Journal of Asian Earth Sciences 34 (2009) 298–309

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Journal of Asian Earth Sciences

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Early cretaceous subduction-related adakite-like rocks of the Gangdese Belt,southern Tibet: Products of slab melting and subsequent melt–peridotiteinteraction?

Di-Cheng Zhu a,*, Zhi-Dan Zhao a, Gui-Tang Pan b, Hao-Yang Lee c, Zhi-Qiang Kang d, Zhong-Li Liao b,Li-Quan Wang b, Guang-Ming Li b, Guo-Chen Dong a, Bo Liu b

a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 29# Xue-Yuan Road, Haidian District, Beijing 100083, Chinab Chengdu Institute of Geology and Mineral Resources, 610082 Chengdu, Chinac Department of Geosciences, National Taiwan University, Taipei 106, Taiwand Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 November 2007Received in revised form 24 April 2008Accepted 19 May 2008

Keywords:SHRIMP zircon datingIn situ Hf isotope analysis of zirconSubduction-related adakiteEarly CretaceousSouthern Tibet

1367-9120/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jseaes.2008.05.003

* Corresponding author. Tel.: +86 10 8232 1115; faE-mail address: dchengzhu@163.com (D.-C. Zhu).

The limited geochronology and geochemistry data available for the Early Cretaceous igneous rocks of thesouthern Gangdese Belt, southern Tibet, has resulted in the proposal of conflicting geodynamic modelsfor the generation of the widespread Cretaceous igneous rocks in the middle and northern parts of thebelt. To explore this issue, we present SHRIMP U–Pb zircon data and geochemical and Sr–Nd–Pb–Hf iso-topic data for the Mamen andesites from the southern margin of the Gangdese Belt. The Mamen ande-sites, emplaced at 136.5 Ma, are sodic (Na2O/K2O = 1.2–2.3) and have geochemical characteristicstypical of adakites (i.e., high Al2O3, high La/Yb ratios and Sr contents, low Y and HREE contents, and posi-tive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial (87Sr/86Sr)t ratiosof 0.70413–0.70513, positive eNd(t) values of 3.7–5.8, and (206Pb/204Pb)t ratios of 18.37–18.51,(207Pb/204Pb)t ratios of 15.59–15.65, and (208Pb/204Pb)t ratios of 38.43–38.72. In situ Hf isotopic analysesof zircons that had previously been dated by SHRIMP yielded positive initial eHf(t) values ranging from+11.0 to +15.5. A model calculation using trace element and Sr–Nd–Pb isotopic data indicates that severalpercent of subducted sediment is required to generate the Mamen andesites, which were derived via thepartial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridizedby peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan oceanic crust was subductednorthward beneath the Gangdese Belt during the Early Cretaceous at a high angle. Our results are incon-sistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan oce-anic crust in generating the widespread Cretaceous magmatism recorded in the Gangdese Belt.

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1. Introduction

It is traditionally accepted that the Gangdese Belt, locatedbetween the Bangong Tso–Nujiang suture zone to the north andthe Yarlung Zangbo suture zone to the south (Fig. 1a), is not onlyan archetype of a collisional orogen related to India–Asia collision,but also a pre-Cenozoic Andean-style convergent margin associ-ated with northward subduction of Neo-Tethyan oceanic crust(Maluski et al., 1982; Xu et al., 1985; Coulon et al., 1986; XBGMR,1991; Copeland et al., 1995; Yin and Harrison, 2000). Numerousstudies in recent decades on Cenozoic magmatism have helpeddevelop an understanding of the India–Asia collision and relatedCenozoic tectonic processes that led to the formation of the Hima-

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layas and the Tibetan Plateau. However, relatively little work hasfocused on pre-Cenozoic magmatism, resulting in conflicting inter-pretations regarding the geodynamic setting of the widespreadCretaceous magmatism present in the middle and northern partsof the Gangdese Belt (Fig. 1a). The magmatism has been inter-preted to have originated from the southward subduction of Ban-gong Tso–Nujiang oceanic crust (Hsü et al., 1995; Mo et al.,2005; Pan et al., 2006; Zhu et al., 2006, 2008a) or the northwardlow-angle or flat-slab subduction of Neo-Tethyan oceanic crust(Ding et al., 2003; Kapp et al., 2003, 2005, 2007; Leier et al., 2007).

The term ‘adakite’ is widely used to represent silica-rich, highSr/Y and La/Yb volcanic and plutonic rocks that form in a varietyof tectonic settings (e.g., subduction zones, continental collisionzones, and extensional environments) via various petrogeneticprocesses (Defant and Drummond, 1990; Atherton and Petford,1993; Xu et al., 2002; Chung et al., 2003; Hou et al., 2004; Wang

Fig. 1. (a) Tectonic outline of the Tibetan Plateau (modified from Pan et al., 2006). (b) Tectonic map of the Gangdese Belt and distribution of the Sangri Group, Zenong Group,Duoni Formation, and Linzizong volcanic rocks (modified from Zhu et al., 2008a). (c) Map showing the distribution of Mesozoic igneous rocks in the southern Gangdese Belt(modified from Zhang et al., 2005 and Zhu et al., 2008a).

D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309 299

et al., 2005; Guo et al., 2007). Although adakites have been recog-nized in southern Tibet for several years, previously reported rocksare all post-collision adakites (26–10 Ma) derived from the partialmelting of lower crust (Chung et al., 2003; Hou et al., 2004; Guo etal., 2007). Their development has typically been discussed in termsof their significance with respect to the timing of uplift of the Tibe-tan Plateau and the onset of east–west extension within the pla-teau (Chung et al., 2003, 2005; Hou et al., 2004; Guo et al., 2007).No adakites older than 100 Ma had been reported from the Gang-dese Belt until the work of Yao et al. (2006), who described the ele-mental geochemistry of Late Jurassic–Early Cretaceous adakitessummarizing from geological survey. However, these adakites havebeen often questioned due to the lack of good quality age data andgeochemical data.

