Lithos 126 (2011) 388–401

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Trace element composition of rutile and the application of Zr-in-rutile thermometryto UHT metamorphism (Epupa Complex, NW Namibia)

Melanie Meyer a,⁎, Timm John b, Sönke Brandt c, Reiner Klemd a

a GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Schlossgarten 5a, D-91054 Erlangen, Germanyb Institut für Mineralogie, Universität Münster, Correnstr. 24, D-48149 Münster, Germanyc Institut für Geowissenschaften, Universität Kiel, Ludewig-Meyn-Str. 10, D-24118 Kiel, Germany

⁎ Corresponding author.E-mail addresses: melanie.meyer@geol.uni-erlangen

timm.john@uni-muenster.de (T. John), brandt@min.uniklemd@geol.uni-erlangen.de (R. Klemd).

0024-4937/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.lithos.2011.07.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 November 2010Accepted 14 July 2011Available online 29 July 2011

Keywords:Rutile trace element compositionLA-ICP-MSEpupa ComplexUHT granulitesZr-in-rutile thermometry

The trace element composition of rutile and Zr-in-rutile temperatures were determined for ultra-hightemperature (UHT) metamorphic rocks from the Epembe Unit of the Epupa Complex in NW-Namibia. Ortho-and paragneisses record Mesoproterozoic peakmetamorphic UHT conditions of 970±40 °C at 9.5±2 kbar, asestimated from conventional thermobarometry and constraints from pseudosection modeling. Rutile exhibitssuperchondritic concentrations of V, Zr, Nb, Hf, Ta, Th and U while rare earth elements (REE) are far lessenriched. Zr and Hf correlate positively with two distinct trends. Nb and Ta as well as Cr and V show a positivecorrelation although with less clear trends. Only Hf correlates with Zr, suggesting a decoupling of Zr and Hffrom the other high field-strength elements (HFSE) probably during retrogression. In general, the non-homogeneous HFSE distribution in rutile indicates that equilibrium trace element distribution achievedduring UHT peakmetamorphic conditions was either almost completely erased or had never been achieved asa common feature of all rutile grains. The retrograde metamorphic evolution of the UHT rocks is interpreted tobe responsible for trace element redistribution under equilibrium conditions restricted to small domains. Thishas affected the trace element composition of the rutile grains investigated here thereby disturbing their UHTsignature, which may cause problems for provenance studies involving such ‘disturbed’ grains. A systematiccomparison of all available Zr-in-rutile thermometer calibrations shows that beside of one, all give similartemperature estimates for the studied samples. No systematic differences regarding the Zr content wereobserved between rutile grains in different textural positions (i.e. matrix grains, those shielded by hostminerals or post-peak grains). However, calculations revealed a broad range of temperatures betweenb400 °C and N1000 °C. The large spread of calculated temperatures is interpreted to result from intergraindiffusion and trace element exchange by fluid-mediated recrystallization during the retrograde metamorphicevolution. This interpretation is supported by the presence of extensively formed retrograde reaction texturesinvolving hydrous phases such as cordierite and biotite in the studied samples. In addition, tiny (≤5 μm) Zr-silica-rich phase separations which occur either homogeneously or heterogeneously distributed in singlerutile grains may cause intergrain Zr variations.

.de (M. Meyer),-kiel.de (S. Brandt),

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Rutile is an important accessory phase in many Ti-bearing rocksbecause of its widespread distribution in a variety of rock types. Rutileoccurs in metamorphic (greenschist- to eclogite- and granulite-faciesrocks) and igneous rocks, mantle xenoliths, lunar rocks, meteoritesand it is commonly found in clastic sediments as a stable heavymineral (e.g., Meinhold, 2010). Depending on the pressure (P) and

temperature (T) conditions at which rocks equilibrate, rutile is animportant host (in addition to zircon) of the HFSE and dominates thebudget of Nb and Ta in crustal rocks (e.g. Rudnick et al., 2000; Schmidtet al., 2009; Zack et al., 2002). Other HFSE are significantly enriched aswell; for example in eclogites approximately onemodal % of rutile cancarrymore than 90% of thewhole-rock content for Ti, Nb, Sb, Ta andWand it may also contain 5–45% of the whole-rock content of V, Cr, Moand Sn (Rudnick et al., 2000; Schmidt et al., 2009; Zack et al., 2002).

Due to analytical advancements, trace element dependent ther-mometers involving accessory minerals such as sphene, rutile andzircon have been introduced in the last years. The application of thesethermometers is a considerable improvement because their closuretemperatures are often much higher than that of conventionalthermometers. The Zr-in-rutile thermometer, introduced by Zacket al. (2004) and subsequently modified, was originally applied to

389M. Meyer et al. / Lithos 126 (2011) 388–401

various rock types across a range of different metamorphic facies.Spear et al. (2006) conducted the first systematic application of theZr-in-rutile thermometer to blueschist-facies rocks. The relatively lowtemperature range between 445 and 505 °C obtained was consistentwith the findings of conventional thermometry and thus it wassuggested that this temperature range indeed reflects the tempera-tures of rutile crystallization (Spear et al., 2006). Miller et al. (2007)applied the Zr-in-rutile thermometer calibrations of Watson et al.(2006) and Zack et al. (2004) to eclogite-facies rocks and suggestedthat the calculated temperatures between 700 to 730 °C and 630 to650 °C, respectively, reflect peakmetamorphic conditions. In addition,Zhang et al. (2010) applied all four available calibrations (Ferry andWatson, 2007; Tomkins et al., 2007; Watson et al., 2006; Zack et al.,2004) of the Zr-in-rutile thermometer to eclogites. They postulatedthat for rocks, which experienced high-pressure (HP) or ultra-highpressure (UHP) conditions, the Tomkins et al. (2007) calibrationwhich includes a pressure-correction gave themost consistent results.Luvizotto et al. (2009) applied the calibration of Tomkins et al. (2007)to medium-grade metasedimentary rocks and concluded that lowtemperatures obtained for some rutile grains may reflect either anearly stage during the prograde path or re-equilibration conditionsduring retrograde metamorphism. Furthermore, Luvizotto and Zack(2009) observed a very large spread of Zr concentrations in rutilegrains in granulite-facies metamorphic rocks from the Ivrea-VerbanoZone. This result was quite surprising because these rocks werethought to contain rather uniform Zr concentrations (due to hightemperatures granulite-facies rocks are generally well equilibrated–Luvizotto and Zack, 2009). Baldwin and Brown (2008) applied thecalibrations of Tomkins et al. (2007), Watson et al. (2006) and Zack etal. (2004) to rutile grains hosted in garnet of UHT rocks of theAnápolis-Itauçu Complex (Brazil) which record UHT temperatures.Harley (2008) reviewed the application of both the Zr-in-rutile andthe Ti-in-zircon thermometer and applied them to UHT samples fromthe Napier Complex (Antarctica). Although for the Zr-in-rutilethermometer the calibration of Watson et al. (2006), Ferry andWatson (2007), and Tomkins et al. (2007) agree well at 10 kbar, rutilein these granulites has Zr contents, which are too low to be inequilibrium with UHT conditions.

