Adakitic rocks from slab melt-modified mantle sources in the continental collision zone of southern Tibet

Lithos 119 (2010) 651–663

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Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s

Adakitic rocks from slab melt-modified mantle sources in the continental collision zone of southern Tibet Yongfeng Gao a,⁎, Zhusen Yang b, M. Santosh c, Zengqian Hou d, Ruihua Wei a, Shihong Tian b a

Shijiazhuang University of Economics, Shijiazhuang, Hebei 050031, China Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China Division of Multidisciplinary Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan d Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b c

a r t i c l e

i n f o

Article history: Received 11 May 2010 Accepted 12 August 2010 Available online 20 August 2010 Keywords: Adakites Slab melt metasomatism Post-collision magmatism Geochemistry Tectonics Southern Tibet

a b s t r a c t Major-trace element and Sr–Nd–Pb isotopic data are presented for newly discovered adakitic rocks in the western Gangdese belt, southern Tibet. The Miocene (26–10 Ma) adakitic rocks from the southern Tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas. These rocks display geochemical affinities similar to those of Cretaceous (136–80 Ma) adakitic rocks derived from the partial melting of subducted Neotethyan slab in southern Tibet. The whole rock geochemical and isotope characteristics of the post-collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the Cretaceous subduction event. An interesting observation is that the E–W trending belt of adakitic rocks along the Yarlung Tsangpo suture zone occupies a fore-arc position, reflecting a geotectonic setting compatible with the genesis of adakitic magmas. The widespread occurrences of Cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite-metasomatized mantle. Our favoured interpretation is that the spatial isotopic variation in the post-collision adakitic rocks is mostly linked to a westward increase in sediment input in the Tibetan mantle region. Under the framework of our paleo-subduction model, a slab break-off event that initiated at around 25 Ma would have allowed an asthenospheric upwelling beneath southern Tibet, which was instrumental in generating the post-collision adakitic magmatism in southern Tibet. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The widespread post-collisional igneous rocks distributed across southern Tibet have long been regarded as a key to understanding the tectonic evolution of Tibetan orogens. Since the first identification of the adakitic porphyries in the Gangdese belt (Gao et al., 2003a), numerous studies have been carried out on Miocene (26–10 Ma) adakitic magmatism (Aitchison et al., 2009; Chung et al., 2003; Gao et al., 2003b, 2007a; F. Guo et al., 2007; Hou et al., 2004; Xu et al., 2010) which demonstrated the presence of an arcuate E–W trending zone of adakitic rocks paralleling the Yarlung Tsangpo suture zone (YTSZ) in southern Tibet. Such rocks typically occur as porphyry intrusions and dikes, rare eruptive rocks and large-volume granitoids along the YTSZ as well as to the north of this suture. Various models have been proposed for the magma generation of these adakitic rocks (Chung et al., 2003; Hou et al., 2004; Gao et al., 2003a, 2007a; F. Guo et al., 2007; Xu et al., 2010).

⁎ Corresponding author. Department of Resources, Shijiazhuang University of Economics, Huaian East Road 136, Shijiazhuang, Hebei 050031, PR China. Tel.: + 86 311 87207857; fax: + 86 311 85882537. E-mail address: [email protected] (Y. Gao). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.08.018

Previous studies on adakitic rocks with high Sr/Y and La/Yb ratios have established that they may form in a variety of tectonic settings through different petrogenetic processes (Defant and Drummond, 1990; Atherton and Petford, 1993; Martin et al., 2005; Moyen, 2009, and references therein; Eyuboglu et al., 2010). A characteristic feature of the Miocene adakitic magmatism is their post-collision geodynamic setting. This implies that the Miocene adakites were probably not related to the active subduction of an oceanic lithosphere beneath the southern Tibet between 26 and 10 Ma. Thus, the model involving partial melting of the thickened Tibetan lower crust (Chung et al., 2003; Hou et al., 2004; F. Guo et al., 2007) appears as a feasible explanation for the post-collision adakitic rocks in this belt. This model, however, could not adequately explain the distinct distribution and geochemistry of the Miocene adakitic rocks in the Lhasa terrane (Gao et al., 2007a). Thus, the nature of magma sources for Miocene adakitic magmatism in this terrane has been a topic of debate (Gao et al., 2007a; Xu et al., 2010), with the tectonic implications remaining equivocal. A number of recent studies have recognized Cretaceous (136.5– 80 Ma) volcanic and plutonic rocks with adakitic geochemical characteristics in the Gangdese arc belt, south Tibet (Yao et al., 2006; Zhu et al., 2009; Wen et al., 2008a; Wei et al., 2007; Kang et al.,

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2009; Zhang et al., 2010). These studies suggest that the adakitic magmas were derived by the partial melting of subducted Neotethyan slab prior to the collision of India with the Asian continent (Yao et al., 2006; Zhu et al., 2009; Zhang et al., 2010). More importantly, the distribution of the pre-collision adakitic rocks spatially overlaps well with those of the post-collision adakitic rocks. Thus, a careful comparison of their chemical and isotopic characteristics can provide an insight into the nature of magma source for the post-collision adakitic magmatism. In this paper, we report geochemical and Sr–Nd–Pb isotopic data for the newly discovered outcrops of adakitic rocks from the western segments of the Gangdese belt. Our study focuses on the geochemical and isotopic characteristics of these rocks to provide robust constraints on their melt sources. In conjunction with previously published information, we attempt to distinguish the petrological and geochemical characteristics associated with the sources, and to elucidate the genetic relationship between the pre- and post-collision adakitic magmas from the Gangdese belt in southern Tibet. 2. Geological setting The Tibetan plateau consists of diverse exotic blocks that were accreted at different time intervals. The Lhasa block is bounded by the YTSZ to the south, and the Bangong-Nujiang suture to the north (Fig. 1). Two magmatic suites have been identified in the Lhasa block, namely the northern magmatic belt and the southern Gangdese belt (Coulon et al., 1986). The latter is generally regarded as an Andeantype volcanic arc, resulting from the northward subduction of the Neotethyan oceanic crust along the YTSZ (Yin and Harrison, 2000; Chung et al., 2005). The voluminous Gangdese batholith and the widespread Linzizong volcanic succession in southern Lhasa terrane formed during the Cretaceous–Early Tertiary (Mo et al., 2007; Wen et al., 2008b, and references therein). A synthesis of the age data for the intrusive and volcanic rocks from the Lhasa terrane shows two distinct stages of Gangdese magmatism, in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 68–43 Ma), with a magmatic gap in between (Wen et al., 2008b, and references therein).

