Interaction between oceanic slab and metasomatized mantle wedge: Constraints from sodic lavas from the Qilian Orogen, NW China

LITHOS 348-349 (2019) 105182

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Interaction between oceanic slab and metasomatized mantle wedge: Constraints from sodic lavas from the Qilian Orogen, NW China Liming Yang a, b, Li Su a, *, Shuguang Song b, Mark B. Allen c, Hengzhe Bi b, Di Feng a, Wufu Li d, Yanguang Li e a

Institute of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China MOE Key Laboratory of Orogenic Belt and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China Department of Earth Sciences, Durham University, Durham DH1 3LE, UK d Qinghai Bureau of Geological Survey, Xining 810012, China e Northwest China Center for Geoscience Innovation, Xi’an Center of Geological Survey, CGS, Xi’an 710054, Shaanxi, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 March 2019 Received in revised form 22 August 2019 Accepted 23 August 2019 Available online 13 September 2019

Generation of arc volcanic rocks is a complicated process, potentially involving some or all of oceanic slab subduction, mantle wedge metasomatism, and melting of both the subducted slab and the mantle wedge. Interaction between melts of different sources plays important roles in the subduction factory and the formation of magmatic arcs and is still controversial. To elucidate the mantle-melts interaction process, we present a study of the sodic adakite-like lavas (445-444 Ma) and basalt-basaltic andesite (basaltic) enclaves (~453 Ma) in the early Paleozoic intra-oceanic arc system from the South Qilian Accretionary Belt, Qilian Orogen. Basaltic enclaves in the Muli adakite-like lavas have high Mg# (62e69) high Cr and Ni, and radiogenic Sr and unradiogenic Nd isotopic compositions (ISr ¼ 0.706327e0.706497; εNd(t) ¼ 3.2~  2.0). They are considered to represent primitive magmas derived from a highly metasomatized mantle wedge. The adakite-like lavas are dacitic with adakite-like compositions, enriched SreNd isotopic compositions (ISr ¼ 0.705597e0.706747; εNd(t) ¼ 1.7~ þ 0.4) and significantly variable zircon Hf isotopic compositions (εHf(t) ¼ þ0.9~ þ 17.8), most likely a hybrid magma produced by mixing primitive basaltic melts of metasomatized mantle wedge and melts of subducted oceanic crust. The Muli adakite-like lavas and basaltic enclaves occurred in an intra-oceanic island arc setting with an unusually hot thermal structure, which induced melting of subducted slab (including sediments and mafic oceanic crust) and metasomatized mantle wedge. The geodynamic mechanism associated with slab melting is plausibly either or both of the juvenile nature of South Qilian Oceanic Crust, and/or the heating of a slab edge by upwelling hot asthenospheric mantle during subduction initiation. © 2019 Elsevier B.V. All rights reserved.

Keywords: South Qilian Accretionary Belt Adakite-like lava Basaltic/basaltic andesitic enclaves Intra-oceanic island arc Slab melting

1. Introduction The wide compositional variability of slab-derived components and melt-mantle interactions play key roles in the diversity of magmas at convergent plate margins (e.g. Kelemen et al. 2014; Yang et al. 2019). Geochemical compositions of these slab-derived components are controlled by both the composition of subducted oceanic lithosphere including overlying sediments, oceanic crust and underlying hydrated mantle lithosphere (e.g. Plank 2014; Schmidt and Poli 1998, 2014) and the thermal structure of the subduction zone (e.g. Schmidt and Poli 1998, 2014; Van Keken et al.

* Corresponding author. E-mail address: [email protected] (L. Su). https://doi.org/10.1016/j.lithos.2019.105182 0024-4937/© 2019 Elsevier B.V. All rights reserved.

2011). Arc magmatism has long been ascribed to the fluxing of mantle wedge by solute-rich (e.g. Ba, Sr and Pb) aqueous fluids that derived from the dehydration of subducted oceanic lithosphere (e.g. Plank et al. 2009). In addition, geochemical evidence for melting of subducted sediment beneath arc systems is widely distributed on global scales (e.g. Cai et al. 2014; Plank 2014). Volcanic rocks in some warm-slab arcs, by contrast, show geochemical evidence for slab melt contributions from oceanic crust, such as SW Japan, Cascades, Mexico (Cai et al. 2014) and Western Aleutians (Yogodzinski et al. 2015). Deciphering the slab melt contributions for arc volcanics in ancient accretionary belts is difficult but, tectonically, of great significance for unravelling the corresponding thermal structure of ancient subduction zones. Interactions between the mantle wedge and the slab melts are complex and thus remain controversial (e.g. Danyushevsky et al.

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L. Yang et al. / LITHOS 348-349 (2019) 105182

2008; Martin et al. 2005; Rapp et al. 1999; Streck et al. 2007; Tatsumi and Hanyu 2003; Yogodzinski et al. 1995). Primitive, magnesium-rich, magmatic rocks are believed to be generated by partial melting of mantle sources metasomatized by subductionderived, volatile-rich component (e.g. Kelemen et al. 2014). Sanukite and the equivalent of sanukitoids (Tatsumi 2006) are one type of primitive magnesium magmatic rocks generated by reaction of a hot mantle wedge with a silicic melt that is derived from partial melting of recycled sediments and/or the oceanic crust (e.g. shimoda et al. 1998; Tatsumi 2006). Adakite is another kind of lava that shows geochemical evidence for slab melt contributions (e.g. Defant and Drummond 1990; Kay 1978). The wide compositional ranges of adakites reflect complex interactions between mantle peridotite and silicic melts. For instance, high-Si adakites (SiO2 > 60 wt%; Martin et al. 2005) are more associated with primitive slab-derived melts and show evidence for interactions between slab melts and peridotite (e.g. Martin et al. 2005; Rapp et al. 1999). It is still controversial, however, whether low-Si adakites (SiO2 < 60 wt%; Martin et al. 2005) should be considered as slab melts that reach equilibrium with mantle peridotitic rocks (e.g. Kay 1978; Tatsumi and Hanyu 2003; Yogodzinski et al. 1995) or partial melts of the metasomatized mantle wedge (e.g. Martin et al. 2005; Rapp et al. 1999), or hybrid magmas generated by complex open-system magmatic processes (e.g. Danyushevsky et al. 2008; Streck et al. 2007). Therefore, these magmatic processes should be further evaluated and these lavas with slab melt contributions can be used as probes of the subarc melt-mantle interaction processes in hot subduction zones. In this paper, we present studies of Muli adakite-like lavas and basaltic enclaves from a well-preserved early Paleozoic intraoceanic arc in the South Qilian Accretionary Belt (SQAB), through in-situ zircon UePb geochronology and LueHf isotope analysis, as well as bulk-rock elemental data and SreNd isotope analysis. In combination with the published Sanukite data in the eastern section of SQAB (Yang et al. 2019), we try to decipher the interaction process between melts from the subducted slab and mantle wedge. We further assess the thermal structure of the early Paleozoic arc system and the possible geodynamic driving mechanism(s) in the SQAB. 2. Geological background The Qilian-Qaidam Orogenic Belt is a wide (exceeding 300 km) orogenic collage, presently exposed on the northern margin of the Tibetan Plateau. Its adjacent areas include the Qaidam Basin to the south, the Alashan Block to the northeast and the Tarim Block to the northwest (Fig. 1a). This whole Qilian-Qaidam region is offset by the strike slip Altyn Tagh Fault in the west. To the east, it merges with the East Kunlun Orogen and continues farther to the eastern Qinling-Dabie Orogenic Belt (Song et al. 2013, 2014). The QilianQaidam Orogenic Belt consists of five tectonic units, from north to south, including the North Qilian Accretionary Belt (NQAB), the Central Qilian Block (CQB), the South Qilian Accretionary Belt (SQAB), the Quanji-Oulongbuluke Block (QOB) and the North Qaidam ultra-high pressure metamorphic (UHPM) belt (Fig. 1a; Song et al. 2014). The NQAB and SQAB are two subparallel oceanic-type accretionary belts that belong to the Qilian Orogen; and the North Qaidam UHPM belt is a continental-type UHPM belt (e.g. Song et al. 2010a, 2013, 2014). The NQAB consists of ophiolitic fragments, arc magmatic sequences including intrusive and volcanic rocks, and subduction-related high-pressure/low-temperature metamorphic rocks (e.g. Song et al. 2013, 2014; Xia et al. 2012; Zhang et al. 2007). The two Precambrian blocks, the CQB and QOB, are interlayered among the three accretionary belts (Fig. 1a). The CQB is located between the NQAB and the SQAB, with Precambrian basement composed of Neoproterozoic granitic intrusions and

Paleoproterozoic granitic gneiss. Rock assemblages and the geochronological spectrum of magmatism indicate that the CQB has affinities with the Yangtze Block (e.g. Song et al. 2010a, 2012; Tung et al. 2013).The Precambrian basement is broadly intruded by early Paleozoic granitoids and mafic rocks (Wang et al. 2017; Xiao et al. 2009). The SQAB occurs along the south margin of CQB, as discontinuous, NW-SE oriented and fault-bounded slivers; it is separated from the QOB by thick and wide Paleozoic volcanic-sedimentary sequences (Fig. 1a). The belt extends 1000 km in length and, from SE to NW, consists of the Lajishan-Yongjing, Gangcha, Muli and Subei-Yanchiwan terranes (Song et al. 2017). The SQAB is identified as a subduction-accretion belt in recent papers (e.g. Pan et al. 2002; Song et al. 2017; Xiao et al. 2009; Yan et al. 2015; Yang et al. 2018, 2019) and predominantly made up of two parallel sequences, including an ophiolite sequence in the north and an arc-volcanic sequence in the south (Zhang et al. 2017). Rock assemblages of the ophiolite sequence include ultramafic rocks (pyroxenites and dunites), cumulate gabbros, pillow and massive basalts and pelagic chert. Pillow lavas of the ophiolite sequence from the LajishanYongjing terrane show E-MORB to OIB affinities with zircon ages of 525e491 Ma (Fu et al. 2018; Zhang et al. 2017). The arc-volcanic sequence within the east section of the SQAB consists of various primitive volcanics, including boninite, an ankaramitic lava series and sanukite; these near-primitive magmas are considered to be simultaneously generated from heterogeneous mantle source(s) fluxed by a wide range of slab-derived fluids/melts within an intraoceanic island arc system (Yang et al. 2019). They are also the products of volcanism in the early stage of the arc system (~460e440 Ma) in response to the collision between the CQB and Lajishan-Yongjing Oceanic Plateau (LYOP) (Song et al. 2017; Yang et al. 2019; Zhang et al. 2017). 3. Field occurrence and petrography The Muli terrane is located in the middle section of the SQAB and occupies an area of ~16  20 km2 (Fig. 1b). It mainly consists of Cambrian ophiolite fragments in the north and an Ordovician arc volcanic-sedimentary sequence in the south, which are unconformably covered by Triassic sedimentary rocks and intruded by arc-related intermediate-acid intrusion with zircon ages of ~470e445 Ma (Fig. 1b; QBGS, Qinghai Bureau of Geological Survey, 2014). The ophiolite sequence occurs as several NW-SE trending, fault-bounded, lensoid tectonic slices between the Precambrian basement in the north and the granitic intrusions in the south (Fig. 1b). Rock assemblages include serpentinized peridotites and pyroxenites, gabbros, and both massive and pillow basalts. Pillow basalts have N-MORB affinity, and zircon from gabbro gives a weighted 206Pb/238U mean age of 491.8 ± 1.2 Ma (QBGS, Qinghai Bureau of Geological Survey, 2014). The Muli arc volcanic-sedimentary sequence predominantly consists of volcaniclastic and volcanic breccia, with minor massive lava flows (Fig. 2a). Dark-green basalt-basaltic-andesite (basaltic) enclaves with varying shape (mostly angular) and size (5e15 cm) are widely distributed in the dark red adakite-like lavas (Fig. 2d) with distinct contact boundaries (Fig. 2b-c). As shown in Fig. 2b and c, all mafic enclaves have reaction rims at the contact borders with the adakite-like lavas. The basaltic enclaves show intersertal textures including plagioclase and pyroxene microlites (Fig. 2e-f), with a few pyroxene and plagioclase phenocrysts (Fig. 2f). Some pyroxene phenocrysts were replaced by amphibole. The adakite-like lavas are generally porphyritic, with high volume (15e20%) of local plagioclase and amphibole phenocrysts, typically in an intersertal-textured groundmass (Fig. 2g). Plagioclase phenocrysts are mostly subhedral to euhedral and underwent variable degrees of saussuritization and epidotization (Fig. 2g and h). Amphibole

