Journal of Asian Earth Sciences 42 (2011) 515–524

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Journal of Asian Earth Sciences

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High density carbonic fluids in a slab window: Evidence from the Gangdesecharnockite, Lhasa terrane, southern Tibet

Zeming Zhang a,b,⇑, Kun Shen c, M. Santosh d, Xin Dong a

a State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Science, No. 26 Baiwanzhuang Road, Beijing 100037, Chinab State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, Chinac Institute of Geological Sciences of Shandong, Jinan 250013, Chinad Division of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan

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

Article history:Received 21 October 2010Received in revised form 12 February 2011Accepted 21 March 2011Available online 17 June 2011

Keywords:CharnockiteCO2-rich fluid inclusionMid-ocean ridge subductionAndean-type orogenyHimalayaTibet

1367-9120/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jseaes.2011.03.017

⇑ Corresponding author at: State Key Laboratory oDynamics, Institute of Geology, Chinese Academy oBaiwanzhuang Road, Beijing 100037, China. Fax: +86

E-mail address: zzm2111@sina.com (Z. Zhang).

Charnockites, occurring in the well-studied Gangdese batholith in the southern Lhasa terrane, easternHimalayan orogen, consist of orthopyroxene, clinopyroxene, plagioclase, and minor quartz and K-feld-spar, and show adakitic affinity. Here we present a systematic study on the fluid inclusions in Gangdesecharnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed andas trails along intragranular fractures in quartz, and that the secondary ones occur along healed mirco-fractures and coexist commonly with mineral inclusions of calcite, magnetite and hematite within thehost plagioclase and quartz. The fluid inclusions contain dominantly near-pure CO2 with traces of N2.Most of the primary fluid inclusions have low CO2 homogenization temperatures and with densities of1.138–1.013 g/cm3 suggesting trapping pressures of 0.7–1.0 GPa at temperatures of 850–950 �C. We pro-pose a model in which the southward subduction of Neo-Tethyan mid-oceanic ridge beneath the Lhasaterrane resulted in the release of heat and CO2 from the upwelling asthenosphere through a slab window,providing high-temperatures (HT) and dry CO2-rich fluids for the formation and stabilization of the adak-itic charnockite as well as the associated HT granulite-facies metamorphic rocks in the Late Cretaceous,prior to the final collision of the Indian plate with the Asian continent. Our study provides new insightsinto the geodynamics of Andean-type orogeny in the southern Tibetan Plateau during the Late Mesozoic.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Fluids in various tectonic settings play a major role in the geo-chemical and tectonic evolution of the Earth as well as material cir-culation on a global scale. The nature and distribution of fluids inthe Earth has largely been reconstructed from studies on crustalfluids based on fluid-related alteration in rocks, fluid-induced min-eral reactions computed from petrologic and phase equilibria stud-ies, direct observation of fluid inclusions in minerals, andgeophysical techniques (Santosh and Omori, 2008; Tsunogaeet al., 2008a; Ickert et al., 2009; Maruyama et al., 2009; Touret,2009). On the basis of the data from exhumed ultrahigh-pressure(UHP) metamorphic rocks and mantle-derived magmas and xeno-liths, the upper crustal fluids are generally considered to be dom-inated by H2O, with subordinate CO2, CH4, and N2, whereas thelower crustal fluids are mostly CO2-rich (e.g., Touret, 2009; Santoshet al., 2009). The growing importance of CO2-rich fluids associated

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f Continental Tectonics andf Geological Science, No. 2610 68994781.

with major orogenic cycles is reinforced by the numerous works inthis topic including those of Harlov (2000), Agrad et al. (2000),Bakker and Mamtani (2000), Bolder-Schrijver et al. (2000), Touret(2001, 2009), Tsunogae et al. (2002), Mohan et al. (2003), Cuneyet al. (2007), Santosh and Omori (2008) and Santosh and Kusky(2009), among others. Some workers have attempted a combina-tion of mineralogic thermobarometry with microthermometricdata of high density carbonic fluid inclusions in anhydrous granu-lites and charnockites to substantiate the model that CO2 has beeninstrumental in the formation and stabilization of the mineralassemblages in these rocks (e.g., Mohan et al., 2003; Santosh andTsunogae, 2003; Cuney et al., 2007; Santosh et al., 2011; Tsunogaeand Santosh, 2011).

