Lithos 239 (2015) 142–156

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Origin and geodynamic significance of the early Mesozoic Weiya LP andHT granulites from the Chinese Eastern Tianshan

Ling-Juan Mao a,b, Zhen-Yu He b,⁎, Ze-Ming Zhang b, Reiner Klemd c, Hua Xiang b,Zuo-Lin Tian b, Ke-Qing Zong c,d

a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, Chinab State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, Chinac GeoZentrum Nordbayern, Universität Erlangen, Schlossgarten 5a, 91054 Erlangen, Germanyd State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

⁎ Corresponding author at: Institute of Geology, CAGS 2100037, China. Tel.: +86 10 68999735; fax: +86 10 6899

E-mail address: ahhzy@163.com (Z.-Y. He).

http://dx.doi.org/10.1016/j.lithos.2015.10.0160024-4937/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 July 2015Accepted 18 October 2015Available online 11 November 2015

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by avariety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamicevolution of the CAOB. In this paper, we present detailed petrological and zircon U–Pb geochronological studiesof the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. Thesegranuliteswere previously considered to be a product of a regionalmetamorphic orogenic event. Due to differentbulk-rock chemistries theWeiya granulites, which occur as lenseswithin the contactmetamorphic aureole of theWeiya granitic ring complex, have a variety of felsic–pelitic and mafic granulites with different textural equilib-rium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granulites, and orthopyroxene–clinopyroxene-bearinggranulites. Average P–T thermobarometric calculations and conventional geothermobarometry indicatesthat the Weiya granulites underwent early prograde metamorphism under conditions of 600–650 °C at3.2–4.2 kbar and peakmetamorphismof 750–840 °C at 2.9–6.3 kbar, indicating a rather high geothermal gradientof ca. 60 °C/km. Zircon U–Pb LA–ICP–MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma,which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that theformation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regionalmetamorphic orogenic event. In addition, a Late Devonianmetamorphic age of ca. 380Mawas recorded in zirconmantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxiametamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircongrains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and wereoverprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for thesouthwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenicevent of the Weiya granulites, need to be viewed with caution.

© 2015 Elsevier B.V. All rights reserved.

Keywords:Central Asian Orogenic BeltChinese Eastern TianshanWeiya granulitesEarly TriassicContact metamorphismGeodynamic setting

1. Introduction

The Central Asian Orogenic Belt (CAOB), located between theSiberian Craton to the north, the East European Craton to the west andthe Tarim and North China Cratons to the south (Fig. 1), is a complexcollage of ophiolitic mélanges, island arcs, oceanic crustal remnantsand microcontinental blocks (Ge et al., 2014; He et al., 2015; Jahn,2004; Jahn et al., 2000; Kröner et al., 2014; Şengör et al., 1993; Tonget al., 2014; Windley et al., 2007; Xiao et al., 2009, 2010). The Chinese

6, Baiwanzhuang Road, Beijing7803.

Tianshan composes the southwestern part of the CAOB (Fig. 1; Şengöret al., 1993; Gao et al., 2009; Xiao et al., 2004, 2010; Klemd et al.,2015) and is characterized by a variety of blueschist/eclogite- andgranulite-facies metamorphic rocks, such as the Akeyazi ultrahigh-pressure (UHP) and high-pressure (HP) rocks (Gao and Klemd, 2003;Gao et al., 1999; Klemd et al., 2011, 2015; Tian and Wei, 2013, 2014),the Yushugou ultrahigh-temperature (UHT) and HP granulites (Wanget al., 1999; Zhang et al., 2015; Zhou et al., 2004), the Tonghuashanblueschists (Liu and Qian, 2003), the Weiya granulites (Chen et al.,1997, 1998; Dong et al., 1996; Shu et al., 1999, 2004) and theXingxingxia granulite-facies metamorphic complex (He et al., 2014)(Fig. 1). These high-pressure or high-temperature metamorphic rocksprovide critical constraints on the geodynamic evolution of the ChineseTianshan.

YM

CTA

CTA

83°E

83°E E°59E°19

N°4

4N°

24

N°2

4N°

44

0 40 80 km

Wenquan

Nalati

Kuluketage

0 800 km

BalticaCraton Siberian

Craton

North ChinaCraton

Central

AsianOrogenic

Belt

TarimCraton

Fig. 2

87°E

Tarim Craton AlatageKorla

KYB

Hami

Junggar Basin

Beishan orogenic collage

Main Tianshan Shear ZoneQubulake Fault

Nalati Fault

Northern Tianshan Fault

Urumqi

Xingxingxia

Xingxingxia Complex~380 Ma

He et al., 2014

Akeyazi (U)HP-LT terranepeak metamorphism

~315 Ma (Klemd et al., 2011)

Yushugou granulite~390 Ma

Zhou et al., 2004

Junggar Basin

NTS

NTS

STAC

STAC

SC

WY

TH

BS

XXWeiya granulite

~538 MaShu et al., 2004

Tonghuashan blueschist~360 Ma

Liu and Qian, 2003

NTS

Late Paleozoic granitoid

Early Paleozoic granitoid

Cenozoic

Mesozoic HP/LT metamorphic rocks

Precambrian

Upper PaleozoicLower Paleozoic

Ultramafic rocks

87°E85°EMesozoic granitoid

Turfan Basin

Jingerquan

Fig. 1. Schematic geological map of the Chinese Tianshan (modified after Xiao et al., 2004; Gao et al., 2009). Locations of some high-pressure or high-temperature metamorphic rocks areafter Liu and Qian (2003), Zhou et al. (2004), Shu et al. (2004), Klemd et al. (2011) and He et al. (2014). CTA: Central Tianshan Arc Terrane; KYB: Kazakhstan-Yili-Block; NTS: NorthernTianshan; STAC: Southern Tianshan Accretionary Complex. The representative Triassic granites in the Chinese Eastern Tianshan are also indicated: BS: Baishitouquan; SC: Shuangchagou;TH: Tianhu; WY: Weiya; XX: Xingxingxia; YM: Yamansubei. Insert figure shows a simplified tectonic map of the Central Asian Orogenic Belt (modified after Jahn et al., 2000; Xiao et al.,2010).

143L.-J. Mao et al. / Lithos 239 (2015) 142–156

Among these metamorphic rocks in the Chinese Tianshan, theWeiya granulites have long been considered to represent a regionalmetamorphic orogenic event, which was interpreted to be related toan early Paleozoic plate subduction process (Shu et al., 1999, 2004)or to display the Paleoproterozoic metamorphic basement of theChinese Eastern Tianshan (Chen et al., 1998; Dong et al., 1996). Forexample, Shu et al. (2004) reported a Sm–Nd isochron age of538 ± 24 Ma and amphibole 40Ar/39Ar plateau ages of 432 ± 1 Maand 435 ± 2 Ma for the peak and retrograde metamorphic stagesof the Weiya granulites, respectively, and suggested a major earlyPaleozoic tectonothermal event in the Chinese Eastern Tianshan.While Chen et al. (1998) calculated low- to medium-pressure meta-morphic condition (P = 6–7 kbar), Shu et al. (2004) argued that theWeiya granulites formed under HP condition of ca. 11 kbar and werelocally overprinted by contact thermal metamorphism based onthe presence of cordierite and wollastonite in the samples nearbythe Weiya granitic ring complex. Therefore, the metamorphicpeak P–T conditions and possible subsequent contact metamorphicoverprints as well as metamorphic ages and the geodynamic settingduring the formation of the Weiya granulites are the topic of thepresent contribution.

In this study, we present a detailed petrology and a zircon U–Pb geo-chronology of the Weiya granulites, in order to constrain their forma-tion ages, metamorphic peak P–T conditions, and discuss the data inthe framework of the tectonometamorphic evolution of the ChineseEastern Tianshan. The results suggest that the studiedWeiya granulitesunderwent an Early Triassic low-pressure (LP) and high-temperature(HT) contact metamorphic event. In addition, zircon mantle domainsof the Weiya granulites reveal a Late Devonian regional metamorphicevent.

