Miocene post-collisional shoshonites and their crustal xenoliths, Yarlung Zangbo Suture Zone southern Tibet: Geodynamic implications

Gondwana Research 25 (2014) 1263–1271

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Miocene post-collisional shoshonites and their crustal xenoliths,Yarlung Zangbo Suture Zone southern Tibet: Geodynamic implications

Réjean Hébert a,⁎, Carl Guilmette a, Jaroslav Dostal b, Rachel Bezard a, Guillaume Lesage a,Émilie Bédard a, Chengshan Wang c

a Département de géologie et de génie géologique, Université Laval, Québec, QC G1K 7P4, Canadab Department of Geology, Saint Mary's University, Halifax, NS B3H 3C3, Canadac Research Center for Tibetan Plateau Geology, China University of Geosciences, 29 Xueyuan Road, Haidian District, 100083 Beijing, China

⁎ Corresponding author. Tel.: +1 418 656 3137; fax:E-mail address: [email protected] (R. Héb

1342-937X/$ – see front matter © 2013 International Ahttp://dx.doi.org/10.1016/j.gr.2013.05.013

a b s t r a c t

Article history:Received 19 June 2012Received in revised form 23 March 2013Accepted 13 May 2013Available online 10 June 2013

Handling Editor: Z.M. Zhang

Keywords:TibetShoshoniteGeochemistryYarlung Zangbo Suture ZoneOphiolite

The convergence between the Indian plate and the southern margin of the Eurasian continent created an ac-tive continental margin from Late Jurassic until about 40 Ma ago, which then evolved to form the Himalayaand the Tibetan Plateau during the continental collision stage. Post-collisional magmatism in southern Tibet,north of the Yarlung Zangbo Suture Zone (YZSZ) has been active since 45 Ma and is related to normal faultingand extensional tectonism. To date no such magmatism was reported within the YZSZ itself. This paper reportson the discovery of Miocene shoshonites within the YZSZ. They are significant because the magma traveled, atleast in part, through oceanic crust, thus limiting interaction with the continental crust to the mid-crustal leveland which affected the post-collisional magmatic rocks occurring in the northern part of the subduction system.In addition, xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocksunderlying the YZSZ at mid-crustal level. The geochemical signatures of the shoshonitic rocks, including Nd andSr isotope systematics, indicate derivation from a garnet-bearing middle continental crustal source. Crustal im-print complicates modeling of the petrogenetic processes which occurred prior to mid-crustal ponding of themagmawhich took place between 11 and 17 Ma at depths of 40 to 50 km. The significant role of crustal contam-ination raises serious concerns about models proposed for similar magmatic activity elsewhere in the Himalayaand the Tibetan Plateau.

© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The continuing collision of the Indian plate with the southernmarginof the Eurasian continent over the past 50 Ma provides the best modernexample of continental collision plate tectonics (Malusky et al., 1982;Allègre et al., 1984; Schärer et al., 1984; Coleman and Hodges, 1995;Debon et al., 1986; Chatterjee et al., 2013). More specifically, the TibetanPlateau plays a key role in various models for the evolution of mountainbelts and Cenozoic climate change (Molnar and Tapponnier, 1975;Chung et al., 1998, 2009; Miller et al., 1999; Hou et al., 2004). However,despite numerous studies (e.g. Allègre et al., 1984; Coleman andHodges, 1995 see Hébert et al., 2012 for a review), the processes respon-sible for the formation of the plateau are still controversial. In fact, eventhe timing of the collision of the plates in southern Tibet which formedthe east–west trending Yarlung Zangbo Suture Zone (YZSZ) is a matterof debate (Allègre et al., 1984; Harrison et al., 1992; Yin et al., 1994;Yin and Harrison, 2000; Aitchison and Davis, 2004; Dupont-Nivet et al.,2010; Najman et al., 2010; Hu, 2011; Guan et al., 2012). The continentalcollision started in the Early Tertiary (~55 Ma) to the west (Pakistan)

+1 418 656 7339.ert).

ssociation for Gondwana Research.

and spread toward the east but not later than 30 Ma. Marine sedimentsin southern Tibet give a maximum age for the presence of theNeo-Tethys basin at 37 Ma (Wang et al., 2002). Recent studies onLiuqu conglomerate (Wang et al., 2010) suggest that erosion of theHimalaya–Tibet orogeny started as early as Middle Eocene. Subsequent-ly, since the Miocene, the extensional tectonic regime led to the devel-opment of north–south-trending graben and a normal fault networkand triggered ultrapotassic to potassic magmatism (Chan et al., 2009;Chung et al., 2009 and references therein). Post-collisional magmatismon the Tibetan Plateau to the north of the YZSZ provides a potentially im-portant window into the thermal and compositional regimes of the deepparts of the India–Asia orogen and of the Tibetan Plateau in particular(Hébert et al., 2007; Lesage et al., 2008; Bezard et al., 2009). However,since these magmas penetrated through a thick continental crust theywere likely contaminated by crustal material, which has some geochem-ical features similar to those of post-collisional magmatism (Platt andEngland, 1994; Chan et al., 2009; Zhao et al., 2009). In fact, the geochem-ical characteristics of the post-collisional magmatic rocks bear a strongcrustal imprint which could hamper petrogenetic modeling needed toconstrain the composition of their mantle sources.