The inconsistent results of the previous studies applying Zr-in-rutile thermometry to UHT rocks arise the question after processes,which may explain the partly observed re-setting of the Zr content inUHT rutile. In the present study, we investigate the reliability of theZr-in-rutile thermometer for UHT rocks by comparing temperaturesgained by all available calibrations to those gained by major elementthermometry. In addition, processes which are possibly related to thepartly observed re-equilibration of the Zr content in rutile will beevaluated. We have applied the different published calibrations of theZr-in-rutile thermometer to UHT granulites from the Epupa Complex(NW Namibia), which equilibrated at extreme temperatures of 970±40 °C as estimated from garnet-orthopyroxene thermometry (Brandtet al., 2003) and constraints from pseudosection modeling onsapphirine-bearing orthopyroxene-sillimanite granulites (Brandtet al., 2007).

2. Geological setting and P–T evolution

The Epupa Complex is exposed in NW Namibia and is situated atthe southwestern margin of the Archean to Palaeoproterozoic CongoCraton of central Africa. The rocks of the Epupa Complex wereprimarily formed during the Eburnian Orogeny which corresponds toa major crust-forming event defined by the formation of Palaeopro-terozoic supracrustal rocks and associated synvolcanic and syn- to latekinematic intrusive rocks (e.g. Cahen et al., 1984), with ages between2250 and 1600 Ma (Carvalho et al., 2000; Seth et al., 2003; 2005). TheEpupa Complex rockswere intruded by the Kunene Intrusive Complex(KIC), which forms one of the largest massif-type anorthosite bodies

in the world (e.g. Ashwal and Twist, 1994; Drüppel et al., 2001, 2007;Gleissner et al., 2010).

The Epupa Complex is situated near the eastern margin of the Pan-African Kaoko Belt of NW Namibia. While the western part of theEpupa Complex was affected by the Pan-African orogeny and becamepartly incorporated into the mobile belt (e.g. Dingeldey et al., 1994),the northeastern sub-area of the Epupa Complex, comprising thestudy area, remained unaffected as a stable part of the Congo Craton asis indicated by overlying un-metamorphosed Neoproterozoic sedi-ments (Brandt et al., 2003). The Epupa Complex is subdivided into theupper amphibolite-facies rocks of the Orue Unit and the UHTgranulite-facies rocks of the Epembe Unit (Brandt et al., 2003;Fig. 1). Both units comprise volcano-sedimentary rocks which wereaccumulated in the early Palaeoproterozoic in a back-arc basin alongthe SWmargin of the Congo Craton (Brandt et al., 2003, 2007). Duringthe Eburnian Orogeny (ca. 2.25 to 1.6 Ga), the volcano-sedimentarysequences were buried to mid-crustal levels, partly accompanied bythe emplacement of granite intrusions during the post-collisionalstage (Brandt et al., 2003). The metamorphic protoliths of the OrueUnit are thought to have formed from 1997 to 1640 Ma (U–Pb onsingle zircon grains; Seth et al., 2005), and the protoliths of theEpembe Unit from 1810 to 1635 Ma (U–Pb on zircon grain cores; Sethet al., 2003).

The E–W-trending Epembe Unit is a granulite-facies fault-bounded terrane of 50 km length and up to 10 km width, exposedto the south of the anorthositic KIC (Fig. 1). It consists of ametamorphosed volcano-sedimentary sequence, comprising ortho-gneisses, such as metavolcanic mafic and felsic granulites, andparagneisses, including metagreywackes, metapelites and quartz-rich garnet-orthopyroxene rocks. Sapphirine-bearing orthopyroxene-sillimanite gneisses and orthopyroxene-garnet rocks occur as rarerestitic domains. This volcano-sedimentary sequence is transected byrare granulite-facies garnet-orthopyroxene granitoids and interme-diate mafic dikes, which are metamorphosed to mafic granulites(Brandt et al., 2003).

The rocks of the Epembe Unit were affected by a multistage UHTmetamorphism during an early Mesoproterozoic event (1520–1447 Ma; Seth et al., 2003). The granulite-facies metamorphism isinterpreted to result from the accretion of anorthositic melt onto thelower crust. The UHT rocks follow an anticlockwise P–T path whichcomprises five metamorphic stages starting with prograde near-isobaric heating to UHT conditions at moderate pressures followed bypressure increase at ultrahigh temperatures (Brandt et al., 2003;2007). Peak-metamorphic P–T conditions of 9.5±2 kbar and 970±40 °C were estimated by garnet-orthopyroxene Fe-Mg-Al thermom-etry (Brandt et al., 2003) as well as constraints from pseudosectionmodeling for orthopyroxene-sillimanite granulites (Brandt et al.,2007). Subsequent decompression started under UHT granulite-faciesconditions and was followed by cooling to amphibolite-faciesconditions accompanied by recrystallization of minerals throughinteraction with the crystallizing melts. The metamorphic evolutionand inferred geodynamicmodel are discussed in detail by Brandt et al.(2003, 2007).

3. Sample description and methods

3.1. Petrography

Nine samples which represent four different rock types (maficgarnet-clinopyroxene granulites, garnet-sillimanite gneisses, garnet-orthopyroxene granulites and sapphirine-bearing orthopyroxene-silli-manite gneisses) were selected for a detailed petrographical investiga-tion. The samples exhibit mineral assemblages and abundant reactiontextures which can be related to several stages of the metamorphicevolution (see Brandt et al., 2003, 2007 for a detailed petrographicdescription including microphotographs). The mineralogy and

Fig. 1. Geological overview: Simplified geological map of the Epupa Complex and the adjacent Kunene Intrusive Complex with sample locations in the Epembe Unit. Adapted andadjusted from Brandt and Klemd (2008); modified and simplified after Köstlin (1967) and Menge (1998).

390 M. Meyer et al. / Lithos 126 (2011) 388–401

(retrograde)mineral assemblages of thedifferent rock types are given inTable 1a and 1b, respectively. During prograde to peak metamorphismbiotite- and/or hornblende-bearing assemblages were almostcompletely replaced by anhydrous mineral assemblages. Decompres-sion is characterized by the development of conspicuous symplectiteand corona textures around the peak phases. Further retrogression ledto the recrystallization of biotite, hornblende, sillimanite and garnet

Table 1aSummarized description of studied samples: Modal proportions are estimates based on visua(2003). Mineral abbreviations follow Whitney and Evans (2010).

Sample Grt Opx Cpx Hbl Pl Qtz

Mafic Grt-Cpx granulite xxx x xx xxx xx –

(311-3-00) Grt2 Opx3 Cpx2 Hbl4 Pl3,4Grt-Sil gneissesQtz-bearing xx(x) x – – x xx(x)(230-F-98, 465-1-99,564-1-99 689-00)

Grt2,4 Opx3 Pl3 Qtz2

Qtz-free xxx – – – x –

(212-A-98) Grt2 Pl3Grt-OpxMetagreywacke-type xx xxx – – xxx xx458-3-99 Grt2 Opx2,3,5 Pl2,3 Qtz2Qtz-rich xxx x – – x xxx(614-1-99) Grt2,4 Opx2,3,5 Pl2,3 Qtz2Spr-bearing Opx-Sil gneiss – xxx – – xx –

(693-00) Opx2,3,(5) Pl3

Modal proportion: xxx: N20 vol.%, xx: 10–20 vol.%, x: 1–10 vol.%, –: b1 vol.%.Metamorphic stages (adapted from Brandt, 2003): 2: peak-metamorphic assemblages, 3: coron

formed at the expense of the symplectitic phases or relict peakmetamorphic phases.