The mineral abbreviations used in this paper are after Whitney andEvans (2010): Alm = almandine, Amp = amphibole, An = anorthite,And = andausite, Ap = apatite, Bt = biotite, Cpx = clinopyroxene,Crd = cordierite, Grs = grossular, Grt = garnet, Ilm = ilmenite,Kfs = K-feldspar, Myr = myrmekite, Opx = orthopyroxene, Or =orthoclase, Pl=plagioclase, Prp=pyrope, Qz=quartz, Sil= sillimanite,Spl = spinel and Sps = spessartine.

2. Geological setting of the Chinese Tianshan

The Chinese Tianshan stretches from the Gansu–Xinjiang borderwestward to Kazakhstan/Kyrgyzstan, separating the Tarim Craton to thesouth and the Junggar Basin to the north (Fig. 1; Windley et al., 2007;Xiao et al., 2010, 2013). Several main tectonic units are distinguishedin the Chinese Tianshan, namely the Northern Tianshan (NTS), theKazakhstan-Yili-Block (KYB), the Central Tianshan Arc Terrane (CTA)and the Southern Tianshan Accretionary Complex (STAC) (Charvetet al., 2007; Gao et al., 1998, 2009; Jiang et al., 2014; Wang et al., 2011;Xiao et al., 2004, 2013; Yuan et al., 2010). The NorthNalati Fault separatesthe KYB and the CTA. The NTS is mainly represented by the Bayingouophiolite mélanges in the western segment of the Chinese Tianshan. Inthe eastern segment, it mainly consists of Carboniferous pyroclastic andfelsic volcanic rocks (Charvet et al., 2007, 2011; Wang et al., 2007; Xiaoet al., 2004, 2013; Xu et al., 2005, 2006). The KYB is a microcontinentwith Neoproterozoic crystalline basements along its southern and north-ernmargins (Hu et al., 1998;Wang et al., 2014). Late Devonian to Carbon-iferous arc-related volcanic rocks and granitoids developed along the Yilibasin edge (Charvet et al., 2007; Wang et al., 2007). The CTA is also amicrocontinental fragment with Meso- to Neoproterozoic basementrocks that are exposed in the Xingxingxia, Weiya, Alatage and Baluntaiareas. The basement rocks were ascribed as the Xingxingxia Group orthe Xingxingxia metamorphic complex, which mainly consists ofgneisses, schists, amphibolite, migmatites and marbles with protoliths'ages of ca. 1400–900 Ma and metamorphic ages of ca. 380 Ma (He et al.,2014, 2015; Hu et al., 1998; Huang et al., 2014a,b; Liu et al., 2004).The STAC comprises a wide ophiolitic mélange zone including theHeiyingshan, Kulehu, Kumux and Hongliuhe mélanges and the LowerCarboniferous Akeyazi (U)HP/LT terrane (Fig. 1; Gao et al., 1999; Wanget al., 1999; Gao and Klemd, 2003; Zhang and Guo, 2008; Wang et al.,2011; Klemd et al., 2011, 2015; Tian and Wei, 2013, 2014).

3. Geology of the studied area and sampling

The here studied Weiya granulites occur as lenses within thePaleozoic gneissic granites and the Xingxingxia metamorphic complex

Weiya ring complex

Weiya

N′5

4°1

4

94°15′E94°05′E 94°25′E

N′0

4°1

4

0 2 4km

Tianhu ringcomplexQuaternary

Carboniferous strata

Paleozoic granitoid

Xingxingxia metamorphicComplex

Weiya granulites

Kawabulak Group

Granitic ring complex

Fault

Sinian strata

Paleozoic gneissic granite

Sample location

Contact metamorphicaureole

X14-1-12X14-1-10

X14-1-8

X14-4-2

X13-7-2X13-7-5

Fig. 2. Sketched geological map of the Weiya area in the Chinese Eastern Tianshan (modified after RGSTX, 1965, 1992). Note that the previously suggested occurrence of the Weiyagranulites also includes the metamorphic lenses outside the contact aureole (outlined by dashed line; RGSTX, 1992), which are, however, amphibolite-facies metamorphic rocks.

144 L.-J. Mao et al. / Lithos 239 (2015) 142–156

close to theWeiya granitic ring complex (Fig. 2). The GPS positions andthemajormineral assemblages of the studied representative samples oftheWeiya granulites are summarized in Table 1. The rock types includeseveral granulite types with varying mineral parageneses due to differ-ent bulk-rock chemistries (Fig. 3). The outcrop area of the granulitelenses ranges from about 2 × 1 km2 to 10 × 5 km2 and the countryrocks include (garnet-bearing) gneisses and gneissic granites. Thegneissic granites are highly deformed and metamorphosed, and cross-cut by basic dykes. Our geochronological studies indicate that thegneissic granites formed during several magmatic events between 440and 280 Ma (unpublished).

The Weiya granitic ring complex, with an outcrop area of ca.200 km2, is composed of syenogranite, quartz diorite, monzograniteand quartz syenite from the central to the rim region (Zhang et al.,2005, 2008). It has been interpreted as typical post-orogenic A-typegranitoid complex, and the different types of rocks in the complexhave consistent crystallization ages (246 ± 6 to 233 ± 8 Ma; SHRIMPzircon U–Pb ages; Zhang et al., 2005). The ring complex intruded intothe surrounding Paleozoic granitic rocks, the Xingxingxia metamorphiccomplex, the Kawabulak Group and Carboniferous strata. A contactmetamorphic aureole hundreds of meters in width formed around theWeiya granitic ring complex (Fig. 2). The Weiya granulite samples,which were previously investigated (Dong et al., 1996; Shu et al.,

Table 1Mineral assemblages of the Weiya granulites.

Sample Rock Minerals(in sequence of relative mo

X13-7-2 Grt–Crd–Sil granulite Crd, Grt, Bt, Pl, Sil, Qz, Kfs, SX13-7-5 Grt–Sil granulite Pl, Kfs, Grt, Bt, Sil, Crd, Spl,X14-4-2 Crd–Sil granulite Qz, Crd, Kfs, Bt, Sil, IlmX14-1-8 Opx–Crd granulite Crd, Opx, Bt, Spl, IlmX14-1-10 Opx–Crd granulite Crd, Opx, Bt, Qz, KfsX14-1-12 Cpx–Opx granulite Pl, Opx, Cpx, Bt, Amp, Qz

2004) and those of this study, all come from the contact metamorphicaureole of the Weiya granitic ring complex (Fig. 2).

It should be noted that the previous studies reported that theWeiya granulites also occur as lenses within the gneisses outsidethe contact aureole (Fig. 2; RGSTX, 1992). However, in contrast tothe granulite lenses within the contact aureole, the lenses outsidethe contact aureole are mainly garnet-orthopyroxene-absent Sil–Crd–Qz schists and amphibolites with mineral assemblages comprisingCrd+ Sil + Bt+ Pl + Qz+ Ilm and Pl+ Amp+Qz+ Bt, respectively,thereby suggesting amphibolite-facies instead of granulite-faciesconditions.

4. Analytical methods

Bulk-rock major element analysis was performed using a Rigaku-3080 X-ray fluorescence spectrometer at the National Research Centerfor Geoanalysis, Chinese Academy of Geological Sciences (CAGS). Theanalytical precision is generally better than 5% for all elements. Crystalwater (H2O+) contents were determined by ignition above 110 °C andthe ferrous oxide content (FeO) by titration of potassium dichromatestandard solution.

Mineral compositions and compositional mapping of the garnetswere analyzed using a JEOL JXA 8100 microprobe with a 15 kV

dal proportions)Protolith GPS position

pl Pelitic N41°43′36.1″, E94°12′41.5″Qz Felsic N41°43′36.1″, E94°12′41.5″

Felsic N41°43′49.9″, E94°12′32.5″Pelitic N41°43′05.0″, E94°12′58.2″Pelitic N41°43′05.0″, E94°12′58.3″Mafic N41°43′14.6″, E94°12′47.0″

c

Opx-Crd granulite

Cpx-Opx granulited

Grt-Sil granulite

Gneissic graniteGneissic granite

Weiya graniticring complex

b

Grt-Crd-Sil granulite

a

Fig. 3. Field occurrences of theWeiya granulites. (a) theGrt–Crd–Sil granulite, occurring as lenseswithin the gneissic granite; (b) theGrt–Sil granulite and the surrounding gneissic granite;(c) the Opx–Crd granulite; also showing the nearby Weiya granitic ring complex; (d) the Cpx–Opx granulite, outcropping as rounded rocks created by spheroidal weathering.

