Cambrian Ultrapotassic Rhyolites from the Lhasa Terrane, South Tibet: Evidence for Andean-Type Magmatism Along the Northern Active Margin of Gondwana

GR-01218; No of Pages 14 Gondwana Research xxx (2014) xxx–xxx

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Cambrian ultrapotassic rhyolites from the Lhasa terrane, south Tibet: Evidence for Andean-type magmatism along the northern active margin of Gondwana

Huixia Ding a,ZemingZhanga,⁎, Xin Dong a,RongYana,YanhaoLinb,HongyingJiangc a State Key Laboratory of Continental Tectonic and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China c School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China article info abstract

Article history: The petrogenesis and tectonic signiﬁcance of early Paleozoic magmatic rocks in south Tibet is debated. In this Received 27 August 2013 paper, we report Cambrian ultrapotassic felsic volcanics from the Lhasa terrane. Petrographic and whole-rock Received in revised form 25 January 2014 geochemical studies indicate that these rocks are rhyolite and rhyolitic ignimbrite, and have varying SiO2 Accepted 17 February 2014 (69.56–82.09 wt.%), Al O (9.52–16.61 wt.%) and Fe Ot (1.0–7.9 wt.%). The rocks are ultrapotassic (high K O Available online xxxx 2 3 2 3 2 4.19–6.90 wt.%, and very low Na2O0.01–0.06 wt.%) and peraluminous (A/CNK = 1.41–2.22). They are enriched Handling Editor: M. Santosh in Rb, Th, U, Pb, Zr and Y, and showed negative Ba, Sr, P, Nb, Ta and Ti anomalies, and slightly fractionated REE patterns with moderately negative Eu anomalies, typical of A-type granitoids. Zircons from six samples yielded ε − Keywords: crystallization ages of ca. 512 Ma. These zircons show Hf(t) values in the range of 4.7 to +1.3, and two- Ultrapotassic rhyolite stage Hf model ages of 1734–1392 Ma. Combining with previous results, we propose that these ultrapotassic Zircon geochronology rhyolites were derived from the partial melting of middle Proterozoic crustal rocks in the lower crust under Andean-type orogeny high-temperature and water-absent conditions in an extensional environment of active magmatic arc. Lhasa terrane Thus, this study provides new insights into early Paleozoic Andean-type orogeny along the northern margin of South Tibet Gondwana supercontinent. Gondwana © 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction magmatism associated with the subduction of the Neo-Tethyan lithosphere (Ding et al., 2003; Hou et al., 2006; He et al., 2007; Wen et al., The Himalayan–Tibetan orogen is the youngest and the most spec- 2008b; Guan et al., 2012) and the India–Asia collision (Chung et al., tacular of all the continent–continent collisional belts on the Earth 2003; Mo et al., 2003; Chung et al., 2005; Mo et al., 2005; Hou et al., (e.g., Yin and Harrison, 2000). The Lhasa terrane, which is the main tec- 2006; Mo et al., 2007, 2008; Chung et al., 2009; W.Q. Ji et al., 2009; tonic component of the orogen, experienced multistage magmatism, in- Zhao et al., 2009; Xia et al., 2011; Zhu et al., 2011; J.L. Chen et al., cluding Neoproterozoic magmatism associated with the opening of the 2012). These discoveries play an important role in evaluating and for- Mozambique Ocean (Zhang et al., 2012a, 2014), Paleozoic magmatism mulating the evolution and geodynamic models for the terrane. related to the ﬁnal assembly of Gondwana (Li et al., 2008a), the subduc- Early Paleozoic magmatic rocks (~530–470 Ma) are ubiquitous tion of the proto-Tethyan lithosphere (Dong et al., 2009; W.H. Ji et al., along the northern margin of Gondwana (Fig. 1a), but their petrogene- 2009; Zhu et al., 2012; Hu et al., 2013), the opening and subduction of sis is still unclear and controversial. Investigations conducted so far have the Paleo-Tethyan Ocean (Dong et al., 2010; Dai et al., 2011; Zhu et al., proposed mainly two scenarios: (1) Pan-African orogenesis in response 2013), the collision between the Lhasa terrane and the northern margin to the ﬁnal assembly of Gondwana (Murphy and Nance, 1991; Meert of Australia (Zhu et al., 2009a), the Mesozoic magmatism derived from and Van Der Voo, 1997; Miller et al., 2001; Xu et al., 2005; Song et al., the subduction of the Meso-Tethyan (Allègre et al., 1984; Zhu et al., 2007; Qi et al., 2010; Xie et al., 2010; Liu et al., 2012; Yang et al., 2012) 2009b, 2011) and the Neo-Tethyan lithosphere (Chu et al., 2006; Wen and (2) an Andean-type orogeny related to the subduction of proto- et al., 2008a,b; Chiu et al., 2009; W.Q. Ji et al., 2009; Zhu et al., 2009c; Tethyan oceanic lithosphere beneath the Gondwana continent (Li Zhang et al., 2010; Guo et al., 2012; W.W. Chen et al., 2012; Zhang and and Powell, 2001; Ramezani and Tucker, 2003; DeCelles et al., 2004; Santosh, 2012; Chen et al., 2014; Meng et al., 2014), and the Cenozoic Gürsu and Göncüoglu, 2005; Cawood et al., 2007; Chen et al., 2007; Hassanzadeh et al., 2008; Horton et al., 2008; Zhang et al., 2008; Dong et al., 2009; W.H. Ji et al., 2009; Saki, 2010; Wang et al., 2011; B.D. ⁎ Corresponding author at: Institute of Geology, Chinese Academy of Geological Wang et al., 2012; Dong et al., 2012; Spencer et al., 2012; X.X. Wang Sciences, No. 26 Baiwangzhuang Road, Beijing 100037, China. Tel.: +86 10 68999735; fax: +86 10 68994781. et al., 2012; Zhang et al., 2012b,c; Zhu et al., 2012; Cai et al., 2013; Dong E-mail address: [email protected] (Z. Zhang). et al., 2013; Hu et al., 2013; Wang et al., 2013; Moghadama et al., 2014).

