Earth and Planetary Science Letters 361 (2013) 238–248

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Earth and Planetary Science Letters

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(Z.C. Zh

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Perovskite and baddeleyite from kimberlitic intrusions in the Tarim largeigneous province signal the onset of an end-Carboniferous mantle plume

Dongyang Zhang a, Zhaochong Zhang a,n, M. Santosh a,b, Zhiguo Cheng a, He Huanga, Jianli Kang a,c

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Chinab Division of Interdisciplinary Science, Kochi University, Kochi 780-8520, Japanc Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China

a r t i c l e i n f o

Article history:

Received 22 June 2012

Received in revised form

12 October 2012

Accepted 13 October 2012

Editor: T.M. Harrisoncorresponding concordia and 206Pb/238U ages, corrected for the common Pb contribution, of about

Available online 22 November 2012

Keywords:

kimberlitic intrusion

geochronology

geochemistry

mantle plume

Tarim

1X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.epsl.2012.10.034

esponding author. Tel.: þ86 10 823 22195; f

ail addresses: zczhang@cugb.edu.cn, zhangzh

ang).

a b s t r a c t

Several tens of kimberlitic pipes and dykes are exposed in the Wajilitag area in the western Tarim large

igneous province. Here we report for the first time secondary ion mass spectrometric U–Pb age data on

perovskite and baddeleyite grains in a kimberlitic pipe and a kimberlitic dyke from the Tarim Craton.

The perovskite yielded a well-defined intercept age of 299.874.3 Ma, which is consistent with its

300 Ma. The baddeleyite separated from two kimberlitic samples from a dyke display identical

concordia U–Pb ages of 300.874.7 Ma and 300.574.4 Ma. Our age data show that the kimberlitic

intrusions were emplaced at ca. 300 Ma, rather than in the late Permian as previously regarded. These

new ages are slightly older than the eruption ages of Tarim flood basalts (291–273 Ma), offering a

critical regional time marker for the onset of Permo-Carboniferous magmatism in the Tarim Craton.

Detailed petrographic observations did not reveal any ultrahigh pressure mineral assemblage in the

Wajilitag kimberlitic intrusions. Phlogopites from these intrusions show eNd(t) values of þ3.7 to þ4.2.

The baddeleyites which are texturally primary and therefore inferred to have crystallized directly from

the kimberlitic magma, yield a range of eHf(t) from þ4.8 to þ8.7. These results combined with

previously reported geochemical data, suggest that the Wajilitag kimberlitic intrusions were most

likely derived from a moderately refractory and depleted subcontinental lithosphere mantle, metaso-

matized by subduction components associated with an early-middle Paleozoic convergent regime. The

kimberlitic magma was generated by small-degree partial melting of the lithospheric mantle in

response to the impingement of the Tarim mantle plume. Thus, our new geochronological data suggest

the arrival of the mantle plume beneath the Tarim lithosphere at least 10 million years before the onset

of Tarim flood basalt volcanism. The end-Carboniferous Wajilitag kimberlitic intrusions, the oldest

known phase associated with Carboniferous magmatism in the Tarim Craton, signals the initial

magmatic pulse triggered by mantle plume impingement.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Kimberlites and related rocks have attracted considerableinterest not only because of their economic potential as the mainsource of diamonds, but also due to their derivation from deepdomains of the Earth’s mantle with a complex evolutionaryhistory, offering valuable insights into the composition of thesub-continental mantle (Coe et al., 2008; Zhang et al., 2010b;Chalapathi Rao et al., 2011). However, owing to their hybridnature and susceptibility to alteration, the origin of kimberliticmagmatism remains enigmatic with a great diversity of different

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source regions and components invoked for the magmas, includ-ing sub-continental lithospheric mantle (SCLM), convective asth-enospheric or sub-asthenospheric mantle and subducted oceaniccrust (Ringwood et al., 1992; Tainton and McKenzie, 1994; Beckerand le Roex, 2006; Paton et al., 2009; Zhang et al., 2010b;Chalapathi Rao et al., 2011). Furthermore, kimberlites provideinformation on the regional tectonics related to their emplace-ment, particularly in the context of lithospheric extension (Bailey,1993; Batumike et al., 2008; Tappe et al., 2008). Several kimber-lite events around the world broadly coincide with the eruption ofcontinental flood basalts now being attributed to impact ofmantle plumes, which led many researchers to invoke mantleplumes for the genesis of kimberlitic magmatism (e.g., le Roex,1986; Haggerty, 1994; Griffin et al., 2005; Kumar et al., 2007;Torsvik et al., 2010; Chalapathi Rao et al., 2011). Additionalsupport for this cause-and-effect relationship is provided by the

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248 239

track-defining age progressions in kimberlite magmatism fromNorth America, which coincides with the continental extension ofthe Great Meteor Hotspot track (Heaman and Kjarsgaard, 2000).This scenario has been interpreted to be the passage of one ormore mantle plumes through this region (Heaman andKjarsgaard, 2000). However, there are some other kimberliteevents, such as those in southern Africa and North China, whichdo not correlate with mantle plume events, suggesting that not allkimberlite events require mantle plume activity and thus alter-native models, such as subduction of oceanic lithosphere andlithospheric extension, have also been considered (Haggerty,1999; Moore et al., 2008; Jelsma et al., 2009; Chalapathi Raoand Srivastava, 2009; Zhang et al., 2010b). The precise determina-tion of the emplacement age and a tight constraint on the natureof the mantle sources are prerequisites for understanding thepetrogenesis of kimberlites and the regional tectonic implications.

