low-temperature eclogites from southwestern Chinese Tianshan, NW China

SIMS U –Pb rutile age of low-temperature eclogites from southwestern Chinese Tianshan,NW China
Qiu-li Li a ,Wei Lin a ,⁎,Wen Su a ,Xian-hua Li a ,Yong-hong Shi b ,Yu Liu a ,Guo-qiang Tang a
a State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China b
Department of Geology,School of Resource and Environment Engineering,Hefei University of Technology,Hefei,Anhui 230009,China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 28July 2010
Accepted 15November 2010Available online xxxx
Keywords:SIMS
Rutile U –Pb age Eclogite
Southwestern Chinese Tianshan Central Asia Orogenic Belt
The geological evolution in the southwestern Chinese Tianshan orogen in NW China is poorly understood because of controversial geochronological results of the low-temperature eclogites.It is likely due to Sm –Nd and Rb –Sr isotopic disequilibrium in different mineral fractions,excess 40Ar in the high-pressure K-bearing minerals,and multi-stage growths of zircon in U –Pb system.Rutile,as a typical eclogite-facies mineral with high U/Pb ratio,is a feasible candidate to constrain the eclogite-facies metamorphic event.In this study,we measured U –Pb rutile age data using SIMS (Cameca IMS 1280)for low-temperature eclogites from southwestern Chinese Tianshan orogen,NW China.We demonstrate that SIMS is a powerful technique for in-situ U –Pb dating of rutile with U content as low as 1ppm.SIMS U –Pb rutile dating yielded an age of 318±7Ma,which may record the time when the rocks cooled down to the closure temperature of Pb in rutile (ca.500°C),but should be very close to the time of eclogite-facies metamorphism in terms of common fast exhumation of coesite-bearing eclogite.This new proof strongly suggests a late Carboniferous collision between Kazakhstan –Yili block and Central Tianshan microcontinent.
©2010Elsevier B.V.All rights reserved.
1.Introduction
The southwestern Chinese Tianshan orogen,composed of blues-chists,eclogites and metapelites,has been given special consideration in understanding the amalgamation of Eurasia and the Phanerozoic continental growth of the Altaïds (Fig.1,e.g.Sengör and Natal'in,1996;Xiao et al.,2004,2009;Charvet et al.,2007;Windley et al.,2007;Kröner et al.,2008;Lin et al.,2009).These rocks experienced high-pressure (HP)to ultrahigh-pressure (UHP)metamorphic conditions (e.g.,Lüet al.,2008),but low peak metamorphic temperature of 480–580°C (e.g.Klemd et al.,2002;Lin and Enami,2006;Wei et al.,2003).These high-pressure low-temperature (HP/LT)metamorphic rocks have provided important information for the tectono-thermal processes,however,the timing of this eclogite-facies metamorphism is still a heated debate,such as ~345Ma (e.g.Gao and Klemd,2003;Klemd et al.,2005;Wang et al.,2010),233–226Ma (Zhang et al.,2007),and ~320Ma (Su et al.,2010).
Integration of isotope geochronology with other geologic parameters such as pressure –temperature,mineral composition or structural –textural information is crucial to quantify the dynamics of geological process (Müller,2003).Generally,rock-forming minerals,rather than accessory phases,de fine metamorphic faces and provide important structural,petrographical and petrological observations.While numer-
ous successful attempts have been made to date high-temperature metamorphic minerals using Sm –Nd,Rb –Sr and Ar –Ar isotopic systems,dating of rock-forming minerals by these methods in the low-temperature metamorphic rocks is often hampered due to the following dif ficulties:(1)These commonly-used isotope chronometers need large amount of pure mineral fractions for precise isotope measurements.However,it is hard to obtain pure mineral concentrates because of intergrowths and small adhering mineral particles in isotopic disequi-librium with the analyzed minerals (e.g.Li et al.,2000;Mezger et al.,1989);(2)initial isotopic disequilibrium among different fractions in low-temperature metamorphic rocks may produce error for Sm –Nd and Rb –Sr ages determined by isochron regression of a suite of whole-rock and/or different mineral fractions (e.g.,Jahn et al.,2005;Li et al.,2000;Luais et al,2001;Thöni and Jagoutz,1992);(3)The occurrence of variable amounts of extraneous argon is an often-described phenome-non in eclogitic minerals (e.g .,Li et al.,1994);(4)Rock-forming minerals in HP/UHP rocks that experienced complicated metamorphic events and/or deformations usually show compositional zonation and/or multi-stage growth history (e.g.Lin and Enami,2006;Zhang et al.,2007).
