Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust

    Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust Hao Wang, Zhaowen Xu, Xiancai Lu, Bin Fu, Jianjun Lu, Xiaonan Yang, Zengxia Zhao PII: DOI: Reference:

S0024-4937(15)00406-5 doi: 10.1016/j.lithos.2015.11.011 LITHOS 3745

To appear in:

LITHOS

Received date: Accepted date:

27 December 2014 4 November 2015

Please cite this article as: Wang, Hao, Xu, Zhaowen, Lu, Xiancai, Fu, Bin, Lu, Jianjun, Yang, Xiaonan, Zhao, Zengxia, Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted PaleoPacific slab and delaminated newly underplated lower crust, LITHOS (2015), doi: 10.1016/j.lithos.2015.11.011

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ACCEPTED MANUSCRIPT Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and

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delaminated newly underplated lower crust

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Hao Wanga,b, Zhaowen Xua,*, Xiancai Lua, Bin Fuc, Jianjun Lua, Xiaonan Yangd, Zengxia Zhaoa State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering,

Nanjing University, Nanjing 210023, China

SOA Key Laboratory of Submarine Geoscience, Second Institute of Oceanography, State

c

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Oceanic Administration, Hangzhou, China

Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200,

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Australia

The Geological Museum of China, Beijing 100034, China

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d

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b

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* Corresponding author at: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Street, Nanjing 210023, China

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Tel: +86 13512519798.

E-mail: [email protected]

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Abstract

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The origin and tectonic setting of Early Cretaceous adakitic rocks from the Luxi terrane in the

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eastern North China Block (NCB) remain debated. To resolve this issue we determined whole-rock geochemistry, zircon U–Pb ages, and in situ Hf–O isotopes of the Mengyin and Liujing adakitic porphyries from the Luxi terrane. Zircon U–Pb dating results reveal that both the

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Mengyin and Liujing plutons were emplaced during the Early Cretaceous, with weighted mean Pb/238U ages of 130 ± 1 Ma (2) and 131 ± 2 Ma (2), respectively. In addition, abundant

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Neoarchean–Paleoproterozoic inherited zircon cores are identified in the Mengyin adakitic 207

Pb/206Pb ages ranging from 2.53 to 2.42 Ga. Rocks of both plutons are silicic

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porphyry with

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(SiO2 = 65.4–70.2 wt.%), metaluminous, and alkaline in composition, comprising mainly quartz

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syenite porphyries. Samples from both plutons are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba), and light rare earth elements (LREEs), depleted in high field

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strength elements (HFSEs) (e.g., Nb, Ta, and Ti), and heavy rare earth elements (HREEs), and have either positive or no Eu anomalies. In addition, both adakitic porphyries have high Mg# values (51–64), high Sr and La contents, low Y and Yb contents, and high Sr/Y (Mengyin = 149–264; Liujing = 58–110) and (La/Yb)N (Mengyin = 32.4–45.3; Liujing = 43.8–53.1) ratios, similar to adakitic rocks worldwide. The Mengyin adakitic porphyry has higher whole-rock Nd(t) values (–5.8 to –4.1), more radiogenic Pb [(206Pb/204Pb)i = 18.35–18.39, (207Pb/204Pb)i = 15.55–15.56, (208Pb/204Pb)i = 38.20–38.23], higher zircon rim Hf(t) values (+3.3 to +8.8) and δ18O values (+6.5‰ to +7.9‰), and lower (87Sr/86Sr)i ratios (0.7049–0.7050) than the Liujing adakitic porphyry [Nd(t) = –12.4 to –12.2, (206Pb/204Pb)i = 17.63–17.72, (207Pb/204Pb)i = 15.56–15.58, 2

ACCEPTED MANUSCRIPT (208Pb/204Pb)i = 37.76–37.94, Hf(t) = –14.8 to –11.2, δ18O = +5.9‰ to +7.1‰, (87Sr/86Sr)i = 0.7090–0.7091]. The Mengyin adakitic porphyry was most likely derived from partial melting of

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subducted oceanic slab with some input of NCB Neoarchean–Paleoproterozoic lower crust

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components. The Liujing adakitic porphyry was probably derived from partial melting of delaminated newly underplated thick lower crust which then interacted with above asthenospheric mantle peridotite. Slab roll-back together with the ridge subduction of the Paleo-Pacific slab was

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the most likely geodynamic mechanism for formation of the Early Cretaceous Mengyin and

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Liujing adakitic porphyries.

Keywords: Geochronology; Sr–Nd–Pb isotopes; Adakite; Zircon Hf–O isotopes; North China

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1. Introduction

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Adakites were originally considered to be produced by partial melting of young and hot subducted oceanic slab (Defant and Drummond, 1990). Geochemically, adakites are characterized

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by high SiO2, Al2O3, and Na2O contents, high Sr/Y and La/Yb ratios, and low Y and Yb contents, indicating an eclogite or garnet amphibolite source without significant amounts of plagioclase in the residue (Martin, 1999; Rapp et al., 1991). However, some rocks with features of adakitic affinity have been considered to have alternative origins, including (1) magma mixing coupled with crustal assimilation and fractional crystallization (AFC) involving basaltic magma (Castillo et al., 1999; Gao et al., 2009; Macpherson et al., 2006); (2) partial melting of a thickened mafic lower crust (Chung et al., 2003, 2009; Hou et al., 2004; Petford and Atherton, 1996; Zeng et al., 2011); (3) partial melting of delaminated mafic continental lower crust in the mantle and subsequent interaction with mantle peridotites (Atherton and Petford,1993; Gao et al., 2004); or (4) 3

ACCEPTED MANUSCRIPT low-degree partial melting of metasomatized mantle (Gao et al., 2010; Jiang et al., 2006; Martin et al., 2005). Recently, many studies have proposed that slab melt can be distinguished from lower

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continental crust melt through chemical characteristics (Ling et al., 2011, 2013; Liu et al., 2010;

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Sun et al., 2012).