L. Yang et al. / LITHOS 348-349 (2019) 105182

3

Fig. 1. (a) Simplified geological map of the Central China Orogenic Belt (modified after Song et al. (2017)) for details). (b) Simplified geological map of the Muli Terrane and sampling locations (modified after QBGS, Qinghai Bureau of Geological Survey (2014)). (c) Cross section of the Muli volcanic rocks.

phenocrysts are euhedral with dark-green colour and clinopyroxene phenocrysts are rare (Fig. 2 g and 2 h). Amphibole phenocrysts also show banded and/or oscillatory zoning under microscope (Fig. 2i). 4. Analytical methods 4.1. Bulk-rock geochemistry analyses The Bulk-rock geochemistry analysis were taken place at the Elemental Geochemistry Lab, the Institute of Earth Sciences, China University of Geosciences, Beijing (CUGB). Major element oxides were determined using a Leeman Prodigy inductively coupled plasma optical emission spectroscopy (ICP-OES) system with high dispersion Echelle optics. The analytical uncertainties are generally <1% for most elements except for P2O5 (~2.0%) and TiO2 (~1.5%) based on rock standards GRS-1 and GSR-3 (national geological standard reference material of China), W-2 (U.S. Geological Survey: USGS) and AGV-2. Loss on ignition (LOI) was determined by placing 1 g sample in the furnace at 1000  C for 3 h before cooling in a desiccator and re-weighing. Bulk-rock trace elements were

measured on an Agilent-7500a inductively coupled plasma mass spectrometry (ICP-MS). The detailed analytical procedures were presented by Song et al. (2010b). Rock standards AGV-2, W-2, and BHVO-2 (USGS) were used to monitor the analytical precision and accuracy. Analytical accuracy, as indicated by relative difference (RE) between recommended and measured values, is better than 5% for most elements, 10% -15% for Sc, Nb, Cu, Er, Th, and U, and 11%e 20% for Ta, Tm, and Gd. Bulk-rock SreNd isotope analyses were carried out at Ministry of Education (MOE) Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. The detailed separation procedures of SreNd isotopes follow Xu et al. (2016). We use rock standard BCR-2 to evaluate the separation and purification process of Rb, Sr, Sm, and Nd. The SreNd isotopic compositions were measured by multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The 87Rb/86Sr and 147Sm/144Nd ratios were determined on the basis of Rb, Sr, Sm, and Nd contents acquired by ICP-MS at CUGB. Mass fractionation corrections for Nd and Sr isotopic ratios were 146 respectively normalized to Nd/144Nd ¼ 0.7219 and 86 Sr/88Sr ¼ 0.1194. Initial 143Nd/144Nd ratios and corresponding εNd (t) values were calculated based on present-day reference

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L. Yang et al. / LITHOS 348-349 (2019) 105182

Fig. 2. Representative field photographs and microphotographs of Muli volcanic lavas: (a) Massive, dark red Muli volcanic lavas with deep green weathering surface; (b-c) Massive, green basaltic enclaves and dark red adakite-like lavas with plagioclase phenocrysts; (d) Dark red Muli adakite-like lavas; (e) Muli basaltic enclaves showing intersertal texture including altered plagioclase and pyroxene microlites; (f) Muli basaltic enclaves with pyroxene phenocrysts partly altered to tremolite; (g-h) Muli adakite-like lavas with abundant plagioclase and amphibole phenocrysts; (d) Fresh green amphibole phenocrysts with oscillatory or banded zoning from Muli adakite-like lavas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

values for CHUR: (143Nd/144Nd)CHUR ¼ 0.512638 and (147Sm/144Nd)CHUR ¼ 0.1967(Jacobsen and Wasserburg 1984). Repeated analyses for the Nd and Sr standard samples (JNdi and 143 NBS987) yielded Nd/144Nd ¼ 0.512121 ± 11 (2s) and 87 86 Sr/ Sr ¼ 0.710251 ± 11 (2s), respectively. 4.2. In-situ zircon UePb dating and Hf isotopic analysis Zircon grains were separated by standard density and magnetic techniques and then handpicked under a binocular microscope. Zircon grains from samples, together with the standard zircon Qinghu, were embedded in an epoxy mount and then polished down to expose the inner structure for analysis. The Cathodoluminescence (CL) examination was carried out to observe the internal structures of zircon grains, by using a CL spectrometer (Garton Mono CL3þ) equipped on a Quanta 200F environmental scanning electron microscope under conditions of 15 kV/120 nA in the School of Earth and Space Sciences, Peking University, Beijing. In-situ zircon U-(Th)-Pb isotope measurements were carried out using an Agilent-7500a quadrupole inductively coupled plasma mass spectrometry coupled with a New Wave SS UP193 laser sampler (LA-ICP-MS) at the Elemental Geochemistry Lab, the

Institute of Earth Sciences, China University of Geosciences, Beijing (CUGB). Laser spot size of 36 mm, laser energy density of 9.5 J/cm2 and a repetition rate of 10 Hz were applied for analysis. The detailed analytical procedures are described in Song et al. (2010b). The NIST 610 glass and zircon standard 91,500 (Wiedenbeck et al. 1995) were used as external standards, Si as internal standard. An in-house zircon standard Qinghu zircon were used as the secondary standard and yield a concordia age of 160.2 ± 0.9 Ma, which is identical within the error to the recommended value of Qinghu zircon (159.5 ± 0.2 Ma; Li et al., 2013a, b). Each spot analysis comprised 5 s of background data and 45 s of sample data acquisition. The software GLITTER (ver. 4.4.4, Macquarie University) was used to process the element concentrations and isotopic ratios of zircons. Common Pb was corrected following Andersen (2002). Weighted mean UePb ages and concordia plots were processed with uncertainties quoted at the 2s and 95% confidence levels, using ISOPLOT 4.15 (Ludwig 2003). In-situ zircon LueHf isotope analyses were carried out on a New Wave UP213 laser-ablation system coupled to a Neptune multiplecollector ICP-MS at the Key Laboratory for the study of focused Magmatism and Giant ore Deposits, MLR, in Xi’an Center of Geological Survey, China Geological Survey. Details of the

Table 1 Major and trace element data for Muli volcanic lavas. Samples

17ML08

17ML09

17ML10

17ML11

Muli B/BA enclaves

17ML02

17ML03

17ML04

17ML05

17ML06

17ML07

17ML-12- 17ML-12- 17ML1 2 13

17ML14

17ML15

17ML17

17ML18

17ML19

17ML20

17ML21

17ML22

Muli adakite-like lavas

%) 50.8 1.04 18.8 7.09 0.18 6.47 7.19 4.14 0.96 0.39 2.89 100 4.32

50.8 0.95 19.2 7.34 0.19 5.19 8.28 4.44 0.49 0.35 3.38 100.6 9.13

51.3 0.99 18.4 7.4 0.19 6.37 6.48 3.65 1.4 0.35 4 100.5 2.61

60.9 0.8 15.9 5.59 0.09 3.4 3.32 5.68 0.96 0.32 2.55 99.5 5.94

58 0.8 16.8 6.12 0.09 4.31 3.73 5.22 1.32 0.35 2.89 99.6 3.96

62.1 0.65 15.7 4.92 0.09 3.18 3.96 4.22 2.79 0.26 1.6 99.4 1.51

61.9 0.65 16 4.89 0.09 2.96 4.21 4.22 2.67 0.27 1.6 99.5 1.58

58 0.78 17.2 6.19 0.09 3.62 3.75 3.02 2.56 0.29 4.4 99.9 1.18

62 0.71 15 5.5 0.08 2.36 5.08 4.26 1.29 0.26 3.34 99.9 3.29

61.1 0.71 14.6 5.47 0.09 2.87 5.53 3.69 1.29 0.25 4.32 99.9 2.86

62.8 0.78 13.5 6.48 0.1 3.14 5.16 4.39 0.81 0.28 2.27 99.7 5.43

62.2 0.79 12.9 7.12 0.08 3.26 5.28 5.08 0.86 0.36 1.75 99.7 5.93

67.5 0.49 14.6 3.94 0.05 1.26 2.51 6.54 1.35 0.2 0.88 99.3 4.85

67.1 0.58 13.8 4.46 0.14 1.69 3.37 4.66 2.08 0.21 1.14 99.3 2.24

65.5 0.63 14.6 4.84 0.21 1.22 2.63 5.17 3.22 0.23 1.55 99.7 1.61

65.4 0.64 13.9 5 0.11 2.01 4.18 4.36 2.3 0.23 1.59 99.7 1.89

64.1 0.63 14.4 4.8 0.1 2.61 5.61 3.66 1.66 0.22 1.94 99.8 2.21

59.7 0.67 15.8 6.15 0.13 1.99 5.58 5.86 1.4 0.24 2.45 100 4.2

64.2 0.57 13.9 5.46 0.1 1.8 4.64 5.26 1.22 0.21 2.38 99.7 4.3

63.5 0.65 14.4 5.93 0.08 3.1 3.7 5.42 1.08 0.2 1.68 99.8 5.03

62.4 0.62 14.3 5.73 0.12 1.3 5.71 4.37 1.57 0.23 3.41 99.8 2.79

68 0.9

62.2 0.84

66.7 0.95

58.6 0.97

62.1 1

60.1 0.92

58.5 0.92

57.7 1.18

50 0.85

55 0.83

53.1 0.77

51.6 0.68

42.6 0.87

46.8 0.86

37 0.87

48.4 0.8

55.9 0.8

43 0.74

43.5 0.75

55 0.86

34.6 0.74

19.12 1375 3866 15.7 6082 156.4 119.4 1429 22.8 36.24 46.26 109.48 27.62 9.96 1045.8 15.55 178.8 13.66 0.41 216.2 29.22 69.46 8.03 30.44 5.54 1.5 4.61

