Lithos 179 (2013) 320–333

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Petrogenesis of Early to Middle Jurassic granitoid rocks from theGangdese belt, Southern Tibet: Implications for early history ofthe Neo-Tethys

Lishuang Guo a,b, Yulin Liu b,c,⁎, Shuwen Liu b, Peter A. Cawood d,e, Zhenghua Wang f, Hongfei Liu g

a Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, CEA, Beijing 100085, Chinab Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, Chinac State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, Chinad Department of Earth Sciences, University of St Andrews, St Andrews, KY16 9AL, UKe School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australiaf WANBAO Mining Limited, Beijing 100053, Chinag Tibet Institute of Geological Survey, Lhasa 850000, China

⁎ Corresponding author at: School of Earth and Space sci100871, China. Tel.: +86 10 13581731303.

E-mail address: ylliu@pku.edu.cn (Y. Liu).

0024-4937/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.lithos.2013.06.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 December 2012Accepted 17 June 2013Available online 11 July 2013

Keywords:The Gangdese beltJurassic granitoid rocksZircon U–Pb chronologyGeochemistryTectonic setting

The Gangdese belt, Tibet, records the opening and closure of the Neo-Tethyan ocean and the resultant colli-sion between the Indian and Eurasian plates. Mesozoic magmatic rocks generated through subduction of theTethyan oceanic slab constitute the main component of the Gangdese belt, and play a crucial role in under-standing the formation and evolution of the Neo-Tethyan tectonic realm. U–Pb and Lu–Hf isotopic data fortonalite and granodiorite from the Xietongmen–Nymo segment of the Gangdese belt indicate a significantpulse of Jurassic magmatism from 184 Ma to 168 Ma. The magmatic rocks belong to metaluminousmedium-K calc-alkaline series, characterized by regular variation in major element compositions with SiO2

of 61.35%–73.59%, low to moderate MgO (0.31%–2.59%) with Mg# of 37–45. These magmatic rocks are alsocharacterized by LREE enrichment with concave upward trend in MREE on the chondrite-normalized REEpatterns, and also LILE enrichment and depletion in Nb, Ta and Ti in the primitive mantle normalizedspidergrams. These rocks have high zircon εHf(t) values of +10.94 to +15.91 and young two-stage depletedmantle model ages (TDM2) of 192 Ma to 670 Ma. The low MgO contents and relatively depleted Hf isotopecompositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basalticlower crust with minor mantle materials injected. In combination with the published data, it is suggestedthat northward subduction of the Neo-Tethyan slab beneath the Lhasa terrane began by the Late-Triassic,which formed a major belt of arc-related magmatism.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Tibet plateau is divisible into four tectonostratigraphic ter-ranes, which from north to south are: Songpan-Ganze, Qingtang,Lhasa, and Tethyan Himalaya. These are separated by the Jinsha,Bangong-Nujiang, and Yarlung-Tsangpo suture zones, respectively(Fig. 1A, Yin and Harrison, 2000). The Lhasa terrane records the open-ing and closing of the Neo-Tethys ocean basin (Chu et al., 2006; Ji etal., 2009; Kapp et al., 2007; Mo et al., 2008; Wen et al., 2008a,2008b; Zhu et al., 2011). The Gangdese belt is located in the southernmargin of the Lhasa terrane, and consists of the Gangdese Batholithand the associated volcanic rocks (Fig. 1B). Previous investigationshave focused on the chronological framework and petrogenesis of

ences, Peking University, Beijing

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Cenozoic and Cretaceous intrusions within the terrane and suggestedthat the Neo-Tethyan ocean slab was undergoing subduction from theCretaceous, until collision of the Indian and Asian continental massesin the Paleocene (Guan et al., 2011; Harrison et al., 2000; Ji et al.,2009, 2012; Jiang et al., 2012; McDermid et al., 2002; Mo et al.,2005a, 2005b; Schärer et al., 1984; Wen et al., 2008a; Xu, 2010; Zhuet al., 2011).

Compared with Cretaceous and Cenozoic magmatism, the EarlyMesozoic magmatic rocks are relatively rare in the Lhasa terrane(Chu et al., 2006; Ji et al., 2009; Qu et al., 2007; Tang et al., 2010;Zhang et al., 2007). However, the presence of the Jurassic YebaFormation volcanic rocks suggests that there is Early Mesozoicmagmatism associated with the subduction of the Neo-Tethys slabin Late Triassic–Jurassic (Dong et al., 2006; Zhu et al., 2008). In fact,the Early Mesozoic magmatism may have been widely developedbased on the study of the Xigaze Forearc Basin (Wu et al., 2010).This study reports new occurrences of the Early Mesozoic magmatic

Fig. 1. The geological maps of the study areas. (A) Tectonic subdivisions of the Tibet plateau (modified from Decelles et al., 2002). (B) The magmatic rocks distribution of Gangdese belt (revised after 1:200,000 geology maps).

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rocks in the Lhasa terrane. New zircon U–Pb isotopic data for the Ju-rassic intrusions from the Gangdese belt are used to constrain the dis-tribution of the Jurassic magmatism. Combined with the zircon in situHf isotopic analyses, and integrated with previous age data, we aim todetermine the distribution and geodynamic processes controlling Ju-rassic magmatic intrusions during the subduction of the Neo-Tethys.