After this quiescent period of magmatism for nearly 10 million years (from 35 to 25 Ma), a renewal of the magmatic activity took place in the period from ~25 Ma to 10 Ma (Turner et al., 1993, 1996; Miller et al., 1999; Williams et al., 2001, 2004; Mahéo et al., 2002; Ding et al., 2003; Chung et al., 2003; Hou et al., 2004; Zhao et al., 2009). This renewed magmatic pulse in the Lhasa terrane produced three types of post-collisional rocks, including potassic–ultrapotassic rocks, adakitic rocks and peraluminous granites (Chung et al., 2005; Mo et al., 2007). An adakitic porphyry belt in an east–west-trending array along the YTSZ in the southern edge of the Gangdese belt has been recognized in several studies (Turner et al., 1996; Miller et al., 1999; Williams et al., 2001, 2004; Gao et al., 2003a,b, 2007a; Chung et al., 2003; Hou et al., 2004; F. Guo et al., 2007; Aitchison et al., 2009; Xu et al., 2010). This array extends for over ~1500 km from Linzhi to Shiquanhe (Fig. 1). Moreover, there is a complete absence of the adakitic outcrops in areas further north of the YTSZ. The adakitic belt is mainly composed of adakitic dikes and small intrusions. Recent studies indicate that felsic tuffs and few large-volume granitoids with adakitic geochemical affinities occur in the post-collision adakitic belt (Aitchison et al., 2009; Xu et al., 2010). Geochronological data constrain the duration of the adakite-like magmatism as 26–10 Ma (Chung et al., 2003; Hou et al., 2004; Aitchison et al., 2009; Xu et al., 2010). Magmatic rocks of adakitic affinity formed during Late Jurassic to Cretaceous (136.5–80.4 Ma) have also been reported in the Gangdese belt in some of the recent studies including those by Yao et al. (2006), Wen et al. (2008a), Zhu et al. (2009), Kang et al. (2009) and Zhang et al. (2010). These rocks consist predominantly of andesites of the Sangri Group and small granitoid intrusions, and are spatio-temporally associated with arc-related calc-alkaline rocks. Sporadic occurrences of the pre-collision adakites constitute an east–west-trending array along the YTSZ. This array extends over 500 km distance from Lilong to Xigaze (Yao et al., 2006). Notably, this zone overlaps well with the post-collision adakitic rock belt (Fig. 1). In comparison with the restricted east–west-trending adakitic belt along the YTSZ, the post-collision potassic–ultrapotassic lavas appear to

Fig. 1. Sample locality map showing the occurrence of Miocene adakitic rocks in the Gangdese belt of southern Tibet (modified from Gao et al., 2007a, and references therein; Aitchison et al., 2009; Xu et al., 2010; and this study). Also shown are locations of ultrapotassic rocks and Cretaceous adakitic rocks (after Yao et al., 2006; Wen et al., 2008a; Zhu et al., 2009; Kang et al., 2009, and Zhang et al., 2010). BNS, Bangong-Nujiang suture; YTSZ, Yarlung-Tsangpo suture zone; STDS, south Tibet detachment surface; MBT, main boundary thrust.

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be distributed in a wide zone throughout the western part of the Lhasa block (Fig. 1). The available age data from these rocks (Miller et al., 1999; Ding et al., 2003; Williams et al., 2001, 2004; Nomade et al., 2004; Zhao et al., 2009) define a magmatic event during 25–12 Ma for the ultrapotassic and associated potassic lavas in the western part of the Lhasa block. Therefore, the duration of ultrapotassic magmatism implies a time window (12–25 Ma) which is coincident with that (10–26 Ma) of the adakitic magmatism. In order to evaluate whether there is an along-strike variation in the Miocene volcanism in the Lhasa block, we divided the Gangdese belt into two segments (i.e. east or west of 89° E). Whereas there have been a number of studies on Miocene adakitic rocks of the eastern Gangdese belt, only limited data are available for the western Gangdese adakites (Turner et al., 1996; Miller et al., 1999; Williams et al., 2001; Chung et al., 2003; F. Guo et al., 2007; Aitchison et al., 2009; Xu et al., 2010). In this study, we investigate two smallvolume porphyries from the Zhunuo (ZN) and Puridazong (PR) in the western Gangdese belt, which lithologically correspond to granodioritic and granitic respectively (Fig. 1). The rocks display prominent porphyritic textures with 10–35% phenocrysts of plagioclase, K-feldspar, quartz and biotite. SHRIMP U–Pb zircon dating for the Zhunuo porphyry yielded a magma crystallization age of 15.6 ± 0.6 Ma (Zheng et al., 2007). 3. Samples and analytical techniques Seventeen fresh samples representative of the Zhunuo (ZN) and Puridazong (PR) porphyries were selected for the present study. The exposed portions and weathered surfaces of the samples were carefully removed, and only the fresh interior domains processed for analyses. Whole rock major and trace element analyses were performed at the Geoanalytical Center of Nuclear Industry (Beijing) with X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Analytical uncertainties are 1–3% for major elements. For trace element and rare earth element (REE) analyses, rock powders (50 mg) were dissolved using mixed acids (HF/HClO4) in capped Savillex Teflon breakers at 120 °C for 6 days, and subsequently dried to wet salt and re-dissolved in 0.5 ml HClO4. The solutions were then evaporated to wet salt at 140 °C and re-dissolved in 1 ml HNO3 and 3 ml water for c. 24 h at 120 °C. The solutions were diluted in 2% HNO3 for analysis. The uncertainties based on the replicate analyses of internal standards are ±5% for REE and ±5–10% for trace elements. Sr, Nd and Pb isotopic analyses were performed at the Geoanalytical Center of Nuclear Industry (Beijing), on a TIMS (Isoprobe-T spectrometer equipped with nine Faraday and one ETP detector and a Wide Angle Retarding Potential). Rock chips (b20 mesh) were leached in purified 0.1 mol L−1 HCl for 24 h at room temperature to avoid the influence of surface alteration or weathering on the Sr–Nd– Pb isotopic samples. Within-run isotope fractionations were corrected by using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. The ShinEtsu standard yield 143 Nd/ 144 Nd = 0.512095 ± 0.000009 (n = 90) and NBS 987 gave 87Sr/86Sr = 0.710229 ± 13 (n = 80) for the last period of 6 months. Pb isotopes are measured by TIMS using single Re filament. Uncorrected results for NBS 981 are during 2008 206 Pb/204Pb = 16.895 ± 0.026 (2σ external reproducibility), 207Pb/ 204 Pb = 15.437 ± 0.029, and 208Pb/204Pb = 36.537 ± 0.121. Consequently, the mass fractionation correction applied to unknowns relative to the NBS 981 values is 1.0 ± 0.1 per mil per amu. The initial Sr, Nd and Pb isotopic ratios were corrected using the age of 15.6 Ma. 4. Results Major and trace element compositions in representative samples of the ZN and PR adakitic porphyries are given in Table 1. The results

