RESEARCH

Petrogenesis and tectonic setting of the Early Cretaceous granitoids in the eastern Tengchong terrane, SW China: Constraint on the evolution of Meso-Tethys

Xiaohu He1, Zheng Liu1, Guochang Wang2, Nicole Leonard3, Wang Tao4, and Shucheng Tan1,* 1DEPARTMENT OF GEOLOGY, SCHOOL OF RESOURCE ENVIRONMENT AND EARTH SCIENCE, YUNNAN UNIVERSITY, KUNMING 650091, CHINA 2YUNNAN KEY LABORATORY FOR PALAEOBIOLOGY, YUNNAN UNIVERSITY, KUNMING 650091, CHINA 3RADIOGENIC ISOTOPE FACILITY, SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, THE UNIVERSITY OF QUEENSLAND, BRISBANE QLD 4072, AUSTRALIA 4YUNNAN GEOLOGICAL SURVEY, KUNMING 650091, CHINA

ABSTRACT

As a result of the evolution of Meso-Tethys, Early Cretaceous granitoids are widespread in the eastern Tengchong terrane, SW China, which is considered as the southern extension of the Tibetan Plateau. These igneous rocks are therefore very important for understanding the tectonic setting of Meso-Tethys and the formation of the Tibetan Plateau. In this paper, we present new zircon U-Pb dating, whole-rock elemental, and Nd isotopic data of granitoids obtained from the eastern Tengchong terrane. Our results show that these granitoids are composed of monzogranites and granodiorites and formed at ca. 124 Ma in the Early Cretaceous. Mineralogically and geochemically, these granitoids display metaluminous nature and affinity to I-type granites, which are derived from preexisting intracrustal igneous source

rocks. The predominantly negative whole-rock εNd(t) values (−10.86 to −8.64) for all samples indicate that they are derived mainly from the partial melting of the Mesoproterozoic metabasic rocks in the lower crust. Integrating previous studies with the data presented in this contribution, we propose that the Early Cretaceous granitic rocks (135–110 Ma) also belong to I-type granites with minor high fractionation. Furthermore, in discriminant diagrams for source, granitoids are mainly derived from the partial melting of metaigneous rocks with minor sediments in the lower crust. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar and mafic enclaves indicate that these Cretaceous igneous rocks were the products of the tectonic evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab rollback. Moreover, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane.

LITHOSPHERE; v. 12; no. 1; p. 150–165 | Published online 30 January 2020 https://doi.org/10.1130/L1149.1

INTRODUCTION batholith which is located in the southern Lhasa terrane (Xie et al., 2016; Qi et al., 2019). However, the dynamic setting of Early Cretaceous magmatism Granites, as results of tectono-thermal events, display great diversity in the eastern Tengchong terrane and its tectonic affinity remain debated. due to the variety of their sources, evolution processes, and emplacement For instance, some researchers suggest that abundant Early Cretaceous within different tectonic regimes and geodynamic environments (Barbarin, magmatism in the Tengchong terrane was related to the southward sub­ 1999). Therefore, granites can provide important insights into tectonic set- duction of the Bangong-Nujiang Meso-Tethyan Ocean (Qi et al., 2011; Xu tings and crust-mantle interaction within orogenic belts (Xu et al., 2008). et al., 2012; Zhu et al., 2015, 2017a, 2017b, 2018; Qi et al., 2019). Others Accordingly, the Early Cretaceous granitoids widespread in the Tengchong propose post-collisional settings to interpret the generation of these igneous terrane, SW China, are prime records for understanding their tectonic envi- rocks (Yang et al., 2006; Luo et al., 2012; Xu et al., 2012; Cao et al., 2014). ronments and Sn mineralization related to synchronous magmatism (Cong Recently, identification of mafic enclaves (Cong et al., 2010; Zhang et al., et al., 2011a, 2011b; Qi et al., 2011; Luo et al., 2012; Xu et al., 2012; Cao 2018; Qi et al., 2019) and the Middle Jurassic Myitkyina ophiolitic suite et al., 2014; Zhu et al., 2015; Xie et al., 2016; Fang et al., 2018; Zhang et suggest that the Bangong-Nujiang Ocean in the Tibetan Plateau extended al., 2018; Qi et al., 2019). On the basis of close similarity in geochemical southward into the Myitkyina-Mogok area in Myanmar, to the west of characteristics and chronology of these igneous rocks, geologists proposed the Tengchong terrane (see Fig. 1B; Liu et al., 2016a, 2016b), rather than that the Tengchong terrane is most likely linked with the Lhasa terrane Gaoligong shear zone (see Fig. 1B). Subsequently, the geodynamic setting and experienced similar tectonic histories since the Early Paleozoic (Xu responsible for the Early Cretaceous magmatism and tectonic affinity of et al., 2008, 2012; Xie et al., 2016; Qi et al., 2019). Compared with the the Tengchong terrane needs to be re-evaluated. magmatism in the Lhasa terrane, the Early Cretaceous magmatism in the In this study we present new zircon laser ablation–inductively coupled Tengchong terrane is considered as the eastern extension of the Gangdese plasma–mass spectrometry (LA-ICP-MS) U-Pb dating data, whole-rock geochemical, and Nd isotopic results for Early Cretaceous granites (124 Ma) from the Tengchong terrane. Combined with previous data He Xiaohu http://orcid.org/0000-0003-0515-3172 Shucheng Tan http://orcid.org/0000-0001-8534-7422 of granitoids and mafic enclaves in this area, we provide new insights *[email protected] into their petrogenesis, and the dynamic setting of the Early Cretaceous

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Legend 115 112110 500km 122 Tarim Basin B ARSZ: Ailaoshan-Red River shear zone 115 C 120130 BNS: Bangong-Nujiang Suture Lushui 124 IBR: Indo-Burman Range N MBT: Main Boundary Thrust N. Q J Legend S. Q ia in MCT: Main Central Thrust 124 ia ngt s ngt ang ha S STDS: South Tibet Detachment System ang N L u e S tu YTS: Yarlung-Tsangpo Suture o- BN S r Te S Mes e LSS: Longmu Co-Shuanghu Suture 119 t o P hy -Tet s hys a l Thrust fault Mingguang M Xigaze Lhasa e 25°40'N o Ophiolite belt Shear zone B YT - T S T e 130 t STDS h 120 y MCT s Tengchong

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Figure 1. (A) Simplified tectonic map of the Tibetan Plateau (modified after Qi et al., 2019). (B) Tectonic division of the eastern Tethyan tectonic domain (modified after Liu et al., 2016a). (C) A simple geological map of the Tengchong terrane, SW China (modified after Xie et al., 2016). Numbers in circles represent zircon U-Pb dating of magmatic rocks from literature data in Table 1.