2. Geological background

The Lhasa terrane, located between the Bangong-Nujiang and theYarlung-Tsangpo suture zones, constitutes the southernmost partof the Asia continent (Fig. 1A). The Gangdese belt, distributed in thesouthern part of the Lhasa terrane, lies immediately north of theYarlung-Tsangpo suture zone, and extends ~2500 km from Kailas inthe west to Linzhi in the east. It is characterized by the widespreadGangdese Batholith, consisting of Jurassic–early Tertiary calc-alkalinegranitoid rocks (Chung et al., 2005; Guynn et al., 2006; Ji et al., 2009;Wen et al., 2008a; Zhu et al., 2011) and the Jurassic Yeba Formation(Dong et al., 2006; Zhu et al., 2008), Cretaceous Sangri Group (Zhu etal., 2009) and Cenozoic Linzizong Group (He et al., 2007; Lee et al.,2007; Mo et al., 2003, 2007, 2008).

The Yeba Formation consists mainly of volcanic and sedimentaryrocks, extending ~250 km from Dazi County in the east to GongboGyamda County in the west. The volcanic rocks are dominated frombottom to top by basalts, felsic lavas and felsic pyroclastic rocks,with some interspersed andesitic layers. Thickness of the volcanicrocks ranges from tens of meters to ~3000 m for the basalts andfrom 2000 m to 7000 m for the felsic lavas and volcaniclastic rocks.Fine-grained sedimentary rocks mainly occur in the upper parts ofthe formation. The Yeba Formation has been metamorphosed togreenschist facies.

TheGangdese Batholith comprises Jurassic, Cretaceous andCenozoicintrusions. The Jurassic intrusions are located in the central andwesternregions of the belt (Fig. 1B), and consist chiefly of tonalite, granodioritesandmonzogranites (Chu et al., 2006; Ji et al., 2009; Qu et al., 2007; Tanget al., 2010; Zhang et al., 2007) (Fig. 2). The Jurassic plutons are in faultcontact with the Cretaceous Sangri Group in the Dazhuqu and Nymoareas. The Cretaceous intrusions include gabbro, diorite, granodiorite,monzodiorite, and tonalite (Fig. 2), and extend from Xietongmen toLilong along both sides of Yarlung-Tsangpo River (Guan et al., 2010,2011; Ji et al., 2009; Wen et al., 2008a, 2008b), and are intruded bythe Cenozoic plutons. Cenozoic intrusions were emplaced into Late Ju-rassic and Early-Cretaceous strata and consist chiefly of granodiorite,quartz-diorite, quartz-monzonite and monzogranite (Guo et al., 2011;Ji et al., 2009, 2012; Wen et al., 2008a).

3. Sample petrology

Ten samples were chosen for zircon U–Pb dating and Hf isotopicanalyses from Xietongmen to Nymo area (Figs. 1B, 2), including 5tonalites (XRX-1, XRX-2, XRX-4, XRX-5 and XN-8) and 5 granodio-rites (XRX-3, XRX-37, XRX-38, XRX-40 and XN-20). The tonalitesare medium to coarse-grained with a mineral assemblage of plagio-clase (50%–60%), quartz (20%–30%), biotite (8%–15%), hornblende(2%–8%) and with minor K-feldspar, and accessory minerals of zirconand apatite (XRX-1, Fig. 3A). Sample XRX-5 (Fig. 3B) shows porphy-ritic texture with phenocrysts of plagioclase and quartz, in a matrixof quartz, biotite, hornblende and minor K-feldspar. Plagioclase inthe sample XRX-4 (Fig. 3C) underwent clay alteration andsericitization. The granodiorites (XRX-3, XRX-37, XRX-38, XRX-40and XN-20) are medium to coarse-grained and consist of plagioclase(40%–55%), quartz (25%–35%), biotite (7%–15%) and hornblende(2%–8%) (XRX-37, Fig. 3D), with accessory phases zircon and apatite.Sample XRX-38 (Fig. 3E) shows a porphyritic texture with the pheno-crysts of plagioclase and quartz, in a matrix of quartz, biotite, horn-blende and minor K-feldspar. Sample XRX-40 (Fig. 3F) underwent

significant alteration, with widespread development of chlorite andepidote.

4. Analytical methods

4.1. Major oxides and trace elements

Major and trace elements, including rare earth elements (REEs),were performed at the Geological Laboratory Center of China Univer-sity of Geosciences (Beijing). Major element measurements were car-ried out using the alkali fusion method and determined by aninductively coupled plasma atomic emission spectrometry (ICP-AES)of the Leeman Labs (type PS-950). The analytical method and proce-dure were described by Liu et al. (2002, 2004).

Trace element concentrations were determined using a MicromassPlatform ICP-HEX-MS (inductively coupled plasma-mass spectrometerwith a hexapole collision cell), analytical accuracy and precision weregenerally better than 5%, and further details of analytical methodswere documented by Han et al. (2007).

4.2. Zircon U–Pb dating

Zircon grains were obtained from crushed rocks using a combinedmethod of heavy liquid and magnetic separation techniques. Separatedzircon grains were hand-picked, then mounted in epoxy resin andpolished to approximately half their thickness. Transmitted and reflectedlight, and cathodoluminescence (CL) images of grains were taken to re-veal their external and internal structures. The U–Pb isotopic analyseswere performed using the laser-ablation, inductively coupled plasmamass spectrometer (LA-ICP-MS). The 206Pb/238U ages are reported withanalytical uncertainties at one-standard deviation (1σ) for one singlespot and two-standard deviation (2σ) for the weighted mean ages.