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are plotted in Figs. 2 and 3, together with the data from previous studies (Turner et al., 1996; Miller et al., 1999; Williams et al., 2001; Gao et al., 2007a; Chung et al., 2003; Hou et al., 2004; F. Guo et al., 2007; Aitchison et al., 2009; Xu et al., 2010; Yao et al., 2006; Wen et al., 2008a; Zhu et al., 2009; Kang et al., 2009). All of the studied samples have high SiO2 (65.10–71.56 wt.%) and Al2O3 (14.14–17.12 wt.%), low MgO (0.83–2.35 wt.%) and CaO (1.81– 3.69 wt.%), and moderate K 2 O (3.19–4.20 wt.%). Most of the porphyries correspond to dacites and rhyodacites (Fig. 2). Compared to typical adakites from active continental arcs (Kay, 1978; Yogodzinski et al., 1995), these samples have relatively high K2O contents with most of the samples plotting in the high-K calcalkaline field in a K2O versus SiO2 diagram (Fig. 2b). In spite of the high K2O contents in the samples studied here, their K2O/Na2O ratios are b1 (Fig. 2c). MgO, TiO2 and CaO in the porphyries correlate positively with SiO2 (Fig. 3a, c and d), whereas Al2O3 remains almost constant (Fig. 3b). All the samples show enrichment in incompatible elements such as large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE). Primordial mantle normalized incompatible trace element patterns (Fig. 4d) are characterized by negative Nb (Ta) and Ti anomalies and peaks at Pb, a typical characteristic of subduction-related magmas. The porphyries have high Sr contents (550–995 ppm), and display distinct positive Sr anomalies (Fig. 4d). The most noticeable feature of the samples is their extraordinarily low HREE and Y content. Thus, the porphyries display high Sr/Y and La/Yb ratios, which are characteristic of adakites (Fig. 5). Sr–Nd–Pb isotope results in this study are summarized in Table 2, and plotted in Figs. 6 and 7. The initial 87Sr/86Sr ratios range from 0.7071 to 0.7096, 143Nd/144Nd ranges from 0.5122 to 0.5124, (206Pb/ 204 Pb)i ranges from 18.417 to 18.682, (207Pb/204Pb)i ranges from 15.646 to 15.722, and (208Pb/204Pb)i ranges from 38.726 to 39.101. 5. Discussion 5.1. Comparison between two generations of adakites Together with the Cretaceous adakites (136.6–80 Ma) in this region which are considered to have formed through Neotethyan subduction, the widespread occurrence of Miocene adakitic rocks (26–10 Ma) defines an east–west-trending array along the YTSZ. Although the timing of initiation age of the India–Asia collision remains unsettled (with an age range of 65–34 Ma; Yin and Harrison, 2000; Mo et al., 2007; Aitchison et al., 2007), the two generations of adakites were clearly produced in pre-collision and post-collision regimes, respectively. Thus, a comparison of their geochemical and isotopic characteristics would provide valuable constraints on the source characteristics and geodynamic processes involved in the generation of Miocene post-collision adakitic magmas in the Gangdese belt. There is no ambiguity in that both suits display chemical features similar to those of modern adakites (Defant and Drummond, 1990). These rocks are characterized by relatively high Al2O3 and Sr, and low Y and HREE, and markedly high Sr/Y and La/Yb ratios (Fig. 5). Such chemical features are considered to be a diagnostic feature of the partial melting of metamorphosed basaltic rocks (amphibolite and eclogite), and are interpreted to reflect both the presence of residual garnet and the absence of plagioclase in the source region (Kay, 1978; Drummond and Defant, 1990; Eyuboglu et al., 2010). The ‘adakitic signature’ (i.e. high Sr/Y and La/Yb ratios) can be achieved by hydrous (involving amphibole) or high-pressure (involving garnet) crystal fractionation of a mafic magma (Richards and Kerrich, 2007; Moyen, 2009). We therefore evaluate the possible role of crystal fractionation in the genesis of the Gangdese adakitic porphyries. In a previous study, Gao et al. (2007a) suggested that

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Table 1 Major and trace elements of the post-collision adakitic rocks from the Gangdese belt, southern Tibet. Location Zhunuo

Puridazong

Sample

ZM-1

ZM-2

ZM-3

ZM-4

ZM-5

ZM-6

ZM-7

ZM-8

ZM-9

ZM-10

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Sum Mg# Sc V Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

66.69 0.64 15.28 4.17 0.02 1.75 2.20 4.16 3.57 0.21 0.90 99.6 49.4 8.31 78.1 23.1 11 15 232 681 12.2 47.2 9 19.1 880 37.4 65.7 8.35 31.3 5.57 1.36 4.46 0.54 2.7 0.49 1.29 0.17 0.99 0.13 1.44 0.69 21 18 2.97

65.96 0.5 15.28 3.07 0.06 1.54 2.61 4.19 3.71 0.18 2.66 99.8 53.9 5.2 66 118 14.4 86.8 202 633 9.21 96.6 9.3 8.47 669 31.5 63.8 7.81 28.3 4.68 1.17 3.34 0.43 1.99 0.34 0.92 0.13 0.82 0.12 4.05 0.79 60.1 27.2 6.64

65.37 67.00 69.09 65.10 63.86 71.56 69.73 69.37 0.60 0.57 0.46 0.60 0.59 0.31 0.40 0.45 15.35 14.56 14.67 15.44 16.01 14.2 14.35 14.14 4.27 4.06 3.18 4.31 3.83 2.39 3.04 3.32 0.08 0.08 0.04 0.09 0.08 0.03 0.04 0.04 1.84 1.60 1.20 1.94 2.35 0.83 1.08 1.34 3.70 2.88 2.06 3.69 3 1.81 2.38 2.52 4.02 3.92 3.81 4.00 4.27 4.32 4.34 3.98 3.25 3.49 4.20 3.19 3.41 4.00 3.85 3.65 0.21 0.19 0.17 0.22 0.27 0.11 0.15 0.16 0.94 1.36 0.86 1.08 1.98 0.22 0.36 0.80 99.6 99.7 99.7 99.7 99.7 99.8 99.7 99.8 50.1 47.9 46.8 51.2 58.8 44.7 45.3 48.5 8.55 7.36 5.72 8.65 5.79 3.6 4.52 5.62 79.9 71.5 56.3 81.5 74.3 36.2 44.9 52.1 22.6 22.6 21.8 24.5 82.6 13.3 18.2 22.2 12.2 10.9 7.27 13 11.5 5.21 6.7 5.24 13.9 12.7 10.5 15.9 17.3 6.41 8.65 14.2 141 158 218 142 153 227 207 191 884 752 550 878 807 567 664 623 17.5 11.0 11.0 11.2 9.57 6.02 7.64 8.98 114 78.6 26.8 64.7 67.5 26.5 46 77.7 9.72 9.14 9.83 9.23 8.98 8.6 8.41 9.77 6.42 6.97 11.1 8.28 6.26 10.8 8.35 14.0 985 912 848 964 741 861 857 838 33.36 37.8 38.4 38.7 33.6 28 37.2 34.8 63.6 68.3 65.9 69.6 66.9 47.4 62.7 61.5 8.32 8.52 8.2 8.65 8.15 5.41 7.45 7.55 31.68 30.8 28.4 31.9 29.2 19.2 26.5 27.8 6.32 5.22 4.79 5.48 4.96 3.02 4.27 4.69 1.1 1.23 1.08 1.36 1.30 0.76 1.00 1.07 4.02 3.98 3.58 4.02 3.54 2.17 3.09 3.36 0.520 0.47 0.47 0.5 0.45 0.25 0.35 0.38 2.72 2.46 2.3 2.5 2.03 1.22 1.64 1.98 0.487 0.43 0.41 0.44 0.38 0.23 0.29 0.35 1.26 1.13 1.13 1.19 0.97 0.61 0.77 0.92 0.16 0.15 0.14 0.15 0.13 0.09 0.11 0.12 1 0.96 0.94 0.91 0.83 0.6 0.67 0.74 0.13 0.14 0.13 0.12 0.11 0.09 0.1 0.1 3.57 2.76 0.94 2.35 2.74 1.11 1.82 2.7 0.77 0.73 0.92 0.69 0.69 0.82 0.76 0.82 34.4 43.9 42.2 34.9 42.4 28 29.7 32.1 24.6 24.5 30.1 21.7 24.6 22.3 26 28.4 5.66 5.91 4.48 5.16 5.39 5.11 4.84 8.33