Here we report the first discovery of the post-collisional Miocene(11.8–17.2 Ma) intrusions emplaced within the YZSZ. Since the magma

Published by Elsevier B.V. All rights reserved.

1264 R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

intruded through, during at least part of its ascent, and crystallizedwith-in oceanic crust, the interaction with continental crust was more limitedwhen comparedwith other Tibetan post-collisional intrusions, making itpotentially easier to recognize various source components involved intheir genesis. Moreover, these magmas carried crustal xenoliths andxenocrysts sampled during their ascent to the surface, constraining thenature of the metamorphic rocks underlying the YZSZ at the time ofthe intrusion and potentially the source of the magmas. Similar Miocenetrachytic rocks were reported in the Xigaze fore-arc flysch and LhasaBlock by Chan et al. (2009) and Chen et al. (2012) respectively.

2. Methodology

From 2006 to 2009, 80 samples from nine different locations werecollected during field missions to Tibet in the Saga and Sangsang areas.Sampleswere selected according to their unaltered condition, xenocrysticand xenolithic contents. Polished thin sections (17) were analyzed byusing a five-spectrometer Cameca SX-100 microprobe at UniversitéLaval (Québec, Canada), with analytical conditions as reported in Hébertet al. (2003). Samples selected for geochemical analyses (19 analyses)were carefully screened to avoid alteration or contamination by xenolithsor xenocrysts andwere prepared for analyses ofmajor and some trace el-ements (Ba, Sr, Rb, Zr, Y, Cr, Ni, Co, V, and Zn) at the Regional GeochemicalCenter of St. Mary's University (Halifax, Canada) by X-ray fluorescenceaccording to the procedure described in Dostal et al. (1986). Additionaltrace elements (rare earths (REE), Hf, Ta, Nb, Th) were analyzed byinductively coupled plasma-mass spectrometry (ICP-MS) using aNa2O2-sintering technique at the Department of Earth Sciences ofthe Memorial University of Newfoundland (St. John's, Canada). Theprecision for the trace elements is between 2 and 8% of the valuescited. Samples were also selected for Nd and Sr isotopic analyses atthe Department of Earth Sciences of Memorial University. Sm andNd abundances and Sr- and Nd-isotope ratios were determined by

Fig. 1. Simplified geological map of the interface between Eura

isotope dilution mass spectrometry. Measured 143Nd/144Nd valueswere normalized to a 146Nd/144Nd ratio of 0.7219. An average valuefor 143Nd/144Nd = 0.511849 ± 9 resulted from replicate analysesof the LaJolla standard, which was analyzed repeatedly throughout.The 87Sr/86Sr was corrected using 86Sr/88Sr = 0.1194. The NBS 987 Srstandard gave an average value of 87Sr/86Sr = 0.710250 ± 11 onreplicate analyses. Six magmatic biotite and amphibole separateswere carefully selected by hand-picking for geochronologic datingby 39Ar/40Ar laser single-grain fusion method at Isotope Laboratory ofUniversity of British Columbia (Vancouver, Canada). Step laser heatingmethod was used to determine plateau ages (2σ) comprising at least59.3% of released 39Ar. Plateau ages include J-error of 5%mean standardweighted deviation 0.45–1.6with probability varying between 0.26 and0.95.

3. Background geology and mineralogy

The studied igneous rocks are exposed in at least nine localitiesalong the ophiolitic beltwhichmarks theYZSZ, in the Saga andSangsangareas about 600 and 500 km west of Lhasa, respectively (Fig. 1). Theserocks form dykes and sub-circular intrusions, from a few cm up to50 m in diameter, cutting through the ophiolitic upper mantle atSangsang and the ophiolitic mélange and ophiolitic crust at Saga(Fig. 2A; Bédard et al., 2009; Hébert et al., 2012; Guilmette et al.,2012). They do not show any systematic orientation nor relationshipwith the surrounding rocks. The porphyritic felsic igneous rocks containubiquitous phenocrysts, xenocrysts, and xenoliths. The magmatic phe-nocrysts comprise needle-shaped to idiomorphic brown amphibole(Ti-ferroan pargasite, Ti-edenitic hornblende, Ti-ferropargasitic horn-blende), F-bearing biotite (Mg# 0.3–0.6; F up to 2.56 wt.%), K-feldspar(Or85–61), normally zoned plagioclase (An77–40) partially replaced by al-bite (An 0–4), pleonaste (Al2O3 55 wt.%, FeO26 wt.%, Fe2O3 8.6 wt.%) andpossibly some garnet (Al79–59Gr5–26Sp5–12Py11–3) which are set in a

sia and India continents. Modified after Mo et al. (2006).

Fig. 3. 39Ar/40Ar dating of magmatic biotite and amphibole separates in shoshonites from Saga and Sangsang areas, Tibet. Age spectra and inverse isochron ages are shown. Plateauages result from step heating methodology.

2 mm5 cm

A B C

Fig. 2. (A) Field occurrence of Miocene shoshonite cutting ophiolitic upper mantle in Sangsang complex; (B) Examples of crustal xenoliths, here garnet-bearing paragneisses,embedded in fine-grained shoshonite. Xenioliths are lined by amphibole–biotite–magnetite dark rim. Magnet is 3 cm in size; (C) Examples of zoned magmatic amphiboles in athin section cut in shoshonite sample 07-200F. Larger amphibole grain is 1.5 mm in diameter.