Table 2Bulk-rock major elements of the Weiya granulites.

Rock Grt–Crd–Silgranulite

Grt–Silgranulite

Crd–Silgranulite

Opx–Crdgranulite

Cpx–Opxgranulite

Sample X13-7-2 X13-7-5 X14-4-2 X14-1-8 X14-1-10 X14-1-12

SiO2 46.91 61.94 69.61 45.13 47.88 53.04TiO2 1.13 0.40 0.72 1.30 1.55 0.93Al2O3 29.28 19.80 16.91 31.79 24.37 19.40FeO 4.77 3.72 8.64 11.98 5.19Fe2O3 0.73 1.02 1.65 1.90 3.48FeOT 13.65 5.43 4.64 10.13 13.69 8.32MnO 0.11 0.26 0.05 0.09 0.12 0.16MgO 4.93 1.03 2.26 7.01 10.39 3.45CaO 0.23 4.49 0.21 0.75 0.20 8.71Na2O 0.46 3.80 0.79 0.59 0.26 3.32K2O 1.49 2.03 3.34 0.73 0.37 1.03P2O5 0.02 0.05 0.03 0.05 0.02 0.42CO2 0.09 0.26 0.34 0.60 0.17H2O+ 1.74 0.76 1.22 0.94 0.82 0.50LOI 1.82 0.26 1.09 0.71 0.23 0.21Total 100.03 99.65 100.01 98.78 99.87 99.51

Note: LOI: loss on ignition.

145L.-J. Mao et al. / Lithos 239 (2015) 142–156

accelerating voltage, 20 nA beam current and 5 μm focused beam at theInstitute of Geology, CAGS. In situ trace element analyses of mineralswere performed at the GeoZentrum Nordbayern of the UniversityErlangen-Nürnberg, Germany, using a single collector quadrupoleAgilent 7500i ICP-MS equipped with an UP193Fx argon fluoride NewWave Research Excimer laser ablation system. The glass reference ma-terial NIST SRM 612 was used as external standard for the silicates.Laser–ICP–MS measurements were conducted using a spot size of50 μm in diameter and a laser frequency of 15 Hz. The Si-content, deter-mined by EMP analysis, was used as internal standard. Reproducibilityand accuracy, which were determined for NIST SRM 610, are usuallyb8% and b6%, respectively. The trace element concentrations werecalculated by GLITTER Version 3.0. Detailed instrument conditions andanalytical procedures are given in Schulz et al. (2006) and Li et al.(2014).

Cathodoluminescence (CL) images of analyzed zircon grains wereobtained using an FEI NOVA NanoSEM 450 scanning electron micro-scope equipped with a Gatan Mono CL4 cathodoluminescence systemat the State Key Laboratory of Continental Tectonics and Dynamics,Institute of Geology, CAGS. Zircon U–Pb dating and trace element anal-yses were carried out synchronously using an Agilent 7500a ICP-MSequipped with GeoLas 2005 laser ablation system at the State Key Lab-oratory of Geological Processes andMineral Resources, ChinaUniversityof Geosciences,Wuhan. Analyseswere conductedwith a beamdiameterof 32 μm, 5 Hz repetition rate. Detailed instrument operating conditionsand data reduction procedures are given in Liu et al. (2010). The zirconstandard 91500 was used as an external standard to calibrate isotopefractionation. The NIST 610 standard was analyzed every tenth analysisin order to correct the time-dependent drift of sensitivity and mass dis-crimination for the trace element analysis. Trace element compositionswere calibrated against NIST 610 using Si as internal standard. Dataprocessing was performed by ICPMSDataCal 7.5 (Liu et al., 2010). Theconcordia diagrams and weighted mean calculations were made usingIsoplot ver. 2.49 (Ludwig, 2001). The age data used in this paper are207Pb/206Pb ages for grains older than 1.0 Ga, and 206Pb/238U ages foryounger grains.

5. Petrology and mineral chemistry

Mineral chemical compositions of five representative samplesare given in Appendix Table 1. The compositional profiles of garnetporphyroblasts from two garnet-bearing granulites (samples X13-7-2andX13-7-5) are listed in Appendix Table 2 and compositionalmappingof garnet from the granulite sample X13-7-5 is illustrated in Fig. 6. Insitu trace element analyses of garnet, plagioclase and cordierite fromthe granulite sample X13-7-2 are given in Appendix Table 3. Bulk-rockmajor elements of the Weiya granulites are listed in Table 2. All fivesamples, with different bulk-rock chemistries and thus varying mineralassemblages, are described in detail below.

146 L.-J. Mao et al. / Lithos 239 (2015) 142–156

5.1. Garnet–cordierite–sillimanite granulite

The Grt–Crd–Sil granulite (sample X13-7-2), which is gray-black incolor and has a massive granoblastic structure, is mainly composedof cordierite (~50 vol.%), garnet (~15 vol.%), biotite (~10 vol.%),plagioclase (~10 vol.%), sillimanite (~5 vol.%), quartz (~3 vol.%), andK-feldspar (~2 vol.%), with spinel, ilmenite, apatite, monazite and zirconas accessory minerals (Fig. 4a). Porphyroblastic garnet is in texturalequilibrium with cordierite, biotite, plagioclase, sillimanite and quartz.Granoblastic cordierite grains mainly form the rock matrix. However,cordierite is also present as inclusions in the garnet porphyroblasts.Some cordierite grains contain mineral inclusions of zircon, spinel andquartz. The garnet porphyroblasts are commonly 1–3 mm in diameterand occur as subhedral grains with inclusion-rich rims and inclusion-poor cores. The inclusions within the garnet porphyroblasts includecordierite, biotite, quartz and ilmenite (Fig. 4a). The bulk-rock major

Bt

Crd

Bt

Opx Crd

Opx

SplCrd

e

a

Crd

Crd

Bt

Bt

QzIlm

Crd

Grt

PlGrt

Qz

Pl

Kfs

Sil

c

pleochroichalos

Bt

Fig. 4. Photomicrographs of theWeiya granulites. (a) Grt–Crd–Sil granulite (X13-7-2), consistinand Crd; (d) Crd–Sil granulite (X14-4-2), containingminerals of Qz, Crd, Kfs, Bt and Sil. The cordulite (X14-1-8), consisting of Crd, Opx, Spl and Bt; (f) Opx–Cpx granulite (X14-1-12), containingelement analysis and the compositional profile of garnet grains illustrated in Figs. 5a and 7, res

element composition of the Grt–Crd–Sil granulite shows a relativelylow SiO2 content (46.91 wt.%), and high Al2O3 (29.28 wt.%), FeOT

(13.65 wt.%) and MgO (4.93 wt.%) contents, suggesting a peliticprotolith.

The matrix cordierite generally shows a lower magnesium value(XMg = 1.00–1.10) than the cordierite (XMg = 1.18–1.28) inclu-sions in the garnet. Biotite has varying TiO2 contents of2.66–6.68 wt.% (Ti = 0.16–0.39 per formula unit). Plagioclase has anandesine–labradorite composition with An = 43–53. K-feldspar ismainly microcline showing a large compositional range of theorthoclase component (Or = 66–86).