Many Early Cretaceous adakitic rocks distributed throughout the Luxi terrane in the eastern North China Block (NCB) have been extensively investigated, and three models have been

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proposed to explain their origin. First, Chen et al. (2013) inferred that the Jinling and Tietonggou

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adakitic rocks were generated by magma mixing between mantle-derived and lower-crust-derived melts. Second, Gu et al. (2013) and Liu et al. (2008b) proposed that the Jinling, Laowa, Yangtao,

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and Fangcheng adakitic rocks originated from partial melting of delaminated lower crust. Third,

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Chen and Zhou (2005) mentioned that the Tietonggou adakitic rocks were originated from partial

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melting of the subducted Paleo-Pacific slab. Therefore, the Luxi terrane is an ideal place to evaluate the genesis of adakitic rocks.

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The geodynamic mechanism controlling the generation of Early Cretaceous magmatic activity in the eastern NCB also remains debated. Zhao and Zheng (2009) and Zhang et al. (2010) proposed that post-collision extension of the Yangtze Block (YB) and the NCB during the Triassic was the main geodynamic mechanism. In contrast, other studies have suggested that the Early Cretaceous magmatism cannot be fully explained by the processes following continental collision between the NCB and the YB because of the time lag of more than 100 Ma between the collision and the magmatism (Jiang et al., 2010; Lan et al., 2011; Ma et al., 2006, 2013). Instead, these studies proposed that slab roll-back together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism that resulted in the Early Cretaceous magmatic 4

ACCEPTED MANUSCRIPT activity (Chen at al., 2005; Jiang et al., 2010; Li et al., 2014; Ling et al., 2013; Ma et al., 2013; Sun et al., 2007; Wu et al., 2005). However, direct evidences for Paleo-Pacific slab subduction

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beneath the eastern NCB during the Late Mesozoic are scarce. Therefore, the discovery of

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subducting oceanic-crust-derived adakite could provide direct evidence for subduction of the Paleo-Pacific slab.

The Mengyin and Liujing adakitic porphyries are two Early Cretaceous plutons in the Luxi

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terrane, but have not yet been studied systematically. Xu et al. (2004a) dated the Mengyin and

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Liujing adakitic porphyries using biotite and hornblende 40Ar–39Ar methods, results of 124.4 ± 0.3 Ma and 124.9 ± 0.2 Ma, respectively. Yang (2007) proposed that the Mengyin and Liujing plutons

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originated from partial melting of enriched lithospheric mantle based on few data. However, the

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precise zircon ages, petrogenesis, and geodynamic setting of these plutons are poorly constrained.

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In this study, we present zircon U–Pb geochronology, in situ Hf–O isotopic compositions, whole-rock geochemistry, and Sr–Nd–Pb isotopic compositions for the Mengyin and Liujing

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adakitic porphyries, with the aim of evaluating the mechanisms that produce adakitic rocks in the region and finding new evidence in the Luxi terrane for subduction of the Paleo-Pacific slab.

2. Geological background The NCB, which has a 3.8 Ga Archean core (Liu et al., 1992), is the largest craton in China. It is bounded on the north by the Central Asian Orogenic Belt (Davis et al., 2001; Sengör et al., 1993) and on the south by the Qinling–Tongbai–Dabie–Sulu orogenic belt (Li et al., 1993; Meng and

Zhang,

2000).

Based

on

lithological,

geochemical,

and

metamorphic

pressure–temperature–time (P–T–t) path studies of the basement rocks, the NCB can be divided into three sub-blocks (Fig. 1a; Zhao et al., 2005): the eastern NCB, the western NCB and the 5

ACCEPTED MANUSCRIPT intervening Trans-North China Orogen (TNCO). The TNCO formed during the collision between the eastern NCB, and the western NCB either at ~1.85 Ga (Zhao et al., 2001, 2005), or at ~2.5 Ga

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(Li et al., 2002; Zhai et al., 2003). The basement of the NCB consists predominantly of Archean to

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Paleoproterozoic tonalite–trondhjemite–granodiorite (TTG) gneisses and greenschist to granulite facies metamorphic rocks, overlain by Sinian to Ordovician marine carbonates and shales, Carboniferous to Permian continental clastic rocks, and Mesozoic continental basin sedimentary

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rocks (Zhao et al., 2001, 2005). Volcanism occurred in the eastern NCB during the Paleozoic,

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whereas magmatism was rare in the western NCB at this time. The Luxi terrane is located in the central part of the eastern NCB and is bounded by the

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Tan-Lu Fault to the east, the Liaocheng–Lankao Fault to the west, the Fengpei Fault to the south,

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and the Qihe–Guangrao Fault to the north (Zhang et al., 2007; Fig. 1b). The typical NCB

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stratigraphic succession (BGMRSP, 1991) comprising Archean–Proterozoic basement, overlain by Paleozoic marine sedimentary rocks and Mesozoic–Cenozoic sedimentary–volcanic rocks, is well

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developed and preserved in the region. Mesozoic intrusive rocks in the Luxi terrane consist of gabbro, diorite, monzonite, granite, and alkaline complexes (Huang et al., 2012; Lan et al., 2011, 2012; Liu et al., 2008a; Wu et al., 2003; Yang et al., 2006, 2008; Zhang et al., 2005; Fig. 1b). In contrast, Mesozoic volcanic rocks are dominantly andesitic (Qiu et al., 1997), and they are located in rift basins such as the Zouping, Fangcheng, and Mengyin basins (Guo et al., 2003; Ling et al., 2009; Qiu et al., 2002; Zhang et al., 2002; Fig. 1b). The 60 km2 Mengyin pluton, located in central-eastern part of Luxi terrane, ~10 km southwest of Mengyin County, intruded the unconformity between the Precambrian basement and Neoarchean diorite (Fig. 1c). The pluton is 2–3 km wide and 20–30 km in length, and 6

ACCEPTED MANUSCRIPT NW-SE-trending. There are also some small stocks (2–3 km2) distributed along the small faults. The intrusion consists mainly of quartz syenite porphyry, minor granodiorite porphyry, and quartz

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diorite porphyry. The volcanic rocks in the Mengyin basin are mainly Qingshan Group andesite,

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with the zircon U–Pb age of 128 ± 2 Ma (Ling et al., 2009). The 25 km2 ellipsoidal Liujing pluton, located ~15 km east of Cangshan county, intruded Cambrian carbonates. The stratums in the Liujing area are mainly Cambrian carbonates and quaternary, with little Ordovician marine

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sedimentary rocks. There are some nearly SN-trending and NNW-trending faults in the area. The

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intrusion consists mainly of quartz syenite porphyry and minor quartz diorite (Fig. 1d).