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magmatism, to understand the evolution of Meso-Tethys and the tectonic Cretaceous–Cenozoic igneous rocks intruded this terrane, which is cov- affinity of the Tengchong terrane. ered by Paleogene–Quaternary volcanic rocks. Magmatism of this period induced the absence of the Cretaceous–Eocene sedimentary strata (Liu REGIONAL GEOLOGICAL SETTINGS et al., 2009). Two stages of magmatism in the Tengchong terrane have been reported in previous studies: (1) Late Cretaceous to early Cenozoic The Tengchong terrane is located in the southern margin of the Tibetan magmatism (75–65 Ma, Xie et al., 2016) and (2) early Cenozoic mag- Plateau (see Fig. 1A) and is bound by the Gaoligong shear zone to the matism (55–47 Ma, Xu et al., 2012; He et al., 2019a). The terrane can east and the Myitkyina suture to the west (Replumaz and Tapponnier, be divided into the eastern and western Tengchong by the Dayingjiang 2003; see Fig. 1B). Previous studies on the paleogeographic evolution fault (Fig. 1C) and is also cut by several ductile shear zones such as the of eastern Tethys (Cocks and Torsvik, 2013; Metcalfe, 2013; Burrett et NNE-trending dextral Nabang shear zone and the prominent N-S–strik- al., 2014) suggest that the Tengchong terrane was located in the northern ing dextral Gaoligong shear zone, which, according to 40Ar/39Ar dating, margin of the Gondwana supercontinent during the early Paleozoic, and separates the Tengchong terrane from the Baoshan terrane and deformed accreted to the Eurasia plate in the late Mesozoic. Before an amalgama- ca. 18–13 Ma (Fig. 1C; Lin et al., 2009; Xie et al., 2016; Cao et al., 2019). tion of Tengchong and Baoshan terranes, there was a Bangong-Nujiang Meso-Tethyan Ocean between the two terranes. This ocean had been SAMPLING AND PETROGRAPHY closed before the Early Cretaceous (e.g., Fan et al., 2018, and references therein) and now is represented by the east-west–trending Bangong-Nuji- Eleven samples for this study were collected from the Xishanjiao ang suture zone, which crosses the central Tibetan Plateau and the west pluton in the central Tengchong terrane (Fig. 2). Samples consist of of the Tengchong terrane and to the south it is correlated with the Shan medium-coarse grained biotite monogranites (XSJ-1, XSJ-2, XSJ-3, boundary in Burma (see Fig. 1B). The closure of the Neo-Tethys (leading XSJ-4, XSJ-5, XSJ-8, XSJ-9) and biotite granodiorites (XSJ-6, XSJ-10,

to the Yarlung-Tsangpo suture) happened after the Late Cretaceous (Xu XSJ-11, XSJ-12). A summary of the locations, lithology, ages, and εHf(t) et al., 2012). The Meso-Neo Proterozoic metamorphic basement, termed of the samples (including this study) from the Tengchong terrane is listed the Gaoligongshan Group, is considered as the oldest geological unit in in Table 1. The representative petrographic features with fine-grained the Tengchong terrane, yielding zircon U-Pb ages of 1053–635 Ma and texture and massive structure for granites of this study are shown in 490–470 Ma, according to Song et al. (2010) and is mainly composed Figure 3. Major minerals are quartz (~30 vol%), K-feldspar (~20 vol%), of amphibolite, gneiss, quartzite, schist, marble, and slate (Zhao et al., plagioclase (~30 vol%), and biotite (~15 vol%). Accessory minerals are 2016a, 2016b). The overlying Paleozoic strata sequence consists of largely hornblende, zircon, and apatite. Plagioclase are partly altered to sericite glacial-marine diamictite, sandstone, and limestone (Fig. 1C). Numerous and kaolinite (see Figs. 3D and 3E).

25°20'N 25°20'N E E ' ' 5 0

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XSJ-9 Quaternary sediment

Zhaojiazhai Shunjiang Permian limestone XSJ-8

XSJ-10 Eocene granite

Carboniferous sandstone/slate XSJ-6 Triassic granite XSJ-11 Xishan Proterozoic Gaoligong XSJ-12 metamorphic rocks XSJ-4 Cretaceous granite XSJ-5 XSJ-3 E XSJ-2 Sampling location Q Xinzhai XSJ-1 E

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° Dacaopo ° 8 8 9 9 25°10'N 25°10'N

Figure 2. Geological map of the Xishanjiao area, in the Tengchong terrane, with sampling locations.

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ge A (Ma) TECT 121 ± 2.0 115 ± 1 110 ± 1 112 ± 1 118 ± 1 116 ± 3 122 ± 1.9 120 ± 0.6 122 ± 1.7 128 ± 1.9 125 ± 1.3 128 ± 1.9 122 ± 3 115 ± 1 116 ± 2 117 ± 1.2 119 ± 2.1 118 ± 2.1 125 ± 1.3 115 ± 5 120 ± 2.0 115 ± 1 114 ± 1 127.4 ± 1 120.4 ± 1.7 124.2 ± 0.5 124.3 ± 0.6 119.6 ± 0.93 124.1 ± 1.6 115.2 ± 1.1 117.6 ± 0.8 114.3 ± 0.6 115.7 ± 0.77 122.2 ± 1.2 122.6 ± 0.8 115.3 ± 0.87 128.9 ± 2.4 128.9 ± 2.4 130.6 ± 2.5 AND , 2 OGY .40 SiO 77 70.31 74.75 73.86 62.90 72.90 70.23 75.30 75.90 71.90 72.90 72.70 65.50 72.96 66.08 76.00 65.39 62.60 61.02 74.48 60.10 74.26 67.30 65.10 67.58 71.35 73.16 70.97 67.99 51.40 51.53 63.03 65.28 49.20 52.19 69.53 73.84 69.73 64.50 (wt%) THOL , LI GES A Locations ′50″ N25°12′35″E98°27 N24°52′56″ E98°42′43″ N24°35′50″ E98°15′00″ N24°30′45″ E98°22′08″ N25°57′03″ E98°44′44″ ′51″ N25°12′40″ E98°27 ′39″ N25°58′35″ E98°47 N25°57′12″ E98°44′29″ ′59″ N25°58′55″ E98°47 N25°58′55″ E98°48′25″ N24°38′01″ E98°31′15″ N25°39′50″ E98°31′27″ N25°51′00″ E98°33′00″ N24°31′06″ E98°17′10″ N25°49′08″ E98°32′04″ N24°14′41″ E98°00′33″ N24°30′00″ E98°17′10″ N24°57′31″ E98°42′44″ N24°26′33″ E98°23′14″ N25°57′13″ E98°44′27″ N24°46′09″ E98°32′35″ N25°57′55″ E98°46′49″ N25°57′03″ E98°44′43″ N24°59′26″ E98°36′01″ N25°57′44″ E98°42′57″ N24°27′45″ E98°16′08″ N24°03′57″ E98°50′14″ N24°57′31″ E98°42′44″ N24°34′20″ E98°17′50″ N25°58′59″ E98°42′26″ N25°16′03″ E98°35′31″ N25°05′41″ E98°30′39″ N25°57′55″ E98°46′49″ N24°31′37″ E98°12′11″ N24°41′48″ E98°34′01″ N24°49′51″ E98°23′39″ N24°42′12″ E98°32′44″ N24°49′51″ E98°23′39″ N25°46′19″ E98°29′31″ U-Pb Y OF ZIRCON e e e e e e v e e e e e e e e e e e e v it e e e e e e e e it it it e e it it it it it it it it anit anit anit anit r r r r e e e e e e e e e anit anit anit anit anit anit es anit r r r r r r r it ypes onit onit ic g R SUMMA odior odior odior ic dior anit anit anit anit anit anit anit anit og og og og og og k t enit e encla oic encla yr y br med g med g med g it Dior Gneiss Gr Gr Gr Gr Gr Gr Gr Gr anodior anodior anodior anodior anodior anodior anodior anodior oc enog S br ph y Monz Monz or or or R eldspar granit Gr Gr Gr Gr Gr Gr Gr Gr Monz Monz Monz S Monz Monz Monz Monz Monz Monz or Gab Dior P Def Def Def Gab K-f ABLE 1. ABLE T -57 -5 -83 -65 -39 -21 ample umber 10QTG-38 QS-1 13TB-48 XSJ-05 XSJ-10 13TB-52 13TB-53 13TB-54 13TB-44 15QMH-1 JJGLZ-340 15QML GD15 15QLL 15QML 13TB-43-A 09Qt-19 13TB-47 13TB-51 13TC09 13TC04 PM2-104 15QML 15QLR-5 GD04 09Qt-33 15QLP DMJ-08 09Qt-20 13TB-50A 15QML lxk-2 D9021H1 lxk-1 D5910H1 DT12 S n

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C D A B

50μm E F

Amp

50μm 50μm

Figure 3. Field outcrop photos, hand specimen photograph, and photomicrographs of Xishanjiao granites from the Tengchong terrane, SW China. (A-B) Field outcrop photos; (C) hand specimen; (D–F) photomicrographs under cross-polarized light. Amp—amphibole; Bt—biotite; Kfs—K-feldspar; Qtz—quartz; Pl—plagioclase.