a) Mafic garnet-clinopyroxene granulite (311-3-00)This sample is dominated by anhedral porphyroblastic garnet (up to3 cm in diameter; 20–22 vol.%) coexisting with clinopyroxene (up to1 cm in length; 12–21 vol.%). Garnet and clinopyroxene are separated

l inspection. The diffentiation between the metamorphic stages is adapted from Brandt

Sil Kfs Crd Bt Ilm Spl Accessories

– – – – x – Rt, Czo, Bt, Zrn, ApIlm2

x(x) xxx x(x) x x x Rt, Zrn, ApSil2,4 Kfs2 Crd3 Bt4 Ilm2,3 Spl2,3

xx xxx x x x x Rt, Zrn, ApSil2,4 Kfs2 Crd3 Bt4 Ilm2,3 Spl2,3

– x x xx – – Sil, Rt, Ilm, Zrn, ApKfs2 Crd3 Bt4

– x x x x – Sil, Rt, Zrn, SplKfs2 Crd3 Bt4 Ilm2,3

x – xx x(x) – x Crn, Zrn, Mnz, Rt, SprSil2,3 Crd3,(5) Bt4 Spl3

a and symplectite formation, 4: re-growth of Bt and Grt 5: formation of late Opx and Crd.

Table 1bSummarized description of retrograde metamorphic conversion textures. The diffentiation between the metamorphic stages is adapted from Brandt (2003).

Sample Retrograde conversion

Stage 3 Stage 4

Mafic Grt-Cpx granulite Corona textures(311-3-00) Opx3, Pl3Grt-Sil gneissesQtz-bearing Symplectite & corona textures Re-growth at the expense of stage 3 phases(230-F-98, 465-1-99, 564-1-99 689-00) Opx3, Crd3, Pl3, Spl3, Ilm3 Bt4, Grt4, Sil4Qtz-free Symplectite & corona textures Re-growth at the expense of stage 3 phases(212-A-98) Crd3, Pl3, Spl3, Ilm3 Bt4, Sil4Grt-OpxMetagreywacke-type Symplectite & corona textures Symplectite phases partly replaced(458-3-99) Crd3, Opx3, Pl3 Bt4Qtz-rich Symplectite & corona textures Symplectite phases partly replaced(614-1-99) Pl3, Ilm3 Bt4, Grt4Spr-bearing Opx-Sil gneiss Fine-grained reaction textures Limited formation(693-00) Opx3, Crd3, Spr3, Spl3, Pl3, Sil3 Bt4

Metamorphic stages (adapted from Brandt, 2003): 3: corona and symplectite formation, 4: re-growth of Bt and Grt.

391M. Meyer et al. / Lithos 126 (2011) 388–401

by orthopyroxene-plagioclase symplectites. A granoblastic corona ofhornblende (50–55 vol.%) and plagioclase (10–12 vol.%) is developedbetween the orthopyroxene-plagioclase symplectite and garnet.Rutile grains occur primarily as inclusions in garnet and clinopy-roxene but also occur in the matrix. Porphyroblastic garnet withabundant cracks filled with fine-grained retrograde minerals pre-serves a prograde zoningwith XMg increasing from core to rim,whileclinopyroxene is almost homogeneous (Brandt, 2003). The investi-gated rutile grains occur either as inclusions in Mg-rich rims ofgarnet, which correspond to the highest metamorphic grade or insmall unzoned garnet.

b) Metapelitic garnet-sillimanite gneissesThe metapelites display a characteristic migmatitic texture, inwhich the restitic domains are dominated by sillimanite (5–23 vol.%) and garnet (13–28 vol.%). Spinel, cordierite, plagioclaseand biotite occur in minor amounts. Accessory mineral phasesinclude ilmenite, rutile, apatite and zircon. The restitic layersalternate with concordant medium- to coarse-grained granoblas-tic leucosomes, which consist of K-feldspar (25–50 vol.%) andquartz (15–30 vol.%).Quartz-bearing garnet-sillimanite gneisses (230-F-98; 465-1-99;564-1-99; 689–00)The samples essentially consist of porphyroblastic garnet (13–21 vol.%), sillimanite (5–18 vol.%), K-feldspar (25–32 vol.%) andquartz (15–30 vol.%). Garnet in sample 230-F-98 is replaced by thinsymplectite coronas of orthopyroxene, cordierite and plagioclasewhich are partly replaced by late biotite. In samples 465-1-99, 564-1-99 and 689–00, garnet is replaced by a thin very fine-grainedsymplectite (mainly consisting of orthopyroxene, cordierite, plagio-clase, spinel and ilmenite, in sample 564-1-99 locally replaced by re-grown biotite). Cracks in garnet are also filled with fine-grainedcordierite and orthopyroxene. Ilmenite and rutile are accessoryphases and occur in thematrix. To some extent, rutile forms skeletalaggregates around relic peak-metamorphic ilmenite. These rutilegrains are interpreted as post-peak phases (Brandt, 2003) and werenot analyzed. A second generation of garnet occurs as fine-grainedcrystals coexisting with biotite and formed at the partial expense ofthe symplectite phases.Quartz-free garnet-sillimanite gneiss (212-A-98)This sample contains porphyroblastic garnet (25–28 vol.%) coexist-ing with perthite (34–50 vol.%) and abundant sillimanite (18–20 vol.%), as well as minor ilmenite, spinel and plagioclase.Furthermore, accessory minerals such as matrix rutile and zircon

occur. Some garnet grains are surrounded by narrow retrogressivemetamorphic textures such as coronas mainly composed ofplagioclase. However, it is particularly noticeable, thatmostmineralsoccur as ‘rounded’ relics, embedded in a fine-grained matrix.

c) Garnet-orthopyroxene granulite and quartz-rich garnet-orthopyr-oxene granuliteMetagreywacke-type garnet-orthopyroxene granulite (458-3-99)This sample predominantly consists of orthopyroxene (~30 vol.%)and subordinate porphyroblastic garnet (12 vol.%) in a quartz-dominated matrix, which additionally contains fine-grained K-feldspar and abundant plagioclase. Rutile and zircon are accesso-ries in the matrix. Symplectites of orthopyroxene, cordierite andplagioclase developed between garnet and matrix quartz. Biotiteplates replace the symplectites.Quartz-rich garnet-orthopyroxene granulite (614-1-99)The sample is dominated by porphyroblastic garnet (~30 vol.%)and subordinate porphyroblastic orthopyroxene (b5 vol.%). Theseminerals are set in a coarse- to fine-grained matrix primarilycontaining quartz (~50 vol.%) with subordinate plagioclase, K-feldspar, and biotite. Accessory matrix minerals include zircon,sillimanite and rare matrix rutile. Garnet is separated from matrixquartz by conspicuous cordierite-orthopyroxene symplectites.

d) Sapphirine-bearing orthopyroxene-sillimanite gneiss (693–00)The rock contains aweak compositional bandingwhich is defined byorthopyroxene-rich layers (~55 vol.%) alternating with sillimanite-rich layers (~10 vol.%). Subordinate amounts of plagioclase, cordi-erite, spinel, biotite and rare K-feldspar are present. Accessoryminerals include rutile and zircon. Conspicuous fine-grainedretrograde reaction textures consist of orthopyroxene, cordierite,sapphirine, spinel, plagioclase, sillimanite and corundum. Biotite islate and was partially replaced by orthopyroxene and cordierite.Rutile occurs in thematrix, is hostedwithin orthopyroxene or occursin direct contact to orthopyroxene.