The garnet grains have end-member compositions of Alm75–83

Prp11–18Grs3–4Sps3–4, showing a relatively large homogenous core do-main while almandine and spessartine increase and pyrope decreasesin the outermost rim (Fig. 7a), suggesting that the garnet core homoge-nized during HT peak metamorphism while the zoning at the outmost

Sil

Bt

Sil

Crd

d

b

GrtGrt

Bt

Kfs

Pl

Qz

Crd

2mm

fOpx

Opx

Qz

Pl

Pl

Cpx

Cpx

Bt

Amp

Amp

pleochroichalos

g of Crd, Grt, Bt, Pl and Sil; (b–c) Grt–Sil granulite (X13-7-5), consisting of Pl, Kfs, Grt, Bt, Silierite is characterized by the pleochroic halos around zircon inclusions. (e) Opx–Crd gran-Pl, Opx, Cpx, and Bt. The solid lines in (a) and (b) denote the locations of LA–ICP–MS tracepectively.

0.01

0.1

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Gar

net/c

hond

rite

(a)

Pla

gioc

lase

/cho

ndrit

e

Core

Rim

Core

Rim

1000

100

10

1La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(b)

Fig. 5. Chondrite-normalized REE patterns of garnet (a) and plagioclase (b) from the Grt–Crd–Sil granulite (sample X13-7-2).

147L.-J. Mao et al. / Lithos 239 (2015) 142–156

garnet rim resulted from intergrain compositional diffusion duringretrograde metamorphism (e.g., Kohn, 2003; Meyer et al., 2013;Spear, 1991; Ulmius et al., 2015). The garnet is relatively rich in HREEand Y and has a distinct zoning of Y, Sr and HREE from the core to therim (Fig. 5a; Appendix Table 3). Chondrite-normalized REE patterns ofthe core domain are characterized by LREE and HREE enrichmentswith negative Eu anomalies. The rim domain, however, shows a relativedepletion of the REE (Fig. 5a). The distinct zoning of HREE and LREEfrom core to rim is thought to have been caused by Rayleigh fraction-ation during prograde garnet growth (Orejana et al., 2011; Otamendiet al., 2002; Rubatto, 2002). The negative Eu anomaly indicates thatgarnet core, mantle and rim domains were in chemical equilibriumwith plagioclase (Fig. 5b). Trace element analyses reveal high Sr andBa, and low Y contents for the plagioclase. The chondrite-normalizedREE patterns are characterized by relative depletions in LREE andHREE in the core domains, and distinctly positive Eu anomalies(Fig. 5b). Cordierite has REE concentrations below the detection limit(Appendix Table 3).

5.2. Garnet–sillimanite granulite

The Grt–Sil granulite (sample X13-7-5) is similar in appearance tothe Grt–Crd–Sil granulite but contains more felsic veins. The rock dom-inantly consists of plagioclase (~30 vol.%), K-feldspar (~25 vol.%), garnet(~15 vol.%), biotite (~14 vol.%), spinel (~6 vol.%), quartz (~5 vol.%), sil-limanite (~4 vol.%), with minor cordierite and zircon (Fig. 4b and c).Porphyroblastic garnet grains vary in size from 3 mm to 10 mm, withmineral inclusions such as plagioclase, K-feldspar, biotite, quartz andspinel in the core regions. Plagioclase and K-feldspar occur as subhedralgrains in thematrix. Myrmekite consists of plagioclase and quartz inter-growths. Reddish-brown biotite displays subhedral flakes, and silliman-ite occurs as fibrous aggregates (Fig. 4c). The Grt–Sil granulite hasrelatively high SiO2 (61.94 wt.%), K2O (2.03 wt.%) and Na2O(3.80 wt.%) contents, as well as low Al2O3 (19.80 wt.%), FeOT

(5.43 wt.%) and CaO (4.49 wt.%) contents, indicating a felsic protolith.The garnet porphyroblasts have the end-member components

Alm70–79Prp11–21Grs4–6Sps4–6 and show compositional zoning(Figs. 6 and 7b) characterized by an increase of pyrope and a decreaseof almandine from the core to the rim domains. The grossular compo-nent is homogenous with a small decrease in the rim domains, whilethe spessartine component shows a slight decrease from the core tothe rim domains. The outermost garnet rim is characterized by an in-crease of the almandine and spessartine components and a decreaseof the pyrope component, representing intergrain diffusion during ret-rogression (e.g., Kohn, 2003; Meyer et al., 2013; Spear, 1991; Ulmiuset al., 2015). Matrix plagioclase is andesine to labradorite with Anvalues of 36–55, while the plagioclase inclusions in garnet have asimilar andesine composition (An = 40–47). K-feldspar has a high

orthoclase component (Or = 80–92), either in the matrix or asinclusion within garnet. Biotite as inclusions in garnet has lower TiO2

contents (2.50–3.76 wt.%) than the matrix biotite (2.65–4.62 wt.%).

5.3. Cordierite–sillimanite granulite

The granoblastic Crd–Sil granulite (sample X14-4-2) is mainlycomposed of quartz (~35 vol.%), cordierite (~25 vol.%), K-feldspar(~20 vol.%), biotite (~10 vol.%), and sillimanite (~8 vol.%) withminor il-menite, zircon and apatite (Fig. 4d). Porphyroblastic cordierite typicallycontains inclusions of biotite, sillimanite, ilmenite and zircon, withpleochoric halos around zircon inclusions. Sillimanite appears as fibrouscrystals and is usually spatially associatedwith biotite. The Grt–Sil gran-ulite shows a felsic composition with SiO2 = 69.61 wt.%, Al2O3 =16.91 wt.%, K2O = 3.34 wt.% and Na2O = 0.79 wt.%.

5.4. Orthopyroxene–cordierite granulite

The Opx–Crd granulite (samples X14-1-8 and X14-1-10) mainlyconsists of cordierite (~55 vol.%), orthopyroxene (~25 vol.%), biotite(~10 vol.%) and spinel (~5 vol.%), and the accessory minerals ilmenite,apatite and zircon (Fig. 4e). Cordierite contains inclusions of spinel, bio-tite, zircon and ilmenite (Fig. 4e). Orthopyroxene is ferro-hypersthenewith the end-member components En45–48Fs51–55 in sample X14-1-8and En45–51Fs49–52 in sample X14-1-10. Biotite has relatively high TiO2

(N4 wt.%), FeO (15.69–20.63 wt.%) and MgO (9.91–13.05 wt.%)contents. Spinel has FeO and MgO contents of 36.53–37.50 wt.% and4.65–5.54 wt.%, respectively. Bulk-rock major element compositions ofthe Opx–Crd granulites show low SiO2 contents (45.13–47.88 wt.%),and high Al2O3 (24.37–31.79 wt.%), FeOT (10.13–13.69 wt.%) and MgO(7.01–10.39 wt.%) contents, indicating a pelitic protolith.

5.5. Clinopyroxene–orthopyroxene granulite

The Cpx–Opx granulite (sample X14-1-12) has amassive granoblastictextures and is mainly composed of plagioclase (~40 wt.%),orthopyroxene (~25 wt.%), clinopyroxene (~20 wt.%), biotite(~10 wt.%), amphibole (~5 wt.%), and minor quartz (Fig. 4f).Orthopyroxene is hypersthene with the end-member componentsEn49–55Fs41–49Wo2–5. Clinopyroxenes have CaO contents from20.30–22.42 wt.% and Na2O contents from 0.22–0.31wt.% correspond-ing to augitewith the end-member components En34–36Fs20–24Wo41–46.Amphibole occurs along the rims of clinopyroxene and orthopyroxene,suggesting a retrograde origin (Fig. 4f). The amphibole has FeO contentsfrom 17.40–18.24 wt.% and CaO contents from 11.23–11.67 wt.%, and isa calcic amphibole according to the classification of Leake et al. (1997).Plagioclase is andesine to labradorite with An values of 49–64. The

a b

Ca

Mg

Mn

Fe

dc

a b

Fig. 6.X-raymapping of garnet from the Grt–Sil granulite (sample X13-7-5): (a) FeO, (b)MgO, (c) CaO and (d)MnOmapping. Note that the “warm” colors display the high concentrationsand the “cold” colors the low concentrations.

148 L.-J. Mao et al. / Lithos 239 (2015) 142–156

granulite has SiO2 = 53.04 wt.%, Al2O3 = 19.40 wt.%, FeOT = 8.32 wt.%,MgO= 3.45 wt.% and CaO = 8.71 wt.%, suggesting a mafic protolith.

