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IntroductionMAGNETITE-ILMENITE ores are a principal source of titanium(e.g., the Panzihua deposits in China: Zhou et al., 2005; Zhanget al., 2006a, 2008). Other sources are rutile-bearing rockssuch as alkalic plutonic rocks and greenschist to eclogite fa-cies rocks, serving as important sources of white titaniumoxide for industrial uses. Meinhold (2010) demonstrated thatpotentially economic concentrations of rutile can occur inmany different geologic environments. Examples includemedium- to high-grade metamorphic rocks (e.g., Goldsmithand Force, 1978; Force, 1991) and low-grade metamorphicrocks (e.g., Banfield and Veblen, 1991; Luvizotto et al., 2009).Some of these ores have Ti concentrations of up to 5.9 wt %(e.g., Huang et al., 2003; Carruzzo et al., 2006; Zhang et al.,2006b; Triebold et al., 2007). Rutile can also be an accessorymineral in granitoid rocks (e.g., Von Quadt et al., 2005), peg-matites (e.g., Okrusch et al., 2003), carbonatites (e.g.,Doroshkevich et al., 2007), kimberlites and xenoliths of meta-somatized peridotite (e.g., Downes et al., 2007). Titaniumminerals are also found in quartz veins (e.g., Watson, 1922),polymetallic ore deposits (e.g., Scott and Radford, 2007; Rab-bia et al., 2009), and in synmetamorphic quartz veins of eclog-ites (Franz et al., 2001; Gao et al., 2007), sometimes withlarge rutile crystals of more than 4 cm in grain size (Watson,1922; Cerný et al., 1999). It is also possible to find rutile in

fine-grained clastic sedimentary rocks. Globally, less than20% of the titanium resource comes from rutile; in China,only 2.0% of the mined Ti comes from rutile deposits (Jia etal., 2006).

Apart from the rutile deposits in sedimentary rocks (e.g.,Roy, 1999; Parnell, 2004; Dill, 2007; Dill et al., 2007) andmagmatic and low-grade metamorphic rock (e.g., Anderson,1960; Valentine and Commeau, 1990; Hébert and Gauthier,2007), most of the rutile deposits occur in metamorphic ter-ranes. Rutile-bearing eclogite, garnet amphibolites, and ret-rograde eclogite (McLimans et al., 1999; Zhang et al., 2006b)are characterized by the presence of garnet, and such materi-als are challenging to mineral process. Fortunately, a newtype of garnet-free metamorphic rutile deposit, the Daixian-type rutile deposit in Daixian County, Shanxi Province (Xu etal., 2009; Pang et al., 2010 and reference therein) has beenfound in Precambrian metamorphic terranes consisting pre-dominately of anthophyllite-rich gneiss.

This deposit contains 300 Mt of rutile ore, or 6 Mt of tita-nium metal. It is one of the largest and potentially the mostvaluable rutile deposits in China (Xu, 2001, Xu et al., 2004; Jiaet al., 2006). Mineral inclusions and growth features of zir-cons in the deposit reveal that the zircons record at least threedistinct geologic events: magmatism, high-grade metamor-phism, and fluid-related events (Xu et al., 2009), but little isknown about the formation age, forming conditions, and par-agenesis of the deposit. Here we report new petrographic

Ion Microprobe U-Pb Age and Zr-in-Rutile Thermometry of Rutiles from the Daixian Rutile Deposit in the Hengshan Mountains, Shanxi Province, China

GUANGHAI SHI,1,2,† XIANHUA LI,2 QIULI LI,2 ZHENYU CHEN,3 JUN DENG,1 YINGXIN LIU,1 ZHIJUAN KANG,1ERCHENG PANG,4 YONGJING XU,1,4 AND XIUMING JIA5

1 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences,

Beijing 100029, China3 Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China4 217 Geological Department of Shanxi Geological Exploration Bureau, Datong 037008, China

5 Taiyuan University of Technology, Taiyuan, Shanxi 030024, China

AbstractThe Daixian rutile deposit is located in the Hengshan Mountains in the Trans-North China Orogen; it is con-

sidered to be one of the largest rutile deposits in China, with 6 million metric tons (Mt) of contained titanium.Most of the rutile ores are hosted in garnet-free anthophyllite gneiss with minor Mg hornblende, feldspar,quartz, phlogopite, rutile, zircon, and titanite. Rutile grains are euhedral, 0.02 to 0.50 mm in size, contain98.649 to 99.784 wt % TiO2, and form chains, thin layers along the foliation, and dense aggregates. Rutiles arecompositionally homogeneous and contain no detectable mineral inclusions except local ilmenite lamellae andzircon. Crystallization temperatures of the rutile are estimated at ~640°C at 0.7 GPa, and ~647°C without pres-sure calibration according to the Zr-in-rutile thermometer, recording amphibolite facies metamorphism of anintermediate P/T ratio series. Variations in Nb versus Cr in rutiles indicate a connection of the ores to maficprotolith; not a pelitic rock derived from aluminous sedimentary rocks. SIMS U-Pb analyses of rutiles from thedeposit yield a mean 207Pb/207Pb age of 1780.2 ± 9.6 Ma. Considering the closure temperature (up to ~650°C),grain sizes and recrystallization of the rutile, this age is more likely to represent closure and/or recrystallizationtime rather than peak metamorphism period, so the rutile deposit formed not younger than ~1780 Ma. Thisunique garnet-free rutile deposit was metamorphosed from mafic rocks in amphibolite facies during the Paleoproterozoic or Archean, being distinct from any other metamorphic rutile deposits, such as the knowneclogite-related types.