29.08 1509 11296 16.53 6586 163.82 124.44 1479 30.3 40.16 36.98 146.56 27.4 40.32 882.6 16.99 191 14.09 1.21 311.6 30.34 69.64 8.33 31.64 5.82 1.55 4.88

11.18 1447 7602 10.6 5010 129.3 81.32 697 19.85 34.4 1.91 108.5 20.56 21.5 634.8 15.64 177 15.12 1.24 427.6 29.34 64.38 7.37 27.08 4.84 1.11 4.14

15.9 1410 9716 11.89 4696 129.48 90.62 683 20.96 39.16 28.18 104.9 21.24 28 563.6 13.86 165 12.55 1.32 715.4 26.5 59.36 6.94 25.92 4.65 1.2 3.89

17.36 2854 61980 14.91 5042 103.54 56.86 706 14.56 26.7 14.34 86.26 24.2 58.72 785.6 13.4 197.4 13.32 1.37 1243.6 32.78 66.96 7.55 25.5 4.36 1 3.01

17.66 3042 58640 14.4 5164 105.36 55.8 693 13.84 25.26 16.39 84.1 24.02 54.24 803.6 13.63 192.5 13.08 1.34 1166 33.36 67.92 7.64 25.9 4.42 1 3.04

19.67 1218 19440 12.34 5038 142.44 69.38 732 19.38 29.28 11.74 76.98 22.06 61.16 525.6 17.93 214.8 15.48 2.06 2294 33.08 69.06 8.05 30.06 5.35 1.92 4.67

12.69 1120 10236 11.47 4700 134.66 66.64 644 16.34 31.7 5.59 73.02 19.63 37.56 653 15.71 206.2 14.5 1.14 563.6 30.5 65.16 7.39 27.54 4.88 1.23 4.12

16.41 1077 9798 11.66 4726 137.24 68.3 772 16.69 31.46 5.19 87.52 19.6 34.36 605.8 15.86 196.2 13.95 1.23 583.4 29.56 61.96 7.21 27.02 4.83 1.18 4.11

12.41 1255 6162 14.59 5200 160.38 99.98 874 20.22 35.14 13.4 59.06 18.54 15.96 756 15.24 153.9 11.37 0.65 458.2 26.58 57.14 6.83 26.44 4.74 1.21 3.94

6.02 1548 6412 17.43 4998 160.4 120.08 676 14.61 19.77 2.65 61.56 12.89 15.92 659.2 17.34 122.4 9.24 0.52 724.4 35.48 75.98 9.1 35.14 5.69 1.39 4.46

7.9 2274 26600 9.64 3518 64.72 45.88 341 9.22 24.42 8.27 52.94 15.68 23.16 434.8 9.56 160 11.36 0.54 794.4 29.68 57.9 6.27 20.48 3.32 0.81 2.3

16.98 2420 41120 12.65 4124 67.44 41.14 1042 17.04 22.68 7.57 79.3 20.52 39.34 419.8 13.19 175.8 11.73 1.11 970 30.1 61.58 7.07 24.24 4.14 0.92 2.94

7.47 1018 24840 9.98 4146 82.62 55.08 1746 16.24 19.12 2.78 69.06 18.33 54.38 576.6 14.29 216.6 15.16 1.63 1257.8 31.62 65.2 7.07 25.3 4.34 1.12 3.61

11.21 1017 18146 10.14 4366 80.64 57.94 940 18.55 25.34 6.01 62.28 19.43 45.96 537.8 16.14 225.4 15.95 1.11 852.2 32.72 68.48 7.5 27.08 4.65 1.18 3.93

15.88 961 12456 10.41 4212 120.42 65.28 847 12.94 26.44 30.5 58.56 19.44 19.99 858.2 15.4 214.4 15.39 0.51 758.4 31.74 65.08 7.3 26.38 4.49 1.12 3.8

7.18 1142 11403 13.42 4767 79.82 107.78 1166 16.26 29.71 2.77 57.06 19.83 35.29 640.2 18.4 179.2 15.88 1.11 926.5 24.35 51.02 6.01 23.18 4.36 1.26 3.98

6.08 880 9028 10.65 3750 64.86 78.78 860 12.24 22.04 2.13 47.42 15.76 27.2 483.2 14.36 143.4 13.07 0.91 725.6 19.16 40.2 4.71 18.08 3.42 0.99 3.12

13.18 927 8410 12.58 4432 123.02 90.38 696 17.25 27.94 3.51 68 15.98 25.66 528 14.69 138.4 13.4 0.92 414.6 23.76 45.42 5.12 19.4 3.67 1.04 3.38

6.37 1130 11891 11.18 4163 117.2 36.41 1041 12.75 10.95 8.15 51.27 17.15 39.43 664.4 15.59 152.9 13.32 1.35 777.9 18.35 39.16 4.67 18.14 3.55 1.03 3.3

Trace elements (ppm) Li 55.24 51.61 P 4476 4355 K 28500 21674 Sc 23.58 23.84 Ti 7994 7781 V 162.14 167.36 Cr 114.56 116.15 Mn 1404 1415 Co 28.72 27.17 Ni 44.04 42.79 Cu 44.58 61.31 Zn 208.8 202.91 Ga 33.64 33.4 Rb 35.28 24.98 Sr 880.8 941.6 Y 15.27 15.07 Zr 185 173.7 Nb 12.58 12.18 Cs 1.08 0.85 Ba 337.8 288.9 La 31.58 32.14 Ce 73.68 72.9 Pr 8.74 8.56 Nd 31.04 30.24 Sm 5.63 5.44 Eu 1.31 1.28 Gd 3.88 3.8

L. Yang et al. / LITHOS 348-349 (2019) 105182

Major elements(wt. SiO2 49.7 TiO2 1.04 Al2O3 19.6 Fe2O3t 7.09 MnO 0.18 MgO 6.67 CaO 6.51 Na2O 4.15 K2O 1.26 P2O5 0.39 LOl 3.4 Total 100 Na2O/ 3.31 K2O Mg# 68.7 A/CNK 0.98

17ML01

(continued on next page)

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L. Yang et al. / LITHOS 348-349 (2019) 105182

0.46 2.69 0.52 1.42 0.21 1.32 0.21 3.39 0.89 12.13 3.8 1.06 18 35.9

17ML21

0.43 2.53 0.5 1.39 0.21 1.35 0.22 3.33 0.78 7.29 3.49 0.58 14.2 33.6

17ML20

0.54 3.22 0.65 1.78 0.28 1.81 0.3 4.19 1.04 9.59 4.35 0.7 13.4 34.8

17ML19

0.51 2.93 0.57 1.56 0.24 1.49 0.24 5.48 1.25 16.63 8.94 2.4 22 33.3 0.46 2.64 0.51 1.38 0.21 1.35 0.22 5.23 1.12 18.75 8.3 2.11 23.4 40.3

0.49 2.8 0.55 1.48 0.23 1.45 0.23 5.21 1.15 27.1 8.65 2.35 21.9 55.7

17ML18 17ML17 17ML15

0.36 2.04 0.39 1.11 0.16 1 0.15 3.95 0.82 11.1 5.47 1.28 30 31.8

17ML14

0.51 2.88 0.56 1.48 0.22 1.37 0.22 3.74 0.67 13.78 5.01 1.26 19.4 49.6

0.55 3.13 0.61 1.67 0.25 1.55 0.24 3.1 0.47 9.12 4.55 1.01 22.9 38

0.28 1.5 0.28 0.79 0.11 0.71 0.11 3.61 0.64 4.56 5.21 1.44 42 45.5

17ML-12- 17ML-12- 17ML1 2 13

0.53 3.06 0.59 1.59 0.24 1.49 0.24 4.77 0.87 11.1 6.68 1.77 19.8 38.2

17ML07

0.54 3.05 0.59 1.56 0.24 1.46 0.23 4.99 0.93 12.35 6.93 1.86 20.9 41.6

17ML06

0.6 3.45 0.67 1.75 0.27 1.65 0.26 5.34 1.05 8.5 7.36 1.95 20.1 29.3

17ML05

0.37 2.09 0.39 1.09 0.15 0.99 0.15 4.17 0.76 12.25 5.66 1.59 33.2 58.6 0.5 2.95 0.56 1.54 0.23 1.42 0.22 3.61 0.72 14.3 5.61 1.51 18.7 40.7 0.61 3.36 0.63 1.63 0.23 1.42 0.22 4.53 0.91 15.6 5.75 1.28 21.3 52

0.54 3.17 0.61 1.7 0.24 1.56 0.24 3.93 0.95 19.73 8.29 2.42 18.8 40.6

Muli adakite-like lavas

0.37 2.07 0.39 1.07 0.15 0.97 0.15 4.07 0.76 12.48 5.46 1.52 34.2 58.9

17ML04

Mg#¼molar 100*Mg/(MgþFe)

0.58 3.2 0.6 1.55 0.22 1.37 0.21 4.4 0.98 19.98 5.55 1.24 21.4 67.2 0.46 2.46 0.44 1.18 0.16 0.96 0.14 3.71 0.71 11.22 3.9 0.9 33.6 62.5

Muli B/BA enclaves

0.47 2.51 0.45 1.2 0.16 0.98 0.14 3.85 0.67 10.53 4.11 0.93 32.1 57.7 Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U La/Yb Sr/Y

17ML03 17ML02 17ML11

17ML01

5. Results 5.1. Bulk-rock geochemistry

17ML10 17ML09 17ML08 Samples

Table 1 (continued )

instrumental conditions and data acquisition procedures are similar to those described by Wu et al. (2006). LueHf isotopic analyses were performed on the same zircon grains that were previously used for LA-ICP-MS UePb dating, with ablation times of 26 s and stationary ablation pits of 44 mm. Zircon GJ-1, as the reference standard, yielded a weighted mean 176Hf/177Hf ratio of 0.282028 ± 12 (2s) during this approach, within error of the recommended value of 0.282013 ± 19 (2s) (Elhlou et al. 2006). Values of 176Lu/175Lu ¼ 0.02658 and 176Yb/173Yb ¼ 0.796218 were used to correct for 176Lu and 176Yb isobaric interferences (Chu et al. 2002). The details on the mass bias correction protocols are given by Iizuka and Hirata (2005) and Wu et al. (2006). Instrumental mass bias correction was accounted for by normalizing Hf isotope ratios to 179 Hf/177Hf of 0.7325 and Yb isotope ratios to 172Yb/173Yb of 1.35274 (Chu et al. 2002) using an exponential law. The calculation methods for εHf(t) values are similar to those of Hou et al. (2007) and Wu et al. (2006), based on zircon UePb ages, 176Lu decay constant (Scherer et al. 2001) and Chondritic values (Blichert-Toft and
de 1997). Albare