1265R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

Table 1Major and trace element compositions of shoshonitic rocks from the Yarlung Zangbo Suture Zone.

Sample 07-SA-21 07-SA-26A 07-SA-26B 07-SG-03 07-SG-04A 07-SG-05 07-SG-06 07-SG-29 07-SG-30

Trachy- Andesite Andesite Andesite Andesite Andesite Andesite Andesite Dacite Dacite

Mg# 61.58 51.29 48.49 47.60 53.19 51.49 52.62 47.12 47.10SiO2 59.03 60.13 59.34 54.82 59.92 60.57 58.90 66.66 67.06TiO2 0.92 0.91 0.89 0.76 0.77 0.79 0.80 0.59 0.58Al2O3 15.86 16.48 16.34 14.93 15.42 16.09 15.98 15.07 15.12Fe2O3 5.57 4.53 4.42 4.29 4.37 4.83 4.51 3.49 3.42MnO 0.08 0.05 0.05 0.06 0.06 0.07 0.07 0.04 0.04MgO 4.51 2.41 2.10 1.97 2.51 2.59 2.53 1.57 1.54CaO 3.61 4.86 5.04 11.30 5.02 4.08 4.17 1.32 2.12Na2O 4.94 8.44 8.34 4.28 5.46 3.86 6.01 4.05 3.40K2O 2.46 0.10 0.07 3.27 4.22 3.45 4.77 4.84 4.73P2O5 0.34 0.31 0.31 0.28 0.29 0.31 0.30 0.22 0.24LOI 2.30 2.70 3.60 3.80 2.10 3.30 1.60 2.10 1.30Cr 178 16 27 25 29 33 25 54 61Ni 127 24 14 12 9 17 9 14 18Co 22 11 15 10 15 16 10 14 8V 102 94 86 93 82 98 86 71 59Pb 36 18 9 6 52 26 38 40 24Zn 84 77 81 49 53 73 72 56 50Rb 49 3 0 151 138 132 159 176 181Ba 854 147 244 860 803 880 948 1288 1229Sr 968 453 1048 1498 295 1061 467 1177 935Ga 20 17 17 19 21 22 19 18 20Ta 0.330 0.340 0.330 0.770 0.880 0.760 0.810 0.570 0.518Nb 4.979 6.356 5.866 12.586 13.827 12.514 13.056 9.854 9.570Hf 4.859 4.493 4.520 5.106 6.080 4.520 5.030 5.050 5.250Zr 182.406 176.927 174.208 202.763 214.917 183.000 198.000 210.000 220.000Y 11.365 6.989 6.772 10.380 10.813 11.936 10.786 12.529 11.993Th 9.311 4.101 4.009 28.669 31.518 29.059 30.025 31.928 32.577La 40.417 27.440 26.438 59.974 63.228 69.225 63.134 59.764 62.761Ce 81.636 55.881 53.583 113.866 120.264 131.116 120.230 113.875 120.213Pr 9.980 6.863 6.560 12.556 13.331 14.459 13.195 13.140 13.892Nd 39.319 26.830 26.305 45.689 49.025 53.110 48.778 48.938 51.267Sm 6.616 4.323 4.275 6.855 7.438 8.242 7.591 7.645 8.001Eu 1.484 1.150 1.117 1.360 1.560 1.765 1.651 1.512 1.556Gd 4.358 2.934 3.036 4.596 5.059 5.331 5.125 4.610 4.839Tb 0.502 0.320 0.361 0.520 0.601 0.634 0.595 0.581 0.570Dy 2.586 1.620 1.699 2.602 2.855 3.019 2.758 2.827 2.735Ho 0.414 0.218 0.244 0.394 0.456 0.494 0.433 0.452 0.429Er 1.061 0.481 0.593 0.939 1.097 1.338 1.049 1.138 1.095Tm 0.150 0.048 0.075 0.105 0.164 0.153 0.139 0.156 0.140Yb 0.989 0.281 0.396 0.724 0.822 0.974 0.844 0.948 0.900Lu 0.151 0.044 0.058 0.110 0.134 0.156 0.121 0.136 0.114K2O/Na2O 0.498 0.012 0.009 0.764 0.773 0.893 0.794 1.195 1.392La/Lu 267.662 623.636 455.828 545.218 471.851 443.750 521.769 439.441 550.535La/Yb 40.867 97.651 66.763 82.837 76.920 71.073 74.803 63.042 69.734La/Ce 0.495 0.491 0.493 0.527 0.526 0.528 0.525 0.525 0.522Nb/La 0.123 0.232 0.222 0.210 0.219 0.181 0.207 0.165 0.152Th/La 0.230 0.149 0.152 0.478 0.498 0.420 0.476 0.534 0.519Th/Ta 28.215 12.062 12.148 37.232 35.816 38.236 37.068 56.014 62.890Ce/Pb 2.268 3.105 5.954 18.978 2.313 5.043 3.164 2.847 5.009Ce/Sr 0.084 0.123 0.051 0.076 0.408 0.124 0.257 0.097 0.129Ba/Sr 0.882 0.325 0.233 0.574 2.722 0.829 2.030 1.094 1.314Rb/Sr 0.051 0.007 0 0.101 0.468 0.124 0.340 0.150 0.194Rb/Ba 0.057 0.020 0 0.176 0.172 0.150 0.168 0.137 0.147Sr/Nd 24.619 16.884 39.840 32.787 6.017 19.977 9.574 24.051 18.238Ba/Nb 171.520 23.128 41.596 68.330 58.075 70.321 72.610 130.708 128.422Sr/Y 85.174 64.816 154.755 144.316 27.282 88.891 43.297 93.942 77.962Zr/Y 16.050 25.315 25.725 19.534 19.876 15.332 18.357 16.761 18.344Ti/Y 0.057 0.090 0.092 0.053 0.050 0.048 0.052 0.033 0.034

