Late Jurassic Sodium-Rich Adakitic Intrusive Rocks in the Southern Qiangtang Terrane, Central Tibet, and Their Implications for the Bangong–Nujiang Ocean Subduction

Lithos 245 (2016) 34–46

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Late Jurassic sodium-rich adakitic intrusive rocks in the southern Qiangtang terrane, central Tibet, and their implications for the Bangong–Nujiang Ocean subduction

Yalin Li a,⁎,JuanHea,b, Zhongpeng Han a, Chengshan Wang a, Pengfei Ma a,AorigeleZhoua, Sheng-Ao Liu a,MingXua a State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China b Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China article info abstract

Article history: The lack of magmatic records with high-quality geochronological and geochemical data in the central segment of Received 31 March 2015 the southern Qiangtang subterrane in central Tibet inhibits a complete understanding of the subduction polarity Accepted 22 October 2015 of the Bangong–Nujiang Ocean lithosphere during the Mesozoic. In this study, we present the zircon U–Pb age as Available online 10 November 2015 well as geochemical and Sr–Nd–Pb isotopic data for the Late Jurassic pluton from the Kangqiong area in the central segment of the southern Qiangtang subterrane. The Kangqiong pluton primarily consists of granodiorites Keywords: (SiO =62.87–65.17 wt.%) and was emplaced in the Late Jurassic (147.6 ± 2.4–149.9 ± 2.1 Ma). The granodio- Adakitic granodiorites 2 – – Late Jurassic rites display high Na2O numbers (Na2O/K2O = 1.75 2.24) as well as high MgO (2.21 3.14 wt.%) and Mg- Bangong–Nujiang Ocean numbers (53–58), are characterized by a low abundance of heavy rare earth elements (e.g., Yb = Northward subduction 1.05–1.92 ppm) and Y (12.63–17.52 ppm), and high Sr/Y (29–61) and La/Yb (14–18) ratios, which are compara- Central Tibet ble in composition to those of slab-derived adakitic rocks. The Kangqiong adakitic granodiorites have initial 87 86 206 204 ( Sr/ Sr)i ratios of 0.70611 to 0.70669, negative εNd(t) values (−1.06 to −0.25), ( Pb/ Pb)t ratios of 207 204 208 204 18.42 to 18.47, ( Pb/ Pb)t ratios of 15.62 to 15.63, and ( Pb/ Pb)t ratios of 38.50 to 38.60. These geochemical signatures indicate that the magmas were most likely derived from the partial melting of the subducted Bangong–Nujiang oceanic crust and minor contaminants from the accretionary complex. Our results, in combination with the coeval magmatism in the western segment of the southern Qiangtang subterrane, indicate that the Bangong–Nujiang oceanic lithosphere was subducted northward beneath the Qiangtang Terrane, forming a west-east magmatic arc over 800 km during the Late Jurassic. © 2015 Elsevier B.V. All rights reserved.

1. Introduction have subducted northward beneath the Qiangtang Terrane (cf. Allègre et al., 1984; Girardeau et al., 1984; Guynn et al., 2006; Kapp et al., The Tibetan–Himalayan orogen, the widest mountain chain on 2005, 2007; Li et al., 2014a,c,d, 2015; Yin and Harrison, 2000; Zhang Earth, has long been considered to be the result of continental collisions et al., 2012; Zhu et al., 2013), the available data mostly come from the and accretions in sequence since the Paleozoic (Yin and Harrison, 2000; western segment (west of Gerze) of the southern Qiangtang subterrane. Zhu et al., 2013). However, the details of such sequential collisional and Additional data, especially the data from the central segment of the accretion events remain unclear. For example, the Bangong–Nujiang southern Qiangtang subterrane in central Tibet, are required to provide suture, located between the Qiangtang Terrane to the north and the vital constraints on this model. Lhasa Terrane to the south in present-day central Tibet (Fig. 1a), is As a special rock type with distinct geochemical signatures (e.g., high generally accepted as representing a good recording of the subduction Sr/Y ratios, low Y and heavy REE), adakitic rocks can form in varying history of the Meso-Tethyan (Bangong–Nujiang) Ocean and subsequent tectonic settings (cf. Cooke et al., 2005; Defant and Drummond, 1990, Qiangtang–Lhasa collision (cf. Dewey et al., 1988; Girardeau et al., 1984; 1993; Jiang et al., 2012; Martin et al., 2005; Oyarzun et al., 2001; Reich Guynn et al., 2006; Kapp et al., 2005; Pearce and Deng, 1988; Zhang et al., 2003; Wang et al., 2008a; Zhu et al., 2009b). However, according et al., 2012, 2014; Zhu et al., 2011, 2013, 2015). Although numerous to the original deﬁnition by Defant and Drummond (1990), if adakitic studies have proposed that the Bangong–Nujiang oceanic lithosphere rocks were derived from the partial melting of the subducted oceanic (slab-derived) crust, they can be considered a vital petrological constraint on the presence of oceanic subduction. However, adakitic rocks ⁎ Corresponding author at: China University of Geosciences (Beijing), 29 Xueyuan Road, ﬁ 100083 Beijing, China. Tel.: +86 10 82321586. that have a Jurassic age have not been identi ed to date in the southern E-mail address: [email protected] (Y. Li). Qiangtang subterrane.

Q a 82° E 92° E Q ALT Songpan-Ganzi Tarim JSS 35° N 32° 168-163 Ma165-163 Ma Qiangtang J2x-J3 s 15´ 162 -160Ma 164-154 Ma SS Fig.1b J2x-J3 s 165-157 Ma 156-150 Ma Gerze BNS Amdo Lhasa 30° N

N 1k IYS 0 200 km Himalaya N k 1 India MCT N 1k J x-J s Q 2 3 J x-J s Kangqiong 2 3 N k 1 JM 036km

Q Quaternary Ultramafic rocks Q 32° Late Jurassic N k 05´ 1 Kangtuo Formation granodiorite Xiali and Suowa JM J2x-J3 s Thrust formations JM E1-2n Niubao Formation Sample localities E 1-2n Location of zircon Jurassic mé lange JM U-Pb sample b 88°10´ Q 88°30´

Fig. 1. (a) Tectonic outline of the Tibetan plateau showing major tectonic units and the localities of the Jurassic arc-related rocks in the western segment of the southern Qiangtang subterrane (after Li et al., 2014a,c; Zhu et al., 2015). IYS, Indus–Yarlung Zangbo suture; BNS, Bangong–Nujiang suture; SS, Longmu–Shuanghu suture; JSS, Jinshajiang suture; ALT, Altyn Tagh fault; MCT, Main Central thrust. (b) Geological map of the Kangqiong area.

