Subduction of Indian continental lithosphere constrained by Eocene-Oligocene magmatism in northern Myanmar

LITHOS 348-349 (2019) 105211

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Subduction of Indian continental lithosphere constrained by EoceneOligocene magmatism in northern Myanmar Jin-Xiang Li a, b, *, Wei-Ming Fan a, b, c, Li-Yun Zhang a, b, Lin Ding a, b, Ya-Li Sun a, b, Tou-Ping Peng b, d, Fu-Long Cai a, b, Qiu-Yun Guan a, c, Kyaing Sein e a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China d State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China e Myanmar Geosciences Society, Yangon, Myanmar b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 July 2019 Received in revised form 12 September 2019 Accepted 12 September 2019 Available online 16 September 2019

Subduction of the Indian continental lithosphere in the eastern Tibet-Himalaya orogenic belt remains unclear. Newly reported zircon U-Pb ages in this study indicate that the Shangalon intermediate-felsic magmatic rocks in northern Myanmar formed at Eocene-Oligocene (~ 40e32 Ma). They have calcalkaline to shoshonitic characteristics, LREE-enriched patterns, enrichments in LILE (e.g., Rb, Cs), and depletions in HFSE (e.g., Nb). Obviously, they show the higher initial Sr and lower Nd-Hf isotopic compositions (87Sr/86Sri ¼ 0.7054e0.7082, εNd(t) ¼ 5.3 to 0.4, and εHf(t) ¼ 3.4 to 10.8) than Cretaceous arc-related mafic-felsic rocks in the West Burma terrane. A positive correlation between Nd isotopic compositions and SiO2 contents indicates that the Eocene Shangalon diorite and andesite with the lowest εNd(t) values (5.3 to 4.0) likely derived from a mantle source contaminated by the subducted Indian continental lithosphere. Coupled with regional coeval OIB-like mafic melts and high temperature metamorphism, the Eocene-Oligocene Shangalon magma might have formed in the Neo-Tethyan oceanic slab break-off setting after the India-Asia collision. In addition, Eocene granodioritic rocks have the adakitic features, which possibly resulted by fractional crystallization of hornblende. The hornblendedominated crystallization in the Shangalon ore-bearing granodioritic rocks is well consistent with magma evolution of H2O-rich melts, which play an important role in the formation of the Eocene Shangalon porphyry Cu-Au deposit. © 2019 Elsevier B.V. All rights reserved.

Keywords: Continental lithosphere subduction Break-off Eocene-Oligocene Adakitic magmatism Myanmar

1. Introduction Subduction of the Indian continental lithosphere is a key process for the formation of the famous Cenozoic Tibet-Himalaya orogenic belt (e.g., Bouilhol et al., 2013; Chapman et al., 2018; Chu et al., 2011; Chung et al., 2005; Ding et al., 2003; Ma et al., 2017; Yin and Harrison, 2000). So the timing of the Indian continental lithosphere subduction should be very important to a deep understanding of collisional orogenesis. Generally, continental lithosphere subducted into the deep mantle beneath overlying lithosphere at where oceanic slab frequently underwent a tear and break-off process (e.g., Ji et al., 2016; van Hunen and Allen, 2011). In

* Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. https://doi.org/10.1016/j.lithos.2019.105211 0024-4937/© 2019 Elsevier B.V. All rights reserved.

turn, the magmatism and metamorphism induced by slab break-off could indirectly constrain subduction of continental lithosphere. In the Tibet-Himalaya orogenic belt, based on OIB-like magmatic and ultra-high pressure rocks, the Neo-Tethyan slab break-off has been defined at Eocene (~45 Ma) in Pakistan and Tibet (Ji et al., 2016; Kohn and Parkinson, 2002), and ~42e40 Ma in Yunnan (China; Xu et al., 2008), respectively. On the other hand, Cenozoic magmatic rocks with lower Nd-Hf isotopic compositions also are successfully used to confine the subduction of the Indian continental lithosphere (e.g., Bouilhol et al., 2013; Chu et al., 2011; Jiang et al., 2014; Ma et al., 2017), because of the overlying lower crust is juvenile (e.g., the Kohistan-Ladakh and Gangdese arc; Zhu et al., 2011; Jagoutz and Schmidt, 2012). However, in the eastern section of the Tibet-Himalaya orogenic belt (especially in Myanmar), the timing of the Indian continental lithosphere subduction still remains unclear, which strongly hampers our understanding on the formation of the entire India-Asia collisional system.

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Fig. 1. (a) Tectonic subdivisions and spatial-temporal distribution of MesozoiceCenozoic mafic-felsic magmatism in northern Myanmar (Gardiner et al., 2018). (b) Geological map for Cretaceous-Oligocene magmatic rocks around the Shangalon area (modified after Htut et al., 2017; Mitchell, 2018). Published zircon U-Pb age data for the Shangalon diorite is from Gardiner et al. (2018).