†Corresponding author: e-mail, shiguanghai@263.net.cn; shigh@cugb.edu.cn

©2012 Society of Economic Geologists, Inc.Economic Geology, v. 107, pp. 525–535

Submitted: October 24, 2010Accepted: September 23, 2011

data of the ores, secondary ion mass spectrometry (SIMS) U-Pb dating, and Zr-in-rutile thermometry study on rutiles fromthe deposit.

Geologic SettingThe deposit lies in the southern limb of the Caoduoshan-

Fenshuiling anticline in the Hengshan Mountains of theTrans-North China Orogen. The Trans-North China Orogenis a S-N−trending zone, 100 to 300 km wide and ~1,200 kmlong (Fig. 1A), separating the Eastern and Western Blocksof the North China craton by the Xingyang-Kaifeng-Shiji-azhuang-Jianping fault and the Huashan-Lishi-Datong-Duolun fault, respectively (Zhao et al., 2007). The Trans-North China Orogen consists of the high-grade metamorphiccomplexes of Taihua, Fuping, Hengshan, Huai’an, and Xuan-hua, and the low intermediate -grade metamorphic complexesof Dengfeng, Zhongtiao, Zanhuang, Lüliang, and Wutai(Zhao et al., 2000). Evidence for collisional tectonics isrecorded in linear structural belts with strike- slip ductileshear zones, large-scale thrusting and folding (Li and Qian,1991; Dirks et al., 1997; Zhang et al., 2007), and sheath foldsand strong mineral lineations (Wu and Zhong, 1998; Zhang etal., 2007). High-pressure granulites and retrograde eclogitesare characterized by clockwise metamorphic P-T paths in-volving near-isothermal decompression (Zhai et al., 1993;

Zhao et al., 2001; Guo et al., 2002; O’brien et al., 2005; Zhanget al., 2006c) and by ancient oceanic fragments and ophioliticmélange (Li et al., 1991; Bai et al., 1992; Polat et al., 2005;Zhao and Kröner, 2007).

The Daixian deposit occurs in the Nianzigou Formation ofthe Wutai Group (AW), which contains greenschists to amphi-bolite facies rocks (Fig. 1B) and is divided into the Taizidi(AWt), Nianzigou (AWn) and Binlingou (AWb) Formations (Jiaet al., 2006). The rutile ores are hosted in anthophyllite gneiss,vermiculite gneiss, albite- clinochlore−bearing anthophyllitegneiss, and vermiculite-anthophyllite−bearing quartzite. Thewall rocks are amphibolite with minor leptynite. A sharpboundary is present between the ore layer and the amphibo-lite (Fig. 2A), and both share similar foliations. According tothe 2007 Prospecting Report by the 211 Geological and Min-eral Branch of the Survey and Developing Bureau of Geologyand Resource of Shanxi Province (p. 77), “the rutile ores occuras layers or lenses in the Nianzigou Formation along a NW-W−SE-E trend (200°−210°/35°−40° upper, 200°−210°/30°−35°lower). Individual orebodies are several hundred meters tomore than 1,000 m long, several meters to more than 100 mthick, and several ten meters to several hundred meters deep.The ores has an average of ~2.0 wt % TiO2, locally up to 6.11wt %, and has been mined since 2005”. From west to eastthere are three mining districts: Hongtang, Nianzigou, and

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100°E 105°E 110°E 115°E 120°E 125°E 130°E

40°N40°N

35°N35°N

125°E

30°N

(Qinling Dabie)-

Jiao

-Lia

o-Ji

Bel

t

Central China Orogen

(-

)S

uL

u

Bayan Obo

Changchun

Songpan

Wuhan

BeijingDaixian

Taiyuan

Duolun

X inyang

Shanghai

X i An'

Western Block Eastern Block

Yinshan Block

Ordos Block

Pyeonrang

Seoul

0 200 400 km

Khondalite Belt

TNCO

Jiayuguan

Central China Orogen

Cen

tral

Chi

naO

roge

n

Fig 1B.

Exposed Archaean to Palaeoproterozoic basement

Hidden basement in the eastern and western blocks

Hidden basement in the palaeoproterozoic orogens

Exposed basement in the Trans-North China Orogen

Exposed basement in the Khondalite beltExposed basement in the Jiao-Liao-Ji BeltMajor fault Rutile deposit area A

FIG. 1. A. Simplified geologic map of the North China craton (after Zhao et al., 2001). B. Geologic map of the Daixiandeposit (modified after the 2007 Prospecting Report by the 211 Geological and Mineral Branch of the Survey and Develop-ing Bureau of Geology and Resource of Shanxi Province). TNCO = Trans-North China Orogen.

Yangtingshi (Fig. 1B). Samples were collected from each ofthe districts and then used to make polished thin sections forpetrographic examination and for electronic microprobe(EMP) analysis. One sample named RZ-1 (Fig. 2B, fromHongtang 39°16.257', E113°01.820'), a kind of the commonrutile-rich ore, was used to separate rutiles for acquiring traceelement contents and SIMS U-Th-Pb measurements.