Fig. 1. (a) Simpliﬁed geological map of the Tibetan Plateau, showing the locations and related ages of early Paleozoic magmatic rocks in the southern Tibet. Data sources for the Southern Qiangtang terrane from the east to west: 476–471 Ma (Pullen et al., 2011), 465 Ma (Hu et al., 2010), 461 Ma (Wang et al., 2008; B.D. Wang et al., 2012), 467 Ma (Li et al., 2008b); Amdo from the east to west: 500 Ma (Zhang et al., 2012b), 488 Ma (Xie et al., 2010), 532 Ma and 483 Ma (Guynn et al., 2012); Lhasa terrane from the east to west: 492 Ma (Zhu et al., 2012), 536 Ma (Panetal.,2012), 501 Ma (W.H. Ji et al., 2009), 525–510 Ma (Hu et al., 2013), 510 Ma (Gehrels et al., 2011), 496 Ma (Dong et al., 2009), 507 Ma (Li et al., 2008a); Himalayas from the east to west: 496 Ma (Miller et al., 2001); 499–473 Ma (X.X. Wang et al., 2012), 471 Ma (Johnson et al., 2001), 484–473 Ma (Gehrels et al., 2006), 470 Ma (Schärer and Allègre, 1983), 475 Ma (Cawood et al., 2007), 499 Ma (Wang et al., 2011), 457 Ma (Visonà et al., 2010), 499 Ma (C. Shi et al., 2010),527Maand506Ma(Quigley et al., 2008), 524 Ma (Qi et al., 2010), 508 Ma (Lee et al., 2000), 500–490 Ma (Zhang et al., 2008); Tengchong–Baoshan Block from the north to south: 487 Ma (Song et al., 2007), 473 Ma (Liu et al., 2012), 489 Ma (Linetal.,2012), 470 Ma (Chen et al., 2007); 492–460 Ma (Wang et al., 2013), 518–502 Ma (Cai et al., 2013), 476–448 Ma (Dong et al., 2013), 502 Ma and 499 Ma (Liu et al., 2009), 499 Ma (Yang et al., 2012), 486 Ma (Dong et al., 2012). JSSZ — Jinsha Suture Zone; LSSZ — Longmu Tso–Shuanghu Suture Zone; BNSZ — Bangong–Nujiang Suture Zone; SNMZ — Shiquan River–Nam Tso Mélange Zone; LMF — Luobadui–Milashan Fault; IYZSZ — Indus-Yarlung Zangbo Suture Zone; NL — Northern Lhasa subterrane; CL — Central Lhasa subterrane; SL — Southern Lhasa subterrane. (b) Geological map of the studied area showing the sampling locations of this study and Hu et al. (2013).

In this paper, we report a suit of Cambrian A-type ultrapotassic rhyo- The northern Lhasa subterrane consists of middle Triassic– lites from the Lhasa terrane, representing one of the microcontinents that Cretaceous sedimentary rocks, abundant early Cretaceous volcanic originated from the northern margin of Gondwana (Allègre et al., 1984; rocks and lower Cretaceous volcano-sedimentary sequence as well as Yin and Harrison, 2000; Metcalfe, 2006, 2009; Zhu et al., 2013; Zhang Cretaceous granitoid batholiths (Pan et al., 2004; Zhu et al., 2011, et al., 2014). Although not common, these rocks are similar to that of 2012; Sui et al., 2013). The central Lhasa subterrane is composed of Pre- lavas reported from Rajasthan, India (Maheshwari, 1987; Sharma, cambrian basement (Lin et al., 2013; Xu et al., 2013), Cambro-Permian 2005), the Yidun, Southeast Tibet (Qu et al., 2003) and the Sinai, Egypt sediments and upper Jurassic–lower Cretaceous strata with abundant (Samuel et al., 2007). The magmatism is correlated to extensional tecton- lavas (Liu et al., 2004; Pan et al., 2004; Zhu et al., 2012), and some ic environments, such as continental rift (Sharma, 2005) and intraplate Mesozoic and Cenozoic metamorphic rocks (Kapp et al., 2005; Dong rift (Qu et al., 2003; Samuel et al., 2007). Here we present the petrology, et al., 2011a,b). The southern Lhasa subterrane is characterized by the zircon U–Pb geochronology and Hf isotopic data on the ultrapotassic growth of juvenile crust (Mo et al., 2008; W.Q. Ji et al., 2009; Zhu rhyolites from the Lhasa terrane. The results provide insights into the et al., 2011) with locally preserved Precambrian crystalline basement early Paleozoic magmatism along the northern margin of Gondwana, (Zhu et al., 2012). This subterrane consists mainly of the Cretaceous– and the Cambrian Andean-type orogeny of the Lhasa terrane. Tertiary Gangdese batholith and the Paleogene Linzizong volcanic succession, with minor Triassic–Cretaceous volcano-sedimentary rocks 2. Geological background and petrography (Pan et al., 2004; Zhu et al., 2012). The present study area is located at the northern part of the central The Tibetan Plateau consists of the Songpan–Ganzi ﬂysch complex, Lhasa subterrane (Fig. 1a), and consists of Cambrian–Carboniferous Northern Qiangtang, Southern Qiangtang, Lhasa terranes and the volcano-sedimentary successions (Fig. 1b). The Cambrian unit is uncon- Himalayas from the north to south (Fig. 1a; Yin and Harrison, 2000; formably overlain by Ordovician strata, and a continuous succession of Tapponnier et al., 2001; Mo et al., 2006). These terranes are separated the Ordovician–Carboniferous strata is preserved. Previous study by the Jinsha, Longmu Tso–Shuanghu, Bangong–Nujiang, and Indus– has assigned the Cambrian strata to the Nyainqêntanglha Group, Yarlung Zangbo suture zones. The Lhasa terrane, located in the southern representing the Precambrian crystalline basement of the Lhasa ter- part of Tibet, is a large crustal segment with a width of 100–300 km and rane (Wang et al., 2003). However, our observation shows that this a length of ca. 2000 km. The Lhasa terrane is divided into the northern, sequence consists mainly of sandstone, mudstone and limestone, with central, and southern subterranes, separated by the Shiquan River–Nam minor interlayered rhyolite and rhyolitic pyroclastic rocks. As described Tso Mélange zone and Luobadui–Milashan fault (Fig. 1a; Pan et al., below, these rhyolitic rocks have a Cambrian crystallization age. Recent 2004; Zhu et al., 2012). studies demonstrated that the Cambrian strata are common in south