ZM-12

ZM-13

PRDZ1

PRDZ2

PRDZ3

PRDZ4

PRDZ5

67.27 65.99 65.61 65.97 65.73 65.02 65.29 0.47 0.5 0.6 0.64 0.65 0.63 0.64 17.12 15.27 15.45 15.4 15.39 15.04 15.18 2.25 3.05 3.46 3.69 3.68 3.54 3.62 0.03 0.06 0.06 0.06 0.06 0.06 0.06 0.97 1.54 1.63 1.75 1.76 1.72 1.7 2.43 2.59 3.27 3.22 3.33 3.16 3.24 5.03 4.13 3.93 3.83 3.78 3.82 3.77 3.75 3.71 3.82 3.84 3.79 3.85 3.8 0.24 0.19 0.26 0.27 0.27 0.26 0.26 0.12 2.76 1.46 1.62 1.5 1.56 1.4 99.7 99.8 99.6 100.3 99.9 98.7 99.0 50.1 54.0 52.3 52.5 52.7 53.1 52.2 2.54 8.96 7.27 7.57 7.28 7.43 8.03 48.8 107 84.8 87.1 83.0 75.6 91.6 112 33.4 24.1 25.5 23.1 377.8 28.7 5.99 13.1 10.6 10.8 10.6 13.4 11.4 6.85 17.1 11.4 11.6 11.5 74.2 12.9 193 40.3 149 143 134 149 155 824 825 1025 987.5 994 950 995 5.06 8.69 12.0 12.1 11.2 11.6 12.9 18.3 166 127 136 127 139 149 7.3 6.45 8.83 8.77 8.07 8.65 9.28 8.42 2.07 3.66 3.79 3.49 3.60 3.92 652 1043 1287 1234 1129 1205 1225 25.4 20.6 42.2 50.8 41.5 46.7 49.7 49.7 41 83.4 94.0 81.0 88.7 95.5 6.65 5.09 9.83 10.42 9.21 9.93 10.70 25.1 19.8 37.5 38.4 34.7 37.2 39.8 4.19 3.45 6.19 6.25 5.53 6.05 6.55 1.04 1.29 1.60 1.57 1.44 1.48 1.60 2.69 2.92 4.64 4.50 4.17 4.38 4.63 0.32 0.41 0.390 0.397 0.359 0.374 0.414 1.22 1.89 2.84 2.79 2.63 2.74 2.91 0.19 0.33 0.504 0.511 0.471 0.491 0.530 0.48 0.96 1.40 1.40 1.30 1.34 1.50 0.06 0.13 0.186 0.185 0.167 0.179 0.188 0.38 0.87 1.032 1.004 0.957 1.004 1.096 0.04 0.12 0.166 0.166 0.157 0.172 0.173 1.34 4.88 3.58 3.84 3.65 3.89 4.15 0.62 0.4 0.582 0.567 0.534 0.573 0.592 23 10.6 32.1 30.8 29.9 31.8 31.9 15 2.95 16.5 17.5 16.0 17.6 17.9 2.83 0.92 2.58 2.43 2.35 2.46 2.57

Mg# = Mg/(Mg + 0.85 ⁎ TFe2+).

although some fractional crystallization must have occurred during the formation of these rocks, crystal fractionation alone cannot account for the adakitic signature of the Gangdese rocks. Considering the presence of a ~ 1500 km long belt of adakitic rocks, a fractionation model would require the existence of an extremely large parent magma body, the evidence for which is lacking. In fact, there is a complete absence of coeval andesitic and basaltic magmatisms in this adakite belt. Recent investigation on the crystallization history of a hydrous primitive andesite composition shows that garnet is stable in andesitic and basaltic bulk compositions only after large degrees of crystallization lead to a decrease of the Mg-number to less than 0.5, and that high Mg-number primitive melts are not garnet saturated at high pressures (Müentener et al., 2001). However, all of the adakitic porphyries from southern Tibet including those with lower Mgnumber show high Dy/Yb ratios and La/Yb ratios (Gao et al., 2007a). The high Sr/Y, Dy/Yb and La/Yb ratios, low heavy REE and Y concentrations of the Gangdese adakites require an adakitic signature in the primary melt source. Overall, the adakitic rocks of different ages in the Gangdese belt display same differentiation trends (Figs. 2 and 3). In both the precollision and post-collision groups, the abundances of MgO (Fig. 3a), TiO2 (Fig. 3c) and CaO (Fig. 3d) decrease with increasing SiO2,

whereas with few exceptions, most samples have nearly constant Al2O3 contents (Fig. 3b). Whereas the MgO and SiO2 contents of the pre-collision adakite show a wide range, most of the post-collision adakites have high SiO2 and low MgO contents, and plot in the high SiO2 adakite field (Fig. 2a). In the two types of adakites, the total alkaline contents (K2O + Na2O wt.%), K2O abundances and K2O/Na2O ratios show a positive correlation with SiO2, displaying the typical differentiation trend of calc-alkaline arc magmas (Fig. 2). However, some of the post-collision adakitic rocks have unusually high K2O contents, yielding abnormally high total alkaline contents and K2O/ Na2O ratios. Consequently, these samples significantly depart from the overall trends (Fig. 2). This suggests that the unusual K2O enrichment was not simply a result of magmatic differentiation. The two generations of adakitic rock in the Gangdese belt show many similarities in terms of distribution of trace elements with typical incompatible trace element fractionation patterns of subduction-related magmas (Fig. 4). Overall, the adakites of different ages display significant positive Pb and Sr anomalies, and negative Nb, Ta and Ti anomalies (Fig. 4), correlating with typical features of adakitic magmas (Martin et al., 2005). Despite their similar trace element patterns, the geochemical signatures of the rocks in different regions show some distinction. Some of the post-collision adakitic rocks from

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collision adakites have low radiogenic Sr and high unradiogenic Nd isotopes similar to those of the pre-collision adakites, and plot in the depleted end of the spectrum. These values partially overlap with those for the basaltic rocks from Yarlung ophiolite. Based on the existing dataset, it is evident that the post-collision adakitic rocks possess a wide range of 87Sr/86Sr and 143Nd/144Nd ratios, resulting in a distinct array, and extending to similar compositions of the co-genetic ultrapotassic lavas from west Gangdese. Overall, the post-collision adakites from the west Gangdese segment plot along an array characterized by high initial 87Sr/86Sr, but low initial 143Nd/144Nd (Fig. 6), reflecting a striking relationship between the isotopic composition and geographical distribution. On the other hand, Pb isotope systematics of the post-collision adakitic rocks show strong correlations with both uranogenic and thorogenic spaces (Fig. 7a and b). Limited Pb isotope data of the adakitic rocks from the Gangdese belt partially overlap the fields of basaltic rocks from the Yarlung ophiolite (Zhang et al., 2005), and extend these fields towards higher 207Pb/204Pb and 208Pb/204Pb values, similar to postcollision ultrapotassic rocks from southern Tibet (Fig. 7a and b). The combined Sr, Nd and Pb isotope compositions of post-collision adakitic rocks from the Gangdese belt furthermore define distinct isotopic arrays from basaltic rocks in Yarlung ophiolite to post-collision ultrapotassic rocks in the western Gangdese belt (Fig. 7c and d). It is worth noting that the post-collision adakitic rocks from the western Gangdese have higher radiogenic Pb isotopes and non-radiogenic Nd relative to the eastern Gangdese adakitic rocks (Fig. 7c and d). Unfortunately, there is limited available Pb isotopic data (Zhu et al., 2009) for the pre-collision adakites for this comparison. In summary, a close examination of the major and trace element systematics allows the recognition of an overall uniform differentiation trend in both the pre-collision and post-collision adakitic rocks from the Gangdese belt. Although the isotopic compositions of the post-collision adakitic rocks show significant variations, some of the rocks belonging to this suite have low initial 87Sr/86Sr and high 143Nd/ 144 Nd ratios similar to those of the pre-collision adakitic rocks (Fig. 6). In conjunction with the similarities of adakitic signatures (i.e. high Sr/Y and La/Yb; Fig. 5), the above feature suggests that the melt source for the post-collision adakitic rocks was ultimately linked to the precollision adakitic magmatism in the Gangdese belt. 5.2. Post-collision adakitic rocks by lower crustal melting?