0.45 2.71 0.54 1.46 0.22 1.42 0.22 3.43 0.81 10.72 3.45 1.21 12.9 42.6

17ML22

6

Bulk-rock elemental and SreNd isotopic data of the Muli adakite-like lavas are listed in Tables 1 and 2. Major element compositions are normalized to 100% on a volatile-free basis in the following discussion and plots. All the Muli volcanic lavas, in the TAS diagram (Fig. 3a), can be subdivided into the andesitic-dacitic lavas and basaltic-basaltic andesitic enclaves, the former of which belong to sub-alkaline series (Fig. 3a and b) and define a strongly calc-alkaline trend in the FeOt/MgO-SiO2 diagram (Fig. 3c). The basaltic enclaves have intermediate alkali contents in the TAS diagram (Fig. 3a). Given the potential influence to the mobile alkalis during alteration, the SiO2 vs. Zr/TiO2 diagram (Fig. 3b) is utilized for rock discrimination, where the basaltic enclaves plot in the transitional domain between sub-alkaline series and alkaline series, consistent with that in TAS diagram (Fig. 3a). Both of them have medium-potassic contents. The sodic character of these lavas is reinforced by high Na2O contents (3.02e6.54 wt%) and high Na2O/K2O ratios of 1.18e9.13 (mostly >2) (Fig. 3d). It should be noted that the Muli adakite-like lavas geochemically exhibit similar major element compositions (Fig. 3a-d), high Sr/Y ratios (Fig. 3e), and unradiogenic bulk-rock SreNd and zircon Hf isotopic compositions (see next) to the Lajishan sanukites (Yang et al. 2019). The Muli adakite-like lavas and basaltic enclaves are distinguishable based on their CaO þ Na2O, TiO2, MgO, Sr, Cr and Ni (Table 1). The Muli adakite-like lavas are characterized by low CaO þ Na2O, TiO2, MgO, Sr, Cr and Ni relative to the basaltic enclaves. Both of them display high LREEs and Sr (419e1045 ppm), low HREEs (e.g. Yb ¼ 0.71e1.81 ppm) and Y (9.56e18.40 ppm), and high Sr/Y (29.3e67.2) and (La/Yb)N (9.3e30.1) (Fig. 3e and f). The basaltic enclaves exhibit relatively high Al2O3 (19.04e20.31 wt%) and MgO (5.34e6.91 wt%), while the adakite-like lavas have moderate to low Al2O3 (13.14e17.98 wt%) and MgO (1.24e4.45 wt%) contents. Chondrite-normalized REE patterns (Fig. 4a) of the adakite-like lavas and the basaltic enclaves are characterized by enrichment of LREEs over HREEs with weak negative Eu anomalies (Eu*/Eu ¼ 0.76e0.92). In the primitive mantle-normalized trace element diagram (Fig. 4b), all the samples are enriched in the large ion lithophile elements (LILEs) and depleted in some high field strength elements (HFSEs) but not Zr and Hf. All of them display narrow range of slightly negative εNd(t) values of 3.2  þ0.4 calculated at 445 Ma using zircon age data, and initial 87Sr/86Sr (ISr) values of 0.705597e0.706747 (Table 2). The adakite-like lavas

Table 2 Bulk-rock Sr-Nd isotopic data for Muli volcanic lavas. Sample No.

17ML-08 17ML-09 17ML-10 17ML-11 17ML-01 17ML-02 17ML-03 17ML-07 17ML-13 17ML-14 17ML-18 17ML-20

T

Rb

Sr

(Ma)

(ppm)

(ppm)

445 445 445 445 445 445 445 445 445 445 445 445

35.3 25 10 40.3 21.5 28 58.7 34.4 23.2 39.3 20 27.2

880.8 941.6 1045.8 882.6 634.8 563.6 785.6 605.8 434.8 419.8 858.2 483.2

87Rb/86Sr

0.113 0.075 0.027 0.129 0.096 0.14 0.211 0.16 0.15 0.265 0.066 0.159

87Sr/86Sr

Isr

Ratio


2¦O

0.707075 0.70691 0.70667 0.70716 0.707211 0.707153 0.708085 0.707495 0.707164 0.708001 0.707084 0.706605

9.01 6.92 9.6 7.28 4.61 5.03 11.9 7.22 5.04 9.79 7.81 10.3

0.706345 0.706426 0.706497 0.706327 0.706605 0.706263 0.706747 0.706479 0.70621 0.706323 0.706667 0.705597

Sm

Nd

(ppm)

(ppm)

5.63 5.44 5.54 5.82 4.84 4.65 4.36 4.83 3.32 4.14 4.49 3.42

31 30.2 30.4 31.6 27.1 25.9 25.5 27 20.5 24.2 26.4 18.1

147Sm/144Nd

0.115 0.114 0.115 0.117 0.113 0.114 0.108 0.113 0.103 0.108 0.108 0.12

143Nd/144Nd Ratio


2¦O

0.512294 0.512289 0.512234 0.512294 0.512309 0.51233 0.512322 0.512326 0.512287 0.512313 0.512312 0.512436

3.42 5.34 5.64 6.54 2.57 4.39 6.8 3.65 5.09 6.17 4.79 10.6

Initial Nd

¦ÅNd(0)

¦ÅNd(t)

0.511952 0.51195 0.511892 0.511948 0.511978 0.511998 0.512006 0.511996 0.511987 0.511997 0.511997 0.512086

-6.7 -6.8 -7.9 -6.7 -6.4 -6 -6.2 -6.1 -6.8 -6.3 -6.4 -3.9

-2 -2 -3.2 -2.1 -1.7 -1.3 -1.1 -1.3 -1.5 -1.3 -1.3 0.4

 (e¦ET-1),  10-11 year-1 (IUPAC; Villa et al., 2015). € € Note: (1) ISr ¼87Sr/86Sr€C87Rb/86Sr ¡A where ¦ERb ¼1.3972 ¡A  (e¦ET  ET  10,000, where ¦ ESm  10-12 year-1; € -1))/(143Nd/144Nd)CHUR €C (147Sm/144Nd)CHUR¡A(e¦ € -1)} -1} ¡A € (2) ¦ÅNd(t) ¼{(143Nd/144Nd €C 147Sm/144Nd ¡A ¼6.54 ¡A (143Nd/144Nd)CHUR ¼0.512638;(147Sm/144Nd)CHUR¼0.1967 (Jacobsen and Wasserburg, 1984).

Fig. 3. (a) TAS diagram (Le Maitre et al., 2005); (b) SiO2- Zr/TiO2 diagram; (c) SiO2- FeOt/MgO diagram; (d) K2O-Na2O diagram; (e) Sr/Y-Y diagram (Defant and Drummond 1993); (f) (La/Yb)N vs. YbN (Defant and Drummond, 1990). Data of Aleutian adakites and Setouchi HMA are respective from Defant and Drummond (1990) and Tatsumi (2006). Data of Lajishan Sanukites for comparison are from Yang et al. (2019). Modelling melts with melt fractions of 20% (white polygons) are from Ma et al. (2015).

8

L. Yang et al. / LITHOS 348-349 (2019) 105182

morphology and geochronology of zircon population indicate that zircon grains in basaltic enclave are likely to be “inherit” zircon from either the recycled sediments or the crustal-level assimilation process. Zircon grains from adakile-like lavas are euhedral crystals with long axis varying between 150 and 250 mm and show straight and wide oscillatory growth bands in CL images (Fig. 5b and c). They have uniform concentrations of Th (32e320 ppm) and U (58e267 ppm) with a small range of Th/U ratios (0.6e1.52) (Table 3). Twenty-eight analyses of zircons from sample 17ML-03 yield apparent 206Pb/238U ages of 435e461 Ma, giving a weighted 206 Pb/238U mean age of 445.1 ± 2.6 Ma (MSWD ¼ 3.1) (Fig. 5b). Twenty-nine zircon grains from sample 17ML-14 give a weighted mean age of 444.4 ± 2.5 Ma (MSWD ¼ 1.4) (Fig. 5c). Zircon Hf isotopic data for two adakite-like samples (17ML-03, 17ML-14) are given in Table 4 and Fig. 6. Zircon grains have a moderate range of initial 176Hf/177Hf ratios (0.282529e0.282999) and the calculated εHf (t) values range from 1.2 to 17.8 (Fig. 6) with respective weighted mean values of 6.4 ± 1.1 (MSWD ¼ 3.8, n ¼ 27) and 5.6 ± 0.88 (MSWD ¼ 2.4, n ¼ 30), more depleted than those of Lajishan sanukites (Yang et al. 2019). 6. Discussion 6.1. Petrogenesis of Muli volcanic lavas

Fig. 4. Chondrite-normalized REE patterns (a) and Primitive mantle (PM)-normalized multi-element patterns (b) for Muli volcanic lavas. Normalization values are from Sun and Mcdonough (1989). Values of typical adakites for comparison are from Defant et al. (1991).

exhibit slightly elevated εNd (t) values of 1.7~ þ 0.4 relative to the basaltic enclaves (3.2~  2.0; Table 2). The Muli andesitic-dacitic lavas show “adakitic signature” including high Sr/Y and La/Yb ratios. Given the slightly high HREE and Y, and radiogenic SreNd isotopic compositions relative to those of typical adakites (e.g. Defant and Drummond 1990), they were termed as adakite-like lavas in this study. The basaltic enclaves, with high Al2O3 (>16.5e17.0 wt%), low SiO2 (<54 wt%) and MgO (<7%), and intermediate alkalis, are closer to the high-alumina basalt/basaltic andesite (e.g. Crawford et al. 1987; Kuno 1960). 5.2. In-situ zircon UePb ages and Hf isotopes One basaltic enclave (17ML-08) and two Muli adakite-like lava samples (17ML-03, 17ML-14) are selected for LA-ICP-MS zircon UePb dating, and the results are presented in Table 3 and Fig. 5. Zircon grains from the basaltic enclave sample are colorless and subhedral crystals with varying long axis lengths up to 50e120 mm and length/width ratios up to 1.0e1.5. CL images show variable luminescence with blurred oscillatory growth bands (Fig. 5). They have various abundances of Th (48-344 ppm) and U (67-559 ppm) with high Th/U ratios of 0.13e0.91(Table 3). Fourteen analyses of zircon from basaltic enclave sample consists of eleven early Paleozoic zircon grains with 206Pb/238U ages ranging from 481 to 445 Ma and three Proterozoic zircon grains with 207Pb/206Pb ages of 2459-912 Ma (Fig. 5a). Ten Paleozoic zircon grains give a weighted 206 Pb/238U mean age of 455.0 ± 4.9 Ma (MSWD ¼ 2.9) (Fig. 5a). The