In this paper, we report the first SHRIMP zircon age data forEarly Cretaceous adakite-like rocks from eastern Zedong, south-ern margin of the Gangdese Belt. We also present new whole-rock elemental, Sr–Nd–Pb isotopic, and in situ zircon Hf isotopicdata with the aim of gaining a better understanding of the pet-rogenesis and subduction history of Neo-Tethyan oceanic crust.Our data provide valuable constraints on the geodynamic pro-cesses involved in the generation of Early Cretaceous magmasin the Gangdese Belt.

2. Geological setting

Tibet is essentially composed of the following four continen-tal blocks or terranes (from north to south): the Songpan–Ganziflysch complex, Qiangtang terrane, Gangdese Belt, and theHimalayan Belt. These blocks are separated by the Jinsha, Ban-gong–Nujiang, and Yarlung Zangbo suture zones, representingPaleo-, Meso-, and Neo-Tethyan oceanic relicts, respectively(Fig. 1a) (cf. Yin and Harrison, 2000). The Yarlung Zangbo suture

zone comprises abundant Jurassic–Cretaceous ophiolites andminor Late Triassic–Middle Jurassic ophiolites (Pan et al.,2006), marking the location where the Neo-Tethyan oceanicdomains were consumed by northward subduction beneath theGangdese Belt during the Early Jurassic to Late Cretaceous (Xuet al., 1985; Harris et al., 1988; Zhu et al., 2008a, and referencestherein).

The Gangdese Belt consists primarily of Paleozoic–Paleogenesedimentary strata and associated igneous rocks (Yin and Harri-son, 2000). The latter include a series of volcanic suites (e.g.,Early Jurassic volcanic rocks of the Yeba Formation, Zhu et al.,2008a; Late Jurassic–Early Cretaceous volcanic rocks of the San-gri Group, Zhu et al., 2006), the voluminous Gangdese batholith(ca. 103–80 Ma, Wen et al., 2008), and the Linzizong volcanicsuccessions (ca. 65–45 Ma, Mo et al., 2006) in the southernGangdese Belt, together with widespread Mesozoic igneous rocks(e.g., Early Jurassic Amdo granitoids, Guynn et al., 2006; LateJurassic–Early Cretaceous volcanic rocks of the Zenong Groupand associated granitoids, Zhu et al., 2006) in the middle andnorthern parts of the Gangdese Belt (Fig. 1b). These igneousrocks define five magmatic episodes that took place at 190–175, 120–110, 100–80, 65–45, and 25–10 Ma, with two mag-matic flare-ups at ca. 110 and 50 Ma (Wen et al., 2008; Zhuet al., 2008b). The Gangdese Belt is traditionally thought to havedetached from Gondwana and then drifted northward, finallyamalgamating with the Qiangtang terrane in the Early Creta-ceous (Kapp et al., 2005). Mesozoic magmatism in the southernGangdese Belt is generally ascribed to the northward subductionof the Neo-Tethyan oceanic crust beneath the Gangdese Belt;however, the geodynamic process of the magmatism in the mid-dle and northern parts of the belt remains a subject of debate(e.g., Kapp et al., 2005, 2007; Zhu et al., 2008b, and referencestherein).

300 D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309

3. Field occurrence and petrography

The Late Jurassic–Early Cretaceous volcano-sedimentarysequences of the Sangri Group, which consists of the underlyingMamuxia Formation (the focus of the present study) and overly-ing Bima Formation, are sporadically exposed in the southernGangdese Belt from Yawa in the west to Sangri County in theeast (Fig. 1c). The general lithological features of the MamuxiaFormation are shown in Fig. 2 and summarized in Table 1. Asa whole, a Late Jurassic–Early Cretaceous age for deposition ofthe Mamuxia Formation is indicated by fossil corals, bivalves,and gastropods observed in bioclastic limestone from the Yawa,Salada, Rongma, Padui, and Mamen sections (Fig. 2; Table 1).Regional comparisons of sedimentary sequence and fossils indi-cate that the major period of volcanism probably started duringthe early stages of deposition of the Mamuxia Formation (Fig. 2),in a shore to shallow sea or continental shelf facies (Zhu et al.,2003). The Mamuxia Formation is concordantly overlain by theEarly Cretaceous Bima Formation, which consists mainly of vol-canic rocks, sandstones and siltstones, slates, and bioclastic crys-talline limestones. The volcanic rocks within the formation(�1500 m thick) vary compositionally from basaltic andesite toandesite and dacite, with typical island-arc geochemical signa-tures (Li and Zhang, 1995).

To constrain the age and geochemical nature of volcanic rockswithin the Mamuxia Formation, samples were collected from theMamen section, where the formation was originally identified(Fig. 2f; Badengzhu, 1979) and is easily accessible. The Mamen sec-tion is located on the south bank of the Yarlung Zangbo River,about 3 km north of the Yarlung Zangbo suture zone (Fig. 2f). Phe-nocrysts within the Mamen andesites are predominantly chlori-tized plagioclase. Minor epidotized amphibole and rareclinopyroxene and magnetite occur. The groundmass is dominatedby abundant plagioclase micro-crystals.

Fig. 2. (a–e) Stratigraphic sections of the Mamuxia Formation, showing the spatial variGeological sketch map of the studied area (Badengzhu, 1979). (g). Entire Mamen sectiMamen andesites, showing sample locations.

4. Analytical techniques

Five of the six samples described in this paper were collected in2003 from the base of the Mamen section; the remaining sample(T203A) was collected in 2005 from the same section (Fig. 2h).Powdered samples were analyzed for major elements by X-rayfluorescence (XRF) at the Analytical Center, Chengdu Institute ofGeology and Mineral Resources, China, with analytical uncertain-ties better than 5%. Trace element concentrations were determinedusing a Perkin Elmer Elan 6000 ICP-MS at the National GeologicalAnalytical Center, Chinese Academy of Geological Sciences, Beijing,China; analytical accuracy and precision were generally better than8%. Further details of analytical methods can be found in Guo et al.(2005). Sample T203A was analyzed for major elements by X-rayfluorescence using a Rigaku RIX-2000 spectrometer and for traceelements by ICP-MS using an Agilent� 7500s, both housed at theDepartment of Geosciences, National Taiwan University, Taiwan;further details can be found in Chung et al. (2003).

