Gondwana Research 17 (2010) 135–144

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The Late Triassic rift-related volcanic rocks from eastern Qiangtang, northern Tibet(China): Age and tectonic implications

Xiu-gen Fu ⁎, Jian Wang, Fu-wen Tan, Ming Chen, Wen-bin ChenChengdu Institute of Geology and Mineral Resources, Chengdu, 610081, China

⁎ Corresponding author.E-mail address: fuxiugen@126.com (X. Fu).

1342-937X/$ – see front matter. Crown Copyright © 20doi:10.1016/j.gr.2009.04.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 September 2008Received in revised form 1 April 2009Accepted 15 April 2009Available online 24 April 2009

Keywords:SHRIMP datingContinental rift settingNadi Kangri volcanic rocksQiangtang basin, northern Tibet

The Late Triassic Nadi Kangri volcanic rocks, with nearly EW trending outcrops within the Qiangtang basin,northern Xizang (Tibet), China, are composedmainly of acid tuff, dacite, rhyolite and minor basic volcanic rocks.There exists a significant depositional hiatus between the Nadi Kangri volcanic rocks and underlying strata.Therefore, the Nadi Kangri volcanic rocks represent a new evolution history of theMesozoic Qiangtang basin. Themagma emplacement age of the Nadi Kangri volcanic rocks in the Geladaindong area is 220.4±2.3 Ma,representing the onset of theMesozoic Qiangtang basin. The Nadi Kangri basalts have high Nb/Zr (0.049–0.058),Ta/Hf (0.12–0.15) andZr/Y (4.95–6.01) ratios. In the tectonic discriminationdiagrams, such asZrvs. Zr/YandTh/Hfvs. Ta/Hf, the Nadi Kangri basaltic rocksmostly plot in the “within-plate” setting field. The geological backgroundand the geochemical characteristics suggest that the Nadi Kangri volcanic rocks were formed in a continental riftsetting.Crown Copyright © 2009 Published by Elsevier B.V. on behalf of International Association for Gondwana

Research. All rights reserved.

1. Introduction

The Late Triassic Nadi Kangri volcanic rocks, with nearly EWtrending outcrops within the Qiangtang basin, northern Xizang(Tibet), China, are composed mainly of acid tuff, dacite, rhyolite, andminor basic volcanic rocks. Regionally, these rocks, covering an area of50 km wide and 300 km long, dominantly occur in the western andeastern parts of the Qiangtang basin.

Due to rapid facies changes and lack of correlatable age data on aregional scale, it is difficult to subdivide and correlate the Nadi Kangrivolcanic–volcaniclastic rocks. Recently, there has been much debateabout the age and subdivision of these strata, and they were assignedto the Middle Jurassic time (Bureau of Geological and MineralResources of Xizang Autonomous Region, 1993; Wang et al., 2001a;Li et al., 2002; Zhu et al., 2005a,b,c) or the Lower Jurassic time (Zhuet al., 1996, 1997). In addition, due to the difficult access to thenorthern Tibet (China), little geochemical study has been carried outon the Nadi Kangri volcanic rocks. Consequently, the tectonic settingsof these rocks are not well understood. Furthermore, most of theavailable data focus only on the felsic rocks from the western part ofthe Qiangtang basin (e.g., Zhu et al., 2002; Li et al., 2007a; Zhai and Li,2007). It is generally believed that the tectonomagmatic affiliations ofbasalts and dolerite dykes are much clear than those of mafic–

09 Published by Elsevier B.V. on

ultramafic intrusions and felsic rocks (Li et al., 2007b; Zi et al., 2008).Therefore, the tectonic settings of these volcanic rocks should befurther studied due to the absence of the geochemical data of basicvolcanic rocks.

In this paper, we present new geochemical data and a SHRIMPzircon U–Pb age of the Nadi Kangri volcanic rocks. Our aims are: (1) todate the onset of the Mesozoic Qiangtang basin; (2) to re-evaluate thetectonic settings of these volcanic rocks based on the basaltic lavas.

2. Geological background

The Qiangtang block, marked by the Hoh Xil–Jinsha River suturezone to the north and the Bangong Lake–Nujiang River suture zone tothe south, respectively, consists of the Northern Qiangtang depression(North Qiangtang sub-basin), the central uplift and the SouthernQiangtang depression (South Qiangtang sub-basin) (Fig. 1a) (Zhaoet al., 2001; Wang et al., 2004). Between the Qiangtang block to thesouth and Eurasia to the north was an old ocean, whose floor wasconsumed by northern subduction beneath the Kunlun terrane duringPermo-Trassic time and southward subduction beneath Qiangtangduring Middle–Late Triassic time (Dewey et al., 1988; Pearce and Mei,1988; Nie et al., 1994; Kapp et al., 2003, Ye et al., 2008). In this interval,most parts of the Qiangtang basin were uplifted into a land.Meanwhile paleo-weathering crusts occurred widely in the JuhuaMountain, Shishui River, Nadigangri and Wuoro Mountain regions in

behalf of International Association for Gondwana Research. All rights reserved.