Tibet (W.H. Ji et al., 2009; Li et al., 2010; Pan et al., 2012; Zhu et al., were analyzed using inductively coupled plasma mass spectrometry 2012). (ICP-MS, TJA-PQ-ExCell). The analytical uncertainties range from 1% to The rhyolite and rhyolitic ignimbrite were collected from the Cam- 5% when the abundance is larger than 1 ppm, and 5–10% when the brian strata in the central Lhasa subterrane (Fig. 1). Hu et al. (2013) abundance is less than 1 ppm. reported some ultrapotassic rhyolites from the adjacent area (Fig. 1b). Zircons were separated using conventional techniques. Zircons, select- But, they speculated that the ultrapotassic characteristics of the rocks ed by examination with a binocular microscope, were mounted in epoxy have originated from alteration and low-grade metamorphism. Our resin and polished to approximately half thickness. Cathodoluminescence field and petrographic observations show that these rocks are fresh or (CL) images, carried out using a HITACHI S2250-N scanning electron with very weak alteration, and have well-preserved magmatic texture, microscope at the SHRIMP unit in the Institute of Geology, Chinese structure and mineral association. The rhyolites are commonly gray or Academy of Geological Sciences, were used to check the internal textures dark gray, with porphyritic texture and fluxion structure, and contain of individual zircon and to guide U–Pb dating and Hf isotope analysis. K-feldspar and quartz as phenocryst and fine quartzofeldspathic U–Pb zircon dating was conducted by LA–ICP-MS on an Agilent 7500 mineral and minor sericite as groundmass (Fig. 2a, b). Phenocrysts of equipped with a 193 nm ArF excimer laser at the State Key Laboratory K-feldspar occur commonly as euhedral platy crystals with tartan of Geological Processes and Mineral Resources, China University of twinning and embayed edge. Phenocrysts of quartz are subhedral grains Geosciences (Wuhan). Operating conditions are the same as described (Fig. 2a, b). Rhyolitic ignimbrite shows welded tuff texture and pearl- by Liu et al. (2010). A beam size of 32 μm was used for all the samples. shaped cracks (Fig. 2c), and consists of quartz and K-feldspar crystal Zircon 91500 and the GSE-1G glass were used as an external standard fragments, glass shards and aphanitic volcanic ash (Fig. 2d). Some of for Pb/U ratio and concentration, respectively. Common Pb correction glass shards and volcanic ash have devitrified to very fine-grained was made utilizing the correction function (Andersen, 2002). Off- quartzofeldspathic minerals (Fig. 2d). line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration were carried 3. Analytical methods out using ICPMSDataCal (Liu et al., 2010). The data were processed by the ISOPLOT program (Ludwig, 2003). Fresh samples were chosen for bulk rock chemical analysis. Approx- In situ zircon Hf isotope analysis was conducted using a Neptune imately 500–1000 g of each sample was crushed to 60 mesh in a steel MC–ICP-MS with a beam size of 44 μm, a hit rate of 8 Hz and laser jaw crusher followed by shaking them evenly; and then about 60 g energy of 60 mJ at the State Key Laboratory of Geological Processes was powdered in an agate ring mill to less than 200 mesh. All the sam- and Mineral Resources, China University of Geosciences (Wuhan). Ana- ples were analyzed in the National Geological Analysis Center of China, lytical spots were performed on the same LA–ICP-MS spots or in the Beijing. Oxides of major elements including LOI were determined by same growth domain as inferred from CL images. To gain the accuracy X-ray fluorescence (XRF) (Rigaku-3080) with an analytical uncertainty results, every 10 sample analyses were followed by analysis of the of b0.5%. Trace elements Zr, Nb, V, Cr, Sr, Ba, Zn, Ni, Rb and Y were ana- zircon standards (GJ-1 and 91500). The instrumental conditions and lyzed by a different XRF instrument (Rigaku-2100) with an analytical data acquisition were described by Hu et al. (2012). During the analysis, uncertainty of b3–5%. Other trace elements and rare earth elements the standard zircons gave 176Hf/177Hf ratios of 0.282014 ± 9 (1σ,

Kfs Q Kfs Q

Q Q Kfs Kfs

Kfs Kfs Q Q

a) 1mm b) 1mm

c) 500 µm d) 1mm

Fig. 2. Photomicrographs of the studied ultrapotassic rhyolites. (a and b) The rhyolite and (c and d) the rhyolitic ignimbrite.

Please cite this article as: Ding, H., et al., Cambrian ultrapotassic rhyolites from the Lhasa terrane, south Tibet: Evidence for Andean-type magmatism along the northern active..., Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.02.003 4 H. Ding et al. / Gondwana Research xxx (2014) xxx–xxx n = 10) for GJ-1 and 0.282317 ± 10 (1σ, n = 11) for 91500, respec- 4. Zircon U–Pb age and Lu–Hf isotope tively, in agreement with the recommended values (0.282015 ± 19 for GJ-1, Elhlou et al., 2006; 0.282308 ± 6 for 91500, Blichert, 2008) 4.1. Zircon U–Pb age 176 177 within analytical errors. The initial Hf/ Hf ratios and εHf(t) values were calculated in reference to the chondritic reservoir (CHUR) at the LA–ICP-MS zircon U–Pb data from six samples are listed in Appendix time of zircon growth from magmas. The decay constant for 176Lu of 1, U–Pb concordia plots and representative cathodoluminescence (CL) 1.867 × 10−11/year (Soderlund et al., 2004), the chondritic 176Hf/177Hf images of zircons are shown in Fig. 3. Most zircon grains are colorless, ratio of 0.282785 and 176Lu/177Hf ratio of 0.0336 (Bouvier et al., 2008) transparent, and show euhedral short prismatic forms (70–200 μm). were used. Depleted mantle model ages (TDM)adoptedforthe In CL images (Fig. 3), zircon crystals display oscillatory zoning, typi- ultrapotassic rhyolitic rocks were calculated in reference to the de- cal of magmatic origin (Corfu et al., 2003). The analyzed zircons have pleted mantle at a present-day 176Hf/177Hf ratio of 0.28325 and Th = 77–545 ppm and U = 112–510 ppm with high Th/U ratios of 176Lu/177Hf of 0.0384 (Grifﬁn et al., 2000). For the volcanics we also 0.69–1.26 (Appendix 1), indicating magmatic origin (Hoskin and calculated the Hf isotope crustal model ages (TDM2) by assuming its Schaltegger, 2003). Zircons from the six samples yielded near- parental magma to have been derived from an average continental concordant and consistent 206Pb/238U ages, with the weighted mean crust with 176Lu/177Hf = 0.015, which derived from the depleted man- ages of 504 ± 15 Ma (MSWD = 6.4), 496 ± 12 Ma (MSWD = 3.9), tle source (Grifﬁnetal.,2002). 505 ± 8 Ma (MSWD = 3.0), 522 ± 9 Ma (MSWD = 2.6), 519 ±