In summary, the studiedWeiya granulites either have a felsic–peliticormafic bulk-rock composition and the textural equilibriummineral as-semblages are Crd +Grt+ Sil + Bt + Pl + Qz+ Spl in the Grt–Crd–Siland Grt–Sil granulites, Crd + Opx+ Bt + Spl in the Opx–Crd granulite,

0.80

0.75

0.70

0.650.60

0.70

0.80

0.90

0.00

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0.15

0.20

0.25

Cor e RimRim

Cat

ion

mol

e fr

actio

n

Cat

ion

mol

e fr

actio

n

(a) X13-7-20.85

0.00

0.05

0.10

0.15

0.20

0.25

Fig. 7. Compositional profile of garnet: (a) Grt–Crd–Sil granulite

Crd+Bt+Sil+Qz in the Crd–Sil granulite and Pl+Opx+Cpx+Bt inthe Cpx–Opx granulite. The presence of cordierite + garnet ± silli-manite ± spinel and the absence of kyanite in the more felsic–peliticgranulites, and the presence of orthopyroxene + clinopyroxene + Ca-plagioclase in the absence of garnet are characteristic for contactmetamorphic conditions (e.g., Miyashiro, 1994).

(b) X13-7-5

Alm

Prp

Grs

Sps

Cor e RimRim

(sample X13-7-2); (b) Grt–Sil granulite (sample X13-7-5).

Table 3Results of the geothermobarometric calculations for the Weiya granulites.

Sample Method Prograde Peak

P (kbar) T (°C) P (kbar) T (°C)

X13-7-2 Average P–T calculation 5.0 ± 0.8 776 ± 54X13-7-5 Average P–T calculation 6.3 ± 1.0 835 ± 76X13-7-2 Garnet–biotite–plagioclase–quartz geothermobarometer 2.9–4.3 746–793X13-7-5 Garnet–biotite–plagioclase–quartz geothermo-barometer 3.2–4.2 604–649 3.6–5.1 793–813X14-1-12 Two-pyroxene geothermometer 802–837a

X14-1-8 Al2O3 in orthopyroxene geothermometer 780a

Average P–T conditions are calculated using THERMOCALC (Powell and Holland, 1994). The garnet–biotite–plagioclase–quartz geothermobarometer is after Wu et al. (2004), the two-pyroxene geothermometer after Taylor (1998), and the Al2O3 in orthopyroxene geothermometer after Berman and Aranovich (1996).

a Calculated at a given pressure of 4 kbar.

149L.-J. Mao et al. / Lithos 239 (2015) 142–156

6. Metamorphic P–T conditions

Average P–T thermobarometric calculations were conducted usingthe approach of Powell and Holland (1994). Peak metamorphic condi-tions were calculated with THERMOCALC (version 3.3.6) using anupdated version (2003) of the internally consistent thermodynamicdata set of Holland and Powell (1998) in the ‘average P–T’ mode(Powell and Holland, 1994). Activities required for the average P–Tcalculation of different endmembers of the mineral assemblage werecalculated using AX (Holland and Powell, 1998) assuming a tempera-ture of 800 °C and a pressure of 4 kbar. The fluid phase was assumedto be pure H2O, which is thought to be a legitimate assumption becausecarbonates, sulfides or carbon minerals are not present in considerableamounts. Mineral compositional data are listed in Appendix Table 1.The calculated P–T conditions are 776 ± 54 °C and 5.0 ± 0.8 kbar forthe assemblage Grt–Crd–Bt–Pl–Kfs–Spl–Sil–Qz–H2O [α(H2O) = 1,cor = 0.813, sigfit = 1.34] of the Grt–Crd–Sil granulite (sample X13-7-2) and 835 ± 76 °C and 6.3 ± 1.0 kbar for the assemblage Grt–Pl–Kfs–Bt–Spl–Crd–Ilm–Qz–H2O [α(H2O) = 1, cor = 0.204, sigfit = 1.31]of the Grt–Sil granulite (sample X13-7-5) (Table 3).

To further constrain the metamorphic P–T conditions of theWeiya granulites, the garnet–biotite–plagioclase–quartz (GBPQ)geothermobarometry for medium- to high-grade metapelites (Wuet al., 2004) were applied to the Grt–Crd–Sil granulite (sampleX13-7-2) and the Grt–Sil granulite (sample X13-7-5) and the two-pyroxene geothermometer (Taylor, 1998) was used for the mafic Opx–Cpx granulite (X14-1-12). The Al2O3 in orthopyroxene geothermometer(Berman and Aranovich, 1996) was used for the pelitic Opx–Crd granu-lite (samples X14-1-8). The used mineral compositional data are listedin Appendix Table 1.

Based on the coexistingmineral assemblage of cordierite+garnet+biotite+ plagioclase+ sillimanite+ quartz+ spinel of the Grt–Crd–Silgranulite (X13-7-2), and to avoid the intergrain diffusion influence onthe rims of the garnet during retrogression, the garnet core composi-tions were used together with the compositions of biotite and plagio-clase in the matrix to calculate minimum peak-metamorphic P–Tconditions. The calculation yielded pressure of 2.9–4.3 kbar and temper-ature of 746–793 °C (Table 3).

The prograde and peak metamorphic P–T conditions of the Grt–Silgranulite (X13-7-5) were estimated using compositions of coexistingGrt (core), Pl (inclusion in garnet) and Bt (inclusion in garnet), and ofGrt (rim), Pl (matrix) and Bt (matrix), respectively. The calculated re-sults display prograde P–T conditions of 3.2–4.2 kbar at 604–649 °C,and peak metamorphic P–T conditions of 3.6–5.1 kbar and 793–813 °C(Table 3).

The two-pyroxene geothermometer (Taylor, 1998) was used for themafic Opx–Cpx granulite (sample X14-1-12). The calculation yieldedtemperatures of 802 to 837 °C at a given pressure of 4.0 kbar(Table 3). Moreover, orthopyroxene of the pelitic Opx–Crd granulite(samples X14-1-8) has Al2O3 content of up to 5.8 wt.%, indicating atemperature of ca. 780 °C at 4 kbar using the geothermometricalapproach of Berman and Aranovich (1996) (Table 3).

In summary, the geothermobarometric calculations suggest that theWeiya granulites underwent prograde metamorphism under P–T con-ditions of 3.2–4.2 kbar and 600–650 °C and peak metamorphism at2.9–6.3 kbar and 750–840 °C. This is in accordance with coexistingorthopyroxene, cordierite and spinel and the absence of garnet in themetapelitic Opx–Crd granulite, since according to the FMAS grid formetapelitic rocks, pressures are b 8 kbar at temperatures of ca. 800 °C(for an extensive discussion see Brandt et al. (2003)). Therefore, it issuggested that the Weiya granulites experienced LP and HT granulite-facies peak-metamorphism at a rather high geothermal gradient of ca.60 °C/km with an isobaric heating P–T path, which is characteristic forcontact metamorphism (cf., Miyashiro, 1994).

7. Zircon U–Pb geochronology

Zircon grains from the six representative samples were analyzed toconstrain the metamorphic and protolith ages of the Weiya granulites.The CL images of representative zircon grains are shown in Fig. 8. TheU–Pb ages and REE compositions of zircon grains are listed in AppendixTable 4 and are graphically shown in Fig. 9.

7.1. Grt–Crd–Sil granulite

Zircon grains from sample X13-7-2 are commonly transparent,colorless with soccerball-like multifaceted crystals, which are similarto zircon of high-temperature granulites (Hoskin and Black, 2000;Rubatto, 2002; Vavra et al., 1999). CL images suggest the presence oftwo zircon grain groups. The first group exhibits patchy or weak zoningwithout inherited cores (Fig. 8a). Most of the zircon grains containmineral inclusions of cordierite, K-feldspar and quartz (analyzed bythe electron probe energy spectrum). The second group has a core-mantle-rim structure. The core has a complicated and variable shape,consistent with that of detrital zircon. Themantle showsweak lumines-cence with a weakly zoned or homogeneous internal structure, whilethe rim is bright luminescent similar to the internal structure of thefirst group zircon.