Fig. 1. (a) Simplified tectonic map of the Qiangtang basin showing the distribution of the Nadi Kangri volcanic rocks. (b) Simplified geological map of the Geladaindong area(modified from the 1:250,000 geological maps, Yao et al., 2003). TR = Tarim basin; KL = Kunlun terrane; SP = Songpan–Ganzi flysch complex; HJS = Hoh Xil–Jinsha River suture;HXP = Hoh Xili piedmont zone; QT = Qiagtang basin; BNS = Bangong Lake–Nujiang River suture; LS = Lhasa terrane; YTS = Yarlung Tsangpo suture; HMLY = Himalayas.

136 X. Fu et al. / Gondwana Research 17 (2010) 135–144

the Qiangtang Basin (Fu et al., 2007; Wang et al., 2007a). Subse-quently, these weathering crusts were overlain unconformably by asuccession of volcanic–volcaniclastic strata, i.e. the Nadi Kangrivolcanic–volcaniclastic strata (Fu et al., 2008; Wang et al., 2008),marking the onset of the Mesozoic Qiangtang basin.

The early sedimentary successions of the Mesozoic Qiangtangbasin, starting with a restricted continental molasse unit, consistmainly of alluvial and fluvial sedimentary facies associated withcontinental volcanic explosion facies (Wang et al., 2004). Thesesuccessions are overlain by littoral to shallow-marine facies associa-tions. Following the littoral to shallow-marine facies association, a

drowned carbonate-platform facies associationwas formed as a resultof rapid differential subsidence and the consequent sea-level rise andtransgression. Therefore, the early sedimentary history of the MesozoicQiangtang basin is characterized by a progradational sequence oftransition from continental to marine facies and reflects a progressiverift extension.

The Mesozoic rift-related volcanic rocks, i.e. the Nadi Kangrivolcanic rocks, with fluidal and amygdaloidal structures, consistmainly of felsic volcanic rocks varying in thickness on differentoutcrops. Additionally minor basalts, with massive and amygdaloidalstructures, are also observed in the Geladaindong area, eastern

137X. Fu et al. / Gondwana Research 17 (2010) 135–144

Qiangtang basin (EQ) (Fig. 1b). The Nadi Kangri volcanic sequences inthe EQ are comprised of a variety of rock types, including basalt,volcanic breccia, rhyolite and dacite. In general, basalts and volcanicbreccias dominate the lower part of the sequences and rhyolites anddacites dominate the upper part. The thickness of the basalts in the EQranges from 208.4 m to 765.2 m, with an average of ~500 m.

Petrographically, the Nadi Kangri volcanic rocks experiencedvariable alteration, however, original textures can still be observed.In hand specimens, most of the Nadi Kangri basalts are aphyric withthe exception of samples JR07 and JR08 that contain microvesiclesfilled with chlorite and carbonate; while the felsic volcanic rocks aremostly phyric. In thin sections, the Nadi Kangri basalts exhibitporphyritic textures (phenocrysts ~8–16%) dominated by labradoritephenocrysts (limited alteration), with a little highly alteredhypersthene. The groundmass consists chiefly of plagioclase andaugite with subordinate pigeonite. Accessory minerals are titanite,ilmenite and variable amounts of zircon, magnetite and marcasite.Phenocrysts (8–11%) in the Nadi Kangri felsic volcanic rocks arecomposed mainly of quartz, potassic feldspar and plagioclase. Thefeldspars are extensively altered (e.g. sericite). The quartz phenocrystsare corroded or embayed crystals of variable size. The groundmassconsists of fine-grained quartz, feldspar, sericite and volcanic ash.Accessory minerals are magnetite, limonite, ilmenite and zircon.

3. Samples and analytical methods

Nineteen samples were collected for this study, with seven basalt(JRN) and three rhyolite (JR) samples from the EQ, and nine rhyoliteand dacite samples from the west Qiangtang basin (WQ).

Zircon grains were separated from a basaltic sample (JRN04) usingstandard density and magnetic separation techniques at the SpecialLaboratory of the Geological Team of Hebei Province, China. The zircongrains, together with the zircon U–Pb standard TEMORA (Black et al.,2003), were mounted in an epoxy disc and then polished to exposetheir centers. The internal structures of the zircons were observedusing cathodoluminescence (CL) images. Under the guidance of zirconCL images, the zircons were analyzed for U–Pb isotopes and U, Th, andPb concentrations using a SHRIMP II ion microprobe at the BeijingSHRIMP Center, Chinese Academy of Geological Sciences. Theprinciple of SHRIMP zircon U–Pb age dating, preparation of the targetzircons and workflow are referenced from Song et al. (2002) andWilliams (1998). Measurements were corrected using a referencezircon standard SL13 (572 Ma) and a reference zircon standardTEMORA (417 Ma). The reference zircon TEMORA was analyzed after

Table 1U–Pb zircon SHRIMP analytical data for the Nadi Kangri volcanic rocks in the Geladaindong