Among the common anhydrous rocks which build the Earth’scrust, charnockites occupy a dominant position forming extensiveorthogneiss plutons in many Precambrian granulite terranes, andalso occurring less commonly as unmetamorphosed plutons in var-ious tectonic settings (Rajesh and Santosh, 2004; Frost and Frost,2008; Clark et al., 2009; Liu et al., in press; Rajesh et al., 2011).Their anhydrous mineralogy and the common association of CO2-rich fluid inclusions require that water activity was buffered tolow levels during their formation (e.g., Santosh, 1992; Santosh

516 Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524

and Yoshida, 1992). The mechanisms of formation of granulites arediverse, and various models have been proposed for the origin ofanhydrous granulite facies assemblages including extraction ofwater in partial melts leaving a granulite residue, metamorphismof anhydrous lithologies, and CO2-induced dehydration (reviewedin Santosh and Omori, 2008). Among these, charnockites in variousregions have provided important clues on the role of CO2-rich flu-ids in buffering water activity and stabilizing anhydrous mineralassemblages, typically orthopyroxene.

In this study, we report high density CO2 fluid inclusions incharnockites from the Southern Lhasa terrane in Eastern Himalaya.A recent petrologic, geochemical and geochronological study onthese charnockites has revealed that they were generated duringthe subduction of the Neo-Tethys beneath the south Lhasa terranein Late Cretaceous (Zhang et al., 2010a). From the nature, composi-tion, and density of the fluid inclusions in these charnockites, weevaluate the fluid regime associated with the Andean-type orogenyand subduction of Neo-Tethyan mid-ocean ridge.

2. Geological setting and samples

The Himalayan orogen is one of the youngest mountain belts onthe Earth created by continent–continent collision and offers anopportunity for understanding the process of mountain building(e.g., Yin and Harrison, 2000 and references therein). The easternHimalayan syntaxis in southeastern Tibet is expressed as a largeantiform and consists of three major tectonic units from south tonorth: the Indian terrane, the Indus–Tsangpo suture zone and theLhasa terrane (Fig. 1). The Indian terrane includes Tethyan Himala-yan- and High Himalayan rocks, of which the former consists oflow grade metasedimentary and sedimentary rocks of Cambrian–Eocene ages that were originally deposited along the passive mar-gin of northern India (e.g., Yin and Harrison, 2000; Zhang et al.,2004; Geng et al., 2006 and references therein), whereas the latterconstitutes the Namche Barwa Group that forms the core of theNamche Barwa antiform in the middle of the syntaxis, and is

Fig. 1. Simplified geological map of the eastern Himalaya (after Zhang et al., 2010a), shITS = Indus–Tsangpo suture zone; ATF = Tltyn Tagh Fault; KJFZ = Karakorum–Jiali Fault Z

composed mainly of amphibolite- to granulite-facies rocks (Liuand Zhong, 1997; Lee et al., 2000; Ding et al., 2001; Gehrelset al., 2003; Geng et al., 2006; Zhang et al., 2007, 2010a,b; Liuet al., in press; Xu et al., 2010). The narrow, 2–10 km-wide In-dus–Tsangpo suture zone consists of highly deformed greenschist,mica-quartz schist and various lenses of a dismembered ophioliticsuite (Geng et al., 2006 and references therein).

The Lhasa terrane, from where the charnockites have beeninvestigated in this study, is located to the north and east of the In-dus–Tsangpo suture zone and is composed mainly of a sequence ofamphibolite- to granulite-facies metamorphic rocks, together withPaleozoic and Mesozoic sedimentary rocks (Fig. 1). The high-grademetamorphic rocks are composed mainly of gneiss, amphiboliteand schist, along with sporadic marble metamorphosed at peakgranulite-facies P–T conditions of 750–850 �C and �1.0 GPa, withevidence for extensive migmatization and polyphase deformation(Wang et al., 2008; Dong et al., 2009, 2010; Zhang et al.,2010a,b). The sedimentary rocks of the Lhasa terrane consist ofCambrian to Eocene strata together with volcanic intercalations(Geng et al., 2006).