0.090 a) T11-14-1 550 b) T11-14-2 471±8 Ma 0.086 530 507±5 Ma 0.086 530 510 0.082 520±6 Ma 510

U 0.082 U

238 238 490

Pb/ 490 Pb/ 0.078 0.078 508±9 Ma

206 206 470 470 Mean =504 ± 15 Ma 0.074 Mean =496 ± 12 Ma 0.074 MSWD=6.4, n=8 MSWD=3.9, n=9 450 450

0.070 0.070 0.5 0.6 0.7 0.8 0.9 0.4 0.5 0.6 0.7 0.8 0.9

0.094 c) T11-14-3 d) T11-14-4 0.090 550 560 486±5 Ma 0.090 521±7 Ma 0.086 530 540

514±8 Ma 0.086 510 U 0.082 U 520

238 238 490 Pb/ Pb/ 0.082 0.078

206 206 500 470 509±7 Ma 0.078 0.074 Mean = 505 ± 8 Ma 480 Mean =522 ± 9 Ma 450 MSWD=3.0, n=10 MSWD=2.6, n=9

0.070 0.074 0.45 0.55 0.65 0.750.85 0.55 0.65 0.75 0.85

0.094 e) T11-14-6 f) T11-14-7 570 0.090 550 0.090 550 502±9 Ma 480±8 Ma 0.086 530 0.086 530

495±9 Ma 510 488±7 Ma U 0.082 U 510

238 238 0.082 490

Pb/ Pb/ 0.078 490 206 206 0.078 470 Mean =519 ± 7 Ma 470 Mean =511 ± 13 Ma 0.074 MSWD=2.6, n=13 0.074 MSWD=6.9, n=12 450 450 0.070 0.070 0.4 0.5 0.6 0.7 0.8 0.9 0.45 0.55 0.65 0.75 0.85 207Pb/235U 207Pb/235U

Fig. 3. Zircon U–Pb age concordia plots of the studied ultrapotassic rhyolites, showing CL images of the representing zircon grains. The circles indicate the locations of U–Pb dating and Hf isotopic analyses. The scale bars in CL images are 100 μm.

7 Ma (MSWD = 2.6) and 511 ± 13 Ma (MSWD = 6.9), respectively of TDM2 model ages, with a major peak at εHf(t) = ca. −2.1 and TDM2 (Fig. 3). These ages are similar within analytical errors. All analytical model age = ca. 1624 Ma (Fig. 4b, c), respectively. These results suggest spots from six samples yielded a weighted mean age of 512 ± 4.1 Ma that these volcanics were derived from a middle Proterozoic crustal (MSWD = 4.6), which is consistent with their peak age in the frequen- source with minor mantle-derived magma contributions (see discus- cy diagram (Fig. 4a). Therefore, the 512 Ma age is interpreted to be the sion below). crystallization age of the ultrapotassic rocks. 5. Whole-rock major and trace elements 4.2. Zircon Lu–Hf isotope 5.1. Major elements In-situ Lu–Hf isotopic analysis was carried out for the representative zircon grains from six samples. The relevant data are listed in Appendix The volcanics have variable SiO2 (69.56–82.09 wt.%), Al2O3 2. Fifty-nine spots yielded εHf(t) values ranging from −4.7 to 1.3 and (9.52–16.61 wt.%) and TFe2O3 (1.0–7.9 wt.%) contents, with high two-stage Hf model ages (TDM2)of1734–1392 Ma and display a K2O (4.19–6.90 wt.%), and very low Na2O (0.01–0.06 wt.%), CaO quasi-unimodal distribution of εHf(t) values and unimodal distribution (0.11–0.24 wt.%) and LOI values (0.79–2.35 wt.%) (Table 1). They are rhyolites based on the total alkali versus silica diagram (Fig. 5),

and are ultrapotassic in terms of the K2O/Na2Oratios(99–419) (Fig. 6a). Their aluminum saturation index (A/CNK = molecular 7 a) ca. 512 Ma Al2O3/(CaO + Na2O+K2O)) shows that they are peraluminous (1.41–2.22) (Fig. 6b). On the compositional variation diagrams 6 (Fig. 7a–e), Al2O3,K2O, TFe2O3,TiO2 and MnO2 decrease with increasing 5 SiO2, indicating the crystallization fractionation of K-feldspar, ferromag- nesian minerals and Ti–Fe oxides. Eight ultrapotassic rhyolites, reported 4 by Hu et al. (2013), from the adjacent area exhibit similar major element features (Figs. 5–7). In general, ultrapotassic rhyolites from Number 3 the Lhasa terrane are ferroan or magnesian (Fig. 6c) and alkali-calcic 2 to calcic (Fig. 6d). Most ultrapotassic rhyolites from the Lhasa terrane plot in or close to A-type granitoid ﬁelds (Fig. 6c, d). Moreover, as 1 shown in Figs. 5 and 6 and Table 1, major element compositions of the ultrapotassic rocks from the Lhasa terrane are generally comparable to 0 460 475 490 505 520 535 550 ultrapotassic rhyolites from other regions of the world (Maheshwari, Age(Ma) 1987; Qu et al., 2003; Sharma, 2005; Samuel et al., 2007).

5.2. Trace elements 9 b) ca.-2.1 8 The studied rocks have variable Zr (127–735 ppm) and Y (23–62.5 ppm), Rb (321–658 ppm) and Sr (6.34–23.1 ppm) concen- 7 trations (Table 1). On the primitive mantle-normalized trace element 6 diagram (Fig. 8a), the ultrapotassic rhyolites from the Lhasa terrane 5 show enrichment in Rb, Th, U, K, Pb, Zr and Hf, as well as pronounced

Number 4 negative anomalies of Ba, Nb, Ta, Sr, P and Ti. These rocks have REE patterns characterized by fractionated LREE [(La/Sm)N =(2.96–5.23)] 3 and ﬂat to depleted HREE [(Gd/Yb)N = (0.56–2.43)], with moderate 2 negative Eu anomalies (Eu/Eu* = 0.41–0.58; Fig. 8b). They are enriched 1 in HFSE (Zr and Y), and have compositional afﬁnities to A-type granit- – 0 oids (Fig. 9a, b). On the element variation diagrams (Fig. 7 e l), trace -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 elements Sr, Nb, U, Rb, Hf and Th decrease, but Ba slightly increases, ε Hf(t) with increasing SiO2, implying fractional crystallization of feldspar, titanite, zircon, amphibole and/or apatite. Such fractional crystallization c) processes have probably resulted in the ultrapotassic rhyolites having 10 ca. 1624 Ma variable major and trace element compositions. But, in general, the ultrapotassic rhyolites from the Lhasa terrane have similar trace ele- 8 ment compositions to other ultrapotassic rhyolites mentioned above (Table 1 and Fig. 8).