07-SG-31A 07-SG-31B 07-SG-48A 07-SG-48B 07-SG-52A 07-SG-66 06-SA-28C 06-SA-48A 06-SG-200 06-SG-201

Dacite Dacite Dacite Dacite Andesite Dacite Andesite Andesite Andesite Andesite

1266 R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

fine-grained matrix mostly made of quartz, K-feldspar and albite–oligoclase with minor apatite and zircon. Amphiboles are locallyzoned, and show a decrease in Al and Ti towards the rim (Fig. 2C).Xenocrystic red garnets (Al67–74Gr5–7Sp0.4–2Py28–19) are surroundedby a coronitic magnetite–biotite intergrowth. The crustal xenoliths aregneisses, schists, quartzite, shales, and calc-silicates all surroundedby amphibole–biotite–magnetite less than 1 mm thick reaction rims(Fig. 2B). Among the xenoliths, the study has been focused on the gneissicones since they show the highest metamorphic grade and, thus thedeepest crustal paragenetic history.

The gneissic xenoliths are composed of various combinations of kya-nite, rutile, plagioclase (andesine to bytownite), pleonaste, sillimanite, co-rundum, brown biotite, quartz, and garnet (Al37–77Gr10–22Sp5–7Py6–32).Although these assemblages are not suitable for extensive geother-mometric and geobarometric calculations, the metamorphic paragenesesand composition of the minerals of the xenoliths, suggest that metamor-phism took place at a depth of about 30–40 km and at a temperature of600–700 °C (Bezard et al., 2009). These results represent a lower P–Tspace portion of high-grade metamorphism and lower depths of extrac-tion than reported for high-grade aluminous xenoliths by Hacker et al.

Table 1 (continued)

07-SG-31A 07-SG-31B 07-SG-48A 07-SG-48B 07-SG-52A 07-SG-66 06-SA-28C 06-SA-48A 06-SG-200 06-SG-201

Dacite Dacite Dacite Dacite Andesite Dacite Andesite Andesite Andesite Andesite

47.90 48.17 44.18 44.01 59.48 49.20 63.56 48.04 56.34 49.3367.28 65.98 65.13 64.88 58.09 65.81 57.20 57.25 57.80 59.200.56 0.58 0.73 0.75 0.95 0.53 0.92 0.88 0.86 0.78

14.86 14.82 16.57 16.70 15.72 16.25 15.39 15.94 17.38 16.153.06 3.17 3.43 3.53 6.14 3.17 5.73 4.41 4.77 4.780.04 0.04 0.04 0.05 0.09 0.04 0.08 0.05 0.06 0.071.42 1.49 1.37 1.40 4.55 1.55 5.05 2.06 3.11 2.351.51 1.97 2.63 2.45 5.42 3.03 3.93 5.91 3.28 4.574.04 3.61 4.40 4.43 2.97 4.28 4.57 7.28 3.47 3.164.52 4.57 3.86 3.97 3.27 3.66 3.03 0.10 4.06 3.390.20 0.22 0.29 0.30 0.37 0.21 0.27 0.30 0.32 0.302.60 3.50 0.90 1.00 2.80 0.90 3.20 4.90 3.80 4.10

38 37 20 24 163 24 167 24 22 2512 11 36 13 103 10 97 68 54 296 11 9 6 24 10 22 11 12 14

59 50 64 70 131 70 125 82 93 9539 36 28 42 49 75 46 14 27 1641 43 61 71 78 75 86 80 64 61

201 186 135 138 135 159 48 0 183 1361131 1162 1112 1152 925 977 703 243 934 866690 627 1034 954 838 884 262 941 969 97320 21 22 22 18 22 15 14 22 210.612 0.584 0.610 0.310 0.610 0.320 0.430 0.430 0.990 0.7909.910 10.066 10.268 4.952 10.431 5.122 8.270 7.520 15.699 13.2244.980 5.530 4.064 4.647 6.006 3.927 3.730 3.650 5.560 5.013

204.000 214.000 186.317 178.198 230.000 148.000 151.000 153.000 223.000 207.24211.961 12.873 8.554 8.678 14.143 6.368 10.500 6.300 12.472 13.45832.801 33.561 22.096 21.817 32.910 9.942 4.860 3.510 35.142 31.02156.163 59.828 52.382 53.387 58.780 25.694 23.680 21.690 72.531 69.778