The kimberlitic intrusions exposed in the Wajilitag area withinthe northwestern part of Tarim Carton (abbreviated TC hereafter)are located in the well-known Tarim large igneous province (LIP).This rare occurrence provides an opportunity to evaluate thegenesis in the light of recent petrogenetic models developed forother kimberlites worldwide, and to assess the importance ofmantle plumes versus tectonic triggers as causal mechanisms forkimberlitic intrusions in the TC. Preliminary studies have sug-gested that the Wajilitag kimberlitic intrusions formed duringlate Permian (�253 Ma) based on 40Ar/39Ar phlogopite dating,although no information on the analytical procedures or details ofthe data were reported (Li et al., 2001). Since kimberlitic rocks aremineralogically quite complex and are easily susceptible tochemical alteration, phlogopite is not an ideal mineral for datingthese rocks. Thus 40Ar/39Ar dating of phlogopite separates couldresult in mixed ages or cooling/resetting ages, as groundmass andxenocrystic phlogopite may be mixed, and is thus too imprecise toconstrain the time of emplacement of kimberlitic intrusions(Batumike et al., 2008; Li et al., 2011a). In addition, consideringthe spatial association of the kimberlitic intrusions with the TarimLIP, many workers broadly interpreted it as a part of the LIP andcorrelated it to mantle plume activity (e.g., Jiang et al., 2004; Liet al., 2011b). Unfortunately, no direct intrusive relationshipbetween the kimberlitic rocks and Tarim flood basalts has beenfound, although several dolerite dykes cut across the kimberliticintrusions. Thus the accurate age of the kimberlite intrusions ispoorly constrained. In this contribution, we report for the firsttime precise isotope ages from perovskite and baddeleyite, as well

Fig. 1. (a) Main tectonic units of China. Abbreviations for terranes: WS: West Siberia

Mongolis-Argun; BJ: Bureya-Jiamusi; SN: Songnen (modified from Wang et al., 2006). (b

of Permian basalts in Tarim. KD: Kuche depression; NTU: northern Tarim uplift; N

depression; STU: south Tarim uplift; SED: southeast depression (modified from Tian et a

kimberlitic intrusions (XJGMR (Xinjiang Bureau of Geology and Mineral Resources), 19

as phlogopite Sr–Nd and baddeleyite Lu–Hf isotopic data for theWajilitag kimberlitic intrusions. Our results provide tight con-straints on the emplacement age and plausible mantle sourceregions of the Wajilitag kimberlitic magma, which in turn haveimportant implications for the geodynamic evolution in thisregion.

2. Geological setting

The TC, located in northwestern China, together with the NorthChina and Yangtze Cratons, defines the fundamental tectoniccollage of China. The Craton is surrounded by the PaleozoicTianshan orogen to the north, the western Kunlun orogen to thesouth, and the Altyn Tagh orogen to the southeast (Fig. 1a, b).The craton has a complex Precambrian basement consistingmainly of late Neoarchean-early Paleoproterozoic tonalitic-trondhjemitic-granodioritic gneisses, late Paleoproterozoic toearly Neoproterozoic marine volcano-sedimentary rocks and lateNeoproterozoic low grade metamorphic volcaniclastic rocks andglacial deposits, mainly exposed along the margins of the craton(Long et al., 2011b; Shu et al., 2011; Zhang et al., 2012a). Thebasement is discordantly overlain by a thick sequence of Phaner-ozoic shallow marine and terrestrial volcano-sedimentary strata(Guo et al., 2005). Since most of the Phanerozoic rocks are buriedby a thick succession of Neogene aeolian deposits in the centralpart of the craton, the reconstruction of geologic and tectonicelements are based on available exposures along the margins ofthe craton, together with drill core and geophysical data. The pre-Permian strata are mainly characterized by carbonates, lime-stones and sandstones (Guo et al., 2005; Tian et al., 2010). ThePermian strata are dominated by clastic rocks, muddy limestonesand volcanic rocks (Zhou et al., 2009). Recent studies haverevealed widespread early Permian (271.772.2–291.972.2 Ma;zircon LA-ICPMS, SHRIMP and Cameca U–Pb ages) basalts acrossthe Tarim region through drilling and seismic studies (e.g., Tianet al., 2010; Yu et al., 2011). It is estimated that the basalts andother cogenetic igneous units presently occupy an estimated areaof more than 300,000 km2 with a maximum total erupted basaltvolume of 3.0�105 km3, leading to the suggestion that thePermian Tarim igneous units represent a typical LIP, termed theTarim LIP (e.g., Tian et al., 2010; Zhang et al., 2010a, 2010c,2012d). Although many studies favor a link between the TarimLIP and early Permian mantle plume (e.g., Zhang et al., 2010a;

n; TM: Tuva-Mongolia, QD: Qaidam; QT: Qiangtang; GD: Gangdise; MA: Central

) Simplified tectonic map of the TC and surrounding areas showing the distribution

TD: northern Tarim depression; CTU: central Tarim uplift; SWD: southwestern

l., 2010). (c) Simplified geological map of the Wajilitag area showing the location of

84).

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248240

Tian et al., 2010; Qin et al., 2011), an alternative geodynamicmodel postulated that the Tarim LIP may be linked to lithosphericextension accompanying passive continental rifting (Yang et al.,2007). The basement and Paleozoic strata are folded and faultedby several major Phanerozoic deformational events, resulting inseveral roughly E-W trending uplifts and depressions (Fig. 1b).

Recent studies have revealed multistage ultramafic-maficintrusions, mafic dyke swarms and granites exposed along themargins of the TC (e.g., Zhang et al., 2008, 2010a, 2012b; Longet al., 2011a; Ge et al., 2012). Voluminous Neoproterozoic (0.83–0.62 Ga) ultramafic-mafic intrusions and dykes, as well as gran-itoids, are considered to be products of a mantle plume within acontinental rifting setting, which has been assigned to thebreakup of the Rodinian continent (Zhang et al., 2009, 2011,2012b; Zhu et al., 2011; Long et al., 2011a). Arc-like Silurian toDevonian granites occur sporadically in the region and areinterpreted to have formed during the southward (present coor-dinates) subduction of the southern Tianshan ocean beneath theTC (Jiang et al., 2001; Zhang and Sun, 2010; Ge et al., 2012).The early Permian intrusive complexes, mafic dikes and granites,widely distributed in the northern Tarim, are also interpretedas a part of the Tarim LIP (Yang et al., 2007; Zhang et al., 2008,2010a).