In this paper, we present the zircon LA-ICP MS age as well as whole- sediments, which are considered to be the result of the Qiangtang– rock geochemical, and Sr–Nd–Pb isotopic data for the Kangqiong pluton Lhasa collision (Li et al., 2013, 2015; Zhang et al., 2012). The Jurassic in the central segment of the southern Qiangtang subterrane (Fig. 1a). (168–150 Ma) magmatic rocks that are currently reported are mainly ex- Our data indicate that the Kangqiong pluton was emplaced at ca. posed in the western segment (west of Gerze) of the southern Qiangtang 149 Ma and displays geochemical affinity with adakitic rocks, most like- subterrane (Fig. 1a). These rocks are dominated by intermediate-silicic ly resulting from the partial melting of the subducting oceanic litho- rocks and show the characteristics of arc-related magmatism, which sphere. Our results reveal the development of a west-east Late Jurassic were interpreted to be the products of the northward subduction of the magmatic arc of over 800 km and provide a robust constraint on the Bangong–Nujiang oceanic lithosphere (cf. Du et al., 2011; Kapp et al., northward subduction of the Bangong–Nujiang oceanic lithosphere in 2005, 2007; Li et al., 2014a,c; Zhu et al., 2015). No arc-related magmatism the central segment of the southern Qiangtang subterrane during the has been observed in the central segment (from Gerze to Amdo) of the Late Jurassic. southern Qiangtang subterrane (Fig. 1a). The Bangong–Nujiang suture is characterized by a N1400 km-long 2. Geological background east–west trending belt that is mainly composed of the accretionary complex and associated ophiolitic fragments (Fig. 1a) (Dewey et al., The Tibetan plateau is essentially composed of four continental blocks 1988; Girardeau et al., 1984; Kapp et al., 2005; Schneider et al., 2003; or terranes: the Songpan–Ganzi, Qiangtang, Lhasa, and Himalaya, from Yin and Harrison, 2000; Zhang et al., 2012; Zhu et al., 2015). Previous north to south (Fig. 1a). These blocks are separated by the Jinsha, studies have shown that most of the ophiolitic fragments have a supra Bangong–Nujiang, and Indus–Yarlung Zangbo suture zones, representing subduction-zone signature (Girardeau et al., 1984; Wang et al., Paleo-, Meso-, and Neo-Tethyan oceanic relicts, respectively (Yin and 2008b), while fewer ophiolites show MORB-like and oceanic island ba- Harrison, 2000). The Qiangtang Terrane can further be divided into north- salt (OIB) characteristics (Zhang et al., 2007; Zhu et al., 2006b). The iso- ern and southern subterranes by the Longmu–Shuanghu suture (Fig. 1a) topic ages of these ophiolitic fragments show a range from 190 to and the Lhasa Terrane can be divided into northern, central, and southern 108 Ma (Wang et al., 2008b; Zhang et al., 2012; Zhu et al., 2006b), and subterranes (Zhu et al., 2009a,b, 2011, 2013). Rock units within the the youngest ages indicate that the Bangong–Nujiang oceanic crust Bangong–Nujiang suture zone and adjacent regions of the southern did not close until the Early Cretaceous. The matrix of the accretionary Qiangtang and northern Lhasa subterranes that are closely associated complex consists of sandstone and siltstone interbedded with shale with the purpose of this study are summarized below. and fossil-rich limestone that yielded a Jurassic to Early Cretaceous The southern Qiangtang subterrane investigated in this study is age (Zhang et al., 2012; Zhu et al., 2015). Geological investigation in dominated by Middle to Upper Jurassic sediments (Yanshiping Group), the Gerze area revealed that the Jurassic sediments (Yanshiping with thicknesses of more than 3000 m (Li et al., 2014b; Wang and Qu, Group) in the southern Qiangtang subterrane have the characteristics 2012), including the Middle Jurassic sequences that mainly consist of clas- of an accretionary complex (Li et al., 2011) with an origin from the tic rocks and the Upper Jurassic sequences that are composed of carbon- southern Qiangtang subterrane (Zeng et al., 2015). The accretionary ates. These Jurassic sequences are unconformably overlain by weakly complex and associated ophiolitic fragments are unconformably over- deformed Upper Cretaceous (i.e., Abushan Formation) continental lain by Late Cretaceous terrestrial sediments that were interpreted to 36 Y. Li et al. / Lithos 245 (2016) 34–46 have been deposited following the initiation of the Qiangtang–Lhasa hornblende (5–10%), quartz (30–35%), and biotite (1–3%), as well as collision (Girardeau et al., 1984; Kapp et al., 2005; Li et al., 2013; minor accessory minerals. Some hornblendes and plagioclases are al- Zhang et al., 2012; Zhu et al., 2015). tered to chlorite and kaolinite (Fig. 2b). The northern Lhasa subterrane is mainly composed of Jurassic–Early Cretaceous shelf sediments with scattered Triassic outcrops, which are 4. Analytical results considered to be the sedimentary cover of a juvenile crust (cf. Zhu et al., 2009a, 2011, 2013). Early Cretaceous (120–110 Ma) magmatic 4.1. Zircon U–Pb geochronology rocks are exposed in the northern Lhasa subterrane (cf. Zhu et al., 2011, 2015). These magmatic rocks include intermediate-silicic intru- In this study, two samples were selected for dating, including sample sions and volcanic rocks and were attributed to the southward subduc- KQU1 from the southern part of the Kangqiong pluton and sample tion of the Bangong–Nujiang oceanic lithosphere beneath the Lhasa KQU2 from the northern part of the Kangqiong pluton (Fig. 1b). Des- Terrane and subsequent slab breakoff and detachment (Ma and Yue, criptions of the analytical methods are presented in Appendix A. The 2010; Zhu et al., 2006a, 2009a, 2011, 2013, 2015). zircon U–Pb analytical data and calculation results are listed in Table 1, and the results for the U–Pb age determinations are shown in 3. Field occurrence and petrography Fig. 3. Cathodoluminescence (CL) images demonstrate that the zircons from the two samples show similar crystal forms. These zircons have Samples were collected from the Kangqiong area (E88° 07′52″,N32° long axes of 80 to 180 μm and length/width ratios of 2:1 to 5:1 and 10′32″) in the southern margin of the southern Qiangtang subterrane have an euhedral morphology. The zircon crystals display clear oscilla- immediately adjacent to the Bangong–Nujiang suture (Fig. 1b). The tory zoning without inherited cores (Fig. 3a and b). These, together lithostratigraphical units exposed in the Kangqiong area include the with the high Th/U ratios (0.80–2.0) of the dated analyses, suggest an Middle Jurassic Yanshiping Group (Xiali and Suowa formations) of the origin of crystallization from magmas. southern Qiangtang subterrane in the north and the Middle to Upper Seventeen zircons form KQU1 yield 206Pb/238U ages ranging from Jurassic accretionary complex and related mafic–ultramaficblocksofthe 142 ± 2 Ma to 158 ± 4 Ma, with a weighted mean age of 147.6 ± Bangong–Nujiang suture in the south. TheseJurassicsedimentsareun- 2.4 Ma (MSWD = 2.3, n = 17) (Fig. 3c). Eighteen zircons from KQU2 conformably overlain by Cenozoic terrestrial sediments (e.g., the Niubao yield 206Pb/238U ages between 143 ± 3 Ma to 161 ± 5 Ma, with a and Kangtuo formations) (Fig. 1b). weighted mean age of 149.9 ± 2.1 Ma (MSWD = 1.3, n = 18) The Kangqiong pluton, which has an area of ca. 60 km2, intrudes into (Fig. 3d). These data suggest that the Kangqiong pluton was emplaced the Middle to Upper Jurassic sequences (i.e., the Xiali and Suowa forma- in the Late Jurassic (148–150 Ma), within the time interval of the mag- tions) (Figs. 1b; 2a). The pluton is primarily composed of granodiorites. matic activities (168–150 Ma) reported from the western segment of The granodiorites are gray and exhibit mid- to fine-grained igneous the southern Qiangtang subterrane (Fig. 1a) (Du et al., 2011; Ran texture, consisting of K-feldspar (10–15%), plagioclase (45–50%), et al., 2015; Zhu et al., 2015).

4.2. Whole-rock geochemistry NE The whole-rock major and trace elemental and Sr–Nd–Pb isotopic analytical methods are presented in Appendix A. The results are listed in Table 2.