Spot Pbc (%) U (×10−6) Th (×10−6) 232Th/238U 206Pb⁎ (×10−6) 238U/206P

JRN-1.1 0.62 504 239 0.49 15.3 28.34JRN-1.2 0.27 632 312 0.51 20.6 26.38JRN-2.1 0.25 672 299 0.46 20.3 28.44JRN-3.1 0.65 903 488 0.56 27.4 28.26JRN-4.1 0.22 1259 685 0.56 37.8 28.60JRN-5.1 2.01 528 244 0.48 16.0 28.41JRN-6.1 0.84 1633 1061 0.67 51.1 27.47JRN-6.2 0.00 1505 1213 0.83 53.3 24.24JRN-7.1 0.11 1428 1063 0.77 42.3 29.01JRN-7.2 0.50 1239 734 0.61 42.0 25.33JRN-8.1 0.20 1604 1315 0.85 46.2 29.80JRN-9.1 0.45 1052 702 0.69 31.1 29.05JRN-10.1 0.39 874 485 0.57 26.9 27.94JRN-12.1 0.70 1068 851 0.82 33.1 27.76JRN-13.1 1.50 793 411 0.54 25.1 27.18JRN-16.1 0.20 670 386 0.59 20.0 28.81JRN-17.1 6.42 566 270 0.49 20.4 23.82JRN-18.1 0.40 515 229 0.46 17.9 24.65

every third analysis. Measured compositions were corrected forcommon Pb using the 204Pb method, and data processing was carriedout using Isoplot (Ludwig, 2001). Uncertainties on individual analysesare reported at the 1σ level, and mean ages for pooled 206Pb/238Uresults are quoted at the 95% confidence level.

Major element data were collected using X-ray fluorescence (XRF)on fused glass beads using a Rigaku ZSX100e spectrometer in theAnalytical Center, Chengdu Institute of Geology and MineralResources. The analytical uncertainty is usually b5%. With the samewhole-rock powders, trace element concentrations were determinedusing a Perkin Elmer Sciex Elan 6000 inductively-coupled plasmamass spectrometer (ICP-MS) at the Guangzhou Institute of Geochem-istry, Chinese Academy of Sciences. The analytical procedures aresimilar to those described by Li (1997). The analytical precision isgenerally within 5%.

4. Results

4.1. SHRIMP zircon U–Pb geochronology

The zircon U–Pb analytical data and calculation results are listedin Table 1. The results for the U–Pb age determinations can be seen inFig. 2.

Zircons from the Geladaindong basalt (JRN04) are euhedral, andhave perfect pyramidal and prismatic crystal forms. The zircons range inlength from ~100 μm to ~250 μmwith length/width ratios of ~2:1. Thezircon cathodoluminescence images exhibit good magmatic oscillatoryzoning (Fig. 3). These zircon grains have high U and Th contents(U=504×10− 6–1633×10−6 and Th=229×10− 6–1315×10− 6,respectively) (Table 1), and low Th/U ratios (Th/U=0.46–0.85). Theanalytical data for zircons may be divided into two groups. The firstgroup includes measured spots 1.2, 6.2, 7.2, 17.1 and 18.1. The five zirconcores give the 206Pb/238U ages from 239.2 to 260.6 Ma, representing thecrystallization ages of inherited zircons. The rest thirteen spots are in thesecond group. These spots have 206Pb/238U ages ranging from 212.3 to229.5 Ma with a weighted mean age of 220.4±2.3 Ma (MSWD=1.5)(Fig. 2). This age of 220.4±2.3 Ma is interpreted as the magmaemplacement age of the Nadi Kangri basalts in the Geladaindong area.

4.2. Major and trace elements

Major and trace elemental data for the Nadi Kangri volcanic rocksare given in Table 2.

area.

b⁎ 207Pb⁎/206Pb⁎ (%) 207Pb⁎/235U (%) 206Pb⁎/238U (%) 206Pb/238U age/±1σ Ma

0.0481±6.4 0.25±5.0 0.0352±3.2 222.2±6.90.0441±4.2 0.25±4.4 0.0380±2.7 239.2±6.30.0510±4.4 0.25±4.3 0.0351±2.7 222.3±5.90.0483±3.3 0.25±4.1 0.0353±2.7 222.7±5.90.0534±2.6 0.25±3.7 0.0349±2.6 221.1±5.70.0478±14.3 0.23±5.6 0.0345±2.8 218.6±6.20.0518±4.7 0.24±4.0 0.0359±2.9 228.6±6.50.0513±2.5 0.27±3.7 0.0411±2.6 260.6±6.60.0520±2.8 0.24±3.7 0.0344±2.6 218.2±5.60.0460±4.8 0.26±4.0 0.0394±2.9 248.4±7.00.0477±2.1 0.24±3.0 0.0336±2.6 212.3±5.40.0460±4.7 0.24±4.2 0.0345±2.6 217.2±5.50.0494±3.8 0.28±3.5 0.0359±2.6 225.9±5.80.0484±3.7 0.24±3.8 0.0358±2.6 226.6±5.80.0456±10.5 0.26±4.8 0.0365±2.7 229.5±6.30.0531±3.8 0.22±4.5 0.0343±2.6 219.5±5.70.0438±26.7 0.29±5.8 0.0398±2.7 248.4±7.40.0514±6.4 0.28±4.9 0.0403±2.8 255.4±6.9

Fig. 2. Zircon U–Pb concordia diagram for the Nadi Kangri volcanic rocks in the Geladaindong area.

138 X. Fu et al. / Gondwana Research 17 (2010) 135–144

The ignition loss of 21 samples is from 0.90% to 11.67% (Table 2).Except for four samples (JRN07, JRN08, JP8-3 and JP9-1) with higherignition loss, the other 17 samples have the ignition-loss rangebetween 0.90% and 4.56%, with an average of 2.97%. It seems thatthese samples are relatively fresh. Based on thin section analysis andNb/Y–Zr/TiO2 diagram for the classification of the altered volcanicrocks, most of the samples from the EQ fall in the fields of rhyolite andbasalt (Fig. 4). In contrast, the samples from the WQ mostly plot intothe field of rhyodacite or dacite.