Whole-rock Nd and Sr isotopic compositions were determinedusing a multicollector Finnigan MAT-261 mass spectrometer oper-ated in static multicollector mode at the Laboratory for RadiogenicIsotope Geochemistry, Institute of Geology and Geophysics, Chi-nese Academy of Sciences, Beijing (IGGCAS), China. Measured87Sr/86Sr and 143Nd/144Nd ratios were normalized to86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively, for massfractionation correction. During the period of data acquisition, themean 87Sr/86Sr ratio of NBS987 standard was 0.710254 ± 16 (n = 8),the mean 143Nd/144Nd ratio of La Jolla standard was 0.511862 ± 7(n = 12), and standard BCR-1 yielded 143Nd/144Nd = 0.512626 ± 9(n = 12). Pb isotopic ratios were measured using a VG354 massspectrometer at the National Geological Analytical Center, ChineseAcademy of Geological Sciences, Beijing, China. The standardNBS981 yielded 204Pb/206Pb = 0.059003 ± 0.000084 (n = 6),207Pb/206Pb = 0.91449 ± 0.00017 (n = 6), and 208Pb/206Pb =

ation of different rock types (Gao et al., 1994; Xie et al., 2003; Zhu et al., 2003). (f)on, showing sample locations (modified from Badengzhu, 1979). (h) Profile of the

Tabl

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2100

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D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309 301

2.16691 ± 0.00097 (n = 6). The average 2r uncertainties for206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were 0.7%, 0.3%, and0.6% per atomic mass unit, respectively. Details of the analyticalprocedures employed in measuring Sr–Nd–Pb isotopes can befound in Guo et al. (2005).

Zircons were successfully separated from a relatively coarse-grained sample (MM02-3) using standard density and magneticseparation techniques at the Special Laboratory of the GeologicalTeam of Hebei Province, China. In situ zircon U–Pb dating was car-ried out using a SHRIMP II at the Beijing SHRIMP Lab, ChineseAcademy of Geological Sciences, China, with analytical conditionsthe same as those reported in Liu et al. (2006). U–Th–Pb isotope ra-tios were measured relative to the zircon standard TEMORA (Blacket al., 2003).

In situ Hf isotope measurements were subsequently performedon the dated spots within the zircons using LA-MC-ICP-MS,equipped with a 193 nm laser, at the IGGCAS. A stationary spotwith a beam diameter of about 63 lm was used for the analyses.Instrumental conditions and data acquisition were generally as de-scribed by Wu et al. (2006). During analyses, the 176Hf/177Hf and176Lu/177Hf ratios of the standard zircon (91500) were0.282322 ± 22 (2rn, n = 28) and 0.000318, consistent with the val-ues (0.282307 ± 31, 2rn, n = 44) obtained previously in this labora-tory (Wu et al., 2006).

5. Results

5.1. Zircon SHRIMP U–Pb data

Zircon SHRIMP U–Pb data are summarized in Table 2 andshown in Fig. 3. Cathodoluminescence (CL) images of zircon dem-onstrate that the grains are mostly 100–250 lm in size (Fig. 3a). Allof the zircons show similar crystal forms, with no resorption orinherited cores. The U and Th contents of analyzed zircons are39–241 and 40–234 ppm, respectively, with Th/U ratios rangingfrom 0.62 to 1.07. These ratios are higher than those of metamor-phic zircons (typically <0.1), but consistent with those of magmaticzircons (Hoskin and Black, 2000). Fourteen U–Pb analyses yieldedages of 141.2 to 119.4 Ma. The concordant curve (Fig. 3b) reflectsrelatively large uncertainties associated with the 207Pb/235U ages,possibly related to correction for common lead, which is difficultto determine precisely. This uncertainty is relatively minor forthe obtained 206Pb/ 238U ages; consequently, we refer to 206Pb/238U ages when considering the crystallization age of the Mamenmagma. With the exception of two discordant spots (13.1 and14.1), 12 analyses yield a weighted mean 206Pb/238U age of136.5 ± 1.7 Ma, with a MSWD of 0.99 at the 95% confidence interval(2r). We therefore conclude that Mamen volcanism occurred inthe Early Cretaceous, consistent with the constraints of age-diag-nostic fossils (Badengzhu, 1979).

5.2. Whole-rock geochemistry

Whole-rock geochemical data of analyzed Mamen andesites arelisted in Table 3. Major element compositions are normalized to100% on a volatile-free basis. Mamen lavas are characterized by alimited range in SiO2 content (56–63%), and plot in the medium-and high-potassic andesite domains on a K2O vs. SiO2 diagram(Rollinson, 1993) (Fig. 4). All Mamen lavas record Al2O3 contentsgreater than 15% and high Na2O concentrations (up to 5.2%). Thesodic character of these lavas is reinforced by Na2O/K2O ratios ashigh as 2.3.

Mamen lavas display low concentrations of heavy rare earthelements (HREEs) and Y (e.g., Yb = 0.71–1.08 ppm; Y = 8.7–13.5 ppm). These characteristics, together with high Sr contents(476–1755 ppm) and Sr/Y ratios (45–73), indicate that the samples

Table 2SHRIMP zircon data of a Mamen adakite-like rock (MM02-3), southern Tibet

Spot U (ppm) Th (ppm) Th/U f206c (%) 206Pb* (ppm) 207Pb*/235U (±1r) 206Pb*/238U (±1r) 206Pb/238U (Ma; ±1r)