U –Pb method has been widely applied to age determinations of zircons in metamorphic rocks.However,as an accessory mineral,zircon can overgrow during different metamorphic stages (Chen et al.,2010;Hoskin and Schaltegger,2003;Xia et al,2009),which makes the interpretation of U –Pb dates unclear,and sometimes even contradictive with each other.For instance,metamorphic zircons in the low-temperature eclogites from southwestern Chinese Tianshan (NW China)were dated producing controversial ages of ~230Ma by Zhang
Lithos xxx (2010)xxx –xxx
⁎Corresponding author.Tel.:+861082998546;fax:+861062010846.E-mail address:linwei@ (W.Lin).
LITHOS-02356;No of Pages 11
0024-4937/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.lithos.2010.11.007
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Lithos
j ou r n a l h o m e pa g e :ww w.e l s ev i e r.c o m/l o c a t e /l i t h o s
et al.(2007)and ~320Ma by Su et al.(2010)using SIMS U –Pb technique,leading to divergent geological interpretations of these U –Pb zircon ages.
As a typical eclogite-facies mineral,rutile has also be used for U –Pb age determination (e.g.,Kooijman et al.,2010;Kylander-Clark et al.,2008;Li et al.,2003;Meinhold,2010;Mezger et al.,1989;Vry and Baker,2006and references therein).However,traditional isotope dilution-TIMS method to date rutile is challenging because rutile often has retrograde rim of titanite,and mineral inclusions with high proportion of common lead.Considered that secondary ion mass spectrometry (SIMS)analyses are performed on sample surface with ca.1μm in depth,it is easy to observe and avoid the possible inclusions and fractures,even based on re flecting images.In this regard,SIMS had been shown as a powerful tool for in-situ U –Pb dating of rutile,however,only applied for Precambrian rutile samples because of low U content (Clark et al.,2000;Ireland and Williams,2003).In this study we carried out SIMS U –Pb age determinations on rutile from the southwestern Chinese Tianshan eclogites.Though with very low U content,rutile gave a U –Pb age of 318±7Ma to constrain the regional eclogite-facies metamorphism in late Carboniferous.2.Regional geology and sample descriptions
The Tianshan Orogenic Belt is the southernmost part of the Central Asian Orogenic Belt (CAOB)and it recorded the process of the accretionary orogenesis of CAOB during the Paleozoic (Sengör and Natal'in,1996;Xiao et al.,2004;Windley et al.,2007;Kröner et al.,2008Fig.1).It extends broadly east –west for over 2500km and exhibits some of the highest relief on Earth,with the present topography being attributed to the far-field tectonic stress related to the Tertiary Asia –India collision (Avouac et al.,1993;Sobel and Dumitru,1997).During the Phanerozoic,accretion of several continental blocks,island arcs and accretionary complexes to the southern margin of Eurasia formed the huge Altaid orogenic collage within which the Tianshan Belt formed by amalgamation of the Tarim,Junggar and Kazakhstan –Yili blocks and intervening microcontinents (Kröner et al.,2008;Wang et al.,1994;Windley et al.,2007).
The SW Chinese Tianshan HP/LT belt connects westward with the Atbashe and Mailisu eclogite –blueschist belt in Kyrgyzstan (Sobolev et al.,1986;Tagiri et al.,1995)and the Karategin blueschist belt in Tajikistan (Volkova and Budanov,1999).Eastward,north of Kumux,it connects with the Gangou –Mishigou mélange (Gao et al.,1995).Blueschists and eclogitic rocks occurred sporadically along convergent plate margins of southern margin of Kazakhstan –Yili block (Fig.2,e.g.Sobolev et al.,1986;Dobretsov et al.,1987;Tagiri et al.,1995;Gao et al.,1999;Klemd et al.,2002;Gao and Klemd,2003;Zhang et al.,2007;Lüet al.,2008).A large number of mineralogical,petrological,and geochemical investigations on these HP/UHP metamorphic rocks have been reported by Gao et al.(1995,1999),Zhang et al.(2002,2005),Wei et al.(2003),Gao and Klemd (2003)and Lin and Enami (2006)and references therein.Mineralogical features,such as polycrystalline quartz aggregates and coesite in garnet,quartz lamellae in omphacite,magnesite-bearing glaucophane –eclogite,magnesite and calcite inclu-sions in dolomite and primary carbonates and possibly coesite remains,suggest that eclogites in this area experienced UHP metamorphism (Lüet al.,2009;Zhang et al.,2002,2005,2007),but see Klemd (2003)for a different view.While there is a consensus that these HP and UHP rocks are derived from oceanic ma fic magmatic and sedimentary rocks (Gao and Klemd,2003and references therein),the timing of the metamorphism and its tectonics are still controversial (Lin et al.,2009and references therein).