The Nadi Kangri basaltic volcanic rocks have a relatively widerange of SiO2 contents and are of basalt to basaltic andesitecomposition (Table 2). Note that samples JRN07 and JRN08 have lowcontents of SiO2 (45.05% and 43.86%, respectively) corresponding tohigh LOI values (7.78% and 8.68%, respectively), which may beascribed to a high degree of alteration and to amygdaloidal structuresfilled with carbonate and chlorite. All basalts have low contents of TiO2

(0.84–1.09%), comparable to representative continental flood basalts(1%, Zhao et al., 2008), while slightly higher than those of the tholeiiticarc basalts (0.8%, Zhao et al., 2008). The basalts are characterized by

Fig. 3. Cathodoluminescence images sho

relatively high contents of MgO (4.16–8.90%) with Mg# range from60.26 to 75.91, which are similar to those of primary magmaticcompositions (Mg#≥65%, Li et al., 2008). The Nadi Kangri felsicvolcanic rocks have also a relatively wide range of SiO2 contents(60.30–78.24%, not including samples JP8-3 and JP9-1 with highdegree of alteration). They are characterized by high A/CNK [Al2O3/(CaO+Na2O+K2O) molecular ratio] values from 0.79 to 1.53 (notincluding samples JP8-3 and JP9-1 with high degree of alteration)(Table 2), showing that they are peraluminous/metaluminous.

All basaltic samples show similar chondrite-normalized rare-earthelement (REE) patterns (Fig. 5a) and have clearly fractionated lightrare-earth (LREE) elements relative to heavy rare-earth (HREE)elements, with (La/Yb)N=5.94–8.35 and slight or negligible Eunegative anomalies (δEu=0.86–0.96) (Table 2). On the primitivemantle-normalized incompatible element spidergram (Fig. 5b; Sunand McDonough, 1989), the Nadi Kangri basalts differ from MORB(mid-ocean ridge basalt), but are generally similar to OIB (oceanisland basalt, Sun and McDonough, 1989) and CFB (continental floodbasalt, Xiao et al., 2004). They have pronounced Nb and Ta negative

wing spot locations (white circle).

Table 2Chemical compositions of the Nadi Kangri volcanic rocks in the Geladaindong area (major: wt.%; trace element: ×10−6, ⁎ after Yao et al., 2003).

Sample location Geladaindong (eastern Qiangtang) Western Qiangtang

Lithology Basalt Basalt Basalt Basalt Basalt Basalt Basalt Rhyolite Rhyolite Rhyolite Rhyolite⁎ Dacite⁎ Dacite Dacite Dacite Dacite Rhyolite Dacite Dacite Rhyolite Rhyolite

Samples JRN04 JRN04-2 JRN05 JRN06 JRN06-2 JRN07 JRN08 JR05 JR06 JR08 2505a SH-2a JP5-1 JP6-1 JP7-1 JP8-1 JP8-2 JP8-3 JP9-1 SL15-3 SL18-1