3. Sample descriptions and analytical methods

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Seven samples were collected from the Mengyin pluton and four from the Liujing pluton.

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The adakitic porphyry samples from the Mengyin pluton are porphyritic and massive. The

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Mengyin porphyry samples comprise predominantly feldspar and minor quartz, and biotite phenocrysts, with accessory magnetite, zircon, apatite, and titanite (Figs. 2a, b). Feldspar

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phenocrysts (0.8–9.1 mm in size, ~45 vol.%) are euhedral–subhedral and are mainly albite and minor K-feldspar (0.7–1.2 mm, ~5 vol.%). Matrix feldspar is subhedral–anhedral and mainly K-feldspar and albite. Quartz is anhedral and occurs mainly in the matrix and to a lesser degree as phenocrysts (1.1–1.5 mm, ~3 vol.%). Euhedral–subhedral biotite occurs mainly as phenocrysts (0.8–3.2 mm, <3 vol.%). The samples of Liujing adakitic porphyry are also porphyritic and massive. Mineralogically, the samples are dominated by feldspar, with subordinate quartz, biotite, hornblende, and accessory magnetite, zircon, apatite, and titanite (Figs. 2c, d). The phenocrysts consist of dominantly euhedral–subhedral albite (0.8–5.1 mm, ~35 vol.%) with subordinate euhedral–subhedral 7

ACCEPTED MANUSCRIPT K-feldspar (0.6–1.0 mm, ~5 vol.%), hornblende (0.5–2.1 mm, ~3 vol.%), biotite (0.8–2.3 mm, ~3 vol.%) and anhedral quartz (0.6–1.5 mm, ~5 vol.%). The matrix consists of predominantly

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subhedral–anhedral K-feldspar, quartz, and albite.

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Rock samples were crushed and processed using standard mineral seperation techniques, and zircons were hand-picked under binocular. Mineral compositions were analyzed using the electron microprobe at the State Key Laboratory for Mineral Deposits Research (SKLMDR), Nanjing

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University (NJU), China. Major and trace elements were analyzed by XRF and ICP-MS of

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whole-rock powders at SKLMDR. Whole-rock Sr, Nd and Pb isotopic analyses were also conducted at SKLMDR. Individual zircon grains of interest were analyzed for U-Th-Pb ages, and

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Lu-Hf isotopic compositions using the LA-ICP-MS at SKLMDR. In situ O isotope analyses of

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zircons were analyzed by SHRIMP SI in Research School of Earth Sciences (RSES), The

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Australian National University (ANU), Canberra, Australia. Detailed descriptions of the analytical methods are included in Appendix A.

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4. Results

4.1. Mineral chemistry Feldspars in the Mengyin and Liujing adakitic porphyries are mostly alkali feldspars enriched in Na2O or K2O and can be classified as albite, oligoclase and orthoclase (Table S1; Fig. 3a). The core and rim of albites from both adakitic porphyries have similar composition (Figs. 3c, d). Hornblendes in the Liujing adakitic porphyry have high Mg/(Mg + Fe2+) ratios, ranging from 0.73 to 0.84 (Table S1), and are mainly tschermakite and magnesiohornblende (Fig. 3b). The compositions of biotite in the Mengyin and Liujing adakitic porphyries are generally characterized by high MgO and low FeO contents with low Fe/(Mg + Fe) ratios of 0.37–0.49 and 0.30–0.37, 8

ACCEPTED MANUSCRIPT respectively (Table S1). According to the classification of Rieder et al. (1998), the studied biotites in the Mengyin and Liujing adakitic porphyries are mainly eastonite and ferro-phlogopite,

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respectively.

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4.2. LA–ICP–MS zircon U–Pb age data

Zircons from the Mengyin adakitic porphyry (sample MY-3) are mostly between 200 and 300 m in size, and have length-to-width ratios of 2:1 to 3:1. The zircons are commonly

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euhedral–subhedral and many have bright cores mantled by gray overgrowths as revealed by CL

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images (Fig. S1a). Both the cores and rims have obvious oscillatory zonation (Fig. S1a). The U–Pb dating results are listed in Table S2 and illustrated in Fig. 4a. Th/U ratios of the CL-dark

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rims are 0.11–0.34 (except for spots MY-3-14.rim = 0.08 and MY-3-29.rim = 0.07), and the ratios

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of the CL-bright cores are 0.33–2.37. Nineteen analyses of zircon rims yield a weighted mean Pb/238U age of 130 ± 1 Ma (2) that reflects the timing of crystallization of the magma. Eleven

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analyses conducted on bright cores yield

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Pb/206Pb ages of 2.53–2.42 Ga, indicating that

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Neoarchean–Paleoproterozoic zircons are present in the Mengyin adakitic porphyry. Zircons from the Liujing adakitic porphyry (sample LJ-1) are mostly between 150 and 300 m in size, and have length-to-width ratios varying from 2:1 to 2.5:1. Most grains are

euhedral–subhedral and display oscillatory zoning without a core–rim structure, as revealed by CL imaging (Fig. S1b). The U–Pb dating results are listed in Table S2 and illustrated in Fig. 4b. Th/U ratios vary from 0.29 to 0.52. Sixteen analyses of zircons yield a weighted mean 206Pb/238U age of 131 ± 2 Ma (2) that represents the timing of crystallization of the magma. 4.3. Whole-rock major and trace elements Major and trace element contents for the Mengyin and Liujing adakitic porphyries are listed 9

ACCEPTED MANUSCRIPT in Table S3. The Mengyin adakitic porphyry has limited ranges in SiO2 (69.7–70.2 wt.%) and total alkali content (K2O + Na2O = 9.90–10.2 wt.%). The Liujing adakitic porphyry has slightly lower

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SiO2 (65.4–67.0 wt.%) and K2O + Na2O (9.09–9.79 wt.%) contents than the Mengyin adakitic

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porphyry. Most data plot within the fields of alkali-feldspar quartz syenite and quartz syenite in the quartz vs. anorthite–orthoclase (Q’–ANOR) diagram (Fig. 5a). Both the Mengyin and Liujing adakitic porphyries are metaluminous, with an alumina saturation index (ACNK) [Al2O3/(CaO +

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Na2O + K2O)] of 0.92 to 0.98 (Fig. 5b). Both adakitic porphyries have high A.R [Al2O3 + CaO +