MM02-3 (N29�15.2100 , N91�58.5310 , 3638 m)MM02-3-1.1 67 47 0.70 9.94 1.37 0.166 (0.075) 0.0216 (0.0009) 137.8 (5.4)MM02-3-2.1 74 53 0.72 4.07 1.36 0.134 (0.042) 0.0206 (0.0006) 131.5 (3.6)MM02-3-3.1 241 234 0.97 2.84 4.51 0.129 (0.022) 0.0212 (0.0004) 135.0 (2.4)MM02-3-4.1 135 114 0.84 1.00 2.52 0.157 0.014) 0.0216 (0.0004) 137.6 (2.6)MM02-3-5.1 183 162 0.89 1.58 3.41 0.185 (0.028) 0.0214 (0.0004) 136.4 (2.7)MM02-3-6.1 86 84 0.98 2.20 1.64 0.241 (0.020) 0.0218 (0.0005) 139.1 (3.0)MM02-3-7.1 194 197 1.02 1.32 3.70 0.178 (0.016) 0.0219 (0.0004) 139.5 (2.5)MM02-3-8.1 130 110 0.84 4.22 2.48 0.150 (0.053) 0.0212 (0.0006) 135.3 (3.7)MM02-3-9.1 121 88 0.73 0.48 2.31 0.211 (0.030) 0.0221 (0.0004) 141.2 (2.8)MM02-3-10.1 174 143 0.82 0.00 3.16 0.217 (0.009) 0.0211 (0.0004) 134.8 (2.3)MM02-3-11.1 65 40 0.62 5.59 1.22 0.253 (0.056) 0.0205 (0.0007) 130.5 (4.1)MM02-3-12.1 65 45 0.69 3.65 1.21 0.237 (0.043) 0.0210 (0.0006) 133.9 (3.6)MM02-3-13.1 210 224 1.07 1.62 3.65 0.169 (0.020) 0.0199 (0.0004) 126.8 (2.2)MM02-3-14.1 39 40 1.04 4.70 0.658 0.328 (0.072) 0.0187 (0.0006) 119.4 (4.0)Weighted mean (without discordant spots 13.1 and 14.1, 95% confidence, MSWD = 0.99) 136.5 (1.7)

f206c denotes the proportion of common 206Pb in total measured 206Pb*. * denotes radiogenic lead.

Fig. 3. Cathodoluminescence image (a) and concordia plot (b) of zircon SHRIMP data for the Mamen andesite (sample MM02-3) in the southern Gangdese Belt, southernTibet. Solid and dashed circles indicate the locations of SHRIMP U–Pb analyses and LA-MC-ICP-MS Hf analyses, respectively. The SHRIMP U–Pb ages and eHf(t) values aregiven for each spot.

302 D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309

can be classified as adakites as defined by Defant and Drummond(1990) (Fig. 5a), although the samples also exhibit relatively highMgO contents (3.53–5.83%), high Mg-numbers (57.8–72.9), andhigh concentrations of compatible elements (e.g., Cr = 176–225 ppm; Ni = 105–143 ppm).

The samples display small positive Eu anomalies (Eu/Eu* = 0.96–1.35) and have steep heavy REE (HREE) patterns (Fig.5b). High (La/Yb)N ratios (17–24) (the subscript ‘N’ denotes thatthe concentration is normalized to chondrite) indicate pronouncedLREE/HREE fractionation. The samples show strong enrichment inlarge ion lithophile elements (LILEs) relative to high field strengthelements (HFSEs) and pronounced negative Nb–Ta anomalies andpositive K and Pb anomalies in primitive-mantle-normalizedincompatible element patterns (Fig. 5c).

The analyzed samples have relatively low (87Sr/86Sr)t values(0.70413–0.70513) and moderately positive eNd(t) values (3.7–5.8) (Table 3) relative to bulk Earth (Fig. 6a), and high (207Pb/204Pb)t

(15.59–15.65) and (208Pb/204Pb)t (38.43–38.72) values at a given(206Pb/204Pb)t (18.37–18.51) (Table 3) compared with the NorthernHemisphere Reference Line (not shown in figures).

Regarding trace elements, the Mamen lavas differ markedlyfrom the Linzizong andesites (Mo et al., 2007) in terms of theirsteep slope in HREEs, and from post-collisional adakites by relativehigh HREE concentrations and low concentrations of Th, U, and Pb(Fig. 5b and c). Isotopically, Mamen lavas plot close to or overlapwith the field of 120 Ma Tethyan basalts (Mahoney et al., 1998)

(Fig. 6a and b), but are distinct from the Linzizong andesites (Moet al., 2007) and the majority of post-collisional adakites in south-ern Tibet (Hou et al., 2004; Guo et al., 2007).

5.3. Zircon Hf isotope

Thirteen in situ Hf isotope analyses were successfully carriedout on zircons within sample MM02-3 (Table 4). The zircons arecharacterized by clearly positive initial eHf(t) values, with mostranging from +11.0 to +12.9; spot 14.1 has the highest initial eHf(t)value of +15.5 (Table 4). The positive initial eHf(t) values are com-parable with those of Indian MORB (Fig. 6c) and are consistent witha long-term depleted mantle source, in good agreement with theNd–Sr isotope compositions of sample MM02-3, which has aeNd(t) value of +5.8 and initial 87Sr/86Sr ratio of 0.70413.

6. Discussion

6.1. Nature of the source region

The primitive-mantle-normalized incompatible element pat-terns of the Mamen adakite-like rocks (Fig. 5c) exhibit considerableenrichment in LILEs and negative Nb–Ta anomalies, suggesting anaffinity with magmas generated in a subduction-related tectonicsetting. Previous studies have identified two components – thepartial melts of subducted sediment and slab-derived fluids – that

Table 3Whole-rock major, trace element and Sr–Nd–Pb isotope data of the Mamen adakite-like rocks, southern Tibet

Sample MM02-2 MM02-3 MM02-4 MM02-5 MM02-6 T203A

XRF – major element (wt.%)SiO2 56.82 55.87 59.86 57.77 62.98 58.05TiO2 0.80 0.96 0.88 0.98 0.95 1.10Al2O3 15.47 18.12 17.57 17.78 16.61 18.97TFe2O3