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Previous works have suggested that the lower crust beneath the West Burma terrane (Myanmar) is also juvenile, analogous to the Kohistan-Ladakh and Gangdese arc (e.g., Gardiner et al., 2018; Li et al., 2019; Lin et al., 2019; Wang et al., 2014). Thus, igneous rocks with more radiogenic isotopic compositions can trace the involvement of the ancient Indian continental lithosphere in their formation. The formation ages of the magmatic rocks further constrain the timing of the Indian continental lithosphere subduction (e.g., Chu et al., 2011; Ma et al., 2017). Additionally, the Shangalon porphyry Cu-Au deposit is located in the West Burma terrane, northern Myanmar. Although limited geochronological results indicated that this deposit formed at Eocene (~38 Ma; Gardiner et al., 2018; Li et al., 2018), petrogenesis and tectonic setting of ore-bearing magmas are still unclear. In this study, zircon U-Pb, whole-rock geochemical, and Sr-Nd-Hf isotopic data for the Eocene-Oligocene Shangalon magmatic rocks are presented to decipher their petrogenesis and further to reveal the timing and geodynamic process of the Indian continental lithosphere subduction (syn-collisional setting) in the eastern Tibet-Himalaya orogenic belt. 2. Geological setting Myanmar is located in southeastern Asia and the southward extension of the Tibetan-Himalayan orogeny through the Eastern Himalayan Syntaxis (Fig. 1a). It mainly consists of the Indo-Burma Range, West Burma terrane, and Shan Plateau from west to east, separated by the two Mesozoic Kalaymyo and Myitkyina sutures (or Sagaing fault; Fig. 1a) (e.g., Cai et al., 2017; Liu et al., 2016a; Mitchell et al., 2012). Multi-period tectonic events were recorded in Myanmar, including PaleozoiceCenozoic Tethyan/Indian oceanic subduction and continental collision among India, West Burma, Sibumasu, and Indochina terranes (e.g., Cai et al., 2017; Lee et al., 2016; Li et al., 2018; Zhang et al., 2018). The Indo-Burma Range is composed of Cretaceous ophiolite (similar to the Yarlung-Zangpo suture in southern Tibet; Liu et al., 2016a), Triassic turbiditic lange, and Cenozoic flysch and sandstone and schist, Cretaceous me molasse (Yao et al., 2017). The West Burma terrane comprises the Mesozoic-Cenozoic WunthoePopa magmatic belt and Cenozoic sedimentary basins on both sides (e.g., Mitchell et al., 2012; Searle et al., 2017). The Shan Plateau (Fig. 1a) is a part of the Sibumasu terrane, which mainly consists of Late Proterozoic metamorphosed turbidite (i.e., Chaung Magyi Group) and PaleozoiceMesozoic carbonate and clastic sedimentary rocks (Mitchell, 2018; Mitchell et al., 2012; Zaw, 2017). The Sibumasu terrane is considered to have been rifted from Australia in Permian and subsequently docked with the Indochina-Simao terrane at Late Triassic (e.g., Cai

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et al., 2017; Gardiner et al., 2018). In addition, the Mogok Metamorphic belt located at the western side of the Shan Plateau (Fig. 1a), mainly consists of PaleoceneeMiocene low- and highgrade metamorphic rocks and JurassiceMiocene undeformed/ deformed magmatic intrusions (e.g., Barley et al., 2003; Mitchell et al., 2012). It is thought to be the southward extension from the Eastern Himalayan Syntaxis to the Phuket in Thailand through the Gaoligong Metamorphic belt in Yunnan (China; Searle et al., 2017). Three magmatic belts are spatially distributed in Myanmar from east to west: PermianeTriassic granite, MogokeMergui, and WunthoePopa (Fig. 1a; Mitchell et al., 2012; Gardiner et al., 2018; Lin et al., 2019). PermianeTriassic (~265e220 Ma) granite in the eastern Shan Plateau is regarded as the northern extension of the Main Range granite province in Peninsular Malaysia. Emplacements of these granites were likely related to PermianeTriassic subduction of the Paleo-Tethyan oceanic lithosphere and subsequent Late Triassic SibumasueIndochina collision (Gardiner et al., 2018). The MogokeMergui magmatic belt (Gardiner et al., 2018; Mitchell et al., 2012) occurs in the Mogok Metamorphic belt and DaweiePhuket area (southern Myanmar and Thailand; Fig. 1a), northward extending to JurassiceEocene magmatic belt in the Tengchong terrane (Yunnan, China; Xie et al., 2016). These intrusions in the MogokeMergui magmatic belt dominantly consist of JurassicMiocene granite and minor diorite (e.g., Mitchell et al., 2012), and occasionally associated with Sn-W deposits (e.g., Hermyingyi and Mawchi; Myint et al., 2017, 2018; Gardiner et al., 2018). The WunthoePopa magmatic belt in the West Burma terrane (Fig. 1a) mainly includes CretaceouseQuaternary maficefelsic volcanic rocks and intrusions (e.g., Gardiner et al., 2018; Lee et al., 2016; Mitchell et al., 2012), regarded as an eastward extension of the Gangdese arc in Tibet through the JurassiceCretaceous Lohit batholith (India; Wang et al., 2014; Lin et al., 2019). In the northern section of the WunthoePopa belt, the Shangalon porphyry Cu-Au deposit (Fig. 1a) is located at the southwest of the Kawlin town. Extensive Cretaceous (~105e90 Ma) mafic-felsic magmatic rocks at the Shangalon area, including gabbro, diorite, granite, and andesitic-dacitic volcanic rocks (Fig. 1b; Gardiner et al., 2018). Importantly, these Cretaceous rocks were intruded by Eocene-Oligocene intermediate-felsic intrusions, which consist of diorite, granodiorite, diorite porphyry, and granodiorite porphyry (Fig. 1b). Meanwhile, Eocene andesite and dacite also occur in this area and overlay Cretaceous granodiorite batholith and andesitic volcanics (Table 1; Htut et al., 2017; Gardiner et al., 2018). Oligocene monzonitic intrusions were located at the east of the Shangalon deposit (Fig. 1b). Eocene granodiorite porphyry intruded into coeval batholith and volcanic rocks and is associated with the Shangalon Cu-Au mineralization. The Eocene-Oligocene Shangalon intrusions and volcanic rocks are focuses of this study.