The Wajilitag kimberlitic intrusions are located 40 km south-east from the county of Bachu, in the western Tarim LIP, and ismade up of several tens of pipe and dyke swarms that generatedsome excitement in the past as potentially the first diamond-bearing rock reported in Xinjiang (Fig. 1b, c; Du, 1983).The kimberlitic cluster trends NWW-SEE within an area of5 km2 and is deeply weathered at the surface to greenish clay.These kimberlitic pipes and dykes intruded the flat-lying andmetamorphosed continental clastic sequences of the upper Devo-nian Keziletag and Yimugangtawu Formations, and were in turncut by late dolerite dykes (Supplementary file 1a–c). Amongthese, pipe 1, with several microdiamonds separated so far, isthe most important diamondiferous pipe in the area and is anoval-shaped body with dimensions of ca. 180 m�100 m (Du,1983; Su, 1991). The Wajilitag kimberlitic intrusions are spatiallyassociated with the Wajilitag early Permian Fe-Ti oxide ore-bearing ultramafic-mafic-syenitic intrusion that is composed ofclinopyroxenite, gabbro and syenite and was emplaced at ca.274 Ma (Zhang et al., 2008), but no direct intrusive relationshipbetween these rock units has been found in the region (Fig. 1b; Liet al., 2001, 2011b). Moreover, numerous felsic and mafic-ultramafic dykes (including some carbonatites) with variablestrikes intrude either upper Devonian strata or the variousintrusions in the area mentioned above (Zhou et al., 2009;Zhang et al., 2010a).

Fig. 2. Back scattered electron (BSE) images of rocks from the Wajilitag kimberlit

Abbreviations: Cpx¼clinopyroxene; Ph¼phlogopite; Gar¼garnet; Hb¼hornblende; Cc

Bd¼baddeleyite; Zr¼zircon.

3. Petrography and geochemistry of the Wajilitag kimberliticintrusions

The petrographic features of the Wajilitag kimberlitic pipesand dykes are markedly similar. The samples examined in thisstudy show brecciated nature and inequigranular porphyritictexture, which is characteristic of kimberlitic rocks. The rockscontain abundant clinopyroxenite, olivine clinopyroxenite, lesserdunite, and sparse amphibolite xenoliths (Supplementary file 1d–f). They also carry some fragments of the country rocks (mainlysandstone), particularly in the samples collected from along theedge of the intrusions (Supplementary file 1g). The xenolith suiteshows variation in size and abundance, ranging from 45 cm to0.1–0.2 mm, and comprises up to 15 vol% of the rock in someplaces, although they can be extremely rare in other domains. Theeuhedral to rounded macrocrysts and phenocrysts within thekimberlitic rocks are dominated by clinopyroxene (10–15 vol%)and subordinate olivine (5–10 vol%), with minor phlogopite(�5 vol%), amphibole (1–3 vol%) and apatite (�1 vol%), and areset in a fine- to micro-grained, interlocking groundmass domi-nated by clinopyroxene, phlogopite, olivine, apatite, perovskite,baddeleyite, garnet, spinel, rutile, magnetite, calcite and graphite(Supplementary file 1e, g–l). These macrocrysts and phenocryststypically range in length from 0.5 to 5 mm, though some olivineand amphibole grains are as large as 10 mm in their longestdimension. Some macrocrystal and phenocrystal grains appearragged, highly strained and display undulose extinction orkink banding, offering compelling evidence for an origin fromdisaggregated xenoliths (Supplementary file 1i–l). Perovskite andbaddeleyite are two important groundmass phases in the Waji-litag kimberlitic rocks. The groundmass perovskite is relativelyabundant, with concentrations up to 3 vol%, while baddeleyite isrelatively rare in the groundmass. Perovskite occurs as brown andeuhedral or rounded discrete grains, and also co-exists withclinopyroxene, phlogopite, magnetite and graphite (Fig. 2a). Bad-deleyite occurs as discrete crystals, but is more commonlyassociated with apatite and magnetite in late-stage mesostasis(e.g., calcite; Fig. 2b). Most baddeleyite grains are surrounded bysecondary zircon crystals (Fig. 2b). The presence of such anirregular zircon rim may suggest a reaction resulting from theinteraction of baddeleyite with deuteric fluids in the kimberliticmelt (Heaman and LeCheminant, 1993). The majority of macro-crysts and phenocrysts and the groundmass are completely orpartially altered to serpentine, chlorite, epidote, Fe-Ti oxides andcarbonate minerals along their fractures, cleavage planes andcrystal margins. Overall, the Wajilitag kimberlitic intrusionsshow unusually high modal abundance of phenocrysts andmicrophenocrysts of clinopyroxene and apatite but a low modal

ic intrusions, showing perovskite and baddeleyite crystals in the groundmass.

¼calcite; Pv¼perovskite; Gr¼graphite; Mag¼magnetite; Gt¼garnet; Ap¼apatite;

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248 241

abundance of olivine macrocrysts, relative to other archetypalkimberlites (e.g., Becker and le Roex, 2006; Coe et al., 2008;Chalapathi Rao and Srivastava, 2009).

Systematic major and trace element investigations on theWajilitag kimberlitic rocks were published in recent studies(Jiang et al., 2004; Li et al., 2010b; Supplementary file 2). Thesekimberlitic samples are all silica-undersaturated (SiO2o39.84 wt%), and their MgO (15.63–21.52 wt%) and Mg# (0.69–0.75) contents are slightly lower than those in typical kimberlites(Becker and le Roex, 2006; Coe et al., 2008; Chalapathi Rao andSrivastava, 2009). Primitive mantle-normalized trace elementpatterns for the kimberlitic samples show systematic negativetroughs in Zr, Hf, Ti, Nb and Ta (Supplementary file 2). All samplesare characterized by a negative slope of the rare earth elements(REE) patterns with significant light REE/heavy REE fractionation[(La/Yb)N¼30–59]. The exact nature of the Wajilitag host magmais controversial and has been loosely described as kimberlite(Du, 1983; Wang and Su, 1987; Su, 1991), brecciated phlogopiteolivine clinopyroxenite (Li et al., 2001) and kimberlitic brecciatedperidotite (Bao et al., 2009). Based on mineralogy and geochem-istry, Wang and Su (1987) proposed that the Wajilitag intrusionsare similar to typical kimberlites with the exception of slightlylower MgO contents. However, in a more recent study, Bao et al.(2009) argued that the brecciated rock is not a typical kimberlite,and may be described as kimberlitic brecciated peridotite,because the rock does not contain pyrope-garnet and picro-ilmenite, which are common in most typical kimberlites. Inconjunction with our petrographic observations and previousstudies, we consider that the Wajilitag intrusions are more akinto kimberlite-like rocks based on the overall texture and themineral assemblage, and we refer to them as kimberlitic rocks inthis study.