108.779 115.402 102.956 104.239 116.870 49.787 48.620 43.700 137.125 128.85412.429 13.219 11.855 12.079 13.920 5.966 6.270 5.460 15.012 14.39046.581 49.837 44.089 45.655 52.940 23.243 25.990 22.300 53.077 52.4397.720 8.050 6.499 6.973 8.855 4.080 4.630 3.770 8.656 8.4071.471 1.566 1.283 1.304 1.850 0.918 1.190 0.950 1.758 1.7624.797 4.907 3.739 3.765 5.585 2.646 3.390 2.700 5.214 5.1880.602 0.599 0.425 0.431 0.663 0.303 0.436 0.314 0.631 0.6492.874 2.924 2.009 2.036 3.446 1.460 2.410 1.500 2.904 3.0810.474 0.479 0.299 0.325 0.585 0.234 0.397 0.232 0.460 0.4961.275 1.257 0.724 0.794 1.550 0.597 1.091 0.545 1.152 1.2980.169 0.190 0.088 0.104 0.225 0.077 0.132 0.069 0.149 0.1741.050 1.110 0.567 0.682 1.385 0.419 0.924 0.423 0.900 1.0610.160 0.176 0.082 0.094 0.203 0.057 0.129 0.057 0.135 0.1641.118 1.265 0.877 0.895 1.101 0.855 0.663 0.014 1.171 1.072

351.019 339.932 638.805 567.947 289.557 450.772 183.566 380.526 537.267 425.47653.489 53.899 92.384 78.280 42.440 61.322 25.628 51.277 80.590 65.7660.516 0.518 0.509 0.512 0.503 0.516 0.487 0.496 0.529 0.5420.176 0.168 0.196 0.093 0.177 0.199 0.349 0.347 0.216 0.1900.584 0.561 0.422 0.409 0.560 0.387 0.205 0.162 0.485 0.445

53.596 57.467 36.223 70.377 53.951 31.069 11.302 8.163 35.497 39.2672.789 3.206 3.677 2.482 2.385 0.664 1.057 3.121 5.079 8.0530.158 0.184 0.100 0.109 0.139 0.056 0.186 0.046 0.142 0.1321.639 1.853 1.075 1.208 1.104 1.105 2.683 0.258 0.964 0.8900.291 0.297 0.131 0.145 0.161 0.180 0.183 0 0.189 0.1400.178 0.160 0.121 0.120 0.146 0.163 0.068 0 0.196 0.157

14.813 12.581 23.453 20.896 15.829 38.033 10.081 42.197 18.256 18.555114.127 115.438 108.298 232.633 88.678 190.746 85.006 32.314 59.494 65.48757.687 48.707 120.879 109.933 59.252 138.819 24.952 149.365 77.694 72.29917.055 16.624 21.781 20.534 16.262 23.241 14.381 24.286 17.880 15.3990.034 0.033 0.061 0.061 0.048 0.059 0.063 0.101 0.051 0.043

1267R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

(2000). The Saga and Sangsang intrusive rocks contain carbonate-filledmiarolitic cavities, which are not present in adakitic (Hou et al., 2004;Wang et al., 2008) and ultrapotassic rocks (Qu et al., 2004) found northof the suture.

4. Geochronology

40Ar–39Ar age datawere obtained onmagmatic amphibole and biotiteseparates. Plateau age determinations span from 11.8 ± 0.08 Ma to17.2 ± 2.1 Ma (Fig. 3). Among these results, the best defined plateau

(hornblende 06SE-201) results from 8 step heating points (Fig. 3). Itgives an age of 13.87 ± 0.87 Ma with an isochron inverse regression of13.4 ± 1.1 Ma. These results point to a Middle Miocene age, implyingthat the shoshonites are the youngest igneous rocks ever reported withinthe YZSZ.

5. Geochemistry

The analyzed rocks typically contain 60–70 wt.% SiO2 (Table 1) andplot into trachyandesite and trachydacite fields on a Na2O + K2O vs.

K2O

Na2O(wt.%)

KO

/Na

O =

2

2

2

K O/Na O = 0.5

2

2

Shoshonitic

Ultra-Potassic

Calc-Alkaline

Localities 1 and 2

Locality 3

Localities 5 to 8

Locality 9

(wt.%)

Xungba(Miller et al., 1999)

Shiquanhe+Maquiang(Turner et al., 1996)

3 4 5 6 7 8 9210

1

2

3

4

5

6

7

8

9

10

Fig. 5. K2O vs. Na2O classification diagram after Foley et al. (1987). Two samples from lo-calities 1 and 2 lost their K through lixiviation. Fields of Xungba and Shiquanhe-Maquianglocations are shown for comparison. See text for interpretation.

1268 R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

SiO2 diagram (Fig. 4; Le Maître, 1987) whereas they belong to theshoshonitic clan in terms of K2O vs. Na2O (Fig. 5) and K2O vs. SiO2 rela-tionships and to ultrapotassic types in terms of CaO vs. Al2O3. Thetrachyandesites and trachydacites, however, have lower K2O/Na2O ra-tios (0.7–1.2; Fig. 5) than 3 Ma shoshonitic trachybasalts and andesitesreported by Hacker et al. (2000). According to the classification ofultrapotassic rocks, the trachyandesites and the trachydacites are simi-lar to group III orogenic ultrapotassic series (Foley et al., 1987; Copelandet al., 1995). Samples 07-SA-21 and 07-SA-28C have the highest MgOand Cr content of the rock collection.