The Kangqiong granodiorites (SiO2 =62.87–65.17 wt.%; normalized to an anhydrous basis) (Fig. 4a) show high contents of Al2O3 (15.78– SuowaSuowa FormationFormation 16.50 wt.%), MgO (2.21–3.14 wt.%), and Mg# [Mg# =100×Mg2+/ limestonelimestone (Mg2+ + TFe2+)] (53–58). These rocks are enriched in sodium with Na O/K O ratios of 1.75 to 2.24 and are metaluminous as indicated by Kangqiong 2 2 the aluminum saturation index (A/CNK) of 0.91–1.05. The rocks are granodiorite middle-K calc-alkaline, in contrast to the other Jurassic arc-related high-K calc-alkaline magmatic rocks (168–150 Ma) (Fig. 4b) reported in the western segment of the southern Qiangtang subterrane (Du 30 m a et al., 2011; Li et al., 2014a,b). The Kangqiong granodiorites are characterized by high contents of Sr (470–1067 ppm), Sr/Y (29–61) and La/Yb (14–18) and low contents of heavy rare earth elements (HREE) (e.g., Yb = 1.05–1.92 ppm) and Y Pl (12.6–19.7 ppm). On a plot of Sr/Y vs. Y (Fig. 4c), the majority of the samples fall into the field of adakitic rocks, as defined by Defant and Drummond (1993). These geochemical signatures are similar to the Kelu (ca. 90 Ma) and Mamen (ca. 136 Ma) adakitic rocks that were interpreted to be a result of slab-derived intrusion in the Gangdese arc Hb (Jiang et al., 2012; Zhu et al., 2009b) and differ significantly from the Mesozoic arc-related intrusions in the western segment of the southern Q Qiangtang subterrane (Fig. 4c). The Kangqiong samples are enriched in Hb Pl light rare earth elements (LREE) and large ion lithophile elements (LILE) and are depleted in HREE and high field strength elements (HFSE) with Q pronounced negative anomalies of Nb and Ti (Fig. 5a and b). 200 um b Whole-rock Sr–Nd–Pb isotopic data are listed in Table 2. All of the samples show homogeneous Sr–Nd–Pb isotopic compositions. The initial Sr, Nd, and Pb isotopic ratios were corrected using the U–Pb zircon Fig. 2. (a) Field photograph showing the Kangqiong granodiorite pluton into the Upper Jurassic Suowa Formation. (b) Photomicrographs showing the texture of the granodiorites. age of sample KQU1 (147.6 Ma). The Kangqiong granodiorites show 87 86 Q, Quartz; Hb, Hornblende; Pl, plagioclase. ( Sr/ Sr)i ranging from 0.70572 to 0.70669, with εNd(t) from −1.06 Y. Li et al. / Lithos 245 (2016) 34–46 37

Table 1 Zircon age data acquired by LA-ICPMS methods for the Kangqiong intrusive rocks.

Samples and anal. no. Th U Th/U Isotopic ratios (±1σ) Ages (±1σ Ma) (ppm) (ppm) Ratio 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U

Sample KQU–1 KQU1-01 287.9 242.8 1.2 0.04896 0.00954 0.04896 0.00954 0.04896 0.00954 146 355 145 26 145 4 KQU1-02 358.4 241.1 1.5 0.04871 0.0090 0.04871 0.0090 0.04871 0.0090 134 340 148 25 149 3 KQU1-03 176.5 196.2 0.9 0.04936 0.01128 0.04936 0.01128 0.04936 0.01128 165 405 149 32 148 4 KQU1-04 497.5 337.8 1.5 0.04926 0.00521 0.04926 0.00521 0.04926 0.00521 160 237 153 15 153 3 KQU1-06 121.1 160.7 0.8 0.04952 0.00973 0.04952 0.00973 0.04952 0.00973 173 363 149 27 148 4 KQU1-07 248.2 226.5 1.1 0.04906 0.01059 0.04906 0.01059 0.04906 0.01059 151 386 145 29 145 4 KQU1-08 98.5 123.8 0.8 0.04919 0.01217 0.04919 0.01217 0.04919 0.01217 157 433 158 36 158 4 KQU1-11 191.2 214.5 0.9 0.0515 0.01112 0.0515 0.01112 0.0515 0.01112 263 404 159 32 152 5 KQU1-12 289.6 267.8 1.1 0.04864 0.00652 0.04864 0.00652 0.04864 0.00652 131 273 151 19 152 3 KQU1-14 105.9 136.1 0.8 0.04958 0.01275 0.04958 0.01275 0.04958 0.01275 175 452 150 36 149 4 KQU1-16 659.6 420.2 1.6 0.04934 0.00411 0.04934 0.00411 0.04934 0.00411 164 189 146 11 145 3 KQU1-17 574.3 438.8 1.3 0.04942 0.00419 0.04942 0.00419 0.04942 0.00419 168 192 144 11 143 2 KQU1-18 275.3 266.8 1.0 0.04913 0.00618 0.04913 0.00618 0.04913 0.00618 154 264 150 17 150 3 KQU1-19 263.3 274.9 1.0 0.04866 0.00582 0.04866 0.00582 0.04866 0.00582 131 253 144 16 145 3 KQU1-20 226.1 225.0 1.0 0.0493 0.01054 0.0493 0.01054 0.0493 0.01054 162 384 148 29 147 4 KQU1-24 1196.9 604.4 2.0 0.04927 0.00331 0.04927 0.00331 0.04927 0.00331 161 153 143 9 142 2 KQU1-25 214.4 207.0 1.0 0.04868 0.00747 0.04868 0.00747 0.04868 0.00747 132 296 154 22 156 3

Sample KQU–2 KQU2-03 112.1 117.7 1.0 0.04771 0.01202 0.16152 0.04035 0.02455 0.00088 85 423 152 35 156 6 KQU2-04 211.8 273.4 0.8 0.04931 0.00512 0.16243 0.01668 0.02389 0.00052 163 233 153 15 152 3 KQU2-05 437.6 416.3 1.1 0.04814 0.00405 0.15808 0.01307 0.02381 0.0005 106 189 149 11 152 3 KQU2-07 255.4 187.1 1.4 0.04894 0.00993 0.16932 0.03395 0.02509 0.00088 145 363 159 29 160 6 KQU2-08 289.8 249.9 1.2 0.04859 0.00648 0.15803 0.02087 0.02359 0.00055 128 272 149 18 150 3 KQU2-09 328.6 230.3 1.4 0.05239 0.00825 0.16808 0.02604 0.02327 0.00075 302 325 158 23 148 5 KQU2-10 194.4 166.1 1.2 0.04924 0.01545 0.15557 0.04837 0.02292 0.00103 159 534 147 43 146 6 KQU2-11 85.6 91.3 0.9 0.04759 0.01469 0.15014 0.04601 0.02288 0.0009 79 512 142 41 146 6 KQU2-12 134.3 162.1 0.8 0.05022 0.00604 0.16225 0.01932 0.02337 0.00054 205 268 153 17 149 3 KQU2-13 119.9 133.6 0.9 0.05042 0.00794 0.1687 0.02628 0.0242 0.00068 214 318 158 23 154 4 KQU2-14 242.0 205.0 1.2 0.04841 0.0047 0.15958 0.01531 0.02384 0.00049 119 216 150 13 152 3 KQU2-15 203.3 163.1 1.2 0.04981 0.00613 0.16468 0.02004 0.02391 0.00057 186 265 155 17 152 4 KQU2-16 202.7 247.6 0.8 0.04977 0.00499 0.15925 0.01572 0.02314 0.00053 184 228 150 14 147 3 KQU2-17 105.1 122.9 0.9 0.04808 0.00823 0.15924 0.02706 0.02395 0.00061 103 315 150 24 153 4 KQU2-18 111.0 119.0 0.9 0.05123 0.00938 0.17895 0.03237 0.02526 0.00081 251 350 167 28 161 5 KQU2-19 94.9 102.1 0.9 0.05022 0.01002 0.15694 0.03103 0.0226 0.00069 205 370 148 27 144 4 KQU2-20 210.2 234.2 0.9 0.04889 0.00423 0.15577 0.01333 0.02304 0.00046 143 196 147 12 147 3 KQU2-22 242.1 209.3 1.2 0.05042 0.00517 0.1563 0.01583 0.02242 0.0005 214 234 147 14 143 3

The strikes represent the discarded analyses from mean calculations.