Zircon U–Pb isotopic analyses of four samples (XRX-1, XRX-5,XN-8, XN-20) were analyzed at the State Key Laboratory of GeologicalProcesses and Mineral Resources, China University of Geosciences(Wuhan). Detailed analytical procedures followed those of Yuan etal. (2004). Zircon 91500 was used as an external standard to normal-ize isotopic fractionation during analysis of unknowns. The NIST610glass was used as an external standard to calculate U, Th, and Pb con-centrations of unknowns. Three samples (XRX-2, XRX-3, XRX-4) wereanalyzed at the Geological Laboratory Center of China University ofGeosciences (Beijing). Zircon Temora was used as an external stan-dard to normalize isotopic fractionation during analysis. Commonlead was corrected according to the methods proposed by Anderson(2002). Data were calculated using GLITTER 4.4 (GEMOC, MacquarieUniversity, Australia). The other three samples (XRX-37, XRX-38,XRX-40) were analyzed at the Key Laboratory of Isotope Geochronol-ogy and Geochemistry, Chinese Academy of Sciences, Beijing. Samplemounts were placed in the two-volume sample cell flushed with Arand He. Laser ablation was operated at a constant energy of 80 mJand at 8 Hz. The ablated material was carried by the He gas to anAgilent 7500a ICP-MS. Element corrections were made for mass biasdrift, which was evaluated by reference to standard glass NIST 610(Pearce et al., 1997). Temora was used as the age standard (206Pb/238U = 416.8 ± 0.2 Ma) (Black et al., 2003). Data were calculatedusing ICP-MS Data Cal 6.7 (Liu et al., 2008).

4.3. Zircon Lu–Hf isotope measurements

In-situ zircon Hf isotope analyses were carried out using a NewWave UP213 laser-ablation microprobe, attached to a NeptuneMulti-collector ICP-MS at the Institute of Mineral Resources, ChineseAcademy of Geological Sciences, Beijing. Instrumental conditions anddata acquisition are described by Hou et al. (2007) and Wu et al.(2006). A stationary spot was used for the present analyses, with abeam diameter of 40 μm. Helium was used as carrier gas to transport

Fig. 2. The geological maps of the study areas. (A) The geological map of Tangbai area (modified after the 1:200,000 geological map of Xietongmen). (B) The geological map of Dazhuqu area (modified after the 1:200,000 geological map ofNanmulin). (C) The geological map of Nymo area (modified after the 1:200,000 geological map of Quxu).

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Fig. 3. Petrographic features of Jurassic granitoid rocks (XRX-1, XRX-37, XRX-5, XRX-38, XRX-40, XRX-3) Q — quartz; Pl — plagioclase; Kfs — K-feldspar; Hb — hornblende; Bi —biotite; Ep — epidote.

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the ablated sample from the laser-ablation cell to the ICP-MS torch. Inorder to correct the isobaric interferences of 176Lu and 176Yb on 176Hf,176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 ratios were deter-mined (Chu et al., 2002). For instrumental mass bias correction Yb iso-tope ratioswere normalized to 172Yb/173Yb of 1.35274 (Chu et al., 2002)and Hf isotope ratios to 179Hf/177Hf of 0.7325 using an exponential law.Themass bias behavior of Luwas assumed to follow that of Yb, andmassbias correction protocol details were described as in Wu et al. (2006)and Hou et al. (2007).

5. Analytical results

5.1. Zircon U–Pb ages

Cathodoluminescence images and concordia plots for the analyzedzircon grains are shown in Figs. 4–6. LA-ICP-MS analyzed zircon U–Pbisotopic data are presented in Supplementary Table A and summarizedin Table 1. The analyzed zircon grains in this study are mostly euhedraland all exhibit oscillatory zoning with high Th/U ratios from 0.3 to 1.5,indicative of their genesis through magmatic crystallization (Corfu et

al., 2003) and the weighted mean ages calculated for each sample areconsidered to represent the age of crystallization of the magma. Onlya single analysis was undertaken for each zircon. Grains were generallyprismatic with sizes between 50 and 250 μmand length/width ratios ofapproximately 1:1 to 2:1 except sample XN-8 which includes columnargrains with length/width ratios of up to 4:1.

A total of 15 analyzed zircon grains from sample XRX-1 (tonalite)display a narrow range of apparent 206Pb/238U ages from181 ± 2 Mato 182 ± 3 Ma (Fig. 6A), yielding a weighted mean 206Pb/238U age of182 ± 1 Ma (MSWD = 0.01).

25 zircon analyses from sample XRX-2 (tonalite) plot on theconcordia with a range of apparent 206Pb/238U ages from 176 ± 3 Mato 186 ± 3 Ma (Fig. 6B), and yield a weighted mean 206Pb/238U age of181 ± 1 Ma (MSWD = 0.45).

For sample XRX-3 (granodiorite) 30 analyses yield apparent 206Pb/238U ages ranging from 172 ± 7 Ma to 204 ± 7 Ma (SupplementaryTable A), and can be divided into two groups. The younger group con-sists of 25 analyses (spots #1–#4, #6, #8–#16, #18, #19, #21–#23)with apparent 206Pb/238U ages from 172 ± 7 Ma to 183 ± 5 Ma,and defines a weighted mean 206Pb/238U age of 178 ± 3 Ma

Fig. 4. CL images of representative zircon grains in samples of XRX-1, XRX-2, XRX-3, XRX-4, XRX-5, andXRX-37, showing internal structures, analytical locations, apparent 206Pb/238U agesand εHf(t) values. Numbers are spot locations in Supplementary Table A and Supplementary Table B.