Fig. 3. Representative CL images of perovskite and baddeleyite grains from

Wajilitag kimberlitic rocks. The fragmented nature of the grains is an artifact of

sample preparation. Circles indicate locations of analyzed sites, with numbers in

the circles representing spot numbers. The calculated 238U/206Pb age for each spot

is given.

4. Analytical techniques and results

The sampling and analytical methods, chemical compositionsof the perovskite and baddeleyite, U–Pb analytical data onperovskite and baddeleyite, Sr–Nd analyses of phlogopiteand Lu–Hf data on baddeleyite are reported in Supplementaryfiles 3–8, respectively.

4.1. Chemical composition of perovskite and baddeleyite

The perovskite grains show limited chemical variation (Supp-lementary file 4) and are composed of TiO2 (51.0–52.5 wt%) and CaO(34.9–35.6 wt%) with minor FeO (1.2–2.4 wt%). The analyzed badde-leyites contain ZrO2 (95.1–97.1 wt%), with minor amounts of TiO2

(0.41–2.0 wt%), Nb2O5 (0.34–1.1 wt%) and HfO2 (0.27–1.0 wt%).

4.2. Perovskite U–Pb geochronology

Perovskite grains extracted from sample DW31-4, are mostlyeuhedral and fresh and range in size from 15 to 50 mm (Fig. 3).Twenty-three grains of perovskite were analyzed for U–Pb age, andthe complete data set is given in the Supplementary file 5. Theperovskites show relatively uniform uranium contents of112729 ppm (1 SD) and Th/U ratio from 8.0 to 35.5. The scattereddata points on the Tera-Wasserburg plot give a well-defined lowerintercept age at 299.877.5 Ma and an upper intercept with207Pb/206Pb¼0.8570.03 for the common-Pb composition (Fig. 4a).Using the terrestrial Pb (Stacey and Kramers, 1975) as an estimateof common-lead composition, the corrected data yield a concordiaU–Pb age of 299.874.3 Ma (Fig. 4b). 206Pb/238U individualdates, following by the 207Pb-based common-Pb correction, yield a

weighted average 206Pb/238U age of 299.274.3 (MSWD¼0.62;Fig. 4c).

4.3. Baddeleyite U–Pb geochronology

Baddeleyite grains analyzed in this study were separated fromsamples DW21-1 and DW21-4, and are mostly subhedral or frag-mental, ranging from 40 to 100 mm in length (Fig. 3). Twenty-onespot analyses were conducted for each of the two samples analyzed,and the data are reported in the Supplementary file 6. The resultsshow variable U contents from 59 to 790 ppm and Th/U from 0.02 to0.20, with the exception of one analysis (DW21-4@01) which yields arelatively high Th/U¼0.37 due to a significantly high Th content(221 ppm) compared to other studied baddeleyite grains. Our datayield identical concordia U–Pb ages of 300.874.7 Ma (MSWD¼1.6,DW21-1) and 300.574.4 Ma (MSWD¼1.3, DW21-4) (Fig. 5).

4.4. Phlogopite Sr–Nd isotope compositions

The Sr–Nd isotopic data of phlogopite are listed in Supplementaryfile 7. The data of the whole-rock kimberlitic samples (Jiang et al.,2004; Yu, 2009) are also shown for comparison. Because thephlogopite separates show markedly high calculated 87Rb/86Sr ratios

Fig. 4. U–Pb age data for the Wajilitag perovskite. Error bars, error ellipses and

uncertainties of weighted average ages are at 2s level.

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248242

(3.703–4.190), their calculated initial 87Sr/86Sr ratios, which yieldedunreasonably low ratios of less than or close to 0.700, may bearvery large uncertainties and are not meaningful to constrain theirpetrogenesis (Wu et al., 2002). The age-corrected eNd(t) values ofphlogopites show little variation and range from þ3.66 to þ4.18 andsingle-stage Nd model ages are within 660–710 Ma, whereas the age-corrected eNd(t) values for whole rock samples are þ3.88 to þ5.63.The slightly wider range of eNd(t) for whole rocks may suggest thatthe Nd isotope composition of these samples has been disturbed bycrustal contamination and/or mantle entrainment, as also indicatedby the petrographic characteristics. Overall, the kimberlitic samples(including phlogopite separates) are similar in Sr–Nd isotopic com-position to those of Group I kimberlites (Fig. 6a).

4.5. Baddeleyite Hf isotope compositions

The Lu–Hf analyses of baddeleyite were obtained using thesame mounts which were previously used for U–Pb dating(Supplementary file 8). The weighted average value of calculatedinitial 176Hf/177Hf ratios for DW21-1 baddeleyite obtained as0.28278770.000013 (2sm, n¼23, MSWD¼5.2), corresponds toeHf (t¼300 Ma) value of þ5.0 to þ8.7. The DW21-4 baddelyiteshave similar values which are 0.28276670.000013 (2sm, n¼21,MSWD¼5.2) and þ4.8 to þ8.5 (Fig. 6b). The two baddeleyitesamples show single-stage Hf model ages of 579–735 Ma, similarto the single-stage Nd model ages of the studied phlogopites.