Eleven spots were located at the core domains, yielding a large agerange of 2205–505 Ma (Fig. 9a). These analyses have variable U(168–1216 ppm) and Th contents (66.0–283 ppm) with Th/U ratios of0.15–0.77, and high REE contents and the REE patterns display a relativeHREE enrichment, with pronounced negative Eu anomalies (Fig. 9b).Some analyses show scattered and high LREE abundances whichmay be due either to submicroscopic, nanoscale mineral inclusions(e.g., Cavosie et al., 2006) or to the modification of zircon domains byinfiltrating fluids as evidenced by their discordant U–Pb ages(e.g., Hoskin and Black, 2000; Sláma et al., 2007). Seven spots from thezircon mantle are nearly concordant and yield consistent 206Pb/238Uages of 397 ± 5 Ma to 356 ± 5 Ma, with a weighted mean age of382 ± 14 Ma (2σ; MSWD = 8.0; Fig. 9a). Their REE patterns are alsocharacterized by high REE contents and a relative HREE enrichment,which, however, is higher than that of the zircon rim (Fig. 9b). Thesespots have relatively low Th contents of 40.0–94.6 ppm and varying U

10 246±6 Ma

32 227±9 Ma

18 247±8 Ma

47 390±7 Ma 50 356±5 Ma

51 505±12 Ma

52 549±25 Ma

21 243±4 Ma

22 243±3 Ma

27 245±3 Ma

18 243±2 Ma

1 317±4 Ma

19 328±10 Ma

24 328±4 Ma

16 328±4 Ma

23 243±4 Ma

37 242±6 Ma

40 244±4 Ma

50 382±6 Ma

53 382±10 Ma

52 382±3 Ma

20 1481±50 Ma

33 1570±50 Ma

4 248±3 Ma

2 243±3 Ma

10 259±5 Ma

12 236±2 Ma

6 227±3 Ma

8 1331±52 Ma

9 1722±42 Ma

11 2684±34 Ma

1 328±4 Ma

2 243±4 Ma 23 244±4 Ma

10 244±7 Ma

30 243±5 Ma

12 328±4 Ma

17 328±4 Ma

19 328±3 Ma

28 328±4 Ma

(a) X13-7-2

(b) X13-7-5

(d) X14-1-8

(e) X14-1-10

(f) X14-1-12

50µm

50µm

50µm

50µm

100µm

15 1395±70 Ma

2 783±5 Ma

3 833±6 Ma

14 833±11 Ma

4 1140±64 Ma

10 1051±52 Ma

13 1028±54 Ma

5 1825±87 Ma

(c) X14-4-2

50µm

Fig. 8. Cathodoluminescence images of representative zircon grains from the Weiya granulites. Solid circles denote U–Pb analysis spot.

150 L.-J. Mao et al. / Lithos 239 (2015) 142–156

contents of 228–723 ppm, and Th/U ratios of 0.06 to 0.27. Twenty-sixspots on the internally patchy zoned zircon grains and the rims of thesecond group zircon grains are nearly consistent and yield a206Pb/238U weighted mean age of 237 ± 3 Ma (2σ; MSWD = 1.11;Fig. 9a). They have relatively low U (78.6–170 ppm) and Th contents(107–184 ppm), and Th/U ratios of 0.64–1.62. These grains have rela-tively low REE contents and fractionated REE patterns with a relativeHREE enrichment and LREE depletion, but the HREE are lower thanthose of the mantle domains (Fig. 9b).

7.2. Grt–Sil granulite

Zircon grains from sample X13-7-5 are subhedral and commonlyshow soccerball-like multifaceted crystals that are 70–120 μm inlength. They also display two groups based on the CL images. Thefirst group zircon is structure-less or patchy zoned. The second

group of zircon grains shows a core-rim structure with a CL-brightand oscillatory-zoned core, and a CL-dark and structure-less rimwhich is similar to the internal structure of the first group of zircongrains (Fig. 8b).

Twenty core analyses cluster on or near the concordia curve andyield 206Pb/238U ages of 321 ± 4 Ma to 328 ± 10 Ma, with a weightedmean age of 327 ± 1 Ma (2σ; MSWD = 0.76; Fig. 9c). These analysesshow U contents of 144–1377 ppm and Th contents of 61.5–619 ppm,and Th/U ratios of 0.37–1.23. Five spots of the first group of zircon grainsand the rimdomains of the secondgroup of zircon grains are nearly con-sistent and yield a 206Pb/238U weighed mean age of 244 ± 1 Ma (2σ;MSWD = 0.16; Fig. 9c). They have relatively high U (747–1726 ppm)and Th contents (220–587 ppm), and Th/U ratios of 0.29–0.35. Boththe core and rim analyses show similar REE patterns with a relativeHREE enrichment and LREE depletion with negative Eu anomalies(Fig. 9d).

151L.-J. Mao et al. / Lithos 239 (2015) 142–156

7.3. Crd–Sil granulite

Zircon grains from sample X14-4-2 are subrounded crystals withlength of 50–100 μm. CL images show that these zircon grains have aweak luminescence and patchy zoned cores mantled by a brightluminescent and thin rim (ca. 5–10 μm thick) (Fig. 8c). Thirteen analy-ses of the core domains yield a large age range of 1909–783 Ma(Fig. 9e). These spots have a large range of U (25.6–1896 ppm) and Th(20.6–262 ppm) contents with Th/U ratios of 0.13–0.80. The analysesshow similar REE patterns characterized by depleted LREE and enrichedHREE (Fig. 9f).

0.00

0.04

0.08

0.12

0.16

0.20

0207 235Pb/ U

Soccerball grains or metamorphic rim237 Ma (2σ); MSWD=1.11; n=26±3

Homogeneous mantle382 4 Ma (2σ); MSWD=8.0; n=7±1

Detrital core( )n=11

(a) X13-7-2

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1 2 3

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240

0.032

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0.060

0.15 0.25 0.35 0.45 0.55

Core32 1 Ma (2σ)MSWD=0.76; n=20

7±

Metamorphic rim or grains244 1 Ma (2σ)MSWD=0.16; n=5

±

207 235Pb/ U

(c) X13-7-5

207 235Pb/ U

1800

1400

1000

600

0.35

0.25

0.15

0.05

(e) X14-4-2

Detrital core( 13)n=

0 2 4 6

Fig. 9. U–Pb concordia diagrams (left column) and chondrite-normalized R

7.4. Opx–Crd granulite

Zircon grains from sample X14-1-8 are subhedral prismatic crystalsthat are 50–100 μm in length. CL images display two groups of zircongrains. The first group exhibits patchy zoned grains. The second grouphas a core-mantle-rim structure with an irregular, weakly oscillatory-zoned core surrounded by a structure-less mantle and a CL-bright rim(Fig. 8d).

Twenty spots, located in the core domains (Fig. 9g), yield 207Pb/206Pbages of 1721 to 1154 Ma. These analyses have variable U and Thcontents of 140–1562 ppm and 42.3–431 ppm, respectively, while

Zir

con/

chon

drit e

(b) X13-7-2

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb

0.01

0.1

1

10

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Detrital coreMantle

Soccerball grains ormetamorphic rim

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb

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CoreMetamorphic rimor grains

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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb

(f) X14-4-2

EE patterns (right column) of zircon grains from the Weiya granulites.

340

300

260

220

0.030

0.034

0.038

0.042

0.046

0.050

0.054

0.058

0.15 0.25 0.35 0.45 0.55 0.65

Core328 1 Ma (2σ)MSWD=0.92; n=14

±

Metamorphic rim or grains24 Ma (2σ)MSWD=1.4; n=16

2±2

207 235Pb/ U

(k) X14-1-12

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb

(l) X14-1-12

CoreMetamorphic rimor grains

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207 235Pb/ U

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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb

Detrital coreMetamorphic rimor grains

Detrital core(n=5)

(i) X14-1-10 (j) X14-1-10

Metamorphic rim or grains24 Ma (2σ)MSWD=2.7; n=5

1±9

1400

1000

600

200

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

0 1 2 3 4207 235Pb/ U

Detrital core(n=20)

(g) X14-1-8

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm LuYb0.01

0.1

1

10

100

1000

10000

Zir

con/

chon

drite

Detrital core

Metamorphic rimor grains

(h) X14-1-8

260

240

220

0.033

0.035

0.037

0.039

0.041

0.043

0.1 0.2 0.3 0.4 0.5

Mean=243±2 Ma (2 )MSWD=0.12, n=17

σ

Metamorphic rim or grains

1800

Mantle 382 5 Ma (2σ)MSWD=0.001; n=3

± Mantle

Fig. 9 (continued).