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(MSWD = 0.24) (Fig. 6C). The other five analyses (spots #5, #7, #17,#20, #24) show apparent 206Pb/238U ages between 191 ± 7 Ma and204 ± 7 Ma, yielding a weighted mean 206Pb/238U age of 198 ± 6 Ma

(MSWD = 0.42) (Fig. 6C). We suggest that the younger group repre-sents the magmatic crystallization age and the older group may reflectthe timing of an earlier magmatic phase.

Fig. 5. CL images of representative zircon grains in samples of XRX-38, XRX-40, XN-8, and XN-20, showing internal structures, analytical locations, apparent 206Pb/238U ages andεHf(t) values. Numbers are spot locations in Supplementary Table A and Supplementary Table B.

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A total of 24 of 25 zircon analyses (except #19) from sampleXRX-4 (tonalite) plot on the concordia curve with apparent 206Pb/238U ages from179 ± 5 Ma to 190 ± 9 Ma (Fig. 6D), and yield aweighted mean 206Pb/238U age of 184 ± 2 Ma (MSWD = 0.26).

17 zircon analyses from sample XRX-5 (tonalite) yield apparent206Pb/238U ages of 179 ± 3 Ma to 181 ± 5 Ma (Fig. 6E), and yield aweighted mean 206Pb/238U age of 180 ± 1 Ma (MSWD = 0.03).

Analyses of 24 zircons from sample XRX-37 (granodiorite) yield206Pb/238U ages of 167 ± 6 Ma to 177 ± 6 Ma (Fig. 6F). One addi-tional analysis (#20) was rejected from the age calculation becauseof the lower Th and U concentrations and the large measurementerror in U–Th–Pb isotopes. The 24 analyses yield a weighted mean206Pb/238U age of 170 ± 2 Ma (MSWD = 0.14).

For sample XRX-38 (granodiorite) 24 analyses plot on theconcordia curve with apparent 206Pb/238U ages ranging from 165 ±5 Ma to 171 ± 7 Ma (Fig. 6G). The 25th analysis (spot #14) wasrejected from the age calculation because of the large measurementerror in the U–Th–Pb isotopes and it plots to the right of the concordiacurve. The 24 analyses give a weighted mean 206Pb/238U age of168 ± 2 Ma (MSWD = 0.08).

A total of 25 analyzed zircon grains from sample XRX-40 (granodi-orite) yielded apparent 206Pb/238U ages from 168 ± 7 Ma to 180 ±

6 Ma (Fig. 6H), which gave a weighted mean 206Pb/238U age of172 ± 2 Ma (MSWD = 0.28).

Analyses of 17 grains from granodiorite sample XN-8(tonalite)plot on the concordia with a narrow range of apparent 206Pb/238Uages from 169 ± 4 Ma to 171 ± 4 Ma (Fig. 6I), and yield a weightedmean 206Pb/238U age of 170 ± 2 Ma (MSWD = 0.04).

For sample XN-20 (granodiorite) 7 zircon grains plot on theconcordia curve with a narrow range of apparent 206Pb/238U agesfrom 179 ± 3 Ma to 182 ± 3 Ma (Fig. 6J), and yield a weightedmean 206Pb/238U age of 180 ± 2 Ma (MSWD = 0.16).

5.2. Zircon Lu–Hf isotopic results

A total of 101 Lu–Hf isotopic analyses are obtained for five samples(XN-8, XRX-1, XRX-3, XRX-4 and XRX-5), and the analytical resultsare listed in Supplementary Table B.

Zircon grains from the Jurassic granitoid rocks have mostly low176Lu/177Hf (b0.003), with 176Hf/177Hf isotopic ratios of 0.282974–0.283165. The calculated εHf(t) values range from +10.94 to+15.91, and the Hf model ages (TDM2) from 192 Ma to 670 Ma. Inthe εHf(t)-age plot (Fig. 7), most of the data fall between the lowercrust trend at 180 Ma and 320 Ma, with some of the data located on

Fig. 6. Zircon U–Pb concordia diagrams of Jurassic granitoid rocks.

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Table 1Summary of the zircon U–Pb isotopic ages of the Mesozoic magmatic rocks in the Gangdese belt.