3.2. Analytical techniques

Trace element analyses were performed using in-situ laserablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the GeoZentrumNordbayern, Universität Erlangen-Nürnberg.The lab is equipped with a Merchantek 266 LUV (λ=266 nm) laser,coupled with an Agilent 7500i ICP-mass spectrometer with a plasmapower of 1350 W using Ar as the carrier, plasma, and auxiliary gaswith a flow of 1.27 l/min, 14.9 l/min, and 0.9 l/min. NIST 610, NIST612 and NIST 615materials (Jackson et al., 1992; Norman et al., 1996;

392 M. Meyer et al. / Lithos 126 (2011) 388–401

Pearce et al., 1997) were used as standard materials for both oxidesand silicates.

In general, ablation followed a certain pattern: First a burst shotwas executed to remove contamination from the surface of thesample. Then, a single spot measurement followed, with the laserrunning with a repetition rate of 10 Hz and a pulse energy between0.59 and 0.74 mJ (46–58 J/cm2 energy density) for a spot size of40 μm. The measurement of one spot, i.e. one analysis, lasted 40 s.The first 20 s were used for acquiring the background-signalwhereas the following 20 s were used for analysis of the sampleby ablation of rutile. As measurement-mode, a time-resolvedanalysis was carried out and measurements were conducted atthe maximum peak.

A total of 90 grains in 14 thin sections were analyzed for Zr-in-rutilethermometry. In addition to the analysis of 90Zr and 91Zr (40 msdwelltime), 29Si, 47Ti, 49Ti, 51V, 53Cr (each with 10 ms dwelltime) and89Y, 93Nb, 178Hf, 181Ta, 232Th and 238U (each 40 ms dwelltime) wereanalyzed. TiO2was used as internal standard. In addition, the REE 139La,140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159 Tb, 163Dy, 165Ho, 166Er,169Tm, 172Yb and 175Luwere measured in selected samples which weresuitable for a measurement of a greater quantity of elements. Theablation conditions such as laser energy and energy density show slightdifferences for the REE analysis (pulse energy: 0.61–0.76 mJ; energydensity on sample: 50–62 J/cm2). Signal quantification was carried outby GLITTER Version 3.0 (On-line Interactive Data Reduction for LA-ICP-MS Program of the year 2000 by Maquarie Research Ltd. (vanAchterbergh et al., 2000)). In order to check trace element zoningwithin single rutile grains, additional electron microprobe (EMP)analyses (backscattered electron (BSE) images, qualitative mapping)were conducted for selected grains (withmore than one LA-spot, biggergrain size, Zr concentration N0.1 wt.%), using a JEOL JXA-8200 electronmicroprobe at the GeoZentrum Nordbayern, Universität Erlangen-Nürnberg operating at 20 kV, 40 nA, and focused beam.

Table 2Overview of mean values and standard deviation of trace element concentrations of rutile

Sample 212-A-98 230-F-98 31

Rocktype

Qtz-free Grt-Silgneiss

Qtz-bearing Grt-Silgneiss

Mgr

Elements[ppm]

n=23 n=25 n=

Mean SD Mean SD M

V 2059 1013 2742 703 13Cr 298 192 735 346 57Y 44 84 13 19 0.7Zr 1111 910 929 1020 46Nb 2459 1476 2528 1597 31Hf 89 61 60 48 21Ta 87 58 111 83 27Th 1.0 0.61 0.09 0.01 0.2U 95 81 25 25 39

Sample 564-1-99 614-1-99

Rock type Qtz-bearing Grt-Sil gneiss Qtz-rich Grt-Opx gra

Elements [ppm] n=14 n=1

Mean SD Mean

V 1776 625 886Cr 522 138 24Y 7.3 22 0.06Zr 2627 1220 2427Nb 2303 826 15713Hf 156 61 175Ta 99 37 408Th 0.83 0.87 b.d.U 57 20 19

b.d. below detection limit.

4. Results

4.1. Rutile chemistry

4.1.1. Trace element characteristics of rutileAll analyzed trace elements in rutile exhibit a significant spread in

concentrations which varies from sample to sample. Table 2 gives anoverview of mean values and standard deviation of trace elementconcentrations of rutile grains of each sample. The spider-diagram(Fig. 2) shows superchondritic values for most analyzed traceelements (V, Zr, Nb, Hf, Ta, Th, U) with the exception of Cr and Ywhich shows both sub- and superchondritic concentrations. Therange of whole-rock data comprises all rock types analyzed in thisstudy (presented in the supplementary material) and is compiledfrom analyses by Brandt (2003).

Compared to the trace elements (Fig. 2), the REE, which weremeasured in selected rutile grains (allowing a second trace elementLA-ICP-MS analysis) are less enriched. LREE are more enriched thanHREE (Fig. 3; LaN/YbN=1.36–22.2 with an average of 18.8). Thisobservation is in agreement with experimentally determined KD's ofREE in rutile that showed that REE are incompatible in rutile incontrast to the HFSE (Klemme et al., 2005).

The HFSE characteristics of the rutile grains from our study exhibitsignificant chemical differences which are interpreted to have beenmanifested during retrogression of the samples. Zr and Hf (Fig. 4A)exhibit a strong positive correlation with two distinct trends. Inaddition to the main trend (slope 0.0548, R2=0.95) a second trendshows a slightly flatter slope (slope 0.0391, R2=0.99)–(Fig. 4A). Themain trend is formed by the analyses of the garnet-orthopyroxenegranulites (greenish colors), and most of the analyses of themetapelitic garnet-sillimanite gneisses (orange and red colors) andthe sapphirine-bearing orthopyroxene-sillimanite gneiss (violetcolor). The second trend is mainly formed by the analyses of the

grains of each sample, n gives the number of analyses of each sample.

1-3-00 458-3-99 465-1-99

afic Grt-Cpxanulite

Metagreywacke-type Grt-Opxgranulite

Qtz-bearing Grt-Silgneiss

34 n=2 n=12

ean SD Mean SD Mean SD

62 667 841 348 2459 8969 484 425 168 708 326

1.0 0.72 0.48 24 604 435 251 249 2541 12960 128 957 913 2797 1536

16 49 25 145 7114 15 13 139 130

1 0.15 b.d. - 0.13 0.0551 37 37 169 81

689-00 693-00

nulite Qtz-bearing Grt-Sil gneiss Spr-bearing Opx-Sil gneiss

n=3 n=22

SD Mean SD Mean SD

– 2068 762 1366 273– 1421 556 296 68– 0.12 0.08 3.1 5.2– 3670 380 2428 1995– 2389 627 6364 2319– 177 29 121 62– 64 24 413 173– b.d. – 0.08 0.02– 10 4.0 37 12

Fig. 2. Trace element characteristics of rutile grains and range of whole rock data (compiled from analyses by Brandt, 2003). Analyzed elements are given with increasing atomicmass. For each rock type represented bymore than one sample, the variation of analyzed samples is indicated by differently shaded ranges (normalizing values fromMcDonough andSun, 1995). Refer to Table 2 for mean values of trace element concentrations of single samples.