Fig. 2. (K2O+Na2O) versus SiO2 (a) (Le Maitre et al., 2002), K2O versus SiO2 (b) (Peccerillo and Taylor, 1976), and K2O/Na2O versus SiO2 (c) diagrams for adakitic rocks from the study area. All data plotted have been recalculated to 100 wt.% on a volatile-free basis. Solid line from Irvine and Baragar (1971) separates alkaline from calc-alkaline lavas (a). Most of the samples define overall trends towards high total alkaline content, K2O and K2O/Na2O with Si2O increase. Some samples with unusually high K2O deviate markedly from the overall trends. Miocene adakitic rocks from Turner et al. (1996), Miller et al. (1999), Williams et al. (2001), Chung et al. (2003), Hou et al. (2004), Gao et al. (2007a), F. Guo et al. (2007), Aitchison et al. (2009), Xu et al. (2010) and this study. Cretaceous adakitic rocks from Yao et al. (2006), Wen et al. (2008a), Zhu et al. (2009), Kang et al. (2009) and Zhang et al. (2010). Data field for southern Tibetan ultrapotassic rocks from Gao et al. (2007b, and references therein). Data field for the Linzizong volcanics from Mo et al. (2007).

the western Gangdese belt show distinct negative Ba anomalies and Th peaks (Fig. 4c and d). A further similarity between the adakites of different ages is displayed by their Sr and Nd isotopic arrays (Figs. 6 and 7). Overall, the adakites from southern Tibet define a hyperbola similar to the ‘mantle array’ in a Sr versus Nd isotope diagram (Fig. 6). Some of the post-

It is evident that the post-collision adakites from the Gangdese belt show distinct differences in geochemical and isotopic compositions from those of adakites in some of the type localities (such as Cook Island). More importantly, their geodynamic setting is associated with the Indo-Asian continental collision, rather than an active subduction. Several studies in the recent years have therefore proposed that these adakitic rocks were derived from partial melting of the thickened Lhasa lower crust (Chung et al., 2003; Hou et al., 2004). F. Guo et al. (2007) suggested that the melt source of the post-collision adakitic rocks was a mafic-intermediate lower crust formed during a preceding stage (153–40 Ma) of active continental margin magmatism. Mo et al. (2007) further suggested the lower portion of the thickened Tibetan crust, a possible magma source of the post-collision adakites, is mafic and is genetically associated with the earlier Linzizong volcanic rocks. Although the Tibetan plateau has been thickened to approximately twice the normal thickness (~70 km; Molnar, 1990), the time at which this was achieved in south Tibet is still under debate. For example, Nomade et al. (2004) considered a much thinner (ca. 35 km) original crust in south Tibet during the Miocene volcanism. Thus, the two times normal thickness in the modern south Tibetan crust is not necessarily an indicator of the partial melting of metabasalt in the lower crust. A key question for this model is whether there was potential old crustal basement or juvenile mafic lower crust for the post-collision adakitic

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Fig. 3. Variation diagrams for selected major elements. Both groups of adakitic rocks with different ages show clear negative correlations in MgO (a), TiO2 (c) and CaO (d) versus SiO2 diagrams, whereas their Al2O3 contents display inflections at around 65 wt.% SiO2. Symbols and data sources are as in Fig. 2.

Fig. 4. Primitive mantle-normalized (Sun and McDonough, 1989) incompatible element diagrams for the Tibetan adakitic rocks of different ages and locations. It is noted that the two generations of adakitic rocks share similar incompatible element patterns with negative Nb–Ta and Ti anomalies and positive Pb and Sr anomalies. Compared with Cretaceous adakitic rocks, some of Miocene adakitic rocks, especially from the western Gangdese belt, show clearly positive Th peaks (c and d).

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from a remelting of the underplated basaltic crust. Moreover, the postcollision adakitic rocks occur along a narrow E–W trending zone along the YTSZ, and this distribution pattern likely reflects the orientation of their source area in south Tibet. It is noteworthy that the post-collision adakitic rocks do not extend beyond this E–W trending, 1500 km long, narrow belt (Fig. 1). In fact, widespread outcrops of voluminous Linzizong volcanic rocks are mainly exposed north of this adakite belt (Fig. 1). In the context of remelting of thickened lower crust, the Lhasa crustal basement should be flatly distributed throughout the Lhasa block, which is inconsistent with the observed distribution characteristics of the post-collision adakitic rocks. Thus, we conclude that partial melting of the Lhasa lower crust cannot provide a reasonable explanation for the post-collision adakitic magmatism. 5.3. An upper mantle source metasomatised by slab-derived melts

Fig. 5. (a) Sr/Y versus Y diagram and (b) chondrite-normalized La/Yb ratios versus Yb. Fields of adakites and arc magmas are from Defant and Drummond (1990) and Martin et al. (2005). Symbols and data sources are in Fig. 2.

magmatism. Recently, Xu et al. (2010) suggested that none of the exposed Lhasa crustal basement or the juvenile crust could be taken as a potential source for the Gangdese adakitic magmatism. Our extended isotope dataset of the post-collision adakites demonstrates that the partial melting of the juvenile crust or the Lhasa crustal basement cannot adequately explain the magma derivation for all the Gangdese adakitic rocks (Figs. 6 and 7). Both the exposed crustal basement represented by the Amdo orthogneiss (Harris et al., 1988) and the Lhasa lower crust assumed by Miller et al. (1999) have high radiogenic Sr and non-radiogenic Nd isotopes. Obviously, these domains cannot be taken as a conclusive source for the post-collision adakitic rocks (Fig. 6). In spite of their wider ranges in Sr–Nd–Pb isotope compositions, some of the post-collision adakitic rocks have higher radiogenic Sr and Pb and non-radiogenic Nd isotopic compositions than those of the Linzizong volcanic rocks (Figs. 6 and 7), which may be the equivalent of the juvenile crust in the Lhasa terrane (Mo et al., 2007). This comparison demonstrates that the post-collision adakitic melts were not derived

In the sequence of the geological events in southern Tibet so far documented, the Miocene adakitic rocks belong to the post-collision stage. Obviously, a direct partial melting of the subducted oceanic slab is not a viable model to account for the adakitic magmatism during the period of 26–10 Ma. The post-collision adakitic rocks, however, exhibit strong enrichment in the incompatible trace element patterns, suggesting geochemical affinities with subduction-related magmas (Fig. 4). Their high Sr/Y and La/Yb ratios (Fig. 5) and distinctively positive Sr anomalies (Fig. 4) allow the recognition of slab-derived melt component in the post-collision adakitic rocks. A previous study (Gao et al., 2007a) has shown that the Tibetan adakitic porphyries display positive Nb and P anomalies, reflecting excess Nb compared with classical calc-alkaline arc magmas. The additional data presented in this study, especially comparing with Cretaceous adakitic rocks from the Gangdese belt (Yao et al., 2006; Wei et al., 2007; Wen et al., 2008a; Zhu et al., 2009; Kang et al., 2009) support the possibility that the post-collision adakitic magma was derived from an upper mantle source metasomatised by slab melts. Many studies have suggested that some volcanic rocks with adakitic signatures result from the partial melting of a mantle metasomatised by slab melts (Schiano et al., 1995; Yogodzinski et al., 1995; Bourdon et al., 2002; Prouteau and Scaillet, 2003; Grove et al., 2003, 2005; Samaniego et al., 2005; Gómez-Tuena et al., 2007; F. Guo et al., 2007). Experimental works demonstrate that melting of an “adakite-metasomatized mantle” indeed yields magmas similar to the low SiO2 adakite, both in terms of major and trace elements (Moyen, 2009, and references therein). The geochemical signature of the slab melts is variably diluted by reactions in the mantle wedge. Therefore, the major element characteristics of these magmas are inherited from a flux melting process in the mantle wedge, whereas the trace element and isotopic signatures of these magmas are inherited from a mixture of fluids and melts from the slab (Grove et al., 2003, 2005). In the context of mantle metasomatized by partial melting, the generation of adakitic magmas does not require the contemporaneous subduction of a young oceanic crust or partial melting of an eclogitic lower crust. All the adakitic rocks of the different ages (i.e. pre-collision and post-collision) exhibit markedly similar trace element patterns with