ANALYTICAL METHODS PW4400 X-ray fluorescence spectrometer on fused glass beads at Wuhan Sample Solution Analytical Technology Co., Ltd., China. The analyses Zircon U-Pb Dating were monitored by international standard references AGV-1, BCR-2, and BHVO-1, which showed that the analytical errors are <2%. Trace elements Sample ages were calculated based on LA-ICP-MS U-Pb dating of were analyzed using ICP-MS at the State Key Laboratory of Geologi- zircons from the Xishanjiao granites (XSJ-05, XSJ-10). Zircon grains cal Processes and Mineral Resources, China University of Geosciences were extracted by heavy liquid and magnetic separation, before being (Wuhan), following procedures established by Liu et al. (2008). handpicked under a binocular microscope for mounting in epoxy resin. To identify their internal structure and to choose potential target sites for the Nd Isotope U-Pb analysis, cathodoluminescence (CL) images (Fig. 4) were obtained using a scanning electron microscope at the Institute of Geochemistry, High-precision Nd isotope measurements were performed on a Triton Chinese Academy of Sciences (IGCAS), Guiyang, China. Measure- thermal ionization mass spectrometer at SKLOG, IGCAS. Samples for Nd ments of U, Th, and Pb isotopes were conducted using an Agilent 7500a isotopic analyses were dissolved in an acidic mixture of 0.5 mL 40 wt%

LA-ICP-MS at the State Key Laboratory of Ore Deposit Geochemistry HNO3 and 1.0 mL HF 40 wt% in Teflon bombs that were steel-jacketed (SKLOG), IGCAS. A GeoLasPro laser-ablation system and an Agilent and placed in the oven at 195 °C for three days. Digested samples were 7700x inductively coupled plasma–mass spectrometry (ICP-MS) were dried down on a hotplate, reconstituted in 1.5 mL of 1.5N HCl before combined for these experiments. A beam size of 33 μm was used for all ion exchange purification. Sample chemical separation was conducted samples; zircon 91500 (1062 Ma) and NIST SRM 610 with Si were used following Pu et al. (2005). Mass fractionation corrections for Nd isotopic as internal standards for external calibration, and Zr was used as internal ratios were based on 146Nd/144Nd = 0.7219. The 143Nd/144Nd ratios of the standard for other trace elements (Liu et al., 2010). GJ-1 (600 Ma) and La Jolla and JNDI-1 Nd standard solutions were 0.511841 ± 3 (2σ) and Plešovice (337 Ma) were treated as quality control for geochronology. 0.512104 ± 5 (2σ) (Tanaka et al., 2000; Weis et al., 2006). Operational and analytical methods are detailed by Li et al. (2009). Mea- sured compositions were corrected for common Pb using non-radiogenic RESULTS 204Pb. As corrections were sufficiently small to be insensitive to the choice of common Pb composition, an average of present-day crustal composi- Zircon U-Pb Age tion (Stacey and Kramers, 1975) was used for common Pb, assuming that it is largely related to surface contamination introduced during sample Two representational samples (XSJ-05, XSJ-10) were selected from preparation. Uncertainties relating to individual analysis in the data tables the Xishanjiao granites in the Tengchong terrane for zircon U-Pb dating. are reported at a 1δ level, and mean ages for pooled U/Pb (and Pb/Pb) Seventeen and sixteen reliable U-Pb age analytical spots were obtained analyses are quoted at a 95% confidence interval. Data reduction was from XSJ-05 and XSJ-10, respectively. Complete U-Pb isotopic data for conducted using the Isoplot/Ex v. 2.49 program (Ludwig, 2001). these zircons are presented in Table 2. LA-ICP-MS data for XSJ-05 and XSJ-10 yielded weighted mean average 206Pb/238U ages of 124.2 ± 0.5 Ma Whole-Rock Major and Trace Elements (1σ, mean square weighted deviation [MSWD] = 0.15) and 124.3 ± 0.6 Ma (1σ, MSWD = 0.11) (Fig. 4), respectively. In the CL images (Fig. 4), most The granites were crushed to less than 200 mesh for whole-rock major zircon grains show oscillatory zoning, typical of igneous zircons (Wu and and trace elemental analyses. Major oxides were measured using an Axios Zheng, 2004). All of the analyzed spots have high U (251–1093 ppm) and

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206Pb/238U weighted age XSJ-05 0.0202 124.2±0.5 Ma 1 2 3 4 n=16, MSWD=0.15 128

0.0198 5 6 7 8 U 8 3 2 /

b 9 10 12 0.0194 124 11 P 6 0 2 15 128 13 14 16 0.0190 126 124 100μm 120 122 120 0.0186 0.115 0.125 0.135 0.145 207Pb/235U

206Pb/238U weighted age XSJ-10 0.0202 124.3±0.6 Ma 1 2 4 n=17, MSWD=0.11 128 3

U 0.0198 8 5 6 7 8 3 2 / b

P 9 10 11 12 13 6 0.0194 124 0 2

15 128 14 16 17 0.0190 126 124 100μm 120 122 120 0.0186 0.11 0.12 0.13 0.14 0.15 207Pb/235U

Figure 4. U-Pb Concordia plots and representative cathodoluminescence images of zircon grains from Xishanjiao granites, SW China. The solid circles represent U-Pb dating targets. MSWD—mean square weighted deviation.

Th (150–1164 ppm) contents with Th/U ratios of 0.49–2.48 (greater than Al2O3/(CaO+Na2O+K2O)] (0.96–1.25; Fig. 5B), and Rittmann indexes 0.1), which suggests that these zircons have a magmatic origin (Belousova (σ) ranging between 1.60 and 2.06, suggesting that these granites are et al., 2002; Wu and Zheng, 2004). Therefore, we interpret these zircon metaluminous to weakly peraluminous and high-K calc-alkaline series U-Pb ages as the timing of crystallization of the Xishanjiao granites. (Fig. 5C). Trace element analysis of the Xishanjiao granites show simi- lar chondrite-normalized rare-earth element (REE) patterns (Fig. 6A) Major and Trace Element Geochemistry inclining to the right, showing relatively high total REE contents of 114–202 ppm, enrichment in light REE (102–179 ppm) with relatively Major and trace elemental data of all granite samples are listed negative Eu anomalies (δEu = 0.32–0.66), light REE/high REE ratios of

in Table 3. In the total alkali versus silica diagram (Fig. 5A), seven 3.82–10.52, (La/Yb)N ratios of 2.41–11.35, and Sr/Y ratios of 2.06–6.97. samples are plotted in the granite area whereas the others are in the The primitive mantle-normalized trace element spider diagram (Fig. 6B)

granodiorite area. The Xishanjiao granites have variable contents of SiO2 shows relatively marked depletions in large ion lithophile elements

(65.8–74.6 wt%), K2O (1.82–4.89 wt%), K2O/Na2O (0.38–1.29 wt%), (LILE) such as Ba and Sr and high field strength elements (HFSE) such T and K2O+Na2O (6.91–8.21 wt%) and relatively high contents of TiO2 as Th, U, Nb, P, Ti. On the Harker diagrams (Fig. 7), the Al2O3, Fe2O3 , T (0.23–0.59 wt%), CaO (0.18–3.41 wt%), P2O5 (0.03–0.14 wt%), Fe2O3 CaO, MgO, TiO2, and P2O5 contents for the Xishanjiao samples exhibit

(1.67–4.51 wt%), and MgO (0.12–1.64 wt%). They also have rela- decreasing trends with increasing SiO2 contents, which is consistent with

tively low Al2O3 contents (12.7–15.1 wt%), A/CNK indexes [molar previously published available data.