PetrographyThe rutile-bearing anthophyllite gneisses mostly comprise

two amphibole species (Table 1): anthophyllite (50−90 vol %)and magnesiohornblende (Mg hornblende, 5−15 vol %), andplagioclase (5−10 vol %), quartz (0-20 vol %) and phlogopite(0-10 vol %) and accessory rutile (1−6 vol %, up to ~30 vol%), zircon (<1 vol %), apatite (<1 vol %), and titanite (<1 vol%). Garnet is not found in the gneiss.

Anthophyllite occurs as elongated and curved prisms orfibers and is 0.2 to 5.0 mm long. The crystals cut through orsurround Mg hornblende and plagioclase grains. Both Mghornblende and plagioclase occur mostly as isolated grainsand appear to have formed earlier than anthophyllite (Fig.

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100

38p

51p

AWb

AWb 1

AWn2

AWn2

AWn2

AWn2

AWn1

AWn1

AWn1

AWn1 AWn1

AWn1

AWn1

AWn1

AWn1

Q

Q

Q

Q

Q Q

Q

Q

Q

Q

Q

Q

AWt AWt AWt

57p

AWb 2

AWn1

AWn1

Yangtingsi

NianzigouHongtang

2

1

0 500 m

Rutile orebodies

Fault

Thrust

Unknown fault

N

39p 16.257s

113p 01.820s

Sample RZ-1 site

Q

Q

AWb 2

Q Quaternary

2 Purple shale of Cambrian

1Limestone and sandstoneof the Lower Cambrian

Upper Binglinggou formationof the Archean Wutai Group

AWb2

Lower Binglinggou formationof the Archean Wutai Group

AWb 1

AWb1

Upper Nianzigou formationof the Archean Wutai Group

AWn2

AWn1

AWt

Lower Nianzigou formationof the Archean Wutai Group

Taizidi formation of theArchean Wutai Group

Q

Q Q

Q

Q

p B

FIG. 1. (Cont.)

Ath

Rt3 0 cm.

Rutile ore body

Amphibolite

1 0 m.

A

B

FIG. 2. A. Field picture showing a distinct boundary between rutile-bear-ing anthophyllite gneiss (orebody) and amphibolite (wall rock). B. Handspecimen of a common rutile (Rt)-rich ore (sample RZ-1).

3A; Xu et al., 2009, fig. 3a). Quartz occurs as aggregates or assmall worm-like grains along anthophyllite foliation. Syn- tolate- stage phlogopite, locally with tiny titanite lamellae, sur-rounds anthophyllite or occurs along anthophyllite foliation.Anthophyllite is cut by vermiculite, chlorite, and epidote ± al-bite (Fig. 3B). Therefore, there seems to be at least threetypes of mineral association: Mg hornblende + plagioclase,anthophyllite + rutile + quartz (?) + phlogopite (?), and ver-miculite + epidote + chlorite + albite+ phlogopite (?).

Most rutiles occur as euhedral tetragonal crystals or frag-ments and are 0.02 to 0.50 mm long. They are mostly translu-cent and dark red to dark brown. Some have ilmenite exsolu-tion lamellae (Fig. 3C). Rutiles occur as chains or thin layersparallel to the foliation, even where the foliation is curved(Fig. 4A), or form dense aggregates (Fig. 4B). In contrast toanthophyllites which show obvious ductile deformation andpronounced shape-preferred orientation, rutile grains exhibitboth brittle and ductile deformation; they are either broken,curved, subgrained, or twinned (Fig. 4C). Zircon inclusionsappear inside rutile (Fig. 5A). Rutile does not display obviousgrowth zoning on the BSE image (Fig. 5B) and has no crys-tallization gaps.

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TABLE 1. Representative Chemical Compositions and Calculated Formulas of Amphiboles of the Rutile Ores

Points 01 02 03 04 05 06 07

SiO2 (wt %) 53.67 52.76 52.45 53.03 47.66 47.95 47.19TiO2 0.18 0.14 0.12 0.11 0.45 0.37 0.45Al2O3 4.77 6.56 6.07 6.18 13.59 12.46 13.34FeO 13.78 13.27 13.25 13.60 7.30 7.06 7.71Fe2O3 1.21 1.12 1.56 1.03 0.00 0.00 0.00Cr2O3 0.56 0.66 0.53 0.56 0.12 0.10 0.08MnO 0.13 0.03 0.11 0.07 0.1 0.12 0.16MgO 23.21 22.79 22.62 22.83 16.18 16.3 16.24CaO 0.46 0.62 0.54 0.52 11.21 11.55 10.98Na2O 0.03 0.01 0.00 0.01 1.47 1.49 1.48K2O 0.03 0.03 0.04 0.05 0.12 0.08 0.09Total 98.03 97.99 97.29 97.99 98.20 97.48 97.72