6 6. Discussion

Number 4 6.1. Petrogenesis

2 Cenozoic post-collisional ultrapotassic magmatic rocks are common and widespread on the Tibet Plateau (Zhao et al., 2009 and references therein). They are intermediate to maﬁc rocks having SiO = 0 2 1350 1450 15501650 1750 1850 45.39–60.98 wt.%, MgO = 2.50–12.30 wt.%, K2O = 4.66–8.83 wt.%, Na O = 1.15–3.92 wt.% and K O/Na O N 2(e.g.,Miller et al., 1999; Ding TDM2(Ma) 2 2 2 et al., 2006; Zhao et al., 2009) and belong to the typical ultrapotassic group (K O/Na O N 2, K O N 3 wt.% and MgO N 3 wt.%) as deﬁned by Fig. 4. Histograms of U–Pb ages (a), εHf(t) values (b) and Hf model ages (c) of zircons from 2 2 2 the ultrapotassic rhyolites. Foley et al. (1987). In contrast, as described above, Cambrian ultrapotassic

Table 1 Whole rock-chemical compositions of the ultrapotassic rhyolites from the natural and experimental settings.

Rock type Rhyolite Ignimbrite Ignimbrite Rhyolite Rhyolite Rhyolite Rhyolite Rhyolite porphyry Rhyolite Experimental data

SampleT11-14-1T11-14-2T11-14-3T11-14-4T11-14-6T11-14-712 3 4567

Major element (wt.%)

SiO2 80.15 69.73 69.56 73.68 82.09 74.72 76.56 74.6 73.36 70.04 70.92 71.41 73.13

TiO2 0.11 0.69 0.48 0.36 0.32 0.35 0.13 0.22 0.27 0.13 0.18 0.28 0.37

Al2O3 10.56 12.79 16.61 12.58 9.52 12.77 13.58 10.1 9.49 14.41 14.27 13.84 13.62

Fe2O3 0.18 2.71 1.59 2.6 0.71 1.67 1.55 2.86 3.37 1.48 1.71 1.98 1.93 FeO 0.74 4.67 1.78 1.42 0.95 1.08 0.16 0.93 MnO 0.01 0.04 0.02 0.01 0.01 0.01 0.03 0.08 0.07 0.03 0.02 0.03 0.03 MgO 0.25 0.73 0.62 0.42 0.34 0.34 0.10 0.07 1.91 0.16 0.19 0.39 0.49 CaO 0.11 0.24 0.18 0.2 0.23 0.17 0.39 0.91 0.02 1.05 0.96 0.15 0.13

Na2O 0.05 0.06 0.04 0.05 0.01 0.04 0.76 0.2 0.16 1.92 1.78 0.87 0.97

K2O 6.67 5.94 6.54 6.74 4.19 6.90 6.80 8.77 6.67 4.3 4.33 6.52 6.92

P2O5 0.04 0.13 0.06 0.06 0.06 0.07 0.07 0.04 0.02 0.02 0.04 0.06 0.10 LOI 0.79 1.44 2.35 1.63 1.17 1.40 0.00 1.55 0.10 + H2O 0.78 1.45 1.62 1.44 0.52 1.10 1.07 3.06 3.49 Total 100.44 100.62 101.45 101.19 100.12 100.62 100.13 99.40 97.42 93.43 94.30 98.56 101.14

TFe2O3 1.00 7.90 3.57 4.18 1.77 2.87 1.73 2.86 4.40 1.48 1.71 1.98 1.93 Fe-number 0.80 0.92 0.85 0.91 0.84 0.89 0.95 0.98 0.70 0.90 0.90 0.84 0.80 A/NK 1.45 1.96 2.32 1.71 2.09 1.69 1.58 1.03 1.27 1.84 1.87 1.63 1.50 A/CNK 1.41 1.84 2.22 1.62 1.92 1.63 1.46 0.88 1.26 1.48 1.52 1.58 1.46

Trace element (ppm) Hf 4.77 14.7 18.4 13.3 12.3 13.5 13.7 Zr 127 579 735 527 499 563 186 528 635 Sc 3.57 17.6 12.4 11.3 11.7 11.9 Co 0.83 5.05 1.83 2.91 0.90 1.82 9.00 2.00 5.25 Ni 6.04 7.16 2.45 4.29 1.34 2.66 6.00 15.0 16.5 Cu 4.33 11.3 5.53 12.0 3.88 8.72 12.0 38.0 30.5 Zn 11.6 162 45.3 84.8 22.6 55.3 107 77.0 151 Ga 15.8 19.5 29.6 17.4 25.5 21.5 20.9 Rb 337 658 602 436 321 460 249 192 Nb 9.85 23.1 25.3 18.0 16.3 19.2 14.0 47.0 64.9 Ba 685 563 586 816 419 826 1282 225 538 Ta 0.92 1.79 1.98 1.43 1.29 1.46 4.51 Pb 13.6 108 11.1 48.3 4.25 32.9 15.0 244 Th 25.2 35.0 38.6 23.3 16.1 24.0 13.0 U 3.03 6.24 4.70 4.66 3.20 4.09 Cr 14.4 5.30 4.19 3.26 2.05 1.86 8.00 14.0 13.9 Sr 17.2 8.05 9.87 23.1 6.34 17.5 53.0 36.0 23.3 Sn 5.89 5.83 10.6 4.99 9.11 6.26 La 26.5 22.1 16.3 22.6 10.5 12.9 67.4 33.6 58.4 Ce 50.9 44.3 27.7 49.7 19.0 16.6 121 82.8 97.5 Pr 5.67 5.08 4.15 5.41 2.36 3.15 12.5 9.71 11.6 Nd 21.2 19.3 15.2 20.4 9.23 12.4 53.8 38.9 40.2 Sm 4.34 4.04 3.31 4.26 2.06 2.82 10.5 8.84 7.56 Eu 0.55 0.81 0.64 0.85 0.41 0.57 1.81 0.57 0.25 Gd 3.79 4.77 4.20 4.75 2.89 3.39 9.62 9.60 7.30 Tb 0.62 1.12 0.99 0.88 0.64 0.74 1.58 1.40 Dy 3.68 8.60 8.22 6.28 5.13 5.50 9.33 10.1 9.86 Ho 0.76 1.98 1.96 1.39 1.21 1.26 2.01 2.22 Er 2.55 6.43 6.55 4.49 4.24 4.42 5.28 5.55 7.27 Tm 0.36 0.90 0.90 0.66 0.63 0.61 0.83 1.13 Yb 2.63 5.97 6.18 4.41 3.98 4.34 5.04 5.41 7.37 Lu 0.41 0.92 0.99 0.71 0.65 0.65 0.62 0.80 1.24 Y 23.0 62.5 60.2 43.3 39.4 40.3 63.0 61.0 62.7 ΣREE 147 189 157 170 102 110 271 253