* 4.94 6.53 5.01 4.34 6.07 5.88MnO 0.08 0.05 0.06 0.06 0.07 0.06MgO 5.81 5.48 3.51 5.83 4.41 4.03CaO 10.05 5.45 7.62 5.33 4.18 6.52Na2O 3.85 4.61 3.30 5.17 2.56 2.75K2O 1.69 2.40 1.87 2.23 1.92 2.37P2O5 0.48 0.53 0.30 0.51 0.26 0.27LOI 2.67 1.57 1.47 1.55 2.22Mg# 70.2 62.6 58.4 72.9 59.2 57.8

ICP-MS – trace element (ppm)Sc 13.4 15.5 11.4 14.8 8.5 19.3V 125 137 100 123 85.2 25Cr 190 176 225 190 225 220Co 21 27 24 20 22 20Ni 130 133 105 143 139 108Cu 5.19 30.3 63.1 23.6 298 220Zn 63.2 48.9 157 57.4 62.3 58.0Rb 40.1 129 62.1 65.4 57.7 86.9Sr 836 530 631 692 476 633Y 11.5 11.9 11.2 11.3 8.7 13.5Zr 125 149 125 158 133 156Nb 9.58 10.6 10.7 10.8 11.0 10.7Cs 10.7 61.0 30.0 17.7 26.8 44.4Ba 1158 577 477 973 475 724La 30.8 26.0 24.1 22.3 20.0 28.5Ce 59.6 50.3 53.1 44.2 44.1 53.4Pr 6.87 5.77 6.43 5.14 5.04 7.21Nd 26.5 22.7 25.0 20.1 20.0 28.3Sm 4.79 4.03 4.52 3.69 3.79 5.03Eu 1.69 1.66 1.26 1.27 1.09 1.38Gd 3.33 3.49 3.21 3.17 2.62 3.89Tb 0.44 0.46 0.45 0.43 0.37 0.54Dy 2.41 2.39 2.45 2.25 1.99 2.76Ho 0.43 0.44 0.44 0.41 0.35 0.49Er 1.19 1.25 1.21 1.23 0.94 1.28Tm 0.16 0.16 0.16 0.16 0.12 0.18Yb 0.97 1.00 0.99 0.96 0.71 1.08Lu 0.14 0.15 0.14 0.14 0.10 0.16Hf 3.25 3.40 3.43 3.68 3.78 3.60Ta 0.49 0.60 0.56 0.61 0.58 0.68Pb 16.1 11.2 15.2 11.5 10.5 12.0Th 2.45 2.85 2.48 3.24 2.22 4.03U 1.32 1.43 0.62 1.29 0.38 1.06Eu/Eu* 1.30 1.35 1.01 1.14 1.06 0.96

Sr–Nd–Pb isotope compositions87Rb/86Sr 0.0961 0.5612 0.2370 0.331487Sr/86Sr (±2r) 0.705320 ± 11 0.705221 ± 13 0.705132 ± 11 0.705207 ± 11(87Sr/86Sr)t 0.70513 0.70413 0.70467 0.70456147Sm/144Nd 0.1081 0.1089 0.1081 0.1122143Nd/144Nd (±2r) 0.512748 ± 11 0.512859 ± 14 0.512763 ± 10 0.512763 ± 12(143Nd/144Nd)t 0.512651 0.512762 0.512666 0.512663eNd(t) 3.7 5.8 4.0 3.9206Pb/204Pb (±2r) 18.6947 ± 12 18.5663 ± 13 18.5725 ± 10 18.6069 ± 12207Pb/204Pb (±2r) 15.6546 ± 10 15.5988 ± 11 15.6519 ± 10 15.6487 ± 10208Pb/204Pb (±2r) 38.7932 ± 27 38.5571 ± 35 38.7642 ± 26 38.7725 ± 26(206Pb/204Pb)t 18.57 18.37 18.51 18.55(207Pb/204Pb)t 15.65 15.59 15.65 15.65(208Pb/204Pb)t 38.72 38.43 38.68 38.67

Major element oxide contents are normalized to 100 wt.% on a volatile-free basis. LOI = loss on ignition; total iron as TFe2O3*, Mg# = 100 �molar Mg2+/(Mg2+ + total Fe2+)],

calculated by assuming total FeO = 0.9 � TFe2O3*. Eu/Eu* = EuN/(SmN � GdN)1/2, N is chondrite-normalized (Sun and McDonough, 1989). T = age-corrected initial isoto-

pic ratios. Corrected formula as follows: (87Sr/86Sr)t = (87Sr/86Sr)m + 8 7Rb/86Sr(ekt � 1), k = 1.42 � 10�11 a�1; (143Nd/144Nd)t = (143Nd/144Nd)m + (147Sm/144Nd)m � (ekt � 1),eNd(t) = [(143Nd/144Nd)m/(143Nd/144Nd)CHUR(t) � 1] � 104, (143Nd/144Nd)CHUR(t) = 0.512638 � 0.1967 � (ekt � 1), k = 6.54 � 10�12a�1. (206Pb/204Pb)t = (206Pb/204Pb)m +238U/204Pb � (ek1t � 1), k1 = 1.55125 � 10�10 a�1; (207Pb/204Pb)t = (207Pb/204Pb)m + 235U/204Pb � (ek2t � 1), k2 = 9.8485 � 10�10 a�1; (208Pb/204Pb)t = (208Pb/204Pb)m +232U/204Pb � (ek3t � 1), k3 = 0.49475 � 10�10 a�1.

D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309 303

may metasomatize and enrich the source region of subduction-related magmas (Elburg et al., 2002; Guo et al., 2005). Slab-derivedfluids are characterized by high contents of Ba, Rb, Sr, U, and Pb,

whereas partial melts of subducted sediment contain high concen-trations of Th and LREE (Hawkesworth et al., 1997; Guo et al., 2005,2007). The Mamen adakite-like rocks exhibit variable Ba concen-

Fig. 4. K2O vs. SiO2 classification diagram of Rollinson (1993) showing data for theMamen adakite-like rocks. All samples are plotted on an anhydrous basis.