Three samples (TS193,ZS3and 07RU3)were collected from the eclogite block along the Akyazhi River,southwestern Chinese Tianshan (Fig.2).The eclogites crop out as metabasic blocks within
the
Fig.1.Geological map of the western Chinese Tianshan (modi fied from Lin et al.,2009).
2Q.Li et al./Lithos xxx (2010)xxx –xxx
greenschist massif similar to those described by Gao et al.(1999),Zhang et al.(2002,2005,2007),Klemd et al.(2002,2005),Wei et al.(2003)and Lin and Enami (2006).The ENE –WSW oriented foliation is parallel to the regional tectonic trend of the HP massif and dips to the N with a nearly NW –SE trending mineral stretching lineation.In the field,foliation can be faintly observed in this metabasic block,and it is much clear in the surrounding greenschist.The NW –SE mineral orientation is de fined by arrangements of acicular and/or prismatic paragonite,glaucophane and omphacite grains,which can be observed around garnet porphyroblast in thin sections also.Lenticular blueschist with eclogite occurred as a “relict ”in the greenschist.The clear foliation in the greenschist demonstrates relatively stronger deformation.The mineral parageneses indicate pressure (P)and temperature (T)conditions of 1.8GPa/520°C and 2.2–2.4GPa/495–520°C for garnet mantle and matrix parageneses,respectively (Lin and Enami,2006).
The sample TS193is an omphacite –glaucophane metabasite,sampled near the Kuldkourla Summer Pasture (Fig.2).It consists mainly of garnet,omphacite,glaucophane,epidote,phengite,para-gonite,rutile,dolomite,and quartz.Garnet commonly includes mineral inclusions such as omphacite,barroisite –taramite –pargasite/tschermakite –glaucophane,epidote,paragonite,chlorite,rutile and quartz (Fig.3A,B).Rutile is observed in the matrix or as inclusions of HP minerals like garnet,omphacite and glaucophane.X-ray mapping indicated that these rutiles have a retrograde rim of titanite.
The sample 07RU3is a rutile-bearing vein in a sharp contact with the host eclogite,collected from the upstream of Akesay River (Gao et al.,2007,Fig.2).The rutile display oriented,acicular centimeter-sized crystal in the irregularly shaped vein.It is partly retrograded into ilmenite,and sometimes contains few inclusions of quartz (Fig.3C).The vein also contains garnet,omphacite,and quartz with minor apatite,amphibole,paragonite,and titanite.The idioblastic garnet typically
contains inclusions including omphacite,amphibole,paragonite,quartz,and rutile.
The sample ZS3is an omphacite –glaucophane eclogite collected from the mouth of the Habutensu River (Fig.2).Clear foliation with unclear lineation demonstrated relatively stronger orientation.The ZS3eclogite is medium-grained,consisting mainly of garnet,omphacite,glaucophane,epidote,phengite,paragonite,rutile,and quartz (Fig.3D).Some of phengite is retrogressed into muscovite.Garnet has clear zoning structure and commonly includes omphacite,barroisite/taramite,epidote,paragonite,chlorite,rutile,and quartz.In the thin section,rutile is observed in the matrix and as inclusion of HP minerals like garnet,omphacite and glaucophane.3.Analytical methods 3.1.Instrumental conditions
Measurements of U –Pb isotopes were performed using the Cameca IMS 1280ion microprobe at the Institute of Geology and Geophysics of the Chinese Academy of Sciences.Rutile crystals were mounted in a transparent epoxy together with the R10rutile standard (~30ppm U,Concordia age =1090±5Ma,Luvizotto et al.,2009),99JHQ-1rutile (highly variable U content with average of 2ppm,206Pb/238U age=218±1.2Ma,Li et al.,2003),and an in-house rutile megacrystal standard (JDX)(~6ppm U,207Pb/206Pb age =521Ma,206Pb/238U age=500–520Ma,unpublished TIMS data).The mount was well polished to expose the fresh interior of the crystals.After thorough cleaning,the mount was vacuum-coated with high-purity gold prior to ion probe analysis.
The O 2−primary ion beam was accelerated at 13kV,with an intensity of ca.15nA.The aperture illumination mode
(Kohler
Fig.2.Geological map of the Chinese Tianshan (ultra)high-pressure metamorphic belt,between the Muzaert and Akyazhi Rivers and available geochronological data of eclogite.Sporadic localities of eclogites are after Gao et al.(1999),Zhang et al.(2002,2005,2007),Gao and Klemd (2003),Klemd et al.(2002,2005),Klemd (2003),Wei et al.(2003),Lüet al.(2008),Su et al.(2010)and our field survey.Available geochronological data for Zr:zircon,Phn:phengite Ms:muscovite,Gln:glaucophane,Cro:Crossite,Mag:Magmatic age,and Meta:metamorphic age (explanations by original authors)(see text for detail).