Location Sample Lithology Longitude Latitude Age (Ma) 2σ Reference

Xietongmen XC5-01 Granodiorite-porphyry vein 88°26.00′ 29°22.00′ 179 5 ②

XC5002 Granodiorite-porphyry vein 88°26.00′ 29°22.00′ 175 5 ②

5036-303 Quartz-augen diorite porphyry 88°26.00′ 29°22.00′ 173 3 ③

Tangbai XRX-1 Tonalite 88°45.54′ 29°21.92′ 182 1 ①

XRX-2 Tonalite 88°45.60′ 29°21.92′ 181 1 ①

XRX-3 Granodiorite 88°41.25′ 29°21.96′ 178 3 ①

XRX-4 Tonalite 88°41.24′ 29°21.94′ 184 2 ①

XRX-5 Tonalite 88°38.19′ 29°21.94′ 180 1 ①

XRX-37 Granodiorite 88°43.45′ 29°21.88′ 170 2 ①

XRX-38 Porphyritic granodiorite 88°43.77′ 29°21.82′ 168 2 ①

XRX-40 Epidotized granodiorite 88°50.67′ 29°22.18′ 172 2 ①

Dazhuqu 06FW164 Monzogranite 89°37.4′ 29°31.32′ 185 4 ④

06FW165 Granodioritic gneiss 89°37.87′ 29°30.2′ 194 4 ④

06FW166 Monzogranite gneiss 89°37.87′ 29°30.2′ 205 3 ④

06FW167 Monzogranite 89°37.92′ 29°26.38′ 156 2 ④

06FW168 Hb-diorite 89°37.92′ 29°26.38′ 174 3 ④

06FW169 Syenogranitic dike 89°37.92′ 29°26.38′ 152 2 ④

XN-8 Tonalite 89°37.85′ 29°29.78′ 170 2 ①

ST134A Biotite granite 89°37.2′ 29°31.2′ 181 1 ⑤

Nymo XN-20 Granodiorite 90°11.47′ 29°21.36′ 180 2 ②

T384 Biotite-monzogranite 90°11.47′ 29°21.36′ 178 1 ⑥

Note: ① = This study; ② = Qu et al. (2007);③ = Tang et al. (2010); ④ = Ji et al. (2009); ⑤ = Chu et al. (2006); ⑥ = Zhang et al. (2007).

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the depleted mantle trend line. This data suggests magma mixingwith initial magmas mainly from the partial melting of both the juve-nile crust and the mantle materials.

5.3. Geochemical results

Whole-rockmajor and trace element data are listed in Table 2. Theanalyzed samples show uniform changed major element composi-tions, with SiO2 contents of 61.35%–75.04%. On the An–Ab–Or plot(Fig. 8A), the analyzed samples plot into the tonalite–granodioritefields. These samples show K2O contents of 1.00%–2.84%, and plotinto the medium-K calc-alkaline series on the K2O–SiO2 diagram(Fig. 8B). Similarly, these samples show Al2O3 contents of 13.27%–16.55%, and plot in the metaluminous range on the A/CNK–A/NK dia-gram (Fig. 8C), and display low MgO (0.31%–2.59%) and higher Mg#(37–45) (Table 2). These rocks all plot in the field of experimentallydetermined partial melts from basalts or amphibolites (Fig. 8D;Martin et al., 2005).

Fig. 7. εHf(t) vs age diagram of Jurassic granitoid rocks.

Rare earth element (REE) patterns decrease towards the heavy REE(Fig. 9A) with (La/Yb)N ratios of 1.86–11.0 on chondrite-normalizedREE diagrams. The majority of samples do not display Eu anomalieswith δEu ratios of 0.92–1.11, except sample XN-20 (granodiorite)which exhibits Eu positive anomaly with δEu ratio of 1.52 (Table 2).The primitive mantle normalized multi-element patterns of thesamples exhibit enriched large ion lithophile elements (LILEs) anddepleted high field strength elements (HFSEs) with Nb, Ta and Tianomalies (Fig. 9B).

6. Discussion

6.1. Jurassic magmatism in the Gangdese belt

The oldest crystalline basement units in the Lhasa terrane are theAndo orthogneisses (885 Ma, Guynn et al., 2006) and NyainqêntanglhaGroup (787 Ma to 748 Ma, Hu et al., 2005), which are exposed in thecentral and northern parts of the terrane. The late Triassic–Jurassicmagmatism extends throughout the Lhasa terrane, and is especiallywell-developed in the central part (Zhu et al., 2011). The previouswork on the Gangdese Batholith suggests that it consists largely ofPaleocene–Eocene intrusions with minor Cretaceous bodies (Coulonet al., 1986; Mo et al., 2005a, 2007; Schärer et al., 1984; Wen et al.,2008a). Recentwork has indicated the presence of some Jurassic bodies,including granitoid rocks, volcanics and sedimentary rocks (Chu et al.,2006; Ji et al., 2009; Qu et al., 2007; Tang et al., 2010; Wu et al., 2010;Yang et al., 2008; Zhu et al., 2009).

Here we identified for the first time the development of Jurassicmagmatism in the Tangbai area, which expands the distribution of theJurassic magmatism. The previously recognized Jurassic magmatismonly occurs in Nymo (XN-20 and Zhang et al., 2007), north of Dazhuqu(Chu et al., 2006; Ji et al., 2009), and Xiongcun areas (Qu et al., 2007;Tang et al., 2010). Our new data in combination with previous worksuggests that there is a nearly continuous E–W-trending Jurassic mag-matic belt between Xietongmen and Nymo, with ages of 152 to205 Ma (Table 1, Fig. 1B). These ages are consistentwith the detrital zir-con ages of 200–150 Ma from the Xigaze Forearc Basin, which has beeninterpreted to be derived from the Gangdese arc (Wu et al., 2010).

Integrated with previous chronological data, our new obtainedzircon U–Pb isotopic data reveal a late Triassic–Jurassic igneous intru-sion belt in the southern margin of the Gangdese belt (Fig. 1B), andthese early Mesozoic granitoid intrusions are chiefly distributed at

Table 2Analytical results of major (%) and trace elements (ppm) for Jurassic granitoid rocks.

