindividual zircons we have analyzed are unknown, Referencesonly the minimum cooling rate can be calculated.If the peak temperature occurred at an age younger Ahall, K.-I., 1995. Crustal units and the role of the Mylonitethan that of the zircon, the cooling rate would be zone system in the Varberg-Horred region, SW Sweden. GFF

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