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TABLE 2. LASER ABLATION–INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY U-Pb ISOTOPIC COMPOSITIONS AND AGES OF ZIRCON FROM XISHANJIAO GRANITES FROM THE TENGCHONG TERRANE, SW CHINA

Spot Concentration (ppm) Isotope ratio Age (Ma) Th/U number Pb* U Th 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/235U 1σ 206Pb/238U 1σ XSJ-7-1 27.9 829 1149 1.39 0.0490 0.0007 0.1313 0.0019 0.0194 0.0001 125.2 1.7 124.0 0.9 XSJ-7-2 17.3 384 726 1.89 0.0484 0.0016 0.1307 0.0043 0.0195 0.0002 124.7 3.9 124.8 1.4 XSJ-7-3 17.8 549 573 1.04 0.0499 0.0011 0.1334 0.0027 0.0194 0.0002 127.2 2.4 124.1 1.0 XSJ-7-4 18.7 338 836 2.48 0.0491 0.0009 0.1315 0.0024 0.0194 0.0001 125.4 2.2 124.0 0.9 XSJ-7-5 17.7 559 571 1.02 0.0486 0.0014 0.1291 0.0037 0.0193 0.0002 123.3 3.3 123.2 1.2 XSJ-7-6 15.1 458 617 1.35 0.0500 0.0009 0.1340 0.0023 0.0195 0.0002 127.7 2.1 124.5 1.0 XSJ-7-7 13.3 555 502 0.90 0.0479 0.0012 0.1293 0.0033 0.0195 0.0002 123.5 3.0 124.8 1.2 XSJ-7-8 27.0 665 1154 1.73 0.0484 0.0008 0.1306 0.0022 0.0195 0.0001 124.6 2.0 124.6 0.9 XSJ-7-9 28.9 789 1214 1.54 0.0493 0.0008 0.1317 0.0022 0.0194 0.0001 125.6 2.0 123.6 0.8 XSJ-7-10 9.23 399 342 0.86 0.0484 0.0015 0.1302 0.0040 0.0195 0.0002 124.2 3.6 124.7 1.2 XSJ-7-11 14.6 517 379 0.73 0.0495 0.0025 0.1326 0.0060 0.0194 0.0002 126.5 5.4 123.8 1.3 XSJ-7-12 29.0 962 1164 1.21 0.0483 0.0010 0.1297 0.0030 0.0194 0.0002 123.9 2.7 124.1 1.2 XSJ-7-13 18.0 586 724 1.24 0.0486 0.0009 0.1305 0.0023 0.0194 0.0001 124.5 2.1 124.1 0.9 XSJ-7-14 25.1 585 1069 1.83 0.0496 0.0008 0.1329 0.0021 0.0194 0.0001 126.7 1.9 124.1 0.9 XSJ-7-15 13.0 549 488 0.89 0.0484 0.0011 0.1296 0.0028 0.0195 0.0002 123.7 2.6 124.3 1.0 XSJ-7-16 17.7 541 563 1.04 0.0485 0.0010 0.1301 0.0026 0.0194 0.0002 124.2 2.4 124.0 1.0 XSJ-10-1 6.11 338 200 0.59 0.0490 0.0027 0.1303 0.0069 0.0195 0.0003 124.4 6.2 124.5 1.8 XSJ-10-2 4.95 307 150 0.49 0.0563 0.0099 0.1303 0.0071 0.0195 0.0002 124.3 6.4 124.6 1.5 XSJ-10-3 9.79 395 359 0.91 0.0487 0.0016 0.1300 0.0040 0.0194 0.0002 124.1 3.6 124.0 1.2 XSJ-10-4 10.5 534 362 0.68 0.0488 0.0017 0.1304 0.0044 0.0194 0.0002 124.5 4.0 124.0 1.1 XSJ-10-5 30.2 1093 852 0.78 0.0510 0.0016 0.1369 0.0042 0.0194 0.0002 130.2 3.8 124.0 1.5 XSJ-10-6 14.7 635 541 0.85 0.0483 0.0010 0.1301 0.0027 0.0195 0.0002 124.2 2.5 124.8 1.0 XSJ-10-7 11.5 485 417 0.86 0.0488 0.0015 0.1302 0.0039 0.0194 0.0002 124.2 3.5 123.8 1.1 XSJ-10-8 18.2 576 573 0.99 0.0475 0.0016 0.1269 0.0039 0.0195 0.0002 121.3 3.5 124.4 1.3 XSJ-10-9 13.4 455 391 0.86 0.0481 0.0022 0.1298 0.0061 0.0195 0.0002 123.9 5.5 124.3 1.3 XSJ-10-10 9.47 453 324 0.71 0.0474 0.0014 0.1265 0.0036 0.0195 0.0002 121.0 3.2 124.8 1.2 XSJ-10-11 9.00 340 331 0.97 0.0498 0.0018 0.1318 0.0041 0.0196 0.0002 125.7 3.7 124.9 1.4 XSJ-10-12 15.4 540 606 1.12 0.0488 0.0016 0.1309 0.0038 0.0195 0.0003 124.9 3.4 124.4 1.7 XSJ-10-13 15.1 442 498 1.13 0.0497 0.0016 0.1324 0.0042 0.0193 0.0002 126.2 3.8 123.5 1.2 XSJ-10-14 7.30 251 286 1.14 0.0511 0.0014 0.1367 0.0039 0.0195 0.0002 130.1 3.5 124.3 1.1 XSJ-10-15 13.3 717 451 0.63 0.0471 0.0015 0.1261 0.0039 0.0194 0.0002 120.6 3.5 123.9 1.2 XSJ-10-16 10.6 482 380 0.79 0.0493 0.0014 0.1308 0.0034 0.0194 0.0002 124.8 3.0 124.1 1.1 XSJ-10-17 11.7 474 438 0.93 0.0488 0.0015 0.1306 0.0039 0.0195 0.0002 124.6 3.5 124.7 1.4

TABLE 3. MAJOR AND TRACE ELEMENT COMPOSITIONS OF XISHANJIAO GRANITES FROM THE TENGCHONG TERRANE, SW CHINA Rock type Monzogranite Granodiorite Sample no. XSJ-1 XSJ-2 XSJ-3 XSJ-4 XSJ-5 XSJ-8 XSJ-9 XSJ-6 XSJ-10 XSJ-11 XSJ-12

Major element oxides (wt%)

SiO2 73.2 73.5 74.2 72.7 72.9 74.6 73.6 65.8 70.2 69.3 68.3

Al2O3 13.1 13.1 12.7 13.4 13.5 13.7 13.7 15.0 14.1 14.7 15.1 CaO 1.84 1.55 1.67 1.80 1.41 0.18 0.31 2.78 2.92 2.87 3.41 T Fe2O3 2.00 1.97 1.67 2.29 1.80 1.51 1.69 4.51 3.09 3.16 3.48

K2O 3.74 4.00 4.08 3.88 4.65 4.24 4.89 1.85 3.24 3.45 2.89 MgO 0.59 0.55 0.51 0.51 0.49 0.12 0.14 1.64 1.07 1.07 1.21 MnO 0.06 0.05 0.05 0.04 0.06 0.06 0.02 0.13 0.07 0.07 0.08

Na2O 3.33 3.27 3.18 3.59 3.22 3.64 3.13 4.87 3.40 3.54 3.81

P2O5 0.06 0.06 0.05 0.07 0.06 0.05 0.03 0.14 0.10 0.10 0.11

TiO2 0.28 0.28 0.23 0.29 0.24 0.26 0.26 0.59 0.41 0.41 0.46 LOI 0.86 0.82 0.83 0.57 0.79 1.26 1.49 1.80 0.70 1.11 0.41 Total 99.1 99.2 99.1 99.1 99.1 99.6 99.3 99.1 99.3 99.8 99.2