(La/Sm)N 3.95 3.54 3.18 3.43 3.29 2.96 4.17 2.46 4.99

(Gd/Yb)N 1.19 0.66 0.56 0.89 0.60 0.65 1.58 1.47 0.82 δEu 0.41 0.56 0.52 0.58 0.51 0.56 0.54 0.19 0.10

Total Fe as Fe2O3.

Fe-number — TFe2O3/(TFe2O3 + MgO). 1: Malani, India (Sharma, 2005); 2: Sinai, Egypt (Samuel et al., 2007); 3: Yidun, Southeast Tibet (average value of 10 samples; Qu et al., 2003); 4 and 5: experimental melt compositions of tonalitic gneiss at 10 kbar and 925 °C, 10 kbar and 950 °C, respectively (Skjerlie and Johnston, 1993); 6–7: experimental melt compositions of pelitic rock at 7 kbar and 950 °C, 10 kbar and 975 °C, respectively (Patiño Douce and Johnston, 1991). rocks in the Lhasa terrane are rhyolites, characterized by high structure and mineral association are well preserved (Fig. 2). Moreover,

SiO2 (69.56–82.82 wt.%), K2O (4.15–6.90 wt.%), and low Na2O some major and trace elements show clear correlations (Fig. 7a–i, k–l), (0.01–0.45 wt.%) and MgO (0.15–0.78wt.%)contents(Table 1). indicating the history of fractionation crystallization rather than alter- Previous studies suggested that weathering, alteration and high- ation. We therefore conclude that the Lhasa ultrapotassic rhyolites did temperature K-metasomatism can result in K2OenrichmentandNa2O not experience chemical alterations after the magmatic crystallization. depletion in acidic igneous rocks (Ruxton, 1968; McLennan, 1993; Available studies show that fractional crystallization of plagioclase Irfan, 1996, 1999; Ennis et al., 2000). However, the present rocks can result in the formation of ultrapotassic magma (Özdemira et al., are fresh and have low LOI values (Table 1). Their magmatic texture, 2006; Shan et al., 2007). The Lhasa ultrapotassic rhyolites show features

16 Lhasa (this study) Yidun (Qu et al., 2003) of crustal rocks in an extensional tectonic environment, such as conti- Lhasa (Hu et al., 2013) Sina (Samuel et al., 2007) Rajasthan (Sharma, 2005; Experimental data(Patiño nental rift (Sharma, 2005) and intraplate rifting setting (Qu et al., Maheshwari,1987) Douce et al., 1991; 2003; Samuel et al., 2007). The ultrapotassic rhyolites in the Lhasa ter- 12 trachyte- Alkalic Skjerlie et al.,1993) trachydacite rane have low Nb/Ta ratios (6–13) that are comparable with crustal rhyolite values (6, Wedepohl et al., 1991;11–12, Taylor and McLennan, 1985;

O (wt%) 2 8 Green, 1995; Rudnick and Fountain, 1995;12–13, Barth et al., 2000), high Th/U ratios (5–10) and low Eu/Eu* ratios (b1) close to the crustal

O+K 2 value (6, Rudnick and Gao, 2003; b1, Rudnick and Fountain, 1995), sug- dacite Na 4 gesting partial melting of a crustal source. Sub-alkalic ε − andesite The zircons from the studied rocks have Hf(t) values between 4.7 and 1.3, corresponding to model ages (TDM2) of 1734 to 1392 Ma 0 60 65 70 75 80 85 (Fig. 4b, c; Appendix 2), indicating the involvement of middle Protero- zoic crustal material in the petrogenesis of the lavas. In addition, the SiO2 (wt%) wide εHf(t) values (Fig. 4b) and TDM2 model ages (Fig. 4c) indicate pos- sibly that minor mantle-derived magmas have been introduced into the Fig. 5. Total alkali vs. silica (TAS) diagram (after Lebas et al., 1986) of the ultrapotassic rhyolites. parental magmas of the ultrapotassic rhyolites. Recent studies show that the Lhasa terrane may likely have middle Proterozoic crystalline basement. Magmatic zircons from Mesozoic of feldspar fractional crystallization, such as negative anomalies of Ba, Sr and Cenozoic magmatic rocks of the Lhasa terrane yielded middle andEu(Fig. 8a, b), negative correlation between Sr and Rb versus SiO2 Proterozoic Hf two-stage model ages (Zhu et al., 2009a, 2011). Xu (Fig. 7f, i), K/Rb ratio versus Rb (Fig. 10a) and Sr versus Rb (Fig. 10b), et al. (2013) and Lin et al. (2013) reported that the gneisses from the and positive correlation between Ba and Sr (Fig. 10c). But, the variation Lhasa terrane have a protolith age of Proterozoic. Therefore, the recog- trend on Ba versus Eu/Eu* diagram (Fig. 10d) shows that alkaline feld- nized Proterozoic crystallization basement is likely to be the source of spar fractionation, instead of plagioclase, played a key role during the the ultrapotassic rhyolites. magmatic crystallization. Experimental investigations indicated that ultrapotassic and Partial melting of crustal materials is an important way to produce peraluminous A-type granitoids can be produced by partial melting A-type granites, especially peraluminous A-type granitoids (Frost under lower crustal pressure, high-temperature and water-poor and Frost, 2011). Partial melting of tholeiitic, tonalitic or granodioritic conditions, such as partial melting of pelitic rocks at 7–10 kbar and rocks (Creaser et al., 1991; Frost and Frost, 1997; Patiño Douce, 1997), 950–975 °C (Patiño Douce and Johnston, 1991), and tonalitic gneiss and partial or complete melting of an alkali-metasomatized crust at 10 kbar and 925–950 °C (Skjerlie and Johnston, 1993). The melts

(Martin, 2006) could generate A-type granites. Previous studies have generated will have high SiO2 (N70 wt.%), K2O/Na2Oratio(≫1), also demonstrated ultrapotassic rhyolites derived from partial melting Al2O3 (ca. 14 wt.%) and low CaO (≤1 wt.%), which are comparable to