TSi 7.49 7.35 7.37 7.39 6.68 6.78 6.67TAl 0.51 0.65 0.63 0.61 1.32 1.23 1.33Sum T 8.00 8.00 8.00 8.00 8.00 8.00 8.00CAl 0.27 0.42 0.37 0.40 0.93 0.85 0.89CCr 0.06 0.07 0.06 0.06 0.01 0.01 0.01CFe3+ 0.13 0.12 0.17 0.11 0.00 0.00 0.00CTi 0.02 0.02 0.01 0.01 0.05 0.04 0.05CMg 4.52 4.37 4.39 4.41 3.38 3.43 3.42CFe2+ 0.00 0.00 0.00 0.00 0.63 0.67 0.64CMn 0.00 0.00 0.00 0.00 0.00 0.00 0.00CCa 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum_C 5.00 5.00 5.00 5.00 5.00 5.00 5.00BMg 0.31 0.36 0.35 0.33 0.00 0.00 0.00BFe2+ 1.61 1.54 1.56 1.58 0.23 0.17 0.28BMn 0.02 0.00 0.01 0.01 0.01 0.01 0.02BCa 0.07 0.09 0.08 0.08 1.68 1.75 1.66BNa 0.00 0.00 0.00 0.00 0.08 0.07 0.04Sum_B 2.00 2.00 2.00 2.00 2.00 2.00 2.00ANa 0.01 0.00 0.00 0.00 0.33 0.34 0.36AK 0.01 0.01 0.01 0.01 0.02 0.01 0.02Sum_A 0.01 0.01 0.01 0.01 0.35 0.35 0.38Sum-cat 15.01 15.01 15.01 15.01 15.35 15.35 15.36Name anthophyllite Mg hornblende

0.2 mm

Rt

Rt

Rt Rt

Am

Ath

Ath

Ath

Am

0.2 mm

Ath

Ep

Ap

Chl

Ep

Chl

Chl

Ath

Ath

A

B

C

30 um

Rt

Ath

Ath

Ilm

FIG. 3. Backscattered electron images of the rutile ores illustrating that(A) anthophyllite (Ath) and rutile (Rt) formed later than Mg amphibole(Am); (B) epidote (Ep) and chlorite (Chl) formed later than anthophyllite;(C) rutile has ilmenite (Ilm) exsolution lamellae (Ap = apatite; all mineral ab-breviations are after Whitney and Evans, 2010).

Analytical Methods and ResultsBackscattered electron (BSE) images and chemical compo-

sitions of amphiboles were acquired at the Institute of Geol-ogy and Geophysics, Chinese Academy of Sciences, using aJXA-8100 Electron Microprobe Analyzer (EMPA) with a voltage of 15 kV, a beam current of 10 nA, and a spot size lessthan 10 µm. EMPA standards include the following minerals:

andradite for Si and Ca, rutile for Ti, corundum for Al,hematite for Fe, eskolaite for Cr, rhodonite for Mn, bunsen-ite for Ni, periclase for Mg, albite for Na, and K-feldspar forK (Shi et al., 2010). All mineral formulas were recalculatedusing the software MINPET 2.0. Amphibole formulas werecalculated by choosing a method based on “15 NK.” The com-positions of amphiboles are reported in Table 1.There are twoamphibole species, anthophyllite and magnesiohornblende,according to the nomenclature of amphiboles (Leake et al.,1997).

Rutile crystals from sample RZ-1 were first concentrated bycrushing, sieving, gravity separation, electromagnetic separa-tion, then by handpicking under a binocular microscope, andthen cast in a transparent epoxy mount together with the R10rutile standard (Luvizotto et al., 2009) and a well-dated99JHQ-1 rutile (Li et al., 2003). The mount was abraded andpolished. After thorough cleaning, the mount was vacuum-coated with high-purity gold prior to ion microprobe analysis.

Measurements of U-Pb isotopes were performed using aCameca IMS 1280 ion microprobe at the Institute of Geologyand Geophysics, Chinese Academy of Sciences, with analyti-cal procedures described by Li et al. (2011). The O–

2 primary

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A

1.0 mm

0.4 mm

0.4 mm

B

C

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Ath

Ath

Ath

Ath

Ath

FIG. 4. Photomicrographs of the rutile ores displaying: (A) that rutilesoccur along the curved foliation formed predominantly by anthophyllite; (B)dense rutile aggregate; (C) ductile and brittle deformed rutiles.

100 mµ

Zrn

Rt

Ath

Zrn

RtIlm

400 mµ

Rt

Rt

A

B

Rt

Rt

FIG. 5. BSE images exhibiting the following: (A) that zircons (Zrn) occurinside and adjacent to rutile grain; (B) no obvious growth zoning inside rutilegrains.

ion beam with an intensity of ca. 15 nA was used to produceeven sputtering over the entire analyzed area of about 20 ×30 µm. The Pb/U ratios were calibrated with a power law re-lationship between Pb/U and UO2/U relative to the R10 rutilestandard dated at 1095 Ma (Luvizotto et al., 2009) and moni-tored by the 99JHQ-1 rutile dated at 218 Ma by ID-TIMS (Liet al., 2003). The standard deviation of Pb/U values of the ref-erence curve was propagated together with errors from theunknowns to give an overall error for the Pb/U ratio of eachanalysis (Li et al., 2010, 2011). U concentrations were calcu-lated by U+ ion yield which is estimated from the R10 stan-dard with 30 ppm U (Luvizotto et al., 2009). As rutile usuallycontains very low concentrations of Th, it is favorable in theU/Pb dating, using the 208Pb-based common Pb correction(Clark et al., 2000; Luvizotto et al., 2009; Li et al., 2011). Un-certainties on individual analyses in data tables are reportedat 1� level; mean ages for pooled U/Pb (and Pb/Pb) analysesare quoted with 95% confidence interval. Data reduction wascarried out using the Isoplot/Ex v. 2.49 Program (Ludwig,2001). Thirty analyses (Table 2) were obtained from 30 rutilegrains or fragments, and all these data form a tight cluster andyield a mean 207Pb/206Pb age of 1780.2 ± 9.6 Ma (Fig. 6A) andconcordant age of 1777 ± 10 Ma (Fig. 6B). The concordanceof the data is a strong indication that no postcrystallization re-mobilization of Pb has occurred.