87 86 to −0.25 (Table 2). Such low εNd(t) and high ( Sr/ Sr)i differ (0.93–1.08) (Table 2; Fig. 8a), similar to the trend of fluid-induced en- from oceanic crust-derived adakitic rocks, which display a positive richment. Woodhead et al. (2001) and Nebel et al. (2007) proposed 87 86 εNd(t) and low ( Sr/ Sr)i (e.g., the Kelu and Mamen adakitic rocks) that fluid-dominated arc environments have Th/Yb b 1, whereas the (Jiang et al., 2012; Zhu et al., 2009b)(Fig. 6). The Kangqiong samples arc settings of the subducted sediments have Th/Yb ratios N 2. The 206 204 207 204 show a ( Pb/ Pb)t of 18.418 to 18.473, ( Pb/ Pb)t of 15.620 to Kangqiong adakitic rocks have Th/Yb ratios ranging from 4.38 to 6.20, 208 204 15.627, and ( Pb/ Pb)t of 38.499 to 38.598 (Table 2), within the suggesting a significant contribution from sediments in their origin. field of the Bangong–Nujiang MORB and close to those of slab-derived On this basis, the linear trend of the Kangqiong adakitic rocks in a plot adakitic rocks in the Gangdese arc (Jiang et al., 2012; Zhu et al., of Th/Yb vs. Th/Sm (Fig. 8b) could be interpreted in terms of two- 2009b)(Fig. 7). component mixing between the MORB-derived melts and the partial melts of their sedimentary components. This inference is also supported 5. Discussion by the flat trend of the Kangqiong samples in the plot of Ba/Th vs. 87 86 ( Sr/ Sr)i (Fig. 8c). 5.1. Nature of the source region Two possibilities can account for the involvement of sediments, including the metasomatism of magma source region by oceanic Calc-alkaline magmatism has long been linked to oceanic sub- crust-derived sediments (Elburg et al., 2002; Jiang et al., 2012; Plank duction occurring at the convergent plate boundary (Condie, 1982; and Langmuir, 1998; Zhu et al., 2009b) or contamination by wall rocks Peccerillo and Taylor, 1976; Wilson, 1989). It follows that the middle- (e.g., accretionary complex) during magma ascending. In the case of K calc-alkaline Kangqiong granodiorites may have been emplaced at the Na-rich Kangqiong samples, we consider the first possibility to be a subduction-related setting. Under such a setting, two types of compo- less likely because if the sediment components identified in these sam- nents, including partial melts of subducted sediments and slab-derived ples are derived from the subducting Bangong–Nujiang oceanic crust, fluids, can metasomatize and enrich the source region of subduction- significant amounts of these sediments would be required to explain related magmas (Elburg et al., 2002; Zhu et al., 2009b). Generally, their negative εNd(t) (−1.06 to −0.25), resulting in the generation of slab-derived fluids exhibit high abundances of Ba, Sr, Rb, U, and Pb, K-rich rather than Na-rich magmas as the sediments are enriched in K whereas partial melts of the subducted sediments are characterized by (Liu et al., 2010). high abundances of Th and LREE (Guo et al., 2007; Hawkesworth Therefore, we argue that the sediments from accretionary com- et al., 1997). The Kangqiong granodiorites show varying Ba abundances plexes are most likely to be involved during magma ascending because (439–849 ppm) coupled with a narrow range of the Nb/Y ratio the Kangqiong pluton intrudes into the Jurassic accretionary complex 38 Y. Li et al. / Lithos 245 (2016) 34–46

170 2 Mean=147.6± 2.4 Ma 1 4 149 ± 3 Ma 0.0 2 6 N=17 MSWD=2.3 153 ± 3 Ma 145±4Ma 160 145±4Ma 7 8 19

8 0.0 2 4 158±4Ma 3

2 147 ± 4 Ma / 150

2 142±2Ma 147±4Ma 20 0.0 2 2 140 24 18 150 ± 3 Ma 100 um KQU1 145 ± 3 Ma 130 17 c 143 ± 2 Ma 16 0.0 2 0 a KQU1 0.060 0.100 0.140 0.180 0.220 207Pb/ 235 U

170 152 ± 3 Ma 5 Mean=149.9±2.1 Ma 3 160 ± 6 Ma 7 0.0 2 6 N=18 156±6Ma MSWD=1.3 150 ± 3 Ma 160 11 2 8 12 140 ± 3 Ma 146±6Ma 149 ± 3 Ma

8 0.0 2 4

19 2

/ 150 13 15 14 b

152 ± 3 Ma 2 154 ± 4 Ma 152 ± 4 Ma 144 ± 4 Ma

146±6Ma 0.0 2 2 140 10 16 147 ± 3 Ma 18 148±5Ma 22 161±5Ma 143 ± 3 Ma 9 KQU2 17 147 ± 3 Ma20 d 130 153±4Ma 100 um 0020. b KQU2 0.060 0.100 0.140 0.180 0.220 207Pb/ 235 U

Fig. 3. CL images showing the internal structure of the analyzed zircon grains from KQU1(a) and KQU2 (b); (c) and (d) U–Pb concordia diagrams of zircons for samples KQU1 and KQU2 respectively.

(Fig. 1b) (including the Yanshiping Group from Gerze that was also 2003; Karsli et al., 2010; Wen et al., 2008) or in the subducted oceanic interpreted to represent the accretionary complex of the Bangong– crust (Defant and Drummond, 1990; Defant and Drummond, 1993; Nujiang Ocean; Li et al., 2011). Given that the basalts from the Escuder et al., 2007; Jiang et al., 2012; Martin et al., 2005; Wang et al., Bangong–Nujiang suture (Zhang et al., 2014) and the Yanshiping 2008a; Zhu et al., 2009b), can produce melts with the geochemical char- Group sandstones from Gerze (Li et al., 2014b)canbetreatedasproxies acteristics of adakitic rocks. Based on the geochronological and geo- for the oceanic crust-derived components and accretionary complex- chemical data, we suggest that the Kangqiong granodiorites were derived sediments, respectively, the Sr–Nd isotopic compositions can most likely generated by slab melting of the Bangong–Nujiang oceanic be interpreted to be the consequence of mixing between these two crust rather than the thickened lower crust. end members (Fig. 6). Pb isotopic data provide additional constraints on the derivation of sediments from the accretionary complex for the (1) The low K2O, high Na2O/K2O and high Al2O3 of the Kangqiong generation of Kangqiong adakitic rocks. As shown in Fig. 7a and b, the samples are consistent with those of adakitic rocks generated Kangqiong samples have radiogenic Pb isotopic compositions that are by partial melting of the subducted oceanic crust. Because am- similar to those of the Jurassic sandstones from the southern margin phibole is a K-bearing mineral and has much higher K2O contents of the southern Qiangtang subterrane (Li et al., 2014b). than those of garnet and clinopyroxene in residual phases during the high-pressure melting of metabasaltic rocks, the presence of 5.2. Petrogenesis amphibole in the residue can produce low-K silicic melts (Liu et al., 2010). Experimental studies have shown that partial As presented above, the Kangqiong samples display a low content of melting of the MORB compositions (e.g., amphibolites) with HREE and Y as well as Sr/Y and La/Yb ratios that are comparable with garnet + clinopyroxene + amphibole in the residual can pro- those of adakitic rocks (cf. Defant and Drummond, 1990; Martin et al., duce Na-enrichment and high Na2O/K2O adakites (Defant and 2005). Previous studies have veriﬁed that the partial melting of Drummond, 1990; Martin et al., 2005; Petford and Atherton, metabasic igneous rocks in the eclogite to amphibolite facies, either in 1996; Rapp and Watson, 1995; Rapp et al., 1991). In contrast, the thickened lower crust (Atherton and Petford, 1993; Chung et al., partial melting of maﬁc lower crust rocks (e.g., eclogites) without Table 2 Bulk-rock major, trace element and Sr–Nd–Pb isotopic data of the Kangqiong intrusive rocks.

sample KQH1 KQH2 KQH3 KQH4 KQH5 KQH6 KQH7 KQH8 KQH9 KQH10 KQH11 KQH12 KQH13 KQH14 KQH15

Major element (wt.%) SiO2 63.76 63.75 63.86 63.81 65.17 63.69 63.93 62.98 63.21 63.56 62.87 63.61 62.95 63.33 63.48 TiO2 0.69 0.72 0.71 0.69 0.63 0.73 0.73 0.72 0.71 0.70 0.74 0.70 0.66 0.70 0.71 Al2O3 16.11 16.24 16.07 16.20 15.78 16.14 16.47 16.34 16.31 16.44 16.11 16.50 16.00 16.23 16.21 Fe2O3 4.38 4.47 4.38 4.30 3.97 4.55 4.47 4.52 4.50 4.26 4.62 4.24 3.93 4.35 4.44 MnO 0.06 0.06 0.07 0.06 0.07 0.07 0.06 0.07 0.07 0.06 0.07 0.07 0.05 0.06 0.07 MgO 2.90 3.09 2.95 2.78 2.62 3.14 2.99 3.04 3.04 2.60 3.10 2.45 2.21 2.95 2.92 CaO 3.41 3.72 3.43 3.26 2.91 3.76 3.98 3.82 4.06 3.72 3.71 3.71 4.80 3.61 3.08 Na2O 4.44 4.44 4.32 4.43 4.29 4.31 4.41 4.61 4.24 4.55 4.38 4.44 4.14 4.40 4.39 K2O 2.15 2.04 2.16 2.21 2.45 2.14 2.00 2.06 2.01 2.06 2.08 2.07 1.95 2.19 2.47