Table 1 Whole rock Sr-Nd isotopic data of the Eocene-Oligocene Shangalon intermediate-felsic magmatic rocks from the West Burma terrane, northern Myanmar. Sample

Lithology

87

Rb/86Sr

87

Sr/86Sr

15M-130A 15M-130B D15-8-3-94A 15M-130E 15M-130F D15-2-5-42 D15-2-8-152 D15-2-3-54 D15-2-3-119 15M-126A 15M-126B D15-2-5-115 D15-2-3-123 15-M-124A 15-M-131A 15M-131B

Andesite Andesite Andesite Dacite Dacite Diorite Diorite Granodiorite Granodiorite Granodiorite porphyry Granodiorite porphyry Granodiorite porphyry Diorite porphyry Granite porphyry Quartz monzonite porphyry Quartz monzonite porphyry

1.2929 2.0331 1.4653 0.6142 0.7792 0.1017 0.2719 0.8384 0.3285 0.7197 0.8105 0.1428 1.2130 0.6683 2.0649 2.0035

0.708174 0.708402 0.707051 0.707557 0.707515 0.707682 0.707475 0.706421 0.706198 0.707424 0.707500 0.706517 0.706050 0.708558 0.706457 0.706502

SE

147

Sm/144Nd

0.000008 0.000008 0.000009 0.000008 0.000006 0.000009 0.000008 0.000009 0.000008 0.000008 0.000008 0.000009 0.000007 0.000008 0.000007 0.000008

0.1087 0.1063 0.1139 0.0987 0.1039 0.1060 0.1278 0.0973 0.0952 0.0998 0.0981 0.1109 0.1287 0.0795 0.1137 0.1132

143

Nd/144Nd

0.512405 0.512412 0.512406 0.512458 0.512468 0.512344 0.512390 0.512527 0.512492 0.512492 0.512498 0.512460 0.512599 0.512413 0.512526 0.512535

SE

87

Sr/86Sri

0.000003 0.000004 0.000005 0.000003 0.000005 0.000003 0.000004 0.000003 0.000003 0.000005 0.000004 0.000003 0.000003 0.000004 0.000004 0.000005

0.7075 0.7073 0.7062 0.7072 0.7071 0.7076 0.7073 0.7060 0.7060 0.7070 0.7071 0.7064 0.7054 0.7082 0.7055 0.7056

143

Nd/144Ndi

0.512377 0.512385 0.512377 0.512433 0.512441 0.512318 0.512358 0.512503 0.512469 0.512468 0.512474 0.512433 0.512568 0.512392 0.512503 0.512513

εNd(t) 4.1 4.0 4.1 3.0 2.9 5.3 4.5 1.7 2.4 2.4 2.3 3.1 0.4 3.8 1.9 1.7

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3. Sampling, analytical methods, and results 3.1. Sampling and petrography Samples were collected from outcrops and drill holes (e.g., D152-3) of undeformed intermediateefelsic intrusions and volcanic rocks in the Shangalon area, northern Myanmar (West Burma terrane; Fig. 1b). The studied diorite and granodiorite have equigranular textures, whereas diorite/granodiorite/granite/quartz monzonite porphyry and volcanic rocks (andesite and dacite) show

porphyritic textures. All the studied samples have relatively similar accessory mineral assemblage, which mainly includes zircon, apatite, and Fe-Ti oxides (each <1 vol%). Diorite is dominantly composed of hornblende (30e40 vol%), plagioclase (40e45 vol%), quartz (0e5 vol%), biotite (5e10 vol%), and minor pyroxene (0e1 vol%). Granodiorite comprises quartz (15e20 vol%), biotite (15e20 vol%), and plagioclase (30e35 vol%), K-feldspar (10e15 vol %), and hornblende (5e10 vol%). Diorite porphyry and granodiorite porphyry have similar mineral proportion to diorite and granodiorite, with phenocryst assemblage of hornblende, plagioclase,

Fig. 2. Zircon U-Pb concordia diagrams and weighted mean ages of the Shangalon diorite (a, b), dacite (c, d), granite porphyry (e), granodiorite (f, g, h), granodiorite porphyry (i, j), diorite porphyry (k), and quartz monzonite porphyry (l).