Sample XRX-1 XRX-2 XRX-3 XRX-4 XRX-5 XRX-37 XRX-38 XRX-40 XN-8 XN-20

Age (Ma) 182 181 178 184 180 170 168 172 170 180SiO2 61.35 70.88 75.04 67.47 65.16 64.23 64.83 64.02 62.65 73.59TiO2 0.68 0.38 0.16 0.33 0.53 0.50 0.47 0.67 0.51 0.26Al2O3 16.55 13.53 13.31 15.89 16.25 15.38 14.9 14.35 16.52 13.27TFe2O3 6.46 3.48 1.04 3.83 4.66 5.37 5.39 6.48 5.66 1.98MnO 0.13 0.1 0.023 0.04 0.11 0.14 0.14 0.14 0.12 0.071MgO 2.59 1.23 0.31 1.36 1.71 1.98 1.90 2.41 2.29 0.67CaO 5.75 4.19 2.14 3.95 5.08 6.06 5.59 5.43 4.79 2.51Na2O 3.55 3.92 4.10 3.41 4.22 3.66 3.55 3.26 3.60 3.83K2O 1.44 1.74 2.64 2.47 1.00 1.78 2.09 1.72 1.34 2.84P2O5 0.14 0.09 0.04 0.09 0.12 0.17 0.15 0.16 0.11 0.08LOI 1.80 0.39 1.07 0.96 1.14 0.57 0.78 1.04 1.46 0.7Total 100.44 99.93 99.88 99.81 99.96 99.84 99.79 99.68 99.04 99. 08Na2O + K2O 5.05 5.66 6.82 5.95 5.28 5.48 5.70 5.05 5.06 6.73A/CNK 0.93 0.85 0.99 1.03 0.94 0.81 0.81 0.84 1.03 0.95Mg# 44.5 41.4 37.4 41.5 42.3 42.4 41.4 42.6 44.7 40.4AFM 1.12 1.79 6.29 1.90 1.58 1.29 1.27 1.00 1.27 3.14CFM 0.71 1.01 1.84 0.86 0.90 0.93 0.87 0.69 0.67 1.08Test units ② ② ③ ① ② ③ ③ ③ ② ③

Sc 15.62 8.78 0.81 3.03 10.04 11.40 10.90 16.12 16.30 4.32V 137.76 80.7 5.35 29.40 94.20 127.00 132.00 115.54 147.00 30.74Cr 6.34 8.34 9.48 9.83 6.53 5.60 4.94 7.46 6.89 2.98Co 17.76 8.81 0.71 4.07 11.77 14.00 14.50 15.24 15.80 3.60Ni 6.66 3.99 2.67 2.32 4.52 5.62 4.92 5.40 6.23 1.44Cu 34.16 5.82 4.38 348.00 541.61 22.40 47.70 21.10 43.40 1.68Zn 62.94 41.6 55.00 97.80 40.79 57.50 52.80 60.80 68.60 30.68Ga 16.30 14.2 6.68 8.15 17.57 17.30 17.00 15.20 17.00 12.58Rb 33.08 28 29.80 22.90 36.03 38.20 41.60 26.08 36.50 53.78Sr 345.80 262 141.00 180.00 395.96 369.00 365.00 429.00 299.00 330.40Y 18.42 14.5 4.12 8.82 17.91 16.80 17.70 18.70 20.30 6.12Zr 108.54 11.7 134.00 111.00 119.11 11.20 27.3 16.04 9.51 59.14Nb 6.46 5.46 3.26 4.65 9.41 4.94 5.93 3.72 5.38 2.48Mo 0.36 0.19 0.67 0.54 2.98 0.27 0.51 0.26 0.19 0.18Cs 1.54 0.66 0.96 0.90 2.15 2.33 1.44 0.94 3.28 3.28Ba 334.60 325 409.00 353.00 318.47 419.00 489.00 314.20 365.00 470.00La 12.44 12.3 7.96 7.90 16.33 14.80 16.40 5.34 20.80 8.98Ce 25.56 23.6 13.50 14.80 31.12 26.10 30.10 13.22 35.50 15.74Pr 3.20 2.65 1.20 1.73 3.61 3.09 3.39 1.98 3.86 1.74Nd 13.00 10.2 4.01 6.73 13.90 12.40 13.60 9.08 14.90 6.30Sm 3.04 2.17 0.63 1.31 2.98 2.56 2.71 2.50 3.13 1.22Eu 0.96 0.83 0.22 0.40 0.97 0.88 0.89 0.80 0.98 0.58Gd 3.20 2.22 0.57 1.22 3.02 2.46 2.67 2.82 2.97 1.12Tb 0.50 0.43 0.10 0.24 0.47 0.49 0.52 0.46 0.60 0.16Dy 3.14 2.45 0.51 1.46 2.98 2.81 2.88 3.02 3.45 0.98Ho 0.66 0.51 0.12 0.30 0.63 0.59 0.59 0.66 0.72 0.20Er 1.96 1.54 0.43 0.94 1.91 1.75 1.93 2.00 2.14 0.62Tm 0.30 0.26 0.07 0.15 0.28 0.302 0.31 0.30 0.357 0.10Yb 1.96 1.8 0.52 1.05 1.95 2.06 2.03 2.06 2.35 0.70Lu 0.30 0.31 0.09 0.18 0.30 0.33 0.33 0.34 0.37 0.12Hf 2.58 0.69 3.42 0.18 2.80 0.77 1.06 0.58 0.59 1.42Ta 0.54 0.54 0.33 0.49 0.71 0.40 0.47 0.36 0.47 0.26Pb 4.86 8.19 6.90 6.30 3.45 6.29 5.34 11.90 6.05 8.80Th 2.48 5.94 3.64 1.62 3.18 3.90 5.11 1.88 5.79 2.86U 0.70 1.66 0.80 1.02 0.91 1.12 1.39 0.74 1.21 0.46δEu 0.94 1.16 1.11 0.96 0.99 1.08 1.01 0.92 0.98 1.52Test units ② ② ③ ③ ② ③ ③ ② ③ ②

① Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University.② Geological Laboratory Center of China University of Geosciences (Beijing).③ Research Institute of Uranium Geology (Beijing).Note: LOI, loss on ignition; A/CNK = molar(Al2O3)/molar(CaO + Na2O + K2O); Mg# = 100 ∗ molar Mg2+/(Mg2++TFe2+),TFeO = TFe2O3 ∗ 0.8998; AFM = molar (Al2O3)/molar (TFeO + MgO); CFM = molar (CaO)/molar (TFeO + MgO); δEu = Eu/0.058/(Sm/0.153 ∗ Gd/0.2055)1/2.