6.1.1. Assessing different models High Sr/Y and La/Yb signatures can be associated with a variety of processes (Moyen 2009), including: (1) high-pressure partial melting of a subducted slab (e.g. Defant and Drummond 1990; Rapp et al. 1999) and thickened (or delaminated) lower crust (e.g. Atherton and Petford 1993; Wang et al. 2005, 2008); (2) fractional crystallization of basaltic magmas (e.g. Castillo et al. 1999; Macpherson et al. 2006) and (3) inheritance from source rocks (e.g. ancient lower continental crust) under low pressure (Ma et al. 2015; Moyen 2009). Commonly used geochemical arguments for a lower crustal origin are relatively potassic-rich, peraluminous compositions, high content of incompatible elements (e.g. Rb, Ba, Th and U) and enriched SreNd isotopic compositions (e.g. Wang et al. 2005, 2008). Such crust-derived adakitic rocks (Castillo, 2012) are usually characterized by low contents of MgO and other compatible elements, identical to those of experimental melts from metabasalt and eclogite (e.g. Rapp et al. 1999). Compared with the crustderived adakitic rocks, all Muli samples show metaluminous compositions (mostly A/CNK <1: Table 1), low K2O/Na2O ratios and Th, and elevated MgO and compatible elements (e.g. Ni and Cr), inconsistent with the lower crust-derived adakitic rocks as well as the experimental melts of metabasalt. All these geochemical features resemble Cenozoic slab-derived adakites from arc settings. Tectonically, the crust-derived adakitic rocks are generally associated with a collisional or intra-continental extensional setting (Wang et al. 2005, 2008). In view of the rock ages and field relationship, the adakite-like lavas occur slightly later than the basaltic enclaves. The basaltic enclaves, as discussed above, have affinity with high-Al basalt, which is an important lavas erupted in many intra-oceanic island arcs, rather than intraplate or collisional orogen. Besides, various volcanic lavas and intrusions in this belt are considered to be formed in arc setting, restricting its subduction time of 460-440 Ma. Besides, the isotopic compositions of Muli volcanic rocks, including bulk-rock SreNd isotopes and zircon Hf isotope, are significantly more depleted than those of intermediatefelsic intrusions from juvenile and/or old continental crust (Huang et al. 2016). In summary, the Muli adakite-like lavas are tectonically unlikely to be derived from lower crust. All the models arguing for the fractional crystallization require

Table 3 In-situ zircon LA-ICPMS U-Pb data for Muli volcanic lavas. Analysis spots

Concentrations(ppm) Pb

U

Th/ U Th

Isotopic ratios

Isotopic ages(Ma)

207Pb/ 206Pb


2¦O

207Pb/ 235U


2¦O

206Pb/ 238U


2¦O

207Pb/ 206Pb


2¦O

207Pb/ 235U


2¦O

206Pb/ 238U


2¦O

81 58 68 39 140 43 235 89 50 172 62 76 48 68 108 72 75 173 160 68 320 82 121 134 314 169 145 119

1.08 0.74 0.78 0.62 1.13 0.77 1.21 0.99 0.7 1.22 0.94 0.92 0.69 1.05 1.18 1.02 1.02 1.01 0.86 0.78 1.52 0.98 1.01 1.19 1.11 1.05 1.17 0.95

0.0563 0.0541 0.0542 0.0555 0.0554 0.0557 0.0552 0.0544 0.0536 0.0562 0.0547 0.0569 0.0553 0.0529 0.0568 0.055 0.0551 0.0558 0.0553 0.0558 0.0541 0.0553 0.0537 0.0542 0.0553 0.0569 0.0575 0.055

0.0029 0.0022 0.0025 0.0039 0.0027 0.0032 0.0019 0.0029 0.0035 0.002 0.0036 0.0035 0.0042 0.0035 0.0031 0.0035 0.0037 0.0027 0.0024 0.0034 0.0018 0.0038 0.0028 0.0021 0.0014 0.0023 0.0026 0.0026

0.554 0.528 0.546 0.561 0.542 0.569 0.544 0.526 0.542 0.542 0.548 0.565 0.538 0.525 0.551 0.542 0.536 0.554 0.552 0.542 0.52 0.542 0.525 0.535 0.543 0.566 0.564 0.563

0.031 0.023 0.025 0.036 0.026 0.036 0.018 0.025 0.035 0.017 0.029 0.029 0.038 0.034 0.03 0.034 0.034 0.025 0.025 0.031 0.017 0.034 0.028 0.018 0.015 0.022 0.024 0.025

0.0713 0.0709 0.0734 0.0733 0.07076 0.0741 0.07158 0.0716 0.0726 0.0703 0.0735 0.0726 0.0711 0.0723 0.0705 0.0719 0.071 0.0723 0.0725 0.0704 0.06987 0.0714 0.0706 0.0715 0.0714 0.072 0.0713 0.0741

0.0012 0.0013 0.0013 0.0017 0.00093 0.0018 0.00091 0.0017 0.0018 0.0011 0.0019 0.0016 0.0018 0.0015 0.0016 0.0016 0.0016 0.0013 0.0015 0.0015 0.00094 0.0014 0.0011 0.0014 0.001 0.001 0.0014 0.0014

420 368 370 380 390 400 402 350 340 437 350 430 350 270 470 360 360 410 394 390 353 370 320 357 410 458 480 380

120 97 110 150 100 130 74 120 150 81 140 130 160 140 130 140 140 110 97 130 73 140 120 86 57 89 100 110

446 429 441 454 438 454 440 428 437 439 442 452 434 426 443 437 433 446 445 437 424 437 426 434 440 454 453 452

20 15 17 22 17 23 12 17 24 11 20 19 25 23 20 22 22 16 17 21 12 23 19 12 10 14 16 17

444 442 457 456 441 461 446 446 452 440 457 452 443 450 439 448 442 450 451 438 435 445 440 445 445 448 444 461

7 8 8 10 6 11 6 11 11 6 11 10 11 9 10 10 10 8 9 9 6 8 7 8 6 6 9 8

Adakite-like sample (17ML-14) #14.01 30 107 #14.02 18 106 #14.03 30 126 #14.04 29 101 #14.05 36 122 #14.06 51 173 #14.07 12 66 #14.08 11 56 #14.09 13 70 #14.10 28 91 #14.11 7 45 #14.12 28 96 #14.13 30 109 #14.14 30 100 #14.15 18 81 #14.16 19 72 #14.17 41 126 #14.18 17 80 #14.19 63 184 #14.20 14 62 #14.21 42 139 #14.22 15 72 #14.23 13 59

116 70 125 110 149 223 48 46 54 104 32 112 128 126 79 84 171 63 260 57 162 57 49

1.05 0.63 0.96 1.06 1.2 1.22 0.72 0.81 0.75 1.12 0.68 1.14 1.13 1.16 0.93 1.13 1.18 0.78 1.38 0.91 1.14 0.78 0.82

0.0571 0.0534 0.0547 0.0557 0.0569 0.0581 0.0553 0.0502 0.0563 0.0553 0.0569 0.0552 0.056 0.0541 0.0586 0.0585 0.0592 0.0552 0.0551 0.0538 0.055 0.0544 0.0542

0.0032 0.0028 0.0024 0.0025 0.0029 0.0024 0.0041 0.0038 0.0037 0.0028 0.0052 0.003 0.0032 0.003 0.0035 0.0036 0.0039 0.0036 0.0023 0.0041 0.0028 0.0028 0.0038

0.556 0.522 0.531 0.545 0.563 0.567 0.535 0.503 0.563 0.528 0.551 0.545 0.528 0.538 0.572 0.561 0.583 0.553 0.532 0.532 0.541 0.528 0.538

0.03 0.026 0.021 0.026 0.027 0.023 0.038 0.036 0.035 0.025 0.048 0.029 0.029 0.029 0.032 0.037 0.039 0.034 0.022 0.04 0.026 0.028 0.038

0.0713 0.0714 0.0704 0.0721 0.072 0.0709 0.0706 0.0732 0.0731 0.0697 0.0708 0.0718 0.0687 0.0725 0.0712 0.0696 0.073 0.0732 0.0702 0.0721 0.0719 0.0707 0.0725

0.0013 0.0014 0.0011 0.0012 0.0011 0.00094 0.0015 0.0015 0.0022 0.0015 0.0018 0.0015 0.0013 0.0013 0.0014 0.0014 0.0021 0.0017 0.0012 0.0016 0.0013 0.0015 0.0017

450 310 372 409 450 505 350 160 410 390 390 380 400 340 500 490 540 360 388 300 370 380 360

120 110 98 99 110 91 150 160 140 110 190 120 130 120 130 140 140 140 93 160 110 120 140

447 425 432 444 452 455 432 411 450 429 440 440 428 435 457 449 476 444 432 429 438 429 434

19 17 14 15 17 15 25 24 23 17 31 20 20 19 21 24 29 22 14 27 17 19 25

444 444 438 449 448 442 440 455 454 434 441 447 428 451 443 434 454 455 437 449 448 440 451

8 9 6 7 7 6 9 9 13 9 11 9 8 8 8 9 12 10 7 9 8 9 10

L. Yang et al. / LITHOS 348-349 (2019) 105182

Adakite-like sample (17ML-03) #3.01 20 73 #3.02 15 77 #3.03 17 82 #3.04 10 61 #3.05 32 118 #3.06 11 54 #3.07 58 188 #3.08 22 86 #3.09 13 68 #3.10 39 136 #3.11 15 64 #3.12 20 74 #3.13 10 68 #3.14 18 62 #3.15 25 89 #3.16 17 68 #3.17 17 71 #3.18 39 167 #3.19 38 182 #3.20 16 84 #3.21 74 203 #3.22 20 81 #3.23 27 110 #3.24 32 110 #3.25 73 267 #3.26 41 152 #3.27 35 119 #3.28 30 120

9

(continued on next page)

6 15 19 9 6 10 10 10 10 11 7 35 40 22 456 481 447 465 447 445 460 460 454 461 460 1379 2755 850 6 15 19 9 6 17 10 10 10 11 7 22 20 26 456 481 447 465 447 447 460 460 454 461 460 1323 2580 866 20 32 22 15 13 17 13 15 19 25 15 34 29 41 459 473 450 452 451 446 456 460 456 440 458 1237 2459 912 0.07333 0.0774 0.0719 0.0749 0.0718 0.0768 0.074 0.074 0.073 0.0741 0.074 0.2387 0.5333 0.141 B/BA enclave (17ML-08) #8.01 37 #8.02 13 #8.03 22 #8.04 43 #8.05 51 #8.06 85 #8.07 66 #8.08 56 #8.09 32 #8.10 15 #8.11 44 #8.12 261 #8.13 73.8 #8.14 54.5

165 67 425 241 298 360 315 268 171 95 225 370 194 559

151 48 55 180 226 344 267 236 134 76 180 345 57 142

0.9 0.67 0.13 0.72 0.75 0.84 0.83 0.84 0.74 0.79 0.79 0.9 0.29 0.25

0.0568 0.0558 0.0567 0.0545 0.0566 0.055 0.0563 0.0565 0.0568 0.0539 0.0561 0.0819 0.1605 0.0694