5. Discussion

5.1. Emplacement age of Wajilitag kimberlitic rocks

Most kimberlites are hybrid rocks and susceptible to secondaryalteration, containing a significant proportion of crustal and mantlexenoliths, and therefore the precise determination of their timingof emplacement has remained a challenge (Mitchell, 1986; Li et al.,2010a, 2011a).

Previous Ar–Ar isotopic analyses of phlogopite reported aplateau age of �252.7 Ma interpreted to represent the emplace-ment age of the kimberlitic intrusions (Li et al., 2001, 2011b).Because no information about the analytical details or detaileddata were given in these reports, it is difficult to evaluate thequality of these data. Notably, the Ar–Ar age is incompatible withthe primary magmatic zircon LA-ICPMS U–Pb age of 27276 Mafor a later dolerite dyke in the same region (Li et al., 2007). Asmentioned in a previous section, the dolerite dyke cuts across thekimberlitic intrusions and is therefore younger. In addition, post-magmatic alteration and/or crustal contamination processes arewidespread in the kimberlitic intrusions. Therefore, the youngerAr–Ar plateau age of the kimberlitic intrusions is likely to bequestionable. Moreover, previous studies suggested that mostkimberlitic rock-borne clinopyroxenite xenoliths are rock frag-ments from the Wajilitag ultramafic-mafic intrusions exposed inthe same region, because these clinopyroxenite xenoliths showsimilar field occurrence and close spatial correlation withthe intrusive complex (Li et al., 2001; Jiang et al., 2004; Liet al., 2011b). If this proposal is correct, the kimberlitic intru-sions should have been emplaced later than the ultramafic-maficintrusion which formed at ca. 274 Ma (Zhang et al., 2008).However, our recent studies suggest that these clinopyroxenegrains from the clinopyroxenite xenoliths have TiO2 contentsbetween 0.39 and 1.32 wt%, clearly distinct from most clinopyr-oxene grains from the ca. 274 Ma ultramafic-mafic intrusivecomplex (1.01–2.30 wt%; Li et al., 2012a; Supplementary file 9),implying that there is no direct genetic relationship betweenthem. The marked contrast in Sr–Nd isotopic compositionbetween clinopyroxenite xenoliths and early Permianultramafic-mafic intrusive complex further suggest that they arenot cogenetic (Fig. 6a). Furthermore, no direct intrusive relation-ship has been found between the kimberlitic intrusions and earlyPermian ultramafic-mafic intrusion, suggesting that they are notcogenetic.

Perovskite is considered to be an excellent candidate todetermine the emplacement ages of kimberlites using the U–Pbisotopic method because the mineral typically contains moderateamounts of uranium (50–300 ppm), is one of the late-phaseminerals to crystallize from kimberlitic magma, and is rare incrustal rocks, precluding a xenocrystic origin (Heaman, 1989;Heaman and LeCheminant, 2000; Batumike et al., 2008; Li et al.,2010a). Unfortunately, many perovskite grains from kimberlites,

Fig. 5. U–Pb age data for the Wajilitag baddeleyites. Error bars, error ellipses and uncertainties of weighted average ages are at 2s level.

Fig. 6. Diagram of ISr versus eNd(t) (a) and eHf(t) versus crystal age (b). Data sources: DM, MORB and OIB (Zindler and Hart, 1986); early Permian basalts (Zhou et al., 2009;

Li et al., 2012b; Yu et al., 2011; Tian et al., 2010; Zhang et al., 2010a, 2012d); Wajilitag kimberlitic intrusion-hosted xenoliths (Jiang et al., 2004; Yu, 2009); Wajilitag

ultramafic-mafic intrusion (Zhang et al., 2008; Li et al., 2012b); Late Neoarchean and early Paleoproterozoic basement (Zhang et al., 2008, 2012a). Fields for northern Tarim

basement are based on data from Long et al. (2011a) and references therein. Data for Group I and Group II kimberlite fields are from Becker and le Roex (2006), Coe et al.

(2008), Chalapathi Rao et al. (2011) and references therein. Initial Sr isotope ratios and eNd(t) for these data in (a) recalculated to 300 Ma.

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248 243

such as those in the Mengyin kimberlites, were subjected todifferent levels of alteration after crystallization, which can resetthe U–Pb isotopic system (Yang et al., 2009). In this study, thenarrow compositional range of the Wajilitag perovskites and themorphology of the crystals (Fig. 2a), suggest that these perovs-kites did not undergo any significant metasomatic alteration aftercrystallization. Therefore, the perovskite U–Pb age of �300 Ma isconsidered to be the best estimate for the emplacement age of theWajilitag kimberlitic intrusions.

Baddeleyite is rare in kimberlites and similar rock types butcan occur as diopside-baddeleyite-zirconolite rims on mantlezircon megacrysts, as subsolidus reaction products along zircon–ilmenite–rutile interfaces in the presence of calcite (Heaman andLeCheminant, 1993), or as discrete crystals (Heaman andLeCheminant, 2000; Wu et al., 2010; Li et al., 2011a). The firsttwo baddeleyite types are interpreted as products formed byreaction between zircon megacrysts and kimberlite melt. In thiscase, baddeleyite forms fine idiomorphic crystals which areoften oriented perpendicular to the zircon grain boundary. Thesecrystals commonly have dark central domains and light marginalparts in BSE images, indicating a higher U content in rims than incores (Heaman and LeCheminant, 1993; Li et al., 2011a). As notedabove, the Wajilitag baddeleyite grains occur as either individualgrains or inclusions within late-stage mesostasis. We have notfound any evidence showing reactions between zircon xenocrystsand kimberlite melt. Our analyses suggest that the U content inthe cores is higher than that of the rims, with no residual zirconcore, contrary to what formed by subsolidus reaction betweenmacrocrystic zircon and kimberlitic magma (Heaman and