P2O5 0.15 0.15 0.15 0.15 0.13 0.15 0.16 0.17 0.15 0.15 0.16 0.15 0.15 0.15 0.14 34 (2016) 245 Lithos / al. et Li Y. LOI 1.61 1.88 1.60 1.59 1.39 1.20 1.18 1.30 1.17 1.54 1.28 1.12 2.76 1.52 1.19 TOTAL 99.66 100.56 99.70 99.48 99.41 99.61 99.88 100.38 99.63 99.47 99.64 99.12 99.06 99.60 99.49 Na2O/K2O 2.07 2.18 2.00 2.00 1.75 2.01 2.21 2.24 2.11 2.21 2.11 2.14 2.12 2.01 1.78 A/CNK 1.02 1.00 1.02 1.04 1.05 0.99 0.99 0.98 0.99 1.00 0.99 1.01 0.91 1.00 1.05 Mg# 57 58 57 56 56 58 57 57 57 55 57 53 53 58 57

Trace element (ppm) Sc 7.76 9.43 11.29 6.74 7.55 8.93 7.26 6.86 7.42 7.35 8.20 10.24 7.39 6.72 9.20 V 56.8 69.3 82.0 47.3 51.5 56.1 46.0 43.4 47.2 45.4 51.8 64.9 46.9 42.7 56.5′ Cr 19.9 25.7 32.0 19.1 19.5 23.7 18.9 18.6 20.7 19.4 22.5 28.6 18.5 19.3 27.4 –

Co 16.6 20.3 24.6 14.6 16.0 18.7 15.5 14.2 15.9 15.4 17.3 22.2 15.4 14.7 19.9 46 Ni 15.5 19.4 23.2 14.4 15.1 18.5 15.2 14.3 15.9 15.5 17.5 22.2 15.1 15.1 20.4 Ga 15.0 18.1 22.5 13.1 15.4 15.8 13.5 12.3 13.6 13.3 14.2 19.3 14.5 12.9 17.4 Rb 67.8 75.8 102.0 63.0 83.0 75.9 61.4 60.4 60.4 68.4 73.9 90.2 61.3 61.1 89.6 Sr 858 879 735 805 698 691 460 489 450 618 512 622 407 725 1067 Y 15.4 18.2 20.3 13.0 14.9 16.7 13.9 12.6 13.8 13.4 15.3 19.7 14.0 13.0 17.5 Zr 162 192 230 137 167 162 151 131 142 157 168 208 152 139 176 Nb 14.5 17.0 20.3 13.5 15.9 16.6 14.1 13.5 14.2 14.5 16.2 20.2 14.5 13.3 17.5 Ba 529 689 819 496 666 579 467 440 473 592 519 675 461 533 849 La 19.8 22.1 27.1 18.3 22.1 22.0 19.2 17.4 19.6 18.7 20.6 27.7 21.2 18.6 25.7 Ce 30.8 37.4 48.0 27.6 32.3 32.7 28.6 30.7 30.6 31.7 32.0 40.8 32.2 32.6 37.5 Pr 3.58 4.36 5.53 3.14 3.69 3.86 3.33 3.01 3.34 3.21 3.60 4.71 3.48 3.12 4.30 Nd 14.0 16.9 21.5 12.2 14.0 15.1 13.0 11.8 13.0 12.6 14.1 18.2 13.4 12.0 16.6

(continued on next page) 39 40

Table 2 (continued)

sample KQH1 KQH2 KQH3 KQH4 KQH5 KQH6 KQH7 KQH8 KQH9 KQH10 KQH11 KQH12 KQH13 KQH14 KQH15

Sm 2.75 3.30 4.17 2.37 2.70 3.02 2.54 2.30 2.51 2.47 2.78 3.58 2.58 2.34 3.24 Eu 0.94 1.16 1.45 0.83 0.97 1.01 0.86 0.78 0.86 0.89 0.90 1.22 0.89 0.85 1.19 Gd 2.74 3.33 4.19 2.35 2.73 3.01 2.52 2.28 2.50 2.43 2.75 3.54 2.56 2.34 3.22 Tb 0.43 0.52 0.65 0.37 0.42 0.47 0.39 0.36 0.39 0.38 0.44 0.56 0.40 0.37 0.50 Dy 2.45 2.95 3.68 2.09 2.39 2.71 2.23 2.04 2.23 2.17 2.48 3.14 2.24 2.10 2.85 Ho 0.47 0.56 0.70 0.40 0.46 0.52 0.43 0.39 0.43 0.42 0.48 0.60 0.43 0.41 0.55 Er 1.38 1.65 2.05 1.16 1.35 1.48 1.25 1.15 1.25 1.22 1.39 1.77 1.26 1.20 1.59 Tm 0.20 0.23 0.29 0.16 0.19 0.21 0.18 0.16 0.18 0.17 0.20 0.25 0.18 0.17 0.23 Yb 1.29 1.54 1.92 1.08 1.29 1.38 1.15 1.05 1.16 1.14 1.29 1.65 1.18 1.12 1.47 Lu 0.20 0.24 0.30 0.17 0.20 0.22 0.18 0.16 0.18 0.18 0.20 0.26 0.19 0.17 0.23 Hf 4.27 4.97 6.11 3.61 4.48 4.26 3.90 3.37 3.71 4.03 4.29 5.41 3.97 3.62 4.78 Ta 0.94 1.05 1.27 0.85 1.06 1.05 0.88 0.82 0.89 0.89 0.99 1.28 0.92 0.83 1.12 Pb 7.92 10.87 12.51 6.61 11.88 11.39 8.79 8.38 9.17 6.29 11.18 14.83 9.02 6.90 12.67 Th 7.07 7.66 9.67 5.79 7.99 6.87 5.31 4.70 5.30 5.15 5.63 7.81 5.86 5.17 7.83

U 1.10 1.17 1.46 0.91 1.25 1.10 0.88 0.78 0.89 0.82 0.90 1.29 1.35 0.86 1.21 34 (2016) 245 Lithos / al. et Li Y. Eu/Eu* 1.05 1.07 1.06 1.07 1.09 1.03 1.04 1.04 1.05 1.11 0.99 1.05 1.06 1.11 1.12 (La/Yb)N 11 10 10 12 12 11 12 12 12 12 11 12 13 12 13 Sr/Y 56 48 36 62 47 41 33 39 33 46 33 32 29 56 61

Sr–Nd–Pb isotope compositions 87 Rb/86 Sr 0.2289 0.2268 0.3446 0.3182 0.3865 0.3201 0.4366 0.2430 (87 Sr/86 Sr)m 0.70714 0.70717 0.70706 0.70689 0.70654 0.70697 0.70703 0.70702 2SE 12 7 7 10 10 12 9 10 87 86 ( Sr/ Sr)i 0.70666 0.70669 0.70634 0.70622 0.70572 0.70630 0.70611 0.70650 147 144