393M. Meyer et al. / Lithos 126 (2011) 388–401

mafic garnet-clinopyroxene granulite (blue color), some analyses ofthe quartz-bearing metapelitic garnet-sillimanite gneisses (red color)and few analyses of the sapphirine-bearing orthopyroxene-sillimanitegneiss (violet color). A third cluster is defined by two grains from thesapphirine-bearing orthopyroxene-sillimanite gneiss (violet color),which exhibit very high Zr and Hf concentrations (Fig. 4A). Nb and Ta(Fig. 4E) show a positive correlation (slope 0.0579, R2=0.88,excluding two outliers). Although with a broader distribution, Crand V also show a positive correlation for each rock type (Fig. 4F).Beside Hf, no other element correlates with Zr (Fig. 4A, B, C, D), whichsuggests a decoupling of Zr and Hf from the other HFSE.

High U contents (N100 ppm) occur in rutile grains from themetapelitic garnet-sillimanite gneiss samples 212-A-98 and 465-1-99and the mafic garnet-clinopyroxene sample 311-3-00 (Fig. 4B).

All rutile compositions display subchondritic Zr/Hf values (chon-dritic value 34.3±0.3, Münker et al., 2003) with a broad range of 1.67to 32.3 (average of 15.8), whereas for Nb/Ta ratios are bothsubchondritic and superchondritic (values range between 4.98 and78.4 with an average of 23.3; chondritic value 19.9 (Münker et al.,2003) – Fig. 5). In fact, only the mafic garnet-clinopyroxene granuliteand the sapphirine-bearing orthopyroxene-sillimanite gneiss (sam-ples 311-3-00 and 693–00) have rutile with dominantly subchondriticNb/Ta values whereas rutile in other samples (e.g. metapelitic garnet-sillimanite gneisses like samples 230-F-98, 465-1-99) displays a broadrange in Nb/Ta ratio (Fig. 5C).

4.1.2. Zr contents of rutileRutile occurs as four different textural types in the studied

samples. The most frequent one in all samples is matrix rutile witha grain size between 80 and 600 μm. In addition, rutile may occur inthe rim region of porphyroblastic, prograde zoned garnet (grain size

Fig. 3. REE characteristics of rutile. For each sample the average is

of rutile: 140–400 μm, larger crystals form intergrowths withilmenite), in small unzoned garnet (grain size of rutile: 90–200 μm)and in pyroxene (grain size of rutile: 200–350 μm). Most porphyro-blastic garnet grains exhibit small cracks containing retrogrademinerals such as chlorite, cordierite and biotite, indicating possiblepathways for trace element transfer during retrogression.

The different textural rutile types in all rock types and theassociated range of Zr concentrations are summarized in Table 3.

4.1.3. Intra-grain Zr variationsAll rutile grains were analyzed for 29Si, because it may indicate

submicroscopic Zr-silica-rich domains. Some grains indeed exhibitelevated Si contents, which may be caused by submicroscopicexsolutions or relictic inclusions from preexisting grains. Analyseswith apparently elevated Si-contents and unusual high Zr contentswere excluded from the data set (Luvizotto and Zack, 2009; Zack et al.,2004).

BSE images of several rutile grains suggest the absence of asystematic compositional zoning. But some rutile grains, which wereanalyzed withmultiple spots, nevertheless exhibit intra-grain variationin Zr content. Based on the BSE images, three grains (one in sample 465-1-99 and two in sample 693–00) with microscopically visible, tiny(≤5 μm) phase separations (Fig. 6) were selected for a qualitativeelement mapping of the Zr distribution. The mapping shows that thesephase separations are enriched in Zr,whereas thehosting grain containsa rather homogeneous Zr concentration (Fig. 6C, F, I). Rutile grainsfrequently have a homogeneous Zr distribution, however, in places therim is significantly enriched in Zr (Fig. 6A–C). High Zr concentrations inrutile usually indicate a rather homogeneous distribution of tiny Zr-richphase separations throughout the grains (see Fig. 6D–F).However, somerutile grains (e.g. Fig. 6G–I) have high Zr concentration in the core

given (normalizing values from McDonough and Sun, 1995).

Fig. 4. Results of trace element determination with LA-ICP-MS for rutile from UHT rocks of the Epembe Unit. Points represent individual analyses. Samples of equal rock types areindicated by equal colors (blue: mafic Grt-Cpx* granulites, red: Qtz-bearing Grt-Sil gneisses, orange: Qtz-free-Grt-Sil gneisses, olive-green: metagreywacke-type Grt-Opx granulites,green: Qtz-rich Grt-Opx granulites, violet: Spr-bearing Grt-Sil gneisses). (A) Zr versus Hf showing a strong positive correlation, but two distinct trends with different slopes aredeveloped. (B) Zr versus U without correlation, some samples show rather constant U concentrations, e.g. sample 465-1-99 with high U contents or sample 693–00 with constantlylow U concentrations. (C) Zr versus Nb showing no correlation. (D) Zr versus Ta showing no correlation. (E) Nb versus Ta showing a strong positive correlation. (F) Cr versus Vshowing a weak positive correlation, which seems to be to some extent rock-type-dependent, a reasonable correlation of single samples is not observable.(* Mineral abbreviationsfollow Whitney and Evans, 2010).

394 M. Meyer et al. / Lithos 126 (2011) 388–401

(3136 ppm) and low concentration near the rim (1451 ppm). Thus, inthese rutile grains, the Zr-rich phase separations are responsible for thestrong variation of Zr concentrations. A replacement by rutile of a pre-existing Zr-rich mineral like ilmenite is a possible explanation for theformation of these Zr-rich phase separations in rutile (Austrheim et al.,2008). On the other hand, the sharp margin of the area comprising Zr-

richphase separations of certain rutile grains (Fig. 6G–I)may also be dueto twinning, which would explain that different orientated inclusionsoccur in only part of the grain. However, mineral twinning was notdetected by optical means or in BSE images. A third possibility is theformation of the phase separations through exsolution in rutile duringretrograde metamorphism. However, a detailed investigation of the

Fig. 5. Element-ratio plots. Points represent individual analyses. Samples of equal rock types are indicated by equal colors (blue: mafic Grt-Cpx granulites, red: Qtz-bearing Grt-Silgneisses, orange: Qtz-free-Grt-Sil gneisses, olive-green: metagreywacke-type Grt-Opx granulites, green: Qtz-rich Grt-Opx granulites, violet: Spr-bearing Grt-Sil gneisses). (A) Nbversus Nb/Ta. (B) Hf versus Zr/Hf. (C) Zr/Hf versus Nb/Ta diagram. The circles in the background represent Earth's major reservoirs, adapted from John et al. (2011), modified afterMünker et al. (2003): yellow: depleted mantle; violet: mid-ocean ridge basalts (MORB); green: Archean greenstones; orange: ocean island basalt (OIB); blue: continental basalts;red: continental crust; rose: adakites (values from Wang et al., 2007); grey: Archean TTGs (values taken from Hoffmann et al., 2009); dashed lines represent chondritic values forNb/Ta and Zr/Hf, respectively and the star indicates bulk silicate Earth. Most samples do not plot in any of the fields with an exception for some analyses of sample 311-3-00, whichagree with the MORB-field.

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formation of these Zr-rich phase separations in rutile is beyond thescope of this paper.

4.2. Zr-in-rutile thermometry

For temperature calculations the following calibrations of the Zr-in-rutile thermometer were applied:

Zack et al. (2004):

T BCð Þ¼ 127:8⁎ln Zrppm in rutile

� �−10

Table 3Zr concentrations in different positions of rutile grains grouped by rock types.