LeCheminant, 1993). Furthermore, these studied baddeleyitegrains have TiO2 contents between 0.41 and 2.0 wt%, quitedistinct from the high-Ti (up to 6 wt% TiO2) baddeleyite thattypically forms as a subsolidus reaction product along zircon–ilmenite interfaces (Heaman and LeCheminant, 1993, 2000).All these features seem to preclude any possibility of subsolidusreactions involved in the formation of the baddeleyite grains inWajilitag kimberlitic intrusions. The last type of baddeleyitewhich occurs as discrete crystals, has been described in Benfontein,Mbuji-Mayi, Wemindji, Mengyin and Fuxian kimberlites (Scatena-Wachel and Jones, 1984; Heaman and LeCheminant, 1993; Schareret al., 1997; Wu et al., 2010; Li et al., 2011a; Zurevinski andMitchell, 2011). Considering the distinct habit and occurrence ofthese baddeleyite grains, they are interpreted as xenocrysts orprimary groundmass crystals. The petrographic evidence suggeststhat most Wajilitag baddeleyite grains are typically small in sizeand are devoid of any twinning, similar to baddeleyites crystallizedfrom kimberlitic magma (Scatena-Wachel and Jones, 1984;Mitchell, 1986), although markedly different from the baddeleyitemega-xenocrysts (up to 2 cm) from the Mbuji-Mayi kimberlite(Heaman and LeCheminant, 1993; Scharer et al., 1997). A xeno-crystic origin from accidental trapping of baddeleyite crystals fromthe mantle can be also ruled out because no fragments of badde-leyite were found in the kimberlitic rock-hosted mantle xenolithsuite. The fact that these baddeleyites are similar in size to the othergroundmass minerals and occur in close spatial association withsome of the groundmass phases such as apatite and magnetite(Fig. 2b) suggests that crystallization of these minerals wasnear-contemporaneous. Moreover, the Wajilitag baddeleyite is

Fig. 7. Histograms of available age data for the Permo-Carboniferous igneous

rocks in the TC. Data sources: Yu et al. (2011), Tian et al. (2010), Zhang et al.

(2010a, 2012d), Qin et al. (2011) and references therein and this study as well as

authors’ unpublished zircon dating results.

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248244

remarkably pure, containing �95–97 wt% ZrO2, similar to thosedescribed in a primary groundmass and that crystallized fromkimberlitic magmas (96–98 wt%; Scatena-Wachel and Jones, 1984;Zurevinski and Mitchell, 2011). Thus, the Wajilitag baddeleyiteappears to be a late-stage primary groundmass phase that crystallizeddirectly from the kimberlite magma. These baddeleyites thereforelikely record the emplacement age of the kimberlitic magma. Twobaddeleyite samples from the kimberlitic rocks yielded consistentconcordia U–Pb ages of 300.874.7 Ma and 300.574.4 Ma, which arein good agreement with the perovskite U–Pb age of 301.174.1 Ma,further suggesting that the U–Pb age data provide a robust constrainton the timing of emplacement of the kimberlitic rocks.

In summary, the consistency of perovskite and baddeleyite U–Pbages reported in our study provides reliable and precise constraints toinfer that the Wajilitag kimberlitic intrusions were emplaced at theCarboniferous/Permian boundary (�300 Ma) rather than in the latePermian as previously regarded. Interestingly, there is no clear recordof magmatic events in the TC from late Devonian to late Carbonifer-ous times (Jiang et al., 2001; Ge et al., 2012). This has been linked to achange of the tectonic environment of the northern Tarim from anactive continental margin to a passive continental margin during lateDevonian (Ge et al., 2012). The ca. 300 Ma Wajilitag kimberliticintrusions represent the oldest known magmatic event of theCarboniferous magmatism, and is therefore estimate prominentsignal for the initiation of Permo-Carboniferous magmatic event inthe TC (Fig. 7).

5.2. Mantle source and processes

Although it has been widely advocated that the source regions ofkimberlitic rocks have a two-stage evolutionary history, comprisinginitial melt-depletion with subsequent metasomatic enrichment inincompatible elements, the origin of these rocks is still under debate.A wide range of sources have been invoked for kimberlitic magmas,including metasomatized SCLM (e.g., Tainton and McKenzie, 1994;Becker and le Roex, 2006; Chalapathi Rao and Srivastava, 2009),convecting (asthenospheric) mantle (Griffin et al., 2000; Price et al.,

2000) and recycled ancient subducted oceanic crust originating fromthe transition zone or lower mantle (Ringwood et al., 1992; Nowellet al., 2004; Gregorie et al., 2006; Paton et al., 2009). A key aspect ofthe controversy is whether kimberlites are derived from the litho-sphere or the sub-lithosphere. The presence of ultra-deep (4400 km)majorite, ferropericlase, and magnesiowustite inclusions in diamondsand ultra-deep xenoliths entrained within some kimberlites, globallywith OIB-like isotopic signatures, led some researchers to suggest thatthey are convecting mantle melts derived from a transition zonesource (Ringwood et al., 1992), or even from the core-mantleboundary (Haggerty, 1994). However, it is widely believed that theisotopic compositions of the kimberlitic rocks are an overprint of thedepleted mantle source and the late metasomatic component, whichis consistent with a two-stage model of kimberlite formation (Gibsonet al., 1995; Becker and le Roex, 2006; Yang et al., 2009; ChalapathiRao and Srivastava, 2009; Chalapathi Rao et al., 2012). Thesecontrasting models show that the identification of a mantle sourcefor kimberlites based on isotope compositions is a formidable task.Indeed, no ultra-deep mineral assemblages (e.g., majorite, ferroper-iclase, magnesiowustite) have been recognized in the Wajilitagkimberlitic intrusions, indicating that the rock was probably derivedfrom the lithospheric mantle.