3
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illumination)was used with a ca.200μm aperture to produce even sputtering over the entire analyzed area.The ellipsoidal spot is about 20×30μm in size.Positive secondary ions were extracted with a 10kV potential.The mass resolution of ~6000was used and the magnet was cyclically peak-stepped though a sequence including the 206Pb +,207Pb +,208Pb +,U +,UO +,ThO +,UO 2+and 49
TiO 4+to produce one set of data.A single ion-counting electron multiplier (EM)was used as the detection device.The 49TiO 4+signal was used as reference peak for centering secondary ion beams because this peak shows strong enough intensity and free of interference from ZrO.Each measurement consists of 10cycles and the total analytical time was ~15min,including 2min rastering prior to the actual analysis in order to reduce the contribution of surface contaminant Pb.The mass fractionations of Pb isotopes and Pb hydrides (requiring a mass resolution N 30,000)were not considered because a number of studies have shown that these two effects are negligible and there appears to be a mutual cancellation (e.g.,Ireland and Williams,2003;Li et al.,2010a,b;Williams,1998).3.2.Calibration protocols
It is common that signi ficant inter-elemental fractionation between Pb and U exists in SIMS measurement.The apparent 206Pb +/238U +values sputtered using a negative oxygen primary beam are typically variable times greater than the true values of the crystalline target in different matrix,such as 2–4times for zircon,30–60times for perovskite (Li et al.,2010a ),7–10times and 0.5–0.8times for baddeleyite with and
without oxygen flooding,respectively (Li et al.,2010b ).Based on statistics on rutile in this study,the apparent 206Pb +/238U +values are 8to 170times greater than the true values.To calibrate this huge inter-element fractionation,the basic assumption used in the SIMS measure-ments is simply described as:206
Pb ÃþU þ
u 206Pb Ãþ
238U þ
st
=206
Pb Ã
U u 206Pb
Ã
238
U st
=e λt u
−1e λt st −1ð1Þ
at the same [UO x ]+/U +(x =1,2),where,u denotes the unknowns,st for the standard and *for radiogenic.The functional relationship between 206Pb +/238U +and [UO x ]+/U +(sometimes Pb +/UO +and UO 2+/UO +
)has been used as linear,quadratic,or power by different workers on different minerals (e.g.Williams,1998).Fig.4shows the highly variable UO 2/U in rutile analyses,which imply that the power law (206Pb +/238U +=A×([UO x ]+/238U +)E )should be the best choice where the exponential E is same for the unknowns and standard as:206
Pb 238
U u Pb U st
=206
Pb þ238U þu Pb þU þ
st =A u × 238UO þx
238U + E A st ×238UO þx U +
=A u st :ð2Þ
Eq.(2)has been widely used for U –Pb dating on many minerals with different exponent by ion microprobe analyses (Li et al.,
2010a
Fig.3.Microphotograph and back-scattered electron (BSE)image showing the textural relationships and the status of rutile from eclogite at SW Chinese Tianshan HP/LT metamorphic belt.(A)and (B)Euhedral garnet porphyroblast and matrix consisting of phengite,glaucophane with rutile as the inclusion and matrix.In the matrix,rutile has a retrogressive rim of titanite;as an inclusion in the garnet,the rutile almost homogeneous (TS193).(C)The texture of rutile in a single megacrystal vein;titanite as a retrograde mineral around the rutile (07RU3).(D)Rutile and its neighboring HP minerals:omphacite,glaucophane and garnet (ZS 3).Abbreviations for minerals are:Rt,rutile;Q,quartz;Omp,omphacite;Gln,glaucophane;Ep,epidote;Tit,titanite;Grt,Garnet;Phn,phengite;Ilm:ilmenite.
4Q.Li et al./Lithos xxx (2010)xxx –xxx
and reference therein).With limited analyses in one session and some certain shortage (such as crystal lattice deformation,U –Pb discon-cordance)in standard,we can't expect a well-de fined constant for the exponential (E)in different sessions.In most cases,the standard and unknowns share same range of UO x /U,which makes the in fluence from little variation of E less important.However,for rutile SIMS U –Pb analyses,UO x /U in different sample can show quite different range,just like R10rutiles with highest values ranging from 64to 94,but 99JHQ-1rutile with lowest values among 6–30(Fig.4A).The cause may be interesting and call for further investigation,but it makes the calibration of Pb/U fractionation dif ficult,that means,a little difference in E deduced from any standard would cause obvious variation for other samples.