Rock type Mafic Grt-Cpxgranulite

Grt-Sil gneiss

Qtz-bearing Qtz free

Samples 311-3-00 230-F-98, 465-1-99, 564-1-99,689-00

212-A-9

Zr [ppm] Zr [ppm] Zr [ppm

Min Max Min Max Min

Rt in matrix b100 788 b100 4388 b100Rt in porphyroblasic Grt 635 1274 – – –

Rt in small Grt b100 839 – – –

Rt in Px b100 b100 – – –

Watson et al. (2006):

T BCð Þ = 4470F120ð Þ= 7:36F0:10−log Zrppm in rutile

� �� i−273

h

Tomkins et al. (2007) for the β-Qtz-field:

T BCð Þ = 85:7 + 0:473⁎Pð Þ= 0:1453−R⁎ln Zrppm in rutile

� �� �h i−273

With R gasconstantð Þ = 0:0083144kJK−1:

Grt-Opx granulite Spr-bearingOpx-Sil gneiss

Metagreywacke-type

Qtz-rich

8 458-3-99 614-1-99 693-00

] Zr [ppm] Zr [ppm] Zr [ppm]

Max Min Max Min Max Min Max

2880 b100 500 2427 2427 b100 3136– – – – – – –

– – – – – – –

– – – – – 1518 6901

Fig. 6.Note the different scales of the images. (A–C) Rutile grain with homogeneous Zr content throughout the grain but with a narrow rim, which is significantly enriched in Zr (Qtz-bearing Grt-Sil gneiss, sample 465-1-99). (A) Plane polarized light, numbers in the ablation craters give the measured Zr content in ppm. (B) BSE image (red rectangle indicatesposition for C) showing some remanents of the analysis by LA-ICP-MS. (C) Qualitative EMP Zr mapping. Reddish colors indicate highest Zr concentrations, bluish colors indicate lowZr concentrations. (D–F) Rutile grain with high Zr concentrations and randomly distributed Zr-silica rich inclusions throughout the grain (Spr-bearing Opx-Sil gneiss, sample 693–00, mineral 004). (D) Plane polarized light, numbers in the ablation craters give the measured Zr content in ppm. (E) BSE image showing the distribution of tiny phase separations inrutile. The larger white crystals are ilmenite. (F) Qualitative EMP Zr mapping. Reddish colors indicate highest Zr concentrations, bluish colors indicate low Zr concentrations. (G–I)Rutile grain with heterogeneously distributed domains of Zr enrichment causing significantly varying Zr concentrations and thus temperatures (Spr-bearing Opx-Sil gneiss, sample693–00, mineral 001). (G) Plane polarized light, numbers in the ablation craters give the measured Zr content in ppm. (H) BSE image. (I) Qualitative EMP Zr mapping. Reddish colorsindicate highest Zr concentrations, bluish colors indicate low Zr concentrations.

396 M. Meyer et al. / Lithos 126 (2011) 388–401

Please note that we used 9.5 kbar as input parameter for P in theequation of Tomkins et al. (2007).

Ferry and Watson (2007):

T BCð Þ = 4530F111ð Þ = 7:420F0:105−logaSiO2ð Þ½ �−log Zrppm in rutile

� �−273

4.2.1. Rutile in different textural positionsMatrix rutile grains and those enclosed and shielded by other

minerals like garnet or pyroxene, record a similar broad range ofcalculated temperatures ranging from b400 to approximately 1200 °C(Fig. 7). The rangeof calculated temperatures is similar for all calibrations,except for that of Zack et al. (2004). These results are in accordance withprevious studies in which similar trace element characteristics in rutile

grains of different textural position have been observed (e.g. Luvizottoand Zack, 2009; Miller et al., 2007; Spear et al., 2006). In contrast, otherstudies reported a good preservation of peakmetamorphic temperaturesin ‘shielded’ rutile grains enclosed in garnet or pyroxene (e.g. Luvizotto etal., 2009; Triebold et al., 2007). However, our results do not substantiatethis observation as both matrix and inclusion rutile grains display asimilar range of Zr contents and thus temperatures (Fig. 7). For instance,in sample 311-3-00 most temperatures derived from rutile inclusions inMg-rich rims of porphyroblastic garnet or small garnet grains range from700 to 800 °C – far below the peak temperature of ca. 1000 °C, whilethose derived from rutile inclusions in clinopyroxene are even lower.Thus, the proposed ‘shielding’ effect is interpreted to be negligible,probably due to extensive retrograde overprinting. Strong retrogressionof this sample, which is exposed in a shear zone, is evident from theextensive formation of hornblende-plagioclase symplectites resorbing

Fig. 7. Range of temperatures calculated with the Zr-in-rutile thermometry using the calibrations of Ferry andWatson (2007), Tomkins et al. (2007),Watson et al. (2006) and Zack etal. (2004) for (A) rutile enclosed by garnet or pyroxene and (B) matrix rutile. The majority of the analyses yield temperatures between 700 °C and 1000 °C, with many data down tob400 °C and very few data N1000 °C. Temperature calculations for all individual analyses are given in the supplementary data.

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garnet along themargins and along cracks. Textural primary rutile of thissample displays a crystallization age of ca. 1247 Ma (Seth et al., 2005),which is significantly younger than the Eburnian ages of the protoliths ofthe Epembe Unit (1810–1635 Ma) and the Mesoproterozoic UHT event(1520–1447Ma). Thus, the rutile dating seems to support the observa-tion of significant chemical re-equilibration during retrogression. Aretrograde modification of the trace element composition of grainsincluded in ‘shielding’minerals is also indicated by a partly pseudomor-phosed (to sphene) rutile grain hosted in garnet (Fig. 8).

Fig. 8. Rutile grain of the mafic Grt-Cpx sample (311-3-00) which is partly replaced bysphene. Note that it is hosted by unzoned garnet.

4.2.2. Comparison of the different calibrationsApart from the empirical calibration of Zack et al. (2004), all

applied calibrations of the Zr-in-rutile thermometers yield a similartemperature range (Fig. 9). For instance, Miller et al. (2007) andBaldwin and Brown (2008) reported that temperatures calculatedwith the calibration of Zack et al. (2004) are higher than thosecalculated by other calibrations. The results of our study confirm thisobservation and indicate that the more recent experimental calibra-tions are more precise than the empirical one.

The good correlation between the pressure corrected anduncorrected calibrations are ascribed to the almost identical pressureconditions of the here investigated samples (9.5 kbar) and of theexperimental calibrations of Ferry and Watson (2007) and Watsonet al. (2006) (~10 kbar). The importance of a pressure correctionshould not be neglected for samples which experienced a morepressure dominated metamorphism and for these samples thecalibration of Tomkins et al. (2007) is probably the best choice totake into account pressure effects.

5. Discussion

5.1. Rutile used for sedimentary provenance

The log(Cr/Nb) versus temperature plot (Fig. 10A), which has beenproposed to delineate mafic (N0) from pelitic source rocks (b0) in

Fig. 9. Difference in calculated temperature range of the Zr-in-rutile thermometer using various calibrations. Good correlation of 3 calibrations at 9.5 kbar (similar to pressure ofexperimental determination of thermometers). The darker grey gives the T uncertainty of the Watson et al. (2006) calibration, lighter grey gives the T uncertainty of the Ferry andWatson (2007) calibration.