K2O+Na2O 7.20 7.39 7.39 7.58 8.00 8.01 8.21 6.91 6.73 7.09 6.78

K2O/Na2O 1.12 1.22 1.29 1.08 1.45 1.16 1.56 0.38 0.95 0.97 0.76 A/CNK 1.01 1.04 1.00 1.00 1.04 1.25 1.25 0.99 0.98 0.99 0.96 A/NK 1.34 1.29 1.25 1.31 1.21 1.25 1.20 1.81 1.57 1.55 1.70

CaO/Na2O 0.55 0.47 0.53 0.50 0.44 0.05 0.10 0.57 0.86 0.81 0.89 Mg# 36.9 35.6 37.8 30.5 35.0 13.4 14.4 41.9 40.6 40.1 40.8 (continued)

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TABLE 3. MAJOR AND TRACE ELEMENT COMPOSITIONS OF XISHANJIAO GRANITES FROM THE TENGCHONG TERRANE, SW CHINA (continued) Rock type Monzogranite Granodiorite Sample no. XSJ-1 XSJ-2 XSJ-3 XSJ-4 XSJ-5 XSJ-8 XSJ-9 XSJ-6 XSJ-10 XSJ-11 XSJ-12 σ 1.64 1.72 1.68 1.87 2.06 1.95 2.09 1.94 1.60 1.84 1.76 Trace element (ppm) Li 37.7 31.2 23.1 18.5 21.8 0.25 0.46 40.2 22.9 24.9 21.2 Be 2.97 3.01 3.19 2.01 2.69 2.55 3.08 8.35 1.60 2.95 2.16 Sc 4.40 4.44 3.94 6.79 3.96 1.97 1.93 9.54 7.52 8.10 8.44 V 23.1 19.9 22.3 23.0 20.0 17.8 21.5 57.3 49.9 49.5 55.7 Cr 3.94 3.82 12.6 4.93 7.67 15.1 3.36 5.78 8.51 6.52 10.3 Co 24.5 36.3 31.4 19.8 21.1 22.0 20.7 14.3 23.5 26.8 22.7 Ni 1.41 2.02 5.43 1.63 3.28 6.67 1.63 3.36 3.66 3.10 4.65 Cu 3.77 6.69 5.83 2.42 2.68 1.97 2.55 7.22 2.83 2.98 3.80 Zn 56.4 50.5 31.1 82.5 25.1 37.4 49.4 209 88.9 53.6 130 Ga 17.9 17.1 16.1 18.3 16.9 13.6 12.7 20.5 17.1 17.8 18.5 Se 1.19 1.41 1.08 1.22 1.09 0.80 0.61 1.22 0.90 0.97 1.04 Rb 245 262 271 193 271 248 296 297 176 239 173 Sr 116 108 108 102 123 54.2 76.1 121 161 166 180 Y 40.1 39.8 36.1 33.8 35.7 22.1 19.5 58.9 24.6 28.9 25.8 Zr 138 128 119 183 118 146 127 175 183 185 194 Nb 17.2 15.9 14.4 10.9 13.1 12.3 11.0 44.6 9.91 10.4 12.0 Mo 0.32 0.30 0.37 0.35 0.34 2.64 0.34 1.11 0.55 0.36 0.37 Sn 5.01 4.88 3.87 2.99 4.25 4.57 6.36 14.1 13.8 5.56 3.62 Cs 5.00 4.93 5.76 3.27 4.71 5.36 4.54 11.5 4.38 8.04 3.90 Ba 211 236 216 634 340 98.4 236 95.5 533 564 434 La 45.0 34.9 29.1 37.7 36.3 35.5 24.7 25.5 37.0 35.3 35.3 Ce 88.5 70.5 54.5 73.7 70.9 70.7 51.0 53.2 69.9 66.2 66.8 Pr 9.17 7.63 6.09 8.02 7.52 6.99 5.11 6.35 7.14 6.78 6.91 Nd 30.6 26.3 21.2 29.5 26.0 23.2 17.6 23.8 25.3 23.6 25.1 Sm 5.65 5.27 4.29 5.52 5.03 4.01 3.20 5.26 4.47 3.99 4.35 Eu 0.61 0.59 0.54 0.92 0.66 0.44 0.40 0.57 0.89 0.85 0.91 Gd 5.28 5.04 4.38 5.49 4.80 3.58 3.05 5.68 4.26 3.97 4.09 Tb 0.88 0.86 0.72 0.86 0.79 0.55 0.49 0.97 0.67 0.61 0.65 Dy 5.52 5.48 4.58 5.27 4.93 3.29 3.03 6.59 3.97 3.82 3.97 Ho 1.22 1.21 1.03 1.09 1.08 0.71 0.67 1.59 0.84 0.82 0.84 Er 3.78 3.77 3.23 3.15 3.32 2.17 2.02 5.37 2.40 2.56 2.48 Tm 0.62 0.64 0.55 0.49 0.54 0.35 0.33 0.99 0.37 0.44 0.40 Yb 4.26 4.31 3.75 3.17 3.67 2.37 2.15 7.57 2.34 3.13 2.71 Lu 0.65 0.65 0.58 0.47 0.56 0.36 0.32 1.27 0.35 0.51 0.43 Hf 4.29 4.00 3.92 5.40 3.61 4.72 3.87 5.90 4.76 5.20 5.24 Ta 2.57 2.33 2.20 1.06 1.89 2.35 2.04 4.92 1.01 1.23 1.37 W 161 217 181 143 133 107 101 124 210 201 175 Pb 31.6 33.0 28.0 41.6 23.8 23.7 47.7 88.9 42.7 29.7 76.7 Th 48.5 42.5 35.5 21.7 37.9 37.9 34.4 17.1 26.1 24.0 21.0 U 9.51 7.07 7.49 2.36 5.93 6.82 5.86 11.8 3.87 4.24 4.59 ΣREE 202 167 134 175 166 154 114 145 160 153 155 LREE 180 145 116 155 146 141 102 115 145 137 139 HREE 22.2 22.0 18.8 20.0 19.7 13.4 12.1 30.0 15.2 15.9 15.6 LREE/HREE 8.09 6.61 6.15 7.76 7.43 10.5 8.46 3.82 9.52 8.62 8.95

LaN/YbN 7.59 5.82 5.57 8.53 7.09 2.41 11.4 9.34 10.7 8.24 3.45 δEu 0.34 0.35 0.38 0.51 0.41 0.32 0.62 0.66 0.36 0.39 0.13 δCe 1.07 1.06 1.00 1.04 1.05 1.03 1.05 1.05 1.10 1.12 0.85 Sr/Y 2.90 2.71 3.00 3.02 3.45 2.46 3.89 2.06 6.56 5.76 6.97 Zr/Hf 32.3 32.1 30.2 33.8 32.8 30.9 32.9 29.7 38.5 35.5 37.1 Nb/Ta 6.68 6.83 6.56 10.3 6.96 5.24 5.38 9.05 9.77 8.47 8.74 Rb/Sr 2.11 2.43 2.50 1.89 2.21 4.58 3.88 2.44 1.09 1.44 0.96

TZr (°C) 819 814 805 844 806 837 823 833 840 841 842

# T T T Note: Mg = 100 molar*MgO/(Mg + FeO ), assuming FeO = 0.9*Fe2O3 ; TZr (°C) is calculated from zircon saturation thermometry (Watson and Harrison, 1983). A/CNK—molar Al2O3/(CaO+Na2O+K2O); A/NK—molar Al2O3/(Na2O+K2O); ΣREE—total rare earth elements; LOI—loss on ignition; LREE—light rare earth elements; HREE—heavy rare earth elements.