SiO2 50.78 47.18 49.10 50.22 49.61 45.05 43.86 73.91 78.24 72.60 72.64 75.19 60.56 60.30 60.80 67.16 62.14 47.24 56.85 72.02 68.73Al2O3 15.01 16.70 16.65 15.84 16.01 16.34 15.98 13.37 10.72 13.80 13.20 12.66 14.65 15.44 15.90 14.62 16.30 14.85 8.88 11.31 11.63Fe2O3 7.20 8.66 3.75 5.78 5.64 8.03 8.12 1.12 1.02 1.23 1.91 1.17 4.37 5.05 4.15 2.62 3.10 2.08 3.26 1.07 2.11FeO 2.31 1.43 3.36 3.17 3.04 1.95 2.16 0.35 0.49 0.38 1.70 1.19 1.88 2.08 2.00 1.14 0.40 1.40 0.89 1.40 1.90CaO 4.51 12.69 7.44 3.80 4.10 3.63 4.69 0.15 0.17 0.17 0.55 0.03 4.16 4.09 1.34 2.76 4.46 14.69 11.90 2.20 1.75MgO 7.39 4.16 8.54 8.55 8.61 8.90 8.72 0.23 0.20 0.29 1.29 0.13 1.88 2.44 3.77 1.89 0.46 1.45 2.21 2.23 5.34K2O 1.38 0.60 1.11 1.88 1.95 2.62 2.52 8.90 7.28 8.97 3.66 7.42 0.75 1.16 1.61 1.22 3.22 2.36 1.52 4.22 1.13Na2O 4.94 3.32 3.77 3.99 4.02 3.15 2.96 0.13 0.11 0.14 3.38 0.10 5.62 4.63 6.38 4.38 5.51 2.98 1.62 1.60 2.44TiO2 1.09 0.84 1.01 0.98 0.97 1.04 1.07 0.09 0.10 0.11 0.23 0.12 0.88 0.90 0.85 0.47 0.45 0.38 0.70 0.17 0.52MnO 0.16 0.20 0.19 0.18 0.18 0.20 0.21 0.01 0.012 0.01 0.08 0.03 0.086 0.11 0.08 0.08 0.04 0.15 0.25 0.09 0.06P2O5 0.29 0.27 0.26 0.30 0.31 0.31 0.31 0.05 0.04 0.06 0.04 0.05 0.27 0.18 0.19 0.14 0.15 0.13 0.17 0.10 0.15LOI 3.94 2.98 3.82 4.36 4.56 7.78 8.68 1.19 1.08 1.50 0.90 1.31 4.25 3.04 2.97 3.23 3.38 11.67 11.33 3.70 4.29Total 99 99.03 99 99.05 99 99 99.28 99.5 99.462 99.26 99.58 99.4 99.36 99.42 100.04 99.71 99.61 99.38 99.58 100.11 100.05A/NKC 1.32 1.28 1.34 1.25 1.53 0.83 0.95 1.08 1.08 0.79 0.43 0.34 1.01 1.38Mg# 71.79 60.26 75.91 73.63 74.43 75.48 74.23 34.00 28.50 37.28 48.42 12.33 48.11 51.57 64.83 60.72 33.34 53.79 64.28 68.65 77.73La 15.89 16.19 15.7 15.07 14.69 17.12 16.92 31.46 42.28 38.96 48.42 58.9 36.4 34.1 24.1 29.1 25.2 32.6 35.9 19.9 16.3Ce 33.98 32.68 33.63 32.51 33.11 34.5 34.94 60.73 80.98 74.42 50.20 84.5 68.9 64.6 44.9 46.0 38.5 47.7 67.0 40.0 31.4Pr 4.481 4.244 4.454 4.375 4.452 4.46 4.57 6.565 8.984 8.223 81.30 9.61 8.90 8.34 5.82 5.78 4.49 5.60 7.24 4.93 3.94Nd 18.7 17.83 18.94 18.08 18.5 18.5 19.05 22.49 30.46 28.18 8.26 39.3 36.1 33.9 24.1 21.8 16.4 20.3 27.7 19.5 15.3Sm 3.712 3.534 3.725 3.517 3.688 3.631 3.751 3.384 4.78 4.528 36.80 7.28 7.31 6.94 4.95 4.07 2.95 3.44 5.11 4.12 3.00Eu 1.095 1.091 1.183 1.005 1.09 1.125 1.191 0.446 0.685 0.634 6.89 1.00 1.53 1.55 1.22 1.16 0.67 1.10 1.10 0.76 0.70Gd 3.734 3.724 3.853 3.519 3.722 3.67 3.797 3.012 4.211 4.06 1.13 5.77 6.66 6.52 4.42 3.78 2.68 3.36 4.87 3.83 2.73Tb 0.593 0.571 0.613 0.547 0.603 0.547 0.581 0.423 0.64 0.648 6.19 1.07 1.03 1.02 0.68 0.56 0.40 0.47 0.70 0.63 0.42Dy 3.395 3.244 3.62 3.192 3.393 2.982 3.271 2.499 3.986 4.259 1.02 5.66 6.05 6.06 3.99 3.17 2.29 2.68 4.00 3.99 2.46Ho 0.703 0.658 0.734 0.668 0.692 0.603 0.655 0.536 0.846 0.894 6.28 1.05 1.10 1.08 0.70 0.62 0.46 0.55 0.77 0.76 0.44Er 1.965 1.829 1.987 1.894 1.937 1.641 1.745 1.715 2.6 2.796 1.39 3.15 3.61 3.59 2.29 1.79 1.43 1.59 2.26 2.56 1.51Tm 0.278 0.258 0.284 0.268 0.273 0.229 0.243 0.287 0.41 0.473 4.18 0.48 0.47 0.46 0.29 0.25 0.20 0.22 0.31 0.36 0.20Yb 1.817 1.626 1.781 1.711 1.773 1.47 1.583 2.112 2.845 3.167 0.66 2.70 3.36 3.20 2.07 1.71 1.36 1.44 1.95 2.53 1.42Lu 0.281 0.253 0.276 0.266 0.284 0.242 0.242 0.35 0.461 0.524 4.11 0.37 0.47 0.45 0.29 0.25 0.21 0.22 0.30 0.36 0.20ΣREE 87.73 90.78 86.62 90.72 92.54 136.01 184.17 171.77 209.06 220.84 181.89 171.81 119.82 120.04 97.24 121.27 159.21 104.23 80.02(La/Yb)N 6.27 7.14 6.32 6.32 5.94 8.35 7.67 10.68 10.66 8.82 8.76 15.65 7.77 7.76 8.35 12.21 13.29 16.24 13.21 5.64 8.23δEu 0.89 0.91 0.95 0.86 0.89 0.93 0.96 0.42 0.46 0.44 0.52 0.46 0.66 0.69 0.78 0.89 0.71 0.98 0.66 0.58 0.73Sc 30.59 26.67 31.54 29.38 30.95 33.05 31.75 2.12 2.42 3.37 5.90 4.70 13.6 17.0 14.9 11.6 9.74 7.73 10.4 8.42 21.3Cr 384.9 324.8 403.3 356.7 384.8 375.4 329.2 2.84 1.50 2.41 3.96 5.46 6.65 21.5 26.3 26.1 57.7 18.4 84.8 6.89 177V 224.1 275.0 223.0 235.7 247.5 255.1 213.7 13.0 11.68 13.33 25.0 15.0 53.6 102 100 46.3 60.3 30.1 71.6 11.9 116Rb 42.27 19.99 44.74 81.26 82 157.2 157.7 277.5 313.1 341.9 268.0 298.0 29.0 27.7 45.5 36.8 77.2 72.9 52.5 116 27.5Ba 593.6 226.3 660.2 1111.8 1128.2 826.1 674.5 2029.1 2743.3 2542.8 610.0 3110.0 227 647 355 675 1199 734 326 761 268Th 6.679 5.796 6.36 6.226 6.348 5.128 4.751 19.8 21.96 23.46 22.0 19.7 10.6 10.9 8.17 8.72 11.1 7.04 8.77 8.57 5.35Nb 6.154 5.496 6.118 5.728 5.94 4.939 4.845 6.785 8.598 8.884 11.0 11.0 10.6 9.15 7.67 8.33 8.91 6.68 13.0 6.62 5.13Ni 207.7 100.7 185.3 141 145.4 132.3 122.4 2.00 1.52 2.67 6.00 6.84 19.20 20.00 6.41 4.04 5.97 35.20 2.83 18.3Ta 0.38 0.342 0.393 0.374 0.384 0.308 0.3 0.704 0.826 0.866 0.97 1.2 0.84 0.76 0.65 0.77 0.84 0.56 0.96 0.55 0.42Sr 245.2 1640.2 501.1 265.4 279.3 204.5 179.3 20.76 26.23 23.82 97.0 25.0 337 358 122 363 315 439 166 43.7 132Y 19.22 19.47 20.07 18.15 19.05 16.38 18.26 15.68 22.88 24.03 26.8 22.9 37.1 37.3 24.1 16.8 12.7 4.45 21.7 26.7 15.0Zr 108.6 96.38 105.6 100.8 103.5 98.52 98.59 130.7 149.4 164.8 200 230 287 243 216 228 215 155 225 153 145Hf 2.716 2.257 2.678 2.544 2.659 2.499 2.426 3.941 4.402 4.899 5.90 7.1 8.15 7.09 6.25 6.46 6.12 4.45 7.18 4.42 4.20U 1.419 1.468 1.325 1.538 1.595 3.132 3.29 2.655 2.947 3.115 4.35 4.82 3.39 3.55 2.78 3.15 6.65 6.75 2.18 2.20 1.43