Fig. 10. (A) Correlation between metamorphic temperature of formation and trace element ratio Cr/Nb in rutile (adapted from Triebold et al., 2007) illustrating the significant protolithtrace element signature change of rutile. Mafic source rocks have been interpreted to plot above zero, pelitic (felsic) ones below, but only sample 311-3-00 represents a mafic protolith.(B) Correlation betweenNb and Cr contents of rutile: Another possibility to discriminatemafic frompelitic source rocks (Meinhold et al., 2008). The 1:1 (log(Cr/Nb)=0) line is accordingto Triebold et al. (2005) and the lower limit betweenmafic and peltic source rocks of rutilewas set at 800 ppm as recommended byMeinhold et al. (2008).Samples of equal rock types areindicated by equal colors (blue: mafic Grt-Cpx granulites, red and orange: Grt-Sil gneisses (Qtz-bearing and Qtz-free), green: Grt-Opx granulites (metagreywacke-type and Qtz-rich),violet: Spr-bearing Grt-Sil gneisses).

398 M. Meyer et al. / Lithos 126 (2011) 388–401

399M. Meyer et al. / Lithos 126 (2011) 388–401

provenance studies (Triebold et al., 2007), shows a scattered patternnot only for the mafic sample 311-3-00, but also for the metapeliticsamples 230-F-98 and 458-3-99. Rutile data of these three samplesplot in both the field for the mafic and the pelitic source rocks(Fig. 10A). Accordingly, 37% of the mafic rutile is misleadinglyclassified as rutile derived from metapelitic rocks, but on the otherhand only 5.5% of the metapelitic rutile wrongly plot in the metamaficfield. In another discrimination diagram (Meinhold et al., 2008),which is also supposed to separate mafic and pelitic source rocks, allrutile grains of the mafic Grt-Cpx sample (311-3-00) are correctlyclassified as metamafic rocks (Fig. 10B). However, in this discrimina-tion diagram a higher proportion (14%) of metapelitic rutile of all rocktypes is misleadingly classified as metamafic rutile (Fig. 10B).Consequently, the above described discrimination diagrams shouldnot be used for provenance studies of rutile in sedimentary rocks forgranulite-facies source rocks.

5.2. Trace element behavior of rutile

The strong correlation between Zr and Hf and Nb and Ta in rutilefrom granulite-facies rocks was already reported from the Ivrea-Verbano Zone in Southern Alps (Luvizotto and Zack, 2009) and fromeclogite-facies rocks from various other localities (John et al., 2011;Schmidt et al., 2009). The former authors concluded that a fractionationof Zr fromHf andNb from Ta did not occur in rutile of the Ivrea-Verbanogranulite-facies rocks. Furthermore, a good correlation between Zr andHf and Nb and Ta, respectively, in rutile grains was described byLuvizotto et al. (2009; their Fig. 25). However, the rutile grains studiedhere reveal two trends with different slopes in the Zr versus Hf plot(Fig. 4A). This is interpreted to be due to bulk rock compositionaldifferences since the second trend with a flatter slope is mainlyexhibited by the trace element composition of rutile from the maficprotolith (sample 311-3-00) and only a few rutile grains from quartz-bearing metapelitic garnet-sillimanite gneisses (samples 230-F-98 and689–00). The Zr concentration is supposed to be controlled by theambient temperature and the Zr/Hf by the bulk rock composition. Thepositive correlation of Nb and Ta as well as Cr and V (Fig. 4E, F) is lesswell defined than that of Zr and Hf. The correlation between Cr and Vseems to be a rock-type specific feature (regression lines for each rocktype are given in Fig. 4F)whichmay indicate that Cr and V still preservebulk rock characteristics.

In general, the Zr/Hf and Nb/Ta ratios are indicative of the wholerock composition. Yet the Nb/Ta versus Ta (Fig. 5A) and Zr/Hf versusHf (Fig. 5B) plots show a wide range of Nb/Ta and Zr/Hf ratios forsimilar Ta and Hf values not only for the whole sample set, but also forsingle samples, even though themetamorphic evolution of all samplesis essentially identical. This indicates that a significant spread of Nb/Taand Zr values is typical for rutile grains even from single samples.Since this phenomenon was also shown for non-retrogressedprograde eclogites (John et al., 2011; Schmidt et al., 2009), it indicatesthat their distribution in the rock is transport limited and that bulkequilibrium is rarely reached. Another possible explanation is that thebulk disequilibrium occurred during the retrograde rutile recrystal-lization or replacement by sphene. Either way, it seems to bedecoupled from the bulk chemical composition of the rocks, thusreflecting small-scale effects. Our findings are in agreement withexperimental results indicating such small-scale heterogeneities inTi-phases (Lucassen et al., 2010b). Additionally, the fact that a formermafic protolith now displays metapelitic trace element signatures(Fig. 10A) and rutile from a pelitic protolith is classified as metamafic(Fig. 10A, B) may indicate that the trace element signature, whichsupposedly defines the provenance of a rock, was disturbed duringrecrystallization. In addition a change in log(Cr/Nb) ratios can beobserved in samples that still preserve their original ‘protolithic’composition, e.g. sample 693–00 (paragneiss) which correctly plots inthe pelitic field (Fig. 10A).

The correlation between Zr/Hf and Nb/Ta (Fig. 5C) shows a ratherheterogeneous distribution of measured element ratios which impliesa change in trace element supply during the retrograde rutileformation or recrystallization (assuming that during peak metamor-phic conditions all rutile grains were in equilibriumwith the bulk rockcomposition). The rutile grains of two samples (sample 311-3-00 and693–00) are dominated by subchondritic Nb/Ta ratios (Fig. 5). Exceptfor sample 311-3-00, none of the samples plot in the basalt field. Thus,the trace element behavior in the Zr/Hf versus Nb/Ta diagram is mostlikely a result of their distribution duringmetamorphism. A significantvariation in Nb/Ta ratios and exceeding chondritic values, as observedin this study, was already reported by Aulbach et al. (2008). Theseauthors investigated rutile from xenolithic eclogites and observedsubchondritic as well as superchondritic Nb/Ta ratios. A diversionfrom the terrestrial array is known so far only for Archean granitoidsof the tonalite–trondhjemite–granodiorite (TTG) suite (Hoffmann etal., 2009). These rocks, which represent the Earth's early continentalcrust, are characterized by larger Nb/Ta variations than any otherreservoir of the Earth. Large variations in bulk rock Nb/Ta may becaused by partial melting of mafic material at lower crustal levelswhere coexisting Ti-phases control the Nb-Ta systematics of theforming melt (John et al., 2011).

In general, the above observations suggest that post-UHT (retro-grade)mineral reactions are responsible for the heterogeneous elementdistribution in the rutile grains. However, the non-homogeneous HFSEdistribution in the rutile grains may also indicate that the chemicalevidence forUHTpeakmetamorphism, includingpervasive equilibrium,was never established as an uniform element concentration among andwithin the rutile grains. Therefore the HFSE patterns for these samplesare the result of limited reaction in progress and remobilization of HFSEamong rutile and the rocks at a local thermodynamic equilibriumdue toinitially dry UHT condition and the limited solubility of rutile inmetamorphic rocks (e.g., Audétat and Keppler, 2005; Manning et al.,2008). However we prefer the former model, since the trace elementcomposition of rutile always correlates with the extent of retrogressionin the host rock (see below).