In the southeastern Lhasa terrane, abundant plutons, referred toas the Gangdese batholith, yielded zircon U–Pb ages ranging from130 to 20 Ma (Mo et al., 2005a,b; Chung et al., 2005; Chu et al.,2006; Pan et al., 2006; Wen et al., 2008a,b; Ji et al., 2009; Zhanget al., 2010c; Zhao et al., 2009; Zhu et al., 2008, 2009). The majorGangdese rock types include gabbro, diorite, granodiorite, monzog-ranite and syenogranite. Except for the gabbro, which containsorthopyroxene, clinopyroxene and amphibole, all the other rocksare coarse-grained with a mineral assemblage of quartz, plagio-clase, potassium feldspar, biotite and hornblende. In addition, thesouthern Lhasa terrane also contains Cretaceous to Tertiary terres-trial volcanic sequences of the Linzizong Formation (Maluski et al.,1982; Coulon et al., 1986; Mo et al., 2003, 2007, 2008; Zhou et al.,2004; Chung et al., 2005; He et al., 2007; Xia et al., in press).

Zhang et al. (2010a) carried out a detailed petrologic, mineralchemical, whole rock geochemical and geochronological study ofthe Gangdese charnockites. These rocks are distributed along the

owing the sampling locations. The inset figure modified after Chung et al. (2005).one; RRF = Red River Fault; WCF = Wang Chao Fault.

Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524 517

eastern part of the Gangdese batholith between Lilong and Milin(Fig. 1), and show a heterogeneous granular texture and a massivestructure. In contrast to the Precambrian charnockites with a darkgreasy green appearance (e.g., Rajesh and Santosh, 2004), theGangdese charnockites exhibit gray-white color. Wherever thecontact is exposed, the charnockites show an intrusive relationshipwith the country gneiss, amphibolite, marble, schist and granulite.Moreover, rounded and irregular xenoliths of the country rocks arecommonly observed in the charnockites. The charnockites were in-truded by the surrounding granite and granodiorite, which occur asmajor constituent of the Gangdese batholith and display adakiticaffinity and with zircon U–Pb ages of 80–83 Ma (Wen et al.,2008a,b).

The Gangdese charnockites consist of plagioclase (Pl), orthopy-roxene (Opx), clinopyroxene (Cpx), quartz (Qz) and K-feldspar(Kfs), with or without biotite (Bt) and amphibole (Amp) (Fig. 2a).The plagioclase occurs as subhedral laths, and rarely containsabundant inclusions of zircon, magnetite and hematite (Fig. 2b).Most of the clinopyroxenes and orthopyroxenes have subhedralshort prismatic forms. Minor anhedral quartz and K-feldspar occuramong other minerals. Biotite and amphibole show irregular grainmorphology and commonly rim the pyroxenes (Fig. 2a), indicatingthat the early anhydrous mineral assemblage of the charnockitehas been partly replaced by the hydrous minerals during the postcrystallization stage. This is a common feature of the igneous

Fig. 2. Photomicrographs of the Gangdese charnockites. (a) Charnockite consistingof orthopyroxene (Opx), clinopyroxene (Cpx), plagioclase (Pl), quartz (Qz) andbiotite (Bt). (b) Charnockitic plagioclase containing abundant mineral inclusions ofmagnetite (Mag), hematite (Hem) and zircon (Zrn).

charnockites (Frost and Frost, 2008). Zhang et al. (2010a) showedthat the Gangdese charnockites are of magmatic origin, with crys-tallization ages of Late Cretaceous (86–90 Ma). Furthermore, theyexhibit geochemical affinities similar to those of adakites and arecomparable to the magnesian charnockites formed within conti-nental magmatic arc. They also suggested that these rocks weregenerated during the subduction of the Neo-Tethyan mid-oceanicridge beneath the south Lhasa terrane.

3. Fluid inclusions

3.1. Analytical procedures

Doubly polished thin sections of the charnockites were pre-pared for fluid inclusion studies, including observations of thephase (s) present in the inclusions, their shape, size, mode ofoccurrence and distribution, following the techniques outlined byRoedder (1984), Touret (2001) and van den Kerkhof and Hein(2001). Representative fluid inclusions were selected for microth-ermometry and laser-Raman microspectrometry. Microthermo-metric measurements were performed on a LinkamTHM600freezing/heating stage at the Fluid Inclusion Laboratory of the Insti-tute of Mineral Resources, Chinese Academy of Geological Sciences(CAGS). Nitrogen was used as cooling medium. The heating-freezing stage was calibrated by measuring the melting points ofpure water inclusion (0 �C) and CO2 inclusions (�56.6 �C). The esti-mated precision of final melting temperature and homogenizationtemperature of CO2 ± N2 inclusions is about ±0.2 �C. Laser Ramanmicroanalysis for identifying the volatile and solid componentswithin the fluid inclusions was performed with a Laser Ramanspectrometer (LABHR-VIS LabRAM HR800) at the Fluid InclusionLaboratory of the Beijing Institute of Geology, Nuclear IndustryCorporation, China.