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(Na2O + K2O)]/[Al2O3 + CaO – (Na2O + K2O)] values (2.91–4.04), with the data plotting within the field of the alkaline rock series on the A.R. vs. SiO2 diagram (Fig. 5c). The Liujing adakitic

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porphyry has higher K2O contents (4.26–4.53 wt.%) and K2O/Na2O ratios (0.52–0.59) than the

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Mengyin adakitic porphyry (K2O = 2.66–3.04 wt.%; K2O/Na2O = 0.35–0.43), and Liujing samples

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plot within the high-K calc-alkaline series field and Mengyin samples straddling the boundary between high-K calc-alkaline series and calc-alkaline series fields on the K2O vs. SiO2 diagram

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(Fig. 5d). The Mengyin and Liujing adakitic porphyries also have relatively high Mg# values [= 100 × Mg/(Mg + Fe), atomic basis] (Mengyin, Mg# = 51–57; Liujing, Mg# = 59–64). In chondrite-normalized rare earth elements (REEs) plots (Figs. 6a), the Mengyin and Liujing adakitic porphyries are enriched in light rare earth elements (LREEs), with (La/Yb)N of 32.4–45.3 and 43.8–53.1, respectively (Table S3). The Mengyin adakitic porphyry has relative low total REE contents (ΣREE = 40.6–65.2 ppm) and obvious positive Eu anomalies (Eu = 1.70–2.84; Fig. 6a), while the Liujing adakitic porphyry has relatively high REE contents (ΣREE = 186–226 ppm) but no obvious Eu anomalies (Eu = 1.03–1.12; Fig. 6a). In the primitive-mantle-normalized spidergram (Fig. 6b), both adakitic porphyries are enriched in large ion lithophile elements (LILEs) 10

ACCEPTED MANUSCRIPT (e.g., Rb, Sr, and Ba) but depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, and Ti), typical of subduction-related igneous rocks. The Mengyin and Liujing adakitic porphyries

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have high Sr contents (740–1452 ppm), Sr/Y (57.9–264), and (La/Yb)N (32.4–53.1) ratios, but low

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Y (3.15–14.6 ppm) and Yb (0.15–0.71 ppm) contents, and the samples plot within the field of the adakite series (Table S3; Figs. 7a, b). 4.4. Whole-rock Sr–Nd–Pb isotopes

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Whole-rock Sr–Nd–Pb isotopic compositions of the Mengyin and Liujing adakitic porphyries

relatively

low

87

Sr/86Sr

ratios

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are presented in Table S4 and illustrated in Figs. 8 and 9. The Mengyin adakitic porphyry has (0.705227–0.705407)

and

high

143

Nd/144Nd

ratios

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(0.512276–0.512307), corresponding to (87Sr/86Sr)i ratios of 0.7049–0.7050 and Nd(t) values of

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Nd/144Nd ratios (0.511911–0.511919) than the Mengyin adakitic porphyry. Initial values

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lower

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–5.8 to –4.1. The Liujing adakitic porphyry has higher 87Sr/86Sr ratios (0.709256–0.709446) and

are (87Sr/86Sr)i = 0.7090–0.7091 and Nd(t) = –12.4 to –12.2. Two-stage Nd model ages of the

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Mengyin and Liujing adakitic porphyries vary from 1.40 Ga to 1.26 Ga and from 1.93 Ga to 1.91 Ga, respectively.

The Mengyin adakitic porphyry has relatively radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.35–18.39, (207Pb/204Pb)i = 15.55–15.56, (208Pb/204Pb)i = 38.20–38.23], similar to Pacific mid-ocean ridge basalts (MORB) (Vervoort et al., 1999; Fig. 9). The Liujing adakitic porphyry has relatively low radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 17.63–17.72, (207Pb/204Pb)i = 15.56–15.58, (208Pb/204Pb)i = 37.76–37.94]. 4.5. In situ zircon Hf–O isotopic compositions In situ zircon Hf–O isotopic data of the Mengyin and Liujing adakitic porphyries are listed in 11

ACCEPTED MANUSCRIPT Table S5 and plotted in Fig. 10a. Rims of zircon grains in the Mengyin adakitic porphyry (sample MY-3) have a narrow range of Hf and O isotopic compositions, with εHf(t) (age corrected using

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U–Pb ages for individual zircon grains) values varying from +3.3 to +8.8, and δ18O values ranging

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between +6.5‰ and +7.9‰ (7.2 ± 0.4‰ on average ± standard deviation (SD)) (Table S5). They have two-stage Hf model ages (TDM2) of 0.98–0.62 Ga. The Neoarchean–Paleoproterozoic inherited zircon cores of sample MY-3 also have a narrow range of Hf and O isotopic compositions, with

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εHf(t) values varying from +1.8 to +6.5 and δ18O values ranging from +4.6‰ to +6.7‰ (5.8 ±

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0.8‰ on average ± SD) (Table S5).

The zircons from the Liujing adakitic porphyry (sample LJ-1) have lower εHf(t) (–14.8 to

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–11.2) and δ18O (+5.9‰ to +7.1 ‰, 6.7 ± 0.3 ‰ on average ± SD) values than the zircon rims

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from the Mengyin adakitic porphyry (Table S5, Fig. 10a). They have Paleoproterozoic TDM2 ages of

5. Discussion

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2.11–1.89 Ga.

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5.1. Timing of magmatism

Based on zircon morphology and CL images, the obtained LA–ICP–MS zircon U–Pb ages for the Mengyin (130 ± 1 Ma; Fig. 4a) and Liujing (131 ± 2 Ma; Fig. 4b) adakitic porphyries in this study are best interpreted to represent the timing of crystallization of the magma. The results show that the Mengyin and Liujing adakitic porphyries are formed in the same time. The zircon U–Pb ages of the Mengyin adakitic porphyry are also similar to those of the Qingshan Group volcanic rocks (128.0 ± 1.6 Ma; Ling et al., 2009) in the Mengyin region. Previous geochronological studies of the Mengyin and Liujing adakitic porphyries, using biotite and hornblende 40Ar–39Ar methods, yielded ages of 124.4 ± 0.3 Ma and 124.9 ± 0.2 Ma, respectively 12

ACCEPTED MANUSCRIPT (Xu et al., 2004a). Our results are slightly older than the previous data. Zircon is chemically resistant mineral to post-crystallization alteration and may preserve the original age of magma

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methods (e.g., Jackson et al., 2004; Zhao and Zheng, 2009).