152 L.-J. Mao et al. / Lithos 239 (2015) 142–156

Th/U ratios range between 0.07 and 0.65. They have highly variable REEcontents with relatively enriched to flat HREE patterns (Fig. 9h). Thelarge spread in LREE concentrations is thought to be due to the analysisof (nanoscale)mineral inclusions (Cavosie et al., 2006) and/or the alter-ation of the respective zircon core domains by infiltrating fluids as sug-gested by their discordant U–Pb ages (Hoskin and Black, 2000; Slámaet al., 2007). Three spots from the zircon mantle are nearly concordantwith a weighted mean 206Pb/238U age of 382 ± 5 Ma (2σ; MSWD =0.001; Fig. 9g). They have U contents of 561–750 ppm and Th contentsof 39.3–133 ppm, and Th/U ratios of 0.06 to 0.24. Seventeen spots on thefirst group of zircon grains and the rims of the second group of zircon

grains yield nearly concordant ages of 246 ± 6 to 236 ± 6 Ma, with aweighted mean of 243 ± 2 Ma (2σ; MSWD = 0.12; Fig. 9g). Theyhave U contents of 190–374 ppm and Th contents of 90.8–200 ppm,and Th/U ratios of 0.31–0.95. Their REE patterns are characterized by ahigher HREE enrichment than those of the mantle and core domains(Fig. 9h).

Zircon grains from sample X14-1-10 are subrounded crystals thatare 60–100 μm in length. They also form two distinct groups based onthe CL images. Thefirst group exhibits a patchy zoned internal structure.The second group shows a core-rim structure. The rims show a similarinternal structure as the first group of zircon grains (Fig. 8e). Five

153L.-J. Mao et al. / Lithos 239 (2015) 142–156

spots, located in the core domains, yield a large age range of 2684 to998 Ma (Fig. 9i). These analyses display U contents of 244–637 ppmand Th contents of 66.1–224 ppm, and Th/U ratios of 0.10–0.68. Fivespots on the first group of zircon grains and the rims of the secondgroup of zircon grains yield 206Pb/238U ages of 255 ± 7 Ma to 223 ±2 Ma, while four relatively consistent analyses give a weighed mean of241±9Ma (2σ;MSWD=2.7; Fig. 9i). The analyses display Th contentsof 71.1–224 ppm and U contents of 421–941 ppm, while Th/U ratiosrange between 0.11 and 0.45. Their REE patterns are characterized bya pronounced HREE enrichment compared to those of the zircon coredomains (Fig. 9j).

7.5. Cpx–Opx granulite

Zircon grains in sample X14-1-12 exhibit subhedral prismatic crystalshapes with lengths ranging between 50 and 120 μm. They exhibitpatchy or fir-tree zoning, which is similar to zircon formed duringgranulite-facies metamorphism (Vavra et al., 1999; Hoskin and Black,2000; Rubatto, 2002). Some zircon grains also have a CL-dark and band-ed core domains (Fig. 8f).

Fourteen analyses on the core plot on or near the concordia curveand yield 206Pb/238U ages of 329–321 Ma, with a weighted mean of328±1Ma (2σ; MSWD=0.92; Fig. 9k). Their REE patterns show a rel-ative HREE enrichment compared to the LREE and small Eu anomalies(Fig. 9l). They have variable Th (44.2–829 ppm) and U contents(123–683 ppm), and Th/U ratios of 0.29 to 1.21. Sixteen spots on thepatchy zircon grains and rims yield 206Pb/238U ages of 244 ± 7 Ma to235 ± 5 Ma with a weighted mean age of 242 ± 2 Ma (2σ; MSWD =1.4) (Fig. 9k). They have relatively low Th (58.2–316 ppm) and U con-tents (131–371 ppm), and Th/U ratios of 0.44 to 1.11. Their REE patternsdiffer from those of the core domains displaying pronounced negativeEu anomalies (Fig. 9l).

8. Discussion

8.1. The significance of the U–Pb zircon dating results

As already mentioned above, the zircon U–Pb data of the Weiyagranulites reveal three age groups. The zircon cores from the Grt–Crd–Sil granulite and the Opx–Crd granulite exhibit the characteristics of de-trital zircon grains (Fig. 8), with varying internal structures, sizes andmorphologies. They further show a large age range (2684–505 Ma)with varying Th/U ratios (0.07–0.77), suggesting that the protolith ofthe host rocks were sedimentary rocks derived from composite sourcesincludingmagmatic or metamorphic origins. This is consistent with thehigh bulk-rock Al2O3 of the Grt–Crd–Sil and the Opx–Crd granulites thatsuggests a pelitic protolith (see above). The age of deposition mustpostdate the age of the youngest detrital zircon, and should also beolder than the metamorphic age of the mantle domains (~380 Ma,see below), suggesting that the host rocks were formed in the earlyPaleozoic (e.g., Miller et al., 2007; Sun et al., 2008). Furthermore, the zir-con cores from the Grt–Sil and the Cpx–Opx granulites have oscillatoryzoning or broad banding (Fig. 8b and f), which is consistent with amag-matic origin. The zircon U–Pb data, yielding ages of 328 ± 1 and 327 ±1 Ma, are interpreted to represent their protolith ages (Fig. 9c and k).Therefore, the Weiya granulites have various protoliths, includingearly Paleozoic sedimentary rocks and late Paleozoic felsic and basicintrusions.

It is worth to note that a zircon mantle with a homogeneous orweakly zoned internal structurewas found in the two sampleswith sed-imentary protoliths, i.e., the Grt–Crd–Sil and the Opx–Crd granulites(Fig. 8a and d). They have relatively low Th/U ratios of 0.06–0.27,which is characteristic for metamorphic zircon (Bröcker et al., 2010;He et al., 2012, 2014; Orejana et al., 2011). Therefore, their ages of382 ± 14 Ma and 382 ± 5 Ma are thought to represent a Devonianmetamorphic event of the Weiya granulites (Fig. 9a and g).

Zircon grains in all Weiya granulite samples have a soccerball-likemultifaceted crystals structure. The zircon rims and some zircon grainsshowpatchy, fir-tree zoning or homogeneous internal structures,whichare the typical characteristics of metamorphic zircon in granulites(Fig. 8; Rubatto, 2002; Bröcker et al., 2010; He et al., 2012, 2014). As re-vealed by electron probe energy spectrum analyses, the zircon grainsfrom the Grt–Crd–Sil granulite contain inclusions of metamorphicminerals, such as cordierite, K-feldspar and quartz. Furthermore, thesezircon grains have low U contents and high Th/U ratios with enrichedHREE patterns, positive Ce- and negative Eu-anomalies indicating thatthey were formed under high temperature conditions and in equilibri-um with a partial melt (Fig. 9; Rubatto, 2002; Bröcker et al., 2010; Heet al., 2012, 2014). The U–Pb dating results of this zircon type from thedifferent granulites show similar ages ranging from 244 ± 1 to 237 ±3Ma(Fig. 9). Therefore,we suggest that the textural equilibriumassem-blages of the Weiya granulites were formed in the early Mesozoic at~240 Ma.