In this study,R10rutile is used as primary standard because it is the only one with highest U content and proved to be reasonable concordant U –Pb system by TIMS method (Luvizotto et al.,2009).The calibration curve was constructed with power law relationship between Pb/U and UO 2/U relative to the R10rutile standard dated at 1090±5Ma (Luvizotto et al.,2009).The exponential E,shown as the slope in linear relationship between ln(Pb/U)vs.ln(UO 2/U),was finely tuned to correct the results of 99JHQ-1rutile,which is dated at 218±1.2Ma by ID-TIMS (Li et al.,2003)(Fig.4B).The external reproduc-ibility (3.4%,1σ)obtained from the R10standard rutile during the
analytical session was propagated together with the precision of the unknowns to give an overall error for the 206Pb/238U ratio of individual analysis (Li et al.,2010a,b ).As a quality-evidence,with the calibration curve,the in-house JDX rutile standard yielded an average U –Pb age of 510±8Ma,which is well consistent with TIMS result (500–520Ma).In addition,a rutile sample from a rutile deposit located in the Hengshan Mountains,Shanxi province,China,was dated at 207Pb/206
Pb age=1779±10Ma,207Pb/235U age=1779±14Ma 206Pb/238Pb age=1775±23Ma (Shi et al.,in preparation for discussing the genesis of rutile Ore).Because the 207Pb/206Pb age is independent from calibration of inter-element fractionation,the concordance of U –Pb ages indicates a well established calibration curve.
Though U,Th and Pb concentrations are not essential for U –Pb dating,they are useful additional pieces of information for character-ization and commonly measured during ion microprobe analysis.In general,the U concentrations were calculated based on a ratio of +
UO x(x =0,1,2)and the intensity of matrix ion,such as 90Zr 2O +for zircon and CaTi 2O 4+for perovskite (Li et al.,2010a,b ).However,we observed that the intensity of 49TiO 4+in rutile varied by up to 400%among different samples at the same analytical condition.The cause for this phenomenon is unclear,but may be related to the structure of crystal.Nevertheless,the method used for zircon or perovskite is unsuitable for rutile.In this study,we estimated the U concentrations by U +ion yield based on the R10standard with 30ppm U (Luvizotto et al.,2009).This method is proven to be effective to within 50%uncertainty by monitoring the JDX rutile megacrystal.
Rutile usually contains very low concentrations of Th making it favorable in U/Pb dating using 208Pb-based common Pb correction (Clark et al.,2000;Luvizotto et al.,2009).We observed that the ThO +/UO +ratios (corresponding to Th/U with a factor of around 1,Williams,1998)in rutile standards are lower than 0.01,mostly b 1E −4.This feature of rutile is quite useful not only in U –Pb dating,but also in judging if it is rutile grains or other mineral.However,most of dated rutile grains in studied eclogites contain so low U contents that the tiny Th content couldn't be ignored.So,the common Pb proportion was calculated by 207Pb-based (Williams,1998).As for the age calculation,assuming that the rutiles are concordant in the U –Pb system,an alternative to the common-lead correction is using the lower and upper intercepts of a regression line of the data points on a Tera –Wasserburg plot to calculate the U –Pb age and the common-lead composition,respectively (Williams,1998).The 207Pb-based correc-tion results using the terrestrial Pb isotope composition (Stacey and Kramers,1975)are listed for reference only due to large uncertainties.4.Results
The U –Pb data of rutile standards used in this study were shown in Table 1and Fig.4,while the results of rutile samples from eclogites were presented in Table 2and Fig.5.U –Pb ages were calculated using the decay constants recommended by Steiger and Jäger (1977)and calculation routines of Isoplot/Ex (Ludwig,2003).The resulting regressions were quoted at 95%con fidence interval.4.1.Rutile used as standards
R10rutile used in this study is a piece of fragment of a large single crystal from Gjerstad,south Norway (Luvizotto et al.,2009).This rutile has a relatively high U concentration (ca.30ppm)and rather constant U –Pb ages at 1090±5Ma.Based on 17analyses in this study,U contents are homogeneous within ±10%(1σ)assuming the same U +ion mon Pb is very low with most of 208Pb/206Pb b 0.001(Table 1).The 238U –206Pb ages are consistent within 2.8%(1σ)(Fig.5A).The 207Pb ⁎/206Pb ⁎ratios are indistinguishable within errors and give an average of 0.07568±0.00024,corresponding to Pb –Pb age of 1085±7.8Ma,which is well identical to TIMS result of 1090±5Ma (Luvizotto et al.,2009
).