0.0028 0.0051 0.0025 0.0023 0.0022 0.0022 0.0021 0.0023 0.003 0.0038 0.0025 0.0014 0.0028 0.0015

0.575 0.591 0.561 0.562 0.561 0.589 0.568 0.575 0.57 0.549 0.571 2.692 11.7 1.356

0.03 0.053 0.033 0.023 0.02 0.028 0.021 0.023 0.03 0.038 0.024 0.082 0.24 0.061

0.00095 0.0025 0.0031 0.0014 0.001 0.0014 0.0017 0.0017 0.0017 0.0019 0.0012 0.0068 0.0096 0.0039

443 451 448 433 437 456 452 25 18 23 23 14 21 20 434 460 445 462 438 432 434 160 110 130 140 75 130 120 360 490 410 570 428 290 320 0.0015 0.0013 0.0015 0.0011 0.0011 0.0017 0.0013 0.0712 0.0724 0.0719 0.0694 0.0702 0.0732 0.0727 0.038 0.029 0.035 0.036 0.022 0.033 0.03 0.538 0.576 0.554 0.581 0.541 0.534 0.537 0.0041 0.0029 0.0036 0.0038 0.0019 0.0032 0.003 66 125 106 81 158 77 112 13 30 23 11 46 20 21 #14.24 #14.25 #14.26 #14.27 #14.28 #14.29 #14.30

Th U Pb

52 132 86 49 204 85 83

0.78 1.03 0.8 0.6 1.28 1.08 0.75

0.0553 0.0579 0.0562 0.0607 0.0559 0.0531 0.0537

206Pb/ 238U
2¦O 207Pb/ 235U
2¦O 207Pb/ 206Pb 207Pb/ 206Pb


2¦O

207Pb/ 235U


2¦O

206Pb/ 238U


2¦O

Isotopic ages(Ma) Isotopic ratios Th/ U Concentrations(ppm) Analysis spots

Table 3 (continued )

9 8 9 7 7 10 8

L. Yang et al. / LITHOS 348-349 (2019) 105182


2¦O

10

tectonically voluminous basic associations derived from the metasomatized mantle wedge in arc settings, such as the Philippine arc (Castillo et al. 1999; Macpherson et al. 2006) and Ecuadorian Andes. In the case of Muli adakite-like lavas, the whole dataset for these lavas lacks a complete compositional spectrum of lavas from the normal arc basalts to high-Sr/Y adakitic rocks on account of fractional crystallization (Fig. 3e and f). Differentiation of an amphibole-garnet assemblage has been proposed as a mechanism to produce elevated Sr/Y and La/Yb signatures (Castillo et al. 1999; Macpherson et al. 2006). The removal of an amphibole-dominated assemblage under low pressure would produce concave-upwards REE patterns and lead to increasing Sr/Y and La/Yb, but decreasing Dy/Yb with increasing SiO2 (Castillo et al. 1999; Macpherson et al. 2006). Garnet fractionation under high pressure would yield a smoothly decreasing HREE pattern with elevated La/ Yb and Sr/Y as well as Dy/Yb in the evolved melts (Macpherson et al. 2006). As shown in Fig. 7a-c, the slightly decreasing Sr/Y, and the constant La/Yb and Dy/Yb with increasing SiO2 indicate that the effect of amphibole-garnet fractionation is insignificant. Besides, plagioclase fractionation would be suppressed during the early stage of the magmatic evolution owing to the high H2O content in hydrous basaltic magmas (Müntener et al. 2001). The Eu/Eu* of the Muli high-Sr/Y lavas, as illustrated in Fig. 7d, remains constant with decreasing Sr (as well as increasing SiO2; not shown), indicating that plagioclase is not a major phase controlling the fractional crystallization process; their weakly negative Eu anomaly may be governed by residual plagioclase in the source. In addition, basalticandesitic liquids may evolve to the high silica compositions with FeOt/MgO < 2 by fractional crystallization under highly oxidizing conditions. However, the evolved liquids commonly contain <1 wt% MgO at about 63e76 SiO2, and this is not the case for the Muli highSr/Y lavas which mostly contain >2 wt% MgO (Table 1). Accordingly, the elevated Sr/Y and La/Yb signatures of the Muli lavas are unlikely to have been produced by fractional crystallization processes. 6.1.2. Subduction-modified mantle source Slab melt contributions to arc volcanic rocks are generally associated with warm subduction zones (e.g. Cai et al. 2014; Yogodzinski et al. 1995). Typical high-magnesian andesites with slab melt contributions in the Aleutian Islands usually show depleted SreNd isotopic compositions inherited from their oceanic crust sources (Yogodzinski et al. 1995, 2015); this is not the case for global arcs. Arc volcanic rocks in hot subduction zone usually show enriched isotopic signatures involving melting of both subducted oceanic crust and overlying sediments (Cai et al. 2014; Tatsumi 2006). In particular, all the arc volcanic rocks in the North Qilian Orogen (Chen et al. 2012) and eastern section of the SQAB (Yang et al. 2019), including boninites, ankaramitic lava series and sanukites, exhibit evidence for the source contamination by recycled sediments(Fig. 9). The Muli volcanic rocks show radiogenic SreNd isotope signature, which can be commonly controlled by crustal contamination or magma mixing during magma ascent process, or source contamination by recycled components. Correlation variations between SreNd isotope and elements (e.g. Mg#, SiO2, Cr and Ni), generally resulting from crustal-level assimilation or magma mixing en route, are not visible from the studied rock (not shown), which indicate a homogeneously enriched source or due to longterm evolution of magma. Muli adakite-like lavas have large variable zircon Hf isotopic composition, reflecting that they are likely related to the crustal-level assimilation or magma mixing en route. The existence of old xenocrystal zircons is an important indicator to crustal-level assimilation. The absence of xenocrystal zircons in adakite-like lavas, along with the presence of a few “relic” zircons in the grain population of basaltic enclaves, lead us to conclude that the crustal-level assimilation can be ignored and basaltic enclaves

L. Yang et al. / LITHOS 348-349 (2019) 105182

11

Fig. 5. Concordia diagrams of zircon UePb isotope data and the representative zircon Cathodoluminescence (CL) images for Muli volcanic lavas. The yellow and blue circles on the CL images are the respective LA-ICPMS UePb dating and Hf isotope analysis sites. The green site numbers on CL images are the same as Tables 3 and 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

are likely derived from an enriched source that have experienced contamination by recycled sediments. This broadly occurs in Cenozoic continental basalts from eastern China, where the relict zircons were incorporated by deep mixing (Xu et al. 2018). Tectonically, these rocks are unlikely to lie on thick continental crust (Song et al. 2017; Yang et al. 2019), and so unlikely to have experienced extensive crustal contamination. Except the recycled sediments, the radiogenic SreNd isotope signature of Muli volcanic rocks may be partly inherited from the South Qilian Oceanic Crust (see Section 6.2 for details), where basalts broadly show E-MORB to OIB affinity with enriched SreNd isotopic compositions (Fu et al. 2018; Zhang et al. 2017). The addition of subducted slab-derived melts rather than aqueous fluids, as shown in Fig. 8, are required for the petrogenesis of the Muli sodic lavas, as well as the Lajishan sanukites (Yang et al. 2019). Binary mixing models of Sr-Nd-Hf isotope for Muli volcanic lavas restrict the amount of additional sediment melts in the mantle source to ~2e5%, while that for Lajishan sanukites (Yang et al. 2019) can be restricted to 5e10% (Fig. 9). 6.1.3. Muli basaltic enclaves It is difficult for primary slab melts to reach the surface without melt-mantle interactions (Rapp et al. 1999) and the peridotitic mantle is either directly or indirectly involved to different degrees (Castillo 2012). According to the experimental work of Rapp et al. (1999), primary slab melts would be fully consumed by peridotitic mantle at low melt/rock ratios (~1:1). Melting of mantle

peridotitic source metasomatized by primary slab melts would then generate low silica, high Mg primitive magmas (e.g. Martin et al. 2005). The field relations between Muli basaltic enclaves and adakite-like lavas, along with their zircon ages, indicates that the Muli basaltic enclaves were generated before the Muli adakitelike lavas, and are likely to be the primary magmas derived from the metasomatized mantle wedge. Early stage slab melts, in the above model, would be mostly consumed by the mantle wedge at low melt/rock ratios. Geochemically, the high and constant Mg# (62e68) reflect their primitive nature (Kelemen et al. 2014) and so differ from highly evolved adakite with Mg# commonly <60 (Defant and Drummond 1990). Additionally, the basaltic enclaves show minimal enrichment in aqueous fluid mobile elements (e.g. Ba/Th and U/La) but high MgO, FeOt, and compatible elements relative to the adakite-like lavas, demonstrating an overall lower slab contribution but higher mantle contribution. Furthermore, the basaltic enclaves resemble, but are not identical, to Nb-enriched arc basalt that originates from a mantle wedge enriched by slab melt (Defant et al. 1992; Defant and Drummond 1993). As illustrated above, the basaltic enclaves also share similar major elemental compositions with high-Al basalt/basaltic andesite (e.g. Crawford et al. 1987; Kuno 1960). The high-Al basalt/basaltic andesite is variable with diverse petrogenesis mainly concerning whether or not these rocks can represent primary magmas or plagioclasedominated, derivative magmas (Crawford et al. 1987). The basaltic enclaves are most close to the aphyric high-Al basalt described by Kuno (1960), which is representative of primary magma derived

Analysis Spots

Age

176Yb/177Hf


O
¡A2¦

12

Table 4 In-situ zircon Hf isotopic data for Muli volcanic lavas. 176Lu/177Hf


O
¡A2¦

176Hf/177Hf


O
¡A2¦

(176Hf/177Hf) i

¦ÅHf(t)


O
¡A1¦

t(Ma) 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 445 444 444 444 444 444 444 444 444 444 444 444 444 444 444 444 444 444 444


O
¡A1¦

(Ga) 0.02286 0.014653 0.020936 0.016965 0.036938 0.022056 0.059834 0.036329 0.016325 0.046632 0.023943 0.024924 0.012787 0.021594 0.028531 0.023305 0.02365 0.057506 0.028584 0.019064 0.059816 0.01645 0.027136 0.036783 0.044644 0.042883 0.03066 0.039357 0.025703 0.017682 0.033023 0.02456 0.044665 0.044233 0.014423 0.018806 0.023143 0.029209 0.017658 0.035732 0.036796 0.036743 0.028947 0.031635 0.043664 0.021601

0.00019 0.000162 0.000421 0.000215 0.000869 0.000352 0.000236 0.001155 0.000131 0.000601 0.000528 0.000708 0.000024 0.000492 0.000275 0.000117 0.000588 0.000656 0.000233 0.000248 0.001966 0.000157 0.000582 0.000238 0.000653 0.001659 0.000273 0.001114 0.000436 0.000435 0.000329 0.000589 0.000782 0.000699 0.000081 0.000192 0.00053 0.0001 0.000518 0.000241 0.001149 0.001751 0.000667 0.000179 0.001827 0.000571