10 3.0 Lhasa (this study) Lhasa (Hu et al., 2013) Metaluminous 8 Rajasthan (Sharma, 2005; 2.5 Ultrapotassic Maheshwari,1987) Sina (Samuel et al., 2007) Yidun (Qu et al., 2003) 6 Experimental data(Patiño 2.0 Douce et al., 1991; K

Skjerlie et al.,1993) /

A O (wt%) 2 4 1.5 K Shoshonitic Peraluminous O=2 2 2 1.0 O/Na Calc-Alkaline 2O=0.5 K 2 O/Na Peralkaline K 2 a) b) 0 0.5 0 2 468 0.5 1.0 1.5 2.0 2.5

Na2O (wt%) A/CNK 1.0 15 A-type A-type 0.9 10

+MgO) 0.8

3 ferroan

O 5

2 alkalic

0.7 magnesian

O-CaO(wt%) 2

/ (TFe 0 3 alkali-calcic

O 0.6 O+K

2 I-type granitoids 2 calc-alkalic

calcic Na TFe -5 0.5 c) I-type granitoids d) 0.4 -10 50 55 60 65 70 75 80 85 50 55 60 65 70 75 80 85

SiO2 (wt%) SiO2 (wt%)

Fig. 6. Compositional variation diagrams of the ultrapotassic rhyolites. (a) K2Ovs.Na2O(afterLe Maitre, 1989) and (b) A/NK vs. A/CNK (after Maniar and Piccoli, 1989); (c) Fe-number

[TFe2O3/(TFe2O3 + MgO)] vs. SiO2 (after Rajesh, 2007) and (d) modiﬁed alkali lime index (MALI) (Na2O+K2O − CaO) vs. SiO2 plots (after Frost et al., 2001).

20 0.8 700

Al2O3 (wt%) MnO (wt%) Rb (ppm)

16 0.6 500

12 0.4

300 8 0.2 a) e) i) 4 0.0 100 8 55 Sr (ppm) 1100 K2O (wt%) Ba (ppm) 45

6 900 35

25 4 700 b) 15 f) 500 j) 2 5 30 TFe O (wt%) 7 2 3 Nb (ppm) 18 Hf (ppm) 25

14 5 20

15 10 3 10 c) g) 6 k) 1 5 3 0.7 7

TiO2 (wt%) U (ppm) Th (ppm) 6 40

0.5 5 30 4

0.3 3 20

d) 2 h) l) 10 0.1 1.3 70 75 80 83 70 75 80 83 70 75 80 83

SiO2 (wt%) SiO2 (wt%) SiO2 (wt%)

Fig. 7. Harker plots of selected major and trace elements of the ultrapotassic rhyolites in the Lhasa terrane. the ultrapotassic rhyolites in the Lhasa terrane (Fig. 6; Table 1). Prefer- and Pisarevsky, 2005; Cawood and Buchan, 2007; Murphy et al., ential melting of K-feldspar of crustal rocks at highly water unsaturated 2011; Nance et al., 2014). The Lhasa terrane was generally considered condition is likely the main mechanism for generating ultrapotassic and to be a part of the Indian Gondwana margin before Permian (e.g., peraluminous melts (Skjerlie and Johnston, 1993; Qu et al., 2003). The Yin and Harrison, 2000; Metcalfe, 2006; Zhang et al., 2014)andexperi- ultrapotassic rhyolites in the Lhasa terrane have high zirconium satura- enced early Paleozoic orogenesis including the Cambro-Ordovician tion temperature (803–1015 °C with an average value of 915 °C), indi- magmatism (e.g., Li et al., 2008a; Dong et al., 2009; W.H. Ji et al., 2009; cating that the rhyolitic magmas had markedly high initial temperature. Gehrels et al., 2011; Pan et al., 2012; Zhu et al., 2012; Hu et al., 2013; Therefore, we propose that the ultrapotassic rhyolites in the Lhasa Fig. 1a), coeval metamorphism (Zhang et al., 2012a, 2014) and deforma- terrane were derived from the partial melting of middle Proterozoic tion (W.H. Ji et al., 2009; Li et al., 2010). However, the style and mecha- crustal rocks at the lower crustal high-pressure, high-temperature and nism of the orogeny in this region is still debated. As mentioned above, water-absent conditions, and subsequently experienced mixing with some workers thought that the early Paleozoic orogeny is related to the mantle-derived magma and fractional crystallization. ﬁnal assembly of Gondwana (Li et al., 2008a, 2010). However, most recent studies demonstrated that almost all the terranes along the 6.2. Tectonic model northern margin of Gondwana experienced early Paleozoic Andean- type orogeny, including Turkey (Gürsu and Göncüoglu, 2005; Ustaömer The late Precambrian–early Paleozoic was an important period in et al., 2009), Iran (Ramezani and Tucker, 2003; Hassanzadeh et al., the geological evolution of Gondwana, as it was at this time that ﬁnal 2008; Horton et al., 2008; Saki, 2010; Moghadama et al., 2014), Qiangtang assembly of the Gondwana supercontinent took place and subduction (Wang et al., 2008; B.D. Wang et al., 2012), Amdo (Zhang et al., 2012b), was initiated along the peri-Gondwana margins (Meert, 2003; Collins Himalaya (e.g., Cawood et al., 2007; Zhang et al., 2008; Wang et al.,

10000 magmatic rocks from most terranes along the northern margin of Lhasa (this study; Hu et al., 2013) Gondwana (e.g., X.X. Wang et al., 2012; Cai et al., 2013; Wang et al., 2013). 1000 Sina (Samuel et al., 2007) It has been recognized that A-type granites can form in geodynamic Yidun (Qu et al., 2003) settings ranging from within-plate settings to plate boundaries 100 (e.g., Collins et al., 1982; Whalen et al., 1987; Sylvester, 1989; Rogers and Greenberg, 1990; Eby, 1992; Nedelec et al., 1995; Barbarin, 1999; 10 Y.R. Shi et al., 2010; Peng et al., 2012). However, A-type granites in convergent margin setting commonly formed in an extensional episode of Andean-type orogeny (e.g., Smith et al., 1977; Stein et al., 1992; 1