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Fig. 4. Zr/TiO2 vs. Nb/Y diagram for the Nadi Kangri volcanic rocks in the Qiangtangbasin.

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anomalies, Sr negative anomalies, weak or negligible Ti negativeanomalies and P positive anomalies.

The EQ felsic volcanic rocks have higher total REE contents(REE=136.01×10−6–220.84×10−6) and stronger Eu negativeanomalies (δEu=0.42–0.52) than the WQ felsic volcanic rocks(REE=80.02×10−6–181.89×10−6; δEu=0.58–0.98) (Table 2). Inchondrite-normalized REE diagram (Fig. 5c, e), both the EQ and theWQ felsic volcanic rocks show coherent, subparallel REE patterns withsignificant enrichment in LREE relative to HREE, with LaN/YbN=5.64–16.24. The EQ felsic volcanic rocks display lower Ni (1.52×10−6–

6.00×10−6) and Cr (1.50×10−6–5.46×10−6) contents than the WQfelsic volcanic rocks (Ni=1.83×10−6–35.20×10−6; Cr=6.65×10−6–

177×10−6) (Table 2). In the primitivemantle-normalized incompatibleelement spidergram (Fig. 5d, f; Sun andMcDonough,1989), both the EQand the WQ felsic volcanic rocks display an overall enriched patternexcept for depletionofNb, Ta, P, Sr and Ti due to fractional crystallization.In addition, both the EQand theWQ felsic volcanic rocks have higher Th/Ce (0.13–0.44), Nb/Zr (0.04–0.06) and Th/Nb (0.67–2.92) ratios thanthose of MORB-source volcanic rocks.

5. Discussion

5.1. Alteration effects on major and trace elements

The Nadi Kangri volcanic rocks were altered to various degreesafter eruption/emplacement, judging from petrographic observationand variable LOI in analyzed samples (0.90–11.67%) (Table 2). Ageneral consensus exists that LILEs (e.g., K, Na, Rb, Ba, Sr) are mobile,whereas transition metals (e.g., Cr, Ni), REEs, and HFSEs as well as Thand Ti in igneous rocks are relatively immobile during low-temperature alteration (Bienvenu et al., 1990; Staudigel et al., 1996).Element mobility, tested by plotting major and trace element dataagainst LOI (not shown), reveals a high degree of mobility for K, Na, Rband U. However, there are no salient correlations between Ba, Sr andLOI. In contrast, visible correlations between Ba, Sr and MgO are note(not shown). These indicate that the concentrations of Ba and Sr in the

Nadi Kangri basalts are not significantly affected by alteration. Inaddition, both the relatively fresh samples and the highly alteredsamples (e.g., JRN07 and JRN08) have subparallel patterns of REE andHFSE contents, indicating that these mafic rocks still preserve theiroriginal REE and HFSE signatures. Thus, only immobile elements suchas the HFSEs, Th and REEs are used in the following discussion toidentify tectonic settings of these altered volcanic rocks.