Table 2 Sr–Nd–Pb isotope compositions of the post-collision adakitic rocks from the Gangdese belt, southern Tibet. Samples

87 86

ZM-2 ZM-3 ZM-4 ZM-5 ZM-6 ZM-7 ZM-9 ZM-10

Rb/ Sr

0.955 0.512 1.625 0.748 0.481 0.508 0.894 0.948

87 Sr/86Sr ± 2σ

0.70805 ± 21 0.70738 ± 25 0.70749 ± 14 0.70976 ± 16 0.70750 ± 13 0.70784 ± 15 0.70752 ± 13 0.70814 ± 16

(87Sr/ Sr)i

147

86

144

0.70785 0.70727 0.70714 0.70960 0.70740 0.70773 0.70733 0.70793

0.1063 0.1369 0.1201 0.1092 0.1162 0.093 0.1084 0.1054

Sm/ Nd

(143Nd/ 144 Nd)i

εNd(t)

2s

143

Nd/144Nd ±

0.51233 ± 9 0.51227 ± 8 0.51243 ± 9 0.51227 ± 8 0.51228 ± 9 0.51226 ± 9 0.51235 ± 15 0.51227 ± 8

0.51232 0.51226 0.51243 0.51226 0.51227 0.51226 0.51234 0.51226

−6.18 −7.37 −4.12 −7.28 −7.18 −7.42 −5.81 −7.39

206 204

Pb/ Pb

18.602 18.482 18.683 18.703 18.442 18.677 18.620 18.710

207 204

Pb/ Pb

15.686 15.662 15.647 15.738 15.664 15.649 15.680 15.723

208 204

Pb/ Pb

38.982 38.828 39.075 39.109 38.760 39.086 39.014 39.150

(206Pb/ Pb)i

(207Pb/ 204 Pb)i

(208Pb/ 204 Pb)i

18.583 18.454 18.662 18.682 18.417 18.655 18.592 18.665

15.685 15.660 15.646 15.737 15.663 15.648 15.679 15.722

38.957 38.788 39.025 39.073 38.726 39.054 38.965 39.101

204

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Fig. 6. Sr–Nd isotope diagram (initial values) for the Tibetan post-collision adakitic rocks. Published data from Gao et al. (2003a, 2007a), Hou et al. (2004), F. Guo et al. (2007) and Xu et al. (2010). Fields for the basaltic rocks in Yarlung ophiolite representative of the subducted Neotethyan oceanic crust from Zhang et al. (2005); the Cretaceous adakitic rocks from Wei et al. (2007), Wen et al. (2008a), Zhu et al. (2009) and Kang et al. (2009); the Linzizong volcanics from Mo et al. (2007); Miocene ultrapotassic rocks in the western Lhasa terrane from Gao et al. (2007b); mafic granulite in east Himalayan syntaxis from Xu et al. (2010); Lhasa lower crust from Miller et al. (1999).

LILE enrichment, HFSE depletion and distinctive positive Sr anomalies (Fig. 4). Compared with the pre-collision adakites (Fig. 4a), however, some of the post-collision adakitic rocks show higher Th and Pb contents (Fig. 4b–d). Especially, they also have high K2O contents and K2O/Na2O ratios, which are considered as a distinctive feature of

‘continental’ adakites or ‘potassic’ adakites (Moyen, 2009, and references therein). It must be noted that some of the adakitic rocks with unusual K2O enrichment and K2O/Na2O ratios significantly depart from the overall trends (Fig. 2), and the Gangdese adakitic rocks show clear differences

Fig. 7. Pb isotope diagrams (a and b) and combined Pb isotopes versus Nd isotope diagrams (c and d) for the post-collision adakitic rocks. Published data from Gao et al. (2007a), Hou et al. (2004), F. Guo et al. (2007) and Zhu et al. (2009). Fields for basaltic rocks in Yarlung ophiolite, Linzizong volcanics and ultrapotassic rocks are in Fig. 6.

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among the different regions in terms of the enrichment of K2O. Detailed observations of the field relations and petrographic features reveal that the unusually high K2O contents are associated with the strongly altered samples carrying Cu mineralization (such as Jiama outcrop), consistent with K metasomatism of primary plagioclase in the adakitic porphyries which resulted in an increase in K2O. Thus, the high K2O content of the adakitic porphyries does not represent primary values and reflect K metasomatism of plagioclase. With few exceptions, the post-collision adakitic rocks define a clear fractional crystallization trend (Figs. 2 and 3). This is consistent with normally zoned phenocrysts and characteristic phenocryst assemblages (Gao et al., 2003a). Obviously, some fractional crystallization must be responsible for the elevated K2O in some highly evolved adakitic rocks. On the other hand, their overall trend towards high K2O contents and K2O/Na2O ratios reflect distinct geochemical features of magmas derived from the mantle source metasomatised by slab-derived melts and fluids. More important is the observation of high Mg number (30–73) in some of the post-collision adakitic rocks, which is generally believed to require re-equilibration of slab melts with peridotitic mantle (Yogodzinski and Kelemen, 1998). It is generally agreed that the primary effect of slab melt reaction with peridotitic mantle is to dampen the strong trace element fractionation that is produced by partial melting of eclogitic slab (Kelemen et al., 2003; Kelemen, 2008; Martin et al., 2005). In all of the post-collision adakitic rocks from the Gangdese, distinctly elevated Ni concentrations and decrease of Sr/Y ratios with Mg numbers suggest contributions of variable proportions from a peridotitic mantle (Fig. 8). Some of the post-collision adakitic rocks with low 87Sr/86Sr but high 143Nd/144Nd ratios show isotopic similarity with the Cretaceous adakitic rocks (Fig. 6). This comparison demonstrates that the adakitic rocks belonging to different ages from the same region have similar isotopic compositions in their magma sources. An important observation is that the Cretaceous adakitic rocks and basaltic rocks from the Yarlung ophiolite plot as a plausible primitive end-member for the Sr–Nd–Pb isotopic arrays of the post-collision adakitic rocks (Figs. 6 and 7). Thus, the strong similarities in the geochemical and isotopic characteristics of the pre-collision and post-collision adakitic rocks offer a robust indication for the presence of mantle sources metasomatised by slab melts during the Neotethyan subduction. 5.4. Along-strike variation in the post-collision adakitic magmatism One of the salient features of the post-collision adakitic rocks is the large range in their isotopic signatures. Overall, both the Sr–Nd and Pb isotopic ratios are bracketed between the Yarlung-Tsangpo ophiolites and mafic ultrapotassic lavas from the western Gangdese (Figs. 6 and 7). The cumulative isotope dataset for post-collision adakitic rocks from the Gangdese belt shows that the majority of post-collision adakitic rocks from the western Gangdese have higher radiogenic Sr and Pb and nonradiogenic Nd relative to those from the eastern Gangdese, suggesting an obvious correlation between the isotopic composition and geographic location. It is interesting that their arrays extend, at the enriched end, into that defined by ultrapotassic lavas from the western Lhasa terrane (Figs. 6 and 7). An additional observation is that in comparison with widespread occurrences of post-collision adakitic rocks along the YTSZ, the ultrapotassic volcanism appears to be restricted to the western part of the Lhasa terrane (Fig. 1). Previous studies proposed that crustal isotopic characteristics of the western Lhasa terrane were caused by a binary mixing between ultrapotassic melts and pristine high Sr/Y melts from partial melting of the Lhasa lower crust (F. Guo et al., 2007), or the subducted Indian mafic lower crust (Xu et al., 2010). If this concept holds true, there should be a general trend which is geographically controlled with adakitic rocks from the western Gangdese showing higher K2O and K2O/Na2O than those from the eastern Gangdese, because the widespread occurrence of