3.2. Fluid inclusion petrography

Four representative charnockite samples were chosen for fluidinclusion study. The type and occurrence of fluid inclusions andthe mineral assemblage of host rocks are listed in Table 1. The car-bonic fluid inclusions in these charnockites occur mainly in quartzand plagioclase, and commonly show rounded and negative crystalmorphologies with size ranging from a few micron to 20 lm(Fig. 3). Based on the nature of occurrence and distribution charac-teristics, the fluid inclusions can be divided into primary and sec-ondary categories. The former occurs as isolated and randomclusters or as trails along intragranular fractures in quartz(Fig. 3a and b); the latter defines trails along healed microfracturesin quartz and plagioclase (Fig. 3c and d). The mineral inclusions ofmagnetite, hematite, zircon and calcite were recognized within thecharnockitic plagioclase, and sometimes are associated with thefluid inclusions (Figs. 2b and 3c and d).

3.3. Microthermometry and laser Raman analysis

The results of microthermometric measurements of primaryfluid inclusions hosted in quartz and secondary fluid inclusionsin plagioclase are compiled in Table 1 and histograms of CO2 melt-ing- and homogenization temperatures are shown in Figs. 4 and 5,respectively.

The CO2 melting temperatures (TmCO2) of most of the studiedinclusions fall in the range between �57.9 to �56.6 �C except afew of which where the TmCO2 shows a slight depression downto �58.3 �C (Table 1 and Fig. 4). These results imply that most ofthe fluid inclusions contain nearly pure CO2 with only traces ofother gas species. On the other hand, the CO2 homogenization

Table 1Mineral assemblage, type and microthermometry of CO2 inclusions within the host quartz and plagioclase of the Gangdese charnockites.

Sample Piece Mineral assemblage Type Size (lm) Occurrence TmCO2 (�C) ThCO2 (�C) Density (g/cm)

TM07-43-8 a Cpx + Opx + Pl + Kfs + Bt + Amp + Qz P 5–4 In trail �57.1 to �57.0 �42.7 to �15.2 1.127–1.010a P 11–18 In trail �57.1 to �57.0 �8.5 to +14.3 0.976–0.828b P 5–12 In trail �56.8 to �56.6 �37.7 to �18.4 1.108–1.025c P 6–12 In trail �57.3 to �56.6 �40.0 to �18.4 1.117–1.025g P 4�10 In trail �56.9 to �56.6 �45.6 to �10.1 1.138–0.984i P 5–22 In trail �56.9 to �56.6 �44.8 to �15.4 1.135–1.011

TM08-8-1 a Cpx + Opx + Pl + Kfs + Bt + Qz P 4–12 Random �57.2 to �56.6 �24.7 to �17.6 1.054–1.021b P 4–10 Random �57.0 to �56.8 �20.6 to �14.6 1.035–1.007c P 4–10 Random �56.9 to �56.7 �24.2 to �15.9 1.051–1.013c P 4–14 Random �57.6 to �57.1 �22.6 to +11.3 1.044–0.852e P 4–8 Random �58.3 to �57.9 �20.7 to �12 1.036–0.994f P 3–13 Random �57.1 to �56.5 �21.4 to �5.3 1.039–0.959g P 4–13 Random/in trail �56.8 to �56.6 �32.0 to �25.7 1.085–1.058

TM08-8-2 a Cpx + Opx + Pl + Kfs + Bt + Qz P 2–12 In trail �57.1 to �56.9 �43.9 to �31.4 1.132–1.082b P 4–6 In trail �57.7 to �57.5 �27.1 to �15.7 1.064–1.012g P 4�18 In trail �56.9 to �56.6 �24.5 to �18.7 1.053–1.026