329L. Guo et al. / Lithos 179 (2013) 320–333

Xiongcun, Dazhuqu, Nymo, and Qulong, from west to east. Similarly,in the same period volcanic rocks of the Yeba Formation extends for~250 km along an E–W trend with a maximum width of 30 kmfrom Dazi County (east of Lhasa) to Gongbo Gyamda County alongthe north side of the Yarlung-Tsangpo River. Overall the late Trias-sic–Jurassic granitoid rocks have a spatially narrower distributionthan Cretaceous–Tertiary granitoid rocks (Ji et al., 2009; Wen et al.,2008a, 2008b). They parallel the Yarlung-Tsangpo suture zone andthe belt of volcanic rocks in the central to eastern section of theGangdese belt.

6.2. Petrogenesis of Jurassic granitoid rocks

These Jurassic granitoid samples display metaluminous andmedium-K calc-alkaline geochemical features (Fig. 8B and C). Mostof these samples have MgO content of 0.31%–2.59% and Mg# valuesof 37–45 (Table 2), plotting within the partial melting range of basalts(PMB) on the MgO versus SiO2 diagram (Fig. 8D, Martin et al., 2005).However, two samples (XRX-2 and XRX-40) plot above the PMB field(Fig. 8D), but are in the partial melting field of basalt on the AFM ver-sus CFM diagram (Fig. 10). All lines of evidence above suggest that the

Fig. 8.Major element compositions of the granitoid gneisses: (A) An–Ab–Or diagram (Barker, 1979); (B) K2O vs. SiO2 classification diagram (after Rollinson, 1993); (C) ANK (molarAl2O3/(Na2O + K2O)) vs. A/CNK (molar Al2O3/(CaO + Na2O + K2O)) (after Maniar and Piccoli, 1989); (D) Mg2O vs. SiO2 plot (PMB: experimental partial melts from basalts oramphibolites; LSA: low silica adakite; HSA: high silica adakite, after Martin et al., 2005).

330 L. Guo et al. / Lithos 179 (2013) 320–333

Jurassic granitoid magma was derived mainly from the partial melt-ing of meta-basalts.

These Jurassic granitoid samples are characterized by LREE en-richment, flat HREE patterns ((Gd/Yb)N ratios of 0.92–1.35), no Eu

Fig. 9. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagram1989).

anomalies (Fig. 9A), and depletion in Nb, Ta and Ti without strongLILE enrichment (Fig. 9B). These features suggest that the Jurassicgranitoid was probably derived from the partial melting of meta-basaltic rocks with hornblende and garnet preserved as residuals

s for granitoid rocks (chondrite and primitive mantle values after Sun and McDonough,

Fig. 10. Molar Al2O3/(MgO + FeOT) (AFM) vs. molar CaO/(MgO + FeOT) (CFM) dia-gram showing the source composition for granitoid rocks (modified from Altherr etal., 2000).

331L. Guo et al. / Lithos 179 (2013) 320–333

during the partial melting. However, no plagioclase is present in theresidual of partial melting based on the absence of negative Eu anom-aly. These lines of evidence indicate that the granitoid magma is de-rived mainly from partial melting of the mafic lower crust, wherethe pressure of partial melting is beyond the stability field ofplagioclase.

The Jurassic granitoid samples XRX-1, XRX-3, XRX-4, XRX-5 andXN-8 have positive εHf(t) values of +10.9 to +15.9, indicatingtheir juvenile source region. This is consistent with the absence of an-cient crustal materials in the Gangdese belt (Ji et al., 2009; Zhu et al.,2011). Some zircon grains have εHf(t) values that are close to thedepleted mantle values, indicating the involvement of mantle compo-nent. This inference is supported by the samples (XRX-2 and XRX-40)with high MgO contents, which plot above the range of partial melt-ing field (Fig. 8D). It is suggested that the Jurassic granitoid magmasare likely produced mainly by partial melting of the juvenile crustalbasaltic rocks with minor input from the depleted mantle.

The Jurassic granitoid rocks range in composition from tonalite tomonzogranite. The coeval volcanic rocks in Yeba Formation consist ofpredominant mafic and felsic volcanic rocks with minor andesites.The mafic volcanic rocks of the Yeba Formation support the mantleinput, which may partially contribute to the genesis of some of theJurassic granitoid.