On a multi-element diagram, the rocks have high contents of Ba,Ce, La, Rb, Sr, and Zr (Table 1 and Fig. 6) but the concentrations of theseelements are lower than those reported for shoshonitic volcanics fromsouthern and northern Tibet (Turner et al., 1993, 1996; Mo et al., 2006).The highest Cr (167 ppm) and Ni (97 ppm) contents correlate withMgO (samples 07-SA-21, 07-SA-52A, 06-SA-28C, Table 1). The rocksshow large variations in Ba/Nb (19–85), Rb/Ba (0.07–0.26) and Sr/Y(27–149). On primitive mantle-normalized and chondrite-normalizedREE diagrams (Figs. 6 and 7; Table 1) show strongly fractionated patterns(LaN = ~90–300) similar to other potassic and ultrapotassic igneousrocks from the Tibetan Plateau (Turner et al., 1996; Miller et al., 1999).However the patterns forMiocene YSZS shoshonites are less fractionatedthan ultrapotassic rocks from SW Tibet (Xungba, Fig. 7) and Shiquanhe(Turner et al., 1996; Miller et al., 1999).

Two types of the mantle-normalized patterns can be distinguished:1. Samples (e.g. 06-SA-28C and 06-SA-48A) with lower trace elementabundances and a moderate negative Nb–Ta anomaly; 2. Samples(e.g. 06-SG-200, 06-SG-201, 07-SA-26A, 07-SG-52A) which havehigher trace element abundances, a larger negative Ta–Nb anomalyand a significant negative Zr–Hf anomaly. These characteristics ofthe shoshonitic trachyandesites and trachydacites are similar tothose of post-collisional volcanic rocks from the Lhasa Block in SWTibet and Maquiang (Fig. 7; Turner et al., 1996; Miller et al., 1999)but they differ in terms of, among others, Ti, Sr and Ba contents.Low Ce/Pb (1.1–8.1) and Ce/Sr (0.05–0.18) but high Sr/Nd (4.6–42)and Th/La (0.16–0.48) suggest that an OIB mantle component wasnot involved in the genesis of the trachyandesites and trachydacites(Turner et al., 1996; Qu et al., 2004). High Ti/Y, Rb/Sr, La/Ce and Th/Laand low Rb/Ba values indicate a significant contribution from thecontinental crust compared to the oceanic slab (Turner et al., 1993,1996; Hou et al., 2004). Fig. 8 shows close similarities with less than13 Ma K lavas reported by Turner et al. (1993). The cluster of La/Ceand Rb/Sr values is adjoining the box defined for crustal source.

These geochemical characteristics suggest that the shoshonitictrachyandesites and trachydacites are mostly derived from post-collisional partial melting of the middle crust with very little lower

Trachydacite

Linzizong volcanics& Gangdese batholith

K-rich magmatic rocks

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

(wt.%

)N

a 2O

+K

2O

SiO2(wt.%)

Picro-basalt

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

Trachy-andesite

Basaltictrachy-andesiteTrachy-

basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

Localities 1 and 2

Locality 3

Localities 5 to 8

Locality 9

Fig. 4. Na2O + K2O vs. SiO2 classification diagram after Le Maître (1987).

crustal material input. A very high K/Nb ratio (2100–3000) associatedwith high Rb/Sr and La/Ce suggests that phlogopite or K-feldspar waspresent in the source (Foley et al., 1987) or that mid-crustal contam-ination was important (Figs. 6 and 8; Hou et al., 2004). Lower Sr/Yand εNd ratios and higher 87Sr/86Sr isotopic ratios are consistentwith larger contributions of mid-crustal components to the formationof Saga and Sangsang shoshonites than reported for 38 Ma Wolongadakitic granitoids by Guan et al. (2012). On the other hand, theyshow strong similarities with Miocene trachytes form Bugasi areareported by Chen et al. (2012) although we present a different modelfor the generation of Saga and Sangsang shoshonites.

In order to test our hypothesis, six representative samples wereanalyzed for Nd and Sr isotopic ratios (Table 2; Fig. 9). Initial Ndisotopic ratios vary from 0.512158 to 0.512430 and correspondingεNd from −3.38 to −9.19 (Saga ophiolitic basaltic sample 07-SA-25

Fig. 6. Primitive-mantle normalized multi-element diagram for shoshonitic rocks fromthe Yarlung Zangbo Suture Zone. Normalizing values are from Sun and McDonough(1989). Shiquanhe, Maquiang ultrapotassic rocks of Turner et al. (1996) and potassic,ultrapotassic and high-K calc–alkaline rocks of Miller et al. (1999) of Tibet Plateauare added for comparison. LCC = Lower Continental Crust, MCC = Middle ContinentalCrust and UCC = Upper Continental Crust average values are from Rudnik and Gao(2004). Samples from localities 1 and 2 show strong leaching of their Th, K, Ba andRb contents.

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Ultra-K SW-Tibet

Potassic and High-K calc-alkalineMiller (1999)

Maquiang (Turner 1996)Shiquanhe (turner 1996)

Locality 3

Localities 5 to 8

Locality 9

Localities 1 and 2

Roc

k/C

hond

rite

1

10

100

1000

Fig. 7. Chondrite normalized REE patterns for shoshonites from Saga and Sangsangareas, Tibet. Some patterns are distinct from other K-rich lavas with respect to lowerHREE contents reported by Miller et al. (1999) and Turner et al. (1996).