Fig. 5. (a) Sr/Y vs. Y discrimination diagram showing data for adakites and normalcalc-alkaline rocks (Defant and Drummond, 1990). (b–c) Chondrite-normalized REEand primitive-mantle-normalized trace element patterns for Mamen adakite-likerocks, Linzizong andesites (Mo et al., 2007), and post-collisional adakites (Chunget al., 2003; Hou et al., 2004; Guo et al., 2007). Data for chondrite-normalized andprimitive-mantle-normalized values and plotting order are from Sun and McDon-ough (1989).

304 D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309

trations (475–1158 ppm) coupled with a narrow range of Nb/Yratio (except sample MM02-6), consistent with fluid-inducedenrichment (Fig. 7a). Present-day arc settings in which significantamounts of sediments are subducted typically show Th/Ybratios P 2, whereas fluid-dominated arc environments showTh/Yb < 1 (Woodhead et al., 2001; Nebel et al., 2007). The Mamenadakite-like rocks have Th/Yb ratios ranging from 2.51 to 3.74, sug-gesting a significant contribution from sediments in their origin. Itis unlikely that bulk amounts of subducted sediment can be addedto the mantle source of subduction-related magmas (Hawkesworthet al., 1997). This argument is supported by the linear trend of Ma-men adakite-like rocks in a Th/Yb vs. Th/Sm plot (Fig. 7b), whichcould be interpreted in terms of two-component mixing betweenthe Dazhu–Langceling basalts from the Yarlung Zangbo suturezone (or a partial melt thereof) and a partial melt of subductedsediment.

The foregoing interpretation is consistent with the modelingcurves defined by Mamen adakite-like rocks in Sr–Nd–Pb iso-tope diagrams (Fig. 6a and b), in which the Dazhu–Langcelingbasalts from the Yarlung Zangbo suture zone (Fig. 1b; Zhanget al., 2005) and Indian Ocean pelagic sediment (Ben Othmanet al., 1989) are treated as proxies for the mantle source compo-nents of the Mamen adakite-like rocks and for Neo-Tethyanoceanic sediment, respectively. The modeling results of two-component mixing indicate that the origin of Mamen adakite-like rocks can be explained by mixing with contributions of5–10% Indian Oceanic sediments to attain the measured Sr–Ndisotopic composition, or 1–3% to attain the measured Nd–Pb iso-topic composition. Previous studies have shown that a smallcontribution of sediment results in a drastic increase in206Pb/204Pb ratios in subduction-related rocks (Vroon et al.,1995; Rolland et al., 2002). Thus, the decreased contribution ofsediments indicated by Nd–Pb isotopic compositions for the Ma-men adakite-like rocks could be attributed to the effect of206Pb/204Pb ratios, which are highly sensitive to any input ofoceanic sediment. In any case, we can infer with confidence thatthe magma source region of the Mamen adakite-like rocks wasat least partly mixed with sediments, as well as fluids driven offfrom the sediments. This interpretation is similar to the obser-vations of Rolland et al. (2002) for the Cretaceous Ladakh arcand of Bignold and Treloar (2003) for the Cretaceous Kohistanisland arc, for which several percent of sediments are proposedto have become entrained into the magma source regions toexplain the measured Sr–Nd–Pb isotopic compositions.

6.2. Petrogenesis

Previous studies suggest that the partial melting of metabasicigneous rocks in the eclogite to amphibolite facies, either in thethickened lower crust or in subducted oceanic crust, can producemelts with the geochemical characteristics of adakites (Defantand Drummond, 1990; Atherton and Petford, 1993; Yogodzinskiet al., 1995; Rapp et al., 1999; Chung et al., 2003; Hou et al.,

D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309 305

2004; Wang et al., 2005, 2008; Guo et al., 2007). In the case of theMamen adakite-like rocks, their Early Cretaceous age is consistentwith slab melting during northward subduction of the Neo-Teth-yan oceanic lithosphere beneath the Gangdese Belt (Xu et al.,1985; Coulon et al., 1986; Harris et al., 1988; Copeland et al.,1995; Yin and Harrison, 2000). This interpretation is supportedby the following lines of evidence.

(1) The high Mg-numbers (57.8–72.9) of the Mamen adakite-like rocks are inconsistent with a slab origin. The obtained Mg-numbers are significantly higher than those for sodium-rich

magmas from newly underplated basaltic crust (Atherton and Pet-ford, 1993) and are distinct from the fields in the Mg# vs. SiO2 dia-gram (Fig. 8a) of proposed lower-crustal melts (Condie, 2005) andof the lower-crust-derived post-collisional adakites of southern Ti-bet (Chung et al., 2003; Hou et al., 2004; Guo et al., 2007).

(2) Trace element signatures for the Mamen adakite-like rocksare more consistent with slab melting than magma originating inthe lower crust. Empirically, adakites generated in the lower crusttend to be K-rich and are distinguished by high contents of stronglyincompatible elements such as Rb, Ba, Th, and U (e.g., Wang et al.,2005, 2007, 2008). Compared with post-collisional adakites in thesouthern Gangdese Belt (Chung et al., 2003; Hou et al., 2004), theMamen adakite-like rocks are sodic (Table 1) and have low Th con-tents and Th/Ce ratios (Fig. 8b), similar to those of Cenozoic slab-derived adakites in arc settings (Wang et al., 2008).

(3) Isotopic evidence also supports a slab origin for the Ma-men adakite-like rocks. The results of recent studies indicate thatthe Mesozoic lower crust of the southern Gangdese Belt is juve-nile and was probably dominated by underplated magmas ofsimilar composition to the Yeba mafic rocks (Chu et al., 2006;Zhu et al., 2008a). The highest eNd(t) value (+5.8), obtained forsample MM02-3, is distinct from those of the Yeba mafic rocks(Fig. 6a and b) in the southern Gangdese Belt. In terms ofSr–Nd–Pb–Hf isotopic compositions, the Mamen adakite-likerocks are comparable with the Tethyan basalts (Zhang et al.,2005) (Fig. 6a and b) and Indian MORB (Fig. 6c), which probablyrepresented newly formed oceanic crust at the time of the Ma-men adakite-like magmatism.