TM08-8-4 b Cpx + Opx + Pl + Kfs + Bt + Amp + Qz P 2–18 In cluster �58.2 to �56.6 �24.7 to �16.9 1.054–1.018f P + S 5–8 In fracture �56.9 to �56.6 �34.6 to +30.1 1.095–0.591a S 3–10 In fracture �57.0 to �56.6 �9.6 to +30.6 0.982–0.562c S 4–6 In fracture �56.7 to �56.6 �9.3 to �0.6 0.980–0.932d S 12 in fracture �56.7 to �56.5 �12.8 to +13.4 0.998–0.835e S 5–10 in fracture �57.3 to �56.5 �6.3 to +11.8 0.964–0.848

Note: P – primary inclusion; S – secondary inclusion; TmCO2 – CO2 melting temperature; ThCO2 – CO2 homogenization temperature.

Fig. 3. Photomicrographs showing CO2 fluid and mineral inclusion occurrence in charnockitic minerals. (a) Randomly distributed primary CO2 inclusions in Qz (sampleTM08-8-1). (b) Primary CO2 inclusions along a gently-dipping intragranular fracture in Qz (TM07-43-8). (c and d) Secondary CO2 inclusions coexisting with mineral inclusionscalcite (Cal), magnetite (Mag) and hematite (Hem) along microfractures in Pl (TM08-8-4).

518 Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524

temperatures (ThCO2, all homogenised to CO2 liquid) of fluid inclu-sions vary considerably (Table 1). The ThCO2 values of primaryinclusions in sample TM07-43-8 vary from ca. �46 �C to 14 �C,with pronounced peaks between �45 �C and �30 �C and a minor

peak at ca. �20 to �18 �C (Fig. 5a). The ThCO2 values of primaryinclusions in sample TM08-8-1 fall in the range between �32 �Cand 11.3 �C with a clear peak at ca. �20 to �18 �C and a subordi-nate peak at around �30 �C (Fig. 5b). The ThCO2 values of primary

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(c) Primary FIs (TM08-8-2)

(d) Primary+secondary FIs (TM08-8-4)

rebmu

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Fig. 4. Histograms for melting temperatures (Tm) of CO2 fluid inclusions (FIs).

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(c) Primary FIs (TM08-8-2)

(d) Primary and secondary FIs (TM08-8-4)

Fig. 5. Histograms for homogenization temperatures (Th) of CO2 fluid inclusions(FIs).

Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524 519

CO2 inclusions in sample TM08-8-2 fall mainly in two ranges of�44 to �31 �C and �25 to �16 �C with a low and a high peak atca. �36 �C and �20 �C, respectively (Fig. 5c). The ThCO2 values ofCO2 inclusions in sample TM08-8-4 cover a wider range, from�35 �C to 31 �C and show a bimodal distribution (�25 to �16 �Cand �10 to 3 �C) (Fig. 5d). In general, the homogenisation temper-atures of the primary inclusions define two major peaks around�40 �C (�45 to �30 �C) and �20 �C (�25 to �15 �C), whereas thehomogenization temperatures of secondary ones are clustered at�10 to 0 �C (Fig. 5 and Table 1).

Raman spectra of representative fluid inclusions are shown inFig. 6. Whereas some of the carbonic inclusions in Qz and Pl con-tain pure CO2 (Fig. 6a), many inclusions in Qz contain dominantlyCO2 with trace amount of N2 (Fig. 6b and c). The solid inclusions inPl include calcite, magnetite and hematite (Figs. 3c and d and 6d).

4. Discussion

Fluid inclusions trapped within minerals provide one of the po-tential tools to characterize the nature of fluids involved in variousmagmatic, metamorphic and metasomatic processes. These micro-geochemical systems offer important clues on the composition and

density of the fluids, fluid-rock interaction processes and the pres-sure–temperature conditions of equilibration of mineral assem-blages. Our present knowledge on the nature of fluids in differentorogenic belts relies heavily on observations of fluid inclusions inmineral assemblages of magmatic and high-grade metamorphicrocks, and mantle xenoliths (e.g., Touret, 1992, 2001, 2009; San-tosh and Tsunogae, 2003; Santosh and Omori, 2008).