0.000578 0.000379 0.000562 0.000443 0.000903 0.000554 0.001554 0.000888 0.000476 0.001159 0.000586 0.000669 0.000335 0.000568 0.000715 0.000603 0.000638 0.00161 0.000784 0.000492 0.001623 0.000428 0.000736 0.000955 0.001242 0.001136 0.000763 0.001067 0.00069 0.000491 0.000916 0.000657 0.001126 0.001121 0.000388 0.000501 0.000638 0.00076 0.000484 0.000966 0.000956 0.000991 0.000736 0.000789 0.001105 0.000624

0.000008 0.000006 0.000012 0.000001 0.000025 0.000003 0.000017 0.000018 0.000009 0.000021 0.000005 0.000013 0.000003 0.000015 0.000005 0.000003 0.000018 0.000008 0.000009 0.000004 0.000059 0.000002 0.000013 0.000001 0.000013 0.000031 0.000011 0.000023 0.000014 0.000008 0.000011 0.000012 0.000012 0.000012 0.000004 0.000007 0.000008 0.000007 0.000009 0.000003 0.000036 0.000036 0.00001 0.000009 0.000041 0.00001

0.282679 0.282571 0.282685 0.282638 0.282705 0.282595 0.282736 0.282705 0.282706 0.28267 0.282585 0.282663 0.282674 0.282692 0.282636 0.282621 0.282693 0.282789 0.282652 0.282596 0.282694 0.28263 0.282797 0.282717 0.282674 0.282757 0.283006 0.282657 0.282701 0.282714 0.282628 0.282658 0.28256 0.282666 0.282679 0.282703 0.28264 0.282637 0.282695 0.282567 0.28278 0.282657 0.282623 0.282682 0.282898 0.282709

0.000037 0.000034 0.000032 0.000035 0.000045 0.000046 0.000044 0.000042 0.000045 0.00005 0.000047 0.000047 0.000043 0.000043 0.000043 0.000041 0.000046 0.000052 0.000041 0.000034 0.000048 0.000041 0.000036 0.000044 0.000038 0.000043 0.000046 0.000043 0.000039 0.000032 0.00004 0.000037 0.000042 0.000044 0.000042 0.000043 0.000049 0.000041 0.000051 0.00005 0.000046 0.000052 0.000047 0.000049 0.00005 0.00005

0.282674 0.282568 0.28268 0.282635 0.282697 0.28259 0.282723 0.282698 0.282702 0.28266 0.28258 0.282657 0.282671 0.282687 0.28263 0.282616 0.282688 0.282776 0.282646 0.282592 0.282681 0.282626 0.282791 0.282709 0.282663 0.282747 0.282999 0.282649 0.282695 0.28271 0.28262 0.282653 0.28255 0.282656 0.282676 0.282699 0.282634 0.282631 0.282691 0.282559 0.282773 0.282649 0.282617 0.282675 0.282888 0.282704

6.3 2.6 6.5 4.9 7.2 3.4 8.1 7.2 7.3 5.8 3 5.7 6.2 6.8 4.8 4.3 6.8 9.9 5.3 3.4 6.6 4.6 10.5 7.6 6 8.9 17.8 5.4 7 7.6 4.4 5.5 1.9 5.7 6.4 7.2 4.9 4.8 6.9 2.2 9.8 5.4 4.3 6.4 13.9 7.4

1.29 1.18 1.13 1.22 1.56 1.6 1.55 1.47 1.59 1.74 1.63 1.64 1.5 1.5 1.49 1.43 1.6 1.83 1.45 1.17 1.67 1.44 1.26 1.53 1.33 1.52 1.6 1.52 1.36 1.13 1.39 1.31 1.47 1.56 1.47 1.51 1.71 1.42 1.77 1.74 1.61 1.81 1.63 1.73 1.76 1.76

0.8 0.95 0.8 0.86 0.77 0.92 0.74 0.77 0.76 0.83 0.93 0.83 0.8 0.79 0.87 0.89 0.78 0.67 0.85 0.92 0.8 0.87 0.64 0.76 0.83 0.71 0.35 0.84 0.78 0.75 0.88 0.83 0.98 0.83 0.8 0.77 0.86 0.87 0.78 0.97 0.67 0.84 0.89 0.8 0.5 0.76

T(DM2)


O
¡A1¦

fLu/Hf

0.05 0.05 0.04 0.05 0.06 0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.06 0.06 0.06 0.06 0.06 0.08 0.06 0.05 0.07 0.06 0.05 0.06 0.05 0.06 0.06 0.06 0.05 0.05 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.06 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07

-0.98 -0.99 -0.98 -0.99 -0.97 -0.98 -0.95 -0.97 -0.99 -0.97 -0.98 -0.98 -0.99 -0.98 -0.98 -0.98 -0.98 -0.95 -0.98 -0.99 -0.95 -0.99 -0.98 -0.97 -0.96 -0.97 -0.98 -0.97 -0.98 -0.99 -0.97 -0.98 -0.97 -0.97 -0.99 -0.98 -0.98 -0.98 -0.99 -0.97 -0.97 -0.97 -0.98 -0.98 -0.97 -0.98

(Ga) 0.05 0.05 0.04 0.05 0.06 0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.06 0.06 0.06 0.06 0.06 0.08 0.06 0.05 0.07 0.06 0.05 0.06 0.05 0.06 0.06 0.06 0.05 0.05 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.06 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07

1.02 1.26 1.01 1.11 0.97 1.21 0.91 0.97 0.96 1.05 1.23 1.06 1.03 0.99 1.12 1.15 0.99 0.79 1.09 1.21 1.01 1.13 0.76 0.94 1.05 0.86 0.29 1.08 0.98 0.94 1.14 1.07 1.3 1.06 1.02 0.97 1.11 1.12 0.99 1.28 0.8 1.08 1.15 1.02 0.54 0.96

L. Yang et al. / LITHOS 348-349 (2019) 105182

#3.01 #3.02 #3.03 #3.04 #3.05 #3.06 #3.07 #3.08 #3.09 #3.10 #3.11 #3.12 #3.13 #3.14 #3.15 #3.16 #3.17 #3.18 #3.19 #3.20 #3.21 #3.22 #3.23 #3.24 #3.25 #3.26 #3.27 #3.28 #14.01 #14.02 #14.03 #14.04 #14.05 #14.06 #14.07 #14.08 #14.09 #14.10 #14.11 #14.12 #14.13 #14.14 #14.15 #14.16 #14.17 #14.18

T(DM1)

 Et  Et € - 1)]/[(176Hf/177Hf)CHUR -(176Lu/177Hf)CHUR¡A(e¦ € 1)] -1}¡A10000 Notes: ¦ÅHf(t) were calculated by the formula:¦ÅHf(t) ¼ {[(176Hf/177Hf)S - (176Lu/177Hf)S¡A(e¦ where (176Lu/177Hf)CHUR ¼ 0.0332, (176Hf/177Hf)  € ¼ 1.865¡A10-11
de, 1997); ¦E CHUR ¼ 0.282772(Blichert-Toft and Albare year-1(Scherer et al., 2001). 2. Abbreviations: S, sample; CHUR, chondritic reservoir.

#14.19 #14.20 #14.21 #14.22 #14.23 #14.24 #14.25 #14.26 #14.27 #14.28 #14.29 #14.30

444 444 444 444 444 444 444 444 444 444 444 444

0.069994 0.020755 0.040633 0.020206 0.017005 0.016666 0.029282 0.026775 0.012647 0.048561 0.022771 0.016186

0.000307 0.000153 0.000501 0.000046 0.000549 0.000058 0.000707 0.000723 0.00007 0.000385 0.000117 0.000076

0.001761 0.000528 0.001024 0.000578 0.000485 0.000431 0.000817 0.000721 0.000345 0.001221 0.000586 0.000423

0.000019 0.000002 0.000019 0.000005 0.00001 0.000005 0.000022 0.000014 0.000002 0.000019 0.000003 0.000004

0.282605 0.282677 0.282629 0.28269 0.282726 0.282588 0.282667 0.282629 0.282532 0.282688 0.282633 0.282635

0.000051 0.000044 0.000044 0.000043 0.000048 0.000041 0.000053 0.000041 0.000041 0.000042 0.000044 0.000041

0.28259 0.282672 0.28262 0.282685 0.282722 0.282584 0.28266 0.282623 0.282529 0.282678 0.282628 0.282631

3.3 6.2 4.4 6.7 8 3.1 5.8 4.5 1.2 6.4 4.7 4.8

1.78 1.53 1.53 1.52 1.68 1.42 1.84 1.45 1.45 1.46 1.55 1.44

0.94 0.81 0.88 0.79 0.74 0.93 0.83 0.88 1 0.8 0.87 0.86

0.07 0.06 0.06 0.06 0.07 0.06 0.07 0.06 0.06 0.06 0.06 0.06

1.21 1.03 1.14 1 0.91 1.22 1.05 1.14 1.35 1.01 1.13 1.12

0.07 0.06 0.06 0.06 0.07 0.06 0.07 0.06 0.06 0.06 0.06 0.06

-0.95 -0.98 -0.97 -0.98 -0.99 -0.99 -0.98 -0.98 -0.99 -0.96 -0.98 -0.99

L. Yang et al. / LITHOS 348-349 (2019) 105182

13

Fig. 6. Zircon εHf (t) vs. UePb age (t) diagram. Data of Lajishan sanukites for comparison are from Yang et al. (2019).

from mantle peridotite. The basaltic enclaves thus are suggested to be primary partial melts of metasomatized mantle wedge (Fig. 10), consistent with the petrogenesis for the low-Si adakites (e.g. Martin et al. 2005; Rapp et al. 1999). 6.1.4. Muli adakite-like lavas In contrast, as the amount of slab melts increases and melt/rock ratios rise as subduction proceeds, slab melts are unlikely to be fully consumed and would produce Mg-rich, high-silica liquids through melt-mantle reaction (Rapp et al. 1999). This means that the following stage slab melts cannot be consumed by the mantle wedge which have been saturated by the early stage slab melts. Thus, most magnesian andesites are actually hybrid magmas that have been produced by open system processes and present evidence for magma mixing between mafic and silicic magmas (e.g. Castillo 2012; Danyushevsky et al. 2008; Streck et al. 2007). The Muli adakite-like lavas exhibit large compositional ranges, low SiO2 and elevated Mg#, Cr and Ni compared to the experimental metabasalt melts (SiO2 > 65 wt%; MgO <1.5 wt%; Martin et al. 2005). The green amphibole phenocrysts from Muli adakite-like lavas have oscillatory or banded zoning (Fig. 2i), presenting direct evidence for magma evolution. The adakite-like lavas, along with basaltic enclaves, also form curved trends on compatible elements (e.g. Cr, Ni, Co and V) verse MgO (Mg#) plots (not shown). These lead us to conclude that moderate degrees of mixing between the following stage slab melts and the primary magmas (basaltic enclaves) can be responsible for the petrogenesis of the Muli adakitelike lavas (Fig. 10). It should be noticed that both the early stage and the following stage slab melts, as demonstrated in Section 6.2, are composite melts with basaltic crust melts and sediment melts. Accordingly, we adopt the scenario that the Muli basaltic enclaves as well as Lajishan sanukites (Yang et al. 2019) are representative of primary magmas derived from a mantle wedge metasomatized by silicic melts, whereas the Muli adakite-like lavas are hybrid magmas, produced by mixing between the late stage slab melts and primary mantle-derived magmas (Fig. 10). If this interpretation is correct, the scenario for the petrogenesis of these lavas should be supported by other chemical evidence, which we explore in the next section. 6.2. Characterizing the end members The above discussion makes the case for contributions from

Fig. 7. (a) Sr/Y vs. SiO2 diagram; (b) La/Yb vs. SiO2 diagram; (c) Dy/Yb vs. SiO2 diagram; and (d) Eu* vs. SiO2 diagram. Trends in (c) are from Macpherson et al. (2006).