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crystallization. Thus, zircon U–Pb ages represent the timing of magmatic rocks better than other

By integrating the results obtained from this study with existing data, the ages of late Mesozoic igneous and volcanic rocks in the Luxi terrane can be divided into three groups: Early

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Jurassic (185–177 Ma; Fig. 10b; Lan et al., 2012; Xu et al., 2004b), Late Jurassic (144–143 Ma;

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Fig. 10b; Liu et al., 2008a), and Early Cretaceous (135–119 Ma; Fig. 10b; Gu et al., 2013; Guo et al., 2013; Huang et al., 2012; Lan et al., 2011, 2013; Ling et al., 2009; Pei et al., 2004; Wang et al.,

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2011; Xu et al., 2004b; Yang et al., 2005, 2006, 2008, 2012a, 2012b; Zhang et al., 2002; Zhong

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and Huang, 2012). The peak age of Early Cretaceous magmatism in the Luxi terrane is 132–128

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Ma (Fig. 10b), and the Mengyin and Liujing adakitic porphyry ages are consistent with this age. The ages of 2.53–2.42 Ga derived from inherited zircon cores from the Mengyin adakitic

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porphyry are similar to the ages of the basement rocks (Wang et al., 2009) throughout the Luxi terrane. The inherited zircon cores in the Mengyin adakitic porphyry also indicate the inclusion of Neoarchean–Paleoproterozoic basement material. 5.2. Petrogenesis 5.2.1 Magmatic processes Based on the narrow range of SiO2 concentrations and limited samples, it’s hard to distinguish the correlations between SiO2 and other major elements for the two adakitic porphyries (Fig. 11). There are no obviously negative Eu anomalies, and absence of negative Sr anomalies in the primitive mantle-normalized trace elements patterns (Figs. 6a, b), suggesting insignificant 13

ACCEPTED MANUSCRIPT plagioclase fractional crystallization in parental magmas for the two porphyries. In addition, in La/Yb vs. La and Th vs. Ni diagrams (Figs. 12a, b), the data from both porphyries are inconsistent

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with the fractional crystallization trend, also indicating that fractional crystallization was limited.

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No correlations are observed between the (87Sr/86Sr)i ratios and SiO2 (Figs. 12c), or between Nd(t) values and SiO2 (Figs. 12d) for the Mengyin and Liujing adakitic porphyries, suggesting that assimilation of continental crust was insignificant during magma crystallization. However, the

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MASH (melting, assimilation, storage, and homogenization) process cannot be excluded. The

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minimal variations in overall zircon εHf(t) values (Fig. 10a) of each adakitic porphyry indicate that magma mixing was not a significant process during magmatism.

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5.2.2 Origin of the adakitic magmas

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Geochemical characteristics of the Mengyin and Liujing porphyries are similar to those of

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adakities (Defant and Drummond, 1990). The Adakite is believed to be generated by the partial melting of subducted young and hot oceanic slab (Defant and Drummond, 1990). However, some

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rocks with features of adakitic affinity have been considered to have alternative origins. There are no contemporaneous basaltic rocks in the Liujing area, and the contemporaneous basaltic volcanic rocks and NCB crust in the Mengyin area have much higher (87Sr/86Sr)i ratios and lower εNd(t) values (Fig. 8) than the Mengyin adakitic porphyry, indicating that the Mengyin and Liujing adakitic porphyries were not generated by the mixing of mantle- and crust-derived magmas. In addition, both adakitic porphyries show limited variations in overall εHf(t) values (Fig. 10a), and the absence of obvious euhedral overgrowths of albite (Figs. 3c, d) further argues against the magma mixing model (Chen et al., 2013). There are no obvious fractional crystallization and crustal assimilation as discussed above, indicating that they were probably not generated from an 14

ACCEPTED MANUSCRIPT AFC-like process involving basaltic magma. The Mengyin and Liujing adakitic porphyries have high SiO2 contents (Mengyin = 69.7–70.2 wt.%; Liujing = 65.4–67.0 wt.%), which are much

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higher than the metasomatized-mantle-derived adakitic rocks (SiO2 <56 wt.%; Martin et al., 2005),

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also excluding the metasomatized mantle origin model.

The magmas derived from the partial melting of basaltic ocean crust are commonly enriched in Na, with K2O/Na2O ratios of ~0.4 and Th/U ratios of ~3 (Defant and Drummond, 1990; Martin

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et al., 1999). The K2O/Na2O ratios (0.35–0.43) and Th/U ratios (2.2–3.2) of the Mengyin adakitic

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porphyry are similar to those of oceanic slab-derived adakites. In contrast, the K2O/Na2O ratios (0.52–0.59) and Th/U ratios (5.15–8.42) of the Liujing adakitic porphyry are higher than those of

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slab-derived adakites, but are similar to those of the lower continental crust or delaminated lower

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continental crust-derived adakitic rocks in the Dabie Orogen (K2O/Na2O > 0.5; Th/U = 3–50; Ling

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et al., 2011; Wang et al., 2007). In addition, in the Sr/Y vs. (La/Yb)N diagram (Fig. 13a), data from the Mengyin adakitic porphyry plot in the field of the oceanic crust-derived adakites wordwide. In

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contrast, data from the Liujing adakitic porphyry plot in the field of lower continental crust-derived adakitic rocks, similar to the Dabie adakitic rocks (Liu et al., 2010; Ling et al., 2011). Therefore, we suppose that the Mengyin adakitic porphyry may generated from oceanic crust, and Liujing adakitic porphyry may originated from lower continental crust. The Pb isotopic compositions of the Mengyin adakitic porphyry are similar to those of MORB, implying the inclusion of MORB material. Note that the Mengyin adakitic porphyry is characterized by higher (87Sr/86Sr)i ratios and lower Nd(t) values than the MORB (Fig. 8). This can be attributed to the involvement of the NCB Neoarchean–Paleoproterozoic crust material when the oceanic-slab-derived adakitic magma rose through the crust. Modeling calculations show 15

ACCEPTED MANUSCRIPT that

the

subducted

oceanic-slab-derived

magma

might

assimilate

~10%

Neoarchean–Paleoproterozoic lower crust materials to produce whole-rock Sr–Nd isotopic

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compositions similar to those of the Mengyin adakitic porphyry (Fig. 8). In addition, the

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preservation of Neoarchean–Paleoproterozoic zircon cores (Fig. S1a) indicates the involvement of Neoarchean–Paleoproterozoic lower crust material. As discussed above, the oceanic-crust-derived magma assimilated lower crust material might through the MASH process.