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quartz, K-feldspar, and biotite. Granite porphyry has a phenocryst assemblage of quartz, biotite, plagioclase, and K-feldspar, and is mainly composed of quartz (30e35 vol%), biotite (5e10 vol%), plagioclase (30e35 vol%), and K-feldspar (20e30 vol%). In addition, andesite has a phenocryst assemblage of hornblende, plagioclase, quartz, and biotite, and shows a similar mineral proportion to diorite. Studied dacitic volcanic rocks have lower proportions of hornblende relative to andesite, with phenocryst assemblage of plagioclase, hornblende, quartz, K-feldspar, and biotite. Quartz monzonite porphyry comprises quartz (10e15 vol%), biotite (15e20 vol%), and plagioclase (30e35 vol%), K-feldspar (25e30 vol %), and hornblende (5e10 vol%). 3.2. Analytical methods and results Analytical methods of major and trace elements, Sr-Nd isotopes, zircon U-Pb ages, and in-situ Hf isotopes are detailed presented in Appendix S1. 3.2.1. Zircon U-Pb ages Zircons from the studied samples are mostly euhedral, with elongated to short prismatic forms and lengths of 100e300 mm (Appendix S2). Most zircons show oscillatory zoning and Th/U ratios (0.16e3.87; Supplementary Data Table 2) higher than 0.1, indicating that the zircon U-Pb ages can be interpreted as the crystallization age of the host rocks (Corfu et al., 2003). Zircon from two diorite samples in the Shangalon area yielded Eocene 206Pb/238U weighted mean ages of 38.7 ± 0.2 Ma (MSWD ¼ 0.8, n ¼ 24; Fig. 2a) and 38.1 ± 0.3 Ma (MSWD ¼ 1.3, n ¼ 22; Fig. 2b), respectively. Consistently, zircon from two dacite samples also yielded Eocene 206Pb/238U weighted mean ages of 38.5 ± 0.3 Ma (MSWD ¼ 1.8, n ¼ 25; Fig. 2c) and 38.2 ± 0.3 Ma (MSWD ¼ 1.2, n ¼ 21; Fig. 2d), respectively. Whereas one granite porphyry obtained a slightly older zircon weighted mean U-Pb age of 39.6 ± 0.2 Ma (MSWD ¼ 1.3, n ¼ 25; Fig. 2e). In comparison with diorite, zircon from three granodiorite yielded relatively younger 206Pb/238U weighted mean ages between 37.1 ± 0.2 Ma (MSWD ¼ 0.4, n ¼ 22) and 37.4 ± 0.2 Ma (MSWD ¼ 1.1, n ¼ 23; Fig. 2f-h). Two granodiorite porphyry obtained consistent

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zircon weighted mean U-Pb ages of 36.5 ± 0.2 Ma (MSWD ¼ 1.3, n ¼ 24; Fig. 2i) and 36.9 ± 0.3 Ma (MSWD ¼ 2.9, n ¼ 22; Fig. 2j), respectively. One diorite porphyry (D15e2e3-106) intruded into granodiorite gave a zircon weighted mean U-Pb age of 36.6 ± 0.2 Ma (MSWD ¼ 0.5, n ¼ 22; Fig. 2k). In addition, one quartz monzonite porphyry (15 M-131A) yielded the youngest 206Pb/238U weighted mean age of 31.5 ± 0.4 Ma (MSWD ¼ 2.4, n ¼ 21; Fig. 2l), indicating Oligocene emplacement. 3.2.2. Major and trace elements The Eocene (~ 40e36 Ma) Shangalon magmatic rocks show gabbroic diorite, diorite, granodiorite, and granite compositions (Fig. 3a; Supplementary Data Table 1). They mostly belong to the calcalkaline series, with a few plotting in the high-K calc-alkaline field (Fig. 3b). Whereas the Oligocene (~32 Ma) Shangalon intrusion has a quartz monzonite composition and shoshonite feature (Fig. 3a, b). Eocene diorite shows enrichments of light rare earth elements (LREE) with (La/Yb)N ratios of 5.82e10.64, and negligible Eu anomalies (Eu/Eu* ¼ 0.83e0.94; Fig. 4a). Eocene andesite has similar REE characteristics with diorite, with (La/Yb)N of 7.81e10.65 and Eu/Eu* ratios of 0.83e0.95. In addition, Eocene dacite, granodiorite, granodiorite/diorite porphyry, and granite porphyry also show LREE enriched patterns with (La/Yb)N of 10.04e35.54, but lower content of heavy rare earth elements (HREE) relative to diorite and andesite. These magmatic rocks have negligible to positive Eu anomalies with Eu/Eu* values of 0.86e1.15 (Fig. 4a). Moreover, Oligocene quartz monzonite porphyry has LREE enrichment patterns ((La/Yb)N of 16.04e19.52) and slightly negative Eu anomalies (Eu/Eu* ¼ 0.73e0.77), but higher REE content than Eocene magmatic rocks in the Shangalon area (Fig. 4a). On extended trace element diagrams (Fig. 4b), all the studied samples show strong enrichments in large ion lithophile elements (LILE: e.g., Cs and Rb) and depletions in high field strength elements (HFSE: Nb, P, and Ti). 3.2.3. Whole rock Sr-Nd and zircon Hf isotopic signatures Two Eocene diorites from the Shangalon area have 87Sr/86Sri ratios of 0.7073e0.7076 and εNd(t) values ranging from 5.3 to 4.5, which are similar to Sr-Nd isotopic compositions of coeval andesite (87Sr/86Sri ¼ 0.7062e0.7075 and εNd(t) ¼ 4.1 to 4.0;

Fig. 3. Compositions of the Eocene-Oligocene Shangalon magmatic rocks plotted on diagrams of (a) K2O þ Na2O versus SiO2 (Middlemost, 1994) and K2O versus SiO2 (Peccerillo and Taylor, 1976).

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Fig. 4. Chondrite-normalized REE patterns (a) and primitive mantle-normalized element diagrams (b) for the Eocene-Oligocene Shangalon magmatic rocks. The normalizing values for REE and trace elements are from Sun and McDonough (1989).