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Fig. 8. (a) Sr/Y ratios and (b) Ni contents versus Mg numbers for the post-collision adakitic rocks from the Gangdese belt. The post-collision adakitic rocks display clearly negative Sr/Y correlation but positive Ni correlation with their Mg numbers. Data sources are in Fig. 2.

the ultrapotassic rocks seems to be restricted in the western Gangdese (Fig. 1). However, such marked regional variation of K2O and K2O/Na2O values is not observed. Within existing dataset, there is a complete absence of spatial variations in K2O and K2O/Na2O of the post-collision adakitic rocks. This implies that a binary mixing between pristine high Sr/Y melts and ultrapotassic melts cannot reasonably account for the crustal isotopic features of the Tibetan adakitic rocks. In the 86Sr/87Sr versus 143Nd/144Nd diagram (Fig. 6), the mafic granulite in the Himalaya terrane is unlikely taken as a plausible non-radiogenic end-member for the mixing array (Fig. 6). Therefore, the strong similarities in geochemical and isotopic features of adakitic rocks of the different ages preclude the possibility that Miocene high Sr/Y melts were derived from subducted Indian mafic granulite (Xu et al., 2010). An alternative mechanism is required to account for their unusual isotopic characteristics. Here, we interpret the spatial isotopic variation in the postcollision adakitic rocks as evidence for the involvement of subducted sediments. Our previous study (Gao et al., 2007a) suggested that substantial crustal contamination is not a feasible explanation for the more ‘enriched’ adakitic porphyries. It has been demonstrated that in cases where the melts are not derived directly from the slab but also involves a contribution from the melting of subducted sediments, the incompatible elements and isotopic composition would be significantly altered (Nelson, 1992). The geochemical and isotopic differences observed in the Tibetan adakitic rocks suggest the participation of different proportions of melts derived from the subducted basaltic slab as well as the subducted sediments, and the interaction of these

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melts with the mantle wedge. Previous studies (Miller et al., 1999; Gao et al., 2007b, 2009) have recorded the Si2O-rich lamproitic affinity of ultrapotassic mafic lavas from the western Gangdese. Their unusual Sr–Nd–Pb isotopic and geochemical characteristics indicate that metasomatic components in the mantle source of the ultrapotassic mafic rocks were principally derived from subducted sediments. In the Sr–Nd and Pb isotope diagrams (Figs. 6 and 7), the overall arrays of the post-collision adakitic rocks extend, at the enriched end, into ultrapotassic lavas from the western Gangdese. The overall radiogenic isotope systematics thus provides robust evidence for contributions from subducted sediments. Partial melting of the Tibetan mantle source metasomatized by hybrid slab melts and sediment melts is supported by key trace element ratios of the post-collision adakitic rocks from the western Gangdese (Fig. 9). The evidence for partial melting of the western Gangdese lithospheric mantle enriched by melts derived from subducted sediments is markedly high Rb/Sr versus Sr/Y ratios (Fig. 9a). The Rb/Sr ratio extending from high values is a characteristic feature of the Tibetan ultrapotassic mafic lavas to typical slab melts (Stern and Kilian, 1996). Their strength of slab melt signature (i.e. Sr/Y ratios) displays obviously negative correlation with initial 87Sr/86Sr (Fig. 9b). However, the Linzizong andesites from southern Tibet (Mo et al., 2007; Yue and Ding, 2006) do not plot along the mixing lines in these diagrams (Fig. 9a and b). In addition, there is an obvious positive correlation between Sr/Nb ratios and Gd/Yb ratios in the western Gangdese adakitic rocks (Fig. 9c), reflecting a slab component in the metasomatic mantle source. In this diagram (Fig. 9c), the Linzizong andesites, representing typical Neotethyan arc lavas, show a vertical

trend, reflecting a mantle source metasomatized by slab fluids. In a plot of Th/Yb versus Th/Sm (Fig. 9d), the linear trend of the western Gangdese adakitic rocks could be interpreted in terms of two component mixing between N-MORB and subducted sedimentderived melts (Elburg et al., 2002; Plank, 2005; F. Guo et al., 2007). All of the Sr–Nd–Pb isotope arrays of the Tibetan adakitic rocks indicate that the ‘crustal’ radiogenic isotope characteristics of some Tibetan adakitic rocks were most likely derived contribution from subducted sediments. In the context of an enriched mantle source, the along-strike isotopic variation in the post-collision adakitic volcanism is mostly related to a westward increase in sediment input in the western Lhasa terrane. This is consistent with coeval and spatially related occurrences of ultrapotassic mafic rocks (Miller et al., 1999; Williams et al., 2001, 2004; Ding et al., 2003; Gao et al., 2007b, 2009). An excellent analogue is the Aleutian island arc. Kelemen et al. (2003) suggest that variation in the Aleutian arc lava compositions might be related to the composition and volume of subducted sediment, and there is a clear positive correlation between Pb isotope ratios of the lavas and sediment flux. 5.5. Geodynamic interpretation Miocene adakitic rocks are an integral part of post-collision volcanism in the Lhasa terrane. Any model for the Tibetan postcollision adakitic rocks must provide a reasonable interpretation for their spatio-temporal character, particularly their being distributed along a narrow and well defined belt as discussed in a previous section. A direct partial melting of young and hot slabs seems unlikely

Fig. 9. (a) Rb/Sr versus Sr/Y, (b) Sr/Y versus 87Sr/86Sr, (c) Sr/Nb versus Gd/Yb and (d) Th/Yb versus Th/Sm diagrams for the post-collision adakitic rocks from the western Gangdese belt. In these diagrams, the post-collision adakitic rocks display clear mixing trends between slab melts and sediments. N-MORB from Sun and McDonough (1989), Linzizong andesite after Mo et al. (2007) and Yue and Ding (2006). Published data are in Fig. 2.