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15 A feldspar Xishanjiao granites 7 B syenite Early Cretaceous granitoids 6 feldspar 12 monzonite syenite

%) 5 series monzonite %)

9 (wt. monzonite 4 quartz Shoshonite O

2 monzo- (wt. series diorite granite O 3 6 2 calc-alkaline K O+Na

2 High-K 2 series K 3 gabbro diorite granodiorite Calc-alkaline diorite 1 series Tholeiite 0 0 50 55 60 65 70 75 80 40 45 50 55 60 65 70 75 80

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

3.5 C 3.0

Figure 5. Plots of (A) (K2O+Na2O) versus SiO2, (B) K2O ver- 2.5

sus SiO2, and (C) A/NK [molar Al2O3/(Na2O+K2O)] versus

A/CNK [molar Al2O3/(CaO+Na2O+K2O)] of Xishanjiao gran- 2.0 Metaluminous Peraluminous ites in the Tengchong terrane, SW China. Data for the Early

Cretaceous granitoids are sourced from previous refer- A/NK 1.5 ences (Yang et al., 2006; Cong et al., 2011a, 2011b; Luo et al., 2012; Cao et al., 2014; Zhang et al., 2018; Zhu et al., 1.0 2015, 2018; Qi et al., 2011, 2019). 0.5 Peralkaline 0.0 0.5 1.0 1.1 1.5 2.0 A/CNK

103 103 Xishanjiao granites Early Cretaceous granitoids 102 2

10

101

101 Sample/Chondrite 100 Sample/Primitive mantle B 0 A 10 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rb Ba Th U K Ta Nb La Ce Pr Sr P Nd Zr Hf Sm Eu Ti Dy Y Ho Yb Lu

Figure 6. (A) Chondrite-normalized rare earth elements patterns and (B) primitive-mantle normalized trace elemental spider diagram for Xishanjiao granites and Early Cretaceous granitoids, SW China. The chondrite and primitive-mantle normalization values are from Boynton (1984) and Sun and McDonough (1989), respectively. Data sources are the same as in Figure 5.

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19 Xishanjiao granites 6 18 Early Cretaceous granitoids 5 17 ) ) 16 4 % %

. . t 15 t w w (

( 3

3 14 O O 2 l a A

13 C 2 12 1 11 10 0 3.0 9 8 2.5 7 ) ) 2.0 6 % %

. . t t 5 w w (

( 1.5 T

3 4 O O 2 g

1.0 e

M 3 F 2 0.5 1 0.0 0 7 6

6 5

5 ) ) %

4 . t %

. w

t 4 (

w 3 ( O

3 2 a O 2 N

K 2 2

1 1

0 0 1.0 0.30

0.25 0.8 ) ) 0.20 %

. % t 0.6 . t w (

w 5 0.15 I ( - t 2 y O p 2 e t O 0.4 i P re T 0.10 nd 0.2 0.05

0.0 0.00 60 63 66 69 72 75 78 60 63 66 69 72 75 78

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

Figure 7. Harker diagrams for Xishanjiao granites and Early Cretaceous granitoids in the Tengchong terrane, SW China. Data sources are the same as in Figure 5.

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Nd Isotope Geochemistry variable SiO2 contents (65.8–74.2 wt%), high Na2O (3.18–4.87 wt%), and A/CNK indexes (0.96–1.04) indicating metaluminous to weakly The results of Nd isotopic compositions of eleven granite samples are peraluminous compositions, similar to the geochemical characteris-

presented in Table 4. The initial εNd(t) values were calculated at t = 124 Ma. tics of I-type granites. Furthermore, various trends of trace elements

In addition, depleted mantle Nd model ages (TDM) were calculated using such as Rb, Y, and Th can be used to distinguish I- or S-type granites. the De Paolo’s model (1981). Nd isotopic compositions of the Xishanjiao In the Rb versus Th and Y diagrams (Figs. 8C and 8D), the plots of

granites are characterized by relatively homogenous εNd(t) values ranging the Xishanjiao samples show a distinct I-type trend, which is also

from −10.86 to −8.64, with corresponding two-stage depleted-mantle Nd evidenced by the decrease of P2O5 with the increase of SiO2 (Fig. 7)

model ages (T2DM) ranging from 1.49 to 1.68 Ga. and the occurrence of amphibole (Fig. 3D). Therefore, based on the mineralogical and geochemical results, we suggest that the Xishan- DISCUSSION jiao granites are I-type granites, which is consistent with the results reported previously for the Tengchong terrane (Yang et al., 2006; Cong Genetic Type of the Xishanjiao Granites et al., 2011a, 2011b; Qi et al., 2011, 2019; Luo et al., 2012; Cao et al., 2014; Zhu et al., 2015, 2018; Fang et al., 2018; Zhang et al., 2018). We Generally, granites are divided into I-, S-, and A-type granites in the have summarized data of the Early Cretaceous igneous rocks from the literature (Chappell and White, 1974; Loiselle and Wones, 1979). The Tengchong terrane and find that they show similar trends and chemical most important feature of S-type granites is that they are always peralu- characteristics. For instance, all samples reveal I-type trends in the plot

minous with high A/CNK (>1.1) and generally accompanied by Al-rich of P2O5 versus SiO2 (Fig. 7) and in the Rb versus Th and Y diagrams minerals such as cordierite or muscovite. The excess Al is considered (Figs. 8C and 8D), suggesting that Early Cretaceous igneous rocks to be hosted in Al-rich biotite (A/CNK = 1.3–1.5 in biotite, according have I-type affinity with variable degrees of fractional crystallization. to Zen, 1986) and the compositions become less peraluminous with In contrast, the anomalous two samples (XSJ‑08, XSJ-09) in this study

increasing SiO2 (e.g., Chappell et al., 1987). In contrast, most I-type have high contents of SiO2 (73.6–74.6 wt%), K2O (4.24–4.89 wt%), granites, which are derived from preexisting intracrustal igneous source A/CNK indexes (1.25), and low contents of CaO (0.18–0.31 wt%) and rocks, are metaluminous (A/CNK < 1.0) and always contain amphibole, MgO (0.12%–0.14%), indicating that they may be the result of highly although some more felsic I-type granites are weakly peraluminous fractionated magma, which often reveal A-type affinity in geochemical (Chappell et al., 1987, 2012). A-type granites are characterized by high- characteristics (Wu et al., 2017). temperature and anhydrous minerals (e.g., pyroxene, arfvedsonite, and

riebeckite), enriched in REE (except Eu), SiO2, K2O, Fe/Mg contents, Petrogenesis of the Xishanjiao Granites and Implications for Zr, Nb, Ga, Y (Zr+Nb+Ce+Y > 350 ppm) and high molar Ga/Al ratios the Early Cretaceous Magmatism in the Tengchong Terrane

(10000*Ga/Al > 2.6) but low Al2O3, CaO, Ba, Sr, and Eu contents (Loiselle and Wones, 1979; Collins, 1982; Chappell et al., 1987). With I-type granites are considered to be derived from intra-crustal igne- the exception of two samples plotted in the field of fractionated granites ous rocks in the continental crust (Chappell, 1998, 1999; Chappell and or A-type granites, the majority of Xishanjiao samples plotted in the White, 1974), and can be used to constrain the nature of magmatic source. T field of unfractionated granites in the FeO +MgO, (K2O+Na2O)/CaO Based on experimental studies and natural research, three main recog- versus Zr+Nb+Ce+Yb diagrams (Figs. 8A and 8B), indicating that they nized models for the generation of I-type granites have been proposed: are probably I- or S-type granites rather than A-type granites. This is (1) partial melting of mafic to intermediate metaigneous crustal rocks further supported by the low contents of REE (114–202 ppm), Nb, Zr with or without contributions of mantle-derived materials in the presence (119–194 ppm, <220 ppm), and lack of mafic alkaline minerals (e.g., of various amounts of water (e.g., Beard and Lofgren, 1989; Roberts and arfvedsonite and riebeckite). However, the Xishanjiao granites have Clemens, 1993; Griffin et al., 2002; Weissman et al., 2013); (2) fractional