8.2. Geodynamic implications

The U–Pb dating of zirconmantle domains from the Grt–Crd–Sil andthe Opx–Crd granulites suggests a Late Devonian (~380 Ma) metamor-phic age (Fig. 9a and g). Recently, a ca. 380 Ma amphibolite- togranulite-facies metamorphic age of the Xingxingxia metamorphiccomplex, which occurs just to the east of the Weiya area, was reported(He et al., 2014). In addition, theGangou schists and the Yushugou gran-ulites from the Chinese Eastern Tianshan also have similarmetamorphicages (Lin et al., 2011;Wang et al., 1999; Zhou et al., 2004).MetamorphicP–T conditions of 720–730 °C at 4–6 kbar were estimated for theXingxingxia metamorphic complex (He et al., 2014) and 750–960 °Cat 9.7–14.5 kbar for the Yushugou HP granulites (Wang et al., 1999;Zhang et al., 2015). All of them were interpreted to represent a LateDevonian orogenic event related to the closure of the South TianshanOcean (He et al., 2014; Lin et al., 2011; Wang et al., 1999; Zhang et al.,2015). The Late Devonian metamorphic age recorded in the Weiyagranulites further confirms the existence of this orogenic event inthe Chinese Eastern Tianshan. Furthermore, the metamorphic age isobviously older than those of the Akeyazi (U)HP/LT rocks (ca. 315 Ma;Klemd et al., 2011) in the Chinese Western Tianshan. Therefore, thisstudy is consistent with the model that the closure of the SouthTianshan Ocean occurred earlier in the eastern part than in the westernpart of the southwestern CAOB (Dong et al., 2011; Gao et al., 2009; Heet al., 2014; Zong et al., 2012).

As stated above, the metamorphic age of the Weiya granulites isconstrained at 244 ± 1 to 237 ± 3 Ma, which is identical -withinerror- to the crystallization age of the Weiya granitic ring complex(246 ± 6 Ma to 233 ± 8 Ma) (Fig. 2; Zhang et al., 2005). During theTriassic, the Chinese Eastern Tianshan was interpreted to have beenpart of an intraplate extensional tectonic setting that developed enor-mous volumes of mafic–ultramafic and granitic rocks (Gu et al., 2006;Lei et al., 2013; Qin et al., 2011). The Triassic granitic rocks in the EasternTianshan generally show characteristics of A-type granites and are oftenemplaced as ring complexes like the Weiya granitic ring complex(246–233 Ma; Zhang et al., 2005, 2008), the Tianhu granites (241 ±3 Ma; unpublished), the Yamansubei granites (228 ± 1 Ma; Lei et al.,2013), the Shuangchagou granites (252 ± 3 Ma) (Zhou et al., 2010),the Xingxingxia granites (224±11Ma) and the Baishitouquan granites(244± 6Ma) (Li and Chen, 2004) (Fig. 1). Furthermore, the occurrenceof the Weiya granulites is restricted to the immediate vicinity of theWeiya granitic ring complex (Fig. 2). The geothermobarometric calcula-tions indicate that theWeiya granulites underwent progrademetamor-phism under conditions of 604–649 °C at 3.2–4.2 kbar and peakmetamorphic conditions of 746–837 °C at 2.9–6.3 kbar, with a ratherhigh geothermal gradient of ca. 60 °C/km and an isobaric heating P–Tpath that is consistent with contact metamorphic conditions. Further-more, the Weiya granitic ring complex had higher initial parent

154 L.-J. Mao et al. / Lithos 239 (2015) 142–156

magma temperatures, as suggested by the zircon saturation tempera-tures (950–1100 °C; Zhang et al., 2006) and thus could have providedthe heat necessary for the high-grade metamorphism of the countryrocks. Therefore, we suggest that the studied Weiya granulites wereformed during contact metamorphic heating of the intruding Weiyagranitic ring complex.

In addition to theWeiya granulites, other amphibolite- to granulite-facies metamorphic rocks occur in the Chinese Eastern Tianshanwhich were related to contemporaneous magmatism. For example,the staurolite and sillimanite-bearing gneisses (Sm–Nd isochron ageof 292 ± 29 Ma; Wang et al., 1992) in the Jingerquan metamorphic–magmatic belt (Fig. 1) from the Chinese Eastern Tianshan are associatedwith coeval mafic–ultramafic rocks and peraluminous granites. Accord-ing to the geological features, Gu et al. (2006) suggested that the meta-morphic rocks in the Jingerquan belt were derived from intraplatemantle-derived magmas rather than being a product of continentalcollision.

In this context it should be noted that low to medium pressuregranulite-facies metamorphism can also be caused by episodic mantlemelting and periodic thermal pulses in an active continental margintectonic setting which is usually spatially associated with abundantmafic igneous rocks (Bhowmik et al., 2014; Dhuime et al., 2009;Zhang et al., 2013). The tectonic setting of these examples is fundamen-tally different to those created during the classical continent–continentcollisional orogeny (Bhowmik et al., 2014; Dhuime et al., 2009; Zhanget al., 2013). Especially, when seeing that conductive thermal modelingsuggests that intraplate thick granitoid sheets in the crust produce vastthermal perturbations, which can explain granulite formation, anatexisand subsequent (nearly) isobaric cooling and thus corroborate theconcept of regional-scale contact metamorphism (Caggianelli andProsser, 2002).

As already mentioned above, the Weiya granulites were previouslybelieved to represent a regional metamorphic orogenic event, whichwas interpreted to be related to early Paleozoic plate subduction in anactive continental margin (Shu et al., 1999, 2004; Xiao et al., 2013), aspart of the Southern Central Tianshan suture related to the collision inthe Tianshan Orogen (Charvet et al., 2007; Dong et al., 2011) or torepresent Paleoproterozoic metamorphic basement rocks of the CTA(Chen et al., 1997; Hu et al., 1998). However, the present study demon-strates that the Weiya granulites were essentially formed under localcontact metamorphic conditions. Therefore, the geodynamic modelsthat are based on ‘regional metamorphic’ Weiya granulites need to beviewed with caution. However, the Late Devonian amphibolite- togranulite-facies metamorphism (ca. 380 Ma) of the XingxingxiaComplex (He et al., 2014), is also displayed by the mantle domainsof detrital zircon grains from the Weiya granulites, which wereoverprinted by contact metamorphism during the Early Triassic.

9. Conclusions

(1) The Weiya granulites, which occur as lenses in the contactmetamorphic aureole of the Weiya granitic ring complex,have a variety of felsic–pelitic and mafic granulites withtextural equilibrium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granu-lites, and orthopyroxene–clinopyroxene-bearing granulites.Their protoliths comprise early Paleozoic sedimentary rocksand late Paleozoic igneous rocks.

(2) Geothermobarometric calculations were performed for thefelsic–pelitic and mafic granulites. The results suggest thatthe Weiya granulites underwent prograde metamorphismunder conditions of 600–650 °C at 3.2–4.2 kbar and peak meta-morphism of 750–840 °C at 2.9–6.3 kbar, indicating that thegranulites underwent contact metamorphism with a high geo-thermal gradient of ca. 60 °C/km.

(3) Zircon U–Pb dating results suggest that the contactmetamorphicage of the Weiya granulites is 244 ± 1 to 237 ± 3 Ma, which isconsistent with the crystallization age of the nearby Weiyagranitic ring complex. Together with the geology, petrographyand the P–T conditions of the Weiya granulites, we suggest thatthe Weiya granulites were formed by a contact metamorphism,associated with the emplacement of the Weiya granitic ringcomplex.

(4) A Late Devonianmetamorphic age of ca. 380Mawas recorded inthe zirconmantle of the Grt–Crd–Sil and the Opx–Crd granulites,confirming the existence of the Late Devonian orogenic event inthe Chinese Eastern Tianshan,whichwas related to the closure ofthe eastern segment of the South Tianshan Ocean.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2015.10.016.

Acknowledgment

We are grateful to Dr. H. Brätz for assistance with the LA–ICPMSanalysis and to H. Rong for help with the EMP analysis. We want tothank Prof. Marco Scambelluri, Dr. Rongfeng Ge and one anonymousreviewer for their constructive comments on the manuscript. Thiswork was supported by the Program of Excellent Young Geologists ofChina Geological Survey (QNYC2-2012-21) and the Chinese GeologicalSurvey Project (12120115069601, 12120115069201). This paper is acontribution to IGCP 592.

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