5.2. Redefinition of tectonic interval: insights from paleo-weatheringcrusts

Little attention has been paid to the division of the evolutionarystages of the Mesozoic Qiangtang basin. Furthermore, most of theavailable data indicate that the Late Triassic Qiangtang basin is anindependent one because the Jurassic Qiangtang basin is a compositebasin developed on the basis of the Late Triassic basin (Li et al., 2002;Gao et al., 2006). As a result, it is difficult to understand the history ofsedimentation and tectonism in this basin, especially for the JurassicQiangtang basin.

In May 2006, pre-Nadi Kangri paleo-weathering crusts, from a fewtens of centimeters to N1 m in thickness, were discovered in theQiangtang basin (Fig. 6). In the outcrops, the paleo-weathering crustsare brown to purplish red with crustification and breccia structures.They can be further divided into three broad units based onsedimentary features which are, in ascending order: paleokarstzone, dissolution breccia and weathering claystone layers. Lowerpaleokarst zone with a thickness of 35 cm to 60 cm records agradational transition from base rocks to weathering sequences.Above the paleokarst zone, there occurs a succession of dissolutionbreccias with a thickness of 30 cm to 70 cm. Dissolution breccia zone isoverlain by a succession of weathering claystones. These claystonesare mainly formed of brown and red mottled clays, sandy in someplaces, with a thickness of 3 cm to 10 cm.

Regionally, the paleo-weathering crusts are widespread in theNorth Qiangtang depression, the central uplift and the SouthQiangtang depression (Fu et al., 2007; Wang et al., 2007a). Thepaleo-weathering crusts occur at the top of the Permian strata in thecentral uplift and the South Qiangtang depression, and at the top ofthe Xiaochaka (Middle–Late Triassic) strata in the North Qiangtangdepression, respectively. They are unconformably underlain by theNadi Kangri volcanic rocks, indicating a significant depositional hiatusbetween the Nadi Kangri and underlying strata. Therefore, judgingfrom the basin evolution, the Norian (the Nadi Kangri depositiontime) deposits should be assigned to Jurassic structural/rectonicinterval but not to Carnian structural/rectonic interval (the Xiaochakadeposition time).

5.3. Onset of the Mesozoic Qiangtang basin

As discussed above, the paleo-weathering crusts are unconform-ably underlain by the Nadi Kangri volcanic rocks. Therefore, the NadiKangri volcanic rocks represent a new history of the MesozoicQiangtang basin, and the age of the volcanic rocks represents that ofthe onset of the Mesozoic Qiangtang basin.

The magma emplacement age of the Nadi Kangri volcanic rocks inthe EQ is 220.4±2.3 Ma, which is assigned to the Late Triassic.Regionally, the Late Triassic volcanism is also recognized in the WQ.For instance in the Juhua Mountain area, the single zircon of the NadiKangri volcanic rocks gives a U–Pb age of 225±1 Ma (Fu et al., 2008).Previously we obtained zircon SHRIMP U–Pb ages of the Nadi Kangrivolcanic rocks in the Nadigangri, Shishui River and Woruo Mountainregions. They are 210±4Ma, 208±4Ma and 216±4Ma (Wang et al.,2007b), respectively. In addition, in the Mayer Gangri region of thecentral Qiangtang, the volcanic rocks from theWanghuling Formation(comparable to the Nadi Kangri Formation) give a SHRIMP zircon U–Pb age of 214±4 Ma (Li et al., 2007a). These data show that large-

Fig. 5. Chondrite-normalized REE (a: basalts from the EQ; c: felsic volcanic rocks from the EQ; e: felsic volcanic rocks from the WQ) and primitive mantle-normalized trace element(b: basalts from the EQ; d: felsic volcanic rocks from the EQ; f: felsic volcanic rocks from theWQ) distribution patterns for the Nadi Kangri volcanic rocks in the Qiangtang basin. Datasources: chondrite values are from Sun and McDonough (1989); primitive mantle values are from Pearce (1982); middle (MC) and upper crust (UC) values are from Taylor andMcleman (1985).

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scale volcanic eruption and volcanic–sedimentary events once tookplace in the Qiangtang basin during the Late Triassic. Therefore, theonset of theMesozoic Qiangtang basin should be the Late Triassic time(about 205 Ma–220 Ma).

5.4. Tectonic implications

The felsic rocks from the WQ and the EQ have the broadgeochemical similarities, and display almost the same eruption age.We can conclude that these rocks probably erupt in the same tectonicenvironment. In this paper, we thus select basaltic volcanic rocks fromthe EQ to evaluate the tectonic settings.