TABLE 4. WHOLE-ROCK Nd ISOTOPIC COMPOSITIONS OF XISHANJIAO GRANITES FROM THE TENGCHONG TERRANE, SW CHINA

147 144 143 144 (143 144 Sample Sm Nd Sm/ Nd Nd/ Nd 2σ Nd/ Nd)I εNd(t) T2DM fSm/Nd no. (ppm) (ppm) (Ma) XSJ-1 5.65 30.6 0.111666 0.512111 0.000002 0.512021 –10.42 1639 –0.43 XSJ-2 5.27 26.3 0.120804 0.512120 0.000002 0.512022 –10.39 1643 –0.39 XSJ-3 4.29 21.2 0.122675 0.512109 0.000002 0.512010 –10.62 1663 –0.38 XSJ-4 5.52 29.5 0.113340 0.512090 0.000002 0.511998 –10.86 1676 –0.42 XSJ-4R 5.52 29.5 0.113340 0.512091 0.000002 0.511999 –10.84 1675 –0.42 XSJ-5 5.03 26.0 0.117065 0.512121 0.000002 0.512026 –10.31 1634 –0.40 XSJ-8 4.01 23.2 0.104608 0.512118 0.000002 0.512033 –10.17 1615 –0.47 XSJ-9 3.20 17.6 0.110011 0.512118 0.000002 0.512029 –10.25 1624 –0.44 XSJ-6 5.26 23.8 0.133682 0.512140 0.000002 0.512031 –10.20 1636 –0.32 XSJ-10 4.47 25.3 0.106951 0.512198 0.000003 0.512111 –8.64 1492 –0.46 XSJ-10R 4.47 25.3 0.106951 0.512196 0.000002 0.512109 –8.68 1495 –0.46 XSJ-11 3.99 23.6 0.102118 0.512189 0.000002 0.512106 –8.75 1497 –0.48 XSJ-12 4.35 25.1 0.104450 0.512183 0.000003 0.512099 –8.89 1510 –0.47

147 144 143 144 Note: Chondrite uniform reservoir values used are Sm/ Nd = 0.1967, Nd/ Nd = 0.512638; λSm = –11 –1 143 144 6.54 × 10 year (Lugmair and Marti, 1978). ( Nd/ Nd)i and εNd (t) are calculated considering granitoid age as 124 Ma; single-(TDM) or two-stage model age (T2DM) calculation method is from Jahn et al. (1999). 147 144 -1 DM—depleted mantle; fSm/Nd—( Sm/ Nd/0.1967) .

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2 3 10 A Xishanjiao granites 10 B Early Cretaceous granitoids O

A a 2 A C / O 10 ) g O M 1 2 / t

10 a O FG N e + 1 FG

F 10 O 2 K ( OGT OGT 100 100 101 102 103 104 101 102 103 104 Zr+Nb+Ce+Y (ppm) Zr+Nb+Ce+Y (ppm)

60 90 C 80 D 50 70 I-type ) 40 60 ) m m

p 50 p

p 30 p 40 I-type ( h ( Y T 20 30 20 10 10 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Rb (ppm) Rb (ppm)

Figure 8. Discrimination diagrams of genetic type for Xishanjiao granites in the Tengchong terrane, SW China. A—A-type granite; I—I‑type granite; S—S-type granite; FG—fractionated felsic granite; OGT—unfractionated M-, I-, and S-type granite. Data sources are the same as in Figure 5.

crystallization of mafic melts, which has similar compositions to the mafic granites are from partial melting of metaigneous crustal rocks with minor xenoliths, producing intermediate and felsic magmas (e.g., DePaolo, 1981; contributions of mantle-derived materials. Chiaradia, 2009; Be’eri-Shlevin et al., 2010; Weissman et al., 2013; Lee Compared to Nb/Ta ratios (13.4, according to Rudnick and Gao, 2003) and Bachmann, 2014; He et al., 2019b); and (3) mixing of crust- and in continental crust, the significantly low Nb/Ta ratios (5.28–9.77) of mantle-derived melts in different proportions to produce various granit- the Xishanjiao granites suggest they experienced appreciable fractional oid magmas (e.g., Roberts and Clemens, 1993; Droop et al., 2003; Kemp crystallization of apatite, which is supported by the observed negative cor-

et al., 2007; Zhu et al., 2009; Weissman et al., 2013; Zhao et al., 2019). relations of P2O5 and TiO2 versus SiO2 (Fig. 7), as well as the depletion of The Xishanjiao granites are enriched in LILEs (Rb, Th, U) and depleted P and Ti in the primitive-mantle normalized trace elemental spider diagram in HFSEs (Ba, Nb, P, and Ti), suggesting the importance of crustal rocks (Fig. 6B). The negative anomaly δEu (0.32–0.66) in chondrite-normalized in their magma sources (Roberts and Clemens, 1993). Except for the REE patterns (Fig. 6A) indicate the fractionation of plagioclase, which

two anomalous samples, the Xishanjiao granites show a wide range of is evidenced by the increasing of K2O with the increase of SiO2 whereas # # Mg values (30.5–41.9) and a metaigneous source in the plot of Mg ver- Na2O slightly decreases (see Fig. 7). Because the removal of non-K miner-

sus SiO2 (Fig. 9A). Meanwhile, they reveal the result of partial melting als (e.g., plagioclase) can cause the increase of K2O content in melt. The T 2 of amphibolites in the plot of (Na2O+K2O)/(FeO +MgO+TiO ) versus flat HREE pattern (Fig. 6A) and relatively high Y content indicate that T Na2O+K2O+FeO +MgO+TiO2 (Fig. 9B). The relatively low Al2O3/TiO2 the source melting occurred at pressures below the garnet stability field ratios (25.3–55.9) suggest that metapelite is also involved in their source (Rapp and Watson, 1995) and that fractional crystallization of amphibole (Fig. 9C; Sylvester, 1998). Nd isotopic compositions can also be used to has taken place (Jaques and Green, 1980). Therefore, we suggest that the provide constraints on their source components (Champion and Bultitude, Xishanjiao granites have experienced extensive fractional crystallization

2013; He et al., 2019a). The negative εNd(t) values (−8.64 to −10.86) with of plagioclase, amphibole, and apatite.

T2DM ranging from 1.49 to 1.68 Ga are similar to previously published According to our summary in Figures 7–9, the Early Cretaceous igneous

Early Cretaceous granites in the Tengchong terrane, with εNd(t) values rocks in the Tengchong terrane show similar geochemical characteristics ranging from −13.26 to 1.51 (see Fig. 9D; e.g., Yang et al., 2006; Zhu et al., and variations with the Xishanjiao granites in this study, implying that the 2015, 2018; Zhang et al., 2018), implying a predominantly Mesoprotero- Early Cretaceous magmatism probably originated from a similar source zoic crustal source (metaigneous or amphibolite) with little involvement and occurred in a similar geodynamic setting to the Xishanjiao granites of mantle-derived materials. Therefore, we suggest that the Xishanjiao in this study.