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The decoupling of Nd(t) and Hf(t) may have been caused by incomplete melting of the

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inherited zircons (Fig. 13b). Hf in the rocks occurs mainly within zircons. If the inherited zircons from the Neoarchean–Paleoproterozoic lower crust were not fully melted, then the Hf content

of

the

primary

magma

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composition

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would have been largely retained within the inherited zircons. Therefore, the Hf isotopic would

be

minimally affected

by assimilated

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Neoarchean–Paleoproterozoic lower crust material. This hypothesis is supported by the abundant inherited zircon cores in the Mengyin adakitic porphyry. The Mengyin adakitic porphyry have

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high Mg# values (51–57, higher than pure crustal melts; Fig. 13c), and also high Cr (17.5–27.9 ppm) and Ni (5.2–9.8 ppm) contents, that are believed to have interacted with mantle wedge (Prouteau et al., 2001; Rapp et al., 1999). The whole-rock Sr–Nd–Pb and zircon Hf isotopic compositions of the Liujing adakitic porphyry are different from those of Neoarchean–Paleoproterozoic basement rocks in the Luxi terrane and crust (Figs 8 and 9; Jahn et al., 1988, 1999). This observation, suggests that the Liujing adakitic porphyry did not originate from the NCB Neoarchean–Paleoproterozoic crust, but might be originate from a newly underplated thick lower crust. The whole-rock Sr–Nd–Pb and zircon Hf isotopic compositions of the Liujing adakitic porphyry are similar to the Luxi EM2-type 16

ACCEPTED MANUSCRIPT lithospheric mantle (Figs 8 and 9) indicating that the newly thick lower crust may originated from the EM2-type lithospheric mantle-derived basaltic magmas underplating. The high Ba and Sr

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contents for the Liujing adakitic porphyry also prefer a newly underplated thick lower crust origin

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model rather than a NCB Neoarchean–Paleoproterozoic lower crust origin model. The Liujing adakitic porphyry have much higher Ba (740–1452 ppm) and Sr (1301–1740 ppm) contents than the traditional I-type (average Ba–538 ppm and Sr–247 ppm), S-type (average Ba–468 ppm and

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Sr–120 ppm), M-type (average Ba–263 ppm and Sr–282 ppm) and A-type (average Ba–352 ppm

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and Sr–48 ppm) granites (Whalen et al., 1987), and also higher than the high-SiO2 adakites (average Ba–721 ppm and Sr–565 ppm, but Sr<1100 ppm; Martin et al., 2005). This suggests that

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the sources of the Liujing adakitic porphyry are more enriched in Ba and Sr than the common

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sources of granites. The EM2-type lithospheric mantle-derived magmas have high Ba and Sr (e.g.

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Fangcheng basalts, average Ba–1067 ppm and Sr–1313 ppm; Zhang et al., 2002) than the average continental lower crust (average Ba–259 ppm and Sr–348 ppm; Rudnick and Gao, 2003).

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Therefore, the partial melts derived from the newly thick lower crust originated from the EM2-type lithospheric mantle-derived basaltic magmas underplating would have very high Ba and Sr contents.

The Liujing adakitic porphyry high Mg# values (59–64, higher than pure crustal melts; Fig. 13c), and high Cr (20.5–27.1 ppm) and Ni (14.9–17.6 ppm) contents indicating the involvement of mantle material. The zircon δ18O values (+5.9‰ to +7.1 ‰, 6.7 ± 0.3 ‰ on average ± SD) of the Liujing adakitic porphyry are also between the asthenospheric mantle (5.3 ± 0.3 ‰, Valley et al., 1998) and EM2-type lithospheric mantle (+7.1‰ to +8.4 ‰, Xu et al., 2013; Fig. 13d). Therefore, we proposed that the Lijing adakitic porphyry was formed by partial melting of delaminated newly 17

ACCEPTED MANUSCRIPT underplated thick lower crust, and the magma was interacted with above asthenospheric mantle peridotite. This model was also proposed for the Early Cretaceous adakitic rocks in the Luxi and

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Jiaodong areas (Gu et al., 2013; Hou et al., 2007; Liu et al., 2008b, 2009; Fig. 8).

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Therefore, we conclude that the Mengyin adakitic porphyry was formed by partial melting of subducted oceanic crust with ~10% assimilation of Neoarchean–Paleoproterozoic lower crust material, and the Liujing adakitic porphyry was formed by partial melting of delaminated newly

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underplated thick lower crust that interacted with above asthenospheric mantle peridotite.

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5.2.3. Source features

The Mengyin and Liujing adakitic porphyries have high Sr contents, high Sr/Y and La/Yb

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ratios, and low Y and HREEs contents without negative Eu anomalies (Figs. 6a, b and 7a, b),

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which suggest the presence of residual garnet and the absence of plagioclase in the source magma

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(Rapp and Waston, 1995; Rapp et al., 2002). However, they also show flat HREE patterns, indicating that amphibole also presence as residual mineral (Huang and He, 2010; Moyen, 2009).

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Thus, the melting residues were most probably garnet-amphibolites or amphibole-bearing eclogite. In addition, the Nb/Ta ratios of the Mengyin and Liujing adakitic porphyries vary from 28.7 to 33.6, with an average value of 31.7 (Table S3), which is much higher than that of continental crust (12–13; Rudnick and Gao, 2003), MORB (16.7 ± 1.8; Kamber and Collerson, 2000), primitive manle (17.5 ± 2.0; McDonough and Sun, 1995), and Cenozoic basalts in the eastern NCB (15.1 ± 0.7; Liu and Gao, 2007; Liu et al., 2008c). The Zr/Hf ratios of the Mengyin adakitic porphyry vary from 27.9 to 30.5, which are lower than chondrite (34.3 ± 0.3; Münker et al., 2003), and the Zr/Hf ratios of Liujing adakitic porphyry (37.3–38.0) are higher than chondrite. Nb and Ta together with Zr and Hf are thought to behave as identical twins in most mantle magmatic processes due to their 18

ACCEPTED MANUSCRIPT similar ionic radii and valence state (Taylor and McLennan, 1985), suggesting that Nb/Ta and Zr/Hf ratios are hardly affected by magmatic processes. However, previous experimental studies

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have revealed that rutile preferentially partitions Nb, Ta and Zr, Hf, and effectively fractionates

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Nb from Ta and Zr from Hf (Foley et al., 2000; Liang et al., 2009; Xiong et al., 2011). Hence, melts in equilibrium with residual rutile will be characterized by high Nb/Ta ratios and low Zr/Hf ratios (Guo et al., 2004; Xiong et al., 2005). The observed high Nb/Ta ratios and lower Zr/Hf

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ratios of Mengyin adakitic porphyry indicates the presence of residual rutile in the magma source.