Sm/ Nd 0.118910 0.117508 0.116125 0.120693 0.117887 0.119047 0.116500 0.118228 – (143 Nd/144 Nd)m 0.512513 0.512507 0.512507 0.512527 0.512537 0.512550 0.512520 0.512519 46 2SE 9 8 12 10 10 13 17 10 εNd (t) −0.97 −1.06 −1.03 −0.73 −0.48 −0.25 −0.79 −0.84 143 144 ( Nd/ Nd)i 0.51240 0.51239 0.51239 0.51241 0.51242 0.51243 0.51241 0.51240 TDM/Ma 1027 1021 1007 1024 978 969 990 1010 TDM2/Ma 1018 1026 1024 999 979 960 1004 1008 206 Pb/204 Pb 18.690 18.693 18.653 18.623 18.634 18.688 18.674 18.620 2SE 18 27 9 24 14 5 16 8 207 Pb/204 Pb 15.639 15.635 15.633 15.631 15.633 15.635 15.633 15.634 2SE 17 22 8 20 13 4 14 7 208 Pb/204 Pb 38.988 38.977 38.949 38.880 38.896 38.979 38.953 38.930 2SE 46 62 22 54 34 12 38 16 206 204 ( Pb/ Pb)t 18.452 18.456 18.473 18.458 18.464 18.464 18.418 18.456 207 204 ( Pb/ Pb)t 15.627 15.623 15.625 15.622 15.625 15.624 15.620 15.626 208 /204 ( Pb Pb)t 38.500 38.499 38.582 38.551 38.567 38.532 38.598 38.594 2+ 2+ 2+ LOI = loss on ignition; Mg# = 100 × Mg /(Mg +TFe ); A/CNK = molecular Al2O3/(CaO + Na2O+K2O). A/NK = molecular Al2O3/(Na2O+K2O); Eu/Eu* = 2*EuN/(SmN + GdN), the subscript of N means normalized to chondrite. m, measured isotopic ratios; t, age-corrected initial isotopic ratios. εNd(t) are initial values; (CHUR = chondritic uniform reservoir), TDM represents the age of crustal material separated from depleted mantle,TDM2 represents the two-stage Nd depleted-mantle 87 86 87 86 87 86 λT −1 87 86 143 144 143 144 147 144 λT −1 147 144 model age. ( Sr/ Sr)i =( Sr/ Sr)m–( Rb/ Sr) × (e − 1), λRb–Sr =0.0142Ga , Rb/ Sr = (Rb/Sr) × 2.8956. ( Nd/ Nd)i =( Nd/ Nd)m–( Sm/ Nd) × (e − 1), λSm–Nd = 0.00654 Ga , Sm/ Nd = (Sm/Nd) × 0.60456. εNd (t) = 143 144 143 144 4 143 144 λt 143 144 147 144 [( Nd/ Nd)Sample (t)/( Nd/ Nd)CHUR (t) − 1] × 10 ,( Nd/ Nd)CHUR (T) = 0.512638 − 0.1967 × (ε − 1). TDM =1/λSm–Nd ×ln{1+[(( Nd/ Nd)m − 0.51315)/(( Sm/ Nd)Sample − 0.2137)]} Y. Li et al. / Lithos 245 (2016) 34–46 41

14 1000 Nepheline a a syenite Kangqiong adakites Alkalic Subducted oceanic 12 Syenite crust-derived adakites

) Syenite 10 Alkali t i 100

r % granite . Gangdese slab-derived

n adakitic rocks

o ( 8 Syenite-diorite Granite h

Ijolite C

Gabbro k

c N 6

o + 10

O 2 Gabbro Quartz diorite K 4 Diorite (granodiorite) Gabbro Jurassic intrusions in 2 the southern Qiangtang subterrane 1 Sub-Alkalic Dy 0 La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu 40 45 50 55 60 65 70 75 1000 SiO2 (wt.%) b Kangqiong adakites Kangqiong adakitic rocks b 5 Mamen adakitic rocks 100

Kelu adakitic rocks B High-K R Subducted oceanic

calc-alkaline O crust-derived adakites 4 Shoshonite M series - 10

)

c t 3

( Middle-K R

O 2 calc-alkaline 1 K 2 series Gangdese slab-derived Jurassic intrusions in the adakitic rocks southern Qiangtang 1 subterrane 0.1 Low-K tholeiite series Rb Th Nb La Sr Hf Sm Ti Dy Er Lu Ba K Ta Ce Nd Zr Eu Gd Y Yb 0 Fig. 5. Chondrite-normalized REE patterns (a) and N-MORB normalized trace element pat- 45 50 55 60 65 70 75 80 terns (b) for the Kangqiong intrusive rocks. The ﬁeld of subducted oceanic crust-derived SiO2 (wt.%) adakites was constructed using data from Defant and Drummond (1990), Stern and Kilian (1996),andMartin et al. (2005).Theﬁeld of the Gangdese slab-derived adakitic rocks was 120 c constructed using data from Zhu et al. (2009b) and Jiang et al. (2012).

(Prouteau et al., 2001; Rapp and Watson, 1995; Rapp et al., 1991; Adakite field Sen and Dunn, 1994), which agrees with the Kangqiong adakitic 80 rocks that display high Al2O3 contents (average 16.21 wt.%).

/ r Jurassic intrusions in the 8 Kangqiong adakitic rocks

S southern Qiangtang Kelu adakitic rocks subterrane 6 Bangong-Nujiang Mamen adakitic rocks 40 Ocean basalt 4 Jurassic sandstones of the southern Qiangtang subterrane 2 Island-arc Andesite Gangdese slab-derived adakitic rocks dacite-rhyolite field 0 0

) Gangdese post-collisional -2 adakitic rocks 0 10203040(t

Y (ppm) ε -4 Subducted continental crust-derived adakitic rocks -6 Fig. 4. (a) Total alkalis vs. silica diagram (Wilson, 1989); (b) SiO2 vs. K2Odiagram (Peccerillo and Taylor, 1976); (c) Sr/Y vs. Y diagram (Defant and Drummond, 1993). -8 Data sources: Kelu and Mamen adakitic rocks in the Gangdese arc (Jiang et al., 2012; Zhu et al., 2009b), Jurassic arc-related intrusions (168–150 Ma) in the western segment Delaminated lower -10 crust-derived adakitic rocks of the southern Qiangtang subterrane (Du et al., 2011; Li et al., 2014a; Zhu et al., 2015). -12

-14 amphibole in the residual can generate high-K melts (Liu et al., 0.700 0.710 0.720 2010). The Kangqiong adakitic rocks have low K2O (1.95– (87Sr/86 Sr) 2.47 wt.%) contents and high Na2O/K2Oratios(1.75–2.24), sug- i gesting that they originated from the partial melting of oceanic ε 87 86 crust (Fig. 9) with residual amphibole during melting. Experimen- Fig. 6. Plot of Nd(t) vs. ( Sr/ Sr)i diagrams for Kangqiong adakitic rocks. Data sources: Bangong–Nujiang Ocean basalts (Zhang et al., 2014), Jurassic sandstones of the southern tal petrology studies have also shown that the high content Al O 2 3 Qiangtang subterrane (Li et al., 2014b), Kelu and Mamen slab-derived adakitic rocks of and Na2O/K2O magmas were generated by the melting of low-K the Gangdese arc (Jiang et al., 2012; Zhu et al., 2009b). Other data sources are from Jiang MORB at pressures of 1 to2 GPa and temperatures b1100 °C et al. (2012) and references therein. 42 Y. Li et al. / Lithos 245 (2016) 34–46

16.0 that the lithospheric stacking or crustal thickening that is required for the lithospheric delamination (cf. Bonin, 2004; Chung et al., Bangong-Nujiang MORB 2005, 2009; Kay and Kay, 1993)oftheLhasa–Qiangtang collision Qiangtang lower crust zone occurred at 113–93 Ma (cf. Kapp et al., 2007; Wang et al., 15.8 2014), postdating the emplacement of the Kangqiong adakitic rocks ca. 35 Ma. All of these observations point to an origin of oce-

)

b anic crust for the generation of the Kangqiong adakitic rocks.