Furthermore, petrographic observations indicate that almostall samples from the Wajilitag intrusions contain abundantmagmatic phlogopite and hornblende. It seems reasonable toconclude that the primary magma of the Wajilitag intrusions wasenriched in volatiles, which might have been derived from amantle source that had been metasomatized prior to the mainmelting event that produced the kimberlitic rocks. Additionalsupport for this argument is provided by the wide range in Hfisotopic compositions (eHf(t)¼þ4.76 to þ8.74) of the Wajilitagbaddeleyites, which can be explained by derivation from thesecondary enrichment of depleted mantle source (Scharer et al.,1997). As noted above, the isotopic characteristics can beexplained by the interaction between the depleted SCLM and anenriched metasomatic component. Thus, the broadly OIB-likeisotopic signatures of the Wajilitag kimberlitic rocks do notnecessarily mean that the metasomatic melt was derived fromthe convective asthenospheric mantle. Instead, the negative Nb,Ta, Hf and Ti anomalies in the kimberlites are commonly inter-preted to represent subduction-related signatures (Coe et al.,2008; Chalapathi Rao et al., 2010), which are also applicable inthe Wajilitag kimberlitic rocks (Jiang et al., 2004; Li et al., 2010b).We therefore consider that metasomatism of the kimberliticsource region was most likely related to subduction-relatedevents rather than the effect of the convective (asthenospheric)mantle. For mantle-derived rocks, if the Nd and Hf model ages areolder than their formation age, it can be inferred that the magmawas derived from enriched mantle sources or was contaminatedby crustal materials (Wu et al., 2007; Chalapathi Rao et al., 2011).Their minimum depleted-mantle Nd and Hf model age couldcorrespond to the age of enrichment of the source (Gibson et al.,1995; Becker and le Roex, 2006; Chalapathi Rao et al., 2011).In the study, the Hf model ages (TDM¼579–735 Ma) of badde-leyites which are apparently older than their U–Pb ages, providean important constraint on the timing of source enrichment.Such an inference is also supported by the Nd model ages(TDM¼538–710 Ma). As these samples display extreme incompa-tible trace elements enrichment (Jiang et al., 2004; Li et al.,2010b), their mantle Lu/Hf and Sm/Nd ratios may have changedduring magma genesis, even if this process involved partialmelting of pre-enriched source mantle (Gibson et al., 1995;Chalapathi Rao et al., 2011). Again, considering that the Nd andHf isotope signatures of Wajilitag kimberlitic rocks are depletedrelative to present-day Bulk Earth, perhaps the most plausibleinference is that the mantle enrichment age does not appear to be

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248 245

very old, such as late Neoproterozoic, and the metasomaticenrichment of their source region which may relate to an early-middle Paleozoic subduction event, occurred relatively recently.The regional geology of the northern TC also supports thishypothesis. Some ophiolite melanges and arc-like magmaticevents recognized along the northern margin of the TC have agesaround 600–418 Ma and 422–363 Ma, respectively, which sug-gests an active convergent margin along the northern margin ofthe TC in the early-middle Paleozoic, with southward subductionpolarity (Ge et al., 2012 and references therein). We thus concludethat the Wajilitag kimberlitic intrusions were derived from litho-spheric mantle that has been metasomatized by subductioncomponents prior to the partial melting.

5.3. Implication for the Tarim mantle plume event

Although thermal perturbation of the ambient mantle isessential to trigger kimberlite magmatism, diverse geodynamicmodels have been proposed for the trigger of kimberlite volcan-ism, involving subduction of oceanic lithosphere (e.g., Sharp, 1974;Zhang et al., 2010b; Currie and Beaumont, 2011), impingement ofmantle plume (e.g., le Roex, 1986; Haggerty, 1994; Heaman andKjarsgaard, 2000; Torsvik et al., 2010; Chalapathi Rao et al., 2011)and regional lithospheric extension (e.g., Batumike et al., 2008;Tappe et al., 2008; Moore et al., 2008; Jelsma et al., 2009).Recently, Han et al. (2011) demonstrated that the complexaccretion-collision processes in the Xinjiang region were termi-nated during the late Carboniferous based on the synthesis ofexisting stratigraphic, geochronologic and geochemical results.Thus, the TC and surrounding regions were in a post-collisionalor intraplate extensional settings during most of the late Carbo-niferous and the Permian, with no evidence for the existence of asubduction system. Thus, a subduction setting for the formation ofthe kimberlite magma at this time is highly improbable.

It has been widely accepted that the post-collisional litho-spheric extension occurred as a natural consequence of continen-tal collision between the Tarim block and Kazakhstan-Yili blockduring late Carboniferous (325–316 Ma; Han et al., 2011; Zhanget al., 2012c). Han et al. (2011) proposed that a significantgeodynamic change from convergence to extension in the TCand adjacent tectonic units was initiated at �300 Ma, andattributed to delamination of the thickened lithospheric root,accompanied by passive upwelling of the asthenosphere, leadingto concomitant decompressional partial melting of the lowercrust and underlying lithosphere. This geodynamic scenarioseems likely for the generation of the small volume of theWajilitag kimberlitic magma. Yang et al. (2007) also used themodel of lithospheric extension in the passive continental riftingto explain the intraplate bimodal magmatism in the NW Tarimduring early Permian. However, the fact that the kimberliticintrusions carry microdiamonds indicates the existence of a thick(4140 km) lithospheric mantle in the TC at the time of emplace-ment. Furthermore, some workers have proposed that the mod-ern lithosphere in the TC has a thicknesses of about 140–180 km(Liu et al., 2004; An and Shi, 2006; Lei and Zhao, 2007). Thus, itseems unlikely that large-scale delamination of the thick litho-spheric mantle took place in the TC during most of the lateCarboniferous and Permian. Again, in accordance with the modelof McKenzie and Bickle (1988), the composition and volume ofmelts is directly related to the amount of lithospheric extensionand the potential temperature of the underlying asthenosphere.Therefore, the large volume of Permo-Carboniferous magmatismin the TC would require a relatively large amount of lithosphericextension above a mantle of normal potential temperature.However, there is no evidence of widespread late Carboniferousand early Permian faulting in the TC. Thus, the available evidence

does not concur with a simple model of post-collisional litho-spheric extension to explain the large volume of the Tarimmagmatic province, generated during a short period. Instead, weargue for a petrogenetic model that envisages the formation ofthe Wajilitag kimberlitic rocks through melts generated from anenriched SCLM, triggered by heat conducted and advected bymelts rising from the asthenosphere with high potential tem-perature. This essentially translates into a mantle plume on thebasis of the following considerations.