Table 1; Fig. 5a-d). The Eocene Shangalon dacite, granodiorite, granodiorite/diorite porphyry show the relatively lower initial Sr isotopic ratios (87Sr/86Sri ¼ 0.7054e0.7072) and higher Nd isotopic values (εNd(t) ¼ 3.1 to 0.4) than coeval diorite and andesite. While Eocene granite porphyry has 87Sr/86Sri ratio of 0.7082 and εNd(t) value of 3.8, similar to Sr-Nd isotopic values of coeval andesite and diorite. In addition, the Oligocene (~32 Ma) Shangalon quartz monzonite porphyry also shows the higher Sr-Nd isotopic values (87Sr/86Sri ¼ 0.7055e0.7056 and εNd(t) ¼ 1.9 to 1.7) than Eocene diorite and andesite (Table 1; Fig. 5a-d). The Eocene Shangalon diorite has a wide range of zircon Hf isotopic values (εHf(t) ¼ 3.4 to 6.9, mean ¼ 2.8 ± 1.1, n ¼ 58), which are slightly lower than and overlapped with Hf isotopic compositions for coeval dacite (εHf(t) ¼ 0.4 to 5.7, mean ¼ 3.1 ± 0.8, n ¼ 20) and granite porphyry (εHf(t) ¼ 2.2 to 6.2, mean ¼ 4.3 ± 0.9, n ¼ 20; Supplementary Data Table 3). Eocene granodiorite and granodiorite/diorite porphyry have relatively consistent zircon εHf(t) values ranging from 2.8 to 10.8 (mean ¼ 7.2 ± 0.9, n ¼ 119), which are higher than those of diorite (Fig. 6a, b). Whereas zircon from the Oligocene Shangalon quartz monzonite porphyry shows

εHf(t) values of 1.0 to 8.7 (mean ¼ 4.1 ± 1.7, n ¼ 20), similar with Hf isotopic compositions of Eocene granite porphyry (Fig. 6b). 4. Discussion 4.1. Eocene-Oligocene magmatism in southern Tibet and northern Myanmar In the Tibet-Himalaya orogenic belt, Eocene-Oligocene (~45e30 Ma) mafic-felsic magmatism has been found in southern Gangdese magmatic belt (Tibet; e.g., Gao et al., 2008; Guan et al., 2012; Wang et al., 2015; Ji et al., 2016; Ma et al., 2017). Especially, ~45 Ma OIB-type mafic dikes with depleted Sr-Nd isotopes provide strong evidence for the Neo-Tethyan slab break-off (Ji et al., 2016). To east, Eocene (~42e40 Ma) OIB-like basaltic dikes also have been reported in the Gaoligong belt (Yunnan, China; Xu et al., 2008). Importantly, zircon U-Pb ages for the Shangalon magmatic rocks indicate that the Eocene-Oligocene (~40e32 Ma) also occurred in the West Burma terrane (northern Myanmar; Barley et al., 2003; Gardiner et al., 2018; this study), which was linked to the southern

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Fig. 5. Whole rock Sr-Nd (a) and mean zircon Hf isotopic compositions (b) as well as Sr-Nd isotopic data as function of SiO2 content (c, d) for the Eocene-Oligocene Shangalon magmatic rocks, northern Myanmar. Data sources: (a) CretaceouseQuaternary arc-related intrusions and volcanic rocks in the West Burma terrane are from Mitchell et al. (2012), Lee et al. (2016) and Li et al. (2019); ~45e35 mafic magmatism in Gangdese and Gaolinggong are Ji et al. (2016), Xu et al. (2008), Ma et al. (2017), and Guan et al. (2012); oceanic sediments are from Plank and Langmuir (1998) and Chauvel et al. (2008); post-collisional ultrapotassic rocks in Gangdese are from Zhao et al. (2009). (b) Mantle array of Hf-Nd isotopes are from Chauvel et al. (2008). The compositions of end-members assumed for mixing calculations are: (1) the depleted mantle wedge (Chauvel et al., 2008; Zhang et al., 2005): εNd(t) ¼ 8, 87Sr/86Sri ¼ 0.703, εHf(t) ¼ 15; (2) the Himalaya leucogranite (Liu et al., 2016b): εNd(t) ¼ 14, 87Sr/86Sri ¼ 0.743, εHf(t) ¼ 20. The mixing curves in figure b were constructed using different Nd/Hf ratios, K ¼ (Nd/Hf)mantle/(Nd/Hf)sediment.

Lhasa terrane (Li et al., 2019; Wang et al., 2014). In addition, previous works have reported that Eocene-Oligocene (~45e30 Ma) intermediate-felsic intrusions also emplaced in the MogokeMergui magmatic belt (e.g., ~42 Ma Mawchi granite and ~31 Ma Mandalay Hill syenite; Barley et al., 2003; Mitchell et al., 2012; Myint et al., 2017). Therefore, the consistent formation ages of magmatic rocks from southern Tibet to Myanmar suggest that a prolonged EoceneOligocene (~45e30 Ma) mafic-felsic magmatism occurred along the Cenozoic India-Asia orogenic belt. 4.2. Petrogenesis of Eocene-Oligocene magmatism in Myanmar 4.2.1. Eocene (~40e36 Ma) magmatism The studied Eocene Shangalon magmatic rocks show enrichments in LILE (e.g., Cs and Rb) and depletions in HFSE (e.g., Nb, P, and Ti; Fig. 4b), which are similar with geochemical characteristics

of the melts derived from oceanic/continental slab-derived fluids/ melts metasomatized mantle (e.g., Ding et al., 2003; Lee et al., 2016; Ma et al., 2017). Importantly, the Eocene Shangalon dacite, granodiorite, and granodiorite/diorite/granite porphyry show high Sr/Y and (La/Yb)N ratios as well as low Y and Yb contents, consistent with adakitic melts. Whereas coeval diorite and andesite have an opposite geochemical feature, indicating normal arc-type magmatic rocks (Fig. 7a, b; e.g., Richards and Kerrich, 2007). All studied Eocene igneous rocks have decreased MgO and V contents with increasing SiO2 contents, indicating fractional crystallization trend (Fig. 7c, d). Many studies have indicated that both garnet and hornblende fractionation can produce adakitic melts with high Sr/Y and (La/Yb)N characteristics (e.g., Castillo et al., 1999; Macpherson et al., 2006). Garnet fractionation should yield distinctly elevated (Dy/Yb)N ratios, while hornblende fractionation obtain constant or slightly reduced (Dy/Yb)N ratios in evolved melts (e.g., Davidson