Fig.4.Calibration curves for SIMS rutile U –Pb analyses.(A)The correlation diagram between secondary 206Pb ⁎/238U +vs.238U 16O 2/238U;(B)The diagram between ln (206Pb ⁎/238U)and ln(238U 16O 2/238U).Error assigned to the symbol is one sigma.A solid line shows best fit of a power law with Y =A*X E ,where E =0.97.
5
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99JHQ-1rutile is isolated grains from an ultrahigh-pressure eclogite located in Jinheqiao,the Dabie Mountains,same sample as what used in Li et al.,2003.This rutile U–Pb age was dated at218±1.2Ma by TIMS(Li et al.,2003).The average U content is quite low at ca.2ppm,but shown highly inhomogeneous from grain to grain as observed by SIMS.However,there is no clue to distinguish the U-rich grains by conventional images,such as CL or BSE.We used U ion intensities as main guideline to select those relative higher U grains for analyses.The analyzed17grains show U content ranging from0.9 to12ppm and almost zero Th content(Table1).The measured208Pb/ 206Pb vary from0.37to0.001,corresponding to206Pb/204Pb from100 to38000,which is much higher than those obtained on mg size
Table1
Rutile standard U–Pb data analyzed by Cameca IMS1280ion microprobe.
Sample spot U a(ppm)208Pb
206Pb 238U
206Pb
b±1σc(%)207Pb
206Pb
b±1σc(%)t
207/206
(Ma)±1σc(Ma)t206/238d(Ma)±1σc(Ma)
R10rutile e
R10@1250.00894 5.60 3.50.075990.54109511105934 R10@2320.00005 5.33 3.60.076020.59109612110836 R10@3330.00005 5.38 3.50.074580.58105712109836 R10@4310.00007 5.43 3.50.076520.58110912109035 R10@5260.00004 5.39 3.50.075490.59108212109736 R10@6370.00004 5.63 3.60.074740.82106116105435 R10@7330.00004 5.59 3.50.075750.50108810106235 R10@8330.00016 5.20 3.50.075310.70107714113437 R10@9310.00014 5.22 3.50.075720.74108815113037 R10@10250.00014 5.23 3.60.075470.73108114112937 R10@11280.00017 5.20 3.50.076590.75111115113437 R10@12280.00024 5.60 4.20.074980.74106815106041 R10@13290.00013 5.55 4.10.075650.73108614106741 R10@14290.00020 5.16 3.50.075080.87107117114237 R10@15300.00006 5.23 3.50.075080.69107114112737 R10@16310.00005 5.35 3.50.075190.61107412110536 R10@17330.00009 5.54 3.50.076120.53109811106935
JDX rutile
JDX@1 6.20.004812.0 3.50.0594 3.55827451818 JDX@28.10.005112.5 3.50.0576 4.951310449617 JDX@3 6.10.004712.2 3.50.0601 6.060812450918 JDX@4 5.70.004212.4 3.60.0590 5.456811449918 JDX@5 5.00.004112.0 3.50.0601 3.46077251418 JDX@6 4.80.005111.6 3.60.0566 4.44779553119 JDX@7130.005411.9 3.50.0594 5.558111452018 JDX@8210.004212.3 4.20.0514 4.22579450320 JDX@98.10.004412.5 5.00.0574 4.35069349527 JDX@108.30.006612.7 3.60.0572 3.95018448718 JDX@117.30.006012.4 3.70.0523 4.23019349919 JDX@12240.006612.1 3.60.0585 5.255010951218 JDX@13 6.10.005712.1 3.60.0557 4.24389251018 JDX@14 5.60.004612.5 3.50.0586 3.95538249518 JDX@159.40.004811.8 5.00.0604 3.66167552627 JDX@16 5.70.006212.5 3.50.0584 3.95458249717 JDX@17150.007312.1 3.50.0579 4.152******** JDX@18130.001711.8 4.00.0580 2.153******** JDX@199.20.004211.9 3.60.0599 4.66009651918 JDX@20 5.60.007512.9 3.60.0613 5.465111248017 JDX@217.30.006011.3 3.50.0565 3.54737554519 JDX@22150.006711.5 3.50.0604 3.76177753619
99JHQ-1rutile
JHQ-1@1120.004630.3 3.50.0457 6.3––2097 JHQ-1@27.40.026527.7 4.20.05107.0––2269 JHQ-1@3 2.30.019329.4 4.20.054912––2139 JHQ-1@4150.012629.7 3.60.0499 6.5––2127 JHQ-1@5 1.60.378823.5 5.20.14289.1––21911 JHQ-1@6 2.70.057627.2 4.20.067212––2279 JHQ-1@7 3.90.044130.1 3.70.07179.6––2068 JHQ-1@8 1.80.265525.7 5.00.170118––21512 JHQ-1@9110.008627.2 3.80.05897.0––2329 JHQ-1@10120.003030.6 3.60.0540 5.8––2077 JHQ-1@11 1.80.014028.1 4.30.064513––2249 JHQ-1@12120.004029.8 3.60.05587.9––2127 JHQ-1@130.90.056128.17.40.056021––22016 JHQ-1@14 4.90.012327.0 4.30.053911––23310 JHQ-1@15 3.30.032028.1 3.50.057313––2228 JHQ-1@169.80.002330.1 3.50.0514 5.1––2107 JHQ-1@178.60.001028.2 3.50.0462 5.3––2258
a Error of U concentrations is±50%estimated by U+yield of R10standard rutile.