Although some workers speculated that the Wajilitag kimber-litic intrusions is a part of the Tarim LIP, and was also interpretedas being related to the early Permian mantle plume (e.g., Li et al.,2001, 2011b), the precise magmatic sequence of the Tarim LIP hadnot been identified because of the lack of precise ages for theWajilitag kimberlitic intrusions. The reliable ages for the kimber-litic intrusions reported in this study indicate a significant pre-flood-basalt event, with magma output in the TC at least10 million years before the massive outpouring of flood basalts(Fig. 7). This argues against the existing models that consider thelatter to mark the initial magmatic manifestation of a hot mantleplume. The lack of any obvious age progression for the varioustypes of magmatic activity also poses limitations on the plumemodel. Isotope geochronological data combined with detailedstratigraphic and volcanological studies have revealed a far morecomplex igneous history for LIPs than previously considered. Twoor three distinct pulses in magmatic output are observable inmany continental LIPs (e.g., North Atlantic, Parana-Etendeka,Kerguelen and Ontong Java; Gibson et al., 2006; Bryan andErnst, 2008). The hiatus between pulses varies from case to case,but can be a few to tens of millions of years. The relative extrusivevolumes of pulses can be varied, and the volume of magmaproduced during the late pulse(s) may exceed the volumeproduced during the first stage. Furthermore, it has been pro-posed that the effect of a mantle plume on a cratonic lithospheredepends on the distance from the plume (Griffin et al., 2005;Qin et al., 2011) and also on the thickness of the lithosphere(Gibson et al., 1995; Chalapathi Rao et al., 2011). When a mantleplume impinges on the base of a thick lithosphere, it may causemelting of the readily fusible volatile-rich parts of the metaso-matized SCLM due to heat penetrating by conduction and advec-tion. Production of some volatile-rich magmas such as kimberlitescould be the only surface expression of mantle melting as in thecase of the Alto Paranaıba and Deccan LIPs (Gibson et al., 1995;Chalapathi Rao et al., 2011). As noted above, the lithosphere at thetime of the kimberlitic magma generation may have been toothick to allow the newly ascended, inflated head of a mantlestarting-plume, to rise sufficiently high to trigger extensivemelting of the lithosphere, and therefore only small-volume ofvolatile-rich magmas such as that of the Wajilitag kimberliticintrusions could be the only surface manifestation of SCLMmelting. Likewise, the large time interval (�10 Ma) that separatesthe early-phase Wajilitag kimberlitic magma activity from Tarimflood basalts could be related to the widespread presence of a thicklithosphere at the time of initial impact of a plume. The extensiveeffusion of magma may be deferred for several million years until themagma chamber was sufficiently large and hot, the channel of theplume magma was unblocked and lithospheric thickness becomesthin enough to permit large-scale melting. Based on these arguments,it is further proposed that a mantle plume most likely triggeredWajilitag kimberlitic magmatism. If this is true, then it would implythat the ca. 300 Ma Wajilitag kimberlitic intrusions mark the earliestmagmatic manifestation associated with the initial impact of theTarim mantle plume. In other words, the Wajilitag kimberliticmagmatism may represent the arrival of the mantle plume at thebase of the Tarim lithosphere at least 10 million years before theonset of Tarim flood basalt volcanism that created the LIP.

Fig. 8. Simplified geodynamic model showing the evolution of the SCLM beneath the TC and the origin of the Wajilitag kimberlitic intrusions. (a) Early-middle Paleozoic

and (b) End-Carbonijerous (�300 Ma).

D.Y. Zhang et al. / Earth and Planetary Science Letters 361 (2013) 238–248246

Fig. 8 illustrates our preferred geodynamic model involvingthe following events. (1) Subduction occurring along the northernmargins of TC during early-middle Paleozoic provided small-fraction of subduction components that invaded and metasoma-tized the overlying SCLM (Fig. 8a). (2) Small-degree partialmelting of this previously enriched SCLM by the impingementof a mantle plume produced the Wajilitag kimberlitic magmas(Fig. 8b). Importantly, the data presented in this study demon-strate that there is no evidence of significant melt involvementfrom a convecting mantle in the Wajilitag kimberlitic sourceregion. Therefore, it is likely that the major contribution of theTarim mantle plume might have been the heat to trigger partialmelting of the metasomatized SCLM, albeit with no substantialmelt input to the Wajilitag magma system (Fig. 8b), similar to thescenario reported for the kimberlites of central India (ChalapathiRao et al., 2011).

6. Conclusions

(1)

SIMS perovskite and baddeleyite U–Pb dating shows that theWajilitag kimberlitic intrusions were emplaced at the Carbo-niferous/Permian boundary (�300 Ma) rather than in the latePermian as previously proposed.

(2)

Based on detailed petrographic observations, in conjunctionwith Nd isotopic data of phlogopite separates and Hf isotopecompositions for baddeleyites, the Wajilitag kimberlitic rocksare inferred to have been derived from the subcontinentallithospheric mantle, metasomatized during early-middlePaleozoic by subduction components.

(3)

The end-Carboniferous kimberlitic magmatism, which isvolumetrically minor relative to the Tarim flood basalts,may represent the initial surface magmatic expression ofthe impingement of a hot mantle plume related to the TarimLIP, and is interpreted to constrain the arrival of mantleplume beneath the thick lithosphere of Tarim.

Acknowledgments

We are grateful to EPSL Editor Prof. Mark Harrison and threereferees for their constructive suggestions which helped inimproving this manuscript. We thank Prof. Xian-Hua Li,Dr. Qiu-Li Li, Dr. Yue-Heng Yang, Dr. Suo-han Tang andDr. Zhen-yu Chen for technical support. This research work wasfinancially supported by 973 program (2012CB416806), 305Project of the State Science and technology Program of China

(Nos. 2007BAB25B05 and 2011BAB06B02-04) and 111 Project(No. B07011).

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2012.10.034.

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