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Fig. 6. Zircon εHf(t) values versus Age (Ma) diagram (a, b) for the Shangalon magmatic rocks, northern Myanmar. Data sources: (a) Detrital zircons in Chindwin Basin are from Wang et al. (2014); Jurassic-Cretaceous arc magmatism in Myanmar and India related to Neo-Tethyan subduction are from Lin et al. (2019) and Gardiner et al. (2018); ~35 Ma Himalaya leucogranites in southern Tibet are from Liu et al. (2016a). (b) ~35 Ma gabbro and ~38 Ma adakitic rock in the southern Gangdese arc are from Ma et al. (2017) and Guan et al. (2012), respectively.

et al., 2007; Dessimoz et al., 2011; Wanke et al., 2019; Xu et al., 2015). The Eocene Shangalon magmatic rocks show decreased Dy contents, constant (Dy/Yb)N ratios, and elevated Sr/Y ratios with increasing SiO2 contents (SiO2 < ~ 65 wt%), indicating that the Eocene adakitic rocks mainly formed by fractional crystallization of hornblende (Fig. 7e, f, g; e.g., Richards and Kerrich, 2007; Davidson et al., 2007). This conclusion is also supported by elevated Eu*/Eu values with increasing Sr/Y ratios (Fig. 7h), because fractional crystallization of hornblende can result in higher Eu*/Eu values in residual melts (Nandedkar et al., 2016). In addition, the Shangalon igneous rocks with SiO2 higher than ~ 65 wt% (including granite porphyry) present deceased Sr/Y with increasing SiO2 contents (Fig. 7g), suggesting that dominant plagioclase fractionation in more evolved melts (Dai et al., 2017; Nandedkar et al., 2016). Previous works have shown that Cretaceous intermediate-felsic intrusions in the West Burma terrane mainly show positive whole rock εNd(t) and zircon εHf(t) values (Figs. 6, 7; Mitchell et al., 2012; Gardiner et al., 2018; Lin et al., 2019; Li et al., 2019). In combination with positive εHf(t) values in detrital zircons from the Chindwin basin (Wang et al., 2014), the lower crust beneath the West Burma terrane should be juvenile, analogous to the well-defined juvenile lower crust in the southern Lhasa terrane (e.g., Zhu et al., 2011). Importantly, most of the Eocene Shangalon igneous rocks have increasing εNd(t) values and decreasing 87Sr/86Sri ratios with increasing SiO2 contents, indicating fractional crystallization and assimilation (AFC) of juvenile crust (Fig. 5c, d). Further, the variations of isotopic compositions strongly reveal that the more mafic rocks (diorite and andesite) in the Shangalon area likely derived from a magmatic source with lower εNd(t) values and higher 87 Sr/86Sri ratios (Fig. 5c, d). Moreover, the Eocene Shangalon diorite and andesite have obviously lower εNd(t) values and higher 87 Sr/86Sri ratios than Cretaceous arc-type magmatism in the West Burma terrane (Fig. 5a), which formed by melting of slab-derived fluids and/or sediment melts metasomatized mantle during subduction of the Neo-Tethyan oceanic lithosphere (e.g., Gardiner et al., 2018; Li et al., 2019; Lin et al., 2019; Mitchell et al., 2012). Low zircon εHf(t) values (mean ¼ 2.8 ± 1.1) of studied dioritic samples are well consistent with those of subducted oceanic sediments (2.0 ± 3.0; Fig. 5b; Chauvel et al., 2008). These can rule out a metasomatized mantle source by oceanic slab-derived fluids and/or sediment melts. Therefore, it is proposed that the Eocene Shangalon diorite and andesite could be generated from a mantle source