4 (3) Trace elemental signatures support an origin of oceanic crust for

2 15.6 / Gangdese slab-

b the generation of the Kangqiong adakitic rocks. The Sr/Y and (La/

P derived adakitic 7 Yb) ratios are the two most important parameters for discrimi- 0 N 2 rocks ( nating adakitic rocks (Defant and Drummond, 1990; Martin NHRL – – 15.4 et al., 2005). The Sr/Y (29 61) and (La/Yb)N (10 13) ratios of the Kangqiong adakitic rocks (Table 2) resemble magmas originating from oceanic slab rather than lower crustal melting (Liu et al., EM 2010; Martin et al., 2005). In general, altered oceanic crust has 15.2 much higher Sr/La ratios than lower continental crust due to the LREE depletion of N-MORB and Sr enrichment by seawater alter- Kangqiong adakitic rocks ation (Liu et al., 2010). The high Sr/La (19.2–44.1, average 31.5) ra- Mamen adakitic rocks tios of the Kangqiong adakitic rocks are consistent with oceanic 40.0 Jurassic sandstones of the adakitic rocks from subduction zones (Fig. 8d). southern Qiangtang subterrane

t ) Taken together, we propose that the Kangqiong adakitic rocks were

P –

4 most likely derived from the partial melting of the subducted Bangong

2 Qiangtang lower crust / 39.0 Nujiang oceanic crust in a transitional zone of amphibolite–eclogite

8 facies. Such melts subsequently interacted with peridotite in the mantle

2 ( NHRL wedge and were then contaminated by accretionary complex sedi- Bangong-Nujiang ments during magma ascending. MORB 38.0 Gangdese slab- 5.3. Tectonic implications for the Bangong–Nujiang Ocean subduction derived adakitic rocks Existing studies have identified that the Bangong–Nujiang Ocean was present at least from the Late Triassic to Early Cretaceous and was 17.0 18.0 19.0 20.0 closed due to the Lhasa–Qiangtang collision before the Late Cretaceous 206 204 ( Pb/ Pb)t (Dewey et al., 1988; Girardeau et al., 1984; Guynn et al., 2006; Kapp et al., 2005; Pearce and Deng, 1988; Zhang et al., 2012; Zhu et al., Fig. 7. Initial Pb isotopic compositions of the Kangqiong adakitic rocks. The Northern 2013, 2015). It has been well established that the Bangong–Nujiang Hemisphere Reference line (NHRL) is after Zindler and Hart (1986).Dateforlowercrust Oceanic lithosphere was subducted northward beneath the Qiangtang of the Qiangtang Terrane are after Lai et al. (2008). Data sources for the Bangong–Nujiang MORB are taken from Zhang (2007). Mamen adakitic rocks of the Gangdese arc are from Terrane (Girardeau et al., 1984; Guynn et al., 2006; Kapp et al., 2005; Zhu et al. (2009b). Li et al., 2014a,d; Pearce and Deng, 1988; Tang and Wang, 1984; Zhang et al., 2012). However, evidence for the establishment of magmatic rocks mainly comes from the presence of calc-alkaline granitoids (2) The high contents of MgO and Mg# of the Kangqiong granodiorites in the western segment of the southern Qiangtang subterrane (Du et al., are compatible with origin from the partial melting of an oceanic 2011; Li et al., 2014a,d; Liu et al., 2013), with little or no information crust rather than lower crust. Adakitic melts derived from from the central segment of the southern Qiangtang subterrane. Our subducted oceanic crust have high Mg# and high abundances of observations and interpretations confirm that the Kangqiong adakitic compatible elements due to reactions with the mantle wedge rocks were most likely derived from the partial melting of the during ascent (Martin et al., 2005), whereas the melts derived subducting Bangong–Nujiang oceanic lithosphere, providing strong ev- from the partial melting of the thickened maficlowercrustusually idence for the presence of oceanic subduction in the central segment of have the characteristics of low-MgO or Mg# (mostly Mg# b 45) the southern Qiangtang subterrane during the Late Jurassic. Therefore, it (Rapp and Watson, 1995; Sen and Dunn, 1994). The high MgO can be concluded that the coeval subduction-related magmatism has al- (2.21–3.14 wt.%) and Mg# (53–58) of the Kangqiong adakitic ready extended to the Kangqiong region, forming a Late Jurassic mag- rocks are similar to the subducted oceanic slab-derived Mamen matic arc of over 800 km (Fig. 1a). and Kelu adakitic rocks in the Gangdese arc (Jiang et al., 2012; Compared to the generation of normal arc magmas that are intrinsi- Zhu et al., 2009b), but differ significantly from those of the adakitic cally associated with the partial melting of metasomatized peridotite in rocks derived from lower crust (Fig. 10a and b). In addition, the the mantle wedge, the generation of oceanic crust-derived adakitic Kangqiong samples display high abundances of compatible ele- magmas requires an additional heat supply (Defant and Drummond, ments (Cr = 18–32 ppm, Ni = 15–23 ppm) relative to the lower 1990). Defant and Drummond (1990) proposed that only oceanic crust-derived adakitic rocks from the Gangdese Batholith (Guan crust younger than 25 Myr is hot enough to initiate melting of the et al., 2011; Wen et al., 2008).IntheNivs.CrandNivs.Mg# dia- slab. According to the experimental results (Sen and Dunn, 1994), the grams (Fig. 10c and d), the Kangqiong samples all plot on the origin of the adakitic rocks is attributed to slab melting under a high field of adakitic rocks derived from the melting of subducting oce- thermal regime (800–1000 °C at depths of 70–85 km). Thus, adakitic anic crust. The Kangqiong adakitic rocks cannot be derived from rocks could be generated in various hot subduction settings, such as the partial melting of a delaminated lower crust that is also capable the subduction of the mid-ocean ridge (Lagabrielle et al., 2000; Zhang of producing Mg-rich magmatism (cf. Wang et al., 2014). This is et al., 2010) and subduction of very young (b5 Ma) oceanic crust because the available geological and geochemical data indicate (Peacock et al., 1994; Sajona et al., 1993). We note that the Y. Li et al. / Lithos 245 (2016) 34–46 43

1000 a b Fluid-related enrichment Sediment melt 100 800

600

Bangong-Nujiang 60. Tb/ Yh 1 MORB

Ba (ppm) 400 b

50. h

Melt-related enrichment T

200 40. Th/ Sm Kangqiong adakitic rocks 30. 15. 2.0 2.5 3.0 0 0.01 00.5 1.0 1.5 2.0 0.01 1 100 Nb/ Y Th/ Sm 500 c 16 Kangqiong adakitic rocks d

Fluid from altered Mamen adakitic rocks 400 oceanic crust 14 Lower crust-derived 12 adakitic rocks Slab-derived adakitic rocks 300 GLOSS 10

Ba/ Th Bangong-Nujiang

Ce/ Pb Seawater alteration 200 MORB 6 GLOSS averge 4 100 2 Continental contribution 0 0 0.702 0.704 0.706 0.708 0.710 0.712 08010 20 30 40 50 60 70 87 86 Sr/ La ( Sr/ Sr)i

87 86 Fig. 8. Ba vs. Nb/Y (a), Th/Yb vs. Th/Sm (b), Ba/Th vs. ( Sr/ Sr)i (c), and Ce/Pb vs. Sr/La (d) (after Liuetal.,2010) for the Kangqiong adakitic rocks. Data for the Bangong–Nujiang MORB are taken from Bao et al. (2007) and Shi et al. (2008). Data for the GLOSS are from Plank and Langmuir (1998). Data for Mamen adakitic rocks are from Zhu et al. (2009b). emplacement of the Kangqiong adakitic rocks (150–148 Ma) postdate slab-derived dacitic adakites in the western Qiangtang subterrane that the subduction-related magmatism (ca. 168–156 Ma) in the western are interpreted to be the result of oceanic ridge subduction (cf. Zhu segment of the southern Qiangtang subterrane (cf. Li et al., 2014a; Zhu et al., 2015), we propose that the Kangqiong pluton was most likely et al., 2015)(Fig. 1a). Considering that the Kangqiong adakitic rocks derived from the partial melting of the hot and young oceanic crust are ca. 5 Ma younger than the 156 ± 2 Ma OIB-type basaltic rocks and just located south of the subducted mid-ocean ridge (Fig. 11). In combination with existing observations and interpretations, a tectonic scenario for the formation of the Kangqiong adakitic rocks can be summarized as follows (Fig. 11). Before ~156 Ma, the Bangong–Nujiang 2.0 Kangqiong adakitic rocks oceanic lithosphere was subducted northward beneath the Qiangtang Adakitic rocks derived from Terrane and resulted in the development of magmatic arc in the southern thickened lower continental Kelu adakitic rocks crust Qiangtang subterrane. When the subduction continued, the Bangong– Mamen adakitic rocks Nujiang mid-oceanic ridge subducted beneath the Qiangtang Terrane 1.5 and produced distinct rock associations of asthenosphere-derived OIB- type basaltic rocks and slab-derived adakitic dacites during 156 ± 2 Ma