Fig. 8. (a) Th/Yb vs. Nb/Yb diagram (Pearce 2008); (b) U/Th vs. chondrite-normalized La/Sm. Data for Lajishan Boninite, Lajishan Ankaramite series and Lajishan Sanukites are all from Yang et al. (2019).

composite melts to the Muli volcanic lavas and sediment melts to Lajishan sanukites (Yang et al. 2019). Here we model the petrogenesis of these rocks. The complexities of both the melting regime and the mechanisms of melt extraction have significant influence on trace element abundances in model compositions but are poorly revealed for subduction systems (e.g. Kelemen et al. 1995). The goal of this approach is to constrain the endmember components in the formation of these arc magmas, and thus it does not address the complexities of subarc mantle magmatism, such as pyroxenite formation process by interactions between silicic melts and peridotitic rocks (e.g. Yogodzinski et al. 2015). This approach relies on bulk partitioning data acquired in recent experimental work to produce reasonable compositions. The determination of the eclogitic oceanic melt models is based on the bulk partitioning data of Kessel et al. (2005) and South Qilian oceanic crust data from Fu et al. (2018). Bulk partitioning data for the amphibolitic oceanic melt models is based on partition coefficients and complementary restites of amphibolite dehydration-melting experiments at pressures of 5e15 kbar (Zhang et al. 2013 and references therein). The calculation of sediment melt models is done by applying enrichment factors estimated by Yogodzinski et al. (2015) to the interlayered sedimentary rocks from the SQAB (Yang et al. 2019). Sr isotopic data are not used for modelling, because of elevated values in some samples, attributed to the seawater alteration and/or hydrothermal fluids. The key relationships have been shown in figures of Nd isotopic compositions plotted against trace element ratios that can be strongly fractionated by metamorphic minerals (e.g. garnet and rutile) of amphibolite and/or eclogite facies (Fig. 11). All the parameters and melt models are listed in Table S1. Sediments are major potential contributors of incompatible elements to the subarc mantle (e.g. Plank, 2014; Chauvel et al. 2008). In this study, source contamination by sediment melts rather than crustal-level assimilation, as mentioned in Section 6.1.2, is considered. Low εNd(t), along with high La/Sm and Th/Nd, and low Ta/Th and Lu/Hf, can also be used as indicators for contributions of

L. Yang et al. / LITHOS 348-349 (2019) 105182

Fig. 9. Sr-Nd-Hf isotopic compositions. Binary mixing parameters are after Yang et al. 2019 except for the average Nd composition (εNd (t) ¼5.47) of South Qilian Oceanic Crust calculated based on the oceanic crust data of Fu et al. 2018. Data sources: Tonga: Falloon et al. 2008; Fiji: Danyushevsky et al. 2008; Bonin: Li et al., 2013a, b; Aoyougou: Chen et al. 2012. Data for South Qilian oceanic crust are from Zhang et al. (2017) and Fu et al. (2018). Data for Lajishan Boninite, Lajishan Ankaramitic series and Lajishan Sanukite are from Yang et al. (2019). MW: mantle wedge; AOC: altered oceanic crust; TS: terrigenous sediments; PS: Pelagic clay sediments.

15

sediment melts (e.g. Cai et al. 2014; Yogodzinski et al. 2015). Fig. 11 shows that the basaltic enclaves have radiogenic Nd isotopic compositions, high ratios of La/Sm and Th/Nd, and low ratios of Ta/ Th and Lu/Hf relative to the South Qilian Oceanic Crust (Fu et al. 2018; Zhang et al. 2017). In these graphs, binary mixing lines between the mantle wedge and the average sediment melts pass through the main cluster of basaltic enclaves and Lajishan sanukites (Yang et al. 2019), indicating that the addition of sediment melts to the mantle wedge is likely (Fig. 11); this is consistent with the scenario whereby they are derived from the metasomatized mantle. In contrast, the binary mixing lines from the mantle to the sediment (melts) fail to encompass the field of adakite-like lavas (Fig. 11), which may indicate a role for a third end-member. We suspect that the third end-member is a partial melt of subducted basaltic crust in the amphibolite or eclogite facies. The amphibolite or eclogite melt models, as mentioned above, are calculated based on the South Qilian Oceanic Crust data (Fu et al. 2018; Zhang et al. 2017), and have uniform εNd(t) values but strongly fractionated trace element ratios (Fig. 11). The Muli adakite-like lavas show higher εNd (t) values coupled with elevated La/Sm, Th/Nd and Nb/Y, and decreased Ta/Th and Lu/Hf relative to the basaltic enclaves (Fig. 11), thus forming rough trends from the basaltic enclaves towards amphibolite and/or eclogite melt. Therefore, we argue that the addition of the less-radiogenic slab melts can account for the petrogenesis of the Muli adakite-like lavas, consistent with the above scenario whereby the adakitelike lavas represent hybrid magmas mixed by the slab melts and the primitive magmas (basaltic enclaves). Trends formed by the basaltic enclaves and adakite-like lavas are not unambiguously towards the amphibolite and/or eclogite melts, and this can be attributed to the evolution of composite melts from these with basaltic crust melt and sediment melt sources (Fig. 11). The slightly high εNd (t) of adakite-like lavas relative to those of basaltic enclaves indicate that the following stage less-radiogenic composite melts have elevated proportion of basaltic crust melts versus sediment melts compared to the early stage composite melts. Interestingly, the Muli adakite-like lavas also define similar trends with the Lajishan ankaramitic lava series (Fig. 11), which originated from a pyroxenite-bearing enriched mantle source, metasomatized either by slab-derived silicic melts during subduction or OIB-type melts prior to subduction (Yang et al. 2019). 6.3. Geodynamic implication

Fig. 10. Schematic petrogenetic model for Muli volcanic lavas.

The occurrence of slab melting relies on a sufficient heat source at a subduction zone with special thermal structure (e.g., Syracuse et al. 2010), such as the subduction of young oceanic crust (e.g., Defant et al. 1992; Defant and Drummond 1990; Tatsumi 2006) and fast/flat subduction (e.g. Gutscher et al. 2000). In addition, slab edges heated by upwelling hot asthenospheric mantle is another geodynamic mechanism (e.g., Gutscher et al. 2000; Yogodzinski et al. 1995). Either one of the above mechanisms or both seems responsible for the slab melting in the SQAB. Firstly, given that ophiolite fragment in SQAB gave zircon ages of ~525e490 Ma (QBGS, Qinghai Bureau of Geological Survey, 2014; Zhang et al. 2017; Fu et al. 2018) and the island arc volcanic rocks yielded zircon ages of 460e440 Ma, the subducted South Qilian Oceanic Crust would have been young and hot oceanic crust (<45 Ma). Secondly, Song et al. (2017) proposed that the initiation of South Qilian intra-oceanic system can be attributed to the collision between the buoyant LYOP and the CQB, akin to the “Oceanic Plateau model” (Niu et al. 2003). If this model is right, the subduction of young South Qilian Oceanic Crust by forced convergence is feasible and the newly-formed subduction system would then extend laterally from the edge of the LYOP to the adjacent oceanic lithosphere. In this context, the South Qilian Oceanic Crust became the

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Fig. 11. Nd isotope variations (εNd(t)) vs. (a) La/Sm; (b) Th/Nd; (c) Ta/Th; (d) Lu/Hf. The eclogite melts and amphibolite melts are model melts calculated based on South Qilian Oceanic Crust data from Zhang et al. (2017) and Fu et al. (2018). The bulk partitioning data for eclogite melts are given by Kessel et al. (2005) and the bulk partitioning data for amphibolite melts are based on the partition coefficients and complementary restites of amphibolite dehydration-melting experiments at pressures of 5e15 kbar (Zhang et al. 2013 and references therein). The sediment melts are model compositions based on the enrichment factors estimated by Yogodzinski et al. (2015) and SQAB sediment data from Yang et al. (2019). The white star represents the assumed following stage less-radiogenic composite melts. Data for Lajishan Boninite, Lajishan Ankaramitic series and Lajishan Sanukite are from Yang et al. (2019).The model compositions used here are in Appendix table S1.

extension-induced slab edge and began to subside. The asthenosphere would then upwell towards the fore-arc region directly above the top of the shallow dipping slab, leading to slab melting. Such a generic model was proposed by Gutscher et al. (2000), where moderately old (10e45 Ma) subducting slab melt at the early stages of flat subduction, when the leading edge of the slab is heated by ambient mantle.

mechanisms responsible for the unusually hot thermal structure are associated with either or both of the juvenile nature of the South Qilian Oceanic Crust or/and the heating of a slab edge by upwelling hot asthenospheric mantle during the initiation of subduction.

7. Conclusions

The authors thank H. Y. Zhang for help with bulk-rock analyses, G.B. Zhang and W. P. Zhu for help with SreNd isotope separations and analyses, Y. G. Li and M. Q. Jin for help with zircon LA-ICP-MS UePb dating and zircon Hf isotope analyses. The authors also thank the two anonymous reviewers for their helpful and constructive review comments that significantly improve the manuscript. This study was supported by the National Natural Science Foundation of China (Grant 41572040), the Major State Basic Research Development Program (2015CB856105) and the Geological Investigation Project of the China Geological Survey (Grant 12120114079701).

The Muli volcanic lavas, consisting of adakite-like lavas and basaltic enclaves, have geochemical characteristics of subducted slab melts. The basaltic enclaves, zircon UePb ages of ~455 Ma, represent primitive magmas derived from mantle wedge metasomatized by early stage radiogenic composite (mostly sediment) melts. The adakite-like lavas, with zircon UePb ages of ~445e444 Ma, represent the hybrid magmas mixed by the primitive magmas and the following stage less-radiogenic composite melts with elevated basaltic crust melt component. These sodic volcanic arc rocks are likely occur in a late Ordovician intra-oceanic island arc system with unusually hot thermal structure, which correlates with a plateau-continent collision and subsequent lateral extension. Geodynamic

Acknowledgements

Appendix A. Supplementary data Supplementary data to this article can be found online at

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