4.1. Composition and density of the fluid inclusions

Petrography, microthermometry and Raman analysis indicatethat the primary fluid inclusions in the Gangdese charnockites con-tain nearly pure CO2 with only minor N2. The compositional dataon the fluid inclusions coupled with their homogenization temper-atures can be employed to calculate the density of the entrappedfluids. We computed the densities using the FLUIDS package devel-oped by Bakker (2003). Most primary inclusions in the charnock-ites with lowest homogenization temperatures (�45.6 to �30 �C)define the highest density group with a density range of 1.138–1.076 g/cm3 (Table 1). Some primary inclusions with homogeniza-tion temperatures of �25 to �15 �C have intermediate densities inthe range of 1.050–1.013 g/cm3. In contrast, the homogenizationtemperatures of secondary inclusions vary from �10 to +30 �C, cor-responding to a wide range of medium to low densities between0.983 g/cm3 and 0.591 g/cm3 (Table 1). Therefore, most primarycarbonic inclusions show high density, similar to the density of

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nsity

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nsity

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nsity

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nsity

Fig. 6. Raman spectra of fluid and mineral inclusions in the charnockites. (a) A randomly distributed and pure CO2 inclusion hosted in Qz (sample TM08-8-1). (b) A clusteredCO2 inclusion with very minor N2 hosted in Qz (sample TM08-8-1). (c) A CO2 inclusion with very minor N2 in trail in Qz (sample TM07-43-8). (d) Mineral inclusion Hem whichcoexists with CO2 fluid inclusions in Pl (sample TM08-8-4). The arrows in the inserted pictures refer to analytical inclusion.

Ec

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EA

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erusserP

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FIs in Gangdese charnockiteFIs in granulite of southern IndiaFIs in charnockite of Eastern Ghats

Isochores:P-T conditions of the Gangdese charnockite

Fig. 7. P–T diagram showing the isochore ranges (the blue region) of primary CO2

fluid inclusions and speculated trapping conditions (the pink box). The faciesboundaries and wet granite solidus (WGS) are after Santosh and Kusky (2009) andreferences therein as compiled from various published sources. Abbreviations formetamorphic facies: AM, amphibolite; EC, eclogite; BS, blueschist; EA, epidoteamphibolite; GR, granulite; GS, greenschist; HGR, high-pressure granulite; UHT,ultrahigh-temperature metamorphism. Isochores of high-density carbonic fluidinclusions (FIs) from the granulite of southern India (Santosh and Tsunogae, 2003)and from the charnockites of Eastern Ghats, India (Mohan et al., 2003) are shown forcomparison. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

520 Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524

CO2-inclusions reported from some of the ultrahigh-temperaturegranulites in different regions (Fig. 7; Santosh and Omori, 2008and references therein, Tsunogae et al., 2008a; Tsunogae and San-tosh, in press).

4.2. Trapping conditions of the fluid inclusions

As described in an earlier section, primary fluid inclusions in thestudied charnockite occur as isolated and random clusters (Fig. 3a)or as trials along intragranular fractures in quartz (Fig. 3b). Theirnature of distribution indicates that these fluid inclusions weretrapped during the charnockite crystallization based on the criteriaproposed by Roedder (1967, 1984).

The isochores of the primary fluid inclusions were constructedby FLUIDS package (Bakker, 2003), FLINCOR software (Brown andLamb, 1989) and on-line calculation of Duan (www.Geochem-model.org), using different equations of state and the results areplotted in Fig. 7. The maximum pressure difference calculated fromthese equations of state is about 0.1 GPa. The intermediate pres-sures at different temperatures, as calculated using Duan’s equa-tions of state (1992), were chosen to construct the isochores forthe CO2 inclusions.

The crystallization temperatures of the Gangdese charnockiteswere estimated to be in the range of 890–960 �C based on theCa-in-orthopyroxene thermometry and 850–950 �C on the basisof two-pyroxene geothermometer (Zhang et al., 2010a). The inter-section of fluid inclusion isochores with these temperature valuesyield trapping pressures of 0.7–1.0 GPa for primary inclusions withdensity ranges of 1.138–1.013 g/cm3 (Fig. 7). The primary carbonicinclusions show a range of homogenisation temperatures whichtranslate into a range of densities, thus yielding variable pressureestimates. Similar observations of wide density variations havebeen reported from charnockites and UHT granulites (Santoshand Omori, 2008; Tsunogae and Santosh, in press).