1269R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

with depleted mantle εNd of +8.65 is shown for comparison). Modelage calculations suggest that Nd depletion of the mantle occurred inthe Proterozoic from 865 to 1125 Ma (Table 2). Negative εNd valuesandmodel age values confirm that the significant contribution of crustalmaterial is coming from a Precambrian reservoir. Initial Sr isotopicratios vary from 0.707252 to 0.712118, being less radiogenic in Sagathan Sangsang. Such high Sr ratios rule out a substantial contributionfrom a MORB-type subducted slab and mantle which has a value of0.703 as proposed by Defant and Drummond (1990). These ratiosare accordingly higher than those for mantle-derived adakites fromsouthern Tibet (Hou et al., 2004). The range for εNd values and modelages suggests that the source material of the trachyandesites is fromthe middle crustal level of the Asian plate rather than the Indian platewhich is characterized by high negative εNd values (e.g. −11 to −18;Miller et al., 1999; Turner et al., 1993; Turner et al., 1996; Zhao et al.,2009; King et al., 2007, 2011; Fig. 9) and recently confirmed by Hu(2011). These results are consistent with the depth of generation ofmagmatic amphiboles and the provenance of crustal xenoliths. Accordingto the composition of zoned magmatic amphibole, phenocrysts werelikely formed at a depth of about 24–30 km. This is in agreement withgeothermobarometric calculations by using GASP barometer andgarnet–biotite thermometer computed with version 2.32 of winTEEWQ2007 revised from Berman (1991). Bezard et al. (2009) reported meta-morphic peak equilibrium conditions for xenoliths 07-SG-48L of 600 °C.

Fig. 8. La/Ce vs. Rb/Sr diagram for shoshonites from Saga and Sangsang areas. These ra-tios form a cluster akin to K-lavas reported by Miller et al. (1999) and crustal sourceaverage composition. Two samples from locations 1 and 2 show low Rb/Sr ratios dueto selective leaching of Rb but unaltered La/Ce ratios.

6. Discussion

Fig. 6 shows the average compositions of lower (LCC), middle(MCC) and upper continental crust (UCC). The trace element patternsof theMiocene Saga and Sangsang shoshonites display a striking resem-blance to that of MCC. Low HREE contents suggest garnet-bearingsources. The Sangsang samples could be generated by a lower degreeof partial melting or derived from more enriched sources than theSaga samples. Negative anomalies of Nb, Ta, and Ti on the mantle nor-malized plots (Fig. 6) and the values of these elements close to unitysuggest that the parent magma inherited a significant proportion ofcontinental mid-crust material. This is corroborated by high Rb/Baratios which are close to MCC values of 0.2 rather than those ofLCC (0.02–04; Zhao et al., 2009). In addition, the garnet-, kyanite-,andesine–bytownite (An30-60), and rutile-bearing xenoliths associatedwith corundum–pleonaste–sillimanite–plagioclase–Al–spinel and thegarnet and kyanite xenocrysts help to further constrain the sourcecomponents involved in the magmatic processes. The almandite-rich(Al59–79) and pyrope-poor (Py3–11) compositions and their idiomorphiccrystal shapes suggest that these garnets could bemagmatic and relatedto the petrogenesis of the trachyandesites and the trachydacites(Hawkesworth et al., 1990). However, corroded Al67–79 Py28–19 garnetssurrounded by reaction rims are probably xenocrystic andmight be de-rived from the source area. This recalls highly fractionated REE of theshoshonitic rocks with their low YbN (2.6–6.4) and the high (La/Yb)Nof 17.9–55.6. Such (La/Yb)N ratios and YbN values suggest the presenceof 8–25% garnet in the mafic amphibolitic or even eclogitic source(Rudnik and Gao, 2004; Guan et al., 2012). Relatively high MgO, Cr, Niand Co contents support this hypothesis. Xenoliths of schists, gneissesand garnet xenocrysts are surrounded by millimeter-wide coronitic re-action assemblages of magnetite, amphibole and biotite. This reactionzone suggests that interaction under oxidizing and wet conditions oc-curred during themelting process or the ascent of themagma. NegativeEu anomalies, high Th/Ta and lowSr/Nd and Ce/Pb ratios are also consis-tent with the involvement of mid-continental crust (and lower crust?)in the genesis of the shoshonitic magma. It is therefore suggested thatthe magma was generated in the crustal garnet stability zone in agree-ment with the observation of garnet xenocrysts, the highly fractionatedREE patterns, and high Sr/Y, Zr/Y and Ti/Y ratios. The aluminous miner-alogy (such as kyanite, sillimanite, pleonaste) of the xenoliths suggeststhat partial melting of aluminous metamorphic continental crust wasinvolved in producing such a refractorymineral assemblage. Our resultsshow that partial melting of the Tibetan deep crust was already ongoingat least by 17 Ma. The ascending magma was probably mixed and con-taminated by partially melted garnet gneiss and/or phlogopite/biotitegarnet schist components. Thismiddle crustalmelt overprinted the geo-chemical characteristics of the primitive magma, likely of lithosphericmantle or lower crust origin. In addition, source heterogeneity can beinvoked to explain some geochemical variations in the Miocene Sagaand Sangsang shoshonitic magmatism (Erlank et al., 1987). Similar re-sults were obtained, though reflecting a hotter portion of the P–T spaceof metamorphism of the continental crust, by Hacker et al. (2000) forrocks in northern Tibet who suggested interaction between shoshoniticmagma with mid-crustal rocks.