Polished thin section of separated rutile sands from thesample RZ1 was prepared to establish the Zr, Nb, Cr, and Fecontents in rutiles. Analyses on rutiles were performed on a

JEOL JXA-8800R EMPA at the Institute of Mineral Re-sources, Chinese Academy of Geological Sciences. In orderto get a lower detection limit than the normal (Zack et al.,2004a), beam current and counting time were accordingly in-creased with analyses conditions of 20 kV accelerating volt-age, 100 nA beam current, and 5 µm beam spot. Countingtimes for Zr, Nb, Cr, and Fe are 300, 400, 150, and 60 s, re-spectively. Spectroscopic crystal for Zr, Nb, Cr, and Fe arePETH, PETJ LIFJ, and LIFJ, with respective detection lim-its of 20, 27, 28, and 43 µg·g−1 (respective detection limits areyielded directly by JXA-8800R EMPA, also see Chen and Li,2008). Rutile was analyzed in two sequences except Nb in thesecond sequence. Zircon standard and a synthetic rutile wereadopted to calibrate the peak position of Zr and the zero con-centration of Zr at the beginning, middle, and the end of eachsequence. Analytical errors are about ±15 µg·g−1 for Zr con-centrations at 1� according to counting statistics (Chen and Li,2008). Chemical compositions and estimated crystallizationtemperatures of the rutiles are listed in Table 3. Rutiles con-tain 98.649 to 99.784 wt % TiO2 with minor ZrO2 (0.023−0.0071 wt %, corresponding to 320 ± 80 ppm Zr), Cr2O3

(0.027−0.056 wt %), Nb2O5 (0.000−0.070 wt %), and variableFeO (0.011−0.816 wt %). Very low SiO2 (<0.007%) concen-trations show that the EMPA data are not affected by zirconinclusions, in other words, EMPA points are not on the inclu-sions. They are much closer to rutile end member than thosereported in eclogites, like at Jinheqiao in the Dabie Orogen hav-ing 93.785 to 99.242 wt % TiO2, with mostly ≤ 98.649 wt %

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TABLE 2. SIMS Rutile U-Th-Pb Analyses of the Daixian Rutile Deposit

U Th/ f206& 208Pb/ 238U / ±1σ 207Pb/ ±1σ 207Pb*/ ±1σ 206Pb*/ ±1σ t207/206 t207/235 t206/238Spot no. (ppm) U (%) 206Pb 206Pb (%) 206Pb (%) 235U (%) 238U (%) Rho (Ma) ±1σ (Ma) ±1σ (Ma) ±1σ