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A B

C D

# T T Figure 9. Plots of (A) Mg versus SiO2, (B) (Na2O+K2O)/(MgO+FeO +TiO2) versus Na2O+K2O+MgO+FeO +TiO2, (C) CaO/Na2O

versus Al2O3/TiO2, and (D) εNd(t) versus U-Pb age for Xishanjiao granites and Early Cretaceous granitoids in the Tengchong terrane, SW China. Data sources of A–C are the same as in Figure 5. (A) Fields shown are as follows: pure crustal partial melts obtained in experimental studies by dehydration melting of low-K basaltic rocks at 8–16 kbar and 1000–1050 °C (e.g., Rapp

and Watson, 1995); pure crustal melts obtained in experimental studies by the moderately hydrous (1.7–2.3 wt% H2O) melting of medium- to high-K basaltic rocks at 7 kbar and 825–950 °C (e.g., Sisson et al., 2005); mantle melts (basalts) and Quaternary volcanic rocks from the Andean southern volcanic zone (e.g., Lopezescobar et al., 1993); melts from metaigneous sources under crustal pressure and temperature conditions of 0.5–1.5 GPa and 800–1000 °C, respectively, which are based on Patiño Douce (1999) and Wolf and Wyllie (1994). (B) Compositional fields of experimental melts are from Patiño Douce and Harris

(1998), Sylvester, (1998), Patiño Douce (1999), and Altherr et al. (2000). (C) CaO/Na2O versus Al2O3/TiO2 diagram referenced by Sylvester (1998). (D) Data sources are from Yang et al. (2006); Zhu et al. (2015, 2018); and Zhang et al. (2018).

Geodynamic Setting of Meso-Tethys in the Tengchong Terrane in this study, combined with previously published data from the region, we proposed a completely different view on the tectonic setting and pro- Previous comparisons of the Tengchong terrane with the Lhasa terrane vide constraints on the dynamic setting and tectonic evolution of the suggested similarities in magmatic activities, stratigraphy, and paleobio- Tengchong terrane. geography. Subsequently, it has been proposed that the Tengchong terrane As described above, the Xishanjiao plutons are metaluminous to is the southern extension of the Lhasa terrane, both of which experienced weakly peraluminous, high-K, calc-alkaline I-type granites (see Figs. 5A similar tectono-magmatic histories since the Early Paleozoic (Xu et al., and 5C). Two tectonic settings have been recognized to interpret the for- 2008, 2012; Xie et al., 2016; Qi et al., 2019). Accordingly, tectonic models mation of high-K, calc-alkaline, I-type granites: (1) subduction-related of Bangong-Nujiang suture zone in Tibet have been adopted to interpret continental arc setting such as Andean type, where a large number of the Early Cretaceous magmatism in the Tengchong terrane. However, igneous rocks generated by hybrid magma derived from the mantle and several different models have been proposed, being: (1) low-angle or flat crust occur, or (2) post-collision extensional setting caused by decompres- northward subduction of Neo-Tethys oceanic lithosphere (Ding, 2003; sion following crustal thickening (Roberts and Clemens, 1993). Here we Kapp et al., 2005; Liu et al., 2017); (2) southward subduction and slab propose that the following observations provide an important constraint break-off of the Bangong-Nujiang oceanic slab (Zhu et al., 2015, 2017a, on the geodynamic setting for the Early Cretaceous magmatism in the 2017b; Fang et al., 2018; Zhang et al., 2018; Qi et al., 2019); or (3) post- Tengchong terrane. (1) The absence of Jurassic and Cretaceous strata collisional tectonic setting following the closure of the Meso-Tethyan in the whole Tengchong terrane (YNBGMR, 1990) likely indicates that Ocean (Xu et al., 2012; Cao et al., 2014). Based on the data presented regional uplift and erosion had been in progress during this time, which

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is also supported by the occurrence of purplish-red sandstone produced in the continental facies environment (Zhang et al., 2018). (2) Some depleted A mantle-derived diabased and mafic enclaves with emplacement ages at ca. 122–115 Ma have been identified recently in both the Tengchong terrane (Cong et al., 2011a; Zhu et al., 2017b; Fang et al., 2018; Zhang et al., 2018; Qi et al., 2019) and the Mogok metamorphic belt of Myan- mar (Chen et al., 2016) indicating that the onset of the extension-related mantle-derived magmatism occurred during the Early Cretaceous. (3) The relatively high crystal temperatures (most zircon saturation temperature

TZr = 805–844 °C; see Fig. 10A) of granitoids also strongly support a mantle contribution to the generation of the Early Cretaceous magmatism, which is also supported by the more variable εHf ranging from -18.88 to 10.26 (see Fig. 10B). Collectively, all the above features point to the fact that the Meso-Tethys Ocean basin had been closed at least in the Early Cretaceous in the Tengchong region and a post-collisional extensional regime related to the upwelling and subsequent partial melting of the depleted asthenospheric mantle had occurred. Hence, the most feasible mechanism responsible for such scenario can be ascribed to the post-col- B lisional lithospheric extension. Recent identification of Jurassic Myitkyina ophiolite (173 Ma; see Fig. 1A) to the west of Tengchong terrane suggests that the eastern belt in Burma represents the southern continuation of the Bangong-Nujiang suture rather than Gaoligong shear zone (Liu et al., 2016a). Accordingly, Early Cretaceous granitoids in the east Tengchong terrane is closely related to the evolution of Meso-Tethys rather than the Gaoligong shear zone between the Tengchong and Baoshan terranes. Therefore, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane rather than the Lhasa terrane. Moreover, we prefer the model of rollback of Meso-Tethys slab (see Fig. 11) to interpret the generation of Early Cretaceous granitoids produced in a post-collisional lithospheric extension. In this model, a flat or low-angle subduction of the Bangong Meso-Tethys oceanic slab—like Andes-type

in the South America—has occurred, which has been supported by the Figure 10. Plots of (A) TZr (°C) versus SiO2 and (B) εHf(t) versus U-Pb previous ages of magmatic rocks from the Tengchong terrane showing a ages of Xishanjiao granites and Early Cretaceous granitoids in the distinct magmatic gap at 142–130 Ma (Zhang et al., 2017). Because of eastern Tengchong terrane, SW China. Data sources of TZr (°C) are the same as Figure 5, data sources of ε (t) and U-Pb ages are from long-term high temperature and pressure beneath the Tengchong terrane, Hf references in Table 1. the subduction Bangong Meso-Tethys slab converted to anomaly heavy eclogite phase and roll-back happened and triggered subduction angle steeper. When the slab rolled back, hot asthenospheric material upwelled East and enhanced decompression melting in the overriding plate, forming Early Cretaceous

various granitoids with involvements of mantle materials in extensional G r a Burma n Baoshan environments. This process is consistent with the formation of magmatic i Tengchong terrane t o terrane i d rocks in central Tibet (Zhang et al., 2017). s Lithosphere CONCLUSIONS Lithosphere Lithosphere

Slab of M Geochronological and geochemical data for the Early Cretaceous eso-Teth Asthenosphere ys Upwell granitoids in the eastern Tengchong terrane, coupled with previous pub- Rollback lications, allow us to reach the following conclusions: (1) The Xishanjiao pluton was emplaced at ca. 124 Ma in the Early Figure 11. Model of tectonic setting for the generation of Early Cretaceous Cretaceous period. magmatism in the eastern Tengchong terrane, SW China, which was related (2) The Xishanjiao granites, similar to most of Early Cretaceous gra- to post-collisional slab rollback and represented the product of the evolu- nitic rocks in the Tengchong terrane, are dominated by I-type tion of the Meso-Tethys Ocean in eastern Myanmar (Burma) rather than granites mainly derived from partial melting of the Mesoprotero- the Bangong-Nujiang Ocean or Gaoligongshan shear zone between the zoic metaigneous rocks with minor sediments in the lower crust. Tengchong and Baoshan terrane. (3) The Early Cretaceous magmatism in the Tengchong terrane was related to post-collisional slab rollback and represented the product of the evolution of the Myitkyina Meso-Tethys Ocean ACKNOWLEDGMENTS in eastern Myanmar (Burma) rather than the Bangong-Nujiang This study was supported by National Natural Science Foundation of China (grant numbers 41702084, 41872089, and 41903032), Yunnan Department of Science and Technology applica- Ocean or Gaoligongshan shear zone between the Tengchong and tion of basic research project (grant number 2017FD063), and Geology Discipline Construction Baoshan terranes. Project of Yunnan University (grant number C176210227).

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