The Mamen adakite-like rocks are enriched in Zr and HREE (Fig.9), with higher Cr, Ni, and MgO contents than typical adakite (De-fant and Drummond, 1990). Experimental results show that duringascent through the mantle wedge, slab melt assimilates peridotiteand undergoes metasomatic reactions involving orthopyroxeneand garnet. This process has the potential to significantly modifySiO2, MgO, Ni, and Cr contents and increase the abundance of traceelements in hybridized slab melts, although most element ratios(e.g., La/Yb, Sr/Y, Sr/Nd, Nb/La, and K/La) remain largely unchanged(Rapp et al., 1999). This process might be invoked to explain thegeochemical characteristics of the Mamen adakite-like rocks, astheir primitive-mantle-normalized incompatible element patternsare consistent with the patterns observed in experimental melt(Fig. 9; Martin et al., 2005). Such an interpretation of the originof the Mamen adakite-like rocks is similar to those of Stern and Kil-ian (1996) and Yogodzinski and Kelemen (1998), who suggestedthat bajaites and related rocks are derived from reactions betweenslab partial melt and overlying mantle peridotite.

In summary, the Mamen adakite-like rocks are interpreted tohave been derived directly from the partial melting of subductedNeo-Tethyan slab (MORB + sediment + fluid), subsequently havingbeen hybridized by peridotite in the mantle wedge.

Fig. 6. eNd(t) vs. (87Sr/86Sr)t, eNd(t) vs. (206Pb/204Pb)t, and eHf(t) vs. eNd(t) diagramsfor Mamen adakite-like rocks. Data sources are as follows: Tethyan basalts (150 Maand 120 Ma; Mahoney et al., 1998), Dazhu–Langceling basalts (including sampleDZ98-1G) from the Yarlung Zangbo suture zone (Nd = 6.66 ppm, eNd(t) = 8.9,Sr = 180.7 ppm, (87Sr/86Sr)t = 0.70354; Zhang et al., 2005), Indian Ocean pelagicsediment (V28-343, Nd = 23.05 ppm, eNd(t) = –9.3, Sr = 119 ppm, (87Sr/86Sr)t =0.71682, Pb = 32.68 ppm, 206Pb/204Pb = 18.99; Ben Othman et al., 1989), Yeba maficrocks (Zhu et al., 2008a), field of Hf–Nd isotopic data for Indian MORB, and JuvenileRock Array (Chauvel and Blichert-Toft, 2001; Ingle et al., 2003). The star at the top of(b) is the average value for Langceling basalts (Nd = 3.53 ppm, eNd(t) = 8.8,Sr = 103.7 ppm, (87Sr/86Sr)t = 0.70451, Pb = 0.43 ppm, 206Pb/204Pb = 17.68; Zhang etal., 2005). Other data are as in Fig. 5. Indian Ocean pelagic sediment is used as aproxy for Neo-Tethyan sediment. Note that the Sr–Nd and Nd–Pb isotopiccompositions of the Mamen adakite-like rocks can be attained by mixing with 5–10% and 1–3% of Indian Oceanic sediments, respectively.

3

Table 4Hf isotopic data for zircons from a Mamen adakite-like rock (MM02-3), southern Tibet

Spot 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 2r 176Hf/177HfT eHf(0) eHf(t) TDM1 (Ma) TDM2 (Ma) fLu/Hf

1.1 0.022640 0.000458 0.283024 0.000026 0.283023 8.9 11.9 318 431 �0.992.1 0.023769 0.000471 0.283033 0.000020 0.283032 9.2 12.1 306 414 �0.993.1 0.044673 0.000844 0.283004 0.000020 0.283002 8.2 11.1 349 479 �0.974.1 0.022723 0.000437 0.283038 0.000015 0.283037 9.4 12.4 298 398 �0.995.1 0.042328 0.000772 0.283029 0.000017 0.283027 9.1 12.0 314 421 �0.986.1 0.042767 0.000784 0.283038 0.000018 0.283036 9.4 12.4 300 398 �0.987.18.1 0.048164 0.000866 0.283040 0.000025 0.283037 9.5 12.4 300 399 �0.979.1 0.020913 0.000376 0.283004 0.000015 0.283003 8.2 11.3 346 474 �0.99

10.1 0.027656 0.000610 0.283052 0.000027 0.283050 9.9 12.8 280 369 �0.9811.1 0.043378 0.000831 0.283004 0.000018 0.283002 8.2 11.0 349 482 �0.9712.1 0.022197 0.000419 0.283020 0.000023 0.283019 8.8 11.7 323 440 �0.9913.1 0.057601 0.001045 0.283060 0.000023 0.283057 10.2 12.9 272 359 �0.9714.1 0.088543 0.001714 0.283140 0.000018 0.283137 13.0 15.5 160 183 �0.95

*: eHf(t) = 10000 � {[(176Hf/177Hf)S � (176Lu/177Hf)S � (ekt � 1)]/[(176Hf/177Hf)CHUR,0 � (176Lu/177Hf)CHUR � (ekt � 1)] � 1}.TDM1 = 1/k � ln{1 + [(176Hf/177Hf)S � (176Hf/177Hf)DM]/[(176Lu/177Hf)S � (176Lu/177Hf)DM]}.TDM2 = TDM1 � (TDM1 � t) � [(fcc � fs)/(fcc � fDM)].fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR � 1, where k = 1.867 � 10�11 year�1 (Soderlund et al., 2004); (176Lu/177Hf)S and (176Hf/177Hf)S are the measured values of the samples;(176Lu/177Hf)CHUR = 0.0332 and (176Hf/177Hf)CHUR,0 = 0.282772 (Blichert-Toft and Albarède, 1997); (176Lu/177Hf)DM = 0.0384 and (176Hf/177Hf)DM = 0.28325 (Griffin et al., 2000);(176Lu/177Hf)mean crust = 0.015; fcc = [(176Lu/177Hf)mean crust/(176Lu/177Hf)CHUR] � 1; fs = fLu/Hf; fDM = [(176Lu/177Hf)DM/(176Lu/177Hf)CHUR]fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR �1; t = crystallization time of zircon.