1 15.1 0.0042 0.12 0.0037 3.03 4.1 0.1100 1.3 4.96 4.34 0.33 4.12 0.95 1783 24 1810 37 1838 662 15.7 0.0063 0.06 0.003 3.05 4.2 0.1108 1.3 4.98 4.38 0.328 4.17 0.95 1804 24 1811 38 1825 673 14.7 0.0001 0.08 0.0016 3.07 4.1 0.1095 1.8 4.88 4.51 0.325 4.12 0.91 1780 33 1800 39 1816 654 14 0.0000 0.12 0.0024 3.04 4.1 0.1115 1.3 5.00 4.33 0.329 4.12 0.95 1807 24 1821 37 1831 665 21.5 0.0342 0.13 0.0128 2.97 4.1 0.1092 1.1 5.02 4.29 0.337 4.12 0.96 1767 22 1791 37 1863 676 9.0 0.0007 0.1 0.0023 2.91 4.1 0.1096 1.2 5.15 4.3 0.343 4.12 0.96 1778 22 1844 37 1903 687 17.5 0.0153 0.13 0.0072 3.1 4.1 0.1111 1.4 4.89 4.38 0.322 4.12 0.94 1798 27 1787 37 1798 658 24.2 0.0024 0.1 0.0027 3.15 4.1 0.1112 1.5 4.83 4.38 0.317 4.12 0.94 1805 27 1789 38 1777 649 18.7 0.0204 0.27 0.0116 3.23 4.1 0.1121 1.7 4.67 4.5 0.309 4.12 0.92 1796 33 1746 38 1730 6310 19.6 0.0038 0.09 0.003 3.2 4.1 0.1114 1.4 4.76 4.37 0.312 4.14 0.95 1810 26 1776 37 1751 6411 22.6 0.0096 0.13 0.0055 3.28 4.1 0.1103 1.5 4.58 4.4 0.304 4.12 0.94 1786 28 1738 37 1709 6212 19.3 0.0115 0.13 0.0061 3.23 4.1 0.1104 1.5 4.66 4.41 0.309 4.12 0.94 1787 28 1750 37 1734 6313 16 0.0000 0.14 0.0029 3.21 4.1 0.1129 1.7 4.80 4.46 0.311 4.12 0.92 1827 31 1785 38 1748 6314 16.5 0.0007 0.16 0.0035 3.29 4.1 0.1102 1.9 4.56 4.55 0.304 4.12 0.91 1781 35 1742 39 1709 6215 16.4 0.0009 0.19 0.0043 3.23 4.2 0.1111 2.6 4.67 4.94 0.309 4.18 0.85 1790 47 1762 42 1738 6416 20.4 0.0045 0.14 0.0043 3.29 4.2 0.1112 1.6 4.60 4.48 0.303 4.16 0.93 1799 30 1746 38 1707 6317 23.6 0.0052 0.24 0.0065 3.16 4.1 0.1151 1.5 4.92 4.4 0.315 4.12 0.94 1849 28 1803 38 1766 6418 11.7 0.0175 0.35 0.0125 2.91 4.1 0.1104 1.2 5.07 4.28 0.342 4.12 0.96 1755 21 1858 37 1904 6819 15.1 0.0001 0.11 0.0023 3.06 4.1 0.1095 2.2 4.89 4.68 0.326 4.12 0.88 1776 40 1801 40 1821 6620 15.4 0.0023 0.1 0.0027 3.02 4.2 0.1101 1.4 4.98 4.44 0.331 4.21 0.95 1787 25 1815 38 1841 6821 18.5 0.0038 0.09 0.0031 3.26 4.1 0.109 1.5 4.58 4.41 0.307 4.12 0.94 1770 28 1742 37 1724 6322 19.1 0.0006 0.08 0.002 3.3 4.2 0.1092 1.5 4.53 4.46 0.303 4.17 0.94 1774 28 1737 38 1706 6323 20.6 0.0003 0.07 0.0015 3.25 4.1 0.1095 1.6 4.61 4.45 0.307 4.14 0.93 1781 29 1752 38 1726 6324 16.2 0.0001 0.07 0.0016 3.28 4.1 0.1092 1.6 4.56 4.42 0.305 4.13 0.93 1775 29 1743 38 1714 6225 12.8 0.0000 0.07 0.0016 3.19 4.1 0.1102 1.6 4.73 4.43 0.313 4.12 0.93 1791 29 1773 38 1756 6426 17.6 0.0171 0.25 0.0101 3.16 4.1 0.109 1.3 4.65 4.33 0.316 4.12 0.95 1746 24 1777 37 1773 6427 16.1 0.0008 0.1 0.0023 3.1 4.2 0.1072 1.3 4.72 4.37 0.322 4.17 0.95 1738 24 1778 37 1800 6628 28.3 0.0252 0.2 0.0115 3.14 4.1 0.1069 1.1 4.61 4.27 0.318 4.12 0.96 1717 21 1766 36 1782 6529 19.3 0.0129 0.2 0.008 3.14 4.1 0.1097 1.4 4.73 4.36 0.318 4.12 0.94 1765 26 1762 37 1775 6430 17.8 0.0125 0.17 0.0072 3.17 4.1 0.1105 1.7 4.74 4.46 0.315 4.12 0.92 1783 31 1764 38 1763 64

Note: f206& is the percentage of common 206Pb in total 206Pb

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FIG. 6. Diagrams of (A) mean age and (B) concordia age of SIMS 207Pb/206Pb analyses on rutiles from the Daixian rutiledeposit.

A B

Age

(Ma)

206 P

b/2

38U

207Pb/236U

TABLE 3. Chemical Compositions of Rutiles of the Daixian Rutile Deposit and Corresponding Estimated Crystallization Temperatures

SiO2 ZrO2 TiO2 Nb2O5 FeO Cr2O3 Zr Nb Cr T1 T2 T3

No. Mineral (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) Total (ppm) (ppm) (ppm) (°C) (°C) (°C)

1 Zrn 32.598 66.363 0.003 0.005 0.100 0.000 99.069 4913522 Rt 0.000 0.060 99.610 0.009 0.035 0.056 99.779 444 63 383 676 668 7693 Rt 0.000 0.056 99.759 0.020 0.032 0.027 99.899 415 140 185 670 662 7604 Rt 0.000 0.057 99.354 0.007 0.019 0.029 99.466 422 49 198 671 664 7635 Rt 0.000 0.049 99.784 0.044 0.346 0.040 100.267 363 308 274 658 651 7436 Rt 0.000 0.045 99.474 0.047 0.443 0.040 100.049 333 329 274 651 645 7327 Rt 0.000 0.049 98.936 0.048 0.324 0.035 99.392 363 336 239 658 651 7438 Rt 0.000 0.059 99.274 0.022 0.011 0.029 99.395 437 154 198 674 666 7679 Rt 0.000 0.053 98.836 0.016 0.035 0.048 98.988 392 112 328 665 658 75310 Rt 0.000 0.059 99.675 0.026 0.039 0.034 99.835 437 182 233 674 666 76711 Rt 0.000 0.031 98.669 0.062 0.326 0.041 99.137 230 433 281 621 616 68512 Rt 0.000 0.040 98.606 0.045 0.283 0.049 99.023 296 315 335 641 635 71713 Rt 0.001 0.071 99.184 0.048 0.322 0.036 99.668 526 336 246 691 682 79114 Rt 0.000 0.041 99.277 0.053 0.368 0.041 99.780 304 370 281 643 637 72115 Rt 0.000 0.036 99.126 0.054 0.359 0.042 99.617 267 377 287 633 628 70416 Rt 0.000 0.035 98.799 0.048 0.296 0.053 99.231 259 336 363 631 625 70017 Rt 0.007 0.050 98.823 0.047 0.551 0.048 99.535 370 329 328 660 653 74618 Rt 0.000 0.033 99.583 0.041 0.816 0.048 100.525 244 287 328 626 621 69319 Rt 0.000 0.036 99.755 0.036 0.270 0.035 100.132 267 252 239 633 628 70420 Rt 0.000 0.037 99.192 0.046 0.302 0.046 99.623 274 322 315 635 629 70721 Rt 0.000 0.040 98.649 0.070 0.325 0.041 99.130 296 489 281 641 635 71722 Zrn 33.140 65.802 0.003 0.000 0.089 0.000 99.034 48719823 Rt 0.000 0.041 99.122 0.035 0.402 0.049 99.651 304 245 335 643 637 72124 Rt 0.000 0.032 98.862 0.049 0.344 0.036 99.323 237 343 246 624 619 68925 Rt 0.000 0.038 98.930 0.030 0.355 0.038 99.400 281 210 260 637 631 71126 Rt 0.000 0.038 99.021 0.046 0.275 0.053 99.438 281 322 363 637 631 71127 Rt 0.000 0.023 99.511 0.010 0.023 0.027 99.594 170 70 185 599 595 64628 Rt 0.000 0.039 98.985 bdl. 0.043 0.069 99.138 289 bdl. 472 639 634 71429 Rt 0.000 0.032 99.483 bdl. 0.037 0.048 99.600 237 bdl. 328 624 619 68930 Rt 0.000 0.050 98.886 0.005 0.027 0.042 99.010 370 35 287 660 653 74631 Rt 0.000 0.042 99.684 0.012 0.027 0.039 99.804 311 84 267 645 639 72432 Rt 0.000 0.039 99.316 0.008 0.045 0.049 99.457 289 56 335 639 634 71433 Zrn 33.756 65.269 0.004 0.000 0.105 0.000 99.134 483252Average ± St. dev. 324 ± 80 647 ± 20 640 ± 19 725 ± 32