b These ratios for R10and JDX are common Pb corrected,while for99JHQ-1,common Pb uncorrected.
c Error assigne
d to th
e ratio is one sigma estimated by counting statistics and calibration.
d t
206/238
is206Pb–238U age calculated by208Pb-based common-lead correction assuming Th=0.
e The results o
f R10are list here only to show its reproducibility.
6Q.Li et al./Lithos xxx(2010)xxx–xxx
sample by TIMS (Li et al.,2003).These rutile grains show lowest UO 2/U,which are far away from what R10rutile behaves.That means,we must use the extrapolation of calibration line constructed by R10rutile and the slope –effect is signi ficant (Fig.4B).A slope of 0.97was used and yielded the 206Pb –238U age of 217±4Ma,calculated by 208
Pb-based common-lead correction assuming Th=0(Fig.5B).
JDX rutile is a single large euhedral crystal about 5cm in length,2.5cm in thickness,probably from Sri Lanka according to the jeweler.Based on three analyses on U –Pb isotope data by TIMS,this rutile is likely slightly disconcordant,with 207Pb/206Pb age of 514–523Ma and 206
Pb/238U age of 500–520Ma.The U content is averaged at 6ppm,consistent with the weighted average of 22analyses by SIMS,while which are mostly around 5–10ppm,but a few up to 24ppm (Table 1).Due to its disconcordant U –Pb age,this rutile may be not a suitable age standard,but can serve as a valuable object to evaluate the calibration line within 2%uncertainty.The independent 207Pb/206Pb ages are weighted at 529±35Ma (MSWD=1.1).With the abovementioned calibration curve,this rutile gave a weighted average 206Pb/238Pb age of 509±8Ma (Fig.5C),which is consistent with the average TIMS results and demonstrate the validity of the calibration method.4.2.Rutile samples
4.2.1.Rutile in TS193eclogite
A total of twenty-three analyses were conducted on 23rutile crystals with diameters about 80μm.The measured U contents range from 0.1to 39ppm,mostly b 1ppm (Table 1).Common lead is variably high,with values of f 206between 6%and 89%.On the Tera –Wasserburg plot linear regression of the data points (MSWD=1.0)gives a lower intercept age of 306±13Ma and the upper intercept with 207Pb/206
Pb=0.77±0.06for the common Pb composition (Fig.6A).A weighted mean 206Pb/238U age is 308±12Ma (MSWD=1.0)using the 207Pb-based common-lead correction (Williams,1998)with the terrestrial Pb isotope composition of Stacey and Kramers (1975),which is consistent with the lower intercept age within errors.4.2.2.Rutile in ZS3eclogite
Most rutile crystals are about 30μm in diameter,with the largest up to 100μm.Twenty-six analyses were performed on 26grains.Uranium content varies from 0.04to 97ppm,mostly b 1ppm (Table 1).Values of f 206range from 2%to 92%,mostly N 30%.Regression of the data points on the Tera –Wasserburg plot (MSWD=1.2)gives a lower intercept age of 321±14Ma and the upper intercept with 207Pb/206Pb=0.81±0.03for the common Pb composition (Fig.6B).The 206Pb/238U ratios (207Pb-based common-lead correction)yield a weighted mean age of 320±14Ma (MSWD=0.75),consistent with the lower intercept age within errors.