metasomatized by the Indian continental lithosphere. Moreover, the Eocene Shangalon diorite has a mantle-like zircon O (d18O ¼ 5.5 ± 0.4‰) and low εHf(t) values (1.9 ± 1.5; Gardiner et al., 2018), also indicating source contamination (Kemp et al., 2007). Modeling results of Sr-Nd-Hf isotopes indicate that addition of less than ~7% of the Indian continental material into mantle wedge likely causes the isotopic compositions for the Eocene Shangalon magmatic rocks (Fig. 5a, b). In addition, the Shangalon granite porphyry has similar Sr-Nd isotopic values with diorite and andesite, likely indicating negligible crustal contamination (Fig. 5c). Combined with their elemental characteristics (Fig. 7), they likely formed in a relatively closed system during magma evolution. 4.2.2. Oligocene (~32 Ma) magmatism The Oligocene (~32 Ma) Shangalon quartz monzonite porphyry mostly shows the similar REE patterns and geochemical characteristics of trace elements with Eocene (~40e36 Ma) magmatic rocks (Fig. 4a, b). However, they have shoshonitic characteristics (Fig. 3b) and higher LREE content (Fig. 4a). In addition, they show the similar Sr-Nd-Hf isotopic compositions (87Sr/86Sri ¼ 0.7070, εNd(t) ¼ 2.4 to 2.3, and εHf(t) ¼ 1.0 to 8.7) with Eocene granodioritic intrusions (Figs. 5, 6), suggesting same magmatic source metasomatized by the Indian continental lithosphere. Moreover, they have higher Rb/Sr (0.66e0.73) and lower Ba/Rb ratios (2.02e2.42) than Eocene granodioritic rocks with similar SiO2 contents, likely suggesting that a phlogopite-bearing source (Lee et al., 2016). Combined with higher La/Yb ratios, REE, and Nb contents (Figs. 4, 7f), the Oligocene Shangalon K-rich porphyry likely formed by low degree of partial melting of phlogopite-bearing mantle source, similar to collision-related K-rich magmatic rocks in southern Tibet (e.g., Chung et al., 2005; Ding et al., 2003; Zhao et al., 2009). 4.2.3. Implication for the Shangalon Cu-Au mineralization It is well known porphyry Cu deposits are closely associated with adakitic magmas because of their hydrous and high oxygen fugacity features (e.g., Richards and Kerrich, 2007; Wang et al., 2018). The hydrous melts can suppress the crystallization of plagioclase but prompt crystallization of hornblende, which led to elevated Sr/Y ratios and low Y content in evolved melts (e.g., Richards and Kerrich, 2007). At Shangalon, Eocene intrusions show a trend of increasing Sr/Y with decreasing Y contents (Fig. 7a),

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Fig. 7. Diagrams of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b; Richards and Kerrich, 2007), SiO2 vs. MgO (c), V (d), Dy (e), (Dy/Yb)N (f), and Sr/Y (g), Sr/Y vs. Eu*/Eu (h), and REE vs. Nb (i) for the Eocene-Oligocene Shangalon magmatic rocks, northern Myanmar.

strongly suggesting that they are water-rich and dominantly underwent hornblende crystallization. The melts can easily exsolve fluids in a shallow magma chamber, which sequestrate ore-forming elements (e.g., Cu and Au) to produce porphyry-type mineralization (e.g., Richards and Kerrich, 2007). Moreover, the Eocene Shangalon ore-bearing granodioritic intrusions have lower initial Sr and higher εNd(t) values than Eocene more mafic rocks (Fig. 5c, d), indicating more addition of juvenile crust. In Tibet, previous works have well shown that this type of juvenile lower crust should be fertile (e.g., H2O-rich, enrichments in Cl and Cu) and formed by early-stage vertical growth of subduction-related melts (e.g., Hou et al., 2015). Many collision-related giant porphyry Cu deposits (e.g., Qulong and Yulong) were mainly derived from partial melting of the fertile lower crust (e.g., Hou et al., 2015; Wang et al., 2018). Similarly, the addition of the fertile juvenile crust should play an important role in the formation of ore-bearing magmas at the Eocene Shangalon porphyry Cu-Au deposit.

4.3. Implication for Eocene-Oligocene tectonic evolution in Myanmar Previous studies have shown that Indian and Eurasian/Asian continent first at ~65e63 Ma collided with Tibet and then spreading westwards and eastwards (~50 Ma at Myanmar; e.g., Wu et al., 2014; Ding et al., 2017; Gardiner et al., 2018). EoceneeOligocene (~45e30 Ma) mafic-felsic magmatism synchronously occurred in the West Burma-southern Lhasa and western Sibumasu-Tengchong terranes (e.g., Gao et al., 2008; Ma et al., 2017; Li et al., 2018; this study). Moreover, OIB-like mafic magmatism occurred at ~45 Ma in the southern Lhasa terrane and at ~42e40 Ma in the Tengchong terrane, respectively. These OIB-like magmas likely formed by melting of asthenosphere during Neo-Tethyan oceanic slab break-off after terrane collision (Ji et al., 2016; Xu et al., 2008). The younger trend for OIB-like mafic magmas from southern Tibet to Yunnan is also consistent with the collision time variations between Indian and Eurasian/Asian continent (Ding

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Fig. 8. A cartoon demonstrating the geodynamic and petrogenetic process for the Eocene-Oligocene Shangalon igneous rocks from the West Burma terrane, northern Myanmar. SCLM ¼ subcontinental lithospheric mantle.

et al., 2017; Wu et al., 2014), suggesting that slab tear and break-off might migrate eastward from southern Tibet to southeastern Asia (Ji et al., 2016; Xu et al., 2008). Meanwhile, ~40e35 Ma maficintermediate rocks with low εNd-εHf values in southern Tibet were derived from mantle sources metasomatized by subducted Indian continental slab in the Neo-Tethyan oceanic slab break-off setting during Indian-Eurasia collision (Guan et al., 2012; Ma et al., 2017). At Myanmar, the lower crust under the West Burma terrane is juvenile based on the dominantly depleted mantle-like Sr-Nd-Hf isotopic values of mafic-felsic igneous rocks (e.g., Gardiner et al., 2018; Li et al., 2019; Lin et al., 2019; Mitchell et al., 2012; Wang et al., 2014). Therefore, the Eocene Shangalon diorite and andesite have the lowest Nd-Hf and highest Sr isotopic compositions, which should ascribe to the addition of the subducted Indian continental material (Fig. 5a, b). This further indicates that the Indian continental crust has subducted beneath the West Burma terrane (northern Myanmar) at Eocene (~40 Ma). Meanwhile, EoceneeOligocene (~43e30 Ma) high-temperature metamorphic rocks had been well identified in the Mogok Metamorphic belt (Barley et al., 2003; Searle et al., 2017). The coeval granite in the western Sibumasu terrane mainly derived from the ancient