The oxygen fugacity during Gangdese charnockite formationwas relatively high as indicated by the predominance of CO2 fluidinclusions and the common occurrence of magnetite and hematitemineral inclusions in the charnockite minerals. This conclusion isalso supported by the absence of graphite and CH4 component inthe CO2 fluid inclusions.

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4.3. Tectonic implications

Similar to the Andean Cordillera, the Gangdese batholith is oneof the best examples of active continental arc magmatism, becausethese rocks were emplaced from the Cretaceous to Eocene insouthern Tibet and has been widely regarded as the major constit-uent of an Andean-type convergent margin before the collision be-tween the India and Asian continents (e.g., Maluski et al., 1982;Allègre et al., 1984; Xu et al., 1985; Copeland et al., 1995; Coulonet al., 1986; Yin and Harrison, 2000; Pan et al., 2006). Subduc-tion-related magmatism is usually characterized by the eruptionor intrusion of ‘‘normal’’ calc-alkaline rocks, which are thought tobe derived from the partial melting of metasomatized peridotitesfrom the mantle wedge, followed by fractional crystallization ofmafic melts often coupled with crustal assimilation (Benoit et al.,2002). Adakites differ from their typical calc-alkaline equivalentsin their depletion in HREEs and higher Sr/Y ratio (Defant andDrummond, 1990). Adakites have been documented from abouthalf of the active volcanic arcs (Martin, 1999), and their origin isattributed to slab melting under high thermal regime (Sen andDunn, 1994). Thus, adakites occur in various hot subduction set-tings, including in regions of active ridge subduction (Bourgoiset al., 1996; Lagabrielle et al., 2000), subduction of very young(<5 Ma) oceanic crust (Peacock et al., 1994; Sajona et al., 1993),and flat slab subduction (Gutscher et al., 2000).

Previous studies have shown that magmatism at consumingplate margins can change markedly when spreading mid-oceanicridge segments intersect a subduction zone (Aguillon-Robleset al., 2001; Thorkelson and Breitsprecher, 2005; Cole and Stewart,2009; Rosenbaum and Mo, in press). These changes in magmatismare largely attributed to the exposure of suboceanic (sub-slab)mantle reservoirs beneath a continental margin through a gaptermed as slab window that forms as a result of separation of theoceanic plates on either side of a subducted spreading ridge(Fig. 8) (Dickinson and Snyder, 1979; Thorkelson and Taylor,1989; Thorkelson, 1996). In this case, high heat flow through a slabwindow can induce melting of the overlying crustal rocks to formintermediate to acidic magmas (DeLong et al., 1979; Bradley et al.,2003). Adakitic magmas might also be generated from the partialmelting of the oceanic crust at slab window edges (Yogodzinskiet al., 2001; Kinoshita, 2002; Thorkelson and Breitsprecher, 2005)and/or from the partial melting of overlying mafic lower crust(Cole et al., 2006). Therefore, the ridge subduction and concomitantslab window formation is considered as a principal cause of igne-ous, metamorphic and structural variations in active continentalmargins (Dickinson and Snyder, 1979; Johnson and O’Neil, 1984;

Fig. 8. Tectonic model for the origin of the Gangdese

Forsythe and Nelson, 1985; Thorkelson and Taylor, 1989; Sissonet al., 1994, 2003; Thorkelson, 1996; Cole and Stewart, 2009;Eyuboglu et al., 2010).

The late Cretaceous (ca. 100–80 Ma) was the main active stageof the Gangdese magmatism. Furthermore, the magmatic rocksgenerated include calc-alkaline diorite, granodiorite and granite(Mo et al., 2005a; Ji et al., 2009) as well as some adakitic graniteand granodiorite (Wen et al., 2008a,b), andesite, dacite and rhyolite(Yao et al., 2006). The occurrence of hybrid magmas in convergentmargin settings is considered as a hallmark of ridge subduction andslab window mechanism (Bourgois et al., 1996; Guivel et al., 1999;Bradley et al., 2003; Thorkelson and Breitsprecher, 2005; Eyubogluet al., 2010, 2011). The country rocks of the Gangdese charnockiteshave been subjected to HT granulite-facies metamorphism at LateCretaceous of 87–89 Ma (Zhang et al., 2010b). The coeval nature ofthe HT magmatism and HT metamorphism is an important mani-festation of the spreading ridge subduction and concomitant slabwindow formation along continental margins (Sisson et al., 1989;Underwood et al., 1999; Iwamori, 2000; Thorkelson and Breitspre-cher, 2005; Cole and Stewart, 2009; Ickert et al., 2009; Santoshet al., 2009, 2011). Santosh and Omori (2008) demonstrated thatboth mantle upwelling and arc magmatism provide an ideal geody-namic setting for the formation of charnockite and associated HT toultrahigh-temperature (UHT) metamorphic orogens. Thus, theseevidences strengthen our inference that the Gangdese charnockiteswere derived from the subduction of Neo-Tethyan mid-ocean ridge(Zhang et al., 2010a).