Table 2
Rutile U –Pb data analyzed by Cameca IMS 1280ion microprobe.Sample spot U a
(ppm)f 206&(%)238
U 206Pb
#
±1σb (%)207
Pb 206Pb
#
±1σ(%)t 206/238
⁎(Ma)±1σ(Ma)ZS3eclogite ZS3@10.0673 5.56140.62817292166ZS3@2 2.162 5.85100.538 4.840267ZS3@30.973 4.38 3.90.631 5.836796ZS3@40.777 3.78 4.50.656 4.1371100ZS3@50.6577.228.50.4989.137167ZS3@6 4.1577.74 3.40.5057.733951ZS3@70.683 3.04 3.90.709 2.4322120ZS3@8 1.2558.4 5.20.4898.532850ZS3@90.0868 3.299.60.5928.5578155ZS3@100.5587.94 5.60.5081332875ZS3@110.0678 3.879.30.66413347197ZS3@12971318.4 4.40.1581429616ZS3@130.580 3.89 5.60.6867299130ZS3@140.0492 1.25140.778 4.6336377ZS3@15 1.12712.4 3.60.2691036523ZS3@160.383 3.65 3.60.703 6.2282133ZS3@170.2548.98 5.20.4751232058ZS3@180.4220.37.10.0682830423ZS3@190.74313.2 6.40.3901227036ZS3@200.92016.9 4.60.2099.229817ZS3@21 3.23512.7 3.40.332 6.131720ZS3@220.81516.8 4.40.1681331918ZS3@23 2.1509.47 3.50.450 6.732535ZS3@240.6577.599.80.5051234678ZS3@250.52614.6 4.40.2561231823ZS3@260.5
20
13.1 5.6
0.208
10
383
25
07RU3eclogite 07RU3@1 1.21915.9 5.90.20583182107RU3@2 1.1818.1 4.30.113113201507RU3@3 1.71917.4 4.80.199122941807RU3@4 1.71216.8 4.40.151113271607RU3@5 1.42113.9 4.30.2199.43552007RU3@6 1.41715.2 4.40.186143422107RU3@70.92816.250.118103551807RU3@80.911614.4 4.70.179103652007RU3@9 1.5917.7 4.40.122133241607RU3@10 1.61119.5 5.30.136162881807RU3@11 1.73413.9 4.40.323132952907RU3@12 1.91017.4 5.60.135113231907RU3@13 1.71019.8 4.60.129132841507RU3@14 2.1658.45 4.20.56372565007RU3@15 1.5716.2 4.40.112103581707RU3@16 1.7815.9 4.20.119153611707RU3@17 1.5917.8 4.40.124103211507RU3@18 1.558 6.99 4.40.508 4.83724607RU3@19 1.32714.7 4.30.269163092807RU3@20 1.62712.8 4.30.264103572407RU3@21 1.5818.8 4.50.118133071507RU3@22 1.51616.150.1768.73311807RU3@23 1.61017.5 4.40.134203221907RU3@24 1.71615.8 4.40.175143351907RU3@25 1.51715.750.183143332107RU3@26 1.71318.1 4.50.154173041807RU3@27 1.61516.860.173153162307RU3@28 1.91219.5 4.20.147172841607RU3@29 1.91219.8 4.50.144132811507RU3@30 1.21117.9 4.20.1381531316
TS193eclogite TS193@10.289 1.24110.755 5.4486389TS193@2 4.61317.5 4.70.1539.231316TS193@30.31716.5 6.40.1832231728TS193@40.8677.26 6.40.5811427899TS193@5 6.13411.17.10.3221236940TS193@60.21320.9 5.90.1542926223TS193@7 2.3717.2 5.10.112333821TS193@80.54514.2100.4061624246TS193@90.2621.3 4.70.09794827922TS193@100.83011.2 5.10.2931738742TS193@110.93511.7110.3261234848TS193@120.21517.860.173430032TS193@13
0.25211.240.462026869
Table 2(continued )
Sample spot
U a
(ppm)f 206&(%)238
U 206Pb
#
±1σb (%)207
Pb 206Pb
#
±1σ(%)t 206/238
⁎(Ma)±1σ(Ma)TS193eclogite TS193@140.3577.52 3.90.5041635092TS193@150.178 6.42100.6723203201TS193@169.2918.140.1221731716TS193@17 2.62614.87.30.2562331440TS193@1839718.1 6.50.1121332322TS193@198.478 1.76170.66812731410TS193@200.162 6.169.20.54418375137TS193@210.21920.1 6.60.2043225331TS193@22 1.4919.3 3.50.1252529617TS193@23
0.1
14
16.78.8
0.162
33
324
38
&
f 206is the percentage of common 206Pb in total 206Pb,calculated by 207Pb-based.⁎t 206/238is 206Pb –238U age calculated by 207Pb-based common-lead correction.#
The ratios are common Pb uncorrected,used for Tera –Wasserburg plot.a
Error of U concentrations is ±50%estimated by U +yield of R10standard rutile.b
Error assigned to the ratio is one sigma estimated by counting statistics and calibration.
7
Q.Li et al./Lithos xxx (2010)xxx –xxx。