Sibumasu crustal source, based on negative εNd-εHf and high zircon O isotopic values (Gardiner et al., 2018; Mitchell et al., 2012). Therefore, a possible model of the Neo-Tethyan oceanic slab tear and break-off after India-Asia collision (Fig. 8a, b) is presented for explaining the EoceneeOligocene (~40e32 Ma) Shangalon magmatism in the West Burma, and coeval felsic magmatism and hightemperature metamorphism in the western Sibumasu terrane. Slab tear and break-off can lead to the upwelling of asthenosphere mantle, which could be melted to form Eocene OIB-like mafic magmas (e.g., mafic dikes in the Gaoligong belt; Xu et al., 2008). Meanwhile, partial melting of the metasomatized mantle by subducted Indian continental lithosphere formed basaltic melts, and then underplated the base of the juvenile West Burma and subsequently underwent MASH process (Hildreth and Moorbath, 1988) to form the Shangalon intermediatefelsic magmatism with low Nd-Hf isotopic compositions (Fig. 8b). On the other hand, mantlederived basaltic melts underplated the base of ancient western Sibumasu crust to produce Eocene-Oligocene high-temperature metamorphism (Searle et al., 2017) and granitic melts with crustlike Sr-Nd-Hf-O isotopic values (Fig. 8b; e.g., Mitchell et al., 2012; Gardiner et al., 2018; Lin et al., 2019). Moreover, the Oligocene (~ 32 Ma) Shangalon K-rich melts likely formed by small-degree

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melting of the phlogopite-bearing metasomatized mantle in a relative extension setting during the late stage of slab break-off (van Hunen and Allen, 2011). 5. Conclusions (1) Our new zircon U-Pb age data indicate that most of the Shangalon intermediate-felsic igneous rocks formed at Eocene (~40e36 Ma), whereas quartz monzonite porphyry emplaced at Oligocene (~32 Ma). In combination with coeval magmatism in the southern and eastern Tibet Plateau, there is a prolonged Eocene-Oligocene magmatism along the India-Asia collision belt. (2) Geochemical and Sr-Nd-Hf isotopic compositions suggest that the Eocene (~40e36 Ma) Shangalon rocks were derived from partial melting of the metasomatized mantle by the subducted Indian continent-derived fluids/melts, and underwent an AFC process that led to more evolved melts with adakitic characteristics. Whereas K-rich Oligocene (~32 Ma) magmatism likely formed by a low degree of partial melting of the phlogopite-bearing metasomatized mantle. (3) The Eocene-Oligocene magmatism from the Shangalon area (northern Myanmar) possibly formed a slab tear and breakoff setting after the Indian-Asian terrane collision. This strongly suggests that the Indian continental lithosphere has subducted beneath the West Burma terrane at EoceneOligocene (~40e32 Ma), and further indicates that the India-Asia collision may have occurred prior to Eocene (~40 Ma) at northern Myanmar. Acknowledgments This article was funded by the Natural Science Foundation Project (Grant No. 41490613; 41672091), and the Ministry of Science and Technology of China (2016YFC0600303). We yielded support for field work and sampling from Seunghan Kim at the Daewoo International Corporation (Myanmar). We also obtained specific guidance and assistance from Ya-Hui Yue, Jing Xie, ShouQian Zhao, Hong-Xia Jiang, and Yu-Qiong Wang concerning analyses of trace element, Sr-Nd-Hf isotope, and zircon U-Pb dating at the Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.lithos.2019.105211. References Barley, M.E., Pickard, A.L., Zaw, K., Rak, P., Doyle, M.G., 2003. Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the IndiaEurasia collision in Myanmar. Tectonics 22 (3). https://doi.org/10.1029/ 2002tc001398. Bouilhol, P., Jagoutz, O., Hanchar, J.M., Dudas, F.O., 2013. Dating the IndiaeEurasia collision through arc magmatic records. Earth Planet. Sci. Lett. 366, 163e175. Cai, F.L., Ding, L., Yao, W., Laskowski, A.K., Xu, Q., Zhang, J., Sein, K.I., 2017. Provenance and tectonic evolution of Lower Paleozoic-Upper Mesozoic strata from Sibumasu terrane, Myanmar. Gondwana Res. 41, 325e336. Castillo, P.R., Janney, P.E., Solidum, R.U., 1999. Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting. Contrib. Mineral. Petrol. 134, 33e51. Chapman, J.B., Scoggin, S.H., Kapp, P., Carrapa, B., Ducea, M.N., Worthington, J., Oimahmadov, I., Gadoev, M., 2018. Mesozoic to Cenozoic magmatic history of the Pamir. Earth Planet. Sci. Lett. 482, 181e192. Chauvel, C., Lewin, E., Carpentier, M., Arndt, N.T., Marini, J.C., 2008. Role of recycled oceanic basalt and sediment in generating the HfeNd mantle array. Nat. Geosci. 1, 64e67. Chu, M.F., Chung, S.L., O’Reilly, S.Y., Pearson, N.J., Wu, F.Y., Li, X.H., Liu, D.Y., Ji, J.Q., Chu, C.H., Lee, H.Y., 2011. India’s hidden inputs to Tibetan orogeny revealed by Hf isotopes of Transhimalayan zircons and host rocks. Earth Planet. Sci. Lett.

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