5.3. Zr-in rutile thermometry

The wide range of calculated temperatures revealed by thedifferent calibrations was somewhat unexpected as all samplesexperienced similar UHT metamorphic conditions of ca. 1000 °Cfollowed by amultistage retrogrademetamorphic evolution. The earlyretrograde evolution of the rocks is characterized by the developmentof decompressional corona and symplectite textures during granulite-facies conditions. The involvement of a fluid phase (H2O and/or CO2)is apparent, since cordierite is an abundant phase in symplectites(EMP analysis with total sums of 97–99%; Brandt, 2003). Subsequentcooling led to the re-growth of biotite under amphibolite-faciesconditions which also required the presence of a fluid phase (Brandtet al., 2007). Therefore the calculated temperature range is inter-preted to be result of proceeding trace element exchange by grain-boundary-dominated, fluid-mediated diffusional transport withinrestricted sample domains. This effect proceeded during rutilerecrystallization under decompressional granulite-facies conditionsand fluid-mediated recrystallization during the subsequent amphib-olite-facies retrogression. The apparent closure temperature of rutilefor volume diffusion of Zr ranges between 560 and 730 °C for grainswith a diameter of 100 μm (Watson et al., 2006), which is consistentwith the average diameter of the majority of the here investigatedrutile grains. In this context it should be noted that Cherniak andWatson (2007) showed that a considerable diffusional exchangebetween a mineral grain and its surroundings may occur attemperatures well below their nominal ‘closure temperature’, whichimplies a progression of diffusion during cooling and retrogrademetamorphism. Furthermore, Lucassen et al. (2010a) showed by

400 M. Meyer et al. / Lithos 126 (2011) 388–401

modeling Zr and Nb diffusion in rutile that in fluid-dominated naturalsystems, diffusion coefficients in rutile should be higher than thosedetermined experimentally.

In some samples the observed strong variation of analyzed Zrcontents can be explained by tiny Zr-silica-rich phase separations(detected by EMP) in the rutile grains, which is interpreted as post-peakexsolution (Fig. 6). As a consequence thegrainsbecome compositionallyinhomogeneous and the measured Zr concentration depends onwhether the analyzed spot comprises such phase separation.

Though most samples exhibit a strong retrograde metamorphicoverprint, approximately 10% of the rutile trace elementmeasurementsshowed Zr concentrations consistent with UHT conditions (Figs. 7 and9). However, the highest temperature of nearly 1000 °C (temperaturecalculation with experimentally determined calibrations) is preservedonly in one rutile grain from sample 693–00. This sample, whichpetrographically shows the lowest degree of retrogression, containsrutile grainswith thehighest Zr concentrationsamongall samples. Thus,the extent of the retrograde overprint is interpreted as the major factorfor the preservation of trace element contents in rutile and hencedocumenting UHT conditions by Zr-in-rutile thermometry. The lowtemperatures (700 °C and below), which are displayed by rutile in allinvestigated samples, are consistent with a less retentive behavior ofrutile during retrogression from UHT conditions. Furthermore, thisresult is consistent with Zr-in-rutile thermometry conducted by Harley(2008) on UHT rocks from the Napier Complex in Antarctica, whoobserved that the temperatures calculated from Zr contents in peak-metamorphic rutile are lower than peak-conditions as constrained byconventional thermometers in these samples.

5.4. Final remarks

As already noted above, besides Hf, the other HFSE do not correlatewith the Zr concentration, thus their incorporation is interpreted to bedependent on bulk rock composition, rather than on temperature.Subsequent modifications during retrograde metamorphism aredominated by small-scale source and sink characteristics in therock. The non-homogeneous HFSE distribution in rutile indicates thatthe chemical evidence for UHT peak metamorphism, includingpervasive equilibrium, was erased among and within rutile grains.Our sample set is thought to represent a sequence of UHT rocks thathas been reset during changing ambient P–T conditions under initiallydry or almost dry peak metamorphic, granulite-facies condition.Subsequent retrograde metamorphic conditions involve fluid infiltra-tion as indicated by the occurrence of hydrous minerals. Under theseretrograde conditions, significant local redistribution of HFSE resultedin large variations in the trace element compositions and Zrconcentrations of rutile grains in single samples. Fluid infiltrationduring retrogression is rather typical for mid- to lower crustal rocks(Jamtveit and Austrheim, 2010; Putnis and Austrheim, 2010) anddefinitively amplifies the observed bulk composition independentredistribution of trace elements (Lucassen et al., 2010a). This impliesthat the use of rutile composition for provenance studies is critical,since rutile is, in contrast to zircon, quite prone to retrogressionrelated HFSE exchangewith thematrix, as shown in the present study.

6. Conclusions

1. Trace element in situ measurements in rutile grains reveal super-chondritic values for V, Zr, Nb, Hf, Ta, Th and U in rutile, REE are bycontrast rather incompatible. Zr and Hf show a strong positivecorrelation with two pronounced trends while Nb and Ta and Crand V, respectively, exhibit a less well-defined positive correlation.

2. Besides Hf, no other element correlates with Zr indicating adecoupling of Zr and Hf from the other HFSE in the hereinvestigated rutile grains. This behavior is thought to have occurredduring retrogression.

3. Rutile discrimination diagrams like the log(Cr/Nb) versus T plot orthe Cr versus Nb plot should be used with caution for provenancestudies seeing that in the present study of UHT granulite-faciesrocks the trace element signature which defines the provenancewas disturbed during the retrograde metamorphic evolution.

4. In addition to the calculated temperature range, the inhomoge-neous HFSE distribution in rutile indicates that the chemicalevidence for UHT peakmetamorphism is either erased or had neverbeen achieved as a uniform element concentration among andwithin rutile grains.

5. All applied calibrations of the Zr-in-rutile thermometer, apart fromthe empirical calibration of Zack et al. (2004), give equivalenttemperatures. The good correlation between pressure correctedand uncorrected calibrations is ascribed to almost identicalpressure conditions of the investigated samples (9.5 kbar) andthose used for the experimental calibrations (~10 kbar). Thus, thepressure effect of the Zr-in-rutile thermometer is negligible for theUHT rocks investigated here (P=9.5 kbar) but should be takeninto account for rocks that experienced amore pressure dominatedmetamorphism.

6. Systematic chemical differences between rutile of differenttextural position are not observed. Matrix rutile grains and thoseenclosed and shielded by other minerals record a similar widerange of temperatures.

7. In some rutile grains tiny (≤5 μm) Zr-silica-rich phase separationsoccur, which have a strong effect on the analyzed Zr concentra-tions. This indicates that prior to the application of Zr-in-rutilethermometry a careful investigation of the internal structure of theanalyzed rutile grains is required.

8. Though all samples are thought to have experienced the samemetamorphic UHT evolution, only 10% of rutile trace elementmeasurements contain sufficient Zr in agreement with UHTconditions. Considering the significantly varying concentrationsof Zr in rutile, the resulting broad scatter of calculated tempera-tures is attributed to diffusion of Zr in rutile during retrograde re-equilibration and limited fluid-mediated recrystallization duringretrogression. As a consequence, results of the Zr-in-rutilethermometry should be treated with caution when dealing withhigh temperature rocks which have undergone pronouncedretrogression.

Acknowledgements

Helene Brätz is thanked for her help with the LA-ICP-MSmeasurements and Inga Osbahr for assistance with the EMPA.

Thoughtful reviewswith constructive comments and suggestions byGerhard Franz and two anonymous reviewers are highly appreciatedand helped to improve the manuscript substantially. In addition, IanBuick and Marco Scambelluri are thanked for their helpful commentsand for the editorial handling of the manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.lithos.2011.07.013.

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