7. Conclusion

Thepresence of bothmantlemelts and crustalmelts at depth south ofthe Gangdese belt under the YZSZ in the Middle Miocene has an impacton further modeling of crustal thermal structure and behavior. Theshoshonites and their transported xenoliths and xenocrysts provide aunique window into the deep metamorphic stack of the Tibetan Plateauunderlying the YZSZ. P–T conditions indicated by the xenoliths andmag-matic amphiboles, the εNd values, Nd model age, the Sr initial isotopicratios as well as their geochemical compositions imply that even thoughthe shoshonitic rocks were emplaced into a mafic/ultramafic host rocks,

Table 2Sm–Nd and Rb–Sr isotopic compositions for eight shoshonitic rocks from the Yarlung Zangbo Suture Zone.

Sample no. 06-SG-200 07-SA-66 07-SA-21 07-SG-48B 07-SG-31B 07-SA-26A 07-SG-52A 07-SA-25

Nd (ppm) 52.56 22.91 39.51 44.14 47.78 26.16 52.04 7.988Sm (ppm) 8.261 4.029 6.601 6.782 7.735 4.453 8.783 2.791143Nd/144Ndm 0.512167 (5) 0.512358 (5) 0.512439 (5) 0.512339 (5) 0.512298 (4) 0.512419 (4) 0.512305 (5) 0.513083 (5)147Sm/144Nd 0.095 0.1063 0.101 0.0929 0.0979 0.1029 0.102 0.2113εNd −9.01 −5.3 −3.71 −5.65 −6.46 −4.1 −6.33 8.65Nd/Ndi 0.512158 0.512348 0.51243 0.51233 0.512289 0.51241 0.512296 0.513064εNdpres −9.19 −5.46 −3.38 −5.83 −6.63 −4.27 −6.5 8.68TDM 1 1125 976 824 893 983 865 1010TDM2 1262 1124.5 962 1024 1122 1006 1153Rb (ppm) 183 49 138 186 3 135Sr (ppm) 969 968 954 627 453 83887Sr/86Srpres 0.711453 0.707283 0.709154 0.712301 0.708139 0.7113722-σ 0.000018 0.000011 0.00001 0.000011 0.000011 0.00001187Rb/86Sr 0.5466 0.1465 0.4186 0.8587 0.0192 0.4663εSrpresent 98.69 39.5 66.06 110.73 51.65 97.54Age (Ma) 15 15 15 15 15 1587Sr/86Srinit 0.711337 0.707252 0.709065 0.712118 0.708135 0.711273

143Nd/144Ndm are measured Nd isotopic ratios. 143Nd/144Ndi are initial Nd isotopic ratios and εNd is the fractional difference between the 143Nd/144Nd of rock and the bulk earth atthe time of crystallization. εNd and 143Nd/144Ndi assume an age of 14 Ma for the rocks. Values of εNd were calculated using modern. 143Nd/144NdCHUR = 0.512638 and 147Sm/144NdCHUR = 0.1967. εNdpresent–present εNd (measured). Note: Sample 07-SA-25 is basalt from nearby Saga ophiolite, Tibet.

1270 R. Hébert et al. / Gondwana Research 25 (2014) 1263–1271

they clearly display the geochemical characteristics of themiddle conti-nental crust of the Asian plate composed of metasedimentary andmeta-igneous rock series. This strong geochemical imprint probably re-flects the extended ponding of the mantle melts (King et al., 2007,2011) in the middle crust. However, Nd and Sr isotope systematics donot rule out a contribution from other sources such as the Himalayasor lithospheric mantle or lower crust because the strong crustal signa-ture might have obliterated signatures of earlier magmatic processes.This work points out the difficulties with an evaluation of magmaticpetrogenetic processes in the Himalayas and the Tibetan Plateau priorto the mid-crustal ponding period. This study shows that Tibetan deepcrust was still partially melting by 17 Ma.

Acknowledgments

This study was supported by the National Science and EngineeringResearch Council of Canada to RH and JD and the National ScienceFoundation, China to CW.

Na-volcanics in N.Tibet

Himalayan Basement (to 0.90)India subduction trend(24-8 Ma)

Amdo orthogneiss (531 Ma)

North Tibet Geochemical Province (46-0 Ma, Potassic and Ultrapotassic)

Adakitic rocks in Lhasa Terrane (18-12 Ma)

Tethyan oceanic crustalsubduction trend (65-10 Ma)

Yarlung Zangbo ophiolite (200-100 Ma)

Nd

87Sr/86Sr

Gabbro xenolithfrom SW Tibet

ModelLower Crust

Localities 5 to 8

Locality 3Localities 1 and 2

-20

-15

-10

-5

0

5

10

15

0.70 0.71 0.72 0.73 0.74 0.75 0.76

Fig. 9. 87Sr/86Sr vs. εNd diagram for shoshonites from Saga and Sangsang areas comparedwith several northern Tibetan rock suites. Fields after Hou et al. (2004) and Zhao et al.(2009). The reader is referred to Zhao et al. (2009) for complete references.

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