Pearce and Norry (1979) showed that the ratio Zr/Y plotted againstthe fractionation index Zr proved an effective discriminant betweenbasalts from ocean island arcs, MORB and within-plate basalts. In thisdiagram, the Nadi Kangri basalts all plot in the field of within-platebasalts or its periphery (Fig. 7a). Additionally, due to the incompatibilityof Th, Ta and Hf, they have the similar behavior during the magmaticprocesses and thus the Th/Hf and Ta/Hf ratios have little change inmantle partial melting and in crystallization differentiation processes.The variations on the Th/Hf vs. Ta/Hf diagram therefore reflect

Fig. 7. Discrimination diagrams of tectonic setting for the Nadi Kangri volcanic rocks.Data sources: ○ Yao et al., 2003; △ this study; Zr–Ti/100-3Y and Zr/Y–Zr diagrams arefrom Pearce and Norry (1979); Th/Hf–Ta/Hf diagram is fromWang et al. (2001). (a) Zr/Y–Zr discrimination diagram: IAB—island-arc basalts; MORB—mid-ocean ridge basalts;WPB—within-plate basalts. (b). Ta/Hf—Th/Hf diagram: I. Plate divergent margin MORB;II. Plate convergent margin basalts (II1. ocean island-arc basalts II2. Continental marginisland-arc+continental margin volcanic-arc basalts); III. Oceanic within-plate basalts(oceanic island+sea mountain basalt+T-MORB+E-MORB); IV. Continental within-plate basalts (IV1. Intracontinental rift+continental margin rift tholeiites IV2.Intracontinental rift alkali basalts IV3. Continental extensional zone/initial rift basalts);V. Mantle plume basalts.

Fig. 6. The images of paleo-weathering crusts showing their characteristics. (a: paleo-weathering characteristics in the trench profile; b: paleo-weathering crusts in theoutcrops).

differences in source composition (Wang et al., 2001b). The Th/Hf vs.Ta/Hf diagram also illustrate that the Nadi Kangri basalts arecharacteristic of within-plate tholeiitic basalts (Fig. 7b).

The within-plate nature of the Nadi Kangri basalts suggests thatthese rocks formed in a rift setting that may be either a continental riftor a back-arc basin (Grajales-Nishimura et al., 1999; Winter, 2001).The Nadi Kangri basalts have high Nb/Zr (0.049–0.058), Ta/Hf (0.12–0.15) and Zr/Y (4.95–6.01) ratios, corresponding to continental riftbasalts (Rollinson,1993;Wang et al., 2001b;Wu et al., 2003; Sun et al.,2007; Xia et al., 2007). Besides the geochemistry of continental riftbasalts, the Nadi Kangri basalts also exhibit the similar geochemistryto the volcanic-arc basalts from their high La/Nb (2.47–3.49), La/Ta(38.3–56.4), Th/Nb (0.98–1.09), and Th/Hf (1.96–2.57) ratios. Apossible explanation for this is that mantle lithosphere was previouslyaffected by subduction processes (Kapp et al., 2003). The Jinsha Riverophiolitic mélange zones are to the north of the Qiangtang basin, and

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represent the subduction setting of the early oceanic basin (Wang etal., 2000). The geochemical similarities to the volcanic-arc basalts aredirectly related to the regional background (Kapp et al., 2000; Kapp etal., 2003). However, occurrences of the Nadi Kangri basalts have nodirect correlation with the island arc, and our argument is based onthe following points. Firstly, as pointed in the geological background,the early sedimentary history of the Mesozoic Qiangtang basin ischaracterized by a progressive extension of the rift. Secondly, the LateTriassic molasse sequences, marking the end of evolutionary historyof subduction along the northern margin of the Qiangtang basin,occurred at ca. 230 Ma (Wang et al., 2000; Li et al., 2003), while themagma emplacement age of the Nadi Kangri volcanic rocks isconcentrated on the range of 220–205 Ma. Thirdly, Wang et al.(2007c) and Pullen et al. (2008) proposed that volcanism derivedfrom subduction-modified mantle was abundant prior to 220 Ma inthe central Qiangtang terrane, while the intracontinental Qiangtangrift started after 220 Ma. Fourthly, there exists a significantdepositional hiatus between the Nadi Kangri and underlying strataas discussed above, with a depositional hiatus time N2 Ma (Zhu et al.,2002). Additionally, the basal conglomerates of the Nadi KangriFormation are poorly rounded, containing eclogite gravels andblueschist fragments derived from underlying beds. Thus, consider-ing the geochemistry, regional tectonic setting and sedimentaryevolution, we conclude that the Nadi Kangri basalts were not formedin an oceanic island setting (e.g., Dewey et al., 1988; Pearce and Mei,1988; Li et al., 1995; Yin and Harrison, 2000; Zhu et al., 2002; Yaoet al., 2004; Bai et al., 2005; Zhai and Li, 2007), but in a continentalrift setting, although the volcanic rocks have similar geochemicalfeatures to OIB.

6. Conclusions

1. The paleo-weathering crusts are unconformably underlain by theNadi Kangri volcanic rocks. Therefore, the Nadi Kangri volcanicrocks offer a new model for the history of the Mesozoic Qiangtangbasin.

2. The magma emplacement age of the Nadi Kangri volcanic rocks inthe Geladaindong area is 220.4±2.3 Ma, which may represent theonset of the Mesozoic Qiangtang basin.

3. The Nadi Kangri basalts have high Nb/Zr (0.049–0.058), Ta/Hf(0.12–0.15) and Zr/Y (4.95–6.01) ratios. Considering the geologicalbackground and sedimentary history, we conclude that the NadiKangri volcanic rocks were formed in a continental rift setting.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (no. 40702020) and National Oil and Gas SpecialProject: Strategic Investigation and Geological Survey on Tibet Oil andGas Resources (XQ200406). We thank two anonymous reviewers fortheir thoughtful comments that led to the improvements of the paper.

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