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The Liujing adakitic porphyry also has high Nb/Ta ratios, which also indicates the presence of residual rutile in the magma source, but it’s hard to explain the superchondritic Zr/Hf ratios. It has

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been suggested that carbonate metasomatism can be responsible for the superchondritic Zr/Hf

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ratios (Dupuy et al., 1992; Rudnick et al., 1993; Jörg et al., 2007). Previous studies show that the

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EM2-type lithospheric mantle may metasomatised by carbonatite-rich magmas or fluids which were derived from the subducted Yangtze block or Paleo-Pacific slab (Guo et al., 2013; Liu et a.,

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2008a). Therefore, the superchondritic Zr/Hf ratios of the Liujing adakitic porphyry might be inherited from the EM2-type lithospheric mantle. The δ18O values of zircon rims in the Mengyin range from +6.5 to +7.9‰ (7.2 ± 0.4‰ on average; Table S5). Taking into account the SiO2 content of the host rock and using the linear relationship between Δ18O(zircon – WR) and SiO2 composition (Lackey et al., 2008): Δ18O(zircon – WR) ≈ –0.0612 × (wt.% SiO2) + 2.5, the δ18O values for the Mengyin adakitic porphyry are calculated to be 8.3‰–9.7‰ (9.0‰ on average). Assuming the protolith of the inherited zircon cores (δ18O = 4.6‰–6.7‰, 5.8‰ on average) in the Mengyin adakitic porphyry is mafic lower crust (assuming SiO2 = 50 wt.%), the δ18O value for the assimilated NCB Neoarchean–Paleoproterozoic lower 19

ACCEPTED MANUSCRIPT crust is calculated to be ~6.4‰. As calculated by the Sr–Nd isotope simulation, the Paleo-Pacific slab-derived adakitic magma assimilated ~10% NCB Neoarchean–Paleoproterozoic lower crust

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material. Therefore, the δ18O value for the primary oceanic-slab-derived adakitic magma is

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calculated to be ~9.3‰. The δ18O values of the primary oceanic-slab-derived adakitic magma are much higher than those of melts from hydrothermally altered gabbros from the interior of oceanic crust, which typically have δ18O values of 2‰–5‰, lower than the values of partial melts of

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basaltic rocks and/or sediments in the upper part of oceanic crust which typically have δ18O values

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of 9‰–20‰ (Bindeman et al., 2005). Therefore, the source of the Mengyin adakitic porphyry may be a mixture of the upper and interior oceanic crust.

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We conclude that rutile-bearing garnet-amphibolites/amphibole-bearing eclogite upper and

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interior oceanic crust, may be the main source of the Mengyin adakitic porphyry, and

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rutile-bearing garnet-amphibolites/amphibole-bearing eclogite newly underplated thick lower crust may be the main source of the Liujing adakitic porphyry.

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5.3 Geodynamic implications

The Mengyin and Liujing adakitic porphyries are contemporaneous and are located in the Luxi terrane, suggesting they formed in a similar tectonic setting. Previous studies proposed that the Luxi terrane was affected by extensional tectonics in the Early Cretaceous (Jiang et al., 2010; Lan et al., 2011). Recent geological and geophysical studies have shown that the slab roll-back and ridge subduction of the Paleo-Pacific slab beneath the eastern NCB, combined with sinistral slip movement of the Tan-Lu Fault induced by Paleo-Pacific subduction, caused the extension of lithospheric mantle and upwelling of asthenosphere during the late Mesozoic (Chen and Zhou,

20

ACCEPTED MANUSCRIPT 2005; Jiang et al., 2010; Lan et al., 2011; Li et al., 2014; Ling et al., 2013; Sun et al., 2007; Zhu et al., 2011).

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The origin of the Early Jurassic Tongshi alkaline complex (185–177 Ma) suggests that the

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Paleo-Pacific slab was subducted beneath the eastern NCB before ca. 185 Ma (Jiang et al., 2010; Ma et al., 2013; Fig. 14a). Thus, the eastern margin of the NCB was a continental arc since then. Until the Late Jurassic–Early Cretaceous (~144 Ma), when slab roll-back caused continental

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arc-rifting in the Luxi terrane, and resulted in delamination of the EM2-type lithospheric mantle

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(Fig. 14b). The upwelling asthenosphere resulted in partial melting of the EM2-type lithospheric mantle to form the Mengyin and Zichuan mafic dykes (144–143 Ma) in the Luxi terrane (Liu et al.,

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2008a), and the basaltic magmas also underplating the lower crust to form the newly thick

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garnet-amphibolites/amphibole-bearing eclogite lower crust. Until ca. 135 Ma, progressive slab

crust

within

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roll-back combined with ridge subduction caused foundering of newly underplated thick lower shallow

lithospheric

mantle

(Fig.

14c).

The

delaminated

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garnet-amphibolites/amphibole-bearing eclogite lower crust and lithospheric mantle were heated by the upwelling asthenosphere and underwent partial melting to generate the Liujing adakitic porphyry (131 Ma) and widespread mafic rocks (135–119 Ma) in the Luxi terrane. At the same time, partial melting of subducted garnet-amphibolites/amphibole-bearing eclogite upper and interior oceanic crust near the slab window caused by the ridge subduction formed the primary adakitic melts. The adakitic melts interacted with mantle peridotite when rising through the mantle wedge, assimilated ~10% NCB Neoarchean–Paleoproterozoic lower crust material, and finally ascended to the shallow crust to form the Mengyin adakitic porphyry (130 Ma; Fig. 14c). Our

21

ACCEPTED MANUSCRIPT research on the Mengyin and Liujing adakitic porphyries provides direct evidence for Paleo-Pacific slab subduction beneath the eastern NCB in the Early Cretaceous.