Note: bdl = below detection limit1, 2, 3 Temperatures were estimated according to the thermometry by Watson et al. (2006), Tomkins et al. (2007) at 0.7 GPa, and Zack et al. (2004a),

respectively

TiO2 (Chen and Li, 2008). Crystallization temperatures wereestimated according to the Zr-in-rutile thermometry by Zacket al. (2004a), Watson et al. (2006), and Tomkins et al. (2007)at 0.7 GPa. Mean temperatures are shown with 1� standarddeviation (Table 3).

Discussion

Crystallization temperatures of the rutiles and the protolith for the deposit

The Zr content of natural rutile coexisting with zircon orother Zr-rich minerals shows a systematic variation attributedto difference in crystallization temperature (Zack et al.,2004a; Watson et al., 2006). In addition, as the expected vol-ume of rutile changes as Zr4+ substitutes for Ti4+ and the Zrcontent in rutile decreases with increasing pressure, sec-ondary pressure effect has been introduced into a thermom-etry function (Tomkins et al., 2007). Currently there are sev-eral Zr-in-rutile thermometers with or without pressurecalibrations (e.g., Zack et al., 2004a; Watson et al., 2006;Tomkins et al., 2007), which can be adopted for estimatingcrystallization temperatures of natural rutiles. In the Daixiandeposit, zircon occurs inside or adjacent to the rutile crystals(Fig. 5A), quartz occurs as aggregates in the ore and as inclu-sions in overgrowth zone of zircon (Xu et al., 2009), indicat-ing that rutile formed in the presence of zircon and quartzand thus allowed for application of the Zr-in-rutile geother-mometry. In this investigation, three Zr-in-rutile thermome-ters:

T (°C) = (4470 ± 120)/((7.36 ± 0.10) − log (Zrrutile)) − 273(Watson et al., 2006)

T (°C) = (85.7 + 0.473 P)/(0.1453 – R ln (Zr in ppm)) − 273(Tomkins et al., 2007)

T (°C) = 127.8 × ln (Zr in ppm) − 10 (Zack et al., 2004a)

are introduced to estimate crystallization temperatures. Thecrystallization temperature of the rutile was 647° ± 20°C (n =30) without applying the pressure dependence calibration byWatson et al. (2006), a higher temperature of 725° ± 31°C (n= 30) based on Zack et al. (2004a), and 640° ± 19°C (n = 30)with a 0.7 GPa calibration (Tomkins et al., 2007; 0.7 GPa wasused as pressure calibration by referring to Zhao et al. (2000)and Xu et al. (2002)). All the estimated temperatures (Table3) indicate an amphibolite metamorphic facies of intermedi-ate P/T ratio facies series.

A compilation of measured rutiles data indicates that varia-tions of Nb versus Cr in rutile can be used to distinguish be-tween rutile formed in metabasites and metapelites (e.g.,Meinhold et al., 2008; Meinhold, 2010). Rutile is extremelycompatible for Nb compared to most metamorphic phases,concentrating >90% of the available Nb and Ti in a givenrock, therefore, Nb/Ti ratio of source rock is mirrored in ru-tile (e.g., Barth et al., 2000). On the other hand, chromium isnot selective for most metamorphic minerals, more or lessequally distributing between available minerals, and conse-quently the Cr content in the source rock is mirrored in rutile(e.g., Zack et al., 2004b). Based on literature data of Nb/TiO2

ratios of whole rock for pelites, rutile in metapelites contains900 to 2,700 ppm Nb (Zack et al., 2004b). A lower limit of Nb

in metapelitic rutiles set at 800 ppm is proposed by Meinholdet al. (2008). Rutiles with Cr < Nb accompanied by Nb >800ppm are interpreted to be derived from metapelitic rocks,whereas rutiles with Cr > Nb and those with Cr < Nb accom-panied by Nb <800 ppm are interpreted to be derived frommetamafic rocks (Meinhold et al., 2008; Meinhold, 2010). Inthis investigation, all the Nb contains of rutiles are less than800 ppm; the highest one is 489 ppm, with two even less thanthe detection limit (27 µg.g−1). Nb versus Cr of all the rutilesthus fall into area on the plot with Cr > Nb and those with Cr< Nb accompanied by Nb <800 ppm (Fig. 7), indicating aconnection to mafic rocks, not pelitic rocks derived from alu-minous sediments.