Our present study reveals abundant high density pure CO2 fluidinclusions and hematite and magnetite occur within the quartzand plagioclase of the Gangdese charnockites, strongly suggestinglow-water-activity and high oxygen fugacity conditions for thecharnockite crystallization. Therefore, similar to the charnockitesand HT–UHT metamorphic rocks from other convergent plate mar-gins (e.g., Ohyama et al., 2008; Santosh and Omori, 2008; Santoshet al., 2008; Tsunogae et al., 2008a,b; Tsunogae and Santosh, inpress), the Gangdese charnockites are also characterized by thepreservation of dry mineral assemblages and the common associa-tion of CO2-rich fluid inclusions.

The fluids in normal subduction regimes are dominantly hy-drous, as also attested by petrological, geochemical and geophysi-cal studies in subduction systems (e.g., Zhao et al., 2007; Zhanget al., 2008; Hasegawa et al., 2009; Maruyama et al., 2009). But,the fluid composition might undergo a drastic change if a mid-oce-anic ridge is subducted (Santosh and Kusky, 2009; Santosh et al.,2011). A slab window that opens during the ridge subductionwould place the hot asthenosphere against the base of the overly-

charnockites (modified after Zhang et al., 2010a).

522 Z. Zhang et al. / Journal of Asian Earth Sciences 42 (2011) 515–524

ing plate and sublithospheric mantle. This zone is essentially drybecause there is no slab and therefore there are no hydrous fluidsexpelled from the slab. The dominant fluids in this zone are thoseliberated from the asthenosphere, which presumably would beCO2-rich, thereby contributing heat and CO2-rich fluids for gener-ating dry magmas to form charnockites (Fig. 8). Our present studydemonstrates further that the subduction of Neo-Tethyan mid-oceanic ridge beneath the Lhasa terrane resulted in heat and CO2

upwelling from the asthenosphere through slab window (Fig. 8).Thus Andean-type orogeny provided high-temperatures and dryCO2-rich fluids for the formation and stabilization of the adakiticcharnockites as well as the associated HT granulite-facies meta-morphic rocks in the Late Cretaceous prior to the Himalayan-typeorogeny derived from the Indian continent colliding with the Asiancontinent during early Cenozoic.

5. Conclusion

Abundant high-density and CO2-rich fluid inclusions weretrapped during the crystallization of the Gangdese charnockites,southern Tibetan Plateau, implying that the Gangdese adakiticcharnockites were generated under high-temperature and anhy-drous conditions. The subduction of Neo-Tethyan mid-oceanicridge might have resulted in heat and CO2 upwelling from theasthenosphere through a slab window in the Late Cretaceous priorto the collision of Indian plate with the Asian continent. This studyprovides new insights into the Andean-type orogeny of SouthernTibetan Plateau during Late Mesozoic.

Acknowledgements

Zeming Zhang thanks Profs. Zhiqin Xu, Zhenmin Jin, JingshuiYang, Lailin Zheng, Quanru Geng, Linsheng Zheng, Xunxiang Qiand Yongsheng Liu for valuable directions and discussions in thework. Master students Jinli Wang, Guansheng Geng, Feng Liu, FeiYu and Wei Wang took part in the fieldwork during this study.We are most grateful to Prof. Liou J.G., Dr. Yener Eyuboglu andan anonymous reviewer for critical and constructive reviews ofthe manuscript. This research was funded by the Chinese NSFCGrants (40772049, 40972055 and 40921001), Chinese GeologicalSurvey Program (1212010918012 and 1212011121269) and theFoundation for Open Projects of State Key Laboratory of GeologicalProcesses and Mineral Resources, China University of Geosciences(GPMR200907).

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