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6. Conclusions

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(1) Zircon U–Pb dating results indicate that the Mengyin and Liujing adakitic magmas were intruded into the Luxi terrane at 131–130 Ma. The Mengyin adakitic porphyry contains abundant Neoarchean–Paleoproterozoic inherited zircon cores with ages of 2.53–2.42 Ga.

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(2) The Mengyin and Liujing porphyries are characterized by high Sr/Y and La/Yb ratios, and low

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Yb and Y contents, similar to adakites. Geochemical and Sr–Nd–Pb–Hf–O isotopic data indicate that the Mengyin adakitic porphyry was derived from partial melting of subducted

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garnet-amphibolites/amphibole-bearing eclogite upper and interior oceanic crust, and the magma

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assimilated ~10% NCB lower crust material. The Liujing adakitic porphyry was derived from

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partial melting of the delaminated newly underplated thick garnet-amphibolites/amphibole-bearing eclogite lower crust, and the melts interacted with asthenospheric mantle peridotite.

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(3) Slab roll-back together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism for the formation of the Early Cretaceous Mengyin and Liujing adakitic porphyries in the Luxi terrane.

Acknowledgements We thank Prof. Yaohui Jiang, Dr. Lei Liu, and Dr. Yan Xia for their constructive suggestions. This work was supported by the Chinese Natural Science Foundation (No. 41173050) and the Chinese Ministry of Education (No. 20110091110043).

22

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Geological maps. (a) Geological and tectonic map of the North China Block, modified

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thick lower crust (Atherton and Petford, 1993; Muir et al., 1995; Petford and Atherton, 1996),

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Cenozoic subducted oceanic-crust-derived adakites (Aguillo’n-Robles et al., 2001; Defant and Drummond, 1990; Kay and Kay, 1993; Sajona et al., 2000), and Early Cretaceous

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al., 2008b, 2009); the field for the lower and upper–middle crust of the NCB and lower crust

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of the YB are from Jahn et al. (1999); the mantle array is from Zhang et al. (2005); the field for P-MORB (Pacific MORB) is from Barry et al. (2003) and Vervoort et al. (1999); and the

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fields for depleted mantle (DM) is from Zindler and Hart (1986). The (87Sr/86Sr)i and Nd(t) values are recalculated to 130 Ma. Also shown are the binary mixing curve between the oceanic-slab-derived adakitic melt (Sr = 565 ppm, Nd = 18.2 ppm, 87Sr/86Sr = 0.7036, Nd(t) = +7; Sajona et al., 2000) and NCB lower crust (Sr = 1400 ppm, Nd = 70 ppm, 87Sr/86Sr = 0.707, Nd(t) = –35; Huang et al., 2004). Fig. 9. (a) (208Pb/204Pb)i vs. (206Pb/204Pb)i and (b) (207Pb/204Pb)i vs. (206Pb/204Pb)i diagrams for the Mengyin and Liujing adakitic porphyries. Data for the EM1-type rocks in the Luxi terrane are defined by the Jinan and Zouping gabbros (Li et al., 2007; Yang, 2007). Data for the EM2-type rocks in the Luxi terrane are defined by the Fangcheng basalts (Zhang et al., 2002) 42

ACCEPTED MANUSCRIPT and Yinan gabbros (Xu et al., 2004b). The fields for P-MORB are from Vervoort et al. (1999). The (208Pb/204Pb)i, (207Pb/204Pb)i and (206Pb/204Pb)i values are recalculated to 130 Ma.

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porphyries, and (b) histogram of ages of late Mesozoic magmatic rocks in Luxi terrane. In (b), dates for the late Mesozoic magmatic rocks are from Gu et al. (2013), Guo et al. (2013), Huang et al. (2012), Lan et al. (2011), Lan et al. (2012), Lan et al. (2013), Ling et al. (2009),

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Fig. 11. Harker diagrams for Mengyin and Liujing adakitic porphyries.

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Fig. 12. (a) La/Yb vs. La, (b) Th vs. Ni, (c) (87Sr/86Sr)i vs. SiO2 and (d) Nd(t) vs. SiO2 of the

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Mengyin and Liujing adakitic porphyries. Fig. 13. (a) Sr/Y vs. (La/Yb)N (modified from Liu et al., 2010), (b) zircon Hf(t) vs. whole-rock

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Nd(t), (c) Mg# [molar ratio Mg/(Mg + FeT) × 100] vs. SiO2, and (d) Zircon δ18O vs. Hf(t) diagrams showing data of the Mengyin and Liujing adakitic porphyries. In (c), also shown are the fields of pure crustal partial melts obtained in experimental studies by dehydration melting of low-K basaltic rocks at 8–16 kbar and 1000–1050 °C (Rapp and Watson, 1995), of moderately hydrous (1.7–2.3 wt.% H2O) medium- to high-K basaltic rocks at 7 kbar and 825–950 °C (Sisson et al., 2005), and of pelitic rocks at 7–13 kbar and 825–950 °C (Patiño Douce and Johnston, 1991). In (d), also shown is the mixing trend between the depleted mantle (εHf(t) = +15, δ18O = 5.3‰) and newly underplated thick lower crust (εHf(t) = –25, δ18O = 8‰), and between oceanic crust (εHf(t) = +10, δ18O = 9‰) and mafic lower crust 43

ACCEPTED MANUSCRIPT (εHf(t) = –35, δ18O = 5.8‰). Hfm/Hfc is the ratio of Hf concentration in depleted mantle (m) to that in newly underplated thick lower crust (c), and Hfo/Hfmc is the ratio of Hf

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concentration in oceanic crust (o) to that in mafic lower crust (mc). The small open circle on

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Fig. 14. Schematic illustration of the generation and emplacement of the Mengyin and Liujing

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Figure 13

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ACCEPTED MANUSCRIPT Highlights

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1. Mengyin and Liujing adakitic porphyries crystallised at 131–130 Ma.

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2. Mengyin adakitic porphyry was originated from subducted oceanic-crust-derived magmas.

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3. Liujing adakitic porphyry was derived from delaminated newly underplated lower crust.

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4. Mengyin and Liujing plutons were generated in a continental arc-rifting setting.

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