Interpretation of SIMS rutile U-Pb dating

Closure temperature (Tc), crystallization temperature,growth patterns, and mineral inclusion of rutile are four keyfactors in the interpretation of the SIMS U-Pb age data. Noother mineral inclusions except ilmenite and zircon occur inthe rutile. Zircon inclusions could potentially be very impor-tant to rutile U-Pb geochronology, as zircons tend to havehigher U + Th contents than rutile. However, no zircon in-clusions affect the SIMS rutile U-Pb dating, neither doesgrowth pattern, since all rutiles yield a tight cluster of coher-ent ages. Therefore, two key constraints remain on the age in-terpretation: closure temperature and crystallization temper-ature. Unfortunately, the closure temperature for U-Pbsystems in rutile is still controversial. Vry and Baker (2006)suggested that even small rutile is extremely resistant to iso-topic resetting, and rutile has a higher Tc (ca. 600°–640°C forrutile 0.1–0.2 mm diameter cooled at 3°C/m.y.), which is inagreement with experimental results of near 600°C (Cher-niak, 2000). In contrast, a much lower Tc (~400−C) for rutileshas been revealed by Mezger et al. (1989). This Tc subse-quently has been verified and used for nearly two decades asa relatively low temperature geochronometer that can pro-vide timing constraints on metamorphic cooling (e.g., Hea-man and Parrish, 1991; Mezger et al.,1991; Santos Zaldueguiet al., 1996; Li et al., 2003; Treloar et al., 2003; Timmermannet al., 2004). However, it is still not clear whether ~400° or~600°C are better estimates of the Tc. Recent investigationshows that this temperature is strongly dependent on grain

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log(Cr/Nb)= 0

Rutile frommetapelitic rocks

0 500 1000 1500 2000 2500 30000

1000

2000

3000

4000

5000

Nb (ppm)

Cr(p

pm)

rutile frommetamafic rocks

FIG. 7. Nb vs. Cr discrimination diagram for rutiles from the Daixian ru-tile deposit according to Meinhold et al. (2008) and Meinhold (2010).

size; general consensus of ca. 600°C is a good estimate for thecores of grains over ca. 200 um in diameter but can be muchlower near the rims due to volume diffusion (see Kooijman etal., 2010).

All three of the calculated crystallization temperatures ofthe rutiles from Daixian using Zr-in-rutile geothermometryare higher than the lower Tc (Tc = ~400°C) of U-Pb systemrecognized in rutile, even a little higher than the highest Tc

(ca. 600°–640°C) revealed by Vry and Baker (2006). In addi-tion, microphotograph and BSE images of the ores show thatthe rutiles can be up to 0.02 to 0.50 mm long, but almost alllarge rutile grains were broken and most of the fragments areless than 150 µm (Figs. 3A, 4B, C, 5A, B). The tight cluster ofall data on the rutiles shows no distinct difference betweenthe core and the rim, suggesting that detectable volume dif-fusion did not take place. Moreover, the extensive brittle-duc-tile deformation of rutile (Fig. 4C) is potentially another fac-tor which could reduce the Tc, since recrystallization inducedby deformation would facilitate diffusions of the incorporatedions across inhomogeneous intracrystal or among neighborcrystals at lower temperature (e.g., Stünitz, 1998; Bestmannet al., 2008; Shi et al., 2009). However, given the analytical(ca. 20°C) and additional calibration uncertainty (ca. 20°Cdifference between calibrations, and pressure estimates),there is still a little possibility that the mean Zr thermometrytemperatures may overlap an estimate of Tc.

Therefore, mean 207Pb/206Pb age (1780.2 ± 9.6 Ma, Fig. 6A)of rutiles in the Daixian deposit are more likely to reflect clo-sure time or recrystallization time relative to the original crys-tallization age, suggesting that the peak metamorphic eventcoeval with formation of the rutile took place not later than~1780 Ma. In summary, this unique garnet-free rutile depositformed at the expense of mafic rocks in amphibolite faciesduring the Paleoproterozoic or even possibly in the Archean,being distinct from any other metamorphic rutile deposits,such as the known eclogite-related types.

AcknowledgmentsWe are indebted to M. X. Yang and M. L. Liang for their

kind support during the field trip. Thoughtful comments, en-couragement, and editorial handling from journal editorsPeter C. Lightfoot and Larry Meinert, constructive com-ments by F. Corfu and P. Ni, and input, corrections fromJames Darling are appreciated, from which the manuscripthas greatly benefited. This research was financially supportedby the National Science Foundation of China (90914008), theProgram for the New Century Excellent Talents in China(NCET-07-0771), and the Fundamental Research Funds forthe Central Universities (2001YXL048).

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