6.3. Geodynamic implications

Some investigations suggested that the subduction of theNeo-Tethyan ocean began from the Cretaceous (Mo et al., 2005a,2005b; Schärer et al., 1984; Wen et al., 2008a; Zhu et al., 2011,2013) based on that the oldest granitoid rocks found are Cretaceous(Mo et al., 2005a, 2005b; Schärer et al., 1984; Wen et al., 2008a)and the earliest radiolarian assemblages of the Ladinian–Carnian age(237–217) from the Yarlung-Tsangpo suture zone (Zhu et al., 2013and reference therein). Zhu et al. (2013) suggested that the southernedge of the Lhasa terrane was a margin of the Neo-Tethyan back-arcbasin rather than a mature active continental arc during the Late Tri-assic. However, The Luobusa ophiolite located in the east part of theYarlung-Tsangpo suture zone shows the characteristics of N-MORB(Xu, 2010; Zhou et al., 2001). The Sm–Nd isochron age of the plagio-clase and pyroxene in gabbro diabase vein and the whole rock is177 ± 31 Ma (Zhou et al., 2001), and the SHRIMP U–Pb age of the di-abase is 163 ± 3 Ma (Zhong et al., 2006). These lines of evidence in-dicate that the Neo-Tethyan ocean has been opened since the Jurassic.

Zircon geochronology and whole-rock geochemical datapresented in this study provide new evidence for the wide distribu-tion of Late Triassic–Jurassic granitoid rocks (205–152 Ma, Table 1,Fig. 1B) in the Gangdese belt, which were formed in a mature activecontinental arc. These intrusions are hornblende-bearing tonalite–granodiorite–monzogranite, similar to the “typical” rock associationformed in convergent margins. They show highly depleted Hf isotopiccompositions (εHf(t) = +10.94 to +15.91), enriched light-REE andlarge ion lithophile elements (LILEs), and depleted high field strengthelements (HFSEs) with Nb, Ta and Ti negative anomalies, suggesting asubduction-related origin. The granitoid rocks also have a large rangein MgO contents (0.31% to 2.59%) and high Mg# (37–45), lower (La/Yb)N ratios (1.86–11.0) and (Gd/Yb)N ratios (0.92–1.35), indicatingthat the Jurassic granitoid magmas originated from the partial melt-ing of the juvenile mafic lower crust with residuals of amphiboleand less garnet, and involving minor input of mantle melt.

Convergent plate boundaries are major sites of crustal growth(Cawood et al., 2009, 2013; Taylor and McLennan, 1985) with sub-duction induced partial melting of the mantle-wedge leading to arcmagmatism. The Jurassic Yeba Formation is a typical representationof the island arc volcanic rocks (Dong et al., 2006; Zhu et al., 2008).The Yeba Formation basalts have depleted Sr–Nd isotopic composi-tions, indicating that the magma came from the depleted mantle(Zhu et al., 2008), and show enriched light REEs and LILEs, and de-pleted HFSEs with identified Nb, Ta, Ti depletions in the primitivemantle normalized spider diagrams, indicative of the characteristicsof the island arc basalt (Zhu et al., 2008). Some basalts show highAl2O3 contents with the typical features of the high-aluminous basaltin the arc lithological assemblages, which led Zhu et al. (2008) to sug-gest that the Jurassic mafic rocks formed in an island arc environ-ment. The basaltic magma, which underplated island-arc crust,resulted in remelting to form the rhyolitic rocks of the Yeba Forma-tion and the contemporaneous granitoid rocks discussed herein.Moreover, the acidic magma came from the partial melting of juvenilebasaltic crust mixed with the upwelling basaltic magma to form theandesitic magma of the Yeba Formation andesitic rocks.

The Late Triassic–Jurassic granitoid rocks and the coeval Yeba For-mation volcanic rocks are distributed in a liner belt along the south-ern margin of the Lhasa terrane adjacent to the Yarlung-Tsangposuture zone. The occurrence of these rock assemblages suggest thatthe Neo-Tethys subducted northward in the Late Triassic–Jurassic(Chu et al., 2006; Ji et al., 2009; Qu et al., 2007; Tang et al., 2010;Zhang et al., 2007).

7. Conclusions

Combining zircon U–Th–Pb and Lu–Hf isotopic systematics withwhole rock geochemistry of Jurassic granitoids in the Gangdese belthas revealed:

(1) A Jurassic medium K calc-alkaline series of magmatism domi-nated by tonalite and granodiorite were produced from 184to 168 Ma. In combination with the published data, we suggestthat magmatic activity may extend from 205 Ma to 152 Ma.

(2) The Jurassic granitoid rocks have a large range in MgO contents(0.31% to 2.59%) and high Mg# (37–45), (La/Yb)N ratios (1.86–11.0) and (Gd/Yb)N ratios (0.92–1.35), with highly depleted Hfisotopic compositions (εHf(t) = +10.94 to +15.91), indicat-ing that the Jurassic granitoid magmas originated from thepartial melting of the juvenile basaltic lower crust with thehornblende and garnet preserved as residuals of the partialmelting, and with minor mantle melt input.

(3) The Neo-Tethyan slab subducted northward beneath the Lhasaterrane from the beginning of the late Triassic, which producedthe Late Triassic–Jurassic arc magmatism.

332 L. Guo et al. / Lithos 179 (2013) 320–333

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2013.06.011.

Acknowledgments

We thank Y.S. Liu and L. Su for their help with LA-ICP-MS dating,and K.J. Hou for his assistance with Lu–Hf isotopic analyses. Thisstudy was supported by a Research Grant from the Institute of CrustalDynamics, CEA, under Contract No. ZDJ2012-07, the “Researches oftectonics, magmatism evolution, and metallogeny in the Gangdesebelt, Tibet” Program of China Geological Survey (1212010818098)and the Open Research Program of State Key Laboratory of GeologicalProcesses and Mineral Resources, China University of Geosciences,Beijing (GPMR201020).

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