Sample/Primitive mantle a) Bonin, 2007; Zhao et al., 2008; Chen et al., 2014). Our study shows that the ultrapotassic rhyolites are characterized by enrichment in 0.1 Rb Th Nb K Ce Pr Nd Sm Hf Ti Tb Ho Er Yb Rb, K, U, Th, Pb and LREE, and negative anomalies of Nb, Ta and Ti Ba U Ta La Pb Sr P Zr Eu Gd Dy Y Tm Lu (Fig. 8a, b), which are consistent with arc magmatic rocks (Taylor and 1000 McLennan, 1985; McCullochet and Gamble, 1991; Muller et al., 1992; Pearce and Peate, 1995; Tatsumi and Eggins, 1995). These rocks also have similar Th/Ta ratios to felsic rocks in active continental margins (Gorton and Schandl, 2000)(Fig. 11). In addition, the Cambrian strata of the study area contain quartz sandstones with graded and cross- 100 beddings, and abundant volcanics, indicating a tectonic setting of active continental margin (W.H. Ji et al., 2009). These features indicate that these early Paleozoic magmatic rocks in the Lhasa terrane formed in

Sample/Chondrite an environment related to subduction, which is consistent with the 10 previous conclusion obtained from the coeval magmatic rocks from b) the Lhasa terrane (Dong et al., 2009; W.H. Ji et al., 2009; Zhu et al., 2012; Hu et al., 2013). LaCe Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Cawood et al. (2007) considered that the Andean-type orogeny along the northern margin of Gondwana began in late Precambrian Fig. 8. (a) Primitive-mantle-normalized trace element patterns and (b) chondrite- and lasted until early Ordovician. During the middle Cambrian, rollback normalized REE patterns. The normalized values are after Sun and McDonough (1989). of subducted proto-Tethyan Ocean lithosphere resulted in the astheno- spheric upwelling, crustal extension and mantle-derived maficmagma underplating (Fig. 12). Therefore, it is possible that such high- 2011; X.X. Wang et al., 2012) as well as Lhasa and Tengchong–Baoshan temperature and water-absent environment has triggered extensive terranes (e.g., Dong et al., 2009; W.H. Ji et al., 2009; Gehrels et al., 2011; partial melting of lower crust of magmatic arc. Recent studies show Yang et al., 2012; Zhu et al., 2012; Cai et al., 2013; Dong et al., 2013; Hu that slab rollback of subducted oceanic lithosphere could induce et al., 2013; Wang et al., 2013). The models that propose the early formation of crust-derived granites in continental magmatic arcs Paleozoic magmatism derived from Andean-type orogeny rely on the (Collins, 2002; Collins and Richards, 2008). The present study indicates following arguments. (1) Most early Paleozoic magmatic rocks from the further that the Cambrian ultrapotassic A-type rhyolites from the Lhasa northern margin of the Gondwana show geochemical affinity to magmat- terrane show geochemical features of arc magmatic rocks and derived ic arc rocks (Garzanti et al., 1986; Brookfield, 1993; Valdiya, 1995; Miller from partial melting of high-temperature and water-poor lower crust. et al., 2001; Ustaömer et al., 2009; B.D. Wang et al., 2012; Pan et al., 2012; Consequently, we consider that the extensional regime derived from X.X. Wang et al., 2012; Yang et al., 2012; Zhang et al., 2012b,c; Zhu et al., the rollback of subducted oceanic slab is a reasonable mechanism for 2012; Moghadama et al., 2014). (2) Pan-African collisional orogenesis re- generating the Lhasa ultrapotassic rhyolites during the early Paleozoic lated to the final assembly of the various continental components within Andean-type orogeny along the northern margin of Gondwana (Fig. 12). the Gondwana supercontinent, not the margin of Gondwana (Cawood and Buchan, 2007; Cawood et al., 2007). (3) The time of the final assem- 7. Conclusions bly of Gondwana (~570–520 Ma) (Meert, 2003; Cawood and Buchan, 2007; Cawood et al., 2007; Zhang et al., 2008) in part overlaps with but The rhyolite and rhyolitic ignimbrite from the central part of the extends to earlier than the crystallization ages (~530–460 Ma) of Lhasa terrane crystallized at middle Cambrian of ca. 512 Ma, and are

1000 1000

800 A-type rocks

A-type rocks 600 100

400 Zr (ppm) M-,I- & S-type

Zr+Ce+Nb+Y rocks 200 I- & S-type rocks a) b) 0 10 0123456 1 10 1000Ga/Al 1000Ga/Al

Fig. 9. Discrimination diagrams of A-type granites. After Whalen et al. (1987).

200 200

100

150 Pl

K/Rb

Sr (ppm) AF 100 10

a) b)

50 2 150 350 550 750 100 1000 Rb (ppm) Rb (ppm)

10000

1000 30 20 Pl 50 40 30 20 Pl

20 20 10 1000 30 30

Ba (ppm)

Ba (ppm) 20

c) Kf Bi d) AF 400 100 2 10 100 2000.1 1 4 Sr (ppm) Eu/Eu*

Fig. 10. Variation diagrams of trace elements of the ultrapotassic rhyolites in Lhasa terrane. Fractional crystallization trends in (b), (c) and (d) are after Huang et al. (2008), Singh et al. (2006) and Eby (1990), respectively. Mineral abbreviations: AF—alkali feldspar, Kf—K-feldspar, Pl—plagioclase and Bi—biotite.

peraluminous, ultrapotassic A-type acidic volcanics. They were derived Acknowledgments from partial melting of middle Proterozoic crustal rocks with minor mantle-derived magma contributions in the lower crust under high We thank M. Santosh, H.M. Rajesh and H.S. Moghadam for construc- temperature and water-absent conditions. The ultrapotassic rhyolite tive comments that have greatly improved this manuscript. We also may have formed in an extensional episode of Andean-type orogeny re- thank Profs. Dicheng Zhu and Xiaoming Qu for valuable discussions lated to the slab-rollback during the subduction of proto-Tethyan Ocean and suggestions on an earlier version of this manuscript. This study beneath Gondwana supercontinent. was ﬁnancially supported by the National Natural Science Foundation Supplementary data to this article can be found online at http://dx. of China (41230205, 41202035, 40972055 and 40921001) and the doi.org/10.1016/j.gr.2014.02.003. China Geological Survey Project (1212011121269).

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Cambrian India Lhasa volcanism Cambrian volcano- sedimentary sequence Proto-Tethyan Ocean

Oceanic crust Continental crust Granitoids Lithospheric mantle Mafic underplating Lithoshperic mantle

Asthenosphere

Asthenosphere Rollback

Fig. 12. Schematic tectonic model of the origin of Cambrian ultrapotassic rhyolites in the Lhasa terrane.

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