O – 2 (Zhu et al., 2015). Later, at 150 148 Ma, the oceanic crust to the south

N ofthemid-oceanicridgewassubductedbeneaththeQiangtangTerrane. / 1.0

O Partial melting of such a young and hot crust at a transitional zone of am-

K phibolite–eclogite facies with depths of 70 to 85 km produced melts that were parents to the Kangqiong adakitic rocks. Subsequently, such melts interacted with the peridotite in the mantle wedge and contaminated it 0.5 by accretionary complex sediments to form the Kangqiong adakitic granodiorites (Fig. 11). Adakites derived from oceanic slab melting 6. Conclusions 0.0 13 14 15 16 17 18 19 20 (1) The Kangqiong pluton in the central segment of the southern Al23 O (wt.%) Qiangtang subterrane was emplaced at 148–150 Ma and exhibits geochemical signatures of adakitic rocks. Fig. 9. K2O/Na2Ovs.Al2O3 diagram (after Liu et al., 2010). Data for Kelu and Mamen adakitic rocks in Gangdese are from Jiang et al. (2012) and Zhu et al. (2009b). (2) The Kangqiong adakitic magmas were most likely derived from 44 Y. Li et al. / Lithos 245 (2016) 34–46

6 90 a Mantle melts b 80 5 Subducted oceanic crust-derived adakites 70 Subducted oceanic crust-derived 4 60 adakites

)

t 50

g ( 3 Lower crust-derived

O adakites 40

M 2 30

Experimental 20 Lower crust-derived 1 metabasalt adakites and eclogite 10 melts (1-4 Ga) 0 0 45 50 55 60 65 70 75 80 50 55 60 65 70 75 80

SiO2 (wt.%) SiO2 (wt.%) 1000 1000 Kangqiong adakitic rocks c d

Kelu adakitic rocks 100 Mamen adakitic rocks 100

10 10

(ppm)

i i Lower crust-derived

N N adakites Subducted oceanic crust-derived adakites Subducted oceanic 1 crust-derived adakites 1 Lower crust-derived adakites

0.1 0.1 0.1 1 10 100 1000 20 30 40 50 60 70 80 Cr (ppm) Mg#

Fig. 10. (a) MgO vs. SiO2 diagram. The field for adakitic rocks generated by thickened lower crust is after Atherton and Petford (1993), Muir et al. (1995), Petford and Atherton (1996),and Smithies (2000). The field for adakitic rocks derived from partial melting of subducted oceanic crust is after Defant and Drummond (1990), Stern and Kilian (1996), Martin (1999), # Smithies (2000),andDefant et al. (2002).Thefield for experimental melts is from Rapp et al. (1999). (b) Mg vs. SiO2 diagram. Fields of mantle melts; lower crust-derived and subducted oceanic crust-derived adakitic rocks are from (Eyuboglu et al., 2011). (c) Cr vs. Ni diagram (Guan et al., 2011 and references therein). (d) Ni vs. Mg# diagram. The data of lower crust are from Guan et al. (2011) and references therein. Data for Kelu and Mamen adakitic rocks are from Jiang et al. (2012) and Zhu et al. (2009b).

the partial melting of the subducting Bangong–Nujiang basaltic Acknowledgments oceanic lithosphere located just to the south of mid-ocean ridge and underwent interaction with mantle wedge and contamina- This paper is dedicated to Prof. Guitang Pan for his outstanding tion with the accretionary complex during magma ascending. contribution to the geology of the Qinghai–Tibet Plateau. We would (3) The identification of the Kangqiong adakitic rocks in the central like to thank three anonymous reviewers for their constructive and segment of the southern Qiangtang subterrane indicates that helpful reviews. We also sincerely thank Kaiyuan Du and Li Yi for their the Bangong–Nujiang oceanic crust was subducted northward assistance in the field and their assistance with U–Pb dating analyses. beneath the Qiangtang terrane, forming a west-east magmatic We are grateful for helpful discussions with Dicheng Zhu, Jingen Dai, arc of over 800 km during the Late Jurassic. and Zhidan Zhao. The research was financially supported by the Na- tional Natural Science Foundation of China (41572188 and 41172129), N Qiangtang Terrane the National Key Project for Basic Research of China (Project

Sea level 2011CB822001) and the China Geological Survey (1212011221103 and 1212011086037). Accretionary wedge Kangqiong adakitic rocks

Lithospheric mantle Appendix A. Descriptions of analytical methods

Slab melting – Lithospheric Zircon U Pb dating mantle

Mid-ocean ridge Zircons were separated from ~5 kg of materials using heavy-liquid Bangong-Nujiang and magnetic methods at the Laboratory of the Geological Team of oceanic crust Hebei Province, China. Cathodoluminescence images were taken at the Chinese Academy of Geological Sciences (CAGS) for the internal struc- tures of individual zircons and for selecting positions for U–Pb isotope analysis. Zircon dating was conducted using the LA-ICPMS methods at the State Key Laboratory for Mineral Deposits Research at Nanjing Uni- Fig. 11. Simplified model for the production of the Kangqiong adakitic rocks. versity using an Agilent 7500a ICP-MS coupled with a UP213 laser- Y. Li et al. / Lithos 245 (2016) 34–46 45 ablation system (made by New Wave). The analytical procedures used Bao, P.S., Xiao, X.C., Su, L., Wang, J., 2007. Petrological, geochemical and chronological – constraints for the tectonic setting of the Dongco ophiolite in Tibet. Science in for the U Pb dating were described in detail by Belousova et al. China Series D 50, 660–671. (2001). The zircon standard 91500 (origin: Ontario, Canada) used for Belousova, E.A., Griffin, W.L., Shee, S.R., Jackson, S.E., O'Reilly, S.Y., 2001. Two age popula- the correction of mass discrimination was analyzed at intervals of 5 to tions of zircons from the Timber Creek kimberlites, Northern Territory, as determined by laser-ablation ICP–MS analysis. Australian Journal of Earth Sciences 48, 757–765. 7 sample zircon analyses. Afterwards, the data from ICP-MS analyses Bonin, B., 2004. Do coeval mafic and felsic magmas in post-collisional to within-plate were processed using the GLITTER software (Version 4.0). Common Pb regimes necessarily imply two contrasting, mantle and crustal, sources? 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Mount St. Helens: potential example of the partial for all major elements. Trace elements were analyzed using a Finnigan melting of the subducted lithosphere in a volcanic arc. Geology 21, 547–550. Element II ICP-MS. Approximately 50 mg of powdered sample was dis- Defant, M.J., Xu, J.F., Kepezhinskas, P., Wang, Q., Zhang, Q., Xiao, L., 2002. Adakites: some – fl variations on a theme. Acta Petrologica Sinica 18, 129 142. solved in the high-pressure Te on bombs using a HF + HNO3 mixture. Dewey, J.F., Shackleton, R.M., Chang, C., Sun, Y., 1988. The tectonic evolution of the Tibetan Rh was used as an internal standard to monitor signal drift during ICP- plateau. Philosophical Transactions of the Royal Society of London A327, 379–413. MS measurement. The USGS rock standards GSP-1 and AGV-2 were cho- Du, D.D., Qu, X.M., Wang, G.H., Xin, H.B., Liu, Z.B., 2011. Bidirectional subduction of the Middle Tethys oceanic basin in the west segment of Bangonghu–Nujiang suture, sen for calibrating element concentrations of the measured samples. Tibet: evidence from zircon U–Pb LAICPMS dating and petrogeochemistry of arc Analytical uncertainties were lower than 10%. Detailed analytical proce- granites. Acta Petrologica Sinica 27, 1993–2002 (in Chinese with English abstract). dures for trace elements were described by Gao et al. (2003). Elburg, M.A., van Bergen, M., Hoogewerff, J., Foden, J., Vroon, P., Zulkarnain, I., Nasution, A., 2002. Geochemical trends across an arc-continent collision zone: magma sources and Whole-rock Sr, Nd and Pb isotopic compositions were measured slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia. using a Finnigan Triton TI TIMS at the State Key Laboratory for Mineral Geochimica et Cosmochimica Acta 66, 2771–2789. Deposits Research, Nanjing University. 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