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to have led to the generation of the post-collision adakitic magmas in south Tibet. Our favoured interpretation is that the post-collision adakitic melts were generated by partial melting of the upper mantle wedge enriched by oceanic crust and sediment-derived melts during the Neotethyan subduction (Fig. 10). This scenario is equally plausible in the Tibetan context and is also viable with respect to the genesis of the post-collision adakitic rocks, as oceanic crust subduction was a major feature of the former continental margin since the Jurassic. Detailed geochemical investigations indicate that rocks with adakitic affinity are not pure slab melts as most share many geochemical features with ordinary island arc magmas (Yogodzinski et al., 1995), and slab melts may possibly be involved in arc magma genesis without being necessarily emplaced at the surface (Maury et al., 1992; Schiano et al., 1995; Kelemen et al., 2003). The widespread occurrences of Cretaceous adakitic volcanism in the Gangdese belt (Yao et al., 2006; Zhu et al., 2009; Zhang et al., 2010) strongly indicate that the slab melt metasomatic events in the Gangdese mantle wedge would have taken place during the Neotethyan subduction (Fig. 10a). Previous studies of the Cretaceous adakitic rocks in the Gangdese belt have emphasized a strong reaction of slab melts with the peridotitic mantle from the subduction zone to the surface (Yao et al., 2006; Zhu et al., 2009; Zhang et al., 2010). This implies that an adakite-metasomatized mantle during the Neotethyan subduction would behave as a potential direct source of the postcollision adakitic rocks. This inference is further supported by the geochemical and isotopic similarities in the adakitic rocks of different ages.

661

Our paleo-subduction model not only explains the geochemical and isotopic characteristics of the post-collision adakitic rocks at the southern edge of the Lhasa terrane, but also provides effective constraints on the Early Miocene tectonic evolution of southern Tibet. The widespread occurrence of the post-collision adakitic rocks in an elongate E–W trending zone clearly suggests that the post-collision adakitic rocks were not necessarily confined to active N–S trending rift zones. This implies that the post-collision adakitic magmatism was initiated under a N–S compressional regime, rather than an E–W extension across the Tibetan plateau (Hou et al., 2004). This is strongly supported by a recent study for adakitic tuffs within the Lower Miocene Gangrinboche conglomerates along the YTSZ (Aitchison et al., 2009). Based on the temporal and spatial distributions of calc-alkaline rocks at the southern edge of the Lhasa terrane, Mahéo et al. (2002) proposed that post-collision volcanism was likely associated with a slab break-off event initiating at around 25 Ma. Aitchison et al. (2009) also favour slab break-off as explanation for adakitic tuffs within the Lower Miocene Gangrinboche conglomerates along and north of the YTSZ. This model is consistent with increased uplift of the Tibetan plateau at around 21 Ma (Harrision et al., 1992). Slab break-off would most likely have initiated near the continent/ocean lithosphere transition on the down-going slab and would therefore have been more or less parallel to the YTSZ (Aitchison et al., 2009). The peculiar linear distribution of post-collision adakitic volcanism is best explained by the former location of slab window. Partial melting of the lithospheric mantle could result from asthenospheric upwelling

Fig. 10. A tectonic–petrogenetic model for post-collision adakitic rocks in the Lhasa terrane. (a) The pre-collision adakitic rocks were derived from partial melting of subducted Neotethyan slab beneath the Lhasa terrane. Slab melts interacted with the mantle wedge during their ascent to the surface and formed adakite-metasomatized mantle domains. (b) A rollback or steepening of the Neotethyan subducting slab around ~65 Ma induced the Linzizong volcanism. (c) The Neotethyan slab break-off from its adherent Indian continental lithosphere at around 25 Ma would have allowed an asthenospheric upwelling beneath southern Tibet, which might have initiated the post-collision adakitic magmatism in south Tibet.

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and thermal erosion following break-off and the creation of a slab window (Davies and von Blanckenburg, 1995), a process that has been assigned for the generation of high temperature magmas and metamorphic assemblages in subduction-collision zones (Santosh and Kusky, 2010; Santosh et al., 2009). Under the framework of our paleo-subduction model, the slab break-off process would have allowed an asthenospheric upwelling beneath southern Tibet, which should have initiated post-collision adakitic magmatism in south Tibet (Fig. 10c). 6. Conclusions Geochemical and isotopic data reveal that the post-collision adakitic rocks from the Lhasa terrane could be related to the partial melting of an upper mantle region metasomatized by slab-derived melts. Our model presented in this study envisages that the sublithospheric mantle was metasomatized during the Cretaceous subduction event and the metasomatized mantle was then remelted after continent–continent collision because of slab break-off. Together with previously reported adakitic rocks, the post-collision adakitic rocks form an E–W trending narrow belt along the YTSZ in south Tibet. This fore-arc position of this belt is compatible with adakitic magmatism. The widespread occurrences of Cretaceous adakitic rocks in this belt indicate that slab melt metasomatism in the southern Gangdese belt indeed took place during the Neotethyan oceanic crust subduction. A slab break-off event initiating at around 25 Ma would have allowed an asthenospheric upwelling beneath southern Tibet, which might have initiated the post-collision adakitic magmatism in south Tibet. Our paleo-subduction model suggests that the spatial geochemical and isotopic variations in the post-collision adakitic rocks were likely related to a westward increase in sediment input in the western Lhasa terrane. Acknowledgements This project is financially supported by the National Scientific and Technologic Project (No. 2006BAB01A04) and the National Natural Science Foundation of China (40672042). We thank Prof. Jonathan C. Aitchison, an anonymous reviewer and Editor Prof. Nelson Eby for the constructive comments that greatly improved this manuscript. References Aitchison, J.C., Ali, J.R., Davis, A.M., 2007. When and where did India and Asia collide? Journal of Geophysical Research, Solid Earth 112, B05423. doi:10.1029/2006JB004706. Aitchison, J.C., Ali, J.R., Chan, A., Davis, A.M., Lo, C.H., 2009. Tectonic implications of felsic tuffs within the lower Miocene Gangrinboche conglomerates, southern Tibet. Journal of Asian Earth Sciences 34, 287–297. Atherton, M.P., Petford, N., 1993. Generation of sodium-rich magmas from newly underplated basaltic crust. Nature 362, 144–146. Bourdon, E., Eissen, J.P., Monzier, M., Robin, C., Matin, H., Cotton, J., Hall, M.L., 2002. Adakite-like lavas from Antisana volcano (Ecuador): evidence for slab melt metasomatism beneath the Andean northern volcanic zone. Journal of Petrology 43, 199–217. Chung, S.L., Liu, D., Ji, J., Chu, M.F., Lee, H.Y., Wen, D.J., Lo, C.H., Lee, T.Y., Qian, Q., Zhang, Q., 2003. Adakites from continental collision zones: melting of thickened lower crust beneath southern Tibet. Geology 31, 1021–1024. Chung, S.L., Chu, M.F., Zhang, Y., Xie, Y., Lo, C.H., Lee, H.Y., Lan, C.Y., Li, X., Zhang, Q., Wang, Y., 2005. Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism. Earth Science Reviews 68, 173–196. Coulon, C., Maluski, H., Bollinger, C., Wang, S., 1986. Mesozoic and Cenozoic volcanic rocks from central and southern Tibet: 39Ar/40Ar dating, petrological characteristics and geodynamical significance. Earth and Planetary Science Letters 79, 281–302. Davies, J.H., von Blanckenburg, F., 1995. Slab breakoff: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth and Planetary Science Letters 129, 85–102. Defant, M.J., Drummond, M.S., 1990. Drivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665. Ding, L., Kapp, P., Zhong, D.L., Deng, W.M., 2003. Cenozoic volcanism in Tibet: evidence for a transition from oceanic to continental subduction. Journal of Petrology 44, 1835–1865.

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