Fig. 7. Ba vs. Nb/Y and Th/Yb vs. Th/Sm plots for the Mamen adakite-like rocks. Other data are as in Fig. 5.

Fig. 8. (a–b) Mg# vs. SiO2 and Th/Ce vs. Th diagrams of the Mamen adakite-like rocks. Data sources: crustal AFC (Stern and Kilian, 1996), TTG (lower-crustal melts) andadakite (slab melts) (Condie, 2005), Cenozoic crust-derived adakite of the Songpan-Ganzi block (intracontinental setting) and Cenozoic slab-derived adakites (arc setting)(Wang et al., 2008). Other data are as in Fig. 5.

306 D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309

Fig. 9. Primitive-mantle-normalized trace element patterns of the Mamen adakite-like rocks, experimental melt (Martin et al., 2005), and adakite (Defant et al., 1991).

D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309 307

6.3. Geodynamic implications

SHRIMP zircon dating and geochemical data presented in thisstudy provide the first solid evidence for the existence of subduc-tion-related adakite-like rocks in the southern Gangdese Belt dur-ing the Early Cretaceous. This arc volcanism significantly predatesIndia–Asia collision, which began at �65 Ma (Mo et al., 2006, andreferences therein), thereby recording the northward subductionof Neo-Tethyan oceanic crust prior to 130 Ma.

Given that adakites can only form at temperatures above 700 �Cand depths greater than 70–85 km, regardless of whether subduc-tion occurs at normal dips or shallower angles (Defant et al., 1992;Sajona et al., 1993; Gutscher et al., 2000), the Neo-Tethyan oceaniccrust is likely to have subducted beneath the southern Gangdesesub-arc mantle to depths of 70–85 km during the Early Cretaceous.This depth, together with the location of the Mamen adakite-likerocks, which are exposed about 3 km to the north of the fossiltrench represented by the Yarlung Zangbo ophiolites (Fig. 2f), indi-cates that the Neo-Tethyan oceanic crust was subducted north-ward beneath the Gangdese Belt at a steep angle, similar to thatseen in the western Aleutians (Yogodzinski et al., 1995). In thiscase, identification of the Mamen adakite-like rocks in the south-ern Gangdese Belt provides valuable constraints on the geodynam-ic process of widespread Early Cretaceous magmatism in themiddle and northern parts of the Gangdese Belt (Fig. 1a).

The widespread nature of Early Cretaceous magmatism in theGangdese Belt led some investigators (Coulon et al., 1986; Cope-land et al., 1995; Ding et al., 2003; Kapp et al., 2003, 2005, 2007;Leier et al., 2007) to suggest that low-angle or flat-slab subduction,analogous to that observed in the modern Andes (Allmendinger etal., 1997), may have occurred in southern Tibet prior to India–Asiacollision. Evidence in support of this model is based on the scarcityof Early Cretaceous igneous rocks in the southern Gangdese Belt(Kapp et al., 2007, and references therein); however, SHRIMP zir-con age date for the Mamen adakite-like rocks reported in thisstudy, along with the regional comparison shown in Fig. 2, indicatethat significant arc volcanism was active in the southern GangdeseBelt at �136 Ma, coeval with the initial volcanism recorded in theZenong Group (�130 Ma; Zhu et al., 2008c) and associated EarlyCretaceous plutonism in the middle and northern parts of theGangdese Belt (�133 Ma; Zhu et al., 2008b). These recently pub-lished age data suggest that extensive magmatism occurred con-temporaneously throughout the Gangdese Belt during the EarlyCretaceous (Fig. 1b). Such an observation is inconsistent with a tec-tonic model that advocates the low-angle or flat-slab subduction ofthe Neo-Tethyan oceanic crust, as a period of volcanic quiescence

would have occurred if flat subduction had continued for severalmillion years (Gutscher et al., 2000). Accordingly, we argue thatthe generation of Early Cretaceous magmatism throughout theGangdese Belt can be attributed to a distinct geodynamic processthat is beyond the scope of this paper will be discussed in a futurestudy.

7. Conclusions

(1) Early Cretaceous Mamen andesites (136.5 ± 1.7 Ma) in thesouthern Gangdese Belt, southern Tibet, are sodic and showgeochemical affinities with adakite.

(2) The Mamen adakite-like rocks were probably derived frompartial melting of subducted Neo-Tethyan slab (MORB + sed-iment + fluid), subsequently hybridized by peridotite in themantle wedge.

(3) The Mamen adakite-like rocks probably resulted from thenorthward subduction of Neo-Tethyan oceanic crustbeneath the southern Gangdese Belt at a relatively steepangle during the Early Cretaceous.

(4) Our data are inconsistent with a tectonic model in which thewidespread Early Cretaceous igneous rocks of the middleand northern parts of the Gangdese Belt were derived fromthe low-angle or flat-slab subduction of Neo-Tethyan oce-anic crust.

Acknowledgements

We thank Q.R. Geng and C.Y. Zhou for their assistance in thefield; H. Tao and B. Song for help with SHRIMP dating; and F.K.Chen, C.F. Li, L.W. Xie, and Y.H. Yang for their assistance withSr–Nd–Hf isotopic analyses. We are grateful for helpful discussionswith Dr. S.L. Chung and Z.F. Guo, constructive reviews by CatherineChauvel and an anonymous reviewer, and insightful commentsand careful editorial handling by Bor-ming Jahn. This study bene-fited from financial support by ongoing NSFC projects (40503005,40572051, and 40473020), the Programme of Excellent YoungScientists of the Ministry of Land and Resources, the NationalKey Project for Basic Research of China (Project 2002CB412600),and the Integrated Study of Basic Geology of Qinghai–TibetanPlateau project.

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